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 article [D 4 811] , which draws particular attention to the importance of the neutral system for the control of ground fault currents and for the quality of supply. We recommend that you read and understand this article before reading this one.

Protection against polyphase faults is mainly based on simple, easy-to-adjust protection devices, such as the constant-time overcurrent protection devices discussed here, which can be used regardless of the neutral system. However, the connection of high-power decentralized generation may require the use of directional protection, as explained in this article.

Protection against earth faults requires devices adapted to the type of neutral system. Overcurrent protection can be used in the case of a neutral directly earthed or with a low-value impedance, but the settings must be carefully determined; the performance of such protection is limited, but the sensitivity of the protection plan can be improved by using directional earth fault protection. In the case of a network with compensated neutral, directional earth protectors are essential, as is a system for adjusting its compensation impedance. This type of protection is also necessary when the neutral is isolated.