Surge voltage stresses across power transformer winding sections provided with metal oxide surge absorber blocks

Voltage distribution across power transformer winding sections to fast front time surge voltages such as lightning overvoltages can be controlled by providing suitable metal oxide surge absorber (MOA) blocks across these sections. The voltage V, relating to current I through a surge absorber block is given by the equation V = KI^beta (K and beta are constants for the MOA blocks). Computer simulations have been carried out to analyze effect of providing MOA blocks on different sections of power transformer winding. High voltage windings having ´alpha´ (Square root of ratio of total ground capacitance to total series capacitance) values 6, 12 and 18 have been analyzed. Initially, several variations were carried out in the computer simulations to investigate whether MOA blocks could be provided across first few sections of windings only. All these investigations indicated that such an arrangement transfers high electrical stresses to sections across which the surge absorber blocks are not provided. In addition these investigations have shown that by providing MOA blocks on each of the sections of the entire winding can only result in considerable reduction of stresses for fast transient voltages. The results have also shown that, for a values 6, 12 and 18 the maximum voltage stress across a section of winding without MOA blocks can be 35%, 51% and 68% respectively and these values with MOA blocks across sections is limited to between 23% to 24%.