Calculating Demand Values (MicroLogic E)

Presentation

The MicroLogic E trip unit calculates and displays:

• the demand values of phase and neutral currents,

• the demand value of the total active power.

The maximum (peak) demand current and power values are stored in the memory. All demand values are updated once every minute.

Definition

The demand value of a quantity is its average value over a given period of time. In electrical power systems, it is used especially for the current and power. The demand value should not be confused with the instantaneous value or the average (or mean) value, which often refers to the average (or mean) of the instantaneous values of the 3 phases.

Calculation Interval

The time interval (or window) over which the average is calculated can be of 2 types:

• Fixed window

  

  At the end of a fixed metering window:

  • the demand value over the window is calculated and updated,

  • the new demand value is initialized over a new window, starting from the end of the last window.

• Sliding window

  

  At the end of a sliding window:

  • The demand value over the window is calculated and updated.

  • The new demand value is initialized over a new window, starting from a given time after the start of the last window (always less than the duration of the window).

The duration of the sliding window can be set separately for current and power demand from 5 to 60 minutes in 1 minute steps (refer to Measurement Settings). The default setting is 15 minutes.

The time shift between intervals is equal to 1 minute.

Calculation Method

MicroLogic E trip units use the quadratic model to calculate both demand current and demand power.

The quadratic demand calculation model represents the conductor heat rise (thermal image).

The heat rise created by the current I(t) over the time interval T is identical to that created by a constant current Ith over the same interval. This current Ith represents the thermal effect of the current I(t) over the interval T.

Calculation of the demand value according to the thermal model must be always be performed on a sliding window.

Peak Demand Values

The MicroLogic E trip unit calculates:

• the maximum (peak) demand values of phase and neutral currents since the last reset,

• the maximum (peak) demand values of total active power since the last reset.

The peak demand values can be accessed and/or reset in the following ways:

• Peak demand current: via the MicroLogic trip unit (see the detailed topic) or the communication option.

• Peak demand power: via the communication option.