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Discussion Starter · #1 ·
Was doing some work at an old factory yesterday when i saw an old transformer that is not used any more, the transformer does not have any name plate nor a name on it, when i asked an electrician who works at the place about the transformer he said that is was an old grounding transformer.


Personally i don't have any experience with such transformers, but what i can picture is that its a WYE setup which is feed from a delta transmission on one side and grounded at the other side ? ANY Ideas ? :whistling2:
 

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Did he mean a grounding reactor for an impedance grounded system?
 

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Discussion Starter · #8 ·
Did he mean a grounding reactor for an impedance grounded system?
Don't think so, its a transformer called a zigzag, any idea what benefit that provides i mean why not just ground one leg and be done with it ?
 

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Don't think so, its a transformer called a zigzag, any idea what benefit that provides i mean why not just ground one leg and be done with it ?
It would give a phase to phase voltage on two of the phases.

zig zags can be used where 277 volt equipment exists and its no longer desirable to keep the system ungrounded. So it brought in to do so. Of course you could change the service transformer to Y grounded, however that is a considerable cost. A zig zag would be a cheaper option, especially if the 277 load was small.


Another one would be similar but the operators decide to keep the system with the ability to operate with a faulted phase but arcing ground faults are a concern. One would then use the zig zag to derive an artificial neutral and before grounding it a resistor is inserted in series. The resistor is usually sized to allow a current slightly over the capacitive charging current that would normally flow. Ie, if without the resistor grounding a phase makes say 5 amps of current flow to ground with all cables energized in the system then one would spec a resistor that alone allows about 6 or 8 amps to flow. This guarantees arcing faults will not be as bad.
 

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If the transformer is actually a zig-zag, then Meadow described it perfectly.

Another common ground-fault transformer is a basic wye/delta.

The wye side is connected to the ungrounded system, and the center of the wye is grounded. The delta side has an open corner with a resistor connected across it.

In this scenario, if there is no ground fault, there will be equal voltages on all 3 legs of the wye, and equal voltages on all 3 windings of the delta and thus, no voltage across the open corner.

If one of the phases becomes grounded, there will be no voltage on one of the wye legs, and no voltage on one of the delta windings. This will result in voltage appearing across the open corner.

The open corner is connected to either an indicator light or a relay that will cause an alarm or a trip.
 

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To add to what's said here, the other version I often run into is just a little single-phase unit in series with the system neutral, with a resistor bank on the transformer secondary (the diagram on the far right).

Because the secondary drops the L-G fault voltage way down, it allows a much smaller resistor bank than would be otherwise be required if the resistors were directly connected between the neutral point and ground.
 

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Was doing some work at an old factory yesterday when i saw an old transformer that is not used any more, the transformer does not have any name plate nor a name on it, when i asked an electrician who works at the place about the transformer he said that is was an old grounding transformer.


Personally i don't have any experience with such transformers, but what i can picture is that its a WYE setup which is feed from a delta transmission on one side and grounded at the other side ? ANY Ideas ? :whistling2:
It possibly is an impedance grounding transformer. An alternative to a resistive type or just ungrounded system.
 

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I spent 18 months working on a wind farm in south Texas. We had grounding transformers installed on the 34.5kV feeder circuits that tied all of the wind turbines back to the collector substation. Zig-zag transformers can be used as grounding transformers, as previously mentioned. The transformers we had were wye-delta. The wye-winding was connected to the 34.5kV feeder circuit, with the neutral point being grounded. The secondary winding (delta) was left open, not connected to anything. The ground reference allows the protective relaying to sense a ground fault and trip the circuit off line.
 
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