57 CHAPTER 3 Experimental Test Set-Up 3.0 Introduction The electrical performance of porcelain insulators in the presence of pollution and moisture is improved by coating with room temperature vulcanizing (RTV) silicone rubber. Till date there is no established test procedure for its ageing performance though researchers around the world are at it. Different research groups have come out with different test procedures [1,11] for determining the ageing performance of RTV coated insulator but none of those really duplicated the field performance of an aged RTV coated insulator. It is felt imminent to fall back upon a test method developed for porcelain and glass insulators because these RTV coated insulators would have to serve the same functions and in the similar outdoor environment as porcelain or glass insulators [43]. In this Chapter, the classical IEC 60507, 1991 standard [44] test parameters/procedure is modified and applied to the Pollution ageing test on RTV coated insulator, conducted in the pollution ageing chamber. The test parameters viz., nozzle dimensions, air pressure at the nozzle tip, flow rate of solution etc., are modified (scale down) according to the distance between the nozzle columns of pollution
58 ageing chamber with respect to the IEC 60507, 1991 dimensions. It is believed that the fog dynamics are almost same for the IEC 60507, 1991 standard artificial pollution test set-up and the ageing chamber. Low conductivity fog is used to conduct the test as it is more representative of field situation. The parameters for the experimental test set up had been repeated from the published work [46].Thus the repeatability had been checked and the test parameters are fixed for the present laboratory research work. 3.1 IEC 60507, 1991 standard test set up of UHV laboratory [45] The entire laboratory is built with RCC and a door of size 12m x 18m has been provided. The laboratory is circular in shape with a diameter of 24 m and a height of 27 m, as shown in Figure 3.1.The interior of the laboratory is treated with anti-corrosive paint to withstand saline fog contamination. An electric monorail and the winch arrangement is provided to suspend the test object in vertical, V or horizontal configuration to facilitate testing of insulator strings of almost all configurations. The spraying system is flexible to adopt to all these configurations. An air compressor with 100 HP motor is installed to deliver oil free compressed air at 900 kpa. Two nozzle columns with 25 nozzle pairs in each column for saline water and compressed air to generate salt fog according to the requirements stipulated in the IEC 60507 have been erected. 2 A
59 Air receivers and pressure regulators are used to maintain the air pressure at 700 kpa 35 kpa at each of the compressed air nozzle tip. Mechanized saline water preparation and pumping arrangement is used to prepare the saline solution of required concentration (salinity). The nozzle dimension and their positions in the column for producing the salt fog are as specified in the IEC 60507 standards [44]. The flow of the solution to each of the salt solution nozzles is controlled to the required rate of 0.5 l/min 0.05 l/min by individual flow meters for the period of the test. The flow of the solution can be either by gravity or by pumping directly from the bottom of the nozzle system. 3.1.1 Spraying system [44] The fog is produced in the test chamber by means of the specified number of sprays which atomize the solution by a stream of compressed air flowing at right angles to the solution nozzle. The nozzle consists of corrosion resistant tubes, the internal diameter of the air nozzles being 1.2 ± 0.02 mm and the internal diameter of the solution nozzle being 2.0 ± 0.02 mm. Both nozzles shall have an outside diameter of 3.0 ± 0.05 mm and the ends of the nozzles shall be square-cut and polished. The end of the solution nozzle shall lie on the axis of the air nozzle to within ± 0.05mm. The distance between the end of the compressed air nozzle and the central line of the solution nozzle shall
60 be 3.0 ± 0.05 mm. The axes of the two nozzles shall lie in the same plane to within ± 0.05 mm. Typical construction of the fog spray nozzle is shown in Figure 4.3 of Chapter 4. The sprays shall be in two columns parallel to and on opposite sides of the insulator which shall have its axis in the same plane as the columns, i.e. a vertical insulator is tested with vertical columns and a horizontal insulator with horizontal columns. In the case of an inclined insulator the plane containing the insulator and the columns shall intersect the horizontal plane in a line at right angles to the insulator axis; in this case, the axis of the solution nozzles is vertical. The distance between the solution nozzles and the insulator axis shall be 3.0 ± 0.05 m. The sprays shall be spaced at 0.6 m intervals, each spray pointing at right angles to the column axis towards its counterpart on the other column and within an angle of 1 0 to the plane of the sprays. This alignment can be checked for vertical sprays by lowering the solution nozzle, passing water through the air nozzle and directing it towards the opposing spray; afterwards, raising the solution nozzle to the operating position. The mid-point of the insulator shall be in line with the mid-points of the columns of sprays. Both columns shall extend beyond each end of the insulator by at least 0.6 m. The minimum number N of sprays per column shall be, for a length H in metres of the insulator:
61 N = (H/0.6) + 3 The sprays shall be supplied with filtered, oil-free air at a relative pressure of 7 kg/cm 2 ± 0.35 kg/cm 2. The flow of solution to each spray is 500 ml/min ± 50 ml/min. for the period of the test and the tolerance on the total flow is ± 5% of the nominal value. 3.2 Pollution ageing chamber for RTV coated insulator [41] The RTV insulator coating system consists of polydimethylsiloxane (PDMS) polymer, alumina tri-hydrate filler for increased tracking and erosion resistance, catalyst and a cross linking agent. This RTV coating is dispersed in naptha, as per the manufacturer s specification. Naptha merely acts as a solvent and acts as a carrier medium to transfer the RTV rubber to the porcelain insulator surface. As the solvent evaporates from the surface, moisture from the air triggers vulcanization forming a solid rubber coating. RTV coating, thickness of 0.2 0.01 mm was obtained by spraying liquid silicone rubber RTV on 11 kv standard disc porcelain insulator and simultaneously, it was sprayed on a uniform glass plate of 4 cm (l) 1 cm (b) with constant thickness. The thickness of the plate with and with out RTV coating is measured at five different
62 places by Micrometer (least count = 0.001 mm). The difference gives the thickness of the RTV coating. Since, coating is sprayed on the insulator and glass plate in the same way, hence, it is believed that thickness of the RTV coating on the glass plate and on insulator are same. The test chamber is made of stainless steel plate of 2 m (l) x 2 m (b) x 2.5 m (h) dimension as shown in Figure 3.2. A 400 V/33 kv 66 kva, single-phase 50 Hz, transformer, which conforms to IEC specifications is used as test source. Applied test voltage to the sample was 10 kv which is 40% more than the maximum line to ground voltage of an 11 kv system voltage. 3.3 Spraying system (scale-down) Based on the physical dimension i.e., the distance between the nozzle columns (2 2 =2.8 and rounded off to 3.0 m), which is approximately one-half of the IEC-60507, 1991 dimension of 6.0 m, the nozzle dimensions of the ageing chamber are scaled down to onehalf of IEC spray nozzle. 3.4 Experimental test procedure RTV coating is made up of polydimethysiloxane (PDMS) polymer with alumina tri-hydrate filler material for increasing tracking and erosion resistance, a catalyst and a cross-linking agent. This coating is dissolved in naphtha solvent and is sprayed on to the insulator
63 surface. The naphtha solvent evaporates from the surface and the moisture in air triggers the vulcanization forming a solid rubber coating. One porcelain and one glass disc insulator are coated with RTV coating. The disc insulators are of 165 mm height having a creepage distance of 330 ± 5 mm. The RTV coating thickness is 0.20 ± 0.01 mm on each disc insulator. RTV coated porcelain and glass disc insulators and one porcelain disc insulator without coating were subjected to an accelerated ageing test in ageing chamber under the combined stress of voltage and salt-fog. Porcelain disc insulator was used as a reference sample. The test voltage is 10 kv RMS, flow rate of saltwater is 250 ml ± 10 ml and salt-fog salinity equal to 0.15 kg/m 3. The test voltage applied is 40 % more than the normal operating phase to ground voltage of a 11 kv system. Higher voltage is used to accelerate the ageing phenomena of the RTV coating in the test chamber. The parameters monitored were leakage current, cumulative charge, peak leakage current pulses etc using PXI (Peripheral Component Interconnect Extensions for Instruments Specification) controller based data acquisition system explained in section 3.2. Intermittently, RTV coating was peeled off and was subjected to EDX (Energy Dispersion X-ray) analysis. The pollution ageing test is performed on the RTV coated insulator samples in Pollution ageing chamber [39] with the scaled down test parameters. The parameters for the test, are summarized in the Table 3.2.
64 3.5 Definitions Cumulative Charge, it (C). It is the summation of product of instantaneous leakage current and sampling period over the period of interval time. Cumulative integral of current squared, i 2 t (C-A) It is the summation of product of square of instantaneous leakage current and sampling period over the period of interval time. Average leakage current, (Amps.) It is defined as the value obtained by summing up the absolute values of all currents sampled during the positive and negative portions of the 50 Hz over a specified length of time, (which is integration time) and then dividing by the number of sampled currents. 3.6 Conclusion The repeatability of the test parameters of the scaled down test parameters have been obtained and found to be within tolerances. The test set-up used for the ageing test is similar to the standard test set up of IEC: 60507 /1991. A low salinity fog is used to simulate real saline atmosphere.
65 Experimental test set-up 24 m Air Inlet 7 kg/cm 2 0.6 m 2 2 Measuring shunt Flow meter 1 1 400 V/33kV/22kV/7.5kV Transformer 27 m height 6 m Figure 3.1. IEC 60507, 1991, standard artificial pollution test set-up.
66 LAN Remote PC Ageing chamber FM 4 Fog nozzles Transformer 400V/33kV/22kV/10kV Figure 3.2. Aging chamber with its infrastructure
67 Table 3.1 Scaled down test parameters of IEC 60507, in ageing chamber. Test parameter Solution conductivity Distance between nozzle columns Scaled down test parameters of ageing chamber Low = 250 μs/cm 2.8 meters (Approx.: 3.0 meters) Pressure 3.3 kg/cm 2 0.2 kg/cm 2 Number of nozzles used Air nozzle diameter Solution nozzle diameter Test voltage Solution flowrate 2 on each side 0.6 mm 0.02 mm 1.0 mm 0.02 mm 10.0 kv 250 ml /min ± 10 ml / min