Medium Voltage DC Distribution Research Articles

Power Generation Optimization for Next-Generation Cruise Ships with MVDC Architecture: A Dynamic Modeling and Simulation Approach

The cruise industry is obliged by economic and environmental initiatives to pursue fuel-efficient solutions and lower ship exhaust emissions. The medium voltage DC (MVDC) distribution with intelligent power management has become a concept for next-generation onboard power systems as its energy-saving feature is to eliminate the frequency constraint and simultaneously optimize engine loads and speed in response to load variations. The incentive for this transition lies on one hand in the fuel efficiency consideration and the reduction of power losses from serial conversion stages. On the other hand, the DC-based technology has been conceived as high-power density design, thus significantly increasing the payload. This study investigates such potential benefits focusing exclusively on large cruise vessels. A highly representative model of the integrated power platform that incorporates all dynamic interactions from the ship hull and essential machinery typically installed on board cruise ships is proposed. The power management strategy also takes account of actual sea conditions and real-time operation requirements. The simulation results demonstrate that the optimization-based MVDC system is able to maximize the opportunity of search agents in finding optimum fuel efficiency areas throughout the scenario time. An analysis of the system structure weight and space reduction of the MVDC architecture is also performed through the utilization of more compact electrical distribution devices and very high power-dense combustion turbines.

Continuous Time Simulation and System-Level Model of a MVDC Distribution Grid Including SST and MMC-Based AFE

Medium-voltage DC (MVDC) technology has gained increasing attention in recent years. Power electronics devices dominate these grids. Accurate simulation of such a grid, with detailed models of switching semiconductors, can quickly became very time-consuming, according to the number of connected devices to be simulated. A simulation approach based on interactions on a continuous time model can be very interesting, especially for developing a system-level control model of such a modern MVDC distribution grid. The aim of this paper is to present all the steps required for obtaining a continuous time modelling of a +/−10 kV MVDC grid case study, including a solid-state transformer (SST)- and modular multilevel converter (MMC)-based active front end (AFE). An additional aim of this paper is to supply educational content about the use of the continuous time simulation approach, thanks to a detailed description of the various devices modelled into the presented MVDC grid. The results of a certain number of simulation scenarios are eventually presented.

An Improved AC-Link Voltage Matching Control for the Multiport Modular Multilevel DC Transformer in MVDC Applications

In this paper, an improved AC-link voltage matching control (IVM) strategy is proposed for the multiport modular multilevel DC transformer (M3DCT), which comprises a single-phase modular multilevel converter (MMC) and a series of cascaded H-bridge units. The objective of the proposed IVM strategy is to address the AC-link voltage mismatch phenomenon. Distinct from existing control methods, such as various phase-shifting control methods and the conventional AC-link voltage matching control strategy, the proposed IVM strategy orchestrates the operation of the M3DCT in an innovative fashion. It allows the sum of inserted submodule (SM) numbers in the upper and lower arms to be flexible, no longer confined to a specific SM number per arm. Consequently, the AC-link voltage of the M3DCT is maintained proximate to the matched operating condition, regardless of the degree of mismatch in the DC side voltage of the M3DCT. This enables the enhancement of the M3DCT’s overall operational performance, particularly under conditions of light loads within medium-voltage DC (MVDC) distribution systems. The correctness and effectiveness of the proposed control strategy and the corresponding analysis are substantiated through simulation results.

Solid-State Circuit Breaker Based on Cascaded Normally-On SiC JFETs for Medium-Voltage DC Distribution Networks

Solid-State Circuit Breaker Based on Cascaded Normally-On SiC JFETs for Medium-Voltage DC Distribution Networks

A Projected Gradient Descent-Based Distributed Optimal Control Method of Medium-Voltage DC Distribution System Considering Line Loss

A Projected Gradient Descent-Based Distributed Optimal Control Method of Medium-Voltage DC Distribution System Considering Line Loss

Enhanced Fault Detection and Localization Strategy for High-Speed Protection in Medium-Voltage DC Distribution Networks Using Extended Kalman Filtering Algorithm

Enhanced Fault Detection and Localization Strategy for High-Speed Protection in Medium-Voltage DC Distribution Networks Using Extended Kalman Filtering Algorithm

Admittance Criterion of Medium-Voltage DC Distribution Power System and Corresponding Small Signal Stability Analysis

Aiming at the stability of a medium-voltage DC network based on a modular multilevel converter (MMC), this paper proposes an admittance stability criterion considering the influence of current-limiting inductors at the medium voltage side, which prevents the complex products and matrix calculations of traditional criteria. The DC admittance model DC transformers (DCTs) under different working modes are then established based on Thevenin/Norton equivalent circuit methods to analyze the stability of the DC system based on the proposed admittance stability criterion, which proves that the voltage resonance problem at the medium voltage side can be improved by adding active damping control strategies on DCTs also proves the effectiveness of the proposed stability criterion. The time-domain simulation and the hardware-in-loop simulation are then built in PLECS and RT Box to further verify the correctness of the system stability analysis and the effectiveness of the proposed admittance criterion, which provides a theoretical basis and technical reserve for the stable operation of the DC distribution power system.

Joint Planning Method of Virtual Power Plant for Distributed Generation and Grid in AC-DC Medium and Low Voltage Distribution Network

The high proportion of virtual power plants (VPPs) used for distributed generation may cause problems such as power fluctuations and voltage distribution. In contrast, AC/DC hybrid power grid has strong transmission capacity due to its power information transmission. However, Its power grid structure and the location and capacity of VPP have an impact on the consumption capacity of the system, and the relationship between them are closely coupled. In addition, Under different operation modes, power grids of different voltage levels also have great differences. Therefore, the cooperative planning of medium and low voltage AC and DC distribution networks can improve the transmission capacity of the power grid Further improve the ability of distribution network to consume distributed power, A method is proposed in this paper about siting and capacity determination and AC-DC distribution grid frame transformation planning for medium and low voltage systems. The upper-layer model mainly focuses on AC-DC transformation scheduling in the power grid, distributed node location in the access grid and transmission line transmission capacity, while the lower-layer model effectively communicates with the low-voltage system through information flow and changes the operation of the power grid to a certain extent.The results show that the distributed generation system can reduce the operation cost and improve the overall operation efficiency of the power grid.

An adaptive reclosing scheme for MVDC distribution networks with mechanical DCCBs

The DC distribution network is an effective solution to distributed renewable energy applications. After a fault occurs in the network, it is necessary to quickly identify and isolate it to protect system components. Subsequent to fault isolation by DC circuit breakers (DCCBs), power transmission should be swiftly restored by reclosing to improve system reliability. However, conventional reclosing schemes risk a second strike on the grids and endanger the safety of the networks. Given those drawbacks, an adaptive reclosing scheme for medium-voltage DC (MVDC) networks is proposed in this paper. An additional signal injection branch with a simple control strategy is appended to the existing commutation branch of a mechanical DCCB for reclosing. The fault nature can be identified by analyzing the magnitudes of one know frequency of the voltage signals injected into the faulty lines while fault locations under permanent faults can also be calculated before reclosing by applying this scheme. Compared with currently widely used methods, this scheme is characterized by its simplicity in structure and affordability in costs. Simulation results by PSCAD/EMTDC verify the efficacy and robustness of the proposed scheme.

A Three-Port PSFB/DAB-MMC PET With Inertia Enhancement Under LVDC Disturbance

DC distribution system has been popular in research and industry applications in recent years. The topology of the three-port modular multilevel converter (MMC) was proposed to interconnect the medium voltage DC (MVDC) distribution system, the medium voltage AC (MVAC) distribution system and the low voltage DC (LVDC). Compared to classical two-port MMC, a three-port MMC substitutes the tranditional submodules with bidirectional isolated converters. There are two kinds of existing three-port MMC topologies. One topology uses dual active bridge (DAB) converters to transfer power from the medium voltage side to the LVDC system, which is called DAB-MMC. The other topology utilizes bidirectional phase shift full bridge (PSFB) converters as submodules, which is called isolated MMC (I-MMC). Compared to the DAB-MMC, the I-MMC reduces the cost of the submodule capacitors and the power switches. But the LVDC inertia of the I-MMC is low due to the common submodule capacitors, especially during disturbances. In this article, a hybrid three-port MMC topology with inertia enhancement is proposed. With the combination of the PSFB and DAB submodules, the proposed MMC increases the equivalent inertia of the LVDC system using a novel virtual control method. The simulation results in MATLAB/Simulink and real-time RT-LAB prove the feasibility and the accuracy of the proposed inertia enhancing method.

High speed protection of medium voltage DC distribution system using modified mathematical morphology

One of the challenging issues in the realization of medium-voltage DC (MVDC) distribution system is high-speed protection scheme design against quickly rising short circuit fault-DC current. This paper proposes a high-speed protection scheme based on the processing of energy signals through mathematical morphology (MM). The voltage and current signals measured at a distribution line section (DLS) are used to obtain the energy signal. The proposed scheme develops a modified MM-based filter to extract the transient information in the energy signal rapidly and distinctly in the form of positive/negative polarity. The polarity detection of any disturbance in the analysed energy signal indicates the occurrence of a sudden disturbance in the system. The sudden disturbance occurrence confirmation triggers the faulty DLS identification and classification algorithms of the proposed scheme. The faulty DLS identification and classification algorithms are based on the detected polarities of the energy signal. The protection scheme requires low bandwidth communication. The proposed protection scheme is evaluated by using a ±2.5 kV radial distribution network containing 4 feeders under different fault conditions in MATLAB/SIMULINK software. Simulation results verify that the proposed scheme can isolate the faulty portion within a few milliseconds after fault inception under a variety of fault cases.

An improved energy management for MVDC distribution system based on exponential droop control

The energy management optimization and DC voltage stability are the main challenges when loads surging and renewable energy sources (RESs) fluctuations occur in the medium-voltage DC distribution system (MVDC-DS). To solve these issues, this paper proposes a dual-time scale energy management method based on improved droop control for the MVDC-DS. Firstly, a ±10 kV three-terminal hybrid modular multilevel converter (MMC)-based ring MVDC-DS with DC fault ride-through capability is constructed, which including electric vehicle charging stations, energy storage systems, multiple loads and RESs. Secondly, the droop control adopted by the hybrid MMC is improved by exp-function to achieve higher power quality. On this basis, a dual-time scale energy management method is proposed to minimize the electricity purchasing cost. The reference power of each controllable unit is optimized in the long time scale, and the parameters of improved droop control are optimized in the short time scale. Finally, a case study is conducted on the ±10 kV three-terminal hybrid MMC-based ring MVDC-DS, and the results indicate that the proposed method can improve the economy and power quality of system, and help to realize the normal system operation under different conditions.

Research of metering scheme key techniques for medium and low voltage DC distribution network

DC power grid has received increasing attentions recently, and with it comes the demand for highly reliable and stable metering solutions. At present, in the DC distribution projects that have been implemented, some DC metering devices have been put into use. However, from the national and industrial level, there is no clear standard for the design of the metering scheme for the medium and low-voltage DC distribution network, and there is a situation of imperfect metering device configuration. Therefore, based on the analysis of DC metering devices, algorithms and location selection, this paper takes the Hangzhou Jiangdong flexible DC distribution network as an example to analyze the error distribution caused by metering algorithm selection and device configuration. In addition, the influence of harmonics on DC measurement is analyzed to provide the suggestions for DC measurement solutions. The analysis has been verified by MATLAB/Simulink, which provides references for DC metering standards.

A Constant-Frequency High-Voltage Gain Resonant Converter Module With Semiactive Phase-Shifted Voltage Multiplier for MVdc Distribution

A constant-frequency step-up LLC resonant converter module that employs a new phase-shifted semiactive voltage multiplier technique is proposed in this article for a medium-voltage dc distribution grid. In the proposed converter, a wide voltage gain for different loading conditions can be realized by adaptively controlling the phase shift of the auxiliary switch in a voltage quadrupler-based semiactive voltage multiplier. Zero-voltage switching turn-on is always achieved for all the primary side switches with a constant-frequency operation. In the proposed semiactive voltage multiplier, zero-current switching (ZCS) turn-on is achieved for the auxiliary switch with ZCS operations provided for all the output diodes over a wide range of voltage gain and loading conditions. The operating principles of the proposed circuit are discussed in this article. Simulation results on a 120-kW, 10-kV output and experimental results on a modular laboratory-scale silicon carbide (SiC)-based 5-kW (per-module), 240–440-V/5-kV prototype are provided to demonstrate the performance of the proposed converter.

A Surgeless Diode-Clamped Multilevel Solid-State Circuit Breaker for Medium-Voltage DC Distribution Systems

In conventional medium-voltage solid-state circuit breakers (SSCBs), series-connected semiconductor switches are used with metal oxide varistors (MOVs) in parallel with each of them to extend the voltage ratings of the breakers without incurring the dynamic voltage unbalance. Besides, for isolating the faults at faster speeds, the clamped surge voltages of the MOVs are always much higher than the system voltages. For those reasons, the voltage ratings of all the switches in the SSCBs have to be overdesigned according to the surge voltage values, which causes high material costs along with nonideal conduction losses in the SSCBs employed for medium-voltage dc distribution systems. To solve this dilemma, a surgeless diode-clamped multilevel SSCB is proposed in this article. It offers an ultrafast isolation speed with no significant surge voltages or dynamic unbalanced voltage across the semiconductor switches. With these attributes, the proposed SSCB can potentially use the semiconductor switches rated at lower voltages compared to conventional SSCBs and have higher efficiency along with faster isolation speed. The simulation and experimental results are presented to validate the technical feasibility and practical values of the proposed SSCB.

A Sharing-Branch Modular Multilevel DC Transformer With Wide Voltage Range Regulation for DC Distribution Grids

As the key equipment in dc distribution grids, dc transformer (DCT) suffers from low power density caused by a large number of submodules (SMs). Hence, this article proposes a novel DCT for the interconnection of medium-voltage dc (MVdc) and low-voltage dc (LVdc) distribution grids, which adopts a series-connected half-bridge SM branch (SSB) on MV-side to achieve low voltage stress of devices and reduce the number of SMs. Quasi square wave modulation strategy and phase-shifted control are employed to realize bidirectional power transmission control and zero-voltage switching (ZVS) for all switches. Furthermore, in order to ensure low current stress and ZVS within wide voltage range, a certain number of SMs are constantly inserted or bypassed to achieve voltage matching. Besides, smooth transition control strategy is employed to avoid current spikes caused by abruptly bypassing or inserting SMs. Based on the theoretical analysis, design consideration and control strategy are elaborated. Finally, a 500 kW/6–12 kV/750 V simulation model and a 2 kW prototype are built to verify the proposed DCT.

Integrated Coordination Improvement of Droop Control for Medium and Low Voltage DC Distribution Network

In view of the traditional droop control, when the DC distribution network is disturbed or faulted, the transient voltage stability of the DC distribution network is poor due to the nonlinearity of the system and the fixed droop coefficient. The actual operation of the converter station margin cannot be considered, which is prone to full load problem. In order to solve the above problem, firstly, we propose an adaptive comprehensive coordinated improvement droop control strategy, including an outer loop adaptive droop control part which considers the power margin and voltage deviation compensation respectively, as well as an inner loop current adaptive discrete sliding mode control part. Moreover, the power margin and voltage deviation of each converter station are considered comprehensively, through a real‐time adaptive adjustment of the droop coefficient, to reasonably distribute the unbalanced power within the DC grid after disturbance and reduce the deviation of the DC voltage. Finally, we use a four‐terminal medium and low voltage DC distribution of the system to attain the simulation results as an example which verifies the effectiveness and correctness of the proposed control strategy. © 2022 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.

Input-Series-Output-Parallel DC Transformer Impedance Modeling and Phase Reshaping for Rapid Stabilization of MVDC Distribution Systems

This paper focuses on the instability problem of the medium-voltage DC (MVDC) distribution system and proposes an impedance phase reshaping (IPR) method. To obtain the load impedance model of the MVDC distribution system, the input impedance of the input-series-output-parallel (ISOP) DC transformer (DCT) is derived by the generalized average modeling (GAM). Based on the obtained model, the traditional ISOP DCT controller optimization (IDCO) approach is discussed and the IPR method is developed. An impedance phase controller is introduced based on the original control method. According to the optimized impedance stability criterion, the parameters of the impedance phase controller are determined. Compared with the IDCO approach, the proposed method weakens the negative resistance characteristic of the load impedance at the resonant frequency. Therefore, the MV bus voltage oscillation is rapidly mitigated. Besides, the dynamic performance of the system using the IPR method can be classified as good. The simulation results show that the mathematical model is correct, and the proposed method is effective for the rapid stabilization of MVDC distribution systems.

Evaluation of high step-up power conversion systems for large-capacity photovoltaic generation integrated into medium voltage DC grids

With the increase of dc based renewable energy generation and dc loads, the medium voltage dc (MVDC) distribution network is becoming a promising option for more efficient system integration. In particular, large-capacity photovoltaic (PV)-based power generation is growing rapidly, and a corresponding power conversion system is critical to integrate these large PV systems into MVDC power grid. Different from traditional ac grid-connected converters, the converter system for dc grid interfaced PV system requires large-capacity dc conversion over a wide range of ultra-high voltage step-up ratios. This is an important issue, yet received limited research so far. In this paper, a thorough study of dc-dc conversion system for a medium-voltage dc grid-connected PV system is conducted. The required structural features for such a conversion system are first discussed. Based on these features, the conversion system is classified into four categories by series-parallel connection scheme of power modules. Then two existing conversion system configurations as well as a proposed solution are compared in terms of input/output performance, conversion efficiency, modulation method, control complexity, power density, reliability, and hardware cost. In-depth analysis is carried out to select the most suitable conversion systems in various application scenarios.

High‐frequency‐link DC solid state transformer based on voltage self‐balancing control for MVDC power distribution application

This paper presents a novel voltage self-balancing converter (VBC) applied to DC solid-state transformer (DCSST), analyzes the operating principle of the proposed VBC, and proves the voltage balance performance. Based on this converter, a novel DCSST topology is proposed, which can be used in medium and low voltage DC distribution application, microgrid and medium and low voltage energy storage system. Compared with the previous work, under the same voltage level, it can effectively reduce the number of transformers and switches, reduce the voltage level of switches, and realize the soft switching of the VBC switches, so as to improve the system efficiency and power density, and then improve the system reliability. Meanwhile, the fault handling ability and fault tolerance ability of DCSST are improved, which makes DCSST more reliable when multiple isolated bi-directional DC/DC Converters (IBDC) are connected in series. In addition, with this topology, the input voltage self-balancing control strategy of the IBDC is not needed because of the voltage self-balancing characteristic of VBC, which simplifies the control complexity and greatly improves the control accuracy of the control system. Experimental results agree well with the accuracy and correctness of the proposed topology.