Paralleling of Alternators - First Edition - 2022

Elstan A. Fernandez
Elstan A. Fernandez
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Equipment and Systems
Paralleling of Alternators
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Equipment
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Paralleling
of
Alternators

About the Author
Elstan A. Fernandez
• Chartered Engineer - Institution of Engineers (India)
• Fellow of the Institution of Engineers (India)
• Member of The Institution of Engineering and
Technology (UK)
• Member of Leaders Excellence at Harvard Square
(USA)
• Specialist in Marine Control Systems and Automation
• Certified Maritime Trainer and Assessor
• Amazon Central Certified Author
• Member of Non-Fiction Authors Association (USA)
Please Visit LinkedIn.com/in/Elstan for career information
A total of 43+ years of learning, hands-on and teach-
ing experience in this field

Equipment and
Systems

To
The Futurists
Of Our Global Maritime Industry

Elstan A. Fernandez
Equipment and
Systems

By Elstan A. Fernandez
Copyright © 2022 – Elstan A. Fernandez
First Edition: October 2022
Print ISBN: 978-93-5542-331-3
E_Book ISBN: 978-93-5542-338-2
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copyright notice may be reproduced or utilized in any form
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Preface to the First Edition
Elstan’s
Pocket Book Series v
This pocketbook is based on an extract from the book titled Marine Electrical
Technology and with reference to various guidelines for paralleling alternators on
board ships. It thus deals with understanding generic systems on board ships.
Electro Technical Officers and Marine Engineers onboard commercial ships and
those undergoing training to qualify for these positions, will find this book useful.
The Pocket Book Series was introduced because there is a changing trend
in the way books are read today. The new normal is that readers and students
prefer to read specific, and not so voluminous content, in the least time, as time
comes at a premium these days.
Hopefully our team of authors will be able to cater to numerous topics from
many relevant subjects.
Any feedback is always welcome!
Elstan A. Fernandez

Acknowledgement
Elstan’s
Pocket Book Series
vi
The opportunity to share my acquired knowledge with thousands of
professionals and students across many countries and organisations has given
me an immense sense of accomplishment and satisfaction. It has also been a
wonderful journey of discovery for me - both while researching for this book and
teaching the subject in India and abroad.
This book is the result of over 40 years of learning and hands-on experience
in this field, including over 20 years of research and collaboration with various
organisations and specialists in the global maritime industry.
I sincerely thank all the wonderful people who have supported me in every
way, ever since I embarked on this project.
I am indebted to many distinguished persons who have have not only
supported my endeavours but also permitted me to publish very valuable content
for education. These articles are relevant to the building, safe operation and
conscientious survey of commercial ships. Many world-class organisations and
manufacturers have extended their invaluable support too. I am grateful for the
updated information from their websites and related literature. These inclusions
have undoubtedly enriched the content.
Numerous students now realize their dream of being educated through a
scholarship program that is funded by the royalty that I receive.
The encouragement from lay people and professionals alike has thus been
a stimulus to my enthusiasm. In order to give back and say “thank you” to the
maritime fraternity, I also host a free educational website –
www.marineelectricity.com.
In this context, I have a beautiful quote to share with my readers:
“Real knowledge, like everything else of value, is not to be obtained
easily.
It must be worked for, studied for, thought for, and, more than all must be
prayed for.”
Thomas Arnold (1795-1842), British Educator, Scholar

Contents
Elstan’s
Pocket Book Series vii
Article No. Article Page No.
1.1 The Basics 1
1.2 Manual Synchronising 5
1.3 Check-synchronising Unit 9
1.4 Automatic Synchronising 9
1.5 Synchronising with the Aid of Lamps 18
1.6 Synchronising with the Aid of a Voltmeter 22
1.7 Parallel Operation 24
1.8 Excitation Control 26
1.9 Throttle Control 29
1.10 Load Sharing 33
1.11 Speed Droop and Power Generation 51

Paralleling of Alternators
Elstan’s
Pocket Book Series
1.1 The Basics
Main generator units (steam turbine, diesel-driven and shaft-driven)
must be run in parallel to share a total load that exceeds the capacity of a
single machine - especially when discharging cargo, loading cargo, tank
cleaning and manoeuvring.
Changeover of main and standby generator units require a brief
parallel running period to achieve a smooth transition without a blackout
situation. For the sake of simplicity and security, it is normally not
possible or advisable to run a main generator in parallel with either the
emergency generator or the shore supply. Circuit breaker interlocks are
incorporated to prevent it. Parallel running is achieved in two stages -
synchronising and load sharing. Both can be carried out automatically
but manual control is still in common use and is generally provided
anyway as a back-up to the auto control mode. The generator already ‘on
the bars’ is called the running machine and the generator to be brought
into service is the incoming machine. To parallel the incomer smoothly,
it must be synchronised with the running generator (or the bus bars); the
following conditions are essential:
1. Same voltage
2. Same frequency
3. Same phase sequence
Note: On a daily basis the phase sequence will not be an issue as the
connections are permanent. However, care must be taken not to

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interchange phases when retrofitting or replacement is carried out
on the generator’s wiring and other related components or circuits.
Figure 1.1 – A Basic Synchronising Circuit
(Selector switches and instrument transformers are not shown)
In practice, one may find it difficult to adjust the speed of the
incoming machine so that the pointer of the synchroscope is stationary at
12 O’clock. Such a condition is not essential, and a more practical
proposition is to have the pointer rotating slowly in the ‘Fast’ direction
and to close the paralleling switch at about 11 O’clock.
V F
F
V
S
Incoming Generator
Incoming Voltmeter
and Frequency Meter
Bus bar Voltmeter
And Frequency Meter
Synchroscope
ACB

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Due to the time lag of the operating mechanism and human response,
actual synchronising will thus take place closer to the 12 O’clock
position, and the machine, running fast will be slowed down slightly
while taking a small proportion of the load.
If the incoming machine is synchronised when it is running slow, it
would slow down further and draw a motoring current, which will operate
its reverse-power relay and ‘trip’ the circuit-breaker of the machine
already on the ‘bars’ due to overloading. If the frequencies are not almost
equal at the time of synchronising, large power transients will occur until
they stabilise at the common frequency.
The likely consequences of attempting to close the incomer’s breaker
when the generators are not in synchronism are that at the instant of
closing the breaker, the voltage phase difference causes a large circulating
current between the machines; this results in a large magnetic force to
‘pull’ the generators into synchronism. This means rapid acceleration of
one rotor and deceleration of the other.
The large forces may physically damage the generators and their
prime movers, which may include deformation of the stator windings,
movement between the stator core and frame, failure of the rotor diodes
in brushless machines, twisted rotor shafts, localised crushing of shaft-
end keyways and broken couplings.
The large circulating current may also trip each generator breaker.
Severe vibration of the generator is also a symptom of loss of
synchronism.

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This will be accompanied by flashover at the slip rings in the case of
alternators that have a rotating armature. The ultimate result is a blackout,
danger and embarrassment!
The ship’s Power Management System (PMS) has two operating
modes: Manual and Automatic. When manual control is selected, the
PMS has no control over the generating sets; the generators and their
prime movers can be operated locally and at the main switchboard. For
example, the diesel generator local control mode is selected by means of
the Local / Remote switch. When the system is set to automatic, the PMS
controls the operation of the main switchboard and the three generators.
To fulfill the requirements for Unmanned Machinery Space (UMS)
operation, the system controls the following features:
1. Automatic starting of the standby generator in case of a blackout
2. Automatic synchronising
3. Automatic frequency control
4. Automatic load sharing
5. Preferential tripping of loads
6. Sequential restarting of essential consumers
7. Automatic generator starting and connection in response to a
heavy consumer start request
8. Automatic generator start / shutdown in response to high / low
load conditions

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9. Pre-selection of the standby generator priority is achieved by
operation of the standby generator selection switch on the
synchronising panel.
1.2 Manual Synchronising
The governor control switch of the alternator is moved to the “raise”
position; this action will raise the no-load speed setting of the governor.
The incomer must be brought up to an appropriate speed to obtain
approximately the same frequency or within 0.2% of the bus-bar
frequency to achieve smooth synchronising.
The incoming generator’s voltage is now ‘trimmed’ to be equal to
within 5% of the bus bar voltage. This may not be possible if the load is
fluctuating. Fine tuning of the speed can now be observed on the
synchroscope or synchronising lamps, the incomer being adjusted so that
the synchroscope pointer rotates slowly (in the “fast” direction) at about
4 to 5 seconds per revolution, counter-clockwise
In the case of synchronising lamps, the lamps would also appear to
rotate clockwise; this of course will be explained later in article 1.5. This
ensures that the incoming machine is slightly fast and it will immediately
assume load. Figure 1.2(a) depicts the four basic stages in synchronising.
The circuit breaker should be made as the pointer approaches ‘12
O’clock’. Making the breaker between ‘5 to and 5 past’ the ‘12 O’clock’
position of the synchroscope is satisfactory if the pointer’s rotation
is slow.

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It is normal to synchronise with the incoming machine running
slightly fast. This prevents the incoming machine’s reverse power
trip protection relay from operating.
The indication available to show the optimum synchronised
condition is that the incoming generator ammeter will display a slight
‘kick’ when correctly synchronised.
A synchroscope is usually short-time-rated (i.e., 15 to 20 minutes) –
do not forget to switch it off after synchronising is complete. The reason
for restricting the time for which a synchroscope is used is mainly
to prevent long-time damage to the moving coil and other
sensitive components that tend to get heated-up if they are too long in
the circuit.
If the synchroscope is malfunctioning, then the frequency
meter should be used to monitor the incoming alternator’s frequency.

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Elstan’s
Figure 1.2(a) – The Four Basic Stages of Synchronising
Figure 1.2(b) depicts a flow chart that helps to understand the manual
mode of starting and synchronising diesel-driven alternators.

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Figure 1.2(b) – Diesel - driven Alternator Starting / Synchronising
(Manual Operation)
Engine Stopped
Local
Control
Available
Remote
Control
Available
Engine
“Start” Push
Pressed
Auto
Standby
Gen “Off”
Engine
Control to
“Start”
Auto
Synchronising and
Power Control
Switch to “Manual”
Engine Start
Command
Engine
Started
Voltage
Established
ACB
Closed
Power Supply Available
WL
Speed Pick-up
(By watching voltmeter)
Yes
Yes
Overdue 3 Sec
WL
AL
Amber Lamp on MSB
indicates ACB not closed
White Lamp on MSB
indicates ACB closed
Yes
No
White Lamp indicates
Remote Control possible
Synchroscope
Switch “On”
ACB Control
Switch
to “Close”
ACB Close
Command
Governor Control
Raise / Lower
Synchronisation
Phase Control

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Elstan’s
Note: A voltage sensing relay independently monitors the voltage and so
does the synchroscope that monitors the voltage and the phase difference
of the incoming generator
1.3 Check-synchronising Unit
This unit uses an electronic circuit to monitor the voltage, phase angle
and speed (frequency) of the incoming generator with respect to the bus
bars i.e., it prevents faulty manual synchronising. This method provides
a useful safeguard against operator error but retains overall watch keeper
control in adjusting voltage and frequency.
‘Check synchronising’ modules are often provided with a manual
over-ride switch for use in an emergency; this can lead to problems if the
over-ride is left activated after the emergency.
However, modern synchroscopes and automatic synchronising units
have these features built-in. The synchroscope indicates whether the
voltage is too high or low and if the incoming generator is in phase with
the busbars - after which a green LED glows and a relay (25) energises.
Also, an independent voltage sensing relay will permit the breaker to
close only after the voltage is greater than about 95%. This prevents
erroneous closing of the breaker of the incoming generator.
1.4 Automatic Synchronising
This does everything an operator would do. It senses and controls the
voltage and frequency then initiates a circuit-breaker ‘close’ signal (of the
incoming alternator) at the correct instant.

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10
The auto-synchronising equipment uses electronic circuits to monitor
the magnitude of voltage, frequency and phase angle difference and then
acts to regulate them until they are equal to the existing parameters of the
bus bar. (Refer Figure 1.3). Automatic synchronising units have a setting
for the incoming frequency to be between 0 and 0.5 Hz higher than the
busbar frequency. This can be set as low as 0.125 Hz. It is understood that
if the frequency is slightly higher, the voltage will also be slightly higher;
this setting can be between 2 and 12% and can be set at around 3 to 5%;
one manufacturer rates the voltage difference to be as low as 0.5% too,
with a frequency difference of 0.1 Hz, with a phase angle at + 10% and
breaker closing time of 50 milliseconds.
Usually, one set of either check or auto synchronising units is
switched between a set of generators as and when required. When an
incoming generator has been successfully synchronised, the
synchronising equipment should be switched off. The total bus bar load
can now be shared between generators or totally transferred to the new
machine. In modern systems, they switch from synchronising to load
sharing once the incomer’s breaker is closed.
In a parallel operation, the governor of the alternator’s prime mover
directly controls power (kW) while its AVR trimmer or hand voltage
regulator controls reactive volt amps (kvar) or power factor. Figure 1.4(a)
is like Figure 1.2(b) except for the fact that for the latter, while
synchronising is automatic, the process must be manually initiated.
However, Figure 1.4(b) depicts a completely automatic system’s flow
chart.

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Elstan’s
Figure 1.3 – Automatic Synchronising
Voltage matching signals to exciter through AVR
Incoming
Voltage /
Frequency
Speed
Control
Signal
Supply for Synchronising Unit
Running
Voltage
and
Frequency
Breaker
Closing
Signal
Governor
Exciter
To Load
To Load
Synchronising
Unit

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Figure 1.4(a) – Diesel-Driven Alternator Starting / Synchronising
(Manually Initiated Sequential Operation)
Engine Stopped
Local
Control
Available
Remote
Control
Available
Engine
“Start” Push
Pressed
Auto
Standby
Gen “Off”
Engine
Control to
“Start”
Auto
Synchronising
and Power
Control Switch to
“Auto”
Engine Start
Command
Engine
Started
Voltage
Established
ACB Close
Command
ACB
Closed
WL
Speed Pick-up
95% - 5 Sec
Yes
Yes
No
Synchronising
Cancelled –
Warning Available WL
Overdue 3 Sec
WL
AL
Amber Lamp indicates ACB
on MSB not closed
ACB on MSB closed and
Auto Synchronising has occurred
Auto Synchronising
command given
Yes
No
Remote Control possible
Auto
Synchronising
Start Push
Pressed
Auto
Synchronising
Command
Auto
Synchronisation

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Elstan’s
Figure 1.4(b) – Diesel-Driven Alternator Starting / Synchronising
(Automatic Operation)
Remote
Control
Available
Stand-by
Generator Engine
Start Command
Engine
Started
Voltage
Established
Auto
Synchronising
Command
Auto
Synchronisation
Stand-by Gen.
ACB Close
Command
ACB
Closed
WL Yes
WL
Overdue 3 Sec
WL
AL
Amber Lamp indicates ACB
on MSB not closed
White Lamp indicates ACB
on MSB closed and that
Auto Synchronising
has occurred
Auto Synchronising
command given
Yes
No
ACB Trip
(Bus Alive)
Overload
Pref. Trip
Auto
Synchronising and
Power Control
Switch to “Auto”
No
No
Yes
Remote Control
Possible To 2nd Stand-by
Generator (same)
Auto
Standby
Generator
“On”
Main
Engine
Abnormal

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Figure
1.4(c)
–
A
Synchronising
Panel
on
an
Older
Vessel
PT
Auto
System
Abnormal
High
Frequency
High
Voltage
EMS
&
Pre
Power
Fail
DC
24
V
Fail
Low
Frequency
Low
Voltage
Hz
Hz
EMS
&
Pre
Power
DC
24
V
Light
Load
20%
Heavy
Load
90%
Governor
Lower
Governor
Raise
Auto
Load
Shift
Auto
Synchronising
ACB
Close
ACB
Open
Run
Ready
to
Start
Auto
Standby
Start
Stop
Remote
Local
Auto
No.1
No.2
No.3
Off
Off
No.1
Start
No.2
Start
No.3
Start
Off
Off
No.1
No.2
No.3
Off
Off
Frequency
Meter
Select
Synchroscope
Frequency
Meter
No.
2
Gen
No.
1
Gen
No.
3
Gen
Manual
Auto
1-2-3
Auto
2-3-1
Auto
3-1-2
Keep
Parallel
Run
Mode
Select
Load
Shift
Manual
Auto
Identical
controls
for
No
1
and
No
3
Generators
Function
Test
Buzzer
Test
Buzzer
Stop
Flicker
Stop
Lamp
Test
Lamp
Function
Test
Buzzer
Stop
Flicker
Stop
Synchronising
Panel

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Elstan’s
Note: To test the automatic starting arrangement of a stand by generator,
the simplest method is to trip the running generator by simulation or
activate the trip itself – this will lead to a blackout; the standby generator
starts in about 10 seconds.
II
III
VI
FS
FI
SY
SYL
MS ES LVS
PF1 PF2
EGS EGR
LT AR
GCM SGS
GRL GRL GRL
AO AC
FSS SSS
VS
G G G
GSP
AO AO
AC AC
I I
I I
I I
I II
III
I
kW kW kW
I I
I
I II
III

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VI Incoming Voltmeter PF2 Preferential Trip Stage 2
Lamp
FI Incoming Frequency Meter EGS Emergency Gen. On Standby
Lamp
VS System Volt Meter EGR Emergency Gen. Running
Lamp
FS System Frequency Meter LT Lamp Test Switch
MS Main Switchboard Source
Lamp
AR Alarm Reset Switch
ES Emergency Sw. board
Source Lamp
GCM Generator Control Mode
Switch
LVS Low Voltage (24 V) Source
Lamp
SGS Standby Generator Selector
Switch
kW Kilowatt Meter GRL Governor Raise / Lower
Switch
GSP Generator Status Panel AC ACB Close Switch
PF1 Preferential Trip Stage 1
Lamp
AO ACB Open Switch
SY Synchroscope SYL Synchronising Lamps
Figure 1.4(d) – A Synchronising Panel on a Tanker

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Elstan’s
Image Courtesy DEIF.com
Figure 1.4(e) – A Generator’s Load Sharing Unit
The Load Sharing Unit in Figure 1.4(e) controls the LSU contacts to
do the same after the synchronisation process.
In the Manual Mode, the Manual Controller is used to raise and lower
the speed. It must be noted that the Manual Controller must not be
operated while Auto Synchronising or Auto Load Sharing are in progress
unless it is deemed necessary to override the process and to manually
control the governor.
In all the above cases, either 65 R (for raise) and 65 L (for lower) will
operate whenever the speed drops or increases.
The limit switches are meant to stop the motor at the lowest and
highest limits of movement for the governor’s bi-directional motor.

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1.5 Synchronising with the Aid of Lamps
As a back-up or alternative to the synchroscope, a set of lamps may
be used. The lamp method of synchronising makes use of filament lamps,
so connected across the contacts of the paralleling switch that the intensity
of the illumination varies continuously i.e., in each case the lamps are
connected between the incoming generator and the bus bars. The correct
synchronised state may be indicated by the ‘Sequence’ method that
utilises 3 lamps.
The ‘Sequence’ method displays a rotation of lamp brightness, which
indicates whether the incoming machine is running fast (clockwise) or
slow (anticlockwise). As with the Synchroscope’s pointer in Figure 1.6,
the lamps’ sequence must appear to rotate slowly clockwise. Correct
synchronisation occurs when the top or ‘key’ lamp is dark and the two
bottom lamps are equally bright.
The error in the frequency of the incoming machine as compared with
bus bar frequency is shown by the rate at which the lamps ‘darken’ or
‘brighten’. Figure 1.5 depicts the usual ‘Sequence’ or 3-lamp method.
For three-phase systems, although the direct connection of three
lamps across the contacts of each line or cross-connecting of the lamps
are methods which can be used, the Siemens-Halske arrangement as
shown in Figure 1.5 and explained in Figure 1.6, is favoured.

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Elstan’s
This method not only indicates the correct instant for synchronising
but also indicates when the incoming alternator is running fast or slow
relative to the bus bar voltage.
From the superimposed phasor diagrams of Figure 1.6 it will be seen
that when running ‘Slow’, the lamps will glow in the order L1, L3, L2 and
so on. If the incoming machine is running ‘Fast’ the lamps will glow in
the order L1, L2, L3 and so on.
When the machines are in phase, then vectors ‘VR’ and ‘VR1’ will be
aligned and therefore ‘L1’ will be dark, ‘VY’ and ‘VB1’ will be 120o
apart
and therefore ‘L2’ will be approaching maximum luminosity, and the
same will be for ‘L3’ with ‘VY1’ and ‘VB’ 120o
apart.
As the lamps are arranged in a triangular pattern they would tend to
brighten in a clockwise direction when the incoming generator is running
faster than that which is already running; in short the frequency will be
slightly higher. The moment for synchronising is with the ‘key’ lamp L1
‘dark’ and the other lamps L2 and L3 glow equally but not at full brilliance.

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Figure 1.5 – Arrangement of Synchronising Lamps
R R1
Y Y1
B B1
Running Alternator Incoming Alternator
Circuit Breaker
L1
L3 L2
Slow Fast
Key Lamp
Bottom Lamps

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Elstan’s
Figure 1.6 – Phasor Rotation while Synchronising
Alternatively, (or in addition) synchronising instruments may be used
as shown in Figure 1.7. After successful synchronisation, the generator
load should be shared equally, provided the alternators are similarly rated.
VR
VY
VB
VR1
VY1
VB1
L1
L2
L3
Slow Fast
L1
L3 L2
Dark
Equal Brilliance
Moment for Synchronising
Phasor Rotation

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Figure 1.7 – Synchronising Instruments
1.6 Synchronising with the Aid of a Voltmeter
To monitor the correct instant for synchronising without the aid of a
synchroscope or synchronising lamps, connect a pair of 600 V voltmeter
probes across one phase of the incoming machine’s supply breaker.
VR
VB VY
R R1
Y Y1
B B1
F S
V F V F
Running Alternator Incoming Alternator
Circuit Breaker
VR
VB VY

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Elstan’s
Adjust the generator speed until the voltmeter slowly fluctuates from
zero to maximum. Close the breaker when the voltmeter parameters are
almost steady and pass through zero. Some safety tips will help:
1) Ensure that there are two qualified people present; they should
possess the knowledge of electrical safety and emergency
response in the case of an electrical shock; they should also be
wearing appropriate personnel protective equipment and
clothing.
2) One person each should stand on either side of the panel door to
monitor the parameters.
3) The person on the front face of the panel will be responsible to
monitor the incoming voltage and frequency and bring it as close
as possible to the busbar parameters and then keep the incoming
generator slightly faster.
4) The person on the other side would have access to the terminals
of the synchroscope. The person must secure the voltmeter on a
clear, safe base, set the Voltmeter selector switch to say 600 V
AC for a 440 V AC system and then connect the probes of the
meter across the R or the S phases of the synchroscope. The
incoming and busbar terminals will be clearly segregated and
identified with labels so care must be taken to connect the probes
across the same phase on each side e.g., Rbus and Rin.

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1.7 Parallel Operation
To parallel generators, the prime movers must be in proper working
order. For example, the diesel engines need to be mechanically sound and
properly tuned. The governors must be set properly. Before you ever
consider major adjustments on the distribution switchboard, you must
consult the operation and maintenance manual of the prime mover.
If the prime movers do not operate with the expected speed
characteristics, then there is no possible way for you to compensate for
their inaccuracies at the switchboard. For a paralleled alternator to take
its share of the load, it is necessary to study the effects of two possible
adjustments possible - namely:
1. Operation of the field regulator i.e., excitation control
2. Operation of throttle or steam valve i.e., speed control.
With two alternators in parallel, an increase in excitation of one
machine raises the generated emf and should tend to make it bear a greater
share of the load. However, the machine cannot slow down since it is
“tied” synchronously to the system and thus the governor of the prime-
mover is unaffected. No action results in causing the machine to bear
greater loads.
As will be seen, the operation of the excitation control system merely
causes a wattless current, which circulates in the paralleled machines and
the bus bar system. This current, lags the generated emf by an angle  and
the load can be equated to E I Cos.

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Elstan’s
The kW load thus remains constant to maintain an unvaried governor
setting. To change the distribution of load between alternators in parallel,
the throttle valves must be manipulated.
We thus see that for two alternators operating in parallel, since the
speeds (frequencies) must be identical, the kW loading on each machine
must be related to the prime-mover’s input power i.e., to the amount of
operation of the throttle valve and cannot be controlled by the excitation.
The effect of excitation and throttle control will now be considered
in detail. The parallel operation of alternators may be studied under two
distinct considerations:
1. The first would be parallel working with an ‘infinite bus bar’, as
constituted by shore-based power stations linked through a
national transmission grid system. An ideal case of infinite bus
bars is one where the system is so large in comparison with a
single alternator, that its voltage and frequency are unaffected by
the behaviour of the alternator.
2. The second consideration is of importance to the marine
engineer, since it relates to working on board a ship. Here, bus
bar voltage and frequency can be altered by local conditions and
the more common case, of two or more alternators running in
parallel is therefore stressed upon in this chapter.

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1.8 Excitation Control
Assume two alternators to have been paralleled correctly. The
voltage, frequency and phase of each are the same and the phasor
diagrams shown in Figure 1.8(a), (b) and (c) represent this condition. V is
the bus bar voltage i.e., that produced by machine No.1, is generating an
e m f of E1 volts and supplying the bus bar load E2, is the e m f of machine
No.2. Note that the electromotive forces of each of the two machines in
parallel are in phase with respect to the external circuit, but in opposition
when considered with respect to each other; in Figures 1.8 (a), (b) and (c)
the local circuit is considered.
For Figure 1.8(a), the voltages are equal and in opposition. Thus, no
current flows in the local circuit between the two alternators since the
resultant voltage is zero.
In Figure 1.8(b), the excitation of machine No 2 is seen to have been
increased. The generated emf E2 is increased to E'2 and gives rise to a
resultant voltage ER acting round the local circuit. The circuit is mainly
reactive and the resultant current I lags ER by nearly 900
. This current
represents no power flow either to or from the bus bars, the prime mover
of alternator No.2 is unaffected and the governor’s position will remain
unchanged. The machine is considered as supplying a lagging current to,
or taking a leading current from the bus bars.
This current will tend to neutralise the effect of any lagging load
current taken from the bus bars, and thus over-excitation of an alternator
improves the power factor of the paralleled system.

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Figure 1.8 – Excitation Control Phasors
Figure 1.8(c) shows the effect of decreasing the excitation of machine
No.2. It is pointed out that altering excitation does not appreciably alter
the bus bar voltage. This is explained by the relative current I, for
condition (b) possessing a lowering effect on the emf E'2. Lagging current
has a demagnetising effect on the alternator field strength.
In a similar manner, E1 – which is the generated e.m.f of machine
No.1, is increased and the circulating current is such as to make E2 and E1
equal to V - the bus bar voltage. In short, to control the power factor, the
excitation of the alternator is adjusted.
V V
V
ER
I
ER
I
E2
E2’
E2’
E2
(a) (b) (c)

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1.8.1 Loss of Excitation
When excitation is lost on a generator, the effect on the system will
depend on whether it is operating independently or in parallel with other
machines. When a generator is operating independently, it is obvious that
the system voltage will collapse thus resulting in a blackout situation.
However, when one or more healthy machines are connected in
parallel, the system voltage may fall only slightly, if at all. This is because
the excitation on these other machines will increase and will offset any
tendency for the voltage to fall.
Reactive current in the form of a large circulating current will flow
between the faulty and the healthy machines. This current could
eventually cause damage to the faulty machine but, more importantly, this
condition could cause tripping of a healthy machine, once again resulting
in a blackout situation! Excitation loss detection equipment should thus
be fitted to trip the main breaker.
For other than brushless sets, a simple undercurrent relay in the field
circuit can be used to detect loss of excitation. However, brushless sets
are now common and the field circuit is not accessible.
To detect any loss of excitation on such a machine it is necessary to
monitor the power factor, the latter being leading under these
circumstances. This condition may occur due to a failure of one or more
diodes in the polyphase rectifier circuit. Also, if a diode is punctured
(short-circuits), there could be an increase in excitation current, which
may result in damage to the field winding.

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Relays are available which will respond to these conditions, a time
delay also being provided to maintain stability under transient conditions.
The operating current of the excitation loss relay should be set below
the normal over-current settings, 75-100% of the full load current being
typical. Similarly, the time delay of the excitation loss relay should be
shorter than the over-current relay’s (operating) delay time.
These settings will ensure that only the generator experiencing loss
of excitation will be tripped, rather than healthy machines tripping due to
over-current conditions. Although not mandatory, the use of such a
protection circuit is recommended by some classification societies.
1.9 Throttle Control
Assume the governor control to be manipulated so that the fuel or
steam valve of machine No. 2 is opened. Alternator No. 2 tends to speed
up and phasor E2 tries to overtake V, as depicted in Figure 1.9. In
connection with the local circuit, a resultant voltage ER immediately
becomes apparent thus producing a lagging current I, as before, by almost
90o
.
This current is nearly in phase with E2, which means that alternator
No. 2 is now developing power and is expressed as E2ICos 2. When this
power output equals the increase of input power, as is brought about by
the actuation of the throttle valve through the governor, the tendency for
prime mover No. 2 to speed up increases, and this alternator set delivers
power to the load.

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Alternator No. 1, thus relieved of the load, speeds up slightly until its
prime mover governor operates to reduce the input power and bring about
stable speed conditions. The final distribution of load on each alternator
is achieved by alternating the operation of both machines’ throttle
controls until the required loading is as shown by each alternator
wattmeter and the voltage and frequency of the system settle down to the
desired condition.
If the driving power of alternator No.2 were removed, because of
some mechanical fault, such as fuel stoppage, then the conditions would
be as shown in Figure 1.10. Voltage E'2 drops back behind the true
synchronism position by an angle '.
There is now a resultant voltage E'R acting round the local circuit, to
produce current I'1, almost 90o
behind E'R – the circuit comprising the
machine armature, being mainly reactive. The bus bars now supply power
equal to EI' Cos ' to the machine and this will keep it running as a
synchronous motor. The drop back of E'2 from the synchronous position
is only momentary and the machine is accelerated back into synchronism.
Note that if an increased mechanical load is added to the alternator No. 2
when it is being motored, the machine’s emf E'2 would drop back still
further. E'R and I'1 would increase so that the total power supplied by the
bus bars increases. This is the basis of operation of a synchronous motor,
although little work will be done on the AC machine when operating in
this manner. It is usually used as a propulsion motor for marine AC
electric propulsion systems. In the preceding text, reference was made to
the synchronising current and this is now taken a step further.

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Consider parallel operation as depicted in Figure 1.11. The alternators
generate E1 and E2 volts to maintain a bus bar voltage of V volts. Although
these voltages are in phase with respect to the load, they are in direct
opposition to each other. Suppose the excitation and power developed by
each of the prime movers are set to cause currents I1 and I2 at power
factors of Cos1 and Cos2. The total load is the phasor sum of I1 and I2.
This could be shown in the phasor diagram but has been omitted in the
interest of clarity.
Figure 1.9 Figure 1.10 Figure 1.11
Alt No. 2 Speeds | Alt No. 2 Slows Down | Increase in Power of No. 2

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Assume that the power input to machine No.2 is increased and the set
tries to accelerate. It advances by a small angle . New load conditions
are set up. Now E'2 and E1 produce ER acting around the local circuit. This
causes the circulating current, which under no load conditions was
designated as the synchronising current IS, lagging ER by almost 90o
. This
current IS can be added by phasors to the original currents. Thus, it
combines with I2 to give the new machine a current I'2. IS is received by
machine No. 1 and lessens the current output giving I'1 the resultant of I'1
and IS.
The increased input to machine No.2 makes it bear a greater load so
that its speed settles to that decided by the governor-actuated throttle-
valve opening. Meanwhile machine No. 1, having been relieved of load,
accelerates to a new speed (and hence a higher frequency), determined in
the final stage by the overall loading of the system.
Therefore, IS is a short time circulating current, brought about by the
transient conditions resulting from the adjustment of the controls. Once
the overall paralleled system settles down, we have operating conditions
like those existing originally, except that I1, I2 and Cos 1, Cos 2 would
have new values.

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1.10 Load Sharing
We will see that increasing the excitation of a machine produces a
wattless circulating current; this means that a change of generated voltage
relative to the bus bars, changes the amount of reactive kVA which the
machine supplies. An overall balance of load sharing for kW and kVAr
can be seen by comparing the power factor meters of each generator.
Varying the power input tends to speed up the machine and power
E2I2 Cos2 would have new values. Load sharing can therefore be
considered from two viewpoints:
1) Sharing of kW.
2) Sharing of reactive kVA.
1.10.1 kW Load Sharing
This is an important aspect of paralleling and often depends upon the
skill of the engineer on watch especially when it is done manually.
1.10.1.1 Prime-mover Characteristics
In general, we know that for two alternators to operate successfully
in parallel, the load-speed characteristics of the prime movers should be
drooping the speed of the prime-mover should decrease slightly with
increasing loads. The speed droop, also called governor droop or speed
regulation, is usually expressed as a percentage of the full-load speed and
is one method of creating stability in a governor. Droop is used to divide
and balance loads during a paralleling operation.

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Speed droop = Nnl – Nfl x 100% i.e., No load speed – Full load speed x 100
Nfl Full load speed
The percentage of droop normally varies from 2 to 5 % from no-
load to full load. Usually the speed-load characteristics are linear. Not
enough droop can cause hunting, surging or difficulty in response
to a load change. Too much droop can result in slow governor response
in picking up or dropping off a load.
The amount of power generated by a machine is determined by its
prime mover. The speed of the prime mover is fixed, but its torque can be
varied. The effect of changing the governor characteristics is shown in
Figure 1.12. Remember that the power output is related to the frequency
of the machine and P = SP(fnoload – fsystem) where:
SP is the Slope (kW / Hz or MW / Hz), fnoload is the no-load frequency
and fsystem is the system frequency

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Figure 1.12 – Shifting of the Speed-load Characteristic
The speed-load characteristic is shifted to a new position parallel to
the initial position. When two alternators are operating in parallel, an
increase in governor set points in one of them:
a) Increases the system frequency and
b) Increases the power supplied by that alternator and reduces the
power supplied by the other alternator.
Initial governor characteristic
Frequency
f
Hz
f0’
f0
Governor adjusted for more power
P1 P2
f l
f l’
Load

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When two alternators are operating in parallel and the field current of
the second alternator is increased, then:
a) The system terminal voltage is increased; and
b) The reactive power Q supplied by that alternator is increased,
while the reactive power supplied by the other alternator is
decreased.
1.10.1.2 Load Sharing by Two Alternators
Let us assume that two alternators are running in parallel. The
frequency-load characteristics of the two machines are depicted in
Figure 1.13(a).
Figure 1.13(a) – Load Sharing of Two Alternators
Total Load
Load shared by
Machine 2
Load shared by Machine 1
fl2
fl1
Common frequency
Frequency
f
Hz
Total Characteristic
f01
f02
f
Load

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Let
W1 = full load power rating of machine 1; W2 = full load power
rating of machine 2
P1 = Power shared by machine 1; P2 = Power shared by
machine 2
P = Power supplied by two machines
f01 = no load frequency of machine 1; f02 = no load frequency
of machine 2
fl1 = full load frequency of machine 1; fl2 = full load
frequency of machine 2
f = common operating frequency when the two machines are
running in parallel
Machine 1
Drop in frequency from no load to full load = f01 – fl1
Drop in frequency per unit rating = f01 – fl1
W1
Drop in frequency for a load of P1 = f01 – fl1 . P1
W1
Operating frequency of machine 1 = no-load frequency – drop in
frequency
f = f01 – f01–fl1 . P1
W1

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Machine 2
Similarly, for alternator No. 2, the same operating frequency is
f = f02 – f02-fl2  P2
W2
Where f is the common frequency
Also, P1 + P2 = P
1.10.1.3 Load Sharing Between Alternators of Equal Capacities and
Different Droop Characteristics
In the following example, the capacity of generator A is 1000 kW
with a droop of 3% and that of generator B is 1000 kW with a droop
4%. The two alternators are operating in parallel and have to share a
total load of 800 kW:
P1 = load taken by generator A in kW
P2 = load taken by generator B in kW
Total power to be shared = P = P1 + P2 = 800 kW
Original frequency at no load f0 = 62Hz
Generator A (capacity of 1000 kW and 3% droop)
For a maximum load of 1000 kW, the drop in frequency
= 3% of f0 = 3 . 62 = 1.86 Hz
100
Now for a load of 1 kW, the drop in frequency is 1.86
1000

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For a load of P1 kW, the drop in frequency is therefore = 1.86 . P1
1000
Operating frequency of generator A = fA = original frequency – drop in
frequency
= 62 – 1.86 . P1
1000
Generator B (capacity of 1000 kW and 4% droop)
For a max load of 1000 kW, the drop in frequency = 4% of f0 = 4 . 62
= 2.48 Hz
100
Now for a load of 1 kW, the drop is 2.48
1000
For a load of P2 kW, the drop is therefore = 2.48 . P2
1000
Operating frequency of generator B = fB = original frequency – drop in
frequency
= 62 – 2.48 . P2
1000
Since fA = fB
62 – 1.86 . P1 = 62 – 2.48 . P2
1000 1000
1.86 P1 = 2.48 P2
r ¾ P1 = P2

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Now P1 + P2 = 800 kW
Substituting for P2 we get
4P1 + 3P1 = 800
4
or 7P1 = 3200
P1 = 3200 = 457.14 kW
7
And P2 = 800 – 457.14 = 342.8 kW
Therefore, we see that generator A with a flatter characteristic (3%
droop) can bear more load as compared to generator B with a steeper
characteristic (4% droop).
Note: In case the droop characteristics of the above generators are the
same (say 3%), then:
1.86 P1 = 1.86 P2; this will result in P1 being equal to P2 or 2P1 = 800 =
400 kW per generator.
1.10.1.4 Load Sharing Between Alternators with Unequal Capacities
and Same Droop Characteristics
In the following example, two three-phase alternators operate in
parallel; the rating of A is 1000 kW and B is 500 kW. The droop setting
of each generator is 4%. The load to be shared is 800kW.
P1 = load taken by generator A in kW
P2 = load taken by generator B in kW
Total power to be shared = P = P1 + P2 = 800 kW

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Original frequency at no load f0 = 62Hz
Drop in frequency at full load = 4% of 62 = 2.48 Hz
In the case of generator A, for a load of 1 kW the drop in frequency is
2.48
1000
1000
In the case of generator B, for a load of 1 kW the drop in frequency is
2.48
500
500
Operating frequency of generator A = fA = original frequency – drop in
frequency
= 62 – 2.48 . P1
1000
Operating frequency of generator B = fB = original frequency – drop in
frequency
= 62 – 2.48 . P2
500
We know that fA = fB
62 – 2.48 . P1 = 62 – 2.48 . P2
1000 500
2.48 . P1 = 2.48 . P2 or P1 = 2P2
1000 500

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Substituting for P1
2P2 + P2 = 800
or P2 = 800 = 266.67 kW
3
P1 = 800 - 266.6 = 533.33 kW
1.10.2 kVAr Load Sharing
The relative internal voltages largely govern the way in which
machines run in parallel and share the reactive kVA. The voltage
regulation characteristics of two machines are as shown in Figure 1.15.
Note that the voltage is plotted against the kVAr load. As for kW load
sharing, the characteristics can also be plotted back-to-back as shown in
Figure 1.14. The position of the characteristics is determined by the
amount of excitation.

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An increase of excitation for one machine, such as machine No.1, will
raise the curve to 11. Machine No. 1 then takes a larger share of the kVAr
load and the bus bar voltage is raised. Condition 12 shows how machine
No. 1 may be operated at a leading power factor even though the total
load is lagging. Figure 1.15 depicts that the machine with the flatter
characteristics takes the largest share of the load.
Figure 1.14 – kvar Load Sharing (Back-to-Back)
Voltage
Total Load
(Lagging kVAr)
Z
I
I1
I2
Voltage
Leading kVAr Leading kVAr

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1.10.3 Manual Load Sharing
This is achieved by raising the governor setting of the ‘incoming’
machine while lowering the setting on the ‘running’ machine. The
balance of power sharing that is dictated by the governor ‘droop’ of each
machine directs the balance of power sharing. If the alternator is operating
out of synchronism it will begin to vibrate severely and eventually trip
with the help of the reverse power relay.
Figure 1.15 – kVAr Load Sharing (Plot Not to Scale)
Total Load
Load on Machine 1
Load on Machine 2
1
2
Common Voltage
Voltage
Lagging kVAr
Leading kVAr
Total Characteristic

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Current (or kVAr) sharing is set by the generator’s voltage ‘droop’
set by the AVR. For equal load sharing of kW and kVAr, each machine
must have similar ‘droops’– typically 2 to 4% as seen in the examples
above.
An overall balance of load sharing kW and kVAr can be seen by
comparing the power factor (Cos) meters of each machine. If two
generators are sharing load equally in parallel when a total loss of
excitation occurs on machine No.2, the likely outcome is that Generator
No. 2 will run as an induction generator drawing its excitation from No.1.
Both generator currents will rise rapidly with No.1 lagging more,
while No.2 runs with a leading power factor (indicated on the power
factor meter).
A ‘loss of excitation’ trip (if fitted) or an overcurrent trip should trip
No. 2 generator possibly causing an overload on No.1. Alternatively,
No.1 trips on overcurrent that deprives machine No.2 of excitation and its
breaker trips due to an under-voltage condition. The result – total power
failure!
1.10.4 Automatic Speed Control and Load Sharing
Automatic load sharing circuits in a power management system
compare the kW loading of each generator (via CTs and PTs) and any
difference results in an error signal that is used to raise or lower the
governor setting of each prime mover as required.

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Such equipment is usually trouble-free, requiring little maintenance
other than an occasional visual inspection and checking the tightness of
connections. Manual load sharing is the obvious alternative if the
automatic control equipment fails.
The speed control section keeps the prime mover at the correct speed
while the load sharing section senses the load carried by its generator and
helps the loads of all generators in the system to be shared proportionally.
1.10.4.1 Speed Control
The electronic speed relay monitors the generator’s low, normal and
high-speed levels.
Once the alternator is running and the protection circuits are
introduced, the load sharing unit’s speed control system takes over. The
speed control system as shown in Figure 1.16 consists of:
1. A device like a proximity sensor to sense the speed of the prime
mover.
2. A frequency to voltage converter.
3. A speed reference to which the prime mover speed can be
compared.
4. A speed summer and amplifier with an output proportional to the
amount of fuel or steam required to maintain the desired speed at
any given load.

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5. An actuator to position the fuel or steam mechanism of the prime
mover. A speed-sensing device, such as a magnetic pickup,
senses the speed of the prime mover, and converts it to an AC
signal with a frequency proportional to prime mover speed. The
frequency-to-voltage converter receives the AC signal from the
speed sensor and changes it to a proportional DC voltage.
Figure 1.16 – Automatic Speed Control System
A speed-reference circuit generates a DC “reference” voltage to
which the speed signal voltage is compared. The speed signal voltage is
compared to the reference voltage at the summing point. If the speed
signal voltage is lower or higher than the reference voltage, a signal is
sent by the control amplifier thereby ensuring an increase or decrease in
speed.
The actuator responds to the signal from the control amplifier by
repositioning the fuel or steam rack, changing the speed of the prime
mover until the speed signal voltage and the reference voltage are equal.
A “failed speed signal” circuit monitors the speed signal’s input.
When no signal is detected, it ensures that minimum fuel is fed.
Frequency to
Voltage
converter
∑
Speed
Reference
Control
Amplifier Actuator
Prime
Mover
Linkages
Load Sharing and Speed Control Block
Generator
Speed Pickup

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The minimum fuel signal is sufficient to cause the actuator to go to
the minimum position if it is not restricted. However, due to linkage
adjustment or other restrictions in the external system, a minimum
actuator position may not permit the prime mover to shut down
completely.
For controls with say an actuator current of 20 to 160 mA, “minimum
fuel” is defined as an actuator current less of than 10 mA for forward-
acting controls and an actuator current greater than 180 mA for reverse-
acting controls.
As there is a wide variety of installations plus system and component
tolerances, the control must be tuned to each system for optimum
performance. The potentiometers for setting and adjusting these circuits
are located as shown in Figure 1.17. They include the following:
1. The Rated Speed potentiometer that is adjusted so that at rated
speed, the converter’s speed voltage and the reference speed
voltage are equal.
2. The Start Fuel Limit potentiometer that provides a means of
limiting the fuel rack’s position when starting the diesel engines.
3. Adjustment of the potentiometer sets the maximum actuator
position desired. This limit position is automatically enabled
prior to a start-up and is turned off when speed control takes over.
4. The Reset, Gain and Actuator compensation potentiometers
adjust the control amplifier to accommodate various types of
prime mover systems.

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5. Reset adjustment affects prime mover reaction time when
recovering after a sudden load change. The magnitude of the
speed change resulting from a sudden change in load is controlled
by adjusting the gain. Actuator compensation compensates for
the time the actuator and prime mover system takes to react to
signals from the control.
6. The time taken by the prime mover to accelerate from idle to rated
speed and the recommended idle speed, are set with the Ramp
Time and Low Idle Speed potentiometers respectively.
Figure 1.17 – Speed Control Adjustments
1.10.4.2 Paralleling System
There are two basic methods used for paralleling namely droop
(where speed decreases with load) and isochronous (where speed remains
constant). The paralleling system as shown in Figure 1.18 consists of a
load matching circuit and a load amplifier circuit.
Low Idle
Speed
Gain
Actuator
Compensation
Rated
Speed
Start Fuel
Limit
Ramp
Time
Reset

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For example, in some cases, with only one unit on load, the generator
picks up the available load and remains at isochronous speed; this is based
on auxiliary contacts in the circuit breakers which may include both the
generators and the bus tie breaker. If additional units are on line, the load
matching circuit corrects the fuel output proportional to the load.
An amplifier in the load sensing circuit computes the load carried by
each phase of the generator. The current load on each phase is multiplied
by the cosine of the phase difference between the current and the voltage
and the three phases are added to determine the total load.
Figure 1.18 – Automatic Load Matching Circuit and Load
Amplifier Circuit
1.10.4.3 Paralleling Adjustments
The output of the load amplifier is adjusted by the load gain
potentiometer shown in Figure 1.19. By setting the load gain voltage on
each unit to the same level at full load, proportional load sharing is
achieved.
Speed
Reference
Load
Matching
Circuit
Load
Amplifier
Actuator
Prime
Mover
Linkages
Load Sharing and Speed Control Block
Generator
Load Sharing
Lines
Droop
Contact
Circuit Breaker
Contact
Circuit Breaker
Contact Speed Pickup

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Elstan’s
Regardless of differences in generator set capacities in the system,
each generator set is loaded to the same percentage of its capacity. A final
adjustment of the individual load gain potentiometers will compensate for
minor differences in the generator sets.
Figure 1.19 – Load Gain and Droop Potentiometers
The droop mode allows the operation of a generator on an infinite bus
or in parallel with other engine generator units using hydro-mechanical
governors. In the droop mode, the speed changes as the load on the
generator changes. An increase in load results in a decrease in speed. The
amount of speed change or droop is expressed in percent and is set by the
droop potentiometer shown in Figure 1.19.
1.11 Speed Droop and Power Generation
The following text is a re-formatted but unedited extract from
Application Note 01302 by Woodward Governor Company titled ‘Speed
Droop and Power Generation’. The figures have been replicated for
clarity and re-numbered for continuity. It has been included with due
permission from Jeff Snowden, Senior Technical Writer, Woodward
Industrial Controls, Fort Collins, Colorado, USA.
Load
Gain
Droop

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Quote
Droop Engine Control for Stable Operation
Speed droop is a governor function which reduces the governor
reference speed as fuel position (load) increases. All engine controls use
the principle of droop to provide stable operation. The simpler mechanical
governors have the droop function built into the control system, and it
cannot be changed.
More complex hydraulic governors can include temporary droop,
returning the speed setting to its original place after the engine has
recovered from a change in fuel position. This temporary droop is called
compensation. The ability to return to the original speed after a change in
load is called isochronous speed control.
All electronic controls have circuits which effectively provide a form
of temporary droop by adjusting the amount of actuator position change
according to how much off-speed is sensed.
Without some form of droop, engine-speed regulation would always
be unstable. A load increase would cause the engine to slow down. The
governor would respond by increasing the fuel position until the reference
speed was attained.
However, the combined properties of inertia and power lag would
cause the speed to recover to a level greater than the reference. The
governor would reduce fuel and the off-speed would then occur in the
under-speed direction. In most instances the off-speed conditions would
build until the unit tripped due to overspeed.

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With droop, the governor speed setting moves toward the off-speed
as the fuel control moves to increase, allowing a stable return to steady
state control. The feedback in the governor is from the output position.
Since a minimal movement of the output position can cause major speed
changes in an unloaded engine, it is sometimes difficult to gain stability
in unloaded conditions.
Actuator linkage requiring more movement of the output to achieve
a given amount of rack movement at the idle settings than at the loaded
settings will often help achieve stability in the unloaded position. Setting
a greater amount of droop in the governor is another solution. In the case
of isochronous (temporary droop) control, the governor speed with which
the engine returns to the predetermined speed reference is adjustable,
allowing greater flexibility in achieving stable operation, even when
unloaded.
The Droop Curve
Droop is a straight-line function, with a certain speed reference for
every fuel position. Normally, a droop governor lowers the speed
reference from 3 to 5 percent of the reference speed over the full range of
the governor output. Thus, a 3% droop governor with a reference speed
of 1854 rpm at no fuel would have a reference speed of 1800 rpm at max
fuel (61.8 Hz at no fuel and 60 Hz at max fuel).
Notice that the feedback is over the full output-shaft rotation or fuel
rod retraction of the governor. If only a portion of the output is used, the
amount of droop will be reduced by the same proportion.

Elstan’s
Pocket Book Series
54
Likewise, the same governor would only have a droop from 1827 to
1800 if half of the full output moved the fuel rack from no fuel to full fuel
(60.9 Hz droop to 60 Hz; probably not enough droop to provide stability.
Figures 1.20 and 1.21 illustrate 3% and 5% droop governor speed
curves, assuming the use of all the servo movement. The speed figures
given are theoretical since servo position and rack position are seldom
linear.
Most complex hydraulic governors have adjustable droop. In these
cases, droop may be set between 0% and 5%. Droop is not adjustable in
most mechanical governors, although some mechanical governors have
provisions for changes in springs which will change the amount of droop.
Five percent droop is common in simple mechanical governors, although
3% and 10% droop is not uncommon.

________________________________________________________________
Elstan’s
Figure 1.20 – 3% Droop Curve
63
62
61
60
59
58
57
0 50 % 100%
61.8 Hz - No load
60.9 Hz - 50% load
60 Hz. - 100% - Full Load
(a) 3% Droop
Droop
Frequency

Elstan’s
Pocket Book Series
56
Figure 1.21 – 5% Droop Curve
Electric Generation
A single engine electrical generator can operate in isochronous,
changing speeds only temporarily in response to changes in load. This
system can also operate in droop, if a lower speed is permissible under
loaded conditions (see Figures 1.22 and 1.23).
59
58
57
0 50 % 100%
(a) 3% Droop
Droop
63
62
61
60
59
58
57
0 50 % 100%
63 Hz - No load
61.5 Hz - 50% load
60 Hz. - 100% load
(b) 5% Droop
Load
Frequency

________________________________________________________________
Elstan’s
Figure 1.22 – Response Curve of an lsochronous Governor
Figure 1.23 – Response Curve of a Droop Governor
63
62
61
60
59
58
57
Time
Frequency
Isochronous response to increase in load
63
62
61
60
59
58
57
Time
Frequency
Droop response to increase in load
63
62
61
60
59
58
57
Time
Frequency
Isochronous response to increase in load
63
62
61
60
59
58
57
Time
Frequency
Droop response to increase in load

Elstan’s
Pocket Book Series
58
Parallel with a Utility
If, however, the single engine generator is connected to a utility bus,
the utility will determine the frequency of the alternator. Should the
governor speed reference be less than the utility frequency, power in the
utility bus will flow to the alternator and motor the unit. If the governor
speed is even fractionally higher than the frequency of the utility, the
governor will go to full load to increase the bus speed. Since the definition
of a utility is a frequency which is too strong to influence, the engine will
remain at full fuel. Isochronous governor control is impractical when
paralleling with a utility because a speed setting above utility frequency,
by however small an amount, would call for full rack, since the actual
speed could not reach the reference speed. Similarly, if the setting were
even slightly below actual speed, the racks would go to fuel-off position.
Governors should not be paralleled isochronously with any system so
big that the governed unit cannot affect the speed of the system. Droop
provides the solution to this problem. Droop causes the governor speed
reference to decrease as load increases. This allows the governor to vary
the load since the speed cannot change (see Figure 1.24).

________________________________________________________________
Elstan’s
Figure
1.24
–
Comparison
of
3%
and
5%
Droop
Speed
Settings
For
50%
and
100%
Load
63
62
61
60
59
58
57
0
50
%
100%
Speed
Setting
for
3%
Droop,
100%
load
Frequency
0
50
%
100%
Speed
Setting
for
5%
Droop,
50%
load
Frequency
Load
Load
63
62
61
60
59
58
57

Elstan’s
Pocket Book Series
60
Governor Speed Setting Determines Load
When paralleled with a bus, the load on an engine is determined by
the reference speed setting of the droop governor. Increasing the speed
setting cannot cause a change in the speed of the bus, but it will cause a
change in the amount of load the engine is carrying.
The graph shows that the amount of load is determined by where the
droop line intersects the speed of the bus. If the location of this line is
moved, either by changing the reference speed or the amount of droop in
the unit, the amount of load will also be moved. Notice that the amount
of droop set in the governor has little effect on the ability of the governor
reference speed setting to determine the amount of load the engine will
carry. The greater the droop the less sensitive engine load will be to speed
setting. However, excessive droop presents the possibility of overspeed
should the engine be removed from the bus, thus becoming unloaded. In
most cases, 4% droop is adequate to provide stability and allow for
precise loading of the engine (see Figure 1.25).

________________________________________________________________
Elstan’s
Figure 1.25 – Speed Setting for 3% and 5% Droop at 70% Load
Identical engines can show different characteristics if droop settings
are not identical. An engine with more droop will require a greater change
in the speed setting to accomplish a given change in load than will an
engine with less droop in the governor.
As explained in the following paragraphs, the amount of droop is also
controlled by the amount of terminal shaft travel used between no load
and full load. Both considerations should be investigated when apparently
identical units show different responses to changes in the reference speed.
63
62
61
60
59
58
57
0 50 % 100%
Speed Setting for 70% load
Frequency
3%
5%
Load

Elstan’s
Pocket Book Series
62
Output Shaft Movement
The amount of droop in a governor is also influenced by the amount
of available output shaft movement used. The governor’s speed reference
is changed by feedback from the position of the governor output.
A governor with 4% droop over the full travel of the output shaft will
have an effective drop of only 2% if only half of the output is used from
minimum to maximum fuel.
Two percent droop is probably not enough to provide stability in
many operations. Using less than the optimum amount of terminal shaft
movement will require a higher droop adjustment (knob or slider) than
other engines, increasing the danger of overspeed should the generator
suddenly become separated from the bus (load). The low amount of
governor travel may also cause the engine to be unstable.
Multiple Engine Isolated Bus
Droop may also be used to parallel multiple engines on an isolated
bus. In this case, the engines can change the frequency of the bus, and if
all engines are operating in droop, the speed of the generator on the bus
will change with a change in load. This is satisfactory only in cases where
variations in the speed are acceptable. Multiple engines can also be
paralleled on an isolated bus with all but one of the engines in droop and
that one engine in isochronous. These systems will be able to maintain a
constant speed if the isochronous engine can accommodate any load
changes (see Figure 1.26).

________________________________________________________________
Elstan’s
Figure 1.26 – Use of lsochronous and Droop Units on an Isolated
System
In these cases, should load decrease below the combined load setting
of the droop engines, the isochronous engine will completely unload, and
the system frequency will increase to the point that load equals the
combined droop setting of the droop engines. The isochronous engine
would be motored in this instance unless it was automatically removed
from the bus.
If the load increases beyond the capacity of the isochronous unit, the
entire system will slow to the point where the combined droop of the other
units meets the droop-speed position. In this case, the isochronous unit
would remain overloaded to a point where it was unable to achieve the
governor reference speed.
63
62
61
60
59
58
57
0 50 % 100%
Load
Frequency
0 50 % 100%
Load
Frequency
5% Droop Governor
Isochronous Governor
Actual Speed set by Isochronous machine
63
62
61
60
59
58
57

Elstan’s
Pocket Book Series
64
Negative Droop
As has been stated, all mechanical governors use droop, either
constantly or in the case of isochronous governors temporarily, to achieve
stable engine control. It is possible to adjust negative droop (speed
reference increases as load increases) into some governors. Satisfactory
governor control (engine stability) cannot be achieved with negative
droop adjusted into a governor.
Unquote
***

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List of Books Available in Elstan’sTM
Marine Engineering Series
Printed Version eBook Version Title of the Book
9789355421517 9789355421524 Marine Electrical Technology 12th Edition
9789355422156 9789355422163 Marine Control Technology 5th Edition
9789355422101 9789355422132 Competency in Marine Electrical Engineering 3rd
Edition
9788175981799 9789385889653 Marine High Voltage Technology
9789352138630 9789385889660 The Explosion Protection Equipment Guide for Mari-
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9789352139194 9789385889677 Applied Marine Control and Automation
9788194710615 9788194710639 Hydraulics for Mariners
9789385889981 9789355421227 ETO & MEO CoC Q & A 2nd Ed
9788194710608 NA Maintenance and Troubleshooting of Marine Electrical
Systems
9789385889837 9789385889707 Testing of Electronic Components on ships and Land
9789385889851 9789385889844 Maintenance and Troubleshooting of Marine Electrical
Equipment Vol 2
9789385889943 9789385889950 Understanding Reefer Containers
9789385889967 9789385889974 Bridge Equipment for Navigation and Control of a Ship
NA 9789355421562 Marine Electrical Maintenance and Troubleshooting
Vol 1 Sec Ed
9789391043476 9789391043551 Marine Electrical Maintenance and Troubleshooting
Vol-2 Engine Room Equipment
Vol-3 Bridge Equipment
Vol-4 Deck and Hull Machinery
9789355421715 9789355420244 Marine Environment Protection Systems
9789355421579 9789355421623 Ship’s Electrical Circuits - Tracing Made Easy Volume 1
9789355421630 9789355421647 Ship’s Electrical Circuits - Tracing Made Easy Volume 2
NA 9789392506024 Basic Electricity for Seafarers (Free eBook - No Paper-
back as yet)
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NA 9789392506123 Leadership and Management for Gen-Z Seafarers (Free
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Print ISBN: 978-93-5542-331-3
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About the Book
This pocketbook is based on an extract from the book titled Marine Electrical
Technology and with reference to various guidelines for paralleling alternators
on board ships. It thus deals with understanding generic systems on board
ships. Electro Technical Officers and Marine Engineers onboard commercial
ships and those undergoing training to qualify for these positions, will find this
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Equipment
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Paralleling
of
Alternators

Paralleling of Alternators - First Edition - 2022

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