Method for improving stability of grid-connected converter under weak power grid condition

Abstract

The invention discloses a method for improving stability of a grid-connected converter under a weak power grid condition, solves the problem of subsynchronous oscillation generated by an existing grid-connected converter control method under a weak power grid, and belongs to the technical field of grid-connected control. The method comprises the following steps: detecting a reference value and an actual value of a direct-current side bus voltage of a grid-connected converter to obtain a grid-forming type synchronization unit: # imgabs0 # theta s represents a grid-connected angle in the grid-forming type synchronization unit, omega s represents an angular frequency in the grid-forming type synchronization unit, Udc and udc represent the reference value and the actual value of the direct-current side bus voltage, KT, KJ and KD represent first-order transfer function link coefficients, and KT, KJ and KD represent first-order transfer function link coefficients; omega0 represents the reference angular frequency of the system, and s is a Laplace operator; and adding the network construction type synchronization unit into a phase-locked loop control link, and controlling the grid-connected angle of the grid-connected converter by adjusting the control coefficient of the phase-locked loop and the control coefficient of the network construction type synchronization unit.

Description

Technical Field

[0001] The invention relates to a method for improving the stability of a grid-connected converter under weak power grid conditions, and belongs to the technical field of grid-connected control. Background Art

[0002] In the context of large-scale renewable energy grid connection, three-phase converters have been widely used as interfaces for renewable 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 causing new converter control and stability problems.

[0003] Traditional grid-connected converter control relies on the voltage and frequency of the external grid to operate. In this technology, the inverter tracks the grid voltage phase angle through phase-locked loop technology to ensure that the voltage and angle of the inverter are synchronized with the AC grid. The characteristic of this technology is that it relies on the stable frequency and voltage reference values ​​provided by the external grid, but due to the long transmission line between the distributed renewable energy generation end and the grid connection point, the system presents weak grid characteristics. Once the grid fluctuates, the system equipment is prone to subsynchronous oscillation accidents, and may even be disconnected from the grid, which has a great impact on the stable operation of the grid system. Therefore, it is necessary to effectively suppress the oscillation.

[0004] At present, the common converter improvement control methods are mainly divided into two directions, one is the control parameter design, and the other is the control system impedance reshaping. Among them, the parameter design method may need to be designed according to the characteristics of the converter itself, and the controller parameters are adjusted accordingly under different working conditions. Although this method is simple, the improvement effect is affected by the stability margin and dynamic performance of the system. For the improved control method of impedance reshaping, it mainly reshapes the unfavorable frequency band of the equivalent impedance in the system, but the structure is mostly complex. Among the existing control strategies, the grid-type control can work independently without external grid phase information to provide frequency and voltage support for the system. Among them, the grid-type synchronization unit can greatly improve the stability of the grid-connected converter under weak grids. Summary of the invention

[0005] In view of the problem of subsynchronous oscillation generated in the existing grid-connected converter control method under weak power grid conditions, the present invention provides a method for improving the stability of the grid-connected converter under weak power grid conditions.

[0006] A method for improving the stability of a grid-connected converter under weak power grid conditions of the present invention comprises:

[0007] The reference value and actual value of the DC bus voltage of the grid-connected converter are detected to obtain the grid-connected synchronous unit:

[0008]

[0009] Among them, θs Indicates the grid connection angle in the grid-type synchronization unit, ω s represents the angular frequency in the network-type synchronization unit, U dc and u dc Indicates the reference value and actual value of the DC bus voltage, K T , K J , K D represents the first-order transfer function link coefficient, ω0 represents the system reference angular frequency, and s is the Laplace operator;

[0010] A grid-connected synchronization unit is added to the phase-locked loop control link, and the grid-connected angle of the grid-connected converter is controlled by adjusting the control coefficient of the phase-locked loop and the control coefficient of the grid-connected synchronization unit.

[0011] As a preferred embodiment, according to the actual value u of the DC bus voltage dc Transfer function G from grid connection angle dc_θ (s) for K T , K J , K D Conduct design;

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

[0013]

[0014] in, k2 represents the control coefficient related to the network-type synchronization unit, U d Indicates the grid voltage signal U abc The reference value of the d-axis DC component, S d represents the d-axis duty cycle of the grid-connected converter, T s Indicates the sampling period of the networking synchronization unit, I d ,I q Respectively represent the grid-connected point current signal I abc The reference value of the d and q axis DC components, R g , L g Respectively represent the grid side resistance and inductance values.

[0015] Preferably, the method of the present invention further comprises: T , K J , K D To perform tuning:

[0016]

[0017] Preferably, the phase-locked loop control link includes:

[0018]

[0019] Where θ represents the actual grid connection angle after the grid-forming synchronization unit is added to the phase-locked loop control link, ω hybrid represents the actual angular frequency after the meshing type synchronization unit is added to the phase-locked loop control link, k1+k2=1, k1 represents the control coefficient related to the phase-locked loop, U q and u q Respectively represent the grid connection point voltage signal U abc The reference value and actual value of the q-axis DC component, k p_pll is the proportional parameter of the phase-locked loop PI control, k i_pll It is the integral parameter of the phase-locked loop PI control.

[0020] The beneficial effects of the present invention are as follows: the present invention aims at the subsynchronous oscillation problem of the grid-connected converter under the weak power grid, directly introduces the grid-connected synchronization unit on the basis of the traditional phase-locked loop synchronization structure, effectively suppresses the subsynchronous oscillation problem of the system under the weak power grid, improves the adaptability of the grid-connected control system to the weak power grid, and enables the control system to maintain stable operation under the experimental condition of low short-circuit ratio. The present invention adjusts the grid connection angle of the actual system by improving the phase-locked loop technology, and the control structure is simple and the effect is obvious. At the same time, this technology not only significantly improves the performance of the control system, but also can ensure that the control system has a higher phase-locked bandwidth and has excellent dynamic performance. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0022] Figure 2 Schematic diagram of the principle of the network-type synchronization unit; Δθ s is the small angle signal component in the synchronization unit, θ ref is the steady-state value of the output angle, θ s is the grid connection angle in the synchronization unit;

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

[0024] Figure 4This is a block diagram of the traditional grid-connected converter phase-locked loop synchronization link structure;

[0025] Figure 5 A block diagram for improving the phase-locked loop technology. DETAILED DESCRIPTION

[0026] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0027] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other.

[0028] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments, but they are not intended to limit the present invention.

[0029] The method for improving the stability of a grid-connected converter under weak grid conditions in this embodiment includes:

[0030] Step 1: Detect the steady-state value and actual value of the DC bus voltage of the grid-connected converter, obtain the relationship between the DC bus voltage and the grid-connected angle based on real-time data, and obtain the grid-connected synchronization unit:

[0031]

[0032] In the above formula, θ s Indicates the grid connection angle in the grid-type synchronization unit, ω s represents the angular frequency in the network-type synchronization unit, U dc and u dc Indicates the reference value and actual value of the DC bus voltage, K T , K J , K D represents the first-order transfer function coefficient, ω0 represents the system reference angular frequency, and s is the Laplace operator.

[0033] According to the real-time data, the relationship between the DC 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 from grid connection angle dc_θ (s) is expressed as:

[0035]

[0036] Among them, k2 represents the control coefficient related to the network-type synchronization unit, U d Indicates the three-phase winding voltage signal U abc The reference value of the d-axis DC component, S d represents the d-axis duty cycle of the grid-connected converter, T s Indicates the sampling period of the networking synchronization unit, I d ,I q Respectively represent the three-phase winding current signal I abc The reference value of the d and q axis DC components, R g , L g Respectively represent the grid side resistance and inductance values.

[0037] Due to T s is much larger than the transfer function cutoff frequency, so G dc_θ (s) can be simplified as:

[0038]

[0039] Adjust the parameters according to the Type II system:

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

[0041]

[0042] Among them, K and τ are the transfer function coefficients that need to be adjusted, and T is the time constant of the first-order inertia link.

[0043] According to the principle of self-control, the intermediate frequency bandwidth determines the response speed of the system, and the phase margin of the control system determines the stability of the system. Determining the intermediate frequency bandwidth and phase margin can adjust the parameters of K and τ in the open-loop transmission function of the type II system. Referring to the above description, in this implementation, the intermediate frequency bandwidth multiple of the system is designed to be 5, and the parameter relationship can be obtained:

[0044]

[0045] Finally, we get:

[0046]

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

[0048] The small signal component of the DC bus voltage of the grid-connected converter is passed through the first-order transfer function link and the integral link in the grid-type synchronization unit, and the phase small signal component χ is obtained according to the calculation, and is introduced into the phase calculation output end of the phase-locked loop;

[0049]

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

[0051]

[0052] Among them, θ PLL Indicates the grid connection angle of the phase-locked loop output, ω PLL Indicates the angular frequency of the phase-locked loop output, U q and u q Indicates voltage signal U abc The reference value and actual value of the q-axis DC component, k p_pll is the proportional parameter of the phase-locked loop PI control, k i_pll It is the integral parameter of the phase-locked loop PI control.

[0053] The small phase signal component χ is introduced into the phase calculation output of the phase-locked loop to obtain the actual grid-connected angle of the system after the improved phase-locked loop, which is specifically expressed as:

[0054]

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

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

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

[0058] Although the present invention is described herein with reference to specific embodiments, it should be understood that these embodiments are merely examples of the principles and applications of the present invention. It should therefore be understood that many modifications may be made to the exemplary embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims. It should be understood that the various dependent claims and features described herein may be combined in a manner different from that described in the original claims. It should also be understood that features described in conjunction with individual embodiments may be used in other described embodiments.

Claims

1. A method for improving the stability of a grid-connected converter under weak grid conditions, characterized in that: include: The reference value and actual value of the DC bus voltage of the grid-connected converter are detected to obtain the grid-connected synchronous unit: Among them, θ s Indicates the grid connection angle in the grid-type synchronization unit, ω s represents the angular frequency in the network-type synchronization unit, U dc and u dc Indicates the reference value and actual value of the DC bus voltage, K T , K J , K D represents the first-order transfer function link coefficient, ω0 represents the system reference angular frequency, and s is the Laplace operator; A grid-connected synchronization unit is added to the phase-locked loop control link, and the grid-connected angle of the grid-connected converter is controlled by adjusting the control coefficient of the phase-locked loop and the control coefficient of the grid-connected synchronization unit.

2. The method for improving the stability of a grid-connected converter under weak grid conditions according to claim 1, characterized in that: According to the actual value of the DC bus voltage u dc Transfer function G from grid connection angle dc_θ (s) for K T , K J , K D Conduct design; Transfer function G dc_θ (s) is: in, k2 represents the control coefficient related to the network-type synchronization unit, U d Indicates the grid voltage signal U abc The reference value of the d-axis DC component, S d represents the d-axis duty cycle of the grid-connected converter, T s Indicates the sampling period of the networking synchronization unit, I d ,I q Respectively represent the grid-connected point current signal I abc The reference value of the d and q axis DC components, R g , L g Respectively represent the grid side resistance and inductance values.

3. The method for improving the stability of a grid-connected converter under weak grid conditions according to claim 2, characterized in that: The method also includes, according to the type II system, T , K J , K D To perform tuning:

4. The method for improving the stability of a grid-connected converter under weak grid conditions according to claim 1, characterized in that: The phase-locked loop control link includes: Where θ represents the actual grid connection angle after the grid-forming synchronization unit is added to the phase-locked loop control link, ω hybrid represents the actual angular frequency after the meshing type synchronization unit is added to the phase-locked loop control link, k1+k2=1, k1 represents the control coefficient related to the phase-locked loop, U q and u q Respectively represent the grid connection point voltage signal U abc The reference value and actual value of the q-axis DC component, k p_pll is the proportional parameter of the phase-locked loop PI control, k i_pll It is the integral parameter of the 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 a processor, the steps of the method for improving the stability of a grid-connected converter under weak grid conditions as claimed in any one of claims 1 to 4 are implemented.

6. A device for improving the stability of 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 according to any one of claims 1 to 4.

7. A computer program product, comprising a computer program, characterized in that When the computer program is executed by a processor, the steps of the method for improving the stability of a grid-connected converter under weak grid conditions as claimed in any one of claims 1 to 4 are implemented.

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