Book, Energy saving by tackling shaft voltage in turbine generators

Energy Saving by Tackling Shaft Voltage in Turbine Generators
By
MAYTHAM S. AHMED
And
RAAD Z. HOMOD
2014

ACKNOWLEDGMENT
First and foremost, we are very grateful to Allah for giving us the strength,
good health and allowing us to complete this book.
We would like also to express our appreciation to many people who helped
significantly in preparing this book. First, we would like to sincerely thank our
friends Dr. Haider A.F. Almurib and Dr. Farrukh Hafiz Nagi for their help and
advice on the subject area of artificial intelligent controls and their application.
And then we would like to thank our friends inside and outside of petroleum
engineering faculty in Basrah University and Universiti Tenaga Nasional.
Finally, we would like to express our great appreciation for our parents and
family for their patience and encouragement. Last but not least, we wish to give
our sincere gratitude and deepest love to our wives and children for their
continuous love and support, which enabled the completion of this book.
Authors

Page
TABLE OF CONTENT
iiDEDICATE
iiiACKNOWLEDGEMENT
ivTABLE OF CONTENT
viiiLIST OF TABLES
ixLIST OF FIGURES
xivLIST OF NOMENCLATURE AND SYMBOLS
CHAPTER 1 INTRODUCTION
1
2
3
3
3
4
1.1 Problem background
1.2 Problems statement and scope of the book
1.3 Objective of the book
1.4 Scope of book
1.5 Addressing problem
1.6 Book outline
CHAPTER 2 DIFFERENT STRATEGIES FOR TACKLING SHAFT
VOLTAGE
62.0 Introduction
62.1 Journal bearing mechanism
7
7
8
2.1.1 Bearing lubrication
2.1.1.1 Lubrication by oil
2.1.1.2 Lubrication by grease
82.1.2 Bearing failure
8
9
2.1.3 Mechanical failure component
2.1.4 Electrical failure component

11
11
2.1.5 Shaft voltage and bearing current mechanisms
2.1.6 Shaft current recognition
122.2 Potential sources of shaft voltage
132.2.1 External voltages supplied to the rotor windings
142.2.2 Electromagnetic induction
162.2.3 Magnetic asymmetries in electrical windings
162.2.4 Electrostatic voltages
172.3 Bearing Discharge Phenomenon
232.4 Effects of shaft voltage on bearings
242.4.1 Frosting/ Fluting
252.4.2 Spark Tracks
262.4.3 Pitting
272.4.4 Welding
272.5 Mitigation of shaft current and shaft voltage
352.6 Summary
CHAPTER 3 EXCITATION-SHAFT-BEARING MODEL TO
INVESTIGATE SHAFT VOLTAGE
363.0 Introduction
363.1 Excitation system
373.1.1 Full-Wave Six Thyristor rectifier system
38
38
39
42
3.2 Development of excitation-shaft-bearing model of a
turbogenerator
3.2.1 Model implementation
3.2.2 Modification of excitation shaft-to-bearing model
3.2.3 Simulation of the Three-Phase excitation system
433.3 Simulation of the model
443.4 Simulation results without grounding brush connection
463.5 Simulation results with grounding brush connection

473.6 Summary
CHAPTER 4 DEVELOPMENT OF EXCITATIONSHAFT
BEARING MODEL FOR A TNB GAS TURBINE GENERATOR
484.0 Introduction
484.1 TNB Gas Turbine Generator shaft-to-bearing model
524.2 Parameter calculation of 113.306 MVA SJSI Paka turbo
generator model
53
54
58
4.2.1 Parameter calculation
4.2.2 Inductance and capacitance calculations for the
excitation winding
4.2.3 Determining the parameter of Journal bearing
capacitance
614.3 Summary
CHAPTER 5 - SIMULATION RESULTS AND ANALYSIS
625.0 Introduction
625.1 Simulation results
635.1.1 Simulation results of AC supply voltage, DC voltage
an common mode voltage of the excitation system
64
5.2 Simulation results of shaft voltage and bearing current
before adding proposed grounding brush filter
67
5.3 Simulation results of shaft voltage and bearing current after
grounding brush filter
695.4 Optimization solver
715.4.1 Optimization of the parameters to model the grounding
brush
745.5 Summary
CHAPTER 6 CONCLUSIONS AND RECOMMENDATIONS
FOR FUTURE WORK

756.1 Conclusions
766.2 Recommendations for future work
77LIST OF REFERENCES
82APPENDICIES
82Appendix A: Specifications of SJSI Paka power station
(section 4.3)

LIST OF TABLES
PageTable No.
54Technical data on the excitation system and
excitation transformer for 113.306 MVA
turbogenerator at SJSI paka power station.
4.1
55Specification of the SJSI paka power station.4.2
55Excitation winding specification of the SJSI
paka power station.
4.3
56Mechanical characteristics of the SJSI Paka
power generator.
4.4
59Journal bearing specification for SJSI Paka
power station gas turbine generator.
4.5

LIST OF FIGURES
PageFigure No.
4Flowchart for development of GT shaft-to-
bearing model to reduce shaft voltage and
shaft current
1.1
7Journal bearing2.1
8Lubrication in bearing2.2
9Edge load journal shell with Babbitt
mechanical fatigue
2.3
10Electrical discharge in journal bearing2.4
12Elementary diagram for considering shaft
currents in typical turbine generator
arrangement
2.5
15Shaft current generation from a residual
magnetic source – Axial shaft magnetization
or “through flux” creates localized bearing
or seal currents
2.6
15Shaft current generation from a residual
magnetic source “through current” created
by rotating element in highly magnetized
casing or housing
2.7
17Shaft current and voltage due to asymmetric
magnetic field.
2.8
19Distribution of electrical field2.9
19Charged particle concentration increases2.10
20The current begins flowing and Voltage
begins to decrease
2.11
20Discharge channel begins to form between2.12

roller and race
21Current continue to rise and voltage drops2.13
21Discharge channel with high current passing
through it
2.14
22Voltage and current drops2.15
22Molten metal solidifies2.16
23Extracted metal scattered in the dielectric oil2.17
25Illustration of frosting2.18
26Illustration of fluting2.19
27Pitting on the bearing and shaft2.20
28Steam turbine generator experimental
system without brushes, or protection
conducted by Nippes et al.
2.21
28Shaft Grounding, Monitoring, Protection and
Warning
2.22
30Equivalent circuit of shaft potential source2.23
31Model excitation winding and turbo
shaft200MVA turbogenerator proposed by
Golkhandan et al.
2.24
32Shaft to ground voltage at (EE) (a) without
grounding filter and (b) with grounding filter
applied
2.25
32DC-DC buck converter2.26
33(a) Voltage applied to the field (b) Shaft to
ground voltage of winding with buck
converter applied
2.27
34Model of excitation winding and shaft for
numerical calculation of shaft voltage
proposed by Ammann et al.
2.28

34Numerical simulation results for a 1200
MVA turbo generator proposed by
Ammann et al.
2.29
37Schematic of Full-Wave (six thyristors)
Rectifier Bridge
3.1
38Thyristors output waveform3.2
39Generation and Transmission of shaft
voltage due to Excitation systems
3.3
41Complete excitation-shaft-bearing model
and excitation winding model created in
MATLAB Simulink for the 1200MVA
turbogenerator
3.4
42Static exciter System of the generator which
provides input voltage to the excitation-
shaft-bearing model
3.5
43(a) Three phase supply AC line-line voltage
of 1732 peak at 50 Hz frequency and (b) The
input DC voltage to the excitation shaft
bearing model
3.6
44Common mode voltage (CMV) component
of the rectified DC voltage from the
excitation system
3.7
45(a) Shaft voltage at turbine end obtained
from the simulation model without
grounding brush connected and (b) zoomed-
in view of the shaft voltage
3.8
45(a) Shaft voltage at the exciter end obtained
from the simulation model without
grounding brush connected and (b) zoomed-
3.9

in view of the shaft voltage
46Shaft voltage at(a) turbine end and (b)
exciter end with grounding brushes
connected in the simulation model
3.10
50Model of excitation winding and turbo shaft
for a 113.306 MVA turbogenerator at SJSI
Paka created using MATLAB Simulink
4.1
51Excitation system for Generator4.2
51Connection of resistor and passive filter to
model grounding brushes in the model
4.3
58Modeling the journal bearing of
turbogenerator
4.4
64(a) three phase supply AC voltage of is 278
volt peak per phase at 50 Hz frequency and
(b) Rectified DC voltage used as input to the
excitation-shaft-bearing model of SJSI
Power station
5.1
64Common mode voltage obtained from the
rectifier of the SJSI Paka Power station
simulation
5.2
65(a) Shaft voltage at the turbine end (TE) of
the simulated SJSI paka power station
generator without applying any grounding
brushes and (b) zoomed-in view of the shaft
voltage
5.3
66(a) Shaft voltage at the exciter end (EE) of
the simulated SJSI paka power station
generator without applying any grounding
brushes and (b) zoomed-in view of the shaft
5.4

voltage
67(a) Bearing current at the turbine end (TE)
and (b) at exciter end (TE) of the simulated
SJSI paka power station generator without
applying any grounding brushes
5.5
68Shaft to ground voltage at the (a) turbine end
(TE) and (b) Exciter end (EE) after applying
proposed grounding brush model
5.6
69Bearing current at the (a) turbine end (TE)
and (b) Exciter end (EE) after applying
5.7
70Optimization solver connected at the exciter
end (EE) of the excitation excitation-shaft-
bearing model for the 113.306 MVA SJSI
Paka turbogenerator
5.8
72(a) Signal constraint (b) the optimized values
of capacitor and resistor to model the
grounding brush obtained from the
optimization process, (c) the location of the
grounding filter and (d) the input given to
the optimization solver
5.9
73(a) shaft voltage at TE (b) shaft voltage at
EE after optimization of the grounding brush
filter components
5.10

LIST OF NOMENCLATURE AND SYMBOLS
1- List of Abbreviations
GT Gas Turbine
SJSI Stesen Janaelektrik Sultan Ismail
TNB Tenaga Nasional Berhad
TE Turbine End
EE Exciter End
DBS Dielectric Breakdown Strength
ESD Electrostatic discharge
EDM Electrical Discharge Machining
PWM Pulse Width Modulation
EMI Electromagnetic Interference
Ef Electrical Field
CMV Common Mode Voltage
2- List of Symbols
surface area,
capacitance of journal bearings, F
capacitance of insulation in the bearing, F
excitation winding coil capacitance, F
excitation winding coil inductance, H
number of coil strands

l length of winding, m
o permeability of free space
relative permeability of the rotor core
permittivity of free space
relative permittivity
L length
distance, m
diameter of the rotor
diameter of the shaft
Shaft length
effective surface area of the journal
bearing, m
effective length of bearing
diameter of journal bearing
oil film thickness of bearing
capacitance of journal bearing at the
compressor inlet, F
compressor discharge, F
generator end, F
capacitance of journal bearing at turbine
end, F
exciter end, F

Hz frequency
v voltage
vsh Shaft voltage

CHAPTER 1 INTRODUCTION
INTRODUCTION
1.0 Problem background
Shaft voltage has become a serious problem in gas turbine (GT) and large power
plants. The stray voltage generated due to shaft rotation in the magnetic field affects
the journal bearing because it is regarded as ground to this voltage. During the
machine operation, an electrical charge initiate in the shaft has the potential to be
damaged by stray shaft currents either in, mechanical or electrical energy. In addition,
the shaft voltages can be produced by shaft rotation involving irregular magnetism of
electrical machinery, by residual magnetism present in a shaft and exciter current-
carrying brushes. The generated current able to damage the bearings, couplings, seals
and gears, which causes the unit to shutdown can be divided into two types of shaft
voltage; one of them is advantageous and the other type is damaging to the machine.
The first type can provide early notification of problem development long before the
problem is apparent on classical monitors and instruments, while the other can cause
possible circulating currents and result in reduced unit efficiency. Any unexpected
fault in the grounding system of the machine causes the shaft voltage to find another
path to pass through to the ground. The path that is closest to the shaft is usually a
bearing or seal [1]. After a period of time, the currents pass through the bearing
started to erode the metal surfaces. This electrostatic voltage's discharge will be
harmed to the bearing or changes in rotor dynamic movement and in the same time
destroy the shaft of the machine [2].
Despite of all the causes above there are others reason can cause many problems to
the bearing as mechanical / thermal causes. One of the famous problems that can
cause bearing damage is the dirt particles mixed in the lubrication system.
Insufficient lubrication can lead to fail in the lubrication system and oil seal failure.

Gradually, increase the temperature thorough the journal bearing due to friction
between surfaces [3].
This book will focus on bearing failures due to shaft voltages and bearing currents in
turbo generators. The following four types of sources are the most common reasons
for generating of shaft voltage and bearing currents:
1- Magnetic unsymmetries due to rotor eccentricity, stator or rotor sagging produce
variable magnetic flux and joints in the stator laminations. This type can damage any
path which has low resistance due to the large current as a result the induced voltage.
In addition the magnetic flux that is related with shaft of rotor can generate bearing
circuit and induce a shaft voltage.
2- Axial shaft flux due to saturation, asymmetrical rotor winding or residual
magnetization. The induced voltage from the axial shaft flux will produce large
currents which run through the bearings and shaft seals.
3- Electrostatic charge due to steam brushing turbine blades.
4- External voltages on rotor windings due to voltage source, winding insulation
asymmetries or static excitation equipment.
For cases two and three listed above, the voltage between the shaft and bearings
(ground) will cause the oil bearing film to collapse and electric discharge will occur
causing problems for the surfaces of bearings and seals [4].
Nowadays, many researchers have been conducted to eliminate the shaft voltage and
minimize the bearing current to protect the bearing from damage.
1.1 Problems statement and scope of the book
TNB Power Plant is looking into the possible causes of shaft current in gas turbine
generators at SJSI Paka and to provide mitigation measures to avoid the factors that
lead to bearing damages in gas turbine generators. So in order to carry out the
investigation, a generic equivalent circuit model for the shaft-to-bearing system of a

gas turbine generator is to be developed and simulated for investigation of shaft
current and voltage phenomena. The purpose of the work presented here is to
investigate shaft voltage problems in power generation station by modeling and
simulation of shaft voltage at (EE) and (TE) with and without grounding.
1.2 Objective of the book
The specific objectives of the work are as follow:
1. To develop a GT shaft-to-bearing model for a TNB gas turbine generator.
2. To investigate the occurrence of shaft voltage and bearing current due to the static
excitation system through simulation of the developed model.
3. To recommend mitigation and remedial action for shaft voltage problem.
1.3 Scope of book
To study and investigate the shaft voltage phenomena due to external voltage on rotor
windings from the static excitation equipment also to develop shaft-to-bearing model
for one particular gas turbine generator at SJSI paka power station and analysis of
results to recommend the remedial action to prevent damage to the shaft bearings.
The book is conducted using MATLAB software for calculation the shaft voltages.
1.4 Addressing problem
1. Develop model for static excitation system (3-ph bridge rectifier model) which
provides input to the GT excitation-shaft-bearing model in Simulink.
2. Develop the GT excitation shaft-to-bearing model in Simulink.
3. Preliminary simulation of the GT shaft-to-bearing model to investigate shaft
current and shaft voltage due to static excitation system.
4. Verify result with literature data.
5. Develop and simulate the GT excitation-shaft-bearing model for TNB power
plant.
6. Analysis of results and recommend mitigation actions for shaft voltage.

Based on the objective of the book, Figure 1.1 shows the methodology adopted to
conduct the book.
Figure 1.1 Flowchart of methodology of the book
1.5 Book outline
Basically, this book presents six chapters which have been categorized as follows:
Chapter One presents a general introduction of the topic investigated. The book
background and problem statement are concisely discussed in the first section. The
following sections list the project objectives and scope of book. The final section in
this chapter presents the book organization and outline.

Chapter Two will provide an overview of the project’s background and also
literature review conducted during the book. It involves the exploration of
information from academic journals, book and thesis. Pertaining to the causes of
bearing failure in particular due to shaft voltage and bearing current
Chapter Three will present detailed explanation of the book methodology conducted
and it is subdivided into six sections. The first section provides an overview of the
excitation system and the exciter of a generator. The following section presents a
complete model of shaft line, field winding, and static excitation system recreate in
MATLAB Simulink based on the previous work conducted by Amman et al [4].
Simulation of the recreated model to investigate shaft voltages at exciter end and
turbine end, with and without grounding brushes will be presented and compared
with the literature to validate the recreated model.
Chapter Four is divided into three sections to explain the detailed work conducted in
the development of a TNB Gas Turbine excitation-shaft-bearing model and its
simulation for measuring shaft voltage at the exciter end (EE) and at the turbine end
(TE). The value of parameters for all component in the model are calculated based on
the specifications of a GT generator at Stesen Janaelektrik Sultan Ismail (SJSI) Paka
Power Station manual and the values are applied to the simulation.
Chapter Five presents the analysis of the simulation results for the developed model
of the excitation winding and turbo shaft for the 113.306 MVA SJSI Paka GT
turbogenerator. The analyses are based on the simulation results obtained under two
cases, with and without grounding brushes. The built in MATLAB optimization
solver was also used to optimize the values of RC components to model the
grounding brushes in order to obtain the best values for the RC grounding brushes.
Chapter Six provides a summary of the conclusions obtained from the study.
Furthermore, the recommendations for future works are provided in this chapter.

CHAPTER 2
DIFFERENT STRATEGIES FOR TACKLING SHAFT VOLTAGE
2.0 Introduction
This chapter provides literature review on bearings and parameters that could cause
direct or indirect problems on the bearing, as well as the effect of shaft voltage on the
journal bearing. The first section provides an overview of the bearing and the causes
of bearing failure from mechanical and electrical contexts. This is followed by a
review on the common causes of shaft voltage and bearing current in turbogenerator.
However, in this book, the focus is on the one cause of shaft voltage which is external
voltages on rotor windings due to static excitation equipment, which will be
discussed in detail. Review on the mitigation of bearing current and shaft voltage, and
other work related to this present work will also be provided. The last section of this
chapter summarizes the relevant previous research works.
2.1 Journal bearing mechanism
Journal or plain bearings consist of a journal or shaft, which rotates freely in a
supporting metal sleeve or shell as shown in Figure 2.1. Even though the design and
structure of the journal bearing are relatively simple, the model and operation of these
bearings can be complex. The bearing surface is separated from the journal surface
by a jacking oil pump, which creates a lubricant film around the periphery. Moreover
while running, the same film is maintained naturally.

Figure 2.1 Journal bearing
2.1.1 Bearing lubrication
The bearing is one of the most critical components in rotating machines and turbo
machinery. Generally bearings reduce rotational friction and support the radial and
axial loads applied to the journal, during rolling. Bearing plays a significant role in
separating the moving part from the stationary part. Friction in contact surfaces can
lead to maximum wear and tear, and hence lubrication becomes vital to reduce these
impacts.
2.1.1.1 Lubrication by oil
For high speed operations, it is essential for the journal bearing to constantly use oil
for lubrication, which has better performance when compared with grease. Oil is used
in bearing when cooling is required, which reduces the heat from journal bearing, as
shown in Figure 2.2. Basically, oil is applied to the bearings by using an oil pump
system; oil ring grooves in the bearings shell are used to spread the oil within the
surfaces of bearings [3].

Figure 2.2 Lubrication in bearing
2.1.1.2 Lubrication by Grease
Greases are mainly used to lubricate ball bearings and occasionally used in journal
bearings, when cooling of the bearing is not a factor. Grease-lubricated bearings have
greater frictional features because of their high viscosity. Grease is also useful during
shock loading or if the bearing frequently starts and stops or reverses direction.
2.1.2 Bearing failure
There are many reasons for bearing damage, which may lead to failure of the bearing.
It is not always easy to calculate the exact cause, but major the causes of bearing
failure can be classified into to broad areas which are mechanical failure components
and electrical failure components.
2.1.3 Mechanical failure component
While investigating the problems associated with bearing, many factors and
parameters were found to be possible causes of direct or indirect problems on the

bearing. Some problems of bearing might result from design and manufacture defects
[6]. Temperature is the most important factor that affects the bearing material;
therefore increase in temperature accelerates and causes fatigue failure in journal
bearings [7]. The presence of dirt elements in the lubrication system is also one of the
most common reasons of bearing failure. Apart from that, insufficient lubrication,
corrosion, erosion, over-loading, cavitation, under-operating conditions, reduced oil
pressure, generating vapour bubbles, also lead to damage on the bearing surface.
Furthermore, fatigue, as shown in Figure 2.3, includes repeated flexing or bending of
the bearing, which quickly damages the bearings with poor bonding. Factors which
can cause fatigue in journal bearing are such as vibration, bent shaft, imbalance,
journal eccentricity and misalignment. Also, there are many other mechanical reasons
that can cause major problems and lead to damage of the bearings [8].
Figure 2.3 Edge load journal shell with Babbitt mechanical fatigue
2.1.4 Electrical failure component
Quite a number of studies have claimed that, bearing currents and shaft voltages
resulting from non-symmetry effects can lead to bearing damage, Electrostatic
discharge (ESD) influence, or homopolar flux influence in electric machines [9,10].
There is a huge possibility that the bearing current damages the bearing, when the

voltages build up through a
The problems of shaft volta
caused by magnetic asymme
S. P. Verma et.al [12], exp
through the shaft due to Ele
could cause a comparatively
can lead to a very high co
reduce the dielectric breakdo
the Electrical discharge in jo
Figure 2.4
Akagi and Tamura [14] h
(EDM) current is passed ins
frame exceeds the dielectric
bearings. Akagi and Tamura
interference (EMI) filter th
terminals can be reduced.
a motor bearing, between the frame an
age circulating bearing currents have
etry in machines [11].
lained that a small voltage might be i
ctrostatic discharge. As a result of this
y high shaft current. The obstruction o
mponent of induced voltage in the b
own strength (DBS) of the oil film [12
ournal bearing.
4 Electrical Discharge in journal bearing
have explained that the electrical dis
side the motor bearings, if the shaft vo
c breakdown strength (DBS) of the thin
a [14] also showed that by using a passi
he frequency of the common voltag
Shaft voltage machines caused by th
nd the motor shaft.
been reputed to be
initiated end-to-end
s, the small voltage
of the shaft current
bearing, which will
]. Figure 2.4 shows
g
scharge machining
oltage related to the
n lubricating oil, in
ive electromagnetic
ge from the motor
he static excitation

systems in turbogenerators, can influence the insulation, which will lead to an
unexpected electrical discharge current.
The topical issues that can cause undesirable effects like shaft voltage and consequent
bearing currents are high voltage change with time (dv/dt) due to rapid switching
transients and common mode voltage produced by pulse width modulation (PWM)
inverters, in induction-motor drives [15].
2.1.5 Shaft voltage and bearing current mechanisms
The voltage measured between the motor shaft and the stator case is called the shaft
voltage. Chen and Lipo [16] have indicated that, the potential source of the bearing
issues is known as shaft voltage, since it appears across the bearing motor. Jean-Eric
Torlay [17] has explained that, the failures can generate an alternating voltage
between the two ends of the shaft, or sometimes it creates a constant voltage between
the shaft of machine and the ground. Especially in some machines, the impedance of
the circuit made by the shaft of machines in the frame and the journal bearing will be
very low, where, very high currents are passed, which causes big problems by
damaging the shaft and bearing.
2.1.6 Shaft current recognition
Shaft current is not a new problem. The pioneer of French and German engineers
worked in manufacturing electrical rotating machines have studied the cases of bearing
failure, and have published their studies in their own languages from 1907 to 1915. In
the year 1924, Alger and Samson [11] have investigated all these information and
compiled and presented them neatly. According to Alger and Samson [11], the
following are the possible causes of shaft current:
1. Electrostatic effects will cause a difference of potential between shaft and ground.
2. Asymmetrical windings will cause alternating flux flowing in the shaft.
3. When clockwise flux is not equal to the counter clockwise, it will cause an
alternating flux linking the shaft.
Other than the above, the shaft voltage can be originated from many other causes.

The best method for determining the presence and severity of shaft current is
probably by inspecting the affected and damaged parts. As mentioned in [18], the
probable causes of shaft currents are:
• A direct or alternating flux flowing in the shaft.
• A difference of potential between the shaft and ground due to electrostatic, or
to grounding of the rotor conductors to the core.
• An alternating flux linking the shaft.
All components of a turbine-generator system can be represented as in Figure 2.5,
which also explains the path of shaft current [19].
Figure 2.5 Elementary diagram for considering shaft currents in typical turbine generator
arrangement
2.2 Potential sources of shaft voltage
Shaft- potentials arise from various sources in electrical machines, and often damage
important machine components, such as, journal bearing, gears, oil pumps, speed
control units, thrust bearing and hydrogen seals. These components are damaged by

electrolysis effects and arcing, which comes from outages of generating units. In
terms of design, all types of turbo generators must be provided with:
(a) Grounding of the shaft at one or both ends of the turbine.
(b)Insulation of the generator outside bearing, bearing-seal housing, gear-case.
Shaft voltages and the damaging currents have been shown to increase with the size
of turbo-generators [20]. Wang Xiao-hua et al. [21] have discussed the causes of
damages and preventive measures of shaft voltage in turbine generator also, they
have explained all sources that cause shaft voltages.
When machine is operated, four sources of shaft voltages have been identified.
However, this book focuses on only one source which is External Voltages on Rotor
Windings due to static excitation equipment.
The four sources of shaft voltages will be explained further in the following sections.
2.2.1 External voltages supplied to the rotor windings
External voltages supplied to the rotor windings are largely related to the excitation
system of electrical machine. The voltages are either principally dc signal or rectified.
The pulse of the rectifier is often seen in the shaft voltage. Furthermore, other sources
involve, rotor winding insulation asymmetries or voltage source, and active rotor
windings. The resulting external voltage raises the potential to the shaft, according to
the insulation of materials resistance and capacitance of the source, and the shaft
against ground and the winding. As a result, the oil film will be stressed due to the
shaft voltage between journal bearing and the shaft. At times of collapse, electric
discharge will occur between the bearing and the shaft, and pitting will destroy the
bearing’s surface [22, 23]. Other researchers have developed a complete model for
shaft line and static excitation system [24].
The turbine blades experience induce voltages, due to the steam flow though the
blades. In this case, when a spark occurs in the bearing or a seal, the capacitive load
generates high voltages that decreases quickly and increases slowly. In order to

reduce the magnitude of this voltage grounding brushes are connected to the turbine
end shaft of the generator [21].
2.2.2 Electromagnetic induction
Shaft voltage can also originate due to rotating a residual magnetic source in housing
similar to the action that occurs in generators [25]. Michael J. Costello [22] has
distinguished electric and mechanical machines, where electric machines have
armature windings for the purpose of carrying the induced currents in the winding,
while the mechanical machines or the secondary winding is the shaft, bearings, seals
and etc. The following causes have to be present, for this mechanism to create shaft
voltages:
The first factor is the source of high residual magnetism possibly the rotor, bearings,
casings and etc. The second is the presence of a closed, low-reluctance magnetic
circuit, and also the presence of high relative surface velocity (as found in turbo-
machinery) and the last factor to be present for generating shaft voltage is very small
clearances across which, the voltage can discharge.
Figures 2.6 and 2.7 show the two important sources for generating shaft potential due
to a residual magnetic source, firstly, an axial shaft flux (through flux) and secondly,
an axial shaft current (through current). It is essential to calculate the kind that is
present, since the insulation techniques can be different. For instance, a metallic
nonmagnetic coupling spacer would be acceptable to avoid a through flux situation;
however, as it is conductive, when the second situation is present, it would easily
transmit current.
When the source is magnetized correctly, the shaft currents that are created must be a
result of a residual magnetic source. Manufacturers produce many different systems
for automatic demagnetizing of machinery components. So, it is very crucial for dc
down cycling with reverse polarity, on large cross-sectioned components [26]. Also,
Costello [2] proposes this process to be conducted when installing new machinery
parts or mechanisms.

Figure 2.6 Shaft current generations from a residual magnetic source – Axial shaft magnetization or
“through flux” creates localized bearing or seal currents
Figure 2.7 Shaft current generation from a residual magnetic source “through current” created by
rotating element in highly magnetized casing or housing
Stator
Stator
Rotor
Bearing
current

2.2.3 Magnetic asymmetries in electrical windings
When a magnetic flux surrounds the component of machine, it will induce alternating
currents in any loop that pass through it, so these components that contain loops, such
as, frame, shaft and bearings of the machines are attentive to magnetic symmetries.
Though the main sinusoidal supply is viewed to be symmetrical, asymmetries can
occur and cause different stray fluxes. Inside the machine, segmented laminations of
the rotor or stator as well as eccentricity can induce this magnetic flux. Normally, the
potential shaft generated, induces shaft voltage between 30 and 60 volts peak to peak
with high harmonic content when the dissymmetry links the rotor shaft. In addition
large current to ground will pass due to the low source impedance [27].
The linkage of the alternating flux with the shaft is regarded as the most significant
aspect that leads to bearing currents. The flux flows vertically to the axis of the shaft,
and also passes through the rotor and stator cores of machine. It leads from
asymmetry in the magnetic circuit of the machines. At the normal case, when the flux
from poles cross the air gap, and when the magnetic path is symmetrical, the flux
splits half and half one mainly in the clockwise direction and another in the
anticlockwise direction. The asymmetries sometimes come from the construction and
design of the machines, and also from incorrect alignment. The potential differences
between the ends of the shaft are established due to the alternating ring flux, as
shown in Figure 2.8. Arcing will occur between the surfaces of bearing, if the
potentials are large enough to cause breakdown in the lubrication oil film [27].
2.2.4 Electrostatic voltages
The output voltage for the excitation systems contain high frequency components due
to the thyristors controlled in the excitation systems. This voltage can also induce
shaft voltages between the shaft and rotor winding, by capacitive coupling [21].

Figure 2.8 Shaft current and voltage due to asymmetric magnetic field
These occur at some special circumstances such as, the nature of application as pulley
driven loads and belt or low humidity environments particularly, and not due to the
basic design of the machine for example. The shaft voltages start to reinforce until a
discharge happens through the bearing. Occasionally, a little friction of a pulley or
belt is needed in order to set up electrostatic charges. Voltage initiating from such
source is not usually a major problem [27].
2.3 Bearing discharge phenomenon
Some new drive motors setup can experience motor bearing damage, within a few
months after start-up. This problem will happen when the bearing current is induced
in the motor shaft and is discharged through the bearing. Under normal conditions,
manufacturing and machine design have carefully reduced bearing failures, in
modern drive systems, the rapid switching of the invertors produce high frequency
voltages that can harm the bearings. The electric current that pass to earth through a
bearing is known as the electric discharge machining (EDM).

Arun Kumar Datta et al.[3] have extensively investigated mechanical and electrical
problems that cause the expected damage in the ball and journal bearing, and also the
bearing lubrication (grease lubrication and oil lubrication). They have also discussed
about shaft voltage, generated from machine during operation, and the shaft discharge
with potential above the ground. The main reason of journal bearing failure comes
from flow of current to the bearing surfaces under the following three conditions:
circulating currents due to shaft voltages, bearing currents due to the discharge of the
air gap capacitor, known as electric discharge machining (EDM) bearing currents.
They have concluded that, designing an adequate treatment method of bearing current
is important in inverter driven system in motors. The corresponding circuit
characteristic varies from a resistor to a capacitor. When high-resistance grease are
used and if the bearings float on the oil film, impurities on the bearing surfaces
sometimes discharge the rotor of machines, and puncture the oil film. In high quality
bearing, low level discharges happen, which permits the rotor to charge for longer
durations of time, and therefore get higher voltage stages. Bearings of low quality
have less charge occurring in them because of the metal-to-metal contact.
S. Chen et al. [28] stated that when the rotor voltage exceeds the threshold voltage of
the oil film, the dielectric strength of the oil film will be exceeded and electric
discharge machining (EDM) currents will be produced which leads to arcing. The
following step by step illustrations explain the identification of electric discharge
machining cycle and what happens during an (EDM) cycle.
The electrical field (Ef) is defined as the electric force per unit charge, and it is
strongest at the point where the distance between the race and the roller is least. The
positive and negative polarities of the field are as shown in Figure 2.9. Usually, at
this point the current is zero but the voltage still increases.

Figure 2.9 Distribution of Electrical field
As the amount of charged particles increases, the insulating properties of the oil films
start to decrease, along a thin channel centred in the strongest portion of the electric
field. The currents are still zero but voltages have reached its peak as seen in Figure
2.10.
Figure 2.10 Charged particle concentration increases
As shown in Figure 2.11, the currents are generated as the oil becomes less in an
insulator. The voltages begin to decrease.

Figure 2.11 The current begins flowing and Voltage begins to decrease
The temperature builds up very rapidly when the current increases, and at the same
time the voltages continue to drop. Some of the fluid has evaporated due to the heat,
and a discharge channel begins from between the roller and the race as seen in Figure
2.12.
Figure 2.12 Discharge channel begins to form between the roller and the race
A vapour bubble attempts to expand, but a presence of discharge channel limits this
expansion. In addition, the ions will attract to each other, due to the electro-magnetic
field. The voltage continues to drop and current rises (refer to Figure 2.13).

Figure 2.13 Current continue to rise and voltage drops
After a period of time, pressure and heat inside the vapour bubble will reach the
maximum values, current and voltage will be stabilized, and consequently some parts
of metal will melt. Furthermore, the layer of bearing below the discharge column is
also supposed to melt, but due to the pressure of the vapour bubble, the metal is held.
Since the plasma is created during the EDM process, the plasma properties are
strongly influenced by the pressure forced by the surrounding liquid [29]. The
discharge channel of superheated plasma consists of dielectric oil, vaporized metal,
and carbon that enable high currents to pass through it as shown in Figure 2.14.
Figure 2.14 Discharge channel with high current passing through it

After that, avalanche phenomenon occurs to the vapour bubble, and then bearing
metal melts in order to exclude from surface. Furthermore, the temperature also
decreases very fast and the voltage and current drop to zero as shown in Figure 2.15.
Figure 2.15 Voltage and current drops
As a result of quenching the surface of the race and the change in rollers, and the
races speed, flushing the conductive elements away lead to additional dielectric fluid
changes as shown in Figure 2.16. Nevertheless, different layers are formed, but
molten metal remains solidified.
.
Figure 2.16 Molten metal solidifies

The remaining vapours rise to the surrounding surface. The extracted bearing metal
form tiny spheres scattered in the dielectric oil. Finally, conductive particles would
accumulate, creating the unstable spark as shown in Figure 2.17.
Figure 2.17 Extracted metal scattered in the dielectric oil
In most of the bearings, installed in frequency converter driven motors, this sequence
is supposed to occur 500 times per second [30, 31].
2.4 Effects of shaft voltage on bearings
As previously stated, the inspection of the affected damaged parts is regarded as the
best method for determining the presence of shaft currents in machines. The usual
procedures of journal bearings maintenance do not always detect all the problems that
cause damage to bearings, therefore it is very crucial to check and inspect all parts of
the machine, after shutting down and disassembling from the source of shaft potential
or other component.
Shaft current can cause four types of bearing damages, and cause many problems to
the bearing such as: welding, pitting, spark tracks, and frost. The pitting, frosting and
spark tracks must be inspected by using a microscope, as they are simply

misdiagnosed as mechanical or chemical damage [22]. In the following sub sections
all the damage types will be explained detail.
2.4.1 Frosting/Fluting
Frosting is the most common type of damage due to shaft current, which destroys a
lot of machine parts, such as, journal bearing, thrust collars and gear. Depending on
the nature of the machine, this damage takes two forms: fluting or frosting. This
occurs when current is passed through the motor bearing, rather than a grounded
source. Deterioration will happen on the bearing race surface for machines, operating
under a high speed range, and as fluting (grooves) in race for motors, running at
comparatively stationary speeds [22].
Electric Discharge Machining (EDM) can cause this kind of damage based on a lot
of factors. For example, when the contacting area between the roller and race touch
each other, it leads to this damage. It is essential to determine how much current can
pass, without overheating the journal bearing. The increase in speed significant
contacting area with low resistance path, the oil film floating in the bearing, the type
of oil and thickness of film, and the imperfections on the bearing surfaces can cause
this effect [32]. Naturally, high quality bearings charge as much as 80% of the time,
because of an unchanging oil film, and low quality bearings charge considerably less,
because of metal-to-metal contact. When the voltage gradually exceeds the dielectric
strength breakdown of the oil film, the arcing will occur, and the EDM current will
destroy the bearing [33, 34].
Frosting damages are not noticeable by the naked eye, so inspection under a
microscope is required to observe the damages. Though, the frosted surface is seen
like small different craters, the crater’s bottom is round and shiny. It sometimes
appears comparable to chemical attacks, but the effects are more severe. Figure 2.18
illustrates the frosting effect.

Figure 2.18 Illustration of frosting
Doyle Busse et .al. [36] have stated that, the electric discharge machining EDM
bearing currents can cause regular microscopic marks in the bearing race of machine,
with the marking period equally spaced, according to the ball spacing.
Fluting is often related with incessant manufacturing procedures that are
simultaneously run at the same speed for a number of hours. Fluting marks generally
happen in the same place on the bearing-race load region, due to continued
deterioration at the bottom of the original race markings. Figure 2.19 depicts the
effect of fluting on the bearing.
2.4.2 Spark Tracks
The impact of damage on the bearing that makes irregular tracks on surfaces of the
bearings is referred as spark trace. At first, this type of damage seems to be similar to
the scratches that appear on the surfaces from seal oil or some particles in the oil film.
Undamaged bearing
Frosting on bearing without
shaft grounding

Figure 2.19 Illustration of fluting [35]
Potential difference can produce sparks between main bearing and journal pin, in
order to complete the circuit between two electrodes having change in potential.
Several examinations have been made to explain that the problem occurs due to
askew of rotational direction. Under magnification, the bottoms of the traces appear
occasionally melted and the corners around the bearings are sharper. In addition, dirt
particles in the oil would look normally clear and leave rounded corners of bearing.
The depth of the spark track is generally identical around its entire surface [22].
2.4.3 Pitting
The frosting damage is similar to pitting, but the later is usually larger in size as the
source is very powerful. Pitting damage actually happens on the backs of bearings or
in the gear teeth, however sometimes occurs between frame splits. Pitting, as shown
in Figure 2.20, happens randomly and it can cause a number of discharges in the
bearing. The presence of the pits is similar to the individual frosting craters and they
Section of inner race of
bearing showing fluting
Micrograph shown
fluting in bearing

often have round shiny bottoms. Sometimes it is difficult to distinguish pitting with
other types, such as, corrosion; therefore it is essential to obtain clarification from a
qualified metallurgist [35].
Figure 2.20 Pitting on the bearing and shaft
2.4.4 Welding
One of the important problems with conventional steel bearings is their tendency to
weld. Generally, this case will occur when the ball and race make contact, and then
weld together. Sometimes, the welding happens between seals and bearing pads when
a high current passes through them, such as splits (welding of parts).
It is very easy to observe welding as spot welded marks by the naked eye, and often
has to be separated by a sledgehammer or other mechanical means. This effect is
typically the result of a trouble in the process permitting a rotor, to rapidly contact the
stator. In this case, a large current will pass through the bearing. The process is
occasionally called self-excitation [35].
2.5 Mitigation of shaft current and shaft voltage
The solutions for preventing shaft voltage include, grounding of the shaft on the drive
end of the electrical machine, or providing symmetrical filters on the dc side of the
rectifier, and avoiding asymmetries in the excitation circuit. The standard

methodology as given in IE
voltmeter, to measure the en
measure the shaft current [3
exists due to magnetic field
experiments on a steam turb
was without shaft grounding
2.21.
Figure 2.21 Steam turbine g
The second experiment wa
brushes to protect the bearin
Carbon brushes are the most
Figure 2.22 Sha
EEE Standard 112 involves the use o
nd-to-end shaft voltage, and a low res
37]. Nippes et al. [38, 39] has proved
d in turbogenerator. This was obtained
bine-generator. The first experiment co
g or protection against shaft current a
generator experimental system without brush
as connected by grounding the shaft
ng from serious electrical fault as sho
t generally used brushes for shaft groun
aft grounding, monitoring, protection and war
of a high resistance
sistance ammeter to
d that, shaft current
d through practical
onducted by Nippes
as shown in Figure
hes, or protection
using two ground
own in Figure 2.22.
nding.
rning

Based on the investigation by Nippes et al. [39] it has been identified that, the shaft
current in the shaft and the reading from the monitor is different for the two
experiments due to the grounding brushes. Similarly for the case of static excitation
system being the source of shaft voltage, it has been advised to reduce this value, and
control the potential problems initiated in the shaft, by either placing grounding
brushes or by injecting opposing current signals to the machine's rotor [38].
Buckley et.al [40] have explained that using the grounding brushes is the most
common solution, to limit the voltage to a safe level in a turbo-generator, by
providing a path for it to discharge. The grounding brushes used to reduce shaft
current should be located at the turbine end, and should not be located at the exciter
end of generator, in order to avoid the circulating currents from shaft to bearing, and
provide typical technique for protecting the components at exciter end of generator
and insulation of the exciter generator coupling. It is essential to choose a suitable
location for the brushes, to facilitate taking readings conduct safety maintenance,
facilitate testing and inspection. Furthermore, it is important to install at least two
brushes at the same location of the shaft. This is ensuring that the brush is always in
contact with the shaft. In order to obtain perfect electrical contact with the shaft, two
things must be taken into account; firstly, installing the brushes at an angle of 90
degrees to enable it to always have contact with the shaft and secondly, the shaft must
be polished with a stone to obtain a soft surface for the brushes, to ride easily on the
shaft. An equivalent circuit of shaft potential was designed by Buckley et.al [40] as
shown in Figure 2.23.
The brushes provide ground or zero reference, and are generally connected at the
turbine end. It has been advised to use carbon composites, in order to reduce the
voltage drop across the oil film, and to minimize all other problems that create issue
to the other components. But the most vital thing that must be taken into account
when using this type of brushes are they must be inspected every week for
maintenance and replacement.

Figure 2.23 Equivalent circuit of shaft potential source
Golkhandan et al. [24] have proposed an appropriate model for numerical simulations
of shaft voltage, shaft line and field winding of a 200 MVA Ansaldo turbo-
generators, as shown in Figure 2.24, to investigate the induced shaft voltage and
bearing current in turbo generators, due to the interaction with the static excitation
systems. The model consists of a field winding in which each turn of the winding is
modeled with an RL circuit. According to their proposed model and their
investigations, the parasitic capacitance of conductors was also included in the model,
which must be calculated. Parasitic capacitances are represented as following:
1. Turn-to-turn capacitances between adjacent turns.
2. Turn-to-core capacitance and turn-to-frame capacitance.
These capacitances are modeled by two lumped values connected to the ends of each
half turn. Each half turn circuit is connected in series to other half turns.
A passive RL circuit was used modeled to represent the shaft line of the generator,
which expresses the frequency dependent behavior of the shaft line. Furthermore,
Golkhandan et al. [24] have also represented other parameters in their proposed
scheme, such as, coupling capacitance between rotor and stator as well journal
bearing capacitance between shaft line and the frame, which can be calculated from

the bearing lubricating oil material and studying bearing dimensions. Finally, they
modeled brush impedance at the exciter end (EE) and turbine end (TE) as a parallel
RC circuit connected, which is used as a solution to decrease shaft voltages. In their
study Golkhandan et al. [24] have found another solution to eliminate high frequency
peaks in rectifier output voltage, by applying symmetrical filters on DC-side of the
rectifier, and this passive filter is connected symmetrically to both terminals on the
DC-side of the rectifier.
Figures 2.25 (a) and (b) presents the simulation results of shaft voltage with and
without the use of passive filter as obtained by Golkhandan et al. [24].
Based on the simulation results they have concluded that, applying passive RC filter
on DC the side of the rectifier reduces the high-frequency peaks of field voltage, and
also reduces the shaft voltage to a harmless value.
Figure 2.24 Model of excitation winding and turbo shaft for 200MVA turbogenerator

(a) (b)
Figure 2.25 Shaft to ground voltage at (EE) (a) without grounding filter and (b) with grounding
filter applied
Another solution discussed by Golkhandan et al. [41] to control the voltage that
originates in the shaft is by applying a DC-DC buck converter to the DC-side of the
rectifier. The DC-DC buck converter is a switched mode converter this is capable of
providing the desired value of DC-voltage on its output terminals by regulating the
value of its parameters as shown in Figure 2.26. The functionality of the DC-DC
buck converter is to compare the output voltage of the converter with the desired
value, and the variance is applied to a PI controller. This controller generates the
signals to be utilized in the PWM pulse generator [41].
Figure 2.26 DC-DC buck converter

According to the simulation results in [41], by applying the buck converter to the DC
side of the rectifier, the high-frequency peaks of field voltage is reduced and shaft
voltage is eliminated to a harmless value, as shown in Figure 2.27 (a) and (b). The
peaks of shaft voltage are reduced from 115 volt to 30 volt, and they have proved
that, shaft voltage is reduced to harmless values.
(a) (b)
Figure 2.27 (a) Voltage applied to the field (b) Shaft to ground voltage of winding with buck
converter applied
Ammann et al. [4] have also proposed a model, in which each excitation winding coil
was model by two capacitances and one inductance, and thereafter this models were
simulated to investigate issues associated to shaft voltage and bearing current. The
model in [4] is corporates the excitation winding of generator and the shaft line for
frequency domain range from 50Hz to 1MHz. In addition, they have proposed a
passive RC filter to model the grounding brush connected at the exciter end of the
generator, to reduce shaft voltage and bearing current. Furthermore, the paper [4]
discussed all four sources of shaft voltage.
Figure 2.28 illustrates the turbo-generator model developed by Ammann et al. [4].
The excitation winding consists of 14 coils with 9 turns in each coil. The first and last
coils of the excitation winding were modelled individually with each turns
represented by three capacitances and two inductances. The remaining coils were
modeled by one inductance and two capacitances for each coil half.

Figure 2.28 Model of excitation winding and shaft for numerical calculation of shaft voltage
The results obtained by Ammann et al. [4] are shown in Figure 2.29, which indicates
the common-mode voltage waveform from static excitation system and the waveform
of shaft voltage, with and without connecting a passive filter to the Exciter end of the
generator (EE) and Turbine end (TE) to model the grounding brushes. From these
results Ammann et al. [4] proved that, the use of the grounding brushes are to reduce
the shaft voltage and the shaft currents, which cause problems to the journal bearings,
shaft, seal and other components of the machine.
Figure 2.29 Numerical simulation results for a 1200 MVA turbo generator

2.6 Summary
Broad reviews of previous literatures related to bearing failure due to shaft current
were presented in this chapter. In addition, the model of excitation winding and
turbo-shaft for numerical simulation of shaft voltage was discussed and the result
from the simulation of the model had been presented. Previous researchers have
shown that of shaft voltage can be reduced by the use of grounding brushes. Based on
the literature review, we had been summarized all causes of shaft voltage, which
damage the journal bearing. However this present study will focus on only one cause
of shaft voltage that damage the journal bearing which is due to the static excitation
system.

CHAPTER 3
EXCITATION-SHAFT-BEARING MODEL TO INVESTIGATE SHAFT
VOLTAGE
3.0 Introduction
This chapter describes the process of recreating an excitation-shaft-bearing model for
a turbogenerator previously researched by Amman et al. [4]. Also, this chapter
discussed the possible causes of the shaft voltage and bearing current due to common
mode voltage (CMV) and the methodology of this book. The model is recreated and
simulated in order to understand the whole components in this model, to understand
how the model gives information on shaft voltage and shaft current due to static
excitation system and to give us understanding to modify the model for a TNB
turbogenerator, which can be used to investigate shaft current problem in power
generation station. The first section of this chapter discusses the excitation system,
and the effects of exciter. The following sections discuss in detail about the
procedures carried out in the various phases of simulation model recreation.
3.1 Excitation system
All types of electric generators work on the principle of Faraday’s electromagnetic
induction. The important part of this principle is the magnetic field. The magnetic
field is the main part for generating electricity, and while producing electricity the
generator also generates a continuous voltage for the electrical system to work
correctly. The voltage output can be controlled by controlling the magnetic field. One
of the most important parts in the generators, which are responsible to produce the
electric power, is the rotor or field coils that generate the magnetic flux. The rotor is
the non-stationary part, and it is a rotating electromagnet. The magnetic flux is
necessary for the production of electric power. The magnetic flux is created from a
DC voltage applied to the rotor or field coils through a static exciter system.

The static exciter system comprises of three components: the power rectifier bridge,
the electronics control system, and the power transformer. Static exciter is used in
most modern generators. The generator output itself provides the DC power for the
production of the magnetic flux. The rectifier diodes rectify the AC voltage to
produce a DC current, which is fed to the rotor by slip rings, another method used in
generator is called as brushless exciter which eliminates the use of slip rings [42].
3.1.1 Full-Wave Six Thyristor Rectifier System
The six thyristor rectifier system is generally used for generators greater than 10
MVA or above 200 amperes on the field. Although the time reaction for three
thyristors system has a good response, its output in the field circuit has limited ceiling
voltage and limits the speed of voltage decay. The six Thyristors Bridge in Figure 3.1
represents the schematic of Full-Wave Rectifier Bridge.
Figure 3.1 Schematic of Full-Wave (six thyristors) Rectifier Bridge
By means of power transformer, the power flows from field to generator, when the
thyristors gates in the negative direction. Figure 3.2 illustrate the variation in the field
output with various angles of the power thyristors, where is the diode firing angle.
The purpose of the firing circuit is to generate pulses with a variable time for the

gating of thyristors, and this will give balanced output of the bridge to control the
signals supplied to the firing circuit [43].
Figure 3.2 Thyristors output waveform
3.2 Development of excitation-shaft-bearing model of a turbogenerator
3.2.1 Model Implementation
There are several models available to simulate shaft voltage and bearing current
which differ based on the components involved in the model. In this project, we have
recreated the proposed excitation-shaft-bearing model, based on Amman et al. [4].
The general block diagram of the excitation system of generator is shown in Figure
3.3. In this figure, there are three major parts, namely the 6-pulse thyristors rectifiers,
rotor or excitation winding of generator and the turbines. In order to investigate shaft
voltage, the shaft line and other capacitances to ground which includes the
capacitance of journal bearings must be included in the model.

Figure 3.3 Generation and transmission of shaft voltage due to excitation systems
where:
1. Transformer.
2. Transformer to ground capacitance
3. Thyristors rectifier
4. Rotor shaft
5. Excitation winding
6. Steam turbines
7. Shaft to ground bearing capacitances
3.2.2 Modification of excitation shaft-to-bearing model
In order to modify excitation-shaft-bearing model presented in the paper [4] for a
TNB generator, the model of the turbine excitation-shaft-bearing presented by
Amman et al. [4]. The recreated MATLAB Simulink of the excitation-shaft-bearing
model is shown in Figure 3.4. The modelled excitation winding consists of 14 coils,
in which each coil consists of 9 turns [4]. All components of excitation shaft-to-
bearing model and the modelling system specification had been explained in the
previous chapter.
The values of each component used in this model:

Excitation winding: Ct = 2.1nF, Lt =2.2 µH, Cc =18.4nF, Lc =20 µH, Rco = 13.8 ,
Rc1 =262.5 , Lc1 =934 µH. Shaft: Lpt = 1µH, Lmpt = 0.5 µH, Lhpt = 0.2 µH, Cins =
16.5 nF, Coil = 100nF, Rio = 1.73 , Ri1 = 5.8 , Ri2 = 36 , Li = 205 µH, Li1 = 51 µH,
Rbrush =0.5 , RCbrush = 500 , 10µF.

Figure3.4Completeexcitation-shaft-bearingmod
Grounding
brushat(EE)
Excitation
winding
delandexcitationwindingmodelcreatedinMATLABSimulink
Shaftlinine
model
Groundingbrbrushat
(TE)
kforthe1200MVAturbogenerator

3.2.3 Simulation of the Three-Phase Excitation System
As discussed earlier, the complete Simulink model of excitation-shaft-bearing
proposed by Amman et al. [4] has been recreated in MATLAB Simulink. The model
obtains its input DC voltage from a three phase supply as shown in Figure 3.5. The
DC voltage placed at the negative (-) and the positive (+) terminals of the excitation
shaft bearing model in Figure 3.4 was obtained from the output of a three-phase
rectifier, fed by a 1732 V peak three-phase AC supply, at 50 Hz frequency shown in
Figure 3.5. This arrangement is similar to the input given to the excitation winding in
a real gas turbine generator system.
Figure 3.5 Static exciter system of the generator which provides input voltage to the excitation-shaft-
bearing model
The 3-phase AC voltage supplied to the rectifier in Figure 3.5, were connected in star
(wye) configuration. The full Control Bridge rectifier with six thyristors was taken
from the MATLAB Simulink library. Furthermore, the bridge rectifier firing angle is
fixed at 50 which is generated from the pulse generator block. A voltmeter has been
used to measure the phase voltages, where the resistance value (R1, R2) of 10 was
used in the model.
To connect to
‘+’terminal of
excitation-shaft-
bearing model
To connect to ‘-’
terminal of
excitation-shaft-
bearing model
Three Phase AC supply

3.3 Simulation of the model
The excitation-shaft-bearing model, recreated in MATLAB Simulink as shown in
Figures 3.4 and 3.5, for the 1200 MVA turbo generator as reported by Amman et al.
[4] was simulated for 0. 1 seconds, with and without grounding brushes at the turbine
end (TE) and exciter end (EE) of the shaft line in the model. The voltages generated
by the excitation system and the shaft voltages at turbine end and the exciter end,
were plotted.
From the simulation, the three phase AC supply line-to-line voltage waveform
supplied by the excitation system is shown in Figure 3.6(a). Figure 3.6(b), shows the
DC output voltage from the bridge rectifier, which is the input applied to the
excitation-shaft-bearing model.
(a)
(b)
Figure 3.6 (a) Three phase supply AC line-line voltage of 1732 peak at 50 Hz frequency and (b)
The input DC voltage to the excitation shaft bearing model.

As expected, the rectified DC voltage contains ripples of six-times the fundamental
AC supply voltage frequency. The common mode voltage of the rectifier is shown in
Figure 3.7 and as expected the common-mode voltage has a frequency of 150 Hz
which is three times of the fundamental. This voltage is regarded as the source of
bearing currents and the shaft voltage in turbo generator [4].
Figure 3.7 Common mode voltage (CMV) component of the rectified DC voltage from the
excitation system
This is similar to the results presented by Amman et al. [4] and proves that the shaft
voltages are contributed by the common-mode voltage (CMV) component from the
output of the three-phase rectifier in the static excitation system.
3.4 Simulation results without grounding brush connection
As depicted in Figure 3.8 and 3.9, shaft voltage measured on turbine end (TE) and
exciter end (EE) of the 1200 MVA turbo generator without grounding brushes are
observed to have a peak to peak magnitude of approximately 45 volts. It is a periodic
wave form, with a frequency of 150 Hz.

Figure3.8 (a) Shaft voltage at turbine end obtained from the simulation model without grounding
brush connected and (b) zoomed-in view of the shaft voltage
Figure3.9 (a) Shaft voltage at the exciter end obtained from the simulation model without grounding
brush connected and (b) zoomed-in view of the shaft voltage
(a)
(b)
(a)
(b)

3.5 Simulation results with grounding brush connection
The purpose of adding grounding brush at the shaft is to mitigate the shaft voltages
and protect the journal bearing from these voltages. So, the model was simulated
again with the grounding brushes connected at turbine end (TE) and the exciter end
(EE) of the shaft, by referring to Figure 3.4. The shaft voltage at turbine end (TE) and
the exciter end (EE) of generator, obtained from the simulation model with grounding
brushes connected are shown in Figures 3.10 (a) and (b) respectively.
Figure 3.10 Shaft voltage at (a) turbine end and (b) exciter end with grounding brushes connected in
the simulation model
The results obtained from simulation model shows that, by connecting the grounding
brushes at the exciter end and turbine end of the shaft, the shaft voltages are reduced
to approximately zero. However, at instances where the common mode voltage
(CMV) experiences a change in sign, a small shaft voltage of approximately 2.9 V at
the exciter end, and 1 V at the turbine end are seen to be present.
(a)
(b)

3.6 Summary
In this chapter a complete model of shaft line, field winding, and static excitation
system had been recreated based on work by Amman et al. [4] together with an
excitation system to provide the input for the model. The recreated model was
simulated with and without the grounding brushes connected. The values of shaft
voltages at exciter end and turbine end, with and without grounding brushes obtained
from the simulation were plotted. The simulation results are in agreement with that
repeated by Amman et.al and had revealed that, by using the grounding brushes, the
shaft voltage can be mitigated.

CHAPTER 4
DEVELOPMENT OF EXCITATION-SHAFT-BEARING MODEL FOR A
TNB GAS TURBINE GENERATOR
4.0 Introduction
The previous chapter discussed the possible cause of the shaft voltage and bearing
current due to common mode voltage (CMV) and the methodology of this book work
along with analysis models to develop and recreate a model of the excitation winding
and turbo shaft line for a turbo generator to investigate the shaft voltage. This chapter
discusses the methods developed for modeling and simulation of a TNB gas turbine
generator system to measure shaft voltage phenomena. This study simulates the gas
turbine model in order to find ways to mitigate the shaft voltage and shaft current.
Shaft voltage can destroy the shaft and the journal bearing as a result of shaft current
effects, leading to added costs and loss of time for maintenance. The simulation of
the developed model is implemented in the computational software package Matlab
2010. The values of shaft voltages on the exciter end and the turbine end are
investigated based on the data that had been obtained from Stesen Janaelektrik Sultan
Ismail (SJSI) Paka Power Station manual.
4.1 TNB Gas Turbine Generator shaft-to-bearing model
The main objective of this study is to develop a gas turbine shaft-to-bearing model
for a TNB gas turbine generator and simulate the model to investigate the shaft
current and shaft voltage. In Chapter Three the model of excitation winding and turbo
shaft for a 1200 MVA turbogenerator was recreated and simulated to get the similar
results as presented by [4]. The proposed model in this chapter is to model the
excitation winding, shaft voltage and shaft line for a 113.306MVA SJSI Paka
turbogenerator as shown in Figure 4.1.

The parameter of the proposed model should be determined according to the
specifications of the TNB gas turbine generator data. The excitation winding of the
generator at SJSI paka has 61 turns (or coils) per pole. Therefore, it is modeled by
two capacitances (Cc) and one inductance (Lc).
Between the exciter end and the turbine end, a RL circuit is utilized to approximate
the frequency dependent behavior of impedance modeling the shaft line in the turbine
area, where the values are taken from [4]. The bearings are journal type with an oil
lubricant film between the bearings. The journal bearing at exciter end and turbine
end are represented as a capacitor (Coil) along the shaft line and the frame, as shown
in the Figure 4.2. The value of coil is calculated by studying bearing specifications
and clearance as well as the oil properties based on the information that had been
provided by SJSI paka power station.

Figure4.1Modelofexcitationwindingandturboshaftfora113.306MVAturbogeneratoratSJSIPakacreatedusingMATLAB
Simulink
Shaftlinemodel
Grounding
brushat(EE)
Excitation
winding
Groundingbrushat(TE)

Figurer 4.2 Excitation system for Generator
The gas turbine connected to the generator contains three other journal bearings.
These are represented in the model by inductances and capacitances as shown in
Figure 4.1.
In order to reduce the shaft voltage and current on the journal bearing, a passive RC
circuit can be connected at the exciter end of generator and, another resistance
connected at the turbine end, these components will model the grounding brushes as
shown in Figure 4.3.
Figure 4.3 Connection of resistor and passive filter to model grounding brushes in the model

4.2 Parameter calculation of 113.306 MVA SJSI Paka turbogenerator model
To determine all the values in the model for turbogenerator at (SJSI) Paka Power
Plant, it is essential to use specific equation to calculate the values of inductances and
capacitances.
There are two equations should use to find the values of the excitation winding coil
inductance (Lc) and excitation winding coil capacitance (Cc), also by using the same
equation we can calculate the capacitance of journal bearings (Coil) and the
capacitance of insulation in the bearing (Cins).
Referring to [44], the excitation winding coil inductance (Lc) can be calculated using
equation below:
µ µ
(4.1)
where:
L= inductance
N= number of coil strands
A= surface area (in m2
)
l= length of winding (in m)
o = permeability of free space equal to 4 x 10-7
Henry per meter
r = relative permeability of the rotor core
Referring to [44], the capacitance of excitation winding coil and bearing can be
calculated using the equation below:
(4.2)

where:
C= the capacitance
o = permittivity of free space= 8.8541878 × 10-12
r = relative permittivity
A= the surface area (in m2
)
d= distance (in m)
4.2.1 Parameter calculation
The three-phase supply AC line to line voltage, described in previous chapter, given
to the excitation winding is rectified to DC voltage before given as the input voltage
to the excitation-shaft-bearing model. In order to simulate the TNB power plant
model, specific data of the excitation system was obtained from (SJSI) Paka Power
Station shown in Table 4.1.
Based on the information given in Table 4.1 above, the value of AC excitation
voltage is 340 volt rms line to line. So the rms phase voltage is:
Rms Phase Voltage
And the peak voltage is:
Peak Amplitude Voltage
Hence, the final value that is to be used as input for the simulation of TNB model is
278 volt peak with frequency of 50 Hz for each phase of the AC supply with
difference in phase of 0 , -120 and +120 .

Table 4.1 Technical data on the excitation system and excitation transformer for the 113.306 MVA
turbogenerator at SJSI Paka Power Station
EXCITATION SYSTEM
Excitation type Static shunt
Excitation power 311 kW
Voltage 146 V DC
Current 2127 A
Maximum 350 V DC
EXCITATION TRANSFORMER
Supplier Alsthom Atlantique
Type ONAN, Oil Immersed
Tap- Changer Off- Load Type
Power 1.2 MVA
Frequency 50 Hz
Voltage 11500 V / 340 V AC
Primary
Connection
Delta
Secondary
Connection
Star
4.2.2 Inductance and Capacitance calculations for the excitation winding
The generator rotor is joined to the turbine shaft by bolted coupling flange. The rotor
is supported by two journal bearings Coil5 at the exciter end and Coil4 at the turbine
end. The rotor shaft includes field winding coils placed inside slots milled into the
rotor body as shown in Figures A1 and A2 of Appendix A. The field winding
connects to the end winding of rotor to shape the coils and also receive field current
from the excitation system.
The information and specifications of the excitation winding are shown in Tables 4.2,
4.3 and 4.4 are used to calculate the inductance of excitation winding coil (Lc).

Table 4.2 Specification of the SJSI Paka Power Station
Type T229-320
Rated speed 3000 rpm
Over speed 3600 rpm
Active Power 96.5 MW
Apparent Power 113.306 MVA
Rated Voltage 11.5 kV
Rated Current 5,688 kA
Frequency 50 Hz
Voltage Variation +5%
Power Factor 0.85 lagging
Number of poles 2
Insulation class F
Cooling Water Temperature Max 42.5 C
Excitation System static
Table 4.3 Excitation winding specification of the SJSI Paka Power Station
WINDING SPECIFICATIONS
Number of wound slots 28
Number of turns per pole 61
Number of strands per slot 16
Shape Rectangular
By using equation 4.1 and the information in the Tables 4.2, 4.3, and 4.4 above,
µ µ
µ x
µ

Table 4.4 Mechanical characteristics of the SJSI Paka Power Station generator
External dimensions:
Length of frame 6030 mm
Overall length including excitation 8415 mm
Overall width 3360 mm
Width of frame along joint plane 3180 mm
External frame height 3147 mm
Magnetic core (stator):
External diameter 2290 mm
Bore diameter 1120 mm
Lamination stacking length 3200 mm
Useful length 2444 mm
Rotor:
Length between coupling 7478 mm
Overall length ( rotor + BH system) 8283 mm
Bearing span 6000 mm
Retaining ring diameter 1052 mm
Shaft body diameter 1000 mm
Fan diameter 1080 mm
Shaft body length 3450 mm
The diameter of the rotor is calculated as below:
The surface-area encompassing each coil of the excitation winding of the
turbogenerator is calculated by equation 4.3.

(4.3)
The length for each winding is obtained as below:
Where, is the length of the shaft and is the diameter of the rotor.
Therefore, the excitation winding coil inductance ( ) is calculated to be:
The capacitance (Cc) between the excitation winding coil and the shaft of the
turbogenerator can be calculated from the equation 4.2 as below
The surface area of the capacitance is calculated as follows:
The distance ( ) between the excitation winding coil and the shaft is given by:

By using equation 4.2 and the information from Table 4.2, 4.3 and 4.4 above, the
value of excitation winding coil capacitance (Cc) is:
4.2.3 Determining the parameter of Journal bearing capacitance
The capacitance to model the journal bearing, Coil, must be determined. It is
estimated as a parallel plate capacitor shown in Figure 4.4. Two journal bearings
(bearing No.4 and No.5) on the generator side are used to support the rotor shaft of
generator. Sectional view of bearing No.4, for the SJSI Paka turbogenerator is shown
in Figure A3 at Appendix A.
Figure 4.4 Modeling the journal bearing of turbogenerator
The turbine side has three journal bearings; No.1 at compressor inlet, bearing No.2 at
the compressor discharge and bearing No.3 at the generator ends.
Dimension and specifications of all the journal bearings shown in Table 4.5 was
obtained from SJSI Paka Power station 113.306MVA gas turbine generator.
By modifying equation 4.2, the capacitance to model for each journal bearing can be
calculated as shown below:

where:
Table 4.5 Journal bearing specification for SJSI Paka Power Station gas turbine generator [47]
SPECIFICATIONS OF END SHIELD BEARINGS
Journal bearings Elliptical
Diameter 340 mm
Useful width 250 mm
Specific pressure 17.2 bars turbine end, 18.4 bars opposite turbine end
Oil flow rate 96 1/min
Losses 69 kW
Max. Journal bearing temperature 105 C
Bearing No. 1 (compressor inlet)
Diameter=400 mm; Effective Length=250 mm, Diametral
Clearance=0.6 mm
Bearing No. 2 (compressor discharge) Diameter=468 mm; Effective Length=389 mm, Diametral
Clearance=0.71 mm
Bearing No. 3 (generator end) Diameter=396 mm; Effective Length=160 mm, Diametral
Clearance=0.53 mm
Bearing No. 4 (turbine end) Diameter=340 mm; Effective Length=250 mm, Diametral
Clearance=0.5 mm
Bearing No. 5 (exciter end) Diameter=340 mm; Effective Length=250 mm, Diametral
Clearance=0.45 mm.
The effective surface area of the journal bearing is given by [48]:

where ( is the effective length of bearing and is the diameter of journal
bearing. The area for bearing No.5 is calculated to be:
The oil film thickness for the same bearing is calculated as below:
Based on the specifications given in Table 4.5 and using equation 4.5, the capacitance
of journal bearing at the exciter end is calculated to be:
By the same way, we calculate the value of capacitance for each journal bearing
based on Table 4.5 data.
The capacitance of journal bearing at turbine end:
The capacitance of journal bearing at the generator end:
The capacitance of journal bearing at the compressor discharge:
The capacitance of journal bearing at the compressor inlet:

Because of incomplete information, the insulation capacitance Cins at the exciter end
bearing, the inductances to model the turbine stage, and the values of the RL circuit
to model the shaft line were kept to be the same as [4]
The parameters are:
Lpt = 1µH, Lmpt = 0.5 µH, Lhpt = 0.2 µH, Cins = 16.5 nF, Ri = 1.73 , Ri1 = 5.8 , Ri2
= 36 , Li = 205 µH, Li1 = 51 µH.
4.3 Summary
This chapter presented the development of the TNB Gas Turbine generator
excitation-shaft-bearing model to investigate shaft voltage at the exciter end (EE) and
the turbine end (TE). The parameters of the model have been calculated according to
the specifications obtained from Stesen Janaelektrik Sultan Ismail (SJSI) Paka Power
Station manual and the values have been applied to the simulation model. The
simulation results to investigate the shaft voltages before and after the proposed filter
is added to the proposed model will be presented in the next chapter.

CHAPTER 5
SIMULATION RESULTS AND ANALYSIS
5.0 Introduction
The models of shaft line, field winding and static excitation system presented in the
previous chapter are simulated to measure shaft voltage at exciter end and turbine end
of the generator. This chapter shows the results of shaft voltages and bearing current
without and with the shaft to ground brushes connected at exciter end and turbine
end. In addition, optimization solver was also employed to find the optimal values of
capacitor and resistor to model the grounding brushes used to eliminate shaft voltage
at exciter end and turbine end of the generator. Simulation was done using MATLAB
software 2010 for this purpose. Lastly, this chapter presents a detailed discussion for
all obtained results.
5.1 Simulation Results
The signal of shaft voltage is implemented in Matlab to analyse the bearing currents
problem. The advantage of using Matlab/Simulink is in the ability to use a graphical
programming language that is based on different block categories with different
properties of each block. Matlab and its toolboxes are adopted to perform all of the
identification processes and simulations in this book, as well as in our previous works
[49-64]. System identification and optimisation toolboxes were used to identify and
build the model, while the partial differential equation toolbox was used for the
model analyse in two-space dimensions (2-D) and time. The obtained models are then
introduced in the Matlab/Simulink environment for simulation. These categories
include the input/output, transfer functions, arithmetic functions, state space models
and data handling. The building model is represented in the form of ordinary
differential equation (ODE) solvers, which are automatically configured during the

Simulink model’s run-time. The algorithm of the voltage controller is designed by
using Matlab m-files, parameter layer memory and S-functions, which are based on
online parameter tuning. The technique for calculating the cooling loads is easily
implementable, whereby the current balance equation is derived from the arithmetic
functions, from which the energy consumption can be obtained.
5.1.1 Simulation results of AC supply voltage, DC voltage and common mode
voltage of the excitation system
The Matlab Simulink excitation-shaft-bearing model for the 113.306 MVA gas
turbine generator system developed in previous chapter was simulated. The output
voltage from bridge rectifiers is a DC voltage and this voltage is the input to the
excitation-shaft-bearing model.
Figure 5.1 (a) shows the three phase supply line to line AC voltage given to the
bridge rectifier of the excitation system and the rectified DC voltage is shown in
Figure 5.1 (b).
The rectified DC voltage contains ripples of six times the fundamental AC supply
voltage frequency and the wave-form of the DC rectifier is similar to the results
obtained for the 1200MVA turbogenerator presented in chapter three.
(a)

(b)
Figure 5.1 (a) three phase supply AC voltage of is 278 volt peak per phase at 50 Hz frequency and
(b) Rectified DC voltage used as input to the excitation-shaft-bearing model of SJSI Power station
The common mode voltage (CMV) of the bridge rectifier is shown in the Figure 5.2.
It is generated due to the switching of rectifier which gives a rectangular wave-form
and the frequency of this wave is three times the main frequency (that is 150Hz) as
seen previously in chapter three.
Figure 5.2 Common mode voltage obtained from the rectifier of the SJSI Paka Power station
simulation
5.2 Simulation results of shaft voltage and bearing current before adding
proposed grounding brush filter
It is important in our study to prove that shaft voltage and bearing current exist in the
both exciter end (EE) and the turbine end (TE). Figure 5.3 and Figure 5.4 show the
shaft voltage obtained from the simulation of the developed SJSI Paka power station

excitation-shaft-bearing model when subjected to the rectified DC voltage shown in
Figure 5.1(b)
(a)
(b)
Figure 5.3 (a) Shaft voltage at the turbine end (TE) of the simulated SJSI paka power station
generator without applying any grounding brushes and (b) zoomed-in view of the shaft voltage
(a)

(b)
Figure 5.4(a) Shaft voltage at the exciter end (EE) of the simulated SJSI paka power station
generator without applying any grounding brushes and (b) zoomed-in view of the shaft voltage
Figure 5.3 and 5.4 show that there are a significant shaft voltage present at exciter
end and turbine end of the Paka turbogenerator shaft line. It is observed that the
maximum peak-to-peak shaft voltage measured on turbine end is 60V. Meanwhile,
for the exciter end of generator the maximum peak to peak shaft voltage observed in
the simulation is 62 V.
Figure 5.5 (a) and (b) show the bearing current at turbine end and exciter end
respectively, obtained from the simulation of the developed SJSI Paka power station
excitation-shaft-bearing model when subjected to the rectified DC voltage shown in
Figure 5.1(b)
(a)

(b)
Figure 5.5 (a) Bearing current at the turbine end (TE) and (b) at exciter end (TE) of the simulated
SJSI paka power station generator without applying any grounding brushes
It is appeared in Figure 5.5 (a) and (b) that there are significant bearing current
present at turbine end and exciter end of the Paka turbogenerator shaft line. It is
observed that the maximum peak-to-peak bearing current measured on turbine end is
3.6 A.
Meanwhile, for the exciter end of generator the maximum peak to peak bearing
current observed in the simulation is 5.4 A.
5.3 Simulation results of shaft voltage and bearing current after grounding brush
filter
To avoid damage, it is necessary to provide suitable earthing paths and allow stray
currents to pass through the ground and protect the bearing. So, the purpose for
adding filter to the simulation model is to model the grounding brush that can be used
reduce the undesirable shaft voltage that cause problem to the journal bearing.
The same simulation for the excitation-shaft-bearing model of the 113.306 MVA
SJSI Paka turbogenerator was repeated with grounding brush filter at exciter end
(EE) and grounding brush filter at turbine (TE) of generator connected to the model.
Simulation results in the case of connecting the proposed grounding brush passive
filter (RC) to the model at exciter end and turbine end of the generator is shown in
Figure 5.6 (a) and Figure 5.6 (b). The shaft voltage at exciter end (EE) and turbine

end (TE) are observed to be close to zero due to connection of the grounding brushes.
Whereas, the shaft current in the Figure 5.7 (a) at the turbine end and (b) at the
exciter end are observed also to be close to zero due to connection of the grounding
brushes.
The values of the grounding brush components (RC filter) used in this simulation are
exactly the same as that used for the 1200 MVA turbogenerator simulation explained
in chapter three. Furthermore, this value of resistance is small enough to reduce the
current at the bearings.
(a)
(b)
Figure 5.6 Shaft to ground voltage at the (a) turbine end (TE) and (b) Exciter end (EE) after
applying proposed grounding brush model.

(a)
(b)
Figure 5.7 Bearing current at the (a) turbine end (TE) and (b) Exciter end (EE) after applying
5.4 Optimization solver
MATLAB’S optimization [65] toolbox contains optimization technique to solve
maximization and minimization problems. Hence, the optimization was employed to
minimize the shaft voltage by choosing the best value for the RC components to
model the grounding brush filter. The model with optimization solver connected to
the exciter end of the model for the 113.306 MVA SJSI Paka turbogenerator was
simulated to obtain the optimized RC values as shown in Figure 5.8.

Figure5.8Optimizationsolverconnectedattheexciterend(EE)oftheexcitationexcitation-shaft-bearingmodelforthe113.306MVASJSIPaka
turbogenerator

5.4.1 Optimization of the parameters to model the grounding brush
The objective function in Simulink program is minimization of shaft voltage by
changing the variables, in this case the grounding filter’s capacitor and resistor (RC).
The optimization technique chosen was ‘Gradient descent’ and the constraint was the
limit in the dialogue box shown in Figure 5.9 (a) [66]. After running the simulation
model with the optimization solver, the values and location of capacitor and resistor
that have been obtained from the optimization process is shown in Figure 5.9 (b), (c)
and the input given to the optimization solver is shown in Figure 5.9 (d) [67].
(a)
(b)

Figure 5.9 (a) Signal constrain
grounding brush obtained from th
(d) th
Grounding brush
at (EE) RC filter
(
c)
(d)
nt (b) the optimized values of capacitor and re
he optimization process, (c) the location of th
he input given to the optimization solver
Grounddiing
brush at (TE) )
RR filter
esistor to model the
he grounding filter and

From Figure 5.9 (a), the optimized value of capacitor and resistor obtained from the
optimization process is 54.5µF and 147.81 , respectively. These values were used
for the proposed grounding brush model at exciter end (EE) of generator in order to
reduce shaft voltage. In addition, the value of grounding brush resistor at the turbine
end (TE) was set at 0.9 .
Figure 5.10 (a) and (b) shows the shaft voltages at turbine end (TE) and exciter end
(EE), respectively, after the optimized RC values for the grounding brush model was
employed. As observed, the shaft voltage is reduced by using the optimized values of
the grounding brush filter.
(a)
(b)
Figure 5.10 (a) Shaft voltage at TE (b) shaft voltage at EE after optimization of the grounding brush
filter components

5.5 Summary
In conclusion, this chapter presented the simulation results and analysis of the
developed excitation-shaft-bearing model for the 113.306 MVA SJSI Paka
turbogenerator. The voltage produced by the excitation system as well as the shaft
voltages and bearing current at turbine end (TE) and the exciter (EE) of the shaft in
the model of generator has been observed. The shaft voltage and bearing current
measured at the exciter end and turbine end of the SJSI Paka turbogenerator shaft line
with and without the grounding brushes were also observed. According to the
simulation results, it can be concluded that by connecting the proposed grounding
brush filter, the value of shaft voltages and bearing currents on turbine end (TE) and
exciter end (EE) of generator were reduced from 60 V and 62 V peak-to-peak and
3.6 A and 5.4 A peak-to-peak, respectively, to zero. Optimization solver was also
used to optimize the value of RC components in the proposed grounding brush filter.
The optimized RC values, when applied to the simulation, reduced the shaft voltage
to 0.02 volt. The optimized RC values are 54.5µF for capacitor and 147.81 for
resistor. Hence, these values have been proven to effectively eliminate the shaft
voltage to a harmless value.

CHAPTER 6
CONCLUSIONS AND RECOMMENDATIOS FOR FUTURE WORK
6.1 Conclusions
This book has successfully developed a GT shaft-to-bearing model for a TNB gas
turbine generator at SJSI paka. The model was used to investigate the phenomena of
shaft voltage. The shaft voltages were found to be present at the exciter end (EE) and
turbine end (TE) of generator when the model was simulated.
A number of conclusions can be drawn from the work conducted and these are
presented below:
1- This project has presented the development of an excitation-shaft-bearing model
to investigate the shaft voltage due to the common-mode voltage from the static
excitation system at Stesen Janaelektrik Sultan Ismail (SJSI) Paka.
2- Simulation of the model showed the presence of shaft voltage and bearing current
at exciter end (EE) and turbine end (TE) when no grounding brush was connected
to the shaft.
3- The simulation displayed a shaft voltage of approximately 60V and bearing
current 5.4A peak-to-peak. However, these values were reduced to zero when a
grounding brush was connected to both the EE and TE side of the shaft.
4- Due to the difficulty in obtaining accurate values of R and C components to model
the grounding brush, an optimization solver was used to obtain optimized values
of RC components for the grounding brush model. The value obtained is 54.5µF
for capacitor and 147.81 for resistor. These values are the best value to mitigate
shaft voltages and bearing currents caused by static excitation systems of the SJSI
Paka gas turbine generator.

5- It is very important to eliminate the shaft voltage and bearing current to a harmless
value in order to protect the journal bearings of the generator which leads to
savings in terms of bearing maintenance.
6.2 Recommendations for Future Work
According to the results of this project, the following recommendations are suggested
for future work relating to investigation of shaft voltages in gas turbine generators.
1- Experimental investigation of shaft voltage and bearing current due to static
excitation voltage under rotating shaft condition.
2- Experimental investigation of shaft vibration effect on bearing oil film breakdown.
3- Experimental investigation of shaft voltage and bearing current due to residual
shaft magnetism under rotating shaft condition.

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