Grid control strategy of wind turbines based on LADRC voltage control

Traditional grid-controlled wind turbines are required to rely on AC power grid operation, but when the power grid fluctuates, it will cause a series of oscillation instability problems in wind turbines, such as low-frequency oscillation, subsynchronous oscillation, etc. The safe and stable operation of the crisis power grid needs to be ensured. In this study, the virtual synchronous machine is supposed to be adopted as the main control strategy for grid-type control, transforming the traditional system into a more robust and reliable one. Additionally, a converter model of the grid-type control network is set up. By implementing grid-type control, voltage support and frequency support can be offered without relying solely on AC power grid operation. This approach effectively addresses issues caused by preventive such as reliance on wind turbines. The stability of the networked control system is dependent on maintaining stable system power. When there are fluctuations in system power, it impacts the stability of the networked control system. To mitigate these effects and enhance overall stability, this paper proposes using linear active disturbance rejection control (LADRC) based voltage and grid controls to suppress voltage fluctuations through linear active disturbance rejection techniques. The overall system performance is demonstrated through simulations conducted using Matlab/Simulink platform.


Introduction
In recent years, grid-type control technology has received widespread attention [1] .This type of control technology has the characteristics of power synchronization or inertia synchronization, which not only eliminates the phase-locked link of the AC power grid but also demonstrates the response and support to the grid frequency and voltage [2] .Based on the above characteristics, grid-type control technology exhibits better performance in weak and low-inertia power grids.Compared with the widely used gridtype control in wind power generation, grid-type control technology shows significant innovation and breakthroughs in methods and characteristics, which meets the requirements of new energy grid connection in new power systems and has important research significance and value [3][4] .
On this basis, the precise control of grid-side converters has been one of the key technologies to support the power grid capacity and stability system.In the network-based control, the voltage and current double closed-loop control strategy is adopted for the current vector control of the grid-side converter, and the controller is the PID controller, which has difficult PI parameter setting, poor antiinterference ability, and slow dynamic response speed, which affects the accuracy of the control system.Yang et al. [7] proposed a rejection technique for phase-locked loops, which suppresses oscillation to a IOP Publishing doi:10.1088/1742-6596/2771/1/012026 2 certain extent but does not address the problem of system stability degradation when the frequency fluctuation of the power grid fluctuates.In [8] , virtual synchronous machine control is proposed, which effectively solves the problem of frequency fluctuation and can actively support the performance of the power grid but does not deal with the problem of power fluctuation and response speed caused by habitual control.
Linear active disturbance rejection control has the advantages of simple structure, strong antiinterference ability, and good robustness [9] .It is a control strategy that combines the benefits of a simple PID structure and modern control theory state estimation.Therefore, this article proposes the introduction of LADRC as a voltage loop controller in the voltage and current closed-loop loop of virtual synchronous mechanism network control and designs a second-order LADRC.In this controller, linear feedback control rate (LSEF) and third-order linear extended state observer (LESO) are first and foremost used.LSEF replaces the PID link and simplifies it to PD.At the same time, the output voltage of the actual system is subtracted from the output voltage of the observer through LESO to strive for an error.This error is constantly fed back and ultimately reaches 0, achieving a perfect state of observation.Finally, simulations were performed for routine operations and sudden load changes.The results were compared to traditional PI control with the introduction of LADRC control to verify the effectiveness of the proposed LADRC-based voltage control strategy in improving system performance.

Grid-based control strategy
The essential difference between the network type and the network type is that they have other synchronization mechanisms.The network-type control is synchronized through the phase-locked loop and the AC power grid, while the network-type control mostly uses power control to achieve synchronization.For the network structure control, the main reason for the loss of synchronization is the power instability of the synchronous control.
The network control strategy is commonly used in droop control and virtual synchronous machine control, and this paper will analyze and study these two control strategies.

The droop control
Droop control is put forward the simplest and one of the most conventional control strategies based on the grid.It simulates the frequency of synchronous generator active and reactive power voltage sag characteristics.
The block diagram of droop control is illustrated in Figure 1, which constructs the negative feedback relationship between active power and frequency and reactive power and voltage.The control reference values of frequency and amplitude are obtained through the instructions and feedback issued by active and reactive power, as indicated in Equation (1).

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) ( However, the traditional droop control still requires a phase-locked loop to synchronize the output voltage of the grid-side converter with the power grid.It does not have the ability of voltage support and frequency support and cannot run in an off-grid environment.The virtual synchro control improves the shortcomings of droop control and enables the grid-side converter to sustain voltage, frequency, and power distribution.

Virtual synchronizer control
This paper makes a run in the network environment of a virtual synchronization network control system.The essence of virtual synchronous machine control is the improvement of the droop control.The structure block diagram of simulated synchronous machine control is shown in Figure 2, which is characterized by the introduction of the rotor motion equation of the simulated synchronous generator in the active power loop of droop control, as shown in Equation (2).

J D J P
where J and D are the virtual inertia and damping coefficients of the virtual synchronous machine, respectively.By integrating Z , the phase reference value of the converter output voltage T can be obtained.Through active-frequency and reactive-voltage control, the electromotive force ref E and the angular frequency Z are obtained.The electromotive force ref E can provide a reference value for the voltage loop control in the double-closed loop, and the angular frequency Z can support the operation of the network construction system and the following grid system connected in parallel.The topology of the grid-side converter circuit is shown in Figure 3 where d u and q u are the components of the voltage on the arm side of the inverter in the d and q axes, respectively.od u and oq u are the output voltage of the inverter in the D and Q axes.Ld i and Lq i are the filter inductor current in the d and q axes.od i and oq i are the output current in the d and q axes, and Z is the angular frequency of the grid voltage.

Design of LADRC
The grid side control mode usually adopts voltage and current dual closed-loop control, and its controller adopts PI control for control.The proposed strategy is that LADRC technology replaces the PI controller of the d-axis and q-axis voltage loops in the voltage and current dual closed loop to achieve zero difference adjustment and stabilize power, increase the stability of virtual synchronous machine control, and support system operation ability [6] .The advantages of this control strategy are fast-tracking response speed, strong anti-interference ability, and the ability to coordinate control with networked control, continuing to improve the performance of the system.
The LADRC controller is mainly composed of three parts: differential tracker (TD), linear extended observer (LESO), and linear error feedback control rate (LSEF).After taking the derivative of Equation (4), Equation ( 3) is substituted into the second-order differential equation [5] : Due to the duality of the equations of the d and q axes, only the d axis will be analyzed in the following paragraphs.
Therefore, the control principle diagram of the LADRC controller is shown in Figure 4: The control part of the second part is a virtual synchronous machine.Its implementation is as follows.First, the three-phase voltage is measured at the load and the three-phase current.The coordinate transforms the d and q axes and then calculates according to the power of the d and q axes voltage and current, active power, and reactive power.Secondly, the phase angle T and the reference electromotive force ref E are provided by the primary frequency modulation, the primary voltage regulation, and the rotor equation of motion controlled by the virtual synchronous machine.
In this paper, using the control strategy of a control strategy based on the grid and the voltage control based on LADRC coordinated control not only solved the grid system operation of power grid volatility and instability of hidden trouble but improved the power quality.

Simulation parameters
The test in this paper is based on the network based on the feasibility and effectiveness of the LADRC voltage control strategy.A three-phase off-grid system simulation model of the converter is built based on the Matlab/Simulink simulation platform.Through the PI controller and the normal operation of the LADRC controller in voltage and current double closed-loop and load mutation, the performance of the voltage loop is verified.The simulation system of main parameters is shown in Table 1 6 shows the instantaneous active power simulation waveforms under the control of the conventional PI controller and the LADRC controller when the grid-side converter is output.The reference value of active power is 270 kW, and the reference value of reactive power is 0 Var.As can be seen from Figure 8, the output power of the traditional PI controller fluctuates greatly, and the stabilization time is slow.In contrast, the output power of the LADRC controller fluctuates less, the stabilization speed is fast, and the control effect is superior.Figures 7(a) and 7(b) are the FFT analysis results of the A-phase current THD values under the two controllers, respectively.As can be seen from Figure 7, compared with the control of the traditional PI controller, the use of the LADRC controller can effectively reduce the harmonic distortion of the threephase current from 2.91% to 1.20%, and the current quality is improved, which is more conducive to the design of the filter.

Case 2 load mutation.
In order to verify the result that the performance of the LADRC link strategy remains unchanged in the case of load mutation, the duration is increased by 1 s on the basis of the original simulation, and the load power is abruptly reduced by 200 kW at 1 s, as shown in Figure 8.
As can be seen in Figure 8, in the case of sudden load changes, the power fluctuations of the system using the LADRC controller are still smaller and the tracking speed is faster than that of the PI controller.

Conclusion
The research on the grid-based control strategy of wind turbines based on LADRC voltage control proposed in this paper improves the problem of system stability degradation caused by grid fluctuation during the operation of grid-following control.The system has good performance in the process of power output.The network control based on LADRC voltage control not only retains the ability of network control to support the operation of the grid unit but also reduces the current ripple at a steady state, effectively reduces the current harmonic content, and gives the system the advantage of fast response.

where p K and q KFigure 1 .
Figure 1.Droop control.

Figure 2 .Figure 3 .
Figure 2. Virtual synchronizer control.3.Modeling of the grid-side converter and design of the LADRC controller

5K
According to Equation (5), the LADRC-controlled object model of the grid-side inverter is represented as follows: is the inverter gain, and u is the d-axis component of the voltage control signal.f is the total disturbance of the system.

Figure 6 .Figure 7 .
Figure 6.Simulated waveforms of active power output under different controllers.

Figure 8 .
Figure 8. Power simulation waveform at sudden load change.
. where La i , Lb i , and Lc i are the three-phase current output of the filter inductor.oa i , ob i , and oc i are the three-phase output current of the inverter.L is the AC side filter inductor.R is the AC side parasitic resistance.Mathematical model of grid-side converter.In the topology of the grid-side converter, d and q coordinates are established according to Kirchhoff's law, and the coordinate transformation of Park transform.The mathematical model of the grid-side converter is obtained as follows: f

Table 1 .
: Main simulation parameters.Normal operation.The simulation time is 1 s. Figure