Stefan P. Engel

Comparison of the Modular Multilevel DC Converter and the Dual-Active Bridge Converter for Power Conversion in HVDC and MVDC Grids

It is expected that in the near future the use of high-voltage dc (HVDC) transmission and medium-voltage dc (MVDC) distribution technology will expand. This development is driven by the growing share of electrical power generation by renewable energy sources that are located far from load centers and the increased use of distributed power generators in the distribution grid. Power converters that transfer the electric energy between voltage levels and control the power flow in dc grids will be key components in these systems. The recently presented modular multilevel dc converter (M2DC) and the three-phase dual-active bridge converter (DAB) are benchmarked for this task. Three scenarios are examined: a 15 MW converter for power conversion from an HVDC grid to an MVDC grid of a university campus, a gigawatt converter for feeding the energy from an MVDC collector grid of a wind farm into the HVDC grid, and a converter that acts as a power controller between two HVDC grids with the same nominal voltage level. The operation and degrees of freedom of the M2DC are investigated in detail aiming for an optimal design of this converter. The M2DC and the DAB converter are thoroughly compared for the given scenarios in terms of efficiency, amount of semiconductor devices, and expense on capacitive storage and magnetic components.

Dynamic and Balanced Control of Three-Phase High-Power Dual-Active Bridge DC–DC Converters in DC-Grid Applications

The three-phase dual-active bridge (DAB) is a dc–dc converter, which provides galvanic isolation, inherent soft-switching capability, and small filter size. In this study, the dynamic behavior of three-phase DAB is analyzed and a dynamic control strategy is developed. Furthermore, a compensation technique is implemented to compensate unbalanced transformer phase currents. The latter is often caused by asymmetric leakage inductances. State space averaging and first harmonic approximation models, both for the steady state and transient analysis, are developed to describe the dynamic behavior of the three-phase DAB. The accuracy of the models is compared with a detailed circuit simulation and the benefits of each model are identified. When the transferred power of the DAB changes fast, the transformer currents can become unbalanced, leading to oscillations in the output current. A unique control method is presented, which allows settling of the transformer currents within one-third of the switching period. Additionally, the transformer currents stay symmetrical and oscillations are avoided. Based on this fast current control, an outer voltage controller is designed. The comparison of the control system using the fast current control and the conventional quasi-steady-state control demonstrates the potential advantages of the new approach under dynamic conditions. In practice, it is difficult to achieve completely symmetrical short-circuit impedances in a high-power medium-voltage transformer. Asymmetric leakage inductances, however, result in unbalanced phase currents and higher dc current ripple in a three-phase DAB. The new control scheme that is developed here can be extended to compensate any unbalances in the transformer. This approach enables effectively the balancing of the three-phase currents. The new control schemes are experimentally verified.

Improved Instantaneous Current Control for High-Power Three-Phase Dual-Active Bridge DC–DC Converters

With the increasing share of renewable and decentralized power sources, the need for power electronics and especially for efficient high-frequency high-power dc-dc converters is expected to grow. The three-phase dual-active bridge is a promising technology, as it has a high-power density and inherently features galvanic isolation. A highly dynamic method to control the current and thus the transferred power for this converter type has recently been published. The published approach is easy to implement and gives excellent results for transformers with a high transient time constant, i.e., low winding resistance. However, the method can be improved for transformers with increased winding resistance. This paper suggests two approaches that reach steady state in one-third of a switching period and half a switching period, respectively. Independent of the winding resistance, the suggested control schemes give superior results and oscillations of the dc current are completely eliminated. The control schemes are investigated in detail and derived mathematically. These exact solutions are linearized for ease of implementation in digital control circuitry. Simulations and an experimental verification on a laboratory prototype confirm the outstanding performance of the developed approach.

Instantaneous current control for the three-phase dual-active bridge DC-DC converter

The three-phase dual-active bridge (DAB) is a dc-dc converter, which provides galvanic isolation, inherent soft-switching capability and small filter size. In this work, the dynamic behavior of three-phase DAB is analyzed and a dynamic control strategy is developed. Furthermore, a compensation technique is implemented to compensate unbalanced transformer phase currents. The latter is often caused by asymmetric leakage inductances. When the transferred power of the DAB changes fast, the transformer currents can become unbalanced, leading to oscillations in the output current. A unique control method is presented, which allows settling of the transformer currents within one third of the switching period. Additionally, the transformer currents stay symmetrical and oscillations are avoided. In practice, it is difficult to achieve completely symmetrical short-circuit impedances in a high-power medium-voltage transformer. Asymmetric leakage inductances, however, result in unbalanced phase currents and higher dc current ripple in a three-phase DAB. The new control scheme that is developed here can be extended to compensate any unbalances in the transformer. This approach enables effectively the balancing of the three-phase currents. The new control schemes are experimentally verified.

Optimization of the pulse-width-modulation strategy for redundant and non-redundant multi-level cascaded-cell converters

Future energy supply will be mainly ensured by renewable energy sources. Measures have to be taken to buffer imbalances in the grid caused by these volatile power sources. Battery energy storage systems (BESS) present a good solution for many applications. Besides efficiency and cost, reliability of these systems becomes increasingly important. Redundancy reduces the hazard of system failures. Highly efficient and redundant BESS can be built with multi-level cascaded-cell converters (MLCCC). Phase-disposition pulse-width modulation (PDPWM) can be applied for generating the sinusoidal output voltage. The optimized PDPWM presented in this paper distributes the losses equally among the switches. This increases the systems ampacity and balances the load of each converter cell. Finally, the application with redundant MLCCC is presented. Hereby, bypass thyristors are additionally used to increase the efficiency of the converter.

Dynamic power control of three-phase multiport active bridge DC-DC converters for interconnection of future DC-grids

Dc grid technology is currently expanding from the high-voltage to the medium-voltage range and is expected to penetrate also low-voltage grids. For interconnection of these grids, dc-dc converters that enable a flexible and highly dynamic control of the power flow between different voltage levels are required. In the scope of this paper a highly dynamic power and current control of three-phase multiport-active bridge (3ph-MAB) converters is presented. The investigations are exemplified for a three-phase triple-active bridge (3ph-TAB) converter, i.e., a 3ph-MAB converter with three ports, which connects a medium-voltage dc grid to two separate low-voltage dc grids. Firstly, the complex relation between the power at the ports and the load angles is investigated and algorithms for on-line determination of the according load angles are derived. Secondly, the instantaneous current control (ICC), which is known from the dual-active bridge converter, is adopted for the triple-active bridge converter. Thereby, a highly dynamic current control with settling times of half a switching period is achieved. Based on these considerations, a closed-loop control structure is proposed which fully utilizes the highly dynamic behavior of the ICC. The theoretic analysis is verified by simulation for a 150 kW SiC MOSFET converter prototype with three ports and nominal port voltages of 5 kV, 380 V and 760 V.

Control of thyristor-based commutation cells

The growing share of photovoltaic generators connected to low-voltage grids leads to increased and more frequent load changes in these grids. Transfer switches are often applied to control the resulting voltage fluctuations. As mechanical transfer switches are not suitable for frequent switching due to the degeneration of the switching contacts, it is essential to develop new apparatus that are capable of frequent switching of high currents. This paper presents a pulse generator for thyristor-based commutation cells that are capable of fulfilling the function of transfer switches. The control is described for different operating points and is developed in a generalized manner in order to minimize the implementation effort. The control successfully avoids short circuits of the windings and minimizes the voltage stress of the thyristors.