Measurements; Protection; 1.Residual Voltage Protection; 2.Overvoltage And Undervoltage Protection - ABB REX 521 Technical Reference Manual, Standard Configurations

REX 521
11.6.2.
11.6.3.
11.6.3.1.
11.6.3.2.
196
Protection Relay
Technical Reference Manual, Standard Configurations

Measurements

Following measurements are supported in this application example:
• True RMS measurement of phase-to-phase voltages
• True RMS measurement of residual voltage
• System frequency measurement

Protection

All the necessary settings for the protection functions can be set by means of the
Relay Setting Tool or the local HMI. The settings can be found in
Main menu\Protection\.
Residual voltage protection
In a compensated or isolated network, a three-stage residual overvoltage protection
can be used for the main earth-fault protection of the busbar system and for the back-
up protection of the network. On the busbar, the residual voltage can be measured
with a set of voltage transformers in an open-delta connection.
The low stage Uo> can be given a sensitive setting and have an alarming function.
The high stage Uo>> can work as back-up for the outgoing feeders and perform trip
on the incoming feeder and outgoing feeders. The instantaneous stage Uo>>> can
be configured as busbar protection and provide trip to incoming feeder and outgoing
feeders.
The settings for the residual voltage protection are made in the settings for the
residual overvoltage functions and can be found in
Main menu\Protection\Uo>\ ... \Uo>>>\ .
Overvoltage and undervoltage protection
Faults in the network or a faulty tap changer or voltage regulator of a power
transformer may cause abnormal busbar voltages. The overvoltage and undervoltage
protection is used to prevent possible damages to the devices connected to the
network.
In this application the overvoltage and undervoltage protection is included to the
functionality of the protection relay of the measurement cubicle. In the overvoltage
or undervoltage situation the circuit breakers of the outgoing feeders will be opened.
The overvoltage protection includes two protection stages: a low stage U> and a
high stage U>>. The low stage can be set to the definite-time or inverse-time
operation. The definite-time operation is used with the high set stage.
Similarly, the undervoltage protection includes two protection stages: a low stage
U< and a high stage U<<. The low stage can be set to the definite-time or inverse-
time operation. The definite-time operation is used with the high set stage.
An operational characteristic example for overvoltage and undervoltage protection
is presented in a picture below.
1MRS 751802-MUM