Intrinsic self-adaptive damping method for network-building converter based on mixed dq-axis virtual power

Abstract

By simulating the physical characteristics and the operation mechanism of the traditional synchronous generator, the network construction type converter based on the control of the virtual synchronous generator has the capability of actively supporting the frequency and the voltage of a power grid, so that the stability of a novel power system is improved. When the disturbance is dealt with, the transient damping control does not introduce additional steady-state power deviation and the output power has no risk of low-frequency oscillation, so that the problem of insufficient equivalent damping adjustment freedom degree caused by control parameter coupling is solved. However, the dynamic adjustment of its control parameters depends on grid impedance parameters, while fixed control parameters may not adapt to different grid impedances. Therefore, the invention discloses a hybrid dq-axis virtual power-based intrinsic self-adaptive damping method for a network-building converter. Under different power grid working conditions, the method uses the same set of control parameters, and the active power step response performance can be improved without measuring power grid impedance information. When the active power reference value changes in a step mode, the output active power dynamic response shows excellent response performance. Meanwhile, the grid-forming converter can adapt to different power grid strengths and can stably operate under different power grid working conditions.

Description

Technical Field

[0001] The invention relates to an intrinsic adaptive damping method of a grid-connected converter based on hybrid dq-axis virtual power, and belongs to the technical field of power electronic converter control. Background Art

[0002] The decreasing proportion of traditional synchronous generators in the power mix, the increasing penetration of renewable energy, and the large-scale integration of power electronic equipment have become the evolutionary trends of modern power systems. This evolution has reshaped the power system's power structure and operating characteristics, resulting in a grid characterized by "low inertia and weak damping," posing unprecedented challenges to the stable operation of the power system. Against the backdrop of declining system stability due to the large-scale integration of renewable energy, virtual synchronous generator control technology is a highly sought-after solution. By integrating the rotor motion equations and reactive power / voltage regulation mechanisms into the converter's control algorithm, grid-connected converters using virtual synchronous generator control technology possess core and external characteristics similar to synchronous generators. Grid-connected converters can actively provide voltage and frequency support to the system, significantly enhancing the anti-interference and autonomous regulation capabilities of power electronic power systems, thereby improving system operational stability.

[0003] The traditional virtual damping link faces problems such as insufficient equivalent damping and control parameter coupling when dealing with disturbances such as active power reference value steps and grid frequency changes, which leads to the introduction of additional steady-state power deviations and the risk of low-frequency oscillation of the output power. In order to solve the problems caused by the traditional virtual damping link, a transient damping term is introduced into the active power control loop to improve the dynamic response and steady-state response characteristics of the active power control loop. Transient damping control solves the problem of insufficient freedom of equivalent damping adjustment caused by control parameter coupling. However, fixed control parameters may not be able to flexibly adapt to strong / weak grid conditions, and the dynamic adjustment of control parameters requires monitoring of grid impedance information, which is relatively cumbersome. In order to improve the control performance of grid-connected converters, it is necessary to propose a damping control strategy that uses the same control parameters and can adapt to different grid strengths, eliminating the dependence on grid impedance information. Summary of the Invention

[0004] In response to the problems existing in the transient damping control strategy, the present invention provides an intrinsic adaptive damping method for a grid-connected converter based on hybrid dq-axis virtual power. According to the characteristics of the d-axis component and the q-axis component of the grid-connected point voltage under different grid strengths, the damping method combines the d-axis voltage and the q-axis voltage to construct virtual power and form a feedback channel, which is equivalent to automatically adjusting the control parameters to suppress oscillations under strong / weak grids without relying on grid impedance information. At the same time, the active power reference value is processed by a differential controller to improve the response speed. The controller used in the proposed strategy is simple, the number of control parameters introduced is small, and the control parameter selection principle given based on small signal model analysis can provide effective guidance. The present invention not only realizes the use of the same set of control parameters under different grid conditions, but also improves the step response performance of the active power output of the grid-connected converter.

[0005] The intrinsic adaptive damping method of grid-connected converter based on hybrid dq-axis virtual power includes the following six parts:

[0006] Part I: Coordinate transformation, positive and negative sequence separation, and power calculation.

[0007] The specific steps include:

[0008] Step (1): Sample the three-phase AC voltage and three-phase AC current at the grid-connected point of the converter to obtain the voltage u oa 、u ob 、u oc and current i oa 、i ob 、i oc , the subscripts "a", "b", and "c" represent the voltage and current phase A, phase B, and phase C respectively. Then, the sampled u oa 、u ob 、u oc and i oa 、i ob 、i oc Perform 3s / 2s transformation respectively to obtain the αβ axis components of voltage and current: u oα 、u oβ and i oα 、i oβ ;

[0009] Step (2): For u oα 、u oβ and i oα 、i oβ Separate the positive and negative sequences and get u αp 、u βp 、u αn 、u βn and i αp 、i βp 、i αn 、iβn , the subscripts "p" and "n" represent the positive sequence component and negative sequence component of voltage and current respectively, the same below. Then, for u αp 、u βp and i αp 、i βp Perform the positive sequence 2s / 2r transformation to obtain the positive sequence dq axis components of voltage and current: u dp 、u qp and i dp 、i qp ; for u αn 、u βn and i αn 、i βn Perform the negative sequence 2s / 2r transformation to obtain the negative sequence dq axis components of voltage and current: u dn 、u qn 、i dn 、i qn ; Phase θ GMF Generated by the active power control loop;

[0010] Step (3): According to u oα 、u oβ and i oα 、i oβ Calculate the active power P separately o and reactive power Q o .

[0011] Part II: Active power control loop.

[0012] The active power output of the converter is controlled in a closed loop by simulating the rotor motion equation of the synchronous generator, thereby generating θ GMF .

[0013] Part III: Reactive power control loop.

[0014] According to the reactive power reference value Q ref And reactive power calculation value Q o , the reactive power-voltage droop equation is used to control the reactive power transmitted between the converter and the grid, thereby generating a virtual internal potential U emf .

[0015] Part 4: Control parameter selection.

[0016] A small signal closed-loop transfer function is established between the output active power and its reference value, and the value of the control parameter is selected according to the position of the closed-loop zero point and the size of the overshoot.

[0017] Part 5: Virtual impedance link and current control inner loop.

[0018] The specific steps include:

[0019] Step (1): Use the phase θ generated by the active power control loop GMF and the virtual internal potential U generated by the reactive power control loop emf Synthetic reference internal potential u emfa ,u emfb ,u emfc ;

[0020] Step (2): The voltage difference between the internal potential generated by the power control outer loop and the grid voltage passes through the virtual impedance link to generate the current reference value i aref ,i bref ,i cref ,;

[0021] Step (3): Perform 3s / 2r transformation on the current reference value to obtain the positive sequence dq axis component i of the current reference value dpref 、i qpref and negative sequence dq axis component i dnref 、i qnref The PI controller is used to control the grid-connected current to follow the change of the current reference value. The output of the PI controller is I dpPI , I qpPI , I dnPI and I qnPI .

[0022] Part 6: Trigger pulse generation link.

[0023] The specific steps include:

[0024] Step (1): Calculate the positive sequence dq axis component u of the modulation voltage command tdref_p 、u tqref_p and negative sequence dq axis component u tdref_n 、u tqref_n ;

[0025] Step (2): Perform 2r / 3s transformation on the positive and negative sequence dq axis components of the modulation voltage command to obtain the modulation voltage command u in the stationary coordinate system. taref_p 、u tbref_p 、u tcref_p and u taref_n 、u tbref_n 、u tcref_n The final modulation voltage instruction u is obtained by adding the positive and negative sequences of the same phase. taref 、u tbref 、u tcref ;

[0026] Step (3): Three-phase modulation voltage command u taref ,u tbref ,u tcref After the duty cycle calculation, the three-phase duty cycle signal g is obtained a 、gb and g c ; g a 、g b and g c Modulation is performed to obtain the trigger pulses of each switching tube.

[0027] The beneficial effects of the present invention are:

[0028] 1. The control strategy disclosed in the present invention constructs transient damping power by combining the d-axis component and the q-axis component of the grid-connected point voltage, and processes the active power reference value through a differential controller, thereby suppressing power oscillations and improving the system response speed.

[0029] 2. The present invention provides a control parameter selection principle through small signal model analysis, eliminating the dependence on grid impedance information.

[0030] 3. The control strategy disclosed in the present invention is easy to implement digitally and is suitable for application in various scenarios such as energy storage converters, wind power converters, and photovoltaic inverters. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 Schematic diagram of the intrinsic adaptive damping method of the grid-connected converter based on hybrid dq-axis virtual power.

[0032] Figure 2 This is the active power control block diagram. DETAILED DESCRIPTION

[0033] The present invention will be further described below with reference to the accompanying drawings.

[0034] like Figure 1 The intrinsic adaptive damping method of grid-connected converter based on hybrid dq axis virtual power includes the following six parts:

[0035] Part I: Coordinate transformation, positive and negative sequence separation, and power calculation.

[0036] like Figure 1 , specifically including the following steps:

[0037] Step (1): Sample the three-phase AC voltage and three-phase AC current at the grid-connected point of the converter to obtain the voltage u oa 、u ob 、u oc and current i oa 、i ob 、i oc , the subscripts "a", "b", and "c" represent the voltage and current phase A, phase B, and phase C respectively. Then, the sampled u oa 、u ob 、u oc and i oa、i ob 、i oc Perform 3s / 2s transformation respectively to obtain the αβ axis components of voltage and current: u oα 、u oβ and i oα 、i oβ , as shown in formula (1) and formula (2) respectively;

[0038]

[0039] Step (2): For u oα 、u oβ and i oα 、i oβ Perform positive and negative sequence separation, and the separation formula is shown in formula (3) and formula (4), and we get u αp 、u βp 、u αn 、u βn and i αp 、i βp 、i αn 、i βn , the subscripts "p" and "n" represent the positive sequence component and negative sequence component of voltage and current respectively, the same below. Then, for u αp 、u βp and i αp 、i βp Perform the positive sequence 2s / 2r transformation, as shown in formula (5), and obtain the positive sequence dq axis components of voltage and current: u dp 、u qp and i dp 、i qp ; for u αn 、u βn and i αn 、i βn Perform negative sequence 2s / 2r transformation, and the transformation formula is shown in Equation (6), and the negative sequence dq axis components of voltage and current are obtained: dn 、u qn 、i dn 、i qn ; Phase θ GMF Generated by the active power control loop;

[0040]

[0041] Where: F is the voltage or current signal, F' oα , F′ oβ F oα 、F oβ The voltage or current signal obtained after a delay of 1 / 4 power frequency period.

[0042] Step (3): According to u oα 、uoβ and i oα 、i oβ Calculate the active power P separately o and reactive power Q o , the calculation expression is shown in formula (7).

[0043]

[0044] Part II: Active power control loop.

[0045] like Figure 1 and Figure 2 , specifically including the following steps:

[0046] Step (1): Set u oα and u oβ Perform 2s / 2r transformation to obtain the d-axis component and q-axis component of the grid connection point voltage: and The specific expressions are shown in formula (8) and formula (9);

[0047]

[0048] Step (2): Utilize and Construct the damping power P respectively d and P q , its specific expressions in the s domain are shown in Equations (10) and (11);

[0049]

[0050] Where: K G is the gain, and its value is positively correlated with the ability to suppress oscillation, ω c For bandwidth.

[0051] Step (3): Using P d and P q Linear combination constructs virtual power P damping , and its specific expression is shown in formula (12).

[0052] P damping =mP d +P q (12)

[0053] Where: m is a constant used to scale K G .

[0054] Step (4): Using the active power reference value P ref The power reference value P with zero point is constructed by the differential controller ref_D , and its specific expression in the s domain is shown in formula (13).

[0055]

[0056] Where: T D is the time constant, ω c_D For bandwidth.

[0057] Step (5): Perform closed-loop control on the converter output active power by simulating the rotor motion equation of the synchronous generator, and then generate θ GMF , as shown in formula (14).

[0058]

[0059] Where: H is the inertia constant, k ω_p is the primary frequency modulation coefficient. ω Δ 、ω N are the error signal and angular frequency rating of the virtual angular frequency, respectively. The above power and angular frequency are per unit values.

[0060] Part III: Reactive power control loop.

[0061] like Figure 1 , according to the reactive power reference value Q ref And reactive power calculation value Q o , the reactive power-voltage droop equation is used to control the reactive power transmitted between the converter and the grid, thereby generating a virtual internal potential U emf , as shown in formula (15).

[0062] U emf =U N +k p_Q (Q ref -Q o ) (15)

[0063] Where: k p_Q is a drooping system, U N is the rated voltage. The above voltages are per unit values.

[0064] Part 4: Control parameter selection.

[0065] like Figure 1 , specifically including the following steps:

[0066] Step (1): Establish a small signal closed-loop transfer function between the output active power and its reference value, as shown in Equation (16);

[0067]

[0068] Where: U emf 、U g are the converter output voltage and grid voltage amplitude respectively, Xv +X g is the equivalent impedance between the converter and the grid. The above voltages are per unit values.

[0069] Step (2): Take ω c_D =250rad / s,ω c =150rad / s and ignore the pole s=-ω introduced by the differential controller c_D and ω c The related terms reduce the original third-order system to a second-order system. The G cp The expression of (s) is shown in formula (17), and the specific expressions of the coefficients b0~b2 of the characteristic equation are shown in formula (18);

[0070]

[0071] Step (3): Select T according to the influence of the closed-loop zero point z0 on the system response speed and damping degree. D The value of z0 needs to be selected based on the position of the dominant pole. The real part expression of the dominant pole s1 close to the imaginary axis is shown in Equation (19) and Equation (20). G When (or m) and SCR change, observe the change of the real part of s1 and roughly estimate its range by combining equations (19) and (20), and then select T D The value of

[0072]

[0073] in:

[0074]

[0075] Step (4): Let z = 1 / T D According to formula (17), the expression of overshoot σ% is calculated as shown in formula (21) and formula (22). Ignore mK in strong power grid (SCR≥3) G sinδ0 term, and cosδ0 is approximated to 1. The simplified b1 is shown in Equation (23). G When the SCR changes, observe the changing trend of σ% by combining equations (20) to (23). According to the characteristics of the changing trend of σ% under strong power grid, select K G The value of is adapted to different grid strengths;

[0076]

[0077] in:

[0078]

[0079] b1≈kω +K G ω N U0(23)

[0080] Step (5): Determine K G After selecting the value of m, we observe the variation trend of σ% in a weak grid (SCR < 3) by combining equations (20) to (22) when m and SCR vary. Based on the characteristics of the variation trend of σ% in a weak grid, we select the value of m as a compromise between oscillation suppression capability and dynamic response performance to adapt to different grid strengths.

[0081] Part 5: Virtual impedance link and current control inner loop.

[0082] like Figure 1 , specifically including the following steps:

[0083] Step (1): Use the phase θ generated by the active power control loop GMF and the virtual internal potential U generated by the reactive power control loop emf Synthetic reference internal potential u emfa ,u emfb ,u emfc , as shown in formula (24);

[0084]

[0085] Where: U mag is the virtual internal potential U emf The named value of .

[0086] Step (2): The voltage difference between the internal potential generated by the power control outer loop and the grid voltage passes through the virtual impedance link to generate the current reference value i aref ,i bref ,i cref , as shown in formula (25);

[0087]

[0088] Where: R v and L V are virtual resistance and virtual inductance respectively.

[0089] Step (3): Perform 3s / 2r transformation on the current reference value to obtain the positive sequence dq axis component i of the current reference value dpref 、i qpref and negative sequence dq axis component i dnref 、i qnref The PI controller is used to control the grid-connected current to follow the change of the current reference value. The output of the PI controller is I dpPI , I qpPI , I dnPI and IqnPI , and its specific expression is shown in formula (26).

[0090]

[0091] Where: K p , K i are the proportional coefficient and integral coefficient of the PI controller respectively.

[0092] Part 6: Trigger pulse generation link.

[0093] like Figure 1 , specifically including the following steps:

[0094] Step (1): Calculate the positive sequence dq axis component u of the modulation voltage command tdref_p 、u tqref_p and negative sequence dq axis component u tdref_n 、u tqref_n , as shown in formula (27);

[0095]

[0096] Where: L m is the filter inductance of the LC filter.

[0097] Step (2): Perform 2r / 3s transformation on the positive and negative sequence dq axis components of the modulation voltage command to obtain the modulation voltage command u in the stationary coordinate system. taref_p 、u tbref_p 、u tcref_p and u taref_n 、u tbref_n 、u tcref_n The final modulation voltage instruction u is obtained by adding the positive and negative sequences of the same phase. taref 、u tbref 、u tcref , as shown in formula (28);

[0098]

[0099] Step (3): Three-phase modulation voltage command u taref ,u tbref ,u tcref After the duty cycle calculation, the three-phase duty cycle signal g is obtained a 、g b and g c , the specific calculation formula is shown in (29); g a 、g b and g c Modulation is performed to obtain the trigger pulses of each switching tube.

[0100]

[0101] Where: U dc is the DC side voltage of the converter.

[0102] The applicant of the present invention has made a detailed explanation and description of the implementation examples of the present invention in conjunction with the drawings in the specification. However, those skilled in the art should understand that the above implementation examples are only preferred implementation plans of the present invention, and the detailed description is only to help readers better understand the spirit of the present invention, and is not a limitation on the scope of protection of the present invention. On the contrary, any improvements or modifications based on the inventive spirit of the present invention should fall within the scope of protection of the present invention.

[0103] The contents not described in detail in this specification belong to the prior art known to those skilled in the art.

Claims

1. A grid-connected converter intrinsic adaptive damping method based on hybrid dq axis virtual power, characterized in that: It specifically includes the following six parts: Part I: Coordinate transformation, positive and negative sequence separation, and power calculation. The specific steps include: Step (1): Sample the three-phase AC voltage and three-phase AC current at the grid-connected point of the converter to obtain the voltage u oa 、u ob 、u oc and current i oa 、i ob 、i oc , the subscripts "a", "b", and "c" represent the voltage and current phase A, phase B, and phase C respectively. Then, the sampled u oa 、u ob 、u oc and i oa 、i ob 、i oc Perform 3s / 2s transformation respectively to obtain the αβ axis components of voltage and current: u oα 、u oβ and i oα 、i oβ ; Step (2): For u oα 、u oβ and i oα 、i oβ Separate the positive and negative sequences and get u αp 、u βp 、u αn 、u βn and i αp 、i βp 、i αn 、i βn , the subscripts "p" and "n" represent the positive sequence components and negative sequence components of voltage and current respectively, the same below. Then, for u αp 、u βp and i αp 、i βp Perform the positive sequence 2s / 2r transformation to obtain the positive sequence dq axis components of voltage and current: u dp 、u qp and i dp 、i qp ; for u αn 、u βn and i αn 、i βn Perform the negative sequence 2s / 2r transformation to obtain the negative sequence dq axis components of voltage and current: u dn 、u qn 、i dn 、i qn ; Phase θ GMF Generated by the active power control loop; Step (3): According to u oα 、u oβ and i oα 、i oβ Calculate the active power P separately o and reactive power Q o . Part II: Active power control loop. The active power output of the converter is controlled in a closed loop by simulating the rotor motion equation of the synchronous generator, thereby generating θ GMF . Part III: Reactive power control loop. According to the reactive power reference value Q ref And reactive power calculation value Q o , the reactive power-voltage droop equation is used to control the reactive power transmitted between the converter and the grid, thereby generating a virtual internal potential U emf . Part 4: Control parameter selection. A small signal closed-loop transfer function is established between the output active power and its reference value, and the value of the control parameter is selected according to the position of the closed-loop zero point and the size of the overshoot. Part 5: Virtual impedance link and current control inner loop. The specific steps include: Step (1): Use the phase θ generated by the active power control loop GMF and the virtual internal potential U generated by the reactive power control loop emf Synthetic reference internal potential u emfa ,u emfb ,u emfc ; Step (2): The voltage difference between the internal potential generated by the power control outer loop and the grid voltage passes through the virtual impedance link to generate the current reference value i aref ,i bref ,i cref ,; Step (3): Perform 3s / 2r transformation on the current reference value to obtain the positive sequence dq axis component i of the current reference value dpref 、i qpref and negative sequence dq axis component i dnref 、i qnref The PI controller is used to control the grid-connected current to follow the change of the current reference value. The output of the PI controller is I dpPI , I qpPI , I dnPI and I qnPI . Part 6: Trigger pulse generation link. The specific steps include: Step (1): Calculate the positive sequence dq axis component u of the modulation voltage command tdref_p 、u tqref_p and negative sequence dq axis component u tdref_n 、u tqref_n ; Step (2): Perform 2r / 3s transformation on the positive and negative sequence dq axis components of the modulation voltage command to obtain the modulation voltage command u in the stationary coordinate system. taref_p 、u tbref_p 、u tcref_p and u taref_n 、u tbref_n 、u tcref_n The final modulation voltage instruction u is obtained by adding the positive and negative sequences of the same phase. taref 、u tbref 、u tcref ; Step (3): Three-phase modulation voltage command u taref ,u tbref ,u tcref After the duty cycle calculation, the three-phase duty cycle signal g is obtained a 、g b and g c ; g a 、g b and g c Modulation is performed to obtain the trigger pulses of each switching tube.

2. The grid-connected converter intrinsic adaptive damping method based on hybrid dq-axis virtual power according to claim 1 is characterized by: The active power control loop described in the second part specifically includes the following steps: Step (1): Set u oα and u oβ Perform 2s / 2r transformation to obtain the d-axis component and q-axis component of the grid connection point voltage: and The specific expressions are shown in formula (1) and formula (2); Step (2): Utilize and Construct the damping power P respectively d and P q , its specific expressions in the s domain are shown in Equations (3) and (4); Where: K G is the gain, and its value is positively correlated with the ability to suppress oscillation, ω c For bandwidth. Step (3): Using P d and P q Linear combination constructs virtual power P damping , and its specific expression is shown in formula (5). P damping =mP d +P q (5) Where: m is a constant used to scale K G . Step (4): Using the active power reference value P ref The power reference value P with zero point is constructed by the differential controller ref_D , and its specific expression in the s domain is shown in formula (6). Where: T D is the time constant, ω c_D For bandwidth. Step (5): Perform closed-loop control on the converter output active power by simulating the rotor motion equation of the synchronous generator, and then generate θ GMF , as shown in formula (7). Where: H is the inertia constant, k ω_p is the primary frequency modulation coefficient. ω Δ 、ω N are the error signal and angular frequency rating of the virtual angular frequency, respectively. The above power and angular frequency are per unit values.

3. The grid-connected converter intrinsic adaptive damping method based on hybrid dq-axis virtual power according to claim 1 is characterized in that: The control parameter selection described in the fourth part specifically includes the following steps: Step (1): Establish a small signal closed-loop transfer function between the output active power and its reference value, as shown in equation (8); Where: U emf 、U g are the converter output voltage and grid voltage amplitude respectively, X v +X g is the equivalent impedance between the converter and the grid. The above voltages are per unit values. Step (2): Take ω c_D =250rad / s,ω c =150rad / s and ignore the pole s=-ω introduced by the differential controller c_D and ω c The related terms reduce the original third-order system to a second-order system. The G cp The expression of (s) is shown in formula (9), and the specific expressions of the coefficients b0~b2 of the characteristic equation are shown in formula (10); Step (3): Select T according to the influence of the closed-loop zero point z0 on the system response speed and damping degree. D The value of z0 needs to be selected based on the position of the dominant pole. The real part expression of the dominant pole s1 close to the imaginary axis is shown in Equations (11) and (12). G When (or m) and SCR change, observe the change of the real part of s1 by combining equations (11) and (12) and roughly estimate its range, and then select T D The value of in: Step (4): Let z = 1 / T D According to formula (9), the expression of overshoot σ% is calculated as shown in formula (13) and formula (14). Ignore mK in strong power grid (SCR≥3) G sinδ0 term, and cosδ0 is approximated to 1, and the simplified b1 is shown in Equation (15). G When the SCR changes, observe the changing trend of σ% by combining equations (12) to (15). According to the characteristics of the changing trend of σ% under strong power grid, select K G The value of is adapted to different grid strengths; in: b1≈k ω +K G oh N U0(15) Step (5): Determine K G After selecting the value of m, we observe the changing trend of σ% in a weak grid (SCR < 3) by combining equations (12) to (14) when m and SCR vary. Based on the characteristics of the σ% changing trend in a weak grid, we select the value of m as a compromise between oscillation suppression capability and dynamic response performance to adapt to different grid strengths.

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