Converter collection system network construction method based on power grid nonlinear oscillation

Abstract

The invention provides a converter collection system networking method based on power grid nonlinear oscillation, and belongs to the field of energy. Firstly, based on a radial feeder collection system, a power capacitor is additionally arranged on the low-voltage side of a boosting transformer, a feeder inductor-power capacitor combined power oscillator is constructed, a voltage-current nonlinear controller is further constructed, and a network construction topological structure is formed; and finally proposing a nonlinear oscillator equation for representing the network construction dynamic characteristics of the converter collection system. According to the invention, the electrical distance between large power grid synchronous machines can be reduced, large power grid electromagnetic induction coupling strength is constructed, and the power grid synchronous stability is enhanced.

Description

Technical Field

[0001] The invention belongs to the field of energy, and in particular relates to a method for networking a converter collection system based on nonlinear oscillation of a power grid. Background Art

[0002] With the high proportion of power electronic devices and new energy connected to the power system, the "stability" of the voltage and frequency parameters of the power grid, as well as the instantaneous "balance" of power generation and power consumption of the new power system are facing challenges. In the traditional power system, when subject to large and small disturbances, the rotor kinetic energy of the synchronous generator, hot standby and speed regulator, excitation regulation system and power system stabilizer can ensure the stability of the grid voltage and frequency to the greatest extent.

[0003] At present, grid-building technology uses power electronics technology equipment, such as energy storage and its converters, static VAR generators (SVG), wind turbines and photovoltaics and their converters, etc., to enable it to actively respond to, support and enhance the grid inertia, frequency, voltage, and short-circuit current, and ultimately become an independent power source that is more friendly to the grid.

[0004] On the other hand, renewable energy power generation has a wide spatial distribution and a large number of units. It is generally connected to the grid by low voltage access and medium voltage collection. It also has multiple operation control levels, a large-scale security control system, and complex strategies. Photovoltaic and other renewable energy power generation is connected to the grid step by step, and the electrical distance from the main grid is 2-3 times that of conventional thermal power and hydropower units, which reduces the grid strength of the AC synchronous grid and weakens the synchronous coupling capability. Generally, it is necessary to structure and regularize the renewable energy collection system, and solve the problems of uncertain output of new energy and weakening grid strength of AC systems with a high proportion of new energy through hierarchical zoning, various energy storage and other technologies. Summary of the invention

[0005] In order to solve the above technical problems, the present invention proposes a method for networking a converter collection system based on nonlinear oscillation of the power grid. Based on the collection system of radial feeders, a power capacitor is installed on the low-voltage side of the step-up transformer to construct an electric oscillator combining feeder inductance and power capacitor, and a voltage-current nonlinear controller is further constructed to form a network topology. Finally, a nonlinear oscillator equation characterizing the dynamic characteristics of the network of the converter collection system is proposed, and nonlinear oscillation operation and control are realized through the converter, thereby forming a network topology of the converter collection system.

[0006] In order to achieve the above object, the present invention adopts the following technical scheme:

[0007] A method for networking a converter collection system based on nonlinear oscillation of a power grid comprises the following steps:

[0008] Step 1: Based on the radial feeder collection system, a power capacitor is installed on the low-voltage side of the step-up transformer to construct a feeder inductor-power capacitor combined power oscillator;

[0009] Step 2, constructing a voltage-current nonlinear oscillation controller;

[0010] Step 3, designing a nonlinear oscillation network modeling method for the converter collection system to form a network topology structure;

[0011] Step 4: Characterize the nonlinear oscillator equations of the converter collection system network dynamic characteristics and design a voltage-current nonlinear controller control method.

[0012] Beneficial effects:

[0013] The present invention helps to optimize the layout and access mode of the converter, thereby improving the overall performance and response speed of the system, and can achieve real-time adjustment of the grid frequency and power flow, thereby improving the flexibility and adaptability of new energy access and promoting the efficient use of renewable energy. That is, the beneficial effects of the present invention are mainly reflected in the following aspects:

[0014] 1) As the proportion of new energy sources such as photovoltaic and wind power connected to the power grid increases, a converter collection system of feeder-type power oscillators is proposed, which can effectively integrate new energy resources, support their stable grid-connected operation, and promote the utilization of clean energy.

[0015] 2) The power oscillator and converter collection system network topology can improve the power quality of the power grid, reduce harmonic distortion by adjusting voltage and frequency, ensure that the power quality of the power system meets the standard requirements, and protect the safe operation of electrical equipment.

[0016] 3) By treating the new energy collection system as an oscillator synchronous grid power supply, the electrical distance between the synchronous machine and the large grid can be reduced, and the synchronous stability of the grid can be enhanced. By optimizing the transmission network design through synchronous grid power supply, the transmission capacity can be further improved, the transmission loss can be reduced, and the economy and reliability of the grid can be enhanced. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a schematic diagram of the structure of the inverter collection system network provided by the present invention.

[0018] Figure 2 It is a schematic diagram of the principle of nonlinear oscillation network modeling of the converter collection system provided by the present invention.

[0019] Figure 3 It is a schematic diagram of a voltage-current nonlinear controller for a phase-shifting transformer side converter and a parallel transformer side converter provided by the present invention.

[0020] Among them, the reference numerals are: high-voltage AC power grid 101, boost transformer 102, large-capacity power capacitor 201, boost transformer low-voltage side bus 202, first phase-shift series transformer 203, nth phase-shift series transformer 204, Nth phase-shift series transformer 205, first voltage source converter 206, nth voltage source converter 207, Nth voltage source converter 208, DC power supply 209, DC microgrid 210, parallel transformer 211, parallel transformer side converter 212, first PWM generator 2 13. Voltage-current inner loop control 214, nonlinear voltage controller 215, second PWM generator 216, current inner loop control 217, nonlinear current controller 218, voltages of each feeder 219, bus voltage on low voltage side of booster transformer 220, first feeder 301, nth feeder 302, Nth feeder 303, first booster transformer 304, nth booster transformer 305, Nth booster transformer 306, first collection converter 307, nth collection converter 308, Nth collection converter 309. DETAILED DESCRIPTION

[0021] In order to make the purpose, technical scheme and advantages of the present invention clearer, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other. The technical scheme of the present invention will be clearly and completely described below in conjunction with the accompanying drawings. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments.

[0022] The components of the embodiments of the present invention generally described and shown in the drawings herein may be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the claimed invention, but merely represents selected embodiments of the present invention.

[0023] Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative work shall fall within the scope of protection of the present invention.

[0024] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other. The present invention is further described below in conjunction with the accompanying drawings and specific embodiments.

[0025] The present invention provides a method for networking a converter collection system based on nonlinear oscillation of a power grid, comprising the following steps:

[0026] Step 1: Design an electric oscillator consisting of a power capacitor and feeder equivalent reactance, including:

[0027] like Figure 1 As shown, the inverter collection system network topology proposed in the present invention converts the photovoltaic and wind power new energy base into a new power plant to achieve various functions equivalent to or even exceeding those of traditional thermal power plants. The new power plant is mainly for 110kV, 220kV, and 500kV high-voltage AC power grids 101, which are generally boosted by step-up transformers 102. A large-capacity power capacitor 201 is installed on the low-voltage side bus 202 of the step-up transformer, and its capacitance is . The large-capacity power capacitor 201 has the function of withstanding high voltage, and has the function of protecting against overvoltage and overcurrent after a certain period, and has passed the overvoltage periodic test to meet the needs of the large-capacity power capacitor 201 as the power oscillator proposed in the present invention to withstand various functions such as overvoltage and overcurrent. The low-voltage side bus 202 of the step-up transformer is the collection point of the feeders of each branch, and each branch contains the first feeder 301, the nth feeder 302, the Nth feeder 303, the first step-up transformer 304 of 10 / 35kV or 1.0 / 35kV, the nth step-up transformer 305, the Nth step-up transformer 306, and the first collection converter 307, the nth collection converter 308, and the Nth collection converter 309. The first collecting converter 307 is connected to the first step-up transformer 304 of 10 / 35kV or 1.0 / 35kV through the first feeder 301; the nth collecting converter 308 is connected to the nth step-up transformer 305 of 10 / 35kV or 1.0 / 35kV through the nth step-up transformer 305; the Nth collecting converter 309 is connected to the Nth step-up transformer 306 of 10 / 35kV or 1.0 / 35kV through the Nth step-up transformer 306. Figure 1 middle, , , They are the AC voltages at the AC ports of the first collection converter 307 , the nth collection converter 308 , and the Nth collection converter 309 .

[0028] In the present invention, the concentrator converter can refer to a high-power converter or an equivalent model of a low-power converter in each branch. Select the nth feeder and assume that the impedance of a single-phase feeder is , the transformer reactance is ,in , are the feeder resistance and inductance respectively, is the angular frequency of the power frequency, is an imaginary number, n is a counting variable, Therefore, it can be inferred that the inductance of each single-phase feeder from the converter to the step-up transformer 102 is . Figure 1 In FIG. 1 , N is the total number of feeder branches, and T1, Tn, and TN represent the first step-up transformer 304, the nth step-up transformer 305, and the Nth step-up transformer 306, respectively.

[0029] Based on the above description of the power oscillator composed of the power capacitor and the feeder equivalent reactance, it is assumed that the equivalent inductance of each feeder is equal, that is, , is the equivalent inductance of each feeder branch, The equivalent oscillation frequency of the power oscillator of the collection system can be derived as and characteristic impedance They are:

[0030] (1)

[0031] Among them, when the equivalent inductance of each feeder is equal, is the equivalent oscillation frequency, is the equivalent characteristic impedance, is the capacitance of the large-capacity power capacitor 201.

[0032] In the power grid at the sending end of the new energy base, the "double high" system, i.e., a high proportion of new energy and a high proportion of power electronic equipment, has many devices and the system parameters are prone to large fluctuations. Assuming that the equivalent inductance of each feeder is not equal, that is, all are not equal, the equivalent oscillation frequency and characteristic impedance of each feeder are defined as:

[0033] (2)

[0034] (3)

[0035] in, is the equivalent oscillation frequency of the first feeder 301 of the collection system, is the equivalent characteristic impedance of the first feeder 301 of the collection system; is the equivalent oscillation frequency of the nth feeder 302, is the equivalent characteristic impedance of the nth feeder 302; is the equivalent oscillation frequency of the Nth feeder 303, is the equivalent characteristic impedance of the Nth feeder 303; , , are the inductances of each feeder respectively.

[0036] When the average value is taken, the equivalent oscillation frequency and characteristic impedance of the collection system are:

[0037] (4)

[0038] For the convenience of calculation, is the equivalent average characteristic impedance, is the equivalent average oscillation frequency.

[0039] Furthermore, define the variables:

[0040] (5)

[0041] (6)

[0042] (7)

[0043] in, , , are the a-phase currents flowing through the first feeder 301, the nth feeder 302, and the Nth feeder 303 of the collection system, respectively. , , are the b-phase currents flowing through the first feeder 301, the nth feeder 302, and the Nth feeder 303 of the collection system, respectively. , , are the c-phase currents flowing through the first feeder 301, the nth feeder 302, and the Nth feeder 303 of the collection system respectively; , , are the a-phase equivalent voltages of the first feeder 301, the nth feeder 302, and the Nth feeder 303, respectively. , , are the b-phase equivalent voltages of the first feeder 301, the nth feeder 302, and the Nth feeder 303, respectively, , , They are the c-phase equivalent voltages of the first feeder 301 , the nth feeder 302 , and the Nth feeder 303 , respectively.

[0044] Step 2: Design a voltage-current nonlinear controller, including:

[0045] like Figure 1As shown, the present invention proposes a voltage-current nonlinear controller, which is mainly composed of a first phase-shifted series transformer 203, an nth phase-shifted series transformer 204, an Nth phase-shifted series transformer 205, a parallel transformer 211, a voltage source converter, a DC power supply 209, and a DC microgrid 210. The first phase-shifted series transformer 203 is connected to the first voltage source converter 206, the nth phase-shifted series transformer 204 is connected to the nth voltage source converter 207, and the Nth phase-shifted series transformer 205 is connected to the Nth voltage source converter 208; the DC power supply 209, the Nth voltage source converter 208, the nth voltage source converter 207, the first voltage source converter 206, and the parallel transformer side converter 212 together form a DC microgrid 210.

[0046] For the voltage source formed by the phase-shifting series transformer, the target voltage on the feeder side of the first phase-shifting series transformer 203 in series can be expressed as:

[0047] (8)

[0048] in, , , are the three-phase target voltages a, b, and c on the feeder side of the first phase-shifting series transformer 203, For a small controllable parameter, is the equivalent oscillation frequency of the first feeder of the collection system, is the input equivalent excitation voltage, , , They are the a, b, and c three-phase voltages of the large-capacity power capacitor 201 respectively.

[0049] The target voltage of the nth phase-shifted series transformer 204 can be expressed as:

[0050] (9)

[0051] in, , , are the three-phase target voltages of a, b, and c on the series feeder side of the nth phase-shifting series transformer 204, For a small controllable parameter, is the equivalent oscillation frequency of the nth feeder of the collection system, is the input equivalent excitation voltage, , , They are the a, b, and c three-phase voltages of the large-capacity power capacitor 201 respectively.

[0052] The target voltage of the Nth phase-shifted series transformer 205 can be expressed as:

[0053] (10)

[0054] in, , , are the three-phase target voltages a, b, and c on the series feeder side of the Nth series-shifted phase transformer 205, For a small controllable parameter, is the equivalent oscillation frequency of the Nth feeder of the collection system, is the input equivalent excitation voltage, , , They are the a, b, and c three-phase voltages of the large-capacity power capacitor 201 respectively.

[0055] The target current flowing through the parallel transformer 211 can be expressed as:

[0056] (11)

[0057] in, , , are the three-phase target currents a, b, and c connected to the parallel transformer 211, For a small controllable parameter, and are the equivalent average characteristic impedance and equivalent average oscillation frequency of the collection system, is the input equivalent excitation voltage, , , They are the a, b, and c three-phase voltages of the large-capacity power capacitor 201 respectively.

[0058] Step 3: Model the nonlinear oscillation network of the converter collection system, including:

[0059] Assuming that the feeder collection system exhibits three-phase balance, the three phases can be completely decoupled into equal circuits of phase a, phase b, and phase c, such as Figure 2 As shown, that is, by Figure 1 Further equivalent is Figure 2 Circuit diagram, TP1, TPn, TPN represent the first phase-shifted series transformer 203, the nth phase-shifted series transformer 204, and the Nth phase-shifted series transformer 205, respectively. This step constructs a nonlinear oscillation network model of the converter collection system. Taking phase a as an example, the voltage-current differential equation can be established as follows:

[0060] (12)

[0061] , (13)

[0062] in, For large-capacity power capacitors, is the a-phase voltage of the large-capacity power capacitor 201, is the current value flowing into the external power grid, To collect the voltage of the AC side of the converter connected to the low-voltage end of the network, is the transformation ratio of the transformer at the low voltage end of the collection network. Formula (13) takes the nth feeder as an example.

[0063] Multiply both ends of equation (13) by , is the equivalent characteristic impedance of the nth feeder 302, we can get:

[0064] , (14)

[0065] Further, consider , is the a-phase voltage of the large-capacity power capacitor 201, and the Substitute into formula (12), and change formula (9) Substituting into equation (14) and rearranging it, we can obtain:

[0066] (15)

[0067] , (16)

[0068] Similarly, the derivation process of the oscillatory differential equations of phase b and phase c is similar to equations (12) to (16). If the oscillatory differential equations of phase b and phase c need to be derived, that is, phase b: all letters a in equations (12) to (16) are replaced by b; that is, phase c: all letters a in equations (12) to (16) are replaced by c.

[0069] Step 4: Perform voltage-current nonlinear controller control, including:

[0070] For the series phase-shifting transformer, the first voltage source converter 206, the nth voltage source converter 207, and the Nth voltage source converter 208 are used to control the output voltage. The present invention sets the instruction of the Nth phase-shifting series transformer 205. The formation of the instruction requires the acquisition of the bus voltage signal of the low-voltage side bus 202 of the step-up transformer, that is, , also need to generate etc.; further, the power filter voltage and current signals of the parallel transformer side converter 212 are collected, compared with the instructions of the Nth phase-shifted series transformer 205, and the carrier signal is obtained through the voltage-current inner loop control to generate a PWM wave to drive the Nth voltage source converter 208.

[0071] For the parallel transformer, the parallel transformer side converter 212 is used to control the output current. The present invention sets the instruction of the nonlinear current controller 218, and the formation of the instruction requires the acquisition of the bus voltage signal of the low-voltage side bus 202 of the step-up transformer, that is, , also need to generate further, the current signal of the port of the parallel transformer side converter 212 is collected, compared with the instruction of the nonlinear current controller 218, and the carrier signal is obtained through the current inner loop control to generate a PWM wave to drive the parallel transformer side converter 212.

[0072] Figure 3 It is a schematic diagram of a voltage-current nonlinear controller for a phase-shifting transformer side converter and a parallel transformer side converter provided by the present invention. For nonlinear voltage control, the present invention collects the voltage and current of each feeder, implements voltage-current inner loop control 214 through a nonlinear voltage controller 215, and further drives the phase-shifting transformer side converter through a first PWM generator 213; for nonlinear current control, the present invention collects the voltage of each feeder 219 and the bus voltage 220 on the low-voltage side of the step-up transformer, implements current inner loop control 217 through a nonlinear current controller 218, and further drives the parallel transformer side converter 212 through a second PWM generator 216.

Claims

1. A method for networking a converter collection system based on nonlinear oscillation of a power grid, characterized in that: The steps include: Step 1: Based on the radial feeder collection system, a power capacitor is installed on the low-voltage side of the step-up transformer to construct a feeder inductor-power capacitor combined power oscillator; Step 2, constructing a voltage-current nonlinear oscillation controller; Step 3, designing a nonlinear oscillation network modeling method for the converter collection system to form a network topology structure; Step 4: Characterize the nonlinear oscillator equations of the dynamic characteristics of the converter collection system network and design a voltage-current nonlinear controller control method.

2. A method for networking a converter collection system based on nonlinear oscillation of a power grid as claimed in claim 1, characterized in that: The step 1 includes designing a converter collection system network topology structure to convert the photovoltaic and wind power new energy base into a new power plant; the new power plant faces 110kV, 220kV, and 500kV high-voltage AC power grids and boosts the voltage through a step-up transformer.

3. A method for networking a converter collection system based on nonlinear oscillation of a power grid as claimed in claim 1, characterized in that: In the step 1, a large-capacity power capacitor is installed on the low-voltage side bus of the step-up transformer; the low-voltage side bus of the step-up transformer is the collection point of the feeders of each branch, and each branch contains the first feeder, the nth feeder, the Nth feeder, the first step-up transformer, the nth step-up transformer, the Nth step-up transformer, the first collection converter, the nth collection converter, and the Nth collection converter; the first collection converter is connected to the first step-up transformer through the first feeder; the nth collection converter is connected to the nth step-up transformer through the nth step-up transformer; and the Nth collection converter is connected to the Nth step-up transformer through the Nth step-up transformer.

4. A method for networking a converter collection system based on nonlinear oscillation of a power grid as claimed in claim 3, characterized in that: In step 1, select the nth feeder, assuming that the impedance of the single-phase feeder is , the transformer reactance is ,in , are the feeder resistance and inductance respectively, is the angular frequency of the power frequency, is an imaginary number, n is a counting variable, ; It is estimated that the inductance of each single-phase feeder from the converter to the step-up transformer is ; Based on the above description of the power oscillator composed of the power capacitor and the feeder equivalent reactance, it is assumed that the equivalent inductance of each feeder is equal, that is, , is the equivalent inductance of each feeder branch, To collect the number of system feeders, the equivalent oscillation frequency of the power oscillator is derived and characteristic impedance They are: （1） Among them, when the equivalent inductance of each feeder is equal, is the equivalent oscillation frequency, is the equivalent characteristic impedance, The capacitance of a large-capacity power capacitor; Assume that the equivalent inductance of each feeder is not equal, that is, all If they are not equal, the equivalent oscillation frequency and characteristic impedance of each feeder are defined as follows: （2） （3） in, is the equivalent oscillation frequency of the first feeder of the collection system, is the equivalent characteristic impedance of the first feeder of the collection system; is the equivalent oscillation frequency of the nth feeder, is the equivalent characteristic impedance of the nth feeder; is the equivalent oscillation frequency of the Nth feeder, is the equivalent characteristic impedance of the Nth feeder; , , are the inductances of each feeder respectively; When the average value is taken, the equivalent oscillation frequency and characteristic impedance of the collection system are: （4） in, is the equivalent average characteristic impedance, is the equivalent average oscillation frequency.

5. A method for networking a converter collection system based on nonlinear oscillation of a power grid as claimed in claim 1, characterized in that: The step 2 includes constructing a voltage-current nonlinear controller, which includes a first phase-shifting series transformer, an nth phase-shifting series transformer, an Nth phase-shifting series transformer, a parallel transformer, a voltage source converter, a DC power supply and a DC microgrid; the first phase-shifting series transformer is connected to the first voltage source converter, the nth phase-shifting series transformer is connected to the nth voltage source converter, and the Nth phase-shifting series transformer is connected to the Nth voltage source converter; the DC power supply and the Nth voltage source converter, the nth voltage source converter, the first voltage source converter and the parallel transformer side converter together form a DC microgrid.

6. A method for networking a converter collection system based on nonlinear oscillation of a power grid as claimed in claim 5, characterized in that: The step 2 comprises: For the voltage source formed by the phase-shifting series transformer, the target voltage on the feeder side of the first phase-shifting series transformer is expressed as: （8） in, , , are the three-phase target voltages a, b, and c on the feeder side of the first phase-shifting series transformer 203, For a small controllable parameter, is the equivalent oscillation frequency of the first feeder of the collection system, is the input equivalent excitation voltage, , , They are the a, b, and c three-phase voltages of the large-capacity power capacitor; The target voltage of the nth phase-shifted series transformer is expressed as: （9） in, , , are the three-phase target voltages of a, b, and c on the series feeder side of the nth phase-shifting series transformer 204, is the equivalent oscillation frequency of the nth feeder of the collection system, is the input equivalent excitation voltage; The target voltage of the Nth phase-shifted series transformer is expressed as: （10） in, , , are the three-phase target voltages a, b, and c on the series feeder side of the Nth series-shifted phase transformer 205, is the equivalent oscillation frequency of the Nth feeder of the collection system, is the input equivalent excitation voltage; The target current flowing through the parallel transformer is expressed as: （11） in, , , They are the three-phase target currents a, b, and c connected to the parallel transformer, and are the equivalent average characteristic impedance and equivalent average oscillation frequency of the collection system, is the input equivalent excitation voltage; , , are the a-phase equivalent voltages of the 1st feeder, the nth feeder, and the Nth feeder, respectively. , , are the b-phase equivalent voltages of the 1st feeder, the nth feeder, and the Nth feeder, respectively. , , They are the c-phase equivalent voltages of the 1st feeder, the nth feeder, and the Nth feeder respectively.

7. A method for networking a converter collection system based on nonlinear oscillation of a power grid as claimed in claim 6, characterized in that: The step 3 comprises: Assuming that the feeder collection system exhibits three-phase balance, the three phases can be completely decoupled into equivalent circuits of phase a, phase b, and phase c; a nonlinear oscillation network model of the converter collection system is constructed. For phase a, the voltage-current differential equation is established as follows: （12） ， （13） in, For large-capacity power capacitors, is the phase a voltage of the large-capacity power capacitor, is the current value flowing into the external power grid, To collect the voltage of the AC side of the converter connected to the low-voltage end of the network, is the transformation ratio of the transformer at the low voltage end of the collection network; Formula (13) represents the formula for the nth feeder; Multiply both ends of equation (13) by , is the equivalent characteristic impedance of the nth feeder 302, and we have: ， （14） Further, consider , is the phase a voltage of the large-capacity power capacitor, and the Substitute into formula (12), and change formula (9) Substituting into equation (14) and rearranging it, we get: （15） ， （16）。 8. A method for networking a converter collection system based on nonlinear oscillation of a power grid as claimed in claim 7, characterized in that: The step 3 comprises: when deriving the oscillatory differential equation of the b phase, all letters a in equations (12) to (16) are replaced by b; when deriving the oscillatory differential equation of the c phase, all letters a in equations (12) to (16) are replaced by c.

9. A method for networking a converter collection system based on nonlinear oscillation of a power grid as claimed in claim 7, characterized in that: The step 4 includes: for the series phase-shifting transformer, using the first voltage source converter, the nth voltage source converter, and the Nth voltage source converter to control the output voltage; setting the instruction of the Nth phase-shifting series transformer, the formation of the instruction requires the acquisition of the bus voltage signal of the low-voltage side bus of the step-up transformer, that is, , also need to generate signal; further, the power filter voltage and current signals of the converter on the parallel transformer side are collected, compared with the instructions of the Nth phase-shifted series transformer, and the carrier signal is obtained through the voltage-current inner loop control to generate a PWM wave to drive the Nth voltage source converter.

10. A method for networking a converter collection system based on nonlinear oscillation of a power grid as claimed in claim 9, characterized in that: The step 4 includes: for the parallel transformer, using the parallel transformer side converter to control the output current; setting the instruction of the nonlinear current controller, the formation of the instruction requires the acquisition of the bus voltage signal of the low-voltage side bus of the step-up transformer, that is, , also need to generate signal; further, the current signal of the converter port on the parallel transformer side is collected, compared with the instruction of the nonlinear current controller, and the carrier signal is obtained through the current inner loop control to generate a PWM wave to drive the converter on the parallel transformer side.

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