Hanno Stagge

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.

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.

Precise modeling and analysis of DQ-frame current controller for high power converters with low pulse ratio

This paper presents a modeling and analysis methodology for the current loop control design for three-phase high power converters with extremely low sampling ratios. It applies the complex transfer function and the double-sided Frequency-Response-Functions to quantitatively characterize the system such as the phase margin, dynamic performance and the noise immunity. Both standard PI control and PI control with a current state feedback are modeled and validated via circuit simulations in SABER, which demonstrate the precision and the intuitiveness of the proposed modeling concept. Based on these precise models, some special phenomena and design challenges of the current control loops for high power converters are summarized followed by some generalized design recommendations. Since the precision of the modeling concept is independent of the sampling ratio, the methodology can also be leveraged to research topics such as the “negative impedance” and “active damping”. Generally, this simple and precise modeling concept is highly attractive for control loop analysis and design of three-phase converters.

Impact of Modulation Schemes on the Power Capability of High-Power Converters with Low Pulse Ratios

The design of modulation schemes at low pulse ratios is a known topic. However, most of previous work has focused on Optimized Pulse Patterns, e.g. Selective Harmonic Elimination (SHE). For industrial applications, standard carrier modulators are still highly attractive in many cases. Unfortunately, few papers presented insightful studies on carrier modulators at extremely low pulse ratios. This paper covers this gap. At extremely low pulse ratios, several special phenomena of carrier modulators are observed. Their mechanisms are explained in detail. Moreover, a quantitative comparison between different carrier-based modulation schemes is presented. Based on that, several design criteria for modulators at low pulse ratios are proposed at the end.

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.

Damping concepts of LCL filter for a multi-level medium voltage adjustable speed drive with low pulse-ratio

This paper presents the investigation of damping concepts of the resonance oscillation of an LCL-filter in a PWM inverter as a variable-speed-drive with low pulse-ratio. The system is designed for general-purpose operation and should be connected to different machines working in mediumvoltage with a power of 22 MVA. Low current THD is required on the machine side, which makes the use of a filter necessary. However, the oscillation resonance of the filter should be properly damped. The standard passive damping concept is not attractive because of high losses. Moreover, the widely-used active damping concepts using capacitor voltage or capacitor current as feedback are not applicable due to insufficient phase margin under low pulse ratios. An improved active damping concept based on a state estimator is proposed. It extends the use of active damping methods to the low-pulse ratio range. Theoretical analysis of the selected methods shows the technical trade-off regarding stability of the two active damping methods. The theory has been verified via simulations.

Control Strategy for Frequency Control in Autonomous Microgrids

Due to the deployment of distributed generation, future grids will show reduced inertia resulting in higher dynamics and reduced frequency stability. This is especially true in microgrids (MGs). In this sense, the virtual synchronous generator (VSG) concept has been proposed as a promising solution. In this paper, the benefits of this concept are further explored studying the frequency stability improvement under simulation of different cases regarding the MG operation. Here, the analysis is carried out looking at the impact of the control parameters on the rate-of-change-of-frequency and on the average frequency deviation. The VSG concept shows a great potential to reduce both parameters improving the frequency stability.

A fast and precise simulation method for performance screening for high power converter designs

For high power converter designs, a performance screening with hundreds or thousands of configuration combinations and operation points is typically required. Standard time-domain based circuit simulators are not efficient for such screening works because of the long computation time. In this paper, a modeling and simulation methodology is presented, which is ultra-fast compared to time-domain based simulators. The paper presents a generalized method to calculate the steady-state load response, even for complicated loads like machines. Afterwards, a powerful simulation program based on MATLAB m-files is introduced, which calculates the voltage / current waveforms, device power loss and thermal stress jointly. Due to the flexible structure, multiple multilevel topologies, modulation schemes, load types and devices can be configured for the performance screening without major modifications. The proposed method reaches the same steady-state accuracy as circuit simulators in about a factor of thousand reduced computation time. Because of these features, the methodology and the program developed in this paper are perfect for a wide-range performance screenings for high power converter designs.