ABB RELION RET670, Applications Manual

Page: 374 / 1016

8.12.2 Application
IP14961-1 v2
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The fast distance protection function ZMFPDIS in the IED is designed to provide sub-
cycle, down to half-cycle operating time for basic faults. At the same time, it is
specifically designed for extra care during difficult conditions in high-voltage
transmission networks, like faults on long heavily loaded lines and faults generating
heavily distorted signals. These faults are handled with utmost security and
dependability, although sometimes with a reduced operating speed.
8.12.2.1 System grounding
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The type of system grounding plays an important role when designing the protection
system. Some hints with respect to distance protection are highlighted below.
Solidly grounded networks
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In solidly grounded systems, the transformer neutrals are connected directly to ground
without any impedance between the transformer neutral and ground.
ANSI05000215 V2 EN-US
Figure 173: Solidly grounded network
The ground-fault current is as high or even higher than the short-circuit current. The
series impedances determine the magnitude of the fault current. The shunt admittance
has very limited influence on the ground-fault current. The shunt admittance may,
however, have some marginal influence on the ground-fault current in networks with
long transmission lines.
The ground-fault current at single phase-to-ground in phase A can be calculated as
equation
258 :
A A
0
1 2 0 f 1 N f
3 V
3I
Z Z Z 3Z Z Z Z
V
×
= =
+ + + + +
EQUATION1710 V2 EN-US
(Equation 258)
Section 8
1MRK 504 163-UUS A
Impedance protection
368 Transformer protection RET670 2.2 ANSI
Application manual