Virtual synchronous generator control method based on dynamic virtual impedance

Abstract

The invention provides a virtual synchronous generator control method based on dynamic virtual impedance, and belongs to the technical field of converter control. According to the method, the virtual synchronous generator is controlled by sampling filtering inductive current and three-phase output voltage, meanwhile, dynamic virtual impedance in a high-pass filter form is designed by simulating the positive damping characteristic of the synchronous generator, and low-frequency oscillation of the virtual synchronous generator is suppressed by showing small inductance in a low-frequency band. Only the current of the filter inductor is sampled, a grid-side current sensor is abandoned, and the cost is reduced. The method gives consideration to suppression of oscillation of the virtual synchronous generator system and reduction of the power coupling degree, and has good robustness under a strong / weak power grid.

Description

Technical Field

[0001] The invention relates to the technical field of converter control, in particular to a control method of a virtual synchronous generator based on dynamic virtual impedance. Background Art

[0002] With the increasing penetration of distributed energy, the spinning reserve capacity and rotational inertia in the power system have been relatively reduced, and the stability of the power grid has been seriously impacted. Therefore, experts and scholars at home and abroad have proposed the virtual synchronous generator (VSG) technology, which can provide inertia support and damping for the system by simulating the external operating characteristics of the synchronous generator.

[0003] The VSG control strategy can effectively improve the control performance of the grid-connected inverter, but its essence is still a power electronic device, and the system bandwidth is flexible and adjustable. When the line resistance is small, it is easy to excite the synchronous frequency resonance phenomenon, causing oscillation in the power transient process. Using dynamic virtual impedance technology to simulate the positive damping effect of the reactance of the synchronous generator to provide a positive damping torque for the system can suppress the low-frequency oscillation of the virtual synchronous generator system and reduce the degree of dynamic coupling between powers.

[0004] The "Virtual Synchronous Generator Control Method Based on Dynamic Virtual Resistor" published in the Chinese invention patent application specification CN201810083940A on January 29, 2018, designs the traditional steady-state virtual synchronous resistor into a dynamic virtual synchronous resistor in the form of a high-pass filter, which reduces the coupling degree of the active power and reactive power output of the virtual synchronous generator in the dynamic process and suppresses the oscillation of the virtual generator in the dynamic process. However, the purely resistive virtual impedance will cause system instability under strong grid conditions.

[0005] The Chinese invention patent application publication specification CN202010797198A published on August 10, 2020, "A control method for the damping effect of a virtual synchronous machine based on dynamic virtual impedance" analyzes the difference in equivalent internal impedance between the impedance model of the synchronous machine and the VSG, and constructs a dynamic virtual impedance with a lagging phase characteristic by imitating the positive damping mechanism of the synchronous machine. However, this control method increases the cost and control complexity of the system, which is not conducive to industrial application.

[0006] Based on the above literature, the existing virtual impedance technology has the following shortcomings:

[0007] 1. It is impossible to simultaneously suppress the oscillation of the virtual synchronous generator system, reduce the degree of power coupling and the system stability under strong grid conditions.

[0008] 2. The use of unnecessary grid-side current sensors and more complex control methods increases costs and deviates from industrial practice. Summary of the invention

[0009] The technical problem to be solved by the present invention is a problem existing in the prior art. The purpose is to provide a virtual synchronous generator control method based on dynamic virtual impedance, which can suppress the low-frequency oscillation of the virtual synchronous generator system and reduce the degree of power coupling, while also abandoning the grid-side current sensor, reducing costs and being closer to industrial practice.

[0010] In order to solve the technical problem of the present invention, the technical solution adopted is:

[0011] A virtual synchronous generator control method based on dynamic virtual impedance, the topological structure of the virtual synchronous generator using the control method comprises a DC voltage source, an inverter, an LCL filter and a grid impedance connected in sequence, the grid impedance is connected to a three-phase AC grid; the LCL filter comprises a source-side filter inductor, a grid-side filter inductor, a filter capacitor and a damping resistor;

[0012] The steps of the control method are as follows:

[0013] Step 1: Get the three-phase output voltage U of the virtual synchronous generator by sampling a ,U b ,U c , the current flowing through the source side filter inductor is recorded as the three-phase filter current i La ,i Lb ,i Lc , after the conversion from the three-phase stationary coordinate system to the two-phase stationary coordinate system, the output voltage αβ axis component U in the two-phase stationary coordinate system is obtained α ,U β and the filter current αβ axis component i Lα ,i Lβ ;

[0014] Step 2: According to the output voltage αβ axis component U obtained in step 1 α ,U β and the filter current αβ axis component i Lα ,i Lβ , calculate the active power P output by the virtual synchronous generator e and reactive power Q e ;

[0015] Step 3, according to the virtual synchronous generator control algorithm, the potential amplitude E and the angular frequency ω of the virtual synchronous generator are calculated, and the phase θ of the virtual synchronous generator is calculated according to the angular frequency ω;

[0016] Step 4: Substitute the output voltage αβ axis component U obtained in step 1 α ,U β and the filter current αβ axis component i Lα ,i LβUnder the phase θ of the virtual synchronous generator obtained in step 3, the two-phase stationary coordinate system is converted to the two-phase rotating coordinate system to obtain the output voltage dq-axis component U in the two-phase rotating coordinate system. d ,U q and the filtered current dq axis component i Ld ,i Lq ;

[0017] Step 5: The dq-axis component i of the filtered current obtained in step 4 Ld ,i Lq , the dq component of the internal reference voltage value of the converter is modified through dynamic virtual impedance control operation to obtain the output voltage dq axis reference value U of the virtual synchronous generator dref ,U qref , as follows:

[0018] Introducing dynamic virtual impedance Z dv , the dynamic virtual impedance Z dv It is a high-pass filter, and its expression is:

[0019]

[0020] Where K is the gain factor of the high-pass filter, ω s is the cutoff frequency of the high-pass filter, and s is the Laplace operator;

[0021] The dynamic virtual impedance Z is calculated dv The dq axis pressure drop U vd ,U vq , and its calculation formula is:

[0022]

[0023] According to the internal potential amplitude E and dynamic virtual impedance Z of the virtual synchronous generator obtained in step 3 dv The dq axis pressure drop U vd ,U vq , calculate the output voltage dq axis reference value U dref ,U qref , U dref =EU vd , U qref =0-U vq ;

[0024] Step 6: The output voltage dq axis component U obtained in step 4 d ,U q and the filtered current dq axis component i Ld ,i Lq , and the output voltage dq axis reference value U obtained in step 5 dref ,U qref, after voltage and current double closed-loop control, the output signal U in the dq coordinate system is obtained id ,U iq ;

[0025] Step 7: convert the output signal U obtained in step 6 into the dq coordinate system id ,U iq , transform the two-phase rotating coordinate system to the three-phase stationary coordinate system under the phase θ of the virtual synchronous generator obtained in step 3, and obtain the three-phase modulation wave U of the inverter bridge arm voltage ma ,U mb ,U mc , and after PWM modulation, it is used as a driving signal to control the on and off of the switch tube.

[0026] Preferably, the active power P output by the virtual synchronous generator in step 2 is e and reactive power Q e The calculation formulas are:

[0027]

[0028] Among them, ω c is the cutoff frequency of the low-pass filter.

[0029] Preferably, the calculation formulas for the potential amplitude E, angular frequency ω and phase θ in the virtual synchronous generator in step 3 are respectively:

[0030] E=U ref +D q (Q ref -Q e )

[0031]

[0032] Among them, P ref is the reference value of active power, Q ref is the reference value of reactive power, U ref is the reference value of the output line voltage, ω ref is the reference value of angular frequency, J P is the moment of inertia of the virtual synchronous generator, D p is the active power droop coefficient, D q is the droop coefficient of reactive power.

[0033] Preferably, the implementation process of step 6 is:

[0034] First, according to the output voltage dq axis component U obtained in step 4 d ,U q And the output voltage dq axis reference value U obtained in step 5 dref ,U qref, after voltage closed-loop control, the filter current dq axis reference value i is obtained Ldref ,i Lqref , whose expression is:

[0035] i Ldref =(k vp +k vi / s)(U dref -U d )

[0036] i Lqref =(k vp +k vi / s)(U qref -U q )

[0037] Among them, k vp is the voltage closed-loop proportional regulator coefficient, k vi is the voltage closed-loop integral regulator coefficient;

[0038] Secondly, according to the filter current dq axis component i obtained in step 4 Ld ,i Lq and the filtered current dq axis reference value i obtained in step 6 Ldref ,i Lqref , after current closed-loop control, the output signal U in the dq coordinate system is obtained id ,U iq , whose expression is:

[0039] U id =(k ip +k ii / s)(i Ldref -i Ld )

[0040] U iq =(k ip +k ii / s)(i Lqref -i Lq )

[0041] Among them, k ip is the current closed-loop proportional regulator coefficient, k ii is the current closed-loop integral regulator coefficient.

[0042] Compared with the prior art, the present invention has the following beneficial effects:

[0043] 1. While reducing power coupling, it suppresses the low-frequency oscillation of the virtual synchronous generator system and improves the stability of the system.

[0044] 2. Only the current of the filter inductor is sampled, and the grid-side current sensor is discarded, which reduces the cost and is more in line with industrial design.

[0045] 3. The implementation method is simple and efficient, and has good robustness in both strong and weak power grids. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 It is a grid-connected topology diagram of a virtual synchronous generator in an embodiment of the present invention.

[0047] Figure 2 It is a control block diagram of the method of the present invention.

[0048] Figure 3 It is a dynamic response simulation diagram of the virtual synchronous generator outputting active power and reactive power after adding virtual resistance.

[0049] Figure 4 It is a dynamic response simulation diagram of the virtual synchronous generator outputting active power and reactive power when dynamic virtual resistance is added. DETAILED DESCRIPTION

[0050] The present invention is further described below in conjunction with the accompanying drawings and specific embodiments.

[0051] Figure 1 : is a virtual synchronous generator grid-connected topology diagram in an embodiment of the present invention. Figure 1 As shown, the topological structure of the virtual synchronous generator to which the control method is applied includes a DC voltage source, an inverter, an LCL filter and a grid impedance connected in sequence, and the grid impedance is connected to a three-phase AC grid. The LCL filter includes a source-side filter inductor, a grid-side filter inductor, a filter capacitor and a damping resistor;

[0052] exist Figure 1 In the figure, L1 is the source side filter inductance, L2 is the grid side filter inductance, C f is the filter capacitor value, R d is the damping resistance value, L g is the grid impedance value, V dc is the voltage at the DC voltage source, V g Represents a three-phase AC power grid. The specific parameter values ​​are as follows: V dc =1500V, L1=0.07mH, L2=0.06mH, C f =669.6μF, R d =20mΩ, L g =0.64mH.

[0053] Figure 2 This is a control block diagram of a virtual synchronous generator with dynamic virtual impedance added in the present invention. As can be seen from the figure, the steps of the control method of the present invention are as follows:

[0054] Step 1: Get the three-phase output voltage U of the virtual synchronous generator by samplinga ,U b ,U c , the current flowing through the source side filter inductor is recorded as the three-phase filter current i La ,i Lb ,i Lc , after the conversion from the three-phase stationary coordinate system to the two-phase stationary coordinate system, the output voltage αβ axis component U in the two-phase stationary coordinate system is obtained α ,U β and the filter current αβ axis component i Lα ,i Lβ .

[0055] Step 2: According to the output voltage αβ axis component U obtained in step 1 α ,U β and the filter current αβ axis component i Lα ,i Lβ , calculate the active power P output by the virtual synchronous generator e and reactive power Q e .

[0056] In this embodiment, the active power P output by the virtual synchronous generator is e and reactive power Q e The calculation formulas are:

[0057]

[0058] Among them, ω c is the cutoff frequency of the low-pass filter, and s is the Laplace operator.

[0059] Step 3: According to the virtual synchronous generator control algorithm, the potential amplitude E and the angular frequency ω of the virtual synchronous generator are calculated, and the phase θ of the virtual synchronous generator is calculated according to the angular frequency ω.

[0060] In this embodiment, the calculation formulas for the potential amplitude E, angular frequency ω and phase θ in the virtual synchronous generator are respectively:

[0061] E=U ref +D q (Q ref -Q e )

[0062]

[0063] Among them, P ref is the reference value of active power, Q ref is the reference value of reactive power, U ref is the reference value of the output line voltage, ω ref is the reference value of angular frequency, J Pis the moment of inertia of the virtual synchronous generator, D p is the active power droop coefficient, D q is the droop coefficient of reactive power.

[0064] In this embodiment, it is set that: ref =1.7325MW, Q ref =0var,U ref =514V,ω ref =314rad / s,J P =15.96Kg·m 2 , D p =1125,D q =3.266e-5.

[0065] Step 4: Substitute the output voltage αβ axis component U obtained in step 1 α ,U β and the filter current αβ axis component i Lα ,i Lβ Under the phase θ of the virtual synchronous generator obtained in step 3, the two-phase stationary coordinate system is converted to the two-phase rotating coordinate system to obtain the output voltage dq-axis component U in the two-phase rotating coordinate system. d ,U q and the filtered current dq axis component i Ld ,i Lq .

[0066] Step 5: The dq-axis component i of the filtered current obtained in step 4 Ld ,i Lq , the dq component of the internal reference voltage value of the converter is modified through dynamic virtual impedance control operation to obtain the output voltage dq axis reference value U of the virtual synchronous generator dref ,U qref , as follows:

[0067] Introducing dynamic virtual impedance Z dv , the dynamic virtual impedance Z dv It is a high-pass filter, and its expression is:

[0068]

[0069] Where K is the gain factor of the high-pass filter, ω s is the cutoff frequency of the high-pass filter, and s is the Laplace operator;

[0070] The dynamic virtual impedance Z is calculated dv The dq axis pressure drop U vd ,U vq , and its calculation formula is:

[0071]

[0072] According to the internal potential amplitude E and dynamic virtual impedance Z of the virtual synchronous generator obtained in step 3 dv The dq axis pressure drop U vd ,U vq , calculate the output voltage dq axis reference value U dref ,U qref , U dref =EU vd , U qref =0-U vq .

[0073] In this embodiment, it is set that: K = 0.1, ω s =0.628rad / s.

[0074] Step 6: The output voltage dq axis component U obtained in step 4 d ,U q and the filtered current dq axis component i Ld ,i Lq , and the output voltage dq axis reference value U obtained in step 5 dref ,U qref , after voltage and current double closed-loop control, the output signal U in the dq coordinate system is obtained id ,U iq .

[0075] In this embodiment, firstly, the output voltage dq axis component U obtained in step 4 is d ,U q And the output voltage dq axis reference value U obtained in step 5 dref ,U qref , after voltage closed-loop control, the filter current dq axis reference value i is obtained Ldref ,i Lqref , whose expression is:

[0076] i Ldref =(k vp +k vi / s)(U dref -U d )

[0077] i Lqref =(k vp +k vi / s)(U qref -U q )

[0078] Among them, k vp is the voltage closed-loop proportional regulator coefficient, k vi is the voltage closed-loop integral regulator coefficient;

[0079] Secondly, according to the filter current dq axis component i obtained in step 4 Ld ,i Lq and the filtered current dq axis reference value i obtained in step 6 Ldref ,i Lqref , after current closed-loop control, the output signal U in the dq coordinate system is obtained id ,U iq , whose expression is:

[0080] U id =(k ip +k ii / s)(i Ldref -i Ld )

[0081] U iq =(k ip +k ii / s)(i Lqref -i Lq )

[0082] Among them, k ip is the current closed-loop proportional regulator coefficient, k ii is the current closed-loop integral regulator coefficient.

[0083] In this embodiment, it is set that: vp =0.48, k vi =195.48, k ip =0.07, k ii =0.8145.

[0084] Step 7: convert the output signal U obtained in step 6 into the dq coordinate system id ,U iq , transform the two-phase rotating coordinate system to the three-phase stationary coordinate system under the phase θ of the virtual synchronous generator obtained in step 3, and obtain the three-phase modulation wave U of the inverter bridge arm voltage ma ,U mb ,U mc , and after PWM modulation, it is used as a driving signal to control the on and off of the switch tube.

[0085] Figure 3 is the active power P output by the virtual synchronous generator when the virtual resistance is added e and reactive power Q e The dynamic response simulation diagram of . It can be seen from the figure that under this operating environment, even if the virtual impedance is added, the system still cannot find a stable operating point, and the active power P e and reactive power Q e Serious low-frequency oscillations occurred in the dynamic process.

[0086] Figure 4 is the active power P output by the virtual synchronous generator when the dynamic virtual resistance is added e and reactive power Q e It can be seen that after adding the dynamic virtual impedance, the low-frequency oscillation of the system is suppressed, and the active power P e It can follow the reference value well to reach steady state.

Claims

1. A virtual synchronous generator control method based on dynamic virtual impedance, wherein the topological structure of the virtual synchronous generator to which the control method is applied comprises a DC voltage source, an inverter, an LCL filter and a grid impedance connected in sequence, wherein the grid impedance is connected to a three-phase AC grid; the LCL filter comprises a source-side filter inductor, a grid-side filter inductor, a filter capacitor and a damping resistor; It is characterized in that The steps of the control method are as follows: Step 1: Get the three-phase output voltage U of the virtual synchronous generator by sampling a , U b , U c , the current flowing through the source side filter inductor is recorded as the three-phase filter current i La ,i Lb ,i Lc , after the conversion from the three-phase stationary coordinate system to the two-phase stationary coordinate system, the output voltage αβ axis component U in the two-phase stationary coordinate system is obtained α , U β and the filter current αβ axis component i Lα ,i Lβ ; Step 2: According to the output voltage αβ axis component U obtained in step 1 α , U β and the filter current αβ axis component i Lα ,i Lβ , calculate the active power P output by the virtual synchronous generator e and reactive power Q e ; Step 3, according to the virtual synchronous generator control algorithm, the potential amplitude E and the angular frequency ω of the virtual synchronous generator are calculated, and the phase θ of the virtual synchronous generator is calculated according to the angular frequency ω; Step 4: Substitute the output voltage αβ axis component U obtained in step 1 α , U β and the filter current αβ axis component i Lα ,i Lβ Under the phase θ of the virtual synchronous generator obtained in step 3, the two-phase stationary coordinate system is converted to the two-phase rotating coordinate system to obtain the output voltage dq-axis component U in the two-phase rotating coordinate system. d , U q and the filtered current dq axis component i Ld ,i Lq ; Step 5: The dq-axis component i of the filtered current obtained in step 4 Ld ,i Lq , the dq component of the internal reference voltage value of the converter is modified through dynamic virtual impedance control operation to obtain the output voltage dq axis reference value U of the virtual synchronous generator dref , U qref , as follows: Introducing dynamic virtual impedance Z dv , the dynamic virtual impedance Z dv It is a high-pass filter, and its expression is: Where K is the gain factor of the high-pass filter, ω s is the cutoff frequency of the high-pass filter, and s is the Laplace operator; The dynamic virtual impedance Z is calculated dv The dq axis pressure drop U vd , U vq , and its calculation formula is: According to the internal potential amplitude E and dynamic virtual impedance Z of the virtual synchronous generator obtained in step 3 dv The dq axis pressure drop U vd , U vq , calculate the output voltage dq axis reference value U dref , U qref , U dref =EU vd , U qref =0-U vq ; Step 6: The output voltage dq axis component U obtained in step 4 d , U q and the filtered current dq axis component i Ld ,i Lq , and the output voltage dq axis reference value U obtained in step 5 dref , U qref , after voltage and current double closed-loop control, the output signal U in the dq coordinate system is obtained id , U iq ; Step 7: convert the output signal U obtained in step 6 into the dq coordinate system id , U iq , transform the two-phase rotating coordinate system to the three-phase stationary coordinate system under the phase θ of the virtual synchronous generator obtained in step 3, and obtain the three-phase modulation wave U of the inverter bridge arm voltage ma , U mb , U mc , and after PWM modulation, it is used as a driving signal to control the on and off of the switch tube.

2. A virtual synchronous generator control method based on dynamic virtual impedance according to claim 1, characterized in that: The active power P output by the virtual synchronous generator in step 2 e and reactive power Q e The calculation formulas are: Among them, ω c is the cutoff frequency of the low-pass filter.

3. The method for controlling a virtual synchronous generator based on dynamic virtual impedance according to claim 1, characterized in that: The calculation formulas for the potential amplitude E, angular frequency ω and phase θ in the virtual synchronous generator in step 3 are: E=U ref +D q (Q ref -Q e ) Among them, P ref is the reference value of active power, Q ref is the reference value of reactive power, U ref is the reference value of the output line voltage, ω ref is the reference value of angular frequency, J P is the moment of inertia of the virtual synchronous generator, D p is the droop coefficient of active power, D q is the droop coefficient of reactive power.

4. A virtual synchronous generator control method based on dynamic virtual impedance according to claim 1, characterized in that: The implementation process of step 6 is: First, according to the output voltage dq axis component U obtained in step 4 d , U q And the output voltage dq axis reference value U obtained in step 5 dref , U qref , after voltage closed-loop control, the filter current dq axis reference value i is obtained Ldref ,i Lqref , whose expression is: i Ldref =(k vp +k vi / s)(U dref -U d ) i Lqref =(k vp +k vi / s)(U qref -U q ) Among them, k vp is the voltage closed-loop proportional regulator coefficient, k vi is the voltage closed-loop integral regulator coefficient; Secondly, according to the filter current dq axis component i obtained in step 4 Ld ,i Lq and the filtered current dq axis reference value i obtained in step 6 Ldref ,i Lqref , after current closed-loop control, the output signal U in the dq coordinate system is obtained id , U iq , whose expression is: U id =(k ip +k ii / s)(i Ldref -i Ld ) U iq =(k ip +k ii / s)(i Lqref -i Lq ) Among them, k ip is the current closed-loop proportional regulator coefficient, k ii is the current closed-loop integral regulator coefficient.

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