Modular multilevel converter synchronous control method and system based on capacitor voltage

Abstract

The invention provides a modular multilevel converter synchronous control method and system based on capacitor voltage, and the method comprises the steps: controlling an outer ring through the capacitor voltage based on a capacitor voltage average per unit value and a capacitor voltage instruction per unit value of a modular multilevel converter, calculating to obtain an active power instruction per unit value of the AC side of the modular multilevel converter; the active power instruction per unit value and the active power measurement per unit value of the AC side of the modular multilevel converter act on a power control inner ring, and synchronous control of the modular multilevel converter and a power grid is carried out; according to the method, the active power instruction per unit value of the AC side of the modular multilevel converter is calculated, so that the average capacitor voltage value of the sub-module as control feedback input does not have frequency doubling fluctuation in the subsequent synchronous control of the converter and the power grid, and an additional filter is not needed; the problem that the dynamic stability is poor due to the fact that a second-order low-pass filter needs to be additionally arranged when capacitor energy control is adopted is solved.

Description

Technical Field

[0001] This invention patent application belongs to the field of modular multilevel converters, and particularly relates to a synchronous control method and system for modular multilevel converters based on capacitor voltage. Background Art

[0002] The modular multilevel converter is the MMC converter. In existing flexible DC transmission systems based on modular multilevel converters, grid-following control is mostly adopted, which is suitable for connecting to strong AC systems. With the construction of new power systems, flexible DC transmission technology has become the preferred method for large-scale long-distance transmission of new energy. The sending-end flexible DC converter station is connected to a new energy power station that can be regarded as a weak AC system; in some cases, the receiving-end AC system is also weak, and the receiving-end converter station will also be connected to a weak AC system. At this time, the modular multilevel converter using grid-following control is difficult to operate stably in a weak AC system. In recent years, grid-forming control has been proposed to improve the operation stability of the converter when connected to a weak AC system.

[0003] Therefore, there is an urgent need for a flexible DC transmission system with grid-forming function to improve the stability of the flexible DC system when connected to a weak AC system, and at the same time improve its ability to provide inertia, frequency and voltage support to the system, thereby improving the utilization rate of the DC transmission channel and realizing the safe grid connection and efficient transmission of large-scale new energy such as wind power, photovoltaic power, and pumped storage. At present, the research on flexible DC transmission systems with grid-forming functions is still in its infancy. In terms of grid-forming control strategies, it is mainly based on the principle of DC voltage or AC-DC side power balance, including droop control, virtual synchronous generator control, power synchronous control, etc.

[0004] The application number is 202111082156.5, and the invention name is: A MMC Synchronous Control Method and System Based on DC Voltage. By constructing a MMC synchronous control model based on DC voltage using the virtual synchronous machine principle, then obtaining the DC voltage on the DC side, taking the difference with the DC voltage reference value to determine the DC voltage deviation value, and finally inputting the DC voltage deviation value into the MMC synchronous control model based on DC voltage to obtain the voltage phase angle of the commutation bus, the accurate acquisition of the AC side phase angle information is realized when connected to a weak AC system, and the phase-locked control is completed. However, this control method needs to simultaneously achieve the grid-connected self-synchronization of the converter and the control of the DC voltage, and there are problems such as difficult design of control parameters and weak stability.

[0005] The application number is 202310519248.8, and the invention title is: A Flexible DC Energy Grid-Forming Control Device and MMC Control System. By utilizing the energy storage of all sub-module capacitors inside the modular multilevel converter, an energy-power control module is constructed to achieve the control of the energy of sub-module capacitors; by constructing a power synchronization control module and simulating the generator rotor motion equation, the control of the AC voltage phase is achieved. However, due to the double-frequency fluctuation of the energy storage of sub-module capacitors, a second-order low-pass filter needs to be adopted in the energy-power control module to extract the DC component of the energy storage of sub-module capacitors, thereby reducing the adjustment speed and dynamic stability of the capacitor energy control and restricting the application of this method in the grid-forming flexible DC system.

[0006] The application number is 202310655057.4, and the invention title is: A Control Method for a Converter in a Photovoltaic Flexible DC Transmission System. By adding a DC equivalent capacitor to the DC side of the receiving-end converter and introducing a virtual DC power damping control strategy on the basis of constant DC voltage control, grid connection self-synchronization is completed while stabilizing the DC bus voltage. When the photovoltaic grid is connected, it can improve frequency oscillation, reduce the overshoot, and has strong anti-interference ability. However, this method requires an additional capacitor on the DC side, increasing the cost of the converter. Especially for high-voltage flexible DC transmission systems with transmission voltage levels tending to ±800 kV, the technical feasibility of adding high-voltage capacitors on the DC side needs to be demonstrated.

[0007] The application number is 202210994111.3, and the invention title is: A Control Method for a Power Supply System of a Phase-Locked Loop-Grid-Forming MMC Converter Station. By enabling the grid-forming MMC converter station to be applicable to both strong power grids with large short-circuits and weak power grids with small short-circuits, and being able to delay the adjustment of power output when the grid frequency changes or the power command value changes, and having good transient operation characteristics during the fault of the power supply system. However, this method is not applicable to the scenario when the MMC converter station supplies power to passive loads.

[0008] Regarding the control strategy of the grid-forming flexible DC transmission system, the existing research results have technical problems such as it being difficult to simultaneously consider the regulation time and overshoot of the control system only by using proportional control for capacitor energy control, and the dynamic stability is further deteriorated because the capacitor energy needs to pass through a second-order low-pass filter; adding a capacitor on the DC side results in an increase in cost, and it is difficult to meet the control requirements of the grid-forming flexible DC transmission system. Summary of the Invention

[0009] To overcome the deficiencies of the above-mentioned prior art, this invention patent application proposes a synchronous control method for a modular multilevel converter based on capacitor voltage, including:

[0010] Based on the per-unit value of the average capacitor voltage and the per-unit value of the capacitor voltage command of the modular multilevel converter, through the outer loop of capacitor voltage control, the per-unit value of the active power command on the AC side of the modular multilevel converter is calculated;

[0011] The per-unit value of the active power command and the per-unit value of the measured active power on the AC side of the modular multilevel converter are applied to the inner loop of power control to perform synchronous control of the modular multilevel converter and the power grid.

[0012] Preferably, the calculating, based on the per-unit value of the average capacitor voltage and the per-unit value of the capacitor voltage command of the modular multilevel converter, through the outer loop of capacitor voltage control, the per-unit value of the active power command on the AC side of the modular multilevel converter includes:

[0013] Using the outer loop of capacitor voltage control, calculate the difference between the per-unit value of the average capacitor voltage and the per-unit value of the capacitor voltage command, and based on the difference, obtain the output value of the outer loop of capacitor voltage control through the controller of the outer loop of capacitor voltage control;

[0014] Based on the sum of the output value of the outer loop of capacitor voltage control and the per-unit value of the active power on the DC side of the modular multilevel converter, calculate the per-unit value of the active power command on the DC side of the modular multilevel converter;

[0015] Wherein, the controller is a proportional derivative controller or a proportional lead compensator.

[0016] Preferably, the proportional derivative controller includes the following transfer function:

[0017]

[0018] Wherein, G1(s) is the transfer function of the proportional derivative controller, k p is the proportional control coefficient, k d is the derivative control coefficient T d is the time constant of the low-pass filter, and s is the Laplace operator.

[0019] Preferably, the applying the per-unit value of the active power command and the per-unit value of the measured active power on the AC side of the modular multilevel converter to the inner loop of power control to perform synchronous control of the modular multilevel converter and the power grid includes:

[0020] Using the inner loop of power control, calculate the difference between the per-unit value of the active power command on the DC side of the converter and the per-unit value of the measured active power on the AC side of the modular multilevel converter;

[0021] Apply the difference to the synchronous generator rotor motion equation of the inner loop of power control to perform synchronous control of the modular multilevel converter and the power grid.

[0022] Preferably, the calculation formula of the per-unit value of the active power measurement on the AC side of the modular multilevel converter is as follows:

[0023]

[0024] Wherein, is the per-unit value of the active power measurement on the AC side of the modular multilevel converter, and \(i\) a is the instantaneous phase current of phase a at the grid connection point, and \(i\) b is the instantaneous phase current of phase b at the grid connection point, and \(i\) c is the instantaneous phase current of phase c at the grid connection point, and \(u\) a is the instantaneous phase voltage of phase a at the grid connection point, and \(u\) b is the instantaneous phase voltage of phase b at the grid connection point, and \(u\) c is the instantaneous phase voltage of phase c at the grid connection point, and \(S\) N is the rated capacity of the converter.

[0025] Preferably, after applying the difference to the synchronous generator rotor motion equation in the inner power control loop for synchronous control of the modular multilevel converter and the power grid, it further includes:

[0026] Calculating the per-unit value of the output voltage angular frequency variation on the AC side of the modular multilevel converter through the transfer function of the synchronous generator rotor motion equation;

[0027] Calculating the per-unit value of the output voltage angular frequency of the modular multilevel converter based on the per-unit value of the output voltage angular frequency variation and the per-unit value of the grid voltage angular frequency;

[0028] Multiplying the per-unit value of the output voltage angular frequency by the obtained angular frequency reference value to calculate the nominal value of the output voltage angular frequency of the modular multilevel converter;

[0029] Integrating the nominal value of the output voltage angular frequency to calculate the output voltage phase of the modular multilevel converter, and verifying the synchronous control of the modular multilevel converter and the power grid according to the output voltage phase.

[0030] Based on the same inventive concept, the present invention patent application also provides a synchronous control system for a modular multilevel converter based on capacitor voltage, including: an active power command per-unit value calculation module and a synchronous control module;

[0031] The active power command per-unit value calculation module is used to calculate the active power command per-unit value on the AC side of the modular multilevel converter through the capacitor voltage control outer loop based on the average per-unit value of the capacitor voltage and the capacitor voltage command per-unit value of the modular multilevel converter;

[0032] The synchronization control module is used to apply the per-unit value of the active power command and the per-unit value of the measured active power on the AC side of the modular multilevel converter to the inner power control loop for the synchronization control of the modular multilevel converter and the power grid.

[0033] Preferably, the per-unit value calculation module of the active power command is specifically used for:

[0034] Adopt the outer capacitor voltage control loop to calculate the difference between the average per-unit value of the capacitor voltage and the per-unit value of the capacitor voltage command, and based on the difference, obtain the output value of the outer capacitor voltage control loop through the controller of the outer capacitor voltage control loop;

[0035] Based on the sum of the output value of the outer capacitor voltage control loop and the per-unit value of the active power on the DC side of the modular multilevel converter, calculate the per-unit value of the active power command on the DC side of the modular multilevel converter;

[0036] Wherein, the controller is a proportional derivative controller or a proportional lead compensator.

[0037] Preferably, the proportional derivative controller of the per-unit value calculation module of the active power command includes the following transfer function:

[0038]

[0039] Wherein, G1(s) is the transfer function of the proportional derivative controller, k p is the proportional control coefficient, k d is the derivative control coefficient T d is the time constant of the low-pass filter, and s is the Laplace operator.

[0040] Preferably, the synchronization control module is specifically used for:

[0041] Adopt the inner power control loop to calculate the difference between the per-unit value of the active power command on the DC side of the converter and the per-unit value of the measured active power on the AC side of the modular multilevel converter;

[0042] Apply the difference to the synchronous generator rotor motion equation of the inner power control loop for the synchronization control of the modular multilevel converter and the power grid.

[0043] Preferably, the calculation formula of the per-unit value of the measured active power on the AC side of the modular multilevel converter of the synchronization control module is as follows:

[0044]

[0045] Wherein, is the per-unit value of the measured active power on the AC side of the modular multilevel converter, i a is the instantaneous phase current of phase a at the grid connection point, ib is the instantaneous phase current of phase b at the grid connection point, i c is the instantaneous phase current of phase c at the grid connection point, u a is the instantaneous phase voltage of phase a at the grid connection point, u b is the instantaneous phase voltage of phase b at the grid connection point, u c is the instantaneous phase voltage of phase c at the grid connection point, S N is the rated capacity of the converter.

[0046] Preferably, after the synchronization control module applies the difference to the synchronous generator rotor motion equation of the inner power control loop for the synchronous control of the modular multilevel converter and the power grid, it further includes

[0047] Calculating the per-unit value of the change in the output voltage angular frequency of the AC side of the modular multilevel converter through the transfer function of the synchronous generator rotor motion equation;

[0048] Calculating the per-unit value of the output voltage angular frequency of the modular multilevel converter based on the per-unit value of the change in the output voltage angular frequency and the per-unit value of the grid voltage angular frequency;

[0049] Multiplying the per-unit value of the output voltage angular frequency by the obtained angular frequency reference value to calculate the nominal value of the output voltage angular frequency of the modular multilevel converter;

[0050] Integrating the nominal value of the output voltage angular frequency to calculate the output voltage phase of the modular multilevel converter, and verifying the synchronous control of the modular multilevel converter and the power grid according to the output voltage phase.

[0051] Compared with the closest prior art, the beneficial effects of this invention patent application are as follows:

[0052] This invention patent application provides a synchronous control method and system for a modular multilevel converter based on capacitor voltage, including: calculating the per-unit value of the active power command of the AC side of the modular multilevel converter through the outer capacitor voltage control loop based on the average per-unit value of the capacitor voltage and the per-unit value of the capacitor voltage command of the modular multilevel converter; applying the per-unit value of the active power command and the measured per-unit value of the active power of the AC side of the modular multilevel converter to the inner power control loop for the synchronous control of the modular multilevel converter and the power grid; the average per-unit value of the capacitor voltage of the modular multilevel converter in this application calculates the per-unit value of the active power command of the AC side of the modular multilevel converter through the outer capacitor voltage control loop, and then performs subsequent synchronous control of the modular multilevel converter and the power grid. During this process, the average value of the sub-module capacitor voltage as the control feedback input has no second-harmonic fluctuation and does not require an additional filter, overcoming the problem of poor dynamic stability caused by the need for an additional second-order low-pass filter in capacitor energy control. Description of the Drawings

[0053] Figure 1 Schematic diagram of a synchronous control method for a modular multilevel converter based on capacitor voltage provided for this invention patent application;

[0054] Figure 2 Capacitor voltage control block diagram using a proportional - derivative controller provided for this invention patent application;

[0055] Figure 3 Capacitor voltage control block diagram using a proportional - lead compensator provided for this invention patent application;

[0056] Figure 4 A power inner - loop control block diagram provided for this invention patent application;

[0057] Figure 5 A synchronous control block diagram based on a proportional - lead compensator provided for this invention patent application;

[0058] Figure 6 A synchronous control block diagram based on a proportional - derivative controller provided for this invention patent application;

[0059] Figure 7 Schematic diagram of a synchronous control system for a modular multilevel converter based on capacitor voltage provided for this invention patent application. Detailed Description of the Invention

[0060] The following further elaborates on the detailed implementation of this invention patent application with reference to the drawings.

[0061] Example 1:

[0062] A synchronous method for a modular multilevel converter based on capacitor voltage provided for this invention patent application is as Figure 1 shown and includes:

[0063] Step 1: Based on the average per - unit value of the capacitor voltage and the per - unit value of the capacitor voltage command of the modular multilevel converter, calculate the per - unit value of the active power command on the AC side of the modular multilevel converter through the outer - loop capacitor voltage control;

[0064] Step 2: Apply the per - unit value of the active power command and the measured per - unit value of the active power on the AC side of the modular multilevel converter to the inner - loop power control to perform synchronous control of the modular multilevel converter and the power grid.

[0065] Specifically, Step 1 includes:

[0066] The average per - unit value of the internal capacitor voltage of the MMC converter as the feedback input in the outer - loop capacitor voltage control is the per-unit value of the arithmetic mean of the capacitor voltages of all sub-modules inside the MMC converter; the outer capacitor voltage control loop is used to control the average per-unit value of the capacitor voltages inside the MMC converter to make it reach the per-unit value of the capacitor voltage command and calculate the per-unit value of the capacitor voltage command and the average per-unit value of the capacitor voltage The difference is passed through the controller of the outer capacitor voltage control loop to obtain the output value of the outer capacitor voltage control loop.

[0067] The controller used in the outer capacitor voltage control loop can be implemented by a proportional-derivative (PD) controller. The proportional-derivative controller includes the following transfer function:

[0068]

[0069] where G1(s) is the transfer function of the proportional-derivative controller, k p is the proportional control coefficient, k d is the derivative control coefficient T d is the time constant of the low-pass filter, and s is the Laplace operator.

[0070] The controller used in the outer capacitor voltage control loop can also be implemented by a proportional lead compensator. The proportional lead compensator includes the following transfer function:

[0071]

[0072] where G2(s) is the transfer function of the proportional lead compensator, k p is the proportional control coefficient, s is the Laplace operator, T1 and T2 are the time constants of the lead compensator respectively, and T1 > T2 is satisfied.

[0073] The per-unit value of the DC-side active power of the MMC converter in the outer capacitor voltage control loop is calculated using the measured values of the DC voltage u dc and the DC current i dc The calculation formula for the per-unit value of the DC-side active power of the MMC converter is as follows:

[0074]

[0075] where is the per-unit value of the DC-side active power of the MMC converter, u dc is the DC voltage, i dc is the DC current, u dc and i dc are both nominal values, and S N is the rated capacity of the converter;

[0076] The output value of the outer loop of the capacitor voltage control is summed with the per-unit value of the active power on the DC side of the MMC converter to obtain the per-unit value of the active power command on the AC side of the MMC converter without clipping After clipping, the per-unit value of the active power command on the AC side of the MMC converter is obtained

[0077] The output part of the outer loop of the capacitor voltage control includes a clipping link, and the clipping value is the per-unit value of the maximum power of the MMC converter The input of the clipping link is the per-unit value of the active power command on the AC side of the MMC converter without clipping The output is the per-unit value of the active power command on the AC side of the MMC converter after clipping This application provides two controllers, namely a proportional derivative controller or a proportional lead compensator. In the control block diagrams as shown in Figure 2 and 3 both controllers are easy to implement phase lead compensation, and the proportional lead compensator overcomes the problem in the existing technology that only proportional control is used and it is impossible to balance the control response speed and overshoot; the control method of this application can achieve the decoupling of the capacitor voltage and the DC voltage. During the synchronization process, only the capacitor voltage changes, while the DC voltage remains unchanged; the average per-unit value of the capacitor voltage of the modular multilevel converter is used to calculate the per-unit value of the active power command on the AC side of the modular multilevel converter through the outer loop of the capacitor voltage control, and then the subsequent synchronization control of the modular multilevel converter and the power grid is carried out. During this process, the average value of the capacitor voltage of the sub-module as the control feedback input has no double-frequency fluctuation and no additional filter is required, overcoming the problem of poor dynamic stability caused by the need for an additional second-order low-pass filter when using capacitor energy control.

[0078] Specifically, step 2 includes:

[0079] The inner loop of the power control is used to simulate the rotor motion equation of the synchronous generator and calculate the per-unit value of the active power command and the per-unit value of the measured active power on the AC side of the MMC converter The difference is applied to the rotor motion equation of the synchronous generator in the inner loop of the power control to carry out the synchronization control of the modular multilevel converter and the power grid;

[0080] The per-unit value of the measured active power on the AC side of the MMC converter in the inner loop of the power control adopts the three-phase instantaneous phase voltages u a , u b , u c measured at the grid connection point and the three-phase instantaneous phase currents i a , i b , i c, obtained through calculation. The measured value of active power The calculation formula is as follows:

[0081]

[0082] Among them, is the per-unit value of the measured active power on the AC side of the MMC converter, u a is the instantaneous phase voltage of phase a, u b is the instantaneous phase voltage of phase b, u c is the instantaneous phase voltage of phase c, i a is the instantaneous phase current of phase a, i b is the instantaneous phase current of phase b, i c is the instantaneous phase current of phase c. The three-phase instantaneous phase voltages u a , u b , u c and the three-phase instantaneous phase currents i a , i b , i c are all nominal values.

[0083] After performing synchronous control on the modular multilevel converter and the power grid, the per-unit value of the change in the output voltage angular frequency of the MMC converter, Δω p.u. , is obtained through the transfer function of the synchronous generator rotor motion equation. The per-unit value of the angular frequency change, Δω p.u. is added to the per-unit value of the power grid voltage angular frequency to obtain the per-unit value of the output voltage angular frequency of the MMC converter The per-unit value of the output voltage angular frequency of the MMC converter is multiplied by the angular frequency reference value ω B to obtain the nominal value of the output voltage angular frequency of the MMC converter, ω pcc ; Integrating the nominal value of the output voltage angular frequency of the MMC converter, ω pcc , gives the output voltage phase θ pcc . And based on the output voltage phase, the synchronous control of the modular multilevel converter and the power grid is verified.

[0084] The rotor motion equation in the inner loop of power control includes the following partial transfer functions:

[0085]

[0086] Among them, G(s) is the partial transfer function of the rotor motion equation in the inner loop of power control, T J is the inertia time constant, D is the damping coefficient, s is the Laplace operator, and T J , D can be set according to the relevant performance indicators of the control system.

[0087] Per-unit value Δω of the angular frequency change in the inner power control loop p.u. The frequency-domain expression of Δω p.u. (s) is as follows:

[0088]

[0089] Where, Δω p.u. (s) is the frequency-domain expression of the per-unit value of the angular frequency change, is the frequency-domain value of the active power command value on the AC side of the MMC converter, p ac (s) is the frequency-domain value of the measured active power on the AC side of the MMC converter.

[0090] The output voltage phase θ of the inner power control loop pcc The expression is:

[0091]

[0092] Where, θ pcc is the output voltage phase, ω pcc is the nominal value of the output voltage angular frequency of the MMC converter, Δω p.u. is the per-unit value of the angular frequency change, is the per-unit value of the grid voltage angular frequency; The control method of the present application can directly act on the inner power control loop with the per-unit value of the active power command and the measured per-unit value of the active power on the AC side of the modular multilevel converter through the process shown in Figure 4 to achieve the synchronous control of the modular multilevel converter and the grid, without the need for an additional low-pass filter, and can achieve fast control of the capacitor voltage.

[0093] In a specific embodiment

[0094] Such as Figure 5 shown is a synchronous control block diagram based on a proportional lead compensator. T1 and T2 in the proportional lead compensator adopted by the outer capacitor voltage control loop can be set to T1 = 20 ms and T2 = 3 ms.

[0095] Embodiment 2:

[0096] Such as Figure 6 shown, the embodiment of the present invention provides a MMC synchronous control block diagram based on a proportional derivative controller. It consists of an outer capacitor voltage control loop and an inner power control loop.

[0097] Set the per-unit value of the capacitor voltage command k p = 6, k d = 0.1, T d = 1 ms, the inertia time constant T J= 2s, damping coefficient D = 30, per-unit maximum power of the MMC converter Base angular frequency ω B = 314.15926535 rad / s.

[0098] Embodiment 3:

[0099] Based on the same inventive concept, this patent application also provides a modular multilevel converter synchronization system based on capacitor voltage as Figure 7 shown, including: a per-unit active power command calculation module and a synchronization control module;

[0100] The per-unit active power command calculation module is used to calculate the per-unit active power command of the AC side of the modular multilevel converter through the outer loop of capacitor voltage control based on the average per-unit value of the capacitor voltage and the per-unit capacitor voltage command of the modular multilevel converter;

[0101] The synchronization control module is used to apply the per-unit active power command and the measured per-unit active power of the AC side of the modular multilevel converter to the inner loop of power control for synchronous control of the modular multilevel converter and the power grid.

[0102] Preferably, the per-unit active power command calculation module is specifically used for:

[0103] Adopt the outer loop of capacitor voltage control to calculate the difference between the average per-unit value of the capacitor voltage and the per-unit capacitor voltage command, and based on the difference, obtain the output value of the outer loop of capacitor voltage control through the controller of the outer loop of capacitor voltage control;

[0104] Based on the sum of the output value of the outer loop of capacitor voltage control and the per-unit active power of the DC side of the modular multilevel converter, calculate the per-unit active power command of the DC side of the modular multilevel converter;

[0105] Among them, the controller is a proportional derivative controller or a proportional lead compensator.

[0106] Preferably, the proportional derivative controller of the per-unit active power command calculation module includes the following transfer function:

[0107]

[0108] Among them, G1(s) is the transfer function of the proportional derivative controller, k p is the proportional control coefficient, k d is the derivative control coefficient T d is the time constant of the low-pass filter, and s is the Laplace operator.

[0109] Preferably, the synchronization control module is specifically used for:

[0110] Adopt a power control inner loop to calculate the difference between the per-unit value of the active power command on the DC side of the converter and the per-unit value of the measured active power on the AC side of the modular multilevel converter.

[0111] Apply the difference to the synchronous generator rotor motion equation of the power control inner loop to perform synchronous control of the modular multilevel converter and the power grid.

[0112] Preferably, the calculation formula for the per-unit value of the measured active power on the AC side of the modular multilevel converter in the synchronous control module is as follows:

[0113]

[0114] Where is the per-unit value of the measured active power on the AC side of the modular multilevel converter, and i a is the instantaneous phase current of phase a at the grid connection point, and i b is the instantaneous phase current of phase b at the grid connection point, and i c is the instantaneous phase current of phase c at the grid connection point, and u a is the instantaneous phase voltage of phase a at the grid connection point, and u b is the instantaneous phase voltage of phase b at the grid connection point, and u c is the instantaneous phase voltage of phase c at the grid connection point, and S N is the rated capacity of the converter.

[0115] Preferably, after applying the difference to the synchronous generator rotor motion equation of the power control inner loop to perform synchronous control of the modular multilevel converter and the power grid, the synchronous control module further includes

[0116] Calculate the per-unit value of the change in the output voltage angular frequency on the AC side of the modular multilevel converter through the transfer function of the synchronous generator rotor motion equation;

[0117] Based on the per-unit value of the change in the output voltage angular frequency and the per-unit value of the grid voltage angular frequency, calculate the per-unit value of the output voltage angular frequency of the modular multilevel converter;

[0118] Multiply the per-unit value of the output voltage angular frequency by the obtained angular frequency reference value to calculate the nominal value of the output voltage angular frequency of the modular multilevel converter;

[0119] Integrate the nominal value of the output voltage angular frequency to calculate the output voltage phase of the modular multilevel converter, and verify the synchronous control of the modular multilevel converter and the power grid according to the output voltage phase.

[0120] Those skilled in the art should understand that the embodiments of this invention patent application can be provided as a method, a system, or a computer program product. Therefore, this invention patent application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Moreover, this invention patent application can take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0121] This invention patent application is described with reference to the flowcharts and / or block diagrams of methods, apparatuses (systems), and computer program products according to the embodiments of this invention patent application. It should be understood that each flow and / or block in the flowchart and / or block diagram, as well as the combination of flows and / or blocks in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to the processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing devices to generate a machine, such that the instructions executed by the processor of the computer or other programmable data processing devices generate means for implementing the functions specified in Figure 1 one or more of the flows Figure 1 or a plurality of flows and / or blocks

[0122] These computer program instructions can also be stored in a computer-readable memory capable of guiding a computer or other programmable data processing devices to work in a specific manner, such that the instructions stored in the computer-readable memory generate a manufactured article including instruction means, and the instruction means implements the functions specified in Figure 1 one or more of the flows Figure 1 or a plurality of flows and / or blocks

[0123] These computer program instructions can also be loaded onto a computer or other programmable data processing devices, such that a series of operation steps are executed on the computer or other programmable devices to generate a computer-implemented process, and thus the instructions executed on the computer or other programmable devices provide steps for implementing the functions specified in Figure 1 one or more of the flows Figure 1 or a plurality of flows and / or blocks

[0124] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this invention patent application and not to limit its protection scope. Although the above embodiments have been described in detail with reference to this invention patent application, those of ordinary skill in the art should understand that after reading this invention patent application, they can still make various changes, modifications or equivalent substitutions to the specific implementation manners of the application. However, these changes, modifications or equivalent substitutions are all within the protection scope of the claims pending approval of the application.

Claims

1. A synchronous control method for a modular multilevel converter based on capacitor voltage, characterized in that Including: Based on the per-unit value of the average capacitor voltage and the per-unit value of the capacitor voltage command of the modular multilevel converter, through the outer loop of capacitor voltage control, calculate the per-unit value of the active power command on the AC side of the modular multilevel converter; Apply the per-unit value of the active power command and the measured per-unit value of the active power on the AC side of the modular multilevel converter to the inner loop of power control to perform synchronous control of the modular multilevel converter and the power grid.

2. The method according to claim 1, wherein The step of calculating the per-unit value of the active power command on the AC side of the modular multilevel converter based on the per-unit value of the average capacitor voltage and the per-unit value of the capacitor voltage command of the modular multilevel converter through the outer loop of capacitor voltage control includes: Adopt the outer loop of capacitor voltage control to calculate the difference between the per-unit value of the average capacitor voltage and the per-unit value of the capacitor voltage command, and based on the difference, obtain the output value of the outer loop of capacitor voltage control through the controller of the outer loop of capacitor voltage control; Based on the sum of the output value of the outer loop of capacitor voltage control and the per-unit value of the active power on the DC side of the modular multilevel converter, calculate the per-unit value of the active power command on the DC side of the modular multilevel converter; Wherein, the controller is a proportional derivative controller or a proportional lead compensator.

3. The method according to claim 2, wherein The proportional derivative controller includes the following transfer function: Among them, G1(s) is the transfer function of the proportional derivative controller, k p is the proportional control coefficient, k d is the derivative control coefficient T d is the low-pass filter time constant, and s is the Laplace operator.

4. The method according to claim 2, wherein The step of applying the per-unit value of the active power command and the measured per-unit value of the active power on the AC side of the modular multilevel converter to the inner loop of power control to perform synchronous control of the modular multilevel converter and the power grid includes: Adopt the inner loop of power control to calculate the difference between the per-unit value of the active power command on the DC side of the converter and the measured per-unit value of the active power on the AC side of the modular multilevel converter; Apply the difference to the synchronous generator rotor motion equation of the inner loop of power control to perform synchronous control of the modular multilevel converter and the power grid.

5. The method according to claim 4, characterized in that The calculation formula of the measured per-unit value of the active power on the AC side of the modular multilevel converter is as follows: Among them, is the per-unit value of the active power measurement on the AC side of the modular multilevel converter, i a is the instantaneous phase current of phase a at the grid connection point, i b is the instantaneous phase current of phase b at the grid connection point, i c is the instantaneous phase current of phase c at the grid connection point, u a is the instantaneous phase voltage of phase a at the grid connection point, u b is the instantaneous phase voltage of phase b at the grid connection point, u c is the instantaneous phase voltage of phase c at the grid connection point, S N is the rated capacity of the converter.

6. The method according to claim 4, wherein After applying the difference to the synchronous generator rotor motion equation of the inner loop of power control to perform synchronous control of the modular multilevel converter and the power grid, it further includes: Calculate the per-unit value of the change in the output voltage angular frequency on the AC side of the modular multilevel converter through the transfer function of the synchronous generator rotor motion equation; Based on the per-unit value of the change in the output voltage angular frequency and the per-unit value of the grid voltage angular frequency, calculate the per-unit value of the output voltage angular frequency of the modular multilevel converter; Multiply the per-unit value of the output voltage angular frequency by the obtained angular frequency reference value to calculate the nominal value of the output voltage angular frequency of the modular multilevel converter; Integrate the nominal value of the output voltage angular frequency to calculate the output voltage phase of the modular multilevel converter, and verify the synchronous control of the modular multilevel converter and the power grid according to the output voltage phase.

7. A modular multilevel converter synchronization control system based on capacitor voltage, characterized in that Including: a per-unit value calculation module of the active power command and a synchronous control module; The per-unit value calculation module of the active power command is used to calculate the per-unit value of the active power command on the AC side of the modular multilevel converter through the outer loop of the capacitor voltage control based on the average per-unit value of the capacitor voltage and the per-unit value of the capacitor voltage command of the modular multilevel converter; The synchronization control module is used to apply the per-unit value of the active power command and the measured per-unit value of the active power on the AC side of the modular multilevel converter to the inner loop of the power control to perform the synchronization control of the modular multilevel converter and the power grid.

8. The system according to claim 7, wherein The per-unit value calculation module of the active power command is specifically used for: Adopting the outer loop of the capacitor voltage control, calculating the difference between the average per-unit value of the capacitor voltage and the per-unit value of the capacitor voltage command, and obtaining the output value of the outer loop of the capacitor voltage control based on the difference through the controller of the outer loop of the capacitor voltage control; Based on the sum of the output value of the outer loop of the capacitor voltage control and the per-unit value of the active power on the DC side of the modular multilevel converter, calculating the per-unit value of the active power command on the DC side of the modular multilevel converter; Wherein, the controller is a proportional derivative controller or a proportional lead compensator.

9. The system according to claim 8, wherein The proportional derivative controller of the per-unit value calculation module of the active power command includes the following transfer function: Among them, G1(s) is the transfer function of the proportional derivative controller, k p is the proportional control coefficient, k d is the derivative control coefficient T d is the low-pass filter time constant, and s is the Laplace operator.

10. The system according to claim 8, characterized in that The synchronization control module is specifically used for: Adopting the inner loop of the power control, calculating the difference between the per-unit value of the active power command on the DC side of the converter and the measured per-unit value of the active power on the AC side of the modular multilevel converter; Applying the difference to the synchronous generator rotor motion equation of the inner loop of the power control to perform the synchronization control of the modular multilevel converter and the power grid.

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