Optimizing Wind Converter Designs

This article is published by EEPower as part of an exclusive digital content partnership with Bodo’s Power Systems.

One of the major challenges of the 21st century is to contain global warming to limit the consequences of climate change. This target will only be reached by dramatically reducing the emission of greenhouse gases during the first half of the century. In 2019 alone, energy and heat production were the biggest contributors to the greenhouse effect, accounting for nearly one-third of global emissions.
 

Image used courtesy of Freepik

 

Wind Technology and the Clean Energy Infrastructure

It is, therefore, essential and urgent to enable a clean energy infrastructure to reduce these contributions. Wind technology is a key player and is already leading the way. In the last 20 years, wind turbines have tripled, and their power output has increased tremendously, soon exceeding the 15 MW mark. As a result, the demand and the need for highly advanced wind power converters continue growing. Operating under harsh environmental conditions, these converters require high reliability and robustness to ensure a long life cycle. Moreover, high power density is needed to maximize power output while limiting the number of components in the cabinet. Furthermore, large production capacities are essential to support the rising demand. Finally, upgrading existing inverter designs helps reduce both risk and development time.

Infineon’s PrimePACK with IGBT5.XT technology (FF1800R17IP5) could meet these challenges. It has become the standard module in wind converters since its release in 2016. Its combination of advanced interconnection technologies and improved chip design provides reliability and high power density.

Nevertheless, in light of global challenges, further improvements are always welcome. Considering the specific operating conditions of modern wind converter systems, Infineon has developed two IGBT power modules specifically optimized for these conditions. The new modules enable a further step towards higher power density while still providing the benefits of the FF1800R17IP5 PrimePACK IGBT power module.

 

 

Trends in Wind Converter Systems

A clear trend toward a modular design of converter stacks and standardization is now discernible for wind converter systems. This approach supports the growing need for power ratings while keeping development cycles short. It also allows the multiple re-use of the same power stack for wind turbines with higher power ratings. However, increasing the number of cabinets might be impossible due to the limited space in a wind turbine nacelle. Hence, the power density of the individual stack needs to be improved.

The electrical requirements of a machine-side converter (MSC) and a line-side converter (LSC) differ significantly in a wind converter system, another key aspect to be considered. This is particularly true for a fully-rated converter system, as depicted in Figure 1. Due to the power flow from the generator to the DC link, the diodes in the MSC are subject to the heaviest load. On the other hand, in the LSC, the power flow towards the AC grid makes the IGBTs the most stressed chips in the power module.

 

Figure 1. Schematic topology of a fully-rated converter-based wind turbine system. Image used courtesy of Bodo’s Power Systems [PDF]

 

In light of these differing requirements, two differently tuned power modules were developed to optimize each module to the generator-side and the grid-side converter’s specific needs, respectively, while building on the reference module FF1800R17IP5. 

 

Wind Converter Switching Frequencies

High switching frequencies are beneficial for wind applications to reduce generator losses and maintain a grid-side filter with a reasonable size in the converter cabinets. Generally, switching frequencies are significantly higher than for general-purpose motor drive inverters with similar power ratings.

In this context, it is important to note that in any given IGBT technology, there is always a trade-off between switching and conduction losses (see Figure 2). In the optimized IGBT5 chip, this trade-off was modified to support higher switching frequencies by shifting the trade-off towards fewer switching losses. As both LSC and MSC suffer from the high switching losses of the IGBT, this trade-off modification was performed for both modules, targeting the two different converter positions. The resulting increase of V_CE,sat was partially compensated by additional design measures. The consequent optimization of the IGBT is depicted in Figure 2.

 

Figure 2. IGBT5 chip trade-off towards reduced dynamic losses for the two LSC- and the MSC-dedicated modules. Image used courtesy of Bodo’s Power Systems [PDF]

 

As an additional step, a converter-dedicated optimization further improved the modules’ performance. To this end, chip sizes were modified to reduce conduction losses and to improve the thermal conductivity of those chips facing the heaviest load. As shown in Figure 3, the chip size ratio was shifted in favor of a bigger IGBT for the LSC-dedicated module, while a bigger diode was included in the MSC-dedicated module. These two design improvements led to the introduction of two modules designated FF2000XTR17IE5, optimized for the LSC, and FF1700XTR17IE5D, optimized for the MSC.

 

Figure 3. Different chip size ratios for LSC- and MSC-dedicated modules compared to the reference module. Image used courtesy of Bodo’s Power Systems [PDF]

 

Figure 4 highlights the benefits of these two dedicated power modules in the context of an exemplary wind converter system. The corresponding operating conditions, upon which this simulation is based, are given in Table 1. It is important to note that the significant increase in converter power concerning the reference module shown in Figure 4 has been done by simply swapping the power modules and leaving all other components in the converter and control untouched.

 

Figure 4. Simulated power improvement of a wind energy system with LSC/MSC-dedicated modules relative to the reference module. Image used courtesy of Bodo’s Power Systems [PDF]

 

Table 1. Reference LSC (left column) and MSC (right column) system parameters. *DPWM nomenclature

DC link voltage VDC	1200 V
Switching frequency fsw	5 kHz (DPWM1)*	5 kHz (DPWM0)*
Modulation index M	0.9	1
Power factor |cos(φ)|	0.9 (capacitive)	0.8 (inductive)
Rated current Ir	900 A	1000 A
Rated active power Pr	1 MW
Thermal resistance heat sink to ambient Rth,ha	21 K/kW
Coolant temperature Tliquid	40°C

 

In this use case, the maximum virtual junction temperature T_vj,max is the limiting design parameter concerning the achievable converter power. For all PrimePACK modules with IGBT5.XT technology, this maximum is 175°C by design. However, this simulation is limited to 145°C to allow for design margins for overload and fault conditions.

Life-cycle requirements, for which the cyclic loading stress of power semiconductor modules has to be considered, are another typical design parameter for wind energy systems. This aspect is particularly important due to the potentially low fundamental generator frequencies and varying wind conditions surrounding a wind turbine, however, for the PrimePACK with IGBT5. XT technology, such as the modules mentioned in this article, is not an issue, even for applications with strongly alternating loads. Here, .XT interconnection and IGBT5 chip technologies enable an outstanding performance in cyclic load robustness.

Concerning the performance benefits of FF2000XTR17IE5 and FF1700XTR17IE5D, we have considered only one steady operating point. Indeed, a complete field of operating points derived from wind speeds and grid requirements is considered, resulting in different converter currents for LSC and MSC for various generator speeds. These other operating conditions result in different junction temperatures T_vj,max. Figures 5 and 6 show how the two dedicated modules extend the operation range considerably. This improvement is due to reducing the maximal temperatures in the LSC IGBT by 25% and the MSC diode temperature by 13% concerning the reference module.

 

Figure 5. LSC semiconductor temperatures for different operating conditions for the reference module (left) and LSC-dedicated module (right). The red area denotes junction temperatures above the design limit [4]. Image used courtesy of Bodo’s Power Systems [PDF]

 

Figure 6. MSC semiconductor temperatures for different operating conditions for the reference module (left) and MSC-dedicated module (right). The red area denotes junction temperatures above the design limit [4]. Image used courtesy of Bodo’s Power Systems [PDF]

 

Wind Converter System Takeaways

Although the newly released power modules highlighted in this contribution are designed to target wind-energy applications, other high-power applications with a regular unidirectional power flow may also benefit if the modules are included in the design. For instance, high switching frequencies are beneficial for reducing motor losses in drive applications. Again, note that both modules have been optimized to support high switching speeds. Moreover, active grid converters for motor applications or electrolyzer applications might also benefit from the large diode in the FF1700R17IE5D.

Infineon’s latest additions to the PrimePACK portfolio, FF2000XTR17IE5 and FF1700R17IE5D, support upgrading wind converter designs to higher power levels. Ultimately, these products will enable customers to reduce development time and costs while still providing the outstanding robustness of the IGBT5.XT technology.

 

This article is co-authored by Marcel Morisse, a field application engineer, and Michael Busshardt, a product definition engineer, both with Infineon. It originally appeared in Bodo’s Power Systems [PDF] magazine.