Low Voltage DC Bus Research Articles

A Multi-port DC Solid-state Transformer for MVDC Integration Interface of Multiple Distributed Energy Sources and DC Loads in Distribution Network

In traditional DC distribution networks with both a low-voltage DC (LVDC) and a medium-voltage DC (MVDC) bus, DC units are connected to the LVDC bus through LVDC converters. In this approach no galvanic isolation is provided and two power conversion stages are needed between the DC units and the MVDC grid, hence leading to a high number of converters and higher costs. To address these shortcomings, this paper proposes a novel multi-port DC solid-state transformer (MDCSST) to interface DC units directly to the MVDC bus. Multiple modules are connected in series on the medium voltage side to the MVDC bus, whereas DC units are independently connected on the low-voltage side of the MDCSST. Compared to a traditional DC distribution network, the proposed scheme connects DC units to the MVDC bus without an extra converter or an LVDC bus, therefore saving cost and reducing the number of converters. In addition, DC units are galvanically isolated by high-frequency transformers. The main challenge of the MDCSST is to resolve the voltage-imbalance problem on the MVDC side which is caused by the DC units power differences. An LC-branch is used to balance the voltages among the modules and to transfer their differential powers.

Modeling a Switching-Regulated Capacitively Coupled Power Supply for Medium-Voltage AC

The use of capacitive coupling has the potential of reducing the size and cost in the implementation of auxiliary power supplies. This is particularly advantageous in applications with medium-voltage AC feeders, where this arrangement can avert the use of a high-voltage transformer. The current standard for voltage control in this type of power supply is shunt regulation. This scheme maintains a low DC bus voltage, which increases the current demand through the input capacitors and hence decreases the power factor. Switching regulation allows operation at a relatively high intermediate DC bus voltage which can improve the power factor, in addition to providing galvanic isolation and achieving higher efficiencies. However, this type of regulation has not been thoroughly explored in the literature of capacitively coupled power supplies. This paper presents a novel mathematical model to describe high-frequency switching DC-to-DC regulation in a power supply capacitively coupled to a medium-voltage input. The model is used in a case study for the design of a 50 W / 12 V (DC) supply fed from a 1 kV (rms) / 50 Hz AC input. The results of the design validate the model and demonstrate the advantages of the novel concept.

Power Scalable Bi-Directional DC-DC Conversion Solutions for Future Aircraft Applications

With the introduction of the more electric aircraft, there is growing emphasis on improving overall efficiency and thus gravimetric and volumetric power density, as well as smart functionalities and safety of an aircraft. In future on-board power distribution networks, so-called high voltage DC (HVDC, typically +/−270VDC) supplies will be introduced to facilitate distribution and reduce the associated mass and volume, including harness. Future aircraft power distribution systems will also very likely include energy storage devices (probably, batteries) for emergency back up and engine starting. Correspondingly, novel DC-DC conversion solutions are required, which can interface the traditional low voltage (28 V) DC bus with the new 270 V one. Such solutions presently need to cater for a significant degree of flexibility in their power ratings, power transfer capability and number of inputs/outputs. Specifically, multi-port power-scalable bi-directional converters are required. This paper presents the design and testing of such a solution, addressing the use of leading edge wide-band-gap (WBG) solid state technology, especially silicon carbide (SiC), for use as high-frequency switches within the bi-directional converter on the high-voltage side.

Four‐port solid‐state transformer based on hybrid MMC with enhanced dual half‐bridge submodules

This study presents a novel solid-state transformer (SST) with four ports that can connect the medium-voltage (MV) DC bus, the MV AC bus, the low-voltage (LV) DC bus, and the LV AC bus, respectively. This SST is suitable for application in the hybrid AC/DC power grid that functions as an energy router between AC and DC systems. Furthermore, the SST submodule is enhanced with two types of dual-half bridge converters, namely DHSM1 or DHSM2, depending on if an additional active switch is used to block the possible short circuit. Thanks to the designed structure of the proposed SST that combines DHSM1 and DHSM2 as the front-end stage, it achieves reduced hardware complexity with the remaining capability to handle the DC short-circuit fault that may occur at the medium- or high-level voltage DC grid. In addition, an optimised overall control strategy that can reduce the measurement demand and control complexity is proposed, with the possible power flow path inside the SST fully analysed. Simulation and experimental results validate the feasibility of the proposed SST and the effectiveness of the overall control scheme with the DC fault handle capability.

Understanding Turn-On Transients of SiC High-Power Modules: Drain-Source Voltage Plateau Characteristics

The SiC (silicon carbide) high-power module has great potential to replace the IGBT (insulated gate bipolar transistor) power module in high-frequency and high-power applications, due to the superior properties of fast switching and low power loss, however, when the SiC high-power module operates under inappropriate conditions, the advantages of the SiC high-power module will be probably eliminated. In this paper, four kinds of SiC high-power modules are fabricated to investigate fast switching performance. The variations in characteristics of drain-source voltage at turn-on transient under the combined conditions of multiple factors are studied. A characteristic of voltage plateau is observed from the drain-source voltage waveform at turn-on transient in the experiments, and the characteristic is reproduced by simulation. The mechanism behind the voltage plateau is studied, and it is revealed that the characteristic of drain-source voltage plateau is a reflection of the miller plateau effect of gate-source voltage on drain-source voltage under the combined conditions of fast turn-on speed and low DC bus voltage, while the different values of drain-source voltage plateau are attributed to the discrepancy of structure between upper-side and lower-side in the corresponding partial path of the drain circuit loop inside the module, with the standard 62 mm package outline.

A Dual-Active-Bridge With Half-Bridge Submodules DC Solid-State Transformer for DC Distribution Networks

High-frequency dc solid-state transformer (DCSST) plays an important role in DC bus connection, voltage conversion, electrical isolation, and bidirectional power transmission in dc distribution networks. In various DSSCT topologies, the dual-active-bridge (DAB) scheme becomes a typical solution for DCSST due to its various advantages. However, the DAB scheme still has drawbacks during practical applications. In this article, a DAB with half-bridge submodules (HSDAB) is proposed. By replacing the switches in front bridge arm with half-bridge submodules (SMs), the proposed HSDAB scheme maintains the advantages of conventional DAB and obtains bidirectional dc fault-handling capability, reducing the application of large dc capacitors and dc circuit breaker; therefore, the volume and construction cost of DCSST can be effectively reduced. Besides, a high-frequency-link (HFL) voltage regulation strategy for HSDAB is also proposed in this article. The HFL voltages can be adjusted to match the HFL transformer voltage ratio during the whole operating range, reducing the current stress, HFL reactive power, and power loss of DCSST. Moreover, the proposed HSDAB scheme can also be applied as a basic circuit to establish multiple modular architecture DCSST, flexibly connecting low-voltage dc, medium-voltage dc, and high-voltage dc buses in the dc distribution network. The topology, operation principle, control structure, corresponding bidirectional dc fault handling, and HFL voltage regulation strategy of the proposed HSDAB scheme are investigated. The experimental results verify the correctness and effectiveness of the analysis and proposed scheme.

Seven‐level power conversion system for solar power generation system

This study proposes a seven-level power conversion system for a solar power generation system. This seven-level power conversion system consists of a DC–DC power converter and a cascade DC–AC inverter. The cascade DC–AC inverter comprises a full-bridge inverter and a T-type inverter. The T-type inverter generates a three-level quasi-square-wave voltage and the full-bridge inverter provides a three-level high-frequency pulse voltage. The amplitude of the three-level quasi-square-wave voltage is double that of the three-level high-frequency pulse voltage, so seven levels of AC voltage are synthesized. Only the full-bridge inverter with lower DC bus voltage uses high-frequency switching so switching losses are effectively reduced. The novelty of the proposed cascade DC–AC inverter is that the output power of T-type inverter is controlled by regulating the DC bus voltage, so no real power is supplied from the full-bridge inverter and an independent power source is not required to provide power to the full-bridge inverter. The negative terminal voltage for solar cell array keeps almost constant to reduce the leakage current of proposed seven-level power conversion system. A hardware prototype with a digital signal processor controller is developed to verify the performance of seven-level power conversion system for a solar power generation system.

Modeling and Voltage Control of Bidirectional Resonant DC/DC Converter for Application in Marine Power Systems

Abstract DC/DC converters have gained popularity in a number of industrial applications like electric vehicles or marine power systems, due to their high efficiency and power density levels. Pulse width modulation (PWM) and resonant converters are two main types of the DC/DC converters. Thereby, the resonant converters happen to be the preferred technology in the design of modern marine power systems since these converters are more suitable for the high and middle voltage DC applications. The resonant converters are, however, highly nonlinear systems, which limits the use of linear control methods. In this study, we propose a comprehensive analysis, modeling and control concept of a DC/DC resonant converter in marine power systems. First, a mathematical model of the DC/DC resonant converter in the so-called CLLC topology is derived based on the generalized state-space averaging method. The model is used to design a dual-loop voltage control, which aims to regulate the voltage level at the low-voltage DC bus of the resonant converter. The dual-loop voltage control consists of the primal linear controller, which directly regulates the voltage and the reference generator, which dynamically modifies the voltage reference of the primal controller. The major advantage of the suggested control concept is the improved performance of the simple controller without the need to substitute it as well as the possibility to realize, if required, a multi-rate control concept. Simulation studies under different load conditions show that the suggested modeling and control concept improves voltage control and the closed-loop system response.

Hot-Swappable Grid-Connected Multilevel Converter for Battery Energy Storage System

The objective of this brief is to propose a hot-swappable grid-connected multilevel converter (HGM-converter) for the battery energy storage system (BESS). The proposed HGM-converter is composed of series-connected bi-directional H-bridge sub-converters and a LCL filter. Advantages of the proposed HGM-converter include: hot-swappable capability, single-stage power conversion, low dc-bus voltage, and individual power control for each battery module. Therefore, the equalization, lifetime extension, and capacity flexibility of the BESS can be achieved. Based on the equivalent model, the power flow of the battery module can be estimated without the need of input current sensor. Also, with the interleaved operation between the sub-converters, the current ripple of the output inductor can be reduced too. The computer simulations and hardware experimental results are shown to verify the performance of the proposed HGM-converter.

A Unified Multimode Control of a DC–DC Interlinking Converter Integrated into a Hybrid Microgrid

DC–DC interlinking converters (ILCs) allow bidirectional energy exchange between DC buses of different voltage levels in microgrids. This paper introduces a multimode control approach of a half-bridge DC–DC converter interlinking an extra-low-voltage DC (ELVDC) bus of 48 VDC and a low-voltage DC (LVDC) bus of 240 VDC within a hybrid microgrid. By using the proposed control, the converter can transfer power between the buses when the other converters regulate them, or it can ensure the voltage regulation of one of the buses, this originating from its three operation modes. The proposed control scheme is very simple and provides a uniform system response despite the dependence of the converter dynamic on the operating point and the selected mode. Simulation and experimental results validated the theoretical development and demonstrated the usefulness of the proposed scheme.

DESIGN AND SIMULATION OF PHASE SHIFTED DC-DC FULL BRIDGE CONVERTER

Electric and hybrid electric vehicle (EV/HEV) architectures require a small DC-DC converter to replace any conventional vehicle’s alternator. The DC-DC converter, also described as the vehicle Auxiliary Power Module (APM), provides power flow between the vehicle high voltage (HV) and low voltage (LV) DC bus. This abstract represents the phase shifted full bridge DC-DC converter scheme to design, simulate DC/DC Converter for a power rating of 80W intended for low power/auxiliary applications in electric vehicles The proposed converter is very beneficial as it can be used for 4 quadrant operation much adaptive to the unknown parameters in the PV system and these unknown parameters are estimated through the adaptation laws in the algorithm which guarantees maximum power extraction possible from the power converter for a PV system. The converter supplies AC current as per requirement. The overall stability of the converter output is analyzed by simulation. The results are compared with other existing converter results for improving power quality even further. The proposed results indicate the robustness of the proposed scheme.

MMC-Based SRM Drives With Decentralized Battery Energy Storage System for Hybrid Electric Vehicles

This paper proposes a modular multilevel converter (MMC) based switched reluctance motor (SRM) drive with decentralized battery energy storage system for hybrid electric vehicle applications. In the proposed drive, a battery cell and a half-bridge converter is connected as a submodule (SM), and multiple SMs are connected together for the MMC. The modular full-bridge converter is employed to drive the motor. Flexible charging and discharging functions for each SM are obtained by controlling switches in SMs. Multiple working modes and functions are achieved. Compared to conventional and existing SRM drives, there are several advantages for the proposed topology. A lower dc-bus voltage can be flexibly achieved by selecting SM operation states, which can dramatically reduce the voltage stress on the switches. Multilevel phase voltage is obtained to improve the torque capability. Battery state-of-charge balance can be achieved by independently controlling each SM. Flexible fault-tolerance ability for battery cells is equipped. The battery can be flexibly charged under both running and standstill conditions. Furthermore, a completely modular structure is achieved by using standard half-bridge modules, which is beneficial for market mass production. Experiments carried out on a three-phase 12/8 SRM confirm the effectiveness of the proposed SRM drive.

Distributed coordination control of hybrid energy resources for power sharing in coupled hybrid DC/AC microgrid using paralleled IFCs/ILCs

This study proposes flexible controllers for the interlinking converter (ILC) and interfacing converters (IFCs) used in coupled hybrid AC/DC microgrids (HMGs). Proposed controllers are specifically designed for the multiple stacked bidirectional DC–AC ILCs/IFCs based microgrid outlays, to omit the droop power flow and system stability issues. The ILC and IFC grid supportive converter controllers focus on the wide-spread AC/DC bus parameters control for both DC and AC bus voltage regulation and superfluous power sharing while operating in the grid forming and feeding modes. Proposed controllers minimise the need for the controller parameter tuning as opposed to the conventional controllers used in zonal HMG systems. This makes the system stable for a much wider operating conditions as opposed to the widely used higher-order PLL integrated PQ and dq 0 control algorithms. The proposed HMG also integrates the centralised battery energy stack through bidirectional dual active bridge DC–DC converter for achieving the high-power transfer efficiency and omitting the isolation issues between medium-voltage and low-voltage DC buses. The HMG system performance is evaluated using the simulation studies for various strategical operational modes. Further, the proposed controllers have also been tested individually on experimental platform.

A Three-Port Converter Based Distributed DC Grid Connected PV System With Autonomous Output Voltage Sharing Control

A distributed dc grid connected photovoltaic (PV) generation configuration based on hybrid-connected three-port converters (TPCs) and its control strategy are proposed in this paper. Distributed maximum power point tracking (MPPT) and autonomous voltage sharing control are achieved with the proposed configuration and control strategy. The proposed system consists of multiple modular pulsewidth modulation plus phase-shift (PPS) controlled TPCs, which feature soft switching and low voltage stress. The input port of each TPC is connected to an independent PV energy source to achieve individual MPPT, and the output ports of these TPCs are connected in series to interface with a high-voltage (HV) dc bus, while the bidirectional ports are in parallel to build a low-voltage (LV) dc bus. The mismatch power of input sources can be transferred through the LV dc bus among these TPCs, and the power and voltage balancing at the HV output side can be realized. By regulating the voltage reference of a bidirectional port in a linear relationship with the output voltage, output voltage sharing is realized with only the module's own voltage and current being sensed. Fully modular design is achieved. System stability with the proposed control strategy is revealed by using the Routh–Hurwitz criterion. Simulation and experimental results are provided to verify the effectiveness and advantages of the proposed configuration and control strategy.

A single phase shift-based isolated bidirectional DC-DC converter for bidirectional energy transfer in DC-microgrid with identification of optimum operating zone

In the modern times, the pertinence of a single power electronics conversion system used in DC microgrids for facilitating energy transfer between energy generating systems and storage elements with inherent bidirectional capability has increased significantly. A dual active bridge (DAB) converter having two DC-AC converters connected back to back through a high frequency transformer is used as a power electronics conversion system for obtaining highly efficient bidirectional power conversion. In this paper, performance of DAB converter with single phase shift (SPS) is evaluated with solar photovoltaic cell on high voltage DC-bus and battery on the low voltage DC-bus. Linear load is connected in this system at the high voltage DC-bus. A step variation in load is applied and at this instant the power flows from the batteries to supply the increased load demand. Optimal operating range of converter along-with efficiency is obtained.

Fault Performance Comparison Study of a Dual Active Bridge (DAB) Converter and an Isolated Modular Multilevel DC/DC (iM2DC) Converter for Power Conversion Module Application in a Breaker-Less Shipboard MVDC System

A breaker-less shipboard medium-voltage dc (MVDC) system is gaining more attention recently due to its advantage in dealing with a dc fault by using switched-mode power modules and disconnectors. The specific power module investigated in this paper is the power conversion module (PCM) that transforms power from a MVDC to a low-voltage dc (LVDC) bus. The PCM plays an important role in a system-level fault management. The recently emerging isolated modular multilevel dc/dc converter and the widely used dual active bridge converter are promising solutions for PCM applications. The control algorithms including fault protection strategy for both topologies have been developed and presented in this paper. The MVDC side fault and LVDC side fault of each topology has been analyzed and evaluated. The simulation and experimental results are provided to demonstrate the fault protection features of two topologies in a breaker-less dc system.

Power management and operational planning of multiport HPCS for residential applications

Renewable energy integration to the power grid has seen a tremendous rise in last few years. A novel hybrid AC and DC bus layout (i.e. medium-voltage DC bus of 380 V and low-voltage DC bus of 48 or 12 V DC) for the residential consumer premises has been proposed in this study. This would enable the consumer to directly connect DC loads to the system, without the need of an individual power conversion device. This study summarises the strategical operational modes of the proposed hybrid power conditioning system (HPCS) for multiple distributed energy resources (DER) integrated residential premises located within urban/rural area. Modified HPCS converter control strategy has been proposed, such that the system operation is efficient and self-adaptive in both, grid forming (or, islanded) and feeding (or, grid connected) modes. System is configured by using a single power conditioning unit, capable of performing wide range of operations, such as, multiple DER interface, battery power management, grid power control and accessibility to AC and DC household loads at the desired voltage levels. Performance of the designed system has been tested via simulation and experimental studies.

A Double-ended Converter System with Two Different DC-buses Using Open Winding Permanent Magnet Machine for Traction Applications

In hybrid electric vehicles (HEVs), there are usually several different DC-buses of different voltage levels to handle different loads. DC–DC converters are used to transfer energy between different DC-buses. This paper proposed a double-ended converter for two different DC-buses with large voltage ratio. The high-voltage DC-bus is 600 V for the traction motor drive, while the low-voltage DC-bus is 48 V for the electronic devices on the vehicle. By using the proposed system configuration, energy could be transferred freely between the generator and the two loads, forming a very flexible powertrain. In addition, the weight, volume, and cost of power electronics components could be reduced. This paper introduced the operation principal and control algorithm of the proposed system. Simulations verified the functionality of the proposed system. The proposed system configuration is not only suitable for automotive applications, but can also be implemented in other applications, such as aerospace, marine, and industrial, where multiple DC-bus voltage levels are required.

Fault Detection and Isolation of a Loop Type Low Voltage DC Bus Microgrid

Fault Detection and Isolation of a Loop Type Low Voltage DC Bus Microgrid

DC power collection system for PV station based on impedance‐source multi‐module DC–DC step‐up converter

In this study, a novel structure of DC power collection system for photovoltaic (PV) station with consideration of impedance network concepts has been proposed to cope with technical challenges in large-scale DC PV Station. An impedance-source multi-module cascaded DC–DC step-up converter serves as a key device for the proposed collection grid, which has an integrated function of multi-maximum power point tracking (MPPT) control and high-gain DC voltage step-up conversion. The low-voltage DC bus and distributed MPPT devices can be omitted to make the system simple and reliable. The system structure is beneficial to realise internal fault isolation and unitary dynamic control at station level. The introduction of impedance network makes it possible to adjust the voltage gain flexibly as well as resist shoot-through of bridge arms, which can improve the operational adaptability against input power mismatch and reliability of the DC power collection grid for PV station. A detailed introduction to system architecture, key DC step-up converter, working principle and control/modulation strategy has been given in this study. All theoretical results have been validated by system simulation under balanced and mismatched power inputs. The proposed DC power collection grid for PV station has a potential application value in the future construction of large-scale DC PV stations.