6 DISTANCE ISOLATED AND COMPENSATED SYSTEMS
6.1 PETERSON COIL EARTHED SYSTEMS
A Petersen Coil earthing system is used in compensated earthing systems, as well as being 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 compensated 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
0
20
40
60
80
100
120
140
11kV 22kV 33kV 65kV 110kV
Earth fault extinction current limits
as per DIN VDE 0228 Part 2
Capacitive fault current—isolated neutral
Residual fault current—compensated neutral
Figure 82: Current level (amps) at which transient faults are self-extinguishing
The following figure depicts a simple network earthed through a Petersen Coil reactance. It can be shown that if
the reactor is correctly tuned, theoretically no earth fault current will flow.
P543i/P545i Chapter 7 - Distance Protection
P54x1i-TM-EN-1 179
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