Genset fuel consumption and/or additional load capacity 1

[rixtalbert]

Dear engineers;

If there is anything such as short and sweet answers I cannot find any data or studies on this one. It may be obvious to the educated in the field.
The situation is that I have a client in the quarry industry that is running a 930 KW CAT genset.The unit is D399. Roughly 20 years old. The rated amps output is 1460 at 460 volts. Normally during curshing I am running about 600 kw and peaking for a minute or so at 750 KW. My power factor is averaging around 70 percent. During different grades of rock and summer time abient temperatures I have been blowing fuses on the genset. The CAT people have recently found the voltage regulator to be a little flaky and was replaced. My average phase current is 1100 amps with peaks to 1450 for 10 second while a big rock may be in the jaw crusher. Now the genset seems to go right on thru this peak without any problem. My concern is when we start crushing the harder aggreate again the genset may not be able to keep with the contiuous demand.

The question is will installing KVAR power factor correction banks on some of my bigger 300 and 200 hP motors relieve the generator from working harder and even lower loading and/or save on fuel consumption.


Thanks to all that can direct me to a link to fill in the blanks. I am concerned that I do not want to over excite the stator on the genset and will make provision to shed the Kvar as motors are taken off line. We may go to auto cap banks if I can calculate any savings in either one of these areas.

Thanks

Rick Talbert
Centrix Industrial

 

[bigamp]

Installing power factor correction on some of the motors as you propose will reduce the reactive (kVAr) load on the generator. The kW load will not change, but because you have reduced the kVAr load, the overall load (kVA) on the generator will reduce.

If you can improve the power factor from 70% to say 80%, then for a load of 750kW the generator kVA reduces from 1071kVA to 938kVA, and the amps from 1345A to 1177A.

Improving the power factor will result in a very minor (essentially zero) improvment in fuel usage. The generator losses will be a little less and there will be fractionally less excitation power.

With having to start 200hp and 300hp motors and considering the poor operating power factor, the generator sounds to be a little undersized. Still, if it has lasted 20 years it can't be that bad.

Regards

 

[busbar]

If I punched the right keys, for a genset rated at 0.80PF and 930kW, it is also good for 1162kVA and, with adequate excitation, can furnish 697kVAR.

If voltage dip is not of concern during a 1450-ampere loading, it may not be a problem to ease into/ramp up the material feed during harder batches. You have the fortunate situation {and curse} of some control over loads and source. An empircal approach can be used to correlate calculations and nameplate values.

A load of 1450 amperes at 0.70PF and 460V corresponds to 1155kVA - 809 kW and 825kVAR, so you might try something like 10x15kVAR in capacitors and see how the system reacts. Motor starting, bus voltage excursions, connected-capacitor VAR support and excitation of the genset will interact to a degree. It sounds like you have a little flexiblity on bus voltage, horsepower loading and capacitor switching. Remember that capacitor reactive-power contribution varies with the line-voltage-squared. Weigh the installed scheme against ecomonic payback and operational flexibility, factoring ANSI limits for power capacitors.

Fused-disconnect switches {and whatever control scheme is desired} are suggested as a minimum-integrally fused capacitors at this voltage seem to have reliabilty problems for internal shorts/harmonic overcurrents. Watch for voltage overshoot in the process-obviously it can shorten electrical-equipment life. Per NEC Article 460, indiviudual 3ø 15 or 17½kVAR units seem to correspond to 30-ampere increments of disconnect-switch sizes.

I cannot comment on fuel consumption.

 

[cuky2000]

Bigamp and Busbar provide good explanation of improving power factor using capacitor.

There will be not significant fuell saving adding capacitors since the fuell consumption is directly related with the KW running load. Capacitor could reduce the KVAR but not the KW.

The Gen manufacture could provide you with the performance curves of the unit were will show you the max KW and KVAR output for each power factor at rated ambient conditions similar to the encloses curves.

Sample of typical generator Reactive Capability Curve:

http://www.marathonelectric.com/generators/faq_data/526622.pdf

Typical fuel consumption curves:

http://www.detroitdiesel.com/public/specs/5SA428ev.pdf

 

[rixtalbert]

I know for a fact that coming from a background of no engineering background that I have gone through my adult life reading everything I can get my hands on from microcontrollers up to high voltage distribution lines. Coming from a modest electrical background I have read several articles that state that improving Kvar with power factor correction capacitors will release additional capacity for additional current in distribution systems for more loads and reduce overheating of conductors and motors caused by low power factor.
I know and have heard a watt is a watt but if I decrease the heating effect caused by low power factor in the wiring and motor stator I would imagine that most of those watts that I am reading at low power factor are just being wasted as heat thru the system and at the motor!
Now my Idea was if I improved the capacitance to say 90 percent PF I would eliminate that heating effect and in turn lower wattage from the source. The losses I speak of my be minimal but I wish I could calculate the loss.
In light of other energy wasting ideal , some wire companies claing that even increasing a 20 amp load on a branch circuit from a # 12 wire to a # 10 conductor will save energy just due to the fact that the resistive element is reduced.
If a larger wire size is equal to inproving PF on some systems then I would expect to see and inproved and reduced wattage in my system....

Just a thought


Thanks
RIX

 

[meihm]

Your comments are all correct and valid, but there are relative factors in operation here.

Regarding the wire sizing and energy saving aspec:

losses in wire is the result (product) of amperage squared X resistance (I^2 X R)

What you have been told regarding wire size is true, but perhaps not relevant. Increasing wire size, reduces resistance and thus losses in wire, thus energy savings.

Regarding power factor improvement: Improving power factor also results in lower amperage for the same amount of work (Kilowatts). This lowering in amperage also result in lower losses in the wire, thus energy savings.

Consider thia: if you have 800 amp load current on 1000 amp wire, and you improve power factor and lower the load current, what you have done is "free up" 200 amps of wire capacity that you did not have before the power factor correction.

Is this relevant? To some degree. Unless you have very small wiring, or long runs, the wire losses probably are not a signifcant part of the total load.

As far as the engine in the generator set is concerned, it does not matter what the load is(motors, wires, lights), only what the load in watts is. The engine is responsible for the work (in watts/kilowatts); reduce the watt losses in the system and that allows more watts to be used for the actual work.

Increasing wire size in the circuit will lower resistance, lessen watt losses and reduce voltage drop (which may also affect (improve) motor efficiency). This will reduce line current to some degree.

Improving (increasing) power factor will lower line current, which will lessen watt losses and reduce voltage drop (which may also affect (improve) motor efficiency). This will reduce line current to some degree.

Lessening line current will also reduce losses in the alternator which will reduce overall voltage drop (which may also affect (improve) motor efficiency). It will definitely reduce heating on the alternator.

 

[cuky2000]

Rixtalbert,

Appear to be some confusion on the impact of generator fuel consumption with the power factor improvement.

Perhaps the following typical scenario could help to quantify the losses in cable do to the PF and roughly compare with the total fuel consumption witch is direct proportional to the running load KW.


Motor : 200 HP (~150 kW), efficiency 90%
Cable: 100- ft of 3-1/C # 350 kcmil (R~0.04 Ohm).
Voltage: 460 Vac
Power factor: from 0.70 to 1.

Motor full load current: I= [746xHP/1.73V.%eff]/PF Amp
Power Loss In Cable: P_loss=3xI².R %P_loss=P_loss/150kW

PF I (Amp) P_loss(KW) %P_loss
1.00 208 5.2 3.5
0.90 231 6.4 5.0
0.80 260 8.1 5.4
0.70 297 10.6 7.1


This scenario shows power loss in conductors exists even for ideal power factor of 100%. For lower than unit power factor, copper losses in cable increase slightly but still is a fraction of the total power consumption.

This case shows that power factor correction from 70% to 90% will only reduce the heat losses in cable (fuel consumption) in only near 2%.

To avoid false expectation with fuel saving improving PF, careful evaluation on the return of the investment is recommended.

 

[jbartos]

Question: When your process is peaking, how is the situation with the speed regulation, i.e. RPM? Does the speed drop?

 

[rixtalbert]

Jbartos....

I have not recorded the acutal RPM speed during runs and loading. Voltage variance is roughly 447 to 471 VAC for very short periods of 10 sec. But a 1 hour recording shows it is continually changing between these ranges and I feel they are directly related to larger aggreates passing thru cone or jaw crushers. I have seen the same type of voltage fluctuations on distributions systems supplied from other utilities. I believe also that trying to maintain a steady voltage regulation on a ROCK CHRUSHER is hard to do. Especially for a gen set. But they are working it's just that this business of crushing stone is very competitive and rock gradation engineers will try to hang every crusher they can off of an existing genset to make a different grade rock. There in lies the problem ---- to squease every bit of KW that the genset can or will deliver.
Even loaded to around 660 KW you can hear that Diesel moan for long periods. You would think that being rated at 930 KW that it would load a little less..

I'll try to record some RPM values for this genset during our test.

 

[Bung]

Something overlooked in replies so far is the blowing of the fuses at high summer ambient temps. Part of the reason for this is that fuses have a lower rating at elevated temperatures. This can be a problem (apart from the nuisance tripping) because if the fuses are a back-up class (or even general purpose) and are trying to interrupt a current below the minimum breaking current, you could end up with catastrophic failure of the fuse (= a big bang). I would have the fuse ratings checked out and make changes if necessary.
Bung
Life is non-linear...