The proper component alone is rarely enough

Norvento Enerxía needed more than just a suitable DC link capacitor to fit a hermetically sealed 8-megawatt inverter into a standard 20-foot container. The developers at the Spanish renewable energy specialist needed certainty that the design would operate stably under all operating conditions. TDK provided both, offering the ModCap UHP as the basis for the DC link and performing detailed simulations. This saved several months of development time.

"The primary design objective for the nXL was full encapsulation, which means there is no air exchange between the inside of the converter and the environment." This is how Ángel Mayor, the head of innovation at Norvento and the chief developer of the converter, describes the key challenge. This type of encapsulation makes sense because central inverters in photovoltaic systems and large battery storage facilities often operate in challenging environments. These include areas with high humidity, salty air, and metal particles, such as mining sites, as well as regions with active volcanism. Additionally, encapsulated systems require minimal maintenance since filter mats do not need to be replaced regularly. Liquid cooling should ensure thermal management.

However, this design objective has a significant impact on key components, such as the DC link capacitor. TDK provided more than just a suitable component; they also provided technical expertise and service.

Figure 1:

The nXL converter from Norvento Enerxía for 7 to 8 MW, including transformer and medium-voltage switchgear

Challenges in the development process

The DC link solution for the nXL had to meet three key requirements:

• Low equivalent series resistance (ESR) across the frequency range to minimize losses,
• low equivalent series inductance (ESL) to prevent excessive voltage overshoot when the semiconductors are turned off, which will become even more important when fast-switching SiC MOSFET modules are used in the future, and
• high operating temperatures.

Figure 2:

TDK's ModCap UHP can deliver full output power up to +105 °C.

“In addition, modularity was another important factor for us, as we needed a solution that could be flexibly adapted to different configurations,” adds Mayor. That’s why ModCap from TDK, a long-standing partner of Norvento, was the obvious choice.

However, film capacitors with polypropylene as the dielectric must be derated in terms of output power at temperatures above +85 °C. This also applies to previous ModCap variants. This is why TDK developed the ModCap UHP. It uses a new type of dielectric called EPN (ethylene propylene norbornene), which enables the component to deliver full power at temperatures up to +105 °C [1]. EPN is a blend of polypropylene (PP), an established, easily processed dielectric, and cyclic olefin copolymer (COC), a dielectric with greater heat resistance. However, COC alone cannot be formed into a film. Therefore, it is blended with polypropylene. The resulting material can be processed like polypropylene while offering the high-temperature resistance of COC.

Nevertheless, questions remained. How is the current distributed among the individual ModCaps on the busbar at higher frequencies? How is the current distributed among the capacitor windings within the component at higher frequencies? How do losses develop over frequency? TDK's simulation expertise helped Norvento.

Shorter time to market

“One of Norvento’s main concerns was how the current would be distributed among the 18 ModCaps—six per phase—that make up the DC link,” recalls Fernando Rodríguez of TDK’s Applications and Development Group in Málaga. Another concern was whether high-frequency oscillations would have a negative effect, such as resonances related to the dimensions of the busbar. To address these concerns, TDK proposed characterizing the entire DC link digitally and using finite element analysis (FEA) to simulate the current distribution. The result: The busbar design, when combined with ModCap UHP capacitors, remains within specifications across the entire operating frequency range of the application. 

The main advantage of using FEA is that the results are available before any prototype is built. This largely eliminates the need for lengthy durability and stress tests. "In this case, the simulation saved an estimated two to three months of development and testing time," says Rodríguez, summarizing the approach.

"By running simulations early in the project, we optimized the power distribution in the DC link and ruled out potential resonances in advance. This shortened the time to market for Norvento and ensured that the design was sound from the beginning.”

Fernando Rodríguez, Applications and Development Group at TDK

Support: the key to success

Only a team of experienced specialists could fit a hermetically sealed 7 to 8 MW converter, including a transformer and medium-voltage switchgear, into a standard 20-foot container. "The key to success," says Álvar Mayor, "was not only the right DC link capacitor, but also the technical support, especially during the simulations." The nXL can operate in grid-following mode or as a grid-forming system. This makes it ideal for microgrids or systems where grid stability is not guaranteed.

“The collaboration with TDK was excellent and always proactive. We learned a lot from each other during this project. The technical support, especially with the simulations, was key to our success.”

Álvar Mayor, Head of Innovation at Norvento and lead developer of the nXL

Outlook

For Norvento, the nXL is a turning point. With an unmatched power density and the ability to integrate 7 to 8 MW into a standard 20-foot container, the company can now compete with major international power electronics manufacturers. However, development is not yet fully complete. Right from the beginning, the nXL's architecture was designed for SiC MOSFET modules — an upgrade that will further challenge the requirements for the DC link. However, thanks to the comprehensive simulations carried out at TDK, this upgrade path has already been paved.

Achieving such a compact, reliable system would not have been possible without the right combination of components and partners at the right time. TDK's ModCap UHP, with its innovative high-temperature dielectric and patented high-frequency internal structure, and the accompanying FEA simulation service, demonstrate this synergy between component-level expertise and system understanding. Those facing similar challenges will find both at TDK.

References

[1] Power Capacitors: The New ModCap Likes It Hot, https://www.tdk-electronics.tdk.com/en/373562/tech-library/articles/applications-cases/applications-cases/modcap/3660166, TDK, retrieved March 16, 2026