Asymmetrical Grid Voltage Research Articles

An Optimized Active Power Backflow Suppression Strategy for Cascaded H-Bridge PV Grid-Connected Inverter During Inter-Phase Short-Circuit Fault

Active power backflow is a unique problem of three-phase isolated cascaded H-bridge (CHB) PV inverter during asymmetric grid voltage fault, resulting in the continuous rise of H-bridge dc-bus voltages and that the inverter will be eventually shut down and off-grid due to voltage out of control. The existing methods are able to completely suppress the active power backflow during single-phase short-circuit fault and two-phase short-circuit fault, but fails in a large operating zone when inter-phase short-circuit fault occurs. For this issue, this paper proposes a novel multiple specific harmonic voltages injection method based on the existing fundamental-frequency zero-sequence voltage compensation strategy, which calculates and injects the optimal third, fifth, seventh and ninth harmonic voltage according to the fault type, drop depth and the normalized power, for minimizing the amplitude of three-phase modulation voltages and then significantly shrinking active power backflow zone of three-phase isolated CHB PV inverter. Then, the area of the active power backflow zones for different control methods is quantitatively calculated and compared by adopting the polynomial fitting to show the advantages of the proposed method. Finally, experimental results are achieved by a low voltage and low power experimental prototype to verify its effectiveness and feasibility.

Design of a Repetitive Control for a Three-Phase Grid-Tied Converter under Distorted Grid Voltage Conditions

The paper presents a design of repetitive control (RC) in the current control system of a three-phase grid-tied converter. The goal of the control system is to provide sinusoidal input filter currents under the conditions of distorted and asymmetrical grid voltage. A novel design of the RC is presented, in which the repetitive part is not excited by sharp and non-periodic changes of the reference signal, but it enables high-quality performance under periodic disturbance conditions. In the proposed system. RC cooperates with a discrete state feedback controller. An innovative approach to tuning is proposed in which parameters of the repetitive, as well as the state feedback controller, are selected as a result of the optimization process with the use of a particle swarm algorithm. The proposed control system is verified experimentally on a laboratory test bench. The achieved results confirm the high-quality system performance.

Flexible control of grid-connected renewable energy systems inverters under unbalanced grid faults

The sustainable energy opportunity of the future lies in the renewable energy-based distributed Power Generation systems (DPGSs). However, there are different concerns and integration issues that surround them. To increase the rate at which renewable energy can be used, this paper focuses on an integrated utility grid system. A novel strategy to control the grid side inverter is developed for a DPGS when considering an unbalanced grid. First, an analysis of the consequences of the occurrence of a fault on the grid is performed. Secondly, a control strategy is proposed to ensure the operation of the DPGS connected to the grid even if the latter is unbalanced. The proposed approach consists in developing control loops for each of the three symmetrical sequences in a specific reference frame to ensure the operation of the grid-connected DPGS even during asymmetrical grid voltage fault. The found results are promising in terms of the guarantee of service continuity during faulty conditions.

Optimum Control of Grid-Connected Solar Power System Under Asymmetrical Voltage Drop

Solar power systems are now gradually dominating in providing clean, environmentally friendly energy and human health. In areas with a large share of solar power, grid connection control plays a key role in ensuring operational quality and stability, especially in the event of a grid failure. In case of asymmetrical voltage drop, the control system needs to maintain operation and create a function to assist in restoring the power grid. This study proposes a method to control the solar power system in the condition of asymmetric grid voltage drop based on the method of controlling symmetrical components. Controllers for each of the forward and inverse components are built to limit the effects of failures. The optimal control parameter calculation method is also proposed to improve the overall quality and minimize the undesired variation of the electromagnetic quantities. The simulation and experimental results are verified to evaluate the effectiveness of the grid-connected control method in converting DC power to three-phase power.

An effective control algorithm for dynamic voltage restorer under symmetrical and asymmetrical grid voltage conditions

Introduction. Voltage sag, which is associated to a transitory drop in the root mean square voltage characterizing an electrical source network. During these perturbations, the corresponding electronic customers and devices will suffer from serious operating troubles causing dangerous damages. Purpose. In order to attenuate this disturbance effects, the Controlled Dynamic Voltage Restorer constitutes a very interesting solution among many others that have been proposed. The novelty of the proposed work consists in presenting an enhanced algorithm to control efficiently the dynamic voltage restorer when voltage sag is suddenly occurred. Methods. The proposed algorithm is based on an instantaneous phase locked loop using a multi variable filter to synthesize unitary signals involved in compensation voltages computation relative to the sag apparition. Practical value. A detailed study concerning typical voltage sag, which is consolidated by simulation and experimental results, is conducted to show the used algorithm’s effectiveness to cancel the corresponding voltage sag.

Design of fractional delay repetitive control with a dead-beat compensator for a grid-tied converter under distorted grid voltage conditions

The paper presents the design of a current control system of a grid-tied converter which provides high dynamic in transients states and symmetrical sinusoidal currents under distorted and asymmetrical grid voltage conditions. The current control loop contains a dead-beat compensator and a plug-in fractional delay repetitive controller. The whole control system is designed with regard to signal processing delay present in the control loop. The tuning procedure of the repetitive control uses particle swarm optimization to determine the optimal gain, phase lead and filter parameter. The resulting control system with optimal parameters is applied on the digital signal controller and verified experimentally on the laboratory test bench, which is the main contribution of the paper.

A Control Strategy for a Three-Phase Grid Connected PV System under Grid Faults

This paper proposes a Low-Voltage Ride-Through control strategy for a three-phase grid-connected photovoltaic (PV) system. At two stages, the topology is considered for the grid-tied system fed by a photovoltaic generator with a boost converter followed by a three-phase voltage source inverter. A flexible control strategy is built for the proposed system. It accomplishes the PV converter operations under the normal operating mode and under grid faults (symmetrical and asymmetrical grid voltage sag). The boost converter is controlled via an incremental conductance maximum power point tracking technique to maximize the PV generator power extraction. In the case of voltage sag, the implemented control strategy provides a switch between MPPT mode and non-MPPT mode to ensure the protection of the power converters. Theoretical modeling and simulation studies were performed, and significant results are extracted and presented to prove the effectiveness of the proposed control algorithm.

Main and auxiliary control strategies of DFIG under asymmetrical grid voltage dips

When the Doubly Fed Induction Generator (DFIG) asymmetrical dips in the grid, the stator current will not only generate transient DC components, but also generate negative sequence components, especially when the motor impedance is small. The negative sequence component is larger, which will further deteriorate the grid voltage. In this paper, the DFIG is taken as the research object. When the grid is unbalanced, the main and auxiliary control strategies are adopted for the rotor-side converter, which can control the rotor current quickly and effectively, and improve the uninterrupted operation of the entire wind turbine. The MATLAB simulation software was used to simulate the 1.5MW wind turbine model, and the test was carried out on the 1.5MW wind turbine test bench. The simulation results and test results verify the feasibility and effectiveness of the control strategy.

Engineering modelling of wind turbine applied in real‐time simulation with hardware‐in‐loop and optimising control

In order to constantly improve the output characteristics of wind turbines, the HIL (Hardware In the Loop) experimental platform of wind turbines is built to optimize control. According to the product parameters of 1.5 MW direct-drive PMSG (Permanent Magnet Synchronous wind Generator) of Dongfang Electric Corporation (DEC), the simulation model of wind turbine is built in RTDS (Real Time Digital Simulator), and optimised by comparing with the wind speed–power curve of products, the simulation model of wind turbine with engineering accuracy can be obtained. Base on the engineering model and actual controller of products, the HIL experimental platform of wind turbines is built to optimise control. In the case of unbalanced or asymmetrical grid voltage, in order to make the wind turbine output be stable, the DDSOGI-PLL (direct Decoupling Double Second-Order Generalized Integral Phase-Locked Loop) is proposed to improve the performance of PLL, the active/reactive current decoupling algorithm based on positive and negative sequence voltage orientation respectively is proposed to eliminate the negative sequence current to maintain a constant output power. The effectiveness of algorithm is verified by the HIL experimental platform, the active power output of wind turbine remains constant, the grid-connected adaptability of wind turbines is improved significantly.

Detailed Investigation and Performance Improvement of the Dynamic Behavior of Grid-Connected DFIG-Based Wind Turbines Under LVRT Conditions

Power generation and grid stability have become key issues in the last decade. The high penetration of large capacity wind generation into the electric power grid has led to serious concerns about its influence on the dynamic behavior of power systems. The low-voltage ride-through (LVRT) capability of wind turbines during grid faults is one of the core requirements to ensure stability in the power grid during transient conditions. The doubly-fed induction generators (DFIGs) offer several advantages when utilized in wind turbines, but discussions about their LVRT capabilities are limited. This paper presents a comprehensive study of the LVRT of grid-connected DFIG-based wind turbines. It provides a detailed investigation of the transient characteristics and the dynamic behavior of DFIGs during symmetrical and asymmetrical grid voltage sags. A detailed theoretical study supported by computer simulations is provided. This paper also provides a new rotor-side control scheme for DFIG-based wind turbines to enhance its LVRT capability during severe grid voltage sags. The proposed control strategy focuses on mitigating the rotor-side voltage and current transients during abnormal grid conditions, without any additional cost or reliability issues. As a result, the DFIG performance is improved and utility company standards are fulfilled. Computer simulations are used to verify the expanded ride-through capability of the novel strategy and its effective performance compared to the conventional control schemes.

Low‐voltage ride through control strategy of virtual synchronous generator based on the analysis of excitation state

Virtual synchronous generator (VSG) possesses the advantage of friendly interaction with power grid by simulating synchronous generator characteristics. However, its low-voltage ride through (LVRT) capability is insufficient. The excessive output current of VSG easily causes wind turbines to break away from the power grid, which will exacerbate the negative impact of grid fault. Thus, a new LVRT control strategy is proposed based on the analysis of excitation state for VSG. The droop characteristic, reactive power loop and active power loop of the VSG are improved, respectively, by specifically analysing the response characteristics of VSG. Moreover, the additional current loop is redesigned to assist the system operating in the under excitation state and suppress unbalanced currents without changing the original VSG characteristics. Furthermore, a new orientation method is adopted to accelerate the transient process and achieve better transient performance. It is worth noting that the proposed control strategy does not need switch control algorithm with smooth handoff algorithm under grid fault, and it can deal with both symmetric and asymmetric grid voltage drop problems at the same time. The correctness and feasibility of proposed scheme are verified by rigorous theoretical deduction and complete simulation verification.

Control Strategy for a Grid-Connected Inverter under Unbalanced Network Conditions—A Disturbance Observer-Based Decoupled Current Approach

This paper proposes a new approach on the novel current control strategy for grid-tied voltage-source inverters (VSIs) with circumstances of asymmetrical voltage conditions. A standard grid-connected inverter (GCI) allows the degree of freedom to integrate the renewable energy system to enhance the penetration of total utility power. However, restrictive grid codes require that renewable sources connected to the grid must support stability of the grid under grid faults. Conventional synchronously rotating frame dq current controllers are insufficient under grid faults due to the low bandwidth of proportional-integral (PI) controllers. Hence, this work proposes a proportional current controller with a first-order low-pass filter disturbance observer (DOb). The proposed controller establishes independent control on positive, as well as negative, sequence current components under asymmetrical grid voltage conditions. The approach is independent of parametric component values, as it estimates nonlinear feed-forward terms with the low-pass filter DOb. A numerical simulation model of the overall power system was implemented in a MATLAB/Simulink (2014B, MathWorks, Natick, MA, USA). Further, particular results show that double-frequency active power oscillations are suppressed by injecting appropriate negative-sequence currents. Moreover, a set of simulation results provided in the article matches the developed theoretical background for its feasibility.

Ride‐through strategy of quasi‐Z‐source wind power generation system under the asymmetrical grid voltage fault

Asymmetrical grid voltage will introduce negative-sequence current components on the grid side and cause large voltage ripple of quasi-Z-source capacitors. It will lead to the torque ripple of generator. This study proposes a novel control method based on super-capacitor (SC) to improve the capability of asymmetric fault ride through in quasi-Z-source wind power generation system (WPGS). The stream of system power, in the condition of unbalanced grid voltage, is discussed in detail in this study. To limit the voltage rise and suppress the 2-order voltage ripple of quasi-Z-source capacitors, the throughput power of SC is controlled by proportional–resonant controller to track the output power subtraction between generator and quasi-Z-source inverter. Meanwhile, the negative-sequence component of the grid-connected current is decreased significantly. Finally, simulation and experimental results are given to verify the theoretical analysis.

Control strategy of MMC battery energy storage system under asymmetrical grid voltage condition

Control strategy of MMC battery energy storage system under asymmetrical grid voltage condition

New current references calculation for dual vector controlled three level active rectifiers under asymmetrical voltage operation

This paper introduces an effective nonlinear current references calculation to enhance the performance of the dual vector control under asymmetrical grid for three level neutral point active (NPC) rectifiers. Maintaining the DC-link voltage at the desired level without oscillation is the main target for such kind of application. The asymmetrical grid voltage or voltage dip introduces negative sequence components with badly penetrates through the rectifier control loop casing DC-link to oscillate with twice grid frequency. Conventional vector control (VC) technique failed to meet the control targets under unbalance operation of grid voltage as twice grid frequency ripple pollutes the DC-link voltage. Dual vector control (DVC) concept is implemented to control the positive and negative sequence currents separately. The conventional DVC control scheme can be enhanced by incorporating line power losses for calculating the reference currents. Maintaining the DC-link voltage at the desired level without any symptom of twice grid frequency ripples is one of the control targets which can be achieved by selecting the proper constrains for calculating the current references. The proposed enhanced DVC scheme performance is verified under asymmetrical voltage dip test. The entire system is modeled and simulated with real system parameters of a 7 MW multilevel NPC-type active rectifier.

The Transient Characteristics Analysis of Doubly-Fed Induction Generator during the Asymmetric Voltage Sag

In order to analyze the control strategy of the low voltage ride through (LVRT) of DFIG during the asymmetric voltage sag, it is necessary to analyze the transient performance of a DFIG during the asymmetric voltage sag. In this paper, analyzed the influence of the asymmetric grid voltage to DFIG and the analysis method of the asymmetric voltage sag, and on the basis of positive and negative sequence mathematical model, analyzed the composition of stator output active and reactive power under the condition of asymmetric grid voltage. And built a DFIG asymmetric voltage drop simulation model of 1.5MW in MATLAB/Simulink, the simulation results shows that the stator voltage, current, active power and reactive power all present a double frequency ripple during the asymmetric voltage sag, consistent with theoretical analysis. It can provide theoretical basis for double-fed motor control strategy of asymmetric LVRT.

The Doubly Fed Wind Power Generator Output Characteristic with the Asymmetrical Grid Voltage

The paper describe the imbalance loads、single phase short circuit to ground or bidirectional circuit to ground in the grid can bring unbalanced fault and the asymmetrical grid voltage. In the DFIG the faint imbalance stator voltage can bring much imbalance current, electromagnetic torque and active power would fluctuate. The DFIG performance will deteriorate, the central reason is the negative sequence component of the stator voltage、current with the asymmetrical grid voltage. The comparison between evaluated results and simulation of stator、rotor transient currents when the grid voltage drops is similar much.

Research on the Control Strategy of DFIG Grid Side Converter under Unbalanced Grid Voltage Conditions

This paper studies on the electromagnetic transient model of doubly-fed wind turbine grid-side converter under the imbalanced grid condition. And on this basis, the paper analysis the impact of doubly-fed converter when grid voltage asymmetric drop. It puts forward a dual PLL and dual current control combination of doubly-fed converter grid side control strategy. This strategy achieves grid voltage positive and negative sequence fast separation when asymmetric grid voltage drop occur, achieves the active output power secondary fluctuate suppression under the imbalanced grid voltage condition, avoids DC voltage rise at the fault moment and also achieves converter reactive power support under the imbalanced grid fault. The simulation and experimental results show that the proposed control strategy is correct and with the application value of engineering