Process of Load Shedding Works

Many companies are not familiar on how to reduce demand peaks, however load shedding is a very simple way to reduce energy costs. By installing a demand controller you can reduce energy costs by shedding unnecessary loads when a demand peak is coming. Shown below are calculations how to determine your current load and how much you can reduce your demand peaks.

Beginning at the start of a Reset Date the Current Load Limit is set to the same value as the Minimum Load Limit, for example 320KW. After a period of time, the system measures Current Demand during a Block to be 340KW.The demand controller begins to do load shedding At this same time Peak Demand becomes 340KW. The system uses the Peak Demand value to calculate if the Current Load Limit should be increased or not. If the Peak Demand is “less than or equal to” the Current Load Limit plus a value called Load Limit Overshoot the Current Load Limit will not increase. The Load Limit Overshoot value is preset to 20KW.

340=320+20  (equals, the Current Load Limit is not increased).

During the next Block, with the system still doing load shedding, the Current Demand increases to 380KW therefore the Peak Demand increases to 380KW (e.g. Bakery starts working). If the Peak Demand is “greater” than the Current Load Limit plus the Load Limit Overshoot the Current Load Limit will increase.

380>320+20 (greater, the current load limit to be increased)

When the Peak Demand is “greater” than Current Load Limit by the Load Limit Overshoot value or more, a new Current Load Limit will be calculated.

Peak Demand – Load Limit Overshoot = Current Load Limit

380KW – 20 = 360KW

The Current Load Limit is calculated after the Block period has ended and the new Peak Demand has been measured. The new Current Load Limit value will be used as the set point for the next Block.

Why this occurs:

The demand controller should not do load shedding at 320KW when the peak demand has already reached 380KW. Otherwise unnecessary shedding for long periods of time would exhaust the facilities thermal capacity. This would cause the loads to turn on and demand to increase. However, it is reasonable to begin shedding when the Current Demand is 360KW to prevent Peak Demand to be above 380KW.

In the above example, if the Fixed Load Limit was set to 340KW the Highest value the Current Load Limit would reach would be 340KW regardless of the Peak Demand. The system will continue to shed too early affecting Demand Control. The Fixed Load Limit is set to a value close to maximum Peak Demand to ensure it is never reached.

The Fixed Load Limit is a set point that is used by the system throughout the billing year.

Automatic Load Limit Change after Long Periods of Shedding of Many Loads

The demand is high at a facility due to environment, climate and temperature. With systems running at full power the Current Demand has reached 420KW. The system sheds loads for the set shedding period but the Current Demand remains higher than Current Load Limit (say 400KW). The system will increase the Current Load Limit by a Specified Load Limit Increment (50KW) if the following conditions are met:

  • If the number of shedding loads are bigger than “n” (e.g. 4).
  • the loads have been shed for more than a specified time (e.g. 1hour).

Current Load Limit + Specified Load Limit Increment = Next Block Current Load Limit

400KW + 50KW = 450KW

The adjusted Current Load Limit will be used in the next Block.

Why this occurs:

Continuous load shedding will disrupt the thermal capacity of the facility. At the end of the shedding period all systems will come back on simultaneously and Peak Demand will increase due to no gradual start up.

1.0             Definitions

Block : Time period demand is measured.

Current Demand [KW] : Measured demand value during a specific Block.

Current Demand = KWh (measured in block) / Block length (in hours)

Example

Cur.Dem. =  15630KWh-15490KWh / 0.5hr = 140/0.5 =  280 KW

Peak Demand [KW] : Maximum value of Current Demand measured during a billing period.

Example

Billing period is 1 month. Maximum Current Demand is 380KW.

Load Shedding : The process when the system closes normally open contacts, which  are interfaced to units ( e.g. HVAC, Lighting, etc) to reduce the facility Demand.

Load Limit : The Demand limit at which the system starts to shed.

Reset Date: The day and hour (minute) when the Peak Demand and KWh are reset to 0. Determined by the electrical provider.

Example

The first of the month at 0:00.

Minimum Current Load Limit : Load Limit in the beginning of a billing month, just after Reset Date. Usually preset to 20% below Recorded Monthly Peak Demand. Specific Monthly value determined from previous monthly bills.

Recorded Monthly Peak Demand – 20% = Minimum Current Load Limit

Example

400KW- 20% = 320KW

Fixed Load Limit : The maximum possible value of a Current Load Limit.

Example

500KW

Current Load Limit : The value of a Load Limit for a specified Block.

Load Limit Overshoot: The comparing value the system uses to determine if the Current Load Limit should be increased or not. Preset at 20KW.

Specified Load Limit Increment : Increment for a Current Load Limit, if specified number of loads are shed for more than a specified period.

Example

Specified Load Limit Increment selected at 50KW

If more than 4 loads are shed for more than 1 hour the Current Load Limit will be increased by 50KW.

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