Phase-locked loop-free current source type inverter control system and method based on frequency support

Abstract

The invention provides a phase-locked loop-free current source type inverter control system and method based on frequency support, and relates to the technical field of power system and inverter control. Comprising an acquisition module, a first coordinate conversion module, a second coordinate conversion module, a power calculation module, an active frequency control module, a reactive current control module, a current control loop module and a pulse width modulation module. According to the invention, the amplitude and frequency of the output current of the inverter are directly constructed through the active frequency control module and the reactive current control module, and then the output current of the inverter is controlled and determined through the current control loop module. The method integrates the advantages of network following type and network constructing type converter control, adopts a three-layer nested control architecture, does not need to depend on a phase-locked loop to provide a power grid phase angle, can adapt to complex working conditions such as a strong power grid, a weak power grid and different line impedances, also avoids current impact during a fault, can provide inertia support and frequency support for the power grid, and is high in reliability. The system stability is improved.

Description

Technical Field

[0001] The present invention relates to the technical fields of power systems and inverter control technologies. Specifically, it relates to a current source inverter control system and method based on frequency support without a phase-locked loop. Background Art

[0002] The large-scale access of new energy sources such as wind and light to the power grid through power electronic converters will become one of the main technical features of the new generation of power systems. Therefore, the penetration rate of power electronic converters in the power grid is increasing rapidly, and traditional synchronous generators are being replaced by them. However, different from the traditional power system dominated by synchronous generators, the high-proportion new energy power system has characteristics such as intermittent fluctuations and uncertainties in new energy output, flexibility in power electronic converter control, and vulnerability of power electronic devices. These different characteristics bring new challenges to the safe and stable operation of the new generation of power systems.

[0003] Existing solutions: Currently, in practical engineering applications, renewable energy sources such as wind energy and solar energy are usually integrated into the power system through converters adopting a grid-following ( GFL ) control architecture. The grid-following converter obtains the phase information of the grid voltage through a phase-locked loop to achieve synchronization with the grid, and precisely regulates the active and reactive powers through vector current control. In terms of voltage and current output characteristics, the grid-following converter can be equivalent to a controlled current source. However, as more and more renewable energy is incorporated into the grid through converters, the proportion of synchronous generators decreases, and the stable operation of the grid-following converter faces challenges, resulting in insufficient support capacity under weak grids. To solve this problem, inspired by the physical principle of synchronous generators, grid-forming converter control ( GFM ) has emerged. Different researchers have proposed various implementations of grid-forming converter control structures. These methods can independently construct the AC-side output voltage and provide voltage support to the grid without relying on the external grid. Although grid-forming converters play an increasingly important role in power systems with high renewable energy integration and high penetration of power electronic devices, grid-forming converters still face many challenges such as limited over-current capacity and decreased stability in the face of strong grids.

[0004] To simultaneously overcome the synchronization and stability problems faced by grid-following and grid-forming converters, some research has proposed a direct power control method that directly controls the active and reactive power of the converter's output without using an inner current loop. However, due to voltage noise and distortion, the use of a band-pass filter in direct power control is inevitable, which will lead to the same stability problems as using a phase-locked loop. Later, some scholars also proposed a power synchronization control scheme without a phase-locked loop. The results show that it can operate stably in both strong and weak power grids because it does not require the acquisition of the common coupling point voltage. Although the above methods are effective, the controller design is complex and cannot cope with the situation of grid frequency change. Any frequency deviation will bring steady-state error, resulting in unstable operation of the converter. In addition, some researchers have proposed a general control strategy that combines power synchronization and phase-locked loop schemes to utilize their advantages simultaneously. However, this structure still requires a phase-locked loop for synchronization.

[0005] Therefore, the existing solutions still have the following problems to be solved urgently: (1) Under weak grid conditions, the phase-locked loop is vulnerable to grid fluctuations, which may lead to phase shift or unstable locking. Therefore, it is difficult to balance the locking speed and anti-interference ability to ensure stable operation under various grid conditions.

[0006] (2) In a weak grid environment, the grid-following converter has limited ability to provide reactive power support and voltage regulation. A stronger grid support control strategy is needed to ensure stable operation during grid disturbances.

[0007] (3) During grid voltage fluctuations or short-term faults, the stable operation and rapid recovery of the inverter are still challenges.

[0008] (4) In a strong grid environment, the grid-forming converter faces the risk of fault impact current, and there is a contradiction between overcurrent protection and dynamic performance. Too strict current limiting will reduce the system response ability, while too loose current limiting may damage the devices. Summary of the Invention

[0009] In view of the above technical problems in the related art, the present invention proposes a phase-locked loop-free current source inverter control system and method based on frequency support.

[0010] In the first aspect, the present invention provides a phase-locked loop-free current source inverter control system based on frequency support. The control system acts on a power grid system, which includes a DC source, an inverter, a filter inductor, a line impedance, and a power grid. The DC source serves as the energy source of the main circuit of the power grid system, and the input DC source bus voltage V dc , after passing through the inverter and the filter inductor, the output three-phase current i abc is incorporated into the power grid through the line impedance. The control system includes: A sampling module, configured to sample three-phase currents on the output side of the inverter i abc and three-phase voltages at the grid connection point u pabc ; A power calculation module, configured to convert the three-phase currents i abc and three-phase voltages at the grid connection point u pabc into current components dq in a synchronous coordinate system I dq and grid connection point voltage components dq in a synchronous coordinate system V pdq , and then calculate active power dq components I dq and grid connection point voltage components dq in a synchronous coordinate system V pdq and reactive power P , and feed the active power Q back to the active frequency control module and feed the reactive power P back to the reactive current control module; Q An active frequency control module, configured to construct a frequency regulation equation to achieve dynamic change of frequency with active power, and calculate and output the current synchronous phase angle to the power calculation module and the current control loop module; A reactive current control module, configured to adjust the reactive power deviation to a control current axis component d using a first proportional integral controller I dref , and set the control current q axis component I qref to 0 and output it to the current control loop module; A current control loop module, configured to compensate the grid connection point dq voltage I dq based on formula (11) and convert it into a three-phase voltage control quantity dq for output to V pdq the control module; formula (11) is as follows: wherein, PWM is used to decouple and compensate the voltage cross-coupling term; (11) where is used to decouple and compensate the voltage cross-coupling term; k Ip The proportional parameter of the inner current loop control k I i represents the integral parameter of the inner current loop control; is the dq component of the control current reference value, which is composed of the d axis component I dref and the q axis component I qref of the control current reference value; ω n is the rated angular frequency, with a value of 2*π*50; L f is the value of the filter inductor; The pulse width modulation module compares the three-phase voltage control quantity with the internal reference level of the module to obtain the duty cycle and generate PWM a signal to drive the inverter to operate.

[0011] Specifically, the control system further includes: The first coordinate conversion module is used to convert the three-phase variables into components in the synchronous rotating coordinate system ( dq ) according to the current synchronous phase angle through the first coordinate axis conversion formula; The first coordinate axis conversion formula is shown in formula (1): (1) where, θ I is the current synchronous phase angle, S a is the A phase component; S b is the B phase component; S c is the C phase component; S d is the d axis component of the synchronous rotating coordinate system; S q is the q axis component of the synchronous rotating coordinate system; T ( θ I ) is the control function, and its expression is shown in formula (2): (2).

[0012] Specifically, the power calculation module specifically includes: According to the three-phase currenti abc Synchronization phase angle with current θ I Calculate the current in the three-phase current synchronization coordinate system ( dq coordinate system) through the first coordinate conversion module dq component I dq , including the current d axis component I d 、Current q axis component I q ; The specific calculation is shown in formula (3): (3) According to the three-phase voltage at the grid connection point u pabc And the synchronization phase angle with current θ I Calculate the grid connection point voltage in the three-phase voltage synchronization coordinate system of the grid connection point through the first coordinate conversion module dq component V pdq , including the grid connection point voltage d axis component V pd 、Grid connection point voltage q axis component V pq ; The specific calculation is shown in formula (4): (4) The calculation of the active power P is shown in formula (5): (5) The calculation of the reactive power Q is shown in formula (6): (6) Specifically, the frequency regulation equation in the active power frequency control module is shown in formula (7): (7) Among them, ω I is the current angular frequency; ω n is the rated angular frequency; J represents the virtual moment of inertia; D represents the virtual damping coefficient; P represents the currently output active power; P ref represents the active power reference value; Specifically, in the active frequency control module, the synchronous phase angle of the output current is calculated through Formulas (8)-(9), which specifically includes: The frequency adjustment equation is discretely implemented by the controller as shown in Formula (8): (8) Then, the current synchronous phase angle is generated through integration θ I and output to the power calculation module and the current control loop module; the integration formula is as shown in Formula (9): (9) wherein, dt represents the tiny increment of the time variable t Δt.

[0013] Specifically, the first proportional-integral controller in the reactive current control module adopts Formula (10): (10) wherein, Q ref Q* is the reference value of the reactive power output by the inverter; k IP and k II Kp and Ki are the proportional-integral controller parameters for reactive-current amplitude control, k IP Kq is the proportional coefficient of reactive current control, k II Ki is the integral of reactive current control.

[0014] Second, the present invention provides a control method for a current-source inverter without a phase-locked loop based on frequency support. Based on the control system for a current-source inverter without a phase-locked loop based on frequency support described in any one of the first aspects, the method includes the following steps: S 1. Collect the three-phase current on the output side of the inverter i abc and the three-phase voltage at the grid connection point u pabc , and transmit them to the power calculation module; S 2. According to the current synchronous phase angle, convert the three-phase current i abc and the three-phase voltage at the grid connection point u pabc into the current dq component I dq and the grid connection point voltage dq component V pdq in the synchronous coordinate system, and then according to the currentdq Component I dq and the grid connection point voltage dq Component V pdq Calculate the active power P and the reactive power Q , and feedback the active power P to the active frequency control module, and feedback the reactive power Q to the reactive current control module; S 3. Construct a frequency regulation equation to realize the dynamic change of frequency with the active power, and calculate and output the synchronous phase angle of the current to the power calculation module and the current control loop module; S 4. Adjust the reactive power deviation to the control current d axis component I dref according to the first proportional-integral controller, and set the control current q axis component I qref to 0 and output it to the current control loop module; S 5. Based on the control current dq component I dq compensate the grid connection point dq voltage V pdq to obtain a compensated voltage control signal , and then convert it into a three-phase voltage control quantity and output it to the pulse width modulation module; Formula (11) is as follows: (11) where is used to decouple the cross-coupling term of the compensated voltage; k I p Current inner loop control proportional parameter, k I i represents the current inner loop control integral parameter; is the dq component of the control current reference value, which is composed of the d axis component I dref of the control current reference value and the q axis component I qref of the control current reference value; ω n is the rated angular frequency; Lf is the value of the filtering inductor; S 6. Compare the three-phase voltage control quantity with the internal reference level of the module to obtain the duty cycle, and generate PWM a signal to drive the inverter to operate.

[0015] Specifically, step S 2 specifically includes the following steps: S 21. According to the three-phase current i abc and the current synchronous phase angle θ I calculate the current components in the three-phase current synchronous coordinate system through the first coordinate conversion module dq components I dq , including the current d axis component I d , the current q axis component I q ; The specific calculation is shown in formula (3): (3); S 22. According to the three-phase grid-connected voltage u pabc and the current synchronous phase angle θ I calculate the grid-connected voltage components in the three-phase grid-connected voltage synchronous coordinate system through the first coordinate conversion module dq components V pdq , including the grid-connected voltage d axis component V pd , the grid-connected voltage q axis component V pq ; The specific calculation is shown in formula (4): (4) S 23. Calculate the active power P and the reactive power Q according to formulas (5)-(6): (5) (6).

[0016] Specifically, the frequency regulation equation described in step S 3 is shown in formula (7), and through the current angular frequency ωI Integral generating current synchronization phase angle θ I ; (7) Wherein, ω I is the current angular frequency; ω n is the rated angular frequency; J represents the virtual moment of inertia; D represents the virtual damping coefficient; P represents the current output active power; P ref represents the active power reference value.

[0017] Specifically, the first proportional-integral controller described in step S 4 adopts formula (10): (10) Wherein, Q ref is the reference value of the reactive power output by the inverter; k IP and k II are the proportional-integral controller parameters for reactive-current amplitude control, k IP is the proportional coefficient of reactive current control, k II is the integral of reactive current control.

[0018] The present invention directly constructs the amplitude and frequency of the inverter output current through the active frequency control module and the reactive current control module, and then determines the inverter output current through the current control loop module. The present invention combines the advantages of grid-following and grid-forming converter controls, adopts a three-layer nested control architecture, does not require a phase-locked loop to provide the grid phase angle, can adapt to complex working conditions such as strong grids, weak grids and different line impedances, also avoids current shocks during faults, can provide inertial support and frequency support for the grid, and improves system stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can also be obtained based on these drawings without creative efforts.

[0020] Figure 1It is the control block diagram of the current source inverter control system based on frequency support provided by the embodiments of the present invention; Figure 2 It is the voltage-current vector diagram provided by the embodiments of the present invention; Figure 3 It is the schematic diagram of the control method of the current source inverter based on frequency support provided by the embodiments of the present invention; Figure 4 It is the simulation result of the control strategy provided by the embodiments of the present invention when the frequency changes. In the figure, ( a ) frequency f ( Hz )), ( b ) active power P ( kW ) - reactive power Q ( kVar )), ( c ) voltage V pdq ( V ) - current I dq ( A )); Figure 5 It is the simulation result of the control strategy provided by the embodiments of the present invention when the grid strength changes. In the figure, ( a ) frequency f ( Hz )), ( b ) active power P ( kW ) - reactive power Q ( kVar )), ( c ) current I abc ( A )), ( d ) voltage V gabc ( V )); Figure 6 It is the simulation result of the control strategy provided by the embodiments of the present invention when the line impedance changes. In the figure, ( a ) frequency f ( Hz )), ( b ) active power P ( kW ) - reactive power Q ( kVar )), ( c ) voltage V pdq ( V ) - current Idq ( A )。 Detailed implementation manners

[0021] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention belong to the scope of protection of the present invention.

[0022] In addition, the terms "first" and "second" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, the features defined with "first" and "second" may explicitly or implicitly include one or more of such features. In the description of the present invention, "a plurality" means two or more, unless otherwise specifically defined.

[0023] Embodiment 1

[0024] This embodiment provides a current source inverter control system based on frequency support without a phase-locked loop, and its control block diagram is as Figure 1 shown. The control system acts on the power grid system; the power grid system includes a DC source, an inverter, a filter inductor L f , a line impedance, and a power grid; the DC source serves as the energy source of the main circuit of the power grid system, and the input DC source bus voltage V dc , after passing through the inverter and the filter inductor L f , the output three-phase current i abc is incorporated into the power grid after passing through the line impedance; the line impedance includes a line inductor L g and a line resistance R g ; the three-phase voltage at the grid connection point incorporated into the power grid is u pabc ; The inverter is a three-phase full-bridge inverter.

[0025] The control system includes an acquisition module, a first coordinate conversion module, a second coordinate conversion module, a power calculation module, an active frequency control module, a reactive current control module, a current control loop module, and PWM a control module; The present invention adopts a "three - layer nested control structure": 1. Current inner loop: quickly tracks the current command; 2. Outer loop 1 - active power / frequency control: based on the virtual synchronous machine model, realizes the generation of the virtual frequency and the synchronous phase angle of the current; Outer loop 2 - reactive power / current control: provides reactive power control ability.

[0026] The acquisition module is used to acquire the three - phase current on the output side of the inverter i abc and the three - phase voltage at the grid connection point u pabc ; The three - phase current i abc includes i a , i b , i c ; The three - phase voltage at the grid connection point u pabc includes u pa , u pb , u pc ; The three - phase current on the output side of the inverter i abc and the three - phase voltage at the grid connection point u pabc are isolated and acquired by adopting high - precision current (voltage) transformers; The first coordinate conversion module is used to convert the three - phase variables into components in the synchronous rotating coordinate system ( dq ) according to the current synchronous phase angle through the first coordinate axis conversion formula; The first coordinate axis conversion formula is shown as formula (1): (1) where θ I is the current synchronous phase angle, S a is A the component under the S b phase; B is S c the component under the C phase; S d is the d axis component of the synchronous rotating coordinate system; S q is the q axis component of the synchronous rotating coordinate system; T (θ I ) is a control function, and its expression is as shown in formula (2): (2) A power calculation module, configured to convert three-phase current i abc and the three-phase voltage at the grid connection point u pabc into the current dq components I dq in the synchronous coordinate system and the grid connection point voltage dq components V pdq . Then, according to the current dq components I dq and the grid connection point voltage dq components V pdq calculate the active power P and the reactive power Q , and feedback the active power P to the active frequency control module, and feedback the reactive power Q to the reactive current control module; The current dq components I dq include the current d axis component I d , the current q axis component I q ; The grid connection point voltage dq components V pdq include the grid connection point voltage d axis component V pd , the grid connection point voltage q axis component V pq ; Specifically, it includes: According to the three-phase current i abc and the current synchronous phase angle θ I calculate, through the first coordinate conversion module, the current dq in the three-phase current synchronous coordinate system ( dq coordinate system) components I dq , including the current d axis component I d , the current qAxis component I q ; The specific calculation is shown in formula (3): (3) Similarly, calculate the components of the three-phase voltage at the grid connection point in the dq coordinate system V pd 、V pq ; Specifically, it includes: According to the three-phase voltage at the grid connection point u pabc and the current synchronous phase angle θ I Calculate the grid connection point voltage in the synchronous coordinate system of the three-phase voltage at the grid connection point through the first coordinate conversion module dq component V pdq , including the grid connection point voltage d axis component V pd , the grid connection point voltage q axis component V pq ; The specific calculation is shown in formula (4): (4) The current synchronous phase angle θ I is obtained through formula (9) from the initial current angular frequency, and there is a preset value of 50 for the initial current angular frequency Hz .

[0027] It can be understood that the current synchronous phase angle θ I is calculated through formula (9) using the initial current angular frequency during initialization, and in the subsequent active power frequency control module, the current synchronous phase angle θ I is calculated through formula (9) after obtaining the current angular frequency in real time.

[0028] The active power P is calculated as shown in formula (5): (5) The reactive power Q is calculated as shown in formula (6): (6) The active power frequency control module is used to construct a frequency regulation equation to realize the dynamic change of frequency with active power, and calculate and output the current synchronous phase angle to the power calculation module and the current control loop module; the frequency regulation equation is shown in formula (7): (7) Among them, ω I is the angular frequency of the current; ω n is the rated angular frequency; J represents the virtual moment of inertia; D represents the virtual damping coefficient; P represents the current output active power; P ref represents the reference value of the active power; The frequency regulation equation is discretely implemented by the controller as shown in formula (8): (8) Among them, dt represents the small increment of the time variable t ; Then, the current synchronization phase angle is generated through integration θ I and output to the power calculation module and the current control loop module; The integration formula is as shown in formula (9): (9) Among them, dt represents the small increment of the time variable t ; The active frequency control module corresponds to the outer loop 1 - active / frequency control of the three - layer nested control structure. By constructing a "virtual moment of inertia + damping" structure through the frequency regulation equation, the frequency is dynamically changed with the active power. The above - mentioned structure replaces the traditional phase - locked loop. The inverter "senses the grid frequency" through the change of its own output active power and has the ability of natural synchronization and inertial response.

[0029] The reactive current control module is used to adjust the reactive power deviation to the control current according to the first proportional - integral controller d axis component I dref , and set the control current q axis component I qref to 0 and output to the current control loop module; The first proportional - integral controller adopts formula (10): (10) Among them, Q ref is the reference value of the reactive power output by the inverter; k IP and k II are the proportional - integral controller parameters for reactive - current amplitude control, k IP is the proportional coefficient of the reactive current control,k II is the integral coefficient for reactive current control; I dref is used for current inner-loop control current d axis component to achieve independent reactive power regulation ability, while I qref is designed to be 0, positioning the synchronous coordinate system d axis on the current.

[0030] The reactive current control module corresponds to the outer loop 2 - reactive power / current control of the three-layer nested control structure, providing reactive power control ability Through the active frequency control module and the reactive current control module, the reactive-current control path is decoupled from the active-frequency control, and the output current can be quickly adjusted to achieve PQ independent control.

[0031] Reference Figure 2 , it can be seen the essential differences between this embodiment and the traditional grid-following and grid-forming control designs. As Figure 2 in ( a ), the traditional grid-following type uses a phase-locked loop, and the grid-forming control constructs a voltage, making V pq = 0, positioning the voltage on the d axis. Subsequently, the grid-following control designs the reference value of the dq axis current component based on the power transmission formula, while the grid-forming control constructs the amplitude and phase of the d axis voltage based on active-frequency and reactive-voltage to achieve the control of the inverter. As Figure 2 in ( b ), in order to simultaneously achieve power synchronization and avoid the problem of transient impact current in this embodiment, by making I qref designed to be 0, making the d axis positioned on the current, establishing the reference of the d axis current frequency and phase, and establishing the amplitude reference of the d axis current through reactive power feedback, thereby achieving the power control of the inverter. This is essentially different from the control designs of the traditional grid-following and grid-forming types.

[0032] The current control loop module is used to compensate the grid connection point dq component I dq based on formula (11) for the dq voltage V pdq to obtain the compensated voltage control signal , and then convert it into a three-phase voltage control quantity Output to PWM the control module; Equation (11) is shown as follows: (11) where , is used to decouple and compensate the voltage cross-coupling term; k I p The proportional parameter of the inner current loop control, k I i represents the integral parameter of the inner current loop control; is the dq component of the control current reference value, which is composed of the d axis component I dref of the control current reference value and the q axis component I qref of the control current reference value; ω n is the rated angular frequency, with a value of 2*π*50 rad / s ; L f is the value of the filter inductor; It can be understood that the dq component I dq of the current d axis component I d , the q axis component I q ; The dq component V pdq of the grid connection point voltage d axis component V pd , the q axis component V pq ; Through Equation (11), the components of the d axis and the p axis are calculated to obtain and , and these two components are combined to form ; That is, the compensation voltage control signal is a column vector (a column vector is a special form of a matrix).

[0033] It should be noted that in this application, the parameters with subscripts containing dq are all column vectors including the d-axis component and the q-axis component. For example Idq , V pdq , , , The parameter subscripts containing abc are all column vectors including the a, b, and c three-phase components. For example, i abc , u pabc , .

[0034] Equation (11) is the proportional-integral controller and voltage feedforward decoupling compensation structure, compensating the voltage control signal After being inversely transformed by the second coordinate transformation module into three-phase voltage control quantities , used for PWM generating, given to the inverter. The precise feedforward decoupling improves the system bandwidth and stability, facilitating the realization of high-performance dynamic regulation.

[0035] The current control loop module corresponds to the inner current loop of the three-layer nested control structure, used for quickly tracking the current command; The second coordinate transformation module, used for converting the components in the synchronous rotating coordinate system ( dq ) into three-phase variables according to the current synchronization phase angle through the second coordinate axis transformation formula; The second coordinate axis transformation formula is shown in Equation (12) as follows: (12) Wherein, θ I is the current synchronization phase angle, S a is A the component under the S b is B the component under the S c is C the component under the S d is the component of the synchronous rotating coordinate system d on the S q is the component of the synchronous rotating coordinate system q on the T -1 ( θ I ) is the inverse control function, which is the inverse of the control function T ( θ I ), and its expression is shown in Equation (13) as follows: (13) The pulse width modulation module is used to compare the three-phase voltage control quantity with the internal reference level of the module to obtain the duty cycle and generate PWM a signal to drive the inverter to operate.

[0036] Specifically, the three-phase voltage control quantity is divided by the internal reference level of the module ( V dc / 2), V dc where dc is the DC bus voltage of the DC source. In the control of a three-phase full-bridge inverter, the three-phase voltage control quantity is limited to [-1, 1] and then input into the PWM module to generate a duty cycle signal.

[0037] It can be understood that the duty cycle is the ratio of the high-level duration to the entire cycle time in a pulse signal, and it is usually used to describe the pulse width modulation (that is, PWM ) signal. PWM The signal switches between high and low levels at a certain frequency to control the conduction or cutoff of the inverter switching tubes.

[0038] Embodiment 2 Referring to Figure 3 , this embodiment provides a control method for a current source inverter without a phase-locked loop based on frequency support. Based on the control system for a current source inverter without a phase-locked loop based on frequency support described in Embodiment 1, the method includes the following steps: S 1. Collect the three-phase current i abc at the output side of the inverter and the three-phase voltage u pabc at the grid connection point; S 2. According to the current synchronous phase angle, convert the three-phase current i abc and the three-phase voltage u pabc at the grid connection point into the current dq component I dq and the grid connection point voltage dq component V pdq in the synchronous coordinate system. Then, calculate the active power dq component I dq and the grid connection point voltage dq component V pdq and calculate the active power P and the reactive power Q . Then, the active power PFeed back to the active power frequency control module and the reactive power Q Feed back to the reactive current control module; the current dq component I dq includes the current d axis component I d and the current q axis component I q ; the grid connection point voltage dq component V pdq includes the grid connection point voltage d axis component V pd and the grid connection point voltage q axis component V pq ; Specifically, it includes the following steps: S 21. Calculate the current i abc in the three-phase current synchronous coordinate system through the first coordinate conversion module according to the three-phase current θ I and the current synchronous phase angle dq component I dq which includes the current d axis component I d and the current q axis component I q ; the specific calculation is shown in formula (3): (3); S 22. Calculate the grid connection point voltage u pabc in the three-phase grid connection point voltage synchronous coordinate system through the first coordinate conversion module according to the three-phase grid connection point voltage θ I and the current synchronous phase angle dq component V pdq which includes the grid connection point voltage d axis component V pd and the grid connection point voltage q axis component V pq ; the specific calculation is shown in formula (4): (4) S23. Calculate the active power according to Formulas (5)-(6) P and the reactive power Q :[[]] (5) (6).

[0039] S 3. Construct a frequency regulation equation to achieve dynamic change of frequency with active power, and calculate and output the synchronous phase angle of the current to the power calculation module and the current control loop module; The frequency regulation equation is as shown in Formula (7): (7) where ω I is the current angular frequency; ω n is the rated angular frequency; J represents the virtual inertia; D represents the virtual damping coefficient; P represents the currently output active power; P ref represents the reference value of the active power; The frequency regulation equation is discretely implemented by a controller as shown in Formula (8): (8) Then, generate the synchronous phase angle of the current through integration θ I and output it to the power calculation module and the current control loop module; The integration formula is as shown in Formula (9): (9) where dt represents the time variable t is the small increment of

[0040] S 4. Adjust the reactive power deviation to the control current d axis component I dref according to the first proportional-integral controller, and set the control current q axis component I qref to 0 and output it to the current control loop module; The first proportional-integral controller adopts Formula (10): (10) where Q ref is the reference value of the reactive power output by the inverter; k IP and kII is the proportional-integral controller parameter for reactive-current amplitude control, k IP is the proportional coefficient of reactive current control, k II is the integral of reactive current control; S 5. According to the control current dq component I dq Based on formula (11), the grid connection point dq voltage V pdq is compensated to obtain a compensation voltage control signal , and then it is converted into a three-phase voltage control quantity and output to the pulse width modulation module; formula (11) is as follows: (11) wherein, is used to decouple the compensation voltage cross-coupling term; k I p and k I i represent the proportional-integral control parameters of the current inner loop control, k I p is the proportional parameter of the current inner loop control, k I i represents the integral parameter of the current inner loop control; is the dq component of the control current reference value, which is composed of the d axis component I dref of the control current reference value and the q axis component I qref of the control current reference value; ω n is the rated angular frequency, with a value of 2*π*50; L f is the value of the filter inductor; It can be understood that the proportional-integral control parameters of the current inner loop control ( k I p and k I i ) are related to the line impedance and can be determined according to the line impedance and the optimal performance requirements of the controller. This is prior art and will not be elaborated here.

[0041] Specifically, according to the current synchronization phase angle, the second coordinate conversion module is used to convert the compensated output voltage into three-phase voltage control quantities .

[0042] S 6. Compare the three-phase voltage control quantities with the internal reference level of the module to obtain the duty cycle and generate PWM a signal to drive the inverter to operate.

[0043] This embodiment realizes synchronization with the power grid through active power - frequency feedback, does not rely on a phase - locked loop, can provide inertia and frequency support when the power grid frequency deviates, and is applicable to both strong and weak power grids.

[0044] This embodiment is essentially still a grid - following control, so it avoids the damage to the system caused by the transient impact current under grid faults.

[0045] This embodiment adopts an improved current control strategy, effectively compensates for the influence of line impedance changes on current control, and enables the converter to operate stably under different line impedance conditions, ensuring grid - connection quality and system reliability.

[0046] Based on the real - time simulation results and analysis in MATLAB / Simulink In order to verify the proposed control method, this embodiment gives three cases in / MATLAB / Simulink to discuss the effectiveness of the proposed control. The implementation case consists of a DC source, an inverter, a filter inductor L f , line impedance, and the power grid, with a voltage level of 380 V , and a power level of 10 kW . Case 1, Case 2, and Case 3 respectively test the performance of the proposed control scheme under power grid frequency dips, different power grid strengths, and different line impedance changes. The system parameters used in the simulation are shown in Table 1, and the control parameters are shown in Table 2.

[0047]

[0048]

[0049] Case 1: Power grid frequency dip test Figure 4 Verified the frequency support ability of the proposed control strategy for the power grid. Referring to Figure 4 in ( a ), when the power grid frequency drops at the 2nd second, the system increases the output active power from 10 kW to 16.3kW , injects active power into the power grid to support the operation of the power grid. When the grid frequency returns to 50 at the 6th second Hz , the active power output by the system also returns to the given value and resumes normal operation mode. During this period, the reactive power is not affected, as shown in Figure 4 ( b ). Figure 4 ( c ) respectively gives the d -axis and q -axis components of the voltage across the converter and the output current. This implementation proves that the proposed control can provide certain frequency support for the power grid and improve the robustness of the system.

[0050] Case 2: Adaptability test under different grid strengths Figure 5 Verifies the adaptability of the proposed control strategy under different grid strengths. Grid strength is generally classified into extremely weak ( SCR ≤2), weak (2 < SCR < 3) and strong ( SCR ≥ 3) according to the short-circuit ratio ( SCRbb ). SCR can be expressed as: In this implementation case, the output power is set to be constant, and different SCR values are changed by changing the line inductance. As shown in Figure 5 , for the system implementing this scheme, at the beginning, it works under the condition of a weak power grid ( SCR = 2.3), with an active power output of 10 kW and a reactive power of 5 kVar , and the system operates normally. When SCR is increased to 4.6 at the 3rd second, there will be a short-term impact on the system frequency and power, but it can finally return to the rated operation. At the 7th second, the line impedance is further reduced, and the power grid changes from weak to strong, but the proposed control scheme maintains the system output unchanged. This implementation verifies that the system can be applicable to various grid strengths.

[0051] Case 3: Adaptability test under complex line impedance conditions Figure 6 Verifies the adaptability of the proposed control strategy under complex line impedance conditions. In this implementation case, it is verified that the line impedance of the system changes from purely inductive ( XL = 3.14 j Ω) to resistive-inductive ( XL = 2.5 + 1.57 j Ω), and then to purely resistive ( XL= 5Ω). The waveform results show that under the proposed control strategy, the system can always maintain the frequency and the output power is stable. It is proved that the proposed scheme can adapt to different line impedance characteristics at the same time.

[0052] As can be seen from the above three cases, the control method of this embodiment can be applied to different grid conditions. Different from the traditional grid-following and grid-forming inverter controls, in this invention, the amplitude and frequency of the inverter output current are directly constructed through the active frequency control module and the reactive current control module, and then the output current of the inverter is determined by the current control loop module. This scheme adopts a three-layer nested control architecture, does not need to rely on a phase-locked loop to provide the grid phase angle, and also avoids the current impact during faults. Therefore, this scheme is applicable not only to strong and weak grids, but also to different line impedance conditions.

[0053] The control method of this embodiment is different from the traditional PLL The grid connection control is compared in four dimensions: synchronization method, grid connection stability, frequency response ability, and reactive power control, as shown in Table 3:

[0054] The control method of this embodiment combines the advantages of grid-following and grid-forming converter controls, adopts a simplified control strategy, avoids the limitations of the phase-locked loop, and can adapt to complex working conditions such as strong grids, weak grids, and different line impedances. In addition, this scheme directly controls the output current, effectively reduces the risk of current impact, and can provide inertial support and frequency support for the grid, improving system stability.

[0055] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A current source inverter control system without a phase-locked loop based on frequency support, the control system acting on a power grid system, characterized in that, The power grid system includes a DC source, an inverter, a filter inductor, a line impedance, and a power grid; the DC source serves as the energy source of the main circuit of the power grid system, and the input DC source bus voltage V dc , and the three-phase current output after passing through the inverter and the filter inductor i abc is incorporated into the power grid through the line impedance; The control system includes: The voltage and current acquisition module is used to collect the three-phase current on the output side of the inverter i abc and the three-phase voltage at the grid connection point u pabc , and transmit them to the power calculation module; A power calculation module, which is used to convert three-phase current according to the current synchronization phase angle i abc and the three-phase voltage at the grid connection point u pabc into the current components in the synchronous coordinate system dq component I dq and the grid connection point voltage dq component V pdq , and then calculate the active power dq component I dq and the grid connection point voltage dq component V pdq and the reactive power P component Q , and feedback the active power P to the active frequency control module, and feedback the reactive power Q to the reactive current control module; An active power - frequency control module, which is used to construct a frequency regulation equation to realize the dynamic change of frequency with active power, and calculate and output the current synchronous phase angle to the power calculation module and the current control loop module; A reactive current control module, which is used to adjust the reactive power deviation to a control current according to a first proportional-integral controller d Axis component I dref , and set the control current q Axis component I qref to 0 and output it to the current control loop module; The current control loop module is used to control the current dq Quantity I dq Based on formula (11), the grid connection point dq Voltage V pdq Compensation is performed to obtain a compensation voltage control signal , and then convert it into three-phase voltage control quantity Output to the pulse width modulation module; formula (11) is as follows: （11）， Among them, , which is used to decouple and compensate the voltage cross-coupling term; k I p The proportional parameter of the inner current loop control, k I i represents the integral parameter of the inner current loop control; is the dq component of the control current reference value, which is composed of the d axis component I dref of the control current reference value and the q axis component I qref of the control current reference value; ω n is the rated angular frequency; L f is the value of the filter inductor; A pulse width modulation module for comparing a three-phase voltage control quantity with an internal reference level of the module to obtain a duty cycle and generate PWM a signal to drive the inverter to operate.

2. The control system according to claim 1, wherein It also includes: A first coordinate transformation module, which is used to transform three - phase variables into components in a synchronous rotating coordinate system according to the current synchronous phase angle through a first coordinate axis transformation formula; The first coordinate axis transformation formula is shown in formula (1): （1）， Among them, θ I is the current synchronization phase angle, S a is A the component under the S b is B the component under the S c is C the component under the S d is the component on the d axis of the synchronous rotating coordinate system; S q is the component on the q axis of the synchronous rotating coordinate system; T ( θ I ) is the control function, and its expression is shown in formula (2): （2）。 3. The control system according to claim 2, characterized in that, The power calculation module specifically includes: According to the three-phase current i abc and the current synchronization phase angle θ I calculate the current in the three-phase current synchronous coordinate system through the first coordinate conversion module dq components I dq , including the current d axis component I d 、the current q axis component I q ; the specific calculation is shown in formula (3): （3）， According to the three-phase voltage at the grid connection point u pabc and the synchronous phase angle of the current θ I The grid connection point voltage in the synchronous coordinate system of the three-phase voltage at the grid connection point is calculated through the first coordinate conversion module dq components V pdq , including the grid connection point voltage d axis component V pd and the grid connection point voltage q axis component V pq ; The specific calculation is shown in formula (4): （4）， The active power P is calculated as shown in Equation (5): （5）， The reactive power Q is calculated as shown in Equation (6): （6）。 4. The control system according to claim 1, wherein The frequency regulation equation in the active power - frequency control module is shown in formula (7): （7）， Among them, ω I is the angular frequency of the current; ω n is the rated angular frequency; J represents the virtual moment of inertia; D represents the virtual damping coefficient; P represents the current output active power; P ref represents the reference value of the active power.

5. The control system according to claim 1, wherein The output current synchronous phase angle is calculated through formulas (8)-(9) in the active power - frequency control module, specifically including: The frequency regulation equation is discretely implemented by a controller as shown in formula (8): （8）， Then, the current synchronous phase angle is generated through integration θ I and output to the power calculation module and the current control loop module; the integration formula is as shown in formula (9): （9）， Among them, dt represents a small increment of t the time variable.

6. The control system according to claim 1, wherein The first proportional - integral controller in the reactive current control module adopts formula (10): （10）， wherein, Q ref is the reference value of the reactive power output by the inverter; k IP and k II are the proportional-integral controller parameters for reactive-current amplitude control, k IP is the proportional coefficient of reactive current control, k II is the integral coefficient of reactive current control.

7. A control method for a current-source inverter without a phase-locked loop based on frequency support, based on the control system for a current-source inverter without a phase-locked loop based on frequency support according to any one of claims 1-6, characterized in that, It includes the following steps: S 1. Collect the three-phase current on the output side of the inverter i abc and the three-phase voltage at the grid connection point u pabc , and transmit them to the power calculation module; S 2. According to the current synchronization phase angle, the three-phase current i abc Three-phase voltage at the grid connection point u pabc Converted into current in synchronous coordinate system dq Quantity I dq And grid connection point voltage dq Quantity V pdq , then according to the current dq Quantity I dq And grid connection point voltage dq Quantity V pdq Calculating active power P Reactive Power Q , and the active power P Feedback to the active frequency control module, the reactive power Q Feedback to the reactive current control module; S 3. Construct a frequency regulation equation to achieve dynamic variation of frequency with active power, and calculate the output current synchronous phase angle to the power calculation module and the current control loop module; S 4. Adjust the reactive power deviation to the control current according to the first proportional-integral controller d Axis component I dref , and set the q Axis component I qref of the control current to 0 and output it to the current control loop module; S 5. According to the control current dq component I dq Based on formula (11), the grid connection point dq voltage V pdq is compensated to obtain a compensated voltage control signal , and then it is converted into a three-phase voltage control quantity and output to the pulse width modulation module; formula (11) is as follows: （11）， Among them, , which is used to decouple and compensate the cross-coupling term of the compensation voltage; k I p The proportional parameter of the inner current loop control, k I i represents the integral parameter of the inner current loop control; is the dq component of the control current reference value, which is composed of the d axis component I dref of the control current reference value and the q axis component I qref of the control current reference value; ω n is the rated angular frequency; L f is the value of the filter inductor; S 6. Compare the three-phase voltage control quantity with the internal reference level of the module to obtain the duty cycle and generate PWM a signal to drive the inverter to operate.

8. The control method according to claim 7, wherein Step S 2 Specifically, it includes the following steps: S 21. According to the three-phase current i abc and the current synchronization phase angle θ I calculate the current in the three-phase current synchronous coordinate system through the first coordinate conversion module dq components I dq , including the current d axis component I d , the current q axis component I q ; the specific calculation is shown in formula (3): （3）， S 22. According to the three-phase voltage at the grid connection point u pabc and the synchronous phase angle of the current θ I The grid connection point voltage in the synchronous coordinate system of the three-phase voltage at the grid connection point is calculated through the first coordinate conversion module dq component V pdq , including the grid connection point voltage d axis component V pd , the grid connection point voltage q axis component V pq ; The specific calculation is shown in formula (4): （4）， S 23. Calculate the active power according to formulas (5)-(6) P and the reactive power Q :[[]]END]] （5）， （6）。 9. The control method according to claim 7, wherein Step S The frequency adjustment equation described in 3 is as shown in formula (7), and the current angular frequency ω I is integrated to generate the current synchronous phase angle θ I ; （7）， Among them, ω n is the rated angular frequency; J represents the virtual moment of inertia; D represents the virtual damping coefficient; P represents the current output active power; P ref represents the reference value of active power.

10. The control method according to claim 7, characterized in that, Step S The first proportional-integral controller described in 4 adopts formula (10): （10）， Among them, Q ref is the reference value of the reactive power output by the inverter; k IP and k II are the proportional-integral controller parameters for reactive-current amplitude control, k IP is the proportional coefficient of reactive current control, k II is the integral coefficient of reactive current control.

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