Chemical analysis of insulator contaminants and Reliability improvement of T&D line for smart operation of grid under chemically polluted environment

Transcription

1 Chennai and Dr.MGR University Second International Conference on Sustainable Energy and Intelligent System (SEISCON 2011), Dr. M.G.R. University, Maduravoyal, Chennai, Tamil Nadu, India. July , Chemical analysis of insulator contaminants and Reliability improvement of T&D line for smart operation of grid under chemically polluted environment Md Farman1, Salman Ahmad2 Afroz Alam3 and Dr. M. P. Sharma4 Alternate Hydro Energy Centre, Indian Institute of technology Roorkee, India md.farman.ee. salmanahmad afroz.iit mpshafah Keywords: Blackout, ESDD, flashover voltage, LCM, ph, power quality, soluble and insoluble metal salts and its ions. Abstract This paper presents a practical issue of blackout & brownouts which normally occurs in T&D lines under chemically polluted environment and adversely effects power quality of line. Different types and sources of chemical pollutants under different atmospheric conditions are discussed. Chemical factors which regulates the magnitude of leakage current (flashover voltage) and methods of measurement of degree of contamination are discussed. Chemical analysis of various salts (soluble as well as insoluble) and ions on the flashover voltage under dry and wet condition is done to see the effectiveness of salts & ions on performance of line. Based on Hall Effect and magnetic fields, a new approach for improving grid reliability and maintenance work for T&D lines is proposed. Effect of ph on the surface conductivity is also analyzed. It is found that by increasing contamination level there is increase in surface conductivity of the insulator and reduces flashover voltage. It is also concluded that as basic or acidic nature of salt increases, in both cases flashover voltage reduces drastically. 1 Introduction In recent years, for meeting consumers increased demand T&D companies are bound to improve the reliability & efficiency of transmission lines. Efficiency of the system is based mainly on the continuity of the service, avoiding faults that minimize economical losses for companies and users. To maintain this continuity, one of the main problems that have been found is the effect produced by pollution on the insulators of electric lines. This pollution is one of the main causes of flashover on the insulators. The insulator begins to fail when the pollutants that exist in the air settle in the surface of the insulator and combine with the the fog, rain or dew. The mixture of pollutants, plus the humidity form a layer that can become conducting and passage of current that will facilitate the conditions of short circuit. In other words, pollution degrades insulators and affects Severely to their electric characteristics, being one of the main causes of miss operation of insulators. Flashover of contaminated insulator in pollution areas proved to be one of the most important factors influencing the design of EHV & UHV transmission line insulation. The mathematical analysis of the phenomenon involved is quite complicated so the majority of investigations in this field are mainly of experimental in nature. Test of contaminated insulators is roughly classified into Natural and Artifi cial types. In artificial tests insulators are dipped in contaminated solution prepared according to the contamination occurring in actual region and flashover tests (in dry and wet conditions) are performed in laboratory. The results obtained by this can be approximated with natural flashover of insulators. The present study has been undertaken to investigate the performance of insulators under artificially polluted conditions. The effects of different salt species present in natural contaminants, quantity of contaminants, wet and dry flashover voltage of contaminated insulators has been checked. This paper is mainly concerned with ESDD (Equivalent Salt Deposit Density) method for determining the degree of contamination. Based on the values of ESDD calculated in the laboratory tests, the detailed chemical analysis of the contaminating salts has been done 2 Literature Review Sundararajan R et al[1] investigated the ac flashover voltages of contaminated insulators at reduced pressure and a formulated an empirical mathematical model for height and FOV so as to see the effect of height on flashover voltage. Subba B.R et al[2] reported the mechanism of dry band formation and temperature distribution along the surface of an energized artificially polluted insulator string. Chrzan K.L et al[3] Based on the experimental investigation of the performance of insulators with spiral shaped sheds under laboratory and field conditions it was shown that the flashover voltage of spiral shaped insulators is lower than that of insulators with standard sheds. Subba B.R et al[4] Studied of Leakage Current Behavior on Artificially Polluted Surface of Ceramic Insulator was done. Khan A.A et al[5] a Study of Flashover Voltage of Artificially Polluted Porcelain Disc Insulator under Natural Fog Conditions for northern part of India carried out by experimental setup. Salam M A et al[6] established a mathematical relationship between the insulator resistance, leakage distance and Equivalent Salt Deposit Density (ESDD) was developed using Dimensional 539

2 Chennai and Dr.MGR University Second International Conference on Sustainable Energy and Intelligent System Analysis technique. Salam M.A et al[9] derived an analytical expression between the creepage distance and Salt Deposit Density for the polluted insulator using Dimensional Analysis technique. S. Venkataraman et al[10] developed a theoretical model to predict flashover voltage of non-ceramic insulators. Dehkordi J.F et al [11] established an improved mathematical relationship between the minimum flashover voltage (VMF) and the insulator dry arcing distance for standard porcelain station post insulators covered with ice. Chisholm et al[12] carried out an accurate measurement of low insulator contamination level. Nabavi S.M.H et al[13] presented several measuring methods to evaluate the pollution levels on outdoor insulators and compared various pollution measurement methods. 3 Contaminants & Sources: Table.1. list of contaminants Geographical location Contaminants Industrial C a Cl 2,C a (NO 3 )2, MgCl 2, NaNO3, NaCl,MgSO 4,CaCO 3, CaSO 4.2H 2 O Fly-ash, industrial smokestacks, cements, fertilizers, burning of fuel Marine MgCl 2,KC1,NaC l,cacl 2, NaNO 3 Salts, dew, smoke, smog Desert N a C l,kcl,caso 4,MgCO 3, KNO 3,Na 2 SO4,Fe 2 O 3,CaCO 3 Process industries & oil refineries products Rural areas Soil dust, fertilizers, etc. Highways Road salt, smoke Other types Bacteria, algae, mushrooms and lichens, oxalic acid, ice Table.1 show different contaminants which are present in different geographical region and pollute insulator surface. In this paper mainly chemical contaminants are considered for artificial pollution test. 4 Flashover regulating factors & contamination level measurement methods: 4.1. Chemical factors which regulates the magnitude of flashover voltage[7, 8]: 1. Size of atom 2. Mobility of the hydrated ions 3. Weight of the ions 4. Solubility 5. ph of the salts 6. Hydration energy of the salts 7. Lattice energy of the salts 8. Structure of crystal Out of these various factors ph, solubility, mobility, size & weight of ions play an important role in deciding flashover voltage of contaminated insulators Measurement methods for degree of contamination: The following measurement methods are used for degree of contamination level: Equivalent Salt Deposit Density(ESDD) Method: By definition, the density of equivalent salt deposit density (ESDD) equals an amount of sodium chloride which, solved in water, will change water s conductivity to the level equal to that resulting from the solution of polluted deposits gathered from insulator surface divided by the insulator s surface area (mg/cm 2 ). This method is generally used for calculating average pollution based on average density of soluble salt The equation used in ESDD calculation is:[9] σ V( ) (1) (T 20) ESDD = 0.55 A Where: σ is the layer conductivity at lab. temperature (in ms/cm V is the volume of solution in conductivity measurement at laboratory temperature (in ml) T is the temperature of the solu tion ( C) A is the area of the insulator surface (in cm 2 ) Leakage Current Method (LCM): The general systematic structure of leakage current measurement method is illustrated in figure 1. A collar-shaped ring is placed in the end of the insulator near to the earth. The leakage current sensor is placed between the insulator and the ring to create a closedloop current. The sensor, which has a high performance speed, works based on Hall s effect current transformer. The leakage current made on the insulator surface passes through the sensor and flows toward the earth. The sensor performance is based on Hall s effect and magnetic fields. Sensor s input impedance is of a very small value. Sensor s output it directly connected to the central unit of the information recording system. This unit consists of the analog - to - digital (A/D) converter and a microprocessor to gather information, which records leakage current indexes in all the insulators being tested. All the adopted and saved information can be transferred through RS232 port serial or modem. For sufficient information, the sampling frequency related to the A/D converter is usually selected for 20 khz. Since the weather conditions like humidity and temperature have an essential role in creating leakage current, it is necessary for the measurement system to be equipped with sensors of temperature and humidity and wind velocity gauges. The output of the sensors also is connected to the central unit of information recording and is saved. This sensor based leakage current scheme can be utilized to insulator condition monitoring. a message can be generated and sent to the maintenance division for insulator cleaning Non Soluble Salt Deposit Density (NSDD) Method: This method is the developed form of ESDD method, in which non-soluble pollution content in available samples is measured Directional Dust Deposit Gauge (DDG) Method: This method was first chosen by research institute of Electricity Supply Commission in 1974 to examine insulator pollution. DDG includes four vertical split pipes and a pot below each pipe to gather pollution. The pipes are placed along the four geographical 540

3 Chemical analysis of insulator contaminants and reliability improvement of T&D line for smart operation of grid under chemically polluted environment directions, north, south, east, and west. To facilitate international Table.2. Solubility trend of various metal ions comparison of the results, the size of DDG cracks must be the same in all places to be tested DDG pollution index, according Metal Chlorides Carbonates Sulpher Nitrates to equation (3), is the average of the four conductivities (μs/cm) NA Most soluble Most soluble Most soluble Most soluble obtained from the four directions for a 30-day month in 500 cc Be normalized washing water. Pollution gathering machine in DDG Mg method Normalized conductivity and average conductivity are Ca worked out from the equations (2) and (3), respectively.[9] Sr Ba Less Soluble Less Soluble Less Soluble Less Soluble Table.3. Mobility trend of various metal ions Fig.1: General systematic structure for LCM σn = C ( V 500 ) (30 N ) (2) Average Conductivity= σn+ σs+ σe+ σw 4 Where: σn, σs, σe, σw = normalized conductivity indexes in north, east, west, and south (μs/cm) C = conductivity (μs/cm) V = volume of distilled water (ml) N = number of days when the insulator has been under investigation 5 Chemical Analysis For Various Salts And Ions: 5. 1 Chemistry Of Ions: In this section, physical properties (solubility, mobility, weight) of various metal ions like chlorides, carbonates, sulphates, and nitrates are discussed which affects electrical properties of contaminant[6,7]: From table 2 & 3 we can conclude that chloride, carbonate, sulphate, and Nitrate of Na have highest solubility and mobility. From table 4 we see that Na salts has least weight among others metal salts. From table 5 we conclude that nitrates and chlorides of metal salts are highly soluble than sulphate and carbonate salts. Combining these all trends we conclude that nitrate and chlorides salts of Na and Mg can adversely affect the electrical properties of insulator surface. The Experimental value of flash over voltage for these salts also supports the above statement as the flash overvoltage of soluble metal salts is lower than the corresponding insoluble salts. (3) Metal Chlorides Carbonates Sulpher Nitrates NA Be Mg Ca Sr Ba Table.4. Weight trend of various metal ions Metal Chlorides Carbonates Sulpher Nitrates NA Be Mg Ca Sr Ba Table.5. List of high & low soluble salts High soluble salts Ca(NO 3 ) 2, NaNO 3, MgCl 2 CaCl 2, NaCl, KCl Low soluble salts MgSO 4, Na 2 SO 4, Na 2 CO 3, K 2 SO 4, CaCO 3,CaSO 4.2H 2 O, MgCO Chemical Analysis Of Flashover Voltages For Various Salts: Effects of chloride salts on flashover voltage and its comparison with experimental values[7, 8]: The tendency of hydrolysis, lattice energy and covalent character are more for MgCl 2 in comparison to CaCl 2 but the solubility of MgCl 2 is more than that of CaCl 2.Hence due to high solubility, high lattice energy and high covalent character MgCl 2 should have lower flashover voltage than CaCl 2. In our observation also MgCl 2 shows higher flashover voltage in wet condition than CaCl 2 (range of Flashover voltage of MgCl 2 is 54 to 48kV and range of Flashover voltage of CaCl 2 is 56to 49kV) Effects of sulphate salts on flashover voltage and its comparison with experimental values[7, 8]: We have used CaSO 4 and MgSO 4 both are alkaline earth metal salt.the solubility of the sulphate salts in water decreases down 541

4 Chennai and Dr.MGR University Second International Conference on Sustainable Energy and Intelligent System the group in periodic table: BeSO 4 > MgSO 4 > CaSO 4 >SrSO 4 >BaSO 4 Solubility of MgSO 4 is greater than CaSO 4 due to the high enthalpy of hydration of smaller Mg 2+ ions than Ca 2+. MgSO 4 is soluble while CaSO 4 is sparingly soluble ionic radius of Mg is 72 pm while Ca is 100 pm. Due to smaller size ions move faster i.e. Mobility of Mg 2+ is higher than that of Ca 2+. Ionic charge density of Mg is higher than Ca. Also melting point of Mg is 649 C while of Ca is 839 C. Thus we conclude that ionic mobility and solubility of MgSO 4 is greater than CaSO 4 and stability of MgSO 4 is more than that of CaSO 4. From the above discussion MgSO 4 should have less flashover voltage than CaSO 4. According to our observation MgSO 4 shows lower flashover voltage than CaSO 4. Wet flashover voltage of MgSO 4 lies in the range (57-49kV) while wet flashover of CaSO 4 lies in the range (66-52kV) Effects of carbonate salts on flashover voltage and its comparison with experimental values[7, 8]: Carbonates are all ionic and are basic compounds. These salts are insoluble due to less no. of ions and have lower molecular mobility because of higher molecular weight. Thus charge carrier per unit area of insulator surface is very low, this reduces the surface conductivity. Also MgCO 3 is less stable than CaCO 3. So flashover voltage is high in case of carbonate salts. According to our observation flashover voltage for CaCO 3 is lies in the range 69kV to 60kV which is higher than other chemical contaminants Effects of nitrate salts on flashover voltage and its comparison with practical values[7, 8]: Nitrates are soluble basic salts. Solubility of nitrates adopts the following sequence: NaNO 3 > Mg (NO 3 ) 2 >Ca (NO 3 ) 2 Molecular weight of NaNO 3 is least and that of Ca (NO 3 ) 2 is highest that s why mobility of ions follows this sequence: Na + NO 3 - > Mg 2+ 2NO 3 - > Ca2+2NO 3 - In short due to high solubility and high mobility flashover voltage for NaNO 3 should be low. When dissolved in water it forms HNO 3 which is highly corrosive acid. ± 1) connected at the primary side of the transformer that reads the low side voltage. The corresponding high voltages were obtained from a calibration curve dawn by using the sphere sphere electrode system having diameter of 25cm (IS-1876, 1961).Figure 1 provides the experimental setup for artificial pollution test. The Tests were carried out using the suspension insulators which are normally used in Overhead networks in India and neighboring countries. The unit diameter is 254mm, its spacing is 146 mm, Top surface area, 691cm2, Bottom surface area: 908cm2, Total surface area 1599cm2, and its total leakage distance is 305mm. Fig. 3: Experimental setup for flashover test 6.1 Insulator Specification Shed diameter: 254mm Unit spacing: 146mm leakage distance: 305mm Top surface area: 691cm 2 Bottom surface area: 908cm 2 Total surface area: 1599cm 2 Fig.4. Disc insulator specification NaNO 3 +H 2 O NaOH+HNO 3 According to our observations Wet flashover voltage of NaNO 3 lies in the range of (51-47 kv). 6 Experimental Setup The high voltages were obtained from a testing transformer of 391V/150kV, 1-phase 30kVA rating using a voltmeter (accuracy Fig.5. Flashover of insulator under test Figure 3 provides the experimental setup for artificial pollution test; figure 4 gives basic data of insulator which is used for calculation of ESDD, and figure 5 shows the flashing condition of insulator under test. 542

5 Chemical analysis of insulator contaminants and reliability improvement of T&D line for smart operation of grid under chemically polluted environment 7 Observations Salts C (gm/ l) T ( C) P (cm of Hg) Dry F.O.V (kv) Wet F.O.V (kv) σ (ms/ cm) ESDD (mg/ cm2) CaSO MgSO MgCl CaCO NaNO NaCl CaCl CaCO NaCl +CaO CaSO NaCl +CaO ph is much lower than dry FOV. From the both Graphs 1 & 2, it can be seen that FOV in dry& wet condition for mixture is lower than pure salt. Graph 3 compares the variation of wet FOV for Mix 1 (CaCO 3 +NaCl+CaO), and Mix 2 (CaSO 4 +NaCl+CaO) and seen that variation in FOV for Mix 1 is large as compared to Mix 2 at lower degree of contamination level. Graph.1. Comparision of dry & wet F.O.V with ESDD for MgSO 4 Graph.2. Comparison of dry & wet F.O.V for Mix1 (NaCl+CaC O 3 +CaO) 7. 1 Results Graph for Flashover voltage with ESDD and ph for different salt compositions in dry and wet conditions are drowned in graph from 1-5. Graph 1 provides a comparison of dry & wet FOV w.r.t. ESDD, It is clear from the graph that dry FOV is much higher than wet FOV for same contamination level while Graph 2 is a comparison of dry & wet FOV for mixture 1 and in this case also wet FOV Graph.3. Comparison of wet F.O.V for Mix 1 and Mix 2 Graph 4 shows the individual variation of FOV w.r.t ESDD for all seven salts used and from this graph it can be seen that chloride and sulphate salts of Na & Ca has lower FOV than other salts. it has also been reported that in case of NaNO 3 variation in FOV is least for wide range of ESDD. Graph 5 shows variation of FOV w.r.t ph of the salts. It has been found that in case of NaNO 3, CaCl 2 FOV is lower than other salts for same ph value. Variation range of FOV in case of NaCl is maximum than other salts for same range of ph. 543

6 Chennai and Dr.MGR University Second International Conference on Sustainable Energy and Intelligent System on the insulator can be used as a pollution severity indicator and can be utilized for deciding maintenance scheduling of overhead line insulators. Leakage current monitoring can be interfaced with communication system and remote controlling can be performed. References Graph.4. Wet F.O.V vs. ESDD characteristics for all 7 salts. Graph.5. F.O.V (wet) vs. ph characteristics of all salts 8 Conclusions The flashover voltage of polluted insulators depends on the kind and quantity of contaminants. Accordingly, both chemical analysis of the contaminants and measurement of contaminant quantity are necessary from the view point of design and maintenance of insulator under contaminated condition. There are different chemical factors which includes (Size of atom, Mobility of the hydrated ions, Weight of the ions, Solubility, ph of the salts, Hydration & Lattice energy of the salts, Structure of crystal) influences the flashover voltage. Variation in dry flashover voltage of contaminated insulator is not too much affected by pollution level while in wet condition FOV is lower than dry as well as variation is large. Flashover voltage in case of contamination by single salt is much higher than in case of mixture of different salts. After analysis of the chemical effects of various salts of alkaline earth metals and observing their flash over voltage for dry and wet condition, we conclude that chloride and nitrates which are soluble in water affects the flash over voltage drastically. As basic or acidic nature of salt increases, in both cases flashover voltage reduces significantly. As reported in this work, the ph of the contaminants plays a vital role in determining the F.O.V of a contaminated insulator. Thus apart from visual inspection, conductivity measurement, ESDD calculation, leakage current monitoring, ph of the deposits [1] Sundararajan R, W. N. Robert, Effect of Altitude on the Flashover Voltage of Contaminated Insulators, IEEE Annual Report - Conference on Electrical Insulation and Dielectric Phenomena, San Francisco, October 1996 pp [2] Subba B.R, Nagabhushana G.R, Study of Temperature Distribution along an Artificially Polluted Insulator String, IISc, Plasma Science & Technology, vo1.5, No [3] Chrzan K.L,Kindersberger I.J, Pollution behaviour of insulators with spiral shaped sheds, IEEE trans, Germany, [4] Subba B.R, Nagabhushana G.R, Study of Leakage Current Behavior on Artifi cially Polluted Surface of Ceramic Insulator, HV Engg, Indian Institute of Science, Bangalore, [5] Khan A.A, Masood A, Husain E, A Study of Flashover Voltage of Artifi cially Polluted Porcelain Disc Insulator under Natural Fog Conditions IEEE Trans, Department Of Electrical Engineering, Aligarh Muslim University, Aligarh, India [6] Salam M A, Nadir Z, Akbar M and Islam M S, Study the Effects of Different types of Contaminants on the Insulator Resistance, ICECE 2002, 26-28, Dhaka, Bangladesh December [7] Gupta R.K, Amit R.K, A Text Book of Inorganic Chemistry, Arihant Prakashan Meerut, 2nd Edison, [8] LEE J. D. Concise inorganic chemistry, oxford press, Fifth edition, 2005 [9] Salam M.A, Ahmad H, Ahmad A. S, Fuad S.A,Paul K.C, Tamsir T, Buntat Z, Piah M. A. M., Study of Creepage Distance of the Contaminated Insulator in Correlation with Salt Deposit Density, IEEE Conference on Electrical Insulation and Dielectric Phenomena, [10] Venkataraman S and Gorur R S Prediction of fl ashover voltage of non-ceramic insulators under contaminated conditions Department of Electrical Engineering Arizona State University Tempe, AZ, USA [11] Dehkordi J.F, Zhang J, Farzaneh M, Experimental Study and Mathematical Modeling of Flashover on EHV Insulators Covered with Ice, 61st Eastern Snow Conference Portland, Maine, USA [12] Chisholm,W. A. Accurate measurement of low insulator contamination levels, IEEE Transaction on Power Delivery, vol. 9, No. 3, July [13] Nabavi S.M.H, Gholami A, Kazemi A, Masoum M.A.S, Evaluation of Leakage Current Measurement for Site Pollution Severity Assessment, Leonardo Electronic Journal of Practices and Technologies, ISSN , Issue 10, January-June 2007, p