A control method, device, equipment and medium of a converter

Abstract

The application discloses a control method, device and equipment of a converter and a medium, and relates to the field of power electronics. When a phase component of a voltage given value is generated by using an active ring, the angular frequency of a target converter is fed forwardly compensated by using the angular frequency of the whole grid-connected system, so that the vibration amplitude of the power angle difference is reduced, and the power angle difference fluctuation between the converters is reduced. The grid-connected system can effectively provide active frequency support when the grid voltage suddenly increases or decreases. Meanwhile, when the converter is switched and modulated, the current modulation strategy of the converter is adjusted in real time according to the switching loss of the target converter, so that the switching loss of the whole grid-connected system is reduced. The switching loss is reduced and the angular frequency compensation in the active ring is introduced to enhance the transient stability and frequency stability of the converter during the grid fault, so that the stability of the converter and the whole grid-connected system is effectively improved.

Description

Technical Field

[0001] This invention relates to the field of power electronics, and in particular to a control method, apparatus, equipment and medium for a converter. Background Technology

[0002] Currently, grid connection of renewable energy sources has become one of the important development directions of the power system. Renewable energy systems achieve grid connection through converters, enabling the application of renewable energy power. Therefore, the transient stability of the converter is crucial for the stable grid connection of renewable energy. If a grid fault occurs, the converter may be unable to maintain stable operation, potentially disrupting the dynamic balance of the power system, leading to drastic fluctuations in voltage and frequency, and even triggering cascading failures such as regional blackouts. Therefore, improving the stability of converter operation has become an important development direction for converters. Summary of the Invention

[0003] The purpose of this invention is to provide a control method, device, equipment, and medium for a converter, which aims to enhance the transient stability and frequency stability of the converter during grid faults and ensure stable operation of the converter when it is connected to the grid.

[0004] To address the aforementioned technical problems, this invention provides a converter control method applied to a grid-connected system. The grid-connected system includes several converters, whose AC sides are connected in parallel and all connected to the power grid through a common coupling point. For any target converter in the grid-connected system, the converter control method includes:

[0005] The angular frequency deviation of the target converter is determined based on the active power error of the target converter;

[0006] Feedforward compensation is performed on the angular frequency deviation of the target converter based on the angular frequency deviation of other converters in the grid-connected system to obtain the phase component of the voltage setpoint of the target converter;

[0007] The amplitude component of the voltage setpoint is determined based on the reactive power error of the target converter.

[0008] The phase component and the amplitude component are synthesized to obtain the voltage setpoint, and the modulation signal of the target converter is generated based on the voltage setpoint;

[0009] The current modulation strategy of the target converter is determined based on the switching losses of the target converter.

[0010] The operation of the power switching devices in the target converter is controlled based on the current modulation strategy and the modulation signal.

[0011] Optionally, feedforward compensation is performed on the angular frequency deviation of the target converter based on the angular frequency deviation of other converters in the grid-connected system to obtain the phase component of the voltage setpoint of the target converter, including:

[0012] Calculate the weighted average of the angular frequency deviations of all converters in the grid-connected system to obtain the center angular frequency of the grid-connected system;

[0013] The sum of the center angular frequency and the angular frequency of the power grid is input into the integrator to obtain the phase component of the voltage setpoint of the target converter.

[0014] Optionally, the modulation strategy of the target converter includes discontinuous pulse width modulation and space vector pulse width modulation; determining the current modulation strategy of the target converter based on its switching losses includes:

[0015] Based on the switching losses of the target converter, the current modulation strategy of the target converter is controlled to switch between the discontinuous pulse width modulation and the space vector pulse width modulation.

[0016] Optionally, if the target converter is operating normally, the current modulation strategy of the target converter is switched between the discontinuous pulse width modulation and the space vector pulse width modulation based on the switching losses of the target converter, including:

[0017] Determine whether the modulation index of the target converter is greater than a first preset threshold;

[0018] If so, the current modulation strategy of the target converter is switched to discontinuous pulse width modulation;

[0019] If not, the current modulation strategy of the target converter will be switched to space vector pulse width modulation.

[0020] Optionally, if a fault occurs in the power grid, the current modulation strategy of the target converter is switched between the discontinuous pulse width modulation and the space vector pulse width modulation based on the switching losses of the target converter, including:

[0021] The switching frequency of the power switching devices in the target converter is reduced to a first preset frequency.

[0022] Determine whether the switching loss of the target converter is greater than a preset value;

[0023] If so, the current modulation strategy of the target converter is switched to discontinuous pulse width modulation.

[0024] Optionally, after switching the current modulation strategy of the target converter to discontinuous pulse width modulation, the method further includes:

[0025] Determine whether the switching loss of the target converter is greater than a preset value;

[0026] If so, the switching frequency of the power switching devices in the target converter is reduced to a second preset frequency.

[0027] Optionally, if the power grid is in the fault recovery phase, the current modulation strategy of the target converter is switched between the discontinuous pulse width modulation and the space vector pulse width modulation based on the switching losses of the target converter, including:

[0028] The switching frequency of the power switching devices in the target converter is restored to its initial value before the power grid fault occurred;

[0029] Determine whether the switching loss of the target converter is greater than a preset value;

[0030] If so, the current modulation strategy of the target converter is switched to discontinuous pulse width modulation;

[0031] If not, the current modulation strategy of the target converter will be switched to space vector pulse width modulation.

[0032] Optionally, controlling the current modulation strategy of the target converter to switch between the discontinuous pulse width modulation and the space vector pulse width modulation includes:

[0033] If it is necessary to switch the modulation strategy of the target converter from space vector pulse width modulation to discontinuous pulse width modulation, the composite vectors of the several discontinuous pulse width modulation signals corresponding to several power switching devices in the target converter are synthesized to obtain the first debugging vector.

[0034] The second debugging vector is obtained by synthesizing the composite vectors of several space vector pulse width modulation signals corresponding to several power switching devices in the target converter.

[0035] Determine the target moment when the first debugging vector and the second debugging vector are closest;

[0036] At the target time, the modulation strategy of the target converter is switched from space vector pulse width modulation to discontinuous pulse width modulation.

[0037] Optional, also includes:

[0038] The range of control parameters is determined with the goal that the deviation between the transient power angle and the steady-state power angle of the target converter is less than a second preset threshold; wherein, the control parameters include the proportional coefficient and the integral coefficient of the reactive power loop;

[0039] Extract several sets of values ​​for the control parameter from the range of values ​​of the control parameter;

[0040] Determine the calculated damping ratios that correspond one-to-one with several sets of values;

[0041] The set of values ​​corresponding to the calculation result that is closest to the optimal damping ratio among several damping ratio calculation results is determined as the optimal control parameters of the target converter.

[0042] Optionally, the range of control parameters is determined with the objective that the deviation between the transient power angle and the steady-state power angle of the target converter is less than a second preset threshold, including:

[0043] Construct the power angle-control parameter relationship of the target converter;

[0044] Substituting the control parameters into the power angle-control parameter relationship yields the transient power angle of the target converter; wherein, the initial value of the control parameters is 0;

[0045] Compare the transient power angle with the steady-state power angle of the target converter;

[0046] If the deviation between the transient power angle and the steady-state power angle is less than the second preset threshold, then the current value of the control parameter is determined as the allowable value of the control parameter, and the range of values ​​of the control parameter is determined based on the allowable value of the control parameter.

[0047] The control parameter is increased by a preset value, and the process jumps to substituting the control parameter into the power angle-control parameter relationship to obtain the transient power angle of the target converter;

[0048] If the deviation between the transient power angle and the steady-state power angle is not less than the second preset threshold, then directly jump to controlling the control parameter to increase the preset value.

[0049] Optional, also includes:

[0050] If the power grid is in the fault recovery phase, detect the AC side voltage of the target converter;

[0051] If the AC side voltage recovers to the preset voltage, the phase component of the voltage setpoint of the target converter is compensated based on the preset phase compensation angle until the grid-connected system reaches a stable operating state.

[0052] Optional, also includes:

[0053] When the power grid is in a fault, determine the phase transition angle of the common coupling point and the degree of voltage drop in the power grid.

[0054] The preset phase compensation angle is determined based on the phase jump angle of the common coupling point and the degree of voltage drop in the grid.

[0055] Optional, also includes:

[0056] If the grid voltage of the power grid is not less than the preset voltage within a preset time period, the grid-connected system is determined to have reached a stable operating state.

[0057] To address the aforementioned technical problems, the present invention also provides a control device for a converter, applied in a grid-connected system. The grid-connected system includes a plurality of converters, the AC sides of which are connected in parallel and all connected to the power grid through a common coupling point. For any target converter in the grid-connected system, the converter control device includes:

[0058] The active power loop unit is used to determine the angular frequency deviation of the target converter based on the active power error of the target converter;

[0059] The compensation unit is used to perform feedforward compensation on the angular frequency deviation of the target converter based on the angular frequency deviation of other converters in the grid-connected system, so as to obtain the phase component of the voltage setpoint of the target converter;

[0060] A reactive power loop unit is used to determine the amplitude component of the voltage setpoint based on the reactive power error of the target converter.

[0061] A modulation signal generation unit is used to synthesize the phase component and the amplitude component to obtain the voltage setpoint, and to generate a modulation signal for the target converter based on the voltage setpoint;

[0062] A modulation strategy determination unit is used to determine the current modulation strategy of the target converter based on the switching loss of the target converter.

[0063] The control unit is used to control the operation of the power switching devices in the target converter based on the current modulation strategy and the modulation signal.

[0064] To address the aforementioned technical problems, the present invention also provides an electronic device, comprising:

[0065] Memory, used to store computer programs;

[0066] A processor for implementing the steps of the converter control method as described above.

[0067] To address the aforementioned technical problems, the present invention also provides a computer-readable storage medium storing a computer program, which, when executed by a processor, implements the steps of the converter control method described above.

[0068] This invention provides a converter control method. For grid-connected systems with multiple converters, when any target converter generates a modulation signal, a virtual synchronization mechanism grid control strategy is used to output the amplitude and phase components of the voltage setpoint. When generating the phase component of the voltage setpoint using the active power loop, the angular frequency of the entire grid system is used to feedforward compensate the angular frequency of the target converter, thereby reducing the amplitude of the power angle difference vibration and the fluctuation of the power angle difference between converters. This allows the grid-connected system to effectively provide active power frequency support when the grid voltage suddenly increases or decreases. Simultaneously, when the converter performs switching modulation, the current modulation strategy of the converter is adjusted in real time according to the switching losses of the target converter, reducing the switching losses of the entire grid-connected system. By reducing switching losses and introducing angular frequency compensation in the active power loop, the transient stability and frequency stability of the converter during grid faults are enhanced, thereby effectively improving the stability of the converter and the entire grid-connected system.

[0069] The present invention also provides a control device, electronic device, and computer-readable storage medium for a converter, which have the same beneficial effects as the control method for the converter described above. Attached Figure Description

[0070] To more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings used in the prior art and embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0071] Figure 1 A flowchart illustrating a converter control method provided by the present invention;

[0072] Figure 2 A schematic diagram of a control block diagram for a converter control system provided by the present invention;

[0073] Figure 3 A schematic diagram of a control strategy for a virtual synchronization network provided by the present invention;

[0074] Figure 4 A schematic diagram of the control block of another converter control system provided by the present invention;

[0075] Figure 5 A schematic diagram of the structure of a converter provided by the present invention;

[0076] Figure 6 A schematic diagram of a modulation strategy switching process provided by the present invention;

[0077] Figure 7A schematic diagram of the switching process for another modulation strategy provided by the present invention;

[0078] Figure 8 A schematic diagram of a control block for fast phase-locking provided by the present invention;

[0079] Figure 9 A waveform diagram of the voltage signal in a converter using an SVPWM modulation strategy is provided for this invention.

[0080] Figure 10 A waveform diagram of the voltage signal in a converter using a DPWM modulation strategy is provided for this invention.

[0081] Figure 11 This is a schematic diagram of the structure of a converter control device provided by the present invention. Detailed Implementation

[0082] The core of this invention is to provide a control method, device, equipment, and medium for a converter. By reducing switching losses and introducing angular frequency compensation in the active power loop, the transient stability and frequency stability of the converter during grid faults are enhanced, thereby effectively improving the stability of the converter and the entire grid-connected system.

[0083] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0084] See Figure 1 As shown, Figure 1 A flowchart illustrating a converter control method provided by the present invention; see also Figure 2 As shown, Figure 2 This invention provides a control block diagram of a converter control method. To solve the above-mentioned technical problems, this invention provides a converter control method applied to a grid-connected system. The grid-connected system includes several converters, whose AC sides are connected in parallel and all connected to the power grid through a common coupling point. For any target converter in the grid-connected system, the converter control method includes:

[0085] S11: Determine the angular frequency deviation of the target converter based on the active power error of the target converter;

[0086] S12: Feedforward compensation is performed on the angular frequency deviation of the target converter based on the angular frequency deviation of other converters in the grid-connected system to obtain the phase component of the voltage setpoint of the target converter;

[0087] S13: Determine the amplitude component of the voltage setpoint based on the reactive power error of the target converter;

[0088] S14: Synthesize the phase component and amplitude component to obtain the voltage setpoint, and generate the modulation signal of the target converter based on the voltage setpoint;

[0089] It is easy to understand that each converter in the grid-connected system is connected to the power grid through a common coupling point to achieve grid connection with the power system. This common coupling point serves as the grid connection point between each converter and the power grid. In order to optimize the grid connection performance of the converter and improve its grid support capability, this application adopts a virtual synchronous network control strategy as the basic control strategy for the converter in the grid-connected system. At the same time, feedforward compensation of angular frequency is used to improve the control strategy of the converter. Any converter in the grid-connected system can be used as a target converter, and the control method of the converter provided by this invention can be configured in the control system of the corresponding target converter for application.

[0090] It should be noted that, see Figure 3 As shown, Figure 3 A schematic diagram of a control block diagram for a virtual synchronous network control strategy provided by the present invention; the DC side of the inverter is connected to a DC voltage U. dc The AC side is filtered by inductor L. f and filter capacitor C f After filtering out high-frequency harmonics, it is connected to the grid connection point PCC, and the output current i of the grid connection point PCC is... pcc The inverter connects to the AC grid via a line, allowing the inverter to run on the grid. The equivalent inductance of the line between the grid connection point and the grid is L. g The grid voltage is u g The control strategy for a typical grid-connected inverter is as follows: Figure 3As shown, the grid-connected inverter uses VSG (Virtual Synchronous Generator) control, which regulates the active and reactive power of the inverter through active and reactive power loops respectively. The reactive power loop generates a voltage amplitude setpoint signal E, and the active power loop generates a voltage phase setpoint signal δ. After the voltage amplitude and phase setpoint signals are combined to form voltage e, voltage e serves as the setpoint for the inner loop voltage control. The inner loop includes a voltage loop and a current loop, which can output a modulation signal based on the given voltage e, the inverter AC side voltage u, and the inverter AC side current i. The output modulation signal is then used by the drive circuit to generate the drive signal S for the power switching devices in the inverter using PWM (Pulse Width Modulation) signal. abcn .

[0091] It should be further explained that, in order to simulate the dynamic characteristics of a traditional synchronous generator in the converter, a virtual inertia control loop is set in the active power loop to simulate rotor inertia, and a damping control loop is set to suppress oscillations. The moment of inertia J in the virtual inertia control loop and the damping coefficient D in the damping control loop simultaneously affect the transient stability and frequency stability of the VSG. Overall, the imbalance of active power is the root cause of VSG transient instability. The root cause of transient instability of the converter based on the VSC control strategy also lies in the design of the active power loop. Therefore, in order to enhance the transient stability of the converter and improve its frequency stability, this application proposes to add a loop with angular frequency deviation feedback in the active power loop to achieve a transient stability enhancement method. See [link to relevant documentation] Figure 4 As shown, Figure 4 A schematic diagram of the control block for another converter control system provided by the present invention; the control block diagram based on center angular frequency and angular frequency deviation feedback is shown below. Figure 4 As shown.

[0092] It is understandable that the process of the converter generating the modulation signal involves two operations. The first operation is to generate the phase component δ of the voltage setpoint through the active power loop. S The first operation involves generating the amplitude component E of the voltage setpoint through the reactive power loop, with the voltage setpoint serving as the reference voltage for the voltage loop. The second operation utilizes the voltage setpoint, along with the voltage and current loops, to generate the converter's modulation signal d(s). Specifically, a proportional-integral (PI) element (K) is incorporated into the reactive power loop. P +K I / s), reactive power error of the target converter (Q e -Q m After being input into the proportional-integral (PI) stage, the voltage change is obtained. U, voltage change The sum of U and the rated voltage U0 of the converter is used as the amplitude component of the voltage setpoint. The active power loop includes an integral element 1 / ω0 and a proportional-integral element (K... P +K I / s), virtual inertial control element 1 / Js and damping control element D, active power error of the target converter (P e -P m The angular frequency deviation of the target converter is generated by sequentially passing through an integral control stage and a virtual inertial control stage, while simultaneously receiving feedback control from a proportional-integral control stage and a damping control stage. ω, the angular frequency deviation, after feedforward compensation, yields the compensated angular frequency deviation. ω ki Angular frequency deviation after compensation ω ki The angular frequency ω obtained by summing the rated angular frequency ω0 of the converter (which is also the angular frequency of the power grid) with the angular frequency ω of the grid. ki After the integration process of 1 / s, the phase component δ, which is used as the voltage reference value, is... S Among them, Q m Q is the reactive power setpoint for the target converter. e P represents the actual reactive power of the target converter. m P is the active power setpoint for the target converter. e The actual active power value of the target converter.

[0093] It should be noted that, as Figure 4 As shown, feedforward compensation for angular frequency deviation can be achieved using a weighted average method. For any converter in the grid-connected system, it will generate an angular frequency deviation. The angular frequency deviations generated by each converter are multiplied by their respective weighting coefficients and then summed to obtain the compensated angular frequency deviation. ω ki Taking a grid-connected system comprising N converters, with the target converter being the first converter in the system, as an example, the angular frequency deviation of the first converter... The weighted value is obtained by multiplying ω1 by the weighting coefficient k1. ω k1 Angular frequency deviation of the second converter The weighted value is obtained by multiplying ω2 by the weighting coefficient k2. ω k2 Angular frequency deviation of the Nth converter ω N Multiply by the weighting coefficient k N Then the corresponding weighted values ​​are obtained. ω kN The compensated angular frequency deviation is obtained by summing the N weighted values. ωki The specific values ​​of the weighting coefficients for each converter can be set according to actual application requirements, and this application does not impose any special restrictions here.

[0094] S15: Determine the current modulation strategy of the target converter based on the switching losses of the target converter;

[0095] S16: Using the current modulation strategy, control the operation of the power switching devices in the target converter based on the modulation signal.

[0096] Understandably, in addition to adding feedforward compensation for angular frequency deviation in the control strategy, considering the impact of switching losses on the operating stability of the converter, this invention also adjusts the modulation strategy of the target converter according to the real-time switching losses of the target converter when generating the drive signal. This allows for the generation of the final drive signal through different modulation strategies, minimizing the impact of switch failure on operational stability by switching the modulation strategy. The modulation signal refers to the signal output to the drive circuit that instructs the drive circuit to generate the corresponding drive signal. The drive circuit has a built-in control strategy that generates the corresponding drive signal for the power switching devices of the target converter under the instruction of the modulation signal. The drive signal refers to the signal that controls the operation of the power switching devices in the target converter, typically implemented using pulse signals (e.g., PWM signals).

[0097] It should be noted that, as Figure 4 As shown, the converter control system can specifically employ a dual-loop control based on quasi-PR control (Proportional-Resonant Control) with both voltage and current loops to generate the drive signal. After obtaining the phase and amplitude components of the voltage setpoint, they are synthesized into the voltage setpoint v through voltage synthesis. oref (s), voltage setpoint v oref (s) and the current output voltage v of the target converter o (s) After subtraction, the transfer function G of the voltage loop is passed. v (s) Generate current given value i ref (s), current setpoint i ref (s) enters the current loop, and after being subtracted from the current i(s) of the target converter, it passes through the proportional element k. p A modulated signal d(s) is generated, and the modulated signal d(s) is processed by the current modulation strategy K of the target converter. pwm After processing, a drive signal u is generated. abc (s).

[0098] It is not difficult to understand that this application does not impose special limitations on the specific types and implementation methods of each converter in the grid-connected system. It can adopt NPC (Neutral Point Clamped) type I three-level converter topologies, T-type three-level converter topologies, or ANPC (Active Neutral-Point-Clamped) three-level converter topologies, etc. Similarly, this application does not impose special limitations on the specific implementation methods of the control systems for each converter. The control system includes drive circuits for driving power switching devices, which can be implemented using PWM generators, etc. The modulation strategy of the converter can be directly built into the drive circuit. The control method provided in this application is applicable to converters of various topologies. For different types of converters, only the generation method of the drive signal needs to be adjusted accordingly. The DC side of the converter can be connected to various types of new energy storage systems to realize the effective application of new energy and improve the comprehensive energy utilization rate. Specifically, the energy storage system can be a photovoltaic system, a battery, etc., which this application does not specifically limit.

[0099] This invention provides a converter control method. In terms of control strategy, a virtual synchronous network control strategy is adopted to control the converter. Self-synchronization is achieved by simulating the generation characteristics and synchronization mechanism of a synchronous generator, and a given voltage amplitude and phase are output. Injected power is controlled by adjusting the amplitude and phase of the point of common coupling (PCC) voltage. Addressing the power oscillation problem in power plant grid-connected systems, this invention proposes a center angular frequency control method when multiple converters are operating in parallel. Based on the characteristics of each converter, power is rationally allocated by configuring appropriate weighting coefficients, achieving more precise dynamic power regulation and improving the dynamic and steady-state performance of the entire grid-connected system. To address the instability problem of converters and grid-connected systems when grid voltage drops are severe, an angular frequency deviation feedback method is introduced to improve the active power loop, effectively enhancing the transient and frequency stability of the converter and grid-connected system during faults. Simultaneously, the switching losses of the converter are reduced by switching the converter's modulation strategy. This achieves a hybrid modulation method for low common-mode voltage and low switching losses in grid-connected converters, as well as a control method to improve grid-connected transient stability.

[0100] Based on the above embodiments: for any converter, the overall control block diagram of its control system is as follows: Figure 2 As shown, center angular frequency control is introduced into the active power loop to achieve feedforward compensation of angular frequency deviation. In the amplitude component of the voltage setpoint output by the active power loop, a preset phase compensation angle β that supports fast phase locking is further introduced. In the generation process of the drive signal, the modulation strategy switching operation is introduced. The transient stability and frequency stability of the converter are effectively improved from the three parts of control, fast phase locking and control.

[0101] As an optional embodiment, feedforward compensation is performed on the angular frequency deviation of the target converter based on the angular frequency deviation of other converters in the grid-connected system to obtain the phase component of the voltage setpoint of the target converter, including:

[0102] Calculate the weighted average of the angular frequency deviations of all converters in the grid-connected system to obtain the center angular frequency of the grid-connected system;

[0103] The sum of the center angular frequency and the grid angular frequency is input to the integrator to obtain the phase component of the voltage setpoint of the target converter.

[0104] It is easy to understand that feedforward compensation for angular frequency deviation can be implemented using a weighted average method. The weighted average of the angular frequency deviations of all converters in the grid-connected system is defined as the center angular frequency. Feedforward compensation for the angular frequency of each converter is then performed using this center angular frequency to reduce the amplitude of the power angle difference fluctuation and reduce the power angle difference ripple between converters. The center angular frequency can also be the weighted average of the angular frequency deviations of multiple converters in the grid-connected system. A preferred embodiment is to define the center angular frequency as the weighted average of the angular frequency deviations of all converters in the grid-connected system; however, this application does not impose any particular limitation on this method.

[0105] As one specific embodiment, see Figure 5 As shown, Figure 5 This invention provides a schematic diagram of a converter structure. Taking a three-phase, three-arm converter as an example, to avoid coordinate transformation and decoupling control, the control of the three-arm inverter can be simplified to three single-phase inverters. To achieve zero steady-state error tracking of the sinusoidal voltage reference, the voltage outer loop in the control system uses a PR controller, and the current inner loop uses proportional control. The transfer function of the PR controller is:

[0106] ;

[0107] in, This is the cutoff angular frequency of the PR controller. The angular frequency of the power grid. For the proportional gain of the PR controller, This represents the resonant gain of the PR controller.

[0108] For converters, when using a virtual synchronous network control strategy to control the converter, the converter can be equivalent to a VSG model. The swing equation of the VSG model is:

[0109] ;

[0110] The reactive voltage equation of the VSG model is:

[0111] ;

[0112] Among them, T m T is the torque given by the VSG model. e Here, J is the actual torque value of the VSG model, D is the moment of inertia of the VSG model, and ω is the damping coefficient of the VSG model. ki Let ω0 be the sum of the center angular frequency and the rated angular frequency of the VSG model, δ be the power angle of the VSG model, and P be the sum of the center angular frequency and the rated angular frequency of the VSG model. m P is the active power given by the VSG model. e Q represents the actual active power value of the VSG model. m Q is the reactive power given by the VSG model. e Here, E represents the actual reactive power value of the VSG model, E represents the output voltage reference amplitude of the VSG model (i.e., the amplitude component of the voltage setpoint), U0 represents the rated output voltage value of the VSG model, and k represents the voltage rating. q This is the reactive power voltage regulation coefficient.

[0113] Specifically, by weighting the angular frequency deviation to improve the control strategy, the converter and grid-connected system can more effectively provide active frequency support when the load (grid) suddenly increases or decreases, thereby enhancing the stability of the converter and grid-connected system. Especially under weak grid conditions, its impedance characteristics can improve the characteristics of the converter, enabling the grid-connected system with multiple machines in parallel to exhibit positive damping characteristics, thereby reducing the risk of subsynchronous oscillation and supersynchronous oscillation.

[0114] As an optional embodiment, the modulation strategy of the target converter includes discontinuous pulse width modulation and space vector pulse width modulation; determining the current modulation strategy of the target converter based on its switching losses includes:

[0115] Based on the switching loss control of the target converter, the current modulation strategy of the target converter is switched between discontinuous pulse width modulation and space vector pulse width modulation.

[0116] Understandably, to balance the utilization of the DC bus voltage and the reduction of switching losses, a hybrid modulation strategy consisting of discontinuous pulse width modulation (SVPWM) and space vector pulse width modulation (SPWM) can be used as the converter's modulation strategy. SVPWM controls the converter's switching state based on the concept of space vectors to generate the desired voltage vector. DPWM reduces switching losses and improves output voltage quality by optimizing the switching modes of power switching devices. During converter operation, the converter's modulation strategy is determined directly by switching between these two modulation strategies.

[0117] As a specific embodiment, such as Figure 5 As shown, taking a three-level, three-phase, three-arm inverter topology as an example, the inverter has four-phase arms (X=A, B, C, N). Each arm consists of four IGBT power devices (VX1, VX2, VX3, VX4) and two diodes. The AC side of the inverter is connected to a filter inductor L in sequence. f Filter resistor R f and filter capacitor C f The output is then connected from the AC side to the common coupling point PCC via the AC circuit breaker QF. The line between the common coupling point PCC and the power grid is equivalent to X. g e a e b e c O represents the three-phase voltage of the power grid, and O is the three-phase midpoint of the power grid. The midpoint between the two DC bus capacitors C1 and C2 is called the neutral point. The diode connected to the neutral point is the clamping diode. The clamping diode forces the output voltage to be equal to the voltage at the midpoint of the DC capacitors, preventing a short circuit in the DC bus capacitors when current flows between the DC side and the load side. The inverter's output voltage u X (X=a, b, c) has three different values: Udc / 2, 0, and -Udc / 2, where Udc is the DC bus voltage, which can be provided by the energy storage system. When u a When u = Udc / 2, only VX1 and VX2 are on, while VX3 and VX4 are off; when u a When u = 0, only VX2 and VX3 are on, while VX1 and VX4 are off; when u a When the voltage is -Udc / 2, only VX3 and VX4 are turned on, while VX1 and VX2 are turned off, thus turning off the inverter's output voltage u. XThe relationship between the bridge arm output states Si (i=a, b, c) and the output state Si is shown in Table 1. Here, 1 indicates IGBT on, 0 indicates IGBT off, P indicates a high-level output state, O indicates a zero-level output state, and N indicates a low-level output state. Generating corresponding drive signals based on the modulation strategy enables adjustment of the converter's output voltage. Simultaneously, the zero-level output of the bridge arm effectively reduces switching losses.

[0118] Table 1 Inverter switching modes

[0119]

[0120] It should be noted that this application does not impose any specific limitations on the specific implementation methods of the two modulation strategies. Both SVPWM and DPWM can be implemented using the third harmonic injection method. In SVPWM with third harmonic injection, a third harmonic component is added to the modulation signal to make the modulation waveform a saddle wave, thereby further improving the utilization rate of the DC bus voltage. In DPWM with third harmonic injection, a third harmonic component is also added to the modulation signal to improve voltage utilization, and the signal is in a level clamping state for 1 / 3 of a cycle, reducing switching losses.

[0121] Specifically, the converter can implement a hybrid modulation strategy based on two modulation methods: discontinuous pulse width modulation (PWM) and space vector pulse width modulation (SVM). This allows the converter to balance the utilization of DC bus voltage and the reduction of switching losses by smoothly switching between PWM and SVM in different application scenarios. It has good adaptability and can reduce switching losses by switching modulation strategies when the converter is under different power conditions and under current overload conditions such as grid faults, thereby improving the transient stability of the converter.

[0122] As an optional embodiment, if the target converter is operating normally, the current modulation strategy of the target converter is switched between discontinuous pulse width modulation and space vector pulse width modulation based on the switching loss of the target converter, including:

[0123] Determine whether the modulation index of the target converter is greater than a first preset threshold.

[0124] If so, the current modulation strategy of the target converter will be switched to discontinuous pulse width modulation;

[0125] If not, the current modulation strategy of the target converter will be switched to space vector pulse width modulation.

[0126] It is easy to understand that during the normal operation of the converter, the modulation strategy can be switched by setting a first preset threshold corresponding to a modulation index, thereby balancing the switching losses and current harmonics of the converter. When the modulation index is lower than the first preset threshold, SVPWM is used preferentially to reduce current harmonics; when the modulation index is higher than the first preset threshold, it switches to DPWM to reduce switching losses. If the power factor of the converter is close to 1, DPWM is used preferentially, as DPWM can clamp the switching transistor when the current is at its maximum, thus minimizing switching losses. This application does not impose any special limitations on the specific value and implementation method of the first preset threshold; it can be set and adjusted according to actual application needs and experience. For example, the first preset threshold can be set to 0.8. Similarly, this application does not impose any special limitations on the specific method of obtaining the modulation index of the converter; it can be selected and set according to the actual application scenario of the converter.

[0127] Specifically, when the converter is running normally, the switching loss of the target converter can be detected directly by comparing the modulation index. The modulation index can be directly used as the control basis for switching the modulation strategy, which is simple, effective and easy to implement.

[0128] See Figure 6 As shown, Figure 6 This invention provides a schematic diagram of a modulation strategy switching process; as an optional embodiment, if a grid fault occurs, the current modulation strategy of the target converter is switched between discontinuous pulse width modulation and space vector pulse width modulation based on the switching loss of the target converter, including:

[0129] The switching frequency of the power switching devices in the target converter is reduced to a first preset frequency.

[0130] Determine whether the switching losses of the target converter are greater than the preset value;

[0131] If so, the current modulation strategy of the target converter will be switched to discontinuous pulse width modulation.

[0132] As an optional embodiment, after switching the current modulation strategy of the target converter to discontinuous pulse width modulation, the method further includes:

[0133] Determine whether the switching losses of the target converter are greater than the preset value;

[0134] If so, the switching frequency of the power switching devices in the target converter is reduced to a second preset frequency.

[0135] It is understandable that during a grid voltage fault, the converter will experience a double current surge, necessitating a switching of the modulation strategy to reduce switching losses under overload conditions. During converter operation, the system continuously monitors grid voltage faults. If a fault occurs, the switching frequency is first reduced. Then, it is assessed whether the switching losses remain high after reducing the frequency. If they are still very high, the modulation strategy is switched to DPWM. If the switching losses remain high after the modulation strategy switch, the switching frequency can be further reduced to decrease the switching losses. This application does not impose specific limitations on the preset values, the first preset frequency, the second preset frequency, or their implementation methods. Figure 6 As shown, when the initial value of the switching frequency is 16kHz during normal operation of the converter, the first preset frequency can be set to 4kHz, and the second preset frequency can be set to 2kHz. There are various methods for detecting whether the switching losses are significant and whether there is a fault in the grid voltage. This application does not impose any particular limitations here. The determination of whether the switching losses are still significant can be achieved by setting preset values, and the determination of whether there is a fault in the grid voltage can be achieved by detecting whether there is a sudden change in the voltage at the common coupling point.

[0136] Specifically, during the grid voltage fault phase, the switching losses in the converter are reduced by coordinating the reduction of the switching frequency and the switching of the modulation strategy. From the perspectives of commissioning method and switching action parameters, the instability caused by excessive switching losses in the converter during grid faults is avoided to the greatest extent.

[0137] See Figure 7 As shown, Figure 7 A schematic diagram of another modulation strategy switching process provided by the present invention; as an optional embodiment, if the power grid is in the fault recovery phase, the current modulation strategy of the target converter is switched between discontinuous pulse width modulation and space vector pulse width modulation based on the switching loss of the target converter, including:

[0138] The switching frequency of the power switching devices in the target converter is restored to its initial value before the grid fault occurred;

[0139] Determine whether the switching losses of the target converter are greater than the preset value;

[0140] If so, the current modulation strategy of the target converter will be switched to discontinuous pulse width modulation;

[0141] If not, the current modulation strategy of the target converter will be switched to space vector pulse width modulation.

[0142] It is easy to understand that, similar to the fault occurrence phase, the recovery phase of a power grid fault also requires reducing switching losses under overload conditions. After a power grid fault occurs, it is determined in real time whether the voltage fault has been restored. If not, the control mode for the power switching devices determined during the fault occurrence phase is maintained. If the fault has been restored, the switching frequency of the power switching devices is first restored to the initial value of the switching frequency before the fault occurred and during normal operation of the converter. Then, it is determined whether the switching losses of the converter are large. If the switching losses are large, the modulation strategy is switched to DPWM; if the switching losses are not large, the modulation strategy is switched to SVPWM. There are various methods for detecting whether a power grid fault has been restored. This application does not impose any particular limitations here. It is possible to determine whether a power grid fault has been restored by detecting whether the voltage at the common coupling point has returned to a preset voltage.

[0143] Specifically, during the grid voltage fault recovery phase, the switching frequency needs to be restored first to ensure the normal operation of the converter, and then the switching losses in the converter during the grid fault phase are reduced by switching the modulation strategy.

[0144] As an optional embodiment, the current modulation strategy of the target converter is switched between discontinuous pulse width modulation and space vector pulse width modulation, including:

[0145] If it is necessary to switch the modulation strategy of the target converter from space vector pulse width modulation to discontinuous pulse width modulation, the composite vectors of several discontinuous pulse width modulation signals corresponding to several power switching devices in the target converter are combined to obtain the first debugging vector.

[0146] The second debugging vector is obtained by combining the composite vectors of several space vector pulse width modulation signals corresponding to several power switching devices in the target converter.

[0147] Determine the target moment when the first and second debugging vectors are closest;

[0148] At the target time, the modulation strategy of the target converter is switched from space vector pulse width modulation to discontinuous pulse width modulation.

[0149] It is understandable that when switching hybrid modulation strategies, it is necessary to prevent voltage surges from affecting converter performance as much as possible. Therefore, a smooth transition is required during modulation strategy switching. Specifically, this can be achieved by synthesizing several DPWM vectors, comparing the resulting first debugging vector with the second debugging vector synthesized from SVPWM, and selecting the SVPWM vector that is closest to the first debugging vector for switching. This minimizes or eliminates the impact of the switching. Determining the target time when the first and second debugging vectors are closest refers to identifying the debugging vector with the smallest angle difference from the first debugging vector among the second debugging vectors synthesized from SVPWM. The target time corresponding to this debugging vector is then used as the modulation strategy switching time, switching the debugging vector from the second debugging vector synthesized from SVPWM to the first debugging vector synthesized from DPWM. If it is necessary to switch the modulation strategy of the target converter from discontinuous pulse width modulation to space vector pulse width modulation, the method for determining the switching time also adopts the method of this embodiment, which will not be elaborated further here.

[0150] Specifically, smooth switching between the two modulation strategies is achieved by comparing the tuning vectors, avoiding the impact of voltage surges caused by modulation strategy switching on the normal operation of the converter and grid-connected system, and further ensuring the stability of the converter and grid-connected system.

[0151] As an optional embodiment, it also includes:

[0152] The range of control parameters is determined with the objective that the deviation between the transient power angle and the steady-state power angle of the target converter is less than a second preset threshold. The control parameters include the proportional coefficient and the integral coefficient of the reactive power loop.

[0153] Extract several sets of values ​​for the control parameter from its range;

[0154] Determine the calculated damping ratios that correspond one-to-one with several sets of values;

[0155] The set of values ​​corresponding to the calculation result that is closest to the optimal damping ratio among several damping ratio calculation results is determined as the optimal control parameters of the target converter.

[0156] As an optional embodiment, with the goal of the deviation between the transient power angle and the steady-state power angle of the target converter being less than a second preset threshold, the range of control parameters is determined, including:

[0157] Construct the power angle-control parameter relationship of the target converter;

[0158] Substituting the control parameters into the power angle-control parameter relationship yields the transient power angle of the target converter; where the initial value of the control parameters is 0.

[0159] Compare the transient power angle with the steady-state power angle of the target converter;

[0160] If the deviation between the transient power angle and the steady-state power angle is less than the second preset threshold, the current value of the control parameter is determined as the allowable value of the control parameter, and the range of values ​​of the control parameter is determined based on the allowable value of the control parameter.

[0161] The control parameters are increased to preset values, and the process jumps to substituting the control parameters into the power angle-control parameter relationship to obtain the transient power angle of the target converter;

[0162] If the deviation between the transient power angle and the steady-state power angle is not less than the second preset threshold, then directly jump to the control parameter increase preset value.

[0163] It is easy to understand that since a smaller power angle δ results in a larger transient stability margin for the converter, a smaller δ requires a larger integral coefficient K. I However, this can exacerbate frequency oscillations in the converter. Therefore, a comprehensive balance between transient stability and frequency stability is needed to adjust the converter's control parameters. During converter operation, the proportional-integral (K) element... P +K I / s) The angular frequency deviation of the VSG is fed back to the active power loop. Therefore, the improved active power loop satisfies:

[0164] ;

[0165] Where J is the moment of inertia of the VSG model, and D is the damping coefficient of the VSG model. ω is the angular frequency deviation, P m P is the active power given by the VSG model. e Here, ω0 is the actual active power value of the VSG model, K is the rated angular frequency of the VSG model, and K is the actual active power value of the VSG model. P K is the proportional coefficient of the proportional-integral element. I Let be the integral coefficient of the proportional integral term. δ represents the power angle deviation.

[0166] Let the voltage phasor e (i.e., the AC side voltage of the converter) in the VSG model be E∠δ, and the grid voltage phasor be U. g In steady-state operation, the voltage phasor of the VSG model is U0∠δ0. Line resistance is typically neglected, and the impedance between the converter and the grid is considered inductive, directly equivalent to the inductive impedance X. g At this point, the expressions for active power and reactive power transmitted between the VSG model and the power grid are:

[0167] ;

[0168] Considering the reactive power loop effect of the VSG model, substituting the reactive power expression from the above equation into the reactive voltage equation of the VSG model yields the quadratic equation for E:

[0169] ;

[0170] Then, substituting the quadratic equation for E, the expression for transmitted active power, and the expression for reactive power into the swing equation of the VSG model, we can obtain the second-order differential equation for δ considering the reactive power loop. Combining this second-order differential equation for δ with the equations satisfied by the improved active power loop, we obtain a set of equations, which represents the power angle-control parameter relationship of the target converter. Let K... P =0,K I =0 is the initial condition, and K P This proportionality coefficient and K I Substituting this integral coefficient into the simultaneous equations, we obtain the transient power angle δ corresponding to this set of control parameter values. By comparing the transient power angle δ with the steady-state power angle δ0 of the converter before the fault, we obtain the power angle deviation. δ, if the work angle deviation If δ is greater than the set second preset threshold, then let K... P =K P +0.1, K I =K I By adding 0.1, a new set of control parameter values ​​is obtained, which are then substituted into the equation system for iterative calculation. Finally, the value that minimizes the power angle deviation is obtained. K that satisfies the deviation condition P and K I Meanwhile, for K P and K I The converter itself also has a corresponding allowable deviation range based on its stability and dynamic performance. The range of control parameter values ​​also needs to satisfy the constraints that both the proportional and integral coefficients must meet the allowable deviation range. Finally, K that satisfies the deviation conditions is obtained. P and K I Then, for each group K P and K I Substituting these values ​​into the damping ratio calculation formula, the damping ratio calculation formula is as follows:

[0171] ;

[0172] After substituting the values, the damping ratio ζ corresponding to each set of control parameters is solved. For the converter, there is an optimal choice for the damping ratio, which is generally 0.707. Therefore, each damping ratio solution is compared with the optimal damping ratio to determine the solution that is closest to the optimal damping ratio, that is, the damping ratio with the smallest difference from the optimal damping ratio. The set of control parameters corresponding to this solution is taken as the optimal control parameters.

[0173] It should be noted that the proportional and integral coefficients in the final control parameters can be applied not only to the reactive power loop but also to the proportional-integral (K) stage in the active power loop. P +K I / s), the specific value of the second preset threshold and the allowable deviation range of the control parameters can be set according to the actual application requirements. This application does not make any special limitations here. There are multiple choices for the specific implementation of several sets of control parameter values, and it is not limited to the fixed increase of 0.1 in this embodiment.

[0174] Specifically, this embodiment provides a design method for control parameters in a VSG model that considers the optimal damping ratio. When applied to a converter, this method can effectively balance the transient stability and frequency stability of the converter. By setting the control parameters, the stability of the converter can be improved, enabling the converter to have good current overload capacity even during grid faults.

[0175] See Figure 8 As shown, Figure 8 A schematic diagram of a control block for fast phase-locked loop provided by the present invention; as an optional embodiment, it further includes:

[0176] If the power grid is in the fault recovery phase, detect the AC side voltage of the target converter;

[0177] If the AC side voltage recovers to the preset voltage, the phase component of the voltage setpoint of the target converter is compensated based on the preset phase compensation angle until the grid-connected system reaches a stable operating state.

[0178] Considering that during the voltage fault occurrence and recovery phases of the power grid, the phase of the voltage at the grid connection point also jumps along with the changes in the grid voltage amplitude. The converter control system uses a phase-locked loop (PLL) to keep its output voltage phase synchronized with the grid voltage phase. In this case, the voltage phase jump at the grid connection point causes the PLL to re-enter the dynamic process of tracking the voltage phase. Especially under weak grid conditions, the PLL has a large time constant, slow response speed, and reduced dynamic performance. The long dynamic response time of the PLL caused by the phase jump results in a long phase deviation time for the converter, significantly impacting the transient characteristics of the converter and the entire grid-connected system. Furthermore, since the converter is required to compensate for a large amount of reactive power during low-voltage ride-through, a long dynamic process of the PLL during the voltage recovery phase can easily lead to untimely reactive power withdrawal by the converter, causing transient overvoltage problems at the grid connection point, and even overcompensation of reactive power. Therefore, this application also proposes a fast phase-locked loop based on phase compensation for the PLL in the converter control system.

[0179] It is understandable that, based on the characteristic analysis of the fault recovery phase, the phase relationship between the converter's internal electromotive force (i.e., the AC side voltage of the converter) and the grid connection point voltage directly affects the power output characteristics of the DC-side energy storage system. The phase of the converter's internal electromotive force is determined by the phase provided by the phase-locked loop (PLL). Therefore, by performing phase compensation on the PLL and changing its output phase, the phase of the converter's internal electromotive force can be instantaneously changed (its magnitude remains constant at the moment of compensation). If the compensated phase is appropriate, rapid reactive power retraction can be achieved by quickly adjusting the position of the internal electromotive force in the phasor diagram, without requiring a dynamic adjustment process by the PLL. Therefore, this application directly compensates for the phase component of the voltage setpoint generated by the PLL during the grid voltage fault recovery phase using a preset phase compensation angle β, thus avoiding the lengthy dynamic process of the PLL during the fault recovery phase. Since the phase compensation is equivalent to directly giving the converter's internal potential an initial position, the subsequent dynamic adjustment process still relies on the performance of each control loop. Therefore, this phase compensation strategy will not affect the original voltage loop, current loop, and phase-locked loop parameter design. It can improve the speed of reactive power retraction control during fault recovery based on the original control, and further improve the control effect based on the optimization of control parameters, thereby enhancing the transient performance of the phase-locked loop and thus strengthening the transient stability of the converter. This application does not make any special limitations on the specific determination method for the grid voltage fault recovery stage and whether the converter has reached stable operation. When the grid voltage fault is recovered, the AC side voltage of the converter will recover to the level of 0.9 pu (Per Unit). Therefore, the fault recovery stage can be detected by detecting the AC side voltage of the target converter. This application does not make any special limitations on the specific value of the preset voltage, and it is not limited to the value of 0.9 pu.

[0180] As a specific embodiment, such as Figure 8 As shown, the phase-locked loop detects the grid connection point voltage and obtains the three-phase grid voltage e. abc After performing a 3 / 2 transformation, the d-axis component u of the grid voltage is obtained. d and q-axis component u q q-axis component u q After passing through a PI converter, the current angular frequency ω of the power grid is obtained. s After summing it with the rated angular frequency ω0 of the power grid, the initial phase angle δ is obtained by integrator 1 / s. ki If the grid voltage recovers to 0.9 pu at this time, the preset phase compensation angle β will be directly compensated to the initial phase angle δ. ki The phase component δ of the final voltage setpoint is obtained. S If the grid voltage does not recover to 0.9 pu, then directly adjust the initial phase angle δ. ki The phase component δ of the final voltage setpoint S .

[0181] Specifically, considering that phase jumps in grid voltage during fault occurrence and recovery can cause the phase-locked loop (PLL) to enter a new dynamic process, thus adversely affecting the PLL accuracy and output characteristics of the converter, this paper proposes a phase-compensated fast PLL method to address the problem of insufficient transient response of the PLL during fault recovery phases (such as low-voltage ride-through). Phase compensation enables rapid PLL recovery, effectively improving the reactive power support speed and accuracy of the converter during fault recovery phases such as low-voltage ride-through. Combining the proposed hybrid modulation strategy, fast PLL, and center-angular frequency-based control strategy, the virtual synchronous grid converter's twice-short-time current overload capacity during grid voltage faults can be effectively optimized, significantly reducing switching losses in the converter and grid-connected system. The converter can quickly and stably output reactive current, providing reactive power support to the grid, helping to stabilize grid voltage, and can quickly withdraw reactive power during fault recovery to avoid overvoltage faults. Through center-angular frequency compensation control, active power frequency support can be provided more effectively, enhancing the stability of the grid-connected system.

[0182] As an optional embodiment, it also includes:

[0183] When the power grid is in a fault, determine the phase transition angle of the common coupling point and the degree of voltage drop in the power grid;

[0184] The preset phase compensation angle is determined based on the phase jump angle of the common coupling point and the degree of voltage drop in the grid.

[0185] It is easy to understand that during the grid voltage recovery phase, after detecting that the AC side voltage has recovered to 0.9 pu, a preset phase compensation angle β is added. The preset phase compensation angle β is a constant. Phase compensation control remains engaged until the converter and grid-connected system are detected to have entered a steady state, at which point it is deactivated. The subsequent phase angle adjustment of the voltage setpoint is automatically adjusted by the phase-locked loop (PLL) to track the grid voltage phase angle. The preset phase compensation angle β is related to both the voltage amplitude and the phase angle jump during the fault. By measuring the AC side voltage phase angle and the degree of voltage drop during the fault phase, the PLL phase compensation angle of the improved strategy can be determined. Specifically, the formula for calculating the preset phase compensation angle is:

[0186] ;

[0187] in, The angle between the internal potential of the converter and the grid connection point voltage. The phase transition angle of the grid connection point voltage. To compensate for the phase angle of the internal potential of the previous converter. and It can be determined by the degree of voltage drop and the phase angle jump angle.

[0188] Specifically, the preset phase compensation angle β is equivalent to adding an initial value to the output phase of the phase-locked loop. This initial value can reposition the position of the converter output voltage, realize rapid reactive power adjustment, and achieve rapid reactive power retraction under appropriate compensation, thereby reducing the risk of overvoltage at the grid connection point and improving system performance.

[0189] As an optional embodiment, it also includes:

[0190] If the grid voltage is not less than the preset voltage within a preset time period, the grid-connected system is determined to have reached a stable operating state.

[0191] It is understood that if the grid voltage or the AC side voltage of the converter can remain stable above 0.9 pu for more than 5 seconds, it can be said that the converter has reached a stable operating state and the grid-connected system has entered a steady state. Therefore, it can be determined whether the grid-connected system has entered a steady state by judging whether the grid voltage or the AC side voltage of the converter can remain above a preset voltage for a preset time period. The specific value of the preset time period is not particularly limited in this application, and is not limited to 5 seconds in this embodiment.

[0192] Specifically, the control method provided in this application can switch modulation strategies according to different operating conditions of the converter, reducing switching losses and common-mode output voltage of the converter, thereby improving the reliability of the grid-connected system. Phase compensation effectively improves the reactive power support speed and accuracy of the converter during fault recovery phases such as low-voltage ride-through. By introducing angular frequency deviation feedback and a control strategy based on the center angular frequency, the transient response of the converter can be effectively improved, power fluctuations reduced, and more effective active frequency support provided to the grid, helping to stabilize the grid voltage.

[0193] Furthermore, to verify the correctness of the modulation strategy in this invention, simulation verification can be performed by building a three-level, three-phase, three-bridge-arm NPC-type topology inverter simulation platform. See also... Figure 9 As shown, Figure 9 This invention provides a waveform diagram of the voltage signal in a converter using an SVPWM modulation strategy; see also... Figure 10 As shown, Figure 10 This invention provides a waveform diagram of the voltage signals in a converter using a DPWM modulation strategy. The voltage signals include the output line voltages of the inverter's A-phase bridge arm, B-phase bridge arm, and C-phase bridge arm, as well as the common-mode voltage output by the converter. The waveforms clearly show that under the DPWM modulation strategy, the common-mode voltage output by the bridge arm is half that under the SVPWM modulation strategy, reducing electromagnetic interference. Furthermore, the DPWM modulation strategy, due to its level clamping for one-third of the switching cycle, results in lower losses and temperature, improving the reliability of the grid-connected system and effectively reducing the converter's switching losses. The simulation results verify the effectiveness of the proposed modulation strategy in reducing switching losses.

[0194] See Figure 11 As shown, Figure 11 This invention provides a schematic diagram of a converter control device. To solve the above-mentioned technical problems, this invention also provides a converter control device applied to a grid-connected system. The grid-connected system includes several converters, whose AC sides are connected in parallel and all connected to the power grid through a common coupling point. For any target converter in the grid-connected system, the converter control device includes:

[0195] Active power loop unit 11 is used to determine the angular frequency deviation of the target converter based on the active power error of the target converter;

[0196] The compensation unit 12 is used to perform feedforward compensation on the angular frequency deviation of the target converter based on the angular frequency deviation of other converters in the grid-connected system, so as to obtain the phase component of the voltage setpoint of the target converter.

[0197] The reactive power loop unit 13 is used to determine the magnitude component of the voltage setpoint based on the reactive power error of the target converter.

[0198] The modulation signal generation unit 14 is used to synthesize phase components and amplitude components to obtain a voltage setpoint, and to generate a modulation signal for the target converter based on the voltage setpoint.

[0199] Modulation strategy determination unit 15 is used to determine the current modulation strategy of the target converter based on the switching loss of the target converter.

[0200] Control unit 16 is used to control the operation of power switching devices in the target converter based on the modulation signal using the current modulation strategy.

[0201] For a description of the converter control device provided by the present invention, please refer to the embodiments of the converter control method described above. The present invention will not be described again here.

[0202] To address the aforementioned technical problems, the present invention also provides an electronic device, comprising:

[0203] Memory, used to store computer programs;

[0204] A processor for implementing the steps of the control method for a converter as described above.

[0205] The processor may include one or more processing cores, such as a quad-core processor or an octa-core processor. The processor can be implemented using at least one hardware form of DSP (Digital Signal Processor), FPGA (Field-Programmable Gate Array), or PLA (Programmable Logic Array). The processor may also include a main processor and coprocessors. The main processor, also known as the central processing unit, is used to process data in the wake-up state; the coprocessor is a low-power processor used to process data in the standby state. In some embodiments, the processor may integrate a GPU (graphics processing unit), which is responsible for rendering and drawing the content to be displayed on the screen. In some embodiments, the processor may also include an AI (Artificial Intelligence) processor, which is used to handle computational operations related to machine learning.

[0206] The memory may include one or more computer-readable storage media, which may be non-transitory. The memory may also include high-speed random access memory and non-volatile memory, such as one or more disk storage devices or flash memory devices. In this embodiment, the memory is used to store at least the following computer program, which, after being loaded and executed by a processor, is capable of implementing the relevant steps of the converter control method disclosed in any of the foregoing embodiments. In addition, the resources stored in the memory may also include an operating system and data, and the storage method may be temporary or permanent. The operating system may include Windows, Unix, Linux, etc. The data may include, but is not limited to, data related to the converter control method.

[0207] In some embodiments, the electronic device may further include a display screen, input / output interfaces, communication interfaces, a power supply, and a communication bus. Those skilled in the art will understand that the structures described in this embodiment do not constitute a limitation on the electronic device, and may include more or fewer components than those described above.

[0208] For an introduction to the electronic device provided by this invention, please refer to the embodiments of the control method for the converter described above; the invention will not be repeated here.

[0209] To address the aforementioned technical problems, the present invention also provides a computer-readable storage medium storing a computer program, which, when executed by a processor, implements the steps of the aforementioned converter control method.

[0210] It is understood that if the methods in the above embodiments are implemented as software functional units and sold or used as independent products, they can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of this application, in essence, or the part that contributes to the prior art, or all or part of the technical solution, can be embodied in the form of a software product. This computer software product is stored in a storage medium and executes all or part of the steps of the methods described in the various embodiments of this application. Specifically, the computer-readable storage medium may include, but is not limited to, any type of disk, including floppy disks, optical disks, and portable hard drives, or any type of media or device suitable for storing instructions or data, etc., and this application does not make any special limitations here.

[0211] For an introduction to the computer-readable storage medium provided by the present invention, please refer to the embodiments of the control method for the converter described above; the present invention will not be repeated here.

[0212] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on its differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. For the apparatuses disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the descriptions are relatively simple; relevant parts can be referred to the method section. It should also be noted that in this specification, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0213] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A control method for a converter, characterized in that, It is applied to a grid-connected system, which includes several converters, the AC sides of which are connected in parallel and all connected to the power grid through a common coupling point; For any target converter in a grid-connected system, the control methods for the converter include: The angular frequency deviation of the target converter is determined based on the active power error of the target converter; Feedforward compensation is performed on the angular frequency deviation of the target converter based on the angular frequency deviation of other converters in the grid-connected system to obtain the phase component of the voltage setpoint of the target converter; The amplitude component of the voltage setpoint is determined based on the reactive power error of the target converter. The phase component and the amplitude component are synthesized to obtain the voltage setpoint, and the modulation signal of the target converter is generated based on the voltage setpoint; The current modulation strategy of the target converter is determined based on the switching losses of the target converter. The operation of the power switching devices in the target converter is controlled based on the current modulation strategy and the modulation signal.

2. The control method for the converter according to claim 1, characterized in that, Feedforward compensation is performed on the angular frequency deviation of the target converter based on the angular frequency deviation of other converters in the grid-connected system to obtain the phase component of the voltage setpoint of the target converter, including: Calculate the weighted average of the angular frequency deviations of all converters in the grid-connected system to obtain the center angular frequency of the grid-connected system; The sum of the center angular frequency and the angular frequency of the power grid is input into the integrator to obtain the phase component of the voltage setpoint of the target converter.

3. The control method for the converter according to claim 1, characterized in that, The modulation strategy of the target converter includes discontinuous pulse width modulation and space vector pulse width modulation; Determining the current modulation strategy of the target converter based on its switching losses includes: Based on the switching losses of the target converter, the current modulation strategy of the target converter is controlled to switch between the discontinuous pulse width modulation and the space vector pulse width modulation.

4. The control method for the converter according to claim 3, characterized in that, If the target converter is operating normally, the current modulation strategy of the target converter is controlled to switch between the discontinuous pulse width modulation and the space vector pulse width modulation based on the switching loss of the target converter, including: Determine whether the modulation index of the target converter is greater than a first preset threshold; If so, the current modulation strategy of the target converter is switched to discontinuous pulse width modulation; If not, the current modulation strategy of the target converter will be switched to space vector pulse width modulation.

5. The control method for the converter according to claim 3, characterized in that, If a fault occurs in the power grid, the current modulation strategy of the target converter is switched between the discontinuous pulse width modulation and the space vector pulse width modulation based on the switching losses of the target converter, including: The switching frequency of the power switching devices in the target converter is reduced to a first preset frequency. Determine whether the switching loss of the target converter is greater than a preset value; If so, the current modulation strategy of the target converter is switched to discontinuous pulse width modulation.

6. The control method for the converter according to claim 5, characterized in that, After switching the current modulation strategy of the target converter to discontinuous pulse width modulation, the method further includes: Determine whether the switching loss of the target converter is greater than a preset value; If so, the switching frequency of the power switching devices in the target converter is reduced to a second preset frequency.

7. The control method for the converter according to claim 6, characterized in that, If the power grid is in the fault recovery phase, the current modulation strategy of the target converter is switched between the discontinuous pulse width modulation and the space vector pulse width modulation based on the switching losses of the target converter, including: The switching frequency of the power switching devices in the target converter is restored to its initial value before the power grid fault occurred; Determine whether the switching loss of the target converter is greater than a preset value; If so, the current modulation strategy of the target converter is switched to discontinuous pulse width modulation; If not, the current modulation strategy of the target converter will be switched to space vector pulse width modulation.

8. The control method for the converter according to claim 3, characterized in that, Controlling the current modulation strategy of the target converter to switch between the discontinuous pulse width modulation and the space vector pulse width modulation includes: If it is necessary to switch the modulation strategy of the target converter from space vector pulse width modulation to discontinuous pulse width modulation, the composite vectors of the several discontinuous pulse width modulation signals corresponding to several power switching devices in the target converter are synthesized to obtain the first debugging vector. The second debugging vector is obtained by synthesizing the composite vectors of several space vector pulse width modulation signals corresponding to several power switching devices in the target converter. Determine the target moment when the first debugging vector and the second debugging vector are closest; At the target time, the modulation strategy of the target converter is switched from space vector pulse width modulation to discontinuous pulse width modulation.

9. The control method for a converter according to claim 1, characterized in that, Also includes: The range of control parameters is determined with the goal that the deviation between the transient power angle and the steady-state power angle of the target converter is less than a second preset threshold; wherein, the control parameters include the proportional coefficient and the integral coefficient of the reactive power loop; Extract several sets of values ​​for the control parameter from the range of values ​​of the control parameter; Determine the calculated damping ratios that correspond one-to-one with several sets of values; The set of values ​​corresponding to the calculation result that is closest to the optimal damping ratio among several damping ratio calculation results is determined as the optimal control parameters of the target converter.

10. The control method for a converter according to claim 9, characterized in that, With the objective of ensuring that the deviation between the transient power angle and the steady-state power angle of the target converter is less than a second preset threshold, the range of control parameters is determined, including: Construct the power angle-control parameter relationship of the target converter; Substituting the control parameters into the power angle-control parameter relationship yields the transient power angle of the target converter; wherein, the initial value of the control parameters is 0; Compare the transient power angle with the steady-state power angle of the target converter; If the deviation between the transient power angle and the steady-state power angle is less than the second preset threshold, then the current value of the control parameter is determined as the allowable value of the control parameter, and the range of values ​​of the control parameter is determined based on the allowable value of the control parameter. The control parameter is increased by a preset value, and the process jumps to substituting the control parameter into the power angle-control parameter relationship to obtain the transient power angle of the target converter; If the deviation between the transient power angle and the steady-state power angle is not less than the second preset threshold, then directly jump to controlling the control parameter to increase the preset value.

11. The control method for a converter according to any one of claims 1 to 10, characterized in that, Also includes: If the power grid is in the fault recovery phase, detect the AC side voltage of the target converter; If the AC side voltage recovers to the preset voltage, the phase component of the voltage setpoint of the target converter is compensated based on the preset phase compensation angle until the grid-connected system reaches a stable operating state.

12. The control method for a converter according to claim 11, characterized in that, Also includes: When the power grid is in a fault, determine the phase transition angle of the common coupling point and the degree of voltage drop in the power grid. The preset phase compensation angle is determined based on the phase jump angle of the common coupling point and the degree of voltage drop in the grid.

13. The control method for a converter according to claim 11, characterized in that, Also includes: If the grid voltage of the power grid is not less than the preset voltage within a preset time period, the grid-connected system is determined to have reached a stable operating state.

14. A control device for a converter, characterized in that, It is applied to a grid-connected system, which includes several converters, the AC sides of which are connected in parallel and all connected to the power grid through a common coupling point; For any target converter in a grid-connected system, the converter's control device includes: The active power loop unit is used to determine the angular frequency deviation of the target converter based on the active power error of the target converter; The compensation unit is used to perform feedforward compensation on the angular frequency deviation of the target converter based on the angular frequency deviation of other converters in the grid-connected system, so as to obtain the phase component of the voltage setpoint of the target converter; A reactive power loop unit is used to determine the amplitude component of the voltage setpoint based on the reactive power error of the target converter. A modulation signal generation unit is used to synthesize the phase component and the amplitude component to obtain the voltage setpoint, and to generate a modulation signal for the target converter based on the voltage setpoint; A modulation strategy determination unit is used to determine the current modulation strategy of the target converter based on the switching loss of the target converter. The control unit is used to control the operation of the power switching devices in the target converter based on the current modulation strategy and the modulation signal.

15. An electronic device, characterized in that, include: Memory, used to store computer programs; A processor for implementing the steps of the control method for a converter as described in any one of claims 1 to 13.

16. A computer-readable storage medium, characterized in that, The computer-readable storage medium stores a computer program that, when executed by a processor, implements the steps of the control method for the converter as described in any one of claims 1 to 13.

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