Current Ripple Reduction Employing the Reactor of DC–DC Converter to Have Variable Inductance through Magnetic Saturation

Pannawan A., Kaewchum T., Somkun S., Hinkkanen M.

Park S., Park J.W., Kim K.H., Kang F.

We propose a circuit topology suitable as a battery charge/discharge tester with a DAB converter and a non-isolated dc-dc converter as a module structure. The module structure can be configured to have a wide input and output voltage range because it is easy to expand. DAB converters are used for bidirectional power transfer and galvanic isolation. By controlling the phase difference of the voltages across the DAB to be almost identical, the generation of circulating current due to the phase difference is minimized, and the ZVS is guaranteed. In addition, by switching at a phase angle that flattens the DAB switch current, the peak current is reduced to diminish the switch’s conduction loss, lowering the switch current rating, and reducing the magnetic core size. The non-isolated dc-dc converter is used to control the battery output voltage. Through the interleaved structure, good dynamic characteristics and current ripple can be reduced, so the effect of improving the battery lifecycle can be expected. We present four series-parallel structures of the proposed converter and compare them with the case of generating an output voltage only with a DAB converter. Analyze the characteristics of automatic voltage balancing due to the difference between the input capacitors’ initial voltage and capacitance connected in series. We analyze the features of the proposed system through simulation and verify the technical feasibility and excellence of the proposed battery charge/discharge tester through charge/discharge experiments using supercapacitors instead of batteries.

Strajnikov P., Kuperman A.

The article presents the analytical derivation of the minimum dc link capacitance in single-phase front ends with active power factor correction (PFC) without hold-up time requirements. Such systems typically employ boost-type rectifiers; operate under restricted total harmonic distortion (THD) of the grid-side current. Moreover, PFC rectifiers must be capable of tolerating step-like zero-to-rated load power variations. It is well known that these two constraints contradict each other, posing nontrivial design challenges. The revealed value of minimum capacitance is expressed by an explicit function of grid voltage and frequency, converter rated power, dc link voltage reference, and grid current THD and voltage loop phase margin set points. Experimental results validate the proposed methodology, closely matching corresponding analytical predictions.

Kim K., Hwang B., Le T., Park S., Lee C., Park S.

In this paper, we propose a new type of smoothing reactor using different types of reactors with different magnetic flux saturation to construct a DC-DC converter with low current ripple under the light load of the bidirectional DC-DC converter. Reduction of current ripple at light load in the limited hybrid reactor can greatly reduce the inrush current ripple of DC-Grid at light load, which is a weakness of existing bidirectional DC-DC converters and is considered to contribute to DC-Grid stabilization. Experiments were conducted to prove the feasibility of the proposed system and hybrid reactor.

Hou N., Gunawardena P., Wu X., Ding L., Zhang Y., Li Y.W.

With some advantages, such as electric isolation, high efficiency, and fast dynamic response, the input-series output-parallel (ISOP) dual-active-bridge (DAB) dc–dc converter has been regarded as one of the most promising candidates for connecting the medium voltage terminal and the low voltage terminal. For the ISOP DAB dc–dc converter, the existing control strategies are mainly focusing on the equivalent power sharing control, but the fast-dynamic response is not included and the decoupling between the regulation of input voltage and the adjustment of output voltage is not eliminated significantly. In this article, the average model of this ISOP DAB dc–dc converter is presented first, which can be employed to analyze the power distributions of this modular topology clearly. Then, an input-oriented power sharing control scheme with fast-dynamic response is proposed for ensuring both the power sharing ability and the fast-dynamic performance of the ISOP DAB dc–dc converter in this article. Compared with the existing methods, this proposed scheme can also significantly reduce the coupling between the power sharing control and the output voltage regulation. In addition, an inductance-estimating method is proposed for ensuring the power sharing performance of the ISOP DAB dc–dc converter. Finally, the experimental results are provided to verify the effectiveness of the proposed input-oriented power sharing control with fast-dynamic response for the ISOP DAB dc–dc converter system.

Heydari-doostabad H., O'Donnell T.

This article proposes a new wide-range bidirectional dc–dc converter that has an improved voltage gain transfer ratio for use in electric vehicle (EV) applications. The converter preserves the common electrical ground between input and output terminals, and presents a low-voltage stress of switches, high utilization factor, and high efficiency. The proposed EV charger performance is evaluated for a bidirectional power flow in grid-connected vehicle-to-grid (V2G) and grid-to-vehicle (G2V) modes. The converter uses a dead-beat current controller in the dc–dc and dc–ac stages, which has a smooth, accurate, and fast response. Finally, experimental results for a 500 W, 40–200 V prototype are provided under a bidirectional power flow in a closed-loop system in the presence of the proposed dead-beat controllers. The obtained results substantiate the theoretical analysis and the applicability of this structure. The converter exhibits the capability for EV battery charging/discharging and demonstrates a peak efficiency of 97.2% and 96.8% in the step-down and step-up modes of operation, respectively.

Calero F., Canizares C.A., Bhattacharya K.

In this paper, a Battery Energy Storage System (BESS) dynamic model is presented, which considers average models of both Voltage Source Converter (VSC) and bidirectional buck-boost converter (dc-to-dc), for charging and discharging modes of operation. The dynamic BESS model comprises a simplified representation of the battery cells, which allows to simulate the effects of battery degradation, dc-to-dc converter, VSC, and the dynamics associated with the filter and transformer connecting the BESS to the grid. A decoupled dq-current control is used for the VSC, allowing the operation of the BESS in several modes, i.e., constant active and reactive power, constant power factor, voltage regulation, frequency regulation, oscillation damping, and a combination of the latter two. The proposed model is implemented in DSATools and tested for different contingencies on a benchmark system, and compared with a industry-grade BESS model used in power system dynamic studies. The importance of modeling the current control and dynamics of the dc-to-dc are demonstrated, especially when the battery cells are degraded due to, for instance, aging.

Hou N., Li Y.W.

The nonresonant single-phase dual-active-bridge (NSDAB) dc-dc converter has been increasingly adopted for isolated dc-dc power conversion systems. Over the past few years, significant research has been carried out to address the technical challenges associated with modulations and controls of the NSDAB dc-dc converter. The aim of this paper is to review and compare these recent state-of-the-art modulation and control strategies. First, the modulation strategies for the NSDAB dc-dc converter are analyzed. All possible phase-shift patterns are demonstrated, and the correlation analysis of the typical phases-shift modulation methods for the NSDAB dc-dc converter is presented. Then, an overview of steady-state efficiency-optimization strategies is discussed for the NSDAB dc-dc converter. Moreover, a review of optimized techniques for dynamic responses is also provided. For both the efficiency and dynamic optimizations, thorough comparisons and recommendations are provided in this paper. Finally, to improve both steady-state and transient performances, a combination approach to optimize both the efficiency and dynamics for an NSDAB dc-dc converter based on the reviewed methods is presented in this paper.

Liu G., Caldognetto T., Mattavelli P., Magnone P.

Droop-controlled distributed energy resource converters in dc microgrids usually show low output impedances. When coupled with ac systems, second-order harmonics typically appear on the dc-bus voltage, causing significant harmonic currents at the converters resource side. In this paper, we show how to reduce such undesired currents by means of notch filters and resonant regulators included in the converters control loops. The main characteristics of these techniques in terms of harmonic attenuation and stability are systematically investigated. In particular, it is shown that the voltage control-loop bandwidth is limited to be below twice the line frequency to avoid instability. Then, a modified notch filter and a modified resonant regulator are proposed, allowing to remove the constraint on the voltage loop bandwidth. The resulting methods (i.e., the notch filter, the resonant regulator, and their corresponding modified versions) are evaluated in terms of output impedance and stability. Experimental results from a dc microgrid prototype composed of three dc-dc converters and one dc-ac converter, all with a rated power of 5 kW, are reported.

Merai M., Naouar M.W., Slama-Belkhodja I., Monmasson E.

Conventionally, standard proportional and integral (PI) controllers with constant PI gains are commonly used for the dc-link voltage control of single-phase grid-connected converters (GCCs). For such controllers, the selection of the PI gains will lead to a tradeoff between two control objectives: 1) the reduction of the dc-link voltage fluctuations caused by random swings of the active power drawn by the single-phase GCC; and 2) the reduction of the grid current harmonics mainly caused by the 2f oscillation of the active power in single-phase applications. To solve this tradeoff, this paper presents a systematic approach for the design of an adaptive PI controller for the dc-link voltage control of single-phase GCCs. The proposed design approach is simple and it provides a convenient method to properly determine the adaptive PI controller parameters. Representative simulation and experimental results are presented and discussed in order to show the effectiveness of the proposed dc-link voltage controller.

Vongkoon P., Liutanakul P., Wiwatcharagoses N.

On the grid side, the half-bridge topology of the microinverter is of interest for solar rooftop applications because of its high efficiency, low component count, and cost-effectiveness. However, it has an inherent double-line frequency ripple voltage on the dc-link, which causes the injection of a third-order harmonic current when the voltage control loop is closed. Furthermore, in practice, different average voltages or different capacitances of the two capacitors at the dc-link produce a second-order harmonic current that flows into the grid. In this paper, the analytical details of these harmonics are comprehensively described, and a simple and effective low-cost technique using the cascaded connection of two modified notch filters is proposed in the voltage control loop to mitigate their effects. The simulation and experimental results of a 300 W microinverter indicate that the proposed filters represent an effective low-cost solution and perform well in accordance with the IEEE 1547 standard, even if the capacitances at the dc-link are mismatched by 20%. Besides, the prototype is tested when occurring of the changing +1% of the line frequency, or appearing of the distorted waveform of grid voltage with the composition of 6% of fifth-order harmonic.

Taghizadeh S., Hossain M.J., Lu J., Karimi-Ghartemani M.

This paper presents a method to enhance the dc-bus voltage-control loop of a single-phase grid-connected dc/ac converter, which improves its responses in terms of oscillation on its dc-bus voltage as well as its output ac current. Conventionally, the double-frequency (2-f) ripple is reduced by using a large electrolyte capacitor, which increases the cost and size of the system. A state-of-the-art approach is to use a notch filter (NF) to block the 2-f ripple in the voltage-control loop. This can significantly reduce the capacitor size. The existing presentations of this method, however, do not integrate the internal dynamics of the NF into consideration. This paper proposes a new way of implementing the NF, which allows integration of its internal variables into the control loop. The resulted system exhibits enhanced transient responses at both the dc-bus voltage and the output ac current. The proposed method is analyzed in detail and its effectiveness is verified through simulations and experimental results.

Mellincovsky M., Yuhimenko V., Zhong Q., Mordechai Peretz M., Kuperman A.

In this paper, an unconventional method of dc link bulk capacitance active reduction without ripple increase in mains-connected power conversion systems is proposed. Even though the adopted power circuitry (a bidirectional dc-dc converter, terminated by a much smaller capacitance) is analogous to solutions proposed up to date, the concept of operation is quite different. Rather than controlling the current, flowing into the dc link (i.e., operating as a current controlled current sink, similarly to an active power filter), suggested solution regulates the ripple by controlling the dc link voltage (i.e., operating as a voltage controlled current sink) thus letting the grid-interfacing converter OFF the task. This allows widening of the dc link voltage loop bandwidth and, as a result, reducing the ripple for the same capacitance utilized or reducing the capacitance while maintaining the same ripple without trading off the power factor. In order to validate the proposed methodology experimentally, it is successfully applied to a single-phase off-the-shelf power factor correction pre-converter.

Zhao B., Song Q., Li J., Wang Y., Liu W.

This paper proposes a multilevel high-frequency-link dc transformer (MDCT) based on dual active phase-shift principle for medium-voltage dc (MVDC) power distribution application. The proposed MDCT employs multilevel and multiplex conversion principle, which brings many advantages and makes the operation of MDCT quite different with the traditional dc transformer (TDCT) and modular multilevel converter. Compared to the TDCT scheme, the proposed MDCT has smaller circulating current and higher power factor; it can operate as a dc breaker to cutoff the connection with the MVDC distribution grid absolutely when a short fault occurs in the distribution grid; the redundant design can be achieved when some submodules failure to improve the reliability. In the paper, the topology, operation principle, modulation method, switching characterization, voltage, and power characterization, and control strategy of MDCT are presented and analyzed comprehensively. At last, a MDCT prototype is built and the experimental results verify the correctness and effectively of the proposed solution.