International Journal of Electrical and Electronics Engineering 3:11 2009
Analysis of the Deterioration Effects of Stator
Insulation on the its Electro-Thermal Property
Diako Azizi, Ahmad Gholami, Abolfazl Vahedi
operate safely under normal stress conditions, it implies that it
has reached the end of its useful life. The causes of aging are
yet to be fully understood, but obviously, the degree of aging
strongly depends on the nature of the involved material and on
the nature and duration of the applied stresses. Indeed, aging
can be induced by a combination of the various stresses
(electrical, mechanical, thermal or environmental) to which
the insulation system is subjected. The simultaneous
application of these stresses leads to the interaction of aging
mechanisms.
Emerge of some signs of wear and aging in the insulation,
such as voids and their delaminating will accelerate its
deterioration. As a result in the location of the voids, the
electric field intensity will increase many times and cause
partial discharge in the voids and thus will increase the void
size and reduce the insulation life.
There are different ways to measure the insulation life. One
of them is the laboratory method [4]. But this method is not
used much, because of being time consuming. Therefore, this
paper is tried to use simulation and numerical analysis of data
which is obtained to estimate the remaining life of insulation
using coupled Electro-thermal analysis based on strong
method of finite element.
Analysis of the cosmetic results of the remaining life shows
that in the adjacent Walls of the voids, the life of the
insulation is reduced severely. This issue necessitates the need
to repair the damaged insulators.
Abstract—Electrical machinery especially electric generators
which are the main parts of any electrical network, through the time
and everyday functions like any other electrical devices experience a
series of electrical, mechanical and thermal stresses. Continuous of
this transient and stable tension over the time, cause lasting effects on
the machine insulation parts, especially the insulation of stator slot
which is the most important part of the insulation in machines. These
events include creation of voids in the various layers of insulation;
creation the gaps between layers of insulation and increasing the
insulation dissipation factor due to change in capacitance of
insulation. Therefore, in this paper we tried to analysis the impact of
these destructive factors on the stator slot insulation electro-thermal
analysis using finite element analysis and finally estimate the
remaining life of the insulation in these cases with regard to the
effect of the electrical and thermal stress simultaneously.
Keywords—Deterioration, estimated life, generators, void
T
I. INTRODUCTION
HE insulating system is one of the main parts of electrical
rotary machine. High operating and reliability demands
require the proper technological steps to manufacture the
insulating system [1].
Much effort has been invested over several decades in
studying the aging characteristics of the various electrical
insulating system designs and insulating materials employed
in high-voltage equipments. These studies aim mainly to allow
reasonable estimates of the service life expectancies of such
equipment and to assess their reliability in operating
conditions after a given number of years in service. The aging
of a polymeric material, or of any other material for that
matter, inherently involves the alterations of the material’s
physical and/or chemical structure which is expected to be
related to the changes in the physical and chemical properties
of the material [2],[3]. When the aging of a dielectric material
is evoked, it usually implies that the alterations in the
properties of the material are detrimental to its service
operation and service reliability. When these properties have
deteriorated to the point where the material can no longer
II. CASE OF STUDY
To study a sample, it is required to determine or design the
sample first. Therefore, in order to do field analysis and to
study the generator insulation Status, a generator with a given
profile has been selected. The selected generator is
synchronous with Regular winding, three phase, two poles
that have 24 slots in stator. Rated frequency, voltage and
power are respectively 50Hz, 13.8 KV and 1 MVA.
III. ELECTROMAGNETIC MODELS
To derive the electromagnetic system equation, we start
with the Ampere’s law,
Diako Azizi is with the University of Science and Technology, Tehran,
Iran (corresponding author to provide phone: +9821-44491750; e-mail:
diako_ee@yahoo.com).
Dr. Ahmad Gholami was with University of Science and Technology,
Tehran, Iran. He is now with the Department of electrical engineering, (e-mail:
Agholami@iust.ac.ir).
Dr. Abolfazl vahedi is with the Electrical Engineering Department,
University of Science and Technology, Tehran, Iran, (e-mail:
Avahedi@iust.ac.ir).
∇× H = J +
∂D
∂D
= σE + σv × B + J e +
∂t
∂t
(1)
These quantities are:
• The electric field intensity, E
• The electric displacement or electric flux density, D
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International Journal of Electrical and Electronics Engineering 3:11 2009
• The magnetic field intensity, H
• The magnetic flux density, B
• The current density, J
• Externally generated current,
• Electrical conductivity, σ
• Velocity, v
Now we include the effect of the time variant-harmonic
fields and use these definitions for the potentials:
B = ∇× A
u=
∂A
∂t
IV. INSULATION LIFE
Arrhenius model usually has been used for estimating the
life of the insulation parts when thermal stress is applied
alone, but with regard to faster deterioration in the sections of
the insulation when placed under the both electrical and
thermal tensions, Arrhenius model must be comprehensive
[5], [6]. Therefore, Hatch and Endicott presented the Eyring
model for taking to account the thermal and voltage stresses
simultaneously. But using special methods for determining the
coefficients of the equation to estimate the life of Eyring
insulation is actually a challenge [7], [8]. Thus Ramu
proposed [9] a model for the estimation of the insulation life
which use the power law and its parameters are temperature
dependent.
When the insulation is only under thermal stress, the life is
obtained using the following relationship:
(3)
And combine them with the constitutive relationships
B = μ 0 ( H + M ) and D = ε 0 E + P to rewrite the Ampere’s law
as:
( jωσ − ω ε )A + ∇ × (μ
2
0
−1
0
∇× A− M
)
(4)
− σv × (∇ × A) + (σ + jωε 0 )∇V = J + jωP
e
In which ω,
ε 0 , μ 0 , M and P respectively refer to Angular
frequency, Relative permittivity, Relative permeability,
magnetization vector and electric polarization vector.
In the case of 2-dimensional-plane, there are no variations
in the z-direction, so the electric field is parallel to the z-axis.
Therefore you can write ∇V as −ΔV/L where ΔV is the
potential difference over the distance L. Now these equations
are simplified to:
⎛
⎡− M y ⎤
2
− ∇.⎜⎜ μ 0−1∇Az − ⎢
⎥ + σv.∇Az + jωσ − ω ε 0
M
⎣ x ⎦
⎝
ΔV
=σ
+ J ze + jωPz
L
(
)⎞⎟⎟ A
⎠
z
⎡E⎤
L = A exp ⎢ ⎥
⎣ kT ⎦
(5)
⎢⎣ kT ⎥⎦
follows:
⎡ E − σξ ⎤
L = A exp ⎢
⎣ kT ⎥⎦
V.
]
(
)
(9)
The ξ is electrically field (stress) applied.
⎛
⎞
⎛
⎡∂u ⎤
⎡ M ⎤ ⎞⎟
⎡z⎤
⎜
− ⎜ ∂ ∂ .⎜⎜ rμ 0−1 ⎢ ∂r ⎥ + μ 0−1 ⎢ ⎥u − ⎢ z ⎥ ⎟⎟ ⎟
⎢∂u ⎥
⎜ ∂r ∂z ⎜
⎣0⎦
⎣− M r ⎦ ⎟⎠ ⎟
⎣ ∂z ⎦
⎝
⎝
⎠
⎛ ⎡∂u ⎤ ⎞
+ rσ ⎜⎜ v.⎢ ∂r ⎥ ⎟⎟ + r σjω − ω 2 ε 0 u + 2σVr u
⎜ ⎢∂u ⎥ ⎟
⎝ ⎣ ∂z ⎦ ⎠
Vloop
=σ
+ J ϕe + jωP
2πr
(8)
E is the activation energy and k is the Boltzmann’s constant.
A detailed analysis of the data which is obtained in the
presence of thermal stress alone or when both electric and heat
are applied shows that activation energy applying both
tensions simultaneously, is less than the case which one stress
is applied. Taking to account this point, the model for
Arrhenius can be corrected by decreasing the activation
energy using the exp ⎡ − σξ ⎤ factor. New model is expressed as
In the ax-symmetric case, another form of the gradient of
− Vloop
, because the
the electric potential has been used, ∇V =
2πr
electric field is only present in the azimuthally direction. The
above equation, in cylindrical coordinates, becomes:
[
(7)
r
The application mode performs this transformation to avoid
singularities on the symmetry axis.
(2)
E = −∇V −
Aϕ
DETERIORATION EFFECTS
A. Void
In this section we assumed that the second insulation
layer has a void with rectangular dimensions, 0.0001
meters. In Fig.1 distribution of the electric field with the air
void is presented. Fig.1 shows the lines of electric field,
Fig.2 compares the electric field distribution in two cases
with and without holes in the second layer of insulation and
Fig.3 presents the remained life of insulation.
(6)
The dependent variable u is the nonzero component of the
magnetic potential divided by the radial coordinate r, so that:
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International Journal of Electrical and Electronics Engineering 3:11 2009
field in voids increases suddenly. These local increasing
causes partial discharge in that location and thus increases the
void size and the insulation will be damaged locally. Fig. 4
shows the distribution of the temperature with respect to 25
percent increasing in the dissipation factor. The results of the
reduction of the insulation life in these cases have been
presented in the Fig.5.
B. Increasing of the dissipation factor
In this section the aging effect has been modeled by the
increasing of the dissipation factor in insulation layers and its
effects on the field distribution has been specific. Here we
assumed that the dissipation factor of the insulation layer is
increased 25 percent.
Fig. 1 Distribution of the electric field and respective lines if the air
void is in the second layer of insulation (V/m)
Fig. 4 The distribution of the temperature with respect to 25 percent
increasing in the dissipation factor (ºC)
Fig. 2 Comparing the electric field distribution in two cases with and
without holes in the second layer of insulation
Fig. 5 The impact of 25 percent increasing of the dissipation factor
on the insulation life with regards to the thermal and electrical
stresses
As can be seen, 25 percent increasing of the dissipation
factor increases the insulation temperature 3 percent. It should
be noticed that certain changes in the distribution of the
electric field has not been detected in two cases.
Fig. 3 Effect of void in the remaining life
As it is obvious from the results, the intensity of the electric
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International Journal of Electrical and Electronics Engineering 3:11 2009
sector is located in the conductor separation curve and
because of the insulation departing in this section; the electric
field intensity is greatly increased as it is shown in the figure
which increases the temperature of the layer. This issue can
cause serious damage to the insulation section.
C. Gap creation between the insulation layers
Defect of good assembly and connection of appropriate
insulation layers and conductors together, makes available to
time, being utilized in long-terms, the shaking and insulation
exhaustion, an air gap will be created in the part of the layers.
Due to the lower air permittivity coefficient in comparison to
the insulation, the electric field intensity will be greatly
increased in these regions and the localized tensions cause’s
trivial discharges which damage the insulation wall [10], [11].
The spread of this destructive phenomenon during the time
can seriously damage the insulation sections. In this section
the distribution of the electric field is analyzed again while
considering the above mentioned phenomenon. Fig. 6 shows
the Electric field distribution assuming air layer creation and
Fig.7 presents the impact of delaminating on the life of
insulation layers considering the thermal and electrical
stresses.
VI. COMPARING THE RESULTS
Table I shows different impacts of the insulation aging on
the distribution of electric and thermal field, and the insulation
life.
TABLE I
Effects of the insulation deterioration on the distribution of electric and
thermal field, and the remained life in the deteriorated area
deterioration
effect on
effect on
effect on
type
electrical field
thermal field
remaining life
with void
55% increasing
1% increasing
20% increasing
20%
increasing in
dissipation
factor
layering
without
variation
3% increasing
30% decreasing
180%
increasing
4% increasing
70% decreasing
Delaminating is the most important effect on the intensity
of electric field and the temperature section. This problem has
caused the greatest effect on the reduction of the insulation
life which is reduced to 70 percent. This issue reveals the need
to use insulators with high quality materials, more accurate
operation and assembly, and ultimately complete seal between
the insulation layers.
VII. CONCLUSION
Any electrical device has a life time that with passing of
this times gradually, the need to repair and maintenance will
be clearer. Generators and motors which are the basic parts of
electrical networks are not exempted from this vital need. For
this purpose different methods have been used. This paper
was focused to the analysis of fields, such as electrical and
thermal fields. These fields cause tensions in different parts of
the insulation which monitor the various scenarios and
determine the common equations for the insulation life
estimation, and the life in different parts of the insulation is
estimated while considering the aging effect. The results
prove that, void creation and delaminating of the insulation
will dramatically decrease the remaining life of the insulator.
Fig. 6 Electric field distribution assuming air layer creation (V/m)
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International Journal of Electrical and Electronics Engineering 3:11 2009
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Diako Azizi was born in 1985. He has received
B.Sc. degree in Electrical Engineering from Tabriz
University, Tabriz, Iran in 2007. And he received the
Master degree in Electrical Power Engineering from
the University of Science and Technology, Tehran,
Iran in 2009. He is presently pursuing the Ph.D.
degree in Electrical Power Engineering, Iran
University of Science and Technology. His research
interests are aging of insulations in electrical machines.
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