Control of wind farm and VSC-HVDC to enhance frequency reserve

This paper deals with inertia issue for offshore wind farm adopting VSC-HVDC transmission system using novel control scheme. The proposed control scheme utilizes charged DC linked voltage on converter to respond quickly for frequency regulation. The proposed control scheme can control two converter, one is converter of PMSG and the other is converter of VSC-HVDC. PMSG controls generator torque to follow maximum power point tracking (MPPT) using motor side converter (MSC). It means PMSG can use rotating kinetic energy of shaft and blade by using MSC torque control which can operate until angular speed of shaft meets minimum level. Thus wind farm based on PMSG can generate extra power instantaneously when operating point is moved using torque control. In typical VSC-HVDC, the active power control is progressed by focusing on the DC voltage variation: the DC voltage is growth with absorbing of active power, and automatically releases the energy to the other side’s converter to keep DC voltage level. To use charged capacitor energy, reference command of DC linked voltage level is decreased and discharged capacitor energy. Control of discharged capacitor energy also considers recovery scheme. Those two control schemes are combined, and coordinated to operate when system frequency is decreased. This paper shows effect of controlling rotational inertia of PMSG, which is generating extra real power instantaneously and effect of its frequency reserve in power system. Also enhancing frequency reserve control is presented using charged DC linked capacitor energy of VSC HVDC. Coordinated PMSG rotating kinetic energy control and VSC HVDC capacitor energy control also shows results, which is related frequency reserve and its effect. Because those control scheme are absorb existing energy, it occur stability problem after using those controls. Thus recovery condition also simulates using PSCAD/EMTDC. Simulation results show that proposed control scheme is a benefit to help frequency reserve for wind farm and to increasing inertia of power system even though interconnecting wind farm.