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6 Protection Functions
2.3.6
Emergency Restart
In an emergency, it may be necessary to restart a hot motor. An emergency restart can be enabled
through an opto input, using the user interface or the remote communication. This feature removes all
start inhibits (Thermal lockout, No of Hots starts, No of cold starts, and the Time between starts).
This feature resets the thermal memory to 90% if it is greater than 90% or stays as it is if less than
90%.
In an emergency restart, the thermal curve during start-up is inhibited, even if this function is not used
during a start-up in normal operation.
2.4
Overcurrent Protection Principles
Most power system faults result in an overcurrent of some kind. It is the job of protection devices,
formerly known as relays but now known as Intelligent Electronic Devices (IEDs), to protect the power
system from such faults. The general principle is to isolate the faults as quickly as possible to limit the
danger and prevent unwanted fault currents flowing through systems, which can cause severe
damage to equipment and systems. At the same time, we wish to switch off only the parts of the grid
that are absolutely necessary, to prevent unnecessary blackouts. The protection devices that control
the tripping of the grid's circuit breakers are highly sophisticated electronic units, providing an array of
functionality to cover the different fault scenarios for a multitude of applications.
The described products offer a range of overcurrent protection functions including:
• Phase Overcurrent protection
• Earth Fault Overcurrent protection
• Negative Sequence Overcurrent protection
• Sensitive Earth Fault protection
• Restricted Earth Fault protection
To ensure that only the necessary circuit breakers are tripped and that these are tripped with the
smallest possible delay, the IEDs in the protection scheme need to co-ordinate with each other.
Various methods are available to achieve correct co-ordination between IEDs in a system.
These are:
• By means of time alone
• By means of current alone
• By means of a combination of both time and current.
Grading by means of current is only possible where there is an appreciable difference in fault level
between the two locations where the devices are situated. Grading by time is used by some utilities
but can often lead to excessive fault clearance times at or near source substations where the fault
level is highest. For these reasons the most commonly applied characteristic in co-ordinating
overcurrent devices is the IDMT (Inverse Definite Minimum Time) type.
The relay is designed for three stages programmable Overcurrent and Earth fault functions. All three
stages of overcurrent and earth fault protection function are programmable as Inverse Definite
Minimum Time (IDMT) or Definite Time (DT) delay.
2.4.1
IDMT Characteristics
All three stages of OC and EF functions are programmable as per IDMT characteristic based on IEC
and IEEE standards. The inverse time delay is calculated with the following mathematical formula:
6-14
P50 Agile P253
P253/EN M/C
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