High-Power DC-DC Converter
The increase of renewable power generation holds new challenges for tomorrow’s energy grid. Instead of the classical unidirectional power flow from central power plants to the customer, already today more and more energy is fed into medium and low voltage grids. A major issue is the efficient distribution of energy throughout the grid and the balance of energy generation and demand. Direct-current (dc) grids not only integrate renewable energy sources and storage systems more efficiently, they also allow a flexible control of the power flows in the grid. A key component of tomorrow’s dc grids is a dc-dc converter that is highly efficient and more compact than an equivalent 50-Hz transformer.
A high-power prototype of a dc-dc converter is being developed at the “Institute for Power Generation and Storage Systems” at E.ON Energy Research Center. The topology of the demonstrator is the three-phase dual-active bridge. Originally developed for space applications, this technology allows high efficiency through its inherent soft-switching capability as well as high power density due to its elevated operating frequency. One of the main research topics is the medium-frequency transformer as a main part of the converter. It is more compact and saves core losses compared to a standard 50-Hz transformer. However, especially in high-power applications and considering increased voltage levels, the design becomes challenging. A single-phase 2 MVA transformer made of laminated silicon steel for an operation frequency of 1 kHz is available at the institute for testing of the concept.
The main power-electronic devices in the converter are so called “integrated gate-commutated thyristors” (IGCT). These thyristor-based switches achieve very low conduction losses. Switching losses, on the other hand, are already reduced by the soft-switched dual-active bridge topology. Other optimizations like the instantaneous current control and the auxiliary resonant commutated pole to extend the soft-switching area are analyzed to further improve the converter.
The combination of soft-switching, IGCTs and the medium-frequency transformer enables a high-efficient dc-dc converter which is perfectly suitable for the future dc grids. The construction of the demonstrator is kindly supported with power electronic devices by ABB Switzerland.