History and recent advancements of electric propulsion and integrated electrical power systems for commercial & naval vessels

Due to developments in power electronics, electric machines, energy storage and control, electric propulsion and integrated electrical power systems have become major trends for commercial and naval vessels. This is mainly due to the fact that the use of electric propulsion and integrated power systems can improve efficiency and fuel consumption while reducing noise and vibration when compared to conventional systems. Such advantages are extremely attractive to vessel owners due to increasingly stringent emission requirements, especially in environmental control areas, from the international maritime organization. This paper aims to summarize the recent advancement of marine power systems including propulsion systems, electrical distribution systems and novel loads

Modeling and Real-Time Scheduling of DC Platform Supply Vessel for Fuel Efficient Operation

DC marine architecture integrated with variable speed diesel generators (DGs) has garnered the attention of the researchers primarily because of its ability to deliver fuel efficient operation. This paper aims in modeling and to autonomously perform real-time load scheduling of dc platform supply vessel (PSV) with an objective to minimize specific fuel oil consumption (SFOC) for better fuel efficiency. Focus has been on the modeling of various components and control routines, which are envisaged to be an integral part of dc PSVs. Integration with photovoltaic-based energy storage system (ESS) has been considered as an option to cater for the short time load transients. In this context, this paper proposes a real-time transient simulation scheme, which comprises of optimized generation scheduling of generators and ESS using dc optimal power flow algorithm. This framework considers real dynamics of dc PSV during various marine operations with possible contingency scenarios, such as outage of generation systems, abrupt load changes, and unavailability of ESS. The proposed modeling and control routines with real-time transient simulation scheme have been validated utilizing the real-time marine simulation platform. The results indicate that the coordinated treatment of renewable based ESS with DGs operating with optimized speed yields better fuel savings. This has been observed in improved SFOC operating trajectory for critical marine missions. Furthermore, SFOC minimization at multiple suboptimal points with its treatment in the real-time marine system is also highlighted

Next-Generation Shipboard DC Power System: Introduction Smart Grid and dc Microgrid Technologies into Maritime Electrical Networks

In recent years, evidence has suggested that the global energy system is on the verge of a drastic revolution. The evolutionary development in power electronic technologies, the emergence of high-performance energy storage devices, and the ever-increasing penetration of renewable energy sources (RESs) are commonly recognized as the major driving forces of the revolution. The explosion in consumer electronics is also powering this change. In this context, dc power distribution technologies have made a comeback and keep gaining a commendable increase in research interest and industrial applications. In addition, the concept of flexible and smart distribution has also been proposed, which tends to exploit distributed generation and pack together the distributed RESs and local electrical loads as an independent and self-sustainable entity, namely a microgrid. At present, research in the area of dc microgrids has investigated and developed a series of advanced methods in control, management, and objective-oriented optimization that would establish the technical interface enabling future applications in multiple industrial areas, such as smart buildings, electric vehicles, aerospace/aircraft power systems, and maritime power systems

Medium voltage DC power systems on ships: An offline parameter estimation for tuning the controllers' linearizing function

Future shipboard power systems using Medium Voltage Direct (MVDC) technology will be based on a widespread use of power converters for interfacing generating systems and loads with the main DC bus. Such a heavy exploitation makes the voltage control challenging in the presence of tightly controlled converters. By modeling the latter as constant power loads (CPLs), one possibility to ensure the bus voltage stability is offered by the linearizing via state feedback technique, whose aim is to regulate the generating DC-DC power converters to compensate for the destabilizing effect of the CPLs. Although this method has been shown to be effective when system parameters are perfectly known, only a partial linearization can be ensured in case of parameter mismatch, thus, jeopardizing the system stability. In order to improve the linearization, therefore, guaranteeing the voltage stability, an estimation method is proposed in this paper. To this aim, offline tests are performed to provide the input data for the estimation of model parameters. Such estimated values are subsequently used for correctly tuning the linearizing function of the DC-DC converters. Simulation results for bus voltage transients show that in this way converters become sources of stabilizing power

Stability Improvement of DC Power Systems in an All-Electric Ship Using Hybrid SMES/Battery

As the capacity of all-electric ships (AESs) increases dramatically, the sudden changes in the system load may lead to serious problems, such as voltage fluctuations of the ship power grid, increased fuel consumption, and environmental emissions. In order to reduce the effects of system load fluctuations on system efficiency, and to maintain the bus voltage, we propose a hybrid energy storage system (HESS) for use in AESs. The HESS consists of two elements: a battery for high energy density storage and a superconducting magnetic energy storage (SMES) for high power density storage. A dynamic droop control is used to control charge/discharge prioritization. Maneuvering and pulse loads are the main sources of the sudden changes in AESs. There are several types of pulse loads, including electric weapons. These types of loads need large amounts of energy and high electrical power, which makes the HESS a promising power source. Using Simulink/MATLAB, we built a model of the AES power grid integrated with an SMES/battery to show its effectiveness in improving the quality of the power grid

Interactions Between Bandwidth Limited CPLs and MMC Based MVDC Supply

With the improvement in the power electronic technologies, medium voltage dc (MVDC) electrical distribution systems are being considered for on-shore and off-shore applications. These future MVDC electrical distribution systems are expected to provide the possibility of easy interfacing of the renewable energy sources, improving the dynamics of the system and also help in reducing the carbon footprint of energy sources. Modular multilevel converters (MMCs) are used in high voltage dc (HVDC) applications and are being considered for MVDC applications as well. In this paper, we present an MVDC electrical distribution system where the source converter is an MMC and the loads exhibit bandwidth limited constant power load (CPL) behaviour. An analysis is carried out on the dynamic interactions between the MMC source converter and CPLs, considering varying distribution cable lengths between the source and the load, the filtering effort at the load end and different loading conditions

Positive Feedforward Control Design For Stabilization Of A Single-Bus DC Power Distribution System Using An Improved Impedance Identification Technique

Due to recent advances in power electronics technology, DC power distribution systems offer distinct advantages over traditional AC systems for many applications such as electric vehicles, more electric aircrafts and industrial applications. For example, for the All-Electric ship proposed by the U.S. Navy the preferred design option is the adoption of a Medium Voltage DC power distribution system, due to the high power level required on board and the highly dynamic nature of the electric loads. These DC power distribution systems consist of generation units, energy storage systems and different loads connected to one or more DC busses through switching power converters, providing numerous advantages in performance and efficiency. However, the growth of such systems comes with new challenges in the design and control areas. One problem is the potential instability caused by the interaction among feedback-controlled converters connected to the same DC bus. Many criteria have been developed in the past to evaluate system stability. Additionally, passive or active solutions can be implemented to improve stability margins. One previously proposed solution is to implement Positive Feed-Forward (PFF) control in the load-side converter; with this technique it is possible to introduce a virtual damping impedance at the DC bus. A recently proposed design approach for PFF control is based on the Passivity Based Stability Criterion (PBSC), which analyzes passivity of the overall bus impedance to determine whether the system is stable or unstable. However, since the PBSC does not provide direct information about system’s dynamic performance, the PFF control design based on PBSC might lead to lightly damped systems. Therefore, a disturbance in the system may result in long-lasting lightly damped bus voltage oscillations. Moreover, in order to study the system dynamic performance it is necessary to know the bus impedance. A method has been proposed that uses digital network analyzer techniques and an additional converter that acts as a source for current injection to perturb the bus. The present work provides original contributions in this area. First of all, the effect of the dominant poles of the bus impedance on the system dynamic performance is analyzed. A new closed-form design procedure is proposed for PFF control based on the desired location of these dominant poles that ensures a desired dynamic response with appropriate damping. Regarding bus impedance identification using a switching converter for perturbation injection, a new technique is proposed that eliminates the need for an external converter to provide the excitation. The technique combines measurements performed by existing converters to reconstruct the overall bus impedance. Additionally, an improved perturbation technique utilizes multiple injections to eliminate the problems of injected disturbance rejection by the converter feedback loop at low frequency and the problem of attenuation due to reduced loop gain at high frequencies. The proposed methods are validated using time domain simulations, in which the bus impedance of a single-bus DC power distribution system is estimated and then utilized for the design of a PFF controller to improve the dynamic characteristics

DC Bus Stabilization and Dynamic Performance Improvement of a Multi-Converter System

Evolution of semiconductor devices is allowing to implement power electronics technology in a variety of applications, moving traditional AC systems to more efficient and reliable DC systems. Some examples are shipboard power distribution systems, DC microgrids, electric vehicles and more electric aircrafts. Even though these types of systems offer significant advantages that make them attractive in different areas, the interconnection of feedback-controlled power converters leads to emergent dynamic behavior that a system designer must take into account to ensure proper operation of the system. Additionally, system reconfiguration is very likely to happen in applications such as a shipboard power distribution, where the loads and sources may change depending on the current mission or in case of contingency. In these cases, system dynamics will change and the overall stability may be compromised. Several criteria can be found in the literature to evaluate system stability. More recently, it was shown that the DC bus impedance can be used for this purpose, overcoming some limitations that other criteria had. Additionally, system identification can be used to monitor the bus impedance. However, there is still a need for a method to evaluate the dynamic performance to guarantee that sudden changes in the system will not affect the DC distribution bus significantly. Active and passive stabilization methods can be implemented to allow for the plug-in capability of new converters to increase the size of the system without compromising the overall stability of the power distribution system. One approach is to implement Positive Feed-Forward control on a load side converter, which introduces a virtual damping impedance at the DC bus. A proper design approach for this damping impedance that will ensure a good stability margin is needed. To address these challenges, firstly a modelling approach that facilitates obtaining the transfer functions of a multi-converter system is presented. Second, a simplified model of the bus impedance will be developed that will allow to evaluate the system when it undergoes reconfiguration and to take fast corrective actions when they are needed. Third, a design procedure for the required damping impedance based on the bus impedance dominant poles will be proposed. And last, an adaptive and distributed stabilizing controller will be presented to account for changes in the load demand