Impedance reshaping method for dc power supply system accessing water production load

Abstract

The application discloses an impedance reshaping method of a direct-current power supply system connected with a water-making load, and the method comprises the following steps: firstly, obtaining a small signal model according to a mathematical model of a VSC; then, deducing a direct-current side impedance model of the VSC according to the small signal model of the VSC, and deducing a direct-current side impedance model of the water-making load according to the working characteristics of the water-making load; again, obtaining an impedance amplitude-frequency characteristic according to the impedance model of the source and load side, and analyzing the impedance amplitude-frequency characteristic according to the impedance stability principle to obtain the mechanism of high-frequency oscillation of the direct-current power supply system; then, designing an impedance reshaping strategy based on direct-current voltage feedback to suppress the high-frequency oscillation of the direct-current power supply system; and finally, designing a complete impedance reshaping method according to the impedance reshaping strategy to reshape the impedance of the direct-current power supply system connected with the water-making load. The application can suppress the high-frequency oscillation of the direct-current water-making load power supply system, and improve the power supply quality and operation stability of the system.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of new energy power supply system control, and particularly relates to an impedance remodeling method of a direct current power supply system connected with water production load. BACKGROUND

[0002] Solving the problem of fresh water supply on the island is the primary problem of island development. Therefore, the power supply of water production load on the island is a problem worth studying. In recent years, renewable energy such as wind power and photovoltaic has developed rapidly, and micro-grid systems can efficiently integrate and utilize these distributed new energy and power electronic devices. When the devices and power sources of the alternating current micro-grid are connected, the power electronic converters in each are in parallel, and the slight differences in the amplitude, frequency or phase of the port voltage of each power electronic converter will cause system circulating current, which will worsen the power supply quality and stable operation of the entire system. Compared with the alternating current micro-grid, the direct current power supply system does not need to consider phase synchronization, frequency stability, reactive power compensation and other problems, and the system control and operation are simpler. Compared with the alternating current water production load, the direct current water production load can save the intermediate AC / DC converter, and can be connected to the DC bus through DC / DC or directly connected to the DC bus, which greatly improves the energy transmission efficiency.

[0003] The "double high characteristics" of high proportion of new energy access and high proportion of power electronic in the direct current micro-grid system result in insufficient inertia of the direct current micro-grid system. The volatility and intermittency of the renewable distributed power supply bring instability factors to the direct current micro-grid, and the direct current bus voltage is prone to fluctuate sharply. And the water production load and other loads usually operate in constant power mode, and the constant power load has a negative damping characteristic in a wide frequency band, which weakens the damping of the system and causes system oscillation.

[0004] The voltage source converter (VSC) can realize power transmission between alternating current and direct current systems as an interconnection interface. When the direct current micro-grid of the island is connected to the alternating current power grid, the alternating current grid usually presents a weak grid characteristic due to the long distance, which will affect the control and further affect the damping characteristics of the system and the oscillation law.

[0005] According to the impedance stability theory, when the phase difference of the two-port impedance at the amplitude intersection point is greater than 180 degrees, small signal instability will occur. Since the VSC direct current port is usually connected in parallel with the direct current bus capacitor, at high frequency, the VSC direct current port will exhibit capacitive characteristics, while the line will exhibit inductive characteristics due to the presence of line inductance, and the control of the VSC and the constant power water production load will introduce high-frequency negative damping. Therefore, when the direct current water production load is connected to the direct current micro-grid, the suppression of high-frequency oscillation needs to be considered.

[0006] Most researches focus on the AC side impedance characteristics and AC side impedance reshaping of VSC, which are not suitable for explaining the DC side oscillation phenomenon and guiding the DC side impedance reshaping of VSC to suppress the DC bus oscillation. Therefore, it is necessary to model the impedance of the power supply system from the DC side, analyze the system oscillation reason based on the modeling, and design a DC side impedance reshaping method of VSC to suppress the oscillation and improve the power supply quality of the system. SUMMARY

[0007] The present application aims at the deficiencies of the prior art, and provides an impedance reshaping method of a DC power supply system connected with water production load.

[0008] The present application is achieved by the following technical scheme: an impedance reshaping method of a DC power supply system connected with water production load, comprising the following steps:

[0009] (1) obtaining a small signal model according to a mathematical model of VSC, wherein the mathematical model comprises a main circuit model and a controller model;

[0010] (2) obtaining a DC side impedance model of VSC according to the small signal model of VSC obtained in the step (1), and obtaining a DC side impedance model of DC water production load according to the working characteristics of the DC water production load;

[0011] (3) obtaining an impedance amplitude-frequency characteristic according to the DC side impedance model of VSC and the DC side impedance model of DC water production load obtained in the step (2), and analyzing the impedance amplitude-frequency characteristic according to the impedance stability theory to obtain the mechanism of high-frequency oscillation of the DC power supply system;

[0012] (4) designing an impedance reshaping strategy based on DC voltage feedback according to the mechanism of high-frequency oscillation obtained in the step (3) to suppress the high-frequency oscillation of the DC power supply system;

[0013] (5) designing a complete impedance reshaping method according to the impedance reshaping strategy obtained in the step (4) to reshape the impedance of the DC power supply system connected with water production load.

[0014] Optionally, the step (1) of obtaining a small signal model according to a mathematical model of VSC specifically comprises:

[0015] The main circuit equation corresponding to the main circuit model of VSC in the system coordinate system is:

[0016]

[0017] wherein s represents the system coordinate, v dq represents the voltage at the grid-connected point PCC, v dq =[vd v q ] T ;d dq Denotes duty cycle, d dq =[d d d q ] T ;i dq Indicates the current on the AC side of the VSC, i dq =[i d i q ] T ;v dc 、i dc are the voltage and current on the DC side of VSC respectively; Z L Represents the line impedance in the main circuit model;

[0018] Take the small signal of the main circuit equation to obtain the main circuit small signal model, which is expressed as:

[0019]

[0020] Where, ^ represents a small signal, D dq Represents the steady-state value of the duty cycle, V dc Indicates the steady-state value of the DC bus voltage, I dq Indicates the steady-state value of the VSC AC side current;

[0021] The controller equation corresponding to the controller model of VSC in the controller coordinate system is:

[0022]

[0023] Where c represents the controller coordinate, H i represents the current controller transfer function, K ip Represents the proportional coefficient of the current controller, K ii represents the integral coefficient of the current controller, a represents the Laplace operator; G ci represents the current control loop matrix, ω represents the rotation speed of the system coordinate system, and L represents the line inductance value; G del represents the control delay matrix, T d To control the delay, i dqref is the current reference value, i dqref =[i dref i qref ] T ;

[0024] Take the small signal of the controller equation to obtain the controller small signal model, which is expressed as:

[0025]

[0026] Under the weak network condition, the voltage at the PCC fluctuates, resulting in a deviation between the control coordinate system obtained by the phase-locked loop and the system coordinate system. According to the three matrices introduced to obtain the variable conversion relationship between the control coordinate system and the system coordinate system; wherein, The expressions of the three matrices are as follows:

[0027]

[0028] wherein, K PLLp represents a proportional coefficient of the phase-locked loop, K PLLi represents an integral coefficient of the phase-locked loop, D q represents a steady-state value of the duty ratio q-axis component, D d represents a steady-state value of the duty ratio d-axis component;

[0029] The variable conversion relationship between the control coordinate system and the system coordinate system is as follows:

[0030]

[0031]

[0032]

[0033] The formula (2), the formula (4) and the formula (6) to the formula (8) are combined to obtain a small signal model of the VSC.

[0034] Optionally, the step (2) of obtaining the VSC DC side impedance model according to the small signal model of the VSC obtained in the step (1) is specifically as follows:

[0035] According to the small signal model of the main circuit, the following formula is obtained:

[0036]

[0037] wherein, Z g represents a grid impedance;

[0038] According to the small signal model of the main circuit and the small signal model of the controller, the following formula is obtained:

[0039]

[0040] wherein, G pll-v represents an influence introduced by the phase-locked loop,

[0041] The formula (9) and the formula (10) are combined to obtain the formula (11):

[0042]

[0043] Under the control of the DC bus voltage, the expression of the small signal current is:

[0044]

[0045]

[0046] wherein H v represents the transfer function of the DC voltage controller, k vp represents the proportional coefficient of the DC voltage controller, k vi represents the integral coefficient of the DC voltage controller.

[0047] According to the formula (11)-formula (13), the VSC DC side admittance is obtained, which is represented as:

[0048]

[0049] wherein Y vc represents the VSC DC side admittance.

[0050] According to the VSC DC side admittance and the filter capacitance C dc of the VSC DC port, the VSC DC side impedance model is obtained, and the expression thereof is:

[0051]

[0052] wherein Z source is the VSC DC side impedance.

[0053] Optionally, the step (2) of obtaining the DC side impedance model of the DC water-making load according to the working characteristics of the DC water-making load is specifically:

[0054] The working characteristics of the DC water-making load is constant power load characteristics, and the expression of the load power is:

[0055] P L =v L i L =V L I L (16)

[0056]

[0057] wherein P L is the load power, v L is the load voltage, V L is the steady-state value of the load voltage, i L is the load current, and I L is the steady-state value of the load current, YL It is the DC port admittance of DC water generation load and wind power and photovoltaic distributed power supply;

[0058] Based on the filter capacitor connected to the DC water generation load port and the DC line impedance when the DC water generation load is connected to the VSC power port, the DC side impedance model of the DC water generation load is obtained, and its expression is:

[0059]

[0060] Among them, Z load R is the DC side impedance of the DC water generation load, line is the line resistance, L line is the line inductance, C L It is the capacitance value of the DC water generation load in parallel with the DC port of the wind power and photovoltaic distributed power supply.

[0061] Optionally, the impedance stability theory is: when the phase difference at the intersection of the source and load side impedance amplitudes is greater than 180°, the system will oscillate; the mechanism of the high-frequency oscillation is: insufficient system damping and negative damping of the source side DC impedance at high frequencies.

[0062] Optionally, the impedance reshaping strategy is specifically as follows: under the original constant DC voltage control of the VSC, a DC voltage feedforward loop is added to change the active current given value I dref , to change the DC impedance characteristics of the VSC.

[0063] Optionally, the DC voltage feedforward loop is -ka 2 V dc , where k is the feedforward coefficient, a is the Laplace operator, V dc Indicates the steady-state value of the DC bus voltage.

[0064] Optionally, the complete impedance reshaping method is specifically as follows: the phase-locked loop obtains the grid connection point voltage phase, and based on the voltage phase, transforms the three-phase voltage and current into a two-phase rotating system coordinate system; the current inner loop performs PI control on the active current and reactive current, and adds current decoupling and AC voltage feedforward; the outer loop control adopts constant voltage control, and performs PI control on the DC voltage to obtain a given active current value; a DC voltage feedforward link is added to reshape the VSC DC side impedance.

[0065] The beneficial effect of the present application is that the present application establishes a model for the DC side impedance of the DC water production load power supply system under weak power grid conditions, analyzes the system oscillation reason according to the impedance stability theory, and obtains the oscillation mechanism of the DC water production load power supply system under weak power grid; a VSC DC side impedance remodeling method based on DC voltage feedback is designed according to the oscillation mechanism of the power supply system, and a complete impedance remodeling method of the DC power supply system connected with the water production load is designed to improve the phase margin of the power supply system in the original oscillation frequency band, which can suppress the high-frequency oscillation of the DC power supply system and is beneficial to improve the stability and power supply quality of the power supply system. BRIEF DESCRIPTION OF DRAWINGS

[0066] Figure 1 The DC power supply system connected with the DC water production load structure diagram is shown in

[0067] Figure 2 The VSC small signal model block diagram is shown in

[0068] Figure 3 The source side and load side DC impedance amplitude-frequency characteristic curve is shown in

[0069] Figure 4 The control block diagram of the power supply system of the DC water production load under weak power grid is shown in

[0070] Figure 5 The source side DC impedance amplitude-frequency characteristic graph after adding the impedance remodeling strategy is shown in

[0071] Figure 6 The simulation running result is shown in DETAILED DESCRIPTION

[0072] The technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the protection scope of the present application.

[0073] The DC power supply system connected with the DC water production load structure diagram is shown in Figure 1 The DC power supply system connected with the water production load includes a main circuit model and a controller model. In the main circuit model, the AC side of the VSC is connected in series to the three-phase AC power grid V g through the line resistance R, the line inductance L, the grid resistance R g and the grid inductance L gabc ; the DC side of the VSC is connected in parallel to the DC bus capacitor C dc, connected in series through the DC line impedance, connecting the DC water load with constant power equipment such as wind power and photovoltaic power. In the controller model, the three-phase voltage and three-phase current are sampled to obtain the grid phase angle. Through the PARK coordinate transformation, the voltage and current in the three-phase stationary coordinate system are transformed into the voltage and current in the two-phase rotating coordinate system; the voltage outer loop performs PI control on the DC bus voltage to obtain the given value of the active current I dref , reactive current given value I qref Set to 0; the current inner loop performs PI control on the active current and reactive current to obtain the duty cycle.

[0074] In this embodiment, the following text refers to two two-phase rotating coordinate systems: the system coordinate system and the control coordinate system. The system coordinate system rotates at a constant speed (ω = 100πrad / s), and the control coordinate system is a coordinate system obtained by phase-locking a phase-locked loop (PLL). The superscripts s and c denote the state variables in the system coordinate system and the controller coordinate system, respectively, and the superscript ^ denotes the small signal of the corresponding variable.

[0075] The impedance reshaping method of a DC power supply system connected to a water production load of the present invention comprises the following steps:

[0076] (1) Obtain the small signal model based on the mathematical model of VSC.

[0077] In this embodiment, the mathematical model of the VSC includes a main circuit model and a controller model, such as Figure 1 shown.

[0078] See also Figure 1 , the main circuit equation corresponding to the main circuit model of VSC in the system coordinate system is:

[0079]

[0080] Among them, v dq Indicates the voltage at the grid connection point PCC, v dq =[v d v q ] T ;d dq Denotes duty cycle, d dq =[d d d q ] T ;i dq Indicates the current on the AC side of the VSC, i dq =[i d i q ] T ;v dc 、i dc are the voltage and current on the DC side of VSC respectively; Z L Represents the line impedance in the main circuit model.

[0081] Further, the line impedance Z L is expressed as:

[0082]

[0083] wherein a represents a Laplace operator, ω represents a rotation speed of a system coordinate system, L represents a line inductance value, and R represents a line resistance value.

[0084] Taking a small signal of the formula (1), a main circuit small signal model can be obtained, and an expression thereof is:

[0085]

[0086] wherein D dq represents a duty ratio steady-state value, V dc represents a direct-current bus voltage steady-state value, I dq represents a VSC alternating-current side current steady-state value.

[0087] Referring to Figure 1 , a controller equation corresponding to a controller model of the VSC in a controller coordinate system is:

[0088]

[0089] wherein, H i represents a current controller transfer function, K ip represents a proportional coefficient of the current controller, K ii represents an integral coefficient of the current controller. G ci represents a current control loop matrix. G del represents a control delay matrix, T d is a control delay, i dqref is a current reference value, i dqref =[i dref i qref ] T .

[0090] Taking a small signal of the formula (4), a controller small signal model can be obtained, and an expression thereof is:

[0091]

[0092] Under a weak network condition, a voltage at the PCC fluctuates, resulting in a deviation between a control coordinate system and a synchronous coordinate system obtained by a phase-locked loop. Three matrices are introduced to describe a variable conversion between the two coordinate systems, expressions of which are respectively:

[0093]

[0094] wherein, K PLLp represents the proportional coefficient of the phase-locked loop, K PLLi represents the integral coefficient of the phase-locked loop, D q represents the duty ratio q-axis component steady-state value, D d represents the duty ratio d-axis component steady-state value.

[0095] The variable conversion relationship between the control coordinate system and the system coordinate system is:

[0096]

[0097]

[0098]

[0099] The small signal model of the VSC can be obtained by combining the above small signal expressions, and the expression including the superscript ^ is the relevant small signal expression. The block diagram of the small signal model of the VSC is shown in Figure 2 .

[0100] (2) Obtain the DC side impedance model of the VSC according to the small signal model of the VSC obtained in step (1), and obtain the DC side impedance model of the DC water load according to the working characteristics of the DC water load.

[0101] Eliminate in formula (3), and the following formula can be obtained:

[0102]

[0103] wherein, Z g represents the grid impedance.

[0104] Further, the grid impedance Z g is represented as:

[0105]

[0106] wherein, L g represents the grid inductance value, and R g represents the grid resistance value.

[0107] According to formula (3) and formula (5), the following formula can be obtained:

[0108]

[0109] wherein, G pll-v represents the influence introduced by the phase-locked loop.

[0110] Further, the phase-locked loop introduces the influence G pll-v is expressed as:

[0111]

[0112] Combining formula (10) and formula (12), the expression of VSC DC side admittance Y

[0113]

[0114] Referring to Figure 2 Under the control of the DC bus voltage, the expression of the small signal current is:

[0115]

[0116] wherein H v represents the transfer function of the DC voltage controller.

[0117] Further, the expression of the transfer function H v of the DC voltage controller is:

[0118]

[0119] wherein k vp represents the proportional coefficient of the DC voltage controller, and k vi represents the integral coefficient of the DC voltage controller.

[0120] Combining formula (14) and formula (15), the expression of the VSC DC side admittance Y

[0121]

[0122] wherein Y vc represents the VSC DC side admittance.

[0123] The source side DC impedance is composed of two parts, one part is the VSC DC port itself, and the other part is the capacitor C dc connected in parallel with the VSC DC port, therefore, by taking into account the filter capacitor C dc of the VSC DC port, the expression of the VSC DC side impedance model is:

[0124]

[0125] wherein Z source is the VSC DC side impedance.

[0126] In this embodiment, the working characteristic of the DC water production load is constant power load characteristic, and for the DC water production load and the distributed power sources such as wind power and photovoltaic power, since the overall presents constant power load characteristic, the load power is PL , the expression is:

[0127] P L = v L i L = V L I L (19)

[0128]

[0129] wherein v L is the load voltage, V L is the steady-state value of the load voltage, i L is the load current, I L is the steady-state value of the load current, and Y L is the admittance of the DC water-making load and the DC port of the distributed power source such as wind power and photovoltaic power.

[0130] The DC water-making load port is connected with a filter capacitor, and when the DC water-making load is connected to the VSC power source port, the DC line has impedance, and the DC side impedance model of the DC water-making load can be obtained, that is, the DC side impedance of the load is:

[0131]

[0132] wherein Z load is the DC side impedance of the load (i.e., the DC side impedance of the DC water-making load), R line is the line resistance, L line is the line inductance, and C L is the capacitance value of the capacitor connected in parallel with the DC port of the distributed power source such as wind power and photovoltaic power.

[0133] (3) Obtain the impedance amplitude-frequency characteristics according to the VSC DC side impedance model (i.e., the DC side impedance of the source) and the DC side impedance model of the DC water-making load (i.e., the DC side impedance of the load) obtained in step (2), and analyze the impedance amplitude-frequency characteristics according to the impedance stability theory to obtain the mechanism of high-frequency oscillation of the DC power supply system.

[0134] According to the expression of the source side DC impedance (corresponding to formula (18)) and the expression of the load side DC impedance (corresponding to formula (21)) obtained in step (2), the corresponding impedance amplitude-frequency characteristic curves are drawn, as shown in Figure 3 .

[0135] In this embodiment, the impedance stability theory is specifically that when the phase difference at the intersection of the source and load side impedance amplitudes is greater than 180°, the system will oscillate. Exemplarily, as shown in Figure 3 , the source side DC impedance and the load side DC impedance amplitudes intersect at 211 Hz, and the phase difference reaches 187°, so the DC side of the system will oscillate at 211 Hz.

[0136] In summary, the analysis can obtain the mechanism of high-frequency oscillation of the DC power supply system: the reason for high-frequency oscillation is that the system damping is insufficient, that is, the source side DC impedance has negative damping at high frequency, thereby causing oscillation instability.

[0137] (4) Design an impedance reshaping strategy based on DC voltage feedback according to the mechanism of high-frequency oscillation obtained in step (3) to suppress the high-frequency oscillation of the DC power supply system

[0138] In this embodiment, the DC side oscillation is due to the phase difference at the intersection of the source and load side impedance being greater than 180°, and further analysis shows that the VSC DC impedance has negative damping at this frequency band. Therefore, the DC impedance of the VSC is reshaped to move the negative damping out of the frequency band near the impedance intersection frequency, so that the DC impedance of the source and load side meets the impedance stability condition, thereby suppressing the high-frequency oscillation of the DC power supply system, that is, suppressing the DC bus oscillation.

[0139] It should be noted that this embodiment adds a control branch on the basis of the original control, thereby changing the DC impedance characteristics of the VSC, so that the system meets the impedance stability theory and can operate stably.

[0140] Specifically, under the original constant DC voltage control of the VSC, a DC voltage feedforward loop is added to change the given value of the active current I dref , so as to change the DC impedance characteristics of the VSC, thereby suppressing the high-frequency oscillation of the DC power supply system and making the power supply system operate stably.

[0141] The DC voltage feedforward loop uses a second-order differential element, wherein each differential element is subjected to first-order low-pass filtering, that is, multiplied by where a is the Laplace operator, ω cut is the low-pass cutoff angular frequency, and ω cut = 2πf cut , the low-pass cutoff frequency f cut is 1000 Hz, and the feedforward coefficient k is 2e-7. It should be understood that the frequency band within 1000 Hz is effective, so f is 1000 Hz; the value of the feedforward coefficient k is obtained by simulation debugging, and a set of parameters with good running effect is obtained, for example, 2e-7.

[0142] In summary, the added DC voltage feedforward loop is -ka 2 V dc .

[0143] (5) Design a complete impedance reshaping method according to the impedance reshaping strategy obtained in step (4) to reshape the impedance of the DC power supply system connected to the water-making load.

[0144] According to the impedance reshaping strategy obtained in step (4), an impedance reshaping method for the complete DC power supply system connected to the water production load is designed. Specifically, as shown in Figure 4 the control block diagram of the complete impedance reshaping of the DC power supply system connected to the water production load, the phase-locked loop obtains the grid connection point voltage phase, and based on the voltage phase, the three-phase voltage and current are transformed into a two-phase rotating system coordinate system; the inner current loop performs PI control on the active current and the reactive current, and adds current decoupling and AC voltage feedforward; the outer loop control adopts constant voltage control, and PI control is performed on the DC voltage to obtain the given value of the active current. It should be understood that PI control is a classical control method, which is a linear controller, and a control error is formed according to the given value and the actual output value, and the control error is linearly combined to form a control quantity to control the controlled object, which will not be described here.

[0145] On the basis of the above, a DC voltage feedforward link is added, and the dashed box is the added reshaping link, that is, the DC voltage feedforward link, wherein the differential element is as described in step (4), and a first-order low-pass filter is added in the implementation. By reshaping the DC side impedance of the VSC, the phase margin of the system is increased, thereby suppressing high-frequency oscillation.

[0146] As shown in Figure 5 , the source side DC impedance amplitude-frequency characteristic diagram after adding the impedance reshaping strategy proposed in the embodiment is given, and Figure 5 it can be seen that the phase difference of the source load measurement DC impedance at 211 Hz is reduced from 187° to 154°, greatly increasing the phase margin of the system, thereby suppressing high-frequency oscillation.

[0147] As shown in Figure 6 , the simulation running result is shown, and when the impedance reshaping strategy is cut in at 1.2s, the oscillation is quickly suppressed. The results show that the impedance reshaping method has good high-frequency oscillation suppression performance.

[0148] In summary, the embodiment can analyze the high-frequency oscillation mechanism of the DC water production load power supply system under a weak power grid, and then design a VSC DC impedance reshaping method based on DC voltage feedback, which moves the negative damping out of the frequency band near the source load measurement impedance intersection, improves the stability of the system, and solves the high-frequency oscillation problem of the DC water production load power supply system under a weak power grid.

[0149] The above embodiments are only used to illustrate the technical solutions of the present application, but not limit it; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that the technical solutions recorded in the foregoing embodiments can be modified, or some technical features can be replaced by equivalents; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A method for reshaping the impedance of a DC power supply system connected to a water production load, characterized in that: The following steps are involved: (1) obtaining a small signal model based on a mathematical model of the VSC, wherein the mathematical model includes a main circuit model and a controller model; (2) obtaining a VSC DC side impedance model based on the small signal model of the VSC obtained in step (1), and obtaining a DC side impedance model of the DC water-generating load based on the operating characteristics of the DC water-generating load; the DC side impedance model of the DC water-generating load obtained based on the operating characteristics of the DC water-generating load in step (2) is specifically: The operating characteristics of the DC water generation load are constant power load characteristics, and the expression of load power is: P L =v L i L =V L I L (16) Among them, P L is the load power, v L is the load voltage, V L is the steady-state value of the load voltage, i L is the load current, I L is the steady-state value of the load current, Y L It is the DC port admittance of DC water generation load and wind power and photovoltaic distributed power supply; Based on the filter capacitor connected to the DC water generation load port and the DC line impedance when the DC water generation load is connected to the VSC power port, the DC side impedance model of the DC water generation load is obtained, and its expression is: Among them, Z load R is the DC side impedance of the DC water generation load, line is the line resistance, L line is the line inductance, C L The capacitance value of the DC water generation load in parallel with the DC port of the wind power and photovoltaic distributed power supply; (3) obtaining impedance amplitude-frequency characteristics based on the VSC DC side impedance model and the DC side impedance model of the DC water-generating load obtained in step (2), and analyzing the impedance amplitude-frequency characteristics based on the impedance stability theory to obtain the mechanism of high-frequency oscillation in the DC power supply system; (4) designing an impedance reshaping strategy based on DC voltage feedback according to the mechanism of high-frequency oscillation obtained in step (3) to suppress high-frequency oscillation of the DC power supply system; (5) Designing a complete impedance reshaping method based on the impedance reshaping strategy obtained in step (4) to reshape the impedance of the DC power supply system connected to the water production load.

2. The impedance reshaping method of a DC power supply system connected to a water production load according to claim 1, characterized in that: The small signal model is obtained according to the mathematical model of VSC in step (1): The main circuit equation corresponding to the main circuit model of VSC in the system coordinate system is: Where s represents the system coordinates, v dq Indicates the voltage at the grid connection point PCC, v dq =[v d v q ] T ;d dq Denotes duty cycle, d dq =[d d d q ] T ;i dq Indicates the current on the AC side of the VSC, i dq =[i d i q ] T ;v dc 、i dc are the voltage and current on the DC side of VSC respectively; Z L Represents the line impedance in the main circuit model; Take the small signal of the main circuit equation to obtain the main circuit small signal model, which is expressed as: Where, ^ represents a small signal, D dq Represents the steady-state value of the duty cycle, V dc Indicates the steady-state value of the DC bus voltage, I dq Indicates the steady-state value of the VSC AC side current; The controller equation corresponding to the controller model of VSC in the controller coordinate system is: Where c represents the controller coordinate, H i represents the current controller transfer function, K ip Represents the proportional coefficient of the current controller, K ii represents the integral coefficient of the current controller, a represents the Laplace operator; G ci represents the current control loop matrix, ω represents the rotation speed of the system coordinate system, and L represents the line inductance value; G del represents the control delay matrix, T d To control the delay, i dqref is the current reference value, i dqref =[i dref i qref ] T ; Take the small signal of the controller equation to obtain the controller small signal model, which is expressed as: Under weak network conditions, the voltage at the PCC fluctuates, resulting in a deviation between the control coordinate system obtained by the phase-locked loop and the system coordinate system. According to the three matrices introduced To obtain the variable conversion relationship between the control coordinate system and the system coordinate system; The expressions are: in, K PLLp Represents the phase-locked loop proportional coefficient, K PLLi Indicates the integral coefficient of the phase-locked loop, D q Denotes the steady-state value of the duty cycle q-axis component, D d represents the steady-state value of the d-axis component of the duty cycle; The variable conversion relationship between the control coordinate system and the system coordinate system is: Combine Equation (2), Equation (4), and Equation (6)-Equation (8) to obtain the small signal model of VSC.

3. The impedance reshaping method of a DC power supply system connected to a water production load according to claim 1, characterized in that: In step (2), the VSC DC side impedance model is obtained according to the small signal model of the VSC obtained in step (1): The following formula is obtained based on the small signal model of the main circuit: Among them, Z g represents the grid impedance; The following formula is obtained based on the main circuit small signal model and the controller small signal model: Among them, G pll-v Indicates the impact of the phase-locked loop, Combining formula (9) and formula (10) to obtain formula (11): Under constant DC bus voltage control, the expression of small signal current is: Among them, H v represents the transfer function of the DC voltage controller, k vp represents the proportional coefficient of the DC voltage controller, k vi It represents the integral coefficient of the DC voltage controller; According to formula (11)-formula (13), the VSC DC side admittance is obtained as: Among them, Y vc represents the VSC DC side admittance; According to the VSC DC side admittance and the filter capacitor C of the VSC DC port dc Obtain the VSC DC side impedance model, which is expressed as: Among them, Z source is the DC side impedance of VSC.

4. The impedance reshaping method of a DC power supply system connected to a water production load according to claim 1, characterized in that: The impedance stability theory states that when the phase difference at the intersection of the source-load side impedance amplitudes is greater than 180°, the system will oscillate. The mechanism of high-frequency oscillation is that the system is insufficiently damped and the source-side DC impedance has negative damping at high frequencies.

5. The impedance reshaping method of a DC power supply system connected to a water production load according to claim 1, characterized in that: The impedance reshaping strategy is as follows: under the original constant DC voltage control of VSC, a DC voltage feedforward loop is added to change the active current set value I dref , to change the DC impedance characteristics of the VSC.

6. The impedance reshaping method of a DC power supply system connected to a water production load according to claim 5, characterized in that: The DC voltage feedforward loop is -ka 2 V dc , where k is the feedforward coefficient, a is the Laplace operator, V dc Indicates the steady-state value of the DC bus voltage.

7. The impedance reshaping method of a DC power supply system connected to a water production load according to claim 1, characterized in that: The complete impedance reshaping method is specifically as follows: a phase-locked loop obtains the grid connection point voltage phase, and based on the voltage phase, transforms the three-phase voltage and current quantities into a two-phase rotating system coordinate system; the current inner loop performs PI control on the active current and reactive current, and adds current decoupling and AC voltage feedforward; the outer loop control adopts constant voltage control, and performs PI control on the DC voltage to obtain a given active current value; and a DC voltage feedforward link is added to reshape the DC side impedance of the VSC.

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