Siemens siprotec 7SD5 User Manual

2.5 Distance Protection
121
7SD5 Manual
C53000-G1176-C169-1
which were declared as being valid in the initial stage, cannot be eliminated by this
stage, even if they have larger impedances.
In this manner unfaulted „apparent impedances“ are eliminated on the one hand, while
on the other hand, unsymmetrical multi-phase faults and multiple short-circuits are
recognized correctly.
The loops that were designated as being valid are converted to phase information so
that the fault detection correctly alarms the faulted phases.
Double Faults in
Effectively Earthed
Systems
In systems with an effectively or low-resistant earthed starpoint, each connection of a
phase with earth results in a short-circuit condition which must be isolated immediately
by the closest protection systems. Fault detection occurs in the faulted loop associat-
ed with the faulted phase.
With double earth faults, fault detection is generally in two phase-earth loops. If both
earth loops are in the same direction, a phase-phase loop may also pick up. It is pos-
sible to restrict the fault detection to particular loops in this case. It is often desirable
to block the phase-earth loop of the leading phase, as this loop tends to overreach
when there is infeed from both ends to a fault with a common earth fault resistance
(Parameter 1521 2Ph-E faults = Block leading Ø). Alternatively, it is also pos-
sible to block the lagging phase-earth loop (Parameter 2Ph-E faults = Block
lagging Ø). All the affected loops can also be evaluated (Parameter 2Ph-E faults
= All loops), or only the phase-phase loop (Parameter 2Ph-E faults = Ø-Ø
loops only) or only the phase-earth loops (Parameter 2Ph-E faults = Ø-E
loops only).
A prerequisite for these restrictions is that the relevant loops indicate fault locations
which are close together and within the reach of the first zone Z1. The loops are con-
sidered to be close together when they have the same direction and have both been
observed in zone Z1. The loops are considered to be close together when they do not
differ by more than a factor 1.5 (largest to smallest impedance). This prevents the
elimination, during multiple faults with separate fault location, of the loop relating to the
closer fault location by the set restriction. Furthermore a phase-to-phase measure-
ment can only be performed if two earth faults as described above are located close
to one another.
In Table 2-9 the measured values used for the distance measurement in earthed
systems during double earth faults are shown.
Table 2-9 Evaluation of the measured loops for double loop faults in an earthed system in
case both earth faults are close to each other
Loop pickup Evaluated loop(s) Setting of parameter 1521
L1-E, L2-E, L1-L2
L2-E, L3-E, L2-L3
L1-E, L3-E, L3-L1
L2-E, L1-L2
L3-E, L2-L3
L1-E, L3-L1
2Ph-E faults = Block leading Ø
L1-E, L2-E, L1-L2
L2-E, L3-E, L2-L3
L1-E, L3-E, L3-L1
L1-E, L1-L2
L2-E, L2-L3
L3-E, L3-L1
2Ph-E faults = Block lagging Ø
L1-E, L2-E, L1-L2
L2-E, L3-E, L2-L3
L1-E, L3-E, L3-L1
L1-E, L2-E, L1-L2
L2-E, L3-E, L2-L3
L1-E, L3-E, L3-L1
2Ph-E faults = All loops
www . ElectricalPartManuals . com

Siemens siprotec 7SD5 Specifications

General

Brand	Siemens
Model	siprotec 7SD5
Category	Relays
Language	English