Peterson Coil Earthed Systems; Figure 37: Current Level (Amps) At Which Transient Faults Are Self-Extinguishing; Figure 38: Earth Fault In Petersen Coil Earthed System - GE MiCOM P40 Agile Technical Manual

P14D
7.5.2

PETERSON COIL EARTHED SYSTEMS

A Petersen Coil earthing system is used in cases of high impedance earthing. Petersen Coil earthed systems (also
called compensated or resonant systems) are commonly found in areas where the system consists mainly of rural
overhead lines. They are particularly beneficial in locations which are subject to a high incidence of transient
faults. In a Petersen Coil earthed system, the network is earthed via a reactor, whose reactance is tuned to be
nominally equal to the total system capacitance to earth. Similar to insulated systems, if a single-phase to earth
fault is applied to a Petersen Coil earthed system, under steady state conditions no earth fault current flows. The
effectiveness of the method in reducing the current to zero is dependent on the accuracy of the tuning of the
reactance value and any changes in system capacitance (for example due to system configuration changes)
require changes to the coil reactance. In practice, perfect matching of the coil reactance to the system
capacitance is difficult to achieve, so that a small earth fault current will flow.
In isolated and compensation earthed systems, if an earth fault current is below a certain level, then the fault will
self-extinguish due to the low current magnitude. It therefore appears as a transient phenomenon. The figure
below shows earth fault current levels, below which they self-extinguish on these types of system. Statistics
demonstrate that around 80% of earth faults in Petersen Coil earthed systems self-extinguish. This, in part,
explains their popularity
V00756

Figure 37: Current level (amps) at which transient faults are self-extinguishing

The following figure depicts a simple network earthed through a Petersen Coil. It can be shown that if the reactor is
correctly tuned, no earth fault current will flow.
Source
V
AN
jX
L
(=I )
L
Petersen
jX
L
Coil
V00631

Figure 38: Earth fault in Petersen Coil earthed system

Consider a radial distribution system earthed using a Petersen Coil with a phase to earth fault on phase C, shown
in the figure below:
P14D-TM-EN-8
Earth fault extinction current limits
as per DIN VDE 0228 Part 2
140
120
100
80
60
40
20
0
11kV 22kV
Residual fault current—compensated neutral
Capacitive fault current—isolated neutral
-I
B
-I
C
V
    AN
I I I
f B C
j X
L
V
 AN  
0 i f I I
B C
I j X
f
L
-jX
Chapter 6 - Current Protection Functions
33kV 65kV 110kV
V V
AB ac
 
jX jX
C C
(=-I ) (=-I )
B C
-jX -jX
C C C
C
-I
C
A
I
L
-I
B
V V
AC AB
N
B
Current vectors for A phase fault
103