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user-deleted-1076
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Join Date: Oct 2010
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VCB Tripping (Siemens)

05/25/2011 12:18 PM

Recently we have commissioned a group VCB, rating is 150A VCB (SIEMENS MAKE) with 7SR110 SIEMENS make multifunction realy (overcurrent & earth fault, etc.,), the outgoing of the breaker is connected to six pole structure from the structure AB switch its goes to individual VCB's of two 750kVA transformers. what was problems we have faced is, when ever supply resumes the above mentioned 150A VCB got tripped showing over current in the different phases example like

Phase A - 0.163kA - Over Current LED glows

Phase B - 0.120kA - NA

Phase C - 0.210kA - Over current LED glows

but we have tharouly checked the all connection, end terminations & cables every thing is fine and found healthy. the CT ratio of VCB is 150:5, maximum load on HT side is 90A (that too in peak hours), one more thing when supply resums there will be no load connected to VCB, still i am wondering about the over current values recorded in the relays?

some times the VCB will work fine at the time of supply resume, some times the relay records some abnormal overcurrent values and leads to the breaker to trip?

if any one find solution please convey me*******

with lot of hopes

shaju

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PeterT
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#1

Re: VCB Tripping (Siemens)

05/25/2011 2:04 PM

You did not provide information on the trip settings (pickup and time delay) programmed into the overcurrent relay. However, with a 150A circuit breaker tripping at 163 or 210 amps, I would assume that the setting must be such to limit current to less than its rating.

Did someone do a coordination study to develop the relay settings? Was the inrush current of the two 750kVA transformers (10-12 x Full Load Amps @ approx 0.1 second) taken into account in the settings? That sounds like what is happening - your breaker trips on the inrush current when you close in on the two transformers. They don't need any downstream load for this to happen - it's mainly the excitation of the core & windings that causes it.

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user-deleted-1076
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#2
In reply to #1

Re: VCB Tripping (Siemens)

05/25/2011 2:17 PM

thank u for your quick response sir

sorry Mr.Peter T, i forget mention that settings, the delay time is 0.001s and the current setting is full load current (Ifx1). wheather it is possible that, transformer inrush curent will go upto 210amps? for 2x750kVA transformer.

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pwr2thepeople
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#3
In reply to #2

Re: VCB Tripping (Siemens)

05/25/2011 4:45 PM

shajahan, I believe PeterT is correct. You indicate the time delay setting is 1 millisecond, which is 1/20 of a cycle for a 50 Hz system. It takes up to 5 cycles for the core iron to become fully aligned with the new magnetic field. The false trip will not occur every time. It resultant current depends on the phase angle between the applied current at the instant the supply returns and the alignment of the residual magnetic field in the core steel.

I understand the desire to protect your equipment with fast trips, but your trip setting of full load current at 1 millisecond is unrealistic and is the cause of the false trips.

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user-deleted-1076
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#4
In reply to #3

Re: VCB Tripping (Siemens)

05/25/2011 10:10 PM

then, shall i increase the time delay settings to 1 second, if i increase the delay time any kind of cause will harm the VCB?

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V.I.Abraham
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#7
In reply to #4

Re: VCB Tripping (Siemens)

05/26/2011 12:04 AM

Shajan,

The advises of Mr.Peter and pwr 2 the people are quite correct and you should go for the relay coordination and proper resetting as there is no other defect noticed in the system. Normally these false trips do not damage the VCB as they are not tripping on prolonged over load condition or instantaneous or permanent earth fault.
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electricalexpert65
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#8
In reply to #4

Re: VCB Tripping (Siemens)

05/26/2011 7:50 AM

I definitely feel that the tripping is due to the transformer inrush current. The inrush current can very well reach even much beyond the values specified by you. But, to avoid tripping during inrush, one cannot increase the current setting or the time setting in the relay which could be harmful during an actual fault in the system.

Pl. check if the relay has got a Auto-Doubling or Cold Load Pick-up Feature. I'm sure that SIEMENS 7SR relays must have it. But, pl. check. If it is available, please enable this feature.
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Tornado
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#5

Re: VCB Tripping (Siemens)

05/25/2011 11:57 PM

On the basis of numbers given so far, 0.1 second of delay would seem sufficient.

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hien.nguyenquoc
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#6

Re: VCB Tripping (Siemens)

05/26/2011 12:03 AM

Wait a minute!

This is so new to me. Do we have kind of 150A VCB?

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electricalexpert65 (1); hien.nguyenquoc (1); PeterT (1); pwr2thepeople (1); Tornado (1); user-deleted-1076 (2); V.I.Abraham (1)

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