Protection Plan for Medium Voltage Public Distribution Networks

History and local conditions (in particular load density and voltage level) have shaped public distribution networks, particularly in terms of how their medium-voltage neutrals are grounded. In North America, neutrals are directly earthed, in Asia they are frequently insulated, and in Europe there are insulated neutrals, directly earthed neutrals, and neutrals earthed via a fixed resistor or an adjustable compensation coil (or Petersen coil).

These networks are liable to be affected by polyphase and single-phase faults, the latter also referred to as earth faults. The aim of the protection plan, which simultaneously implements protection and automation, is to detect these faults and eliminate them, while at the same time being reliable and fast to limit their consequences, selective to limit as far as possible the number of users cut off, and as simple as possible. All these criteria can be contradictory: a successful protection plan is the expression of a good compromise between these different requirements.

While polyphase faults always result in high fault currents, which are easily dealt with using either overcurrent protection devices or fuses, the same cannot be said for single-phase faults. Ground faults, which account for around 80% of the total number of faults, have different characteristics depending on the neutral system, which leads to the adoption of different detection principles.

Even if each distributor has its own rules for constructing protection plans, it is still possible to propose general principles for each case.

By analyzing the behavior of radial medium-voltage distribution networks when affected by faults, the article [D 4 812] shows how to build and set up a protection plan, particularly according to its neutral regime, and indicates the pitfalls to avoid.