A virtual synchronous machine based flexible dc converter, control method and system

Abstract

The present application relates to a kind of virtual synchronous machine-based flexible direct current converter, control method and system, belong to electrical engineering technical field.When AC system fails, the active limiting coefficient, reactive limiting coefficient and current limiting coefficient are obtained by AC voltage amplitude;The upper and lower limits of the active power reference value of the active virtual speed control loop of virtual synchronous machine control are obtained by multiplying the positive and negative active limiting coefficients with the rated power of converter respectively;The upper and lower limits of the reactive power reference value of the reactive droop control loop of virtual synchronous machine control are obtained by multiplying the positive and negative reactive limiting coefficients with the upper limit of system reactive output respectively;The upper limit of the current reference value of the current inner loop of virtual synchronous machine control is obtained by multiplying the current limiting coefficient with the maximum allowable current setting value in normal operation.Make the converter also be able to run in network configuration type control mode during fault, play the maximum operating capacity of equipment to support AC power grid transient and steady-state stable operation.

Description

Technical Field

[0001] This invention relates to a flexible DC converter based on a virtual synchronous motor, a control method, and a system, belonging to the field of electrical engineering technology. Background Technology

[0002] Intermittent energy sources, such as photovoltaics and wind power, account for an increasingly larger proportion of the total installed capacity of the system. However, the large and frequent random fluctuations in the output of intermittent energy sources impact the active power balance of the system, affecting the system's frequency regulation characteristics. Moreover, unlike the rotating generators of conventional power plants, intermittent energy sources connected to the grid through power electronic devices are non-rotating stationary components and do not possess the rotational inertia of conventional units. Their large-scale grid connection will reduce the system's equivalent rotational inertia, weaken the system's ability to cope with power fluctuations, and affect the system's frequency transient stability.

[0003] To address this issue, large-capacity centralized energy storage can be added to wind and solar power plants to mitigate power output fluctuations. Furthermore, the integration of energy storage with virtual synchronous motor control technology has become a research hotspot in recent years. Virtual synchronous motor control technology incorporates the mathematical model of the synchronous generator and its controller into the control algorithm of the energy storage converter, enabling the power plant to possess inertia and the ability to actively participate in primary frequency and voltage regulation, effectively suppressing frequency oscillations at the grid connection point and enhancing the voltage strength at the grid connection point.

[0004] Virtual synchronous motor control simulates the motion characteristics of a synchronous generator, applying the rotor's motion equations to the active power controller of a flexible DC converter. This couples the converter's active power output with the grid frequency. Under the influence of the virtual rotor's motion equations, changes in the external system cause variations in the converter's output power, thereby increasing or decreasing the virtual speed to maintain synchronization with the grid. Reactive power is controlled by adjusting the droop of reactive power relative to the AC voltage.

[0005] A typical control block diagram is as follows: Figure 1 As shown, the active power reference value and real-time active power are output as phase angles through the active virtual speed control loop. The phase angle is then combined with the AC voltage through voltage dq transformation to output d-axis voltage and q-axis voltage to the outer loop voltage control. The phase angle is then combined with the AC current through current dq transformation to output 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 real-time reactive power are output as d-axis voltage reference values ​​through reactive power droop control. These are then input together with 0 to the outer loop voltage control to output d-axis current reference values ​​and q-axis current reference values ​​to the current limiting module. After passing through the inner loop current control, the modulated wave is output through dq inverse transformation.

[0006] Traditional virtual synchronization control has two drawbacks:

[0007] 1. Active virtual speed relies on the power difference between input and output to accelerate and decelerate the rotor. Since the flexible DC converter based on IGBT has a very small overcurrent capacity, the allowable overload is generally less than 1.2 times the rated power. When the output is at full power, if the grid frequency drops suddenly, sufficient power difference is required to make the system decelerate quickly to match the grid frequency, which can easily cause the converter to trip due to overload.

[0008] 2. When a short-circuit fault occurs in the power grid, fault ride-through is usually achieved by limiting the inner loop current reference value. However, when the current reference value is limited, the outer voltage loop and the active virtual speed loop cannot receive correct feedback, and the speed cannot keep in line with the grid, gradually leading to instability. The traditional approach is to switch the system from grid-based control based on virtual synchronization control to grid-following control when an AC system fault is detected, thereby reducing its frequency support for the power grid during the fault period. Summary of the Invention

[0009] The purpose of this invention is to propose a flexible DC converter, control method, and system based on a virtual synchronous motor, which addresses the problem of reduced grid frequency support caused by the converter switching to grid-following control under transient system conditions and AC system faults, while also solving the problem of converter overload caused by system frequency fluctuations under high-power conditions.

[0010] To achieve the above objectives, the present invention includes:

[0011] This invention discloses a flexible DC converter control method based on a virtual synchronous motor. The method includes: during the control of the converter 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 power limiting coefficient, reactive power limiting coefficient, and current limiting coefficient to obtain the active power limiting coefficient, reactive power limiting coefficient, and current limiting coefficient; multiplying the positive and negative active power limiting coefficients by the converter's rated power to obtain the upper and lower limits of the active power reference value of the virtual speed control loop of the virtual synchronous motor control; multiplying the positive and negative reactive power limiting coefficients by the upper limit of the system's reactive power output to obtain the upper and lower limits of the reactive power reference value of the reactive power droop control loop of the virtual synchronous motor control; and 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 inner current loop of the virtual synchronous motor control.

[0012] Furthermore, the specific method for obtaining the positive correlation between the AC voltage amplitude and the active power limiting coefficient, reactive power limiting coefficient, and 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, reactive power limiting coefficient, and current limiting coefficient, and iterating through tests to find the maximum values ​​of the active power limiting coefficient, reactive power limiting coefficient, and current limiting coefficient for safe and stable system operation as the active power limiting coefficient, reactive power limiting coefficient, and current limiting coefficient corresponding to that AC voltage amplitude.

[0013] Furthermore, the method also includes: performing inertial element integral limiting processing on the result of the integral of the inertial element in the active virtual speed control loop, wherein the upper limit of the inertial element integral limiting is the sum of the real-time frequency filter value and the preset frequency deviation setting, and the lower limit of the inertial element integral limiting is the difference between the real-time frequency filter value and the preset frequency deviation setting.

[0014] Furthermore, the method also includes: performing overload limiting processing on the active power reference value of the inertial link integral limiting link of the input active virtual speed control loop; if the integral link reaches the upper limit of the inertial link integral limiting, the upward limiting signal is 1, otherwise it is 0; if the integral link reaches the lower limit of the inertial 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 overload limiting, Min_limit3 is the lower limit of overload limiting, Poverset is the overload margin setting value, K is the overload limiting proportional braking coefficient, EN1(t) and EN2(t) are the current running cycle, and whether the inertial integral element is upward and downward limited, respectively, and m is the total number of running cycles within the set time window.

[0017] Furthermore, the method also includes: performing angle correction on the output terminal angle in the active virtual speed control loop to make the output phase angle range as follows:

[0018] θ S -Dset << θ << θ S +Dset

[0019] Where, θ S denoted as the grid voltage phase angle, and Dset as the angle deviation correction setting value.

[0020] Furthermore, when the AC system fails, the regulator used in the reactive power droop control loop is a proportional-integral regulator; otherwise, the regulator used in the reactive power droop control loop is a proportional regulator.

[0021] Furthermore, the method for determining whether an AC system fault has occurred is as follows: The effective values ​​of voltage and current are calculated based on the AC voltage and current at the grid connection point. The effective values ​​of voltage and current are then judged. An AC system fault is determined to have occurred when either criterion one or criterion two is met. Criterion one is: the effective voltage value is less than a preset first voltage judgment value and continuously meets a preset first delay value, and the rate of change of the effective current value is greater than a preset current judgment value. Criterion two is: the effective voltage value is less than a preset second voltage judgment value and continuously meets a preset second delay value; the first voltage judgment value is greater than the second voltage judgment value.

[0022] Furthermore, the relationship between AC voltage amplitude and active power limiting factor, reactive power limiting factor and current limiting factor is presented in tabular form.

[0023] The beneficial effects of this invention are as follows: This invention proposes a flexible DC converter control method based on a virtual synchronous motor. When an AC system fault occurs, the active power limiting coefficient, reactive power limiting coefficient, and current limiting coefficient are obtained through the AC voltage amplitude. The positive and negative active power limiting coefficients are multiplied by the rated power of the converter to obtain the upper and lower limits of the active power reference value of the virtual speed control loop controlled by the virtual synchronous motor. The positive and negative reactive power limiting coefficients are multiplied by the upper limit of the system's reactive power output to obtain the upper and lower limits of the reactive power reference value of the reactive power droop control loop controlled by the virtual synchronous motor. The current limiting coefficient is multiplied 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 controlled by the virtual synchronous motor. The traditional approach is to switch the system from grid-based control based on virtual synchronous control to grid-following control when an AC system fault is detected, which reduces its frequency support to the grid during the fault period. This invention enables the converter to operate in grid-based control mode during the fault period based on its own equipment capabilities, maximizing the equipment's operating capacity to support the transient and steady-state stable operation of the AC grid.

[0024] The present invention also provides a flexible DC converter control system based on a virtual synchronous motor, including a processor for executing a computer program to implement the steps of the flexible DC converter control method based on a virtual synchronous motor as described above.

[0025] The flexible DC converter control system based on the virtual synchronous motor can achieve the same beneficial effects as the aforementioned flexible DC converter control method based on the virtual synchronous machine.

[0026] The present invention also provides a flexible DC converter based on a virtual synchronous motor, including a converter controller, the converter controller including a processor, the processor being used to execute a computer program to implement the steps of the flexible DC converter control method based on a virtual synchronous motor as described above.

[0027] This flexible DC converter based on a virtual synchronous motor can achieve the same beneficial effects as the aforementioned flexible DC converter control method based on a virtual synchronous machine. Attached Figure Description

[0028] Figure 1 This is a control block diagram of a flexible DC converter based on a virtual synchronous motor, which is a current technology.

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

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

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

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

[0033] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be described in detail below with reference to the accompanying drawings and embodiments.

[0034] The present invention is conceived as follows: This invention provides a flexible DC converter based on a virtual synchronous motor, a control method, and a system. The method includes: during the control of the converter using virtual synchronous motor control, if an AC system fault occurs, obtaining the AC voltage amplitude, and combining the preset positive correlation between the AC voltage amplitude and the active power limiting coefficient, reactive power limiting coefficient, and current limiting coefficient to obtain the active power limiting coefficient, reactive power limiting coefficient, and current limiting coefficient; multiplying the positive and negative active power limiting coefficients by the converter's rated power to obtain the upper and lower limits of the active power reference value of the virtual speed control loop of the virtual synchronous motor control; multiplying the positive and negative reactive power limiting coefficients by the upper limit of the system's reactive power output to obtain the upper and lower limits of the reactive power reference value of the reactive power droop control loop of the virtual synchronous motor control; and 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 inner current loop of the virtual synchronous motor control. This enables the converter to operate in grid-type control mode during faults, based on its own equipment capabilities, and maximizes the equipment's operating capacity to support the transient and steady-state stable operation of the AC power grid.

[0035] Example of a flexible DC converter control method based on a virtual synchronous motor:

[0036] This implementation provides a flexible DC converter control method based on a virtual synchronous motor, which is based on... Figure 1 The control block diagram shown includes an active virtual speed control loop, a reactive droop control loop, an outer loop voltage control, and an inner loop current control. Finally, it outputs a modulated wave and adds fault identification logic, so that the system can still maintain grid-type control during AC faults and ensure the stability of the grid frequency.

[0037] The control block diagram of this embodiment is as follows: Figure 2 As shown, fault identification logic has been added to the control block diagram of the existing technology to collect the AC voltage U at the grid connection point. ac and AC voltage I ac .

[0038] Specifically, the fault identification logic is as follows: Figure 3 As shown, the AC voltage U at the grid connection point is collected. ac and AC voltage I ac The effective values ​​of voltage and current, Urms and Irms, are calculated. First, the amplitude of Urms is less than the preset first voltage judgment value Uset1 and continues to meet the preset first delay value T1, and the rate of change of Irms is greater than the preset current judgment value DIset, thus satisfying criterion one. Then, the amplitude of Urms is less than the preset second voltage judgment value Uset2 and continues to meet the preset second delay value T2, thus satisfying criterion two. The first voltage judgment value is greater than the second voltage judgment value. Based on experience, the first voltage judgment value Uset1 is generally taken as 0.9pu, the second voltage judgment value Uset2 is taken as 0.8pu, the first delay value T1 is taken as 200us, the second delay value T2 is taken as 100us, and the current judgment value Diset is taken as 0.1pu / ms.

[0039] Finally, an OR logic is performed on criteria one and criteria two. When either criterion is satisfied, the fault identification logic outputs a fault ride-through enable signal Fault_tri. When Fault_tri is 1, fault ride-through is enabled; when Fault_tri is 0, fault ride-through is disabled. When fault ride-through is enabled, a limiting strategy is entered. The limiting strategy uses a lookup table method. The curve of the lookup table method is obtained by analyzing and processing system simulation data. Based on multiple practical experiences, the optimal output active power limiting coefficient Kpl, reactive power limiting coefficient Kql, and current limiting coefficient Kil corresponding to the voltage amplitude are obtained through system simulation to obtain the curve of the lookup table method.

[0040] Kp1, Kq1, and Ki1 are all positively correlated with the voltage amplitude. During a 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, while Ki1 is generally greater than the per-unit value of the voltage.

[0041] The specific method for obtaining these values ​​in this embodiment is as follows: Electromagnetic transient simulation software is used to model the converter. Different degrees of AC faults are simulated at the AC grid connection point to obtain different AC voltage amplitudes. In this embodiment, the AC grid voltage drops by 10% at intervals, and different AC voltage amplitudes from 0% to 90% are tested. The values ​​of Kp1, Kq1, and Ki1 are repeatedly modified, and the maximum values ​​of Kp1, Kq1, and Ki1 that allow the system to operate safely and stably are obtained through repeated testing and used as the corresponding values ​​of Kp1, Kq1, and Ki1 for the voltage amplitude.

[0042] The fault identification logic inputs the active power limiting coefficient to the active virtual speed control loop, the reactive power limiting coefficient to the reactive power droop control loop, and the current limiting coefficient to the current limiting loop.

[0043] (1) Active virtual speed control loop

[0044] The fault identification logic inputs the active power limiting coefficient Kpl into the active power virtual speed control loop. The specific implementation method in the active power virtual speed control loop is as follows: Figure 4 As shown, the active virtual speed control loop designed in this embodiment, based on the synchronous motor, adds active power reference value input limiting (i.e., the upper and lower limits of the active power reference value of the active virtual speed control loop controlled by the virtual synchronous motor), overload limiting, inertial element integral limiting, and angle correction logic. The frequency real-time filter value Fs and the grid voltage phase angle θ at the inertial element integral limiting and angle correction logic are also included. S The phase-locked loop (PLL) obtains the AC voltage U ac The measurement was obtained at the location.

[0045] Specifically, Pref is the set active power reference value. First, the limit value calculated by the active power limiting coefficient Kpl is output by the fault identification logic and then used as the active power reference value input limit in the active power virtual translation control loop. After the damping component of the virtual speed loop is superimposed, the limit is applied by the load limiting module.

[0046] The active power virtual relay control loop's active power reference value input limit upper and lower limits, Max_limit1 and Min_limit1, 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 flexible DC converter.

[0050] The difference between the active power reference value Pref after overload limiting and the real-time active power Pe is used to enter the inertial loop. The integrator of the inertial loop is set with upper and lower limits that 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 frequency value measured by the phase-locked loop, and Fset is the set value of the frequency deviation, which is generally taken as 0.25 to 0.5 Hz.

[0054] When the integral element reaches its upper limit, the output upward limiting signal EN1 is 1, and vice versa; when the integral element reaches its lower limit, the output downward limiting signal EN2 is 1, and vice versa.

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

[0056]

[0057] In the formula, Poverset is the overload margin setting value, which is set according to the requirement of retaining a certain equipment margin for overload energy; K is the overload limiting proportional braking coefficient, which takes a value between 0 and 1; EN1(t) and EN2(t) are the current operating cycle and whether the inertial integral element is limited; and m is the total number of operating cycles within the set time window. This solves the problem of converter overload caused by system frequency fluctuations under high power conditions.

[0058] The angle correction logic limits the angle generated autonomously by the active virtual speed control loop to prevent instability caused by large angle deviations when the external power grid phase changes rapidly and cannot be synchronized. The output phase angle should satisfy the following formula:

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

[0060] In the formula, θ S The phase angle is the grid voltage, and Dset is the angle deviation correction setting value, which is recommended to be 30-60°.

[0061] (2) Reactive power droop control loop

[0062] The fault identification logic also inputs the reactive power limiting coefficient Kql into the reactive power droop control loop. The specific implementation method in the reactive power droop control loop is as follows: Figure 5As shown, this embodiment adds a 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 controlled by the virtual synchronous motor) at the input of the reactive power droop control loop. Qref is the set reactive power reference value. First, the limit value calculated by the reactive power limiting coefficient Kql output by the fault identification logic is used to limit the input of the reactive power droop control loop. The difference between the limited reactive power reference value and the real-time reactive power Q is calculated. First, the difference is compared with the coefficient k. q Multiplication: During normal operation, the control loop integrator is disabled, and the difference is multiplied by the coefficient k. q The product of the two values ​​is compared with the AC voltage reference value U. ac_ref Adding them together, we obtain the d-axis voltage reference value Ud. ref When the system detects an AC system fault (i.e., fault ride-through enabled, Fault_tri signal is 1), the control loop integrator is activated to superimpose the integral input, entering the reactive power closed-loop control mode, and the difference is compared with the coefficient k. q The multiplied value, the control loop integrator output value, and the AC voltage reference value U ac_ref Adding these three values ​​together yields the d-axis voltage reference value Ud. ref .

[0063] The reactive power droop control loop's input limit upper and lower limits, Max_limit4 and Min_limit4, 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's reactive power output.

[0067] (3) Current limiting

[0068] The current limiting module uses proportional limiting logic, with an upper limit of Imax_limit.

[0069] Imax_limit = Ki1 * Imax (10)

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

[0071] This embodiment improves the active virtual speed control loop based on the traditional virtual synchronous control strategy to solve the overload problem under frequency fluctuations. It modifies the fault ride-through logic, abandons the traditional strategy of switching to grid-type control during faults, and no longer relies on limiting the inner loop current to achieve fault ride-through. Instead, it uses limiting the active and reactive power of the outer loop to achieve fault ride-through, so that the system can still maintain grid-type control during AC faults and support grid frequency stability.

[0072] Example of a flexible DC-DC converter control system based on a virtual synchronous motor:

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

[0074] Since the specific implementation process and principle of the flexible DC converter control system based on the virtual synchronous motor in this embodiment have been described in detail in the embodiment of the flexible DC converter control method based on the virtual synchronous motor, they will not be repeated here.

[0075] Example of a flexible DC converter based on a virtual synchronous motor:

[0076] This embodiment provides a flexible DC converter based on a virtual synchronous motor, including an active virtual speed control loop, a reactive power droop control loop, and a current limiting module. The active virtual speed control loop adds an active virtual transcribing control loop input limiting module (i.e., an active power limiting coefficient Kpl-based limiting module for fault crossing limiting), an overload limiting module, an inertial element integral limiting module, and an angle correction logic module. It also adds a phase-locked loop (PLL) for auxiliary stability control. The PLL obtains the AC voltage frequency and phase at the current grid connection point, which is used to limit the virtual speed (i.e., the inertial element 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 element. The reactive power droop control loop adds a reactive power droop control loop input limiting module (i.e., a reactive power limiting coefficient Kql-based limiting module for fault crossing limiting) and a control loop integrator.

[0077] Since the specific implementation process and principle of the flexible DC converter based on the virtual synchronous motor in this embodiment have been described in detail in the embodiment of the flexible DC converter control method based on the virtual synchronous motor, they will not be repeated here.

Claims

1. A method of controlling a VSC based on a virtual synchronous machine, 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 an AC voltage amplitude, combining a preset positive correlation between the AC voltage amplitude and active limiting coefficients, reactive limiting coefficients and current limiting coefficients to obtain the active limiting coefficients, the reactive limiting coefficients and the current limiting coefficients; multiplying the positive and negative active limiting coefficients by the rated power of the converter 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 system reactive power 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 in 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 virtual synchronous machine based line commutated converter control method of claim 1, wherein, The specific method for obtaining the positive correlation between the AC voltage amplitude and the active limiting coefficients, the reactive limiting coefficients and the current limiting coefficients comprises: using electromagnetic transient simulation software to model the converter, simulating different degrees of AC faults at the AC grid access to obtain different AC voltage amplitudes; for a certain AC voltage amplitude, repeatedly modifying the values of the active limiting coefficients, the reactive limiting coefficients and the current limiting coefficients to test the maximum values of the active limiting coefficients, the reactive limiting coefficients and the current limiting coefficients for safe and stable operation of the system as the active limiting coefficients, the reactive limiting coefficients and the current limiting coefficients corresponding to the AC voltage amplitude.

3. The virtual synchronous machine based line commutated converter control method of claim 1, wherein, The method further comprises: performing inertia integral limiting processing on the result of inertia integral in the active virtual speed control loop, the upper limit of the inertia integral limiting being the sum of the real-time filtered value of the frequency and the preset frequency deviation setting value, and the lower limit of the inertia integral limiting being the difference between the real-time filtered value of the frequency and the preset frequency deviation setting value.

4. The virtual synchronous machine based line commutated converter control method of claim 3, wherein, The method further comprises: performing overload limiting processing on the active power reference value of the inertia integral limiting element input into the active virtual speed control loop, the upward limiting signal being 1 if the integral element reaches the upper limit of the inertia integral limiting, and 0 otherwise, and the downward limiting signal being 1 if the integral element reaches the lower limit of the inertia integral limiting, and 0 otherwise; the overload limiting upper and lower limit calculation formulae being as follows: wherein Max_limit3 is the overload limiting upper limit, Min_limit3 is the overload limiting lower limit, Poverset is the overload margin setting value, K is the overload limiting proportional brake coefficient, EN1(t) and EN2(t) are respectively the current operating cycle, the upward limiting signal and the downward limiting signal of the inertia integral element, and m is the total number of operating cycles in the set time window.

5. The virtual synchronous machine based line commutated converter control method of claim 1, wherein, The method further comprises: performing angle correction on the output end angle in the active virtual speed control loop to make the output phase angle range as follows: where θ s is the grid voltage phase angle, and Dsetis the angle deviation correction setting value.

6. The virtual synchronous machine based line commutated converter control method of claim 1, wherein, When the AC system fails, 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.

7. The virtual synchronous machine based line commutated converter control method of claim 1, wherein, The method for judging whether the 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 position, the voltage effective value and the current effective value are judged, when the criterion one or the criterion two is satisfied, it is judged that the AC system fault occurs; the criterion one is: the voltage effective value is less than a preset first voltage judging value and continuously satisfies a preset first delay value, and the current effective value change rate is greater than a preset current judging value; the criterion two is: the voltage effective value is less than a preset second voltage judging value and continuously satisfies a preset second delay value; the first voltage judging value is greater than the second voltage judging value.

8. The virtual synchronous machine based line commutated converter control method of claim 1, wherein, The relationship between the AC voltage amplitude and the active current limiting coefficient, the reactive current limiting coefficient and the current limiting coefficient is represented in a table form.

9. A virtual synchronous machine based flexible HVDC converter control system comprising a processor, characterized in that, The processor is configured to execute a computer program to implement the steps of the virtual synchronous machine based VSC control method according to any one of claims 1-7.

10. A VSC based on a virtual synchronous machine, comprising a converter controller, the converter controller comprising a processor, characterized in that, The processor is configured to execute a computer program to implement the steps of the virtual synchronous machine based VSC control method according to any one of claims 1-7.

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