What happens to L-G voltage in a 4W system if you lose your transformer neutral reference?

[death900]

If you have a 3 phase 4-wire system 480Y/277V, and you lose your neutral reference at the transformer, what happens to the L-G voltage?

My understanding is that because you have lost the transformer's neutral reference, any single phase system downstream would instantly see 480V L-G instead of 277V.
Is there a delay in this process? The transformer would turn into a floating neutral, but if the system is operating i am wondering how or if it would change, and how fast.

 

[jim dungar]

If you loose the neutral connection at the transformer, the L-G voltage becomes undefined.
In reality the L-G voltage will depend on things like coupling capacitance, but it likely would hover around 277V unless you have substantial unbalanced line currents.

 

[charlie b]

My understanding is that because you have lost the transformer's neutral reference, any single phase system downstream would instantly see 480V L-G instead of 277V.

Not true. I wonder what would make you think that.

Jim has the right answer.

 

[death900]

If you loose the neutral connection at the transformer, the L-G voltage becomes undefined.
In reality the L-G voltage will depend on things like coupling capacitance, but it likely would hover around 277V unless you have substantial unbalanced line currents.

Yeah we will have maybe around 10-15% unbalanced line currents.

 

[jim dungar]

Yeah we will have maybe around 10-15% unbalanced line currents.

You will notice unbalanced L-G voltages.

However, if you develop a ground fault your L-G voltage will jump up towards 480V fairly quickly. The actual voltage will depend on the resistance of the fault. There are situations where you could even approach 960V if you have the unfortunate conditions.

 

[jaggedben]

Probably more interesting than the L-G voltage is the L-N voltage seen by your line-to-neutral connected loads.

 

[death900]

You will notice unbalanced L-G voltages.

However, if you develop a ground fault your L-G voltage will jump up towards 480V fairly quickly. The actual voltage will depend on the resistance of the fault. There are situations where you could even approach 960V if you have the unfortunate conditions.

Yeah that definitely makes sense. My overall concern is downstream loads from the neutral forming transformer are single phase running on 277V, and if the neutral forming transformer disconnected from the circuit for any reason, what would happen to the voltage? I understand we have the ITIC curve to account for sags/swells and for how much time but voltage swells seem to be intolerable above 1ms.

So that is the justification for why im wondering what happens to the L-G voltage, assuming 10-15% unbalanced loads.

The actual voltage will depend on the resistance of the fault

Am i understanding this statement correctly, that if each of the three phases are seeing a different current, the voltage they will see could be different? Is there any way to tell without doing some kind of live test?

 

[wwhitney]

There are situations where you could even approach 960V if you have the unfortunate conditions.

Could you expand on that a little?

Thanks, Wayne

 

[winnie]

There are separate issues being discussed here: 1) What happens if the transformer neutral gets disconnected from _ground_ and 2) What happens if the transformer neutral gets disconnected from the supplied systems. 3) What does the OP mean by a 'neutral forming transformer'? Just an ordinary wye secondary on the supply, or a 'delta' supply with some sort of separate device deriving the neutral?

If the system gets ungrounded, then all of the L-L and L-N voltages stay the same, but the L-G voltages become undefined, and the L-G voltages act like what you see in an ungrounded system. Typically because of capacitive coupling and leakage current, the neutral of an ungrounded system hovers near 0V to ground, but it is _not_ well defined and could take any value. The unbalanced load is not directly related to where the neutral voltage hovers, but rather unbalanced capacitive coupling and unbalanced leakage current.

If you have an ungrounded system with a solid L-G fault, then that line drops to 0V to ground, the N line ends up at 277V to ground, and the other phase lines end up 480V to ground.

In an ungrounded system, another sort of fault is a 'restriking' fault where the fault makes and breaks for some reason (for example, a fault in a spinning motor or a fault caused by vibration). Restriking ground faults can interact with the capacitance of an ungrounded system to 'pump' the voltages up in excess of the normal L-L voltage. I've read a report of a restriking ground fault damaging multiple other motors on the same system.

In the second scenario, if the neutral gets disconnected from the system, then the L-N voltages will change. If you know all of the loads connected to the system you can calculate the resulting L-N voltage, but this isn't trivial; you need to know all of the load impedance values (and especially with electronic loads the impedance may _change_ when voltage changes), calculate all of the L-N impedances on the three phases, and then solve the resulting voltage divider.

You can get a rough guess if you know the L-N voltage and L-N current for each phase, and use that to estimate each phase impedance, and then use those three impedances to solve the voltage divider. This is only an approximation because the impedance values are not constant.

-Jonathan

 

[death900]

What does the OP mean by a 'neutral forming transformer'?

Using a Zig-Zag transformer to form a neutral creating a 4-wire system on a 3-wire system. Im not exactly sure what would happen to the L-G voltage, nor the unbalanced current. It would probably flow through the ground, but it wouldnt be able to balance out in the transformer. It seems like it would trigger the ground fault.

 

[jim dungar]

Could you expand on that a little?

Thanks, Wayne

See Winnie's response, about resonance
This was a big issue in the 40s. I haven't seen any articles since the 80s.

 

[winnie]

Using a Zig-Zag transformer to form a neutral creating a 4-wire system on a 3-wire system. Im not exactly sure what would happen to the L-G voltage, nor the unbalanced current. It would probably flow through the ground, but it wouldnt be able to balance out in the transformer. It seems like it would trigger the ground fault.

There are two types of zig-zag transformers, the zig-zag grounding autotransformer and a delta:zig-zag isolation transformer.

The zig-zag autotransformer is connected to a 3 wire delta secondary and derives the 4th (neutral) wire. This neutral is galvanically connected to the 3 wire supply, and is used to ground the 3 wire supply. Typically these are only used for grounding the 3 wire system, not for supplying L-N loads.

The delta:zig-zag transformer is a variation of the common delta:wye transformer, just with different primary:secondary phase angle relationship.

Which sort of transformer are we discussing?

Are you asking what happens if one of the L connections to the transformer gets opened?

-Jonathan

 

[death900]

There are two types of zig-zag transformers, the zig-zag grounding autotransformer and a delta:zig-zag isolation transformer.

The zig-zag autotransformer is connected to a 3 wire delta secondary and derives the 4th (neutral) wire. This neutral is galvanically connected to the 3 wire supply, and is used to ground the 3 wire supply. Typically these are only used for grounding the 3 wire system, not for supplying L-N loads.

The delta:zig-zag transformer is a variation of the common delta:wye transformer, just with different primary:secondary phase angle relationship.

Which sort of transformer are we discussing?

Are you asking what happens if one of the L connections to the transformer gets opened?

-Jonathan

Yeah we are using the zig-zag transformer for providing a neutral point for grounding purposes. We're using section 450.5 to use a zig-zag transformer for the purposes of creating a 3-phase, 4-wire distribution system connected to a 3-phase 3-wire system.