Grounding for CTÆs/PTs in LTC controller scheme.
Grounding for CTÆs/PTs in LTC controller scheme.
(OP)
A while back there had been thread238-174486: Differential Trip 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?
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?






RE: Grounding for CTÆs/PTs in LTC controller scheme.
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.
RE: Grounding for CTÆs/PTs in LTC controller scheme.
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
RE: Grounding for CTÆs/PTs in LTC controller scheme.
Thanks for your responses.
RE: Grounding for CTÆs/PTs in LTC controller scheme.
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.