Download 4 Parallel Operation of Synchronous Generators

Satakunta Polytechnic
Electrical Power Engineering and Automation
Laboratory work
WOUNDED ROTOR SALIENT POLE SYNCHRONOUS
MACHINE
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Test yourself first
Test Questions
The synchronous reactance of a typical salient pole
generator is between 4% and 9%.
Right
Wrong
( )
( )
If a two-pole generator operates at 3600 rpm, 60 Hz
frequency is generated.
( )
( )
A synchronous motor has higher efficiency compared with
an induction motor of similar power and voltage ratings.
( )
( )
The propulsion motors in the latest Finnish icebreakers are
( )
( )
Salient pole synchronous machines.
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Synchronous Generator Supplying an Isolated System
Measurements
Measure the following curves, when the generator is not connected to the 3-phase
network.

The no load magnetisation curve at the rated speed.

The no load stator voltage as a function of speed at constant field current.
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Synchronising the Generator
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Wounded Rotor Salient Pole Synchronous Machine 30/04/2017 HP/sync.doc
3.1 Manual Synchronising
What are the conditions needed to synchronise a generator /1, p.431/?
Synchronise the generator. Continue carefully. Use normal glow bulbs to monitor the
phase sift and voltage and frequency differences between the generator and the 3-phase
line /2, p.275/. The bulbs should be placed in a triangle for better visualising the
phenomenon.
Remove the glow bulbs and add a synchronoscope to the system. Synchronise the
generator with the help of the synchronoscope.
3.2 Automatic Synchronising
Automatic synchronising is a separate exercise.
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Parallel Operation of Synchronous Generators
4.1 Parallel Operation with the Supply Net
Run the generator parallel with the supply net.
Examine, how
 Reactive power
 Active power
varies as the magnetising current varies. When examinig the active power, put a load to
the generator.
4.2 Voltage Control
Examine, how reactive power varies, when line voltage changes.
4.3 Active Power and Torque
Examine how active power varies, when the torque delivered by the dc-motor varies.
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Wounded Rotor Salient Pole Synchronous Machine 30/04/2017 HP/sync.doc
Use a stroboscope to light the end of the shaft of the generator. When the frequency of the
stroboscope is an exact multiple of the line frequency, the torque angle can be monitored
/1, pages 212, 233/. Monitor the torque angle, when the active power is increased.
Examine what is the effect of the field current to the torque angle, when the active power
is kept constant.
Examine the dynamic behaviour of the rotor, when the generator is synchronised or the
torque is suddenly removed.
4.4 Parallel Operation of two Generators
Stabile operation of two or more isolated generators requires that both active- and
reactive powers will be properly controlled. The controller of the turbine or the diesel
controls active power. The voltage controller determines the production of reactive
power. The voltage control must be provided with a voltage droop control. Stable
operation requires that the terminal voltage of the generators will be lowered with
increasing reactive power.
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Synchronous Motor
The family of the synchronous motors is many-sided. There are permanent magnet acservos, very special pancake motors for elevators and huge, electrically excited
synchronous motors. The latest ones are either constant speed motors in pulp and paper
industry, or speed and torque controlled motors in steel mills or propulsion drives in
modern ships
In the laboratory a conventional synchronous generator is used as a synchronous motor.
The students should remember that in real life a constant speed synchronous motor
usually has a squirrel gage winding for starting purposes. Also a brushless excitation
system is in common use.
5.1 Speed
Examine, how the speed of the motor varies as the torque of the load varies. Monitor also
the torque angle.
5.2 Power Factor Correction
Show that an overexcited motor can be used to improve the power factor in addition of
delivering mechanical load.
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Wounded Rotor Salient Pole Synchronous Machine 30/04/2017 HP/sync.doc
5.3 Pull-out Torque
Find out the pull-out torque and the pull-out torque angle of the motor.
Decrease the excitation current and repeat the measurements. Are the results according to
the theory?
5.4 Zero Position of the Rotor
Vector control is very sophisticated speed and torque control principle of ac-machines. It
may be applied on both cage rotor induction motors and synchronous motors. Vector
control is based on space phasor theory, where the electrical quantities of the machines
(such as current, voltage, and flux linkage) are treated as rotating vectors. Those vectors
are based on some coordinate system. The coordinate system may be firm, fixed to the
stator or it may rotate within the rotor. In vector control it is essential, that the position
encoder of the motor shaft is synchronised with the base coordinate system. The zero
position of the rotor referred to the fixed stator coordinates can be find in the following
way:
First the rotor must be excited. For a permanent magnet motor this happens by itself;
electrically excited rotor needs dc-current with correct polarity.
The zero position of the rotor shaft is found by supplying to the stator windings a, b and c
direct current having exact the values of the following table:
Winding
Current (Per Unit)
a
1
b
-0.5
c
-0.5
The rotor now turns accurately to the zero position.
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Advanced Operations
7
Short Circuit Current
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Wounded Rotor Salient Pole Synchronous Machine 30/04/2017 HP/sync.doc
Set the excitation current so, that the no-load voltage has the rated value.
 Record the three phase short circuit current /1, p.453/.
Repeat the test several times.
The results are not identical, why?
 Determine the value of the maximum peak current and the rms-value of the continuous
short circuit current.
 Calculate the synchronous reactance of the generator.
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Study Questions
When starting the synchronous motor, it is desirable to short-circuit the field circuit
trough a discharge resistor, why?
What are the main parts of a brushless synchronous motor?
Sketch the cross section of the rotor of a four-pole generator.
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References
1. Leander W. Match, J.Derald Morgan Electromagnetic and
Electromechanical Machines John Wiley & Sons, 1987
2. Martti Paavola, Pekka Lehtinen Sähkötekniikan oppikirja
WSOY, 1982
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Wounded Rotor Salient Pole Synchronous Machine 30/04/2017 HP/sync.doc