In view of the weak moment of inertia for power system with high penetration of renewable energy and limited transient voltage support of power electronics, a synchronous motor-generator pair (MGP) is proposed as a possible solution for renewable energy integration to improve grid stability. In order to realize the large-scale renewable energy integration, parallel inverters drive MGP is an ideal control scheme. Firstly, the working principle and mathematical model of MGP are introduced. Then based on a comparative analysis of control of parallel inverters and torque control of synchronous motors, the decoupling vector control method of MGP is proposed. Considering the linear relation between q-axis current and the active power in the rotor flux-oriented mode, the strategy of parallel drive for MGP with vector control is established. Different control loop for offgrid and grid-connected operation is designed, and effectiveness of this control strategy is verified by simulation. Finally, a 5 kW MGP experimental platform with dual inverters is built, the parameters are selected to test stability of the system, and power distribution and efficiency are studied. The results indicate that the parallel operation of MGP is stable under dynamic conditions, the strategy can realize the independent control of each inverter.

With the increase of renewable energy sources (RES) in power systems, the number of traditional synchronous generators (SGs) declines gradually, which leads to a decrease in total inertia and frequency instability in extreme situations. The synchronous motor-generator pair (MGP) system was proposed as a new grid-connection method to improve the inertial response and frequency stability of grids.To study the effects of the MGP on the frequency stability and penetration rate of RES in power grids, first the frequency response equations of a single MGP in a single-machine infinite bus system are derived; then, the system frequency response (SFR) model for a multi-machine system with the MGP integrated is built. By comparing key critical indices of the frequency response after step disturbance with this model, we discover that the MGP system can increase the upper limit penetration rate of RES by quite a percentage. And time domain simulations are used to compare the frequency characteristics for the penetration rates of RES at 25%, 50% and 75% with and without MGP. Finally, the MGP test bench is built for relevant experimental verification.INDEX TERMS Renewable energy sources (RES), motor-generator pair (MGP), inertial response, dynamic frequency response characteristics, upper limit penetration rate of RES. NOMENCLATURE A. ACRONYMS