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Differential trip 1
- Thread starter khadisa
- Start date
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1
- #2
RalphChristie
Electrical
Taking the lack of info in consideration, I suppose one of your CTs saturate under heavy current conditions. Do a mag-curve test on your CTs.
What kind of CTs are you using?
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What kind of CTs are you using?
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You have posted just enough information to tell us absolutely nothing useful about your installation.
Describe the scheme properly. Particularly CT locations, and ratings. Describe the transformer, at least in winding configuration and ratio. Remember that the only information we have about your problem is what you post, so if you don't expressly tell us about something, then we don't know.
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Your body might be a temple. Mine is an amusement park...
Describe the scheme properly. Particularly CT locations, and ratings. Describe the transformer, at least in winding configuration and ratio. Remember that the only information we have about your problem is what you post, so if you don't expressly tell us about something, then we don't know.
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- Thread starter
- #4
Iam sorry for my late response.
Actually the transformer is 15MVA, 132/11KV and Y/Y. The primary CT is a bushing CT of ratio 100/1A and the secondary is a panel CT of ratio 1200/0.577. We have sets of interposing CTs on both primary and secondary wired in star/delta. There is REF on the primary circuit.
Actually the transformer is 15MVA, 132/11KV and Y/Y. The primary CT is a bushing CT of ratio 100/1A and the secondary is a panel CT of ratio 1200/0.577. We have sets of interposing CTs on both primary and secondary wired in star/delta. There is REF on the primary circuit.
Have you got a copy of the manual for this relay? You didn't say which of the C21 class you were using, but they are all fairly similar. You will need to register on the website.
The 13th page of the 15 page booklet on the 2C21 / 3C21 shows a typical connection for a combined differential / REF scheme sharing the same primary CTs. My guesses based on the symptoms you describe are: CT saturation as Ralph has already identified, or your neutral CT might be reversed. I am cautious because you don't state whether or not the REF relay also trips when the diff relay trips, and you also don't specify the nature of the through fault. Tell us whether the fault is phase-phase, single-phase-earth, 3-phase-earth, etc. This is very significant because if the scheme is stable for certain faults and unstable for others then that gives a great deal of help diagnosing the problem.
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Your body might be a temple. Mine is an amusement park...
The 13th page of the 15 page booklet on the 2C21 / 3C21 shows a typical connection for a combined differential / REF scheme sharing the same primary CTs. My guesses based on the symptoms you describe are: CT saturation as Ralph has already identified, or your neutral CT might be reversed. I am cautious because you don't state whether or not the REF relay also trips when the diff relay trips, and you also don't specify the nature of the through fault. Tell us whether the fault is phase-phase, single-phase-earth, 3-phase-earth, etc. This is very significant because if the scheme is stable for certain faults and unstable for others then that gives a great deal of help diagnosing the problem.
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- Thread starter
- #7
My experience is that, if the relay is set properly, and the CT's are all matched and set up properly (wye-delta vs. delta-wye for transformer), none damaged, one is probably wired backwards. See if you can take a voltage across the operating coil on standard operating conditions - there should be close to zero if properly wired, but if there is a voltage (say over 20 volts??? depends on the load current, etc.) you probably have a CT wired backwards.
All of the above are possibilities. Begin by taking in-service load reading at maximum possible load. Read current magnitude and angle with respect to any reference voltage. Compare all 6 reading to see if the they are the proper magnitude and direction given the CT ratios and connections. Also the ratio of the taps should equal the ratio of the currents if its an electromechanical relay.
If all this proves correct, do a saturation test of the CT's
If all this proves correct, do a saturation test of the CT's
RalphChristie
Electrical
khadisa:
You still do not supply much info.
From the kind of relay you are using I suppose it is an old installation. How long do you experience this fault? Is it a recent fault?
Combined REF and differential schemes are not unusual. If the scheme is stable on load currents, but unstable on through-fault currents, I would still suspect a wrong CT - most probably a CT that saturates under heavy current conditions. Also remember that these CTs have to be class X CTs. I would not suspect a reversed CT, because then your scheme would be unstable under normal load conditions.
Number of factors that need consideration for a differential scheme on two winding transformer:
a) Transformer vector group (phase shift between HV and LV)
b) Mismatch of HV and LV CTs
c) Varying currents due to on-Load Tap changer (OLTC)
d) Magnetizing in-rush currents (from one side only)
e) Possibility of zero-sequence current de-stabelizing the differential for an external earth fault.
Factor (a) need no consideration, because there is no phase shift between HV and LV. (Transformer Y/y)
[red]Can you confirm that your main HV and LV-CTs are connected both in star? Interposing CTs (both star/delta) seems to be fine.[/red]
The delta connection of the interposing CTs provides a path for circulating zero-sequence current, thereby stabilizing the protection for an external earth fault as required by factor (e)
[red]Seems to be fine[/red]
As the magnetic inrush is predominately 2nd harmonic, filters are then utilized to stabelise the protection for this condition (d)
[red]No inrush trips, seems to be fine[/red]
It is necessary to bias the relay to overcome current unbalances caused by (b) mismatch of CTs and (c) OLTC. The desired setting is dictated by the operating range of the OLTC which is responsible for the biggest current unbalance under healthy conditions.
[red] Seems to be fine because transformer operate under full-load conditions[/red]
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You still do not supply much info.
From the kind of relay you are using I suppose it is an old installation. How long do you experience this fault? Is it a recent fault?
Combined REF and differential schemes are not unusual. If the scheme is stable on load currents, but unstable on through-fault currents, I would still suspect a wrong CT - most probably a CT that saturates under heavy current conditions. Also remember that these CTs have to be class X CTs. I would not suspect a reversed CT, because then your scheme would be unstable under normal load conditions.
Number of factors that need consideration for a differential scheme on two winding transformer:
a) Transformer vector group (phase shift between HV and LV)
b) Mismatch of HV and LV CTs
c) Varying currents due to on-Load Tap changer (OLTC)
d) Magnetizing in-rush currents (from one side only)
e) Possibility of zero-sequence current de-stabelizing the differential for an external earth fault.
Factor (a) need no consideration, because there is no phase shift between HV and LV. (Transformer Y/y)
[red]Can you confirm that your main HV and LV-CTs are connected both in star? Interposing CTs (both star/delta) seems to be fine.[/red]
The delta connection of the interposing CTs provides a path for circulating zero-sequence current, thereby stabilizing the protection for an external earth fault as required by factor (e)
[red]Seems to be fine[/red]
As the magnetic inrush is predominately 2nd harmonic, filters are then utilized to stabelise the protection for this condition (d)
[red]No inrush trips, seems to be fine[/red]
It is necessary to bias the relay to overcome current unbalances caused by (b) mismatch of CTs and (c) OLTC. The desired setting is dictated by the operating range of the OLTC which is responsible for the biggest current unbalance under healthy conditions.
[red] Seems to be fine because transformer operate under full-load conditions[/red]
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Do a simple test to find out whether unbalance current is flowing through any one of the phases for the relay to operate. Put a short on the secondary side of the transformer. Inject low voltage on the primary side and measure through each phase and current through neutrals.
This will ensure CT ratio, connections etc
Subramanian
This will ensure CT ratio, connections etc
Subramanian
- Thread starter
- #12
I wish to thank every body.
What we observed is that the primary CTs saturates earlier than the secondary CTs. The secondary CTs are of X class while there is no name plate for the primary CT but we believe the class to be 5P10. Our relay is an old one and does not have provision for altering the bias setting. We are now trying to replace it with one whose bias can be adjusted. Will this help us in any way?
What we observed is that the primary CTs saturates earlier than the secondary CTs. The secondary CTs are of X class while there is no name plate for the primary CT but we believe the class to be 5P10. Our relay is an old one and does not have provision for altering the bias setting. We are now trying to replace it with one whose bias can be adjusted. Will this help us in any way?
RalphChristie
Electrical
Khadisa:
Thanks for the feedback - we can all learn from such a problem. I am glad you found it.
Regarding your last question:
If you want to replace it with a new microprocessor relay you can leave the main-CTs just as they are. Most of these relays have also REF-funtions (low impedance) incorporated into the relay. With the new era of relays there are much lower burdens imposed on the CTs and the need for dedicated (class X) CTs are not necessary anymore.
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Thanks for the feedback - we can all learn from such a problem. I am glad you found it.
Regarding your last question:
If you want to replace it with a new microprocessor relay you can leave the main-CTs just as they are. Most of these relays have also REF-funtions (low impedance) incorporated into the relay. With the new era of relays there are much lower burdens imposed on the CTs and the need for dedicated (class X) CTs are not necessary anymore.
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davidbeach
Electrical
Ok, someone want to explain what a Class X CT might be? Obviously it is something from outside the ANSI realm, and I recognize something like a 5P10 is designating a protection class CT, but an X? Thanks.
RalphChristie
Electrical
David:
Sorry, I should have been more descriptive.
Class X CTs are included in the old British Standards, BS 3938:1973. (I think it changed to Class PX in the new IEC-standards, but I am more familiar with the old term) It cover protective CTs for the use in applications where the required characteristics of the CTs cannot be conveniently expressed in the terms used for classes 5P and 10P. (like in dedicated protective schemes, such as differential scheme)
Class X CTs are spesified in the following terms:
a) rated primary current
b) turns ratio
c) knee point emf
d) exciting current at the knee point emf and/or at a staded percentage thereof.
e) resistance of the secondary winding, corrected to 75°C or the maximum service temperatue, whichever is the greater.
Regards
Ralph
Sorry, I should have been more descriptive.
Class X CTs are included in the old British Standards, BS 3938:1973. (I think it changed to Class PX in the new IEC-standards, but I am more familiar with the old term) It cover protective CTs for the use in applications where the required characteristics of the CTs cannot be conveniently expressed in the terms used for classes 5P and 10P. (like in dedicated protective schemes, such as differential scheme)
Class X CTs are spesified in the following terms:
a) rated primary current
b) turns ratio
c) knee point emf
d) exciting current at the knee point emf and/or at a staded percentage thereof.
e) resistance of the secondary winding, corrected to 75°C or the maximum service temperatue, whichever is the greater.
Regards
Ralph
A Class X CT is a 'special' with a defined (usually high) kneepoint voltage. They're typically found in high-Z relaying applications. 'Class X' on its own doesn't really tell you much about it: the rest of the spec would typically detail the following, in addition to the usual ratio, voltage class, etc.
Vkp - Voltage knee point
Io - Maximum magnetising current at Vkp
Rs - Maximum resistance of the secondary winding
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I don't suffer from insanity. I enjoy it...
Vkp - Voltage knee point
Io - Maximum magnetising current at Vkp
Rs - Maximum resistance of the secondary winding
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Can anyone comment on the requirement for matching of Class X CT's?
We are having to replace the Class X neutral CT in an LV generator REF protection scheme.
I have nameplate details of the original CT, but the CT that has been offered by my supplier has slightly different characteristics - i.e. Resistance @75deg C was 13.3ohm, CT offered has 9.8ohm, Vk was 600V, CT offered has Vk of 605V etc
Any comments would be appreciated!
Allan
We are having to replace the Class X neutral CT in an LV generator REF protection scheme.
I have nameplate details of the original CT, but the CT that has been offered by my supplier has slightly different characteristics - i.e. Resistance @75deg C was 13.3ohm, CT offered has 9.8ohm, Vk was 600V, CT offered has Vk of 605V etc
Any comments would be appreciated!
Allan
Hi Allan,
Gut feeling in the absence of all the design data is that the alternative CT will be ok. The differences in spec that you've listed are all in the 'right' direction, so you should be ok for those two parameters as far as I can see. Presumably the ratio is an exact match? Can you confirm that the CT resistances are both quoted at the same temperature? Also consider the magnetising current for the CT and see how this compares with the original.
Out of interest, was it not possible to save the core of the original CT and rewind it? What is the primary rating?
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I don't suffer from insanity. I enjoy it...
Gut feeling in the absence of all the design data is that the alternative CT will be ok. The differences in spec that you've listed are all in the 'right' direction, so you should be ok for those two parameters as far as I can see. Presumably the ratio is an exact match? Can you confirm that the CT resistances are both quoted at the same temperature? Also consider the magnetising current for the CT and see how this compares with the original.
Out of interest, was it not possible to save the core of the original CT and rewind it? What is the primary rating?
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