Siemens 7SR10 Argus Series, Manual

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7SR10, 7SR11, 7SR12 Applications Guide
© 2018 Siemens Protection Devices Limited Chapter 7 Page 21 of 41
2.9 High Impedance Restricted Earth Fault Protection (64H)
Restricted Earth Fault (REF) protection is applied to Transformers to detect low level earth faults in the
transformer windings. Current transformers are located on all connections to the transformer. During normal
operation or external fault conditions no current will flow in the relay element. When an internal earth fault occurs,
the currents in the CTs will not balance and the resulting unbalance flows through the relay.
The current transformers may saturate when carrying high levels of fault current. The high impedance name is
derived from the fact that a resistor is added to the relay leg to prevent relay operation due to CT saturation under
through fault conditions.
The REF Trip output is configured to provide an instantaneous trip output from the relay to minimise damage from
developing winding faults.
The application of the element to a delta-star transformer is shown in
Figure 2.18 . Although the connection on the
relay winding is more correctly termed a Balanced Earth-Fault element, it is still usually referred to as Restricted
Earth Fault because of the presence of the transformer.
Figure 2-18 Balanced and Restricted Earth-fault protection of Transformers
The calculation of the value of the Stability Resistor is based on the worst case where one CT fully saturates and
the other balancing CT does not saturate at all. A separate Siemens Protection Devices Limited Publication is
available covering the calculation procedure for REF protection. To summarise this:
The relay Stability (operating) Vs voltage is calculated using worst case lead burden to avoid relay operation for
through-fault conditions where one of the CTs may be fully saturated. The required fault setting (primary operate
current) of the protection is chosen; typically, this is between 10 % and 25 % of the protected winding rated
current. The relay setting current is calculated based on the secondary value of the operate current, note,
however, that the summated CT magnetising current @ Vs must be subtracted to obtain the required relay
operate current setting.
Since the relay operate current setting and stability/operating voltage are now known, a value for the series
resistance can now be calculated.
A check is made as to whether a Non-Linear Resistor is required to limit scheme voltage during internal fault
conditions – typically where the calculated voltage is in excess of 2kV.
The required thermal ratings for external circuit components are calculated.
Balanced
Earth Fault
Restricted
Earth Fault