Transformer and Generator neutral Grounding 1

[power2020]

Dear Folks,
In our Project, we have 3.3kv substation, getting supply from Grid and a stand by DG set 3.3kv, 2MW, which is connected to this 3.3KV switchgear for emergency loads.

We have Two distibution transformers (3.3/0.4KV )connected to this 3.3kv switchgear and feeding to 400v Switchgears. These two transformers are DYN1 and solidly grounded.

Can somebody discuss pros and cons of this system?
1) Transformers (3.3kv/0.4kv, DYN1) are solidly earthed.
2) stand by generator (3.3kv 2MW) is RESISTANCE EARTHING NGR.

We are not planning for parallel operation.

What are we need to take care (relay coordination etc.) for GRID & Generator operation?

Thanks in advance..

 

[power2020]

Dear Folks:
Inadvertently, i have missed some information in my above post.

We have 1)
1) 11kv switchborad - Getting supply from GRID
2) 3.3KV switchboard - getting supply from 11kv switchboard and also from 3.3KV,2 MW generator for emergency loads.

3.3KV switchgear is getting supply from 11KV substation (GRID) via 11/3.3KV transformers, Dyn1, neutral is solidly earthed.
2MW 3.3kv Generator star point is earthed through resistor (NGR).

This 3.3kv switchgear is feeding to LV switchgear(400V switchboard via 3.3KV/400v, Dyn1 transformers.

Can somebody discuss pros and cons of this system?
1)Transformers (11kv/3.3kv, Dyn1) are solidly earthed.
2) stand by generator (3.3kv 2MW) is RESISTANCE EARTHING NGR.

We are not planning for parallel operation.

How do we coordibate for earth faults in case of diferent modes of operations ( Grid or Generator).

Thanks

 

[rbulsara]

What is your real question? You ask a very broad base question and the reply could be a whole engineering book. The system you describe is very common. It appears that ones who designed or planned it know what they are doing.

As for coodination you need to performa coodination study. If you do not know how, you need to hire someone who knows. This forum is no substitute for hiring professinals.

What is your role in this project.

 

[JensenDrive]

Looks like all your 3.3kV loads need be line to line connected since the generator is ungrounded. Surge arrestors need to be line to line rated. You end up needing two types of ground fault detection schemes at 3.3kV; one based on current when operating from the transformer, and another based on line to ground voltage when running from the generator. Ground faults, when operating off of the transformer are isolated via coordinated ground overcurrent elements. You will not know where the ground fault is when running the voltage based ground fault detection scheme. Like rbulsara was getting at, books and papers are written on the topic. You might google search on "ground fault detection."

 

[power2020]

Thanks rbulsara (Electrical) & Thanks to JensenDrive (Electrical).

I am working for a client (for this project).
My requirement:
As per our initial design of our project, all the transformers & the generator star points grounding system is desinged for solid grounding.
We have given design clearance for the rest of the electrical equipements such as Switchgear, transformers, motors & others based on solid grounding, and we have approved 11/3.3kv & 3.3kv/400v transformers for solid grounding and 11kv, 3.3kv and 400v switchgears system grounding as solid grounding.

And we also have formally approved 3.3kv generator for solid grounding.

Now the real problem is , when we are finalising Generator order, the supplier has come with NGR ( Neutral Grounding resistor). He said and insisting us to inlcude NGR, otherwise, he won't give warranty. As we know NGR limits fault currents, we understood that they want to protect their generator and reduce manufacturing cost of the generator.

My lead engineer said that there will not be any problem and he has given clearance for generator with NGR & Transformers without NGR, since there is no parallel operation.
But i am not convinced.


As JensenDrive rightly pointed out, that
(1)we need to change/ upgrade the voltage ratings of Switchgear devices such as surge arrestors, motor windings, and other power components shall be rated for line to line values.

(2) We need to have two ground fault detection schemes as the fault currents are diferent in this two modes of Tranformer & generator individual operations.

Now My Questions are:

1) Is this system safe, without upgrading voltage ratings of electrical components to line values, since the generator will be on line for very short periods and since we have already placed order for all electrical equipments with solid grounding?

2) In case if really required, which are the componenets we must really upgrade to line to line values?

3) Is it compulsory to have two ground detection systems? or can we manage the relay coordination some where in between (for Solid grounding faults & NGR faults) for both grid and Genartor operations.

4) Final question : whether it is good to oppose the NGR for the generator. I mean, Is the Generator with Solid grounding best solution?.

I am really looking for experts advice to upgrade my self as this foroum is for sharing information / knowledge.

Many thanks to all..

 

[ScottyUK]

Downsides of solidly earthing the generator neutral:

A generator with a solidly grounded neutral will require heavier bracing than standard to withstand a line-ground fault. This could be a significant cost or virtually impossible depending on the machine design.

A stator earth fault will probably write off the core, or at least require replacement of a significant section of the core.

It is difficult to provide 100% stator winding protection with a solidly earthed machine, although a good differential scheme will protect the majority of the winding.

Are all your 3.3kV loads 3-wire?


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Sometimes I only open my mouth to swap feet...

 

[slavag]

Hi Power2020.
What is a level of NGR fault? 10A, 20A, 50A, 100A?
What is your 3.3kV swg, it's last point or you have connection to other 3.3kV swg?
I'm try ask you about how steps of ground faults protection you need: one, two or more?

If it's one or two, I'm not see any problem in your case.
You can work with same ground protection scheme in both of cases(grid, generator). Maybe ( need more information) you can used two setting parameters of ground fault protection, depending on CB position. I think, important add ring type CT ( toroid) for ground fault protection.

I don't see any reason change your primary EQ. to line to line voltage, generator with NGR.
Good Luck.
Slava

 

[JensenDrive]

How often has anyone applied what I will term "Small impedance grounding"? It seems it would be a useful approach in this application. I am wondering if you could put only enough impedance in the neutral to make Z0 = Z". Since impedance in the neutral has a 3x factor on Zo, this means Zneutral = Z"/3. If you do this, then a ground fault is less than a three phase fault, so now the generator mfr only needs to brace to I3ph, and a line to ground fault is not the worst case. Further, the generator, as a voltage source, is still an effectively grounded source and all the myriad of issues of having an ungrounded source are bypassed.

During a slg fault
Islg= 3/(Z1+Z2+Z0+3Zn)
If Z1=Zd", and Zn= Zd"/3 then
Islg = 3/(2Zd"+Z2+Z0).
Since Z2+Z0 is typically > Zd", then
Islg < I3ph (i.e., Islg < 1/Zd").

 

[power2020]

Thank you all.
our 3.3kv loads are 3phase 50hz and 3 wire only.
It is not the last point. we have one more step down system i.e. 400V, (3.3KV/400 transformer), which is a solid grounding.

We are planning to go for 150A NGR (30% of Generator Full load current). We would like to use 2 level of ground fault protections.

Is there any basis for selecting generator NGR grounding?

Hi,slavag (Electrical): We have already proposed CBCT'S for 3.3kv switch gear consumers (for MV motors only).
where else we need to use these CBCT's (toroidal CTS)?
Are you talking about transformer primary side i.e on 3.3kv side of 3.3kv/400v Dyn1 transformer?


One question:
What grounding system do we need to specify for ordering 3.3kv motors? whether solid grounding or Low resistance grounding, as these MV motors are connected to the one switchgear whose busbars are fed sometimes by Transformer (solidly grounded star) and some times by generator (Low impedance grounded star)?

Thanks again

 

[RRaghunath]

With the earth fault current limited as indicated, the 3.3kV system is non-effectively grounded. This needs to be included in all the equipment specifications at 3.3kV level.
However, the insulation cost may not go up as long as the earth faults in the system are reliabily detected and the feeder isolated without any delay 9other than required for coordination).

 

[ScottyUK]

The probability of a stator core wreck is much increased due to the relatively high available fault current. Pay attention to stator insulation condition. Routine PD monitoring, if not continuous online monitoring, will help but when a stator earth fault occurs, expect a very long outage and a massive bill.


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Sometimes I only open my mouth to swap feet...