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This letter proposes a closed-loop black start-up control for the dual-active-bridge (DAB) converter. With a simple combination of the extended phase-shift and triple phase-shift modulation schemes, the DAB converter can start up with the maximum allowable precharge current while realizing soft-switching operation in the trapezoidal current mode simultaneously. Compared to the conventional start-up procedure that usually requires multiple steps of operation and parameter fine-tuning, the proposed start-up method can be implemented simply into the existing closed-loop controller and can shorten the start-up transients of the DAB converter significantly. The presented methods are validated experimentally on a downscaled DAB converter prototype. [ABSTRACT FROM AUTHOR]

The three-phase dual-active-bridge (DAB) converter is a promising topology for high-power dc–dc conversion in dc grids and industrial applications. However, the conventional single-phase-shift modulation shows a limited soft-switching range and large reactive currents under light-load operation especially with large voltage variations. This article proposes an asymmetrical duty-cycle control method for the three-phase DAB to extend the soft-switching range and reduce reactive currents by realizing three-phase triangular- and trapezoidal-current-mode operation. Closed-form solutions of the proposed modulation schemes are derived that enable a simple online calculation of the control parameters. The proposed modulation schemes can substantially reduce losses of both power semiconductor devices and the medium-frequency transformer. Thus, the three-phase DAB converter can realize high efficiency over a wide operation range, especially under light-load conditions. Considering the practical implementation, a compensation method is presented to correct the distorted current waveforms caused by the dead time and ensure the desired soft-switching operation. Besides, the power loss imbalance between top and bottom switches induced by the asymmetrical duty cycle is addressed by an active thermal balancing strategy. The effectiveness of the proposed method is validated by simulation results and experiments on a downscaled converter prototype. [ABSTRACT FROM AUTHOR]

The modular-multilevel-based isolated dc–dc converter (MMDC) is an attractive solution for the interconnection of medium-voltage (MV) and low-voltage (LV) dc grids. With a modular-multilevel converter (MMC) on the MV side and full bridges (FBs) on the LV side, the MMDC can be flexibly scaled to different voltages and power ratings. However, the existing MMDC is limited to the two-port operation where the dc-link voltages and powers of FBs must be always kept identical. This letter proposes an operation scheme that enables fully independent three-port bidirectional operation of MMDC capable for the interconnection of MVDC and two isolated LVDC grids with different nominal voltages. A comprehensive operation strategy is proposed, which contains two operation modes with different modulation schemes for the MMC to optimize the transformer current in the full load range. Compared with the most popular three-port isolated dc–dc converter triple active bridge, the power flow among three ports follows a simple mathematical rule and can be regulated bidirectionally with small computational efforts as well as intrinsically eliminated circulating power. The experimental results validate the feasibility and effectiveness of the proposed operation scheme. [ABSTRACT FROM AUTHOR]

The purpose of this study is to present the concept and properties of a dual-active bridge dc–dc converter with a modified auxiliary-resonant commutated (MARC) pole concept, and to discuss preliminary test results of a high-power setup. In this approach, the dual-active bridge based on IGCT is considered. First, the soft-switching boundaries for the converter with lossless snubbers and high switching delays are introduced. Second, the MARC pole is presented to overcome this border to receive full soft-switching capability in the entire operation range of the dc–dc converter. This is of crucial importance since IGCTs will be destroyed in case of a snubber dump. The advantage of the modified circuit is the guaranteed switching under zero-voltage conditions without applying an additional boost current as needed in the classic auxiliary-resonant commutated pole (ARCP) approach. Thus, complexity is reduced and reliability is increased. Measurement results of the MARC are presented from a mega-watt dc–dc converter where the concepts are fully implemented. The results of large-scale systems impressively show how the ARCP reduced the impact of parasitic elements, which would normally affect the switching transitions tremendously. [ABSTRACT FROM AUTHOR]

The three-phase dual-active bridge converter is a promising topology for the high-power high-frequency dc–dc conversion, due to the soft-switching capability and inherent galvanic isolation. With the state-of-the-art instantaneous current control (ICC) based on the single phase-shift (SPS) operation, a bidirectional power flow can be controlled dynamically between two dc ports without inducing overshoots in the transformer currents. However, a transient dc-bias flux may still occur in the transformer, which may cause the transformer core saturation in the dynamic operation such as start-up, power-off, and abrupt load changes. To analyze and address this issue, this paper presents a set of model-based methods to avoid inducing the dc-bias flux in various transient situations. The proposed double-side SPS operation could nullify the dc-bias flux in abrupt load changes and even instant power-flow reversals. Furthermore, the soft-magnetizing and soft-demagnetizing techniques are presented to eliminate the initial and residual dc-bias flux during start-up and power-off instantly. The proposed flux control method can be integrated with the ICC. Thus, the transformer magnetizing flux and the winding currents can be controlled simultaneously and instantaneously in transient situations. Simulations and experiments on a down-scaled hardware prototype validate the effectiveness of the proposed methods. [ABSTRACT FROM AUTHOR]

Power electronics-based medium-voltage direct current (MVdc) microgrids consist of several interconnected feedback-controlled switching converters. Such systems experience bus voltage stability challenges owing to the negative incremental resistance of constant power loads and converter control loop interactions. To tackle the stability challenges, this paper presents the application of the interconnection and damping assignment passivity-based control (IDA-PBC) approach to the port-controlled Hamiltonian model of dual active bridge (DAB) source-side converters in an MVdc microgrid. For the DABs, a fundamental average model approach considering phase shift modulation is provided and used for deriving the corresponding IDA-PBC control law. We analyze the effectiveness of the controller on large signal scenarios considering disturbances such as load step up, and DAB disconnection. Hardware-in-the-Loop experiments using Opal-RT and Labview field programmable gate arrays (FPGAs), as well as, low power prototype tests are carried out to demonstrate the validity and feasibility of the proposed approach. [ABSTRACT FROM AUTHOR]

This paper presents a novel maximum output power point tracking (MOPPT) strategy for a three-phase dual-active bridge (DAB3) converter to integrate a large-scale photovoltaic (PV) energy generation into medium-voltage dc grids. The DAB3 converter is employed as a single-stage solution to step up dc voltage levels and perform the maximum power point tracking of the PV farm at the same time. In order to achieve a high weighted average efficiency over a wide operation range, design considerations of the stray inductance and transformer's turns ratio are presented to shape the soft-switching region of the DAB3 converter in order to match the area of the maximum power points for the PV panels in the given environmental conditions. To further maximize the energy yield of the entire PV power generation system, a novel MOPPT method is proposed to track the maximum output power point of the DAB3 converter. The MOPPT is implemented by a cascaded closed-loop voltage control with the PV current feedforward compensation to improve the dynamic performance. The effectiveness of the proposed control strategy is validated by both simulations and the experimental verification of a down-scaled laboratory prototype. [ABSTRACT FROM AUTHOR]

This paper presents a detailed comparison between the conventional ac collector-grid configuration and two proposed dc collector-grid configurations for large photovoltaic (PV) parks. One complete year of measured weather data at two different locations, one very sunny and the other with relatively less sun, is used for the evaluation. An already validated PV performance model is used to estimate the expected energy yield of the PV park for the given environmental and connection conditions. Loss mechanisms in different grid components in the three configurations are discussed based on the developed models. These components include the inverters, transformers, cables, and dc–dc converters. A comparison of the total losses in these components helps to identify the configuration that has the potential to operate at higher efficiency and, hence, generate maximum revenue. The economic viability metrics are calculated and a sensitivity analysis is carried out using the Monte Carlo method with a \pm \mathbf 5\% uncertainty in the considered cost and energy yield parameters. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Three-phase dual active bridge (3p-DAB) dc-to-dc converters are typically avoided in low-power applications especially for wide voltage and power ranges. Even so, the 3p-DAB do offer a means to reduce filter costs and volume. The aim of this study is to propose the triangular and trapezoidal modulation for the 3p-DAB to address the problem of poor partial load efficiency. The proposed modulation schemes were compared with two conventional DAB concepts. It was found that the efficiency of the 3p-DAB increased substantially. Moreover, the 3p-DAB showed a considerably lower filter volume than that of the single-phase dual active bridge converter (1p-DAB). In conclusion, a modulation strategy combining the two proposed modulation schemes with the phase-shift modulation is ideal, because they boost efficiency and take most benefit from the inherent low filter volume. Ultimately, the three-phase dual active bridge may offer a promising solution to miniaturize galvanically isolated dc-to-dc converters for electric vehicles. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]