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Detection of Partial Discharges in Cable Splices and TerminationsBy Mark Goodman, VP Engineering, UE Systems, Inc., Elmsford, NYABSTRACT Airborne ultrasound can be considered an ideal integrating technology in that these instruments can stand alone to detect a variety of potential problems or they can be used to support RF and infrared inspection programs. Usually portable, these instruments detect partial discharge in cable connections, terminations, and transformers; leaks in both pressurized gas systems or vacuum systems and related equipment such as transformer housings, SF6 insulated circuit breakers and insulated buss bars. Additional applications include inspection of high voltage apparatus for corona, arcing and tracking. A brief overview of the technology its applications and suggested inspection techniques are explained. BACKGROUND AND INTRODUCTION In many companies and institutions, there has been an almost compelling need to find areas of cost reduction. For many this has meant the cutting of budgets which unfortunately included a cutting of personnel. In many departments, this has resulted in many doing more with less. This has also impacted on many maintenance and production departments around the world. In order to maintain an efficiently running operation, there has been a trend towards various forms of Predictive Maintenance. Along with this trend has come a need for effective technologies and instruments that enable personnel to become more and more productive in terms of equipment inspection and problem solving. Opportunities in predictive maintenance have lead to improvements in production, reduced maintenance costs, reduced energy consumption, efficient use of personnel and increased profitability. One of the main reasons for the advances in the area of predictive maintenance has been the development of many new technologies such as vibration, infrared and ultrasound. Of the various technologies, one of the least understood is airborne ultrasound. This technology can be considered an integrated technology since it can be used with both infrared and vibration inspections as well as stand alone to perform a multiplicity of inspection activities. Instruments based on this technology apply to a wide range of plant operations and yet are simple enough to be used with little training. This makes them ideally suited for such programs as TPM (Total Productive Maintenance) in which they may be used by both maintenance and production personnel. Lightweight and portable, these instruments may be used to inspect potential problems in practically every type of equipment and system in a plant. Some typical applications include: Partial discharge in cable splices, terminations, and in transformers, leak detection in pressure and vacuum systems such as, transformers and circuit breakers, specialty gas systems, bearing inspection, steam trap inspection, detection of valve blow-by, detection of cavitation in pumps, detection of corona in switch gear or in connections, and even the integrity of seals and gaskets in transformers. What makes airborne ultrasound so effective? All operating equipment and most leakage problems produce a broad range of sound. The high frequency ultrasonic components of these sounds are extremely short wave in nature. A short wave signal tends to be fairly directional. It is therefore easy to isolate these signals from background plant noises and to detect their exact location. In addition, as subtle changes begin to occur in mechanical equipment, the subtle, directional nature of ultrasound allows these potential warning signals to be detected early, before actual failure. Airborne ultrasound instruments, often referred to as "ultrasonic translators", provide information two ways: qualitative through the ability to "hear" ultrasounds through a noise isolating headphone and quantitative via incremental readings on a meter. Although the ability to gauge intensity and view sonic patterns is important, it is equally important to be able to "hear" the ultrasounds produced by various equipment. That is precisely what makes these instruments so popular. They allow inspectors to confirm a diagnosis on the spot by being able to clearly discriminate among various equipment sounds. This is accomplished in most ultrasonic translators by an electronic process called "heterodyning" that accurately converts the ultrasounds sensed by the instrument into the audible range where users can hear and recognize them through headphones. The high frequency, short wave characteristic of ultrasound enables users to accurately pinpoint the location of a leak or of a particular sound in a machine. Most of the sounds sensed by humans range between 20 Hertz and 20 kilohertz (20 cycles per second to 20,000 cycles per second). The average human threshold is actually 16.5 kHz. These frequencies tend to be relatively gross when compared with the sound waves sensed by ultrasonic translators. Low frequency sounds in the audible range are approximately 1.9 cm (3/4") up to 17 m (56') in length, where-as ultrasounds sensed by ultrasonic translators are only 0.3 cm (1/8") up to 1.6 cm (5/8") long. Since ultrasound wave lengths are magnitudes smaller, the "ultrasonic environment" is much more conducive to locating and isolating the source of problems in loud plant environments. The basic advantages of ultrasound and ultrasonic instruments are: 1. They are directional and can be easily located 2. They provide early warning of impending mechanical failure 3. Instruments are easier to use in a loud, noisy environment 4. They support and enhance other PDM technologies or can stand on their own in a maintenance program. INSTRUMENTATION Airborne Ultrasound translators are relatively simple to use. They consist of a basic hand held unit with headphones, a meter, a sensitivity adjustment, and (most often) interchangeable modules that are used in either a scanning mode or a contact mode. Some instruments have the ability to adjust the frequency response from between 20 to 100 kHz. An ultrasonic transmitter called a tone generator is often included. Many of these features are useful in helping a user adapt to a specific test situation. As an example, should a ultrasound source be too difficult to locate due to an intense signal, a downward adjustment of the sensitivity, will help a user focus in on the exact site. In another instance, should a low level partial discharge occur, the frequency tuning can be adjusted to help a user hear the discharge through the insulation. The interchangeable modules allow users to adjust for different types of inspection problems. The scanning mode is used to detect ultrasounds that travel in the atmosphere such as a pressure leak or a corona discharge, while the contact mode is used to detect ultrasounds generated within a casing such as in an insulated connection or in a termination, bearing, or pump. APPLICATIONS Generically, applications for ultrasonic translators fall under three basic categories: electrical inspection, mechanical inspection and leak detection.
ELECTRIC INSPECTION Electrical problems are detected with ultrasonic translators. When partial discharge, arcing, tracking or corona discharges occur, they ionize air molecules producing ultrasound. Partial discharges in cables or terminations, loose connections in buss bars, junction boxes, etc., can be listened to for these high frequency sounds. This will usually be heard as a “buzzing” or “frying” sound in the headphones. Over the past several years, tests were performed to determine the correlation between signals measured with portable ultrasonic detectors and standard electrical measurements for different partial discharge levels in cables, molded joints and transformers. The faults were created on cables containing artificial voids and the fault that caused the internal partial discharge on the transformer was from a unit taken from the field. Results of what levels of partial discharge that are detectable with ultrasound are as follows:
The data in this paper was obtained utilizing several instrument types, and does not represent the high sensitivity instruments available today. Further testing during the 2nd quarter of 1998 should reflect the sensitivity with the current state of the art technology. Another area of inspection for ultrasonic detectors is in switchgear and overhead high voltage lines for location of corona or partial discharge problems. Although infrared has often been used to locate electrical problems, it has been found that these instruments are often "blind" to corona and tracking in high voltage systems (13 KV and up). Ultrasonic detectors "hear" the sound of corona and enable users to locate them quite quickly. For this reason, many inspectors now use ultrasonic translators to support their infrared electric monitoring programs. In fact, those inspectors that use both technologies often relate that they prefer to screen enclosed switchgear with ultrasound instruments to detect the possibility of corona, arcing or tracking by scanning around door seals and air vents. Some utilities use an ultrasonic detector as a screening device to monitor for severe partial discharge or arcing before entry into manholes and cable vaults. One of the most impressive new developments in ultrasonic scanning is the ultrasonic wave form concentrator, a parabolic reflector that is used to detect leakage at a distance. These accessories are useful in situations where it is difficult to reach an area, such as in a high ceiling, or where the leak might prove dangerous, as in manholes. MECHANICAL INSPECTION Operating mechanical equipment produces a "normal" sound signature while operating effectively. As components begin to fail a change in the original sonic signature occurs. This change can be noted on a meter or a recording device such as an oscilloscope, portable PC, portable vibration analyzer or chart recorder. The sound quality will be heard through headphones. The key to mechanical inspection relies on a consistency factor. Variables should be kept to a minimum. To accomplish this, whether trouble shooting or trending equipment, a test point should be established. This test point can then be used for comparison with other test points on similar equipment or compared with itself over time. As an example, for bearing inspection, in order to determine whether a bearing is in a good or failed mode, touch the bearing housing using the contact probe at one point, usually the grease fitting, and adjust the sensitivity to get a specific meter reading. Compare this reading at the same sensitivity setting on a similar reference point on a bearing operating under the same conditions. The meter reading and the sound quality should be similar. This same reading can then be used to trend each bearing over time to determine lack of lubrication or failure mode. According to NASA research, ultrasonic monitoring of bearings provides the earliest warning of bearing failure. They noted that an increase in amplitude of a monitored ultrasonic frequency of 12-50 times over baseline will indicate the initial stages of bearing failure. This change is detected long before it is indicated by changes in vibration or temperature. With the above described procedure, problems such as cavitation in pumps, leakage, faulty gears, excessive rubbing, poor connections, etc., can be determined in their early stages before break down. Should an existing vibration program already exist for bearing analysis, it should be understood that an ultrasonic bearing monitoring program works very well with on-going vibration programs. Ultrasound translators can be used to aid a diagnosis. The high frequency, short wave characteristic of ultrasound allows the signal to be isolated so that a user can hear and determine if, in fact, a bearing has been correctly diagnosed as failed. At times there can be false signals generated by equipment connected to a particular bearing. By adjusting the sensitivity, the frequency and listening to the sound, it can be determined whether it is the bearing or a rotor or something else that is the root of the problem. The ability to hear what is going on can prove very important. Ultrasound detectors work well on slow speed bearings. In some extreme cases, just being able to hear some movement of a bearing through a well greased casing could provide information about potential failure. The sound might not have enough energy to stimulate classic vibration accelerometers, but will be heard via ultrasonic translators, especially those with frequency tuning. LEAK DETECTION The category of leak detection covers a wide area of plant operations. It can be looked on as a way of keeping a system running more efficiently. Some plants include it as part of an energy conservation program while others refer to it as fugitive emissions. No matter what, leaks can cost money, effect product quality and reek havoc with the environment. Leakage can occur in liquid or gas systems. The greatest advantage of ultrasonic detection is that it can be used in a variety of leak situations since it is sound sensitive and not "gas" specific. The reason ultrasound is so versatile is that it detects the sound of a leak. When a fluid (liquid or gas) leaks, it moves from the high pressure side of a leak through the leak site to the low pressure side where it expands rapidly and produces a turbulent flow. This turbulence has strong ultrasonic components. The intensity of the ultrasonic signal falls off rapidly from the source. For this reason the exact spot of a leak can be located. The method of generalized gas leak detection is quite easy. All one does is scan an area, listening for a distinct rushing sound. With continued adjustments of the sensitivity, the leak area is scanned until the loudest point is heard. Some instruments include a rubber focusing probe which narrows the area of reception so that a small emission can be pinpointed. The rubber focusing probe is also an excellent tool for confirming the location of a leak by pressing it against the surface of the suspected area to determine if the sound of the leak remains consistent. If it decreases in volume, the leak is elsewhere. Vacuum leaks may be located in the same manner. The only difference is that the turbulence will occur within the vacuum chamber. For this reason, the intensity of the sound will be less than that of a pressurized leak. Even though it is most effective with low-mid to gross leaks, the ease of ultrasound detection makes it the instrument of choice for most vacuum leak problems. Liquid leaks are usually relegated to determining leakage through valves and steam traps although there have been some reports of success in locating water leaks from pressurized pipes buried underground. The key to determining whether a product can be checked for leakage is its ability to produce some turbulence as it leaks. Valves are usually checked for leakage with the contact probe. The downstream side is used to determine leakage. This is accomplished by first touching the upstream side and adjusting the sensitivity to read about 50% of scale. The downstream side is then touched and the sound intensity is compared. If the signal is lower than upstream, the valve is considered closed. If it is louder than upstream and is accompanied by a typical rushing sound, it is considered to be leaking. There will be some instances where it is difficult or too time consuming to inspect under pressure or vacuum. In this instance a test unique to ultrasound is incorporated. This method uses an ultrasonic transmitter called a "tone generator". The "tone generators" are placed in the various access ports or fittings to produce an intense, uniform ultrasound within the item under test. Since the sound waves are high frequency, short wave, they will tend to deflect off the solid surface of solid, intact tubes but will penetrate through the leak site of a tube. By scanning the outside with the ultrasonic scanner an operator listens for a distinct high frequency signal indicating the leaking gasket. This is a good pretest for either the vacuum drying or the nitrogen blanket fill. CONCLUSION Airborne ultrasound instruments are becoming an important part of Predictive Maintenance, and service safety programs. Their versatility, ease of use and portability enable managers to effectively plan and implement inspection procedures. By locating leaks, detecting high voltage electrical emissions and sensing early warning of mechanical failure, these instruments contribute to cost reductions, improved system efficiencies and reduced downtime. For optimum effectiveness, it is recommended that the major technologies, Ultrasound and Infrared, be used as part of a comprehensive inspection program.
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