4Pole Circuit Breakers in LV Substation

[EEIre]

Consider a packaged substation as follows:

Two 2.0MVA 10kV - 0.4kV Dyn11 vacuum cast resin transformers TA and TB (close bus coupled to the switchboard).
Two main incomer ACBs to off-load the transformers T9A and T9B
Low voltage switchgear section split into two buses Bus A (normally fed by TA) and Bus B (normally fed by Bus B)
Normally open Bus-tie ACB
400V bus is 4Wire (TN-CS system) with a solid busbar link between the nuetral bar and the earth bar



This is a pretty typical redundant unit substation. I have been asked for advantages in specifying the incomers and bus-tie as 4 pole circuit breakers.

Does anyone care to comment on the advantages/disadvantages of this?

 

[RajT]

Somewhat difficult to comment without the sight of the SLD.

 

[fundam]

if the distributed transformers are not operated in parallel and connections between the substations are only used as transfer connections, the connections may be configured as a five-phase TN-S system with L1-L2-L3-N and PE. Four phase switching devices (L1-L2-L3 and N) are ,however, required in this case. A Four phase transfer device is also required if satellite substations are supplied by two system infeeds in a TN-S system.

Ref:Electrical Installations Handbook(Third Edition) Wiley press(ISBN 3-89578-061-8) or Publicis press. ISBN 0-471-49435-6 page:35 see also figure there...

regards...

 

[rbulsara]

1. There is absolutely no adavantage in using a 4-pole breaker in main CBs (between TX and switchboard). You always want your trasformer neutral and swithboard (system) neutral firmly connected and grounded at all times . It is Code in the USA.

2. While may be permissible is some cases, there is no good reason to use 4 pole tie circuit breaker. Even if you do so, the Neutral contact shall be over lapping type meaning while closing the 4p CB, the N closes before any phase contacts close and while opening the N opens after all phase contacts have opened. This is again to maintain continuity of system grounding. Even momentary ungroudned system can cause funny behaviour in sensitive equipments.

3. Since the two neutrals are already interconnected thorugh the grounding connection, there is no point in trying not to solidy connect the neutral bus. A removalble link is sufficicent to isolate a transformer completely during a maintenance.

4. 4 pole breaker is an odd entity.

 

[alehman]

It would simplify ground fault detection (if that's applicable to your situation). However the 3 pole tie breaker is a very common configuration and most manufacturers can handle it with some version of modified differential ground fault sensing.