Research On Zero-sequence Impedance Measurement Based On Field Disconnection Test And Capacity Detection Of Transformer

As an indispensable power transmission and distribution equipment in the power system,power transformer plays an important role in the power system.Zero-sequence impedance and rated capacity of the transformer are the key parameters to judge the performance of the transformer,which play a vital role in the safe and economic operation of the transformer and the determination of other technical parameters.Accurate zero-sequence impedance is an important basis for power system analysis,calculation of zero-sequence component of short-circuit current,analysis of short-circuit resistance of transformer,selection of relay protection devices and calculation of setting value.However,the measurement and test of zero-sequence impedance of transformer stipulated in the existing standards are complex and harsh,which is not suitable for field test of transformers that have been put into operation.The rated capacity determines the carrying capacity of the transformer.Regular detection of the transformer can timely detect high loss and nominal capacity of the transformer,which can play a role in timely detection and investigation of potential risks and maintenance of power market order.Based on this,this paper studies the measurement of transformer zero-sequence impedance and the detection method of rated capacity.The main research contents are as follows :(1)The equivalent model of power transformer and the standard measurement method of sequence impedance are studied.Firstly,the basic principle and equivalent circuit of power transformer are introduced,and the measurement and calculation method of impedance parameters of power transformer are introduced,and the standard test methods of positive and zero sequence impedance are expounded in detail.Secondly,the main technical parameters and measurement methods for power transformer capacity detection are introduced,which lays a theoretical foundation for subsequent research.(2)The zero-sequence impedance equivalent circuit model of three-phase three-column power transformer is established.Aiming at the single measurement method of high voltage side disconnection test of isolated transformer in existing literature,the zero-sequence impedance test of isolated boost transformer is simplified.Moreover,since the structure of the autotransformer is different from that of the isolated transformer,there is a direct electrical connection between the windings on both sides,so this measurement method is no longer applicable to the autotransformer.Therefore,a zero-sequence impedance field test method of the autotransformer based on the disconnection test is proposed.The unbalanced test condition is formed by applying three-phase power supply to the low voltage side of the autotransformer and constructing the disconnection condition.According to the composite sequence network diagram obtained under different disconnection conditions and the measured data of the difference between the two-phase line current and the line voltage measured at the low voltage side,the calculation formula suitable for the zero sequence impedance of the autotransformer is deduced,and the field measurement of the zero sequence impedance of the autotransformer with different wiring modes is realized.Finally,the simulation model of the autotransformer is built,and the zero-sequence impedance field measurement method of the proposed transformer is simulated and verified.The simulation results are compared with the standard method to prove the effectiveness of the proposed method.(3)Aiming at the problems of high detection cost,complex operation and single reference index of traditional transformer capacity detection methods,a transformer capacity detection method based on zero-sequence sweep impedance is proposed in combination with the above research content and sweep impedance method.Firstly,the zero-sequence impedance sweep reference curves of conventional transformers with different capacities are obtained by the traditional sweep impedance method.Secondly,the zero-sequence impedance sweep curve of the measured transformer is measured by the breaking test method of zero-sequence impedance of the transformer with the sweep power supply.Then,the similarity between the zero-sequence impedance sweep curve and the reference curve is compared to obtain the matching rated capacity value.Finally,a simulation model of 10 k V distribution transformers with different capacity levels is built,and the proposed transformer capacity detection method based on zero sequence sweep impedance is simulated and verified by experiments.The experimental data are compared with the simulation results to verify the effectiveness of the method.