Formation & Characterization of Dry Bands on Polluted Insulators

724 IEEE Transactions on Dielectrics and Electrical Insulation
Vol. 6 No. 5. October 1999
Formation and Characterization of Dry
Bands in Clean Fog on Polluted Insulators
D. L. Williams, A. Haddad, A. R. Rowlands, H. M. Young
and R. T. Waters
Electrical Division, Schwi of Engineering, University ol Wales Cardill, Cardill, UK
ABSTRACT
The formation and development of dry bands can best be studied by modifying the standard
test procedures. When such controlled behavior is allied with synchronized optical and elec-
trical recordings, then characterization of the pre-formative leakage current, the transient phe-
nomena associated with partial arcs across dry bands, the location of partial arcs and the volt-
age drop across dry bands can be determined. Interpretation of test data is greatly aided by
finite element computation of insulating structures with a conducting surface layer. When this
layer is continuous, this allows straightforward prediction of dry band formation under wet-
ting conditions. Following formation, dry bands can be represented by discontinuities in this
layer. Simulation of dry bands with various lengths, when combined with the test data, enables
partial arc voltage gradients to be quantified. These results will be discussed in the context of
previous work on the pollution flashover mechanism of ceramic insulators.
1 INTRODUCTION
a uartial arc is simulated in series with a resistor reoresentine the ~011~-
AILURES due to environmental pollution on electricity networks
have increased with network sizes and industrial emissions. In-
creasingly, generation is being created in coastal sites for easy access to
cooling water and transport olfuel (gas). This has led to severe saline
pollution and sea fog threats. When first installed, an insulator is essen-
tially non-conductive, but with deposited air-borne particles a pollution
layer will be formed on its surface. Such a layer is composed of largely
inert and often conductive compounds such as carbon, metal oxides and
salts. The layer may form a conductive path, usually in the presence of
moisture. The resulting current flow under system voltages will cause
dry banding, partial arcing and may lead to complete flashover.
Thereis anextensive and well-established literature [l-311 on the per-
formance of ceramic insulators under polluted conditions. Investiga-
tions have been conducted under direct, alternating and impulse volt-
age conditions [Z-6,251. However, repeatability of the earlier tests was
difficult. For this reason, a Joint Task force of IEEE and CIGRE work-
ing groups was formed to “define more rigidly a clean fog test proce-
dure which would be adequate for widespread use and adoption as stan-
dard”. A final report was published by the task force in 1987 (71, sum-
marizing test results obtained at nine different laboratories on polluted
ceramic insulators using the clean fog method. This document was used
in the revision of IEC 507 - 1991 [E]. This standard is used as a basis for
the work presented in this paper.
Many researchers have formulated models to evaluate the process
of electrical discharge leading to a complete flashover of the insulator.
These models can be categorized according to the type of voltage consid-
ered. One of the first dc models was described by Obenaus [9] in which
F
YL
tion. The model was then improved by Neumarker [IO] who assumed a
uniform pollution layer. Sundararajan and Gorur [ll] further developed
this approach to consider non-uniform pollution layers and to include
Hampton’s criterion [I21 for arc propagation. Recent work (131 on this
concept of gradient criterion claims that Hampton’s criterion (that the
field in the pollution layer is higher than that in the arc Ep 2 Ea) is not a
sufficient condition for complete flashover to take place. Rizk [14] gives
an excellent review of the mathematical approaches used to model insu-
lators under polluted conditions. Under alternating voltages, the arcing
of polluted insulators is different from that under direct voltages since
arc re-ignition is required after current zeros. Rizk 1141 summarized ac
models under energy re-ignition models and dielectric re-ignition mod-
els and indicated good agreement with experiments. The dc approach
yielded much lower values for minimum flashover voltages. It is clear
that existing models provide a means of rationalizing experiments but
cannot substitute for testing; many parameters (temperature, nature of
pollutant, particle size, wetting, etc.) are inevitably neglected for sim-
plification of models.
Other analytical models have been published also. In [4], the charge
simulation method was used to study the electrical field around a pol-
luted insulator under ac and impulse stresses. Similar high stresses are
shown at the edges of dry bands irrespective of their location on the
insulator surface. Power frequency voltages caused higher stress than
switching and lightning impulses. Cline et al. [15] used a neural net-
work technique to predict imminence of flashover on substation insu-
lators. The system uses input from an ultrasound monitor and a leak-
age current transducer. These authors [I51 showed that 89% correct pre-
1070-98781991 $3.00 0 1999 IEEE

IEEE Transactions on Dielectrics and Electn’cal Insulation Vol. 6 No. 5, October 1999 725
dictions were given by their system under safe operation of the system
but only 75% correct predictions were made under hazardous operation
when the voltage level is closer to flashover voltage. Naito et al. [16]
used a ‘static probabilistic’ approach to evaluate the contamination per.
formance of insulators based upon the equivalent salt deposit density
(ESDD) and the relative humidity
a
I -
Figure 1, Schematic layout of fog chamber. (1) HV conductor, (2) Fog
chamber shell, (3) Pressure outlet, (4) Spray nozzles; (5) Viewing door, (6)
Test insulator, (7) Isolating insulator; (8) Leakage current: to shunt resistor;
(9) grounding mesh; (10) Sump, (11) Water purge nozzles.
Figure 2. Circuit diagram of the test setup. (1) Voltage regulator, (2) Iso-
lating transformer, (3) Low-pass filter, (4) Test kansformer, (5) HV arm of re-
sistive divider, (6) Capacitive divider, (7) Current limiting resistor, (8) Test
insulator; (9) Variable measuring resistance, (10) Protection circuit: gas dis-
charge tube, metal oxide varistor, fast transient suppressor; (11) to digital
storage oscilloscope; (12) to rms voltmeter.
The oresent exoerimentation is desimed to vield not onlv flashover
taneously with the applied voltage is possible. It is shown that simple
partial arc models can account for the measured dependence of dry band
length on the applied voltage. In addition, electrodynamic finite ele-
ment modeling (FEM) software [17] was used to simulate the tested in-
sulator with its pollution layer and to predict dry band formation and
development. The critical conditions of voltage and dry band length for
insulator flashover provide data for comparison with dc and ac arc mod-
els.
2 TEST FACILITY
Since artificial pollution tests were first devised in the 1960’s, util-
ities and insulator companies throughout the world have constructed
fog chambers ranging in size from 10 to 1000 m3.
The fog chamber used for the tests described here is shown in Fig-
ure 1. The chamber is manufactured from polypropylene and has a vol-
ume of 12 m3. The electrode arrangement within the chamber was de-
signed to provide an axi-symmetric system to aid computational mod-
eling.
The test and measurement circuit is shown in Figure 2. The test volt-
age is supplied by a 220/100 kV, 7.5 kVA single phase test transformer.
For flashover tests, limitations in the power source ratings can be over-
come by additional shunt capacitance [30]. Voltage and leakage current
waveforms were obtained via a capacitive divider (83001) and measur-
ing resistance. Both were recorded on a digital storage oscilloscope.
In addition to electrical measurements, a visual analysis is also con-
ducted using a high-resolution camera and video recording system. The
camera can operate in low illumination levels and has a 15 to 150 mm
motorized zoom lens. Images seen by the camera are recorded on an
S-VHS time-lapse video recorder. Periods of up to 483 h can be moni-
tored to record any discharge activity The visual measurement system
continuously monitors the test object.
A system has been developed [18] to synchronize the electrical and
visual signals recorded throughout each test (see Figure 3).
3 CLEAN FOG POLLUTION
TESTS
Several artificial methodologies exist, two of which have become
standardized [8]. One uses a salt fog and the other a clean fog on a pre-
applied pollution layer. This work was conducted using clean fog pro-
cedures for maximum controllability (see Flow Chart, Figure 13).
3.1 PREPARATION
A solid laver uollutant of Kaolin composition is audied to the test
voltage‘s but to foilow the formation anYd the development &dry bands
during clean fog tests on polluted ceramic insulators. The solid layer
method was adopted in the test program. In addition to monitoring the
voltage and current signals, a synchronizedvisualmonitoringsystem is
used, which allows the study of dry bands and their arcing in a unique
way Consequently the measurement of the arc (dry band) length simul-
samples by ;dipping method. Table 1 gik details ofpollutant quanti-
ties used in these tests. The wetting rate was set prior to the layer con-
ductivity assessment. A moderate wetting rate was employed for these
tests (water-flow rate of 3.5 l/h and nozzle air pressure of 0.14 MPa).
Table 2 summarizes the setting for the fog chamber. Figure 4 shows the
measured wetting rates.

726
Williams et al.: Formation and Characterization of Dry Bands
material
type
Table 1, Characteristics of inert materials used in solid layer suspension.
Composition in %wt, size in pm.
Composition Gran do met cy
A1203 I Si02 I Fez03 I Other oxides particle size
Kaolin
0.5% >IO
0.01% >55
38 48 0.G 13.4 98%>44
Table 2. Input rates for wetting action tests.
4
4
2
high v0Itase
+----1
+l+l I
4 ?cl I
t
3A
Figure 3. Synchronization system (1) Fog chamber; (2) Test insulator;
(3) Fiber optic line; (4) Light emitting diode (5) Signal (6) Measuring unit
(7) Camera, (8) Video recorder (9) monitor (10) Digital storage oscilloscope
(11) Computer for electrical data acquisition; (12) Computer for visual data
acquisition.
3.2 HVTESTS
To investigate dry band formation and development, a modified
method of test voltage application was adopted. The test voltage is ap-
plied in 2% steps from zero to the point at which insulator flashover is
imminent. Each step is maintained for a period of 1 min. Ifno dry band
discharge (partial arc) is observed within this time, then the voltage is in-
creased. If partial arcs are observed, then the test voltage is maintained
until the partial arcs cease for a further minute or the partial arc length
appears constant. This procedure was used specifically to investigate
the relationship between dry band length and the test voltage.
4 DRY BAND LENGTH
Fieure 5 shows the results of drv band measurements conducted on
MEASUREMENTS
0 2C 40 60 80 iM IZO
The ( t I mimites
Figure 4. Wetting ratecurves,
and DB2. A linear relationship exists between the test voltage and dry
band length. The dry band length was approximated from the surface
creepage length over which partial arcs extended. The points at which
partial arc discharges terminated were noted from the video sequences,
and the partial-arc length was deduced geometrically The total creep-
age length of this insulator was 32 cm.
.-
,o . . . . . . . . . . . ..
0 "" ''''1',,'l',,,J,,',I,,/,
0 2 4 8
6 10 I2
Drybandlength (x) em
Figure 5. The variation of partial arc voltage with dry band length,
5 COMPARISON WITH
PARTIAL-ARC MODELING AND
FLASHOVER CRITERIA
The experimental data of Figure 5 were acquired along with record-
ing of the partial-arc peak current (typically 50 mA) with low-voltage
leakage-current measurements. At the pollution levels used in the tests,
leakage currents were -1 mA at 0.5 kV.
These studies provide a direct opportunity to quantify the dc and
ac partial arc theories referred to in Section 1. Thus, the dc theory of
Obenaus [9] and Nenmarker [lo] correlates the applied voltage U, the
partial-arc length 2, the arc current i and the leakage path resistance.
The resistance per unit length rP is assumed to be constant over the
whole insulator leakage path L. Then a static-arc characteristic, reduced
here for present purposes to a simple inverse form
(1)
Ax
U,,, = -
i
with A constant. willvield theNeumarker relationshivbetween aDolied
U ., II
two identical standard-profile cap-and-pin insulators referred to as DB1
voltage and the maximum sustainable partial-arc length (simplified for

IEEE Transactions on Dielectrics and Electrical Insulation
Vol. 6 No. 5, October 1999 727
the given U,,, function)
A critical flashover voltage criterion U = U, can be obtained by
setting dU/dx = 0. This provides several verifiable relationships for
U,, the critical partial-arc length z,, and the critical current i,.
5.1 PARTIAL-ARC LENGTH
The straightforward criterion
(3)
L
zc = -
2
compares with the measured values zc = 11 cm and L = 32 cm.
5.2 PARTIAL-ARC CURRENT
(4)
The average peak arc current for the polluted insulator was -50 mA.
Since thevalue~fr~fromthelow-voltageleakagetestswas-15 kWcm,
therelationship fori, willbesatisfiedbyachoiceof A-37. Thisisrea-
sonable but not directly justifiable. It can, however, be used to calculate
a flashover voltage value as follows.
5.3 FLASHOVER VOLTAGE
U, = L& (5)
Using A = 37, a voltage of U, = 24 kV is obtained, which underes-
timates the value of -40 kV generally found here.
Rizk [14] has already noted the underestimate of flashover voltage
usually yielded by dc arc models. He pointed out that maintenance of
the low-current partial arc under alternating voltages requires sufficient
voltage to re-ignite the arc after current zeros. Such maintenance by re-
ignition has been found in present tests. Rizk analyzed the minimum
requirement for dielectric breakdown of the arc path for a uniformly pol-
luted insulator, and obtained a relationship
This gives U, = 35 kV for the present data, which is closer to the mea-
sured values than the dc model.
6 FINITE ELEMENT MODELING
U, = 23Lrj.4 (6)
OF DRY BAND DEVELOPMENT
It is now well known that the flashover mechanism of an insulator can
be described in four basic stages: conduction, dry band formation, dry
band arcing and insulator flashover. During these four stages, both elec-
tric field and voltage distributions along the insulator surface change
significantly
Measurement of electric field distribution along insulator lengths has
been the subject of many investigations [19]. Metallic probes are known
to distort the field causing anerror in the measurement. ODtical urobes.
1201 that their usage in fog/wet environment is affected by surface wet-
ting of the optical crystal.
Computation of field and voltage changes can be accurately achieved
by commercially available software provided that correct input data are
provided. Billings and Storey [21] were among the first investigators to
use numerical techniques to compute the voltage distribution on pol-
luted insulators. They showed that following the formation of a dry
band, local stresses rose tenfold from 0.4 kV/cm to 4 kV/cm which was
suggested as an explanation of some punctures observed on insulators.
Two recent studies [22,23] use the finite element technique to calculate
voltage and field on porcelain bushings. They both provide valuable in-
formation on local changes and their correlation to observed phenomena
and measured quantities. However, it should be emphasized that since
pollution of insulators is not uniform [31], only idealized conditions can
be simulated accurately Here, we model the growth of the dry band us-
ing a commercially available finite element software [17] and compare
the results with the measurements shown in earlier Sections.
6.1 INSULATOR PROFILE
A cap-and-pin insulator as shown in Figure 6 is used in the test pro-
gram and modeled to study the dry band development. For consistency
with the artificial pollution tests conducted in the fog chamber, the cap
electrode was modeled as the HV terminal and the pin as the low voltage
terminal.
On the recommendation of the insulator manufacturer [24], the sand-
cement sections, which secure the electrodes to the insulating body,
were not considered as separate regions. Instead, they were modeled
as an extension of the adjacent electrode regions. The pollution layer is
considered to be uniform and of 0.5 mm thickness.
,,,
$1 52 SI
(7) 0.32 m (CAP)
(6) 0.27 m
(21 0.05 m'
(11 n m (PIN)-
010.10m'~
(410.17m
Figure 6. Insulator profile and dimensions
6.2 MATERIAL PROPERTIES
The electrical urouerties of the materials used in this comuutation
" .I
on the other hand, have an advantage in that they do not interfere with
the field distribution when introduced in a circuit. However, it is found are summarized in Table
3 below These values are similar to those
used in [23]. When conducting artificial pollution tests, the HV stress

728
Williams et al.: Formation and Characterization of Dry Bands
Table 3. Material properties used for modeling the cap and pin insulator.
is applied to the insulator during the period of maximum conductiv-
ity, which is determined from the wetting rate tests. At this instant, the
leakage current on the insulator surface is mainly resistive. The max-
imum conductivity is obtained when the dry pollution layer saturates
withwaterhencetheuseofrelativepermittivityof 80. Aconductanceof
6~10-~ S/m was measured during the tests with a salt deposit density
(SDD) of 0.07 mgicm'.
6.3 VOLTAGE AND ELECTRIC FIELD
PROFILES
The field and voltage distributions were first computed for a clean
and uniformly polluted insulator surface without dry bands. A unit
voltage is applied to the cap while the pin is held at zero potential. Fig-
ures 7(a) and 7(b) show the computed equipotentials for the two cases.
The resulting voltage and modulus of the electric field profiles along the
surface are shown in Figure 8.
Figure 7. Computed equipotential lines (a) clean insulator surface, (b)
polluted insulator surface.
From the above Figures, it can be seen that more than 75% of the volt-
age fallisconcentrated between thepinelectrodeand the first inner skirt
(S1 onFigure 6), viz. over 40% of the total creepage path. Almost 10%
of the voltage fall is concentrated around the cap electrode extending
- El-" runare
- VDnVMSYrioir
'A'^
.....,, .~ ..,...,..,...
1.0 "T " ' " " I ' ' ~ " ' , " " ~ ' ' '1 7
, ..
.. ...
. .. .. .
-
0 0.05 0.10 0,lS 0.20 0.21 0.30 0.35
Crespagepach (XI in
Figure 8. Computed voltage distributions for clean and polluted insu-
lators
over 10% of the creepage path. When the insulator is polluted, the resis-
tive leakage current redistributes the voltage and smoothes, as expected,
the undulations seen on the clean insulator voltage profile. However,
the stress at both the cap and pin terminals is reduced only marginally
As can be seen in Figure 8, this very non-uniform voltage distribution
will result in extremely high electric field magnitudes around the cap
and pin regions. These magnitudes can be sufficient for the formation
of dry bands.
For the clean insulators, individual equipotential lines cross the insu-
lator profile, hence causing the undulations seen on the voltage profile.
These undulations are accompanied by electric field reversal along the
surface at the insulator skirts, i.e. lines can be directed either from air
into porcelain or from porcelain out to air (see Figure 9).
The cause of the peaks on the field curve of the polluted surface is not
immediately apparent from the equipotential plot but they arise because
the field lines at these locations (5,lO and 17 cm) lie along the surface.
7 PREDICTION OF DRY BAND
LOCATIONS
7.1 POWER CONSIDERATIONS
It is now widely accepted that, as shown on simplified flat plate sys-
tems [25-271, non-uniform moisture loss and pollution layers of IOW
resistance promote dry band formation. The energy necessary for dry
band formation is determined by surface Conductivity and the applied
test voltage [28].
Dry band formation is essentially governed by the power dissipated
in the pollution layer due to ohmic heating as against the rate of mois-
ture deposition on the polluted surface. The ohmic heating is the source
of the rate of energy dissipation needed for evaporation. Since the evap-
oration rate must not be less than the wetting rate, we have [29]
P, = uE2t, (7)
P, = lw (8)
where P, is the power dissipated per square meter along the pollution
layer (Wim'). Pe is the power per square meter necessary to evaporate
a unit mass of water (W/m2). E is the electric field strength along the
pollutionlayer (V/m). U is theconductivity of the pollutionlayer (S/m).