Index Terms - Booster shed (BS), ice-covered insulator, modeling and simulation, finite element method (FEM), potential and electric-field distribution.
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S. M. Ale-Emran and M Farzaneh: Dimensioning of Booster Sheds for Icing Protection of Post Station Insulators
Dimensioning of Booster Sheds for Icing Protection of Post Station Insulators S. M. Ale-Emran and M. Farzaneh NSERC / Hydro-Quebec / UQAC Industrial Chair on Atmospheric Icing of Power Network Equipment (CIGELE) and Canada Research Chair on Engineering of Power Network Atmospheric Icing (INGIVRE), (www.cigele.ca) at Université du Québec à Chicoutimi 555, Boulevard de l'Université Chicoutimi, Québec, Canada, G7H 2B1 ABSTRACT The main objective of this paper is to introduce an analytical and numerical method to calculate the diameter and position of booster sheds (BSs) on post station insulators under heavy icing conditions. Numerical simulations using the finite element method (FEM), implemented by the commercial software Comsol Multiphysics, were performed to calculate the influence of BSs on voltage drops along two units of a standard post insulator under heavy icing conditions. The shape of ice accretion and icicles used in the present study were based on laboratory tests carried out at CIGELE. The presented simulations and analytical calculations seem to be efficient options to quantify the effects of the electric field and ice bridging on the dimensioning of the BSs along the post insulator. Based on the obtained results, the electric field strength would be the main factor involved for BSs close to the HV electrode whereas ice-bridging (icicle length) would be the main one for those close to the ground electrode. Index Terms - Booster shed (BS), ice-covered insulator, modeling and simulation, finite element method (FEM), potential and electric-field distribution.
1 INTRODUCTION IN many cold climate countries, atmospheric icing is a major problem for overhead electrical power systems causing power outages with important associated costs. Two famous ice storms which clearly illustrate the disastrous consequences of that problem happened in Eastern Canada and US in 1998, and in Southern China in 2008. The economic consequences in the first case were estimated at nearly US $6 billion. Almost 1.6 million customers in Quebec and Ontario were left without power for periods of 3-30 days. For the second ice disaster, frozen rain and snow lasted for more than three weeks, causing a large-scale outage in several provinces of China, and leading to economic losses of US $7.9 billion [1–5]. Based on field experience, EHV post insulators are more susceptible to flashover rather than line insulators [6, 7]. It is mainly due to their higher electric field stress, smaller shed spacing, larger diameter (or lower leakage distance) and the presence of many parallel post insulators concentrated in small areas [8]. Thus, this research is concerned mainly with post insulators. Over the past decades, numerous studies have been carried out on icing test methods, icing flashover performance, mitigation options, and so on. Several complex factors affect the dielectric strength of ice-covered insulators such as the type and density of ice, length of icicles, freezing water conductivity, Manuscript received on 18 November 2013, in final form 3 May 2014, accepted 22 June 2014.
altitude, profile and type of insulator, and the position and length of ice-free zones (also called air gaps) [8–13]. Most researchers believed that one of the main reasons causing significant decrease in flashover voltage is ice-bridging of the insulator sheds and the presence of a highly conductive water film on the ice surface. Some approaches have been proposed to reduce the probability of ice-bridging, for example by using booster sheds (BSs), creepage extenders, semiconducting glaze insulators, and insulators with greater shed-to-shed distances [4, 8, 9]. As booster shed (BS) has been found to be an easy-to-use and efficient accessory on already installed insulators, it seems to be a promising solution to this problem [14–17]. The previous studies on BSs were generally without modeling. Also, in [18] a modeling method to determine the proper positions and diameters of three BSs along with one unit of a standard icecovered post station insulator was presented. However, the calculated values were not suitable compared to the BS test results in [1]. The main problems of this modeling method are: simulation of simple air gaps rather than BSs, improper approximation of the icicle lengths along BSs, imprecise definitions of insulator diameter, BS radius, etc., in the model, inaccurate estimation of breakdown voltage as a function of the air gap length. Hence, to advance the previous studies on BSs, this paper presents a new approach of BS dimensioning under heavy ice
DOI 10.1109/TDEI.2014.004469
IEEE Transactions on Dielectrics and Electrical Insulation
Vol. 21, No. 6; December 2014
conditions based on previous laboratory results at CIGELE [1], and two-dimensional axisymmetric simulations by Comsol Multiphysics. It deals with two parameters of BSs (position and diameter) in the installation of 4, 5, and 6 BSs along two units of an ice-covered EHV ceramic post station insulator.
2 LABORATORY OBSERVATIONS Two units of a three-unit post station insulator, as used in 735kV substations in Quebec, were tested in order to evaluate the flashover performance of 4-, 5- and 6-BS configurations [1]. Each unit had a 3500-mm leakage distance and a 1390-mm dry arcing distance. The BSs had an external diameter of 65.5 cm. Heavy ice tests were carried out with an applied water conductivity of 30μS/cm. Figure 1 shows the test results for the BS configurations in a CIGELE climate room. The tests were carried out under melting regime or with wet-grown ice. Wet-grown ice is characterized by the presence of a water film with relatively low resistivity on the ice surface. It is the most dangerous type of ice causing insulator flashover [19, 20].
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three withstands in four tests at one voltage level, and two flashovers at 5% higher voltage level. Also, Maximum withstand voltage (VWS): is the voltage level that gives three withstands out of four tests. Minimum flashover voltage (VMF): is the voltage level that is 5% higher than VWS and gives two flashovers out of a maximum of three tests. The use of BSs allows the creation of some air gaps along an ice-covered insulator. The air gaps and icicles play an important role in the flashover process as it is where partial arcs appear [22]. Therefore, an effective study on the characteristics of the formed air gaps and icicles in the BS configurations helps acquiring a better understanding of BS effects. The test was repeated 5-6 times for each configuration. Tables 1 and 2 present the average length of the air gaps and icicles of the BS experiments (Figure 1). These lengths mainly depend on the accumulation time and electrical field strength at the area of icicle growth. There is some tendency for longer icicle length on the BSs that are in regions of lower electrical stress, towards the bottom of the insulator. These values will be used in the dimensioning approach of the BS parameters. Table 1. The average lengths of the air gaps (cm) during the BS tests. 4 BSs
5 BSs
6 BSs
BS1
23.2
23.2
21.8
BS2
16.2
16.2
13.8
BS3
9.6
9
8.4
BS4
11
9.6
7
BS5
-
11
2.6
BS6
-
-
9.2
Table 2. The average length of the icicles (cm) during the BS tests.
(a)
(b)
VWS = 285 kV VMF = 300 kV
VWS = 300 kV VMF = 315 kV
(c)
4 BSs
5 BSs
6 BSs
BS1
17.4
17.4
19.8
BS2
20
20
24
BS3
30
32.2
29.8
BS4
37
29.6
33
BS5
-
26.8
37
BS6
-
-
44
VWS = 315 kV VMF = 330 kV
Figure 1. Test results for standard post insulators in heavy icing conditions (30 mm ice accretion on rotating cylinder) with a) 4 BSs, b) 5 BSs and c) 6 BSs [1].
The test processes of ice accretion, hardening, and melting sequences followed IEEE Standard 1783 [21]. The ice accretion period was 140-min long with an applied voltage of 285 kV (i.e. 105-kV per meter of dry arcing distance) and temperature of -12 ºC. Then, during the hardening period, the applied voltage was set to zero for around 30 min to harden the accreted ice layers on the insulator as well as to take pictures and measurements of air gaps. After that, during melting period, the applied voltage was set at 285 kV and the temperature was raised in order to produce water films on the ice surfaces. Finally, during the evaluation period, the applied voltage was increased by 5% steps (15 kV) and was fixed during at least 15 min to observe a withstand or flashover. Based on the standard [21], the tests are finished after observation of
3 NUMERICAL SIMULATIONS The simulations were carried out using the FEM commercial software COMSOL MultiphysicsTM. With this software surface conductivity and open boundary can be easily implemented. Also, for faster comparative calculations, 2D axisymmetric modeling was used instead of 3D modeling (Figure 2). The potential and the electric field distributions were calculated on the vertical and radial plane (zr-plane). The accuracy of this approach was verified in the previous studies [20, 23]. The simulation parameters are summarized in Table 3. Also, in Table 4, the voltage drops (V (%)) for 4-, 5- and 6-BS tests are compared. The voltage drops were computed by using reference lines located along the air gaps. Detailed explanations on the modeling method were presented in the previous papers [24, 25]. These calculated voltage drops will be used in the proposed approach of BS dimensioning.
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S. M. Ale-Emran and M Farzaneh: Dimensioning of Booster Sheds for Icing Protection of Post Station Insulators Table 3. Simulation parameters. Porcelain
Air
Ice
Water film
Booster Shed
6
1
75
81
2-15
0
0
0
30
0
-
-
-
0.15
5
Relative permittivity Conductivity s (S/cm) at 20C Thickness (mm)
α (°): is the slope angle of the icicles (α ≈ 8°, except αBS1= 15°). Li (cm): is the maximum length of the icicles on BSi obtained from experimental results. Wi (cm): is the total insulator length protected from precipitation by BSi. Pi (cm): is the distance between the extremities of two BSs which determines the position (P) of the BSs. xi (cm): is the minimum required length of each air gap to prevent electrical breakdown (between the tip of the icicles and the opposite surface of ice). The AC breakdown voltage in the air gap of BSij (Vb-i) as a function of xi (cm) can be stated as equation (1) or equation (2): Vb-i (kVrms) = axi+b Vb-i (kVrms) = cln(xi)+d
(1) (2)
Where a, b, c, and d: are real constant values. If we obtain the values of Vb-i, a, and b, or Vb-i, c, and d, then we can calculate xi as equation (3) or equation (4): xi = (Vb-i - b)/a xi =exp((Vb-i - d)/c)
(b) (a) (c) Figure 2. Equipotential line distributions of the post insulator under heavy wet-grown ice condition with a) 4 BSs, b) 5 BSs, c) 6 BSs in 2D axisymmetric view Table 4. Comparison of the calculated voltage drops (V (%)) for the 4-, 5and 6-BS tests. Air gap no. 4 BSs 5 BSs 6 BSs 1
52.5
51.2
50.3
2
23.1
20.0
20.8
3
9.8
7.9
6.9
4
14.0
8.1
6.7
5
-
12.2
3.1
6
-
-
11.8
Total
99.4
99.4
99.4
(3) (4)
According to the geometric model (Figure 3), the minimum BS diameter (DBSi-min) required to prevent electrical breakdown is: DBSi-min (cm) = 2(Rho-i)/cos(β) DBSi-min (cm) =2((dext-i/2)+einij+xi+Li.sin(α))/cos(β)
(5) (6)
4 PROCEDURE FOR BOOSTER SHED DIMENSIONING Figure 3 illustrates the geometric model of the standard post insulator with BSs taking into account the direction of precipitation. In Figure 3: DBSi (cm): is the diameter of BSi. RBSi (cm): is the radius of BSi. Rho-i (cm): is the horizontal distance between the center axis of the insulator and the tip of BSi. BSij: signifies the zone between BSi and BSj. einij (cm): is the ice thickness on the insulator surface in BSij. ebsj (cm): is the ice thickness (≈ 3cm) on the upper surface of BSj. dext-i (cm): is the average exterior diameter of the insulator shed under BSi. β (°): is the inclination angle of BSi (β = 24.5° that is equal to the upper shed angle of the insulator shed). θ (°): is the precipitation incidence angle (0 ≤ θ and at CIGELE Laboratory: θ = 53±5°).
Figure 3. Geometric model of the standard post insulator with booster sheds.
IEEE Transactions on Dielectrics and Electrical Insulation
Vol. 21, No. 6; December 2014
Moreover, the minimum required distance between the tips of two consecutive BSs (Pi min) is (Figure 3): Pi-min(cm)= Li cos(α) + ebsi+ xi
(7)
To calculate Dbsi-min and Pi-min, all of the parameters are known in equations (6) and (7) except xi. In other words, the challenging issue in this method is the estimation of xi. Thus, in this method, the estimation of the breakdown voltage equations in the BS air gaps is required to calculate the proper diameters and positions of BSs. In the next section, two estimation approaches are explained.
5 ESTIMATION OF AC BREAKDOWN VOLTAGE EQUATIONS IN BS AIR GAPS 5.1 FIRST APPROACH Air gaps formed during the ice accumulation in a wet regime can be assimilated to a rod-plane configuration. The rod and the plane respectively represent the icicle on the BS and the surface of the ice accumulated on the upper surface of the next BS (Figure 4).
(b)
(a)
(c)
Figure 4. Schematic diagrams of a) icicle on BS, b) icicle-plane configuration, and c) icicle dimensions.
The high value of the freezing water conductivity (σ=340 µS/cm) of the icicles used in the previous icicle-plane experimental tests were based on field experience [6, 7]. In fact, during the freezing process impurities are rejected from the solid part toward the liquid portion of drops or droplets [7, 26]. Therefore, it causes the surface conductivity of ice to increase and enables it to reach values as high as ten times those of the freezing water conductivity (σ= 30 µS/cm) in the BS tests [1, 27]. This is the reason that why σ=340 µS/cm corresponds to the BS test conditions. In reality, freezing water conductivity has a wide range of natural values but levels of 30 S/cm are more typical in Quebec, Canada. Values at this high level are influenced by road salt [28].
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By curve fitting of the obtained results in [27], we attain the AC breakdown voltage (Vb) as a function of minimal air gap length (x) for the corresponding temperatures (temp) and (σ=340 µS/cm and 1 cm ≤x≤4 cm): temp = 0 C (8) Vb (kVrms) = 6.3x (cm) + 8.0 , temp = -4 C (9) Vb (kVrms) = 7.3x (cm) +12, Moreover, the BS tests were performed at a temperature (temp) of -1 C. Using a linear estimation between equations (8) and (9), leads in: Vb (kVrms) = 6.55x (cm) + 9.0, temp = -1 C (10) The presence of a water film at the icicle tip can result in about 27% reduction in the breakdown voltage [29]. Thus, multiplying (10) by 73% results in the final required breakdown voltage (σ=340µS/cm and 1cm ≤x≤4cm): Vb (kVrms) = 4.78 x (cm) + 6.57 (temperature = -1 C with water film effect) (11) 5.2 SECOND APPROACH The AC breakdown voltage (Vb) obtained from (1) can be compared with the values of the voltage drop at air gap no. i (Vag-i) under two conditions. First, when the applied voltage equals the flashover voltage (VMF), and second, when it equals the withstand voltage (VWS). In short, the following inequality must be satisfied: (12) Vag-i-WS < Vb-agi < Vagi-MF Where Vagi-WS: is the voltage drop at the air gap no. i (Vagi) when the applied voltage equals the flashover voltage. Vagi-MF : is the voltage drop at the air gap no. i when the applied voltage equals the withstand voltage. Vb-agi: is the breakdown voltage of air gap no. i. Actually, the objective of this second approach is the estimation of the breakdown voltage equations in the BS air gaps considering their average air gap lengths and their variation length (Lag (cm) ≠ 0). To this end, it is sufficient to focus only on the first air gap (close to the HV electrode). In other words, air gap 1 has the most determinant role in the electrical performance of the ice-covered insulator. The reasons for this are as follows: 1) It was shown that following the addition of 4, 5 or 6 BSs, the majority (more than 50%, Table 4) of the applied voltage was dropped along the first air gap closest to the HV electrode. 2) In addition, based on the previous BS tests, partial arcs usually appear first in the first air gap. Table 5 shows the voltage drops on the first air gaps (obtained from the simulations, Figure 2) for the 4-, 5- and 6-BS tests when the applied voltage is equal to VMF or VWS. In addition, Lag1-ave is the average length of air gap 1 during the BS tests. Table 5. Analysis of the voltage drops along the first air gaps for the 4-, 5and 6-BS tests when the applied voltage is equals to VMF or VWS. 4 BSs
5 BSs
VWS
285
300
6 BSs 315
VMF
300
315
330
Vag1-WS
149.62
153.6
158.44
Vag1-MF
157.50
161.28
165.99
Vag1-MF - Vag1-WS
7.88
7.68
7.55
Lag1-ave (cm)
23.2
23.2
21.8
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S. M. Ale-Emran and M Farzaneh: Dimensioning of Booster Sheds for Icing Protection of Post Station Insulators
The AC breakdown voltage equations used for icicle-plane configurations in [1, 25, 27, 30, 31] suggest that the typical values for a and b can be in the ranges below: 3.5 < a
