Distortion and faults detection in shipboard AC/DC power distribution system

Nowadays electric propulsion has become a valid alternative to mechanical propulsion for large ships that require high speed. The electric propulsion advantages are well known and widely documented in the literature: higher dynamic performance of the electric propulsion motors; internal combustion engines separation from shafts; increased flexibility in space/zones subdivision; increased efficiency through the modulation of number of running generators; noise and vibration reduction; increasing in automation, with a consequent crew reduction. The use of electric propulsion along with the progressive increase, in number and power, in electrical loads used for ship services, led to the development of the All Electric Ship (AES) concept. Over the last years, the All Electric Ships (AESs) concept has begun to be adopted by the most important Navies, principally by the U.S. Navy, giving a boost to the technological research. An AES is a ship where all onboard electrical loads (including propulsion) are powered by a single electrical system, called Integrated Electrical System (IPS). The IPS requires careful design and management in order to ensure both high Power Quality standard and the continuity of the service. With the technological progress, the shipboard electrical systems have changed considerably, rising from few MW of installed power to values of the order of hundred MW, both in cruises and military ships. Especially in military vessels, considering the number of special devices that are present on board (weapon systems, communication equipment, radar, sonar, and missile guidance systems), a performing and reliable electrical systems is required. Moreover, it is necessary to notice that some of the new electrical pulsed loads specific to military applications (e.g. radar, electromagnetic launchers, etc.) together with electric drives for propulsion engines can cause strong disturbances to the system, thus causing the malfunction of other electric utilities that may endanger the continuity of the service. The penetration of power electronics converters is the main issue for the contribution of harmonic distortion in AC grids, which must be limited not to increase system power losses, and to allow the correct operation of system and user devices. Standards dictate the maximum admissible values of the total voltage harmonic distortion and of the individual harmonics amplitudes, as a function of the rated system voltage. The relatively limited short-circuit power available on board also exposes the IPS to significant voltage sags and flickers caused by switching and/or intermittent loads. In this scenario, DC electrical distribution systems can be very attractive, thanks to their intrinsic immunity to harmonic problems. If DC micro-grids are interfaced to AC networks by means of Front End Converters (FECs), both AC/DC grid decoupling and considerable AC-side harmonic distortion reduction can be achieved. In addition, they simplify the power supply of converter-fed loads and the interfacing of storage systems. The latter can perform several tasks, including ensuring power supply in case of AC grid loss, peak-shaving and levelling pulsating loads further improving both the quality and the continuity of supply to DC islands loads. In the light of the above, it is evident that the electric power system is of primary importance for a modern ship. Moreover, if high-performance is required, careful analysis of the disturbances in the power system is mandatory. In fact, in order to achieve a reliable and performing power system, together with a high-Power Quality, it is necessary to assess this situation and propose guidelines to be observed for the solution of various problems. The definition and evaluation of possible IPS architectures should take into account AC/DC protection devices in order to carry out an integrated analysis of the system. Different MVAC/MVDC electrical distribution layouts coupling with all-electric or hybrid propulsion (electric/diesel/gas turbine) needs to be accurately investigated to show its advantages in terms of reliability, safety and quality of power. The thesis focusses on the Naval Smart Grid (NaSG) research project completed in partnership with the University of Trieste and the Polytechnic University of Milan. The aim of the research is to produce useful results for the design of a new ship, equipped with the following innovative features: modular power system; subsystem flexible integration; efficiency improvement; security improvement; new weapon systems; survivability improvement and high Power Quality standard. The main focus was the study of methodologies/solutions able to improve and define the onboard Power Quality (PQ). The research project reports Power Quality analysis about aspects of continuity of service, harmonic disturbances, pulsed power loads impact on the system, electromechanical transient evaluation and use of power and energy storage systems. An exhaustive investigation was carried out on system architectures in frequency domain to identify resonances and non-linear loads to detect disturbance frequencies. Moreover, the guidelines for the correct coordination of all the elements of the power system design affecting system performance (protections, converters, control systems, energy storage systems, etc.) are reported. A brief abstract for each Chapter is reported. Chapter 1 and 2 - Overview of Electrical Naval Systems and Integrated Power System in Military Ships The chapter reports the complete state of the art on naval electrical system and a brief description of naval classification, showing technological improvements and historical evolution. Details about electric propulsion, electrical generation on board, energy distribution and network layout are carried out. A complete description of the main IPS military ships with their own architecture and features is reported. Chapter 3 - Methodologies for Harmonic Disturbances Analysis and Power Quality (Service Continuity) In the field of Power quality (harmonic content, asymmetries, voltage sags, power factor), methodologies applied for the analysis/detection of harmonic disturbances are reported with an overview of electrical systems dependability in order to evaluate the service continuity of the system. Harmonic distortion could affect equipment on shipboard causing its outages, consequently, in an island system, power distribution network should ensure high re-configurability after faults, damage or untimely switch off. However, the increased interest in system’s safety and resilience generates, in turn, an increase in design burden necessary to analyze the consequences of faults and demonstrate the system’s compliance with the relevant regulations. The chapter presents the models and calculation code used for simulation activities. A Simulink model for time domain analysis and for time varying non-linear load, as well as a Fortran model for harmonic domain are described. Chapter 4 and 5 - Characterization of a military aircraft carrier and Aircraft Cavour – Measurement campaign A measurement campaign onboard the ship Cavour was carried out with the aim to characterize the relevant electric loads on board military vessel and to validate the models of the system’s components to be used. The analysis of data collected, allows to model the behavior of loads in terms of time and frequency domains, thus permitting their use for the required studies. Some specific electrical loads, such as new electrically pulsed loads specific for military applications (e.g. radar, electromagnetic launchers, etc.) with high distorted current absorption were identified. Their characterization was carried out in order to define their contribution to harmonic disturbances and their impact on the network. A model validation based on a measurement campaign is carried out. Chapter 6 - A New layout for an Integrated Power System Naval Unit-All Electric/Hybrid Different IPS architectures are defined: a full MVAC (Medium Voltage Alternate Current) power system, a hybrid MVAC plus MVDC/LVDC islands (Medium/Low Voltage Direct Current) and a MVAC 50-60 Hz, with a hybrid (electric/diesel/gas turbine) propulsion. In the architecture of the latter, the power of the installed engines is much lower than the first two cases. Chapter 7 - Network Equivalents in Harmonic Domain The needs to easily represent a complex network with high accuracy, lead to the development of a methodology based on aggregation of loads, creating a simplified network to carry out harmonic analysis. Different equivalent network models have been proposed that show their accuracy, through network impedances, and compare them with the overall representation of the network. The influence of cables was also studied. The best radial equivalent network was identified. Chapter 8 - Harmonic Analysis In order to propose appropriate solutions designed to improve power quality, the study of system impedance and power systems in frequency domain were studied. This analysis, carried out on the basis of the schematics and data load obtained in cooperation with the IT Navy, revealed some criticalities in the frequency range for both the systems architectures. As to full MVAC (Medium Voltage Alternate Current) power system and hybrid MVAC plus MVDC/LVDC islands, the aim was to evaluate whether or not the inclusion of capacitors (on shore, for power factor correction in shore connections) or filters (onboard, to reduce harmonic disturbances produced by propulsion systems) cause special issues, because of the high power of installed propulsion engines. Moreover, the advantages of DC island on electrical distribution in order to ensure high reliability and quality of service, in addition to the need to increase the efficiency of the ships’ power systems are highlighted. For the MVAC 50-60 Hz layout, the goal is to show how the use of hybrid (electric/diesel/gas turbine) propulsion where the power of engines is significantly reduced as compared to previous cases could solve some issues relating to power quality aspects. Chapter 9 - Reliability Analysis Preliminary studies about dependability, re-configurability and some top-events relevant for the vessel, were evaluated for all electric MVAC/MVAC "hybrid" models. The analysis of electrical disconnection of load areas due to a fault or an untimely tripping of the switches caused by harmonic disturbances was carried out. Chapter 10 - Three-Phase Short Circuit Analysis For MVAC 50-60 Hz Layout Preliminary evaluations were performed by analyzing the system within the perspective of given faults to perform system analysis in both permanent and short-circuit conditions. To highlight possible protection issues, the steady state condition and the three-phase short-circuit faults were studied and simulated under different load conditions for the MVAC architecture plus rotary converters, with hybrid (electric/diesel/gas turbine) propulsion.