Bi Daqiang

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.

Quasi-Z source inverter based pole-phase modulation machine drive system

This paper presents a motor drive system based on the quasi-Z source inverter (qZSI) to control the pole-phase modulation (PPM) machines. With the inductor-capacitor (LC) impedance network, the qZSI can advantageously use the shoot-through state to obtain any desired output ac voltage. In this paper, the capacitor voltage of the quasi-Z source (qZS) network is controlled to obtain the constant dc-link peak voltage. The double vector control algorithms are developed for the PPM induction machine (IM), where one for 3-phase with 12-pole mode, and another for 9-phase with 4-pole mode. The drive system, including the PPM-IM model and the proposed control scheme, is built in MATLAB/ SIMULINK. Three operating modes of the system, i.e., 3-phase with 12-pole, 9-phase with 4-pole, and pole-changing transient process are tested in the simulations. Simulated results verify the proposed drive system.

The out-of-step protection with prediction function for generators

This paper puts forward a developed scheme against the deficiencies of the out-of-step protection based on the three-impedance element principle, which is widely applied at present. Compared with the original scheme, the developed scheme can predict the out-of-step caused by steady stability disruption or transient instability by using the least square method in advance and monitor the limit of steady stability. So the performance of the out of step protection is improved. The paper gives the whole developed protection scheme. The results of dynamic simulation experiments verify the scheme.

Magnetic analysis and design method for solid rotor induction motors based on directly coupled field-circuit

A new method is presented for analysis and design of solid rotor induction motors in this paper. The main idea is illustrated as follows. Based on the terminal voltage as known condition and considering the end effect, a field-circuit directly coupled mathematical model of solid rotor motors is set up under sine stable condition. The method treats all the nodal potential and three currents of stator windings as unknown variables. By means of the results of the electromagnetic field analysis, the electromagnetic torque can be directly calculated by the numerical method, further other performances are calculated. The new method not only avoids the reiteration of terminal voltage, which is needed in the finite element method using current resource as known condition, but also cuts the complicated calculation of rotor parameters, which must be used in the equivalent circuit method. The finite element method is embedded in the design program so the accuracy of calculation is improved and the design programmer is adapted to solid rotor induction motors with deferent structure. The practical applications validate the new method.

An improved MPPT method for photovoltaic power generation system

To overcome the shortcomings of existing Maximum Power Point Tracking methods for PV power generation system, an improved method is proposed. The method is a combination of Perturb&Observe Algorithms, Quadratic Interpolation Method, and Constant Voltage Tracking. Simulation and experiments are carried out in a prototype to validate the proposed method.

Analysis and improvement on stator earth-fault protection by injecting 20 Hz signal

By analyzing the sensitivity and its influencing factors of stator earth-fault protection by injecting a 20 Hz signal, this paper puts forward an improved generator protection scheme based on the impedance angle and its mutation component. The simulation results verify this kind of protection scheme has higher reliability and sensitivity under generator neutral grounding through a distribution transformer with a secondary resistor and arc-extinguishing coil compared with the conventional criterion. Against different grounding modes, the sensitivity of the protection scheme is further improved by correspondingly adjusting the frequency of the injected signal.

Transient simulation for stator ground fault in generators based on ML-FEM coupled model

This paper puts forward the ML-FEM coupled model for analyzing the transient behavior of stator ground fault in generators, which possesses the distinguished capability to accurately take account of not only the distributed capacitances of the stator windings to the ground, but a variety of generator nonlinearities. The numerical simulation of stator ground fault in generators, along with the related experiments, has been completed. The simulation and experiment results explicitly demonstrate that the ML-FEM coupled model is superior to the ML model presented previously, and more preferable for transient analysis of stator ground fault in generators. As paves the way to develop and improve the relay protection scheme against this type of fault.

A new selective transient protection for the ground fault of large unit-connected generators

Based on the multi-loop circuit method of AC machine a mathematic model is established for the large unit-connected generators, namely the outlet terminal of generators connected to a common busbar, and the transient zero-sequence current and voltage of these generators are simulated and analyzed at different grounding location. Further, making use of the wavelet transform sensitively detecting the singularity of signal, the properties of their value of wavelet transform are analyzed, and a selective transient protection is proposed according to the sign and value of the modulus maxima of wavelet transform. The results of simulation and experiment verify the new protection can correctly detect the generator with ground fault, and discriminate between the internal and external faults.