Wye secondary Transformer operated ungrounded

[NickParker]

Observing a one-line diagram where the wye configuration's - (secondary side) operates without grounding (ungrounded), why isn't the delta connection employed instead?
What benefits does the wye secondary offer over delta?

 

[RRaghunath]

Y-configuration allows single phase loads to be connected, between a phase and neutral.
Earthing or not earthing the neutral is a matter of design philosophy of the client for the plant.

R Raghunath

 

[NickParker]

Y-configuration allows single phase loads to be connected, between a phase and neutral.

here, the secondary L-L Voltage is 690, therefore the L-N comes around 400V; not much single phase equipment operates at 400V.

 

[che12345]

I have the following opinion for your consideration.
"...here, the secondary L-L Voltage is 690, therefore the L-N comes around 400V; not much single phase equipment operates at 400V".
1. In this case, it is beneficial to have in Star with L-L=690V. All standard IEC LV motors with 6 terminals wound for delta 400V are suitable for 690V in Star connection. See 3 below.
2. The motor wound in delta 400V can be connected in star 690V without any change in the output Power kW rating and the direction of rotation. No rewinding is required. The current is reduced by the 400/690 = 0.579 ratio.
3. Info: For Star 690V, connect [L1-U1, L2-V1, L3-W1, link W2-U2-V2], or [L1-W2, L2-U2, L3-V2, link U1-V1-W1].
Che Kuan Yau (Singapore)

 

[NickParker]

The secondary neutral (Wye) is left open (not grounded).

 

[cranky108]

Because with an ungrounded wye, the open Xo bushing can be used for ground detection, with a single PT, or voltage measurement. A delta system would require three PTs or three voltage measurements.

Actually, here most larger buildings use 277V lighting, but some lights can use 480V. So 400V lighting seems plausible if it is manufactured.

I think also that larger motors could be made to work on this voltage, and if the motors are delta, they don't need the ground.

 

[bacon4life]

Check out the Whys of Wyes for a comprehensive explanation of both configurations.

 

[electricuwe]

With 5800 kVA it is unlikely that these transformers power public distribution, lighting or drives. Maybe the load is some process equipment. In this case it is beneficial that with an ungrounded secondary there are no large fault currents in case of an insulation failure. However, insulation has to be designed matching to this condition.

Nick, can you provide more background information ?

 

[che12345]

"...The secondary neutral (Wye) is left open (not grounded)".
1. I am of the opinion that it is favourable to Ground the Y secondary Neutral, irrespective of whether it is distributed/not distributed or is there any single-phase balanced/unbalanced load.
2. A 690/400V Y secondary transformer
a) with Neutral isolated from ground. When say L1 is grounded, L1-G =0V, but L2-G = L3-G = 690V. The whole installation including all motors insulation would be over-stressed. It may not be apparent/noticeable at the initial stage, until the damage is wide-spread.
b) with Neutral grounded. When say L1 is high resistance grounded, it may not cause the over-current protection to trip. Now L1-G = L2-G = L3-G = 400V. Insulation is not over-stressed.
Che Kuan Yau (Singapore)