Showing papers on "Voltage sag published in 2019"

Analysis of the Impact of Electric Vehicle Charging Station on Power Quality Issues

Abstract: Although, number of Electric Vehicles (EV) in Bangladesh are limited compared to conventional vehicles but in near future, expecting that the EV penetration will be increased significantly. Owing to environmental & socio-economic benefits with reduction of fossil fuel use, EV markets are expanding. These EV have a major impact on the power gird & distribution networks and due to the consequences of huge power demand to recharge their batteries. Large number of EV charging station when integrates with the utility grid, it produces harmonics, affect the voltage profile, finally affects the power quality. In this paper, the impact of electric vehicle charging station in Bangladesh on power grid and distribution networks is analyzed in terms of power demand, harmonics, Voltage sag & swelling and transformer power loss. Also, the mitigation technique for reducing power quality disturbances is analyzed in this paper.

Super-Twisting Sliding Mode Control for Gearless PMSG-Based Wind Turbine

Abstract: In recent years, the complexities of wind turbine control are raised while implementing grid codes in voltage sag conditions. In fact, wind turbines should stay connected to the grid and inject reactive power according to the new grid codes. Accordingly, this paper presents a new control algorithm based on super-twisting sliding mode for a gearless wind turbine by a permanent magnet synchronous generator (PMSG). The PMSG is connected to the grid via the back-to-back converter. In the proposed method, the machine side converter regulates the DC-link voltage. This strategy improves low-voltage ride through (LVRT) capability. In addition, the grid side inverter provides the maximum power point tracking (MPPT) control. It should be noted that the super-twisting sliding mode (STSM) control is implemented to effectively deal with nonlinear relationship between DC-link voltage and the input control signal. The main features of the designed controller are being chattering-free and its robustness against external disturbances such as grid fault conditions. Simulations are performed on the MATLAB/Simulink platform. This controller is compared with Proportional-Integral (PI) and the first-order sliding mode (FOSM) controllers to illustrate the DC-link voltage regulation capability in the normal and grid fault conditions. Then, to show the MPPT implementation of the proposed controller, wind speed is changed with time. The simulation results show designed STSM controller better performance and robustness under different conditions.

A Novel Dual-DC-Port Dynamic Voltage Restorer With Reduced-Rating Integrated DC–DC Converter for Wide-Range Voltage Sag Compensation

Abstract: A novel dual-dc-port dynamic voltage restorer (DDP-DVR) is proposed. One low-voltage dc port is directly connected to the energy storage, while the other high-voltage dc port is connected to the intermediate dc bus. An integrated dc–dc converter is interfaced with these two dc ports which is aimed at providing a high-dc-link voltage and maintaining the voltage injection capability even if the energy-storage voltage is lower than the peak value of the injected ac-line voltage. Compared with the traditional DVR that uses the dc–dc converter for all voltage sag compensation, the proposed DDP-DVR can compensate shallow voltage sags without using the dc–dc converter, and for deep voltage sags, only partial active power needs to be processed by the dc–dc converter. Therefore, the power rating of the dc–dc converter can be reduced significantly. Meanwhile, much lower power losses can be achieved by the proposed DDP-DVR due to reduced power conversion stages. The operation principles, modulation strategies, and characteristics of the proposed DDP-DVR are analyzed in detail and verified with experimental results.

Multiobjective Dynamic Voltage Restorer With Modified EPLL Control and Optimized PI-Controller Gains

Abstract: This paper describes a control algorithm based on modified enhanced phase-locked loop (MEPLL) in dynamic voltage restorer (DVR). It compensates distortions and unbalances in the supply voltage along with voltage sag/swell. Three-phase MEPLL extracts the fundamental positive-, negative-, and zero-sequence components from the distorted/unbalanced signals. Further fundamental positive-sequence components are used in the reference load voltage calculations. In addition to track the angle of the input signals similar to the conventional PLLs, the proposed algorithm (MEPLL) offers features of getting fundamental and sequential components in case of distorted or unbalanced grid voltage for all the three phases simultaneously. Optimization approach named as autonomous groups particle swarm optimization, a variant of PSO is used for the calculation of PI-controller gains. The integrated time square error is used as a cost function for optimization of an error between the reference and actual values. This approach of tuning PI gains improves the performance by eliminating the manual process. The proposed control algorithm is implemented in DVR system using MATLAB software and validated in a laboratory environment. The performance shows that the proposed control algorithm gives time effective and satisfactory solution for the unpredictable issues mentioned.

Dynamic voltage restorer with an improved strategy to voltage sag compensation and energy self-recovery

Abstract: In this paper, an improved control scheme is proposed to improve the voltage quality of sensitive loads using dynamic voltage restorer. The existing control strategies either put emphasis on the optimal control during steady operation stage of compensation or correct the phase angle jump in the initial stage of compensation. In current researches, the impact of phase jump characteristic of voltage sag on the load side after voltage sag recoveries is widely ignored, further, there are still drawbacks in existing energy self-recovery strategies of DVR. Therefore, to improve the overall voltage compensation time while correcting the phase jump and accelerate the energy recovery of dc link side, this paper aims to 1) Propose the strategies to minimum active power consumption during voltage compensation stage and maximum active power absorption during energy self-recovery stage. 2) Deliver smooth transition in dynamic process to ensure the flexible switching between two stages. Theoretical analysis of proposed strategy is been validated through simulation and experimental results.

An On-Line Operational Optimization Approach for Open Unified Power Quality Conditioner for Energy Loss Minimization of Distribution Networks

Abstract: This paper presents an on-line operational optimization approach to determine the optimal reactive power/volt-ampere reactive (VAr) set points for open unified power quality conditioner (UPQC-O) under varying load demand of a distribution network. The UPQC-O consists of series and shunt inverters. The modeling of UPQC-O is done to provide VAr support to a network, to mitigate supply voltage sag to the downstream buses, and to eliminate harmonics present in the line current. A non-linear optimization problem is formulated to determine the optimal VAr set points for the inverters of UPQC-O in each loading condition. The objective function is the minimization of energy loss. The limits on bus voltage magnitude, line current flow, and maximum VAr support capacities of inverters are considered to be the operational constraints. The optimization is performed with the data of hourly varying load demand for different seasons in a year. The optimization problem is solved using CONOPT solver of general algebraic modeling system. The cost-benefit analysis and the infrastructure required for the real time implementation of the proposed methodology are also provided. The simulation results show the effectiveness of the proposed approach in energy loss reduction.

Enhancing Transient Voltage Quality in a Distribution Power System With SMES-Based DVR and SFCL

Abstract: Conceptual design and performance evaluation of a voltage sag compensation scheme based on the superconducting fault current limiter (SFCL) and a MW-class dynamic voltage restorer (DVR) system integrated with superconducting magnetic energy storage (SMES) are presented and investigated. A pre-sag compensation strategy is introduced to lock the instantaneous magnitudes and phase angles of real-time line voltages for compensating the improper voltage components completely. A 0.3-H/1.76-kA SMES magnet is structurally designed for MW-class power exchange operations, and the SFCL's resistance is also estimated in detail. Various simulation cases with regard to voltage sag are carried out. The simulations have demonstrated that the proposed voltage sag compensation scheme is able to maintain the stabilizations of root-mean-square voltages, and mitigate the adverse effects of voltage sag on sensitive load. Furthermore, the compensated power injected to sensitive load by DVR is decreased owing to the additional voltage improvement provided by SFCL, thereby reducing the total capital costs of DVR.

A Multi-Mode Flexible Power Point Tracking Algorithm for Photovoltaic Power Plants

Abstract: Flexible power point tracking (FPPT) is the control of active power generated by grid-connected photovoltaic power plants (GCPVPPs) to provide grid-support functionality. An FPPT algorithm for the reduction of the extracted power from photovoltaic (PV) strings during voltage sags was previously proposed by the authors. An advantage of this algorithm, compared to conventional FPPT algorithms, was its fast dynamics facilitated by use of a simple PI controller that dynamically modifies the PV voltage reference. The previously proposed scheme could only be employed for the short duration in which the power system experiences a voltage sag. A novel modification to this algorithm with multi-mode operation is introduced in this letter, which provides FPPT capability for continuous operation of GCPVPPs. Unlike the previous algorithm, which was able to only move the operation point to the right-hand side of MPP, the proposed algorithm in this letter is able to move the operation point to both right- and left-hand sides of the MPP that provides the flexibility to operate in the optimum operation region for both single- and two-stage GCPVPPs. Experimental results are provided to demonstrate the performance of the proposed algorithm under dynamic irradiance conditions.

Adaptive shunt filtering control of UPQC for increased nonlinear loads

Abstract: In a distribution system, to compensate for the effect of voltage sag/swell and mitigate harmonic currents, a unified power quality conditioner (UPQC) is used. Conventionally, the power rating of UPQC is determined based on the expected sag/swell, linear and nonlinear loads. However, with the shift towards usage of electronic loads and electrical drives, the composition of nonlinear loads is increasing. Hence, the UPQC deployed earlier may not be capable of handling increased harmonic currents due to its limited current rating. While trying to compensate for both harmonic currents and voltage sag/swell, the shunt converter (of UPQC) may reach its current limit and would result in undesirable behaviour. To address this issue, a technique to prioritise sag/swell compensation over active filtering is proposed in this study. The remaining power capacity of UPQC (after sag/swell compensation) is utilised for harmonic filtering. Adaptive determination of harmonic compensation current also allows prioritising the various harmonics to be compensated. This helps in case passive tuned filters are installed for some harmonic frequencies for which UPQC need not respond. Detailed simulation studies are performed on MATLAB/SIMULINK platform and results are included in this study. A laboratory prototype of single phase UPQC is developed to validate the performance of the proposed scheme.

Droop Control for DC Multi-Microgrids Based on Local Adaptive Fuzzy Approach and Global Power Allocation Correction

Abstract: A new droop control strategy is proposed for dc multi-microgrids including constant power load (CPL) and resistance load. The droop control is designed by taking advantage of local adaptive fuzzy approach and global power allocation correction (GPAC). The local adaptive fuzzy droop control based on the CPL compensator with disturbance observer (DO) is designed for distributed generations in individual dc microgrid, which can eliminate the effect of line resistance, CPL and other large-signal disturbances. The CPL compensator can automatically generate a reference voltage compensation signal for converter inner ring control by the CPL DO. The adaptive fuzzy inference system can dynamically regulate the droop coefficient, increase the accuracy of power allocation, and enhance the system stability margin. The GPAC is constructed for an individual dc microgrid to restore voltage sag caused by the droop coefficient and achieve global power allocation among dc multi-microgrids, which is implemented by a normalized method. Furthermore, the power exchange control law is designed for GPAC, which can avoid unnecessary global operation. By using sparse communication, the proposed strategy can ensure the power allocation accuracy and reduce the complexity of the control system. The simulation results of MATLAB/Simulink platform and the experimental results of RTDS show the effectiveness of the proposed strategy.

Modeling and Control of a Multiport Converter based EV Charging Station with PV and Battery

Abstract: As an environmental friendly vehicle, the increasing number of electrical vehicles (EVs) leads to a pressing need of widely distributed charging stations, especially due to the limited on-board battery capacity. However, fast charging stations, especially super-fast charging stations may stress power grid with potential overload at peaking time, sudden power gap and voltage sag. This paper discusses the detailed modeling of a multiport converter based EV charging station integrated with PV power generation, and battery energy storage system, by using ANSYS TwinBuilder. In this paper, the control scheme and combination of PV power generation, EV charging station, and battery energy storage (BES) provides improved stabilization including power gap balancing, peak shaving and valley filling, and voltage sag compensation. As a result, the influence on power grid is reduced due to the matching between daily charging demand and adequate daytime PV generation. Simulation results are presented to confirm the benefits at different modes of this proposed multiport EV charging circuits with the PV-BES configuration. Furthermore, SiC devices are employed to the EV charging station to further improve the efficiency. For different modes and functions, power losses and efficiency are investigated and compared in simulation with conventional Si devices based charging circuits.

Dynamic Voltage Restorer With Quasi-Newton Filter-Based Control Algorithm and Optimized Values of PI Regulator Gains

Abstract: In this paper, a three-phase three-wire dynamic voltage restorer (DVR) system has been chosen with quasi-Newton (QN) filter-based control algorithm. It uses Hessian matrix iteratively which is also named as QN based on modified Broyden–Fletcher–Goldfarb–Shanno (MBFGS) in each phase. The main features of this algorithm are the fundamental frequency estimation, rejection of harmonics its components, and tracking of the peak value of voltage or current. As DVR in this application has been implemented for compensation of voltage disturbances like imbalance and distortions along with voltage sag and swell, the fundamental component (FC) of the supply voltage is essential components to design control algorithm. The QN technique using MBFGS has been used for exaction of the FC from each disturbed supply voltage, which has been utilized in generating the reference load voltage. For proportional integrator (PI) regulator gains estimation, the population-based optimization technique, called as multiverse optimization (MVO), has been introduced in this paper. The MVO algorithm has adaptive coefficients which gives the benefits of smoothly maximizing the possibility of wormholes in the universe and increases the accuracy of the local search. The validation of control algorithm with optimization techniques-based PI tuning for DVR has been done using MATLAB software and real-time implementation.

Delayed-Signal-Cancellation-Based Sag Detector for a Dynamic Voltage Restorer in Distorted Grids

Abstract: Dynamic voltage restorers (DVRs) are a cost-effective solution to protect sensitive loads against voltage sags in medium- and high-power applications because the power required for the compensation is only a fraction of the load rated power. Voltage sag compensation with DVRs requires a robust sag detector that has to be able to address distorted grid conditions. Also, sags should be detected fast in order to guarantee adequate load protection. In this paper, a sag detection algorithm for a DVR is proposed and comprehensively analyzed. It is shown that this algorithm provides harmonic cancellation properties when the parameters are adequately tuned. Implementation aspects are addressed carefully and a method to adapt the algorithm when the grid frequency fluctuates is explained. The proposed algorithm was experimentally tested in a 5 kVA prototype of a DVR that protected a sensitive load (linear and non-linear) against voltage sags and voltage harmonics. The proposed algorithm was compared with three alternatives already proposed in the literature in terms of detection time, performance against voltage harmonics, and computational burden.

DC-Link Voltage Control of a Grid-Connected Solar Photovoltaic System for Fault Ride-Through Capability Enhancement

Abstract: The high penetration level of solar photovoltaic (SPV) generation systems imposes a major challenge to the secure operation of power systems. SPV generation systems are connected to the power grid via power converters. During a fault on the grid side; overvoltage can occur at the direct current link (DCL) due to the power imbalance between the SPV and the grid sides. Subsequently; the SPV inverter is disconnected; which reduces the grid reliability. DC-link voltage control is an important task during low voltage ride-through (LVRT) for SPV generation systems. By properly controlling the power converters; we can enhance the LVRT capability of a grid-connected SPV system according to the grid code (GC) requirements. This study proposes a novel DCL voltage control scheme for a DC–DC converter to enhance the LVRT capability of the two-stage grid-connected SPV system. The control scheme includes a “control without maximum power point tracking (MPPT)” controller; which is activated when the DCL voltage exceeds its nominal value; otherwise, the MPPT control is activated. Compared to the existing LVRT schemes the proposed method is economical as it is achieved by connecting the proposed controller to the existing MPPT controller without additional hardware or changes in the software. In this approach, although the SPV system will not operate at the maximum power point and the inverter will not face any over current challenge it can still provide reactive power support in response to a grid fault. A comprehensive simulation was carried out to verify the effectiveness of the proposed control scheme for enhancing the LVRT capability and stability margin of an interconnected SPV generation system under symmetrical and asymmetrical grid faults.

Fuel cell integrated unified power quality conditioner for voltage and current reparation in four-wire distribution grid

Abstract: Electrical and electronic devices, when exposed to one or more power quality problems, are prone to failure. This study aims to enhance the quality of power in three-phase four-wire distribution grid using fuel cell integrated unified power quality conditioner (FCI-UPQC). The proposed FCI-UPQC has a four-leg converter on the shunt side and three-leg converter on the series side. A combination of a synchronous reference frame and instantaneous reactive power theories is utilised to generate reference signals of the FCI-UPQC. Also, this study proposes an adaptive neuro-fuzzy inference system (ANFIS) controller to maintain the DC-link voltage in the FCI-UPQC. The ANFIS controller is designed like a Sugeno fuzzy architecture and trained offline using data from the proportional–integral controller. The obtained results proved that the proposed FCI-UPQC compensated power quality problems such as voltage sag, swell, harmonics, neutral current, source current imbalance in the three-phase four-wire distribution grid. The presence of fuel cell in this work makes more effectiveness of the proposed system by providing real power support during supply interruption on the grid side.

A Multiple Improved Notch Filter-Based Control for a Single-Stage PV System Tied to a Weak Grid

Abstract: In this paper, a control scheme based on improvement in a generalized integrator is implemented on a three-phase single-stage grid-tied solar photovoltaic system with the distribution static compensator capabilities under grid abnormal conditions of voltage distortion and voltage unbalance. The photovoltaic voltage-source converter system compensating for the reactive power consumed by nonlinear load at point of common coupling provides load balancing and mitigates harmonics. The proposed multiple-improved-notch-filter-based quadrature signal generator control approach extracts the load current fundamental component, independent of the grid voltage. This control has better dc offset and harmonics component rejection capability in comparison to a conventional second-order generalized integrator algorithm. The perturb and observe-based maximum power point tracking algorithm is applied for the extraction of maximum power from the photovoltaic array. The system is analyzed under different abnormal conditions of voltage distortions, voltages unbalance, voltage swell, voltage sag, load currents unbalance, and insolation change on a prototype developed in the laboratory. The system performance is found to be satisfactory, within limits as described in an IEEE-519 standard while feeding active power to distribution network and connected loads.

Sensitivity of Programmable Logic Controllers to Voltage Sags

Abstract: This paper discusses the sensitivity of programmable logic controllers (PLCs) to voltage sags based on mechanistic analysis and numerous tests First, the sag sensitivity caused by different values of rated working voltage is shown through an analysis of the power supply and discrete input/output modules of PLCs Second, a detailed sag-tolerance experimental scheme is proposed, and PLC sensitivity curves that differ from early curves are established through several experimental tests Experiments are conducted with nine typical PLCs The sensitivity of the power supply and discrete input modules are researched first, and then, the effect of the key characteristics of voltage sag, namely, magnitude, duration, points-on-wave of sag initiation, odd harmonics, multiple sags, and continuous sags, is tested The uncertain area of the PLC sag tolerance is identified after intensively processing the extensive experimental data and curve fitting A more accurate sag influence assessment for PLCs can be made with the given curves compared to the early curves Furthermore, experiments have revealed that certain PLCs may work in an uncertain status during voltage sags The reason and the method of elimination with the proper choice of dc bus capacitance for PLCs are also analyzed

A sliding mode approach to enhance the power quality of wind turbines under unbalanced voltage conditions

Abstract: An integral terminal sliding mode-based control design is proposed in this paper to enhance the power quality of wind turbines under unbalanced voltage conditions. The design combines the robustness, fast response, and high quality transient characteristics of the integral terminal sliding mode control with the estimation properties of disturbance observers. The controller gains were auto-tuned using a fuzzy logic approach. The effectiveness of the proposed design was assessed under deep voltage sag conditions and parameter variations. Its dynamic response was also compared to that of a standard SMC approach. The performance analysis and simulation results confirmed the ability of the proposed approach to maintain the active power, currents, DC-link voltage and electromagnetic torque within their acceptable ranges even under the most severe unbalanced voltage conditions. It was also shown to be robust to uncertainties and parameter variations, while effectively mitigating chattering in comparison with the standard SMC.

A novel low voltage ride through strategy for cascaded power electronic transformer

Abstract: In view of the operating characteristics for voltage sags of AC side of the power electronic transformer(PET), a low-voltage ride through(LVRT) strategy adapted to bidirectional power exchange of PET is proposed for the purposes of maintaining the system stability, assisting the system voltage recovery and protecting PET safety. During the asymmetric voltage sag, the negative sequence current of PET is eliminated to ensure the symmetry of the injected current. According to the degree of positive sequence voltage sag, the reactive current injection is provided to assist in voltage recovery. According to the PET active power condition before the voltage sag, the level and direction of which are maintained as far as possible without exceeding the limit, for which the disturbance to the AC and DC grids is reduced. Finally, the effectiveness of the proposed LVRT strategy is verified by simulation model.

A Discrete-Time Control Method for Fast Transient Voltage-Sag Compensation in DVR

Abstract: This paper presents a discrete-time domain control scheme for balanced voltage sag compensation using a Dynamic Voltage Restorer (DVR), which is recognized to be an appropriate and economical power electronic device with which to ameliorate these disturbances. The proposed control method is implemented in the synchronous reference frame (SRF), with two nested regulators, one of which includes an integral action. This algorithm has some advantages with respect to other control algorithms, such as the fact that the proposed methodology permits all the closed-loop poles of the DVR system to be placed in the desired locations in order to define the dynamical behavior with a reduction in the number of the electrical magnitudes to be measured and without the need for state observers, as occurs in traditional control methods. What is more, the well-known inner current loop implemented in other control schemes, which is employed to attenuate the resonance of the plant, is unnecessary. Furthermore, the unbalanced voltage sag compensation can be achieved by adding a “plug-in” controller and following the same methodology presented for balanced voltage sags to design the controller. The good performance of the proposed control scheme is validated by means of simulation and experimental results carried out with a 5 kW DVR laboratory prototype. The discrete-time control method is also compared with two control schemes previously proposed in literature.

Stochastic Characterization of Voltage Sag Occurrence Based on Field Data

Abstract: Computational methods for predicting voltage sags consider their occurrence as a Poisson process, although without confirmation from monitoring data, until now. In this paper, the stochastic nature of voltage sags is analyzed and discussed using monitoring data from 60 sites of the Portuguese Transmission Network, covering the years 2011–2015 (a total of 17 157 recorded voltage sags). A mathematical model to describe the voltage sag occurrence as a stochastic process is presented. The assumption of constant failure rates of the network elements implies that the occurrence of voltage sags is a Poisson process. However, that assumption is not valid if voltage sag clusters are included, as these imply considering time-dependent failure rates. Then, the time between voltage sags is described by an exponential distribution, if clusters are not included, and may be described by the gamma distribution, if including clusters. The boundaries of the adequacy of exponential and gamma distributions are assessed, based on monitoring data. The time of occurrence of monitored voltage sags are analyzed and results confirm that the Poisson process describes the occurrence of voltage sags when voltage sag clusters are disregarded. The gamma distribution fitting is also confirmed when clusters are included in the analysis.

Optimal control of novel fuel cell-based DVR using ANFISC-MOSSA to increase FRT capability of DFIG-wind turbine

Abstract: Although Doubly fed induction generator (DFIG)-based wind turbine has several significant advantages, any voltage fluctuation can severely disturb its normal and safe operation, i.e., it is characteristically very sensitive. Hence, fault ride-through is determined as a prominent benchmark for DFIG which must be enhanced by a voltage compensator. Dynamic voltage restorer (DVR), a voltage fluctuation mitigator, is connected in-series with distribution and sub-transmission lines so that DFIG can appropriately operate during any type of disturbance. In this paper, a new two switches boost converter (TSBC) coupled with quinary multilevel inverter (QMLI) is proposed which is fed by fuel cell to produce staircase sinusoidal voltage for DVR. The proposed step-up DC/DC presents two important advantages: low current ripple and low switch stress, while QMLI can create high-step staircase sinusoidal voltage without considering a number of switches. TSBC-QMLI-based DVR has been controlled by neuro fuzzy inference system controller (ANFISC). As the control problem is multi-objective and complex, Multi-Objective Salp Swarm Algorithm (MOSSA) has been applied to optimally tune the ANFISC’s parameters, and then compared with Non-dominated Sorting Genetic Algorithm II (NSGA-II). To verify and validate the compensation capability of proposed DVR, it has been thoroughly evaluated under different voltage disturbances. Eventually, the simulation results have validated the compensation capability of the suggested TSBC-QMLI-based DVR which is accurately controlled by MOSSA-based ANFISC to mitigate the balanced and unbalanced voltage sag and swell along with the flicker and harmonic voltage.

Power Quality Disturbances Assessment during Unintentional Islanding Scenarios. A Contribution to Voltage Sag Studies

Abstract: This paper presents a novel voltage sag topology that occurs during an unintentional islanding operation (IO) within a distribution network (DN) due to large induction motors (IMs). When a fault occurs, following the circuit breaker (CB) fault clearing, transiently, the IMs act as generators due to their remanent kinetic energy until the CB reclosing takes place. This paper primarily contributes to voltage sag characterization. Therefore, this novel topology is presented, analytically modelled and further validated. It is worth mentioning that this voltage sag has been identified in a real DN in which events have been recorded for two years. The model validation of the proposed voltage sag is done via digital simulations with a model of the real DN implemented in Matlab considering a wide range of scenarios. Both simulations and field measurements confirm the voltage sag analytical expression presented in this paper as well as exhibiting the high accuracy achieved in the three-phase model adopted.

GI-Based Control Scheme for Single-Stage Grid Interfaced SECS for Power Quality Improvement

Abstract: This paper presents an improved generalized integrator (GI)-based control with a frequency locked loop for multifunctional three-phase single-stage grid interfaced solar energy conversion system for power quality enhancement of the distribution network under abnormal grid conditions. The perturb and observe-based maximum power point tracking technique is utilized to obtain peak power from solar photovoltaic array under varying atmospheric conditions. This control scheme provides unity power factor operation, load balancing, harmonics mitigation, and reactive power compensation. The improved GI control algorithm has an advantage of better dc offset and harmonics rejection capabilities as compared to a conventional second-order GI algorithm. To substantiate the control scheme, tests are performed on a paradigm in the research laboratory for manifold operating conditions. Test results show the satisfactory behavior under steady state and dynamic operating scenario such as unbalanced load, solar irradiations variation, voltage sag, and swell and distorted voltage grid. The total harmonic distortions of the grid voltages–currents are achieved within constraints of grid code compliance of an IEEE 519 and 1564 standards.

A Novel Low Voltage Ride-Through Technique of Three-Phase Grid-Connected Inverters Based on a Nonlinear Phase-Locked Loop

Abstract: In this paper, a novel low voltage ride-through (LVRT) technique for three-phase grid-connected inverters is proposed. The proposed technique consists of two parts: a nonlinear phase locked loop based on complex-coefficient filters (NLCCF-PLL) and an LVRT control scheme. Generally, the synchronization process of three-phase grid-connected inverters is performed via PLL with a relatively low bandwidth, which delays the detection of voltage sag and recovery during the LVRT process. To accelerate the synchronization process, the NLCCF-PLL with adaptive controller gains is proposed to improve both the filtering capability and dynamic performance of PLL at the same time. The stability of the NLCCF-PLL is validated by the second method of Lyapunov in the nonlinear model, and the superiority of its operating performance is verified. The proposed LVRT control scheme consists of a reference current calculation block to effectively suppress the power ripples and an inner loop controller with strong robustness as well as fast dynamic response. By comparing the proposed LVRT technique with the existing LVRT technique on the basis of experimental results, the superiority of the proposed LVRT technique is confirmed.

An Algorithm for Fast Flexible Power Point Tracking in Photovoltaic Power Plants

Abstract: Flexible power point tracking (FPPT) is control of active power generated by grid-connected photovoltaic power plants (GCPVPPs) to provide various grid-support functionalities, such as frequency response and voltage support. Fast transient response is required during grid voltage and frequency variations. An algorithm, based on dynamic voltage reference design and model predictive control for the dc-dc converter in GCPVPPs is introduced in this paper. The main novelty of the proposed algorithm is fast dynamic response based on the maximum capacity of the system. This feature enables fast FPPT operation in GCPVPPs. The proposed control algorithm results in several features for the FPPT operation, including: higher bandwidth, low power oscillations during steady-state, flexibility for adding constraints based on the operation conditions, and ease of implementation without requiring any complex control parameters tuning and design procedure. The transient performance of the proposed algorithm is evaluated under conditions of voltage sag and frequency deviation. Experimental results are also provided to demonstrate the effectiveness of the proposed algorithm when subjected to rapid changes in PV reference power.