Generator Grounding Method 3

[volkanksr]

We have 2*62.5 MVA Transformer, 2*12 MW Generator and lots of big motors and loads in our power plant. Transformers are YNd,generators are connected to D side and we are discussing about grounding of generators. Should we use hybrid, solid or high resistance ground for generators? Or should we use a zigzag grounding transformer on D side of Main transformer? Is it common to use both Zigzag grounding transformer and HRG generator at same time? Which is common grounding method for generators for such big power plants ?
Best regards

 

[slavag]

Hi volkanksr .
Could you please send more information.
1. All trafo's and generators connect on common bus or you have couplers?
2. What is a voltage rating on generator side?
3. Please write about operation options:
3.1. only two trafo's connected.
3.2. two generators and one trafo in parallel.
3.3. only two generators, etc.
Very important point: what is type of protection you use for
generator and trafo'sagainst E/F (51N, 59GN, REF)
Best Regards.
Slava

 

[volkanksr]

Hi Slava,

1. I have 154/11 kV YNd TR and a generator connected at 11 kV side. And other set of TR and generator is coupled in 11 kV side with a bus coupler.
2. Generator side is 11 kV
3. Conditions:
a. Two transformer and two gen parallel
b. One TR and one gen parallel with coupling open
c. One TR with 2 gen paralle with copuling closed and other
TR open.
4. At the moment i didn't decide how to protect as i don't decide earthing of Generators yet.
Best regards,

 

[slavag]

Hi Volkan.
In your case you have several options.
You need check also cost of each solution.
We start from best technical solution.
1. Two zigzag transformer each one on separte bus.
zigzag transformer will be earted (grounded) via high speed earht switch. ( with grounding resistor or without, is depends on earht fault calculation).
what is a benefit of this solution.
very flexible system with logic of coupler position, all time in all configuration you can choice only one virtual earht point. Good option for ET maintanance.
But you need two aditional feeders ( CB, CT, control and protection )for these earth trafo's.
Possible also connect them to bus via disconectors, in this case some short circuits or bucholz on ET open all infeeds on this bus.
2. Four earhing points , HRG connect to generators via high speed grounding switch, two ET you connect to main trafo (between trafo and CB), grounding of ET neutral point via high speed grounding switch (with resistor or without).
You add logic for choice only one virtual earth point.
More equipment, ET in differential protection zone.
3. Same to point 2 , but without any high speed grounding switch and without logic.
It's also work without any problem, we have more or lees same system on one industrial power plant (but without coupler)more from 20 years. E/f current flow between 4 points ( actually is not big problem, all TR and GEN in the same place). Protection system against E/F it's 51N (you put toroid CT on each grounding point).But I think possible discuss about protection after your choice of grounding method.
I'm sure you recieve many other recommendations.
Best Regards.
Slava

 

[volkanksr]

Thanks Slava

 

[slavag]

Hi Volkan.
I forgot about one important point.it's not connect to your Q,but..
Please pay attention on 3ph short circuit current in case of two TR and two GEN operate in parallel with coupler closed. From my point of view you have in this cases more from 40kA. Check again your MV swg.
Regards.
Slava

 

[rcw retired EE]

Look at IEEE Standard C37.101- 1993 "IEEE Guide For Generator Ground Protection."

I recommend using high resistance grounding of generators to limit stator iron damage due to a ground fault. Using a transformer/resistor sized for 10-20 amps is typical. This low current may create some relaying problems for the other circuits. Also, the fault current value will change based on the number of generators on line.

The zig-zag approach has the benefit of being easier to coordinate the relaying, but the generators will suffer more damage on ground faults.

 

[RRaghunath]

If the generator is only 12MW rated and is connected to the MV bus that supplies the loads, preferred arrangement, I think, is low/medim resistance grounding (limiting the e/f current to a max of generator rated current).
It is also important to ensure that only one earth connection is in service at any given point of time in the MV system.

 

[slavag]

Hi Raghun.
Could you please explain your recommendation.
It's very important states
Regards.
Slava

 

[volkanksr]

Hi Raghun,

I plan to use zigzag grounding at D side of main transformer and hybrid grounding at generators. What do you mean by "It is also important to ensure that only one earth connection is in service at any given point of time in the MV system.". ?f i only ground generator neutrals then when these feeders are OFF, i will have isolated system that has many disadvantages .

Regards,
Volkan

 

[RRaghunath]

Sorry, I wasn't visiting the site offlate.

The grounding method preferred in case of MV systems is low/medium resistance grounding. In the present case, the earth fault current can be limited to the rated current of the generator.

At any given point of time, it is important to ensure that not more than one earth connection is present in the system.
This is to prevent circulation of currents (triplen harmonics especially) in the system that can cause undesirable heating in the neutral circuits and in the windings.
To achieve this, load break switches are provided in the neutral circuits of generators and alarm configured in case more than one switch is closed. This is normal practice when more than one generator is connected to the same bus bars.

Trust I have been helpful.

 

[slavag]

Dear Raghun.
Thank you.
Regards.
Slava

 

[slavag]

Dear All.
What are levels of high/medium/low resistance?
Regards.
Slava

 

[dpc]

This should give you background on methods of grounding and some definition of terms:

http://standards.ieee.org/colorbooks/sampler/Greenbook.pdf

After that, you should read up on recommendations for generator grounding.

 

[slavag]

Dear DPC.
Thank you.
I'm sorry, my Q was not correct,
I asked about definition of terms,point 1.2 in the greenbook, whatI'm looking for.
Regards.
Slava