DESCRIPTION WIND TURBINE GENERATOR AND ITS CONTROL METHOD Technical Field The present application is based upon and claims the benefit of priority from Japanese Patent Application No. 2007-103059, filed on 10 April, 2007, the disclosure of which is incorporated herein by reference. The present invention relates to a wind turbine generator system and a control method therefor, and in particular, relates to yaw control of the wind turbine generator system. Background Art One important ^control for improving the efficiency of a wind turbine generator system is yaw control in which the direction of the wind turbine rotor is controlled in accordance with the wind direction. The wind turbine generator system, which provides highest efficiency when the wind turbine rotor faces the front with respect to the wind, requires direction control of the wind turbine rotor by performing a yaw rotation of the nacelle which supports the wind turbine rotor in accordance with the wind direction. Various, approaches have been made for yaw rotation mechanisms and yaw control techniques; for example, Japanese Laid Open Patent Application No. P2004-2 8 5858A discloses a technique in which the wind direction and wind power are detected with the use of a laser anemovane and yaw control is performed based on the detected wind direction and wind speed. Additionally/ Japanese Laid Open Patent Applications Nos. P2005-113899A and P2001-289149A disclose a configuration of a drive mechanism for the yaw rotation of the nacelle. One important issue of the yaw control of the wind turbine generator system is to minimize the number of times of yaw rotations of the nacelle. Due to the large weight of the nacelle, a large number of times of yaw rotations of the nacelle cause increased mechanical loads of the rotation mechanism which rotates the nacelle* and the braking mechanism which stops the rotations of the nacelle, increasing mechanical wear of these mechanisms. In order to reduce the wear of the rotation mechanism and braking mechanism, it is desirable that the number of times of yaw rotations be reduced. The most general control logic of the yaw control used to satisfy such need is a control logic in which, when a state in which the absolute value of the wind direction deviation, that is, the deviation between the wind turbine direction (i.e. the direction of a wind turbine rotor) and the actual wind direction, is greater than a predetermined threshold value continues for a predetermined duration time (e.g. 20 seconds), a yaw rotation of the nacelle is performed such that the wind direction deviation is zero (i.e. such- that the v^ind turbine direction agrees with the most recent wind direction), as shown in FIG. 1. Such a control logic, in which a yaw rotation is not performed unless the absolute value of the wind direction deviation exceeds a threshold value, reduces the number of times of yaw rotations by setting an appropriate threshold value. One problem of. such control logic is that the value of the wind direction deviation is not reduced averagely under a condition where the wind direction gradually changes over a long time (over several hours under some wind conditions), as shown in FIG. 2. Depending on locations of mountains, valleys and seas, there is a case where a wind condition at a certain point shows random changes in the wind direction with high degree of randomness during the daytime but does not show random changes in the wind direction at nighttime. In other words the wind condition at nighttime often exhibits wind direction changes over a long time. The use of the above-mentioned control logic averagely reduces the value of the wind direction deviation close to zero under a condition where the wind direction randomly changes with high degree of randomness. However, when the wind direction gradually changes over a long time (over several hours under some wind conditions) (indicated by "A" at the top of FIG. 2) as shown in FIG. 2, the wind direction deviation becomes zero only for a moment (C at the bottom of FIG. 2) even if yaw rotations are repeated (indicated by "B" at the middle of FIG. 2) in the case of the - 4 - above-mentioned control logic. Therefore, the average value of wind direction deviations is not reduced. This is not preferable in terms of improvement of the efficiency of a wind turbine generator system. Disclosure of Invention Therefore, an overall object'of the present invention is to improve the efficiency of a wind turbine generator system, and more in detail, an object of the present invention is to achieve at least one of the following two aims : First aim: To provide a yaw control technique for a wind turbine generator system which reduces the value of the wind direction deviation even when the wind direction gradually changes over a long time while preventing the increase in the number of times of yaw rotations; and Second aim: To provide a yaw control technique for a wind turbine generator system which improves the efficiency of a wind turbine generator system by catching transitional change in the wind direction in an early stage to perform a yaw rotation at an appropriate timing. In one aspect of the present invention, a wind turbine generator system is provided with: a nacelle supporting a wind turbine rotor; a rotation mechanism performing a yaw rotation of the nacelle; a vjind direction measurement apparatus measuring a wind direction; and a control apparatus controlling said rotation mechanism. Said - D - control apparatus calculates a wind direction deviation from the wind direction measured by said wind direction measurement apparatus and a direction of said wind turbine rotor, and performs a yaW rotation of said nacelle by said rotation mechanism when any of conditions (1) and (2) is satis fled, wherein the condition (1) is a condition under which a state where an absolute value of said wind direction deviation is equal to or more than a first threshold value {or exceeds the first threshold value) continues for a first duration predetermined, and wherein the condition (2) is a condition under which a state where the absolute value of said wind direction deviation is equal to or more than a second threshold value larger than said first threshold value {or exceeds the second threshold value) continues for a second duration shorter than said first duration. Preferably, said control apparatus performs the yaw rotation of said nacelle such that said wind direction deviation is reduced to zero, when any of said conditions (1) and (2) is satisfied. It is also preferable that said control apparatus alternatively performs the yaw rotation of said nacelle such that a sign of said wind direction deviation is reversed betvjeen before and after the yaw rotation and the absolute value of said wind direction deviation after the yavj rotation is larger than zero and equal to or less than said second threshold value/ when said condition (2) is satis fied. In this case, ■ it is preferable that said control apparatus performs the yaw rotation of said nacelle such that the absolute value of said wind direction deviation after the yaw rotation is equal to said second threshold value, when said condition (2) is satisfied. In another aspect of the present invention, a wind turbine generator system is provided with: a nacelle supporting a wind turbine rotor; a rotation mechanism performing a yaw rotation of the nacelle; a wind direction measurement apparatus measuring a wind direction; and a control apparatus controlling said rotation mechanism. Said control apparatus (a) calculates a wind direction deviation from the wind direction measured by said wind direction measurement apparatus and a direction of said wind turbine rotor, (b) decide which of first and second conditions is a present wind condition, the first condition being a condition where a wind direction randomly changes with high degree of randomness, and the second condition being a condition where a wind direction gradually changes over a long time, and (c) performs a yaw rotation of said nacelle by said rotation mechanism such that a sign of said wind direction deviation is reversed between before and after the yaw rotation and the absolute value of said wind direction deviation after the yaw rotation is larger than zero and equal to or less than said first threshold value, - 7 - when said control apparatus decides said present wind condition is said second condition and an absolute value of said wind direction deviation is equal to or more than a first threshold value larger than a first threshold value predetermined (or exceeds the first threshold value} continues for a first duration predetermined. Preferably, said control apparatus performs the yaw rotation of said nacelle such that the absolute value of said wind direction deviation after the yaw rotation is equal to said first threshold value, when said control apparatus decides said present wind condition is the second condition and a state where the absolute value of said wind direction deviation is equal to or more than said first threshold value {or exceeds the first threshold value) continues for said first duration. It is also preferable that said control apparatus performs the yaw rotation of said nacelle such that said wind direction deviation after the yaw rotation is reduced to zero, when said control apparatus decides said present wind condition is said first condition and a state where the absolute value of said wind direction deviation is equal to or more than a second threshold value predetermined (or exceeds the second threshold value) continues for said first duration. In still another aspect of the present invention, a wind turbine generator system is provided with: a nacelle supporting a wind turbine rotor; a rotation mechanism performing a yaw rotation of the nacelle; a wind direction measurement apparatus measuring a wind direction; and a control apparatus controlling said rotation mechanism. Said control apparatus calculates a wind direction deviation from the wind direction measured by said wind direction measurement apparatus and a direction of said wind turbine rotor, and performs a yaw rotation of said nacelle by said rotation mechani sm such that said wind direction deviation is reduced to zero, when said wind direction deviation satisfies a predetermined condition for every time t of t^-T < t ^ tr., where tr, is a present time and T is a predetermined value, and wherein said predetermined condition is (Ae(t) [ > eTH(t) , or I AG (t) I > eTH(t) . Note that |Ae(t)| is an absolute value of said wind direction deviation for the time t, and 9TH(t) is a function monotonically non-decreasing in tr,-T ^ t ^ to- Preferably, the derivative deTH(t)/dt of eTH(t) with respect to the time t is monotonically non-increasing for t.-,-T ^ t ^ t.;. excluding the time t at which the derivative cannot be defined, and more preferably, the derivative deTH(t)/dt is monotonically decreasing for t.:,-T < t ^ to- The present invention improves the efficiency of a wind turbine generator system. More specifically, one embodiment of the present .invention provides a yaw control technique for a wind turbine generator system which reduces the value of the wind direction deviation- even when the wind direction gradually changes over a long time while preventing the increase in the number of times of yaw rotations. Another embodiment of the present invention provides a yaw control technique for a wind turbine generator system which improves the efficiency of a wind turbine generator system by catching transitional change in the wind direction in an early stage to perform a yaw rotation at an appropriate timing. Brief Description of Drawings FIG. 1 is a graph showing a conventional control logic; FIG. 2 is a graph illustrating a problem of the conventional control logic; FIG. 3 is a diagram showing a configuration of a wind turbine generator system in a first embodiment of the present invent ion; FIG. 4 is a sectional view showing the configuration of a nacelle rotation mechanism in the first embodiment of the present invention; FIG. 5 is a block diagram showing the structure of a yaw control system in the first embodiment of the present invention; FIG. 6A is a; graph showing changes in the. wind direction deviation according to the conventional control logic; FIG. 6B is a graph showing changes in the wind direction deviation according to a control logic of the first embodiment; FIG. 5C is a graph showing change in a wind direction deviation according to the control logic of the first embodiment; FIG. 7A is a graph showing changes in the wind turbine direction according to a control logic of the second embodiment; FIG. 7B is a graph showing changes in the wind turbine direction according to the conventional control logic; FIG. 8 is a graph showing the efficiency of a wind turbine generator system achieved by the control logic of the second embodiment and the conventional control logic when the change rate of the wind direction is fixed and the change amplitude of the wind direction deviation is constant; FIG. 9 shows graphs indicating the timing of the start of the yaw rotation according to the conventional control logic and the timing of the start of the yaw rotation according to the control logic of the third embodiment; FIG. 10 is a graph showing an example of the function GTH (t) ; and FIG. 11 is a graph showing an example of the distinction betvjeen "the condition in which the wind direction randomly changes with high degree of randomness" and "the condition in which the wind direction gradually changes over a long time". Best Mode for Carrying Out the Invention (First Embodiment) FIG. 3 is a side view showing a configuration of a wind turbine generator system 1 in one embodiment of the present invention. The wind turbine generator system 1 is provided with a tower 2 and a nacelle 3 provided on the top of the tower 2. The nacelle 3 is rotatable in the yaw direction and directed towards a desired direction by a nacelle rotation mechanism 4. The nacelle 3 is provided with a wound-rotor induction generator 5 and gears 5. The rotor of the wound-rotor induction generator 5 is joined to a rotation shaft 7a of a wind turbine rotor 7 through the gears 6. The wind turbine rotor 7 is provided with a hub 8 connected to the rotation shaft 7a and blades 9 attached to the hub 8. The nacelle 3 is further provided ' with an anemovane 10 for measuring the wind speed and direction. FIG. 4 is a sectional view showing an example of the configuration of the nacelle rotation mechanism 4. The nacelle rotation mechanism 4 is provided with a yaw motor ) 11, reduction gears 12, a pinion 13, an internal gear 14, a yavj braking mechanism 15, and a brake disc 16. The yaw motor 11, the reduction gears 12, the pinion 13, and the yaw braking mechanism 15 are provided on the nacelle 3, and movable with uhe nacelle 3. On the other hand, the internal gear 14 and the brake disc 16 are fixed to the tower 2. The rotor of the yaw motor 11 is mechanically connected to the pinion 13 through the reduction gears 12, and the pinion 13 and the internal gear 14 are engaged with each other. When electricity is supplied to the yaw motor 11, the pinion 13 turns around to perform a yaw rotation of the nacelle 3. The yaw rotation of the nacelle 3 is braked by the braking mechanism 15. After brake shoes 17 of the yaw braking mechanism 15 puts the brake disc 16 therebetween, the yaw rotation of the nacelle 3 is braked or stopped. FIG. 5 is a block diagram showing an example of the structure of the control system for the yaw control. In this embodiment, the yaw control system is provided with a control unit 21, a motor drive unit 22, and a braking mechanism drive unit 23. The motor drive unit 22 supplies drive power to the yaw motor 11 in response to a control signal from the control unit 21. The braking mechanism drive unit 23 presses the brake shoes 17 of the braking mechani sm 15 against the brake disc 16 in response to a control signal from the control unit 21. The control unit 21 determines a desired direction of the wind turbine rotor 7 from the wind speed and direction measured by the anemovane 10 and performs a yaw rotation of the nacelle 3 such that the -vjind turbine rotor 7 is directed towards the desired direction by operating the yaw motor 11. Further, the control unit 21 stops a yaw rotation by operating the braking mechanism 15 after the wind turbine rotor 7 is directed towards the desired direction as a result of the yaw rotation. Next, a "description is given of yaw control of the wind turbine generator system 1 of this embodiment. In this embodiment, yaw control is performed in response to the wind direction measured by the anemovane 10. In detail, the control unit 21 performs yaw control as follows: The anemovane 10 measures the wind direction at each time point at predetermined sampling intervals and supplies wind direction data indicative of the wind directions at the respective times, to the control unit 21. In the wind direction data, the wind direction is defined as the angle with respect to a predetermined reference direction. The control unit 21 generates control-oriented wind direction data, which are actually used for yaw control, by performing low-pass filter processing on the measured wind direction data (most easily by averaging a series of wind direction data which are adjacent in the time domain) , and calculates the difference between the wind direction indicated by the control-oriented wind direction data and the wind turbine direction as the wind direction deviation. In this embodiment, the wind turbine direction is defined as the angle betvjeen the direction of the rotation shaft - 14 - 7a of the wind turbine rotor 7 and a predetermined reference direction. The wind direction deviation is data.allowed to take any of a positive value, a negative value, and zero, and in one embodiment, the value of the wind direction indicated by the control-oriented wind direction data subtracted by the angle of the wind turbine direction is defined as the wind direction deviation. Further, the control unit 21 controls the motor drive unit 22 and the braking mechanism drive unit 23 in response to the calculated wind direction deviation to- perform a yaw rotation of the nacelle 3. In this embodiment, the control unit 21 performs a yaw rotation such that the wind direction deviation is reduced to zero (i.e. to the wind direction indicated by the most recent control-oriented wind direction data) when at least one of the following two conditions is satisfied: (1) A state where the absolute value of the wind direction deviation is equal to or more than a threshold value 9THI (or exceeds 9THI) continues for Ti seconds. (2) A state where the absolute value of the wind direction deviation is equal to or more than a threshold value STHI (>eTHi) (or exceeds STH:) continues for T; ( eTH{t) , " ... (la) where to is the present time. Here, lAe(t) I is the absolute value of the wind direction deviation at a time t, T is a predetermined value, and 9TH(t) is a threshold value used for judgment of the start of a yaw rotation, which is a function which is monotonically non-decreasing with respect to t,-.-T eTH(t) . ■.-.-... (lb) ■ Such a control! logic, as shown in the bottom of FIG. 9,, allows detecting' a transitional change in the wind direction and performing a yaw rotation at as an early timing as possible.' In detail, the control logic of the third embodiment, where the threshold value at which a yaw ,1 rotation is started increases with time, allows taking int.o account the change in the wind direction deviation in an initial period when the absolute value of the wind direction deviation'.is small in the judgment of the start of a yaw rotation. ^. Therefore, the control logic of ^the !■ . It third embodiment allows detecting transitional changes in the wind direction and performing a yaw rotation at a proper timing. This is effective in terms of improving the efficiency of the wind turbine generator system 1. . It is preferable that the function 0TH (t) is such a function,that the derivative d0TH{t)/dt thereof with respect to the time; is a function which is monotonically non-increasing in the,whole area of t.:,-T eTH(t) , or I AG (t) 1 > eTH(t) , where jA9{t) I is an absolute value of said wind direction deviation for the time t, and eTH(t) is a function monotonical ly non-decreasing in tr.-T < t ^ t,-,- 9. The wind turbine generator system according to claim 8, wherein a derivative d9TH(t)/dt of GTH (t) with respect to the time t is monotonically non-increasing for tr,-T ^ t ^ t.j excluding the time t at which the derivative cannot be defined. 10. The wind turbine generator system according to claim 8, wherein said derivative d0TH{t)/dt is monotonically decreasing for t; -T ^ t ^ t,.. 11. A control method for a wind turbine generator sys'tem - 34 - including a nacelle on which a wind turbine rotor is provided, said method comprising: a step of measuring a wind direction; a step of calculating a wind direction deviation from said measured wind direction and a direction of said wind-turbine rotor; and • performing a yaw rotation of said nacelle by said rotation mechanism when any of conditions (1) and (2} is satisfied, wherein the condition (1) is a condition under which a state where an absolute value of said wind direction deviation is equal to or more than a first threshold value (or exceeds the first threshold value) continues for a first duration predetermined, and wherein the condition (2) is a condition under'which a state where the absolute value of said wind direction deviation is equal to or more than a second threshold value larger than said first threshold value (or exceeds the second threshold value) continues for a second duration shorter than said first duration. 12. A control method for a wind turbine generator system comprising: a step of measuring a wind direction; a step of calculating a wind direction deviation from said measured wind direction and a direction of said wind turbine rotor; and ■ - 35 - a step of deciding which of first and second conditions is a present wind condition, the first condition being a condition where a wind direction randomly changes with high degree of randomness and the second condition being a condition where a wind direction gradually changes over a long time; and a step of performing a yaw rotation of said nacelle by said rotation mechanism such that a sign of said wind direction deviation, is reversed between before and after the yaw rotation and the absolute value of said wind direction deviation'after the yaw rotation is larger than zero and equal to or less than said first threshold value, when said control apparatus decides said present wind condition is said second condition and an absolute value of said wind direction deviation is equal to or more than a first threshold value larger than a first threshold value predetermined (or exceeds the tirst threshold value) continues for a first duration predetermined. 13. A control method for a wind turbine generator system comprising: a step of measuring a wind direction; a step of calculating a wind direction deviation from the wind direction measured by said wind direction measurement apparatus and a direction of said vjind turbine rotor; and ■ ■} ■ a step of performing.a yaw rotation of said nacelle. by said rotation mechanisiri such that said wind direction deviation is reduced to zero, when.said' wind direction deviation satisfies^ a predetermined condition for every time t of tn-T < t < to/ where to is a present time and -T is a predetermined ^value, and ■ ■. wherein said predetermined condition is ue (t) I > eTH(t), or , . ■ ' I Ae(t) ! > eTH(t), ■ ■ ■■ ■ ■ : where {A0(t) I is an absolute value of said wind-direction deviation for the time t, and Ginlt)- is ,a function monotonically non-decreasing in tu-T ^ t < to.