Medium Voltage DC integrated power systems for large all electric ships

The Medium Voltage Direct Current (MVDC) distribution represents a promising technology for future shipboard power systems. In such a topic, during the last years, universities and reserch centers have proposed technical solutions to achieve the important targets of MVDC technology, for instance fuel saving, reducing power system weight/space, reconfigurability in case of fault and enhanced power quality. Conversely, the main challenge to face regards voltage control, which has to be capable for guaranteeing the paramount requirement of stability. In regards to this aspect, a possible instability may arise due to the presence of high-bandwidth controlled load converters, modeled as Constant Power Loads (CPLs). Such non-linear loads are seen from the system as negative incremental resistances which are the cause of voltage instability in presence of a perturbation (e.g. load connection, generating system disconnection). The thesis has been realized in the Laboratory of Grid Connected and Marine Electric Power Generation and Control (EPGC Lab.), at the University of Trieste. The aim is to develop voltage control strategies to solve the CPL issue in a realistic multi-converter MVDC Integrated Power System, which is conveniently designed considering a real cruise line MVAC distribution. In such a system, voltage instability may be engage by different approaches, exploiting plant solutions (addition of dedicated filters, addition of energy storage devices) or control solutions. The latter is followed in this thesis: in this case voltage actuators (DC/DC power converters) are used to compensate for the voltage instability: therefore, on one hand (load side) power converters are responsible for the non-linear loads’ issue but, on the other (generators side), they may be utilized to contribute in its solution, thus ensuring a stable behavior. The stabilizing approach foresees the employment of different control techniques, whose theory is focused in the thesis. Starting from the simplier State Feedback (SF), two techniques are mostly studied in the multi-converter arrangement, i.e the Active Damping (AD) and the Linearization via State Feedback (LSF). The AD is a control method to transiently increase the filter resistances in order to damp the voltage oscillations: one of the main pros is the simple implementation on digital controllers, whereas the drawback regards its limited stabilizing action. Therefore, strategies based on Active Damping are to be used to stabilize non-critical systems. Conversely, LSF is a well-performing technique to obtain a notable cancellation of the non-linearities related to CPLs, by exploiting the DC/DC converters to apply a proper non-linear control function. Against the notable capability in stabilizing critical systems, great attention is to be paid in control function’s estimation: inaccurate system parameters or errors in controller’ feedbacks may invalidate the LSF approach, determining a partial loop-cancellation, therefore a non-linear resulting power system. Final simulations are aimed in testing AD and LSF, implemented in global and local control strategies: the former strategy has the purpose to solve the instability directly on CPLs, whereas the second one ensures the bus stability

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

Inland waterway gas-fueled vessels: CASM-based electrification of a pushboat for the European network

The peculiarities of the European inland waterway transport are analyzed, and a novel design of a pushboat for barges convoys is proposed and optimized for the Rhine-Danube corridor. To this aim, a hybrid parallel electric propulsion system is adopted with the perspective to define an eco\u2013friendly vessel. The present work is to be intended as the early-stage in the proof-of-concept (POC) for commercial technologies useful for the electrification of pushboats employed in inland waterway navigation. Specifically, the optimal design solution is highlighted by evaluating of proper attribute weights, which determine the degree of closeness among possible solution and the design target. In particular, CASM methodology to minimize CAPEX and OPEX of a pushboat is adopted

The Role of Voltage Controls in Modern All-Electric Ships: Toward the all electric ship

Ships have witnessed an astounding evolution in the last 200 years. The introduction of the combustion engine has started an ever-faster change, both in the performance and functionality given by the ships. From the steam-powered ships of the early 1800s to the modern diesel-electric ships, the improvements were significant and increasingly rapid. In particular, in the last 30 years, the design of ships has made a huge leap ahead, both in terms of efficiency of the entire vessel and new functions given to the owners. This is due to the progressive electrification that has occurred

A Multi-objective Optimization for the Power Management of Shipboard Zonal DC Microgrids

In the marine industry, flexibility on load supply and efficiency on prime movers are the key elements to develop advanced shipboard systems. To achieve these outcomes, the DC zonal distribution is preferable, as it can enable the most suitable power transfer from sources and storage to loads. In these systems, the high-level control logics are integrated in the Power Management System (PMS), which coordinates the power converters to guarantee the ship mission. The paper is aimed at proposing an optimization algorithm to lead the PMS actions on the controlled DC system. By setting the power setpoints on the interface converters, the optimized PMS can ensure the best use of prime movers, while preserving the duration of energy storage support. By switching from single objective to multi-objective, the optimization can purse the best operating points both in single and in separated buses configuration, thus enhancing the use of energy onboard and the autonomy along the ship route

A Reduced Order Model for the Stable LC-Filter Design on Shipboard DC Microgrids

In advanced ships, the power request is increasing as most of the loads, including propulsion, are electrically supplied. This aspect represents a challenge when focusing on the dynamics performance in balancing the power flows and when considering the large amount of energy to be managed. To this issue, the DC technology is an effective solution to prioritize the flexibility of shipboard power systems and for a better use of onboard energy. A DC shipboard microgrid is a complex system, incorporating several controlled converters. Between the main priorities, certainly the system stability has a prominent role. Indeed, unstable interactions among controlled converters and their filtering stages can arise during the ship operation, thus eventually compromising the ship mission. As the analytical stability assessment on this controlled system is not feasible, then equivalent models or order reductions help when investigating on stability performance. In this work, a cascade-connected shipboard DC system is studied, and an analytical expression of its stability boundaries is attained. The stability results are verified by means of circuital simulations to identify when the load filters are negligible, then achieving the chased order reduction

Renewable Power Routing from a DC Microgrid to an Industrial Cluster

Nowadays, the exploitation of renewable energy sources is the most effective way to force a drastic abatement in greenhouse gas emissions. This consideration is even more strategic when envisaging the green supply of large power industrial loads, which are indeed responsible for an important quota of CO2 production. To foster the sustainability revolution in the industrial sector, an appealing option is to combine battery storage systems and photovoltaic power plant into a DC microgrid architecture. Such a power system is characterized by the presence of multiple power electronics converters, enabling its full controllability. The converters can be coordinated in order to direct the green energy towards the industrial factories. The paper wants to discuss about the smart DC power routing to support the energy demand of an industrial cluster

A Stability-Aimed PMS for Shipboard Zonal DC Microgrids: The C-HIL Tests on Real-Time Platform

In future vessels, high reliability and flexibility in loads supply will be the main drivers in the power system design. To these aims, the zonal DC distribution is the most promising technology, as it enables the suitable and safe transfer of onboard power. Conversely, the large exploitation of filtered DC converters opens challenges, such as system stability. As large bandwidth controls on converters can reduce the stability margin of DC grids, the centralized Power Management System (PMS) can be programmed to supervise the stability status. In this paper, the Weighted Bandwidth Method (WBM) is implemented to configure the PMS when unstable conditions are foreseen. The smart PMS can tune the control gains in order to constantly ensure the stable operation of power distribution, regardless the system configuration. The capability of the reconfiguration algorithm is verified by performing a C-HIL test on a real-time platform

Hybrid-electric solutions for the propulsion of a luxury sailing yacht

The application of hybrid-electric solutions for the propulsion of pleasure crafts is increasing in recent years. The more restrictive regulations on pollutants and the growing sensibility of the people on greenness make this solution even more attractive. In case the craft is stationing in a marine equipped with renewable energy sources, the attractiveness of such a solution increases even more. Besides, a hybrid-electric configuration can reduce the consumptions and, consequently, the operative costs against an initial higher investment. Therefore, hybrid-electric propulsion could give a benefit to raise a stationary market, as the pleasure craft one. This study compares in terms of consumptions three different solutions, besides conventional diesel one, for the propulsion of a sailing yacht

The User Experience as a Demand Response Action in Large All Electric Ships

In the global warming context, the newest All Electric Ships are to be designed to pursue efficiency and system sustainability. As in land power grids, also in shipboard applications the Demand Response is an efficient approach to achieve these important goals. Indeed, it can ensure a considerable reduction in the real-time demand of onboard loads, thus helping in avoiding power unbalances and consequent black-outs. From the digital world, the User Experience (UX) is coming as innovative strategy to smartly train the Demand Response (DR) actions. By several mechanisms, like incentives or gamification, the UX can greatly support the Demand Response in limiting the power requests from onboard users. The paper is conceived to give a short recap on DR, while raising the curtain on new UX methodology