Nils Soltau

The Balance of Renewable Sources and User Demands in Grids: Power Electronics for Modular Battery Energy Storage Systems

The continuously growing amount of renewable sources starts compromising the stability of electrical grids. Contradictory to fossil fuel power plants, energy production of wind and photovoltaic (PV) energy is fluctuating. Although predictions have significantly improved, an outage of multi-MW offshore wind farms poses a challenging problem. One solution could be the integration of storage systems in the grid. After a short overview, this paper focuses on two exemplary battery storage systems, including the required power electronics. The grid integration, as well as the optimal usage of volatile energy reserves, is presented for a 5- kW PV system for home application, as well as for a 100- MW medium-voltage system, intended for wind farm usage. The efficiency and cost of topologies are investigated as a key parameter for large-scale integration of renewable power at medium- and low-voltage.

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.

Development and demonstration of a medium-voltage high-power DC-DC converter for DC distribution systems

In many low-voltage or high-voltage applications, the electrical energy transmission and distribution is enhanced using direct current (dc) already today. To use the advantages of dc also in medium-voltage applications a highly efficient dc-dc converter is needed that has a power capability of several megawatts. At the E.ON Energy Research Center of the RWTH Aachen University a demonstrator for a 5 MW medium-voltage dc-dc converter is constructed, which is presented in this work. Firstly, some general considerations about the three-phase dual-active bridge dc-dc converter are made, with a focus on medium-voltage high-power applications. Whether IGBTs or IGCTs are the optimal switching devices is discussed and a further improvement using a Dual-ICT is evaluated. The construction and the design of the mentioned demonstrator are presented. Besides the demonstrator itself, the medium-voltage high-power transformer that is operated in the ac link is discussed in detail. Finally, the advantages of a three-phase dual-active bridge over the single-phase variant are discussed using the experience gained from the commissioning.

Iron Losses in a Medium-Frequency Transformer Operated in a High-Power DC–DC Converter

The three-phase dual-active bridge is a dc-dc converter, which is highly suitable for high-power applications. Among others, this is due to the medium-frequency transformer in the ac link, which provides galvanic isolation. The transformer is operated with a square-shaped voltage waveform. The flux density in the transformer core is piecewise linear. However, for the sake of simplicity, the magnetic flux is often assumed sinusoidal. Thereby, the actual iron losses generated in the core material are misinterpreted. This paper discusses the difference between the exact piecewise linear and the sinusoidal course in terms of iron losses. Silicon steel with a thickness of 0.18 mm is measured at a frequency of 1000 Hz, comparing the sinusoidal excitation with the actual one. The measurements are validated using the improved generalized Steinmetz equation. Finally, the transformer core losses are evaluated when the dc-dc converter is operated under load. The results are confirmed through measurement.

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.

Design and implementation of a 5 kW photovoltaic system with li-ion battery and additional DC-DC converter

The integration of photovoltaic (PV) generators in the gird is still an ongoing topic. The peak efficiency of todays power electronics is already above 98 %, therefore innovative aspects can grow from new ways of grid integration. This paper focuses on the combination of a 5 kW PV-generator with a li-ion battery. The temporal decoupling of the generation and injection of energy into the European low voltage grid is presented. Compared to a classic PV system, an additional bidirectional DC/DC-converter is required to ensure the charge and discharge of the battery. The power management of the system can be controlled by the grid operator or according to the best profit defined in a feed-in tariff law for renewable energy. Special attention has to be paid to the efficiency of this DC/DC-converter, due to the bidirectional energy passing before a grid injection is possible in case of energy buffering. After a brief introduction and system overview, this paper focuses on the DC/DC-Converter.

Comprehensive modeling and control strategies for a three-phase dual-active bridge

The dual-active bridge (DAB) is a dc-dc converter, which offers several advantages especially in high-power applications. Besides bidirectional power flow and small filter size, the converter provides galvanic isolation via a medium-frequency transformer. The converters inherent soft-switching capability promises very high efficiency due to the reduction of switching losses. In this paper, different modeling approaches are derived that cover the dynamic behavior of the three-phase dual-active bridge. The first approach utilizes a state-space averaging method in conjunction with state-variable averaging. The second modeling approach applies a first-harmonic approximation. After the comparison of these two different models with a circuit simulation, a control design method is developed based on those models. The control is evaluated for different converter parameters. Finally, the developed control is implemented using a DSP-controlled laboratory prototype of the converter and the results are presented.

Ensuring soft-switching operation of a three-phase dual-active bridge DC-DC converter applying an auxiliary resonant-commutated pole

An auxiliary resonant-commutated pole (ARCP) ensures soft switching in the entire operation range of a three-phase dual-active bridge dc-dc converter. This work evaluates the design and the resulting boost in system efficiency for different semiconductor materials and devices. Afterwards, a full-scale medium-voltage prototype of an ARCP is constructed. The subsequent measurements are presented within this work, before the economic feasibility is discussed.

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.