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EE 295 Smart Grid and the Transmission System Dr. Paul Hines University of Vermont School of Engineering NY City, Nov. 9, 1965 © Bob Gomel, Life Where are we? • • • • • • 1. Electromechanical grid 2. SG & power supply, renewables, etc 3. Basics of comm systems, scada, etc. 4. SG & retail electricity 5. SG & distribuJon, microgrids, storage, etc. 6. SG & Transmission 2 Main topics • Synchronized Phasor Measurement Units • Flexible AC Transmission Systems • Improved operaJons centers, controlling cascading failures 3 4 5 6 Basics of PMU measurement • Sinusoidal AC • Measure magnitude • Measure phase • Methods of measurement • hTps://www.naspi.org 7 Phasor data concentrators 8 F-Net • hTp://fnetpublic.utk.edu 9 Key concepts • PosiJve sequence • Purpose: get an average phasor representaJon of the three-‐phase system. • There are also zero sequence and negaJve sequence components, but we’ll focus on the posiJve sequence. 10 11 PMU Applications (model validation) • Power systems live and die by their models. • Dynamic models exist. • However, it was previously extremely difficult to impossible to validate them • A “scan” of convenJonal RTU data could take 30 seconds to 5 minutes (or more). • Dynamics are much faster. • A few PMUs allow one to determine if your model works or not. • Many PMUs should allow you to calibrate your model. 12 PMU Applications (state estimation) • To be redundant: • A “scan” of convenJonal RTU data could take 30 seconds to 5 minutes (or more). • State esJmaJon is the process of combining measurements, each with some error, to esJmate the “true” state of the system. • The state esJmator is key to idenJfying overloads, as well as conJngency analysis. • In power systems, voltage magnitudes and angles are typically considered the state variables. • Once these are known, all other values can be computed from the power flow equaJons. • Voltage angles were previously only available from the state esJmator. • Now they can be “measured” • There is some conversaJon about “state measurement” rather than “state esJmaJon” but this is misleading. All measurements have error, requiring esJmaJon. • With a few phase angle measurements, state esJmaJon can be improved greatly. 13 PMU Applications (wide area monitoring) • The angles between areas tend to drij, indicaJng large power flows. • Monitoring these angles can be a useful measure of stress in the system. 14 15 PMU Applications (protection) • Zones of protecJon 16 Example… 17 PMU Applications (protection) • Out-‐of-‐step relays • Look for major angle differences (generators that our out-‐of-‐phase with the system) • What do you do with them when you find the problem? 18 PMU Applications (real time stability) 19 Real-time coordination of relay actions 20 Open PMU problems • PMU placement • Where do we place the PMUs to get the most informaJon? • ExtracJng informaJon from noisy data. • PrevenJng instability from large-‐scale stochasJc sources. 21 Nature, Sept. 2009: As large, complex systems approach “collapse” they shows signs of criJcal slowing down. Ecological models, climate models, epilepJc seizure Hines, May 23, 2012 From Scheffer et al., 2009 • Signs of CSD: noise amplified, lower-‐frequency elements in noise filtered, autocorrelaJon Eigenvalues far from the origin Hines, May 23, 2012 Eigenvalues near the origin Autocorrelation and variance in a single machine infinite bus system Low stress case Hines, May 23, 2012 High stress case 1-machine, infinite bus model Bus 1 Infinite bus, but noisy voltage Bus 2 Hines, May 23, 2012 2 bus network model Time -‐> Hines, May 23, 2012 9 bus network model Hines, May 23, 2012 What about the WSCC on August 10, 1996? • Lines sagged into trees, triggering a cascading failure • 7.5 million customers lost power. 7 states + Canada. Hines, May 23, 2012 WSCC (WECC, BPA) on August 10, 1996 Hines, May 23, 2012 Can we use this to predict distance to transition? Use a simple regression model to esJmate parameters. Test on separate data Hines, May 23, 2012 Flexible AC transmission systems • The problem • We cannot (easily) control the flow of power in a transmission system. • Do 2-‐bus example. • AcJvely controlling voltages requires • Switched capacitors, which are relaJvely expensive and can only be used in discrete chunks. • Could be rather slow • Synchronous condenser • A generator that doesn’t produce acJve power • Expensive. • Can we control voltages and currents using power electronics? 31 Distributed FACTS 32 Concepts from the readings • Loop flow • CongesJon 33 High Voltage DC 34 HVDC 35 36 Celilo Converter Station in 1989 37 38 Advantages • Advantages • No skin effect=> less resistance • about 3.5% per 1,000 km [wikipedia] • Smaller capaciJve losses to ground • Fully controllable • Transmission lines are less expensive (2-‐wires) • Disadvantages • Circuit breakers • Lower reliability • Expensive converter staJons 39