[fo_function87GHApplication&SettingsFunction2, 1, en_US]
Operation Requirement
For internal faults the relay will operate at the calculated voltage setting (V
set
). This operating voltage will also
be applied across the CT secondary windings of all the CT secondaries connected in parallel with the relay. This
voltage will drive a magnetizing current in each of the CT secondary windings and this must be added to the
relay operate current when calculating the operate current of the high-impedance protection scheme.
[fo_function87GHApplication&SettingsFunction3, 1, en_US]
Consideration of Component Thermal Ratings
When the relay circuit operates for an internal fault the circuit‑breakers are opened and the flow of fault
current ceases.
Where a CB fails to trip then fault current will flow in the high‑impedance circuit until the fault is cleared by
the operation of CB failure or back-up protection. The thermal rating of the relay circuit components should
then be considered.
Alternatively the high‑impedance circuit can be arranged to short circuit the external components after opera-
tion.
Establishing the Required Stabilizing Resistor Value
The relay burden need not be considered as it is effectively negligible relative to the burden of the stabilizing
resistor. The setting (operate) voltage (V
set
) across the relay and stabilizing resistor at the relay operating
current (I
set
).
[fo_function87GHApplication&SettingsFunction4, 1, en_US]
Stabilizing resistor power rating must be sufficient for continuous operation at the circuit operate voltage
(V
set
).
[fo_function87GHApplication&SettingsFunction5, 1, en_US]
Short time rated to withstand I
Fint
for the maximum fault clearance time. For a failed circuit‑breaker condition
the back up protection clearance time is considered, typically a 1 second rating is sufficient.
[fo_function87GHApplication&SettingsFunction6, 1, en_US]
Where:
[fo_function87GHApplication&SettingsFunction7, 1, en_US]
Where I
Fint
is not known, the breaking capacity current of the circuit‑breaker can be used.
Limiting Circuit Over-Voltages (Metrosils)
Non-linear resistors are connected in parallel with the relay circuit to limit the peak voltage developed across
the high‑impedance components during internal faults to a safe level below 3 kV peak. Where a Metrosil is not
connected in circuit the peak voltage can be calculated from:
Protection and Automation Functions
5.21 87GH Restricted Earth-Fault Protection – High-Impedance
298
Reyrolle 7SR5, Transformer Protection Device, Device Manual
C53000-G7040-C015-1, Edition 11.2019
