Power Factor Correction on Highly Variable Loads 5

[rawelk]

Prologue
Our facility uses a 10 MVA, 69kV:12470V transformer to feed a mixture of 12470:480 transformers (mostly delta-wye, but with one 3750 KVA delta-delta, and a 2500 KVA wye-wye). The utility doesn't charge for low power factor, but we're planning several process line additions, and must either significantly improve power factor (nominally 78 to 80%) to free up capacity, or do extensive work to increase KVA.

One complicating factor is, while we have a number of high power across-the-line, and delta-wye started motors a significant portion of our load are DC motors ranging from 250 HP to 700 HP driven from S-6 bridges. Newer machines - about 1200 HP worth per nameplate - are using AC motors driven with ABB ACS800 drives, but, so far, none were built using the -ULH ultra low harmonic option. Our newer AC and DC drive installations are equipped with 3% line reactors or isolation transformers, but those on older installations are connected directly to the bus.

One proposed solution is to install controlled capacitor banks in the 12470 transformer yards to operate at nearly unity. While this would free up KVA and eliminate the need to re-work the 10 MVA transformer and lines to it I'm feeling incredibly queasy at the prospect of potential harmonics and resonance issues.

I'm thinking a better scheme is to install properly sized capacitor banks at larger (30 HP and up) across-the-line and delta-wye started motors, install line reactors on DC drives that aren't already so equipped (eventually we'll be phasing these out in favor of AC drives), and to specify ABB ACS800 drives with the -ULH option for new installations.

I've read previous Eng-Tips threads, but haven't yet run across one on this specific question ...

In several cases - process pumps, cooling tower fans, and air compressors (where we use a single VSD compressor for trim control, and our sequenced fixed capacity compressor motors run fully loaded most of the time)KVAR sizing isn't very difficult.

However, each of our process lines use a 75 HP or 100 HP reclaim granulator, and on these the load profile changes dramatically. During normal process operations they run lightly loaded, but must be sized to allow for periods of heavy scrap generation.

It seems to me using a controlled PFC bank for these would be problematic (in addition to costly) due to the very 'peaky' load profile. If I size to correct PF at nominal loading then it'll over-correct under heavier loads.

I'm leaning towards KVAR sizing to achieve 95% PF from whatever the power factor is at peak recorded loads (in this case, from 85% to 95%), and am interested in any opinions pro or con, and/or if there are rules-of-thumb when sizing KVAR caps for highly variable loads.

Trending for a typical installation is attached.

 

[rawelk]

I'm still grappling with understanding this. In my case, KVAR caps are across the motor leads at the load side of 1M contactor, and stay connected to the line supply throughout transition from wye to delta.

I can see why one wouldn't want to connect KVAR caps in an open transition arrangement (or closed transition, for that matter) at the 2M contactor load side since they'd be effectively shorted while the wye contactor is closed, and start charging through the motor windings until 2M contactor seals.

Another hazard with this wiring would be if the motor were shut off, then restarted before the caps discharged ... I'd imagine a big bang would ensue when the 3M wye contactor closed during the restart with a good chance of welding it's contact sets together, and possibly destroying the caps.

That leaves open two questions:

1). Why do Siemens (and - I've been looking around - Square D, ABB, Cutler-Hammer, and several others) caution against using KVAR capacitors on open transition starter configurations?

They recommend placing the caps at the contactor line side, and I don't see any difference (except for not requiring the MTOL be set at a lower trip setpoint, and higher line current through the contact sets) between placing them there versus at the 1M load side.

2). Why do these sources specifically mention open transition as being a problem, but not closed transition starting?

The issues arise when the capacitors are connected to the motor in such a way that when one contactor opens, there is a circuit through the capacitors and the motor.

A motor equipped with closed transition starting uses a set of high power, low ohm resistors and yet another contactor to maintain motor connection throughout the transition.

I'm wondering why these different manufacturers don't mention closed transition starting as causing difficulties?

It's advantage over open transition is to provide enough motor current via these resistors to keep the line supply and rotating motor field in sync. When KVAR caps are added to the mix, and connected to 1M they would be supplying magnetizing current throughout the wye-to-delta process.

 

[LionelHutz]

"there is a circuit through the capacitors and the motor".

I suspect they mean a setup where the caps would effectively replace the closed transition resistors. In that case, the open motor leads would be shifted some amount from the line phasing. I suspect it would be close to 90 degrees out.

Also, your last paragraph doesn't tell the whole story. When the shorting contactor opens those motor leads are not in phase with the line power. The resistors pull them in phase before they are connected at full-voltage.

 

[wcarter]

rawelk,

Rbulsara is correct in utilizing a PFCC at the transformer. But at lower subs, IE 480V might be another option. At least one of the manufacturers has active switching on their LV PFCC's. IE, below a setpoint, it activates a 75 or 150KVAR stage to correct the pf. If this is not enough, it activates another stage, etc. Once pf raises above another setpoint, for a predetermined amount of time, it will begin dropping out stages and monitoring the pf. This would aleviate your over-correction concerns.

 

[edison123]

Bill

A lps for your 16 Jan 11 11:23 excellent post.

Muthu

http://www.edison.co.in