Low-voltage ride-through stability control method for grid-following type grid-connected converter under weak power grid condition

Abstract

The invention provides a grid-following type grid-connected converter low voltage ride-through stability control method under a weak power grid condition, and belongs to the technical field of grid-connected converter low voltage ride-through stability control. A grid-following type grid-connected converter system equivalent model under a weak power grid is established based on a grid-following type three-phase two-level grid-connected converter system topology; a transient stability criterion of the following grid type grid-connected converter system under the weak power grid is obtained by combining a mixed potential function theory; and according to a transient stability criterion, an active current and reactive current reference value calculation formula capable of transmitting the maximum active power to the power grid on the premise of meeting stability is obtained, and comparison verification is carried out by adopting a traditional low voltage ride through control method, so that stable grid connection of the new energy power generation unit during the voltage drop period of the power grid is ensured.

Description

Technical Field

[0001] This invention belongs to the field of low voltage ride-through stability control technology for grid-connected converters, specifically a low voltage ride-through stability control method for grid-connected converters under weak grid conditions. Background Technology

[0002] Profound transformation of the energy system is a core driving force supporting the evolution of modern civilization and a strategic element for maintaining ecological security and industrial upgrading. Electricity, as the core carrier of new energy development and utilization, forms a crucial support for the modern energy system through its diversified acquisition methods. The development of new energy has become key to solving energy security, addressing climate change, and achieving sustainable development. With the continuous increase in the penetration rate of new energy, the switching on and off of new energy equipment has an increasingly significant impact on the stable operation of the power grid. When a fault occurs on the grid side causing a voltage drop, it can easily lead to system instability or trigger system protection devices, resulting in large-scale disconnection of new energy equipment from the grid, causing a significant impact on the power grid. This not only hinders fault recovery but can also lead to the expansion of the fault, and in severe cases, cause the entire power grid to collapse. Therefore, my country's grid connection guidelines stipulate that new energy grid-connected converters must have low-voltage ride-through capability. When a fault causes a voltage drop at the grid connection point, the new energy unit should remain connected to the grid for a period of time, while simultaneously outputting a certain amount of reactive power to support the recovery of the grid connection point voltage, ensuring the normal operation of the power grid. Grid-following control strategy is the most commonly used control method when new energy power generation units are connected to the grid.

[0003] Grid-connected converters, as a common type of grid-connected equipment, play a crucial role in grid operation. When grid voltage drops, grid-connected converters can react quickly, entering a low-voltage ride-through control mode. This mode allows the converter to maintain grid connection even when grid voltage is low, ensuring the continuous operation of renewable energy generation units and preventing large-scale grid disconnection due to voltage drops. Under strong grid conditions, grid-connected converters demonstrate strong robustness due to their excellent performance. They can interact stably with the grid, maintaining operational stability even during significant grid voltage fluctuations, providing strong support for the safe and stable operation of the grid. However, in weak grid environments, due to the typically lower short-circuit capacity and weaker voltage support, converters face more challenges during grid-connected operation. The control strategies for converters need to be more refined and flexible to ensure stable system operation. Therefore, researching low-voltage ride-through stability control strategies for grid-connected converters under weak grid conditions and the transient stability of grid-connected converters during faults is of profound significance for improving the stable operation of the grid.

[0004] Current literature on transient stability analysis of grid-connected converters mainly focuses on specific control loops. Some researchers, based on the traditional second-order equal-area criterion, have studied the transient stability of grid-connected converters considering only phase-locked loops (PLLs) and proposed methods to improve transient stability. Others have studied transient stability analysis methods for multi-machine systems connected to wind farms composed of doubly-fed induction generators (DFIGs), proposing a method to convert the influence of wind power on the system's power angle into the influence of electrical quantities between synchronous generators, thereby analyzing its impact on system transient stability. This research method can be used as a reference. Still others have analyzed the impact of different system structural parameters and converter control parameters on the transient stability of grids containing large-scale photovoltaic (PV) energy storage systems, finding that reducing the active current coefficient during low-voltage ride-through (LVRT) can reduce the probability of PV grid disconnection and improve system transient stability. Currently, various transient stability analysis methods have been widely studied and applied, but most assume connection to strong grids. For weak grids, the transient stability analysis of grid-connected converters and low-voltage ride-through optimization control strategies have not been thoroughly studied.

[0005] my country's grid connection standards have clearly defined the requirements for low-voltage ride-through control of grid-connected converters. However, existing research on grid-connected low-voltage ride-through mainly focuses on control optimization and small-disturbance stability analysis to improve low-voltage ride-through performance under strong grid conditions. There is little mention of low-voltage ride-through transient stability analysis under weak grid conditions, which requires further in-depth research and analysis. Summary of the Invention

[0006] This patent proposes an effective low-voltage ride-through stability control method for grid-connected converters of new energy power generation units under weak grid conditions. This method ensures that the new energy power generation units maintain stable grid connection during grid voltage drops, effectively supports grid voltage recovery, and improves the grid's stable operation capability.

[0007] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0008] A method for low-voltage ride-through stability control of a grid-connected converter under weak grid conditions includes the following steps:

[0009] Step 1: Establish an equivalent model of the grid-connected converter system under a weak power grid based on the topology of the grid-connected three-phase two-level grid-connected converter system;

[0010] Step 2: Based on the equivalent model of the grid-connected converter system under weak power grid conditions, and combined with the mixed potential function theory, the transient stability criterion of the grid-connected converter system under weak power grid conditions is obtained.

[0011] Step 3: Based on the transient stability criterion, combined with the grid connection criteria and converter capacity constraints, we obtain the calculation formulas for the reference values ​​of active current and reactive current that can transmit the maximum active power to the grid under the premise of stability.

[0012] Step 4: Set the SCR of the weak grid to 1.5 and 3 respectively, and compare and verify the proposed method and the traditional low voltage ride-through control method.

[0013] Furthermore, the grid-connected three-phase two-level converter system is transformed from the three-phase stationary coordinate system abc to the dq synchronous rotating coordinate system, resulting in the following mathematical model of the dq synchronous rotating coordinate coefficients for a grid-connected converter under weak grid conditions:

[0014]

[0015] Where L is the line inductance, R is the line resistance, and C is the line resistance. dc Where ω is the DC bus capacitance, e is the angular frequency, and ω is the DC bus capacitance. d and e q These are the d-axis and q-axis components of the grid-side electromotive force; i d i q For the d-axis and q-axis components of the grid-side current, i L For inductor current, s d and s q It refers to the switching components of the converter on the d-axis and q-axis, v dc This is the DC bus voltage.

[0016] Furthermore, the weak grid impedance is the series impedance of the AC voltage source.

[0017] Furthermore, the current and voltage variables of the grid-connected system under weak grid conditions are defined as follows:

[0018]

[0019] Among them, i g For grid current, v g This refers to the grid voltage.

[0020] Based on the transient stability criterion, the hybrid potential function model P(i,v) of a grid-connected system under weak grid conditions is:

[0021]

[0022] Among them, e g R is the electromotive force of the power grid. g v is the equivalent line impedance of the power grid. d The grid-side voltage i0 is the DC-side current, and P is the DC-side voltage. G For new energy power value, v g This represents the grid voltage.

[0023] Furthermore, the active current reference value i for a grid-connected converter system in a weak power grid that satisfies the system transient stability constraints. dref The range of values ​​is:

[0024]

[0025] Among them, i dref The active current reference value is given. The SCR (Short-Circuit Ratio) represents the strength of the power grid, determined by the short-circuit ratio. rated k is the rated capacity of the new energy power generation system connected to the grid. vp Voltage loop proportionality coefficient, k ip It is the proportional coefficient of the current loop.

[0026] Furthermore, the active current reference value i satisfies transient stability constraints, grid connection rule constraints, and its own capacity limitations. dref and reactive current reference value i qref The generative formula is:

[0027]

[0028] Among them, i N For the rated current of the grid-connected converter, I max To account for the maximum limiting current obtained by considering the capacity of the grid-connected converter, I qref This is the q-axis current reference value.

[0029] Furthermore, the new energy power source is a wind turbine array or a photovoltaic array.

[0030] Furthermore, the grid-connected system model used for verification was built using Matlab-Simulink software based on the topology of a grid-connected three-phase two-level grid-connected converter system.

[0031] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0032] 1) For the first time, the hybrid potential function theory was applied to the transient stability analysis of grid-connected converters under weak power grids. Stability criteria and current distribution strategies were constructed to effectively solve the system instability problem caused by traditional control strategies under weak power grids and ensure continuous grid-connected operation during voltage dips.

[0033] 2) Dynamically adjust the active / reactive current reference values ​​according to the grid short-circuit ratio to maximize active power transmission while ensuring stability;

[0034] 3) Simultaneously satisfy multiple constraint compatibility: Simultaneously satisfy transient stability constraints, grid connection criterion constraints and converter capacity constraints, and consider non-ideal factors such as line impedance and phase-locked loop dynamics, so that the model covers actual operating conditions;

[0035] 4) By optimizing active current output, losses in renewable energy generation are reduced, and power oscillations under weak grid conditions are eliminated using traditional strategies, thus ensuring power quality.

[0036] 5) No need to modify the converter hardware structure, the function can be realized only by upgrading the control algorithm, reducing deployment costs. At the same time, the control parameters are automatically adjusted for different short-circuit ratios to adapt to diverse scenarios from extremely weak grids to strong grids.

[0037] 6) After the fault is cleared, the system can quickly return to a steady state, avoiding the collapse problem of traditional strategies;

[0038] 7) Under the premise of meeting the grid connection criteria and the capacity limit of the converter itself, this method can dynamically adjust the reference values ​​of active current and reactive current. When the grid voltage drops, it can provide sufficient reactive power to ensure the stability of the system, and can transmit as much active power as possible to the grid. Attached Figure Description

[0039] Figure 1 This is a flowchart of the low-voltage ride-through stability control method for grid-connected converters under weak grid conditions, according to an embodiment of the present invention.

[0040] Figure 2 This is a topology diagram of a three-phase two-level grid-connected converter system according to an embodiment of this application;

[0041] Figure 3 This is a block diagram of the low-voltage ride-through control of the grid-connected converter in an embodiment of this application;

[0042] Figure 4 This is a schematic diagram of the equivalent grid-connected converter in the embodiments of this application;

[0043] Figure 5 This is a vector diagram of a grid-connected converter in a weak power grid environment, as described in this application embodiment.

[0044] Figure 6 This is a vector diagram of a grid-connected converter in a weak power grid environment according to an embodiment of this application;

[0045] Figure 7 This is a low-voltage ride-through vector diagram (non-steady state) of a grid-connected converter in a weak power grid according to an embodiment of this application.

[0046] Figure 8 This is an equivalent model diagram of the AC side of a weak current grid according to an embodiment of this application;

[0047] Figure 9 This is a topology diagram of a DC / AC converter according to an embodiment of this application;

[0048] Figure 10This is an equivalent model diagram of the DC / AC converter in the dq rotating coordinate system according to an embodiment of this application;

[0049] Figure 11 This is an equivalent model diagram of a grid-connected converter system with a weak power grid under the embodiments of this application;

[0050] Figure 12 The active current i that decreases continuously during the SCR operation in the embodiments of this application. d With grid voltage v g Relationship diagram;

[0051] Figure 13 This is a schematic diagram of the feasible domain for the active current id reference value that satisfies the grid connection criteria and converter capacity constraints in the embodiments of this application.

[0052] Figure 14 This is a schematic diagram of the feasible domain for the active current id reference value that satisfies the three constraints under different SCRs in the embodiments of this application.

[0053] Figure 15 A schematic diagram of a grid-connected low-voltage ride-through stability control simulation model for SCRs in this application embodiment;

[0054] Figure 16 This is a simulation waveform diagram of conventional low-voltage ride-through control used in a weak power grid according to an embodiment of this application;

[0055] Figure 17 The following is a simulation waveform diagram of the proposed low-voltage ride-through stability control strategy applied under a weak power grid in the embodiments of this application;

[0056] Figure 18 This is a simulation waveform diagram of conventional low-voltage ride-through control under extremely weak power grid conditions according to an embodiment of this application;

[0057] Figure 19 The above is a simulation waveform diagram of the proposed low-voltage ride-through stability control strategy under extremely weak power grid conditions in the embodiments of this application. Detailed Implementation

[0058] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0059] Example

[0060] Figure 2 The system topology diagram for a grid-connected three-phase two-level grid-connected converter is shown below. From left to right, the DC power supply is shown below.dc DC side capacitor C dc A three-phase full-bridge converter, LC filter, and power grid. The DC-side capacitor C... dc Used to smooth DC-side voltage fluctuations and achieve voltage stability, S1 to S6 are power semiconductor switching devices used in the three-phase full-bridge converter. f and C f A three-phase LC filter is formed to filter out harmonics generated by power electronic conversion. f e is the parasitic resistance of the filter inductor. gm (m=a,b,c) represents the voltage of the three-phase AC power grid, L g and R g i is the equivalent line impedance of the power grid. oabc To measure the inductor current of the converter, i abc For three-phase grid-connected current, v abc This is the three-phase grid-connected voltage.

[0061] The main characteristic of a grid-connected converter is that the voltage and frequency after inverterization closely follow the voltage and frequency of the AC power grid, and it cannot actively maintain the grid's voltage and frequency. The control section includes modules such as a phase-locked loop (PLL), a voltage-current dual closed-loop system, and PWM modulation. Specifically, the control circuit samples the three-phase grid-connected voltage and current signals from the main circuit's grid connection point, and inputs the voltage signals into the PLL to obtain the phase θ of the grid voltage. PLL The frequency f ensures that the converter output remains synchronized with the grid. The voltage and current signals at the grid connection point are transformed using Park to obtain voltage and current signals in the dq rotating coordinate system. Then, the voltage and current feedback values ​​are compared with the voltage and current reference values ​​to generate an error signal. This error signal is then adjusted through dual closed-loop regulation to regulate the converter's output voltage and current.

[0062] According to the requirements of the grid connection guidelines, new energy power generation equipment such as wind turbines must have the corresponding low voltage ride-through capability during grid faults. That is, when the grid voltage suddenly drops, the grid-connected converter needs to maintain grid-connected operation and provide a certain amount of reactive power support to help restore the grid voltage. Figure 3A low-voltage ride-through control strategy for a grid-connected converter system under grid voltage dip faults is presented. Under normal conditions without faults, the d-axis current inner loop reference value is generated from the voltage outer loop. Since the converter only supplies active power to the grid, the q-axis current inner loop reference value is set to 0. When a grid voltage dip occurs, the voltage dip detection module detects the voltage dip depth and outputs a switching signal s. This switching signal switches the system strategy from normal control to low-voltage ride-through control. Simultaneously, during the voltage dip caused by a grid fault, to ensure grid voltage stability, the grid-connected converter must prioritize injecting reactive current to assist in voltage recovery at the grid connection point (PCC). According to the technical specifications for grid connection of new energy power plants, the reactive current setpoint i of the grid-connected converter is... oq-lvrt (Negative values ​​represent the supply of inductive reactive power to the grid) and the active current setpoint i od-lvrt It needs to be dynamically generated based on the voltage at the PCC point using a preset algorithm. According to the grid connection specifications, the relationship between the current command value and the PCC voltage can be described as follows:

[0063]

[0064] Among them, i odref 0 This is the reference value for the d-axis current component of a grid-connected converter under normal control. N To match the rated current of the grid-connected converter, i max To account for the maximum limiting current obtained by considering the capacity of the grid-connected converter, i is usually... max Set to 1.2i N u t The grid connection point PCC voltage, u B This represents the system voltage reference value, and k is the dynamic reactive power proportional coefficient, which can be determined according to the actual situation of the power system, and is generally not less than 1.5.

[0065] I. Analysis of Low Voltage Ride-through Instability Mechanism of Grid-Connected Converters in Weak Power Grids

[0066] Grid-connected converter guidelines typically assume a strong AC grid (i.e., short-circuit ratio SCR > 3), requiring the converter to have a certain low-voltage ride-through capability. However, my country's renewable energy grid connection is characterized by long distances, large scale, and high concentration. The grid side is usually a weak grid (i.e., short-circuit ratio SCR < 3). Unlike voltage dip faults at the near-end common point of connection (PCC) in a strong grid, voltage dip faults in weak grids often occur at the far end of the grid. Due to the high inductive reactance of the lines, typical low-voltage ride-through control strategies are prone to instability.

[0067] For grid-connected converters, since they cannot directly control the grid voltage, the closed-loop control design is based on current, ultimately exhibiting controlled current source characteristics. Therefore, they can be equivalent to a power frequency AC current source. The equivalent model of a grid-connected converter under weak grid conditions is as follows: Figure 4 As shown. Where i g It is the grid-connected current output by the converter, ∠a is the phase angle of the grid-connected current, v pcc ∠b represents the grid connection point voltage and the phase angle between the grid connection point voltages, z g ∠θ represents the line impedance and the line impedance angle, respectively. g ∠0 represents the grid voltage; the angle of the grid voltage is assumed to be 0. Because the equivalent impedance of a grid line is typically resistive-inductive, the line impedance z... g It can be written as R g +jX g R g X is the line resistance. g This refers to the line inductance.

[0068] First, when the grid-connected converter operates under normal conditions (i.e., the converter only provides active power), the grid-connected current and the voltage at the PCC remain in phase, i.e., ∠a=∠b. Therefore... Figure 4 The vector relationships of various physical quantities in the system are as follows: Figure 5 As shown. Where i g , Given the vectors of grid-connected current, grid-connected point voltage, and grid voltage, the expression for the grid-connected point PCC voltage is:

[0069]

[0070] Because my country's grid connection standards require grid-connected converters to inject reactive power into the grid connection point to raise the voltage at the PCC (Power Control Center) when the grid voltage drops, a vector diagram of the system's physical quantities during the low-voltage ride-through process of a grid-connected converter under weak grid conditions can be obtained, such as... Figure 6 As shown. Where k is the voltage dip depth of the grid, when a voltage dip occurs, k∈[0,0.9]. At this time, the voltage expression at the grid connection point PCC during the low-voltage ride-through process can be obtained as:

[0071]

[0072] From equation (3), we know that there exists a radical if i in the equation d If the value is too large, the square root part will be less than 0, and the PCC voltage at the grid connection point will lose its steady-state value. For example... Figure 7 As shown, It is the voltage at the grid connection point PCC in the initial state, when i d When it is too big, arrive State, at this time i dand There is a phase difference, due to the effect of the system's phase-locked loop (PLL), i d Will follow The phase, however at this time A new state is reached. In this new state, the vector relationships cannot form a closed triangle and do not satisfy Kirchhoff's laws. At this point, the system has no stable equilibrium point and becomes unstable.

[0073] For grid-connected converters for renewable energy, it is desirable to inject as much active power as possible into the grid during low-voltage ride-through (LVRT) to improve the efficiency of renewable energy generation. Under strong grid conditions, LVRT typically transmits as much active power as possible according to its maximum capacity limit or by maintaining the rated active current before the fault voltage drop. However, based on the above analysis, in weak grid conditions, grid-connected converters face limitations during LVRT. When the active current reference value i... d When the voltage is too high, the grid-connected system will become unstable. To address this issue, this patent proposes a low-voltage ride-through stability control based on new energy grid connection standards and the hybrid potential function method. This ensures that the system meets grid connection standards under weak grid conditions and outputs the maximum possible active power to the grid while maintaining system stability.

[0074] II. Low Voltage Ride-Through Stability Control Methods for Converters in Weak Grids

[0075] 1. Mixed potential function theory

[0076] The method for constructing nonlinear models based on the theory of mixed potential functions can be summarized into the following key stages:

[0077] 1) For non-energy storage components such as resistors in the circuit, construct their voltage potential function or current potential function respectively to describe the energy dissipation characteristics.

[0078] 2) For energy storage elements such as capacitors, calculate the stored electric field energy by integrating the relationship between capacitor voltage and charge.

[0079] 3) Combining the results of the above two parts, a unified energy expression is obtained, and it is organized into a standardized mathematical model to facilitate subsequent analysis and control design.

[0080] Based on the above steps, the mixed potential function P(i,v) related to the system structure is obtained as follows:

[0081] P(i,v)=-A(i)+B(v)+(i,γv-α) (2)

[0082] Where A(i) is the current potential function, B(v) is the voltage potential function, and (i, γv-α) is determined according to the system topology.

[0083] First, define the current potential function P(i) and the voltage potential function B(v), and then take the second-order partial derivatives of each:

[0084]

[0085] When the magnitudes of current and voltage increase significantly (i + v → ∞), if the system meets specific stability conditions, its dynamic behavior will be confined to a certain stable region and gradually tend towards a preset steady-state equilibrium point. In this case, all dynamic responses of the system will eventually converge to the region near the equilibrium point, thus ensuring that the overall operation tends to be stable.

[0086] 2. Equivalent Model of Grid-Connected Converter System

[0087] As more and more new energy power generation systems are connected to the grid, the grid strength is gradually transitioning from a strong grid to a weak grid. The strength of the grid is represented by the short-circuit ratio (SCR).

[0088]

[0089] Among them, P rated The rated capacity of the new energy power generation system connected to the power grid; S sc This is expressed as the system's short-circuit capacity. S sc This can be expressed as:

[0090]

[0091] Among them, v g Represents grid voltage; z g This is expressed as the grid impedance. Therefore, we can obtain:

[0092]

[0093] Where z g =R g +jωL g R g L is the resistance of the power grid. g For the mains inductance, jωL g Represents inductive reactance, ω is the angular frequency, and j is the imaginary unit. The equivalent impedance relationship of the power grid is usually R. g <<ωL g For inductive circuits, we can obtain:

[0094]

[0095] Strong power grids are typically modeled as an infinitely large bus with constant voltage and zero impedance. However, weak power grid modeling must consider impedance effects. This patent equates a weak power grid to an AC voltage source in series with its impedance, such as... Figure 8As shown, the wind turbine renewable energy generation unit consists of a wind turbine array and a Boost converter. Because the output of the wind turbine array is affected by its location and wind speed, there will be fluctuations. To ensure maximum wind utilization, the wind turbine array typically operates in Maximum Power Point Tracking (MPPT) mode. The control principle of MPPT is to continuously adjust the pitch angle to adjust the output power P of the wind turbine array. pv The power gradually increases and reaches the maximum power point. The turbine-side converter connects the wind turbine to the DC bus. Under normal circumstances, from the DC side perspective, the wind turbine power generation unit under the MPPT control strategy can be regarded as a constant power source. The renewable energy power value P G Power P at the maximum power point of the wind turbine array pv Correspondingly.

[0096] The mathematical model of a DC / AC grid-connected converter is highly complex due to its multivariable coupling characteristics. To simplify the analysis process, an abc / dq coordinate system transformation is typically used. When constructing the mathematical model of a DC / AC converter, a detailed analysis of the port characteristics on both the DC and AC sides is first required, followed by the establishment of an equivalent circuit model, such as... Figure 9 As shown. The AC side contains three-phase electromotive force, the instantaneous value of which reflects the input characteristics of the power grid or power source; the input filtering stage consists of inductor L. s Composition, resistor R s The equivalent parasitic loss resistance of the inductor is characterized, and its magnitude directly affects the energy conversion efficiency of the system. The DC side includes the filter capacitor C. dc and load R L C dc Used to stabilize DC bus voltage V dc To suppress voltage fluctuations caused by sudden power changes; R L This represents the equivalent resistance of the load. Additionally, the model introduces the electromotive force e of the DC power supply. L As the DC-side energy input or output port, the reference ground point N provides a unified potential reference for the circuit.

[0097] Introducing the switching function s k The general mathematical model of a DC / AC converter can be expressed by the following formula:

[0098]

[0099] Because the converter model involves the coupling of three-phase AC and DC variables, direct analysis is highly complex. Therefore, a coordinate transformation method is needed to simplify the model. By converting the three-phase stationary coordinate system abc to the dq synchronous rotating coordinate system, the time-varying AC quantities can be converted into DC quantities, thus significantly reducing the mathematical dimension of the control model. After the transformation, the DC / AC converter model can be converted to the dq synchronous rotating coordinate system, such as... Figure 10As shown. The mathematical model of the DC / AC converter after coordinate transformation is:

[0100]

[0101] Where e d and e q These are the d-axis and q-axis components of the grid-side electromotive force; i d i q For the d-axis and q-axis components of the grid-side current, s d and s q These are the switching components of the converter on the d-axis and q-axis, i L Let v be the inductor current. d =s d v dc v q =s q v dc According to the principle of power conservation, the DC side current i0 can be expressed as:

[0102]

[0103] In summary, the simplified model of a grid-connected converter system in the dq coordinate system under weak power grid conditions can be derived from... Figure 11 As shown. 3. Signal stability criteria for grid-connected converters.

[0104] based on Figure 2 The topology of the grid-connected converter system shown is used to establish an equivalent model of the grid-connected converter system in the dq rotating coordinate system under a weak power grid. Next, the transient stability criterion of the grid-connected converter system is derived using the mixed potential function theory. First, based on... Figure 11 Define the current and voltage variables of the grid-connected system:

[0105]

[0106] The derivation yields the relationship with current i d The relevant energy function P(i) and voltage v dc The relevant energy functions P(v) are as follows:

[0107]

[0108] DC capacitor C dc The energy on C is represented by P. Cdc P C Represented as:

[0109]

[0110] The mixed potential function model P(i,v) of the system is:

[0111]

[0112] The derived mixed potential function model P(i,v) is verified as follows:

[0113]

[0114] Equation (18) shows that it satisfies Kirchhoff's voltage and current laws. Therefore, the mixed potential function model given by equation (17) is correct. Based on the mixed potential function model obtained above, the current potential function A(i) and the voltage potential function B(v) of the system can be obtained as follows:

[0115]

[0116]

[0117] The first derivative of the current potential function with respect to the current variable is:

[0118]

[0119] The first derivative of the voltage potential function with respect to the voltage variable is as follows:

[0120]

[0121] In order to solve L -1 / 2 A ii (i)L -1 / 2 To find the smallest eigenvalue μ1, we need to further derive the second-order partial derivative A of the current potential function. ii (i) is:

[0122]

[0123] Under the dual closed-loop PI control of the voltage outer loop and current inner loop, the current inner loop responds rapidly, much faster than the voltage outer loop. Therefore, when determining the stability criterion, the d-axis current i can be considered as... d Compared with reference value i dref They are equal, and ∫(i odref -i od )dt、∫(u dcref -u dc )dt is a constant. Therefore, we can obtain

[0124]

[0125] Similarly, in order to solve C -1 / 2 B vv (v)C -1 / 2 To find the smallest eigenvalue μ2, we need to further derive the second-order partial derivative B of the voltage potential function. vv (v):

[0126]

[0127] L is obtained according to the mixing potential function theorem. -1 / 2 A ii (i)L -1 / 2 and C -1 / 2 B vv (v)C -1 / 2 The smallest eigenvalues ​​μ1 and μ2 are respectively:

[0128]

[0129] In summary, the stability criterion for a grid-connected converter system considering grid SCR in a weak power grid can be obtained as follows:

[0130]

[0131] Further analysis yields the active current reference value i for a grid-connected converter system in a weak power grid that satisfies the system's transient stability constraints. dref The range of values ​​is:

[0132]

[0133] The control equations for a grid-connected converter can be expressed as:

[0134]

[0135] Where, k vp k vi These are the proportional and integral parameters of the voltage outer loop PI controller, k ip k ii These are the proportional and integral parameters of the inner-loop PI controller, i. od i oq i abc The d-axis and q-axis components after the abc-dq transformation, e d e q Let be the d-axis and q-axis components of the AC grid voltage, respectively, and ω = 2πf be the angular frequency of the system, where ωL f i od ,ωL f i oq This is a coupling term.

[0136] 4. Analysis of Low Voltage Ride-Through Stability Control Strategy for Converters Considering SCR

[0137] The above analysis shows that in weak power grids, excessive active current output by grid-connected converters during grid voltage dips can affect system stability. During grid voltage dips, the reference values ​​for active and reactive current of the grid-connected converter need to be comprehensively set, taking into account both the converter's capacity limitations and grid connection criteria, while ensuring system stability. In analyzing the allocation of active and reactive current reference values, this paper comprehensively considers three important constraints: active current i... d Transient stability constraints, grid-connected converter capacity constraints, and converter grid connection criterion constraints.

[0138] 1) Active current i d Transient stability constraints. Ensuring system stability is a prerequisite for achieving active and reactive current distribution. The active current i is plotted as the SCR continuously decreases. d With grid voltage v g Relationships, such as Figure 12 As shown in the figure. The horizontal axis represents the grid voltage v. g Per-unit value, with the vertical axis representing active current i. d Per-unit value. Required active current i d Reference values ​​at different grid voltages v g The drop is consistently below this curve. As the SCR continuously decreases, the active current i d The smaller the maximum limit of the reference value under the same grid voltage drop level, the better.

[0139] 2) Grid connection criteria constraints. Taking Part 6, Fault Ride, of GB / T19964—2024 "Technical Regulations for Wind Turbine Power Stations Connected to the Power System" as an example, it requires that when the voltage at the grid connection point of a wind farm is lower than 90% of the nominal voltage, the wind farm should be able to inject dynamic reactive current into the grid to support voltage recovery. The increment of dynamic reactive current at the wind farm should respond to changes in the grid connection point voltage and satisfy formula (1).

[0140] 3) Grid-connected converter capacity constraints. Grid-connected converters have limited current output capacity; overload can jeopardize their safe operation. Therefore, current capacity constraints must be considered. Taking into account these constraints, the active current reference value i during low-voltage ride-through is... dref The following relationship should be satisfied:

[0141]

[0142] Among them, I max To account for the capacity constraints of grid-connected converters, the maximum output current is typically 1.2 times the rated current, i.e., 1.2 pu; dref.mTo consider the maximum active current under the capacity constraint of the grid-connected converter, combining equations (32) and (1), the relationship satisfying the grid connection criterion constraint and the converter capacity constraint can be obtained as follows:

[0143]

[0144] The active current i that satisfies the grid connection criterion and converter capacity constraint is plotted according to equation (33). d Reference value feasible region, such as Figure 13 The shaded area is shown.

[0145] Based on active current i d Transient stability constraints, grid-connected converter capacity constraints, and grid connection criterion constraints, while also considering when 0 <v g When the voltage is less than 0.9 pu, the system adopts a low-voltage ride-through control strategy and i dref For positive properties, by obtaining the intersection region satisfying the three constraints, the system active current reference command i during grid voltage dips can be obtained. dref The feasible region interval, such as Figure 14 As shown in the figure. It can be seen from the figure that the active current i varies with different SCR values. d The stability constraint curve, the grid connection rule constraint curve, and the grid-connected converter's own capacity limit constraint curve intersect at a point, such as when SCR = 1.5, point A is the active current i. d The intersection of the stability constraint curve, the grid connection rule curve, and the grid-connected converter's own capacity limit constraint curve, and the active current i satisfying the three constraints. d The stable interval is shown in the shaded area. Looking at the x-axis, the active current i satisfies the condition before point A. d The stability constraint curve, after point A, satisfies both the grid connection rule curve and the grid-connected converter's own capacity limit constraint curve. As the SCR (scaling strength) increases (stronger grid strength), the curve intersection point is closer to the y-axis. For example, in the SCR=7 curve, the system should satisfy the grid connection rule and the grid-connected converter's own capacity limit constraint curve for most of the voltage dip depth. Active current i d The stability constraint region is relatively small. The smaller the SCR (the weaker the grid strength), the more the stability is affected by the active current i. d The greater the impact.

[0146] Based on equations (30), (31), (32), and (33), when the SCR is different, the active current i must satisfy transient stability constraints, grid connection rule constraints, and its own capacity limit at different voltage drop depths. d reactive current i q The reference value generation strategy is as follows:

[0147]

[0148] The proposed strategy not only satisfies the three constraints mentioned above, but also transmits the maximum active power to the grid while maintaining stability.

[0149] III. Method Validation

[0150] To verify the low-voltage ride-through stability control algorithm considering SCR under weak power grid conditions proposed in this patent, based on... Figure 2 Following the grid-connected converter system topology, build a system in the software Matlab-Simulink as follows: Figure 15 The grid-connected system model is shown below. The main simulation parameters are shown in Table 1.

[0151] Table 1 Simulation parameters of a grid-connected low-voltage ride-through stable control system considering SCR

[0152]

[0153] Two sets of comparative simulation cases with different SCRs were set up to verify the proposed stability control strategy. The parameters that differ between the two sets of simulation cases are shown in Table 2.

[0154] Table 2 Simulation parameters of the active synchronous droop low-voltage ride-through system

[0155]

[0156] according to Figure 15 It can be seen that when the system SCR=3, in order to verify the effectiveness of the low-voltage ride-through stability control strategy, simulations were performed using parameters A, causing the voltage to drop to 0.4 pu. The proposed method and the traditional low-voltage ride-through control strategy were then compared. Since the active current reference value of the traditional low-voltage ride-through control strategy only satisfies the grid connection criteria and the converter capacity limit curve, i dref The parameter is 0.9 pu. According to... Figure 14 Equation (34) satisfies the low voltage ride-through stability control strategy. dref The parameter must be less than 0.6 pu and set to 10A. In group A simulation, the grid resistance R is set. g =0.27Ω, grid inductance L g =8.55mH, ​​set the start time of the grid voltage drop fault to 0.2s, the clearing time of the voltage drop fault to 0.4s, and the duration of the fault to 0.2s.

[0157] like Figure 16As shown, under the system SCR=3 (weak grid) condition, before the voltage dip fault, the grid-connected converter outputs 10kW of active power and no reactive power. At 0.2s, the voltage dips from 311V to 124.4V, triggering the conventional low-voltage ride-through control switching. During the fault duration, it outputs 3.8kW of active power with power oscillations and provides 3.1kvar of reactive power to the grid to support the grid voltage. After the voltage dip fault is cleared at 0.4s, the converter... Figure 16 It can be seen that the grid-connected current waveform, output active power, and reactive power oscillate and collapse, leading to system instability. Therefore, under weak grid conditions (SCR=3), when a voltage drop to 0.4 pu fault occurs, the traditional low-voltage ride-through control strategy will prevent the system from returning to a stable operating state after the fault ride-through, ultimately causing the grid-connected system to collapse.

[0158] Simulations were performed on a grid-connected converter system employing the proposed low-voltage ride-through stability control strategy, such as... Figure 17 As shown, after a 0.2s voltage dip fault, the system output current and power do not fluctuate significantly, and the system can return to a stable operating state after the fault is cleared in 0.4s. During the low voltage ride-through, the grid-connected converter outputs 3.1kvar reactive power and 2.5kW active power, which not only meets the grid connection requirements, providing reactive power to support the grid voltage dip, but also outputs as much active power as possible while ensuring stability.

[0159] To verify the effectiveness of the low-voltage ride-through stability control strategy considering the system's SCR, a simulation case of SCR = 1.5 (extremely weak grid) was set up for group B. The simulation was set so that the voltage drops to 0.6 pu and the grid resistance R... g =0.47Ω, grid inductance L g =17.1mH, with the grid voltage dip fault start time set to 0.2s, voltage dip fault clearing time to 0.4s, and fault duration to 0.2s. The proposed method and the traditional low-voltage ride-through control strategy are compared in simulations. The simulation results are as follows: Figure 18 , Figure 19 As shown.

[0160] like Figure 18As shown, under the condition of SCR=1.5 (extremely weak grid), using the traditional low-voltage ride-through control strategy, the grid-connected converter outputs 10kW of active power and no reactive power before the voltage sag fault. At 0.2s, the voltage drops from 311V to 186V, triggering the traditional low-voltage ride-through control switching. During the low-voltage ride-through, the grid voltage and current exhibit severe distortion, with 6.8kW of active power output and severe power oscillations, providing 2.7kvar of reactive power to the grid to support the grid voltage. After the voltage sag fault is cleared at 0.4s, the grid voltage, current, output active power, and reactive power oscillate and collapse, leading to system instability. Therefore, it can be concluded that under the condition of SCR=1.5 in an extremely weak grid system, when a voltage sag to 0.6pu occurs, the grid-connected converter using the traditional low-voltage ride-through control strategy will experience severe oscillations, leading to system collapse.

[0161] like Figure 19 As shown, a simulation of a system employing a low-voltage ride-through stability control strategy under an extremely weak power grid with an SCR of 1.5 was conducted. The system did not exhibit significant oscillations after a 0.2s voltage dip fault, and recovered to a stable operating state after the fault was cleared in 0.4s. During the low-voltage ride-through period, the grid-connected converter output 2.7kvar of reactive power and 5kW of active power, satisfying the grid connection criteria requirement of outputting reactive power to support the faulty grid voltage while simultaneously outputting as much active power as possible while ensuring stability.

[0162] Finally, it should be noted that the above embodiments are merely preferred embodiments of the present invention and are only used to illustrate the technical solutions of the present invention, not to limit them. Any modifications, improvements, or other modifications or equivalent substitutions made by those skilled in the art to the technical solutions of the present invention, as long as they do not depart from the spirit and scope of the technical solutions of the present invention, should be included within the protection scope of the present invention.

Claims

1. A method for low-voltage ride-through stability control of a grid-connected converter under weak power grid conditions, characterized in that, Includes the following steps: Step 1: Establish an equivalent model of the grid-connected converter system under a weak power grid based on the topology of the grid-connected three-phase two-level grid-connected converter system; Step 2: Based on the equivalent model of the grid-connected converter system under weak power grid conditions, and combined with the mixed potential function theory, the transient stability criterion of the grid-connected converter system under weak power grid conditions is obtained. Step 3: Based on the transient stability criterion, combined with the grid connection criteria and converter capacity constraints, we obtain the calculation formulas for the reference values ​​of active current and reactive current that can transmit the maximum active power to the grid under the premise of stability. Step 4: Set the SCR of the weak grid to 1.5 and 3 respectively, and compare and verify the proposed method and the traditional low voltage ride-through control method.

2. The low-voltage ride-through stability control method for grid-connected converters under weak power grid conditions according to claim 1, characterized in that: The three-phase two-level grid-connected converter system is transformed from the three-phase stationary coordinate system abc to the dq synchronous rotating coordinate system, resulting in the following mathematical model of the dq synchronous rotating coordinate system for a grid-connected converter in a weak power grid: Where L is the line inductance, R is the line resistance, and C is the line resistance. dc Where ω is the DC bus capacitance, e is the angular frequency, and ω is the DC bus capacitance. d and e q These are the d-axis and q-axis components of the grid-side electromotive force; i d i q For the d-axis and q-axis components of the grid-side current, i L For inductor current, s d and s q It refers to the switching components of the converter on the d-axis and q-axis, v dc This is the DC bus voltage.

3. The low-voltage ride-through stability control method for grid-connected converters under weak power grid conditions according to claim 1, characterized in that: The impedance of the weak power grid is the series impedance of the AC voltage source.

4. The low-voltage ride-through stability control method for grid-connected converters under weak power grid conditions according to claim 1, characterized in that: Define the current and voltage variables of the grid-connected system under weak grid conditions as follows: Among them, i g For grid current, v g This refers to the grid voltage. Based on the transient stability criterion, the hybrid potential function model P(i,v) of a grid-connected system under weak grid conditions is: Among them, e g R is the electromotive force of the power grid. g v is the equivalent line impedance of the power grid. d Let i0 be the d-axis component of the grid-side voltage, i0 be the DC-side current, and P be the DC-side current. G For new energy power value, v g Represents the grid voltage.

5. The low-voltage ride-through stability control method for grid-connected converters under weak power grid conditions according to claim 4, characterized in that: The active current reference value i for a grid-connected converter system in a weak power grid that meets the system transient stability constraints. dref The range of values ​​is: Among them, i dref The active current reference value is given, and SCR is the grid short-circuit ratio, indicating the strength of the grid. p rated k is the rated capacity of the new energy power generation system connected to the grid. vp Voltage loop proportionality coefficient, k ip It is the proportional coefficient of the current loop.

6. The low-voltage ride-through stability control method for grid-connected converters under weak power grid conditions according to claim 1, characterized in that: Active current reference value i that satisfies transient stability constraints, grid connection rule constraints, and self-capacity limitations dref Reactive current reference value i qref The generative formula is: Among them, i N For the rated current of the grid-connected converter, I max To account for the maximum limiting current obtained by considering the capacity of the grid-connected converter, I qref This is the q-axis current reference value.

7. The low-voltage ride-through stability control method for grid-connected converters under weak power grid conditions according to claim 1, characterized in that: The new energy power source is a wind turbine array or a photovoltaic array.

8. The low-voltage ride-through stability control method for grid-connected converters under weak power grid conditions according to claim 1, characterized in that: The grid-connected system model used for verification was built using Matlab-Simulink software based on the topology of a grid-connected three-phase two-level grid-connected converter system.

Images