Abstract
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.
Highlights
With the increasing depletion of fossil fuels and environmental problems, the development of renewable energy generation to replace traditional thermal power plants has become an effective solution
According to the above analysis, disturbances caused by grid faults are weakened greatly by the shaft of the motor-generator pair (MGP) system which plays an important role in protecting the renewable energy units
To verify the isolation and protection functions of the MGP system for renewable energy units, three kinds of grid faults are set with the power grid emulator according to the China National Standard and the 9.28 blackout in South Australia
Summary
With the increasing depletion of fossil fuels and environmental problems, the development of renewable energy generation to replace traditional thermal power plants has become an effective solution. The auxiliary hardware circuit methods includes energy storage system [9], rotor crowbar circuits and DC chopper circuit [10], STATCOM, SVC, DVR, and other FACTS devices [11], [12], and stator series impedance [13] These devices can achieve FRT in renewable energy units, there are still many cases of secondary failure due to unreasonable switching during faults. The sixth fault caused a 505 MW wind turbine disconnection, resulting in a blackout lasting 50 hours This incident occurred because renewable energy plants do—es not have the capability to realize successive FRTs over a very short time, and no effective solution has been reported yet. The isolation effects of MGP are verified and analyzed by simulation and experiment
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