sizing a capacitor in an unusual situation

[VINSA01]

I would like your comments on the following case:


1.- In a new comercial building (like a hotel for eldery people) the maintenance people detected a money penalty of the electric utility for ?low power factor? ( 0.34 ).

2.- The building is new and by the moment almost no electric load. They have a 750 KVA substation (23 Kv to 220 / 127 V) and from 6 to 11 kW m?ximum demand.

The ?low power factor? sense by the utility was due to the transformer magnetization current

The load of 6 to 11 kw was almost lightning and some contacts so NO induction loads


The Capacitor vendor is saying that in order to shift fro 0.34 to 0.95 the p.f. he will recomend a 30 KVAR capacitor capacity ( not automatic)


K=TAN(ACOS(0.34))-TAN(ACOS(0.95))= 2.437


KVAR= 2.437 X 13 KW = 31.7 KVAR


Questions

1.- Whats your opini?n about the size of the fix capacitor ? . Incidently this low load situation will probably last 1 ? 2 years

2.- We are planning to put to work just half of the 30 KVAR capacity and check the p.f. in the utility meter.

3.- If so, is it advisable to design and install now an automatic capacitor and add some modules as they are needed . Say start with 30 KVAR up to 60 KVAR ?

Regards

 

[petersonra]

At a 6kw load, and a 0.34 PF, you have about a 16.5 kvar inductive load.

At 11kw and .34 PF, its about 30 kvar.

Does the hotel get penalized for having a leading PF? If not, I would be inclined to accept the manufacturer's suggestion.

Another solution to the problem might be to take the huge substation you don't need offline and install a smaller transformer that more closely matches the expected load for the next few years.

 

[zbang]

Also consider the cost and payback time of the capacitor installation verses the low-PF charges.

Also, where is this? 220/127 V isn't common in the US.

 

[VINSA01]

Thanks for your commnts

It is not feasible to use a small transformer temporatry

I guess there is no penalty for p.f leading

the 220 / 127 V is one of the standard voltages in Mexico

I will put half of the 30 kVAR capacitor because I expect to have around 5 kW max at day hours and around 3 kW at night

I will put the other half in service as the load increases

Thanks again

 

[winnie]

Another solution to the problem might be to take the huge substation you don't need offline and install a smaller transformer that more closely matches the expected load for the next few years.

IMHO you should be giving strong consideration to this point. That huge substation doesn't just have low power factor; it also has significant real power consumption. For the duration you are paying for all of the losses associated with keeping that large transformer magnetized. Of course, you may want to keep it energized for any number of reasons, including self heating to keep it dry, or keeping it energized to discourage theft.

-Jon

 

[broadgage]

As posted above it may be desirable to keep the large transformer energised to keep it dry or deter theft.
If this is not the case, I would seriously consider taking it off line in order to avoid paying for the losses.

Since the load at present is so small, there may not be any need to install a smaller transformer.
A supply of only a few KW may be available at your ultilisation voltage of 127/220, direct from the utility.
Unless your building is in an isolated area, I would expect modest capacity 127/220 volt supplies to be readily available for homes and smaller busines premises.

If you do decide to keep the transformer energised, then I would select the larger capacitor bank (provided that there is no penalty for leading power factor)
As the load increases in future years, the power factor should increase since the transformer magnetising current will be a smaller part of the load.
However the larger load at a better power factor will still need more correction than the present small load.
A larger capacitor will therefore be more future proof.

 

[petersonra]

I will put half of the 30 kVAR capacitor because I expect to have around 5 kW max at day hours and around 3 kW at night

I don't know what you are paying for electricity in mexico but at an average load of 4 kW, here in the US it is likely you would be paying less then $2500 a year for the electricity. I can't imagine the PF penalty is high enough that there would be a positive payback on a small capacitor bank like this.

 

[VINSA01]

test of the capacitor

test of the capacitor

Today we put in service the 30 KVAR capacitor

From the Utility meter there was no change . As it is located on the primary of the 750 KVA transformer the p.f reading 0.33 remains

On the secondary side ( 220 / 127 V) the display indicates p.f. 0.95 ( no inductive loads and power change 3 , 6 and 12 KW

After conecting the capacitor (full 30 KVARS) the secondary display went capacitive

Fase A 0.09 to 0.11
Fase B 0.10 to 0.09
Fase C 0.05

That means a leading angle > 85? !

Question : If the utility do not charge for leading p.f , is it convinient to put the capacitor on service ? or later when inductive loads require of it-

After that we switch off the capacitor

Tomorrow we will turn on all the inductive load

With and with out capacitor

If the Utility meter still doesnt change we may ask for a meter check and if it is the case to change it

Regards

 

[VINSA01]

almost end of the story

almost end of the story

Today we put on service like 1/4 of all the A/C equipment . The load increased from 12 to 45 kW and still the Utility meeter stayed indicating the same numbers

35 kwh
100 kVarh

The Capacitor vendor recomended to leave the 30 kVAR capacitor on despite the fact of f.p. readings "capacitive" 0.010 averarge

He says ( and I think he is right) that we will see changes in the differencial readings of the utility meeter one month from now as follows


Actual readings 35 KWH y 100 KVARH
Forecast readings one month from now 50 KWH y 105 KVARH
Differences: 15 KWH y 5 KVARH

The expected p.f. sensing from the utility meter

Cos((angtan(5/15))= 0.95

Coments ?

 

[chircapma]

Also consider the cost and payback time of the capacitor installation verses the low-PF charges.

Also, where is this? 220/127 V isn't common in the US.

I have the same idea with you. Great!
Glad to hear you're using this: I plan to keep it much more aggressively up-to-date than has been the case in the past, but don't hesitate to let me know if you find errors or need clarifications.

 

[jrannis]

I would like your comments on the following case:


1.- In a new comercial building (like a hotel for eldery people) the maintenance people detected a money penalty of the electric utility for ?low power factor? ( 0.34 ).

2.- The building is new and by the moment almost no electric load. They have a 750 KVA substation (23 Kv to 220 / 127 V) and from 6 to 11 kW m?ximum demand.

The ?low power factor? sense by the utility was due to the transformer magnetization current

The load of 6 to 11 kw was almost lightning and some contacts so NO induction loads


The Capacitor vendor is saying that in order to shift fro 0.34 to 0.95 the p.f. he will recomend a 30 KVAR capacitor capacity ( not automatic)


K=TAN(ACOS(0.34))-TAN(ACOS(0.95))= 2.437


KVAR= 2.437 X 13 KW = 31.7 KVAR


Questions

1.- Whats your opini?n about the size of the fix capacitor ? . Incidently this low load situation will probably last 1 ? 2 years

2.- We are planning to put to work just half of the 30 KVAR capacity and check the p.f. in the utility meter.

3.- If so, is it advisable to design and install now an automatic capacitor and add some modules as they are needed . Say start with 30 KVAR up to 60 KVAR ?

Regards

Eaton has an interesting calculator on its site:

http://www1.eatonelectrical.com/calculators/PowerFactorROI/index.html