Case Study on Effect of Transformer Rating on Impulse Voltage Distribution in Windings

The design of Power transformers are basically governed by certain vital parameters: Transformer rating in kilo Volt Amperes (kVA), frequency, Voltage ratings and ratio, tapping range, Impedance values, Losses, Temperature rises, Insulation levels, Sound levels etc. The domain of this paper is specifically focused on difference in winding design of same voltage class based on different kVA ratings of transformer. The major change is related to design and type of high and intermediate voltage windings with respect to impulse voltage distribution characteristics across windings. The impulse voltage distribution which is initially based on series and ground capacitance of windings is relatively more non-linear for Low kVA transformers as compared to high kVA transformers for same voltage classes. The results of impulse distribution validating relatively poor safety margins for intermediate voltage winding in low kVA transformer have been discussed. The challenges and results are based on example of 10,000 kVA 225/132/33kV Power Transformer taking reference of 132kV winding and comparison has been established with 60,000 kVA Power Transformer of same voltage class.