Directional Earth-Fault (50/51G, 50/51N, 50/51Sef); Compensated Coil Earthing Networks - Siemens 7SR10 Argus Series Manual

2.8

Directional Earth-Fault (50/51G, 50/51N, 50/51SEF)

The directional earth-fault elements, either measured directly or derived from the three line currents the zero
sequence current (operate quantity) and compare this against the derived zero phase sequence voltage
(polarising quantity). Chapter 1 of the Technical Manual 'Description of Operation' details the method of
measurement. The required setting is entered directly as dictated by the system impedances.
Example: Expected fault angle is -45° (i.e. residual current lagging residual voltage) therefore 67G Char Angle =
-45°
However directional earth elements can be selectable to use either ZPS or NPS Polarising. This is to allow for the
situation where ZPS voltage is not available; perhaps because a 3-limb VT is being used. Care must be taken as
the Characteristic Angle will change if NPS Polarising is used.
Once again the fault angle is completely predictable, though this is a little more complicated as the method of
earthing must be considered.
0
O
V
RES
I
RES
Resistance
Earthing Transformer
Earthed
Systems

2.8.1 Compensated Coil Earthing Networks

In compensated networks the Resonant Coil (Petersen coil) is tuned to match the capacitive charging currents
such that when an earth fault occurs, negligible fault current will flow. However, resistive losses in the primary
conductors and also in the earthing coil will lead to resistive (wattmetric) components which can be measured by
the 50/51SEF elements and used to indicate fault position. Core balance CTs are recommended for this
application to achieve the necessary accuracy of residual current measurement.
I
L
Resonant
coil
Figure 2-13
© 2018 Siemens Protection Devices Limited
MTA (I wrt V)
0
-15
Resistive Neutral
V
RES
I
RES
with Resistor
Distribution System
- Solidly Earthed
Figure 2-12 Earth Fault Angles
Feeder 1
Feeder 2
Feeder 3
I
F
Earth fault current distribution in Compensated network
7SR10, 7SR11, 7SR12 Applications Guide
0
-45 -65
Reactive Neutral
V
RES
I
RES
Transmission System
- Solidly Earthed
I
01
I
03
0 0
-90
V
V
RES
RES
I
RES
I
RES
Reactance
Earthed
Systems
I
02
Chapter 7 Page 18 of 41