Reduction of voltage stresses in power transformer windings due to surges

Behaviour of HV power transformer winding insulation to fast front time surge voltages such as lightning overvoltages for initial stress distribution is dependent upon the square root of the ratio of total ground capacitance to total series capacitance of the winding which is termed alpha (α). Suitably designed metal oxide surge absorber blocks can maintain voltage across its terminals at certain level of voltage magnitudes (approximately) while passing large values of surge currents. In the present theoretical investigations, analysis has been made to identify how the initial electric stress distribution across highly stressed portion of transformer winding can be reduced by providing metal oxide surge absorber shocks across these sections of windings. High voltage windings with α values 10 and 20 have been analysed. The analysis has indicated that maximum electrical stress across highly stressed portion of winding is reduced by 20.3% by providing surge absorber blocks across 20% length of winding from line terminal. For α=20, the reduction is 19.6%. However, it may be noted here that initial maximum voltage stress across winding sections are considerably higher for winding with α=20 than for winding with α=10. In view of the reduction in initial voltage distribution for windings provided with metal oxide surge absorber blocks, the envelope of maximum voltages about uniform voltage distribution is also considerably reduced.