Grounding for CTÆs/PTs in LTC controller scheme. 1

[bacon4life]

A while back there had been thread238-174486 on single point grounding of protection CT circuits. Do the same rules apply for PT/CT circuits involved in transformer paralleling applications rather than protection applications?

1. I have two transformers that will be paralleled with the circulating current method. The application guide shows ground symbols for everything normally at ground instead of drawing the interconnecting wires. While this looks great schematically, there is equipment in both transformers as well as in several cubiles in the control house. Can I really just connect everything shown as ground to the nearest ground block? If not, I am not sure which transformer to ground at nor can I convince myself that both CTs will still be grounded when the bus tie breaker is opened.

2. In a substation application, what will happen if station service is grounded multiple times? For example, once at the dry type SS transformer, and again at each breaker and substation transformer cabinet? One possible answer: Return current will split between the neutral and the ground grid. How much of a problem is that since everything is tied together by the ground grid? Are there any other problems it would cause?

3 Similarly if PT neutrals circuits have inadvertently been tied ground at multiple LTC regulators, would it cause problems or it is just not best practice?

 

[davidbeach]

1. I don't know.

2. While the NEC doesn't apply in the substation that would be a violation of the NEC. The neutral should be the conductor carrying the return current but not fault current and the ground should be the fault return path but not a load current carrying path. Any fault current flows on your ground grid will also be imposed on your 120/240V circuit, I doubt you really want that. Also, any GFCI devices will misoperate.

3. See 2.

 

[subtech]

1. No, I wouldn't. We use Beckwith controllers and have several circulating current/paralleling schemes in use. Our practice is to supply 1 grounded conductor from one transformer control cabinet, and carry that conductor to all other points (such as the a and b contacts from the main and bus tie cubicles in the switchgear) where it is necessary. After several years of use, we've had no problems.

2. No, I wouldn't. As davidbeach explains, placing more than one earth connection on the station power neutral places that nuetral conductor in parallel with the station ground mat. One good hard ground fault and you may lose your station power neutral conductor. If there is one place where you don't want to lose a neutral conductor, it's your station power circuit.

3. Again, agree with davidbeach

 

[bacon4life]

I am not planning on a new installation with multiple grounds, but rather am trying to prioritize removing them from past installations.

Thanks for your responses.

 

[jghrist]

You will typically find that power transformer manufacturers and circuit breaker manufacturers will connect the neutral terminal of the customer ac auxiliary power circuit to the ground bus. The ground buses will be bonded to the station ground grid. While this violates the one ground rule, I'm not sure if I would remove all of these factory ground connections. GFCI protection is usually not a concern in these applications. With a very substancial ground grid path in parallel with the neutral, and an equipment grounding conductor also run with the auxiliary power circuit, I don't think much fault current would flow in the neutral.

I would not ground protection VT and CT circuits at more than one place. This can cause protection problems.

I think that the multiple ground symbols in the current paralleling application guide control schematics are to avoid having to show the ground wire. You would connect all of these points to a grounded wire, but the grounded wire would be grounded only in one place.