Dynamic Demand Control

A scheme is being investigated that uses the already existing stored energy in millions of consumer appliances and requires the installation of dynamic demand control (DDC). These monitor system frequency and switch the appliance on or off, striking a compromise between the needs of the appliance and the grid. Initially fridges and freezer applications have been investigated. Refrigerators are 'on' in all seasons, throughout the day and night, and are therefore available to participate in...

Renewables

Figure 1.10 provides an overview of the earth's main energy paths that can be tapped to generate sustainable electricity. The main source of easily accessible renewable energy Figure 1.9 Geological storage options for CO2. (Source World Coal Institute) Figure 1.9 Geological storage options for CO2. (Source World Coal Institute) Figure 1.10 Renewable energy flow paths Figure 1.10 Renewable energy flow paths is the sun. On average the rate of solar radiation intercepted by the earth's surface is...

Limits of Penetration from Nondispatchable Sources

Early on in the development of renewables, the UK's Central Electricity Generating Board (CEGB) carried out a number of extensive simulation studies to estimate the impact of large Figure 3.17 CEGB simulation study of large penetration. (From Grubb, M.J., IEE Proceedings C, V. 138(2), March 1991, reproduced with permission of IET) Figure 3.17 CEGB simulation study of large penetration. (From Grubb, M.J., IEE Proceedings C, V. 138(2), March 1991, reproduced with permission of IET) penetrations...

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...

Harmonics Distortion

Both loads and generators can cause distortion of network voltages, and such distortion is usually expressed in terms of harmonics. A common source of harmonics is the widespread use of cheap power supplies found in computers, televisions and other appliances. The voltage waveform in a modern office building or in a residential distribution network can be badly distorted by harmonics created by such loads, long before any distributed generators are even contemplated. The distortion of the...

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...

References

Circuits, Devices and Systems, 5th edition, John Wiley amp Sons, Ltd, Chichester. 2 Gross, C.A. Power System Analysis, John Wiley amp Sons, Ltd, Chichester, 1986. 3 Lander, C.W. PowerElectronics McGraw-Hill, Maidenhead, 1987. 4 Slootweg, H. and De Vries, E. 'Inside wind turbines fixed vs variable speed'. Renewable Energy World, January-February 31-40. 5 Pedersen, T.K. 'Semi-variable speed operation - a compromise ' Vestas-Danish Wind Technology A S, Denmark, EWEA...

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...

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...

Transformer Basics

Transformer Basics

The transformer is an indispensable part of any power system operating at a range of voltages. The transformer mode of operation is included here as it provides a useful aid to the under- Figure 4.9 The Transformer. Reproduced from Reference 1 with permission of John Wiley amp Sons, Inc. Figure 4.9 The Transformer. Reproduced from Reference 1 with permission of John Wiley amp Sons, Inc. standing of the mode of operation of induction generators which are used extensively in wind turbines. The...

Optimum Economic Dispatch

The aim is to load generating sets in response to demand changes so as to minimize the cost of supply. This is known as optimum economic dispatch OED and over the years sophisticated OED methods have been developed to identify the minimum cost solution and at the same time satisfy numerous engineering and operational requirements and constraints. Here the basic ideas that provide guidance in the task of adjusting the load on individual conventional or RE generators will be investigated in order...

Load Flow and Power System Simulation [

Solar Power System Diagram Pdf

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...

Steady State Voltage Rise

The connection of a distributed generator usually has the effect of raising the voltage at the PCC and this can lead to overvoltages for nearby customers. The need to limit this voltage rise, rather than exceeding the thermal capacity of the line, often determines the limiting size of generator that may be connected to a particular location. An initial estimate of the voltage rise caused by connection of a generator can be obtained from analysis of the system as represented in a simplified form...

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...

The Asynchronous Generator

Asynchronous Generator

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...