Relationship between Maximum demand and size of a diesel generator

[T.M.Haja Sahib]

mivey:

The relationship does exist. It seems you complicated it. Take, for example, your own post below.

For a simplified calculation (although not technically exact-a-mundo because of starting p.f., inrush, etc.):

( 3s * (480kW + 1kW) + (840s - 3s) * (80kW + 1kW) + 60s * 1kW ) / 900s = 77 kW

Here (480kW + 1kW) is the maximum KW and this should be approximately ( because you stated the above relationship approximate) the minimum size of the diesel engine of the gen set corresponding to the maximum demand of 77 KW.

 

[mivey]

mivey:

The relationship does exist. It seems you complicated it. Take, for example, your own post below.



Here (480kW + 1kW) is the maximum KW and this should be approximately ( because you stated the above relationship approximate) the minimum size of the diesel engine of the gen set corresponding to the maximum demand of 77 KW.

The demand reading does not give you information about the 480 kW.

For example: I have a demand reading of 102 kW. What size generator do you think I need?

 

[bob]

Help me out.

Why do all the links I posted say the highest demand for 15 minutes?

Iwire, basically you are correct. The method of how the demand is reached is the question. During the 15 or 30 minute window
the microchips are taking hundreds/thousands of readings. The readings are averaged over the time interval. If the demand
is reading 50 kw and a motor starts causing an additional 250 kw for 5 seconds to be recorded, that 250 kw is averaged in with all
of the 50 kw readings so that it has just a small affect on the total demand. The question regarding the snake oil products to reduce demand do not work
because of the way the demand is recorded. This is why the demand reading may give you are error when trying to use it to determine
the size of a generator backup.

 

[mivey]

Iwire, basically you are correct.

No, he was not.

The method of how the demand is reached is the question. During the 15 or 30 minute window
the microchips are taking hundreds/thousands of readings. The readings are averaged over the time interval. If the demand
is reading 50 kw and a motor starts causing an additional 250 kw for 5 seconds to be recorded, that 250 kw is averaged in with all
of the 50 kw readings so that it has just a small affect on the total demand.

First, the meter is recording chunks of energy readings, not averaging the demands. To get the interval demand it divides the sum of the energy consumed over the interval by the interval time.

Second, the mistake Iwire made is the same one you just over-looked: The motor that started at 250 kW also ran for 14 minutes at an added 40kW+ making the average demand across the 14 minutes about 50 kW + 40 kW or about 90 kW. It is the 14 minute run time of the motor that made the 1 kW in Iwire's answer wrong, not the starting kW. If the motor had started up then immediately shut down, the approximate 1 kW would be correct.

The question regarding the snake oil products to reduce demand do not work because of the way the demand is recorded. This is why the demand reading may give you are error when trying to use it to determine the size of a generator backup.

Another way to say it is that we do not record instantaneous demands or that we do not have very short demand intervals. The snake oil products do not work because, at least for residential, the meter measures the energy consumption (kWh) and ignores the leading and lagging varh.

 

[bob]

Mivey, I believe your are the one mistaken. My comments had nothing to do with any of your posts.

No, he was not.

First, the meter is recording chunks of energy readings
, not averaging the demands. The meters records the highest average demand during the 15/30 minute period. To get the interval demand it divides the sum of the energy consumed over the interval by the interval time.

Second, the mistake Iwire made is the same one you just over-looked: The motor that started at 250 kW also ran for 14 minutes at an added 40kW+ making the average demand across the 14 minutes about 50 kW + 40 kW or about 90 kW. It is the 14 minute run time of the motor that made the 1 kW in Iwire's answer wrong, not the starting kW. If the motor had started up then immediately shut down, the approximate 1 kW would be correct. I made no comment with regards to this statement. Iwire's answer was wrong but his basic concept was not.

Another way to say it is that we do not record instantaneous demands or that we do not have very short demand intervals. The snake oil products do not work because, at least for residential, the meter measures the energy consumption (kWh) and ignores the leading and lagging varh. I agree. residential customers are not billed a demand charge. Some products do address power factor and others try to state other reasons.

Understanding Electrical demand




This is hard to see but it states that the demand charge will be the highest average KW
measured during the 15 minute interal during the billing period and not chunks of energy..

 

[mivey]

Mivey, I believe your are the one mistaken. My comments had nothing to do with any of your posts
.

Since you quoted Iwire's response to my post, it is only natural to think that it had something to do with my post. If you are talking about something completely different, then so be it and carry on.

This is hard to see but it states that the demand charge will be the highest average KW
measured during the 15 minute interal during the billing period and not chunks of energy..

You are misunderstanding what they are saying. They are saying the kW is an average obtained by taking the energy divided by the time. This gives you the average kW for the 15 minute interval. In other words, the thousands of readings are accumulating chunks of energy in registers. Each chunk represents the area under the kW vs time graph and is a chunk of energy.

To state it a different way: there is only one average kW for any 15 minute interval and that is the 15-minute energy divided by time. There are not multiple averages within the 15 minutes that you can search through to find a highest average. The "highest" comparison is made by comparing different 15 minute periods.

You could also say each chunk is the average demand over the fraction of a second that it represents. So you can see it does not make sense to refer to the highest average demand as being the highest of a bunch of averages within an individual interval.The "highest" they refer to is not the highest average inside the interval as that makes no sense and it not the way a meter works. By "highest" they mean the meter has a register that stores the highest 15 minute demand which is the average demand for the interval which is the energy in the interval divided by the time in the interval.

When the demand register is reset, the highest demand is zero. The first 15 minutes of energy is divided by the time and the register stores this as the highest demand (the average for the 15 minute period) because it is higher than zero. At the end of the second 15 minutes, the average demand is again calculated and compared to the register value. If the new average is higher, it replaces the old value in the register. This continues every 15 minutes until the demand register is reset.

National Grid's "the demand charge will be the highest average kW" means they do not use an instantaneous demand. They don't use one of the thousand of readings made per second. Supposed they used a 5-second average within the 15 minute interval. Then if they said they used the highest average within a 15-minute interval, that would simply mean they are using a 5-second demand interval across the whole billing period and the 15-minutes means nothing. Just think about the math. FWIW, even a 5 second demand is the Joules of energy consumed in 5 seconds divided by 5 seconds to give you J/s or watts.

Iwire's answer was wrong but his basic concept was not.

His basic concept considered the start time but not the motor run time he specified.

 

[mivey]

FWIW, you can have a sliding window instead of a fixed window as well. For a 15-minute demand interval and a 5-minute calculation rate, this would calculate the average demand over the previous 15-minutes and would re-calculate this every 5 minutes. However, this is not the same as a 5-minute demand.

A fixed window is a special case of the sliding window where the interval and calculation rate is the same. You would have, for example, a 15-minute demand interval and a 15-minute calculation rate.

 

[bob]

Since you quoted Iwire's response to my post, it is only natural to think that it had something to do with my post. If you are talking about something completely different, then so be it and carry on.

You are misunderstanding what they are saying. They are saying the kW is an average obtained by taking the energy divided by the time. This gives you the average kW for the 15 minute interval. In other words, the thousands of readings are accumulating chunks of energy in registers. Each chunk represents the area under the kW vs time graph and is a chunk of energy. Totally agree.

To state it a different way: there is only one average kW for any 15 minute interval and that is the 15-minute energy divided by time. There are not multiple averages within the 15 minutes that you can search through to find a highest average. The "highest" comparison is made by comparing different 15 minute periods. Agree

You could also say each chunk is the average demand over the fraction of a second that it represents. So you can see it does not make sense to refer to the highest average demand as being the highest of a bunch of averages within an individual interval.The "highest" they refer to is not the highest average inside the interval as that makes no sense and it not the way a meter works. By "highest" they mean the meter has a register that stores the highest 15 minute demand which is the average demand for the interval which is the energy in the interval divided by the time in the interval. I agree

When the demand register is reset, the highest demand is zero. The first 15 minutes of energy is divided by the time and the register stores this as the highest demand (the average for the 15 minute period) because it is higher than zero. At the end of the second 15 minutes, the average demand is again calculated and compared to the register value. If the new average is higher, it replaces the old value in the register. This continues every 15 minutes until the demand register is reset.Agree

Agree

 

[T.M.Haja Sahib]

The demand reading does not give you information about the 480 kW.

Yes. But we are talking about an equation between maximum demand and size of a diesel gen set.

For example: I have a demand reading of 102 kW. What size generator do you think I need?

You need to supply more information such as size of starting load, its starting time, stopping time in a demand interval in which the maximum demand occurs etc., to work out the relationship.

 

[mivey]

Agree

I'm glad we are in agreement. My apologies if I did not read what you meant earlier.

 

[mivey]

So we have journeyed from thinking the demand reading will reflect the motor start information needed and will give us a demand vs size equation:

the 30KVA of demand does not reflect motor starting requirements

I believe 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.

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.

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.

To realizing that the demand does not tell us what we need and that we need other information (the "other limiting factors"):

Yes. But we are talking about an equation between maximum demand and size of a diesel gen set.

Do you now realize that the maximum demand meter reading tells you nothing significant about the "instantaneous" load?

You need to supply more information such as size of starting load, its starting time, stopping time in a demand interval in which the maximum demand occurs etc., to work out the relationship.

It seems you are complicating it. Remember my statement about the "other limiting factors"? You get it now?

In case you don't: In the process of developing an equation, you have to include so many other calculations to restrict the applicability of the equation that the equation becomes specific for the site. In other words, why not just calculate the generator need using the recommended method from the manufacturer? What good does the equation relating the demand and size do you when you will have to create a different equation for a different site?

 

[T.M.Haja Sahib]

It seems you are complicating it.

I think not.

In the process of developing an equation, you have to include so many other calculations to restrict the applicability of the equation that the equation becomes specific for the site.

If the building is equipped with an energy management system, size and time of loads in a maximum demand period can easily be found out to make up the equation.

why not just calculate the generator need using the recommended method from the manufacturer?

What good does the equation relating the demand and size do you when you will have to create a different equation for a different site?

I think the method may be useful in finding out the minimum size of the gen set.

At the same time, I want to know where this method fails.

 

[mivey]

I think not.

So if I say you need more info then I'm over-complicating it but if you say you need more info you are not over-complicating it? That sounds about right.

If the building is equipped with an energy management system, size and time of loads in a maximum demand period can easily be found out to make up the equation.

If the energy management system can give you that level of detail, it kind of makes deriving an equation for metered demand vs gen size useless.

I think the method may be useful in finding out the minimum size of the gen set.

At the same time, I want to know where this method fails.

Then have at it and let us know what you find out.

 

[T.M.Haja Sahib]

So if I say you need more info then I'm over-complicating it but if you say you need more info you are not over-complicating it?

Which one is complicating the issue obviously depends on the purpose of this thread: to find out the relationship between MD and size of the gen set. I think my asking for details is more in line with that purpose.

If the energy management system can give you that level of detail, it kind of makes deriving an equation for metered demand vs gen size useless.

No. It is because

The method may be useful in finding out the minimum size of the gen set.

Then have at it and let us know what you find out.

Sure. Will do it. Meanwhile, please give your valuable comments on

I want to know where this method fails.

 

[mivey]

Meanwhile, please give your valuable comments on

I want to know where this method fails.

That has been covered in previous posts.

I guess the alternate way for you to find out is for you to conduct your own experiments on different sites, assuming you have enough diversity in your sample set. Without site diversity, you have one of those "limiting factors" discussed previously.

 

[MIEngineer]

Not to belabor the point but I find this thread very intersting. What is the advantage then of staggering motor loads within a building? My understanding was that it lowers the demand. If I have 2 motors what difference does it make if they both start simultaneously or separately within the same 15 minute window if the demand metering doesn't account for the instantaneous starting?

 

[mivey]

Not to belabor the point but I find this thread very intersting. What is the advantage then of staggering motor loads within a building?

Power quality (like volt drop). Reduced capacity need and/or less encroachment on relay/breaker curves if the starting time is a factor.

My understanding was that it lowers the demand. If I have 2 motors what difference does it make if they both start simultaneously or separately within the same 15 minute window if the demand metering doesn't account for the instantaneous starting?

If they start in the same interval window, it does not matter for interval demand. I would have to think about the thermal demand emulation as it has been a while since we used that type.

PS: The instantaneous is accounted for, it just does not show up in the significant digits. Starting together or separately (assuming the same starting kW) will give the same demand either way if all is in the same window.

 

[T.M.Haja Sahib]

That has been covered in previous posts.

I do not agree, because, unfortunately, you did not give a counter demonstration to my demonstration of relationship between MD and gen set size as in

mivey:

The relationship does exist. It seems you complicated it. Take, for example, your own post below.



Here (480kW + 1kW) is the maximum KW and this should be approximately ( because you stated the above relationship approximate) the minimum size of the diesel engine of the gen set corresponding to the maximum demand of 77 KW.

 

[T.M.Haja Sahib]

The instantaneous is Starting together or separately (assuming the same starting kW) will give the same demand either way if all is in the same window.accounted for, it just does not show up in the significant digits.

It is simply because the size of the starting loads is small or the frequency of the same in the demand interval is not high.
So the following is not always correct.

Starting together or separately (assuming the same starting kW) will give the same demand either way if all is in the same window.

 

[mivey]

I do not agree, because, unfortunately, you did not give a counter demonstration to my demonstration of relationship between MD and gen set size as in

You did not give me a gen size for the question below so it should be obvious to you:

The demand reading does not give you information about the 480 kW.

For example: I have a demand reading of 102 kW. What size generator do you think I need?

You need to supply more information such as size of starting load, its starting time, stopping time in a demand interval in which the maximum demand occurs etc., to work out the relationship.

The failing is that there is not an empirical formula that makes the demand vs size equation particularly useful because there is so much diversity between sites and operation characteristics. If you have a fairly uniform set of sites with the same operation characteristics, then you can apply some "other limiting factors" like I discussed earlier.


As for empirical data: A PQ meter (or other high-resolution meter) that collects data during typical site operations would give you what you need to size the generator. Obviously you would need a different data set for each site (unless the sites are essentially twins). A revenue meter does not give this information (very fancy ones can).


You started out discussing whether the calculation or the metered demand was the way to go:

...I calculated the MD of a building...The maximum demand worked out 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 ?

When you were told that the metered demand would not work you wanted to argue the point. It should have been obvious to you long ago that a detailed calculation is the only way to go. There is no simple empirical relationship that will work for the general case.

It should be obvious to you that developing an empirical formula for a site is silly when you will have to develop a formula for each site (unless you apply the "other limiting factors" that you also wanted to argue about). The manufacturers already have a method for making the size calculation and that is what you should use.

What is obvious and what you are willing to accept without argument seems to be two different things. That is why I think making your own experiments would probably best the best way for you to learn from this point forward.