Full-Scale Medium-Voltage Converters for Wind Power Generators up to 7 MVA 1 Introduction

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: +41 58 58 9 32 35 Fax: +41 58 58 9 26 18 E-mail: [email protected] [email protected] [email protected] [email protected] URL: www.abb.com „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 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 Vrms) 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. 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. 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. 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 full- scale 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.

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