ABB RET670 Applications Manual page 141

1MRK504116-UUS C
Application manual
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ANSI05000738 V2 EN
Figure 35: 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. The operating characteristic 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 its high impedance, normally in
the range of hundreds of ohms and sometimes above kiloohm. When an internal fault
occurs the current cannot circulate and is forced through the differential circuit causing
operation.
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Section 3
IED application
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