Medium Voltage Direct Current Power 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.

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.

High Temperature Insulation Materials for DC Cable Insulation—Part II: Partial Discharge Behavior at Elevated Altitudes

Utilizing hybrid electric propulsion systems in the next generation of aircrafts with elevated DC voltage level has raised the concern about the degradation of insulation subjected to partial discharge at high altitude and harsh environment. In this work, the surface discharge behavior of commonly used high-temperature insulation materials in aviation systems (FEP, ETFE, and PEEK) under positive and negative DC and ramp voltage was studied, including the pulse waveform, frequency spectrum, PD count and magnitude. Pressure changes as a significant factor influencing surface discharge phenomenon was thoroughly studied. Dust figure technique was employed after voltage ramping tests for both positive and negative polarities at low pressure to investigate the surface charge accumulation and surface discharge traces, and unveil the involved physical mechanisms at different stages of streamer propagation. The content of this paper provides a reference for surface discharge studies and evaluation of high temperature materials for medium voltage direct current power distribution in the future aviation systems.

Design of an HTS Fault Current Limiting Module for an MVDC Power Grid

We have studied a DC circuit breaker with an HTS fault current limiting (FCL) module for MVDC(Medium-Voltage Direct Current) power grid. The attached FCL module can reduce the burden of the circuit breaker to make the system break the fault current faster as well as safer. In this paper, we will present the design of an HTS fault current limiting module with 2G REBCO conductor for the MVDC power grid with the rated voltage of 15 kV. Among several different type of 2G REBCO conductor from different suppliers, we selected the most suitable conductor, which was SuNAM's stabilizer free REBCO conductor, based on the overcurrent test results. And we performed the circuit simulation to verify the module's validity for rated current of 3 kA in 15 kV class DC grid.

Electrical faults in high temperature superconducting power cables for MVDC power systems of all-electric ships

High temperature superconducting (HTS) power cables are expected to be used in the future in electric ships that will have integrated power and energy systems (IPES). There have been a few studies on understanding electrical faults in power systems consisting of HTS cables; however, there are no comprehensive studies on the response of HTS cables for various types of electrical faults. We have recently initiated a research project on understanding various electrical faults in shipboard medium voltage direct current (MVDC) power systems and the duration of the fault, maximum voltage, and maximum current that the HTS cables will encounter during the fault. This paper presents investigations on the potential cable architectures being developed for MVDC power systems, and the type of faults that HTS cables will encounter in such systems. The paper will assess the relative merits of these architectures in terms of their suitability for accommodating the limitations of cryogenically cooled HTS cables and offer design suggestions for resilient power systems. Electrical and cryogenic thermal models of HTS cables that are suitable for assessing the response of HTS cables for electrical faults, as well as HTS cable designs that can endure electrical faults without catastrophic damage will also be discussed.

Multi-time Scale Optimal Power Flow Strategy for Medium-voltage DC Power Grid Considering Different Operation Modes

Direct current (DC) power grids based on flexible high-voltage DC technology have become a common solution of facilitating the large-scale integration of distributed energy resources (DERs) and the construction of advanced urban power grids. In this study, a typical topology analysis is performed for an advanced urban medium-voltage DC (MVDC) distribution network with DERs, including wind, photovoltaic, and electrical energy storage elements. Then, a multi-time scale optimal power flow (OPF) strategy is proposed for the MVDC network in different operation modes, including utility grid-connected and off-grid operation modes. In the utility grid-connected operation mode, the day-ahead optimization objective minimizes both the DER power curtailment and the network power loss. In addition, in the off-grid operation mode, the day-ahead optimization objective prioritizes the satisfaction of loads, and the DER power curtailment and the network power loss are minimized. A dynamic weighting method is employed to transform the multi-objective optimization problem into a quadratically constrained quadratic programming (QCQP) problem, which is solvable via standard methods. During intraday scheduling, the optimization objective gives priority to ensure minimum deviation between the actual and predicted values of the state of charge of the battery, and then seeks to minimize the DER power curtailment and the network power loss. Model predictive control (MPC) is used to correct deviations according to the results of ultra short-term load forecasting. Furthermore, an improved particle swarm optimization (PSO) algorithm is applied for global intraday optimization, which effectively increases the convergence rate to obtain solutions. MATLAB simulation results indicate that the proposed optimization strategy is effective and efficient.

Small-signal model predictive control based resilient energy storage management strategy for all electric ship MVDC voltage stabilization

Abstract Medium Voltage Direct Current (MVDC) power system will revolutionize the future of the marine all-electric ships (AES). However, with the presence of the constant power load (CPL), which behaves as incremental negative resistance, and the presence of heavy pulsed loads (HPL), that consume a massive amount of pulsed power, the voltage stability of the system arises as a critical problem. This problem needs to be addressed appropriately. This work presents a small signal model for a MVDC power system and design of a model predictive controller that maintains the voltage stability and ensures proper power sharing among the resources on the ship. Unlike previous works, the small signal model of the system is embedded inside the controller, which increases the flexibility of the controller to adapt to the different circumstances on the shipboard. The model and the controller are simulated through MATLAB/Simulink and validated using a processor in the loop. The results showed the ability of the controller to maintain the voltage of the MVDC under the different operational circumstances. Also, the controller was able to ensure the adequate operation of the resources and warrant proper power sharing.

Complex Affine Arithmetic-Based Power Flow Analysis for Zonal Medium Voltage Direct Current Shipboard Power Systems in the Presence of Power Variation

The medium-voltage direct-current (MVDC) ship power system has been extensively investigated and discussed in recent years. This paper presents an alternating current/direct-current (AC/DC) power flow algorithm based on complex affine arithmetic for the zonal MVDC shipboard power system in the presence of power variation. The power converter effect is considered, and an affine power converter model is proposed in the power flow model. An affine-analysis-based sequential method is adopted to solve AC/DC power flows. With the proposed algorithm, the bounds of bus voltages can be effectively obtained. A relative influence factor is defined to quantitatively assess the effect of power variation on voltage profile. Four cases are studied in detail to demonstrate the applicability of the proposed algorithm. The proposed algorithm is useful in network architecture design, planning, and online operations of MVDC power systems when such decisions are subject to power variation.

Modeling and Control of the Isolated DC–DC Modular Multilevel Converter for Electric Ship Medium Voltage Direct Current Power System

The dc–dc modular multilevel converter (dc–dc MMC) with a medium-frequency ac-link transformer is a promising candidate for the modern electric ship medium voltage dc power system to provide galvanic isolation, flexible voltage regulation, and fault ride through ability. However, the steady states and dynamics of such an isolated dc–dc MMC are rather complicated because it contains many state variables. Moreover, the on-ship requirements further complicate the system design. This paper thus proposes the modeling and control of such a dc–dc MMC. Based on the fact that such a dc–dc MMC is controlled by the phase-shifted operation, the fundamental period averaging (FPA) method is applied to obtain the steady-state and small-signal models, and based on the on-ship requirements, the modulation index control as well as the simple but efficient competitive dual closed loops (a voltage loop for normal condition and a current loop for overload condition) are designed. It is proved that the FPA method can preserve the necessary model information about the dc–dc MMC, and the proposed controllers can be well designed based on the steady-state and small-signal FPA models by compromising the on-ship requirements. The analysis and design results are verified by the simulations and experiments.

Guest Editorial Special Issue on Emerging Electric Ship MVDC Power Technology

Increased demand for electric power in general and emerging requirements for servicing highly dynamic loads at reduced size and weight of the overall electric power system and its associated components on board ships, especially for naval surface combatants, motivates the consideration of a variety of new power systems architectures and technologies. While there are still advances in the alternating current (ac) system, the advent of wide bandgap (WBG) power electronics for high-power applications specifically drives the development of medium-voltage direct current (MVDC) power systems. Those are expected to provide significant operational and economic benefits for future ships. However, several technical challenges arise with MVDC systems. The main objective of this Special Edition is to collect the latest developments from industry and academia to address these technical challenges from different aspects.

Notional all-electric ship systems integration thermal simulation and visualization

This work presents a simplified mathematical model for fast visualization and thermal simulation of complex and integrated energy systems that is capable of providing quick responses during system design. The tool allows for the determination of the resulting whole system temperature and relative humidity distribution. For that, the simplified physical model combines principles of classical thermodynamics and heat transfer, resulting in a system of three-dimensional (3D) differential equations that are discretized in space using a 3D cell-centered finite volume scheme. As an example of a complex and integrated system analysis, 3D simulations are performed in order to determine the temperature and relative humidity distributions inside an all-electric ship for a baseline medium voltage direct current power system architecture, under different operating conditions. A relatively coarse mesh was used (9410 volume elements) to obtain converged results for a large computational domain (185m×24m×34m) containing diverse equipment. The largest computational time required for obtaining results was 560 s, that is, less than 10 min. Therefore, after experimental validation for a particular system, it is reasonable to state that the model could be used as an efficient tool for complex and integrated systems thermal design, control and optimization.

Controller Optimization for Bidirectional Power Flow in Medium-Voltage DC Power Systems

This paper focuses on the control of bidirectional power flow in the electric shipboard power systems, especially in the Medium-Voltage Direct Current (MVDC) shipboard power system. Bidirectional power control between the main MVDC bus and the local zones can improve the energy efficiency and control flexibility of electric ship systems. However, since the MVDC system contains various nonlinear loads such as pulsed power load and radar in various subsystems, the voltage of the MVDC and the local zones varies significantly. This voltage variation affects the control performance of the bidirectional DC-DC converters as exogenous? disturbances. To improve the control performance regardless of uncertainties and disturbances, this paper proposes a novel controller design method of the bidirectional DC?C converters using L1 control theory and intelligent optimization algorithm. The performance of the proposed method is verified via large-scale real-time digital simulation of a notional shipboard MVDC power system.