Several types of generators can be coupled to wind power turbines: parallel and compound generators, dc and ac types, and especially, induction generators (see Chapter 10).
Once the installation site of a wind power plant has been selected, the next steps are to select the turbine rating, the generator, and the distribution system. In general, the distribution transformer is sized to the peak capacity of the generator according to the available distribution network capacity. As a practical rule, the output characteristics of a wind turbine power do not exactly follow those of the generator power, and they must be matched in the most reasonable way possible. Based on the maximum speed expected for a wind turbine, and taking into account the cubic relationship between wind speed and power, the designer must select the generator and gearbox to match these limits. The most sensitive point is the correct selection of the rated turbine speed for the power plant. If it is too low, generation for high-speed winds will not be possible. If it is too high, the power factor will be too low. There is an iterative design process on the match of the characteristic of commercially available wind turbines and generators with regard to cost, efficiency, and the maximum power generated. The maximum value of Cp should occur at approximately the same speed as that of the maximum power in the power distribution curve. So the tip speed ratio must be kept optimally constant at the maximum speed possible to capture the maximum wind power. This feature suggests that to optimize the annual energy capture at a given site, it is necessary that the turbine speed (the tip speed ratio) should vary to keep Cp maximized, as illustrated in Figure 4.15 for a hypothetical turbine. Obviously, the design stress must be kept within the limits of the turbine manufacturer’s data, since torque relates to the instantaneous power as P = To.
It should be pointed out that in the case of parallel generators connected to batteries, small rotation increments are linked to large increments in the output current. When the terminal voltage of the battery stays constant (increasing the nominal rotation by a determined percentage), the current suffers an increase of k times its initial value. As a result, the power also suffers an increase of k times its initial value.
If the voltage drop of the generator occurs at higher rates than the current increment, a dc compound generator of three brushes should be used rather than a parallel generator. For a compound generator, the design is executed so that the decrease in the characteristic is at the same rate as the number of demagnetization coil turns. For three-brush generators (due to the increase in coil current), both field voltage and magnetic field are reduced.
Due to the advantages of working as a motor or generator (and its low cost with respect to other generators), the induction machine offers enhanced conditions for wind power plants. Only an induction generator connected to an infinite bar is considered in this chapter; other types are discussed more fully in Chapter 10.
The aerodynamic efficiency of a wind power machine of very small size varies, at most, between 40 and 45%, but in practice it is, on average, 35%. The efficiency of a dc generator is on the order of 55 to 60%. Therefore, the total efficiency of a small system (a few hundred watts) is on the order of 20%. One of the main causes of loss is the excitation demanded by the generator. In that case it is advisable to choose permanent magnet-based dc generators. It is possible to eliminate the switches in alternators of six or more poles by the use of permanent magnets. It is also advisable to use rectifiers to charge batteries. The generator should present voltage and frequency proportional to the rotation so as to provide current regulation by the inductive reactance of the circuit.
In conclusion, the designer needs the following specifications before purchasing a wind power turbine:
1. Wind intensities in the area and the duration curve
3. Purposes of the energy generated
4. Present and future energy needs for the generator and turbine according to the
area’s wind capacity (otherwise, any specification becomes useless)
5. Determined need for a turbine isolation valve
6. Specifications for the type of regulator
7. Specifications for the generator (see Chapter 10) according to:
a. Type: induction, synchronous, or direct current
b. Electric output: alternating or direct current, maximum power, voltage level, number of phases, frequency
c. Climate: temperature and humidity
d. Automation level: manual, automatic, semiautomatic, or remote control
e. Operation: stand-alone or for injection in the network (in which case a sale, purchase, or exchange commercial agreement should exist with the electric power company)
f. Number of phases: single – or three phase transmission and distribution
8. Other equipment: control panels, protection, and drives
9. Location of the machine house (if there is one, distance and dimensions)
10. Access possibilities to the site: boat, bike, highway, rails, walking distance,
suitability for land transportation of equipment and maintenance planning
The cost of wind power plants has been changing in the last few years. By the year 2004, they cost around $0.50 per watt and $0.06 per kilowatthour.