Transfix On-Line DGA & Moisture

A New Generation in Transformer Monitoring

Michael Cunningham Dr. Colin McIlroy

Introduction

The modern world is increasingly dependant on electricity, both for economic prosperity and maintaining our way of life. Recent high profile incidents in the US and Europe have shown how failures in the electrical networks can be have very serious consequences. Without doubt the single most important piece of equipment in electrical networks is the power transformer. However the average age of transformers around the world is increasing rapidly with transformer failures rising exponentially, and to replace all of the aged power transformers at risk would be prohibitively expensive. Electrical companies most try to avoid these expensive failures and also maximise their transformer asset. Hence accurately monitoring the condition of these transformers is becoming evermore critical. Dissolved Gas Analysis (DGA) has long been recognised as being the singular most important test for transformer condition monitoring, providing the vast majority of the information required. The traditional method of DGA consists of taking an oil sample from the transformer and sending this to a laboratory to be measured using a gas chromatograph (GC). This can cause problems with time delays when the equipment could be in critical condition. It also means that developing problems in transformers can often be missed because of the potentially long sampling intervals, which are typically annual. Sampling procedures and handling can often also add inconsistencies, meaning the information gathered can often be misleading. A DGA result is only is good as the sample taken (Dr F. Jacob, ACTI Labs, USA). In addition it has been shown that there can be large differences between laboratories, and even with the same GC. Electrical companies have long recognised the value of on-line monitoring of their power transformers. Until now reliable, accurate and efficient DGA could not be implemented in an on-line system. It has been attempted to bring the GC to the field by utilizing an on-line version. These systems have many inherent problems, including high maintenance requirements and the need for consumables of carrier and calibration gasses. This has left electrical companies with very little choice but to install monitoring equipment that provides very little condition information, but merely acts as a warning flag. Kelman Ltd have now released the Transfix, using new, field proven technology to provide reliable on-line DGA, including moisture analysis, in a package that is extremely easy to install requiring minimal maintenance and no consumables or regular calibration.

Kelman have shown how Photo-Acoustic Spectroscopy can by used for extremely reliable and accurate DGA by developing the successful Transport X portable unit (see EW magazine, June 04, and Dr. McIlroy, EPRI 2004, USA). The Transfix uses the same core technology and represents a new generation of on-line condition monitor.

The Photo-Acoustic Effect The photo-acoustic effect is caused by the ability of a gas to absorb electromagnetic radiation (e.g. infrared light). In absorbing the radiation the temperature of the gas will increase and, if the gas is held in a sealed container, this temperature rise will produce a proportional rise in pressure. If the light source is pulsed the pressure of the gas fluctuates in sympathy and these pressure waves can then be detected using sensitive microphones. There are two key facts that permit this effect be used for analytical measurements. The first is that each gas has a unique absorption spectrum, thereby allowing the frequency of the infrared source to be tuned to excite a given substance. The second is that the level of absorption is directly proportional to the concentration of the given gas. Therefore by selecting an appropriate wavelength and measuring the level of the resultant signal it is possible to detect, not only the presence but also the concentration of any given gas, even in a complex cocktail of other compounds. This forms the central principle of Photo-Acoustic Spectroscopy (PAS).

Photo-Acoustic Spectroscopy for DGA The conceptual design of a practical PAS measurement module is shown below. A simple hot wire source produces broad band radiation across the IR range that is focused into the measurement cell using a parabolic mirror. The chopper wheel rotates at a constant speed giving a stroboscopic effect to the light source. Before reaching the measurement cell the radiation is passed through one of a number of optical filters. These filters are designed to transmit the specific wavelengths chosen to excite one of the compounds under investigation. The extracted gas sample is introduced into the measurement cell and the signal level is recorded from the microphones as each optical filter is indexed into the light path. The series of readings produced then gives the concentration of the desired compounds in the sample.

Figure 1. Illustration of Photo-acoustic Spectroscopy

Kelman has developed the Photo-acoustic Gas Analysis (PGA) module, shown in Figure 2 which is used in both the Transport X and the Transfix.

Figure 2. Kelman PGA module

Transfix and the PGA Module The Transfix is an on-line, self-contained DGA system, providing measurements of all the fault gases plus oxygen and moisture. There are no consumables required, such as tanks of carrier gases or calibration gases. Figures 3 and 4 show the system and illustrate the connection to the transformer.

Figure 3. Transfix Unit Figure 4. Connection to the Transformer

The Transfix has a programmable sampling rate and can test the transformer every hour. This allows the electrical company to have an in depth view into the condition of their transformer, viewing important behaviour such as load and temperature dependence.

Figure 5 & 6. Transfix Installation

The Transfix does not require vacuum extraction, and the gases are extracted from the oil sample by controlled excitation, with all oil being returned to the transformer after sampling. The extracted gases are analysed using the Kelman PGA module, with the internal memory able to store up to 10,000 records. Wide-ranging local and remote communications options mean that the Transfix is easily adapted into any system. These include modem, GSM and Bluetooth options. The Transfix also has extensive alarm capabilities, allowing flexibility in configuration. There are six independent alarm screens available, each settable by the operator. These are based on gas levels, rates-of-change, total gas levels or gas ratios. Each screen can activate the front panel LEDs (see Figure 3), one of three alarm relays or send warning messages, even via SMS messages (optional). There will also be detailed history kept of all alarm settings. Another important part of the system is the TransComm PC software. This allows the user to configure all alarms and sampling rates, offering password protection for all settings if required. The operator can interrogate the Transfix (remotely or locally, depending on the comms options selected) and download DGA data. TransCom is also used to graph and trend results in order to aid the diagnosis of transformer condition, see Figure 7.

Figure 7. TransCom Software

Transfix Specification Compound Lower Detection Limit

ppm Calibrated Range ppm

Hydrogen H2 10 10 2,000 Carbon Dioxide CO2 10 1 50,000 Carbon Monoxide CO 1 1 50,000 Methane CH4 1 1 50,000 Ethane C2H6 1 2 50,000 Ethylene C2H4 2 1 50,000 Acetylene C2H2 1 1 50,000 Oxygen O2 100 100 50,000 Water H2O 3ppm 1 100%RH Accuracy: 10% of reading + 1LDL

(at 25C, dependent on partition coefficient of oil in use)

Water: 1 100% RH Accuracy 5% of displayed reading

Environmental: Operating Temperature: -40 to +55C (-10 to 55C at power up, 1 hour warm up) Transformer Top Oil Temperature : -10 to +110C Oil Inlet Oil Temperature: -10 to +80C Operating Humidity: 10% to 95% RH non-condensing Enclosure rating: NEMA IP56 Oil Pressure at in-let connector Operating: 0 3 bar (0 45psi) Non-operating: -1 6 bar (-15 87psi) Analysis Interval: User settable - 1/hour to 1/day in 1 hour steps

(1 hour interval for ambient temperature of -10 - +45C, 2 hour interval for ambient temperatures between -40 - +55C)

Alarms: - 2 sunlight visible panel LEDs, 3 alarm relay contacts, each user configurable - All alarms programmed locally or remotely via TransCom PC Software - 6 alarms settings screens or scenarios available - 3 based on gas levels, TDCG, rate of change - 2 based on gas ratios - Each screen can activate1 of 3 relays, red or yellow LED or send SMS (if GS modem option elected) - All alarm screens independent of each other - Detailed history of alarm settings kept - Password protection available Electrical: Mains Voltage:

85-130Vac OR 200-250Vac (factory configured) 47-63Hz. 8A max single phase

Alarm Relays: NO and NC provided, 5A 250Vac, 5A 30Vdc Communications RS-2332, USB, PSTN Modem, other options

available

Laboratory and Field Testing As the Kelman Photo-Acoustic Gas Analyser (PGA) module is common between the successful Transport X product and the Transfix laboratory and field testing of the technology can apply to both products. Extensive laboratory and field testing of the core Kelman PGA module has taken place in the US, Canada, Mexico, South Africa, Europe and Asia to investigate the accuracy and repeatability of the DGA results. The extensive testing includes analysing laboratory prepared samples as well as many taken from transformers, tap changers, circuit breakers and bushings. These tests encompassed wide ranging climate conditions, from sub-arctic to tropical. While being too voluminous for the article, the findings of the results are outlined in a paper by Dr Colin McIlroy for EPRI 2004 the US1.

Figure 8 & 9. Laboratory and Field Testing of Kelman PGA module. Repeatability: In these trials it has been independently shown that for all fault gases the Kelman PGA module is more repeatable than lab based GC units. The repeatability (spread of results) of the Kelman PGA module unit has been shown to be 2% or 2ppm, whichever is greater. Accuracy: Measured using lab prepared samples it was shown that for almost all gases the Kelman PGA module was noticeably more accurate than the GCs. None of the gases show the GCs having consistently better results. One of the most critical issues in DGA for transformer maintenance is the ability to reliably detect low levels of acetylene, C2H2. The Kelman PGA module has consistently shown that it can accurately identify trace levels of C2H2 down to 1ppm.

The extensive laboratory testing has shown that the Transfix can provide DGA and moisture results that are reliable and repeatable, and it is more than acceptable for analysis of electrical equipment.

Conclusions The results of controlled lab experiments and practical field trials from around the world clearly demonstrate that the Transfix technology can provide accurate and reliable DGA results. With an ever-aging transformer population, the monitoring of transformers is increasingly vital. The Transfix from Kelman, with continuous DGA and moisture analysis combined with the sophisticated alarm system, communications options and TransCom software, represents an invaluable tool for modern Asset Management.

References1. Dr.McIlroy,Colin.PhotoAcousticSpectroscopy,ANewTechniquefor

DissolvedGasAnalysisinOil.EPRISubstationEquipmentDiagnosticsConference,NewOrleans,LA,USA.February2326,2003

2. Dr.McIlroy,Colin.AcomparisonofPhotoacousticSpectrometerandGasChromatographTechniquesforDissolvedGasAnalysisofTransformerOil.EPRISubstationEquipmentDiagnosticsConference,NewOrleans,LA,USA.February15182004

Author Biographies: MichaelCunninghamKelmanLtdMichaelCunninghamisProductManagerfortheAssetManagementdivisionofKelmanLtd.ThisdivisionincludesCBanalysisandTransformerDGAequipment.Theroleinvolvesallaspectsoftheproduct,fromprojectmanagementfornewdevelopmentthroughtosales.MrCunninghamhasbeenwithKelmanfor7years.Forthefirst3yearsheworkedasaDesignEngineerinthedevelopmentofproductsfortheelectricalutilities.MrCunninghamreceivedaBEngdegreeinElectronicandElectricalEngineeringfromNottinghamUniversityinEnglandandanMScinElectronicsandDigitalSignalProcessingfromtheUniversityofUlsterDrColinMcIlroyKelmanLtdDr.ColinMcIlroyisDirectorofAppliedEngineeringatKelmanLtdspecializingininstrumentationandswitchgearfortheelectricpowerindustry.Heisresponsiblefortheapplicationofnewtechnologiesandtechniques,particularlyintheareaofconditionmonitoring.BeforejoiningKelmanin1996,Dr.McIlroywasVicePresidentofEngineeringforHathawayinDenver.PriortothatpositionhewasemployedbyCSDintheUKandUSA,

withparticularexpertiseinthedesignanduseofcircuitbreakertimingandconditionmonitoringsystems.Dr.McIlroyreceivedB.Sc.andPh.D.degreesinElectrical&ElectronicEngineeringfromtheQueensUniversity,Belfastin1981and1984respectively.