Medium Voltage DC Power System Research Articles

Reprint of: Adaptive supertwisting sliding mode control of multi-converter MVDC power systems

Medium Voltage DC (MVDC) integrated power systems are anticipated to feed the submarines and surface combatants. The MVDC based power systems use dc/dc converters tightly regulated via high bandwidth controllers act as constant power loads (CPL’s). The negative incremental impedance of the CPL’s may result in voltage instability in the system under disturbance. Thus for the efficient and robust control of the MVDC based power system, this paper proposes a high order sliding mode control (HOSMC) paradigm based on model-free adaptive super twisting sliding mode control (ASTA-SMC). This paper initially designs a first-order sliding mode control (FOSMC) and super twisting sliding mode control (STA-SMC) based on HOSMC DC link voltage controllers. The difficult task of gain tuning in timely varying conditions is resolved by incorporating adaptive control to STA-SMC. The proposed controller is validated and compared with pre-presented control schemes under two test cases using simulation and experimental workbench carried out in dSPACE based hardware in the loop (HIL) workbench. The adaptive STA-SMC (ASTA-SMC) is compared with FOSMC and feedback linearization-based control schemes. The stability of ASTA-SMC in finite time is validated using the Lyapunov stability theorem.

An improved risk prioritization method for propulsion system based on heterogeneous information and PageRank algorithm

Failure mode and effect analysis (FMEA) has been widely applied as a powerful reliability analysis technique to identify and eliminate system failures in various industries. Though many contributions have been made to improve the traditional FMEA, some challenges still exist. For example, how to deal with different types of risk information involved in FMEA due to the heterogeneity of risk factors. Additionally, the influence of the relationship between failure modes and the attenuation effect on the final risk priority deserves further study. Motivated by these challenges, we propose an improved risk prioritization method. First, heterogeneous information expression structures (i.e., crisp numbers, interval numbers, triangular fuzzy numbers, linguistic Z-numbers) are utilized to describe risk factors from quantitative and qualitative aspects. Then, the initial risk priority numbers (RPNs) of failure modes are determined without transforming different representation structures into a unified one. Next, the total relation influence matrix is generated considering attenuation effects. Afterward, the PageRank algorithm is extended to obtain the final risk priority by analyzing the mutual influence between failure modes and their initial RPNs simultaneously. Finally, a case study of FMEA for a shipboard medium voltage DC power system is presented to verify the effectiveness of the proposed method. The results indicate that our method can process the heterogeneous risk information more flexibly and prioritize the failure modes more reasonably.

Static state power smoothing and transient power quality enhancement of a DC microgrid based on multi-function SMES/battery distributed hybrid energy storage system

Using hybrid energy storage system (HESS) to solve the issues of power regulation and fault protection of DC microgrids has attracted extensive attention. To realize multiple functions of power flow control and power quality enhancement of the DC microgrid, a concept of distributed HESS based on superconducting magnetic energy storage (SMES) and battery energy storage (BES) is introduced in this paper. A SMES-based energy-storage-composited DC transformer (ESDCT) is designed to interconnect the low-voltage DC microgrid into a medium-voltage DC power system. It is with the functions of DC voltage transformation, and grid fault isolation. The BES is selected to regulate the unstable output power of the LVDC microgrid caused by renewable energy generators and load variations. The modeling of the SMES-based ESDCT and the cooperative control of the BES and SMES is analyzed. Finally, the performance of the SMES/BES distributed HESS is verified via a simulation, and the feasibility of the proposed SMES-based ESDCT is demonstrated via a scaled-down experiment.

Full Simulation Modeling of All-Electric Ship with Medium Voltage DC Power System

This paper proposes the full simulation model for the electrical analysis of all-electric ship (AES) based on a medium voltage DC power system. The AES has become popular both in the commercial and the military areas due to a low emission, a high fuel consumption efficiency, and a wide applicability. In spite of many advantages, it is complex and difficult to construct the whole system with many mechanical and electrical components onboard. Full electrical analysis is essentially required to simplify the design of the AES, a control and optimization of a ship electric system. The proposed full simulation model of the AES includes the mechanical and the electrical elements by using the MATLAB/Simulink. The mechanical elements are comprised of a steam turbine and a hydrodynamic model of a ship which is adopted by an average value model that is based on the characteristic equation of the mechanical system. The electrical elements are developed by full detailed models which consist of generators, a propulsion motor, a battery, and a power electronics system. In order to design the distribution of the ship, the presented simulation model combined the mechanical and the electrical systems. The consistency of the developed individual models and the integrated AES was verified through the results of the presence or absence of the energy storage system for the speed acceleration and deceleration, loss of prime mover, and full propulsion load rejection.

A Novel High-Power Hybrid DC Breaker Based on Compound Power Electronic Switch With Integrated Commutation Ability

Medium voltage dc power system has the characteristics of low line impedance, resulting in high peak and fast rise rate of the fault current. For example, the current in an onboard 10 kV dc grid can rise to more than 40 kA in 3 ms after the failure. Therefore, it is necessary to configure dc circuit breaker with high current breaking capacity. In this letter, a novel high-power hybrid dc breaker based on integrated commutation compound power electronic switch is proposed. By giving full play to the complementary advantages of insulated gate bipolar transistor (IGBT) and integrated gate-commutated thyristor (IGCT), and skillfully integrating the commutation function into the power electronic switch, the proposed dc breaker has the characteristics of low cost, compact, high performance, and can also play a role in current limiting. The working principle of the proposed scheme is described in detail, and a 10-kV prototype with 40 kA current breaking ability is built to verify its feasibility.

Multiagent Distributed Power Management of DC Shipboard Power Systems for Optimal Fuel Efficiency

Medium-voltage dc power systems have been recognized as a promising electrical architecture for marine vessels. Such shipboard power systems (SPSs) require advanced control architectures to ensure reliability and flexibility, as well as achieve fuel-efficient operation. With this regard, this article presents a multiagent distributed power management scheme to achieve voltage regulation and economic power-sharing of diesel generators (DGs) for optimal fuel efficiency in dc SPSs. Based on the power management signal, each DG will adjust its operating speed to minimize fuel oil consumption. All functions are realized in a distributed manner with only neighboring communications, which offers more reliability and flexibility compared to conventional centralized approaches. The proposed power management scheme is validated by the controller hardware-in-the-loop (CHIL) experiment. A dc SPS test system is developed in the OPAL-RT-based real-time simulator. The B&R programmable logic controllers and network switches are set up to realize the multiagent-based distributed control. The CHIL experimental results under different operation conditions demonstrate the feasibility and advantages of the proposed method.

Fault diagnosis of shipboard medium-voltage DC power system based on machine learning

This study proposed a fault diagnosis method of a shipboard medium-voltage DC (MVDC) power system based on Noise-Assisted Multivariate Empirical Mode Decomposition (NA-MEMD) and Multilevel Iterative – LightGBM (MI-LightGBM), which overcomes the limitations of the existing fault diagnosis methods in this regard, such as relies heavily on the relay or slow training process. MI-LightGBM is proposed to solve the problem of unbalanced training samples caused by the difficulty in obtaining fault samples in practical engineering. First, NA-MEMD was adopted to pre-process the voltage signals, which were decomposed into a set of components called Intrinsic Mode Functions (IMFs) according to the local characteristic time scales of the original signals. The energy moment of each order IMF was calculated as fault feature vector to train the MI-LightGBM model, which led to the development of a high-precision fault classifier. A model of a shipboard MVDC power system was established using the AppSIM Real-Time Simulator. Simulations were performed on earth fault and short-circuit fault at the generator output and DC cable. Compared with the existing fault diagnosis methods, the proposed method is simple to use and save more than half of the training time while maintaining high diagnostic performance, which is more suitable for engineering applications.

Simulation study of harmonic characteristics of modular multilevel converter in medium voltage DC power system on ship

Simulation study of harmonic characteristics of modular multilevel converter in medium voltage DC power system on ship

Study of Liquid Metal Fault Current Limiter for Medium-Voltage DC Power Systems

Fault current limitation is an urgent but challenging technology for medium-voltage dc (MVDC) power systems. For a liquid metal current limiter (LMCL), the arc discharge characteristics often determine its current-limiting properties. The evolving arc in liquid metal is measured by contacting voltage probes, and the properties of the formed dumbbell-like arc column are studied and discussed in detail. It indicates that the total arc voltage almost behaves as a linear function of arc current for an LMCL. Besides, a simplified numerical arc model based on magnetohydrodynamics theory is built and solved by the software FLUENT. It is concluded that the temperature rather than the slightly fluctuant pressure dominates the arc properties, which coincides well with test results to some extent. Finally, a fault current limiter topology including serial LMCLs and parallel resistors is put forward, whose feasibility for MVDC application is discussed on the basis of its current-limiting behaviors.

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.

Fuzzy Logic Based Energy Storage Management System for MVDC Power System of All Electric Ship

In this paper, for supporting the medium voltage dc (MVDC) shipboard power system, an energy storage management (ESM) system based on fuzzy logic (FL) has been proposed and its performance with a proportional-integral (PI) control based ESM system is compared. In order to support the peak demand and pulsed load, a hybrid energy storage system incorporating high energy density storage (battery) and high power density storage (supercapacitor) is proposed. For energy transfer among the energy storages and the MVDC system, bidirectional dc–dc converters with dual active bridge (DAB) configuration are used. With the change of the bus voltage and load power demand, the ESM systems provide instantaneous reference powers for charging or discharging of the battery and supercapacitor. The reference powers for the battery and supercapacitor are sent to the respective controllers of the DAB converters. Two power sharing strategies are designed to share power among multiple energy storages. The MVDC shipboard power system with the generators, loads, battery, and supercapacitor with DAB converters are modeled in SimPowerSystems. Simulation results are used to make a comparison of performances of the FL and PI controller based ESM systems. Finally, controller hardware-in-the-loop based experimental results are added to demonstrate the effectiveness of the controller.

Generation control system for improving design and stability of medium-voltage DC power systems on ships

Voltage stability in medium-voltage DC (MVDC) power systems on ships is a key design goal. MVDC bus voltage stability can be impaired because of the presence of power electronic converters that can induce negative incremental resistance instabilities. This study presents a control strategy that stabilises MVDC bus voltage in the presence of destabilising constant power load converters. A state-feedback adaptive control is designed by linearisation and implemented using controlled interface converters connecting the MVAC alternators to the MVDC bus. Large signal stability of the control system is assessed using Lyapunov techniques. The proposed control is verified against both average-simplified and detailed time-domain numerical simulations, thus providing a two-way comparison.

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.