Dynamic stability enhancement control method for grid-connected converter under asymmetric short-circuit fault in weak power grid

Abstract

The present invention discloses a dynamic stabilization control method for a grid-connected converter under an asymmetric short-circuit fault in a weak power grid. The damping of the negative-sequence phase-locked loop of the system is increased by adding an adaptive damping controller to the negative-sequence current loop, thereby improving the small signal stability of the grid-connected converter during an asymmetric short-circuit fault in a weak power grid. The method redesigns the control strategy of the grid-connected converter without adding hardware equipment. The dynamic stabilization control strategy proposed by the present invention is used during an asymmetric short-circuit fault in a weak power grid to enhance the dynamic stability of the system during the asymmetric fault and improve the success rate of crossing an asymmetric short-circuit fault in a weak power grid.

Description

Technical Field

[0001] The present invention belongs to the field of new energy grid-connected converter control, and in particular relates to a dynamic stabilization control method for a grid-connected converter under an asymmetric short-circuit fault in a weak power grid. Background Art

[0002] Power electronic converters are grid-friendly and are an important interface between new energy power generation systems and the grid. In new energy power generation bases represented by solar power stations and wind farms, more and more renewable energy is connected to the grid through power electronic converters. The "double-high" power system, that is, a high proportion of new energy and a high proportion of power electronic equipment, has become an important trend in the development of the power system. In addition, my country's new energy power generation field has two significant characteristics: large-scale development and long-distance transmission. This development method is also very likely to cause safety failures in the operation of new energy grid connection.

[0003] Under the mode of centralized development and long-distance transmission, as the scale of grid connection of new energy power generation bases continues to increase, the number of power electronic power generation equipment has increased sharply, resulting in the weakening of the dominant characteristics of synchronous machines, the weakening of the power grid strength, and the influence of transmission line impedance on system stability has become increasingly significant. Especially during the asymmetric short-circuit fault of weak power grid, the control of the new energy power generation system is complicated. Affected by the strong nonlinearity of the system and the coupling of positive and negative sequence impedance, the dynamic coupling degree between the new energy power generation base and the AC power grid is further aggravated. The dynamic response of the system presents the characteristics of multiple time scales, and the oscillation presents the characteristics of wide band, which makes the stability problem of the grid-connected converter under the asymmetric short-circuit fault of weak power grid worse.

[0004] Therefore, the small signal stability enhancement control strategy of the grid-connected converter during the asymmetric short-circuit fault of the weak grid has attracted more and more attention. At present, scholars at home and abroad have conducted research on the stability enhancement control technology applicable to the grid-connected converter under the asymmetric short-circuit fault of the weak grid, such as the following published documents:

[0005] (1) XIONG Hao, DU Xiong, SUN Pengju, JI Yongliang. Active power safe operation area of ​​three-phase grid-connected converter during asymmetric grid faults[J]. Proceedings of the CSEE, 2018, 38(20): 6110-6118.

[0006] (2) Zhao Lei, Yang Hui. Optimal control of grid-connected converter under grid voltage asymmetry fault[J]. Micromotors, 2018, 51(05): 52-58.

[0007] Reference (1) proposed an optimization control method to minimize the maximum phase current of the grid-connected converter during an asymmetric short-circuit fault in the power grid, which maximizes the injection of active power while ensuring the safety of the converter itself, thereby improving the maximum active power output capability of the system and the control stability during the fault. However, this method does not consider how to enhance the small signal stability of the grid-connected converter during an asymmetric short-circuit fault in a weak power grid.

[0008] Reference (2) designed an optimization control strategy for grid-connected converters under asymmetric short-circuit faults in the power grid. By suppressing the double-frequency oscillation of the DC side voltage and the grid-side reactive power, the grid-connected converter's ability to operate safely and stably under asymmetric grid voltage was improved. The rationality of the capacitor current resonant closed loop suppressing the double-frequency pulsation of the DC side voltage and the reactive power resonant closed loop suppressing the double-frequency pulsation of the grid-side reactive power was theoretically analyzed. However, this method does not consider the small signal stability of the grid-connected converter system during asymmetric short-circuit faults in weak power grids, and does not conduct in-depth research on how to improve the small signal stability of the system during faults. Summary of the invention

[0009] In view of the above-mentioned deficiencies in the prior art, the purpose of the present invention is to propose a dynamic stabilization control method that can improve the small signal stability of a grid-connected converter during an asymmetric short-circuit fault in a weak power grid. This method redesigns the control strategy of the grid-connected converter without adding hardware equipment. Using the dynamic stabilization control strategy proposed in the present invention, the small signal stability of the system during an asymmetric fault can be enhanced, and the success rate of crossing an asymmetric short-circuit fault in a weak power grid can be improved.

[0010] The technical solution of the present invention is achieved in this way:

[0011] A dynamic stabilization control method for a grid-connected converter under an asymmetric short-circuit fault in a weak power grid is characterized in that: during the fault period, an adaptive damping controller is added to the negative-sequence current loop to increase the damping of the system's negative-sequence phase-locked loop, thereby improving the small signal stability of the grid-connected converter during the asymmetric short-circuit fault in the weak power grid.

[0012] The specific control steps are as follows:

[0013] 1) When an asymmetric short-circuit fault is detected in a weak grid, the system switches to a fault ride-through control strategy and uses a negative-sequence current loop to control the negative-sequence current. And use the negative sequence phase locked loop to detect the negative sequence voltage phase angle θ pll－ ;

[0014] 2) Detect the location of the asymmetric short-circuit fault in the power network, measure the fault impedance, and calculate the sequence impedance and inter-sequence coupling impedance of the faulty power grid; and are the positive sequence impedance and negative sequence impedance of the fault power grid;

[0015] 3) Calculate the damping rate ξ of the negative sequence phase-locked loop according to the following formula, where k p2 and k i2 are the proportional coefficient and integral coefficient of the PI controller in the negative sequence phase locked loop; is the negative sequence inductive reactance of the faulty grid, is the negative sequence reactive current during the fault period; is the negative sequence d-axis component of the grid connection point voltage;

[0016]

[0017] 4) If ξ is less than 0, an adaptive damping controller is designed to be added to the negative sequence current loop; the adaptive damping controller is configured on the negative sequence d axis current loop; by adjusting the proportional coefficient k of the PI controller applied in the adaptive damping controller cp and the integration coefficient k ci , so that the damping rate of the negative sequence phase-locked loop after the application of the adaptive damping controller is ξ * is a constant greater than 0; the PI controller applied in the adaptive damping controller is used to adjust the phase-locked loop output angular velocity ω pll－ and the system target angular velocity ω g－ The deviation between pll－ Negative feedback is given to the output of the negative sequence d-axis current loop; once ω pll－ During the fault period, the deviation g－ , the converter will be based on Δω pll－ The q-axis current component of its output negative sequence current is automatically adjusted; thereby enhancing the dynamic stability of the system during a fault and improving the asymmetric short-circuit fault ride-through capability of the system.

[0018] The present invention adjusts the proportional coefficient k of the PI controller used in the adaptive damping controller according to the following formula: cp and the integration coefficient k ci , so that the damping rate of the negative sequence phase-locked loop after the application of the adaptive damping controller is ξ * is a constant greater than 0, where is the negative sequence resistance value of the faulty power grid;

[0019]

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] The method redesigns the control strategy of the grid-connected converter without adding hardware equipment. The dynamic stabilization control strategy proposed by the present invention can enhance the small signal stability of the system during asymmetric faults and improve the success rate of weak grid asymmetric short circuit fault crossing. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is an asymmetric short-circuit fault ride-through control strategy for grid-connected converters.

[0023] Figure 2 It is a dual-sequence phase-locked loop control structure.

[0024] Figure 3 This is the dynamic stabilization control strategy proposed in the present invention.

[0025] Figure 4 The figure is a comparison chart of simulation waveforms with and without the application of dynamic stabilization control strategy. DETAILED DESCRIPTION

[0026] The specific embodiments of the present invention are described in detail below with reference to the accompanying drawings.

[0027] like Figure 1 As shown, the grid-connected converter needs to control the positive-sequence reactive current and the negative-sequence reactive current at the same time. Because the response speed of the external control loop (power control) is slower than that of the internal control loop (current control), in order to ensure the timeliness of current control, the grid-connected converter generally only uses the internal control loop during fault ride-through. During an asymmetric fault, the system generally uses a dual-sequence current loop to control the positive-sequence current and the negative-sequence current—introducing a negative-sequence current loop to control the negative-sequence current. When the system detects an asymmetric short-circuit fault in the power grid, Flag = 2, and the system immediately switches to the fault ride-through control strategy in the box: the outer power loop is removed, and the reactive current instruction It is given by direct calculation based on the fault degree and (1).

[0028]

[0029] The relevant symbols involved in the figures and formulas of the present invention are explained as follows: and They are positive-sequence / negative-sequence reactive current instructions respectively. and They are positive sequence / negative sequence active current instructions respectively. g and Q g are the system active power and reactive power, P gref and Q gref They are active power command and reactive power command respectively. and They are the positive / negative sequence d-axis components of the grid-connected point voltage respectively. and The positive and negative sequence currents of the system output detected by the phase-locked loop. and The positive and negative sequence voltages at the grid connection point detected by the phase-locked loop. and It is the positive sequence voltage and negative sequence voltage of the system output terminal detected by the phase-locked loop. f Represents the filter inductance on the output side of the converter. G and Z G Represent the grid voltage and grid impedance respectively. The grid is divided into remote line Z g1 and near-end line Z g2 In addition, U t Represents the system grid-connected point voltage. V g and I g Represent the output voltage and current of VSC respectively. pll+ and θ pll－ It is the positive sequence phase locked loop output angle and the negative sequence phase locked loop output angle.

[0030] Depend on Figure 2 It can be seen that during an asymmetric short circuit fault, the system will use a dual-sequence phase-locked loop to detect the phase angle of the positive-sequence and negative-sequence grid-connected point voltages. tαβ First, it is decomposed into a positive sequence reference voltage by the positive and negative sequence voltage separation algorithm and negative sequence reference voltage Subsequently, the q-axis components of the positive-sequence reference voltage and the negative-sequence reference voltage will enter the independent positive-sequence phase-locked loop and the negative-sequence phase-locked loop to obtain the phase angle θ of the positive-sequence and negative-sequence grid-connected point voltages. pll+ and θ pll－ .

[0031] Figure 3 The adaptive damping controller added to the negative sequence current loop is designed for the dynamic stabilization control strategy. The adaptive damping controller is configured on the negative sequence d-axis current loop, which converts the phase-locked loop output angular velocity ω through the PI controller. pll－ and the system target angular velocity ω g－ (-50Hz) Deviation Δω pll－ Negative feedback is given to the output of the negative sequence d-axis current loop. Once ω pll－ During the fault period, the deviation g－ , the converter will be based on Δω pll－ Automatically adjust the q-axis current component of its output negative sequence current.

[0032] The specific implementation steps of the present invention are as follows:

[0033] 1) When an asymmetric short circuit fault is detected in a weak grid, the system switches to a fault ride-through control strategy and uses a negative sequence current loop to control the negative sequence current. And use the negative sequence phase locked loop to detect the negative sequence voltage phase angle θ pll－ ,like Figure 1 and Figure 2 .

[0034] 2) Use fault location technology to detect the location of the asymmetric short-circuit fault in the power network, measure the fault impedance, and calculate the sequence impedance and inter-sequence coupling impedance of the faulty power grid; and are the positive sequence impedance and negative sequence impedance of the fault power grid;

[0035] 3) Calculate the damping rate ξ of the negative sequence phase-locked loop according to the following formula (2), where k p2 and k i2 are the proportional coefficient and integral coefficient of the PI controller in the negative sequence phase locked loop; is the negative sequence inductive reactance of the faulty grid, is the negative sequence reactive current during the fault period; It is the negative sequence d-axis component of the grid-connected point voltage.

[0036]

[0037] 4) If ξ is less than 0, then design an adaptive damping controller to be added to the negative sequence current loop, such as Figure 3 The adaptive damping controller is configured on the negative sequence d-axis current loop, which converts the phase-locked loop output angular velocity ω through the PI controller. pll－ and the system target angular velocity ω g－ (-50Hz) Deviation Δω pll－ Negative feedback is given to the output of the negative sequence d-axis current loop. Once ω pll－ During the fault period, the deviation g－ , the converter will be based on Δω pll－ Automatically adjust the q-axis current component of its output negative sequence current. The dynamic stability of the system during faults will be enhanced, and the asymmetric short-circuit fault ride-through capability of the system will be improved. cp and k ci are the proportional coefficient and integral coefficient of the PI controller used in the adaptive damping controller. is the negative sequence resistance value of the faulty power grid. Adjust k according to the following formula (3): cp and k ci To * is a constant greater than 0, where ξ * is the damping rate of the negative sequence phase locked loop after applying the adaptive damping controller.

[0038]

[0039] Compared with the prior art, the present invention has the following beneficial effects:

[0040] The method redesigns the control strategy of the grid-connected converter without adding hardware equipment. The dynamic stabilization control strategy proposed by the present invention can enhance the small signal stability of the system during asymmetric faults and improve the success rate of weak grid asymmetric short circuit fault crossing.

[0041] Effect description of the present invention:

[0042] Figure 4 The following is a comparison of the simulated waveforms of the grid-connected converter system before and after the dynamic stabilization control strategy of the present invention is applied after an asymmetric short circuit occurs in the weak grid. In the figure, a short circuit occurs between phases a and b of the system at 1.5s to 2.5s. gabc is the output three-phase current, U tabc is the three-phase voltage at the grid connection point. It can be seen from Figure A that during the asymmetric short-circuit fault, the grid-connected converter system suffers from small signal instability, and the harmonic content in the output current and the grid-connected voltage is too high. The dynamic stability of the system during the fault is very poor, which reduces the success rate of asymmetric short-circuit fault crossing. Figure B is a simulation waveform diagram after applying the dynamic stabilization control strategy of the present invention. It can be seen from the figure that during the fault, by applying the dynamic stabilization control strategy, the small signal stability of the grid-connected converter during the asymmetric short-circuit fault of the weak power grid is effectively improved, the harmonic content in the output current and the grid-connected voltage of the system is reduced, and the success rate of asymmetric short-circuit fault crossing is improved.

[0043] In summary, the dynamic stabilization control strategy of a grid-connected converter under an asymmetric short-circuit fault in a weak power grid described in the present invention has the following advantages: it improves the damping of the grid-connected converter system during an asymmetric short-circuit fault in a weak power grid, effectively improves the small signal stability of the grid-connected converter system during an asymmetric short-circuit fault, and improves the success rate of the converter system's asymmetric short-circuit fault crossing.

[0044] Finally, it should be noted that the above examples of the present invention are merely examples for illustrating the present invention, and are not intended to limit the embodiments of the present invention. Although the applicant has described the present invention in detail with reference to the preferred embodiments, for those of ordinary skill in the art, other different forms of changes and modifications can be made based on the above description. It is impossible to list all the embodiments here. Any obvious changes or modifications derived from the technical solution of the present invention are still within the scope of protection of the present invention.

Claims

1. A dynamic stabilization control method for a grid-connected converter under asymmetric short-circuit fault in a weak power grid. Features: During the fault period, the damping of the system negative sequence phase locked loop is increased by adding an adaptive damping controller to the negative sequence current loop, thereby improving the small signal stability of the grid-connected converter during asymmetric short circuit faults in weak grids. The specific control steps are as follows: 1) When an asymmetric short-circuit fault is detected in a weak grid, the system switches to a fault ride-through control strategy and uses a negative-sequence current loop to control the negative-sequence current. And use the negative sequence phase locked loop to detect the negative sequence voltage phase angle θ pll－ ; 2) Detect the location of the asymmetric short-circuit fault in the power network, measure the fault impedance, and calculate the sequence impedance and inter-sequence coupling impedance of the faulty power grid; and are the positive sequence impedance and negative sequence impedance of the fault power grid; 3) Calculate the damping rate ξ of the negative sequence phase-locked loop according to the following formula, where k p2 and k i2 are the proportional coefficient and integral coefficient of the PI controller in the negative sequence phase locked loop; is the negative sequence inductive reactance of the faulty grid, is the negative sequence reactive current during the fault period; is the negative sequence d-axis component of the grid connection point voltage; 4) If ξ is less than 0, an adaptive damping controller is designed and added to the negative sequence current loop; The adaptive damping controller is configured on the negative sequence d-axis current loop; by adjusting the proportional coefficient k of the PI controller applied in the adaptive damping controller cp and the integration coefficient k ci , so that the damping rate of the negative sequence phase-locked loop after the application of the adaptive damping controller is ξ * is a constant greater than 0; the PI controller applied in the adaptive damping controller is used to adjust the phase-locked loop output angular velocity ω pll－ and the system target angular velocity ω g－ The deviation between pll－ Negative feedback is given to the output of the negative sequence d-axis current loop; once ω pll－ During the fault period, the deviation g－ , the converter will be based on Δω pll－ Automatically adjust the q-axis current component of its output negative sequence current; thereby enhancing the dynamic stability of the system during faults and improving the system's asymmetric short-circuit fault ride-through capability; The proportional coefficient k of the PI controller used in the adaptive damping controller is adjusted according to the following formula: cp and the integration coefficient k ci , so that the damping rate of the negative sequence phase-locked loop after the application of the adaptive damping controller is ξ * is a constant greater than 0, where is the negative sequence resistance value of the faulty power grid;

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