Protective Elements
Settings for the Phase Distance Protection Z[2]
A remote backup protection is defined as a backup protection for faults on the remote protected object, in this case
the faults on the longest transmission line connected to the generating unit high-voltage bus. A distance protection
element for this usage should detect faults on whole line and isolate the generator with the faulted system (only) if
faults are not tripped by line protection functions because of relay failure.
The phase distance element Z[2] is selected for this remote backup protection. The impedance reach setting should
be set such that it sees at least 120% of the longest line connected to the generating station high-voltage bus:
Z = (X
TG
+ 120% ⋅ Z
LLG1
) ⋅ Z
B,secondary
= (j0.11607 + 120% ⋅ (0.04404 + j0.46437)) ⋅ 17.56 Ω
= (0.93 + j11.82) Ω
Hence it follows:
Therefore the distance protection element Z[2] is set as follows:
• »Mho Pos.Imp.Reach« = 11.9 Ω
• »Mho Pos.Imp.Angle« = 85°
• »Mho Offs.Imp.Reach« = 0 Ω
• »Mho Offs.Imp.Angle« = 0°
The time delay for Z[2] should be set greater than the tripping time of zone 2 of the longest line distance protection
(0.8 s) plus the tripping time of the used breaker failure function (0.1 s), as well as the CB opening time (e.g. 0.1 s).
With an additional security margin of 0.1 s the resulting time setting is:
•
»Trip delay time«
= (0.8 + 0.1+ 0.1 + 0.1) s = 1.1 s
The setting calculation above is only an example calculation to illustrate the
setting procedure in a simplified manner.
In fact, there are many more setting possibilities and relevant factors, such as
in-feed effects during system faults, maximum loading conditions to be
expected, stable power swings; all these may have significant impacts on the
distance backup protection settings.
It is therefore recommended that the setting of these protections be evaluated
between the generator protection engineers and the system protection
engineers to optimize coordination while still protecting the generator. System
stability studies may be necessary to help determine settings which optimize
protection and coordination.
832 MCDGV4 DOK-HB-MCDGV4-2E
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