Simulation And Monitoring On DC Bias Of Power Transformer

The phenomenon that DC current through transformer winding generates DC magnetic flux in the core and leads core half saturation is called DC bias. DC bias causes problems include harmonic increasing, the escalation of transformer vibration and noise and so on, and then affects the safe operation of transformer and power system. In this thesis, simulation and monitoring of power transformer DC bias are studied; the main contents are as follows:(1) The analysis on characteristics of normal operation and DC bias of transformer indicates that hysteresis characteristic of core material is the origin of DC bias. The hysterisis phenomenon could be simulated by JA model, but it is difficult to evaluate the parameters of JAmodel. Equidistant feature points on hyteresis loop are got based on curve integral and the degree of agreement between hysteresis loops is quantified by the sum of distances between feature points. An adaptive genetic algorithm for JA model parameters optimization is proposed in the thesis.With the algorithm, simulation and actual silicon steel hysteresis loop parameters are optimized, and satisfactory results are obtained.(2) The simulation model which can reflect the characteristics of transformer core hysteresis is established based on JA theory. The methods to evaluate structure parameters of tranformer and initial magnetization are studied. The comparison between simulation results and experiment data of model transformer shows that the simulation method is valid. The relationship between the JA model parameters and the exciting current harmonics is studied with the simulation model.(3) As to the phenomenon that harmonic currents flowing through reactive compensation capacitors overflow when DC bias occurs, the simulation model of transformer and reactive compensation capacitors under DC bias is established based on JA theory. Simulations are performed for two cases, and the simulation results are in good agreement with measuring data. The harmonic current of the capacitors under different DC bias conditions is calculated, and the results show that all the harmonic components increase nearly linearly with DC current. The causes of excessive total rms current contain small 4th order harmonic impedance of reactive compensation capacitors and 4th order harmonic amplification between capacitors. The increase of reactance rate can make the 4th order harmonic impedance increasing and avoid 4th order harmonic amplification through increasing the series reactance, and augments DC bias withstand ability of reactive power compensation capacitors(4) Asymmetry operation of DC transmission system is one of the main reasons causing DC bias. The DC current’s flowing cuicurt in AC transmission system is analyzed. AC transmission system is simplified to a circuit in which substations are taken as nodes and all resistance branches exist only between substations. The underground parts are described by self-resistance (ground resistance) and mutual resistance. The unified calculation model of ground current of DC transmission system distribution in AC system is established. DC current distribution of Three Gorges Power Station is calculated with the calculation model.(5) Monitoring methods of DC current on transformer neutral point, vibration of tank surface and noise are studied. A transformer DC bias on-line monitoring system is designed to monitor the current of transformer neutral point, vibration and noise signals. The system can save data automatically according to the variation of monitoring data for future analysis. The monitoring system has been used in the Three Gorges Power Station.