Off shore Wind Farm (Sweden)

The concept of large scale wind farms is to augment existing supplies. The concept in wind farming is to harvest the wind to its maximum potential, which is utilising all of the wind captured in the farming region and to take up as much of the utility load that is possible, without loss of supply integrity. This differs from most line generators where the aim is to minimise fuel use and spinning reserve. There is a number of practical limitations, primarily, dynamic stability off the grid, which limits the percentage of wind energy which can be applied to the grid at any one time. Modern technology and developments are pushing these boundaries promising new horizons.

Selection of a suitable site

Section of a suitable site involves a number of both technical and non-technical aspects that are outside the design criteria but would include historical data collected from the site, environmental impacts and legislative requirements.

Technical issues

Large Scale Wind Turbine

Most importantly the selection criterion for a suitable site is the available wind resource that is in terms of both reliability and wind speed. Remembering that, the aim of the turbine is to convert the kinetic energy of the wind into electrical energy. Locations that suffer from blustery conditions are far less suited than areas which have a regime where there is a constant wind at perhaps lower speeds. Wind turbines being used in areas where there are lower consistent speeds will tend to have a larger wing span, which makes the turbine more vulnerable in adverse conditions. A reliable resource is essential when planning a renewable component on the transmission grid.

Blade design will reflect a number of elements;

  • expected wind conditions
  • turbine design
  • turbine size, including turbine RPM

An area with multiple turbines offers a flywheel effect through the collective mass of the rotor, turbine and blade mass of each turbine. This can be both advantageous as well as detrimental. The major concerns with turbines in changing wind conditions are;

  • frequency changing
  • variable voltages and associated AVR response times
  • changed line transmission status including;
  • lead and lag angle
  • harmonic distortion
  • reflected harmonics
  • circulating currents and unwanted transients

Where non synchronous machines are used, most of the above mentioned problems are alleviated. Most non synchronous machines perform very much like a maximum power point tracking inverter, where the actual line condition is monitored by the inverter (converter) and maximum energy is transferred to the grid within the parameters set by the inverter. These turbines are called variable speed turbines and generate AC which is some cases is rectified and then inverted.

Most modern wind turbines utilising this technique tend to use insulated gate bipolar transistors IGBT in their switching circuits. This initially posed a problem because the generation of IGBT switching voltage was low, typically elected at 240 volts or 600 volts, requiring very large switching currents on large machines. Nevertheless, these units are growing in popularity and proving to be very reliable. Today the largest wind turbine is a variable speed unit rated between 6 and 7 MW.

Synchronous machines have large gearboxes to achieve the appropriate alternator speed and rely on blade furling, load excitation and system electrical mass to maintain the appropriate frequency. In a large field of turbines the collective mass of the turbines can help to stabilise the frequency by effectively absorbing gusting wind conditions. That is when all units are locked in synchronism, (see image above, right) the units will respond as if they are all connected on one shaft. Any variation in turbine speed must be achieved by all turbines in the wind field as well as other generators on the grid. The very nature of mechanical mass requires that for a substantial variation in turbine speed the wind conditions must be present for some time.

 

Synchronous Turbines

Synchronous Wind Turbines

Where the wind field has been carefully designed and located, the design solution may have the turbines spread such that it is unlikely that all turbines will be subject to the blustery conditions at the same time. Nevertheless, blustery wind conditions still pose a significant problem to transmission quality. It is important at this stage to remember that the outer tip of the blade is often moving at speeds much greater than the wind. The centrifugal forces increase with wind speed, hence making it critical to employ some form of braking as well as speed control.

Most synchronous turbines use multi-pole machines, a 4 or 6 pole machine will help to reduce wind turbine speed. Typical wind turbine blade speed is around 21 RPM. The supply frequency is achieved by using a gear box and multi-pole alternators. Salient pole constructions are common in this type of alternator because of the ease in construction and maintenance. An advantage of using a synchronous machine is that the generation voltage can be at transmission level reducing the need for additional transformation and the need for inverters. In the past larger machines have tended to be synchronous machines, but this is changing as semiconductor technology continues to develop. In addition these types of turbines generate a sine wave with low level transients and limited harmonic distortion without the need for sophisticated electronics.

Disadvantages;

  • maximum efficiency is achieved at only one wind speed or a narrow band of wind speeds
  • synchronous machines are more prone to fatigue than variable speed wind turbines
  • power quality is dependent upon wind conditions
  • greater turbine and tower fatigue stress

Early synchronous machines controlled the frequency of the turbine’s generation by reducing excitation in low wind speeds which reduced the load on the rotor and increased excitation as the wind strength increased. When maximum wind speed was experienced, the turbine would yawl away from the wind and if necessary apply brakes. Some manufactures experimented with pole switching techniques using 6 pole configurations for low speeds and moving to a 4 pole as wind conditions improved. This gave the unit two possible maximum power points, although efficiency was lost through a range of wind strengths. Another concept is to adjust the pitch of the turbine blades using the inertial of the blade assembly to act as a cushion to changing speeds and allowing time for the blade pitch to change to the new wind speed.

There is a trend towards variable speed wind turbines with a capture range that is still virtually a synchronous alternator by introducing either positive or negative slip. These machines offer greater flexibility in a wind park and effectively improve the transmission quality by reducing the effect of changing wind speeds. The double feed rotor alternator uses an AC to AC converter where the rotor excitation is at a slip speed.

Slip speed is the difference between the generated electrical waveform and the mechanical rotating magnetic field. For example; a 4-pole machine to generate 50 Hz, needs to have a mechanical rotor speed of 1500 RPM, effectively producing 720° electrical with every rotation.

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