Phase to ground voltage imbalance 4

[jes1983]

I recently installed a new 2400v/480 v delta -delta transformer with a new motor control center. The MCC operates 5 -75 hp cooling tower fan motors. My ground detection meters are reading 270v on phase A, 300 on phase B, and 270v on phase. What can be causing this imbalance on my phase to ground?

Regards
Jes

 

[dpc]

If the 480 V delta isn't grounded, there is no good reference to ground. The neutral point will not be reliably at ground potential resulting in the kind of imbalance you're seeing. Essentially you have a very high impedance capacitively-grounded system, so any slight difference in the physical arrangement of the phases and differences in load currents can make the neutral point go all over the place.

Ungrounded 480 V systems are bad new, even though there are sitll a lot of them around.

 

[jes1983]

Thanks DPC, what do you mean bad news? safety or reliabiltity wise? , The MCC I replaced was 40 yrs, I terminated to the earth ground provided, however I do not know if it is connected to our grid ground. I should look into testing my ground.
Regards

Jes

 

[waross]

Hi dpc and jes1983
If your transformers are 240/480 volt transformers you may consider grounding one of the mid points. A condition known as a "Discontinous ground fault or arcing ground fault" can occur on ungrounded systems and can superimpose a high frequency, high voltage on your 480 volt system. This can quickly destroy your motors and/or transformers one after the other. The condition is very rare but very costly.
yours

 

[davidbeach]

I'd never (maybe that's a bit harsh, but I certainly can't think of any exceptions) use an ungrounded 480V delta secondary. Bad, bad news. The thought behind them was to have an ungrounded system, but there really isn't any such thing and now you will be subjected to all sorts of voltage problems during any faults. Far better would be a wye secondary with a high impedance (high resistance) ground. An arcing fault on a system grounded only through the system capacitance can develop voltages to ground several times the line-to-line voltage; limited when the system flashes-over somewhere else.

The voltages you report are certainly not unexpected with that type of system. What to do about it: 1) nothing, that type of system has been used for years; 2) not very likely, but it could be grounded as waross suggests (never seen a transformer that would allow that, but that doesn't mean they don't exist); 3) corner ground the system; or, 4) add a zig-zag or wye-delta grounding transformer and either solidly or impedance ground it that way. Option 1 leaves you with what you've always had. Options 2 and 3 provide a solidly grounded system, which many industrial user want to avoid for system reliability/availability reasons. Option 4 would provide the best solution, but certainly costs much more than any of the other options. I suppose an option 5) would be replace your new transformer with an even newer transformer and impedance ground it, but that would be far more expensive than option 4.

The advantages of option 4 (or 5) would be stable voltages to ground during normal operation, limitation of voltage rise during arcing faults, and the ability to trace the location of a single ground fault. In the ungrounded system it is far more difficult to trace down a ground fault without sequentially turning off portions of the system.

 

[waross]

Hi davidbeach.
If this is a three phase transformer, you're probably right.
I see quite a few single phase distribution transformers with dual voltage secondaries at 240/480 volts. I'm in an area where a lot of delta is used. Some transformers have three secondary bushings, some have four bushings.
Years ago when I was in areas where virtually all three phase was wye, I saw only two bushing, 277 volt transformers on 480 volt systems.
jes1983 Are you using three transformers or one three phase transformer? Do any of your 480 volt windings have a center tap?
respectfully

 

[davidbeach]

waross, you may well be right if the transformer is actually three single phase transformers, but I have some real concerns.

First concern centers on the operational changes associated with the increased ground fault currents if the system becomes solidly grounded. Generally use of an ungrounded system is because there is a desire for continued operation while a single ground fault is located and repaired.

Second concern is that I've never seen anything about the operation of high impedance grounded systems where the grounding is anywhere other than the center of a wye or zig-zag. The response of those systems to ground faults is well understood, but what about the system response to an impedance grounded phase or mid-point of a winding? Particular concern would be what happens to the unfaulted phases if the ground fault is on the wild-leg. Maybe there is no particular problem, but I've just never heard of it being done.