Improved reactive power support method for AHO during asymmetric low voltage ride through

Abstract

The invention discloses an improved reactive power support method for an AHO (Active High Voltage Over) during asymmetric low voltage ride through, and relates to the technical field of network construction type inverter control. On the basis of a traditional AHO control framework, a positive sequence AHO and a negative sequence AHO are constructed, negative sequence voltage and current components of a power grid are counteracted in real time through the negative sequence AHO, negative sequence interference suppression is achieved, and the operation stability of a GFI under an asymmetric fault is guaranteed; a self-adaptive current amplitude limiting strategy is integrated in the positive sequence AHO, when it is detected that GFI output current exceeds a preset safety threshold value, an amplitude limiting mechanism is automatically started, and the fault current amplitude is restrained to be within a safety range; and in the LVRT period, dynamic adjustment is carried out based on the positive sequence AHO control parameters, so that the GFI outputs reactive power to the power grid, and the voltage of the auxiliary power grid is quickly recovered and kept synchronous. According to the method, the technical problems that the fault current is out of control and the power is out of step in the LVRT period of the GFI based on the AHO can be solved.

Description

Technical Field

[0001] This invention relates to the field of grid-connected inverter control technology, specifically to an improved reactive power support method for AHO during asymmetric low voltage ride-through. Background Technology

[0002] In recent years, significant progress has been made in grid formation control technology based on virtual oscillators (VOCs). This technology initially achieved automatic coordination and distributed control of inverters without communication using nonlinear limit-loop oscillators. Subsequent refinement of key control components led to the emergence of the Andronov-Hopf oscillator (AHO). Implemented in the time domain, it not only fully covers the steady-state functions of traditional time-domain control strategies but also boasts advantages such as superior dynamic response, fast response speed, strong disturbance rejection capability, and harmonic-free output waveforms, while overcoming the inherent limitations of other virtual oscillators.

[0003] However, existing AHO-related technologies still have many unresolved issues: First, traditional AHOs are first-order, inertia-free, and undamped controllers, which are susceptible to frequency disturbances and do not integrate active-frequency, reactive-voltage, and voltage-current control links, making them difficult to apply directly to fault ride-through scenarios. Second, although AHO control strategies with nested virtual inertia and damping coefficients possess virtual inertia, their low-voltage ride-through (LVRT) capability is insufficient, and significant current overruns are likely to occur when the grid voltage drops, potentially damaging the inverter. Current limiting strategies are needed to control the fault current within a safe range. Third, asymmetrical faults such as single-phase grounding and two-phase grounding are more common in the grid. These faults generate negative-sequence components in the current, causing three-phase imbalance. At the same time, voltage drops increase the voltage difference between the AHO and the grid, limiting the active power transmission capability. Furthermore, the lack of negative-sequence suppression can lead to equipment overload, grid imbalance, insufficient reactive power support, synchronization instability, power coupling, and energy loss, seriously affecting the operational stability of the inverter grid-connected system. Summary of the Invention

[0004] In view of this, the present invention provides an improved reactive power support method for AHO during asymmetric low voltage ride-through, aiming to solve the technical problems of fault current runaway and power synchronization loss that are prone to occur in grid-connected inverters (GFI) based on traditional AHO control during grid low voltage ride-through (LVRT).

[0005] The technical solution of this invention is:

[0006] An improved method for reactive power support during asymmetric low-voltage ride-through (AHO) of an AHO system, comprising the following steps:

[0007] S1. The three-phase voltage and current output from the grid connection point are decomposed into positive sequence components and negative sequence components in real time.

[0008] S2. Construct positive-order AHO and negative-order AHO, and use negative-order AHO to achieve active suppression of negative-order components;

[0009] S3. Introduce a current limiting strategy in the positive sequence AHO to limit the fault current during low voltage ride-through.

[0010] S4. Based on the active current reference value of GFI, the reactive current reference value of GFI and the voltage reference value of positive sequence AHO, the reactive power support during low voltage ride-through is achieved using positive sequence AHO.

[0011] Compared with the prior art, the present invention has the following beneficial effects:

[0012] (1) This invention can specifically suppress the negative sequence components of voltage and current, solve the problem of three-phase current and voltage imbalance from the root, and avoid equipment overload and power transmission disorder caused by negative sequence components; at the same time, it can stably maintain the voltage source characteristics of AHO, ensure that its power synchronization relationship with the power grid is not interrupted, and provide core support for the stable operation of the system during faults.

[0013] (2) This invention makes full use of the fault IV characteristics of AHO itself. Without the need to add a complex control module, it can actively provide reactive power support, directly participate in alleviating the voltage imbalance of the power grid, improve the voltage support capability of the power grid in fault scenarios, reduce the adverse effects of voltage drop on the power grid and associated loads, and enhance the overall anti-disturbance performance of the system.

[0014] (3) The present invention can accurately suppress fault current, avoid GFI damage due to overcurrent, and ensure equipment safety; at the same time, it can stably maintain the voltage source characteristics of AHO, ensure that it continues to play its grid-building characteristics during fault transients and recovery, and does not interrupt the support for the power grid, further improving the reliability and stability of fault ride-through of distributed power systems. Attached Figure Description

[0015] Figure 1 This is a flowchart of the improved AHO reactive power support method during asymmetric low voltage ride-through in this embodiment.

[0016] Figure 2 This is a control principle diagram of the improved AHO reactive power support method during asymmetric low voltage ride-through in this embodiment. Detailed Implementation

[0017] To facilitate understanding of this application, a more comprehensive description of this application will be provided below with reference to the accompanying drawings.

[0018] The core idea of ​​this invention is as follows: First, a dual second-order generalized integrator is used to separate the positive and negative sequences of the grid connection point voltage and current in real time, and a negative-sequence AHO and a positive-sequence AHO are constructed, with negative-sequence component suppression performed in the negative-sequence AHO. Next, a quasi-proportional resonant controller is nested before the positive-sequence AHO as an inertial element, so that the input and output signals of the positive-sequence AHO remain in phase under steady state and power oscillation is suppressed. Then, a current limiting strategy is introduced in the positive-sequence AHO to limit the fault current during low-voltage ride-through. At the same time, a voltage-current loop is constructed, and the active current reference value and reactive current reference value of GFI are adaptively adjusted according to the degree of voltage drop at the grid connection point. Then, the voltage reference value expression of the positive-sequence AHO is improved to generate the desired reactive current without switching the current control mode. Finally, the power expression is improved based on the active current reference value of GFI, the reactive current reference value of GFI, and the voltage reference value of the positive-sequence AHO, thereby realizing reactive power support during low-voltage ride-through using the positive-sequence AHO.

[0019] Figure 1 and Figure 2 These are the flowchart and control principle diagram of the improved AHO reactive power support method during asymmetric low voltage ride-through in this embodiment. The power stage includes a DC voltage source, a GFI, and an LCL filter. and These represent the filter inductor and filter capacitor on the GFI side, respectively. This represents the AC grid-side filter inductor, where PCC is the grid connection point. For resistance, For AC power grids. For example... Figure 1 and Figure 2 As shown, the improved AHO reactive power support method during asymmetric low voltage ride-through includes the following steps:

[0020] S1. Real-time detection of the three-phase voltage at the grid connection point, and decomposition of the three-phase voltage and current output at the grid connection point into positive sequence components and negative sequence components;

[0021] This implementation uses the positive-sequence separation method of dual second-order generalized integrators (DSOGI) to decompose the three-phase voltage and current at the grid connection point into positive-sequence and negative-sequence components in real time, obtaining the positive-sequence voltage component V. zα V zβ The negative sequence component of voltage V fα V fβ Positive sequence component of current I zα I zβ and the negative sequence component of current I fα I fβ .

[0022] S2. Construct positive-order AHO and negative-order AHO, and use negative-order AHO to achieve active suppression of negative-order components;

[0023] S2.1: Construct a negative sequence AHO and set both the negative sequence voltage reference value and the negative sequence current reference value to 0. Use the negative sequence AHO to achieve active suppression of the negative sequence component.

[0024] The governing equation for the negative-order AHO in this implementation is:

[0025]

[0026] in, , This is a negative-sequence voltage control command output by the negative-sequence AHO. , This refers to the actual detected negative sequence voltage; , This is the negative sequence reference current for GFI; It is the convergence rate of the negative-order AHO; It is the current gain; It is the reference amplitude of the negative sequence voltage component; It is the rated angular frequency of the negative sequence component; and This represents the difference between the negative sequence reference current and the negative sequence current output by GFI.

[0027] The negative-sequence AHO dynamically adjusts its output to force the negative-sequence voltage component to converge to zero, while simultaneously driving the negative-sequence current component to track changes in the reference value, thus suppressing the negative-sequence current.

[0028] S2.2: Construct a positive sequence AHO, and introduce a quasi-proportional resonant controller as an inertial element before the positive sequence AHO;

[0029] This implementation introduces a quasi-proportional resonant controller as an inertial element before the positive-sequence AHO. The error signal of the input current of the positive-sequence AHO is obtained through this inertial element, thus giving the GFI adjustable inertial damping and suppressing power oscillations. The transfer function of the quasi-proportional resonant controller... for:

[0030]

[0031] in The damping angular frequency; This is the proportionality coefficient; It is the natural angular frequency.

[0032] In this embodiment, the governing equation for the positive-order AHO is:

[0033]

[0034] in, , This is the positive sequence voltage control command output by the positive sequence AHO. , This refers to the actual detected positive sequence voltage; It is the convergence rate of the forward-order AHO; It is the current gain; It is the reference amplitude of the positive sequence voltage component; It is the positive-sequence component angular frequency; , The positive sequence reference current for GFI; (I zβ * -I zβ ) and (I zα * -I zα This represents the difference between the positive sequence reference current and the positive sequence current output by GFI.

[0035] S3. Introduce a current limiting strategy in the positive sequence AHO to limit the fault current during low voltage ride-through.

[0036] The positive sequence AHO limits the fault current during low voltage ride-through according to a preset current limiting strategy; the current limiting strategy is: to limit the output current of GFI using boundary equations, and to limit the output current of GFI to within 1.2 times the rated current of GFI by a preset threshold.

[0037] The GFI output current is limited using the boundary equation shown below. Specifically, the preset threshold is 1.2 times the rated output current of the GFI. When the absolute value of the current reference value is not greater than the preset threshold, the output current of the GFI tracks the current reference value. When the absolute value of the current reference value is greater than the preset threshold, the output current of the GFI is clamped to the preset threshold and maintains the same phase as the current reference value.

[0038]

[0039] in, This is the reference value for reactive current in GFI; This is the active current reference value for GFI; This represents the maximum permissible output current amplitude of the GFI. This is the rated output current of GFI.

[0040] S4. Utilize positive sequence AHO to achieve reactive power support during low voltage ride-through.

[0041] S4.1. Adjust the active current reference value of GFI adaptively according to the voltage drop at the grid connection point. and reactive current reference value It further provides reactive power support to help alleviate grid voltage imbalance without switching control modes.

[0042] Power grid specifications require that once the power grid's rated voltage... If the voltage drops below 90%, low-voltage ride-through control is activated, and reactive power is injected to support the grid voltage. In this implementation, the active current reference value of GFI is adjusted according to the following formula. and reactive current reference value :

[0043]

[0044] Where e=V pcc / V g The percentage of the grid voltage drop; This refers to the voltage at the grid connection point. This is the rated voltage of the power grid; This is the reactive current reference value for GFI. This is the active current reference value for GFI; This is the maximum allowable current amplitude for GFI.

[0045] S4.2. Map the reactive current output by GFI to the positive-sequence AHO output voltage reference value through the IV characteristic. :

[0046]

[0047] in, This is the reference value for the output voltage of the positive sequence AHO. The voltage at which the power grid experiences a deep voltage drop; Line resistance; This refers to the line reactance.

[0048] S4.3, Output voltage reference value based on positive sequence AHO The active power reference value of GFI is obtained. and reactive power reference value ;

[0049]

[0050] in, This is the reference value for the output voltage of the positive sequence AHO. This is a reference value for reactive current. For the angle of attack; This is the rated active power output of GFI when the grid voltage is stable;

[0051] S4.4 Add a proportional-derivative (PI) controller to the positive-sequence AHO to provide a reference value for the output voltage of the positive-sequence AHO. Control measures: Set the reactive power reference value... With the reactive power output of GFI The difference is calculated by obtaining the output voltage deviation reference value of the positive sequence AHO through the PI controller and comparing it with the output voltage reference value. Summing yields the reference amplitude of the output voltage of the positive sequence AHO. :

[0052]

[0053] in, and These are the proportional coefficient and the integral coefficient, respectively.

[0054] This invention provides a highly efficient fault current limiting method during asymmetrical grid voltage faults. Based on the severity of the fault, it converts current reference values ​​into power control reference values ​​and voltage control values ​​to meet grid specifications. It also provides reactive power support for the grid voltage and reduces active power to mitigate the fault current, improving the voltage source characteristics of the AHO (Active Power Source). The output current maintains three-phase balance and does not exceed limits, and no control mode switching is required throughout the process. It exhibits minimal transient oscillations and rapid recovery, meeting the technical requirements of "fault-free grid connection and active support."

[0055] It should be understood that, inspired by the technical concept of this invention, those skilled in the art can make various improvements or modifications based on the above content without departing from the scope of this invention, and these modifications still fall within the protection scope of this invention.

Claims

1. An improved method for reactive power support during asymmetric low-voltage ride-through using an AHO (Active Power Controller), characterized in that, The method includes the following steps: S1. The three-phase voltage and current output from the grid connection point are decomposed into positive sequence components and negative sequence components in real time. S2. Construct positive-order AHO and negative-order AHO, and use negative-order AHO to achieve active suppression of negative-order components; S3. Introduce a current limiting strategy in the positive sequence AHO to limit the fault current during low voltage ride-through. S4. Based on the active current reference value of GFI, the reactive current reference value of GFI and the voltage reference value of positive sequence AHO, the reactive power support during low voltage ride-through is achieved using positive sequence AHO.

2. The reactive power support method according to claim 1, characterized in that, In S1, the positive and negative sequence separation method of dual second-order generalized integrators is used to decompose the three-phase voltage and current output at the grid connection point into positive sequence components and negative sequence components in real time.

3. The reactive power support method according to claim 1, characterized in that, The governing equations for negative-order AHO are: ； in, , This is a negative-sequence voltage control command output by the negative-sequence AHO. , This refers to the actual detected negative sequence voltage; It is the convergence rate of the negative-order AHO; It is the current gain; It is the reference amplitude of the negative sequence voltage component; It is the rated angular frequency of the negative sequence component; I fα I fβ The negative sequence component of the current output by GFI; , This is the negative sequence reference current for GFI.

4. The reactive power support method according to claim 3, characterized in that, The method for actively suppressing negative sequence components using negative sequence AHO as described in S2 is as follows: both the negative sequence voltage reference value and the negative sequence current reference value are set to 0. By dynamically adjusting the output of negative sequence AHO, the negative sequence voltage component is forced to converge to zero, while the negative sequence current component is driven to track the change of the reference value, thereby achieving active suppression of negative sequence components.

5. The reactive power support method according to claim 4, characterized in that, The orthogonal AHO includes: The governing equations for the positive-sequence AHO are: ； in, , This is the positive sequence voltage control command output by the positive sequence AHO. , This refers to the actual detected positive sequence voltage; It is the convergence rate of the forward-order AHO; It is the current gain; It is the reference amplitude of the positive sequence voltage component; It is the positive-sequence component angular frequency; I fα I fβ This refers to the positive-sequence component of the current output by GFI. , This is the positive sequence reference current for GFI; A quasi-proportional resonant controller is nested before the positive-sequence AHO as an inertial element. The transfer function of this quasi-proportional resonant controller is... for: ； in This is the proportionality coefficient; The damping angular frequency; It is the natural angular frequency.

6. The reactive power support method according to claim 5, characterized in that, In S3, the current limiting strategy is as follows: ;in, This is the reference value for reactive current in GFI; This is the active current reference value for GFI; This represents the maximum permissible output current amplitude of the GFI. This is the rated output current of GFI.

7. The reactive power support method according to claim 6, characterized in that, S4 further includes: S4.

1. Based on the voltage drop at the grid connection point, adaptively adjust the active current reference value of GFI according to the following formula. and reactive current reference value ; ； Where e=V pcc / V g The percentage of the grid voltage drop; This refers to the voltage at the grid connection point. This is the rated voltage of the power grid; S4.

2. Map the reactive current output by GFI to the positive-sequence AHO output voltage reference value through the IV characteristic. ; S4.3, Output voltage reference value based on positive sequence AHO Determine the active power reference value of GFI. and reactive power reference value expression; S4.4 Add a PI controller pair in the ascending sequence AHO Control: Set the reactive power reference value of GFI to... With the reactive power output of GFI The difference is calculated by obtaining the output voltage deviation reference value of the positive sequence AHO through the PI controller and then comparing it with... Summing yields the reference amplitude of the output voltage of the positive sequence AHO. .

8. The reactive power support method according to claim 7, characterized in that, Output voltage reference value of positive sequence AHO The expression is: ； in, The voltage at which the power grid experiences a deep voltage drop; Line resistance; This refers to the line reactance.

9. The reactive power support method according to claim 7, characterized in that, The active power reference value of the GFI and reactive power reference value The expression is as follows: ;in, This is the reference value for the output voltage of the positive sequence AHO. This is the reference value for reactive current in GFI; For the angle of attack; This is the rated active power output of GFI when the grid voltage is stable.

10. The reactive power support method according to claim 7, characterized in that, The output voltage reference amplitude of the positive sequence AHO The expression is as follows: ;in, and These are the proportional coefficient and the integral coefficient, respectively.

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