Effects of leading power factor? 1

[haze10]

Need to add 200KVAR of capacitors to my 2000KVA 12.4KV-480/277 Padmount transformer. Trying to add small KVAR units to each individual motor just isn't practical as MCC space is almost non-existent, and motors are all small, like 30hp. Thinking the easiest, cheapest approach is to add two 100KVAR fused Nema 3R Cap units right onto the transformer on the 480 side. Run the leads (under 10ft) directly to the xfrm bushings.

My question, or concern, is those times when the building is down for repairs. Not often as our load factor is 86%, but when all I have is lighting load the pf may go leading. I know this will have a slight raising of voltage but other than that don't see how this would cause any harm. Am I missing something?

Also, is there a way to calculate the rise of voltage across the transformer with a leading power factor. Is it just the reduction in amps proportional to the xfrm impedance, or is there something more?

Working on the 12KV side is out, just too expensive for the MV caps, and 2each 100KVAR 480 fused cap units are relatively inexpensive and easy to install.

Comments?

 

[dpc]

It's a phasor calculation - the effect is greater than the reduced voltage drop. There's no fundamental issue with operating at a leading power factor if voltages don't go too high. Some utilities now charge for leading and well as lagging vars, so that could be a consideration. It should be simple enough to switch off the caps during planned downtime.

 

[waross]

" Run the leads (under 10ft) directly to the xfrm bushings. "
I take it you won't be calling for an inspection on this.
I have at times connected capacitors to the terminals of motor starters, ahead of the O/L relay.
If the capacitor current does not pass through the O/L relay, you can over correct the individual motors.
Another trick used in sawmills and similar plants is to over correct the largest motors on the basis that when a large motor is running you can count on a number of smaller motors to be running also.
If any motors are interlocked, so that A must be running before B may run and B must be running before C may run, you can put a lot of capacity on the last motor to start.
How have you determined that you need 200 KVA of capacity?
Back in the day when no-one would pay for a power factor controller and creative power factor correction was more art than science, we would often correct the average PF to just over 90%, 11 months out of 12, to get away from penalties.


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

 

[dpc]

I missed that bit. You have to have overcurrent protection for the capacitors.

 

[iceworm]

If it is per NEC 460:
Conductors are 135% minimum. For 100kvars, 120A x 1.35 = 162A 2/0 Cu
Disconnect is required (at least 135%) 200A
OCP is required (as low as practicable) 120A x 1.25 = 150A

So, 200A disconnect, 150A fuses.

I have only done a couple of these (refinery). The loading was pretty consistent. The only time they were turned off was when the refinery was shutdown for turn-around.

carl

Harmless flakes working together can unleash an avalanche of destruction

 

[waross]

The reason I asked about the needed capacity;
I got a call from a customer.
He was getting a power factor penalty on his power bill.
Not much but he wanted it to go away.
He already had two prices for a PF controller.
$2000 and $4000 (The second contractor would have probably subbed out to the first contractor. I guess that he sometimes got lucky.)
The actual amount of capacity needed was so little that an $8 capacitor hidden and permanently connected to a 15 Amp breaker got rid of his penalty.
dpc and iceworm, I haven't checked recently, however under the CEC motor capacitors were limited to enough to correct the PF to unity, if the capacitor current passed through the O/L relay.
If the capacitors were connected between the contactor and the O/L rely, the amount of capacity was limited by the capacity of the contactor and the disconnect.
What is currently accepted by the NEC.
Thanks in advance.

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

 

[iceworm]

Bill had a pretty good question: "How have you determined that you need 200 KVA of capacity?" I guessing you want to get the pf up over .9 to reduce penalties.

However, your numbers look okay.
50% load (1000kw) and .86pf give ~600kvars (lagging). Adding 200kvars leading will pull up the pf to over .9
75% load (1500kw) and .86pf give ~900kvars (lagging). Again, adding 200kvars leading will pull up the pf to over .9

Voltage rise is linear with transformer impedance.

%V(rise) = (C(kvars) * %Z)/(base Kva)

With a 2000kva xfm and 5% to 8% Z, and 200kvars, %V(rise) less than 1%.

(edit to add) Whoops, forgot to include a link. I like this guys paper.
Link

carl

Harmless flakes working together can unleash an avalanche of destruction

 

[iceworm]

... under the CEC motor capacitors were limited to enough to correct the PF to unity, if the capacitor current passed through the O/L relay. If the capacitors were connected between the contactor and the O/L rely, the amount of capacity was limited by the capacity of the contactor and the disconnect.
What is currently accepted by the NEC.

I'm not sure exactly what you are asking. Maybe:
NEC 2017

460.9 Rating or Setting of Motor Overload Device. Where a motor installation includes a capacitor connected on the load side of the motor overload device, the rating or setting of the motor overload device shall be based on the improved power factor of the motor circuit.

The effect of the capacitor shall be disregarded in determining the motor circuit conductor rating in accordance with 430.22.
​

The overloads are reduced, but the conductor size is still the same.

I've never seen the caps connected between the contactor and overload block. I guess one could, however one would likely not want to go leading. In this case it likely would not matter much.

carl
Harmless flakes working together can unleash an avalanche of destruction

 

[RRaghunath]

I would suggest adding a disconnector (along with fuses) and switching-off the capacitors under light load conditions.
As we know, capacitors can cause ferro-resonance in combination with the inductance of the transformer and it is a destructive phenomenon.

 

[waross]

That was the art of PF correction, Ice.
First determine how many KVARHrs per month you needed to bring the PF above 90%.
Then start looking for places to connect it.
Sometimes we could use bulk correction, connected 24/7.
That was the cheapest.
KVAR times running hours until the total reached the target.
Look for large motors and loads that typically ran 24/7, or continuously during plant running hours.
The largest motors would get 200% correction, but bypassing the O/Ls.
Every plant was different and a new challenge.
I worked in one plant where an over excited synchronous motor was used as part of the PF correction scheme.
An electrician would start it manually every morning, and manually adjust the field current. There were no numbers left on the ammeter face, just a felt pen mark to show where to set the current.
about a half hour later, the field would have heated up and the resistance would have increased, lowering the current.
An electrician would go back to the motor after it had been running for a half hour and adjust the field current back up to the felt pen mark.

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

 

[LionelHutz]

I would recommend installing a proper setup with a main disconnect. Otherwise, how can you check/replace fuses or capacitors without taking the whole transformer and plant offline?

You could also put a current sensor onto one of the phase wires to the MCC's and switch the capacitors on at a certain current level. With the new smart meters, I seen demand billing which is based on the demand kVA x a factor or the demand kW, whichever is higher. The factor is typically 0.9 so 90% of the KVA demand or 100% of the kW demand whichever is higher. So, all you need to do is switch the capacitors on-line before the kVA demand x 0.9 reaches the same level as the kW demand. It can save putting operation hours on the capacitors if your plant loading varies enough.

You should also investigate the harmonics on the system. With enough harmonics you need to de-tune the capacitors. To be safe I would be looking at installing de-tuned capacitors regardless.

You should look at some method of monitoring and flagging a failed capacitor or open fuse too. No fun getting the bill with increased demand charges before realizing you need to repair the capacitor.

I'd want to be careful taking a motor leading with capacitors. There's a big jump to go from correcting power factor to 90% lagging vs reaching a unity or leading power factor. For example, a 600hp motor might require 75kVAR to reach -0.9 power factor but about 250kVAR to reach unity power factor.

 

[waross]

For example, a 600hp motor might require 75kVAR to reach -0.9 power factor but about 250kVAR to reach unity power factor.

When applying the art of PF correction we may rephrase that.
One motor corrected to 100% PF may provide enough capacity to bring two more similar motors to 90% PF.
One motor with double the capacity needed for 100% PF may provide enough capacity to bring five more similar motors to 90% PF.
Lots of caps on one large motor will provide compensation to 90% PF for a lot of smaller motors.
It was all about getting enough KVAHrs without spending money on extra contactors and using fewer, larger caps instead of more, smaller caps.
Not too much art left.
Most plant budgets allow PF controllers, and more utilities are monitoring real time PF instead of using a monthly average.
And with the high cost of power these days, the ROI for a PF controller is less than it used to be.

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

 

[waross]

Savvy old electricians would drag their hand lightly on the side of any capacitors or capacitor banks that they passed in the plant.
If a cap was cool, it was checked for proper functioning.

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

 

[haze10]

Some are asking for the particulars so here they are:

Transformer: 12.4KV-480/277, 2000 KVA, Imp 5.5%
This is primary metered, we own the transformer (not the utility).
Manufacturing, 24hrs/day 365 days per year, with a one week shutdown per year for maintenance.
From utility bill:
KW=1000KW
KVA= 1220 KVA
Load Factor = 86%
existing pf = 0.82
Desired pf = 0.90 (pf lower than 0.9 incur penalties)
Capacitor KVAR required= 214 KVAR (200KVAR is close enough as penalty is progressive and linear further away from 0.9)

Current plan is to put two each 100KVAR Nema 3R rate 480V capacitor unit at the transformer for a total of 200KVAR. Originally, since I have a primary 12.4KV breaker feeding the transformer I was just going to attach a fused cap unit directly to the secondary bushings. On suggestions from the replies, I can see now a disconnect for each cap unit is advised. This will also permit not only routine maintenance, but also allow me to turn off the cap units during our one week annual outage. These padmounts are not typical in that there is one cabinet with high and low sides. These padmounts have the high voltage cabinet on the back end, the middle tank section, and the low voltage compartment on the front end. There is concrete pad space to mount the 100KVAR unit to the left and the right of the front low voltage ccompartment. So I'll mount a 200A Fused Disconnect on the outside with 175A fuses, supplying each 100KVAR cap unit. Wire sized per NEC 310.


Reasons for installing as described:
Motors are all small, but there are a lot of them. There is no single large motor that is significant to the low pf, its the summation of all the small ones. Trying to add KVAR to any one motor, is just not practical as at even 200% oversized, the KVAR value would be too low and too many of them would be required to make it cost effective. On top of that, bucket space and MCC space has long since been maximized. I understand that this makes a better install that is self compensating by only turning on KVAR when the motor is running, but just not practical here. Plus, we the high load factor of 86% and a 360 day/yr operation, only goal is to reduce utility penalty.
12.4KV Cap unit: Don't want to deal with medium voltage cable in this scenario as 480V side is so much easier. If the cap KVAR needed was much larger and space was an issue I would consider it. But in this case two 100 KVAR 480V caps, one on either side of the terminal compartment will work fine.

How does this sound as a plan?

 

[iceworm]

... How does this sound as a plan?

Sounds pretty good. Couple of minor points, that I think you know (refer to my posts, May 2, May 3)

If this is per the NEC, Article 460 is the guide for selecting Conductor Ampacity, Disconnect, and fuses.
And of course 240.21.B tap conductors

The numbers you have picked out meet Art 460. The 175A fuse selection is one step higher than I would have picked, but certainly well within reason.

I think you got it.


Harmless flakes working together can unleash an avalanche of destruction

 

[waross]

Welcome to the art of PF correction.
One thing, It may be required in this instance, but I never calculated PF correction based on demand values.
I used KWHrs and KVARHrs.
I couldn't depend on accurate information from the customer, so I would request the power bills for the last 12 months or more.
I would calculate the KVARHrs needed for each month to achieve a 0.90 PF. Then I would divide by the number of hours in each billing period.
Then, depending on the results and the customer/s wishes, I would add enough capacity to avoid the penalty in 11 of 12 months, or for 12 of 12 months.
You may want to do a KVARHr sanity check on one or two months.
Or not.
My first big PF job was similar in that there were dozens of small motors, yes about 30 HP.
My solution was also bulk correction.

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

 

[RRaghunath]

With the capacitors connected to the LV terminals of transformer, the current seen by HV will be smaller when compared to that of LV incomer in the switchboard (after applying the transformer turns ratio factor, of course). This is because the LV incomer sees the load current whereas, the current reflected on to HV side is after compensation of some of kVARs by capacitor banks.
The kVAR is small when compared to the transformer kVA rating and hopefully, this will not affect any of the transformer protection settings. Just wanted to mention.