With its pollution-free and renewable nature, wind power has been widely recognized by countries all over the world and has developed rapidly in recent years. The variable-speed constant-frequency wind power generation system with double-fed motor has significant advantages compared with the traditional constant-speed constant-frequency wind power generation system. For example, the wind energy utilization coefficient is high, and it can absorb the energy fluctuation caused by the sudden change of wind speed to avoid the spindle and the transmission mechanism. Tolerate excessive torque and stress, as well as improve the power factor of the system.

The core technology of the variable speed constant frequency doubly-fed wind power generation system is based on power electronic and computer controlled AC excitation control technology. Although theoretical analysis and computer simulation can be used to study the control technology of variable-speed constant-frequency wind power generation system, due to the non-authenticity of the simulation model and its parameters and the non-real-time nature of the control algorithm, simulation research is often difficult to replace the experimental research of the simulation system. Based on the analysis of the operating principle and excitation control method of the doubly-fed motor, the excitation control test system of VSCF doubly-fed wind turbine based on 80C196MC single-chip microcomputer is designed and constructed, and the control technology is systematically studied.

The working principle of 2VSCF wind turbine 2.1 The principle of VSCF control of doubly-fed electric machine VSCF wind power generation system is mainly composed of wind turbine, speed increasing box, doubly-fed generator, bidirectional converter and controller. The stator winding of the doubly-fed generator is connected to the grid, and the rotor winding is excited by a three-phase power source with an adjustable frequency, and is generally powered by an AC-AC converter or an AC-DC-AC converter. The doubly-fed generator can be operated at different speeds, and its speed can be adjusted appropriately with the change of wind speed, so that the operation of the wind turbine is always in an optimal state to improve the utilization of wind energy. When the load and speed of the motor change, by adjusting the current fed into the rotor winding, not only can the voltage and frequency of the stator output be kept constant, but also the power factor of the generator can be adjusted.

According to the principle that the rotating magnetic field generated by the induction motor and the rotor winding current is relatively static, it can be known that the VSCF wind turbine speed stator voltage stator current rotor voltage rotor current, the relationship between the machine speed and the stator and rotor winding current frequency is as follows: flow frequency, generator Speed ​​and pole logarithm.

It can be known from equation (1) that when the rotational speed n changes, if the corresponding change of the adjustment /2 is made, it can be kept constant, that is, consistent with the grid frequency, and the VSCF control of the wind power generator can be realized. When the wind turbine is running at sub-synchronous speed, the formula (1) takes a positive sign; when the wind turbine is at super-synchronous speed, the formula (1) takes a negative sign; at the synchronous speed, /2=0, the variable current The device supplies a DC excitation current to the rotor.

2.2 Rotor winding power flow in different operating modes When the motor loss is neglected and the stator is the generator convention and the rotor is the electric motor convention, the stator output electric power A of the generator is equal to the sum of the input power of the rotor and the input mechanical power on the motor shaft. That is, according to the operating principle of the induction motor, the electric power of the rotor winding and the mechanical power on the motor shaft can be expressed as expressed by equations (2) to (4), respectively. When the generator is running at subsynchronous speed, 5>0, it is necessary to The rotor winding feeds electric power, and the electromagnetic power transmitted from the rotor to the stator is the electric power transmitted by the wind turbine to the stator. Only when the generator is running at super synchronous speed, s<0, the rotor winding is powered outward, that is, the stator and rotor simultaneously generate electricity. At this time, the power supplied by the wind turbine to the generator is increased to the input-output power relationship of the (l+Lyl)iV doubly-fed generator in different operating modes below and above the synchronous speed, and the available power flow is schematically represented. Since the power flow of the rotor windings is different at different operating modes below and above the synchronous speed, a bidirectional converter is required.

3 Excitation control system hardware: 3.1 The basic function of the excitation control system is to meet the requirements of various conditions of the doubly-fed generator below synchronous speed, synchronous speed and higher than synchronous speed operation, and bidirectional converter feeding to the rotor winding The output voltage (or current) amplitude, frequency, phase, and phase sequence should be adjustable. The reactive power of the generator can be adjusted by controlling the amplitude and phase of the excitation current; the active power of the generator can be adjusted by controlling the frequency of the excitation current; and the optimal combination of the pitch control of the wind turbine and the excitation control of the generator can be optimally The mode of operation regulates the speed of the generator.

3.2 Excitation Control System Basic Composition The block diagram of the VSCF doubly-fed wind turbine simulation test system is shown. The system consists of a wound rotor induction motor with rated power of 2.8kW, DC drag motor, voltage regulator, IGBT cross-connected bidirectional converter, photoelectric encoder, current and voltage sensor, 80C196MC microcontroller, PC and parameter display, etc. composition.

4 Excitation control technology research 4.1 Variable speed constant frequency control The variable speed constant frequency control of the doubly-fed wind generator is to control the frequency of the rotor excitation current according to the change of the speed of the wind turbine, so that the voltage frequency of the output of the doubly-fed generator is consistent with the grid. . There are two methods for achieving variable speed constant frequency control, that is, variable speed constant frequency control with speed sensor and no speed sensor. The former is relatively easy to control, but requires a photoelectric encoder; the latter control technique is slightly more complicated.

The excitation control system shown uses variable speed constant frequency control with a speed sensor. The pole number of the motor / 7 = 2, the stator current frequency / l = 50Hz. Substituting the p and /; values ​​into the equation (1), the relationship between the excitation current frequency /2 and the motor speed detection signal can be obtained.

The current frequency fed into the rotor at the subsynchronous speed is the current frequency fed to the rotor at the supersynchronous speed as the number of output pulses. The relationship between the motor speed and the motor speed can be calculated according to the output of 2000 pulses per revolution of the photoelectric encoder.

It is the waveform of the rotor winding current with the speed adjustment frequency when the doubly-fed generator is lower than the synchronous speed. It can be seen from the figure that the frequency of the rotor current changes according to the law of the formula (1), and the variable speed constant frequency control of the doubly-fed generator is realized.

Rotor winding current frequency variation waveform 4.2 Constant voltage control When the stator winding is open and the doubly-fed generator is operated at no load, the effective value of the open-phase voltage of the stator winding is the number of turns and the winding coefficient of each phase of the stator winding. Each pole flux = only / 2) is determined by the rotor winding field current.

It can be known from equation (7) that when the stator winding voltage frequency is a constant value, the stator winding terminal voltage can be kept constant at different speeds as long as the rotor winding excitation current value is kept constant. However, when the generator load is running, due to the stator winding resistance and leakage voltage drop, and due to the influence of the stator current armature reaction magnetic field, the voltage of each pole flux and stator winding terminal is no longer constant even if the rotor excitation current is constant. . In order to keep the generator terminal voltage constant under different operating conditions, it is necessary to adjust the rotor excitation current through voltage feedback to achieve closed-loop constant voltage control. The test shows that after the closed-loop control of the output voltage of the doubly-fed generator, the speed increases from 1300r/min to 1480r/min, and the output voltage of the stator winding only changes by 0.2V. 4.3 The grid-connected control of the doubly-fed generator is more than that of the traditional wind turbine. The use of asynchronous generators has a greater impact on the grid when connected to the grid. The doubly-fed generator can achieve soft grid connection by adjusting the rotor excitation current, avoiding current surge and excessive voltage fluctuations during grid connection.

In the excitation control system, the voltage, amplitude, phase and phase sequence of the grid and generator voltage are respectively detected by the voltage sensor before the grid connection, and the rotor excitation current is adjusted by the bidirectional converter to make the generator output voltage correspond to the grid. The voltage frequency, amplitude and phase are consistent, and it is automatically connected to the grid when the grid conditions are met. It can be seen that the stator current has an oscillation phenomenon after grid connection. This is because the closed loop control of active and reactive power is not used in the grid connection test. After the closed loop control, the power angle of the generator remains unchanged to solve the current oscillation. problem.

As shown in the stator grid voltage and current waveforms, the generator voltage is slightly higher than the grid voltage before the grid connection, and the generator voltage is the grid voltage after the grid connection. Before the grid connection, the generator current is the current of the auxiliary load, and the current after the grid connection is the current fed into the grid. The auxiliary load is used for generator voltage and current monitoring before grid connection, and the auxiliary load is cut off after grid connection. In order to facilitate the comparison of the voltage and current of the generator stator winding before and after grid connection, the auxiliary load is used to detect the voltage and current of the stator winding before the grid connection. In the actual VSCF system, the auxiliary load is not necessarily needed, and the grid can be detected and compared. And the terminal voltage of the generator to determine whether the grid connection condition is met.

4.4 Three-state switching control When the sub-synchronous speed is running, the converter feeds the AC excitation current to the rotor winding. When the synchronous speed is running, the converter feeds the DC winding to the rotor winding, while the super-synchronous speed operation changes the output of the rotor winding. The flow device is fed into the grid. The dynamic conversion of three different operating states of subsynchronization, synchronization and supersynchronization is a key technology for the excitation control of variable speed constant frequency doubly-fed wind turbines.

Due to the instability of wind speed changes, wind turbines are difficult to operate at synchronous speed for a long time. In order to avoid the difficulty of repeatedly crossing the synchronization point and the small slip zone near the synchronous speed, in the actual variable-speed constant-frequency wind power generation system, the stable operation working point is always selected to avoid the small slip zone near the synchronous speed. .5> Naturally, crossing the synchronization point is inevitable.

The conversion of the three operating states across the synchronization point can be carried out in two different ways, one using the "AC-DC-AC" control mode and the second using the "AC-AC" control mode. The “crossing-crossing” control mode is to gradually reduce the frequency of the rotor winding current as the generator speed increases. When the speed is close to the synchronous speed, the rotor winding is supplied with DC (the rotor three-phase winding is “two-to-one series” connection mode). The converter controls the three power switching devices of different bridge arms to be turned on or off at the same time by PWM, and outputs a controllable DC excitation current). When the speed exceeds the synchronous speed, the converter stops the DC power supply, and at this time, the rotor winding outputs the alternating current of the slip frequency to the converter. The measured waveform of the rotor current across the synchronous speed of the generator using the "AC-DC-AC" control mode is as shown. The "intersection-crossing" control mode eliminates the need to supply DC power to the rotor windings. The control is slightly easier, but the smoothness of the three operating state transitions is slightly worse, and the rotor current test is as dauntous.

5 Conclusions (1) Crossing synchronous speed is one of the key technologies for excitation control of variable-speed constant-frequency doubly-fed wind turbines. Sub-synchronization, synchronization and super-synchronization can be realized by adopting the "AC-DC-AC" or "AC-AC" control mode. Conversion between modes of operation.

C2) Grid-connected operation is another key technology that needs to be solved in the excitation control of variable-speed constant-frequency doubly-fed wind turbines. Different grid-connected parties can be used, but the current surge and voltage fluctuations in the grid-connecting process need to be solved.

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