Surge arrester MCOV 1

[AggiesNM]

Hi,

I need to specify a surge arrester in the neutral circuit of a 99MVA 138kv-34.5kv (14.4kv tertiary) auto transformer. The arrester will be located in the neutral circuit of the secondary winding (34.5kV) and is placed between the transformer neutral and a neutral grounding reactor (1ohm, 600A 10sec).

How would one select the MCOV rating of a station class surge arrester in this application? The X0 winding BIL is 200kV.

Regards.

 

[magoo2]

I'd be tempted to go with an MCOV rating just higher than the I X drop across the reactor under fault conditions for highest current.

If your reactor rating is any indication, 600 A and 1 ohm gives you 600 V. Above 600 V, I'm not sure what the next available arresters are. I've used some 3 kV duty cycle ratings and they have an MCOV of 2.4 kV I believe.

 

[AggiesNM]

What is I in the I x drop across reactor? Continuous rating of the reactor?

 

[magoo2]

Sorry, I meant the I * X across the reactor. X is 1 ohm and I is 600 A.

 

[AggiesNM]

I am a little bit confused now. I came across this GE publication that recommends sizing the arrester rating at 70% of line-to-line system operating voltage.

I am thinking 24.4kV MCOV station class in the neutral.I have based this on the recommendation shown on page 16 (of 19) in the attached file. See "selection of surge arrester" at the bottom right.

Your choice of 2.4kV MCOV against system overvoltage due to fault current seems reasonable too.

 

[magoo2]

The 24.4 kV MCOV will certainly work and adequately protect 200 kV BIL insulation, but I think you can go lower. I looked at page 16 and must have missed the reference to 70% so forgive my comments.

Normally, the line arresters are based on around 80% of the line-to-line voltage or 1.25 times the line to neutral voltage. For the neutral to ground, particularly with a neutral reactor, it's a different story. I think you can go much lower. There's an IEEE standard on the subject of neutral grounding so I'll have to check to see if it says anything different than my opinion.

 

[Marmite]

The attached document tells you how to specify arresters including for transformer neutrals. I couldn't find it online, although I originally downloaded it, so I've attached the document. It's a useful reference to keep hold of.

Regards
Marmite

 

[odlanor]

AggiesNM
GE publication that recommends sizing the arrester rating at 70% of line-to-line system operating voltage for solidly grounded system. In ungrounded system it will be 100%.