Supplying a Grounded Wye Transformer From An Ungrounded Source

[3winding]

I am trying to determine why it is not advisable to supply a grounded wye transformer from an ungrounded source? IEEE c57.105-1978 and other literature states this configuration is not advisable.

 

[RRaghunath]

If source is ungrounded, I don't see any merit in grounding the neutral at load end.

The source cannot any way supply currents for earth faults on the secondary side (presuming secondary side windings also to be Y-connected). This will keep the earth fault detection and clearance a difficult job.

Further, the 3rd harmonics required to maintain the transformer voltages sinusoidal will not come from the source, as a result having the voltage form distorted.

 

[3winding]

To add more detail. It is a DG site with multiple generators connected to a 480V GrdY/ 13.8kV Grd/Y step up xfmr which is then connected to a 13.8kV Delta/ 69kV Wye step up. When the system is not generating the 480V system consumes power. We are seeing distorted voltage waveforms whether the DG's are generating or not, and also mysterious overvoltages are occuring on the 13.8kV system. The DG's are line commutated induction generators which may be a reason that the distortion is not eliminated when they are online.

 

[jghrist]

How are the generators connected and grounded?

 

[3winding]

The generators wye connected and solidly grounded.

 

[alehman]

What does the voltage waveform look like? Do you know the harmonic components?

 

[AntonNel]

I have a similar problem. I have to design a 11kV/415V, YNd11 - mobile substation. The LV will be connected to 1000 - 2000 kVA generators, Connected Y - grounded. When the HV breaker is closed with the LV breaker open. No earthing point exist on the LV. The LV will be proteced by a residual voltage relay (open delta VT + Voltage relay).

My main concern: is this unearthed delta allowed.

Should I go for aYNyn0 transformer?

Any help or suggestions will be appreciated.

 

[RalphChristie]

3winding,

For the distorted waveforms, I would also suspect harmonics. When do the overvoltages occur? The whole time or just when you are operating in a certain configuration?

Anton,

I assume you'll have the source just on the 415V side of the trsf?

It is convenient that the supply winding of a trsf feeding a system to be connected to earth (like in star connected and thereby providing a neutral for connecting to earth) It is also desirable to have a delta winding to provide a path for third-harmonic currents. Can't you rather specify a delta-HV/star-LV trsf? (Dyn11)

Regards
Ralph

 

[RRaghunath]

3winding,

When the power is being drawn from the grid, I can see that your 13.8kV system is unearthed as the secondary of 69/13.8kV system is delta connected.

To me, it appears that your 13.8kV system is not designed for working long hours without the in-house generators being on.

Providing earthing transformer, connected to the 13.8kV bus bars, should solve the problem. Until then, you need to watch out for any earth faults (when the generators are not running) in the 13.8kV system and quickly remove them (not an easy task!) to avoid failures and shock hazard.

Further, do you have neutral disconnecting switches in 480V generator and transformer connections and do you keep them all open except one to provide earth fault current source? If you do, this will help improve reliability of your system.

 

[3winding]

rraghunath,

Could you elaborate on the 3rd harmonic content required to keep the voltages sinusoidal?

The overvoltages seem to be occuring randomly.

alehman, the waveforms have a distinctive notch near at about 70 - 80 degrees and again 180 deg later.

 

[RRaghunath]

About 3rd harmonic,
what you said about having a distinct notch in the wave form near about 70 to 80degrees could be confirming the presence of 3rd harmonic in the voltage.

This happens when the circuit doesn't provide path for the 3rd harmonics to circulate.

The no-load or magnetising currents of a transformer will have substantial amounts of 3rd harmonics and are a necessity to ensure that the output voltage waveform remains sinusoidal. You may refer J&P Transformer manual for more details.

Regarding overvoltages, are you sure they are occuring randomly!! Actually over voltages could be occuring only there is an uncleared earth fault in the system.

When the system is unearthed, the voltage of healthy phase to ground raises to phase - phase voltage level whenever there is an earth fault involving one of the phases.

Depending on the capacitance of the system (that includes capacitance of transformer windings, generator / motor windings, cables / bus bars in the system), this can give rise to arcing grounds and pulsating overvoltage surges of magnitudes exceedings 5xtimes the rated value.

 

[busbar]

 
I can’t quote specific clauses, but for ammunition you might sift through IEEE C62.92.4 …Neutral Grounding in Electrical Utility Systems… — Distribution

This may be setting up significant problems with transient/resonant overvoltage, leading to {hideous} insulation and surge-device coordination—particularly when ungrounded and grounded portions are called separate through protective-device operation.

 

[PWR]

A wye wye transformer, even if solidly grounded on one or both sides, will have an unstable neutral if it is supplied from a source that has no connection (direct or via ground) to the neutral point of the transformer. Therefore, when the DGs are off line, the neutral is unstable on the 480V side as well as the 13.8kV side. If you have any line to neutral loads on the 480V side you will have serious problems. You don't need a ground fault to have unusual voltages to ground on one or more phases.