Download assessment of dielectric properties of epoxy resin

CHAPTER 4
ASSESSMENT OF DIELECTRIC PROPERTIES OF EPOXY RESIN
COMPONENTS UNDER HUMID CONDITIONS.
4.1
INTRODUCTION
s’
The previous chapters established a foundation for the presence of moisture on
the electrical Insulation In a humid environment. The moisture ingression Into the material
was also seen to be significant, and it was also shown that the absorption of water by epoxy
resin composites significantly altered their dielectric properties. Therefore the assessment of
the increase in the dielectric loss factor and other attributes like leakage current,partial
discharge and volume conductivity would be a valuable Information for designing reliable
switchgear components. This assumes more importance in the case of medium voltage
switchgear components which were shown In the earlier chapter to be prone to moisture
condensation under aggravated service conditions.
This chapter reports on the investigations carried out on medium voltage
switchgear components, such as instrument transformers and bus support bushings, which
were subjected to various humidity levels and the dielectric parameters such as loss factor,
leakage current and partial discharges were monitored. Based on this data, a new
deterioration Index termed as 'Cumulative Characteristic Factorhas been developed to make
an assessment on the rate of Increase in these parameters In the presence of high humidity
conditions. Also a three dimensional relief model was developed for pictorially depleting the
variations of loss factor with humidity and voltage.
-\
4.2. THEORY OF MOIST DIELECTRICS
The quantification of the environmental effect on the switchgear components is
based mainly on measurable quantities,l.e loss tangent ( tan 8 ), surface leakage current (li)
and partial discharges. The other determinable parameters such as volume conductivity,loss
index^ancf ^electric power loss are also used for the assessment of effects of humidity on the
dielectric properties of the switchgear components. The understanding of the basic
mechanism of moist dielectrics Is necessary to establish the usage of these factors in the
64
quantification of environmental effect on epoxy cast equipments.
4,2,1
Theory of dielectric losses in epoxy composites
When a dielectric is subjected to an alternating electric field, the orientation of
the dipoles, and therefore also the polarisation, will tend to reverse each time when the field
polarity reverses. Their ability to follow the field depends on the inertia of the polarising
mechanisms. At very high frequencies (1015Hz) only the lightest of the mechanisms can
follow the field and hence the electronic and atomic polarisations dominate. At frequencies
less than 107Hz, contributions to the dielectric constant come from the movement of
permanent dipoles (Debye relaxation) and interfacial polarisation which is due to electronic
charges accumulating at the boundaries between different phases in the material.
The result of these polarisation mechanisms is that it is necessary to define a
complex relative permittivity for a dielectric.subjected to an alternating electric field and is
given by
e* = e*-je"
(4.D
in which £’ is simply known as the dielectric constant, E " is the loss
tand j = V - 1.
The loss tangent, and therefore the loss angle,are defined by the equation
tan 5 = £ "
/£*
(4.2)
at any given frequency .the loss factor will be the sum of several mechanisms, and their
relative importance will depend on the frequency of the applied field, so that for example:
E " = £ " (Debye) + £ " (conduction) + £ " (interfacial)
(4.3)
Thinking in the same lines in order to explain the dielectric losses in epoxy
composites we have to recognise that any specific epoxy re§ln formulation will be affected by
the type of filler and the hardening process employed. In the case of a filled resin the
degradation observed is dependent on the filler and the cavities present at the
electrode-resin interface. Based on this it has been proposed that the dielectric constant of a
composite material can be expressed as that of mixtures so that its complex permittivity can
be expressed as.
2
£ = £f Vf + £r Vr + £i Vi
(4.4)
65
where the subscripts, f. r and 1 refer respectively the filler, resin and lnerface,
and V is the volume fraction of each component. It Is considered that interfaclal region plays
an Important role In water absorption, and therefore the dielectric properties of the
composite must also depend on it. It follows from Equation (4.4) that the relative permittivity
and the loss factor for the moist dielectric can also be given by similar relatlons.so that:
E' = E'f Vf + E’r Vr + E’l Vt
(4.5)
and
£" = £”f Vf + E'Y Vr + £”i Vj
(4.6)
The perusal of the Equation (4.6) shows that the dielectric losses in epoxy are
contributed by the resin, filler and the Interfaces. In addition to these,cavities present at the
interfaces due to manufacturing processes and water ingress during the synthesis of the
resin are all contributory to discharges and high dielectric losses. In presence of moisture.
Based on this principle, loss langortwas chosen as one of the parameters for the assessment
of the dielectric behaviour of switchgear components under high relative humidity
conditions.
Further, the experimental results due to H. Egger and G.Praxl (401 also subscribe
to the above outlined theoretical reasoning. It was shown by these authors during
experimentation on epoxy insulation coll overhang.that loss factor increases to a high value
within half an hour after exposure to high relative humidity condition. This is illustrated in
Figure 4. la. The field investigations reported by Milne and Blower |4J show an increase in
the loss factor of about six percent after exposure to high ambient humidity. Hence the
choice of losstangcntfor quantification is Justified theoretically as well as experimentally.
4.2.2
Theory of leakage current due to surface conduction
Surface resistivity Is affected by moisture absorbed in the bulk of the material
and also by moisture available from the surrounding atmosphere. Surface conduction
results from moisture or other contaminants present on the surface of a dielectric. Water
has a high conductivity and even, a fine film of moisture on the dielectric surface is enough
to cause a conduction of current whose magnitude depends on the thickness of the film. The
resistance of the moisture film adhered to the surface is dependent on the nature of the
surface, and hence surface conduction is treated as the property of the dielectric itself.
The absorption of moisture by the surface of the dielectric Is highly dependent on
66
(a )
FIG. 4.1 (A) LOSS TANGENT VARIATIONS WITH TIME
67
the relative humidity of the surrounding medium. Hence the ambient environmental
conditions are the most important factors that determine the surface conductivity of a
dielectric. The lower the polarizability of a substance, the cleaner the surface and the better
Its finish,the smaller is the surface conductivity. Polar dielectrics have a lower surface
resistivity which greatly diminishes In a moist atmosphere. The epoxy resin being a polymer
and a polar dielectric and water also being a polar dielectric it can be inferred that
adherence of water on the epoxy resin components would significantly affect the surface
resistivity. The leakage current ii can be expressed as,
U=f{V,A,a)
Where
V = Voltage
(4.7)
A = Cross - sectional area of the water film
a = Conductivity of the water film
The factors A and O are functions of relative humidity (RH)
Hence
ii = f(V,RH)
(4.8)
The experimental investigations due to Eagger and Praxl (401 established an
increase in surface conductance at a relative humidity of 95 percent,which is shown in
Figure 4.1b . Based on the above theoretical reasoning and experimental results, leakage
current was chosen as one of the important parameters for quantification of the
environmental effect on the epoxy cast resin components.
4.2.3
Theory of Partial Discharges
Honda and others (101 have characterised the behaviour of epoxy mould
Insulation under sustained AC Voltage. They have attempted to study the deterioration of
epoxy moulded insulation even under low partial discharge levels as low as 0.1 pC. The
studies indicated that, even on practically void - free moulded insulation the deterioration is
developed under the application of high electrical stress. The above said deterioration is
closely related to an occurrence of small partial discharge over the boundary between the
embedded electrodes and the epoxy resin. The discharge magnitude that actually effects
deterioration is approximately 0.1 pC. Among possible causes of small partial discharges,
electric field concentrate around micro protrusions over embedded electrode surfaces is
dominant In creating the phenomenon. On the other hand, fine epoxy debonding from the
electrode surfaces caused by different thermal expansion coefficients of electrode material
and resin could serve as another reason for creating the small partial discharges. Earlier to
68
20
RH - 93 V.
r
15
CONDUCTANCE
(
J1S
t
10
5
___________ I__________________L
0
12
TIME ( Hrs) —►
18
(b)
FIG. 4.1(B) SURFACE CONDUCTANCE VARIATION
WITH TIME
24
69
this work. Dieter Kind and Konig [23J have established that partial discharges, in voids in
epoxy resin insulations subjected to long-term voltage stressing, can lead to breakdown of
the insulation. For identical electrode spacing and void dimensions, the resistance to
breakdown is dependent upon the epoxy resin system, the condition of the surface of the
void and the size of the void. Epoxy resin filled with powdered quartz was found to be more
resistant than the corresponding unfilled resin. Small and narrow voids were found to be
particularly dangerous to the dielectric even though the magnitudes of the partial discharge
quantities were small. Therefore partial discharge levels were also chosen as one of the
parameters for observation during characterisation of epoxy cast components under
aggravated service condition.
4.3
4.3.1
EXPERIMENTAL SET-UP AND PROCEDURE
Specimen Selection
Before deciding upon the experimental methodology the choice of specimen is
very important. The characterisation of dielectric materials reported in literature usually
makes use of models or thin samples of the original insulation system. However it was felt
that from the practical view point, it would be better if the component themselves, such as
instrument transformers and bus support insulation are used as test specimens. The
equipments, namely, instrument transformer and bushings are either impregnated or cast
with bisphenol resin composites. Figure 4.2 gives the details of the specimen used for
characterisation. The composition of the bisphenol epoxy resin is provided in Table 4.1. The
mechanical, physical and electrical properties of the bisphenol resin is tabulated in Table
4.2.
4.3.2
Methodology
The characterisation of the dielectric properties of switchgear components under
aggravated service conditions requires the following experimental conditions:
—
Variation of relative humidity from 45 percent to 95 percent
—
Variation of the temperature from 35 to 45°C
—
Measurement of loss factor, leakage current and partial discharges.
Variation of relative humidity: The experiments were carried out in a thermal
humidity chamber shown in Figure 4.3. The humidity chamber had the facility for changing
11
FIG . 4.2 DETAILS
kV ENTRANT BUSHING
11
kV PT
OF SPECIMEN
QVOT t o
ID d u jvs /0 0 Z ' A>Ul
3-3 kV CT
sc ale
;
70
71
TABLE 4.1 EPOXY RESINS FORMULATION FOR EXPERIMENT
Resin formulation
Curring Schedule
Casting .
SPC 20
-
40 gms
2 hrs. at room
Silica
-
600 gms
temperature
Hardener
-,
40 gms
Impregnation
Resin CY 205
300 gms
Hardener HY 905
Silica
300 gms
750 gms
Plastizier 061
.06 gms
Accelerator 040
.004 gms
110°C at 6 hours
TABLE 4.2. ELECTRICAL PROPERTIES
Properties
Temperature
Electric strength,
2mm plague, 20-s value,
50 Hz. Electrodes:
plate-to- plate, 25mm
and 75mm, In castor oil
23°C
Arc resistance
Electrolytic corrosion
Loss factor tanS
23°C
Standard
SI Units
Values
IEC 243
kV/mm
18-20
ASTM-D 495
s
185-195
DIN 53484
grade
LI
DIN 53480
grade
KA2
IEC 112
V
475
DIN 53489
grade
A-l
DIN 53483
%
2-3
60°C
Dielectric constant
23°C
3-4
DIN 53483
3.9-4.3
DIN 53482
4.3-4.7
1016
60°C
Volume relstlvlty
23°C
60°C
£2cm
1015
72
MEATER
X
FIG. 4.3
THERMAL HUMIDITY CHAMBER
73
the relative humidity from 40 percent to 95 percent. The special feature of the chamber was
the provision to raise the temperature of the air to a required level for a pre-set relative
humidity. This enables the experiments to be conducted at dilferent temperature levels. In
Chapter 2 it was shown that the maximum temperature raise on a current transformer was
42°C. Hence during experiments conducted on current transformer the temperature was
maintained at 42°C. This condition simulated the temperature raise likely to occur during
t
normal operations of the equipment. The losses occurring in potential transformer and Bus
support insulators are not considerable. Therefore for these components the temperature
was maintained at 38°C which was the average ambient temperature of Southern part of
India.
Pre-conditioning of the epoxy components: The experimental investigations
referred in earlier literature show^ that within one hour of moisturing time the values of loss
factor and leakage current attain stabilization. Based on this the samples were conditioned
in the humidity chamber for three hours, at each level of humidity. During this conditioning
the voltages were maintained at nominal rated values. The secondary circuit of the potential
transformer was connected to a resistance equivalent to that in the service condition. The
secondary windings of the current transformer were short circuited. The samples were
initially dried and cleaned before putting into the chamber. All the samples were tested as
per IEC specifications before the commencement of the characterisation. The samples not
passing the routine test were not used for the experimental studies. No micro-discharges or
corona were allowed during bulk property characterisation.
Measurement of Dielectric parameters: For the measurement of capacitance and
loss factor (tan 8 ) of the test specimens a high voltage Schering Bridge of M/s TETTEX AG
INSTRUMENTS make. Type 2801 was used with an electronic null detector and an
automatic guard potential regulator. Screened leads were used for connecting the test
specimen and the standard capacitor. The Schering Bridge was kept sufficiently away from
the high voltage source so as to minimise its influence on the measurement. The circuit
adopted for measurement Is shown In Figure 4.4
Partial discharge measurements were carried out using a Discharge
Detector.Model 5, Type 500, Bonar Instruments, U.K. The instrument incorporates two
particular features that facilitated routine discharge testing, a discharge magnitude meter
and a gating unit which allows meaningful discharge magnitude meter reading to be taken
in the presence of persistent electrical interferences. Of the various test configurations, the
standard test circuit shown in Figure 4.5. was chosen for measurement of partial discharge
in this study. The single phase test voltage derived from mains supply is applied through a
z
FIG. 4.4
(a) SCHEMATIC DIAGRAM
LOSS
MEASURING CIRCUIT
(b) MEASUREMENT CIRCUIT DIAGRAM
74
FIG. 4.5
Cb
Cx
-
BUSHING
FREE
UNDER
DISCHARGE
TEST
CAPACITOR
PARTIAL DISCHARGES DETECTION CIRCUIT
75
76
regulator to the primary of 230V/150kV high voltage transformer. The output of this high
voltage transformer Is connected to a discharge free capacitor, the test specimen and a
voltmeter resistor. The test voltage Is monitored on the discharge detector via the voltmeter
resistor. For a better accuracy !n the measurement the input unit must match the tuning
capacitance provided by the test circuit. This Is the total external capacitance appearing
across the terminals A and B, and in the circuit chosen it is the series combination of the
specimen and blocking capacitances. Precautions were taken to eliminate discharges In the
external circuit. Also the bus bars were cleaned and polished to reduce the surface
Irregularities to a minimum. The discharge detector was kept In a shielded cubicle to reduce
external interference to a minimum.
The leakage current was measured using a 3-1/2 digit precision digital
multimeter. The characteristic of the current was monitored using oscilloscope. The
measurement of current was taken after application of the voltage for 30-60 minutes.
4.4. CASE STUDIES
The assessment of dielectric properties of the switchgear components under
varying humidity was undertaken in the following manner:
—
Case study I:Variations of dissipation factor changes with relative humidity. The
voltage stress was kept below the Ionization point.
—
Case study II:Variations of leakage current with relative humidity,at nominal rated
voltage.
—
Case study HkVariations of partial discharge with relative humidity.
—
Case study IV:Variatlons of dissipation factor with Voltage at various levels of
humidity.
—
Case study V:Variatlon of leakage current with Voltage under various humidity
levels.
—
Case study VkVariatlon of volume conductivity with humidity.
The above six case studies Involved totally experimental Investigation of twenty
five variations. The characteristics obtained for various equipment under various conditions
are listed In Table 4.3.
No.
C ase
Voltage w as held
at llk V RMS
%
Support
b u sh in g s
ttnn
3
iv)
?
a
Jj
Voltage w as held
at llk V RMS
%
a
-U
llk V
5)
05
c
a
-p
5
E n tran t
b u sh in g
T em perature w as
m aintained a t 42°C
Hum idity variable
3.3kV 100/5A
JJ
iii)
T em perature w as
m aintained a t 42° C
Hum idity variable
llk V 200/5A
E
C u rren t
T ransform er
held constant
hum idity variable
Single Phase
llk V
Si
Voltage a t
P aram eter for
O bservation
-U
ii)
llk V
held constant
hum idity variable
Voltage a t
C ondition for
Experim en t
5
llk V /llO V
33K V /110 V
Single Phase
Voltage C lass
-o
c
*)
CD
c
a
•P
Potential
T ransform er
Potential
T ransform er
i)
E q u ip m en ts
TABLE 4 .3 CASE STUDIES
77
a
§
mU
r
J.
CO
CO
CO
CO
5
3
c
t)
<T>
z
4J
CO
3
CO
CO
CO
TABLE
i)
a
cu rren t
Voltage was
held at 1 lkV RMS
B
llk V
llk V
E n tran t B ushings
Partial
discharges
Partial
discharges
s
-3£
3
cn
V)
eS
At each level of
hum idity, Voltatge
w as varied
At each level of
hum idity, Voltage
w as varied
Hum idity
w as varied
llk V
Potential T ransform er
B ushings
Hum idity
w as varied
llk V
cu rren t
Leakage
cu rren t
Leakage
cn
11)
1)
111)
Potential T ransform er
3.3KV
Support B ushings
iv)
1)
llk V
o
3
E n tran t B ushings
u
ill)
l
Voltage was
held a t 3.3 kV RMS
Leakage
Voltage w as
held a t 3.3 kV RMS
T em perature held
at 42° C
uV
C urrent
3.3kV
Leakage
Voltage w as held
a t 11 kV RMS
llk V
Potential T ransform er
4 .3 CASE STUDIES (Contd.)
78
r
u
£0
%
5
>
llk V
6.6kV
llk V
E n tran t B ushings
B us S upport
iii)
E n tran t B ushing
ii)
i)
llk V
Potential T ransform er
i)
TABLE 4 .3 CASE STUDIES (C ontd.)
Relative Humidity
w as varied
w as varied
At each level of
hum idity, Voltage
w as varied
At each level of
hum idity, Voltage
w as varied
At each level of
hum idity. Voltage
Leakage
Volume
Conductivity
cu rren t
Leakage
cu rren t
Leakage
cu rren t
79
£
80
4.5. ANALYSIS OF THE CHARACTERISTICS CURVES
4.5.1
Variation of loss tangent with relative humidity
The characteristics curves of the loss factor variations with humidity for the
various equipments under constant voltage stress are shown in Figures 4.6 to 4.9. The loss
factor for all the equipment were fairly constant upto 60 percent relative humidity and
thereafter they increased exponentially towards higher values. The increase in loss factor
was six to ten times the initial values obtained at 50 to 55 percent relative humidity. In
absolute terms the loss tAngentof the complete equipment at an average humidity of eighty
five percent was about 10 to 13 percent for instrument transformers. While in case of
bushings it was ranging from 3.5 percent to 6 percent. The specific values of loss factor as
per IEC 243, for filled resins was 3 to 7 percent. But it was found that during high humidity
condition the loss factor value rose upto 18 percent. Further it was found that instrument
transformers have a higher loss factor value than bushings.
The above characterisation leads to a thought, that the micro voids on the
surface increase in size and number,due to differential expansion and.contraction brought
about due to heating and cooling taking place on the surface,because of the increase and
decrease in the values of loss kmqerttduring high and low humidity conditions. Physically this
will weaken the matrix structure of the resin composite. Though it may not cause infant
mortality of the equipment but it will nevertheless lead to surface degradation with time.
Further dielectrics with higher losses will introduce substantial heat in the-insulating
material which would result in the progressive degradation of the bulk property of the
insulation system.
4.5.2.
Variation of loss-fcongenfcwith Voltage
The characteristics curves showing the variation of loss factor with voltages at a
fixed level of humidity are shown in Figures 4.10, 4.11 and 4.12. At lower percentage of
relative humidity loss fcaaqenfc values gradually increase upto 75 percent of the rated voltage
and thereafter increases exponentially to higher values. At higher relative humidity the
initial values of the lossfangcntare high even for low voltages and steadily increases to higher
values more linearly. A perusal of the characteristics shows that at high relative humidity
conditions the loss toyngeofc are at elevated levels even for low voltage stresses and they
become purely dependent on humidity. This type of characteristics may be due to increase in
absorption current because of the Increase in the surface conductivity. Secondly there is a
definite point of transition termed as ‘Ionization Point* beyond which there is a steep
DISSIPATION t a n g e n t
81
RELATIVE HUMIDITY
(V.) ---- ►
(a)
FIG. 4.6(a) VARIATION OF LOSS TANGENT
WITH RELATIVE HUMIDITY
FOR 11KV POTENTIAL TRANSFORMER
LOSS
82
RELATIVE
HUMIDITYC/.) —►
(b)
FIG. 4.6(b) VARIATION OF LOSS
WITH RELATIVE HUMIDITY
FOR 33KV POTENTIAL TRANSFORMER
LOSS T A N
ft EN T.
83
T—J)L_______ IIII1I—
40
50
60
70
80
90
RELATIVE HUMIDITY (•/.)—*►
FIG. 4.7
VARIATION OF LOSS TA NftFNT
WITH RELATIVE HUMIDITY
FOR 3.3KV CURRENT TRANSFORMER.
100
LOSS
TA N G E N T —
*
84
FIG. 4.8
VARIATION OF LOSS TANGENT
WITH RELATIVE HUMIDITY FOR 3.3KV BUS SUPPORT
LOSS TANffiEMT**.. »
85
FIG. 4.9
VARIATION OF LOSS TANGENT
WITH RELATIVE HUMIDITY FOR 11KV ENTRANT BUSHING
A- VOLTAGE STRESS
AT
7 kV BELOW
IONISATION POINT
86
(
l
A>
)
39 V110A
<
h>
-aid
5S01
DNiHsna uawaaa unouio aod aovnoA hum
XNasNVj- ssoi do noiiviuva
——
fo
To
-JN'3?9toVl. SS01
FIG. 4.11
3000
7000
VO LTAG E
( kV
9000
)
J_____ L
1100
13000
I
15000
FOR 11KV, 200/5 AMP. CURRENT TRANSFORMER
VARIATION OFLOSSTAN(?»ENTWITH VOLTAGE
5000
17000
87
LOSS
LOSS
88
FIG. 4.12
VARIATION OF LOSS TAMCP.ENT
FOR POTENTIAL TRANSFORMER
A - Relative Humidity 80%
B - Relative Humidity 50%
89
increase in loss factor. This transition point takes place at lower voltages under high
humidity conditions.
4.5.3
Variation of leakage current with relative humidity
The variations of leakage current with relative humidity is shown in Figures
4.13, 4.14 and 4.15. The leakage current was found to be fairly constant upto a certain low
level of humidity (50% -60%) and thereafter it steadily increased to higher values of about 7
times the initial value for instrument transformer. Similar change was also observed for
bushings. It was further found that the leakage current increased linearly with voltage at 50
percent relative humidity but at higher percentages of relative humidity the Increase in the
current was more pronounced. This may be attributed to the onset of micro-discharges and
more ionisation at the surface due to an increase in humid layer thickness which ranges
from 100 to 600 )i m .
4.5.4
Variation of Partial Discharges with relative humidity
The initial values of partial discharges of instrument transformer were observed
to be very high. As the humidity increased the values rose to 2 to 3 times the initial value.
So at 95 percent humidity the values, did not drastically Increase. But the partial discharge
inception Voltage decreased sharply with increase In humidity as shown in Figure 4.16. For
example, partial discharge inception voltage came down from an initial value of llkV to
7.5KV at 85 percent humidity.
4.6. STATISTICAL ANALYSIS OF THE DATA
Explanation outlined in the previous sections were of more qualitative in nature.
In order to have a meaningful interpretation of the results, statistical methods were used.
Statistical regression theory presumes that a functional relationship exists between the
variable to be predicted and the variables which can be observed and that this relationship
can be determined from the observed data. There are many types of relationships that might
be exhibited by the data points. They can be listed as given below:
Y = ko + kih (linear fit)
Y = kohki (power law fit)
Y = koekih (exponential fit)
(4.9)
(4.10)
(4.11)
where Y is dielectric parameter, h is relative humidity, ko and ki are constants.
O
K>
90
CURRENT
LEAKAGE
4O
0
t YF
40
JL
JL
JL
50
60
70
80
90
RELATIVE HUM I DITY (V.)—►
FIG. 4.13 VARIATION OF LEAKAGE CURRENT
WITH RELATIVE HUMIDITY
FOR 11KV POTENTIAL TRANSFORMER
100
>
in
cr
V)
6O
0
( JU.A )
O
O
40
( JJ. A )
a
o
O
O
CURRENT
W
Ki
-*
LEAKAGE
0
>
_ _ _ _ _ _ _ _ _ _
►
o
**
o
91
0
ss----- >SJ
40
JL
_L
50
60
RELATIVE
JL
JL
70
60
90
HUMIDITY (V.) -
FIG. 4.14 VARIATION OF LEAKAGE CURRENT
WITH RELATIVE HUMIDITY
FOR CIRCUIT BREAKER ENTRANT BUSHING
100
LEAKAGE CURRENT (JUA
)
92
0
40
50
60
70
RELATIVE HUMIDITY
80
90
100
(V.) —►
FIG. 4.15 VARIATION OF LEAKAGE CURRENT WITH RELATIVE
HUMIDITY FOR 3.3KV CURRENT TRANSFORMER
T A N fl& N T
LOSS
FIG. 4.16 VARIATION OF PARTIAL DISCHARGES
WITH RELATIVE HUMIDITY FOR BUSHING
A - Partial Discharge Inception Voltage
B - Loss tangent
C - Partial Discharge (pC)
t
94
After fitting the curve the regression co-efficients were determined.
4.6.1
Analysis of Loss tan^enlData
The loss factor variations with humidity at constant voltage and time was first
analysed. To this data set, all the above listed curve fittings were tried. It was found that
exponential fit was the best fit for the data.
The regression model for various equipments are listed In Table 4.4. The
closeness of the magnitude of the regression to unity is the indication of the correctness of
the fit.
The establishment of exponential fit is supportive of the physical fact that the
values are fairly constant upto 50 to 60 percent humidity and thereafter Increases steeply to
higher values with Increase In relative humidity. Hence the variation of lossfcangentcan be
expressed as
tan8 = koekih
4.6.2
'
(4.12)
Variation of Leakage Current with Humidity
Having derived an expression for the variation of tan8 with humidity it was
proceeded in the same manner to derive an expression for leakage current. The variation of
leakage current with humidity when voltage and time were held constant was found to be
following a power law.
Thus ii = kohki
(4.13)
The Table 4.5 shows the regression coefficient ‘r’ for the different data sets. This
shows that the leakage current is very much dependent upon humidity condition when
voltage and time were held constant.
4.6.3
Variation of Partial Discharge with humidity
The similar techniques of curve fitting was adopted for the partial discharge
variations with relative humidity. The Table 4.6 shows the regression coefficient and the
constants for an exponential model.which was found to be the best fit. Further it can be
Potential T ransform er
CB E n tran t B ushing
B us Support
Potential T ransform er
llk V
33kV
33kV
ao
llk V
M
- 4.60566
- 8.70286
- 12.6063
- 6.73088
- 5.94963
COt
o
H
£
o
in
05
05
05
T
o
H
X
co
■'*
(N
CO
CD
9
o
eg
CD
O
m
co
CO
io
»—■<
eg
CO
o
**H
<g
CO
05
r—i
CO
o
CO
cs
t—i
0.0437897
0.0625452
0.187167
0.0615491
0.0366777
«—«
C urrent T ransform er
TANGENT
Average
0.958872
0.912369
0.957318
0.980323
0.950099
0.994251
M
3.3kV
Equipm ent
TABLE 4.4 REGRESSION MODEL FOR LOSS
95
Potential Transform er
CB E ntrant bushing
Bus support
Potentlial Transform er
Transformer (llk V test)
Transformer (IkV test)
Transformer (6kV test)
Transformer (4kV test)
Transformer (2kV test)
llkV
llkV
33kV
33kV
llkV
llkV
llkV
llkV
llkV
I"*-*
(N
CD
CO
O)
<N
r00
in
1—1
CO
5 3o
o
H
H
1
0
o
I-t
<N
in
01—<
CO
o
<N
CD
0.300645
T
o
H
£
- 1.64365
- 3.44293
o
-2 .1 5 0 8 2
eg
CO
1.88941
r—4
-
inb
- 9.74302
- 22.465
- 7.37446
- 1.20183
<N
05
in
00
c4
10.4648
-
•a
OV
Current Transform er
•3
33 kV
0.0319708
0.913273
0.116389
0.537907
0.276072
0.471034
0.508149
0.393029
2.69408
5.91228
3.03401
2.39605
3.39939
•o
1911510
Equipment
POWER LAW MODEL FOR LEAKAGE CURRENT
0.993494
0.859393
0.952606
0.990622
0.950325
0.998433
0.99342
0.960378
0.957994
0.964862
u
Z01ZV9-
TABLE 4.5
96
41.78896956
76.83428278
20.824474751
27.19034214
47.47715433
Potential Transformer
Potential Transformer
Bus support
current Transformer
E ntrant Bushing
33kV
33kV
33kV
1 lkV
0.01013336
0.01241795
0.013530219
0.01188156
0.016868036
EXPONENTIAL LAW MODEL FOR PARTIAL DISCHARGES
11 kV
Equipment
TABLE NO. 4.6
0.864455009
0.84210749
0.8674293
0.854700601
0.751625558
Y = acbx
Y = a*bx
Y = a*bx
1
Y = acbx
Y = aebx
Model
97
98
seen from the table the regression coefficients are not as close to unity as in the case of
leakage current and loss factor. This shows that increase in partial discharge values with
humidity is not so pronounced as In the case of loss factor and leakage current. The values
of partial discharges at higher values of humidity are mostly due to surface discharges
occurring on the humid layer of the components.
4.7. DEVELOPMENT OF A NEW DETERIORATION FACTOR TERMED AS
CUMULATIVE CHARACTERISTIC FACTOR
Various parameters measured during experimentation were brought under a
single specification by coining a new factor termed as ‘Cumulative Characteristic Factor’
involving loss factor, leakage current and partial discharge levels. Based on the regression
analysis each of these parameters were given weightages. It can be seen that the loss factor
and leakage current have good dependency on relative humidity conditions. In accordance
p
^>\e
with the principal^ loss tangent and the leakage current were given higher weights while
partial discharge level, capacitances and partial discharges inception voltage were given a
lower weightage. The various weights are shown below:
Leakage current
...
Loss
...
Parlial Discharge
...
Capacitances
...
Partial discharge Inception voltage
...
...
...
...
=.40
= .35
=.15
= .05
= .05
The weighted values were normalised to a base of one at the minimum humidity
level (50 Percent Relative Humidity Optimal Condition). The optimal condition of 50 percent
was chosen on the basis that the average minimum relative humidity for the Southern part
of India is 50 percent. Further Headley, Milne and Blower |4] while evaluating the insulation
characteristics of medium voltage switchgear found that loss factor values show a good
recoveiy characteristics when subjected to an atmosphere of 50 percent relative humidity at
21°C. In a CIGRE paper [41] by Kamer it was demonstrated that at 50 percent humidity the
values of dielectric parameters are at lower levels. Muller [42J and others working on the
behaviour of organic Indoor insulation under electrical and climatic stresses found that at
50 percent humidity the layer conductivity for hydroscopic surface was as low as 10'5 micro
Simens. Based on the above facts reported in literature and observations during current
investigation the 50 percent humidity level was decided as optimal operating condition.
Thereafter with increase in humidity this base value at 50 percent relative humidity was
lowered depending upon the increase in the factors. The sum of all the weighted attribute
99
values was defined as the rate Index ‘R\ The reciprocal of rate Index was termed as
‘Cumulative Characteristic Factor’ (CCF). The expression for this factor can be given as
below
CCF = 1 / IR
where
R
=ZW x M
W = weights of the attributes and M = the parametric values
The Cumulative Characteristic Factor was plotted against the relative humidity.
Further regression analysis was done to establish the correlation between this factor and
relative humidity. Tables 4.7 and 4.8 list the aggregated characteristic factor for various
switchgear components. Figures 4.17 and 4.18 give the variation of Cumulative
characteristic factor with relative humidity. It can be seen from regression analysis that CCF
has good correlation with relative humidity variations. Hence it would be a good Index for
the assessment of variation of dielectric properties with relative humidity.
4.7.1
Utility of Cumulative Characteristic Factor
This factor will be a valuable tool in the hands of the insulation designer to
assess new material and designs for aggravated service conditions. If 70 percent relative
humidity is chosen as the base for aggravated service conditions, CCF above 2 indicates
steep increases in loss factor, leakage current and partial discharges. Similarly at 95 percent
humidity the CCF ranges from 2 to 5. Based on thls.a procedure is prescribed for the
assessment of the materials and components under aggravated service conditions.
—
Take measurements of loss factor, leakage current and partial discharges at 60, 70
and 90 percent humidity for the new system under observation.
—
Assign the weights as given In the previous paragraph.
—
Aggregate the values of these parameters according to the procedure outlined earlier.
—
Determine the cumulative characteristic factor and obtain its variation with
humidity.
—
Intercept the curve at 70 percent and 90 percent relative humidity and obtain CCF
for the design and compare with initial values.
—
In general if the CCF is below 2 for 70 percent relative humidity and below 3 for 90
100
J________ I________ 1________ 1________ I________I________ L_
AO
50
60
70
80
90
100
RELATIVE HUMIDITY (•/.)—►
FIG. 4.17
VARIATION OF CUMULATIVE CHARACTERISTIC FACTOR
WITH RELATIVE HUMIDITY
FOR 11KV POTENTIAL TRANSFORMER
CCF - Cumulative Characteristic Factor
101
FIG. 4.18
VARIATION OF CUMULATIVE CHARACTERISTIC FACTOR
WITH RELATIVE HUMIDITY
FOR 3.3KV CURRENT TRANSFORMER
CCF - Cumulative Characteristic Factor
humidity
Relative
TABLE 4.7a.
0s
t
o
rH
H
>P
o
in
w o o o o
to
00 O) 0)
0.658434595
195 (0.56)
200 (0.55)
251 (0.44)
k i = 0.012569843
r = 0.976705042
Ko =
H
CCF = KoeklU
d
110(1)
105 (0.95)
'o
(pf>
10.49 (1).
10.79 (0.97)
11.31 (0.92)
14.3 (0.73)
17.7 (0.59)
21.2 (0.49)
”
10.4 (0.98)
10.1 (0.95)
9.5 (0.89)
8.9 (0.83)
8.2 (0.77)
10.6(1).
1.07
1.184
1.8868
2.4213
3.125
0.413
0.3205
C.C.F
0.933
tn CO
00 in
o' o'
TABLE 4.7b Regression Co-efficients
0.045(1)
0.048 (0.93)
0.057 (0.78)
0.095 (0.47)
0.135 (0.33)
0.195 (0.23)
s'
Partial discharge
inception voltage
Pi
103.6 (0.36)
128 (0.29)
( )IA)
Loss factor
Capacitance
I
38(1)
42 (0.9)
45.1(0.84)
73.2 (0.51)
current
Leakage
Partial
discharges
CUMULATIVE CHARACTERISTIC FACTOR FOR POTENTIAL TRANSFORMER.
102
Humidity
(%)
Relative
Loss Factor
£7
00
in
o
o
23.8
38.6
56.2
96.0
in
00
q
<N
2.2 (0.78)
2.0 (0.71)
0.478
0.383
0.2835
CCF
1.156
1.538
2.619
3.527
o
cs
(0.13)
2.8 (1)
2.7 (0.96)
2.6 (0.92)
Partial Discharge
Inception (kV)
Voltage <[
o
00
(0.22)
(0.32)
(0.52)
52 (0.96)
70 (0.71)
85 (0.58)
80 (0.62)
102 (0.49)
31.8 (1)
36.3 (0.87)
42.23 (0.75)
62.43 (0.509)
68.4 (0.46)
73.13(0.43)
tance
(PC)
50(1)
Capaci-
Partial
Discharge
»—■
0.062 (0.93)
0.08 (0.72))
0.102 (0.56)
0.145 (0.4))
0.198 (0.291)
12.5 (1)
16.3 (0.76)
Leakage
Current
(micro amp)
CUMULATIVE CHARACTERISTIC FACTOR FOR CURRENT TRANSFORMER
890
9980
TABLE 4-8
103
o
o o 00
o o
05 in
05
in CO
104
TABLE 4.8b
REGRESSION ANALYSIS FOR CUMULATIVE
CHARACTERISTIC FACTOR
Model
tan5
li
ccf
ccf
=aebh
= ahb
=aebh
= aebU
a
b
r
1.662423x1O'4
0.6615491
.95009911
2.1922xl0'2
2.39605
.957994
0.132418
0.035104
.936399
0.6162240
0.156045
.972526119
105
percent relative humidity the new designs can be said to be an Improvement over the
existing system when exposed to aggravated service conditions.4.8
DEVELOPMENT OF THREE DIMENSIONAL MODEL
>
Based on the data on the variations of the dielectric properties with voltage and
humidity, a three dimensional relief model has been developed. The loss bo^y^eakage
Current are funcUons of voltage and humidity at a given time. Using multiple regression
technique, polynomial power lit was attempted. The analysis of the data base established a
power fit for the variation of the two factors with respect to changes in humidity and voltage.
The generalised power model for the variation was estimated as,
yi = aix iblX2cl
y2 = a2Xib2 iX2c2
(4.14)
(4.15)
where
yi = loss tangent
y2 = leakage current
xi = relative humidity
X2 = voltage
ai, a2, bi, b2. ci and C2 are constants.
The models for the various equipments are given in the Table 4.9.
4.8.1
Three dimensional surface chart
The Multiple regression model was used to create a surface chart. The software
programme 'ENER GRAPHICS’ developed by IBM was used to create the surface chart. The
surface chart has three dimensions. X axis corresponds to relative humidity. Y axis
corresponds to voltage and Z axis represents loss factor. The humidity ranged from 10
percent to 95 percent. The three dimensional relief model obtained for two equipments
namely a potential transformer and an entrant bushing Is shown In the Figure 4.19 and
Figure 4.20. The three dimensional surface chart clearly shows abnormal increase of
dielectric losses with high relative humidity even at low voltage stresses. It also shows a
change In behaviour In the case of Impregnated systems (Instrument transformers) and the
cast systems (bushings). For example beyond sixty percent humidity, loss factor increases to
higher values with Increase In voltage. While In the case of bushings, the surface profile
gradually increases upto 70 percent of voltage, thereafter there is a steep increase in the
value of loss factor. This finding Is In conformity with the physical fact that In the case of
Impregnated component there would be an Increase In the number of cavities at the
106
FIG. 4.19 3-D SURFACE CHART FOR VARIATION OF LOSS TANGENT
WITH RELATIVE HUMIDITY
FOR POTENTIAL TRANSFORMER
107
FIG. 4.20 3-D SURFACE CHART FOR VARIATION OF LOSS TANGENT
WITH RELATIVE HUMIDITY
FOR ENTRANT BUSHING
4.945
7.705
18.59
-20.144
11 kV Entrant Bushing
7.0344
11 kV Current Transformer
1.7140210
Log a
x>
11 kV Potential Transformer
Equipment
o
I8SS0
TABLE 4.9 MULTIPLE REGRESSION MODEL
8
o
°o
H
2.175
Model
108
> > >
8
s
II
ii
ii
>
><
<
N
109
resin-filler interface and the windings. These cavities have more affinity to draw moisture
and hence the degradation of the dielectric properties are much m6re pronounced in the
presence of humidity and voltage stress conditions. It can also be noted here that the
physical variation portrayed in these diagrams have similar to that of characteristic factor
(CCF) variations with humidity described in the previous section. Hence a relief model
developed for the first time in this thesis work gives a good physical picture of increase in
loss factor for an equipment working at high humidity conditions.
4.9. DIELECTRICS PARAMETER CHART
To have a ready reckoner of the various attributes of the equipment effected by
humidity, a dielectric parametric chart was compiled using determinable factors such as'
—
Loss factor index
—
Volume conductivity
—
Dielectric power loss
'
The value of these factors for 50 percent and 80 percent (RH) are shown in Table
4.10.
4.9.1
Loss factor Index
This is a determinable quantity describing the effect of combination of
permittivity and loss factor.
Loss factor Index =£. tan 8
(4.16)
This helps one to have a better insight into the phenomenon of dielectric losses
since one of the basic properties of the material namely permittivity is also taken along with
loss factor. The permittivity chosen for this calculation was 5.6 since its rise was found
during experimentation to be 5.6 due to moisture ingress. At 50 percent level,permittivity
chosen was 3.5. The Table provides information about this factors at 50 percent level and 80
percent level of humidity.
4.9.2
Volume conductivity •
The volume conductivity was calculated by using the formula given below:
.126
.18
50% RH
80% RH
CB
50% RH
BUSHING
GO
BUSHING
•
vP
80% RH
50% RH
21
.21
0 -0
CN
TR
\J
00
O
CT
H
.042
.39
it
PT
c*
o
o
80% RH
Loss Tangent
Index
.276
Condition
DIELECTRIC PARAMETERS CHART
H
50% RH
Specimen
4 .1 0
.0864
.0102
?
a
7.00
3.5
5.833
.583
1.16
1.32
.11
21.66
10.83
.055
15.2
7.6
10"12m ho/cm
Vol. Conductivity
?
OS
li
p
0001
(X, ft
o
301'
TABLE
CO
CO
H
H
11.66
110
r
Ill
Active volume conductivity = f E tan 8 / 1.8 x 1012 mho / cm
(4.17)
where
f
= supply frequency
E
= relative permittivity of epoxy resin cast material.
The variation of volume conductivity with relative humidity is shown In Figures
4.21 and 4.22. The active volume conductivity Is a good measure of the property of
dielectric’s responses to high humidity environmental stress. The Increase In the volume
conductivity seen In the graph has been attributed to an Increase In a charge carrier with
the concentration of the absorbed moisture. The volume conductivity also increased
exponentially to higher values after 60 percent relative humidity. Specifically the Increase is
about 8 to 10 times the Initial values at 55 percent relative humidity. Such a steep Increase
in the value is due to the Increase In conductivity of the moist dielectric because of high
activation energy of the charge particles of water, concentration of charge carriers and the
diffusion coefficients.
4.9.3 Dielectric Power loss
The losses In the dielectric was computed using the formula given below:
P = E2 f £ tan 8 / 1.8 x 1012 W / cm3
where
E = Electrical field intensity at the Electrode resin interface,
f = supply frequency
E = relative permittivity
(4.18)
The value of E, estimated using charge simulation method, was found to be
around 15 to 25kV/cm at the electrode-resin interface. The details of the method adopted
are outlined in a later chapter. From the perusal of the Table 4.10 it can be seen that the
dielectric power losses are more in instrument transformers than In bushings. It can also be
pointed out In the case of Instrument transformer that the dielectric power losses are more
pronounced even at 50 percent relative humidity level. These losses contributed to the faster
surface deterioration of the epoxides under high humidity hazards.
4.10.
CONCLUSION
The characterisation of the bulk properties of switchgear components under high
humidity condition has resulted in the following findings:
Hit)
It) 3.
A
T
AT 6b0B U
if
i;
0
AT
113V 2 5 U
/
U O L . C O N T H JC T I U I T Y
I N w h o . 'C N x 1 0
123.
1RU
93
VU.
G3.
4U_
33.
■—i—
■■n"
]
MJ
I. * f
til
V«i
> i Lit 1 ] U L IIUMiMiy
FIG. 4.21
Vv
IN X
~r
UU
UL
—#►
VARIATION OF VOLUME CONDUCTIVITY
WITH RELATIVE HUMIDITY
FOR 11KV POTENTIAL TRANSFORMER
T
UU
T
u\»
16
..
14
A
AT 6508 U
□
AT
10725 U
/
/
12.
t
1A
0
\
o
£tU . 04.
>6.04
r*
/*
r-4.84.
D
fi
z0
U2.04.
... —oA-
1—
nr
mi
t.'.>
v»»
KLi.ritU’i: mifiiunv
~T~
bt)
v:»
in
u:»
1
‘JO
1
■J'j
mo
*
FIG. 4.22 VARIATION OF VOLUME CONDUCTIVITY
WITH RELATIVE HUMIDITY FOR ENTRANT BUSHING
114
The dielectric parameters were fairly constant upto Sixty percent relative
humidity and thereafter they rose exponentially. For example, the Increase In loss factor
was six to ten times the initial value obtained at fifty percent relative humidity.
—
Experiments demonstrate that the humidity hazard is continuously present beyond
sixty five percent relative humidity.
—
Regression models were used to quantify the characteristic changes In the
parameters. A multiple regression analysis on the data base established pqwer fit for
the two factors namely loss factor and leakage current with respect to humidity and
voltage.
—
This relation was used to plot a three dimensional relief model Involving loss factor,
voltage and humidity. The three dimensional surface chart clearly shows abnormal
Increase of dielectric losses with high relative humidity. It also shows a change of
behaviour In the case of impregnated systems (Instrument transformers) and the cast
systems (bushings). For example beyond sixty percent humidity, loss factor Increase
towards higher values with Increase in voltage. While in the case of bushings, the
surface profile gradually Increases upto 70 percent of voltage, thereafter there is a
steep Increase In the value of the loss factor. This finding Is in conformity with the
physical fact that In the case of impregnated components there would be an Increase
In the number of cavities at the resin filler interface and the windings. These cavities
have more affinity to draw moisture and hence the degradation of the dielectric
properties are much more pronounced In the presence of humidity and voltage stress
conditions.
—
The various properties measured during experimentation were brought under a
single specification by coining a new factor termed as ‘Cumulative Characteristics
Factor1 involving loss factor,leakage current and partial discharge levels. Based on
this factor new designs for equipment and resin formulations can be assessed for
their performance under high humidity conditions.
—
Further, based on the above principle a ready reckoner of dielectric parametric chart
Involving loss index, volume conductivity and dielectric power loss was compiled for
usage In the design of equipments under aggravated service conditions.