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Capacitor Bank Connected Parallel to Motor

08/18/2014 11:09 AM

Dear Experts,

We have a capacitor bank connected parallel to the Motor for PF correction.The capacitor bank has fuse protection.The Motor Protection Relay is provided in the Motor feeder.If the fuse(s) of the capacitor fails, what will be the effect on the Motor? what will be unbalanced currents / voltages produced? what should be the settings of the Negative sequence protection / unbalance protection of the motors? On searching the Internet, the normal unbalance protection is set as 15% of FLA. On what basis it is set at 15%? I think even if fuses for PF Capacitors are not provided, the Motor protection relay will act in case of faults in capacitor bank. Does unbalance happen if two or three fuses of capacitor bank are blown?

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#1

Re: Capacitor Bank connected parallel to Motor

08/18/2014 11:13 AM

What is the capacitor bank there for, again?

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#2

Re: Capacitor Bank Connected Parallel to Motor

08/18/2014 1:03 PM

Please provide the details of Motor and capacitor bank

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#4
In reply to #2

Re: Capacitor Bank Connected Parallel to Motor

08/18/2014 9:33 PM

Motor Output: 1950KW

Motor rated voltage: 6KV

FLA: 243A

Capacitor Bank rating: 600Kvar (200 x 3) with 6% series reactor

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#3

Re: Capacitor Bank Connected Parallel to Motor

08/18/2014 9:31 PM

From experience I'll tell you what happens if a single fuse blows. The motor will trip on un-balanced load. Added to which the motor will sound awful.

If two or three blow then the motor is liable to trip on O/C but that depends on how sensitive you have the O/L's set.

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#5

Re: Capacitor Bank Connected Parallel to Motor

08/19/2014 10:24 AM

CApacitor bank fuse will not make any effect on Motor. Unbalance protection of 15% is as per international standard (IEC)for the Voltage and Motor manufacturing. How did you you say unbalance if fuses of capacitor bank blown.

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#7
In reply to #5

Re: Capacitor Bank Connected Parallel to Motor

08/19/2014 11:03 AM

Dear Sir,

Could i know the IEC reference code relating to the unbalance setting?

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#8
In reply to #5

Re: Capacitor Bank Connected Parallel to Motor

08/19/2014 11:11 AM
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#10
In reply to #5

Re: Capacitor Bank Connected Parallel to Motor

08/23/2014 7:22 AM

I said because the line current differs on account of vars reduction from the capacitor.

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#6

Re: Capacitor Bank Connected Parallel to Motor

08/19/2014 10:37 AM

It is quite likely each capacitor has internal fuses apart from fuses at the main connection for the entire bank.

Internal fuse blows in case of a defect in individual capacitor unit and the main fuses blow only in case of a major fault (which is very rare). A few faulty units (typically 3nos) should not cause much concern except a slight increase in voltage across the remaining units and the capacitors are sized with adequate margins to withstand overvoltages.

Thermal overload protection of the motor protects the capacitor bank as well assuming the CTs are located upstream of motor/capacitor bank connection.

Blowing of main fuse is definitely a major concern and will give rise to operation of negative phase sequence protection to isolate the motor/capacitor bank.

Setting of 15% for negative phase sequence protection of motor has the basis in the motor withstand capability and can be verified with the motor data sheet / vendor information.

Trust the above is helpful.

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#9
In reply to #6

Re: Capacitor Bank Connected Parallel to Motor

08/23/2014 4:07 AM

Could you please tell me what happens to the current and voltage at the Motor terminals when

1) Single fuse blown

2) Two or three fuses blown and

How the Motor Protection Relay located at the Motor feeder sees it?

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