Daniele Bosich

Multiconverter Medium Voltage DC Power Systems on Ships: Constant-Power Loads Instability Solution Using Linearization via State Feedback Control

Bus voltage stability is a key issue in future medium-voltage DC (MVDC) power systems on ships. The presence of high-bandwidth controlled load converters (Constant Power Load, CPL) may induce voltage instabilities. A control design procedure is presented which starts at the modeling level and comes to control implementation. A control method based on a Linearization via State Feedback (LSF), is proposed to face the CPL destabilizing effect and to ensure the MVDC bus voltage stability. A multiconverter shipboard DC grid is analyzed by means of a new comprehensive model, which is able to capture the overall behavior in a second-order nonlinear differential equation. Exploiting DC-DC converters that interface power sources to the bus, LSF technique is able to compensate for system nonlinearities, obtaining a linear system. Then, traditional linear control techniques can be applied to obtain a desired pole placement. With reference to system parameters mismatch, LSF control design is verified by means of a sensitivity analysis, evaluating the possibility of an over-linearization strategy. Time-domain numerical simulations are used to validate the proposed control, in presence of relevant perturbations by means of a two-way comparison (average value model and detailed switching model).

Linearizing control of shipboard multi-machine MVDC power systems feeding Constant Power Loads

Voltage stability in Medium-Voltage DC (MVDC) power systems on ships is a key design goal. MVDC bus voltage stability can be impaired due to the presence of high-bandwidth controlled Constant Power Load (CPL) converters, which can induce negative incremental resistance instabilities. A linearization via state-feedback control is presented in this paper, to stabilize a MVDC bus voltage in presence of destabilizing CPLs. Once a linear control system is obtained, a traditional stable pole placement is operated. The control is designed to be implemented using the controlled DC/DC converters that interface the system power sources to the MVDC bus. The proposed control is verified by means of time-domain numeric simulations based on both an average state space model and a detailed power electronics circuit model, thus providing a two-way comparison.

Considerations on the design of voltage control for multi-machine MVDC power systems on large ships

Medium Voltage Direct Current (MVDC) distribution is an enabling technology for future large ships, e.g. cruise liners or military vessels. In MVDC systems, shipboard loads are normally fed through power-converters directly connected to the MVDC bus. For such systems a key design goal is voltage stability, impaired by the presence of high-bandwidth controlled loads (Constant Power Loads, CPLs). The paper proposes an approach to stabilize the MVDC bus using the generating systems as sources of stabilizing power. Fast controlled DC/DC converters, interfacing generators to MVDC bus, are employed to control it in a stable way and to provide power sharing among the generators. To this aim, an Active Damping method is exploited. A supplementary Linearization via State Feedback control is utilized to stabilize DC/DC load converters feeding particularly impacting CPLs. Proposed controls are verified by means of time-domain numerical simulations. Shipboard feasibility and performance of the proposed control systems are most considered in the work as conclusions.

Shore-to-Ship Power

This contribution starts with a review of the state of the art of existing high-voltage shore connection (HVSC) systems in terms of principles, rules, publications, technologies, and relevant installations. Then, tutorial sections present the main technical aspects of HVSC systems as ship-to-shore interface, shore equipment (transformers, converters, etc.), onboard devices (cubicles, shore switchboard, etc.), operating sequences, and feasibility aspects, for both commercial and military applications. Finally, some technical challenges are presented, concerning intentional/unintentional bonding, interactions between HVSC bonding and cathodic protection systems, bonding opportunity, and electrical safety aspects related to bonding issues in case of large earth fault currents in port facilities.

Linearizing voltage control of MVDC power systems feeding constant power loads: Stability analysis under saturation

Voltage stability in Medium Voltage DC (MVDC) power systems can be impaired due to the presence of tightly regulated power converters in the points of load. Such converters behave as instantaneous Constant Power Loads (CPLs) and can induce negative incremental resistance instabilities. Linearization via State Feedback (LSF) is a control technique which has proven to be effective to prevent CPL instability in MVDC systems. Theoretically, LSF can compensate any amount of CPL, but in practice it works until the interface converter output voltage is saturated. In this work, a controlled converter is employed to interface a generator to an MVDC bus feeding a CPL. A voltage stability study is presented, which applies in case of interface converter saturation. The study is suitable to assess voltage stability of the MVDC system, and to represent it graphically using state plane trajectories.

Voltage control on a refitted luxury yacht using hybrid electric propulsion and LVDC distribution

The adoption of a hybrid electric propulsion system is a promising solution for luxury ship refitting, as it makes it possible to preserve the yacht architecture while introducing innovative technological elements to optimize energy use. Flexibility, comfort and efficiency can be effectively improved using electrical applications in ships. Low Voltage Direct Current (LVDC) distribution can represent a valid solution to achieve these important targets. A luxury ship refitting, based on the introduction of a hybrid electric propulsion and a LVDC distribution, is presented in this paper. Three different power conversion architectures are discussed. One architecture is adopted and analyzed, which utilizes fast DC/DC buck converters to interface generating systems to the grid. Buck converters operate as actuators of the LVDC bus voltage control system. The latter provides bus voltage regulation, load sharing and Active Damping of destabilizing Constant Power Loads.

Active front-end for shaft power generation and voltage control in FREMM frigates integrated power system: Modeling and validation

Italian Navy is being equipping its fleet with the new FREMM class of frigates, developed under a joint program by Italian and French Navies. They are conceptually new war ships endowed with a hybrid propulsion system, composed by a gas turbine and two electric drives, which can be operated also as shaft-generators. University of Trieste has been charged with developing a software simulator of the FREMM integrated electrical power system and its propulsion drives, which has been recently delivered. The simulator will be employed by Italian Navy for virtual tests and personnel training activities. The paper is focused on the shaft-generator operation of the electric propulsion drives and their simulation. Numeric results are displayed in the paper and compared with factory tests in order to validate the simulator.

Toward the future: The MVDC large ship research program

Nowadays, the Medium Voltage Direct Current (MVDC) technology represents a new possibility to renew the shipboard power system of large All Electric Ships (e.g. cruise liners or military vessels.). This new concept is capable of achieving valued advantages, such as simplified system integration, modularity and system efficiency, together with an expected reduction of the space devoted to technical system. For a shipyard interested in keeping a leading position in the market, such benefits will be the main strength points. Therefore, the MVDC Large Ship research program (presented in the paper) has been carried out trying to meet with the industrial needs of one of the partners, whose expertise deeply influenced the research and was deemed indispensable: Fincantieri Shipyard.

MVDC power system voltage control through feedback linearization technique: Application to different shipboard power conversion architectures

MVDC distribution systems are proving to be a very promising research topic, given the large number of studies proposed in the academia and in the navies worldwide. In this paper authors present some multi-machine MVDC distribution system architectures, exploiting and comparing a range of choices on the topic of voltage control. Three network topologies are presented and analyzed, applying a series of tests which include load connections and fault events. Considerations about engineering feasibility, control complexity, performance and possible improvements are given as conclusions.

More Electric Vehicles DC power systems: A large signal stability analysis in presence of CPLs fed by floating supply voltage

This paper deals with voltage stability of DC power systems in More Electric Vehicles (MEVs) in presence of Constant Power Loads (CPLs). Large signal voltage stability is studied according to Lyapunov theory. Original analytical developments are presented to evaluate a Region of Asymptotic Stability (RAS) around a systems stable equilibrium point. Analytical forms are found, which show how the RAS boundaries vary as a function of the systems supply voltage, when the latter is floating. The validation has been carried out by means of the numerical continuation analysis. This result makes it possible to point out how supply voltage fluctuations can decrease the RAS, up to impairing voltage stability not only for large, but even in case of small perturbations.