Maximum Demand

Stats isn't my best subject, but you need to look at:

• What is the distribution of usage - does it vary widely from month to month, or stay near the average with occasional excursions?
• What factors affect your usage, and are these likely to vary in future?
• What are the consequences of exceeding your declared maximum demand?

Just a few thoughts...

JohnDG:

1. The usage is is not vary too much. The value is at around 44000 kW to 46500 kW. Basically, it will not exceed it during our normal operation. Except, if we have a plant turnaround/shutdown, the power intake will be much much lower.

2. Our plant operation. But as per last August and September, we are running at full swing, and the value is 46,483.0 kW & 45,218.00 kW respectively. Our higher consumption was 48,279.00 kW, which is way back on May 2004.

3. There are some penalty if we exceed 70,000 kW (our maximum declared demand).

Demand is a measured thing and your electric meter will have a demand pointer that goes up as you place a load on your service, but it does not go down.

The electricity company charges a demand fee based on that peak. The meter also counts all the kilovoltamps you use in a month and they charge you for that.

So what makes your demand go high?

The answer is the large load of startup of large machines, after which they fall back to a lower value.

The solution to this lies in several methods.

1 phased startup. Do not start all machines at once, start them slowly over one hour, even if you must hire someone to come in early, the demand saving may offset the wage costs.

2 Smart electronic starters for large motors that start with a low voltage ramp instead of a huge current inrush.

3 machine scheduling with shifts etc. See if you can avoid operating all the large machines at the same time. That might lead to a rule that machine "A" is never run while machine "B" is also running.

4 replace one large machine with several small machines that are brough into operation as production demand increases.

Now some of these things may not be practical, but some will.

There are "demand consultants" who earn a living from coming to places like yours and doing a demand audit. Once they leave you pay them and the savings are permanent as long as you follow their rules of demand useage.

I'm a little confused . . . When you say, " . . . our maximum ever usage per month is around 49,000 kW," do you mean 49,000 kWh?

What is the maximum instantaneous demand? It may average 49,000/(30 X 24) kW, but that will not necessarily be anywhere near the peak load during any one hour. Whatever the peak load is is what you need from the power company.

Your maximum usage appears to be monthly demands, not hourly. 49000 is your monthly averaged peak demand, what is your actually short-term peak demand? What is the highest peak hour you have used? your electrical engineers that designed the facility should have calculated the peak demands based on the equipment and proposed operations. You should be able to get the peaking factor from your electrical design engineer and see how you current average demand compares to that estimated by the EE , use the peaking factor to estimate peak demands.

I spent over 30 years with one of the largest electric utilities in the US and here are my thoughts on your question. First 70,000 kW is a very large load. Second, if your peak demand has historically been 49,000 kW there is really no reason to contract for any larger demand. You should set your contract capacity as close to your actual needs as possible. Rates and terms of service are different in every state, even for a utility that serves many states, because retail rates are governed by the state utility commisions. In my state, for my company, the peak demand period was 15 minutes and you got billed for your highest 15 minute peak, subject to a minimum of 60% of your contract capacity, or 60% of your highest previous demand during the last 11 months. Contracting for 70,000 kW would simply set your minimum demand at 42,000 kW, but would not cause you to be billed for 70,000 kW. Contracting for 70,000 kW will not give you any assurance that the utility will supply you with 70,000 kW of capacity. Perhaps they would initially install a substation large enough for that kind of load, but if other growth in the area uses up transmission or generation capacity, you can only get what they can supply. Lastly, the comment someone else made about start up is not correct and is one of the most common misconceptions. Inrush current for motor startup lasts only a matter of seconds and the demand period is 15 minutes. Staggering start up for motors does not lower your demand. The only time start up really has an affect is if your are starting thermostatically controlled loads, such as melting furnaces or heat treating where the start up time is long. If that is the case, wait until one is up to temperature before starting any thing else.

this is strange. A place where I worked had 60 3 roll ink mills that took close to a minute to get up to speed and they had no clutch. They varied from 2 HP to 20 HP, 5550 VAC 3 phase, and we had a start up procedure staggered over more than an hour to reduce Demand peak.

That would make for about one a minute. Not to bad. Another reason for the start up of the mills being staggered, could be how "stiff" the supply from the utility was. What was the HP demand of the mills upon start up, 2 or 20 HP?

550 volts AC 3 phase, no phase correctors, so there might have been phase angle penalties as well.

This was in 1960 =summer student, so I am not up on these details. It was an old factory, now converted to lofts.

I expect the supply was not that stiff, so that could have been a reason for the slow startup. I recall I was told it was to keep the demand low.

The demand period is 24hrs. Every month, the utility company will gives us the daily demand (together with the invoice) and the 49000 kW is the maximum ever usage in a single day since 2002.