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Protection Relays Settings Chapter | 19 289

Subchapter 19.4

Setting of Distance Protection

19.4.1 INTRODUCTION

Distance protection is widely used as the main protection—primary
protection—in overhead transmission lines and has the advantage that it
gives backup protection for adjacent lines. However, it has the disadvantage
that it does not have accurate measurements for close-up faults near the
remote end of the line due to the errors in CT and PT measurements and the
transient nature of the short circuit current, also due to the tower arc fault
resistance—what we call distance protection overreaching and underreach-
ing—that it is not accurate in measurement in short transmission lines due to
the effect of the system impedance ratio.

19.4.2 WORKED EXAMPLES
Worked example 1
Most distance protection is set to have three zones and one backup start zone.
The setting of the first zone is as follows for a relay at end A:
Z 1 5 0.85 Z L , t 1 5 0 instantaneously
where Z L 5 line impedance.
Setting of the second step zone 2 is as follows for a relay at end A
(Fig. 19.4.1):
The Z 2 setting is the lower value of the following:

ð
Z 2 5 0:85 Z L11 0:85 Z L2 Þ
ð
Z 2 5 0:85 Z L1 1 Z T Þ

FIGURE 19.4.1 Distance protection application setting: Example 1.