Modeling of UHV power transformer and analysis of electromagnetic transient

To correctly apply transformer differential protection in the environment of UHV (ultra high voltage), it is necessary to model the UHV power transformer reasonably and carry out corresponding electromagnetic transient simulations. Therefore, according to the equivalent circuit of three winding auto-transformer, we firstly set up the three winding autotransformer model by means of UMEC (Unified Magnetic Equivalent Circuit) transformer model provided by EMTDC software. Then, the parameters of UHV transformer are converted to those of the UMEC model. In this way, the UHV transformer model is built. Based on this model, the internal faults model of UHV transformer is set up. Under the UHV environment, the energization and internal fault of UHV power transformer are simulated, and the simulated data are utilized to investigate the operation reliability of the well-applied differential protection combined with the criterion of second harmonic blocking. It is shown with the simulation results that the second harmonic ratios of three phase inrush of the UHV power transformer are all below 10% in the case of some energization scenarios. In this case, the mal-operation of the differential protection with 15%~20% second harmonic restraint ratio cannot be avoided, even though it adopts such a blocking strategy that the protection will be blocked as the second harmonic ratio of any one phase exceeds the threshold. Besides, in some light fault conditions, the second harmonic ratio of the fault current is relatively high in the beginning of fault inception, leading to the short time delay of operation of protection.