ABB Switzerland Ltd REG 316*4 1MRB520049-Uen / Rev. B
4-4
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
N
and 5 I
N
).
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.
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