Low Voltage Ride Through Control Strategy Research Articles

A Genetic Algorithm-Based Low Voltage Ride-Through Control Strategy for Grid Connected Doubly Fed Induction Wind Generators

This paper proposes a new computational intelligence-based control strategy, to enhance the low voltage ride-through capability of grid-connected wind turbines (WTs) with doubly fed induction generators (DFIGs). Grid codes world-wide require that WTs should supply reactive power to the grid during and after the fault, in order to support the grid voltage. The conventional crowbar-based systems that were initially applied in order to protect the rotor-side converter at the occurrence of grid faults, do not fulfill this requirement, as during the connection of the crowbar, the DFIG behaves as a squirrel cage machine, absorbing reactive power from the grid. This drawback led to the design of control systems that eliminate or even avoid the use of the crowbar. In order to conform to the above-mentioned requirement, this paper proposes a coordinated control strategy of the DFIG converters during a grid fault, managing to ride-through the fault without the use of any auxiliary hardware. The coordination of the two controllers is achieved via a fuzzy controller which is properly tuned using genetic algorithms. To validate the proposed control strategy, a case study of a 1.5-MW DFIG supplying a relatively weak electrical system is carried out by simulation.

자속 추종을 통한 DFIG 시스템의 LVRT 기법

Doubly Fed Induction Generator(DFIG) systems occupy the largest proportion of worldwide wind energy generation market. DFIG systems are very sensitive to grid disturbances especially to voltage dips due to the structure of the stator connected to grid. In the past, when a grid fault occurs generators are separated from grid(trip method) in order to protect the systems. Nowadays, due to the growing penetration level of wind power, many countries have made some requirements that wind turbines are required to have Low Voltage Ride Through(LVRT) capability during grid faults. In this paper, a flux tracking LVRT control strategy based on system modeling equations is proposed. The validity of the proposed strategy is verified through computer simulations.

An LVRT Control Strategy Based on Flux Linkage Tracking for DFIG-Based WECS

For doubly fed induction generator (DFIG)-based wind energy conversion systems (WECSs), large electromotive force will be induced in the rotor circuit during grid faults. Without proper protection scheme, the rotor side of DFIG will suffer from overcurrents, which may even destroy the rotor-side converter (RSC). To mitigate this problem, a new flux-linkage-tracking-based low-voltage ride-through (LVRT) control strategy is proposed to suppress the short-circuit rotor current. Under the proposed control strategy, the rotor flux linkage is controlled to track a reduced fraction of the changing stator flux linkage by switching the control algorithm of RSC during grid faults. To validate the proposed control strategy, a case study of a typical 1.5-MW DFIG-based WECS is carried out by simulation using the full-order model in SIMULINK/SimPowerSystems. In the case study, a comparison with a typical LVRT method based on RSC control is given, and the effect of the control parameter on the control performance is also investigated. Finally, the validity of the proposed method is further verified by means of laboratory experiments with a scaled-size DFIG system.

Grid-connected photovoltaic system based on switched-inductor quasi-Z-source inverter and its low voltage ride-through control strategy

Compared with traditional Z-source and quasi-Z-source inverter, switched-inductor quasi-Z-source has higher booster multiple, lower capacitance stress, and higher conversion efficiency. Therefore, it is very suitable for grid-connected photovoltaic power generation system. This paper analyzes the voltage boosting theory of the switched-inductor quasi-Z-source inverter which is applied to grid-connected photovoltaic power generation system. For the imbalanced power grid voltage sag situation, this paper puts forward a low voltage ride-through (LVRT) control strategy. System realizes the unity power factor running and has a good LVRT ability under imbalanced power grid voltage sag. At last, the simulation and experimental results are shown to verify the correctness of the control method.

Low Voltage Ride-through Analysis and Control for Doubly Fed Wind-Power Induction Generator Using Finite State Machine Method

Low Voltage Ride through (LVRT) technology of Doubly Fed Wind-Power Induction Generator (DFIG) is a very important issue for grid-connection of wind power system. The paper analyzed and simulated dynamic response of DFIG under balanced and unbalanced gird voltage dip. Then proposed a whole set of LVRT control strategies based on finite state machine method for DFIG and carried out a detailed experimental validation on a 22KW DFIG driving system test platform on balanced grid voltage dip, which was connected to a voltage-dip generator. The experimental waveforms indicated that the proposed control strategy could effectively met LVRT requirement under different dip depth and time.

An Improved Low-Voltage Ride-Through Control Strategy of Doubly Fed Induction Generator During Grid Faults

This paper presents a control strategy to improve the low-voltage ride-through capability of a doubly fed induction generator (DFIG); since the stator of a DFIG is directly connected to a grid, this sort of machine is very sensitive to grid disturbance. Grid voltage sag causes overcurrents and overvoltages in rotor windings, which can damage the rotor-side converter (RSC). In order to protect the RSC, a classical solution based on installation of the so-called crowbar is adopted; however, as the DFIG absorbs reactive power from the grid, this type of solution deteriorates grid voltage sags and cannot meet the requirements of a new grid code. An improved control strategy which uses virtual resistance to limit rotor side overcurrents is proposed in this paper, which can make a crowbar inactive and supply reactive power to fulfill the latest grid code requirement during voltage sags. In order to validate the proposed strategy, simulations and experiments have been carried out, and the results demonstrate the effectiveness of the proposed strategy.

계통 연계형 풍력 발전 시스템의 LVRT 제어 전략

본 논문은 계통 전압 사고 상황에서 계통 연계형 풍력 발전 시스템이 만족시켜줘야 할 LVRT(Low Voltage Ride Through) 제어 전략을 제안한다. LVRT 규정은 계통 전압 사고 시 풍력 발전 시스템이 지켜야 할 부분들을 전압 감소율과 사고 시간에 대해 나타내고 있다. 특히 전압 감소율이 10% 이상일 경우에는 풍력 발전 시스템은 규정된 무효 전류를 전력 계통에 제공하여 계통 전압 확보에 이바지해야 한다. 본 논문에서의 LVRT 규정은 세계적으로 가장 엄격한 규정인, 독일 계통 연계 규정(German Grid Code)을 기준으로 하고 풀 스케일(Full-scale) 가변 속도 전력 변환 시스템을 고려하여 제어 전략을 수립한다. 본 LVRT 제어 전략은 계통 사고 시 LVRT 규정을 모두 만족시킴과 동시에 직류단 전압 제어의 추가적인 알고리즘으로 직류단 전압의 제어를 통하여 전체 풍력 발전 시스템의 전력 균형을 기할 수 있다. 3상 전압 지락 사고의 경우 계통으로의 전력 제어가 불가능하여 계통 측 컨버터가직류단 전압을 제어할 수 없으므로, 전력 제어의 기능을 발전기 측 인버터로 이행 시켜 상황에 따라 유연한 직류단 전압 제어가 가능함을 보였다. 시뮬레이션과 실험을 통해 LVRT 제어 전략의 타당성을 검증하였다. This paper proposes a LVRT (Low Voltage Ride Through) control strategy which should be satisfied by grid-connected wind power system when grid faults occur. The LVRT regulation indicates rules or actions which have to be executed according to the voltage dip ratio and the fault duration. Especially the wind power system has to support the grid with specified reactive current to secure the grid stability when voltage reduction ratio is over 10%. The LVRT regulation in this paper is based on the German Grid Code and full-scale variable speed wind power conversion system is considered for LVRT control strategy. The proposed LVRT control strategy satisfies not only LVRT regulation but also makes power balance between wind turbine and power system through additional DC link voltage regulation algorithms. Because it is impossible to control grid side power when the 3-phase to ground fault occurs, the DC link voltage is controlled by a generator side inverter using the DC link voltage control strategy. Through the simulation and experiment result, the proposed LVRT control strategy is evaluated and its effectiveness is verified.