Power System Fundamentals

1
Power System Fundamentals

• ECE 0909.402.05
• ECE 0909.504.05 - Lecture 9
• 1 April 2003
• Peter Mark Jansson PP PE MScEng

2
Aims

• Next Weeks Tour
• Mid-Term Feedback
• Chapter 5 Synchronous Machines
• Chapter 6 Parallel Operation of Synchronous
  Generators

3
Next Monday

• April 7, 2003
• 730 am
• Princeton Plasma Physics Laboratory
• Princeton University
• www.pppl.gov
• Identification/security requirements

4
Mid-Term Performance

• Very Good Performance To-Date
• Will provide a mid-term review for each student
  after HW 4 is graded
• Quiz 1 April 8, 2003
• Quiz 2 Finals Week

5
Chapter 5

• Synchronous Machine Construction
• Speed of Rotation
• Voltage of a Synchronous Generator
• Phasor Diagrams of a Synchronous Generator
• Synchronous Generator Operating Alone
• Synchronous Motors
• Synchronous Machine Construction

6
Synchronous Machines

• Motors and generators whose magnetic field
  current for the rotor is supplied by a separate
  DC power source
• Synchronous generators are used to produce nearly
  all the electric power produced in the world

7
Construction
STATOR or Armature Windings
S
ROTOR or Field Windings
S
N
N
8
Field windings

• Salient pole constructed in a manner that it
  protrudes from the surface of rotor (see Figure
  5-2, p. 194)
• Nonsalient pole constructed flush with the
  surface of the rotor (see Figure 5-1, p. 193)

9
How we create the DC current for the Rotor
Magnetic field

• External Source DC currents created by slip
  rings and brushes (lead to higher maintenance and
  power/voltage drop across brushes)
• Brushless Exciter small AC generator with field
  circuit mounted on stator and the armature
  mounted on the rotor creating 3-? AC currents. A
  3-? rectifier changes AC to DC for the main field

10
Completely independent

• Pilot Exciter uses permanent magnets on rotor
  to induce 3-? AC currents in the armature which
  then produce exciter fields in armature leading
  to 3-? AC currents in the rotoretc.
• Redundancy many synchronous generators that use
  brushless exciters also have slip rings and
  brushes so that an auxiliary method for making DC
  is available in emergencies

11
Exciter circuit diagrams

• See board

12
What is relationship?

• Of electrical frequency and speed of the
  mechanical (prime mover) device?
• Where
• fm electrical frequency in Hz
• nm mechanical speed of field in rpm (rotor
  speed)
• P number of magnetic poles

13
Voltage of a Sync. Gen.

• From chapter 4
• Simplifying for what is controllable during
  operation

14
Equivalent circuit of a sync. gen.

• The internal voltage EA is not usually equal to
  the output voltage V? of a synchronous generator
  due to 4 factors
• Armature reaction
• Self-inductance of armature coils
• Resistance of armature coils
• Effects of salient pole rotor shapes
• The revised equation for output voltage V?

15
Phasor Diagrams of Sync. Machines

• See board

16
Power and Torque in sync. gen.
17
Measuring sync. gen. parameters

• The model equation for sync gen output voltage V?
  
• To model the overall sync gen we need to know
• Relationship between field current and flux
• Synchronous reactance (XS) of the generator
• Armature resistance (RA)

18
Measuring the model parameters

• Open-circuit test loads are disconnected
  (terminals are open), field current is zero,
  construct plot of EA V? vs. field current IF
  this determines air-gap line and overall OCC
• Short-circuit test loads are disconnected
  terminals are shorted, field current is zero,
  construct plot of IA vs. field current IF this
  determines the overall SCC

19
Modeling sync. gen. parameters

• The model equation for sync gen armature current
  IA
• Since R is much smaller than X we can approximate
  X at any given point by the following process
• Get EA from OCC at given field current
• Find short circuit current flow (IA) from SCC at
  field current
• Calculate Synchronous reactance (XS) of the
  generator

20
Equations
21
Limitations

• This approximation only is accurate up to the
  knee in the saturation curve of the OCC, its
  value as a true approximation of X reduces as
  saturation increases

22
example
23
Sync. gen. Operating alone

• What happens as load (of constant power factor)
  is increased on generator?
• (a) lagging power factor (inductive loads)
• (b) unity power factor
• (c) leading power factor (capacitive loads)

24
Generator response

• (a) V? and VT decrease significantly
• (b) slight decrease in V? and VT
• (c) a rise in V? and VT

25
Voltage regulation

• Normally desirable to keep the voltage out of a
  generator constant even when loads are varying.
• How can this be done?
• Since EA K?? which one do you think we can
  most easily vary?
• Why?
• and How?

26
Changing the Field Resistor FR

• 1. Decreasing field resistance increases field
  current
• 2. Increases in field current increase flux
• 3. Increase in flux increases EA
• 4. An increase in EA leads to increase in V? and
  VT
• PROCESS IS REVERSIBLE

27
Comparing voltage regulation

• the model equation for voltage regulation is
  defined as
• Since R is much smaller than X we can approximate
  X at any given point by the following process
• Get EA from OCC at given field current
• Find short circuit current flow (IA) from SCC at
  field current
• Calculate Synchronous reactance (XS) of the
  generator

28
Synchronous motors

• Same as generators
• All the same equations apply
• Only differences are in phasor diagrams
• Also when maximum torque is exceeded rotor will
  start to slip

29
Starting synchronous motors

• No net starting torque therefore
• Reduce stator frequency to safe speed
• Use external prime mover
• Install damper or amortisseur windings to bring
  it up to synchronous speed before applying DC
  field current to field windings

30
Chapter 6

• Rationale for paralleling
• Conditions for paralleling
• Procedure for paralleling
• Characteristics of a Synchronous Generator
• Operation with an Infinite Bus
• Operating with another of similar size

31
Paralleling generators

• Why?
• Higher loads
• Increased reliability under failure
• Maintenance
• More efficient operation of the fleet

32
Conditions for paralleling

• Rms line voltages must be equal
• Same phase sequence
• Phase angles must be equal
• Frequency of new generator (oncoming unit) must
  be slightly higher the frequency of the running
  system

33
procedure

• First verify terminal voltage of oncoming
  generator equals line voltage of system
• Second verify that the phase sequence of the
  oncoming generator is the same as the phase
  sequence of the running system (motor, bulbs,
  synchroscope)
• Third adjust the frequency of the oncoming unit
  to be slightly higher than the frequency of the
  running system

34
Ch. 7 - Induction Machines

• Motors and generators whose magnetic field
  current is supplied by magnetic induction
  (transformer action) into the field windings of
  the rotor (a DC power source is not required)
• Although induction machines can be motors or
  generators they have many disadvantages as
  generators. Thus, they are referred to typically
  as induction motors. Most popular type of AC
  motor
• See pages 289-291