With the increasing dependence on electricity supplies, in both developing and developed countries, the need to achieve an acceptable level of reliability, quality and safety at an economic price becomes even more important to customers. A further requirement is the safety of the electricity supply. A priority of any supply system is that it has been well designed and properly maintained in order to limit the number of faults that might occur. Associated with the distribution networks themselves are a number of ancillary systems to assist in meeting the requirements for safety, reliability and quality of supply. The most important of these are the protection systems that are installed to clear faults and limit any damage to distribution equipment. Among the principal causes of faults are lightning discharges, the deterioration of insulation, vandalism, and tree branches and animals contacting the electricity circuits. The majority of faults are of a transient nature and can often be cleared with no loss of supply, or just the shortest of interruptions, whereas permanent faults can result in longer outages. To avoid damage, suitable and reliable protection should be installed on all circuits and electrical equipment. Protective relays initiate the isolation of faulted sections of the network in order to maintain supplies elsewhere on the system. This then leads to an improved electricity service with better continuity and quality of supply.

A protection relay is a device that senses any change in the signal it is receiving, usually from a current and/or voltage source. If the magnitude of the incoming signal is outside a pre-set value, the relay will carry out a specific operation, generally to close or open electrical contacts to initiate some further operation, for example the tripping of a circuit breaker.

Very high current levels in electrical power systems are usually caused by faults on the system. These currents can be used to determine the presence of faults and operate protection devices, which can vary in design depending on the complexity and accuracy required. Among the more common types of protection are thermomagnetic switches, moulded-case circuit breakers (MCCBs), fuses and overcurrent relays. The first two types have simple operating arrangements and are principally used in the protection of low-voltage equipment. Fuses are also often used at low voltages, especially for protecting lines and distribution transformers. Overcurrent relays, which form the basis of this chapter, are the most common form of protection used to deal with excessive currents on power systems. They should not be installed purely as a means of protecting systems against overloads which are associated with the thermal capacity of machines or lines since over current protection is primarily intended to operate only under fault conditions. However, the relay settings that are selected are often a compromise in order to cope with both overload and overcurrent conditions.

Directional overcurrent protection is used when it is necessary to protect the system against fault currents that could circulate in both directions through a system element, and when bidirectional overcurrent protection could produce unnecessary disconnection of circuits. This can happen in ring or mesh-type systems and in systems with a number of infeed points.

Distance protection is a non-unit type of protection and has the ability to discriminate between faults occurring in different parts of the system, depending on the impedance measured. Essentially, this involves comparing the fault current, as seen by the relay, against the voltage at the relay location to determine the impedance down the line to the fault. The main advantage of using a distance relay is that its zone of protection depends on the impedance of the protected line that is a constant virtually independent of the magnitudes of the voltage and current. Thus, the distance relay has a fixed reach, in contrast to overcurrent units where the reach varies depending on system conditions.

To assist in restoring the equilibrium, frequency relays are employed. These disconnect the less important loads in stages when the frequency drops to a level that indicates that a loss of generation or an overload has occurred. This type of scheme is very useful in industrial plants where in-house generation is synchronised to the public grid system. The concepts, criteria and conditions applicable to the design of an automatic load shedding system for industrial plants, based on frequency relays, are set out in this chapter.

This chapter presents the communication networks for power systems automation. It presents the IEC 61850 standard for the the information exchange intelligent electronic device within automated substation and a remote control link. It also discusses the substation IT networks and the process bus as defined by the IEC 61850 standard.

This appendix contains the solution to the exercises set in the rest of the book.