ABB Relion 670 series, Applications Manual

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IEC05000163-4-en.vsd
IEC05000163 V4 EN-US
Figure 31: Different applications of a 1Ph High impedance differential protection HZPDIF
function
6.1.2.1 The basics of the high impedance principle
SEMOD54734-153 v9
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
Section 6 1MRK 505 343-UEN B
Differential protection
94
Application manual