Directional Overcurrent Protection; Time Delay Vts; Setting Guidelines - GE MiCOM P40 Agile Technical Manual

Application Notes
MiCOM P40 Agile P442, P444
The timer hold facility can be found for the first and second overcurrent stages as settings
I>1 tRESET and I>2 tRESET. Note that these cells are not visible if an inverse time reset
characteristic has been selected, as the reset time is then determined by the programmed
time dial setting.
4.3.3

Directional Overcurrent Protection

If fault current can flow in both directions through a relay location, it is necessary to add
directional control to the overcurrent relays to obtain correct discrimination. Typical systems
that require such protection are parallel feeders and ring main systems. Where I>1 or I>2
stages are directionalised, no characteristic angle needs to be set as the relay uses the
same directionalising technique as for the distance zones (fixed superimposed power
technique).
4.3.4

Time Delay VTS

Should the Voltage Transformer Supervision function detect an ac voltage input failure to the
relay, such as due to a VT fuse blow, this will affect operation of voltage dependent
protection elements. Distance protection will not be able to make a forward or reverse
decision, and so will be blocked. As the I>1 and I>2 overcurrent elements in the relay use
the same directionalising technique as for the distance zones, any directional zones would
be unable to trip.
To maintain protection during periods of VTS detected failure, the relay allows an I> Time
Delay VTS to be applied to the I>1 and I>2 elements. On VTS pickup, both elements are
forced to have non-directional operation, and are subject to their revised definite time delay.
4.3.5

Setting Guidelines

I>1 and I>2 Overcurrent Protection
When applying the overcurrent or directional overcurrent protection provided in the P442 and
P444 relays, standard principles should be applied in calculating the necessary current and
time settings for co-ordination. In general, where overcurrent elements are set, these should
also be set to time discriminate with downstream and reverse distance protection. The I>1
and I>2 elements are continuously active. However, tripping is blocked if the distance
protection function starts. An example is shown in Figure 64.
Reverse
Figure 64: Time grading overcurrent protection with distance protection (DT
example)
I>1 and I>2 Time Delay VTS
The I>1 and I>2 overcurrent elements should be set to mimic operation of distance
protection during VTS pickup. This requires I>1 and I>2 current settings to be calculated to
approximate to distance zone reaches, although operating non-directional. If fast protection
is the main priority then a time delay of zero or equal to tZ2 could be used. If parallel current-
based main protection is used alongside the relay, and protection discrimination remains the
priority, then a DT setting greater than that for the distance zones should be used. An
example is shown in Figure 65.
Time
I>2
Z4, tZ4
Z2,tZ2
Z1,tZ1
I>1
Z3,tZ3
Zp,tZp
Forward
P44x/EN AP/Hb6
(AP) 5-95
P3069ENa