open circuit breaker via auxiliary contacts feeding the opto-inputs or it detects a combination of both
under
current and undervoltage on any one phase.
2.3 APPLICATION NOTES
2.3.1 UNDERVOLTAGE PROTECTION
Principle
Under
voltage conditions can occur on a power system as a result of increased loading, fault conditions or
incorrect regulation. Transient voltage dips can allow successful motor re-acceleration. However, sustained
undervoltage conditions will result in the motor stalling. Time delayed undervoltage protection is commonly
applied.
The undervoltage protection included in the P24DM relays consists of three independent measuring stages.
Multiple stages are included to provide both alarm and trip stages, where required. Depending on the severity of
the voltage dip different time settings may be needed so that motor loads are able to withstand a small voltage
depression for longer than if a major voltage excursion occurs. Therefore, two stages can be used; one with a
higher setting and a longer time delay and vice versa for the second stage.
Setting guidelines
The voltage threshold setting for the undervoltage protection should be set at some value below the voltage
excursions that may be expected under normal system operating conditions. This threshold is dependent on the
system in question but typical healthy system voltage excursions may be -10% of nominal value.
The same applies to the time setting. The required time delay is dependent on the time for which the system is able
to withstand a reduced voltage. A typical time setting may be 0.5 seconds.
The setting Inhib.During St. must be set to Enabled to allow the voltage decrease during motor starting.
This feature must be interlocked with the motor control gear to ensure that it is disabled when the motor is
stopped. The interlock is made by the CB Close signal.
Chapter 10 - Voltage Protection Functions P24xM
206 P24xM-TM-EN-2.1
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