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6 Protection Functions P50 Agile P153
6-4 P153/EN M/B
2 PROTECTION FUNCTIONS
2.1 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
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 OC and EF functions. All three stages of
overcurrent and earth fault protection function are programmable as Inverse Definite Minimum Time
(IDMT) or Definite Time (DT) delay.
2.1.1 IDMT Characteristics
All three stages of Overcurrent and Earth fault functions are programmable as per IDMT characteristic
based on IEC and IEEE standards. The inverse time delay is calculated with the following
mathematical formula:
t = T *
K
I
I
s
α
- 1
+ L