The power system
Introduction
All countries now have a power system which transports electrical energy from generators to consumers. In some countries several separate systems may exist, but it is preferable to interconnect small systems and to operate the combination as one, so that economy of operation and security of supply to consumers is maximized. This integrated system (often known as the ‘grid’) has become dominant in most areas and it is usually considered as a major factor in the well-being and level of economic activity in a country.
All systems are based on alternating current, usually at a frequency of either 50 Hz or 60 Hz. The 50 Hz is used in Europe, India, Africa and Australia, and 60 Hz is used in North and South America and parts of Japan.
Systems are traditionally designed and operated in the following three groupings:
● the source of energy – generation
● bulk transfer – transmission
● supply to individual customers – distribution
Generation
Generators are required to convert fuels (such as coal, gas, oil and nuclear) and other energy sources (such as water, wind and solar radiation) into electrical power. Nearly all generators are rotating machines, which are controlled to provide a steady output at a given voltage. The main types of generator and the means of control are described in Chapter 5.
The total power output of all operating generators connected to the same integrated system must at every instant be equal to the sum of the consumer demand and the losses in the system. This implies careful and co-ordinated control such that the system frequency is maintained, because the majority of generators in an ac power system are synchronous machines and their rotors, which produce a magnetic field, must lock into the rotating magnetic field produced by alternating currents in the stator winding. Any excess of generated power over the absorbed power causes the frequency to rise, and a deficit causes the frequency to fall. As the demand of domestic, commercial and industrial consumers varies, so the generated power must also vary, and this is normally managed by transmission system control which instructs some generators to maintain a steady output and others (particularly hydro and gas turbine plant) to ‘follow’ the load; load ‘following’ is usually achieved by sensitive control of the input, dependent upon frequency. It is desirable to run the generating plant such that the overall cost of supplying the consumer at all times is a mini- mum, subject to the various constraints which are imposed by individual generator characteristics.
In de-regulated, or privatized power systems this is achieved by competition among generators combined with additional regulated markets for ancillary services and use of the transmission system.
Distributed and renewable generation
The worldwide imperative to reduce greenhouse gases, particularly CO2, and to secure energy supplies for the long-term future has prioritized the development of electricity generation from renewable energy sources. Renewable generators and other high-efficiency schemes, such as Combined Heat and Power (CHP) are relatively small in capacity compared with large thermal power stations. Consequently, these generators are often embedded or distributed in the network at voltages, such as 11 kV or 33 kV.
As the penetration of renewables into the system increases there are major issues for the planning and operation of the power system. Some renewables, such as biomass can be regarded as providing firm capacity, or others, such as tidal power may be predictable but periodic, but most, including wind, wave and photovoltaic, have to be regarded as intermittent. Hydroelectric power is also a renewable and apart from the run-of-river plant, it offers a valuable energy storage capability. The various plant characteristics can have a significant impact for the system operator, especially in determining the required level of spinning reserve, and/or demand management, required within the system to cover the increased intermittence of supply. At present, the higher capital cost of renewable generation needs to be compensated through government subsidy to seek the total levels of renewable energy desired.
Distributed Generation (DG) also poses serious technical issues for the distribution network. These include power quality problems, such as harmonic current injection, and the inability of many DGs to ‘ride-through’ voltage dips (thereby exacerbating the problem). The DGs also affect fault levels in networks, either by contributing excess fault current in the case of directly connected induction generators or by not contributing sufficient fault current, where generators are connected through power electronic converters. Further issues include the possibility of bi-directional power flow in low-voltage networks and whether ‘islanded’ sections of the network could become a safe and acceptable operational option in the future. These technical issues are certainly solvable using present day technology, but they pose an interesting and important challenge for power system engineers.