Multiple Electrical Faults at Plant in Island Mode

Sounds like a lightning strike....

I would guess a now vaporized critter gained access to something they shouldn't have touched. After that a cascade of faults happened.

We need to investigate root cause of this cascaded failure . .

The termination of one of cables of Left most phase of Plant-II feeder had sign of flash.

The EPR insulation had crack at location near core copper screen end which caused electrical flash at Plant-II feeder.

The transformers were badly damaged but the link between termination failure and transformers failure needs to established . .

<...need to investigate root cause of this cascaded failure...>

CR4 is not a permission-giving entity nor does it embrace <...need...>. The <...We...> can do whatever it wants without further reference.

OK. Something unintended gained access to the <...Plant-II “B Bus-bar” feeder...>. Whether it was a creature, a tool or a loose engine dropped off a passing Boeing 747 aircraft cannot be seen from here. That's why this answer is an <...estimate...>.

Quite a spectacular firework party, one imagines?

Can you please estimate the cost to repair.

We have done new termination of MV cables for Plant - II Feeder, replaced current transformers. One of 1000KVA transformers is beyond repair. If core is healthy then we will get other 1000KVA transformer rewinded

Undefined: <...We...>; this particular CR4 subscriber is not involved.

That will be difficult without knowing the currency in which the repair is to be embraced!

1. The 1-line diag does not show any protective circuit breakers for the secy of the transformers.

2. The tripping by Buchholt was likely caused by vaporisation of trfmr oil by unprotected high current within your trfmr secy that got them burnt. Buchholt does not protect against fault current!

3. Your trfmr secy high current can happen after Fault 1 only if somehow they are supplying the short ccted fault (the 1-line diag does not suggest any explanation) and were unable to trip because there were no protective cbr.

4. If the 1-line diag is exactly as shown, the design of the protection of your installation is suspect. Any shortcircuit fault on your services will cause your trfmrs to blow up. Your system is not protected against this failure mode because there are no cbr to do so. Knowing whether it was a squirrel or a spanner that caused Fault 1 will only help you prevent them from entering the switchboard in the future; not going to protect your trfmrs.

5. I would be interested to hear more about your further investigations.

Thank you for your reply.

Fault 1 is just failure of termination of MV cable. The flash over between conductor and core copper screen . .

The detailed single line is as follow:

Can't see any detail in the diagram. I m expecting you to take my suggestions and run with it, assuming you are an electrical engineer. If you are not, then you really need a electrical power engineer on site to work with you.

Cheers

Upgrade your transformer protection scheme...

https://electrical-engineering-portal.com/download-center/books-and-guides/power-substations/transformer-differential-protection-scheme

https://www.gegridsolutions.com/smartgrid/Mar07/article5.pdf

Good one Solar Eagle.

Would you recommend deferential protection for 1000KVA distribution transformer . . it does not appear to be economically justifiable protection

Thank you for your prompt reply, our comments are :

1) You are right. The transformer secondary side does not have a protective circuit breaker. The individual outgoing three phase motor feeders at transformer secondary have their own protection (fuses). Any fault which is at primary, secondary or anywhere upstream of the outgoing feeders is isolated through the primary side circuit breaker which has Overcurrent (51), Short Circuit (50) and Ground Fault protections (51N). We understand that a ground fault at transformer LV side will not be seen by the MV breaker ground fault protection because of the delta connection on MV side. What is the general practice for protection against ground faults at LV side with Dyn11 vector group transformers?

2) Yes. Buchholz relay probably tripped as a result of vaporization / explosion which was too slow to protect the transformer from permanent damage.

Thx for the extra info. Now i am curious: How come the fuses didnt protect the trfmrs?

One general principle: cbr/fuses only protect items above it.

Yeah, you need to find out why the fuses fail to protect the trfmr secy. Are the fuses far away from the trfmrs? If the high fault current is before the fuses, that can explain why the fuses didnt blow and the trfmr secy got fried.

Hmmm!

Buchholz/Sudden pressure relays are the most sensitive transformer protection for oil filled transformers, they did not work as they should have.

One thing to remember about 51N relays is that due to CT saturation, you cannot reliably detect fault currents below the CT primary rating, regardless of relay setting. So a 51N relay using 600/5A CTs will not reliably trip for fault currents less than 600 amperes primary, even if set at 0.5A pickup, which you would expect to pick up on 60 amperes of fault current.

The upstream 50/51 relays need to be set to protect the transformer for through faults, typically called the ANSI point, by design to be for your Class II transformers, 25 times rated winding current (primary) for 2 seconds, your relays need to clear the fault before this. The slope of the thermal damage line is linear plotted on log-log paper, the next point is 11.3 x rated current for 10 seconds.

Most industrials have converted to high resistance grounding for balanced 3 phase loads, where one phase can be grounded and not interrupt operation. A ground resistor sized to allow slightly more than the system charging current is inserted in the neutral line. The resistor has taps, so that a portion of the resistor can be shunted, using a pulsing contactor. A large clamp on galvanometer can be used to trace the location of the ground fault by observing the amplitude of the meter, the conduit or cable that you clamp with the highest fluctuation is the feeder with the fault.

A voltage relay across the resistor tells you whether you have a ground fault, so you know to go looking, and repair it before it goes phase to phase, or double phase to ground. A GE Engineer, I think Walter Bloomquist(?) came up with this, so sometimes his name is used in conjunction with the design.

At higher voltages, the charging current becomes high enough so this is not a practical method, but on your low voltage side, it is an easy conversion that I have done several times on existing systems, and have operated the system afterwards.

Sudden pressure relays ANSI 63 are the most sensitive fault protection available, so it is likely that all of the subsequent failures were a result of that catastrophic transformer failure. Usually the Bucholz relay pops and there is no other indication of damage until you TTR or hipot the windings, or test the oil. High voltages would not be unusual as a result of clearing the fault, or, as lightning, the cause of the fault.

i didn’t see single line with the protection shown, it would be unusual for anything but 2 or 3 phase over current relays on a 1000kVA transformer feeder, set to protect for through faults, and coordinate with downstream devices.

Arcing faults can appear as load, so the potential for a huge amount of damage before the protection reacts is easily possible. You need just the right conditions to cause great damage. A good thing to recognize is it takes about 400 amperes to sustain an arc at 480V. The damage from that before it devolves into a fault that can be sensed can be enormous.

If a large amount of your fault current is from local generation, that can also complicate things if the generator trips off early, or doesn’t have field forcing for the excitation to support external fault sourcing. The fact that you have pilot wire protection implies distance between components, which then can attenuate current flow under fault conditions, increasing the potential for arcing faults.

nice work with your documentation.

Estimate: <...Fault 1 is just failure of termination of MV cable...>

The ring shaped crack in EPR insulation at point near copper core screen was observed . . this may be initiation of cascaded flash/faults.

Kashif, as much asr your problem is fun to conjecture, it would seem to me that you are not an engineer and therefore tend to jump at a "solution" without engineering analysis. We cannot risk giving you such "solutions".

If your system can be in island mode, i would conjecture that it is not extensive distance-wise. You cannot NOT protect your 1MVA trfmrs. You already know the repair/replacement cost of the trfmrs. Protection is always only a fraction of their cost. An electrical engineer on site will be able to decide whether you need more than the minimum, which is merely an ACB with the right fault level. If your services are exposed freqly to phase to ground faults eg trees falling across the lines, you may need the protection shown by SolarEagle, including the circuit breakers of the right fault level!

In short, as lyn had suggested, get on-site professional advice; this forum can only do so much. You have to protect your generators and trfmrs because failures are bound to occur and destroy unprotected expensive equipment.

Happy new year.

I find it hard to believe that the Bucholz sudden pressure relays cleared the fault, your damage of the transformers should be undetectable from the outside. The flag on the diaphragm may be showing, but it did not do the job it is designed to do., for whatever reason. Misoperation is not unheard of, so they may have been disconnected.

Also, you need to look at your relay coordination and your generator over-current protection, when your available fault current is sharply reduced, islanded. Voltage restrained overcurrent relays allow higher currents if the terminal voltage is low, to provide enough fault current to assure positive tripping of downstream protection. Remember the 400A arcing fault current, that's why the excitation and overcurrent protection is important when islanded.

Impossible to determine the cause of the failures, however I would be interested to know why a Buchholz relay tripped a Tx!

I would first go around the plant using the two sense you were born with, you eyes and sense of smell.

sound like old cables and they have failed, maybe rats!!
Best of luck sorting this one out!

As much as your concern is amusing to guess, it would appear to me that you are not a designer and along these lines will in general seize an "answer" without building examination. We can't chance giving you such "arrangements".

On the off chance that your framework can be in island mode, I would guess that it isn't broad separation astute. You can't NOT ensure your 1MVA trfmrs. You definitely know the fix/substitution cost of the trfmrs. Security is in every case just a small amount of their expense. An electrical architect on location will have the capacity to choose whether you require more than the base, which is only an ACB with the correct blame dimension. In the event that your administrations are presented freqly to stage to ground flaws eg trees falling over the lines, you may require the assurance appeared by Solar Eagle, including the circuit breakers of the correct blame dimension!

To put it plainly, as lyn had recommended, get nearby expert exhortation; this discussion can just do as such much. You need to secure your generators and trfmrs on the grounds that disappointments will undoubtedly happen and decimate unprotected costly hardware.