High Voltage Test Equipment

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32NETA WORLD Fall 2007 www.netaworld.org

N

o-OutageInspe

ctionCorner

by Don A. GenutisGroup CBS

Which No-Outage ToolShould You Choose?

Tis column focuses on electrical inspection methods and technologies that are performed

while the electrical system remains energized. Although no-outage inspections can be very

valuable tools, always remember to comply with proper safely guidelines when conducting

energizedon-lineinspections.

Although this is just a partial list oobservations and decisions to make, idoes display the wide array o consid-erations that the technician must aceExperience helps make the decision-making process more likely to be suc-cessul in regard to optimizing saetymeasurement integrity, and eciencyBut every situation is unique, and thetechnician must always be alert andthinking ahead. All this is done while

trying ones best to make a good rsimpression on the customer and to ap-pear cooperative and knowledgeable.

No matter how many eorts havebeen made to provide the customerwith adequate sales and technical inormation beorehand, the customestill usually asks, What is the di-erence between inrared and partiadischarge? I am always happy to an-swer this (and any other) question, buater nearly ten years o applying new

no-outage technologies to customerassets, I eel that displaying the dier-ences pictorially may better help iden-tiy the dierences in the technologiesRemember the old saying, a picture isworth a thousand words.

Beore jumping ahead with the pic-tures, I would like to rst present theollowing relatively unsuccessul (wel

Oten when I begin setting up or a partial discharge, ultrasonic, or

daylight corona survey at a customers site, I nd mysel attempt-

ing to answer some customer questions regarding the viability o

the new technologies. At the same time I am doing my best to ocus on the

important aspects o perorming the job properly, including:

What the general layout o the equipment to be surveyed is like What type o enclosures are present What the general condition o the equipment is like based upon the

external appearance

Observation o any unusual noises or smells Determining the necessary clearances Finding the best place to set up Finding a nearby power source Locating at least two exits Determining the best way to get the test equipment to the testing

location

Identiying any obstructions or trip hazards Identiying additional saety considerations that I need to make or

the customers personnel saety

Insuring that my partner is ocused on his/her role and is not beingdistracted by the customer


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Figure 1Tis picture displays the test setup. Te thicker conductor is an unshielded, singleconductor cable, and the thinner conductor is at ground potential. Te ac high

potential test set is at the far left of the picture.

we did somehow get the p.o.) verbal or written eorts that Ihave used to describe the dierences in the technologies.

Inrared technology detects heat that is generated by cur-rent fow or current related problems that are oten caused byloose connections. So, inrared detects the condition o theconductor. Partial discharge, ultrasonic, and corona surveysdetect voltage problems that occur rom the partial ailureo the insulation. Tese technologies detect the conditiono the insulation. Unortunately, a signicant voltage must

be exceeded or these partial discharges or corona to occur.Tis voltage is typically greater than 2,000 volts. Tereore,o these technologies, only inrared is eective or low-volt-age applications.

Infrared vs. PDPrimarilyforLowVoltage MediumandHighVoltage

CurrentistheCulprit VoltageistheVillain

Now, back to the pictures. I created simple, surace dis-charges in the laboratory that were produced by applying apotential rom an ac high potential test set to a medium-voltage unshielded conductor that was placed upon agrounded conductor. Several types o tests were perormedas shown below.


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Figure 3his infrared image of the same event does not show any appreciativtemperature rise.

Figure 2Te yellow areas of this photo show the ultraviolet light associated with thecorona that is occurring between the energized and grounded conductor.


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Figure 4Partial discharge test results clearly show a dangerous problem. Te largersignal was obtained using a high frequency current transformer placed aroundthe ground conductor and the smaller signal was obtained using a capacitivesensor placed on the test set control unit.

Figure 5Magnitude vs. time and magnitude vs. frequency graphs obtained fromrecording the audio output f rom an ultrasonic test set. In this case, the graphsare not very useful but listening to the audio output certainly is.


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High Voltage Test Equipment