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Whatever be the considered country, as far as it is large enough, the systems of electrical energy supply are meshed. They contain then three parts:

• Generating units, i. e. the power plants

• High voltage transmission networks moving large quantities of energy to distant consumers and maintaining synchronisation of the power system

• Medium voltage and low voltage distribution networks, supplying the customers.

In one part or another, each may be the target of incidents such as insulation failure, incorrect operation of circuit breakers and shunts, short circuits and open circuits. Thus a panoply of devices exist whose aim is to avoid the destruction of assets, or the loss of energy supply. Among them, we can distinguish 3 classes:

• Protective relays control the tripping of the circuit breakers surrounding the faulted part of the network

• Automatic operation, such as auto-reclosing or system restart

• Monitoring equipment which collects data on the system for post event analysis

The operating quality of these devices, and especially of the protective relays, is always capital. However it can be appraised differently as well as we consider either a part or another of the system.

Protection of the generator sets

In a power plant, the protective relays have to avoid the deterioration of the alternators or of the transformers in case of bad condition operation, due to internal failures, as well as insulating shortcomings or regulation disfunctioning. Such failures are unusual, and on well designed, wel manufactured, well installed, well maintained and well employed gears the protective relay have to operate very rarely. If a protective relay fails to detect as disfunctioning, the damage to the alternator or to the transformer may have important financial consequences. But these consequences remain internal to the utility: repair of the damaged asset , and loss of production, which must be replaced by more expensive production means - as well as the utility has available production means already synchronised to the network-.

Protection of the assets of the high voltage transmission network

On the high voltage transmission network, the issue is quite different : Firstly, an overhead line, which runs on the public domain, is periodically the target of short circuits, due to lightnings, unpruned trees, cranes or great height engins working in the neighbourhood, wind, pollution. A good design of the line may lower their probability, but not remove them. E. g. , on the network of the Electricité de France, an average of 7 short circuits per year and per 100 km may be observed. Secondly, when the temperature of a line increases, it becomes longer, and its lowest point, between two towers, fall away. If the line is not tripped in time, it becomes dangerous for the persons. The consequences must be then evaluated no more in millions of euros, but in lost of human lives ! And that is why the protection systems must include back-up functions : The failure of a relay is palliated by the back-up operation of relays may be located on other points of the network. This can lead to the tripping of many assets. The supply of a whole region can be jeopardised. A false operation of a protective relay may then provoke the tripping of one or several customers, may be a whole town, comprising the prioritar customers (hospitals, traffic lights, …). So, when a manufacturer consuming 10 MW is cut during 6 minutes, e.g., this coresponds not only to 1 MW of non selled energy, but top the non-selled energy during the time, may be several hours, necessary to restart the manufacture. But this corresponds to a dissatisfied customer, which lost several hours of his production, and may be underwent some deterioration on his equipment. If the matter is the tripping of a town, the supplyer must account for the failure, as public utility, to local, or even regional or national, authorities

Protection of the assets of the middle and low voltage networks

On these networks, the same considerations may be applied, but the consequences of a maloperation are not so important.

To have more information

You may consult the website •* protection and monitoring of the electrical energy transmission network on which the operating principles of the different devices used on the transmission networks are described, as well as their installation modality and the reckoning of their setting. The information contained in this website is essentially issued from an experience gathered on the network of the Electricité de France. But you can find also some information coming from audits in other countries, as well as from international working groups (CIGRE, IEEE). A few information is also available on the protection of generators. On the other hand, the protection of the middle and low voltage networks is not approached.