GE P24DM, Техническое руководство

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3.1.1 IEC 60255 IDMT CURVES
There are four well-known variants of this characteristic:
● Standard Inverse
●
Very inverse
● Extremely inverse
●
UK Long Time inverse
These equations and corresponding curves governing these characteristics are very well known in the power
industry.
Standard Inverse
This characteristic is commonly known as the 3/10 characteristic, i.e. at ten times setting current and TMS of 1 the
relay will operate in 3 seconds.
The characteristic curve can be defined by the mathematical expression:
t T
I
I
op
s
=






−
0 14
1
0 02
.
.
The standard inverse time characteristic is widely applied at all system voltages – as back up protection on EHV
systems and as the main protection on HV and MV distribution systems.
In general, the standard inverse characteristics are used when:
● There are no co-ordination requirements with other types of protective equipment further out on the
system, e.g. Fuses, thermal characteristics of transformers, motors etc.
●
The fault levels at the near and far ends of the system do not vary significantly.
● There is minimal inrush on cold load pick up. Cold load inrush is that current which occurs when a feeder is
energised after a prolonged outage. In general the relay cannot be set above this value but the current
should decrease below the relay setting before the relay operates.
Very Inverse
This type of characteristic is normally used to obtain greater time selectivity when the limiting overall time factor is
very low, and the fault current at any point does not vary too widely with system conditions. It is particularly
suitable, if there is a substantial reduction of fault current as the distance from the power source increases. The
steeper inverse curve gives longer time grading intervals. Its operating time is approximately doubled for a
reduction in setting from 7 to 4 times the relay current setting. This permits the same time multiplier setting for
several relays in series.
The characteristic curve can be defined by the mathematical expression:
t T
I
I
op
s
=





 −
13 5
1
.
Extremely Inverse
With this characteristic the operating time is approximately inversely proportional to the square of the current. The
long operating time of the relay at peak values of load current make the relay particularly suitable for grading with
fuses and also for protection of feeders which are subject to peak currents on switching in, such as feeders
supplying refrigerators, pumps, water heaters etc., which remain connected even after a prolonged interruption of
supply.
P24xM Chapter 6 - Current Protection Functions
P24xM-TM-EN-2.1 85