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Electrical Tester Oct 2009 from Megger Print

Published by Megger
October 2009
The industry's recognised information tool
ELECTRICAL
TESTER
In this issue
CAPITALISE
Testing multi-ratio CTs
Nurturing expertise in the Gulf
Magnetic losses and efficiency in magnetic materials
CAPITALISE
Joe Tremblay
Product Manager
In order to succeed, every business needs to extract maximum value from its costly fi xed assets, and there’s no shortage of asset management software available to help. However useful and powerful this software may be, it is unlikely to fully address the requirements associated with major items of electrical plant.
There are very good reasons, therefore, for augmenting standard asset management systems with specialist software that has been specifically designed with electrical assets in mind. Before discussing the requirements for software of this type, and the benefits it can be expected to provide, it is useful to recap some of the key objectives of asset management in general.
These can be summarised as maximising productivity, performance and service levels while minimising costs; ensuring that safety and regulatory requirements are fully met; and satisfying the universal demand for reliability. It can be seen at once that these objectives go right to the core of business operations, irrespective of the type of organisation involved.
As a result, failure to manage assets properly can have devastating effects, and this is certainly true in the case of electrical plant.
A failed power transformer in a utility’s transmission network can, for example, leave hundreds of consumers without a supply, and a similar problem in an industrial installation can shut down a whole manufacturing site.
And there’s no quick fi x for problems of this type – if a replacement transformer is needed, a delivery time of several months can be expected, and the situation is not much better for other major items of electrical plant. Clearly, there is a need to manage assets in such a way that the risk of such failures is kept to an absolute minimum.
The key to success in this endeavour is to monitor the condition of the equipment so that impending problems can be detected and tackled before they develop into full-blown failures. In successfully achieving this for electrical plant, there are two issues to be tackled.
The first is that it is not always easy to determine the condition of electrical equipment – visual inspection, for example, usually reveals few if any clues about possible problems. The monitoring regime must, therefore, be based on testing, but this gives rise to the second issue: how can the data which testing yields be turned into a useful asset management tool?
The answer may seem to be readily apparent – simply store and process the data using the software provided by the supplier of the instruments used to carry out the tests. That’s a good answer as far as it goes, but in reality it’s only a starting point. Even if there is only one item of equipment to test – say a transformer – to fully evaluate its condition, several different tests are likely to be needed, each using a different instrument.
This is where things begin to become complicated, because each instrument is likely to have its own software for processing data and each is, therefore, likely to produce results in its own particular format. Collating these disparate results and transferring them effectively to an enterprise asset management system is almost certainly to prove a nightmarishly complicated task.
What’s needed is a software package that offers compatibility with a wide range of different types of test set and that ideally, is not tied to test equipment from a single supplier.
Such a package might be expected not only to be configurable to produce reports in almost any format that is convenient for the user, but also to be pre-confi gured to produce reports in industry-standard formats for equipment such as batteries, cables,
For in-situ testing of wind turbines, the low resistance test set is typically located near the tip of the blade with a long test lead running down the side of the tower to connect it the earth conductor in the tower base. The duplex probe is then used to make connection to the turbine blade lightning receptors.
Testing multi-ratio CTs
David Milner
Product Manager
Because of the crucial role that current transformers (CTs) play in metering and protection schemes, CT testing is a routine task for every power engineer. It is also a task that can be very time consuming, particularly when multi-ratio CTs are involved.
It’s not hard to see why. Testing CTs in line with IEC or IEEE requirements involves measuring several key characteristics, including ratio, knee point, saturation and polarity. In addition, the CT will typically have to be demagnetised as part of the test procedure. Carrying out all of the tests that are needed on a single-ratio CT takes a significant amount of time and, for multi-ratio CTs, the situation is even worse as the test leads have to be repositioned and the tests repeated for every ratio.
That’s the position with traditional CT test sets. The latest generation of automated test sets, however, make life much easier for power engineers. With these new test sets, the full test sequence, including all of the prescribed tests, is initiated by a single button press. Even better, the best of these test sets can be directly connected to multi-ratio CTs and will automatically carry out the tests on all taps without the need to change the test connections.
The CTs can be tested in their normal equipment configuration – that is, while mounted in transformers, oil circuit breakers or other switchgear – which saves even more time.
A good example of a modern CT test set that offers all of these facilities and more is the new MCT1605 from Megger. Based on the proven technology used in the MCT1600 range of instruments, this new test set performs a CT saturation test and calculates the rated knee point at the touch of a button. Tests can be performed at 50 Hz or 60 Hz, and the knee point calculations can be carried in line with IEEE C57.13.1, IEC 60044-1 or IEC 60044-6.
While the saturation test is being performed, the tester plots the CT saturation curve on a large graphical display. To cater for CTs with multi-ratio secondaries, up to 10 saturation curves can be plotted and displayed simultaneously.
To accurately determine the ratio of the CT, the test set injects a voltage into the secondary, and measures resulting voltage in the primary. Normally, this test is performed automatically at the same time as the saturation test, but provision is also made for it to be carried out manually.
The polarity of the CT is displayed as a simple “correct” or “incorrect” indication, accompanied by the measured phase angle.
The MCT1605 also offers the option of automatically demagnetising the CT under test. Demagnetisation prior to saturation testing helps to ensure that accurate results are obtained, and this procedure is recommended in ANSI C57.13.1.
As a further aid to fast, convenient and comprehensive testing of CTs, the MCT1605 incorporates a 500/1000 V insulation resistance test system, which ensures that the CT secondary winding and wiring are properly insulated as required by ANSI C57.13.1. The test set automatically switches the test connections to perform all of the required insulation tests, including H-L, H-G, and L-G. For maximum user convenience, the MCT1605 catalogues and stores all of the test results for later retrieval. Configurable test plans can be associated with individual CTs and stored along with the results, which greatly simplifies ongoing monitoring and profiling.
The instrument stores results in special xml schema so that PowerDB can read it. A particularly versatile option, however, is to transfer the data to Megger’s PowerDB Lite software package, which has powerful facilities for generating reports in industry-standard fomats. PowerDB Lite can also be used to control the MCT1605 without the need for operator intervention, thereby allowing it to be used as a fully automated computer-controlled CT test system.
Today’s ever-increasing commercial pressures mean that power engineers now have less time available than ever before for carrying out routine yet essential tasks such as testing CTs. Fortunately, as we’ve seen, the latest CT test sets deliver big time savings complemented by greatly enhanced convenience, thereby providing at least a partial solution to this enduring problem.
Nurturing expertise in the Gulf
Dennis Neitzel CPE
Director of AVO Training Institute, Inc.
The Gulf is one of the world’s fastest developing economic regions, and has an insatiable need for expertise. This need is particularly acute in the electrical sector, where the rapid growth of the electricity supply infrastructure and the dynamism of the construction industry mean that skilled workers are always in short supply.
For many years, AVO Training Institute Inc, a Megger subsidiary with its headquarters in Dallas, TX, has helped to nurture expertise in the Gulf by providing electrical maintenance, testing, engineering and safety training for electrical technicians and engineers at customers’ sites.
In the summer of 2009, however, the AVO Training Institute set up a new dedicated training centre in Bahrain, which will allow it to provide even more support for the development of this dynamic region. As would be expected from an organization with an enviable record of leadership in the provision of electrical training, the institute has spared neither expense nor effort in ensuring that the new centre operates to the highest standards.
Dr Sadeq Yusuf Alabdulwahab, heading the operations in Bahrain holds a doctorate in organizational leadership from the University of Phoenix, and a Master’s Degree in Engineering Management from the University of Bahrain. A member of the Bahrain Society of Engineers, he also has a BSc in Electrical Engineering from the University of Bahrain, and he is a Cisco Certified Network Associate.
Working in conjunction with his colleagues at the AVO Institute in Dallas, Dr Alabdulwahab has put in place a comprehensive programme of courses for the new centre.
Initially, the courses available will include basic electricity, arc flash compliance, electrical safety for industrial facilities, electrical safety for utilities, protective device coordination, short-circuit analysis and electrical print reading.
In the near future, this range of courses will be expanded to cover further important topics such as substation maintenance, power transformer testing, circuit breaker maintenance and cable testing.
The courses have been individually developed on the basis of expert analysis, resulting in well defined objectives that are relevant to current industry needs. Effective language, illustrations, supplementary materials and logical presentation sequence are integral parts of every course, which ensures that students leave with the skills and tools they need to do the job.
For further information please contact AVO Institute on 00973 17 720 286 or visit the website www.avotraining.com/me.
Magnetic losses and efficiency in magnetic materials
Dr Stan Zurek
Magnetics Technical Specialist
Where it all started...
Before we delve deep into our topic let us take a step back in time. As is well known, in the 19th century great scientists like Faraday and Maxwell developed the electromagnetic theory. Around the same time, the first electric motors and transformers were invented, but there was a need for them to be made more powerful, more efficient, smaller and lighter. This required a deeper understanding of magnetic phenomena and studies were undertaken to solve the engineering problems concerned.
As early as 1851, Foucault discovered that “eddy currents” are generated in any conductor exposed to a varying magnetic field. The next advancement came from Steinmetz in 1891. He described “hysteresis law” relating the level of magnetisation to hysteresis loss under quasi-static conditions, where the magnetisation is varied very slowly in order not to induce the eddy currents, for example at the rate of a cycle per minute rather than a few milliseconds.
First mathematical model
It soon became apparent that “magnetic loss” is actually a manifestation of more than one phenomenon and can be divided into sub-components. One of the first mathematical models of losses combined the losses resulting from the Foucault currents (Ee) and those resulting from the Steinmetz hysteresis law (Eh) into one equation: Etotal = Eh + Ee. This model worked quite well for most magnetic materials, which at the time were mainly mild steel.
Practical implications
It can be calculated that the power loss resulting from the eddy currents is directly proportional to the square of frequency and square of magnetisation and inversely proportional to the thickness of the material. For this reason, the copper and aluminium bars in medium voltage substations working at power frequencies are usually designed to be no thicker than 10-12 mm, as otherwise they would be more lossy due to the magnetic field generated by neighbouring conductors carrying high currents.
Also to reduce eddy current losses, the magnetic cores of transformers, motors and generators are invariably made out of thin laminations (Fig. 1). Unfortunately, thinner sheets are more expensive to manufacture, so a compromise has to be made between the cost and efficiency of the device. However, in the USA iron-based amorphous metallic ribbon (e.g. Metglas) is widely used in transformer cores. This provides significantly better efficiency – the magnetic losses can be reduced by up to 70%. One of the decisive factors is the fact that the effective thickness of Metglas is around one-tenth that of conventional electrical steel – around 0.02 mm compared with 0.27 mm.
Fig.1. Cross sections made up of thinner elements reduce eddy current losses
Turning now to the Steinmetz component of losses, this is linearly proportional to frequency and roughly to the square of magnetisation (the actual exponent can vary between 1.6 and 2 depending on material).
Yet another component
In 1950s, a new type of soft magnetic material was developed – grain oriented electrical steel. It quickly became apparent that the previous model was not working well for these materials, and also later for amorphous and nano-crystalline materials. The scientists quickly found a solution to this problem; the equation was modified to
Etotal = Eh + Ee + Ea, where the Ea stands for “anomalous” losses. The model was known to work reasonably well before, so if the material was not behaving as expected clearly the material must be anomalous rather than there being a problem with the equation!
It was empirically found that the “anomalous” component is proportional to frequency raised to the power of 1.5. Further studies confirmed that it can be linked to the complex magnetic domain structure and other phenomena like skin depth, and so on.
Today, the widely adopted model of magnetic losses is still used: Etotal = Eh + Ee + Ea = Ch + Ce.f + Ca.f 1/2 (where C denotes constant).
Nowadays, the “anomalous” component (Ea) is termed as “excess” loss as the mechanism behind it is quite well understood, so it is no longer “anomalous”. However, as can be seen from Fig. 2 it can be rather significant, and in modern materials like amorphous or nano-crystalline cores it can be THE major component of losses, whereas the hysteresis loss (Eh) is almost negligible for higher frequencies.
Fig. 2. Energy loss separation for electrical steel
Points worth remembering
It can be calculated that the power loss resulting from the eddy currents is directly proportional to the square of frequency and square of magnetisation and inversely proportional to the thickness of the material. For this reason, the copper and aluminium bars in medium voltage substations working at power frequencies are usually designed to be no thicker than 10-12 mm, as otherwise they would be more lossy due to the magnetic field generated by neighbouring conductors carrying high currents.
Fig. 3. Total power losses rise with frequency (Hz) and flux density (T)
However, magnetic saturation and cooling limits are implacable regardless of the price of the device. And for that reason we will not see power transformers being shrunk to the size of a matchbox any time soon!
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