Research Of Low-frequency Measuring Method For Ferromagnetic Elements Excitation Characteristics Based On Core Loss Compensation

With the rapidly development of national economy and increase of electrical load,the voltage level and transfer ability of power system has a rapidly increase. It has become a great significance for ensuring the whole safe operation of the power grid. There has a lot of ferromagnetic elements such as power transformer, current transform and reactor,which transmit, convert and measure the electricity. The excitation characteristic is one of most useful ways to check the winding and core fault. But with the element's capacity increase, it's become more difficult to measure the excitation characteristic at 50 Hz. It can lower the volume and quality of the test power supply while we lower the power frequency. So exploring how to measure the excitation characteristics at a low frequency is very important and necessary.In this paper, the structure and core loss of power and current transformer are analyzed in detail, which include the composition of core loss and excitation current, the influence of winding connection, capacitive current on excitation current, the influence of supply waveform, and frequency on core loss. All of those become to be basis for low frequency test study.The equivalent circuit model of transformer is established. With the analysis of excitation current, this paper proposed a low frequency test approach to calculate the eddy loss resistance and excitation characteristic of current transformer. The low frequency test can reduce the power supply voltage from 2000 V to 200 V, and the power capacity from 6kVA to 600 VA.The test result shows that this approach can accurately calculate the power frequency excitation characteristic with the low frequency data.In order to correct the voltage error caused by leakage inductance at low frequency test, the leakage inductance should be measured accurately. So the principle of leakage inductance is analyzed in detail. On the one hand, the theory and numerical calculation and finite element are used to calculate the leakage inductance. On the other hand, a parameter identification method is carried out to measure the leakage inductance. The leakage inductance calculation method of single phase and three phase transformer are respectively proposed.The single phase transformer excitation characteristic test and leakage inductance calculation method are carried out by a variable frequency power supply. The leakage inductance calculated results has 14% difference compared with short circuit test results. Considering the transformer impedance voltage is about 10%, the calculate error has little effect to low frequency test. By measuring the leakage inductance at different frequency and different saturation depth, low frequency is unchanged during the low frequency excitation characteristic test.Finally, the excitation characteristic of transformer is measured by sine wave and square wave. The results show that the low frequency test methods can apply to measure transformer excitation characteristic. Comparing with power frequency test results proves that this method is in reason.