DC Port Research Articles

Hybrid DC–AC Zonal Microgrid Enabled by Solid-State Transformer and Centralized ESD Integration

Solid state transformer (SST) integrated hybrid dc–ac microgrid (HMG) has wide operating capabilities in presence of dc–ac loads, renewable energy resources, and energy storage. This paper proposes control strategies for localized control of zonal hybrid microgrid, enabled by the SST and centralized energy storage devices (ESD) integrated using dual active bridge (DAB) converter. The control of DAB dc–dc converter is modified for zero circulating power flow (ZCPF) operation in both SST and ESD. Power management and coordination control used in this paper ensures the power supply reliability of the zonal HMG integrated through the SST. Microgrid system, along with its proposed controllers, provides a stabilized low-voltage dc and ac network ports, in both grid feeding and forming modes. The performance of the configured HMG system is evaluated under various power management scenarios using the MATLAB software platform. Further, the SST architecture, with its proposed controller, is tested on FPGA-based hardware platform.

Capacitors Energy Strategy Based Cascaded H-Bridge Converter for DC Port Failures

Capacitors Energy Strategy Based Cascaded H-Bridge Converter for DC Port Failures

Improved capacitor voltage balancing control for multimode operation of modular multilevel converter with integrated battery energy storage system

Modular multilevel converter with integrated battery energy storage system (MMC-BESS) has been proposed for energy storage requirements in high-voltage applications with large-scale renewable energy resources. The MMC-BESS is essentially a three-port converter which can transfer energy between any two of the ac port, dc port, and BESS, constituting multimode operation of the system. Different from conventional MMCs, the state-of-charge (SOC) inconsistency among batteries would magnify the submodule capacitor voltage unbalance issue in the MMC-BESS. In this study, an improved capacitor voltage balancing method applicable for multimode operation of MMC-BESS is proposed by adjusting ac and dc modulation indexes simultaneously. After that, the ratio of ac and dc modulation indexes is optimised to enhance the tolerance of unbalanced power among SMs within phase arm, greatly improving the SOC equalisation rate. Based on the proposed capacitor voltage balancing method, the control structure is given and the dynamic model is conducted for the analytic design of the closed-loop controller. Finally, the simulations and experimental results validate the effectiveness and feasibility of the proposed control strategy.

A Multiport Power Electronic Transformer Based on Modular Multilevel Converter and Mixed-Frequency Modulation

Power electronic transformer (PET) is a key apparatus in smart grid system, due to the advantages of power controllability and ac/dc grids interconnection. The four-stage PETs have multiple output ports, good controllability, and flexibility, but are limited by multiple power conversion stages, complex structure, and low efficiency. Therefore, a novel modular multilevel converter (MMC)-based PET topology with mixed-frequency modulation is proposed to reduce the power conversion stage. The power decoupling circuits are designed to realize multiple output ports, including medium voltage ac (MVAC) port, medium voltage dc (MVDC) port, and low voltage dc (LVDC) port. The equivalent circuit and controllability of proposed PET are analyzed. The proposed PET topology provides the same multiple output ports by only three power conversion stages, therefore, the PET structure is simplified. In addition, simulation results are provided to verify the effectiveness of proposed PET.

Droop-Controlled Rectifiers That Continuously Take Part in Grid Regulation

In this paper, a generic framework for a rectifier to continuously take part in the regulation of grid voltage and frequency is proposed. It can automatically change the power consumed to support the grid, without affecting the normal operation of the load. Such a rectifier has a built-in storage port, in addition to the normal ac and dc ports. The flexibility required by the ac port to support the grid is provided by the storage port without affecting the dc port. The grid support of the ac port is achieved through the recently proposed universal droop controller (UDC). A dc-bus voltage controller cascaded to the UDC is proposed to regulate the dc-bus voltage of the storage port within a wide range so as to provide the flexibility needed, while the dc-port voltage is maintained constant. An illustrative example is presented with the recently proposed $\theta$ -converter, which consists of three ports with only four switches. Experimental results are provided to verify the capability of the rectifier to regulate the grid voltage and frequency without affecting the load.

A Novel Dual-DC-Port Dynamic Voltage Restorer With Reduced-Rating Integrated DC–DC Converter for Wide-Range Voltage Sag Compensation

A novel dual-dc-port dynamic voltage restorer (DDP-DVR) is proposed. One low-voltage dc port is directly connected to the energy storage, while the other high-voltage dc port is connected to the intermediate dc bus. An integrated dc-dc converter is interfaced with these two dc ports which is aimed at providing a high-dc-link voltage and maintaining the voltage injection capability even if the energy-storage voltage is lower than the peak value of the injected ac-line voltage. Compared with the traditional DVR that uses the dc-dc converter for all voltage sag compensation, the proposed DDP-DVR can compensate shallow voltage sags without using the dc-dc converter, and for deep voltage sags, only partial active power needs to be processed by the dc-dc converter. Therefore, the power rating of the dc-dc converter can be reduced significantly. Meanwhile, much lower power losses can be achieved by the proposed DDP-DVR due to reduced power conversion stages. The operation principles, modulation strategies, and characteristics of the proposed DDP-DVR are analyzed in detail and verified with experimental results.

Double Voltage Rectification Modulation for Bidirectional DC/DC Resonant Converters for Wide Voltage Range Operation

It is difficult to achieve wide voltage range for full-bridge-based bidirectional resonant dc/dc converter due to the large resonant current caused by the small magnetizing inductance. A half-bridge modulation was proposed to extend the output voltage range by offering an additional step-down voltage step of 0.5 normalized gain. But in backward operation, the converter cannot provide the two-times step-up voltage gain to match the forward 0.5 step-down voltage gain, which disables the converter to truly work bidirectionally with large voltage range. This paper proposes a double voltage rectification (DVR) modulation on the rectification side to achieve twice the voltage gain for the backward direction to effectively widen the operation voltage range, which matches the ratio of the two dc port voltages where the half-bridge modulation is used in the forward direction. In the DVR modulation, the operation of the input-side resonant tank is the same as that of the conventional modulation. The switches on the input side still turn on with zero-voltage switching (ZVS), and the body diodes on the rectification side still turn off with zero-current switching. Moreover, the turning on and off of the mosfets on the rectification side are also ZVS, which causes no additional losses and guarantees high-frequency operation. The rotational operation of the rectification-side switches improves the thermal balance condition, enlarging the power capacity and improving the reliability of the converter. A smooth transition method between the two modulations is also proposed, which still guarantees ZVS turning on for the rectification-side switches. The symmetrical CLLLC resonant converter is taken as an example to illustrate the operation principle and features of the proposed modulation. And other bidirectional resonant converters for the DVR modulation are also presented. Finally, a 3-kW bidirectional CLLLC resonant converter prototype with 100-kHz resonant frequency is built to verify the proposed modulation.

Instantaneous Flux and Current Control for a Three-Phase Dual-Active Bridge DC–DC Converter

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.

A Hybrid MMC-Based Photovoltaic and Battery Energy Storage System

This paper proposes a new configuration and its control strategy for a modular multilevel converter (MMC)-based photovoltaic (PV)-battery energy storage (BES) system. In the MMC-based PV-BES system, each PV submodule is interfaced from its dc side with multiple PV generators using isolated dual active bridge (DAB) dc–dc converters. One BES system is embedded into each arm of the converter and is connected to the dc port of the associated BES submodule using multiple isolated DAB converters. The embedded BES systems are used to smooth the output power of the PV generators and limit the rate of change of the power delivered to the host grid. Moreover, they enable compensation of power mismatches between the arms and legs of the system by exchanging power with the arms of the converter. This paper then proposes a hybrid power mismatch elimination strategy using a combination of power exchange with the arms of the converter and internal power flow control of the MMC. The proposed hybrid power mismatch elimination strategy employs BES systems and differential currents to compensate power mismatches and transfer power between the arms and legs of the converter, respectively. The effectiveness of the proposed power smoothing technique using the embedded BES systems and hybrid power mismatch elimination strategy is demonstrated using time-domain simulations conducted on a switched model of the PV-BES system in PSCAD/EMTDC software environment.

Steady‐state model of multi‐port electric energy router and power flow analysis method of AC/DC hybrid system considering control strategies

This study proposes a steady-state model of multi-port electric energy router (EER) and a power flow analysis method for AC/DC hybrid system considering control strategies. Firstly, a circuit topology model of EER is put forward, including AC and DC port equivalent parts. Secondly, considering external control characteristics of EER, the control strategies including PQ mode, V/f mode, and droop mode are established, which is suitable for system analysis. Then, using the extended sub-network model, an AC/DC power flow model containing multi-port EER is established. In terms of the power flow algorithm for AC/DC hybrid system, AC/DC network admittance matrix and the extended AC/DC network modification equations are applied to solve the problem. Two examples are designed and show that EER can be used for flexible power regulation and voltage improvement. Besides, power flow analysis method of AC/DC hybrid system is demonstrated to be effective.

Stochastic Monte Carlo‐based voltage variation analysis for low‐voltage hybrid DC/AC radial distribution feeders interfaced with DERs

This study proposes a hybrid DC/AC radial distribution feeder architecture for the future microgrids. The architecture includes multiple distributed energy resource (DER) integrated household premises with AC and DC ports. Voltage unbalance (VU) and voltage drop (VD) in AC or DC distribution feeders are a research area that needs to be revisited in the light of the DERs integration. This study analyses the voltage variations occurring as a result of household DERs integration in the proposed low-voltage hybrid DC/AC distribution feeders. The impact of DERs rating and its location on DC or AC distribution feeder voltage is analysed by varying these parameters stochastically, via Monte Carlo algorithm. Sweep backward–forward load flow analysis technique is implemented to gain knowledge of distribution node voltage profiles for different source and load scenarios. The impact of cross-feeder VU, sag, and swell conditions is also evaluated here. The test case studies are primarily conducted for static conditions which are further extended to dynamic test profiles. This study would help in predicting a failure index of voltage fluctuations in the proposed hybrid DC/AC radial feeders in the presence of the DERs. The results are obtained through the MATLAB and PSCAD/EMTDC-based simulation studies.

Analysis and Optimization of Multi-DC Current Flow Controllers With Different Configuration Positions in M2TDC Grid

In multi-terminal HVDC (MTDC) grids, the voltages and currents at the converter DC terminals can be accurately regulated by the control of converter stations. For the DC line currents in meshed MTDC (M 2 TDC) grids, they are naturally distributed based on the line resistance and the voltage difference at DC ports, which greatly affects the controllability and optimal distribution of the DC power/current flow. In this paper, regarding different configurations of positioning (COP) DC CFCs in M 2 TDC grids, the dynamic behaviors and optimized configuration scheme are investigated. Firstly, topological structure of a generalized M 2 TDC is presented. Two different COP schemes for double DC CFCs in a four-terminal M 2 TDC grid are proposed according to control objectives of the DC current flows. Secondly, in order to achieve the coordinated control of DC line currents and capacitor voltage of the DC CFC, a generalized voltage-droop control strategy is applied. It can be applicable to multiple DC CFCs under different COP without the need of communication. Apart from that, considering the impact of DC CFCs under specific COP on the control characteristics of DC current flows and control margins, an auxiliary control strategy, which is capable of mitigating the confliction of multiple DC currents, is proposed. It can effectively improve the control performance of the DC CFC. Based on the time-domain simulation PSCAD/EMTDC, the M 2 TDC grids with different COP of DC CFCs are established. The merits and drawbacks of different COP, in terms of the complexity and implementation of control systems, dynamic behaviors in mitigating transient voltage spikes and current surges, control margins and robustness of the controller design, are discussed. In addition, the effectiveness of the proposed generalized voltage-droop control and auxiliary control strategies is verified with an optimized COP scheme of multiple DC CFCs proposed.

Three-Port-Converter-Based Single-Phase Bidirectional AC–DC Converter With Reduced Power Processing Stages and Improved Overall Efficiency

Single-phase bidirectional ac-dc converters based on a three-port converter (TPC) are proposed for energy storage applications. Three power interfacing ports, i.e., a dc-bus port, a dc input port, and an ac port, are provided by the proposed converter. A battery, whose voltage is lower than the peak amplitude of the ac voltage, is connected to the dc input port of the TPC, with which energy exchange between the battery and grid can be achieved directly. Therefore, power processed by the down-stream dc-dc converter is reduced, which can help to reduce the power losses induced by the down-stream dc-dc converter. In addition, switching losses of the TPC can be reduced as well due to multilevel characteristic. As a result, the overall efficiency of the entire bidirectional ac-dc converter is improved significantly. Topologies, operation principles, modulation, and control of the proposed TPC-based bidirectional ac-dc converter are analyzed in detail. A 2-kW prototype is built and tested to verify the effectiveness and advantages of the proposed solution.

A Single-Phase Inverter/Rectifier Topology With Suppressed Double-Frequency Ripple

This paper introduces a new bidirectional single-phase inverter topology. The proposed topology has three ports: a dc port, an ac port, and a ripple port. The ac and dc ports are bidirectional to support rectifier or inverter operation. A small inductor, which is of alternating current, exchanges power between different ports of the single-phase system. The proposed topology is capable of accomplishing voltage step-up or step-down, suppressing the ripple power, and performing inversion or rectification operation in one stage of power conversion. The proposed configuration offers an active decoupling function, which not only eliminates the double-frequency ripple power at the dc port but also achieves minimum capacitance requirements to minimize the size of the decoupling capacitor. This facilitates the use of a very small thin-film capacitor, which offers a much longer life-cycle and higher reliability compared with a bulky electrolytic capacitor. A control approach is also developed to regulate the dc-port and ac-port currents and manage the ripple power through proper distribution of power between each port. Moreover, a very small capacitor can be placed in parallel with the inductor to reinforce the proposed configuration with soft-switching operation, which enhances the overall efficiency and minimizes the voltage stress over the semiconductor devices. This converter is capable of accommodating an arbitrary number of dc or single-phase ac sources and/or loads configuring a multiple-input multiple-output inverter without introducing any additional passive elements or sacrificing the performance of the inverter. The primary focus of this paper is dc-to-ac conversion mode of operation, in spite of the fact that it can easily be configured to serve as a rectifier. Experimental and simulation results are presented to validate the operation of the proposed topology and its control strategy.

A Power Mismatch Elimination Strategy for an MMC-Based Photovoltaic System

This paper proposes a power mismatch elimination strategy for a medium-voltage modular multilevel converter (MMC) -based photovoltaic (PV) system. In the MMC-based PV system, each submodule of the MMC is energized by multiple PV generators, each interfaced with the dc port of the submodule by a corresponding isolated dual active bridge dc–dc converter. This configuration allows for a transformerless connection to the host grid and independent maximum power point tracking for the PV generators. The paper then proposes a power mismatch elimination strategy that ensures that the current delivered to the host grid is balanced in spite of unbalanced PV generator outputs. The proposed power mismatch elimination strategy employs a dc differential current to equalize the leg powers, and an ac differential current to stabilize the dc voltages of the submodules. The effectiveness of the proposed power mismatch elimination strategy is demonstrated by time-domain simulations conducted on a model of the PV system in PSCAD/EMTDC software environment.

Modified SVPWM-Controlled Three-Port Three-Phase AC–DC Converters With Reduced Power Conversion Stages for Wide Voltage Range Applications

Three-port three-phase rectifiers (TPTPRs) are proposed by introducing a new low-voltage (LV) dc power port into the three-phase six-switch boost rectifier. An ac input port, an LV dc load port, and a high-voltage (HV) dc bus port are provided simultaneously by TPTPRs. A dc load, whose voltage is lower than the peak amplitude of the ac line voltage and varies over a wide range, can be connected to the LV dc load port of the TPTPR directly. A modified space vector pulse width modulation (SVPWM) strategy is proposed for the TPTPR to accomplish voltage and current regulations, based on which part of the ac input power can be directly fed to the LV dc load within single power conversion stage. As a result, the power rating and losses of the downstream dc-dc converter, which is connected to the HV dc bus port of the TPTPR, are reduced significantly. The conversion efficiency of the TPTPR-based ac-dc converter benefits not only from the reduced power conversion stages, but also from the reduced switching losses due to the multilevel characteristics of the TPTPR. Operation principles and characteristics of the proposed TPTPRs and modified SVPWM strategies are analyzed in detail and verified with experimental results.

New three-port DC-AC inverter outputting a single-phase and a set of three-phase AC voltages

ABSTRACTA conventional DC-AC inverter can only output either a single-phase AC voltage or a set of three-phase AC voltages. A new three-port DC-AC inverter which can simultaneously output a single-phase AC voltage and a set of three-phase AC voltages is proposed in this paper. This three-port DC-AC inverter is based on the three-port T-type multi-level power converter which is composed of three T-type power electronic legs, a decoupling transformer set, a filter inductor set, a single-phase filter capacitor, and a three-phase filter capacitor set. The DC port of the proposed power converter is connected to a DC power source to act as the input port, and the single-phase AC port and the three-phase AC port serve as two output ports to supply power to the single-phase load and the three-phase load, respectively. The zero-sequence transformer is used to decouple the single-phase and three-phase AC components, which are generated by the three T-type power electronic legs. The operation principle of this three-port DC-AC inverter is analyzed, and a hardware prototype is established to verify the performance of the proposed three-port DC-AC inverter. The experimental results are as expected.

Three-Port Bridgeless PFC-Based Quasi Single-Stage Single-Phase AC–DC Converters for Wide Voltage Range Applications

Quasi single-stage single-phase ac–dc power converters based on a three-port bridgeless power factor corrector (TPB-PFC) are proposed in this paper. The TPB-PFC, which is able to provide an ac input port, a dc load port, and a dc bus port simultaneously, is derived by introducing a new dc load port into a conventional bridgeless PFC. The load, whose voltage can be either lower or higher than the peak amplitude of the ac line voltage, is connected to the dc load port of the TPB-PFC directly. Therefore, the ac input power can be fed to the dc load with the single-stage power conversion when the instantaneous ac voltage is lower than the voltage of the dc load. Although another dc–dc converter is required to accomplish the power transmission between the dc bus port and the dc load port of the TPB-PFC, the power rating and losses of the dc–dc converter are reduced significantly, in comparison with a two-stage ac–dc converter. Furthermore, the switching losses of the TPB-PFC are reduced thanks to the three-level characteristic of the proposed converter. The topology derivation, operation principles, modulation strategies, and characteristics of the proposed ac–dc converter are analyzed and verified experimentally with a 2-kW prototype.

A 5.8-GHz Bidirectional and Reconfigurable RF Energy Harvesting Circuit With Rectifier and Oscillator Modes

A bidirectional circuit that can be reconfigured as a receiver for rectifying radio-frequency (RF) power for energy harvesting, and as a power oscillator for transmission, is proposed. The circuit is compact and suitable for integration in time-division multiplexed wireless sensors. A design concept based on time-reversal duality theory is employed to reuse the same circuit elements for both the oscillator and the rectifier modes by reconfiguring power flow between the dc and RF ports. A proof-of-concept prototype circuit using a high-efficiency class-DE circuit topology is implemented in a 65-nm CMOS technology. The circuit operates at a frequency of 5.8 GHz, and the measured peak efficiency is greater than 50% in both operating modes.

Active Suppression of Selected DC Bus Harmonics for Dual Active Bridge DC–DC Converters

AC coupled dual active bridge (DAB) dc–dc converters typically use phase shifted square wave (PSSW) modulation to manage the power flow between two dc sources. With this scheme, the current flowing between the converter bridges injects high-magnitude current harmonics into each dc port at multiples of the primary switching frequency, which can excite resonances in the $LC$ circuits created by parasitic second-order impedances such as wiring inductances in the dc-link connections. This can cause substantial dc bus voltage and current oscillations, particularly with the higher switching frequencies that are used with wide bandgap devices, leading to excessive electromagnetic interference, significant filter stress, and eventual component operational failure. Conventionally, a relatively large dc bus filter capacitor (or inductor) helps to suppress these dc bus harmonic dynamics. However, the use of adaptive three-level modulation for a single phase DAB provides a much greater solution space to achieve a desired power transfer condition, with the three PSSW control angles that are available. This paper now explores the additional use of these angles to selectively suppress particular dc bus current harmonics across the entire operating range of the converter and thus allow the size of the DAB dc bus bridge capacitors to be minimized. This new active harmonic suppression (AHS) strategy is validated by theory, simulation, and matching experimental results.