Isolated Networks; Minimum Polarising Voltage - Siemens 7SR10 Argus Series Manual

2.8.2 Isolated Networks

During earth faults on isolated distribution networks there is no fault current path to the source and subsequently
no fault current will flow. However, the phase-neutral capacitive charging currents on the three phases will
become unbalanced and the healthy phase currents will create an unbalance current which flows to earth.
Unbalanced charging current for the whole connected network will return to the source through the fault path. This
will produce a current at the relay which can be used to detect the presence of the fault. On each healthy circuit
the unbalanced capacitive currents appear as a residual current which LAGS the residual voltage by 90º. On the
faulted circuit the charging current creates no residual but the return of the charging current on the other circuits
appears as a residual current which LEADS the residual voltage by 90º. The characteristic angle should be set to
+90º.
INCOMER
Some customers prefer to use only the sine (reactive) component of the residual current which can be easily
achieved by setting 'Ires Select' to 'Ires Real' to select the operating current to I
characteristic angle Ø to +90º.

2.8.3 Minimum Polarising Voltage

The correct residual voltage direction must be measured to allow a forward/reverse decision to be made.
Minimum polarising voltage setting can be used to prevent tripping when fault conditions are such that significant
residual voltage is not generated and the directional decision would be unreliable. The setting must allow for error
in voltage measurement due to VT inaccuracy and connection. It can be used to improve stability under non-fault
conditions during unbalanced load, when earth fault elements with very sensitive current settings are applied. This
is ensured by selecting a setting which is near to the minimum expected residual voltage during fault conditions.
High impedance earthing methods, including compensated and isolated systems, will result in high levels of
residual voltage, up to 3 times normal phase to neutral voltage, during earth faults. The minimum polarising
voltage can therefore be increased to allow very low residual current settings to be applied without risk of
operation during unbalanced load conditions.
© 2018 Siemens Protection Devices Limited
Figure 2-17 Earth fault current in isolated network
7SR10, 7SR11, 7SR12 Applications Guide
I +I +I +I +I +I
B1 C1 B2 C2 B3 C3
I
B1
I
C1
Circuit 2
I
B2
I
C2
Circuit 3
I
B3
I
C3
Cos(θ-Ø) and setting the
RES
Circuit 1
Chapter 7 Page 20 of 41