With the increasing dependency of our society on electricity supplies, the need to achieve an acceptable level of reliability, quality and safety at an economic price becomes important to customers. The power system as such is well designed and also adequately maintained to minimize the number of faults that can occur. Protection systems are installed to clear faults, like short circuits, because short-circuit currents can damage the cables, lines, busbars and trans- formers. The voltage and current transformers provide measured values of the actual voltage and current to the protective relay. The relay processes the data and determines, based on its settings, whether or not it needs to operate a circuit breaker in order to isolate faulted sections or components.
The classic protective relay is the electromagnetic relay which is constructed with electrical, magnetic and mechanical components. Nowadays computerized relays are taking over as they have many advantages, such as: computerized relays can perform a self-diagnosis, they can record events and disturbances in a data base and they can be integrated in the communication, measurement and control environment of the modern substations.
A reliable protection is indispensable for a power system. When a fault or an abnormal system condition occurs (such as: over/under-voltage, over/under-frequency, overcurrent and so on) the related protective relay has to react in order to isolate the affected section while leaving the rest of the power system in service. The protection must be sensitive enough to operate when a fault occurs, but the protection should be stable enough not to operate when the system is operating at its maximum rated current. There are also faults of a transient nature, a lightning stroke on or in the vicinity of a transmission line for instance, and it is undesirable that these faults would lead to a loss of supply. Therefore, the protective relays are usually equipped with autoreclosure functionality. Autoreclosure implies that the protective relay, directly after hav- ing detected an abnormal situation leading to the opening of the contacts of the circuit breaker, commands the contacts of the circuit breaker to close again in order to check whether the abnormal situation is still there. In case of a fault of a transient nature, the normal situation is likely to be restored again so that there is and was no loss of supply. When the abnormal situation is still there, the protective relay commands the circuit breaker to open its contacts again so that either the fault is cleared or consecutive autoreclosure sequences can follow. In most cases, so-called back-up protection is installed in order to improve the reliability of the protec- tion system.
When protective relays and circuit breakers are not economically justifiable in certain parts of the grid, fuses can be applied. A fuse combines the ‘basic functionality’ of the current trans- former, relay and circuit breaker in one very simple overcurrent protection device. The fuse element is directly heated by the current passing through and is destroyed when the current exceeds a certain value, thus leading to an isolation of the faulted sections or components. After the fault is repaired/removed, the fuse needs to be replaced so that the isolated grid section can be energized again.