Study On The Measurement Of The No-load Characteristics With Low-frequency Method Of Ferromagnetic Components Under Nonsinusoidal Excitation

With the rapid development of the power system in our country,the capacity of the single generator and the voltage level of the power transmission grid continues to increase.As the main equipment in the power system,the capacity of ferromagnetic components,such as transformer,mutual inductor,reactor,etc,is increasing.Its performance directly affects the safety and economic operation of the power system.The no-load characteristics measurement is one of the most effective methods to detect the winding and core faults of these devices.However,with the improvements of the voltage and capacity of the ferromagnetic elements,the requiredvoltage and capacity of the equipment to measure the no-load characteristics using the nominal frequency(NF)method are larger which its test process is complicated.Whereas the existing measurement method using low frequency(LF)of no-load characteristics has a low conversion precision,and the required large-capacity frequency conversion power supply of sine wave is expensive.So it is not suitable to popularize the engineering applications.In order to reduce the volume and capacity of the test equipment and cut the test cost to have a wider application range of the LF method,it is the measurement of the no-load characteristics with LFmethod of ferromagnetic components under nonsinusoidal excitation that is presented.Firstly,an improved method for separation of core loss under nonsinusoidal excitation(square wave and triangular wave)is proposed in this paper.The core loss data of multiple frequencies under nonsinusoidal excitation and combined with the least square method is used to calculate the core loss converted to the same freq uency and nominal frequency(NF)sine wave.Furthermore,the excitation characteristic curve and hysteresis loop under LF nonsinusoidal excitation,combined with spline interpolation and polynomial fitting,are utilized to compensate for the magnetizing current and eddy current loss current.After that,the excitation characteristic curve converted to NF sine wave is obtained.Experiments are conducted on the single-phase transformer,voltage regulator,current regulator and electromagnetic current transfor mer usingprogrammable variable frequency.The results show that the average relative error of the core loss calculated by the method described in this paper is about 5%,compared with the NF measurement method.The calculation error of the excitation volta ge is within 1%.For the transformer,voltage regulator,current regulator excitation with large excitation current,the calculation error of excitation current is less than 3%,while for the electromagnetic current transformer with high turn ratio,small excitation current,the excitation current calculation error is about 5%.Compared with the results of Omicron CT analyzer,the accuracy of the proposed method in essay is higher.In addition,the required power capacity decreases proportionally with the decrease of frequency and avoids the use of high cost large capacity variable frequency sine wave power supply instead of frequency conversion square wave or triangle wave power supply.Therefore,it has a practical value of engineering application in some extent.