Fundamentals of Discrimination and Selectivity in Power System
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| Protective Relay Panel | Source: Siemens |
Protections comprise a coherent whole in relation to the structure of the system and its grounding. They should be looked upon as a system based on the principle of discrimination which consists of isolating as quickly as possible the part of the system affected by the fault and only that part, leaving all the fault-free parts of the system energized.
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| Protective Relay Panel | Source: Siemens |
Various means can be implemented to ensure proper discrimination in electrical system protection:
- current discrimination
- time discrimination
- discrimination by data exchange referred to as logic discrimination
- discrimination by the use of directional protection devices
- discrimination by the use of differential protection devices.
Current Discrinimation
Current discrimination is based on the fact that within a system, the further the fault is from the source, the weaker the fault current. Current-based protection is installed at the starting point of each section where its setting is set at a value lower than the minimum value of short-circuit current caused by a fault in the monitored section, and higher than the maximum value of the current caused by a fault located downstream (beyond the monitored area).
Set in this way, each
protection device operates only for faults
located immediately downstream from it, and
is not sensitive to faults beyond.
In practice, it is difficult to define the settings
for two cascading protection devices (and
still ensure good discrimination) when there
is no notable decrease in current between
two adjacent areas (medium voltage
system).
However, for sections of lines
separated by a transformer, this system can
be used advantageously as it is simple,
economical and quick (tripping with no
delay).
Related Article: What is the Advantage of IDMT in Protective Relaying?
Time Discrimination
Time discrimination consists of setting
different time delays for the current-based
protection devices distributed throughout the
system. The closer the relay is to the
source, the longer the time delay.
The fault shown in the diagram below is
detected by all the protections (at A, B, C,
and D).
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| Phase to Phase Fault | Time Discrimination |
From the figure, the following condition is established:
- IRA ≥ IRB ≥ IRC ≥ IRD
- Where IR is the setting of the overcurrent protection
The time-delayed protection at D closes its contacts more quickly than the one installed at C, which is in turn faster to react than the one at B, etc. Once circuit breaker D has been tripped and the fault current has been cleared, protections A, B, and C, which are no longer required, return to the stand-by position.
Related Article: Power System Selectivity: The Basics Of Protective Coordination
The time delays set for time discrimination
are activated when the current exceeds
the relay settings.
The settings must be coherent.
There are 2 types of time-delayed current-based relays:
Definite time relays,
the time delay is constant regardless of the
current provided it is higher than the setting.
- IrA > IrB > IrC.
- where tA > tB > tC
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| Definite Time Tripping Curve |
IDMT relays,
the stronger the current, the shorter the time
delay. If the settings are set to In, overload
protection is ensured at the same time as
short-circuit protection and setting
coherency is guaranteed.
- InA > InB > InC
- IrA = InA
- IrB = InB
- IrC = InC
The time delays are set for the
discrimination interval ∆t of the maximum
current detected by the upstream protection
relay.
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| IDMT Tripping Curve |
Logic Selectivity
This principle is used when a short fault
clearing time is required. The exchange of logic data between
successive protection devices eliminates
the need for discrimination intervals.
In a radial system, the protections located
upstream from the fault point are activated;
those downstream are not. The fault point
and the circuit breaker to be controlled can
therefore be located without any ambiguity.
Each protection activated by a fault sends:
- a blocking input to the upstream stage (order to increase the upstream relay time delay),
- a tripping order to the related circuit breaker unless it has already received a blocking input from the downstream stage. Time-delayed tripping is provided for as back-up.
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| Phase to Phase Fault | Logic Selectivity System |
Directional Discrimination
In a looped system, in which faults are fed
from both ends, it is necessary to use a
protection system that is sensitive to the direction of the flow of fault current in order
to locate and clear it.
Example of the use of directional
protections (refer to the diagram below):
- D1 and D2 are equipped with instantaneous directional protections;
- H1 and H2 are equipped with time-delayed overcurrent protections.
In the event of a fault at point 1, only the protections on D1 (directional), H1 and H2 detect the fault. The protection on D2 does not detect it (because of the direction of its detection system). D1 breaks. The H2 protection de-energizes and H1 breaks.
- t H1 = tH2
- t D1 = tD2
- t H = tD + ∆t
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| Directional Discrimination |
Reference:
- Protection Guide and Control
- Publisher: Merlin Gerin | Download
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