Medium Voltage Direct Current System Research Articles

Spacecraft Medium Voltage Direct-Current (MVDC) Power and Propulsion System

This paper introduces a medium voltage direct-current (MVDC) system for large spacecraft megawatt-scale (MW) power and propulsion systems intended for interplanetary transport, including missions to the Moon and Mars. The proposed MVDC system includes: (i) A nuclear electric propulsion (NEP) that powers a permanent magnet (PM) generator whose output is rectified and connected to the MVDC bus. (ii) A solar photovoltaic (PV) source that is interfaced to the MVDC bus using a unidirectional boost DC-DC converter. (iii) A backup battery energy storage system (BESS) that connects to the MVDC bus using a bidirectional DC-DC boost converter. (iv) A dual active bridge (DAB) converter that controls the power to the spacecraft’s electric thruster. The NEP serves as the main power source for the spacecraft’s electric thruster, while the solar PV and BESS are intended to provide power for the payload and spacecraft’s low-voltage power system. The paper will (i) provide a review of the spacecraft MVDC power and prolusion system highlighting state-of-the-art main components, (ii) address the control of boost converters for the PV and BESS sources and the DAB converter for the thruster, and (iii) propose an uncertainty and disturbance estimator (UDE) concept based on current control algorithms to mitigate MVDC instability due to unpredictable factors and external disruptions. The proposed UDE can actively estimate and compensate for the system disturbance and uncertainty in real time, and thus, both the system tracking performance and robustness can be improved. Simulation studies have been conducted to substantiate the efficacy of the proposed schemes.

Research on fast frequency response to AC grid of a battery‐free MVDC system incorporating a high proportion of distributed photovoltaic generation units with reserve capacity

AbstractWith the fast development of renewable energy, system operators intend to require renewable energy generation to have more grid support capabilities. This paper studies the grid‐connected control strategy of a battery‐free medium‐voltage direct current (MVDC) system with distributed photovoltaic generation units (DPVGUs), aiming to realize the rapid grid support function to AC grid frequency. Firstly, a scheme for the MVDC system can keep some reserve capacity is provided. At this moment, DPVGUs execute a master‐slave coordinated control strategy, that is, the master DPVGU adopts an improved DC constant voltage control strategy to maintain the MVDC bus voltage, and the remaining DPVGUs adopt an improved droop control strategy to quickly provide corresponding power. Secondly, when the frequency deviation of AC grid exceeds the deadband of primary frequency control, fast frequency response (FFR) control strategy of the modular multilevel converter is proposed, aiming to actively transmit some or all of the reserved power of the MVDC system to the AC grid and can quickly provide frequency support to AC grid. In FFR control strategy, an additional control strategy is added on the vector control strategy. Finally, simulation studies are carried out in RTDS to verify the effectiveness of the proposed control scheme.

Medium Voltage Direct Current Distribution System for an Electric Vehicle Fast Charging Park

There is an increasing shift towards the electrification of automobiles to meet zero-emission standards set by many nations. As electric vehicles become more common, their power demand on the power system becomes greater. A substantial modernization or upgrade of the current distribution power grid is required to meet such demand. Since most Level 3 fast chargers utilize DC power, medium voltage direct current (MVDC) provides a feasible alternative to the present AC distribution infrastructure. This study proposes an MVDC distribution model for powering a large EV park consisting of 40 EV charging stations with a 9.6-MW total power demand. Calculation and simulation are used to evaluate the model and compare it with an equivalent MVAC system. The outcomes show that implementing an MVDC distribution system is an efficient approach to meeting the increasing power demand for electric vehicles. The proposed 40-kV MVDC system power loss (13.1kW) is six times lower than that of the equivalent MVAC system (89.74kW). Further, since MVDC systems do not require AC step-down transformers and AC/DC converters at the equipment end, they can be a lower-cost option for powering large EV charging parks. The findings help enhance EV charging infrastructure, which expedites the adoption of EVs for reducing carbon emissions in the transportation sector.

Parameter Optimization Design of the Commutation Circuit of a Hybrid DC-Current-Limiting Circuit Breaker

Aimed at the optimization design of the parameters of the commutation circuit of a hybrid DC-current-limiting circuit breaker (HDCCLCB), a parameter selection model considering the short-time withstand of the thyristor and the volume of the commutation circuit is proposed by simplifying the object, and the commutation circuit parameters were preliminarily obtained. In order to verify the correctness of the method of selecting the commutation circuit parameters, the circuit simulation model of the HDCCLCB was built. The experimental platform was built, and the breaking experiment was completed under the fault condition with the current rising rate of 20 A/μs. The correctness of the simulation model and parameter design method was verified by comparing the circuit model simulation results with the experimental results. In order to further optimize the parameters of the commutation circuit, a mathematical model for optimization design was established. Taking the maximum critical breakdown voltage under unit capacitance energy as the objective function, the arcing time before commutating and the current-limiting inductance and capacitance are enumerated to obtain the optimized commutation circuit parameters. By comparing this result with the result of the preliminary design, the objective function is improved by 20.7%, laying a solid foundation for further research and the development of current-limiting circuit breakers for medium-voltage DC systems.

Analysis and Modeling of Self-Powered Cascaded Resonant Switched Capacitor Power Supply for Medium-Voltage DC Systems

Analysis and Modeling of Self-Powered Cascaded Resonant Switched Capacitor Power Supply for Medium-Voltage DC Systems

Analysis and Design of the High Current Rising Rate Hybrid DC Current Limiting Circuit Breaker

To solve the problem of the high rising rate and large peak value of the expected current of the short-circuit current in marine DC power system faults, a hybrid DC current limiting circuit breaker scheme based on a high-speed electromagnetic repulsion mechanism is proposed. A parameter selection model is constructed by comprehensively considering the short-time withstand of the thyristor, the volume of the commutation circuit, and capacitor energy, and the optimal value of the commutation circuit parameters at a certain voltage level is obtained. The finite element mathematical model of the high-speed electromagnetic repulsion mechanism is established by coupling the electromagnetic force field, which enables the deformation process of the mechanism under the condition of high acceleration to be considered. The von Mises yield criterion is adopted as the mechanical boundary condition in the design of a high-speed electromagnetic repulsion mechanism, which solves the problem of the long inherent time of opening. The experiment platform is built, and the experiment under the fault condition with a current rising rate of 20 A/μs is completed. The arcing time, commutation time, zero-voltage recovery time, and contact movement characteristics are obtained, which meet the design requirements, verify the effectiveness of the analysis, and lay a solid foundation for further research and development of the current limiting circuit breakers for medium voltage DC systems.

Thyristor-Based DCCB With Reliable Fast Reclosing Protection Ability by Restoring Capacitor Polarity

Lots of dc circuit breakers have been proposed for protecting medium voltage dc (MVDC) systems. However, they normally have one or more following limitations: low operational reliability, lack of controllability, lack of reclosing protection ability, power supply required, and high construction cost. To overcome these problems, a novel thyristor-based dc circuit breaker (TCB) is proposed in this paper. The proposed TCB could break the bi-directional current actively and reliably. In the reclosing process, the polarity of capacitor in the counter current loop could restore automatically to the initial state, and the auxiliary thyristor could be turned <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> naturally. As a result, the proposed TCB guarantees reliable fast reclosing protection ability on the premise of no additional pre-charge power supply. Moreover, all thyristors of the proposed TCB could be turned <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> by voltage drop of the anti-parallel diodes, and the overvoltage problem caused by reverse recovery process of thyristors is eliminated, which further enhances the reliability of TCB. The detailed topology introduction, parameters designing guidelines, scaled-down experimental results, and comparative study of the proposed TCB are all presented.

A Multiport Current Flow Controller for Meshed Multiterminal DC Grids

In the meshed multiterminal dc (MTDC) grid, the individual line currents cannot be controlled precisely without additional equipment. To address this issue, the current flow controllers (CFCs) that can regulate the line current and avoid line overloads are adopted. A new multiport current flow controller topology with independent regulation capability has been proposed in this article. This CFC topology is composed of the main circuit and auxiliary circuit, which are connected via the coupled capacitors. The proposed CFC topology can regulate one or more line currents in the MTDC grids and can realize line currents regulation independently. Furthermore, it can be applied in medium voltage or high voltage dc systems due to the modular structure. The power losses of the proposed CFC are minimized because the auxiliary circuit transmits small power and operates in resonant mode. Meanwhile, the operation principle and the control strategy of the proposed CFC topology are illustrated in this article. The comparison with other CFC topologies previously proposed proves the advantages of the proposed topology. Simulations performed in MATLAB/Simulink and experiments based on a down-scaled prototype are completed to verify the operation principle and control strategy.

A Novel Oscillating-Commutation Solid-State DC Breaker Based on Compound IGCTs

Medium voltage dc system has fast rise rate in the event of a short-circuit fault. To avoid high-peak fault current, it is necessary to configure dc circuit breaker with fast operation. Solid-state dc circuit breaker has short breaking time and high reliability, which is suitable for Dc system. In this letter, a novel oscillating-commutation solid-state dc circuit breaker based on compound integrated gate-commutated thyristors (IGCTs) is proposed. By combining IGCTs with different advantages, such as high current breaking and high withstand-voltage capability, the proposed dc breaker has the advantages at less breaking time, high performance, low cost, and small volume. The working principle of the proposed scheme is described in detail, and an 8 kV prototype with 7 kA current breaking ability is built to verify its feasibility.

Buck-Boost Type High Step-Down Low Power Modular Converter for Medium Voltage DC Systems

The medium-voltage dc (MVDC) system are intensely investigated by both industry and academy. However, how to realize a lower power circuit directly fed by MVDC is an important but easily neglected problem, which seriously affects the reliability of the dc system. The current high step-down circuits are mostly suitable for high-power applications, while the circuits for low-power applications are designed case by case. To address the obstacle, a generalized buck-boost type high step-down converter (BHSC) structure is proposed in this article. The submodules of BHSC are derived from basic dc–dc converter. As a result, the features of high modularity, simple structure, and low cost can be obtained, and each submodule operates independently without communication and synchronization to achieve voltage balance. Furthermore, a submodule strategy with Bang-Bang control, instead of hardware modification, is proposed to overcome the limitation of BHSC generality in medium-voltage (MV) and high-voltage (HV) applications, due to the operating current difference in different submodules of the cascaded structure. Therefore, BHSC can be easily extended to MV or HV by increasing the number of submodules. Finally, an experimental evaluation of a 1 kV/80 W prototype converter is presented, which can be applied as auxiliary power supply for high-power converter.

A design optimisation tool to minimise volume and failure rate of the modular multilevel converter and the thyristor-controlled rectifier

The unfolding of MVDC (Medium Voltage DC) systems has the prospects to enable the incorporation of power electronic converters with higher power density and reliability. A tool with an integrated design approach is required to minimise the overall system volume by identifying optimal components. In this paper, a component-level early-stage design tool has been developed to attain the minimum achievable volume and failure rate for MVDC power converters. The developed tool optimises the choice of semiconductor switching devices, required heatsink, and other passive components (including dc-link filters and inductors) to minimise failure rate and overall converter volume. The optimisation algorithm employs the non-dominated sorting genetic algorithm (NSGA-II) to evaluate designs based on developed fitness functions. The design tool demonstrates the trade-off when evaluating multiple converter topologies and helps make an informed decision. A comparative study between two converter topologies shows the outcomes in terms of targeted metrics (volume and failure rate). This tool is expected to benefit early-stage design to perform trade-off studies among power electronic converter topologies based on key metrics like volume and failure rate.

Interconnecting Power-Electronic Buck Converter Modules in a Novel High-Power Test Bench for MVDC Circuit Breakers

Advances in medium voltage direct current (MVDC) technologies and the penetration of extended MVDC systems are still significantly hindered by the lack of adequate direct current (DC) switching equipment. The fundamentally different fault current behavior in case of a DC fault, compared to faults in alternating current (AC) systems, with regard to the characteristics and development of fault currents and their interruption make dedicated test procedures necessary. One testing approach is the application of a power-electronic buck converter (PEBC) to simulate relevant stresses on DC switching equipment during a DC fault current interruption. Since the associated requirements, especially regarding current ratings of several kiloamperes, cannot be fulfilled by using a singular PEBC, a modularization becomes necessary. However, particularly in high-power applications, the interconnection of several PEBC modules poses significant challenges. In this article, a demonstrator PEBC-based high-power test circuit for the provision of relevant testing parameters is presented. The underlying challenges and respective solutions with regard to the interconnection of, in total, 120 individual PEBC modules are discussed. It can be shown that the harmonization of connection busbar inductances is the main contributor towards a stable and safe test circuit operation.

Coordinated constant voltage control strategies of a battery‐free medium‐voltage direct current system incorporating distributed photovoltaic generation units

AbstractTraditional distributed photovoltaic generation units (DPVGUs) with maximum power point tracking (MPPT) control strategies are generally regarded as current sources without voltage regulation capability. Due to the lack of voltage regulation capability of DPVGUs, this paper proposes two control strategies to realise the voltage regulation capability of a battery‐free medium‐voltage DC (MVDC) system incorporating DPVGUs in the off‐grid mode. A primary control strategy used on an individual DPVGU is proposed first, which is called an improved DC constant voltage control (CVC) strategy. Under this strategy, the maximum dynamic output characteristics of a photovoltaic (PV) panel and the boost capability of a DC/DC boost converter can be fully utilised to control the MVDC system bus voltage. Secondly, the sequential coordinated constant voltage control (SCCVC) strategy is then proposed on the condition that the limitation of the voltage regulation capability of an individual DPVGU is reached. Under the SCCVC strategy, according to the sequence of the voltage regulation capability of the DPVGUs from large to small, the DPVGUs take turns to operate in the improved DC CVC strategy to realise the voltage control function. Furthermore, simulations using the RTDS are conducted to verify the effectiveness of the proposed control strategies in actual applications.

Parameter Matching and Structural Optimization Design of H-SFCL in MMC Ship MVDC System

As a high-density and high-load system, ship DC system has the characteristics of short power supply line, low system damping. When a short-circuit fault occurs, it will cause serious damage and the direct current circuit breaker (DCCB) is required to cut off the fault current quickly. In our previous paper, a hybrid-type superconducting fault current limiter (H-SFCL) has been proposed, which can help DCCB reliably remove faults and realize fast reclosing. On this basis, the H-SFCL is selected and applied in ship medium voltage direct current (MVDC) system. In this paper, not only the current limiting principle of H-SFCL is introduced, but also its current limiting characteristics in modular multilevel converter (MMC) ship MVDC system are simulated and analyzed. In addition, the paraments selection and coil structural design of H-SFCL are optimized to improve the economy and ensure the safety and stability of the coil. Firstly, the parameter matching method is proposed based on the optimization algorithm in Matlab, and then the electromagnetic force coupling model in COMSOL is built to complete the structural optimization of the superconducting coil. The effectiveness of the optimization results is verified by stress analysis.

A study on technical specifications of voltage and current measurement apparatuses for [formula omitted]10 kV MVDC systems

The medium-voltage direct-current (MVDC) electrical energy system has attracted increasing attention because of its high energy density and low carbon potential. Accurately characterizing, measuring, and monitoring the DC voltage and current are crucial for the control, protection, and energy metering of the MVDC system. Nonetheless, so far, neither industry nor academia has reached no consensus on the technical specifications of voltage and current measurement apparatuses for the MVDC systems. This paper analyzes the technical specifications of voltage and current measurement apparatuses for typical ±10kV MVDC systems considering the systems requirements and compares the adaption of the voltage and current sensors. Typical two-terminal and four-terminal MVDC system simulation models are constructed. The events that would generate the worst transient waveforms are modeled respectively to figure out the technical specifications for the control, protection, and metering systems. The closed-loop tunnel magnetoresistance (TMR) based current sensor and the resistors-based voltage divider are proposed as the most suitable apparatuses for MVDC system measurements. The research results can guide the research of sensors for MVDC systems and provide references for engineering applications.

Unit Protection of Tapped Line DC Microgrid

DC microgrids are attractive options for being more efficient and less complex than ac microgrids. Several protection methods exist for ring main and radial dc microgrid architecture, but the development of protection techniques for tapped line-based dc microgrids is still in the beginning stage. Usually, the loads and distributed energy resources (DERs) are tapped at any section of the line in the microgrid. When there is no communication link from the tapped line to any bus in the microgrid, the existing unit protection schemes fail to identify the fault in the tapped line-based medium-voltage direct current (MVDC) microgrid. In that case, the existing unit protection scheme fails to identify the fault in the tapped line-based MVDC microgrid. Therefore, this article proposes a new unit protection scheme for the tapped line MVDC system based on the superimposed resistance. The proposed method identifies the tapping status using the prefault current data. The technique is robust during load changes and power generation variation at the tapping station. The proposed scheme is validated using a two-bus tapped dc microgrid for distinct fault scenarios like high-resistance fault, varying fault location, and different modes of distributed generation operation under MATLAB/Simulink environment. Also, the performance of the proposed scheme is verified on a lab-implemented dc microgrid hardware bed and tested for different types of fault scenarios.

Modelling and Control Development of a Cascaded NPC-Based MVDC Converter for Harmonic Analysis Studies in Power Distribution Networks

Today’s power distribution networks are predicted to incorporate more Power Electronics (PE)-based power conversion systems, widely acknowledged as harmonic sources. Concerns about power harmonic severity in the distribution networks can arise, especially with the growing numbers of Medium Voltage Direct Current (MVDC) systems, which are also facilitated by such power converters. Yet, an accurate harmonic model of the MVDC converter is required to investigate its harmonic emissions, propagations, effects, and solutions in today’s distribution networks. This article is devoted to the development of a detailed model of a cascaded Neutral Point-Clamped (NPC)-based MVDC converter for accurate harmonic analysis studies. An appropriate control system with a simple Proportional Integral (PI) controller tuned using the loop-shaping technique is developed. An interleaved Sinusoidal Pulse-Width Modulation (SPWM) scheme aiming to improve the harmonic performance of such an application is introduced. The detailed model of the MVDC system was developed using the Simulink/MATLAB simulation environment, for which the concept of operation was validated, control system performance was investigated, and the effectiveness of the harmonic reduction method was analysed. The key finding is that the interleaved SPWM technique has significantly reduced the Total Harmonic Distortion (THD) to 2% with no significant even-order harmonic components in comparison to the reported models.

Multiobjective Optimization Based Framework for Early Stage Design of Modular Multilevel Converter for All-Electric Ship Application

The Medium Voltage DC (MVDC) architecture for All Electric Ships (AES) has the potential to provide superior features compared to traditional 60-HZ AC distribution systems in terms of power density, power quality, and system stability. The MVDC system introduces extensive use of power electronics equipment into the shipboard power system (SPS) design that brings complexity to the system design. These power electronics equipment connect the power sources and load centers to the MVDC bus and play a major role in handling system faults. This paper focuses on developing a framework to determine the volume and failure rate of a Modular Multilevel Converter (MMC) for early stage ship design. Two different methodologies (Taguchi method and a genetic algorithm) were used to determine the best design from a robust set of design options. Once the design parameters have been identified, the Taguchi method forms orthogonal array to explore and evaluate designs. At the end of the design cycle, it identifies the best parameters from a large set of design parameters to achieve lower volume and failure rate. These parameters are used as input to the optimization process. This helps to narrow out the number of inputs for optimization algorithm. The Nondominated Sorting Genetic Algorithm II (NSGA-II) has been integrated with converter design tool to minimize the volume and failure rate of MMC. The results show that the optimization algorithm coupled with Taguchi Method provides the lowest volume and failure rate for MMC. One of the goals of early-stage ship design is to develop preliminary design and evaluation of trade space to narrow it down. This paper is expected to aid early-stage ship design of power electronics converter design for MVDC systems in SPS.

Mechanism of Power Quality Deterioration Caused by Multiple Load Converters for the MVDC System

Medium-voltage direct current (MVDC) systems are widely used to ship power-distributed systems, wind farms, and photovoltaic power plants. With the increase of load converters interfacing into the MVDC system, the power quality deteriorates. Few research studies focused on the factors affecting the MVDC power quality, and effects caused by multiple load converters are often neglected. In this study, the mechanism of power quality deterioration caused by interfacing multiple load converters on the MVDC system has been discussed. The impedance model of the MVDC system is developed with the state-space averaging method and the small-signal analysis method. A three-level H-bridge DC/DC converter is employed as the load converter. The results by the analysis of the impedance model show that the more the load converters connect to the MVDC system, the more fragile the MVDC system is to background harmonics. Simulation cases are implemented to verify this conclusion.

LC DC Circuit Breaker based on Pre-charged Capacitor Commutation

To overcome limitations of the peak DC fault current with previous LC DC Circuit Breakers, this paper studies using a pre-charged capacitor in the auxiliary branch which enables rapid commutation of the fault current from the disconnector in the main branch. The analysis on the theoretical model concludes that the pre-charged capacitor should ideally have high capacitance and low voltage. The design for the practical test case of 5 kV, 2 kA LC DC Circuit Breaker is presented and shows that the pre-charged capacitor voltage requirement is low, at 10% of the DC CB rating. Both, film and electrolytic (lower size and cost) pre-charged capacitor are considered, and the experimental tests demonstrate successful DC current commutation of 2 kA at 5 kV. Successful breaking is also demonstrated on hardware for negative DC fault current and for very low currents, which represent difficult operating conditions. The experimental study further evaluates robustness to the change of timing of the current injection and the pre-charged capacitor voltage. The mechanical LC DC CB commutates DC fault current in very short time of 320 µs, and the use of pre-charged capacitor makes this topology good candidate for applications in medium voltage DC systems.