Selection of Lightning Arrestor for outdoor switchyard

Because surge arrester ratings are maximum phase to ground voltages based on the required protective levels or margins.

For a 66kV system we provide a lightning arrestor with a rating of 60kV. This does not match with the value of phase to ground voltage. Please throw some more light.

It is taken as ~ 80% of the highest system voltage.

"The selection is done at 80% of highest voltage", this statement is wrong.For 400 KV system we can select either 360 KV or 390 KV. It depends on the insulation co ordination study.

1.5 times of Phase to Earth Voltage. (i.e) System Voltage/root 3 x 1.5

Lightning arrester name plate voltage rating depends upon the following calculation-

for 400kv nominal system voltage, the maximum system voltage may be 420KV. Then peak voltage 420x1.414=593.88= 594(approx)

then phase to ground voltage = 594/1.732 = 343 (approx).

for gap-less type ZnO arrester,there are some definite standard ratings depending upon the material of LA. therefore we hve to choose the LA voltage which is equal or greater than 343KV. since 390KV is the standard rating just after 343KV, therefore you have to select 390KV rating arrester in your 400KV ss.