Over-Capacitance and Electrical Power Systems

I don't think that the capacitors caused the problem. They merely keep the power in phase as to voltage and current. That would be necessary in a plant that has many motors running. If anything the capacitors would help to reduce load on the wires. Motors tend to cause the voltage and current to be out of phase with each other that makes the power meters used by the electrical company to under report the usage, actually you get more power for less cost. The capacitors only bring the power back into phase. In an area where there were a lot of motors running if no one used capacitors then the line power would be very much out of unity causing overheating of the inductive loads such as motors.

Dear Aikhh,

Electrical installations with power factor correction capacitors installed cannot be treated as simple as you described.

Sizing the capacitors is critical. The sizes must be chosen such that it does not create a resonant condition which will increase the current in the cable beyond its limits. This will cause heating , hardening of insulation and eventual failure.

Secondly, you must ensure that there are no harmonics on the power system.This can be verified with a power quality meter.Sometimes the harmonics are not just the 3rd . If you have variable frequency drives or other thyristor controlled equipment (which distorts the sine wave) on the plant , this can create harmonics all the way up to the 11th and 13th times the power frequency.You may even have harmonics present which are generated by neighbouring plants.

Power factor correction application cannot be treated as simply adding capacitor to remove the lagging volt-ampere reactive . Correcting to unity is a bad idea if the capacitors are always in parallel to the motor. Since during motor coastdown, there is a residual voltage generated in the motor which will decrease in frequency with the possibility of a resonant condition which will increase the current significantly.

OVERCAPACITANCE WOULD NOT EASILY OVERLOAD YOUR LINE AS REPORTED UNLESS THE DEGREE OF EXCESS WAS THAT MUCH. THEREFORE OVERLOADING WAS THE MAJOR CAUSE OF YOUR OVERLOADING. OR NOT EFFECTIVE LUGGING I.E. LOOSE CONNECTION. ALSO ENSURE THAT YOUR CAPACITOR BANKS WERE CONNECTED TO LOAD/LINE IN PARALLEL.

I don't think, "over capacitance" is the problem for insulation failure, as u are using automatic power factor controller. Over loading may be the reason, also check the cable size.

Dear spghadei,

If 75kw motor is connected parallel with 50Kvar, i am sure it is 'over capacitance' . I am not really good in precious capacitance value,normally i will use the thumb rule by kw x 0.25 .Please advise me if i am wrong.

Thank you and Best Regards

50KVAR looks to be approximately OK

Assuming that the motor is at 0.85 pf (most common)

the reactive power = √ (1-0.85²) { sinΦ = √(1-cos²Φ)}

this comes out to be approx 0.53

Also if you see from the other angle,

75KW at 0.85 pf means the total KVA = 75/0.85 = 88.235

The KVA_active = 75

so KVA_r = √(88.235² - 75²) = 46.48 KVA

In fact the reactive power is what we call the tanδ component = 0.53/0.85 = 0.62*KW

Dear Guru sb,

Thank you very much and best regards

Dear aikhh and sb,

i think 50 kvar for 75 kw motor is not the right selection.

75 kw motor 6 pole – At full load pf is around 0.86

Cosθ₁ = 0.86 (initial)

e.g. if we have to improve the power factor to 0.95

Cosθ₁ = 0.86

θ₁ = Cos^-1 ( 0.86 ) = 30.68

Cosθ₂ = 0.95

Θ₂ = Cos^-1 ( 0.95 ) = 18.19

Tan θ₁ – Tan θ₂ = 0.593 –0.328

= 0.265

KVAR required = Kw x 0.265

= 75 x 0.265

= 19.87 KVAR

REQUIRED Capacitor Specification should be 20 KVar.

Dear spghadei,

Thank you very much.

Best Regards

You have teken the second Φ₂ as pf=0.95 where as my calculation was based on expected full compensation. that's why the difference has come. Put pf2=1 and you will get the value as mentioned. .

The bank capacitors might have been designed to near unity compensation and not to 0.95.