DESIGN REACTIVE POWER COMPENSATION PANEL

[Joaco M]

Hello all,

I have some doubts regarding the sizing of the protection and cables for a reactive compensation. This is a 3phase system 50hz.

The reactive power I have to compense is 100Kvar, for which im planning to put 4 capacitors of 25kvar each.

For each capacitor i will put special contactor por capacitors rated for 25kvar and a thermomagnetic circuit breaker of (25000/(400*1.73))*1.5=54A. As I have read in many articles, the breaker should be sized 1.5 the nominal current of the capacitor. The available in market is 63A so im putting those ones.

The cable for the circuit breaker and contactor and capacitor will be of 10mm2, as i have read in many acticles the cables should be sized 1.36 of nominal current of capacitor. The datasheet of the manufacturer of the cable states that on air the nominal current admisible is 60A. As its inside the cabinet and not inside any duct I assume this..

all 4 circuit breakers of the capacitors will be feed of a breaker sized (54*4)=216A and will use a infeed cable of 70mm2. The main circuit breaker is it necesary to be sized *1.5 of all nominal current ??

Is all this correct ?? make sense ?? The size of the cables are correct ?? what about the circuit breakers ??

One thing i cant understand is that if I choose the circuit breaker for a current of 1.5 the capacitor, why the cables have to be 1.36 and not 1.5 as the circuit breaker ?? because if eventually the current raises, the circuit breaker will trip later and the cable might get damaged..

Thanks!

 

[7anoter4]

See-for instance:

https://www.eaton.com/ecm/groups/public/@pub/@electrical/documents/content/sa02607001e.pdf

and NEC 2017 Art.460

 

[7anoter4]

The cable ampacity has to be more than the circuit breaker rating current.

 

[7anoter4]

IEEE Std C37.99-2000, IEEE Guide for the Protection of Shunt Capacitor Banks
7.2.3 Bank overcurrent protection
Time-overcurrent relays can be applied with normal settings without encountering false operations due to inrush currents. The desirable minimum pickup is 135% of nominal phase current for grounded wye banks or 125% for ungrounded banks. Instantaneous relays, if used, should be set high to override inrush or outrush transients.

 

[waross]

Some snippets from the Canadian Code:
26-208 Conductor size for capacitors
(1) The ampacity of capacitor feeder circuits and branch circuits shall be not less than 135% of the rated
current of the capacitor.
26-210 Overcurrent protection (see Appendix B)
An overcurrent device, rated or set as low as practicable without causing unnecessary opening of the circuit, but
not exceeding 250% of the rated current of the capacitor, shall be provided in each ungrounded conductor of a
capacitor feeder or branch circuit, unless a deviation has been allowed in accordance with Rule 2-030.
26-214 Rating of the disconnecting means for capacitor feeders or branch circuits
The disconnecting means for a capacitor feeder or branch circuit shall be rated not less than 135% of the rated
current of the capacitor.
26-216 Rating of contactors for capacitor feeders or branch circuits
Contactors used for the switching of capacitors shall have a current rating not less than the following percentage
of the rated capacitor current:
(a) open-type contactor: 135%; and
(b) enclosed-type contactor: 150%.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Joaco M]

Thanks everyone for the replies. But still having the same doubts..

Could someone give me a specific response to my case ?? and not generic..

Im using normal thermomagnetic circuit breakers curve C.. magnetic fixed to 10In

Circuit breaker for each capacitor will be rated 135% nominal current of the capacitor.

And the main line that feed all the capacitors will be protected with a circuit breaker of 135% of the sum of all capacitors.

the cable will also be sized 140% for each case.

Is this correct ??

Thanks.

 

[waross]

That will satisfy Canadian standards.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Joaco M]

Thanks Bill, one more question,

When should the circuit breaker be 135%? or 150%? or 180% ?, etc, of the nominal current ??

I mean what do I have to take into account in order to design it by 135% or another value greater without exceeding 250% ??

Thanks!!

 

[waross]

When should the circuit breaker be 135%? or 150%? or 180% ?, etc, of the nominal current ??

An overcurrent device, rated or set as low as practicable without causing unnecessary opening of the circuit,

Your 25 KVA capacitor bank will draw 36 Amps.
The available 63 Amp rated breaker is 175% of the capacitor current.
That is within the maximum 250%.
Capacitors are one of the loads for which the Canadian Code does not require overload protection.
They do require overcurrent protection, that is short circuit protection.
I understand that the 63 Amp rated breaker will withstand an inrush of 630 Amps.
That should handle the charging current of the caps.
In Canada I would use a 70 Amp breaker.
63 Amps is not a common size breaker here.


Bill
--------------------
"Why not the best?"
Jimmy Carter