Patent Description:
Wind turbines may be required to operate in geographical areas which can experience very cold climatic conditions for at least parts of the year. Although the system components of a wind turbine are designed to operate in a variety of adverse climatic conditions, it is a well-known issue that icing of wind turbine blades can have a detrimental impact on the power generation efficiency of wind turbines. Ice accretion on the wind turbine blades increases blade mass but it also changes the aerodynamic profile of the blades therefore making them less efficient aerofoils. This results in the rotor producing less torque from a given wind speed so the power generation potential is reduced.

Various approaches have been developed to remove ice from wind turbine blades. For example, shaking devices can be installed on blades in an effort to impart violent oscillations to the blade structure in order to physically shake the ice from the blades.

Another approach is to implement an electrical heating system in or within the blades. For example, it is known to blow heated air into the hollow interior of the blade in order to raise the temperature of the blade surface sufficiently to prevent or reduce ice accretion. A further approach is to integrate electrical heating elements into the blade surface, and such a system is generally successful at preventing ice accretion. However, electrical heating elements tend to be extremely power hungry which is a challenge in terms of controlling them effectively to prevent ice build-up whilst keeping the power flow within limits determined by the power transfer system within the hub. Moreover, typically a blade will be equipped with several different heating devices spaced about the blade surface in order to provide heating in selected zones on the blade. Due to manufacturing issues, the heating devices will have electrical and mechanical characteristics which vary between devices, which adds a further challenge to the way in which the anti-ice system is controlled. Documents <CIT> and <CIT> are prior art examples of heating devices on wind turbine blades.

Is it with a view to addressing some of these problems that the embodiments of the invention have been devised.

Against this background, the invention provides a method of controlling a wind turbine blade anti-ice system, according to claim <NUM>, comprising a power supply and a plurality of electrical blade heating devices. The method comprises - energising a selected one of the blade heating devices; measuring one or more electrical parameters associated with the selected one of the blade heating devices; determining the power of the selected one of the blade heating devices based on the one or more measured electrical parameters; and controlling the selected one of the blade heating devices based on the determined power draw, wherein controlling the selected blade heating device may include determining a maximum on-time for the selected blade heating device based at least in part on the determined power draw.

Beneficially the method of the invention provides for calibration of the electrical heating devices in situ during operation of the anti-ice system. Therefore, the performance of the heating devices may be assessed under a predetermined input electrical supply profile which enables the anti-ice system to be controlled more effectively.

The steps of energising, measuring and determining may be performed in sequence for each one of a plurality of blade heating devices. In this way, calibration may be performed for each one of the heating devices separately using a reduced set of sensors. The calibration process may be run for each heating devices sequentially, and substantially immediately, one after another. Alternatively, the calibration process may be scheduled for each heating device at a predetermined time.

The step of energising a selected one of the blade heating devices may include activating a switch to provide power to that blade heating device. Therefore, the calibration process may be focussed on one of the heating devices at a time.

The step of measuring the electrical parameters associated with the selected blade heating device may include changing the energisation of the blade heating device to cause a change in the one or more measured electrical parameters. For example, the applied voltage of a heat device may be varied in order to assess the performance of the heating device across a range of current loads.

The measured electrical parameters may include the voltage applied to the selected blade heating device and the current associated with the selected heating device. Likewise, measuring the current through the selected one of the blade heating devices may include taking an input current measurement in respect of the blade heating device and an output current measurement.

The step of controlling the selected blade heating device may include determining a maximum on-time for the selected blade heating device based at least in part on the determined power draw. Alternatively, or in addition, the step of controlling the selected blade heating device may include analysing the determined electrical parameters and/or the determined power over an extended time period in order to provide a diagnostic function.

The invention also extends to a wind turbine, according to claim <NUM>, comprising a plurality of wind turbine blades, a blade anti-ice system including a plurality of blade heating devices, and a control system to control the plurality of blade heating devices and to provide power thereto, and further comprising a sensing arrangement to sense electrical parameters associated with the plurality of blade heating devices, and wherein the control system is operable to energise a selected one of the blade heating devices, to measure the electrical parameters thereof, to determine the power of the heating device based on the measured electrical parameters, and to control the heating device based on the determined power thereof, wherein controlling the selected blade heating device may include determining a maximum on-time for the selected blade heating device based at least in part on the determined power draw. The control system includes a switching module comprising a plurality of switch devices, each being associated with a respective blade heating device, wherein the control system is operable to selectively operate one of the switch devices in order to selectively energise an associated blade heating device.

The sensing arrangement may include a voltage sensor to measure the applied voltage across a current supply line associated with the blade heating devices and a current return line from the blade heating devices. The sensing arrangement may further include a current sensor on the current supply line and/or a current sensor on the current return line.

A specific embodiment of the invention will now be described in which numerous features will be discussed in detail in order to provide a thorough understanding of the inventive concept as defined in the claims. However, it will be apparent to the skilled person that the invention may be put in to effect without the specific details and that in some instances, well known methods, techniques and structures have not been described in detail in order not to obscure the invention unnecessarily.

In order to place the embodiments of the invention in a suitable context, reference will firstly be made to <FIG>, which shows a portion of a typical horizontal-axis wind turbine <NUM> (HAWT) in which the embodiments of the invention may be implemented.

In overview, the wind turbine <NUM> comprises a nacelle <NUM> rotatably coupled to the top of a tower <NUM> so that the nacelle <NUM> can slew left and right with respect to the tower <NUM>. Although not shown in <FIG>, the wind turbine <NUM> is mounted to a suitable foundation, which may be on-shore or off-shore depending on the requirements of the installation. Both installations would be familiar to the skilled person.

In the usual way, the nacelle <NUM> supports a rotor hub <NUM> to which is coupled a set of wind turbine blades <NUM>. The illustrated wind turbine includes three blades, although it will be appreciated that this is merely an example and that a different number of blades is acceptable. In the usual way, a flow of wind acts on the blades <NUM> to create torque on the rotor hub <NUM>, thereby rotating the rotor hub <NUM> and transmitting rotational movement to power generating equipment housed within the nacelle <NUM>. The nacelle <NUM> houses many of the generating components of the wind turbine <NUM>, including the generator, gearbox, drive train and brake assembly, as well as convertor and other electrical equipment for converting the mechanical energy of the wind into electrical energy for provision to the grid. Such components are conventional and so further detail has been omitted so as not to obscure the invention to the reader.

The wind turbine includes an electrical system and part of this electrical system is shown in <FIG> as a blade anti-icing system <NUM>. At this point it should be appreciated that the electrical system of the wind turbine is shown in schematic form and so is simplified for the purposes of this description. In particular, the focus of this discussion is on the anti-ice system, which is emphasised in <FIG> in favour of other electric systems and components of the wind turbine, which are not shown or discussed here for the sake of clarity, but which would nevertheless be present in a real-life implementation.

As has been discussed above, cold climatic conditions may cause ice to gather onto the blades during operation and this phenomenon can have a detrimental effect on power generation. Principally this is because the build-up of ice changes the aerofoil profile of the blade which makes it less efficient at generating torque for a given wind speed. As a result, the amount of power generated by the wind turbine steadily reduces as ice builds up. This gradual loss of power generation is detectable by the control system of the wind turbine and can act as a trigger for activation of the anti-ice system.

The anti-ice system <NUM> provides a means to combat the build-up of ice on the surface of the blade. To this end the anti-ice system <NUM> may be operable to apply heat to the blade surface under various triggering conditions using one or more heating devices. For example, activation of the anti-ice system may occur when a certain ice threshold is detected. The supply of power to the heating devices may be regulated in various ways to achieve the required reduction in icing. For example, power supply may be applied to the heating devices gradually based on the detected or modelled temperature at the exterior blade surface. Alternatively, power may be applied to the heating devices at a maximum level for a predetermined period of time in order to achieve ice reduction. Such a power strategy may be applied periodically in order to keep the blade clear of ice.

In overview, the anti-ice system <NUM> comprises a plurality of electrical surface heating devices <NUM> that are electrically connected to a power and control system <NUM>. The heating devices or 'elements' <NUM> may be in the form of electrically conductive mats, panels, nets or pads, which are generally known in the art. Alternatively, the electrical heating devices <NUM> may be a run of electrically resistive cable that is wound or routed in appropriate regions of the blade surface in order to generate heat at desired locations. A type of suitable electrical heating device is known from <CIT>, which discloses electrical heating devices in the form of glass fibre mat coated with electrically conductive carbon.

In the illustrated embodiment, three heating devices <NUM> are provided in spaced apart locations along the blade. As shown in <FIG> the spacing of the heating devices <NUM> is for convenience only and does not indicate a particular spacing within the blade. Typically more than three heating devices would be provided, although this is not essential, and the heating devices may have an optimised spacing that is different to what is shown here. For example, between <NUM> and <NUM> heating devices may be incorporated in the blade, distributed between windward and leeward surfaces. Certain arrangements of heating devices may be devised in which less critical or vulnerable areas of the blade are not provided with heating devices, for example regions near to the blade root. However, it should be noted at this point that the spacing of the heating devices is not central to the invention and the previous arrangements are only provided by way of example.

Remaining with the schematic system view of <FIG>, the power and control system <NUM> for the heating devices <NUM> includes, in overview, a blade power control module <NUM>, a hub power control module <NUM>, a power transfer arrangement <NUM>, a power supply or source <NUM> and a system controller <NUM>. It should be appreciated at this point that the anti-ice system <NUM> is only shown here for one of the blades and that, in practice, each blade would have suitable components to apply heating to the blade. The various electrical and electronic components referred to above are coupled together as appropriate by suitable power and control cables and/or busbars so that power and control signals may be transferred between the respective components as required.

The power transfer arrangement <NUM> is a rotating interface between the rotating hub <NUM> and the stationary nacelle <NUM>. Such a component is conventional and so a full discussion is not required here. However, such a component typically takes the form of a slip ring arrangement which is able to transfer electrical power from a nacelle-based power input and provide a power output into the rotating structure of the hub for supplying power to whichever power consumers are located in the hub. One such power consumer is the hub-based components of the anti-ice system <NUM>.

The power transfer arrangement <NUM> may transfer DC and/or AC power. Typically, AC power will be transferred either as a single phase or as three phases.

The power transfer arrangement <NUM> is also able to transfer electronic signals across the rotating interface. As such the power transfer arrangement <NUM> is shown here as receiving a first input from the power source <NUM> and a second input from the system controller <NUM>.

Conversely, the power transfer arrangement <NUM> provides an output <NUM> to the hub power control module <NUM>. The output <NUM> provides power but also may provide electronic signals. It should be noted that although electronic control signals may be transferred by a hardwired network, it is also possible for those electronic control signals to be transferred by way of a wireless network.

The system controller <NUM> controls the operation of the anti-ice system <NUM> and may be embodied in different ways. As shown here, the system controller <NUM> is based in the nacelle <NUM>. However, this is not essential and the system controller <NUM> may instead be based in the rotor hub <NUM>. The functionality provided by the system controller <NUM> may, moreover, be provided by a main wind turbine controller (not shown) or the system controller <NUM> may be a dedicated control unit for controlling the anti-ice system only. Irrespective of the physical location of the system controller <NUM> it will be appreciated that it provides a suitable processing environment and associated computational, memory and input/output components in order to control and communicate with the various components of the anti-ice system.

The system controller <NUM> receives inputs (not shown) regarding the operation of the wind turbine, and its operational environment, and performs processing to determine the desired activation strategy for the electrical heating devices <NUM>. For example, based on the measured or the modelled outside air temperature and power generation values, the system controller <NUM> may determine that at least one of the electrical heating devices <NUM> should be activated.

Further determination may be made about the timings for which the one or more heating devices <NUM> are activated. For example, the system controller <NUM> may command the electrical heating devices to be operated for a predetermined time period in order to generate a predetermined thermal power output. Furthermore, the system controller <NUM> may command an activation duty cycle to be implemented in order to maintain a predetermined average power output. Such a strategy is described in <CIT>.

The hub power control module <NUM> is located in the hub <NUM> and provides a power distribution function for the anti-ice system components that are located in the blades. The hub power control module <NUM> receives an input from the power transfer arrangement <NUM> and distributes electrical power to the heating devices <NUM>. The precise configuration of the hub power control module <NUM> is not central to the invention, and the general form is conventional. However, it should be capable of selectively distributing power to the heating devices within the blades.

The blade power control module <NUM> has the functionality of receiving a power input from the hub power control module <NUM> and distributing power between the electrical heating devices <NUM> within the respective blade. Therefore, each blade power control module <NUM> incorporates the required switching units and sensors in order to selectively activate and monitor the electrical heating devices <NUM>.

<FIG> illustrates features of the blade power control module <NUM> and the electrical heating devices <NUM> in more detail. Here, the blade power control module <NUM> includes a switching unit <NUM> that selectively powers the three blade heating devices <NUM>.

One challenge relating to the objective of powering and controlling such electrical heating devices is that the electrical characteristics can vary between devices. Typically this is due to material variances that arise during manufacture. For example, heating mats using carbon fibre-based conductive material are observed to exhibit around <NUM>-<NUM>% variation in their electrical resistance between devices. Such a variation can have a significant impact on the electrical current drawn by each heating device given a predetermined applied voltage. When considered in the context of an entire anti-ice system that may have a nominal power draw potential of around 50kW or more, a <NUM>-<NUM>% variation in electrical resistance of the heating mats is significant. This variation would normally be considered in designing the system to ensure the system does not exceed safe limits. This, however, results in a system that is sub-optimal in performance and non-uniform in application.

To guard against these issues, the embodiments of the invention provide a system and a control process that is configured to quantify and adapt to the changes in electrical characteristics to ensure proper operation.

Referring to <FIG>, the switching unit <NUM> is shown as being under the control of the system controller <NUM> and therefore receives a control signal input from it. Based on these control inputs the switching unit <NUM> is operable to control the current flow to each of the heating devices <NUM> that are connected to it. In addition to communicating control signals to the switching unit <NUM>, the system controller <NUM> also receives data signals from sensing devices associated with the anti-ice system <NUM>. Here, the sensing devices sense electrical parameters associated with the input voltage and current for the electrical heating devices <NUM>.

As can be seen in <FIG>, the blade power control module <NUM> includes an input current line <NUM>, and an output current or 'ground' line <NUM>. The input current line <NUM> is connected, via the switching unit <NUM>, to electric power inputs <NUM> of each one of the electrical heating devices <NUM>. Only two of the power inputs <NUM> are indicated on <FIG> for clarity. Electrical switches <NUM> are provided by the switching unit <NUM> to provide control over the activation of each of the electric heating devices <NUM>. In the illustrated embodiment, each switch <NUM> is associated with a single electric heating device <NUM>. However, it should be noted that a switch <NUM> could also be associated with a group of heating devices. It is envisaged that most suitable would be an electronically controlled semiconductor switch, although other switch configurations would be acceptable. It should be noted at this point that the system controller <NUM> may control the switches <NUM> through a suitable communications link to the blade power control module <NUM>.

The output current line <NUM> is connected to a power return connection <NUM> (only one of which is shown here, for clarity) of each electrical heating device <NUM> and therefore provides a neutral connection. It will therefore be appreciated that current is able to flow along input current line <NUM>, through the switching unit <NUM> and to the electric power inputs <NUM> of the heating devices. From there, current flows from the power return connection <NUM> to the output current line <NUM>.

In order to monitor the power flow to the electrical heating devices <NUM>, the blade power control module <NUM> is provided with a first current sensor <NUM> and a second current sensor <NUM>. It should be noted at this point that the two current sensors are not needed to monitor the current flowing through the devices, as only a single current sensor on either the input or output current lines <NUM>,<NUM> would be sufficient. However, provision of both current sensing devices provides redundancy and also a means for the control system to verify correcting functioning of both sensors.

The first current sensor <NUM> is configured and arranged to monitor the current flowing through the input current line <NUM>. Similarly the second current sensor <NUM> is associated with the output current line <NUM> for monitoring the current flow therethrough. Any suitable current sensor/transducer would be acceptable, for example based on Hall effect, inductive or magnetoresistive sensing principles.

In addition to the two current sensors, there is also provided a voltage sensor <NUM>. The voltage sensor is arranged and configured to measure the applied voltage between the input current line <NUM> and the output current line <NUM>. In a similar manner to the current sensors, any suitable voltage sensor may be used, for a resistive or capacitive voltage sensor.

Although not shown here, it should be appreciated that the first and second current sensors <NUM>,<NUM> and the voltage sensor <NUM> are configured to communicate with the system controller <NUM> in order to provide it with measurements about the performance of the heating devices <NUM>. To this end, the sensors may be coupled to the system controller <NUM> by a wired or wireless connection.

By virtue of the provision of the current sensor <NUM>,<NUM> and the voltage sensor <NUM>, the system controller <NUM> is able to measure the actual current that is flowing through the heating devices <NUM> and therefore also calculate the actual electrical power that is being dissipated which will be proportional to the heating power. Beneficially, therefore, the system controller <NUM> is able to carry out a calibration process to determine the actual power drawn by the electrical heating devices <NUM> compared to the expected power draw based on their theoretical electrical specification.

The system controller <NUM> is configured to operate each of the switches <NUM> of the switching unit <NUM> in order to activate selected ones of the electrical heating devices <NUM> for calibration. For example, in order to calibrate the first electrical heating device <NUM> (identified specifically by the label Pz), the system controller <NUM> would configure the uppermost switch <NUM> into a closed position and configure the middle switch <NUM> and the lower most switch <NUM> into the open position. Since only one of the electrical heating devices <NUM> is activated, the applied voltage is across only that single switch and so electrical current will flow from the input current line <NUM>, through a single switch <NUM> and electrical heating device <NUM>, and to the output current line <NUM>. The system controller <NUM> is therefore able to monitor the voltage sensor <NUM> and either one of the two current sensors <NUM>,<NUM> and determine the electrical power consumed by the electrical heating device (based on the relationship P=VI) and also the power dissipated by the electrical heating devices as thermal energy (based on the relationship P=I<NUM>R).

The system controller <NUM> may carry out a calibration process at repeated intervals. For example, the process may be carried out when the wind turbine is started following a maintenance event. Alternatively, the process may be carried out based on a predetermined schedule, or on command as triggered manually e.g. over a SCADA interface with the wind turbine main control centre. By virtue of the above discussion, therefore, the system controller <NUM> applies a dynamic calibration process to the electric heating devices <NUM> in which selected heating devices can be selected in turn so as to gather data of the associated electrical characteristics through which process the devices can be controlled more effectively.

The calibration process may involve all of the electrical heating devices <NUM> being calibrated during a single session. However, since this may take a significant period of time, one option is to implement a 'rolling calibration process' in which the system controller <NUM> cycles the calibration of the electrical heating device <NUM>.

Using the calibrated electrical parameters of the electrical heating devices <NUM>, the system controller <NUM> is able to calculate an accurate measurement of the electrical power drawn by each electrical heating device <NUM> when it is activated with a given supply voltage. The system controller <NUM> may therefore store the calibrated electrical parameters in a suitable processor memory for use in subsequent calculations relating to safe operation of the electrical heating devices <NUM>.

The above procedure may be expressed as a process flow diagram. Therefore, with reference to <FIG>, the system controller <NUM> may be considered to implement the functionality expressed by a control process <NUM> with which the system controller is able to determine electrical characteristics of the anti-ice system more accurately, and as a result is able to control the anti-ice system more effectively.

The control process <NUM> beings at step <NUM> at which the system controller <NUM> energises a selected heating device <NUM>. As has been mentioned above, it is envisaged that a single one of the electrical heating devices <NUM> would be activated at a time in order for the voltage and current to be measured in respect of that heating device. However, it should be noted that a more complex and therefore more costly and possibly less reliable sensing mechanism could be envisaged that would enable multiple electrical devices to be analysed simultaneously. For example, current and voltage sensors could be installed on the respective power return connection <NUM> associated with each heating device or on the individual connection lines 58a extending to the main power return line <NUM>.

Once the selected heating device <NUM> has been activated, the system controller <NUM> is then able, at step <NUM>, to determine the voltage applied to the selected electrical heating device <NUM> and the current drawn by it. Suitably, those parameters may be stored in memory for later use. Moreover, those determined parameters may be stored in an appropriate log, as this may be useful in long term diagnosis of the health of the heating devices.

Once the voltage and current associated with the selected electrical heating device <NUM> have been determined, the system controller <NUM> then is able to calculate, at step <NUM>, the necessary electrical characteristics of the heating device <NUM>. For example, this may be the power draw in respect of the predetermined voltage. During the calibration process therefore, the system controller <NUM> may activate the heating device with a range of discrete applied voltages and to determine the power draw for each voltage level.

Finally, the system controller <NUM> is operable to control, at step <NUM>, the anti-ice system based on the determined characteristics of the selected electrical heating device. It should be noted that the process <NUM> may be repeated for different ones of the electrical heating devices.

In terms of control over the electrical heating devices, the system control <NUM> may use the determined electrical characteristics in the following non-limiting use cases:.

Claim 1:
A method of controlling a wind turbine blade anti-ice system, the system comprising a power supply and a plurality of electrical blade heating devices, wherein the method comprises:
energising a selected one of the blade heating devices;
measuring one or more electrical parameters associated with the selected one of the blade heating devices;
determining the power of the selected one of the blade heating devices based on the one or more measured electrical parameters;
controlling the selected one of the blade heating devices based on the determined power,
wherein controlling the selected blade heating device includes determining a maximum on-time for the selected blade heating device based at least in part on the determined power draw, and
wherein the determined maximum on-time is calculated and implemented as an auto-reset threshold time period so that rest of the selected electrical heating device is triggered if the threshold time period is exceeded.