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# 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
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
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
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
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)
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
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
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
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)
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
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
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
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)
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
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
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
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)
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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