Modelling of DFIG wind turbine with consideration for converter behaviour during supply fault conditions

Abstract

The Doubly-Fed Induction Generator (DFIG) is widely used for large grid-connected, variable-speed wind turbines. As the amount of installed wind power increases, it is increasingly important that turbine generators remain connected and support the grid transmission network during transient system disturbances: so-called fault ride-through (FRT), as specified by various grid codes. To study the FRT capability of the DFIG, an accurate model of the system is needed. This must be able to take into account the switching behaviour of the rotor circuit diodes and IGBTs if it is to simulate the converter and the DC-link capacitor current and voltage waveforms during supply fault conditions when vector control is lost, inverter IGBTs are switched off and the inverter appears as a simple 3-phase rectifier. In this thesis, a Simulink model for a vector controlled DFIG is developed to investigate drive fault through characteristics, allowing for the switching effects of all IGBTs and anti-parallel diodes. The model is used to predict machine and converter current and voltage waveforms during network fault conditions, represented by a 3-phase supply voltage dip, and thus assess the FRT performance of the DFIG in accordance with the transmission system grid codes. Four case studies during normal conditions and three fault scenarios during fault conditions are investigated and validated by the 7.5kW DFIG Test Rig. The simulation and experimental results are in very close agreement. The simulation shows that transient rotor currents can obviously damage the converter IGBT devices and DC-link capacitor if no protective action (using Crowbar and DC chopper) is taken. Moreover, the developed model can be used to investigate the transient behaviour of the DFIG drive system during supply fault conditions when the drive IGBTs are switched off and the rotor converter appears to be a simple diode bridge rectifier. Also, the developed model including both FRT devices (Crowbar and DC-brake chopper) will be employed to investigate the DFIGs FRT performance and design the minimum value of crowbar resistance.