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

Operation and protection of DC shipboard power system

Marine vessels integrated with electrical propulsion have conventionally been based on fixed-voltage, fixed-frequency (50/60 Hz) ac generation and distribution system. In recent years, dc power system in the marine vessels has been proposed primarily to take advantage of the fuel-efficient operation, which is enabled through the integration of the variable frequency diesel generators. Such dc shipboard power system (SPS) also enables interconnection of the alternative power generation and energy storage technologies, which helps in peak shaving of the generators in the event of wide load variation. In spite of the advantages, one of the impediments to the widespread adoption of dc SPS is the lack of comprehensive short-circuit fault management strategies. These vessels are dominated by a significant number of active loads and a finite number of dc generation sources. As a result, the network configuration is expected to be dynamically altered to fulfil the required generation and load demands to cater for the desired marine mission. Such varying network configurations make the transient responses significantly different from the conventional ac grids and the prospective dc grids and hence making the fault management strategies more challenging. Thus, the modeling and control of dc generation sources, loading scenarios, and system operation become important aspects to effectively understand and analyze transient responses. The aim of this thesis is to address the modeling and control aspects of dc shipboard power systems and devising protection algorithms. This thesis considers the platform supply vessel (PSV) as the target dc marine vessel and covers detailed investigation on the challenges in the modeling and control and the solutions of the dc generation and load systems. PSV is taken as an example of the marine vessel due to its complex operating scenarios and wider applicability in the marine industry. Voltage source converter (VSC) based dc generation system is chosen owing to its improved operational benefits. The real-time transient framework and operation of the dc PSV are discussed along with the possible contingency scenarios, such as the outage of the generation systems, abrupt load changes, effect of the energy storage systems and so on. The disadvantage of the dc system is the lack of zero current crossing which worsens the problem of arc quenching. In dc power system, the converters will be used as interfaces between the generators and the marine loads. During the fault, the dc-link capacitor will discharge rapidly, releasing high current. This capacitive discharge current represents a serious challenge in fault detection and protection as the current profile depends on the circuit parameters. Moreover, the time required to detect the dc fault current is very low. It is thus required to devise suitable protection algorithm for fault detection and isolation. Proper operation of the dc SPS calls for high fidelity control and modeling of the system components. After the modeling and operation, this thesis also covers systematic transient studies to devise the short-circuit fault detection technique for the dc PSV. The transient response of the VSC-based dc generation system in the dc PSV is generally characterized by rapidly rising capacitive discharge current which is different from the ac counterpart. The limitations of the traditional time-domain based fault detection techniques for the varying network conditions of the dc PSV are also discussed. The rapidly rising fault current is expected to have high-frequency components which could be an effective indicator of the transient condition. With this regard, this thesis also covers a short-time Fourier transform (STFT) based quantitative analysis of the high-frequency components in the dc fault currents. Detailed operating principles, factors affecting the STFT operation and the sensitivity analysis are also discussed. For the enhanced selectivity in the dc PSV, a novel directional protection is also proposed which uses directional zonal interlocking as a directional element and STFT as fault detector. The efficacy of this proposed directional protection is also presented which is verified against a range of fault impedances initiated at the generator terminals, load terminals, lines and buses of the dc PSV. The thesis is concluded by discussing the future work and recommendations on the fault-tolerant architectures to external short-circuit faults.Doctor of Philosoph

Protection of MVDC shipboard power system using Rogowski coil

One of the major challenges faced in the design of DC shipboard power systems (SPS) is the provision of effective DC protection. The selection of the current sensors plays an important role in the detection of fault current in DC systems. The Rogowski coil has emerged as a popular choice of current sensor for use in DC SPS protection system primarily because of its fast response and non-saturable property. This paper presents the modelling and operation of the Rogowski coil and its integration into protection algorithms to achieve effective protection. With this method, the transient conditions experienced during faults can be effectively identified and differentiated.NRF (Natl Research Foundation, S’pore)Accepted versio

Protection Strategies for LVDC Distribution System

DC power system is gaining popularity. Research is being conducted worldwide on implementation of LVDC (low voltage direct current) and MVDC (medium voltage direct current) in applications such as ship power systems. One major hurdle to this path is the immaturity of the protection systems. This paper deals with the modeling of LV/MVDC power system with different marine loads and faults. Since LV/MVDC power systems are still in preliminary stages, their modeling and fault analysis will help in addressing the requirements of various protection strategies. The system modeling and fault simulations are done in simulation software Power Systems Computer Aided Design (PSCAD). The components of DC ship power system are also described.Accepted versio

Short-circuit fault management in DC electric ship propulsion system : protection requirements, review of existing technologies and future research trends

Ease of integration of the variable speed diesel generators resulting in substantial reduction of the fuel consumption is the key motivation for the development of the dc shipboard power system (SPS). One of the impediments to the widespread adoption of the dc SPS, however, has been the lack of comprehensive fault management strategies during the short-circuit faults. Such strategies comprise of fault detection, fault isolation, and reconfiguration of dc SPS. In the existing literature, all these aspects of fault management are dealt independently and mostly assuming ideal conditions. All the strategies are of utmost importance and it is needed to study them under a common framework which is the aim of this paper. This paper starts with a brief discussion on the characteristics of dc SPS along with recent modeling techniques. Subsequently, this paper describes the short-circuit fault studies, fault characteristics, and protection requirements. Finally, this paper outlines the working principle, advantages, and limitations of the fault detection, isolation, and reconfiguration strategies developed for the dc power system and analyzes their suitability to the dc SPS. This paper is concluded by identifying the future research trends needed for the development of the short-circuit fault management strategies of dc SPS for critical marine missions.Accepted versio

Power systems frequency estimation using amplitude tracking square wave for low-end protective relays

Growing emphasis on the disturbance-free electrical power distribution demands for economical and efficient smart estimation techniques in the automation architectures of the future medium voltage (MV)/ low voltage (LV) distribution networks. Unlike high-end protective relays which are typically used in the high voltage transmission systems, low-end economical protective relays are suitable for the MV/LV distribution networks. Frequency estimation is one of the pre-requisites of the electrical protective relays in such systems. The low-end protective relays demand for computationally efficient signal processing algorithms for frequency estimation thus eliminating the need to integrate sophisticated hardware components and advanced software routines. In this paper, time-domain based frequency estimation is proposed, which is focused for the implementation in the low-end relays for MV/LV distribution networks. Frequency estimation is based on the novel amplitude tracking square wave (ATSW), which is fast and sensitive to disturbances in the monitored signal. Performance of the ATSW for frequency estimation is analyzed on the recorded real-world disturbance signals as well as on the various simulated case studies. Simplicity of the ATSW for low-end relays is substantiated by comparative analysis with the existing frequency estimation techniques. In addition to the frequency estimation, versatility of the ATSW is substantiated by application for transient state detection in the AC and DC power systems.Accepted versio

Design, analysis, and comparison of automatic flux regulator with automatic voltage regulator-based generation system for dc marine vessels

The generation system in the dc marine vessel is expected to be comprised of wound rotor synchronous generator (WRSG) which is interfaced with an active front-end (AFE) rectifier. For such system, WRSG flux and dc voltage at the AFE rectifier output are controlled independently. WRSG flux is maintained by the field excitation circuit of WRSG for which automatic voltage regulator (AVR) has been traditionally employed. AVR operates by regulating the WRSG terminal voltage, thus demands additional tuned filter for the application in the dc marine vessel. This filter increases the size of the generation system along with additional cooling requirements at higher load. With this regard, this paper introduces an automatic flux regulator (AFR)-based field excitation system where the WRSG flux is maintained directly and is independent of the measurement of the terminal voltage. Small-signal analysis is done to design the control loop of the AFR-based field excitation circuit. To verify its efficacy, comparison with the AVR-based dc generation system is conducted. The comparison has been done on the basis of control response and marine operation.Accepted versio

Improved S transform-based fault detection method in voltage source converter interfaced DC system

The short circuit fault in the voltage source converter-based dc power system typically generates rapidly rising transient current which may have serious repercussions on dc grid operation and health of the integrated power electronic devices. Thus, the dc grid requires a high speed and robust fault detection for reliable system operation. With this regard, this article proposes a fault detection method based on the S transform (ST) with adaptive adjustment. This improved ST is based on frequency-domain and is able to detect the fault condition within 0.3 ms. It consists of high-frequency detection, which is responsible for fast response due to high time resolution, and low-frequency screening which is used to differentiate faults from other transient conditions. Introducing a correction factor into a Gaussian function when computing ST could extract the high-frequency spectrum, while the low frequency spectrum information is still retained. The proposed method is validated with the multiterminal dc system developed in the OPAL-RT-based real-time simulator. Additionally, its performance is tested with the point-to-point experimental dc test bed. Comparative analysis with other popular frequency-domain fault detection methods, namely, wavelet transform and short-time Fourier transform substantiates the effectiveness of this method

Modelling of DC shipboard power system

Modelling and simulation of emerging DC shipboard power system (SPS) is required to validate the hardware design, converter control and protection algorithms. As DC SPSs may be large and complicated, the simulation of such systems using high fidelity models generally consumes much time and the validation of performance using multiple test scenarios becomes impractical. An alternative method is to conduct such simulations in real-time using a multi-core simulation platform. Real-time simulation also allows protection relays (running embedded protection algorithms) to be verified through `hardware in the loop' techniques. In this paper we discuss the simulation of DC SPSs in real-time using Opal-RT simulation platform. Firstly the system is modelled for non-real-time simulation. Subsequently these models are converted for real-time simulation and executed using RT-Lab on the Opal-RT target. Modelling and control approaches of DC SPS and some of the attributes of Opal-RT are also discussed.NRF (Natl Research Foundation, S’pore)Accepted versio

Short-time fourier transform based transient analysis of VSC interfaced point-to-point DC system

The transient response of the voltage source converter (VSC) interfaced dc system is significantly different from the ac counterpart. The rapid discharge current from the dc-link capacitors and the vulnerability of the freewheeling diodes during the short circuit in dc grid demand that the transient detection algorithm executes within a few milliseconds. The rapidly rising fault current in the dc grid is expected to have high-frequency components which might be an effective indicator of the transient condition. This paper presents quantitative investigation of the high-frequency components utilizing short-time Fourier transform (STFT) during transient conditions. Detailed operating principles with various factors affecting the STFT operation such as ripple content of the input dc signal and window type and length have been thoroughly investigated. STFT algorithm is able to detect low-impedance faults within 1 ms and high-impedance faults in 2 ms. Moreover, it is able to distinguish between short-circuit fault and less severe transient conditions such as sudden load change. The STFT algorithm is evaluated analytically and subsequently applied to a MATLAB/Simulink based dc test system. It is further validated and substantiated with the real fault current data obtained from a scaled-down experimental testbed. Sensitivity analysis and comparison with the existing frequency-domain-based fault-detection method are done to support the efficacy of the proposed method.Accepted versio