Presentation on theme: "Module G1 Electric Power Generation and Machine Controls"— Presentation transcript:
1 Module G1 Electric Power Generation and Machine Controls James D. McCalley
2 Overview Energy transformation into electrical form Generation operationRevolving magnetic fieldPhasor diagramEquivalent CircuitPower relationshipsGenerator pull-out powerExcitation controlTurbine speed control
3 Energy Transformation Transformation processes:ChemicalphotovoltaicelectromechanicalElectromechanical: conversion of energy from coal, petroleum, natural gas, uranium, water flow, geothermal, and wind into electrical energyTurbine-synchronous AC generator conversion process most common in industry today
4 Click on the below for some pictures of power plants and synchronous generatorsISU Power PlantISU Power Plant synchronous generatorAmes Power PlantAmes Power Plant synchronous generator
5 Feedback Control Systems for Synchronous Generators Turbine-generator basic formGovernor and excitation systems are known as feedback control systems; they control the speed and voltage respectively
6 Synchronous Machine Structure Phase ASTATOR(armaturewinding)ROTOR(fieldwinding)+NPhase B+DCVoltageSThe negative terminalfor each phase is180 degrees fromthe correspondingpositive terminal.+Phase CA Two Pole Machine (p=2)Salient Pole Structure
8 Synchronous Machine Structure A Four Pole Machine (p=4)(Salient Pole Structure)
9 Generation OperationThe generator is classified as a synchronous machine because it is only at synchronous speed that it can develop electromagnetic torque= frequency in rad/sec= machine speed in RPMp = number of poles on the rotor of the machine
11 For 60 Hz operation (f=60)No. of Poles (p) Synchronous speed (Ns)
12 Fact: hydro turbines are slow speed, steam turbines are high speed.Do hydro-turbine generators have few poles or many?Do steam-turbine generators have few poles or many?Fact: salient pole incurs significantmechanical stress at high speed.Do steam-turbine generators have salient poles or smooth?Fact: Salient pole rotors are cheaper to build than smooth.Do hydro-turbine generators have salient poles or smooth?
13 Generation OperationA magnetic field is provided by the DC-current carrying field winding which induces the desired AC voltage in the armature windingField winding is always located on the rotor where it is connected to an external DC source via slip rings and brushes or to a revolving DC source via a special brushless configurationArmature winding is located on the stator where there is no rotationThe armature consists of three windings all of which are wound on the stator, physically displaced from each other by 120 degrees
14 Synchronous Machine Structure Phase Avoltage induced in phasewdgs by flux fromfield wdgcurrent in phase wdgsproduces flux thatalso inducesvoltage in phasewdgs.+NPhase B+DCVoltageS+Phase C
16 Generation Operation: The revolving magnetic field = flux associated with the revolving magnetic field which links the armature windings. It will have a flux density of B.At any given time t, the B-field will be constant along the coil.By Faraday’s Law of Induction, the rotating magnetic field will induce voltages phase displaced in time by 120 degrees (for a two pole machine) in the three armature windingsLet’s consider just the A-phase.
20 Generation Operation: The revolving magnetic field (cont’d) If each of the three armature windings are connected across equalimpedances, balanced three phase currents will flow in themproducing their own magnetic fields = == resultant field with associated flux obtained as the sum of the three component fluxes is the field of armature reactionThe two fields represented by and are stationary with respect to each otherThe armature field has “locked in” with the rotor field and the two fields are said to be rotating in synchronismThe total resultant field =
21 Generation Operation: The phasor diagram From Faraday’s Law of Induction, a voltage is induced in each of the three armature windings:where N = number of winding turnsAll voltages, ,lag their corresponding fluxes, ,by 90 degreesThe current winding a, denoted by , is in phase with the flux it produces
22 Generation Operation: The phasor diagram (cont’d) Note:
23 Generation Operation: The equivalent circuit model Develop equivalent model for winding a only; same applies to winding b and c with appropriate 120 degree phase shifts in currents and voltages given a balanced load
24 Generation Operation: The equivalent circuit model (cont’d)
25 Generation Operation: The equivalent circuit model (cont’d) IajXsEfZloadVt
26 Generation Operation: The equivalent circuit model (cont’d) You can perform per-phase equivalent analysisor you can perform per-unit analysis.In per-phase, Ef and Vt are both line to neutral voltages,Ia is the line current, and Z is the impedance of theequivalent Y-connected load.In per-unit, Ef and Vt are per-unit voltages,Ia is the per unit current, and Z is the per unit loadimpedance.
27 Leading and Lagging Generator Operation LetFrom the equivalent circuit,So here we see that
28 Leading and Lagging Generator Operation Assign Vt as the reference:Then,So here we see thatThis gives an easy way to remember the relationbetween load, sign of current angle, leading/lagging,and sign of power angle.
29 Leading and Lagging Generator Operation Circle the correct answer in each columnInductive load Capacitive load
30 Phasor Diagram for Equivalent Circuit jXsEfZloadVtFrom KVL:
31 Phasor Diagram for Equivalent Circuit This equation gives directions forconstructing the phasor diagram.1. Draw Vt phasor2. Draw Ia phasor3. Scale Ia phasor magnitude by Xs androtate it by 90 degrees.4. Add scaled and rotated vectorto VtEfjXsIajXsIaVtIaTry it for lagging case.XsIa
32 Phasor Diagram for Equivalent Circuit This equation gives directions forconstructing the phasor diagram.1. Draw Vt phasor2. Draw Ia phasor3. Scale Ia phasor magnitude by Xs androtate it by 90 degrees.4. Add scaled and rotated vectorto VtYou do it for leading case.
33 Phasor Diagram for Equivalent Circuit Let’s define the angle that Ef makes with Vt asFor generator operation (power supplied by machine),this angle is always positive.For motor operation, this angle is negative.