Renewable energy units are required to have reliable fault ride-through capability to prevent large-scale disconnection of renewable energy plants due to grid faults. Since synchronous generators have better voltage and current resistance capabilities, one of the advantages of renewable energy connected to the grid through the motor-generator Pair (MGP) system is the isolation of grid faults, which is totally different from existing fault ride-through methods. First, the structure and mathematical model of the MGP system are introduced in this paper. Then, a novel control method for the MGP system based on DC link voltage feedback is given, which is very important for fault ride-through in the control power balance on both sides of the DC bus. The isolation mechanism, damping and reactive power support functions of the MGP system on grid faults are analyzed by means of the rotor motion equation and phasor diagram of synchronous machine. A 5-kW MGP simulation and experimental platforms are built to verify the isolation and protection effects of the MGP system on a renewable energy generation system during the transient processes of grid voltage sag, overvoltage and multiple voltage sags. The results demonstrate that the MGP system can isolate grid faults on the generator side, while the oscillation amplitude on the motor side is much smaller, and the generator can provide strong reactive power support to the grid during three grid faults. INDEX TERMS Fault ride-through, high proportion renewable energy grid, motor-generator pair (MGP), multiple voltage sags, fault isolation.

With the increase of renewable energy in power system, the usage of a large number of converters brings safety and stability issues to power system, including lack of reliable inertia response, insufficient abilities for supporting current, voltage and frequency and lack of reliable damping to restrain oscillation. To solve these instability issues, the synchronous motor-generator pair (MGP) is proposed as a new method of grid-connection. Firstly, the main structure and application scene of MGP are analyzed. Secondly, the classical model for the MGP small signal stability is derived. Theoretical deduction proves that the MGP can provide more damping than a single generator with the same mass block. Then, 5kW MGP experimental system is built. The calculation parameters of the small signal model for the experimental system are determined by fitting experimental data repeatedly. On the basis, the calculated and measured frequency responses are compared under the same disturbance. The agreements between calculations and measurements show that the MGP small signal model can reflect damping and inertia level of MGP system. Finally, same disturbance experiment is carried out on a single-generator experimental system with the same mass block. The frequency response curves of the two modes under the same disturbance show that the MGP is more conducive to system stability. INDEX TERMS High penetration of renewable energy, synchronous machine, motor-generator pair (MGP), small signal stability, damping characteristic.

The purpose of this paper is to improve low voltage ride through (LVRT) capability of doubly fed induction generator (DFIG).First, this paper gives expressions of the stator flux and rotor current during the grid fault occurrence and clearing by solving the mathematical model of DFIG. The results of simulation conducted on a 1.5MW DFIG in the PSCAD/EMTDC verified the correctness of the derivation. Through qualitative analysis of stator flux expression, different fault durations will lead to the difference of transient during fault clearing and ignoring the process of fault clearing will lead to excessive crowbar resistance. Therefore, this paper optimizes the Crowbar resistance by considering both the process of grid fault and fault clearing, and then presents a Crowbar resistance adaptive setting principle based on Crowbar structure with parallel resistor. Simulation confirms this method can improve LVRT of wind farms.

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The increasing penetration of renewable energy leads to the decrease of inertia and damping for replacing conventional power plants. The motor–generator pair (MGP) is proposed to improve inertial response and frequency regulation capability for the renewable energy. First, the mathematical model of the MGP and the control method of DC‐link voltage are given. Then, the inertial characteristics of the MGP are analysed, and a mass block is designed by analysing the key factor affecting the moment of inertia. The transient processes in two machines are discussed by comparing the variations of the voltage and current vectors. It turns out that the electromagnetic–mechanical coupling is the key to realise the primary frequency regulation without frequency acquisition. The photovoltaic (PV) is chosen to illustrate the mechanism of achieving a better primary frequency response with DC‐link voltage control methods based on the de‐loading and additional frequency deviation strategies. A grid‐scale simulation model is built to verify the effectiveness of the proposed scheme. Furthermore, six groups of experiments are designed and performed on an experimental bench. The simulation and experimental results demonstrate that the inertial response and primary frequency response of PV can be improved effectively by adopting MGP for grid connection.