Y-Y PT's 2

[CuriousElectron]

Hello,
I apologize if this question was already asked on this forum, but is it a common(industry) practice to provide ground connection for Y-Y grounded PT's at the switchgear where the PT's are installed? If the secondary wiring(4-wire "Y") leaves the switchgear to control room, we should avoid grounding the the neutral again in the control room, correct?
Thanks,
EE

 

[davidbeach]

Every instrument transformer secondary circuit must be grounded EXACTLY once. No less, no more.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[waross]

What is the practice in your organization, David. Ground in the field or at the instruments.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[davidbeach]

In general, ground at the first point in the control house. There are a few usage cases that push the grounding point further into the control house.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[stevenal]

See IEEE C57.13.3. Ground at first point of application. As the industry moves to merging units in the switchgear, the single point ground location will be moving out of the control house again. Dinosaurs like David and I ground in the control house.

 

[davidbeach]

Circuit has to come into the control house to be grounded in the control house. Circuits that never go beyond the breaker cabinet obviously get grounded at the breaker. Didn't even think that might be an issue. Circuits that don't leave the breaker could be free standing distribution breakers where nothing leaves the breaker or it could be a breaker with a merging unit and a bit of fiber leaves the breaker. But for conventional transmission design, see above.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[bacon4life]

[Anecdote]
We used to trouble with transmission capacitor banks tripping off occasionally for faults 20+ miles away. Eventually I found the the PT had been grounded both out in the control yard and inside the control house. During remote ground faults, enough ground fault current would flow through the PT neutral wire to cause the capacitor bank voltage differential relay to operate.
[Anecdote off]

 

[RRaghunath]

Interesting anecdote.
I know of transformer differential protection maloperation due to multiple earthing of CT secondary star point.

 

[prc]

Good inputs from bacon &Raghu. Thanks.