Directional protection equipment

Implementation of network configuration

This “Cahier Technique” aims to give the reader a better understanding of so called “directional” protection: a very useful technology for HV networks and machines. Advances in digital technology and the integration of logical selectivity, has enabled it to make great progress in terms of reliability, simplicity of incorporation and even costs.

Its use allows to implement network configuration and selectivity system improving the dependability of power supplies. After reviewing their operating principle, the author goes on to present their many applications and gives some useful information on their incorporation.

The role of directional protection equipment

Protection equipment has the basic role of detecting an electrical fault and disconnecting that part of the network in which the fault occurs limiting the size of the disconnected section as far as possible. Directional protection enables better discrimination of the faulty part of the network than with overcurrent protection.

It is necessary to use it in the following conditions:

• in a system with several sources,
• in closed loop or parallel-cabled systems,
• in isolated neutral systems for the feedback of capacitive currents,
• and to detect an abnormal direction of flow of active or reactive power (generators).

Figure 1 illustrates a situation in which both power sources would be tripped if overcurrent protections were used. Directional current protection equipment is capable of only tripping the faulty incomer. The direction in which the fault occurs is detected by measuring the direction of current flow, or in other words the phase displacement between the current and voltage.

AUTHOR: Schneider Electric expert | Pierre BERTRAND