MFAC BB Prot Relay

[stegall]

Hi,

We have a new system with a bus bar protection scheme comprising of an Areva MFAC Relay (BBP), MVTP Relay (CTS), Metrosil & Variable Resistors all in parallel with each other.
The CTS has operated on a couple of occasions, and measurements were taken that highlighted ~20mA of 'spill' current.
The settings on the BBP are set fairly low (as per the spec) - would it be feasible to increase the value of the shunt resistor to de-sensitise the CTS?
My senior doesn't want to arrange a needless shut down and I am a recent graduate with not a lot of experience in the field.
Other thoughts are faulty CT somewhere on the scheme, or mismatched CTs?

Kind Regards

 

[Kiribanda]

Stegall,

Did the relay opearte for a through fault?

MFAC relay is a high impedance differential relay working as a voltage relay. It is NOT a low impedance differential relay working as a current relay for spill current. Therefore, you have to calculate the voltage setting of the MFAC relay and set it little above the calculated value so that it will not operate for through faults (stability is ensured). The voltage can be calculated by taking into consideration the CT resistance value, maximum through fault current etc. Then based on the set voltage setting, no. of parallel CTs connected, CT saturation curve shape etc, you can then calculate the minimum operating current that the relay required to opearte. For calculation please refer manufacturer's catalogue or IEEE Buff book.

So, first you have to calculate the voltage setting and compare it with the existing setting. (During normal healthy conditions no voltage is applied across the relay)

May be you get a better picture then.

Metrosil resister is to clamp the induced voltage across the relay during a through faults.

If I am correct, MTVP relay serves another purpose. It is a relay to check the voltage loss of the buses.

Therefore,MFAC and MTVP are two different relays.

Hope this helps!

Kiribanda

 

[ScottyUK]

kiribanda,

Per the Areva T&D website:

These provide continuous supervision of the buswires in high impedance type busbar protection schemes, detecting open-circuited buswires as well as open-circuited main current transformers.

It's not a voltage sensing relay per se, but a monitoring relay for a high impedance differential relay scheme. I'm not familiar with this model, but the scheme as stegall describes it seems reasonable after looking at the brochure on Areva's website.

stegall,

You need to investigate where the spill current is originating from. Mismatched CTs in a new board seems unlikely. Perhaps a stray earth connection? The CT group should be earthed at one, and only one, location. Additional earth paths cause wierd things to happen. A sensitive current probe should able to see 20mA without difficulty - check out some of those offered by LEM, or you could hire a Tektronix AM503 (better yet, hire a pair) from one of the test equipment rental companies.

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