The invention relates to the control of the lightning current injection to carbon laminates to avoid local overcurrents and the lightning current and voltage distribution between different conductive paths in lightning protection system for wind turbine blades.
Lightning protection systems for wind turbine blades usually comprise lightning receptor arrangements having an external metallic receptor element and an electrically conductive internal block connected to a down conductor of a grounding arrangement of the wind turbine. Once captured lightning currents by the receptor element must be transmitted to the electrically conductive internal block that connect the receptor element to the down conductor.
The evolution in the development of wind turbines towards increased power output had led to larger wind turbines both in tower height and rotor diameter.
As an increasing blade length involves and increased rigidity wind turbine blades incorporating carbon fiber laminates are needed. As carbon fiber laminates are conductive they must be connected in parallel with the down conductor to prevent the generation of internal arcs between the down conductor and the carbon fiber laminates and to avoid direct lightning impacts over them.
WO 2006/051147 A1 describes a lightning protection system comprising means for equipotentializing the carbon fiber laminates with the down conductor including auxiliary cables to connect directly the down conductor with the carbon fiber laminates. These auxiliary cables are connected using a joint screwed to a metal plate in direct contact with the layers of carbon fiber. The electrical connection can be improved by adding conductive resins to the joint area.
If the wind turbine blade has, for example, one carbon fiber laminate, the lightning protection system is converted into a two-branched circuit in parallel: one branch formed by the down conductor, of low resistance and high inductance, and the other branch formed by the carbon fiber laminate, having high resistance and low inductance. When lightning strikes on one receptor element, the lightning protection system must evacuate the current, whose waveform is characterized by having a first phase in which the current rises steeply, followed by a second phase where the current drops slowly. When this current is injected into the circuit formed by the carbon fiber laminate connected in parallel to the down conductor, the current is distributed as follows:
With the current distribution described above, the carbon fiber laminate undergoes a large current peak at the beginning of the discharge. On the other side as the size of the blades increases, the inductance of the carbon fiber laminates (wider and thicker) decreases causing that the fraction of the current conducted by the carbon fiber laminates increases raising a problem as the carbon fiber laminates contain resins that degenerate at temperatures between 100° C. and 200° C.).
To solve this problem, ES 2 396, 839 A1 discloses the use of a high-inductance device placed in the connection between a carbon fiber laminate and a down conductor to reduce the passage of current through the carbon fiber laminate and favour its conduction through the down conductor.
A problem of lightning protection system of wind turbine blades with carbon fiber laminates is that local lightning current injection to carbon laminates may not be properly distributed producing damage in the injection area. This is caused by the transient nature of the lightning strike together with differences in material conductivities which reduce the effective lightning current injection area of the connection.
Another problem is that the lightning current and voltage distribution between the down conductors and the carbon fiber laminates may not be balanced due to variations of the real values of the impedance of the carbon fiber laminates used in a wind turbine blade with respect to their expected values when the lightning protection system is designed.
This invention is directed to the solution of these problems.
The invention provides a lightning protection system for a wind turbine blade comprising at least a lightning receptor connected to one or more down conductor of a grounding arrangement of the wind turbine blade and one or two carbon fiber laminates. The down conductors are equipotentialized with the carbon fiber laminates by auxiliary cables which are connected to conductive plates embedded in the carbon fiber laminates at various points along the wind turbine blade. The lightning protection system further comprises at least one local lightning current injection area in the carbon fiber laminates associated to one auxiliary cable having one or more additional parallel conductive paths comprising secondary cables derived from said auxiliary cable and connected to conductive devices embedded in the carbon fiber laminates which are configured for avoiding overcurrents in the carbon fiber laminates.
The configurations of said additional parallel conductive paths include several means for controlling the current injected by each of them on the carbon fiber laminates such as resistors in the secondary cables, placing the conductive paths at a given distance between them, using secondary cables and/or conductive devices of different resistances or using conductive devices of different materials or geometries.
The lightning protection system of the invention further comprises means for achieving a balanced distribution of currents and voltages between the down conductors and the carbon fiber laminates.
In the case of a wind turbine blade with one carbon fiber laminate, if there is a negative difference exceeding a predetermined threshold between the impedance of the carbon fiber laminate and the reference value considered in the design of the lightning protection system, the lightning protection system further comprises one or more impedance devices in said auxiliary cables for achieving a balanced distribution of currents and voltages between the down conductors and the carbon fiber laminate.
In the case of a wind turbine blade with two carbon fiber laminates, if the difference between the impedances of the carbon fiber laminates exceeds a predetermined threshold, the lightning protection system further comprises one or more impedance devices in said auxiliary cables configured for achieving a balanced distribution of currents and voltages between the down conductors and the carbon fiber laminates.
Other features and advantages of the present invention will be understood from the following detailed description in relation with the enclosed figures.
Wind turbine blades with multiple conductive elements along its length can be found with inclusion of new technologies or materials as part of its structure. This is the case of wind turbine blades using carbon fiber laminates as part of its structure or electric and electronic equipment along the blade or at its tip. In all of these cases, there are multiple conductive paths to ground lightning current. The existence of multiple conductive paths implies that the different paths must transmit an amount of lightning currents during the event of a lightning strike to a lightning receptor.
In the case, for example, of a wind turbine blade 10 with a carbon fiber laminate 11 (see
To improve the known lightning protection systems of wind turbine blades with carbon fiber laminates the invention proposes in the first place a local lightning injection area 14 associated to one auxiliary cable 19 and close, typically, to one lightning receptor which is configured for avoiding overcurrents in the carbon fiber laminates. The auxiliary cable 19 is typically connected to a conductive plate 31 embedded in a carbon fiber laminate.
In one embodiment (see
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In the second place the invention proposes achieving a balanced lightning current and voltage distribution between the down conductors and the carbon fiber laminates in the wind turbine blade.
With the use of multiple conductive paths, a lightning current distribution is achieved which is a function of the materials and geometry of these. However, equipotential points do not permit the control of lightning current waveforms and voltages found between different elements at different blade radius. As shown in
To control and balance the lightning currents along the conductive paths of a wind turbine blade the invention proposes incorporating impedance devices 25 in one or more auxiliary cables 19 if the difference between the impedance of the carbon fiber laminate and the impedance considered in the design of the system exceeds a predetermined threshold.
The features of the impedance devices 25 are determined in function of the impedance of the carbon fiber laminate 11 measured after its manufacturing.
In the case of a wind turbine blade 10 with three conductive paths: an upper carbon fiber laminate 11, a lower carbon fiber laminate 13 and a down conductor 17 (see
The impedances of the three conductive paths are represented by, respectively, circuits with resistors and inductors of resistance and inductance values Rcap1, Lcap1; Rcap2, Lcap2; Rcond, Lcond, being Rcap1, Lcap1 lower than Rcap2, Lcap2.To balance the current distribution in the three conductive paths, an impedance device 25 having a resistor and an inductor of resistance and inductance values Rcontrol, Lcontrol is incorporated to the first path 11. The values of Rcontrol, Lcontrol thus depend on the differences between Rcap1, Lcap1 and Rcap2, Lcap2, being the resistance the main factor.
If, for example Rcap1=200 mΩ and Rcap2=400 mΩ) the distribution of currents between carbon fiber laminates 11, 13 would be (supposing a lineal distribution) ⅔ by carbon fiber laminate 11 and ⅓ by carbon fiber laminate 13. Then to achieve a balance current distribution the resistance Rcontrol of the impedance device 25 incorporated to the lightning protection system should be of 200 mΩ.
The lightning protection system of a wind turbine blade 10 may comprise the use of multiple impedance devices 25 located at different equipotential points as shown in
Although the present invention has been described in connection with various embodiments, it will be appreciated from the specification that various combinations of elements, variations or improvements therein may be made, and are within the scope of the invention as defined by the appended claims.
Number | Date | Country | Kind |
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201500449 | Jun 2015 | ES | national |