Cost Effective High r Converter for Windmills Norbert Pluschke SEMIKRON (Hong Kong) Co., Ltd., Hong Kong Abstract:The por demand for windmills is getting higher and higher and so as the price pressure. The price for windmill por is dropping year by year. The requirements for windmill converter are getting more tough (LVRT/HVRT) and the price pressure as ll. In our paper, will introduce several new design possibilities of high por windmill converter (above 5MW). We will compare the construction (modular), the location (Nacelle, bottom of the tor), the low voltage solution (1700V) and the high voltage solution (4500V) in windmill converter design.medium voltage converter in a wide voltage range (up to 20KV) with low voltage device in a new topology will be introduced. This new topology is also suitable for high voltage DC transmission.the comparison beten high voltage IGBT and low voltage IGBT in different topologies will be another topic in our presentation. A price competitive solution will be introduced with this new technology. Efficiency and redundancy will be discussed and a comparison beten high voltage solution (4500V-IGBT) and low voltage solution (1700V-IGBT) will be shown.modularity and maintenance will be another topic in the presentation. Keywords: medium voltage converter with 1700V IGBT, HVDV transmission, modularity, SKiiP3600A 0 Topologies for Windmill Converter Today the market is changing from Doubly Fed Induction Generator (DFIG) to Full Size or Full Scale Windmill Converter. Several topologies are in discussion but it is too early to identify a real market trend. Some companies are playing with the new 3-level-topologies and other companies are trying to use standard 2-leveltopologies in modular design. Medium voltage converters are in discussion but it is not 100% clear which topology will dominate. As shown in Fig. 1, there are several topologies in discussion for medium voltage converter: Medium voltage converter with high voltage IGBTs Medium voltage converter with low voltage device Medium voltage converter in 3-level technology Another important topic is the location of the windmill converter. Installation on the ground is more convenient but may will generate higher filter and cable cost. New solutions are required. One possibility is to split the DC bus and to use only a DC cable connection beten the line side and generator side converter which can be located in different locations. Offshore wind application is getting more and more important for the future. Cost will be a very important factor to select the right converter topology and the right location in the windmill. a. Full Size converter
b. DFIG converter c. 3-level converter d. Medium Voltage converter Fig. 1: Several Topologies are still under the research 1 Full Size Windmill Converter Full size windmill converters are already in the market with the por up to 8 MW. The design of this huge por isnot possible with simple paralleling of high por IGBTs. The combination of compact modular windmill converter and smart paralleling topologies are the key for a successful design. The cost can be decreased and the maintenance can be reduced. This is a very important issue for offshore wind application. Modular windmill converters with 1.5MW or 2MW por unitsare easy to parallel and maintain.fig 2 shows a compact 8 MW windmill converter with 8 x 2MW windmill converter. One full size windmill converter needs 2 x 2MW converter. The paralleling of these 2MW unitsis very easy and a redundancy is possible. High reliability is very important. This compact windmill converter is using high por SKiiP-IGBTs with 3600A/1700V. A simple and cost effective converter can be designed. Experience is another important factor. SKiiP-IGBTs have been in the market for more than 15 years and been mainly used in windmill and solar converter for high por. Fig 2: 8 MW full converter designed by 2 MW converter units including reactor. size windmill
Fig 3 shows a high por SKiiP-IGBT with compact DC bus. The SKiiP technology is optimized for high por load cycle capability and low thermal resistance. This will improve the life time of the IGBT module and the whole converter. Furthermore, the SKiiP is designed for a low stray inductance DC bus design. It is very easy to connect capacitors to this por module. In the design of the por terminals, the lifetime of the capacitors is also considered. A homogenious current distribution beten the capacitors and the por module is very important to guarantee an excellent temperature balance. Fig 3: single half bridge module with SKiiP-IGBT (3600A/1700V), simple and cost effective, 3 x half bridge module for 2 MW converter The design of a windmill converter can be made by any company in a very short time. Time to market is also very important and in this case it can be helpful if the windmill company can buy a complete por unit that is ready for use, fully qualified, cost optimized, burn-in tested and ll-proven. Fig 4 shows some por units that are ready for use.skiiprack and SEMIStackhave been in the marketsince many years ago. Combined with a controller board, it is a very fast solution to design a windmill converter and to enter the market. Fig 4:SKiiPRACK for 1.5MW Full Size Converter (left) and SEMISTACK for 3 MW full Size converter (right) 2 3-level Windmill Converter This topology is already in the market but not with the latest technology. New por modules which already contain all IGBTs and diodes for 3-level half bridges are in the market. These por modules are not so porful compared to standard IGBTs or SKiiP-IGBTs. Paralleling is necessary. Furthermore, a very detailed analysis of the right 3-level-topology is necessary. NPC and T-NPC have different behaviors (efficiency and losses) by different switching frequencies. For low switching frequency application like wind, T-NPC is the best choice. A combination of 3-level and 2-level topologies is also possible. Moreover, a combination of uncontrolled rectifier on the generator side and a 3-level-converter on the line is possible. Many combinations
are suitable for modern windmill converter. We must always keep an eye on the cost and efficiency. Fig 5 shows different topologies and available device. DC+ N D2 T2 T1 T3 D3 T4 DC- D1 AC D4 Fig 5: NPC topology and T-NPC topology, 600A/1200V 3-level NPC device Fig 6: Uncontrolled rectifier (12 pulse) and 3 Level converter on the line side, high efficiency Fig 6 shows the mentioned topology with uncontrolled rectifier and 3-level-NPC converter on the line side.this new technology will take some time to penetrate the market for windmill applications according our experience. This topology is more preferred for solar application because of the line side converter por. 500KVA and 1MW are common but it is too low for windmill application. A new trend is coming for low por windmill application like 250kW and 500kW. This can be a new opportunity. Paralleling of this new technology is another alternative but in this case the price will be too high. This technology cannot compete with standard 2- level topologies in price by high por. 3 Medium Voltage Windmill Converter Medium voltage windmill converter can be designed with high voltage device like 3300V, 4500V or 6500V. All these devices will have a very high forward voltage Vceand high switching losses. Many papershad already compared the cooling conditions and efficiencies of these high voltage devices to standard 1700V devices. Furthermore, these devices are very expensive and sometimes with only single source. Medium voltage invertersalready with 1700V standard IGBTs have been in the market for many years. This topology is suitable for motors but not for windmill generators. A small change is necessary to convert a medium voltage inverter to a medium voltage converter for windmill generators. Fig 7 shows both topologies and one single por unit with 1700V IGBTs. These units are connected in series. The uncontrolled rectifier prevents a regenerative operation. This rectifier must be changed to an IGBT rectifier.
Fig 7: Medium voltage topology concept, uncontrolled rectifier and H-bridge for motors, H-bridge on input and output for generators. We will introduce a new concept that will reduce the total cost. Also, it is possible to split the converter into 2 parts; generator side in the nacelle and line side converter on the bottom. This will reduce the filter cost as ll. Fig 8 shows thisnew topology. F ilt Topology with uncontrolled rectifier in the nacelle, H-bridge- and 3-phase topology units Fig 8: Fig 9 shows the construction of the new medium voltage converter. The uncontrolled rectifier can be installed in the nacelle and the converter on the bottom. Fig 9: Uncontrolled rectifier in the nacelle, converter at the bottom
The big advantage of this new topology is the use of standard IGBTs and standard por units. This topology connects standard converters in series. The chopper in front of the converter will work as a bypass to improve the efficiency and the reliability in a failure case. This new topology can also be used for high voltage transmission. It is necessary to remove the uncontrolled rectifier and replace it by the same topology like have on the line side. This high voltage transmission can work bidirectional. In Fig 10 shows a high voltage transmission converter with this new topology.it is important that standard IGBTs with 1700V can always be used. Fig 10: High voltage transmission converter with new topology 4 Conclusion Several possibilities for high por windmill converter design are introduced. The cost can be reduced if standard components are available and suitable for high por converter. Cable cost can be reduced with the right construction. The user of high por windmill converter has the opportunity to decide which kind of development steps he likes to do. The easiest way is to buy a complete por unit (such asskiiprack or SEMISTACK) and use his own controller board. This is the fastest way to enter the market. The other alternative is to design a por unit by using the SKiiP-IGBT. It is also a fast way because Semikron will support the customer and optimize the cost. A controller board is necessary. The last alternative is to design a por unit by using standard IGBT modules. This will take time and has no big advantage compared to the mentioned solutions. In sum, it is important to have an overview of the complete system to decide -which topology -which IGBT modules -which location of the converter Literature: [1] Backhaus, K.: Intelligent por module family SKiiPPACK 3 rd generation, PCIM Europe, 2000 [2] Pluschke, N. : Converter topologies for offshore windmills, Offshore Wind China 2009 conference [3] Pluschke, N. : High por density design for windmill application CWP 2009, China [4] Prof. Schumacher, TU Braunschig,; Concepts for Converter in parallel [5] SEMIKRON; SKiiPRACK application note