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2. Application Notes
2.1 Overcurrent and Undercurrent Protection
2.1.1
Non-directional Overcurrent Protection
GRD140 provides distribution network protection with four-stage phase fault and earth fault
overcurrent elements OC1 to OC4, EF1 to EF4, sensitive earth fault elements SEF1 to SEF4, and
two-stage negative sequence overcurrent elements NOC1 and NOC2 which can be enabled or
disabled by scheme switch setting. The OC1, EF1 and SEF1 elements have selective inverse time
and definite time characteristics. The protection of local and downstream terminals is coordinated
with the current setting, time setting, or both.
The characteristic of overcurrent elements are as follows:
2.1.1.1 Inverse Time Overcurrent Protection
In a system for which the fault current is practically determined by the fault location, without
being substantially affected by changes in the power source impedance, it is advantageous to use
inverse definite minimum time (IDMT) overcurrent protection. This protection provides
reasonably fast tripping, even at a terminal close to the power source where the most severe faults
can occur.
Where ZS (the impedance between the relay and the power source) is small compared with that of
the protected section ZL, there is an appreciable difference between the current for a fault at the far
end of the section (ES/(ZS+ZL), ES: source voltage), and the current for a fault at the near end
(ES/ZS). When operating time is inversely proportional to the current, the relay operates faster for
a fault at the end of the section nearer the power source, and the operating time ratio for a fault at
the near end to the far end is ZS/(ZS + ZL).
The resultant time-distance characteristics are shown in Figure 2.1.2 for radial networks with
several feeder sections. With the same selective time coordination margin TC as the download
section, the operating time can be further reduced by using a more inverse characteristic.
Note: NOC provides two stage overcurrent elements.
Figure 2.1.1
Characteristic of Overcurrent Elements
⎯
10
Stage 4
Stage 1
0
I
⎯
6 F 2 S 0 7 5 8
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