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Published by Megger
December 2009 |
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| The industry's recognised information tool |
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ELECTRICAL
TESTER |
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In this issue |
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Vibration testing of circuit breakers |
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Taking the lead in wind turbine testing |
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Investing in the Middle East |
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New battery test guide |
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Grounds for Fault |
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Vibration testing of circuit breakers |
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| Romain Douib |
| Product Marketing Manager |
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| The objective of all circuit breaker diagnostic testing is to detect defects before they cause damage and system outages. Many test techniques are used to achieve this, but one that receives less attention than it deserves is vibration testing. |
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| A recent CIGRÉ study on circuit breakers in active use revealed that 70% of malfunctions were caused by mechanical rather than electrical faults. What better basis could there be for diagnostic tests, therefore, than looking directly at the mechanical behaviour of the breaker? And this is exactly what vibration testing and more accurately an acoustic signature does. |
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| The test method employed is easy to understand. Accelerometers, which behave in much the same way as microphones, are attached to the circuit breaker under test. As the breaker opens and closes, the vibrations they pick up are recorded in much the same way as an MP3 file might be captured for a voice or music recording. The recordings can subsequently be replayed and analysed at will. |
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| The results are easiest to interpret if a reference recording – often referred to as a reference signature or “footprint” – is available. This is simply a recording made when the breaker was known to be in good condition and operating correctly. The reference signature can be readily compared with subsequent recordings, with any significant changes indicating potential problems. |
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| If no reference signature is available for the specific breaker under test, the comparisons can instead be made with the reference signature of another breaker of the same type. This is however a compromise, not least because it is very different to be certain that the parameters for tests on two individual breakers – such as the exact placement of the accelerometers – are identical. |
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| While it is possible for experienced users of vibration analysis to display the recordings on a computer screen and compare them by eye, dedicated software will always provide more detailed and more reliable results. The analysis looks in particular at two aspects of the comparison, which are usually referred to as time shift and deviation (magnitude). Particular vibration patterns are associated with specific mechanical events, such as the closing of the breaker’s arcing contacts or main contacts. The time shift analysis looks for differences in the timing of these vibration patterns between the reference signature and the test recording. Deviation analysis looks for differences in the amplitude and frequency spectrum of the events. |
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| While experience is undoubtedly the best guide to the interpretation of the results, it is possible to provide some rules of thumb. While these illustrate the usefulness of vibration testing however, it must be understood that they are neither absolute nor universal criteria and they must not be used without careful consideration of the specific application. |
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| According to these rules of thumb, if the time shift is less than 4 ms, and the deviation curves never diverge by more than 10 dB, it is reasonable to assume that the circuit breaker is in the same mechanical condition as it was when the reference signature was generated. Note that successive tests on the same breaker will typically show differences of between 5 to 7 dB, and this should be regarded as normal. |
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| In cases where the time shift is greater than 4 ms but less than 8 ms, but the deviation curves still never diverge by more than 10 dB, this is likely to mean that the breaker is in the same mechanical condition, but that it is operating more slowly. |
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| This may be due to inadequate lubrication but, if the test is the first operation of the breaker for some considerable time, it is worth repeating it. If the second test gives normal results, it is very probable that the slow operation was caused by poor lubrication and/or corrosion friction. Note that a difference in ambient temperature can also produce a |
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| DTW analysis (Magnitude vs Time & Time vs Time) |
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| time shift as, in general, the colder the mechanism the slower the operating speed. This is particularly noticeable with hydraulic operating mechanisms. |
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| Finally, in cases where the deviation curves diverge by more than 10 dB or the time shift is greater than 8 ms, this is a strong indication that the breaker is no longer in the same mechanical condition, and that there is a severe risk of it malfunctioning if it is put back into service. At this point, the vibration data can be further analysed to provide a guide to which part of the breaker’s mechanical system is behaving differently. This is a good starting point for further diagnostic and remedial work. |
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| This article has, until now, concentrated on offering a simple explanation of vibration testing and the type of diagnostic information it can produce. Care must be taken in setting up and carrying out the tests if accurate and dependable results are to be obtained. |
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| In particular, the positioning of the accelerometers that pick up vibrations from the breaker is critical - changes in position of just a centimetre or two can make a big difference to the results. This also means that the positions used for the reference test must be clearly and indelibly marked, so that the accelerometers can be mounted in exactly the same positions for subsequent tests. |
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| Another issue that often needs to be addressed is that of electrical interference such as induction, which is always present in electrical substations. The best instruments, such as the TM1600 and TM1800 from Megger, are well protected against interference but, nevertheless, it is still important to inspect the recordings for evidence of its influence. Fortunately, this is in most cases straightforward, as genuine vibration signals are always symmetrical around the zero line, whereas interference signals are not. |
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| Other factors that affect the quality of the results obtained from vibration testing include the method of attachment of the accelerometers to the circuit breaker, the level of signal pre-amplification used, and the time window used for the result comparisons. Manufacturers of test sets that support vibration analysis can be expected, however, to provide practical and reliable advice in all these areas. |
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| Nothing in this article should be interpreted as meaning that vibration analysis is the only type of testing needed for circuit breakers – it should always be used in conjunction with more conventional motion, timing, DRM (Dynamic Resistance Measurement), and coil current tests to give an overall picture of breaker operation and condition. Vibration analysis is, however, a very useful weapon in the circuit breaker testing armoury – it is convenient to apply and it can often uncover incipient problems that other test techniques would miss until they had become far more serious. |
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Taking the lead in wind turbine testing |
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| Paul Swinerd |
| Product Manager |
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| In the manufacture and maintenance of wind turbines, low resistance testing is essential to verify the continuity of the lightning protection conductors in the turbine blades. In principle, this testing is straightforward but, in practice, there’s a problem: the turbine blades are invariably tens of metres long and sourcing quality test leads of sufficient length is difficult. |
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| The traditional solution is for test engineers and technicians who work with wind turbines to fabricate their own test leads, but this is time consuming and inconvenient, and the results are often uncertain. In particular, leads fabricated on an ad-hoc basis can be hard to handle and prone to tangling during use. |
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| At the request of leading manufacturers of wind turbines, Megger has developed its KC series of test leads specifically to address these problems. Believed to be the world’s first commercially available products of their type, the leads are available in a range of different lengths and are equally suitable for use on site or in a manufacturing plant where very large equipment must tested. |
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| For convenience and ease of handling, the leads are supplied as standard on a heavy-duty cable reel that is fitted with a friction brake to minimise the risk of tangles when paying out the cable. |
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| The leads are terminated with large, robust Kelvin clips that have been specially designed to offer ease of use while providing the consistently reliable connections needed to ensure accurate and repeatable test results. |
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| Included with each lead set is a cable fitted with a duplex handspike for probing the lightning receptors on the tips of the turbine blades. |
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| KC test leads are compatible with most types of low resistance ohmmeter but, when selecting an instrument to use with them, there are many benefits to choosing a model that, like the Megger DLRO10HD, combines robust construction to ensure reliable operation in the field, with a high test current capability to reveal faults that might otherwise go undetected. |
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| Where frequent outdoor use is anticipated, an ingress protection rating of at least IP54 when the instrument is in use is also highly desirable, as this will allow it to be used even in wet weather. |
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| Susan Welch |
| Marketing Communications Manager, Bahrain |
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| Investing money is always risky, even more so during a global recession! Every part of the world has been hit to some extent, and the Middle East is no exception. In the boom years, rapid expansion and huge numbers of real estate and infrastructure development projects marked the region. But as oil revenues and availability of foreign investment have fallen, so the rate of growth in the region has also slowed. In a recent study conducted by a Middle East research company it has been estimated that of around 3000 development projects across the GCC, 729 have been postponed or cancelled, a reflection of the new financial reality. Another study taken by a leading recruitment company claims that the hiring of skilled workers in certain countries throughout the GCC has declined by 40% from last year’s figures. |
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| Aside from the headline reduction in major projects in certain states however, most countries in the region have suffered just a marginal dent in their economies. Admittedly smaller businesses and local companies have suffered financially, and the buying of property in the region has dropped, but in many cases ongoing projects have not been majorly affected; work continues and projects are being completed. Places such as Qatar, Abu Dhabi and Oman are still seeing strong growth on previous years (and indeed a rise in recruitment levels), and in Saudi Arabia the commitment to maintaining the infrastructure development has not been affected. Despite the fall in oil prices and delays to many new projects, Saudi Aramco also continue to invest in their own infrastructure, so the outlook is generally positive. |
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| Multinational companies are looking at the bigger picture and know that investing now will have rewarding implications in the near future, with the realization that the recession won’t last forever. Megger is no exception and has continued its investment in local sales and technical support operations, recently opening a regional office in Bahrain to service the Middle East region. Nick Parton, sales manager in the Middle |
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| East says “Megger have had a presence in the region for many years, but we recognized that to maintain and develop the demand for our product range we needed to be more active and prominent locally. The Middle East team has doubled in the past 6 months, and our technical staff ’s knowledge of the products we sell and of the region is paramount to our growing success.” |
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| The Middle East Office now comprises Mohammed Tariq and Hussain Al-Juffairi as applications engineers, Ali Dawood as Regional Sales Manager and Susan Welch as Marketing Communications Manager. Also supported from the Bahrain office is Abduladhim Al-Hassan looking after sales in the Kingdom of Saudi Arabia, and Sadiq Alabdulwhab of the AVO Institute. |
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| Bahrain is a tiny archipelago in the Arabian Gulf and has become a major financial hub of the Middle East. It is recognized as a business friendly location, and with excellent road and air links to neighbouring states it lends itself as a base from which to cover the region. This continuing investment in sales and technical support is further evidence of the business opportunities available to companies that are prepared to invest in the region. |
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New battery test guide |
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| Elsa Cantu |
| Marketing Services Manager, Americas |
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| Storage batteries play an essential role in many sectors of industry and commerce, not least as convenient sources of standby power. All too often however, stationary battery systems receive little attention after installation – in many cases, testing is carried out infrequently, if at all. A new publication from Megger, “The Battery Testing Guide” explains the risks associated with neglecting battery installations and describes practical, cost-effective measures that can be taken to minimise these risks. |
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| The 32-page A5 format publication is comprehensive in its scope. After discussing why back-up batteries are needed and where they are used, it gives a concise overview of the main battery types and the various failure modes associated with each type. Battery maintenance philosophies are then examined and contrasted, with particular reference to the recommendations embodied in the IEEE 450, IEEE 1188 and IEEE 1106 standards. |
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| The largest section of the publication is devoted to practical methods of testing, and is designed to help battery users to optimise the performance, reliability and life of their installations. |
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| Discharge testing, which is the only definitive way of establishing battery capacity, is fully covered, as are impedance test methods, which often provide a faster and more convenient approach for regular routine testing. Techniques for locating ground faults rapidly and conveniently without the need to sectionalise the battery installation are also described. |
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| The guide concludes with a useful selection of frequently asked questions, and a brief overview of Megger products that have applications in the testing of battery installations. |
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| With its new Guide to Battery Testing, Megger has been successful in bringing together, in a single compact and clearly presented publication, all of the essential information needed to understand, test and maintain stationery battery systems. The Guide will, therefore, undoubtedly prove to be an indispensible aid for all of those who work with batteries. |
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Grounds for Fault |
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| Andrew Sagl |
| Product Marketing Engineer |
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| In many applications, ranging from circuit breaker tripping systems to uninterruptible power supplies for data and telecommunication installations, storage batteries are used as a standby power source. And, in most cases, the battery systems are designed to float – that is, to operate without an earth connection. A fault is usually indicated on a control panel and may create an alarm condition. |
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| A common problem however, is that a ground fault develops somewhere in the battery system, typically as a result of dirt and moisture. Such faults can be very difficult to locate, especially in large battery installations for example power stations and large substations. |
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| The most obvious approach to ground fault location is to disconnect the battery from its load, and to split it into sections so that the fault can be localised. While this approach undoubtedly works, it is inconvenient as the equipment powered by the battery will have to be taken out of service while the tests are performed. |
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| Several alternative testing techniques have been proposed in an attempt to solve these problems. One of these is to inject a lowfrequency test pulse, typically around 5 Hz, and use a signal tracker to determine how this propagates through the battery system, thereby locating the fault. |
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| This arrangement has the benefit that the tests can, at least in theory, be carried out while the battery remains in service, and that no removal of connecting straps is needed. However, the use of low-frequency pulses has a major drawback, as the pulses are very likely to upset the operation of protection relays associated with the battery, leading to spurious tripping. The designers of some test battery ground fault location test sets have sought to avoid this problem by using test signals of much higher frequency, in the 5 kHz range. This eliminates the adverse effects on protection relays, but it introduces another problem. |
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| At these frequencies, the capacitors that are often used in battery systems for surge suppression present a low impedance and, therefore, during testing they appear as “phantom” ground faults, making it much harder to localise the real ground faults. |
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| The most effective solution, and the one adopted in Megger’s BGFT battery ground fault tester, is to use a test signal frequency in the range 20 Hz to 30 Hz. This is high enough to prevent interaction with protective relays, but low enough to minimise the phantom ground fault problems associated with capacitors. |
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| The technique of locating ground faults with test sets operating at these frequencies is straightforward. The test equipment comprises a signal generator, which is mains powered, and a separate battery-powered probe, which is |
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| essentially a clamp meter that has been tuned so that it discriminates against signals at frequencies other than that of the test signal. |
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| The transmitter incorporates a resistance bridge; it shows the resistance of all the parallel circuits to ground from the point where it is clamped. A capacitance bridge is also provided as an additional aid to discrimination against phantom faults. |
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| To locate a fault, the signal generator is connected between earth and either the positive or the negative pole of the battery system, the correct choice being the pole that has the ground fault. The probe is then used to track the test signal from the source, always following the circuit branches that show the lowest resistance. Systematically applied, this procedure leads quickly and positively to the fault location. Note that tests can be carried out equally well on batteries that are in service and those that have been isolated. |
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| While this method of locating battery ground faults is simple in principle, it nevertheless makes substantial demands on the test equipment used. To ensure success, the equipment must, for example, be immune to distributed noise in the battery system and it must also be able to deliver consistent results even in the presence of large ripple currents. |
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| With good quality equipment, however, there is no doubt that testing at frequencies in the 20 Hz to 30 Hz range provides a fast, dependable and convenient solution to the perennial problem of locating ground faults in battery systems. |
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