Chapter 5 PFC Setup and Operation
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Protection against High Harmonics
High THD protection prevents damage to capacitors at high levels of voltage
harmonics, which can be caused by voltage resonance at a higher frequency.
The effect of the high THD setpoint protection is limited progressive disconnection
of the capacitor banks until resonance conditions disappear, and then, after a
long safety delay, returning the PFC into normal operation.
The setpoint can operate at high THD of 3 to 300% depending on the network
conditions with a delay of 0.3 to 300 seconds. The setpoint is released when the
voltage THD drops to a level of 10-20% of the setpoint operate limit. To avoid un-
necessary disconnecting of the banks, the setpoint is released without a delay.
From the Event/Alarm Setpoints Menu, set the setpoint parameters as follows (example):
Trigger
=
r.thd.U
Operate Limit =
5-10%
Release Limit =
4-5% of the Operate Limit
Operate Delay =
20 sec
Release Delay =
0
Action
=
SFt.OF
After the alarm, the PFC will not return to automatic operation for a period of time that is
50 times the power factor setpoint operate delay.
Protection by External Triggers
A protection setpoint can also be used to disconnect the capacitor banks in
response to an external contact status provided through the instrument status
input. This may be useful in substations with generators or large synchronous
motors that generate a large amount of reactive power. The contact should be
closed if generators or motors are working and opened if they are not working.
The control signal can be provided from normally open auxiliary contacts of the
start-up contactors.
From the Event/Alarm Setpoints Menu, set the setpoint parameters as follows (example):
Trigger
=
St. On
Operate Limit =
none
Release Limit =
none
Operate Delay =
5 sec
Release Delay =
1 sec
Action
=
Hrd.OF
or
SFt.OF