Hydrogenerators behavior under transient conditions

665
Hydrogenerators behavior
under transient conditions
Working Group
A1.35
September 2016

HYDROGENERATORS
BEHAVIOUR UNDER TRANSIENT
CONDITIONS
WG A1.35
Members
R. N. Bedi, Convenor (IN), Edgar Robles (MX),
T Pujal (AR), C. Roth (AR), M. Membrive (AR), D Vaughan (AU), G Schacher (AUT),
E. Ruppert (BR), F. Renno (BR), V. Pamplona (BR), A. Campos (BR), A. Tètrault (CA),
S. Yutian, (CN), T. Hildinger (DE), B. O’Sullivan (EI), J.J. Ahtiainen (FI), J. Kangas (FI),
Z. Milojković (HR), M. Brčić (HR), T. Aso (JP), I. Kukovski (MK), J. Amundsen (NO), D. Zlatanovici (RO),
R. Zlatanovici (RO), C. Cicirone (RO), D.E. Comanescu (RO), P. Mladjenovic (RS), J. Ritonja (SI),
J.J. Perez (SP), O. Martinez (SP), D. Tarrant (ZA), M. Bruintjies (ZA), R. Tremblay (CA)
Copyright © 2016
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ISBN: 978-2-85873-368-2

HYDROGENERATORS BEHAVIOUR UNDER TRANSIENT CONDITIONS
Page 2
HYDROGENERATORS BEHAVIOUR UNDER
TRANSIENT CONDITIONS
Contents
EXECUTIVE SUMMARY........................................................................................................ 3
1. Introduction.....................................................................................................................................3
2. Transient Events of Generators...................................................................................................3
2.1 First Level of Transient Events:...............................................................................................3
2.1 Second Level Of Transient Events...................................................................................... 13
3. On Line Monitoring Systems..................................................................................................... 16
4. Type of Information for Analysis............................................................................................. 19
4.1 ‘Off-line’ Inspections ............................................................................................................ 19
4.2 ‘On-line’ measurements ....................................................................................................... 20
5. Conclusion.................................................................................................................................... 20
ANNEXURE..................................................................................................................... 21
RESPONSES TO THE QUESTIONNAIRE:.......................................................................................... 21
SUMMARY OF GENERATORS COVERED IN THE SURVEY:.......................................................... 22
Information On 1st Level Of Transient Events Received:............................................................... 25
Completed Questionnaire: CROATIA 01 (HPP-A)................................................................... 29
Completed Questionnaire: CROATIA 02 (HPP-B).................................................................... 33
Completed Questionnaire: CROATIA 03 (HPP-C)................................................................... 38
Completed Questionnaire: CROATIA 04 (HEV)....................................................................... 42
Completed Questionnaire: CROATIA 05 (BULB A).................................................................. 48
Completed Questionnaire: CROATIA 05 (BULB B).................................................................. 53
Completed Questionnaire: INDIA............................................................................................. 58
Completed Questionnaire: India 02 (Chamera 1)................................................................. 65
Completed Questionnaire: India 03 (Indira Sagar 4) .......................................................... 68
Completed Questionnaire: India 04 (Tanakpur/Utharakand)............................................. 72
Completed Questionnaire: Ireland 01 (ESB)........................................................................... 77
Completed Questionnaire: Norway 01 (Statkraft Svartisen) .............................................. 82
Completed Questionnaire: Mexico 01 (MP4)......................................................................... 87
Completed Questionnaire: Mexico 02 (PE1).......................................................................... 92
Completed Questionnaire: Mexico 03 (PE4).......................................................................... 99
Completed Questionnaire: Mexico 04 (ZMN1)....................................................................105
Completed Questionnaire: Serbia 01 (HPP Bistrica G2)....................................................112
Completed Questionnaire: Spain 01 .....................................................................................117

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EXECUTIVE SUMMARY
Transient recorders and digital relays provide important information to understand hydro electric generator
behaviour under abnormal operating conditions. Some variables can be recorded under actual failure conditions,
for instance the generator and excitation current and voltages or stator winding neutral currents. On the other hand,
modern advanced monitoring systems, such as stator winding partial discharges or vibration levels can help to
determine the degree of deterioration that a generator have suffered after a transient event.
Abnormal operating conditions can result in catastrophic failures or can accelerate the ageing process of a
hydroelectric generator. Information gathering and analysis under both circumstances are relevant to understand
generator behaviour. Potential damages to the generator under transient conditions will be analysed for power
export systems where a circuit breaker is installed as well as for power export systems that do not have a generator
circuit breaker included in the generator power export circuit.
1. Introduction
Generators abnormal operating conditions will accelerate ageing, or in extreme cases can cause severe failures.
Particularly, hydroelectric generators require functioning under load cycling, from no load to full load in a short
period of time. In some cases hydroelectric generators are used to supply peak loads, and therefore required to be
connected daily. Consequently, the possibility of an out-of-step synchronization or a failure in the generator circuit
breaker is greater in this type of generators.
One of the most critical conditions is the operation under short circuit that can occur within or outside the generator.
As most of the generators neutrals are grounded with a high resistance, a phase to ground failure is limited to a few
amperes and generally produces minor damages. Failures in the generator lubricating or cooling systems might
contaminate the stator windings with oil or water and produce a phase to phase short circuit in the stator end
windings that are more deleterious.
Primary windings of the step up transformers are delta connected; then a failure occurring in the high voltage side
of the transformer or in the substation equipment will give rise to phase to phase circulating currents in the
generator. Phase to phase currents in the stator windings also affect the rotor windings and the excitation system,
also they produce negative sequence currents that enhance mechanical stresses. Other type of abnormal
operation and its effects on the hydro generators that will be considered are loss of field and load rejection.
In the other hand, hydroelectric generators can be synchronized to the network with a circuit breaker in the high
voltage side of the transformer unit or at the generator output. Both schemes operate properly under steady state
conditions. Under fault conditions; both on the generator, on the transformer unit or in the substation, there might
be instances where the generator cannot be isolated from the failure and severe damages can be produced.
Special consideration should be taken into account according to the general arrangement of the power plant. If a
breaker is provided at the generator terminal (between generator & the Generator Transformer) then the fault
current feeding from generator to generator transformer (when there is fault at HV bushing or HV winding of
Generator Transformer or in Issolated Phase Busbar Ducting) during de-excitation period can be avoided by
isolating the breaker.
2. Transient Events of Generators
In practice there are a number of abnormal /transient operating conditions which generators are subjected to. The
abnormal conditions to be recorded are:
2.1 First Level of Transient Events:
a) Short Circuit failures within the generator
Stator ground faults are the most common winding failure in generators, and this kind of fault occurs due to
stator winding insulation breakdown and electrical contact between the active phase winding and the

Page 4
grounded stator core. Following a stator ground fault and protection relay trip, the generator performance
and the damage caused to the stator core, depend on how the stator winding is grounded. A stator phase-
to-ground fault produces two effects: overcurrent on the affected phase and an overvoltage on the
undamaged phases. The phase-to-ground fault current flows from the damaged winding to ground through
the stator core and returns to the winding through the grounding impedance which must be designed to
keep the damage at a non-severe level. Three types of phase to ground faults can be identified:
i. Single phase to ground
Only one phase makes contact with the stator core. No visual secondary damage results from such
a contact and the generator earth fault protection should disconnect the generator from the
Generator Transformer or Grid.
ii. Phase to phase leads to ground fault
Where more than one phase makes contact with stator grounding, severe stator bar damage and
core damage can result. Following two examples of during which more than one phase made
contact with stator grounding, resulting in destructive circulating currents.
Figure 1: Single phase to ground and phase to phase fault in generator
iii. Three phase fault in generator
Faults can start as a single phase to ground fault and develop into a phase to phase ground fault. A
single phase to ground fault will result in a voltage increase in the two healthy phases which can
then lead to a second failure on one of the pre-fault healthy phases. Phase to phase faults can also
develop into phase to phase ground faults due to the severe destruction of insulation material
during such a fault. Following some examples of faults during which multiple phases were involved
during failures. Fault current and voltage wave forms indicate pre-fault and post fault conditions.

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Figure 2 is an example of a three phase failure which occurred when the field circuit breaker was
operated when the generator was already disconnected from the power system resulting in a stator
bar failure:
Figure 2a: Pre- and post-fault voltage and current signals for the failure of A-phase
stator bar.
Figure 2b and 2c: Failure on Phase A stator bar.
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T1: T2: TD: ms-94.27 25.95 120.23
Va
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Phase A
Phase B
Phase C

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The example in Figure 3 shows a short circuit in stator winding, beginning as a phase-to-phase fault and
ending as a three-phase short circuit, causing severe damage to the stator winding:
Figure 3a and 3b: Damage resulting from a three phase short circuit
Figure 3c: Pre- and post-fault current and voltage wave forms recorded
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INTENSIDADES
TIERRA ESTATOR
Electrotek Concepts® TOP, The Output Processor®
5_?-IfaseC(Mag)6_?-IfaseB(Mag)7_?-IfaseA(Mag)8_?-Ineutro(Mag)
Time (ms)
00004100210063448974_0353>5_?-I fase C(A) 00004100210063448974_0353>6_?-I fase B(A)
00004100210063448974_0353>7_?-I fase A(A) 00004100210063448974_0353>8_?-I neutro(A)
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TENSIONES
TIERRA ESTATOR
1_?-UfaseC(Mag)2_?-UfaseB(Mag)3_?-UfaseA(Mag)4_?-Uneutro(Mag)
Time (ms)
00004100210063448974_0353>1_?-U fase C(V) 00004100210063448974_0353>2_?-U fase B(V)
00004100210063448974_0353>3_?-U fase A(V) 00004100210063448974_0353>4_?-U neutro(V)

Page 7
b) Short circuit in the isolated phase bus
The example in Figure 4 show damages resulting from an inadequate braiding connection to the
step up transformer which produced a short circuit to ground. The external phase to ground fault
induced a two phase to ground fault within the generator.
Figure 4a: Initial failure at
the interconnecting
braiding of the step up
transformer, Phase A
Figure 4b: Phase B failure
at the endwinding in the
generator
Figure 4c: Phase C failure
in the back region of the
endwinding
Figure 4: Pre- and post-fault voltage and current wave forms showing the initial fault and
the development of secondary faults within the generator stator.

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The generator voltage and current waveforms shown in Figure 5 was of a generator running unloaded
when suddenly a short circuit took place in the isolated phase bus close to unit transformer
Figure 5: Pre- and post-fault voltage and current recordings of an unloaded generator
exposed to a three phase short circuit in the phase isolated busbar.
c) Short circuit in the step-up transformer
When there is fault in the primary/HV winding or secondary/LV winding of the Generator Transformer,
Generator-fed short-circuit currents are interrupted within a maximum of four cycles whereas the reduction
of the fault current by the de-excitation equipment requires a number of seconds.
d) Short circuit in the HV substation
Short circuit in HV substation causes the generator to feed fault current. If the HV breaker operates then the
fault gets isolated within four cycles.
Figure 6 shows failure of Phase ‘B’ on a 230 kV machine circuit breaker. This breaker failure resulted a
sudden collapse of Phases B and C of the generator.
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00002111027132615660_3626>1_C-IC Lado Neutro(A) CURRENTS
1_C-ICLadoNeutro(Mag)2_B-IBLadoNeutro(Mag)3_A-IALadoNeutro(Mag)4_?-INeutroTrafoPot(Mag)
Time (ms)
00002111027132615660_3626>1_C-IC Lado Neutro(A) 00002111027132615660_3626>2_B-IB Lado Neutro(A)
00002111027132615660_3626>3_A-IA Lado Neutro(A) 00002111027132615660_3626>4_?-I Neutro Trafo Pot(A)
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00002111027132615660_3626>5_C-UCN Lado 15kV(V) VOLTAGES
5_C-UCNLado15kV(Mag)6_B-UBNLado15kV(Mag)7_A-UANLado15kV(Mag)8_?-UNeutroGenerador(Mag)
Time (ms)
00002111027132615660_3626>5_C-UCN Lado 15kV(V) 00002111027132615660_3626>6_B-UBN Lado 15kV(V)
00002111027132615660_3626>7_A-UAN Lado 15kV(V) 00002111027132615660_3626>8_?-U Neutro Generador(V)

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Figure 6a: Failure of a 230 kV circuit breaker, initially on Phase B
Figure 6b: Sudden collapse of Phases B and C of the generator
Figure 6c: Short circuit currents circulate on Phases B and C, Phase B
current is three times larger than Phase C

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Figure 6d: Failure was cleared up after 18.8 s
e) Inadvertent energization
Operating errors, breaker head flash overs, control circuit mal-functions or a combination of these problems
have been some of the main causes of inadvertent energization. Inadvertent energization results in severe
damage to the generator in lieu of flow of severe fault current. Damper winding and field winding may suffer
the most severe effects.
f) Loss of excitation
The Generator delivers both Active and Reactive Power to the grid. The Active power comes from the
Turbine while the Reactive power is due to Field Excitation. When Field Excitation is lost while the
Mechanical Power remains intact, it would attempt to remain synchronized by running as an Induction
Generator . As an Induction Generator, the machine speeds up slightly above the synchronous speed and
draws Excitation from the grid.
When Excitation is lost, rotor current (If), Internal voltage (E) and terminal voltage (Vt) falls. Due to reduced
voltage, Stator current increases for the same ‘Pe’. As V/I ratio become smaller, the Generator Positive
Sequence Impedance (Z+) as measured at its terminals will reduce and enter the 4th Quadrant of the R-X
plane.
g) Out of phase synchronization
The likelihood of out-of-phase reclosing of a distributed generator is very low, but not impossible. The main
cause of out of phase synchronization is due to wiring errors caused during maintenance or commissioning
when voltage transformer and synchronising equipment are connected or reconnected wrongly. The wiring
errors can cause 180° or 120º out of phase synchronization. The wrong setting of the synchronizing system
may also cause out of synchronization.
180° or 120º out of phase synchronization may cause severe damage to Generator, Generator Transformer
and the associated equipment.
The example in Figure 7 show the current and voltage wave forms for a 180° out of phase synchronisation
This generator is connected in a back-to-back process with another generator (pump mode). An out of
phase synchronization of both machines on the HV network took place.

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Figure 7a: Current waveforms for a 180° out of phase synchronisation
Figure 7b: Voltage waveforms for a 180° out of phase synchronisation
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I_G5>1_A-Ig fase1(A) CURRENTS
1_A-Igfase1(Mag)2_B-Igfase2(Mag)3_C-Igfase3(Mag)4_?-Ineutro(Mag)
Time (ms)
I_G5>1_A-Ig fase1(A) I_G5>2_B-Ig fase2(A)
I_G5>3_C-Ig fase3(A) I_G5>4_?-I neutro(A)
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I_G5>5_A-Ug 1-2(V) VOLTAGES
5_A-Ug1-2(Mag)6_B-Ug2-3(Mag)7_C-Ug3-1(Mag)8_?-Uneutro(Mag)
Time (ms)
I_G5>5_A-Ug 1-2(V) I_G5>6_B-Ug 2-3(V)
I_G5>7_C-Ug 3-1(V) I_G5>8_?-U neutro(V)

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h) Generator Guide bearing (Radial/axial) fault
A variety of important malfunctions such as rough load zone, unbalance, shear pin failure, bearing
problems, and wicket gate problems.
i) Generator runaway
The runaway speed of a water turbine is its speed at full flow, and no shaft load. The turbine and the
generator are designed to survive the mechanical forces of this speed.
The example in Figure 8 show the speed curve when by mistake the reset signal from Auto sync panel was
connected at the speed raise / lower terminals of the governing system. When the Auto synchronizer was
switching ON, the high signal was opening the guide vanes to full position which resulted in the increase in
speed from 500RPM to 815 RPM.
Figure 8: Shaft speed increase due to faulty wiring
j) Short-circuit of 50% of rotor poles
Nowadays, generators have to be designed to resist to the worst rotor poles short-circuit. This fault is really
severe for rotor spider to rim tangential connections and also for generator guide bearings and their
supports.

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2.1 Second Level Of Transient Events:
a) Difference of expansion between rotating and stationary parts (Eccentricity and ellipticity)
Eccentricity is a measurement of the amount of sag or bow in a rotor. It may also provide indication of a
bent shaft.
Differential expansion is the difference between the thermal growth of the rotor compared to the thermal
growth of the frame (casing).
Figure 9: Fracture of the torque transmitter on rotor poles due to
eccentricity and ellipticity.
b) Operating with excessive vibration
As large hydro turbine generator units play important roles in power grids, rotor shaft system vibrations of a
hydro turbine generator unit is one of the most important factors that affect the stability and safety of these
important machines. The phenomenon of excessive vibration frequently happens in hydro power plants, and it
greatly influence the operating stability and safety of the unit and even can even result in malfunctions of the
unit’s components, which will lead to accidents. The shaft system of a unit might introduce axial, torsional and
lateral vibration, and of all the vibration characteristics, lateral vibration is of the most common occurrence.
There are many factors that will exert influence on lateral vibration and dynamic behavior, such as changes in
the guide bearing stiffness and the unit’s rotational speed, and unbalanced magnetic pull, unbalanced hydraulic
force and unbalanced mechanical force during operating, as well as the eccentricity of unit shaft system that is
caused by a combination of defects on all the rotational parts originating from manufacturing. Vibration response
analysis of a hydro turbine generator unit is very complex in an operational hydro power plant because
measured vibrations can be influenced by boundary conditions and external excitation and are usually
associated with varying operating conditions such as the rotational speed, load and excitation of the unit.
Boundary conditions such as the guide bearing oil film forces are related to rotational speed and eccentricity. In
respect of external excitation, the unbalanced mechanical force is proportional to the rotational speed, while
unbalanced hydraulic force is connected with load and unbalanced magnetic pull is related to excitation current.

Page 14
c) Over fluxing
Of the known over fluxing events, several occurred during machine run-up or at rated speed, but not when
connected to the grid system. A common feature of these events is a failure of the voltage feedback circuit
to the Automatic Voltage Regulator (AVR), which may ramp up the generator field current in an attempt to
achieve the desired generator terminal voltage.
The voltage feedback signal can fail for several reasons, including:
 failure to restore voltage transformer (VT) fuses following generator maintenance,
 failure to reinstate VT connections if maintenance work is performed on the copper work or bus
duct connecting the VTs to the generator,
 internal failure within the AVR,
Failure of the voltage feedback signal may also render the overfluxing (Volts per Hertz - V/Hz) protection
inoperative.
Generator over fluxing has caused serious damage to generator stator cores and stator winding insulations
for many utilities and manufacturers worldwide. Over fluxing damage to the stator core is likely to occur
towards each end of the core. Bore inspection may reveal overheating on the top of the core teeth.
Increased core losses will release heat into the core, and the temperature will increase with time. Core
temperatures can rapidly increase to the point where the inter laminar insulation breaks down, allowing
circulating currents to flow axially within the core, further increasing the heat input. Temperatures can then
continue to increase until the core steel melts. If this area of the core is adjacent to a stator winding
conductor bar, the bar insulation may become heat-damaged and break down, possibly resulting in a stator
earth fault.
d) Rotor distortion
Rotor winding distortion caused by poor end turn blocking support design or by foreshortening of the rotor
coils. Foreshortening is caused by thermal forces which compress rotor coils. Rotors should be tested for
turn to turn shorts at operating speed.
e) Core lamination faults
Stator core looseness can occur over time as pre-tensioned through bolts relax. A loose core results in
insulation wear to coils and laminations resulting in hot spots and core-to-coil failures. Foreign objects
entering the air gap can cause severe damage to the stator core laminations as shown in Figure 10.
Figure 10: Stator core mechanical stroke and winding insulation problems after a screw
felled between rotor and stator.

Page 15
f) Operation under abnormal conditions
iv. Unbalanced load
An unbalanced 3 phase load is one in which the load is not equally distributed between the three
phases. Unbalanced load causes more current flow in the common return line and thus a greater
I2
R power loss in the resistance of the return line. If a 3 phase star connected system is perfectly
balanced then the net current in the return line is equal to zero.
When the load on the generator becomes unbalanced, negative phase sequence currents flows.
The negative sequence components produce a rotating magnetic field which rotates at
synchronous speed in a direction opposite to the direction of rotor field. Hence effectively the
relative speed between the two is double the synchronous speed. Thus double frequency currents
are induced in the rotor. These currents cause sever heating of the rotor and can cause damage to
the rotor. The unbalanced stator currents also cause sever vibration and heating of stator.
v. Stator buckling
Owing to the temperature difference between the core and frame of the stator, the frame will
restrain the expansion of the core, which will lead to buckling of the stator core in case of a high-
stiffened frame. In order to prevent electromagnetic vibration of the stator core, the stator frame will
be required to have an appropriate supporting stiffness for the stator core.
vi. Overloads
It is important that generators never be overloaded without an investigation of the limitations of all
associated equipment. Equipment such as cables, buses, reactors, circuit breakers, disconnecting
switches, current transformers, and power transformers should be checked. Any one of these may
constitute the practical limit in load carrying ability of the unit. Overloading of generators causes
severe temperature rise which is detrimental to the life of Generator winding insulation.
vii. Non-linear loads
Many electrical power applications require continuous and high quality power. Nonlinear loads,
including uninterruptable power supply (UPS), variable frequency drives (VFD), adjustable speed
drives (ASD), switched mode power supplies, computers, laser printers, smps, reactifiers, plc’s,
electronic ballasts, refrigerators, TV’s etc. present a special challenge to successful delivery of high
quality power under all operating conditions. Nonlinear loads cause voltage distortion. The nature
of non-linear loads is to generate harmonics in the current waveform. This distortion of the current
waveform leads to distortion of the voltage waveform. Under these conditions, the voltage
waveform is no longer proportional to the current.
viii. Loss of synchronism
If load angle δ becomes larger than 70° for a salient pole generator (90° for a cylindrical rotor) due
to an attempt to obtain more than Pmax, increase in δ results in less power output and the machine
becomes unstable and loses synchronism. Loss of synchronism results in the interchange of
significant current surges between the generator and network as the poles of the machine pull into
synchronism and then out again.

Page 16
g) Rotor windings failures
i. Ground failures
A sensitive rotor earth fault protection is required for large generators. Two protective stages, one
for alarm only and one for trip are necessary due to the risk of a double earth fault and the
possibility of big damages on the generator side.
ii. Turn to turn short circuits
While a generator is operating or converting from the static state to the dynamic state, due to the
abrasion of the generator rotor turn-to-turn insulation or the relative dislocation caused by the
relative motion between the turns in the rotor, the turns may contact each other. When this fault
has developed to a certain extent, a turn-turn shot circuit will happen. As a result of these faults,
the effective magnetic field of the generator will decrease, and the generator reactive power will be
affected. This leads to imbalance in the magnetic circuit which causes vibration, and then
“monopole potential” and “monopole current” will be produced which will magnetize the generator
shaft. In addition, partial overheating at the turn to turn fault point may grow to a ground fault in the
rotor windings.
3. On Line Monitoring Systems
Condition monitoring plays a vital role in preventing generator failures. Having adequate condition monitoring
systems connected to the generator, consisting of continues or periodic information recording and expert
analysis, can inform plant engineers of eminent failures. Condition monitoring systems will also assist with
critical decision making after transient events as the information obtained prior to the incident can be compared
with equipment condition information after the incident.
The following on-line monitoring is possible on hydro-generators if the correct monitoring field devices are
installed:
a) Partial discharges of the stator winding
Partial discharges are small electrical sparks that occur within the voids of high-voltage insulation systems.
By monitoring these partial discharges, a variety of winding-related problems can be detected, allowing
maintenance to be planned and serious failures to be avoided. Monitoring can be done as follows:
i. Continuous monitoring
ii. Periodic monitoring
b) Rotor eccentricity
Eccentricity is a measurement of the amount of sag or bow in a rotor. It may also provide indication of a
bent shaft. This measurement is used by the operator to indicate when the machine can safely be brought
up to speed without causing rubs or damage to the seals.
Differential expansion is the difference between the thermal growth of the rotor compared to the thermal
growth of the casing. Differential expansion monitoring is most critical during a turbine "cold" start-up.

Page 17
c) Vibration monitoring
i. Bearings
By measuring vibrations on the generator and turbine guide bearings, a variety of important
malfunctions can be detected such as rough load zones*, various sources of rotor unbalance,
shear pin failure, bearing problems, and wicket gate problems.
ii. End-windings
Vibration, especially stator end-winding vibration, is an extremely important index of the overall
soundness of rotating electrical machines and can be used for the early detection of abnormalities
and trouble. High vibration can lead to loosening of the entire end-winding support system,
deterioration of supports, insulation wear, rupture of coil conductors or fatigue cracking of
conductors. All of these conditions require extensive out-of-service repairs. Fibre Optic Vibration
Systems have been developed to measure the stator end-winding vibrations in high voltage
generators where conventional hardwired transducers cannot be safely mounted. Because of their
importance, vibration process data are frequently integrated into the on-line monitoring system of
many generators. A modern Fibre Optic Vibration System provides a data base which is helpful in
anticipating generator end-winding vibration problems and predicting future maintenance needs,
which can result in extending inspection intervals and minimizing down time for maintenance. The
generator stator end-winding experience forced mechanical vibrations during operation. The
frequency of vibrations excited by the stator current is twice the electrical synchronous frequency
of the generator (120 Hz for 60 Hz systems and 100Hz for 50Hz systems).
iii. Stator core
Vibration of the stator core and frame can cause fretting and damage to the winding insulation. An
uneven air gap can also cause the stator core to vibrate. By mounting an appropriate seismic
vibration transducer on the stator core/frame, such problems can be detected before serious
damage occurs.
d) Neutral current or voltage
The Generator Neutral is grounded through a Neutral Grounding Transformer (NGT) and resistor.
Continuous measurement of the current/voltage in the secondary circuit is used for detecting an earth fault
in the winding.
e) Shaft current or voltage
Stray voltages occur on rotating shafts in magnitudes ranging from micro-volts to hundreds of volts. The
former may be generated from shaft rotation in the earth’s magnetic field, or induced from
electromagnetic communication signal induction. The latter can be induced by shaft rotation linking
asymmetric magnetism of electrical machinery, by residual magnetism present in a shaft or in adjacent
stationary members and by induction from switching of power electronics, exciters and/or current-carrying
brushes.
Shaft voltages can be either “friend” or “foe”. As “friend”, they can warn, at an early stage, of problem
development long before the problem is apparent on traditional monitors and instruments. As “foe”,
they can, as a minimum, generate circulating currents, reducing unit efficiency and, as a maximum, the
generated current can damage bearings, seals, gears and couplings, often forcing unit shut down.
Control of shaft voltages can minimize the potential for damage. This control can be either passive, by
simply placing grounding brushes, or active by injecting counteracting current signals onto the rotor. In both
cases strategic brush placement and consideration is essential to satisfactory shaft grounding and signal

Page 18
sensing. Very important to the success of shaft grounding and signal sensing is the choice and reliability of
plant grounding.
f) Rotor Winding temperature
The resistance of the field at operating temperature can be determined by measuring the voltage
impressed on the field by the generator exciter and the resulting DC current through the field as a result of
the exciter voltage. Since these relationships independently determine the field winding resistance, by
combining the ohms law relation with the properties of the material relation, an operating temperature can
be determined using the actual field voltage and shunt current.
g) Exciter diode fault detector
Rectifier diodes mounted on the rotor of a brushless generator are a common cause of faults in brushless
synchronous machines. The diodes fail by either going open circuit or short circuit. If a rectifier shorts, a
very high current flows through the associated exciter armature winding, thus causing excessive heating
and probable failure of the exciter. If a rectifier opens, the voltage regulator will substantially increase the
excitation to maintain the operating level.
This constant high level of excitation could lead to failure of the regulator. The pro-active approach is to
detect both types of faults in the shortest delays to prevent additional damage being done. An accurate and
easy way to do this is to monitor the ripple content of the exciter field current. With this method we are able
to detect immediately if a diode is open or shorted and enable different clearing actions depending on the
type of failure.
h) Rotor air gap sensor
Special capacitive sensors mounted around the bore of the stator measure the distance between the
rotating and stationary parts in the generator. Air gap measurement is important because the stator is a
flexible assembly that can become distorted or off centre. The monitor is able to provide instantaneous,
minimum, maximum, and average air gap measurements along with the rotor pole, to which min and max
measurements coincide.
i) Stator temperature sensor
Temperature sensors are installed in locations such as in stator slots, air cooler inlet and outlet, water inlet
and outlet, etc., providing important information on stator condition. The monitor provides alarming
functions, alerting operators when temperatures are outside of acceptable limits. The monitor can also
supply temperature information to System 1 software where it can be trended and correlated with other
measurements for a more complete picture of unit health
j) Air gap flux monitor
Rotor flux monitoring involves measuring the magnetic flux in the generator air-gap to determine if field
winding shorts have occurred in the rotor poles. The radial magnetic flux is detected by means of a flat coil
(or probe) consisting of several dozen turns that is glued to stator teeth . As each rotor pole sweeps by the
flux probe, a voltage is induced in the coil that is proportional to the flux from the pole that is passing the
coil. The voltage is measured by electronic instruments such as a digital oscilloscope or analog-digital
(A/D) converter. In a salient pole machine, the radial magnetic flux profile across each rotor pole depends
on the MW and MVAR loading of the machine. Any change in the flux profile within a pole at a given load
may be due to shorted turns. As each pole in the rotor passes, there will be a peak in the induced voltage
caused by the change in magnetic flux from the pole. The voltage can then be recorded and the “average”
flux across one rotor pole can be calculated. Any turn shorted turn in a pole reduces the effective ampere-
turns of that pole and thus the signal from the flux probe associated with that pole. The recorded waveform
data can then be analyzed to locate the poles containing the fault, as long as one has calibrated the pole
location from a ‘start’ location marked on the rotor shaft.

Page 19
k) Absolute vibration on turbine wheel and access shaft
Radial vibration measures the radial motion of the rotating shaft relative to the case. This measurement
gives the first indication of a fault, such as unbalance, misalignment, cracked shaft, oil whirl or other
dynamic instabilities. Vibration measurements can be made in a single plane or a two plane (X-Y)
arrangement where the sensors are 90 degrees apart and perpendicular to the shaft. Eddy current probes
are usually installed in a hole drilled through the bearing cap and are held in place by either a bracket or a
probe holder.
Absolute Shaft Vibration is a measure of the shaft’s motion relative to free space. The measurement is
typically applied when the rotating assembly is five or more times heavier than the case of the machine.
Absolute shaft motion is proportional to the vector addition of the casing absolute motion and the shaft
relative motion.
l) Digital relays/Numerical relays
A digital protective relay uses a microcontroller with software-based protection algorithms for the detection
of electrical or process faults. Such relays are also termed as microprocessor type protective relays. The
digital protective relay, or numeric relay, is a protective relay that uses a microprocessor to analyze power
system voltages, currents or other process quantities for the purpose of detection of faults in an industrial
process system.
m) Transient recorder/Disturbance recorder
A short history of the entire sampled data is kept for oscillographic records. The event recording would
include some means for the user to see the timing of key logic events, relay I/O (input/output) changes, and
see, in an oscillographic fashion, at least the fundamental component of the incoming analogue
parameters.
4. Type of Information for Analysis
Type of information to be analysed after the occurrence of a major event in generator:
i. Information extracted from digital relays/Numerical relays
ii. From transient recorder/Disturbance recorder
iii. Sequence of events recorder
iv. Vibration recorder
4.1 ‘Off-line’ Inspections
Type of “off line” inspections and testing to be carried out to determine the integrity of a generator:
i. IR measurement of Stator winding
ii. IR measurement of rotor winding
iii. Tan delta measurement of stator winding
iv. Off line partial discharge measurement
v. HV test of stator winding
vi. HV test of rotor winding
vii. Voltage balance test of rotor pole
viii. Impedance measurement of rotor

Page 20
4.2 ‘On-line’ measurements
Type of “on-line” measurements to be done before synchronization of the generator into the system:
i. Secondary current measurement of CT during SCC
ii. Secondary Voltage measurement of VT during OCC
iii. Vibration measurement
iv. 3-Phase rotation measured and compared between generator VTs and system VTs
v. 3-Phase sequence measured and compared between generator VTs and system VT’s
5. Conclusion
Abnormal operating conditions can produce catastrophic failures and can accelerate the ageing process
of a hydroelectric generator. Information gathering and analysis under both circumstances are relevant to
understanding generator behaviour. Potential damages to the generator under transient conditions should
be analysed with and without a circuit breaker at the generator output.
Transient recorders and digital relays provide important information to understand hydro electric generators
behaviour under abnormal operating conditions. Some variables can be recorded under actual failure
conditions, like generator stator current, stator voltage, excitation current, excitation voltage, stator
winding neutral currents, etc. In the other hand, modern monitoring systems, like stator winding partial
discharges, air gap flux monitor, vibration levels, etc. can help to determine the degree of deterioration that
a generator have suffered after a transient event.
Based on the on-line recorded measurement results prior and during a fault, decisions can be taken to
make further off-line testing to ascertain the fault.

Page 21
ANNEXURE
RESPONSES TO THE QUESTIONNAIRE:
A questionnaire was prepared and sent to the Working Group, the following countries responded:
Country
No of
generators
Croatia 6
Finland 15
India 4
Ireland 1
Norway 1
Mexico 4
Spain 4
Serbia 1
Total 36

Page 22
SUMMARY OF GENERATORS COVERED IN THE SURVEY:
CROATIA
Generator Power Output
(MVA)
Rated Voltage
(kV)
Speed
(RPM, 50 Hz)
Manufacturing
Year
CR-01 23 10.5 600 1968
CR-02 155 15.7 600 1984
CR-03 35 10.5 500 2003
CR-04 47 10.5 125 1975
CR-05 42 6.3 125 1989
CR-06 42 6.3 125 1982
FINLAND
(MVA)
Rated Voltage
(kV)
Speed
(RPM, 50 Hz)
Manufacturing
Year
FI-01 33 10.5 100 1992
FI-02 14 6.3 750 2001
FI-03 33 10.5 93.8 1990
FI-04 17.5 6.3 125 1987
FI-05 12 6.3 750 1995
FI-06 46 10.5 88.3 2008 (1965)
FI-07 16 10.5 214.3 2004 (1962)
FI-08 85 13.8 115.4 2010 (1959)
FI-09 45 10.5 150 1980
FI-10 45 10.5 150 1980
FI-11 70 12.2 100 2011 (1966)
FI-12 78 13.5 115.4 2004 (1963)

Page 23
FI-13 50 13.8 100 1971
FI-14 26 6.3 135.4 1984
FI-15 39 10.5 75 2001 (1961)
INDIA
(MVA)
Rated Voltage
(kV)
Speed
(RPM, 50 Hz)
Manufacturing
Year
IN-01 35 11.0 500 2011
IN-02 180 13.8 214 1996
IN-03 125 11.0 166 2003
IN-04 31.4 11.0 125 2007
IRELAND
(MVA)
Rated Voltage
(kV)
Speed
(RPM, 50 Hz)
Manufacturing
Year
IE-01 87.5 10.5 500 1970
NORWAY
(MVA)
Rated Voltage
(kV)
Speed
(RPM, 50 Hz)
Manufacturing
Year
NO-01 410 20 333 1993
MEXICO
(MVA)
Rated Voltage
(kV)
Speed
(RPM, 60 Hz)
Manufacturing
Year
MX-01 218 15 128.57 1966 (2006)
MX-02 110 13.8 112.5 1984
MX-03 110 13.8 112.5 1984
MX-04 153.7 16 300 1994

Page 24
SPAIN
(MVA)
Rated Voltage
(kV)
Speed
(RPM, 50 Hz)
Manufacturing
Year
SP-01 255 15 115.4 1969
SP-02 210.5 G
207.4 M
15 200 1986
SP-03 210.5 G
207.4 M
15 200 1986
SP-04 136.6 15 150 1988
SERBIA
(MVA)
Rated Voltage
(kV)
Speed
(RPM, 50 Hz)
Manufacturing
Year
RS-01 54 10.5 600 1959

Page 25
Information On 1st Level Of Transient Events Received:
CROATIA
Generator File Event Led to a failure
CR-01 HPP-A
Phase to ground short circuit in the isolated bus No
Short circuit in the HV substation No
Generator runaway No
Turn to turn short circuits in the rotor windings No
CR-02 HPP-B
Short Circuit failures within the generator. Single Phase to ground.
Phase to Phase. Three Phases.
Yes
Loss of excitation No
Out of Phase Synchronization No
Guide Bearing Fault No
CR-03 HPP-C
Short Circuit failures within the generator. Single Phase to ground.
Phase to Phase. Three Phases.
Yes
CR-04 HPP-C
Short Circuit failures within the generator. Single Phase to ground. Yes
Short circuit in the step-up transformer. Primary winding. Secondary
winding.
No
Out of phase synchronization No
CR-05 BULB- A
Guide Bearing Fault No
CR-06 BULB- B
Short Circuit failures within the generator. Phase to Phase. Yes
Short circuit in the step up transformer. Secondary winding. No

Page 26
FINLAND
FI-01 KKI Single Phase to ground Yes
FI-02 KLK None
FI-03 KOS None
FI-04 KUT Single Phase to ground Yes
FI-05 MAT None
FI-06 OS Single Phase to ground Yes
FI-07 PER None
FI-08 PI None
FI-09 POR None No
FI-10 POR None No
FI-11 PT Single Phase to ground. Two phases to ground Yes
FI-12 SK Inadvertent energization Yes
FI-13 VA None
FI-14 VAJ None
FI-15 VL None
INDIA
IN-01 Budhil 01
Rotor was undergoing run away speed during synchronization
by auto synchroniser
No
IN-02 Chamera 1 Phase to earth and phase to phase short circuit Yes
IN-03 Indira Sagar sator earth fault Yes
IN-04 Tanakpur Stator earth fault Yes

Page 27
IRELAND
IE-01 ESB
Single phase to ground Yes
Phase to phase Yes
Inadvertent energization No
MEXICO
MX-01 MP4
Rotor winding failure. Turn to turn short circuits. Ground failure. Yes
Difference of expansion between rotating and stationary parts
(Eccentricity, during failure)
Yes
Guide bearing fault (as a consequence) Yes
MX-02 PE1
Three phases of stator windings to ground Yes
Short circuit in the HV lines (several times with single pole reclosing) No
MX-03 PE4 Short circuit at the machine breaker on the HV substation No
MX-04 ZMN1
Short circuit in the isolated bus (step up connection to the transformer)
Induced a two phases to ground within the generator Yes
NORWAY
NO-01 Svartisen
Short circuit in the HV lines (6 times with automatic re-enerization) No
Core lamination faults (triggered by previous event) No
Stator overheating failure (7 months later) Yes

Page 28
SPAIN
Generator Event Led to a failure
SP-01
Short circuit within the generator, phase to phase evolved to a three phase failure Yes
Short circuits in the HV substation No
SP-02
Short circuit within the generator, single phase to ground, phase to phase Yes
Overspeeding
Operating with excessive vibration
SP-03
Overspeeding No
Operating with excessive vibration No
SP-04
Short circuit in the isolated bus No
SERBIA
RS-01 001
Short circuit within the generator, phase to ground and phase to phase Yes
Guide bearing fault No

Page 29
Completed Questionnaire: CROATIA 01 (HPP-A)
1. Classification of generators under study
Specify your main generator data:
Power output (MVA) 23
Rated Voltage (kV) 10,5
Rated Current (A) 1265
Power Factor 0,8
Speed (rpm) 600
Frequency (Hz): 50
Manufacturing Year: 1968.
Operating Time (hours):
Type of Excitation
System
Static X
AC Brushless
DC Generator
Other
Excitation System
characteristics:
Voltage (V) 120
Current (A) 567
Ceiling voltage (V) 200
Ceiling Current (A) 1000
Stator windings
cooling type:
Indirect air cooled X
Direct water cooled
Rotor windings
Direct water cooled
Stator core
Direct water cooled
Insulation Class:
Stator B
Rotor B
2. Classification according to operation type:
Base load X
a. How many times do you synchronize the generator?
Twice a day X

Page 30
Total number of start/stop cycles 8000
b. Average generator operating hours:
Dry season Rainy season
Per day 8 24
c. What configuration do you use to synchronize the generator?
Circuit breaker at the HV side of the step up transformer X
3. In this work, transient events are classified according to the degree of damage that might cause to the
generator. The most critical ones can lead to a failure and the less severe might just accelerate the ageing
process of a particular component of the generator.
a. Fill the following table specifying the frequency of occurrence (never experienced, once a year,
every 5, 10 or more than 10 years.
Major Event, 1st Level
Frequency of
occurrence
Lead to a generator
failure
Short Circuit failures within the generator
Single phase to ground never experienced Yes
Phase to phase never experienced Yes
Three phases never experienced Yes
Short circuit in the isolated bus
Phase to ground
once in more than 10
years
No
Short circuit in the step-up transformer
Primary winding never experienced
Secondary winding never experienced
Short circuit in the HV substation every 5 years No
Inadvertent energization never experienced
Loss of excitation once a year No
Out of phase synchronization never experienced
Generator runaway once a year No
Guide bearing (Radial/axial) fault never experienced

Page 31
Any other major event (Please make a comment)
Comments on additional major events:
No comments
2nd Level Transient Event
Frequency of
occurrence
Length of time
under the specified
condition
Overspeeding never experienced
Difference of expansion between rotating and stationary
parts (Eccentricity and ellipticity, air gap)
never experienced
Operating with excessive vibration never experienced
Overflux never experienced
Rotor distortion never experienced
Core lamination faults never experienced
Operation under abnormal conditions
Unbalance load never experienced
Stator buckling never experienced
Overloads never experienced
Non-linear loads
Loss of synchronism never experienced
Rotor windings failures
Ground failures never experienced
Turn to turn short circuits once in more than 10
years
No
Any other abnormal event on the operation of the
generator (Please make a comment)
Comments on 2nd Level additional transient events:
No comments
4. Specify the type of grounding the neutral stator winding
High impedance neutral grounding X

Page 32
Comment on the grounding scheme of your generator:
No comments
5. Is your generator equipped with on-line monitoring?
Variable Yes No
Partial discharges of the stator winding
Continuous
Periodic X
Rotor eccentricity X
Vibrations monitor
Bearings X
End-windings X
Stator core X
Neutral current or voltage X
Shaft current or voltage X
Rotor Winding temperature X
Exciter diode fault detector X
Rotor air gap sensor X
Stator temperature sensor X
Any other variable being monitored (Please specify)
Bearings temperatures X
6. Is your generator protected with a digital relay, do you have a transient recorder?
Equipment for transient data analysis Yes No
Digital relays/Numerical relays X
Transient recorder/Disturbance recorder X
7. What type of information do you analyse after the occurrence of a major event in your generator?
a. Information extracted from digital relays/Numerical relays
b. From transient recorders/Disturbance recorder
c. Sequence of events recorder
d. Vibration recorder

Page 33
8. What kind of analysis do you perform after the occurrence of a major event
a. What kind of “off line” inspection do you carry out to determine the integrity of the generator?
b. Do you take any register of “on-line” measurements before you synchronize the generator into the
system?
9. If you have on-line monitoring system of critical variables as specified in 5, do you compare the results after
and before the occurrence of a major event?
a. Do you usually obtain the same results? Yes
b. Do you experience a significant difference with respect to the previous trending?
Completed Questionnaire: CROATIA 02 (HPP-B)
Rated Voltage (kV) 15,75±7,5%
Rated Current (A) 5682±7,5%
Power Factor 0,89
Speed (rpm) 600
Frequency (Hz): 50
Operating Time (hours) – until 12/2012: 78.800
Type of Excitation
System
Static x
AC Brushless
DC Generator
Other
Excitation System
Voltage (V)

Page 34
characteristics: Current (A)
Ceiling voltage (V) x
Ceiling Current (A)
Stator windings
cooling type:
Indirect air cooled x
Direct water cooled
Rotor windings
Direct water cooled
Stator core
Direct water cooled
Insulation Class:
Stator F
Rotor F
2. Classification according to operation type
Peak load x
Pumped storage x
Prepared for reactive power
compensation
x
Times per month 40
Total number of start/stop cycles 500/year
b. Average generator operating hours
Per month 180 540
Circuit breaker at the HV side of the step up transformer x

Page 35
Frequency of
occurrence
Lead to a generator
failure
Short Circuit failures within the generator 10-15 years
Single phase to ground Yes
Phase to phase Yes
Three phases Yes
Short circuit in the isolated bus Never experienced
Phase to ground
Short circuit in the step-up transformer Never experienced
Primary winding
Secondary winding
Short circuit in the HV substation Never experienced
Inadvertent energization
Loss of excitation 10-15 years Yes
Out of phase synchronization 10-15 years Yes
Generator runaway Once a year Yes
Guide bearing (Radial/axial) fault 10-15 years Yes
Any other major event (Please make a comment) Yes
Damaged caused by strange object (water for example).
Frequency of
occurrence
Length of time
under the specified
condition
Overspeeding Once a year, caused
by any electrical fault
10 min per year
(Eccentricity and ellipticity, air gap)
Never experienced
Operating with excessive vibration Never experienced
Overflux Never experienced
Rotor distortion Never experienced

Page 36
Core lamination faults Never experienced
Operation under abnormal conditions Never experienced
Unbalance load
Stator buckling
Overloads
Non-linear loads
Loss of synchronism Never experienced
Rotor windings failures Never experienced
Ground failures
Turn to turn short circuits
Any other abnormal event on the operation of the generator
(Please make a comment)
10-15 years
Fault caused by badly adjust protection of other objects of Electrical power system (busbars, overhead line, etc.).
Low resistance neutral grounding x
No comments
Variable Yes No
Continuous
Periodic x
Rotor eccentricity
Vibrations monitor
Bearings
End-windings

Page 37
Stator core
Neutral current or voltage
Shaft current or voltage
Rotor Winding temperature
Exciter diode fault detector
Rotor air gap sensor
Stator temperature sensor
Digital relays/Numerical relays x
Transient recorder/Disturbance recorder x
system?
a. Do you usually obtain the same results?

Page 38
Completed Questionnaire: CROATIA 03 (HPP-C)
Specify your main generator data
Power Factor 0,9
Speed (rpm) 500
Frequency (Hz): 50
Manufacturing Year: 2003
Operating Time (hours): 67933
Type of Excitation
System
Static
AC Brushless x
DC Generator
Other
Excitation System
characteristics:
Voltage (V) 98
Current (A) 640
Ceiling voltage (V)
Ceiling Current (A)
Stator windings
cooling type:
Direct water cooled
Rotor windings
Direct water cooled
Stator core
Direct water cooled
Insulation Class:
Stator F
Rotor F
Base load
Peak load x

Page 39
Times per week 5
Per month 450 450
Circuit breaker at the HV side of the step up transformer x
Frequency of
occurrence
Lead to a generator
failure
Single phase to ground 0 Yes
Phase to phase 0 Yes
Three phases 0 Yes
Short circuit in the isolated bus 0
Phase to ground 0
Short circuit in the step-up transformer 0
Primary winding 0
Secondary winding 0
Short circuit in the HV substation 0
Inadvertent energization 0
Loss of excitation 0
Out of phase synchronization 0
Generator runaway 0
Guide bearing (Radial/axial) fault 0

Page 40
No comments
Frequency of
occurrence
Length of time
under the specified
condition
Overspeeding 0
0
Operating with excessive vibration 0
Overflux 0
Rotor distortion 0
Core lamination faults 0
Unbalance load 0
Stator buckling 0
Overloads 0
Non-linear loads 0
Loss of synchronism 0
Rotor windings failures 0
Ground failures 0
Turn to turn short circuits 0
No comments
Low resistance neutral grounding x
No comments

Page 41
Variable Yes No
Partial discharges of the stator winding x
Continuous
Periodic
Rotor eccentricity x
Vibrations monitor
Bearings x
End-windings x
Stator core x
Neutral current or voltage x
Shaft current or voltage x
Rotor Winding temperature x
Exciter diode fault detector x
Rotor air gap sensor x
Stator temperature sensor x

Page 42
system?
Completed Questionnaire: CROATIA 04 (HEV)
Power output (MVA) 2x47
Power Factor 0.85
Speed (rpm) 125
Frequency (Hz): 50
Operating Time (hours): A: 227.396h
B: 232.396h
Type of Excitation
System
Static Yes
AC Brushless
DC Generator
Other
Excitation System Voltage (V) 250

Page 43
characteristics: Current (A) 936
Stator windings
cooling type:
Indirect air cooled Yes
Direct water cooled
Rotor windings
Direct water cooled
Stator core
Direct water cooled
Insulation Class:
Stator F
Rotor B
2. Classification according to operation type:
Base load X (rainy season)
Peak load X
Times per week 4,5
Times per month 18
Times per year 220
Total number of start/stop cycles A:9069/B:8548
Per day 12 h 24 h
Per Week 94 h 168 h
Per month 2820 h 5040 h
Circuit breaker at the HV side of the step up transformer Yes

Page 44
Frequency of
occurrence
Lead to a generator
failure
Short Circuit failures within the generator More than 20 years
Single phase to ground More than 20 years Yes
Phase to phase Never Yes
Three phases Never Yes
Short circuit in the isolated bus Never No
Phase to ground Never No
Short circuit in the step-up transformer More than 20 years No
Primary winding More than 20 years No
Secondary winding More than 20 years No
Short circuit in the HV substation More than 20 years No
Inadvertent energization Never
Loss of excitation Never
Out of phase synchronization More than 20 years No
Generator runaway Never
Guide bearing (Radial/axial) fault Never
Generator A is replaced in year 1996 because of high vibration. Since then there have been no problems with it.

Page 45
Frequency of
occurrence
Length of time
under the specified
condition
Overspeeding Never
Never
Operating with excessive vibration Gen. A till 1996. Unknown
Overflux Never
Rotor distortion Never
Core lamination faults Never
Operation under abnormal conditions Never
Unbalance load More than 20 years 1 h
Stator buckling Never
Overloads Never
Non-linear loads Never
Loss of synchronism Never
Rotor windings failures Never
Ground failures Never
Turn to turn short circuits Never
No comments
Low resistance neutral grounding Yes
No comments

Page 46
Variable Yes No
Partial discharges of the stator winding No
Continuous No
Periodic No
Rotor eccentricity Yes
Vibrations monitor Yes
Bearings Yes
End-windings No
Stator core Yes
Neutral current or voltage Yes
Shaft current or voltage No
Rotor Winding temperature Yes
Exciter diode fault detector No
Rotor air gap sensor Yes
Stator temperature sensor Yes
Magnetic induction Yes
Manpower/reactive power generator Yes
Excitation characteristics ( U, I) Yes
Generator characteristics (U, I) Yes
Mesh characteristics (U, I) Yes
Digital relays/Numerical relays Yes
Transient recorder/Disturbance recorder Yes

Page 47
a. Information extracted from digital relays/Numerical relays +
b. From transient recorders/Disturbance recorder +
c. Sequence of events recorder +
d. Vibration recorder +
v. HV test of stator winding +
vi. HV test of rotor winding +
vii. Voltage balance test of rotor pole +
viii. Impedance measurement of rotor +
system?
and before the occurrence of a major event? Yes
b. Do you experience a significant difference with respect to the previous trending? Yes

Page 48
Completed Questionnaire: CROATIA 05 (BULB A)
Power Factor 0.95
Speed (rpm) 125
Frequency (Hz): 50
Operating Time (hours): A:126.989h
B:132.271h
Type of Excitation
System
Static
AC Brushless Yes
DC Generator
Other
Excitation
System
characteristics:
Voltage (V) 281
Current (A) 1185
Ceiling voltage (V) 370,3
Stator windings
cooling type:
Direct water cooled
Rotor windings
Direct water cooled
Stator core
Direct water cooled
Insulation Class:
Stator F
Rotor F
a. Classification according to operation type
Base load X /rainy season)
Peak load X

Page 49
b. How many times do you synchronize the generator?
Times per week 1
Times per month 4
Times per year 50
Total number of start/stop cycles A:5.007
B:4.965
c. Average generator operating hours
Per day 20h 24h
Per Week 140h 168h
Per month 4.200 5040h
d. What configuration do you use to synchronize the generator?
Frequency of
occurrence
Lead to a generator
failure
Single phase to ground Every 10 years Yes
Phase to phase Never Yes
Phase to ground More than 20 years Yes
Short circuit in the step-up transformer Never
Primary winding Never
Secondary winding Never
Short circuit in the HV substation Never
Inadvertent energization Never

Page 50
Loss of excitation Every 10 years No
Out of phase synchronization Every 10 years No
Generator runaway
Guide bearing (Radial/axial) fault
No comments
Frequency of
occurrence
Length of time
under the specified
condition
Overspeeding Never
Never
Overflux Never
Core lamination faults Never
Unbalance load Every 10 years
Overloads Never
Loss of synchronism Every 10 years 1h
Ground failures Every 10 years

Page 51
No comments
No comments
Variable Yes No
Partial discharges of the stator winding Yes
Continuous Yes
Periodic No
Bearings Yes
End-windings No
Stator core Yes
Neutral current or voltage No
Rotor Winding temperature Yes
Exciter diode fault detector Yes
Excitation characteristics (U, I) Yes
Relative rotor vibrations Yes
Synchronous speed Yes
Synchronization inputs Yes

Page 52
Magnetic induction Yes
Manpower/reactive power generator Yes
Generator characteristics (U, I) Yes
Absolute vibration on turbine wheel Yes
Insulation resistance Yes
iii. Tan delta measurement of stator winding +
v. HV test of stator winding +
vi. HV test of rotor winding +
viii. Impedance measurement of rotor +
system?
b. Do you experience a significant difference with respect to the previous trending? Yes

Page 53
Completed Questionnaire: CROATIA 05 (BULB B)
Rated Voltage (kV) 6,3 +7%/-7%
Rated Current (A) 3849 +5%/-5%
Power Factor 0.95
Speed (rpm) 125
Frequency (Hz): 50
Operating Time (hours): A:176.105h
B:171.041h
Type of
Excitation System
Static
AC Brushless Yes
DC Generator
Other
Excitation System
characteristics:
Voltage (V) 281
Current (A) 1185
Ceiling voltage (V)
Ceiling Current (A)
Stator windings
cooling type:
Direct water cooled
Rotor windings
Direct water cooled
Stator core
Direct water cooled
Insulation Class:
Stator F
Rotor F
Base load X (rainy season)
Peak load X

Page 54
Times per week 5
Times per month 17
Times per year 200
Total number of start/stop cycles A:6.971
B:7.126
Per day 17h 20h
Per Week 88h 140h
Per month 4.500h 7200h
Frequency of
occurrence
Lead to a generator
failure
Single phase to ground Every 5 years Yes
Phase to phase More than 20 years Yes
Phase to ground Never
Primary winding Never
Secondary winding More than 20 years No
Short circuit in the HV substation Never
Inadvertent energization More than 20 years No

Page 55
Loss of excitation More than 30 years No
Out of phase synchronization Never
Generator runaway Every 5 years No
Guide bearing (Radial/axial) fault Never
No comments
Frequency of
occurrence
Length of time
under the specified
condition
Overspeeding Never
Never
Operating with excessive vibration Every start 2 min
Overflux Never
Core lamination faults Every 10 years
Unbalance load Never
Overloads Never
Loss of synchronism Every 10 years
Rotor windings failures Every 5 years
Ground failures Every 5 years

Page 56
No comments
No comments
Variable Yes No
Partial discharges of the stator winding Yes
Continuous Yes
Periodic No
Bearings Yes
End-windings No
Stator core Yes
Rotor Winding temperature No
Exciter diode fault detector Yes
Stator temperature sensor yes
Magnetic flux Yes
Synchronous speed Yes
Absolute vibration on turbine wheel and access shaft Yes

Page 57
‐ Information extracted from digital relays/Numerical relays
‐ From transient recorders/Disturbance recorder
‐ Sequence of events recorder
‐ Vibration recorder
iii. Off line partial discharge measurement
system?
12. Do you usually obtain the same results? Yes
13. Do you experience a significant difference with respect to the previous trending? Yes

Page 58
Completed Questionnaire: INDIA
Incident occurred in May 2012.
Budhil/ Himachal Pradesh- Unit-2
Unit capacity: 2x35MW
Terminal Voltage: 11KV
Supplier: DEC/China
Gen cooling: Air cooled with air to Water Heat exchanger
Problem encountered: During voltage build up after making the Excitation system ON, the unit got tripped on rotor
Earth fault.
Investigation: Rotor IR measurement was done. The IR value was zero. On thorough checking the insulation of one
of the leads from slip rings to rotor winding was found damaged. This had resulted touching of bare cable with rotor
body and hence there was rotor earth fault
Remedy: The cable was replaced.
Incident ocurred on 13 May 2012
Budhil/ Himachal Pradesh- Unit-1
Supplier: DEC/China
Problem encountered: During initiation of speed raise command by Auto synchronizer, the speed of turbine was
going to run away speed.
Investigation: It was due to wrong wiring in governor circuit. By mistake the reset signal from Auto sync panel was
connected at the speed raise / lower terminals of the governing system. When the Auto synchronizer was getting
ON, the high signal was opening the guide vanes to full position which was resulting in the increase in speed from
500RPM to 815 RPM.
Remedy: The wiring was rectified.

Page 59
Power output (MVA) 38.89MVA
Rated Voltage (kV) 11 KV
Rated Current (A) 2040 A
Power Factor 0.9 Lag
Speed (rpm) 500 RPM
Frequency (Hz): 50 Hz
Manufacturing Year:
Type of Excitation
System
Static Static
AC Brushless No
DC Generator No
Other
Excitation System
characteristics:
Voltage (V)
Current (A)
Ceiling voltage (V)
Ceiling Current (A)
Stator windings
cooling type:
Direct water cooled No
Rotor windings
Stator core
Insulation Class:
Stator F
Rotor F
Peak load Yes
Twice a day Yes

Page 60
Per day 4 24
Circuit breaker at the generator output Yes
Frequency of
occurrence
Lead to a generator
failure
Short Circuit failures within the generator No
Single phase to ground Yes No
Phase to phase
No
Three phases
No
Phase to ground No
Primary winding
No
Secondary winding
No
Short circuit in the HV substation
No
Inadvertent energization
No
Loss of excitation
No
Out of phase synchronization
No
Generator runaway Yes yes
Guide bearing (Radial/axial) fault

Page 61
 During voltage build up after making the Excitation system ON, the unit got tripped on rotor Earth fault.
Rotor IR measurement was done. The IR value was zero. On thorough checking the insulation of one of the leads
from slip rings to rotor winding was found damaged. This had resulted touching of bare cable with rotor body and
hence there was rotor earth fault
Frequency of
occurrence
Length of time
under the specified
condition
Over speeding yes
No
No
Overflux
No
Rotor distortion
No
Core lamination faults
Unbalance load
No
Stator buckling
No
Overloads
No
Non-linear loads
No
Loss of synchronism
No
No
Ground failures
No
Turn to turn short circuits
No
No
 During initiation of speed raise command by Auto synchronizer, the speed of turbine was going to run away
speed and the same was due to wrong wiring in governor circuit. By mistake the reset signal from Auto
sync panel was connected at the speed raise / lower terminals of the governing system. When the Auto
synchronizer was getting ON, the high signal was opening the guide vanes to full position which was
resulting in the increase in speed from 500RPM to 815 RPM.

Page 62
High impedance neutral grounding Yes
Generator is grounded through NGT
Variable Yes No
Continuous No
Periodic No
Rotor eccentricity No
Vibrations monitor
Bearings Yes
End-windings No
Stator core No
Shaft current or voltage Yes
Rotor Winding temperature No
Exciter diode fault detector No
Rotor air gap sensor No

Page 63
iv. Voltage balance test of rotor pole
v. Impedance measurement of rotor
system?
(in case of major repair)
Yes, measured values are compared with factory test/previous test results

Page 64

Page 65
Completed Questionnaire: India 02 (Chamera 1)
Chamera-1
June-1996/Unit-3
Terminal Voltage: 13.8KV
Supplier: GE Canada
Problem encountered: The winding bar joint end cap material (fiber glass) was getting heated while the generator
was catering load. The unit tripped on differential and earth fault protection relay operation.
Investigation: On checking phase to phase short circuit was observed in the stator over hang. The same was at
the end joints associated with end caps. This had resulted phase to phase short circuit. On checking it was found
that the end caps were getting heated up and were undergoing cracking. Sort circuit occurred due to exposed
brazed bars on overhang.
Remedy: The End caps were replaced by GE
Rated Voltage (kV) 13.8
Power Factor 0.9
Speed (rpm) 214.3
Frequency (Hz): 50
Type of Excitation
System
Static √
AC Brushless
DC Generator
Other
Excitation System
characteristics:
Voltage (V) 384
Current (A) 1400
Stator windings
cooling type:
Indirect air cooled √
Direct water cooled

Page 66
Rotor windings
Direct water cooled
Stator core
Direct water cooled
Insulation Class:
Stator F
Rotor B
Peak load √
Twice a day √
Per day 4 24
Circuit breaker at the HV side of the step up transformer √
Frequency of
occurrence
Lead to a generator
failure
Single phase to ground Yes Yes
Phase to phase yes Yes
Three phases yes Yes
The winding bar joint end cap material (fiber glass) was getting heated while the generator was catering load. The
unit tripped on differential and earth fault protection relay operation.

Page 67
On checking phase to phase short circuit was observed in the stator over hang. The same was at the end joints
associated with end caps. This had resulted phase to phase short circuit. On checking it was found that the end
caps were getting heated up and were undergoing cracking. Short circuit occurred due to exposed brazed bars on
overhang.
Through NGT
Variable Yes No
Continuous
Periodic x
Rotor eccentricity x
Vibrations monitor
Bearings x
End-windings
Stator core
Neutral current or voltage x
Shaft current or voltage x
Rotor Winding temperature x
Exciter diode fault detector
Rotor air gap sensor x
Stator temperature sensor x

Page 68
system?
Yes , the results after and before the occurrence of a major event are compared.
Completed Questionnaire: India 03 (Indira Sagar 4)
Year 2003
Unit capacity: 8X125MW, Unit-4
Supplier: BHEL, Bhopal
Problem encountered: Machine tripped on stator earth fault.
Investigation: On investigation the Bakelite cover of generator terminals was found burnt. The burning of Bakelite
was due to heating of clamping bolts which were magnetic. Due to heating the terminal lead insulation got burnt
and had developed earth fault.
Remedy: Magnetic bolts were replaced by non magnetic bolts

Page 69
Power output (MVA) 138.89 MVA
Rated Voltage (kV) 11KV
Rated Current (A) 7290 A
Power Factor 0.9 Lag
Speed (rpm) 166.67
Frequency (Hz): 50Hz
Manufacturing Year: 2011-12
Type of Excitation
System
Static Static
AC Brushless
DC Generator
Other
Excitation System
characteristics:
Voltage (V)
Current (A)
Ceiling voltage (V)
Ceiling Current (A)
Stator windings
cooling type:
Rotor windings
Stator core
Insulation Class:
Stator F
Rotor F
Base load Yes
Twice a day Yes

Page 70
Per day 4 24
Frequency of
occurrence
Lead to a generator failure
Short Circuit failures within the generator:
Single phase to ground Once
On investigation, the Bakelite cover
of generator terminals was found
burnt. The burning of Bakelite was
due to heating of clamping bolts
which were magnetic . Due to
heating, the terminal lead insulation
got burnt and had developed
generator earth fault
Machine tripped on stator earth fault. On investigation the Bakelite cover of generator terminals was found burnt.
The burning of Bakelite was due to heating of clamping bolts which were magnetic. Due to heating the terminal
lead insulation got burnt and had developed earth fault.
Generator Neutral is grounded through NGT
Variable Yes No
Continuous No

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