A general network-forming type control system of a modular multilevel converter

Abstract

The application provides a universal network configuration type control system of a modular multilevel converter, comprising: an alternating current frequency reference generation module for obtaining reference values of output alternating current frequency and phase, realizing autonomous frequency construction of sub-module energy-alternating current frequency equivalent mapping; a power calculation module for obtaining active power and reactive power; an alternating current voltage reference generation module for obtaining output alternating current voltage reference values; a virtual admittance current limiting module for obtaining converter alternating current voltage modulation waves, realizing multi-target control based on the virtual admittance principle on the alternating current side; a direct current voltage reference generation module for obtaining output direct current voltage reference values; a circulating current suppression module for obtaining circulating current suppression voltage reference values; and a compensation modulation module for obtaining switch signals of each sub-module; realizing multi-target control based on compensation modulation on the direct current side; the application uses the same set of control systems, can cover the control target demand in most scenarios, and can be flexibly and seamlessly switched between different control targets.

Description

Technical Field

[0001] This invention relates to the field of power grid technology, and more specifically, to a general grid-type control system for a modular multilevel converter. Background Technology

[0002] The "3060" carbon reduction target elevates the scale, geographical scope, and depth of renewable energy development to a higher level. Addressing the challenges of diverse energy types in large-scale renewable energy bases, the wide distribution of new energy clusters, and relatively weak power grids, the use of AC systems to aggregate fluctuating power over broad areas faces technical bottlenecks related to voltage stability and grid connection stability of wind and solar power. Therefore, flexible DC aggregation and grid connection solutions have attracted widespread attention. By coupling multiple energy clusters through multi-terminal flexible DC systems and aggregating fluctuating power via DC, wide-area multi-energy complementarity can be achieved, becoming a future development trend for large-scale renewable energy bases.

[0003] With the rapid increase in the scale of new energy grid connection, higher requirements are being placed on the control strategies of flexible DC converters. On the new energy side, since renewable energy bases are usually far from the AC main grid, the grid strength at the connection point is relatively weak, and the short-circuit current is low. If grid-following control based on traditional phase-locked loop synchronous grid connection is adopted, it may cause oscillation and instability problems under weak grid conditions. On the AC main grid side, with the rapid increase in the penetration rate of new energy, the overall inertia of the grid decreases, and the inertia-free and damped characteristics of flexible DC converters under grid-following control will further affect the frequency stability of the grid. Therefore, adopting grid-following control is an important development trend for future flexible DC converters.

[0004] Currently, grid-based control of flexible DC converters is mainly designed based on a specific type of control objective. For example, islanded flexible DC converters typically employ constant Vf grid-based control; grid-connected flexible DC converters controlling active power generally use virtual synchronization control, power synchronization control, and synchronous power control; while flexible DC converters controlling DC voltage employ matching control, inertial synchronization control, and analog synchronization control. These grid-based control strategies often have significantly different structures, making it difficult to achieve a unified approach.

[0005] In multi-terminal flexible DC transmission systems, there is a switching between control modes. For example, when the master station controlling the DC voltage is disconnected due to a fault, the slave station controlling active power needs to switch to constant DC voltage operation. When a line connected to the AC main grid is disconnected due to a fault, the flexible DC converter that was originally operating in grid-connected mode needs to switch to islanded operation, switching to Vf grid-connected control mode. This switching between different grid-connected control strategies may cause oscillation problems. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the purpose of this invention is to provide a universal grid-type control system for modular multilevel converters.

[0007] According to one aspect of the present invention, a universal grid-type control system for a modular multilevel converter is provided, comprising:

[0008] The AC frequency reference generation module obtains reference values ​​for the output AC frequency and phase, enabling autonomous frequency construction of energy-AC frequency equivalence mapping for all sub-modules of the converter.

[0009] A power calculation module that obtains active power and reactive power based on the reference value of the phase;

[0010] An AC voltage reference generation module obtains a reference value for the output AC voltage based on the reactive power.

[0011] A virtual admittance current limiting module is provided. This module obtains the AC voltage modulation wave of the converter based on the reference values ​​of the phase and the AC voltage. Based on the virtual admittance current limiting module, multi-objective control based on the virtual admittance principle is realized on the AC side of the converter.

[0012] A DC voltage reference generation module, which obtains a reference value for the output DC voltage based on the active power;

[0013] The circulating current suppression module obtains a reference value for the circulating current suppression voltage.

[0014] The compensation modulation module acquires the switching signals of each sub-module of the converter based on the AC voltage modulation wave of the converter, the reference value of the DC voltage, and the reference value of the circulating current suppression voltage; based on the compensation modulation module, multi-objective control based on compensation modulation is realized on the DC side of the converter.

[0015] Based on the AC frequency reference generation module, the AC voltage reference generation module, and the DC voltage reference generation module, seamless switching between network and island operation modes based on submodule energy determination is achieved.

[0016] Preferably, in the AC frequency reference generation module, the process of obtaining reference values ​​for the output AC frequency and phase includes:

[0017] Calculate the total energy W of the converter submodules MMC With rated total energy W MMC_nom The difference:

[0018] Where N represents the number of submodules in each arm of the converter; the converter has six arms, and there are a total of 6N submodules; C SM The size of the submodule capacitor; USMi U represents the actual amplitude of the capacitor voltage of the i-th submodule. SM_nom This refers to the rated voltage of the submodule capacitor.

[0019] Calculate the reference value for the converter output AC frequency:

[0020]

[0021] H v ω0 is the virtual inertia coefficient, and ω0 is the rated frequency;

[0022] For ω c Integrating, we obtain the reference value θ of the AC phase of the converter output.

[0023] Preferably, the autonomous frequency construction for realizing the energy-AC frequency equivalence mapping of the submodule includes:

[0024] The deviation between the submodule's energy and its rated value is directly reflected in the output AC frequency;

[0025] When connected to the network, this mapping relationship can be used to simulate the rotor motion equation of a synchronous generator, achieving phase-locked loop-free self-synchronization while maintaining the energy stability of the submodule.

[0026] When isolated, the AC frequency output of the converter is kept stable to achieve constant frequency control.

[0027] Preferably, the process of obtaining active power and reactive power in the power calculation module includes:

[0028] Detect the grid-connected AC voltage u on the AC system side of the converter's connecting transformer. pcc With the current i on the converter side o ;

[0029] will u pcc and i o Perform a PARK transform at the reference phase θ to obtain u pcc and i o The dq axis components are u pccd u pccq i od and i oq ;

[0030] Calculate the output active power P on the AC side of the converter. g With reactive power Qg:

[0031] P g =u pccd i od +u pccq i oq

[0032] Qg =u pccq i od -u pccd i oq .

[0033] Preferably, in the AC voltage reference generation module, the process of obtaining a reference value for the output AC voltage includes:

[0034] The AC voltage at the grid connection point u pcc The measured value and the given value u pccref Subtract the difference and multiply by the AC voltage control coefficient K. V ;

[0035] The measured value of reactive power Q g With a given value Q ref Divide by the reactive power control coefficient K Q ;

[0036] K V and K Q Add them together, after an integration process, and then add the rated value u of the AC voltage. ac_nom Obtain the reference value u of the output AC voltage. acref .

[0037] Preferably, in the virtual admittance current limiting module, the process of acquiring the AC voltage modulation wave of the converter includes:

[0038] The AC voltage reference value u acref with u pccd Subtract the values ​​and add the first reference value i of the q-axis current. *q0 Multiply by the rated frequency ω0, then divide by a virtual admittance R v +sL v , where R v For virtual resistance, L v Let s be a virtual inductor, s be the Laplace operator, and obtain the first reference value i of the d-axis current. * d0 ;

[0039] Connect 0 and u pccq Subtract i from the difference. * d0 Multiply by the rated frequency ω0, then divide by a virtual admittance R v +s Lv Obtain the first reference value i of the q-axis current. * q0 ;

[0040] will i * d0 and i * q0After passing through a current-limiting circuit, the second reference values ​​i of the d-axis and q-axis output currents are obtained. * d and i * q ;

[0041] will i * d The measured value of the d-axis current i od The difference is calculated, then passed through a proportional-integral controller, and added to u. pccd -ω0L c i oq L c The converter output filter reactance is used to obtain the reference value E of the d-axis AC modulation voltage. d ;

[0042] will i * q The measured value of the q-axis current i oq The difference is calculated, and then passed through a proportional-integral controller, along with upccd+ω0L. c i od L c The converter output filter reactance is used to obtain the reference value E of the q-axis AC modulation voltage. q ;

[0043] E d and E q Perform an inverse PARK transform at the reference phase θ to obtain the AC modulation voltage u. sabc .

[0044] Preferably, the multi-objective control based on the virtual admittance principle on the AC side of the converter includes:

[0045] When K Q ≠0, K V =0, the converter operates in constant reactive power control mode;

[0046] When K Q =0,K V ≠0, the converter operates in constant AC voltage control mode;

[0047] When K Q ≠0, K V ≠0, the converter operates in AC voltage-reactive power droop control mode.

[0048] Preferably, in the current limiting step,

[0049] If i * d0 and i * q0 The square root of the sum of squares is less than the current limiting amplitude i.lim , then i * d =i * d0 i * q =i * q0 ;

[0050] If i * d0 and i * q0 The square root of the sum of squares is greater than the current limiting value i. lim Then i * d0 and i* q0 Scalculate proportionally by a factor of n until i * d0 / n and i * q0 / n The square root of the sum of squares equals i lim At this time i * d =i *d0 / n i * q =i * q0 / n .

[0051] Preferably, in the DC voltage reference generation module, the process of obtaining a reference value for the output DC voltage includes:

[0052] DC voltage setpoint U dcref DC voltage measurement value U dc Divide and add to the DC voltage control coefficient K dc Multiply; multiply the active power given value P ref With active power measurement value P g Divide the difference and compare it with the active power control coefficient K. p Multiply;

[0053] The actual value of the total energy of the submodule (W) MMC Total energy rating of submodules (W) MMC_nom Divide the result and compare it with the submodule energy control coefficient K. W Multiply;

[0054] The three product results are added together, passed through a proportional-integral controller, and then added to the measured DC voltage value to obtain the reference value E of the output DC voltage. dc .

[0055] Preferably, multi-objective control based on compensation modulation is implemented on the DC side of the converter, including:

[0056] When connected to the internet, set KW =0,

[0057] If K dc =0,K P ≠0, the converter operates in constant active power mode;

[0058] If K dc ≠0, K P =0, the converter operates in constant active power mode;

[0059] If K dc ≠0, K P ≠0, the converter operates in DC voltage-active power droop control mode;

[0060] In an isolated scenario, set K dc =K P =0,K W ≠0;

[0061] At this time, the converter operates under the energy control of the stator module, and due to the equivalent mapping relationship between the energy of the sub-module and the output AC frequency, the converter operates under the control of a constant AC frequency.

[0062] Preferably, in the circulating current suppression module, the process of obtaining the reference value of the circulating current suppression voltage includes:

[0063] Measurement of the second harmonic circulating current component i of the three phases cira i cirb with i circ After subtracting from 0, the values ​​are passed through a proportional-resonant controller to obtain the reference value u of the three-phase second-harmonic voltage. cira u cirb and u circ .

[0064] Preferably, in the compensation modulation module, the process of acquiring the switching signal of each submodule includes:

[0065] The modulation voltage of each bridge arm is obtained, and the switching signal of each submodule is obtained after sorting by the nearest level.

[0066] in:

[0067]

[0068] u pj U is the modulation voltage of the upper arm of phase j. nj The modulation voltage of the lower arm of phase j;

[0069] will u pj and u nj Divide by the average voltage U of each submodule SM_av Obtain the insertion N of the three-phase upper and lower bridge arms.pj and N nj ,in:

[0070]

[0071] The submodule capacitor voltages of the upper and lower bridge arms are sorted to obtain the switching signals of the submodules: when the bridge arm current is positive, the m submodules with the smallest capacitor voltage are switched on; when the bridge arm current is negative, the m submodules with the largest capacitor voltage are switched on, where m is equal to the insertion index of the bridge arm.

[0072] Preferably, the seamless switching between network-isolated operation modes for energy determination of the submodule includes:

[0073] When the converter switches from grid mode to islanded operation mode, the energy of the submodules will deviate from the rated value. When the deviation exceeds the threshold, it will automatically switch to islanded operation mode.

[0074] The control objective of the DC voltage reference generation module becomes maintaining constant energy of the submodule;

[0075] The control objective of the AC voltage reference generation module becomes constant AC voltage control.

[0076] The AC frequency reference generation module remains unchanged;

[0077] Based on the equivalent mapping relationship between submodule energy and output AC frequency, when the submodule energy is constant, the converter output AC frequency is also constant.

[0078] Compared with the prior art, the embodiments of the present invention have at least one of the following beneficial effects:

[0079] This invention provides a universal grid-type control system for a modular multilevel converter. Using the same control structure, it can cover the control target requirements in most scenarios and can flexibly and seamlessly switch between different control targets.

[0080] This invention provides a general grid-type control system for a modular multilevel converter, applicable to various application scenarios such as grid connection, islanding, strong grid, and weak grid. Attached Figure Description

[0081] Other features, objects, and advantages of the invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings:

[0082] Figure 1 This is a schematic diagram of a general grid-type control system for a modular multilevel converter according to an embodiment of the present invention;

[0083] Figure 2This is a schematic diagram of the AC frequency reference generation module in a preferred embodiment of the present invention;

[0084] Figure 3 This is a schematic diagram of the AC voltage reference generation module in a preferred embodiment of the present invention;

[0085] Figure 4 This is a schematic diagram of the virtual admittance current limiting module in a preferred embodiment of the present invention;

[0086] Figure 5 This is a schematic diagram of the DC voltage reference generation module in a preferred embodiment of the present invention;

[0087] Figure 6 This is a schematic diagram of the circulating current suppression module in a preferred embodiment of the present invention;

[0088] Figure 7 This is a schematic diagram of the compensation modulation module in a preferred embodiment of the present invention;

[0089] Figure 8 This is a schematic diagram of the simulation system structure during the simulation experiment of this invention;

[0090] Figure 9 This is a schematic diagram showing the changes in DC voltage, power of converter 1, and power of converter 2 under operating condition 1 during the simulation experiment of this invention.

[0091] Figure 10 This is a schematic diagram showing the changes in DC voltage, power of converter 1, and power of converter 2 under operating condition 2 during the simulation experiment of this invention. Detailed Implementation

[0092] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention. These all fall within the scope of protection of the present invention.

[0093] This invention provides an embodiment of a general-purpose grid-type control system for a modular multilevel converter, the structure of which is as follows: Figure 1 As shown, it mainly includes:

[0094] The AC frequency reference generation module obtains reference values ​​for the output AC frequency and phase, enabling autonomous frequency construction of energy-AC frequency equivalence mapping for all sub-modules of the converter.

[0095] The power calculation module obtains active and reactive power based on the phase reference value;

[0096] An AC voltage reference generation module obtains a reference value for the output AC voltage based on reactive power.

[0097] The virtual admittance current limiting module acquires the AC voltage modulation wave of the converter based on the reference values ​​of the phase and AC voltage; based on the virtual admittance current limiting module, multi-objective control based on the virtual admittance principle is realized on the AC side of the converter.

[0098] A DC voltage reference generation module, which obtains a reference value for the output DC voltage based on active power;

[0099] The circulating current suppression module obtains a reference value for the circulating current suppression voltage.

[0100] The compensation modulation module acquires the switching signals of each sub-module of the converter based on the AC voltage modulation waveform, the reference value of the DC voltage, and the reference value of the circulating current suppression voltage. Based on the compensation modulation module, multi-objective control based on compensation modulation is realized on the DC side of the converter.

[0101] Based on the AC frequency reference generation module, AC voltage reference generation module, and DC voltage reference generation module, seamless switching between networked and islanded operation modes is achieved based on submodule energy determination.

[0102] This embodiment addresses the problem in current flexible DC transmission systems where modular multilevel converters using grid-based control suffer from a single control objective, making it difficult to flexibly switch control objectives and achieve seamless switching between networked and isolated systems. It utilizes a single control system to cover the control objective requirements of most scenarios and enables flexible and seamless switching between different control objectives.

[0103] In a preferred embodiment of the present invention, an optimized process is provided for the AC frequency reference generation module to calculate and output reference values ​​of AC frequency and phase, which can be found in [reference needed]. Figure 2 Specifically:

[0104] Calculate the total energy W of the submodule of the flexible DC converter. MMC With rated total energy W MMC_nom The difference is calculated as follows:

[0105]

[0106] Where N represents the number of submodules in each arm of the converter; since the converter has six arms, there are a total of 6N submodules; C SM The size of the submodule capacitor; U SMi U represents the actual amplitude of the capacitor voltage of the i-th submodule. SM_nom This refers to the rated voltage of the submodule capacitor.

[0107] Divide the difference by 2H v H v The virtual inertia coefficient, typically around 100ms, is added to the rated frequency ω0 (usually 50Hz) to obtain the reference value ω of the converter output AC frequency. c For ω c Integrating, we obtain the reference value θ of the AC phase of the converter output.

[0108] Furthermore, in this embodiment, an autonomous frequency construction strategy based on the equivalent mapping of submodule energy and AC frequency is formed. An equivalent mapping relationship between submodule energy and output AC frequency is established on the AC side, directly reflecting the deviation of submodule energy from its rated value in the output AC frequency. When connected to the grid, this mapping relationship can simulate the rotor motion equation of a synchronous generator, achieving phase-locked loop-free self-synchronization while maintaining the stability of submodule energy. In islanded operation, the module can also stabilize the AC frequency output by the converter at 50Hz, achieving constant frequency control.

[0109] In a preferred embodiment of the present invention, an optimized process for the power calculation module to calculate the active power and reactive power output is provided, specifically:

[0110] First, detect the voltage u on the AC system side of the connecting transformer of the flexible DC converter. pcc With the current i on the converter side o ;

[0111] Next, u pcc and i o Perform a PARK transform at the reference phase θ to obtain u pcc and i o The dq axis components are u pccd u pccq i od and i oq ;

[0112] Finally, calculate the output active power P on the AC side of the converter. g With reactive power Qg:

[0113] P g =u pccd i od +u pccq i oq

[0114] Q g =u pccq i od -u pccd i oq .

[0115] In a preferred embodiment of the present invention, an AC voltage reference generation module is provided to obtain a reference value u of the output AC voltage. acref For the optimization process, please refer to Figure 3 Specifically:

[0116] The AC voltage at the grid connection point u pcc The measured value and the given value u pccref Difference multiplied by K V (K V (This is the AC voltage control coefficient), and then the measured reactive power value Qg is compared with the given value Q. ref Divide by K Q (K Q (This is the reactive power control coefficient). The results are summed, and after an integration process, the rated AC voltage u is added. ac_nom Obtain the reference value u of the output AC voltage. acref .

[0117] In a preferred embodiment of the present invention, a virtual admittance current limiting module is provided to obtain the AC voltage modulation waveform u of the converter. sabc For the optimization process, please refer to Figure 4 Specifically:

[0118] First, u pcc and i o The obtained reference phase θ is subjected to PARK transform to obtain u pcc and i o d q Axis components, upccd, u pccq i od and i oq .

[0119] Secondly, the obtained AC voltage reference value u acref with u pccd Subtract the values ​​and add the first reference value i of the q-axis current. *q0 Multiply by the rated frequency ω0, then divide by a virtual admittance R v +sL v , where R v For virtual resistance, L v Let s be a virtual inductor, s be the Laplace operator, and obtain the first reference value i of the d-axis current. * d0 .

[0120] At the same time, subtract i from 0 and upccq. * d0 Multiply by the rated frequency ω0, then divide by a virtual admittance R v +sL v Obtain the first reference value i*q0 for the q-axis current.

[0121] will i * d0 and i * q0 After passing through a current-limiting circuit, the second reference values ​​i of the d-axis and q-axis output currents are obtained. * d and i * q , change i * d The measured value of the d-axis current i od The difference is calculated, then passed through a proportional-integral controller (PI controller), and added to u. pccd -ω0L c i oq L c The converter output filter reactance is used to obtain the reference value E of the d-axis AC modulation voltage. d .

[0122] will i * q The measured value of the q-axis current i oq The difference is calculated, then passed through a proportional-integral controller (PI controller), and added to u. pccd +ω0L c i od L c The converter output filter reactance is used to obtain the reference value E of the q-axis AC modulation voltage. q .

[0123] E d and E q Perform an inverse PARK transform at the reference phase θ to obtain the AC modulation voltage u. sabc .

[0124] In a preferred embodiment, the principle of the current limiting mechanism is provided, if i * d0 and i * q0 The square root of the sum of squares is less than the current limiting amplitude i. lim (Generally 1.2 to 1.5 times the rated current), then i * d =i * d0 i * q =i * q0 . If i * d0 and i * q0 The square root of the sum of squares is greater than the current limiting value i. lim Then i * d0and i * q0 Scaling down proportionally by a factor of n until (i * d0 / n) and (i * q0 The square root of the sum of the squares of (n) equals i. lim At this time, i * d =(i * d0 / n), i*q=(i * q0 / n).

[0125] In a preferred embodiment, a multi-objective control strategy based on the virtual admittance principle is proposed on the AC side. Under the same control structure, reactive power control, AC voltage control, and reactive power-AC voltage droop control can be achieved by adjusting parameters, and seamless and flexible switching between several control modes is possible. Specifically,

[0126] When K Q ≠0, K V =0, the converter operates in constant reactive power control mode;

[0127] When K Q =0,K V ≠0, the converter operates in constant AC voltage control mode;

[0128] When K Q ≠0, K V ≠0, the converter operates in AC voltage-reactive power droop control mode.

[0129] In a preferred embodiment of the present invention, a DC voltage reference generation module is provided to obtain a reference value E of the output DC voltage. dc For the optimization process, see Figure 5 Specifically:

[0130] DC voltage setpoint U dcref DC voltage measurement value U dc Divide and add to the DC voltage control coefficient K dc Multiply;

[0131] The active power setpoint P ref With active power measurement value P g Divide the difference and compare it with the active power control coefficient K. p Multiply;

[0132] The actual value of the total energy of the submodule (W) MMC Total energy rating of submodules (W) MMC_n The difference between om and the submodule energy control coefficient K is calculated. W Multiply;

[0133] The three products are added together, passed through a proportional-integral controller, and then added to the measured DC voltage value to obtain the reference value E of the output DC voltage. dc .

[0134] In a preferred embodiment of the present invention, a reference value u for obtaining the circulating current suppression voltage through a circulating current suppression module is provided. cir For the optimization process, please refer to Figure 6 Specifically:

[0135] The second harmonic circulating current components i of the three phases were measured respectively. cira i cirb with i circ After subtracting from 0, the voltage is passed through a proportional-resonant controller (PR controller) to obtain the reference value u of the three-phase second-harmonic voltage. cira u cirb and u circ .

[0136] In a preferred embodiment of the present invention, a preferred process for obtaining the switching signal of each submodule through a compensation modulation module is provided, which can be found in [reference needed]. Figure 7 , specifically

[0137] The modulation voltage of each bridge arm is obtained, and the switching signal of each submodule is obtained after sorting by the nearest level, where:

[0138]

[0139] u pj U is the modulation voltage of the upper arm of phase j. nj This is the modulation voltage of the lower arm of phase j. (The last part, "u", appears to be a typo and can be left as is.) pj and u nj Divide by the average voltage U of each submodule SM_av Obtain the insertion N of the three-phase upper and lower bridge arms. pj and N nj ,in:

[0140]

[0141] Next, the capacitor voltages of the submodules in the upper and lower bridge arms are sorted to obtain the switching signals for the submodules. When the bridge arm current is positive, the m submodules with the smallest capacitor voltage are switched on; when the bridge arm current is negative, the m submodules with the largest capacitor voltage are switched on, where m is equal to the insertion exponent of that bridge arm.

[0142] In a preferred embodiment, a multi-objective control strategy based on compensated modulation is proposed on the DC side. Under the same control structure, multiple control objectives such as DC voltage control, active power control, DC voltage-active power droop control, and stator module energy (islanding only) can be achieved by adjusting parameters, and seamless and flexible switching between several control modes is possible. Specifically,

[0143] When connected to the internet, K is generally set. W =0, at this time, if K dc =0,K P ≠0, the converter operates in constant active power mode, if K dc ≠0, K P =0, the converter operates in constant active power mode, if K dc ≠0, K P ≠0, the converter operates in DC voltage-active power droop control mode.

[0144] In an isolated scenario, K is typically set. dc =K P =0,K W ≠0. At this time, the converter operates under the energy control of the stator module, and due to the equivalent mapping relationship between the energy of the neutron module and the output AC frequency, the converter operates under the control of the constant AC frequency.

[0145] The above embodiments, through the flexible combination of AC / DC side control targets, can cover most operating scenarios and control requirements. Furthermore, since they share the same control structure, they occupy fewer controller resources and experience less impact during the switching process between different control modes.

[0146] In a preferred embodiment, a seamless switching strategy between networked and islanded operation modes based on submodule energy determination is proposed. Specifically, when the modular multilevel converter switches from networked mode to islanded operation mode under certain emergency conditions (such as AC tie line disconnection), the submodule energy will deviate from the rated value. When the deviation exceeds ±10%, the switch to islanded operation mode will be automatically determined. At this time, the control objective of the DC voltage reference generation module becomes maintaining constant submodule energy, the control objective of the AC voltage reference generation module becomes constant AC voltage control, and the AC frequency reference generation module remains unchanged. Due to the equivalent mapping relationship between submodule energy and output AC frequency, when the submodule energy is constant, the converter output AC frequency is also constant. At this time, the flexible DC converter can seamlessly switch from any networked control mode to Vf control under islanded mode.

[0147] The following simulation experiment verifies the effectiveness of the technical solution in the embodiments of the present invention. (See also...) Figure 8 Create the simulation system shown in the figure below in PSCAD / EMTDC. The system parameters are as follows:

[0148] Rated active power / MW 1100 Rated grid voltage / kV 500 Rated voltage on the wind farm side / kV 220 Rated DC voltage / kV ±400 Number of sub-modules per bridge arm 400+50 (redundancy) Submodule capacitor / mF 11 Bridge arm reactance / mH 150 Connecting transformer capacity / MVA 1400 Connecting transformer leakage reactance / pu 0.14 Converter 1 Transformer Turns Ratio 416 / 500 Converter 2 Transformer Turns Ratio 416 / 220

[0149] In addition, the load is 400MW+60Mvar, and the wind farm's output power is 60MW.

[0150] Simulations were performed under two operating conditions:

[0151] Operating Condition 1: Control Mode Switching under Networked Conditions

[0152] See Figure 9 Initially, converter 1 operates in constant DC voltage control mode (i.e., Kdc ≠ 0, KP = kW = 0), controlling the DC voltage amplitude at 800kV. Converter 2 operates in constant active power control mode (i.e., KP ≠ 0, Kdc = kW = 0), controlling the active power at 200MW. At t = 1s, converter 1 switches to constant active power control mode, and 0.1s later, converter 2 switches to constant DC voltage control mode. The DC voltage and power responses at this time are shown in the figure below. It can be observed that during the switching process, the fluctuations in the system's DC voltage and active power are very small, essentially achieving seamless switching.

[0153] Working Condition 2: Switching between Networked and Isolated Modes

[0154] See Figure 10 Initially, converter 1 operates in constant DC voltage control mode (i.e., Kdc≠0, KP=KW=0), controlling the DC voltage amplitude to 800kV. Converter 2 operates in constant active power control mode (i.e., KP≠0, Kdc=KW=0), controlling the active power to 200MW. At t=0.5s, the circuit breaker opens, disconnecting the AC tie line. After detecting the energy deviation of the submodule, converter 2 automatically switches to islanded operation mode. The response results of DC voltage and power during the switching process are shown in the figure below. It can be found that during the grid-to-island switching process, the DC voltage fluctuation is less than 2.5%, and after switching to islanded mode, converter 2 can quickly maintain the voltage stability of the islanded grid and provide sufficient active and reactive power to the islanded grid.

[0155] Specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention. The above preferred features can be used in any combination without conflict.

Claims

1. A universal network-type control system for a modular multilevel converter, characterized in that, include: The AC frequency reference generation module obtains reference values ​​for the output AC frequency and phase, enabling autonomous frequency construction of energy-AC frequency equivalence mapping for all sub-modules of the converter. A power calculation module that obtains active power and reactive power based on the reference value of the phase; An AC voltage reference generation module obtains a reference value for the output AC voltage based on the reactive power. A virtual admittance current limiting module is provided. This module obtains the AC voltage modulation wave of the converter based on the reference values ​​of the phase and the AC voltage. Based on the virtual admittance current limiting module, multi-objective control based on the virtual admittance principle is realized on the AC side of the converter. A DC voltage reference generation module, which obtains a reference value for the output DC voltage based on the active power; The circulating current suppression module obtains a reference value for the circulating current suppression voltage. The compensation modulation module acquires the switching signals of each sub-module of the converter based on the AC voltage modulation wave of the converter, the reference value of the DC voltage, and the reference value of the circulating current suppression voltage. Based on the aforementioned compensation modulation module, multi-objective control based on compensation modulation is realized on the DC side of the converter. Based on the AC frequency reference generation module, the AC voltage reference generation module, and the DC voltage reference generation module, seamless switching between network-to-island operation modes based on submodule energy determination is achieved; The process of obtaining a reference value for the output AC voltage in the AC voltage reference generation module includes: The AC voltage at the grid connection point u pcc The measured value and the given value u pccref Subtract the difference and multiply by the AC voltage control coefficient K. V ; The measured value of reactive power Q g With a given value Q ref Divide by the reactive power control coefficient K Q ; K V and K Q Add them together, after an integration process, and then add the rated value u of the AC voltage. ac_nom Obtain the reference value u of the output AC voltage. acref ; The implementation of multi-objective control based on the virtual admittance principle on the AC side of the converter includes: when K Q ≠ 0, K V = 0, the converter operates in constant reactive power control mode; when K Q = 0, K V ≠ 0, the converter operates in constant AC voltage control mode; when K Q ≠ 0, K V ≠ 0, the converter operates in AC voltage-reactive power droop control mode; The process of obtaining a reference value for the output DC voltage in the DC voltage reference generation module includes: DC voltage setpoint U dcref DC voltage measurement value U dc Divide and add to the DC voltage control coefficient K dc Multiply; multiply the active power given value P ref With active power measurement value P g Divide the difference and compare it with the active power control coefficient K. p Multiply; The actual value of the total energy of the submodule (W) MMC Total energy rating of submodules (W) MMC_nom Divide the result and compare it with the submodule energy control coefficient K. W Multiply; The three product results are added together, passed through a proportional-integral controller, and then added to the measured DC voltage value to obtain the reference value E of the output DC voltage. dc ; Implementing multi-objective control based on compensated modulation on the DC side of the converter, including: When connected to the internet, set K W =0, like K dc = 0, K P ≠ 0, the converter operates in constant active power mode; like K dc ≠ 0, K P = 0, the converter operates in constant active power mode; like K dc ≠ 0, K P ≠ 0, the converter operates in DC voltage-active power droop control mode; In an isolated situation, set K dc = K P =0, K W ≠ 0; At this time, the converter operates under the energy control of the stator module, and due to the equivalent mapping relationship between the energy of the sub-module and the output AC frequency, the converter operates under the constant AC frequency control. The seamless switching between network-isolated operation modes for energy determination in the submodule includes: When the converter switches from grid mode to islanded operation mode, the energy of the submodules will deviate from the rated value. When the deviation exceeds the threshold, it will automatically switch to islanded operation mode. The control objective of the DC voltage reference generation module becomes maintaining constant energy of the submodule; The control objective of the AC voltage reference generation module becomes constant AC voltage control. The AC frequency reference generation module remains unchanged; Based on the equivalent mapping relationship between submodule energy and output AC frequency, when the submodule energy is constant, the converter output AC frequency is also constant.

2. The universal grid-type control system for a modular multilevel converter according to claim 1, characterized in that, The process of obtaining reference values ​​for the output AC frequency and phase in the AC frequency reference generation module includes: Calculate the total energy W of the converter submodules MMC With rated total energy W MMC_nom The difference: ; Where N represents the number of submodules in each arm of the converter; the converter has six arms, and there are a total of 6N submodules; C SM The size of the submodule capacitor; U SMi U represents the actual amplitude of the capacitor voltage of the i-th submodule. SM_nom This refers to the rated voltage of the submodule capacitor. Calculate the reference value for the converter output AC frequency: + ； H v ω0 is the virtual inertia coefficient, and ω0 is the rated frequency; For ω c Integrating, we obtain the reference value θ of the AC phase of the converter output.

3. The universal grid-type control system for a modular multilevel converter according to claim 2, characterized in that, The autonomous frequency construction for realizing the energy-AC frequency equivalence mapping of the submodule includes: The deviation between the submodule's energy and its rated value is directly reflected in the output AC frequency; When connected to the network, this mapping relationship can be used to simulate the rotor motion equation of a synchronous generator, achieving phase-locked loop-free self-synchronization while maintaining the energy stability of the submodule. When isolated, the AC frequency output of the converter is kept stable to achieve constant frequency control.

4. A general-purpose network control system for a modular multilevel converter according to claim 1, characterized in that, The process of obtaining active power and reactive power in the power calculation module includes: Detect the grid-connected AC voltage on the AC system side of the converter's connecting transformer. u pcc Current on the converter side i o ; Will u pcc and i o Perform a PARK transform at the reference phase θ to obtain u pcc and i o The dq axis components are respectively u pccd , u pccq , i od and i oq ; Calculate the output active power on the AC side of the converter. P g With reactive power Q g: 。 5. A general-purpose network control system for a modular multilevel converter according to claim 4, characterized in that, The process of acquiring the AC voltage modulation wave of the converter in the virtual admittance current limiting module includes: The AC voltage reference value u acref with u pccd Subtract the values ​​and add the first reference value of the q-axis current. q0 Multiply by the rated frequency ω0, then divide by a virtual admittance R v +sL v , where R v For virtual resistance, L v Let 's' be a virtual inductance, 's' be the Laplace operator, and obtain the first reference value of the d-axis current. d0 ; Connect 0 and u pccq Subtract the difference. d0 Multiply by the rated frequency ω0, then divide by a virtual admittance R v +s Lv Obtain the first reference value of the q-axis current. q0 ; Will d0 and q0 After passing through a current-limiting circuit, the second reference values ​​of the d-axis and q-axis output currents are obtained. d and q ; Will d The measured value of the d-axis current i od The difference is calculated, then passed through a proportional-integral controller, and added to u. pccd - ω 0L c i oq L c The converter output filter reactance is used to obtain the reference value E of the d-axis AC modulation voltage. d ; Will q The measured value of the q-axis current i oq The difference is calculated, and then passed through a proportional-integral controller, along with upccd+ω0L. c i od L c The converter output filter reactance is used to obtain the reference value E of the q-axis AC modulation voltage. q ; E d and E q Perform an inverse PARK transform at the reference phase θ to obtain the AC modulation voltage u. sabc .

6. A universal grid-type control system for a modular multilevel converter according to claim 5, characterized in that, In the aforementioned flow limiting process, like d0 and q0 The square root of the sum of squares is less than the current limiting amplitude i. lim， but d = d0 , q = q0 ; like d0 and q0 The square root of the sum of squares is greater than the current limiting value i. lim Then d0 and q0 Scale down proportionally by a factor of n until... d0 / n and q0 / n The square root of the sum of squares equals i lim， at this time d = d0 / n , q = q0 / n .

7. A general-purpose network control system for a modular multilevel converter according to claim 2, characterized in that, The process of obtaining the reference value of the circulating current suppression voltage in the circulating current suppression module includes: Measurement of the second harmonic circulating current component i of the three phases cira i cirb with i circ After subtracting from 0, the values ​​are passed through a proportional-resonant controller to obtain the reference value u of the three-phase second-harmonic voltage. cira u cirb and u circ .

8. A universal grid-type control system for a modular multilevel converter according to claim 7, characterized in that, The process of acquiring the switching signal of each submodule in the compensation modulation module includes: The modulation voltage of each bridge arm is obtained, and the switching signal of each submodule is obtained after sorting by the nearest level. in: ； u pj The modulation voltage of the upper arm of phase j ，u nj The modulation voltage of the lower arm of phase j; Will u pj and u nj Divide by the average voltage U of each submodule SM_av Obtain the insertion N of the three-phase upper and lower bridge arms. pj and N nj ,in: ； The submodule capacitor voltages of the upper and lower bridge arms are sorted to obtain the switching signals of the submodules: when the bridge arm current is positive, the m submodules with the smallest capacitor voltage are switched on; when the bridge arm current is negative, the m submodules with the largest capacitor voltage are switched on, where m is equal to the insertion index of the bridge arm.

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