Limiting peak demand with IF-THEN rules on a power meter concentrator

Many electricity contracts for commercial and industrial sites include a charge based on the highest demand in the billing period, or a contracted capacity that should not be exceeded. The peak is often caused by a short overlap of loads that could have waited a few minutes, such as water heaters, ventilation or EV chargers. Limiting peak demand means detecting that overlap in time and switching some of those loads off for a while. This article explains how demand is measured and how to build staged load shedding with the IF-THEN-ELSE logic on an ICP DAS power meter concentrator, with a worked example.

Demand, peak demand and the integration period

Instantaneous power changes from second to second. Demand is the average power over a fixed integration period, often 15 minutes; the exact period and the way it is billed are set by the supply contract or the grid operator. Peak demand is the highest of these averages in the billing period. Two consequences matter for control:

• A short spike counts only in proportion to its length. An extra 100 kW for 3 minutes raises a 15-minute average by 20 kW, not by 100 kW.
• There is time to react. If consumption is heading above the limit early in the period, switching off a few loads for the rest of it can keep the average below.

Load shedding mainly moves consumption in time. Choose loads that can be interrupted for some minutes without harming the process, product quality or comfort.

What the concentrator provides

The ICP DAS PMC and PMD series power meter concentrators have a built-in IF-THEN-ELSE logic engine for power demand management. Rules are edited on web pages without programming, and the specifications list an unlimited number of rules, timers and schedules, plus internal registers (70 on the PMC-5231). The concentrator reads the power meters and adjusts device operation according to the power status through Modbus I/O modules, which is how load shedding is carried out. Alarms can be sent by email, and on the IIoT models also by LINE or Telegram.

Which values can serve as rule conditions, and how demand is calculated, depends on model and firmware; check the user manual before fixing the design.

Outputs for switching loads

• M-7000 RS-485 I/O modules, for example the M-7061 with 12 Form A power relays rated 5 A at 250 VAC. Configurable safe values and a dual watchdog define what each output does when communication is lost.
• XV-board expansion boards on the PMC-5231, PMC-2241M and PMC-2841M, or XW-boards on the PMC-5151.
• Standard Modbus TCP or Modbus RTU slave modules, which the concentrators can also use.

These relays switch contactor coils or enable inputs, not the load itself. Wiring them into a distribution board is work for a qualified electrician.

Load shedding logic for limiting peak demand

1. Measure the total with a three-phase meter on the main incomer, for example a PM-3133-RCT with Rogowski coils for large cables.
2. List the sheddable loads by priority, with the power of each, how long it may stay off and how often it may be switched.
3. Set staged thresholds: a warning level, then one threshold per stage. Shed the lowest-priority load first.
4. Add delays and hysteresis. Shed only when a threshold has been exceeded for some time, and restore only when power has fallen clearly below it, otherwise loads cycle around the limit.
5. Limit the off-time and restore in reverse order. Bring each load back after its maximum off-time, and restore the last load shed first.
6. Use schedules to enable shedding only when it is needed, and provide a manual override.
7. Define the fail-safe state. Decide whether each load should run or stay off if the concentrator or bus fails, and set the safe values of the output module to match.

Worked example

A building has a target maximum demand of 250 kW. All figures here are example values; use your own contract and load data. A PM-3133-RCT500P meter on the main incomer is read by a PMC-5231 over RS-485. An M-7061 relay module on the same RS-485 line drives contactors or enable inputs for three sheddable loads.

The rules in plain form (the web configuration looks different):

• IF total kW > 225 for 60 s THEN send email "demand warning"
• IF total kW > 235 for 60 s THEN relay 1 OFF
• IF total kW > 245 for 60 s THEN relay 2 OFF
• IF total kW > 250 for 30 s THEN relay 3 OFF
• IF total kW < 190 for 5 min THEN restore the last relay switched off
• IF relay n OFF longer than its max. off-time THEN relay n ON

The delays and off-time limits can be implemented with the timer function. The delay stops motor starts and other short spikes, which hardly affect a 15-minute average, from triggering a shed. Test the rules outside working hours with lowered thresholds, and check in the logged data that each stage switches off and restores as intended.

Which ICP DAS products fit

• Concentrators: PMC-5231, PMC-5231M, PMC-2241M, PMC-2841M, PMC-5151-EN, PMD-2201-EN and PMD-4201-EN all have the IF-THEN-ELSE logic engine for power demand management. The PMD models add a local touch panel.
• Meters: the PM-3133 series, the PM-3133-RCT for large conductors, and the PM-4324 where many feeders share one board.
• Outputs: M-7000 relay modules such as the M-7061, or XV-board expansion.
• Single meter: where only one incomer needs watching, the PM-5133 shows a 1-hour demand trend on its 3.5" touch screen, gives a contracted capacity warning and has two relay outputs triggered by defined alarms. For more outputs or staged rules, use a concentrator.

Send technical sales your demand limit, a load profile and a list of loads that can be interrupted, and we will help you plan meters, concentrator and outputs.