Distorted Grid Conditions Research Articles

Scalable Single-Phase Multi-Functional Inverter for Integration of Rooftop Solar-PV to Low-Voltage Ideal and Weak Utility Grid

Integration of rooftop solar-PV (RTSPV) systems and extensive use of nonlinear loads in the low-voltage distribution system (LVDS) leads to poor power quality (PQ). Therefore, it is necessary to address the issues leading to poor PQ at the point of common coupling of the LVDS. In this article, a multi-band hysteresis current control (MB-HCC) for the multi-functional inverter (MFI) is proposed which improves the efficiency of the MFI and also enhances the PQ of the LVDS. The MB-HCC uses simple switching logic and outperforms in its multi-functional tasks such as active power injection and power conditioning. MB-HCC offers better efficiency over variable double-band HCC (VDB-HCC) as it operates at a lower switching frequency. The performance of the proposed system is simulated by using MATLAB/Simulink and validated by OPAL-RT based real-time simulation studies. During the variation of solar irradiation, the proposed MFI has an average efficiency of 98.5% under the ideal grid and 97.34% under the distorted grid. Moreover, the percentage of Total Harmonic Distortion under ideal and distorted grid conditions is brought down to below 5%, and also, reactive power compensation maintains unity power factor operation complying with the IEEE-519-2014 and 1547 standards. These results substantiate the hypothesis of scalability of the single-phase MB-HCC-based MFI for an LVDS contributing to economy and ecology.

Adaptive Observer Based Control for Rooftop Solar PV System

In this paper, the multifunctional adaptive observer-based control scheme is proposed for double-stage grid interacted solar energy conversion system with distribution static compensator (DSTATCOM) abilities. The adaptive observer based control scheme facilitates the manifold power quality (PQ) functionalities in the distribution grid such as grid currents balancing, unity power factor, harmonics compensation, adaptive dc-link voltage regulation, etc. The dc-link voltage is varied adaptatively in proportion to the common coupling point voltage to reduce switching losses in the voltage source converter and to evade its tripping in weak distribution grid. The global asymptotic stability of the adaptive observer is demonstrated with zero steady-state error using passivity, Lyapunov stability theory, and LaSalle's invariance principle. To verify the effectiveness of the proposed control scheme, an experimental prototype is realized in the laboratory under manifold dynamic conditions such as load currents imbalancing, variation of solar irradiations, DSTATCOM-SPVA (solar photovoltaic array) mode, SPVA-DSTATCOM mode, under voltage, over voltage distorted grid conditions. The experimental comparative performances of the system, are demonstrated to validate the effectiveness of the proposed control strategy over conventional control scheme under distorted grid voltages and dynamic operating conditions. Moreover, the loss analysis between adaptive and fixed dc-link voltages is analyzed for double stage SPVA system.

Dual Role CDSC-Based Dual Vector Control for Effective Operation of DVR With Harmonic Mitigation

For the effective operation of a dynamic voltage restorer (DVR), a control strategy plays a significant role. This paper presents an enhanced control strategy for DVR using dual role cascaded delay signal cancellation (CDSC)-based dual vector control (DVC) under unbalanced and distorted grid conditions. Based on the numerical analysis, it is found that the CDSC prefilter is a promising solution when grid voltage is distorted by symmetric, asymmetric harmonics, and unbalanced sag. Mainly, the CDSC prefilter extracts instantaneous symmetrical components of the grid voltage required for voltage sags detection and generation of fundamental component of reference voltage for the DVR. A CDSC-based DVC algorithm with inductor current and capacitor voltage feedback is implemented in a synchronous reference frame, which tracks the fundamental DVR reference voltages. An extractor based on the modified CDSC strategy is designed to extract harmonics from load voltage during distorted grid conditions. These extracted harmonic components (nonfundamental) are added in phase opposition with fundamental component and fed to pulse width modulation block to generate reference voltages. Experimental studies are conducted on scaled down (100 V, 0.5 kVA) laboratory prototype DVR to verify the effectiveness of the proposed control algorithm under unbalanced and distorted grid conditions.

Application of Multi-Resonator Notch Frequency Control for Tracking the Frequency in Low Inertia Microgrids Under Distorted Grid Conditions

Frequency variations are much faster and more significant in microgrids with low-inertia inverters and other control devices for distributed generation. Accordingly, it is imperative to utilize a concise frequency estimation method for analyzing the transient behavior of microgrids. This paper proposes a robust algorithm which is based on multi-resonator and adaptive notch filter and is referred to as the multi-resonant notch frequency control for improving the frequency estimation of perturbed power system. In general, power system distortions such as harmonics, inter-harmonics, and faults may affect the accuracy of frequency estimation. In contrast to conventional methods for frequency estimator, the proposed method is less sensitive to distortions and unbalanced power system conditions. The proposed method uses the information in all three-phase signals which is proved to be more robust against signal variations than single-phase methods. Simulation results demonstrate the accuracy and the speed of the proposed method for frequency estimation in both grid-connected and islanded mode of low inertia microgrids under grid faults.

Frequency Adaptive Prefiltering Stage for Differentiation-Based Control of Shunt Active Filter Under Polluted Grid Conditions

A differentiation-based control method for a shunt active power filter is used in this paper for functioning in the presence of grid voltages distortion and imbalances. To enable the fast and accurate estimation of the variables in the presence of unbalancing or distortion in the grid voltages, a prefiltering scheme is considered based on the dual second-order generalized integrator approach. The signal recomposition performed by the prefilter isolates the harmonics from the voltage and current waveforms, and the differentiator frequency locked loop (dFLL) is able to estimate the frequency of the waveforms, which is used as a feedback for the prefilter, thus making the system frequency adaptive. The control algorithm is validated with both simulation studies. The prototype of an active power filter is developed, which carried out current compensation robustly, demonstrating the speed and accuracy of the system even under distorted grid conditions by achieved total harmonic distortions (THDs) of the grid currents within limits (less than 5%) as dictated by IEEE Std. 519-2014.

Comparison of Synchronization Techniques Under Distorted Grid Conditions

In grid-connected power converter applications, the phase-locked loop (PLL) is probably the most widely used grid synchronization technique, owing to its simple implementation. However, in power grids some very common problems, such as voltage distortion, voltage unbalance, and frequency instability make synchronization a challenging task. The performance of the conventional synchronous reference frame PLL (SRF-PLL) is greatly reduced in the presence of distorted grid conditions. For a polluted grid some advance PLL techniques have been proposed, such as moving average filter PLL (MAF-PLL) and cascaded delayed signal cancellation PLL (CDSC-PLL). These techniques have been mostly evaluated in the presence of odd and even harmonics but the effects of interharmonics on these synchronization techniques still needs to be investigated. In this paper, a detailed performance comparison has been made between SRF-PLL, MAF-PLL, and CDSC-PLL for grid voltages contaminated with interharmonics in the presence of different grid disturbances, such as frequency jump, phase angle jump, and dc offset. The techniques are simulated using Matlab/Simulink. The CDSC-PLL shows excellent performance as compared with other techniques in terms of dynamic response as it settles to frequency step change in a half cycle but the presence of interharmonics greatly reduces its filtering capability. On the other hand, MAF-PLL gives a ripple free behavior in frequency estimation but with a much slower dynamic response as it settles to a frequency step change in more than three cycles. SRF-PLL only performs well under harmonics free grid voltages.

Internal Model-Based Active Resonance Damping Current Control of a Grid-Connected Voltage-Sourced Converter With an LCL Filter

Three-phase passive harmonic filters (e.g., LCL filters) are often utilized to attenuate harmonic currents caused by pulsewidth modulation of voltage-sourced converters (VSC) to achieve high power quality. However, LCL filters may jeopardize the system stability due to their inherent susceptibility to resonance. This paper proposes an active damping current controller based on the internal model principle for a grid-connected VSC equipped with an LCL filter. This controller can be considered a high-order multi-input multi-output transfer function, implemented in the $dq$ reference frame, including decoupling terms for improved transient behavior. Simulation case studies are performed in the PSCAD/EMTDC environment to evaluate the transient behavior of the controller and test its robustness against parameter variations and unbalanced and distorted grid conditions. Experimental results verify the performance of the proposed controller and confirm its ability to operate at low switching frequencies. The results are compared with two existing active damping strategies (virtual RC and notch filter methods), confirming that the proposed controller shows significantly improved robustness with no degradation of transient behavior, has the ability to operate at low switching frequencies, and does not need additional sensors.

An LQR-Based Controller Design for an LCL-Filtered Grid-Connected Inverter in Discrete-Time State-Space under Distorted Grid Environment

In order to alleviate the negative impacts of harmonically distorted grid conditions on inverters, this paper presents a linear quadratic regulator (LQR)-based current control design for an inductive-capacitive-inductive (LCL)-filtered grid-connected inverter. The proposed control scheme is constructed based on the internal model (IM) principle in which a full-state feedback controller is used for the purpose of stabilization and the integral terms as well as resonant terms are augmented into a control structure for the reference tracking and harmonic compensation, respectively. Additionally, the proposed scheme is implemented in the synchronous reference frame (SRF) to take advantage of the simultaneous compensation for both the negative and positive sequence harmonics by one resonant term. Since this leads to the decrease of necessary resonant terms by half, the computation effort of the controller can be reduced. With regard to the full-state feedback control approach for the LCL-filtered grid connected inverter, additional sensing devices are normally required to measure all of the system state variables. However, this causes a complexity in hardware and high implementation cost for measurement devices. To overcome this challenge, this paper presents a discrete-time current full-state observer that uses only the information from the control input, grid-side current sensor, and grid voltage sensor to estimate all of the system state variables with a high precision. Finally, an optimal linear quadratic control approach is introduced for the purpose of choosing optimal feedback gains, systematically, for both the controller and full-state observer. The simulation and experimental results are presented to prove the effectiveness and validity of the proposed control scheme.

Robust Control Scheme for Three-Phase Grid-Connected Inverters With <italic>LCL</italic>-Filter Under Unbalanced and Distorted Grid Conditions

The grid voltage, especially under unbalanced and harmonically distorted grid conditions, often distorts the injected currents of grid-connected inverters. To address this problem, a robust control scheme of grid-connected inverters is presented in this paper. The proposed scheme is achieved by an internal model (IM)-based current controller and a robust phase-locked loop (PLL) scheme. The robust PLL scheme employs open-loop filtering technique to offer exact steady-state performance as well as fast transient response. The IM-based current controller is stabilized using the linear matrix inequality approach to optimize the feedback gains, and implemented in the synchronous reference frame to reduce computational efforts. A state estimator is also introduced into current controller for the purpose of reducing the number of current sensors, which are often required in state feedback controller to damp the unstable dynamics of LCL filters. The validity of the proposed control scheme is demonstrated through comparative simulations and experimental results using a prototype grid-connected inverter.

Grid Synchronization With Selective Harmonic Detection Based on Generalized Delayed Signal Superposition

Grid synchronization has always been an important challenge for grid-connected converters under extremely distorted grid conditions. Moreover, how to quickly and accurately extract multiple required harmonics is also essential for control systems. In this paper, two types of generalized delayed signal superposition (GDSS) operators capable of extracting any arbitrary harmonic component out of an input signal are derived. In order to show the benefits of GDSS operators, a new grid information estimation concept based on multiple GDSS operators (referred to as MGDSS-PLL) is proposed to track the fundamental and multiple targeted harmonics under single-phase and three-phase adverse grid conditions. MGDSS-PLL can be flexibly tuned to extract any harmonic components according to specific requirements within one fundamental period, and it also exhibits great robustness to grid disturbances. Simulations and experimental results are presented for verifying the performance of the MGDSS-PLL.

A Hybrid Filtering Technique-Based PLL Targeting Fast and Robust Tracking Performance under Distorted Grid Conditions

In most grid-connected power converter applications, the phase-locked loop (PLL) is probably the most widespread grid synchronization technique, owing to its simple implementation. However, its phase-tracking performance tends to worsen when the grid voltage is under unbalanced and distorted conditions. Many filtering techniques are utilized to solve this problem, however, at the cost of slowing down the transient response. It is a major challenge for PLL to achieve a satisfactory dynamic performance without degrading its filtering capability. To tackle this challenge, a hybrid filtering technique is proposed in this paper. Our idea is to eliminate the fundamental frequency negative sequence (FFNS) and other harmonic sequences at the prefiltering stage and inner loop of PLL, respectively. Second-order generalized integrators (SOGIs) are used to remove FFNS before the Park transformation. This makes moving average filters (MAFs) eliminate other harmonics with a narrowed window length, which means the time delay that is caused by MAFs is reduced. The entire hybrid filtering technique is included in a quasi-type-1 PLL structure (QT1-PLL), which can provide a rapid dynamic behavior. The small-signal model of the proposed PLL is established. Based on this model, the parameter design guidelines targeting the fast transient response are given. Comprehensive experiments are carried out to confirm the effectiveness of our method. The results show that the settling time of the proposed PLL is less than one grid cycle, which is shorter than most of the widespread PLLs. The harmonic rejection capability is also better than other methods, under both nominal and adverse grid conditions.

FPGA‐based implementation for improved control scheme of grid‐connected PV system with 3‐phase 3‐level NPC‐VSI

SummaryThe large scale penetration of renewable energy resources has boosted the need of using improved control technique and modular power electronic converter structures for efficient and reliable operation of grid‐connected systems. This study investigates the performance of a grid‐connected 3‐phase 3‐level neutral‐point clamped voltage source inverter for renewable energy integration by using improved current control technique. For medium or high‐voltage grid interfacing, the multilevel inverter structure is generally used to reduce the voltage stress across the switching device as well as the harmonic distortion. The neutral‐point clamped voltage source inverter is controlled by using decoupling technique along with the proper grid synchronization via moving average filter–based phase‐locked loop. The moving average filter–based phase‐locked loop is used to reduce the delay in grid angle estimation under balanced as well as distorted grid conditions. A Lyapunov‐based approach for analysing the stability of the system has also been discussed. In this study, the hardware‐in‐loop (HIL) simulation of the control algorithm and the grid synchronization technique is realized using Virtex‐6 FPGA ML605 evaluation kit. The performance of the system is analyzed by conducting a time‐domain simulation in the Matlab/Simulink platform and its performance is examined in the HIL environment. The simulation and the hardware cosimulation results are presented to validate the effectiveness of the proposed control scheme.

An Adaptive Digital-Control Scheme for Improved Active Power Filtering Under Distorted Grid Conditions

The operation of active power filters (APFs) under nonideal grid conditions, such as grid-frequency fluctuation and voltage harmonics, can lead to significant degradation in harmonic compensation performance. This paper proposes an adaptive digital-control scheme for a three-phase APF for use in harmonically distorted and variable-frequency grid conditions. This scheme is comprised of a grid-frequency adaptive resonant current controller and an enhanced synchronous-reference-frame phase-locked loop (SRF-PLL). The PLL uses an inherently stable adaptive-filtering stage to improve grid phase and frequency estimates in the presence of voltage harmonics. The improved PLL frequency estimate is used to update the resonant gains of a PI + vector-proportional-integral current-control scheme, implemented in the SRF. This enables the APF to maintain optimal performance in distorted grid conditions. The performance of the proposed APF control scheme is evaluated in a test microgrid, with a 15-kVA three-phase voltage-source converter configured as the APF, a 90-kVA grid emulator utilized to replicate distorted grid conditions, and a load emulator implemented to draw harmonic currents. The control scheme presented here is shown to demonstrate significant performance improvements under nonideal grid conditions compared with equivalent adaptive and nonadaptive methods.

A Novel Multifrequency Current Reference Calculation to Mitigate Active Power Fluctuations

The rise of distributed power generation systems comes along with an energy storage systems increment. Moreover, the expensiveness of batteries and dc capacitors generate interest on extending their lifetime, directly related with dc current ripple. Unlike other multifrequency approaches, in this paper, the harmonic reference currents are calculated to reduce the low-frequency active power fluctuations, obtaining superior results in contrast with existing techniques and effectively alleviating associated harmful effects. As the harmonic currents are not set to zero in this system, an approach that minimizes current total harmonic distortion while eliminates the main harmonic fluctuations is developed, leading to an optimized performance in terms of power quality and power filtering. Finally, simulation and experimental results in unbalanced and distorted grid conditions prove the advantages of the proposed technique.

Hybrid VSS–LMS–LMF based adaptive control of SPV‐DSTATCOM system under distorted grid conditions

This study presents a solar photovoltaic distribution static compensator (SPV-DSTATCOM) system feeding a non-linear load connected at point of common coupling. A hybrid variable step size–least mean square–least mean fourth (VSS–LMS–LMF) algorithm is proposed for the control of voltage source converter, which acts as DSTATCOM. A weighing factor is used so that LMS and LMF work simultaneously, and a varying step-size helps in obtaining an adaptable system. The proposed control shows high convergence rate and low value of mean square error. The SPV array uses perturb and observe technique for maximum power extraction. The transition of SPV-DSTATCOM to DSTATCOM and vice-versa is also studied. In SPV-DSTATCOM mode, the SPV feeds power to the load and the grid, resolving the power quality issues of the system, whereas in DSTATCOM mode, the grid feeds power to the load at unity power factor. The system is first simulated in MATLAB environment and then these results are experimentally verified on the developed prototype in the laboratory. The system is studied for both power factor correction and zero voltage regulation modes and under distorted grid conditions. The obtained results adhere to an IEEE-519 standard.

A Comparative Performance Evaluation of Extended AANF with Different Parameter Estimation Techniques for Renewable Energy Integration

This paper presents real time performance evaluation of three phase extension of an amplitude adaptive notch filter (AANF) for online estimation of frequency, amplitude, and sequence components of the input grid voltage signal. The performance of an extended AANF is compared with conventional synchronous reference frame-phase lock loop (SRF-PLL), enhanced phase lock loop (EPLL), and existing adaptive notch filter (ANF). Comparative analysis has been carried out based on their ability in extracting frequency and amplitude of input grid voltage signal under balanced and unbalanced voltage sag/swell, frequency shift and distorted grid condition. Three phase AANF method provides high degree of accuracy than SRF-PLL, EPLL, and ANF in extracting appropriate signal information for unbalanced and harmonically distorted grid condition. The important feature of this method is its amplitude adaptability, which improves its speed of response and accuracy when grid signal is of variable amplitude. OPAL-RT’s (OP4500) real time controller with an in-built Xilinx Kintex-7 FPGA processor is used for real time implementation. Experimental results validate fast and accurate performance of an extended AANF in extracting frequency, amplitude, and sequence components of the utility grid voltage signal, which can be further used for performance improvement of grid connected renewable energy systems, custom power devices, and flexible ac transmission systems (FACTS) devices.

Grid Interfaced Distributed Generation System With Modified Current Control Loop Using Adaptive Synchronization Technique

This paper presents real-time implementation of a grid interfaced distributed generation (DG) system with modified current control loop using three phase amplitude adaptive notch filter (AANF) based synchronization tool. A grid current feedback based modified $dq$ -current control technique for interfacing inverter is developed in order to achieve constant loading on the grid, transient-free operation, and power factor improvement close to unity power factor (UPF) of the utility grid during sudden load variations. This technique does not require separate calculation of reference reactive component and harmonics component of currents hence reduces control circuit complexity. In addition, it requires only three voltage and three current sensors. Three phase AANF is developed and is used for online extraction of utility voltage phase angle to generate synchronized reference current signals for interfacing inverter. AANF is used because of its adjustable accuracy and amplitude adaptability even under unbalanced voltage sag and swell, frequency variation, and distorted grid conditions. Fast and accurate behavior of three phase AANF improves the dynamic response of entire DG system control performance for sudden load variations. The dynamic behavior of the proposed grid interfaced DG system is experimentally evaluated in maintaining constant loading on grid, transient-free operation, and power factor improvement close to UPF operation of the utility grid, by compensating total reactive power and harmonic current demanded by variable linear as well as nonlinear load.

Finite Control Set–Model Predictive Control with Modulation to Mitigate Harmonic Component in Output Current for a Grid-Connected Inverter under Distorted Grid Conditions

This paper presents an improved current control strategy for a three-phase grid-connected inverter under distorted grid conditions. In terms of performance, it is important for a grid-connected inverter to maintain the harmonic contents of inverter output currents below the specified limit even when the grid is subject to harmonic distortion. To address this problem, this paper proposes a modulated finite control set–model predictive control (FCS-MPC) scheme, which effectively mitigates the harmonic components in output current of a grid-connected inverter. In the proposed scheme, the system behavior in the future is predicted from the system model in the discrete-time domain. Then, the cost function is selected based on the control objective of system. This cost function is minimized during the optimization process to determine the control signals that minimize the cost function. In addition, since the proposed scheme requires pure sinusoidal reference currents in the stationary frame to work successfully, the moving average filter (MAF) is employed to enhance the performance of the traditional phase lock loop (PLL). Due to the control performance of the FCS-MPC scheme as well as the harmonic disturbance rejection capability of the MAF-PLL, the proposed scheme is able to suppress the harmonic distortion even in the presence of distorted grid condition, while retaining fast transient response. Comparative simulation results of different controllers verify the effectiveness of the proposed control scheme in compensating the harmonic disturbance. To validate the practical feasibility of the proposed scheme, the whole control algorithm is implemented on a 32-bit floating-point digital signal processor (DSP) TMS320F28335 to control a 2 kW three-phase grid-connected inverter. As a result, the proposed scheme is a promising approach toward improving the current quality of a grid-connected inverter under distorted grid conditions.

Frequency-Adaptive Modified Comb-Filter-Based Phase-Locked Loop for a Doubly-Fed Adjustable-Speed Pumped-Storage Hydropower Plant under Distorted Grid Conditions

The control system of a doubly-fed adjustable-speed pumped-storage hydropower plant needs phase-locked loops (PLLs) to obtain the phase angle of grid voltage. The main drawback of a comb-filter-based phase-locked loop (CF-PLL) is the slow dynamic response. This paper presents a modified comb-filter-based phase-locked loop (MCF-PLL) by improving the pole-zero pattern of the comb filter, and gives the parameters’ setting method of the controller, based on the discrete model of MCF-PLL. In order to improve the disturbance resistibility of MCF-PLL when the power grid’s frequency changes, this paper proposes a frequency-adaptive modified, comb-filter-based, phase-locked loop (FAMCF-PLL) and its digital implementation scheme. Experimental results show that FAMCF-PLL has good steady-state and dynamic performance under distorted grid conditions. Furthermore, FAMCF-PLL can determine the phase angle of the grid voltage, which is locked when it is applied to a doubly-fed adjustable-speed pumped-storage hydropower experimental platform.

A Hybrid Filtering Stage Based Quasi-type-1 PLL under Distorted Grid Conditions

For three-phase synchronization applications, the synchronous reference frame phase-locked loop (SRF-PLL) is probably the most widely used technique due to its ease of implementation and satisfactory phase tracking performance under ideal grid conditions. However, under unbalanced and distorted grid conditions, its performance tends to worsen. To deal with this problem, a variety of filtering stages have been proposed and used in SRF-PLLs for the rejection of disturbance components at the cost of degrading the dynamic performance. In this paper, to improve dynamic performance without compromising the filtering capability, an effective hybrid filtering stage is proposed and incorporated into the inner loop of a quasi-type-1 PLL (QT1-PLL). The proposed filtering stage is a combination of a moving average filter (MAF) and a modified delay signal cancellation (DSC) operator in cascade. The time delay caused by the proposed filtering stage is smaller than that in the conventional MAF-based and DSC-based PLLs. A small-signal model of the proposed PLL is derived. The stability is analyzed and parameters design guidelines are given. The effectiveness of the proposed PLL is confirmed through experimental results.