Excitation Control Of Asynchronized Synchronous Generator And Stability Analysis Of The Control System

As an advanced power generation, an asynchronized synchronous generator (ASG) has multiphase windings in the rotor. The rotor speed of ASG is adjustable when the rotor windings are excited by variable frequency alternating current. According to the methods of excited current control, there are two operating modes: generalized synchronous operating mode and generalized asynchronous operating mode. When the ASG operates at generalized asynchronous operating mode, the frequency of excited current is decided by the rotor speed and the generator can produce constant frequency even though rotor speed varies. When the ASG operates at generalized synchronous operating mode, the frequency of excited current is determined by an AC excited device and the rotor speed is dependent on the frequency of the alternating current to keep the MMF of stator and the MMF of rotor relatively stillness and to change the rigid linkage into flexible coupling. A power electronic device provides the excited alternating current of the machine. The machine can be operated at adjusting rotor speed, or the active power and the reactive power of the machine can be controlled independently by controlling the amplitude, the frequency, the phase angle and the phase sequence of the excited alternating current. When applied to power system, this generator can enhance the dynamic stability of the power system if appropriate control strategy is adopted.Owing to the advantage of adjustable rotor speed, the ASG can increase the efficiency of the generating system and damp the vibrancy, decrease gas erode and friction losses of turbine when an ASG is applied to hydropower generating stations. Applied to wind turbine system, wind turbine speed is adjusted as a function of wind speed to maximize output power. Operating at the maximum power point can be realized over wide power range. So, ASG can improve system efficiency and reduce mechanical stresses because gusts of wind can be absorbed.Excited alternating current control is the key of the ASG operation. The fuzzy-PID control technology is applied to the ASG system. The theoretic analysis and experimental researches of the control system are taken in the paper. This paper analyzes the stability of the control system alsoThis paper derives the ASG's basic mathematic modes when the rotor is three-phase symmetrical windings, two-phase windings in detail, and gives the basic per-unit equations in dqo coordinates. The equations are the base of research in strategy of exciting control, analysis of static stability and the stability of exciting control system.The basic equations, equivalent circuit, phase-vector diagram and characteristics of the ASG under steady operation are discussed in the paper. Distinct from a synchronous generator and variable frequency timing of induction motor, the power flow of ASG's rotor windings varies with different operating modes. The power flow of the rotor windings is discussed in this paper.The active power of the rotor is absorbed when the machine operates at sub synchronous speed. However, when the machine operates at supper synchronous speed, the active power of the rotor may be absorbed or output. A critical slip is introduced. Analysis indicates that the active power of the rotor is absorbed from excited device when the slip is greater than critical slip and is feedback to the excited device when the slip is lower critical slip. These analyses will provide the bases of the design of the excited device.This paper analyses the effect of limitations of rotor current on the active power and reactive power regulating. And the relation between the active power and reactive power under constant rotor current isgiv$n out. The components of ASG's electro-magnetic torque is analyzed, the relations between the electro-magnetic torque and slip, voltage phase angle is discussed also.The static stability of an ASG at different operating modes is analyzed in this paper. Analysis indicates that the static stability is enhanced when the machine operates at generalized asynchronous operating mode by adjusting the phase angle of alternating excited voltage.The theory of stator-flux-oriented control of the ASG is investigated. The dynamic mathematic model of stator-flux-oriented control of the ASG is presented in this paper. The dual-loop control scheme is adopted; in which the inner-loop is current-loop, and out-loops is power-loop. The active power and the reactive power of the ASG independently control is achieved.The conversion angle of rotor variables acts an important role. When the given rotor voltages in dqo coordinates are transformed into the variables in rotor three-phase coordinates, and the measured rotor variables are transformed into ones in dqo coordinates, the angle will be involved. The methods to determine the angle are discussed in detail. And feed forward term is added into the q-axis control variable to eliminate the tracking error.The fuzzy-PID control technology is applied \o the stator-flux-oriented control system. The fuzzy-PID controller gets over the limitations of conventional PID controller, and combines the superiorities of conventional PID controller and ones of fuzzy logic control. Numeric emulation and test research proves that the fuzzy-PID controller applied to the inner current-loop and out-power-loop can improve the adjusting characteristic and let the ASG change smoothly one operating state into another state.This paper analyzed in detail the observing of stator flux and rotor speed. The open-loop observer is easy to be implemented, but there are some limitations, such as that the observer can't eliminate the original error of flux and rotor speed, the precision of the observer will be effected by the ASG's parameters. After comparing open-loop observer and closed-loop observer of stator flux, this paper adopts the least-order flux observer. The observer is implemented in a DSP excited device. Analysis and experimental researches indicate that the closed-loop observer can eliminate the original error of flux and restrain the error arising from the deviation of machine's parameters. The rotor speed observer is constructed based on Lypunov's stability theory at the same time. The observer has good astringency proven by numeric analysis and experimental researches. The observer with Kalman filter of stator flux and rotor speed is constructed. Numeric analysis indicates that this kind estimator has better astringency, but is difficult to be achieved.Stability is important to a control system. A control system can be applied only after the system being stable. The stability analysis is important for the design of a control system. There are many methods to analyze the stability of the control system, such as small gain condition, Lypunov's direct method and indirect method, circle criteria for stability, Popov's hyper-stability Criterion and passive stability analysis method. Some methods have their own limitations or rigid precondition. For example, for Lypunov's direct method, it's difficult to find out an adjuvant positive definite function, which its differential function is minus. For a high-order control system, Lypunov's indirect method is effective to analyze the stability.The dynamic equations of the closed-loop control system of ASG are given out, the relations between the real part of eigen roots and the parameters of the machine are analyzed based on Lypunov's indirect method. The stable region of the control system can be influenced by the ASG's parameters, such as stator resistance, inductance. Analysis indicates that to decrease the gain of PID regulator is beneficial to the stability when the given stator resistance is larger than real stator resistance.The influences of different balanced points to the eigen roots of dynamic equations are analyzed. The change of given reactive power, i.e. d-axis current, has little influence to the stability region. This is owing to that the rotor speed will not be changed with the change of reactive power. But there are some influencesto the stable region when the given active power is regulated because the rotor speed will be changed with regulation of active power. The PID regulators' parameters of stable region will decrease with the increase of given q-axis current.This paper emphasizes the influences of the PID controllers' parameters on the stability of the system, and finds out the stable region and the best range of PID controllers' parameters. Analysis results indicate that when the integral coefficient increase, the control system will tend to be unstable even the eigen root of system is larger than zero and the system will lose stability. But it not that the smaller of integral coefficient is, the better the control performance is. When the integral coefficient is small, the absolute value of the max eigen root is small and the convergent rate will tend to be laggard. Considering if the control system is stable, the value region of regulators' gain is large. But similarly, the value region of regulators' gain is too large or too small, the absolute value of the max eigen root is small and the convergent rate will trend to be slow. Based on the control system stability, the appropriate regulating ranges of the PID controllers' parameters were given out, which conducts the design of the control system.In practical research, an alternating current excitation device based on the digital signal processor (DSP) and a fuzzy-PID controller was designed. The paper introduces the construction of system hardware and the implementation of software, and carries out abundant experimentations. Full and accurate results are given out.The excitation device is composed of a microcomputer, a DSP controller, sensors, AC quantities pretreating devices and an AC-DC-AC cycloconvertor. The fuzzy-PID controller controls the output voltage and the frequency of the cycloconvertor. The microcomputer mainly acts as giving orders and monitoring. The function of AC quantities pretreating devices is deflecting, magnifying or diminishing the sensors outputs and converting to the value that is adapt to the DSP device. The function of DSP controller is to finish control operation and giving out the PWM pattern based on the orders and feedbacks. The cycloconvertor supplies the three phase exciting current based on the PWM signals. The system software includes a data-collecting module, a data output module, PID regulators modules, a fuzzy controller, reference axis-transferring module, SVPWM driving module, observing of stator's flux and rotor speed module. This paper discussed the numeric filter technology and the implement of PWM drives.Based on the experimental set-up, series of experimental researches, including experimentations of open-loop control, PID control and fuzzy-PID control AC excitation, were conducted.A series of tests under variable PID regulator's parameters are conducted in this paper. The results indicate that the parameters have much effect on the dynamic responds.With the increasing of PID's integral coefficient, the dynamic responds of control system speed up, but at the same time, the overshoot increase and the system tend to oscillate. The step responds will be slow when the PID's gain increase. Experimentations indicate that sometimes, the ascending responds are good but the descending responds are bad at same PID's parameters. This demonstrates that it is necessary to introduce the fuzzy logic control, an advanced control technology, into the AC exciting control system.The experimentations under fuzzy logical control are conducted. The results prove that the fuzzy PID controllers, which combine the rubust of fuzzy logical control and the accuracy of PID controllers, will speed up the responds of the system and enhance the stability of the control system.The experimental results demonstrate that the fuzzy-PID controllers are effective, Not only that the active power and reactive power can be controlled independently, but also the controllers can improve the dynamic performances of the generator.