Motor-generator pair: a novel solution to provide inertia and damping for power system with high penetration of renewable energy

Increasing penetration rate of renewable energy sources makes it a trend for power system to be dominated by power electronics, which brings challenges for reliability and stability: first, unlike synchronous generator, converters do not have inherent inertia which is important for frequency response. Second, more complex interaction induced by renewable energies causes problems of power system dynamics, for example damping oscillation. In addition, converters cannot support high fault current, leading to limitation of some existing control schemes during transient events. However, compared with converters, synchronous generator has some advantages to solve aforementioned problems. Therefore, this study proposes a synchronous motor-generator pair (MGP) system as a possible grid-connection way for high penetration of renewable energies to improve stability. State equations for analysis of small signal stability are established based on proposed equivalent rotor angle, which is verified by experiment. Results of mode shape analysis and transient simulation show that MGP can effectively enhance frequency response, damp oscillation and isolate and weaken the negative effects brought by wind farm. Effects on damping ratio with different proportions of MGP are also investigated in different penetration rates.