Active power flow direction effect on stability in multi-terminal VSC-HVDC transmission system in integrating wind farm

Variation of wind power affects the stability of a multi-terminal HVDC (MT-HVDC) system installed in the purpose of integrating offshore wind farms and supplying an offshore constant power load in oil and gas plat form. This paper investigates the stability of such MT-HVDC system. In order to determine the stability, an impedance-based stability method is adopted. The stability is determined by checking the Nyquist plot of minor loop gain. It has been observed that a high control bandwidth of the dc voltage control loop has better system performance; however the stability margin is very poor and vice versa. Moreover, the dc voltage controlled-converter needs to operate both as an inverter and a rectifier to provide the flexibility of having the power flows both directions and with a high control bandwidth of the dc voltage control loop, the converter cannot operate stably in rectifier mode, on the other hand with a low control bandwidth, the converter operates stably both as an inverter and rectifier; however the system performance is very poor. In order to improve the system performance, a droop control is introduced and the performance of the droop is validated in simulation. The detailed analysis is presented for different power levels of the wind farm and is shown how it affects the stability of the interconnected system.