Generator Transformer Vector and earthing options 2

[Rodmcm]

Hi We have an application for two 11kV generators to supply a local load and export to a 33kV system. The specification for supply says that the generators must operate in island mode so in this mode the 33kV will have to be earthed. We expect the 11kV should also be earthed for the local supply . The generators will have neutral resistors. So options
a) Star Star with 33kV star switched to earth in island mode, 11kV star earthed
b) Delta Star, with a switched zig zag on 33kV for island mode, 11kV star earthed
c) Star Delta with 33kV star switched to earth and zig zag on 11kV
d) Star Delta with 33kV star switched to earth and 11kV floating

I would be interested in this groups comments

 

[davidbeach]

Frankly, I'd throw in a 11kV-11kV transformer, high impedance ground the generators, have them look at the delta windings of the 11/33 and 11/11 transformers with a grounded wye on both transformers away from the generation. But that's just me.

 

[collies99]

My preference is for a neutral grounding xformer for the generator. Dyn11 for the step up xformer, distribute at 33 kv to load, one line. Dyn1 for local service.

 

[Rodmcm]

So what you think is Option B?

 

[ScottyUK]

Agree with DavidBeach that your generators really want a delta winding and a high impedance neutral earth to allow maximum protection to the machine stator. The generator will be the most expensive electrical item on this system by a fair margin, exceeded only perhaps by the gas turbine driver.

How about one dYn* GSU transformer per generator onto a 33kV busbar, with 33/11 Dyn* station aux transformer for the local 11kV auxiliaries? Apologies to collies99 if that was what he described - 'neutral earthing transformer' wasn't clear if it means a single-phase transformer with a resistor burden at the generator neutral point or a 3-phase earthing transformer on the main bus.

 

[collies99]

If I understand option b correctly, you have a delta tertiary, wye grounded 11 kv with high impedance grounded generator and wye 33 kv ungrounded with ziz-zag.

What is the size of the generators and type ? I think its best to know first.

If you are also connected to utility supply, then a generator stator ground will likely be severe, fault sourced from utility. If this is operated strictly for island mode, that should be ok.

more after unit capability is known.

 

[Rodmcm]

The unit sizes are 9MVA and 5MVA, so the ecomomics of two unit transformers has an impact. Normally the 33kV is connected to a 220/33kV dist system with the 33kV earthed. When we island the 33kV from the hydro has to supply about a 6MW local load from the 33kV line. I also am of the thought that a 33/11kV Dyn is the best way to go with each generator high resistance earthed and an 11/400V Dyn for local supply. This would then requre a switchable earth on the 33kV for the island mode...probably by zigzag.
If I had unit transformers then Ynd would be the way to go, but the two units makes this option costly. Hence the questions. From my reading of this site there seems to be reluctance to YnYn transformers due to the earth fault discrimination

 

[Marmite]

What do you mean by operating in island mode, in this context?
Is it:-

1. Continuing to operate whilst isolated from the grid by supplying only the parasitic loads of the plant.

2. Continuing to operate by supplying both the parasitic loads of the plant plus the needs of a 33kV distribution system, which has itself become isolated from the wider network.

Regards
Marmite

 

[Marmite]

Sorry, posts crossed over. The page crashed the first time I submitted.

Regards
Marmite

 

[odlanor]

with these low MVA generators and hydroplant I have seen more economical solution: generators of 6.9kV -ungrounded and connect to transformer 14MVA-6.9-33kV delta-wye grounded. Local loads are supplied by 6MVA-6.9/0.44kV Dyn.
- 6.9kV bus is grounded by zig-zag transformer.
- generators ungrounded are good solution for civil works.

 

[collies99]

The picture is much clearer.

I do not see the need for the ziz-zag if you install a 33kv high side grounded wye, it is similar to a 33kv main bus supplied from your transformers and the utility. 33 kv is non-radial with 3 ground sources. All breakers trip for fault on 33kv side.

As an option with you can get a 3-winding unit, DDy11; units feed into Delta side, each with its own winding and unit breaker and a high side breaker. Unit maintenance flexibility is maintained. Transformer maintenance can be an issue needing both unit out of service.

local supply as you noted looks ok.

 

[Kiribanda]

Rodmcm,
If you are thinking to export power, when the generator is islanded, your 33kV network shall continue to be "EFFECTIVELY EARTHED" to avoid earth fault issues. Normally any Utility will not allow you to export power without an EFFECTIVELY EARTHED 33kV network when the gen is islanded. That means your 33kV transformer should have an earthed WYE on the Utility side. Now your 11kV side should be DELTA and the 11 kV generators should be EARTHED WYE with a suitably sized earthing resister. By having the above connections, if you find that your 11kV network go UNEARTHED due to any operation requirement, use a 59N scheme for earth fault protection for the unerathed 11kV network when the generators are off-line.

 

[Rodmcm]

Marmite Your No 2 applies
Kiribanda
Your solution is correct if you can run the 11kV un earthed. But what if you have to also Effectively Earth the 11kV when the generators are not running?

Collies99
I understand your solutions, but with every solution comes a cost, especially in terms of voltage regulation and protection of three winding transformers. That is the reason, for this size of plant, that I am leaning towards two winding transformers

Odlaner
While your arrangement at 6.9kV may be cheaper 6.9kV is an unusal voltage here, most local service transformers are 11kV, Your suggestion, voltage aside, still does not earth the 33kV for effectively earthed island operation.

 

[odlanor]

my suggestion is generators ungrounded ,each one connected by CB to 6.9kV busbar. Windings transformer are 6.9kV delta and 33kV star grounded.
- 6.9kV bus is grounded by zig-zag transformer.
- generators ungrounded.

I understood local loads are 400V not 11kV.

 

[davidbeach]

I'd strongly urge avoiding ungrounded operation of the generators. If there's ever a stator ground fault, there will be no ground reference once the breaker opens and the arcing ground fault can produce extremely high voltage and more damage than there'd be if the stator is grounded through a resistance.

 

[sakaran51]

Hi,
In the context of the OP, I have a related issue I had come across.Please throw some light on this.
At 33kV level,on the Delta secondary side of Utility transformer the impedance earthing is adopted at the Utility substation. A 10MW 11kV Generator is to be connected to the 33kV Utility substation bus with a step up Generator Transformer by running a DEDICATED 33kV overhead line.The 11kV generator neutral will be earthed by 100 amps resistor.What will be the best combination of GT vector group and earthing arrangement of the GT? Is it Ynd configuration with HV side star point impedance earthed? Or Dyn configuration with star point on the LV side resistance earthed,with DELTA floating?33kV side of the Generating plant will NOT have any virtual grounding.I am bit concerned about Dyn vector group with star side of the GT on the generator side.

 

[sakaran51]

HI,
In continuation of my query,can we go for another option,namely,GT with Ynd vector group,leaving the HV Star side neutral unearthed,floating?

 

[collies99]

Sakaran:

"At 33kV level,on the Delta secondary side of Utility transformer the impedance earthing is adopted at the Utility substation" ? Please check if this is factual, my guess it that it is WYE, solidly grounded distribution.

GT: Delta 11KV, WYE soldily grounded 33 KV, Gen neutral grounded through 100 Amp resistor.

 

[odlanor]

davidbeach
The operation of the ungrounded generators requires a connnection to a 6.9kV bus grounded by zig-zag transformer. They will never operate ungrounded!

 

[davidbeach]

They'd still be ungrounded any time the breaker is open.

Also, can we just forget about the whole primary/secondary designation when dealing with transformers that have sources on both sides? What is the primary of a GSU? Is it the generator side since that's where the power flow through the transformer normally comes from or is it the other side with the higher voltage? On the other hand, if reference is made to just the high-side and low-side there will no confusion. I don't know how the bushings on an IEC transformer would be designated, but in the ANSI world we could even talk of the H side and the X side (and occasionally the Y side) without any ambiguity. But, please, lets leave primary and secondary to radially fed transformers with a source only ever on one side and load always on the other side; if, that is, we can't just do away with primary and secondary all together.