Non-isolated high turns ratio bidirectional AC / DC converter control optimization method and system

Abstract

The present invention provides a control optimization method and system for a non-isolated high turns ratio bidirectional AC / DC converter, including: introducing a k-th harmonic notch filter to the feedback signal of the DC-side resonant circulating current of a high-voltage series voltage divider to eliminate the k-th current harmonic on the DC side of the converter; obtaining the quasi-PR control parameters of the voltage equalization inner loop to control the resonant circulating current; obtaining the PI control parameters of the voltage equalization outer loop, the voltage stabilization inner loop and the voltage stabilization outer loop, and introducing a resonant frequency notch filter in the feedback link to filter out the resonant current interference; and / or, adopting a double closed-loop to control the voltage and frequency at the PCC point of the wind farm grid connection, and introducing a current inner loop; using a k-th harmonic notch filter to filter out the k-th harmonic current of the DC-side resonant circulating current of the high-voltage series voltage divider to obtain a low-harmonic voltage equalization inner loop resonant current feedback signal; obtaining the PI control parameters of the voltage outer loop and the current inner loop, and introducing k-th and 2k-th harmonic notch filters in the feedback link to eliminate the influence of k±1 and 2k±1 voltage and current harmonics.

Description

Technical Field

[0001] The present invention relates to the field of power electronics technology, and specifically, to a control optimization method and system for a non-isolated high turns ratio bidirectional AC / DC converter for hybrid commutation. Background Art

[0002] In order to achieve high-voltage power extraction, a converter capable of realizing the conversion from high-voltage DC to low-voltage three-phase AC is required. The modular multilevel converter (MMC) is constructed by sub-modules and is a high-voltage and large-capacity AC / DC converter. However, when the DC side of the MMC is at a high voltage, limited by its modulation ratio, it is difficult to obtain low-voltage three-phase alternating current on the AC side, and a transformer must be added. Moreover, the traditional MMC has a large number of modules, so the cost is relatively high when it is used for the conversion from high-voltage DC to low-voltage AC.

[0003] After retrieval, it is found that:

[0004] The Chinese patent application for invention "Hybrid Offshore Wind Farm DC Converter" with the application number 201711288924.6 uses a high-voltage capacitor to resonate with the AC port (arm inductor). The converter designed by this method has low ability to suppress circulating current, relatively strict working conditions, difficult tuning of control parameters, and is not conducive to engineering implementation. In addition, the asymmetric structural characteristics of the converter may cause a DC offset in the AC side output voltage. The AC port needs to be connected to a power frequency transformer for isolation, and has high requirements for the insulation strength of the power frequency transformer.

[0005] The Chinese patent application for invention "AC / DC Converter and Control Method of Hybrid Offshore Wind Farm DC Converter" with the application number 201711080781.X is based on the above-mentioned "Hybrid Offshore Wind Farm DC Converter". The low-voltage AC port adopts open-loop control, so the system has a risk of instability and cannot adjust the disturbance at the output end. The control of the high-voltage series voltage divider does not consider the input power on the DC side and completely relies on stabilizing the capacitor voltage of the sub-module to achieve the purpose of power control. This results in very high requirements for the control accuracy of the circulating current command generation and also has a risk of system instability.

[0006] The Chinese invention patent application "High-ratio Bidirectional AC / DC Converter and Its Control Method, Pre-charging Method" with the application number 202010951461.2 improves the topology and control structure of the AC / DC converter based on the above-mentioned "AC / DC Converter and Control Method of Hybrid Offshore Wind Farm DC Converter". First, for the inner loop of the circulating current control of the high-voltage series voltage divider, only proportional control is adopted, which will generate static errors. Second, due to the existence of resonance current, in addition to the DC component, the average voltage of the sub-module also has an AC component with the resonance frequency. As the feedback signal for the voltage stabilization control of the sub-module, it will introduce an AC component, affecting the effect of its PI control. Third, the control structure of the AC / DC converter is complex, but the basis for the value of the control parameters is not given, which is not conducive to guiding the design of the actual system.

[0007] In summary, the existing AC / DC converters have defects such as poor control effect and difficult design of control parameters. Currently, no description or report of similar technologies to the present invention has been found, and no similar materials at home and abroad have been collected. Summary of the Invention

[0008] In view of the above deficiencies in the prior art, the present invention provides an optimized control method and system for a non-isolated high-ratio bidirectional AC / DC converter for hybrid commutation applicable to a wind power hybrid transmission scheme. At the same time, a corresponding terminal and a computer-readable storage medium are provided.

[0009] According to one aspect of the present invention, an optimized control method for a non-isolated high-ratio bidirectional AC / DC converter is provided, including: optimizing the control of the high-voltage series voltage divider and / or optimizing the control of the low-voltage AC port; wherein:

[0010] The optimization of the control of the high-voltage series voltage divider includes:

[0011] According to the number of pulses k of the uncontrolled rectifier, a notch filter with the kth harmonic as the center frequency is introduced into the feedback signal of the DC-side resonance circulating current of the high-voltage series voltage divider to eliminate the kth current harmonic on the DC side of the converter;

[0012] Obtain the quasi-PR control parameters of the voltage equalization inner loop, and use a quasi-PR controller to control the resonance circulating current;

[0013] Obtain the PI control parameters of the voltage equalization outer loop, the voltage stabilization inner loop, and the voltage stabilization outer loop, and introduce a notch filter with the resonance frequency into the feedback links of the voltage equalization outer loop, the voltage stabilization inner loop, and the voltage stabilization outer loop of the sub-module capacitor of the AC / DC converter to filter out the interference generated by the resonance current;

[0014] The optimization of the control of the low-voltage AC port includes:

[0015] The voltage and frequency at the PCC point of the wind farm grid connection are controlled by a double closed-loop, and a current inner loop is introduced to add virtual damping to enhance the stability of the system;

[0016] According to the number of pulses k of the uncontrolled rectifier, the corresponding k-th harmonic trap is used to filter the k-th harmonic current of the DC-side resonant circulating current of the high-voltage series voltage divider, and a low-harmonic equalizing inner-loop resonant current feedback signal is obtained;

[0017] The PI control parameters of the voltage outer loop and the current inner loop are obtained, and according to the number of pulses k of the rectifier converter station, traps centered on the k-th and 2k-th harmonics are respectively introduced into the three-phase output voltage and current feedback signals of the low-voltage AC port to eliminate the k±1 and 2k±1 harmonics of the voltage and current on the AC side of the uncontrolled rectifier converter station.

[0018] According to another aspect of the present invention, a non-isolated high-voltage ratio bidirectional AC / DC converter control optimization system is provided, including: a high-voltage series voltage divider control optimization part and / or a low-voltage AC port control optimization part; wherein:

[0019] The high-voltage series voltage divider control optimization part includes:

[0020] A trap module in the high-voltage series voltage divider control structure, which introduces a trap centered on the k-th harmonic to the feedback signal of the DC-side resonant circulating current of the high-voltage series voltage divider according to the number of pulses k of the uncontrolled rectifier to eliminate the k-th current harmonic on the DC side of the converter;

[0021] A quasi-PR control module, which obtains the quasi-PR control parameters of the equalizing inner loop and uses a quasi-PR controller to control the resonant circulating current;

[0022] A PI control module in the high-voltage series voltage divider control structure, which obtains the PI control parameters of the equalizing outer loop, the voltage stabilizing inner loop and the voltage stabilizing outer loop, and introduces a trap of the resonant frequency in the feedback links of the equalizing outer loop, the voltage stabilizing inner loop and the voltage stabilizing outer loop of the AC / DC converter sub-module capacitor to filter out the interference generated by the resonant current;

[0023] The low-voltage AC port control optimization part includes:

[0024] A double closed-loop control module, which uses a double closed-loop to control the voltage and frequency at the PCC point of the wind farm grid connection, and introduces a current inner loop to add virtual damping to enhance the stability of the system;

[0025] A trap module in the low-voltage AC port control structure, which filters the k-th harmonic current of the DC-side resonant circulating current of the high-voltage series voltage divider by using the corresponding k-th harmonic trap according to the number of pulses k of the uncontrolled rectifier to obtain a low-harmonic equalizing inner-loop resonant current feedback signal;

[0026] The PI control module in the low - voltage AC port control structure obtains the PI control parameters of the voltage outer loop and the current inner loop, and according to the pulse number k of the rectifier - inverter station, notch filters centered on the k - th and 2k - th harmonics are respectively introduced into the three - phase output voltage and current feedback signals of the low - voltage AC port to eliminate the voltage and current harmonics of the (k±1) - th and (2k±1) - th orders on the AC side of the uncontrolled rectifier - inverter station.

[0027] According to the third aspect of the present invention, a terminal is provided, including a memory, a processor, and a computer program stored on the memory and executable on the processor. When the processor executes the program, it can be used to execute the method described in any one of the above.

[0028] According to the fourth aspect of the present invention, a computer - readable storage medium is provided, on which a computer program is stored. When the program is executed by a processor, it can be used to execute the method described in any one of the above.

[0029] Due to the adoption of the above - mentioned technical solutions, compared with the prior art, the present invention has at least one of the following beneficial effects:

[0030] The non - isolated high - turn - ratio bidirectional AC / DC converter control optimization method and system provided by the present invention optimize the high - voltage series voltage divider control structure of the existing non - isolated high - turn - ratio bidirectional AC / DC converter, and improve the control effects of the AC / DC converter on sub - module voltage sharing, voltage stabilization, and circulating current.

[0031] The non - isolated high - turn - ratio bidirectional AC / DC converter control optimization method and system provided by the present invention perform small - signal modeling on the circuit structure of the existing non - isolated high - turn - ratio bidirectional AC / DC converter, give the design basis for the control parameters of the high - voltage series voltage divider and the low - voltage AC port, and further guide the actual system.

[0032] The non - isolated high - turn - ratio bidirectional AC / DC converter control optimization method and system provided by the present invention can effectively reduce the influence of the resonant circulating current on the steady - state control effect of the high - voltage series voltage divider, and at the same time reduce the influence of the (k±1) - th and (2k±1) - th harmonics generated by the k - pulse uncontrolled rectification on the control effect of the low - voltage AC port.

[0033] The non - isolated high - turn - ratio bidirectional AC / DC converter control optimization method and system provided by the present invention can facilitate the tuning of the control parameters of the high - voltage series voltage divider and the low - voltage AC port. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] By reading the detailed description of the non - restrictive embodiments with reference to the following drawings, other features, objects, and advantages of the present invention will become more apparent:

[0035] Figure 1It is the flowchart of the control optimization method for the non-isolated high turns ratio bidirectional AC / DC converter in an embodiment of the present invention; wherein, (a) is the flowchart for optimizing the control of the high-voltage series voltage divider, and (b) is the flowchart for optimizing the control of the low-voltage AC port.

[0036] Figure 2 It is the circuit diagram of the non-isolated high turns ratio bidirectional AC / DC converter in a preferred embodiment of the present invention.

[0037] Figure 3 It is the control block diagram of the high-voltage series voltage divider in a preferred embodiment of the present invention.

[0038] Figure 4 It is the control block diagram of the low-voltage AC port in a preferred embodiment of the present invention.

[0039] Figure 5 It is the average value circuit diagram of the non-isolated high turns ratio bidirectional AC / DC converter in a preferred embodiment of the present invention.

[0040] Figure 6 It is the control block diagram of the sub-module voltage equalization in a preferred embodiment of the present invention; wherein, (a) is the inner loop control block diagram, and (b) is the outer loop control block diagram.

[0041] Figure 7 It is the voltage stabilization equivalent circuit of the inner voltage stabilization loop in a preferred embodiment of the present invention.

[0042] Figure 8 It is the closed-loop transfer function block diagram of the voltage stabilization loop in a preferred embodiment of the present invention; wherein, (a) is the inner loop block diagram, and (b) is the outer loop block diagram.

[0043] Figure 9 It is the circuit structure of the low-voltage AC port in a preferred embodiment of the present invention.

[0044] Figure 10 It is the closed-loop transfer function block diagram of the current inner loop in a preferred embodiment of the present invention.

[0045] Figure 11 It is the circuit structure of the control object of the outer loop of the low-voltage AC port in a preferred embodiment of the present invention.

[0046] Figure 12 It is the closed-loop transfer function block diagram of the AC voltage outer loop in a preferred embodiment of the present invention.

[0047] Figure 13 It is the waveforms of various variables in the wind farm black start stage in a preferred embodiment of the present invention.

[0048] Figure 14 It is the waveforms of the dynamic characteristics of each outer loop control quantity in a preferred embodiment of the present invention.

[0049] Figure 15 Schematic diagram of the composition structure for optimizing the control of a non-isolated high-ratio bidirectional AC / DC converter according to an embodiment of the present invention; among them, (a) is the control optimization part of the high-voltage series voltage divider, and (b) is the control optimization part of the low-voltage AC port. Detailed implementation manners

[0050] The embodiments of the present invention will be described in detail below: These embodiments are implemented on the premise of the technical solution of the present invention, and detailed implementation manners and specific operation processes are given. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can be made, and these all belong to the protection scope of the present invention.

[0051] Figure 1 Flowchart of the working process of the method for optimizing the control of a non-isolated high-ratio bidirectional AC / DC converter provided by an embodiment of the present invention.

[0052] As Figure 1 shown in (a) of [], the method for optimizing the control of a non-isolated high-ratio bidirectional AC / DC converter provided by this embodiment may include: optimizing the control of the high-voltage series voltage divider; among them: S100, optimizing the control of the high-voltage series voltage divider, which may specifically include the following steps:

[0053] S101, according to the number of pulses k of the uncontrolled rectifier, introduce a notch filter with the kth harmonic as the center frequency into the feedback signal of the DC-side resonant circulating current of the high-voltage series voltage divider to eliminate the kth current harmonic on the DC side of the converter;

[0054] S102, obtain the quasi-PR control parameters of the voltage-sharing inner loop, and use a quasi-PR controller to control the resonant circulating current to achieve static zero error of the resonant circulating current;

[0055] S103, obtain the PI control parameters of the voltage-sharing outer loop, the voltage-stabilizing inner loop, and the voltage-stabilizing outer loop, and introduce a notch filter with the resonant frequency into the feedback links of the voltage-sharing outer loop, the voltage-stabilizing inner loop, and the voltage-stabilizing outer loop of the capacitors of the sub-modules of the AC / DC converter (that is, the feedback signal channels of the PI control module) to filter out the interference generated by the resonant current.

[0056] As Figure 1 shown in (b) of [], the method for optimizing the control of a non-isolated high-ratio bidirectional AC / DC converter provided by this embodiment may include: optimizing the control of the low-voltage AC port; among them: S200, optimizing the control of the low-voltage AC port, which may specifically include the following steps:

[0057] S201, adopt double closed-loop control for the voltage and frequency at the PCC point of the wind farm grid connection, and introduce a current inner loop to increase the current inner loop for facilitating the control of additional virtual damping and enhancing the stability of the system;

[0058] S202. According to the number of pulses k of the uncontrolled rectifier, the corresponding k - th harmonic trap is used to filter the k - th harmonic current of the DC - side resonance circulating current of the high - voltage series voltage divider, and a low - harmonic voltage - equalizing inner - loop resonance current feedback signal is obtained.

[0059] S203. Obtain the PI control parameters of the voltage outer - loop and current inner - loop, and according to the number of pulses k of the rectifier converter station, notch filters centered on the k - th and 2k - th harmonics are respectively introduced to the three - phase output voltage and current feedback signals of the low - voltage AC port to eliminate the voltage and current harmonics of k±1 and 2k±1 times on the AC side of the uncontrolled rectifier converter station.

[0060] The control optimization method of the non - isolated high - turns - ratio bidirectional AC / DC converter provided by the embodiment of the present invention may also include: optimizing the control of the high - voltage series voltage divider and optimizing the control of the low - voltage AC port. The specific implementation methods can refer to the above two corresponding embodiments and will not be elaborated here.

[0061] In S102 of this embodiment, as a preferred embodiment, obtaining the quasi - PR control parameters of the voltage - equalizing inner - loop may include the following steps:

[0062] S1021. Construct the quasi - PR controller H of the voltage - equalizing inner - loop PR ;

[0063] S1022. According to the topology structure of the non - isolated high - turns - ratio bidirectional AC / DC converter, obtain the average - value circuit model of the non - isolated high - turns - ratio bidirectional AC / DC converter.

[0064] S1023. According to the average - value circuit model, determine the DC - side circuit relationship of the average - value circuit model.

[0065] S1024. According to the DC - side circuit relationship, obtain the transfer function H(s) of the control object of the voltage - equalizing inner - loop p1 (s);

[0066] S1025. According to the transfer function H(s) of the control object of the voltage - equalizing inner - loop and the control structure of the quasi - PR controller H of the voltage - equalizing inner - loop p1 (s) and the quasi - PR controller H of the voltage - equalizing inner - loop PR control structure, obtain the open - loop transfer function Hopen1(s) of the closed - loop system structure of the voltage - equalizing inner - loop.

[0067] S1026. According to the open - loop transfer function Hopen1(s) of the closed - loop system structure of the voltage - equalizing inner - loop, determine the bandwidth of the voltage - equalizing inner - loop, so that the bandwidth of the voltage - equalizing inner - loop is greater than the resonance frequency, to complete the acquisition of the quasi - PR control parameters of the voltage - equalizing inner - loop.

[0068] In a specific application example of S1021, constructing the quasi - PR controller H PR is:

[0069]

[0070] In Equation (1), K P is the proportional parameter of the H PR controller, and K R is the resonant gain parameter of the H PR controller. ζ is the resonant damping of the H PR controller, and ω n is the resonant frequency of the H PR controller. s is the complex frequency.

[0071] In a specific application example of S1022, according to the non-isolated high-ratio bidirectional AC / DC converter topology, the average value circuit model of the non-isolated high-ratio bidirectional AC / DC converter is obtained, as Figure 5 shown. Among them:

[0072] As Figure 2 shown, the non-isolated high-ratio bidirectional AC / DC converter topology may include: a resonant branch 101, a first high-voltage series voltage divider 102, a second high-voltage series voltage divider 103, and a three-phase low-voltage AC port 104; the resonant branch 101 is composed of a resonant capacitor C bp and a resonant inductor L bp connected in series, serving as the resonant energy channel of the first high-voltage series voltage divider for balancing the input and output powers of the high-voltage sub-module string; the first high-voltage series voltage divider 102 and the second high-voltage series voltage divider 103 are respectively composed of a plurality of cascaded sub-modules and are respectively connected in series at both ends of the three-phase low-voltage AC port 104 for DC voltage division to reduce the DC-side voltage of the three-phase low-voltage AC port 104; each phase bridge arm of the three-phase low-voltage AC port 104 is composed of a series connection of several half-bridge structure sub-module strings and bridge arm inductors for inverting the low DC-side voltage; among them, the sub-module topology adopts a half-bridge structure 105.

[0073] The topology of the non-isolated high-ratio bidirectional AC / DC converter is the actual circuit structure proposed in this embodiment. By performing mathematical modeling and simplification on this actual physical structure, the average value circuit model of the non-isolated high-ratio bidirectional AC / DC converter is obtained. In terms of the average value circuit model, the cascaded sub-module strings of the high-voltage series voltage divider and the bridge arm sub-module strings of the low-voltage AC port in the topology provided in this embodiment are both equivalent through controlled sources, and the equivalent method is the same as that of the traditional modular multilevel converter, which is well known in the art and will not be elaborated here.

[0074] In a specific application example of S1023, according to the average value circuit model, the DC-side circuit relationship of the average value circuit model can be determined, which may include the following steps:

[0075] Referring to the average value circuit model, the following equations can be established:

[0076]

[0077] In Equation (2), m1, i dc , u dc1 , N1, U1, u res and Δu res are respectively the modulation ratio, DC-side injection current, equivalent capacitor voltage, number of sub-modules, rated DC voltage, reference resonant voltage and resonant current control voltage of the high-voltage series voltage divider; m ux , m lx , i ux , i lx , u ux , u lx are respectively the modulation ratios, currents and equivalent capacitor voltages of the upper and lower bridge arms of the x-phase on the low-voltage AC port, where x is the phase and can take a, b, c. For example, Figure 5 corresponds to "a" in x N2, L, U2 and u dc are respectively the number of bridge-arm sub-modules, bridge-arm inductance, rated DC voltage and current modulation voltage of the low-voltage AC port; U dc and C are respectively the total DC-side voltage and sub-module capacitance of the non-isolated high-ratio bidirectional AC / DC converter; Figure 5 The explanations of the variables marked in bp and not involved in Equation (2) are as follows. L bp and C dc are the resonant inductor and resonant capacitor on the DC side of the AC / DC converter; I dc is the DC injection current of the non-isolated high-ratio bidirectional AC / DC converter, and i

[0078] In a specific application example of S1024, obtaining the transfer function H p1 (s) of the control object of the voltage-sharing inner loop according to the DC-side circuit relationship may include the following steps:

[0079] Make the following transformation of Equation (2), as Equation (3)

[0080]

[0081] In Equation (3), i res is the resonant circulating current of the AC / DC converter.

[0082] Combining Equation (2) and Equation (3) can obtain Equation (4), as follows

[0083]

[0084] In Equation (4), ires is the DC-side resonant circulating current of the non-isolated high turns-ratio bidirectional AC / DC converter, L is the arm inductance of the low-voltage AC port, and Δu res is the resonant current control voltage of the high-voltage series voltage divider.

[0085] The transfer function H p1 (s) of the control object of the voltage-sharing inner loop is obtained according to Equation (4) as follows:

[0086]

[0087] In Equation (5), the tilde above the variable represents a small perturbation of the variable.

[0088] In a specific application of S1025, according to the transfer function H p1 (s) of the control object of the voltage-sharing inner loop and the quasi-PR controller H PR control structure of the voltage-sharing inner loop, the open-loop transfer function Hopen1(s) of the closed-loop system structure of the voltage-sharing inner loop can be obtained through the following steps:

[0089] According to Equation (5) and combined with the control structure of the quasi-PR controller of the voltage-sharing inner loop, as Figure 3 shown in 203, where the control variables i dc , I dc , i res and Δu res are the DC-side injection current of the high-voltage series voltage divider, the DC injection current of the non-isolated high turns-ratio bidirectional AC / DC converter, and the resonant circulating current respectively, and the closed-loop system structure of the voltage-sharing inner loop is obtained, as Figure 6 shown in (a), where the variables in the figure are consistent with the previous explanations, and its open-loop transfer function H open1 (s) expression is:

[0090] H open1 (s) = H PR (s)H P1 (s)H bs (s) (6)

[0091] In Equation (6), H PR (s) is the transfer function of the quasi-resonant controller, and its expression is well known in the industry and will not be given here. H p1 (s) is as shown in Equation (5), and H bs (s) is the transfer function of the band-stop filter, and the specific expression of the transfer function is as shown in Equation (7):

[0092]

[0093] In Equation (7), ω nis the center frequency of the band-stop filter, where ζ is the damping and s is the complex frequency.

[0094] In a specific application example of S1026, the bandwidth of the inner equalizing voltage loop is determined according to the open-loop transfer function of the inner equalizing voltage loop, so that the bandwidth of the inner equalizing voltage loop is greater than the resonance frequency, and the acquisition of the quasi-PR control parameters of the inner equalizing voltage loop is completed.

[0095] In S103 of this embodiment, as a preferred embodiment, obtaining the PI control parameters of the outer equalizing voltage loop may include the following steps:

[0096] S1031, constructing the sub-module voltage state equation of the high-voltage series voltage divider and the AC port;

[0097] S1032, according to the control structure of the outer equalizing voltage loop and the sub-module voltage state equation of the high-voltage series voltage divider and the AC port, obtaining the transfer function H p2 (s);

[0098] S1033, according to the transfer function H p2 (s) of the control object of the outer equalizing voltage loop and the closed-loop system structure, obtaining the open-loop transfer function H open2 (s);

[0099] S1034, according to the open-loop transfer function H open2 (s) of the outer equalizing voltage loop, determining the bandwidth of the outer equalizing voltage loop so that the bandwidth of the outer equalizing voltage loop is within a preset threshold to complete the acquisition of the PI control parameters of the outer equalizing voltage loop. In a specific application example, the preset threshold may be 50 Hz.

[0100] In a specific application example of S1031, constructing the sub-module voltage state equation of the high-voltage series voltage divider and the AC port may include the following steps:

[0101] According to the power conservation of the controlled source, referring to the average value circuit model of the non-isolated high-voltage-ratio bidirectional AC / DC converter, as Figure 5 shown, the following equations are established:

[0102]

[0103] In Equation (8), the variables U1, u res , N1, U2, N2, I dc , i res and C have the same meanings as in Equation (2), and are respectively the DC voltage of the high-voltage series voltage divider, the resonant voltage, the number of series sub-modules, the DC voltage of the low-voltage AC port, the number of sub-modules, the DC injection current of the non-isolated high-voltage-ratio bidirectional AC / DC converter, the resonant circulating current, and the sub-module capacitance; in addition, u c1is the average voltage of the sub-module capacitor of the high-voltage series voltage divider, u c2 is the average voltage of the sub-module capacitor of the low-voltage AC port, P ac is the current output power of the low-voltage AC port.

[0104] In a specific application example of S1032, according to the voltage equalization outer-loop control structure and the sub-module voltage state equations of the high-voltage series voltage divider and the AC port, the transfer function H p2 (s) of the control object of the voltage equalization outer-loop can be obtained through the following steps:

[0105] The voltage equalization outer-loop is as Figure 3 shown in the control structure of Figure 202, and the output is the reference amplitude I res of the resonant circulating current i * res , I * res continues to be multiplied by the unit sine signal of the resonant frequency, and then the reference resonant circulating current i * res is obtained. The feedback is the difference u c1 -u c2 between the average capacitor voltages of the sub-modules of the high-voltage series voltage divider and the low-voltage AC port. Then, the transfer function H P2 (s) of the control object of the voltage equalization outer-loop is:

[0106]

[0107] The tilde superscript in Equation (9) represents the small perturbation of the variable.

[0108] Since U1I dc , U2I dc and P ac are all regarded as constants and their small perturbations are 0, by simultaneously solving Equations (8) and (9), the transfer function H p2 (s) of the control object of the voltage equalization outer-loop can be obtained, as shown in Equation (10):

[0109]

[0110] In Equation (10), U res , U c1 , U c2 are the steady-state operating points of the amplitudes of u res , u c1 , u c2 respectively.

[0111] In a specific application example of S1033, according to the transfer function H p2 (s) of the control object of the voltage equalization outer-loop and the closed-loop system structure, the open-loop transfer function H open2(s) may include the following steps:

[0112] According to Equation (10) and in combination with the control structure of the outer voltage equalization loop, the transfer function of the outer voltage equalization loop is obtained.

[0113] Since the bandwidth of the outer voltage equalization loop is much lower than that of the inner voltage equalization loop, within the bandwidth of the outer voltage equalization loop, the closed-loop transfer function of the inner voltage equalization loop is equivalent to 1, and the closed-loop system structure of the outer voltage equalization loop system can be obtained, as Figure 6 shown in (b) in the figure. The variables in the figure have been given previously, so the open-loop transfer function H open2 (s) expression is:

[0114] H open2 (s) = H PI1 (s)H P2 (s)H bs (s) (11)

[0115] In Equation (11), H PI1 (s) is the PI controller of the outer voltage equalization loop, H P2 (s) is the transfer function of the controlled object of the inner voltage equalization loop as shown in Equation (10), and H bs (s) is the transfer function of the band-stop filter as shown in Equation (7).

[0116] In a specific application example of S1034, the bandwidth of the outer voltage equalization loop is determined according to the open-loop transfer function of the outer voltage equalization loop, so that the bandwidth of the outer voltage equalization loop is within the preset threshold, and the acquisition of the PI control parameters of the outer voltage equalization loop is completed. In a specific application example, the preset threshold can be 50 Hz.

[0117] In S103 of this embodiment, as a preferred embodiment, obtaining the PI control parameters of the inner voltage stabilization loop may include the following steps:

[0118] S103i, according to the inner voltage stabilization loop control structure and the DC equivalent circuit of the non-isolated high-ratio bidirectional AC / DC converter, obtain the transfer function H p3 (s) of the controlled object of the inner voltage stabilization loop;

[0119] S103ii, according to the transfer function H p3 (s) of the controlled object of the inner voltage stabilization loop and the closed-loop system structure, obtain the open-loop transfer function H open3 (s);

[0120] S103iii, according to the open-loop transfer function H open3 (s) of the inner voltage stabilization loop, determine the bandwidth of the inner voltage stabilization loop, so that the bandwidth of the inner voltage stabilization loop is within 100 - 200 Hz, and the acquisition of the PI control parameters of the inner voltage stabilization loop is completed.

[0121] In a specific application example of S103i, according to the voltage-stabilizing inner-loop control structure and the DC equivalent circuit of the non-isolated high-ratio bidirectional AC / DC converter, the transfer function H p3 (s) of the control object of the voltage-stabilizing inner loop is obtained, including:

[0122] According to the control structure of the voltage-stabilizing inner loop, as shown in the 205 structure in Figure 3 , the output quantity is the DC modulation voltage u1 of the high-voltage series voltage divider, and the feedback quantity is the DC injection current I of the non-isolated high-ratio bidirectional AC / DC converter dc . In addition, the intermediate control variable ΔU1 is the output adjustment voltage of the voltage-stabilizing inner-loop PI controller, and U * 1 is the feedforward reference voltage. Thus, the transfer function H p3 (s) of the control object of the voltage-stabilizing inner loop can be determined in the form of;

[0123]

[0124] In Equation (12), the tilde superscript represents a small perturbation of the variable.

[0125] According to the DC equivalent circuit of the non-isolated high-ratio bidirectional AC / DC converter, as shown in Figure 7 , where L, U1, U2, L bp , C bp , R bp , L dc , R dc , U dc and I dc are respectively the arm inductance of the low-voltage AC port, the DC voltage of the series high-voltage voltage divider, the DC voltage of the low-voltage AC port, the resonant inductance, the resonant capacitance, the stray resistance of the resonant branch, the equivalent inductance of the system DC side, the equivalent resistance of the system DC side, the equivalent voltage of the system DC side, and the DC injection current of the non-isolated high-ratio bidirectional AC / DC converter. Combining with the form of the transfer function H p3 (s) of the control object, the transfer function H p3 (s) of the control object of the voltage-stabilizing inner loop can be obtained as shown in Equation (13):

[0126]

[0127] In Equation (13), the tilde superscript represents a small perturbation of the variable, and s is the complex frequency. Other variables have been explained earlier.

[0128] In a specific application example of S103ii, according to the transfer function H p3 (s) of the control object of the voltage-stabilizing inner loop and the closed-loop system structure, the open-loop transfer function H open3(s) may include the following steps:

[0129] According to Equation (13) and in combination with the closed-loop system structure of the inner voltage stabilization loop, as Figure 8 shown in (a) therein, where the variables have been explained above, the open-loop transfer function H open3 (s) is obtained, and the expression is (14):

[0130] H open3 (s) = H PI2 (s)H P3 (s)H bs (s) (14)

[0131] In Equation (14), H PI2 (s) is the PI controller of the inner voltage stabilization loop, H P3 (s) is the transfer function of the controlled object of the inner voltage stabilization loop, as in (13), H bs (s) is the band-stop filter transfer function, as in Equation (7).

[0132] In a specific application example of S103iii, the bandwidth of the inner voltage stabilization loop is determined according to the open-loop transfer function of the inner voltage stabilization loop, so that the inner loop bandwidth is 100 - 200 Hz, and the acquisition of the PI control parameters of the inner voltage stabilization loop is completed.

[0133] In S103 of this embodiment, as a preferred embodiment, to obtain the PI control parameters of the outer voltage stabilization loop, the following steps may be included:

[0134] S103I, according to the control structure of the outer voltage stabilization loop and the power relationship of the non-isolated high-ratio bidirectional AC / DC converter, the transfer function H p4 (s) of the controlled object of the outer voltage stabilization loop is obtained;

[0135] S103II, according to the transfer function H p4 (s) of the controlled object of the outer voltage stabilization loop and the closed-loop system structure, the open-loop transfer function H open4 (s) of the outer voltage stabilization loop is obtained;

[0136] S103III, according to the open-loop transfer function H open4 (s) of the outer voltage stabilization loop, the bandwidth of the outer voltage stabilization loop is determined so that the bandwidth of the outer voltage stabilization loop is within a preset threshold, and the acquisition of the PI control parameters of the outer voltage stabilization loop is completed. In a specific application example, the preset threshold may be 50 Hz.

[0137] In a specific application example of S103I, according to the control structure of the outer voltage stabilization loop and the power relationship of the non-isolated high-ratio bidirectional AC / DC converter, the transfer function H p4 (s) of the controlled object of the outer voltage stabilization loop is obtained, and the following steps may be included:

[0138] According to the control structure block diagram of the voltage stabilization outer loop, as Figure 3 shown in 204, the output quantity is I * dc the DC injection current of the non-isolated high turns ratio bidirectional AC / DC converter, and the feedback quantity is the average voltage u of all sub-modules of the non-isolated bidirectional AC / DC converter c , and the transfer function H p4 (s) is in the form of (15);

[0139]

[0140] In formula (15), the wavy line superscript represents the small perturbation of the variable.

[0141] Through the power relationship of the AC / DC converter and the form of the transfer function of the control object of the voltage stabilization outer loop, the transfer function H P4 (s) is as shown in formula (16):

[0142]

[0143] In formula (16): u c1 , N1 and i dc are the capacitor voltage of the high-voltage series voltage divider sub-module, the number of sub-modules, and the DC-side injection current, u c2 , N2 and P ac are the capacitor voltage of the low-voltage AC port sub-module, the number of sub-modules, and the AC output power, which are equivalent to constants; U dc , L dc and R dc are respectively the internal electromotive force of the DC source, the internal reactance of the DC source, and the resistance of the non-isolated high turns ratio bidirectional AC / DC converter; the average voltage u c = u c1 = u c2 ; I dc and U c are respectively the steady-state operating points of i dc and u c , and s is the complex frequency.

[0144] In a specific application example of S103II, according to the transfer function H p4 (s) of the control object of the voltage stabilization outer loop and the closed-loop system structure, the open-loop transfer function H open4 (s) of the voltage stabilization outer loop can be obtained, and the following steps can be included:

[0145] Since the bandwidth of the voltage stabilization outer loop is much smaller than the bandwidth of the voltage stabilization inner loop, the closed-loop transfer function of the voltage stabilization inner loop is equivalent to 1 within the bandwidth of the voltage stabilization outer loop, and the closed-loop system structure of the voltage stabilization outer loop is asFigure 8 as shown in (b), where the variables have been explained previously, and the open-loop transfer function H open4 (s) is expressed as:

[0146] H open4 (s) = H PI3 (s)H P4 (s)H bs (s) (17)

[0147] In Equation (17), H PI3 (s) is the PI controller of the voltage stabilization outer loop, and H P4 (s) is as shown in Equation (16), and H bs (s) is the transfer function of the band-stop filter as shown in Equation (7).

[0148] In a specific application example of S103III, the bandwidth of the voltage stabilization outer loop is determined according to the open-loop transfer function of the voltage stabilization outer loop, so that within a preset threshold, the acquisition of the PI control parameters of the voltage stabilization outer loop is completed. In a specific application example, the preset threshold can be 50 Hz.

[0149] In S202 of this embodiment, as a preferred embodiment, obtaining the PI control parameters of the current inner loop may include the following steps:

[0150] S202i, according to the control structure of the current inner loop and the equivalent circuit of the low-voltage AC port, obtain the transfer function H p5 (s) of the controlled object of the current inner loop;

[0151] S202ii, according to the transfer function H p5 (s) of the controlled object of the current inner loop and the closed-loop system structure, obtain the open-loop transfer function H open5 (s) of the current inner loop;

[0152] S202iii, according to the open-loop transfer function H open5 (s) of the current inner loop, determine the bandwidth of the current inner loop so that the bandwidth of the current inner loop is within 100 - 200 Hz, and complete the acquisition of the PI control parameters of the current inner loop.

[0153] In a specific application example of S202i, according to the control structure of the current inner loop and the equivalent circuit of the low-voltage AC port, obtaining the transfer function H p5 (s) of the controlled object of the current inner loop may include the following steps:

[0154] According to the control structure block diagram of the current inner loop, such as Figure 4 shown in 302, where the variables related to the current inner loop include the feedback quantities i′ d and i′ q, which is the dq component \(i\) of the alternating current d and \(i\) q after filtering; \(u\) * d is the d-axis voltage feedforward, and the output \(u\) dm and \(u\) qm are the reference dq-axis modulation voltages. Taking the d-axis as an example, the transfer function \(H\) p5 (s) of the control object of the current inner loop is in the form shown in Equation (18):

[0155]

[0156] where the tilde superscript represents small perturbations.

[0157] According to the equivalent circuit of the low-voltage AC port, as Figure 9 shown, the transfer function \(H\) p5 (s) of the control object of the current inner loop is:

[0158]

[0159] In Equation (19), \(u\) j and \(i\) j are the three-phase output voltage and current of the low-voltage AC port, \(j\) represents the phase (such as a, b, c), and \(u\) WFj is the three-phase input voltage of the wind farm, \(j\) represents the phase (such as a, b, c); \(u\) d and \(u\) q and \(i\) d and \(i\) q are the dq-axis output voltage and current of the low-voltage AC port, \(u\) WFd and \(u\) WFq are the dq-axis input voltages of the wind farm; \(L\) and \(R\) are the arm inductance and resistance of the low-voltage AC port bridge; \(L\) T and \(R\) T are the equivalent inductance and resistance of the connecting transformer; \(\omega_1\) is the power frequency angular frequency, and \(s\) is the complex frequency.

[0160] In a specific application example of S202ii, according to the transfer function \(H\) p5 (s) of the control object of the current inner loop and the closed-loop system structure, the open-loop transfer function \(H\) open5 (s) of the current inner loop can be obtained, and the following steps may be included:

[0161] According to Equation (19), the closed-loop system structure of the current inner loop is obtained, as Figure 10 shown, where the variables have been explained previously. The expression of the open-loop transfer function \(H\) open5 (s) is as shown in (20):

[0162] \(H\) open5 (s) = \(H\) PI4(s)H P5 (s)H lp (s)[1-H bs1 (s)-H bs2 (s)] (20)

[0163] In formula (20), H PI4 (s) is the current-loop PI controller, H P5 (s) is as in (19), H lp (s) is the low-pass filter, H bs1 (s), H bs2 (s) is the band-stop filter, as in formula (21), T lp is the low-pass filtering time constant, ω n1 is the angular frequency of the k-th harmonic (k is the number of pulses of the uncontrolled rectifier), ω n2 is the angular frequency of the 2k-th harmonic, ζ is the damping coefficient, and s is the complex frequency.

[0164] Among them, the band-stop filters H bs1 (s), H bs2 (s) are as follows:

[0165]

[0166] In a specific application example of S202iii, the bandwidth of the current inner loop is determined according to the open-loop transfer function of the current inner loop, so that the bandwidth is 100 - 200 Hz, and the PI control parameters of the current inner loop are obtained.

[0167] In S202 of this embodiment, as a preferred embodiment, obtaining the PI control parameters of the voltage outer loop may include the following steps:

[0168] S202I, according to the control structure of the voltage outer loop and the equivalent circuit of the low-voltage AC port and the wind farm interconnection system, obtain the transfer function H p6 (s) of the control object of the voltage outer loop;

[0169] S202II, according to the transfer function H p6 (s) of the control object of the voltage outer loop and the closed-loop system structure, obtain the open-loop transfer function H open6 (s) of the voltage outer loop;

[0170] S202III, according to the open-loop transfer function H open6 (s) of the voltage outer loop, determine the bandwidth of the voltage outer loop so that the bandwidth of the voltage outer loop is less than a preset threshold, and complete the acquisition of the PI control parameters of the voltage outer loop. In a specific application example, the preset threshold may be 50 Hz.

[0171] In a specific application example of S202I, according to the control structure of the voltage outer loop and the equivalent circuit of the low-voltage AC port and the wind farm interconnection system, the transfer function H of the controlled object of the voltage outer loop is obtained. p6 (s), which may include the following steps:

[0172] Combining the output and feedback of the current inner loop, and at the same time according to the control structure of the voltage outer loop, such as Figure 4 shown in the 301 structure in, where the variables related to the voltage outer loop include: input variables dq-axis reference voltages u dr 、u qr , feedback quantities u′ d 、u′ q , which are the dq-axis components u WFd 、u WFq of the wind farm input voltage after being processed by the filter, and the output quantities are dq-axis reference currents i dr 、i qr . Taking the d-axis as an example, the form of the transfer function H p6 (s) is (22);

[0173]

[0174] wherein, u WFd is the wind farm input d-axis voltage, i dr is the dq-axis reference input current of the current loop, and the tilde superscript represents a small perturbation;

[0175] According to the circuit structures of the low-voltage AC port and the wind farm, such as Figure 11 shown, the transfer function H p6 (s) of the controlled object of the voltage outer loop is as shown in Equation (23).

[0176]

[0177] wherein, u WFd 、u WFq are the wind farm input dq-axis voltages, i dr 、i qr are the dq-axis reference input currents of the current loop, and the tilde superscript represents a small perturbation;

[0178] In a specific application example of S202II, according to the transfer function H p6 (s) of the controlled object of the voltage outer loop and the closed-loop system structure, the open-loop transfer function H open6 (s) of the voltage outer loop is obtained, which may include the following steps:

[0179] Within the bandwidth of the voltage outer loop, the magnitude of the closed-loop transfer function of the current inner loop is equivalent to 1, then the closed-loop system structure of the voltage outer loop is as Figure 12As shown, where the variables have been explained previously, the open-loop transfer function H open6 (s) is expressed as:

[0180] H open6 (s) = H PI5 (s)H P6 (s)H lp (s)[1 - H bs1 (s) - H bs2 (s)] (24)

[0181] In Equation (24), H PI5 (s) is the voltage outer-loop PI controller, H P6 (s) is as shown in (23), H lp (s) is the low-pass filter, H bs1 (s), H bs2 (s) are band-stop filters, as shown in Equation (21).

[0182] In a specific application example of S202III, the voltage outer-loop bandwidth is determined according to the open-loop transfer function of the voltage outer-loop, such that the bandwidth is less than a preset threshold, which can be 50 Hz, to complete the acquisition of the PI control parameters of the voltage outer-loop.

[0183] The following further describes in detail the technical solutions provided in the above embodiments of the present invention with reference to the accompanying drawings.

[0184] The control of the non-isolated high-ratio bidirectional AC / DC converter includes the control of the high-voltage series voltage divider and the control of the low-voltage AC port. The optimized overall control structure is as shown in Figure 3 、 Figure 4 . The structures of the high-voltage series voltage divider, the low-voltage AC port, and the resonant branch are as shown in Figure 2 . In the figure, part 101 is the resonant branch, which is composed of the resonant capacitor C bp and the resonant inductor L bp connected in series. Its function is to serve as the resonant energy channel of the high-voltage series voltage divider 102 to balance the input and output powers of the high-voltage sub-module string; parts 102 and 103 are the high-voltage series voltage divider, which is composed of a large number of cascaded sub-modules. Its function is to divide the DC voltage, greatly reducing the DC-side voltage of the three-phase low-voltage AC port 104, playing a role in step-down; part 104 is the three-phase low-voltage AC port, and each phase bridge arm thereof is composed of a small number of cascaded sub-module strings and bridge arm inductors. Its function is the inversion of the low DC-side voltage; among them, the sub-module topology adopts a half-bridge structure, as shown in Figure 2 at 105.

[0185] In the control of the high-voltage series voltage divider: First, to eliminate the influence of the kth current harmonic corresponding to the DC side generated by the k-pulse uncontrolled rectifier on the resonant circulating current control, according to the pulse number k of the uncontrolled rectifier, a notch filter with the kth harmonic as the center frequency is introduced into the feedback signal of the DC-side resonant circulating current of the high-voltage series voltage divider to eliminate the kth current harmonic on the DC side of the converter; Second, to achieve static zero error of the resonant circulating current, a quasi-PR controller is used for control. The quasi-PR controller is shown in Equation (25). In the equation, K P is the proportional parameter of the H PR controller, K R is the resonant gain parameter of the H PR controller, ζ is the resonant damping of the H PR controller, ω n is the resonant frequency of the H PR controller, and s is the complex frequency; Third, the resonant circulating current will have an adverse impact on the PI control of the voltage equalization and voltage stabilization of the sub-modules, manifested as harmonic components with the resonant frequency in the average capacitor voltage and DC-side current of the sub-modules, which will introduce AC interference to the DC output reference of the PI. Therefore, a notch filter with the resonant frequency is introduced into the feedback links of the voltage equalization outer loop, voltage stabilization inner loop, and voltage stabilization outer loop to filter out the interference generated by the resonant current.

[0186]

[0187] In the control of the low-voltage AC port: First, a double closed-loop control is adopted for the voltage and frequency at the PCC point of the wind farm grid connection. An inner current loop is introduced to facilitate the addition of virtual damping and further enhance the stability of the system; Second, according to the pulse number k of the uncontrolled rectifier, a corresponding kth harmonic notch filter is used to filter out the kth harmonic current of the DC-side resonant circulating current of the high-voltage series voltage divider to obtain a low-harmonic resonant current feedback signal for the voltage equalization inner loop; Third, since the k-pulse uncontrolled rectification introduces harmonic components of k±1 and 2k±1 times into the dq-axis voltage and dq-axis current at the PCC point after the park transformation, which will have an adverse impact on the double closed-loop PI control, therefore, k and 2k harmonic notch filters are introduced into the feedback links of the voltage outer loop and current inner loop to eliminate the influence of the k-pulse uncontrolled rectification on the constant voltage control.

[0188] In the design of the control parameters of the high-voltage series voltage divider, it includes: the design of the quasi-PR parameters of the voltage equalization inner loop, the design of the PI parameters of the voltage equalization outer loop, the design of the PI parameters of the voltage stabilization inner loop, and the design of the PI parameters of the voltage stabilization outer loop. The design methods are as follows:

[0189] For the design of the quasi-PR parameters of the voltage equalization inner loop, the transfer function H p1 (s) of the control object of the voltage equalization inner loop needs to be established, as shown in the following Equation (26).

[0190]

[0191] In the formula, i res is the DC side resonant circulating current of the non-isolated high-ratio bidirectional AC / DC converter, Δu res is the resonant current control voltage of the high voltage series voltage divider. The tilde superscript represents a small disturbance of the variable.

[0192] according to Figure 5 The average value model corresponding to the topological structure can be written as the equation group shown in formula (27), where m1, i dc 、u dc1 、N1、U1、u res and Δu res are the modulation ratio of the high-voltage series voltage divider, the DC side injection current, the equivalent capacitor voltage, the number of submodules, the rated DC voltage, the reference resonant voltage and the resonant current control voltage; m ux 、m lx 、i ux 、i lx 、u ux 、u lx They are the x-phase modulation ratio, current and equivalent capacitor voltage of the upper and lower bridge arms of the low-voltage AC port, N2, L, U2 and u x are the number of bridge arm submodules, bridge arm inductance, rated DC voltage and current modulation voltage of the low voltage AC port; U dc and C are the total voltage on the DC side of the non-isolated high-ratio bidirectional AC / DC converter and the submodule capacitance, respectively;

[0193] Make the assumption of (27) as (28), and then combine (27) and (28) to get (29). Through transformation, we can get the transfer function of the controlled object (30). The wavy line subscript represents a small disturbance of the variable. Combined with the control structure, we can get the transfer function block diagram of the pressure equalization inner loop, as shown in Figure 6 As shown in (a), the open-loop transfer function of the pressure-equalizing inner loop is shown in equation (31), where H bs (s) is the transfer function of the band-stop filter. The specific expression of the transfer function is shown in (32). In formula (32), ω n is the center frequency of the band-stop filter, where ζ is the damping and s is the complex frequency. The bandwidth of the voltage-equalizing inner loop can be designed according to the open-loop transfer function. Since the voltage-equalizing inner loop mainly controls the AC component of the resonant frequency, the inner loop bandwidth should be greater than the resonant frequency.

[0194]

[0195]

[0196] H open1 (s)=H PR (s)HP1 (s)H bs (s) (31)

[0197]

[0198] Design of the PI parameters of the voltage-sharing outer loop. Similarly, first establish the mathematical model of the controlled object. According to the power conservation of the controlled source, referring to the average value circuit model of the non-isolated high-ratio bidirectional AC / DC converter, as Figure 5 shown, Equation (33) can be obtained. In the equation, the variables U1, u res , N1, U2, N2, I dc , i res and C have the same meanings as in (2), which are the DC voltage of the high-voltage series voltage divider, the resonant voltage, the number of series sub-modules, the DC voltage of the low-voltage AC port, the number of sub-modules, the DC injection current, the resonant circulating current and the sub-module capacitance of the non-isolated high-ratio bidirectional AC / DC converter respectively; in addition, u c1 is the average voltage of the sub-module capacitance of the high-voltage series voltage divider, u c2 is the average voltage of the sub-module capacitance of the low-voltage AC port, and P ac is the AC output power of the low-voltage AC port.

[0199] The voltage-sharing outer loop is like the control structure shown in 202 in Figure 3 , and the output is the reference amplitude I res of the resonant circulating current i * res . I * res continues to be multiplied by the unit sine signal of the resonant frequency, and then the reference resonant circulating current i * res is obtained. The feedback is the difference u c1 -u c2 between the average capacitance voltages of the sub-modules of the high-voltage series voltage divider and the low-voltage AC port. Then the transfer function H P2 (s) of the controlled object of the voltage-sharing outer loop is as shown in Equation (34). In Equation (34), the tilde superscript represents small perturbations. By linearizing through Equation (34), Equation (35) can be obtained. In Equation (35), U res , U c1 , U c2 are the steady-state operating points of the amplitudes of u res , u c1 , u c2 respectively. Furthermore, combining the control structure of the voltage-sharing outer loop, the transfer function block diagram of the voltage-sharing outer loop can be obtained, as shown in (b) in Figure 6 . Considering that the bandwidth of the outer loop is much lower than that of the inner loop, within the bandwidth of the outer loop, the closed-loop transfer function of the inner loop is approximately equal to 1 (equivalent to 1). Therefore, the open-loop transfer function of the voltage-sharing outer loop can be listed as shown in Equation (36). In Equation (36), HPI1 (s) is the PI controller for the voltage equalization outer loop, H P2 (s) is as shown in Equation (35), H bs (s) is the transfer function of the band-stop filter as shown in Equation (32); the PI parameters of the voltage equalization outer loop can be designed according to the bandwidth of the voltage equalization outer loop within 50 Hz.

[0200]

[0201] H open2 (s) = H PI1 (s)H P2 (s)H bs (s) (36)

[0202] The design of the PI parameters for the voltage stabilization inner loop is as Figure 3 shown in the 205 structure, the output is the DC modulation voltage u1 of the high-voltage series voltage divider, and the feedback is the DC injection current I of the non-isolated high-ratio bidirectional AC / DC converter dc , in addition, the intermediate control variable ΔU1 is the output adjustment voltage of the voltage stabilization inner loop PI controller, U * 1 is the feedforward reference voltage, and thus the transfer function H of the controlled object of the voltage stabilization inner loop can be determined p3 (s) is in the form shown in Equation (37), the tilde superscript represents the small perturbation of the variable, and the DC equivalent circuit of the system is as Figure 7 shown, the low-voltage AC port and the DC side voltage of the system are constants, where L, U1, U2, L bp , C bp , R bp , L dc , R dc , U dc and I dc are respectively the arm inductance of the low-voltage AC port, the DC voltage of the series high-voltage voltage divider, the DC voltage of the low-voltage AC port, the resonant inductor, the resonant capacitor, the stray resistance of the resonant branch, the equivalent inductor of the system DC side, the equivalent resistance of the system DC side, the equivalent voltage of the system DC side, and the DC injection current of the non-isolated high-ratio bidirectional AC / DC converter. Combining the transfer function H p3 (s) form, the transfer function H of the controlled object of the voltage stabilization inner loop can be obtained p3 (s) is as shown in Equation (38), the tilde on the variable in Equation (38) is the small perturbation, s is the complex frequency, and other variables have been explained previously. According to Equation (38), and combining the closed-loop system structure of the voltage stabilization inner loop, as Figure 8 shown in (a) therein, where the variables have been explained in the previous text, the open-loop transfer function H of the voltage stabilization inner loop is obtained open3 (s), and the expression is as shown in Equation (39). In Equation (39), H PI2(s) is the PI controller of the inner voltage stabilization loop, H P3 (s) is the transfer function of the controlled object of the inner voltage stabilization loop, as shown in Equation (38), H bs (s) is the transfer function of the band-stop filter, as shown in Equation (32). The bandwidth of the inner voltage stabilization loop can be designed to be 100 - 200 Hz according to it, and the PI parameters can be designed.

[0203]

[0204] H open3 (s) = H PI2 (s)H P3 (s)H bs (s) (39)

[0205] The design of the PI parameters of the outer voltage stabilization loop is as Figure 3 shown in 204, and the output quantity is I * dc the DC injection current of the non-isolated high-ratio bidirectional AC / DC converter, and the feedback quantity is the average voltage u of all sub-modules of the non-isolated bidirectional AC / DC converter c , and the transfer function H of the controlled object of the outer voltage stabilization loop p4 (s) is in the form shown in Equation (40), where the wavy line superscript represents the small perturbation of the variable. Through the power relationship of the AC / DC converter, the transfer function H of the controlled object of the outer voltage stabilization loop can be derived P3 (s), as shown in Equation (41), where: u c1 , N1 and i dc are the capacitor voltage of the high-voltage series voltage divider sub-module, the number of sub-modules, and the DC-side injection current, u c2 , N2 and P ac are the capacitor voltage of the low-voltage AC port sub-module, the number of sub-modules, and the AC output power, which are equivalent to constants; U dc , L dc and R dc are the internal electromotive force, internal reactance, and resistance of the DC source of the non-isolated high-ratio bidirectional AC / DC converter respectively; the average voltage u c = u c1 = u c2 ; I dc and U c are the steady-state operating points of i dc and u c respectively, s is the complex frequency, and H p3 (s) is the transfer function of the controlled object of the outer voltage stabilization loop.

[0206] Here, based on the assumption that the sub-modules are evenly voltage-divided V c = u dc1 = u dc2 , u dc1is the capacitor voltage of the sub-module of the high-voltage series voltage divider, u dc2 is the capacitor voltage of the sub-module of the low-voltage AC port, P ac is the AC output power of the AC / DC converter, which can be equivalent to a constant without considering the wind farm power change. Since the bandwidth of the voltage regulation outer loop is much smaller than that of the voltage regulation inner loop, the closed-loop transfer function of the voltage regulation inner loop is equivalent to 1 within the bandwidth of the voltage regulation outer loop. Then the closed-loop system structure of the voltage regulation outer loop is as Figure 8 shown in (b) in which the variables have been explained previously. The open-loop transfer function H open4 (s) is expressed as shown in (42), where H PI3 (s) is the PI controller of the voltage regulation outer loop, H P4 (s) is as shown in Equation (41), H bs (s) is the band-stop filter transfer function as shown in Equation (32);

[0207] Design the bandwidth of the voltage regulation outer loop according to the open-loop transfer function of the voltage regulation outer loop, so as to complete the design of the PI control parameters of the voltage regulation outer loop within 50Hz.

[0208]

[0209] H open4 (s) = H PI3 (s)H P4 (s)H bs (s) (42)

[0210] In terms of the design of the control parameters of the low-voltage AC port, it includes: the design of the PI parameters of the current inner loop and the design of the PI parameters of the voltage outer loop.

[0211] For the design of the PI parameters of the current inner loop of the low-voltage AC port, according to the control structure block diagram of the current inner loop, as Figure 4 shown in 302, where the variables related to the current inner loop include the feedback quantities i′ d and i′ q , which are the outputs after filtering the dq components i d 、i q of the AC current; u * d is the d-axis voltage feedforward, and the output quantities u dm 、u qm are the reference dq-axis modulation voltages. Taking the d-axis as an example, the transfer function H p5 (s) of the controlled object of the current inner loop is in the form shown in Equation (43). According to the equivalent circuit of the low-voltage AC port, as Figure 9 shown, the transfer function H p5 (s) of the controlled object of the current inner loop is as shown in Equation (44), where u j 、i jFor the three-phase output voltage and current of the low-voltage AC port, j represents the phase (such as a, b, c), u WFj is the three-phase input voltage of the wind farm, j represents the phase (such as a, b, c); u d 、u q 、i d 、i q are the dq-axis components of the output voltage and current of the low-voltage AC port, u WFd 、u WFq are the dq-axis voltage components of the wind farm voltage u WFj ; L and R are the arm inductance and resistance of the low-voltage AC port; L T 、R T are the equivalent inductance and resistance of the connecting transformer; ω1 is the power frequency angular frequency, and s is the complex frequency. Taking the d-axis as an example, the closed-loop system structure of the current inner loop is obtained, as shown in Figure 10 . Among them, the variables have been explained in the previous text. The open-loop transfer function H open5 (s) is expressed as in Equation (45). In the formula, H PI4 (s) is the current loop PI controller, H P5 (s) is as in (44), H lp (s) is the low-pass filter, H bs1 (s)、H bs2(s) are the band-stop filters, as in Equation (46), T lp is the low-pass filtering time constant, ω n1 is the kth harmonic angular frequency (k is the pulse number of the uncontrolled rectifier), ω n2 is the 2kth harmonic angular frequency, ζ is the damping coefficient, and s is the complex frequency. Design the bandwidth of the current inner loop according to the open-loop transfer function of the current inner loop, so that the bandwidth is 100 - 200 Hz, and complete the design of the PI control parameters of the current inner loop.

[0212]

[0213] H open5 (s) = H PI4 (s)H P5 (s)H lp (s)[1 - H bs1 (s) - H bs2 (s)] (45)

[0214]

[0215] Design the PI parameters of the AC voltage outer loop of the low-voltage AC port. Combine the output and feedback of the current inner loop, and at the same time, according to the control structure of the voltage outer loop, as shown in the 301 structure in Figure 4 . Among them, the variables related to the voltage outer loop include: the input variables dq-axis reference voltages u dr 、uqr , the feedback quantity u' d 、u' q , which is the dq-axis component u of the wind farm input voltage WFd 、u WFq The output after being processed by the filter, and the output quantity is the dq-axis reference current i dr 、i qr , taking the d-axis as an example, the transfer function H of the controlled object p6 (s) is in the form shown in Equation (47), where u WFd is the d-axis voltage input of the wind farm, i dr is the dq-axis reference input current of the current loop, and the tilde superscript represents small perturbations. According to the circuit structures of the low-voltage AC port and the wind farm, as Figure 11 shown, the transfer function H of the controlled object of the voltage outer loop is obtained p6 (s) as shown in Equation (48). Within the bandwidth of the voltage outer loop, the magnitude of the closed-loop transfer function of the current inner loop is equivalent to 1, then the closed-loop system structure of the voltage outer loop is as Figure 12 shown, where the variables have been explained previously. The open-loop transfer function H open6 (s) is expressed as shown in Equation (49), where H PI5 (s) is the voltage outer loop PI controller, H P6 (s) is as in (48), H lp (s) and H bs1 (s), H bs2 (s) are respectively the low-pass filter and the band-stop filter, as shown in Equation (46).

[0216]

[0217] H open6 (s) = H PI5 (s)H P6 (s)H lp (s)[1 - H bs1 (s) - H bs2 (s)] (49)

[0218] Next, in combination with a specific application example, the technical solutions provided in the above embodiments of the present invention will be further described in detail.

[0219] First, establish as Figure 2The non-isolated high-ratio bidirectional AC / DC converter shown has an effective value of the phase voltage at the AC port of 14 kV, a DC bus voltage on the DC side of 100 kV, a DC voltage of 70 kV for the high-voltage series voltage divider, and a DC voltage of 30 kV at the DC side of the AC port. A step-up transformer with a capacity of 50 MVA and a turns ratio of 14√3 kV / 220 kV is connected in parallel at the converter outlet and is incorporated into the 220 kV PCC point of the wind farm. The arm reactance of the non-isolated AC / DC converter is 10 mH, the sub-module capacitance is 10 mF, the resonant capacitance is 180 μF, the resonant inductance is 1.6 mH, the number of sub-modules of the high-voltage series voltage divider is 116, and the number of arm sub-modules at the low-voltage AC port is 42. The circulating current voltage is 8 kV, and the reference value of the voltage outer loop is 107.7775 kV (the voltage amplitude corresponding to 0.6 times the line voltage of 220 kV). According to the aforementioned controller parameter design methods for the high-voltage series voltage divider and the low-voltage AC port. The bandwidths of each link and the corresponding PI parameters are as follows:

[0220] Table 1 Controller Bandwidth Design and Parameters

[0221]

[0222] The non-isolated high-ratio AC / DC converter is built using SIMULINK to simulate the black start of the wind farm and the reverse power transmission after startup. The wind farm starts a black start at 0.1 s of the simulation, increasing the required active power at a slope of 40 MW / s and reaching 4 MW at 0.2 s and remaining constant. Power generation starts at 1.5 s of the simulation, with the required power decreasing at a slope of 40 MW / s and a small amount of active power being reverse transmitted, reaching -4 MW (reverse transmitted power) at 1.7 s and remaining constant. The circulating current, AC voltage / current, and the transmitted power of the AC / DC converter during this process are as Figure 13 shown.

[0223] In addition, the dynamic characteristics of the voltage equalization outer loop, voltage stabilization outer loop, and AC voltage outer loop are also verified through SIMULINK simulation. At 2 s of the simulation, the voltage equalization reference value of the voltage equalization outer loop steps by 1% of the rated voltage (from 0 V to 9 V). At 2.2 s of the simulation, the voltage stabilization reference value of the voltage stabilization outer loop steps by 1.01% of the rated voltage (from 900 V to 909 V). At 2.5 s of the simulation, the AC voltage reference value at the low-voltage AC port steps by 0.3333 pu (the effective value of the phase voltage steps from 8.4 kV to 11.2 kV). During this dynamic process, the waveforms of the dynamic characteristics of the control quantities of each outer loop are as Figure 14 shown. The transient times of the control quantities of each outer loop are all within the order of magnitude of 0.1 s, which is consistent with the theoretical design effect.

[0224] Figure 15 This is a schematic diagram of the composition structure of the control optimization system for the non-isolated high-ratio bidirectional AC / DC converter provided by an embodiment of the present invention.

[0225] like Figure 15 As shown in (a), the non-isolated high-ratio bidirectional AC / DC converter control optimization system provided in this embodiment may include: a high-voltage series voltage divider control optimization part; wherein:

[0226] The high-voltage series voltage divider control optimization part can include the following modules:

[0227] A trap module in the control structure of the high-voltage series voltage divider, which introduces a trap with the kth harmonic as the center frequency into the feedback signal of the DC side resonant ring current of the high-voltage series voltage divider according to the pulse number k of the uncontrolled rectifier, so as to eliminate the kth harmonic of the current on the DC side of the converter;

[0228] A quasi-PR control module, which obtains the quasi-PR control parameters of the pressure-equalizing inner loop and uses a quasi-PR controller to control the resonant circulating current to achieve static zero difference of the resonant circulating current;

[0229] The PI control module in the high-voltage series voltage divider control structure obtains the PI control parameters of the voltage balancing outer loop, the voltage stabilization inner loop and the voltage stabilization outer loop, and introduces a resonant frequency trap in the feedback link (in the feedback signal channel) of the capacitor voltage balancing outer loop, the voltage stabilization inner loop and the voltage stabilization outer loop of the AC / DC converter submodule to filter out the interference caused by the resonant current;

[0230] like Figure 15 As shown in (b), the non-isolated high-ratio bidirectional AC / DC converter control optimization system provided in this embodiment may include: a low-voltage AC port control optimization part; wherein:

[0231] The low voltage AC port control optimization part can include the following modules:

[0232] Dual closed-loop control module, which uses dual closed-loop control of the voltage and frequency of the PCC point of the wind farm grid connection. The current inner loop is used to control the additional virtual damping to enhance the stability of the system;

[0233] The trap module in the low-voltage AC port control structure uses a corresponding k-th harmonic trap to filter out the k-th harmonic current of the DC-side resonant ring current of the high-voltage series voltage divider according to the pulse number k of the uncontrolled rectifier, and obtains a low-harmonic equalizing inner ring resonant current feedback signal;

[0234] The PI control module in the low-voltage AC port control structure obtains the PI control parameters of the voltage outer loop and the current inner loop, and introduces notch filters with k and 2k harmonics as the center frequency to the three-phase output voltage and current feedback signals of the low-voltage AC port according to the pulse number k of the rectifier and converter station, so as to eliminate the k±1 and 2k±1 voltage and current harmonics on the AC side of the uncontrolled rectifier and converter station.

[0235] The non-isolated high turns ratio bidirectional AC / DC converter control optimization system provided by the embodiments of the present invention may further include: a high-voltage series voltage divider control optimization part and a low-voltage AC port control optimization part. The specific implementation manners can refer to the above two corresponding embodiments and will not be elaborated herein.

[0236] In the embodiments of the present invention, according to the signal flow direction, the notch filter is the previous-stage controller of the PI controller, and it can optimize the input signal of the PI controller.

[0237] It should be noted that the steps in the method provided by the present invention can be implemented by corresponding modules, devices, etc. in the system. Those skilled in the art can refer to the technical solution of the method to implement the composition of the system. That is, the embodiments in the method can be understood as the preferred examples for constructing the system and will not be elaborated herein.

[0238] The non-isolated high turns ratio bidirectional AC / DC converter control optimization method and system provided by the above embodiments of the present invention optimize the high-voltage series voltage divider control structure of the existing non-isolated high turns ratio bidirectional AC / DC converter, and improve the control effects of the AC / DC converter on the equal voltage, steady voltage and circulating current of the sub-modules; through small-signal modeling of the circuit structure of the existing non-isolated high turns ratio bidirectional AC / DC converter, the design basis of the control parameters of the high-voltage series voltage divider and the AC port is given, and then the actual system is guided; it can effectively reduce the influence of the resonant circulating current on the steady-state control effect of the high-voltage series voltage divider, and at the same time reduce the influence of the k and 2k harmonics (such as the 12th and 24th harmonics) generated by the k-pulse (such as 12-pulse) uncontrolled rectification on the control effect of the low-voltage AC port; it can facilitate the tuning of the control parameters of the high-voltage series voltage divider and the low-voltage AC port.

[0239] Matters not described in detail in the above embodiments of the present invention are all well-known technologies in the art.

[0240] The specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art can make various deformations or modifications within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A control optimization method for a non-isolated high turn ratio bidirectional AC / DC converter, characterized in that include: The control of the high voltage series voltage divider is optimized and the control of the low voltage AC port is optimized; wherein: Non-isolated high-ratio bidirectional AC / DC converter topology, comprising: a resonant branch (101), a first high-voltage series voltage divider (102), a second high-voltage series voltage divider (103), and a three-phase low-voltage AC port (104); the resonant branch (101) is composed of a resonant capacitor C bp and a resonant inductor L bp connected in series, serving as the resonant energy channel of the first high-voltage series voltage divider for balancing the input and output power of the high-voltage sub-module string; the first high-voltage series voltage divider (102) and the second high-voltage series voltage divider (103) are respectively composed of a plurality of cascaded sub-modules and are respectively connected in series at both ends of the three-phase low-voltage AC port (104) for voltage division of the DC voltage to reduce the DC-side voltage of the three-phase low-voltage AC port (104); each phase arm of the three-phase low-voltage AC port (104) is composed of a cascade of several half-bridge structure sub-module strings and phase arm inductors for inversion of the low DC-side voltage; The optimization of the control of the high-voltage series voltage divider includes: According to the pulse number k of the uncontrolled rectifier, a notch filter with the kth harmonic as the center frequency is introduced into the feedback signal of the DC side resonant ring current of the high-voltage series voltage divider to eliminate the kth harmonic of the DC side current of the converter; Obtain the quasi-PR control parameters of the voltage-equalizing inner loop, and use a quasi-PR controller to control the resonant circulating current; Obtain PI control parameters of the voltage balancing outer loop, voltage stabilization inner loop, and voltage stabilization outer loop, and introduce a resonant frequency notch filter into the feedback links of the capacitor voltage balancing outer loop, voltage stabilization inner loop, and voltage stabilization outer loop of the AC / DC converter submodule to filter out interference caused by the resonant current; The optimization of the control of the low voltage AC port includes: The voltage and frequency of the PCC point at the wind farm grid connection are controlled by a double closed loop, and the inner current loop is introduced to add virtual damping to enhance the stability of the system. According to the pulse number k of the uncontrolled rectifier, the corresponding k-th harmonic trap is used to filter the k-th harmonic current of the DC side resonant ring current of the high-voltage series voltage divider to obtain a low-harmonic voltage-equalizing inner ring resonant current feedback signal; The PI control parameters of the voltage outer loop and the current inner loop are obtained, and according to the pulse number k of the rectifier converter station, notch filters with k and 2k harmonics as the center frequencies are introduced into the three-phase output voltage and current feedback signals of the low-voltage AC port respectively, so as to eliminate the k±1 and 2k±1 voltage and current harmonics on the AC side of the uncontrolled rectifier converter station.

2. The control optimization method for a non-isolated high turns ratio bidirectional AC / DC converter according to claim 1, characterized in that, The obtaining of the quasi-PR control parameters of the pressure-equalizing inner loop includes: Construct the quasi-PR controller H for the equalizing inner loop PR ; According to the topology structure of the non-isolated high-ratio bidirectional AC / DC converter, an average value circuit model of the non-isolated high-ratio bidirectional AC / DC converter is obtained; Determining a DC side circuit relationship of the average value circuit model according to the average value circuit model; Obtain the transfer function H(s) of the control object of the voltage equalization inner loop according to the DC-side circuit relationship p1 (s); According to the transfer function H of the controlled object of the voltage equalizing inner loop p1 (s) and the quasi-PR controller H of the voltage equalizing inner loop PR control structure, the open-loop transfer function Hopen1(s) of the closed-loop system structure of the voltage equalizing inner loop is obtained; According to the open-loop transfer function Hopen1(s) of the closed-loop system structure of the inner pressure equalizing loop, the inner pressure equalizing loop bandwidth is determined so that the inner pressure equalizing loop bandwidth is greater than the resonant frequency to complete the acquisition of the quasi-PR control parameters of the inner pressure equalizing loop.

3. The control optimization method of the non-isolated high turns ratio bidirectional AC / DC converter according to claim 2, wherein, Also includes any one or more of the following: The quasi-PR controller H of the equalizing inner ring PR is as follows: In Equation (50), K P is the proportional parameter of the H PR controller, K R is the resonant gain parameter of the H PR controller, ζ is the resonant damping of the H PR controller, ω n is the resonant frequency of the H PR controller, and s is the complex frequency; The step of determining the DC side circuit relationship of the average value circuit model according to the average value circuit model includes: Referring to the average value circuit model, the following equation is established: In formula (51), m1, i dc , u dc1 , N1, U1, u res and Δu res are respectively the modulation ratio, DC - side injected current, equivalent capacitor voltage, number of sub - modules, rated DC voltage, reference resonant voltage and resonant current control voltage of the high - voltage series voltage divider; m ux , m lx , i ux , i lx , u ux , u lx are respectively the modulation ratio of the upper - arm x - phase, modulation ratio of the lower - arm x - phase, current of the upper - arm x - phase, current of the lower - arm x - phase, equivalent capacitor voltage of the upper - arm x - phase and equivalent capacitor voltage of the lower - arm x - phase on the low - voltage AC port, N2, L, U2 and u x are respectively the number of sub - modules of the bridge arm, bridge - arm inductance, rated DC voltage and AC x - phase modulation voltage of the low - voltage AC port; U dc and C are respectively the total DC - side voltage and sub - module capacitor of the non - isolated high - turn - ratio bidirectional AC / DC converter; The transfer function H(s) of the control object of the voltage equalization inner loop obtained according to the DC-side circuit relationship includes: p1 (s), including: The following transformation is made to equation (51): In Equation (52), i res is the resonant circulating current of the AC / DC converter; Combining equation (51) and equation (52) yields equation (53): In formula (53), i res is the DC-side resonant circulating current of the non-isolated high turns ratio bidirectional AC / DC converter, L is the arm inductor of the low-voltage AC port, and Δu res is the resonant current control voltage of the high-voltage series voltage divider; The transfer function H(s) of the controlled object of the inner equalizing voltage loop is obtained according to Equation (54): p1 (s) is as follows: In formula (54), the wavy lines above the variables are small perturbations of the variables; The transfer function H of the controlled object according to the equalizing inner loop p1 (s) and the quasi-PR controller control structure of the equalizing inner loop to obtain the open-loop transfer function H of the closed-loop system structure of the equalizing inner loop open1 (s), including: According to Equation (54) and combined with the quasi-PR controller control structure of the voltage equalization inner loop, the open-loop transfer function H of the closed-loop system structure of the voltage equalization inner loop is obtained open1 (s) is expressed as: H open1 (s) = H PR (s)H P1 (s)H bs (s) (55) In Equation (55), H PR (s) is the transfer function of the quasi-resonant controller, and H bs (s) is the transfer function of the band-stop filter. The transfer function expression is as shown in (56): In Equation (56), ω n is the center frequency of the band-stop filter, where ζ is the damping and s is the complex frequency.

4. The control optimization method for the non-isolated high turns ratio bidirectional AC / DC converter according to claim 1, wherein The step of obtaining the PI control parameters of the pressure-sharing outer loop includes: Construct the submodule voltage state equations for the high voltage series divider and the AC port; According to the voltage state equations of the equalizing outer loop control structure, the high-voltage series voltage divider, and the sub-module of the AC port, the transfer function H p2 (s) of the control object of the equalizing outer loop is obtained; According to the transfer function H p2 (s) of the controlled object of the voltage equalizing outer loop and the closed-loop system structure, the open-loop transfer function H open2 (s) of the voltage equalizing outer loop is obtained; According to the open-loop transfer function H open2 (s) of the voltage equalizing outer loop, determine the bandwidth of the voltage equalizing outer loop so that the bandwidth of the voltage equalizing outer loop is within a preset threshold value, so as to complete the acquisition of the PI control parameters of the voltage equalizing outer loop.

5. The control optimization method for the non-isolated high turns ratio bidirectional AC / DC converter according to claim 4, wherein Also includes any one or more of the following: The submodule voltage state equation for constructing the high-voltage series voltage divider and the AC port includes: According to the controlled source power conservation, referring to the average value circuit model of the non-isolated high-ratio bidirectional AC / DC converter, the following equation is established: In Equation (57), the variables U1, u res , N1, U2, N2, I dc , i res and C are the DC voltage, resonance voltage, number of series sub-modules of the high-voltage series voltage divider, the DC voltage, number of sub-modules of the low-voltage AC port, the DC injection current, resonance circulating current and sub-module capacitance of the non-isolated high-ratio bidirectional AC / DC converter; u c1 is the average voltage of the sub-module capacitance of the high-voltage series voltage divider, u c2 is the average voltage of the sub-module capacitance of the low-voltage AC port, and P ac is the AC output power of the low-voltage AC port; The transfer function H(s) of the control object of the outer voltage equalization loop is obtained according to the voltage state equation of the sub-module of the voltage equalization outer loop control structure, the high-voltage series voltage divider, and the AC port, and includes: p2 (s), including: The output of the voltage equalization outer loop control structure is the resonant circulating current i res of the reference amplitude I * res wherein the reference amplitude I * res continues to be multiplied by the unit sine signal of the resonant frequency, and then the reference resonant circulating current i * res is obtained. The feedback of the voltage equalization outer loop control structure is the difference u c1 -u c2 between the average capacitor voltages of the high-voltage series voltage divider sub-module and the low-voltage AC port sub-module. Then, the transfer function H P2 (s) of the controlled object of the voltage equalization outer loop is as follows: The tilde superscript in (58) represents a small perturbation of the variable; Since U1I in Equation (57) dc , U2I dc and P ac are all regarded as constants and their small perturbations are 0, by combining Equation (57) and Equation (58), the transfer function H p2 (s) of the controlled object of the voltage-sharing outer loop is obtained as follows: In Equation (59), U res , U c1 , U c2 are the steady-state operating points of the amplitudes of u res , u c1 , and u c2 respectively; The transfer function H of the controlled object according to the voltage equalizing outer loop p2 (s) and the closed-loop system structure are used to obtain the open-loop transfer function H open2 (s) of the voltage equalizing outer loop, including: According to formula (59), combined with the pressure balancing outer loop control structure, the transfer function of the pressure balancing outer loop is obtained; Since the bandwidth of the outer voltage equalization loop is lower than that of the inner voltage equalization loop, within the bandwidth of the outer voltage equalization loop, the closed-loop transfer function of the inner voltage equalization loop is equivalent to 1, and the closed-loop system structure of the outer voltage equalization loop system is obtained. Therefore, the open-loop transfer function H open2 (s) expression is as follows: H open2 (s) = H PI1 (s)H P2 (s)H bs (s) (60) In Equation (60), H PI1 (s) is the PI controller of the outer voltage equalization loop, and H P2 (s) is the transfer function of the controlled object of the inner voltage equalization loop, and H bs (s) is the transfer function of the band-stop filter.

6. The control optimization method of the non-isolated high turns ratio bidirectional AC / DC converter according to claim 1, wherein, The obtaining of the PI control parameters of the voltage stabilization inner loop includes: According to the voltage stabilization inner loop control structure and the DC equivalent circuit of the non-isolated high turns ratio bidirectional AC / DC converter, the transfer function H p3 (s) of the controlled object of the voltage stabilization inner loop is obtained; According to the transfer function H p3 (s) of the controlled object of the voltage stabilization inner loop and the closed-loop system structure, the open-loop transfer function H open3 (s) of the voltage stabilization inner loop is obtained; According to the open-loop transfer function H open3 (s) of the voltage stabilizing inner loop, determine the bandwidth of the voltage stabilizing inner loop so that the bandwidth of the voltage stabilizing inner loop is within 100 - 200 Hz, and obtain the PI control parameters of the voltage stabilizing inner loop.

7. The control optimization method for the non-isolated high turns ratio bidirectional AC / DC converter according to claim 6, wherein It also includes any one or any combination of the following: The control object transfer function H(s) of the voltage stabilization inner loop is obtained according to the voltage stabilization inner loop control structure and the DC equivalent circuit of the non-isolated high turns ratio bidirectional AC / DC converter, and includes: p3 (s), including: According to the control structure of the voltage stabilizing inner loop, its output quantity is the DC modulation voltage u1 of the high-voltage series voltage divider, and its feedback quantity is the DC injection current I of the non-isolated high-ratio bidirectional AC / DC converter dc , that is, to determine the transfer function H of the control object of the voltage stabilizing inner loop p3 (s) is in the form of; In formula (61), the wavy line superscript represents small perturbations of variables. According to the DC equivalent circuit of the non-isolated high turns ratio bidirectional AC / DC converter and combining with the transfer function H p3 (s) form, the transfer function H p3 (s) of the control object of the voltage stabilizing inner loop is as follows: In Equation (62), L, U1, U2, L bp , C bp , R bp , L dc , R dc , U dc and I dc are respectively the arm inductor of the low - voltage AC port, the DC voltage of the series high - voltage divider, the DC voltage of the low - voltage AC port, the resonant inductor, the resonant capacitor, the stray resistance of the resonant branch, the equivalent inductor on the DC side of the system, the equivalent resistance on the DC side of the system, the equivalent voltage on the DC side of the system, and the DC injection current of the non - isolated high - turn - ratio bidirectional AC / DC converter. The wavy line superscript is a small perturbation of the variable, and s is the complex frequency; The transfer function H of the control object of the said voltage stabilizing inner loop p3 (s) and the closed-loop system structure are used to obtain the open-loop transfer function H open3 (s) of the voltage stabilizing inner loop, including: According to Equation (62) and combined with the closed-loop system structure of the voltage stabilization inner loop, the expression of the open-loop transfer function H open3 (s) is as follows: H open3 (s) = H PI2 (s)H P3 (s)H bs (s) (63) In Equation (63), H PI2 (s) is the PI controller of the inner voltage stabilization loop, and H p3 (s) is the transfer function of the controlled object of the inner voltage stabilization loop, and H bs (s) is the transfer function of the band-stop filter.

8. The control optimization method of the non-isolated high turns ratio bidirectional AC / DC converter according to claim 1, wherein The obtaining of the PI control parameters of the voltage stabilization outer loop includes: According to the power relationship between the voltage stabilization outer loop control structure and the non-isolated high-ratio bidirectional AC / DC converter, the transfer function H p4 (s) of the controlled object of the voltage stabilization outer loop is obtained; According to the transfer function H p4 (s) of the controlled object of the voltage stabilizing outer loop and the closed-loop system structure, the open-loop transfer function H open4 (s) of the voltage stabilizing outer loop is obtained; According to the open-loop transfer function H open4 (s) of the voltage stabilization outer loop, determine the bandwidth of the voltage stabilization outer loop so that the bandwidth of the voltage stabilization outer loop is within a preset threshold value, and complete the acquisition of the PI control parameters of the voltage stabilization outer loop.

9. The control optimization method of the non-isolated high turns ratio bidirectional AC / DC converter according to claim 8, wherein, It also includes any one or any combination of the following: According to the power relationship between the regulated outer loop control structure and the non-isolated high-ratio bidirectional AC / DC converter, the transfer function H p4 (s) of the control object of the regulated outer loop is obtained, including: According to the control structure of the voltage-stabilizing outer loop, its output is I * dc The DC injection current i of the non-isolated high-ratio bidirectional AC / DC converter dc , and its feedback quantity is the average voltage u of all sub-modules of the non-isolated bidirectional AC / DC converter c , and the transfer function H p4 (s) is expressed as; In formula (64), the wavy line superscript represents small perturbations of control variables. The transfer function H(s) of the control object of the voltage stabilization outer loop is obtained through the power relationship of the AC / DC converter and the form of the transfer function of the control object of the voltage stabilization outer loop. p4 (s) is as follows: In Equation (65): u c1 , N1, and i dc are the capacitor voltage of the high-voltage series voltage divider sub-module, the number of sub-modules, and the DC-side injection current. u c2 , N2, and P ac are the capacitor voltage of the low-voltage AC port sub-module, the number of sub-modules, and the AC output power, which are equivalent to constants; U dc , L dc , and R dc are the internal electromotive force, internal reactance, and resistance of the DC source of the non-isolated high-ratio bidirectional AC / DC converter, respectively; the average voltage u c = u c1 = u c2 ; I dc and U c are the steady-state operating points of i dc and u c respectively, and s is the complex frequency; The transfer function H p4 (s) of the control object according to the voltage stabilization outer loop and the closed-loop system structure are used to obtain the open-loop transfer function H open4 (s) of the voltage stabilization outer loop, including: Since the bandwidth of the voltage regulation outer loop is smaller than that of the voltage regulation inner loop, the closed-loop transfer function of the voltage regulation inner loop is equivalent to 1 within the bandwidth of the voltage regulation outer loop. Then, the expression of the open-loop transfer function H open4 (s) of the closed-loop system structure of the voltage regulation outer loop is as follows: H open4 (s) = H PI3 (s)H P4 (s)H bs (s) (66) In Equation (66), H PI3 (s) is the PI controller of the voltage-stabilizing outer loop, and H P4 (s) is the transfer function of the controlled object of the voltage-stabilizing outer loop, and H bs (s) is the transfer function of the band-stop filter.

10. The control optimization method for the non-isolated high turns ratio bidirectional AC / DC converter according to claim 1, wherein, The obtaining of the PI control parameters of the current inner loop includes: According to the control structure of the inner current loop and the equivalent circuit of the low-voltage AC port, the transfer function H p5 (s) of the controlled object of the inner current loop is obtained; According to the transfer function H p5 (s) of the controlled object of the current inner loop and the closed-loop system structure, the open-loop transfer function H open5 (s) of the current inner loop is obtained; According to the open-loop transfer function H open5 (s) of the current inner loop, determine the bandwidth of the current inner loop such that the bandwidth of the current inner loop is within 100 - 200 Hz, and complete the acquisition of the PI control parameters of the current inner loop.

11. The control optimization method of the non-isolated high turn ratio bidirectional AC / DC converter according to claim 10, characterized in that It also includes any one or any combination of the following: The transfer function H(s) of the controlled object of the inner current loop is obtained according to the control structure of the inner current loop and the equivalent circuit of the low-voltage AC port, including: p5 (s), including: According to the control structure of the inner current loop, the transfer function H of the controlled object of the inner current loop is obtained p5 (s) is in the form of: where \(i\) d is the d-axis feedback of the current loop, \(U\) d is the d-axis output of the current loop, and the tilde superscript represents small perturbations; According to the equivalent circuit of the low-voltage AC port, the transfer function H(s) of the controlled object of the inner current loop is obtained as follows: p5 (s) is: In Equation (68), u j , i j are the three-phase output voltage and current of the low-voltage AC port, j represents the phase, u WFj is the three-phase input voltage of the wind farm, j represents the phase; u d , u q , i d , i q are the dq-axis components of the output voltage and current of the low-voltage AC port, u WFd , u WFq are the dq-axis voltage components of the wind farm voltage u WFj ; L and R are the arm inductance and resistance of the low-voltage AC port; L T , R T are the equivalent inductance and resistance of the connecting transformer; ω1 is the power frequency angular frequency, s is the complex frequency; The transfer function H of the controlled object of the current inner loop p5 (s) and the closed-loop system structure are used to obtain the open-loop transfer function H open5 (s) of the current inner loop, including: According to Equation (68) and the closed-loop system structure of the inner current loop, the open-loop transfer function H open5 (s) is expressed as: H open5 (s) = H PI4 (s)H P5 (s)H lp (s)[1 - H bs1 (s) - H bs2 (s)] (69) In Equation (69), H PI4 (s) is the current-loop PI controller, H p5 (s) is the transfer function of the controlled object of the inner current loop, H lp (s) is the low-pass filter, H bs1 (s), H bs2 (s) is the band-stop filter, T lp is the low-pass filtering time constant, ω n1 is the angular frequency of the kth harmonic, k is the number of pulses of the uncontrolled rectifier, ω n2 is the angular frequency of the 2kth harmonic, ζ is the damping coefficient, s is the complex frequency; among them, the band-stop filters H bs1 (s), H bs2 (s) are as follows:

12. The non-isolated high turns ratio bidirectional AC / DC converter control optimization method according to claim 1, wherein The obtaining of the PI control parameters of the voltage outer loop includes: According to the control structure of the voltage outer loop and the equivalent circuit of the low-voltage AC port and the wind farm interconnection system, the transfer function H p6 (s) of the controlled object of the voltage outer loop is obtained; According to the transfer function H p6 (s) of the controlled object of the voltage outer loop and the closed-loop system structure, the open-loop transfer function H open6 (s) of the voltage outer loop is obtained; According to the open-loop transfer function H open6 (s) of the voltage outer loop, determine the voltage outer loop bandwidth such that the voltage outer loop bandwidth is less than a preset threshold, and complete the acquisition of the PI control parameters of the voltage outer loop.

13. The optimization method for controlling a non-isolated high turns ratio bidirectional AC / DC converter according to claim 12, characterized in that, It also includes any one or any combination of the following: The transfer function H(s) of the controlled object of the voltage outer loop is obtained based on the control structure of the voltage outer loop and the equivalent circuit of the low-voltage AC port and the wind farm interconnection system, and includes: p6 (s), including: Combining the output and feedback of the voltage outer loop, and at the same time according to the control structure of the voltage outer loop, the transfer function H of the controlled object of the voltage outer loop is obtained p6 (s) is in the form of: In Equation (71), u WFd is the d-axis voltage input of the wind field, and i dr is the reference input current of the dq axes of the current loop. The tilde superscript represents small perturbations; According to the circuit structures of the low-voltage AC port and the wind farm, the transfer function H(s) of the control object of the voltage outer loop is obtained as follows: p6 (s) is: In Equation (72), u WFd , u WFq are the dq-axis voltages input by the wind field, and i dr , i qr are the dq-axis reference input currents of the current loop. The tilde superscript represents small perturbations; The transfer function H of the controlled object according to the voltage outer loop p6 (s) and the closed-loop system structure are used to obtain the open-loop transfer function H open6 (s) of the voltage outer loop, including: Within the bandwidth of the voltage outer loop, if the magnitude of the closed-loop transfer function of the current inner loop is equivalent to 1, the open-loop transfer function H of the closed-loop system structure of the voltage outer loop is obtained. open6 (s) is expressed as: H open6 (s) = H PI5 (s)H P6 (s)H lp (s)[1 - H bs1 (s) - H bs2 (s)] (73) In Equation (73), H PI5 (s) is the PI controller of the outer voltage loop, and H P6 (s) is the transfer function of the controlled object of the outer voltage loop, and H lp (s) is the low-pass filter, and H bs1 (s), H bs2 (s) is the band-stop filter.

14. A non-isolated high turns ratio bidirectional AC / DC converter control optimization system, characterized in that, It includes: A high-voltage series voltage divider control optimization part and a low-voltage AC port control optimization part; where: Non-isolated high-ratio bidirectional AC / DC converter topology, comprising: a resonant branch (101), a first high-voltage series voltage divider (102), a second high-voltage series voltage divider (103), and a three-phase low-voltage AC port (104); the resonant branch (101) is formed by connecting a resonant capacitor C bp and a resonant inductor L bp in series, serving as the resonant energy channel of the first high-voltage series voltage divider for balancing the input and output power of the high-voltage sub-module string; the first high-voltage series voltage divider (102) and the second high-voltage series voltage divider (103) are respectively formed by cascading a plurality of sub-modules and are respectively connected in series at both ends of the three-phase low-voltage AC port (104) for DC voltage division to reduce the DC-side voltage of the three-phase low-voltage AC port (104); each phase bridge arm of the three-phase low-voltage AC port (104) is formed by cascading a plurality of half-bridge structure sub-module strings and a bridge arm inductor for inverting the low DC-side voltage; The high-voltage series voltage divider control optimization part includes: A notch filter module in the high-voltage series voltage divider control structure, which introduces a notch filter with the kth harmonic as the center frequency for the feedback signal of the DC-side resonant circulating current of the high-voltage series voltage divider according to the pulse number k of the uncontrolled rectifier to eliminate the kth current harmonic on the DC side of the converter. A quasi-PR control module, which obtains the quasi-PR control parameters of the voltage equalization inner loop and uses a quasi-PR controller to control the resonant circulating current. A PI control module in the high-voltage series voltage divider control structure, which obtains the PI control parameters of the voltage equalization outer loop, the voltage stabilization inner loop, and the voltage stabilization outer loop, and introduces a notch filter with the resonant frequency in the feedback links of the voltage equalization outer loop, the voltage stabilization inner loop, and the voltage stabilization outer loop of the AC / DC converter sub-module capacitors to filter out the interference generated by the resonant current. The low-voltage AC port control optimization part includes: A double closed-loop control module, which uses double closed-loop control to control the voltage and frequency at the PCC point of the wind farm grid connection, and introduces a current inner loop for additional virtual damping to enhance the stability of the system. A notch filter module in the low-voltage AC port control structure, which filters out the kth harmonic current of the DC-side resonant circulating current of the high-voltage series voltage divider using a corresponding kth harmonic notch filter according to the pulse number k of the uncontrolled rectifier to obtain a low-harmonic voltage equalization inner loop resonant current feedback signal. A PI control module in the low-voltage AC port control structure, which obtains the PI control parameters of the voltage outer loop and the current inner loop, and introduces notch filters with the kth and 2kth harmonics as the center frequencies for the three-phase output voltage and current feedback signals of the low-voltage AC port respectively according to the pulse number k of the rectifier converter station to eliminate the k±1 and 2k±1th voltage and current harmonics on the AC side of the uncontrolled rectifier converter station.

15. A computer device, comprising a memory, a processor, and a computer program stored on the memory and executable on the processor, characterized in that, When the processor executes the program, it can be used to execute the method described in any one of claims 1-13.

16. A computer-readable storage medium having a computer program stored thereon, characterized in that, When the program is executed by the processor, it can be used to execute the method described in any one of claims 1-13.

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