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Busbar short circuit

04/17/2022 8:19 AM

Dear all

on medium voltage busbar when three phase short circuit to earth happened all

voltage transformer on three phase are damage,my question is "if the voltage at earth

fault become zero why the voltage transformer damaged "

regards

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#1

Re: busbar short circuit

04/17/2022 9:25 AM

"...why the voltage transformer damaged "

Many amperes flowing...

All the voltage drop is in the transformer windings, I = V/RWindings

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#3
In reply to #1

Re: busbar short circuit

04/17/2022 11:37 AM

but when voltage decrease the current also decrease Based on the law I = V/RWindings

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#4
In reply to #3

Re: busbar short circuit

04/17/2022 4:44 PM

Your bus bar is not made of superconducting material.

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#10
In reply to #3

Re: busbar short circuit

04/18/2022 8:21 AM

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#2

Re: busbar short circuit

04/17/2022 9:32 AM

Look at it this way. All of the energy that the transformer could provide downstream was instead briefly released into the transformer. Usually, only a few percent of that energy is released into the transformer.

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#5

Re: busbar short circuit

04/18/2022 12:15 AM
Re: busbar short circuit
You can simplify it this way: just like the terminal voltage, V of a battery is less than the EMF, E by the internal voltage drop i.e. V = E - I*r (r is the battery internal resistance), for V to become near zero, I (current) would have greatly increased. Apply this analogy to the transformer and you'll see that the low output voltage is due to a huge flow of current caused by the short circuit. The transformer windings cannot withstand this current, that is why they burn if no protective device activates. Note that the short-circuit is happening downstream of the transformer i.e. after the transformer; if it happens before the transformer then damage can only occur if there are major fault current contributors downstream of the transformer (motors, other generators) since their contribution will flow through the transformer to the fault point.

Hope this helps.

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#6

Re: busbar short circuit

04/18/2022 12:47 AM

The huge thermal and mechanical forces from the short circuit may have damaged the PTs. In addition, you may also want to check if rhe short circuit was not caused by PT failure in the first place.

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#7
In reply to #6

Re: busbar short circuit

04/18/2022 1:30 AM

thanks for all

the short circuit happened on primary side of voltage transformer (ON BUSBAR) and

this voltage transformer feed the protection relay

regards

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#11
In reply to #7

Re: busbar short circuit

04/18/2022 9:04 AM

You forgot CR4's FAQ #14:

  • Provide as much information as possible. The most frustrating questions on CR4 are like this one: "I need a sensor to test water flow - which is the best one." What are you testing for and what are you trying to determine? Is it in a stream, a pipe, or an aquifer? Are you looking to measure velocity or mass? Are you testing for particulates, mineral concentration, or pollution? Is it wastewater, potable water, slurry, etc.? The more you let us know, the better the chance that someone on the site will be able to help you find an answer.

The devil regularly resides in the unspecified details. How far away from the step-down transformer did the primary short circuit occur? Was it on the busbar terminals of the transformer, a mere few centimeters from the transformer core? Maybe the short circuit happened at another busbar in the same switchyard or was it even further upstream? What made the short circuit path and did it suffer more or less damage than this transformer? What other safety or system protection devices tripped when this failure happened? Did rain or vermin enter the wrong space?

Lastly, like a fuse, protection relay systems are installed to minimize and localize the equipment damaged by an accidental fault. They do not prevent all secondary damage from happening somewhere.

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#12
In reply to #11

Re: busbar short circuit

04/18/2022 4:03 PM

Thanks for all

I am very sorry if the information is not enough, but now I am attaching a diagram

that explains the issue.

regards

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#13
In reply to #12

Re: busbar short circuit

04/18/2022 5:40 PM

So you had an earth fault on an ungrounded system, except it was 3 phase fault to ground, otherwise no fault current. I assume your 132kV breaker cleared the fault. There is a very high voltage generated by the magnetic flux passing down through the transformer windings, as the fault current is interrupted, and nothing to limit the voltage, such as surge arresters on your generator terminals, or the generator stator windings themselves. Your PTs would be nominal 95kV BIL, so you would have had to surge arresters on the transformer secondary to protect the PTs. That would be very uncommon practice, since fault protection would normally assume your generator was running.

I doubt that any typical installation would have allowed the PTs to survive in this somewhat unusual situation.

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#8

Re: busbar short circuit

04/18/2022 2:52 AM

By voltage transformer do you mean the step down transformers that measure the line voltage and convert it to a lower metering voltage?

One probable cause is that the short circuit starts out between phases and the arcing can generate high frequency spikes which exceed the voltage ratings of the VTs. Think pilot arc transformers where a combination of capacitance, inductance and a spark gap provide a tank circuit where many times the voltage and frequency can be generated. This can be far in excess for the insulation of the VT can handle.

What starts out a a phase fault ionises the air in the fault area and propagates to other phases and earth.

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#9

Re: busbar short circuit

04/18/2022 2:53 AM

The damage to the transformers likely came from the voltage generated at the time of the fault interruption. You can look up switching transients for similar events occurring during normal power system operation. It may be that your voltage transformers were rated too low for the system operation and grounding effectiveness. For instance, on a 35kV system you might have PTs rated for 150kV BIL where the system is assumed to be solidly grounded. If the system is not well grounded, you would select 200kV BIL potential transformers.

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#14

Re: Busbar short circuit

04/19/2022 3:17 AM

The most important thing about <...three phase short circuit to earth...> is preventing recurrence.

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#15

Re: Busbar short circuit

04/19/2022 4:29 AM

<...all voltage transformer on three phase are damage...>

It would be surprising if that were all the <...damage...>.

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#16

Re: Busbar short circuit

04/23/2022 3:56 AM

I believe the fault started as a single line to ground fault that remained uncleared.

It is well known that single line to ground fault creates voltage rise on unfaulted phases stressing the insulation.

Further, an uncleared earth fault in an unearthed system can lead to arcing ground in which case, the voltage can rise cyclically to many times more than the rated (like 5 to 6 times if I remember right).

This led to failure of insulation on healthy phases too and finally causing three phase to earth fault.

I think there is need to review the generator neutral earthing philosophy and (single line to) earth fault protection scheme that is provided for 15kV system.

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