Three unbalanced loads connected line-to-line from a wye supply with floating neutral.

[CanadianDave]

I have a situation that is a first for me, and I'm looking for some confirmation and confidence boosting.

I have a three-phase supply fed from a grounded neutral wye secondary, but the neutral conductor is not brought out from the transformer. This secondary feeds into a motor control center that currently feeds only three-phase motor loads (that drive various pumps in a water treatment plant). Engineering has designed a circuit to feed three exterior buildings, one Power Distribution Center (PDC, or e-house) and two battery storage buildings (all three buildings associated with a new solar array). This circuit will provide base building power for these three buildings, but will not connect to the array. Each building is designed to be supplied by two phases of the three phase supply, connected line-to-line and will supply single phase primary, split phase secondary transformers to create 120/204 for lighting, receptacles, HVAC, fire alarm, battery monitoring equipment (but not battery charging), and other, general building loads. The PDC load is approximately two times the load of one of the battery buildings. My questions are as follows:
1. Are the line-to-line voltages going to be affected by the unbalanced loads and floating neutral, given that there are no line-to-neutral loads in the feeder circuitry? If so, to what extent?
2. The MCC will be bonded to the metering cabinet in the PDC (first point of termination for the supply circuit) with the bonding conductor in the feeder cable (the neutral conductor is to be capped and spared). Under fault conditions, will the bonding connection be sufficient to activate the overcurrent device in the MCC, given that there is no neutral in this portion of the circuit?
3. Would a line-to ground fault cause a situation where equipment enclosures could become energized, while the system continues to operate, such as in an ungrounded delta circuit?

My gut is telling me that the neutral needs to be continuous from the first transformer star point, through the MCC and into the metering cabinet, but the design engineers are telling me that I'm worried about nothing. Any suggestions?

 

[zbang]

Barely into my coffee here, but-
1- yes, they will be, but single-phase loads on a delta three-phase system isn't usually a problem.
2- metering cabinet? This all sounds like customer-owned equipment subject to NEC, so all equipment is bonded to the EGC. You'll also need a grounding electrode system at each building bonded to the EGC from the service.
3- is the MCC ungrounded?

The transformer in each building is an SDS, so it has it's own bonding/grounding requirements.

A one-line would be helpful here, since it's not clear (to me) what's being treated as a service and what as a feeder.

 

[texie]

Under the NEC this is a big time violation as it creates a major shock hazard and short circuits have no return path. The neutral must be brought to the first disconnect and bonded.
I'm sure the CEC has similar rules as the laws of physics don't change across the border.

 

[zbang]

The OP says they have a "grounded neutral wye secondary" so there's the return path.

 

[electrofelon]

The OP says they have a "grounded neutral wye secondary" so there's the return path.

I agree, it sounds fine to me.... if I am understanding the setup correctly. There should be an EGC, ultimately connected to the grounded XO secondary. Think about it: if the neutral was extended and run everywhere, what would it be doing? It would just be sitting there with nothing connected to it.

 

[wwhitney]

I have a three-phase supply fed from a grounded neutral wye secondary
. . .
floating neutral

Right, these two parts of the OP are in contradiction.

Cheers, Wayne

 

[electrofelon]

Right, these two parts of the OP are in contradiction.

Cheers, Wayne

Right, I assume by floating he means not used, but definitely should be clarified.

 

[CanadianDave]

Right, these two parts of the OP are in contradiction.

Cheers, Wayne

Barely into my coffee here, but-
1- yes, they will be, but single-phase loads on a delta three-phase system isn't usually a problem.
2- metering cabinet? This all sounds like customer-owned equipment subject to NEC, so all equipment is bonded to the EGC. You'll also need a grounding electrode system at each building bonded to the EGC from the service.
3- is the MCC ungrounded?

The transformer in each building is an SDS, so it has it's own bonding/grounding requirements.

A one-line would be helpful here, since it's not clear (to me) what's being treated as a service and what as a feeder.

So to clarify, the transformer supplying the MCC has a grounded center tap, but no neutral conductor is installed to the MCC. The MCC happens to have a four-bus configuration, but the neutral bus is not connected to anything. The EGC exists at all equipment, but the neutral does not exist outside of the single connection at the first transformer, I'm trying to figure out how to paste a jpeg or pdf of the SLD

 

[winnie]

I have a three-phase supply fed from a grounded neutral wye secondary, but the neutral conductor is not brought out from the transformer.

Is the neutral properly bonded to ground, and is an EGC brought out of the transformer through to all the other equipment?

1. Are the line-to-line voltages going to be affected by the unbalanced loads and floating neutral, given that there are no line-to-neutral loads in the feeder circuitry? If so, to what extent?

As you've described, you have single phase L-L loads, but _zero_ L-N loads. The unbalanced single phase L-L loads will unbalance the 3 phase voltages, to an extent determined by voltage drop. Since there are no L-N loads, the 'floating neutral' won't make a difference.

2. The MCC will be bonded to the metering cabinet in the PDC (first point of termination for the supply circuit) with the bonding conductor in the feeder cable (the neutral conductor is to be capped and spared). Under fault conditions, will the bonding connection be sufficient to activate the overcurrent device in the MCC, given that there is no neutral in this portion of the circuit?

Since the neutral is _not_ supposed to be used for bonding, the presence or lack of the neutral makes no difference. What is _key_ is that the EGC be bonded to the transformer neutral.

3. Would a line-to ground fault cause a situation where equipment enclosures could become energized, while the system continues to operate, such as in an ungrounded delta circuit?

Not if the enclosures are correctly bonded back to the transformer neutral point.

My gut is telling me that the neutral needs to be continuous from the first transformer star point, through the MCC and into the metering cabinet, but the design engineers are telling me that I'm worried about nothing. Any suggestions?

Your gut has gotten confused by the color of the wire There _must_ be a continuous conductive path from the transformer star point to any metal that needs to carry fault current (equipment enclosures, conduit, etc). But since these items are not in general supposed to carry current as circuit conductors, the continuous conductive path is the EGC.

You only need a neutral if you have actual L-N loads, _or_ if the neutral is being used as the bonding conductor. For example in a residential service the neutral is used as the bonding conductor through the meter pan to the service equipment.

-Jon

 

[CanadianDave]

First attempt at SLD upload

 

[texie]

The OP says they have a "grounded neutral wye secondary" so there's the return path.

I'm thinking this is a POCO transformer that is bonded but the grounded conductor (neutral in this case) is not brought to the service disconnect. In that case there would be no fault return path.

 

[wwhitney]

I'm thinking this is a POCO transformer that is bonded but the grounded conductor (neutral in this case) is not brought to the service disconnect. In that case there would be no fault return path.

If the POCO only provides you with 3 ungrounded phase conductors, can't you just treat it like as an ungrounded delta service? Does it matter whether their transformer's secondary is actually a delta, or a wye with the neutral point earthed?

It would helpful to know the location of the service point in the SLD.

Cheers, Wayne

 

[winnie]

I'm thinking this is a POCO transformer that is bonded but the grounded conductor (neutral in this case) is not brought to the service disconnect. In that case there would be no fault return path.

Quite possibly.

The drawing in post #10 shows for each cable callout, the number and size of the circuit conductors (including 'white conductor isolated and spared') along with the size of the GND conductor, _except_ for the red line from the utility to the 'emergency MCC'.

The red line is described as '3x1C conductors in conduit'. If the conduit is metallic and properly bonded at each end, then there is the needed EGC. If the conduit is non-metallic, then there is an incomplete fault current path.

-Jon

 

[wwhitney]

The red line is described as '3x1C conductors in conduit'. If the conduit is metallic and properly bonded at each end, then there is the needed EGC. If the conduit is non-metallic, then there is an incomplete fault current path.

Right, but if that conduit is the service conduit, and the "emergency MCC" is the service disconnect, then it could be an ungrounded service, with the EGC originating in that service disconnect, connected to GES there, and with ground monitors.

Cheers, Wayne

 

[texie]

If the POCO only provides you with 3 ungrounded phase conductors, can't you just treat it like as an ungrounded delta service? Does it matter whether their transformer's secondary is actually a delta, or a wye with the neutral point earthed?

It would helpful to know the location of the service point in the SLD.

Cheers, Wayne

If the POCO secondary is grounded you cannot have an ungrounded system at the customer end. This is a common mistake that I have seen too often and is very dangerous.

 

[wwhitney]

If the POCO secondary is grounded you cannot have an ungrounded system at the customer end. This is a common mistake that I have seen too often and is very dangerous.

Right, thank you. The first ground fault would cause current flow through the earth which would be dangerous. While on a true ungrounded system, the first ground fault would cause not current flow (except through the ground detectors), it would just reference the system voltages conductively to ground (as opposed to the incidental prior capacitive ground reference).

So 2 follow up questions:

(1) Say you have a proper ungrounded delta service (delta secondary) with ground detectors and all, and the POCO came along and replaced the transformer with one with a wye secondary but left the neutral point floating. Would that difference in the secondary configuration matter? Would you be able to tell by observations/measurements after the service point?

(2) Say they also messed up and grounded the wye neutral point and created the dangerous situation above. Would all your ground detectors light? Are they typically sensitive enough to show a high impedance fault like that?

As to the OP, the post specifies a grounded neutral point, but the SLD is silent on the issue. The question is whether that secondary neutral point is grounded or not.

Thanks,
Wayne