Y-Y transformer bank with LV grounding resistor 4

[ffont]

We are looking at installing a three phase transformer bank made out of 3 single phase oil filled transformers rated 167kVA each, to feed an small industrial load. The HV is a 4 wire system 24940GrdY14400V

The LV neutral will be connected to ground via an NGR

If we choose a Y-Y connection, I understand that the primary should not be solidly grounded? Is it correct? and if so, why can it not be solidly grounded?

Would we be better off by using a Delta-Y connection? (suppression of harmonics)

Thanks!

 

[davidbeach]

If the high side wye is not solidly grounded, you won't have an effectively grounded system on the low side. A wye-wye is not a ground source the way that a delta-wye is; the wye-wye can connect the low side to the high side ground source, but only if both wye-points are grounded.

 

[jghrist]

As David said, you have to ground the HV neutral to get a ground source. With the HV neutral grounded, the LV NGR will reduce ground fault current.

 

[odlanor]

if your HV is a 4 wire system you can use delta.
If your HV is a 3 wire sistem and you are sure that remote source of your high voltage is solidly grounded, you can use a Delta-Y connection.

 

[waross]

A wye:wye transformer bank should have the primary neutral connected to the source neutral to stabilize the secondary voltages and to provide a path for harmonic currents. Most wye distribution systems carry a multiple grounded neutral conductor. A connection to this conductor is referred to as "grounding" and it is by way of the multiple grounds, but the important part is the connection back to the source neutral.
Grounding the wye point of a transformer bank fed from an ungrounded delta will not accomplish much as there is no return path back to the delta source. It may cause interesting issues with the utilities protection schemes.
The voltages that you give imply a four wire system so you should be safe with a wye:wye transformer bank, but remember, "grounding" the primary wye point means connecting it to the system neutral, NOT just driving a ground rod or connecting it to your grounding grid.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[odlanor]

waross,
I am not understand.You mean that there is not fault to ground at LV- starGround because of the MV-delta?

 

[ffont]

David Beach

Thanks for the answer! the medium voltage system feeding the bank of 3 - 167kVA is a solidly multigrounded 24.9kV.

So for this appliaction (ie a NGR in the LV) the H0 must be solidly grounded (as Waross indicates, tied to the system ground).

The LV X0 must then be an insulated bushing and will be connected to ground via the Neutral Grounding Resistance.

Besides blocking the triplens, is there any advantage in using a Delta-GrdY connection instead of staying with a GrdY-GrdY bank? (the LV GrdY being in both cases grounded via a low impedance Grounding resistor)

Thanks again!

 

[waross]

No. What I was trying to explain is that an ungrounded delta primary has no ground return path. Installing a transformer bank with a primary wye point grounded may lead to unexpected results should a primary phase go to ground. The wye point is not a stable neutral point but it is not floating either as saturation of the transformers on the unfaulted phases will limit the voltage rise on the unfaulted phases. The phantom delta will also help limit the voltage rise of the unfaulted phases. These effects may seriously compromise the ground detection scheme of the supply grid.
In this instance the ffont has indicated that the primary supply is a four wire system with a multiple grounded system neutral conductor. My comments concerning a floating primary wye point do not apply on ffont's system.
One of the big advantages of a wye primary in some areas may be availability of transformers. If the local practice is to connect individual transformers line to neutral you may find that there is a good selection of 14,400 Volt transformers available on the suppliers shelf while 24,940 Volt transformers are a special order which have to be located and shipped into your area.
In a delta:wye transformer or bank, secondary line to neutral loads or faults become line to line loads or faults on the primary.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[prc]

Thank you waross for your explanation.

Then in case the primary of wye-wye transformer is fed from an isolated 3 wire system, by providing a neutral grounding transformer at sending end, will the problems listed by you be solved?

Can you comment on odlanor's response of 6th. I find some contradictions.

 

[odlanor]

OK. Thank you Waross.

 

[waross]

When a wye transformer is fed from a three wire source, The wye may be stabilized by a grounding transformer connected to the wye point. The grounding transformer may be installed adjacent to the wye transformer. A wye:delta transformer or transformer bank will inherently stabilize the wye point.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[prc]

odalanor,can you explain your response of 6th. Or you would like to revise it? Iam little confused.

 

[odlanor]

I only have familiarity with systems delta / wye 3-wire. I have never worked on system design with wye / wye.Nao have to say.