Multi-level converter reactive power and bridge arm current combined control system and method

Abstract

The application discloses a kind of reactive power and bridge arm current joint control system and method of multilevel converter, it is related to power system operation and control technical field.The application is based on the traditional reactive double-loop control of MMC (Modular Multilevel Converter, MMC) inverter, and introduces the pre-control link mainly by voltage control and its buffer module and bridge arm current control module, to the bus voltage of the common connection point of MMC inverter and AC power grid as the first control quantity, and the second control quantity is the current of MMC bridge arm.The control method provided by the application can stably and flexibly adjust the steady-state voltage of AC power grid when the AC power grid is normally operated under the condition that the MMC inverter is connected to a weak AC power grid;Compared with the traditional reactive double-loop control of MMC inverter, the transient overcurrent of MMC bridge arm can be effectively suppressed when low-voltage ride-through fault occurs in the AC power grid, so that the MMC inverter can provide maximum reactive support for the AC power grid, effectively improve the fault voltage of the AC power grid, and significantly shorten the transient process of the AC power grid after the fault.

Description

Technical Field

[0001] This invention relates to the field of power system operation and control technology, and in particular to a method for joint control of reactive power and arm current of a multilevel converter. Background Technology

[0002] Modular multilevel converters (MMCs) are AC / DC power conversion devices based on power electronic components. They feature low switching losses, high waveform quality, strong fault handling capabilities, and rapid adjustment of active and reactive power. They are widely used in flexible DC transmission scenarios and power supply scenarios such as passive networks or weak AC grids. With the increasing application of MMC-based flexible DC (MMC-HVDC) transmission systems in urban power grid expansion, their transmission capacity accounts for a continuously increasing proportion of the urban power grid load capacity. This leads to a gradual decrease in the short-circuit ratio of traditional strong AC grids such as urban power grids, revealing weak AC grid characteristics that are detrimental to the stable operation of MMC-HVDC systems. The strength of the AC grid at the receiving end of the MMC-HVDC directly affects the dynamic and stability characteristics of the entire system. On the one hand, the lower the strength of the AC grid at the receiving end, the greater the impact of the reactive power outer loop of the MMC inverter controller on the stability of the AC grid operation, and the more likely stability problems will occur. On the other hand, compared with a strong AC grid, when subjected to disturbances, the MMC inverter needs to adjust reactive power over a larger range to stabilize its point of common coupling (PCC) bus voltage with the AC grid.

[0003] In MMC-HVDC systems supplying power to weak AC grids, using constant AC voltage control for the outer loop reactive power control of the MMC inverter controller can improve the active power delivery limit of the MMC-HVDC system. However, the lag characteristic of constant AC voltage control can lead to negative damping in the system, affecting its stability. Due to the insufficient active and reactive power regulation and support capabilities of the equivalent power source of weak AC grids, the voltage often drops to even lower levels when a short-circuit fault occurs. For weak AC grids with a large proportion of induction motor loads, under the influence of induction motor characteristics, the faster the MMC inverter generates reactive power after an AC grid short-circuit fault, the more significant its support for the grid voltage, and the lower the degree of grid voltage drop.

[0004] In summary, compared to strong AC grids, weak AC grids require more and faster reactive power support. MMC inverters supplying power to weak AC grids need to adjust their reactive power output more quickly and flexibly to maintain stable AC grid operation. Existing MMC-HVDC receiving-end converters mostly employ dual-loop control for reactive power control, typically selecting either the MMC inverter's reactive power or the AC grid voltage as the control variable. When the MMC inverter is connected to a weak AC grid, existing control methods cannot enable the MMC inverter to participate in AC grid reactive power regulation and provide maximum reactive power support.

[0005] For reactive power dual-loop control that uses the reactive power of the MMC inverter as the control variable, without droop control, the MMC inverter cannot generate additional reactive power to support the AC grid voltage during a low-voltage ride-through fault. With droop control, the MMC inverter can generate additional reactive power according to the degree of AC grid voltage drop during a low-voltage ride-through fault, determined by a droop coefficient. However, this does not fully utilize the reactive power support capability of the MMC inverter, and it cannot suppress transient overcurrents in the MMC inverter arms. For reactive power dual-loop control that uses AC grid voltage as the control variable, a large AC grid voltage drop during a low-voltage ride-through fault will generate a large impact signal within the control system, causing greater fluctuations or even instability in the faulty AC grid. Furthermore, after the fault disappears, the AC grid requires a longer transient process to transition to a steady state.

[0006] With the widespread application of MMC-HVDC in urban power distribution network expansion leading to a trend of urban power distribution networks gradually exhibiting weak AC grid characteristics, research on reactive power control methods for MMC-HVDC receiving-end converters is crucial for maximizing the potential of MMC inverters as reactive power sources to regulate AC voltage. This research is of great significance for stable grid operation and improved power supply quality.

[0007] Therefore, a method for joint control of reactive power and arm current of a multilevel converter is needed. Summary of the Invention

[0008] To address the problem that existing MMC-HVDC receiving-end converter control methods cannot fully utilize the MMC's ability to rapidly regulate reactive power, support the weak AC grid voltage at the receiving end under short-circuit faults, and improve its operational stability, this invention provides a joint control method for reactive power and arm current of a multilevel converter. This method can cope with the system instability factors that may arise from connecting an MMC-HVDC receiving-end converter to a weak AC grid. The specific technical solution is as follows:

[0009] A multilevel converter reactive power and arm current joint control system includes a pre-control loop and a reactive power dual-loop control loop. The pre-control loop is connected to the reactive power dual-loop control loop and controls the PCC bus voltage of the MMC inverter and the AC grid through cascading.

[0010] The pre-control stage includes a fault and auxiliary fault signal extraction module, a constant voltage control and buffer module, and a bridge arm current control module. The constant voltage control and buffer module includes a constant AC voltage control module and a buffer module. The output of the constant AC voltage control module is a control signal Q″. ref The buffer module is used to buffer the impact generated by the reset of the constant AC voltage-controlled integrator, and its output is a buffer signal Q. buff Control signal Q″ ref With buffer signal Q buff After accumulation, the output Q′ of the constant voltage control and its buffer module is obtained. ref The arm current control module can output a correction signal Q when a low-voltage ride-through fault in the AC power grid causes a transient overcurrent in the MMC arm. rec This is used to reduce the reactive power output of the MMC, thereby suppressing transient overcurrent in its bridge arms, Q′ ref With the correction signal Q rec After accumulation, the output of the pre-control loop is generated, which is the command value Q of the reactive power dual-loop control loop. ref ;

[0011] The reactive power dual-loop control system includes an outer loop constant reactive power control module and an inner loop constant reactive current control module, which are connected to each other.

[0012] Preferably, the fault and auxiliary fault signal extraction module includes a hysteresis module. The hysteresis module takes the effective value of the PCC bus voltage and its rate of change as input, and outputs the fault signal and auxiliary fault signal according to the high and low thresholds corresponding to the fault signal and auxiliary fault signal, specifically:

[0013] When the effective value of the PCC bus voltage is lower than the low threshold of the hysteresis circuit corresponding to the auxiliary fault signal, the auxiliary fault signal is set to fault value 0; when it is higher than the high threshold of the hysteresis circuit corresponding to the auxiliary fault signal, the auxiliary fault signal is set to normal value 1.

[0014] When the effective rate of change of the PCC bus voltage is lower than the low threshold of the hysteresis circuit corresponding to the fault signal, the fault signal is set to fault value 0; when it is higher than the high threshold of the hysteresis circuit corresponding to the fault signal, the fault signal is set to normal value 1.

[0015] Preferably, the fault and auxiliary fault signal extraction module further includes a lockout module. The lockout module takes the fault signal and auxiliary fault signal output by the hysteresis loop as input and output. When the input fault signal and auxiliary fault signal change, the lockout loop will maintain its output as the new value of the fault signal and auxiliary fault signal within a set time.

[0016] Preferably, the constant AC voltage control module adopts a PI controller. After the AC grid fault connected to the MMC inverter disappears, the PI controller integrator is reset to the steady-state integral value before the fault. The integrator reset action is jointly controlled by the fault signal and the auxiliary fault signal. Specifically, the integrator is reset when the values ​​of the fault signal and the auxiliary fault signal change from not all being normal values ​​of 1 to all being normal values ​​of 1.

[0017] Preferably, the buffer module uses the difference between the output value of the constant AC voltage control module at the instant the fault disappears and the output value under steady-state conditions before the fault as its initial value, and transitions to zero after a set time. Assuming the buffer module is engaged at time zero (t=0), the time-domain expression of the buffer module's output value is as follows:

[0018] Q buff =(Q PI_fault -Q PI_0 )e -At

[0019] In the formula, Q buff Q is the output value of the buffer module. PI_fault The AC voltage control output value, Q, is set at the instant the fault disappears. PI_0 The AC voltage control output value is the steady-state operating condition before the fault, and A is the time constant.

[0020] The buffer module is activated by an auxiliary fault signal, which is activated when the auxiliary fault signal changes from a fault value of 0 to a normal value of 1.

[0021] Preferably, the arm current control module includes an arm current acquisition module, which is used to calculate the MMC arm current value, and the calculation formula is as follows:

[0022]

[0023] In the formula, I arm I represents the MMC bridge arm current value. dc I is the effective value of the DC side current of the MMC. ac This represents the effective value of the AC side current of the MMC.

[0024] Preferably, the bridge arm current control module adopts a PI controller, whose input signal is the MMC bridge arm current value; its command value is generated based on the steady-state value of the MMC bridge arm current and the maximum allowable value of the transient current, specifically by:

[0025] Taking the steady-state value of the MMC arm current as the initial value, after a first-order inertial delay, it transitions to the maximum allowable transient current value of the MMC arm. Assuming the arm current control module is activated at time zero (t=0), the time-domain expression of its command value is as follows:

[0026] I arm_ref =I arm_max -(I arm_max -I arm_0 )e -At

[0027] In the formula, I arm_ref For instruction value, I arm_max I is the maximum permissible value of transient current in the MMC arm. arm_0 Here, A represents the steady-state value of the MMC bridge arm current, and A is the time constant.

[0028] The switching of the arm current control module is controlled by an auxiliary fault signal. The arm current control module is activated and put into operation the instant the auxiliary fault signal changes from a normal value of 1 to a fault value of 0. The arm current control module is deactivated when the auxiliary fault signal changes back to a normal value of 1.

[0029] Preferably, the bridge arm current control module outputs a correction signal Q. rec The range is from zero to the maximum transient reactive power rating of the MMC.

[0030] A method for joint control of reactive power and arm current in a multilevel converter, applied to the joint control system of reactive power and arm current in a multilevel converter as described above, is characterized by comprising the following steps:

[0031] S1: Establish MMC reactive dual-loop control with outer loop constant reactive power control and inner loop constant reactive current control;

[0032] S2: Obtain the effective value and rate of change of the PCC bus voltage of the MMC inverter and the connected AC grid, and generate fault signals and auxiliary fault signals through hysteresis and blocking links;

[0033] S3: Obtain the effective values ​​of the AC and DC currents of the MMC, calculate the MMC arm current based on them, and generate the command value of the arm current control module when a fault occurs in the AC power grid.

[0034] S4: Based on S1, a combination of constant voltage control and its buffer module, bridge arm current control module, and fault signal and auxiliary fault signal extraction module is used as the pre-control loop. Under steady-state and transient conditions, the pre-control loop and reactive power dual-loop control loop are cascaded to generate the PWM modulation signal U for controlling the MMC. qref This is to control the reactive power output of the MMC.

[0035] Preferably, based on the traditional reactive dual-loop control of the MMC inverter, a pre-control loop mainly composed of constant voltage control and its buffer module and bridge arm current control module is introduced, with the PCC bus voltage of the MMC inverter and the AC grid as the first control quantity and the MMC bridge arm current as the second control quantity.

[0036] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0037] 1. This invention connects a pre-control loop before the reactive power dual-loop control of the MMC inverter. When the MMC inverter is connected to a weak AC grid, the steady-state voltage of the AC grid can be stably and flexibly adjusted when the AC grid is operating normally. When the AC grid experiences a low voltage ride-through fault, the reactive power can be rapidly increased, effectively improving the fault voltage of the AC grid.

[0038] 2. Based on the gain effect 1, compared with the traditional reactive dual-loop control of MMC inverters, the control method provided by this invention can effectively suppress transient overcurrent of MMC bridge arms, enabling the MMC inverter to provide reactive power support to the AC grid to the greatest extent.

[0039] 3. Compared with the traditional reactive dual-loop control of MMC inverters, when the MMC inverter is connected to a weak AC grid, the control method provided by this invention can reduce the amplitude of AC grid voltage and power fluctuations caused by AC faults and shorten the transient process of the AC grid after the fault disappears. Attached Figure Description

[0040] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the accompanying drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. In all the drawings, similar elements or parts are generally identified by similar reference numerals. In the drawings, the elements or parts are not necessarily drawn to scale.

[0041] Figure 1 This is a schematic diagram of the overall structure of the present invention;

[0042] Figure 2 This is a schematic diagram of the constant voltage control and its buffer module structure of the present invention;

[0043] Figure 3This is a schematic diagram of the bridge arm current control module structure of the present invention;

[0044] Figure 4 This is a schematic diagram of the fault signal and auxiliary fault signal generation process structure of the present invention;

[0045] Figure 5 This is a topology diagram of the simulation system;

[0046] Figure 6 MMC response diagram under step AC voltage command value;

[0047] Figure 7 MMC response diagram under AC voltage drop to 0.9 pu;

[0048] Figure 8 MMC response diagram under AC voltage drop to 0.8 pu;

[0049] Figure 9 MMC response diagram under AC voltage drop to 0.7 pu;

[0050] Figure 10 The diagram shows the MMC response when the arm current command value is the same as the arm current under constant reactive power. Detailed Implementation

[0051] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0052] It should be understood that, when used in this specification and the appended claims, the terms "comprising" and "including" indicate the presence of the described features, integrals, steps, operations, elements and / or components, but do not exclude the presence or addition of one or more other features, integrals, steps, operations, elements, components and / or collections thereof.

[0053] It should also be understood that the terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to limit the invention. As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” are intended to include the plural forms unless the context clearly indicates otherwise.

[0054] It should also be further understood that the term "and / or" as used in this specification and the appended claims refers to any combination of one or more of the associated listed items and all possible combinations, and includes such combinations.

[0055] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. The terminology used herein is for the purpose of describing embodiments of this disclosure only and is not intended to be limiting of this disclosure.

[0056] This invention provides an embodiment of a method for joint control of reactive power and arm current in a multilevel converter. The modular multilevel converter method for joint control of reactive power and arm current specifically includes the following:

[0057] S1: Establish a dual-loop reactive power control system for MMC inverters, consisting of an outer loop constant reactive power control and an inner loop constant reactive current control.

[0058] Among them, the outer loop constant reactive power control uses the reactive power of the MMC inverter as the input signal and outputs the reactive current command value of the inner loop constant reactive current control.

[0059] The inner-loop constant reactive current control uses the reactive current of the MMC inverter as the input signal and the output signal of the outer-loop constant reactive power control as the command value, outputting a PWM modulation signal U for controlling the MMC. qref .

[0060] S2: Obtain the effective value and rate of change of the PCC bus voltage between the MMC inverter and the AC grid, and generate fault signals and auxiliary fault signals through hysteresis and blocking circuits;

[0061] The hysteresis circuit takes the effective value and rate of change of the PCC bus voltage as input, and outputs fault signals and auxiliary fault signals according to the high and low thresholds corresponding to the fault signals and auxiliary fault signals:

[0062] When the effective value of the PCC bus voltage is lower than the low threshold of the hysteresis circuit corresponding to the auxiliary fault signal, the auxiliary fault signal is set to fault value 0; when it is higher than the high threshold of the hysteresis circuit corresponding to the auxiliary fault signal, the auxiliary fault signal is set to normal value 1.

[0063] When the effective rate of change of the PCC bus voltage is lower than the low threshold of the hysteresis circuit corresponding to the fault signal, the fault signal is set to fault value 0; when it is higher than the high threshold of the hysteresis circuit corresponding to the fault signal, the fault signal is set to normal value 1.

[0064] The interlocking module takes the fault signal and auxiliary fault signal output by the hysteresis circuit as input and output. When the input fault signal and auxiliary fault signal change, the interlocking circuit will maintain its output at the new values ​​of the fault signal and auxiliary fault signal within the set time.

[0065] S3: Obtain the effective values ​​of the AC and DC currents of the MMC, calculate the MMC arm current based on them, and generate the instruction value of the arm current control module based on the steady-state value and the maximum allowable value of the transient current of the MMC arm current.

[0066] The MMC arm current is calculated using the following formula:

[0067]

[0068] In the formula, I arm I represents the MMC bridge arm current value. dc I is the effective value of the DC side current of the MMC. ac This represents the effective value of the AC side current of the MMC.

[0069] The specific method for generating the instruction values ​​for the bridge arm current control module is as follows:

[0070] Taking the steady-state value of the MMC arm current as the initial value, after a first-order inertial delay, it transitions to the maximum allowable transient current value of the MMC arm. Assuming the arm current control module is activated at time zero (t=0), the time-domain expression of its command value is as follows:

[0071] I arm_ref =I arm_max -(I arm_max -I arm_0 )e -At

[0072] In the formula, I arm_ref For instruction value, I arm_max I is the maximum permissible value of transient current in the MMC arm. arm_0 Here, A represents the steady-state value of the MMC bridge arm current, and A is the time constant.

[0073] S4: Based on S1, a combination of fault signal and auxiliary fault signal extraction module, constant voltage control and its buffer module, and bridge arm current control module is used as the front-end control link. The front-end control link is connected to the reactive power double-loop control link and controls the PCC bus voltage of the MMC inverter and the AC grid through cascading. The front-end control link is the first control link of the cascaded control, and the reactive power double-loop control link is the second control link of the cascaded control.

[0074] The output of the constant voltage control and its buffer module is the control signal Q′. ref The arm current control module can output a correction signal Q when a low-voltage ride-through fault in the AC power grid causes a transient overcurrent in the MMC arm. rec Q′ ref With the correction signal Q rec After accumulation, the output command value Q of the reactive power dual-loop control loop is obtained. ref .

[0075] S41: The constant AC voltage control is the main control in the front-end control loop. It adopts a PI controller and is fully engaged under both steady-state and transient conditions. It takes the PCC bus voltage as the input signal and outputs the control signal Q″. ref ;

[0076] The constant voltage control and its buffer module are jointly controlled by the fault signal generated by S2 and the auxiliary fault signal. After the fault in the AC grid connected to the MMC inverter disappears, the integral value of the PI controller integrator is reset to the steady-state integral value before the fault. Specifically, at the instant when the fault signal and the auxiliary fault signal all become the normal value 1, the PI controller integrator is reset to the steady-state integral value before the fault.

[0077] The moment the auxiliary fault signal changes from a fault value of 0 to a normal value of 1, the buffer module engages to buffer the sudden change in the reactive power dual-loop control command value caused by the reset of the PI controller integrator; the output value Q of the buffer module... buff The output value transitions to zero from the initial value after a set time. The initial value is the difference between the steady-state output value before the AC voltage control fault and the transient output value during the fault. The specific formula is as follows:

[0078] Q buff =(Q PI_fault -Q PI_0 )e -At

[0079] In the formula, Q buff Q is the output value of the buffer module. PI_fault The AC voltage control output value, Q, is set at the instant the fault disappears. PI_0 The AC voltage control output value is the steady-state operating condition before the fault, and A is the time constant.

[0080] The control signal Q″ output by the constant AC voltage control module ref With the output value Q of the buffer module buff The sum is the output Q′ of the constant voltage control and its buffer module. ref .

[0081] S42: The arm current control is an auxiliary control in the pre-control loop, using a PI controller. It is only activated under transient conditions. The input signal is the MMC arm current value generated by S3, and the command value generated by S3 is used as its command value. The output correction signal Q is used. rec ;

[0082] The switching of the arm current control module is controlled by the auxiliary fault signal generated by S2. The arm current control module is activated after initialization when the auxiliary fault signal changes from a normal value of 1 to a fault value of 0. The arm current control module deactivates when the auxiliary fault signal changes back to a normal value of 1, and the correction signal Q... recThe value ranges from zero to the maximum transient reactive power rating of the MMC, and is used to suppress transient overcurrent in the MMC arm.

[0083] S43: In addition to using the pre-control loop to generate reactive power command values, in order to improve the operational flexibility of the MMC inverter, the control method proposed in this invention can also directly use the received external command values ​​as the reactive power command values ​​for reactive dual-loop control, so that the MMC inverter operates in a constant reactive power state.

[0084] This embodiment also provides a joint control system for reactive power and arm current of a multilevel converter, including a pre-control loop and a reactive power dual-loop control loop. The pre-control loop is connected to the reactive power dual-loop control loop and controls the PCC bus voltage of the MMC inverter and the AC grid through cascading. The pre-control loop includes a fault and auxiliary fault signal extraction module, a constant voltage control and buffer module, and an arm current control module. The constant voltage control and buffer module includes a constant AC voltage control module and a buffer module. The constant AC voltage control module is used to generate a control signal Q″. ref The buffer module is used to buffer the impact generated by the reset of the integrator of the constant AC voltage controlled PI controller, and outputs a buffer signal Q. buff Control signal Q″ ref With buffer signal Q buff The sum Q′ ref The output of the constant voltage control and buffer module is used; the arm current control module is connected in parallel with the constant AC voltage control module. When a transient overcurrent occurs in the MMC arm under transient operating conditions, the arm current control module outputs a correction signal Q. rec This is used to reduce the reactive power output of the MMC to suppress transient overcurrent in its bridge arms; the output Q′ of the constant voltage control and its buffer module is... ref With the output Q of the bridge arm current control module rec sum Q ref The output of the pre-control loop is used as the reactive power command value input to the reactive power dual-loop control loop; the reactive power dual-loop control loop includes an outer loop constant reactive power control module and an inner loop constant reactive current control module, and the output of the reactive power dual-loop control loop is a PWM modulation signal U used to control the MMC. qref The outer loop constant reactive power control module is connected to the inner loop constant reactive current control module.

[0085] It should be noted that the pre-control loop and reactive power dual-loop control loop mentioned here correspond to the pre-control loop and reactive power dual-loop control loop in the embodiment of the multi-level converter reactive power and arm current joint control method provided above. The other modules also correspond to the links in the embodiment of the multi-level converter reactive power and arm current joint control system. Each module in the system implements the function of the corresponding link in the method.

[0086] To further illustrate the technical solution of this invention, simulation verification is provided below. See [link / reference]. Figure 5 The model shown is a 35kV passive distribution network powered by an MMC inverter and an external infinite power supply. The load model consists of ZIP loads and induction motor loads connected in parallel. The short-circuit ratio of the infinite power supply to this distribution network at the PCC bus after the tie line is less than 3.

[0087] The total active power of the 35kV passive distribution network load is 50MW, and the rated apparent power of the MMC inverter supplying it is 50MVA. To simulate the system capacity redundancy in the real MMC-HVDC project, the rated active power of the MMC inverter is designed to be 40MW.

[0088] The MMC inverter adopts a constant active power dual-loop control with active power-frequency droop control. To facilitate the demonstration of the gain effect of the proposed control method, the reactive power control of the MMC inverter in the simulation model adopts the combined reactive power and bridge arm current control proposed in this invention and the traditional constant reactive power dual-loop control.

[0089] Specifically, firstly, the flexibility and smoothness of the joint control of reactive power and bridge arm current are illustrated by the step of the AC voltage command value under steady-state conditions;

[0090] Then, under the condition that the MMC inverter is generating rated active power, a three-phase short-circuit fault lasting 500 milliseconds is simulated on the PCC bus. When the MMC inverter adopts reactive power dual-loop control with constant reactive power, the lowest PCC bus voltage during the fault period is 0.9 pu, 0.8 pu, and 0.7 pu, respectively. The response and system operating parameters of the MMC inverter under the same fault are compared when using constant reactive power control and the combined control of reactive power and arm current. The effectiveness of the designed combined control method of reactive power and arm current in increasing reactive power to raise the AC grid voltage, suppressing transient overcurrent in the arm, and shortening the transient process of the AC grid during the fault period is verified.

[0091] Finally, the maximum transient value of the MMC arm current under the joint control of reactive power and arm current was set as the transient value of the arm current when the MMC inverter was operating in a constant reactive power state and the PCC bus voltage was at least 0.8 pu. The effect of the designed joint control method of reactive power and arm current on increasing the reactive power of the MMC inverter was compared with that under the same operating conditions when the arm current was the same.

[0092] The simulation results and their analysis are as follows:

[0093] 1. Steady-state step response of AC voltage in MMC inverter under combined control of reactive power and arm current:

[0094] When the MMC inverter employs the aforementioned combined reactive power and arm current control method, a step signal is applied to its AC voltage command value. The response curves of PCC bus voltage (Upcc), MMC reactive power (Q), and MMC current (Immc) are as follows: Figure 6 As shown.

[0095] Within hundreds of milliseconds after the step signal was applied, the PCC bus voltage transitioned to the new command value quickly and smoothly, demonstrating the flexibility and smoothness of the designed reactive power and arm current joint control method in responding to sudden changes in AC voltage command values ​​and adjusting the steady-state voltage of the AC grid.

[0096] 2. When the MMC inverter is operating in constant reactive power mode, the combined control response of reactive power and bridge arm current under the condition that the PCC bus fault voltage drops to 0.9 pu is as follows:

[0097] The response curves of PCC bus voltage (Upcc), MMC reactive power (Q), and MMC arm current (Iarm) under this operating condition are as follows: Figure 7 As shown.

[0098] As shown in the figure, when constant reactive power control is used, the lowest fault voltage of the PCC bus is 0.9 pu. When reactive power and arm current are controlled together, after the same fault occurs, the fault voltage of the PCC bus gradually increases as the reactive power output of the MMC inverter increases. Finally, due to the sufficient reactive power capacity of the MMC inverter, the fault voltage of the PCC bus rises back to the command value. After the fault disappears, the reactive power output of the MMC inverter quickly recovers to the steady-state value before the fault, avoiding severe overvoltage in the AC power grid.

[0099] 3. When the MMC inverter is operating in constant reactive power mode, the combined control response of reactive power and bridge arm current under the condition that the PCC bus fault voltage drops to 0.8 pu is as follows:

[0100] The response curves of PCC bus voltage (Upcc), MMC reactive power (Q), and MMC arm current (Iarm) under this operating condition are as follows: Figure 8 As shown.

[0101] As shown in the figure, when constant reactive power control is used, the lowest PCC bus fault voltage is 0.8 pu. When reactive power and bridge arm current are used for joint control, after the same fault occurs, the PCC bus fault voltage gradually increases as the reactive power output of the MMC inverter increases. Due to the insufficient reactive power capacity of the MMC inverter, the PCC bus voltage fails to rise to the command value. Finally, the PCC bus voltage rises to the maximum transient value when the reactive power of the MMC inverter rises to the maximum value. After the fault disappears, under the action of the buffer module, the reactive power output of the MMC inverter smoothly transitions to the steady-state value before the fault.

[0102] It should be noted that under this operating condition, the auxiliary fault signal is already at a fault value of 0, and the arm current control module is put into operation under its control; however, the MMC arm current has not yet shown a trend of transient overcurrent, so the output value of the arm current control module is always 0, and it does not play the role of suppressing the reactive power output of the MMC inverter.

[0103] 4. When the MMC inverter is operating in constant reactive power mode, the combined control response of reactive power and bridge arm current is as follows when the fault voltage of the PCC bus drops to 0.7 pu:

[0104] The response curves of PCC bus voltage (Upcc), MMC reactive power (Q), and MMC arm current (Iarm) under this operating condition are as follows: Figure 9 As shown.

[0105] As shown in the figure, when constant reactive power control is used, the lowest PCC bus fault voltage is 0.7 pu. When combined reactive power and arm current control is used, after the same fault occurs, the PCC bus fault voltage gradually increases as the reactive power output of the MMC inverter rises. Under this condition, the AC grid fault voltage is too low, causing transient overcurrent in the MMC arm. The output of the arm current control module is no longer 0, but outputs a negative value, reducing the reactive power output of the MMC inverter to suppress its arm overcurrent. Under the action of the arm current control module, the MMC inverter arm current during the fault period remains below the maximum allowable transient current value.

[0106] After the fault disappeared, the AC grid voltage began to rise, and the MMC arm current decreased, causing the output of the arm current control module to gradually rise back to 0. The suppression of reactive power of the MMC inverter disappeared, and the reactive power output of the MMC inverter increased, causing the under-voltage inductors in the load to absorb more reactive power and accelerate the recovery of the AC grid voltage. After the AC grid voltage recovered to the threshold value, the reactive power output of the MMC inverter smoothly transitioned to the pre-fault steady-state value under the action of the buffer module.

[0107] 5. Set the maximum allowable value of the transient current of the MMC arm under the combined control of reactive power and arm current to the transient value of the MMC arm current when operating under constant reactive power in simulation condition 3, and re-simulate condition 3:

[0108] The response curves of PCC bus voltage (Upcc), MMC reactive power (Q), and MMC arm current (Iarm) under this operating condition are as follows: Figure 10 As shown.

[0109] As shown in the figure, when constant reactive power control is used, the lowest fault voltage of the PCC bus is 0.8 pu, and the reactive power output of the MMC inverter remains basically unchanged during the fault. When reactive power and bridge arm current are controlled together, after the same fault occurs, the bridge arm current of the MMC inverter does not change much compared with the constant reactive power control, but its output reactive power increases to a certain extent, which causes the AC grid voltage to increase accordingly.

[0110] It should be noted that there is a certain proportion of inductor load in the AC grid load model, and the MMC inverter capacity accounts for a large proportion in this simulation model. This causes the active power fluctuations caused by the grid voltage drop in the inductor to generate large current fluctuations in the MMC inverter arm. The control method provided by this invention focuses on suppressing the overcurrent of the MMC arm during the AC grid short-circuit transient process in order to avoid device damage caused by long-term overcurrent. However, it cannot suppress the subtransient overcurrent of the MMC arm caused by the grid load characteristics.

[0111] In summary, this invention constructs a joint control system for reactive power and arm current of a multi-level converter by cascading a pre-control loop before the traditional reactive power dual-loop control of the MMC. This system enables the MMC inverter to flexibly and smoothly adjust the steady-state voltage of the AC grid when connected to a weak AC grid during steady-state operation. Furthermore, in the event of a short-circuit fault in the AC grid, it rapidly and maximally increases the reactive power output of the MMC inverter while avoiding transient overcurrent in the arm, effectively raising the fault voltage of the AC grid and shortening the transient process. Therefore, the technical solution proposed in this invention solves the problems mentioned in the background art.

[0112] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention, and they should all be covered within the scope of the claims and specification of the present invention.

Claims

1. A joint control system for reactive power and arm current of a multilevel converter, characterized in that, It includes a front-end control loop and a reactive power dual-loop control loop. The front-end control loop is connected to the reactive power dual-loop control loop and controls the point of common coupling (PCC) bus voltage between the MMC inverter and the AC grid through a cascading method. The pre-control stage includes a fault and auxiliary fault signal extraction module, a constant voltage control and buffer module, and a bridge arm current control module; the constant voltage control and buffer module includes a constant AC voltage control module and a buffer module; the output of the constant AC voltage control module is a control signal Q. r " ef The buffer module is used to buffer the impact generated by the reset of the constant AC voltage-controlled integrator, and its output is a buffer signal Q. buff Control signal Q r " ef With buffer signal Q buff After accumulation, the output Q of the constant voltage control and its buffer module is obtained. r ′ ef The arm current control module can output a correction signal Q when a low-voltage ride-through fault in the AC power grid causes a transient overcurrent in the MMC arm. rec This is used to reduce the reactive power output of the MMC, thereby suppressing transient overcurrent in its bridge arms; Q r ′ ef With correction signal Q rec After accumulation, the output of the pre-control loop is generated, which is the command value Q of the reactive power dual-loop control loop. ref ; The reactive power dual-loop control system includes an outer loop constant reactive power control module and an inner loop constant reactive current control module, which are connected together.

2. The multilevel converter reactive power and arm current joint control system according to claim 1, characterized in that, The fault and auxiliary fault signal extraction module includes a hysteresis module. The hysteresis module takes the effective value of the PCC bus voltage and its rate of change as input, and outputs the fault signal and auxiliary fault signal according to the high and low thresholds corresponding to the fault signal and auxiliary fault signal. Specifically: When the effective value of the PCC bus voltage is lower than the low threshold of the hysteresis circuit corresponding to the auxiliary fault signal, the auxiliary fault signal is set to fault value 0; when it is higher than the high threshold of the hysteresis circuit corresponding to the auxiliary fault signal, the auxiliary fault signal is set to normal value 1. When the effective rate of change of the PCC bus voltage is lower than the low threshold of the hysteresis circuit corresponding to the fault signal, the fault signal is set to fault value 0; when it is higher than the high threshold of the hysteresis circuit corresponding to the fault signal, the fault signal is set to normal value 1.

3. The multilevel converter reactive power and arm current joint control system according to claim 2, characterized in that, The fault and auxiliary fault signal extraction module also includes a locking module. The locking module takes the fault signal and auxiliary fault signal output by the hysteresis link as input and output. When the input fault signal and auxiliary fault signal change, the locking link will maintain its output as the new value of the fault signal and auxiliary fault signal within a set time.

4. The multilevel converter reactive power and arm current joint control system according to claim 1, characterized in that, The constant AC voltage control module adopts a PI controller. After the fault in the AC power grid connected to the MMC inverter disappears, the integral value of the PI controller integrator is reset to the steady-state integral value before the fault. The integrator reset action is jointly controlled by the fault signal and the auxiliary fault signal. Specifically, the integrator is reset when the values ​​of the fault signal and the auxiliary fault signal change from not all being normal values ​​of 1 to all being normal values ​​of 1.

5. The multilevel converter reactive power and arm current joint control system according to claim 1, characterized in that, The buffer module uses the difference between the output value of the constant AC voltage control module at the instant the fault disappears and the output value under steady-state conditions before the fault as its initial value. After a set time, it transitions to zero as a first-order element. Assuming the buffer module is engaged at time zero (t=0), the time-domain expression of the buffer module's output value is as follows: Q buff =(Q PI_fault -Q PI_0 )e -At In the formula, Q buff Q is the output value of the buffer module. PI_fault The AC voltage control output value, Q, is set at the instant the fault disappears. PI_0 The AC voltage control output value is the steady-state operating condition before the fault, and A is the time constant. The buffer module is activated by an auxiliary fault signal, which is activated when the auxiliary fault signal changes from a fault value of 0 to a normal value of 1.

6. The multilevel converter reactive power and arm current joint control system according to claim 1, characterized in that, The arm current control module includes an arm current acquisition module, which is used to calculate the arm current of the MMC. The calculation formula is as follows: In the formula, I arm I represents the MMC bridge arm current value. dc I is the effective value of the DC side current of the MMC. ac This represents the effective value of the AC side current of the MMC.

7. A multilevel converter reactive power and arm current joint control system according to claim 6, characterized in that, The bridge arm current control module adopts a PI controller, and its input signal is the MMC bridge arm current value; its command value is generated based on the steady-state value of the MMC bridge arm current and the maximum allowable value of the transient current, specifically as follows: Taking the steady-state value of the MMC arm current as the initial value, after a first-order inertial delay, it transitions to the maximum allowable transient current value of the MMC arm. Assuming the arm current control module is activated at time zero (t=0), the time-domain expression of its command value is as follows: I arm_ref =I arm_max -(I arm_max -I arm_0 )e -At In the formula, I arm_ref For instruction value, I arm_max I is the maximum permissible value of transient current in the MMC arm. arm_0 Here, A represents the steady-state value of the MMC bridge arm current, and A is the time constant. The switching of the arm current control module is controlled by an auxiliary fault signal. The arm current control module is activated and put into operation the instant the auxiliary fault signal changes from a normal value of 1 to a fault value of 0. The arm current control module is deactivated when the auxiliary fault signal changes back to a normal value of 1.

8. A multilevel converter reactive power and arm current joint control system according to claim 7, characterized in that, The bridge arm current control module outputs a correction signal Q. rec The range is from zero to the maximum transient reactive power rating of MMC.

9. A method for joint control of reactive power and arm current of a multilevel converter, applied to a joint control system for reactive power and arm current of a multilevel converter as described in any one of claims 1-8, characterized in that, Includes the following steps: S1: Establish MMC reactive dual-loop control with outer loop constant reactive power control and inner loop constant reactive current control; S2: Obtain the effective value and rate of change of the PCC bus voltage of the MMC inverter and the connected AC grid, and generate fault signals and auxiliary fault signals through hysteresis and blocking links; S3: Obtain the effective values ​​of the AC and DC currents of the MMC, calculate the MMC arm current based on them, and generate the command value of the arm current control module when a fault occurs in the AC power grid. S4: Based on S1, a combination of constant voltage control and its buffer module, bridge arm current control module, and fault signal and auxiliary fault signal extraction module is used as the pre-control loop. Under steady-state and transient conditions, the pre-control loop and reactive power dual-loop control loop are cascaded to generate the PWM modulation signal U for controlling the MMC. qref This is to control the reactive power output of the MMC.

10. A method for joint control of reactive power and arm current of a multilevel converter according to claim 9, characterized in that, Under steady-state conditions and transient conditions with high AC grid fault voltage, the control method of cascading the pre-control loop and reactive power dual-loop control uses the PCC bus voltage as the control quantity and receives the PCC bus voltage command value input to the control system. Under transient conditions with low AC grid fault voltage, based on the above two conditions, the MMC arm current is introduced as a second control quantity, and the command value of the second control quantity is generated according to the steady-state value of the MMC arm current and the maximum allowable value of the transient current to suppress the transient overcurrent of the MMC arm.

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