17,260 research outputs found

    Control Loop Interactions and Their Mitigation Schemes in VSC-HVDC Systems

    Get PDF
    In line with two goals of the United Nations, i.e., providing affordable and clean energy as well as combating climate change, various converter-interfaced renewable energy sources (RESs) are being integrated into the power systems. The transfer of renewable power generated by the RESs such as offshore wind farms to remote load centers may require the use of direct current (DC) lines, which are connected to the alternating current (AC) grid via AC-DC converters. In addition to facilitating the reliable connection of RESs to the power grid, high-voltage DC (HVDC) lines may be used for the transcontinental exchange of power to transfer power over long distances. One of the major challenges in the evolution of AC systems to hybrid AC-DC systems is the control of converters. Each converter station owns various control loops that require proper tuning in their stand-alone mode of operation. Furthermore, control loops of adjacent converters may also impact one another, and as a result, there must be coordination among the control design of converters to guarantee stability and appropriate dynamic response of the entire grid. The control loop interactions among the converters worsen with increasing the size of the system and the number of converters, especially when one converter station is already in operation and re-tuning the converter's controllers is not an option. Another important aspect of future AC-DC power grids is the employment of converters built by multiple vendors, who will take part in the development of converter controllers with unique designs and know-how. These independently designed controllers will form a part of the grid control system. In this scenario, the stability of the entire system is of great importance and needs to be verified due to control loop interactions. This thesis studies both internal and external control loop interactions in voltage-sourced converters (VSCs) embedded in AC-HVDC systems. This thesis, first, studies the internal control loop interactions, where the control loops within one single converter interact with one another, and develops a method to design the individual control loops within a VSC such that the converter stability is ensured. A metric is proposed to measure interaction levels, and the impact of interactions on set-point tracking capability is also investigated. This thesis, next, considers the connections among various converters either from the AC side or the DC side and studies the external control loop interactions among the adjacent converters. Regarding the external control loop interactions caused by DC side connections, suitable system models are introduced to enable individual control design for the converters in a multi-terminal DC (MTDC)-HVDC grid. As for the AC side external control loop interactions, two scenarios are considered: 1) the converters are in the grid-following (GFL) mode of operation, and 2) the converters are in the grid-forming (GFM) mode of operation. Regarding the GFL mode of operation, the impact of control modes on the interactions is studied, and the control modes causing the highest interaction levels are identified. A novel control design framework is designed to relate the control design of each converter to the interconnected system stability. The multi-vendor issue then is considered, and the interactions are mitigated by designing individual robust controllers or by employing interaction filters. The interaction analyses are then extended to the parallel connection of GFM converters and hybrid connections of GFL and GFM converters. Stability and coupling analyses are performed among GFL and GFR converters. small-signal stability of parallel GFM converters is proved, and real-time simulations and hardware-in-the-loop-test are performed for validating the studies

    Stability Testing and Analysis of a PMAD DC Test Bed for the Space Station Freedom

    Get PDF
    The Power Management and Distribution (PMAD) DC Test Bed at the NASA Lewis Research Center is introduced. Its usefulness to the Space Station Freedom Electrical Power (EPS) development and design are discussed in context of verifying system stability. Stability criteria developed by Middlebrook and Cuk are discussed as they apply to constant power DC to DC converters exhibiting negative input impedance at low frequencies. The utility-type Secondary Subsystem is presented and each component is described. The instrumentation used to measure input and output impedance under load is defined. Test results obtained from input and output impedance measurements of test bed components are presented. It is shown that the PMAD DC Test Bed Secondary Subsystem meets the Middlebrook stability criterion for certain loading conditions

    The development of test beds to support the definition and evolution of the Space Station Freedom power system

    Get PDF
    Since the beginning of the Space Station Freedom Program (SSFP), the Lewis Research Center (LeRC) and the Rocketdyne Division of Rockwell International have had extensive efforts underway to develop test beds to support the definition of the detailed electrical power system design. Because of the extensive redirections that have taken place in the Space Station Freedom Program in the past several years, the test bed effort was forced to accommodate a large number of changes. A short history of these program changes and their impact on the LeRC test beds is presented to understand how the current test bed configuration has evolved. The current test objectives and the development approach for the current DC Test Bed are discussed. A description of the test bed configuration, along with its power and controller hardware and its software components, is presented. Next, the uses of the test bed during the mature design and verification phase of SSFP are examined. Finally, the uses of the test bed in operation and evolution of the SSF are addressed

    An Efficiency-Focused Design of Direct-DC Loads in Buildings

    Get PDF
    Despite the recent interest in direct current (DC) power distribution in buildings, the market for DC-ready loads remains small. The existing DC loads in various products or research test beds are not always designed to efficiently leverage the benefits of DC. This work addresses a pressing need for a study into the development of efficient DC loads. In particular, it focuses on documenting and demonstrating how to best leverage a DC input to eliminate or improve conversion stages in a load’s power converter. This work identifies how typical building loads can benefit from DC input, including bath fans, refrigerators, task lights, and zone lighting. It then details the development of several prototypes that demonstrate efficiency savings with DC. The most efficient direct-DC loads are explicitly designed for DC from the ground up, rather than from an AC modification

    Description of the PMAD DC test bed architecture and integration sequence

    Get PDF
    NASA-Lewis is responsible for the development, fabrication, and assembly of the electric power system (EPS) for the Space Station Freedom (SSF). The SSF power system is radically different from previous spacecraft power systems in both the size and complexity of the system. Unlike past spacecraft power system the SSF EPS will grow and be maintained on orbit and must be flexible to meet changing user power needs. The SSF power system is also unique in comparison with terrestrial power systems because it is dominated by power electronic converters which regulate and control the power. Although spacecraft historically have used power converters for regulation they typically involved only a single series regulating element. The SSF EPS involves multiple regulating elements, two or more in series, prior to the load. These unique system features required the construction of a testbed which would allow the development of spacecraft power system technology. A description is provided of the Power Management and Distribution (PMAD) DC Testbed which was assembled to support the design and early evaluation of the SSF EPS. A description of the integration process used in the assembly sequence is also given along with a description of the support facility

    Overview and evolution of the LeRC PMAD DC Testbed

    Get PDF
    Since the beginning of the Space Station Freedom Program (SSFP), the Lewis Research Center (LeRC) has been developed electrical power system test beds to support the overall design effort. Through this time, the SSFP has changed the design baseline numerous times, however, the test bed effort has endeavored to track these changes. Beginning in August 1989 with the baseline and an all DC system, a test bed was developed to support the design baseline. The LeRC power measurement and distribution (PMAD) DC test bed and the changes in the restructure are described. The changes includeed the size reduction of primary power channel and various power processing elements. A substantial reduction was also made in the amount of flight software with the subsequent migration of these functions to ground control centers. The impact of these changes on the design of the power hardware, the controller algorithms, the control software, and a description of their current status is presented. An overview of the testing using the test bed is described, which includes investigation of stability and source impedance, primary and secondary fault protection, and performance of a rotary utility transfer device. Finally, information is presented on the evolution of the test bed to support the verification and operational phases of the SSFP in light of these restructure scrubs

    NASA Tech Briefs, June 2011

    Get PDF
    Topics covered include: Wind and Temperature Spectrometry of the Upper Atmosphere in Low-Earth Orbit; Health Monitor for Multitasking, Safety-Critical, Real-Time Software; Stereo Imaging Miniature Endoscope; Early Oscillation Detection Technique for Hybrid DC/DC Converters; Parallel Wavefront Analysis for a 4D Interferometer; Schottky Heterodyne Receivers With Full Waveguide Bandwidth; Carbon Nanofiber-Based, High-Frequency, High-Q, Miniaturized Mechanical Resonators; Ultracapacitor-Based Uninterrupted Power Supply System; Coaxial Cables for Martian Extreme Temperature Environments; Using Spare Logic Resources To Create Dynamic Test Points; Autonomous Coordination of Science Observations Using Multiple Spacecraft; Autonomous Phase Retrieval Calibration; EOS MLS Level 1B Data Processing Software, Version 3; Cassini Tour Atlas Automated Generation; Software Development Standard Processes (SDSP); Graphite Composite Panel Polishing Fixture; Material Gradients in Oxygen System Components Improve Safety; Ridge Waveguide Structures in Magnesium-Doped Lithium Niobate; Modifying Matrix Materials to Increase Wetting and Adhesion; Lightweight Magnetic Cooler With a Reversible Circulator; The Invasive Species Forecasting System; Method for Cleanly and Precisely Breaking Off a Rock Core Using a Radial Compressive Force; Praying Mantis Bending Core Breakoff and Retention Mechanism; Scoring Dawg Core Breakoff and Retention Mechanism; Rolling-Tooth Core Breakoff and Retention Mechanism; Vibration Isolation and Stabilization System for Spacecraft Exercise Treadmill Devices; Microgravity-Enhanced Stem Cell Selection; Diagnosis and Treatment of Neurological Disorders by Millimeter-Wave Stimulation; Passive Vaporizing Heat Sink; Remote Sensing and Quantization of Analog Sensors; Phase Retrieval for Radio Telescope and Antenna Control; Helium-Cooled Black Shroud for Subscale Cryogenic Testing; Receive Mode Analysis and Design of Microstrip Reflectarrays; and Chance-Constrained Guidance With Non-Convex Constraints

    Description of a 20 Kilohertz power distribution system

    Get PDF
    A single phase, 440 VRMS, 20 kHz power distribution system with a regulated sinusoidal wave form is discussed. A single phase power system minimizes the wiring, sensing, and control complexities required in a multi-sourced redundantly distributed power system. The single phase addresses only the distribution link; mulitphase lower frequency inputs and outputs accommodation techniques are described. While the 440 V operating potential was initially selected for aircraft operating below 50,000 ft, this potential also appears suitable for space power systems. This voltage choice recognizes a reasonable upper limit for semiconductor ratings, yet will direct synthesis of 220 V, 3 power. A 20 kHz operating frequency was selected to be above the range of audibility, minimize the weight of reactive components, yet allow the construction of single power stages of 25 to 30 kW. The regulated sinusoidal distribution system has several advantages. With a regulated voltage, most ac/dc conversions involve rather simple transformer rectifier applications. A sinusoidal distribution system, when used in conjunction with zero crossing switching, represents a minimal source of EMI. The present state of 20 kHz power technology includes computer controls of voltage and/or frequency, low inductance cable, current limiting circuit protection, bi-directional power flow, and motor/generator operating using standard induction machines. A status update and description of each of these items and their significance is presented

    Remote power control strategy based on virtual flux approach for the grid tied power converters

    Get PDF
    The control of active and reactive power for the Renewable Energy Sources (RES) based power plants are very important. The injection of active and reactive power to the grid is normally controlled at the Point of Common Connection (PCC) where this point is typically far away from the power converter station. This paper proposed a controlling principle which is based on virtual flux approach that permits to control remotely the power injected at the PCC. The results will show that the Virtual Flux (VF) estimation is capable to estimate the grid voltage in any point of the network as well as the capability of the control principle to inject the specific amount of active and reactive power at a point that can be some kilometers away. In this paper, the basic principle for the remote power control is presented and the effectiveness of the proposed system has been validated by experimental studies.Postprint (published version

    Electrodynamic tether system study

    Get PDF
    The purpose of this program is to define an Electrodynamic Tether System (ETS) that could be erected from the space station and/or platforms to function as an energy storage device. A schematic representation of the ETS concept mounted on the space station is presented. In addition to the hardware design and configuration efforts, studies are also documented involving simulations of the Earth's magnetic fields and the effects this has on overall system efficiency calculations. Also discussed are some preliminary computer simulations of orbit perturbations caused by the cyclic/night operations of the ETS. System cost estimates, an outline for future development testing for the ETS system, and conclusions and recommendations are also provided
    corecore