Parallel Operation of Alternators

1. Electrical Machines-II
6th Semester, EE and EEE
By
Dr. Binod Kumar Sahu
Associate Professor, Electrical Engg.
Siksha ‘O’ Anusandhan, Deemed to be University,
Bhubaneswar, Odisha, India
Lecture-15

2. 2
Learning Outcomes: - (Previous Lecture_14)
 To understand the concept of active power & reactive power in a
cylindrical pole and salient pole type alternator.
 To analyse the power angle characteristic of a cylindrical pole and salient
pole type alternator.
 To solve numerical related to power equation and power angle
characteristic.
 To understand the significance of synchronizing power coefficient and
synchronizing torque coefficient.

3. 3
Learning Outcomes: - (Today’s Lecture_15)
 To understand the need of parallel operation of alternators.
 To analyse the various conditions to synchronize an alternator with
another alternator or an infinite bus.
 To analyse the various methods of synchronization.

4. 4
Advantages of parallel operation: -
 Continuity of service: - When there is maintenance or an
inspection, one machine can be taken out from service and the other
alternators will maintain the continuity of supply.
 Reduced operating cost: - Alternators are designed to give
maximum efficiency at full load. If we use a single large unit for
supplying the load, it will be uneconomical under light load
condition. While if we use smaller individual units we can switch on
or switch off the generators as per the load requirements in order to
meet the maximum efficiency and the required power demand can
be generated with minimum cost.
L
O
A
D
100
MW
G1 G2 G3
100
MW
100
MW
L
O
A
D
300
MW
G
 Economy and Utility: - Interconnection of several power stations by grid is
economical and advantageous. Interconnection of thermal and hydroelectric power plants
enables the optimum utility of alternators.
 Expansion Plans: - Suppose the current capacity of the power plant is 500 MW. If after
2 year, it plans to expand the capacity to 700 MW, a 200 MW alternator can be added in
parallel with the existing system.

5. 5
Advantages of parallel operation (Summary): -
 Several alternators can supply a bigger load than a single alternator.
 One or more alternators may shut down during the period of light loads. Thus, the
remaining alternator operates at near or full load with greater efficiency.
 When one machine is taken out of service for its scheduled maintenance and inspection,
the remaining machines maintain the continuity of the supply.
 If there is a breakdown of one generator, there is no interruption of the power supply.
 New machines can be added without disturbing the initial installation according to the
requirement to fulfil the increasing future demand of the load.
 Parallel operation of the alternator, reduces the operating cost and the cost of energy
generation.
 It ensures the greater security of supply and enables overall economic generation.

6. 6
Evolution of National Grid: -
 Grid management on regional basis started in sixties.
 Initially, State grids were interconnected to form regional grid and India was
demarcated into 5 regions namely Northern, Eastern, Western, North Eastern and
Southern region.
 In October 1991 North Eastern and Eastern grids were interconnected.
 In March 2003, WR and ER-NER were interconnected .
 August 2006, North and East grids were interconnected thereby 4 regional grids
Northern, Eastern, Western and North Eastern grids are synchronously connected
forming central grid operating at one frequency.
 On 31st December 2013, Southern Region was connected to Central Grid in
Synchronous mode with the commissioning of 765 kV Raichur-Solapur Transmission
line thereby achieving 'ONE NATION'-'ONE GRID'-'ONE FREQUENCY'.
 There are also
a. Synchronous interconnections to Bhutan.
b. Asynchronous links with Bangladesh, Myanmar, and Nepal.
c. An undersea interconnection to Sri Lanka.

7. 7
Northern (NR) Eastern (ER) Western (WR) North-Eastern (NER) Southern (SR)
Chandigarh Bihar Chhattisgarh Arunachal Pradesh Andhra Pradesh
Delhi Jharkhand Gujarat Assam Karnataka
Haryana Orissa Daman & Diu Manipur Kerala
Himachal Pradesh West Bengal Dadar & Nagar
Haveli
Meghalaya Tamil Nadu
Jammu & Kashmir Sikkim Madhya Pradesh Mizoram Pondicherry
Punjab Andaman &
Nicobar
Maharashtra Nagaland Lakshadweep
Rajasthan Goa Tripura
Uttar Pradesh
Uttarakhand
National Grid: -

8. 8
National Grid (Contd..): -
Northern Grid
Eastern Grid
Southern Grid
Western Grid
North-Eastern Grid

9. 9
Conditions for parallel operation/Synchronization: -
 Two DC sources can be connected in parallel, if:
a. Their voltage ratings are same and
b. They must be in phase opposition within the loop formed as a result of their
parallel connection.
 Violation of any of the above stated condition results in a circulation of large current
from one source another at no load. This may damage both the sources.
1I
90V 90V 0.4LR  
0.1 0.1
2I
LI
1I
90V 90V 0.4LR  
0.1 0.1
2I
LI
At no load
I1 = I2 = IL = 0
Under loaded condition
VL = 80 V
I1 = I2 = 100 A
IL = 200 A

10. 10
At no load
I1 = I = (90-60)/0.2 = 150 A
and I2 = -I = -150 A
Under loaded condition
VL = 66.667 V,
I1 = (90-66.667)/0.1 = 233.33 A,
I2 = (60-66.667)/0.1 = -66.67 A and
IL = 66.667/0.4 = 166.67 A
1I
90V 0.4LR  
0.1 0.1
2I
LI
I
60V
1I
90V 0.4LR  
0.1 0.1
2I
LI
60V
When the voltage rating of the two sources are not equal: -
a. At no load, a heavy circulating follows within the loop formed by interconnection of
the two sources.
b. Under loaded condition, one of the source may become load on the other instead of
sharing power to the load.

11. 11
 Two single phase AC sources can be connected in parallel, if:
a. Their voltage ratings are same,
b. Their frequencies are same,
c. The phase difference between the two sources must be zero.
d. They must be in phase opposition within the loop formed as a result of their
parallel connection.
 Violation of any of the above stated condition results in a circulation of large current
from one source another at no load. This may damage both the sources.
L
O
A
D
Zs1 Zs2
1 1V  2 2V 
CI
1I 2I
LI
1f 2f
Waveforms are plotted by
taking f1 = 50 Hz; f2 = 60
Hz; and RMS value of the
voltage as 230 V.

12. 12
Waveforms are plotted by
taking f1 = f2 = 50 Hz;
δ1 = 100; δ2 = 300; and RMS
value of the voltage as 230 V.
 The above discussion, clearly
indicates that for satisfactory
parallel operation:
Waveforms are plotted by taking f1 = f2 = 50 Hz;
δ1 = 100; δ2 = 100; and RMS value of the voltage as 230 V.1 2 1 2 1 2, ,V V f f   

13. 13
 Two three phase AC sources can be connected in parallel, if:
a. Their voltage ratings are same,
b. Their frequencies are same,
c. Their phase sequences are same.
d. The phase angle of their phase voltages/line voltages must be same.

14. 14
Synchronization of Alternators: -
 The process of connecting an alternator in parallel with another or with common bus
bars (constant voltage and constant frequency) to which many alternators are already
connected is called synchronization of an alternator.
 The alternator which is to be connected in parallel is called as the incoming alternator.
 Since a three phase alternator is nothing but a three phase source, the conditions to be
satisfied before synchronizing it are:
a. The terminal voltage of the incoming alternator must be same as that of bus
bar voltage.
b. The frequency of the incoming alternator must be same as that of the bus bar
frequency.
c. The phase sequence of the three phases of the incoming alternator must be
same as phase sequence of bus bars.
d. The phase difference between the voltage generated by incoming alternator
and voltage of bus bar must be zero.

15. 15
G1 G2 Incomming
Alternator
Gn…………
R1
Y
B
1
1
V
f
2
2
V
f
fI
Field Winding
NS
Armature
Terminals
Y1
B1
R2
Y2
B2
R

16. 16
 Conditions for synchronization:
a. The terminal voltage of the incoming alternator must be same as that of bus bar
voltage.
b. The frequency of the incoming alternator must be same as that of the bus bar
frequency.
c. The phase sequence of the three phases of the incoming alternator must be same
as phase sequence of bus bars.
d. The phase difference between the voltage generated by incoming alternator and
voltage of bus bar must be zero.
 First condition can be confirmed by simply using a voltmeter. If the terminal voltage of
incoming alternator (V2) is less than that of the bus bar voltage (V1), then V2 can be
made equal to V1 by increasing the field current of the alternator and vice versa.
 Frequency of the incoming alternator and that of bus bar can be measured by using a
frequency meter. Frequency of the incoming alternator can be adjusted and made
equal to that of bus bar by varying the speed of the rotor with the help of its prime
mover. Synchroscope can be used to know whether the incoming alternator is running
faster or slower than the required speed.

17. 17
 Phase sequence can be confirmed by using a voltmeter or a phase sequence indicator.
After matching the terminal voltages, if a voltmeter is connected between the identical
phases (R-phase of bus bar with the R-phase of incoming alternator), it will show zero. If
the voltmeter is not connected between the identical phases, it will read the line
voltage.
 The instant of zero phase difference can be observed by using two bright one dark
lamp method of synchronization.

18. 18
Two Bright One Dark Method of Synchronization: -
 Lamp L1 is connected between the same phases
(R1 & R2), whereas L2 is between B1 & Y2 and L3
between Y1 & B2.
 By observing the sequence of lamps L1,L2, L3
becoming dark and bright, we can decide whether
the incoming frequency of voltage is higher or
lower than bus-bar frequency.
 If the sequence of bright and dark of lamps is L1-
L2-L3 then the frequency of voltage of alternator is
higher than the bus bar voltage and if the sequence
is L3-L2-L1, the frequency of voltage of alternator
is less than that of bus bar frequency.
 At the instant of zero phase difference, lamp L1
will go dark and lamps L2 and L3 will go equally
bright. At this instant, the incoming alternator is
synchronized with the bus-bar.
R1 R2
Y1 Y2
B1 B2
L1
L2L3
Bus bar
terminals
Alternator
terminals
VR ER
VY
EY
VR
ER
VRR
VYY
VBB

19. 19
Thank you