VSG-based low-voltage ride-through hybrid control method for network construction type converter

Abstract

The invention discloses a VSG-based low-voltage ride-through hybrid control method for a network-building converter. The method comprises the following steps: acquiring grid-connected point voltage and output current of a grid-forming converter in real time, and judging whether to enter a low-voltage ride-through state according to a grid voltage drop degree; in the initial stage of a fault, a current reference saturation mode is adopted to quickly limit the output current of the converter so as to prevent a power device from being damaged by overcurrent; in the fault duration stage, a dynamic self-adaptive virtual impedance and reactive power reference coordinated regulation mechanism is introduced, so that the converter gradually recovers the reactive power injection capability while limiting the fault current, thereby realizing effective support for the grid-connected point voltage; when the voltage of the power grid drops seriously, the capacity of the inverter can be reserved for the reactive power support by temporarily reducing the output of the active power. According to the method, the current protection and voltage support requirements during the fault period are considered, and the operation stability and the power grid adaptive capacity of the grid-forming converter in the low voltage ride-through process are improved.

Description

Technical Field

[0001] This invention belongs to the field of power electronics technology, specifically relating to a hybrid control method for low voltage ride-through of grid-connected converters based on VSG (Virtual Synchronous Generator). Background Technology

[0002] With the rapid development of new energy power generation technologies such as wind power and photovoltaics, and the widespread application of distributed power sources, traditional synchronous generators are being replaced by grid-connected converters. Grid-connected converters based on virtual synchronous machine control strategies can simulate the dynamic characteristics of traditional synchronous generators, achieving autonomous voltage and frequency regulation, and providing necessary support and regulation services for the power grid.

[0003] During grid faults, especially under low-voltage ride-through conditions, grid-connected converters not only face the risk of output overcurrent due to the sharp drop in grid voltage, but also need to maintain effective grid voltage support during the fault period to ensure overall system stability. Traditional control strategies during low-voltage ride-through often focus on suppressing fault current, using current-limiting protection measures to prevent damage to the converter from transient overcurrents. However, simple current suppression may cause the converter to lose its ability to support grid voltage during the fault, leading to system frequency and voltage fluctuations, and potentially even synchronous instability.

[0004] The main fault current limiting techniques for grid-connected converters under low-voltage ride-through conditions include direct current reference saturation control, switch-level current limiting, and current limiting methods based on virtual impedance. Current limiting techniques based on virtual impedance can maintain the voltage source characteristics of the converter while protecting the equipment by dynamically adjusting the equivalent impedance at the output. However, research on voltage support during low-voltage ride-through is relatively limited. Some literature even focuses on how to achieve fault suppression by reducing reference power, neglecting effective compensation for voltage support capability. Therefore, how to balance fault current suppression and voltage support under low-voltage ride-through conditions has become a key research topic in grid-connected converter control. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the technical problem this invention aims to solve is to provide a hybrid control method for low-voltage ride-through of grid-connected converters based on VSG, thereby resolving the technical problem in existing technologies where grid-connected converters struggle to simultaneously achieve fault current suppression and voltage support during low-voltage ride-through.

[0006] The present invention solves the aforementioned technical problem by adopting the following technical solution: A hybrid low-voltage ride-through control method for a VSG-based grid converter, characterized by the following steps: Step 1: Detect the grid voltage. When the grid voltage is detected to be lower than the preset threshold, enter the low voltage ride-through state and start the hybrid control strategy. Step 2: In the early stage of the fault, use the direct current limiting method to quickly suppress the current and control the converter output current within a safe range; Step 3: During the fault period, the voltage support function is smoothly restored by adjusting the virtual impedance and reactive power reference value; Step 4: Based on the recovery of the power grid voltage, gradually adjust the control parameters to restore the system to normal operation.

[0007] Furthermore, in step three, when a fault or increased load causes the current to exceed the current threshold, the virtual impedance increases proportionally according to the magnitude of the current exceeding the threshold; the adjustment coefficient is gradually reduced through an adaptive algorithm, enabling the system to re-inject reactive power into the grid; Define virtual impedance for: (4) in, This indicates the magnitude of the current output current. To trigger the current threshold for virtual impedance compensation, and These are the adjustment coefficients.

[0008] Furthermore, in step three, when the grid connection point voltage is lower than a preset threshold, the system enters a low-voltage state and initiates voltage support control; the controller increases the priority of reactive power output and linearly adjusts the reactive power reference value according to the degree of grid voltage drop; the linear adjustment formula is: (5) in, This is a reference value for reactive power. Maximum reactive power, This is the per-unit value of the grid connection point voltage.

[0009] Compared with the prior art, the beneficial effects of the present invention are: 1. The hybrid control strategy proposed in this invention quickly controls the overcurrent within a safe range in the early stage of a fault through direct current limiting; during the duration of the fault, it maintains the converter output voltage near a predetermined reference value by reducing virtual impedance and adjusting reactive power, thus providing necessary voltage support to the power grid. This achieves the dual objectives of rapidly suppressing overcurrent in the early stage of a fault and smoothly restoring voltage support during the duration of the fault.

[0010] 2. This invention combines direct current limiting and dynamic adjustment technology based on virtual impedance to effectively limit the output current and support the voltage of grid-connected converters during low voltage ride-through. Compared with simple current suppression or voltage support strategies, the hybrid strategy can better balance the dual needs of equipment protection and grid support. Attached Figure Description

[0011] Figure 1 This is a schematic diagram of the grid-connected system of the grid-type converter of the present invention; Figure 2 This is a schematic diagram of the topology of the mesh-type VSG of the present invention; Figure 3 This is a schematic diagram of the active power-frequency control principle of the present invention; Figure 4 This is a schematic diagram of the reactive power-voltage control principle of the present invention; Figure 5 This is an overall flowchart of the present invention; Figure 6 This is the schematic diagram of the adaptive virtual impedance control principle of this invention; Figure 7 This is a schematic diagram illustrating the linear setting of the reactive power reference value in this invention; Figure 8 The waveform of the line inductor current under unlimited current measures; Figure 9 The waveform diagram of the line inductor current under direct current limiting measures; Figure 10 The waveform diagram of the line inductor current under hybrid control; Figure 11 The voltage waveform at the grid connection point under the unlimited current measure; Figure 12 The voltage waveform at the grid connection point under direct current limiting measures; Figure 13 This is a waveform diagram of the grid connection point voltage under hybrid control. Detailed Implementation

[0012] Specific embodiments are given below with reference to the accompanying drawings. These specific embodiments are only used to describe the technical solution of the present invention in detail and are not intended to limit the scope of protection of this application.

[0013] like Figure 1As shown, a grid-connected converter system mainly consists of a DC power supply, a grid-connected SVG (Static Var Generator), and the receiving-end grid. The DC power supply provides energy to the grid-connected SVG, which employs a dual closed-loop control strategy. The inner loop current control achieves fast response and fault current suppression, while the outer loop voltage control generates a reactive power reference, providing voltage support to the grid. At the grid end, the grid-connected SVG is connected in parallel to the busbar via a filter reactor to track and regulate the voltage at the grid's point of common coupling (PCC). Line inductance ensures the system's dynamic performance and stability under various load and fault conditions.

[0014] like Figure 2 As shown, the topology of a grid-type VSG consists of a three-phase full-bridge inverter and filter inductors and capacitors forming the output circuit. PWM modulation converts the DC-side voltage into a controllable AC voltage. The output voltage of each phase arm is filtered by R, L, and C and then connected to the grid's point of common coupling (PCC), simultaneously supplying power to local loads. The power calculation module measures the output power and grid voltage information in real time. Based on the principle of a virtual synchronous machine, the VSG controller adjusts active and reactive power, providing grid frequency and voltage support. Figure 3 , Figure 4 This is a schematic diagram of active-frequency control and reactive-voltage control.

[0015] like Figure 5 As shown, this invention provides a hybrid low-voltage ride-through control method for grid-connected converters based on VSG, comprising the following steps: Step 1: Detect the grid voltage. When the grid voltage is detected to be lower than the preset threshold (e.g., 0.8 pu), the system enters the low voltage ride-through state and starts the hybrid control strategy. Step 2: In the early stage of the fault (the first few cycles after the fault occurs), use the direct current limiting method to quickly suppress the current and control the converter output current within a safe range; In this embodiment, the direct current limiting method employs a current reference saturation method. When the current reference value generated by the voltage controller exceeds the preset maximum allowable value, it is scaled using a saturation function to ensure that the current reference value sent to the inner loop controller does not exceed the preset maximum allowable value; specifically defined as: (1) (2) in, This represents the reference current value after saturation treatment. This indicates the current reference value generated by the voltage controller. Indicates the scaling factor. This indicates the preset maximum allowed value.

[0016] Step 3: During the fault period, the voltage support function is smoothly restored by dynamically and adaptively adjusting the virtual impedance and reactive power reference value. That is, the virtual impedance is dynamically reduced according to the real-time current amplitude, and the reactive power reference value is increased according to the voltage drop depth. When a low-voltage fault occurs in the grid voltage, since the terminal voltage remains unchanged, it can be deduced that the fault current will rise rapidly. This method introduces a virtual impedance module in the voltage control loop or inner loop, which increases the equivalent output impedance of the system during the fault, thereby limiting the current flowing through the converter. (3) in, It is the fault current. and These are the grid voltage and the generator terminal voltage, It is an adaptive virtual impedance; Define adaptive virtual impedance For a parameter that depends on the current amplitude, its mathematical expression is: (4) in, Indicates the magnitude of the current output current; The current threshold that triggers virtual impedance compensation is set below which no additional compensation is performed. and These are the adjustment coefficients for the increments of virtual resistance and virtual inductance when the current exceeds the threshold, respectively.

[0017] When a fault or increased load causes the current to exceed the current threshold, the virtual impedance increases proportionally to the magnitude of the excess current, thereby increasing the system's equivalent output impedance, limiting the fault current, and protecting power devices from overload. During the fault duration, an adaptive algorithm gradually reduces the regulation coefficient. and This allows the system to re-inject reactive power into the grid, thereby supporting the voltage.

[0018] like Figure 6 As shown, the low voltage state is identified by voltage detection and threshold judgment. When the grid connection point (PCC) voltage Vpcc is lower than 0.8pu, the system determines that it has entered a low voltage state and starts the voltage support process. At this time, the controller will increase the priority of reactive power output to offset the impact of voltage drop.

[0019] like Figure 7 As shown, the reactive power reference setting adopts a linear adjustment method. When the grid voltage drops to 0.8 pu, the reactive power reference value begins to increase, and when the grid voltage drops to 0 pu, the reactive power reference value reaches its maximum. The specific formula is: (5) in, This is a reference value for reactive power. Maximum reactive power, This is the per-unit value of the grid connection point voltage.

[0020] When the grid voltage drops significantly, the active power output can be temporarily reduced. If the grid voltage drops significantly, reducing the active power output will allow the inverter to have more capacity for reactive power support, thus preventing the active power from consuming too much inverter capacity and weakening the reactive power injection capability.

[0021] Step 4: Based on the recovery of the power grid voltage, gradually adjust the control parameters to restore the system to normal operation.

[0022] Example This embodiment verifies the method of the present invention through simulation. The main simulation parameters are shown in Table 1. Table 1. Main Simulation Parameters

[0023] Figure 8-10 The figures show the line inductor current waveforms under three conditions: unlimited current control, direct current limiting, and hybrid control. <signal> :1、 <signal>:2 and <signal>:3 represent the three phases, and Constant3 and Constant4 represent the baseline. When the grid voltage drops to 0 p.u., Figure 8 The data shows that without any current limiting measures, the line inductance current can reach up to ten times the rated current, far exceeding the capacity of the switching devices in the converter. Figure 9 Direct current limiting is employed. It can be seen that before 0.15 seconds, the current limiter operates, suppressing the current to a low range. After 0.15 seconds, it begins to reduce the virtual impedance and increase the reference value of reactive power, resulting in... Figure 10 The waveform shown.

[0024] Figure 11-13 The figures show the grid connection point voltage waveforms under the following conditions: unlimited current control, direct current limiting, and hybrid control. Figure 11 The data shows that, without any current limiting measures, the grid connection voltage can reach 80% of the rated voltage. Figure 12 After the direct current limiting measures were taken, the voltage support capacity decreased, and the voltage amplitude at the grid connection point dropped to about 170V. Figure 13 A hybrid strategy was adopted. It can be seen that the grid connection point voltage was raised to about 200V after 0.15 seconds. If the virtual impedance is further reduced or the reactive power reference value is increased, the waveform of the grid connection point voltage will show sawtooth waves due to current limiting, which will affect the power quality.

[0025] Any aspects not covered in this invention are applicable to existing technologies.< / signal> < / signal> < / signal>

Claims

1. A hybrid low-voltage ride-through control method for a VSG-based grid converter, characterized in that, Includes the following steps: Step 1: Detect the grid voltage. When the grid voltage is detected to be lower than the preset threshold, enter the low voltage ride-through state and start the hybrid control strategy. Step 2: In the early stage of the fault, use the direct current limiting method to quickly suppress the current and control the converter output current within a safe range; Step 3: During the fault period, the voltage support function is smoothly restored by adjusting the virtual impedance and reactive power reference value; Step 4: Based on the recovery of the power grid voltage, gradually adjust the control parameters to restore the system to normal operation.

2. The low-voltage ride-through hybrid control method for VSG-based grid converters according to claim 1, characterized in that, In step three, when a fault or increased load causes the current to exceed the current threshold, the virtual impedance increases proportionally according to the magnitude of the current excess; the adjustment coefficient is gradually reduced through an adaptive algorithm, enabling the system to re-inject reactive power into the grid. Define virtual impedance for: （4） in, This indicates the magnitude of the current output current. To trigger the current threshold for virtual impedance compensation, and These are the adjustment coefficients.

3. The low-voltage ride-through hybrid control method for VSG-based grid converters according to claim 1 or 2, characterized in that, In step three, when the grid connection point voltage is lower than the preset threshold, the system enters a low-voltage state and initiates voltage support control; the controller increases the priority of reactive power output and linearly adjusts the reactive power reference value according to the degree of grid voltage drop; The linear adjustment formula is: （5） in, This is a reference value for reactive power. Maximum reactive power, This is the per-unit value of the grid connection point voltage.

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