Control of wind turbines

Abstract

A method of controlling a wind turbine is described. The method involves forecasting the temperature evolution of a component of the wind turbine based upon the current operating parameters of the wind turbine and upon a required power output; predicting from the temperature forecast a future alarm event caused by the temperature of the component exceeding a first threshold level or falling below a second threshold level; and adjusting the operating parameters of the wind turbine to control the temperature evolution of the component thereby to avoid or delay the predicted alarm event.

Claims

1. A method of controlling a wind turbine, the method comprising: forecasting, using at least a first operating parameter of the wind turbine and a required power output of the wind turbine, a temperature evolution of a temperature of a component of the wind turbine; predicting, using the temperature evolution, a future alarm event caused by the temperature of the component exceeding a first threshold level or falling below a second threshold level; and while operating the wind turbine to provide the required power output, adjusting at least a second operating parameter of the wind turbine to control the temperature evolution to avoid or delay the predicted future alarm event, wherein adjusting the second operating parameter comprises operating an auxiliary heating system or an auxiliary cooling system of the wind turbine prior to the predicted future alarm event.

2. The method of claim 1, wherein adjusting the second operating parameter further comprises adjusting a rotor speed, a torque, or the power output of the wind turbine prior to the predicted future alarm event.

3. The method of claim 1, wherein forecasting the temperature evolution comprises: forecasting an effect of a demanded increase in the power output of the wind turbine upon the temperature evolution.

4. The method of claim 3, wherein forecasting the temperature evolution further comprises: forecasting an effect upon the temperature evolution of a first overrating strategy and a second overrating strategy for achieving the demanded increase in power, wherein the first overrating strategy comprises increasing a rotor speed of the wind turbine and the second overrating strategy comprises increasing a torque of the wind turbine; and selecting, between the first overrating strategy and the second overrating strategy, an overrating strategy that is less likely to result in the predicted future alarm event occurring.

5. The method of claim 3, further comprising: determining, when the demanded increase in the power output is predicted to trigger the predicted future alarm event, a maximum overrated power at which the wind turbine can operate without triggering the predicted future alarm event, and operating the wind turbine to output the maximum overrated power.

6. The method of claim 1, wherein the component comprises one of a gear oil, a hydraulic fluid, a converter, a cooling water system, and one or more generator phases.

7. The method of claim 1, wherein the first operating parameter of the wind turbine include measured temperature values.

8. The method of claim 1, wherein the first operating parameter include estimated temperature values.

9. The method of claim 1, wherein forecasting the temperature evolution is performed responsive to receiving an increased power demand.

10. The method of claim 1, wherein operating an auxiliary cooling system of the wind turbine comprises one of: operating a water-cooled heat exchanger configured to cool gear oil; and operating one or more fans to cool air within a nacelle of the wind turbine.

11. The method of claim 10, wherein operating the auxiliary cooling system comprises: selecting, based on the temperature of the component, a cooling mode of a plurality of predefined cooling modes, wherein different cooling modes of the plurality of predefined cooling modes correspond to different temperature ranges; and operating the auxiliary cooling system in the selected cooling mode.

12. The method of claim 11, wherein the different cooling modes correspond specify different rates of operating the auxiliary cooling system.

13. The method of claim 1, wherein the required power output corresponds to a rated power of the wind turbine, and wherein adjusting at least the second parameter to control the temperature evolution comprises avoiding de-rated operation of the wind turbine.

14. A wind turbine control system comprising: a turbine controller configured to control operation of a plurality of components of a wind turbine; and a temperature observer module configured to: forecast, using at least a first operating parameter of the wind turbine and a required power output of the wind turbine, a temperature evolution of a temperature of a component of the plurality of components; predict, using the temperature evolution, a future alarm event caused by the temperature of the component exceeding a first threshold level or falling below a second threshold level; and while the turbine controller operates the wind turbine to provide the required power output, adjust at least a second operating parameter of the wind turbine to control the temperature evolution to avoid or delay the predicted future alarm event, wherein adjusting the second operating parameter comprises operating an auxiliary heating system or an auxiliary cooling system of the wind turbine prior to the predicted future alarm event.

15. The wind turbine control system of claim 14, further comprising: a production controller configured to determine, using one or more measured operating parameters of the wind turbine, a power reference and a speed reference, wherein the temperature observer module is further configured to: receive information indicative of the temperature of the plurality of components; and receive the power reference and the speed reference determined by the production controller.

16. The wind turbine control system of claim 14, wherein the temperature observer module is further configured to: receive estimates of the temperatures of the plurality of components.

17. The wind turbine control system of claim 14, wherein adjusting the second operating parameter further comprises: adjusting a rotor speed, a torque, or the power output of the wind turbine prior to the predicted future alarm event.

18. The wind turbine control system of claim 14, wherein forecasting the temperature evolution comprises: forecasting an effect of a demanded increase in the power output of the wind turbine upon the temperature evolution.

19. The wind turbine control system of claim 18, wherein forecasting the temperature evolution further comprises: forecasting an effect upon the temperature evolution of a first overrating strategy and a second overrating strategy for achieving the demanded increase in power, wherein the first overrating strategy comprises increasing a rotor speed of the wind turbine and the second overrating strategy comprises increasing a torque of the wind turbine; and selecting an overrating strategy that is less likely to result in the predicted future alarm event occurring.

20. The wind turbine control system of claim 18, wherein the temperature observer module is further configured to: determine, when the demanded increase in the power output is predicted to trigger the predicted future alarm event, a maximum overrated power at which the wind turbine can operate without triggering the predicted future alarm event, and wherein the turbine controller is further configured to operate the wind turbine to output the maximum overrated power.

21. A wind turbine, comprising: a tower; a nacelle disposed on the tower; one or more of an auxiliary heating system and an auxiliary cooling system; and a wind turbine control system configured to: forecast, using at least a first operating parameter of the wind turbine and a required power output of the wind turbine, a temperature evolution of a temperature of a component of a plurality of components of the wind turbine; predict, using the temperature evolution, a future alarm event caused by the temperature of the component exceeding a first threshold level or falling below a second threshold level; and while operating the wind turbine to provide the required power output, adjust at least a second operating parameter of the wind turbine, wherein adjusting the second operating parameter controls the temperature evolution to avoid or delay the predicted future alarm event, wherein adjusting the second operating parameter comprises operating the auxiliary heating system or the auxiliary cooling system prior to the predicted future alarm event.

22. A wind farm, comprising a plurality of wind turbines, at least one wind turbine of which comprises: a tower; a nacelle disposed on the tower; one or more of an auxiliary heating system and an auxiliary cooling system; and a wind turbine control system configured to: forecast, using at least a first operating parameter of the wind turbine and a required power output of the wind turbine, a temperature evolution of a temperature of a component of a plurality of components of the wind turbine; predict, using the temperature evolution, a future alarm event caused by the temperature of the component exceeding a first threshold level or falling below a second threshold level; and while operating the wind turbine to provide the required power output, adjust at least a second operating parameter of the at least one wind turbine, wherein adjusting the second operating parameter controls the temperature evolution to avoid or delay the predicted alarm event, wherein adjusting the second operating parameter comprises operating the auxiliary heating system or the auxiliary cooling system prior to the predicted future alarm event.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 which shows a power de-rate scenario in case of failure of the gear oil cooling system has already been described above by way of background to the present invention.

(2) In order that the invention may be more readily understood, examples of the invention will now be described with reference to the following drawings, in which:

(3) FIG. 2 shows a wind turbine in accordance with an embodiment of the present invention;

(4) FIG. 3 is a schematic representation of the nacelle and rotor of the wind turbine shown in FIG. 2;

(5) FIG. 4 is a schematic representation of a wind turbine control system for controlling the wind turbine of FIG. 2;

(6) FIG. 5 is a plot of the temperature evolution of a wind turbine component showing the situation when a demanded power boost triggers an alarm event;

(7) FIG. 6 is a plot showing the temperature evolution of the component when cooling is activated in advance to prevent the alarm event from being triggered; and

(8) FIG. 7 is a simulated plot showing a temperature evolution forecast of the gear oil of a wind turbine when a power boost is demanded.

(9) FIG. 8 illustrates a wind farm comprising a plurality of wind turbines.

DETAILED DESCRIPTION

(10) FIG. 2 shows a wind turbine 10 according to an embodiment of the present invention. The wind turbine 10 comprises a tower 12 supporting a nacelle 14 at its upper end. A rotor 16 is mounted to the nacelle 14. The rotor 16 comprises three rotor blades 18, which extend from a central hub 20. A plurality of actuators (not shown) are located inside the hub 20. The actuators are part of a pitch control system, and are configured to turn the blades 18 about their respective longitudinal axes in order to control the rotational speed of the rotor 16 depending upon the wind conditions and power demand. In this example the actuators are hydraulically operated, although in other examples the actuators may be operated by other means, for example electrically.

(11) FIG. 8 illustrates a wind farm 800 comprising a plurality of wind turbines 10 coupled with a grid 805.

(12) FIG. 3 is a simplified schematic representation of the nacelle 14 and rotor 16. Referring to FIG. 3, the nacelle 14 houses a generator 22 and a gearbox 24. A low speed gear 26 intermeshes with a high speed gear 28 inside the gearbox 24. The low speed gear 26 is mounted at one end of a low speed shaft 30, and the rotor 16 is mounted at the other end of the said shaft 30. The high speed gear 28 is mounted on one end of a high speed shaft 32 and the other end of the high speed shaft 32 connects to the generator 22, which converts the rotational energy of the high speed shaft 32 to electrical energy. The gearbox 24 includes a low speed bearing 34 for supporting the low speed shaft 30 and a high speed bearing 36 for supporting the high speed shaft 32.

(13) The gearbox 24 further includes an oil sump 38 containing gear oil, in which the high-speed gear 28 and the high speed bearing 36 are shown partially immersed. An oil inlet 40 is located at the top of the gearbox 24. The nacelle 14 also houses a number of auxiliary cooling systems. In this example, there is shown a water-cooled heat exchanger 42 associated with the gearbox 24 for cooling the gear oil, and one or more fans 44 for cooling the air inside the nacelle 14.

(14) One or more controllers 46 for controlling the wind turbine 10 are also located inside the nacelle 14. The controllers 46 receive input signals from a plurality of sensors associated with the various components. In this example, a plurality of first temperature sensors 48 are configured to sense the respective temperatures of the three phases of the generator 22; a second temperature sensor 50 is configured to sense the temperature T.sub.W of the cooling water of the heat exchanger 42; a third temperature sensor 52 is configured to sense the temperature of the hydraulic fluid of the pitch control system; a fourth sensor 54 is configured to sense the temperature T.sub.N of the air inside the nacelle; and a fifth sensor 56 is configured to sense the temperature T.sub.ambient of the ambient air outside the nacelle.

(15) It should be appreciated that FIG. 3 is a simplified representation of the nacelle 14. In reality there would be more components inside the nacelle 14 and the components may be configured differently. For example, the gearbox 24 may comprise any suitable number and arrangement of gears or it might have a dry sump, where the oil is pumped over the gears before being collected in a main tank where it is filtered and cooled.

(16) FIG. 4 is a schematic representation of a wind turbine control system 58 for controlling the wind turbine 10 of FIG. 2. The control system 58 comprises a production controller 60 and a turbine controller 62. The turbine controller 62 in turn comprises a plurality of turbine control modules 64, 66, 68, 70 associated respectively with the gearbox, the generator, the hydraulic fluid of the pitch control system, and the auxiliary cooling systems such as the heat exchanger and the fans mentioned above. The production controller 60 and the turbine controller 62 are configured to communicate directly, and indirectly via a temperature observer module 72. The temperature observer module 72 is a novel feature of the control system of the present invention, and its operation will be described in more detail later.

(17) The various turbine control modules 64-70 each include a respective temperature sensor and a respective temperature estimator. As mentioned above, the temperature sensors are configured to measure the temperature of the components with which they are associated. The measured temperature of the components is communicated to the production controller 60 via the turbine controller 62 across a data bus 74.

(18) The temperature estimators are configured to estimate the temperature of their associated components based upon the operating parameters of the wind turbine, the ambient conditions and from thermodynamic relationships. An example of the gear oil temperature estimator is described in detail later, and further details of the temperature estimators can be found in applicant's co-pending patent application WO2012/025121, which was described above by way of background to the present invention.

(19) As is well known in the art, the production controller 60 determines a power reference and a speed reference for operating the wind turbine on the basis of a demanded power and measured external parameters such as wind speed. The power reference and the speed reference are communicated to the turbine controller across the data bus 74, which operates the various turbine components in accordance with predetermined control strategies in order to follow the speed reference and power reference values set by the production controller 60.

(20) If the temperature of the gearbox, generator or hydraulic fluid exceeds a predetermined threshold level, then an alarm event is generated. In response to the alarm event, the production controller 60 determines a de-rated mode of operation by reducing the power reference and/or the speed reference provided to the turbine controller 62. If de-rating the turbine is not sufficient to reduce the temperature of the component(s) to a safe level, then the production controller 60 may take action to shut down the wind turbine.

(21) The temperature observer module 72 of the present invention is concerned with increasing the availability of the wind turbine by predicting the possibility of an alarm event occurring in the future and taking proactive steps to avoid or delay the alarm event occurring. The temperature measurements from the turbine controller 62 and the estimated temperature values from the various turbine control modules 64-70 are communicated to the temperature observer module 72 via a data bus 76. The temperature observer module 72 also receives the power reference and the speed reference from the production controller 60, and other inputs such as the ambient air temperature T.sub.ambient and the temperature T.sub.N inside the nacelle 14. The temperature observer module 72 is also connected to the auxiliary cooling systems 70 and is configured to receive the current status of the auxiliary cooling systems and to control the auxiliary cooling systems as described further below.

(22) Based upon the temperature measurements from the various control modules 64-70, the temperature observer module 72 forecasts the temperature evolution of the gearbox, the various generator phases, the hydraulic fluid, the converter and any other temperature-critical components and determines if an alarm event is likely to occur at a future time. The temperature observer module 72 communicates the temperature evolution forecast to the production controller 60 together with the estimated time to alarm. If the temperature observer module 72 determines that an alarm event is likely to occur in the future, it may take proactive action to prevent the alarm event from occurring, for example by controlling heating or cooling systems and/or adjusting rotor speed or torque. This guarantees continuous operation of the wind turbine in both cold and hot ambient conditions and enables the wind turbine to be overrated without triggering alarm events. An example will now be described with reference to FIGS. 5 and 6.

(23) FIG. 5 is a plot of the temperature of a wind turbine component versus time. The component may be the generator, the gear oil or the hydraulic fluid for example. The solid line 100 between time t.sub.0 and time t.sub.1 shows the measured temperature of the component obtained from the sensor readings, whilst the dotted line 102 from time t.sub.1 onwards shows the estimated temperature evolution of the component determined by the temperature observer module 72.

(24) From time t.sub.0 to t.sub.1 the wind turbine is operating normally and the measured temperature of the component is below an alarm threshold temperature T.sub.A. At time t.sub.1, a power boost is demanded by the grid operator and the production controller 60 calculates an increased power reference value. Based upon the increased power reference value and the current operating parameters of the wind turbine, the temperature observer module 72 calculates a predicted temperature evolution of the component (dotted line 102), which is also referred to herein as a temperature forecast.

(25) From the temperature forecast, the temperature observer module 72 predicts that the temperature of the component will reach the alarm threshold temperature T.sub.A at a future time t.sub.2. In practice, once the alarm threshold temperature T.sub.A is reached, the production controller 60 starts to de-rate the wind turbine. The temperature observer module 72 factors this predicted de-rated operation following time t.sub.2 into the forecast. Allowing for de-rated operation, the temperature observer module 72 predicts that at future time t.sub.3 the temperature of the component will reach an extended alarm threshold temperature T.sub.EA, at which point the production controller 60 will shut down the wind turbine.

(26) As mentioned above, the temperature observer module 72 is able to control the auxiliary cooling systems 70 proactively to prevent an alarm event occurring, as will now be described with reference to FIG. 6.

(27) Referring to FIG. 6, this shows two trajectories for the forecasted temperature evolution of the component. The first trajectory is indicated by the dotted line 102 and corresponds to the dotted line in FIG. 5 described above. The second trajectory is indicated by the dashed line 104, and represents the situation where cooling of the component is activated at time t.sub.1 when the power boost is demanded. Referring first to the first trajectory 102 (i.e. without cooling activated) the temperature observer module 72 predicts that the alarm threshold temperature T.sub.A, will be reached at time t.sub.2, and will continue to rise thereafter between time t.sub.2 and time t.sub.3. Referring now to the second trajectory 104, the temperature observer module 72 predicts that if cooling is activated at time t.sub.1, then the temperature of the component will still increase but will remain below the alarm threshold temperature T.sub.A at time t.sub.2 and will stabilise at a level still below the alarm threshold temperature T.sub.A at time t.sub.3.

(28) Accordingly, by activating or otherwise controlling one or more auxiliary cooling systems 70 in advance of a predicted alarm event, the temperature observer module 72 is able to prevent the temperature of the component(s) from rising above the relevant alarm threshold temperature T.sub.A and therefore avoiding the need to de-rate or shut down the wind turbine. By forecasting the temperature evolution of the temperature-critical components, and pro-actively controlling the auxiliary cooling systems 70, the wind turbine is able to accommodate requests for a power boost even when the wind turbine is operating close to its maximum limits and in high ambient temperatures.

(29) There now follows an explanation of the theory underpinning one of the temperature estimators, namely the gear oil temperature estimator, and an explanation of how the temperature observer module forecasts the temperature evolution based upon the estimated temperature values.

(30) Referring again to FIG. 3, the gear oil temperature T.sub.O is affected by the temperature T.sub.B of the high speed bearing, by the flow and temperature T.sub.W of the cooling water through the heat exchanger, and by the temperature T.sub.N inside the nacelle. During operation of the wind turbine 10, heat is transferred from the rotating gears 26, 28 to the oil in the sump 38. The hot oil is then extracted from the sump and filtered before being pumped through the heat exchanger 42. The cooling water in the heat exchanger cools the oil and the cooled and filtered oil is pumped back into the gearbox 24 through the oil inlet 40.

(31) To calculate the gear oil temperature T.sub.O an estimate of the total energy added to or removed from the gear oil has to be found. By using Newton's law of cooling, it is possible to calculate the heat transfer between two components with different temperatures using the following equation:

(32) $\begin{array}{cc}\frac{\mathrm{dQ}}{\mathrm{dt}}=h.Math.A.Math.\left(T_1-T_2\right)& \mathrm{Equation}1\end{array}$

(33) Where Q is the lost heat flow (W), h is the heat transfer coefficient (W/m.sup.2K) A is the heat transfer surface area between the components (m.sup.2) and T1T2 is the temperature difference between the components (K)

(34) In the following calculations, a single constant, K.sub.bearing (W/K) is used as the constant for the energy transfer between the high speed bearing and the gear oil, where
K.sub.bearing=h.Math.AEquation 2

(35) The energy transfer from the bearing to the gear oil is then calculated using the following equation:
Q=K.sub.bearing.Math.(T.sub.BT.sub.O)dtEquation 3
where T.sub.B is the temperature of the bearing and T.sub.O is the temperature of the gear oil.

(36) The total energy transfer to the gear oil requires the subtraction of the energy transferred away from the gear oil to the air inside the nacelle and to the cooling water of the gearbox heat exchanger. Accordingly, the total energy transferred to the gear oil can be expressed using the following equation:
Q.sub.total=(T.sub.BT.sub.O).Math.K.sub.bearingdt(T.sub.OT.sub.N).Math.K.sub.Nacelledt(T.sub.OT.sub.W).Math.K.sub.coolingdtEquation 4
where T.sub.N is the air temperature inside the nacelle, T.sub.W is the temperature of the cooling water of the heat exchanger, K.sub.Nacelle is the constant for heat transfer from the gear oil to the air inside the nacelle, and K.sub.cooling is the constant for heat transfer from the gear oil to the cooling water.

(37) The temperature of the gear oil increases proportionally to the total energy added to the gear oil. Accordingly, the estimated temperature of the gear oil can be expressed by the following equation:
T.sub.O=Q.sub.total.Math.K.sub.oil+T.sub.OinitEquation 5

(38) Where T.sub.O is the estimated temperature of the gear oil; K.sub.oil is the constant of proportionality between the gear-oil temperature and the energy added to the gear oil; and T.sub.Oinit is the initial temperature of the gear oil.

(39) Equation 5 above can be written in the form of a temperature predictor in the time domain as follows:

(40) $\begin{array}{cc}\mathrm{To}\left(t+t\right)=\mathrm{To}\left(t\right)+K_{\mathrm{oil}}\left(\begin{array}{c}\left(T_B\left(t\right)-T_O\left(t\right)\right).Math.e^{\left(-K_{\mathrm{bearing}}.Math.t\right)}-\left(T_O\left(t\right)-T_N\left(t\right)\right).Math.\\ e^{\left(-K_{\mathrm{Nacelle}}.Math.t\right)}-\left(T_O\left(t\right)-T_W\left(t\right)\right).Math.e^{\left(-K_{\mathrm{cooling}}.Math.t\right)}\end{array}\right)& \mathrm{Equation}6\end{array}$

(41) Where t is the current time and t is the prediction interval.

(42) The next task is to determine the various heat constants, which requires an understanding of how the cooling system operates. In this example, the cooling system operates in three different modes depending on the gear oil temperature. In a first no cooling mode, the fans are turned off and hence no auxiliary cooling of the water in the heat exchanger takes place. In a second low cooling mode, the fans operate at a relatively low speed if the temperature of the gear oil reaches 55 C., and the fans turn off when the temperature of the gear oil drops to 47 C. In a third cooling mode, the fans operate at a relatively high speed when the temperature of the gear oil reaches 60 C., and the fans turn off when the temperature drops to 52 C.

(43) This means that the constant K.sub.cooling will have three different values depending upon which mode the cooling system operates in.

(44) Whilst the skilled person will appreciate that there are several ways to determine the different heat constants, a convenient method is to use logged data of T.sub.B, T.sub.N, T.sub.W and T.sub.O together with the discretised version of Equation 6 and a MATLAB script. The MATLAB script sweeps through different values of all the heat transfer constants and for every change in the constant it compares the gear oil temperatures calculated using Equation 6 with the measured temperature using the Least Square method. The set of heat transfer constants that gives the minimum least square error between the estimated and measured data is then chosen.

(45) Once the heat transfer constants are known, the temperature observer module 72 can forecast the temperature evolution of the gear oil using Equation 6 above. The forecast calculation assumes that the variables T.sub.B, T.sub.N and T.sub.W remain constant during the prediction time interval t. The equilibrium temperature can be evaluated by calculating the time domain solution to the differential equation.

(46) If a power boost is required by the control system at a future point in time, a new set point rotor speed is determined. The bearing temperature T.sub.B varies in proportion to the rotor speed, and the temperature observer module 72 accesses a look-up table that correlates the bearing temperature with rotor speed. This allows the temperature observer module 72 to forecast the temperature evolution of the gear oil when a power boost is required, as will now be explained further by way of example with reference to FIG. 7.

(47) Referring to FIG. 7, this is a simulated plot showing the forecasted temperature evolution of the gear oil (T.sub.o) calculated using Equation 6 above. The figure also includes plots of the forecasted temperature evolution of the high speed bearing (T.sub.B), the cooling water (T.sub.W) and the nacelle (T.sub.N). The initial parameters are as follows: T.sub.O(t)=49 C. (initial temperature of the gear oil sump) T.sub.B=50 C. (initial temperature of the high speed bearing) T.sub.N=30 C. (nacelle temperature) T.sub.W=40 C. (water temperature)

(48) Between time t=0 to t=10 seconds, the wind turbine is operating at a first set point rotor speed. The gear oil temperature T.sub.O falls from its initial value of 49 C. at t=0 to an equilibrium temperature of approximately 46 C. at approximately t=8 seconds. The forecast assumes that T.sub.N and T.sub.W remain constant for the duration of the forecast time period, which is twenty seconds in this example.

(49) The temperature observer module 72 receives a signal from the production controller 60 that a power boost is required at t=10 seconds. The power boost will require the rotor speed to increase to a second set point level, which is higher than the first set point. The temperature observer module 72 determines from the look-up table the predicted temperature T.sub.B of the high speed bearing corresponding to the second set point rotor speed. The temperature observer module 72 also determines from previously logged data that the second set point rotor speed will be reached at approximately time t=15 seconds, and that the high speed bearing temperature T.sub.B rises linearly during this period. The temperature observer module 72 can then predict the temperature evolution of the gear oil based upon this predicted rise in temperature of the high speed bearing T.sub.B. Accordingly, it is forecast that the gear oil temperature T.sub.O will rise steadily from about 46 C. at t=10 seconds, up to a new steady state temperature of approximately 48.5 C. at t=19 seconds.

(50) It will be appreciated that the heat transfer constants will change in value if changes are made to the system, for example if the position of the sensors is changed or if the modes of operation of the cooling system are varied. To make the temperature forecaster more robust against real measurements, standard approaches based on Kalman filters or equivalent methods can be utilised. In such cases, the current measurements may be used to update old predictions and to correct future predictions according to the new information about the system. Furthermore temperature predictors, due to the simple dynamics involved, can be built in the form of autoregressive equations, directly identified from measured data. Hence the predictors may be formulated using least square methods along with model order identification methodologies.

(51) Various modifications may be made to the above examples without departing from the scope of the present invention as defined by the following claims.