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Shunt reactor differential protection and CT sizing

Shunt reactor differential protection and CT sizing

Shunt reactor differential protection and CT sizing

(OP)
hi friends,

I have two questions:
1) Which is better for a 60 MVAR air gap iron cored shunt reactor connected to a 220kV Bus; high impedance differential protection or low impedance differential protection? I went through the Alstom application handbook for P643 relay and it recommends high impedance diff over low impedance differential protection due to better performance during CT saturation. What is the utility industry trend in this case?

2) For CT sizing of shunt reactor diff protection, the current consideration should be inrush current ( 4-5 times rated reactor current) or the bus fault level of the connected bus (considering a HV terminal bushing fault) ?

Thanks

Tinyprot

RE: Shunt reactor differential protection and CT sizing

Does the 220kV bus have high impedance bus protection? If so, you can probably get away with a normal differential scheme on the reactor. This assumes your protection zone for the reactor is defined by its own breaker w/ CTs on both sides (one set for the bus, one for the reactor).

Size your CT based on available fault current from the bus. You can use 2nd harmonic blocking to block non legitimate trips on energization of the reactor.

RE: Shunt reactor differential protection and CT sizing

(OP)
Thanks Mark,
The BB protection is low impedance. All CTs (including BBP) are only on the reactor side of the CB. What is the problem if I size the CT based on inrush current alone ?

RE: Shunt reactor differential protection and CT sizing

You might have coordination issues with have CTs on only one side of the reactor breaker, you may be in a situation where you trip the bus zone for a reactor zone fault, which isn't ideal. With CTs only on the reactor side of the breaker your protection zones aren't going to overlap, which is typically a bad thing for a high voltage station. Depending on the available fault currents at the station the low impedance bus protection may be ok, but it is a little bit surprising to hear that it isn't high impedance. That being said, if your bus protections are low impedance than it won't gain anything by using high impedance differential protection on the reactor. If a CT ends up saturating for an out of zone fault the bus protection will take out the reactor regardless. Has anyone completed a fault study at this station?

CT sizing should always be based on available fault current. Transformer inrush current has nothing to do with sizing your CT. You want to set your relaying up so you block protection operation when it detects inrush current.

Also - I'm not quite sure (although I can guess) what you are referring to with 'BB' and 'BBP'.

RE: Shunt reactor differential protection and CT sizing

(OP)
" if your bus protections are low impedance than it won't gain anything by using high impedance differential protection on the reactor. If a CT ends up saturating for an out of zone fault the bus protection will take out the reactor regardless"
This gives a reasonable logic to avoid high impedance diff protection for the reactor and go for low impedance only.

Well the client utility specifications calls for low impedance Bus Bar (BB) protection only. Bus fault level is 50kA for 1 s. High impedance is limited to only for Power transformer restricted earth fault and 11kV Bus bar protection. Above 33kV it is only low impedance bus bar protection.
Also, location of CTs on the load side of the CB is also bound by specifications only. Long time ago, they had CTs on either side of the CB.
The reactor CT sizing concern was primarily due to low turns ratio as the reactor load current is very less and calls for 400:1 A CT whereas achieving the required knee point voltage at 50kA fault level is difficult and it is easier to size for the inrush current of the reactor.

Thanks Mark for the guidance

RE: Shunt reactor differential protection and CT sizing

CT sizing will be a factor to your decision as well.

There are plenty of installations for high voltage systems (230kV, 500kV, and higher) where you want high impedance protection. It really has more to do with CT sizing and available out of zone fault contributions. A system study and fault analysis will give you good information.

RE: Shunt reactor differential protection and CT sizing

(OP)
Thanks Mark

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