Application of Equivalent Instantaneous Inductance Algorithm to the Y-Δ Three-Phase Transformer

The key issue of current differential protection is the correct reorganization inrush current and the internal fault current. The algorithm based on equivalent instantaneous inductance can correctly discriminate inrush current from the internal fault current since it makes full use of the measured voltage information. For the Y-Delta connected three-phase transformer, the phase current of the delta side can be decomposed into the two components, i.e. the non-circulating component and the circulating component. The non-circulating component is derived from the fine current of the delta side of the transformer but the circulating current can not be obtained by this way. The equivalent instantaneous inductance is largely affected by the circulating current of delta side, the accuracy of equivalent instantaneous inductance can be guaranteed only when the impact of circulating current is taken into account. At present, there is no accurate estimation algorithm for obtaining the circulating current, therefore, the computation accuracy of equivalent instantaneous inductance is affected. A new algorithm for accurately calculate the circulating current of the transformer delta side is proposed in this paper, based on which the phase current and the difference current equivalent instantaneous inductance can be derived in sequence. By using this algorithm, not only the equivalent instantaneous inductance can be calculated accurately to avoid the impact of the circulating current, but also the "dead zone" eliminated in the differential protection. At the same time, the differential protection can operate sensitively and reliably. The calculation of equivalent instantaneous inductance when the transformer is under the conditions of the inrush current and internal fault is simulated by using the electromagnetic transient simulation program (EMTP). The correctness of the identification results are also verified on a hardware platform based on a digital signal processor (DSP). The results show that when inrush occurs, the equivalent instantaneous inductance undergo an obvious alternative process from large to small and then from small to large. This signifies that the iron core of the transformer operates alternatively at the linear and the saturation area of the excitation curve. However, when the internal fault occurs, equivalent instantaneous inductance is very small. This method can effectively identify the inrush current and internal fault current. The phase current of delta winding is not required to be measured therefore it is not necessary to change the configuration of CTs. Furthermore, the technique has a high value for practical application and can be applied to transformers at voltages of UHV/EHV level and under 220 kV.