Stability improvement method for grid-connected converter under weak grid condition

Abstract

A method for improving the stability of grid-connected converters under weak grid conditions solves the problem of subsynchronous oscillations caused by weak grid conditions in existing grid-connected converter control methods, belonging to the field of grid-connected control technology. This invention includes: detecting the reference value and actual value of the DC-side bus voltage of the grid-connected converter to obtain the grid-type synchronization unit: θ s ω represents the grid connection angle in a grid-type synchronous unit. s U represents the angular frequency in a network-type synchronization unit. dc and u dc K represents the reference and actual values ​​of the DC-side bus voltage. T K J K D ω0 represents the system reference angular frequency, and s is the Laplace operator. A grid-type synchronization unit is added to the phase-locked loop control loop. By adjusting the control coefficients of the phase-locked loop and the control coefficients of the grid-type synchronization unit, the grid connection angle of the grid-connected converter can be controlled.

Description

Technical Field

[0001] This invention relates to a method for improving the stability of grid-connected converters under weak grid conditions, belonging to the field of grid-connected control technology. Background Technology

[0002] Against the backdrop of large-scale grid connection of new energy sources, three-phase converters have been widely used as interfaces for new energy transmission. Modern power systems have a high proportion of power electronic equipment, which greatly changes the stability and dynamic characteristics of the system, thus giving rise to new converter control and stability issues.

[0003] Traditional grid-connected converter control relies on the voltage and frequency of the external power grid for operation. In this technology, the inverter tracks the grid voltage phase angle using phase-locked loop (PLL) technology, ensuring that the inverter's voltage and angle are synchronized with the AC grid. This technology is characterized by its dependence on stable frequency and voltage reference values ​​provided by the external power grid. However, due to the long transmission lines between distributed renewable energy generation and the grid connection point, the system exhibits weak grid characteristics. Once the grid fluctuates, the system equipment is prone to subsynchronous oscillations, and may even disconnect from the grid, significantly impacting the stable operation of the power grid system. Therefore, effective oscillation suppression is necessary.

[0004] Currently, common converter control improvement methods mainly fall into two categories: control parameter design and system impedance reshaping. Parameter design requires designing based on the converter's own characteristics and adjusting controller parameters under different operating conditions. While simple, this method's effectiveness is affected by the system's stability margin and dynamic performance. Impedance reshaping improvement methods primarily target the unfavorable frequency bands of the system's equivalent impedance, but their structures are often quite complex. In existing control strategies, grid-based control can operate independently without external grid phase information, providing frequency and voltage support to the system. Grid-based synchronization units can significantly improve the stability of grid-connected converters under weak grid conditions. Summary of the Invention

[0005] To address the problem of subsynchronous oscillations caused by weak power grids in existing grid-connected converter control methods, this invention provides a method for improving the stability of grid-connected converters under weak power grid conditions.

[0006] The present invention provides a method for improving the stability of a grid-connected converter under weak power grid conditions, comprising:

[0007] By comparing the reference value and the actual value of the DC bus voltage of the grid-connected converter, the grid-type synchronization unit is obtained:

[0008]

[0009] Where, θs ω represents the grid connection angle in a grid-type synchronous unit. s U represents the angular frequency in a network-type synchronization unit. dc and u dc K represents the reference and actual values ​​of the DC-side bus voltage. T K J K D denoted by the element coefficients of the first-order transfer function, ω0 represents the system's reference angular frequency, and s is the Laplace operator;

[0010] By adding a grid-type synchronization unit to the phase-locked loop (PLL) control loop and adjusting the control coefficients of the PLL and the grid-type synchronization unit, the grid connection angle of the grid-connected converter can be controlled.

[0011] Preferably, based on the actual value u of the DC side bus voltage dc Transfer function G to grid connection angle dc_θ (s) for K T K J K D Design;

[0012] Transfer function G dc_θ (s) is:

[0013]

[0014] in, k2 represents the control coefficient associated with the network-type synchronization unit, U d Represents the grid connection point voltage signal U abc The reference value of the d-axis DC component, S d T represents the d-axis duty cycle of the grid-connected converter. s I represents the sampling period of the network-type synchronization unit. d I q These represent the grid connection point current signal I, respectively. abc The reference values ​​for the DC components of the d and q axes, R g L g These represent the grid-side resistance and inductance values, respectively.

[0015] Preferably, the method of the present invention further includes, according to the type II system, K T K J K D Perform the adjustment:

[0016]

[0017] As a preferred option, the phase-locked loop control circuit includes:

[0018]

[0019] Where θ represents the actual grid connection angle after adding the grid-type synchronization unit to the phase-locked loop control loop, and ω hybrid This represents the actual angular frequency after adding the network-type synchronization unit to the phase-locked loop control loop, k1+k2=1, where k1 represents the control coefficient related to the phase-locked loop, U q and u q These represent the grid connection point voltage signal U. abc The reference and actual values ​​of the q-axis DC component, k p_pll k is the proportional parameter for phase-locked loop PI control. i_pll These are the integral parameters for phase-locked loop PI control.

[0020] The beneficial effects of this invention are as follows: Addressing the subsynchronous oscillation problem of grid-connected converters under weak power grid conditions, this invention directly introduces a grid-type synchronization unit into the traditional phase-locked loop (PLL) synchronization structure. This effectively suppresses the subsynchronous oscillation problem under weak power grid conditions, improves the adaptability of the grid-connected control system to weak power grids, and enables the control system to maintain stable operation under experimental conditions with low short-circuit ratios. This invention adjusts the grid connection angle of the actual system by improving PLL technology, resulting in a simple control structure and significant effects. Furthermore, this technology not only significantly improves the performance of the control system but also ensures a high PLL bandwidth, exhibiting excellent dynamic performance. Attached Figure Description

[0021] Figure 1 The structural block diagram of a method for improving the stability of grid-connected converters under weak power grid conditions; u abc For the grid connection point voltage, i abc Z is the grid connection point current, L is the filter inductance, and Z is the current at the grid connection point. g Let θ be the line impedance, θ be the actual output angle of the phase-locked loop improved control, and u be the line impedance. dcref Given the DC voltage value, i ref The given value for the grid-side current loop, u ref The grid-side converter output voltage reference value is given by g, which is the modulation signal generated by the modulation module, and u is the modulation signal generated by the modulation module. dc This refers to the DC-side bus voltage.

[0022] Figure 2 A schematic diagram of the principle of a network-type synchronization unit; Δθ s For the small angular signal component in the synchronization unit, θ ref To output the steady-state value of the angle, θ s The grid connection angle in the synchronization unit;

[0023] Figure 3 for Figure 2 Equivalent Type II system control block diagram;

[0024] Figure 4A block diagram of the phase-locked loop synchronization element in a traditional grid-connected converter;

[0025] Figure 5 A structural block diagram for improving phase-locked loop technology. Detailed Implementation

[0026] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0027] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0028] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but this is not intended to limit the scope of the invention.

[0029] The stability improvement method for grid-connected converters under weak grid conditions in this embodiment includes:

[0030] Step 1: Detect the steady-state and actual values ​​of the DC-side bus voltage of the grid-connected converter. Based on the real-time data, obtain the relationship between the DC-side bus voltage and the grid connection angle to obtain the grid-type synchronization unit.

[0031]

[0032] In the above formula, θ s ω represents the grid connection angle in a grid-type synchronous unit. s U represents the angular frequency in a network-type synchronization unit. dc and u dc K represents the reference and actual values ​​of the DC-side bus voltage. T K J K D Let ω0 represent the coefficients of the first-order transfer function, ω0 represent the system reference angular frequency, and s be the Laplace operator.

[0033] Based on real-time data, the relationship between the DC-side bus voltage and the grid connection angle is obtained, and the coefficient K in the first-order transfer function is determined. T K J K D :

[0034] DC bus voltage u dc Transfer function G to grid connection angle dc_θ (s) is represented as:

[0035]

[0036] Where k2 represents the control coefficient associated with the network-type synchronization unit, U d Represents the three-phase winding voltage signal U abc The reference value of the d-axis DC component, S d T represents the d-axis duty cycle of the grid-connected converter. s I represents the sampling period of the network-type synchronization unit. d I q These represent the three-phase winding current signals I, respectively. abc The reference values ​​for the DC components of the d and q axes, R g L g These represent the grid-side resistance and inductance values, respectively.

[0037] Because of T s Much greater than the cutoff frequency of the transfer function, therefore G dc_θ (s) can be simplified to:

[0038]

[0039] The parameters were tuned according to the Type II system:

[0040] The following is a typical open-loop transfer function G for a Type II system:

[0041]

[0042] Where K and τ are the transfer function coefficients that need to be tuned, and T is the time constant of the first-order inertial element.

[0043] According to the principles of automatic control, the intermediate frequency bandwidth determines the system's response speed, and the phase margin of the control system determines the system's stability. Determining the intermediate frequency bandwidth and phase margin allows for tuning the parameters K and τ in the open-loop transfer function of a Type II system. Referring to the above explanation, in this embodiment, the intermediate frequency bandwidth is designed to be a multiple of 5, and the parameter relationships can be obtained as follows:

[0044]

[0045] The final result is:

[0046]

[0047] Step 2: Add the network-type synchronization unit to the phase-locked loop control loop:

[0048] The small-signal component of the DC bus voltage of the grid-connected converter is passed through the first-order transfer function and integral element in the grid-type synchronous unit. The phase small-signal component χ is calculated and introduced into the phase calculation output terminal of the phase-locked loop.

[0049]

[0050] The original phase-locked loop output angle expression of the grid-connected converter is:

[0051]

[0052] Where, θ PLL ω represents the grid connection angle of the phase-locked loop output. PLL U represents the angular frequency of the phase-locked loop output. q and u q Represents voltage signal U abc The reference and actual values ​​of the q-axis DC component, k p_pll k is the proportional parameter for phase-locked loop PI control. i_pll These are the integral parameters for phase-locked loop PI control.

[0053] By introducing the small-signal component χ of the phase into the phase calculation output of the phase-locked loop, the actual grid connection angle of the system after the phase-locked loop is improved is obtained, specifically expressed as:

[0054]

[0055] k = k1 + k2 = 1 (11)

[0056] Where θ represents the actual grid connection angle after adding the grid-type synchronization unit to the phase-locked loop control loop, and ω hybrid k represents the actual angular frequency after adding the network-type synchronization unit to the phase-locked loop control loop, k1 represents the control coefficient related to the phase-locked loop, k2 represents the control coefficient related to the network-type synchronization unit, and the sum of the two coefficients is k.

[0057] For different converter operating conditions, the ratio between control coefficients k1 and k2 can be adjusted, and a grid-type control synchronization unit can be directly introduced into the grid-connected control system to effectively suppress the subsynchronous oscillation problem of the system under weak grid conditions, thus realizing a method to improve the stability of grid-connected converters under weak grid conditions.

[0058] While the invention has been described herein with reference to specific embodiments, it should be understood that these embodiments are merely examples of the principles and applications of the invention. Therefore, it should be understood that many modifications can be made to the exemplary embodiments, and other arrangements can be designed without departing from the spirit and scope of the invention as defined by the appended claims. It should be understood that different dependent claims and features described herein can be combined in ways different from those described in the original claims. It is also understood that features described in conjunction with individual embodiments can be used in other described embodiments.

Claims

1. A method for improving the stability of grid-connected converters under weak power grid conditions, characterized in that, include: By comparing the reference value and the actual value of the DC bus voltage of the grid-connected converter, the grid-type synchronization unit is obtained: Where, θ s ω represents the grid connection angle in a grid-type synchronous unit. s U represents the angular frequency in a network-type synchronization unit. dc and u dc K represents the reference and actual values ​​of the DC-side bus voltage. T K J K D denoted by the element coefficients of the first-order transfer function, ω0 represents the system's reference angular frequency, and s is the Laplace operator; By adding a grid-type synchronization unit to the phase-locked loop (PLL) control loop and adjusting the control coefficients of the PLL and the grid-type synchronization unit, the grid connection angle of the grid-connected converter can be controlled.

2. The method for improving the stability of grid-connected converters under weak power grid conditions according to claim 1, characterized in that, Based on the actual value u of the DC side bus voltage dc Transfer function G to grid connection angle dc_θ (s) for K T K J K D Design; Transfer function G dc_θ (s) is: in, k2 represents the control coefficient associated with the network-type synchronization unit, U d Represents the grid connection point voltage signal U abc The reference value of the d-axis DC component, S d T represents the duty cycle of the d-axis DC component of the grid-connected converter. s I represents the sampling period of the network-type synchronization unit. d I q These represent the grid connection point current signal I, respectively. abc The reference values ​​for the DC components of the d and q axes, R g L g These represent the grid-side resistance and inductance values, respectively.

3. The method for improving the stability of a grid-connected converter under weak power grid conditions according to claim 2, characterized in that, The method also includes applying K according to the Type II system. T K J K D Perform the adjustment:

4. The method for improving the stability of grid-connected converters under weak power grid conditions according to claim 2, characterized in that, Phase-locked loop (PLL) control circuitry includes: Where θ represents the actual grid connection angle after adding the grid-type synchronization unit to the phase-locked loop control loop, and ω hybrid This represents the actual angular frequency after adding the network-type synchronization unit to the phase-locked loop control loop, k1+k2=1, where k1 represents the control coefficient related to the phase-locked loop, U q and u q These represent the grid connection point voltage signal U. abc The reference and actual values ​​of the q-axis DC component, k p_pll k is the proportional parameter for phase-locked loop PI control. i_pll These are the integral parameters for phase-locked loop PI control.

5. A computer-readable storage device storing a computer program, characterized in that, When the computer program is executed by the processor, it implements the steps of the stability improvement method for grid-connected converters under weak grid conditions as described in any one of claims 1 to 4.

6. A stability improvement device for a grid-connected converter under weak grid conditions, comprising a storage device, a processor, and a computer program stored in the storage device and executable on the processor, characterized in that, The processor executes the computer program to implement the steps of the method for improving the stability of a grid-connected converter under weak grid conditions as described in any one of claims 1 to 4.

7. A computer program product, comprising a computer program, characterized in that, When executed by a processor, the computer program implements the steps of the stability improvement method for grid-connected converters under weak grid conditions as described in any one of claims 1 to 4.

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