MVDC Grid Research Articles

Fault detection and mitigation in MVDC grids using a Non-Unit Voltage Ratio Derivative Algorithm

Future MVDC grids will be used to reinforce the grid infrastructure enhancing transmission capabilities, minimizing losses, and providing capability to compensate reactive power as well as to control load flows. This paper addresses the protection of MVDC grids, which is one of the main challenges to be overcome, before taking advantage of all the inherent benefits that this technology will bring. The main objective of this paper is to validate a non-unit voltage derivative fault detection algorithm in a MVDC grid with green hydrogen production. The performance is analyzed with different fault cases and fault locations. As a main conclusion, the distance is the most relevant factor, which affects detection and tripping times.

PV integrated shipboard MVDC grid with reduced bus voltage ripple

Integration of renewable energy sources for power generation is a proven solution to reduce the increasing greenhouse gas emissions towards decarbonisation onboard ships. Also, it is easy to integrate a variety of energy sources in an all-electric ship under medium voltage direct current distribution system. In such distribution system, high frequency switching power converters are greatly involved on the source and the load sides, which are massive source of harmonics. In this paper, a grid connected photovoltaic module is designed and paralleled with the generator, as a step towards reducing greenhouse gas emissions. Additionally, to take care of the voltage ripple and noise created by the high frequency switching power converters in the system, a shunt direct current electric spring with a control strategy is incorporated. The proposed system eliminates the use of heavy passive filters, commonly used at the rectifier end. MATLAB/Simulink results substantiate that the increased voltage ripple with the inclusion of energy sources and loads, that goes around 10.91% under fully loaded condition, is reduced to 1.27% with the shunt direct current electric spring, hence well within the IEEE 1709 std., which is less than 5%.

(Invited) Recent Progress in Wide-Bandgap Semiconductor Devices for a More Electric Future

Wide-bandgap (WBG) semiconductors, with their excellent electrical properties, offer breakthrough performance in power electronics enabling low losses, high switching frequencies, and high temperature operation. WBG semiconductors are likely candidates to replace silicon-based semiconductors in the near future seeing as Silicon is fast approaching its performance limits for high power requirements. Wide-bandgap power semiconductor devices offer breakthrough circuit performance enabling low losses, high switching frequencies, and high temperature operation which will allow for enormous energy efficiency gains in a wide range of potential applications. In the past ten years, the U.S. Department of Energy’s Advanced Research Project Agency - Energy (ARPA-E), which was established to fund creative, out-of-the-box, transformational energy technologies that are too early for private-sector investment, has invested in WBG semiconductors including material and device-centric programs along with application specific programs targeting the barriers to widespread adoption in power electronics. Under these ARPA-E programs, medium voltage (10-20kV) WBG device development has commenced to push the voltage boundaries of the devices including the development of WBG super-junction devices. Light triggered photoconductive WBG devices are also being investigated for MV applications. The WBG MV devices will enable MVDC grid distribution applicable to markets including electrified transportation, renewable interconnections, and offshore oil, gas, and wind production. Other WBG device ideas are also being explored under ARPA-E programs including WBG integrated circuits and neutron detectors. The progress and challenges of the WBG devices being developed under ARPA-E programs will be reviewed along with thoughts on the future trends of WBG device development.

(Invited) Recent Progress in Wide-Bandgap Semiconductor Devices for a More Electric Future

Wide-bandgap (WBG) semiconductors, with their excellent electrical properties, offer breakthrough performance in power electronics enabling low losses, high switching frequencies, and high temperature operation. WBG semiconductors, such as silicon carbide and gallium nitride, are likely candidates to replace silicon in the near future for high power applications as silicon is fast approaching its performance limits. Wide-bandgap power semiconductor devices enable breakthrough circuit performance and energy efficiency gains in a wide range of potential applications. The U.S. Department of Energy’s Advanced Research Project Agency - Energy (ARPA-E) has invested in WBG semiconductors over the past ten years targeting the barriers to widespread adoption of WBGs in power electronics including material and device development. Under ARPA-E projects, medium voltage (10-20kV) WBG device development has commenced to push the voltage boundaries of WBGs. This includes super-junction devices and light triggered photoconductive devices for MV applications. The WBG MV devices will enable MVDC grid distribution applicable to markets including electrified transportation, renewable interconnections, and offshore oil, gas, and wind production. Advanced WBG device ideas are additionally being explored including 3D device structures, WBG integrated circuits, and neutron detectors The progress and challenges of the WBG devices being developed under ARPA-E programs will be reviewed along with thoughts on the future trends of WBG device development.

Current-Source Solid-State DC Transformer Integrating LVDC Microgrid, Energy Storage, and Renewable Energy Into MVDC Grid

Solid-state dc transformer to integrate low-voltage dc (LVdc) microgrid, wind turbine (WT) generator, photovoltaic (PV), and energy storage (ES) into medium-voltage (MV) direct-current (MVdc) distribution grids is attractive. This article proposes current-source dc solid-state transformer (SST) for MVdc collection system in WT, PV, and ES farms or as an interface between the MVdc grid and the LVdc microgrid. Compared to conventional current-source converter (CSC) based SSTs, a switch reduction scheme on reverse-blocking device bridges is proposed to reduce device count and the number of devices on the dc-link current path. Importantly, the proposed switch reduction scheme is generic and can be applied to the dc ports of dc–ac, ac–dc, or dc–dc hard-switching or soft-switching CSC-based SSTs. Based on this scheme, the proposed current-source dc SSTs are derived, which have reduced electrolytic-capacitor-less dc-link. The proposed dc SSTs also achieve single-stage isolated dc–dc or dc–ac conversion, full-range zero-voltage switching (ZVS) for main switches, zero-current switching (ZCS) for resonant switches, and controlled <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ dv/dt $</tex-math></inline-formula> . The proposed dc SSTs, operating principles, predictive control method, the ZVS, and the controlled <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ dv/dt $</tex-math></inline-formula> under voltage buck-boost ranges are verified with MV simulations and an experimental prototype based on SiC <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mosfets</small> , diodes, and a nanocrystalline transformer.

SiC-Based 5-kV Universal Modular Soft-Switching Solid-State Transformer (M-S4T) for Medium-Voltage DC Microgrids and Distribution Grids

Medium-voltage dc (MVdc) grids are attractive for electric aircraft and ship power systems, battery energy storage system (BESS), fast charging electric vehicle (EV), etc. Such EV or BESS applications need isolated bidirectional MVdc to low-voltage dc (LVdc) or LVac converters. However, the existing Si-based solutions cannot fulfill the requirements of a high-efficiency and robust converter for MVdc grids. This article presents a 5-kV SiC-based universal modular solid-state transformer (SST). This universal current-source SST can interface either an LVac or LVdc grid with an MVdc grid in single-stage power conversion, while the conventional dual-active bridge (DAB) converter needs an additional inverter. The proposed SST module using 3.3-kV SiC <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">MOSFET</small> s and diodes is bidirectional and can serve as a building block in series or parallel for higher voltage higher power systems. The topology of each module is based on the soft-switching solid-state transformer (S4T) with reduced conduction loss, which features reduced electromagnetic interference electromagnetic interference (EMI) through controlled dv/dt, and high efficiency with full-range zero-voltage switching for main devices and zero-current switching for auxiliary devices. Operation principle of the modular S4T (M-S4T), capacitor voltage balancing control between the cascaded modules, design of components including a medium-voltage (MV) medium-frequency transformer (MFT) to realize a 50-kVA, 5-kV dc to 600 V dc or 480 V ac M-S4T are presented. Importantly, the MV MFT prototype achieves very low leakage inductance (0.13%) and 15-kV insulation with coaxial cables and nanocrystalline cores. The proposed universal modular SST is compared against the DAB solution and verified with dc–dc and dc–ac simulation and 4-kV experimental results. Significantly, the MV experimental results of a modular dc transformer with each module at MVdc are rarely covered in the literature and reported for the first time.

Hybrid 전력망의 모델링 및 사고 해석

In order to increase an acceptance of renewable energy sources and to lay out an efficient power grid, a hybrid power grid model design and faults analysis must be preceded. This paper is a basic study for a hybrid power grid configuration, fault analysis, and protection coordination. Using PSCAD software, modeling of a hybrid power grid and faults analysis through short circuit are performed. First, a hybrid power grid consisting of an AC 154[kV] power source, a 50[MVA] transformer, a MVDC and LVDC power grid through bidirectional power converters, PV, and load was constructed. Each components of the selected hybrid power grid was modeled with PSCAD software and the controllers were designed and simulated. And it is intended to analyze the faults characteristics by grasping the fault currents during the short circuit and +pole to -pole faults while changing each fault inception location. Finally, the simulation results of the faults analysis by the maximum value and the current fluctuation rate of the fault currents were compared.

Parallel Nine-Phase Generator Control in a Medium-Voltage DC Wind System

In this article, we propose a variable voltage control strategy for parallel connected hybrid generators (HGs) in a medium-voltage dc (MVdc) array grid. In the presented scheme, the outputs of HGs are rectified and interfaced to an offshore MVdc array grid. The MVdc grid, whose voltage is varied but controlled, connects the wind turbines in the farm to an offshore substation. Using dc–dc converters, the voltage is stepped up at the offshore substation and sent to a terrestrial grid via undersea high-voltage dc (HVdc) export cables. The MVdc voltage is varied in a prescribed control strategy, whereas the HVdc voltage is maintained fixed at all times. The HG in the scheme presented in this article utilizes two rotor elements, a permanent-magnet rotor with fixed excitation and a wound field (WF) rotor with controllable excitation. The proposed control strategy includes a WF control scheme local to the HGs coordinated with a wind farm supervisory control system (SCS). A simulation platform is developed to evaluate the effectiveness of the control strategy in a large wind farm. Simulation results are validated on a laboratory prototype HG system showing a good agreement.

Stable and Passive High-Power Dual Active Bridge Converters Interfacing MVDC Grids

Dual active bridge (DAB) is a topology that is receiving more and more attention as a potential solution to interface dc grids of different voltage levels. From a system level, the implications of DABs on the stability of complex power systems are addressed in this paper. Dynamics modeling and stability assessment for a DAB implementation aimed to interface low-voltage energy resources with medium-voltage dc (MVdc) collection and distribution grids are presented. The DAB admittance is analytically derived and assessed in order to describe its dynamics and anticipate its behavior when integrated in a complex MVdc grid. The model considers the low-frequency range, mostly dominated by the controller action, and the high-frequency range, described by a nonlinear operation. The theoretical analysis is verified by hardware-in-the-loop emulation, with the controller running on a digital signal processor. The proposed implementation is proved to achieve passivity in the whole spectrum, which undoubtedly is a desired feature for massive power electronics integration in the future MVdc grids.

Hybrid Energy Storage System With Active Filter Function for Shipboard MVDC System Applications Based on Isolated Modular Multilevel DC/DC Converter

This paper proposes an isolated modular multilevel dc/dc converter (iM2DC)-based hybrid energy storage system (ESS) for shipboard MVDC grid application. The cell capacitors of the iM2DC are applied to implement the dc active power filter capability, which improves the MVDC bus power quality without auxiliary devices or sacrificing the battery lifetime. In addition, the proposed ESS achieves superior fault response with fault current limiting function, which benefits the fault localization and fast recovery. The operation principle of the proposed system is introduced first and the converter equivalent averaged model is derived. Then the multifunctional control method is presented, including the virtual-impedance-based ripple distribution strategy. Both offline simulation and controller hardware-in-the-loop test results are provided to validate the control strategy as well as the ESS performance.

Prioritized Load Balancing and Power Quality Improvement in MVDC Grid

Micro grids are maturing technically as well as commercially over the past few years. Micro grids are now used for islanded operations where conventional energy sources are not available. This is very helpful to remote mine sites and also for electrifying isolated islands. Maintaining reliability is a crucial matter for islanded micro grids. Hence load and supply balancing is very essential. This paper suggests a prioritized load balancing technique which ensures reliability of service. The loads are studied and prioritized based on which balancing is done. The micro grid is powered by renewable sources like wind energy, solar PV panel and a backup DC generator. This approach always ensures continuity of crucial services while shutting down unnecessary loads during shortage of power. In the advanced stage, in order to improve power quality FACTS devices are installed. Also to harness maximum energy, Maximum power point tracker (MPPT) is installed for wind mill and solar PV panel.