Ground fault in parallel operated Generators.

[power2020]

Dear Folks,
If three same rated generators are connected in parallel and only one generator neutral is connected to ground through NGR (neutral grounding resistor, ground fault current restricted to 300A). Let us say Generator no.1 neutral is connected to ground.

For generator No.2 terminal fault (L-G) or internal fault (L-G),
what is the individual fault current contribution of three generators? And how earth fault protection can be configured for these generators with out differential protection?
Advanced thanks for replies.

 

[power2020]

In addition to my previous post, all generators are not connected to utility grid and are operating in droop mode.

 

[slavag]

Hi Power2020.
From my point of view, you can use 67N function.
You need install CBCT ( toroid CT around all of 3 phases) and VT with broken delta connection.

Assume, you have only two generators one with NGR connected.
L-G fault in one of the generators, I0 is eqv. in both of generators, only direction is different.

Additional advantages of this system. Second stage of 59N
you can use as BB earth(ground) fault protection and backup of generator earth(ground) fault protection .

For example only:
300A is current limit.
Put, I don't know, 5-10% of pick-up setting of I0 and about
20V ( secondary voltages) of U0.
Regards.
Slava

 

[burnt2x]

Specific comments:
1. Since NGR limits ground fault within 300A, set directional ground relay at 30A (10%).
2. If the grounding potential trafos (broken-delta) is rated at 110V, set polarizing voltage (also called reference, supervising) at 11 volts (10% of rated voltage output 3Vo also).
3. Shunt trip contact to each genset breakers can be made more selective by inserting a series contact from an auxiliary relay driven by the 59N (overvoltage to ground, installed to monitor bus voltage to ground) on the trip contact from the 67N element. Any sero-sequence current (probably caused by unbalanced loading) will not trip the genset breakers since there will be no voltage rise wrt ground (monitored by 59N) and trip signal supervised by 59N aux contact.

4. Set trip to definite time not to exceed 1 sec

Respectfully.

 

[slavag]

Hi Burnt2x.
Actually, new 67N relays ( what I know) include this option .
We usaully use option: I0>---> > from set point and U0> > from U0 base and direction.

Next point, I'm not agree with you: unbalance load is not reason for the zero-seq. current ( of course if you use CBCT and not rsidual connection)
Regards.
Slava

 

[power2020]

Dear Mr.Slavag & Mr.Burnt2x,
Thank you for your suggestions,
If only one generator neutral is connected to ground thru NGR, is there any chances to circulate 3rd harmonic currents to ground if switching loads (variable frequency drives) are connected on distribution board ( no transformer connected between generators and distribution board).
If residual connected CTs and Open delta PT are used to find directional earth fault , is there any chance to trip on earth fault due to switching loads?
Mr. Burnt2x suggestion seems interesting to use 59N aux. contact in series with shunt trip to avoid unwanted trip.
Advanced thanks for your discussion.

 

[slavag]

Hey Power2020.
Mr. Slavag is so official, simple Slava is OK.
I try help you for fun, not for salory ;-).

Please send us more information about your generator ( size and voltages) or what is your CT and after this we think together, what prefer use residual connection or CBCT.

BTW, price one CBCT 100/1 10P10 5-10VA is about 300-400Euro not so expensive.

New relays with 67N are include 59N into relay and don't need additional wiring.

What relay you'll use for this generators.

Regards.
Slava

 

[slavag]

And additional.
In case of CBCT and Broken delta VT connection ( don't use
open delta term, our US partners dosen't agree with this term ) and right setting and testing: Not chance ( 0f course 99.999999%) to unwanted trip. In case of residual connection I don't now exectly, but also very small chance.
Regards.
Slava

 

[power2020]

Hi Slavag,
Generator rating is 1030kVA, 0.38kV &60HZ, 0.8 pf, 1800rpm. Proposed CT rating is 15VA, 5P20, 2500/1 A for O/C relay and same CTs will be used for detecting earthfaults by residual connection.

 

[slavag]

Hi Power2020.
We have today good synchronization with you.

Oops. There are low voltage generators and size less then 1MW. I prefer wait some recommendation from Waross or Catserveng (or others). These folks have a very good experience with those type of applications. I'm not sure and don't want give you some "stuipid" answers.
From my point of view:
This is a clasical on-shore application with several brushless low-voltage genertors are running in parallel.Are better used solid grounding for all generators and simple
50, 51, 50N, 51N protection function ( in all cases better also add some toroid CT on the neutral conductor). Don't forget it's 380V application and 380V system "must" be solid grounding..
Sorry, Power2020, but in this case I'm very weak.
Regards.
Slava

 

[power2020]

Hi Slava,
Sorry I did not mention before. All generators are located on barge mounted (non-sailed).
Total 9-generators (6no. of 670mW & 3no. of 830mW) are proposed by contractor. And all these generator neutrals are solidly grounded. As fault current (L-E)is high i.e.100kA, I would like to propose NGR however the system will be designed for 3-phase bolt fault. As most of the faults are L-E and Arc flash hazard can be minimised.
If all neutrals are solidly grounded, there is a chance of circulating currents due to different rating of generators and switching loads.
Contractor proposed relays 50/51 & 50N/51N. I am studying to use NGR with protection co.ordination.
Pl. discuss
Thanks

 

[slavag]

Hi Power2020.
There are only my theoretical opinions.
I haven't any practical experience.

1.Possible use off-shore application, that mean,
isolted (ungrounded) network system with only one ,per BB, relay with 59N function. One stage for alarm, one for common trip. 67N in this case are not applicable ( or used it for outgoing cables).
I think, possible separate SWG to 3 sections with 59N on each one. Disadvantage of this system is only second ground fault.

2. Possible also use one zig-zag xfr with NGR ( may be better high impedance application)per BB.
In this case possible use 67N function, but I'm strongly recommend use CBCT on the cable ( size about 100-200/1A).

3. Your idea are request additional 9 CB's and 9 additional NGR's ( or 9 CB's and one common NGR) and you need build additional logic. Probably it's cheaper solution.

Regards.
Slava

 

[power2020]

Hi Slavag,

Your idea No.2 looks intersting and feasible. In this case, no need of generator NGR and neutral circuit breakers. Always all generators operate un-grounded and it will avoid 9-NGR and 9-neutral circuit breakers.
And it is easy to co.ordinate for ground faults by using 67N for generators and CBCTs for load feeders.
what are the general specifications of ZIG-ZAG transformers for generator (parallel operation) bus?

 

[slavag]

Hi Power2020.
Please find attached.
Hope it help.
thread238-158419
thread238-151636
thread238-164608

Regards.
Slava

 

[slavag]

I'm not familar with LV generators.
But for MV generators: