Flexible DC converter based on virtual synchronous motor, control method and system

Abstract

The invention relates to a flexible DC converter based on a virtual synchronous motor, and a control method and system, and belongs to the technical field of electrical engineering. When an AC system fails, an active amplitude limiting coefficient, a reactive amplitude limiting coefficient and a current amplitude limiting coefficient are obtained through an AC voltage amplitude; multiplying the positive and negative active amplitude limiting coefficients by the rated power of the converter to obtain upper and lower limits of an active power reference value of an active virtual rotating speed control ring controlled by the virtual synchronous motor; multiplying the positive and negative reactive power amplitude limiting coefficients by a reactive power output upper limit of the system to obtain an upper limit and a lower limit of a reactive power reference value of a reactive power droop control ring controlled by the virtual synchronous motor; and multiplying the current amplitude limiting coefficient by the maximum allowable current set value during normal operation to obtain the upper limit of the current reference value of the current inner loop controlled by the virtual synchronous motor. The converter can also operate in a network construction type control mode during a fault period, and the maximum operation capability of equipment is exerted to support transient and steady stable operation of an alternating current power grid.

Description

Technical Field

[0001] The present invention relates to a flexible direct current converter, a control method and a system based on a virtual synchronous machine, belonging to the technical field of electrical engineering. Background Art

[0002] The installed capacity of intermittent energy sources represented by photovoltaic and wind power accounts for an increasingly large proportion of the total installed capacity of the system. However, the large-scale and frequent random volatility of the output of intermittent energy sources impacts the active power balance of the system, affects the frequency regulation characteristics of the system, and is different from the rotating machines of conventional power plants. The intermittent energy sources connected to the power grid through power electronic devices are non-rotating static components and do not have the moment of inertia of conventional units. Their large-scale connection to the power grid will reduce the equivalent moment of inertia of the system, weaken the system's ability to cope with power fluctuations, and affect the frequency transient stability level of the system.

[0003] To address this problem, large-capacity centralized energy storage can be added to wind power and photovoltaic power stations to suppress the output fluctuations, and the combination of energy storage and virtual synchronous machine control technology has been a research hotspot in recent years. The virtual synchronous machine control technology introduces the mathematical models of the synchronous generator body and its controller into the control algorithm of the energy storage converter, enabling the power station to have inertia and actively participate in primary frequency regulation and voltage regulation, effectively suppressing the frequency oscillation at the grid connection point and enhancing the voltage strength at the grid connection point.

[0004] Virtual synchronous machine control simulates the motion characteristics of a synchronous generator, applies the rotor motion equation to the active power controller of the flexible direct current converter, realizes the coupling of the active power output of the converter and the grid frequency. Under the action of the virtual rotor motion equation, when the external system changes, it causes the change of the converter output power, and then increases or decreases the virtual speed to maintain synchronization with the grid. The reactive power adopts the droop control of reactive power and AC voltage.

[0005] A typical control block diagram is as Figure 1 shown. The active power reference value and the real-time active power pass through the active virtual speed control loop to output a phase angle. The phase angle and the AC voltage pass through the voltage dq transformation to output the d-axis voltage and q-axis voltage to the outer-loop voltage control. The phase angle and the AC current pass through the current dq transformation to output the d-axis current and q-axis current to the inner-loop current control. The phase angle is also input to the dq inverse transformation; the reactive power reference value and the real-time reactive power pass through the reactive power droop control to output the d-axis voltage reference value, which is then input to the outer-loop voltage control together with 0 to output the d-axis current reference value and q-axis current reference value to the current limiting module, and then through the inner-loop current control, and through the dq inverse transformation to output the modulation wave.

[0006] Traditional virtual synchronous control has two disadvantages:

[0007] 1. The active virtual speed realizes the acceleration and deceleration of the rotor by relying on the power difference between the input and output. Due to the flexible DC converter based on IGBT, its overcurrent capacity is very small, and the allowable overload is generally lower than 1.2 times the rated power. When the output reaches full power, such as when the grid frequency suddenly drops, a sufficient power difference is required to quickly decelerate the system to match the grid frequency, which easily causes the converter to trip due to overload.

[0008] 2. When a short - circuit fault occurs in the power grid, the fault crossing is usually achieved by limiting the reference value of the inner - loop current. However, when the reference value of the current is limited, the voltage outer - loop and the active virtual speed loop cannot obtain the correct feedback, and the speed cannot be kept consistent with the power grid, and it will gradually become unstable. The traditional method is to switch the system from the grid - forming control based on virtual synchronous control to the grid - following control when an AC system fault is detected, which reduces the frequency support for the power grid during the fault period. Summary of the Invention

[0009] The object of the present invention is to propose a flexible DC converter, a control method and a system based on a virtual synchronous machine, which are used to solve the problem that when the converter is in the transient condition of the system, the converter switches to the grid - following control under the AC system fault, resulting in a reduction in the frequency support for the power grid. At the same time, it solves the problem that the system frequency fluctuation causes the converter to be overloaded under high - power conditions.

[0010] To achieve the above object, the solution of the present invention includes:

[0011] A control method for a flexible DC converter based on a virtual synchronous machine of the present invention includes: during the process of controlling the converter by using virtual synchronous machine control, if an AC system fault occurs, obtain the AC voltage amplitude, and combine the positive - correlation relationship between the preset AC voltage amplitude and the active - limit coefficient, reactive - limit coefficient and current - limit coefficient to obtain the active - limit coefficient, reactive - limit coefficient and current - limit coefficient; multiply the positive and negative active - limit coefficients by the rated power of the converter respectively to obtain the upper and lower limits of the active - power reference value of the active - power virtual - speed control loop of the virtual synchronous machine control; multiply the positive and negative reactive - limit coefficients by the upper limit of the system reactive - power output respectively to obtain the upper and lower limits of the reactive - power reference value of the reactive - power droop control loop of the virtual synchronous machine control; multiply the current - limit coefficient by the maximum allowable current set value during normal operation to obtain the upper limit of the current reference value of the current inner - loop of the virtual synchronous machine control.

[0012] Further, the specific method for obtaining the positive correlation relationship between the AC voltage amplitude, the active power limiting coefficient, the reactive power limiting coefficient, and the current limiting coefficient includes: using electromagnetic transient simulation software to model the converter, simulating different degrees of AC faults at the AC grid connection point to obtain different AC voltage amplitudes; for a certain AC voltage amplitude, repeatedly modifying the values of the active power limiting coefficient, the reactive power limiting coefficient, and the current limiting coefficient, and traversing and testing the maximum values of the active power limiting coefficient, the reactive power limiting coefficient, and the current limiting coefficient for the safe and stable operation of the system as the active power limiting coefficient, the reactive power limiting coefficient, and the current limiting coefficient corresponding to the AC voltage amplitude.

[0013] Further, the method further includes: performing an inertia link integral limiting process on the result of the integral of the inertia link in the active virtual speed control loop, where the upper limit of the inertia link integral limiting is the sum of the real-time frequency filtering value and the preset frequency deviation fixed value, and the lower limit of the inertia link integral limiting is the difference between the real-time frequency filtering value and the preset frequency deviation fixed value.

[0014] Further, the method further includes: performing an overload limiting process on the active power reference value of the inertia link integral limiting link input to the active virtual speed control loop. If the integral link reaches the upper limit of the inertia link integral limiting, the upward limiting signal is 1, otherwise it is 0; if the integral link reaches the lower limit of the inertia link integral limiting, the downward limiting signal is 1, otherwise it is 0; the calculation formulas for the upper and lower limits of the overload limiting are as follows:

[0015]

[0016] Where Max_limit3 is the upper limit of the overload limiting, Min_limit3 is the lower limit of the overload limiting, Poverset is the set value of the overload margin, K is the overload limiting proportional braking coefficient, EN1(t) and EN2(t) are the current operating cycles, the upward limiting signal and the downward limiting signal of whether the inertia integral link is limited respectively, and m is the total number of operating cycles within the set time window.

[0017] Further, the method further includes: performing an angle correction on the output angle at the output end of the active virtual speed control loop so that the output phase angle range is as follows:

[0018] θ S -Dset << θ << θ S +Dset

[0019] Where, θ S is the phase angle of the grid voltage, and Dset is the fixed value of the angle deviation correction.

[0020] Further, when there is an AC system fault, the regulator used in the reactive droop control loop is a proportional-integral regulator; otherwise, the regulator used in the reactive droop control loop is a proportional regulator.

[0021] Further, the method for determining whether there is an AC system fault is as follows: Calculate the effective voltage value and the effective current value based on the grid-connected AC voltage and AC current, and judge the effective voltage value and the effective current value. When it is determined that criterion one or criterion two is satisfied, it is determined that there is an AC system fault; criterion one is that the effective voltage value is less than the preset first voltage determination value and continuously satisfies the preset first delay value, and the change rate of the effective current value is greater than the preset current determination value; criterion two is that the effective voltage value is less than the preset second voltage determination value and continuously satisfies the preset second delay value; the first voltage determination value is greater than the second voltage determination value.

[0022] Further, the relationship between the AC voltage amplitude, the active power limiting coefficient, the reactive power limiting coefficient, and the current limiting coefficient is represented in tabular form.

[0023] The beneficial effects of the present invention are as follows: The present invention proposes a control method for a flexible DC converter based on a virtual synchronous machine. When there is an AC system fault, the active power limiting coefficient, the reactive power limiting coefficient, and the current limiting coefficient are obtained through the AC voltage amplitude; the upper and lower limits of the active power reference value of the active virtual speed control loop for virtual synchronous machine control are obtained by multiplying the positive and negative active power limiting coefficients by the rated power of the converter respectively; the upper and lower limits of the reactive power reference value of the reactive droop control loop for virtual synchronous machine control are obtained by multiplying the positive and negative reactive power limiting coefficients by the upper limit of the system reactive power output respectively; the upper limit of the current reference value of the current inner loop for virtual synchronous machine control is obtained by multiplying the current limiting coefficient by the maximum allowable current setting value during normal operation. The traditional method is to switch the system from the grid-forming control based on virtual synchronous control to the grid-following control when an AC system fault is detected, which reduces the frequency support for the power grid during the fault. The present invention enables the converter to operate in the grid-forming control mode during the fault based on its own equipment capabilities, giving full play to the maximum operating capacity of the equipment to support the transient and steady-state stable operation of the AC power grid.

[0024] The present invention also provides a flexible DC converter control system based on a virtual synchronous machine, including a processor, which is used to execute a computer program to implement the steps of the above-mentioned flexible DC converter control method based on a virtual synchronous machine.

[0025] The flexible DC converter control system based on a virtual synchronous machine can achieve the same beneficial effects as the above-mentioned flexible DC converter control method based on a virtual synchronous machine.

[0026] The present invention also provides a flexible DC converter based on a virtual synchronous machine, which includes a converter controller. The converter controller includes a processor, and the processor is configured to execute a computer program to implement the steps of the flexible DC converter control method based on a virtual synchronous machine as described above.

[0027] The flexible DC converter based on a virtual synchronous machine can achieve beneficial effects consistent with the above flexible DC converter control method based on a virtual synchronous machine. Description of the Drawings

[0028] Figure 1 is a control block diagram of a flexible DC converter based on a virtual synchronous machine in the prior art;

[0029] Figure 2 is a control block diagram of the flexible DC converter based on a virtual synchronous machine of the present invention;

[0030] Figure 3 is a schematic diagram of the fault identification logic of the present invention;

[0031] Figure 4 is a schematic diagram of the active virtual speed control loop of the present invention;

[0032] Figure 5 is a schematic diagram of the reactive power droop control loop of the present invention. Detailed Embodiments

[0033] To make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be described in detail clearly and completely below with reference to the drawings and embodiments.

[0034] The concept of the present invention is as follows: The present invention provides a flexible DC converter, a control method and a system based on a virtual synchronous machine. The method includes: during the process of controlling the converter by using virtual synchronous machine control, if an AC system fault occurs, obtaining the AC voltage amplitude, and combining the positive correlation relationship between the preset AC voltage amplitude and the active power limit coefficient, the reactive power limit coefficient and the current limit coefficient, to obtain the active power limit coefficient, the reactive power limit coefficient and the current limit coefficient; multiplying the positive and negative active power limit coefficients by the rated power of the converter respectively to obtain the upper and lower limits of the active power reference value of the active virtual speed control loop of the virtual synchronous machine control; multiplying the positive and negative reactive power limit coefficients by the upper limit of the system reactive power output respectively to obtain the upper and lower limits of the reactive power reference value of the reactive power droop control loop of the virtual synchronous machine control; multiplying the current limit coefficient by the maximum allowable current setting value during normal operation to obtain the upper limit of the current reference value of the current inner loop of the virtual synchronous machine control. So that the converter can also operate in the grid-forming control mode during the fault period based on its own equipment capabilities, and exert the maximum operating capacity of the equipment to support the transient and steady-state stable operation of the AC power grid.

[0035] Embodiment of the control method for the flexible DC converter based on the virtual synchronous machine:

[0036] This embodiment provides a control method for the flexible DC converter based on the virtual synchronous machine, which is based on the control block diagram as shown in Figure 1 . It includes an active virtual speed control loop, a reactive droop control loop, an outer-loop voltage control, and an inner-loop current control, and finally outputs a modulation wave. A fault identification logic is added to enable the system to still maintain grid-forming control during AC faults and ensure the stability of the supported grid frequency.

[0037] The control block diagram of this embodiment is as shown in Figure 2 . A fault identification logic is added to the control block diagram of the existing technology. The AC voltage U at the grid connection ac and the AC current I ac are collected.

[0038] Specifically, the fault identification logic is as shown in Figure 3 . The AC voltage U at the grid connection ac and the AC current I ac are collected, and the effective values Urms and Irms of the voltage and current are calculated. First, the amplitude of Urms is less than the preset first voltage determination value Uset1 and continuously satisfies the preset first delay value T1, and the change rate of Irms is greater than the preset current determination value DIset, satisfying criterion 1. Then, the amplitude of Urms is less than the preset second voltage determination value Uset2 and continuously satisfies the preset second delay value T2, satisfying criterion 2. The first voltage determination value is greater than the second voltage determination value. Among them, according to experience, the first voltage determination value Uset1 is generally taken as 0.9 pu, the second voltage determination value Uset2 is taken as 0.8 pu, the first delay value T1 is taken as 200 us, the second delay value T2 is taken as 100 us, and the current determination value Diset is taken as 0.1 pu / ms.

[0039] Finally, an OR logic discrimination is performed on criterion 1 and criterion 2. That is, when any of the above criteria is satisfied, the fault identification logic outputs a fault ride-through enable signal Fault_tri. When Fault_tri is 1, the fault ride-through is enabled; when Fault_tri is 0, the fault ride-through is not enabled. When the fault ride-through is enabled, the clipping strategy is entered. The clipping strategy uses a look-up table method. The curve of the look-up table method is obtained through analysis and processing of system simulation data. The optimal output active power clipping coefficient Kpl, reactive power clipping coefficient Kql, and current clipping coefficient Kil corresponding to the voltage amplitude obtained from multiple practical experiences are obtained through system simulation to obtain the curve of the look-up table method.

[0040] Among them, Kp1, Kq1, and Ki1 are all positively correlated with the voltage amplitude. During the fault, the lower the voltage amplitude, the smaller the values of Kp1, Kq1, and Ki1. Kp1 and Kq1 are generally less than the per-unit value of the current voltage, and Ki1 is generally greater than the per-unit value of the voltage.

[0041] Specific calculation method in this embodiment: Use electromagnetic transient simulation software to model the converter, simulate different degrees of AC faults at the AC grid connection point to obtain different AC voltage amplitudes. In this embodiment, with a 10% interval of grid AC voltage sag, test different AC voltage amplitudes when the voltage sags to 0 - 90%, repeatedly modify the values of Kp1, Kq1, and Ki1, and traverse to test the maximum values of Kp1, Kq1, and Ki1 for which the system can operate safely and stably as the values of Kp1, Kq1, and Ki1 corresponding to the voltage amplitudes.

[0042] Input the active power limit coefficient into the active virtual speed control loop, the reactive power droop control, and the current limit through the fault recognition logic respectively.

[0043] (1) Active virtual speed control loop

[0044] The fault recognition logic inputs the active power limit coefficient Kpl into the active virtual speed control loop. The specific implementation method in the active virtual speed control loop is as Figure 4 shown. The active virtual speed control loop designed in this embodiment adds the upper and lower limits of the active power reference value input limit (i.e., the upper and lower limits of the active power reference value of the active virtual speed control loop in the virtual synchronous machine control), overload limit, integral limit of the inertia link, and angle correction logic on the basis of the synchronous motor. The real-time filtered frequency value Fs and the grid voltage phase angle θ at the integral limit of the inertia link and the angle correction logic S are measured by the phase-locked loop PLL from the AC voltage U ac at that point.

[0045] Specifically, Pref is the set active power reference value. First, it undergoes the limit of the active power reference value input to the active virtual speed control loop calculated by the output active power limit coefficient Kpl of the fault recognition logic. After adding the damping component of the virtual speed loop, it is limited by the load limit module.

[0046] The upper and lower limits of the active power reference value input limit to the active virtual speed control loop are Max_limit1 and Min_limit1, which satisfy the following formula:

[0047] Max_limit1 = Kp1 * Pn (1)

[0048] Min_limit1 = -Kp1 * Pn (2)

[0049] In the formula, Pn is the rated power of the VSC-HVDC converter.

[0050] The difference between the active power reference value Pref after overload limiting and the real-time active power Pe is taken and enters the inertia link. The integrator of the inertia link sets the upper and lower limit values of the change, and the upper and lower limit values satisfy the following formula:

[0051] Max_limit2 = Fs + Fset (3)

[0052] Min_limit2 = Fs - Fset (4)

[0053] In the formula, Fs is the real-time filtered value of the frequency measured by the phase-locked loop, and Fset is the set value of the frequency deviation, generally taking values from 0.25 to 0.5 Hz.

[0054] When the integration link reaches the upper limit, the output upward limit signal EN1 is 1, otherwise it is 0; when the integration link reaches the lower limit, the output downward limit signal EN2 is 1, otherwise it is 0.

[0055] The upper and lower limits of the overload limiting of the overload limiting module are Max_limit3 and Min_limit3 respectively, and they satisfy the following formula:

[0056]

[0057] In the formula, Poverset is the set value of the overload margin, which is set according to a certain equipment margin reserved for the overload energy. K is the overload limit proportional braking coefficient, taking a value from 0 to 1. EN1(t) and EN2(t) are the signals indicating whether the inertia integration link is limited in the current operating cycle, and m is the total number of operating cycles within the set time window. It solves the problem of the converter being overloaded due to the fluctuation of the system frequency under high-power conditions.

[0058] The angle correction logic limits the angle autonomously generated by the active virtual speed control loop to avoid instability caused by large angle deviations when it is unable to synchronize due to a rapid phase mutation of the external power grid. The output phase angle should satisfy the following formula:

[0059] θ S -Dset << θ << θ S +Dset (7)

[0060] In the formula, θ S is the phase angle of the grid voltage, and Dset is the set value of the angle deviation correction, recommended to take 30 - 60°.

[0061] (2) Reactive power droop control loop

[0062] The fault identification logic also inputs the reactive power limit coefficient Kql to the reactive power droop control loop. The specific implementation method in the reactive power droop control loop is as Figure 5As shown in the figure, in this embodiment, a reactive power droop control loop reactive power reference value input limit (i.e., the upper and lower limits of the reactive power reference value of the reactive power droop control loop of the virtual synchronous machine control) is added at the input of the reactive power droop control loop. Qref is the set reactive power reference value. First, it passes through the limit value calculated by the fault identification logic output reactive power limit coefficient Kql for reactive power droop control loop input limiting. The limited reactive power reference value is subtracted from the real-time reactive power Q. First, the difference is multiplied by the coefficient k q When operating normally, the control loop integrator is not enabled, and the value obtained by multiplying the difference by the coefficient k q is added to the AC voltage reference value U ac_ref to obtain the d-axis voltage reference value Ud ref ; When the system detects an AC system fault (i.e., fault ride-through is enabled and the Fault_tri signal is 1), the control loop integrator is input with superposition integration, entering the reactive power closed-loop control mode. The value obtained by multiplying the difference by the coefficient k q , the output value of the control loop integrator, and the AC voltage reference value U ac_ref are added together to obtain the d-axis voltage reference value Ud ref .

[0063] The upper and lower limits Max_limit4 and Min_limit4 of the reactive power droop control loop reactive power reference value input limit satisfy the following formula:

[0064] Max_limit4 = Kq1 * Qmax (8)

[0065] Min_limit4 = -Kq1 * Qmax (9)

[0066] In the formula, Qmax is the upper limit of the system reactive power output.

[0067] (3) Current limiting

[0068] The current limiting module adopts an equal-proportion limiting logic, and the upper limit of the amplitude limit is Imax_limit

[0069] Imax_limit = Ki1 * Imax (10)

[0070] Imax is the maximum allowable current setting value during normal operation.

[0071] Based on the traditional virtual synchronous control strategy, this embodiment improves the active virtual speed control loop to solve the overload problem under frequency fluctuations, modifies the fault ride-through logic, abandons the traditional strategy of switching to grid-following control during faults, no longer relies on current limiting of the inner loop to achieve fault ride-through, and adopts limiting of the outer loop active and reactive power to achieve fault ride-through, enabling the system to still maintain grid-forming control during AC faults and support the stability of the grid frequency.

[0072] Embodiment of a control system for a flexible DC converter based on a virtual synchronous machine:

[0073] This embodiment provides a control system for a flexible DC converter based on a virtual synchronous machine, including a processor configured to execute a computer program to implement the steps of the embodiment of the control method for a flexible DC converter based on a virtual synchronous machine.

[0074] Since the specific implementation process and principle of the control system for a flexible DC converter based on a virtual synchronous machine in this embodiment have been described in detail in the embodiment of the control method for a flexible DC converter based on a virtual synchronous machine, no further elaboration will be provided here.

[0075] Embodiment of a flexible DC converter based on a virtual synchronous machine:

[0076] This embodiment provides a flexible DC converter based on a virtual synchronous machine, including an active virtual speed control loop, a reactive droop control loop, and a current limiting module. The active virtual speed control loop adds an input limiting module for the active virtual speed control loop (i.e., the active limiting coefficient Kpl of the fault ride-through limiting acting limiting module), an overload limiting module, an inertial link integral limiting module, and an angle correction logic module. It also adds a phase-locked loop for auxiliary stability control. The phase-locked loop extracts the AC voltage frequency and phase at the current grid connection point, which is used for limiting the virtual speed (i.e., the inertial link integral limiting module) and the virtual phase angle (i.e., the angle correction logic module), serving as the basis for the strategies of each limiting link. The reactive droop control loop adds an input limiting module for the reactive droop control loop (i.e., the reactive limiting coefficient Kql of the fault ride-through limiting acting limiting module) and a control loop integrator.

[0077] Since the specific implementation process and principle of the flexible DC converter based on a virtual synchronous machine in this embodiment have been described in detail in the embodiment of the control method for a flexible DC converter based on a virtual synchronous machine, no further elaboration will be provided here.

Claims

1. A control method for a flexible DC converter based on a virtual synchronous motor, characterized in that: The method comprises: in the process of controlling the converter by using virtual synchronous motor control, if an AC system fault occurs, obtaining the AC voltage amplitude, combining the preset AC voltage amplitude with the positive correlation between the active limiting coefficient, the reactive limiting coefficient and the current limiting coefficient, to obtain the active limiting coefficient, the reactive limiting coefficient and the current limiting coefficient; multiplying the positive and negative active limiting coefficients by the rated power of the converter respectively to obtain the upper and lower limits of the active power reference value of the active virtual speed control loop of the virtual synchronous motor control; multiplying the positive and negative reactive limiting coefficients by the upper limit of the reactive output of the system respectively to obtain the upper and lower limits of the reactive power reference value of the reactive droop control loop of the virtual synchronous motor control; multiplying the current limiting coefficient by the maximum allowable current setting value during normal operation to obtain the upper limit of the current reference value of the current inner loop of the virtual synchronous motor control.

2. The method for controlling a flexible DC converter based on a virtual synchronous motor according to claim 1, characterized in that: The specific method for obtaining the positive correlation between the AC voltage amplitude and the active limiting coefficient, the reactive limiting coefficient and the current limiting coefficient includes: using electromagnetic transient simulation software to model the converter, simulating different degrees of AC faults at the AC grid, and thus obtaining different AC voltage amplitudes; for a certain AC voltage amplitude, repeatedly modifying the values ​​of the active limiting coefficient, the reactive limiting coefficient and the current limiting coefficient, and traversing and testing the maximum values ​​of the active limiting coefficient, the reactive limiting coefficient and the current limiting coefficient for safe and stable operation of the system as the active limiting coefficient, the reactive limiting coefficient and the current limiting coefficient corresponding to the AC voltage amplitude.

3. The method for controlling a flexible DC converter based on a virtual synchronous motor according to claim 1, characterized in that: The method also includes: performing inertia link integral limiting processing on the result of the inertia link integration in the active virtual speed control loop, the upper limit of the inertia link integral limiting is the sum of the real-time frequency filtering value and the preset frequency deviation constant, and the lower limit of the inertia link integral limiting is the difference between the real-time frequency filtering value and the preset frequency deviation constant.

4. The method for controlling a flexible DC converter based on a virtual synchronous motor according to claim 3, characterized in that: The method also includes: performing overload limiting processing on the active power reference value of the inertia link integral limiting link of the input active virtual speed control loop, if the integral link reaches the upper limit of the inertia link integral limiting, the upward limiting signal is 1, otherwise it is 0; if the integral link reaches the lower limit of the inertia link integral limiting, the downward limiting signal is 1, otherwise it is 0; the upper and lower limits of the overload limiting are calculated as follows: Among them, Max_limit3 is the upper limit of overload limit, Min_limit3 is the lower limit of overload limit, Poverset is the overload margin setting value, K is the overload limit proportional braking coefficient, EN1(t) and EN2(t) are the current operation cycle, whether the inertia integral link is upward limited signal and downward limited signal, respectively, and m is the total number of operation cycles in the set time window.

5. The method for controlling a flexible DC converter based on a virtual synchronous motor according to claim 1, characterized in that: The method further includes: performing angle correction on the output terminal angle in the active virtual speed control loop so that the output phase angle range is as follows: θ S -Dset<<θ<<θ S +Dset Among them, θ S is the grid voltage phase angle, and Dset is the angle deviation correction setting value.

6. The method for controlling a flexible DC converter based on a virtual synchronous motor according to claim 1, characterized in that: When the AC system fails, the regulator used by the reactive power droop control loop is a proportional-integral regulator, otherwise the regulator used by the reactive power droop control loop is a proportional regulator.

7. The method for controlling a flexible DC converter based on a virtual synchronous motor according to claim 1, characterized in that: The method for judging whether an AC system fault occurs is: the voltage effective value and the current effective value are calculated according to the AC voltage and the AC current at the grid-connected point, and the voltage effective value and the current effective value are judged. When it is judged that the first criterion or the second criterion is met, it is judged that an AC system fault occurs; the first criterion is: the voltage effective value is less than the preset first voltage judgment value and continuously meets the preset first delay value, and the current effective value change rate is greater than the preset current judgment value; the second criterion is: the voltage effective value is less than the preset second voltage judgment value and continuously meets the preset second delay value; the first voltage judgment value is greater than the second voltage judgment value.

8. The method for controlling a flexible DC converter based on a virtual synchronous motor according to claim 1, characterized in that: The relationship between the AC voltage amplitude and the active limiting factor, reactive limiting factor and current limiting factor is expressed in tabular form.

9. A flexible DC converter control system based on a virtual synchronous motor, comprising a processor, characterized in that: The processor is used to execute a computer program to implement the steps of the flexible DC converter control method based on a virtual synchronous motor according to any one of claims 1 to 7.

10. A flexible direct current converter based on a virtual synchronous motor, comprising a converter controller, the converter controller comprising a processor, characterized in that: The processor is used to execute a computer program to implement the steps of the flexible DC converter control method based on a virtual synchronous motor according to any one of claims 1 to 7.

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