Spanish technology company Ingeteam reveals details of its Ingecon CleanPower series of doubly fed generators. Eize de Vries spoke with the company’s hardware R&D manager, David Sole, about the new system and its promise of substantial power quality improvement.
Conventional geared wind systems with fast-running generators have dominated the wind market for many years. That comfortable status quo was again confirmed in 2007 when the cumulative world market share of geared wind systems remained unchanged at about 86%. Direct drive turbines made up almost the entire remainder, while low-speed Multibrid-type turbines, featuring a single-stage transmission and currently available from two suppliers are just getting off the starting blocks commercially.
Also important in terms of progress in wind technology is that – for different largely technical reasons – variable speed, pitch-controlled wind turbines started becoming the dominant technology from about 2000-2002 onwards. When these variable speed installations are coupled to a generator, they produce electric power with variable frequency and therefore require a converter to deliver grid compliant power.
For many years, conventional variable speed wind turbines featuring a doubly fed induction generator (DFIG) have enjoyed a leading position in the global wind market, if not a position as an industry semi-standard.
Commercial applications for the wind sector first took place during 1996 with the development of a 1.5 MW Tacke prototype, now GE Energy. This was followed a little later by a 500 kW DeWind prototype and in the US a 750 kW former Zond turbine was also equipped with a similar generator/power converter solution at around that time. Ever since, DFIG applications have rapidly gained popularity within the industry and are now in use by a wide range of suppliers including Acciona, Dewind, Ecotecnia, Fuhrländer, Gamesa, GE, Mitsubishi, Nordex, REpower, and Vestas.
The single technical feature that has perhaps contributed most to the DFIG success story is the fact that only 25%-30% of the electricity generated is fed into the grid through a power converter. This percentage is identical to the share of total power produced in the generator’s rotor, and it also corresponds to the pre-defined wind turbine rotational speed range. As an inherent part of DFIG systems, the generator stator feeds the remaining 70%-75% of total power directly into the grid, usually at 690V via a step-up transformer. This system can either be a 50Hz or 60Hz, suitable for either Europe or North America respectively, but each grid situation requires a generator solution adapted to the specific operational circumstances.
Compared with figures from 1996, the costs of power electronics have fallen by a factor of 8-10, but according to wind industry sources, the smaller frequency converter possible with DFIG technology still represents an overall system investment saving.
New grid rules for wind turbines came into effect in Germany from January 2004 following a pioneering initiative of the German utility giant E.ON. As part of these compulsory new rules, wind turbines within its supply area – and in an analogy to conventional power plants – have to remain grid-connected in the case of a major voltage dip. This grid rule model is now widely regarded as a valuable case of forward thinking and is being followed by several other countries with major wind markets, although often it is introduced with country-specific modifications. A second common requirement of these tailored grid rule packages is that grid-connected wind turbines should have a built-in capacity to actively support the grid. Both measures are designed to avoid a worst-case scenario, whereby instantly switching off a large chunk of wind generating capacity during an emergency could cause catastrophic grid failure and a widespread blackout.
A minority of geared variable speed wind turbine suppliers, for example Siemens, apply asynchronous generators while others, such as the new GE 2.5xl machines, use synchronous generators. Both these generator types in variable speed mode operate in combination with a full converter system in which 100% of the generated power is fed into the grid through a frequency converter. Siemens for instance uses a 690 V, three-phase squirrel-cage type generator in all its variable speed wind turbine types up to 3.6 MW. These machines were for decades also the preferred choice for all the company’s fixed-speed models. Induction-type generators – known to be relatively simple, robust and cost-effective – are widely used, while one often-mentioned advantage of a brushless induction generator is that the problem of slip rings is avoided.
One claimed benefit for synchronous generators is that these machines can be applied in 50 Hz or 60 Hz grids without any need for technical modifications as the system power converter is capable of handling the grid variables.
Furthermore, compared with full power converters, DFIG equivalents show more problematic grid behaviour. For example, during grid failure situations, such as a voltage dip, potentially harmful large peak currents are introduced to the system. This issue has been dealt with effectively by the wind industry by incorporating a so-called crowbar into the generator rotor which short circuits the generator rotor in the event of a grid failure. This enables the evacuation of these peak currents, explains David Solé, R&D manager at Ingeteam. However, this fact is also regarded as an advantage from the point of view of the grid operator, because it contributes to clear the fault by tripping over-current switches, he adds.
Stray currents and other disadvantages
A disadvantage related to DFIG is the occurrence of internal stray currents in the generator. These currents are infamous in the wind industry as a contributing factor in premature generator bearing failure. Protective counter measures include the application of special generator bearings and/or seals, that shield them against the negative impact of stray currents. Furthermore, in contrast to both synchronous and asynchronous generators, DFIGs are inherently incapable of acting as electric brakes for emergency or alternative uses. Nonetheless, while the use of such an electric braking function is as yet not very common in the wind industry, within DFIG topologies, if the stator power decreases abruptly due to a grid fault or a disconnection from the grid, there is no control over the drive train.
However, within the Ingecon® CleanPower System an electrical excitation machine is included together with the DFIG. By using this so-called xDFM® technology, the exciter machine power can be used to drive an electric brake. The electric brake may also be used together with aerodynamic braking, minimizing peak torque loads and undesired accelerations that might cause premature damage to some components of the wind turbine.
Harmonic currents, flicker, and the existence of ripple in generator power delivered are well-known electric power quality distorting problems associated with the majority of today’s grid-connected variable speed wind turbines. Such problems are not exclusively confined to DFIGs, however. They also occur in synchronous as well as asynchronous electric machines operating with variable speed and connected to the grid with the aid of a power converter. As a counter measure, suppliers add harmonic filters to the electrical system.
A typical state-of-the-art wind turbine power converter consists of two parallel AC-DC converter units. Although these sister components act together, they are not necessarily fully identical with regard to power rating. In a standard DFIG-type power conversion system the AC input side of first converter unit is connected to the generator rotor output wires, – see Figure 1. The input to the unit is a generator current characterized by a continuously varying frequency of 15-55 Hz that is rectified into DC current in the device. Both DC-sides of the twin sister units are interconnected by a so-called DC-link bus. Finally, the AC output side of the second converter unit is connected to the grid system, where it feeds in grid-compliant power of the right ‘quality’ and with a stable frequency of either 50 Hz or 60 Hz depending on the local grid standards.
In contrast, in an asynchronous or synchronous generator connected to a ‘full’ AC-DC DC-AC power converter, the AC input side of first converter unit is connected to the stator output wires, as shown in Figure 2, above.
One of the few exceptions is the DeWind D8.2 turbine, a variable speed, pitch-controlled wind turbine that features a hydrodynamic WinDrive® transmission supplied by Voith of Germany. As part of a novel power conversion concept, the rotor turns with a variable speed, while the WinDrive unit continuously adjusts rotor speed fluctuations into a fixed generator speed. The D8.2 features a synchronous generator that feeds power directly into either a 50 Hz or 60 Hz grid. Consequently, a frequency converter is no longer necessary.
Positive features summarized
The most prominent added hardware feature as part of the Ingecon CleanPower System is the inclusion of an exciter electrical machine together with the DFIG – the so-called xDFM®. This is in essence a permanent magnet-type synchronous machine that is mechanically coupled to the drive train, but its application is rather unique in a wind turbine generator. And, in analogy to a ‘standard’ DFIG system, an Ingecon CleanPower System unit comprises an AC-DC DC-AC power converter with a 25%-30% of generator rated power handling capacity. The rated capacity of the exciter machine is about 16%-18% of the generator power rating, about 500 kW for a 3 MW wind turbine. The AC input side of the first converter unit is again connected to the generator rotor output wires. However, in the new Ingecon system the output side of the second converter unit is not directly connected to the grid, but is instead attached internally to the exciter machine. This is illustrated in Figures 3 and 4.
The inclusion of the excitation machine makes it possible to isolate the power converter in such a manner that it is in no way directly connected to the grid. As the illustration shows, in the xDFM technology solution, the stator is the only grid-connected output, a solution that is different from a standard DFIG grid-connection, in which the generator rotor power is fed into the grid via a power converter. An Ingecon CleanPower System can either operate at a sub-synchronous speed, at synchronous speed – 1500 rpm at 50 Hz for a four-pole machine – or at super-synchronous speeds. During sub-synchronous operation, power flows from the exciter machine to the DFIG rotor, an operational condition in which the excitation machine acts as a generator. At synchronous speed no power flows between the DFIG rotor and the exciter machine. However, when the generator operates with super-synchronous speed, power flows in the opposite direction – from the DFIG rotor to the excitation machine, and the latter acts as an electric motor. The power balance during the entire speed range is such that power ratio generated/consumed in the excitation machine fully offsets the power ratio consumed/generated within the DFIG rotor, except for minor losses within the different system elements.
Solé says that the Ingecon CleanPower System provides an effective solution to the most common grid-related problems caused by grid-connected variable speed wind turbines. ‘Our tests showed a remarkable improvement in output power quality,’ he says, adding: ‘xDFM technology offers an outstanding LVRT behaviour, as the converter is always active, due to the fact that there is power supply from the exciter machine, the Ingecon CleanPower does not need any LVRT device and keeps total control of the system, limiting torque peaks and reducing mechanical loads that typically occur during a fault or in emergency situations.’
Solé acknowledges that the Ingecon CleanPower System technology with an excitation machine does slightly add to overall system investment costs but says: ‘On the other hand substantial savings can be achieved by the fact that the harmonic switching filters are no longer needed as a result of the excellent power quality with the new system. In addition, the crowbar can be eliminated from the system, as the need to short-circuit the generator rotor in case of a major grid failure does not exist anymore. One very cost effective solution could be using a medium-voltage generator stator of around 12 kV. This could eliminate the costs for a step-up transformer.’
As far as the current project status is concerned, Ingeteam has already built a prototype and has tested and optimized it for a prolonged period in its facilities. The next envisaged step is to install an Ingecon CleanPower System in a multi-megawatt class wind turbine for actual field tests, to take place before the end of the year. Solé is very confident in these field test results: ‘Our novel xDFM innovation is perhaps not revolutionary, but it provides an effective solution to the most common grid-related problems caused by variable speed wind turbines. In fact it combines, in our opinion, all advantages of standard DFIGs – including lower system costs – with specific benefits of full converter generator systems,’ he says.
Finally, Ingeteam expects a lot from the patented development in the US market, where General Electric holds an important patent on variable speed power conversion technology.
Says Solé: ‘Perhaps even more important is that our new system is perfectly equipped to deal with the new grid internationally valid codes that will come into effect soon.’
Eize de Vries is Wind Technology Correspondent for Renewable Energy World.
You can contact Elize de Vries at firstname.lastname@example.org
Figure 1. Standard DFIG-type power conversion system
Figure 2. Asynchronous or synchronous generator system
Figure 3. The new Ingecon system. A new electrical topology for wind turbines; variable speed with converter isolated from the grid
Figure 4. The excitation machine
Ingeteam is one of the world’s leading DFIG and control system suppliers, with an estimated 16% of 2007 market share on a MW basis. Besides DFIGs, the company also supplies asynchronous machines and synchronous generators with external field excitation or permanent magnets.
The fast-growing Ingeteam Group comprises 28 companies and employs a total staff of 3000. Its energy division is a leading independent supplier of electric power conversion and control systems for wind turbines. Other groups include industry, marine, traction, basic technologies, and services. Between May 1997 and today, the energy group – which focuses on renewable energy sources – has supplied over 11,200 Ingecon type wind power converter units with a cumulative capacity of 11,900 MW. Factory output for 2007 alone amounted to 2992 MW, and manufacturing capacity is being expanded again this year to help meet strong global market demand. Besides different kinds of electric machines and power converter units, Ingeteam also supplies PLC type controllers, pitch control technology, plus local and remote controls. The division is subdivided into three business segments: wind power, photovoltaic energy – which had approximately 300 MW of capacity in 2007 – and hydroelectric, biomass and biofuels, with a station capacity of some 2200 MW total by the end of 2007.