High Voltage Capacitors for Power Factor Improvement 2

[sunilji]

I need to improve the power factor of a steel induction melting furnace plant from 0.95 to 0.99. The total kW load is 1700 kW ( out of which 1500 kW is the furnace load ).
The furnace is fed through a 11kv/550V,1750 KVA Trafo and rest of the auxillary load ( pumps, cranes, lights )is fed through a 11kv/415V,250 KVA Trafo. What is the optimum point to install power factor improvement capacitors ? I am in favour of 11 kV capacitors.

 

[redtrumpet]

Optimum placement of capacitors depends on what you are trying to achieve. Your power factor is already quite high at .95, so it doesn't sound like you are trying to reduce utility charges or free up transformer capacity. Are you trying to gain voltage support for a weak utility supply? Have you thought about a static VAR compensator for this application? Induction furnaces usually have dynamic load profiles beyond the capability of a traditional switched capacitor bank.

 

[electricpete]

It sounds like redtrumpet covered it pretty well.

I might add some obvious comments. If you add the capacitors at the 11kv bus, then there is no reduction in load (and losses) through the 550 and 415v transformers and associated cables. By adding the caps at the lower voltage buses, you slightly reduce load on the transformers...5% reduction in current if you go from 0.95 to 1.0 pf... This equates to roughly 10% reduction in temperature rise and possibly may increase the life/reliability of your transfomrers. Also you'll reduce the I^2*R losses in your transformers by approx 10% (but it's probably a fairly small number to begin with).

I'm sure there are lots of other cost/complexity factors in the decision.

 

[sunilji]

Thanks redtrumpet and electricpete. I think I should have revealed the motive for aiming at unity power factor. The electric utility is offering a rebate in energy charges for a p.f. above 0.95. The incentive is 1% reduction in energy charges for every percentage point improvement in p.f. beyond 0.95, with a max. rebate of 5% at unity p.f. There are already capacitors installed on LV side (415V). The 550V volt level is likely to be crowded with harmonics generated by the induction furnace (converter/inverter). Hence I was thinking of the 11 kV level, where one large bank could do the job. Agree ?

 

[redtrumpet]

Agree. If it is only to reduce utility charges, then placing the capacitors at the primary bus makes good sense. Still wouldn't hurt to consider the effects of harmonics on the 11 kV bus, you may need a detuned capacitor bank to avoid resonance.

 

[electricpete]

Sounds reasonable to me that you wouldn't want to get those caps too close to your nonlinear loads. Since you brought it up, I'm sure you've figured out that capacitors are low impedance to high-frequency harmonics. You'd probably have to apply a pretty hefty derating which would jack up the cost.

I really don't have much insight into other cost/complexity/reliability differences between the two options. (I'm sure there's others out there that can provide more info)

 

[pyppyp]

you have a problem, which I have been tackling for 5 years now. the required capacitors approx. can not be installed on your auxilary transformer lest it will be overloaded. 11 kV capacitors need isolators, breakers, etc. this compensation is also Fixed. the furnace load is varying. in some cases this will give rise to leading power factor, the RKVAH lead being measured by utility as KWH increasing your consumption.

the best way is to install capacitors with properly de-tuned reactors somewhere else, not on furnace transformer. since i am doing this activity on commercial basis and installed about 20 such equipment rest detials, will be furnished if you can please contact me on ambikharmonic@vsnl.com

 

[GRPatel]

You can perhaps use dynamic compensation using dynamic switching. These are in the standard range of reputed manufacturers.

 

[SPI]

FIRST CALCULATE REACTIVE POWER TO BE REQUIRED FOR IMPROVING
POWER FACTOR FROM 0.95 TO 0.99 FOR 1700 KW LOAD. @ 242.23 KVAR REQUIRED FOR .99 PF FOR 1700 KW. MY SUGGESTION IS FOR HIGH VOLTAGE CAPACITOR BANK TO FIX CAPACITOR & TRY INSTALLED ON LT SIDE BECAUSE FURNACE ACTIVE & REACTIVE LOAD ALWAYS FLUCTUATE DUE TO THAT RESULTANT PF ALSO FLUCTUTATE.HIGH VOLTAGE CAPCITOR BREAKER RATING 2 TIMES OF NORMAL VOLTAGE (AROUND 22 KV) DUE TO SWITCHING OF CAPCITOR . i.e COST ALSO INCRESED.

 

[SPI]

FOR PF IMPROVEMENT FROM 0.95 TO 0.99 FOR 1700 KW LOAD AROUND REACTIVE POWER TO BE REQUIRED 242.23 KVAR. MY SUGGESTION IS GO THROUGH SATAIC VAR COMPANSATOR FOR INDUCTIVE FURNACE ( ITS COSTLY ) OTHER SOLUTATION FIX CAPACIOR BANK INSTALLED IN 11 KV SIDE ( BRAEKER COST ALSO INCRESED ITS RATING 2 TIME OF NORMAL VOLTAGE .VARIABLE CAPICTOR BANK WITH POWER FACTOR CORRECTION RELAY BECAUSE OF FURNACE ACTIVE & REACTIVE LOAD CONTINUES CHANGING WITH MELTING & DURING TAPPING TIME FURNACE LOAD OFF i.e PF
CONTINUES CHANGE WITH FURNACE LOAD.

 

[redtrumpet]

SPI - 1700 kW load is 1789 kVA at .95 power factor and 1717 kVA at .99 power factor. At .95 power factor there is 557 kvar of reactive power supplied by the system. At .99 power factor there is 241 kvar of reactive power supplied by the system. The difference is 557 minus 241 kvar, or 316 kvar, to be supplied by the power factor correction capacitors. How did you arrive at 242.23 kvar?

 

[jbartos]

Suggestion: The harmonic content can be analyzed. The power factor calculation is different for power supplies and loads experiencing harmonics.
PFtotal=PFfundamendal/[1 + (THDi in %/100%)**2]**0.5
where
PFtotal is total power factor where current harmonics appears
PFfundamental is your normal 60Hz or 50Hz power factor, normally used if the harmonics are not considered or present
THDi is the current total harmonic distortion (see IEEE Std 519, or IEC counterpart)
which means that your current power factor is lower due to harmonic contamination, if applicable.