High Impedance Restricted Earth Fault Protection (87Ref) - Siemens 7SR17 Rho Manual

Chapter 7) 7SR17 Rho Applications Guide
2.6 H I
IGH MPEDANCE
Restricted Earth Fault (REF) protection is applied to motors to detect low level earth faults in the stator windings.
Current transformers are located as shown in Figure 2.6-1. During normal operation 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 REF function is configured to provide an instantaneous trip output to minimise damage from developing
winding faults.
The calculation for the value of the Stabilising Resistor (Rstab) is based on the worst case where for the
maximum current one CT fully saturates and the other balancing CT does not saturate at all.
The required fault setting (primary operate current) of the protection is chosen; typically, this is less than 10% of
the motor rated current.
Additional external Non-Linear Resistor and stabilising resistor components are required.
See separate publication 'Technical Guidance Note (TGN) High Impedance Restricted Earth Fault Protection'.
Page 22 of 36
R E
ESTRICTED ARTH
Motor
A
B
C
NLR
Figure 2.6-1 Restricted Earth-fault protection
F P
AULT ROTECTION
Rstab
7SR17
©2014 Siemens Protection Devices Limited
(87REF)