The objectives  of the protection system are:

–  limit damage to people and to the plant;

–  permit different  service conditions;

–  guarantee maximum service continuity for the plant not affected  by faults;

–  activate the automatisms provided.

The characteristics of the protection system of an electric networks are:

–   Dependence: it can be called on to work after either a short or long period after installation. In any case, it must work when it is called on to operate;

–   Safety: it must not operate when is not required (it must not operate during transients). It must allow the various service conditions and activate the automatism provided;

–   Selectivity: it must operate only and when necessary, guaranteeing maximum service continuity with minimum disconnection of the network;

–   Speed: represented by the minimum fault time and by damage to the machinery;

–   Simplicity: measured by the number of pieces of equipment needed to protect the network;

–   Economy: assessed as the cost of the protection system in relation to the cost of malfunctioning.

The protection system is the ensemble of the instrument transformers and the relays with adequate settings. The relay is only one of the components making up the protection system. Selection of  the type of function and of the functions required to adequately protect a machine or a plant must be made on the basis of:

–   Standards;

–   interface with the external network;

–   acceptable risk (consequences of the fault);

–   short-circuit currents (maximum and minimum);

–   status of the neutral;

–   presence of self-production in a plant;

–   coordination with the existing system;

–   configurations and network running criteria;

–   practices.

The aim is to achieve the best technical economic compromise which allows adequate protection against “faults” with “significant” probability and to verify that the investment is commensurate with the importance of the plant.

The electric protections are of different types and have different applications:

–   zone protections (e.g. differential or with impedance);

–   machine protections (e.g. reverse power);

–   selective protections (e.g. over current);

–   non-selective protections (e.g. Under voltage, frequency);

–   protections in support (e.g. fuses, overcurrent, under voltage);

–   interface protections (e.g. under voltage protections; under/over and rate of change of frequency; overcurrent for a disconnection between the plant network and the utility network);

–   protections for making automatism

(e.g. synchronism check).

The criterion which is followed when the setting of a protection is calculated is to efficaciously protect the machine or plant and then look for trip selectivity. Trip selectivity means isolating the smallest area of a plant in the case of a fault in the shortest time possible (selectivity) and then to ensure a reserve (back-up) in the case of failure of the primary protection. There are various different selectivity criteria which can be used in plants.

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