selection of station insulators

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FARZANEHet al.: SELECTION OF STATION INSULATORS WITH RESPECT TO ICE AND SNOWPART I 265

Fig. 1. Density function of melted-ice conductivity [15].

be wettable. This also may present a criticalflashover conditionin cases of heavily polluted snow or pre-contaminated insulator

surfaces.

2) Amount and Distribution of Ice: The amount of ice has

a significant influence onflashover performance [4], [28], [30],[31]. Normally, a thickness of 530 mm, measured on a standardrod [28], is recommended for ice testing. The thickness should

be decided by utilities depending on their service experience in

exposure [55] and desired reliability [28].

The distribution of ice along the insulators is another impor-

tant parameter. For typical precipitation intensities and duration,

ice and icicles cover the windward side of the insulator and the

opposite side is free of ice. The level of ice bridging between the

sheds determines the size and number of air gaps where initial

discharges and partial arcs are triggered. This level of bridging is

mainly influenced by the electricfield distribution along the in-

sulator, which in turn is governed by the level of applied voltage,the presence of grading rings and grading devices on apparatus

insulators (e.g., capacitors), bus bars, support structures and in-

sulator orientation.

3) Electrical Conductivity of Water: Ions in cloud droplets,

rain, and wet snow may stem from sea salt, nitrates, sulphates

and various industrial pollutants [15].

As concerns insulator icing tests, three types of conductivity

can be identified: (i) applied water conductivity (also calledfreezing water conductivity), (ii) dripping water conductivity,

and (iii) melted ice conductivity. The conductivity of applied

wateris always measured and usually reported.However, it is the

conductivity of the waterfilm on the ice surface, best correlatedwith dripping water conductivity that mainly determines theflashover voltage. This is similar to the insulator surface conduc-tivity in the caseofflashover mechanisms of polluted insulators.

The Norwegian Power Grid Company (STATNETT) pro-

vided information about the conductivity of 122 ice samples

taken from service, between 1987 to 1995 [16]. The probability

density function of the conductivity of these melted-ice samples

is shown in Fig. 1.

These samples, mostly of rime ice, were taken from racks and

towers close to the ground, which thus corresponds to applied

water conductivity. The samples are assumed to be more con-

taminated than snow on the ground, because of the higher con-

tent of rime. The results were statistically evaluated and it wasfound that a log-normal distribution, with mean value of 3.2 (av-

Fig. 2. Relationship between ESDD and distance from seacoast.

TABLE IPOLLUTIONLEVELS ANDRECOMMENDEDSPECIFIC LEAKAGE DISTANCE

ACCORDING TOIEC 60815 AND ESTIMATEDESDD IN-SERVICE LEVELS

erage conductivity 25.4 S/cm) and standard deviation of 0.8

(6.4 S/cm) gave the bestfit. The log-normal distribution for

conductivity can be approximated by (1), as follows:

(1)

B. Clean Ice Deposit on Contaminated Surfaces

Fig. 2 shows monthly rates of increase of contamination

levels as a function of distance from seacoast in Japan [61];

superimposed are three increase rates in ESDD observed for

urban, suburban, and rural areas in Ontario.

IEC 60 815 [56], currently under revision, has defined fourpre-existing pollution levels and specific leakage distance re-

quirements for insulators, as shown in Table I. As well, theIEEE TF has added a recommended range of pollution levels,

expressed as ESDD values, to Table I.

The pre-existing ESDD will dominate the electrical conduc-

tivity of the ice for any (generally thin) layer of ice, freezing

fog or drizzle. Only the surfaces in direct contact with ice ac-

cretion, typically top surfaces and vertical portions of insulators,

will contribute to the conductivity of ice. The contribution of the

ESDD to the ice conductivity can be calculated using [57]

ESDD Area

Volume (2)

Area is in cm , ESDD in g/cm , volume in ml, conductivityin S/cm.


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266 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 20, NO. 1, JANUARY 2005

C. Combined Pollution: Ice and Snow on Contaminated

Surface

One of the most difficult winter conditions for insulators isassociated with build-up of contamination over a period of a

month, as shown in Fig. 2, combined with a moderate amount

of ice or wet-snow accretion. Accretion onto a pre-contami-

nated surface will draw in the surface ions, enhancing or dom-inating the natural conductivity of the ice. Through the process

of freeze-thaw partitioning, these ions will migrate to the ice

surface, lower the surface impedance and reduce theflashovervoltage further as described in Part II of the paper.

An estimate of the increase in ice conductivity is given by

(2), using the ESDD on the top surface of the sheds, the surface

area and volume of the ice cap. For complex situations, where

different surfaces have different ESDD levels, it is suggested

that the ice conductivity and weight be measured directly or

inferred from the dripping water properties.

D. Methods for Evaluating the Electrical Performance ofInsulators and Ranking Under Icing Conditions

Laboratory test methods should simulate a wide range of ice

accretion types and regimes. Two common accretion methods

are (i) by freezing precipitation, where generally icicles are

formed by run-off of supercooled water; and (ii) through con-

solidation of snow or rime in melting conditions. In both cases,

a waterfilm may appear during melting caused by a rise in airtemperature above 0 C or by solar input.

It is generally agreed that the simultaneous presence of a

highly conductive water film and initial air gaps are neededforflashover to occur [32]. Thus, for the evaluation tests, theexperimental procedure should reproduce these conditions

as closely as possible. The procedure for icing test methods

including preparation, exposure, quantification, and evaluationfeatures have been reported in detail in the previous position

paper [28]. The two following methods for evaluating and

comparing the electrical performance of ice-covered insulators

have already been recommended in that work [28].

i) Compare the icing performance of the insulators for a

given test severity.

ii) Compare the icing performance of different insulators at

the maximum possible ice severity (e.g., maximum pos-

sible level of bridging), giving the worst case icing test

performance for each insulator.

The approaches for the evaluation and ranking of insulator

performance following this outline have already been proposed

and discussed in [28].

III. CLIMATE ANDENVIRONMENTAL CONTEXT

A. Climate

The risks offlashover on station insulators are correlated withthe occurrence, in general, of freezing rain, drizzle or wet snow.

Hence, meteorological statistics may provide relevant informa-

tion for the estimation of risk levels. However, it is also impor-

tant to note that the pollution level of such precipitation mayvary significantly, as a result of both long-range [15] and short-

Fig. 3. Annual number of hours with freezing rain.

Fig. 4. Annual number of hours of freezing drizzle.

Fig. 5. Number of hours per year with fog or freezing fog with

C

[33].

range transport of contamination. It is also important to be aware

of enhanced salt content in wet snow, stemming from rough sea

and heavy production of sea spray in the upwind area, prior

to the deposition. Two seemingly equal wet snow or freezing

rain events may therefore have different impacts on electrical

equipment.

Figs. 35 show periodic observations of the annual number ofhours with freezing rain [33], freezing drizzle [58], and freezing

fog, respectively. This data can be used further for the estimationof a number of icing events.


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B. Environment

Insulator pollution standards, such as IEC 60 815 [56],

currently under revision, provide some guidance in Table I re-

garding the leakage distance needed for reliable long-term use

as a function of insulator pollution level. The present IEEE TF

recommends the use of four pollution levels in Table I as ranges

of equivalent salt deposit density (ESDD). A revised standardIEC 60 815 will also take into account another important

pollution parameter, i.e., nonsoluble deposit density (NSDD).

Equivalent advice is given below for icing and cold-fog con-

ditions. However, there is little guidance for establishing the

anticipated ESDD level, except for test values near the sea [13].

Winter exposure is somewhat similar to desert exposure in

the sense that the top surfaces are not exposed to rain for ex-

tended periods of weeks or months, depending on the local am-

bient temperature. Short-term exposure of several weeks tends

to have a linear relationship between ESDD and duration, while

the ESDD levels off for long-term exposure of 6 to 18 months.

Under winter conditions, top-surface ESDD values increase tolevels higher than bottom surfaces.The rate of increase of ESDD

with time must be established from the local pollution environ-

ment, and used in conjunction with climate records for the me-

dian or extreme duration of dry periods.

A model for pollution exposure at substations can be de-

rived from the massflux of nearby point, line, or area pollutionsources. An example of a point source would be a chimney lo-

cated several kilometers upwind of the station. Road salting on

expressways ata level of metrictons ofNaClper lanekm is

a common line source, and generating stations located near the

sea are exposed to a half-plane source whenever the wind blows

in from the ocean. The source strength of the half-plane formed

by the sea is strongly influenced by wind speeds and wave ac-tion [13].

Thewind rosette,a polar plot of wind speed and directionover the exposure period, is used in common surface deposi-

tion models. The probability of being directly downwind from

a point source is fairly low, so the exposures from line and area

sources usually dominate the calculation. When such models are

applied on a sea surface area, it is important to consider the dif-

ferences in surface winds and the wind direction at 5001000 mlevels, where the main transportation of the pollution, as well

as the release of precipitation, takes place. Due to the veering

of wind with height, the local surface wind direction may be as

much as 90 (anticlockwise on the northern hemisphere) off thetransport direction of sea salt aloft, depending on the roughness

of the terrain.

IV. EFFECTS OF ATMOSPHERICICE ANDPOLLUTION WITH

RESPECT TOINSULATORPARAMETERS

A. Exposed Surface to Icing

The surface exposed to icing is normally a product of insu-

lator dry arcing distance and diameter, with some adjustment for

different shed profiles. Whenever the leakage distance is fullybridged with ice or snow, the insulation strength is established

mainly by the electrical properties of the ice or snow, and the ex-posed surface area. Under the most severe meteorological con-

ditions that lead to slow melting, (3) and (4) have approximated

the insulation strength [60]:

kV m dry arc for ice (3)

kV m dry arc for snow. (4)

ISP is the product of the ice layer weight (g/cm of dry arcing

distance) and the ice layer conductivity ( S/cm). Reference [8]

also notes a nonlinear relationship for EHV insulators.

B. Diameter

North American experience during heavy ice storms suggests

that station insulators of greater diameter are more prone to

iceflashover than neighboring post insulators of smaller diam-eter [34]. The relationship between ice accretion diameter and

flashover strength for afixed shed profile was recently studied[35] and can be expressed as:

V kV m dry arc W (5)

where W is the ice width (525 cm).A rough relation, as recently established during the experi-

ment at the CIGELE laboratory (UQAC), can be made between

the ice thickness on the reference cylinder and the weight of ac-

creted ice per meter of 300-mm uniform shed profile station postinsulators, as follows:

Weight Thickness (6)

During this experiment the applied-water conductivity was

80 S/cm and the maximum ice thickness was 20 mm on a

rotating monitoring cylinder.

C. Shed Spacing

Typically, station post insulators differ from line insulators of

the same voltage class in their shed spacing. Cap-and-pin line

insulators with 146-mm construction height have a spacing of

about 100 mm from the bottom of one skirt to the top surface

of the next insulator. Sheds on typical ceramic post station insu-

lators are about 50 mm apart. It is obvious that, with the same

ice exposure and similar diameter, ice bridging will occur much

more rapidly on station insulators. Once the insulators are fully

bridged, the accumulation rates on station and line insulators are

almost the same, and their electrical performance tends to con-

verge. This means stations will have more problems than lines,

especially in areas where there are several moderate ice storms

per year.

D. Shed Profile

Additional leakage distance is provided on insulators by, for

example, forming bottom parts of the sheds with deep under-

ribs. The leakage distance thus obtained is 2 or 3 times longer

than the dry arcing distance. This parameter improves pollution

performance for a wide range of pollution conditions when no

icing occurs.

Many different shed profiles can be obtained for the samedry-arcing distance. Under conditions of heavy icing, deeper

sheds tend to pack more ice and snow, before bridging, leadingto a greater ice weight (g/cm of dry arcing distance) for the same


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Fig. 6. Comparison of Clean-Fog and Cold-Fog flashover strength as afunction of ESDD.

icing exposure, but this effect is less important than insulator di-

ameter. Alternating diameters of sheds and multi-cone profilescan improve ice and snowflashover performance in some con-ditions by delaying the onset of ice bridging, compared to de-

signs with uniform shed profile [1], [18], [21], [24], [36], [42].Booster sheds of various designs have also been evaluated for

this function.

Tests on a wide range of shed profiles are reported for 24-kVclass insulators [10]. These results suggest that the influence ofshed profile onflashover voltage is relatively limited comparedto the parameters of the ice deposit.

E. Leakage Distance

Leakage distance and the so-calledprotected leakage dis-tance play an important role in pollutionflashover strength forvery light levels of freezing rain, drizzle, and fog accumulation,

but are less important when ice or snow bridging of insulator

shed spacing occurs. In the worst case, the effective leakage dis-

tance is reduced to the dry arcing distance.

Generally, there is a reasonable agreement between the results

of cold-fog tests versus clean-fog tests, according to IEC 60507,

with cold-fog giving strengths that are 1020% lower than cleanfog results [40]. Empirically the electricalflashover strength ofleakage distance in cold-fog conditions, as a function of ESDD,

is given by (7).

V kV/m leakage ESDD (7)

ESDD in g/cm .

Fig. 6 shows the strength predicted by (7), superimposed on

a summary of conventional clean-fog strength, replotted from

data in [40] using a rough conversion factor of 2.5x for the re-

lation between leakage and arcing distance.

F. Orientation

Testing experience for iced station insulators has generally

been for vertical orientations, since these are most common.

Results for V-strings of suspension insulators [37] suggest an

improvement in strength for oblique angles, compared to ver-tical. Operational and testing experience with wetflashover of

Fig. 7. Icing test on a T-shaped circuit-breaker at (a) open position and(b) closed position.

wall bushings [41] suggest that horizontal orientation may per-

form in unexpected ways compared to vertical post insulators.

Ice or snow accretion on the top of the horizontal insulator will

bridge the dry arcing distance at a lower precipitation amount.

However, overall ice thickness will not build up as fast because

icicles will develop vertically downward, not across the shed

spacings, see example in Fig. 7.

The placement of two station insulators in close proximity

will tend to degrade the performance, compared to the single

insulator. This is true for both heavy snow and ice accretion and

was previously noted for line insulators [38].

A special question for orientation concerns the ice perfor-

mance of T-shaped circuit-breakers, or disconnecting circuit-

breakers. To simulate the service case, it was decided to perform

ice accretion on both chambers (horizontal part) and support in-

sulator (vertical part). The Ice Progressive Stress Method (IPS)

was used for testing [20], [21]. The test results of the breaker

shown in Fig. 7 demonstrated a drastic difference in ice distri-

bution at the breaker in open and closed operating positions (see

examples). This also resulted in differentflashover voltages inopen and closed positions.

G. Surface Material and Finish

The ice accretion onset and rate tend to be relatively unaf-

fected by insulator surface materials, whether ceramic or non-

ceramic. However, the electrical performance of nonceramic

or silicone-coated ceramic insulators is somewhat better, es-

pecially under conditions of partial ice bridging [2], [39]. An-

other surface that improves performance under icing conditions

is a semi-conductive glaze with a surface resistivity of about

1030 M [63]. This surface suppresses the onset of arcing[39], [42][46], [59] and is more effective under heavy ice con-ditions than silicone.


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The surfacefinish of the insulator affects performance inan indirect way. Test results show that ice slides off more

easily from smooth, new ceramic surfaces than from aged

porcelain, silicone-coated porcelain or nonceramic sheds of the

same shape. This shortens the time that melting ice is on the

insulators and proportionally reduces risk offlashover.

V. CONCLUSION

This first part of the paper has provided the physical andchemical background needed to describe the electricalflashoveron iced insulators. Several ice and snow parameters, including

type and density, amount and distribution, water conductivity,

as well as certain insulator parameters, such as diameter, shed

spacing and profile, insulator orientation, and exposed surfaceto ice, influenceflashover mechanisms and performance.

The most important aspect of icing performance is specif-

ically related to the partial or complete bridging of insulator

leakage distance by icicles extending from shed to shed. Under

fully bridged conditions, the insulator leakage distance is re-duced nearly to the dry arcing distance. This drastic reduction

in leakage distance (by a factor of about three) can lead to se-

vere but sporadic ice-inducedflashovers at a number of affectedutilities.

Flashover can also occur along the leakage distance of the sta-

tion insulator under conditions of light or moderate icing com-

bined with pre-deposited pollution. For many cold regions, the

maximum levels of insulator contamination (ESDD) are reached

in the winter. The exposure can be aggravated from the effects

of high winds and road salting. An additional margin of 1020%over the existing design recommendations for specific leakagedistance for adequate clean-fog performance is needed for suffi-cient cold-fog performance on vertical ceramic post insulators.

PartII ofthe paper [62] applies the environmental parameters,

as described in Part I, to the selection process and mitigation

options.

ACKNOWLEDGMENT

The Chairman would like to thank Dr. W. A. Chisholm espe-

cially for his enthusiasm, dedication, and contributions, to Tom

Grisham for helping with IEEE administration and to Dr. Igor

Gutman for links to the IEC 60 815 revision process.

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