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Member

Join Date: Oct 2012
Posts: 7

Temporary Increase in Fault Level

10/11/2012 12:59 AM

There are two 40MVA 220kV/6.6kV transformers feeding 6.6kV loads separately. The Bus Coupler is normally OPEN. To maintain system availability , the Bus Coupler is closed for a few seconds ( 5 seconds) before tripping any Incomer breaker during maintenance outages. This increases the fault level temporarily. My query is - Is this a safe practice ? Is there any Rule (India) that allows /prevents this ? If we do this , what should be the equipment rating ?

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Guru

Join Date: Oct 2007
Location: India
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#1

Re: Temporary Increase in Fault Level

10/11/2012 3:35 AM

Mine is a commonsense answer. The Bus Coupler should close (300 Milli seconds) before tripping any Incomer breaker during maintenance outages. This time (300 Milli seconds) should be so set as to be less than fault level breaker setting. Regret, I am not knowledgeable about code in India.

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Guru
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#2

Re: Temporary Increase in Fault Level

10/11/2012 5:28 AM

You have to work out the consequences and chances of a major fault in those 5 seconds against the consequences of several power outages during the switching procedure. I've all ways done the switching live because of the chaos an outage would cause.

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Guru

Join Date: Jun 2007
Location: chennai,India
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#3

Re: Temporary Increase in Fault Level

10/11/2012 11:00 PM

Please provide the Z of 40 MVA transformer and the rupturing rating of 6.6 kv switchgear. Without checking the capacity of switch gear and cables closing the bus coupler will be risky

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Power-User

Join Date: Nov 2011
Posts: 131
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#4

Re: Temporary Increase in Fault Level

10/12/2012 12:16 AM

Why 5 seconds? During momentary parallelling (say for 1 sec) when both the incomers and the bus coupler will remain closed, you can trip the selected incomer through a 'trip selector' switch. This will ensure uninterrupted power supply to both the bus sections while taking one incoming feeder for maintenance.

As a good engineering practice the breaking capacity of the 6.6 kV circuit breakers should be in line with the 6.6 kV bus fault level considering parallel operation. However, some people take calculated risk considering the rare possibility of occurrence of fault during momentary parallel operation.

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Member

Join Date: Oct 2012
Posts: 7
#5

Re: Temporary Increase in Fault Level

10/12/2012 2:45 AM

@ramvinod

%Z is 12 % and Isc making is 63kA and Isc breaking is 25kA

@debata

Trip selector is like taking the Bus coupler close contact to trip a pre-selected Incomer or something else ??

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Power-User

Join Date: Nov 2011
Posts: 131
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#6
In reply to #5

Re: Temporary Increase in Fault Level

10/12/2012 8:03 AM

Yes, you are right. The NO contact of the bus coupler breaker is used in the tripping circuit of the pre selected incomer.

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Active Contributor

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#7

Re: Temporary Increase in Fault Level

10/13/2012 12:50 PM

This is a normal practice. However, to find the ACTUAL RATINGS OF THE CIRCUIT BREAKERS, the fault levels on the 6.6kv bus-bars for two options( paralleling and non paralleling of the transformers) will have to be used.

Once you are able to determine the CURRENTS flowing during through the CIRCUIT BREAKERS during fault conditions, this can then be used to consequently compute the SHORT TIME CURRENT and FAULT LEVEL (in MVA) on each CIRCUIT BREAKER(220kv or 6.6kv).

These FAULT LEVEL computations are necessary to determine correct EQUIPMENT SPECIFICATIONS.This is needed so that damage to equipment during fault conditions could be prevented.

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Guru

Join Date: Oct 2009
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#8

Re: Temporary Increase in Fault Level

10/21/2012 9:31 AM

Momentary Paralleling is NOT allowed in India, if the Breakers & the Bus bars are not sized for the combined short circuit contribution from both the sources. I have had problems in the past.

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Users who posted comments:

akpanda74 (1); debata07 (2); electricalexpert65 (1); Joshi (1); Olu Gyibrilla (1); ramvinod (1); TonyS (1)

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