Underload Generators? 6

[Mechathan]

We have 3 gensets used as back up in a high rise building. 2 gensets were tagged as prime power at 1.6MW each rated capacity. The other is at 1MW rated capacity and plated stand by. Our billing from the power utility suggested a 3.5 MW demand (max) last month and the building is 80% occupied. Our technical group has audited the power system and concluded that the generators are not sufficient in running the building, which is quite contradicting with the utility provider's billing. We have synchronization panels & load sharing equipment to evenly distribute the load. My questions are:
1. Can we run the prime power to 110% its capacity? Is this be possible? Dowe need to purchase another generator based on the details above?
2. Is it safe to assume that our utility provider has left some switchgears unmetered (i know this sounds funny)? Can we rely on their billing or just ask them to make an audit themselves to verify our findings?
3. How do we go about a full scale power audit in a typical building? Has anyone done this? Any tips?

Thank you.

 

[waross]

I suggest operating the two prime rated sets as "Base load" sets at 100% rated output and using the standby rated set to supply the balance. The standby set can respond to load swings.
Standby rated sets may be identical to prime rated sets except for the ratings. However in the larger sizes the prime rated sets may have options fitted to the engine to facilitate continuous running at high loads. Some common options may be larger oil sumps and oil coolers.
The engine will be over sized to allow loading above nameplate rating. The same engine may, in some instances, be used for standby sets. The engine may have the power output "turned down" during the manufacturers load test.
Metering.
Utility metering is commonly done at one point of the supply, rather than on individual feeders.
However, if the tenants are metered individually there is a possibility that some feeders may have been accidentally or intentionally missed on the metering scheme.
3.5 MW demand on an 80% occupied building suggests a demand of 4.375 MW with full occupancy, but such a simple extrapolation is very unreliable. Based on an admittedly unreliable extrapolation the 1.6MW+1.6MW+1Mw=4.2
MW capacity is low.
Prime rated sets allow 10% overload for defined durations. Typically one hour in ten hours.
Standby rated sets do not allow overloading.
Your numbers are so close that a complete power survey is probably required. The survey should include an allowance for full occupancy.
You may consider a N+1 scheme. This may be 3 x 1.6 MW sets for a total capacity of 4.8 MW. A fourth 1.6 MW set would be installed for reliability. In some plants the "+1" set is available, in some plants all 4 sets will be operated together so that the failure of any one set will not cause an outage.
respectfully

 

[apowerengr]

What does your building audit show for the facility's load? Perhaps your audit does not consider the time coincidence of the individual service peaks, thus the difference? Assuming the Utility is correct it still looks like you've got a reliability issue as loss of any one genset results in outages without the Utility source available.

 

[Mechathan]

We bill the tenants as we are in the primary metering system. Also, I assume that the utility company will not in any way make mistakes in metering since this will have a big effect in their bottomline. N+1 might be a good idea but considering the cost to purchase one and will just be used for back up, may not be economical to the bosses. The group has not shown me any data regarding the audit. This is the same group that told management that the generators are sufficient for emergency use but later changed what they said. I also noticed that this group is wasting a lot of energy running the building. Any checklist/procedures on how to do an energy audit quickly? Thanks for the responses waross and apowerengr.

 

[itsmoked]

Lot of waste? A better solution might be reducing the load intelligently. Perhaps shedding really low use loads. This would have the added benefit of allowing some up front decisions during a utility failure. Like if it was an earthquake that shattered the utility then a serious curtailment would allow greatly extended run times.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[sslobodan]

I don't know what systems your building has, but I would suggest that if you have climatisation for entire building you should put it out of the emergency power load. Considering that power factor is near the amount you need, maybe you can save up enough by turning of corridor light out of equitation leaving only emergency light (that should have self sufficient power reserve for 2h if projected well) that should save at least 4 - 5% of power . By calculations above you need to save up 200 - 300 KW in emergency regime with peak load. Also you need to check what is power factor of genset. and than to adjust it to pf of building. If it is lower set than pf you can also gain some power from there. If you cant not do anything from above you should consider purchasing standby diesel generator from 200-300KW when load reaches dangerous level (100% of consumers) In most cases any project should have at least 2 zones of consumer demand so you can save up some power in emergency regime, and that is not 100% of load. It would be smart to print for your tenants emergency procedure bulletin that would explain that in emergency they should not have all their gear turned on because that would shut down emergency power generators, and they would be left without power so they should save power by
1. If they cook something they should turn of air conditioning and water heat boilers.
If you take those 3 consumers you will see that they are taking 60% or more of each flat consumption. If people are aware of problem and know what to do they will comply willingly. Even if there is % of people that will not comply you will have your load at 80% or less in emergency if you send each tenant a bulletin that costs 1cent or less

 

[Mechathan]

Waste, such as overcooling. The Building Management system is still not futioning well. They are still trying to repair it including sensors that controls the compressor. Thanks for the tips on conserving energy during emergency runs. Is there a website that can give us some information on how we can balance the pf of the genset to the building? Thanks for all the help.

 

[sslobodan]

I posted the formula for PF in
Power factor correction
thread238-195278
its recent.
And you can get the reading with power meter, you measure, active and reactive power from consumers, than you get angle from that calculus P/Q=S*ctg(f) -> arc(ctg(f)) is a phase angle you need to know, after that you just calculate cos(f) genset power factor should be known from SCADA system, and maybe it can even calculate consumer consumption. If you have any measurement for power company for few MV object you should get that data about pf from that device (Simeas P for instance has that monitorings built in)or from power company.

 

[jghrist]

You can't base too much on a single month's billing. If A/C is a large portion of the load, and the last month had above normal temperatures, then this would give you some more confidence. If heating will be a larger load than A/C, then you can't tell from a summer month billing what the load will be in the winter.

Typically, the demand determined by the engineers will be considerably higher than actual because not enough diversity is considered. Utilities sometimes use a factor of 30% - 50% of service entrance size to size transformers for commercial customers.

Providing full backup power for a calculated maximum demand could be very expensive. Better to leave some interruptible loads off of the backup system or use some load shedding scheme on selected feeders. Leaving some loads off may not be feasible without extensive changes in switchgear. Maybe underfrequency relaying could be applied to some feeders for load shedding, coordinated with the generator protection.

 

[Mechathan]

Thanks for the formula. We'll use this for our pf requirements.

Our average demand is at 3.2MW spread over a year. It is still safe to say that it is within capacity. But with tenants moving in at a faster phase, we might hit 100% occupancy by mid 2008, which is our worry right now. We can do load shedding, great idea, but you know tenants. They just get in your nerves when they see a small portion of their units not being lit. In any case, we need to identify areas where we can shed some in case of emergency.

Any methodology and checklist for power audit... anyone? Thanks.

 

[sreid]

If the yearly average demand is 3.5 MW, wouldn't peak day time demand be much higher?

 

[itsmoked]

Makes sense.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jtkirb]

1. Can we run the prime power to 110% its capacity?

110% is possible, but not suggested. Most generators have some headroom, but you may have warranty issues.

2. Is it safe to assume that our utility provider has left some switchgears unmetered (i know this sounds funny)?

You can put a recorder on the service and get a better idea of your power useage. Most people don't understand utility billing (kwh) and instant kw fluxuations.

3. How do we go about a full scale power audit in a typical building? Has anyone done this? Any tips?

I would put a KW recorder or electronic metering package that will track this information for you.