transformer engineering design and practice 1_phần 1 pdf

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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%, Z0=17.0%

LV/TV:Z1=Z2=6%, Z0=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 (X0/X1 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:

Copyright © 2004 by Marcel Dekker, Inc.