A Flexible LCC-HVDC System and Its Anti-Inversion Subversion Control Method

Abstract

The present invention discloses a flexible LCC-HVDC system and an anti-inverter subversion control method thereof, comprising the following steps: a cascaded H-bridge consisting of IGCTs is installed between the inverter valve and the transformer on the inverter side of the LCC-HVDC system. The IGCT's current shock resistance can help the cascaded H-bridge achieve better operating performance; the cascaded H-bridge uses unipolar frequency-doubled carrier phase-shift modulation to output an AC voltage that lags the fundamental wave of the current on the converter valve side by 90 degrees to prevent commutation failure, thereby achieving decoupling control of the DC voltage and arc extinction angle, while increasing capacitor voltage utilization and reducing switching losses; and the inverter adopts a CFPREV control mode, which can reduce the number of submodules required for the system to prevent inverter subversion, further improving the anti-inverter subversion performance of the flexible LCC while controlling costs.

Description

Technical Field

[0001] The present invention belongs to the technical field of suppressing high-voltage direct current (HVDC) commutation failure, and in particular relates to a flexible LCC-HVDC system and an anti-inverter subversion control method thereof. Background Art

[0002] LCC-HVDC systems are of great significance to the development of power grids due to their significant advantages in long-distance, high-capacity power transmission. However, in traditional system topologies, the converter valves rely on external voltage to drive their switching, and the negative voltage on the inverter side is relatively short. This makes inverter upset very likely when faults such as voltage sags and short circuits occur. Inverter upsets can generate significant current surges, shortening the service life of the converter valves and causing significant economic losses to the power grid. Continuous commutation failures on the inverter side can significantly impact the stable operation of the power grid. Therefore, research is needed on technologies to mitigate inverter upsets in LCC-HVDC systems.

[0003] Between 2004 and 2018 alone, HVDC sites in China recorded 1,353 commutation failures, with an average of 9.1 commutation failures per DC transmission line per year. Compared to other faults, commutation failures occur most frequently, and the majority of these failures are caused by inverter upsets. Scholars have conducted extensive research to address the inverter upset issue in LCC-HVDC systems. Numerous papers have analyzed and proposed solutions to mitigate inverter upset in LCC-HVDC systems. These solutions fall into three main categories: the first involves installing reactive power compensation devices on the AC bus to improve bus voltage stability; the second involves modifying the converter control strategy; and the third involves modifying the converter topology by adding auxiliary commutation devices on the AC side.

[0004] For the first solution, common reactive power compensation devices include static var generators (SVGs), static var compensators (SVCs), and phase regulators. This type of solution is less effective in suppressing commutation failures in the early stages of a fault. At the same time, as the DC transmission capacity increases, the reactive power compensation capacity required by the system increases, which will reduce the economic efficiency of system operation. The second type of solution is to adjust the control system setpoint in a timely manner according to the fault conditions, increasing the arc extinction margin of the thyristors, thereby preventing inverter subversion. However, this type of solution often increases the arc extinction angle of the converter valve by advancing the trigger angle, thereby increasing the reactive power demand of the converter. The increased reactive power demand of the converter on the grid during the fault period will cause the AC side fault to worsen, further affecting the normal commutation of the converter. The third type of solution is to use additional equipment to superimpose an additional voltage of a certain waveform on the original sinusoidal commutation voltage. After the valve exits conduction, the system can still have a longer phase angle under the action of reverse voltage to ensure the recovery of blocking capability. This auxiliary commutation solution is better than the above two solutions in suppressing the commutation failure problem, and it solves the mechanistic causes of commutation failure more thoroughly. Although there is the problem of increasing system construction cost and complexity, it still shows more valuable application prospects.

[0005] In summary, there is an urgent need to improve the LCC-HVDC system to prevent inverter subversion and propose an accurate control method to control the system and reduce the number of commutation failures. Summary of the Invention

[0006] The object of the present invention is to provide a flexible LCC-HVDC system and an anti-inverter subversion control method thereof, so as to solve the problem that inverter subversion is prone to occur in the LCC system.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions:

[0008] A flexible LCC-HVDC system includes a rectifier-side converter transformer, wherein the grid side of the rectifier-side converter transformer is connected to the AC system AC1, and the valve side is connected to the AC side of the rectifier. The DC side of the rectifier is connected to the DC side of the inverter via a DC transmission line. The AC side of the inverter is connected to the input end of a cascaded H-bridge, the output end of the cascaded H-bridge is connected to the valve side of the inverter-side converter transformer, and the grid side of the inverter-side converter transformer is connected to the AC system AC2. The cascaded H-bridge is composed of full-bridge submodules connected in series.

[0009] Furthermore, the controllable switch of the full-bridge submodule adopts IGCT.

[0010] A flexible LCC-HVDC system anti-inverter subversion control method includes the following steps:

[0011] Step 1: The cascaded H-bridge outputs a voltage that lags behind the fundamental wave of the inverter current by 90°, providing additional valve voltage drive for the converter in the LCC-HVDC system.

[0012] Step 2: A commutation failure prevention control method is used for the inverters in the LCC-HVDC system. When a single-phase or three-phase voltage drop occurs in the system, the inverter angle is adjusted according to the voltage drop amplitude. The adjusted inverter angle is used to synchronously trigger the converter valves to prevent inverter failure.

[0013] Furthermore, in step 1, the cascade H-bridge is controlled by a unipolar frequency multiplication modulation method.

[0014] Furthermore, in step 1, under normal operating conditions, the output voltage of the cascaded H-bridge is solved using a model iteration method.

[0015] Furthermore, in step 1, when an AC power grid fault occurs, the cascaded H-bridge switches to maximum output voltage control according to the single-phase voltage drop amplitude or the three-phase voltage drop amplitude.

[0016] Furthermore, in step 2, the calculation process of the single-phase voltage drop amplitude includes the following steps:

[0017] S1. Construct the zero-sequence voltage rotating phasor. The formula is:

[0018]

[0019] Where: U0(t) is the instantaneous value of the zero-sequence voltage measured at the current moment, U0(t-Δt) is the instantaneous value of the zero-sequence voltage measured last time, Δt is the time interval between the two measurements, and ω is the angular frequency on the AC side;

[0020] S2, the three-phase symmetrical voltage of the virtual zero-sequence voltage rotating phasor and the complex domain abc / q transformation are performed to obtain the time domain d-axis component U of the zero-sequence voltage rotating phasor d and the time domain q-axis component U of the zero-sequence voltage rotating phasor q , the complex domain abc / q transformation formula is:

[0021]

[0022] Where: α is the rotation factor,

[0023] S3. Calculate the single-phase voltage drop amplitude based on the instantaneous value of the zero-sequence voltage amplitude. The formula is:

[0024]

[0025] Where: U dp1 is the single-phase voltage drop amplitude, and U0 is the instantaneous value of the zero-sequence voltage amplitude.

[0026] Furthermore, in step 2, the calculation formula for the three-phase voltage drop amplitude is:

[0027]

[0028] in,

[0029] Where: U s * is the rated AC bus phase voltage amplitude, U dp3 is the three-phase voltage drop amplitude, U α is the α-axis component obtained by three-phase voltage transformation, U β is the β-axis component obtained by three-phase voltage transformation, U a (t), U b (t), U c (t) are the phase voltages of phase a, phase b and phase c at the AC bus respectively.

[0030] Furthermore, in step 2, when adjusting the inverter angle according to the single-phase voltage drop amplitude or the three-phase voltage drop amplitude, there are upper and lower thresholds. When the single-phase voltage drop amplitude or the three-phase voltage drop amplitude is between its upper and lower thresholds, there is a linear relationship between the single-phase voltage drop amplitude and its inverter angle adjustment amount; when the drop amplitude is lower than the lower threshold, the inverter angle adjustment amount is zero; when the drop amplitude is higher than the upper threshold, the inverter angle adjustment amount is the inverter angle adjustment amount corresponding to the upper threshold; the inverter angle adjustment amounts obtained by the single-phase voltage drop amplitude and the three-phase voltage drop amplitude are compared, and the maximum value is taken as the final inverter angle adjustment amount.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] In the present invention, a cascaded H-bridge is used to provide the converter with an AC voltage that lags behind the fundamental wave of the AC side output current by 90°, providing an additional valve voltage drive for the LCC, increasing the voltage value received by the converter valve in the commutation state, reducing the commutation time, delaying the valve voltage zero crossing point, thereby increasing the arc extinction angle, achieving decoupling of the DC voltage and the arc extinction angle, and preventing inverter subversion. At the same time, the capacitor voltage utilization rate of the cascaded H-bridge is relatively high, and the switching loss is relatively low. The inverter adopts a commutation failure prevention (CFPREV) control method. When a single-phase or three-phase voltage drop occurs in the system, the inverter angle is adjusted in time according to the drop amplitude, further preventing the occurrence of inverter failure. It can reduce the cascaded H-bridge voltage required to prevent inverter subversion, thereby reducing the number of required cascaded H-bridge submodules, and further improving the anti-inverter subversion performance of the flexible LCC while controlling costs.

[0033] Furthermore, since a large amount of surge current will be generated when a traditional LCC-HVDC system undergoes inverter overturning, when IGCT is applied to the cascade H-bridge of the flexible LCC, its current shock resistance can help the cascade H-bridge achieve better performance. Since the current of the IGCT is controlled by the LCC, there will be no short circuit phenomenon, and no additional series impedance is required, which can greatly reduce the loss of system operation. The cascade H-bridge is composed of multiple sub-modules connected in series. Each sub-module does not need to have high-frequency switching performance, which can effectively improve the operating performance of the cascade H-bridge.

[0034] Furthermore, the output voltage of the cascaded H-bridge during normal operation is calculated using a model calculation method, which solves the difficulty of measuring the arc extinction angle, is not affected by integral saturation, and ensures rapid regulation. The threshold for determining the maximum voltage input control of the cascaded H-bridge is set relatively low, allowing sufficient compensation voltage to be quickly applied before the voltage drops to a critical value, improving response speed and reducing the risk of commutation failure.

[0035] In CFPREV control, the zero-sequence instantaneous symmetrical component transformation and three-phase abc / αβ transformation are used to obtain the voltage drop amplitude, which can quickly obtain the inverter angle adjustment amount, thereby maximizing the anti-inverter subversion effect of CFPREV.

[0036] Furthermore, the inverter angle adjustment amount of CFPREV has upper and lower limits to ensure that the inverter angle is adjusted within a reasonable range, avoiding excessive adjustment of the inverter angle when the voltage fluctuation is slight, which may cause an increase in DC current. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art; obviously, the drawings described below are some embodiments of the present invention, and for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0038] Figure 1 This is a topology diagram of the flexible LCC-HVDC system;

[0039] Figure 2 This is a schematic diagram of the topology of the cascaded H-bridge submodule;

[0040] Figure 3 It is the principle of unipolar frequency-multiplied carrier phase-shift modulation;

[0041] Figure 4 This is the model iteration flow chart of the cascade H-bridge during normal operation;

[0042] Figure 5This is the anti-inverter subversion control block diagram of the cascade H-bridge part;

[0043] Figure 6 This is the block diagram of the LCC partial anti-inverter subversion control strategy. DETAILED DESCRIPTION

[0044] To make the purpose, technical effects, and technical solutions of the embodiments of the present invention more clear, the technical solutions of the embodiments of the present invention are clearly and completely described below in conjunction with the accompanying drawings of the embodiments of the present invention; it is obvious that the described embodiments are only part of the embodiments of the present invention. Based on the embodiments disclosed in the present invention, other embodiments obtained by ordinary technicians in this field without making any creative efforts should fall within the scope of protection of the present invention.

[0045] Reference Figure 1 A flexible LCC-HVDC system includes a rectifier-side converter transformer, wherein the grid side of the rectifier-side converter transformer is connected to the AC system AC1, the valve side is connected to the AC side of the rectifier, the DC side of the rectifier is connected to the DC side of the inverter through a DC transmission line, the AC side of the inverter is connected to the input end of the cascade H-bridge, the output end of the cascade H-bridge is connected to the valve side of the inverter-side converter transformer, and the grid side of the inverter-side converter transformer is connected to the AC system AC2.

[0046] See also Figure 2 The cascaded H-bridge is constructed from multiple full-bridge submodules connected in series. Each full-bridge submodule includes two bridge arms and a capacitor Cd connected in parallel with the two arms. One arm consists of IGCT T1 and IGCT T2, and the other consists of IGCT T3 and IGCT T4. IGCTs T1, T2, T3, and T4 are connected in anti-parallel with diodes VD1, VD2, VD3, and VD4, respectively. The IGCTs also have peripheral control circuitry. As high-power, turn-off power electronic devices with improved current surge resistance, IGCTs can replace IGBTs and combine with the cascaded H-bridge to achieve better operating characteristics.

[0047] An anti-inverter subversion control method for a flexible LCC-HVDC system based on CFPREV according to an embodiment of the present invention includes the following steps:

[0048] Step 1: Control the cascaded H-bridge to use unipolar frequency-doubled carrier phase-shift modulation to output a voltage that lags the fundamental wave of the inverter square wave current by 90°, providing additional valve voltage drive for the LCC, increasing the arc extinction angle margin of the converter valve, preventing commutation failure, and achieving decoupling of the DC voltage and the arc extinction angle.

[0049] The cascaded H-bridge adopts a unipolar frequency-doubled phase-shifted carrier modulation method to control the switching action of each IGCT, so that the output voltage of the superimposed cascaded H-bridge during the period from the end of the flexible LCC commutation to the voltage zero crossing is negative, thereby increasing the voltage utilization of the cascaded H-bridge.

[0050] Reference Figure 3 In the figure, C1, C2, and C3 are the phase-shifted carriers of the three full-bridge submodules. Usually, the phase shift angle is 180° / N (N is the number of submodules), which corresponds to 60° in the figure; u r 、-u r It is a positive and negative modulation wave; u1, u2, and u3 are the output voltages of the three submodules respectively. For a single submodule, the maximum output level is the submodule capacitor voltage U dc u is the total output voltage of the cascaded H-bridge. As can be seen in the figure, this modulation scheme reduces the switching frequency of a single IGCT and increases the overall switching frequency of the cascaded H-bridge. This reduces the harmonics in the output voltage of the cascaded H-bridge and improves the waveform quality.

[0051] During normal operation: the output voltage of the cascaded H-bridge arc extinction angle control is solved using the model iterative calculation method.

[0052] Reference Figure 4 The specific method of model iteration operation is: measure the three-phase voltage amplitude of the AC busbar U S , the actual trigger angle α of LCC, the DC current I d , and set an assumed value of the cascaded H-bridge output voltage U cp * (usually it can be assumed to be 0), the assumed value U cp * , three-phase voltage amplitude U s , DC current I d The actual trigger angle α of the LCC is substituted into the mathematical model of the flexible LCC for iterative solution operation to calculate the arc extinction angle γ. If the arc extinction angle γ is less than the arc extinction angle setting value γ*, the assumed value U of the cascade H-bridge output voltage is increased. cp *Continue the iterative operation until the calculated arc extinction angle γ is greater than or equal to the arc extinction angle setting value γ*. At this time, the corresponding assumed value is the output voltage of the cascade H-bridge arc extinction angle control.

[0053] In the event of an AC grid fault, the cascaded H-bridge has a maximum voltage input mechanism. Based on the relationship between the single-phase or three-phase voltage drop and the set threshold, it switches between maximum voltage input and normal operation control. When the single-phase or three-phase voltage drop exceeds the set threshold, the cascaded H-bridge inputs the maximum compensation voltage; otherwise, it maintains normal arc extinction angle control.

[0054] Reference Figure 5 The cascaded H-bridge first samples the valve side line current ia 、i b The phase angle θ of the converter valve side line current is obtained through the phase-locked loop PLL. i Then the phase angle θ i As a reference, it is divided into d-axis and q-axis for separate control, where the q-axis output voltage u iq is the main output voltage of the cascade H-bridge. The control quantity is negative, which means the output voltage of the cascade H-bridge lags behind the converter valve side line current by 90 degrees. The d-axis output voltage u id Small, its main function is to measure the average capacitance voltage of the full-bridge submodule V capav and the set value V capav * Perform overall capacitor voltage control of the submodule.

[0055] Specifically, the setting value V of the average capacitor voltage of the full-bridge submodule is used capav * Subtract the measured value of the average capacitor voltage of the full-bridge submodule V capav , then perform PI adjustment to obtain the d-axis output voltage u id In the normal operation state, the q-axis control uses the arc extinction angle control of the model iteration calculation method to measure the AC bus three-phase voltage amplitude U s , DC current I d and the actual trigger angle α of the LCC to calculate the set value U of the cascaded H-bridge output voltage cp *. The q-axis control is also combined with the maximum voltage input control. The single-phase voltage drop amplitude and the three-phase voltage drop amplitude calculated by the LCC control part are compared with their respective judgment thresholds. If any of them is greater than the judgment threshold, the q-axis control voltage u iq From the normal output voltage setting value U cp * Switch to maximum output voltage u iqmax Otherwise, the output voltage remains at the set value U cp * Cascade H-bridge d / q axis control voltage u id 、u iq After dq / abc conversion, the three-phase modulation wave u of the cascade H bridge is generated i , and then after the sub-module capacitor voltage balance control, a phase-shifted carrier for carrier phase-shift modulation is generated to control the switching actions of the cascaded H-bridge.

[0056] Step 2: The commutation failure prevention (CFPREV) control method is used for the LCC-HVDC system inverter. When the LCC is in normal operation, the DC voltage control is used to generate the inverter angle control variable β of the converter valve. *When a single-phase or three-phase voltage drop occurs on the inverter side of the system, CFPREV control is used, that is, the inverter angle adjustment amount Δβ is generated according to the calculated single-phase or three-phase voltage drop amplitude, and the inverter angle adjustment amount Δβ is compared with the conventional inverter angle control amount β * The sum of the two values is used as the final inverter angle control variable, which is used to trigger the commutation valve synchronously. This control method helps prevent commutation failure.

[0057] The specific steps include:

[0058] S2.1. Calculate the single-phase voltage drop amplitude U dp1 And the three-phase voltage drop amplitude U dp3 ;

[0059] (1) The amplitude of the zero-sequence component of the AC bus voltage is solved by the instantaneous symmetrical component transformation method, and the single-phase voltage drop amplitude U is obtained. dp1 , specifically including the following steps:

[0060] Step 1. Construct the rotating phasor of zero-sequence voltage. The formula is:

[0061]

[0062] Where: is the zero-sequence voltage rotating phasor, U0(t) is the instantaneous value of the zero-sequence voltage measured at the current moment, U0(t-Δt) is the instantaneous value of the zero-sequence voltage measured last time, Δt is the time interval between the two measurements, and ω is the angular frequency on the AC side.

[0063] Step 2: Virtually rotate the three-phase symmetrical voltage of the zero-sequence voltage phase and perform a complex domain abc / q transformation. The formula is:

[0064]

[0065] Where: is the complex domain q-axis phasor of the zero-sequence voltage rotating phasor, U d is the time domain d-axis component of the zero-sequence voltage rotating phasor, U q is the time domain q-axis component of the zero-sequence voltage rotating phasor, α is the rotation factor, The virtual three-phase symmetrical voltage is obtained by multiplying the matrix with the column vector containing α.

[0066] Step 3. Calculate the single-phase voltage drop amplitude based on the instantaneous value of the zero-sequence voltage amplitude. The formula is:

[0067]

[0068] Where: U dp1 is the single-phase voltage drop amplitude, and U0 is the instantaneous value of the zero-sequence voltage amplitude.

[0069] (2) The three-phase voltage drop amplitude is obtained by performing abc / αβ transformation on the three-phase voltage at the AC bus to obtain the instantaneous amplitude of the three-phase voltage, and then subtracting it from the rated AC voltage amplitude. The specific solution process is:

[0070]

[0071] Where: U α is the α-axis component obtained by three-phase voltage transformation, U β is the β-axis component obtained by three-phase voltage transformation, U a (t), U b (t), U c (t) is the three-phase voltage at the AC bus.

[0072]

[0073] Where: U s * is the rated AC bus phase voltage amplitude, U dp3 is the three-phase voltage drop amplitude.

[0074] S2.2, according to the single-phase voltage drop amplitude U dp1 Calculate the single-phase voltage drop inverter angle adjustment amount, according to the three-phase voltage drop amplitude U dp3 Calculate the three-phase voltage drop inverter angle adjustment;

[0075] There is a linear relationship between the single-phase voltage drop amplitude and the three-phase voltage drop amplitude and their respective inverter angle adjustment amounts, which can be expressed as:

[0076]

[0077] Where: Δβ1 is the inverter angle adjustment amount for single-phase voltage drop, Δβ3 is the inverter angle adjustment amount for three-phase voltage drop, k1 and k2 are proportional coefficients, U th1h 、U th1l They are the upper and lower thresholds of single-phase voltage drop, U th3h 、U th3l They are the upper and lower thresholds of the three-phase voltage drop respectively.

[0078] When the voltage drop amplitude is below the lower threshold, the inverter angle adjustment amount is zero. When the voltage drop amplitude is above the upper threshold, the inverter angle adjustment amount is maintained at the maximum limit of the above formula. The inverter angle adjustment amount obtained by the single-phase voltage drop amplitude and the three-phase voltage drop amplitude is compared and the maximum value is used as the final inverter angle adjustment amount.

[0079] S2.3, the inverter angle adjustment amount and the conventional inverter angle control amount β *The sum is used as the final inverter angle control quantity to further prevent the occurrence of inverter failure.

[0080] In summary, the control method of the present invention directly solves the single-phase voltage drop amplitude and the three-phase voltage drop amplitude through calculation, and generates the inverter angle adjustment amount of CFPREV based on the linear relationship between the two voltage drop amplitudes and their respective inverter angle adjustment amounts. Compared with the traditional CFPREV method, it has the characteristics of simple algorithm and small calculation amount. In addition, adding a cascaded H-bridge to the traditional LCC topology can provide an additional valve voltage drive for the LCC, thereby realizing the decoupling of the DC voltage and the arc extinction angle. The controllable switch of the full-bridge submodule adopts IGCT. As a high-power turn-off power electronic device with better current shock resistance, IGCT can replace IGBT and combine with the cascaded H-bridge to obtain better working characteristics.

[0081] The present invention can be applied to high-voltage direct current power systems. By adopting the method proposed in the present invention, combined with topology transformation and the control mode of CFPREV, the system can solve the inverter subversion caused by single-phase or three-phase drops. This has a very positive significance for preventing the system from inverter failure, improving the operational reliability of the direct current transmission system, and ensuring good economic benefits of the power system.

[0082] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the same. Although the present invention has been described in detail with reference to the above embodiments, a person skilled in the art may still modify or make equivalent substitutions to the specific implementations of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention are within the scope of protection of the claims of the present invention to be approved.

Claims

1. A flexible LCC-HVDC system anti-inverter subversion control method, characterized in that: It includes a rectifier-side converter transformer, the grid side of which is connected to the AC system AC1, the valve side of which is connected to the AC side of the rectifier, the DC side of which is connected to the DC side of the inverter via a DC transmission line, the AC side of the inverter is connected to the input end of a cascaded H-bridge, the output end of the cascaded H-bridge is connected to the valve side of the inverter-side converter transformer, and the grid side of the inverter-side converter transformer is connected to the AC system AC2; the cascaded H-bridge is composed of full-bridge submodules connected in series; The control method comprises the following steps: Step 1: The cascaded H-bridge outputs a voltage that lags behind the fundamental wave of the inverter current by 90°, providing additional valve voltage drive for the converter in the LCC-HVDC system. Step 2: A commutation failure prevention control method is used for the inverters in the LCC-HVDC system. When a single-phase or three-phase voltage drop occurs in the system, the inverter angle is adjusted according to the voltage drop amplitude. The adjusted inverter angle is used to synchronously trigger the converter valves to prevent inverter failure.

2. The anti-inverter subversion control method for a flexible LCC-HVDC system according to claim 1, characterized in that: In step 1, the cascaded H-bridge is controlled by a unipolar frequency multiplication modulation method.

3. The anti-inverter subversion control method for a flexible LCC-HVDC system according to claim 1, characterized in that: In step 1, under normal operating conditions, the output voltage of the cascaded H-bridge is calculated using a model iterative calculation method.

4. The anti-inverter subversion control method for a flexible LCC-HVDC system according to claim 1, characterized in that: In step 1, when an AC power grid fault occurs, the cascaded H-bridge switches to maximum output voltage control according to the single-phase voltage drop amplitude or the three-phase voltage drop amplitude.

5. The anti-inverter subversion control method for a flexible LCC-HVDC system according to claim 1, characterized in that: In step 2, the calculation process of the single-phase voltage drop amplitude includes the following steps: S1. Construct the zero-sequence voltage rotating phasor. The formula is: Where: U0(t) is the instantaneous value of the zero-sequence voltage measured at the current moment, U0(t-Δt) is the instantaneous value of the zero-sequence voltage measured last time, Δt is the time interval between the two measurements, and ω is the angular frequency on the AC side; S2, the three-phase symmetrical voltage of the virtual zero-sequence voltage rotating phasor and the complex domain abc / q transformation are performed to obtain the time domain d-axis component U of the zero-sequence voltage rotating phasor d and the time domain q-axis component U of the zero-sequence voltage rotating phasor q , the complex domain abc / q transformation formula is: Where: α is the rotation factor, S3. Calculate the single-phase voltage drop amplitude based on the instantaneous value of the zero-sequence voltage amplitude. The formula is: Where: U dp1 is the single-phase voltage drop amplitude, and U0 is the instantaneous value of the zero-sequence voltage amplitude.

6. The anti-inverter subversion control method for a flexible LCC-HVDC system according to claim 1, characterized in that: In step 2, the calculation formula for the three-phase voltage drop amplitude is: in, Where: U s * is the rated AC bus phase voltage amplitude, U dp3 is the three-phase voltage drop amplitude, U α is the α-axis component obtained by three-phase voltage transformation, U β is the β-axis component obtained by three-phase voltage transformation, U a (t), U b (t), U c (t) are the phase voltages of phase a, phase b and phase c at the AC bus respectively.

7. The anti-inverter subversion control method for a flexible LCC-HVDC system according to claim 1, characterized in that: In step 2, when adjusting the inverter angle according to the single-phase voltage drop amplitude or the three-phase voltage drop amplitude, there are upper and lower thresholds. When the single-phase voltage drop amplitude or the three-phase voltage drop amplitude is between the upper and lower thresholds, there is a linear relationship between the single-phase voltage drop amplitude and its inverter angle adjustment amount; when the drop amplitude is lower than the lower threshold, the inverter angle adjustment amount is zero; when the drop amplitude is higher than the upper threshold, the inverter angle adjustment amount is the inverter angle adjustment amount corresponding to the upper threshold; the inverter angle adjustment amounts obtained by the single-phase voltage drop amplitude and the three-phase voltage drop amplitude are compared, and the maximum value is taken as the final inverter angle adjustment amount.

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