Wind power and other renewable decentralized energy resources play a bigger part in our energy mix than ever before, this increases the demand on a more complex power system and increases the demand on the power grid on every voltage level. Direct drive the benefits still heavily overweigh disadvantages connected to direct drive, such as the Full grid compliance. In this paper three different kind of variable-speed wind turbines has been described, the frequency converted, the so called DFIG system and the so called full-power converter system. Power electronics might still be expensive but if one considers the lack of a gearbox and the decrease on maintenance cost that follows it’s a reasonable expensive solution. New grid codes and a more complex energy mix and the ability to control reactive power and voltage will have e more centralized role in the power system than ever before, power electronic is one sustainable solution to these problems.
Wind power is one of the fastest growing renewable energy source in our time, because of this the development of the technology has been push forward at tremendous speed. From simple constructions in the early 70 and 80’s to a multi-million business with research being done in aerodynamics, mechanics, electronics and generator construction not the least (Szarks, Cowell, Ellis, Strachan, & Warren, 2012).
Wind turbines today is drive with either a fixed speed generator or a variable speed generator. The main advantage with fixed-speed wind turbines is that they have a well-established technique. On the other hand, the system has a lot of disadvantages such as large power variations, no ability to control reactive power and it requires a relatively strong structure.
While some of the advantages with variable-speed wind turbines are considerable lower trust forces, no over torques, less power variations and possibility to control reactive power. This leads to lower flicker emission and lighter structure for a variable speed wind turbine.
Here, three different kind of variable-speed wind turbines has been described, the frequency converted, the so called DFIG system and the so called full-power converter system. During normal operation the systems behaves in a similar way. However, the full-power converter System are able handle grid disturbances better than the DFIG and frequency converter system. The increasing amount of renewables creates more demands on stable and flexible electrical grids on all voltage levels (more of the different voltage levels in next chapter).
Most countries have a leveled power grid (As shown in the Figure 5) with high voltage powerlines distributing electricity from the different energy resources available in the specific country.
Figure 1: Simplified grid overview of voltage levs in a power grid. Source. Texas Instruments
In Sweden main grid covers 220 and 400 kV systems and main components of the relations between the neighboring Nordic countries. The main grid functions mainly as a transmission and transfer a major impact on long distances, particularly from hydropower in the north to the consumption areas in the south. Power producers and industrial uses grid to transport electricity from their own plants or purchased electricity to the consumption of the country.
The regional networks typically have a voltage level of 30-130kV and is owned and operated by power companies. The networks linking the national grid with large beneficiaries of power distribution companies and major consuming industry.
The local network that normally has a voltage level of not more than 20 kV owned by the distribution companies. Because of the lower voltage levels have local networks rather limited transmission capacity, the bulk of the country’s electricity consumers are connected to the local network via the distribution network. In the distribution areas power is transformed down to a working voltage of 400 / 230V.
On every of the described voltage level the grid owner has certain demands on electric quality and stability, this is what pushes the development of power electronics solutions with the ability do contribute to a smart power grid by handling voltage, frequency and power.
Variable speed turbines
In order to extract the maximum amount of energy in the wind a turbine with variable speed is ideal, because of its ability to follow the wind speed it can be programed to produce the optimum energy at every certain wind speed. But there is not only the flexible power output that makes the variable speed turbine superior to its sin gel speed concurrent, the variable speed turbines ability to absorb and produce reactive effect is a critical advantage with rising demands on efficiency on the grid.
Elimination of the gearbox allows for higher part load system efficiencies, improving your return on investment. Providing a reduction in overall maintenance, these solutions are particularly well adapted for offshore applications. However, this technology is still relatively new ad therefor relatively expensive compered to single speed gearbox turbines (Andreas Petersson, 2006).
But when it comes to direct drive the benefits still heavily overweigh disadvantages connected to direct drive, such as the Full grid compliance: Fully-fed with low-speed direct drive. Advanced resin insulation system provides increased insulation performance (Sorensen & Sorensen, 2012). Some other pros of a direct drive system that might give it an edge over single speed systems is:
- Higher power density, due to extra cooling system.
- Increased efficiency, due to permanent magnets and sturdier stator
- Increased reliability.
- Reduces maintenance and increased generator availability due to the use of permanent magnets.
Direct drive systems has also the advantage of being able to be customize for lover voltage levels which in turn allows the system to be tailored to several requirements.
But direct drive also has some drawbacks, the power electronics in the system is still relatively expensive and sensitive for voltage peaks and the generators is usually bigger and heavier which in turns increases the demand on the tower and also increases the demand on the crane used to lift the nacelle. This in turns may drastically increase the investment cost, especially offshore (Wikipeida, 2014) (Wizelius, 2007).
Multi-pole synchronous generators, ore ring generators has been used a long time in hydropower stations. The advantage of this generator type is that is can operate with a low rotational speed. In wind power applications the ring generator can be driven directly by the rotor without gearbox. This is achieved by using power electronics in the form of a frequency converter do adapt the power output to the grid (Wizelius, 2007).
The frequency converter is used to change the frequency and magnitude of the constant grid voltage to a variable load voltage. Frequency converters are especially useable in variable frequency AC motor drives. In order to use a synchronous generators as a variable speed turbine the output of the generator must first be rectified by converting it from AC to DC and then back to AC to match the grid frequency, this is done by using a Frequency converter (in this case a DC converter shown in Figure 1).
Figure 2: DC Converter connected to multi-pole synchronous generator. Source Elforsk rapport 06:04
Most typical frequency converter topology is the three-phase two-level voltage source inverter. The phase voltages are controlled using power semiconductor switches and PMW . Semiconductor switching devices and anti-parallel connected freewheeling diodes form a bridge, which can connect each motor phase to the positive or negative DC-link potential. The PWM changes the connections of the phases between the positive and the negative DC-link potentials so the wave voltage has the desired frequency and amplitude. The generator reacts primarily to the fundamental frequency and filters out the effects of harmonic frequencies. (Manwell, Mcgowan, & Rogers, 2009).
By using this type of converter active power can be transferred in both directions AC-DC ore DC-AC, and the reactive power demand on respective AC side can be delivered by the PMW (Sorensen & Sorensen, 2012).
Double-Fed Induction Generator
One of the current situation usual Access System turbines with variable speed is the DFIG Where the rotor circuit is fed from an inverter and stator is connected directly to the mains shows in Figure 2. The reason why the system of DFIG has become so popular is that the power electronic inverter need only handle a small portion of the power, typically 20-30 % of the turbine’s rated power. This means that the losses in the inverter itself can be reduced compared to a system where the inverter is forced to handle the full effect.
DFIG is very popular for modern wind turbines and is used I most of the bigger developer’s direct turbines. The generator uses multiple phases rotor and slip ring with brushes connected to the rotor windings.
The rotor circuit of a DFGI normally uses a back-to-back converter which consist of two bidirectional converter with a shared DC-link which in turns controls both the rotor and the grid currents by having on conation to the grid and one to rotor.
By using the back-to-back setup the rotor frequency can freely differ from the grid frequency. The power electronic converter for this sort of variable speed generator has the ability to control both active and reactive power delivered to the grid. To control this there are mainly two principle used, either the two-axis current vector control ore the Direct Torque Control called DTC. DTC has shown to have a better stability than current vector control this is especially the case when the generator has a high reactive current requirement. This happens when de generator needs to magnetize itself in the startup process.
The DFIG rotor are typically wound with 2-3 times the number of turns of the stator, this means that the rotor voltages will be higher and the rotor current lower (Wkipedia, 2015). The nominal power of the converter is around 30% of the turbine power. This enables the rotor speed to vary ± 30% operational from the speed range around the synchronous speed. Slip is changed by the rate of power flowing through the generator circuit and by controlling the active power in the converter it is possible to vary the speed of the generator and thus the turbines rotor (Sorensen & Sorensen, 2012). The drawback of this system is that a multistage gearbox is needed in the turbines drive train in order to handle the speed range, and thus giving the system all the drawback associated with gearboxes. Another drawback is that in order for the power converter to control the rotor it requires electrical conation between the rotating system and the stationary system, such connection is usually given by carbon brushes on the stationary system. These brushes require regular maintenance and are a crucial part that can cause machine failure. Also in the case of voltage dips on the grid, the stator and rotor currents may increase drastically for short amount of time (~ 100 ms) this can cause high loads on the drive train due to high increases in torque which may shorten the lifetime expectancy for the generator (Sorensen & Sorensen, 2012).
to summary’s the DFIG, several advantages but also some drawbacks in wind power applications. the greatest advantage is that the electronic converter is able to both import and export reactive power which is important in a time of tougher grid codes. Control of the rotor voltages and currents that allows the generator to remain synchronized with the grid even duo wind speeds varies. A variable speed wind turbine can utilizes wind resource more efficiently than a fixed speed wind turbine, this gives it an extra edge during light wind conditions (Wizelius, 2007). Cost is lower compared with other variable speed solutions because of only a fraction of the power is fed through the converter, the rest of the power being fed to grid directly from the stator, See Figure 2. The efficiency of the DFIG is very good for the same reason (Andreas Petersson, 2006) (Manwell, Mcgowan, & Rogers, 2009).
Figure 3: illustration of DFIG operated wind turbine. Elforsk rapport 06:04
The converter consists of two multiphase windings with similar pole-pairs and are placed on the rotor and the stator. Since the windings of the rotor is actively converting energy whit the winding in the stator utilization of the magnetic fields in the core are optimized. This single it out from other electric machine types where the magnetic estate is not as effectively used.
Traditionally a multi slip ring rotation system together with carbon brushes used to transfer power to the rotating windings and to allow an independent control of rotor winding. As mentioned earlier the slip ring assembly requires maintenance and causes a uncertainty in the systems reliability, cost and efficiency (Sorensen & Sorensen, 2012).
Development of modern wind turbine generators has evolved through the years, in the aspects of output power, dimensions, and the technology used. Conceptual scheme of wind turbines with full scale power converter; with variable speed with a synchronous generator, is shown in Figure 3. This type of turbines nowadays may or may not include a gearbox and wide range electrical generators types can be employed, for example, induction, wound-rotor synchronous or multi-pole permanent magnet synchronous. All the power generated by the generator passes through the convertor, thus isolating the dynamic operation of the generator from the grid. Electric frequency of the generator can vary following the change in the wind’s speed, otherwise power frequency injected in the grid remains unchanged; thus allowing variable speed operation of the wind turbine.
Figure 4: Full-power converter consisting of IGBT modules. Source. Elforsk rapport 06:04
The power converters can be arranged in different ways. Whereas the generator side converter can be a diode rectifier or a PWM voltage source converter, the grid side converter is typically a PWM source converter. The control strategy of the generator operation and active and reactive power flow to the grid depends on the type of power converter arrangement. The rotor side converter ensures the rotational speed being adjusted within a large range, whereas its grid side converter transfers the active power to the grid and attempts to cancel the reactive power consumption.
This transistor inverter can be used both as rectifier and inverter, namely it is possible to drive the power in both directions (from DC voltage to AC voltage or vice versa, just as in the IDFG). It is also possible to control the active and reactive power independently of each other, which the IDFG could not. Therefore, one can also use this type of inverters towards the generator. Figure 4 shows how a typical transistor inverter can be configured.
Figure 5: IGBT thyristor converter setup for full-power conversion. Source Elforsk rapport 06:04
The transistors in the drive can be switched on and off with a very high frequency (several kHz). Then transistor rectifier connected to the mains supply a filter between the inverter and the electric grid as shown above. In cases the transistor rectifier is coupled to a generator it may suffice to use the generator leakage inductance as a filter, in these cases there is no need for an extra filter. The filter is required because the drive controls a voltage of the AC – voltage side, and two voltage- system can’t be connected without a filter in between. The filter will also filter the voltage so that power will not contain as much high frequency harmonics.
In a power system with an increasing contribution from renewable resources like wind, solar, and tidal power the demand on more flexible solutions to balance the grid will also increase. Non-direct drive turbines are still a working concept and the thing making the biggest differs between direct drive and single drive gearbox turbines is the obdurate cost around the gearbox and the extra maintenance costs due to more moving mechanical parts. The more effective harnessing of wind energy that is the case of direct drive systems is also a factor that will increase production and there for might increase the interest for direct drive. Now producers still really on single speed turbines in most of the turbines this might come to change if and when de demands set by the grid codes is changed to include tougher rules on voltage and power control, if and when that happens the need for more advance power electronics will be much greater than it already is.
Power electronics might still be expensive and have some cons that still needs to be sorted out but if one considers the including of a gearbox to be optional and the decrease on maintenance cost that follows it’s a reasonable expensive solution. Whit sharpened grid codes the ability to control reactive power and voltage will be of great importance, there is two ways of doing this, power electronics in the turbine ore external reactive power and voltage compensation equipment. So however method one chose to go with power electronics is going to play a major part in the future power system, this is mostly due to renewables being more decentralized than fossil based energy has been during the past decades and we now face a more complex energy mix.
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