Nick Jenkins

Tutorial IV: Power systems

The power system converts mechanical energy into electrical energy using generators, then transmits the electricity over long distances and finally distributes it to domestic, industrial and commercial loads. Generation is at a low voltage (400 V to around 25 kV) and then the voltage is stepped up to transmission voltage levels (e.g. 765 kV, 400 kV, 275 kV) and finally stepped down to distribution voltages (e.g. 13.8 kV, 11 kV or 400 V). Each of these conversion stages takes place at a substation with a number of different pieces of equipment to: (a) transform the system voltage (power transformers), (b) break the current during faults (circuit breakers), (c) isolate a section for maintenance (isolators) after breaking the current, (d) protect the circuit against lightning overvoltages (surge arresters) and (e) take voltage and current measurements (voltage transformers VT and current transformers CT). In addition to this primary plant, which carries the main current, secondary electronic equipment is used to monitor and control the power system as well as to detect faults (short-circuits) and control the circuit breakers. Practical AC power systems use three phases that are of the same magnitude and displaced 120° degrees electrical from each other. When the three phases are thus balanced, no current flows in the neutral and so at higher voltages only three phase conductors are used and the neutral wire is omitted. In order to represent the power system in control diagrams and reports, a single line representation is used; the three-phase lines are shown by a single line. Typical single line diagram symbols used for a balanced power system.

Tutorial III: Power electronics

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.