Protection plan for medium-voltage public distribution networks — Principles

History and the local context (in particular load density and voltage level) have shaped public distribution networks, particularly in terms of the grounding method for their medium-voltage neutral. In North America, neutrals are connected directly to earth, in Asia they are frequently isolated, and in Europe there are isolated neutrals, neutrals connected directly to earth, and neutrals earthed via a fixed resistor or an adjustable compensation coil (or Petersen coil).

These networks are susceptible to polyphase and single-phase faults, i.e. 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, fast to limit their consequences, selective to limit as far as possible the number of users cut off, and simple; all criteria which can be contradictory: a successful protection plan is the expression of the right compromise between these different requirements.

While polyphase faults always result in high fault currents, easily dealt with either by fuses or overcurrent protection, 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, and require very 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.

The implementation of these protections is covered in the following file [D 4 812] . A third file, [D 4 813] , enables us to study ways of supplementing and developing existing equipment to improve the quality of supply and adapt to current network developments.