Energy Storage Systems For Electrical Microgrids With Pulsed Power Loads

Pulsed power loads (PPLs) are highly non-linear and can cause significant stability and power quality issues in an electrical microgrid. According to the present invention, many of these issues can be mitigated by an Energy Storage System (ESS) that offsets the PPL. The ESS can maintain a constant bus voltage and decouple the generation sources from the PPL. For example, the ESS specifications can be obtained with an ideal, band-limited hybrid battery and flywheel system.https://digitalcommons.mtu.edu/patents/1158/thumbnail.jp

Model Predictive Control for Dual Active Bridge in Naval DC Microgrids Supplying Pulsed Power Loads Featuring Fast Transition and Online Transformer Current Minimization

Pulsed power loads (PPLs) are commonly incorporated in medium voltage dc microgrids on naval vessels. To mitigate their detrimental effects, dedicated energy storage systems can be installed and their converters need to have excellent disturbance rejection capability. To facilitate this objective, a moving-discretized-control-set model-predictive-control (MDCS-MPC) is proposed in this letter and applied on a dual-active-bridge converter. Fixed switching frequency is maintained, enabling easy passive components design. The proposed MDCS-MPC has a small number of calculating points in each switching period, which enables the implementation in standard commercial control platforms. The operating principle of the MDCS-MPC is introduced in development of a cost function that, on one hand, provides stiff voltage regulation; on the other hand, minimizes transformer current stress online. Theoretical claims are verified on a 20 kHz 1 kW dual active bridge

Model-Predictive-Control for Dual-Active-Bridge Converters Supplying Pulsed Power Loads in Naval DC Micro-grids

Pulsed-Power-Loads (PPLs) are becoming prevalent in medium-voltage naval DC micro-grids. To alleviate their effects on the system, energy storages are commonly installed. For optimal performance, their interface converters need to have fast dynamics and excellent disturbance rejection capability. Moreover, these converters often need to have voltage transformation and galvanic isolation capability since common energy storage technologies like batteries and super-caps are typically assembled with low voltage strings. In order to address these issues, a Moving-Discretized-Control-Set Model-Predictive-Control (MDCS-MPC) is proposed in this paper and applied on a Dual-Active-Bridge converter. Fixed switching frequency is maintained, enabling easy passive components design. The proposed MDCS-MPC has a reduced prediction horizon, which allows low computational burden. The operating principle of the MDCS-MPC is introduced in development of a cost function that provides stiff voltage regulation. Resonance damping and sampling noise resistance can also be achieved with the proposed cost function. An adaptive step is introduced to enable fast transition. Assessments on the performance of the proposed MDCS-MPC are conducted. Comparisons with other control methods are also provided. Experimental validations on a 300V/300V 20kHz 1kW Dual-Active-Bridge converter are carried out to verify the theoretical claims. Index Terms-Isolated DC/DC converter, Dual-Active-Bridge (DAB), Model Predictive Control (MPC)

Sliding mode control for pulsed load power supply converters in DC shipboard microgrids

Pulsed power load (PPL) is a special load type in shipboard microgrids (SMGs), which consists of the generation module, energy storage system, and various types of loads. Having a reliable power supply to shipboard loads is a challenge as the SMG operates in islanded mode in most cases. Particularly, the PPLs require high transient power transfer with fast dynamics and strong robustness. Conventional solution to supply for the PPL is based on proportional-integral (PI) control, which can be used by linearizing the system around the equilibrium operation point. However, for a pulsed power supply (PPS) system, the load demand drastically changes in a short time, usually in millisecond level, making the operating point changes when the pulsed power is triggered or terminated. To supply the PPL with fast dynamics and robustness, an improved PPS control method is presented in this paper. By adopting a nonlinear sliding mode control (SMC) method, fast voltage regulation and robust pulse power tracking can be achieved. In the PPS, the PPL power demand is divided into two terms: one is the average power that is supplied by the SMG and the other is the fast pulsed power that is supplied by the storage capacitor. The size and cost of the storage capacitor are reduced as it is intentionally driven to a deep discharge. The PPS system configuration and coordination principle, SMC controllers, and sizing of passive elements in the PPS are analyzed in detail. The effectiveness of the presented PPS is verified by simulation results.Peer ReviewedPostprint (published version

Methods for Dynamic Stabilization, Performance Improvement, and Load Power Sharing In DC Power Distribution Systems

Modern DC power distribution systems (DC-PDS) offer high efficiency and flexibility which make them ideal for mission-critical applications such as on-board power systems of All-Electric ships, electric vehicles, More-Electric-Aircrafts, and DC Microgrids. Despite these attractive features, there are still challenges that need to be addressed. The two most important challenges are system stability and load power sharing. The stability and performance are of concern because DC-PDS are typically formed by the interconnection of several feedback-controlled power converters. The resulting interactions can lead to destabilizing dynamics. Likewise, in a DC-PDS there are several source converters that are operating in parallel to supply the total load power. This improves the system reliability through structural redundancy. Improper load sharing, however, leads to overloading of some of the source converters which might result in cascaded failures. Several stability criteria are proposed in the literature. Among all, the impedance-based approaches are well accepted for stability analysis and stabilizing controllers design. These methods are based on evaluating the system impedances using linear control theory and small-signal dynamic analysis. So, using such methods, stabilization is accomplished in an intuitive and design-oriented manner. However, an important disadvantage of linear methods is that their range of effectiveness is limited to a small-signal region around an operating point of the system wherein the non-linear system can be approximated by a linear one. Likewise, DC-PDS often experience large-signal transients and operating point variations. Thus, linear controllers may fail to preserve the stability and performance for large-signal transients. Therefore, there is a need to develop new methods that guarantee system stability and performance during such large-signal transients. To solve the problem of load power sharing in DC-PDS, various methods can be found in the literature. Load sharing mechanisms can be categorized as Droop methods and active sharing techniques. In the conventional Droop method, a virtual resistance is added to the output impedance of the source converter and a decentralized load sharing is achieved. Although simple and effective, Droop control causes a variable bus voltage drop which requires additional control measures to achieve tight voltage regulation. Active methods, on the other hand, manage to achieve load sharing at the cost of additional control requirements such as high bandwidth communication links among the source converters which increase the complexity and cost. Thus, it is desirable to develop new methods to solve the problem of proper load sharing in a simple, efficient, and inexpensive manner. To address the above challenges, in this dissertation, a generic DC-PDS is considered and the system dynamics is studied for small-signal and large-signal operations. Based on this analysis, novel stabilizing control methods are proposed that are implemented in a source converter. The proposed approach manages to guarantees stability and performance for various operating scenarios. Additionally, to solve the load-sharing problem, a novel communication-less current-sharing control scheme is proposed. This method guarantees proper distributed load sharing among several source converters without any bus voltage drop and requiring any physical communication network

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