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Published by Megger
September 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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Turbine protection |
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Investing in North East USA |
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When worms find fault! |
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Turbine protection |
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| Paul Swinerd |
| Product Manager |
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| In the early 1990s, the largest wind turbines had a generating capacity of around 150 kW. The corresponding figure today is 6 MW. This increase in generating capacity has been paralleled by an increase in the physical size of the turbines – where 60 m high was once the norm, heights in excess of 160 m are now common. |
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| This has resulted in a much higher risk of lightning strikes and in many cases, that risk is further increased by the locations that are now favoured for the siting of wind turbines. These include open land, coastal regions, mountain ridges and offshore, all of which mean that the turbines are likely to be the highest structure in the area. |
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| Since lightning strikes have the potential to cause severe damage, leading to a need for costly repairs as well as losses in revenue resulting from the downtime until those repairs can be made, manufacturers of wind turbines go to great lengths to design lightning protection into their products. By and large, their efforts have been very successful, but there is nevertheless a need for the lightning protection measures to be tested to verify that they will perform correctly |
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| Lightning protection measures for wind turbines are detailed in IEC 61400-24 Wind Turbine Generator Systems, Part 24 Lightning Protection Systems. The most common test for wind turbine lightning protection however is the use of low resistance measurement to confirm the continuity of the conductors that will carry lightning currents. The tests are typically carried out during manufacture, as part of the commissioning process, following blade tip repairs and during maintenance. |
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| Long test leads |
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| Because it is invariably necessary to use very long test leads in this application, the four-terminal method of measurement is used, as this automatically eliminates the effect on the results of the comparatively high test lead resistance. Typical resistance values expected from the blade tip to the earth connection at the base of the tower are in the range 20 to 50 milliohms and in the majority of cases, the value is less than 25 milliohms. |
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| Irrespective of the type of instrument used, those involved with the testing of wind turbines always face one particular problem – finding test leads of sufficient length. Commonly 30 m and 50 m leads are needed for testing during turbine manufacture, and for on-site use, 100 m leads are normal. Users can, of course, make their own test leads but this is both time consuming and inconvenient. |
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| To satisfy the requirements of leading wind turbine manufacturers and others for a more convenient solution, special test lead sets, |
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| such as those in the Megger KC range, are now appearing on the market. These comprise a 100 m, 50 m or 30 m lead mounted on a high quality cable reel and terminated with a large robust Kelvin clip. |
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| The cable reel should ideally be provided with a friction brake to avoid tangles when paying out the cable, and each lead set should include a 5 m cable fitted with a duplex handspike for probing the lightning receptors on the tips of the turbine blades. |
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| 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. |
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| 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. |
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Investing in North East USA |
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| Greg Wolfe |
| VP and General Manager, Megger Valley Forge, USA |
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| When a recession strikes, the instinctive response for businesses is to slash expenditure to the bone, and that invariably means putting all investment plans on hold. For some businesses that are in poor financial health, such a response may be unavoidable, but it’s perfectly possible that companies in this category would have struggled to survive even without a recession. |
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| But what about the majority of companies that are in a stronger financial position? Are they right to stop investing? Anyone who believes the answer to be yes is ignoring one very important fact – the recession will ultimately come to an end. The timing may be uncertain, but end it most certainly will. |
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| Now if all companies adopted the same strategy, when the economy improves there would still be a level playing field. But that’s not how it works – some enlightened organisations, while taking care to eliminate unnecessary expenditure, will have maintained their commercial momentum by continuing to invest throughout the recession. When it ends, therefore, they will be in a much stronger position than their competitors to profit from the upswing. |
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| For this compelling reason, continuing investment is the strategy that has been adopted by all parts of the Megger organisation across the globe. In the UK and Sweden, major capital projects are ongoing, and now it’s the turn of the USA to demonstrate how a dynamic, confident and forward-thinking business deals with a recession and plans for the future. |
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| At the company’s major manufacturing site in Valley Forge, PA, a new and highly automated surface mount line has just been commissioned. This allows printed circuit boards to be manufactured in-house, reducing costs and lead times, and allowing even better control over product quality. Note that all of these factors enhance the company’s competitive position during the recession and, possibly even more important, after it. |
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| At the same site, the opportunity has been taken to lease an additional 400 sq. m of building space, which has allowed six new engineering laboratories – three for HV equipment and three for LV – to be set up. These will facilitate the further development of new and existing products, once again paving the way for enhanced success in the future. To cope with the anticipated increase in demand, the production area has also been increased by 20%. |
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| The laboratories are complemented by a newly installed machine shop that provides in-house metal fabrication facilities. As a result product prototypes can be produced faster than ever and at reduced cost. That means a faster response to customer requirements as they develop and change. |
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| Of course as well as making investments in house, a company with its eye on future growth needs to make sure that its customers can see the benefits that this |
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| expenditure is bringing to them. For this reason, the company has created a new product showroom, which is co-located with the North-East USA AVO Training facility. |
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| This facility is also a recent addition, and provides existing and potential customers with the opportunity to do a little investing of their own to enhance the expertise of their employees. Internal restructuring to bring together teams of co-workers on related products, and a rather large investment in the back-room computer systems that underpin the business were also necessary. |
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| This story is not intended as a catalogue of what Megger has done, but as an example of how a business with a genuine enthusiasm for the future can, even in these difficult times, lay sound foundations for future success and profitability. Without doubt, it’s time for businesses of all sizes to shake off that recessionary torpor, and to remember the old adage that fortune favours the brave! |
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When worms find fault! |
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| Allen Joyce |
| Training and Technical Manager |
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| Unlikely as it may seem, worms can be a big help in accurately pinpointing the location of faults in buried cables. It has been reported that two groups of worm – annelids and aschelminths – are the most useful, but for those cable test technicians who are possibly a little lacking in worm expertise, it will probably be a relief to know any healthy ground worms will do. |
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| The basis for cable fault locating with worms is the feeding habits of birds, many of which have a strong liking for eating fresh healthy juicy worms. When they are hungry, these birds stamp on the ground. In response, the worms come to the surface where, of course, the birds eat them. |
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| Cable fault location using worms has many similarities except the engineer performing the test is not actually required to eat the worms unless, of course, they really want to! This is how the technique works. |
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| Once the fault has been pre-located using conventional high or low voltage techniques, its precise location needs to be pinpointed so that it can be excavated, repaired and the cable put back into service. Most often, faults are pinpointed acoustically. This method involves using a high voltage pulse from a surge generator (sometimes called a thumper) to produce a fl ashover at the site of the fault. The fl ashover generates noise, which is detected using a ground microphone and an acoustic receiver. This method allows the location of the fault to be pinpointed very accurately. |
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| So where do the worms come in? |
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| The answer is that they can take the place of the ground microphone and acoustic receiver! |
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| With the worm technique, a surge generator is still used to create the fl ashover but, instead of listening for the noise this creates, the test engineer looks for worms coming out of the ground. The worms respond to the noise from the fl ashover just as they would if a bird was tapping the ground – in other words, they rise to the surface. |
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| This method is almost as accurate as the conventional technique. It also has the advantage of working best in ground that’s wet or boggy, conditions that can often hamper the pinpoint location of faults using standard acoustic methods. |
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| Best of all, if you use this technique, you don’t even have to bring your own worms – they’re already there, in the ground, patiently waiting to help you to locate that elusive fault! |
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