Study On The Measurement Of The No-load Characteristics With Low-frequency Method And Residual Flux Of Ferromagnetic Components

With the development of the power system in our country,the voltage level and capacity continue to increase.Meanwhile,the transmission range become more and more widely.Power transformers,electromagnetic instrument transformers and reactors and other ferromagnetic components are needed to power transmission,transformation and measurement in power grids.The safe and stable operation of ferromagnetic components is critical to the reliable operation of power grids.The no-load characteristics measurement is one of the effective methods to detect the winding and core faults of these devices.However,with the increase of the voltage and capacity of the ferromagnetic elements,the voltage and capacity of the required equipment to measure the no-load characteristics using the nominal frequency(NF)method are large.And,the test process is complicated.In addition,the residual flux may cause the transformer to suffer a large inrush current,and cause measurement error increasing or malfunction of the instrument transformer.So,exploring the no-load characteristics measuring method,residual flux measuring and eliminating methods of ferromagnetic components are very essential.In order to reduce the size and weight of the test equipment and make the test equipment portable,a low-frequency(LF)power supply was used to conduct the no-load test and residual flux measuring and eliminating of ferromagnetic components.Based on the domestic and foreign researches,a circuit mode of ferromagnetic components(single-phase transformer,instrument transformer)on no-load condition is built in this paper.The composition of iron core loss and its relationship with frequency are analyzed.Then,the LF measurement method of core loss based on the principle of least squares and Steinmetz core loss separation formula is proposed.Excitation current simulations of single-phase transformer are conducted using Matlab Simulink.Then,the frequency spectral of excitation current is then obtained by discrete FFT.The results show that the fundamental of the excitation current increase with increasing frequency,whereas the high-order harmonics remain unchanged.Thus,a LF measurement method of no load current harmonic and excitation characteristics for ferromagnetic element is firstly put forward in this paper,where the eddy current is compensated on the fundamental of excitation current.Experiments were conducted on the single-phase transformer and current transformer(CT)using programmable variable voltage variable frequency(VVVF)power supply.These results show that the relative error of the core loss and the excitation characteristic of the ferromagnetic elements are less than 5%,and the measurement error of no-load current harmonic content is within 1%.The test power supply capacity is basically proportional to the frequency,so the test power capacity is reducing with the decreasing frequency.Thus,the LF test method proposed in this paper can approximately replace the NF test.In addition,the experiment studies of the measuring and eliminating methods of the residual flux are carried out on the CT.The remanence measuring method based on the alternating polarity DC source and several demagnetization methods based on the LF principle are analyzed.The feasibility of these methods are proved by conducting the experiments on the CT using programmable power supply.Compared with the open-circuit NF demagnetization method,the demagnetization method described in this paper has the advantages of better demagnetization effect,shorter time,lower voltage and capacity.