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DETERMINATION OF A CALIBRATION CURVE FOR AN INSULATOR POLLUTION MONITORING RELAY W.H. Schwardt J.P. Holtzhausen W. L. Vosloo Department of Electrical Engineering TSI University of Stellenbosch ESKOM Private Bag X1 Matieland 7602 ABSTRACT: An Insulator Pollution Monitoring Relay (IPMR) was developed to measure the surface conductance of a naturally polluted insulator to determine the pollution severity. The measured conductance must be converted to an Equivalent Salt Deposit Density (ESDD) value using the IPMR calibration curve. The calibration curve is determined by performing both artificial and natural pollution tests. Various devices were tested to apply the artificial wetting during tests. Calibration tests were performed and a suitable calibration curve was determined. KEYWORDS: Insulator, Pollution, Monitoring 1. INTRODUCTION 2. THE INSULATOR POLLUTION MONITORING RELAY Flashovers on high voltage insulators due to natural pollution cause problems in the performance and 2.1 IPMR Construction reliability of overhead lines in polluted areas. The IPMR is a portable, self-contained device that can Pollution that is deposited on the insulator surface easily be installed in a substation to determine the site becomes a conductive electrolyte when the insulator severity. The IPMR’s main components are: surface is wetted by rain or fog. This allows leakage currents to increase over the insulator surface and • one test insulator, decreases the electrical withstand voltage of the • a mechanical arm to raise or lower the insulator, insulator. It is therefore of great importance to study • a humidifier producing artificial wetting, the effect that environmental conditions have on the • an air dryer, performance of insulators. • 220/3000V test transformer, • a microprocessor to control the tests and to log the The IPMR was therefore designed to determine the measured values. pollution severity of the site at which it is installed. The IPMR can serve these very important functions: The IPMR only requires 220V (50Hz) supply and a pressurized water point at the testing location. • defining of the pollution severity of an area during the planning of an overhead line or station, • determining of maintenance intervals on surrounding insulation, • triggering of an alarm when measured values exceed maximum permissible values. The IPMR was developed by ESKOM after experience on a similar device called the Insulator Pollution Monitoring Apparatus (IPMA). The University of Stellenbosch, in conjunction with ESKOM, designed the first IPMA in the early 1990’s , . In the late 1980’s CESI in Italy also designed a device to measure surface conductance called the Pollution Monitoring Equipment (PME) . It was shown by these Figure 1: Image of the IPMR. forerunners to the IPMR that the surface conductance approach was a very effective technique in the 2.2 IPMR Measurements determination of the site severity. The IPMR is utilised to monitor the pre-deposited pollution as well as the instances when instantaneous pollution deposits can occur. Pre-deposited pollution occurs at a natural rate and the surface conductance is dependent on the degree of wetting on the insulator surface. Instantaneous pollution deposits occur when The reference conductance is logged after being highly conductive fog moves into the area causing measured by applying five cycles of the 3kV wave. flashovers but leaves a very low resultant pollution The flow chart of the test is illustrated in Figure 2.The level on the insulator. This phenomenon is a serious flow chart of the measurement cycle is shown in threat to insulation since this condition typically Figure 3. During the measurement cycle the humidifier occurs within less than an hour. raises the humidity levels to allow moisture absorption by the pollution layer. The humidifying process is The IPMR was thus designed to monitor: repeated at a set time delay. A voltage of 3kV is applied for five cycles to the insulator after every • surface conductance on pre-deposited pollution moisture cycle and the resultant leakage currents are with natural wetting (Measurement A), logged. The test is repeated when the leakage currents • surface conductance on pre-deposited pollution stay within a set tolerance (x%) of I max. If the with artificial wetting (Measurement B), measured leakage current is higher than Imax, it will be • leakage currents on pre-deposited pollution with stored as the new I ax value. When the measured m natural wetting as well as monitoring for the onset current is smaller than x% of Imax, the test is stopped of instantaneous pollution deposits (Measurement and the dryer dries the insulator. The test insulator will C). be raised to expose the insulator to the natural environment. Measurement A Start The test insulator is energizing at set intervals for five Measurement Cycle cycles to assess the surface conductance during natural pre-deposited pollution and wetting conditions. The Generate Artificial Wetting 3kV voltage is only applied to the insulator for five consecutive cycles from the 50 Hz wave to avoid the Drying Cycle formation of dry bands on the insulator surface. Time Delay Set Measured Measurement B Current = Imax Switch ON The test insulator is enclosed in the test chamber Transformer whereby surface conductance is measured if critical Measure PEAK wetting should occur on the pre-deposited pollution. Current Over 5 Cycles Critical wetting will be discussed later in the paper. YES Switch OFF The IPMR measures the surface conductance on one Transformer test insulator. The test insulator is mounted on a Is Measured Log Measured Current > Imax? mechanical arm that can lower the test insulator into NO Current the test chamber. The test chamber acts as a controlled environment that shields the insulator from the Compare Measured surrounding environmental conditions during the tests. Current With I max S T A R T NO O F Is Measured M E A S U R E M E N T B C u r r e n t < x % o f Im a x ? L O W E R YES T E S T I N S U L A T O R Stop Measurement D R Y I N G C Y C L E Cycle M E A S U R E R E F E R E N C E Figure 3: Flow Chart of the Measurement Cycle C O N D U C T A N C E During Measurement B. M E A S U R E M E N T C Y C L E The conductance of an insulator is a function of D R Y I N G C Y C L E leakage current through the contamination layer due to the applied voltage. Conductance values are unique to R A I S E T E S T I N S U L A T O R the leakage currents flowing over the specific insulator profile. It is therefore difficult to compare different E N D O F M E A S U R E M E N T B insulators to their conductance values. To overcome this effect, the conductivity value can be used to Figure 2: Flow Chart of Measurement B. compare different insulators since it is independent of the insulator’s geometry. The measured surface At the beginning of the artificial wetting test, the conductance (Gs ) is multiplied by the form factor (F) insulator is lowered into the test chamber by the of the test insulator to determine the layer conductivity mechanical arm. An air dryer heats the air inside the (σs ) of the pollution layer. chamber drying the pollution layer on the insulator. σs = F . Gs …(1) 3.1 Method of Artificial Wetting The form factor (F) identifies each insulator shape in The accuracy and repeatability of the Measurement B terms of the insulator radius (r) as a function of greatly depends on the device used for the generation creepage length (L) . of the artificial wetting. Four different devices were tested to determine a suitable method of artificial L wetting. ∫ 2.π.r (s ).ds 1 F= …(2) O 3.1.1 Acoustic Cell It was decided to relate the IPMR Measurement B The first device tested was an acoustic cell. This values to ESDD values, as it existed as a standard device is used as humidifiers for domestic use. The defining the characteristics of the pollution layer. The cell is capable of creating a very fine mist in the ESDD can be defined as the equivalent deposit of chamber but was not suitable for use in harsh NaCl on the insulator surface that will have the same environments due to its fragile construction. electrical conductance as that of the actual deposit dissolved in the same amount of water. 3.1.2 High Pressure Nozzles Measurement C The second artificial wetting device tested were high- The leakage currents are measured on up to three pressure nozzles connected to a 7,5 bar high-pressure energised insulators in the nearby vicinity. The leakage pump. As wetting is required, the pump is simply current sensors can be connected up to 100m from the switched on and small blades in the nozzles “chopped” IPMR via fibre optic cable. the water into a mist between 10 – 50 m. The benefit of using this configuration was that the IPMR could be Verma published the well-known Imax theory in the used in areas where no pressurized water is available late 1970’s. Imax was defined as the minimum amount since the water could be pumped from an installed of leakage current that was necessary to cause water tank. Unfortunately the configuration proved flashover. Imax was independent of the insulator problematic due to the clogging of nozzles and the shape, pollutant or test procedure. The only governing difficulty controlling the precise amounts of wetting. factor was the specific creepage distance (mm/kV) of This was due to pressure left in the system after the the insulator. pump was switched off. 2 3.1.3 Kettle mm / kV Imax = …(3) 15 ,32 The third artificial wetting device tested was a small 1,1 litre steam generator. The steam generator makes Imax can thus be used to predict the actual risk of use of a 2 kW kettle element to minimise the need for flashover on a real-time basis. The Ihighest is used as a specialized spares in the case of repairs. Water is criterion since the calculated Imax value was too close supplied via one 220V solenoid valve connected onto a to the actual flashover. pressurised water supply. The inlet valve is opened before the start of every steam generation cycle to 2 ensure that the steam generator is filled with sufficient mm / kV Ihighest = Ih factor . …(4) water during the test. The element is then switched on 15 ,32 and left to boil. When the steam is released a thin layer of moisture is deposited on the insulator surface. The Ihighest value gives an indication of excessive However since the kettle was an open looped system, leakage current rise that can lead to flashover. it had no way of assessing whether the kettle contained sufficient water. 3. LABORATORY CALIBRATION OF IPMR 3.1.4 Miniature Boiler The IPMR needed to be calibrated in terms of surface conductance and ESDD before field installation. The A miniature 5-litre boiler capable of generating a calibration curve is used to relate the pollution severity steam output of 3kg/h was constructed. The steam to the conductance measured on the test insulator. output was obtained by simply opening the output solenoid valve. The boiler kept the steam pressure high during the day and simplified the testing procedure and reduced the time required for a test. The only problem with the boiler as wetting device is the high manufacturing cost involved IPMR Calibration Curve for Porcelian Post Type Insulator 70 60 IPMR Surface Conductance (uS) 50 40 30 20 10 0 0.000 0.100 0.200 0.300 0.400 0.500 0.600 ESDD (mg/cm2 ) Figure 4: Calibration Curve for the IPMR. 3.2 Artificial Pollution Process approximate guide was developed from data obtained from standard cap and pin insulators tested vertically The artificial pollution process was done according to using the kaolin as the inert material. the solid layer method prescribed in the IEC 507 document . The solution consisted of 40 grams of 4. CONCLUSIONS kaolin per litre water. By adding different amounts of NaCl to the solution simulated the different pollution An Insulator Pollution Monitoring Relay (IPMR) was levels. The kaolin in the solution is a non-dissolving designed to assess site severity and to give alarm inert material used as a bonding agent for the NaCl on messages when pollution levels exceed pre-determined the insulator surface. The kaolin simulates inert values. Four different devices were tested to produce materials, e.g. cement, lime, dust, clay, etc., that the artificial wetting during the ESDD measurement. A performs the same bonding function when insulators calibration curve was determined and the curve has are exposed to natural conditions. The test insulator is shown similarities with an approximate guide of dipped in the solution ensuring that a uniform correspondence between the degree of pollution and pollution layer is applied to the surface. The insulator the volume conductivity of the suspension. is then allowed to dry before it is placed in the IPMR. REFERENCES 3.3 IPMR calibration curve  Van Wyk L., “Insulator Pollution Monitoring: The test insulator is polluted using the solid layer Evaluation of Various Methods of Severity method and left to dry. The insulator is placed in the Measurements At A Coastal Site”, Thesis, IPMR and a test is performed. The insulator is washed University of Stellenbosch, December 1996. after the test to determine the ESDD of the deposit on  Van Wyk L., Holtzhausen J.P., Vosloo W.L., the insulator surface. By using the surface conductance “Relation Between Surface Conductivity, and ESDD values of each test, a calibration curve Leakage Current and Humidity of Ceramic (Surface Conductance ( S) vs. ESDD (mg/cm2 )) was Insulators”, 31st UPEC, Iraklio, Crete, drawn up. September 1996.  Bertrazzi A., Perego G., Sampaoli G., The calibration curve was determined by plotting the Vachiratarapadorn Y., Eamsa-as V., “A values obtained from the tests and performing Device For The Automatic Measurement Of regression analysis to determine the relationship of the Surface Conductivity Of Insulators”, Paper parameters. 47.41, 6th ISH, New Orleans, USA, Aug./Sept. 1989. The pollution range in the calibration curve ranges  IEC Publication 507, “Artificial Pollution from light (0,03 – 0,06 mg/cm2 ), medium (0,10 – 0,20 Tests on HV Insulators to be Used On AC mg/cm2 ) and heavy (0,30 – 0,60 mg/cm2 ) . Systems”, Technical Committee No. 36: Insulators, January 1987. The solid line is the IPMR calibration curve for the  IEC Publication 815, “Guide For The porcelain post type insulator and the dashed line is an Selection Of Insulators in Respect of Polluted approximate guide given by the IEC507 . The Conditions”, 1986.
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