Voltage Protection Functions

The following voltage protection functions are available with the MicroLogic P trip unit.

Minimum voltage
Maximum voltage
Voltage unbalance

The voltage protection functions are set using the keypad. The settings are listed in Threshold and Time-Delay Settings.

Operating Principle

Protection tripped by a minimum value

Protection tripped by a maximum value

Pickup threshold
Pickup time delay
Dropout threshold
Dropout time delay

For protection tripped by a minimum or maximum value, it is possible to set:
- A pickup threshold (1) that activates an alarm, a contact and/or tripping.
- A pickup time delay (2) that steps in when the pickup threshold (1) is reached.
- A dropout threshold (3) corresponding to deactivation of the alarm and/or contact.
- A dropout time delay (4) that steps in when the dropout threshold (3) is reached.
For protection tripped by a minimum value, the dropout threshold is always greater than or equal to the pickup threshold.
For protection tripped by a maximum value, the dropout threshold is always less than or equal to the pickup threshold.
If both the minimum and maximum protection functions are activated at the same time, the minimum threshold is automatically limited to the value of the maximum and vice versa.

Minimum-Voltage Protection U min

This function calculates the minimum rms value of the three phase-to-phase voltages.
Protection is activated when at least one of the three phase-to-phase voltages (U12, U23, U31) is below the threshold set by the user.
This protection function does not detect phase failure.

NOTE: If the voltage protection functions are activated and the voltage measurement inputs are still energized, it is impossible to reset and close the circuit breaker.

Maximum-Voltage Protection U max

This function calculates the maximum rms value of the three phase-to-phase voltages.
Protection is activated when the three phase-to-phase voltages (U12, U23, U31) are simultaneously above the threshold set by the user.

Voltage-Unbalance Protection U unbal

This protection is activated by an adjustable level of unbalance between the rms values of the three phase-to-phase voltages.

This function calculates the rms value of the unbalance between the three phase-to-phase voltages.

U unbal = |E max|/U avg

Where:

U avg is the average value of the rms voltages of the three phases.

U avg = (U12 + U23 + U31)/3
E max is the maximum difference between the voltage of each phase and U avg.