Research Of The Power Equipment Test System Based On Voltage Source Inverter

With the development of power electronics technology, high-power power electronic devices are widely applied, which brings challenges to the test technique of power equipment. Due to the lack of experiment method, only the general functions can be tested in the factory inspection, while more grid connected operating performance, especially under varies power quality disturbance, can only be tested in real power grid. This not only hinders the development and application of power equipment, but also affects the safe operation of the grid, as the grid is served as a "test platform" indirectly. In order to solve this problem, this paper proposes a test system for power equipment based on Voltage Source Inverter (VSI). The system consists of three parts:digital system, VSI devices and equipment to be tested. The digital system simulates the test environment mathematically, while the VSI devices serve as a power amplifier, enlarging the output of digital system to the required level for power equipment test. The test conditions can reach almost the same voltage and capacity levels as those in reality.Firstly, the theory of power equipment test system is studied. According to the different ways of simulation, three methods are proposed to realize the test system.1) Power system simulation based on waveform characteristics.2. Power system simulation based on system equivalent model.3. Power system simulation based on Real Time Digital Simulation (RTDS).Secondly, AC output waveform control strategies for VSI are studied. A method of frequency division observation of differential element is proposed in this paper: derivative feedforward of inductor current is implemented for waveform control of voltage, and derivative feedforward of reference current is also used for waveform control of current. The feedforward element mentioned above effectively improves the output performance of VSI. A modified reduction method based on weighted current is proposed combined with active damping aimed at LCL filter, which transforms the three-order system into one-order system, beneficial to design of control strategy.Then, design and control of the dc-side of VSI is studied. State observer of dc voltage based on analysis of power coupling is proposed.With the observer of fluctuation discipline, new control methods are proposed to solve the average DC voltage calculation problem under the operating modes of variable frequency output and sub-synchronous current output, which guarantees the balance of DC voltage and performance of the rectifier side under above two operating mode. The phenomenon of source-type harmonic energy unbalance in cascaded topology is initiated, and the restraint strategy is provided. Moreover, a multiwinding-transformer-based rectifier is presented for the back-to-back cascaded H-bridge multilevel VSI, and carrier phase-shifted modulation is applied in the multiwinding-transformer-based rectifier side to enhance the high frequency harmonic performance without adding a filter.At last, combined with the demand of practical projects, two kinds of typical electric power equipment test system are developed. With the cascaded-multilevel high-voltage/high-power test system, the simulation of power system based on waveform characteristics is achieved. With the RTDS-based low-voltage test system, the simulation of power system based on RTDS is realized. The research of testing system provides a feasible scheme for complete performance test of electric power equipment under the laboratory environment.