Grid-following Inverter Phase Locking Method and System Based on Composite Voltage Regulation

Abstract

The present application discloses a grid - following inverter phase - locking method and system based on composite voltage regulation, belonging to the technical field of inverter control. The method of the present application includes: detecting the grid current and the grid - connected point voltage, and obtaining the grid current and the output voltage in the dq coordinate system through coordinate transformation; obtaining the active power of the grid - following inverter according to the output voltage and the grid current in the dq coordinate system; obtaining the reference current of the output current according to the active power of the grid - following inverter; obtaining the transformation angle in the dq coordinate system according to the error between the output voltage and the voltage reference value; establishing a model of the composite synchronous dominant loop to obtain the composite synchronous loop gain; updating the transformation angle and the sampled current according to the composite synchronous loop gain; and generating a PWM signal through a current regulator to drive the IGBT. The method of the present application can achieve the composite voltage synchronous control of the grid - connected inverter, and significantly improve the robustness of the grid - following inverter under strong and weak grids.

Description

Technical Field

[0001] The present invention relates to the technical field of inverter control, and in particular to a grid-following inverter phase-locking method and system based on composite voltage regulation. Background Art

[0002] Distributed power generation systems based on renewable energy sources such as wind and solar energy are developing on a large scale. Grid-connected inverters, as the energy conversion interface between distributed power generation units and the power grid, play a vital role in the safe, stable and efficient operation of distributed power generation systems. Centralized new energy power generation clusters are mostly located at the end of the power grid (such as the Northwest Wind and Solar Base), lacking conventional power sources such as thermal power and hydropower, and showing weak power grid characteristics. The grid impedance cannot be ignored and may vary over a wide range. The grid-following inverter achieves synchronization with the grid voltage through a phase-locked loop. In a weak power grid, the phase-locked loop and the grid impedance are interactively coupled, which may cause synchronization stability problems. Summary of the invention

[0003] The purpose of this application is to overcome the defects of the prior art and provide a phase-locking method and system for a grid-following inverter based on composite voltage regulation, aiming to improve the stable operation of the grid-following inverter under a wide range of grid impedance.

[0004] In a first aspect, the present application provides a grid-following inverter phase locking method based on composite voltage regulation, comprising the following steps:

[0005] Detect the grid current and perform coordinate transformation on the grid current to obtain dq The grid current in the coordinate system is dq The grid current in the coordinate system includes the d-axis grid current and the q-axis grid current; the grid connection point voltage is detected, and the grid connection point voltage is transformed into a coordinate system to obtain dq The output voltage in the coordinate system is dq The output voltage in the coordinate system includes the d-axis output voltage and the q-axis output voltage;

[0006] according to dq The output voltage and grid current in the coordinate system are used to obtain the active power output by the grid-following inverter;

[0007] Obtaining a reference current of a grid current according to the active power output by the grid-following inverter and the d-axis output voltage, wherein the reference current of the grid current includes a d-axis reference current and a q-axis reference current;

[0008] The dq The output voltage in the coordinate system is compared with the corresponding output voltage reference value to obtain dq The error between the output voltage in the coordinate system and the corresponding output voltage reference value is obtained by passing the error through the phase-locked loop controller.dq Transformation angle in the coordinate system;

[0009] Establish a model of the composite synchronization master loop, obtain the loop gain from the grid voltage phase angle to the grid-connected voltage phase angle, and obtain the updated dq Transformation angle in the coordinate system;

[0010] Send the updated dq Transformation angle in the coordinate system into the grid current coordinate transformation path to obtain a new sampled current, where the new sampled current includes a d-axis sampled current and a q-axis sampled current;

[0011] Send the new sampled current and the error between the new sampled current and the reference current into the current controller, and obtain the PWM modulation signal through current regulation to achieve the control of the grid-following inverter.

[0012] Optionally, the expression for the active power output by the grid-following inverter is:

[0013]

[0014] where P is the active power output by the grid-following inverter, is the d-axis output grid voltage, is the q-axis output voltage, i g_d is the d-axis grid current, i g_q is the q-axis grid current.

[0015] Optionally, the matrix expression for the reference current of the grid current is:

[0016]

[0017] where, i gref_d is the d-axis reference current of the grid current, i gref_q is the q-axis reference current of the grid current, is the active power loop controller, is the voltage loop controller, P ref is the active power reference value, P is the active power output by the grid-following inverter, U ref_d is the d-axis output voltage reference value, U d is the d-axis output voltage actual value.

[0018] Optionally, comparing the output voltage in the dq coordinate system with the voltage reference value to obtain dqThe error between the output voltage and the voltage reference value in the coordinate system is passed through a phase-locked loop controller to obtain dq the transformation angle in the coordinate system, including:

[0019] Compare the q-axis grid voltage with the q-axis output voltage reference value to obtain the q-axis output voltage error, and pass the q-axis output voltage error through a q-axis PI controller to obtain the q-axis angular frequency variation;

[0020] Compare the d-axis grid voltage with the d-axis output voltage reference value to obtain the d-axis output voltage error, and pass the d-axis output voltage error through a d-axis PI controller to obtain the d-axis angular frequency variation;

[0021] Add the q-axis angular frequency variation, the d-axis angular frequency variation, and the rated angular frequency and multiply by a Laplace integrator to obtain the phase angle of the output voltage.

[0022] Optionally, establish a model of the composite synchronization dominant loop to obtain the loop gain from the grid voltage phase angle to the grid-connected voltage phase angle, and obtain the updated dq transformation angle in the coordinate system, including:

[0023] In dq coordinate system, establish an equivalent circuit of the grid-following inverter;

[0024] According to the equivalent circuit of the grid-following inverter, obtain the LCL dynamic equation;

[0025] Use the q-axis phase-locked loop controller to determine dq the rotational angular frequency of the coordinate system, and transform the grid voltage to dq coordinate system according to the rotational angular frequency of the dq coordinate system, and obtain the dq vector linearized representation of the grid voltage, and obtain the dq vector;

[0026] According to the dq vector of the grid voltage, obtain the simplified grid-connected voltage;

[0027] According to the simplified grid-connected voltage, obtain the composite synchronization dominant loop gain transfer function;

[0028] Use the composite synchronization dominant loop gain transfer function to obtain the updated dq transformation angle in the coordinate system.

[0029] Optionally, the dq vector linearized expression of the grid voltage is:

[0030]

[0031] wherein, is the grid voltage amplitude, is the dq vector V gdq between the grid-connected voltage dq vector U dq steady-state value of the power angle, is the linearized small-signal power angle quantity, is the linearized grid voltage d axis component, is the linearized grid voltage q axis component.

[0032] Optionally, the composite synchronization dominant loop gain transfer function is:

[0033]

[0034] wherein, is the d axis component of the composite synchronization dominant loop gain transfer function, is the q axis component of the composite synchronization dominant loop gain transfer function.

[0035] Optionally, the dq rotation angular frequency of the coordinate system is expressed as:

[0036]

[0037] wherein, is the small-signal quantity of the grid-connected angular frequency, is the rated angular frequency.

[0038] Optionally, the small-signal quantity of the grid-connected angular frequency is expressed as:

[0039]

[0040] wherein, is the q axis phase-locked loop transfer function, is the q-axis component of the grid-connected voltage, is the q axis PI controller, and s is the Laplace operator.

[0041] In a second aspect, the present application further provides a grid-following inverter phase-locking system based on composite voltage regulation. The grid-following inverter phase-locking system based on composite voltage regulation executes the steps of the grid-following inverter phase-locking method based on composite voltage regulation according to any one of the first aspects, including:

[0042] A data acquisition module, which is used to detect the grid current and the grid connection point voltage, perform coordinate transformation on the grid current and the grid connection point voltage, and obtain the grid current and the output voltage;

[0043] A power calculation module, which is used to calculate the active power output by the grid-connected inverter according to the grid current and the output voltage;

[0044] A reference current calculation module, which is used to determine the reference current of the output current according to the active power output by the grid-connected inverter and the output voltage;

[0045] A transformation angle module, which is used to obtain dq the transformation angle in the coordinate system according to the output voltage and the voltage reference value;

[0046] A composite synchronous dominant loop and loop gain module, which is used to construct a composite synchronous dominant loop model, and use the composite synchronous dominant loop model to obtain the loop gain and a new dq coordinate system transformation angle;

[0047] A sampled current update module, which is used to update the sampled current according to the new dq coordinate system transformation angle;

[0048] A PWM modulation module, which is used to obtain a PWM modulation signal according to the new sampled current and control the grid-connected inverter.

[0049] The present application provides a grid-connected inverter phase-locked method and system based on composite voltage regulation. By introducing d axis voltage control to track the grid connection point voltage phase in real time, combined with q-axis voltage error correction, a composite synchronous dominant loop model is constructed, which can dynamically adjust the rotation angular frequency of the dq coordinate system, effectively suppress the influence of LCL filter resonance on stability, and improve the anti-interference ability of the phase-locked loop under weak grids; by establishing the loop gain, the balance between the phase-locked loop bandwidth and stability is achieved, which can not only ensure the phase-locked accuracy, but also enhance the robustness of the system to grid impedance changes.

[0050] To make the above features and advantages of the invention more obvious and understandable, specific embodiments are given below and detailed descriptions are made in conjunction with the accompanying drawings as follows. Description of the Drawings

[0051] In order to more clearly illustrate the technical solutions in the embodiments of the present application or related technologies, the following will briefly introduce the accompanying drawings required for the description of the embodiments or related technologies. Obviously, the accompanying drawings in the following description are only some embodiments of the present application. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0052] Figure 1 This is the control block diagram of the grid - following inverter phase - locking method based on composite voltage regulation provided in an embodiment of the present application.

[0053] Figure 2 This is the flow chart of the grid - following inverter phase - locking method based on composite voltage regulation provided in an embodiment of the present application.

[0054] Figure 3 This is the flow chart of step S4 in the grid - following inverter phase - locking method based on composite voltage regulation provided in an embodiment of the present application.

[0055] Figure 4 This is the flow chart of step S5 in the grid - following inverter phase - locking method based on composite voltage regulation provided in an embodiment of the present application.

[0056] Figure 5 This is the equivalent circuit diagram of the grid - following inverter in the grid - following inverter phase - locking method based on composite voltage regulation provided in an embodiment of the present application.

[0057] Figure 6 This is the block diagram of the transfer function of the composite synchronization dominant loop in the grid - following inverter phase - locking method based on composite voltage regulation provided in an embodiment of the present application.

[0058] Figure 7 This is the amplitude - frequency characteristic curve diagram of the sweep - frequency verification of the closed - loop transfer function of the synchronization dominant loop of the grid - following inverter before improvement in the grid - following inverter phase - locking method based on composite voltage regulation provided in an embodiment of the present application.

[0059] Figure 8 This is the amplitude - frequency characteristic curve diagram of the sweep - frequency verification of the closed - loop transfer function of the synchronization dominant loop of the grid - following inverter based on composite voltage regulation after improvement in the grid - following inverter phase - locking method based on composite voltage regulation provided in an embodiment of the present application.

[0060] Figure 9 This is the simulation waveform diagram of the grid - connected voltage, current and frequency of the grid - following inverter before improvement under weak grid conditions in the grid - following inverter phase - locking method based on composite voltage regulation provided in another embodiment of the present application.

[0061] Figure 10 This is the simulation waveform diagram of the grid - connected voltage, current and frequency of the composite - synchronization grid - following inverter after improvement under weak grid conditions in the grid - following inverter phase - locking method based on composite voltage regulation provided in another embodiment of the present application.

[0062] Figure 11 This is the structural schematic diagram of the grid - following inverter phase - locking system based on composite voltage regulation provided in another embodiment of the present application. Detailed implementation manners

[0063] To make the objectives and technical solutions of the embodiments of the present application clearer, the technical solutions of the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the described embodiments of the present application without creative efforts shall fall within the scope of protection of the present application.

[0064] Figure 1 is the control block diagram of the grid-following inverter phase-locking method based on composite voltage regulation provided by the present application. From Figure 1 it can be seen that the grid-following inverter includes: a power supply module 1, a three-phase inverter bridge module 2, an LCL filter module 3, a common power grid module 4, and a control module 5. Among them, the power supply module 1 is connected to the first end of the three-phase inverter bridge module 2, the second end of the three-phase inverter bridge module 2 is connected to the first end of the LCL filter module 3, the second end of the LCL filter module 3 is connected to the common power grid module 4 through the point of common coupling PCC, the input end of the control module 5 is connected to the point of common coupling PCC and the third end of the LCL filter module 3, and the output end of the control module 5 is connected to the third end of the three-phase inverter bridge module 2.

[0065] As an example, the power supply module 1 may include a DC power supply V dc ; the three-phase inverter bridge module 2 may include switching tubes and freewheeling diodes; LCL the filter module 3 may include a filter resistor R f , a filter inductor L f1 , L f2 and a filter capacitor C ; the common power grid module 4 may include a three-phase voltage source v g and an equivalent power grid impedance Z g , and the equivalent power grid impedance Z g is equivalently obtained by a grid resistor R g and a grid inductor L g ; the controller module 5 may include a phase-locked loop controller, an output current controller, a capacitor current controller, an active loop controller, a voltage loop controller, power calculation, and an abc / dq converter.

[0066] In one embodiment, please refer to Figure 2, this application provides a grid - following inverter phase - locking method based on composite voltage regulation. The grid - following inverter phase - locking method based on composite voltage regulation may include the following steps: Step S1 to Step S7.

[0067] Step S1: Detect the grid current, and perform coordinate transformation on the grid current to obtain dq the grid current in the dq coordinate system. The grid current in the dq coordinate system includes the d - axis grid current and the q - axis grid current; Detect the grid - connection point voltage, and perform coordinate transformation on the grid - connection point voltage to obtain dq the output voltage in the

[0068] coordinate system. The output voltage in the dq coordinate system includes the d - axis output voltage and the q - axis output voltage.

[0069] Step S2: According to the d output voltage and the grid current in the q coordinate system, obtain the active power output by the grid - following inverter.

[0070] Step S3: According to the active power output by the grid - following inverter and the d - axis output voltage, obtain the reference current of the grid current. The reference current of the grid current includes dq the dq axis reference current and dq the transformation angle of the

[0071] coordinate system obtained by passing the error between the output voltage in the dq coordinate system and the corresponding output voltage reference value through a phase - locked loop controller.

[0072] Step S5: Establish the model of the composite synchronous dominant loop, obtain the loop gain from the grid voltage phase angle to the grid - connection voltage phase angle, and obtain the updated dq transformation angle of the d coordinate system. The new sampled current includes q the

[0073] axis sampled current and

[0074] In the grid - following inverter phase - locking method based on composite voltage regulation of the present application, the coordinate transformation angle is corrected in real time through a phase - locked loop controller, which can effectively eliminate the grid - connection impact current; the reference current is calculated collaboratively using the active power and the d - axis voltage, which can establish a direct power - current mapping relationship; by introducing d axis voltage control to track the grid - connection point voltage phase in real time, generating the reference angle of the synchronous rotating coordinate system, and combining with the power calculation module to generate the reference signal of the current loop, the composite voltage synchronization control of the grid - connected inverter is realized. It can achieve higher power density and lower losses while ensuring the grid stability, significantly improving the robustness of the grid - following inverter under strong and weak grids, and has significant engineering application value.

[0075] In step S1, refer to Figure 2 step S1 therein, detect the grid current, and perform coordinate transformation on the grid current to obtain the dq grid current in the dq coordinate system. The dq grid current in the dq coordinate system includes the d - axis grid current and the q - axis grid current; detect the grid - connection point voltage, and perform coordinate transformation on the grid - connection point voltage to obtain the dq output voltage in the dq coordinate system. The dq output voltage in the dq coordinate system includes the d - axis output voltage and the q - axis output voltage.

[0076] Specifically, detect the grid current through a current sensor i g , and perform coordinate transformation on the grid current i g to obtain the dq grid current in the dq coordinate system. The dq grid current in the dq coordinate system includes the d - axis grid current i g_d and the q - axis grid current i g_q .

[0077] Furthermore, detect the grid - connection point voltage through a voltage sensor (i.e., the output voltage ), and perform coordinate transformation on the output voltage to obtain the dq output voltage in the dq coordinate system. The dq output voltage in the dq coordinate system includes the d - axis output voltage and the q - axis output voltage .

[0078] As an example, synchronous sampling technology can be used to ensure the phase alignment of current and voltage signals.

[0079] In step S2, refer to Figure 2In step S2, based on dq the output voltage and grid current in the coordinate system, the active power output by the grid-connected inverter is obtained.

[0080] Specifically, based on dq the d-axis output voltage 、the q-axis output voltage 、the d-axis grid current i g_d and the q-axis grid current i g_q , the active power P output by the grid-connected inverter is obtained, and the expression is:

[0081]

[0082] where is d the d-axis output voltage, is q the q-axis output voltage, i g_d is d the d-axis grid current, i g_q is q the q-axis grid current.

[0083] In step S3, please refer to Figure 2 step S3 therein. Based on the active power output by the grid-connected inverter and the d-axis output voltage, the reference current of the grid current is obtained. The reference current of the grid current includes d the d-axis reference current and q the q-axis reference current.

[0084] Specifically, based on the active power P, the d-axis output voltage , the d-axis output voltage reference value U ref_d , the active power reference value P ref , the reference current of the grid current is obtained. The reference current of the grid current includes d the d-axis reference current i gref_d and q the q-axis reference current i gref_q .

[0085] As an example, the matrix expression of the reference current of the grid current is:

[0086]

[0087] where is the active power loop controller, is the voltage loop controller,P ref is the reference value of the active power, U ref_d is the reference value of the d-axis output voltage, U d is the d-axis output voltage actual value, and P is the active power output by the grid-connected inverter.

[0088] As an example, the active power loop controller has the following expression:

[0089]

[0090] where is the proportional coefficient, is the integral coefficient, and s is the Laplace operator.

[0091] As an example, the voltage loop controller has the following expression:

[0092]

[0093] where is the proportional coefficient, is the integral coefficient, and s is the Laplace operator.

[0094] As an example, the reference value of the active power P ref can be set to 10 KW.

[0095] As an example, the reference value of the d-axis output voltage U ref_d can be set to 311 V.

[0096] In step S4, refer to Figure 2 step S4 in, and compare the output voltage in the dq coordinate system with the corresponding output voltage reference value to obtain dq the error between the output voltage in the coordinate system and the corresponding output voltage reference value. Pass the error through the phase-locked loop controller to obtain dq the transformation angle of the coordinate system.

[0097] As an example, refer to Figure 3 , step S4 may include the following steps: steps S41 to S43.

[0098] Step S41: Compare the q axis output voltage with the q axis output voltage reference value to obtain q the axis output voltage error. Pass the q axis output voltage error through the q axis PI controller to obtain qAxis angular frequency variation.

[0099] Step S42: Compare the d axis output voltage with the d axis output voltage reference value to obtain the d axis output voltage error. Pass the d axis output voltage error through the d axis PI controller to obtain the d axis angular frequency variation.

[0100] Step S43: Add the q axis angular frequency variation, the d axis angular frequency variation, and the rated angular frequency, and multiply the sum by 1 / s to obtain the phase angle of the output voltage.

[0101] Specifically, in Step S41, compare the q axis output voltage with the q axis output voltage reference value U ref_q to obtain the q axis output voltage error. Pass the q axis output voltage error through the q axis phase-locked loop controller F q (s) to obtain the q axis angular frequency variation .

[0102] As an example, the expression of the q axis phase-locked loop controller F q (s) is:

[0103]

[0104] where is the proportionality coefficient, is the integral coefficient, and s is the Laplace operator.

[0105] As an example, q the axis output voltage reference value U ref_q can be set to 0V.

[0106] Furthermore, in Step S42, compare the d axis output voltage with the d axis output voltage reference value U ref_d to obtain the d axis output voltage error. Pass the d axis output voltage error through the d axis phase-locked loop controller F d(s), obtain d Axis angular frequency variation .

[0107] As an example, the d Axis phase-locked loop controller F d (s) The expression is:

[0108]

[0109] Wherein, Is the proportionality coefficient.

[0110] As an example, d Axis output voltage reference value U ref_d Can be set to 311V.

[0111] Furthermore, in step S43, the q Axis angular frequency variation , d Axis angular frequency variation , Rated angular frequency Are added and multiplied by the Laplace integrator 1 / s to obtain dq Transformation angle in the coordinate system θ .

[0112] As an example, the rated angular frequency The expression is:

[0113]

[0114] Wherein, Is the grid rated frequency and can be set to 50Hz.

[0115] As an example, the rated angular frequency Can be set to 100p rad / s.

[0116] In step S5, please refer to Figure 2 In step S5, establish the model of the composite synchronization dominant loop to obtain the loop gain from the grid voltage phase angle to the grid-connected voltage phase angle, and obtain the updated dq Transformation angle in the coordinate system.

[0117] As an example, please refer to Figure 4 , Step S5 may include the following steps: Step S51~Step S56.

[0118] Step S51: In dq Coordinate system, establish the equivalent circuit of the grid-following inverter.

[0119] Step S52: Obtain the LCL dynamic equation according to the equivalent circuit of the grid-following inverter.

[0120] Step S53: Use the q-axis phase-locked loop controller to determine dq the rotational angular frequency of the coordinate system, and transform the grid voltage according to dq the rotational angular frequency of the coordinate system to dq the coordinate system, and obtain the dq vector of the grid voltage.

[0121] Step S54: Obtain the linearized representation of the vector of the grid voltage to obtain the simplified grid-connected voltage. dq vector of the grid voltage, and obtain the dq vector linearized representation of the grid voltage to obtain the simplified grid-connected voltage.

[0122] Step S55: Obtain the composite synchronous dominant loop gain transfer function according to the simplified grid-connected voltage.

[0123] Step S56: Use the composite synchronous dominant loop gain transfer function to obtain the updated dq transformation angle in the coordinate system.

[0124] Specifically, in Step S51, simplify the circuit topology of the grid-following inverter to obtain dq the simplified circuit of the grid-following inverter in the coordinate system, as Figure 5 shown. The simplified circuit of the grid-following inverter includes: the output voltage of the grid-following inverter port, the machine-side inductance impedance Z f1 the grid-side inductance impedance Z f2 the grid-side impedance Z g the three-phase voltage source v g the capacitance admittance Y c . Among them, the first end of the machine-side inductance impedance Z f1 is connected to the output voltage of the grid-following inverter port, and the second end of the machine-side inductance impedance Z f1 is connected to the first end of the grid-side inductance impedance Z f2 , the second end of the grid-side inductance impedance Z f2 is connected to the first end of the grid-side impedance Z g , the second end of the grid-side impedance Z g is connected to the three-phase voltage source v g the capacitance admittance Yc The first end of Z f1 is connected to the machine-side inductive impedance Y c The second end of

[0125] As an example, the machine-side inductive impedance Z f1 may include a filter inductor L f1 and a filter resistor R f The first end of the filter resistor R f is connected to the output voltage of the grid-connected inverter port The filter resistor R f The second end is connected to the first end of the filter inductor L f1 ; The grid-side inductive impedance Z f2 may include a filter inductor L f2 The first end of the filter inductor L f2 is connected to the second end of the filter inductor L f1 The second end of the filter inductor L f2 is connected to the first end of the grid-side impedance Z g ; The grid-side impedance Z g may include a grid resistor R g and a grid inductor L g The first end of the grid resistor R g is connected to the second end of the filter inductor L f2 The second end of the grid resistor R g is connected to the first end of the grid inductor L g The second end of the grid inductor L g is connected to the three-phase voltage source v g ; The capacitive admittance Y c may include a filter capacitor C The filter capacitor C The first end is connected to the second end of the filter inductor L f1 The second end of the filter capacitor CThe second end of [filter capacitor] is grounded. The capacitor voltage of the filter capacitor C is U c , and the filter capacitor C has a capacitor current of I c . The grid-side impedance Z g has a grid current of I g , and the output current of the grid-following inverter is I .

[0126] As an example, in step S52, to suppress the influence of LCL filter resonance on stability, active damping of the capacitor current is added to control the LCL filter module 3, and the LCL dynamic equation expression can be obtained as follows:

[0127]

[0128]

[0129]

[0130] where is the vector of the capacitor voltage U c , dq the vector of is the grid-connected voltage U , dq the vector of is the output voltage at the port of the grid-following inverter , dq the vector of is the grid current I g , dq the vector of is the output current of the grid-following inverter I , dq the vector of is the capacitor current I c , dq the vector of

[0131] As an example, the vector U c of the capacitor voltage dq is expressed as:

[0132]

[0133] where is the capacitor voltage U c , dThe axial component, is the capacitor voltage U c 's q axial component.

[0134] As an example, the grid-connected voltage U 's dq vector expression is:

[0135]

[0136] Wherein, is the grid-connected voltage U 's d axial component, is the grid-connected voltage U 's q axial component.

[0137] As an example, the output voltage of the grid-following inverter port 's dq vector expression is:

[0138]

[0139] Wherein, is the output voltage of the grid-following inverter port 's d axial component, is the output voltage of the grid-following inverter port 's q axial component.

[0140] As an example, the grid current I g 's dq vector expression is:

[0141]

[0142] Wherein, is the grid current I g 's d axial component, is the grid current I g 's q axial component.

[0143] As an example, the output current of the grid-following inverter I 's dq vector expression is:

[0144]

[0145] Among them, is the d-axis component of the output current of the grid-following inverter I of d the is the q-axis component of the output current of the grid-following inverter I of q the

[0146] As an example, the vector I c of dq the capacitor current

[0147]

[0148] Among them, is the d-axis component of I c the d capacitor current is the q-axis component of I c the q capacitor current

[0149] Furthermore, based on the active damping G AD (s) of the capacitor current and the LCL dynamic equation, the output voltage dq vector of the grid-following inverter port can be obtained, and the expression is:

[0150]

[0151] Among them, is the PWM gain of the grid-following inverter, is the reference voltage dq vector of the output voltage of the grid-following inverter port is the grid current I g of dq the reference value of I g the dq grid current is the vector I c of dq the capacitor current is the 1.5-beat digital control delay, is the output current controller of the grid-following inverter,

[0152] As an example, the reference voltage dqVector The expression is:

[0153]

[0154] Wherein, is the reference voltage of the grid voltage d axis component, is the reference voltage of the grid voltage q axis component.

[0155] As an example, the PWM gain of the grid-connected inverter The expression is:

[0156]

[0157] Wherein, is the amplitude of the triangular carrier wave, is the DC power supply.

[0158] As an example, the triangular carrier wave amplitude can be set to 3V or 4.58V.

[0159] As an example, the transfer function expression of the 1.5-beat digital control delay is:

[0160]

[0161] Wherein, T s is the sampling period, s is the Laplace operator, represents the approximate continuous domain of the 1.5-beat digital control delay.

[0162] As an example, the output current controller of the grid-connected inverter The expression is:

[0163]

[0164] Wherein, is the proportional coefficient, is the integral coefficient, s is the Laplace operator.

[0165] As an example, the capacitor current controller The expression is:

[0166]

[0167] Wherein, is the proportional coefficient, is the integral coefficient, s is the Laplace operator.

[0168] Further, in step S53, using a q-axis phase-locked loop controller, determine dq the rotational angular frequency of the coordinate system as:

[0169]

[0170] wherein, is the small-signal quantity of the grid-connected angular frequency, is the rated value of the angular frequency.

[0171] As an example, the expression of the small-signal quantity of the grid-connected angular frequency is:

[0172]

[0173] wherein, is the q transfer function of the q-axis phase-locked loop, is the q-axis component of the grid-connected voltage, is the q q-axis PI controller, and s is the Laplace operator.

[0174] As an example, q the transfer function of the q-axis phase-locked loop is expressed as:

[0175]

[0176] wherein, is the proportional gain, is the integral gain, is the q q-axis PI controller, and s is the Laplace operator.

[0177] Further, in order to form the closed-loop dynamics of the entire system, it is necessary to transform the grid voltage (i.e., the three-phase voltage source v g ) to the dq coordinate system to obtain the dq vector V gdq whose expression is:

[0178]

[0179] wherein, is the d d-axis component of the grid voltage, is the q q-axis component of the grid voltage.

[0180] Further, in step S54, the dq vector Vgdq , grid-connected voltage dq vector U dq , grid current dq vector I gdq The relationship between them is:

[0181]

[0182] Among them, R g is the grid resistance, L g is the grid inductance, is dq the rotational angular frequency of the coordinate system, and s is the Laplace operator.

[0183] Furthermore, the dq vector V gdq of the grid voltage can be linearly represented at the steady-state operating point of the system as:

[0184]

[0185] Among them, is the grid voltage amplitude, is the dq vector V gdq of the grid voltage and the dq vector U dq of the grid-connected voltage, is the steady-state value of the power angle, is the linearized small-signal quantity of the power angle, d is the x-axis component of the linearized grid voltage, is the y-axis component of the linearized grid voltage. q

[0186] As an example, the expression of the linearized small-signal quantity of the power angle is:

[0187]

[0188] Among them, is the small-signal quantity of the grid-connected voltage phase angle, is the small-signal quantity of the grid voltage phase angle.

[0189] As an example, the simplified grid-connected voltage obtained is:

[0190]

[0191] Among them, is the simplified grid-connected voltage​d The d-axis component, is the simplified grid-connected voltage q The q-axis component, R g is the grid resistance, L g is the grid inductance, is the rated angular frequency, is q the d-axis PI controller, of the grid voltage dq vector V gdq and the grid-connected voltage dq vector U dq the steady-state power angle value between them, is the grid voltage amplitude, is the grid-connected current q the steady-state value of the d-axis component, of the grid voltage dq vector V gdq and the grid-connected voltage dq vector U dq the small-signal power angle between them, , is the identity matrix.

[0192] As an example, the matrix expression is:

[0193]

[0194] where, is the PWM gain of the grid-following inverter, is the filter capacitor, is the filter inductor, is the filter inductor, is the filter resistor, is the filter inductor L f1 equivalent resistance of, is the filter inductor L f2 equivalent resistance of, is the rated angular frequency, is the 1.5 beat digital control delay, is the Laplace operator, is the grid-connected voltage d steady-state value of the d-axis component, is the active power loop controller, is the grid-following inverter output current controller, is the capacitor current controller.

[0195] As an example, the matrix expression is:

[0196]

[0197] where is the steady-state value of the grid-connected current d axis component, is the PWM gain of the grid-following inverter, is the filter capacitor, is the filter inductor, is the filter inductor, is the filter resistor, is the filter inductor L f1 equivalent resistance, is the filter inductor L f2 equivalent resistance, is the rated angular frequency, is the 1.5-beat digital control delay, is the Laplace operator, is the active loop controller, is the output current controller of the grid-following inverter, is the capacitor current controller, and E is the identity matrix.

[0198] Furthermore, in step S55, according to the simplified grid-connected voltage, taking as the input, the simplified grid-connected voltage q axis component transfer function is:

[0199]

[0200] where is the simplified grid-connected voltage q axis component, R g is the grid resistance, L g is the grid inductance, is the rated angular frequency, is q axis PI controller, is the dq vector V gdq of the grid voltage dq vector U dq between the power angle steady-state value and the grid-connected voltage is the grid voltage amplitude, is the steady-state value of the in-phase component of the grid-connected current q , is the dq vector V gdq of the grid voltage dq vector U dq and the small-signal quantity of the power angle between the grid-connected voltage , is the identity matrix

[0201] Similarly, the simplified transfer function of the in-phase component d of the grid-connected voltage can be obtained .

[0202] Furthermore, the transfer function of the composite synchronous dominant loop gain is as follows

[0203]

[0204] where is the in-phase component q of the transfer function of the composite synchronous dominant loop gain and d is the quadrature component of the transfer function of the composite synchronous dominant loop gain

[0205] Specifically, the expression of the in-phase component q of the transfer function of the composite synchronous dominant loop gain is as follows

[0206]

[0207] where is the transfer function of the in-phase component q of the grid-connected voltage , and q is the transfer function of the quadrature-axis phase-locked loop

[0208] As an example q the transfer function of the quadrature-axis phase-locked loop is expressed as

[0209]

[0210] where is the q quadrature-axis PI controller, and s is the Laplace operator

[0211] As an example, the expression of the in-phase component d of the transfer function of the composite synchronous dominant loop gain is as follows

[0212]

[0213] wherein, is the grid-connected voltage d axis component transfer function of, is d axis phase-locked loop transfer function.

[0214] As an example, d axis phase-locked loop transfer function expression is:

[0215]

[0216] wherein, is d axis PI controller, s is the Laplace operator.

[0217] As an example, d axis PI controller The proportional coefficient value can be obtained according to the parameter root locus analysis under different grid strength conditions K PLLdpmax , K PLLdpmin constraint curve enclosed area, wherein, K PLLdpmax , K PLLdpmin respectively represent the maximum and minimum values under this SCR (short circuit ratio), that is, K PLLdpmax , K PLLdpmin respectively represent the maximum and minimum values that the proportional term of the d-axis phase-locked loop controller can take.

[0218] As an example, the d-axis phase-locked loop controller can be a proportional controller.

[0219] At this time, dq coordinate system rotation angular frequency is:

[0220]

[0221] wherein, is the small-signal quantity of the grid-connected angular frequency, is the rated value of the angular frequency.

[0222] Specifically, the grid-connected angular frequency expression is:

[0223]

[0224] wherein, is the grid-connected voltage q axis component, For the grid-connected voltage d axis component, is q axis phase-locked loop transfer function, is d axis phase-locked loop transfer function, is q axis PI controller, is d axis PI controller, where s is the Laplace operator.

[0225] Furthermore, in step S56, Figure 6 is the block diagram of the composite synchronization dominant loop transfer function. From Figure 6 it can be seen that the small-signal quantity of the input grid voltage phase angle , after passing through the composite synchronization dominant loop gain transfer function , obtains the small-signal quantity of the grid-connected voltage phase angle . Based on the composite synchronization dominant loop gain and according to the Nyquist stability criterion, the parameters of the phase-locked loop controller can be optimized to determine the updated dq transformation angle in the coordinate system θ’ , where θ’ is the small-signal quantity of the grid-connected voltage phase angle obtained after passing through the composite synchronization dominant loop gain transfer function superimposed with the steady-state quantity, which is used to control the abc / dq transformation.

[0226] Figure 2 In step S6, please refer to step S6 in Figure 2 . Send the updated dq transformation angle in the coordinate system into the grid current coordinate transformation path to obtain the new sampled current, and the new sampled current includes d axis sampled current and q axis sampled current.

[0227] Specifically, send the updated dq transformation angle in the coordinate system θ into the grid current coordinate transformation path to control the transformation angle of the abc / dq converter, and obtain the new sampled current in the dq coordinate system. It can improve the sampling accuracy. The real-time updated transformation angle ensures the accuracy of the coordinate transformation, provides a high-precision input for the current loop control, and enhances the current tracking ability.

[0228] As an example, the new sampled current in the dq coordinate system includes: the new d axis sampled current i gd reflecting the active component and the new q axis sampled current igq .

[0229] In step S7, refer to step S7 in Figure 2 . Send the new sampled current and the error between the new sampled current and the reference current to the current controller. After current regulation, a PWM modulation signal is obtained to achieve the control of the grid-following inverter.

[0230] Specifically, send the new d axis sampled current i gd , the new q axis sampled current i gq , the new d axis sampled current i gd and d axis reference current i ref_d error, the new q axis sampled current i gq and q axis reference current i ref_q error to the current controller G i ( s ), and then together with the I c axis component d of the capacitor current and the I c axis component q of the capacitor current pass through the capacitor current active damping G AD (s) . Then, combined with the updated dq transformation angle in the θ’ coordinate system, a modulation signal is generated through dq current decoupling, and a PWM control signal is generated through sinusoidal pulse width modulation to achieve the control of the grid-following inverter.

[0231] In one example, to verify the accuracy of the method of this application, the frequency sweep verification of the closed-loop transfer function of the grid-following inverter synchronous dominant loop is performed on the PLECS platform using the frequency sweep method, and a comparison diagram of the amplitude-phase curves before and after improvement is obtained. Figure 7 is the amplitude-frequency characteristic curve diagram of the frequency sweep verification of the closed-loop transfer function of the grid-following inverter synchronous dominant loop before improvement, Figure 8 is the amplitude-frequency characteristic curve diagram of the frequency sweep verification of the closed-loop transfer function of the grid-following inverter synchronous dominant loop after improvement.

[0232] In another example, a simulation model of a three-phase grid-following grid-connected inverter was built based on the PLECS simulation platform to verify the correctness of the concept of this application and the rationality of the closed-loop parameter design. Figure 9 The simulation waveform diagrams of the grid-connected voltage, current, and frequency of the grid-following inverter before improvement under a weak grid are shown in Figure 10 The simulation waveform diagrams of the grid-connected voltage, current, and frequency of the composite synchronous grid-following inverter after improvement under a weak grid are shown in K PLLd The composite synchronization coefficient of the composite synchronous grid-following inverter can be set to be equal to 0.85. Combining Figure 9 and Figure 10 It can be seen that the phase-locked method of the grid-following inverter with composite voltage regulation proposed in this application can improve the stability of the grid-following inverter under a weak grid, demonstrate the effectiveness of the control strategy of the composite synchronous grid-following inverter, and verify the accuracy of the parameter design.

[0233] In the phase-locked method of the grid-following inverter based on composite voltage regulation of this application, by introducing d-axis voltage control to track the phase of the grid connection point voltage in real time and combining q-axis voltage error correction, a composite synchronous dominant loop model is constructed; by dynamically adjusting the rotation angular frequency of the dq coordinate system through the composite synchronous dominant loop model, the influence of the LCL filter resonance on stability can be effectively suppressed, and the anti-interference ability of the phase-locked loop under a weak grid can be improved; through the cascade control of the active power loop and the voltage loop, the accurate calculation of the reference current is realized, and the stability of the power output and the dynamic response speed are significantly improved; by correcting the coordinate transformation angle in real time, the grid connection inrush current is eliminated, and the current sampling accuracy is improved; the updated transformation angle ensures the phase alignment of the current and voltage signals in the dq coordinate system, provides a high-precision input for the current loop control, and significantly improves the current tracking ability. The method of this application reduces the switching loss while ensuring the grid stability, improves the power density, especially shows stronger adaptability under weak grid conditions, significantly improves the phase-locked accuracy, power control ability, and system robustness of the grid-following inverter under different grid strengths, and provides an efficient and stable solution for new energy grid connection.

[0234] It should be understood that although the steps in the flowchart of the accompanying drawings are shown in sequence according to the arrows, these steps are not necessarily executed in the order indicated by the arrows. Unless otherwise clearly stated in this article, the execution of these steps has no strict order limit, and these steps can be executed in other orders. Moreover, at least a part of the steps in the accompanying drawings may include multiple sub-steps or multiple stages. These sub-steps or stages are not necessarily executed at the same time, but can be executed at different times. The execution order of these sub-steps or stages is not necessarily sequential, but can be executed alternately or alternately with at least a part of other steps or sub-steps or stages of other steps.

[0235] In another embodiment, refer to Figure 11 , this application also provides a grid-following inverter phase-locked system based on composite voltage regulation, which may include: a data acquisition module 10, a power calculation module 20, a reference current calculation module 30, a transformation angle module 40, a composite synchronization dominant loop and loop gain module 50, a sampled current update module 60, and a PWM modulation module 70. Among them, the data acquisition module 10 is used to detect the grid current and the grid-connected point voltage, perform coordinate transformation on the grid current and the grid-connected point voltage, and obtain the grid current and the output voltage; the power calculation module 20 is used to obtain the active power output by the grid-following inverter according to the grid current and the output voltage; the reference current calculation module 30 is used to determine the reference current of the output current according to the active power output by the grid-following inverter and the output voltage; the transformation angle module 40 is used to obtain dq the transformation angle in the coordinate system according to the output voltage and the voltage reference value; the composite synchronization dominant loop and loop gain module 50 is used to construct a composite synchronization dominant loop model, and use the composite synchronization dominant loop model to obtain the loop gain and a new dq coordinate system transformation angle; the sampled current update module 60 is used to update the sampled current according to the new dq coordinate system transformation angle; the PWM modulation module 70 is used to obtain a PWM modulation signal according to the new sampled current and control the grid-following inverter.

[0236] In the above grid-following inverter phase-locked system based on composite voltage regulation, the data acquisition module 10 accurately obtains the grid current and the output voltage, providing a basis for subsequent calculations; the power calculation module 20 and the reference current calculation module 30 cooperate to establish an accurate power-current mapping; the transformation angle module 40 corrects the coordinate transformation angle in real time to eliminate the grid-connected impact current; the composite synchronization dominant loop and loop gain module 50 constructs a model to optimize the phase-locked loop parameters; the sampled current update module 60 ensures the sampling accuracy; the PWM modulation module 70 generates a modulation signal to control the inverter. The system of this application can improve the robustness of the grid-following inverter under strong and weak grids, reduce losses, and increase the power density, having significant engineering application value.

[0237] The technical features of the above embodiments can be combined arbitrarily. For the sake of brevity of description, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, it should be considered to be within the scope described in this specification.

[0238] Although the present application has been disclosed above by way of examples, it is not intended to limit the present application. Any person with ordinary knowledge in the relevant technical field may make some modifications and refinements without departing from the spirit and scope of the present application. Therefore, the scope of protection of the present application shall be subject to that defined by the appended patent application scope.

Claims

1. A phase-locked method for a grid-following inverter based on composite voltage regulation, characterized in that, It includes the following steps: Detect the grid current, and perform coordinate transformation on the grid current to obtain dq the grid current in the dq coordinate system, where the grid current in the dq coordinate system includes the d-axis grid current and the q-axis grid current; Detect the grid-connected point voltage, and perform coordinate transformation on the grid-connected point voltage to obtain dq the output voltage in the coordinate system, where the output voltage in the coordinate system includes the d-axis output voltage and the q-axis output voltage; According to dq the output voltage and grid current in the coordinate system, the active power output by the grid-connected inverter is obtained; According to the active power output by the grid-connected inverter and the d-axis output voltage, obtain the reference current of the grid current, where the reference current of the grid current includes the d-axis reference current and the q-axis reference current; Compare the output voltage in the dq coordinate system with the corresponding output voltage reference value to obtain dq the error between the output voltage in the dq coordinate system and the corresponding output voltage reference value. Pass the error through a phase-locked loop controller to obtain Build a model of the composite synchronization master loop to obtain the loop gain from the grid voltage phase angle to the grid-connected voltage phase angle, and obtain the updated dq transformation angle in the coordinate system; Send the updated dq transformation angle under the coordinate system into the grid current coordinate transformation path to obtain a new sampled current, where the new sampled current includes a d-axis sampled current and a q-axis sampled current; Send the new sampled current and the error between the new sampled current and the reference current into the current controller, and obtain the PWM modulation signal through current regulation to achieve the control of the grid-connected inverter.

2. The phase-locking method of the grid-following inverter based on composite voltage regulation according to claim 1, wherein The expression of the active power output by the grid-connected inverter is: Among them, P is the active power output by the grid-connected inverter, is the d-axis output voltage, is the q-axis output voltage, i g_d is the d-axis grid current, i g_q is the q-axis grid current.

3. The phase-locking method of the grid-connected inverter based on composite voltage regulation according to claim 2, wherein The matrix expression of the reference current of the grid current is: Among them, i gref_d is the d-axis reference current of the grid current, i gref_q is the q-axis reference current of the grid current, is the active power loop controller, is the voltage loop controller, P ref is the active power reference value, P is the active power output by the grid-connected inverter, U ref_d is the d-axis output voltage reference value, U d is the d-axis output voltage actual value of.

4. The phase-locking method of the grid-following inverter based on composite voltage regulation according to claim 3, characterized in that Compare the output voltage in the dq coordinate system with the voltage reference value to obtain dq the error between the output voltage in the dq coordinate system and the voltage reference value. Pass the error through a phase-locked loop controller to obtain Compare the q-axis output voltage with the q-axis output voltage reference value to obtain the q-axis output voltage error, and pass the q-axis output voltage error through the q-axis PI controller to obtain the q-axis angular frequency change; Compare the d-axis output voltage with the d-axis output voltage reference value to obtain the d-axis output voltage error, and pass the d-axis output voltage error through the d-axis PI controller to obtain the d-axis angular frequency change; Add the q-axis angular frequency change, the d-axis angular frequency change, and the rated angular frequency and multiply by the Laplace integrator to obtain the phase angle of the output voltage.

5. The phase-locking method of the grid-following inverter based on composite voltage regulation according to claim 4, wherein Build a model of the composite synchronization master loop to obtain the loop gain from the grid voltage phase angle to the grid-connected voltage phase angle, and obtain the updated dq transformation angle in the coordinate system, including: In dq a coordinate system, an equivalent circuit of a grid-connected inverter is established; According to the equivalent circuit of the grid-connected inverter, obtain the LCL dynamic equation; Using a q-axis phase-locked loop controller, determine dq the rotational angular frequency of the coordinate system, and according to the dq rotational angular frequency of the coordinate system, transform the grid voltage to dq the coordinate system to obtain the dq vector of the grid voltage; According to the dq vector of the grid voltage, obtain the dq vector linearized representation of the grid voltage to obtain a simplified grid-connected voltage; According to the simplified grid-connected voltage, obtain the composite synchronous dominant loop gain transfer function; Using the composite synchronization dominant loop gain transfer function, an updated dq transformation angle in the coordinate system is obtained.

6. The phase-locking method of the grid-following inverter based on composite voltage regulation according to claim 5, characterized in that, of the grid voltage dq The vector linearization expression is: Among them, is the grid voltage amplitude, is the dq vector V gdq of the grid voltage dq vector U dq and the steady-state value of the power angle between the grid-connected voltage is the linearized small-signal quantity of the power angle, is the linearized grid voltage d axis component, is the linearized grid voltage q axis component.

7. The phase-locking method of the grid-following inverter based on composite voltage regulation according to claim 5, wherein The composite synchronous master loop gain transfer function is as follows: Among them, is the d axis component of the composite synchronization dominant loop gain transfer function, is the q axis component of the composite synchronization dominant loop gain transfer function.

8. The phase-locking method of the grid-connected inverter based on composite voltage regulation according to claim 5, characterized in that The dq angular frequency of rotation of the coordinate system is expressed as: Among them, is the small-signal quantity of the grid-connected angular frequency, is the rated value of the angular frequency.

9. The phase-locking method of the grid-following inverter based on composite voltage regulation according to claim 8, wherein The grid-connected angular frequency small-signal quantity The expression is: Among them, is q the shaft phase-locked loop transfer function, is the q-axis component of the grid-connected voltage, is q the q-axis PI controller, and s is the Laplace operator.

10. A phase-locked system for a grid-following inverter based on composite voltage regulation, characterized in that, The grid-connected inverter phase-locked system based on composite voltage regulation executes the steps of the grid-connected inverter phase-locking method based on composite voltage regulation according to any one of claims 1 to 9, including: A data acquisition module for detecting the grid current and the grid connection point voltage, performing coordinate transformation on the grid current and the grid connection point voltage to obtain the grid current and the output voltage; A power calculation module for obtaining the active power output by the grid-connected inverter according to the grid current and the output voltage; A reference current calculation module for determining the reference current of the output current according to the active power output by the grid-connected inverter and the output voltage; A transformation angle module, configured to obtain a transformation angle in a coordinate system according to the output voltage and a voltage reference value dq ​ The composite synchronization dominant loop and loop gain module are used to construct a composite synchronization dominant loop model. By using the composite synchronization dominant loop model, the loop gain and a new dq coordinate system transformation angle are obtained. A sampling current update module, which is used to update the sampling current according to the new dq coordinate system transformation angle; A PWM modulation module for obtaining the PWM modulation signal according to the new sampled current to control the grid-connected inverter.

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