Fault ride-through control method for network construction type converter

Abstract

The invention provides a fault ride-through control method for a network construction type converter, and belongs to the technical field of energy storage converters and the like. The method comprises the following steps: firstly, measuring PCC voltage and power grid side current, calculating amplitudes of the PCC voltage and the power grid side current, and then judging whether the PCC voltage drops below a set threshold value or not; if not, keeping the active power reference and the voltage amplitude reference unchanged; and if so, performing active power reference modification: judging whether a current limiting strategy is triggered, if not, locking the PCC voltage reference as a measured value, and if so, gradually decreasing the PCC voltage reference by an increment value based on the measured value. According to the invention, accurate evaluation of the existence of the stable equilibrium point is realized; meanwhile, an improved fault ride-through control method is provided, active power reference and voltage amplitude reference are dynamically modified, so that the grid-forming converter is transited to a new stable balance point during a fault period, and current limiting, transient synchronization stability improvement, PCC voltage source characteristic maintenance and maximum fault current output can be taken into account.

Description

Technical Field

[0001] This invention relates to the technical fields of new energy grid connection, energy storage converters, "source-grid-load-storage" integration and power electronic energy conversion, specifically a fault ride-through control method for grid-type converters. Background Technology

[0002] With the widespread integration of renewable energy, grid-connected converters, due to their ability to simulate synchronous generator characteristics and provide key grid support functions such as system inertia and damping, are becoming an important component in building new power systems. Compared to traditional grid-connected converters, grid-connected converters exhibit stronger stability under extremely weak grid conditions. However, due to the voltage source nature of grid-connected converters, they are prone to overcurrent problems when encountering grid faults, thus necessitating the introduction of current limiting strategies to ensure safe system operation. Existing current limiting strategies mainly include direct current limiting strategies, virtual impedance technology, voltage limiting strategies, and power reference correction strategies. Although these current limiting strategies can effectively limit fault current, they still face two key challenges: first, improving the transient synchronous stability of the system under current limiting constraints; and second, achieving maximum fault current output while maintaining the voltage source characteristics at the PCC (Power Source Control).

[0003] To address the transient synchronization stability issue during grid faults, previous research, through analysis of the power angle characteristic curve, revealed that current-limiting strategies constrain the maximum value of active power transmission. In other words, the introduction of current-limiting strategies reduces the peak value of the power angle characteristic curve, directly affecting transient synchronization stability. To solve this problem, existing technologies introduce virtual resistors to suppress transient overcurrents and propose a strategy for adjusting the outer-loop power reference value. These existing technologies dynamically adjust the active and reactive power references based on the voltage drop during the fault to limit the steady-state fault current, enabling the grid-connected converter to establish a new stable equilibrium point during the fault and maintain the voltage source characteristics at the PCC (Power Source Control Center).

[0004] However, the method of analyzing the power angle characteristic curve ignores the influence of the internal potential amplitude on the existence of the stable equilibrium point. Regarding the problem of achieving maximum fault current output while maintaining the voltage source characteristics at the PCC, existing solutions limit the steady-state fault current by adjusting the outer loop power reference. However, this solution only considers improving transient synchronization stability and maintaining the voltage source characteristics at the PCC, employing a relatively conservative current-limiting strategy. This solution dynamically reduces active and reactive power based on the voltage dip depth, thus limiting the output current amplitude of the grid-connected converter to a low level during faults. It cannot provide maximum fault current at all voltage dip levels, thereby limiting the grid-connected converter's ability to support the grid during faults. Summary of the Invention

[0005] The technical problem to be solved by this invention is: how to deliver the maximum reactive fault current to the grid while maintaining the voltage source characteristics at the PCC, so as to improve the grid support capability of the grid-type converter during the fault period.

[0006] To achieve the above technical objectives, the present invention adopts the following technical solution: A fault ride-through control method for a grid-type converter includes: 1) Measure the PCC voltage and grid-side current and calculate their amplitudes; 2) Determine if the PCC voltage has dropped below the set threshold; 3) If the result of step 2) is negative, the active power reference and voltage amplitude reference remain unchanged, and the method ends; if the result of step 2) is positive, proceed to step 4). 4) Active power reference modification; 5) Determine if a rate limiting policy has been triggered; 6) If the judgment result of step 5) is negative, lock the PCC voltage reference to the measured value and end the method; if the judgment result of step 5) is positive, the PCC voltage reference gradually decreases based on the measured value by an incremental value and end the method.

[0007] Preferably, the method is executed cyclically with a fixed sampling period.

[0008] Preferably, in this method, the active power reference is modified as follows: ; in, This indicates that there is no need to lower the PPC voltage threshold for the active power reference; In the above formula, when the PCC voltage is not less than When the active power reference remains unchanged, the PCC voltage drops to [a certain value]. At the following times, the active power reference is reduced to zero.

[0009] Preferably, in this method, the voltage amplitude reference is modified through a state machine: reference voltage amplitude Set to the nominal value at the beginning. If no overcurrent occurs, the state remains unchanged. If a voltage dip occurs in the grid, the current limiting strategy is activated; at this time, the state transitions to... , based on The current limiting mode is gradually reduced by an increment ∆V; when the PCC reference voltage decreases, the controller exits the current limiting mode and reaches a new stable equilibrium point within the stable operating domain of the grid-type converter. The state has switched to PCC reference voltage It is locked to the measured value.

[0010] Preferably, the method is implemented through a power and voltage amplitude reference modification module, which is a component of the power outer loop control. The power and voltage amplitude reference modification module only modifies the active power and voltage amplitude reference during faults.

[0011] Preferably, the power outer loop control, together with the inner potential synthesis module, the current-limiting virtual resistor, the voltage circuit control loop, and the PWM modulation module, constitute the overall control strategy.

[0012] Preferably, in the power outer loop control, the active power control loop adopts droop control with a low-pass filter, and the control loop equation is: ; in, This represents the droop coefficient of the active power control loop. This is a reference value for active power. This is the nominal value of the angular frequency. This is the cutoff frequency of the low-pass filter in the active power control loop; The reactive power control loop uses a PI controller to control the PCC voltage amplitude. Adjust to its reference value The control loop equation is: ; in, and These are the proportional gain and integral gain of the PI controller, respectively. Current-limiting virtual resistor Designed as follows: ; in, The proportional coefficient of the current-limiting virtual resistor. This represents the amplitude of the grid-side current. The threshold for triggering the rate limiting policy, Slightly smaller than the maximum output current amplitude ; The inner loop control adopts dual closed-loop control of voltage and current. The outer voltage loop maintains the stability of the inverter output voltage and provides a reference value for the inner current loop. The inner current loop adjusts the output current and outputs a modulated wave signal, which is output through the PWM modulation module to a signal used to precisely control the conduction timing of the power switching transistors.

[0013] The expressions for the active and reactive power supplied to the grid by the grid-connected converter using the aforementioned control strategy, without triggering the current limiting strategy, are as follows: ; ; in, Indicates the internal potential of a grid-type converter With grid voltage The angle between them is called the work angle.

[0014] Preferably, in this method, the operating domain of the grid-type converter is defined as: ; The existence of a stable equilibrium point without triggering a current-limiting strategy is determined by observing whether the contour lines intersect within the operating domain of the grid-type converter defined in the above equation, i.e.: ; Preferably, in this method, upon detecting a voltage drop in the mains power grid, the following settings are immediately configured: ,make sure To maintain transient synchronization stability; to exit current limiting mode by dynamically adjusting the PCC voltage reference, maintain voltage source characteristics at the PCC, and provide maximum reactive fault current during voltage dips.

[0015] This invention addresses the issues of transient synchronization stability and voltage source characteristic maintenance at the point of common coupling (PCC) of grid-connected converters during grid faults. It proposes a fault ride-through control method based on grid-connected converter operating domain analysis. By reconstructing the stable equilibrium point, the transient synchronization stability of the grid-connected converter during faults is improved. While maintaining the voltage source characteristics at the PCC, the maximum reactive power fault current is output, thereby enhancing the grid-connected converter's support capability for the grid during faults.

[0016] In the above technical solutions, the influence of internal potential amplitude on the existence of a stable equilibrium point is considered. An operating domain framework is proposed to maintain the normal operation of the converter in grid-type mode without triggering current-limiting protection, achieving accurate assessment of the existence of the stable equilibrium point. Based on the proposed operating domain analysis framework for grid-type converters, an improved fault ride-through control method is proposed. By dynamically modifying the active power reference and voltage amplitude reference, the grid-type converter can transition to a new stable equilibrium point during faults, improving transient synchronization stability and enabling the transmission of maximum reactive fault current to the grid while maintaining the voltage source characteristics at the PCC.

[0017] In summary, this invention addresses the issues of improving transient synchronization stability and maintaining voltage source characteristics at the PCC during grid faults by proposing an operating domain framework for maintaining normal operation of the converter in grid-connected mode without triggering current-limiting protection. Compared to existing analysis methods, the proposed framework considers the influence of internal potential amplitude on the existence of a stable equilibrium point. This operating domain analysis framework can accurately assess whether a stable equilibrium point exists for the grid-connected converter in the operating domain without triggering current-limiting strategies.

[0018] Based on the operating domain analysis framework of grid-connected converters, this invention proposes a fault ride-through control method for grid-connected converters. Compared with existing technologies, the fault ride-through control method proposed in this invention can deliver the maximum reactive fault current to the grid while maintaining the voltage source characteristics at the PCC, thereby improving the grid support capability of grid-connected converters during faults. Attached Figure Description

[0019] Figure 1 This is the main circuit topology diagram of the grid-type converter and the power grid connection in this invention.

[0020] Figure 2 This is a block diagram of the overall control strategy of the present invention.

[0021] Figure 3 This is the equivalent circuit diagram of the grid-type converter in this invention.

[0022] Figure 4 This is the phasor diagram of the grid-type converter considering current limiting in this invention.

[0023] Figure 5 This is a stable equilibrium point analysis diagram of the operating region concept of grid-type converter under different grid voltage drop depths in this invention.

[0024] Figure 6 This is a flowchart of the method of the present invention.

[0025] Figure 7 This is a diagram illustrating the method for modifying the voltage amplitude reference of a grid-type converter in this invention.

[0026] Figure 8 The figure shows the simulation results of the method of this invention under a severe voltage drop fault. Detailed Implementation

[0027] The specific embodiments of the present invention will be described in detail below. To avoid excessive and unnecessary detail, well-known structures or functions will not be described in detail in the following embodiments. The approximate language used in the following embodiments is for quantitative purposes, indicating that a certain degree of variation in quantity is permissible without changing the basic function. Unless otherwise defined, the technical and scientific terms used in the following embodiments have the same meaning as commonly understood by those skilled in the art to which this invention pertains.

[0028] Figure 1 This is a topology diagram of the main circuit connecting the power converter and the power grid in a fault ride-through control method based on the operating domain analysis of a grid-connected converter, according to the present invention. Phasors are represented by bold letters, and their magnitudes are represented by regular uppercase letters. This power circuit primarily performs DC-to-AC power conversion, and can also perform AC-to-DC power conversion, possessing bidirectional power flow capability. The circuit topology of this power converter mainly consists of three parts: a power conversion section, an LC filter section, and a grid connection interface. (Figure shows...) It is a DC voltage source; the power conversion section adopts a three-phase bridge topology, and the three-phase full-bridge power switches are respectively Each bridge arm consists of two IGBT switching devices, and the upper and lower switches operate in complementary mode. , and These are the filter inductor, filter capacitor, and line inductance, respectively; the AC power grid consists of its line impedance. Ideal AC voltage source in series express; For inverter-side current, The voltage at PCC. For grid-side current, and Used to calculate active power P and reactive power Q.

[0029] Figure 2 The diagram shows the overall control strategy of the fault ride-through control method proposed in this invention. It mainly consists of an outer power loop control, an inner potential synthesis module, a current-limiting virtual resistor, a voltage circuit control loop, and a PWM modulation module. Specifically, the power and voltage amplitude reference modification module in the outer power loop control only modifies the active power and voltage amplitude reference during fault periods.

[0030] In the power outer loop control, the active power control loop adopts traditional droop control with a low-pass filter, and the control loop equation can be written as follows: ; in, This represents the droop coefficient of the active power control loop. This is a reference value for active power. This is the nominal value of the angular frequency. This is the cutoff frequency of the low-pass filter in the active power control loop. The reactive power control loop uses a PI controller to adjust the PCC voltage amplitude. Adjust to its reference value The control loop equation can be written as ; in, and These represent the proportional gain and integral gain of the PI controller, respectively. To suppress transient overcurrents during grid faults and keep the output current within a safe range, a current-limiting virtual resistor is used. Designed as follows: ; in, This is the scaling factor for the current-limiting virtual resistance. This represents the amplitude of the grid-side current. The threshold for triggering the rate limiting policy, Slightly smaller than the maximum output current amplitude .

[0031] The inner loop control adopts dual closed-loop control of voltage and current. The outer voltage loop is responsible for maintaining the stability of the inverter output voltage and providing a reference value for the inner current loop. The inner current loop is responsible for adjusting the output current and outputting a modulation wave signal, which is output through the PWM modulation module to precisely control the conduction timing of the power switching transistors.

[0032] The grid-connected equivalent circuit diagram of the grid-type converter using the above control strategy is as follows: Figure 3 As shown, when the current limiting strategy is not triggered, the expressions for the active power and reactive power delivered to the grid by the grid-connected converter are as follows: ; in, Indicates the internal potential of a grid-type converter With grid voltage The angle between them is called the work angle.

[0033] To address the issue of transient synchronization stability and voltage source characteristics at the PCC caused by the introduction of current limiting strategies during grid faults, this invention considers the impact of internal potential amplitude on the existence of a stable equilibrium point and proposes an operating domain framework to maintain the normal operation of the converter in grid-connected mode without triggering current limiting protection. This operating domain analysis framework can accurately assess whether a grid-connected converter can establish a stable equilibrium point under a given power reference without triggering current limiting strategies.

[0034] The phasor diagram of a GFM converter considering current limiting is as follows: Figure 4 As shown. Grid voltage phasor Extending from vertex A to vertex O. Phasor The endpoint is located at vertex B, and the voltage difference between the two phasors is Due to overcurrent limitations, the current amplitude must not exceed its maximum value. Therefore, to avoid triggering the current limiting strategy, impedance... The pressure drop must be located centered at O ​​with a radius of... Inside the circle. According to and The relationship between them needs to be analyzed in two scenarios. Due to symmetry, the focus of the analysis should be on... Above. (As shown) Figure 4 As shown in (a), if Then vertex A is located outside circle O. from Change to At that time, always satisfied But when hour, This will activate the rate limiting policy. For example... Figure 4 As shown in (b), for This is a situation that typically occurs during a deep voltage dip in the power grid, with vertex A inside circle O. A stable equilibrium point can be reached at... Internally established, because From 0 to Within the range, it is always permissible to keep the vertex B containing phasor E on circle O.

[0035] like Figure 4 As shown, the area of ​​triangle AOB is Therefore, when When the maximum value is reached, the converter can output the maximum power. That is, when phasor With grid voltage phasor When orthogonal, the maximum value of the output active power It can be represented as: ; From formula (6), we can obtain that when hour, Will follow The increase in power angle leads to a decrease in power angle, thus disrupting the negative feedback of active power control. Therefore, the maximum power angle is: ; Therefore, the operating domain of the grid-type converter proposed in this invention is defined as follows: ; The determination can be made based on formulas (4) and (5). and Contour lines on a three-dimensional surface and projected onto On the plane. The existence of a stable equilibrium point without triggering a current-limiting strategy can be determined by observing whether the contour lines intersect within the operating domain of the grid-type converter as defined by formula (8), i.e.: ; Figure 5 The existence of stable equilibrium points under three different voltage drop depths was analyzed, with the short-circuit ratio (SCR) set to 2.5 and the grid voltage amplitude... Set to respectively , and The light blue area in the figure represents the operating domain of the grid-type converter as defined in formula (8). According to formula (2), the grid-type converter will inject different reactive power at different grid voltage dip depths to restore the PCC voltage to its rated voltage. Figure 5 As shown in (a), when hour, The range is approximately , The range is approximately Active power reference setting Inject reactive power To adjust the PCC voltage; and The contour lines intersect within the operating domain of the grid-type converter, indicating the existence of a stable equilibrium point within that domain. For example... Figure 5 As shown in (b), when hour, Shift the range to the left , The scope has been expanded to Active power reference maintained at At this point, more reactive power needs to be injected to restore the PCC voltage; and The contour lines intersect outside the operating domain of the grid-type converter, indicating that there is no stable equilibrium point within the operating domain of the grid-type converter. For example... Figure 5 As shown in (c), when hour, The scope was further narrowed to , The scope has been expanded to Active power reference maintained at ,and Figure 4 (b) Consistent with the analysis; with Further reduction, The contour lines shift out of the operating domain of the grid-type converter, therefore, there is no stable equilibrium point within the operating domain of the grid-type converter.

[0036] Based on the aforementioned operating domain analysis framework for grid-connected converters, this invention proposes a dynamic fault ride-through control method for grid-connected converters that enhances transient synchronization stability, maintains voltage source characteristics at the PCC (Power Control Center), and can output the maximum reactive power fault current. Its working principle is as follows: upon detecting a voltage dip in the grid, immediately set... ,make sure To maintain transient synchronization stability; to exit current limiting mode by dynamically adjusting the PCC voltage reference, maintain voltage source characteristics at the PCC, and provide maximum reactive fault current during voltage dips.

[0037] The fault ride-through control method proposed in this invention uses... Figure 2 The power and voltage amplitude reference modification module shown is implemented as follows: (See flowchart below) Figure 6 As shown, it is executed cyclically with a fixed sampling period.

[0038] The active power reference is modified as follows: ; in, (For example ) indicates that the PPC voltage threshold does not need to be reduced for the active power reference. In formula (10), when the PCC voltage is not less than At that time, the active power reference remains unchanged. If the PCC voltage drops to When the active power reference is reduced to zero, the maximum transient power angle stability margin is ensured, thereby improving transient synchronization stability.

[0039] Figure 7 This invention proposes an innovative method for modifying a voltage amplitude reference using a state machine. (Reference voltage amplitude) Set to the nominal value at the beginning. If no overcurrent occurs, the state remains unchanged. If a voltage dip occurs in the grid, the current limiting strategy is activated. At this point, the state transitions to... , based on The current limiting mode is gradually reduced by an increment ∆V. When the PCC reference voltage decreases, the controller exits the current limiting mode and reaches a new stable equilibrium point within the stable operating domain of the grid-type converter. The state has switched to PCC reference voltage The voltage is locked at the measured value. At this point, the output of the voltage amplitude PI controller remains unchanged, establishing a new stable equilibrium point. The current limiting strategy exits, and the voltage source characteristics at the PCC are maintained; moreover, the new stable equilibrium point is located on the boundary of the grid-connected converter's operating domain, ensuring that the grid-connected converter delivers the maximum reactive fault current to the grid.

[0040] Finally, the effectiveness of the proposed method was verified through simulation. Taking a severe voltage dip fault as an example, the voltage dip depth was set to 0.2 pu and the fault duration to 2.5 s.

[0041] Figure 8 Simulation waveforms of the grid-type converter using the proposed fault ride-through control method under severe voltage dip fault conditions are shown. The simulation results show that when a voltage dip fault occurs, the grid-type converter, acting as a voltage source, rapidly increases its output current to support the grid. The active power reference is set to zero according to formula (10), and reactive current is injected to boost the PCC voltage. When the grid-side current... Exceeding the threshold for triggering rate limiting policies At this time, the current-limiting virtual resistor is activated to limit the fault current. PCC voltage reference. Figure 7 The modification method shown is dynamically adjusted, gradually decreasing during the rate limiting process until it exits the rate limiting mode and converges to a new stable equilibrium point within the runtime domain. At the new stable equilibrium point, In other words, without triggering the current limiting strategy, the grid-connected converter maintains its voltage source characteristics and provides the maximum fault current. After the fault is cleared, the active power reference is restored, and the grid-connected converter returns to its pre-fault state through adjustment of the power outer loop control circuit. The simulation results demonstrate the effectiveness of the fault ride-through method proposed in this invention, which can enhance transient synchronization stability, maintain the voltage source characteristics at the PCC, and output the maximum reactive fault current.

[0042] The embodiments of the present invention have been described in detail above, but the content described is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the scope of the present invention should be included within the protection scope of the present invention.

Claims

1. A fault ride-through control method for a grid-type converter, characterized in that, include: 1) Measure the PCC voltage and grid-side current and calculate their amplitudes; 2) Determine if the PCC voltage has dropped below the set threshold; 3) If the result of step 2) is negative, the active power reference and voltage amplitude reference remain unchanged, and the method ends; if the result of step 2) is positive, proceed to step 4). 4) Active power reference modification; 5) Determine if a rate limiting policy has been triggered; 6) If the result of step 5) is negative, lock the PCC voltage reference to the measured value and end the method; When the judgment result of step 5) is yes, the PCC voltage reference gradually decreases based on the measured value by an incremental value, and the method ends.

2. The fault ride-through control method for a grid-type converter according to claim 1, characterized in that, This method is executed cyclically with a fixed sampling period.

3. The fault ride-through control method for a grid-type converter according to claim 1, characterized in that, In this method, the active power reference is modified as follows: ； in, This indicates that there is no need to lower the PPC voltage threshold for the active power reference; In the above formula, when the PCC voltage is not less than When the active power reference remains unchanged, the PCC voltage drops to [a certain value]. At the following times, the active power reference is reduced to zero.

4. The fault ride-through control method for a grid-type converter according to claim 1, characterized in that, In this method, the voltage amplitude reference is modified through a state machine: reference voltage amplitude. Set to the nominal value at the beginning. If no overcurrent occurs, the state remains unchanged. If a voltage dip occurs in the grid, the current limiting strategy is activated; at this time, the state transitions to... , based on The current limiting mode is gradually reduced by an increment ∆V; when the PCC reference voltage decreases, the controller exits the current limiting mode and reaches a new stable equilibrium point within the stable operating domain of the grid-type converter. The state has switched to PCC reference voltage It is locked to the measured value.

5. The fault ride-through control method for a grid-type converter according to claim 1, characterized in that, This method is implemented through a power and voltage amplitude reference modification module, which is a component of the power outer loop control. The power and voltage amplitude reference modification module only modifies the active power and voltage amplitude reference during faults.

6. The fault ride-through control method for a grid-type converter according to claim 5, characterized in that, The power outer loop control, together with the internal potential synthesis module, the current-limiting virtual resistor, the voltage circuit control loop, and the PWM modulation module, constitute the overall control strategy.

7. The fault ride-through control method for a grid-type converter according to claim 6, characterized in that, In the power outer loop control, the active power control loop adopts droop control with a low-pass filter, and the control loop equation is: ； in, This represents the droop coefficient of the active power control loop. This is a reference value for active power. This is the nominal value of the angular frequency. This is the cutoff frequency of the low-pass filter in the active power control loop; The reactive power control loop uses a PI controller to control the PCC voltage amplitude. Adjust to its reference value The control loop equation is: ； in, and These are the proportional gain and integral gain of the PI controller, respectively. Current-limiting virtual resistor Designed as follows: ； in, The proportional coefficient of the current-limiting virtual resistor. This represents the amplitude of the grid-side current. The threshold for triggering the rate limiting policy, Slightly smaller than the maximum output current amplitude ; The inner loop control adopts dual closed-loop control of voltage and current. The outer voltage loop maintains the stability of the inverter output voltage and provides a reference value for the inner current loop. The inner current loop adjusts the output current and outputs a modulated wave signal, which is output through the PWM modulation module to a signal used to precisely control the conduction timing of the power switching transistors.

8. The fault ride-through control method for a grid-type converter according to claim 7, characterized in that, The expressions for the active and reactive power supplied to the grid by the grid-connected converter using the aforementioned control strategy, without triggering the current limiting strategy, are as follows: ； ； in, Indicates the internal potential of a grid-type converter With grid voltage The angle between them is called the work angle.

9. The fault ride-through control method for a grid-type converter according to claim 8, characterized in that, In this method, the operating domain of the grid-type converter is defined as: ； The existence of a stable equilibrium point without triggering a current-limiting strategy is determined by observing whether the contour lines intersect within the operating domain of the grid-type converter defined in the above equation, i.e.: 。 10. The fault ride-through control method for a grid-type converter according to claim 1, characterized in that, In this method, upon detecting a voltage drop in the mains power grid, a setting is immediately implemented. ,make sure To maintain transient synchronization stability; By dynamically adjusting the PCC voltage reference to exit current-limiting mode, the voltage source characteristics at the PCC are maintained, and maximum reactive fault current is provided during voltage dips.

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