The UK Electricity Supply Industry ESI

The State-Owned Central Electricity Generating Board Before 1990, the entire UK ESI was in the state sector and was centrally planned and operated by the Central Electricity Generating Board (CEGB). The remit of the CEGB was to provide a secure supply of electricity to its consumers. All generating plant, transmission lines, distribution lines, substations and metering were owned and operated by the CEGB as a monopoly. During the days of the CEGB, electricity was supplied to consumers through...

Active and Reactive Power from Renewable Energy Generators

As mentioned earlier, the large generators of conventional power systems have their individual AVRs set to maintain the generator bus voltage virtually constant. Generators fed from renewable energy sources are substantially smaller in rating and are, in general, connected to the distribution rather than the transmission network. For these reasons, conventional generator control schemes have not been considered appropriate for small embedded renewable energy supplied generators. For example, a...

Building Integrated Wind Turbines

There has been considerable interest of late, particularly in Northern Europe, in the possibility of fitting small wind turbines on the roof tops of individual houses. Enthusiasts for this approach claim that small wind turbines sited within the urban environment have the potential to make a significant contribution to the reduction of CO2 levels. A typical wind turbine of this type is illustrated in Figure 8.4. The difficulties facing the designers and manufacturers of such machines are...

Example Load Flow

Node Load Flow

As discussed already, renewable energy generation can affect both line loadings and voltages throughout the system. Load flow is a technique that allows the flows of real and reactive current throughout the network to be calculated, based on the location of the loads and sources and the line impedances. The network of Figure 6.1 is used here to illustrate the way in which load flow analysis is applied at a distribution level to assess the effect of connecting a renewable energy generator at a...

Capacity Factor and Capacity Credit [

When renewables displace significant amounts of conventional generation plant, an extra conventional plant capacity margin is required to maintain system supply reliability. It is important to recognize that an additional plant margin will only be required at times of low electricity demand and high input from variable sources. At other times an additional margin will not be required as there will be sufficient conventional plant available to meet demand irrespective of the contribution from...

Photovoltaic Systems

At the heart of a PV system is the PV module. Detailed descriptions of the different PV technologies and the basics of solar cell operation can be found in a wide range of textbooks, for example References 13 and 14 . PV modules produce output determined mainly by the level of incident radiation. They are characterized for given external conditions, by an I-V curve of the type shown in Figure 2.13 . The power, IV, depends on the operating point and is maximized for operation near to the knee of...

Applications to Renewable Energy Generators

Direct Drive Enercon Diagram

All PV systems generate direct current and so power electronic converters, known as inverters, are required to connect them to power systems. As described in Chapter 2, PV modules have an I-V characteristic that depends on incident radiation intensity and to a lesser extent the solar spectrum and module temperature. Different types of PV have different shapes of I-V curve, characterized in simple terms by a fill factor, as explained in Chapter 2 . To maximise the output of any PV module, or...

The Synchronous Generator Equivalent Circuit

Generator Equivalent Circuit

To analyse the power flows in electrical systems, component representations are required that can be incorporated into network or circuit models. To use the available circuit analysis tools described in Chapter 5 , it is necessary to build up these representations from basic circuit elements, namely resistors, inductors, capacitors and voltage or current sources Appendix A . Electrical power engineers over the years have developed a range of what are known as equivalent circuits for network...

Wind Turbines

Vestas V90 Power Curve

The power in the wind than can be extracted by a wind turbine is proportional to the cube of the wind speed and is given in watts by Figure 2.6 The Vestas V9G, Systems A S 3MW wind turbine. Reproduced with permission of Vestas Wind where p is the air density, A is the rotor swept area, U is the wind speed and Cp is the power coefficient that represents the aerodynamic efficiency of the rotor. The variability in power output from one wind turbine would therefore be expected to substantially...

Reactive Power Management

Reactive Power Management

The principle of conservation of reactive power described in Section A.13 requires that reactive power is scheduled so that as the consumer demand of Q varies over a day, appropriate Q resources are available to provide it. In Chapter 3 it was explained that a discrepancy between demand and supply of active power is manifested by rises or falls in frequency, a parameter that is common to the whole interconnected power system. In contrast, reactive power deficits or excesses manifest themselves...

Fault Level Increase

As noted previously, fault level can be used as a measure of grid strength and can be compared with proposed generator capacity to give an indication of likely effects on line voltage. However, as shown in Figure 6.6, the fault level can itself be increased by the connection of embedded generators, particularly where synchronous machines are involved. This does not invalidate the previous comments on voltage effects, but it does represent another factor to be considered in the connection of a...

Offshore Wind Farm Development

An offshore installation has advantages over and above the reduction in turbine costs from the resulting economies of scale. The marine environment is windier resulting in an increased energy yield, and the winds are more persistent and less turbulent making system-wide integration easier. Very large wind turbines that might cause an unacceptable visual intrusion onshore may well be acceptable if sited away from the shore. Additionally, the transportation of exceptionally large or heavy items...

Load Flow and Power System Simulation [

Power System Flow

Section 5.4 looked at the maths describing an individual overhead line or underground cable. Mature power systems are likely to have thousands of such lines, all interconnected. The same basic maths applies to each and every line, but now the equations must be solved simultaneously. Structured procedures for such calculations are known as load flow. A basic load flow calculation provides information about the voltages and currents and complex power flows throughout a network, at a particular...

Wind Variability

Wind Variability Graphs

The wind speed at a given location is continuously varying. There are changes in the annual mean wind speed from year to year annualI changes with season seasonal , with passing weather systems synoptic , on a daily basis diurnal and from second to second turbulence . All these changes, on their different timescales, can cause problems in predicting the overall energy capture from a site annual and seasonal , and in ensuring that the variability of energy production does not adversely affect...

Solar Thermal Electric Systems

Solar thermal electricity generation systems most commonly use solar concentrators to produce high temperatures that can drive heat engines with acceptable conversion efficiency. Two main types of generator suitable for large scale generation have so far been demonstrated, both requiring direct or beam radiation. Climates with cloudy or overcast conditions, where most of the radiation is diffuse, are thus unsuited to this technology. The two technologies are Large collections or solar farms of...

Large Wind Turbines

Recent progress in wind turbine technology has been breathtaking. At the time of writing 5MW wind turbines are commercially available. Ten years ago, the largest wind turbine on the market was typically an 800 kW machine with most manufacturers offering 600 kW units. At present, most wind turbines are fitted with rotors of 60-90 m compared to the 35-45 m of the mid-1990s. Due to economies of scale, larger machines generate electricity at a lower cost per kW h , particularly in offshore...

Equal Incremental Cost Dispatch

Input Output Curve Generating Unit

The merit order philosophy is adequate and convenient for rough scheduling but as a precise tool of economic dispatch it is inaccurate. The reason for this is that cost curves are not quite linear as assumed above they are better described by a quadratic function as in the example of Figure 7.5 . Such curves can be used to determine the overall efficiency of the plant at various output levels. As previously, it can be assumed that only two plants A and B are supplying a demand Pd. Following the...

ZLu f T

Figure 5.8 Resistance, inductance and capacitance of a three-phase overhead line or underground cable Figure 5.9 Single-phase equivalent circuit of a transmission line Figure 5.9 Single-phase equivalent circuit of a transmission line short length, the capacitance is substantial. For such cases, the circuit of Figure 5.9 can still be used for the first stage of the analysis. The capacitance can be lumped at the two ends of the line and its effect taken into account at a second stage in the...

The Asynchronous Generator

Torque Converter Negative Slip

Asynchronous or induction machines operating as motors are the most widely used electromechanical converters. In an induction machine the stator is identical to the one for synchronous machines shown in Figure 4.3 in which three-phase currents supplied to the stator produce a rotating magnetic field RMF . The rotor, however, is radically different and it has neither an external magnetizing source nor permanent magnets. Instead, alternating currents are injected in the rotor from the stator...