1. Module G1 Electric Power Generation and Machine Controls
James D. McCalley

2. Overview Energy transformation into electrical form
Generation operation
Revolving magnetic field
Phasor diagram
Equivalent Circuit
Power relationships
Generator pull-out power
Excitation control
Turbine speed control

3. Energy Transformation
Transformation processes:
Chemical
photovoltaic
electromechanical
Electromechanical: conversion of energy from coal, petroleum, natural gas, uranium, water flow, geothermal, and wind into electrical energy
Turbine-synchronous AC generator conversion process most common in industry today

4. Click on the below for some pictures of power plants
and synchronous generators
ISU Power Plant
ISU Power Plant synchronous generator
Ames Power Plant
Ames Power Plant synchronous generator

5. Feedback Control Systems for Synchronous Generators
Turbine-generator basic form
Governor and excitation systems are known as feedback control systems; they control the speed and voltage respectively

6. Synchronous Machine Structure
Phase A
STATOR
(armature
winding)
ROTOR
(field
winding) + N
Phase B + DC
Voltage S
The negative terminal
for each phase is
180 degrees from
the corresponding
positive terminal. +
Phase C
A Two Pole Machine (p=2)
Salient Pole Structure

7. Salient Pole
Construction
Smooth rotor
Construction

8. Synchronous Machine Structure
A Four Pole Machine (p=4)
(Salient Pole Structure)

9. Generation Operation
The 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 RPM
p = number of poles on the rotor of the machine

10. For 60 Hz operation (f=60)

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 significant
mechanical 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 Operation
A magnetic field is provided by the DC-current carrying field winding which induces the desired AC voltage in the armature winding
Field 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 configuration
Armature winding is located on the stator where there is no rotation
The 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 A
voltage induced in phase
wdgs by flux from
field wdg
current in phase wdgs
produces flux that
also induces
voltage in phase
wdgs. + N
Phase B + DC
Voltage S +
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 windings
Let’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 equal
impedances, balanced three phase currents will flow in them
producing their own magnetic fields = =
= resultant field with associated flux obtained as the sum of the three component fluxes is the field of armature reaction
The two fields represented by and are stationary with respect to each other
The armature field has “locked in” with the rotor field and the two fields are said to be rotating in synchronism
The 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 turns
All voltages, ,lag their corresponding fluxes, ,by 90 degrees
The 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)Ia
jXs Ef
Zload Vt

26. Generation Operation: The equivalent circuit model (cont’d)
You can perform per-phase equivalent analysis
or 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 the
equivalent 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 load
impedance.

27. Leading and Lagging Generator Operation
Let
From the equivalent circuit,
So here we see that

28. Leading and Lagging Generator Operation
Assign Vt as the reference:
Then,
So here we see that
This gives an easy way to remember the relation
between load, sign of current angle, leading/lagging,
and sign of power angle.

29. Leading and Lagging Generator Operation
Circle the correct answer in each column
Inductive load Capacitive load

30. Phasor Diagram for Equivalent Circuit
jXs Ef
Zload Vt
From KVL:

31. Phasor Diagram for Equivalent Circuit
This equation gives directions for
constructing the phasor diagram.
1. Draw Vt phasor
2. Draw Ia phasor
3. Scale Ia phasor magnitude by Xs and
rotate it by 90 degrees.
4. Add scaled and rotated vector
to Vt Ef
jXsIa
jXsIa Vt Ia
Try it for lagging case.
XsIa

32. Phasor Diagram for Equivalent Circuit
This equation gives directions for
constructing the phasor diagram.
1. Draw Vt phasor
2. Draw Ia phasor
3. Scale Ia phasor magnitude by Xs and
rotate it by 90 degrees.
4. Add scaled and rotated vector
to Vt
You do it for leading case.

33. Phasor Diagram for Equivalent Circuit
Let’s define the angle that Ef makes with Vt as
For generator operation (power supplied by machine),
this angle is always positive.
For motor operation, this angle is negative.