Transformer Fault Analysis

[etgbccc2003]

We have a transformer fault on a 33kV/6kV secondary linkbox of a 15MVA transformer.

The event recorder at the differential protection (using GE multilin 745) shows that the secondary CTs shows about 4000A (not the same value for all 3 pahses but about 4000A while the primary CTs shows about 1750A (not the same value for all 3 pahses but about 1750A).

If we use these values to estimate, a total outzone fault should have caused a secondary CTs to measure about 9625A (1750 X 5.5). Since this transformer measures 4000A at the secondary side, there seems to us that there is also an inzone fault besides outzone fault at all 3 phases. In addition, this transformer have a 8.2% impedence and by calculation, a fault could have caused a 17000A fault current. This suggest that the fault is a higher impedence fault than true fault.

Finally , this linkbox is certainly ingressed by moisture as we can see a lot of rust inside the linkbox. The windings of the transformer wasn't damaged too.

I'd like to ask if the above analysis is correct? Also, what could be the likelihood of the cause of this kind of failure (both inzone and outzone fault occurs at the same time? Could it have been moisture? If there's high humidity in the linkbox, could it have caused a flashover allover?

I find it difficult to explain the coincidence of fault happening before and after the secondary CTs and at all 3 phases.


Thanks.

 

[Bung]

What does the relay say in the event record about the differential and bias currents? Check the times of each event - sometimes the time differences can be very useful! Also, have you factored in the effects of load currents? 15MVA at 6kV is 1440A on the LV side about 30% of the measured current. (this assumes a fully loaded transformer) You should also be able to tell something from the relative angles of the measured currents in and out.

It is probably better to examine all the facts you have, and then draw a conclusion, rather than start with a conclusion (simultaneous fault in and out of zone) and trying to fit the facts to it.



Bung
Life is non-linear...

 

[jghrist]

I would suspect saturation of the secondary CTs. Does the oscillography show a distorted waveform (more current in positive half of the cycle than in the negative half)? Was there a high second harmonic? What are the CT ratings and burdens?

 

[etgbccc2003]

The CTs readings are
IA = 1710 deg 0
IB = 2180 deg 142
IC = 1642 deg 241

Ia = 4267 deg 89
Ib = 3915 deg 215
Ic = 3586 deg 329

The transformer vector is Dy1 and when measured in GE 745's convention, there's a 180deg phase difference. Thus, it seems like the phase shift during flashover is -120deg in all phases.

Sorry for jumping to conclusion. I indeed suspect this analysis that's why I am posing this question here for discussion.

The CT ratings are
HV: 300/5 10VA 5P10
LV: 1500/5 10VA 5P10

Is that high possiblity for CT saturation? I didn't see the CT saturation portion yet. Thanks for reminding. Hormally what specifications determine if a CT can satuarate more easily.

Thanks a lot.

 

[etgbccc2003]

I think the CT saturation is unlikely as the fault current is only 3 times the CT current (1500).

Could the current in the CT be due to motor contribution?
How do we calculate motor contribution?

Thanks.

 

[stevenal]

Has relay been secure for previous through faults?

 

[etgbccc2003]

stevenal,

there isn't any previous failures before.
However, I checked some sites saying that typically motor contribution is about 4 times the running current. My motor is running at >900A. Thus, I think the readings at the secondary CT might be only motor contribution and there could only be a fault at the bushings (before the secondary CTs).

Thanks.

 

[rbulsara]

Before jumping to conclusion and into deep theories, lets clairfy some basic preliminary facts.

1. What are the trip 'flags' shown on the protection relay? 87? 51? 51G?

2. What are, if any, visual signs of the 'damage' and where?

3. What is fed by this transformer? (It appears that only 900A motor? HP?)

4. What were pre-fault conditions? Was the motor running? or did the fault occur when energizing the transofmer? etc.
This may help clarify if the primary current included any tranformer inrush current!

5. The recored current appear to be 'balanced' 3 phase current, I seriously doubt that there is any internal 3 phase fault. If any it would have been a phase to phase or a phase to G fault.

6. At the same time its difficult to comperhend a 3 phase load side fault also....so physical signs of damage is a key here.

7. What test have been done after the fault? Meggaring the tranformer, cables, motor?

There may be more info required, but answers to the above will be a good start.

 

[stevenal]

etg,

What did you mean by "no previous failures?" Was system secure for through faults, or were there none? I'm having a difficult time imagining a fault that would cause a 120 degree shift. Current would need to go from leading to lagging. Fault impedance is usually resistive. I suspect a CT wiring error at installation, but a history of secure through faults would argue against that.

Any motor contribution should sum to zero for faults outside the differential zone (defined by CT location), and not to zero for faults inside. Presence of a motor should not fool the relay.

 

[RalphChristie]

I agree with stevenal regarding the differential zone and the comments of rbulsara. A diff-scheme should not operate for faults outside the zone. To add some more questions...

Can you confirm:


Trip indication
If it was a diff-trip can you confirm:

If the linkbox (fault location) is inside/outside the diff zone.
CT-polarity
Confirm relay settings (I'm not familiar with this relay, but with most of this type of relays, you'll have to set primary and secondary CT-ratio's and have to set the vector-group of the transformer.)

Regards
Ralph

 

[etgbccc2003]

rbulsara

Red and yellow bushings porcelain craked. PT cables and fuses burnt.
51G picked up. 87T picked up.
The transformer is a captive transformer running only 1 14000HP motor.
Motor was running for many days before failure.
Transformer windings, CT and PTs were not damaged. Motor meggar was also good.
A lot of rust seen in the box. Busbar of red phase slightly melted only. 3 isolator supports holding the busbar near the outgoing cables broke.

stevenal
I suspect fault happened at the bushings which is inside diff zone. All relay settings and CT wiring is correct.

Thanks.

 

[alehman]

It seems like the motor contribution could account for the difference.

Was there only one event recorded? What were the phase differential and ground current values recorded? Is the transformer secondary solidly grounded? Is the motor synchronous or induction?

 

[RalphChristie]

My opinion with a few assumptions...

It seems as if the fault was inside the differential zone - should have been a Diff trip. Any differential trip for an inzone fault is very fast, normally just a few cycles. (Doesn't have to coordinate)
I assume it was not a solid fault, but rather a flashover between the phases. The arc impedance and source impedance should limit the current seen from the primary side flowing into the fault. This is your current reading on IA, IB and IC. Your secondary CTs won't see this current because it is not a through-fault.

But the motor will also contribute to the fault, it acts for a very short period like a generator because it is turning, and some current will flow from the motor into the fault. This current will just be seen by the secondary CTs.(900x4=3600A)

Regards
Ralph

 

[digitrex]

The fault was in the transformer's secondary linkbox with Red and Yellow phase porcelain bushing damaged. Then the ground fault relay 51G at secondary side operated, in addition to the diff protection 87T.

However, from the event recorder's current readings, it seems that more zero sequence current is detected in primary side of the transformer, which indicates that an earth fault had occured at primary side.

I am confused and need more data.

 

[RalphChristie]

For any type of earth-fault in the zone I would suspect a REF-trip. Normally this type of relays have a combined REF and Diff scheme/setting. Can you confirm it?

Diff-protection is much quicker than E/F protection, but for a trip on both (51G and 87) I would suspect an instantanious setting on the 51G.

The Primary and Secondary currents seems almost balanced in all three phases, so I think the 51G-trip was due to a slight unbalance between the phases,(51G normally in the residual connection) which was above the pick-up setting of the 51G.

Regards
Ralph

 

[etgbccc2003]

sorry,

As the event was deleted by someone, I couldn't get the ground fault values in time. This is a synchronous motor.

Now, we may assume it as a fault at the bushing and assume that the current in the secondary CTs are due to motor contribution.
However, what's the significance of the phase shift of -120deg and additional 20deg shift in yellow phase? I think it may tell it all but I can't really explain.

Thanks a lot for all your comments.

 

[etgbccc2003]

Also,
the transformer is DY1 with NGR at the star point of secondary.


There's no signs of damage in the primary linkbox. (Clean and dry)

Thanks.

 

[davidbeach]

When looking at the phase shift, consider that the motor became a generator during the fault and that there were two sources feeding the fault. The nature of the fault and its impedance could account for the additional phase shift.

 

[RalphChristie]

Am I missing something here?

etgbccc2003:

The values you've given were:
IA = 1710 deg 0
IB = 2180 deg 142
IC = 1642 deg 241

Ia = 4267 deg 89
Ib = 3915 deg 215
Ic = 3586 deg 329

Primary
Now for normal phase-rotation, ABC, anticlockwise:
A=0°, B=120° and C=240° which are almost the values you've given. (Your values are A=0°, B=142° and C=241° - B is 20° lagging from the normal position which is a little strange)

Secondary
For a Dy1 transformer, the secondary lags the primary with 30°
Thus:
a=30°, b=150° and c=270° and your values are a=89°, b=215° and c=329°
All the phases are 60° lagging from the normal position.

fault conditions
Under normal conditions the current is lagging the voltage between 0° and 30°. (cos 30° = 0.866)
For a three phase fault on a distribution system the current will lag the voltage with between 35° and 60°, this value is even higher for transmission systems (up to 80° lagging)

This values you've provided seems to be fine for a three phase fault????

Regards
Ralph

 

[etgbccc2003]

Thanks all for your valuable discussion with this topic.