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Methods to reduce the risk of wooden pole ignition
Electric Power Systems Research 134 (2016) 213–221
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Electric Power Systems Research journal homepage: www.elsevier.com/locate/epsr
Methods to reduce the risk of wooden pole ignition M. Islam a,∗ , H. Greg a,b a b
Advanced Grid Engineering, National Grid USA Service Co., Syracuse, NY, 13202, USA Advanced Grid Engineering, National Grid USA Service Co., Waltham, MA, 02451, USA
a r t i c l e
i n f o
Article history: Received 22 May 2015 Received in revised form 9 July 2015 Accepted 29 July 2015 Available online 31 December 2015 Keywords: Wooden poles Insulation degradation Leakage current Bolt–wood junction
a b s t r a c t A number of reported wooden pole ignition incidences show evidence that the source was located near the pole bolt/pin mounting hardware. This has been observed for environmentally stressed insulators, as well as for larger overhead equipment in situations where the wooden pole has become the dominant part of the leakage conduction path through loss/deterioration of a case ground conductor. In both of these cases, the bolt-to-wood interface is the focal point for the leakage current. A mechanism which can cause pole ignition in these situations is examined with comprehensive details. This paper suggests a novel method to reduce the risk of wooden pole ignition. The mathematical basis of the suggested method is also explained. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Wooden poles are common all over the world as part of the physical transmission and distribution network. While wooden poles are insulators by nature, the conductance of wooden pole surfaces exposed to the environment can increase based on airborne contaminations. In addition, airborne contamination and weather driven cyclic stresses can deteriorate the porcelain used for the insulation of conduction wires. Medium and high voltage conductors are insulated from the wooden pole by way of pin glass/ceramic insulators. In this situation, the wooden pole is deliberately part of the leakage path, but the insulator is sufficiently specified to prevent significant leakage currents. In addition, utilities design to have a minimum distance of wood between primary and other ground/secondary conductors. This distance concept, as well as the typical construction for overhead pin insulators is shown in Fig. 1. Overhead equipment with metallic cases, such as reclosers and capacitors, are installed with a physical down-pole wire between the metallic equipment case, secondary neutral, and earth ground. This equipment has insulating devices, between the primary voltages and case, specified to prevent significant leakage current. However, the deterioration of the ground wire over time, theft, or other means of absence, can create a failure condition which allows the wooden pole itself to be the primary conduction path for leakage current, where it normally would not. Missing ground
∗ Corresponding author. Tel.: +1 9792299703. E-mail addresses: [email protected], [email protected] (M. Islam), [email protected] (H. Greg). http://dx.doi.org/10.1016/j.epsr.2015.07.017 0378-7796/© 2015 Elsevier B.V. All rights reserved.
conductor, combined with insulator degradation, can lead to the effect being examined similar to the above pin insulator scenario, conducting through the mounting bolts of the device. In addition, many of these devices have internal components, such as power supplies or other capacitive coupled devices, which can present the full voltage on the case when the ground cable is removed. The mounting bolts and typical installation can be seen in Fig. 2. With prolonged exposure the insulation integrity of both of these examples can become compromised, and therefore the magnitude of the leakage current would also increase. An IR camera captured the persistent incandescent glow of the bolt caused by the heat produced as result of the leakage current flow in each wood specimen. The threshold value at which the leakage current caused smoldering and glow in wood was measured as 3.2 mA in specimen 1 and 4.1 mA in specimen 2. These observations validated some of the anecdotal evidence which suggests that the ignition of pole fires usually takes place in the junction between the metallic accessories such as king bolts or insulator base-pins and the timber pole (Figs. 3 and 4). This junction forms a hotspot in the wooden structure and is prone to smoldering. The potential mechanisms behind ignition will be explained in the Section 2. Analysis of past wooden pole ignition events suggests that the ignition started at the bolt metal–wood junction. The photographs shown below further support this concept through visual inspection. It is also found to be consistent in laboratory tests that the metal–wood junction is the most susceptible to the pole ignition process [1] contingent to wood smoldering due to localized high temperature. It is worth noting that, in the absence of a valid case ground connection, overhead equipment also poses the risk of accidental electrocution to workers. Lack of ground connections allow stray voltages to be present
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on what otherwise would be grounded surfaces. While most utility workers are trained to always assume that these surfaces are ‘hot’ when first approaching them, there is still an increased safety risk. Unfortunately, no utilities today are actively monitoring for a valid ground connection to each overhead device, as this would be an expensive proposition. 2. Mechanisms behind ignition 2.1. Smoldering followed by high voltage arc at the bolt–wood junction Older wooden pole surfaces exposed to harsh weather conditions and seasonal stress can develop enough conductivity from surface contamination often assist pole ignition [1]. Brine precipitation can be built up on the wood surface providing a lower impedance path down the pole. These surface contaminants allow sufficiently large tracking current to develop in the wooden pole. Close to the point of origin, near the wood metal junction, high temperatures are created. This process can begin with a corona discharge that ionizes the dry air in the vicinity of the bolt head. Wind can also intermittently blow ionized air, exposing its high resistivity to the leakage current. Due to high impedance, it can create a high voltage in the bolt–wood junction. It is shown in the subsequent analysis that the leakage current passed through this small area poses a high enough thermal energy density per unit area to ignite the pole. After ignition, this fire is sustained by the dry, due to the
subsequent temperature rise, wood surrounding the heated area. At a high enough temperature to smolder the dry wood surface, a subsequent electric arc can lead the dry wood surface (hydrocarbon) to sustain ignition. The intensity of the liberated energy from the arc would be sustained by the magnitude of the leakage current. The ignition process is strongly aided by the wood-through metal bolt that attaches through the pole. It continually forms the mechanical vulnerable junction with the wooden pole. Metal exposed to open environment is subject to oxidation, which means that metals such as iron loose electrons in the presence of environmental catalyst such as sunlight, temperature, water vapor and airborne impurity such as positive ions. The rate of oxidation of metal surface due to its exposure to varying environmental condition varies from one kind of metal or metal alloy to another kind. Regardless of how long a certain metal or metal alloy may take to show a visible oxidation symptom it is well observed that the reaction starts with loosing electrons to air containing water molecule, acid or salt within short period of time, typically a few weeks. This oxidation process causes the metal to be positively charged and act as cathode in the electrical sparking process [2]. 2.2. Voltage surge at the ungrounded pole mounted equipment Pole top equipment including transformers, reclosers, line charging capacitors or isolators have a grounding contact. However, grounding is deemed optional by most of the electrical regulatory body for the pole mounted equipment more than 8 feet above
Fig. 1. Typical overhead pin construction, showing leakage path.
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Fig. 2. Typical recloser overhead installation.
Fig. 3. Fire started at the wood-metal junction (Source: http://incident-prevention.com/ip-articles/responding-to-pole-fires).
the ground. Transient voltage surges can originate from different phenomena including lightning, surge on metallic enclosures of the equipment, open circuit faults, or from cosmic failure of solid state switches. In such situations, an electric spark is a common
incident that leads to pole fires. There is also good possibility of consecutive occurrences of internal faults and natural hazards (tree falls, heavy rain) leading to electrical fires. The voltage build up at the metal–wood junction will seek lower impedance path to
Fig. 4. Evidence of burn near the bolt-eye from the pole through bolt.
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Fig. 5. Electrical arc mechanism in wooden poles.
neutralize unless a ground path is provided. If the transient current flows through the wooden pole surface with elevated conductance due to aging resistive heating can lead to pole fire.
ionized air intermittently. Removal of the ionized air from the junction switches the high resistance back on the circuit model. The leakage current through this high resistance dissipates a large amount of heat.
2.3. Model of pole fire inception process 2.4. Equivalent circuit model The model of the pole fire inception process will be explained in sub Section 2.4. A junction capacitance is formed by polarization of the metal with positive charge and the pole surface close to the metal–wood junction with negative charge. However, the capacitance formed will exhibit high loss tangent due to the contaminated air dielectric and will be variable due to random variation in the level of contamination. The tracking layer on the wood surface is considered as galvanic contact to the model of the capacitance. Tracking is a high conductive layer formed due to water, air-born pollutant on the wooden surface. The ionized air in the vicinity of the metal provides a varying low resistive path parallel to metal–wood junction capacitance. This low impedance path prevents heat build-up since it bypasses the high resistance of dry wood area. However, blowing wind can wipe out the
Many research papers have been published on pole top ignition in the last decade. Some of the authors proposed their own electrical equivalent circuit models for the simulation purpose. For example, rain water modified simple resistive network in [3] and ladder network model [4–7]. All of the models are perception based and not justified through lab tests, but supported by the engineering judgments of the authors. In the model presented here, the same principle was followed–resistive network energized by the leakage current and terminated at the ground impedance. The unique nature of the model in this paper is that it can explain the voltage stress at the bolt–wood junction and is consistent with an electrical sparking mechanism that the authors thought to be the most likely case. In the equivalent circuit the exposed surface of the wooden
Fig. 6. Equivalent circuit of pole leakage current.
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Using (3) and solving (2) for the transformed voltage VC (s)is given by: VC (s) =
⇒ VC (s) =
ωIl 1/ (2RW ) + sCJ
s2 + ω 2
ωIl /CJ
1/ 2RW CJ + s
s2 + ω 2
(4)
To find inverse Laplace transform of (4) the following identity [9] can be used 1 (s+a)(s2 +b2 ) tan−1 ba
Fig. 7. Equivalent circuit prior to ignition.
e−at +
1 a2 +b2
=
1 b
a2 + b2 sin bt −
, where =
Applying the identity the time domain expression for the junction voltage can be found as: VC (t) =
ωIl /CJ a2
+ ω2
e−at +
1 2 a + ω2 sin ωt − ω
(5)
where, = tan−1 2RW ωCJ indicates that the metal–wood junction voltage will lag the leakage peak current by the angle and a = 1/ 2RW CJ . Expanding (5): VC (t) =
2 4ωCJ Il RW 2 1 + 4ω2 CJ 2 RW
e−at + 2Il RW
1
sin ωt −
2 1 + 4ω2 CJ2 RW
(6) and evaluating (6) at t = 0 it can be shown Fig. 8. Equivalent circuit after heating process of ignition.
VC (0) =
pole (Fig. 5) is modeled by resistance RW (Fig. 6) which is usually in the order of several hundred ohms to a few kilo ohms based on the level of exposure to environmental impurity [8]. The tracking layer (electrolytic precipitation) exhibits high conductivity in moist weather. It is shown as Rtr in Fig. 6. In series with leakage current source Il (t). The ionized air in the dry wood area provides a low resistive path through Rion effectively bypassing the dry wood resistance Rdw and the metal–wood junction capacitance CJ . The resistance of the Rion is of the order of few ohms whereas dry wood exhibits from several thousands to a few mega ohms range based on the moisture content [8]. Even though the mechanism involves non linear processes it can be explained using linear transform theory such as Laplace transform for short term dynamics of the system expressed by the differential Eq. (1). Fig. 7 is used to show the initial status of the equivalent circuit before the ignition process begins. Fig. 8 depicts the stage right after inception of ignition process. VC (t) dVC (t) + CJ = il (t) 2RW dt
(1)
Taking Laplace transformation of Eq. (1) can be written as VC (t) + sCJ VC (s) = Il (s) 2RW
(2)
The leakage current alternates at sinusoidal power angular frequency ω, with magnitude of Il . However, it should be kept in mind the polarization taking place on the wooden surface due to the electric field is nonlinear and inhomogeneous [8]. It can be shown that the Laplace transform of the periodic leakage current should be: IL (s) =
ωIl s2 + ω 2
(3)
2 4ωCJ Il RW 2 1 + 4ω2 CJ2 RW
1
− 2Il RW
sin
(7)
2 1 + 4ω2 CJ2 RW
Assuming that no capacitance exists at the metal–wood junction the transient portion of (6) becomes trivial and the power 2I R I dissipated will be purely resistive ( √l W × √l = Il Rw ). However, if 2
2
a large capacitance is created then (6) can be simplified by using the approximation: 2 = 4ω2 CJ2 RW
VC (0) =
2RW ZCJ
2
Il 1 − sin ωCJ
>> 1. This will reduce (7) to:
(8)
In practice the transient process of (6) is likely to be repeated due to wind intermittency. The second or the steady state value of VC (t) can be reduced to: VC (t) =
Il sin ωt − ωCJ
(9)
Therefore, it can be postulated from (8) and (9) that the voltage across the dry wood area can be reduced by increasing the capacitance. Higher capacitance would suppress the real power dissipation at the edge of the bolt head. On the other hand, lowering the voltage at the varying junction impedance will significantly reduce the probability of arc voltage to build in the small area. Obviously it will not be smart to install a physical capacitor parallel to the dry wood resistance as it will create more metal–wood junctions that are vulnerable to catch fire. Secondly it is not possible to locate the connection point for the capacitor on the wood area. A new concept increasing the capacitance is discussed under Section 3. From the time domain plot of the power wave at the metal–wood junction the existence of asymmetry due to the transients parts of equation will be evident. Just to illustrate the case some of the typical values were used to plot Fig. 9. In this case the following typical values were used:
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100
Cj = 0.5 uF
Power flow through the junction impedance (Watts)
Cj = 0 uF
Cj = 2.2 uF
50
0
-50
0
0.01
0. 02
0.03
0.04
0.05
0. 06
0.07
0.08
0.09
0.1
Time (seconds) Fig. 9. Power wave at the bolt–wood junction for different value of capacitance.
• Leakage current, Il = 10 mA. • Junction capacitance (three scenarios)=0 F, 0.5 F, 2.2 F. • Wet wood resistance (throughout the pole surface) = 8.5 k. In Fig. 9, six cycles of power wave after the equivalent transient effect at the metal–wood junction is plotted using (6). For the repeated occurrence of such event, without any arrangement of suppressing the temperature, it can be elevated high enough to smolder the wood. In the example case the generated heating power could be as high as 850 W without any capacitance. 3. Prevention of pole top ignition at the metal–wood junction A number of power distribution companies undertook various pole fire prevention schemes including gradually phasing out the wooden poles by replacing them with concrete ones. A utility in Colorado Springs, Colorado found fiber–glass cross arms more resilient to pole ignitions compared to wooden cross arms [10]. However, both of the aforementioned strategies will incur high capital expenditure and time and therefore retarding to most of the regulated utility companies operating in the USA. Some other techniques such as insulator washing [11], painting the metal-wood connection with conducting paints [12] are also used. The discussion presented in [13,14] on the effect of chemical treatment of woods in pole fire inception did not result in reportedly effective prevention methods. Possible reasons for that might be the focus area of the investigation—the leakage current variation due to the variation of wood surface resistance but not the mechanism of the pole fire inception. In the next few subsections some other relatively economical and rugged methods are proposed. The methods are then compared based on their economic viability.
junction capacitance is random and widely varied with temperature and surrounding airborne components, wood surface physical properties etc. Therefore, it will be more useful if the capacitance can be increased by mechanical design of the bolt. CJ =
2εeff ln
b Farad per unit length
The capacitance formed at the geometry of the metal–wood junction, refers to Fig. 6 that resembles the capacitance formed along a small strip of a coaxial cable. In Fig. 10 the capacitance per unit length of the coaxial cable can be calculated from (10). There are three parameters in (10)—a, b and εeff. However, in the case described here the parameter ‘b’ is unknown and parameter ‘a’ is design standard. Therefore, choosing a dielectric with desired dielectric properties can be a viable solution to increase the capacitance. If the bolt could be coated with some dielectric with the following properties then it would be possible to prevent the bolt from acting as a cathode in the ignition process: • High dielectric constant to increase capacitance thus leading leakage current to junction voltage. • High thermal shock resistance. • High fracture toughness thus lower brittleness.
3.1. Increasing the metal–wood junction capacitance One can easily find resemblance of the cross section of a coated bolt in Fig. 10 with that of a coaxial cable except for a slight irregularity of the shape. If a capacitor could be connected physically in parallel it would increase the capacitance. However, two factors will make it complicated. First, the geometry of the metal junction capacitor is unknown or fictitious. Secondly, the value of
(10)
a
Fig. 10. Iron reinforced glass fiber bolt (top), section of coaxial cable.
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• Strongly adhesive to the metal surface (cohesive). • High elongation to rapture or elasticity so that it can support the variation of the substrate volume under the influence temperature variation. It may be hard to harness all of those expected features from a single material but many advances in material science leveraging manufacturing composite materials can fairly meet the expectation.
3.1.1. Dielectric coating on the wood-through bolt It is important to ensure that metal expansion due to temperature rise must not lead to the rupture of the dielectric coating. Choosing a particular coating technology consistent with the material is as important as the selecting right dielectric. For example plasma spraying is a much more expensive process compared to oxygen acetylene rod [15]. Perhaps, the second method can be used for coating bolts economically. Ceramic matrix composite (CMC) materials in which ceramic fibers are embedded with a specific coating exhibits superior qualities to the parent material. For example CMC material overcomes the major disadvantages of conventional ceramics. Common ceramic material coating suffers brittle failure, low fracture toughness and limited thermal shock resistance. In contrast, CMCs such as chemical vapor infiltrated silicon carbide with embedded silicon carbide fiber (CVI-SiC/SiC) [16] poses all of the features expected from a coating material for the bolt as mentioned in sub Section 2.1. A cheaper solution could be glass fiber coating. Glass fiber possesses excellent physical properties. Physical properties of a typical glass fiber material [17,18]: • • • • • • •
Tenacity—6.3–6.9 g/den (cohesion). Density 2.5 g/cc. Elongation at break—3%. Moisture Regain (MR%)—0%. Resiliency—Excellent. Dielectric constant—4.3–4.7. Insensitive to organic solvents, bleach, alkali, mildew etc.
3.1.2. Use of non-metallic mounting bolt Thermal conductivity of glass fiber is very low. Therefore, it should suppress the heat transfer through the metal–wood junction. Some other types of coating, e.g., Epoxy-401 [19], have long been used for metal surface coating exposed to the environment. However, even though coating the bolt might suppress the possibility of electrical arc it may not suppress the smoldering of the wood near the bolt–wood junction by the leakage. Smoldering could weaken wood fiber and compromise the structural integrity of the pole. The entire bolt can be made of dielectric such as Al2 O3 type of ceramic [20]. Some physical properties of Al2 O3 , such as durability shear force, bending moment etc. need to be verified before field deployment can be considered due to the weight it needs to carry safely. Physical properties of a typical Al2 O3 type of ceramic [20] • • • • • • •
Hard, wear-resistant. Excellent dielectric properties from DC to GHz frequencies. Resists strong acid and alkali attack at elevated temperatures. Good thermal conductivity. Excellent size and shape capability. High strength and stiffness. Available in purity ranges from 94% to 99.8% for the most demanding high temperature applications.
Fig. 11. Reducing energy density from resistive hitting using metallic strap.
3.2. Some other potential retrofits 3.2.1. Equipment grounding Observance of some utility case history/experience suggests that grounding the metal bolt should reduce the risk of ignition. The metal bolt is electrically connected to the equipment body through a small contact resistance. If the bolt is connected to ground, potential it should ground any transient voltage surges. More research work is required to justify the claim through lab test/simulation process. However, grounding of pole top equipment close to HV lines poses risk of unexpected flashover between live line and metallic enclosure of the equipment. 3.2.2. Metallic strap as a mean of bypassing bolt–wood junction The smoldering of wood is supported by high leakage current leading to high enough temperature that dries up wooden surface around the metal bolt much quicker than the area exposed to rain water or humid air. A technique that has the potential to bypass the vulnerable bolt–wood junction is depicted in Fig. 11. A perforated metal strap around the periphery of the wooden pole that is shorted to the mounting bolt and positioned below the cross arm which is otherwise exposed to air can potentially serve the purpose. First, it should eliminate the excessive voltage build up around the metal bolt which is physically surrounded by the dry wood area whereas the strap just acting as a jumper for the two segments of the wooden pole it divides above and below the strap. Secondly the current density of the discharge current (not the leakage current) due to voltage surge should be reduced. Note that perforated metallic strap is forced to be as the same potential as the metal bolt which is now the same as the pole surface potential. However, because of different polarization characteristics of metal and wood some heat energy should be dissipated at the metal–wood interface. Perforation reduces heat dissipation area but it will allow the wood surface under the strap to be more conductive in humid weather. 3.2.3. Reducing the pole surface conductance to suppress leakage current All of the methods described in the subsections before was aimed to prevent creation of arc voltage at the metal–wood junctions. The leakage current would remain fairly unaltered by those methods. It is known that the amplitude of leakage current dictates the rate of temperature rise of the wood surface. Therefore,
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M. Islam, H. Greg / Electric Power Systems Research 134 (2016) 213–221
Fig. 12. Increasing leakage current path impedance.
increasing the wooden pole surface resistivity is an obvious solution. For example an epoxy patch tightly wrapped/bonded around a portion of the wooden pole close to the ground should prevent airborne pollutant to alter the original conductance of that portion significantly. The high resistivity of weather shielded portion will come in series with the exposed pole surface resistance. The outer surface of the patch should be protected against continuous flow of water as shown in Fig. 12. 4. Comparison of the methods All of the methods can be combined. However, economic viability, simplicity and time for deployment likely dictate prudency in opting for any or combination of those methods. In this subsection a comparative discussion is presented as support for good decision making experience. At first the following cases are introduced: • Case 1: High voltage overhead lines as source of leakage current. • Case 2: Equipment on cross arm as the source of leakage current. • Case 3: Both the overhead lines and equipment on cross arm is the source of leakage current. As it sounds Case 3 depicts the worst case scenario when a massive arc flash is fairly certain. In this case the phase difference of the leakage sources may cause unrepairable damages to equipment, pole and even the overhead lines. In fact none of the methods
described in this paper can be used to prevent this to happen. Therefore, in the comparative discussion Case 3 will not be considered. The following metrics will be used to justify each of the methods:
• Retrofit—Yes (1) or No (0). • Cost index—(1 through 5) 5 is for lowest cost and 1 is for highest. • Reliability—(1 to 5) 5 for highly reliable. Comment on associated risk if necessary. • Deployment simplicity—(1 through 5) 1 is for very complex and 5 is for very simple. • Case(s) of leakage current it can handle—Number of cases. Maximum 2.
Method 1: Dielectric coating on the bolt (total score: 8)RetrofitCost indexReliabilitySimplicityCases01[3] Will reduce smoldering of wood and prevent ignition risk as well.22 Method 2: Use of nonmetallic bolt (total score: 7)RetrofitCost indexReliabilitySimplicityCases01[2] Prevent ignition but structural integrity may be compromised.22 Method 3: Equipment grounding (total score: 12)RetrofitCost indexReliabilitySimplicityCases13[3] Will reduce smoldering and ignition of wood. Proper grounding necessary for safety.41 (case 2) Method 4: Use of metallic strap (total score: 15)RetrofitCost indexReliabilitySimplicityCases15[3] Will reduce chance of smoldering and risk of ignition by removing voltage stress.51 (case 1)
M. Islam, H. Greg / Electric Power Systems Research 134 (2016) 213–221
Method 5: Epoxy patch (total score: 11)RetrofitCost indexReliabilitySimplicityCases12[3] Reduce leakage current therefore smoldering. Will not reduce ignition risk.32 (case 1) It should be kept in mind that the score for each method can vary from a viewpoint to another viewpoint and therefore is not suggestive. The intention behind this comparison chart was to show how researches at National Grid USA viewed the solution methods for adoption. It is very clear from the score column that Method 4 (use of metallic strap) was opted for deployment. However, it is obviously not the best choice for all cases but merely indicates the most favorable choice within the department’s scope of work. 5. Conclusion The pole fire ignition mechanism is explained with comprehensive laws of natural physics. A circuit model for the mechanism is analyzed using Laplace transformation theory that indicates that the pole fire is sustained by the leakage current but the ignition is incepted from the randomly varying high voltage at the metal–wood junction. It was concluded based on the finding that increasing the capacitance at the metal–wood junction should significantly reduce the risk electrical arc leading to pole fire. Some of the retrofit options discussed may be viable in reducing the arc voltage. A method is also described as to impede the leakage current which should reduce resistive heating and therefore smoldering of the wood. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.epsr.2015.07.017. References [1] P.J. Sokolowski, A. Dwivedi, S. Pathak, F. Buratto, Y. Xinghuo, Investigating the impedance of a wooden power pole after a pole fire, in: Proceedings of the Australasian Universities Power Engineering Conference (AUPEC’08), IEEE Xplore Conference Proceeding, Sydney, Australia, 2008 [2009].
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[2] L.B. Loeb, J.M. Meek, The mechanism of the electric spark, Stanford University Press, Redwood, USA, 1941, pp. 180. [3] K.L. Wong, S. Pathak, X. Yu, Aging effect on leakage current flow in wooden pole, IEEE Trans Dielectr Electr Insul 16 (1 (February)) (2009). [4] K.L. Wong, M.F. Rahmat, Investigation of leakage current in wooden pole using ladder network model, in: Proceedings of the Australian Power Engineering Conference (OPEC’08), IEEE, Piscataway, NJ, 2008. [5] K.L. Wong, M.F. Rahmat, Study of leakage current distribution in wooden pole using ladder network model, in: IEEE Trans Power Deliv. 25, 2010, pp. 995–1000, 2 (April). [6] K.L. Wong, M.F. Rahmat, Feasibility study of leakage current shunting method based on the ladder network model, in: IEEE Trans Power Deliv. 25, 2010, pp. 1133–1137, 2 (April). [7] A.E. Vlastos, T. Orbeck, Outdoor leakage current monitoring of silicone composite insulators in coastal service conditions, in: IEEE Trans Power Deliv. 11, 1996, pp. 1066–1070, 2 (April). [8] J.L. William, Dielectric properties of wood and hardboard: variation with temperature, frequency, moisture content and grain orientation, Forest Products Laboratory, USDA Forest Service, Madison, USA, 1975 [research paper]. [9] D.K. Cheng, Analysis of linear system, Narosa Publishing House, New Delhi, 2001, pp. 400. [10] S. Torres, Colorado Springs Utilities, Utility extinguishes risk of pole-top fires, T&D World Mag (2014) 52–53 [27.01.14]. [11] D. Perin, A. Pigini, I. Visintainer, C.M. Ramamoorty, Live line insulator washing: experimental investigation to assess safety and efficiency requirements, IEEE Trans Power Deliv 10 (1995) 518–525. [12] M. Darveniza, G.J. Limbourn, S.A. Prentice, Line design and electrical properties of wood, IEEE Trans Power Appar Syst 86 (1967) 1344–1356. [13] A.R. Katz, D.G. Miller, Efforts of some preservatives on the electrical resistance on the electrical resistance of the red pine, Am Wood-Preserv Assoc 59 (1963) 204–217. [14] R. Filter, The influence of wood pole preservatives on wood fire and electrical safety, IEEE Trans Power Appar Syst 103 (1984) 3089–3095. [15] Available: http://www.thomasnet.com/articles/chemicals/ceramic-coatingmetals [accessed 28.10.14]. [16] Hinoki T, Lara-Curzio E, Snead LI, “Mechanical properties of high purity SiC fiber-reinforced CVI-SiC matrix composites”. Oak Ridge National Laboratory, Oakridge TN37831. Available: http://web.ornl.gov/∼webworks/cppr/ y2001/pres/116946.pdf [accessed on 28.09.14]. [17] Available: http://textilefashionstudy.com/glass-fiber-physical-and-chemicalproperties-of-glass-fiber/. [18] Wallenberger FT, Watson JC, Hong Li, PPG Industries Inc., “Fiber Glass”; ASM Handbook, Vol. 21: Composites (#06781G). Available from: www. asminternational.org. [19] Datasheet can be available from: http://www.globalspec.com/ds/3476/ areaspec/feature dielectric. [20] Available from: http://accuratus.com/alumox.html.
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Electric Power Systems Research journal homepage: www.elsevier.com/locate/epsr
Methods to reduce the risk of wooden pole ignition M. Islam a,∗ , H. Greg a,b a b
Advanced Grid Engineering, National Grid USA Service Co., Syracuse, NY, 13202, USA Advanced Grid Engineering, National Grid USA Service Co., Waltham, MA, 02451, USA
a r t i c l e
i n f o
Article history: Received 22 May 2015 Received in revised form 9 July 2015 Accepted 29 July 2015 Available online 31 December 2015 Keywords: Wooden poles Insulation degradation Leakage current Bolt–wood junction
a b s t r a c t A number of reported wooden pole ignition incidences show evidence that the source was located near the pole bolt/pin mounting hardware. This has been observed for environmentally stressed insulators, as well as for larger overhead equipment in situations where the wooden pole has become the dominant part of the leakage conduction path through loss/deterioration of a case ground conductor. In both of these cases, the bolt-to-wood interface is the focal point for the leakage current. A mechanism which can cause pole ignition in these situations is examined with comprehensive details. This paper suggests a novel method to reduce the risk of wooden pole ignition. The mathematical basis of the suggested method is also explained. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Wooden poles are common all over the world as part of the physical transmission and distribution network. While wooden poles are insulators by nature, the conductance of wooden pole surfaces exposed to the environment can increase based on airborne contaminations. In addition, airborne contamination and weather driven cyclic stresses can deteriorate the porcelain used for the insulation of conduction wires. Medium and high voltage conductors are insulated from the wooden pole by way of pin glass/ceramic insulators. In this situation, the wooden pole is deliberately part of the leakage path, but the insulator is sufficiently specified to prevent significant leakage currents. In addition, utilities design to have a minimum distance of wood between primary and other ground/secondary conductors. This distance concept, as well as the typical construction for overhead pin insulators is shown in Fig. 1. Overhead equipment with metallic cases, such as reclosers and capacitors, are installed with a physical down-pole wire between the metallic equipment case, secondary neutral, and earth ground. This equipment has insulating devices, between the primary voltages and case, specified to prevent significant leakage current. However, the deterioration of the ground wire over time, theft, or other means of absence, can create a failure condition which allows the wooden pole itself to be the primary conduction path for leakage current, where it normally would not. Missing ground
∗ Corresponding author. Tel.: +1 9792299703. E-mail addresses: [email protected], [email protected] (M. Islam), [email protected] (H. Greg). http://dx.doi.org/10.1016/j.epsr.2015.07.017 0378-7796/© 2015 Elsevier B.V. All rights reserved.
conductor, combined with insulator degradation, can lead to the effect being examined similar to the above pin insulator scenario, conducting through the mounting bolts of the device. In addition, many of these devices have internal components, such as power supplies or other capacitive coupled devices, which can present the full voltage on the case when the ground cable is removed. The mounting bolts and typical installation can be seen in Fig. 2. With prolonged exposure the insulation integrity of both of these examples can become compromised, and therefore the magnitude of the leakage current would also increase. An IR camera captured the persistent incandescent glow of the bolt caused by the heat produced as result of the leakage current flow in each wood specimen. The threshold value at which the leakage current caused smoldering and glow in wood was measured as 3.2 mA in specimen 1 and 4.1 mA in specimen 2. These observations validated some of the anecdotal evidence which suggests that the ignition of pole fires usually takes place in the junction between the metallic accessories such as king bolts or insulator base-pins and the timber pole (Figs. 3 and 4). This junction forms a hotspot in the wooden structure and is prone to smoldering. The potential mechanisms behind ignition will be explained in the Section 2. Analysis of past wooden pole ignition events suggests that the ignition started at the bolt metal–wood junction. The photographs shown below further support this concept through visual inspection. It is also found to be consistent in laboratory tests that the metal–wood junction is the most susceptible to the pole ignition process [1] contingent to wood smoldering due to localized high temperature. It is worth noting that, in the absence of a valid case ground connection, overhead equipment also poses the risk of accidental electrocution to workers. Lack of ground connections allow stray voltages to be present
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on what otherwise would be grounded surfaces. While most utility workers are trained to always assume that these surfaces are ‘hot’ when first approaching them, there is still an increased safety risk. Unfortunately, no utilities today are actively monitoring for a valid ground connection to each overhead device, as this would be an expensive proposition. 2. Mechanisms behind ignition 2.1. Smoldering followed by high voltage arc at the bolt–wood junction Older wooden pole surfaces exposed to harsh weather conditions and seasonal stress can develop enough conductivity from surface contamination often assist pole ignition [1]. Brine precipitation can be built up on the wood surface providing a lower impedance path down the pole. These surface contaminants allow sufficiently large tracking current to develop in the wooden pole. Close to the point of origin, near the wood metal junction, high temperatures are created. This process can begin with a corona discharge that ionizes the dry air in the vicinity of the bolt head. Wind can also intermittently blow ionized air, exposing its high resistivity to the leakage current. Due to high impedance, it can create a high voltage in the bolt–wood junction. It is shown in the subsequent analysis that the leakage current passed through this small area poses a high enough thermal energy density per unit area to ignite the pole. After ignition, this fire is sustained by the dry, due to the
subsequent temperature rise, wood surrounding the heated area. At a high enough temperature to smolder the dry wood surface, a subsequent electric arc can lead the dry wood surface (hydrocarbon) to sustain ignition. The intensity of the liberated energy from the arc would be sustained by the magnitude of the leakage current. The ignition process is strongly aided by the wood-through metal bolt that attaches through the pole. It continually forms the mechanical vulnerable junction with the wooden pole. Metal exposed to open environment is subject to oxidation, which means that metals such as iron loose electrons in the presence of environmental catalyst such as sunlight, temperature, water vapor and airborne impurity such as positive ions. The rate of oxidation of metal surface due to its exposure to varying environmental condition varies from one kind of metal or metal alloy to another kind. Regardless of how long a certain metal or metal alloy may take to show a visible oxidation symptom it is well observed that the reaction starts with loosing electrons to air containing water molecule, acid or salt within short period of time, typically a few weeks. This oxidation process causes the metal to be positively charged and act as cathode in the electrical sparking process [2]. 2.2. Voltage surge at the ungrounded pole mounted equipment Pole top equipment including transformers, reclosers, line charging capacitors or isolators have a grounding contact. However, grounding is deemed optional by most of the electrical regulatory body for the pole mounted equipment more than 8 feet above
Fig. 1. Typical overhead pin construction, showing leakage path.
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Fig. 2. Typical recloser overhead installation.
Fig. 3. Fire started at the wood-metal junction (Source: http://incident-prevention.com/ip-articles/responding-to-pole-fires).
the ground. Transient voltage surges can originate from different phenomena including lightning, surge on metallic enclosures of the equipment, open circuit faults, or from cosmic failure of solid state switches. In such situations, an electric spark is a common
incident that leads to pole fires. There is also good possibility of consecutive occurrences of internal faults and natural hazards (tree falls, heavy rain) leading to electrical fires. The voltage build up at the metal–wood junction will seek lower impedance path to
Fig. 4. Evidence of burn near the bolt-eye from the pole through bolt.
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Fig. 5. Electrical arc mechanism in wooden poles.
neutralize unless a ground path is provided. If the transient current flows through the wooden pole surface with elevated conductance due to aging resistive heating can lead to pole fire.
ionized air intermittently. Removal of the ionized air from the junction switches the high resistance back on the circuit model. The leakage current through this high resistance dissipates a large amount of heat.
2.3. Model of pole fire inception process 2.4. Equivalent circuit model The model of the pole fire inception process will be explained in sub Section 2.4. A junction capacitance is formed by polarization of the metal with positive charge and the pole surface close to the metal–wood junction with negative charge. However, the capacitance formed will exhibit high loss tangent due to the contaminated air dielectric and will be variable due to random variation in the level of contamination. The tracking layer on the wood surface is considered as galvanic contact to the model of the capacitance. Tracking is a high conductive layer formed due to water, air-born pollutant on the wooden surface. The ionized air in the vicinity of the metal provides a varying low resistive path parallel to metal–wood junction capacitance. This low impedance path prevents heat build-up since it bypasses the high resistance of dry wood area. However, blowing wind can wipe out the
Many research papers have been published on pole top ignition in the last decade. Some of the authors proposed their own electrical equivalent circuit models for the simulation purpose. For example, rain water modified simple resistive network in [3] and ladder network model [4–7]. All of the models are perception based and not justified through lab tests, but supported by the engineering judgments of the authors. In the model presented here, the same principle was followed–resistive network energized by the leakage current and terminated at the ground impedance. The unique nature of the model in this paper is that it can explain the voltage stress at the bolt–wood junction and is consistent with an electrical sparking mechanism that the authors thought to be the most likely case. In the equivalent circuit the exposed surface of the wooden
Fig. 6. Equivalent circuit of pole leakage current.
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Using (3) and solving (2) for the transformed voltage VC (s)is given by: VC (s) =
⇒ VC (s) =
ωIl 1/ (2RW ) + sCJ
s2 + ω 2
ωIl /CJ
1/ 2RW CJ + s
s2 + ω 2
(4)
To find inverse Laplace transform of (4) the following identity [9] can be used 1 (s+a)(s2 +b2 ) tan−1 ba
Fig. 7. Equivalent circuit prior to ignition.
e−at +
1 a2 +b2
=
1 b
a2 + b2 sin bt −
, where =
Applying the identity the time domain expression for the junction voltage can be found as: VC (t) =
ωIl /CJ a2
+ ω2
e−at +
1 2 a + ω2 sin ωt − ω
(5)
where, = tan−1 2RW ωCJ indicates that the metal–wood junction voltage will lag the leakage peak current by the angle and a = 1/ 2RW CJ . Expanding (5): VC (t) =
2 4ωCJ Il RW 2 1 + 4ω2 CJ 2 RW
e−at + 2Il RW
1
sin ωt −
2 1 + 4ω2 CJ2 RW
(6) and evaluating (6) at t = 0 it can be shown Fig. 8. Equivalent circuit after heating process of ignition.
VC (0) =
pole (Fig. 5) is modeled by resistance RW (Fig. 6) which is usually in the order of several hundred ohms to a few kilo ohms based on the level of exposure to environmental impurity [8]. The tracking layer (electrolytic precipitation) exhibits high conductivity in moist weather. It is shown as Rtr in Fig. 6. In series with leakage current source Il (t). The ionized air in the dry wood area provides a low resistive path through Rion effectively bypassing the dry wood resistance Rdw and the metal–wood junction capacitance CJ . The resistance of the Rion is of the order of few ohms whereas dry wood exhibits from several thousands to a few mega ohms range based on the moisture content [8]. Even though the mechanism involves non linear processes it can be explained using linear transform theory such as Laplace transform for short term dynamics of the system expressed by the differential Eq. (1). Fig. 7 is used to show the initial status of the equivalent circuit before the ignition process begins. Fig. 8 depicts the stage right after inception of ignition process. VC (t) dVC (t) + CJ = il (t) 2RW dt
(1)
Taking Laplace transformation of Eq. (1) can be written as VC (t) + sCJ VC (s) = Il (s) 2RW
(2)
The leakage current alternates at sinusoidal power angular frequency ω, with magnitude of Il . However, it should be kept in mind the polarization taking place on the wooden surface due to the electric field is nonlinear and inhomogeneous [8]. It can be shown that the Laplace transform of the periodic leakage current should be: IL (s) =
ωIl s2 + ω 2
(3)
2 4ωCJ Il RW 2 1 + 4ω2 CJ2 RW
1
− 2Il RW
sin
(7)
2 1 + 4ω2 CJ2 RW
Assuming that no capacitance exists at the metal–wood junction the transient portion of (6) becomes trivial and the power 2I R I dissipated will be purely resistive ( √l W × √l = Il Rw ). However, if 2
2
a large capacitance is created then (6) can be simplified by using the approximation: 2 = 4ω2 CJ2 RW
VC (0) =
2RW ZCJ
2
Il 1 − sin ωCJ
>> 1. This will reduce (7) to:
(8)
In practice the transient process of (6) is likely to be repeated due to wind intermittency. The second or the steady state value of VC (t) can be reduced to: VC (t) =
Il sin ωt − ωCJ
(9)
Therefore, it can be postulated from (8) and (9) that the voltage across the dry wood area can be reduced by increasing the capacitance. Higher capacitance would suppress the real power dissipation at the edge of the bolt head. On the other hand, lowering the voltage at the varying junction impedance will significantly reduce the probability of arc voltage to build in the small area. Obviously it will not be smart to install a physical capacitor parallel to the dry wood resistance as it will create more metal–wood junctions that are vulnerable to catch fire. Secondly it is not possible to locate the connection point for the capacitor on the wood area. A new concept increasing the capacitance is discussed under Section 3. From the time domain plot of the power wave at the metal–wood junction the existence of asymmetry due to the transients parts of equation will be evident. Just to illustrate the case some of the typical values were used to plot Fig. 9. In this case the following typical values were used:
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100
Cj = 0.5 uF
Power flow through the junction impedance (Watts)
Cj = 0 uF
Cj = 2.2 uF
50
0
-50
0
0.01
0. 02
0.03
0.04
0.05
0. 06
0.07
0.08
0.09
0.1
Time (seconds) Fig. 9. Power wave at the bolt–wood junction for different value of capacitance.
• Leakage current, Il = 10 mA. • Junction capacitance (three scenarios)=0 F, 0.5 F, 2.2 F. • Wet wood resistance (throughout the pole surface) = 8.5 k. In Fig. 9, six cycles of power wave after the equivalent transient effect at the metal–wood junction is plotted using (6). For the repeated occurrence of such event, without any arrangement of suppressing the temperature, it can be elevated high enough to smolder the wood. In the example case the generated heating power could be as high as 850 W without any capacitance. 3. Prevention of pole top ignition at the metal–wood junction A number of power distribution companies undertook various pole fire prevention schemes including gradually phasing out the wooden poles by replacing them with concrete ones. A utility in Colorado Springs, Colorado found fiber–glass cross arms more resilient to pole ignitions compared to wooden cross arms [10]. However, both of the aforementioned strategies will incur high capital expenditure and time and therefore retarding to most of the regulated utility companies operating in the USA. Some other techniques such as insulator washing [11], painting the metal-wood connection with conducting paints [12] are also used. The discussion presented in [13,14] on the effect of chemical treatment of woods in pole fire inception did not result in reportedly effective prevention methods. Possible reasons for that might be the focus area of the investigation—the leakage current variation due to the variation of wood surface resistance but not the mechanism of the pole fire inception. In the next few subsections some other relatively economical and rugged methods are proposed. The methods are then compared based on their economic viability.
junction capacitance is random and widely varied with temperature and surrounding airborne components, wood surface physical properties etc. Therefore, it will be more useful if the capacitance can be increased by mechanical design of the bolt. CJ =
2εeff ln
b Farad per unit length
The capacitance formed at the geometry of the metal–wood junction, refers to Fig. 6 that resembles the capacitance formed along a small strip of a coaxial cable. In Fig. 10 the capacitance per unit length of the coaxial cable can be calculated from (10). There are three parameters in (10)—a, b and εeff. However, in the case described here the parameter ‘b’ is unknown and parameter ‘a’ is design standard. Therefore, choosing a dielectric with desired dielectric properties can be a viable solution to increase the capacitance. If the bolt could be coated with some dielectric with the following properties then it would be possible to prevent the bolt from acting as a cathode in the ignition process: • High dielectric constant to increase capacitance thus leading leakage current to junction voltage. • High thermal shock resistance. • High fracture toughness thus lower brittleness.
3.1. Increasing the metal–wood junction capacitance One can easily find resemblance of the cross section of a coated bolt in Fig. 10 with that of a coaxial cable except for a slight irregularity of the shape. If a capacitor could be connected physically in parallel it would increase the capacitance. However, two factors will make it complicated. First, the geometry of the metal junction capacitor is unknown or fictitious. Secondly, the value of
(10)
a
Fig. 10. Iron reinforced glass fiber bolt (top), section of coaxial cable.
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• Strongly adhesive to the metal surface (cohesive). • High elongation to rapture or elasticity so that it can support the variation of the substrate volume under the influence temperature variation. It may be hard to harness all of those expected features from a single material but many advances in material science leveraging manufacturing composite materials can fairly meet the expectation.
3.1.1. Dielectric coating on the wood-through bolt It is important to ensure that metal expansion due to temperature rise must not lead to the rupture of the dielectric coating. Choosing a particular coating technology consistent with the material is as important as the selecting right dielectric. For example plasma spraying is a much more expensive process compared to oxygen acetylene rod [15]. Perhaps, the second method can be used for coating bolts economically. Ceramic matrix composite (CMC) materials in which ceramic fibers are embedded with a specific coating exhibits superior qualities to the parent material. For example CMC material overcomes the major disadvantages of conventional ceramics. Common ceramic material coating suffers brittle failure, low fracture toughness and limited thermal shock resistance. In contrast, CMCs such as chemical vapor infiltrated silicon carbide with embedded silicon carbide fiber (CVI-SiC/SiC) [16] poses all of the features expected from a coating material for the bolt as mentioned in sub Section 2.1. A cheaper solution could be glass fiber coating. Glass fiber possesses excellent physical properties. Physical properties of a typical glass fiber material [17,18]: • • • • • • •
Tenacity—6.3–6.9 g/den (cohesion). Density 2.5 g/cc. Elongation at break—3%. Moisture Regain (MR%)—0%. Resiliency—Excellent. Dielectric constant—4.3–4.7. Insensitive to organic solvents, bleach, alkali, mildew etc.
3.1.2. Use of non-metallic mounting bolt Thermal conductivity of glass fiber is very low. Therefore, it should suppress the heat transfer through the metal–wood junction. Some other types of coating, e.g., Epoxy-401 [19], have long been used for metal surface coating exposed to the environment. However, even though coating the bolt might suppress the possibility of electrical arc it may not suppress the smoldering of the wood near the bolt–wood junction by the leakage. Smoldering could weaken wood fiber and compromise the structural integrity of the pole. The entire bolt can be made of dielectric such as Al2 O3 type of ceramic [20]. Some physical properties of Al2 O3 , such as durability shear force, bending moment etc. need to be verified before field deployment can be considered due to the weight it needs to carry safely. Physical properties of a typical Al2 O3 type of ceramic [20] • • • • • • •
Hard, wear-resistant. Excellent dielectric properties from DC to GHz frequencies. Resists strong acid and alkali attack at elevated temperatures. Good thermal conductivity. Excellent size and shape capability. High strength and stiffness. Available in purity ranges from 94% to 99.8% for the most demanding high temperature applications.
Fig. 11. Reducing energy density from resistive hitting using metallic strap.
3.2. Some other potential retrofits 3.2.1. Equipment grounding Observance of some utility case history/experience suggests that grounding the metal bolt should reduce the risk of ignition. The metal bolt is electrically connected to the equipment body through a small contact resistance. If the bolt is connected to ground, potential it should ground any transient voltage surges. More research work is required to justify the claim through lab test/simulation process. However, grounding of pole top equipment close to HV lines poses risk of unexpected flashover between live line and metallic enclosure of the equipment. 3.2.2. Metallic strap as a mean of bypassing bolt–wood junction The smoldering of wood is supported by high leakage current leading to high enough temperature that dries up wooden surface around the metal bolt much quicker than the area exposed to rain water or humid air. A technique that has the potential to bypass the vulnerable bolt–wood junction is depicted in Fig. 11. A perforated metal strap around the periphery of the wooden pole that is shorted to the mounting bolt and positioned below the cross arm which is otherwise exposed to air can potentially serve the purpose. First, it should eliminate the excessive voltage build up around the metal bolt which is physically surrounded by the dry wood area whereas the strap just acting as a jumper for the two segments of the wooden pole it divides above and below the strap. Secondly the current density of the discharge current (not the leakage current) due to voltage surge should be reduced. Note that perforated metallic strap is forced to be as the same potential as the metal bolt which is now the same as the pole surface potential. However, because of different polarization characteristics of metal and wood some heat energy should be dissipated at the metal–wood interface. Perforation reduces heat dissipation area but it will allow the wood surface under the strap to be more conductive in humid weather. 3.2.3. Reducing the pole surface conductance to suppress leakage current All of the methods described in the subsections before was aimed to prevent creation of arc voltage at the metal–wood junctions. The leakage current would remain fairly unaltered by those methods. It is known that the amplitude of leakage current dictates the rate of temperature rise of the wood surface. Therefore,
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Fig. 12. Increasing leakage current path impedance.
increasing the wooden pole surface resistivity is an obvious solution. For example an epoxy patch tightly wrapped/bonded around a portion of the wooden pole close to the ground should prevent airborne pollutant to alter the original conductance of that portion significantly. The high resistivity of weather shielded portion will come in series with the exposed pole surface resistance. The outer surface of the patch should be protected against continuous flow of water as shown in Fig. 12. 4. Comparison of the methods All of the methods can be combined. However, economic viability, simplicity and time for deployment likely dictate prudency in opting for any or combination of those methods. In this subsection a comparative discussion is presented as support for good decision making experience. At first the following cases are introduced: • Case 1: High voltage overhead lines as source of leakage current. • Case 2: Equipment on cross arm as the source of leakage current. • Case 3: Both the overhead lines and equipment on cross arm is the source of leakage current. As it sounds Case 3 depicts the worst case scenario when a massive arc flash is fairly certain. In this case the phase difference of the leakage sources may cause unrepairable damages to equipment, pole and even the overhead lines. In fact none of the methods
described in this paper can be used to prevent this to happen. Therefore, in the comparative discussion Case 3 will not be considered. The following metrics will be used to justify each of the methods:
• Retrofit—Yes (1) or No (0). • Cost index—(1 through 5) 5 is for lowest cost and 1 is for highest. • Reliability—(1 to 5) 5 for highly reliable. Comment on associated risk if necessary. • Deployment simplicity—(1 through 5) 1 is for very complex and 5 is for very simple. • Case(s) of leakage current it can handle—Number of cases. Maximum 2.
Method 1: Dielectric coating on the bolt (total score: 8)RetrofitCost indexReliabilitySimplicityCases01[3] Will reduce smoldering of wood and prevent ignition risk as well.22 Method 2: Use of nonmetallic bolt (total score: 7)RetrofitCost indexReliabilitySimplicityCases01[2] Prevent ignition but structural integrity may be compromised.22 Method 3: Equipment grounding (total score: 12)RetrofitCost indexReliabilitySimplicityCases13[3] Will reduce smoldering and ignition of wood. Proper grounding necessary for safety.41 (case 2) Method 4: Use of metallic strap (total score: 15)RetrofitCost indexReliabilitySimplicityCases15[3] Will reduce chance of smoldering and risk of ignition by removing voltage stress.51 (case 1)
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Method 5: Epoxy patch (total score: 11)RetrofitCost indexReliabilitySimplicityCases12[3] Reduce leakage current therefore smoldering. Will not reduce ignition risk.32 (case 1) It should be kept in mind that the score for each method can vary from a viewpoint to another viewpoint and therefore is not suggestive. The intention behind this comparison chart was to show how researches at National Grid USA viewed the solution methods for adoption. It is very clear from the score column that Method 4 (use of metallic strap) was opted for deployment. However, it is obviously not the best choice for all cases but merely indicates the most favorable choice within the department’s scope of work. 5. Conclusion The pole fire ignition mechanism is explained with comprehensive laws of natural physics. A circuit model for the mechanism is analyzed using Laplace transformation theory that indicates that the pole fire is sustained by the leakage current but the ignition is incepted from the randomly varying high voltage at the metal–wood junction. It was concluded based on the finding that increasing the capacitance at the metal–wood junction should significantly reduce the risk electrical arc leading to pole fire. Some of the retrofit options discussed may be viable in reducing the arc voltage. A method is also described as to impede the leakage current which should reduce resistive heating and therefore smoldering of the wood. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.epsr.2015.07.017. References [1] P.J. Sokolowski, A. Dwivedi, S. Pathak, F. Buratto, Y. Xinghuo, Investigating the impedance of a wooden power pole after a pole fire, in: Proceedings of the Australasian Universities Power Engineering Conference (AUPEC’08), IEEE Xplore Conference Proceeding, Sydney, Australia, 2008 [2009].
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[2] L.B. Loeb, J.M. Meek, The mechanism of the electric spark, Stanford University Press, Redwood, USA, 1941, pp. 180. [3] K.L. Wong, S. Pathak, X. Yu, Aging effect on leakage current flow in wooden pole, IEEE Trans Dielectr Electr Insul 16 (1 (February)) (2009). [4] K.L. Wong, M.F. Rahmat, Investigation of leakage current in wooden pole using ladder network model, in: Proceedings of the Australian Power Engineering Conference (OPEC’08), IEEE, Piscataway, NJ, 2008. [5] K.L. Wong, M.F. Rahmat, Study of leakage current distribution in wooden pole using ladder network model, in: IEEE Trans Power Deliv. 25, 2010, pp. 995–1000, 2 (April). [6] K.L. Wong, M.F. Rahmat, Feasibility study of leakage current shunting method based on the ladder network model, in: IEEE Trans Power Deliv. 25, 2010, pp. 1133–1137, 2 (April). [7] A.E. Vlastos, T. Orbeck, Outdoor leakage current monitoring of silicone composite insulators in coastal service conditions, in: IEEE Trans Power Deliv. 11, 1996, pp. 1066–1070, 2 (April). [8] J.L. William, Dielectric properties of wood and hardboard: variation with temperature, frequency, moisture content and grain orientation, Forest Products Laboratory, USDA Forest Service, Madison, USA, 1975 [research paper]. [9] D.K. Cheng, Analysis of linear system, Narosa Publishing House, New Delhi, 2001, pp. 400. [10] S. Torres, Colorado Springs Utilities, Utility extinguishes risk of pole-top fires, T&D World Mag (2014) 52–53 [27.01.14]. [11] D. Perin, A. Pigini, I. Visintainer, C.M. Ramamoorty, Live line insulator washing: experimental investigation to assess safety and efficiency requirements, IEEE Trans Power Deliv 10 (1995) 518–525. [12] M. Darveniza, G.J. Limbourn, S.A. Prentice, Line design and electrical properties of wood, IEEE Trans Power Appar Syst 86 (1967) 1344–1356. [13] A.R. Katz, D.G. Miller, Efforts of some preservatives on the electrical resistance on the electrical resistance of the red pine, Am Wood-Preserv Assoc 59 (1963) 204–217. [14] R. Filter, The influence of wood pole preservatives on wood fire and electrical safety, IEEE Trans Power Appar Syst 103 (1984) 3089–3095. [15] Available: http://www.thomasnet.com/articles/chemicals/ceramic-coatingmetals [accessed 28.10.14]. [16] Hinoki T, Lara-Curzio E, Snead LI, “Mechanical properties of high purity SiC fiber-reinforced CVI-SiC matrix composites”. Oak Ridge National Laboratory, Oakridge TN37831. Available: http://web.ornl.gov/∼webworks/cppr/ y2001/pres/116946.pdf [accessed on 28.09.14]. [17] Available: http://textilefashionstudy.com/glass-fiber-physical-and-chemicalproperties-of-glass-fiber/. [18] Wallenberger FT, Watson JC, Hong Li, PPG Industries Inc., “Fiber Glass”; ASM Handbook, Vol. 21: Composites (#06781G). Available from: www. asminternational.org. [19] Datasheet can be available from: http://www.globalspec.com/ds/3476/ areaspec/feature dielectric. [20] Available from: http://accuratus.com/alumox.html.