The Doubly-fed Wind Power Generator Virtual Synchronous Control

Given the high proportion of wind power frequency/voltage stability problems, the power system based on synchronized coordinates (DFIG) mathematical model of doubly-fed motor, studies a current source containing inertial link type wind turbine virtual synchronous control strategy, and the virtual synchronous control loop and the current control design of the inner ring. Established in the Matlab/Simulink simulation model of doubly-fed wind power generator, the virtual synchronous generator realized virtual synchronous generator inertia. Simulation results confirm the validity of the theoretical analysis and the effectiveness of the proposed control strategy.


Introduction
Doubly-fed induction generator (DFIG) has been widely used in wind turbines.The grid frequency or voltage changes, based on the traditional vector control of DFIG does not have traditional synchronous generator inertia, supported frequency and voltage support.To improve the stability of the power grid, increasing the permeability of the DFIG wind power generating unit is very important to enhance the capacity of DFIG system support for the grid.
The power generation equipment adopts a control strategy based on the virtual synchronous generator, and it provides frequency and voltage through the simulation of the traditional synchronous generator running inertia.Li et al. [1] proposed the concept of a virtual synchronous machine, by controlling the phase current to simulate a synchronous generator.Therefore, VISMA like the current source.Wen et al. [2] proposed a VSG control scheme based on a swing equation.Peng and Yuan [3] proposed control can inherit VSG droop control advantages, to provide a power grid of inertia.
At present, many research institutions, universities, and scholars at home and abroad have studied virtual synchronization control technology.Different virtual synchronization techniques have been proposed, such as a virtual synchronous motor (VISMA), synchronous generator, and virtual synchronous generator (VSG).From the point of external characteristics, there are two categories: current control and voltage control virtual synchronous generator [4][5] .From the point of view of outlets, the former is more suitable for the low permeability of distributed power supply, which is suitable for the high permeability of the environment [6] .
This paper constructs the wind power system simulation model based on a virtual synchronous machine and corresponding experiments, to verify the wind power converter can stable power grid frequency and voltage change.Results are effective.

Wind turbine virtual synchronous machine model and design principle
Doubly-fed wind power generator cannot adjust the frequency and voltage of the converter fault of power electronic devices.Therefore, wind power converter is used to simulate the electromechanical transient characteristic of the synchronous generator, a synchronous generator inertia, FM, reactive power controller, and other functions of the network operation.
In order to achieve the basic characteristic of the synchronous generator, the simplified model, Equation (1).
where e is the excitation electromotive force; u is the armature terminal voltage or generator output voltage; The s R , s L are armature resistance and inductance respectively.The damping characteristics is ignored and to 0 D  , synchronous generator rotor motion Equation (2).
where m

T , e
T respectively represent synchronous generator mechanical and electromagnetic torque, m P , e P are mechanical and electromagnetic power, respectively, Angle  is for a synchronous generator,  represents synchronous generator mechanical and electrical angular velocity, and 0  is rated electrical angular velocity.

Virtual synchronization control strategy 2.1.1Frequency control mechanism
Speed regulation equation of single generator is under Equation (3).
  where m is to adjust the coefficient of frequency, which shows that when the unit appears to add and subtract load change generated when the corresponding rotational speed deviation; 0 P is the original power of the initial load, m P is generator active power instructions, 0  is rated electrical angular velocity, the actual electrical angular velocity for  synchronous generator.
In Figure 1.-active control block diagram for the VSG frequency, its characteristic is to droop control of synchronous motor and rotary inertia.

Voltage control mechanism
Adopting droop control to adapt to the reactive power changes caused by voltage changes, Equation (4).

 
where m Q is real-time reactive power instruction; 0 Q is the original power of the initial load; 0 U is rated voltage; U is the actual voltage value and n is the coefficient of regulating voltage.
In Figure 2. voltage-reactive power control block diagram, the rated voltage of the stator is different from the actual voltage of the stator, the reactive power is added by the voltage regulating coefficient, and the internal potential is controlled by the PI controller output three-phase reference current of the rotor.
The virtual synchronization control structure of the machine-side converter composed of virtual synchronization control outer loop and rotor current closed-loop is displayed in Figure 3.  of machine side converter.

Simulation waveform A. Frequency disturbance
In Figure 4., slope change is adopted to simulate the frequency decline of a weak power grid.When t=2 s, the frequency decreases from 50 Hz to 48 Hz with a slope of 1 Hz/s, and recovers to 50 Hz at the same rate when t=13 s.As shown in Figure 4(a)., when traditional vector control is adopted, rotor speed and stator side output active and reactive power do not change.As shown in Figure 4(b)., when the virtual synchronous control strategy is adopted, the rotor speed decreases from 1020 r/min to 1014 r/min after 2 s.At this time, the rotor releases kinetic energy to provide energy.When the frequency has a slope change, the active power increases to 1310 W. Finally, it stabilizes to 1250 W, and it can be analyzed that the primary frequency modulation provides 250 W of energy, while the inertial support provides part of the energy.A frequency modulation coefficient is p 20   , a frequency modulation power of 250 W. Therefore, the actual measured values are consistent with the theoretical analysis values.By comparing the simulation waveforms of two control strategies, the virtual synchronous control strategy can provide frequency support for the entire system, capable of primary frequency modulation and inertial support.

B. Under the condition of voltage disturbance
The response of traditional vector control and virtual synchronous control were investigated.The voltage regulation dead band was set to 1%, and the coefficient of one-time voltage regulation was q 10   .Results are shown in Figure 4. T=3 s, the terminal voltage decreases by 5%, and when t=5 s, it returns to 311 V.In Figure 4(c)., the active and reactive power of the traditional vector control strategy remain unchanged; in Figure 4(d), a virtual synchronous control strategy is adopted to increase the output reactive power to 155 Var to maintain voltage stability.
By comparing the simulation waveforms of two control strategies, the virtual synchronous control strategy can adjust the voltage of the entire system and has the ability to adjust the voltage in one go.

Experimentation
Build a 5.5 kW DFIG system and compare the traditional vector control strategy of the doubly-fed wind turbine with the simulated frequency and voltage regulation experiments of the virtual synchronous control of the doubly-fed wind turbine.
The 5.5Kw DFIG system is composed of a recycled analog power supply 61830, a doubly-fed machine side converter cabinet, a doubly-fed grid side converter cabinet, a doubly-fed motor, and an asynchronous motor.The upper computer of the computer observes the transmission instructions and control variable waveform process, and observes the voltage and current experimental waveforms through a waveform recorder.The experimental platform is composed of the above devices, as shown in Figure 5.

Experimental waveform
Traditional vector control and virtual synchronous control are compared and analyzed under frequency and voltage disturbances, respectively.
Virtual Synchronous Control of Speed and Output Power under Frequency Disturbance.(i) Traditional vector control voltage and stator side output reactive power under voltage disturbance.(g) Virtual synchronous control of voltage and stator side output reactive power under voltage disturbance.
From Figure 6(c), the amplitude of the rotor current has no change, and the stator current has a certain variation with the drop in stator voltage.As shown in Figure 6(i), when t=63 s, the voltage drops by 5%; When t=74 s, the voltage recovers to 221 V, and the stator side reactive power remains almost unchanged.The experimental and theoretical results are consistent, and traditional vector control cannot provide primary voltage regulation.From Figure 6(d), the amplitude of the rotor current slightly increases when the voltage drops and slightly decreases when the voltage rises.The stator current has a certain change with the drop in the stator voltage.As shown in Figure 6(g), when t=36 s, the voltage drops by 5%, and the stator side reactive power increases to 155 Var; When t=66 s, the voltage recovers to 221 V, and the stator side reactive power drops back to 0 Var.In theory, there is a change in the rotor current.When the voltage regulation coefficient is selected as q 10   , the theoretical output reactive power should be 155.56Var.The experimental and theoretical results are consistent, so theoretically, virtual synchronous control has a primary voltage regulation effect.

Conclusion
Most power electronic devices are used in the grid following devices.This article focuses on the gridconnected application of DFIG, and addresses the problem that traditional vector-controlled doubly-fed wind turbines cannot provide inertia and frequency support for the power grid.A current-type virtual synchronization control strategy built on a phase-locked loop active frequency loop and reactive voltage loop is studied.In the case of frequency disturbance, the virtual synchronous control strategy provides inertial support and primary frequency regulation for the power grid.In the case of voltage disturbance in the grid, the virtual synchronous control strategy plays a role in voltage regulation.

Figure 4 .
Comparison of the waveform between traditional vector control and virtual synchronous control under frequency and voltage disturbances.(a) (b) are respectively the traditional vector control and virtual synchronous control speed and output power when the frequency is disturbed.(c) (d) are respectively traditional vector control and virtual synchronous control output power under voltage disturbances.