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September 2009
Turbine protection

Turbine protection

30 September 2009

Paul Swinerd - Product manager

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.

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.

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

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.

Long test leads

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.

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.

To satisfy the requirements of leading wind turbine manufacturers and others for a more convenient solution, special test lead sets, 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.

DLRO10HD_KC100_I7_P03.gifThe 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.

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.

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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