143 / 2018-08-25 14:13:18

Analysis on the Adaptability of Asymmetric Single-phase Substitution for 500kV Main Transformer

500kV main transformers,PSCAD/EMTDC,single-phase substitution,unbalanced parameters

500kV main transformers occupy a very important position in power system, these transformers are always paired with a standby transformer in case failure happens. However, as the amount of high-voltage transformer is increasing, it’s uneconomical to install a standby transformer for each one. A standby transformer working as the backup of many running transformers is considered. And on the other hand, with the growth of the transformer’s service life, a standby transformer may break down because of aging which cannot be used in the emergency. And then the inconsistent transformer may be used to substitute the broken one in case the huge losses on the entire grid. And these situations would lead to the problem of asymmetric substitution. Especially when it comes to single-phase replacement, the situation would be much more complicated, because not only the imbalance among the transformers in parallel but also the inter-phases asymmetry should be considered; So combined with the practical project that using the standby A-phase transformer of No.1 main transformers to substitute the B-phase transformer of No.2 main transformers in a 500kV substation, this paper studied the adaptability of the asymmetric single-phase substitution for the main transformers.
In order to figure out validity of the substitution, a preliminary simulation model was established in PSCAD/EMTDC which mainly considered unbalanced winding impedance of three-winding autotransformer. Based on the model, the imbalance of voltage and current were analyzed from aspect of transient and steady operation. According to the results of the simulation, the value of the voltage unbalance factor and current unbalance factor during steady operation changed little after the replacement and transient current, voltage in short-circuit failure and inrush current were almost same which means that a certain degree of asymmetry is allowed for impedance of windings and the inconsistent substitution was rational. However, the practical situation shows that there actually was unbalanced voltage when the transformer was first charged after the replacement. For further research, a detail model was built which took distributed capacitance into account. The unbalanced winding to winding capacitance and winding to ground capacitance were both considered which are always ignored at low frequency. Besides according to the equivalent model, vector analysis method was adopted to study the influence of the distributed capacitance on the unbalanced voltage. And the quantitative relation between the distributed capacitance and unbalanced voltage was presented. It was found that the unbalanced distributed capacitance had a significant impact on the unbalance voltage factor. And the simulation results in PSCAD were consistent with the analysis which proved that the characteristics of distributed capacitance would affect the balanced operation in 500kV autotransformer. Finally, based on the analysis, some compensators were installed on the transformers and the practical operation was good which further validated the conclusion.
Combined with practical operation and experimental simulation, this paper analyzed the influence of unbalanced parameters in 500kV autotransformer on unbalanced voltage and current. The results show that a certain degree of asymmetry is allowed for impedance of windings, but the characteristics of distributed capacitance would affect the balanced operation which should be considered in the operation of power transformer. And the conclusion provided reference for the further study on the mutual reserve among different transformers.