Another Transformer Question

[brian john]

On a 480-Delta - 208/120 Wye Transformer back-fed 208 3-phase on the wye primary, no neutral but the neutral bond was left intact. The neutral bond in one case was heat damaged but still connected in another circumstance burnt open.

I THINK, the secondary current (Delta H side) was single phase 480 (unit heaters), so loads were not always balanced.

As always any input would be appreciated.


Also from a proper terminology stand point, in this case is the 208 considered the primary, my contention is it is? I have had this discussion more than once.

 

[kingpb]

Forget primary and secondary, too confusing. Simply use HV and LV, and say which each is hooked to, i.e. H or X windings. That explains it all without the confusion.

 

[brian john]

KingPB


But does not answer my main question, the terminology issue was and remains a a point of discussion, until we (YOU AND ME) make a formal ruling and then make others adopt our ways.

 

[hillbilly1]

Even though a neutral was not brought to the transformer on the 208 volt side, it effectively was connected because the XO bond was still connected to ground, which in turn is connected to the neutral back at the service.

 

[jim dungar]

The delta side (secondary) unbalanced currents created unbalanced currents on the wye (primary)side.
For wye-delta step up connections the unbalanced currents are normally accomodated by the L-N voltages changing while the L-L voltages try to stay constant (this is basically the effect you see with a 120/240V single phase service that loses the neutral connection resulting in the L-N voltages varying with the current imbalance).

Because of the internal N (X0)-G bond in the transformer, the L-N voltages, as well as the L-L voltages, were both fixed, so the only way the way to handle the secondary imbalance was to have current flow to/from the X0 point of the source to the X0 point of this transformer. In your case the factory, internal, bonding jumper was not sufficient for the current.

 

[hardworkingstiff]

What is the best way to wire the input (LV) side of a step down (480 Delta x 208Y/120 Wye) transformer being used as a step up transformer?

Bring a neutral with the 208 3-phase and connect to the XO, or

Isolate the XO from ground, and don't bring a neutral to the XO (leave it floating)?

Also, what happens to the currents? Seems to me they would act differently in the 2 different configurations.

 

[don_resqcapt19]

If we forget the terms "primary" and secondary and replace them with HV and LV, how do we apply the overcurrent protection rules in 450.3?
In my opinion the primary is always the supply side and the secondary is always the load side.

 

[Dennis Alwon]

If we forget the terms "primary" and secondary and replace them with HV and LV, how do we apply the overcurrent protection rules in 450.3?
In my opinion the primary is always the supply side and the secondary is always the load side.

I agree

I am not a transformer guy but to me primary means the main voltage (primary) into the trany and secondary is the voltage that we desire to have from that primary voltage used. I can't see it any other way.

 

[brian john]

Even though a neutral was not brought to the transformer on the 208 volt side, it effectively was connected because the XO bond was still connected to ground, which in turn is connected to the neutral back at the service.

.

I know this, I want to know why the neutral bond connection experienced heat damage

 

[brian john]

If we forget the terms "primary" and secondary and replace them with HV and LV, how do we apply the overcurrent protection rules in 450.3?
In my opinion the primary is always the supply side and the secondary is always the load side.

And that is my opinion as well, but I was called to tadsk for describing it as such.

 

[Dennis Alwon]

And that is my opinion as well, but I was called to tadsk for describing it as such.

Probably because the primary is connected to the secondary windings of the trany... I assume that is what you had. I still would call it the primary connection.

 

[brian john]

The delta side (secondary) unbalanced currents created unbalanced currents on the wye (primary)side.
For wye-delta step up connections the unbalanced currents are normally accomodated by the L-N voltages changing while the L-L voltages try to stay constant (this is basically the effect you see with a 120/240V single phase service that loses the neutral connection resulting in the L-N voltages varying with the current imbalance).

Because of the internal N (X0)-G bond in the transformer, the L-N voltages, as well as the L-L voltages, were both fixed, so the only way the way to handle the secondary imbalance was to have current flow to/from the X0 point of the source to the X0 point of this transformer. In your case the factory, internal, bonding jumper was not sufficient for the current.

Thats what I thought, second I was always told no neutral on the primary.

This was not our install but an issue I was trouble shooting years ago. The neutral on the primary comes up every so often. Myself I prefer to get a 208 delta to 480/277 wye.

 

[jim dungar]

Isolate the XO from ground, and don't bring a neutral to the XO (leave it floating)?

Unless you are a utility, you should never connect, any conductor or bonding jumper, to the neutral point of a transformer with a wye primary (incoming).

 

[hardworkingstiff]

Unless you are a utility, you should never connect, any conductor or bonding jumper, to the neutral point of a transformer with a wye primary (incoming).

I believe you, but can you tell me why? What happens if you do? Why is it OK for PoCo to do it?

Thanks,

 

[jim dungar]

I believe you, but can you tell me why? What happens if you do? Why is it OK for PoCo to do it?

Thanks,

As I previously explained, one issue is the neutral conductor is going to try to be a source of balancing current.

Utilities can connect to the neutral point of a wye connection for many different reasons, among them are:
Their transformers are usually step down so the primary currents are relatively easy to accommodate.
They rarely supply a wye-delta connection. Their normal connection is wye-wye which handles unbalanced currents differently.

 

[hardworkingstiff]

As I previously explained, one issue is the neutral conductor is going to try to be a source of balancing current.

Is that a bad thing? Balancing current? Wouldn't that relieve some stress from the coil?

 

[brian john]

HWS, If it exceeds the capacity of the neutral bond jumper I would have to say yes.

 

[jim dungar]

Is that a bad thing? Balancing current? Wouldn't that relieve some stress from the coil?

What stress?

Being a source of 'balancing current' can be a major problem. It is possible to overload the neutral conductor yet have the individual phase conductors underloaded. This means the 'standard' primary protective devices might not adequately protect the neutral conductor, if you used a 200% neutral this probably would not be an issue.

Please note:I have purposely tried to keep this discussion on a non-technical basis, so some liberties have been taken.

 

[hardworkingstiff]

Please note:I have purposely tried to keep this discussion on a non-technical basis, so some liberties have been taken.

Thanks, because I guess I'm asking questions that the technical answers would be above my education level.

 

[mike_kilroy]

.

I know this, I want to know why the neutral bond connection experienced heat damage

very confusing but seems you are asking why the nuetral burned thru when connected. to help answer, tell where the 208 3ph come from? what source? delta? wye? where was ground on the power source feeding the 208? was it 208 or 240v?