Can Someone Explain Separate Transformer/Service Grounds?

[Brad-X]

I understand from both relevant IEEE standards and in my case the Canadian Electrical Code that services should typically have a grounding electrode for the main service switchboard which is distinct from the ground grid on the main transformer secondary. What I don't fully understand, however, is the purpose of keeping these grids separate. Typically in the smaller commercial/industrial facilities I'm in, the service transformer and main service switchboard are quite close (<25' apart).

Furthermore, I've seen several installations where the switchboard uses the close-by transformer ground grid as it's own grounding electrode via the neutral. Can someone explain the danger of this kind of setup?

For Context:
-I'm typically dealing with primary voltages of 27.6kV and under, and secondary voltage of 600/347V.
-Service transformers are either grounded wye-grounded wye or delta-grounded wye.

 

[dpc]

Not quite clear on your terminology, but...

In NEC land,

If "Main Transformer" is owned by the utility, their service conductors will consists of three phase conductors and a ground/neutral conductor. This ground/neutral is bonded to the utility grounding system (we hope). At the customer service entrance equipment, the neutral and ground are bonded together once only. Downstream of the service panel, the ground and neutral conductors must be separate.

So the utility ground is not isolated from the customer ground.

If you're talking about a customer-owned transformer, that is a separately-derived system with its own neutral, but the grounding systems should be bonded.

 

[Mbrooke]

Not exactly sure of the scenario you have in mind so I might be way off- but sometimes MV grounds are kept separate from LV grounds in the event that there is a medium voltage L-G fault. This prevents MV voltage from showing up on the secondary grounding system.


An example of this are 3 phase 3 wire ungrounded delta distribution systems. Because a phase to ground fault will not open an OCPD, the transformer case and lightning arrestors are connected and run down to a grounding electrode, while the secondary neutral bushing is connected to an insulated conductor and run to another separate electrode (ground rod) 25 feet away from the pole.

 

[Brad-X]

Sorry, I realize my question may be a bit general, as I'm trying to understand the grounding situation for both a utility-owned, and customer-owned transformers.

If "Main Transformer" is owned by the utility, their service conductors will consists of three phase conductors and a ground/neutral conductor. This ground/neutral is bonded to the utility grounding system (we hope). At the customer service entrance equipment, the neutral and ground are bonded together once only. Downstream of the service panel, the ground and neutral conductors must be separate.

I completely agree- no neutral-to-ground connections after the service entrance.

If you're talking about a customer-owned transformer, that is a separately-derived system with its own neutral, but the grounding systems should be bonded.

So, if I'm reading you correctly, you're talking about customer-owned transformers that are delta-wye? My understanding is that in wye-wye transformers, the two center points are bonded together, and thus the downstream system is not truly separately-derived. This gets back to my question of if and why a grounding electrode (i.e. ground grid) should be installed at the main switchboard in addition to the transformer, when in other separately-derived systems (e.g. distribution transformers) there is one grounding electrode at either the transformer OR the first switch in the system.

Not exactly sure of the scenario you have in mind so I might be way off- but sometimes MV grounds are kept separate from LV grounds in the event that there is a medium voltage L-G fault. This prevents MV voltage from showing up on the secondary grounding system.

The following diagram from the CEC may clarify my question (hopefully this lines up with NEC requirements too). Even when the grids are kept separate, they're still interconnected by the grounded conductor (i.e. neutral), so a portion of the fault-current on the MV side will still flow to the LV ground grid.

Basically, I'm trying to understand why the "Service Grounding" electrode shown in this diagram is required.

 

[waross]

The utility owned transformer ground is part of the multiple grounded neutral scheme.
Nevertheless I understand the reason is redundancy of grounding protection.
If either ground is lost the other ground will still provide grounding protection.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[leafmelly]

For the example of your attached diagram, when a L-G fault happend in the source side (MV side),there will be a voltage liftup for the grounding, and do harm to LV service device, By seperating the service grounding to different electrod and to a distance,the harm can be reduced to a safe level.But when a good Equipotent bonding is taken everywhere,this seperation is not necessary since the grounding voltage will be lift together and do no harm.

 

[Mbrooke]

The following diagram from the CEC may clarify my question (hopefully this lines up with NEC requirements too). Even when the grids are kept separate, they're still interconnected by the grounded conductor (i.e. neutral), so a portion of the fault-current on the MV side will still flow to the LV ground grid.

Not necessarily. That graphic shows the LV neutral grounded- not the tank or other parts. If the POCO decides to do so they can ground the tank via one set of electrodes and the LV through another set.


If an MGN system (multigrounded neutral) is used, NESC requires that the MV neutral, LV neutral, tank, arrestors, ect all be connected together and grounded via a single electrode system.

Basically, I'm trying to understand why the "Service Grounding" electrode shown in this diagram is required.

Redundancy and basically how the POCO operates. They are often governed by the NESC in the US and the service via the NEC.

NESC requires that the neutral be grounded down at the transformer when taken over to the customer, and the NEC requires that the service neutral be grounded down. No harm in doing both ways.

 

[Brad-X]

Many thanks for the replies guys.