Application Guide - NP900 Series
79 (504
)
A996A
P
ETERSEN COIL GROUNDED
(
COMPENSATED
)
NETWORK
There are many benefits with Petersen coil grounded network. Amount of automatic
reclosing is highly decreased and therefore maintenance of breakers is diminished. Arc
faults die on their own and cables and equipment suffer less damage. In emergency
situations line with earth fault can be used for certain time.
Figure 3.2.5.2-40 Angle tracking of DEF function. Petersen coil grounded network model.
When Petersen coil grounded (compensated) network mode is chosen the IED expects
the fault current to be opposite direction to zero sequence voltage. Healthy phases of
healthy feeders and faulty feeder produce capacitive current similar to unearthed network.
Inductance of Petersen coil is compensating the capacitive current and therefore residual
current in fault location is close to zero. Size of the inductance is chosen according the
prospective earth fault current of the network. Desired compensation grade is achieved
when k-factor is close to 1.0 and network is fully compensated. Network is over
compensated when k-factor is greater than one and under compensated if the factor is
smaller than one.
Inductance connected to the star point of incoming transformer or like in most cases to
grounding transformer compensates the capacitance of the network but due this the
capacitive fault current cannot be measured anymore. Fault detection is handled by
connecting a resistance on parallel with the inductance. This resistance concludes the
amplitude of fault current. In under –or over compensated situations the resistive
component during the fault does not change, therefore selective tripping is ensured even if
the network is bit under –or over compensated.
U0
I
0
F2 current
I
0
F1 current
Over
compensated
Under
compensated
