Medium Voltage Direct Current System Research Articles

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.

Study on Overvoltage Rise Rate Suppression Method for DC Interruption

DC circuit breaker is the key equipment for control and protection in the medium and high voltage DC system. Mechanical DC circuit breakers have transient overvoltage in breaking, which may lead to re-breakdown of the breaker or even breaking failure in severe cases. In this paper, the transient overvoltage in the breaking process of typical mechanical DC circuit breakers is studied. An improved topology which can supress the transient voltage is propose. The effect of different capacitance and resistance parameters on weakening the transient voltage is investigated, and the adaptability of the proposed method to different breaking currents is studied.

Impact of MV Standards on Shipboard DC Cable System Size

There is rising interest in medium-voltage direct current (MVDC) power systems for several reasons, including compatibility with DC loads and avoidance of alternating current (AC) frequency synchronization issues when combining multiple source outputs [1]. However, few MVDC systems are currently in operation, and there is relatively little experience compared to the knowledge available for medium-voltage alternating current (MVAC) systems. Presently, IEEE Std. 1709 is the only standard that provides guidance for MVDC systems [2]. This work’s objective is to harmonize the information within MVAC standards to produce prospective design and test values for shipboard MVDC cables and evaluate their impact on cable system sizes. Standards for MVAC cable design and test parameters including Basic Lightning Impulse Insulation Level, Withstand Voltage, and cable insulation thickness are compiled to understand the range of parameter values deemed acceptable for MVAC cables. Based on existing shipboard MVAC standards and IEEE Std. 1709, this study produces prospective design and test values for a shipboard MVDC system with a 12 kV Nominal System Voltage. Additionally, a tradeoff was identified with the thickness of cable insulation where a small reduction in cable system size can be achieved but at the expense of substantially greater electric stresses within the insulation.

Comparative Assessment of Multi-Port MMCs for High-Power Applications

Multi-port converters can interconnect different dc and ac systems while reducing semiconductor requirements and losses by eliminating redundant power conversion stages. Modular multilevel converter (MMC)-based multiport systems are well suited for application in mixed ac-dc grids containing high-voltage dc (HVDC) and medium-voltage dc (MVDC) systems. This paper conducts a detailed comparative assessment of multi-port dc-dc-ac MMCs for high power applications. Four representative topologies are chosen for study due to their contrasting internal power processing characteristics. Three different network scenarios are investigated that include HVDC and MVDC applications, covering several different power flow cases. The multi-port MMCs are compared in terms of losses, semiconductor effort, internal energy storage and magnetics requirements. The results are extensively discussed and general conclusions are summarized.

Piezoelectrically Actuated Fast Mechanical Switch for MVDC Protection

Medium voltage direct current (MVDC) power systems are in their early phase of development, but the lack of efficient and reliable DC circuit breakers is currently one of the most significant bottlenecks for widespread adoption of MVDC applications. Compared to protection devices in high voltage direct current (HVDC) systems, MVDC circuit breakers are expected to interrupt fault currents with higher rates of rise in more power-dense and more controllable structures. A hybrid approach with a piezoelectrically actuated fast mechanical switch in parallel to a solid-state circuit breaker is expected to meet the above requirements. A prototype of a 15 kV-class piezoelectrically actuated fast mechanical switch is presented in this paper. The prototype demonstrates an actuator opening time of 1 ms, a breakdown voltage of over 20 kV, and a continuous current rating of 500 A. After integrating the fast mechanical switch into a hybrid circuit breaker testbed, the fast mechanical switch was able to open within 1.5 ms and the fault was cleared within 5 ms. Therefore, piezoelectrically actuated fast mechanical switches can enable fast, reliable and controllable protection for various MVDC systems like electric ships, electric aircraft and microgrids that integrate distributed energy resources.

Hierarchical Modeling Scheme for High-Speed Electromagnetic Transient Simulations of Power Electronic Transformers

The extensive application of the power electronic transformers (PETs) in power systems poses a challenge as the accurate and high-speed electromagnetic transient (EMT) simulation of PET has been a critical issue. The computational time of the detailed EMT simulation of PET on EMT programs is unacceptable due to the large electrical node count, microsecond-range simulation steps, and high switching frequency of the devices and transformers. This article proposes a hierarchical modeling scheme for PET. Unlike the existing modeling methods, the proposed technique recursively decreases the dimension order of the admittance matrix to obtain the generalized Norton equivalent of each phase leg. The final admittance matrix overlaid onto the external system admittance matrix has a dimension order of magnitude remarkably smaller than that of the unreduced structure. By comparison with a detailed EMT model of a medium-voltage dc system, the performance of the proposed scheme has been assessed in PSCAD/EMTDC under various working conditions. With negligible loss of accuracy, approximately one to two orders of magnitude speedup over a straightforward EMT program is achieved.

Fault Protection Considerations for MVDC Shipboard Power Systems Operating with Pulsed-Power Loads

Medium Voltage Direct Current (MVDC) power distribution architectures are of immense interest for various shipboard power applications due to their advantages over classical MVAC distribution systems with respect to power quality, power density, and efficiency. However, MVDC are far away from maturity when compared to MVAC with respect to fault detection and isolation. Currently, there are no standards available for applying MVDC protection systems in shipboard applications. Furthermore, due to the absence of zero crossings in DC waveforms and unique transient fault signatures, it is challenging to design effective protection system schemes to isolate faults via conventional protection systems. This paper investigates and analyses various types of shipboard MVDC dynamic fault behaviours and signatures under different DC bus disturbances such as: bus to ground, bus to bus to ground, and impact of Pulsed-Power Load (PPL) with and without faults on a shipboard MVDC distribution system. Furthermore, a communication-based fault detection and isolation system controller that improves upon a directional ac overcurrent relay protection system is proposed offering additional protection discrimination between faults and PPLs in MVDC systems. To validate the effectiveness of the proposed protection controller, different bus current disturbances are simulated within a time-domain electromagnetic transient simulation of a shipboard power system including a PPL system operating with different ramp rate profiles, pulse widths, peak powers, and fault locations

Adaptive Solid-State Circuit Breaker Without Varistors in VSC-Interfaced DC System

The rapid discharge of the dc-link capacitors during the short-circuit fault would cause the transient current with fast propagation speed in the voltage-source-converter-based dc system. Therefore, it is necessary to limit the dc fault current and lower the requirement of fault-detection speed. However, the traditional fault current limitation method with directly installed inductors in the dc line will prolong the fault-isolation time and result in the system instability during the power flow shifting. In this article, an adaptive solid-state circuit breaker (SSCB) is proposed for the low- and the medium-voltage dc system, which can bypass the installed inductors during the normal state and connect them into the dc line when the fault occurs. Besides, the energy dissipation circuit and the snubber circuit are applied to release the energy stored in the installed inductors and the dc line inductance after tripping. Hence, the fault-isolation speed can be accelerated, and it is not necessary to use the metal-oxide varistors in the proposed SSCB. By comparing with other SSCB schemes with the fault current limitation function, the effective fault current limitation, the instantaneous fault isolation, and the valid fault ride-through of the health part of the proposed SSCB are verified in the OPAL-RT-based simulation and the hardware test.

Innovative Energy Management System for MVDC Networks with Black-Start Capabilities

Medium voltage DC (MVDC) networks are attracting more attention amid increased renewables penetration. The reliability of these DC systems is critical, especially following grid contingencies to maintain critical loads supply and provide ancillary services, such as black-start. This paper proposes an innovative energy management system (EMS) to maintain reliable MVDC network operation under prolonged AC grid contingencies. Similar EMS designs in literature tend to focus on limited operating modes and fall short of covering comprehensive elongated blackout considerations. The proposed EMS in this paper aims to preserve the distribution network functionality of the impacted MVDC system through maintaining a constant DC bus voltage, maximizing critical load supply duration, and maintaining the MVDC system black-start readiness. These objectives are achieved through controlling generation units between Maximum Power Point Tracking (MPPT) and Voltage Regulation (VR) modes, and implementing a smart load shedding and restoration algorithm based on network parameters feedback, such as storage State of Change (SoC) and available resources. Practical design considerations for MVDC network participation in AC network black start, and the following grid synchronization steps are presented and tested as part of the EMS. The proposed system is validated through simulations and scaled lab setup experimental scenarios.

Topology and Control Strategy of PV MVDC Grid-Connected Converter with LVRT Capability

This paper proposes an isolated buck-boost topology and control strategy for the photovoltaic (PV) medium-voltage DC (MVDC) converter with low-voltage ride through (LVRT) capability. The proposed isolated buck-boost topology operates on either boost or buck mode by only controlling the active semiconductors on the low-voltage side. Based on this topology, medium-voltage (MV) dc–dc module is able to be developed to reduce the number of modules and increase the power density in the converter, which corresponds to the first contribution. As another contribution, a LVRT method based on an LC filter for MVDC converter is proposed without additional circuit and a feedback capacitor current control method for the isolated buck-boost converter is proposed to solve the instability problem caused by the resonance spike of the LC filter. Five kV/50 kW SiC-based dc–dc modules and ±10 kV/200 kW PV MVDC converters were developed. Experiments of the converter for MVDC system in the normal and LVRT conditions are presented. The experimental results verify the effectiveness of the proposed topology and control strategy.

Efficient Protection Scheme Based on Y-Source Circuit Breaker in Bi-Directional Zones for MVDC Micro-Grids

A new bi-directional circuit breaker is presented for medium-voltage dc (MVDC) systems. The Y-source impedance network topology is used to implement the breaker. The current transfer function is derived to show the frequency response and the breaker operation with the high frequencies. Mathematical analysis is achieved with different conditions of coupling among the breaker inductors. The minimum level of the magnetic coupling is determined, which is represented by the null condition. The effect of the turns-ratio on this condition is investigated as well. The breaker is designed with two types of fault conductance slope rates. The Y-source breaker is simulated, and the results verify the breaker operation during the fault condition and the load change. The results also demonstrate the effect of the coupling level on the minimum values of the source current when the fault occurs. Based on the expected fault type in the MVDC systems, the proposed breaker is developed to interrupt the overcurrent due to any of these fault types. A protection scheme is proposed for a 12-bus, two-level micro-grid, where the Y-source breakers are used in the bi-directional zones. The results verify the ability of the breaker to conduct and interrupt the current in both directions of the power flow.

A DC Solid-State Transformer With DC Fault Ride-Through Capability

This article investigates a dc solid-state transformer (DCSST) with dc fault ride-through (FRT) capability for medium-voltage dc (MVDC) systems. The FRT requirement for DCSST in dc grid is investigated. Short-circuit fault clearance on the medium-voltage (MV) side is realized by introducing full-bridge converters with cascaded dc converter modules on the MV side of the DCSST. The FRT strategy under typical voltage sags is analyzed. Factors affecting the fault clearing and system recovery are considered, including the number of full-bridge converters, the values of dc inductance, and the dc-link bus voltage. The concept is verified with both simulations and experiments.

Analysis of the Energy Dissipation Characteristic in an MMC-Based MVDC System With the Liquid Metal Current Limiter

Utilizing DCCBs to realize selective protection is a promising solution for dc fault isolation. While the large fault current and high fault energy make the dc fault interruption a crucial problem in the medium-voltage dc (MVDC) distribution system. A novel fault isolation strategy based on the liquid metal current limiter (LMCL) is proposed in this article to reduce the breaking current and dissipated energy of the dc circuit breakers (DCCBs). Based on a three-terminal MVDC distribution system, the fault current and energy dissipation characteristics for the reactor fault current limiter (L-FCL) and the LMCL are analyzed theoretically. The simulation results show that under the same breaking current, the dissipation energy and the dissipation time in the LMCL case are reduced by 48% and 41%, respectively. Besides, the influence of system parameters on breaking current and energy dissipation characteristics under the LMCL case is further analyzed. Finally, an experiment circuit is designed to verify the proposed scheme.

Prototype Current-Limiting Hybrid DC Circuit Breaker Incorporating a Fuse and a Semiconductor Device

Owing to the increased demand for DC interruption, a prototype current-limiting hybrid DC circuit breaker incorporating a fuse and a semiconductor device was developed, based on a completely novel design concept. Recently, hybrid DC breakers have been attracting attention because of their low conduction loss and high-speed current interruption capability. The superiority of this hybrid DC circuit breaker lies in the simultaneous use of the high-speed current-limiting performance of the fuse and the highly reliable current-interruption performance of the semiconductor device. Furthermore, the hybrid DC circuit breaker performs the current interruption without any mechanical contact, thus preventing any delay in operation induced by the mechanical contact. The feasibility of the proposed concept was demonstrated by the successful application of the circuit breaker in a 1 kA fault current interruption procedure. Hence, the current-limiting hybrid DC circuit breaker is suitable for low voltage and medium voltage DC system protection. As the fuses and semiconductor modules progress, the hybrid DC circuit breaker may also adapt to HVDC systems.

Si/SiC hybrid 5-level active NPC inverter for electric aircraft propulsion drive applications

Medium voltage DC(MVDC) system is considered as a promising technology to improve the efficiency and power density of electric aircraft propulsion(EAP) drives. To adapt to the MVDC voltage level and achieve high drive performance, a five-level active neutral point clamped(5L-ANPC) inverter consisting of three-level ANPC and flying capacitor circuits is investigated, which possesses higher voltage capability, lower output harmonics, as well as mitigated dv/dt and common-mode voltage. To fulfill the requirements of high-speed operation and pursue further enhanced efficiency and power density of the inverter for the next-generation EAP drives, Silicon Carbide(SiC) semiconductor devices are considered for implementing the 5L-ANPC inverter. However, the large commutation loops associated with certain switching states of the inverter lessen the benefits of configuring all the switches as SiC devices. As a result, a hybrid Si/SiC 5L-ANPC inverter is developed with a synchronous optimal pulse(SOP) width modulation strategy for controlling the switches in cell 2 and finite-control-set model predictive controller(FCS-MPC) for those in cell 3 of the inverter. Consequently, in the proposed topology, the SiC devices are merely used for the high-frequency switches in cell 3 and the rest of the low-frequency switches are configured with Si IGBTs. This Si/SiC hybrid ANPC inverter concurrently provides high efficiency and low implementation cost at high-speed operation mode. Simulation and experimental results are provided to verify the effectiveness of the proposed hybrid inverter.

Disturbance Rejection and Control Design of MVDC Converter with Evaluation of Power Loss and Efficiency Comparison of SiC and Si Based Power Devices

With direct current (DC) power generation from renewable sources, as well as the current relocation of loads from alternating current (AC) to DC, medium-voltage DC (MVDC) should fill gaps in the areas of distribution and transmission, thereby improving energy efficiency. The MVDC system is a platform that interconnects electric power generation renewables (solar, wind) with loads such as data centers, industrial facilities and electric vehicle (EV) charging stations (also using MVDC technology). DC–DC power converters are part of the rising technology for interconnecting future DC grids, providing good controllability, reliability and bi-directional power flow. The contribution of this work is a novel and efficient multi-port DC–DC converter topology having interconnections between two converters, three-level neutral point clamping (NPC) on the high-voltage (HV) side and two converters on the low-voltage (LV) side, providing two nominal low voltages of 400 V (constant) and 500 V (variable), respectively. The design of this new and effective control strategy on the LV side has taken into condition load disturbances, fluctuations and voltage dips. A double-closed-loop control topology is suggested, where an outside voltage control loop (in which the capacitance energies are analyzed as variable, and the inside current loop is decoupled without the precise value of boost inductance) is used. The simulation results show the effectiveness of the proposed control system. In the second part of this study, wide-bandgap SiC and Si devices are compared by using comprehensive mathematical modeling and LT-spice software. Improving power loss efficiency and overall cost comparisons are also discussed.

Interaction characteristics between multi‐port hybrid DC circuit breaker and MVDC distribution system under diversified working conditions

Due to the progressive penetration and usage of renewable sources and loads based on power electronics, medium voltage direct current (MVDC) distribution system is getting broad attention. Direct current circuit breakers (DCCBs) are of vital importance for the reliability and flexibility of power system. With features of low cost and micro-operating losses, multi-port hybrid DCCB with negative voltage source (NVS) has been proposed by the authors and might be a better choice. To further promote its industry application in MVDC system, interaction characteristics between DCCB and power system are investigated in this study. The structure of multi-port hybrid DCCB is briefly introduced. Then, considering the diversified working conditions, e.g. single fault, multiple faults and switching load current with random direction, the cooperation sequence of components in multi-port DCCB under all these working conditions is proposed, respectively. Then, based on simulation model established in PSCAD/EMTDC, transient current/voltage distribution pattern inside multi-port DCCB and its mechanism are discussed, and simulation results have verified the superiority and effectiveness of multi-port hybrid DCCB with NVS in MVDC system.

20kV급 방사형 MVDC 배전계통의 보호협조 운용알고리즘에 관한 연구

Recently, MVDC(medium voltage direct current) distribution system has been interested worldwide due to introduction of renewable energy sources, an increase of DC load and customer’s demands for high quality and high reliability of the power system. Although these MVDC systems have technologies to improve and modernize the performance of the distribution system with high reliability, high flexibility and high efficiency, they have faced challenges and difficulties in the early stages of research and development. In particular, since power converter devices to supply DC sources have extremely high sensitivity, protection method of the MVDC distribution system is being required in order to protect and operate the MVDC system in a stable manner. Therefore, this paper proposes operation algorithm of protection coordination in MVDC distribution system to rapidly separate fault sections by using slope characteristics of fault currents and presents prevention method for shutdown of the main power source based on the CLR(current limit resistor). And also, this paper performs modeling of 20kV MVDC distribution system which are composed of substation, main and section converter based on the PSCAD/EMTDC S/W. From the simulation results of fault characteristics the at primary and secondary side in MVDC, it is confirmed that the main converter in primary feeder cannot be shutdown due to keeping fault currents within allowable limits of main converter, if the proper capacity of the CLR is installed at secondary side of section converter. In addition, it is found that the proposed method can separate outage sections in primary feeder and minimize them, if slope ranges of fault current are properly determined at each protection section.