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ECE 476 POWER SYSTEM ANALYSIS

ECE 476 POWER SYSTEM ANALYSIS. Lecture 2 Complex Power, Reactive Compensation, Three Phase Professor Tom Overbye Department of Electrical and Computer Engineering. Reading and Homework. For lectures 2 through 3 please be reading Chapters 1 and 2 HW 1 is 2.7, 12, 21, 26; due Thursday 9/4.

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ECE 476 POWER SYSTEM ANALYSIS

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  1. ECE 476POWER SYSTEM ANALYSIS Lecture 2 Complex Power, Reactive Compensation, Three Phase Professor Tom Overbye Department of Electrical andComputer Engineering

  2. Reading and Homework • For lectures 2 through 3 please be reading Chapters 1 and 2 • HW 1 is 2.7, 12, 21, 26; due Thursday 9/4

  3. Generation Transmission Distribution Customer Service Vertical Monopolies • Within a particular geographic market, the electric utility had an exclusive franchise In return for this exclusive franchise, the utility had the obligation to serve all existing and future customers at rates determined jointly by utility and regulators It was a “cost plus” business

  4. Vertical Monopolies • Within its service territory each utility was the only game in town • Neighboring utilities functioned more as colleagues than competitors • Utilities gradually interconnected their systems so by 1970 transmission lines crisscrossed North America, with voltages up to 765 kV • Economies of scale keep resulted in decreasing rates, so most every one was happy

  5. Current Midwest Electric Grid

  6. History, cont’d -- 1970’s • 1970’s brought inflation, increased fossil-fuel prices, calls for conservation and growing environmental concerns • Increasing rates replaced decreasing ones • As a result, U.S. Congress passed Public Utilities Regulator Policies Act (PURPA) in 1978, which mandated utilities must purchase power from independent generators located in their service territory (modified 2005) • PURPA introduced some competition

  7. History, cont’d – 1990’s & 2000’s • Major opening of industry to competition occurred as a result of National Energy Policy Act of 1992 • This act mandated that utilities provide “nondiscriminatory” access to the high voltage transmission • Goal was to set up true competition in generation • Result over the last few years has been a dramatic restructuring of electric utility industry (for better or worse!) • Energy Bill 2005 repealed PUHCA; modified PURPA

  8. State Variation in Electric Rates

  9. The Goal: Customer Choice

  10. OFF OFF The Result for California in 2000/1

  11. WA ME MT VT ND MN OR NH ID SD WI NY MA WY MI RI PA CT IA NV NE NJ OH IN DE IL UT DC W VA MD CO VA KS CA MO KY NC AZ TN OK NM AR SC GA MS AL TX LA AK FL HI electricity restructuring suspended restructuring delayed restructuring no activity Source : http://www.eia.doe.gov/cneaf/electricity/chg_str/regmap.html The California-Enron Effect

  12. August 14th, 2003 Blackout

  13. 2007 Illinois Electricity Crisis • Two main electric utilities in Illinois are ComEd and Ameren • Restructuring law had frozen electricity prices for ten years, with rate decreases for many. • Prices rose on January 1, 2007 as price freeze ended; price increases were especially high for electric heating customers who had previously enjoyed rates as low as 2.5 cents/kWh • Current average residential rate (in cents/kWh) is 10.4 in IL, 8.74 IN, 11.1 WI, 7.94 MO, 9.96 IA, 19.56 CT, 6.09 ID, 14.03 in CA, 10.76 US average

  14. Review of Phasors Goal of phasor analysis is to simplify the analysis of constant frequency ac systems v(t) = Vmax cos(wt + qv) i(t) = Imax cos(wt + qI) Root Mean Square (RMS) voltage of sinusoid

  15. Phasor Representation

  16. Phasor Representation, cont’d (Note: Some texts use “boldface” type for complex numbers, or “bars on the top”)

  17. Advantages of Phasor Analysis (Note: Z is a complex number but not a phasor)

  18. RL Circuit Example

  19. Complex Power

  20. Complex Power, cont’d

  21. Complex Power (Note: S is a complex number but not a phasor)

  22. Complex Power, cont’d

  23. Conservation of Power • At every node (bus) in the system • Sum of real power into node must equal zero • Sum of reactive power into node must equal zero • This is a direct consequence of Kirchhoff’s current law, which states that the total current into each node must equal zero. • Conservation of power follows since S = VI*

  24. Conversation of Power Example Earlier we found I = 20-6.9 amps

  25. Power Consumption in Devices

  26. Example First solve basic circuit

  27. Example, cont’d Now add additional reactive power load and resolve

  28. Power System Notation Power system components are usually shown as “one-line diagrams.” Previous circuit redrawn Arrows are used to show loads Transmission lines are shown as a single line Generators are shown as circles

  29. Reactive Compensation Key idea of reactive compensation is to supply reactive power locally. In the previous example this can be done by adding a 16 Mvar capacitor at the load Compensated circuit is identical to first example with just real power load

  30. Reactive Compensation, cont’d • Reactive compensation decreased the line flow from 564 Amps to 400 Amps. This has advantages • Lines losses, which are equal to I2 R decrease • Lower current allows utility to use small wires, or alternatively, supply more load over the same wires • Voltage drop on the line is less • Reactive compensation is used extensively by utilities • Capacitors can be used to “correct” a load’s power factor to an arbitrary value.

  31. Power Factor Correction Example

  32. Distribution System Capacitors

  33. Balanced 3 Phase () Systems • A balanced 3 phase () system has • three voltage sources with equal magnitude, but with an angle shift of 120 • equal loads on each phase • equal impedance on the lines connecting the generators to the loads • Bulk power systems are almost exclusively 3 • Single phase is used primarily only in low voltage, low power settings, such as residential and some commercial

  34. Balanced 3 -- No Neutral Current

  35. Advantages of 3 Power • Can transmit more power for same amount of wire (twice as much as single phase) • Torque produced by 3 machines is constrant • Three phase machines use less material for same power rating • Three phase machines start more easily than single phase machines

  36. Three Phase - Wye Connection • There are two ways to connect 3 systems • Wye (Y) • Delta ()

  37. Vcn Vab Vca Van Vbn Vbc Wye Connection Line Voltages -Vbn (α = 0 in this case) Line to line voltages are also balanced

  38. Wye Connection, cont’d • Define voltage/current across/through device to be phase voltage/current • Define voltage/current across/through lines to be line voltage/current

  39. Ic Ica Ib Iab Ibc Ia Delta Connection

  40. Three Phase Example Assume a -connected load is supplied from a 3 13.8 kV (L-L) source with Z = 10020W

  41. Three Phase Example, cont’d

  42. Delta-Wye Transformation

  43. Delta-Wye Transformation Proof

  44. Delta-Wye Transformation, cont’d

  45. Three Phase Transmission Line

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