DK180378B1 - System to measure the relative length of a conductive wire in wind turbine blades - Google Patents

Abstract

The invention is a system to be mounted in a wind turbine to monitor that the lightning down conductor within a wind turbine blade is fully connected throughout the blade. This is achieved via use of a spark gap, by measuring the charge flow through each blade down conductor of a wind turbine, and comparing measurements across all blades of the turbine. The measurement principle make use of naturally occurring electrical charge and voltage potential differences of the atmosphere surrounding the wind turbine blades during normal operation. If a blade has comparatively less charge flow during normal operation than other blades, an alarm is generated, indicating a defective down conductor.

Description

DK 180378 B1 Introduction Wind turbine blades are very exposed to lightning strikes due to their height compared to the immediate surroundings. In order to control the path of a lightning strike discharge most blades make use of conductive lightning receptors placed on the surface of the blade. The receptors are connected to a lightning down conductor inside the blade, which in turn connects to the nacelle and from there the tower down to ground. Mechanical or other environmental stresses may lead to defects in the down conductor inside the blade, resulting in the down conductor cable becoming detached between one or more receptors along the length of the wind turbine blade. This results in loss of a controlled lightning path, and a severely increased risk of blade damage during a lightning strike event. A means for detecting if a lightning down conductor has been disconnected at any point towards the final receptor near the tip of the blade, is needed. Ideally, such a solution should support online monitoring, i.e. the measurement apparatus used should be fixed to the individual blade.

Areas of use The invention is a system to be mounted in a wind turbine to supervise the state of the lightning down conductor within each wind turbine blade.

Prior Art Several methods exist to measure if a conductive wire is broken somewhere along its intended path. A simple resistive measurement as well as high frequency (pulse) wave instruments are typical choices, where the first require a conductive loop to be established (see later comments to CN 105545615 A), and the latter require complex equipment which carries a comparatively high cost seen with respect to the present invention. Measurement of the charge of the air, with the purpose of detecting and measuring lightning discharges is e.g. described in the U.S. Patent to Murtha, Jr., et al., issued Nov. 2, 1999, (U.S. Pat. No. 6,061,216) for a Lightning Detection Apparatus and Methodology which makes use of an antenna and amplifier configuration for receiving electrostatic discharges of lightning and electrical noise to output an analog electrical signal representative of the discharge. This analog signal is applied to a threshold detection circuit whose output is activated when the input signal rises above a predetermined level. Herein a computer or similar device examines the output to determine an open event window. It is the pulses of the event window that are given signatures which are compared. If Page 2 of 7

DK 180378 B1 the pulse signature correlates with that of lightning, then an alert signal is generated. Measurement of the charge from a lightning running through a down-conducting system is described in 'Method for detection of charge originating from lightning (US patent no.

US9450392B2)'. The invention relates to a detection apparatus of a wind turbine wherein said wind turbine comprises a down-conducting system arranged to conduct current induced from lightning, wherein said detection apparatus comprises a charge measurement apparatus for establishment of a charge representation, wherein said charge representation represents charge induced into said down- conducting system by lightning, and wherein said detection apparatus further comprises an estimator for estimating deterioration induced by lightning of one or more components in a wind turbine on the basis of said charge representation. An example of electric field measurement in the air is described in Korean patent number KR101813851B1. The patent describes a non-rotating type electric field sensor based on electronic circuits. The most concrete example of a patent seeking to achieve the same goal as the present, can be found in CN 105545615 A, which through use of a conductive loop enables a direct ohmic measurement at the root of a wind turbine blade. The patent addresses one of the major concerns when creating such a conductive loop. Since it will be subjected to extreme electrical currents during a lightning strike, induced voltages will create a significant risk of arching between the wires of the loop. CN 105545615 A describe how to make use of spark gaps distributed down through the blade between the wires of the conductive loop, to control said arching. As such, the method and application mentioned in patent CN 105545615 A is fundamentally different to that of the present patent. This can also be exemplified in the differences in properties and requirements between the two; e (CN 105545615 A is primarily an option during turbine blade production, while a retrofit solution would generally be considered impractical. The present invention is easily applicable in both scenarios, as there are no requirements to wiring changes or additions in the blade itself.

e A safe application of CN 105545615 A would require multiple spark gaps to be used throughout the blade structure, whereas the present use a single spark gap at the root of the blade.

e The present invention relies on continuous monitoring via a measuring unit, and subsequent Page 3 of 7

DK 180378 B1 processing in a logic/determination unit, as it relies on atmospheric phenomena, and the rotation of the wind turbine blades during normal operation.

The above references, while relevant in terms of measuring charge or electric fields in relation to air or the actual down conductor in a wind turbine, does not present ideas or tools with which a person skilled in the relevant arts would be able to combine to the present invention, where the measurements does not focus on lightning discharge events, and where measurement of the voltage potential is targeting measurement of the differences between conducting wires, and not a measurement of the voltage potential in free air, or conductivity of a wire loop.

The technical problem to be solved Due to mechanical stress and poor connections, internal down conductors in turbine blades are sometimes broken. It is hard to identify those broken down conductors, leading to a need for a practical detection method. Detecting a broken down conductor via resistive measurement requires establishment of a conductive loop on a turbine blade. It is practically expensive, in particular for retrofit solutions, as additional wires needs to be attached to the blades for the measurement, likely on the outside of the blade. Application of a fixed return conductor will additionally present a significant risk of electrical flashover/arching due to the induced voltages during a lightning strike event, which may both lead to increased damage risk of the turbine blade, and also a high risk of the return conductor itself breaking as a result of a lightning discharge. Adding a return conductor is therefore not a preferable solution. High frequency (pulse) wave measurement systems do not need a return conductor, however such systems are complex, and therefore costly.

An online monitoring solution is preferred by wind turbine operators to be able to make focused preventive maintenance and repair. This drives the need for a low cost, fairly simple fixed method for determining if a down conductor is broken.

The new technology A system for measuring if a down conductor (fig. ref. 1), ensuring interconnection of lightning receptors (fig. ref. 2) is broken, may be based on a relative measurement (fig. ref. 3) of the conductor length between the three blades of a typical wind turbine design. This method assumes that in a failed state, at least one of the down conductors presents a different galvanically connected Page 4 of 7

DK 180378 B1 length compared to the others, and also that the turbine is in normal operation (i.e. turbine blades are rotating). A relative measure of the connected length of a down conductor can be obtained by removing the galvanic connection at the connection point at the root of the blade (fig. ref. 4) towards the blade tip (fig. ref. 2), and inserting a spark gap over which measurement wires (fig. ref.

5)interface to measurement electronics (figure 3) required for the present invention. Said electronics consisting of the input from the spark gap (fig. ref. 5), a protection circuit (fig. ref. 6), further signal conditioning (fig. ref. 7), and the conditioned output (fig. ref. 8).

Said electronics will measure the charge flow resulting from the voltage potential difference between the down conductor of the blade and the nacelle, and thereby a measure for the potential difference itself. The nacelle is assumed well connected to the ground voltage potential. Comparing the voltage potential measurement across a full revolution of the blades will reveal if one or more blades are characterized by a low voltage potential difference. If that is the case, the down conductor is damaged, and a need for repair activities can be indicated.

As the measurement methodology relies on a certain level of electric field being present in the air surrounding the wind turbine, as well as the turbine to be rotating, it is not possible to guarantee instant readout of the health state of the down conductors. However, in most practical cases a turbine will fulfil both measurement criteria on a daily or at least weekly basis. Detection logic (figure 4) is therefore required to both categorize at which times the relative measures for the turbine blades are valid, and for valid periods then detect if the relative measurements appear statistically different across the down conductor of each wind turbine blade. The detection logic receives input from the measurement electronics attached to each of the blades (fig. ref. B1, B2, B3), is processed in the unit itself (fig. ref. L), with direct alarm output for each blade (fig. ref. A1, A2,A3), and/or using a more advanced communications readout (fig. ref. C). When a lightning strike hits the receptors and down conductor, the conduction path needs to be established via the spark gap. This can be achieved, using a robust and well specified spark gap (e.g. 2kV arching threshold), and designing the measurement electronics to be able to withstand such voltage potential levels, including a reasonable safety margin. This will enable full functionality of the intended purpose of the receptors and down conductor, while leaving the measurement electronics of the present invention capable of surviving lightning strikes, and otherwise continue detecting if a down conductor is broken. Page 5 of 7

DK 180378 B1 The technical effect Relative measurement of the charge flow through a turbine blade, when oriented towards the sky, can be used to determine if a down conductor is broken in the blade. The electrical field between the sky and the earth potential varies significantly over time, in particular in connection with thunderstorms. Often thunderstorms pass by without any direct lightning strikes, but the increase in the electric potential is still clearly detectable. This voltage potential can be measured using existing techniques, such as KR101813851B1 described in the chapter 'Prior art. If such potential differs between the blades on a turbine in normal operation (when the blades are revolving), it is a strong indicator for a damaged down conductor system.

Construction example To be able to measure the charge building up in the blade, it is required to have the blade nearly isolated from the ground potential, i.e. with an impedance presenting relatively high resistivity (in the MOhm range). Hereby it is practically possible to measure the voltage potential difference between the blade compared to the ground potential.

When separating the blade from a strong ground connection, it is important to have a controlled path for the strike current. This is obtained using a spark gap.

The measurement circuit electronics must ensure that the potential difference does not exceed the spark gap arching voltage during normal operation. This will also ensure that the receptors continue to present a voltage potential capable of attracting lightning discharges.

Placement of a spark gap and measurement electronics for a single blade is illustrated in figure 1.

The down-conductor attachment is illustrated in figure 2. The signal conditioning circuit for one blade is illustrated in figure 3. The logic unit that analyse the conditioned signals and produce an evaluation result, is illustrated in figure 4.

The complete measurement system consists of a) the down-conductor attachment, b) the blade signal conditioning, c) a logic unit calculating the condition of the blades. Description of figures Figure 1 illustrate a blade having receptors (1) at locations on the surface of the blade placed from Page 6 of 7

DK 180378 B1 the tip and down the length of the blade towards the root. The receptors are interconnected in series by a down-conductor (2). At the root end of the blade, the down-conductor is attached to the hub for further connection towards the tower and down to earth (or sea). The measurement system per blade is positioned at the root end of the blade as illustrated by (3).

Figure 2 illustrate the spark gap positioned in series with the down conductor. The spark gap ensure a conduction path for the strike and as an isolation for measurements during normal operation. Figure 3 illustrate the measurement unit in the blade which ensures that the receptors remain close to earth voltage potential and which measures the charge condition of the blade conductor system. Measurement wires enter from the left (see figure 2), while the conditioned signal which can be processed by the logic unit exits on the right (see figure 4). Figure 4 illustrate the logic unit, which is connected to all 3 blades of a turbine. The unit compares the charge condition of the blades to each other and to other blade specific data and generates an alarm if a service visit is needed to ensure proper connection of the down conductor system. The alarm may e.g. be optical, relay driven or via a digital communication line. The communication line may be wired or wireless.

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Claims (3)

DK 180378 B1 Patent claim A measuring system capable of detecting the relative length of a conductive wire (Figure 1) located within a plurality of wind turbine blades, the system comprising: - a number of receptors interconnected in series with a conductive wire, in each single of the set of blades, where the receptors are configured to receive lightning strikes, - a spark gap connected at one end to the conductive wire and the other end of the spark gap connected to the ground connection to each of the set of blades (Figure 2), characterized by a measuring unit configured to measure the voltage potential in the ambient atmosphere, between each of the conductive wires in the amount of wind turbine blades to the ground connection (Figure 3), and a determining unit configured to determine whether a conductive wire in a blade is damaged, by comparing the measured voltage potential with the conductive wires among the set of wind turbine blades (Figure 4). A measuring system according to claim 1, wherein the potential difference is measured either continuously or at discrete times via electronics which enables measurement of the full build-up of voltage potential since the previous discrete measurement. A measurement system according to claim 2, wherein the evaluation of the potential difference is based on statistical analysis of a time series of recorded data. Page 1 of 1