Feature Test & measurement Testing turbine protection systems
With wind turbine installations becoming more and more vulnerable to lightning strikes, the protection of their electronics and electrical systems is becoming ever more important. The protection systems, however, need to be properly tested and maintained. John McNab, assistant product manager at the Seaward Group, comments
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s successive governments press ahead with their green agendas to produce more and more of the UK’s power needs from renew- able energy sources, we are seeing a pro- liferation in the number of wind farms. The increasing number of turbine installations and their height (upwards of 60m), however, means they are becoming more vulnerable to lightning strikes. This has resulted in an incidence of damage greater than anticipated, with repair costs escalat- ing to the point where it’s almost economically unsustainable.
It is therefore becoming important for those responsible for turbine design, build and maintenance pro- grammes to incorporate control wiring systems capable of withstanding the most severe of lightning strikes – one which could potentially contain one billion volts and between 10,000 and 200,000 amperes of current.
Approaches to protection must fulfil two tasks: the provision of a low imped- ance path for lightning current to flow to the ground; and the protection of electronics and electrical equipment from damage induced voltage effects and transferred potentials.
The hazard to equipment tends to arise from two sources. The large magnetic fields produced by a lightning strike may result in large magnitudes of induced voltages in wiring. A rise in earth potential also results in the possibility of high potentials present at one wind turbine, causing damage to the electronics/electrical systems. Induced voltage effects can be avoided by sensible routing of wiring, the use of bonding wires and the provi- sion of cable trays bonded to the turbine at both ends. In fact, cable trays may result in induced voltage levels being reduced by a factor of ten.
Although transient suppression devices can be used for protection of components from large transferred potentials, it is important to ensure that such devices are located close to good earthing points. Wind farms are commonly situated in areas with high soil resistivities, so earthing systems are geographically distributed over a large area – 5km of earth electrode or more is not uncom- mon. Unlike compact earthing systems
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A digital
microhmmeter can be used to carry out low resistance measurement
conventionally used for lighting protec- tion purposes, the earthing system of a wind farm has a high inductive reactance which needs to be included when assessing its performance during a lightning strike. It is also important that equipotential bonding is provided within a wind turbine to ensure that no hazardous touch voltages will occur during a strike.
Using the right equipment Protection systems must be designed to carry lightning current to earth and protect the turbine from damage. While the overall protection of key assets like blades is important, so is the protection of other smaller, cheaper components. For example, electronic control systems must be protected from damage through induced voltage and large transferred potentials. The failure of the control system is one of the biggest contributors to wind turbines being offline or not functioning properly.
As protection systems are continu- ally improved, however, these need to be properly tested and maintained, and this includes earth bonding measurement and related safety test- ing applications.
For such applications, the latest gen- eration of digital microhmmeter can be used. The rugged Cropico DO7010 from Seaward, for example, ensures accurate and reliable low resistance measure- ment. This has a measuring range of 600µΩ to 60Ω with 0.1 and 10mΩ reso- lution respectively, and can measure with switchable current levels of 1A or 10A on all except the highest range.
As the instrument can be used with 90m long test leads without any adverse effect on accuracy, it is possible to test the safety bonding of wind turbines from top to bottom with one instrument. Furthermore, a test current capability of up to 10A allows for bonding checks to be made through any surface corrosion that can develop in outdoor locations. Wind turbines often incorporate an embedded Faraday cage of shield built into the tower structure – an enclosure formed by conducting material or by a mesh of such material. These provide protection against a lightning strike for service crew undertaking maintenance and repairs or to prevent damage to mechanical, electrical and electronic equipment. Here, the instrument can be used to check the cage is functioning correctly and afford the requisite levels of protection.
Full information on measurement configuration is displayed on the LCD panel, and the ability to trigger measurements by lead contact to the test device gives fast single-person operation. A remote hand terminal is also available to fully control the tester at a distance of up to 15m.
Other features include true four- terminal measurement to eliminate lead resistance errors, auto averaging of forward/reverse measurement and sequence testing with memory for 1000 readings. A data logging function enables downloading of test sequences from an XL spreadsheet and uploading of test results to the spreadsheet.
Cropico
www.cropico.co.uk
Enter 698 FEBRUARY 2011 Instrumentation
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