Showing papers by "Juan Carlos Balda published in 2015"

A Semi-Markov Model for Control of Energy Storage in Utility Grids and Microgrids With PV Generation

Abstract: Photovoltaic (PV) penetration levels in the power grid have significantly increased during the last years. However, issues such as cloud-induced intermittency in PV generation forces equipment on the electrical grid to cycle excessively, preventing PV generation from being considered as a reliable or dispatchable source of power, particularly by utilities. A semi-Markov process model is proposed to model PV power. Unlike existing models of PV power, the proposed model has a wide range of applicability across both small and large timescales. These applications include simulating PV power, short-term forecasting of PV power, design of rule-based controllers for energy storage units (ESU), and stochastic scheduling of ESU in conjunction with other resources. The model is applied to study two cases of coordinating ESU with PV generation. In the first case, the model serves to design a coordination scheme for a hybrid battery-ultracapacitor (UC) ESU where the UC serves to extend the lifetime of a lead-acid battery. In the second case, the model allows probabilistic scheduling in a standalone PV/diesel/battery-ESU microgrid. In both cases, the model improves performance in terms of either battery lifetime or fuel consumption.

An evaluation of selected solid-state transformer topologies for electric distribution systems

Abstract: Multiple topologies of solid-state transformers (SST) were proposed for locomotive traction systems, and more recently, for electric power distribution systems because of advantages such as reduced volume and weight, and functionalities not always possible with traditional fundamental-frequency transformers. The main goal of this paper is to evaluate selected SST topologies based on dual-active bridge (DAB) converters as well as a new SST topology based on boost converters in order to establish their suitability for emerging applications in electric power distribution systems. The evaluation is based on the number of power semiconductor devices, requirements imposed upon the high-frequency (HF) transformers, system losses, and a simplified cost comparison. The new boost-based SST topology has the highest system efficiency among the evaluated topologies, but requires twice the number of power semiconductor devices when using the same high-voltage device in all topologies.

The effects of flooding attacks on time-critical communications in the smart grid

Abstract: Many smart grid communications are delay sensitive and have very strict timing requirements for message deliveries. For example, trip protection messages must be delivered to the destination within 3 ms according to IEC 61850. Such time-critical communications are vulnerable to flooding attacks which attempt to increase message delivery delay through congesting the network channel and exhausting the computation resources of the communicating nodes. However, there is a lack of understanding on how much flooding attacks affect message delivery delays. In this paper, we conduct experimental studies to investigate how flooding attacks affect message delivery delays for time-critical communications in smart grid. Our experiments are based on both wireless networks in a lab and wired networks in a real, industry-standard electric power facility. Experimental results show that even low-rate flooding attacks can significantly increase the message delivery delays, especially when wireless networks are used.

A 13.8-kV 4.75-MVA microgrid laboratory test bed

Abstract: Microgrid concepts and applications have become more promising during recent years because of the potential for improving system operation, particularly, under emergency conditions. The design and hardware realization of a microgrid laboratory test bed enabling research and development for microgrids, smart-grid systems, and distributed energy integration is presented in this paper. The state-of-the-art regenerative back-to-back voltage source converter topology is used for implementing and emulating the testing capabilities in the MVA range which are defined by IEEE Std 1547.7TM-2011. An overview of the proposed 13.8-kV microgrid test bed functions and goals are shown. Detailed power electronic circuit hardware, experimental prototype design, and control algorithms are addressed. Hardware test results of case study are shown which display the uniqueness of the proposed test bed which is planned for the research of future microgrids.

A new SST topology comprising boost three-level AC/DC converters for applications in electric power distribution systems

Abstract: Interest in integrating distributed generation into existing power distribution systems and achieving a more efficient, modern, reliable and versatile grid is leading to more research efforts on developing grid-interfaced converters such as the solid-state transformer. As current and voltage ratings of commercially available power semiconductor devices are normally below power ratings required in distribution systems (e.g., 13.8 kV), multiple modules must be connected in cascade configuration at the high-voltage side to reach higher voltage ratings as well as in parallel on the low-voltage side of to achieve high current levels. A new SST topology consisting of modular boost-based three-level AC-DC converters, high- or medium-frequency transformers with two secondary windings and four-leg ac-dc converters is presented in this paper. This topology when compared to similar approaches has fewer power conversion stages, lower voltage applied across the semiconductor devices on the high-voltage side (which reduces the number of modules), and lower current flowing through each device on the low-voltage side, leading to fewer parallel-connected devices. The feasibility of the proposed topology is experimentally validated on a 2 kW, 120 Vpk/ 10 Vdc scaled-down prototype.

A scaled-down microgrid laboratory testbed

Abstract: The demand for microgrids is growing rapidly since it is able to integrate distributed generation, reduce peak-load profile and transmission power losses, and increase customers' power reliability. Microgrid infrastructures and testbeds are built or under construction globally. A 13.8-kV 4.75-MVA microgrid laboratory testbed is being built and initially tested at the University of Arkansas. However, applying and debugging new control schemes and algorithms directly in a large-scale high-power testbed poses considerable safety risks and could even damage testbed equipment. Computer-based simulations (e.g., using Matlab/SimulinkTM, PSCADTM) of a microgrid including several power electronic converters are time consuming and sometimes the degree of modeling may not be able to reveal some hardware experimental problems. Therefore, a low-cost, scaled-down microgrid laboratory testbed is proposed in this paper for pre-validation of new microgrid concepts before full-power implementation. Power electronic circuit hardware design, the differences with the high-power prototype and hardware test results are presented to demonstrate the feasibility and shortage of the proposed ideas.

Resonance propagation of ac filters in a large-scale microgrid

Abstract: Research into microgrids has become more promising during recent years. It has the potential of leading to a more resilient electric power delivery system in the face of system disturbances. Low pass ac filters are frequently adopted in microgrid power electronic interfaces that convert dc power to ac power because most of today's distribution grids have ac voltages. Compared to a simple L filter, higher order filters, such as LC or LCL filters, are preferred due to their more effective reduction of switching-frequency harmonics and smaller sizes. However, the resonance problems caused by paralleling multiple converters with LCL filters, especially those converters having large power ratings (e.g., in the MVA range), must be accounted for when designing the entire microgrid. The analyses and designs should consider not only the resonant stability of a single converter but also the resonance propagation between multiple converters. A microgrid resonance propagation circuit model is built and investigated in this paper. Experimental waveforms validate the proposed analyses.

Control of a flyback converter operating in BCM using the natural switching surface

Abstract: The derivation and implementation of natural switching surface (NSS) control for a flyback converter operating in boundary conduction mode (BCM) is the main focus of this paper. The flyback converter has been wildly utilized in the area of power electronics and BCM operation has been proven to be successful in attaining high efficiencies. The NSS presents many benefits for the control of non-linear systems such as fast transient response under load-changing conditions. The NSS control technique has previously been implemented in non-isolated (e.g., buck and boost converters) and isolated (dual active bridge) converter topologies demonstrating excellent performances. The analytical derivation of the proposed switching surfaces is presented and validated through simulations using MATLAB/Simulink™ and a 65W prototype was experimentally tested.

750-kW interleaved buck converter dc supply control implementation in a low-cost FPGA

Abstract: The digital control of a 750-kW dc supply that converts a 480-V three-phase ac input voltage to an adjustable dc output voltage which has the maximum voltage value of 600 V is presented in this paper. Digital control is important for achieving precise data acquisition and proper operation of power electronic equipment. In addition to power semiconductor switching signals, several fault and warning signals from the power converter must be considered. Providing sufficient inputs/outputs (I/Os) and high-speed parallel processing, the fieldprogrammable gate array (FPGA) is one of the best candidates to accomplish the tasks mentioned above. In the proposed FPGA application, all functions including analog-to-digital conversion (ADC), power electronic converter algorithms, data interface, and protection logic for the dc supply are implemented in one single cheap FPGA which operates at a 25 MHz clock. All programming of the FPGA is completed by manual coding of hardware description language - the Very high speed integrated circuit Hardware Description Language (VHDL). A small-scale prototype is built and tested to provide further verification of the VHDL code prior to final implementation of the 750-kW prototype. The validity of control algorithms and FPGA functions are verified by simulations using Matlab/SimulinkTM Xilinx ISETM, respectively, prior to implementation in the hardware prototype.

Complexity analysis and verification of real-time operation for a semi-Markov model of photovoltaic intermittency

Abstract: Photovoltaic (PV) penetration levels in the power grid have significantly increased during the last few years. However, issues such as cloud-induced intermittency in PV generation forces equipment on the electrical grid to cycle excessively, preventing PV generation from being a reliable or dispatchable source of power, particularly for utilities. To mitigate this intermittency, the PV must be coordinated with either dispatchable generation, energy storage, or demand response. In order to do so economically, it is necessary to develop a model of PV power that can be included in design and control problems. A semi-Markov process model is proposed for PV power. Unlike existing models of PV power, the proposed model has a wide range of applicability across both small and large timescales. These applications include simulating PV power, short-term forecasting of PV power, design of rule-based controllers for energy storage units (ESU), and stochastic scheduling of ESU in conjunction with other resources. This paper investigates the suitability of the model implemented in algorithms in real-time. To this end, the complexity of four algorithms involving the model is analyzed in terms of the number of operations required vs. number of samples processed. This is validated by implementing the algorithms on a representative target platform, demonstrating that they can complete within reasonable deadlines.