High Voltage Incidents on a Y-Delta Transformer Bank

[boyaxski]

good day everyone,

i'm working in an electric distribution utility and hope you can help me with a situation i can't yet support, technically, how it happens. almost always, our company is deluged with claims for damaged appliances whenever the secondary lead wire from one of the transformers in an ungrounded Y-Delta connection becomes detached from the transformer bushing, i.e. when a break occurs at the secondary delta loop (note: the primary source is a wye multi-grounded feeder). thanks.

boyaxski
philippines

 

[waross]

Hello boyaxski;
Yes, you can and, with unbalanced loads probably will, have high voltages when a floating wye/closed delta goes to floating wye/broken delta.
Think of a set of resistors connected in wye with the wye point floating. If the resistors are unequal, the voltages will be unequal.
If two resistors have equal impedance and the third one has a much higher impedance, The voltage across each of the equal resistors will be close to, but slighly more than 1/2 line voltage. The voltage across the third resistor will approach half of root three times line voltage, or 87% of line voltage.
The normal voltage on a balance set of resistors will be 57.7% of line voltage.
With one resistor with relatively high impedance, the voltage across the resistor with the higher impedance will rise to a limit of 87% of line voltage.
The voltages across the two equal resistors with relatively low impedance will drop to a limit of 50% of line voltage.

Now if two resistors have equal impedance and the third one has a much lower impedance, the voltage across each of the equal resistors will be close to, but slighly less than line voltage. The voltage across the third resistor will approach zero.
The voltage vectors in the first instance will look like a "T" with a slight dip in the center of the top line.
The voltage vectors in the second instance will look like a "V" with a slight tail on the bottom of the "V", or a very short "Y".

When a floating wye/delta bank goes broken, (Note, this is not the same as an open delta) the voltages of the transformer primaries will be similar to the voltages of unequal resistors with a floating wye point. The transformer impedances will be determined in part by the loading on each one. The voltage will drop on the heavier loaded transformers, and rise on the lightly loaded transformers.
To make it more interesting, as the voltage rises, the transformers may approach saturation. This will lower the impedance and limit the voltage somewhat, but the voltage is already too high when this happens.

I believe that some of your customers may have valid claims for damage caused by high voltage. Probably more customers have valid claims for damage caused by low voltage. In the case that you have described, I would expect that as many or more refrigerators, freezers and air conditioners are damaged by low voltage as by high voltage.

As I understand your post, one transformer looses a connection on the secondary. I would expect the voltage on this transformer to go high and the voltage on the other transformers that are still connected to the load to go low.
Damage to refrigeration compressors and other motors will be the result of low voltage.
respectfully