Negative Sequence Grid Voltages Research Articles

Negative Sequence Current Regulation Based Power Control Strategy for Vienna Rectifier Under Unbalanced Grid Voltage Dips

When Vienna rectifier operates at unbalanced grid voltage dips, the conventional power control strategy suffers from the active power reduction and the input current distortion. To solve these problems, a novel negative sequence current regulation based power control strategy is proposed. On the basis of analyzing maximum input current and maximum phase difference between the gird voltage and the input current, the maximum active power and the available operation region of the conventional strategy are obtained. The proposed strategy introduces the current harmonic coefficient and the active power coefficient. The current harmonic coefficient can change the phase angle characteristic of the grid voltage and the input current. The active power coefficient can reduce the effect of the negative sequence grid voltage on active power and realize the maximum input current limitation. According to the synthesis of the active power and the coefficients information, the negative sequence current regulation is realized, which can reduce the input current harmonics and increase the active power by injecting a small amount of input power ripple. In addition, the coefficients of the negative sequence current regulation are selected according to the unbalance degree of the grid voltage and the DC-link voltage. Finally, the effectiveness of the proposed strategy is verified on a Vienna rectifier platform.

A Control Method to Minimize Negative-Sequence Grid Voltage Considering Battery Lifespan in Battery Energy Storage Systems

A Control Method to Minimize Negative-Sequence Grid Voltage Considering Battery Lifespan in Battery Energy Storage Systems

Analysis and Suppression of Active Power Backflow of Three-Phase Common DC-Bus Cascaded H-Bridge PV Grid-Connected Inverter During LVRT

Featured with the expandable modular structure, three-phase isolated cascaded H-bridge (CHB) inverters are capable of directly connecting to medium voltage power grid without bulky and heavy line-frequency transformer, which has outstanding advantages applied in large-scale photovoltaic (PV) power plants. However, different from traditional PV inverters, three-phase CHB topology is essentially composed of three single-phase CHB inverters. Affected by negative-sequence grid voltage, the active powers of some phases may inversely flow from ac grid to PV inverter during low voltage ride through (LVRT), resulting in that dc-bus voltages of all H-bridges in these phases continue to rise, and that system will shut down due to over-voltage protection. Aiming at this issue, this article studies the mechanism of active power backflow during LVRT, and deduces the quantitative relationship between active current and reactive current to be necessarily injected under different types of voltage faults and different degrees of voltage sags. In addition, combined with the relevant standard of LVRT, the feasibility of suppressing active power backflow by positive-sequence active power current injection is analyzed in detail. The validity of the proposed method is verified by simulation and experimental results.

A Co-ordinated Ride Through Capability and Power Quality Enhancement Scheme for Grid tied PMSG based Wind Energy

The quality of injected current into the grid and low voltage ride-through capability (LVRT) for a grid tied PMSG wind energy is essential for grid operators. Under unbalanced fault conditions and grid-distorted conditions, the negative sequence current injects into the power electronic converter and leads to the wind turbine's disconnection. However, as per the latest grid codes, wind turbines must not disconnect from the grid system. The power electronic converter on the generator side completely decoupled from disturbances on-grid side. Therefore, the effect of unbalanced and grid distorted current has less effect on machine side converter. However, the grid side converter is greatly affected since it is directly coupled to an external grid. The negative sequence grid voltage under unbalanced and distorted grid conditions introduces an unbalanced grid current, which results in fluctuations in dc-link voltage, rotor speed, torque active, and reactive power. Therefore, this paper suggests a Proportional Resonant controller (PR) for Grid side inverter of PMSG to enhance the ride-through capability and enhances the quality of injected grid current under adverse grid faults. The proposed scheme for grid-tied PMSG wind turbine system is simulated in MATLAB/SIMULINK platform. The usefulness of the proposed scheme is validated by comparing it with a typical PI control scheme https://dorl.net/dor/20.1001.1.13090127.2021.11.2.4.3

Three-phase phase-locked loop algorithm and application to a static synchronous compensator

This work presents a phase-locked loop (PLL) algorithm for the detection of positive-sequence and negative-sequence grid voltages in three-phase systems. Since it employs an adaptive filter (AF), it is possible to track the fundamental component of the grid voltages accurately even in the presence of harmonics, subharmonics, and interharmonics. This aspect is of major concern for practical applications, especially because some popular algorithms present limited performance in such scenarios, e.g., the decoupled double synchronous reference frame PLL (DDSRF-PLL) and double second-order generalized integrator frequency-locked loop (DSOGI-FLL). A thorough comparison among the proposed method and other similar structures is presented in terms of steady-state and dynamic behaviors to show that the introduced PLL presents high immunity to power quality disturbances. Unfortunately, a possible drawback lies in the slow dynamic response due to the AF. In order to verify the applicability of the proposed approach, it is also incorporated to the control structure of a grid-connected power converter operating as a static synchronous compensator (STATCOM). This converter aims at regulating the ac voltage at the point of common coupling (PCC) and power factor correction. A detailed mathematical analysis is presented and experimental results are also discussed to validate the theoretical assumptions.

Improved virtual synchronous control for grid-connected VSCs under grid voltage unbalanced conditions

This paper presents an improved virtual synchronous control (VSynC) for the grid-connected voltage source converter (VSC) so as to continuously operate under the grid voltage with steady unbalance. The improved VSynC introduces the negative sequence power controls on basis of conventional VSynC. The improved VSynC is capable of regulating the negative sequence internal voltage to reduce the negative-sequence injected currents and oscillated powers of the VSC aroused by the negative-sequence grid voltage. Three alternative local control objectives for the VSC itself under steady state unbalanced grid conditions and their corresponding power references are deduced and computed. Simulated and experimental results are presented to validate the correctness and effectiveness of the proposed improved VSynC to enhance the continuous operation performance of VSynC-based VSCs during grid voltage steady-state unbalance.

Coordinated control of a hybrid wind farm with PMSG and FSIG during asymmetrical grid fault

This paper investigates a coordinated control of a hybrid wind farm with permanent magnet synchronous generator (PMSG)-based and fixed-speed induction generator (FSIG)-based wind turbines during asymmetrical grid fault. According to the boundary conditions of asymmetrical grid fault, the coupling relationship between positive- and negative-sequence grid voltages is presented in detail. Furthermore, the impact of the operation and equivalent impedance of the hybrid wind farm on the grid voltage components during asymmetrical grid fault are analyzed. A coordinated low voltage ride through (LVRT) control strategy for a hybrid wind farm without communication equipments is then developed based on the impact analysis. The proposed strategy supports the grid voltage recovery with the reactive power provided from the PMSG systems. Meanwhile, by using the extra current capacity of the PMSG systems to provide the negative-sequence current, the negative-sequence grid voltage can be suppressed. As a result, performance and output power quality of the FSIG-based wind farm are both improved with the coordinated control of PMSG-based wind farm. Finally, the simulation results of a hybrid wind farm under different asymmetrical grid fault conditions and a phase-phase short circuit fault experimental results of laboratory-scale test rig both validate the proposed coordinated control strategy.