Hi,
We have a 31.25 MVA, 11kV, 50 Hz generator driven by frame 5 GE gas turbine. The generator operates in Islanding mode. Today , a three phase short circuit happened at one the outgoing feeders at 11 kV. This resulted in high short circuit of magnitude -4400 Amp, and the circuit breaker of faulty feeder had tripped on high set overcurrent protection in 450 ms as desired and thus cleared fault. Simultaneously, under voltage happened (to the level of 610V - phase voltage) at generator terminal which was sensed by generator protection relay.
The generator under voltage protection is set at 86 % i.e. 5461V with a 2 seconds definite time delay.

After fault has cleared, generator terminal voltage should be maintained by AVR & control system with fast response to avoid tripping on undervoltage.

Approx, 1500 miliseconds time was left before generator under voltage tripping occurs. But AVR & control system could not build up the required voltage upto undervoltage drop off value.

I want to discuss here what could be the possible reasons for inadequate response in voltage build up.

Attached is the detailed files mentioning voltage rising rate .

 

Mohit.Sharama...
1) Please provide a simple Single-Line-Diagram showing Gen'r & Fdr !
2) Do you know the cause of the Feeder CB trip: Cable, Bus? Other?
3) Did other two Gen'r phases show near-matching current lists?
4) Does Unitrol provide a listing of corresponding corrective output signals to the Gen'r excitation system?
5) Has ABB provided an exclamation?
Regards, Phil Corso

 

Hi

looks like original poster did not reply since last post from @PhilCorso

I was ready and happy to support remotely but since no reply ...no support..

Cheers
James

 

Thanks @PhilCorso @ControlsGuy25 for responding on the thread.
@PhilCorso @ControlsGuy25 Sorry for the delayed response. Just collecting field data & prepared a preliminary report. @PhilCorso Kindly find below information yet collected:
1) Please provide a simple Single-Line-Diagram showing Gen'r & Fdr !
Kindly refer attached preliminary report

2) Do you know the cause of the Feeder CB trip: Cable, Bus? Other?

Physical inspection of fault feeder CB and feeder compartment was done, Flashover was observed on lower poles of CB inside breaker compartment. No physical material was found that could have caused three phase fault on CB lower poles. Pictures are attached in report. IR of CB (after cleaning of arc carbon), feeder components and cable was found just fine (> 400 Mohm at 5kV DC).
However, one critical observation made after fault is that the other incomer feeder of this switchgear panel had moisture (condensation ) inside cable and breaker compartment. Hence presently we are suspecting moisture as the prime factor.
Your suggestions for other probable causes for this type of fault are most welcome.

3) Did other two Gen'r phases show near-matching current lists?

Yes, the report mentions the trip current of the feeder relay. Apart from it, I have taken out the disturbance record of generator protection relay REG670 and presented here in pdf file. I have mentioned the pre-fault rms current and rms current at different at different fault stages. It seems fault had cleared at about 450-490ms.

4) Does Unitrol provide a listing of corresponding corrective output signals to the Gen'r excitation system?
ABB AVR engineer has not visited site yet and AVR transient response is presently not available. ABB team most probably visit next week. Will share data as soon as I get it.

5) Has ABB provided an exclamation?
ABB said that they can only comment after analyzing AVR transient records.

Thanks
Mohit

 

Thank you @Mohit.Sharama for sharing such details on this incident.

First question is:

Is That 11KV Circuit breaker is USED to get the unit synchronized..or it is used to get the switchboard connected to the Unit transformer/Generator..
Looks like it is a Vacuum Circuit breaker do you confirm? I used to see Vacuum breaker till 6.6KV not 11KV (SF6 UNIT )

Is that CB also called 52 G or other denomination like 52S ( S would stand for switchgear)

Something is missed here I will review closer the files that you shared here and come back with comments..

Regards,
James

 

@Mohit.Sharama...
My reason for asking about an SLD and the question about additional phase-fault currents was to determine how the 11-kV system was earthed/grounded, and was the fault symmetrical ! Furthermore. it is my opinion that fault-clearing-time was excessive ! Then... try to determine how the ABB AVR reacted to such conditions. My position was similar to ABB's !
Regards, Phil Corso
Ps: I'm used to waiting a long time for answers to questions !

 

Mohit.Sharama...
6) Forgot to ask earlier. Had the ABB-6080 on other units worked when responding to previous faults?
Phil Corso

 

Thank you @Mohit.Sharama for sharing such details on this incident.

First question is:

Is That 11KV Circuit breaker is USED to get the unit synchronized..or it is used to get the switchboard connected to the Unit transformer/Generator..
Looks like it is a Vacuum Circuit breaker do you confirm? I used to see Vacuum breaker till 6.6KV not 11KV (SF6 UNIT )

Is that CB also called 52 G or other denomination like 52S ( S would stand for switchgear)

Something is missed here I will review closer the files that you shared here and come back with comments..

Regards,
James

Hi James,
Kindly find below comments.

Yes, this is Vacuum Circuit breaker. This VCB under fault is used to get the switchboard connected to generator distribution panel. which is approx 1.5 km away from this substation. The distribution is being done by underground cables. Flashover happend in 52S (located 1.5 km away) and 52S tripped on high set OC fault and 52G(generator breaker) tripped on generator undervoltage.

Thanks,
Mohit

 

@Mohit.Sharama...
My reason for asking about an SLD and the question about additional phase-fault currents was to determine how the 11-kV system was earthed/grounded, and was the fault symmetrical ! Furthermore. it is my opinion that fault-clearing-time was excessive ! Then... try to determine how the ABB AVR reacted to such conditions. My position was similar to ABB's !
Regards, Phil Corso
Ps: I'm used to waiting a long time for answers to questions !

@PhilCorso
The generator neutral is grounded through NGR limiting ground fault to 200 Amps. The fault was almost symmetrical. I completely agree with your observation on excessive fault clearing time.

 

Mohit.Sharama...
6) Forgot to ask earlier. Had the ABB-6080 on other units worked when responding to previous faults?
Phil Corso

@PhilCorso Only one Unit was running . Other is standby.

 

Mohit.Sharama...
7) James' identification of system components was helpful. Did the Ground-Fault-Protective-Relay on 52S operate ?
8) Who is the VCB manufacturer?
9) If the VCB is mounted on a Pull-Out Frame, then how is it grounded/earthed?
Phil Corso

 

Mohit.Sharama...
7) Did the Ground Fault Protective Relay on 52S operate ?

No it hasn't operated.

 

Mohit.Sharama...
Please provide more about Chamber Pics:
10) The one showing debris in the corner is definitely similar to that of a 3-phase-Fault, i.e., debris blown away from location of fault.
11) My reason for asking how the breaker-carriage, if retractable from the chamber, was properly grounded ! I had investigated a similar problem, but the client had depended on ground-fault-current passing thru the carriage wheels !
Phil Corso

 

@PhilCorso
Kindly find attached one pic of chamber of faulty breaker. Other photos of a healthy breaker and chamber is attached for reference. For earthing there is a dedicated copper earth strip which engages during rack in of breaker.

 

Hello Mohit Sharama,


Something get me confused when i read the posts here..
Can you clarify on Generator Voltage ( undervoltage ) level as you mentionning :
The generator under voltage protection is set at 86 % i.e. 5461V with a 2 seconds definite time delay.

And you said that this is a 11KV Generator...

Otherwise did you have a electrical study on the installed system that you got...

I mean how about protection selectivity...

Meanwhile for Voltage built up We do not know if there is kind of Excitation transformer installed in MCC or other method like connected to auxiliary transformer ... a SLD Would be welcome here to get a better approach on your issue..

Do you know if there is kind of field boosting/flashing option in your plant..

This is what i can add for the moment until we can see the SLD and electrical system study result..

Regards,
James

 

Mohit Sharama...
Haven't forgotten you! Please provide the names of the "electrical function" assigned to each channel. My new questions are:,
12) GT Phase Currents, Ph-Ph Voltages, Phase Angles.
13) GT Rotor Current and Voltage.
14) Ground Fault Current if any.
15) Any other Channels associated with carrying Fault-Current to CB-52S.
16) The GT's parameters, X''d, X'd, and Xd.
I also understand why you are reluctant to provide name of VCB Mfg.
Regards, Phil Corso

 

Mohit Sharama...
Haven't forgotten you! Please provide the names of the "electrical function" assigned to each channel. My new questions are:,
12) GT Phase Currents, Ph-Ph Voltages, Phase Angles.
13) GT Rotor Current and Voltage.
14) Ground Fault Current if any.
15) Any other Channels associated with carrying Fault-Current to CB-52S.
16) The GT's parameters, X''d, X'd, and Xd.
I also understand why you are reluctant to provide name of VCB Mfg.
Regards, Phil Corso

Phil Corso

I guess that VCB manufacturer /maker is Siemens as we can see on the pics that Mohit Sharma shared...

You asked for :
The GT's parameters, X''d, X'd, and Xd. you mean Generator parameters ..right..

I asked Mohit for system electrical study if available with few other questions that can help to troubleshoot about AVR Voltage Build up during such event


We are waiting for answers from Mohit now.

Regards,
James

 

Thank you James !
Yes ! I need GT Parameters to calculate fault-currents for the usual case! BTW, Mohit, another question, Q17) what are cable parameters?)

Another thought... I asked the wrong question in message #7 ! I should have asked Q18) Had the ABB-6080 (in question) operated properly for a previous fault, or was it the 1st time?

I have some ideas I'd like to pursue... once I can interpret the Chart information ! My eyesight is not as good as it once was. My "wish" is that Mohit could reproduce the charts, but with non-related channels eliminated. My biggest problem at the moment is finding 8.5" by 14" (legal) size paper !

Regards to all and stay well,
Phil Corso

 

Hello Mohit Sharama,


Something get me confused when i read the posts here..
Can you clarify on Generator Voltage ( undervoltage ) level as you mentionning :
The generator under voltage protection is set at 86 % i.e. 5461V with a 2 seconds definite time delay.

And you said that this is a 11KV Generator...

Otherwise did you have a electrical study on the installed system that you got...

I mean how about protection selectivity...

Meanwhile for Voltage built up We do not know if there is kind of Excitation transformer installed in MCC or other method like connected to auxiliary transformer ... a SLD Would be welcome here to get a better approach on your issue..

Do you know if there is kind of field boosting/flashing option in your plant..

This is what i can add for the moment until we can see the SLD and electrical system study result..

Regards,
James

@ControlsGuy25
The relay setting and coordination is as per commissioning ( year1992). For electrical study , I'll get back to you, once I get hold of such document.
Plant SLD is attached. Generator AVR get's supply from shaft mounted PMG. No other excitation transformer is there.

Apart from AVR, no field boosting/flashing option is there.

Regards
Mohit

 

Thank you James !
Yes ! I need GT Parameters to calculate fault-currents for the usual case! BTW, Mohit, another question, Q17) what are cable parameters?)

Another thought... I asked the wrong question in message #7 ! I should have asked Q18) Had the ABB-6080 (in question) operated properly for a previous fault, or was it the 1st time?

I have some ideas I'd like to pursue... once I can interpret the Chart information ! My eyesight is not as good as it once was. My "wish" is that Mohit could reproduce the charts, but with non-related channels eliminated. My biggest problem at the moment is finding 8.5" by 14" (legal) size paper !

Regards to all and stay well,
Phil Corso

@PhilCorso
For Q12 to Q15, I'll try to reproduce charts along with channel function for required channels only, it is taken from ABB's wavemin software.
Q16 - For generator X''d, X'd, and Xd , please refer attached file.
Q17- Cable parameters:
Size : 3CX 240 sqmm Aluminium
Number of runs - 2 runs
Length of each run - 1.4 km
Laying method - Overhead tray
DC Resistance @20 deg - 0.125 Ohm per km
Reactance - 0.106 Ohm per km
Q18- Three phase fault had earlier not occurred at our site and but for phase to earth fault in past, AVR has responded perfectly well.