• 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.
Failures may occur in each part, such as insulation failure, fallen or broken transmission lines, incorrect operation of circuit breakers, short circuits and open circuits. Protection devices are installed with the aims of protection of assets, and ensure continued supply of energy. The three classes of protective devices are:
• 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
While the operating quality of these devices, and especially of the protective relays, is always critical, different strategies are considered for protecting the different parts of the system. Very important equipment may have completely redundant and independent protective systems, while a minor branch distribution line may have very simple low-cost protection.
In a power plant, the protective relays are intended to prevent damage to alternators or of the transformers in case of abnormal conditions of operation, due to internal failures, as well as insulating failures or regulation malfunctions. Such failures are unusual, so the protective relay have to operate very rarely.
If a protective relay fails to detect a fault, the damage to the alternator or to the transformer may have important financial consequences for the repair or replacement of equipment and the value of the energy that otherwise would have been sold.
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, drops. 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 loss 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 which 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, or 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·h of non sellable energy, but top the unsold 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.
On these networks, the same considerations may be applied, but the consequences of a maloperation are not so important.