Studies On Stator Ground Fault And Its Protection For Large-Sized Hydro-Generators

The ground faults in the generator stator are by far common faults to which generators are subjected. Internal short-circuit faults, which may immediately damage generator, are always preceded by the ground faults. The possibility of internal fault and damages caused by it can be decreased largely if the stator ground fault protection reliably and sensitively operates. Therefore, the stator ground fault protection plays an important role in the protection system of generator. At present, the large-sized hydro-generators, such as the generators in Three Gorges Power Station, require that the stator ground protection must be with very high sensitivity, so that it is beyond the capacity of available protection schemes.In order to analyze the characteristics of stator ground fault and develop its protection schemes, a mathematical model for large-sized hydro-generators is established based on the multi-loop theory of AC machines and considering the capacitance to ground and actual construction of the stator windings. Modifying the fault-conductance matrix can simulate the ground faults and other kinds of fault in the stator windings.To resolve the situation that the sensitivity of present ground fault protection schemes is low for the large-sized hydro-generators, a kind of protection scheme based on the fault component of third-harmonic voltage and two kinds of protection scheme based on the fault component of zero-sequence voltage at the terminals and neutral are developed, and they can obtain higher sensitivity and reliability and provide 100% protection for the stator windings.The influence of noise on the stator ground fault protection based on the local modulus maximum of wavelet transform is analyzed. To overcome the adverse effect of noise, a new kind of spectral energy protection scheme with high sensitivity and anti-interference is put forward in the thesis by using the multi-scale filter of wavelet transform. Further, extending the simulation model tothe large unit-connected generators, a selective protection scheme is presented by analyzing the characteristics of the variation of zero-sequence transient voltage and current caused by ground faults at different locations.To accurately calculate the value of grounding transient resistance, the thesis compares and analyzes the effect of the equivalent parameters of grounding equipment at the neutral on the admittance criterion of injecting sub-harmonic voltage. In addition, choosing the different value of inductance and resistance of Petersen-coil at the neutral may influence the operation performance of the zero-sequence fundamental frequency voltage protection.A large number of experiments on different generators verify that the mathematical model is correct and effective, and that the protection schemes are sensitive and reliable.