A review of power electronics equipment for all-electric ship MVDC power systems

Medium Voltage DC (MVDC) distribution Power Systems for all-electric ships (AES) can be regarded as functionally composed of three subsystems, namely the power sources, the load centers and the distribution network. Extensive use of power electronics is required for connecting power sources and load centers to the MVDC bus and for protecting the MVDC power system through properly placed DC circuit breakers. In this paper, an overview is given of the power electronics equipment found in the literature and on the market that could be suitable for use in future AES MVDC power systems. Some industrial experiences regarding DC generator systems, energy storage apparatus and solid-state DC circuit breaker prototypes are reported in the paper as examples of state-of-the-art realizations. Different DC/DC converters, which can be employed as solid-state transformers, are also discussed and a structure obtained by combining them is proposed

Fault Discrimination Using SiC JFET Based Self-Powered Solid State Circuit Breakers in a Residential DC Community Microgrid

This thesis validates the use of ultra-fast normally-on SiC JFET based self-powered solid state circuit breakers (SSCBs) as the main protective device for a 340Vdc residential DC community microgrid. These SSCBs will be incorporated into a radial distribution system so that line to line short circuit faults and other types of faults can be isolated anywhere within the microgrid. Because of the nature and characteristics of short circuit fault inception in DC microgrids, the time-current trip characteristics of protective devices must be several orders of magnitude of faster than conventional circuit breakers. The proposed SSCB detects short circuit faults by sensing its drain-source voltage rise, and draws power from the fault condition to turn and hold off the SiC JFET. The new two-terminal SSCB can be directly placed in a circuit branch without requiring any external power supply or additional wiring. To achieve the coordination between upstream and downstream SSCBs in the DC community microgrid, a little change has been made to the proposed SSCB. A resistor in the schematic of SSCB has been changed to a potentiometer to have a different response time to short circuit fault. In order to figure out the value of that potentiometer to get the best coordination, a transfer function is derived. LTspice VI and PLECS are used to verify the analytical work in the design. In the simulation layout, the DC community microgrid has been simplified to a radial system and 5 SSCBs are connected in series. Short circuit fault is applied at different locations in the DC system to test the effectiveness of the coordination scheme

Solid state fault interruption devices in medium voltage distribution systems with distributed generators

Faults in electric power grids often result in long term interruption of electricity supply. The frequent and the sudden occurrences of faults undermine the reliability and continuity of electric networks. However, the new age digital economy demands a continuous supply of electricity of high power quality. This demand of continuous supply and the effort to increase the penetration of renewable power sources into the electric grid has led to a need of highly efficient and flexible systems with the ability to provide power of very high quality. The solid state fault interruption device (SSFID) is one amongst those devices. It is used to quickly isolate sections of the networks where permanent faults have occurred. The development and simulated testing of a SSFID to validate its use in future medium voltage transmission systems is discussed. Also, the effects of using these SSFIDs in such systems during faults are investigated --Abstract, page iii

Passive Clamping Circuit for Reduced Switch Count in Solid State Circuit Breakers

DC Solid-State Circuit Breakers (SSCB) play an important role in the protection of DC and hybrid AC/DC systems. As they operate mostly in on-state, the use of wide-bandgap devices, with low conduction losses, is an attractive solution. These devices need to be selected based on the rare occasion of fault breaking, with associated overcurrent and overvoltage intervals. In this work a novel clamping circuit, consisting in its simplified form of Metal-Oxide Varistors and capacitors, is proposed to be used in a DC SSCB. This circuit has the objective to reduce the breaker overvoltage during protection, enabling a lower number of required semiconductors in series and consequent overall reduced losses. An analysis of the idea and operation principle of the system is presented, alongside its stages of operation and defining equations. To evaluate its benefits, the system is compared with one typical passive solution. At the end a scaled prototype is tested, validating the benefits and theoretical analysis of the proposed approach

Enhancement of fault current contribution from inverter-based resources

The reduction in levels of fault current infeed as inverter-based resources (IBR) displace synchronous machines undermines the ability of a conventional protection system to identify and isolate faults in an effective manner and is therefore a concern for system operators (SOs). This observation provided the motivation to investigate the limitations of IBRs when injecting fault current and to explore how these limitations might be overcome. This thesis investigates techniques aimed at significantly increasing Fault Current Contribution (FCC) from an IBR system so that renewable energy resources can continue to be deployed without compromising the protection system. The techniques for enhancing FCC are at three different levels of an IBR system: at semiconductor or device level, circuit level and system level. The first study uses phase change materials (PCM) to provide a short-term overload rating to insulated-gate bipolar transistors (IGBTs) and found them to have very limited potential to provide FCC. A Finite Element Analysis (FEA) of heat-flow concluded that, although the PCM was useful for dealing with short over-load currents, it was unsuitable for facilitating large fault currents of several times normal load current. The view was that if the fault current cannot be created at device level through better thermal management, then a circuit level innovation would be required. The second study investigates series/parallel switching of submodules in modular converters. This takes advantage of the fact that during a fault, the line voltage is reduced, and if it falls below 0.5 pu then half of the sub-modules (SMs) can be put into parallel with the other half to double the FCC (2 pu) at half the voltage (0.5 pu). Similarly, if the voltage drops below 0.25 pu, parallel connection of four groups of SMs would enable 4 pu current capability. A model of a static synchronous compensator (STATCOM) was developed, inspired by the alternate arm converter (AAC), with the director switch of the AAC used as part of the reconfiguration circuit. The conclusion of this study was that the penalty paid in power losses in the additional semiconductor devices used for reconfiguration is reasonable for the 2 pu FCC case but not at the 4 pu FCC case. The third study was based on circuit reconfiguration but beyond the converter itself and in this case the windings of the coupling transformer of a STATCOM. Sections of winding were switched using thyristors to tap-change the transformer by a large factor. Using the proposed thyristor-based electronic tap-changer (eTC), the number of turns of the grid-side winding was reduced during a voltage dip, so that larger current can be delivered to the network for the same converter current. The STATCOM was controlled in the natural frame (abc frame) and this control is used to actively drive the currents in the tap-changer thyristors to zero when needed so that they can be commuted rapidly. The transformer was configured to give a normal ratio of 1:4 and be able to tap-down to 1:2 and 1:1 to increase FCC to 2 pu or 4 pu. Theoretical analysis of, and operating principles for, the proposed eTC, together with their associated control schemes, are verified by time-domain simulation at full-scale. The case-study circuit demonstrates delivery of substantial fault current contribution (FCC) of up to 4 pu at the point of common coupling (PCC) in less than half a cycle (10 ms) after detection of three- and single-phase faults. The results demonstrate that the proposed eTC is a good candidate for the enhancement of fault current from IBR systems that employ coupling transformers, allowing them thereby to make a contribution to future electricity networks dominated by IBR.Open Acces

On the performance evaluation of lithium-ion battery systems for dynamic load functions in warship hybrid power and propulsion systems

Battery technology has developed to a juncture where high power and high energy density characteristics can be exploited for a common use battery energy storage system (ESS) for warship power systems to improve system steady state and dynamic performance. A critical review of previous research has exposed a lack of knowledge in performance assessment of battery ESS to operate as power reserve, to load level generator sets and supply laser directed energy weapons (LDEW) in a warship hybrid power and propulsion system. This research explores the performance impact of using a battery ESS in a candidate hybrid power and propulsion system. A simulation model of a lithium-ion nickel manganese cobalt based ESS was developed and validated against high rates of charge and discharge. Three system models were developed to explore the steady state, quasi-steady state and dynamic performance of the candidate power system when the battery is integrated. Three key investigations were conducted using the respective system models. The first explored the effects of ESS on the candidate power system performance when the ESS is operated as power reserve. Analysis showed that a 40% reduction in exhaust greenhouse gas (GHG) emissions was potentially achievable from the candidate warship compared to conventional operating practice. The second explored power system performance when operating the ESS operates to load level a diesel generator under quasi-steady state conditions. A 2% droop limit is suggested to mitigate against adverse quality of power supply (QPS) conditions for electrical consumers. The third investigation, and key contribution to the field of naval power systems, explored the impact of LDEW demands on the transient response of the ESS and system quality of power supply. The research findings show that the battery ESS is capable of high rates of fire for extended periods subject to state of charge operating limitations. To mitigate against adverse QPS conditions and provide operators with a realistic operating envelope to power the laser with the battery ESS, it is recommended that the power limit of the laser load should be 1.75 MW peak power

Hybrid Energy Storage Implementation in DC and AC Power System for Efficiency, Power Quality and Reliability Improvements

Battery storage devices have been widely utilized for different applications. However, for high power applications, battery storage systems come with several challenges, such as the thermal issue, low power density, low life span and high cost. Compared with batteries, supercapacitors have a lower energy density but their power density is very high, and they offer higher cyclic life and efficiency even during fast charge and discharge processes. In this dissertation, new techniques for the control and energy management of the hybrid battery-supercapacitor storage system are developed to improve the performance of the system in terms of efficiency, power quality and reliability. To evaluate the findings of this dissertation, a laboratory-scale DC microgrid system is designed and implemented. The developed microgrid utilizes a hybrid lead-acid battery and supercapacitor energy storage system and is loaded under various grid conditions. The developed microgrid has also real-time monitoring, control and energy management capabilities. A new control scheme and real-time energy management algorithm for an actively controlled hybrid DC microgrid is developed to reduce the adverse impacts of pulsed power loads. The developed control scheme is an adaptive current-voltage controller that is based on the moving average measurement technique and an adaptive proportional compensator. Unlike conventional energy control methods, the developed controller has the advantages of controlling both current and voltage of the system. This development is experimentally tested and verified. The results show significant improvements achieved in terms of enhancing the system efficiency, reducing the AC grid voltage drop and mitigating frequency fluctuation. Moreover, a novel event-based protection scheme for a multi-terminal DC power system has been developed and evaluated. In this technique, fault identification and classifications are performed based on the current derivative method and employing an artificial inductive line impedance. The developed scheme does not require high speed communication and synchronization and it transfers much less data when compared with the traditional method such as the differential protection approach. Moreover, this scheme utilizes less measurement equipment since only the DC bus data is required