Unbalanced Grid Voltage Conditions Research Articles

An Improved Collaborative Control Scheme to Resist Grid Voltage Unbalance for BDFG-Based Wind Turbine

This article presents an improved collaborative control to resist grid voltage unbalance for brushless doubly fed generator (BDFG)-based wind turbine (BDFGWT). The mathematical model of grid-connected BDFG including machine side converter (MSC) and grid side converter (GSC) in the αβ reference frame during unbalanced grid voltage condition is established. On this base, the improved collaborative control between MSC and GSC is presented. Under the control, the control objective of the whole BDFGWT system, including canceling the pulsations of electromagnetic torque and the unbalance of BDFGWT’s total currents, pulsations of BDFGWT’s total powers are capable of being realized; therefore, the control capability of BDFGWT to resist unbalanced grid voltage is greatly improved. Moreover, improved single-loop current controllers adopting PR regulators are proposed for both MSC and GSC where the sequence extractions for both MSC and GSC currents are not needed any more, and hence the proposed control is much simpler. In addition, the transient characteristics are also improved. Moreover, in order to achieve the decoupling control of current and average power, current controller also adopts the feedforward control approach. Case studies for a two MW BDFGWT system are implemented, and the results verify that the presented control is capable of effectively improving the control capability for BDFGWT to resist grid voltage unbalance and exhibit good stable and dynamic control performances.

Control of DFIG-based wind power generation system under unbalanced grid voltage conditions

Doubly Fed Induction Generator (DFIG)-based wind turbines have become increasingly popular in recent years due to their capacity to operate at varying speeds. Weaknesses in the DFIG system can arise from issues with the power grid due to the stator's direct connection and the excitation converter's power rating limitation. Under situations of unbalanced grid voltage, this study aims to explore the efficacy of the Direct Power Control (DPC) approach in managing wind turbine systems based on DFIG. Throughout the experimental investigation, we evaluated the system in standard and unbalanced grid voltage settings. MATLAB/SIMULINK simulations implement DPC, specifically tailored for a 9 MW DFIG-based wind farm. The results of these simulations show that the changed control method effectively reduces torque oscillations by making it possible to create active and reactive power references for the rotor-side converter. This eliminates the requirement for sequence component excitation, which was previously necessary. Furthermore, the research highlights the intrinsic link between control techniques and grid circumstances, showing this connectivity's crucial role in improving wind energy systems' stability and operational efficiency based on DFIG.

DSVPWM-Based SSO-PIDR Control Scheme for Indirect Matrix Converter with Unity Power Factor Under Unbalanced Grid Voltage Condition

DSVPWM-Based SSO-PIDR Control Scheme for Indirect Matrix Converter with Unity Power Factor Under Unbalanced Grid Voltage Condition

Hardware-in-the-loop testing of brushless doubly fed reluctance generator under unbalanced grid voltage conditions

An extended vector control method for the operation of a brushless doubly fed reluctance generator-based wind energy conversion system under unbalanced grid voltage conditions is proposed. The modified scheme has the capability to control the positive and negative sequences of the inverter-fed winding (secondary) currents independently. The primary aim of this work is to evaluate the effectiveness of the control algorithm in case of asymmetrical grid voltages. Since large-scale generators of this type are not yet available, Hardware-in-the-Loop testing is considered the most suitable viable alternative to true experiments to validate the wind turbine performance using the optimized design parameters. The presented test results for different control targets have revealed the promising potential of the underlying control strategy and the controller itself to mitigate the oscillations and improve the quality of the grid-connected winding (primary) and secondary currents, electro-magnetic torque, reactive and active power waveforms, being of utmost importance for the grid integration of this emerging generator technology.© 2017 ElsevierInc.Allrightsreserved.

Optimal Decoupled Discontinuous Modulation for StatComs Based on the Cascaded H-Bridge Converter

Conventional discontinuous pulse width modulation (DPWM) strategies for cascaded H-bridge static compensators suffer from poor inter-phase capacitor voltage balance control during unbalanced grid voltage conditions. Specifically, the logic-based algorithm that calculates the zero-sequence voltage (ZSV) for discontinuous operation interacts with the closed-loop capacitor voltage control. This paper proposes an optimal DPWM strategy based on a finite-set optimization that chooses the best ZSV candidate that minimizes the control interactions while clamping a converter voltage. Furthermore, the effect of the different ZSV candidates on the switching loss is also considered within the optimization. Extensive experimental study is provided to demonstrate the effectiveness of the proposed approach.

Control Strategy of Three-level Active Neutral Point Clamped Grid Connected Inverters in Unbalanced Power Grid

Traditional phase-locked loops in three-level active neutral point-clamped inverter grid-connected systems for photovoltaic power generation exhibit a deficiency in the decoupling of positive and negative sequences under unbalanced power grid conditions. To address this issue, this study proposes the implementation of the fourth-order generalized integrator into the phase-locked loop to facilitate positive and negative sequence decoupling. Initially, a 3L-ANPC inverter grid connection system is established. However, under unbalanced grid voltage conditions, the PLL effect of the Decoupled Double Synchronous Reference Frame Phase-Locked Loop is found to be suboptimal, leading to considerable fluctuations and distortions in the output frequency signal. To address these complications, we propose an optimized phase-locked loop structure. In this improved design, the PLL first utilizes the FOGI to filter out multiple harmonics in the grid voltage prior to decoupling the positive and negative sequence voltage. This process effectively mitigates the effect on the PLL, thereby enabling rapid and precise tracking of the grid voltage. Simulation analysis is conducted by using Matlab/Simulink to substantiate the effectiveness of the proposed PLL, demonstrating its ability to reliably lock onto the frequency and phase of the fundamental voltage under conditions of unbalanced power grid voltage.

A model predictive control of three‐phase grid‐connected current‐source inverter based on optimization theory

AbstractIn the three‐phase grid‐connected current‐source inverters (CSIs), the resonance result from the AC‐side CL filter and the quality of the grid‐current waveform under the unbalanced and harmonic grid voltage conditions are two issues deserving attention. To solve the two problems, a continuous control set‐model predictive control (CCS‐MPC) method based on the optimization theory is proposed in the two‐phase synchronous coordinate frame in this paper. The constructed cost function takes grid‐connected current and output current of the inverter as control target, and obtains its optimal solution under the least‐squares method, where the optimal inverter‐side current reference is composed of inductor‐current proportional feedback, capacitor‐voltage proportional feedback, and inverter‐side current steady‐state value feedforward. The proposed method can easily realize the normal operation of three‐phase grid‐connected CSIs because the method can be determined by only one parameter under the unbalanced and distorted grids. Furthermore, in the proposed method, the proportional feedback of capacitor voltage can effectively suppress the CL filter resonance. Finally, simulation and experimental results verify the effectiveness of the proposed control strategy.

Predictive Power Control of PMSG based WECS: Development and Implementation for Smooth Grid Synchronisation, Balanced and Unbalanced Grid

In this paper, the distributed generation unit is controlled to operate in both stand-alone (SA) and grid-connected (GC) modes and offers a smooth transition between these modes using the model predictive control scheme. The permanent magnet synchronous generator (PMSG), along with a 2-level back-to-back converter, is named a DGU. A simple and intuitive approach using predictive control is presented in a stationary reference frame for a load-side converter (LSC). A voltage control loop is used during SA mode to fix the voltage and frequency at the load end. The smooth synchronization technique is developed without a phase-locked loop. During GC, a predictive power control regulates active and reactive power during the symmetry and unbalanced grid voltage conditions. The designed strategy makes the power factor improvement, reduction of grid current harmonics, and elimination of twice grid frequency ripple from power during grid unbalance possible. This work has proposed a solution to reduce the overall computational burden on the processor by eliminating the state prediction in each sampling period. The machine-end converter control is formulated in the rotor flux reference frame to keep the power flow across the dc-link capacitor constant. Comprehensive experiments are conducted with the designed scaled laboratory prototype to successfully validate the control’s theoretical claim.

Virtual Flux Direct Power Control for Vienna Rectifier Under Unbalanced Grid

The Vienna rectifier is being broadly utilized in many industrial applications, which makes it essential to address its challenges. This article conducts the virtual flux-based direct power control and proposes an improved switching table for the Vienna rectifier under a balanced or unbalanced grid's voltage. Accordingly, the proposed method improves the THD of the three-phase input current when the grid voltage is unbalanced. Moreover, employing the virtual flux concept eliminates using three-phase input voltage. This results in reducing the cost of the converter. One of the challenges in the Vienna rectifier is to consider the current direction in the control design. Accordingly, the proposed switching table is designed based on the input current's sector. Furthermore, the positive and negative sequences of the input currents and virtual flux are utilized to correctly estimate the instantaneous input active and reactive power under the unbalanced grid voltage condition. Moreover, the proposed method can balance the output capacitors in balanced and unbalanced grid voltage conditions. This method possesses a high dynamic response due to hysteresis bands. Finally, a 430-watt laboratory prototype of the Vienna rectifier is implemented, and the accuracy of the proposed VFDPC is verified through the experimental results.

An optimal initialisation for robust model reference adaptive PI controller for grid-tied power systems under unbalanced grid conditions

This paper presents an Optimal Robust Model Reference Adaptive Proportional–Integral controller for grid-tied power systems, which is automatically parametrised using a Genetic Algorithm. The task of choosing the adaptation rate and the controller’s initial gains set is now performed in a computational and automatic way, saving design time and ensuring satisfactory transient regimes.The use of a Genetic Algorithm for complete parametrisation of robust adaptive controllers applied to renewable energy power generation systems with LCL filter is first here explored. Five cost functions are considered in the optimisation process. A comparison of simulation results applying the optimised controller in a grid-tied Voltage Source Inverter with LCL filter points out the benefits and drawbacks of using each cost function. A framework for performance analysis and an in-depth discussion are provided. The function determined as the best one, which is integral of absolute error, is used in the controller implemented experimentally, ensuring suitable tracking of grid current references and rejection of grid voltage harmonics, even under unbalanced grid voltage conditions. An experimental comparison in a three-phase 6.75 kW power converter with the non-optimised controller is also provided, where the optimised controller obtains a reduction in the overshoot in all transient regimes and smaller tracking error, resulting in a reduction in the mean error of 14.31% and 60.30% in α and β coordinates, respectively. In addition, the optimised controller reduces total harmonic content, improving energy quality. This parametrisation procedure can be immediately extended to the control of any other power converter systems.

New Modeling and Improved Current Control Strategy to Eliminate the Impact of Synchronization Method and Parks Transformation for Grid-connected Four-leg PWM Inverter

The synchronization method’s ability is one of the fundamental guarantees for the stability and accuracy of injected current control for grid-connected converter-based distribution resources. These synchronization methods, such as phase locked loop (PLL), are generally based on Park transformation and grid voltage regulation, which may affect an unstable phenomenon under distortion and unbalanced grid voltage conditions and result in more computational complexity. In this present paper, analogous to the traditional voltage-oriented control strategy (VOC) in the synchronous rotating frame (dq0-frame) based on PLL and Parks transformation, an improved voltage-oriented control strategy (IVOC) without synchronization methods and Parks transformation is proposed for grid-connected four-leg inverters (GC-FLVSI) to achieve accurate current control with high-quality performance in the dq0-frame. This proposed strategy is not only used for controlling the GC-FLVSI but also to provide the module of GC-FLVSI in the dq0-frame based on the instantaneous active and reactive powers theory (DPC). The proposed IVOC strategy has the same properties and identical performance as the traditional VOC when the grid phase angle is correctly detected by any synchronization method, with the advantages of both traditional DPC and VOC at the same time. In order to validate the superiority and excellent dynamic and steady-state performances of the proposed IVOC strategy in comparison with the traditional VOC strategy, some simulation scenarios using MATLAB/Simulink under different operations and grid conditions have been performed and presented.

Negative-Sequence Current Capability in Low-Capacitance Cascaded H-Bridge Static Compensators With Optimal Third- Harmonic Circulating Current Injection

The cascaded H-bridge (CHB) low-capacitance static compensator (LC-StatCom) has a limited negative-sequence current injection capability compared to a conventional CHB StatCom, due to the comparatively larger oscillations on the capacitor voltages. Placing limits on the ability to provide negative-sequence current to the grid for a given capacitor size is required to prevent overmodulation. This article shows that these limits can be maximized by injecting an optimal third-harmonic circulating current. The effects of injecting the third-harmonic current on the capacitor voltages, both in balanced and unbalanced grid voltage conditions, are analyzed. Specifically, the procedure allows the knowledge of the potential capability of the LC-StatCom when injecting negative-sequence currents. The procedure is based on a linear programming approach. Although the procedure is applicable to any CHB StatCom, regardless of the submodule capacitor size, it is particularly important in LC-StatComs. To corroborate the proposed analysis, simulation and experimental results under balanced and unbalanced grid voltage conditions are presented.

Enhanced Operation of a Virtual Synchronous Machine Under Unbalanced Grid Voltage Condition Based on Lyapunov Energy Function

Three-phase grid connected inverters integrated with renewable energy sources like photovoltaic systems require to provide inertia to damp out oscillations in the frequency. However, during grid voltage sag or unbalance classical virtual synchronous machines (VSMs) cannot maintain oscillation free power supplied to the grid. Therefore, the maximum power point tracking of the photovoltaic system cannot be maintained, and optimal usage of the available photovoltaic power cannot be accomplished. To cater this problem, in this article a Lyapunov energy function-based augmented controller architecture has been proposed alongside a VSM during unbalanced grid voltage to effectively suppress these double frequency oscillations in the power. Identifying the dynamics of these oscillations followed by detailed stability analyses for the proposed architecture have been presented. In comparison to classical VSMs, the augmented system provides better transient response as well as double frequency power oscillation damping during grid voltage sag or unbalance. To verify the efficacy and understand the challenges of practical implementation of the proposed architecture, experimental verification on a reduced scale laboratory prototype is performed and various important case study results are obtained and presented in this article.

Power Decoupling Method for Voltage Source Inverters Using Grid Voltage Modulated Direct Power Control in Unbalanced System

The grid voltage modulated direct power control (GVM-DPC) is a novel power regulation method for grid-connected voltage source inverters (VSI). However, interactions between real and reactive powers still exist in GVM-DPC under unbalanced grid voltage condition, which may deteriorate its transient performance. This article proposes a power decoupling method for GVM-DPC-based VSI in unbalanced systems based on the dynamic feedforward power compensation strategy, which can reduce the coupling magnitudes of real and reactive powers in the transient stages. First, the power coupling mechanism of positive and negative sequence real and reactive powers of VSI under the unbalanced voltage condition is analyzed. Then, the power coupling magnitudes (PCM) are derived according to relationships among the positive sequence components of real power, reactive power and point of common coupling (PCC) voltages. Next, the PCM are compensated into the GVM-DPC for the better decoupling performance. Furthermore, a stability analysis of the proposed control system is studied based on the impedance model, and a comparison with the traditional power decoupling method based on virtual impedance is conducted to show the superiority of the proposed method. Finally, simulations, hardware-in-loop test and experiment are conducted to validate the effectiveness of the proposed method.

Optimal Suppression Strategy for Capacitor Voltage Ripples of Hybrid MMCs Under Unbalanced Grid Voltages

Submodule (SM) capacitor voltage ripples in modular multilevel converters (MMCs) can be suppressed by harmonic injection strategy, which can reduce the volume and cost of MMCs. The existing methods using the dual harmonic injection strategy for hybrid MMCs only consider steady-state conditions. Harmonic injection strategies for unbalanced grid voltage conditions only use single harmonic injection and can be applied for single-phase faults. To address the above issues, this paper proposes an optimal dual harmonic injection of second-order harmonic current and third-order harmonic voltage strategy for hybrid MMCs. Firstly, the mathematical model of MMC under unbalanced grid voltages is established. Then the ripple characteristics of SM capacitor voltage are analyzed to reveal the relationship between the capacitor voltage and the arm power ripples. Based on the developed instantaneous arm power model, optimal harmonic injection parameters are calculated. A hybrid MMC simulation model is built in PSCAD/EMTDC to verify the effectiveness of the proposed strategy. The results show the excellent performance of the proposed strategy in suppressing the voltage ripples under unbalanced voltage conditions.

An adaptive phase‐locked loop‐less control strategy for LCL‐filtered grid‐connected inverter under complex grid conditions

SummaryThe phase‐locked loop, as a method to extract grid voltage phase angle, is commonly used for grid‐connected inverter to synchronize the injected grid current with the grid. However, the phase‐locked loop will also bring some problems. The current loop controller, the phase‐locked loop, and the grid impedance are coupled together under the weak grid. Then, the phase angle of the inverter output impedance will be reduced, causing the system to be unstable. Moreover, the output phase angle of the phase‐locked loop participates in the coordinate transformation under the unbalanced grid voltage condition, which degrades the quality of the injected grid current generated by the inverter with high harmonic content. Therefore, a vector current control strategy without phase‐locked loop derived from direct power control is used in this paper. An adaptive linear Kalman filter algorithm is also applied to observe the voltage information and avoid the harmonic problems caused by power calculation. Finally, simulations and experiments are conducted to verify that the proposed method is effective, and the results are consistent with the theoretical analysis.

Advanced Vector Controller for a Four-Switch Three-Phase Rectifier under Unbalanced Grid Voltage

One of the main challenges of a four-switch three-phase rectifier (FSTPR) is a DC imbalance in capacitor voltages. On the other hand, under unbalanced grid voltage conditions, unbalanced three-phase input current and the DC-link voltage affect the performance of the FSTPR. Although many papers focus on designing a controller to balance DC-link capacitor voltage, a few papers are available to cope with the imbalance of DC-link capacitor voltages and input current simultaneously under unbalanced grid voltage. In this paper, first, the operation of the FSTPR under unbalanced grid voltage conditions is investigated. It can be seen that under these conditions, the oscillatory parts of the active and reactive input power, i.e., sin and cos components, are the leading cause of the problems that can severely degrade the FSTPR performance of the controller. Therefore, this paper presents a promising control technique to eliminate the mentioned oscillation components. Aiming at this purpose, the current control loops in the dq axis are divided into two positive and negative sequences, i.e., idq+ and idq−. Simulation results in MATLAB/SimPowerSystem™ show that the proposed controller can reduce the output voltage ripple, the total harmonic distortion, and the unbalancing of input current compared to a conventional controller. Under these conditions, the DC-link capacitor voltages are more balanced, significantly reducing the voltage limiter of the FSTPR.

A Direct Discrete-Time Output Feedback-based LS-RMRAC for Grid-tied Current Control Loop of a Static 3-Wire Converter under Unbalanced Grid Voltage Conditions

Nowadays, renewable energy generation is a worldwide tendency. These clean sources are generally tied to the grid by voltage-fed static converters. In this work, a direct discrete-time Output Feedback-based LS (Least Squares)-RMRAC (Robust Model Reference Adaptive Control) is proposed for grid-side current loop control of a static 3-wire converter connected to the grid through LCL filter. The proposed control method mitigates the LCL filter resonance,rejects grid disturbances without needing Resonant Controllers implementation, and deals properly with grid uncertainties and parametric variations, being suitable for grids with unbalanced voltage conditions. Simulation results of proposed controller are presented. Keywords: Robust Adaptive Control. Grid-tied converter. LCL filter.

Conductance Emulating Control Strategy for Three-Phase Current Source Rectifier Under Unbalanced Grid Voltages

Power oscillation is a technical challenge for the grid-connected three-phase current source rectifiers (3ph-CSRs) under unbalanced grid voltage conditions. To manipulate the instantaneous power adapted to different grid requirements under unbalanced grid voltage conditions, a conductance emulating control method is proposed in this brief. The proposed method constructs a fictitious voltage pair with two individually adaptable parameters. One parameter regulates the distribution of oscillating power between active power and reactive power, and another parameter controls the average reactive power. Then, the rectifier imitates variable conductance under the fictitious voltage pair, regulating the average active power. As a result, the power control of the grid-connected 3ph-CSRs becomes flexible and adaptable to various grid requirements. The correctness and effectiveness of the proposed control scheme are verified by experimental results.

A control strategy for DFIG-based systems operating under unbalanced grid voltage conditions

An integral control strategy for DFIG-based systems operating under unbalanced grid-voltage conditions is presented in this article. First, dynamic models of the DFIG and the back-to-back converter needed to design the system’s current control-loops are presented. Then, a strategy to control the generator’s electromagnetic torque and reactive power is introduced. The strategy is implemented by using a stator-flux aligned reference frame, where the stator-flux signals are obtained from an observer. Next, a control strategy for sinusoidal current injection under unbalanced voltage conditions is presented. This strategy generates the current references for the grid side converter from the desired mean values of the output active and reactive powers and the positive-sequence component of the grid voltage. Also, a DC-Link voltage control that works in combination with the sinusoidal current injection strategy is proposed. The design of a non-linear passivity-based controller to track the rotor- and grid-side converters current reference signals is presented. The controller allows the implementation of the integral control strategy without negative-sequence detectors which increases robustness and simplicity. Finally, simulation results are presented to validate the proposal and a stability analysis is performed.