With the increasing efforts worldwide to de-carbonise energy supply, a wide variety of generating plant types is being connected to electrical distribution networks. Examples include the well-established technologies of combined heat and power (CHP), wind turbines and photovoltaic systems. In addition there are many newer technologies, such as fuel cells, solar thermal, micro-CHP and the marine renewable technologies as well as flywheel and flow battery storage that are at various stages of demonstrating their commercial viability. In deregulated electricity supply systems the owners of the distributed generation) plant (who will not be the distribution utility in most cases) will respond to pricing and other commercial signals to determine whether to invest in such plant and then how to operate it. As distributed generation displaces large central generating plant, it will increasingly take over the ancillary services (e.g. voltage and frequency control) that are necessary for the operation of the power system and so distributed generation will develop from being a source only of energy to being an integral contributor to the supply of electrical power.

This chapter discusses the fault current and electrical protection scheme in distributed generators. Electrical faults, caused by the breakdown of insulation, are inevitable in any electrical power system. Faults may be caused by mechanical damage to the equipment or created by the degradation of insulation over time. Electrical protection is then used to operate circuit breakers that isolate the faulty equipment rapidly. Distributed generators must be protected against internal electrical faults, with fault current flowing from the network, and conversely the distribution network must be protected against fault current from the distributed generators. Islanded operation of smaller distributed generators is not generally permitted and so this condition is detected by protection systems designed to detect islanding or loss of mains and the generator then disconnected. Finally, the addition of distributed generation to a distribution network may alter the flows of network fault current in subtle ways and so lead to maloperation of conventionally designed distribution network protection systems.

For distributed generation to compete successfully with central generation in a competitive environment, network pricing arrangements are critically important. Distributed generation, depending on the technology, operating pattern and exact connection point, may reduce the need for upstream network reinforcements and when located closer to the load, can reduce losses and potentially contribute to increasing local reliability of supply. The vehicle for realising this additional value is efficient network pricing. In general, in a deregulated electrical power system, network prices should send signals to users reflecting the benefits they bring and costs they impose on network operation and/or development. Efficient pricing distinguishes between different locations and between different times of use, thus avoiding cross-subsidies and facilitating a level-playing field between distributed and central generation. For distribution network with distributed generation, it is essential to take account of the contribution that the generation makes to network security, i.e. the ability of distributed generation to substitute for network capacity. Otherwise the value of distributed generation in this regard is not recognised and hence cannot be rewarded.

In all public electricity supply systems, the mains voltage alternates at 50 or 60 cycles per second (Hz) and when a load is connected it draws an alternating current. An alternating current or voltage changes its polarity periodically to give a wave like shape on an oscilloscope trace. Sine, square and even saw-tooth waves are used in different electronic circuits, but this chapter considers only the sine wave, which is the shape of the AC mains. A generator produces a three-phase alternating voltage and this voltage is increased (stepped-up) for long distance transmission. The transmission voltages are then stepped-down and the power distributed to loads. Low voltage final distribution circuits typically use four wires (in Europe, three phase wires each at 230 V and a neutral wire that provides the zero volt reference). Single-phase loads (e.g. houses) are connected across two wires (to one phase wire and the neutral wire) and three-phase loads (industry and commercial buildings) are connected to all four wires.

Power electronic converters are presently used to interface many forms of renewable generation and energy storage systems to distribution networks, while the use of power electronics is likely to increase in the future as this technology is also an important element of SmartGrids and active distribution networks. The development of high power electronic converters benefits from recent rapid advances in power semiconductor switching devices and in the progress being made in the design and control of variable speed drives for large motors.One obvious application of a power electronic converter is to invert the DC generated from some energy sources (e.g. photovoltaics, fuel cells or batteries) to 50/60 Hz AC. Converters may also be used to de-couple a rotating generator and prime mover from the network and so allow it to operate at its most effective speed over a range of input powers. This is one of the arguments put forward in favour of the use of variable speed wind turbines but is also now being proposed for some small hydro generation. Another advantage of variable speed operation is the reduction in mechanical loads possible by making use of the flywheel effect to store energy during transient changes in input or output power. However, large power electronic converters do have a number of disadvantages including significant capital cost and complexity, electrical losses (which may include a considerable element independent of output power) and the possibility of injecting harmonic currents into the network.