Non-ideal Grid Conditions Research Articles

Comparative study of single-phase phase-locked loops for grid-connected inverters under non-ideal grid conditions

In the renewable power generation systems, ensuring the synchronization of the inverter and the power grid is crucial for the stable operation of grid-connected inverters. Nowadays, the phase-locked loop (PLL) technology has become a widely used grid synchronization method because of its simple implementation and robustness under various grid conditions. Even though a lot of PLLs have been proposed, the overview and comparative analysis of multiple PLLs can be helpful for the practical applications. In addition, the weak grid condition is a great challenge for the system. Therefore, this study firstly presents an overview of the existing PLLs together with their general structures and basic working principles. Depending on the implementation of the phase detector, the PLL can be divided into three categories: power-based PLL (pPLL), orthogonal-signal-generator-based PLL (OSG-PLL) and adaptive-filter-based PLL (AF-PLL). Then, from the above classification, seven typical single-phase PLLs are selected for further study. Finally, some test results are given, and a comprehensive evaluation of the selected PLLs under different grid conditions is conducted.

A CM Filter Configuration for Grid-Tied Voltage Source Converters

Common mode (CM) filters play a crucial role in determining adherence to conducted emissions (CE) standards for grid-tied voltage source converters. Design and implementation of such filters can be challenging since they depend on several factors like identification of CM noise paths, fidelity of passive components and discerning the frequency and amplitude of CM noise sources. In this article, a CM filter has been proposed, which uses passive components along with the converter's heat sink as a circulating return path for the CM currents. Based on the presented CM circuit models with the proposed filter, a detailed design process has been delineated for selecting the passive components required to realize the CM filter. Additionally, the heat-sink potential has been shown to be touch safe during both ideal and nonideal grid conditions with the proposed CM filter. CE spectral results measured using a commercially procured line impedance stabilization network and captured time domain converter operational wave forms for a 2-level, 3π grid-tied voltage source converter validate the functionality and effectiveness of the presented design.

A novel hybrid coordinates multiple decoupled phase‐locked loop under nonideal grid conditions

AbstractA novel hybrid coordinates multiple decoupled phase‐locked loop (HCMD‐PLL) is presented to improve the performance of the PLL under nonideal grid conditions. Based on the Clark transform and the first‐order low‐pass filter, the complex coefficient filter is derived to analyze phase‐locked loop. It is different from the phase‐locked loop with a complex coefficient filter. On this basis, the multiple‐complex‐coefficient band‐pass filter of HCMD‐PLL is derived, and the equivalent transfer function of the decoupling of the fundamental positive sequence, negative sequence and harmonic components is obtained, and the magnitude and phase frequency responses are analyzed. The fundamental positive sequence component is extracted without phase shift and amplitude attenuation under the nonideal grid conditions, eliminating the effect of specific‐order harmonics components because the amplitude is attenuated to zero. And the specific‐order harmonic component can also be extracted precisely. The interaction from the various specific components is eliminated. The stability of the system is analyzed by the pole distribution of the closed‐loop system. The stationary αβ coordinate system for PLL is adopted to suppress the change of the grid voltage. Simulation and experimental results verify the effectiveness of the HCMD‐PLL. © 2019 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Grid Integration of Three-Phase Single-Stage PV System Using Adaptive Laguerre Filter Based Control Algorithm Under Nonideal Distribution System

The increasing integration of utility with renewable sources of energy has resulted into serious issues of power quality (PQ) deterioration primarily in a scenario of weak distribution grid. In order to improve the PQ, efficient implementation of a control algorithm for the solar energy conversion system interfaced to the grid is paramount. This article illustrates the usage of an adaptive Laguerre filter based control technique for optimal operation providing distribution static compensator capabilities along with load balancing, power factor correction and harmonics mitigation. Moreover, it provides the active power transfer to the grid together with feeding the nonlinear and linear loads. In addition, the system performance according to an IEEE-519 standard has been verified, hence it is proficient in maintaining the PQ. The Laguerre filter offers the benefits of reduced computational time in addition with the alleviation in model complexity predominant during abnormal grid conditions. By utilizing maximum power point tracking, the efficient utilization of solar PV array is accomplished with a perturb and observe method. For validating performance of the proposed system, it is examined through simulation results. Furthermore, a prototype is developed for validation and test results corroborate reliable operation under nonideal grid conditions comprising of wide range of load variations, voltage sag, swell, distortion, and unbalance conditions.

Control of a Three-Phase Grid-Connected Inverter Under Non-Ideal Grid Conditions With Online Parameter Update

Three-phase grid-connected inverter modeling depends on the equivalent resistance and inductance between the inverter and the grid. However, these parameters are not fixed during the operation of the inverter and vary with the operating conditions. In this paper, a new globally stable adaptive controller is proposed to estimate these values online and update the controller during its operation. A model reference adaptive system (MRAS) based on two fictitious quantities with no physical significance is utilized, namely $M = v \cdot i^*$ and $N = v^* \times i$ , where $v$ and $i$ are the steady-state values of voltage and current in synchronously rotating frame of reference. Detailed stability analysis of the proposed estimation technique has been presented using a Lyapunov energy function-based approach proving the global stability of the technique. Small-signal modeling and analysis of the proposed scheme has also been presented so as to understand the local dynamics of the system. The guidelines to select the parameter of the proposed MRAS system along with updating the controller are elaborated in this paper. Detailed simulation based on MATLAB/Simulink and PLECS, along with experimental studies through a Texas Instruments TMS320F28377S microcontroller, validates the feasibility of the proposed control architecture.

An optimal operation strategy for an active power filter using cascaded H-bridges in delta-connection

An optimal strategy is presented to determine the combined control reference for the terminal and circulating currents of a shunt active power filter (APF) based on the delta-connected cascaded H-bridge (CHB) topology, hereby minimizing the operational power under ideal and non-ideal grid voltages, where the ideal grid voltages contain only the positive-sequence fundamental frequency component and the non-ideal grid voltages contain unbalance and/or harmonic components. The optimization is carried out to simultaneously satisfy the desired grid current distortion limits, the source current imbalance characteristics and the power factor as well as power balance requirement. By properly coordinating terminal current control and circulating current injection of the delta-connected CHB and taking possibly unbalanced and/or distorted grids the total power rating of the APF can be kept at minimum, leading to an economical operation. The optimization problem is formulated using a convex quadratic objective function and non-convex quadratic constraints and solved by an iterative procedure. The simulation results show that the proposed strategy is applicable and effective for reactive power and harmonic compensation with an optimal rating of the APF under ideal and non-ideal grid conditions.

Delta-Bar-Delta Neural-Network-Based Control Approach for Power Quality Improvement of Solar-PV-Interfaced Distribution System

A serious concern regarding deterioration in power quality has emerged with the increasing integration of solar photovoltaic (PV) energy sources to the utility primarily in the scenario of a weak distribution grid. Therefore, power quality improvement of the grid-tied solar energy conversion system is paramount by implementation of a robust control technique. This paper deals with a delta-bar-delta neural network (NN) control for operating optimally by feeding active power to the loads and remaining power to the grid as a function of distribution static compensator capabilities, such as mitigating harmonics, balancing of load, and improving power factor. The control algorithm provides the ability to adjust weights adaptively in an independent manner, and hence, it offers alleviation in model complexity predominant during abnormal grid conditions along with reduction in computational time. Moreover, the NN-based control technique offers enhanced accuracy due to the combinational neural structure in the estimation process. In addition, the system performance according to the IEEE-519 standard has been verified; hence, it is proficient in maintaining the power quality. The solar-PV-array-efficient utilization is accomplished through an incremental-conductance-based maximum power point tracking technique. For validating the behavior of the proposed system, its performance is studied using simulation results. Moreover, a prototype is developed for validation, and experimental results corroborate reliable operation under nonideal grid conditions comprising of a wide range of load variations, voltage sag, and varying solar insolation conditions.

Real-time Neural Input–Output Feedback Linearization control of DFIG based wind turbines in presence of grid disturbances

This paper proposes a Neural Input–Output Feedback Linearization (N-IOFL) controller for a Doubly Fed Induction Generator (DFIG) prototype connected to the grid. Under unbalanced grid voltages, the existing control strategies need to be modified, becoming very complicated. By using an identifier based on Recurrent High Order Neural Network (RHONN), trained on-line with an Extended Kalman Filter (EKF), an adequate model of the DFIG and of the DC-link can be obtained, which helps the control law based on feedback linearization to reject grid disturbances appearing under non-ideal grid conditions without decomposition process. Based on such identification, the proposed controller is used to track a desired Direct Current (DC) voltage reference at the output of DC-link, to maintain constant the electric power factor controlled by the Grid Side Converter (GSC), and to force independently the rotor currents to track a specified reference defined from the required stator powers, controlled by the Rotor Side Converter (RSC), under both balanced and unbalanced grid conditions. Real-time results illustrate the effectiveness of the proposed controller even in presence of non-ideal grid conditions.

Multivector Model Predictive Power Control of Three-Phase Rectifiers With Reduced Power Ripples Under Nonideal Grid Conditions

Model predictive power control (MPPC) is a promising control scheme for bidirectional ac–dc rectifiers. However, the control performance of conventional MPPC (C-MPPC) deteriorates under nonideal grid conditions. Furthermore, the one-switching-vector-per-control-interval characteristic of C-MPPC leads to high ripples of control variables. To improve the steady-state performance of rectifiers under nonideal grid voltage conditions, a multivector MPPC (MV-MPPC) scheme is proposed. The proposed method presents a constant switching frequency and better steady-state control performance without increasing its sampling frequency. By selecting two active vectors and one zero vector, the range of optimal vector for active and reactive power regulation can be extended from fixed phase and magnitude to arbitrary phase and magnitude. Incorporated with a second-order generalized integrator for obtaining the quadrature component of grid voltage, the proposed method is applicable to the grid conditions where low-order harmonics exist. The preliminary calculation is adopted to avoid repetitive computation of the predicted values in the implementation of MV-MPPC, which reduces the calculation burden of the proposed scheme. A thorough experimental evaluation of the proposed scheme with the C-MPPC and duty-optimal MPPC has been conducted to validate the superiority of the proposed MV-MPPC solution.

Research on the Neutral-Point Voltage Balance for NPC Three-Level Inverters under Non-Ideal Grid Conditions

In order to solve the neutral-point voltage unbalancing problem for neutral-point clamped (NPC) three-level inverters under non-ideal grid conditions, a novel zero-sequence component injection method for modulation signals was proposed in this paper. Based on the mathematical expressions of neutral-point voltage unbalancing, the fluctuation of the neutral-point voltage under different grid conditions was studied. The proposed method dynamically calculates the zero-sequence component for modulation signals by region selections based on sinusoidal pulse width modulation (DCR-SPWM). Under non-ideal grid conditions, this DCR-SPWM method can control the neutral-point voltage balance for NPC three-level inverters with different grid and output power conditions, and improves the grid current quality. The simulation results verified the correctness of the theoretical analysis in this paper, and the feasibility and effectiveness of this method were verified by the experiments in the experimental platform of the NPC three-level grid-connected inverter based on DSP-CPLD controllers.

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.

Improved Two-Phase Stationary Frame EPLL to Eliminate the Effect of Input Harmonics, Unbalance, and DC Offsets

The effect of input harmonics, unbalance, and dc offsets on the two-phase stationary frame enhanced phase-locked loop (αβ-EPLL) is theoretically analyzed first. It indicated that the nonideal grid conditions cause the periodic ripples in the estimated amplitude, frequency, and phase angle. An improved αβ-EPLL by introducing two pure integrators and moving average filter (MAF) into both the amplitude and frequency estimation loops of αβ-EPLL is proposed to eliminate the effect of the aforementioned nonideal grid conditions. An improved implementation of MAF to eliminate the effect of the variation of the grid frequency is proposed as well. The linear model of the improved αβ-EPLL is built and the design rule of the control parameters is determined. Detailed experimental results are presented to verify the validity and feasibility of the improved αβ-EPLL.

Multiple DSC Filter-Based Three-Phase EPLL for Nonideal Grid Synchronization

This paper originally analyzes the effect of the harmonics, dc offsets, and unbalance on the three-phase enhanced phase-locked loop (3P-EPLL). It proves that 3P-EPLL is not able to accurately estimate the amplitude, phase angle, and frequency of the grid in such cases. In order to correctly estimate the grid synchronous information even under the nonideal grid conditions, an improved 3P-EPLL combined with the multiple delayed signal cancellation (DSC) filters is proposed. A DSC operator is set at the each input of 3P-EPLL to eliminate the input dc offsets and even-order harmonics. And the cascaded DSC (CDSC) module is plugged in both the frequency and amplitude loops to remove the effect of most of the odd-order harmonics and unbalance. The relationship between the delay constant of the DSC operator and the order of the harmonics hoping to eliminate is derived to design the structure of the CDSC module and determine the delay constant of each DSC operator in the CDSC module. Detailed experimental results of the standard 3P-EPLL and proposed one validate the theoretical analysis and the proposed phase-locked loop.

IDA-PBC controller design for grid connected Front End Converters under non-ideal grid conditions

A passivity-based non-linear controller design for three-phase front-end converters used to connect renewable energy sources to the grid is proposed in this paper. The control objective is to inject all the generated power to the grid, while adjusting the reactive power exchanged with the power system. Besides, generated currents must have very low distortion, in order to satisfy the standards, even when grid voltages are distorted or unbalanced. The proposed controller is first designed using the Interconnection and Damping Assignment strategy, which allows obtaining controller parameters while ensuring the closed-loop system stability. In order to ensure that the output currents have very low harmonic distortion, the obtained control laws are modified based on the fundamental positive sequence grid voltage, obtained from a positive sequence detector. The proposal allows controlling both injected powers and DC-link voltage, from a unique controller design. Simulation and experimental results are presented to validate the proposed controller.

Reconfiguration of Three Phase MAF-SRF-PLL as Single Phase PLL

Improved SRF-PLL using moving average filter (MAF) is an established phase locked loop method for operation of three phase grid connected VSI under non-ideal grid conditions. This three phase PLL method can also be used in a single phase system without changing its structure. Hence, in applications where the grid connected VSI has to work under both three phase and single phase source, a MAF-SRF-PLL is useful for maintaining proper synchronism. In this letter, first an already available design of three phase MAF-SRF-PLL has been evaluated experimentally under various single phase grid condition in terms of unit vector quality. Next the same PLL structure has been tested for situations where circuit reconfiguration from three phase to single phase and vice versa are necessary.

Efficiency enhancement scheme of cascaded multilevel grid-connected inverter and its improvement to eliminate effect of non-ideal grid conditions

Abstract In this paper, an online efficiency enhancement scheme of a cascaded multilevel grid-connected inverter for renewable energy generation systems is researched in detail. Taking the special grid current standard as the limitation condition, an efficiency enhancement scheme with adjusting carrier frequency in real time is proposed. Furthermore, the effect of the harmonics and DC offset of the tied grid on the enhanced phase locked loop (EPLL) is analyzed and a hybrid filter based EPLL is proposed to eliminate the former effect. With the improved EPLL, the amplitude, phase angle and frequency of grid are all estimated correctly and the performance of the efficiency enhancement scheme is maintained even under non-ideal grid conditions. Detailed experimental results validate the accuracy and practicability of the improved EPLL and efficiency enhancement scheme.

A Novel Phase Locked Loop for Grid-Connected Converters under Non-Ideal Grid Conditions

Grid synchronization is one of the key techniques for the grid-connected power converters used in distributed power generation systems. In order to achieve fast and accurate grid synchronization, a new phase locked loop (PLL) is proposed on the basis of the complex filter matrixes (CFM) orthogonal signal generator (OSG) crossing-decoupling method. By combining first-order complex filters with relation matrixes of positive and negative sequence voltage components, the OSG is designed to extract specific frequency orthogonal signals. Then, the OSG mathematical model is built in the frequency-domain and time-domain to analyze the spectral characteristics. Moreover, a crossing-decoupling method is suggested to decouple the fundamental voltage. From the eigenvalue analysis point of view, the stability and dynamic performance of the new PLL method is evaluated. Meanwhile, the digital implementation method is also provided. Finally, the effectiveness of the proposed method is verified by experiments under unbalanced and distorted grid voltage conditions.

Performance evaluation of three phase SRF-PLL and MAF-SRF-PLL

Synchronous reference frame phase locked loop (SRF-PLL) is a well established technique used for maintaining synchronism of a grid connected VSI with an electric grid. Many methods of PLL are present in the literature to achieve improved performance under nonideal grid condition. These solutions are based on SRF-PLL structure along with some modification to achieve improved performance. It is observed that faster and better performance are achieved at the expense of more computations. Moving average filter (MAF) SRF-PLL structure is one such solution that consumes less resources and gives a reasonably fast response. In this work the performance of a MAF-SRF-PLL structure is evaluated in terms of unit vector distortion and settling time under various nonideal grid conditions. Its performance is compared with that of two differently designed SRF-PLLs. This evaluation gives a clear idea about possible utilizations of these PLL structures under different possible nonideal grid conditions.

Impedance Shaping of the Grid-Connected Inverter with LCL Filter to Improve Its Adaptability to the Weak Grid Condition

The current-controlled grid-connected inverter with LCL filter is widely used in the distributed generation system (DGS), due to its fast dynamic response and better power quality features. However, with the increase of power injected into the grid, control performances of the inverter will be significantly influenced by the nonideal grid conditions. Specifically, the possible wide variation of the grid impedance challenges the system stability. Meanwhile, background harmonics of the grid can greatly distort the injected current. Therefore, the control of the inverter should be designed with strong stability-robustness and high harmonic-rejection-ability, both of which correlate closely with the inverter output impedance. However, it is difficult to shape the output impedance into the one with a desirable characteristic simply by adjusting the current loop gain. In this paper, an impedance shaping method is proposed with virtual impedances, and the current control loop can be designed independently. The implementation and parameter design of the virtual impedances are studied under the practical considerations. With this proposed method, the grid-connected inverter can work stably over a wide range of the typical inductive-resistive grid impedance and exhibit strong rejection ability of grid-voltage harmonics. Experimental results from a 6-kW single-phase grid-connected inverter confirm the effectiveness of the proposed method.

Roentgenogram of the Month

This paper proposes a Neural Input–Output Feedback Linearization (N-IOFL) controller for a Doubly Fed Induction Generator (DFIG) prototype connected to the grid. Under unbalanced grid voltages, the existing control strategies need to be modified, becoming very complicated. By using an identifier based on Recurrent High Order Neural Network (RHONN), trained on-line with an Extended Kalman Filter (EKF), an adequate model of the DFIG and of the DC-link can be obtained, which helps the control law based on feedback linearization to reject grid disturbances appearing under non-ideal grid conditions without decomposition process. Based on such identification, the proposed controller is used to track a desired Direct Current (DC) voltage reference at the output of DC-link, to maintain constant the electric power factor controlled by the Grid Side Converter (GSC), and to force independently the rotor currents to track a specified reference defined from the required stator powers, controlled by the Rotor Side Converter (RSC), under both balanced and unbalanced grid conditions. Real-time results illustrate the effectiveness of the proposed controller even in presence of non-ideal grid conditions.