Life Expectancy Determination of Form-wound Coil
Isolation of High-voltage Motor
Ilija Jeftenic, Koviljka Stankovic
Nenad Kartalovic
Faculty of Electrical Engineering
University of Belgrade
Belgrade, Serbia
[email protected]; [email protected]
Institute of Electrical Engineering “Nikola Tesla”
Belgrade, Serbia
[email protected]
Boris Loncar
Faculty of Technology and Metallurgy
University of Belgrade
Belgrade, Serbia
[email protected]
Abstract— Isolation capability of organic and inorganic
isolation materials is considerably different in respect to time.
There is a relatively poor dependence on time in case of inorganic
isolation materials. It is very significant in case of organic
materials (that are more often in use). This paper considers life
expectancy of form-wound coil isolation of high-voltage motor
depending on the part of form-wound coil where it is located.
Namely, although the sleave of a high-voltage motor is uniformly
isolated, it appears that breakdown in a form-wound coil
happens mainly in certain points. The paper starts from the
presumption of Weibull distribution of random variables
breakdown voltage and breakdown time. Certain relation
between parameters of these distributions and life expectancy
exponent has been specified on basis of this presumption.
Expectancy life exponent of form-wound coil isolation of highvoltage motor has been determined on basis of this relation and
adequate experimentally determined parameters of Weibull
distributions (depending on the position on the form-wound coil).
Quantile dependence of breakdown probability on form-wound
coil depending on the position on the form-wound coil has been
determined on basis of knowing the expectancy life exponent.
Keywords— high-voltage motor; form-wound coil isolation; life
expectancy.
I. INTRODUCTION
A most common problem for the operation of high voltage
asynchronous machines is well known as interconductor
insulation breakdown [1,2,3]. It is present in lower high
voltage asynchronous machines, i.e. when a high voltage
asynchronous machine has a large number of windings in the
groove [4,5]. The first several windings at the high voltage
asynchronous machine`s input could be charged from 80% of
the maximum value of interconductor insulation overvoltage.
The nominal value of interconductor voltage for high voltage
asynchronous machines is around 100 V, and it is exceeded by
overvoltage quite often. This leads to a need for a proper
dimensioning of overvoltage protection so the maximum
overvoltage stays within boundaries of the interconductor
insulation nominal limit. When a fault occurs it is almost
impossible to detect the cause of the short-circuiting between
the windings in the asynchronous machines [6].
There is a real necessity to develop better technical
solutions for high voltage asynchronous machines` windings
and to develop better interconductor insulation quality. In this
paper the accent will be on problems for detecting weak points
in interconductor insulation.
II. MOTOR’S INSULATION
In the 1 MW – 10 MW power range, all high-voltage
rotating machines have to be build using form-wound multiturn
coils at the stator side. The a.c. armature winding may be
defined as one in which, when in motion relative to a
heteropolar magnetic field, electromagnetic field`s (E.m.f.`s)
are generated in a number of sections or phases. The winding is
usually composed of small group of conductors, in slots spaced
round the periphery of the armature, and connected together
with the ends brought out to form a phases winding
independent of other similar phases [7,8].
Question of mechanical balance is not an issue in the a.c.
windings, so a.c. winding are different from commutator
winding of d.c. machine. The closed connection and the
commutator of the d.c. armature winding are required so that
the e.m.f. between the commutator brushes will be in effect. an
integration of the flux wave between corresponding points on
the armature surface, and will remain a virtually constant e.m.f.
although the conductors contributing to it are continually
replaced by others. Still a.c. winding, do produce an alternating
e.m.f. that corresponds to the actual space distribution of the
alternate N- and S-polar flux in the airgap. It thus resolves
itself into separate phase groups and could in fact be reduced to
a single conductor or turn in each phase [9].
Typical double-layer windings for medium-size machines
are illustrated in Figure 1.
a
Fig. 1. Double-layer winding; Partly-wound; lap multi-turn coils: open slots.
Figure 2 shows the scheme of conductor coated with tape
that contains mica particles. These particles are used in order to
prevent partial discharges.
b
Fig. 4. a) Scheme of investigated windings; b) the cross section of double
layer wingding (1 –conductor, 2 – inner insulation, 3 – outer insulation).
Fig. 2. The scheme of conductor coated with tape that contains mica
particles.
All steps in production the conductor are shown in Figure 3
The last step includes coating with mica tape.
a
b
c
d
e
f
III. DISTRIBUTION FUNCTIONS OF BREAKDOWN TIME AND
BREAKDOWN VOLTAGE
If a constant-voltage test at voltage ud1 is considered and
performed on n test pieces, then the n realizations of the
variable breakdown time are obtained [11]. The empirical
distribution function of the breakdown time obtained from the
test can be advantageously described by a Weibull distribution:
g
Fig. 3. All steps in production the conductor.
Above shown type of insulation is commercially known as
LGGL (Lacquer-Glass-Glass-Lacquer). The LGGL insulation
if tested on two conductors placed side by side, can withstand
breakdown voltages in range 9 kV - 12 kV [10]. Exact value of
voltage between turns depends on the power of the motor, but
nominal voltage is usually around 100V. This kind of
insulation provides high reliability because insulation of such a
winding is overprotected by 100 times.
Scheme of investigated windings is shown in Figure 4a.
Figure 4b shows the cross section of double layer wingding.


 
 t
td
  
F t d ; u d1   1  exp  
  t d 63 u d1   



The breakdown–voltage/breakdown-time diagram, the socalled ‘life characteristic’, can be constructed from selected
quantiles of this distribution; experience has shown that it
forms a straight line on a double-logarithmic scale. If the
confidence regions are known for the quantiles concerned, they
can be transferred to the life characteristic. For each p-order of
the breakdown time, the life characteristic is described by

1 r

udp  k dpt dp

where kdp is a constant characterizing the geometry of the
arrangement and r is the life exponent mainly dependent on the
insulating material. Inflexions in the life characteristic indicate
changes in the ageing mechanism (breakdown process).
All measurement were done in accordance IEC standards and
ISO recommendations [12,13,14].
If, by analogy to equation (1), a Weibull distribution

  u

d

F u d ; t d1   1  exp  
  u d 63 t d1  

u





is also assumed for the breakdown voltage ud with a fixed
breakdown time td1, then, for the same breakdown probabilities
F t d ; ud1   F ud ; t d1  :

ud 63 t d1 t d1 t
u
 ud1 t d 63 ud1 t
u


With the life law, given with equation (2), and assuming
the exponent r is applicable to all quantiles, the following
expression holds for the same relationship and a pair of value
ud1  ud 63; t d1  :

ud 63 t d1 t d1 1 r  k d 63 
Fig. 5. The schematic representation of the testing circuit (U – HV load, Z –
filter, Ca – investigated object, Cs –capacitor, Zmi –instrument impedance, CC
– cable, MI –instrument for measuring and recording).

Comparing the coefficients on the left-hand sides of
equations (4) and (5) provides a relationship between the
Weibull exponents for breakdown time and breakdown voltage
 t ; u  , and the life exponent r, in the form:

r
u

t

This relationship is only correct if both variables
breakdown time and breakdown voltage are Weibulldistributed. It should be expressly pointed out that equation (6)
and the model presented can only be used if r applies equally
well to all quantiles. It is consequently a more reliable
approach to determine the performance function of the
breakdown voltage directly by experiment.
IV. THE EXPERIMENT
For the experiment sample we have used new windings of a
high voltage asynchronous motor, without the outer micaresin-polyester insulation. Samples approximately 30 cm long
were cut from each zone of these windings.
The experiments were performed using constant voltage.
The parameters and the sizes of the samples were determined
by the preliminary examination. The total number of voltage
steps was 10, and the difference between two consecutive steps
was 1 kV. The initial amplitude of the ac voltage was 5 kV
while the last amplitude was 16 kV.
The schematic representation of the testing circuit is shown
in Figure 5. Ten identical samples were tested at the same time.
Fig. 6. Determination of life characteristic in constant-voltage test:
a) Distribution function of breakdown time (Weibull paper); b) Life
characteristic u d  k d t d 1 / r ; for zone I.
V. RESULTS AND DISCUSSION
It is founded out that experimentally obtained statistical
samples, of the random variables breakdown voltage and
breakdown time, belong to the Weibull’s distribution
(graphical view, using chi-square test and Kolmogorov test).
The functions of the generatrix of the random value of
breakdown time on the Weibull’s probability paper for the
samples of the zone I, zone II and zone III are shown in
Figures 6a, 7a and 8a. Figures 6b, 7b and 8b show the
corresponding lifetime characteristics. Table 1 provides the
values of the coefficients of the lifetime that are determined
based on the lifetime characteristics (ra) and on the 
parameters of the statistical samples of the random variables
breakdown voltage and breakdown time (rb).
Fig. 7. Determination of life characteristic in constant-voltage test:
a) Distribution function of breakdown time (Weibull paper); b) Life
characteristic u d  k d t d 1 / r ; for zone II.
The obtained results indicate that the insulations from the
zones I, II and III are different. Namely, the technological
process of the forming of the laminated conductor leads to a
deformation of the insulation. The insulation that is damaged in
such a way shows different insulation characteristics, and its
lifetime expectancy is different. From that reason, the
technological process of the forming of the laminated
conductor should be changed or to strengthen the insulation in
the zones II and III. The second solution is simpler, and it
provides the homogenous insulation of the new laminated
conductor.
TABLE I.
LIFETIME COEFFICIENT VALUES
Zone
I
II
III
ra
10.4
11.3
12.4
rb
10.9
11.7
12.9
Fig. 8. Determination of life characteristic in constant-voltage test:
a) Distribution function of breakdown time (Weibull paper); b) Life
characteristic u d  k d t d 1 / r ; for zone III.
VI. CONCLUSION
In this paper, the lifetime expectancy of the high voltage
laminated conductor was discussed. It is shown that lifetime
expectancy of the laminated conductor is determined by the
lifetime expectancy of its weakest part. That weakest part is the
part of the laminated conductor that was bent during the
production process. That bending, of the previously insulated
laminated conductor, leads to a change in insulation (of the
parts that were bent). In order to avoid this adverse effect,
those parts of the laminated conductor should have multiple
layers of insulation.
[4]
[5]
[6]
[7]
[8]
ACKNOWLEDGMENT
[9]
This work was supported by The Ministry of Education,
Science and Technological Development of The Republic of
Serbia under contract no. 171007.
[10]
REFERENCES
[11]
[12]
[1]
[2]
[3]
L. Vereb, P. Osmokrović, M. Vujisić, Z. Lazarević, N. Kartalović,
"Effect of insulation construction bending on stator winding failure",
IEEE Trans. Dielect. Electr. Insul., Vol. 14, pp. 1302-1307, 2007.
L. Veréb, P. Osmokrović, M. Vujisić, Ć. Dolićanin, K. Stanković,
"Prospects of constructing 20 kV asynchronous motors", IEEE Trans.
Dielect. Electr. Insul., Vol. 16, pp. 251-256, 2009.
M. Leijon, L. Gertmar, H. Frank, B. Dahlstrand, "Powerformer TM
based on etablished products and experiences from T&D", Invited Paper
for IEEE T&D Committee Summer Meeting, Edmonton, Alberta
Canada, July 18-22, 1999.
[13]
[14]
J. L. Rylander, A High Frequency Voltage Test for Insulation of
Rotating Electrical Apparatus, Trans. of AIEE, 1926.
B. K. Gupta, B. A. Lloyd and D. K. Sharma, “Degradation of turn
insulation in motor coils under repetitive surges”, IEEE Trans. Energy
Conver., Vol. 5, pp. 320-326, 1990.
M. G. Say, The Performance and Design of Alternating Current
Machines; Transformes, Three-Phase Induction Motors and
Synchronous Machines, Pitman Paperbacks, 3rd ed., 1968.
T. E. Goodeve, G. C. Stone, I. M. Culbert, "Important considerations for
specifying new hydrogenerator stator windings or rewinds", Iris
Rotating Machine Conf. Rec., 2005.
H. Meyer, F. J. Pollmeier, Transiente Beanspruchung der Spulenwindungsisolierung Von Elektrischen Maschinen Mit Leistung über
1MW, Siemens Energie Technik, 11-12/1981.
K. Delaere, R. Belmans, K. Hameyer, "Influence of rotor slot wedges on
stator currents and stator vibration spectrum of induction machines: a
transient finite element analysis", IEEE Trans. Magn., Vol. 39, pp. 1492
– 1494, 2002.
T. Dakin, "Electrical Insulation Deterioration Treated as a Chemical
Rate Phenomenon", AIEE Trans., Vol. 67, pp. 113 – 122, 1948.
W. Hauschild and W. Mosch, Statistical Techniques for High-Voltage
Engineering, IEEE Power Series 13, Peter Peregrinus Ltd., 1992.
ISO Guide to the Expression on Uncertainty in Measurement,
International Organization for Standardization, Geneve, 1993.
A. Kovačević, D. Despotović, Z. Rajović, K. Stanković, A. Kovačević,
U. Kovačević, "Uncertainty evaluation of the conducted emission
measurements", Nucl. Technol. Radiat., Vol. 28, pp. 182-190, 2013.
A. Kovačević, A. Kovačević, K. Stanković, U. Kovačević, "The
combined method for uncertainty evaluation in electromagnetic
radiation measurement", Nucl. Technol. Radiat., Vol. 29, pp. 279-284,
2014.