8.13.2 Application
Sub-transmission networks are being extended and often become more and more
complex, consisting of a high number of multi-circuit and/or multi terminal lines of very
different lengths. These changes in the network will normally impose more stringent
demands on the fault clearing equipment in order to maintain an unchanged or increased
security level of the power system.
The high speed distance protection function (ZMFCPDIS) in the IED is designed to
provide sub-cycle, down to half-cycle operate 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 outmost security and
dependability, although sometimes with reduced operating speed.
8.13.2.1 System grounding
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
In solidly grounded systems, the transformer neutrals are connected directly to ground
without any impedance between the transformer neutral and ground.
ANSI05000215 V2 EN
Figure 186: 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
104:
Section 8 1MRK 506 369-UUS -
Impedance protection
384 Line distance protection REL670 2.2 ANSI
Application manual
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