The Basics Of The High Impedance Principle - ABB Relion REC670 Applications Manual

1MRK 511 358-UEN A
Figure 31:
6.1.2.1

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.
Id
IEC05000164 V3 EN-US
Figure 32: Example for the high impedance restricted earth fault protection application
For a through fault one current transformer might saturate when the other CTs still will feed
current. For such a case a voltage will be developed across the measuring branch. The
Application manual
Different applications of a 1Ph High impedance differential protection HZPDIF
function
R
Metrosil
IEC05000164-2-en.vsd
Section 6
Differential protection
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