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Generation of Electrical Energy: Sources and Generators

Generation of Electrical Energy: Sources and Generators. References: Schavemaker , P. & van der Sluis , L., Electrical Power Systems Essentials , 2 nd Ed, Wiley, 2017, pp. 53 et seq. http :// www.npr.org/2009/04/24/110997398/visualizing-the-u-s-electric-grid

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Generation of Electrical Energy: Sources and Generators

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  1. Generation of Electrical Energy:Sources and Generators References: Schavemaker, P. & van der Sluis, L., Electrical Power Systems Essentials, 2nd Ed, Wiley, 2017, pp. 53 et seq. http://www.npr.org/2009/04/24/110997398/visualizing-the-u-s-electric-grid http://www.asope.org/pdfs/AC_Electrical_Generators_ASOPE.pdf HitachiReview Vol. 56 (2007), No. 4 pp. 99 et seq. at www.hitachi.com/rev/pdf/2007/r2007_04_102.pdf

  2. Energy Sources by Fuel Type Europe in 2006 – Bracketing Practices New England in 2009 – In-state Generation

  3. Thermal to Mechanical Energy Conversion Temperature 450 to 650 deg. C Superheater Temperature 900 to 1450 deg. C Nuclear and old coal plants use only Rankine cycle because combustion temperature is too low.

  4. Efficiency – Endoreversible Thermodynamics • Carnot Cycle: • Endoreversible: See: https://en.wikipedia.org/wiki/Endoreversible_thermodynamics for an introduction to this subject. The endoreversible result is the same as the Carnot efficiency between a source at the geometric mean of T_H and T_C and the low temperature T_C.

  5. Smooth and Salient Rotors with Stator Windings for Three-Phase Generation

  6. Exciter

  7. Exciting Wiring Customer Demand Generator Output Turbine Fuel Valve Control and Rankine Exciter Control

  8. The Unexciting Way – Slip Rings

  9. Generator with Single Slot-Pair on Both Stator and Rotor • B, Flux, Voltage • Bad Idea Three Ways

  10. Designing Rotor Windings to Give a Sine Wave • Spread the slots with the rotor wires unevenly around the rotor to form a stepwise approximation to a magnetic field that is sinusoidal around the circumference.

  11. General Electric Smooth-rotor Generator Fully Assembled

  12. Why so Big? Size, Power, Mechanical Design Constraints • Power is product of torque and angular velocity • Torque is force times radius at which it is applied. Force on circumference is proportional to product of B of the rotor with NISTAT of the stator and the length of the stator/rotor assembly • Let , called the “Electric loading,” be the ratio of NISTAT to the circumference of the rotor, then: • This expression is the product of the rotor volume, the operating frequency, the rotor peak field and the ampere turns per meter of rotor circumference. The frequency is fixed, the B field has practical limits from materials, and the electric loading also has practical limits from heat transfer and the properties of copper. As a result, the rotor volume is the simplest and principal design parameter to determine generator capacity.

  13. Threading a Four-pole Cylindrical Rotor into the Stator of a 1525 MVA Generator

  14. Schematic Representation of a Stator Structure – Provides Return Flux Path, Winding Retention and Ventilation Slots

  15. Rotor with Bearings, End Caps, Winding and Ventilation Slots….

  16. Structure of a Rotor with Slip-ring Excitation and Axial Fans for Cooling Windings

  17. Poor Picture – Cross-section of Gas Cooling System: Gas Enters Through Rotor Shaft and Gap – Exits Through Slots in Stator

  18. Cooling Stator and Rotor: • Water used to cool stator coils – copper tubing among wires or bar with pipe through it. High purity water (deionized) to avoid shorting output. • Gas cooling is required for the rotor and either gas or water cools the stator magnetic structure. • Hydrogen is used as coolant for generators over 100 MW • Advantages of hydrogen: • Density is 1/14 th that of air which implies the velocity of flow is times faster for a given pressure difference. • Specific heat by volume is the same for air and hydrogen, so if heat transfer is similar, hydrogen is more than 3 times as efficient • Filling above atmospheric pressure trades off viscosity and specific heat. Higher density pressurization improves heat transfer, that is, watts per deg. temperature difference is but at an increase in viscous drag. • Speed of sound is inversely proportional to the square root of molecular mass so speed of sound is 1270 m/sec vs. 330 m/sec for air • Disadvantages of hydrogen: • Flammability and explosive potential • Need to seal the generator and to reinforce its structure to withstand the pressure

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