Insulator Leakage Distance Dimensioning in Areas of ... - IEEE Xplore

IEEE Transactions on Dielectrics and Electrical Insulation

Vol. 14, No. 6; December 2007

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Insulator Leakage Distance Dimensioning in Areas of Winter Contamination using Cold-fog Test Results William A. Chisholm Kinectrics 800 Kipling Ave. KL206 Toronto, ON, M8Z 6C4, Canada

ABSTRACT Electrical utilities have adapted insulator leakage distances to a wide range of contamination environments. This has led to development of guidelines for unified specific creepage distance, for example in IEC 60815. Under freezing conditions, a number of factors change. The surface conductivity of ice is much lower than that of water, but there is the countervailing possibility of accumulating a significant amount of ice. One of the crossover points between applications occurs when a very light layer of hoar frost or freezing fog, simulated in a “cold fog” test, fully wets a precontaminated insulator. The leakage distance requirements for these conditions are compared with those suggested for conventional clean-fog requirements. The possibility of treating fog accretion as a form of non-soluble deposit density is explored. Index Terms - Insulator contamination, surface cleaning, dielectric breakdown, conductivity.

1 INTRODUCTION THE problem of setting a suitable insulator leakage distance for local conditions has interested electrical power utilities for more than 100 years. Stress, described by insulator surface pre-contamination, is coordinated with flashover strength for reference pollution levels in this process. Guidelines from service experience, supported by lab tests, have been refined recently in IEC 60815 [1] and related documents. This paper reviews the cold-fog flashover process, consolidates test results and compares them with recommended unified specific creepage distances [1].

Sampling methods including the rag-wipe technique are described in standards for pollution testing of insulators [1,2]. Extra care is necessary when sampling low pollution levels (less than 0.03 mg/cm2 or 30 μg/cm2) for EHV studies. 1.2 EXISTING CLEAN-FOG DESIGN PROCESS The most up-to-date design process for leakage distance classifies local measurements of ESDD and NSDD into five rough categories, ranging from very light to very heavy, as shown in Figures 1 and 2.

1.1 PRE-CONTAMINATION TERMS Surface pre-contamination on insulators is measured by equivalent salt deposit density (ESDD) for water-soluble ions and refined with values of non-soluble deposit density (NSDD) for insoluble material. Under clean or salt fog conditions, the non-soluble deposit (NSD) tends to stabilize wetting of the electrically conductive equivalent salt deposit (ESD). Models for electrical flashover of the resistive layer formed when ESD is wetted are well developed and these models describe laboratory testing experience with acceptable accuracy. Manuscript received on 9 December 2006, in final form 29 January 2006.

Figure 1: Relation between NSDD / ESDD and Site Pollution Severity for Cap-and-Pin Disc Insulators [1]. The values of 0,1 mg/cm2 have commas in IEC notation that represent decimal points in IEEE practice.

1070-9878/07/$25.00 © 2007 IEEE

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W. A. Chisholm: Insulator Leakage Distance Dimensioning in Areas of Winter Contamination using Cold-fog Test Results

•

Wind speeds are higher in winter, and pollution accumulation varies as the third power of wind speed in one model [5].

•

Pollution accumulates rapidly on top surfaces, bringing their ESDD levels above bottom-surface levels.

•

Rate of increase of ESDD is increased by road salt.

Balancing these negative factors are two strong influences of reduced temperature on the resistance of the polluted layer: •

Ions lose their electrical conductivity with temperature as shown in Table 1. A 60-70% increase in resistance of a contamination layer from 20°C to 0°C should give a 20% increase in electrical flashover strength.

•

At temperatures below 0ºC, the conductivity of the ice falls by several orders of magnitude. Sea water has a median electrical conductivity of 3300 mS/m; arctic ice varies from 0.01 to 0.3 mS/m [3].

Figure 2: Relation between NSDD / ESDD and Site Pollution Severity for Long-Rod Insulators [1]. The values of 0,1 mg/cm2 have commas in IEC notation that represent decimal points in IEEE practice.

Figure 2 describes the relation for long-rod line insulators, which are closer in shape and performance to station post insulators than strings of cap-an-pin insulators considered in Figure 1. The dividing lines between zones in Figures 1 and 2 have ±20% uncertainty in ESDD. Also, in both figures, the presence of heavy non-soluble deposits can influence the classification. Long-rod ceramic insulators, and station posts, tend to have lower pollution accumulation and lower flashover strength and this is reflected in the revised classifications found in Figure 2. Once a pollution zone has been established, the IEC standard [1] describes recommended “unified specific creepage distance” (USCD) values, expressed in mm of leakage distance per kV of line-to-ground voltage. Laboratory test results are expressed as a 50% flashover gradient V50% in units of kV rms, line to ground, per meter of leakage distance. The conversion of V50% (kV per m) to USCD flashover (mm per kV) is given by:

USCD50% = 1000

V50%

For modest 0.05 mg/cm values of NSDD, associated with the standard 40 g/l kaolin added to precontamination slurries [2], the relation between critical flashover voltage stress V50% (kV rms line to ground per m of leakage distance) on cap-and-pin insulator strings can be described as a function of ESDD (mg/cm2) by [22]: −0.36

*Strength increase estimated using exponent of -0.36.

(1)

2

V50% = 12.8 ⋅ (ESDD )

Table 1. Equivalent Conductance of Ions at 0° and 20°C [4] Strength Ratio, μS/cm per μS/cm per Increase* Ion 20°C to μEq/L at μEq/L at at 0°C 0°C 0°C 20°C Hydrogen (H+) 0.239 0.326 1.36 12% Chloride (Cl -) 0.041 0.069 1.67 20% Sodium (Na+) 0.026 0.045 1.69 21% Potassium (K+) 0.041 0.067 1.64 19% Magnesium (Mg 2+) 0.029 0.047 1.63 19% Ammonium (NH4 +) 0.040 0.066 1.65 20% Carbonate (CO3 2-) 0.036 0.063 1.75 22% Nitrate (NO3 -) 0.040 0.065 1.62 19% Sulfate (SO4 2-) 0.041 0.072 1.75 22% Calcium (Ca 2+) 0.031 0.053 1.75 21% Hydroxyl (OH-) 0.105 0.178 1.70 21%

(2)

1.3 TEMPERATURE EFFECTS NEAR 0ºC Some factors may decrease the electrical strength of insulation in winter conditions. In the North American winter climate, these may include the following aspects. •

Pressure and absolute humidity (98.5 kPa, 1 g/m3) are low in freezing rain events, compared to standard conditions of 101.3 kPa, 11 g/m3 and 20°C [2]. This should decrease electrical strength of air slightly.

•

The longest duration of exposure without rain is often in the winter when snow is on the ground.

1.4 ICE AS A NON-SOLUBLE DEPOSIT Thin accumulation of fog as frost in freezing conditions has some aspects in common with a non-soluble deposit. Below freezing, the electrical conductivity of the frost is very low. Even at the melting point, the contribution of ions from the fog to the electrical conductivity of a surface pollution layer can be neglected. At the melting point, what matters is that the layer is fully wetted but also stabilized. The sublimation time of the frost layer is long enough that arcs form and propagate without the dry-banding process typical of conventional contamination activity. A thin frost layer of 50 μm thickness has a deposit density of 5 mg/cm2. Figure 3 shows the measured changes in withstand voltage with NSDD using inert Tonoko [5]. If an ice deposit of 5 mg/cm2 is considered to be just as insoluble (because it is frozen), it would also reduce the withstand voltage of an insulator by about 30% in moderate or heavy levels of ESDD, compared to the strength with the reference density of 0.1 mg/cm2.



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swept through the area, causing repeated flashovers leading to a total of 4.3 system-minutes of outages. This was a significant fraction of the annual target reliability level of 15 system-minutes per year for a large utility in 1989. Comparisons of the natural ice in tipping-bucket rain gauges with ice caps and drip water from insulators led to the conclusion that surface precontamination ions were the source of more than 90% of the total electrical conductivity.

3 TEST RESULTS WITH COLD FOG 3.1 COLD-FOG TEST METHOD A specialized test method was developed to reproduce line voltage flashovers. This method [6] consists of: •

Figure 3: Reduction in Withstand Voltage of Contaminated Long-Rod Insulators as a function of Non-Soluble Deposit Density (NSDD) [5]

2 FIELD EXPERIENCE IN COLD FOG A series of EHV flashovers in the 1980s and 1990s led to the realization that station posts did not perform as well as hoped in ice and winter fog conditions [6]. The first evidence of this actually was noted in the UK 275 kV transmission system, with widespread flashovers after hoar frost accumulated on insulators that had been exposed without rain for a month [7]. Ice flashover was also tested at Project UHV using water of 2.9 mS/m conductivity [8]. The electrical performance of insulators under heavy ice or snow conditions, leading to a reduction in the effective dry arc distance, has widespread utility interest and has received detailed study, including IEEE and CIGRE summaries [9-15]. In contrast, selection of leakage distance for winter conditions is generally not as restrictive. 2.1 275 KV HOAR-FROST EVENT, 1962-1963 After an abnormally dry autumn in 1962, Jolly [7] noted that accumulation of hoar frost, followed by melting on the evening of January 23, 1963, caused 130 flashovers on the UK 275-kV system, causing isolations that dropped 17% of the total system load. Overall, there were 281 line flashovers and 122 station flashovers in December 1962 and January 1963 as a result of the pollution accumulation and winter fog. A system flashover rate of 3 per year had been observed in the period from 1957 to 1961. 2.2 230 kV COLD-FOG EVENT, 1989 In the area of Toronto, Ontario, Canada, December of 1989 was so cold that it eliminated the average upward trend of global warming through the entire decade. Pollution accumulation from road salt reached critical levels at several substations. On the morning of December 31, 1989, a freezing rain storm with limited accumulation

Pre-contamination using flow coating of kaolin/salt mixtures or dry-spray methods • Chilling of insulator to -2°C • Application of line-voltage stress • Circulation of fog with 10μm volume mean diameter and 3 m/s wind speed, similar to natural conditions, giving dew point depression of less than 2 C° • Controlled increase in temperature and dew point • Increase of voltage in 5% steps to flashover at measured dew-point temperature intervals • Interpolation of results to establish 50% flashover voltage (CFO) for 30 min at dew point of 0 °C. The relative standard deviation of CFO in most cold-fog tests was estimated based on a linear decline in measured flashover voltage with increasing dew point temperature. Typical relative standard deviations of CFO were found to be less than 4%, indicating a well controlled test method. The ability to reproduce the same ESDD deposit with slurry coating [2] or dry spray methods is sensitive to the relative humidity at the time of application. This remains as a large source of test-to-test variations in the cold-fog test method. 3.2 COLD-FOG TESTS ON 44-500 kV INSULATORS In the course of a large number of investigations to evaluate insulator performance and to compare remedial measures, many reference test results using the cold-fog method have been accumulated. Table 3 shows results of cold-fog tests on insulator strings and stacks of three or five deep-skirt apparatus insulators. Table 4 gives results measured on post insulators, classed as “long rod” insulators in IEC terms. In all cases, a nonsoluble deposit associated with 40 g/L of kaolin (0.05-0.06 mg/cm2) was used on the porcelain surfaces. Generally, for each type of insulator, V50 flashover voltage decreases with increasing ESDD. The trend is stronger for post insulators in Table 4 than for cap-and-pin insulators in Table 3.


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Table 3. Cold-Fog Test Results for Cap-and-Pin Insulators [6] Insulator Type

Dry Arc Distance

Leakage Distance

3 unit stack 3 unit stack 3 unit stack 3 unit stack 3 unit stack 3 unit stack 3 unit stack 3 unit stack 3 unit stack 3 unit stack 3 unit stack 3 unit stack 3 unit stack 3 unit stack

1100 mm 1100 mm 1100 mm 1100 mm 1100 mm 1100 mm 1100 mm 1100 mm 1100 mm 1100 mm 1100 mm 1100 mm 1100 mm 1100 mm

5 unit stack 5 unit stack 5 unit stack 5 unit stack 5 unit stack 5 unit stack 5 unit stack 5 unit stack 5 unit stack 14 unit string 14 unit string 14 unit string 14 unit string 14 unit string

Flashover 50% kV l-g

2730 mm 2730 mm 2730 mm 2730 mm 2730 mm 2730 mm 2730 mm 2730 mm 2730 mm 2730 mm 2730 mm 2730 mm 2730 mm 2730 mm

ESDD, mg/cm2 0.067 0.088 0.119 0.129 0.181 0.183 0.219 0.225 0.260 0.371 0.447 0.455 0.768 0.799

1850 mm 1850 mm 1850 mm 1850 mm 1850 mm 1850 mm 1850 mm 1850 mm 1850 mm

4820 mm 4820 mm 4820 mm 4820 mm 4820 mm 4820 mm 4820 mm 4820 mm 4820 mm

0.097 0.098 0.130 0.156 0.167 0.171 0.217 0.306 0.314

212 149 142 205 212 145 165 138 149

2040 mm 2040 mm 2040 mm 2040 mm 2040 mm

4680 mm 4680 mm 6410 mm 6410 mm 6410 mm

0.148 0.230 0.150 0.332 0.736

203 198 219 213 170

23 unit string* 3360 mm 23 unit string* 3360 mm 23 unit string* 3360 mm 23 unit string* 3360 mm * Not included in empirical fit

5870 mm 6010 mm 6050 mm 6660 mm

0.035 0.035 0.033 0.031

251 280 246 280

169 107 86 86 104 88 73 107 72 75 62 53 57 51

Table 4. Cold-Fog Test Results for Post Insulators [6, 17] Dry Arc Leakage Flashover ESDD, Insulator Type Distance Distance 50% kV l-g mg/cm2 44-kV Post 44-kV Post 44-kV Post

240 mm 240 mm 240 mm

559 mm 559 mm 559 mm

0.043 0.052 0.072

41.0 38.0 32.5

69-kV Post 69-kV Post 69-kV Post 69-kV Post 69-kV Post 69-kV Post

680 mm 680 mm 680 mm 680 mm 680 mm 680 mm

1940 mm 1940 mm 1940 mm 1940 mm 1940 mm 1940 mm

0.050 0.100 0.220 0.220 0.350 0.350

70.5 59.5 33.6 33.8 32.0 28.9

500 kV Post 2870 mm 6560 mm 0.016 500 kV Post 2870 mm 6560 mm 0.018 500 kV Post 2870 mm 6560 mm 0.020 500 kV Post 2870 mm 6560 mm 0.021 500 kV Post 2870 mm 6560 mm 0.025 500 kV Post 2870 mm 6560 mm 0.026 500 kV Post 2870 mm 7620 mm 0.025 500 kV Post, Sheds* 2870 mm 8540 mm 0.025 500 kV Post 3330 mm 10300 mm 0.045 500 kV Post 3330 mm 10300 mm 0.050 500 kV Post 2870 mm 9790 mm 0.035 500 kV Post 2870 mm 9880 mm 0.027 500 kV Post 2870 mm 10220 mm 0.035 * 6560 mm post fitted with six leakage distance extender sheds

365 350 360 320 330 350 303 303 303 303 303 303 303

3.3 EMPIRICAL MODEL OF RESULTS The relation between Cold-Fog flashover voltage stress V50% (kV rms line to ground per m of leakage distance) can be described as a function of ESDD (mg/cm2) by:

V50% = 14.5 ⋅ (ESDD )

−0.36

(3)

This expression is fitted to the results of flashover voltage calculations on a 300-mm cylinder using the ac arc model for ice described in [18]. The calculation was carried out on a tube with leakage distance of 10 m but the result is essentially linear with leakage distance in the range 0.5 m to 15 m. Equation (3) is plotted as a unified specific creepage distance flashover level as a reference line in Figures 4 and 5.

Figure 4: Unified Specific Creepage Distance Critical Flashover versus Equivalent Salt Deposit Density (ESDD) in Cold-Fog Tests, segregated by Insulator Type. Farzaneh/Zhang/ChenTheory: [18].

Empirical fits to critical flashover voltage V50% in Table 3 and Table 4 were evaluated, fixing the exponent to match n=-0.36 in Equation (3). The segregated results by insulator type (pin or post) are given in Equations (4) and (5).

PIN TYPE : V50% = 18.6 ⋅ (ESDD )

(4)

POST TYPE : V50% = 12.7 ⋅ (ESDD )

(5)

−0.36

−0.36

In Equations (3),(4) and (5), ESDD is expressed in mg/cm2 and the resulting V50% is in kV rms, line to ground, per meter of leakage distance. There is a significant difference in the flashover levels of the post insulators compared to cap-and-pin types in coldfog conditions. This difference of at least 20% in strength per unit leakage distance at constant ESDD (or factor of two differences in ESDD level for the same strength) matches to the IEC classifications given in Figures 1 and 2. The data from Tables 3 and 4 were also sorted according to the ratio of leakage distance to dry arc distance and plotted in two groups. Figure 5 shows that there is no strong influence of this insulator characteristic on the coldfog performance.



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3.4 COMPARING COLD-FOG, SNOW AND ICE For a wide (100:1) range of ice conductivity and thickness, an icing stress product (ISP) formed from the conductivity of the melted ice (μS/cm at 20ºC) and the ice weight per cm of dry arc distance (g/cm) can be used to predict the flashover strength. The exponent relating strength and ISP is normally n=-0.19 compared to n=-0.36 in Equation (3) for the cold-fog case. Snow behaves more like cold fog with an exponent of n=-0.26. There are matching differences in the impulse flashover characteristics on humid and dry ice surfaces: Humid surfaces were found to have an exponent of n=-0.20 and dry surfaces have n=-0.31 [19]. Figure 5: Unified Specific Creepage Distance Critical Flashover versus Equivalent Salt Deposit Density (ESDD) in Cold-Fog Tests, segregated by Ratio of Leakage to Dry-Arc Distance. Farzaneh/Zhang/Chen theory [18].

3.4 COMPARING CLEAN- AND COLD-FOG RESULTS It is particularly interesting that the empirical fit of the conventional clean-fog flashover strength with ESDD, described in equation (2), has the same exponent as that fitted to the theoretical results in equation (3). This means that, even considering all of the factors that would tend to make them different, the overall result is that the theoretical cold-fog flashover strength is only 13% higher than the clean-fog strength. When separated into requirements for post and cap-andpin insulators, Figure 6 shows that the cold-fog leakage distance requirements are more onerous than clean-fog requirements for the post insulators. The standard design practice from equations (1) and (2) gives a median 32% margin for cap-and-pin insulators.

Figure 6. Ratio of Cold-Fog to Clean-Fog Flashover Gradient for Pin and Post Insulators (Tables 3,4 versus Equation (2) [22]).

The median ratio of cold-fog to clean-fog V50 in Figure 6 is 1.32 for cap-and-pin insulators and 0.92 for post insulators. The need for additional leakage distance on post insulators in cold-fog conditions may, along with the estimates of increased contamination exposure, help to explain why stations that perform adequately in summer still tend to fail in winter conditions.

4 SILICONE MATERIALS AND COATINGS The possibility of contamination flashover on silicone insulators in clean-fog or salt-fog conditions is considered negligible in the laboratory when they are applied with the same specific stress (kV per meter of leakage distance) as porcelain insulators. Tests show that the specific flashover levels can be increased for cap-and-pin apparatus insulators, as shown in Table 5. Table 5. Cold-Fog Test Results for Silicone-Coated Insulators [20] Dry Arc Leakage Flashover ESDD, Insulator Type Distance Distance 50% kV l-g mg/cm2 3 unit stack 3 unit stack

1110 mm 1110 mm

2890 mm 2890 mm

0.022 0.022

137 157

4 unit stack 4 unit stack 4 unit stack

1480 mm 1480 mm 1480 mm

3860 mm 3860 mm 3860 mm

0.022 0.440 0.440

263 168 177

5 unit stack 5 unit stack 5 unit stack 5 unit stack 5 unit stack 5 unit stack 5 unit stack

1850 mm 1850 mm 1850 mm 1850 mm 1850 mm 1850 mm 1850 mm

4820 mm 4820 mm 4820 mm 4820 mm 4820 mm 4820 mm 4820 mm

0.022 0.080 0.113 0.380 0.380 0.440 0.440

>270 >270 >270 >300 175 >300 250

The flashover strength tends to decline if the pollution remains on the insulator surface for several days, leading to the 175 kV and 250 kV flashovers in Table 5 for high 0.4 mg/cm2 values of contamination. Flashovers occurred on these heavily contaminated silicone-coated insulators at unified specific creepage distance levels of between 19 and 28 mm/kVrms l-g. In comparison, the uncoated insulators follow equation (3), which when converted from kV/m to mm/kV would indicate critical flashover at 41 mm/kVrms l-g. Silicone coating is normally applied using an automotive paint gun. The coating process can be carried out on-site to improve the performance of existing porcelain transformer and breaker bushings. Quality of surface preparation and coating application can be assessed using the water immersion test for non-ceramic insulators in IEC 1109/1992 [21]. This test calls for 42 h immersion boiling deionized water with 0.1% NaCl by weight. The boiling

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test has also proved to be suitable for evaluating the adhesion of silicone coatings on porcelain, EPDM, silicone and other insulating substrates. With these satisfactory installation procedures, it has been possible to obtain more than ten years of satisfactory field operating experience with an RTV coating in an area of high winter contamination [20].

5 APPLICATION 5.1 DAYS WITHOUT RAIN PRIOR TO COLD FOG In a desert, pollution accumulation is linear with time for a few months of exposure, and then eventually levels off as the deposit and blow-off rates reach equilibrium. Pollution accumulates more quickly on surfaces that face upwards or into the wind. Rain also removes the ESDD and NSDD from these surfaces more effectively than from those parts of the insulator facing towards the ground. Areas prone to winter contamination problems generally have temperate climates with a high frequency of rain washing. Measurements in Ontario have shown that rain accumulation of 0.4 mm in an hour is generally sufficient to wet and wash porcelain post surfaces. This means that, in many areas of concern, the ESDD is normally increasing linearly with exposure duration from the most recent rain event. The onset and disappearance dates of snow cover are good indicators of the duration of “winter desert” conditions in a given location. The median data in Figure 7 range from 75 days (Jan. 1 to Mar. 15) to 135 days (Dec. 1 to Apr. 15).

Figure 7: Onset and End Dates of Snow Cover in Ontario, Canada [16]

5.2 RATE OF CHANGE OF ESDD In winter conditions, there are two components to the ESDD that accumulates on insulator surfaces. A background of industrial pollution, consisting of sulphate, nitrate and some ammonium ions, occurs yearround. Surveys of local pollution indicate the deposit rates on upward-facing surfaces. In polluted areas, individual annual ion deposit rates can exceed 3000 kg/km2/yr or 30 kg/ha/y [23]. These values both convert to ESDD increase rates of 0.025 mg/cm2 per month per ion species using the relation that 1 hectare = 108 cm2.

In the winter months, a high quantity of sodium chloride accretion can be superimposed for stations that are relatively close to roadways. Urban road salting leads to ESDD escalation rates of 0.2 mg/cm2/month adjacent to elevated expressways. This will tend to dominate industrial pollution. General urban exposure tends to result in lower rates of increase of 0.02 mg/cm2 per month [12], which will add to, but not dominate, the general accumulation rate. 5.3 SELECTING AN APPROPRIATE RISK LEVEL The season-maximum ESDD can be estimated by multiplying the median number of months of snow cover by the monthly rate of change of ESDD. This means that, for 75 days of exposure, a season-peak ESDD of (75/30 x 0.2) = 0.5 mg/cm2 would be expected near an elevated expressway. Climate records will indicate the weather conditions at the end of long-duration cold spells. Any occurrence of icing or fog in the hours prior to a transition from -2°C to +2°C is a high-risk occasion. The frequency of these occurrences can be established in a suitable climate review. The flashover conditions are similar to those that cause distribution pole fires from high leakage currents. The number of years between widespread pole-fire outbreaks can also be used as an estimate of return period for cold-fog risks. Moderate climates, where there is some probability of washing from rain in the middle of the winter, have shorter duration of pollution accumulation. Rain will effectively reset the ESDD to zero. Electrical facilities located near expressways should use the appropriate 0.2 mg/cm2/month ESDD escalation rate with the reduced number of months of exposure. 5.4 LEAKAGE-DISTANCE COORDINATION In areas where a high fraction of winter dry spells end in freezing rain or fog, the insulator design should use a leakage distance suitable for the season-maximum contamination level. As discussed above, leakage distance from the IEEE guideline for clean fog flashover performance in equation (2) needs to be increased by a median 8% to meet cold-fog requirements for post insulators. Equation (2) gives a median 32% margin for cap-and-pin insulators in the same conditions. The overall 8% increase in leakage distance requirement for post insulators in cold-fog conditions can be considered to be composed of two simultaneous effects: • A decrease of 20% related to the dependence of ion conductivity with temperature in Table 1 • An increase of 30% related to the NSDD associated with the (inert) ice layer in Figure 3 This hypothesis could be explored further by studying cold-fog flashover of insulators with acid contamination. The insulation coordination should select a design probability level that recognizes the fact that a large number of insulators will be exposed in the small area of a station. Procedures for selecting this probability level,



based on both the median flashover level and the standard deviation, are found in test and selection guides [1,11-14]. The flashover probability of a number of insulators in parallel at a common location is sensitive to line voltage. Cold-fog conditions are not associated with peak loads in winter (-30°C, high wind speed) and thus the possibility of using 5% voltage reductions to manage the events is open to some utilities to operate around problem events.

6 CONCLUSION The selection of a suitable insulator leakage distance for cold-fog accretion on pre-contaminated insulators should follow the same approach and rules as the coordination for clean-fog conditions. The process spelled out in IEC and IEEE recommendations relies mainly on developing a good estimate of the equivalent salt deposit density (ESDD) on the insulators. In winter climates, the period of snow cover on the ground gives a good indication of the duration of the ESDD accumulation period without rain. The ac flashover voltage in cold-fog conditions scales linearly with leakage distance from 0.5 to 10 m, covering system voltages from 44 kV to 500 kV. The ac flashover voltage gradient in cold-fog conditions, expressed in kV rms, line to ground, per meter of leakage distance, varies with ESDD raised to the exponent of (-0.36). This is the same exponent as found for clean-fog conditions. Selecting an insulator using clean-fog criteria will not satisfy cold-fog requirements for station post insulators. An additional margin of up to 20% will be needed, depending on the local ESDD levels.

ACKNOWLEDGMENT I acknowledge the roles of Emeritus Professor Reuben Hackam and Prof. Masoud Farzaneh in organizing this special issue of DEIS Transactions. Prof. Ed Cherney reviewed the manuscript and provided a number of helpful comments before submission. At Kinectrics, Hydro-One and IESO, many colleagues have given important insights and opportunities to study winter flashover problems. Tests have also been sponsored by TVA and Raychem.

[7] [8] [9] [10] [11]

[12]

[13]

[14]

[15]

[16] [17] [18] [19] [20] [21] [22]

REFERENCES [1] [2] [3] [4]

[5] [6]

IEC 36/220/NP, “Selection and Dimensioning of High-Voltage Insulators for Polluted Conditions, Part I and Part II”, 2004. IEEE Standard 4 – 1995, IEEE Standard Techniques for HighVoltage Testing (IEEE: Piscataway, NJ), 1995. CCIR Recommendation 832, “World Atlas of Ground Conductivities”. A. Wüest, G. Piepke and J.D. Halfman, “The Density Stratification of Lake Malawi, Appendix A, Equivalent Conductance as a Function of Temperature”, in The Limnology, Climatology and Paleoclimatology of the East African Lakes, Johnson, T.G. and E.O. Odada (eds), Toronto, Gordon and Breach, 1996. CIGRE Task Force 33.04.01, “Polluted Insulators: A Review of Current Knowledge”, CIGRE Brochure 158, 2000. W.A.Chisholm, K.G. Ringler, C.C. Erven, M.A. Green, O. Melo, Y. Tam, O. Nigol, J. Kuffel, A. Boyer, I.K. Pavasars, F.X. Macedo, J.K. Sabiston, R.B. Caputo, “The Cold Fog Test” , IEEE Trans. Power Delivery, Vol. 11, pp. 1874-1880, 1996.

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William A. Chisholm (M’78-SM’90-F’07) was born in Plattsburgh, New York in 1955. He received the B.A.Sc. and M.Eng degrees from the University of Toronto in 1977 and 1979, respectively and the Ph.D. degree from the University of Waterloo in 2004. He has worked at Kinectrics, formerly the Ontario Hydro Research Division, since 1977 and is an Adjunct Professor At the University of Quebec at Chicoutimi. He chaired the IEEE Power Engineering Society Lightning and Insulator Subcommittee from 1997 to 2005 and also contributes to CIGRE work in lightning protection and selection of weather parameters for transmission line thermal rating.