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Trong phụ lục này, lỗi tính toán không đối xứng cho máy biến áp được đưa ra. Một lỗi-line-to-mặt đất duy nhất trên LV bên được xem xét. Các trung lập của LV và cuộn dây HV căn cứ vững chắc. Hai trường hợp riêng biệt được coi là, tức. có và không có lỗi thức ăn từ bên thứ cấp (LV) | Appendix A Fault Calculations In this appendix asymmetrical fault calculations for a transformer are given. A single-line-to-ground fault on LV side is considered. The neutrals of LV and HV windings are solidly grounded. Two distinct cases are considered viz. with and without fault in-feed from the secondary LV side. For each of these two cases two conditions are considered viz. with and without stabilizing tertiary winding TV . The effects of omitting tertiary winding are illustrated with these cases. Transformer Data Rating S 25 MVA Core construction three-phase three-limb Voltage ratio HV LV TV 132 11 11 KV star star delta Leakage sequence impedances based on 25 MVA base HV LV Z1 Z2 12 Z0 10.2 HV TV Z1 Z2 20 Zo 17.0 LV TV Z1 Z2 6 Zo 5.4 132 KV system fault level SF 5000 MVA The positive-sequence system impedance on the HV side is Z1HS 0.5 calculated by equation 6.2 . The HV and LV side systems are assumed as effectively grounded systems Xfj Xx not greater than 3 . Here in particular the zero-sequence system impedances on the LV and HV sides are assumed to be equal to the corresponding positive-sequence system impedances for simplicity. As mentioned earlier two alternatives are considered for 11 KV LV system i Without any fault in-feed from 11 KV system. ii With fault in-feed from 11 KV system 453 Copyright 2004 by Marcel Dekker Inc. 454 Appendix A 11 KV system fault level SF 833.33 MVA which gives the positivesequence system impedance on LV side Z1LS 3.0 by equation 6.2 . The equivalent impedances in the star equivalent circuit can be computed by using the theory given in Section 3.5 Chapter 3 as ZH ZHL ZHT-ZLT 2 ZL ZHL ZLT-ZHT 2 ZT ZHT ZLT-ZHL 2 Therefore the corresponding positive- negative- and zero-sequence impedances of HV LV and TV windings are computed as follows Z1H Z2H 12 20-6 2 l3 ZOh 10.2 17-5.4 2 10.9 Z1L Z2L 12 6-20 2 -l Zol 10.2 5.4-17 2 -0.7 Z1T Z2T 20 6-12 2 7 ZOt 17 5.4-10.2 2 6.1 0.5 13 -1 -mm--------ƠĨT----------mm- Positive-sequence network .
