Overview

Protective devices form a coherent whole in relation to the network structure and its earthing system. They should be looked upon as a system based on the principle of selectivity that consists of isolating the part of the network affected by the fault, and only that part, as quickly as possible, while all the other unaffected parts of the network remain energized.

There are various ways of ensuring proper selectivity in electrical network protection:

• amperemetric selectivity (using current);

• time-graded selectivity (using time);

• selectivity via information exchange, referred to as logic selectivity;

• selectivity via use of directional or differential protection.

9.1 Amperemetric selectivity

This is based on the fact that, within a network, the further the fault is from the source, the weaker the fault current.

Amperemetric protection is installed at the starting point of each section. Its threshold is set at a value lower than the minimum short-circuit current caused by a fault occurring on the monitored section, and higher than the maximum value of the current caused by a fault located downstream (beyond the monitored area).

Once set, each protection is only activated for faults located immediately downstream of its position (inside the monitored zone), and is insensitive to faults occurring beyond this point.

Nevertheless, in practice, it is difficult to define the settings of two cascading protection devices (while continuing to ensure proper selectivity), when the current does not notably decrease between two neighboring zones.

However, for line sections separated by a transformer, this system can be...