Areva MiCOM P441, Technical Manual

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Application Notes P44x/EN AP/E33
MiCOM P441/P442 & P444 Page 75/220
2.14.1 Application of Timer Hold Facility
The first two stages of overcurrent protection in the P441, P442 and P444 relays are
provided with a timer hold facility, which may either be set to zero or to a definite time value.
(Note that if an IEEE/US operate curve is selected, the reset characteristic may be set to
either definite or inverse time in cell I>1 Reset Char; otherwise this setting cell is not visible
in the menu). Setting of the timer to zero means that the overcurrent timer for that stage will
reset instantaneously once the current falls below 95% of the current setting. Setting of the
hold timer to a value other than zero, delays the resetting of the protection element timers for
this period. This may be useful in certain applications, for example when grading with
upstream electromechanical overcurrent relays which have inherent reset time delays.
Another possible situation where the timer hold facility may be used to reduce fault clearance
times is where intermittent faults may be experienced. An example of this may occur in a
plastic insulated cable. In this application it is possible that the fault energy melts and reseals
the cable insulation, thereby extinguishing the fault. This process repeats to give a
succession of fault current pulses, each of increasing duration with reducing intervals
between the pulses, until the fault becomes permanent.
When the reset time of the overcurrent relay is instantaneous the relay may not trip until the
fault becomes permanent. By using the timer hold facility the relay will integrate the fault
current pulses, thereby reducing fault clearance time.
Note that the timer hold facility should not be used where high speed autoreclose with short
dead times are set.
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 this cell is not visible if an inverse time reset
characteristic has been selected, as the reset time is then determined by the programmed
time dial setting.
2.14.2 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 in order to obtain correct discrimination. Typical
systems which 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).
2.14.3 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.
2.14.4 Setting Guidelines
I>1 and I>2 Overcurrent Protection
When applying the overcurrent or directional overcurrent protection provided in the P441,
P442 and P444 relays, standard principles should be applied in calculating the necessary
current and time settings for co-ordination. For more detailed information regarding
overcurrent relay co-ordination, reference should be made to AREVA’s ‘Protective relay
Application Guide’ - Chapter 9. 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 42.