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IEC05000738-3-en.vsd
IEC05000738 V3 EN
Figure 58: Different applications of a 1Ph High impedance differential protection
HZPDIF function
7.2.2.1 The basics of the high impedance principle
The high impedance differential protection principle has been used for many years
and is well documented in literature publicly available. Its operating principle
provides very good sensitivity and high speed operation. One main benefit offered by
the principle is an absolute stability (that is, no operation) for external faults even in
the presence of heavy CT saturation. The principle is based on the CT secondary
current circulating between involved current transformers and not through the IED
due to high impedance in the measuring branch. This stabilizing resistance is in the
range of hundreds of ohms and sometimes above one kilo Ohm. When an internal fault
occurs the current cannot circulate and is forced through the measuring branch
causing relay operation.
It should be remembered that the whole scheme, its built-in components and wiring
must be adequately maintained throughout the lifetime of the equipment in order to be
able to withstand the high voltage peaks (that is, pulses) which may appear during an
internal fault. Otherwise any flash-over in CT secondary circuits or any other part of
the scheme may prevent correct operation of the high impedance differential relay for
an actual internal fault.
Section 7 1MRK 502 071-UEN -
Differential protection
150 Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Application manual
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