ABB Relion 650 Series Applications Manual page 66

Section 6
Differential protection
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.
87N
ANSI05000164 V1 EN-US
Figure 24: 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 calculations
are made with the worst situations in mind and a minimum operating voltage V
according to equation
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Busbar protection REB650
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