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Section 7
Differential protection
7.2.2.1
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ANSI05000738 V2 EN-US
Figure 57:
Different applications of a 1Ph High impedance differential protection
HZPDIF (87) function
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.
Generator protection REG670 2.2 ANSI and Injection equipment REX060, REX061, REX062
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ANSI05000738-2-en.vsd
SEMOD54734-153 v9
Application manual
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