36-WG11/Stafford/109

DETERMINATION OF A CALIBRATION CURVE FOR AN INSULATOR POLLUTION MONITORING RELAY W.H. Schwardt

J.P. Holtzhausen

W. L. Vosloo

Department of Electrical Engineering University of Stellenbosch Private Bag X1 Matieland 7602

TSI ESKOM

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 Flashovers on high voltage insulators due to natural pollution cause problems in the performance and reliability of overhead lines in polluted areas. Pollution that is deposited on the insulator surface becomes a conductive electrolyte when the insulator surface is wetted by rain or fog. This allows leakage currents to increase over the insulator surface and decreases the electrical withstand voltage of the insulator. It is therefore of great importance to study the effect that environmental conditions have on the performance of insulators. The IPMR was therefore designed to determine the pollution severity of the site at which it is installed. The IPMR can serve these very important functions: • • •

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 [1], [2]. In the late 1980’s CESI in Italy also designed a device to measure surface conductance called the Pollution Monitoring Equipment (PME) [3]. It was shown by these forerunners to the IPMR that the surface conductance approach was a very effective technique in the determination of the site severity.

2. THE INSULATOR POLLUTION MONITORING RELAY 2.1 IPMR Construction The IPMR is a portable, self-contained device that can easily be installed in a substation to determine the site severity. The IPMR’s main components are: • • • • • •

one test insulator, a mechanical arm to raise or lower the insulator, a humidifier producing artificial wetting, an air dryer, 220/3000V test transformer, a microprocessor to control the tests and to log the measured values.

The IPMR only requires 220V (50Hz) supply and a pressurized water point at the testing location.

Figure 1: Image of the IPMR. 2.2 IPMR Measurements 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 highly conductive fog moves into the area causing flashovers but leaves a very low resultant pollution level on the insulator. This phenomenon is a serious threat to insulation since this condition typically occurs within less than an hour. The IPMR was thus designed to monitor: • • •

surface conductance on pre-deposited pollution with natural wetting (Measurement A), surface conductance on pre-deposited pollution with artificial wetting (Measurement B), leakage currents on pre-deposited pollution with natural wetting as well as monitoring for the onset of instantaneous pollution deposits (Measurement C).

The reference conductance is logged after being measured by applying five cycles of the 3kV wave. The flow chart of the test is illustrated in Figure 2.The flow chart of the measurement cycle is shown in Figure 3. During the measurement cycle the humidifier raises the humidity levels to allow moisture absorption by the pollution layer. The humidifying process is repeated at a set time delay. A voltage of 3kV is applied for five cycles to the insulator after every moisture cycle and the resultant leakage currents are logged. The test is repeated when the leakage currents stay within a set tolerance (x%) of I max. If the measured leakage current is higher than Imax, it will be stored as the new Imax value. When the measured current is smaller than x% of Imax, the test is stopped and the dryer dries the insulator. The test insulator will be raised to expose the insulator to the natural environment.

Measurement A The test insulator is energizing at set intervals for five cycles to assess the surface conductance during natural pre-deposited pollution and wetting conditions. The 3kV voltage is only applied to the insulator for five consecutive cycles from the 50 Hz wave to avoid the formation of dry bands on the insulator surface.

Start Measurement Cycle

Generate Artificial Wetting

Drying Cycle

Time Delay Set Measured

Measurement B The test insulator is enclosed in the test chamber whereby surface conductance is measured if critical wetting should occur on the pre-deposited pollution. Critical wetting will be discussed later in the paper.

Current = Imax Switch ON Transformer

Measure PEAK Current Over 5 Cycles

Switch OFF Transformer

YES

The IPMR measures the surface conductance on one test insulator. The test insulator is mounted on a mechanical arm that can lower the test insulator into the test chamber. The test chamber acts as a controlled environment that shields the insulator from the surrounding environmental conditions during the tests. S T A R T O F M E A S U R E M E N T

Is Measured Current > Imax?

Log Measured NO

Compare Measured Current With I

NO B

Current

max

Is Measured C u r r e n t < x % o f Im a x ?

YES

L O W E R T E S T I N S U L A T O R Stop D R Y I N G

Measurement Cycle

C Y C L E

Figure 3: Flow Chart of the Measurement Cycle During Measurement B.

M E A S U R E R E F E R E N C E C O N D U C T A N C E

M E A S U R E M E N T C Y C L E

D R Y I N G

R A I S E

C Y C L E

T E S T

I N S U L A T O R

E N D O F M E A S U R E M E N T

B

Figure 2: Flow Chart of Measurement B. At the beginning of the artificial insulator is lowered into the test mechanical arm. An air dryer heats chamber drying the pollution layer

wetting test, the chamber by the the air inside the on the insulator.

The conductance of an insulator is a function of leakage current through the contamination layer due to the applied voltage. Conductance values are unique to the leakage currents flowing over the specific insulator profile. It is therefore difficult to compare different insulators to their conductance values. To overcome this effect, the conductivity value can be used to compare different insulators since it is independent of the insulator’s geometry. The measured surface conductance (Gs ) is multiplied by the form factor (F) of the test insulator to determine the layer conductivity (σs ) of the pollution layer.

σs = F . Gs

…(1)

The form factor (F) identifies each insulator shape in terms of the insulator radius (r) as a function of creepage length (L) [4]. L

F=

∫ 2.π.r (s ).ds 1

3.1 Method of Artificial Wetting The accuracy and repeatability of the Measurement B greatly depends on the device used for the generation of the artificial wetting. Four different devices were tested to determine a suitable method of artificial wetting.

…(2) 3.1.1 Acoustic Cell

O

It was decided to relate the IPMR Measurement B values to ESDD values, as it existed as a standard defining the characteristics of the pollution layer. The ESDD can be defined as the equivalent deposit of NaCl on the insulator surface that will have the same electrical conductance as that of the actual deposit dissolved in the same amount of water.

The first device tested was an acoustic cell. This device is used as humidifiers for domestic use. The cell is capable of creating a very fine mist in the chamber but was not suitable for use in harsh environments due to its fragile construction.

Measurement C The leakage currents are measured on up to three energised insulators in the nearby vicinity. The leakage current sensors can be connected up to 100m from the IPMR via fibre optic cable.

The second artificial wetting device tested were highpressure nozzles connected to a 7,5 bar high-pressure pump. As wetting is required, the pump is simply switched on and small blades in the nozzles “chopped” the water into a mist between 10 – 50 m. The benefit of using this configuration was that the IPMR could be used in areas where no pressurized water is available since the water could be pumped from an installed water tank. Unfortunately the configuration proved problematic due to the clogging of nozzles and the difficulty controlling the precise amounts of wetting. This was due to pressure left in the system after the pump was switched off.

Verma published the well-known Imax theory in the late 1970’s. Imax was defined as the minimum amount of leakage current that was necessary to cause flashover. Imax was independent of the insulator shape, pollutant or test procedure. The only governing factor was the specific creepage distance (mm/kV) of the insulator.  mm / kV  Imax =    15 ,32 

3.1.3 Kettle

2

…(3)

Imax can thus be used to predict the actual risk of flashover on a real-time basis. The Ihighest is used as a criterion since the calculated Imax value was too close to the actual flashover.  mm / kV  Ihighest = Ih factor .    15 ,32 

3.1.2 High Pressure Nozzles

2

…(4)

The Ihighest value gives an indication of excessive leakage current rise that can lead to flashover.

The third artificial wetting device tested was a small 1,1 litre steam generator. The steam generator makes use of a 2 kW kettle element to minimise the need for specialized spares in the case of repairs. Water is supplied via one 220V solenoid valve connected onto a pressurised water supply. The inlet valve is opened before the start of every steam generation cycle to ensure that the steam generator is filled with sufficient water during the test. The element is then switched on and left to boil. When the steam is released a thin layer of moisture is deposited on the insulator surface. However since the kettle was an open looped system, 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 calibration curve is used to relate the pollution severity to the conductance measured on the test insulator.

A miniature 5-litre boiler capable of generating a steam output of 3kg/h was constructed. The steam 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

IPMR Surface Conductance (uS)

60

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 The artificial pollution process was done according to the solid layer method prescribed in the IEC 507 document [4]. The solution consisted of 40 grams of kaolin per litre water. By adding different amounts of NaCl to the solution simulated the different pollution levels. The kaolin in the solution is a non-dissolving inert material used as a bonding agent for the NaCl on the insulator surface. The kaolin simulates inert materials, e.g. cement, lime, dust, clay, etc., that performs the same bonding function when insulators are exposed to natural conditions. The test insulator is dipped in the solution ensuring that a uniform pollution layer is applied to the surface. The insulator is then allowed to dry before it is placed in the IPMR.

approximate guide was developed from data obtained from standard cap and pin insulators tested vertically using the kaolin as the inert material. 4. CONCLUSIONS An Insulator Pollution Monitoring Relay (IPMR) was designed to assess site severity and to give alarm messages when pollution levels exceed pre-determined values. Four different devices were tested to produce the artificial wetting during the ESDD measurement. A calibration curve was determined and the curve has shown similarities with an approximate guide of correspondence between the degree of pollution and the volume conductivity of the suspension. REFERENCES

3.3 IPMR calibration curve [1] The test insulator is polluted using the solid layer method and left to dry. The insulator is placed in the IPMR and a test is performed. The insulator is washed after the test to determine the ESDD of the deposit on the insulator surface. By using the surface conductance and ESDD values of each test, a calibration curve (Surface Conductance ( S) vs. ESDD (mg/cm2 )) was drawn up.

[2]

[3] The calibration curve was determined by plotting the values obtained from the tests and performing regression analysis to determine the relationship of the parameters. The pollution range in the calibration curve ranges from light (0,03 – 0,06 mg/cm2 ), medium (0,10 – 0,20 mg/cm2 ) and heavy (0,30 – 0,60 mg/cm2 ) [5].

[4]

The solid line is the IPMR calibration curve for the porcelain post type insulator and the dashed line is an approximate guide given by the IEC507 [4]. The

[5]

Van Wyk L., “Insulator Pollution Monitoring: Evaluation of Various Methods of Severity Measurements At A Coastal Site”, Thesis, University of Stellenbosch, December 1996. Van Wyk L., Holtzhausen J.P., Vosloo W.L., “Relation Between Surface Conductivity, Leakage Current and Humidity of Ceramic Insulators”, 31st UPEC, Iraklio, Crete, September 1996. Bertrazzi A., Perego G., Sampaoli G., Vachiratarapadorn Y., Eamsa-as V., “A Device For The Automatic Measurement Of Surface Conductivity Of Insulators”, Paper 47.41, 6th ISH, New Orleans, USA, Aug./Sept. 1989. IEC Publication 507, “Artificial Pollution Tests on HV Insulators to be Used On AC Systems”, Technical Committee No. 36: Insulators, January 1987. IEC Publication 815, “Guide For The Selection Of Insulators in Respect of Polluted Conditions”, 1986.