Figure 2.8-2 Composite Overcurrent And Restricted Earth-Fault Protection - Siemens 7SR21 Manual

7SR21 & 7SR22 Argus Applications Guide
Composite overcurrent and REF protection can be provided using a multi-element relay as.
overcurrent
elements
series
stabilising
REF
resistor
25
element
non-linear resistor

Figure 2.8-2 Composite Overcurrent and Restricted Earth-fault Protection

Although core-balance CTs are traditionally used with elements requiring sensitive pickup settings, cost and size
usually precludes this on REF schemes. Instead single-Phase CTs are used and their secondary's connected in
parallel.
Where sensitive settings are required, the setting must be above any line charging current levels that can be
detected by the relay.
On occurrence of an out of zone earth fault the elevation of sound phase voltage to earth in a non-effectively
earthed system can result in a zero sequence current of up 3 times phase charging current flowing through the
relay location.
The step change from balanced 3-phase charging currents to this level of zero sequence current includes
transients. It is recommended to allow for a transient factor of 2 to 3 when determining the limit of charging
current. Based on the above considerations the minimum setting of a relay in a resistance earthed power system
is 6 to 9 times the charging current per phase.
High impedance differential protection is suitable for application to auto transformers as line currents are in phase
and the secondary current through the relay is balanced to zero by the use of CTs ratios at all three terminals.
High impedance protection of this type is very sensitive and fast operating for internal faults.
©2010 Siemens Protection Devices Limited Chapter 7 Page 22 of 39