Protection plan for medium-voltage public distribution networks — Implementation

The principles governing the construction of a protection plan for a medium-voltage public distribution network are set out in the [D 4 811] file, which draws particular attention to the importance of the neutral system for the control of ground fault currents and for the quality of supply.

Protection against polyphase faults is mainly based on simple, easy-to-adjust protection devices, such as the constant-time current protection devices discussed in this dossier [D 4 812], which can be used whatever the neutral system, but protection against earth faults requires devices adapted to the type of neutral system. Amperometric protection can be used in the case of a neutral earthed by a low-value impedance, but the settings must be carefully determined.

The performance of these amperometric protections is limited, but the sensitivity of the protection plan can be improved by making them directional. In the case of a network with compensated neutral, directional earth protectors are essential, as is a system for adjusting their compensation impedance.

Current developments in public distribution networks, notably the arrival of decentralized generation and the need to improve quality of supply, require additional protection in primary substations and new developments in existing equipment, such as fault-transmission indicators. All these additions and developments are covered in the third dossier [D 4 813] .