Full-Scale Medium-Voltage Converters for Wind Power Generators up to 7 MVA

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1 Full-Scale Medium-Voltage Converters for Wind Power Generators up to 7 MVA Philippe Maibach, Alexander Faulstich, Markus Eichler, Stephen Dewar ABB Switzerland Ltd CH-5300 Turgi, Switzerland Phone: Fax: philippe.maibach@ch.abb.com alexander.faulstich@ch.abb.com markus.eichler@ch.abb.com stephen.dewar@ch.abb.com URL: Abstract The ongoing increase in the share of wind power in installed generation capacity continuously increasing number of wind power plants brought into operation forces the transmission system operators (TSO) to tighten their grid connection rules in order to limit the effects on network quality. These new rules demand that wind power plants and farms support the electricity network throughout their operation. Wind turbines using full-scale converters include several advantages and are most suitable to be adapted flexibly to different grid requirements without the need for additional reactive power compensation equipment. With larger turbine unit sizes, medium-voltage converter systems are suitable for the corresponding high power. Based on platforms widely used for industrial drives applications, ABB has successfully applied reliable medium-voltage converter technology to wind power. Keywords: IGCT, Permanent magnet generator, Wind generator systems, Voltage Source Inverters, Grid code 1 Introduction In the early days of the wind power industry, wind turbine manufacturers developed lower power wind turbines, which were installed as single units or in small groups at a given location. Increasingly higher power turbines are installed and grouped into large farms or wind power stations. The turbine manufacturers are faced with a number of challenges such as larger and heavier mechanical structures, more severe safety issues, environmental compatibility issues, handling high electrical power within the nacelle and tower, and last but not least the set-up of wind parks and connecting them to the distribution or transmission grid. Integration of ever larger wind farms to the utility grids is increasingly challenging because Distribution and Transmission System Operators (DSO and TSO) require a wind power station to behave similarly to a conventional power station. This is the background to the grid code modifications seen during the last few years. This paper proposes full-scale medium-voltage converters for high-power wind turbines as a contribution to facilitate the integration of large wind turbines into an existing grid.

2 2 Field of application Growing wind turbine power ratings come along with larger gearboxes, generators, transformers and power electronics. The doubly-fed induction generator is one preferred solution with the advantage of little required power electronics. However, fault ride-through requirements can increase the power electronic effort needed. Another widely used solution is a synchronous generator combined with a full-scale converter, i.e. the generator is connected to the grid via a power electronic converter rated for the full generator power. Both solutions are currently realized at low-voltage level (below 1000 V rms ) for low power up to even the highest power ratings, i.e. < 1 MW up to some of the existing 5 MW pilot installations. As wind turbine powers increase the corresponding increase in current to be handled requires the connections become bulkier. Especially at low-voltage level this is a disadvantage, but it can be overcome if the transformer is placed inside the nacelle. However, this has the disadvantage of a considerable increase in the nacelle weight, with implications on logistics and mechanics. An interesting compromise which allows a lower nacelle weight and reduced interconnection effort is to apply a full-scale medium-voltage converter in conjunction with a medium-voltage permanent magnet synchronous generator. For the avoidance of doubt, in this paper the term medium-voltage describes electric equipment with a voltage rating of more than 1 kv but less than 5 kv. The transformer does not need to be placed inside the nacelle; and the cabling effort is comparable to a medium power low-voltage wind turbine installation. As a major additional advantage, the system can make use of the well-defined fault-ride-through behavior of the converter. 3 Grid Code Requirements The German grid codes have been amongst the most important driving documents regulating the connection of large wind farms to the transmission grid. The most recent grid codes require the wind farm among other things to contribute to Reactive power exchange and voltage control Fault-ride through support in the case of balanced faults and sometimes Defined behavior in the case of unbalanced faults 3.1 Reactive power exchange and voltage control The Grid Codes specify the power factor at nominal active wind park power output over a defined connection voltage range. Figure 1 shows the VDN requirement for the transmission voltage level. If an on-load tap changer transformer is installed between transmission system and distribution system, different power factor requirements may be applicable if the wind park is connected to the distribution grid.

3 Figure 1: VDN power factor specification Figure 2 shows an example valid for the British grid. The black lines apply for transmission system, the red lines for the distribution system. Figure 2: British Grid Code reactive power requirements Some grid codes require the implementation of a voltage control functionality as it is well known for other generators: Reactive power has to be provided depending on the system voltage following a droop characteristic. Thereby, the provided reactive power is a linear function of the system voltage. Parameters such as target voltage and droop slope have to be altered remotely by the overriding controller. 3.2 Fault-ride-through behavior at balanced faults In case of a three-phase balanced short circuit in the grid, the wind farm has to supply reactive current for the duration of the short circuit. The grid codes define the range of voltage dips at which the wind farm must not trip offline but supply reactive current. Some grid codes as the British Grid Code require fault ride through capability down to 0% remaining voltage at the transmission level. Figure 3 shows the requirements according to VDN. There are considerable differences in the specification of how much reactive current is expected during voltage dips. Figure 4 shows the VDN requirement in terms of reactive current supply during grid faults.

4 Figure 3: VDN Fault-ride-through requirement Figure 4: VDN reactive current requirement during faults 3.3 Behavior at unbalanced grid faults Some grid codes such as the British Grid Code explicitly require that the wind park must ride through unbalanced faults without tripping offline. During such faults, reactive current needs to be injected as well.

5 4 Low-Voltage Technology Versus Medium-Voltage Technology For turbine ratings up to around 2 MW, the converter-less structure has been applied successfully in the past and resulted in a simple, effective system. High performance turbines have been built with variable speed systems; either using doubly-fed induction generators with a small converter or gearless systems with fullscale converters. Low-voltage technology has been applied successfully at all power levels. At converter power levels in excess of around 500 kva, a parallel connection of converter modules is typically used to fulfill the technical requirements. An indicator of the cabling and connection costs of such a system is the effective current, which loads the connections between nacelle and tower bottom. In a 690 V system a phase current of 1700 A is reached at about 2 MW. This already results in a parallel connection of multiple cables per phase and a substantial voltage drop. This disadvantage can be mitigated by placing the electrical conversion system, including the transformer, into the nacelle. Due to the necessity to connect low-voltage converter modules in parallel, the space needed by the converters increases roughly in proportion to its power. The nacelle dimensions and weight increase considerably and complicate the mechanical stability and the logistics during turbine erection. In industrial power conversion it is well known that low-voltage is most cost-efficient at low power levels, while medium-voltage is superior at high power levels. The limit between these two ranges is dependent on the application. As the power ratings of wind turbines increase, medium-voltage converters become more competitive. Compared to low-voltage converters they employ fewer components, which is an inherent advantage with respect to reliability. Although the costs of cables and connections are reduced, those for the transformer and generator are barely affected. 5 Full-Scale Converter Figure 5 shows the basic diagram of a full-scale four-quadrant wind power converter. The main building blocks of the converter are the two inverter modules connected by the dc link and the grid filter module. IGCTs (Integrated Gate Commutated Thyristors) are used as semiconductor switches. These elements are a further development of the Gate Turn-Off Thyristor (GTO). IGCTs are inherently robust semiconductor elements like thyristors. They have considerably better switching behavior than GTOs and have been proven to be an excellent semiconductor switch for a number of different applications as e.g. industrial medium-voltage drives or frequency converters for railway grids. In all these applications, excellent field experience can be reported. A three-level topology is used for this power range and kind of application. Three-level inverters are commonly used in medium-voltage industrial converters. In [1] a more detailed description of the three-level topology is given. The main advantages of this topology are lower output current ripple and better harmonic performance compared to a two-level topology operated at the same semiconductor switching frequency. The basic circuit diagram also shows auxiliary circuits like the clamping circuits and the slope filter on the generator side. This filter is used to slow down the voltage slopes on the generator side and thus to avoid excessive over-voltage in the generator caused by reflection on the interconnection of converter to generator. The grid filter is a LC filter in combination with a damping circuit for the lowest order harmonic. This filter is designed to meet the stringent VDEW requirements [4]. It also meets IEC [3] and IEEE [2] requirements.

6 Figure 5: Basic circuit diagram of the 4-quadrant wind power converter In [1] a benchmark with alternative converter solutions is presented. A four-quadrant converter would allow for a bi-directional power flow which is not necessarily required for this application. However, the comparison shows that a converter topology as proposed is the optimum for the whole system because of the following reasons: The elevated semiconductor switching frequency allows a very low current ripple on the generator side to be achieved, resulting in very low torque ripple stress in the shaft and/or gear box. A four-quadrant converter is able to draw reactive power from the generator. Doing so, it is possible to keep the converter terminal voltage and the DC link voltage constant over the whole speed range. Constant DC link voltage is important to best utilize the converter and be able to deliver full rated power at reduced speed. All these properties could not be realized with a simple diode rectifier on the generator side. Although most of today s turbines built for full-scale power conversion are equipped with synchronous generators, there are also opportunities to use the robust and cheap induction generator. Induction generators require the converter to be able to deliver the magnetization current, i.e. a 4Q converter is required to run an induction generator with a full-scale power conversion. 6 Harmonics at the Point of Connection As shown in Figure 5 the output of the converter is connected through a grid filter to a feeder transformer, which adjusts to higher utility voltages by employing an appropriate turns-ratio. An important challenge in the converter design is meeting the harmonic requirements according to standards, as e.g. [2], [3] or even [4]. In order to meet these specifications by means of the chosen converter topology and an optimally sized filter, the approach of optimized pulse patterns is used. This technique enables the converter to eliminate low order harmonics. The passive grid filter eliminates the remaining higher order harmonics. Therefore the wind power converter is operated with an optimized pulse pattern on the grid side (9 50 Hz, 7 60 Hz). Figure 6 illustrates the converter compliance with VDEW and IEEE current harmonic limits at the point of connection (PC). Figure 7 illustrates the converter compliance with IEC voltage harmonic limits at the PC. These results are based on studies done when designing the grid filter. This study was performed to verify the connection of a 5 MW wind turbine to a distribution grid.

7 Figure 6: Current harmonics at the Point of Connection and applicable limits Figure 7: Voltage harmonics at the Point of Connection and applicable limits

8 7 Power Rating The power rating of wind turbines of interest for the use with full-scale medium-voltage converters starts in the 3-4 MW range and currently ends at 5 MW. Larger wind turbines are in discussion for the future. ABB have developed and successfully commissioned full-scale medium-voltage converters for a 2 MW wind turbine manufacturer [1]. At the time of writing this paper a prototype converter for the Multibrid M MW wind turbine is being commissioned [6]. Both designs are based on IGCT converters for industrial medium-voltage applications. In order to meet the requirements of grid codes as discussed above, the converter rating needs to be considerably higher than the nominal wind turbine power rating. The converter rating should also be adapted to the wind turbine overload capability. This leads to a continuous converter power rating well in excess of 120% of the turbine nominal rating. Compared to other semiconductor switches applied to the same power range, IGCTs produce lower losses. This allows the converters to be operated at an elevated switching frequency, and therefore the use of a reduced grid filter size. Furthermore the low losses result in a high conversion efficiency: the losses of the 5 MW wind turbine converter are in the order of 2% to 2.5%.

9 8 Converter Behavior Following, simulation results are presented, valid for the converter for a 5 MW wind turbine. Positive reactive power corresponds to over-excited operation. Figure 8: Power capability at nominal grid voltage (99.6% of nominal voltage) Figure 9: Power capability at reduced grid voltage (87% of nominal voltage)

10 Reactive power/current capability at different balanced faults. The grid voltage is measured at the connection point of the wind turbine to the distribution system. Figure 10: Grid voltage, current, active and reactive power during a single phase voltage dip to 70%

11 Figure 11: Grid voltage, current, active and reactive power during a single phase voltage dip to 18%

12 Behavior at different unbalanced faults: Figure 12: Grid voltage, current, active and reactive power during a two phase voltage dip to 50%

13 Figure 13: Grid voltage, current, active and reactive power during a two phase voltage dip to 25%

14 9 Conclusion ABB has successfully applied reliable and efficient medium-voltage converter technology to wind power applications. The combination of powerful hardware and flexible control topology, supported by enhanced simulation facilities, is best suited to serve the wind power industry and to integrate even the largest wind turbines into grids with demanding connection requirements. 10 References [1] A. Faulstich, J. K. Steinke, F. Wittwer, Medium Voltage Converter for Permanent Magnet Wind Power Generators up to 5 MW, EPE 2005 Dresden [2] IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems, IEEE [3] Electromagnetic compatibility (EMC) Part 2-12: Environment Compatibility levels for lowfrequency conducted disturbances and signalling in public medium-voltage power supply systems, IEC [4] Verband der Netzbetreiber VDN e.v. beim VDEW (German Electricity Association), REA generating plants connected to the HV and EHV networks, Berlin, Germany, August [5] British Grid Code, Issue 3, Revision 16, 30th May 2006 [6] Multibrid Entwicklungsgesellschaft mbh, ZAB3BHS253163E01