Grid-connected converter dynamic voltage supporting method facing power grid balance disturbance

Abstract

The invention provides a dynamic voltage support method for a grid-connected converter facing power grid balance disturbance. The method comprises the following steps: Step 1, modeling a grid-connected system; step 2, carrying out phase-locked loop dynamic modeling; 3, a dynamic current compensation branch circuit is designed; dynamic currents induced by balance disturbance are respectively compensated to a current inner ring reference value of a grid-connected controller, and the influence of the grid-connected inverter on the stability of the system when the grid-connected inverter is disturbed is reduced by compensating the dynamic currents. According to the method, the mathematical model of the slow dynamic characteristic of the phase-locked loop is considered in network tracking control for the first time, and a dynamic voltage support basis is provided based on feed-forward decoupling, so that the stability of the port voltage of the converter when the system is subjected to equilibrium disturbance is realized. According to the invention, a voltage compensation branch considering the dynamic behavior of the phase-locked loop is established based on a network following control strategy, and the oscillation of the voltage of the output port when the converter grid-connected system is subjected to balance disturbance is suppressed to a certain extent.

Description

Technical Field

[0001] This invention relates to the field of converter grid connection technology, and specifically to a dynamic voltage support method for grid-connected converters oriented towards grid balance disturbances. Background Technology

[0002] In the converter field, different decoupling strategies are employed for different control structures in grid-based control. For power decoupling strategies in Voltage Vector Oriented Control (VOC), the traditional classic decoupling strategy is the inductor current state feedback method. Because the differential current signal needs to undergo Park transformation in the dq coordinate system, this process introduces mutual coupling between the dq currents, which exists in both transient and steady states. The ICSF method subtracts the respective coupling current components from the dq axis currents, i.e., multiplying the dq current by a coefficient ωL, where ω is the frequency obtained from the PLL, and L is the inductance of the inverter output filter. This method solves the dq current coupling problem in steady state, thus perfectly achieving steady-state power decoupling. To address the dq current coupling problem caused by sampling and communication delays in VOC, decoupling strategies are divided into two categories: current loop control parameter optimization methods based on PI control and novel current loop control structures.

[0003] Some studies have proposed a preprocessing current prediction method to improve the dynamic performance of control systems and achieve dq current decoupling. By analyzing the current loop control closed-loop transfer function, the impact of sampling delay on dq current coupling was analyzed, and the effectiveness of the proposed method for dq current decoupling was verified. However, none of the above studies have investigated the transient dq current coupling mechanism caused by the slow dynamic characteristics of the PLL in the VOC in a weak power grid.

[0004] Existing control technologies still have limitations when dealing with voltage disturbances and power coupling. These problems can lead to output current distortion and power fluctuations, thus affecting power quality and overall system stability. Researchers are actively exploring new control schemes, including advanced technologies such as sliding mode control and model predictive control. These new methods can not only improve the dynamic response performance of converters but also enhance power control characteristics under unbalanced grid conditions. However, current control strategies do not consider the transient grid voltage phase orientation error caused by the slow dynamic characteristics of phase-locked loops under grid balance disturbance conditions. Consequently, power coupling effects persist, exacerbating disturbance-induced converter output power oscillations. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a dynamic voltage support method for grid-connected inverters in response to grid balance disturbances, thereby improving the response capability and system stability of grid-connected inverters under grid disturbance conditions.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: Dynamic voltage support methods for grid-connected converters to address grid balance disturbances include: Step 1: Grid-connected system modeling; Select the inverter flow to the system side as the positive direction, mainly including collecting the three-phase voltage and three-phase current of the common coupling point PCC, as well as the three-phase voltage of the inverter output port, to model the grid-connected inverter and obtain the dynamic model of the AC side filter circuit. Step 2: Dynamic modeling of the phase-locked loop (PLL); The system is linearized at the stable operating point of the inverter to obtain the dynamic relationship between the phase of the PLL output and the phase of the input voltage; After linearization, it can be represented by a transfer function, which expresses the influence of the input voltage disturbance on the phase of the PLL output; Step 3: Dynamic current compensation branch design; The dynamic current induced by the balance disturbance is compensated to the current inner loop reference value of the grid-connected controller. By compensating for the dynamic current, the impact of the grid-connected inverter on the system stability when subjected to disturbance is reduced.

[0007] In Step 3 above, in order to achieve compensation for dynamic current, it is first necessary to perform linearization analysis on the power outer loop. The power outer loop outputs the current reference value in the inverter's dq coordinate system. The current inner loop controls the inverter's output current based on these reference values. Therefore, by linearizing the current inner loop at the inverter's stable operating point, its dynamic characteristics can be obtained. In the compensation strategy, the amplitude and phase disturbance of the grid connection point voltage are first measured. Then, through the designed compensation channel, the voltage disturbance is injected into the reference value of the inner current loop to adjust the output voltage reference value of the inverter. This method enables the inverter to respond quickly under grid disturbance conditions, improves the port voltage support capability, and reduces the impact of grid disturbance on the normal operation of the inverter.

[0008] In Step 1 above, the grid-connected system modeling process includes: To control the output of the grid-connected inverter, the controller needs to collect electrical parameters from the system side at the common coupling point (PCC) and establish a dynamic model of the system filter circuit. For a three-phase system, the dynamic model of the AC side filter circuit is obtained in the abc coordinate system and then transformed to the dynamic model in the dq coordinate system.

[0009] The aforementioned electrical parameters acquired from the system side via the common coupling point PCC include three-phase voltage. V pcc,abc Three-phase current i abc and inverter output voltage V inv,abc .

[0010] The dynamic model of the AC side filter circuit of the above system is represented in the abc coordinate system as follows: ; in, L f It is the inverter-side filter inductor, used to filter out high-frequency components; R f It is the series resistance of the filter inductor on the inverter side. It is usually small, but it can reflect the loss of the filter inductor. i a , i b , i c It is the three-phase current output by the inverter; V inv,a , V inv,b , V inv,c It is the three-phase voltage at the output of the inverter; V pcc,a , V pcc,b , V pcc,c It is the three-phase voltage at the common coupling point.

[0011] The specific steps for transforming the dynamic model to the dq coordinate system described above are as follows: The dynamic model of the AC side filter circuit in the three-phase system described above is transformed into a synchronous rotating coordinate system, namely the dq coordinate system. In the dq coordinate system, the inverter's voltage and current can be expressed as DC quantities, instead of sinusoidal quantities that vary with time. Through Clarke and Park transformations, the dynamic model of the filter circuit can be expressed as: ; v d , v q These are the d-axis and q-axis components of the PCC voltage vector in the dq coordinate system, respectively. i d and i q These are the current components in the dq coordinate system; V inv,d and V inv,q These are the inverter output voltage components in the dq coordinate system; V pcc,d and V pcc,q These are the voltage components at the common coupling point in the dq coordinate system; ω is the grid angular frequency, used to describe the rotational speed of the dq coordinate system. R andL These are the Thevenin equivalent resistance and inductance on the grid side, respectively.

[0012] In Step 2 above, the phase-locked loop (PLL) uses a single-synchronous coordinate system software PLL (SRF-PLL). The dynamic modeling process of the PLL is as follows: Linearization at the stable operating point yields the following relationship between the phase-locked loop output phase and the input voltage phase: ; k p pll These are the parameters of the proportional element in a phase-locked loop. k i pll These are the parameters of the integral element in the phase-locked loop; s For the Laplace operator, To provide a small phase disturbance for the PLL output. This represents a small perturbation in the phase of the PCC voltage vector. The voltage vector at the common coupling point PCC in the dq coordinate system is expressed as: ; v pcc , d , v pcc , q These are the d-axis and q-axis components of the PCC voltage vector in the dq coordinate system, respectively. θ v The phase of the PCC voltage vector; θ pll The PLL output phase; Combining the above two equations, the dynamic transfer function of the phase-locked loop can be obtained: ; H pll The phase-locked loop transfer function describes the dynamic behavior of the phase-locked loop.

[0013] The dynamic current compensation branch design in Step 3 above compensates the dynamic current induced by the balance disturbance to the inner current reference value of the grid-connected controller. V dref 、V qref First, linearization analysis needs to be performed on the outer loop of the grid control power and the inner loop of the current separately: The dq coordinate system current reference value output by the power outer loop is a reference signal for the active and reactive power of the grid-connected inverter and the power grid. The inner current loop targets the reference signals of active and reactive power and tracks the grid voltage by adjusting the inverter's output current. First, the inner current loop needs to be linearized. At the inverter's stable operating point, the inner current loop can be linearized to obtain its dynamic characteristic model.

[0014] The dynamic model after linearization of the current inner loop described above is based on the current inner loop controlling the inverter output current according to the dq current reference value output by the power outer loop. Linearizing the current inner loop at the inverter's stable operating point yields: ; Where, k IP With k ii Parameters of the proportional and integral elements of the converter current inner loop, Δ i d With Δ i q Δ represents the small disturbances in the d-axis and q-axis components of the converter output current. i dref With Δ i qref Small disturbances to the d-axis and q-axis reference values ​​of the converter output current; ω 0 represents the power frequency angular frequency; L f This is a filter inductor.

[0015] In Step 3 above, after performing linearization analysis on the grid control power outer loop and current inner loop, the relationship between the PCC voltage amplitude, phase angle, and current in the dq coordinate system is analyzed to further understand the impact of grid disturbances on the inverter output current. At the steady operating point, the relationship between the PCC voltage magnitude, phase angle, and current in the dq coordinate system is obtained through linearization: ; Where Δ V Gd , Δ V Gq =0 represents the dq-axis voltage component of the Thevenin equivalent voltage source of the power grid, which is assumed to be constant here; Z g Let the impedance matrix be the transmission line impedance matrix; substituting the above equation into the voltage vector expression in the dq coordinate system, we get: ; This model describes the dynamic coupling relationship between current disturbances and voltage changes; Substituting the above equation into the dynamic transfer function of the phase-locked loop, we can obtain the expression for calculating the compensation voltage: ; By measuring the voltage amplitude and phase disturbance at the grid connection point, and adding a reference value to the converter modulation voltage through the compensation channel, rapid support of the converter port voltage under disturbance conditions can be achieved.

[0016] The dynamic voltage support method for grid-connected converters accommodating grid balance disturbances mentioned in this invention has the following beneficial effects: 1. Enhance system immunity: By dynamically compensating for the current caused by disturbances, the system can effectively reduce the impact of grid voltage fluctuations, phase jumps and other disturbances on the inverter output current and power, ensuring the stable operation of the system under complex grid conditions.

[0017] 2. Enhanced voltage support capability: This compensation strategy can quickly adjust the voltage reference value at the inverter output port, enabling the inverter to respond promptly to voltage changes during grid disturbances, thereby enhancing the inverter's support for grid voltage and reducing grid-side pressure.

[0018] 3. Improved system stability: By linearizing the inner current loop and dynamically compensating for it, this technology reduces the negative impact of grid-connected inverter output power coupling on system stability, effectively improves the dynamic performance of the grid-connected system, and avoids system instability or oscillation.

[0019] 4. Improve power quality: The combination of filters and dynamic compensation reduces high-frequency harmonic interference, while compensating for dynamic current, reducing harmonic current and voltage distortion, and significantly improving the power quality of the grid-connected system.

[0020] In summary, this technology effectively solves the inverter response problem under grid disturbances, and provides a guarantee for the safe, stable and reliable operation of grid-connected inverters in complex grid environments. Attached Figure Description

[0021] The present invention will be further described below with reference to the accompanying drawings and embodiments: Figure 1 This is a schematic diagram of the grid-connected system topology in this invention; Figure 2 This is a block diagram of the phase-locked loop SRF-PLL control in this invention; Figure 3 This is a control block diagram of the grid voltage-oriented control converter in this invention; Figure 4 This is a schematic diagram of dynamic current compensation in this invention. Detailed Implementation

[0022] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings and embodiments.

[0023] Example 1: Dynamic voltage support methods for grid-connected converters to address grid balance disturbances include: 1. Grid-connected system modeling: In modeling the grid-connected inverter, the inverter flow towards the system side is selected as the positive direction. This mainly involves collecting the three-phase voltage and current at the common coupling point (PCC), as well as the three-phase voltage at the inverter output port. The overall goal of the modeling is to obtain a dynamic model of the AC-side filter circuit.

[0024] In this structure, the inverter typically employs an L-type filter to filter high-frequency harmonics in the inverter output, resulting in a smoother output current. Within the system, the voltage and current at the PCC (Power Control Center) serve as critical feedback signals input to the grid-connected control, typically in a Voltage Oriented Control (VOC) loop. The power grid utilizes the Thevenin equivalent impedance model, represented by a series equivalent inductance (Lg), resistance (Rg), and an ideal voltage source.

[0025] For the DC side, the inverter's DC voltage source can come from various scenarios. For example, in a photovoltaic system, the DC power is provided by the photovoltaic panels; in a wind power system, the DC power can be provided by the rectified DC voltage from the wind turbine; and in a DC microgrid scenario, the DC power comes from the interconnected DC bus. However, since this patent focuses on analyzing the impact of the inverter's AC side output power coupling on system stability, it assumes that the DC side voltage is constant and treats it as an equivalent constant DC voltage source, ignoring the dynamic effects of the DC side voltage.

[0026] 2. Dynamic modeling of phase-locked loops: A phase-locked loop (PLL) is used for phase synchronization in grid-connected inverters. Its main function is to keep the inverter's output current synchronized with the phase of the power grid. In this process, the PLL extracts the phase information of the voltage based on the acquired PCC voltage signal, compares it with a reference phase, and adjusts the phase of the inverter's output current through feedback.

[0027] Based on the control block diagram of a phase-locked loop (PLL), the system can be linearized at the inverter's stable operating point, thus obtaining the dynamic relationship between the PLL output phase and the input voltage phase. The linearized PLL can be represented as a transfer function, expressing the influence of input voltage disturbances on the PLL output phase. In practical applications, the dynamic characteristics of the PLL have a significant impact on the system's dynamic response and stability, especially under conditions of grid voltage fluctuations or phase jumps.

[0028] 3. Dynamic current compensation branch design: The core idea of ​​this technical solution is to reduce the impact of disturbances on system stability by compensating for dynamic current. Specifically, the dynamic current induced by balance disturbances is compensated into the inner current loop reference value of the grid-connected controller, i.e., V. dref and V qref middle.

[0029] To achieve this compensation, a linearization analysis of the power outer loop is first required. The power outer loop outputs the current reference value in the inverter's dq coordinate system. The current inner loop controls the inverter's output current based on these reference values. Therefore, by linearizing the current inner loop at the inverter's stable operating point, its dynamic characteristics can be obtained.

[0030] The compensation strategy first measures the amplitude and phase disturbance of the grid connection point voltage. Then, through a designed compensation channel, the voltage disturbance is injected into the reference value of the inner current loop to adjust the inverter's output voltage reference value. This method enables the inverter to respond quickly to grid disturbances, improves the port voltage support capability, and reduces the impact of grid disturbances on the normal operation of the inverter.

[0031] In summary, the design of the dynamic current compensation branch enhances the inverter's response to disturbances, thereby providing stronger support for system stability.

[0032] Example 2: The core idea of ​​this technical solution for dynamic voltage support of grid-connected inverters in response to grid balance disturbances is to reduce the impact of disturbances on system stability by compensating for dynamic current. Specifically, the dynamic current induced by the balance disturbance is compensated into the inner current loop reference value of the grid-connected controller, i.e., V. dref and V qref In modeling grid-connected inverters, the power transfer direction from the inverter to the system side is typically chosen as the positive direction. This setting ensures consistency in the power flow direction during analysis, helps establish a unified reference coordinate system, and facilitates the design and analysis of control strategies. The following section details grid-connected system modeling: To control the output of the grid-connected inverter, the controller needs to acquire key electrical parameters from the system side at the point of common coupling (PCC). The specific signals acquired include: Three-phase voltage V pcc,abc This voltage reflects the operating status of the inverter after it is connected to the grid. By collecting the three-phase voltage, we can understand the changes in the grid-side voltage and then use it for voltage outer loop control.

[0033] Three-phase current i abcThe controller also collects the three-phase current output by the inverter. These current signals are feedback quantities of the current inner loop control and can reflect the current status of the inverter's supply to the grid.

[0034] Inverter output voltage V inv,abc The three-phase voltage at the inverter output is a crucial parameter for controlling the inverter's output. By adjusting the inverter's output voltage, precise control over the inverter's output power and current can be achieved.

[0035] 1. Dynamic model of the system filter circuit A filter circuit is typically connected to the output of a grid-connected inverter to eliminate current and voltage harmonics introduced by high-frequency switching operations. A common filter is an L-type filter, which consists of an inductor Lf and a resistor Rf (which may be negligible or contain a small resistor to account for losses in the inverter's output inductor coil). This filter is connected between the inverter output and the point of common coupling (PCC) to smooth the voltage and current at the inverter output. A schematic diagram of the system topology is shown below. Figure 1 As shown.

[0036] For a three-phase system, in the abc coordinate system, the dynamic model of the AC side filter circuit can be expressed as: (1) in, L f It is the inverter-side filter inductor, used to filter out high-frequency components; R f It is the series resistance of the filter inductor on the inverter side. It is usually small, but it can reflect the loss of the filter inductor. i a , i b , i c It is the three-phase current output by the inverter; V inv,a , V inv,b , V inv,c It is the three-phase voltage at the output of the inverter; V pcc,a , V pcc,b , V pcc,c It is the three-phase voltage at the common coupling point.

[0037] This equation describes the dynamic behavior of the filter inductor. The filter ensures that the current delivered by the inverter to the grid is as close as possible to an ideal sine wave by suppressing the high-frequency harmonics generated by the inverter's switching frequency.

[0038] Dynamic model transformed to dq coordinate system To facilitate better control, the three-phase model described above is typically transformed into a synchronous rotating coordinate system (dq coordinate system), which simplifies system analysis and control design. In the dq coordinate system, the inverter's voltage and current can be represented as DC quantities, rather than sinusoidal quantities that vary with time. Through Clarke and Park transforms, the dynamic model of the filter circuit can be expressed as: (2) v d , v q These are the d-axis and q-axis components of the PCC voltage vector in the dq coordinate system, respectively. i d and i q These are the current components in the dq coordinate system; V inv,d and V inv,q These are the inverter output voltage components in the dq coordinate system; V pcc,d and V pcc,q These are the voltage components at the common coupling point in the dq coordinate system; ω is the grid angular frequency, used to describe the rotational speed of the dq coordinate system.

[0039] In the dq coordinate system, the dynamic changes in current are affected not only by inductance and resistance, but also by the cross-coupling terms ωLfiq and ωLfid generated by the coordinate system rotation. These terms represent the mutual coupling effect between the d-axis and q-axis components due to the coordinate system rotation.

[0040] This model allows for the design of current controllers that decouple the currents on the d-axis and q-axis, ensuring that the inverter's output current is phase-synchronized with the grid voltage and enabling precise control of active and reactive power output.

[0041] Grid-connected control of an inverter involves acquiring voltage and current signals from the point of common coupling and ensuring the quality of the output current through filtering circuits. Based on these signals, a dynamic model of the filtering circuit can be established, and precise adjustment and control of the grid-connected inverter can be achieved through decoupling control in the dq coordinate system. This dynamic model provides a foundation for subsequent control design and system analysis, and is particularly important in handling grid disturbances and dynamic responses.

[0042] 2. Dynamic Modeling of Phase-Locked Loops A phase-locked loop (PLL) in a grid-connected inverter serves to synchronize the phase of the current with the grid voltage. The single-synchronous-frame software PLL (SRF-PLL) is one of the most commonly used types. It extracts the phase information of the point of common coupling (PCC) voltage to ensure that the phase of the inverter's output current matches the phase of the grid voltage, thereby guaranteeing the stability and synchronization of grid-connected operation.

[0043] Basic working principle of SRF-PLL Input signal acquisition: The phase-locked loop acquires the three-phase voltage signal from the common coupling point (PCC) and converts the three-phase voltage into voltage on the axes of the two-phase stationary coordinate system through Clarke transformation.

[0044] Coordinate transformation to the dq coordinate system: The voltages in the two-phase stationary coordinate system are transformed to the synchronous rotating coordinate system dq using the Park transformation, where Vd represents the voltage component synchronized with the power grid, and Vq represents the orthogonal voltage component. The core control objective of the SRF-PLL is to make the Vq component zero, that is, to achieve phase synchronization of the voltage.

[0045] Phase error correction: The phase-locked loop (PLL) obtains the phase error signal by comparing the Vq component with the reference phase. This error signal is then regulated by a proportional-integral (PI) controller to adjust the phase of the inverter's output current, ensuring it aligns with the phase of the grid voltage.

[0046] Feedback mechanism: The adjustment signal output by the PI controller is used as the phase compensation amount of the phase-locked loop and fed back to the phase-locked loop controller to adjust the synchronization phase of the voltage, thereby maintaining the phase lock between the grid voltage and the inverter output current.

[0047] Linearization of the phase-locked loop control block diagram At the stable operating point, the control block diagram of the phase-locked loop (PLL) can be linearized, simplifying the analysis and design process. Linearization allows us to derive the dynamic relationship between the PLL output phase and the input voltage phase. This relationship can be described by a transfer function, showing how voltage disturbances affect the dynamic response of the PLL output phase.

[0048] Based on the phase-locked loop (PLL) control block diagram, linearization at the steady operating point yields the following relationship between the PLL output phase and the input voltage phase: (3) k p pll These are the parameters of the proportional element in a phase-locked loop. ki pll These are the parameters of the integral element in the phase-locked loop; s For the Laplace operator, To provide a small phase disturbance for the PLL output. The phase vector of the PCC is a small disturbance in phase. The dynamic characteristics of the phase-locked loop (PLL) are crucial to the system's fast response and stability, especially during grid voltage fluctuations or phase abrupt changes. The parameters of the PI controller determine the PLL's response speed and steady-state error. A larger proportional gain can improve the system's response speed, while a larger integral gain can reduce the steady-state error.

[0049] The expression for the PCC voltage vector in the dq coordinate system is: (4) v pcc , d , v pcc , q These are the d-axis and q-axis components of the PCC voltage vector in the dq coordinate system, respectively. θ v The phase of the PCC voltage vector; θ pll The PLL output phase; Combining the above two equations, the dynamic transfer function of the phase-locked loop can be obtained: (5) H pll This is the transfer function of the phase-locked loop (PLL) to describe its dynamic behavior. This transfer function describes the dynamic behavior of the PLL, reflecting the dynamic response characteristics of the system, especially how the PLL adjusts its phase to maintain synchronization when the grid voltage is disturbed or undergoes a phase jump.

[0050] 3. Dynamic current compensation branch design: The core idea of ​​this invention is to compensate the dynamic current induced by the balancing disturbance to the reference value of the output voltage of the inner loop of the grid control current. V dref 、V qref First, linearization analysis needs to be performed on the outer power loop and inner current loop of the grid-connected control system. The outer power loop outputs the current reference value in the inverter's dq coordinate system, while the inner current loop controls the inverter's output current based on these reference values. Therefore, by linearizing the outer power loop and inner current loop at the inverter's stable operating point, their dynamic characteristics can be obtained. The control block diagram of the grid voltage-oriented control converter is as follows: Figure 3 As shown in the image.

[0051] Linearization analysis of control systems To achieve dynamic current compensation, a linearization analysis of the grid-connected control system is first required, specifically including the linearization of the power outer loop and the current inner loop. The function of the power outer loop is to generate the current reference value of the inverter in the dq coordinate system, while the current inner loop controls the output current of the inverter based on these reference values.

[0052] Power outer loop: The current reference value in the dq coordinate system output by the power outer loop is a reference signal for the active and reactive power of the grid-connected inverter and the power grid.

[0053] Inner Current Loop: The inner current loop targets these reference values ​​and tracks the grid voltage by adjusting the inverter's output current. To precisely control the current, the inner current loop first needs to be linearized. At the inverter's stable operating point, the inner current loop can be linearized to obtain its dynamic characteristic model. This linearized model represents the dynamic response relationship between the current reference value and the output current.

[0054] Linearization model of the inner current loop: The inner current loop controls the inverter output current based on the dq current reference value output by the outer power loop. Linearizing the inner current loop at the inverter's stable operating point yields: ; (6) Where, k IP With k ii Parameters of the proportional and integral elements of the converter current inner loop, Δ i d With Δ i q Δ represents the small disturbances in the d-axis and q-axis components of the converter output current. i dref With Δ i qref Small disturbances to the d-axis and q-axis reference values ​​of the converter output current; ω 0 represents the power frequency angular frequency; L f This is a filter inductor.

[0055] PCC Voltage Amplitude, Phase Angle, and Current Relationship In grid-connected systems, the magnitude and phase angle of the point of common coupling (PCC) voltage play a crucial role in the stable operation of the system. To further understand the impact of grid disturbances on the inverter output current, it is necessary to analyze the relationship between the PCC voltage magnitude, phase angle, and current in the dq coordinate system.

[0056] At the steady operating point, the relationship between the PCC voltage magnitude, phase angle, and current in the dq coordinate system is obtained through linearization (see Equation (7)). In this analysis, it is assumed that the system voltage (i.e., the grid-side voltage) remains constant. This assumption helps to simplify the system model, allowing the focus to be on current dynamics and the response characteristics of inverter control.

[0057] After linearization at the system's stable operating point, we can obtain: ; (7) Where Δ V Gd , Δ V Gq =0 represents the dq-axis voltage component of the Thevenin equivalent voltage source of the power grid, which is assumed to be constant here; Z g The impedance matrix of the transmission line is given; substituting (7) into (4) yields: (8) This model describes the dynamic coupling relationship between current disturbances and voltage changes.

[0058] In the further derivation process, the model is incorporated into the framework of the compensation control strategy to obtain the final compensation voltage calculation expression (see formula (9)). This expression can provide the system with a dynamic compensation value for the modulation voltage, and effectively improve the voltage support capability of the inverter under grid disturbances by adjusting the voltage reference value at the inverter output in real time.

[0059] Substituting (8) into (5) yields the expression for calculating the compensation voltage: (9) By measuring the voltage amplitude and phase disturbance at the grid connection point, and adding a reference value to the converter modulation voltage via the compensation channel as shown in equation (9), rapid support of the converter port voltage under disturbance conditions can be achieved. The compensation diagram is shown below. Figure 4 As shown in the image.

[0060] The implementation of the dynamic current compensation branch relies on real-time measurement of the voltage amplitude and phase at the grid connection point (PCC). When the system detects voltage disturbances or phase shifts in the grid, it modulates the inverter's output voltage reference value through the compensation channel. The main task of the compensation channel is to promptly increase the appropriate compensation voltage reference value based on the measured voltage changes, thereby enabling the inverter output port voltage to respond quickly to disturbances and enhancing the system's voltage support capability.

[0061] This invention is the first to consider the mathematical model of the slow dynamic characteristics of the phase-locked loop in grid-connected control, and proposes a dynamic voltage support basis based on feedforward decoupling, thereby achieving the stability of the converter port voltage when the system is subjected to balance disturbances.

[0062] Existing control technologies still have limitations when dealing with problems such as voltage imbalance and power coupling. These problems can lead to output current distortion and power fluctuations, thus affecting power quality and overall system stability. Current control strategies do not consider the transient grid voltage phase orientation error caused by the slow dynamic characteristics of the phase-locked loop (PLL) under grid balance disturbance conditions, resulting in the continued existence of power coupling effects and exacerbating disturbance-induced converter output power oscillations. Therefore, this invention, based on a grid-following control strategy, establishes a voltage compensation branch that considers the dynamic behavior of the PLL, which to some extent suppresses the output port voltage oscillation of the converter grid-connected system under balance disturbances.

Claims

1. A method for dynamic voltage support of grid-connected converters to address grid balance disturbances, characterized in that, include: Step 1: Modeling the grid-connected system; The inverter flow to the system side is selected as the positive direction. The main functions include collecting the three-phase voltage and three-phase current at the common coupling point PCC, as well as the three-phase voltage at the inverter output port, to model the grid-connected inverter and obtain the dynamic model of the AC side filter circuit. Step 2: Dynamic modeling of the phase-locked loop; The system is linearized at the stable operating point of the inverter to obtain the dynamic relationship between the phase of the phase-locked loop output and the phase of the input voltage; the linearized result is represented by a transfer function, which expresses the influence of the input voltage disturbance on the phase of the phase-locked loop output. Step 3: Dynamic current compensation branch design; The dynamic current induced by the balance disturbance is compensated to the current inner loop reference value of the grid-connected controller. By compensating for the dynamic current, the impact of the grid-connected inverter on the system stability when subjected to disturbance is reduced.

2. The method for dynamic voltage support of grid-connected converters accommodating grid balance disturbances according to claim 1, characterized in that, In Step 3, to achieve compensation for dynamic current, it is first necessary to perform linearization analysis on the power outer loop; the output of the power outer loop is the current reference value in the dq coordinate system of the inverter. The inner current loop controls the inverter output current based on these reference values; therefore, its dynamic characteristics can be obtained by linearizing the inner current loop at the inverter's stable operating point. In the compensation strategy, the amplitude and phase disturbance of the grid connection point voltage are first measured, and then the voltage disturbance is injected into the reference value of the inner current loop through the designed compensation channel to adjust the output voltage reference value of the converter.

3. The method for dynamic voltage support of grid-connected converters accommodating grid balance disturbances according to claim 2, characterized in that, In Step 1, the grid-connected system modeling process includes: To control the output of the grid-connected inverter, the controller needs to collect electrical parameters from the system side at the common coupling point (PCC) and establish a dynamic model of the system filter circuit. For a three-phase system, the dynamic model of the AC side filter circuit is obtained in the abc coordinate system and then transformed to the dynamic model in the dq coordinate system.

4. The method for dynamic voltage support of grid-connected converters accommodating grid balance disturbances according to claim 3, characterized in that, The electrical parameters acquired from the system side via the common coupling point PCC include three-phase voltage. V pcc,abc Three-phase current i abc and inverter output voltage V inv,abc .

5. The method for dynamic voltage support of grid-connected converters accommodating grid balance disturbances according to claim 4, characterized in that, The dynamic model of the AC side filter circuit of the system is represented in the abc coordinate system as follows: ； in, L f It is the inverter-side filter inductor, used to filter out high-frequency components; R f It is the series resistance of the inverter-side filter inductor; i a , i b , i c It is the three-phase current output by the inverter; V inv,a , V inv,b , V inv,c It is the three-phase voltage at the output of the inverter; V pcc,a , V pcc,b , V pcc,c It is the three-phase voltage at the common coupling point.

6. The method for dynamic voltage support of grid-connected converters accommodating grid balance disturbances according to claim 5, characterized in that, The specific steps for transforming the dynamic model to the dq coordinate system are as follows: The dynamic model of the AC side filter circuit in the three-phase system described above is transformed into a synchronous rotating coordinate system, namely the dq coordinate system. In the dq coordinate system, the inverter's voltage and current can be expressed as DC quantities, instead of sinusoidal quantities that vary with time. Through Clarke and Park transformations, the dynamic model of the filter circuit can be expressed as: ； v d , v q These are the d-axis and q-axis components of the PCC voltage vector in the dq coordinate system, respectively. i d and i q These are the current components in the dq coordinate system; V inv,d and V inv,q These are the inverter output voltage components in the dq coordinate system; V pcc,d and V pcc,q These are the voltage components at the common coupling point in the dq coordinate system; ω is the angular frequency of the power grid, used to describe the rotational speed of the dq coordinate system. R and L These are the Thevenin equivalent resistance and inductance on the grid side, respectively.

7. The method for dynamic voltage support of grid-connected converters accommodating grid balance disturbances according to claim 6, characterized in that, In Step 2, the phase-locked loop (PLL) uses a single synchronous coordinate system software PLL (SRF-PLL). The dynamic modeling process of the PLL is as follows: Linearization at the stable operating point yields the following relationship between the phase-locked loop output phase and the input voltage phase: ； k p pll These are the parameters of the proportional element in a phase-locked loop. k i pll These are the parameters of the integral element in the phase-locked loop; s For the Laplace operator, To provide a small phase disturbance for the PLL output. This represents a small perturbation in the phase of the PCC voltage vector. The voltage vector at the common coupling point PCC in the dq coordinate system is expressed as: ； v pcc , d , v pcc , q These are the d-axis and q-axis components of the PCC voltage vector in the dq coordinate system, respectively. θ v The phase of the PCC voltage vector; θ pll The PLL output phase; Combining the above two equations, the dynamic transfer function of the phase-locked loop can be obtained: ； H pll The phase-locked loop transfer function describes the dynamic behavior of the phase-locked loop.

8. The method for dynamic voltage support of grid-connected converters accommodating grid balance disturbances according to claim 1, characterized in that, The dynamic current compensation branch design in Step 3 compensates the dynamic current induced by the balance disturbance to the inner current reference value of the grid-connected controller. V dref 、V qref First, linearization analysis needs to be performed on the outer loop of the grid control power and the inner loop of the current separately: The dq coordinate system current reference value output by the power outer loop is a reference signal for the active and reactive power of the grid-connected inverter and the power grid. The inner current loop targets the reference signals of active and reactive power and tracks the grid voltage by adjusting the inverter's output current. First, the inner current loop needs to be linearized. At the inverter's stable operating point, the inner current loop can be linearized to obtain its dynamic characteristic model.

9. The method for dynamic voltage support of grid-connected converters accommodating grid balance disturbances according to claim 8, characterized in that, The dynamic model after linearization of the current inner loop is obtained by controlling the inverter output current based on the dq current reference value output by the power outer loop. The current inner loop is linearized at the inverter's stable operating point as follows: ； Where, k IP With k ii Parameters of the proportional and integral elements of the converter current inner loop, Δ i d With Δ i q Δ represents the small disturbances in the d-axis and q-axis components of the converter output current. i dref With Δ i qref Small disturbances to the d-axis and q-axis reference values ​​of the converter output current; ω 0 represents the power frequency angular frequency; L f This is a filter inductor.

10. The method for dynamic voltage support of grid-connected converters accommodating grid balance disturbances according to claim 9, characterized in that, In Step 3, after performing linearization analysis on the grid control power outer loop and current inner loop, the relationship between the PCC voltage amplitude, phase angle, and current in the dq coordinate system is analyzed to further understand the impact of grid disturbances on the inverter output current. At the steady operating point, the relationship between the PCC voltage magnitude, phase angle, and current in the dq coordinate system is obtained through linearization: ； Where Δ V Gd , Δ V Gq =0 represents the dq-axis voltage component of the Thevenin equivalent voltage source of the power grid, which is assumed to be constant here; Z g Let the impedance matrix be the transmission line impedance matrix; substituting the above equation into the voltage vector expression in the dq coordinate system, we get: ； This model describes the dynamic coupling relationship between current disturbances and voltage changes; Substituting the above equation into the dynamic transfer function of the phase-locked loop, we can obtain the expression for calculating the compensation voltage: ； By measuring the voltage amplitude and phase disturbance at the grid connection point, and adding a reference value to the converter modulation voltage through the compensation channel, rapid support of the converter port voltage under disturbance conditions can be achieved.

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