Cumulative Maximum Demand

hi,

can u distinguish between cumulative maximum demand and only maximum demand ?

divyesh patel

Hi,

I don't know if this is right, but how I understand it is that Maximum Demand is the demand measured over a period (usually 30 minutes) and Cumulative Maximum Demand is the total Maximum Demand. But according to my knowledge electrical billing is based on you maximum demand over a period.

One of the answers is very close ... First off Demand is measured in Kilowatts or KW. Electric meters measure in KWH (Kilowatt hours). So for KW the time must be taken into the equation. The time interval used must be continuous. Today that time period is usually 15 minutes. So KW is equal to the highest average 15 minutes usage during the month. So over a 15 minute period if the customer used 10 KW for the first 5 minutes and then 20 KW for the next five minutes and 30KW for the third 5 minutes the for the month they would be build for 20 KW the average over the 15 continuous minutes.

Now for the Cumulative part: Meter reader resets demand and then reads the register. When the MR resets the demand the present monthly demand is added to last demand reading. The office subtracts the last read from the present read and bills the difference. Why do this? Customers from time to time like to reset their own demand if this happens the customer will be billed for maybe double or triple what the monthly demand really was. The thief is caught. Cumulative maximum demand is not any different than maximum demand except the meter is not reset to zero.

Non cumulative demand if the customer resets demand it is reset to zero and the information is lost forever if the company is still using real live meter readers over electronic reads. Then the actual demand is recorded electronically and the customer can play all they like.

Dear Guest,

Just a point for you on the interval. The standard interval of time is 1 hour. If a shorter interval than 1 hour is used to measure kW demand, the kWh value is multiplied to equate to what it would be if it stayed at that level for 1 hour.

Many different interval schemes are employed, 5 min, 15 min, 30 min, 60 min, rolling intervals of varying length etc, but the base kWh measurement interval is in terms of 1 hour. This lends itself to kWh/h=kW formulas and is the industry standard.

To clarify, say for instance the demand is measured in an interval of 15 minutes in length. This is 1/4 of the hour interval. You will find that the value displayed or recorded for the 15 minutes is actually the kWh value measured for the 15 minutes X 4 (kWh X 4) divided by 1 hour. Thus the value; (kWh X 4)/1h = kW and so forth. The meter is designed to do the math before displaying or recording the value.

Back when meters had dials driven by gears, a 15 minute demand gear ratio was 4 times higher than the kWh gear ratio. The demand gears were turned by the kWh measuring disc through a pushing mechanism that mechanically reset to the starting position at 15 minutes, thus it displayed what the value would be for 1 hour.

With the advent of digital meter reads, the intervals could be arranged in the variety of ways mentioned above to ascertain a clearer picture of short bursts of very high usage. The kW calculation changes accordingly, so a five minute interval meter would display the (kWh X 12)/1hr value.

Also, the tariff is applied only to the highest recorded value, not to each interval.

I bring this up since many in our audience are students, and many questions about metering show confusion about the differences in kWh, kW, kVAR, kVARh and kVA. It should be made clear to them that modern electronic meters only measure kWh. All other values are calculated from the kWh measurement, even though the display can be programmed to display a wide variety of the other values.

Also about the mention of the meter reader resetting the demand before the meter is read; In my Utility the meter is read first, and the reset is the last operation since it clears some of the dials or registers to zero. If the meter is read after the reset operation, the demand reads would always be zero.

I noted that you posted as Guest, but you seem to have a lot to offer this forum. You should join and contribute more often.

Demand refers to the power drawn by an electrical system from it's source. Knowing the maximum power demanded from the source aids in sizing and maintaining the source, especially when there are several systems drawing from the same source. Usually the maximum demand is measured in kWs (kilo Watt)s, and is per 1 hour by standard. There are several ways of interpolating to get this value, but that is not the question here.

Maximum demand is the measured/interpolated value for the highest power usage for an hour of an electrical system in kW for a given period such as a meter read cycle.

Cumulative Maximum demand is a validation method for the values. Cumulative maximum demand is the sum of all the maximum demands recorded at the time of reset to zero. Usually this reset to zero to begin measuring maximum demand for the next period occurs at the time the meter reading is collected.

Say for instance a system has a maximum demand of +/-50kW on a regular basis. The value of the MaxDem would be 50kW, and the cumulative MD 0.0kW until the demand reset is actuated. Then the MD would show 0.0kW and the CumMaxDem = 50kW after the first reset. 30 days or so later the MD = 50kW again, and CMD = 50kW until after reset. Then CumDem = 100kW while the MD is back to 0.0kW ready to measure the next period. CMD is a running total of MD.

Validation occurs this way. If the meter read cycle passes and the demand is not reset, then the next cycle would be 50 kW short in the CumDem value, indicating that the MD shown may be questioned since the device was apparently not reset to zero, advancing the CMD value. If the CMD value is 50Kw or 100Kw higher at the next meter read cycle, that not only indicates the device was reset more than once during the cycle, but can also be used to interpolate the values that were indicated at the extra occurrences of reset.

There is a great deal of detail that could be supplied but this is in keeping with the topic.

Isn't MD measured in VA?

VoltAmps is not power, it is apparent power in an AC system. Yes, the demand on a power source has elements of both voltage and amperage, but more importantly these two elements depend entirely on timing in order to transfer power. Technically if the voltage value and the actual electron action occur 90 electrical degrees (1/4 cycle) apart, there is zero power produced or transferred. The major forces that cause timing differences are induction and capacitance. The other major feature is resistance, which by comparison to the other two tends to be fairly stable in values.

Watts is power, and the measurement of watts states the true power used, or 'demanded' by an AC system. Watts are the net results; the useful, functional interaction of volts, amps and their timing.

With these two points in mind, the answer to the clarification question "Isn't MD measured in VA?" is - Both kVA and kW are used, and others such as Q metering can measure demand. Which way it is measured and stated depends on the purpose, and somewhat on the method reasonably available. Since the majority of metering is in kWhr, most tariffs delineate kW demand values. There are special foumulae utilized by the utility to derive useful values for amperage, heat loss, load percentages etc. that are used in maintenance and planning based on kW.

Originally, demand measurement was accomplished by measuring the heat produced in a specially designed circuit. Though heat is measured in watts, only the voltamp and resistance features of the AC circuit was clearly represented. This is apparent power.

With the advent of other methods of metering demand, kW was measured. Now, and especially since digital electronics arrived in metering, a full range of measurement and display of values is available.

So to bring this into the range of the original question, no matter what demand is measured, or how it is stated, cumulative demand is the sum of the demand values that get erased when the measurement device is reset to zero. Purpose = validation.