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WORLD'S FIRST 5 MW-DC CONVERTER

Posted by Björn Riemer

Schaffner Innovation Moves Energy Revolution Forward

Industrial revolution requires innovative solutions.  In the energy industry, revolution requires a solution that provides significant advancement of high efficiency electrical power transmission.  Innovative transmission solutions have been challenged by the increasing complexity of distribution systems.  Power generation is decentralizing, and the availability of renewable power generation is variable.  These factors have increasingly shifted the focus of research to DC transmission, which previously was too expensive to serve as a comprehensive solution.  In cooperation with the Flexible Electrical Networks (FEN) Research Campus Aachen, Schaffner has developed a high-power DC-DC converter.  This development catapults the current state of transmission technology and the energy revolution takes a decisive step forward.

Advantages of the DC Transmission Technique

High voltage DC transmission systems have been used for more than 50 years.  The advantages of DC systems over AC systems in the area of high voltage transmission are well known.  These benefits extend to medium voltage as well.  DC transmission results in a significant reduction of line losses through the control of load flow with reactive power and increases the capacity of existing distribution systems up to three times.  Additionally, DC transmission can further reduce losses by eliminating redundant conversions.  When generation produces DC power (solar, wind, etc.), DC grids don’t require a conversion to AC for transmission.  And when DC grids deliver power to consumers with DC loads (IT equipment, motor drives, electric vehicle chargers, etc.), again no conversion to AC is necessary.

The Revolution Has Become A Reality

Previously, implementation of DC transmission in medium voltage applications was unrealistic due to high costs and the technology limitations of operating equipment.  However, new possibilities exist with the development of a MV DC-DC converter with power capacity of 5 MW.  This achievement is the result of a five-year intensive research project by the Schaffner Group in cooperation with the Flexible Electrical Networks (FEN) Research Campus Aachen.  The FEN Research Campus is an association of institutes of the RWTH Aachen University, Schaffner Group, and other industrial partners.  The Schaffner Group, as a global leader of solutions for shaping electrical power, made a significant contribution in solving the primary problem for the progress of MV DC-DC converters:  the development and optimization of a medium voltage, medium frequency transformer.

Operating Principle of the 5 MW DC-DC Converter

Schaffner's 5 MW DC converter
 

The 5 MW DC-DC converter operates according to the three-phase dual active bridge principle (DAB) and generally consists of three components:  an input three phase inverter, an output three phase inverter, and the transformer developed by Schaffner.  The transformer is a critical component as it ensures galvanic isolation of the inverters.  From a 5 KV DC voltage supply, the input inverter generates a stepped medium frequency voltage of 1 kHz that is transferred by the transformer to the output inverter which produces a 5 KV DC voltage.

 

The newly developed DC-DC converter supports a bidirectional, flexible, and reliable power flow that is the solution for transmission grid congestion caused by the increasing decentralization of power generation. It provides a foundation for the efficient connection of renewable power generation like wind and solar.  It also permits the integration of electric vehicles that can consume and generate power.

Schaffner Transformer Was the Key to Success

Conventional three phase transformers operate at 50 or 60 Hz sinusoidal voltages.  The Schaffner transformer is designed to operate with a 1000 Hz stepped voltage waveform supplied from the inverters. This much higher frequency increases the gravimetric power density and allows for a huge reduction of physical size.  The Schaffner transformer is 15 to 20 times smaller than 50 or 60 HZ transformers of the same power rating and substantially reduces the requirement for raw materials.  This transformer development yields many benefits:  reduced consumption of valuable real estate, reduced consumption of diminishing raw material resources, large cost savings.  Instead of nanocrystalline or amorphous cores, the Schaffner design uses special electrical steel sheet material which is easier to process and has improved price stability.  The design is also compatible with the modern SiC-power semiconductors used to reduce inverter losses and ensures reliable operation with the resulting steep voltage waveforms from the inverter that can cause damage to traditional transformer designs.  Through intensive research and development, Schaffner successfully optimized a transformer design for this unique application that provides economic, manufacturing, and performance advantages.

Successful Research and Development Work

A fundamental component of transformer designs is the insulation material placed within the transformer windings and on the iron core, which prevents short circuits within the transformer.  The insulation system for the new transformer is specially developed for medium frequency applications and increased stress from the very steep voltage waveforms.  The development work for the insulation system was required to consider contradictory design factors.  High cost and limitation of heat transfer within the windings favor minimal usage of insulation material.  Conversely, the high frequencies and steep voltage waveforms demand higher dielectric strength that requires increased usage of insulation material.  Schaffner developed new calculation models for the transformer insulation system and winding design to optimize the usage of insulation material.  This development was critical in allowing the DC-DC converter to reach a higher power density and achieve a 5 MW power rating.

Schaffner Makes the Energy Revolution Possible

Schaffner makes the energy revolution possible
 

Schaffner’s technical solution for the medium voltage, medium frequency transformer makes higher power capacities possible for DC-DC converters and lays the foundation for continued development.  In cooperation with the Research Campus FEN Aachen, Schaffner will pursue additional optimization of the transformer component and converter.

 

As research and geopolitics continue to shift the focus of the energy revolution to DC transmission, the development of a 5 MW DC-DC converter is a step forward for the implementation of larger DC grids.  The next objective is the development of isolated island networks near large consumers of DC power:  data centers, wind farms, solar collector arrays, or quick charging electric vehicle stations.

If your project or development teams encounter similar challenging problems that require customized technical solutions, please contact Schaffner’s competent team of experienced technical experts.  If you would like to join our research and development team to participate in the technical solutions for challenging and exciting projects, please contact Schaffner’s Human Resources Department.

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