Wye-Wye Transformer on Low Resistance Grounded System 1

[T247]

We have a 4160 V system that is low resistance grounded. The transformer supplying the 4.16 kV system is a 10 MVA, 13.8 kV Delta-Wye with a 12 Ohm neutral ground resistor. There is a temporary feeder being installed to supply some 480 V equipment along with a 4160-480 V transformer, however, the transformer that is being provided is a Wye-Wye transformer. My questions are:

1. What is the effect on the 4.16 kV system, specifically the ground fault protection, if the transformer is installed and the Wye-Wye transformer is solidly grounded on both the primary and secondary? Can this transfomer feed only three-phase loads?
2. What happens if the neutrals of both the primary and secondary are left floating? I know that there are the typical issues that occur when a system is left ungrounded and that some form of ground detection on the 480 V system would be requireded, but are there any other limitations?

 

[waross]

The 4.16 kV to 480 V transformer.
Primary; Do not ground, connect to the XO of the 13.8 kV:4.16 kV transformer.
Secondary; This is a separately derived system. The secondary XO may be grounded to safely supply line to neutral loads. (277V)

 

[T247]

We can't access the X0 bushing to the 13.8-4.16 kV transformer since the equipment is energized.

 

[wcaseyharman]

My initial thought, if you can’t take Waross’s suggestion, is ground the primary Ho to prevent neutral shift. Leave the secondary neutral open and install a properly sized grounding transformer - you’ll need a good sized one if you want to carry single phase loads, or a small one if you just want to pair it with a resistor for a resistance grounded system.

 

[waross]

We can't access the X0 bushing to the 13.8-4.16 kV transformer since the equipment is energized.

You may have to take an outage to do the job properly.

 

[RRaghunath]

We have a 4160 V system that is low resistance grounded. The transformer supplying the 4.16 kV system is a 10 MVA, 13.8 kV Delta-Wye with a 12 Ohm neutral ground resistor. There is a temporary feeder being installed to supply some 480 V equipment along with a 4160-480 V transformer, however, the transformer that is being provided is a Wye-Wye transformer. My questions are:

1. What is the effect on the 4.16 kV system, specifically the ground fault protection, if the transformer is installed and the Wye-Wye transformer is solidly grounded on both the primary and secondary? Can this transfomer feed only three-phase loads?
2. What happens if the neutrals of both the primary and secondary are left floating? I know that there are the typical issues that occur when a system is left ungrounded and that some form of ground detection on the 480 V system would be requireded, but are there any other limitations?

If you keep 4160V winding of LV transformer ungrounded, it behaves like Delta for ground faults on 480V side. Ungrounded Wye means the neutral voltage may rise to Line-to-Ground voltage level when there is an earth fault. But, this may not be a concern as the 410V voltage level the windings would have been uniformly insulated.
If you earth 4160V star point, any ground fault on 480V side will reflect on to 4160V side and the NGR connected to the Star point of the source transformer will limit the ground fault current that can circulate in 4160V system and in turn in 480V system. But this should not pose any problem as the protection study can take care through appropriate settings. The problem lies in supplying the single phase loads in 480V system and hence the solution is appropriate only if the 480V system has no single phase loads (it is OK if the single phase loads are connected through Dy connected isolation transformers).

 

[dpc]

If you keep 4160V winding of LV transformer ungrounded, it behaves like Delta for ground faults on 480V side.

This isn't really correct. The zero sequence impedance will be much higher and the 480 V ground fault current much lower than a delta winding. There can be a "phantom delta" effect depending on the transformer core when the flux leaves the core during a ground fault, but it isn't really going to be the same as an actual delta winding.

The temporary wye-wye transformer should work, but you need to make sure appropriate protection is provided for 4160 V ground faults since the fault current will be much higher without the grounding resistor.

You don't want to leave the neutrals floating. If you have 277V loads, the neutral must be solidly grounded anyway.

 

[waross]

Single phase loads may be line to line or line to neutral.
If any single phase loads are anticipated, LL or LN, then the primary neutral must be connected to the supply X0, not to ground.
If the primary neutral of the 4160/480 transformer is not connected to the supply X0, but rather to ground, then any unbalanced phases will cause a voltage rise on the lighter loaded phase(s).
This voltage rise will be limited by the onset of saturation, or the neutral grounding resistor current, or a combination of the two effects.
With a wye-wye transformer it is normal practice to connect the primary wye point to the source neutral.
In the absence of a source neutral, the primary of the wye-wye transformer should be connected to an artificial neutral.

 

[RRaghunath]

This isn't really correct. The zero sequence impedance will be much higher and the 480 V ground fault current much lower than a delta winding. There can be a "phantom delta" effect depending on the transformer core when the flux leaves the core during a ground fault, but it isn't really going to be the same as an actual delta winding.

The temporary wye-wye transformer should work, but you need to make sure appropriate protection is provided for 4160 V ground faults since the fault current will be much higher without the grounding resistor.

You don't want to leave the neutrals floating. If you have 277V loads, the neutral must be solidly grounded anyway.

Thanks for correcting me, Sir!