Power System Fundamentals
EE 317
Lecture 9
27 October 2010
Aims
 Chapter 6 – Parallel Operation of Synchronous
Generators
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
Paralleling generators
 Why?
Higher loads
Increased reliability under failure
Maintenance
More efficient operation of the fleet
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
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)
 Third – adjust the frequency of the oncoming unit to be
slightly higher than the frequency of the running system
(synchroscope)
 Close circuit breaker when 1-3 are satisfied and the
generator is in phase with the power system

synchroscope
 Measures the difference in phase angle between
the phases of two systems
 Dial shows the difference between two a or b or
c phases
 Faster (desirable) means clockwise from straight
up (which means in phase)
Characteristics of a
Synchronous Generator
 When operating alone its P&Q supplied will
depend entirely on the P&Q demanded by the
load
 Governor – sets the frequency and resulting
Real Power output of the synch. Generator
 Field Current – controls the output terminal
voltage VT and resulting Reactive Power
Operation with an Infinite Bus
Infinite bus – a power system so large that any draw of
reactive and real power will not affect frequency and
voltage
 Result: no reasonable action on the part of one
generator will cause an observable change in system
overall
 Imperative that frequency of connected devices be
higher than system frequency when connecting

Reverse-power trip
 Most real generators are equipped with a
reverse power trip so that if they do begin
to consume power they will disconnect
automatically.
Operating with another of similar
size
 Sum of load P&Q is supplied by the n generators
 Increase in governor set-point on one generator:
Increases system frequency
Decreases power supplied by other, up on this one
 Increase in field current of one generator:
System terminal voltage is increased
Reactive power supplied by other is decreased
Paralleling Generators
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
