Chapter 8 Results and discussions

Evaluation of in-service machines

Chapter 8
Evaluation of In-service machines
8.0 INTRODUCTION
The progressive deterioration of high voltage machine insulation is assessed
through non-destructive techniques like measurement of Insulation
Resistance, PI, tan  and Capacitance, PD measurements, mainly for trend
analysis. Though a single test cannot indicate the condition of the stator
winding insulation, a reasonable assessment can be made by analysing all
the test results together. Dielectric spectroscopy and RVM techniques could
not be used on these machines as these techniques are still not accepted
techniques among the utilities and hence there were apprehensions in the use
and interpretations of the results of these techniques. Therefore
Spectroscopic techniques were limited to laboratory studies only. Based on
electrical evaluation techniques several motors and generators were tested at
some industries, hydro, thermal and nuclear power stations. This chapter
presents the test results and analysis made on several machines at site. Case
studies are employed to illustrate the usefulness of measurements on the
stator windings in service. The machines tested are shown in chart 8.1.

In service machines evaluated

1. 3.2 kV, 210 kW Class B, Motors

2. 6.6 kV, 200 to 280 kW Motors
3. 6.6 kV, 5100 kW Motors 1. Insulation
4. 11 kV, 1.3 – 6.2 MW Evaluation
resistance
Synchronous motor techniques
2. Polarisation index
5. 11 kV, 6.1 MW motor 3. Tan delta &
6. 11 kV, 50 MW Hydro Generator Capacitance
7. 11 kV, 89 MW Turbo Generator 4. Partial discharge,
8. 11 kV, 7000 HP, Class B Motors 5. IDE
9. 11 kV, 5 MW Hydro Generator
10. 11 kV, 17 MW Generators
11. 11 kV, 21.25 MW Generators
12. 11 kV, 37.5 MW Hydro Generator
13. 11 kV, 144 MVA Hydro Generator
14. 15.5 kV,210 MW Turbo Generator
15. 15.75 kV, 210 MW Turbo
Generator

Chart 8.1 In service machine evaluation

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Chapter 8 Results and discussions
Evaluation of in-service machines
8.1 RESULTS AND DISCUSSIONS

Case (i) Effect of moisture and dirt on tan delta of stator winding:
11 kV, 50 MW Hydro generator:

As discussed in chapter 3, tan  values are influenced by the presence of dirt

and moisture either on the surface or in the bulk of the stator windings. It is
therefore necessary that before commencement of the measurements the
machine winding should be thoroughly cleaned and externally dried with
space heater or by passing current so that good analysis of the winding
insulation is possible. Normally, it is a practice that during maintenance shut
down period and when the machine is idle, space heaters are used to
minimise ingress of moisture and if necessary drying process is carried out by
passing suitable current through stator windings. A case study of hydro
generators is considered and discussed below where the presence of
moisture has highly influenced tan delta values.

Two generators both of 50 MW,11 kV, Class F insulation were newly

commissioned in one of the hydro power stations located in hilly terrain in
eastern part of India. The history is that just before the machines could be put
into operation, they were accidentally submerged in water due to wall collapse
caused by flash floods. However, the machines were refurbished by the
Operation and Maintenance personnel of the Power Station and diagnostic
testing activity carried out to assess the condition of stator winding insulation
and to put the Generators back into operation. Tests were conducted on
these machines and it was observed that the measured IR and PI values were
low. Tan δ measurements carried out upto the rated voltage 11 kV, showed
higher tan δ base values (tan δ @ 20% VL), though the tip-up values were
well within the permissible limits. This could possibly be because of presence
of moisture either on the surface or bulk of the insulation as the machines
were shut down for quite a long time. Drying process was initiated by passing
suitable current through the windings until the IR values were better. The tan
δ measurements were repeated after adequate drying process and the data
is as shown in the Figure 8.1. Curves I (R,Y & B) indicate the tan δ values
obtained on these machines before adequate drying and curves II (R*, Y* &

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Chapter 8 Results and discussions
Evaluation of in-service machines
B*) the values obtained were satisfactory after drying. This clearly
demonstrates that the presence of moisture affects the base tan δ values,
however the trend in tan  variation with voltage is more or les the same with
only a shift on the ordinate and no change in tip-up values.

I I

II

Figure 8.1. Variation of dielectric loss with voltage for 50 MW hydro generators
before and after drying process

Case (ii) Effect of winding temperature on Dissipation factor

11 kV, 89 MW Turbo Generator

Besides studying the increments of tan δ of the complete windings at ambient

temperature it is necessary that measurements at higher temperatures of the
winding should also be made wherever possible. If the insulation condition is
found to be unsatisfactory, considerable change in values at higher
temperature of the winding would be expected. Turbo Generators, 11 kV, 89
MW capacity comparatively new winding, were tested at one of the utilities.
Measurements were carried out immediately after shutting down the
generator from spinning. At the first measurement, the winding temperature
was 61 oC and the measurements were repeated at intervals till it reached
the ambient temperature. The data obtained at different temperature is
shown in Figure 8.2. It is observed that the base tan  values are relatively

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Chapter 8 Results and discussions
Evaluation of in-service machines
higher at higher temperature, however maintaining the same trend and tip-up
values. This clearly demonstrates that tan  tip-up values are more promising
tool than base tan  values in assessment of condition of stator windings.
This supports the laboratory findings on model coils discussed in this study.

Figure 8.2. Variation of tan δ with voltage for 89 MW, 11 kV Generator

Case (iii) Non synthetic (Bitumen) vs Synthetic (Epoxy) insulation

15.5 kV 210 MW Turbo Generator :

As discussed in chapter 1 the insulation system of machines have undergone

changes in the last few decades. With the advent of new synthetic materials
having better properties and manufacturing technology, the development of
larger capacity machines are made worldwide. Large capacity machines are
being manufactured indigenously in the country and special measurements
were introduced for the new machines with the specific purpose of monitoring
the characteristics of the machine as made, for providing the basic data for
future reference and for effecting improvements in the manufacture process
whenever necessary. The characteristics of mica epoxy bonded system are
far superior when compared with that of mica bitumen/asphalt system. As an

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Chapter 8 Results and discussions
Evaluation of in-service machines
illustration the comparative data in respect of new machines with two types of
systems are shown in figure 8.3. It is seen that the tan δ variation with voltage
are much smaller for mica epoxy system. Such characteristics result in a
better performance of the machines during service.

Figure 8.3. Variation of tan δ with voltage for 210 MW, 15.75 kV Generators

Case (iv) Interchanging Line end and neutral end coils of the motor of
6.6 kV, 5100 kW machines

The stator winding evaluation were carried out on two identical 12 year old 6.6
kV 5.1MW synchronous motors installed in a petro chemical plant. The results
obtained on the machines are summarized in table 8.1. As it can be seen from
the table, tan δ tip up, capacitance tip up & IDE values were high for class B
insulation. These results indicate the severe deteriorated condition of the
stator insulation of both the machines. During IDE measurements on R-phase
of motor A, the loop trace obtained on the screen of dielectric loss analyzer
was unstable at about 3.7 kV onwards. It was suspected that the instability of
pattern may be due to the presence of severe slot or end winding discharges

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Chapter 8 Results and discussions
Evaluation of in-service machines
in the winding. These discharges are detrimental to the insulation as they
develop quickly and ultimately lead to the failure. Further the discharge
inception voltage was low for both the machines. The analysis of the data
show that the condition of stator winding is not healthy and failure risk factor
was high.
Table 8.1
Results: 6.6 kV, 5100 kW motors A & B before refurbishment
Motor Tan δ Capacitance I.D.E. Vi
Phase Tan δ
tip-up tip-up% µJ/pF/Cycle kV
R 0.0694 0.03515 10.3 1.79 2.10
A Y 0.0697 0.0326 8.5 1.98 2.10
B 0.0709 0.0385 11.7 1.96 2.15
R 0.0848 0.0296 8.5 1.52 2.11
B Y 0.0826 0.0305 8.6 1.7 2.13
B 0.0861 0.0299 7.5 1.65 2.10

The Company was advised about the precarious condition of the machine and
the need for replacement of coils. However, to gain time for arrangement of
new coils and to plan the outages, it was recommended to interchange the
phase end and neutral end coils, so that tip-up range from minimum at the
neutral end of the winding. The coils at the neutral end would have
experienced least stresses and would have deteriorated to a lesser extent
compared to line end coils. Tests were conducted on a group of coils located
at different position in the ascending order, with position varying from 1 to 25.
Position 1 indicated the coils on line end side and position 25 indicated the
stator coil at neutral end. Figure 8.4 shows the variation of tan δ with coil
position. It is evident from this graph that the tan values are higher for the
coils at the line end compared to coils at the neutral end.

Rework of the stator was carried out and varnish applied after interchanging
line & neutral end coils. The Diagnostic tests were repeated on the completed
stators Table 7.2 gives the test data obtained on the machines after the
refurbishment. Figure. 8.5 represents dissipation factor data obtained on
these machines before and after rewinding. Curves A,B and A*, B* shown in
the graph depict the values before and after phase end reversal and after

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Chapter 8 Results and discussions
Evaluation of in-service machines
reconditioning. From the figure it is seen that there is shift in the trend of
curves indicating that the overall tan δ values have reduced with
interchanging of line end and neutral end coils.

5.1 MW, 6.6 kV, Motors

0.12

0.1

0.08 A
tan delta (abs)

B
0.06 C
D
0.04 D

0.02

0
-4 1 6 11 16 21 26
Coil position

Figure 8.4 Variation of tan δ with coil position for 5.1 MW, 6.6 kV motors

Table 8.2
Results: Results: 6.6 kV, 5100 kW motors A & B after refurbishment
Motor Phase Tan δ Tan δ Capacitanc I.D.E. Vi
tip-up e tip-up% µJ/pF/Cycle kV
R 0.0405 0.03150 7.10 2.9 2.10
A Y 0.0430 0.02895 6.95 2.5 2.15
B 0.0440 0.03350 8.57 2.9 2.10
R 0.0540 0.02660 7.09 2.2 2.15
B Y 0.0550 0.02975 8.30 2.1 2.15
B 0.0545 0.03375 11.6 2.4 2.10

However there was no improvement in the state and quality of the insulation.
The DLA pattern obtained on R-phase of Motor A was again unstable. In the
present case non destructive tests demonstrated that, even after phase
reversal the tip-ups were not satisfactory indicating that insulation had
deteriorated to such an extent that a high risk of electrical failure existed
which was confirmed by failure a month later.

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Chapter 8 Results and discussions
Evaluation of in-service machines

5.1 MW, 6.6 kV, Class B Insulation Motors

Before phase
0.16 reversal
0.14

0.12
Tan delta (abs)

0.1 A
B
0.08
After phase A*
0.06 reversal B*

0.04

0.02

0
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
Test voltage in kV

Fig. 8.5 Variation of tan δ with voltage for 5.1 MW, 6.6 kV motors before and
after phase reversal of coils.

Case (v) Variation of dielectric losses due to ageing of the machine

insulation of 6.6 kV, 200 to 280 KW motors

The machines rated between 200 to 280 KW, Class B, which had served for
10 to 15 years were evaluated to obtain an overall assessment. The dielectric
loss data obtained on these machines is depicted in Figure 8.6. From the
figure it is observed that dielectric losses increase steeply with voltage for
machines which are much older than the others. For machines which are in
service for 10 -12 years the tip up values are marginal.

Case (vi) Comparison of dielectric losses with thermal B & F class of

insulation 6.6 kV, 700 to 770 KW motors

Several motors rated 6.6. kV, 700 to 770 kW motors with non synthetic (class
B) and synthetic (Class F) insulation motors were tested. The age of these
machines also ranged from 12 years to 18 years at the time of measurement.
Figure 8.7 shows the trend for both class B and class F insulation motors. For
class B insulation motors, there is steep rise in the dielectric losses beyond

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Chapter 8 Results and discussions
Evaluation of in-service machines

6.6 kV, 200 KW to 280 KW motors

0.12
280 KW

0.1 275 KW
275 KW
275 KW
0.08
Tandelta (abs)

275 KW
250 KW
0.06
250 KW
250 KW
0.04
235 KW
250 KW
0.02
250 KW
200 KW
0
0 1 2 3 4 5
Test voltage in kV

Fig. 8.6 Variation of tan δ with voltage for 200 KW to 280 KW, 6.6 kV, class B motors

6.6 kV, 700 kW - 770 kW MOTORS

0.08

0.07 700 KW
Class B
710 KW
0.06
770 KW
Tandelta (abs)

0.05 770 KW
750 KW
0.04
760 KW
0.03 750 KW
750 KW
0.02
735 KW
0.01 735 KW
Class F
0
0 1 2 3 4 5
Test Voltage in kV

Fig. 8.7 Variation of tan δ with voltage for 700 KW to 770 KW, 6.6 kV, class B and class F
motors

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Chapter 8 Results and discussions
Evaluation of in-service machines
2.5 kV resulting from onset of partial discharge activity. However in case of
class F synthetic insulation motors the incremental values are much flatter.
This demonstrates that class F insulation motors are much superior than class
B insulation motors.

Case (vii) Studies on efficacy of rewinding: 11kV, 6.1 MW motor

The 11 kV, 6.1 MW motor is insulated with bitumen insulation system and
located in fertiliser plant and has served nearly 20 years of operation. The
diagnostic tests were carried out as part of a routine maintenance exercise.
The tests revealed alarmingly high discharge energy values and tan δ tip-up
and capacitance tip-up values showed appreciable deterioration of the
insulation over the years of operation. The user was advised that early failure
was likely and that a further rewind or replacement should be provided. This
diagnostic was confirmed by failure three months later.

Non-destructive tests were carried out on a newly rewound machine. Figure.

8.8 shows results obtained on 20 year old machine before and after
rewinding.

11 kV, 6100 KW, Class B Motor

0.12

Before rewinding
0.1

R-Phase
0.08
tan deleta (abs)

Y-Phase
B-Phase
0.06
R-Phase
Y-Phase
0.04
B-phase

0.02

After rewinding
0
0 2 4 6 8 10 12
Te st volta ge in kV

Figure. 8.8 Variation of tan δ with voltage for 11 kV, 6100 KW, class B motor
before and after rewinding

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Chapter 8 Results and discussions
Evaluation of in-service machines
Case (viii) 11 kV 1.6 MW - 6.1 MW synchronous motors :
These class B motors situated in a petro-chemical plant, were operating for
the last 26 years. Diagnostic tests were carried out on these machines since
1981. Fig.8.9 shows the plot of loss tangent characteristics obtained on these
machines. From the data obtained on these machines it was observed that
there is no incremental values of tan δ and capacitance. Similar trend is also
observed in case of IDE values. It was concluded that the insulation condition
of the stator is satisfactory and still in opertion.

Figure. 8.9 Variation of tan δ with voltage for 11 kV, 1.6 MW – 6.2 MW class B motors

Case (ix) Effect of distillate in water cooled Generator

15.75 kV, 210 MW, Turbo Generator

The stator winding of a large turbo generator was constructed from series
connected, water cooled conductor bars. The operating conditions, magnitude
of bar forces and nature of overall duty may differ substantially. Thus, a water
cooled steam turbine generator set will not be operated normally above 60
degree C, and the dominant stresses experienced by the insulation are
mechanical / electrical. Diagnostic tests on large water cooled generators are
normally carried out during shutdown periods. During normal operation the
generator winding is maintained in a dry hydrogen environment at a positive
pressure relative to the cooling water circuit to prevent discharge erosion of

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Chapter 8 Results and discussions
Evaluation of in-service machines
the exposed insulation of stator windings. However during major overhaul
there is a risk that moisture will have ingressed into interstices in the stator
insulation. Measurement of IR, PI which is commonly used as an index of
dryness of the winding is complicated in the presence of leakage path to earth
through the insulating hoses connected to the cooling water circuit. Drying
out the water cooling circuit allow direct measurement of the winding
resistance but is time-consuming and may extend the shutdown period. A
test method by measuring the ac impedance at low frequencies has been
proposed by Burney K.G & et al [63] for estimating the dc resistance and
polarisation index of water cooled stator winding without any need to dry out
the cooling water circuit, but expresses a need for further work about the
preferred test voltage and reliability of results over a practical range of winding
conditions.

Discharge measurements provide a sensitive, non destructive means of

detecting minute defects in the insulation. For water cooled generators it is
very essential that a complete dry out be given to the water circuit and
measurements carried out with and without distillate flowing. Table 8.3
presents the data obtained on 15.75 kV, 210 MW water cooled generator with
and without distillate flowing. From the table it is seen that the tan  values
are higher when the distillate starts flowing in the winding bars. This is
because, the distillate acts as a resistive component in parallel with the
capacitance of the stator insulation as shown in figure 8.10. However the
incremental tan δ and capacitance values remains almost unaltered (table
8.3). Hence utmost care must be taken to interpret the results as the values
become complicated due to the presence of water circuit path in parallel with
the winding insulation.

RR

Figure 8.10 Schematic representation of stator capacitance with resistive component of distillate

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 Chapter 8 Results and discussions
Evaluation of in-service machines
Table 8.3
Results: 15.75 kV, 210 MW, Water cooled Generator, 10 year old
Without distillate flowing With distillate flowing
R-Phase Y-Phase B-Phase R-Phase Y-Phase B-Phase
Insulation 900 1250 1250 - - -
Resistance (MΩ)
Polarisation 5 3.6 2.8 - - - #
Index
Tan δ at 0.2 VL 0.0195 0.0197 0.0200 0.0533 0.0459 0.0580
0.0327 0.0302 0.0234 0.0533 0.0459 0.0580 #

Capacitance at 142.30 142.90 142.30 142.89 143.99 143.07

0.2 VL (nf) 149.59 142.12 141.51 142.39 142.97 142.39 #

Ohm-Farad 134.63 187.65 176.88 - - -

Tan δ tip-up % 0.35 0.30 0.26 0.27 0.35 0.66

0.31 0.43 0.56 0.27 0.35 0.66 #
Capacitance tip- 1.01 0.90 0.98 0.85 1.34 1.52
up 0.84 0.90 0.98 0.85 1.34 1.53 #
I D E at Vph 5.22 4.64 4.64 5.6 4.9 5.6
µJ/pF/cycle 5.60 4.90 5.60 5.6 4.9 5.6 #
# - Measurement conducted one year later - indicate not measured

Case (x) 11 kV, 135 MW and 110 MW, Turbo Generator

Table 8.4 and 8.5 presents the data obtained on 11 kV, 135 MW and 11 kV,
110 MW, class B, Steam Turbine Generators respectively. These generators
have been in operation for several years. As a routine maintenance,
diagnostic tests are carried out periodically. Though tan  tip-up in case of 135
MW generator is higher than 110 MW generator, it is still satisfactory, as the
insulation is Class B and the limits are generally higher than for Class F
insulation machines. Over the years of operation there has been no
appreciable deterioration in the machine insulation and are in operation
satisfactorily.
Table 8.4
Results: 11kV, 135 MW Turbo Generator
R-Phase Y-Phase B-Phase
Insulation Resistance 450 M ohm 475 M ohm 425 M ohm
Polarisation Index 2.9 3 3.2
Tan δ at 0.2 VL 0.0189 0.01869 0.01855
Tan δ tip-up % 1.2 1.26 1.235
Capacitance at 0.2 VL 717.27 720.10 717.98
(nF)
Capacitance tip-up % 0.296 0.397 0.212
Ohm-Farad 322.77 342.04 305.14
IDE at Vph 0.72 0.7 0.675

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 Chapter 8 Results and discussions
Evaluation of in-service machines
Table 8.5
Results: 11 kV, 110 MW Turbo Generator
R-Phase Y-Phase B-Phase
Insulation Resistance 1800 M ohm 1530 M ohm 975 M ohm
Polarisation Index 3 4.12 2.3
Tan δ at 0.2 VL 0.00929 0.0089 0.00869
Tan δ tip-up % 0.04 0.0445 0.031
Capacitance at 0.2 VL 276.05 278.76 278.9
(nF)
Capacitance tip-up % 0.09 0.0717 0.0538

Ohm-Farad 496.89 425.50 271.93

IDE at Vph 0.2175 0.145 0.145

Case (xi) 11 kV,7000H.P. Synchronous Motors (class B)

This is another interesting case study where class B thermo plastic insulation
could regain its properties even after overheating of insulation. These two
identical, 18 years old class B motors have been operating in the ammonia
plant of a fertilizer company. The machines are located close to each other in
the plant and operating conditions are same. The diagnostic tests were
conducted on the machines in order to assess the insulation condition of the
stators. The results of the test obtained are presented in the table 8.6
Table 8.6
Results: 7000H.P. Synchronous Motors (class B)
Test Parameter Motor 1 Motor 2
Tan δ 0.0686 0.036

Tan δ tip-up 0.00615 0.00148

Capacitance tip-up% 0.3535 0.3325

I.D.E. µJ/pF/Cycle 0.64 0.32

Discharge inception 4.0 5.0

Voltage kV

The results of the tests indicate significant variations in the general conditions
of these two machines. The Motor No.1 showed comparatively higher values
of base tan , tan  and capacitance tip-ups & IDE, than the other motor even
though the stress levels and other operating conditions were same. Normally
identical machines situated at a particular location exhibit comparable results.
In this case the results obtained were contradictory. Tan δ, tan δ tip-up, IDE &
Vi clearly indicated that the stator insulation of the Motor No.1 was not in good

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 Chapter 8 Results and discussions
Evaluation of in-service machines
condition. Reviewing the operational history of motor No. 1, it is found that
the cooling system of the motor No.1 had failed just few months prior to the
date of tests and stator winding got over heated. The shut down of the
machine was taken only when the smoke emanating from the enclosure of the
machine was noticed. Here the interesting fact was that the capacitance tip-up
values of the machines were comparable while wide variation existed in tan 
tip-up. The experience obtained in the laboratory on the investigation of
ageing phenomena of HV machine insulation has proved that the deterioration
of the insulation, which is an irreversible process, is invariably associated with
a large capacitance tip-up along with other parameters. As both the machines
exhibited comparable values of capacitance tip-up, it was concluded that the
insulation condition of stator of motor No.1 was also satisfactory, even though
its present state was not comparable with that of the other. The results of the
subsequent measurements on both the machines two years later confirmed
the predicted failure. The results are presented in table 8.7. The class B
insulation system being thermoplastic in nature could regain its insulating
properties at the operating temperature in the course of its service. Both the
machines are operating satisfactorily thus demonstrating the usefulness of
these tests.

Table 8.7
Results: 11 kV,7000 H.P. Synchronous class B Motors
(Measurements carried out two years later)
Test Parameter Motor 1 Motor 2
Tan δ 0.0395 0.0396
Tan δ tip-up 0.00253 0.00265
Capacitance tip-up% 0.37 0.38
I.D.E. µJ/pF/Cycle 0.85 0.825
Discharge inception Voltage kV 4.4 4.4

Case (xii) 3.3 kV,210 kW Induction Motor ( class B)

This is another case study where the migration of slot wedges could lead to
higher discharge activity and hence higher tan  and capacitance tip-ups. The
evaluation studies made on two 18 years old, 3.3 kV, 210 kW Induction Motors
installed in a Fertilizer Company are tabulated in table 8.8. Tan δ tip-up &
capacitance tip-up values were quite high for such insulation even though they

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 Chapter 8 Results and discussions
Evaluation of in-service machines
have not reached an alarming level. Based on these results it was concluded
that the condition of the stator insulation of both the machines was not
satisfactory. It was recommended for refurbishment of the stators in order to
increase the remaining useful service lives of the machines. The user company
accepted the recommendations and initiated action to give a refurbishment of
the stators.

The physical inspection of the stators revealed that the wedges were loose and
damaged and had become black due to accumulation of carbon particles. The
stators were given a refurbishment with new wedges and varnish. The
diagnostic tests were again carried out on the stators after refurbishment in
order to check the state and quality of the insulation. The results are furnished
in table 8.9.
Table 8.8
Results: 3.3 kV,210 kW Induction Motor ( class B)
Motor Tan δ Tan δ tip- Capacitance I.D.E. Vi
up tip-up% µJ/pF/Cycle kV
C 0.0084 0.0163 2.103 0.54 1.80

D 0.0058 0.01725 3.44 0.473 1.82

Table 8.9
Results: 3.3 kV,210 kW Induction, Class B Motor(after revarnishing)
Motor Tan δ Tan δ Capacitance I.D.E. Vi
tip-up tip-up% µJ/pF/Cycle kV
C 0.00353 0.00175 No More than 2
0.67
discharges kV
D 0.00287 0.00225 No More than 2
0.44
discharges kV

These results revealed that the insulation condition of the stators has
improved considerably after rewedging and varnishing of the stators. Thus,
before judging the aged condition of the insulation, it is necessary to use other
information like condition of wedges (whether loose, firm, or migrated below),
winding surface cleanliness etc by physical/visual inspection whichever is
necessary to make meaningful conclusions and not misled by tan  tip-up and
capacitance tip-up alone.

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Chapter 8 Results and discussions
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Case (xiii) 11 kV,17 MW and 21.25 MW Generators
A case study to discuss the use of application of DC voltage test, Surge
voltage tests in addition to IR, PI, tan  and capacitance measurements is
presented and discussed. The brief history of machines operating in the
hydro-plant are as follows.

a) Plant history:

Six generators were commissioned during 1955 and 1959 in two stages.
These Generators are of Class B insulation and are in service for more than
50 years operating at high load factor. Station Records indicate the average
annual generation of power station is 675 MU as against 525 MU and has
been delivering 114.5 MW on Cumulative Monthly Rate basis. As reported by
the station authorities and as per the records available in the power station,
there have been frequent failures of generator stator coils and some of the
coils have been replaced and the machines are put in operation.

(b) Evaluation:
Condition assessment tests were conducted the on generator insulation in
order to assess the insulation conditionn when the machines were 34 years
(1994) and 53 (2008) year old. Based on the test results obtained when the
machines were 34 year old, it was opined that the stator insulation has aged
quite considerably. As an initial corrective measure, it was recommended to
carry out thorough visual inspection and examination of the entire stator
winding which will help in detecting visible symptoms of deterioration such as
mechanical damage to coil surface and end windings, looseness of coils and
wedges, deterioration due to thermal effects and the like. Accordingly, station
authorities initiated action for conducting the visual inspection of the
Generators. The visual inspection & examination revealed that the generators
were very soiled, particularly in the core portion of the stator bore. The slot
wedges have become loose and the wedge material dusty due to rubbing of
the wedges against the core iron and leakage of oil has formed a sticky black
paste. Further, rust was found on the back side of the core, in the air ducts
and on individual segments. Based on the findings it was recommended to
thoroughly clean and rewedge.

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Chapter 8 Results and discussions
Evaluation of in-service machines

Figure 8.11. A view of Generator hall commissioned in 1952-1954

(c) Discussion: 11 kV, 17 MW Generators

The data obtained on these machines when the machine had served 53 years
are presented tables 8.10 to 8.14. Table 8.10 and 8.11 present the data
obtained on generator units 1 and 2 respectively. From these tables it is
evident that the first stage Generators, Unit No 1. and 2 show moderate
ageing of the insulation.. The IR & PI values of unit 1 & 2 lie in the acceptable
range for a 51 years old machine indicating that the stator winding is clean
and dry. The DC leakage current characteristics obtained on the stator
winding showed the magnitude of the leakage current was small and its
variation with the test voltage does not exhibit steep slope as shown in the
Figures 8.12. These results further confirm that surface conditions of the
stator winding are dry.

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Evaluation of in-service machines

DC Leakage Current Characteristic DC Leakage Current Characteristic

14 16
12 14
Leakage Current

Leakage Current
12

(micro Amps)
(micro Amps)
10
R-Phase 10 R-Phase
8
Y-Phase 8 Y-Phase
6 B-Phase
B-Phase 6
4
4
2 2
0 0
0.0 2.0 4.0 6.0 0.0 2.0 4.0 6.0
Voltage (KV dc) Voltage (KV dc)

Unit No. 1 Unit No.2

Figure 8.12. DC leakage current characteristics of unit No 1 and 2

Table 8.10
Results of the IR , Tan δ and partial discharge tests obtained on each phase of 17
MW GENERATOR UNIT #1 stator winding
IR PD PD PD
Tan δ
Test (in inception extincti magnitude
(%) C
conne MΩ) PI T (%) voltage on at phase
@ 2.2 (%)
ction Year (60 (kV) voltage voltage (pC)
kV
sec) (kV)
1994 190 2.95 0.2 0.39 0.21 -- -- **
R
phase No Partial
2008 387 3.59 3.083 0.130 0.22 -- --
discharge
1994 170 2.44 3.39 0.27 0.49 -- -- **
Y
phase No Partial
2008 388 3.80 3.721 0.125 0.20 -- --
discharge
1994 190 3.26 3.46 0.31 0.63 -- -- **
B
phase No Partial
2008 432 3.34 3.758 0.128 0.19 -- --
discharge
** PD not measured

The tan δ values which are the figure of merit of the insulation lie in the
normal permissible range for an in-service aged machine. The observed
results indicate normal dielectric losses in the stator winding insulation
system. The Tan δ tip-up (T) and Capacitance tip-up (C) which indicate the
void content are low in the stator insulation system. The surge comparison
test showed normal patterns without any distortion indicating no inter turn fault

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Table 8.11
Results of the IR , Tan δ and partial discharge tests obtained on each phase of 17
MW GENERATOR UNIT #2 stator winding
IR PD PD PD
Tan δ
Test (in inception extinct magnitude
(%) C
connect MΩ) PI T (%) voltage ion at phase
@ 2.2 (%)
ion Year (60 (kV) voltag voltage (pC)
kV
sec) e (kV)

R 1994 310 2.55 3.34 0.41 0.49 -- -- --

phase 2008 323 2.98 4.377 0.112 0.19 2.2 2.0 8000
1994 275 2.10 3.20 0.41 0.42 -- -- --
Y
phase 2008 340 3.29 4.343 0.143 0.22 2.6 2.2 6000
1994 275 2.18 3.08 0.46 0.63 -- -- --
B
phase 2008 228 4.61 4.362 0.156 0.23 2.3 2.2 6400

in the stator winding insulation. The winding resistance values of all the three
phases lie in the normal permissible range. Based on the above findings it is
concluded that insulation condition is quite healthy and it was recommended
to repeat the measurements after two years to monitor the trend in ageing as
it is an ongoing process.

(d) Discussion: 11 kV, 21.25 MW Generators

Further the data obtained on Generator units No. 4 to 6 are shown in tables
8.12 to 8.14 respectively. From these tables it is evident that Generator Units
4 to 6 of II stage have high initial values of tan δ and starting of corona
discharges at 30% of rated voltage. i.e. about half the phase voltage. Tan δ
values obtained on these machines are abnormally high for an in service
class B machine. As compared to the base tan δ values obtained in the year
1994, the tan δ increments are too high indicating substantial ageing
/deterioration of the stator winding insulation system. The Tan δ tip-up (T)
and Capacitance tip-up (C) values are very high for an in service class B
machine and indicate substantial ageing of the stator insulation. These high
T and C represent higher void content in the insulation. As the machine is
51 years old, the void content have considerably increased with ageing,
contributing to increase in T and C.

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Table 8.12
Results of the IR , Tan δ and partial discharge tests obtained on each phase of 21.25
MW GENERATOR UNIT #4 stator winding
PD PD PD
IR Tan δ inceptio extincti magnit
Test
Year (in MΩ) (%) T C n on ude at
connec PI voltage voltage phase
(60 @ 2.2 (%) (%)
tion (kV) (kV) voltage
sec) kV
(pC)

R 1994 210 2.6 7.57 1.84 5.02 -- -- **

phase 2008 258 -- 10.84 2.36 6.31 -- -- 5000O
1994 230 2.5 8.04 1.59 5.21 -- -- **
Y
phase 2008 85.6 1.9 10.78 2.41 6.50 -- -- 30000
1994 200 2.9 7.10 2.11 5.25 -- -- **
B
phase 2008 75.1 2.2 10.42 2.54 6.75 -- -- 24000
** not measured
Table 8.13
Results of the IR , Tan δ and partial discharge tests obtained on each phase of
21.25 MW GENERATOR UNIT #5 stator winding
PD PD PD
Tan δ inceptio extincti magnit
Test IR
(%) C n on ude at
connect (in MΩ) PI T (%)
Year @ 2.2 (%) voltage voltage phase
ion (60 sec)
kV (kV) (kV) voltage
(pC)

R 1994 275 3.11 8.18 1.84 5.53 -- -- --

phase 2008 117 2.31 10.1 2.26 6.27 3.593 3.357 20000
1994 265 3.13 7.56 2.01 5.81 -- -- --
Y
phase 2008 129 2.40 9.26 2.30 6.46 4.991 4.200 30000
1994 260 3.17 7.57 1.96 5.31 -- -- --
B
phase 2008 125 2.35 9.44 2.31 6.45 4.282 3.657 50000

The partial discharge magnitude at operating phase voltage is abnormally

high for an in-service machine. These results indicate highly deteriorated
condition of the stator winding insulation. PD inception and extinction
voltages are lower than 50% of the operating phase voltage indicating slot
end discharges in addition to PD. This is a clear indication that the winding
insulation has reached a stage of deterioration where a sudden breakdown
can occur and the operational reliability and the availability are very much
endangered.

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Evaluation of in-service machines
Table 8.14
Results of the IR, Tan δ and partial discharge tests obtained on each phase of
21.25 MW GENERATOR UNIT #6 stator winding
PD PD PD
Tan δ inceptio extincti magnit
Test IR
(%) C n on ude at
connect (in MΩ) PI T (%)
Year @ 2.2 (%) voltage voltage phase
ion (60 sec)
kV (kV) (kV) voltage
(pC)

R 1994 85 2.6 8.16 1.4 3.81 -- -- --

phase 2008 147 3.9 8.56 1.90 5.31 2.5 2.0 80000
1994 110 2.3 8.19 1.46 4.08 -- -- --
Y
phase 2008 311 5.1 8.44 1.97 5.58 2.4 1.9 70000
1994 90 2.3 8.20 1.55 4.40 -- -- --
B
phase 2008 393 3.1 8.05 2.20 6.09 2.4 2.0 80000

As it is evident in the above Tables – 8.12 to 8.14, majority of the Generators

exhibit substantial ageing / deterioration of their insulation system and have
come close to the end of their life. As the equipments are already 51 – 53
years old and in view of persistent problems associated with both electrical &
mechanical equipment as recorded in the O & M history illustrated above, it
was recommended to consider the power station for renovation and
modernization with latest technology, refurbishment and retrofitting. With
modern class F insulation system, the Generator output power rating can also
be enhanced to the extent of 20%. The R & M program not only improves
reliability and availability of the equipment but also enhances the station
power output. A year later the recommendations were implemented by the
utility.

Case study (xiv): 11 kV, 5 MW Hydro Generators:

These generators have been commissioned during 1956 and operating
satisfactorily in a dam power house. Non-destructive tests were carried out for
the first time during 1991. The user company was contemplating to update the
rating of the power station by replacing the existing class-B insulation with
class-F system. However with a view to ascertain the present condition of the
machines the company decided to carryout the non-destructive tests on the
stators. The results of the tests are tabulated in Table 8.15.

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Chapter 8 Results and discussions
Evaluation of in-service machines
Table 8.15
Results: 11 kV, 5 MW Hydro Generator
Machine No. 1
Phase Tan δ @ ΔT % ΔC % IDE Maximum
0.2 V (%) µJ/pF/cycle Vi
R 6.97 0.02 1.07 0.42 6.0
Y 6.92 0.02 0.25 0.35 6.0
B 7.09 0.03 0.25 0.28 6.0
Machine No. 2
Phase Tan δ @ ΔT % ΔC % IDE Maximum
0.2 V (%) µJ/pF/cycle Vi
R 6.55 0.07 0.15 0.35 6.0
Y 6.50 0.075 0.46 0.42 6.0
B 6.53 0.095 0.44 0.42 6.0

The results indicated low level of deterioration. It was concluded that the
insulation conditions of stators of both the machines are healthy. Even now
the machines are operating satisfactorily and thus the diagnostic tests gave
the user company confidence in the reliable operation of the machines .

Case study (xv):11 kV, 37.5 MW Hydro Generator

This class B machine commissioned during 1957 has been working
satisfactorily in a dam power house. The non-destructive tests were carried
out for the first time with a view to assess the condition of the insulation. The
results of the tests are tabulated in Table 8.16.

Table 8.16
Results: 11 kV, 37.5 MW Hydro Generator
Phase Tan δ 0.2 ΔT % ΔC % IDE Maximum
Vl (%) µJ/pF/cycle Vi
R 12.82 0.93 2.54 11.24 3.0
Y 12.20 1.09 2.86 8.72 3.0
B 12.84 1.08 2.70 10.19 3.0

From the table it can be seen that the base value of tan δ is high for such an
insulation system. This may be attributed to the substantial amount of
contamination of the insulation. The low value of Vi and high values of IDE
indicate high void content in the insulation. These results indicate
considerable amount of deterioration of the insulation. However, with a view to

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Chapter 8 Results and discussions
Evaluation of in-service machines
assess the rate of deterioration, it was recommended to carryout diagnostic
tests after a year.

Case study(xvi): 11 kV, 144 MVA Hydro Generators

With a view to evaluate the condition of these two class F generators
commissioned during 1976 and 1985, detailed visual examination/ inspection
and testing of the stator windings were carried out. Typically the results of R -
Phase of the winding are summarized in table 8.17.

Table 8.17
Results: 11 kV, 144 MVA Hydro Generators
Test Parameter Generator No.1 (1976) R Generator No.2 (1985) R
phase phase
Tan δ (%) 0.95 0.87
ΔT % 0.094 0.29
ΔC % 0.36 0.36
IDE µJ/pF/cycle 1.37 1.01
Vi (kV) 4.5 5.3
Max. Magnitude of slot 160 165
discharge (millivolts)
Max. PD Magnitude at 5000 3600
Vph. pC
Visual inspection / Stator winding was Slight carbon contamination
Examination moderately contaminated of the stator winding. No
with carbon dust. No symptoms of deterioration
symptoms of deterioration due to time-temperature
due time-temperature effects, corona /mechanical
effects, corona damage, wedges were found
/mechanical damage, to be tight.
wedges were found to be
tight.

Tan δ, ΔT, ΔC, IDE and Vi indicate low level of deterioration of the stator
insulation. The overall structural integrity of the stator insulation is found to be
healthy as no symptoms of deterioration due to thermal effects and
mechanical damage were observed.

Conclusions:
This chapter presents and discusses several case studies which are taken up
from machines in service. The findings of the laboratory investigations have
been applied in assessing the condition of stator winding insulation of these

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Chapter 8 Results and discussions
Evaluation of in-service machines
machines. Based on field testing of in service machines the following
conclusions are drawn
 Periodic diagnostic tests such as tan delta, capacitance, PD
measurements, Integrated Partial Discharge Energy (DLA)
measurements give useful information on the relative condition of
winding insulation.
 Measurement of IR, PI is complicated by the presence of leakage path
to earth through the insulation hoses connected to the cooling water
circuit in case of water cooled generator.
 A careful interpretation and analysis of results of measurements would
be very helpful for meaningful evaluation
 Dissipation factor and capacitance are particularly helpful in detecting
defects that are global in the winding.
 Partial discharge is more effective than tan or tan  and capacitance
tip-ups in detecting localized damage.

183