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Power-User

Join Date: Jul 2011
Posts: 133

### Bus Under Voltage

07/27/2012 5:59 PM

we have a network of 6.6KV distribution system fed from 132 KV grid station through a 132/6.6KV power transformer.the 6.6KV bus has an undervoltage relay whose setting is 85% of 6.6kv i.e U/V setting =5.61KV.

the transformer tap changer does regulation of 6.6KV secondary voltage between 118KV and 145KV primary grid voltage ( i.e. between taps 1 to 27).below 118KV on the primary side of transformer, the secondary 6.6KV side voltage decreases linearly according to the V1/V2=N1/N2. relation. now according to the 85% setting of 5.61KV on our 6.6KV bus under voltage relay, the primary voltage should be 112KV i.e. 85% of 132 KV. but we have seen that even if the primary voltage falls as low as 103KV , no tripping of the 6.6KV bus load feeders and incoming feeder occurs which is very strange. v have checked our under voltage relays and replaced them but no problem is found with them. i do not know the justification of no bus tripping below 112KV on primary side of transformer to provide.

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Guru

Join Date: Mar 2007
Location: at the beach in Florida
Posts: 5427
#1

### Re: Bus Under Voltage

07/27/2012 6:20 PM

What is the duration of time set at? How long were you at 103kv? Is this instantaneous trip? Was it working before?

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Guru

Join Date: Jun 2010
Posts: 531
#2

### Re: Bus Under Voltage

07/27/2012 7:29 PM

Maybe you can educate a dumb ole mechanical engineer, but it seems to me that if the tap changer regulates the 6.6KV down to 118KV on the primary, that the trip would not switch until 85% of 118KV. 118KV * .85 = 100.3KV

Good Answer (Score 3)
Guru

Join Date: May 2010
Location: Liverpool, NY
Posts: 567
#3
In reply to #2

### Re: Bus Under Voltage

07/27/2012 9:44 PM

Actually, bigg is onto it. If your tap changer's 27 taps will compensate from 118kV to 145kV, then you will have 6.6kV on the secondary throughout that range of primary voltage. You need to get down to 85% of the lowest tap setting for your relay to operate. But throughout the range of the tapchanger, if it's doing its job you will have normal secondary voltage.

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Power-User

Join Date: Jul 2011
Posts: 133
#4
In reply to #2

### Re: Bus Under Voltage

07/28/2012 9:18 AM

time set for undervoltage relay is 6 second and is definite time.the grid voltage remains at 103KV for even 30 minutes.

85% of 118KV ?? bigg think u do not understand what does the rating 132/6.6KV mean for a transformer.

Guru

Join Date: Aug 2010
Location: Under the Major Oak
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#5
In reply to #4

### Re: Bus Under Voltage

07/28/2012 10:25 AM

I don't think you understand the function of a tap changer.

85% of 118KV is 100.3KV which is lower than your observed 103KV. The 6.6KV U/V relay should not operate.

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Associate

Join Date: May 2011
Posts: 42
#6
In reply to #4

### Re: Bus Under Voltage

07/29/2012 12:07 AM

Dear Coolyaar

Why are you not monitoring the secondary voltage of transformer? I think your transformer is designed to provide 6.6 KV while the tap changer is in its lowest taping and input voltage is 118 kv. Your relay will operate only if the secondary voltage goes below 5.61 kv.

Guru

Join Date: Jun 2011
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#7

### Re: Bus Under Voltage

07/29/2012 1:22 AM

Like TonyS said and Satendarkumart suggests.

If your secondary voltage isn't getting below 5.61kV then the U/V relay won't trip.

Measure the actual voltage that is being presented to the relay.

Maybe you want it to trip earlier in which case change the set point.

"Sensible" measurements will tell more. Plenty of times when I've kicked myself for not checking something basic (especially when getting to the location was 90% of the battle)

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Active Contributor

Join Date: Mar 2011
Posts: 17
#8

### Re: Bus Under Voltage

07/29/2012 4:21 AM

As Mr.bigg and Mr.satendrakumart pointed out the UV relay wont trip above 5.61kv.I will put it this way.When tap changer @ position 27,with primary votage of 118 KV the secondary voltage will be 6.6 KV When primary voltage is 103 kv and @tap setting at 27 the secondary voltage is 5.76 KV.As the relay is set to trip below 5.61KV the relay will not trip. If you want to really test the relay fucntion, you may set the tap switch at higher tap when the voltage is low so that you get secondary voltage below 5.61 kv and then observe

Member

Join Date: Dec 2011
Posts: 7
#9

### Re: Bus Under Voltage

07/29/2012 9:51 PM

Mr.coolyar please send the schematic lay out of the protection being used so that we can give you sure remedy since we are using the same bus system with same bus protection ****** kathirasan engineer electrical mtce.

Active Contributor

Join Date: Dec 2010
Location: visakhapatnam, India
Posts: 13
#10

### Re: Bus Under Voltage

07/31/2012 9:20 AM

Can you provide the following details.

what is the PT ratio?

Do you have ON Load Tap Changer controlled by a suitable device? if so what is the settings in that device?

Off Topic (Score 5)