8/8/2019 Transformer Oil Monitoring

http://slidepdf.com/reader/full/transformer-oil-monitoring 1/2

n OUTLINE

n EXECUTIVE SUMMARY

Transformer Oil Monitoring

A pulsating sensor has been developed for monitoring transformer oil degradation. The in-house developed device of newkind has been tested at CPRI, Bangaluru with a large number of service oil samples from different transformers, andcompared with the conventional techniques for oil characterisation. The device qualified for its use as the rapid screeninginstrument by the maintenance personnel.

Slow build up of oxidation products as a result of chemical reactions with oxygen and moisture fromenvironment under the prevailing service conditionsand physical ingress of polar molecules such as watercause the dielectric permeability of the service oil in any transformer to increase with time. A pulsating sensorhas been developed for monitoring transformer oildegradation. The sensor is basically capable of detectingminor shift in dielectric permeability of the oil duringservice. A laboratory made uniformly spaced multiplenumber of disc type stainless steel electrode assembly,immersed in the mineral oil of interest, serves as the

timing capacitor of an oscillator circuit which is drivenby a dc supply. With all other factors remainingunchanged, the digital pulse frequency at the output isgoverned by the dielectric permeability of the oil thatacts as the dielectric medium in the capacitor assembly.

  The pulse frequency changes sensitively with thechange in dielectric permeability and, hence, thereduction in measured pulse frequency with respect tothe fresh oil is a measure of dielectric degradation of theused oil during service.

  The measurement set up is shown schematically inFigure 1. Frequency can be measured by any of thecommercially available hand-held frequency meter.However, for a detailed study, the frequency data as afunction of time canalso be recorded through a PC usinga laboratorydeveloped interface hardware and software.

 The digital pulse frequency data as a function of time forfresh oil as well as for a slightly degraded oil using atypical device is shown in Figure 2. Decrease in pulsefrequency for the used oil corresponds to increase in thedielectric permeability by about 1.6 %. Thus, precisefrequency data permits monitoring of very minute shiftsin dielectric permeability, hence enabling early detection of onset and progress of degradation in thequality of service oil.

Under an MoU between IGCAR and Central PowerResearch Institute (CPRI), the devices made in IGCAR

  were tested at CPRI along with the conventionalstandard techniques for degradation assessment. Twohundred oil samples from power transformers of different ratings were collected and evaluated by the

Dielectric Laboratory of CPRI for the standardcharacteristics viz., interfacial tension, neutralizationvalue, dielectric dissipation factor, resistivity,breakdown voltage and water content following theprescribed procedures. The digital pulse frequencies

 were also recorded by CPRI on each sample using thepulsating sensor based oil monitoring devices. Thus,seven parameters were determined experimentally onevery oil sample.

It was observed that the pulse frequency, as measuredby the IGCAR devices, reduced by about seven percentfor service oils nearing rejection compared to that for thefresh oil. As the devices permit precise measurement of frequency shifts less than 0.2%, onset of degradation isdetectable at an early stage.

Fig 1 : Schematic representation of the oil monitoring device

Fig. 2 : Pulse frequencies recorded as a function of time for fresh oil and asampleof service oil

214

TECHNOLOGY-23

8/8/2019 Transformer Oil Monitoring

http://slidepdf.com/reader/full/transformer-oil-monitoring 2/2

n ACHIEVEMENT

Features such as (i) compact and rugged probe design, (ii) ease of pulsed digital signal generation at the probe head, (iii) noise freesignal transmission, and (iv) reliable means of pulse frequency measurement with high precision, led to design and developmentof on-line probes.

A flange mounted prototype on-line device is in operationfor over two years with an 11-kV transformer in IGCAR. Troublefree and stable field operation of the probe demonstratedusefulness of this unique product for continuous vigil on thequality of transformer oilin service.

  The sensing element is a capacitor assembly which isconstructed out of circular polished stainless steel discs profiledby wire cut EDM. The discs are closely spaced with uniform gapsand areelectrically separatedfrom theadjacentones by flat teflon

rings over thesupporting rod.

  The miniature pulse generation electronics is enclosed within asmall cylindrical enclosure placed close to the sensing elementand fixed on the same side of the mounting flange. A dielectrictype pulsating temperature sensor is also provided close to thesensor. Thus the sensors and the associated signal generationelectronics remain immersed within oil for the on-line device.

(Fig. 3)

Fig. 3 : Photograph of off-line and on-line transformer oildegradationmonitoring probes

 The correlations among the conventionally determined standard parameters, ie among interfacial tension, neutralizationvalue, dielectric dissipation factor, resistivity, breakdown voltage and water content, were comparable with those betweenthe digital pulse frequency measured by IGCAR devices and the standard parameters. The conclusion of the jointevaluation by IGCARand CPRI wasthat, as the pulsating sensor based new technique of IGCARis simple, the transformermaintenance personnel may use it as a screening test for deciding whether the oil sample should be sent to a centrallaboratory for full evaluation towards further actions with respect to treatments forreuseor disposal.

Hydrocarbon molecules constituting the transformer oil experience strong electric field in presence of occluded specieslike oxygen and moisture. Progressive damage to the oil is on account of build-up of polar/polarisable oxygenatedhydrocarbon molecules generated by electric field assisted oxidative chemical reactions. Dielectric permeability of the oil,thus, increases at a very slow rate. The capacitance of a device, with such oil as the dielectric medium, is a function of the

extent of contamination. Hence, the pulse frequency at the output of an oscillator, configured with this capacitor in thetiming circuit, decreases as the oil quality deteriorates.

1) Newkind of inexpensive devices foroff-line andon-line monitoring of transformer oil.2) Realisation of a field monitoring kitfor rapid andnon-destructive assay of ethanol in petrol (asa spin-off )

Further inquiries:Shri B. Saha, Innovative Instrumentation Section

Electronics and Instrumentation Group, IGCAR, E-mail: saha@igcar.gov.in

215

n AS A NON-DESTRUCTIVE AND RAPID OIL SCREENING DEVICE

n ADAPTATION FOR ON-LINE MONITORING

n BRIEF DESCRIPTION OF THEORETICAL BACKGROUND

TECHNOLOGY-23