Protection Functions; High-Impedance; General - ABB REG316 4 Series Operating Instructions Manual

ABB Switzerland Ltd
4.2.

Protection functions

4.2.1.

High-impedance

4.2.1.1.

General

In a high-impedance scheme, the measurement circuit repre-
sents a high-impedance across a circulating current circuit. High-
impedance protection is used for
phase and possibly earth fault protection for generators, mo-
·
tors and compensators
restricted E/F protection for power transformers.
·
The main advantage of a high-impedance scheme compared
with a normal differential scheme is its better stability for rela-
tively low through-fault currents (between I
The disadvantages are
the high voltage across the circulating current circuit during
·
internal faults
the special requirements to be fulfilled by the c.t's.
·
A high-impedance scheme is used to advantage where
fault currents are relatively low
·
discrimination is absolutely essential.
·
This is the case for phase faults on air-cored compensators and
earth faults on power transformers that are grounded via an im-
pedance. In some instances solidly grounded transformers can
also be protected.
Either an overcurrent function with a series resistor or an over-
voltage function can be used in a high-impedance scheme. The
restricted earth fault protection (R.E.F.) described in the next
Section is an example for the application of an overcurrent func-
tion.
The required level of through-fault stability is determined by the
value of the high impedance presented by the protection meas-
uring circuit.
4-4
REG 316*4 1MRB520049-Uen / Rev. B
and 5 I
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