The doubly-fed generator is also called AC excitation generator. It is similar in structure to the wound-type induction motor. The three-phase stator of the stator is connected to the power frequency grid. The static frequency converter provides low-frequency AC excitation to the rotor winding, which can realize the active power of the generator. Independent adjustment of reactive power and speed. The generator has good stability and strong phase-in-phase operation capability; it has the characteristics of realizing the constant frequency of the stator under the condition of the change of the prime mover speed, that is, the power generation capability of the variable-speed constant-frequency power generation, the wind power generation, the pumped water storage, and the improvement of the power system The field of stability and other fields have broad application prospects and are gradually receiving attention and attention.
At present, many application vector control techniques have been applied to study the decoupling control of active and reactive power of doubly-fed generators. The vector control technique based on air-gap magnetic field orientation is used to derive the active and reactive decoupling excitation control model for the doubly-fed generator steady state. However, due to the neglect of the stator leakage impedance and the rotor leakage inductance in the derivation, The accuracy of the excitation control model is degraded. The vector control technology based on stator magnetic field or stator voltage orientation is used. The simulation shows that the excitation control model has higher precision, but it also has the disadvantage of poor dynamic response. However, the above-mentioned vector control technology using air gap field orientation, stator field orientation or stator voltage orientation is based on the traditional vector control technology concept, that is, the measurement of stator and rotor current and speed is required as feedback of the excitation controller. The signal, and its measurement accuracy and real-time, largely determine the dynamic response performance and control accuracy of the doubly-fed generator.
The control equation of the dynamic synchronous shafting system is established under the dual channel, and the steady-state decoupling control is realized. However, the detection of the stator quantity, the rotor voltage and the rotational speed quantity is still needed in the feedback signal of the active channel. Although a robust control strategy for active and reactive power of doubly-fed generators without measuring rotor current based on stator voltage orientation is proposed, the algorithm is only suitable for steady-state regulation of stator active and reactive power, and cannot be used for speed. Independent adjustment, even difficult to study problems such as short circuit faults.
Starting from the motor motion equation, this paper proposes a novel control strategy based on infinite grid voltage orientation for grid-connected doubly-fed generators, and establishes the corresponding excitation control equation. The effects of active, reactive and steady-state regulation of doubly-fed generators and the effects of rotor parameters changes during motor operation were simulated. Finally, the transition process of three-camera-to-ground sudden short-circuit was also studied.
2Doubly-fed generator model and excitation control model 2.1 The mathematical model of the doubly-fed generator assumes that the stator voltage and current of the doubly-fed generator are in the positive direction according to the generator convention. When the positive direction of the rotor voltage and current is in accordance with the motor convention, the synchronous rotation dq can be written. The voltage and flux linkage equations of the doubly-fed generator of the three-phase symmetric system under the shafting are voltage vectors; / is the current vector; represents the flux linkage; the ruler is the resistance; i is the inductance; 1 is the mutual inductance between the stator and the rotor; D = d/d (for differential operators; (where Fanhe 厶 includes grid line resistance and inductance), (motor synchronous angular velocity and slip angular velocity, respectively, and satisfies equation (5).
Taking stator current and rotor flux as state variables, simultaneous (1) (4), can obtain the mechanical torque of the doubly-fed generator; Tem is the electromagnetic torque; = 4 (1 - 2.2 based on the infinite grid voltage The directional control model takes the direction of the infinite grid voltage vector as the d-axis, and the constraint condition is that the active and reactive power expression of the stator-to-system output of the doubly-fed generator is expressed by equation (13). When doubly-fed power generation is required When the machine outputs a certain amount of active or reactive power, the command value (indicated by the dq axis component of the stator current synchronous shaft system is used. Therefore, the active and reactive power adjustment of the doubly-fed generator can be realized by controlling the excitation voltage of the rotor. The adjustment of the stator current dq axis component. The relationship between the rotor excitation voltage and the dynamic adjustment process is as follows. The command value of the excitation voltage can be obtained from the motor state equation (8) (9) in order to obtain the rotor flux linkage command value and the stator current command value. The relationship between the stator current, the rotor flux and the rotor angular velocity in the adjustment process is as follows. The dynamic variation of the stator current can be obtained as follows. From equation (20) In order to obtain the rotor flux linkage command value, the dynamic variation of the rotor excitation voltage during the adjustment process can be obtained by the following formula: wherein the proportional adjustment coefficient; >0. Thus, equations (15)(17), (21) (22) together constitute an excitation control model based on the voltage orientation of the infinite grid.
In order to study the short-circuit fault of the system, the circuit equation between the stator terminal voltage of the grid-connected doubly-fed generator and the infinite bus voltage is 0.394Q/km as shown in equation (23). The double-loop outlet, the length of the line between the power plant and the infinite bus is 100km. 3.2 Simulation study of steady-state regulation characteristics The doubly-fed generator is implemented by the above-mentioned excitation control strategy for the simulation study of stator active, reactive and speed regulation. Specific simulation results are given separately. Among them, it is the simulation result that the active power is adjusted from 0.8 to 0.9, the reactive power is reduced to 0.5, and the slip rate is unchanged. It is the simulation result that the reactive power is adjusted from 0.4 to 0.5, the active power is 0.9, and the slip rate is unchanged. The difference rate is adjusted from 0.05 to 0.055, and the simulation result of 0.9 is active and 0.5 is unchanged. The simulation result is that the active power is adjusted from 0.8 to 0.9, the slip rate is adjusted from 0.05 to 0.055, and the reactive power is unchanged. In addition, in order to consider the influence of the change of the rotor parameters of the motor on the steady-state regulation characteristics, the simulation results when the rotor resistance and the rotor leakage resistance value of the generator are in error are added. In the figure, curve 1 shows the simulation result when the motor operating parameters have not changed, and curve 2 shows the simulation result of the motor running rotor resistance and leakage reactance value increased by 10%. Comparing curves 1 and 2, it can be seen that the excitation control model is still effective after the motor is running, such as the change of the rotor parameters, and the decoupling control effect of active power, reactive power and speed can still be achieved.
3.3 Simulation analysis of transient characteristics The doubly-fed generator has good transient stability in theory. The premise is that the frequency, amplitude and phase of the excitation voltage can be quickly quasi-main research direction as double-feeding, mainly engaged in motor and its control. By accurately tracking the change in motor speed, a large electromagnetic torque can be obtained after the system is short-circuited and returned to normal to maximize the increase in rotor speed. In order to study whether the excitation control strategy proposed in this paper can meet this requirement, this paper simulates the sudden short circuit of the three-camera of the grid-connected doubly-fed generator. And the simulation curves of the active, reactive and slip transition processes of the doubly-fed generator after the short-circuit of the three-camera generator in the two states of the reactive power and the reactive power are given. At 0.1 s, the short-circuit duration is set to 0.25 s (assuming that the input power of the prime mover remains unchanged during the short-circuit transition). Among them, the transient simulation results of the active corps s=0.9, reactive power gs=0.5, 00.5 before the short circuit fault; the transient simulation results of the active corps s=0.9, reactive power æ¡=-0.5, 00.5 before the short circuit fault.
Transient characteristic curve of doubly-fed generator under reactive power state Transient characteristic curve of doubly-fed generator under reactive state 4 Conclusion By establishing the dynamic equation of control variable, this paper proposes doubly-fed power generation based on voltage orientation of infinite grid. The excitation control strategy of the machine does not require the measurement of the rotor current and the feedback signal of the rotational speed, which simplifies the complexity of the control system to some extent.
In addition, the simulation study of the stator active, reactive and rotational speed steady-state characteristics of the grid-connected doubly-fed generator and the influence of the rotor parameters of the motor are carried out. The results show that the doubly-fed generator can be realized by using the control strategy. Stator active, reactive and rotational speed independent adjustment or simultaneous adjustment of active and rotational speed, with good stability. The simulation analysis of the transient characteristics of the three-phase sudden short circuit from the machine end shows that after the short-circuit fault is removed, the initial state of the generator is either reactive or reactive, and the system can quickly stabilize, and after the transition process is over, Its active, reactive and slip rates can return to the original set value for stable operation, with good dynamic quality and dynamic tracking ability. In summary, the simulation results from steady state and dynamic characteristics show that the excitation control strategy proposed in this paper is correct.
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