Overcurrent Protection Principles; Idmt Characteristics - GE MiCOM P40 Agile Technical Manual

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Chapter 6 - Current Protection Functions
2

OVERCURRENT PROTECTION PRINCIPLES

Most electrical power system faults result in an overcurrent of one kind or another. 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 faults. The general principle is to isolate the faults as quickly as possible to limit the danger and
prevent 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 power grid that are absolutely necessary, to prevent
unnecessary blackouts. The protection devices that control the tripping of the power 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
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 alone 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.
2.1

IDMT CHARACTERISTICS

There are two basic requirements to consider when designing protection schemes:
All faults should be cleared as quickly as possible to minimise damage to equipment
Fault clearance should result in minimum disruption to the electrical power grid.
The second requirement means that the protection scheme should be designed such that only the circuit
breaker(s) in the protection zone where the fault occurs, should trip.
These two criteria are actually in conflict with one another, because to satisfy (1), we increase the risk of shutting
off healthy parts of the grid, and to satisfy (2) we purposely introduce time delays, which increase the amount of
time a fault current will flow. With IDMT protection applied to radial feeders, this probem is exacerbated by the
nature of faults in that the protection devices nearest the source, where the fault currents are largest, actually
need the longest time delay.
IDMT characteristics are described by operating curves. Traditionally, these were defined by the performance of
electromechanical relays. In numerical protection, equations are used to replicate these characteristics so that
they can be used to grade with older equipment.
The old electromechanical relays countered this problem somewhat due to their natural operate time v. fault
current characteristic, whereby the higher the fault current, the quicker the operate time. The characteristic typical
of these electromechanical relays is called Inverse Definite Minimum Time or IDMT for short.
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P14x
P14xEd1-TM-EN-1

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