In this article, we propose a novel, model-independent, unified detection strategy based on disagreement Laplacian potential for effective identification of cyber anomalies in interconnected autonomous direct current (dc) microgrid (MG) clusters. This interconnection enables inter and intra-microgrid power exchange between them, thereby rendering efficient and maximum utilisation of the distributed energy resources. However, these clusters are vulnerable to cyber intrusion that disrupt the power sharing schemes. As such, we have investigated the effect of false data injections at various vulnerable points, including both voltage and current sensors in primary, and data transmission points in the distributed secondary, local, and global tertiary layers. Simultaneously, a logical evaluation in the form of data-integrity indices are formulated from the outcomes of the proposed detection scheme to provide for their timely mitigation. Ultimately a fault-ride through operation of the converters is achieved instead of alienating the misbehaving component, unlike previously reported works.
The perturb & observe (P&O) algorithm is very popular for maximum power point tracking (MPPT) for solar photovoltaic (PV) systems. However, it has tracking problems during varying irradiations as well as the nuisance of oscillations around the maximum power point (MPP). This work introduces a circle center-line concept based P&O (CCCP&O) algorithm for MPPT, where, the concept of circle and its center are combined with the P&O algorithm. This algorithm tends to reduce the number of iterations taken to reach the MPP, which reduces settling time. Moreover, the problem of large oscillations around the MPP is eliminated by using the concept of flexible step size. The algorithm initializes with standard P&O, but utilizes a approach of diameter equivalence of a circle as a procedure to reach next operating point on the power-voltage plot. Therefore, the iterations required to get to the MPP are reduced substantially. The MPP changes with change in solar irradiance, therefore, a concept of artificial envelope around the P-V curve is used to improve tracking of the algorithm during varying irradiances. The overall performance of the algorithm is demonstrated and compared in simulation using SIMULINK MATLAB as well as also shown experimentally in a developed hardware prototype.
This article deals with a dual second-order generalized integrator phase-locked loop (DSOGI-PLL) with in-loop filter-based control approach for a single-stage, three-phase three-wire solar-grid-interfaced system under abnormal grid voltage conditions, unbalanced load conditions, and varying solar insolation levels. The DSOGI with in-loop filter algorithm with enhanced filtering capability, employed for both voltages and currents, helps to attenuate the harmonics, eliminates dc offset, and estimates the sequence components. This algorithm elicits the fundamental component of highly nonlinear load current required for calculating the reference magnitude of the grid currents. Even during unbalanced load conditions, these fundamental components of currents are free from the dc offset and dominant harmonics of double frequency. In order to maintain the sinusoidal and balanced grid currents, the positive sequence voltages are estimated to get the accurate unit templates during the unbalanced and abnormal grid voltages conditions. Moreover, these positive sequence voltages are used by PLL to compute the phase required for the magnitude and the angle calculation of the currents and voltages. The dc-link voltage is maintained at maximum power point by an incremental conductance based technique. This system is simulated in MATLAB/Simulink environment. Test results on a developed laboratory prototype are observed in accordance with the standard of the IEEE-519.
This paper proposes a multifunctional grid-tied solar photovoltaic (PV) system with a multifeatured adaptive control using a two-leg voltage source converter (VSC). This topology provides multifunctions such as injection of real power output from a PV array and improvements of power quality. It provides reactive power compensation; hence, unity power factor (UPF) is realized at the grid. Moreover, it performs various functions such as balancing of grid currents, harmonics mitigation and compensation of neutral current. The maximum power point tracking (MPPT) algorithm is used for harvesting the peak PV array power. The proposed topology is simulated using MATLAB/SIMULINK toolbox. Simulated responses are validated through experimental results.
The conventional over/under voltage, over/under frequency based anti-islanding protection scheme presents significant nondetection zone (NDZ) under critical loading conditions of distribution networks. To overcome this challenge, a unique hybrid technique has been proposed in this article for the anti-islanding protection of distributed generators (DGs). The algorithm requires the injection of an active oscillatory disturbance signal of very small magnitude through the current control loop along the direct axis of the synchronously rotating reference frame of the converter. Small signal stability analysis of the system is carried out to analyze the effect of such active signal injection having different frequencies. The anti-islanding protection algorithm first involves the superimposition of d-axis voltage. Thereafter, two novel indexes are proposed based on which the trip signal logic is developed for the protection scheme. The methodology has been found to detect an unintentional islanding scenario within 90 ms from the initiation instant. The efficacy of the proposed hybrid anti-islanding protection scheme is tested under various abnormal operating conditions by performing simulations on the CIGRE LV test system. Experimental validation of the proposed methodology has also been carried out in Controller Hardware-in-the-Loop (CHIL) platform using Typhoon HIL 602+ and Speedgoat baseline real-time target machine.
This letter presents a theoretical approach to investigate the cause of marginal stability/instability of an islanded system following unintentional islanding event when DGs are enabled with P(f) and Q(V) regulation. Non-detection zone being a traditional cause as per previous investigation, a small-signal model of the standard UL 1741 anti-islanding test system is developed in this work under islanded condition to illustrate the problem of undamped oscillations. Dynamic model validation has also been performed to verify the correctness of the developed linearized state space model. The eigenvalue analysis of the model successfully illustrates the reason behind sustained oscillations of the system under loss of mains scenario. Finally, timedomain simulations are performed in PSCAD/EMTDC platform to demonstrate the effect of such instability/marginal stability of power island.
Synchronverter (SV) control has emerged as a popular method for distributed energy resources (DERs), to emulate response of a synchronous generator. In this work, a simple gradient descent based pre‐synchronization control for SV scheme is proposed that varies the reference frequency in SV control alone. Thus, local load connection to DER can remain intact during synchronization with proposed pre‐synchronization method, unlike virtual current based methods. Normally, phase‐locked loop used for synchronization purpose, uses a first order loop filter such as a PI controller. In the proposed pre‐synchronization control, the inherent low pass filter of SV scheme itself is used as a loop filter. Transient response analysis is presented in this work, based on small signal transfer functions derived from the proposed method. From the theoretical analysis of proposed pre‐synchronisation control, design of the parameters is presented. Thus, there is no trial and error basis for parameters tuning in the proposed method, as compared to virtual current based methods. Validation of proposed pre‐synchronisation control through experiments are presented for all initial conditions. The transient response analysis, effectiveness of proposed method during local load changes and grid integration are verified by experimental results.
Standalone DC microgrids (SDCMGs) are emerging as prominent solutions to remote customers. As these SDCMGs mainly depend on renewable energy sources to curb carbon emission, reliability of the system is lessened due to their intermittent nature. The battery could offer a solution to this problem as it is inherently enclosed by DC grid for power balancing, but the purpose may not be served completely due to high capital cost and maintenance. Thus, the diesel generator (DG) becomes a major alternative to overcome this issue because of low investment, high compatibility, and flexibility. However, DG inhibits with cranking delay, sluggish response and low fuel efficiency under frequent switching and variable loading scenarios. To suppress the issues, a new power management strategy (PMS) is developed to ensure proper coordination between different sources, storage devices and loads in SDCMG. In addition to this, an effective control scheme is also proposed to achieve seamless regulation of DC bus voltage even under extreme conditions. In this study, different SDCMG configuration is considered for testing and simulation of system is carried out in real‐time digital simulator to prove their viability. A scaled prototype is developed to validate simulation results and authenticate proposed PMS and control scheme.
This paper introduces a cascaded packed U cell (CPUC) multilevel converter (MLC) to achieve a higher-level count in converter voltage with a minimum number of switches. Here, two five-level PUC topologies are connected in a cascaded manner to obtain twenty-five levels in its output converter voltage. The switch count in CPUC is reduced to 12, as compared to the number of semiconductor devices used for obtaining 25 levels in converter output. A binary-quintuple progression is used for selection of voltage ratios between DC voltage sources, and capacitors. CPUC is operated at low-frequency switching, using the nearest level modulation technique (NLMT). The fundamental switching frequency ensures reduced switching losses as compared to pulse-width modulation (PWM) schemes. The converter performance is analyzed for grid-tied and standalone applications. The performance parameters such as total harmonic distortion (THD) of converter voltage and THD of grid/load current are examined. The CPUC configuration is modeled and simulated in MATLAB/Simulink, and test results are taken using OPAL-RT test bench. The acquired simulation and test results confirm viability, practicability, acceptability, and cost-effectiveness of CPUC-MLI converter over existing MLC topologies for efficient power conversion.
With the integration of intermittent renewable energy sources in the distribution network, the number of network reconfiguration events has increased significantly. In a medium voltage distribution network, most of the critical circuit breakers' (CBs') statuses are monitored by remote terminal units (RTUs). However, in many cases, some of the CBs' status may not be correctly updated by the supervisory control and data acquisition (SCADA) system because of communication failure and data packet loss issues. Thus, the distribution system operator (DSO) cannot solely rely on CB status provided by the SCADA for topology detection. This paper proposes a data-driven topology tracking algorithm for active distribution networks. The topology of the distribution network is represented with a time-varying connectivity matrix. The changes in network topology are detected by estimating the elements of the bus connectivity matrix using voltage phase angle measurements provided by a sparse set of micro phasor measurement units (micro-PMUs). The algorithm extracts information from previous network topology using an l1 norm regularization on the difference of consecutive connectivity matrices. The changes in topology can be detected by observing a few micro-PMU phasor samples. The algorithm is tested on IEEE benchmark test feeders with real load profiles.
Currently, in the literature, the slow responding device like on-load tap changer transformer (OLTC) is better operated by introducing the time delay in the operation of fast-acting converters. Although the literature also indicates an improvement in the transient condition, it keeps the converters idle during the waiting period. Henceforth, there is a lot of scope for improving the reactive power reserve. In this article, we propose a coordinated voltage control (CVC) scheme that employs a unique approach where the converters are allowed to absorb reactive power (of a specific voltage range from the grid) during the waiting period. A modified IEEE 33-bus distribution system is modeled in the real-time digital simulator platform and the simulation results are compared with the existing and without CVC schemes. The simulation results justify the effectiveness of the proposed CVC scheme in terms of improving the reactive power reserve and voltage profile up to 100%/164% and 0.0093 pu/0.014 pu against existing/without CVC scheme, respectively. Furthermore, the postfault voltage recovery time in the case of proposed CVC is decreased by 20 ms/28 ms with respect to the existing/without CVC schemes. These results imply that the proposed CVC scheme performs better in all the operating conditions of the grid.
This article addresses a voltage control and energy management strategy of active distribution systems with a grid-connected dc microgrid as well as for an islanded dc microgrid with hybrid energy resources. In the islanded mode, a control and management strategy using a backup diesel generator (DG), a renewable energy source (RES), and an energy storage system plays a vital role in maintaining the microgrid bus voltages within the limits. However, operating backup diesel generator (DG) has its own challenges including startup delay, frequent switching, and uneven loading when operated along with a RES. Additionally, fuel efficiency and emission characteristics vary with loading since most of DGs are driven by constant-speed diesel engines. Hence, an exhaustive power management scheme (PMS) is proposed by utilizing the hybrid energy storage system. Real-time simulation and experimental validation of the proposed scheme are provided using a real-time digital simulator (RTDS) and a laboratory-scale prototype, respectively. Extreme scenarios including DG failure/scheduled maintenance, low power generation, and battery charge are analyzed in the islanded mode. Furthermore, a dc microgrid is connected to an IEEE active distribution system feeder to analyze control and management challenges for the grid connected mode with contribution from a microgrid and with no contribution from a microgrid. These scenarios resemble more realistic unbalanced utility grid conditions. A centralized optimization problem is formulated at an advanced distribution management system level to maintain all the node voltages within limits in the IEEE test system. RTDS is used to simulate dc microgrid connected with the IEEE test system and an optimization algorithm is implemented in MATLAB. Superior performance of the developed algorithms are demonstrated and validated for coordination between centralized optimization at ADMS and the microgrid energy management system.
A photovoltaic (PV) system for reliable interconnection of a PV array to the grid is presented in this article. A boost converter ensures operation of the PV array at its maximum power point, and it also enables the adaptive adjustment of the dc link voltage of the interfacing voltage source converter according to the variations in the grid voltages. This prevents system tripping during the voltage sag, and helps to maintain the grid current power quality at voltage swell conditions. Even in the weak grid scenarios of unbalanced or distorted voltages, the grid currents are maintained distortion free and balanced. Furthermore, the system eliminates the dc offset in the sensed grid voltages, and ensures absence of dc offset in the grid currents. The fast dynamic response of the system to sudden load variations is obtained by employing a total least squares-based control technique, which swiftly extracts the fundamental active weights from the distorted load currents. Moreover, the PV system performs grid power quality conditioning, such as neutral current mitigation, harmonics reduction, power factor correction, and balancing of grid currents, even under the absence of PV power, such as during nights. The system performance is validated by test results at unbalanced load currents, unbalanced grid voltages, distorted grid voltages, grid voltage sag and swell, PV power variations, and dc offset in the sensed grid voltage.
An efficient control of a grid integrated microgrid with wind energy generating system (WEGS) and a battery storage, is implemented in this work. The quality of power injected to the utility is deteriorated with the level of unbalance and/or distortions present in the grid voltages. To compensate for the abnormalities, such as unbalance and distortions in the grid voltages, positive sequence components (PSCs) of grid voltages are extracted from their filtered αβ components. This work implements hybrid generalized integrator (HGI) filters for filtering the αβ components of grid voltages and also provides its 90 ∘ delayed signals, which are useful in generating PSCs. Phase and frequency of the grid currents are decided by the unit active templates derived from the PSCs of the grid voltages, therefore, high-quality grid currents are injected into the grid even for abnormal grid conditions. For faster dynamics during variations in load power, HGI filters are used for quick and accurate estimation of fundamental components of load currents. Intermittent power generation from WEGS is compensated for by integrating a battery bank into the system. Along with high-quality power injection, the grid side converter operates the microgrid in off-grid mode (during grid outages) and provides smooth grid integration once the grid reappears. A 3.7 kW speed sensorless synchronous reluctance generator is used in WEGS, which is controlled by the machine side converter. The control techniques are validated in MATLAB /Simulink platform, and experimental validation is done on a prototype developed in the laboratory.
Autonomous microgrids supply power to large remote areas, where access to the grid is infeasible. The generation of these microgrids is highly dominated by renewable energy sources equipped with a storage battery. Due to the uncertainty associated with the renewables, the sustainability and reliability of supply become the prime areas of focus. The battery reduces the uncertainty during demand rise, power deficiency, and overloading with their faster response than diesel generators and microturbines. In this article, the power generation of the isolated microgrid is considered from solar and wind energy sources along with a battery. As the load decreases/increases suddenly, the point of common coupling (PCC) voltages are disturbed. So, as a remedy, a new control strategy for enhancing the voltage profile at PCC is introduced here. The improved adjustable step continuous norm adaptive control strategy filters the constituent of the load current, providing enhanced power quality and assurance of the system reliability in feeding sensitive loads. The intelligently controlled proportional integral controller enhances the speed regulation of the synchronous generator, whereas the enhanced perturb and observe technique provides the effective wind maximum power point (MPP) tracking. The load side power converter regulates the dc-link voltage and the boost converter allows the extraction of solar MPP. The batteries are deployed as the residential storage systems for backing up the power firming renewable production. Moreover, it optimizes the power flow and operation of the microgrid. It provides the required power based on fluctuating load demand. The performance improvement obtained on a laboratory developed test setup verifies the microgrid effectiveness under renewable generation variability and load alterations.
Autonomous microgrids supply power to large remote areas where access to the grid is infeasible. The generation of these microgrids is highly dominated by renewable energy resources (RESs) equipped with a storage battery. Due to the uncertainty associated with RESs, the sustainability and reliability of supply become the prime areas of focus. The battery reduces the uncertainty during demand rise, power deficiency, and overloading with their faster response than diesel generators and microturbines. In this article, the power generation of the isolated microgrid is considered from the wind energy source along with a battery. For the favorable speed regulation of the wind turbine driven generator, the traditional proportional integral (PI) controller is replaced by natural genetics [genetic algorithm (GA)] and evolution inspired PI controller. The issues identified by the use of a fixed gain PI controller such as unsuitable regulation due to incorrect tuning and discontinuous tuning are addressed by the use of a GA tuned speed controller. The performance of the PI controller degrades while dealing with the ac quantities, so in terms of dealing with sinusoidal signals, here an adaptive proportional resonant controller with a combined generalized integrator based phase locked loop is utilized. It tracks the reference ac voltage and provides frequency adaptation in the presence of harmonics pollution with zero steady state error. The batteries deployed through a bidirectional converter, act as the residential storage for backing up the power firming renewable production. The maximum power point (MPP) extraction of the wind generator is carried by a perturb and observe based MPP technique. The performance improvement obtained from the laboratory developed test setup verifies the microgrid effectiveness under renewable generation variability and load alterations.
Owing to the rising power demand, depleting conventional energy sources and recent advancement in incorporating deeper injections of renewable energy resources into the grid, the existing distribution system will have to take into account DC injections/withdrawals, thus giving rise to AC-DC distribution system. The load-flow in the aforementioned systems is an exigent task because of the presence of power converters. This work presents a novel and computationally efficient load-flow algorithm for AC-DC radial distribution network utilizing the notion of graph-theory with matrix-algebra. The remarkable trait of the proposed methodology lies in the formulation of path impedance matrix, loads beyond branch matrix, path drop matrix, slack bus to other buses drop matrix and load flow matrix which will remain unaffected for the entire load-flow operation. The per-unit equivalent model of power converters has been developed for solving load-flow equations in per-unit. Various models of distributed generations are also incorporated in the proposed load-flow study. The developed method is capable of addressing the aforementioned modeling challenges. The proposed technique has been tested on several AC-DC test networks that include different operating modes of power converters and various models of DGs, which proves feasibility and legitimacy of the proposed technique.
Penetration of distributed generations (DGs) enable microgrid to operate in different mode such as grid-connected and islanded mode with radial and mesh topology. The fault current magnitude and direction varies according to the operating conditions which imposes serious protection challenges and may lead to protection system failure and nonselective relay operation. Thus, the impact of operating conditions on the existing overcurrent relay coordination has been extensively studied in this paper. The overcurrent relay coordination is implemented using genetic algorithm (GA) considering different topology and operating modes of the microgrid. Extensive test results indicate that the existing overcurrent relay coordination may fail during some critical operating conditions and thus, a differential relay coordination scheme is proposed which provides improved performance for microgrid operating in different modes and different network topologies with both synchronous and inverter-based DGs.
A three-phase four-wire (TPFW) microgrid comprising of a solar photovoltaic (PV) array- battery energy storage (BES)- a diesel engine generator set (DEGS) is presented in this paper. Here an enhanced adaptive filter (EAF) control and an incremental conductance (INC) maximum power point tracking (MPPT) algorithm are used to improve the power quality and to extract the maximum power from the PV array. The EAF control provides higher disturbance rejection capability over other controls. The EAF control is applied to the voltage source converter (VSC) to improve the power quality, such as compensation of harmonics, reactive power, and load unbalance. The INC control is used to harvest the PV array maximum power. For controlling the voltage output of the DEGS, an electronic automatic voltage regulator (AVR) is provided at the synchronous generator (SG) field winding. The neutral current compensation is achieved by controlling the fourth-leg of VSC. This microgrid is simulated using MATLAB/Simulink tool. The robustness of the EAF control strategy is tested on a developed system prototype in the laboratory.
In this paper, a cyber-physical system (CPS) is considered, whose state estimation is done by a central controller (CC) using the measurements received from a wireless powered sensor network (WPSN) over fading channels. An adversary injects false data in this system by compromising some of the idle sensor nodes (SNs) of the WPSN. Using the WPSN for transmitting supervision and control data, in the aforementioned setting, makes the CPS vulnerable to both error and false data injection (FDI). The existing techniques of launching stealthy FDI attack are not applicable to the aforementioned network due to the random nature of wireless channels, which is used for both transmitting control and false data. The objectives of the adversary and the CC to launch stealthy FDI attack and to detect the same, respectively, are found to be depending on the powers they use for transmitting data over wireless channels. The transmit powers of the CC, and the adversary that fulfill their respective objectives are derived by modeling their interaction as a Bayesian Stackelberg game. Based on their objectives, novel utility functions are defined for the CC and the adversary. Subsequently, the equilibrium of the proposed game is obtained by solving a non-convex bi-level quadratic-quadratic program. Finally, the analytical results are verified and compared with other state-of-art techniques by applying them in a realistic smart grid simulations.
Abstract:The incorporation of ancillary services on dis-tributed generators (DGs) at utility level has imposed severalchallenges on the existing distribution networks. To address oneof such problem, this paper discusses the impact of ancillaryservices, mainly the voltage ride through (VRT) capability andactive power curtailment control (APCC) of DGs on their anti-islanding protection scheme (AIPS). In this study, different typesof load models (viz. ZIP, induction motor (IM) and compositeloads) have been considered and non-detection zone (NDZ)characteristics are obtained for a modified IEEE 1547/ UL 1741test system for analyzing the problem. Further demonstrationof this issue has been carried out by conducting time domainsimulations in PSCAD/EMTDC platform on modified IEEE 1547and CIGRE low voltage benchmark test system having compositeloads at each of the load buses. Finally, an appropriate mitigationframework has been proposed by incorporating a hybrid island-ing detection technique to enable simultaneous implementation ofVRT, APCC and AIPS in DGs. The online implementation of theproposed framework is realized in MATLAB/Simulink platformand real-time simulations are performed in OPAL-RT (OP5600)test bed on the modified IEEE 1547/ UL 1741 test system forvalidating its efficacy.
Abstract:This study deals with a single-stage photovoltaic (PV) grid-tied system along with enhanced momentum least mean square (EMLMS) control algorithm, which extracts the fundamental components from the non-linear load currents. Three-phase voltage-source converter with the EMLMS technique, feeds the solar PV power into the grid. Moreover, it is used to reduce the harmonics, load unbalancing and it compensates the reactive power to achieve the unity power factor of the grid currents. Moreover, this control provides the good dynamic response and fast convergence speed with less steady-state error. The main objective of this control algorithm is to provide the sinusoidal and balanced grid currents under steady-state and dynamic conditions. Test results of EMLMS technique are validated on a developed laboratory prototype.
This paper implements a new control method for a microgrid comprising wind energy generation system, solar PV array, the battery, and the utility grid, which provides a seamless transition of power during on-grid to off-grid operations and vice versa. This microgrid uses synchronous reluctance generator controlled by a mechanical position sensor-less field-oriented control, for the electric power generation from a variable speed wind turbine. The output of the solar PV array is connected to the DC-link of the microgrid through a DC-DC boost converter, whose output voltage is regulated at the desired value by the battery bank. For smooth synchronization and fast dynamic response, fourth-order generalized integrator based frequency-locked-loop with pre-filter (C-FGI-FLL) is implemented in this work. The use of C-FGI-FLL enhances the filtering capabilities of FGI-FLL under weak grid conditions, without affecting the computational burden. The C-FGI-FLL generates filtered input and its quadrature component, and these are further used for the extraction of positive sequence components of the grid voltages. For faster dynamic response during load variations, C-FGI filters are used in each phase so that fundamental peaks of load currents are extracted accurately without any delay. This microgrid has been tested both for dynamic and steady-state conditions on a prototype developed in the laboratory.
This article presents a green energy solution to a microgrid for a location dependent on a diesel generator (DG) to meet its electricity requirement. This microgrid is powered by two renewable energy sources, namely wind energy using doubly fed induction generator (DFIG) and solar photovoltaic (PV) array. The solar PV array is directly connected to common dc bus of back-to-back voltage source converters (VSCs), which are connected in the rotor side of DFIG. Moreover, a battery energy storage is connected at the same dc bus through a bidirectional buck/boost dc-dc converter to provide a path for excess stator power of DFIG. The extraction of maximum power from both wind and solar is achieved through rotor side VSC control and bidirectional buck/boost dc-dc converter control, respectively. A modified perturb and observe algorithm is presented to extract maximum power from a solar PV array. Moreover, the control of load side VSC is designed to optimize the fuel consumption of DG. A novel generalized concept is used to compute the reference DG power output for optimal fuel consumption. The microgrid is modeled and simulated using SimPowerSystems toolbox of MATLAB for various scenarios such as varying wind speeds, varying insolation, effect of load variation on a bidirectional converter, and unbalanced nonlinear load connected at point of common coupling. The DFIG stator currents and DG currents are found balanced and sinusoidal. Finally, a prototype is developed in the laboratory to validate the design and control of it.
—This article presents the sharing of reactive power between two converters of a doubly fed induction generator (DFIG) based wind energy conversion system interacting with the grid. The rotor side converter (RSC) control of DFIG is designed for sharing of reactive power at below rated wind speeds, which essentially reduces the amount of rotor winding copper loss. However, at rated wind speed, the RSC control is designed to maintain the unity power factor at stator terminals and to extract rated power without exceeding its rating. Further, the reduction in rotor winding copper loss due to reactive power distribution is demonstrated with an example. Moreover, the grid side converter (GSC) control is designed to feed regulated power flow to the grid along with reactive power support to DFIG and to the load connected at point of common coupling. Moreover, the GSC control is designed to compensate load unbalance and load harmonics. The battery energy storage connected at dc link of back-to-back converters, is used for maintaining the regulated grid power flow regardless of wind speed variation. The system is modeled and its performance is simulated under change in grid reference active power, varying wind speed, sharing of reactive power, and unbalanced nonlinear load using SimPowerSystems toolbox of MATLAB. Finally, a prototype is developed to verify the system steady state and dynamic performance. Moreover, system voltages and currents are found sinusoidal and balanced, and their total harmonic distortions are as per the IEEE 519 standard.
This study investigates an applicability of a reweighted l 1 norm penalized least mean square fourth (RL 1 -LMSF) algorithm and an improved multivariable filter based frequency locked loop (IMVF-FLL) through the performance demonstration of a three-phase solar-grid interfaced system in a double stage topology. The effectiveness of the system is ascertained by considering grid voltages unbalance, voltage sag/swell, unbalanced dc offset, unbalance in loads, and variation in solar irradiance. This RL 1 -LMSF algorithm adaptively extracts fundamental component of nonlinear load currents for estimation of reference grid currents with fast convergence and least steady-state error. An IMVF-FLL is used to elicit positive sequence components of grid voltages at abnormal grid conditions. The adaptability to input frequency deviations, harmonics suppression, dc offset rejection, and fast response of IMVF-FLL, during dynamic conditions, enables accurate estimation of unit templates. These unit templates maintain sinusoidal and balanced reference grid currents at varying conditions. The harmonic distortions in grid currents are strictly following the standard of IEEE-519. The photovoltaic array is kept at maximum power point using a boost converter together with a technique based on incremental conductance. The system is simulated in MATLAB environment, and test results recorded using a hardware setup emphasize its relevance.
The objective of this work is to develop a framework for carrying out the locational marginal pricing (LMP) in a financially consistent manner for a generalized AC-DC system by considering the presence of multi-terminal high-voltage DC (MHVDC) links. Here, the financial consistency is sought in terms of having revenue adequate settlement within the multi-settlement framework in the presence of financial transmission rights. A suitable DC optimal power model is developed to perform the financially consistent locational marginal pricing for the generalized AC-DC system without compromising significantly with the power flow accuracy. Power losses in both the AC and DC networks are duly taken into account by using an upgraded version of the popular marginal loss modelling (MLM) approach. Apart from the incorporation of MHVDC links in the LMP based market clearing, provision is also created to receive bids and offers from entities that are directly connected to the DC network. Thus, separate sets of LMPs are generated for AC and DC networks in the AC-DC system. Ample case studies are performed with several differently sized systems to verify the effectiveness of the proposed MLM-based DCOPF framework for the LMP-based market clearing in a generalized AC-DC system.
Security of various cyber–physical systems is a major concern for researchers worldwide. Nowadays, microgrids also form such cyber–physical system to achieve a number of objectives such as enhanced frequency regulation, economic active power sharing, proper reactive power sharing, etc. Communication links are essential in such microgrids to ensure bilateral flow of data to be fed to the secondary controllers of the distributed generators integrated to the power network. This article, therefore, studies the impact of various kinds of cyberattacks viz., false data injection, denial-of-service, and replay attack on the performance of these communication enabled secondary controllers having potential vulnerabilities. Additionally, it presents a unique framework to ensure system stability when communication links are subjected to such attacks. The analysis has been carried out in a microgrid operating in islanded mode where the controller area network (CAN) bus communication network is utilized to form the cyber–physical system. Simulations are performed in MATLAB/Simulink platform and real-time evaluation of the proposed framework has been carried out in a test bed having OPAL-RT (OP5600) simulator and physical CAN devices to form the communication links.
This work presents a linear state-feedback controller using a backstepping design approach for output voltageregulation of 3φ voltage-sourced converters feeding to customers' loads in a stand-alone AC microgrid system. Irrespective ofload type and its variations, parameter uncertainties, and other disturbances, the controller is robust enough to achieve aregulated voltage magnitude within the prescribed bounds and exact frequency tracking. The criterion for gain selection usingLyapunov analysis is given. With global exponential convergence feature and better coordination among inner current and outervoltage control loops, improved transient response is achieved. Controller features are examined by eigenvalue analysis andextensive simulation studies. Experimental results demonstrate the feasibility of the proposed controller implementation.
Abstract: This article proposes an optimal charging and dis-charging schedule for a hybrid photovoltaic-battery system con-nected in the premises of a residential customer. The schedulingstrategy is formulated to minimize the electricity bill of the cus-tomer. The proposed scheme uses the data obtained from short-term load, weather, and solar forecasting. A time-of-use tariffscheme is considered to be implemented by the utility. The proposedmethod is tested on real residential load and solar generationscenario. The test results verify that the implementation of theoptimal battery scheduling algorithm can significantly increase thenet saving in the electricity bill of the customer.
Active Power Decoupling (APD) circuits enable the use of long lifetime capacitors (film or ceramic capacitors) in single phase power converters. Owing to the inclusion of the APD circuits, the literature reports (1.5 to 1.8)% drop in efficiency of single phase converter at rated power. This reduction in conversion efficiency is one of the significant challenges in the practical use of APD circuits. This paper proposes an approach to reduce the power loss in the bidirectional buck converter based APD circuit. This approach is presented with the help of analytical calculations and graphical representation of operation of APD circuit. The proposed approach requires rapid variation in the average voltage of the buffer capacitor with a change in inverter power. To achieve this, an enhanced control technique is suggested with a duty ratio injection controller. The steady state and transient response of the proposed control technique is validated with simulation and experimentation. Further, the reduction in power losses realized by the proposed approach is verified with the help of a developed laboratory prototype. The proposed approach obtains up to 1% improvement in efficiency of single phase converter at rated power, when compared with existing APD approaches.
The prime facets for the control of grid integrated voltage source converters (VSC) during abnormal grid variations, are the control of voltage as well as power quality. In this work, a unique control strategy is presented for the control of solar photovoltaic (PV) system interfaced to the grid utilizing an interweaved generalized integrator (IGI). A single stage three phase topology is considered. The primary purpose of control is to deliver the PV power to grid even during various abnormal grid variations. During normal operation, the system delivers power at unity power factor (UPF). However, during variations in the grid voltage, the profile of the PCC voltage is maintained within prescribed limits by reactive power injection. Moreover, LVRT operation is undertaken during severe voltage sags. The utilization of the system is increased in the absence of PV during night, the VSC and DC link capacitor act as a distribution static compensator (DSTATCOM). Contrary to traditional control techniques, power quality of the system is not compromised. The achievements of the control are demonstrated through simulation as well as with hardware implementation. Furthermore, a comparative analysis with the state of the art techniques, is highlighted, which shows the efficacy of the presented control.
In this paper, a single-phase solar photovoltaic (PV)-BES (Battery Energy Storage) based microgrid is presented, which functions in grid connected and islanded modes seamlessly. The PV-BES microgrid extracts the optimal power from the available solar energy and feeds the nonlinear loads through a load side voltage source converter (LVSC). The maximum power from the solar PV array is extracted by controlling the boost converter connected between a PV array and the common DC link. A perturb and observe (P&O) based maximum power extraction technique (MPPT) is utilized for maximum power extraction. The BES is connected at the DC link. A control scheme is presented, which operates the system in grid connected as well as islanded modes. An M-MSOF based filtering technique is used to improve the power quality at the grid, while mitigating the harmonics from the grid current, as per the IEEE-519 standard.
This article aims to develop a multiobjective optimization portfolio for real time energy management in a smart home equipped with battery associated rooftop solar panels, lighting loads, air conditioners, and other smart home appliances. The energy management problem is framed for simultaneous minimization of the monetary energy cost and total dissatisfaction due to regulation in power consumption. The entire optimization portfolio is designed as a time average stochastic problem, which is simplified by the combination of queueing theory and Lyapunov optimization. The revised problem takes form of a mixed integer convex nonlinear programming, which is further solved using outer approximation approach. The proposed real time home energy management framework needs only the current data regarding the random input parameters like renewable generation, energy price, and aggregated load demand, and does not call for their probabilistic estimation. Case study is carried out on a practical home data to proof efficacy of the proposed strategy and also the simulation outcomes are compared with one of the popular real time energy management process named greedy algorithm.
Abstract: In this article, the presented work proposes a power management scheme in a dc microgrid by utilizing integrated device level control designs. Dynamics of nondispatchable energy sources, energy storage systems, and critical loads while synthesizing the control scheme are extensively studied. The presented work focuses on the integrated operation of local converter controls and power control unit during load fluctuations, environmental changes, accidental islanding, state of charge (SOC) breach, etc., without any time-critical information sharing. Low bandwidth communication (LBC) assisted control actions are enabled only in response to specific events such as battery management system (BMS) operation in autonomy, onset of peak hours, and load shedding. This article examines small signal analysis of individual interfacing converters to conduct eigenvalue studies of the closed-loop control systems. Successful control transitions are achieved in real-time digital simulator to validate the proposed management strategy under various system conditions. Hardware-in-the-loop setup validates the implement-ability of the control strategy in dc microgrids.
This paper deals with a novel swarm optimization algorithm (NSOA) based multi-objective optimized drift-free maximum power point tracking (MODF-MPPT) technique for a photovoltaic (PV) array fed grid connected multilevel converter (MLC) system. The conventional MPPT algorithms experience drift in PV voltage with sudden irradiation changes. The nature-based multi-objective NSOA integrates the concept of swarm intelligence and pressure gradient force to achieve optimal drift-free operating point. Unlike particle swarm optimization (PSO), the acceleration of swarm particles depends on the pressure difference and distance between them. The comparative analysis of the voltage drift over existing MPPT algorithms is presented. Simulated results analyze the performance of MODF-MPPT over existing MPPT techniques. Later, this algorithm is experimentally validated for power quality improvement of the PV array fed grid-tied multilevel converter. The algorithm has minimum voltage drift, enhanced system efficiency and improved power quality.
The requirement for high capacity power conditioning unit (PCU) has increased for application in large-scale solar photovoltaic (PV) plant for optimisation of the balance of system cost. For high power applications, one of the most important criteria for designing of PCU is to have lower switching losses to minimise the heat generated in the power switching devices. Conventionally, PCU utilises traditional two-level or three-level voltage source converter (VSC) topology with high-frequency switching techniques and due to this, they have lower conversion efficiency and higher device switching losses. In this study, fundamental frequency switching (FFS) modulated three-level neutral point clamp (3L-NPC) VSC is used for MW scale PCU, as they have lower switching losses, higher conversion efficiency and higher AC/DC voltage ratio. The 3L-NPC VSC generates lower order current harmonics, which are mitigated by using different phase-shifted 12-pulse PCU transformers located at different pooling location inside solar PV plant. The PV plant configuration for a 40 MW (AC) plant capacity is developed in the Matlab/Simulink environment and implemented in real-time simulator OPAL-RT to validate the proposed concept. Harmonics, steady-state and dynamic performances are demonstrated at constant and changing solar irradiance levels.
Conjoint participation of wind generation with conventional power plant, necessitate wind-farms to participate in automatic generation control (AGC) for frequency regulation. This implicates wind farms operator to monitor commands, received from the transmission system operator (TSO). Consequently, advanced control techniques are deployed, which assuredly helps in power tracking, but increases pitch angle controller dynamics. This has resulted in mechanical stress on equipment involved. In order to improve wind-farms response, a synergistic frequency regulation control mechanism (SFRCM) is proposed by the authors. The scheme considers the response time and reserve availability depending on forecasted wind data and also examine load curve pattern to obtain the reference signal for the wind-farm controllers. The work provides a distinct solution to address the following: 1) Optimal pitch dynamics regulated operating point tracking with revised-pitch angle control (R-PAC), 2) Maximization of rotational kinetic energy viz attuned-rotor speed control (A-RSC), to increase stored kinetic energy in the rotor. Case studies at constant and variable wind speed are presented to show the effectiveness of proposed algorithm. To test robustness of the technique, transient operation is conducted as well as forecasted data prediction error is also invoked in the study. The simulations, performed in the real time environment, depict the fulfillment of the above objective in an efficient manner compared with other conventional approaches.
Abstract:System inertia plays a vital role in controlling theangular stability of the system during a disturbance. Due toincreased penetration of power electronic interfaced sources, suchas Solar Photovoltaic (SPV) source, the overall system inertiareduces and varies depending on their operating conditions. Inthis paper, an approach for online inertia estimation in thepower system network with SPV sources is proposed, using thesynchronized measurements from Phasor Measurement Units(PMUs). An equivalent swing equation is used to emulate thenetwork dynamics. A relationship between the inertia constantand the roots of this equation is determined. In order tonumerically obtain the roots, the Estimation of Signal Parametervia Rotational Invariance Techniques (ESPRIT) method is firstused to find the modes present in the frequency signal. A newformulation is proposed to extract an equivalent mode from allthe obtained modes. Also, to avoid phase step error, Rate OfChange Of Frequency (ROCOF) is estimated from the equivalentmode of the frequency signal. Results obtained for the39busNew England system for various test cases, using Real-TimeDigital Simulator (RTDS), prove the efficacy and superiorityof the proposed approach over the existing approaches in theliterature.
This article deals with a phase-locked loop (PLL)-based novel control for wind turbine driven doubly fed induction generator interfaced to utility grid with a battery energy storage (BES) connected at the dc link. The control of grid-side converter (GSC) is modified to export/import constant power to/from the grid. The state of charge of BES helps in deciding the reference export power to the grid apart from the manual selection using averaged wind power in a particular period of time. An off maximum power point tracking logic is incorporated in the rotor-side converter (RSC) control to operate the BES within its constraints and, moreover, to feed constant power to the grid. In addition, the energy management scheme of the system is presented in the form of flowchart for both exporting and importing power to/from the grid. The RSC and GSC have taken care of unity power factor operation at stator terminals and to mitigate harmonics and grid currents balancing, respectively. The system performance is found robust as the PLL response is not affected even under grid voltages with dc offset. The system is modeled and simulations are carried out in MATLAB using SimPowerSystems tool box. Moreover, the control scheme performance is compared with conventional control algorithms both in terms of PLL and converter controls. To validate the effectiveness of the control scheme, a prototype of the system is developed. Test results demonstrate the satisfactory performance of the system under various operating conditions.
Abstract:This article elucidates a real-time energy management strategy for a smart residential apartment building having nonidentical occupants at the dwelling units (DUs). The aim of the present article is to design a distributed energy management algorithm, which can optimize the real-time demand of the entire building against abruptly updated rooftop solar generation and real-time price (RTP) of energy. The proposed energy management strategy differentiates among the DUs by considering a new parameter named load criticality level, which is defined as the value imposed by the DU residents to their power consumption. The optimization portfolio is developed as a novel bilevel, stochastic, multiobjective optimization problem where the maximization of utility of the consumed power is considered simultaneously with the cost minimization. To this end, a virtual energy trading platform is designed in this article between central building management system and the DUs, where they interact with each other by following the directives of single-leader multifollower Stackelberg game. The solution strategy is proposed as a Lyapunov optimization, which needs only the current values of the uncertain parameters, such as load variation, renewable generation, and energy price, and do not require any knowledge about their probabilistic variation, to eliminate the complexities regarding time average stochastic equations. Strenuous simulation on real-time data of four DUs, it is proved that the proposed framework can track the abrupt change in RTP and solar generation efficiently. Comparing with two benchmark methods viz. centralized process and greedy algorithm, the superiority of the designed energy management portfolio is established.
This article presents a protection scheme based on the magnitude-phase plane of impedance difference (ID) using wide-area positive sequence components of current and voltage signals for the microgrid. The computation of the ID of the line is performed by extracting signals from the phasor measurement units complied with IEEE C37.118 standards. The composite magnitude-phase plane obtained from the magnitude of the ID and the angle of ID is considered as the key index for the detection of faults. The feasibility to include the proposed microgrid protection technique is examined by the extensive fault cases simulation with substantial variation in fault conditions, including types of fault and fault at different line length and for the grid-connected and islanding modes of microgrid operation. The fault study is carried out on a medium-voltage 15 bus test microgrid and a standard IEEE 34 bus microgrid testbed. The impact of several critical no-fault conditions is also investigated to analyze the possibility of false operation. The proposed ID-based scheme is extensively tested on MATLAB/Simulink platform and validated on the real-time digital simulator. The test results indicate that the magnitude-phase plane of the ID-based scheme can provide a dependable microgrid protection measure.
This article deals with the adaptive control algorithm to maximize the distributed generations (DGs) in islanded microgrid. The approximate multiplier least mean square (AMLMS) control scheme is used for the extraction of fundamental load component with reduced steady state error and faster convergence to provide better performance than conventional LMS control. This control focuses on reduction of complexity of LMS by adding multipliers in the finite impulse response filter and updating the coefficients. The AMLMS control algorithm for a voltage source converter is also effective in compensating harmonics for power quality improvement of islanded microgrid system while feeding nonlinear load. In an islanded microgrid at ac side, a synchronous reluctance generator based pico-hydropower generator is used because it is economical than other conventional generators. At dc side, a photovoltaic array and a bidirectional dc-dc converter (BDC) are used. The BDC with a storage battery maintains the optimal power flow to provide the power equilibrium between load and sources through dc-link voltage regulation. The BDC also mitigates the second order harmonic from the battery charging and discharging current. Test and simulation results validate the effectiveness of AMLMS control under steady and dynamic conditions of solar insolation change and load unbalance.
In the literature, the fault ride through (FRT) studies of the doubly fed induction generator (DFIG)-based wind system consider fault to occur after the fault current limiter (FCL) with respect to the DFIG system. In this article, the fault is considered before FCL and the power hardware in the loop (PHIL) experimentation is performed. Results demonstrate that the occurrence of the fault before FCL can worsen the FRT of the DFIG system. To address this, a novel arrangement of FCL with four power electronic switches in a particular fashion has been suggested. This arrangement allows the perception of fault occurring before FCL as after FCL due to the alternate path of power flow thereby improving the FRT of the DFIG. Furthermore, a quick fault detection and switching technique has been developed which is able to detect the fault by comparing the instantaneous value of the phase current with the threshold. Thereby, it is possible to achieve on/off of the switches upon the fault occurrence with negligible time delay. In order to validate the effectiveness of the proposed technique, the PHIL experimentation is performed at sub/supersynchronous mode of operations (±0.1 slip), different fault resistances (6, 20 Ω), delay time (40 ms), and various fault perception instants (negative, zero, positive). The results justify the effectiveness of the proposed arrangement in improving FRT of the DFIG when a fault occurs before the FCL with respect to the DFIG in comparison to the existing literature and in the absence of FCL in the system.
Abstract:The relative share of renewable energy, specifically the solar photovoltaic (PV), is increasing exponentially in the world electric energy sector. This is a cumulative result of reduction in the cost of solar panels, improvement in the panel efficiency, and advancement in the associated power electronics. Among different types of PV plants, installation of small-scale rooftop PV is growing rapidly due to direct end-user benefits and lucrative governmental incentives. There are various standards developed in regards to grid integration of PVs and other distributed generations (DGs). Different power converter topologies are developed to interface the PV panel with the utility grid. To keep up with the stringent regulations imposed by the standards, various control strategies and grid synchronization methods have been developed. This review article amalgamates and summarizes all of the aforementioned aspects of a grid-integrated PV system including various standards, power stage architectures, grid synchronization methods, operation under extreme events, and control methodologies, pertaining to small-scale PV plants. This article will help freshman researchers to gain some familiarity with the topic and introduce them to some of the key issues encountered in this field.
Abstract:A solar photovoltaic (PV)-battery energy storage-based microgrid with a multifunctional voltage source converter(VSC) is presented in this article. The maximum power extractionfrom a PV array, reactive power compensation, harmonics mit-igation, balancing of grid currents and seamless transition fromgrid connected (GC) mode to standalone (SA) mode and vice versa,are performed in this system. Whenever the grid fails, this systemoperates in SA mode automatically, thereby without causing anyinterruption in supplying the load. Similarly, it automatically shiftsto the GC mode, when the grid is restored. The VSC functions incurrent control for GC mode, and it operates in voltage controlfor SA mode of operation. This system is capable of extracting themaximum power from the solar PV array irrespective it is operatingin the GC mode or SA mode. The charging and discharging of thebattery are controlled by using a bidirectional dc–dc converter. Itregulates the dc-link voltage to the maximum power point voltageof the PV array. If the absence of the battery is detected, thenthe control is automatically shifted to VSC for performing theextraction of the maximum power of the PV array.
Matrix is one of the convenient means for depicting/illustrating a graph on the computer. In this study, the notion graph-theory in conjunction with matrix algebraic operations has been adopted for solving the load-flow problem of three-phase distribution systems (radial and meshed). Five significant matrices, path impedance (PI), loads beyond branch (LB), path drop (PD), slack bus to other buses drop (SBOBD), load flow matrix (LFM), and straight-forward matrix operations have been utilised to attain the load-flow solutions. The aforementioned matrices reveal the system's topology and pertinent information about the operating characteristics of the distribution system during LF studies. This algorithm is formulated entirely on various matrices formulation and computations, even at the stage of upgrading the voltage at every individual bus. Owing to the aforementioned reasons, this LF methodology is computationally efficient for large-sized distribution systems. Moreover, the distributed generations (DGs) modelled as PQ and PV buses are incorporated into the proposed load-flow algorithm. A generalised breakpoint matrix has been derived to compute the mesh breakpoint and PV breakpoint injections simultaneously. The effectiveness of the proposed methodology has been tested on several standard distribution systems. The test outcome shows the viability and accuracy of the proposed method.
Abstract:This paper presents a microgrid composed of photovoltaic (PV) array‐battery energy storage (BES) and a diesel generator (DG) set. A bidirectional DC–DC converter (BDDC) is used to integrate the BES to the DC link of the voltage source converter. The BDDC controls the voltage at the DC link to the output voltage of the maximum power point tracking (MPPT) controller, thereby PV array always operates at its maximum power point (MPP). Another function of the BDDC is to block the second‐order harmonic from the BES, when the microgrid is feeding with unequal loads in the three phases. The magnitude of DG set terminal voltages is regulated by an electronic automatic voltage regulator (AVR) that controls the excitation to the synchronous generator. A fast, accurate and oscillations‐less modified variable step size least mean square based adaptive control is used here for improving the power quality. The main features of this microgrid topology are the operation of the PV array at its MPP in all operating scenarios, regulation of DG set voltages, harmonics elimination, balancing of DG set currents and reactive power compensation for the unity power factor operation.
Abstract:In this study, a proportionate power sharing among the parallel inverters operating in an islanded microgrid is achieved using droop and virtual inductance control. The same droop coefficients are used to achieve the frequency regulation as well. The frequency changes which are inevitable in droop control are measured and used to emulate the behaviour of damping and inertia to the DC link voltage using hybrid energy storage system consisting of battery and supercapacitor units. The proposed DC link voltage regulator restores the DC link voltage quickly by providing power corresponding to the rate of change of frequency and frequency deviation. This reduces the impact of voltage variations on the DC-load and keeps modulation index within the linear range for voltage source inverter. The design aspects of DC link voltage regulator, damping and inertia constants, selection of battery and supercapacitor units based on rating of the DC link voltage are discussed. The proposed decentralised droop control and DC link voltage restoration methods are validated through detailed simulation and experimental studies.
Accommodating increased penetration of renewable energy resources like solar Photo-Voltaics (PV), imposes serious challenges on voltage regulation for the traditionally designed distribution system. Battery Energy Storage Systems (BESS) can mitigate voltage regulation issues, as they can act quickly in response to the uncertainties introduced due to solar PV. However, if there is no coordination between existing devices such as On Load Tap Changing Transformers (OLTC) and BESS, then BESS takes all the burden and is generally over utilized. The uncoordinated control schematic shall also lead to under utilization of OLTC. Hence, in this paper, a coordinated control strategy to control BESS along with OLTC is proposed to warrant acceptable voltage magnitudes across the distribution feeder. The formulated optimization problem aims to mitigate the voltage deviation from its required values while reducing the number of changes in tap positions and also enhancing the battery life. The improvement in voltage regulation and optimal utilization of resources by using the proposed coordinated scheme over the traditional uncoordinated scheme is demonstrated for the IEEE 13 bus and 33 bus distribution systems in MATLAB/ Simulink.
Abstract: This study presents the control of distribution static compensator (DSTATCOM) using band-dependent variable step size (BD-VSS) individual weighting factor with sign error based adaptive filter for power quality improvement in a weak distribution grid. Here, the proposed algorithm is used for estimation of fundamental active weight components from the distorted load currents in order to generate the reference grid currents to mitigate the grid currents power quality issues. This new control algorithm is proposed for fast and accurate estimation of active weight components with low steady-state error, without dynamic oscillation at fast convergence speed. Moreover, the frequency locked loop-double self-tuning second-order generalised integrator based voltage filter is used to extract the positive sequence voltages of the distorted grid voltages for estimating the harmonics, and noise-free voltage unit templates. Therefore, the DSTATCOM with the proposed control algorithm is capable of mitigating the harmonic currents, providing reactive power compensation and operating at unity power factor in a weak distribution grid. Test results demonstrate the viability and robustness of the proposed control algorithm under balanced and unbalanced non-linear load conditions.
The nonlinearity in the consumption does not maintain steady voltage or frequency; therefore, in this paper, a CLO-FLL-WPF (Circular Limited Cycle Oscillator Frequency Locked Loop with Pre-Filter) based control technique for voltage source converter (VSC) control is used to achieve voltage and frequency regulation in an islanded microgrid. This control is implemented to compensate local load reactive power demand, harmonic currents, and load unbalance. Besides, the active power demand of all local loads, is shared between the multiple energy sources. The CLO-FLL-WPF control technique performance is validated experimentally and through simulations by comparing it with the existing control algorithm in a microgrid. This microgrid is a combination of the solar photovoltaic (PV) array, pico-hydro turbine-driven SyRG (Synchronous Reluctance Generator), PMBLDCG (Permanent Magnet Brushless DC Generator) based WEC (Wind Energy Conversion) and a battery storage. In traditional battery control, a proportional-integral (PI) control approach is used, which can cause a stability problem. Thereby, in this paper, the bidirectional DC-DC converter (BDC) control method is used, which provides improved stability and makes the controller design and computation straightforward. Test results validate the effectiveness of the implemented control algorithm under different dynamic and steady-state conditions.
Abstract:The presence of phasor measurements in ac systems provides a range of protection techniques based on sequence components and phase comparison to discriminate the fault. However, the absence of phasor measurements in dc system reduces the available alternatives for fault detection. In addition, the presence of low fault tolerant converters, large range of fault impedance and varying grid conditions demands sensitive and selective protection schemes. In this regard, several recent works have suggested a communication-based primary protection due to its high sensitivity to faults. However, with a failure in the communication network, the primary protection will also fail to detect a fault. This paper proposes a backup scheme to isolate the faulty section, even in the case of a communication failure. In the literature, overcurrent- and undervoltage-based backup protection schemes are suggested along with unit primary protection. In the presence of low fault tolerant converters and variable fault resistances, the traditional backup schemes may not work well. This paper proposes a new fault detection method for backup schemes, which utilizes only the locally measured current signal, and uses both derivative and integral characteristics of current to ascertain the occurrence of a fault. The proposed method is capable of detecting the fault accurately and within the required time. The performance of the proposed scheme has been assessed on a � 600 V TN-S grounded dc microgrid under various conditions using hardware-in-loop simulations on real-time digital simulator.
Abstract:Switched-mode power converters are used to interface equipments andconsumer electronic devices over a wide range of power levels. The switch technology ofany power converter is a signature of its power rating. The article discusses differentpower electronic switch technologies used in modern power supply implementation andevaluates their impact in the system design. From the design prospective, power loss ismore important than efficiency. It can be established that as power level of converter goesup, it is indispensable to have higher efficiency due to thermal design considerations.A trend between the switching frequency and its efficiency with respect to the powerrating of the converter has been inculcated in the article.
Abstract:In this paper, the effect of jamming in free space optical (FSO) link, is evaluated by deriving closed-form expressions of the bit error rate (BER) and outage probability (OP) for single-input single-output (SISO) and multiple-input single-output (MISO) FSO systems. The effects of partial-band jamming and broadband jamming over the error performance of the considered FSO systems are also analyzed. The jammer behaves as a random noise source, following the negative exponential distribution-due to the atmospheric turbulence. Therefore, in the presence of jammer, the error performance of FSO systems is governed by the additive negative exponential noise. It is shown by a rigorous analysis that a MISO FSO system can significantly mitigate the effect of jamming. Specifically, we consider a 2×1 FSO system for analysis, and demonstrate the improvement in BER and OP performance an FSO system can gain by using an additional spatial dimension, in the presence of a random jammer. The mitigation of jamming, due to implementation of arbitrary transmit apertures, is also verified through simulation results; which lies in complete agreement with the analytical results. Also, the error performance under the jamming effect is studied numerically over the Gamma-Gamma fading channel incorporated with pointing error effect.
An increase in the utilization of renewable energy has called for a cost-effective and reliable solutions to overcome their intermittency. With this regard, this paper offers a unique way of interfacing dc microgrid (DCMG) to the doubly fed induction generator (DFIG) wind system during grid-connected, fault, and isolated working conditions making it economical. The control flow strategy proposed here uses the grid-side converter (GSC) of the DFIG system to serve multiple purposes; first, the GSC is used to regulate the DCMG voltage during the grid-connected mode and avoids the need for an additional converter for the DCMG to connect it to the grid. Second, the GSC is allowed to function as a simple diode bridge rectifier thereby allowing the DFIG machine to feed DCMG during isolated conditions hence avoiding the need for replacing the converter. This paper also improves the fault ride through performance of the DFIG wind system without any additional investment. This is possible due to the usage of already available ultracapacitor in the DCMG. Hence, the DFIG wind system need not have to finance separately to protect its dc link during the fault. Furthermore, a small-signal-based stability analysis performed also indicates the robustness of the proposed control strategy. A 2.2-kW hardware prototype has been developed and the results obtained from the elaborate experimentation justify that this paper is economical, efficient, reliable, and offers better power quality compared to the existing work.
Abstract:Synchronous reference frame (SRF) control strategy for solar photovoltaic (SPV) sources is widely used to delivermaximum power to the grid. However, poor inertia support just after a disturbance and improper phase angle tracking inpresence of the harmonics and system unbalance are noticed when the conventional phase-locked loop (PLL) based SRFcontrol structure is used. In this study, an inertia enhancement method for inverter interfaced SPV sources is proposed, whichadjusts only the PLL parameters with a notch filter (NF) in the SRF controller. NF in PLL is used due to its disturbance rejectionpotential and accurate phase angle tracking, even during system unbalance. The dynamic equation of pseudo induced voltage(PIV) vector with respect to the point of common coupling is derived. The kinematic equation of the PIV angle vector correlatesinertia contribution and inverter terminal voltage. The impact of the change in PLL parameters on inertia enhancement isanalyzed by validating the proposed technique on a test system in the real-time digital simulator. The frequency response by theproposed method has been compared with two state-of-art methods to prove the superiority of the proposed approach inenhancing the inertia of the SPV source in a microgrid.
Abstract:Point wind velocity measurement given by nacelle-top mounted sensor may not be same as effective wind velocity, which strikes the wind turbine rotor blades. This article proposes a new method for the computation of effective wind velocity by inverting the turbine's aerodynamic model after estimating the turbine torque and its rotor speed. Nonlinear control theory-based sliding mode observers are used to estimate the wind turbine generator induced speed emfs, rotor speed, and aerodynamic torque imparted to generator shaft. While satisfying the Lyapunov inequality condition, design aspects of sliding mode observers are discussed in detail. The performance of the proposed wind velocity estimation method is evaluated by using permanent magnet synchronous generator-based wind turbine emulator in laboratory. Simulation and experimental studies confirm that the estimated wind velocity under different wind profile conditions is accurate and can be used in various control algorithms in wind energy conversion systems.
Abstract: Utility-interfaced power electronic systems use a grid synchronizing framework, known as phase locked-loop and need transformation of sinusoidal signals to rotating dq reference frame, for control purpose. The voltage or current signal parameters including instantaneous fundamental frequency, phase angle, amplitude need to be captured in presence of harmonics, noise and DC offset. This work proposes an adaptive estimation scheme for the same, using recursive least squares with time-varying covariance gains. Proposed adaptation of gain presents faster transient response and noise-tolerant steady-state response, achieving optimal trade-off between the two. Covariance resetting mechanism is presented for better dynamic profile during step-changes in the signal. The scheme for single phase signal transformation is extended for transforming three phase unbalanced sinusoids to decoupled double synchronous reference frame. Stability analysis, design guidelines and discrete-time realization of proposed methods is provided for reproducibility. Theoretical deductions of proposed method are supported with several comparative test cases simulated in MATLAB/Simulink as well as the experimental results.
Abstract: This paper develops a robust control strategy forvoltage regulation of customer-end AC loads, where the lowvoltage multi-terminal DC network has been used for powerdistribution purpose. The control law for load-end DC-ACconverter is synthesized by Lyapunov’s direct method usingbackstepping design process. The resultant control action issimply obtained by linear combination of states and referenceset-points, and do not add extra dynamics into the systemavoiding complexity. The proposed method does not require loadcurrent measurement. Simulation studies in MATLAB/Simulinkshow better dynamic performance compared to conventional PIcontrollers. The experimental results from a laboratory-scaleprototype demonstrate robustness of proposed controller againstvariations in type and amount of load profile as well as DCvoltage.
Abstract:In this paper, the groundwork on jamming effect and its alleviation mechanismsin free space optical (FSO) communication are studied over Gamma-Gamma (GG) fadingchannels along with the pointing error (PE) effects. A closed-form expression of the biterror rate (BER) is evaluated analytically for a single-input single-output (SISO) FSO systemin the presence of jammer. The worst case jamming condition for maximization of erroris calculated numerically. The jammer channel is considered to be GG distributed. Beingthe most dominating noise, the jamming signal is acted as the only noise source in theconsidered FSO set-up. Therefore, this study is performed over additive GG noise channel.The error performances are investigated for different atmospheric turbulence (AT) regimes(weak to strong) and for different PE parameters of jamming noise. Moreover, to combatthe jamming effect, a multiple-input single-output (MISO) FSO system is considered. Aclosed-form expression of the BER of the multiple transmit channels over the GG noiseis analytically calculated. The analytical results for both SISO and MISO FSO systems areverified with the help of the simulation results obtained by MATLAB software. It is establishedthat MISO FSO performs better than SISO FSO system in terms of BER performance underthe influence of jamming noise. Furthermore, many important observations are made uponthe BER performances for different AT and PE parameters for SISO and MISO FSO systems.
Abstract: In this work, the authors consider the communication network of a power substation, where multiple intelligent electronic devices (IEDs) transmit their delay sensitive control and monitoring messages to a common receiver over a wireless network; in the presence of a broadband jammer, which is capable of jamming multiple channels simultaneously. The objective of the IEDs is to successfully transmit their messages within a specified time, whereas the jammer wants to obstruct IED's transmission. A novel utility function is designed for the players, that addresses the time-critical nature of communication. Due to the conflicting interest of the IEDs and the jammer, they model the interaction between them as a repeated Bayesian zero-sum game, which also addresses the repeated interaction among the IEDs and the jammer, and the unavailability of exact information about the jammer. The equilibrium strategies for both the scenarios of perfect and imperfect monitoring are derived and verified through simulation results. Further, the performance of the proposed game model in various scenarios is thoroughly compared in the result section. Finally, the efficacy of the proposed defence strategies is tested in a practical communication network of a power substation under jamming, which is simulated in Optimised Network Engineering Tool (OPNET).
Abstract:This paper deals with the design and stability analysis
of a DC microgrid with battery-supercapacitor energy storage
system under variable supercapacitor operating voltage. The
conventional design method reported in the literature considers
the rated supercapacitor voltage in the modeling and design of
controllers. However, the supercapacitor unit can discharge as
low as 10% of its rated voltage due to self discharge. It is observed
that the conventional method of controller design can potentially
make the system unstable or introduce ringing in the DC link
voltage at low supercapacitor voltage. In this work, the sensitivity
of DC microgrid stability with respect to supercapacitor voltage
variation is analyzed, an optimal supercapacitor voltage to be
considered in the design is calculated and a design method is
proposed to ensure the stability of DC microgrid in all operating
modes. The stability of the DC microgrid with controllers
designed using the proposed method is evaluated with digital
simulation and experimental studies.
Conventional distribution network is a radial network with a single power source. Usually, overcurrent protection schemes are employed for such system protection for their simplicity and low cost. With the introduction of renewable generations, the existing protection coordination needs to be upgraded. Provision of directional feature and the requirement of high capacity circuit-breakers at certain points for the protection scheme demands considerable investment. The renovation cost required for upgrading the protection scheme will significantly impact the network cost requirement and consequently distribution use of system (DUoS) charges. This study aims to investigate the impact of renewable generations on the DUoS charges considering the cost associated in revamping the protection scheme. A power flow based MW + MVAr-miles DUoS charging method, that considers used capacity of the network, is proposed to carry out the DUoS charging calculations. The proposed charging mechanism appraise/penalise the users in accordance they are affecting system power factor. Accordingly, the proposed pricing algorithm may encourage users to act based on the economic signal generated at each location. The proposed charging algorithm has been tested on IEEE-33 and IEEE-69 bus distribution systems to examine the impact of renewable generations on the use of network costs.
With increased integration of wind energy systems, an accurate wind speed forecasting technique is a must for the reliable and secure operation of the power network. Statistical methods such as Auto-Regressive Integrated Moving Average (ARIMA) and hybrid methods such as Wavelet Transform (WT) based ARIMA (WT-ARIMA) model have been the popular techniques in recent times for short-term and very short-term forecasting of wind speed. However, the contribution of the forecasting error due to different decomposed time series on the resultant wind speed forecasting error has yet not been analyzed. This paper, thus explores this shortcoming of the ARIMA and WT-ARIMA models in forecasting of wind speed and proposes a new Repeated WT based ARIMA (RWT-ARIMA) model, which has improved accuracy for very short-term wind speed forecasting. A comparison of the proposed RWT-ARIMA model with the benchmark persistence model for very short-term wind speed forecasting, ARIMA model and WT-ARIMA model has been done for various time-scales of forecasting such as 1min, 3min, 5min, 7min and 10min. This comparison proves the superiority of the proposed RWT-ARIMA model over other models in very short-term wind speed forecasting.
Abstract:This paper deals with the protection of critical
loads from voltage-related power quality issues using a
dynamic voltage restorer (DVR). A generalized control algorithm
based on instantaneous space phasor and dual P -Q
theory has been proposed to generate the instantaneous
reference voltages to compensate the load voltages with
direct power flow control. The proposed algorithm adapts
energy-optimized series voltage compensation, which results
in a reduction of energy storage requirement. The
proposed DVR control scheme can support the load from
voltage-related power quality issues irrespective of the load
current profile. Each leg of the three-phase three-leg split
capacitor inverter is used to inject series compensation voltage
in respective phases of the system. Model-based computer
simulation studies and real-time experimental results
validate the effectiveness of the proposed control algorithm.
Abstract:This paper proposes a centralized control strategy
for power management of hybrid microgrid connected to the
grid using a parallel combination of grid side converters (GSCs).
An improved version of instantaneous symmetrical component
theory (ISCT) is developed and is used for the control of parallel
operated GSCs, which results in reduced sensor requirement,
control complexity, and communication bandwidth. In addition,
a simple power management algorithm is developed to test the
efficacy of the proposed parallel grid side converter control
strategy for all the microgrid modes considering state of charge
(SOC) limits of hybrid energy storage system (HESS), load
changes, and renewable power variations. In the proposed system,
a better dc link voltage regulation is achieved and usage of
supercapacitor reduces the current stresses on the battery. With
the proposed control strategy, the essential features of grid
side converters like power quality, power injection, bidirectional
power flow and proportional power sharing are achieved. The
effectiveness of the developed control strategy for the proposed
system is tested using MATLAB based simulink environment and
validated experimentally using a laboratory prototype.
Abstract:The AC-DC distribution systems have recently gained huge popularity due to advancements in power converters, high penetration of renewable energy resources and wide usages of DC loads. However, load flow in such systems is a challenging task due to non linear characteristics of power converters. This paper presents a novel load flow algorithm for AC-DC distribution systems, utilizing the concept of graph theory and matrix algebra. Four developed matrices, loads beyond branch matrix [LB], path impedance matrix [PI], path drop matrix [PD], slack bus to other buses drop matrix [SBOBD] and simple matrix operations are utilized to obtain load flow solutions. These matrices reveal the network topology and relevant information about the behaviour of AC-DC distribution network during load flow studies. In contrast with traditional load flow methods for HVDC systems, the proposed technique does not require any lower upper (LU) decomposition, matrix inversion and forward-backward substitution of Jacobian matrix. Because of the aforementioned reasons, the developed technique is computationally efficient. The proposed method has been tested using several case studies of AC-DC distribution network which includes different operating modes of various power converters. Results show feasibility and authenticity of the proposed method.
Abstract: Home energy management systems (HEMS) encourage
participation of residential consumers into the demand response
programs. This paper proposes a robust-CVaR (Conditional Value at
Risk) optimization approach for day ahead HEMS to reduce the effect
of risk of real time exposure to energy price and solar power
generation uncertainties. Initially the CVaR method is integrated
with the Two-point Estimation (2PE) analysis to approximate the
solar power, modelled as Beta probability distribution function, in
low computation effort compared to conventional Monte Carlo
Simulation (MCS) based CVaR approach. Then the optimization
constraints are revised to their robust counterparts by accounting a
certain amount of uncertainty in the energy prices from their nominal
values. Unlike previous literatures, the optimization problem is
developed to minimize the risk value of the energy cost. Again to
maximize the life of the plug in electric vehicle (PEV) a pseudo cost
function for the PEV battery degradation is proposed. The entire
optimization portfolio is developed as a mixed integer linear
programming (MILP) for its easy execution. Simulation is
demonstrated on a smart home, designed as an AC-DC microgrid
(MG), having practical appliance data sets, to prove the efficacy of the
Abstract: In this paper, we study the physical layer secrecy performance of a hybrid satellite
and free-space optical (FSO) cooperative system. The satellite links are assumed to follow
the shadowed-Rician fading distribution, and the channel of the terrestrial link between
the relay and destination is assumed to experience the gamma–gamma fading. For the
FSO communications, the effects of different types of detection techniques (i.e., heterodyne
detection and intensity modulation with direct detection) as well as the pointing error are
considered. We derive exact analytical expressions for the average secrecy capacity and
secrecy outage probability (SOP) for both cases of amplify-and-forward (AF) and decodeand-
forward (DF) relaying. The asymptotic analysis for the SOP is also conducted to provide
more insights on the impact of FSO and satellite channels on secrecy performance. It is
found that with the AF with fixed gain scheme, the secrecy diversity order of the investigated
system is only dependent on the channel characteristics of the FSO link and the FSO
detection type, whereas the secrecy diversity is zero when the relay node employs DF or
AF with variable-gain schemes.
With the rapid proliferation of the advanced metering infrastructure, the smart grid is evolving towards increased
customer participation. It is now possible for a utility to influence the customer demand profile via
demand side management techniques such as real-time pricing and incentives. Energy storage devices play a
critical role in this context, and must be optimally utilized. For instance, the peak power demands can be shaved
by charging (discharging) the batteries during periods of low (high) demand. This paper considers the problem
of optimal battery usage under real-time and non-stationary prices. The problem is formulated as a finite-horizon
optimization problem, and solved via an online stochastic algorithm that is provably near-optimal. The proposed
approach gives rise to a class of algorithms that utilize the battery state-of-charge to make usage decisions in
real-time. The proposed algorithms are simple to implement, provably convergent for a wide class of nonstationary
prices, easy to modify for a variety of use cases, and outperform the state-of-the-art techniques, such
as those based on the theory of Markov decision processes or Lyapunov optimization. The robustness and
flexibility of the proposed algorithms is tested extensively via numerical studies in MATLAB and real time digital
Abstract: Quick fault detection and isolation of faulty section are desired in DC microgrid due to the presence of power
electronic converters and low cable impedances. Owing to need of fast disconnection, limited time and data are available for
online fault distance estimation. Some of the existing techniques consider source capacitors connected at only one end of the
cable; therefore, assume that the fault current is contributed by only one end of the cable. This may not be true in the case of
multi-source DC microgrids, where fault current would be supplied from both the ends. Further, existing communication-based
techniques require either data synchronisation or fast communication network. To address these issues, this study proposes an
online fault location method for multi-source DC microgrid without using communication. The mathematical model of faulted
cable section connected to sources at both the ends is derived. This model is used along with the measurements to determine
the fault distance. The model consistency with the measurements is quantified using the confidence level based on the residual
analysis. A ring-type multi-source DC microgrid system is considered and simulated on real-time digital simulator to demonstrate
the effectiveness of the proposed algorithm.
Abstract: Real-time visualisation of large power systems, by tracking the system states, is a challenging task as it involves
processing a large measurement set to obtain the system states. This study proposes a hierarchical parallel dynamic estimation
algorithm to estimate the states of a large-scale interconnected power system. The power system is decomposed into smaller
subsystems, which is processed in parallel to obtain a reduced order state estimate. This information is then transmitted to the
central processor, which collates the individual reduced order estimates to obtain the global estimates. Each processor uses
state matrix of smaller dimension, thereby reducing the computational burden. The low-level processors utilise only a fraction of
the global measurements in the proposed approach, and there is no need for any information exchange from the central
processor to the low level processors, which helps in reducing the communication requirements. Moreover, detection of
anomalies can also be carried out at the local processors without the need for any separate bad data detection at the central
processor. IEEE 30- and 118-bus systems are used as test beds to study the proposed approach.