Supplying a Grounded Wye Transformer From An Ungrounded Source
Supplying a Grounded Wye Transformer From An Ungrounded Source
(OP)
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.






RE: Supplying a Grounded Wye Transformer From An Ungrounded Source
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.
RE: Supplying a Grounded Wye Transformer From An Ungrounded Source
RE: Supplying a Grounded Wye Transformer From An Ungrounded Source
RE: Supplying a Grounded Wye Transformer From An Ungrounded Source
RE: Supplying a Grounded Wye Transformer From An Ungrounded Source
RE: Supplying a Grounded Wye Transformer From An Ungrounded Source
My main concern: is this unearthed delta allowed.
Should I go for aYNyn0 transformer?
Any help or suggestions will be appreciated.
RE: Supplying a Grounded Wye Transformer From An Ungrounded Source
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
RE: Supplying a Grounded Wye Transformer From An Ungrounded Source
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.
RE: Supplying a Grounded Wye Transformer From An Ungrounded Source
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.
RE: Supplying a Grounded Wye Transformer From An Ungrounded Source
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.
RE: Supplying a Grounded Wye Transformer From An Ungrounded Source
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.
RE: Supplying a Grounded Wye Transformer From An Ungrounded Source