Separately Derived Sources and Multiple Grounding of Neu

[adhazarika]

Q1. We have two or more 13.2kV delta primary, 480/277V, 3-phase, 4-wire wye solidly grounded transformers associated with unit substations tied together with tie circuit breakers. All circuit breakers including main and tie circuit breakers are 3-pole. Each unit substation transformer is solidly grounded locally and is a separately derived power source. This required that the neutrals from all unit substations tied together via tie circuit breakers be connected together resulting in multiple grounding of neutrals – once at its source and next at the other source via neutral bus. Is this code compliant?

Q2. We are planning to install Partial Differential Ground Fault Protection schemes to account for all the ground current flowing back to the source via different parallel paths created by multiple grounding of neutrals during a ground fault condition, so that the faulted section can be isolated quickly. Will this protection scheme make the above electrical system code compliant?

Q4. Should we install 4-pole main and tie circuit breakers to prevent multiple grounding of neutrals instead?

Ajoy D.

 

[bennie]

Re: Separately Derived Sources and Multiple Grounding of Neu

Ajoy D. Your questions are best handled by a power distribution engineer. The items of concern are not in the NEC scope.

Where are you located?

 

[bennie]

Re: Separately Derived Sources and Multiple Grounding of Neu

Ajoy D. You are changing from a distribution system to a utilization system. This transition is not a separately derived system, the neutral/ground conductors are solidly connected.

Monitoring ground current flow is done often for coordination of overcurrent devices, usually with time overcurrent relays, and power circuit breakers with external logic.

[ July 14, 2003, 07:07 PM: Message edited by: bennie ]

 

[brian john]

Re: Separately Derived Sources and Multiple Grounding of Neu

I need to draw this out and will on Tuesday, but if four pole switches are an option they sure simplify this distribution system. Four poles are expensive and usally not used for this reason. A properly designed and coordinated ground fault system will work.

As bennie stated this is a design issue than a distribution enginner should design.

 

[hbendillo]

Re: Separately Derived Sources and Multiple Grounding of Neu

Bennie has a point. Although you could probably find help from someone browsing this forum you may be off topic a little. Try some of the forums here:

http://www.eng-tips.com/

They have forums specifically related to electrical design in several categories.

 

[adhazarika]

Re: Separately Derived Sources and Multiple Grounding of Neu

Thank you very much. I will check the reference. According to NEC a separately derived system is a premises wiring system that derives its power from battery, solar photovoltaic system, a generator, transformer, or converter winding. Separately derived systems have no direct electrical connection to supply conductors of another system [Article 100 and 250-20(d)].

That is the problem. Because, without a 4-pole tie breaker I will have to connect neutrals of all separately derived systems. That means the neutral will be grounded more than once, since all separately derived systems are solidly grounded.