Power control method and circuit for LCC-MMC parallel system

Abstract

The invention 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 comprises the following steps: obtaining an inverter side bus voltage value, an active power value and a reactive power value; calculating the difference between the rated voltage value of the inverter side bus and the voltage value of the inverter side bus to obtain a voltage drop 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 AC voltage reference value; and generating a pulse width modulation signal based on the AC voltage reference value so as to perform switching control on a switching tube of the MMC inverter. According to the invention, power can be effectively controlled to maintain AC bus voltage.

Description

Technical Field

[0001] The present invention relates to the field of power electronics technology, and in particular to a power control method and circuit for an LCC-MMC parallel system. Background Art

[0002] Line Commutated Converter High Voltage Direct Current (LCC-HVDC) system is an important part of the HVDC transmission system. In the LCC-HVDC system, the converter valve is composed of multiple thyristors connected in series, which are used to realize the conversion from AC to DC and the inversion process from DC to AC. However, due to the semi-controlled characteristics of the thyristors in the system, when the AC / DC system fails and the voltage drops, it may not meet the commutation conditions, thereby causing commutation failure. Commutation failure refers to the valve that has just exited conduction and has not yet restored its blocking ability and has become conductive again. Commutation failure will cause the thyristor to fail to shut down normally, which is prone to short circuit. If effective suppression measures are not 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 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 that needs to be solved urgently. Summary of the invention

[0003] The purpose of the embodiments of the present invention is to provide a power control method and circuit for an LCC-MMC parallel system, which can effectively control 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 object, an embodiment of the present invention provides a power control method for an LCC-MMC parallel system, which is applied to an MMC inverter and includes:

[0005] Obtaining 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;

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

[0007] Compare 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 difference between the active power reference value and the active power value to obtain an active power control component;

[0010] Integrating the difference between 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 AC voltage reference value;

[0012] A pulse width modulation signal is generated based on the AC voltage reference value, so as to control the switching of the switch tube of the MMC inverter according to the pulse width modulation signal.

[0013] Compared with the prior art, the power control method for an LCC-MMC parallel system disclosed in the present invention obtains a voltage drop value of an inverter-side bus voltage value and an inverter-side bus rated voltage value by first calculating, and then compares the voltage drop value with a preset threshold value to obtain a first comparison result to determine whether a thyristor in the LCC-HVDC system has a commutation fault; further, based on the first comparison result, a suitable active power reference value is selected from an active power data set; further, the active power reference value and the active power value are differentially 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 differentially integrated to obtain a reactive power control component to adjust the active power value output by the MMC inverter. The output reactive power value is adjusted; further, the current is reversely transformed according to the active power control component and the reactive power control component to obtain an AC voltage reference value. The present invention adopts a decoupling control method to achieve independent regulation of active power and reactive power control; in this way, a pulse width modulation signal is generated based on the AC voltage reference value, and the switch tube of the MMC inverter is switched according to the pulse width modulation signal, so that the technical effect of adjusting the inverter side bus voltage can be achieved, and the problem that the thyristor in the prior art is easily affected by the voltage drop and causes the commutation failure is solved, and the active power and reactive power are coordinated to control 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.

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

[0015] Performing numerical comparison on the AC voltage reference value and the inverter-side bus voltage value to obtain a second comparison result;

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

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

[0018] 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,

[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, a second pulse width modulation signal is generated; wherein the duty cycle of the first pulse width modulation signal is smaller than the duty cycle of the second pulse width modulation signal.

[0020] 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 is less than the second active power value, and selecting an active power reference value in the active power data set based on the first comparison result includes:

[0021] 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,

[0022] 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.

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

[0024] A subtractor module is connected to one side of the inverter-side bus, and 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 a voltage drop value;

[0025] A comparator module, connected to the subtractor module, and configured to perform a numerical comparison between the voltage drop value and a preset threshold value to obtain a first comparison result;

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

[0027] An active power integration module, connected to the selector module, and configured to perform a difference integration on the active power reference value and the active power value to obtain an active power control component;

[0028] A reactive power integration module is connected to one side of the inverter-side busbar, and is used to perform a difference integration on the voltage drop value and the reactive power value to obtain a reactive power control component;

[0029] A current control loop module, connected to the active power integration module and the reactive power integration module, the current control loop module is used 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 is connected to the current control loop module, and is used to generate a pulse width modulation signal based on the AC voltage reference value, so as to control the switch tube of the MMC inverter according to the pulse width modulation signal.

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

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

[0033] A generating unit is connected to the comparing unit, and is used to generate a pulse width modulation signal based on the second comparison result, so as to control the switching tube of the MMC inverter according to the pulse width modulation signal.

[0034] In some embodiments, the circuit further includes an LCC rectifier composed of at least one LCC, an MMC rectifier composed of at least one MMC sub-module, and a rectifier-side bus, one side of the rectifier-side bus is respectively connected to the AC side of the LCC rectifier and the AC side of the MMC rectifier, the other side of the rectifier-side bus is connected to a 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.

[0035] In some embodiments, the circuit further includes a first converter transformer and a second converter transformer, one side of the rectifier-side 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-side bus through the second converter transformer.

[0036] In some embodiments, the circuit further includes a third converter transformer and a fourth converter transformer, one side of the rectifier-side 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-side bus through the fourth converter transformer.

[0037] In some embodiments, the circuit further includes a first filter and a second filter, wherein one side of the first filter is connected to 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 to the other side of the inverter side bus, and the other side of the second filter is grounded. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 is a flow chart of a power control method for an LCC-MMC parallel system according to an embodiment of the present invention;

[0039] Figure 2 is a schematic diagram of reactive power flow in an embodiment of the present invention;

[0040] Figure 3 yes Figure 1 A flowchart of step S108 in FIG.

[0041] Figure 4 A schematic diagram of a module structure of a power control circuit for an LCC-MMC parallel system according to an embodiment of the present invention

[0042] Figure 5 The present invention is a schematic diagram of a circuit structure of a power control circuit for an LCC-MMC parallel system according to an embodiment of the present invention.

[0043] Fig. 6A It is a schematic diagram of a curve of changes of AC voltage-time and DC voltage-time of an LCC-HVDC system in an embodiment of the present invention;

[0044] Figure 6B It is a schematic diagram of a curve of changes of AC voltage-time and DC voltage-time of an LCC-MMC parallel system in an embodiment of the present invention;

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

[0046] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0047] In the description of the present application, it should be understood that the terms "center", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", etc., indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the present application.

[0048] The terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of the features. In the description of this application, unless otherwise specified, "plurality" means two or more.

[0049] In the description of this application, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in this application can be understood according to specific circumstances.

[0050] First, some nouns involved in this application are analyzed:

[0051] Line Commutated Converter (LCC): A commutation technology used in high-voltage direct current (HVDC) transmission systems that relies on the AC voltage of the grid to control the on and off of the thyristors in the converter, thereby achieving conversion between AC and DC.

[0052] High Voltage Direct Current (HVDC): is a power transmission technology that converts AC power into DC power for long-distance transmission and then converts it back into AC power 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 commutation devices. The LCC-HVDC system converts AC power into DC power through a rectifier station, then transmits it through a DC transmission line, and finally converts DC power back to AC power at an inverter station.

[0054] Line-commutation: refers to the process of transferring the current flowing through the converter from one current path to another by opening and closing the converter valve. In the converter, if the valve that has exited conduction fails to restore its blocking capacity within a period of time under the action of reverse voltage, this is called commutation failure.

[0055] Modular Multilevel Converter (MMC): It is an important technology in the field of high-voltage direct current (HVDC) transmission that inserts an inductor between each module to achieve multi-level voltage smoothing, and controls the switching state of each module through high-frequency carrier phase-shift modulation technology to achieve output voltage control. MMC has the characteristics of modular design, multi-level structure, high voltage capacity, etc., 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: is a high voltage direct current transmission technology based on voltage source converter (VSC). The MMC-HVDC system adopts a modular design and is composed of multiple cascaded sub-modules. Each sub-module contains several half-bridge or full-bridge arms. By accurately controlling the switching state of each sub-module, the output voltage can be accurately controlled.

[0057] Per-unit normalization: is a method of converting the actual parameters of a power system or motor (such as voltage, current, power, impedance, etc.) into dimensionless relative values. Per-unit normalization involves dividing the actual value of a physical quantity by a selected value of the same unit, which is called the reference value. Per-unit normalization eliminates the conversion between units, and all values ​​are normalized to a unified standard, so that motors of different capacities or different ratings can be directly compared through the per-unit value.

[0058] Proportional-Integral (PI) controller: It is a commonly used closed-loop control strategy for achieving error regulation of the system. The PI controller combines proportional control (P) and integral control (I). In power systems, PI controllers are often used to adjust active power and reactive power to maintain voltage and frequency stability. Specifically, the PI controller calculates the error value by comparing the actual output of the system with the expected output (reference value), and then generates a control signal through the calculation of the proportional term and the integral term to adjust the behavior of the system to achieve the desired performance. This method can effectively eliminate steady-state errors, reduce system oscillations, and improve control stability and accuracy.

[0059] In the LCC-HVDC system, the commutation valve is composed of multiple thyristors connected in series, which are used to realize the conversion from AC to DC and the inversion process from DC to AC. However, due to the semi-controlled characteristics of the thyristors in the system, when the AC / DC system fails and the voltage drops, it may not meet the commutation conditions, thereby causing commutation failure. Commutation failure refers to the valve that has just exited conduction and has not yet restored its blocking ability and has become conductive again. Commutation failure will cause the thyristor to fail to shut down normally, which is prone to short circuit. If effective suppression measures are not 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 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 that needs to be solved urgently.

[0060] Because the LCC-HVDC system is susceptible to commutation failure problems, the VSC-HVDC technology using fully controlled devices can independently control active and reactive power, four-quadrant operation and flow reversal capabilities, can achieve switching operations without relying on the power grid, has a voltage support function, and can effectively resist commutation failures. Therefore, by connecting the LCC and the VSC in parallel, and utilizing the VSC's independent active and reactive power control characteristics, reactive support can be provided during grid disturbances, thereby enhancing the LCC's ability to resist commutation failures. In an embodiment of the present invention, the MMC and LCC are connected to the power grid in parallel (i.e., the LCC-HVDC system and the MMC-HVDC system are connected in parallel to form an LCC-MMC parallel system), and a coordinated control strategy is designed to stabilize the bus voltage when the AC system fails, avoid commutation failures, and increase the transmission capacity.

[0061] See also Figure 1 , Figure 1 is an optional flow chart of a power control method for an LCC-MMC parallel system provided by an embodiment of the present invention, the method is applied to an MMC inverter, Figure 1 The method may include but is not limited to steps S101 to S108.

[0062] Step S101, obtaining 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;

[0063] Step S102, calculating the difference between the rated voltage value of the inverter-side bus and the voltage value of the inverter-side bus to obtain a voltage drop value;

[0064] Step S103, performing a numerical comparison between the voltage drop value and a preset threshold value to obtain a first comparison result;

[0065] Step S104, selecting an active power reference value from the active power data set based on the first comparison result;

[0066] Step S105, integrating the difference between the active power reference value and the active power value to obtain an active power control component;

[0067] Step S106, integrating the difference between the voltage drop value and the reactive power value to obtain a reactive power control component;

[0068] Step S107, performing current inverse transformation on the active power control component and the reactive power control component to obtain an AC voltage reference value;

[0069] Step S108, generating a pulse width modulation signal based on the AC voltage reference value, so as to control the switching of the switch tube of the MMC inverter according to the pulse width modulation signal.

[0070] In steps S101 to S108 shown in the embodiment of the present invention, by comparing the voltage drop value of the inverter-side bus voltage value and the rated voltage value of the inverter-side bus voltage value with a preset threshold value, it is determined whether the LCC inverter has a commutation failure fault according to a first comparison result; further, based on the state of the LCC inverter corresponding to the first comparison result, an active power reference value expected to be output by the MMC inverter in the current state is selected from the active power data set; further, the active power reference value and the active power value are differentially integrated to obtain an active power control component; the voltage drop value and the reactive power value are differentially integrated to obtain a reactive power value. The active power control component is used to adjust the active power value and reactive power value output by the MMC inverter respectively; further, the active power control component and the reactive power control component are subjected to current inverse transformation to obtain an AC voltage reference value, thereby realizing the decoupling control of active power and reactive power; thus, a pulse width modulation signal is generated based on the AC voltage reference value, and the switch tube of the MMC inverter is controlled according to the pulse width modulation signal, so that the MMC inverter can actively inject reactive power into the inverter-side bus to provide voltage support, which can effectively suppress the commutation failure caused by the voltage drop of the AC power grid and increase the stable operation capability of the system.

[0071] In step S101 of some embodiments, the inverter-side bus voltage value may be a per-unit value obtained by normalizing the grid amplitude of the inverter-side bus, denoted by V rmsm The active power value output by the MMC inverter can be the per-unit value obtained by normalizing the measured and calculated AC side active power, expressed as P m The reactive power value output by the MMC inverter can be the reactive power per unit value of the MMC-HVDC system multiplied by the coefficient, expressed as Q pu ·K drp Here Q pu Refers to the per unit value of reactive power, and K drp It is a coefficient related to reactive power control and is usually used to adjust reactive power output to maintain the stability of AC voltage. rmsm The acquisition method of can be selected according to the actual situation and specific needs, and is not limited here. For example, the inverter side bus voltage value V rmsm The voltage of the AC bus can be directly measured by installing a voltage sensor on the AC bus of the inverter and then normalized. In addition, the active power value P output by the MMC inverter is m The reactive power value Q output by the MMC inverter pu ·K drp The method of obtaining P can be selected according to the actual situation and is not limited here. m With Q pu ·Kdrp Both can be obtained by measuring the voltage and current values ​​at the output of the MMC inverter using professional equipment, and then calculating and normalizing them.

[0072] In step S102 of some embodiments, the rated voltage value of the inverter-side busbar may be a per-unit value obtained by normalizing the effective value of the rated voltage of the inverter-side grid-side, and V rmsr Specifically, the rated voltage of the inverter-side busbar is an important parameter for the design and operation of the inverter, and is usually determined by the manufacturer based on the specifications and performance of the inverter. The voltage drop value can be calculated by calculating the difference between the rated voltage of the inverter-side busbar and the voltage of the inverter-side busbar, that is, the rated voltage of the inverter-side busbar V rmsr Subtract the inverter side bus voltage value V rmsm The obtained value, so the voltage drop value can be used to detect AC system faults and determine the degree of voltage drop.

[0073] In step S103 of some embodiments, the preset threshold is a numerical value pre-set for comparison with the voltage drop value. The first comparison result may be a comparison result obtained by numerically comparing the voltage drop value with the preset threshold. Specifically, the preset threshold may be determined by calculating the difference between the rated voltage value and the actual voltage value of the inverter-side bus when a commutation failure occurs in the LCC inverter. Thus, when the voltage drop value exceeds the preset threshold, it can be considered that a commutation failure occurs in the LCC inverter, and the system will trigger a corresponding response or measure.

[0074] In step S104 of some embodiments, the active power data set may be a pre-built data set including a plurality of active power reference values. The active power reference value may be an 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 is less than the second active power value, and the process of selecting the active power reference value in the active power data set in step S104 may include the following two cases:

[0076] Case 1: 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,

[0077] Case 2: 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.

[0078] For case 1, the first active power value can be 0. When the voltage drop value is greater than the preset threshold, it is determined that a commutation failure occurs at this time, and the MMC inverter needs to output more reactive power through control to provide support for the voltage of the inverter-side busbar and help maintain the voltage at the desired level. Because the reactive output capacity of the MMC-HVDC system is limited by the active transmission power, in order to quickly increase the reactive power at this time, it is necessary to reduce the active power value output by the MMC inverter. Therefore, 0 is selected as the active power value that the system expects the MMC inverter to output so that the MMC inverter can output more reactive power values.

[0079] For case 2, the second active power value can be the per-unit value of the rated power when the system has no commutation failure fault, and P r When the voltage drop value is less than or equal to the preset threshold, it is determined that no commutation failure occurs at this time, so 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 drops and the voltage drop value is greater than the preset threshold, if the original control value output by the MMC inverter is maintained, for the LCC-MMC parallel system, the reactive power generated by the MMC may not be sufficient to maintain the bus voltage and thus cannot resist the occurrence of a commutation failure. Therefore, when a commutation failure is detected, the embodiment of the present invention sets the active power reference value of the MMC-HVDC system to 0. At this time, the MMC-HVDC system gradually stops generating active power and can output more reactive power, thereby achieving the purpose of rapidly increasing reactive power by the MMC inverter. Figure 2 As shown, the reactive power ΔQ generated by MMC MMC It can simultaneously compensate reactive power to the LCC inverter, reduce the probability of commutation failure, and improve system stability. LCC +Reactive power ΔQ received by the receiving grid AC The sum of the reactive power ΔQ sent by the MMC inverter MMC + Reactive power ΔQ sent by the second filter C The sum is equal to ΔQ LCC +ΔQ AC =ΔQ MMC +ΔQ C .

[0081] In step S105 of some embodiments, the active power control component may be a control component generated by integrating the difference between the active power reference value and the active power value, and is represented by I drefThe difference integration may be performed by using the difference between the two values ​​and integrating the difference, and the difference integration may be implemented by a PI controller. For example, when the active power reference value is 0, the PI controller integrates the active power value P m The power difference between the active power reference value and the active power value P m ) to perform integration, and select the current reference value I of the d-axis in the rotating coordinate system in the integration result dref As an active power control component, it ensures that the active power changes in the expected manner.

[0082] In step S106 of some embodiments, the reactive power control component may be a control component generated by integrating the difference between the opposite of the voltage drop value and the reactive power value, and the reactive power value may be represented by I qref In the MMC-HVDC system, active power and reactive power can be controlled independently, which is more flexible. The active power control component I dref and reactive power control component I qref The power regulation flexibility in this solution can be enhanced. It should be noted that the active power control component I dref and reactive power control component I qref All are non-standard values.

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

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

[0085] See also Figure 3 In some embodiments, step S108 may include but is not limited to steps S301 to S302.

[0086] Step S301, performing numerical comparison between the AC voltage reference value and the inverter-side bus voltage value to obtain a second comparison result;

[0087] Step S302: Generate a pulse width modulation signal based on the second comparison result, so as to control the switching of the switch tube of the MMC inverter according to the pulse width modulation signal.

[0088] In step S301 of some embodiments, the second comparison result may 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 may be a signal for performing switching control on a switch tube of the MMC inverter generated according to the second comparison result.

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

[0091] Case 1: 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 2: 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 smaller than the duty cycle of the second pulse width modulation signal.

[0093] For situation 1, the duty cycle refers to the proportion of the high level duration in the pulse width modulation signal (PWM signal) cycle. 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 that reduces the duty cycle to reduce the output voltage of the MMC inverter.

[0094] For situation 2, 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] See also Figure 4 , Figure 4It is a block diagram of a power control circuit for an LCC-MMC parallel system provided by an embodiment of the present invention. Specifically, the circuit includes an LCC inverter 410 composed of at least one LCC, an MMC inverter 420 composed of at least one MMC submodule, an inverter-side bus 430, and a receiving-end power grid 440. Among them, the MMC inverter 420 also includes a subtractor module 421, a comparator module 422, a selector module 423, an active power integration module 424, a reactive power integration module 425, a current control loop module 426, and a pulse width modulation module 427. Specifically, the AC side of the LCC inverter 410 is connected in parallel with the AC side of the MMC inverter 420 and then connected to one side of the inverter-side bus 430, and the other side of the inverter-side bus 430 is connected to the receiving-end power grid 440; the subtractor module 421 is connected to one side of the inverter-side bus 430, and the subtractor module 421 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 a voltage drop value; the comparator module 422 is connected to the subtractor module 421, and the comparator module 422 is used to perform a numerical comparison between the voltage drop value and a preset threshold value to obtain a first comparison result; the selector module 423 is connected to the comparator module 422, and the selector module 423 is used to select an active power reference value in the active power data set based on the first comparison result; the active power integration module 424 is connected to the selector module 423, and there is The active power integration module 424 is used to perform a difference integration between the active power reference value and the active power value to obtain an active power control component; the reactive power integration module 425 is connected to one side of the inverter-side bus 430, and the reactive power integration module 425 is used to perform a difference integration between the voltage drop value and the reactive power value to obtain a reactive power control component; the current control loop module 426 is connected to the active power integration module 424 and the reactive power integration module 425, and the current control loop module 426 is used to perform a current inverse transformation on the active power control component and the reactive power control component to obtain an AC voltage reference value; the pulse width modulation module 427 is connected to the current control loop module 426, and the pulse width modulation module 427 is used to generate a pulse width modulation signal based on the AC voltage reference value, so as to control the switching tube of the MMC inverter 420 according to the pulse width modulation signal.

[0096] In the embodiment of the present invention, the subtractor module 421 calculates the voltage drop value of the inverter-side bus voltage value and the inverter-side bus rated voltage value, and then compares the voltage drop value with a preset threshold value through the comparator module 422 to generate a first comparison result; further, the selector module 423 determines whether a commutation failure occurs in the LCC inverter according to the first comparison result, and selects the active power reference value expected to be output by the MMC inverter 420 in the current state from the active power data set based on the state of the LCC inverter corresponding to the first comparison result; further, the active power integration module 424 integrates the difference between the active power reference value and the active power value to obtain an active power control component; correspondingly, the reactive power integration module 425 integrates the voltage drop value and the reactive power value. The active power value is differentially integrated 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 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 to obtain an AC voltage reference value, thereby realizing the decoupling control of the active power and the reactive power; thus, the pulse width modulation module 427 generates a pulse width modulation signal based on the AC voltage reference value, so as to 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, which can effectively suppress the commutation failure caused by the voltage drop of the AC power grid and increase the stable operation capability of the system.

[0097] It should be noted that, after the LCC inverter is judged to have a commutation failure according to the first comparison result, the LCC-HVDC system maintains a shutdown angle higher than the minimum shutdown angle. At the same time, the MMC-HVDC system is equipped with a PWM controller in the MMC-HVDC system according to the voltage drop value and the active power transmitted by the LCC inverter at the time of the fault, and the reactive power emitted by the MMC is controlled by calculating the reactive power through the PWM controller.

[0098] It should be noted that a power control circuit for an LCC-MMC parallel system provided in an embodiment of the present invention can be embedded in an 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 to the current control loop module 426, and the comparison unit is used to perform a numerical comparison between the AC voltage reference value and the inverter-side bus voltage value to obtain a second comparison result; the generation unit is connected to the comparison unit, and the generation unit is used to generate a pulse width modulation signal based on the second comparison result to control the switch 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 switch tubes on each bridge arm submodule in the MMC and the twelve-pulse thyristor converter in the LCC.

[0099] See also Figure 5 , Figure 5 It is a circuit diagram of a power control circuit for an LCC-MMC parallel system provided by an embodiment of the present invention. The circuit also includes an LCC rectifier 411 composed of at least one LCC, an MMC rectifier 428 composed of at least one MMC submodule, and a rectifier-side bus 431, one side of the rectifier-side bus 431 is respectively connected to the AC side of the LCC rectifier 411 and the AC side of the MMC rectifier 428, the other side of the rectifier-side bus 431 is connected to the sending-end power grid 441, the DC side of the LCC rectifier 411 is connected to the DC side of the LCC inverter 410, and the DC side of the MMC rectifier 428 is connected to the DC side of the MMC inverter 420. The LCC and MMC operating in parallel can suppress the circulating current, that is, the unnecessary current flow between the two converters, through a specific control strategy, so that the utilization rate of the DC bus can be improved and the energy loss can be reduced. Furthermore, the parallel operation of LCC and MMC also improves the voltage support capability: MMC has good reactive power control capability and can provide fast reactive power support. When operating in parallel with LCC, it can enhance the voltage support capability of the entire system.

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

[0101] Correspondingly, the reactive power integration module 425 includes an adder, a subtractor and a PI controller, wherein the adder converts the AC voltage reference value V acref (It is a per unit value, which is the same as the rated voltage of the inverter side busbar V rmsr has the same meaning) and reactive power value Q pu ·K drp Add (equivalent to V rmsr +Q pu ·K drp), then the subtractor converts the inverter bus voltage value V rmsm Subtract the above added value (V rmsr +Q pu ·K drp ), can be combined into (V rmsm -V rmsr )-Q pu ·K drp , where (V rmsm -V rmsr ) is the opposite of the voltage drop value. Finally, the PI controller integrates the difference between the opposite of the voltage drop value and the reactive power value to obtain the reactive power control component I qref The embodiment of the present invention can adjust the active power value and reactive power value output by the MMC inverter 420 respectively through the active power integration module 424 and the reactive power integration module 425; the current control loop module 426 includes a current control loop and the pulse width modulation module 427 includes a PWM controller.

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

[0103] It should be noted that the circuit also includes a first converter transformer 463 and a second converter transformer 460. One side of the rectifier-side bus 431 is connected to 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 to one side of the inverter-side bus 430 through the second converter transformer 460. Both the first converter transformer 463 and the second converter transformer 460 can be converter transformers. In addition to transmitting electric energy and converting the AC side working voltage into the commutation voltage of the converter in the DC power transmission system, the converter transformer can also provide two sets of three-phase symmetrical commutation voltages with equal amplitudes and a phase difference of 30° for the two series-connected converters to achieve twelve-pulse commutation. This configuration helps to reduce the generation of harmonics and improve the operating efficiency and reliability of the system.

[0104] It should be noted that the circuit further includes a third converter transformer 462 and a fourth converter transformer 461, one side of the rectifier-side bus 431 is connected to 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 to one side of the inverter-side bus 430 through the fourth converter transformer 461. Both the third converter transformer 462 and the fourth converter transformer 461 can be converter transformers, which can convert the AC system voltage into the commutation voltage required by the converter to meet the working requirements of the converter.

[0105] It should be noted that the circuit also includes a first filter 451 and a second filter 450. One side of the first filter 451 is connected to 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 to the other side of the inverter side bus 430, and the other side of the second filter 450 is grounded.

[0106] See also Fig. 6A , 6B and 6C, Fig. 6A The curves of AC voltage-time and DC voltage-time of LCC-HVDC system when commutation failure occurs are shown. Fig. 6A It can be seen that in the interval from fault occurrence to fault recovery, the change amplitudes of the AC voltage and DC voltage are large, the AC voltage-time curve waveform is obviously distorted after the fault occurs, and the system recovery time after the fault occurs is 0.2 seconds (s). Figure 6B The curves of AC voltage-time and DC voltage-time of the LCC-MMC parallel system when commutation failure occurs are shown. Fig. 6A and Figure 6B In the middle curve, it can be seen 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 does not show distortion after the fault occurs. The system recovery time is about 0.1s, achieving rapid recovery. That is, the power control method for the LCC-MMC parallel system provided in the embodiment of the present invention successfully resists commutation failure. Figure 6C The power-time curves of the LCC-HVDC system and the LCC-MMC parallel system when commutation failure occurs are shown. Figure 6CIt can be seen that when the power control method of the embodiment of the present invention is not used, that is, when the LCC-HVDC system has a phase-change failure fault, 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 LCC-MMC parallel system fails, 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 1 second after the phase-change failure fault occurs, the reactive power output by the LCC-MMC parallel system is greatly improved compared to the reactive power output by the LCC-HVDC system, proving that a power control method for an LCC-MMC parallel system provided by an embodiment of the present invention can improve the reactive power generation capacity. Fig. 6A , 6B 6C demonstrate the system's ability to resist commutation failure when a fault occurs and after adding additional control without additional control, and at the same time demonstrate the improvement of reactive power generation capability by the power control method for the LCC-MMC parallel system provided in an embodiment of the present invention.

[0107] A person skilled in the art can understand that all or part of the processes in the above-mentioned embodiments can be implemented by instructing the relevant hardware through a computer program, and the program can be stored in a computer-readable storage medium, and when the program is executed, it can include the processes of the embodiments of the above-mentioned methods. The storage medium can be a disk, an optical disk, a read-only memory (ROM) or a random access memory (RAM), etc.

[0108] The above is a preferred embodiment of the present invention. It should be pointed out that a person skilled in the art can make several improvements and modifications without departing from the principle of the present invention. These improvements and modifications are also considered to be within the scope of protection of the present invention.

Claims

1. A power control method for an LCC-MMC parallel system, characterized in that: Applied to MMC inverters, including: Obtaining 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; Calculate the difference between the rated voltage value of the inverter-side bus and the voltage value of the inverter-side bus to obtain a voltage drop value; Compare the voltage drop value with a preset threshold value to obtain a first comparison result; selecting an active power reference value in the active power data set based on the first comparison result; Integrating the difference between the active power reference value and the active power value to obtain an active power control component; Integrating the difference between 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 AC voltage reference value; A pulse width modulation signal is generated based on the AC voltage reference value, so as to control the switching of the switch tube of the MMC inverter according to the pulse width modulation signal.

2. The method according to claim 1, characterized in that The generating of a pulse width modulation signal based on the AC voltage reference value to control the switch tube of the MMC inverter according to the pulse width modulation signal comprises: Performing numerical comparison on the AC voltage reference value and the inverter-side bus voltage value to obtain a second comparison result; A pulse width modulation signal is generated based on the second comparison result, so as to control the switching of the switch tube of the MMC inverter according to the pulse width modulation signal.

3. The method according to claim 2, characterized in that The step of generating a pulse width modulation signal based on the second comparison result comprises: 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 smaller than the duty cycle of the second pulse width modulation signal.

4. The method according to claim 1, characterized in that The active power data set includes 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 an active power reference value from the active power data set 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 busbar, wherein the AC side of the LCC inverter is connected in parallel with the AC side of the MMC inverter and then connected to one side of the inverter-side busbar, and the other side of the inverter-side busbar is connected to a receiving-end power grid; wherein the MMC inverter further includes: A subtractor module is connected to one side of the inverter-side bus, and 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 a voltage drop value; A comparator module, connected to the subtractor module, and configured to perform a numerical comparison between the voltage drop value and a preset threshold value to obtain a first comparison result; a selector module connected to the comparator module, the selector module being configured to select an active power reference value from the active power data set based on the first comparison result; An active power integration module, connected to the selector module, and configured to perform a difference integration on the active power reference value and the active power value to obtain an active power control component; A reactive power integration module is connected to one side of the inverter-side busbar, and is used to perform a difference integration on the voltage drop value and the reactive power value to obtain a reactive power control component; A current control loop module, connected to the active power integration module and the reactive power integration module, the current control loop module is used to perform current inverse 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, and is used to generate a pulse width modulation signal based on the AC voltage reference value, so as to control the switch tube of the MMC inverter according to the pulse width modulation signal.

6. The circuit according to claim 5, characterized in that The pulse width modulation module comprises: A comparison unit connected to the current control loop module, the comparison unit is used to perform a numerical comparison between the AC voltage reference value and the inverter-side bus voltage value to obtain a second comparison result; A generating unit is connected to the comparing unit, and is used to generate a pulse width modulation signal based on the second comparison result, so as to control the switching tube of the MMC inverter according to the pulse width modulation signal.

7. The circuit according to 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 busbar, one side of the rectifier-side busbar is respectively connected to the AC side of the LCC rectifier and the AC side of the MMC rectifier, the other side of the rectifier-side busbar 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 according to claim 7, characterized in that The circuit also includes a first converter transformer and a second converter transformer, one side of the rectifier-side 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-side bus through the second converter transformer.

9. The circuit according to claim 8, characterized in that The circuit also includes a third converter transformer and a fourth converter transformer. One side of the rectifier-side 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-side bus through the fourth converter transformer.

10. The circuit according to claim 7, characterized in that The circuit also includes a first filter and a second filter, one side of the first filter is connected to 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 to the other side of the inverter side bus, and the other side of the second filter is grounded.

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