Frequency-Support-Based Current-Source Inverter Control System and Method without Phase-Locked Loop

Abstract

The present invention provides a phase-locked loop-free current source inverter control system and method based on frequency support, which relates to the fields of power systems and inverter control technology, and 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 a pulse width modulation module. 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 controls and determines the output current of the inverter through the current control loop module. The present invention combines the advantages of grid-following and grid-forming converter control, adopts a three-layer nested control architecture, does not need to rely on 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, and also avoids current shocks during faults. It can provide inertial support and frequency support for the grid, thereby improving system stability.

Description

Technical Field

[0001] The present invention relates to the technical field of power systems and inverter control, and in particular to a frequency-support-based phase-locked loop-free current source inverter control system and method. Background Art

[0002] Large-scale grid integration of renewable energy sources such as wind and solar power via power electronic converters will become a key technical feature of the next-generation power system. Consequently, the penetration of power electronic converters in the power grid is rapidly increasing, and they are replacing traditional synchronous generators. However, unlike traditional power systems dominated by synchronous generators, power systems with a high proportion of renewable energy are characterized by intermittent and fluctuating output from renewable energy sources, the control flexibility of power electronic converters, and the fragility of power electronic devices. These differences pose new challenges to the safe and stable operation of the next-generation power system.

[0003] Existing solutions: At present, in actual engineering applications, renewable energy such as wind energy and solar energy are usually achieved by adopting grid following ( GFL ) control architecture is integrated into the power system. The grid-following converter obtains the phase information of the grid voltage through a phase-locked loop to achieve synchronization with the grid, and accurately adjusts the active and reactive power 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 integrated 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. In order to solve this problem, the grid-building converter control ( GFM Various researchers have proposed various control structures for grid-type converters. These methods can independently build the AC output voltage and provide voltage support to the grid without relying on the external grid. Although grid-type converters are playing an increasingly important role in power systems with high levels of renewable energy integration and widespread use of power electronics, they still face numerous challenges, such as limited overcurrent capability and reduced stability in the face of strong grid conditions.

[0004] To simultaneously overcome the synchronization and stability issues faced by both grid-following and grid-forming converters, some researchers have proposed direct power control methods, which bypass the current inner loop and directly control the converter's output active and reactive power. However, due to voltage noise and distortion, the use of a bandpass filter is unavoidable in direct power control, leading to the same stability issues as using a phase-locked loop (PLL). Subsequently, other researchers proposed a PLL-free power synchronization control scheme. Results showed that, since it does not require voltage acquisition at the point of common coupling, it can operate stably in both strong and weak grids. While effective, this approach suffers from complex controller design and is unable to cope with grid frequency fluctuations. Any frequency deviation will introduce steady-state errors, resulting in unstable converter operation. Furthermore, researchers have proposed a general control strategy that combines power synchronization and PLL schemes to leverage their respective advantages. However, this structure still requires a PLL for synchronization.

[0005] Therefore, the existing solutions still have the following problems that need to be solved urgently:

[0006] (1) Under weak grid conditions, the phase-locked loop is susceptible to grid fluctuations, which may cause phase shift or unstable locking. Therefore, it is difficult to balance locking speed and anti-interference ability to ensure stable operation under various grid conditions.

[0007] (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.

[0008] (3) When the grid voltage fluctuates or there is a short-term fault, the stable operation and rapid recovery of the inverter remain challenges.

[0009] (4) In a strong power grid environment, the grid-type converter faces the risk of fault surge current, and there is a contradiction between overcurrent protection and dynamic performance. Excessive current limiting will reduce the system response capability, while too loose current limiting may damage the device. Summary of the Invention

[0010] In response to 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.

[0011] In the first aspect, the present invention provides a phase-locked loop-free current source inverter control system based on frequency support, wherein the control system acts on a power grid system, wherein 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 , the three-phase current output after passing through the inverter and filter inductor i abcAfter passing through the line impedance, it is connected to the power grid; the control system includes:

[0012] Acquisition module, used to collect the three-phase current on the output side of the inverter i abc And three-phase voltage at the grid connection point u pabc ;

[0013] The power calculation module is used to convert the three-phase current into 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 and 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;

[0014] Active frequency control module, used to construct frequency regulation equation to achieve dynamic frequency change with active power, and calculate the output current synchronization phase angle to the power calculation module and current control loop module;

[0015] Reactive current control module, used to adjust the reactive power deviation to control current according to the first proportional integral controller d Axis component I dref and will control the current q Axis component I qref Set to 0 and output to the current control loop module;

[0016] Current control loop module, used to control current according to dq Quantity I dq Based on formula (11) dq Voltage V pdq After compensation, it is converted into three-phase voltage control quantity Output to PWM Control module; Formula (11) is as follows:

[0017] (11)

[0018] in, , used to decouple the compensation voltage cross-coupling term; k I p Current inner loop control proportional parameters, k I i Indicates the integral parameter of the current inner loop control; To control the current reference value dq component, controlled by the current reference value d Axis component I dref and control current reference value q Axis component I qref composition; ω n is the rated angular frequency, which is 2*π*50; L f is the filter inductance value;

[0019] Pulse width modulation module, which controls the three-phase voltage Compare with the module's internal reference level to get the duty cycle and generate PWM signal to drive the inverter to operate.

[0020] Specifically, the control system further includes:

[0021] The first coordinate conversion module is used to convert the three-phase variables into a synchronous rotating coordinate system ( dq ) under the weight;

[0022] The first coordinate axis conversion formula is shown in formula (1):

[0023] (1)

[0024] in, θ I is the current synchronization phase angle, S a for A Lower weight; S b for B Lower weight; S c for C Lower weight; S d Synchronous rotating coordinate system d Axis component; S qSynchronous rotating coordinate system q Axis component; T ( θ I ) is the control function, and its expression is shown in formula (2):

[0025] (2).

[0026] Specifically, the power calculation module includes:

[0027] According to the three-phase current i abc Synchronous phase angle with current θ I The three-phase current synchronous coordinate system is calculated by the first coordinate conversion module ( dq Current in the coordinate system) dq Quantity I dq , including current d Axis component I d , current q Axis component I q The specific calculation is shown in formula (3):

[0028] (3)

[0029] According to the three-phase voltage of the grid connection point u pabc Synchronous phase angle with current θ I The grid connection point voltage in the three-phase voltage synchronization coordinate system is calculated by the first coordinate conversion module dq Quantity V pdq , including 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):

[0030] (4)

[0031] The active power P The calculation of is shown in formula (5):

[0032] (5)

[0033] The reactive power Q The calculation of is shown in formula (6):

[0034] (6)

[0035] Specifically, the frequency adjustment equation in the active frequency control module is shown in formula (7):

[0036] (7)

[0037] in, 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 Indicates the current output active power; P ref Indicates the active power reference value;

[0038] Specifically, the active frequency control module calculates the output current synchronization phase angle through formulas (8)-(9), which specifically includes:

[0039] The frequency regulation equation is discretely implemented by the controller as shown in formula (8):

[0040] (8)

[0041] Then generate the current synchronization phase angle by integration θ I Output to the power calculation module and the current control loop module; the integral formula is shown in formula (9):

[0042] (9)

[0043] in, dt Represents time variables t of small increments.

[0044] Specifically, the first proportional-integral controller in the reactive current control module adopts formula (10):

[0045] (10)

[0046] in, Q ref Output reactive power reference value for inverter; k IP and k II are the proportional-integral controller parameters for reactive-current amplitude control, k IP is the reactive current control proportional coefficient, k II is the reactive current control integral.

[0047] In a second aspect, the present invention provides a frequency-supported phase-locked loop-free current source inverter control method, based on the frequency-supported phase-locked loop-free current source inverter control system described in any one of the first aspects, comprising the following steps:

[0048] S 1. Collect the three-phase current on the inverter output side i abc And three-phase voltage at the grid connection point u pabc , transmitted to the power calculation module;

[0049] 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 and 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;

[0050] S 3. Construct a frequency regulation equation to achieve dynamic changes in frequency with active power, and calculate the output current synchronization phase angle to the power calculation module and current control loop module;

[0051] S 4. According to the first proportional integral controller, the reactive power deviation is adjusted to control current d Axis component I dref and will control the current q Axis component I qref Set to 0 and output to the current control loop module;

[0052] S 5. According to the control current dq Quantity I dq Based on formula (11)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:

[0053] (11)

[0054] in, , used to decouple the compensation voltage cross-coupling term; k I p Current inner loop control proportional parameters, k I i Indicates the integral parameter of the current inner loop control; To control the current reference value dq component, controlled by the current reference value d Axis component I dref and control current reference value q Axis component I qref composition; ω n is the rated angular frequency; L f is the filter inductance value;

[0055] S 6. Control the three-phase voltage Compare with the module's internal reference level to get the duty cycle and generate PWM signal to drive the inverter to operate.

[0056] Specifically, steps S 2 specifically includes the following steps:

[0057] S 21. According to the three-phase current i abc Synchronous phase angle with current θ I The current in the three-phase current synchronous coordinate system is calculated by the first coordinate conversion module dq Quantity I dq , including current d Axis component I d , current q Axis component I q The specific calculation is shown in formula (3):

[0058] (3);

[0059] S 22. According to the three-phase voltage of the grid connection point u pabc Synchronous phase angle with current θ I The grid connection point voltage in the three-phase voltage synchronization coordinate system is calculated by the first coordinate conversion module dq Quantity V pdq , including 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):

[0060] (4)

[0061] S 23. Calculate active power according to formulas (5)-(6) P and reactive power Q :

[0062] (5)

[0063] (6).

[0064] Specifically, steps S The frequency adjustment equation described in 3 is shown in formula (7), through the current angular frequency ω I Integration generates current synchronization phase angle θ I ;

[0065] (7)

[0066] in, 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 Indicates the current output active power; P ref Indicates the active power reference value.

[0067] Specifically, steps S The first proportional-integral controller in 4 adopts formula (10):

[0068] (10)

[0069] in, Q ref Output reactive power reference value for inverter; k IP and k II are the proportional-integral controller parameters for reactive-current amplitude control, k IP is the reactive current control proportional coefficient, k II is the reactive current control integral.

[0070] This invention directly constructs the amplitude and frequency of the inverter's output current through an active frequency control module and a reactive current control module, and then controls the inverter's output current via a current control loop module. This invention combines the advantages of both grid-following and grid-forming converter control, employing a three-layer nested control architecture. It eliminates the need for a phase-locked loop (PLL) to provide grid phase angle, adapting to complex operating conditions such as strong and weak grids and varying line impedances. It also mitigates current surges during faults, provides inertial support and frequency support for the grid, and improves system stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0071] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0072] Figure 1 This is a control block diagram of a phase-locked loop-free current source inverter control system based on frequency support provided by an embodiment of the present invention;

[0073] Figure 2 This is a voltage and current vector diagram provided by an embodiment of the present invention;

[0074] Figure 3 Schematic diagram of a frequency-support-based phase-locked loop-free current source inverter control method provided by an embodiment of the present invention;

[0075] Figure 4 This is the simulation result of the control strategy provided by the embodiment 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 );

[0076] Figure 5 This is the simulation result of the control strategy provided by the embodiment of the present invention when the power 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 );

[0077] Figure 6 This is the simulation result of the control strategy provided by the embodiment 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 I dq ( A ). DETAILED DESCRIPTION

[0078] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention are within the scope of protection of the present invention.

[0079] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of the technical features being referred to. Thus, a feature identified as "first" or "second" may explicitly or implicitly include one or more of the features. In the description of the present invention, "plurality" means two or more, unless otherwise specifically defined.

[0080] Example 1

[0081] This embodiment provides a phase-locked loop-free current source inverter control system based on frequency support, and its control block diagram is as follows: Figure 1 As 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 , line impedance and power grid; DC source is the energy source of the main circuit of the power grid system, and the input DC source bus voltage V dc , through the inverter and filter inductor L f The three-phase current output i abc After being incorporated into the grid through line impedance; the line impedance includes line inductance L g and line resistance R g ; The three-phase voltage of the grid connection point is u pabc ;

[0082] The inverter is a three-phase full-bridge inverter.

[0083] 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 Control module;

[0084] The present invention adopts a "three-layer nested control structure": 1. Current inner loop: quickly tracks current commands; 2. Outer loop 1 - active power / frequency control: based on a virtual synchronous machine model, realizes the generation of virtual current frequency and current synchronous phase angle; Outer loop 2 - reactive power / current control: provides reactive power control capability.

[0085] Acquisition module, used to collect the three-phase current on the output side of the inverter i abc And three-phase voltage at the grid connection point u pabc The three-phase current i abc include i a、 i b 、 i c ; The three-phase voltage of the grid point u pabc include u pa 、 u pb 、 u pc ;

[0086] Three-phase current on the inverter output side i abc And three-phase voltage at the grid connection point u pabc Signal isolation and acquisition are achieved by using high-precision current (voltage) transformers;

[0087] The first coordinate conversion module is used to convert the three-phase variables into a synchronous rotating coordinate system ( dq ) under the weight;

[0088] The first coordinate axis conversion formula is shown in formula (1):

[0089] (1)

[0090] in, θ I is the current synchronization phase angle, S a for A Lower weight; S b for B Lower weight; S c for C Lower weight; S d Synchronous rotating coordinate system d Axis component; S q Synchronous rotating coordinate system q Axis component; T ( θ I ) is the control function, and its expression is shown in formula (2):

[0091] (2)

[0092] The power calculation module is used to convert the three-phase current into i abc Three-phase voltage at the grid connection point u pabc Converted into current in synchronous coordinate systemdq 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 and 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;

[0093] The current dq Quantity I dq Including current d Axis component I d , current q Axis component I q ; The grid connection point voltage dq Quantity V pdq Including grid connection point voltage d Axis component V pd , grid connection point voltage q Axis component V pq ;

[0094] Specifically include:

[0095] According to the three-phase current i abc Synchronous phase angle with current θ I The three-phase current synchronous coordinate system is calculated by the first coordinate conversion module ( dq Current in the coordinate system) dq Quantity I dq , including current d Axis component I d , current q Axis component I q The specific calculation is shown in formula (3):

[0096] (3)

[0097] Similarly, calculate the three-phase voltage at the grid connection point dqComponents under coordinates V pd 、V pq ; Specifically include:

[0098] According to the three-phase voltage of the grid connection point u pabc Synchronous phase angle with current θ I The grid connection point voltage in the three-phase voltage synchronization coordinate system is calculated by the first coordinate conversion module dq Quantity V pdq , including 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):

[0099] (4)

[0100] Current synchronization phase angle θ I The initial current angular frequency is obtained by formula (9), and the initial current angular frequency has a preset value of 50 Hz .

[0101] It is understandable that the current synchronization phase angle θ I The initial current angular frequency is calculated by formula (9) during initialization, and the current synchronization phase angle in the subsequent active frequency control module is θ I It is obtained by calculating the current angular frequency in real time using formula (9).

[0102] The active power P The calculation of is shown in formula (5):

[0103] (5)

[0104] The reactive power Q The calculation of is shown in formula (6):

[0105] (6)

[0106] The active frequency control module is used to construct a frequency adjustment equation to achieve dynamic changes in frequency with active power, and calculate the output current synchronization phase angle to the power calculation module and the current control loop module; the frequency adjustment equation is shown in formula (7):

[0107] (7)

[0108] in, 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 Indicates the current output active power; P ref Indicates the active power reference value;

[0109] The frequency regulation equation is discretely implemented by the controller as shown in formula (8):

[0110] (8)

[0111] in, dt Represents time variables t Small increments of

[0112] Then generate the current synchronization phase angle by integration θ I Output to the power calculation module and the current control loop module; the integral formula is shown in formula (9):

[0113] (9)

[0114] in, dt Represents time variables t Small increments of

[0115] The active power frequency control module corresponds to outer loop 1 of the three-layer nested control structure - active power / frequency control. It uses a frequency regulation equation to construct a "virtual moment of inertia + damping" structure to achieve dynamic frequency variation with active power. This structure replaces the traditional phase-locked loop (PLL). The inverter "senses grid frequency" through changes in its own output active power, providing natural synchronization and inertial response capabilities.

[0116] Reactive current control module, used to adjust the reactive power deviation to control current according to the first proportional integral controller d Axis component I dref and will control the current q Axis component I qref Set to 0 and output to the current control loop module; the first proportional integral controller uses formula (10):

[0117] (10)

[0118] in, Q refOutput reactive power reference value for inverter; k IP and k II are the proportional-integral controller parameters for reactive-current amplitude control, k IP is the reactive current control proportional coefficient, k II is the reactive current control integral coefficient;

[0119] I dref Used for current inner loop control current d axis components to achieve independent reactive power regulation capability, and I qref Set to 0 to synchronize the coordinate system d The shaft is positioned on the current.

[0120] The reactive current control module corresponds to the outer loop 2 of the three-layer nested control structure - reactive / current control, providing reactive control capabilities

[0121] The active frequency control module and the reactive current control module decouple the reactive current control path from the active frequency control, which can quickly adjust the output current and achieve PQ Independent control.

[0122] refer to Figure 2 , we can see the essential difference between this implementation case and the traditional grid-following and grid-building control designs. Figure 2 In ( a ) As shown in the figure, the traditional grid-following type uses a phase-locked loop, and the grid-forming type controls the voltage by constructing it. V pq =0, set the voltage to d On-axis, then grid-type control based on power transfer formula design dq The reference value of the axis current component is constructed based on the active-frequency and reactive-voltage d The amplitude and phase of the shaft voltage are used to control the inverter. Figure 2 In ( b ), in order to achieve power synchronization and avoid the problem of transient impact current, this embodiment is I qref Designed to 0, d The axis is positioned on the current, establishing d Shaft current frequency and phase reference, established through reactive power feedback d The shaft current amplitude reference is used to achieve inverter power control. This is fundamentally different from traditional grid-following and grid-forming control designs.

[0123] The current control loop module is used to dq Quantity I dq Based on formula (11) 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 PWM Control module; Formula (11) is as follows:

[0124] (11)

[0125] in, , used to decouple the compensation voltage cross-coupling term; k I p Current inner loop control proportional parameters, k I i Indicates the integral parameter of the current inner loop control; To control the current reference value dq component, controlled by the current reference value d Axis component I dref and control current reference value q Axis component I qref composition; ω n is the rated angular frequency, which is 2*π*50 rad / s ; L f is the filter inductance value;

[0126] It is understandable that the current dq Quantity I dq Including current d Axis component I d , current q Axis component I q ; The grid connection point voltage dq Quantity V pdq Including grid connection point voltage d Axis component V pd , grid connection point voltage q Axis component V pq ; Through formula (11), d Axis and pThe axis components are calculated to obtain and , these two components are synthesized ; That is, the compensation voltage control signal is a column vector (a column vector is a special form of matrix).

[0127] It is worth noting that in this application, all parameters with the subscript dq are column vectors containing d-axis components and q-axis components, for example I dq , V pdq , , , the parameter subscripts containing abc are all column vectors containing the three-phase components a, b, and c, for example i abc , u pabc , .

[0128] Formula (11) is the proportional-integral controller and voltage feedforward decoupling compensation structure, the compensation voltage control signal After being inverted into three-phase voltage control quantity by the second coordinate conversion module , used for PWM Generate and feed it to the inverter, and precise feedforward decoupling improves system bandwidth and stability, facilitating high-performance dynamic regulation.

[0129] The current control loop module corresponds to the current inner loop of the three-layer nested control structure, which is used to quickly track the current command;

[0130] The second coordinate conversion module is used to convert the synchronous rotating coordinate system ( dq ) is converted into three-phase variables;

[0131] The second coordinate axis conversion formula is shown in formula (12):

[0132] (12)

[0133] in, θ I is the current synchronization phase angle, S a for A Lower weight; S b for B Lower weight; S c for C Lower weight; S d Synchronous rotating coordinate systemd Axis component; S q Synchronous rotating coordinate system q Axis component; T -1 ( θ I ) is the inverse control function, is the control function T ( θ I ), its expression is shown in formula (13):

[0134] (13)

[0135] Pulse width modulation module is used to control the three-phase voltage Compare with the module's internal reference level to get the duty cycle and generate PWM signal to drive the inverter to operate.

[0136] Specifically, the three-phase voltage control quantity Divide by the module internal reference level ( V dc / 2), V dc is the DC source bus voltage. In the three-phase full-bridge inverter control, the three-phase voltage control quantity is Limit it to [-1,1], and then input PWM The duty cycle signal is generated in the module.

[0137] It is understood that the duty cycle is the ratio of the high level duration in the pulse signal to the entire cycle time, which is usually used to describe pulse width modulation (also known as PWM )Signal, PWM The signal switches between high and low levels at a certain frequency to control the inverter switch to turn on or off.

[0138] Example 2

[0139] refer to Figure 3 This embodiment provides a frequency-support-based phase-locked loop-free current source inverter control method, based on a frequency-support-based phase-locked loop-free current source inverter control system described in Example 1, including the following steps:

[0140] S 1. Collect the three-phase current on the inverter output side i abc And three-phase voltage at the grid connection point u pabc ;

[0141] S 2. According to the current synchronization phase angle, the three-phase current iabc 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 and 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; the current dq Quantity I dq Including current d Axis component I d , current q Axis component I q ; The grid connection point voltage dq Quantity V pdq Including grid connection point voltage d Axis component V pd , grid connection point voltage q Axis component V pq ;

[0142] The specific steps include:

[0143] S 21. According to the three-phase current i abc Synchronous phase angle with current θ I The current in the three-phase current synchronous coordinate system is calculated by the first coordinate conversion module dq Quantity I dq , including current d Axis component I d , current q Axis component I q The specific calculation is shown in formula (3):

[0144] (3);

[0145] S 22. According to the three-phase voltage of the grid connection point u pabc Synchronous phase angle with current θ I The grid connection point voltage in the three-phase voltage synchronization coordinate system is calculated by the first coordinate conversion module dq Quantity V pdq , including 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):

[0146] (4)

[0147] S 23. Calculate active power according to formulas (5)-(6) P and reactive power Q :

[0148] (5)

[0149] (6).

[0150] S 3. Construct a frequency regulation equation to achieve dynamic changes in frequency with active power, and calculate the output current synchronization phase angle to the power calculation module and current control loop module;

[0151] The frequency adjustment equation is shown in formula (7):

[0152] (7)

[0153] in, 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 Indicates the current output active power; P ref Indicates the active power reference value;

[0154] The frequency regulation equation is discretely implemented by the controller as shown in formula (8):

[0155] (8)

[0156] Then generate the current synchronization phase angle by integration θ I Output to the power calculation module and the current control loop module; the integral formula is shown in formula (9):

[0157] (9)

[0158] in, dt Represents time variables t of small increments.

[0159] S 4. According to the first proportional integral controller, the reactive power deviation is adjusted to control current d Axis component I dref and will control the current q Axis component I qref Set to 0 and output to the current control loop module; the first proportional integral controller uses formula (10):

[0160] (10)

[0161] in, Q ref Output reactive power reference value for inverter; k IP and k II are the proportional-integral controller parameters for reactive-current amplitude control, k IP is the reactive current control proportional coefficient, k II is the reactive current control integral;

[0162] S 5. According to the control current dq Quantity I dq Based on formula (11) 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:

[0163] (11)

[0164] in, , used to decouple the compensation voltage cross-coupling term; k I p andk I i Indicates the proportional-integral control parameter of the current inner loop control, k I p Current inner loop control proportional parameters, k I i Indicates the integral parameter of the current inner loop control; To control the current reference value dq component, controlled by the current reference value d Axis component I dref and control current reference value q Axis component I qref composition; ω n is the rated angular frequency, which is 2*π*50; L f is the filter inductance value;

[0165] It can be understood that the proportional integral control parameters of the current inner loop control ( k I p and k I i ) is related to the line impedance and can be determined based on the line impedance and the design parameters required for the optimal performance of the controller. This is existing technology and will not be described in detail here.

[0166] Specifically, the compensation output voltage is converted into Converted into three-phase voltage control quantity .

[0167] S 6. Control the three-phase voltage Compare with the module's internal reference level to get the duty cycle and generate PWM signal to drive the inverter to operate.

[0168] This embodiment achieves synchronization with the power grid through active power-frequency feedback, does not need to 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.

[0169] This embodiment is essentially still a grid-following control, thus avoiding damage to the system caused by transient surge currents under grid faults.

[0170] This embodiment adopts an improved current control strategy to effectively compensate for the impact of line impedance changes on current control. The converter can maintain stable operation under different line impedance conditions, ensuring grid connection quality and system reliability.

[0171] Based on MATLAB / Simulink Real-time simulation results and analysis

[0172] In order to verify the proposed control method, this embodiment gives three MATLAB / Simulink The effectiveness of the proposed control is discussed in the following example. The implementation example consists of a DC source, an inverter, a filter inductor, L f , line impedance, grid composition, voltage level is 380 V , power level is 10 kW Cases 1, 2, and 3 test the performance of the proposed control scheme under conditions of grid frequency drops, varying grid strengths, and varying line impedances. The system parameters used in the simulations are shown in Table 1, and the control parameters are shown in Table 2.

[0173]

[0174]

[0175] Case 1: Grid frequency drop test

[0176] Figure 4 The frequency support capability of the proposed control strategy for the power grid is verified. Figure 4 In ( a ), when the grid frequency drops in the second second, the system will output active power from 10 kW Increased to 16.3 kW , injecting active power into the grid to support grid operation. When the grid frequency recovers to 50 in the 6th second Hz When the system output active power also returns to the given value and returns to normal operation mode. During this period, the reactive power is not affected. Figure 4 ( b ) as shown. Figure 4 ( c ) respectively give the voltage across the converter and the output current d Axis and q This implementation demonstrates that the proposed control can provide certain frequency support for the power grid and improve the robustness of the system.

[0177] Case 2: Adaptability test under different grid strengths

[0178] Figure 5The adaptability of the proposed control strategy under different grid strengths is verified. Grid strength is generally measured by short circuit ratio ( SCR ) is divided into very weak ( SCR ≤2), weak (2< SCR <3) and strong ( SCRbb 0 3). SCR It can be expressed as:

[0179]

[0180] In this implementation case, the output power is set to be constant, and the line inductance is changed to change different SCR Value. Figure 5 As shown, the system implementing this solution starts with a weak grid ( SCR =2.3) under the condition of output active power 10 kW , reactive power 5 kVar , the system works normally. SCR At the third second, the frequency rises to 4.6, resulting in a brief surge in system frequency and power, but ultimately returning to rated operation. At the seventh second, the line impedance is further reduced, and the grid transitions from weak to strong, but the proposed control scheme maintains system output unchanged. This implementation demonstrates the system's adaptability to various grid strengths.

[0181] Case 3: Adaptability test under complex line impedance conditions

[0182] Figure 6 The adaptability of the proposed control strategy in complex line impedance conditions is verified. In this implementation case, the system line impedance is verified from pure induction ( XL =3.14 j Ω) to resistance and inductance ( XL =2.5+1.57 j Ω), then purely resistive ( XL =5Ω). The results show that under the proposed control strategy, the system can maintain stable frequency and output power. This demonstrates that the solution can adapt to different line impedance characteristics.

[0183] From the above three cases, it can be seen that the control method of this embodiment can be applied to different grid conditions. Unlike the traditional grid-following and grid-forming inverter control, this method 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 controls the output current of the inverter through the current control loop module. This solution adopts a three-layer nested control architecture, which does not need to rely on the phase-locked loop to provide the grid phase angle, and also avoids the current shock during faults. Therefore, this solution is not only applicable to strong and weak grids, but also to different line impedance conditions.

[0184] The control method of this embodiment is different from the conventional PLL The grid-connected control is compared from four dimensions: synchronization mode, grid-connected stability, frequency response capability, and reactive power control, as shown in Table 3:

[0185]

[0186] This control method combines the advantages of both grid-following and grid-forming converter control. It employs a simplified control strategy, avoids the limitations of phase-locked loops, and adapts to complex operating conditions, including strong and weak grids and varying line impedances. Furthermore, this solution directly controls the output current, effectively reducing the risk of current surges and providing inertia and frequency support for the grid, improving system stability.

[0187] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A phase-locked loop-free current source inverter control system based on frequency support, the control system acting on a power grid system, characterized in that: The grid system includes a DC source, an inverter, a filter inductor, a line impedance and a grid; the DC source serves as the energy source of the main circuit of the grid system, and the input DC source bus voltage V dc , the three-phase current output after passing through the inverter and filter inductor i abc After passing through the line impedance, it is connected to the grid; The control system includes: Voltage and current acquisition module, used to collect the three-phase current on the output side of the inverter i abc And three-phase voltage at the grid connection point u pabc , transmitted to the power calculation module; The power calculation module is used to convert the three-phase current into 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 and 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; Active frequency control module, used to construct frequency regulation equation to achieve dynamic frequency change with active power, and calculate the output current synchronization phase angle to the power calculation module and current control loop module; Reactive current control module, used to adjust the reactive power deviation to control current according to the first proportional integral controller d Axis component I dref and will control the current q Axis component I qref Set to 0 and output to the current control loop module; Current control loop module, used to control current according to dq Quantity I dq Based on formula (11) 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）， in, , used to decouple the compensation voltage cross-coupling term; k I p Current inner loop control proportional parameters, k I i Indicates the integral parameter of the current inner loop control; To control the current reference value dq component, controlled by the current reference value d Axis component I dref and control current reference value q Axis component I qref composition; ω n is the rated angular frequency; L f is the filter inductance value; Pulse width modulation module is used to control the three-phase voltage Compare with the module's internal reference level to get the duty cycle and generate PWM signal to drive the inverter to operate.

2. The control system according to claim 1, characterized in that: Also includes: A first coordinate conversion module is used to convert the three-phase variables into components in a synchronous rotating coordinate system through a first coordinate axis conversion formula according to the current synchronous phase angle; The first coordinate axis conversion formula is shown in formula (1): （1）， in, θ I is the current synchronization phase angle, S a for A Lower weight; S b for B Lower weight; S c for C Lower weight; S d Synchronous rotating coordinate system d Axis component; S q Synchronous rotating coordinate system q Axis component; 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 Synchronous phase angle with current θ I The current in the three-phase current synchronous coordinate system is calculated by the first coordinate conversion module dq Quantity I dq , including 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 of the grid connection point u pabc Synchronous phase angle with current θ I The grid connection point voltage in the three-phase voltage synchronization coordinate system is calculated by the first coordinate conversion module dq Quantity V pdq , including 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 active power P The calculation of is shown in formula (5): （5）， The reactive power Q The calculation of is shown in formula (6): （6）。 4. The control system according to claim 1, characterized in that: The frequency regulation equation in the active frequency control module is shown in formula (7): （7）， in, 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 Indicates the current output active power; P ref Indicates the active power reference value.

5. The control system according to claim 1, characterized in that: The active frequency control module calculates the output current synchronization phase angle through formulas (8)-(9), which specifically includes: The frequency regulation equation is discretely implemented by the controller as shown in formula (8): （8）， Then generate the current synchronization phase angle by integration θ I Output to the power calculation module and the current control loop module; the integral formula is shown in formula (9): （9）， in, dt Represents time variables t of small increments.

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

7. A frequency-support-based phase-locked loop-free current source inverter control method, based on the frequency-support-based phase-locked loop-free current source inverter control system according to any one of claims 1 to 6, characterized in that: The following steps are involved: S 1. Collect the three-phase current on the inverter output side i abc And three-phase voltage at the grid connection point u pabc , transmitted 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 and 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 changes in frequency with active power, and calculate the output current synchronization phase angle to the power calculation module and current control loop module; S 4. According to the first proportional integral controller, the reactive power deviation is adjusted to control current d Axis component I dref and will control the current q Axis component I qref Set to 0 and output to the current control loop module; S 5. According to the control current dq Quantity I dq Based on formula (11) 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）， in, , used to decouple the compensation voltage cross-coupling term; k I p Current inner loop control proportional parameters, k I i Indicates the integral parameter of the current inner loop control; To control the current reference value dq component, controlled by the current reference value d Axis component I dref and control current reference value q Axis component I qref composition; ω n is the rated angular frequency; L f is the filter inductance value; S 6. Control the three-phase voltage Compare with the module's internal reference level to get the duty cycle and generate PWM signal to drive the inverter to operate.

8. The control method according to claim 7, characterized in that: step S 2 specifically includes the following steps: S 21. According to the three-phase current i abc Synchronous phase angle with current θ I The current in the three-phase current synchronous coordinate system is calculated by the first coordinate conversion module dq Quantity I dq , including current d Axis component I d , current q Axis component I q The specific calculation is shown in formula (3): （3）， S 22. According to the three-phase voltage of the grid connection point u pabc Synchronous phase angle with current θ I The grid connection point voltage in the three-phase voltage synchronization coordinate system is calculated by the first coordinate conversion module dq Quantity V pdq , including 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）， S 23. Calculate active power according to formulas (5)-(6) P and reactive power Q : （5）， （6）。 9. The control method according to claim 7, characterized in that: step S The frequency adjustment equation described in 3 is shown in formula (7), through the current angular frequency ω I Integration generates current synchronization phase angle θ I ; （7）， in, ω n is the rated angular frequency; J represents the virtual moment of inertia; D represents the virtual damping coefficient; P Indicates the current output active power; P ref Indicates the active power reference value.

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

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