Undervoltage Protection (ANSI 27)

Presentation

Undervoltage protection (ANSI 27) constantly monitors the system voltage. If the voltage level of an installation goes out of its acceptable limits, the information provided by undervoltage protection can be used to initiate appropriate action to restore good operating conditions in the installation.

The information provided by undervoltage protection is used to generate alarms and circuit breaker tripping when required. In addition, the constant monitoring of phase-to-phase or phase-to-neutral voltages enables appropriate action to be initiated to safeguard the operation of the installation during abnormal or critical situations, for example, load shedding, source change-over, and emergency generator starting.

Prerequisites

Undervoltage protection requires an external 24 Vdc power supply.

Undervoltage protection is compatible with:

• MicroLogic Active E/EP/Ei control units for IEC standard.

• MicroLogic Active E/EP/Ei control units with firmware version greater than or equal to 004.004.000. Earlier firmware versions need to be updated.

Operating Principle

Undervoltage protection monitors either three phase-to-phase voltages (V12, V23, V31), or three phase-to-neutral voltages (V1N, V2N, V3N). Undervoltage protection has two protection modes: Per phase: each voltage is monitored independently. The undervoltage protection function detects when one of three monitored voltages reaches the threshold Vmin. All phases: the three voltages are monitored together. The undervoltage protection function detects when all three monitored voltages reach the threshold Vmin. The action performed by the undervoltage protection function is delayed by a settable time delay, tVmin. Each undervoltage protection mode, per phase or all phases, can be enabled or disabled separately.

Voltage Measurement

For undervoltage protection set in tripping mode, the voltage must be measured on the power source side to allow closing of the circuit breaker. As standard, the MicroLogic Active E/EP/Ei voltage input is directly connected to the internal pickup voltage (PTI) on the bottom side of the circuit breaker. So:

• If the circuit breaker is bottom fed, the internal pickup voltage (PTI) is suitable for undervoltage protection and circuit breaker closing.

• If the circuit breaker is top fed, an external voltage input is required. The external voltage tap (PTE) option must be used to measure the voltage on the power source side.

Setting the Protection

The settings for undervoltage protection are:

• Activation. Disable, or enable and configure the undervoltage protection mode:

  • OFF: disable undervoltage protection

  • Per phase: undervoltage protection is activated when one of three phase voltages reaches the threshold Vmin (ANSI 27-1)

  • All phases: undervoltage protection is activated when all three phase voltages reaches the threshold Vmin (ANSI 27-2)

  • Both: undervoltage protection is activated when one or all of three phase voltages reaches the threshold Vmin

• Action. Select the action to take when undervoltage protection is activated:

  • Trip: the circuit breaker trips when both voltage threshold and time delay are exceeded

  • Alarm: alarms are generated without the circuit breaker tripping

• Type. Select the type of voltage to monitor:

  • VLL phase-to-phase voltage selection

  • VLN phase-to-neutral voltage selection (this setting should be selected only with 4-pole circuit breakers or 3-pole circuit breakers with ENVT wired and configured)

• Vmin: threshold of undervoltage protection

• tVmin: time delay of undervoltage protection

They can be set on the MicroLogic Active display screen, by navigating from the tree navigation menu to Protection > Advanced Protection > Undervoltage and pressing Edit settings.

Only the fine adjustment mode is available to configure undervoltage protection settings.

Protection Settings

Protection Characteristics

Characteristics of undervoltage protection:

• Definite time delay

• Instantaneous reset time

• Hysteresis: fixed 98%

• Maximum breaking time 140 ms with time delay set to 1 s

Predefined Events

The function generates the following predefined events:

Predefined events cannot be modified by the user. For general information about events, refer to Event management.

Protection events are generated as follows:

• The start event is generated when the protection is activated.

• The operate event is generated when the protection time delay elapses.

• The trip event is generated when the circuit breaker tripping voltage release (MITOP) activates. The trip event is not generated when the protection action parameter is set to Alarm.

Recommended Actions

Resetting a Trip Event

For information about resetting the circuit breaker after a trip due to an electrical fault, refer to the relevant document:

• MasterPacT MTZ1 IEC Circuit Breakers with MicroLogic Active Control Unit - User Guide

• MasterPacT MTZ2/MTZ3 IEC Circuit Breakers with MicroLogic Active Control Unit - User Guide