Lightning Monitoring System - Acquire and evaluate lightning currents. Dipl.-Wirt.-Ing. Achim Zirkel, Trabtech Power and Signal Quality, Phoenix Contact GmbH & Co. KG, Blomberg in The Windfair Newsletter

The availability of wind power plants plays an important role since unscheduled plant downtimes cause high costs. Lightning strikes are often the reason for plant failure. Electrical and electronic components should therefore be protected with an effective surge protection concept.

However, the external structures, particularly the rotor blades, continue to be subject to the high energy of lightning strikes. In order to evaluate the current state of the plant, information on the actual load from lightning strikes is required. The more detailed this information is, the more exact the evaluation of the state of the plant and optimized control of maintenance costs can take place.

Number of lightning strike events is not decisive

Currently, two methods for evaluating lightning strikes are widespread. One of them is the quantitative evaluation by means of event counters. This counting unit records the number of lightning strikes. In the case of this device class, the qualitative evaluation is generally restricted to a statement on the response range of the counter.
The second widespread measuring method is a system consisting of a magnetic card and a separate evaluation unit. In this case, the strongest interference is evaluated via a card reader. A statement on the number of lightning strike events is not possible using this method.
Both methods are not adequate for a detailed analysis of the load caused by lightning currents.

Detailed evaluation of lightning currents

The polarimetric LM-S lightning monitoring system from Phoenix Contact acquires each lightning current that flows through an arrester and evaluates it on the basis of lightning current-typical characteristics. The measuring system is based in the Faraday Effect and is therefore a pure optical measuring principle.
The system was specially developed for wind power (image 2) consists of an evaluation unit and up to three optical sensors mounted directly on the lightning current-conducting arresters and is extremely robust as far as environmental conditions are concerned.

Interference of a light signal is measured continually and evaluated. In the case of a lightning strike event, the figures are calculated from these values that characterize the lightning current.
The lightning current amplitude, the maximum current slope as well as the load and the specific energy of the lightning current is then stored in the evaluation unit together with date and time.

Availability of the data

In order to prevent the need for local reading and evaluating of offshore plants, it is necessary to have remote access to the measured data. The evaluation unit of the measuring system is equipped with both a relay contact for informing on the lightning strike event and an RJ45 interface for enabling remote access to the data. This allows the use of either an existing network structure or a standalone solution with separately installed modem.

Holistic consideration of the lightning current

Consideration of the load in wind power plants from lightning currents is mainly restricted to the number of currents and possibly their maximum amplitude. However, the charge and the rise time of the lightning current are the essential indicators for possible damage. The damage to be expected from a sustained lightning strike with low amplitude (long duration strike) can be just as large as the damage from a lightning strike with much higher amplitude but of considerably shorter duration. Frequent lightning strikes with lower amplitude also represent a high load on mechanical structures. If this is to be taken into account, the cumulative load must be considered.

Substantiated evaluation increases efficiency

A detailed lightning current analysis allows exact assessment of the plant state. Using the available information, it is possible to make a substantiated assessment on whether and when maintenance is necessary. In the event of serious damage, it is possible to initiate a controlled shutdown of the plant to prevent any further damage. Local visual checking of the plant or reading the measured values is therefore not necessary. The costs for traveling and inspection are reduced; causes for follow-up damage are detected early and can be eliminated.