Double ended switchgear design

[cduranph]

We are a switchgear manufacturer and we are currently designing a selective type (main-tie-main) switchgear for a secondary unit substation. Now, our problem is whch type of tie CB are we going to use? 3P or 4P? The nominal LV side is 480Y/277V, 3-phase, 5W (T-N-S). Type of application is for semiconductor's firm wherein non-linear types of loads are very common.

The firm is following the ANSI standard, in which single point grounding is very common, However, contrary to that they specify on their specifications that the single point grounding is not acceptable which we agreed and seconded. However, one of their engrs says that, 3P is okay because they have no L-N loads and the 4th conductor has no longer use. Can we use 3P tie CB though we have a 4th conductor? Because of that comment, nothing is fixed right now, can you help us somehow?

 

[wanderer20001us]

Re: Double ended switchgear design

250.24 (3) Dual Fed Services.

Common grounding electrode connection to the tie point of the grounded conductors permitted.

250.30 (2)(b) Multiple SDS

common grounding electrode conductor sized to 250.66

I have never seen a 4 pole tie breaker used in a M-T-M configured switchboard.

 

[rcwilson]

Re: Double ended switchgear design

Use a 3-pole breaker for the tie breaker. But you will have to do some special connections to make the ground fault system work correctly, whether you have a single point ground for both transformers or have each transformer connected to its own ground.

When a ground fault occurs, some fault current can return via the other transformer's ground connection and the neutral bus, since the neutrals are connected together. That current will be seen as a phase-neutral load and not be measured by the GF protection.

The major breaker manufacturer's have instructions on how to wire your ground fault systems to prevent these problems.

 

[cduranph]

Re: Double ended switchgear design

Sam & Bob,

We appreciate your replies. Thank you.

What about during sharing of loads? I mean if section A is out of service and section B will share its capacity to section A, and we have a 3P tie CB, is it also correct that all harmonics and unbalance currents will flow to the neutral of transformer at section A while the supply is coming from transformer at section B? In fact the neutral bus is rated at 100% of the phase buses just to mitigate the effect of harmonics? What would happen if we don't have a connecting link to the neutrals of section A & B since we have only 3P tie CB?

Would appreciate your reply.

thanks.

 

[ron]

Re: Double ended switchgear design

The gear would not be loaded to more than 50% of non-continuous load. With 3P breaker, the neutrals of each side need to be tied across.
The neutral current wants to go back to the source.

 

[wanderer20001us]

Re: Double ended switchgear design

If only three phase loads are utilized, the neutral will not be part of the circuit and, therefore, not impacted by harmonics.

If four wire loads are connected, the neutrals must be sized to accomidate the combined netrual loading, including harmonics, for both sections when operating in a M-T situation, and the neutrals from each section must be connected.

The neutral will be continuous between Section A and Section B (basically a common neutral buss). As was pointed out, the ground fault protection will require special attention to insure proper opertion