16-08-2012, 04:40 PM
Coordination tables
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Problems and requirements for coordinating protection devices
Selection of the system for protecting an electric installation is of paramount importance
both to ensure correct economic and functional operation of the whole installation and
to reduce any problems caused by anomalous operating conditions and actual faults
to a minimum.
This analysis deals with coordination between the different devices dedicated to
protection of zones and specific components in order to:
– guarantee safety for people and the installation at all times;
– identify and rapidly exclude only the zone affected by a given problem, instead of
taking indiscriminate action thereby reducing the energy available in areas unaffected
by the fault;
– reduce the effects of a fault on other sound parts of the installation (voltage drops,
loss of stability in rotating machines);
– reduce the stress on components and damage in the zone involved;
– ensure service continuity with good quality power supply voltage;
– guarantee adequate backup in the event of any malfunction of the protection device
responsible for opening the circuit;
– provide maintenance personnel and the management system with the information
needed to restore the service as rapidly as possible and with minimal disturbance to
the rest of the network;
– achieve a valid compromise between reliability, simplicity and cost effectiveness.
To be more precise, a valid protection system must be able to:
– understand what and where an event has occurred, discriminating between situations
that are anomalous but tolerable and genuine faults within a given zone of influence,
avoiding unwarranted trips which lead to unjustified stoppage of a sound part of the
installation;
– act as rapidly as possible to limit damage (destruction, accelerated ageing, etc.),
safeguarding continuity and stability of the power supply.
The solutions stem from a compromise between the following two opposing needs
- precise identification of the fault and rapid intervention - and are defined according
to which requirement takes priority.
Types of coordination
Influence of the electrical parameters of the installation (rated current and shortcircuit
current)
If the analysis is restricted to the behavior of the protection devices with tripping based
on overcurrent releases, the strategy used to coordinate the protection devices mainly
depends on the rated current (In) and short-circuit current (Ik) values in the part of
installation concerned.
Generally speaking, the following types of coordination can be classified:
– current type selectivity;
– time type selectivity;
– zone selectivity;
– energy selectivity;
– back-up.
Now let us examine these various solutions in detail.
Current type selectivity
This type of discrimination is based on the observation that the closer the fault is to
the power supply of the installation, the higher the short-circuit current will be. We can
therefore pinpoint the zone where the fault has occurred can therefore be discriminated
simply by setting the protection devices to a limit value so that this does not generate
unwarranted trips due to faults in the zone of influence of the protection device
immediately to the load side (where the fault current must be lower than the current
threshold set on the protection device on the supply side).
Total discrimination can normally only be obtained in specific cases where the fault
current is not very high or where a component with high impedance is placed between
the two protection devices (e.g. a transformer, a very long cable, or a cable with reduced
cross-section, etc.) giving rise to a great difference between the short-circuit current
values.
This type of coordination is therefore mainly used in end distribution (with low rated
current and short-circuit current values and high impedance of the connection
cables).
The device time-current trip curves are generally used for the study.
This solution is intrinsically rapid (instantaneous), easy to implement and inexpensive.
On the other hand: