Zhang Qixue

A small single-phase switched reluctance generator for wind power generation

This paper discusses an 8/8 configuration single-phase switched reluctance generator (SRG), which is applied for wind power generation. It describes the current-chopping control (CCC) method of generating and the speed feedback control method of the whole system. The hardware implementations and some of experimental results are represented.

Comprehensive simulation for faults in the stator windings of salient-pole synchronous generators

Based on the multi-loop circuit method of calculating the inductance parameters of AC machine windings, a comprehensive and transient simulation model is established for various faults in the stator windings of a salient-pole synchronous generator. In term of different fault styles, the fault-associated matrix in the state equations is modified, and all of faults in stator windings including internal faults and ground faults can be simulated in this unified model. Simulation results are compared with those of experiments to verify the proposed method. Using the model to study the variation of current and voltage is conductive to analyze the features of faults, and design the relevant protection scheme.

Research on new principle of pole discrepancy protection based on voltage phasor difference

The pole discrepancy fault of hydropower units and gas-turbine units may easily occur due to its frequent open and close operations. This cannot be detected by the traditional protection during the initial period of grid-connection and in the normal operation of grid-disconnection. This paper applies the symmetrical component method to analyze the voltage characteristics of both sides of breaking point in a single-machine infinite system. A PSCAD digital simulation model based on the parameters of NR (NR Electric Co., Ltd) dynamic simulation unit is established. The single-phase and dual-phase pole discrepancy faults are simulated to verify the correctness of theoretical calculation. Furthermore, a new principle of pole discrepancy protection based on the voltage phasor difference is proposed. The corresponding protection equipment is developed and the dynamic experiment is carried out on the NR dynamic simulation unit to verify the correctness of the new criterion. Independent of the current variable values and the three-phase discrepancy contact status, the new protection principle can availably detect the pole discrepancy fault during the initial period of grid-connection and normal operation of grid-disconnection for large-scale units.

Analysis and improvement of the biased stator earth fault 100% protection

A kind of high resistance grounding methods of generators is few applied by a 3-phase displacement transformer, which is called Pohl (or Bütow) transformer. The secondary asymmetry of the Pohl transformer yields a fundamental zero-sequence displacement voltage so that the biased stator earth fault (SEF) 100% protection can be applied. The law of the fundamental zero-sequence voltage variation under the normal and SEF conditions is derived from an equivalent circuit. For protecting 100% stator windings, the grounding resistance should be not great than √3 times of the capacitive impedance. The characteristic surface of the biased SEF relay is analyzed, and a novel calculating method of the operational voltage via the generator voltage and frequency compensation is proposed. The miss operation zone in the low voltage and low frequency could be reduced by the improved scheme.