We have installed a very expensive Maximum Demand Controller from a reputed manufacturer, but still paying demand penalty by the electric utility. We don’t understand- why?
A general comment from the industries on Maximum Demand Controller (MDC)
Maximum demand by definition is the maximum power value, usually, the average of 15 minutes or 30 minutes, reached during the billing period or commonly known as the integration period. Once the value is higher than the contracted power, the customer will pay a penalty. The measure of power can be either in kVA or kW.
In a typical flow electric supply by the utility, the utility meter is the interface point, beyond which the industry installs its own HT / LT panels with Energy Management System (EnMS) and Maximum Demand Controllers (MDC). In general, the MDC computes the raising demand and predicts the likely demand that would be reached at the end of the integration period – say 15 minutes or 30 minutes, as per the applicable tariff policy. Especially in India, in most utilities, consumers with contract demand more than 1000 kVA have an integration period of 15 minutes, while all other HT and LT consumers are being billed with 30 minutes integration period.
The MDC also has relay outputs, by which the non-essential load can be tripped, in case the predicted demand is likely to exceed the contract demand during the current integration period.
Consider an industry with an integration period of 30-minutes for its utility billing/demand calculations. Let the industry’s actual load profile at a 5-minutes intervals, with reference to an absolute time reference like from Global Position System (GPS) based clock, be as shown in Fig.
Both the utility meter and the MDC see the same profile at their input points, but depending on their internal Real-Time Clock (RTC), each would start the integration period at the integer multiples of say 30-minutes period. In this case, if both the devices, utility meter, and the Maximum Demand Controller (MDC), have their RTC synchronized with the GPS Clock, then the demand profile for 30-minutes integration period would be as shown in Table 1, which is computed as the average of the 6 sets of five-minute demand values. Thus, for the considered period of 10:00 hrs to 14:00 hrs, the maximum demand occurs in the interval 10:30 to 11:00 with a value of 213 kVA.
The MD values as recorded by both the devices, the utility meter, and MDC, will match if their RTCs are synchronized to the GPS Clock or any other datum. However, this is seldom true. In many cases, the MDC being part of the industry, its RTC would be synchronized with a standard clock or it is possible to set the RTC, to the correct time, in case of any drift. On the other hand, any RTC drift in the utility meter has to be corrected only by the utility engineers and is not a high-priority task. In some cases, meter manufacturers demand a separate fee for correcting the RTC offset.
Given this practical situation, let us see the impact of RTC drift in the recordings of MD values. For simplicity, let us assume a very nominal difference of 5 minutes, between the utility meter and the MDC, as shown in Fig. 3.
For the same 5-minutes demand profile as in Fig. 2, now the demand computed over a 30-minutes integration period is shown in Fig. 4 and Table. 2.
As seen, the demand values as recorded by both devices are not the same. In fact, for the period 10:30 to 11:00, while the utility meter has recorded a demand of 213 kVA, the MDC has recorded only 192 kVA. Similarly, for the given period of 1000 hrs to 1400 hrs, the MD value as per utility meter is 213 kVA during the period 10:30 to 11:00, while as per MDC the MD value is 193 kVA during the period 13:30 to 14:00 hrs.
With a contract demand of 200 kVA for the industry, the utility bill will show a penalty for the 213 kVA, while as per MDC recordings the industry would claim that it never exceeds the contract demand of 200 kVA. If this is the problem with a drift of 5 minutes, one can imagine the difference in a recording by the two devices if the difference is a few hours.
Table 2
An innovative solution is to implement the functions of MDC combined with the Bill Audit (Check meter). This solution besides overcomes the limitations of the existing MDC also provides a complete bill audit of the utility meter. More details are available on request.
