Abstract:
A wind power excitation synchronous generation system having a maximum power determining unit and a control method thereof are disclosed. In this control method, dual input shafts and a single output shaft of a gear transmission mechanism are used, and two kinds of inputted power, such as wind power and servo motor control power, are integrated, so as to allow the output shaft to drive an excitation synchronous generator to generate electric power. In this system, a rotation speed and a phase of a servo motor are controlled, so as to allow the excitation synchronous generator to output the electric power with a frequency and a phase identical to the utility grid.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part application of U.S. patent application Ser. No. 13/345,706, filed on Jan. 7, 2012, the disclosure of which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a wind power excitation synchronous generation system and a control method thereof, and more particularly, to a control method for driving the generator at a constant speed, stable voltage, frequency, and a phase equal to the utility grid by using a motor servo control and an excitation current control of the excitation synchronous generator. 
       BACKGROUND OF THE INVENTION 
       [0003]    Generally, in a wind power generation system with a permanent magnet generator or an induction generator, a wind power is transmitted by using a transmission mechanism to transmit a rotational power to a generator. A rotation speed and a torque of the generator are determined according to the magnitude of the wind power. Therefore, the rotation speed thereof is required to be limited for ensuring that the rotation speed varies in a specific range. When the wind power is higher or lower than a standard range, the generator is turned off until the wind power is in the standard range. In this passive power generation system, an AC-to-DC converter and a DC-to-AC converter are required to output the wind power. However, this converting method will result in a power loss of the power conversion, hence deteriorating a power conversion efficiency and increasing the cost of the generation equipment. 
         [0004]    Besides, in the induction wind generator, when the inputted power is altered, or when the load of the utility grid is raised, the induction generator cannot control the excitation current thereof. Thus, when the power required for a load terminal is increased, a voltage from an output terminal of the generator cannot be constant, resulting in a reduction of an output power quality. 
       SUMMARY OF THE INVENTION 
       [0005]    Therefore, an aspect of the present invention is to provide a control method of an excitation synchronous generator for maximum power tracking. With use of a motor servo control and an excitation current control of the excitation synchronous generator, a rotation speed of a transmission mechanism can be adjusted. When an input rotation speed is too high or low due to a variation of a power source, such as a wind power, the motor servo control is used, so as to allow the transmission mechanism to rotate at a constant rotation speed, and to control the phase thereof. Therefore, the excitation synchronous generator can be rotated at a constant speed for stably outputting power with a frequency and a phase. Moreover, a maximum power determining unit can integrate an reference wind power and a fine-tuning power for determining a power command, and can feed back an output power of the synchronous generator for generating an excitation current command to control an output voltage and a current of the excitation synchronous generator, so as to allow the excitation synchronous generator to obtain the maximum power. 
         [0006]    In the present invention, with use of the motor servo control for frequency stabilization and an excitation current control of the excitation synchronous generator for maximum power tracking, when the wind power of the power generation system varies, the output of the transmission mechanism can be stabilized for controlling the voltage, frequency and phase thereof. Furthermore, by using a power feedback and an excitation current control, the power generation system can generate the maximum power to a utility grid. 
         [0007]    According to a preferred embodiment of the present invention, the control method of the wind power excitation synchronous generation system comprises the following steps: detecting an output voltage, a current and a power of the excitation synchronous generator; controlling an excitation current of the excitation synchronous generator according to the output voltage, the current and the power, so as to allow the excitation synchronous generator to output a power to a utility grid; and performing a servo control of a motor according to an information of an encoder, so as to allow a transmission mechanism to drive the excitation synchronous generator at a predetermined speed, thereby generating a three-phase alternating-current (AC) power supply with a phase equal to the utility grid, wherein the three-phase AC power supply is allowed to be connected to the utility grid in parallel. 
         [0008]    According to another embodiment of the present invention, the wind power excitation synchronous generation system comprises: a wind power; an excitation synchronous generator; a transmission mechanism configured to use the wind power to drive the excitation synchronous generator; an excitation controlling unit configured to provide an excitation current signal to the excitation synchronous generator, so as to allow the excitation synchronous generator to output an electric power to the utility grid; a motor configured to control the driving of the transmission mechanism; a digital signal processing controller configured to determine a duty cycle width of a pulse width modulation (PWM) controlling unit according to a phase information of the utility grid and a position information of an armature of the excitation synchronous generator; and a power driving inverter configured to receive a power switch timing transmitted form the PWM controlling unit for driving the motor. 
         [0009]    According to a preferred embodiment of the present invention, a control method of a wind power excitation synchronous generation system for generating power from a wind power comprises the following steps of:
       detecting an output voltage, a current and a power of an excitation synchronous generator by using a power detector, so that a real-time output power information is obtained;   using a motor to control a driving of a transmission mechanism, wherein the transmission mechanism is connected between the wind power and the excitation synchronous generator for using the wind power to drive the excitation synchronous generator, an encoder transmits a position information of the excitation synchronous generator to a digital signal processing controller, and the digital signal processing controller uses a phase detector to obtain a phase information of a utility grid as a present position command for comparing with the position information of the excitation synchronous generator, so as to determine a duty cycle width of a PWM controlling unit, and to output a power switch timing to the power driving inverter for driving the motor; performing a position servo control of the motor according to the position information of the excitation synchronous generator from the encoder, so as to allow the transmission mechanism to drive the excitation synchronous generator at a predetermined speed, thereby generating a three-phase alternating-current (AC) power to the utility grid in parallel;   using a maximum power determining unit to determine a predetermined power according to a reference wind power and a fine-tuning power, wherein the reference wind power is determined according to a wind speed, the fine-tuning power is determined by detecting a motor input current, and a power command for tracking the predetermined power is generated according to a sum of the reference wind power and the fine-tuning power; and   adjusting an excitation current of the excitation synchronous generator, so as to allow the excitation synchronous generator to output the predetermined power to the utility grid;   wherein the real-time output power information is fed back to a power controller to compare with the power command, and the power controller is used for generating an excitation current command to an excitation controlling unit, and the excitation controlling unit is used for generating the excitation current for controlling an excitation field of the excitation synchronous generator.       
 
         [0015]    In one embodiment of the present invention, the control method further comprises the following steps: when the wind power decreases, the motor input power is a positive compensatory power for maintaining the excitation synchronous generator at the predetermined speed, and simultaneously adjusting an excitation controlling unit for reducing the excitation current to ensure that the excitation synchronous generator fully absorbs the wind power and converts the wind power into the utility grid, hence reducing a power consumption which is used to drive the excitation synchronous generator by the motor. 
         [0016]    In one embodiment of the present invention, the control method further comprises the following steps: when the wind power increases, the motor input power is a negative compensatory power for maintaining the excitation synchronous generator at the predetermined speed, and simultaneously adjusting an excitation controlling unit for raising the excitation current to ensure that the excitation synchronous generator fully absorbs the wind power, and converts the wind power into the utility grid, hence reducing a power consumption which is used to drive the excitation synchronous generator by the motor. 
         [0017]    According to a preferred embodiment of the present invention, a wind power excitation synchronous generation system, comprises:
       a wind power;   an excitation synchronous generator;   a transmission mechanism connected between the wind power and the excitation synchronous generator and configured to use the wind power to drive the excitation synchronous generator;   an excitation controlling unit configured to provide an excitation current to the excitation synchronous generator, so as to allow the excitation synchronous generator to output an electric power to a utility grid;   a motor configured to control the driving of the transmission mechanism;   a digital signal processing controller configured to determine a duty cycle width of a pulse width modulation (PWM) controlling unit according to a phase information of the utility grid and a position information of an armature of the excitation synchronous generator, wherein the duty cycle width is determined by comparing the phase information of the utility grid and the position information of the armature of the excitation synchronous generator; and   a power driving inverter configured to receive a power switch timing transmitted from the PWM controlling unit for driving the motor, wherein the power switch timing is determined according to the duty cycle width of the PWM controlling unit;   a maximum power determining unit configured to determine a power of the wind power excitation synchronous generation system according to a reference wind power and a fine-tuning power, wherein the reference wind power is determined by a wind speed, the fine-tuning power is determined by a motor input current, a power command for tracking the power is provided to the excitation synchronous generator according to a sum of the reference wind power and the fine-tuning power, and the excitation current is controlled and applied to the excitation synchronous generator to track the power; and   an encoder configured to transmit the position information of the armature of the excitation synchronous generator to the digital signal processing controller;   wherein the power command is generated by the maximum power determining unit, the maximum power determining unit uses a power detector to obtain a real-time output power information from an output terminal of the excitation synchronous generator, and the real-time output power information is fed back and compared with the power command, and a power controller is used to generate an excitation current command to the excitation controlling unit for generating the excitation current.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
           [0029]      FIG. 1  is a schematic diagram showing a system using the control method of a wind power excitation synchronous generation system according to one embodiment of the present invention; 
           [0030]      FIG. 2  is a block diagram showing the control method of a wind power excitation synchronous generation system according to the embodiment of the present invention; and 
           [0031]      FIG. 3  shows the experimental results according to the embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0032]    In order to make the illustration of the present invention more explicit and complete, the following description is stated with reference to  FIG. 1  through  FIG. 3 . 
         [0033]    In the drawings, like reference numerals indicate like components or items. 
         [0034]    Referring to  FIG. 1 , a schematic diagram showing a system using the control method of a wind power excitation synchronous generation system according to an embodiment of the present invention is illustrated. The method of the present invention can be applicable to a wind power generation system which is described below. However, the method may be applicable to other power systems, such as waterpower, firepower and tidal power system, but not limited to the above description. The present invention can be used for a control technique of a renewable energy relating to any power generation systems. 
         [0035]    Referring to  FIG. 1  again, the generation system of the present invention can comprise a wind power  10 , a transmission mechanism  20 , an excitation synchronous generator  30 , a utility grid  40 , a power driving inverter  50 , a pulse width modulation (PWM) controlling unit  51 , a motor  60 , an encoder  61 , a current detector  62 , a excitation controlling unit  70 , a voltage-current-power detector  71 , a phase detector  72  and a digital signal processing controller  80 . 
         [0036]    Referring to  FIG. 1  again, when the wind power  10  is inputted, the transmission mechanism  20  drives the excitation synchronous generator  30  to work by inputting the wind power  10 . The excitation controlling unit  70  provides an excitation current, so as to allow the excitation synchronous generator  30  to generate an electrical power outputted to the utility grid  40 . 
         [0037]    Referring to  FIG. 1  again, the encoder  61  transmits a position information of the excitation synchronous generator  30  to the digital signal processing controller  80 . The digital signal processing controller  80  uses the phase detector  72  to obtain a phase information of the utility grid as a present position command for comparing with the position information of an armature of the excitation synchronous generator  30 , so as to determine a duty cycle width of the PWM controlling unit  51 , and to output a power switch timing to the power driving inverter  50  for driving the motor  60 . With use of the position servo control of the motor, the transmission mechanism  20  can drive the excitation synchronous generator  30  at a constant speed. Therefore, the frequency of the voltage outputted by the generator  30  can be stable, and the phase of the outputted voltage is equal to the utility grid. When the excitation synchronous generator  30  works, a signal fed from the voltage-current-power detector  71  is used to detect the voltage, current and power of the excitation synchronous generator  30 . According to an information of the voltage-current-power detector  71 , the digital signal processing controller  80  can provide an excitation current control to excitation controlling unit  70  for adjusting an excitation current of the generator, so as to allow the generator to output a constant voltage and current. 
         [0038]    Referring to  FIG. 1  again, when the wind power  10  decreases, and the rotation speed of the transmission mechanism  20  slows down, for maintaining the rotation speed thereof, the digital signal processing controller  80  can adjust the PWM controlling unit  51  according to the information of the encoder  61  and the current detector  62 , so as to adjust a duty cycle of the motor and drive the motor  60  to follow the position command which is fed back by the phase detector  72  for reducing the position error and maintaining the excitation synchronous generator  30  at a constant rotation speed. At the same time, the digital signal processing controller  80  can adjust the excitation current provided by the excitation controlling unit  70  for reducing the excitation current to the generator. 
         [0039]    Referring to  FIG. 1  again, when the wind power  10  increases, and the rotation speed of the transmission mechanism  20  speeds up, for maintaining the rotation speed thereof, the digital signal processing controller  80  can adjust the PWM controlling unit  51  according to the information of the encoder  61  and the current detector  62 , so as to adjust the duty cycle of the motor and drive the motor  60  to follow the position command which is fed back by the phase detector  72  for reducing the position error and maintaining the excitation synchronous generator  30  at a constant rotation speed. At the same time, the digital signal processing controller  80  can adjust the excitation current provided by the excitation controlling unit  70  for raising the excitation current to the generator. 
         [0040]    Referring to  FIG. 2 , the power generation system further comprises a maximum power determining unit  81  which is used to determine a predetermined power of the power generation system according to a reference wind power P W (V W ) and a fine-tuning power ΔP(I m ), i.e. P W (V W )+ΔP(I m ). The reference wind power P W (V W ) is determined according to a wind speed (V W ). For raising the efficiency of the power generation system, the outputted power thereof is required to follow the wind power, and it is also required to reduce the power used by the motor, so as to achieve a constant speed control. Therefore, a motor input current (I m ) comprising three-phase currents I m , I v , and I w  is detected to obtain a motor input power P m , and a motor command P mc  is used for comparing with the motor input power P m  to determine the fine tuning power ΔP(I m ) through a proportional-integral controller  83  (PI controller). Preferably, the motor command P mc  is zero, therefore, the fine tuning power ΔP(I m ) can be determined directly according to the motor input power P m , i.e. the motor input current (I m ). A power command P −  for tracking the predetermined power is provided to the generator according to the sum of the reference wind power P W (V W ) and the motor fine tuning power ΔP(I m ). 
         [0041]    Referring to  FIG. 2  again, the power command P* is generated by the maximum power determining unit  81 . The maximum power determining unit  81  uses the power detector  71  to obtain three-phase voltages (E R , E S , E T ), and currents (I R , I S , I T ) of the excitation synchronous generator  30  and generate a real-time output power information P 0  from the output terminal of the excitation synchronous generator  30 , and feeds back this information P 0  to a power controller  82  to compare with the power command P*. The power controller  82  can generate an excitation current command I E * to the excitation controlling unit  70  according to the real-time output power information P 0  and the power command P*, and thus the excitation controlling unit  70  can generate an excitation current I E  for controlling an excitation field of the excitation synchronous generator  30 , and the excitation current I E  can be controlled to generate the predetermined power to the utility grid  40 . 
         [0042]    Furthermore, when the wind power decreases, the motor input power P m  is a positive compensatory power for maintaining the excitation synchronous generator  30  at the predetermined speed, and simultaneously adjusting the excitation controlling unit  70  for reducing the excitation current I E  to ensure that the excitation synchronous generator  30  fully absorbs the wind power and converts the wind power into the utility grid  40 , hence reducing a power consumption which is used to drive the excitation synchronous generator  30  by the motor  60 . 
         [0043]    In addition, when the wind power increases, the motor input power P m  is a negative compensatory power for maintaining the excitation synchronous generator  30  at the predetermined speed, and simultaneously adjusting the excitation controlling unit  70  for raising the excitation current I E  to ensure that the excitation synchronous generator  30  fully absorbs the wind power, and converts the wind power into the utility grid  40 , hence reducing a power consumption which is used to drive the excitation synchronous generator  30  by the motor  60 . 
         [0044]    Referring to  FIG. 3 , which shows the experimental results for demonstrating the wind power excitation synchronous generation system and the control method according to the embodiment of the present invention. The system is assumed to have a stable point at a wind power of 2500 W in the time period beginning in  FIG. 3 . However, the measured generator power output is only 2000 W. The generator output power is about 500 W less than the wind power, which is mainly due to the power consumptions of mechanical friction and moment of inertia at rotation speed of 1800 rpm, and the partial power loss comes from power conversion efficiency of the generator. According to  FIG. 3 , the wind is sine wave change; the system converts the wind power into electricity. A slight amount of power provided by the servo motor can maintain the generator shaft speed constantly (1800 rpm) and achieve excellent power quality. 
         [0045]    In a natural environment, the wind power varies with time. In order to stabilize the output voltage, output current and output power of the generator, the output power thereof has to track the input power variation and react immediately by adjusting the excitation current. The present invention provides a wind power excitation synchronous generation system having a maximum power determining unit and a control method thereof can minimize the consumption of the servo motor power, and most of the input power (wind power) can be transferred to the utility grid by the generator, as shown in  FIG. 3 . 
         [0046]    As is understood by a person skilled in the art, the foregoing embodiments of the present invention are strengths of the present invention rather than limiting of the present invention. It is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims, the scope of which should be accorded the broadest interpretation so as to encompass all such modifications and similar structures.