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SMJE 2103
Synchronous Generator
Synchronous Generator
• Scope of discussion
1) Construction
2) Rotation speed
3) Generated voltage
4) Equivalent circuit
5) Power and Torque
6) Testing
7) Parallel operation
8) Rating
Synchronous Generator
-construction-
Synchronous Generator
-speed of rotation• The electrical frequency is synchronized with
the mechanical rate of rotation
• Relationship between magnetic field speed
and electrical frequency,
Synchronous Generator
-generated voltageInduced voltage,
Example 1
At a location in Europe, it is necessary to supply
300 kW of 60-Hz power. The only power sources
available operate at 50 Hz. It is decided to
generate the power by means of a motorgenerator set consisting of a synchronous motor
driving a synchronous generator. How many
poles should each of the two machines have in
order to convert 50-Hz power to 60-Hz power?
Synchronous Generator
-equivalent circuitThere are some factors that affected in
forming of an equivalent circuit:
1.Armature reaction
2.Self inductance of the armature coil
3.Resistance of the armature coil
4.The effect of salient pole rotor shapes
Armature
Reaction
Synchronous Generator
-equivalent circuit-
Synchronous Generator
-equivalent circuit-
Full
equivalent
circuit
Example 2
Assume that the field current of the generator in
Example 5 is adjusted to achieve rated voltage (2300 V)
at full load conditions in each of the questions below.
(a) What is the efficiency of the generator at rated load?
(b) What is the voltage regulation of the generator if it is
loaded to rated kilovoltamperes with 0.8-Pflagging loads?
(c) What is the voltage regulation of the generator if it is
loaded to rated kilovoltamperes with 0.8-Pfleading loads?
(d) What is the voltage regulation of the generator if it is
loaded to rated kilovoltamperes with unity-powerfactor
loads?
Synchronous Generator
-power and torque-
Synchronous Generator
-power and torque-
Synchronous Generator
-power and torque-
- RA assumed to be zero
-  Torque angle
- Maximum torque due to
max power when sin  is 1
Synchronous Generator
-power and torqueBasic torque equation:
From power expression Pout = Pconv = indm
Example 3
Assume that the field current of the generator in Example 5
has been adjusted to a value of 4.5 A.
(a) What will the terminal voltage of this generator be if it is
connected to a Δ-connected load with an,impedance of
20∠30° Ω?
(b) Sketch the phasor diagram of this generator.
(c) What is the efficiency of the generator at these conditions?
(d) Now assume that another identical Δ-connected load is to
be paralleled with the first one. What happens to the phasor
diagram for the generator?
(e) What is the new terminal voltage after the load has been
added?
(f) What must be done to restore the terminal voltage to its
original value?
Example 4
Assume that the field current of the generator in
Example 5 has been adjusted so that it supplies rated
voltage when loaded with rated current at unity power
factor. (You may ignore the effects of RA when
answering these questions.)
(a) What is the torque angle δ of the generator when
supplying rated current at unity power factor?
(b) When this generator is running at full load with unity
power factor, how close is it to the static stability limit of
the machine?
Synchronous Generator
-testingPurpose of test is to determine these
parameters:
a) Field current and flux relationship
b) Synchronous reactance
c) Armature resistance
Open circuit and short circuit tests should be
performed in order to get the purpose.
Synchronous Generator
-testing (open circuit)Procedures:
1) Generators is rotated at the rated speed
2) No load is connected at the terminals
3) Field current is increased from 0 to maximum
4) Record values of the terminal voltage and
field current value
Synchronous Generator
-testing (open circuit)-
The iron saturated, mmf getting
slow down due to increasing
reluctance of the iron
IA = 0, so EA = V, possible to plot EA or VT vs IF graph. It
is possible to find internal generated voltage for any
given field current
Synchronous Generator
-testing (short circuit)Procedures of short circuit test:
1) Generator is rotated at rated speed
2) Adjust field current to 0
3) Short circuit the terminals
4) Measure armature current or line current as
the field current is increased.
Synchronous Generator
-testing (short circuit)-
The net magnetic field is very small, the iron is
not saturated, so the relationship is linear
Synchronous Generator
-testing (short circuit)From the both tests. EA from OCC while IA from SCC
Capability Curve
Problem statement;
• Notice that for some possible current angles the
required EA exceeds EA,max. If the generator were
operated at the rated armature current and these
power factors, the field winding would burn up.
• Based upon these limits, there is a need to plot
the capability of the synchronous generator. This
is so that it can be shown graphically the limits of
the generator.
• A capability diagram is a plot of complex power
S=P+jQ. The capability curve can be derived back
from the voltage phasor of the synchronous
generator.
On the voltage axes, the origin of the phasor diagram is at -Vf on
the horizontal axis, so the origin on the power diagram is at:
Q
3V
XS
 V   
3V
XS
The field current is proportional to the machine’s flux, and the
flux is proportional to EA = Kfw. The length corresponding to EA
on the power diagram is:
3E AV
DE 
XS
The armature current IA is proportional to XSIA , and the length
corresponding to XSIA on the power diagram is 3VfIA.
Example 5
A 2300-V 1000-kVA 0.8-PF-lagging 60-Hz two-pole Yconnected synchronous generator has a synchronous
reactance of 1.1 Ω and an armature resistance of 0.15 Ω. At
60 Hz, its friction and windage losses are 24 kW, and its core
losses are 18 kW. The field circuit has a dc voltage of 200 V,
and the maximum IF is 10 A. The resistance of the field circuit
is adjustable over the range from 20 to 200 Ω. The OCC of
this generator is shown in Figure.
(a) How much field current is required to make VT equal to
2300 V when the generator is running at no load?
(b) What is the internal generated voltage of this machine at
rated conditions?
(c) How much field current is required to make VT equal to
2300 V when the generator is running at rated conditions?
(d) How much power and torque must the generator’s prime
mover be capable of supplying?
(e) Construct a capability curve for this generator.
Synchronous Generator
-parallel operationReasons for operating in parallel:
a) Handling larger loads.
b) Maintenance can be done w/t power
disruption.
c) Increasing system reliability.
d) Increased efficiency.
Synchronous Generator
-parallel operationCondition required:
1) RMS line voltage must be equal.
2) Both have same phase sequence.
3) Output phase angles are same.
4) Must have a slightly higher frequency for
new generator. It will change slowly.
Synchronous Generator
-ratings• Frequency rating: the frequency at the system.
• Voltage rating: voltage generated that depend
on flux and speed.
• Apparent power rating: maximum power with
maximum armature current.
• Power factor rating:
Example 6
A 480-V 400-kVA 0.85-PF-lagging 50-Hz four-pole Δ-connected
generator is driven by a 500-hp diesel engine and is used as a standby
or emergency generator. This machine can also be paralleled with the
normal power supply (a very large power system) if desired.
(a) What are the conditions required for paralleling the emergency
generator with the existing power system? What is the generator’s
rate of shaft rotation after paralleling occurs?
(b) If the generator is connected to the power system and is initially
floating on the line, sketch the resulting magnetic fields and phasor
diagram.
(c) The governor setting on the diesel is now increased. Show both by
means of house diagrams and by means of phasor diagrams what
happens to the generator. How much reactive power does the
generator supply now?
(d) With the diesel generator now supplying real power to the power
system, what happens to the generator as its field current is increased
and decreased? Show this behavior both with phasor diagrams and
with house diagrams.
Example 7
A 13.8-kV 10-MVA 0.8-PF-lagging 60-Hz two-pole Yconnected steam-turbine generator has a synchronous
reactance of 12 Ω per phase and an armature resistance of
1.5 Ω per phase. This generator is operating in parallel with a
large power system (infinite bus).
(a) What is the magnitude of EA at rated conditions?
(b) What is the torque angle of the generator at rated
conditions?
(c) If the field current is constant, what is the maximum power
possible out of this generator? How much reserve power or
torque does this generator have at full load?
(d) At the absolute maximum power possible, how much
reactive power will this generator be supplying or consuming?
Sketch the corresponding phasor diagram. (Assume IF is still
unchanged.)