Small scale embedded generation

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Small scale wind energy

Small scale embedded generation is defined as "any source of electrical energy rated up to, and including, 16 A per phase, single or multi-phase, 230/400V AC" [1]. In the case of wind energy it is quite common to consider small scale wind turbines as those rated less than 100kW. Small scale wind turbines offer several advantages over their large-scale counterparts although as a general rule of thumb the total cost of power generation decreases with the size of the turbine. Large scale wind energy production requires a big capital investment not only due to the equipment cost but also due to the very large windy sites required for installation. This makes smaller wind turbines more suitable for applications such as "isolated islands, single dwellings, remote cabins, and street lights" requiring much less capital investment although the cost of per generated watt increases [2].

The low power rating of small wind turbines allows the use of technologically advanced solutions which would be difficult to implement in the case of e.g. a 5MW wind turbine. This makes the use of a number of smaller wind turbines with significant cumulative output power much more attractive. Moreover, the power rating of a wind turbine increases with size causing environmental problems and significant noise. This makes the installation of high power rated wind turbines unsuitable for urban areas but mostly suitable for remote areas where the connection to the grid is weak [3].

In the White Paper on Energy published by the UK government it was estimated that at 2007, in UK alone, there were 20000 installed small scale wind turbines with a total output power of 7MW. This shows the significant proportion of small scale wind power generation in the renewable sector compared to the estimated 1300 photovoltaic UK installations. Furthermore, the average cost of 7p/kWh makes small wind technology much more attractive than solar PV energy with an average cost of 24p/kWh but still considerable expensive compared to large scale systems having an average cost of 3p/kWh [4].

Wind turbine types

There are two basic types of small scale wind turbines depending on the position of the rotor: horizontal axis and vertical axis turbines. The advantage of the vertical axis, also called Savonius or Darrieus rotor, over the horizontal type is the fact that it operates irrespective of the direction of the wind. Although most large scale wind turbines nowadays use horizontal type wind turbines, in small scale systems the vertical axis orientation is still quite common. The horizontal types can be subdivided into axial and transversal depending on their orientation with respect to the direction of the wind. Axial wind turbines are widely used in both small and large scale systems. Small wind turbines are offered with 2, 3 or more blades. Orientation is provided via either a tail or shaped blades [5].

The big difference of small scale wind turbines compared to large scale systems is that the blades are fixed and protection is offered through the aerodynamics of the rotor. This is often referred to as stall control since the rotor stalls at high wind speeds offering the advantages of simplicity and less power fluctuations. The disadvantages over the pitch control method employed in large scale systems, where the angle of attack is controlled by changing the pitch angle of the blades, are that less power is extracted from the wind at low wind speeds while no assistance is offered at start-up. Further, variations in air density and the frequency of the grid can cause variations in the output power [6].

In rural areas small scale wind turbines are usually mounted on a mast close to a dwelling. However in urban environments where not much space is available, they can be installed on the roof of a building. The disadvantage of this installation is the turbulence of the air created by the orientation of the space surrounding the building. This can be overcome by modifying the structure of the building in order to guide the air to flow through the turbine blades. Very small wind turbines are also mounted on sailboats. Finally note that with small scale wind turbines no connection in the grid substation is required; the turbine can directly be connected in the local distribution system [5].

Another classification of wind turbines is made with respect to the speed of the rotor. Most small scale systems nowadays use variable speed turbines in which the rotational speed of the generator changes in order to maximise the power extracted from the wind. As a result, gusts of wind are mostly absorbed by variations of the rotational speed of the generator keeping the torque, and thus the output power, relatively constant. In contrast, fixed speed turbines keep the rotational speed of the rotor constant irrespective of the wind speed while controlling the extracted power by varying the winding sets. Fixed speed turbines use induction generators and offer simplicity, reliability, robustness and low cost at the expense of reactive power consumption for the excitement of the generator and increased mechanical stress and line losses. These losses are caused by voltage variations that may occur due to changes in power as a result of wind speed fluctuations. On the other hand variable speed wind turbines use either synchronous or induction generators and are connected to the grid via a converter which controls the speed of the generator. Although fluctuations in output power are smaller and mechanical stress in the aerodynamic system is less than in the case of fixed speed turbines, the introduction of power electronic devices increases the complexity and cost of the system while additional losses occur in the converter [6].

Generators

In general two types of generators are used widely in wind turbines: induction and synchronous generators each with its relevant advantages and disadvantages. Induction generators have governed the industry especially in large scale wind systems. However small scale wind turbines have started to be dominated by permanent magnet synchronous machines. The biggest difference of a synchronous and an induction generator is that the former operates at the frequency of the grid while the latter at a higher frequency. In both generators the stator is made of a laminated iron core fitted with a three phase winding producing a rotating magnetic field with constant speed. However the rotors in the two machines are different. In a synchronous generator the field winding of the rotor is fed with a DC current creating a magnetic field. The interaction between the two fields causes the rotor to rotate in synchronism with the stator field. In an induction generator the rotor is not fed with current but the currents are induced due to the relative movement of the rotor with respect to the magnetic field of the stator. The difference between the synchronous speed and the rotational speed of the rotor is called slip [7].

Induction generators can be either squirrel cage or wound rotor type. Squirrel cage generators are very efficient and require little maintenance but a gearbox between rotor and generator must be used since they rotate at different speeds. Their ability to slightly change their rotational speed for large variations of wind speeds makes them ideal for use in fixed speed wind turbines. However their steep torque-speed characteristic together with the high inrush currents can cause severe voltage depressions and make necessary the use of a soft starter. The torque speed characteristic can be modified with the use of a wound rotor where the resistance of the rotor winding can change but the overall cost of the rotor increases [5 page66]. If for example the generator has high inrush currents, the resistance of the winding can be increased at start-up thus producing high starting torque with low current. However this resistance must be decreased at high speeds to prevent large variations of speed with relevant torque changes caused by the modification of the torque speed characteristic. Wound rotor generators are usually used with variable speed wind turbines and in conjunction with an optically controlled converter that modifies the resistance of the rotor winding.

In general induction generators are efficient, although less efficient than synchronous generators, and robust while there are minimum requirements for maintenance. Moreover their large production has dropped down the cost of manufacture. Another advantage is that they can simply be connected to the grid either by bringing the rotor to rated speed and then connect the generator to the grid or by connecting the generator to the grid and use it as a motor to bring the rotor in the rated speed [9 page 229]. Either case synchronism is not required. The big disadvantage of induction generators it the requirement for reactive power to excite the stator core which must be provided either by the grid or a power converter. The corresponding reduction in load power factor can be compensated with the use of capacitor banks [6 page67].

Synchronous generators are expensive and require maintenance but they are very efficient and have the big advantage of control over reactive power flow through control of the field winding [8 p121]. This gives full control over the voltage at the terminals of the generator. A disadvantage of synchronous machines is that when connected to the grid special synchronization equipment is needed to match the electrical angle of the AC power with the angular position of the rotor. Another disadvantage is that they are relatively stiff machines compared to induction generators due to their constant speed characteristic. As a result they respond to sudden gusts of wind or faults in an oscillatory way by varying only the load angle which can cause instability and loss of synchronism.

Permanent magnet Synchronous machines

Power electronics

Open circuit faults

  1. AC and DC aggregation effects of small-scale wind generators p.124
  2. Suitable design...p.1
  3. Comparison of power converter topologies p.2359
  4. the feasibility of building p.11
  5. small wind turbines in the built.. p.1
  6. wind power in power systems p.55
  7. wind energy technology john f. walker page 46
  8. Renewable energy in power systems
  9. wind energy explained

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