Distributed Generation

[PRSharma]

Distributed Generation: What should be the preferred interface transformer's winding configuration (utility side Delta, Star solid grd, or Star effectively grd. a) preferred by utility; b) preferred by generating facility. However I am interested in utility's preference particularly when utility's distribution is 3 p 4 w.
I understand that this is highly debateable issue and I am familiar with over-voltage and ground fault coordination issues.

 

[ScottyUK]

I'll comment from the perspective of a generating facility:

A generating facility would almost always prefer an ungrounded delta on their side in order to allow effective protection of the generator stator. This typically requires a single high resistance grounded connection at the generator stator neutral point. Anything which limits the fault level the generator is exposed to in the event of a system fault is beneficial in the eyes of the generating facility, so either a resistance grounded or ungrounded utility side would suit the generator. At distribution voltages the additional cost of uprating the GSU transformer insulation won't be huge, but to multiply that cost across an entire utility network would find little favour with the utility.


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If we learn from our mistakes I'm getting a great education!

 

[PRSharma]

I apologize for the late reply. I was pretty busy in other commitments. However, thanks for your input and I would appreciate your comments on following arrangement: Interface transformer: utility side- delta and generating facility side- star solid ground. This releives utility from extermenly complex ground fault cordination and neutral reactor sizing. But leaves risks of overvoltages on healthy phases when there is single line to ground fault in distribution line. To mitigate the overvoltages the utility side breaker is tripped after tripping of generating facity breaker. This is achieved in following manner: Ground faults are sensed by relay (at utility station), simultaneously a transfer trip signal is sent to generating facility and the time cordination is ensured that generating facility breaker opens prior to breaker at utility station. Do you see any probelem in such arrangement.

 

[slavag]

Hi PRSharma.
Could you please send us more information.
What size of generator, xfr, voltage level of gen and utilities side.
Better, if possible send us some scheme of primary connection with your protection coordination.
Actually, Scotty are right, Delta is preferred connection on
gen side and it's strongly recommended.
Reason, see attached:
thread238-201719

Regards.
Slava

 

[PRSharma]

Hello Slavag
Thanks for the thread reference. However, I could not find any materialistic debate there.
The details of system - as under:
Utility supply: 27.6 kV 3 phase 4 wire, multigrounded
Generating Facility interface transformer: HV delta 27.6 kV; LV star 600 V solid ground
Generator: 8 MW; 600 V; 3 phase; synchronous
Utility feeder: load = 3 MW; traditional o/c and e/f feeder protection. [1/2 cycle relay, 3 cycle tripping relay, 5 cycle breaker at utility; 1 cycle transfer trip time, 1/2 cycle relay, 1/2 cycle tripping unit, 5 cycle breker at generating facility. Hope this explains a bit.

 

[slavag]

Hello PRSharma.
I'm not familiar with LV generators.
From my point of view: it's possible and practic used
LV generator with solid grounding neutral.
More from this, I think it's better for you used solid grounding of generator. In case of island mode.( I assume according to your data, you are have load on 600V and works is co-generation mode).
BTW, your generator with rotaded diodes or brushless.
See attached example

Regards.
Slava

 

[slavag]

Of course, for this size of generatore, you must use 87G
differntial protection.
In you have block trafo-generator, I preffer used low or medium resistance neutral of generator

 

[slavag]

And, next example.
I'm hope, it's help you.
Good luck.
Slava

 

[davidbeach]

For a generator of that size, I'd step the generator output up to 27.6kV using a transformer with a delta on the 600V side and a grounded wye on the 27.6kV side.

 

[Kiribanda]

PRSharma,

Normally the Utility prefers to have the following interfacing transformer connection.

Utility (HV) side - STAR solidly grounded
Generator (LV) side - DELTA

This satisfies the important condition of "Effectively
Grounded" for the Utility side to trip on possible ground faults on their side even with the generator connected.

Generator Star point - To be Low resistance (nearly 400-500 A) grounded.
Your generator is 8 MW at 600 V (FLC = 7698 A!!!). So you have enough current to use for ground fault tripping if you use low resistance grounding.

I also have a question about your 8 MW/ 600 V generator.
What is the current rating of the generator CB?
Who is the manufacturer?

Kiri

 

[ScottyUK]

Low resistance earthing for a generator which does not have any load other than the GSU transformer is poor design because it does little to limit damage to the stator in the event of an earth fault. It is entirely possible to reliably detect an earth fault current limited to 5A or less, a level which is low enough to avoid damage to the core. This method is employed on almost all large utility class machines where a stator assembly typically has a capital value of several millions of pounds, quite apart from the value of lost revenue while repairs take place. Typically earthing is accomplished through the HV winding of a small distribution transformer transformer burdened with a low resistance across the LV winding rather than using a direct connected earthing resistor.

If the generator is embedded into an existing distribution network serving other loads then it may not be possible to utilise high impedance earthing.


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If we learn from our mistakes I'm getting a great education!

 

[slavag]

Hi Kiribanda.
As, I saied, I'm not familiar with LV, but from my partners
I heard about standard LV CB with interuption about 6000-6500A and fault interuption up to 200kA.

 

[davidbeach]

8MW @ 600V, assuming a single machine, requires a 10,000A CB. Probably not realistic. Probably best to look at a higher voltage. At 4160V you would only need a 1600A breaker.

 

[ScottyUK]

Unless you have other undisclosed reasons to generate at 600V I concur with David - generate at a higher voltage and bring the current down to something more manageable.

If you must generate at LV then a dedicated GSU transformer with a close-coupled transformer and HV side breaker equipped for synchronising would be a reasonable solution. Use of an LV isolated phase duct would minimise the chances of an interphase fault and keep the magnetic fields around the busbar under control. This is the technique used on big machines where the LV side breakers can become impractically large and external flux levels reach troublesome levels.


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If we learn from our mistakes I'm getting a great education!

 

[slavag]

Hi.
I think it's clear for all of us, that 8MW 600V generator
is so strange ( but who know?) . I assume, that mean several generators in parallel with sum. of 8MW. We'll wait to PRSharma explain.
PRSharma;please send us your connection scheme.
Regards.
Slava
BTW, for HV generator we have GCB for 10kA.

 

[PRSharma]

Hi Folks
Apologies: I meant: 8 MW to be cummulative generation of DG (4 units of 2 MW each @ 600 V). However, the question remains the same even if we consider 8 MW generator (single unit)@ 13.8 kV for utility feeder voltage at 13.8 kV.

HV gnd for GSU, is traditional and well practised throughout the world for genertaors connected to transmission system. I am talking about distributed generation which is supposed to be connected at distribution voltages (< 50 kV). The typical L-G faults in the distribution system are limited to: 44 kV (19 kA); 27.6 kV (12 kA). Furthermore distribution system has line re-closers, lateral fuses, and branches throughout the network, which makes the ground fault cordination a real challange (usually not a problem in transmission system unless you come across a Tee line).

As the penetration of DG increases at distribution level utilities ground fault protection is screwed if you increase the ground sources such as connecting GSUs with HV gnd. If you opt for GSUs HV ungrounded you have overvoltages on healthy phases on L-G faults. Please refer my first & second thread. I will appreciate your comments.

 

[slavag]

Hi PRSharma.
From my point of view, you haven't any problem.
You can use xfr. with Delta connection on the HV side.
But may be better, use Star connection with high speed grounding switch ( as option with resistor).
Yes, your GSU are ungrounded, but your system isn't.
In case of : than only your GSU will feed distribution network, you'll connect this grounding switch.
LV side of xfr. possible several options.
I think good option, Delta connection of GSU, ungrounded generators and one zig-zag xfr. on the generation bus.
Conclusion: you'll use standard Y/D step-up xfr. Y with
grounding switch ( normal mode it's in the open position).
LV side is Delta, in second thinking, w/o any zig-zag xfr.,
generators with solid grounding or low or medium resistive grounding ( according to genset supplier recommendation).
Regards.
Slava.
BTW, next time, send us Q's with full data, you see what happend.

 

[slavag]

Hey, forgot.
We have several such applications ( but generators are HV).
Via SGU are connected to distribution network 22kV or 36kV
( petersen coil grounding). No problem.
Good Luck.
Slava

 

[PRSharma]

Slavag
Ground switch is not an option. When DG is operating in parallel with grid and if utility side breaker opens for line to ground fault at matching loads it is extremely difficult for DG to sense any significant change in parameters to realize that utility breaker has opened.The over voltages due to the neutral shift will be greater than TOV ratings of Surge arrestors rated line to ground.

Yes, impedance grounding is a option. What is the size (ratings)of peterson coils with respect to size of Xmer and size of generator.

 

[slavag]

Hi PRSharma.
Oh, it's not simple case, and need serios evaluation.
I'm not understand, were is utilities CB, it's same station or some remote station? If it's same station you can use contacts of utilities CB for build logic.: open CB---- close grounding switch (high-speed).
Please pay attention, I'm wrote: grounding switch with resistor. Choice this resistor for the 400-500A.
Don't think about Petersen coil, it's separeted issue.
Next point, you work in parallel with distribution network.
Your generator are feed your plant only or possible
transmit energy to the grid?
Are you have autorecloser on distribution grid or not?
AR are 3ph or single phase?
For my pinion you need some expert, eng-tips it's only tips.
Here, you need design.
Regards.
Slava