138kV arrester sizing 2

You need to consider the grounding of the 138 kV system. If it is not effectively grounded, during a ground fault the Ø-grd voltage of the unfaulted phases will increase to as much as 138 kV.

Below is a frequent range of arrester rating recommended by NEMA.
As a suggestion, an underhung mounted surge arrester could simplify the design reducing steel, installation cost and avoid potential clearance issues.

Thanks for the reply guys. Putting more context to my original question, table posted by Cuky (thanks for the table btw) has a range for solidly grounded system i-e 84 MCOV or 98 MCOV. I am curious if its only TOV capabilities of the arrester that drives the decision to choose between these two or there are other variables to look at?

This is outside my scope but we had 138kv wye gnd sources feeding 138kv delta customers. I understand customer arrestor failures occurred because offset voltages from SLG faults had not been accounted for. That suggests that the definition of solidly grounded depends on the customer's transformer more than the utility's.

Bill

Yes, temporary overvoltage (TOV) is an important factor to consider since this condition imposed overstresses (thermal overrun) in the arrester. Under any circumstances, the arrester MCOV should never be decreased below the steady-state line-to-ground voltage of the system. Due to the specific TOV level, the arrester MCOV may need to be increased accordingly. A simplified way to address this issue is by adding an adequate protective margin varying from >15% for switching surge & > 20 % for FOW and lightning surge.

Among other factors, the arrester's energy capability needs to be taking into consideration. If more accuracy is desired to select the arrester, and insulation coordination study could be an option but difficult to justify the cost for a small project.
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Hi Bill: If the system impedance is known, it is possible to determine the " Coefficient of Grounding". Solidly grounded is defined as 0< X0/X1<3.0 and 0<R0/X0<1.0.

The table posted by cuky doesnt seem to provide a range for arresters ratings but more likely just suggests two consecutive catalog ratings (for different security margins I assume).

I dont know about ANSI/IEEE standards, but IEC states that the minimum MCOV for a solidly ground application should be at least: 1.05*Umax/√3 (for the 138 kV example it means 1.05*145/√3 = 88 kV aprox).

The 1.05 "safety factor" covers possible voltage wave distortion and unbalances.

Thats the first step, you later check TOV capabilites and finally verify you got an adequate protective margin as cuky said.

Do you guys think I need to worry about TRV of the breaker when sizing the arrester? The breaker on the load side of arrester has a TRV peak magnitude of 284kV when it clears the fault. Does it mean anytime, breaker operates arrester will see TOV of 284kV for few micro seconds (duration of TRV)

You don't need to worry.

The breaker TRV does not represent a Temporary Over Voltage (TOV), wich is sustained in time for seconds, instead look at it as a Switching Impulse type of transient overvoltage.

What you see in the catalog screenshot you posted, in the column "Maximum Switching Discharge Voltage", is not the admissible switching voltage for the arrester, but the voltage that will arise across between its top and ground terminals, in case of a current discharge of switching type (current that flows through the arrester).

The arrester will protect against this kind of voltage surge (same as with lightnings) and one of the protective margins you can calculate its the ratio between this arrester Switching Discharge Voltage and the Switching Withstand Voltage of the equipment to be protected (when proper selected: arrester SDV < equipment SWV).

See the graph cuky linked in his second post above to see this more clearly.

Hope it helps.

1)For a comprehensive coverage of selection of arresters, refer to the Application Guide ( IEC 60099-5-2013; IEEE C62.22-2009) and select the rating from IEC 60099-1-2012 ;IEEE C62.11-2012)You can also refer to CIGRE TBs287 &441 for MV circuits. ABB, Siemens also offer application guides through their sites.

2) Voltage rating selected based on max line to ground voltage coming on arrester during LG fault conditions + safety margin, based on the coefficient of earthing. Check the BIL of transformer and maximum residual voltage of arrester shall be less than (1-protecive margin) x BIL of transformer.