Demand Power

[js1960]

Simple question, but I can't seem to find the answer to it.

What is the definition of demand? I believe that it is the "average" power used over a user defined period of time (i.e., KWh/hours). Also, I have seen the term "peak demand" which I understand to be the maximum power consumed over a user defined period.

Can someone pleas verify (or correct) these definitions for me.

Thanks,

JS

 

[Marmite]

The IEEE definition, cut and pasted from IEEE 100
The Authoritative Dictionary of
IEEE Standards Terms:-

demand (1) (electric power systems in commercial buildings)
The electrical load at the receiving terminals averaged over a
specified interval of time. Demand is expressed in kilowatts,
kilovoltamperes, kilovars, amperes, or other suitable units.
The interval of time is generally 15 minutes, 30 minutes, or
60 minutes. Note: If there are two 50 hp motors (which drive
45 hp loads) connected to the electric power system, but only
one load is operating at any time, the demand load is only 45
hp but the connected load is 100 hp. Synonym: demand load.
(IA/PSE) 241-1990r, 141-1993r
(2) The average value of power or a related quantity over a
specified interval of time. Note: Demand is expressed in kilowatts,
kilovolt-amperes, kilovars, or other suitable units.
(ELM) C12.1-1988
(3) The rate at which electric energy is being used.
(PE/PSE) 858-1993w
(4) The rate of consumption, e.g., power, volume/hour.
(AMR/SCC31) 1377-1997

Regards
Marmite

 

[js1960]

Marmite,

Thanks for the information. Is the IEEE 100 dictionary on line? If so, could you post the link?

Thanks,

JS

 

[waross]

Demand over a time period is used in a the case of revenue metering and transformer heating.
Demand factors are also used to take advantage of the diversity of intermittent loads to size services, feeders and other equipment. In these cases the peak demand may be considered rather than the time weighted demand.
respectfully

 

[stevenal]

IEEE 100 is available on line for those who have paid the subscription fee.

A meter actually measures energy used. The demand is then the kWh accumulated during the demand interval divided by the interval. The result is the average power during the interval, or the demand. You will have a number of these intervals over the duration of a billing cycle, so the peak demand over the billing cycle will be the maximum of these demand values. Or you may look at annual or longer peak demands to see if equipment capacity is sufficient. Either way, two time periods should be specified for clarity: ie "monthly peak 15m demand"

"peak" is a little misleading that way, since it is not an instantaneous peak, but the maximum of average values.

 

[jghrist]

There is also a thermal demand which follows a thermal heating curve instead of a straight average. This was the basis of electromechanical demand meters and can be emulated in electronic meters.

 

[stevenal]

Shall we further confuse the issue with sliding window demand and the sub-intervals involved? Now three time periods are needed.

 

[js1960]

Why not ..... my question was answered long ago, but I am enjoying the opportunity to broaden my knowledge.

 

[waross]

The billing demands that I have worked with have been based on the heating curves of a sample transformer. With a step change in load, the temperature of a transformer will rise to about 63% of the terminal temperature in about 20% of the time taken to reach the terminal temperature.
If we assume a time interval of 15 minutes, then a 10KW step increase in loading that lasts for 20% of 15 minutes or 3 minutes will be billed at 63% or 6.3 KW additional demand. If the load persists for the full 15 minutes it will be billed at 100%.
I have never seen an average applied to a metering demand. I have seen more than a couple of demand meters and investigated ways to reduce the peak charges.
Another point of clarification. The demand curve is based on transformer heating curves but you and I don't get to develop those curves. The utility decides what time interval they wish to use. It may be 15 minutes, 20 minutes, 30 minutes or some other interval. You have to check the local tariffs.
I understood that the "sliding window" was another way of expressing the same information. Most of my meter work has been with older meters and I am willing to learn here.
This is billing demand and is based on heating of transformers and other equipment.
There are also demand factors applied to services and feders that are not based on heating but on diversity. These demand factors do not factor in time.
Check the CEC or the NEC for more information on these demand factors.
respectfully