Mechanical regulation of electrical frequency in an electrical generation system

There is provided an electrical generation system for producing an alternating electric current with a regulated frequency from motive power with variable speed. The rotor of an alternator is mechanically coupled to the motive power and thus rotates with a variable speed. In order to compensate for the rotor speed variation, the alternator stator is rotated about the rotor such that the relative speed between the stator and the rotor is regulated. The stator speed is controlled such that the frequency of the produced alternating current is regulated.

TECHNICAL FIELD

The invention relates to the electrical generators and more specifically to the production of an alternating electric current with regulated frequency from a motive power with variable speed.

BACKGROUND

An electrical generator produces alternating current from a motive power typically produced by the rotation of a prime mover such as a gas turbine, a water turbine or a wind turbine for example. When the electrical generator is used as a power production generator to be connected to a large power grid, the frequency of the produced alternating current must match the utility frequency of the power grid. The power production generator will need to be synchronized with the power grid before it is connected.

In typical electrical generators, the rotor is mechanically coupled to the prime mover such that when the speed of the prime mover varies, so does the frequency of the generated electric current. In cases where the speed of the prime mover may vary, such as with typical wind turbines, the speed of the rotor needs to be regulated or the frequency variation of the produced electric current to be corrected.

One solution is to use power electronics to correct the frequency of the produced electric current. A high-power rectifier is used to rectify the produced variable-frequency alternating current to provide direct current. Direct current is then converted back to alternating current with the required frequency using a high-power inverter (AC to DC to AC conversion). Power electronics is energy consuming and decreases the overall efficiency of the electric generator.

The rotation speed of the turbine may also be regulated by adjusting the opening of the supply valve in the case of a water turbine or by adjusting the angle of attack of the blades in the case of a wind turbine. However, the angle of attach often cannot be adjusted with a sufficient time response in cases of a gust of wind.

SUMMARY

There is provided an electrical generation system for producing an alternating electric current with a regulated frequency from motive power with variable speed. The rotor of an alternator is mechanically coupled to the motive power and thus rotates with a variable speed. In order to compensate for the rotor speed variation, the alternator stator is rotated about the rotor such that the relative speed between the stator and the rotor is regulated. The stator speed is controlled such that the frequency of the produced alternating current is regulated.

According to one aspect, there is provided a method for producing an alternating electric current with a regulated frequency from a prime mover having a variable speed. The method comprises: actuating an alternator rotor by transmitting a rotation motion of the prime mover to the rotor, a rotation speed of the rotor varying with the variable speed of the prime mover; producing the alternating current by the rotation of the rotor relative to an alternator stator, a frequency of the alternating current being given by a relative speed between the rotor and the stator; rotating the stator relative the rotor to regulate the relative speed between the rotor and the stator, the rotor and stator rotating about a common axis; and controlling the rotation of the stator to maintain the frequency to the regulated frequency while the rotation speed of the rotor varies.

According to another aspect, there is provided an electrical generation system for producing an alternating electric current with a regulated frequency from a prime mover having a variable speed. The system comprises an alternator having a rotor and a rotative stator, mounted concentrically from one another about a rotation axis, the rotor to be mechanically coupled to the prime mover such that a rotation speed of the rotor varies with the variable speed of the prime mover, an electromagnetic interaction between the rotor and the stator upon a relative rotation motion of the rotor to the stator producing the alternating current in the stator, the frequency of the alternating current being given by a relative speed between the rotor and the stator. The system further comprises an auxiliary machine drivingly connected to the stator to drive a rotation of the stator, and a controlling unit connected to the auxiliary machine for controlling the rotation of the auxiliary machine and thereby of the stator to regulate the relative speed between the rotor and the stator while the rotation speed of the rotor varies, thereby regulating the frequency.

According to another aspect, there is provided an electrical generation system for producing an alternating electric current synchronised with a power grid to which it is to be connected, from a prime mover having a variable speed. The system comprises an alternator having a rotor and a rotative stator mounted concentrically from one another about a rotation axis, the rotor to be mechanically coupled to the prime mover such that a rotation speed of the rotor varies with the variable speed of the prime mover, an electromagnetic interaction between the rotor and the stator upon a relative rotation motion of the rotor to the stator producing the alternating current in the stator, an alternator synchronous speed being defined by a relative speed between the rotor and the stator. The system further comprises an auxiliary machine drivingly connected to the stator to drive a rotation of the stator, and a controlling unit connected to the auxiliary machine for controlling the rotation of the auxiliary machine and thereby of the stator to regulate the alternator synchronous speed to the power grid synchronous speed required by the power grid while the rotation speed of the rotor varies.

According to another aspect, there is provide a method for producing an alternating electric current synchronised with a power grid to which it is to be connected, from a prime mover having a variable speed. The method comprises: actuating an alternator rotor by transmitting a rotation motion of the prime mover to the rotor, a rotation speed of the rotor varying with the variable speed of the prime mover; producing the alternating current by the rotation of the rotor relative to an alternator stator, an alternator synchronous speed being defined by a relative speed between the rotor and the stator; rotating the stator relative the rotor to regulate the relative speed between the rotor and the stator, the rotor and stator rotating about a common axis; and controlling the rotation of the stator to maintain the alternator synchronous speed to a power grid synchronous speed required by the power grid while the rotation speed of the rotor varies.

DETAILED DESCRIPTION

FIG. 1illustrates an electrical generation system100for producing alternating electric current84with a regulated frequency from motive power with variable rotation speed. A synchronous alternator10having a rotor12and a rotative stator14mounted concentrically from one another is mechanically coupled to a prime mover through the rotor shaft18such that rotation of the prime mover drives the rotation of the rotor12. As the rotation speed of the prime mover varies, so does the rotation speed of the rotor12. The alternator10is typically a three-phase brushless alternator with a permanent magnet rotor12and a four-pole electrical winding stator14. The principles presented herein can also be applied to other alternators such as single-phase or four-phase alternators for example. It is noted that the term stator is used herein by analogy to conventional alternators in which the stator is fixed, i.e. static. In the embodiments presented herein the stator14has an electric function which is in all aspects similar to conventional stators, but for the fact that it is allowed to rotate. The stator14is mechanically coupled to a rotative stator shaft20that rotates with the stator14. Slip ring connectors16located on the stator shaft20allows the electric current produced in the electrical windings of the stator14to be collected while the stator14rotates. As will be explained below, the stator14is allowed to rotate in both directions about its rotation axis.

Rotation of the prime mover drives the rotation of the rotor12and the electromagnetic interaction between the rotor12and the stator14generates an alternating electric current84in the electrical windings of the stator14. The frequency of the alternating current is related to the relative rotation speed between the rotor12and the stator14.

By controlling the rotation of the stator14about the rotor12, the relative speed, and thereby the frequency of the generated electric current, can be regulated. For example, in a typical wind turbine generator, a 60-Hz alternating current is generated in a four-pole three-phase alternator that rotates at 1800 rotations per minute (rpm). When the wind is strong, the speed of the prime mover, i.e. the wind turbine, may rotate faster, at 2000 rpm for example. In order to compensate for such a higher rotation speed of the rotor12, the stator14is rotated at 200 rpm in the direction of rotation of the rotor. The relative speed between the rotor12and the stator14is thus 1800 rpm [2000 rpm−200 rpm=1800 rpm]. If the speed of the rotor12decreases due to weak winds for example, e.g. at 1500 rpm, the stator14is rotated at 300 rpm in the direction opposite to the rotor12. The relative speed is thus 1800 rpm (1500 rpm+300 rpm=1800 rpm).

Rotation of the stator is driven by an auxiliary electric machine40which is a synchronous machine with a rotor42and a stator44. The stator44of the electric machine40is however static, i.e. it is not allowed to rotate. The central shaft46of the rotor42is drivingly connected to the rotative stator14through its shaft20to mechanically drive its rotation. In the examples illustrated herein, the rotor shaft46and the stator shaft20are connected using a belt and pulleys arrangement (seeFIGS. 2 and 3) but it is noted that a roller chain and sprocket arrangement, a gear arrangement or any other power transmission arrangement50may also be used. The electric machine40comprises a variable speed drive62that is used to energize the stator windings in such a manner that the rotation speed of the electric machine40can be controllably varied. The variable speed drive62receives a control signal76from a controlling unit60and energizes the electric machine40accordingly. The variable speed drive62also receives feedback from an encoder66which senses the rotor position, or the rotor speed, in the electric machine40. The controlling unit60is used in a closed loop configuration to control the rotation of the electric machine40and consequently of the rotative stator14to regulate the relative speed between the rotor12and the stator14, thereby regulating the frequency of the produced alternating current84.

In the illustrated system100, the controlling unit60receives a feedback signal72from an encoder64which senses the position, or the speed, of the rotor12in order to control the rotation speed of the stator14. In this case, the encoder64is positioned on the rotor12to sense the position, and thereby the speed, of the rotor12. The controlling unit also reads the produced alternating current84as a feedback. From the received feedback signal72and/or alternating current84, the controlling unit60produces the control signal76which is inputted to the variable speed drive62to control the rotation of the electric machine40and thereby of the stator14. As will be described below, the controlling unit60may use feedback from the feedback signal72, the reading of the alternating current84, or a combination of both. The controlling unit60can be provided as a programmable logic controller, a computer or any other processing unit for example. As described herein below, the control of the electric machine40can be performed in speed or in torque.

The variable speed drive62is typically powered using the electric current84produced by the alternator10and the frequency regulation consequently consumes part of the produced power but the total balance of produced electric power remains positive.

It is noted that the encoder64may sense, the relative position, or speed, between the rotor12and the stator14as well. Accordingly, in another embodiment, the encoder64senses the relative position between the rotor12and the stator14to produce the feedback signal72.

FIGS. 2 and 3show the mechanical components of the electrical generation system100ofFIG. 1.FIG. 4shows the mechanical components mounted in a nacelle200of a wind turbine300. The rotor12and stator14are mounted in a cylindrical casing80. Both rotor12and stator14are mounted to be rotatable about a common rotation axle. The rotor shaft18and the stator shaft20are mounted inline one at the end of the other, a proximate end of the stator shaft20embracing a proximate end of the rotor shaft18with a rotary bearing joint90in-between, thereby allowing both shafts18and20to rotate from one another and about the common rotation axle. The distal end of the stator shaft20is mounted about a first end of the casing80using rotary bearings94and the distal end of the rotor shaft18is mounted about a second end of the casing80using rotary bearings92. Each of the rotor and stator shafts18and20is then allowed to rotate independently about the casing80. The rotor12is mounted concentrically over the rotor shaft18. The stator14is mounted concentrically outside of the rotor12, the stator14comprising stator windings86supported by a cylindrical stator frame82. A first end of the stator frame82is fixed over the stator shaft20at junction of the rotor and stator shafts18and20, and is rotatively mounted about the second end of the casing80using rotary bearings96on its second end. The stator frame82rotates with the stator shaft20and the stator windings86are fixed to the interior of the stator frame82such that they are located in close relationship with the rotor12for electromagnetic interaction. The stator windings86are electrically connected to the slip rings16affixed to the stator shaft20between bearings90and94. Brushes (not shown) are used to collect the electric current produced in the stator windings and available on the slip rings16.

Inside the stator frame82, an exciter generator102is also mounted besides the rotor12on the rotor shaft18. A rotor portion104of the exciter generator102is affixed to the rotor shaft18, and a stator portion106of the exciter generator102is affixed to the stator frame.

The distal end19of the rotor shaft18extending outside the casing80is mechanically coupled to the prime mover (not shown).

The electric machine40is mounted in a casing48affixed on top of the casing80using assembling means98comprising brackets and bolts such that the output shaft47of the electric machine40is aligned in parallel relationship with the stator shaft20. The output shaft47of the electric machine40and the stator shaft20are drivingly connected using a timing belt56and pulleys52and54. The pulley52is fitted to the distal end of the stator shaft20extending outside the casing80and the pulley54is fitted to the output shaft47of the electric machine40such that both pulleys52,54are vertically aligned from one another. The timing belt56links the two pulleys52,54for one to drive the other.

The following describes the operation of the electrical generation system100when connected to a large power grid. When a synchronous alternator is connected to a large power grid, the power grid should be considered as infinitively large since such a power grid is made up of hundreds of alternators and submitted to thousands of charges. The power grid thus fixes a voltage, a frequency and a phase. Accordingly, as the alternator10is connected to the power grid, the voltage E0at the stator is given by the voltage of the power grid Eb, i.e. both voltages are equal in magnitude value and in phase. The alternator10still requires to be synchronized with the power grid so that it produces useful electric power.

The electric power produced by a synchronous alternator is given by:

P=E0⁢EbXs⁢sin⁢⁢δ,(1)
where Xsis the synchronous reactance per phase of the alternator10and δ is the electric phase between the rotor electric field and the stator electric field. The rotor electric field is given by the position of the rotor in the alternator and the stator field is given by the phase of the voltage Ebin the case of a static stator and by a combination of the phase of the voltage Eband the stator position in the case of a rotative stator. According to equation (1), the maximum power produced by the alternator should be 90° but for stability reasons, the nominal electric phase is fixed to 30°.

In a 60-Hz four-pole alternator the rotation speed of the stator electric field, which is also called the synchronous speed, is 1800 rpm. In a synchronous alternator, the rotation speed of the stator field should be equal to the rotation speed of the rotor field so that the stator and rotor field are stationary relative to one another and so that the nominal electric phase is maintained. In a mechanical point of view, the synchronous speed is given by:
nsync=nrotor−nstator,  (2)
where nsyncis the synchronous speed, nrotoris the rotor rotation speed and nstatoris the stator rotation speed. Since the stator field is governed by the power grid to which it is connected, in order for the stator and rotor field to be stationary relative to one another, we should have:
nstator=nrotor−nsync,  (3)
where a negative value of nstatoris for a stator that rotates in a direction opposite to the direction of the rotor.

Accordingly, in a conventional alternator, the stator is fixed while the rotor rotates. Consequently, in a 60-Hz four-pole alternator, the rotor speed should be held constantly to 1800 rpm. As explained above, in the configuration presented herein, in order to maintain a synchronous speed at 1800 rpm when the rotor speed is 1650 rpm, the stator is rotated at 150 rpm in the opposite direction so as to maintain a relative speed of 1800 rpm. When the rotor speed is 1800 rpm, the stator is held mechanically stationary. When the rotor speed is 1950 rpm, the stator is rotated at 150 rpm in the same direction.

Feedback control loop illustrated inFIG. 1is based both on the monitoring of the rotation speed of the rotor12in the alternator10using the encoder64and the monitoring of the produced alternating current84. Before connecting the alternator10to the power grid, the system100should be synchronized. A synchroscope (not shown) will allow the power grid connection only when the alternator10is synchronized with the power grid. In order to synchronise the electric generation system100to the power grid when planning a connection, the control unit60receives a feedback signal72which represents the rotation speed of the rotor14and provides a control signal76in speed to the variable speed drive62. In this stage, feedback from the produced alternating current84is not used and the control of the auxiliary electric machine40is performed in speed. The synchroscope connects the alternator10to the power grid when synchronisation conditions are met.

Thereafter, the tension and frequency of the electric power produced by alternator10are fixed by the power grid. The control of the auxiliary electric machine40then switches in torque instead of speed, i.e. the control signal76is applied in torque instead of speed. According to Newton's reaction law, the torque generated by the rotor12(action) is equal in magnitude but opposite in direction to the torque applied to the stator14. Electric power produced by the alternator10is directly related to the rotor torque and thereby to the stator torque. The rotor torque generated by a wind turbine for example is quite variable since it is subject to the wind fluctuations. The control signal76acting on the torque applied to the stator minimizes the impact of rotor torque fluctuations on the alternator10. The controlling unit60therefore optimises the torque generated by the rotor12by adjusting the torque applied by the auxiliary electric machine40to the stator14. In this stage, the control unit60primarily uses feedback from the produced alternating current, but feedback from the feedback signal72may still be used for diagnosis or other monitoring functions. The control unit60uses an algorithm that adjusts the torque applied to the auxiliary electric machine40so as to that maximises the power of the produced alternating current84, i.e. the control unit60uses a maximum power searching algorithm based on feedback from the produced alternating current84.

It is noted that the resultant of the latter control scheme is that the relative rotation between the rotor12and the stator14will be regulated to the synchronous speed fixed by the power grid and that the frequency of the produced electric current will be maintained to a desired nominal frequency of the power grid while the rotation speed of the rotor varies.

It is noted that in another embodiment, the auxiliary electric machine40remains controlled in speed after connection to the power grid. In still another embodiment, the control unit60uses feedback from the encoder64only, even after connection to the power grid. Other control schemes are also possible.

It is also noted that while a synchronous alternator10is used in the generation system100, an asynchronous alternator may also be used. The synchroscope may then be omitted.

FIG. 5illustrates another example of an electrical generation system200in a configuration allowing reclaiming of an electrical power generated in the auxiliary electric machine40when the rotor speed is above the synchronous speed. Most components are equivalent to the corresponding components of the system100ofFIG. 1and the description of like elements will therefore not be repeated. The variable speed drive62of the system10is replaced by a variable speed drive/regenerator262in the system200. When the rotor speed is below the synchronous speed, the variable speed drive/regenerator262works as a variable speed drive and when the rotor speed is above the synchronous speed, the variable speed drive/regenerator262works as a regenerator. The variable speed drive/regenerator262then receives the electric current produced by the auxiliary electric machine40and converts its frequency in order to reclaim the produced auxiliary electric current to the power grid.

As described above, when the rotor speed is above the synchronous speed, the stator14is rotated in the direction of the rotor12to regulate the relative speed between the rotor12and the stator14to the synchronous speed. In fact, the electromagnetic interaction between the rotor12and the stator14drags the stator14to effectively rotate in the direction of the rotor12. Accordingly, the electric machine40which is then driven by the rotation of the stator14acts as an alternator and produces electric current. The produced auxiliary electric current can be output as an auxiliary source of electric power. It is however noted that the produced auxiliary electric current is not necessarily synchronised with the frequency of the power grid. Before being connected to the power grid, the variable speed drive/regenerator262rectifies and inverts the auxiliary electric current to the nominal frequency of the power grid.

It is noted that, in another embodiment, a separate inverter is used instead of a variable speed drive/regenerator for reclaiming the electrical power generated in the auxiliary electric machine40.

It should be noted that the principles presented herein are especially useful in the case of wind turbine generators but may also find applications in other types of generators such as water turbine generators for example.

The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without departing from the scope of the invention disclosed. For example, the described generator can be adapted of any type of electrical generator including water turbine and gas turbine generators. The stator windings and rotor permanent magnets can also be interchanged to provide an electrical winding rotor12and a permanent magnet rotative stator14be a permanent magnet stator. Slit rings should then be used on the rotor12instead of the stator14. The auxiliary electric machine may also be replaced by any motor such as a hydraulic motor for example. The embodiments described above are intended to be exemplary only. The scope of the invention is therefore intended to be limited solely by the appended claims.