Grid-connected equipment virtual synchronous machine control method for eliminating grid-connected voltage harmonic disturbance

Abstract

A grid-connected equipment virtual synchronous machine control method for eliminating grid-connected voltage harmonic disturbance comprises the following steps that a virtual synchronous machine VSG power outer loop controller is designed, and the output of the VSG power outer loop controller serves as the input of a VSG voltage inner loop; the method comprises the following steps: establishing a state space mathematical model of a grid-side converter GSC of grid-connected equipment, designing a state feedback SF controller adopted by a VSG voltage inner loop, and obtaining a VSG voltage inner loop SF control closed-loop system; establishing a generalized integral GI controller based on a harmonic disturbance signal model; and a compensator of the GI controller is selected, and if the control gain of the GI controller meets the judgment condition, the GI controller is added to a control loop of the VSG voltage inner loop SF control closed system, so that the control system can be still kept stable. And the influence of large-scale new energy access on the power grid can be better dealt with, and the dynamic supporting capability of the network construction type flexible grid-connected equipment is enhanced.

Description

Technical Field

[0001] The invention relates to a virtual synchronous machine control method for grid-connected equipment for eliminating grid-connected voltage harmonic disturbance. Background Art

[0002] With the large-scale, high-penetration, and high-density access to the grid of distributed renewable energy generators (DG) and various loads using power electronic converter grid-connected interfaces, the penetration rate of conventional thermal power units based on synchronous generators (SG) continues to decline. However, the power electronic grid-connected interface does not have the characteristics of rigid body rotational inertia. When the system has a small disturbance and a power shortage, the DG unit cannot provide additional power dynamic support; while the conventional SG unit can respond to the process in real time due to its large rotational inertia. The decline in the penetration rate of SG units means that the power system will deteriorate in operational stability due to the lack of sufficient inertia support. In order to cope with the lack of system inertia and damping caused by the high-density and high-penetration renewable energy access, and to improve the grid frequency, voltage support capacity and system stability, the grid control technology based on virtual synchronous generators (VSG) came into being. By simulating the dynamic adjustment characteristics of synchronous generators, VSG can provide the necessary inertia and damping to suppress the fluctuations of grid frequency and output power during system disturbances.

[0003] Virtual synchronous generator (VSG) control provides grid control functions such as grid frequency support and droop control by simulating the virtual inertia and droop characteristics of synchronous generators to enhance the operational stability of the power system. VSG control includes an outer power control loop and an inner voltage control loop. The inner voltage control loop of conventional VSG control mostly uses a synchronous rotating reference frame (SRF) proportional integral (PI) controller, which cannot guarantee high-quality grid-connected (PCC) voltage under harmonic distortion disturbances, such as nonlinear rectifier charging loads, distorted grid voltage, etc. The distorted PCC voltage will increase the risk of oscillation of grid-connected wind turbines. In addition, the parameter design and setting of the PI controller parameters are usually completed by trial and error, and the parameter optimization and debugging process of the voltage controller is very time-consuming. Summary of the invention

[0004] The purpose of the present invention is to solve the deficiencies of the above-mentioned prior art, thereby providing a virtual synchronous machine control method for grid-connected equipment that eliminates grid-connected voltage harmonic disturbances, which can better cope with the impact of large-scale new energy access on the power grid and enhance the dynamic support capability of grid-connected flexible grid-connected equipment.

[0005] A method for controlling a virtual synchronous machine of a grid-connected device for eliminating grid-connected voltage harmonic disturbances comprises the following steps:

[0006] Step 1: Design a virtual synchronous machine VSG power outer loop controller, and use the VSG power outer loop controller output as the input of the VSG voltage inner loop;

[0007] Step 2: Establish a state space mathematical model of the grid-side converter GSC of the grid-connected equipment, design a state feedback SF controller used in the VSG voltage inner loop based on the state space mathematical model of GSC, and obtain the VSG voltage inner loop SF control closed-loop system;

[0008] Step 3: Establish a generalized integral GI controller based on the harmonic disturbance signal model;

[0009] Step 4: Select the compensator of the GI controller to fully compensate for the phase delay of the VSG voltage inner loop SF control closed loop system at the hth harmonic frequency. If the control gain of the GI controller meets the judgment condition, adding the GI controller to the control loop of the VSG voltage inner loop SF control closed loop system can keep the control system stable.

[0010] The virtual synchronous machine VSG power outer loop controller is:

[0011] (1)

[0012] (2)

[0013] Formula (1) represents the active power P of VSG and the frequency w m Relationship, where P ref is the input active signal of the VSG power outer loop, P is the active output signal of the grid-connected device, J is the moment of inertia of the VSG, and D is the damping coefficient; m is the mechanical angular frequency of VSG, w n is the grid angular frequency; Formula (2) represents the reactive power Q and voltage v of VSG m Droop characteristics, where v m is the grid connection point voltage reference value in the synchronous rotating dq axis coordinate system, v0 is the rated voltage, k q is the droop coefficient, Q ref is the input reactive signal of the VSG power outer loop, Q is the reactive output signal of the grid-connected equipment;

[0014] The output of the VSG power outer loop controller is used as the input of the VSG voltage inner loop, specifically:

[0015] The grid connection point voltage reference value v output by the VSG power outer loop m , after dq / abc coordinate transformation, it will be converted into the three-phase grid connection point voltage reference v in the stationary abc coordinate system ref,j (k), its voltage phase number j = a, b, c, three-phase grid voltage reference vref,j (k) will serve as the reference input of the VSG voltage inner loop.

[0016] Step 2 specifically includes the following steps:

[0017] The differential equation mathematical model of wind turbine GSC is established as follows:

[0018] (3)

[0019] Discretizing (3) yields the following discrete state space mathematical model of GSC:

[0020] (4)

[0021] in, is the state variable, is the control variable, , and is the coefficient matrix, i 1,j (k) is the j-phase inductor current on the GSC side, v o,j (k) is the voltage at the grid point of phase j, u j (k) is the control signal of the j-phase GSC, i o,j (k) is the output current of GSC flowing to the grid;

[0022] Based on the discrete state space mathematical model of GSC, the SF controller used in the VSG voltage inner loop is as follows:

[0023] (5)

[0024] in , j is the voltage phase number, j = a, b, c;

[0025] Substituting equation (5) into equation (4), the VSG voltage inner loop SF control closed-loop system can be obtained as follows:

[0026] (6)

[0027] By selecting the feedback coefficient matrices F and Y, the SF controller can directly configure the poles of the VSG voltage inner loop SF control closed loop system, and by optimizing the pole configuration within the unit circle, a VSG voltage inner loop SF control closed loop system with good rapidity and stability can be obtained;

[0028] For the VSG voltage inner loop SF control closed loop system, the grid-connected point voltage reference Voltage to the grid point v o,j The transfer function of (k) is written as .

[0029] The GI controller is:

[0030] (7)

[0031] in, The frequency is w h The hth harmonic disturbance signal is expressed as:

[0032] (8)

[0033] k h The frequency is w h The control gain of the GI controller for the hth harmonic disturbance, F h (z) is the frequency w h The compensator for the hth harmonic disturbance, q h At frequency w h The phase compensation angle at S is the sampling frequency of the control system.

[0034] The compensator of the GI controller is selected to fully compensate the phase delay of the VSG voltage inner loop SF control closed-loop system at the hth harmonic frequency, which is expressed as:

[0035] .

[0036] The judgment condition of the GI controller control gain is:

[0037] .

[0038] The VSG control method for grid-connected equipment proposed in the present invention can quickly, robustly and accurately adjust the grid-connected voltage, eliminate the harmonic distortion of the grid-connected voltage caused by harmonic disturbances, reduce the risk of system oscillation caused by the VSG control method, and better adapt to the development requirements of future new energy-friendly grid-connected technologies.

[0039] The present invention is aimed at grid-connected devices, and proposes a virtual synchronous machine voltage inner loop controller composed of a state feedback (SF) controller based on a grid-side converter (GSC) model and a generalized integral (GI) controller based on a harmonic disturbance signal model, which is used to quickly, robustly and accurately adjust the grid-connected voltage and eliminate the harmonic distortion caused by harmonic disturbances, thereby improving the power quality output by the grid-connected devices and the system stability.

[0040] The present invention provides a simple and feasible controller parameter design and selection method for a proposed virtual synchronous machine voltage inner loop controller composed of a state feedback controller based on a grid-side converter model and a generalized integral controller based on a harmonic disturbance signal model. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 It is the topology diagram of the VSG control system of the grid-type wind turbine under the condition of harmonic distortion disturbance;

[0042] Figure 2 It is the control block diagram of VSG power outer loop controller;

[0043] Figure 3 It is the control block diagram of the conventional VSG voltage inner loop PI controller;

[0044] Figure 4 It is the control block diagram of the VSG voltage inner loop state feedback SF controller based on the GSC model;

[0045] Figure 5 The voltage and current waveforms of the GSC grid connection point when the VSG voltage inner loop adopts the PI voltage controller;

[0046] Figure 6 The voltage and current waveforms of the grid connection point when the VSG voltage inner loop designed by the present invention adopts the SF+GI composite voltage controller. DETAILED DESCRIPTION

[0047] The virtual synchronous machine control method of the grid-connected equipment (such as a wind turbine) for eliminating grid-connected voltage harmonic disturbance of the present invention comprises the following steps:

[0048] Step 1, design the VSG power outer loop controller as follows:

[0049] (1)

[0050] (2)

[0051] In the above formula, formula (1) is used to design the active power (P)-frequency (w m ) relationship, where P ref is the input active signal of the power outer loop, P is the active output signal of GSC, J is the moment of inertia of VSG, and D is the damping coefficient; m is the mechanical angular frequency of VSG, w n is the grid angular frequency; Formula (2) is used to design the reactive power (Q)-voltage (v m ) droop characteristics. Where v m is the reference value of the grid connection point voltage, v0 is the rated voltage, k q is the droop coefficient, Q ref is the input reactive signal of the VSG power outer loop, and Q is the reactive output signal of the GSC.

[0052] The grid connection point voltage reference value v output by the VSG power outer loop m, after dq / abc coordinate transformation, it will be converted into the three-phase grid connection point voltage reference v in the stationary abc coordinate system ref,j (k), its voltage phase number j = a, b, c. Three-phase grid connection point voltage reference v ref,j (k) will serve as the reference input of the VSG voltage inner loop.

[0053] Step 2, establish a mathematical model of the grid-connected converter GSC on the grid side of the grid-connected device, then design and establish a state feedback SF controller of the VSG inner loop voltage loop based on the mathematical model of GSC, and obtain the VSG voltage inner loop SF control closed-loop system;

[0054] The differential equation mathematical model of wind turbine GSC is established as follows:

[0055] (3)

[0056] Discretizing (3) yields the following discrete state space mathematical model of GSC:

[0057] (4)

[0058] in, is the state variable, is the control variable, , and is the coefficient matrix, i 1,j (k) is the j-phase inductor current on the GSC side, v o,j (k) is the PCC voltage of phase j, u j (k) is the control signal of the j-phase GSC, i o,j (k) is the output current flowing to the grid.

[0059] Based on the state space mathematical model of GSC, the VSG inner voltage loop adopts the SF controller as follows:

[0060] (5)

[0061] in is the grid connection point voltage reference of phase j = a, b, c.

[0062] Substituting equation (5) into equation (4), the VSG voltage inner loop closed-loop system using the SF controller can be obtained as follows:

[0063] (6)

[0064] By selecting the feedback coefficient matrices F and Y, the SF controller can directly and arbitrarily configure the poles of the VSG voltage inner loop SF control closed loop system, and obtain a VSG voltage inner loop SF control closed loop system with good rapidity and stability by optimizing the pole configuration within the unit circle. For the VSG voltage inner loop SF control closed loop system, from the grid connection point voltage reference Voltage to the grid point v o,j The transfer function of (k) is written as .

[0065] Step 3: Based on the harmonic disturbance signal model, a generalized integrator (GI) controller is established as follows:

[0066] (7)

[0067] in, The frequency is w h The h-th harmonic disturbance signal is modeled as follows

[0068] (8)

[0069] k h The frequency is w h The control gain of the GI controller for the hth harmonic disturbance, F h (z) is the frequency w h The compensator for the hth harmonic disturbance, q h At frequency w h The phase compensation angle at S is the sampling frequency of the control system.

[0070] Step 4: Design and select F h (z) It can completely compensate the phase delay of the voltage inner loop SF control closed loop system at the hth harmonic frequency, that is, , if the control gain of the GI controller satisfies

[0071] ,

[0072] After adding the GI controller to the VSG voltage inner loop SF control closed-loop system, the control system will remain stable.

[0073] The VSG voltage inner loop composite controller, which consists of an SF controller based on the GSC model and a GI controller based on the harmonic disturbance signal model, will be able to eliminate the grid connection point voltage distortion caused by harmonic disturbances while maintaining the rapidity and robustness of the SF controller, thereby reducing a series of risks such as system harmonic oscillations.

[0074] Example:

[0075] The present invention implements and verifies the effectiveness of the VSG voltage inner loop voltage control method designed according to the present invention when there is a nonlinear rectifier load disturbance at the grid connection point of the grid-connected equipment (such as a wind turbine) and harmonic distortion in the grid voltage. The present invention selects the conventional PI control method for the VSG voltage inner loop as a comparison scheme to verify the superiority of the control method proposed by the present invention. The specific situation is as follows:

[0076] Figure 1 (a) is the block diagram of the VSG control system for a grid-type wind turbine. Figure 1 (b) is the circuit topology diagram of the VSG control system of the grid-type wind turbine. The wind turbine includes a wind rotor, a permanent magnet synchronous generator, a machine-side converter RSC, and a grid-side converter GSC. RSC is a three-phase AC / DC converter, GSC is a three-phase DC / AC converter, the permanent magnet synchronous generator is connected to the AC side of the machine-side converter, the wind rotor is installed on the input shaft of the permanent magnet synchronous generator, the permanent magnet synchronous generator and the wind rotor form a wind turbine, the DC side of RSC and GSC are connected to capacitors in parallel, the AC side of the three-phase GSC is connected to the grid, a nonlinear rectifier load is connected at the grid connection point (PCC), and the equivalent inductance of the grid line is L g,j , j=a, b, c Grid voltage v g,j , j=a, b, c contains harmonic distortion.

[0077] When the wind turbine is working in the grid-building mode, the GSC adopts the VSG control mode. The VSG control includes an outer power control loop and an inner voltage control loop.

[0078] like Figure 2 As shown, the outer power control loop defines the grid frequency support and droop control of the GSC by simulating the virtual inertia and droop characteristics of the synchronous generator as follows:

[0079] (1)

[0080] (2)

[0081] like Figure 3 As shown, the conventional inner voltage control loop mostly adopts a model-free dq coordinate system PI controller.

[0082] Since the PI controller cannot effectively suppress the harmonic distortion of the PCC voltage, and the trial-and-error method to design and adjust the parameters of the model-free PI controller is time-consuming and laborious, and it is difficult to optimize the controller performance. Therefore, according to the method proposed in the present invention, the model information of the GSC and the harmonic disturbance signal information of the PCC point are fully utilized to design and construct a VSG voltage inner loop controller based on the system model.

[0083] First, the differential equation mathematical model of the wind turbine GSC is established as follows:

[0084] (3)

[0085] Discretizing (3) yields the following discrete state space mathematical model of GSC:

[0086] (4)

[0087] in, is the state variable, is the control variable, , and is the coefficient matrix, i 1,j (k) is the j-phase inductor current on the GSC side, v o,j (k) is the voltage at the grid point of phase j, u j (k) is the control signal of the j-phase GSC, i o,j (k) is the output current flowing to the grid.

[0088] Based on the discrete state space mathematical model of GSC, the VSG voltage inner loop adopts a state feedback (SF) controller as follows:

[0089] (5)

[0090] in is the PCC reference voltage.

[0091] like Figure 4 As shown, by substituting equation (5) into equation (4), the VSG voltage inner loop SF control closed-loop system can be obtained as follows:

[0092] (6)

[0093] By selecting the feedback coefficient matrices F and Y, the SF controller can directly and arbitrarily configure the poles of the VSG inner loop SF closed-loop feedback voltage loop control system within the unit circle, and obtain a VSG inner loop voltage feedback control loop with good speed and stability by optimizing the pole configuration. For example, select F and Y so that the poles of the SF control system are configured at the origin, that is, =0 and HY=1, then , that is, a fast control response without beat is achieved And because the poles of the deadbeat control system are located at the origin, far away from the unit circle stability boundary, the SF control system generally has good robust stability.

[0094] Furthermore, in order to eliminate the PCC voltage distortion caused by harmonic disturbance, a voltage inner loop additional generalized integrator (GI) controller based on the harmonic disturbance signal model is constructed as follows:

[0095] (7)

[0096] Where h∈N represents the order of the main harmonic in the harmonic distortion disturbance; The frequency is w h The h-th harmonic disturbance signal is modeled as follows

[0097] (8)

[0098] k h The frequency is w h The control gain of the GI controller for the hth harmonic disturbance, F h (z) is the frequency w h The compensator for the hth harmonic disturbance, q h At frequency w h The phase compensation angle at S is the sampling frequency of the control system.

[0099] Design Selection F h (z) to fully compensate for the phase delay of the SF control system at the hth harmonic frequency, so that If the control gain of the GI controller satisfies

[0100] ,

[0101] Then the control system can remain stable after the VSG inner loop SF voltage control loop is added with the GI controller.

[0102] The simulation analysis uses the following system parameters:

[0103] When VSG adopts the conventional PI voltage inner loop controller, the waveforms of PCC voltage and current are as follows: Figure 5 As shown, Figure 5 (a) is the voltage waveform of the GSC grid connection point when the conventional PI voltage inner loop controller is used. Figure 5 (b) is the current waveform of the GSC grid-connected point when the conventional PI voltage inner loop controller is used, in which the total harmonic distortion (THD) of the PCC voltage is as high as 7.26%, exceeding the 5% specified in the grid-connected regulations.

[0104] When the VSG adopts the voltage controller based on the system model proposed in the present invention, the waveforms of the PCC voltage and current are as follows: Figure 6 As shown, Figure 6 (a) is a voltage waveform diagram of the GSC grid connection point using the method proposed by the present invention. Figure 6 (b) is a current waveform diagram of the GSC grid-connected point using the method proposed in the present invention, in which the total harmonic distortion (THD) of the PCC voltage is only 1.8%, which is much lower than the 5% specified in the grid-connected specification.

Claims

1. A method for controlling a virtual synchronous machine of a grid-connected device for eliminating grid-connected voltage harmonic disturbances, characterized in that: The following steps are involved: Step 1: Design a virtual synchronous machine VSG power outer loop controller, and use the VSG power outer loop controller output as the input of the VSG voltage inner loop; Step 2: Establish a state space mathematical model of the grid-side converter GSC of the grid-connected equipment, design a state feedback SF controller used in the VSG voltage inner loop based on the state space mathematical model of GSC, and obtain the VSG voltage inner loop SF control closed-loop system; Step 3: Establish a generalized integral GI controller based on the harmonic disturbance signal model; Step 4: Select the compensator of the GI controller to fully compensate for the phase delay of the VSG voltage inner loop SF control closed loop system at the hth harmonic frequency. If the control gain of the GI controller meets the judgment condition, adding the GI controller to the control loop of the VSG voltage inner loop SF control closed loop system can keep the control system stable.

2. A method for controlling a virtual synchronous machine of a grid-connected device for eliminating grid-connected voltage harmonic disturbance according to claim 1, characterized in that: The virtual synchronous machine VSG power outer loop controller is: (1) (2) Formula (1) represents the active power P of VSG and the frequency w m Relationship, where P ref is the input active signal of the VSG power outer loop, P is the active output signal of the grid-connected device, J is the moment of inertia of the VSG, and D is the damping coefficient; m is the mechanical angular frequency of VSG, w n is the grid angular frequency; Formula (2) represents the reactive power Q and voltage v of VSG m Droop characteristics, where v m is the grid connection point voltage reference value in the synchronous rotating dq axis coordinate system, v0 is the rated voltage, k q is the droop coefficient, Q ref is the input reactive signal of the VSG power outer loop, and Q is the reactive output signal of the grid-connected device.

3. A method for controlling a virtual synchronous machine of a grid-connected device for eliminating grid-connected voltage harmonic disturbance according to claim 1, characterized in that: The output of the VSG power outer loop controller is used as the input of the VSG voltage inner loop, specifically: The grid connection point voltage reference value v output by the VSG power outer loop m , after dq / abc coordinate transformation, it will be converted into the three-phase grid connection point voltage reference v in the stationary abc coordinate system ref,j (k), its voltage phase number j = a, b, c, three-phase grid voltage reference v ref,j (k) will serve as the reference input of the VSG voltage inner loop.

4. A method for controlling a virtual synchronous machine of a grid-connected device for eliminating grid-connected voltage harmonic disturbance according to claim 1, characterized in that: Step 2 specifically includes the following steps: The differential equation mathematical model of wind turbine GSC is established as follows: （3） Discretizing (3) yields the following discrete state space mathematical model of GSC: (4) in, is the state variable, is the control variable, , and is the coefficient matrix, i 1,j (k) is the j-phase inductor current on the GSC side, v o,j (k) is the grid-connected voltage of phase j, u j (k) is the control signal of the j-phase GSC, i o,j (k) is the output current of GSC flowing to the grid; Based on the discrete state space mathematical model of GSC, the SF controller used in the VSG voltage inner loop is as follows: (5) in , j is the voltage phase number, j = a, b, c; Substituting equation (5) into equation (4), the VSG voltage inner loop SF control closed-loop system can be obtained as follows: (6) By selecting the feedback coefficient matrices F and Y, the SF controller can directly configure the poles of the VSG voltage inner loop SF control closed loop system, and by optimizing the pole configuration within the unit circle, a VSG voltage inner loop SF control closed loop system with good rapidity and stability can be obtained; For the VSG voltage inner loop SF control closed loop system, the grid-connected point voltage reference Voltage to the grid point v o,j The transfer function of (k) is written as .

5. A method for controlling a virtual synchronous machine of a grid-connected device for eliminating grid-connected voltage harmonic disturbance according to claim 1, characterized in that: The GI controller is: (7) in, The frequency is w h The hth harmonic disturbance signal is expressed as: (8) k h The frequency is w h The control gain of the GI controller for the hth harmonic disturbance, F h (z) is the frequency w h The compensator for the hth harmonic disturbance, q h At frequency w h The phase compensation angle at S is the sampling frequency of the control system.

6. A method for controlling a virtual synchronous machine of a grid-connected device for eliminating grid-connected voltage harmonic disturbance according to claim 1, characterized in that: The compensator of the GI controller is selected to fully compensate the phase delay of the VSG voltage inner loop SF control closed-loop system at the hth harmonic frequency, which is expressed as: 。 7. A method for controlling a virtual synchronous machine of a grid-connected device for eliminating grid-connected voltage harmonic disturbance according to claim 1, characterized in that: The judgment condition of the GI controller control gain is: 。

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