Relationship between Maximum demand and size of a diesel generator

[T.M.Haja Sahib]

I look for any empirical relationship between the maximum demand of an installation and size of a diesel generator to serve it.
Incidentally, I calculated the MD of a building based on connected loads and applicable diversity factors for sizing up a diesel generator per official guidelines. The maximum demand worked out to be 100 KVA. So the size of the diesel generator may also be taken to be 100 KVA. But per the digital energy meter of the building the recorded MD is around 30 KVA only. So which value will serve as correct size for the Diesel generator to be selected ? Thanks.

 

[Shoe]

I say it depends. If you have a full year or two of utlity data that shows a peak demand of 30 KVA, you could probably get away with a generator less than the maximum calculated demand. This is assuming the Owner is not interested in future capacity or will change the building-use drastically. Some automatic load shedding controls would be recommended.

The other issue is what loads will the generator be serving? The starting capacity of the unit needs to be taken into account, as the 30KVA of demand does not reflect motor starting requirements, and the generator is not a "stiff" source like the utility. The generator will not only need to be sized for load but also for starting of the largest motor load while loaded. High harmonic loads would also affect the specifications.

 

[JoeStillman]

In addition to the motor-starting premium, you have to allow for extra UPS loads. The generator has to support the UPS load plus an extra battery charging load after power has failed. You can be sure that battery charging load isn't in your utility peak demand.

I like to use the Cummins generator sizing program. It does all that for you.

 

[T.M.Haja Sahib]

Thanks, shoe and joestillman for your insights.......

One clarification regarding the point below is in order before proceeding further.

....... the 30KVA of demand does not reflect motor starting requirements,

I beleive the maximum demand meter will record a corresponding increase in reading, though of small value, even for instantaneous load above its recorded value.

 

[don_resqcapt19]

...
I beleive the maximum demand meter will record a corresponding increase in reading, though of small value, even for instantaneous load above its recorded value.

Most utility demand readings are based on a 15 or 30 minute sliding window and will not reflect the motor starting current.

 

[Hameedulla-Ekhlas]

I look for any empirical relationship between the maximum demand of an installation and size of a diesel generator to serve it.
Incidentally, I calculated the MD of a building based on connected loads and applicable diversity factors for sizing up a diesel generator per official guidelines. The maximum demand worked out to be 100 KVA. So the size of the diesel generator may also be taken to be 100 KVA. But per the digital energy meter of the building the recorded MD is around 30 KVA only. So which value will serve as correct size for the Diesel generator to be selected ? Thanks.

Hi Haja Sahib,

Just want to mention, diversity factor is applying not just for " a building" but for "buildings". If it is just one building, just apply demand factors, and size the generator for maximum demand. 30 kVA could be maximum demand for a specific time period not always.

 

[T.M.Haja Sahib]

Most utility demand readings are based on a 15 or 30 minute sliding window and will not reflect the motor starting current.

If what you say were true, energy conservation advices such as not to start all the motor loads in the same demand period so as not to increase the maximum demand due to starting current of motors do not hold true. So I adhere to my earlier statement to continue further.

 

[T.M.Haja Sahib]

Hi Haja Sahib,

Just want to mention, diversity factor is applying not just for " a building" but for "buildings". If it is just one building, just apply demand factors, and size the generator for maximum demand. 30 kVA could be maximum demand for a specific time period not always.

Thanks, Hameedulla Ekhlas. By demand factor, do you mean load factor?

 

[iwire]

If what you say were true, energy conservation advices such as not to start all the motor loads in the same demand period so as not to increase the maximum demand due to starting current of motors do not hold true. So I adhere to my earlier statement to continue further.

http://www.northwesternenergy.com/documents/E+Programs/E+demandcharges.pdf

The electric utility uses demand meters that measure flowing electricity as the board in the above example measures flowing water. Demand meters register the highest rate of electrical flow (or current) during a billing period. It?s actually a little more complicated than that, because the meter records an average flow for every 15 minute interval. The customer is billed for the highest average 15 minute flow during the billing period.

 

[T.M.Haja Sahib]

iwire;
Your web reference is elementary. What I stated is also true and verifiable.

 

[iwire]

Unlike a speedometer needle, demand meters record the highest average kilo- watts reached and maintained in a 15- minute interval within the billing period.
If your demand reaches 50 kW, for example, and stays there for 15 minutes, the meter needle remains at 50 kW unless or until your demand exceeds that level. If your demand later reaches 55 kW and stays there for 15 minutes, the needle will then stay at 55. The new index point is maintained, even when you are using electricity at below 55 kW, until the meter reader comes to record the demand and resets the meter back to zero.


For example, suppose you have a 10 kW motor in one part of the building and a 15 kW motor in another. If you operate both units simultaneously, the demand meter will record 25 kW.

http://www.nationalgridus.com/niagaramohawk/non_html/eff_elec-demand.pdf

 

[iwire]

Central Electric Cooperative monitors demand over a 15-minute period. The customer is charged for the highest 15-minute average recorded on the demand meter.

http://www.centralelectriccoop.com/Understanding Demand .pdf

 

[iwire]

Your electric meter also registers electric demand or the maximum amount of energy you use at one specific time (typically a 15 minute period). Demand is measured in kilowatts (kW) of electricity.

http://www.focusonenergy.com/files/.../understandingelectricbill_technicalsheet.pdf

 

[T.M.Haja Sahib]

iwire:
Thanks for your inputs. But none of your web references contradict my statement that maximum demand meter reading can be influenced and increased by instantaneous loads, because my statement is also based on sound sources.

 

[T.M.Haja Sahib]

Okay. Let me try to make it clear. MD is the highest average of power recorded in every fifteen minute interval. Suppose it is 'X'. Suppose during the next 15 minute interval, only motor starting load of 'Y' power lasting for 'Z' minutes occurs. So the average demand during the demand period of 15 minutes is [X* (15-Z)+ Y*Z]/15. If it is greater than X , the MD recorded by the meter would be '[X* (15-Z)+ Y*Z]/15'. Otherwise, it is 'X' only.

Note:-Even 'instantaneous' loads last for a definite time period and so have an average power.

 

[iwire]

Imagine a pump house, normally the load is a fairly steady 1KW.

Once every hour a 100 HP motor starts up adding about 80 KW to the 1 KW. The motor never runs more than 14 minutes.

Based on my power company links above, what would be approximate demand meter reading?

 

[T.M.Haja Sahib]

Imagine a pump house, normally the load is a fairly steady 1KW.

Once every hour a 100 HP motor starts up adding about 80 KW to the 1 KW. The motor never runs more than 14 minutes.

Based on my power company links above, what would be approximate demand meter reading?

The demand meter will read both steady load and starting KW of the motor and so the reading will be a little bit more than 75 KW.

 

[iwire]

The demand meter will read both steady load and starting KW of the motor and so the reading will be a little bit more than 75 KW.

No, the reading will be 1 KW.

 

[T.M.Haja Sahib]

No, the reading will be 1 KW.

Unbelievable.

 

[Hameedulla-Ekhlas]

Thanks, Hameedulla Ekhlas. By demand factor, do you mean load factor?

No, if you are sizing a generator just for a building, I think demand factors are enough, if you are sizing a generator which will feed many different buildings in that case we will use diversity factor. load factor is another case for large power plant, it plays an important part on the cost of generation per unit.