Parallel Operation of Generators 1 Chapter 4: Parallel Operation of Generators Notes | EduRev

: Parallel Operation of Generators 1 Chapter 4: Parallel Operation of Generators Notes | EduRev

 Page 1


Parallel Operation of Generators 
1 
Chapter 4: 
Parallel Operation of Generators 
 
 
In modern power systems isolated generators are very rare. Power systems are highly 
interconnected and many generators share the load. The first problem of an engineer is 
connecting a synchronous generator on an existing bus. 
 
Generator 1
Generator 2
System
Load
3 phase
switch
Synchronizing
 lamps
 
Figure 4-1 
 
CONNECTING a GENERATOR to a BUS 
 
The above figure 4-1 illustrates a generator G1 which is already connected to a power 
grid under load. Generator 2 has to be connected or “brought on line” 
 
1. The prime mover of the generator has to bring the speed of the shaft close to the 
rated speed of the generator. 
2. The excitation of the generator has to be increased to give a no-load output 
voltage as close as possible to the existing bus voltage 
 
We want to create a phasor rotation for generator 2 output similar to the 
bus voltage phasor. 
 
3. Observe the lights which are connected across the switches: they should beat, first 
get brighter and then dim as the phasors for generator and bus respectively shift. 
If the 3 lights beat concurrently, the phase sequence is correct, else if lights beat 
out of phase, one pair of phases should be reversed. 
4. Adjust now the prime mover to slowly increase/decrease the speed of generator 2. 
One should observe a slow beat of the light brightness.  
5. When the lamps beat slowly, the switches should be closed when the lights are 
extinguished (line-line voltage at minimum). 
 
Page 2


Parallel Operation of Generators 
1 
Chapter 4: 
Parallel Operation of Generators 
 
 
In modern power systems isolated generators are very rare. Power systems are highly 
interconnected and many generators share the load. The first problem of an engineer is 
connecting a synchronous generator on an existing bus. 
 
Generator 1
Generator 2
System
Load
3 phase
switch
Synchronizing
 lamps
 
Figure 4-1 
 
CONNECTING a GENERATOR to a BUS 
 
The above figure 4-1 illustrates a generator G1 which is already connected to a power 
grid under load. Generator 2 has to be connected or “brought on line” 
 
1. The prime mover of the generator has to bring the speed of the shaft close to the 
rated speed of the generator. 
2. The excitation of the generator has to be increased to give a no-load output 
voltage as close as possible to the existing bus voltage 
 
We want to create a phasor rotation for generator 2 output similar to the 
bus voltage phasor. 
 
3. Observe the lights which are connected across the switches: they should beat, first 
get brighter and then dim as the phasors for generator and bus respectively shift. 
If the 3 lights beat concurrently, the phase sequence is correct, else if lights beat 
out of phase, one pair of phases should be reversed. 
4. Adjust now the prime mover to slowly increase/decrease the speed of generator 2. 
One should observe a slow beat of the light brightness.  
5. When the lamps beat slowly, the switches should be closed when the lights are 
extinguished (line-line voltage at minimum). 
 
Parallel Operation of Generators 
2 
Recap: 
• Phase sequence must be the same 
• Voltages must have same magnitude 
• Frequency must be the same 
• Phasors must be aligned 
 
Note that in modern installations a ”synchroscope” is used. The synchroscope will 
instruct the governor of the prime mover to set the speed, and instruct the exciter to 
produce a voltage. When the phasors are detected within 5 degrees match, the 
synchroscope will close the switch. 
 
PARALLEL OPERATION of GENERATORS 
 
        Figure 4-2 
When the prime mover of a 
generator is set to deliver a certain 
power on the shaft, and the voltage 
is set to deliver that power to an 
electrical load, a certain operating 
point is reached [speed, Voltage, 
Power]. If the load increases, the 
generator speed (governor) will 
decrease (not enough power to 
move the shaft). Hence we can see 
the typical prime mover/governor 
characteristic. The characteristic starts at the “no load speed”, and droops. The droop rate 
is a parameter of the generator: 
rated
load full load no
P
f f
P
f
GD
-
= =
?
?
  [4-1] 
 
Since the power is related to the speed, a very useful formula is used as: 
 
( )
sys nl p output
f f S P - =  [4-2] 
 
Where: S
p
 is the slope of the curve in kW/Hz 
 f
nl
 is the no-load frequency of the generator 
 f
sys
 is the operating frequency of the system 
 
This shows that the power generated by a generator is a function of its frequency (or 
speed). 
 
 
Example:  a single generator’s characteristic is 1MW/Hz and its no-load frequency is 
61Hz. What is the load connected when the bus frequency is 60Hz? 
60Hz
3600
62
500 Power(kW)
Nominal
F(Hz)
Speed(rpm)
No load speed (frq)
Page 3


Parallel Operation of Generators 
1 
Chapter 4: 
Parallel Operation of Generators 
 
 
In modern power systems isolated generators are very rare. Power systems are highly 
interconnected and many generators share the load. The first problem of an engineer is 
connecting a synchronous generator on an existing bus. 
 
Generator 1
Generator 2
System
Load
3 phase
switch
Synchronizing
 lamps
 
Figure 4-1 
 
CONNECTING a GENERATOR to a BUS 
 
The above figure 4-1 illustrates a generator G1 which is already connected to a power 
grid under load. Generator 2 has to be connected or “brought on line” 
 
1. The prime mover of the generator has to bring the speed of the shaft close to the 
rated speed of the generator. 
2. The excitation of the generator has to be increased to give a no-load output 
voltage as close as possible to the existing bus voltage 
 
We want to create a phasor rotation for generator 2 output similar to the 
bus voltage phasor. 
 
3. Observe the lights which are connected across the switches: they should beat, first 
get brighter and then dim as the phasors for generator and bus respectively shift. 
If the 3 lights beat concurrently, the phase sequence is correct, else if lights beat 
out of phase, one pair of phases should be reversed. 
4. Adjust now the prime mover to slowly increase/decrease the speed of generator 2. 
One should observe a slow beat of the light brightness.  
5. When the lamps beat slowly, the switches should be closed when the lights are 
extinguished (line-line voltage at minimum). 
 
Parallel Operation of Generators 
2 
Recap: 
• Phase sequence must be the same 
• Voltages must have same magnitude 
• Frequency must be the same 
• Phasors must be aligned 
 
Note that in modern installations a ”synchroscope” is used. The synchroscope will 
instruct the governor of the prime mover to set the speed, and instruct the exciter to 
produce a voltage. When the phasors are detected within 5 degrees match, the 
synchroscope will close the switch. 
 
PARALLEL OPERATION of GENERATORS 
 
        Figure 4-2 
When the prime mover of a 
generator is set to deliver a certain 
power on the shaft, and the voltage 
is set to deliver that power to an 
electrical load, a certain operating 
point is reached [speed, Voltage, 
Power]. If the load increases, the 
generator speed (governor) will 
decrease (not enough power to 
move the shaft). Hence we can see 
the typical prime mover/governor 
characteristic. The characteristic starts at the “no load speed”, and droops. The droop rate 
is a parameter of the generator: 
rated
load full load no
P
f f
P
f
GD
-
= =
?
?
  [4-1] 
 
Since the power is related to the speed, a very useful formula is used as: 
 
( )
sys nl p output
f f S P - =  [4-2] 
 
Where: S
p
 is the slope of the curve in kW/Hz 
 f
nl
 is the no-load frequency of the generator 
 f
sys
 is the operating frequency of the system 
 
This shows that the power generated by a generator is a function of its frequency (or 
speed). 
 
 
Example:  a single generator’s characteristic is 1MW/Hz and its no-load frequency is 
61Hz. What is the load connected when the bus frequency is 60Hz? 
60Hz
3600
62
500 Power(kW)
Nominal
F(Hz)
Speed(rpm)
No load speed (frq)
Parallel Operation of Generators 
3 
 
The power generated therefore is:  () kW
Hz
kW
P
output
1000 60 61
1
1000
= - = 
If one connects another 1000kW load to the bus  
what is the frequency drop? 
 
Hz
Hz kW
kW
S
P
f f
p
nl sys
59
/ 1000
2000
61 =
÷
ø
ö
ç
è
æ
- = - = 
In order to bring the system frequency back to 60Hz: 
() 60
1
1000
2000 - =
nl
f
Hz
kW
kW >> Hz f
nl
62 = 
and the governor has to increase its no-load set point to 62Hz 
 
 
If two generator characteristics are shown, and they are connected in parallel on the same 
bus, they must have the same frequency of operation, hence the operating point. In 
figure4-3 we can see that Generator A delivers twice the power of generator B. 
PB
PA
frq
Fixed Frequency
  
Figure 4-3 
 
In order to change the power in a generator for a given frequency of operation, one has to 
change the prime mover (change the value of the no-load frequency, or set point). 
Changing the governor will cause the characteristic to move up and down with the same 
slope. 
 
NOTE: if the governor and exciter are unchanged, any change of speed of one generator 
will cause a circulating current between the 2 machines in such a way as to oppose the 
change, hence it is called a “synchronizing torque”. This torque can be enormous and 
will always make sure that the machines are in synchronism (same frequency). 
 
Page 4


Parallel Operation of Generators 
1 
Chapter 4: 
Parallel Operation of Generators 
 
 
In modern power systems isolated generators are very rare. Power systems are highly 
interconnected and many generators share the load. The first problem of an engineer is 
connecting a synchronous generator on an existing bus. 
 
Generator 1
Generator 2
System
Load
3 phase
switch
Synchronizing
 lamps
 
Figure 4-1 
 
CONNECTING a GENERATOR to a BUS 
 
The above figure 4-1 illustrates a generator G1 which is already connected to a power 
grid under load. Generator 2 has to be connected or “brought on line” 
 
1. The prime mover of the generator has to bring the speed of the shaft close to the 
rated speed of the generator. 
2. The excitation of the generator has to be increased to give a no-load output 
voltage as close as possible to the existing bus voltage 
 
We want to create a phasor rotation for generator 2 output similar to the 
bus voltage phasor. 
 
3. Observe the lights which are connected across the switches: they should beat, first 
get brighter and then dim as the phasors for generator and bus respectively shift. 
If the 3 lights beat concurrently, the phase sequence is correct, else if lights beat 
out of phase, one pair of phases should be reversed. 
4. Adjust now the prime mover to slowly increase/decrease the speed of generator 2. 
One should observe a slow beat of the light brightness.  
5. When the lamps beat slowly, the switches should be closed when the lights are 
extinguished (line-line voltage at minimum). 
 
Parallel Operation of Generators 
2 
Recap: 
• Phase sequence must be the same 
• Voltages must have same magnitude 
• Frequency must be the same 
• Phasors must be aligned 
 
Note that in modern installations a ”synchroscope” is used. The synchroscope will 
instruct the governor of the prime mover to set the speed, and instruct the exciter to 
produce a voltage. When the phasors are detected within 5 degrees match, the 
synchroscope will close the switch. 
 
PARALLEL OPERATION of GENERATORS 
 
        Figure 4-2 
When the prime mover of a 
generator is set to deliver a certain 
power on the shaft, and the voltage 
is set to deliver that power to an 
electrical load, a certain operating 
point is reached [speed, Voltage, 
Power]. If the load increases, the 
generator speed (governor) will 
decrease (not enough power to 
move the shaft). Hence we can see 
the typical prime mover/governor 
characteristic. The characteristic starts at the “no load speed”, and droops. The droop rate 
is a parameter of the generator: 
rated
load full load no
P
f f
P
f
GD
-
= =
?
?
  [4-1] 
 
Since the power is related to the speed, a very useful formula is used as: 
 
( )
sys nl p output
f f S P - =  [4-2] 
 
Where: S
p
 is the slope of the curve in kW/Hz 
 f
nl
 is the no-load frequency of the generator 
 f
sys
 is the operating frequency of the system 
 
This shows that the power generated by a generator is a function of its frequency (or 
speed). 
 
 
Example:  a single generator’s characteristic is 1MW/Hz and its no-load frequency is 
61Hz. What is the load connected when the bus frequency is 60Hz? 
60Hz
3600
62
500 Power(kW)
Nominal
F(Hz)
Speed(rpm)
No load speed (frq)
Parallel Operation of Generators 
3 
 
The power generated therefore is:  () kW
Hz
kW
P
output
1000 60 61
1
1000
= - = 
If one connects another 1000kW load to the bus  
what is the frequency drop? 
 
Hz
Hz kW
kW
S
P
f f
p
nl sys
59
/ 1000
2000
61 =
÷
ø
ö
ç
è
æ
- = - = 
In order to bring the system frequency back to 60Hz: 
() 60
1
1000
2000 - =
nl
f
Hz
kW
kW >> Hz f
nl
62 = 
and the governor has to increase its no-load set point to 62Hz 
 
 
If two generator characteristics are shown, and they are connected in parallel on the same 
bus, they must have the same frequency of operation, hence the operating point. In 
figure4-3 we can see that Generator A delivers twice the power of generator B. 
PB
PA
frq
Fixed Frequency
  
Figure 4-3 
 
In order to change the power in a generator for a given frequency of operation, one has to 
change the prime mover (change the value of the no-load frequency, or set point). 
Changing the governor will cause the characteristic to move up and down with the same 
slope. 
 
NOTE: if the governor and exciter are unchanged, any change of speed of one generator 
will cause a circulating current between the 2 machines in such a way as to oppose the 
change, hence it is called a “synchronizing torque”. This torque can be enormous and 
will always make sure that the machines are in synchronism (same frequency). 
 
Parallel Operation of Generators 
4 
CHANGES of OPERATING PARAMETERS 
 
Assume a generator is connected to an INFINITE BUS. This means that the bus has a 
CONSTANT FREQUENCY and a CONSTANT VOLTAGE.  Furthermore it can absorb 
power (active and reactive) and can provide power (active and reactive) as needed. 
 
Assume an operating point (speed/excitation) of the governor and exciter which delivers 
an active power that is the power delivered by the prime mover (minus losses) 
wt = P  mechanical power provided on the shaft 
If the excitation i
f
 produces E so that the power factor is unity: 
E
I
jXI
Pactive
V
 
Figure 4-4 
 
The excitation remains constant and the prime mover increases the torque, hence the 
power output increases 
E
jXI
V
I
Pactive NEW
 
 
Figure 4-5 
It can be seen that as the power increases at the prime mover the internal angle increases 
and therefore I increases also. At the same time the current starts to lead, which means 
that the generator also provides excess of reactive power. If one wants to bring the power 
factor back (without touching the prime mover), one would have to decrease the 
excitation accordingly as shown in the next figure: 
Page 5


Parallel Operation of Generators 
1 
Chapter 4: 
Parallel Operation of Generators 
 
 
In modern power systems isolated generators are very rare. Power systems are highly 
interconnected and many generators share the load. The first problem of an engineer is 
connecting a synchronous generator on an existing bus. 
 
Generator 1
Generator 2
System
Load
3 phase
switch
Synchronizing
 lamps
 
Figure 4-1 
 
CONNECTING a GENERATOR to a BUS 
 
The above figure 4-1 illustrates a generator G1 which is already connected to a power 
grid under load. Generator 2 has to be connected or “brought on line” 
 
1. The prime mover of the generator has to bring the speed of the shaft close to the 
rated speed of the generator. 
2. The excitation of the generator has to be increased to give a no-load output 
voltage as close as possible to the existing bus voltage 
 
We want to create a phasor rotation for generator 2 output similar to the 
bus voltage phasor. 
 
3. Observe the lights which are connected across the switches: they should beat, first 
get brighter and then dim as the phasors for generator and bus respectively shift. 
If the 3 lights beat concurrently, the phase sequence is correct, else if lights beat 
out of phase, one pair of phases should be reversed. 
4. Adjust now the prime mover to slowly increase/decrease the speed of generator 2. 
One should observe a slow beat of the light brightness.  
5. When the lamps beat slowly, the switches should be closed when the lights are 
extinguished (line-line voltage at minimum). 
 
Parallel Operation of Generators 
2 
Recap: 
• Phase sequence must be the same 
• Voltages must have same magnitude 
• Frequency must be the same 
• Phasors must be aligned 
 
Note that in modern installations a ”synchroscope” is used. The synchroscope will 
instruct the governor of the prime mover to set the speed, and instruct the exciter to 
produce a voltage. When the phasors are detected within 5 degrees match, the 
synchroscope will close the switch. 
 
PARALLEL OPERATION of GENERATORS 
 
        Figure 4-2 
When the prime mover of a 
generator is set to deliver a certain 
power on the shaft, and the voltage 
is set to deliver that power to an 
electrical load, a certain operating 
point is reached [speed, Voltage, 
Power]. If the load increases, the 
generator speed (governor) will 
decrease (not enough power to 
move the shaft). Hence we can see 
the typical prime mover/governor 
characteristic. The characteristic starts at the “no load speed”, and droops. The droop rate 
is a parameter of the generator: 
rated
load full load no
P
f f
P
f
GD
-
= =
?
?
  [4-1] 
 
Since the power is related to the speed, a very useful formula is used as: 
 
( )
sys nl p output
f f S P - =  [4-2] 
 
Where: S
p
 is the slope of the curve in kW/Hz 
 f
nl
 is the no-load frequency of the generator 
 f
sys
 is the operating frequency of the system 
 
This shows that the power generated by a generator is a function of its frequency (or 
speed). 
 
 
Example:  a single generator’s characteristic is 1MW/Hz and its no-load frequency is 
61Hz. What is the load connected when the bus frequency is 60Hz? 
60Hz
3600
62
500 Power(kW)
Nominal
F(Hz)
Speed(rpm)
No load speed (frq)
Parallel Operation of Generators 
3 
 
The power generated therefore is:  () kW
Hz
kW
P
output
1000 60 61
1
1000
= - = 
If one connects another 1000kW load to the bus  
what is the frequency drop? 
 
Hz
Hz kW
kW
S
P
f f
p
nl sys
59
/ 1000
2000
61 =
÷
ø
ö
ç
è
æ
- = - = 
In order to bring the system frequency back to 60Hz: 
() 60
1
1000
2000 - =
nl
f
Hz
kW
kW >> Hz f
nl
62 = 
and the governor has to increase its no-load set point to 62Hz 
 
 
If two generator characteristics are shown, and they are connected in parallel on the same 
bus, they must have the same frequency of operation, hence the operating point. In 
figure4-3 we can see that Generator A delivers twice the power of generator B. 
PB
PA
frq
Fixed Frequency
  
Figure 4-3 
 
In order to change the power in a generator for a given frequency of operation, one has to 
change the prime mover (change the value of the no-load frequency, or set point). 
Changing the governor will cause the characteristic to move up and down with the same 
slope. 
 
NOTE: if the governor and exciter are unchanged, any change of speed of one generator 
will cause a circulating current between the 2 machines in such a way as to oppose the 
change, hence it is called a “synchronizing torque”. This torque can be enormous and 
will always make sure that the machines are in synchronism (same frequency). 
 
Parallel Operation of Generators 
4 
CHANGES of OPERATING PARAMETERS 
 
Assume a generator is connected to an INFINITE BUS. This means that the bus has a 
CONSTANT FREQUENCY and a CONSTANT VOLTAGE.  Furthermore it can absorb 
power (active and reactive) and can provide power (active and reactive) as needed. 
 
Assume an operating point (speed/excitation) of the governor and exciter which delivers 
an active power that is the power delivered by the prime mover (minus losses) 
wt = P  mechanical power provided on the shaft 
If the excitation i
f
 produces E so that the power factor is unity: 
E
I
jXI
Pactive
V
 
Figure 4-4 
 
The excitation remains constant and the prime mover increases the torque, hence the 
power output increases 
E
jXI
V
I
Pactive NEW
 
 
Figure 4-5 
It can be seen that as the power increases at the prime mover the internal angle increases 
and therefore I increases also. At the same time the current starts to lead, which means 
that the generator also provides excess of reactive power. If one wants to bring the power 
factor back (without touching the prime mover), one would have to decrease the 
excitation accordingly as shown in the next figure: 
Parallel Operation of Generators 
5 
E
V
Pactive NEW
I
jXI
 
Figure 4-6 
EXCITER CHARACTERISTIC 
 
In a generator connected to an infinite bus, one can see from the previous figures that the 
magnitude of the armature current varies extensively as the excitation-power operating 
point varies. It is important to make sure that the generator does not exceed the rated 
values during an operating point setting.  Figure 4-7 illustrates this point. Assume an 
operation under unity power factor with a power of P0 and excitation E0, and rated 
current I0. The locus of the end of jXI must be on the circle as shown. 
V
P0
E0
P1
jXI0
I1
E1
(1)
(1')
I0
 
 
Figure 4-7 
 
If  the governor changes to a new setting, say decrease its mechanical power, if one wants 
to maintain rated current, there would be 2 operating points  (1) and (1’). (1) corresponds 
to a lagging current (inductive load), the other to a leading current. Hence the exciter has 
to assume the corresponding excitation to maintain stability. It is important to understand 
that the operating point of a generator has 2 control parameters: excitation (provided by 
the exciter) and real power (provided by the prime mover) = 2 degrees of freedom. 
However the operating point is also defined by the load as another degree of freedom: 
either PF, or magnitude of the current can be chosen. 
 
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