Surge Arresters Electrical Ratings 1

[Engineer1916]

Eng Tips Family,
Can you guys please help me with an easy reference to how to calculate the required MCOV rating surge arresters for different pieces of equipment. For example, how to determine the MCOV of required surge arrester for a transformer or for an underground feeder etc.

Thank you guys in advance.

 

[dpc]

In the US, ABB has some good reference material. Basically, it will depend on whether or not your system is reliably grounded. If always solidly grounded, you can take the expected maximum Line-to-ground voltage, add about 10 or 15% safety factor and pick the next standard MCOV above that. For ungrounded or resistance-grounded system, same idea, but you have to use the full line-to-line voltage.

 

[Engineer1916]

dpc,
Thank you very much. I know a grounding factor k is used to include the effect of different grounding types. Could you please give names of some easy references?

 

[dpc]

https://library.e.abb.com/public/837fb693e1d19a1dc1257b130057b22a/Buyers guide Selection (C).pdf

http://apps.geindustrial.com/publibrary/checkout/Arresters?TNR=White Papers|Arresters|generic

The grounding factor is just a measure of the effectiveness of the ground, based on the per unit voltage of the unfaulted phases for a single line-to-ground fault. So for a solidly grounded system, it will be close to 1.0. For an ungrounded system it is 1.732.

 

[Engineer1916]

dpc,
Thank you very much. As always, i really appreciate the help.

 

[KillBill7]

another thing to keep in mind is how quickly your protection operates (either clears the fault or trip the breaker). Arresters have temporary over voltage capabilities for duration of time. You don't necessarily have to use exact line to ground voltage for solidly grounded system or line to line for resistively grounded system if you have fast operting protection in place.

For example, looking at temporary over voltage curve of UltraSIL polymer-housed VariSTAR surge arresters, they are rated for 1.6 MCOV at 0.1s and 1.51 MCOV at 1s. Let say you have 25kV solidly grounded system and your protection operates within 1 sec. LG voltage is 14.5kV, adding 10% buffer will make it 16kV. So based purely on MCOV ratings, you can choose 12.7MCOV arrester, as it's overvolatge withstanding capability is 12.7*1.51= 19 MCOV for 1s (until your protection operates and either clears the fault or trip the upstream breaker).

However, you may not be able to use 12.7MCOV arrester on 25kV system as the distance between hot end and cold end may not be enough to provide adequate air gap which is required for 25kV system.

Hope this helps.

Asad

 

[cuky2000]

Below is an MCOV table for a different nominal system voltage in accordance with the system grounding. This could be used in conjunction with a simplified deterministic method as posted above providing satisfactory results in many applications. Be aware that there are more rigorous approaches selecting arresters applying insulation coordination calculations modeling the system with transient programs such as EMTP, ATP or PSCAD.

For underground cable protection, two distinct approaches are used:
1) Arrester OH/UG Cable mounted in a Riser Pole to protect the cable insulation.
2) Arrester sheath voltage limiter (SVL) mounted in a UG link box to protect the cable outer jacket of the cable from electrical stresses caused during transient events. 

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BillKill7: could you please clarified if there is overlapping between the surge arrester operation
In the microsecond range vs. slower protective device that operate in the millisecond range.
For reference, see the link for a thread
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[prc]

For selecting surge arresters, please refer to the following Arrester application guides -IEC60099-5-2013 or IEEE C62.22-2009. For specifications and standard ratings refer to IEC 60099-4-2009 or C62.11-2012. For MV &LV surge arresters, CIGRE Brochures 287-2006& 441-2010 also may be referred to.