A power control method and circuit for LCC-MMC parallel system

Abstract

The application discloses a power control method and circuit for an LCC-MMC parallel system, and belongs to the technical field of power electronics. The method is applied to an MMC inverter, and the method comprises the following steps: acquiring an inverter-side bus voltage value, an active power value and a reactive power value; calculating a voltage drop value by means of the difference between an inverter-side bus rated voltage value and the inverter-side bus voltage value; obtaining a first comparison result according to the voltage drop value and a preset threshold value; selecting an active power reference value; performing difference integration on the active power reference value and the active power value to obtain an active power control component; performing difference integration on the voltage drop value and the reactive power value to obtain a reactive power control component; performing current inverse transformation on the active power control component and the reactive power control component to obtain an alternating-current voltage reference value; and generating a pulse width modulation signal based on the alternating-current voltage reference value, so as to perform switching control on a switching tube of the MMC inverter. The application can effectively control power to maintain the alternating-current bus voltage.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of power electronics, in particular to a power control method and circuit for LCC-MMC parallel system. BACKGROUND

[0002] Line Commutated Converter High Voltage Direct Current (LCC-HVDC) system is an important part of high voltage direct current transmission system. In the LCC-HVDC system, the converter valve is composed of a plurality of thyristors in series, which is used to realize the conversion of alternating current to direct current and the inversion of direct current to alternating current. However, due to the half-controlled characteristic of the thyristor in the system, when the voltage drops due to the fault of the AC-DC system, the thyristor may not meet the commutation condition, thereby causing commutation failure. Commutation failure refers to the state that the valve just out of conduction has not recovered the blocking capability and becomes conducting state again. Commutation failure will cause the thyristor to fail to normally turn off, thereby easily causing short circuit phenomenon. If effective inhibition measures are not taken after the first commutation failure, the short circuit of the thyristor may cause continuous commutation failure, and eventually may cause the power transmission of the LCC-HVDC system to be interrupted. Therefore, how to control the power to maintain the AC bus voltage, thereby reducing the possibility of thyristor commutation failure and improving the reliability and stability of the LCC-HVDC system has become a technical problem to be solved. SUMMARY

[0003] The purpose of the embodiment of the present application is to provide a power control method and circuit for LCC-MMC parallel system, which can effectively control the power to maintain the AC bus voltage, thereby reducing the possibility of thyristor commutation failure and improving the reliability and stability of the LCC-HVDC system.

[0004] To achieve the above purpose, the embodiment of the present application provides a power control method for LCC-MMC parallel system, which is applied to an MMC inverter, and includes the following steps.

[0005] Obtaining an inverter side bus voltage value, an active power value output by the MMC inverter and a reactive power value output by the MMC inverter;

[0006] Calculating the difference value of the inverter side bus rated voltage value and the inverter side bus voltage value to obtain a voltage drop value;

[0007] Comparing the voltage drop value with a preset threshold value to obtain a first comparison result;

[0008] Selecting an active power reference value in the active power data set based on the first comparison result;

[0009] integrating the active power reference value and the active power value to obtain an active power control component;

[0010] integrating the voltage drop value and the reactive power value to obtain a reactive power control component;

[0011] performing current inverse transformation on the active power control component and the reactive power control component to obtain an alternating voltage reference value;

[0012] generating a pulse width modulation signal based on the alternating voltage reference value, to switch control the switching tube of the MMC inverter according to the pulse width modulation signal.

[0013] Compared with the prior art, the power control method for the LCC-MMC parallel system disclosed in the present application calculates the voltage drop value of the inverter bus voltage value and the inverter bus rated voltage value first, then compares the voltage drop value with a preset threshold to obtain a first comparison result, to determine whether the thyristor in the LCC-HVDC system has a commutation failure; further, based on the first comparison result, a suitable active power reference value is selected in the active power data set; further, the active power reference value and the active power value are integrated to obtain an active power control component, to adjust the active power value output by the MMC inverter; the voltage drop value and the reactive power value are integrated to obtain a reactive power control component, to adjust the reactive power value output by the MMC inverter; further, according to the active power control component and the reactive power control component, current inverse transformation is performed to obtain an alternating voltage reference value, the present application adopts a decoupling control method, and realizes independent adjustment of active power and reactive power control; in this way, a pulse width modulation signal is generated based on the alternating voltage reference value, to switch control the switching tube of the MMC inverter according to the pulse width modulation signal, so that the technical effect of adjusting the inverter bus voltage can be realized, the problem that the thyristor in the prior art is easily affected by voltage drop and causes commutation failure is solved, the beneficial effects of coordinating the control of active power and reactive power to maintain the alternating bus voltage are obtained, so that the possibility of thyristor commutation failure is reduced, and the reliability and stability of the LCC-HVDC system are improved.

[0014] In some embodiments, the generating a pulse width modulation signal based on the alternating voltage reference value, to switch control the switching tube of the MMC inverter according to the pulse width modulation signal, comprises:

[0015] performing numerical comparison on the alternating voltage reference value and the inverter bus voltage value to obtain a second comparison result;

[0016] generate a pulse width modulation signal based on the second comparison result, so as to switch control the switch tube of the MMC inverter according to the pulse width modulation signal.

[0017] In some embodiments, the generating the pulse width modulation signal based on the second comparison result comprises:

[0018] if the second comparison result indicates that the AC voltage reference value is less than the inverter-side bus voltage value, generating a first pulse width modulation signal; or,

[0019] if the second comparison result indicates that the AC voltage reference value is greater than or equal to the inverter-side bus voltage value, generating a second pulse width modulation signal; wherein the duty cycle of the first pulse width modulation signal is less than the duty cycle of the second pulse width modulation signal.

[0020] In some embodiments, the active power data set comprises a first active power value and a second active power value, the first active power value is less than the second active power value, and the selecting the active power reference value in the active power data set based on the first comparison result comprises:

[0021] if the first comparison result indicates that the voltage drop value is greater than the preset threshold value, taking the first active power value as the active power reference value; or,

[0022] if the first comparison result indicates that the voltage drop value is less than or equal to the preset threshold value, taking the second active power value as the active power reference value.

[0023] To achieve the above-mentioned purpose, a second aspect of the embodiment of the present application proposes a power control circuit for an LCC-MMC parallel system, the circuit comprises at least one LCC composed of an LCC inverter, an MMC inverter composed of at least one MMC sub-module, and an inverter-side bus, the AC side of the LCC inverter is connected in parallel with the AC side of the MMC inverter, and then connected with one side of the inverter-side bus, and the other side of the inverter-side bus is connected with a receiving end power grid; wherein the MMC inverter further comprises:

[0024] a subtracter module connected with one side of the inverter-side bus, the subtracter module being used for difference calculation of an inverter-side bus rated voltage value and the inverter-side bus voltage value to obtain a voltage drop value;

[0025] a comparator module connected with the subtracter module, the comparator module being used for numerical comparison of the voltage drop value with a preset threshold value to obtain a first comparison result;

[0026] a selector module connected with the comparator module, the selector module configured to select an active power reference value in an active power data set based on the first comparison result;

[0027] an active power integration module connected with the selector module, the active power integration module configured to integrate the active power reference value and the active power value by difference to obtain an active power control component;

[0028] a reactive power integration module connected with one side of the inverter-side bus, the reactive power integration module configured to integrate the voltage drop value and the reactive power value by difference to obtain a reactive power control component;

[0029] a current control loop module connected with the active power integration module and the reactive power integration module, the current control loop module configured to perform current inverse transformation on the active power control component and the reactive power control component to obtain an AC voltage reference value;

[0030] a pulse width modulation module connected with the current control loop module, the pulse width modulation module configured to generate a pulse width modulation signal based on the AC voltage reference value, and to perform switching control on switching tubes of the MMC inverter according to the pulse width modulation signal.

[0031] In some embodiments, the pulse width modulation module comprises:

[0032] a comparison unit connected with the current control loop module, the comparison unit configured to perform numerical comparison on the AC voltage reference value and the inverter-side bus voltage value to obtain a second comparison result;

[0033] a generation unit connected with the comparison unit, the generation unit configured to generate a pulse width modulation signal based on the second comparison result, and to perform switching control on switching tubes of the MMC inverter according to the pulse width modulation signal.

[0034] In some embodiments, the circuit further comprises an LCC rectifier composed of at least one LCC sub-module, an MMC rectifier composed of at least one MMC sub-module, and a rectifier-side bus, one side of the rectifier-side bus being connected with an AC side of the LCC rectifier and an AC side of the MMC rectifier respectively, the other side of the rectifier-side bus being connected with a sending-end power grid, a DC side of the LCC rectifier being connected with a DC side of the LCC inverter, and a DC side of the MMC rectifier being connected with a DC side of the MMC inverter.

[0035] In some embodiments, the circuit further comprises a first converter transformer and a second converter transformer, one side of the rectifier side bus is connected with the AC side of the LCC rectifier through the first converter transformer, and the AC side of the LCC inverter is connected with one side of the inverter side bus through the second converter transformer.

[0036] In some embodiments, the circuit further comprises a third converter transformer and a fourth converter transformer, one side of the rectifier side bus is connected with the AC side of the MMC rectifier through the third converter transformer, and the AC side of the MMC inverter is connected with one side of the inverter side bus through the fourth converter transformer.

[0037] In some embodiments, the circuit further comprises a first filter and a second filter, one side of the first filter is connected with the other side of the rectifier side bus, and the other side of the first filter is grounded; one side of the second filter is connected with the other side of the inverter side bus, and the other side of the second filter is grounded. BRIEF DESCRIPTION OF DRAWINGS

[0038] Figure 1 is a flowchart of a power control method for an LCC-MMC parallel system in an embodiment of the present application;

[0039] Figure 2 is a flowchart of a power control method for an LCC-MMC parallel system in an embodiment of the present application;

[0040] Figure 3 is a flowchart of a power control method for an LCC-MMC parallel system in an embodiment of the present application; Figure 1

[0041] Figure 4 is a flowchart of a power control method for an LCC-MMC parallel system in an embodiment of the present application;

[0042] Figure 5 is a flowchart of a power control method for an LCC-MMC parallel system in an embodiment of the present application;

[0043] Figure 6A is a flowchart of a power control method for an LCC-MMC parallel system in an embodiment of the present application;

[0044] Figure 6B is a flowchart of a power control method for an LCC-MMC parallel system in an embodiment of the present application;

[0045] Figure 6C ​is a power-time curve diagram of an LCC-HVDC system and an LCC-MMC parallel system in an embodiment of the present application. DETAILED DESCRIPTION

[0046] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by a person of ordinary skill in the art without creative effort belong to the scope of protection of the present application.

[0047] In the description of the present application, it should be understood that the terms "center", "upper", "lower", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer" and the like indicate the orientation or positional relationship shown in the drawings based on the orientation or positional relationship shown in the drawings, and are only for the purpose of facilitating the description of the present application and simplifying the description, and therefore cannot be understood as indicating or implying that the device or element referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation on the present application.

[0048] The terms "first", "second" are only for descriptive purpose, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the technical features indicated. Therefore, the features defined with "first", "second" can explicitly or implicitly include one or more of the features. In the description of the present application, the meaning of "a plurality of" is two or more, unless otherwise specified.

[0049] In the description of the present application, it should be noted that, unless otherwise explicitly specified and limited, the terms "mounting", "connection", "connection" should be understood in a broad sense, for example, it can be fixed connection, or detachable connection, or integrally connected; it can be mechanical connection, or electrical connection; it can be directly connected, or indirectly connected through an intermediate medium, or the communication inside two elements. For a person of ordinary skill in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0050] Firstly, the terms involved in the present application are analyzed:

[0051] Line Commutated Converter (LCC): It is a kind of commutation technology used in high voltage direct current (HVDC) power transmission system, which relies on the AC voltage of the power grid to control the conduction and turn-off of thyristors in the converter, so as to realize the conversion between AC and DC.

[0052] High Voltage Direct Current (HVDC): A power transmission technology that converts alternating current (AC) to direct current (DC) for long-distance transmission, and then converts it back to AC at the receiving end.

[0053] Line Commutated Converter High Voltage Direct Current (LCC-HVDC) system: A high voltage direct current transmission technology that uses thyristors as the main switching device. LCC-HVDC system converts AC to DC through the rectifier station, then transmits through the DC transmission line, and finally converts DC back to AC at the inverter station.

[0054] Line-commutated: Refers to the process of transferring the current flowing through the converter from one current path to another by means of the turn-on and turn-off of the converter valve. In the converter, if the valve that exits conduction fails to recover blocking capability within a certain period of time under the action of reverse voltage, this situation is called commutation failure.

[0055] Modular Multilevel Converter (MMC): An important technology in the field of high voltage direct current (HVDC) transmission that realizes multi-level smoothing of voltage by inserting an inductor between each module and controls the switching state of each module through high-frequency carrier phase-shifted modulation technology to control the output voltage. MMC has modular design, multi-level structure, high voltage capacity and other characteristics, and is widely used in high voltage direct current transmission systems, energy storage systems, motor drive systems and other fields.

[0056] Modular Multilevel Converter Based High Voltage Direct Current (MMC-HVDC) system: A high voltage direct current transmission technology based on voltage source converter (VSC). MMC-HVDC system adopts modular design and is composed of multiple sub-modules in cascade, each sub-module contains several half-bridge or full-bridge arms, and the switching state of each sub-module is accurately controlled to realize accurate control of the output voltage.

[0057] Normalization: A method of converting actual parameters (such as voltage, current, power, impedance, etc.) of a power system or electric machine into relative values without dimension. Normalization involves dividing the actual value of a physical quantity by a selected value of the same unit, which is called the reference value. Normalization eliminates the conversion between units, and all values are normalized to a unified standard, so that electric machines of different capacities or different rated values can be directly compared through normalized values.

[0058] Proportional-Integral (PI) Controller: A commonly used closed-loop control strategy for error adjustment of a system. PI controller combines proportional control (P) and integral control (I) two control methods. In power systems, PI controller is often used to regulate active power and reactive power to maintain voltage and frequency stability. Specifically, PI controller compares the actual output of the system with the desired output (reference value), calculates the error value, and then generates control signals through proportional and integral calculations to adjust the behavior of the system to achieve the desired performance. This method can effectively eliminate steady-state error and reduce system oscillation, improve control stability and accuracy.

[0059] In LCC-HVDC systems, the converter valve is composed of multiple thyristors in series, which is used to realize the conversion of AC to DC and the inversion of DC to AC. However, due to the half-controlled characteristics of the thyristor, when the AC-DC system fails and the voltage drops, the thyristor may not meet the commutation conditions, causing commutation failure. Commutation failure refers to the state where the valve that has just exited conduction has not recovered the blocking capability and becomes conducting again. Commutation failure will cause the thyristor to fail to normally turn off, which may easily cause short circuit. If no effective suppression measures are taken after the first commutation failure, the short circuit of the thyristor may cause continuous commutation failure, which may eventually lead to the interruption of power transmission in the LCC-HVDC system. Therefore, how to control power to maintain the AC bus voltage and reduce the possibility of thyristor commutation failure, and improve the reliability and stability of the LCC-HVDC system has become a technical problem to be solved.

[0060] Because the LCC-HVDC system is vulnerable to commutation failure, and the VSC-HVDC technology using full-controlled devices can realize switching operation independent of the power grid, has voltage support function, and can effectively resist commutation failure due to its ability of independent control of active and reactive power, four-quadrant operation and flow reversal. Therefore, by connecting the LCC and the VSC in parallel, the active and reactive power independent control characteristics of the VSC can be used to provide reactive power support when the power grid is disturbed, and the ability of the LCC to resist commutation failure is enhanced. The embodiment of the application connects the MMC and the LCC in parallel to the power grid (i.e., the LCC-HVDC system and the MMC-HVDC system are connected in parallel to form an LCC-MMC parallel system), and designs a coordinated control strategy to stabilize the bus voltage during AC system faults, avoid commutation failure, and improve the power transmission capacity.

[0061] Reference is made to Figure 1 , Figure 1 is an optional flowchart of a power control method for an LCC-MMC parallel system provided by the embodiment of the application, the method is applied to an MMC inverter, Figure 1 The method in the step S101 to the step S108 can include but is not limited to the steps.

[0062] In the step S101, an inverter-side bus voltage value, an active power value output by the MMC inverter, and a reactive power value output by the MMC inverter are obtained.

[0063] In the step S102, a difference value between an inverter-side bus rated voltage value and the inverter-side bus voltage value is calculated to obtain a voltage drop value.

[0064] In the step S103, a numerical comparison between the voltage drop value and a preset threshold value is performed to obtain a first comparison result.

[0065] In the step S104, an active power reference value is selected from an active power data set based on the first comparison result.

[0066] In the step S105, a difference value between the active power reference value and the active power value is integrated to obtain an active power control component.

[0067] In the step S106, a difference value between the voltage drop value and the reactive power value is integrated to obtain a reactive power control component.

[0068] In the step S107, a current inverse transformation is performed on the active power control component and the reactive power control component to obtain an AC voltage reference value.

[0069] In the step S108, a pulse width modulation signal is generated based on the AC voltage reference value, and switching control is performed on the switching tubes of the MMC inverter according to the pulse width modulation signal.

[0070] The steps S101 to S108 shown in the embodiments of the present application, by comparing the voltage drop value of the inverter side bus voltage value and the inverter side bus rated voltage value with the preset threshold value, judging whether the LCC inverter fails to commutate according to the first comparison result; further, selecting the active power reference value expected to be output by the MMC inverter under the current state in the active power data set based on the state of the LCC inverter corresponding to the first comparison result; further, integrating the difference value of the active power reference value and the active power value to obtain the active power control component; integrating the difference value of the voltage drop value and the reactive power value to obtain the reactive power control component, so as to adjust the active power value and the reactive power value output by the MMC inverter respectively; further, performing current inverse transformation on the active power control component and the reactive power control component to obtain the alternating current voltage reference value, realizing the decoupling control of the active power and the reactive power; in this way, the pulse width modulation signal is generated based on the alternating current voltage reference value, so as to switch control the switching tube of the MMC inverter according to the pulse width modulation signal, so that the MMC inverter can actively inject reactive power into the inverter side bus and provide voltage support, which can effectively suppress the commutation failure caused by the voltage drop of the alternating current power grid and increase the stable operation ability of the system.

[0071] In step S101 of some embodiments, the inverter side bus voltage value can be a normalized value obtained by normalizing the amplitude of the inverter side bus, denoted as V rmsm . The active power value output by the MMC inverter can be a normalized value obtained by normalizing the measured and calculated alternating current side active power, denoted as P m . The reactive power value output by the MMC inverter can be a normalized value obtained by multiplying the reactive power normalized value of the MMC-HVDC system by a coefficient, denoted as Q pu ·K drp . Here, Q pu refers to the normalized value of the reactive power, and K drp is a coefficient related to the control of the reactive power, which is usually used to adjust the reactive power output to maintain the stability of the alternating current voltage. Wherein, the acquisition method of the inverter side bus voltage value V rmsm may be selected according to actual conditions and specific requirements, which is not limited here. Illustratively, the inverter side bus voltage value V rmsm may be obtained by directly measuring the alternating current bus voltage and normalizing it by installing a voltage sensor on the alternating current bus of the inverter. In addition, the acquisition method of the active power value P m output by the MMC inverter and the reactive power value Q pu ·K drp may be selected according to actual conditions, which is not limited here. For example, P m and Q pu ·Kdrp The voltage and current values at the output end of the MMC inverter can be measured by using professional equipment, and then the voltage and current values can be calculated and normalized to obtain the voltage and current values.

[0072] In step S102 of some embodiments, the inverter-side bus voltage value can be a normalized value of the inverter-side grid-side voltage effective value, denoted as V rmsr Specifically, the inverter-side bus voltage value is an important parameter for the design and operation of the inverter, and is usually determined by the manufacturer according to the specifications and performance of the inverter. The voltage drop value can be obtained by subtracting the inverter-side bus voltage value V rmsr from the inverter-side bus voltage value V rmsm Therefore, the voltage drop value can be used to detect the fault of the AC system and determine the voltage drop degree.

[0073] In step S103 of some embodiments, the preset threshold value is a value that is preset for comparison with the voltage drop value. The first comparison result can be a comparison result obtained by comparing the voltage drop value with the preset threshold value. Specifically, the preset threshold value can be determined by calculating the difference between the rated voltage value and the actual voltage value of the inverter-side bus when the LCC inverter fails to commutate. In this way, when the voltage drop value exceeds the preset threshold value, it can be considered that the LCC inverter fails to commutate, and the system will trigger the corresponding response or measure.

[0074] In step S104 of some embodiments, the active power data set can be a data set including a plurality of active power reference values that is constructed in advance. The active power reference value can be the active power value that the system expects the MMC inverter to output in the state represented by the first comparison result.

[0075] In some embodiments, the active power data set includes a first active power value and a second active power value, the first active power value being less than the second active power value. The process of selecting the active power reference value in the active power data set in step S104 can include the following two cases:

[0076] Case one: if the first comparison result indicates that the voltage drop value is greater than the preset threshold value, the first active power value is selected as the active power reference value; or

[0077] Case two: if the first comparison result indicates that the voltage drop value is less than or equal to the preset threshold value, the second active power value is selected as the active power reference value.

[0078] In scenario one, the initial active power value can be 0. When the voltage drop exceeds a preset threshold, a commutation failure is identified. The MMC inverter needs to output more reactive power to support the voltage on the inverter bus and maintain it at the desired level. Since the reactive power output capability of the MMC-HVDC system is limited by the active power transmission capacity, the active power output of the MMC inverter needs to be reduced to quickly increase reactive power. Therefore, 0 is chosen as the system's desired active power output value from the MMC inverter to enable it to output more reactive power.

[0079] For scenario two, the second active power value can be a per-unit value normalized to the rated power when the system has no commutation failure fault, denoted by P. r This indicates that when the voltage drop is less than or equal to the preset threshold, it is determined that no commutation failure has occurred, and therefore the original control value P of the MMC inverter output can continue to be maintained. r .

[0080] It should be noted that when the inverter side bus voltage value V rmsm When the voltage drop exceeds a preset threshold, maintaining the original control value of the MMC inverter output may not be sufficient to maintain the bus voltage for an LCC-MMC parallel system, thus failing to withstand commutation failure. Therefore, in this embodiment, upon detecting a commutation failure, the active power reference value of the MMC-HVDC system is set to 0. At this time, the MMC-HVDC system gradually stops generating active power while simultaneously outputting more reactive power, achieving the goal of rapidly increasing reactive power generation by the MMC inverter. Figure 2 As shown, the reactive power ΔQ generated by the MMC MMC It can simultaneously compensate reactive power to the LCC inverter, reducing the probability of commutation failure and improving system stability. Specifically, the reactive power ΔQ received by the LCC inverter... LCC +Reactive power ΔQ received by the receiving-end grid AC The sum of these and the reactive power ΔQ sent by the MMC inverter MMC + The reactive power ΔQ transmitted by the second filter C The sums are equal, i.e., ΔQ LCC +ΔQ AC =ΔQ MMC +ΔQ C .

[0081] In step S105 of some embodiments, the active power control component can be a control component generated by integrating the difference between the active power reference value and the active power value, denoted by I. drefThe difference integral can be performed by using the difference between the two values and integrating the difference. The difference integral can be performed by a PI controller. Illustratively, when the active power reference value is 0, the PI controller integrates the power difference (the active power reference value minus the active power value P m ) between 0 and the active power value P m , and selects the current reference value I dref of the d-axis in the rotating coordinate system as the active power control component to ensure that the active power changes in the expected manner.

[0082] In step S106 of some embodiments, the reactive power control component can be a control component generated by differentiating and integrating the opposite of the voltage drop value and the reactive power value, denoted as I qref In the MMC-HVDC system, the active power and the reactive power can be independently controlled, and the control is more flexible. The active power control component I dref and the reactive power control component I qref can enhance the flexibility of power regulation in the present scheme. It should be noted that the active power control component I dref and the reactive power control component I qref are both non-unity values.

[0083] In step S107 of some embodiments, the AC voltage reference value can be a value obtained by current inverse transformation of the active power control component and the reactive power control component. The current inverse transformation can be implemented by a current control loop and a Park inverse transformation. Specifically, by generating I dref and I qref , the MMC-HVDC system can achieve decoupled control of active power and reactive power. I dref and I qref After the current control loop and the Park inverse transformation, the AC voltage reference value required by the MMC inverter can be obtained, and then the pulse width modulation module (PWM module) is controlled to generate a pulse width modulation signal to control the switching tube in the MMC inverter.

[0084] In step S108 of some embodiments, the pulse width modulation signal can be a signal for switching control of the switching tube of the MMC inverter generated according to the AC voltage reference value. In this way, the AC voltage reference value can effectively control the PWM module to generate a corresponding pulse width modulation signal, and then control the MMC inverter to output the required AC voltage.

[0085] Referring to Figure 3 , in some embodiments, step S108 can include but is not limited to steps S301 to S302.

[0086] Step S301, a numerical comparison is performed between the AC voltage reference value and the inverter-side bus voltage value, and a second comparison result is obtained.

[0087] Step S302, a pulse width modulation signal is generated based on the second comparison result, and the switching tube of the MMC inverter is switched controlled according to the pulse width modulation signal.

[0088] In step S301 of some embodiments, the second comparison result can be a comparison result obtained by numerically comparing the AC voltage reference value and the inverter-side bus voltage value.

[0089] In step S302 of some embodiments, the pulse width modulation signal can be a signal for switching control of the switching tube of the MMC inverter generated according to the second comparison result.

[0090] In some embodiments, the process of generating the pulse width modulation signal based on the second comparison result in step S302 can include the following two cases:

[0091] Case one: if the second comparison result indicates that the AC voltage reference value is less than the inverter-side bus voltage value, a first pulse width modulation signal is generated; or,

[0092] Case two: if the second comparison result indicates that the AC voltage reference value is greater than or equal to the inverter-side bus voltage value, a second pulse width modulation signal is generated; wherein the duty cycle of the first pulse width modulation signal is less than the duty cycle of the second pulse width modulation signal.

[0093] For case one, the duty cycle refers to the proportion of the high level duration in the pulse width modulation signal (PWM signal) period. When the AC voltage reference value is less than the inverter-side bus voltage value, the first pulse width modulation signal can be a signal for reducing the duty cycle to reduce the output voltage of the MMC inverter.

[0094] For case two, when the AC voltage reference value is greater than or equal to the inverter-side bus voltage value, the second pulse width modulation signal can be a signal for increasing the duty cycle to increase the output voltage of the MMC inverter. After increasing the output voltage of the MMC inverter, the inverter-side bus voltage value will increase accordingly, achieving the purpose of maintaining the inverter-side bus AC voltage.

[0095] Please refer to Figure 4 , Figure 4is a module diagram of a power control circuit for an LCC-MMC parallel system provided by an embodiment of the present application. Specifically, the circuit comprises at least one LCC constituted LCC inverter 410, at least one MMC submodule constituted MMC inverter 420, inverter side bus 430 and receiving end power grid 440. Wherein, the MMC inverter 420 further comprises subtracter module 421, comparator module 422, selector module 423, active power integration module 424, reactive power integration module 425, current control loop module 426 and pulse width modulation module 427. Specifically, the AC side of the LCC inverter 410 is connected with one side of the AC side of the MMC inverter 420 in parallel, and the other side of the inverter side bus 430 is connected with the receiving end power grid 440; the subtracter module 421 is connected with one side of the inverter side bus 430, and the subtracter module 421 is used for difference calculation on the inverter side bus rated voltage value and the inverter side bus voltage value to obtain the voltage drop value; the comparator module 422 is connected with the subtracter module 421, and the comparator module 422 is used for numerical comparison on the voltage drop value and the preset threshold value to obtain the first comparison result; the selector module 423 is connected with the comparator module 422, and the selector module 423 is used for selecting the active power reference value in the active power data set based on the first comparison result; the active power integration module 424 is connected with the selector module 423, and the active power integration module 424 is used for difference integration on the active power reference value and the active power value to obtain the active power control component; the reactive power integration module 425 is connected with one side of the inverter side bus 430, and the reactive power integration module 425 is used for difference integration on the voltage drop value and the reactive power value to obtain the reactive power control component; the current control loop module 426 is connected with the active power integration module 424 and the reactive power integration module 425, and the current control loop module 426 is used for current inverse transformation on the active power control component and the reactive power control component to obtain the AC voltage reference value; the pulse width modulation module 427 is connected with the current control loop module 426, and the pulse width modulation module 427 is used for generating the pulse width modulation signal based on the AC voltage reference value to perform switching control on the switching tube of the MMC inverter 420 according to the pulse width modulation signal.

[0096] The voltage drop value of the inverter side bus voltage value and the inverter side bus rated voltage value is calculated by the subtracter module 421, then the voltage drop value is compared with the preset threshold value by the comparator module 422, and a first comparison result is generated; further, the selector module 423 judges whether the LCC inverter fails to commutate according to the first comparison result, and selects the active power reference value expected to be output by the MMC inverter 420 under the current state in the active power data set based on the state of the LCC inverter corresponding to the first comparison result; further, the active power integral module 424 integrates the difference between the active power reference value and the active power value, and obtains an active power control component; correspondingly, the reactive power integral module 425 integrates the difference between the voltage drop value and the reactive power value, and obtains a reactive power control component, so as to adjust the active power value and the reactive power value output by the MMC inverter 420 respectively; further, the current control loop module 426 performs current inverse transformation on the active power control component and the reactive power control component, and obtains an alternating voltage reference value, so as to realize decoupling control of the active power and the reactive power; in this way, the pulse width modulation module 427 generates a pulse width modulation signal based on the alternating voltage reference value, so as to switch control the switching tube of the MMC inverter 420 according to the pulse width modulation signal, so that the MMC inverter 420 can actively inject reactive power into the inverter side bus and provide voltage support, and the commutation failure caused by the AC grid voltage drop can be effectively inhibited, and the system stable operation capability can be increased.

[0097] It should be noted that when it is judged according to the first comparison result that the LCC inverter fails to commutate, the LCC-HVDC system keeps the turn-off angle higher than the minimum turn-off angle, and at the same time, the MMC-HVDC system is equipped with a PWM controller, and the reactive power control component is calculated by the PWM controller to control the MMC to output the reactive power.

[0098] It should be noted that the power control circuit for the LCC-MMC parallel system provided by the embodiment of the application can be embedded in the MMC inverter 420. The pulse width modulation module 427 includes a comparison unit and a generation unit (not shown in the figure), the comparison unit is connected with the current control loop module 426, and the comparison unit is used for numerical comparison of the alternating voltage reference value and the inverter side bus voltage value to obtain a second comparison result; the generation unit is connected with the comparison unit, and the generation unit is used for generating a pulse width modulation signal based on the second comparison result, so as to switch control the switching tube of the MMC inverter according to the pulse width modulation signal. Specifically, in order to achieve the control effect and purpose, the pulse width modulation signal can act on the switching tube of each bridge arm sub-module in the MMC and the twelve pulse thyristor converters in the LCC.

[0099] Please refer toFigure 5 , Figure 5 is a circuit diagram of a power control circuit for a LCC-MMC parallel system provided by an embodiment of the present application. The circuit further comprises an LCC rectifier 411 composed of at least one LCC, an MMC rectifier 428 composed of at least one MMC sub-module, and a rectifier-side bus 431, one side of the rectifier-side bus 431 being connected to the alternating current side of the LCC rectifier 411 and the alternating current side of the MMC rectifier 428 respectively, the other side of the rectifier-side bus 431 being connected to a sending end power grid 441, the direct current side of the LCC rectifier 411 being connected to the direct current side of an LCC inverter 410, and the direct current side of the MMC rectifier 428 being connected to the direct current side of an MMC inverter 420. The parallel operation of the LCC and the MMC can suppress circulating current, i.e. unnecessary current flow between the two converters, through a specific control strategy, so as to improve the utilization rate of the direct current bus and reduce energy loss. Further, the parallel operation of the LCC and the MMC also improves the voltage support capability: the MMC has good reactive power control capability and can provide fast reactive power support, and when it is operated in parallel with the LCC, the voltage support capability of the entire system can be enhanced.

[0100] Specifically, the subtracter module 421 comprises a subtracter, which subtracts the inverter-side bus voltage value V rmsr from the inverter-side bus rated voltage value V rmsm to obtain a voltage drop value; further, the comparator module 422 comprises a comparator, which compares the voltage drop value with a preset threshold to generate a first comparison result; further, the selector module 423 comprises a selector, which judges whether the LCC inverter has a commutation failure fault according to the first comparison result, and selects the active power reference value (0 or P r ) expected to be output by the MMC inverter 420 under the current state in the active power data set based on the state of the LCC inverter corresponding to the first comparison result; the active power integral module 424 comprises a subtracter and a PI controller, wherein the subtracter subtracts the active power reference value from the active power value P m , and the PI controller then integrates the difference to obtain an active power control component I dref .

[0101] Correspondingly, the reactive power integral module 425 comprises an adder, a subtracter and a PI controller, wherein the adder adds the alternating current voltage reference value V acref (which is a per unit value and has the same meaning as the inverter-side bus rated voltage value V rmsr ) and the reactive power value Q pu ·K drp , i.e. V rmsr + Q pu ·K drp), after which the subtracter will inverse side bus voltage value V rmsm Subtract the aforementioned addition value (V rmsr +Q pu ·K drp ), that is, can be combined as (V rmsm -V rmsr )-Q pu ·K drp , wherein (V rmsm -V rmsr ) is the opposite number of voltage drop value, and finally the PI controller carries out difference integration on the difference between the opposite number of voltage drop value and the reactive power value, and obtains the reactive power control component I qref The active power integral module 424 and the reactive power integral module 425 can realize the adjustment of the active power value and the reactive power value output by the MMC inverter 420 respectively.

[0102] It should be noted that the MMC-HVDC inverter side adopts fixed AC voltage control, when the reactive power integral module 425 detects that the AC voltage reference value V acref changes, the current control loop module 426 and the pulse width modulation module 427 will increase the reactive power to maintain the AC bus voltage, in the LCC-MMC parallel system, the fixed AC voltage control strategy of the MMC-HVDC system has certain ability to reduce the risk of commutation failure fault.

[0103] It should be noted that the circuit further comprises a first converter transformer 463 and a second converter transformer 460, one side of the rectifier side bus 431 is connected with the AC side of the LCC rectifier 411 through the first converter transformer 463, and the AC side of the LCC inverter 410 is connected with one side of the inverter side bus 430 through the second converter transformer 460. The first converter transformer 463 and the second converter transformer 460 can be converter transformers. In addition to transmitting power and converting the AC side operating voltage into the commutation voltage of the converter, the converter transformer can also provide two groups of three-phase symmetrical commutation voltages with equal amplitude and 30° phase difference for the two converters in series to realize twelve-pulse commutation. This configuration helps to reduce the generation of harmonics and improve the operation efficiency and reliability of the system.

[0104] It should be noted that the circuit further comprises a third converter transformer 462 and a fourth converter transformer 461, one side of the rectifier side bus 431 is connected with the AC side of the MMC rectifier 428 through the third converter transformer 462, and the AC side of the MMC inverter 420 is connected with one side of the inverter side bus 430 through the fourth converter transformer 461. The third converter transformer 462 and the fourth converter transformer 461 can be converter transformers, which can transform the AC system voltage to the commutation voltage required by the converter to adapt to the working requirements of the converter.

[0105] It should be noted that the circuit further comprises a first filter 451 and a second filter 450, one side of the first filter 451 is connected with the other side of the rectifier side bus 431, and the other side of the first filter 451 is grounded; one side of the second filter 450 is connected with the other side of the inverter side bus 430, and the other side of the second filter 450 is grounded.

[0106] Please refer to Figure 6A , 6B and 6C, Figure 6A show the change curves of AC voltage-time and DC voltage-time when the commutation failure occurs in the LCC-HVDC system, from Figure 6A It can be seen that the change amplitudes of the AC voltage and the DC voltage are large in the interval from the fault occurrence to the fault recovery, the waveform of the AC voltage-time curve is distorted obviously after the fault occurs, and the recovery time of the system is 0.2 seconds (s) after the fault occurs. Figure 6B show the change curves of AC voltage-time and DC voltage-time when the commutation failure occurs in the LCC-MMC parallel system. Comparing the curves in Figure 6A and Figure 6B , it can be seen that Figure 6B the change amplitudes of the AC voltage and the DC voltage are greatly reduced, and the waveform of the AC voltage-time curve is not distorted after the fault occurs, and the recovery time of the system is about 0.1 s, which realizes fast recovery, that is, the power control method for the LCC-MMC parallel system provided by the embodiment of the present application successfully resists the commutation failure. Figure 6C show the power-time curves when the commutation failure occurs in the LCC-HVDC system and the LCC-MMC parallel system. From Figure 6CIt can be seen that when the power control method of the embodiment of the present application is not used, that is, when the commutation failure fault occurs in the LCC-HVDC system, the active power value output by the LCC-HVDC system is almost unchanged, and the reactive power value is increased by about 50 megawatts (MW). However, after the fault occurs, the active power value output by the LCC-MMC parallel system is greatly reduced, and the reactive power value is increased by about 100 megawatts (MW). Within 1s after the commutation failure fault occurs, the reactive power output by the LCC-MMC parallel system is greatly increased compared with the reactive power output by the LCC-HVDC system, which proves that the power control method for the LCC-MMC parallel system provided by the embodiment of the present application can improve the reactive power output capability. Figure 6A 、 6B and 6C demonstrate the ability of the system to resist commutation failure under the condition of no additional control after the fault occurs and after the additional control is added, and at the same time demonstrate the improvement of the power control method for the LCC-MMC parallel system provided by the embodiment of the present application to the reactive power output capability.

[0107] A person of ordinary skill in the art can understand that all or part of the processes in the above-mentioned embodiment methods can be completed by a computer program instructing related hardware, and the program can be stored in a computer readable storage medium. When the program is executed, it can include the processes of the above-mentioned embodiment methods. The storage medium can be a magnetic disc, an optical disc, a read-only memory (ROM) or a random access memory (RAM), etc.

[0108] The above is the preferred embodiment of the present application. It should be noted that for those of ordinary skill in the art, without departing from the principles of the present application, a number of improvements and refinements can be made, and these improvements and refinements are also considered within the protection scope of the present application.

Claims

1. A power control method for an LCC-MMC parallel system, characterized in that, Applications in MMC inverters include: Obtain the inverter-side bus voltage value, the active power value output by the MMC inverter, and the reactive power value output by the MMC inverter; The voltage sag value is obtained by calculating the difference between the rated voltage value of the inverter side bus and the voltage value of the inverter side bus. A first comparison result is obtained by comparing the voltage drop value with a preset threshold. Based on the first comparison result, select an active power reference value from the active power dataset; The active power control component is obtained by integrating the difference between the active power reference value and the active power value. The reactive power control component is obtained by integrating the difference between the voltage drop value and the reactive power value. The active power control component and the reactive power control component are subjected to inverse current transformation to obtain an AC voltage reference value. A pulse width modulation signal is generated based on the AC voltage reference value, and the switching transistors of the MMC inverter are controlled to switch on and off according to the pulse width modulation signal.

2. The method as described in claim 1, characterized in that, The step of generating a pulse width modulation signal based on the AC voltage reference value, and controlling the switching transistors of the MMC inverter according to the pulse width modulation signal, includes: A second comparison result is obtained by numerically comparing the AC voltage reference value and the inverter side bus voltage value. A pulse width modulation signal is generated based on the second comparison result, and the switching transistors of the MMC inverter are switched according to the pulse width modulation signal.

3. The method as described in claim 2, characterized in that, The generation of the pulse width modulation signal based on the second comparison result includes: If the second comparison result indicates that the AC voltage reference value is less than the inverter-side bus voltage value, a first pulse width modulation signal is generated; or... If the second comparison result indicates that the AC voltage reference value is greater than or equal to the inverter side bus voltage value, a second pulse width modulation signal is generated; wherein, the duty cycle of the first pulse width modulation signal is less than the duty cycle of the second pulse width modulation signal.

4. The method as described in claim 1, characterized in that, The active power dataset includes a first active power value and a second active power value, wherein the first active power value is less than the second active power value. The step of selecting an active power reference value from the active power dataset based on the first comparison result includes: If the first comparison result indicates that the voltage drop value is greater than the preset threshold, the first active power value is used as the active power reference value; or... If the first comparison result indicates that the voltage drop value is less than or equal to the preset threshold, the second active power value is used as the active power reference value.

5. A power control circuit for an LCC-MMC parallel system, characterized in that, The circuit includes an LCC inverter composed of at least one LCC, an MMC inverter composed of at least one MMC submodule, and an inverter-side bus. The AC side of the LCC inverter and the AC side of the MMC inverter are connected in parallel and then connected to one side of the inverter-side bus. The other side of the inverter-side bus is connected to the receiving-end power grid. The MMC inverter further includes: The subtractor module is connected to one side of the inverter-side bus. The subtractor module is used to calculate the difference between the rated voltage value of the inverter-side bus and the voltage value of the inverter-side bus to obtain the voltage drop value. A comparator module is connected to the subtractor module. The comparator module is used to compare the voltage drop value with a preset threshold value to obtain a first comparison result. The selector module is connected to the comparator module, and the selector module is used to select an active power reference value from the active power dataset based on the first comparison result. An active power integration module is connected to the selector module. The active power integration module is used to integrate the difference between the active power reference value and the active power value to obtain the active power control component. A reactive power integration module is connected to one side of the inverter side bus. The reactive power integration module is used to integrate the difference between the voltage drop value and the reactive power value to obtain the reactive power control component. A current control loop module is connected to the active power integration module and the reactive power integration module. The current control loop module is used to perform inverse current transformation on the active power control component and the reactive power control component to obtain an AC voltage reference value. A pulse width modulation module is connected to the current control loop module. The pulse width modulation module is used to generate a pulse width modulation signal based on the AC voltage reference value, so as to control the switching transistors of the MMC inverter according to the pulse width modulation signal.

6. The circuit as described in claim 5, characterized in that, The pulse width modulation module includes: A comparison unit is connected to the current control loop module. The comparison unit is used to compare the AC voltage reference value and the inverter side bus voltage value to obtain a second comparison result. A generation unit, connected to the comparison unit, is used to generate a pulse width modulation signal based on a second comparison result, so as to control the switching transistors of the MMC inverter according to the pulse width modulation signal.

7. The circuit as described in claim 5, characterized in that, The circuit also includes an LCC rectifier composed of at least one LCC, an MMC rectifier composed of at least one MMC submodule, and a rectifier-side bus. One side of the rectifier-side bus is connected to the AC side of the LCC rectifier and the AC side of the MMC rectifier, respectively. The other side of the rectifier-side bus is connected to the sending-end power grid. The DC side of the LCC rectifier is connected to the DC side of the LCC inverter, and the DC side of the MMC rectifier is connected to the DC side of the MMC inverter.

8. The circuit as described in claim 7, characterized in that, The circuit also includes a first converter transformer and a second converter transformer. One side of the rectifier bus is connected to the AC side of the LCC rectifier through the first converter transformer, and the AC side of the LCC inverter is connected to one side of the inverter bus through the second converter transformer.

9. The circuit as described in claim 8, characterized in that, The circuit also includes a third converter transformer and a fourth converter transformer. One side of the rectifier bus is connected to the AC side of the MMC rectifier through the third converter transformer, and the AC side of the MMC inverter is connected to one side of the inverter bus through the fourth converter transformer.

10. The circuit as described in claim 7, characterized in that, The circuit further includes a first filter and a second filter. One side of the first filter is connected to the other side of the rectifier bus, and the other side of the first filter is grounded. One side of the second filter is connected to the other side of the inverter bus, and the other side of the second filter is grounded.

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