DETERMINATION OF A CALIBRATION CURVE FOR AN INSULATOR POLLUTION by gyvwpsjkko

VIEWS: 39 PAGES: 4

									    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 [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                                  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) [4].                                     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 [4]. 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
                                                                                                     [1]       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                                               [2]       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.
                                                                                                     [3]       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                                                  [4]       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 ) [5].                                                                  Systems”, Technical Committee No. 36:
                                                                                                               Insulators, January 1987.
The solid line is the IPMR calibration curve for the                                                 [5]       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 [4]. The                                                                 Conditions”, 1986.

								
To top