CN113839407A - Power flow reversal control method and control device of direct-current power transmission system - Google Patents
Power flow reversal control method and control device of direct-current power transmission system Download PDFInfo
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- CN113839407A CN113839407A CN202111044864.XA CN202111044864A CN113839407A CN 113839407 A CN113839407 A CN 113839407A CN 202111044864 A CN202111044864 A CN 202111044864A CN 113839407 A CN113839407 A CN 113839407A
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/66—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal
- H02M7/68—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters
- H02M7/72—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/79—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
- H02M7/797—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/60—Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]
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Abstract
The application provides a power flow reversal control method and a control device, because the control modes of converter stations in a flexible direct current transmission system are exchanged before the power is reduced to zero, the operation modes of the two converter stations can be rapidly and smoothly switched after the direct current power crosses zero, and because the control modes and the operation modes of the converter stations are correspondingly exchanged, the corresponding relation between the control modes and the operation modes of the converter stations is not changed after the power flow is turned over, so that the problem of overvoltage cannot easily occur in different power flow directions.
Description
Technical Field
The invention relates to the technical field of power electronics, in particular to a power flow reversal control method and a control device.
Background
A flexible dc transmission system typically comprises two or more converters, each converter being divided into a rectifier and an inverter according to its role in the flexible dc transmission system. The rectifier is used for converting alternating current into direct current for output, and the inverter is used for converting direct current in direct current transmission into alternating current for output, so that the load center can use the alternating current. Therefore, the side where the rectifier is located is referred to as the transmitting side of the flexible dc power transmission system, and the side where the inverter is located is referred to as the receiving side of the flexible dc power transmission system.
In a flexible direct current transmission system, in some application situations, the flexible direct current transmission system is required to perform power flow reversal, also called power reversal. During a reversal of the power flow of a flexible direct current transmission system, the direction of the transmission power may change, which may cause a change in the operation mode of the converter stations in the transmission system.
After the current reversal of the current of the existing flexible direct current transmission system occurs, the constant voltage control mode and the constant power control mode of the converter do not change correspondingly with the change of the operation mode of the converter, so that the corresponding relation between the control mode and the operation mode of the converter may change in the process of the current reversal, and the problem that the overvoltage level of the direct current system becomes high is caused.
Disclosure of Invention
Based on the above, the application provides a power flow reversal control method and a control device, so that the corresponding relation between the control mode and the operation mode of the converter is not changed after the power flow is turned over, and the problem of overvoltage is not easy to occur in different power flow directions.
A power flow reversal control method of a flexible direct current transmission system comprises the following steps:
when a direct current inversion instruction is received, controlling a first converter station in a power control mode to reduce the direct current power of the direct current flexible direct current transmission system at a preset speed, and controlling a second converter station of the direct current transmission system in a direct current voltage control mode;
after the magnitude of the direct current power is reduced to a threshold value, switching the control modes of the first converter station and the second converter station;
and controlling the second converter station in a power control mode to control the direct current power to be reduced to zero and reversely increased to a set value at the preset speed, and controlling the first converter station of the direct current transmission system in a direct current voltage control mode.
In some embodiments, after performing the step of reducing the magnitude of the dc power to the threshold and before reducing the dc power to zero, the method further includes:
and judging whether the current flexible direct current transmission system is normal, if so, continuing to execute the step of adopting the power control mode to control the second converter station so as to control the direct current power to be reduced to zero and then reversely increase to a set value at the preset speed, otherwise, executing the step of interchanging the control modes of the first converter station and the second converter station.
In some embodiments, before the dc transmission system requires power flow reversal, the operation mode of the first converter station is an inverter operation mode, and the operation mode of the second converter station is a rectifier operation mode.
In some embodiments, when the dc power transmission system needs to perform power flow reversal, the step of controlling the first converter station in the power control mode to decrease the magnitude of the dc power of the dc flexible dc power transmission system at a preset rate, and controlling the second converter station in the dc power transmission system in the dc voltage control mode includes:
obtaining a first direct current power control instruction according to a first power difference value between the direct current power and a first target direct current power;
obtaining a first direct-current voltage control instruction according to a first voltage difference value between the direct-current voltage and a first target direct-current voltage;
controlling the switching state of a power switch in the first converter station through the first direct current power control instruction so as to control the direct current power to follow the change of the first target direct current power;
controlling the switching state of a power switch in the second converter station through the first direct-current voltage control instruction so as to control the direct-current power to follow the change of the first target direct-current voltage;
when a power flow reversal instruction is received, the first target direct-current power begins to be reduced from the set value at the preset speed, and the first target direct-current voltage is kept constant.
In some embodiments, the step of controlling the second converter station in a power control mode to control the dc power to decrease to zero and increase to a set value at the preset rate in a reverse direction, and the step of controlling the first converter station of the dc power transmission system in a dc voltage control mode includes:
obtaining a second direct current power control instruction according to a second power difference value between the direct current power and a second target direct current power;
obtaining a second direct-current voltage control instruction according to a second voltage difference value between the direct-current voltage and a second target direct-current voltage;
controlling the switching state of a power switch in the first converter station through the second direct current power control instruction so as to control the direct current power to follow the change of the second target direct current power;
controlling the switching state of a power switch in the first converter station through the second direct-current voltage control instruction so as to control the direct-current power to follow the change of the second target direct-current voltage;
after the magnitude of the second target direct-current power is reduced to zero, the magnitude of the second target direct-current power starts to rise from zero to the set value at the preset rate, and the second target direct-current voltage is kept constant.
In some embodiments, the step of switching the control modes of the first converter station and the second converter station after the magnitude of the dc power drops to the threshold value comprises:
comparing the direct current power with the threshold value;
when the direct current power is reduced to the threshold value, switching a switching state control instruction of a power switch in the first converter station from the first direct current power control instruction to the second direct current voltage control instruction so as to switch a control mode of the first converter station from a direct current power control mode to a direct current voltage control mode;
and switching the switch state control command of the power switch in the second converter station from the first direct-current voltage control command to the second direct-current power control command so as to switch the control mode of the second converter station from the direct-current voltage control mode to the direct-current power control mode.
In some embodiments, the step of obtaining a first dc power control command according to a first power difference between the dc power and a first target dc power is:
and performing proportional integration on the first power difference value through a PI controller to obtain the first direct current power control direct current.
In some embodiments, further comprising:
and when the magnitude of the direct current power is reduced to the threshold value, carrying out zero clearing operation on the integral of the PI control device.
A power flow reversal control device in a direct current power transmission system, comprising:
the controller is used for controlling the first converter station in a direct-current power control mode to reduce the direct-current power of the direct-current flexible direct-current transmission system at a preset rate when a power flow reversal instruction is received, controlling the second converter station of the direct-current transmission system in a direct-current voltage control mode, controlling the second converter station in the direct-current power control mode after the control modes of the first converter station and the second converter station are switched, controlling the direct-current power to be reduced to zero and reversely increased to a set value at the preset rate, and controlling the first converter station of the direct-current transmission system in the direct-current voltage control mode;
and the mode switcher is used for switching the control modes of the first converter station and the second converter station after the magnitude of the direct current power is reduced to a threshold value.
In some embodiments, a switching signal generator is also included;
the controller is used for obtaining a first direct current power control instruction according to a first power difference value between the direct current power and a first target direct current power when the controller receives a power flow reversal instruction, obtaining a first direct current voltage control instruction according to a first voltage difference value between the direct current voltage and a first target direct current voltage, obtaining a second direct current power control instruction according to a second power difference value between the direct current power and a second target direct current power after the control modes of the first converter station and the second converter station are switched, and obtaining a second direct current voltage control instruction according to a second voltage difference value between the direct current voltage and a second target direct current voltage;
before the control modes of the first converter station and the second converter station are switched, the first direct current power control command and the first direct current voltage control command are respectively input to a switching signal generator through the mode switcher, so that switching signals of power switches in the first converter station and the second converter station are respectively generated through the triggering signal generator according to the first direct current power control command and the first direct current voltage control command;
when the direct current power is reduced to the threshold value, the mode switcher switches the input of the switching signal generator from the first direct current power control instruction and the first direct current voltage control instruction to the second direct current voltage control instruction and the second direct current power control instruction respectively, so that the switching signals of the power switches in the first converter station and the second converter station are generated through the trigger signal generator according to the second direct current voltage control instruction and the second direct current power control instruction.
According to the power flow reversal control method and the control device, the control modes of the converter stations in the flexible direct-current transmission system are exchanged before the power is reduced to zero, so that the operation modes of the two converter stations can be switched rapidly and smoothly after the direct-current power crosses zero, and the corresponding relation between the control mode and the operation mode of the converter stations is not changed after the power flow is turned over due to the fact that the control modes and the operation modes of the converter stations are exchanged correspondingly, so that the problem of overvoltage cannot occur easily in different power flow directions.
Drawings
In order to more clearly illustrate the technical solutions in the embodiments or the conventional technologies of the present application, the drawings used in the descriptions of the embodiments or the conventional technologies will be briefly introduced below, it is obvious that the drawings in the following descriptions are only some embodiments of the present application, and for those skilled in the art, other drawings can be obtained according to the drawings without creative efforts.
Fig. 1 is a flow chart illustrating a power flow reversal control method according to an embodiment of the present application;
fig. 2 is a schematic flow chart illustrating a power flow reversal control method according to another embodiment of the present application;
FIG. 3 is a schematic diagram illustrating a partial structure of a power flow reversal control device according to the present application;
fig. 4 is a schematic diagram of a converter topology of a flexible dc power transmission system according to an embodiment of the present application;
fig. 5 is a schematic structural diagram of a sub-module of fig. 4.
Detailed Description
To facilitate an understanding of the present application, the present application will now be described more fully with reference to the accompanying drawings. Embodiments of the present application are set forth in the accompanying drawings. This application may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete.
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein in the description of the present application is for the purpose of describing particular embodiments only and is not intended to be limiting of the application.
It will be understood that, as used herein, the terms "first," "second," and the like may be used herein to describe various elements, but these elements are not limited by these terms. These terms are only used to distinguish one element from another. For example, a first resistance may be referred to as a second resistance, and similarly, a second resistance may be referred to as a first resistance, without departing from the scope of the present application. The first resistance and the second resistance are both resistances, but they are not the same resistance.
It is to be understood that "connection" in the following embodiments is to be understood as "electrical connection", "communication connection", and the like if the connected circuits, modules, units, and the like have communication of electrical signals or data with each other.
As used herein, the singular forms "a", "an" and "the" may include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises/comprising," "includes" or "including," etc., specify the presence of stated features, integers, steps, operations, components, parts, or combinations thereof, but do not preclude the presence or addition of one or more other features, integers, steps, operations, components, parts, or combinations thereof.
In order to ensure the stable operation of the flexible direct current transmission system, coordination control should be performed between the rectifier and the inverter, and generally, the direct current voltage in the flexible direct current transmission system is controlled by the rectifier on the transmitting end side, and the active power of the flexible direct current transmission system is controlled by the inverter on the receiving end side. When the receiving end has an alternating current system fault, the control of the sending end rectifier on the direct current voltage cannot be influenced by the limited power of the receiving end inverter, and the overvoltage level of the direct current system can be reduced under the control mode. However, in the power flow inversion application, since the control modes of the rectifier and the inverter are not switched correspondingly after the operation modes are switched, the control method has the problems that the receiving end controls the direct-current voltage and the transmitting end controls the direct-current power, and an overvoltage phenomenon may occur in the power flow inversion process. In order to solve the problem, the application provides a power flow reversal method of a flexible direct current transmission system, a controller and the flexible direct current transmission system.
Fig. 1 illustrates a power flow reversal control method of a flexible dc power transmission system according to the application, which mainly includes S1-S3 in the present embodiment, which is described in detail as follows.
S1: and when a direct current inversion instruction is received, controlling the first converter station by adopting a power control mode, reducing the direct current power of the flexible direct current transmission system at a preset speed, and controlling the second converter station of the direct current transmission system by adopting a direct current voltage control mode.
When the power flow reversal of the flexible direct current transmission system is needed, a controller (such as a computer or a single chip microcomputer) receives a power flow reversal command to control a first converter station of the flexible direct current transmission system to perform a power control mode so as to reduce the power of the flexible direct current transmission system, and in the process, in order to accelerate the power flow reversal speed, a direct current voltage control mode is adopted to control a second converter station of the flexible direct current transmission system, and the direct current voltage of the flexible direct current transmission system is maintained at a constant value through the control of the second converter station.
Further, in this embodiment, before the power flow reversal instruction is received, that is, when the direct current transmission system does not currently need to perform power flow reversal and is in the normal transmission state, the operation mode of the first converter station is the inverter operation mode, that is, when the direct current transmission system is in the normal transmission state, the first converter station is the inversion converter station of the direct current transmission system. And when the direct current transmission system is in a normal working state, the operation mode of the second converter station is a rectification operation mode, namely the second converter station is the rectification converter station of the direct current transmission system when the direct current transmission system is in a normal transmission state. During the normal transmission state of the direct current transmission system, the first converter station which is used as the inversion converter station is controlled by adopting the power control mode, and the second converter station which is used as the rectification converter station is controlled by adopting the voltage control mode, so that the control of the second converter station on the direct current voltage cannot be influenced when the output power of the first converter station is abnormal, and the problem of overvoltage of the direct current transmission system can be avoided.
During normal transmission of the dc power transmission system, the first converter station is in a dc power control mode to control the dc power of the dc power transmission system to be maintained at a desired power, which is generally constant. And the second converter station is in a dc voltage control mode to control the dc of the dc transmission system to maintain a constant voltage. The power control mode is to generate a power control command for controlling the switching state of the power switch in the converter station according to the difference between the dc power of the dc transmission system and the desired power (also referred to as a target power), and then generate an existing switching signal according to the power control command to drive the power switch in the converter station to turn on (switch on or switch off) and turn off (switch off or switch off).
In order to increase the speed of the power flow reversal, the second converter station is controlled in S1 by means of the dc voltage control mode to control the dc voltage of the dc transmission system to be maintained at a constant value. Therefore, S1 further includes the steps of:
s11, the controller obtains a first DC power control command according to a first power difference between the DC power and a first target DC power, and obtains a first DC voltage control according to a first voltage difference between the DC voltage and a first target DC voltage.
In this embodiment, the control stations of the first converter station and the second converter station are respectively configured, and the controller includes a first controller and a second controller respectively located in the control station of the first converter station and the control station of the second converter station, where the first controller obtains the first dc power control instruction, and the second controller obtains the first dc voltage control instruction. In other embodiments, the control stations of the first converter station and the second converter station may also be integrated, i.e. the controller comprises only one set of control computer or single chip for obtaining the first dc power control command and the first dc voltage control command.
S12: and controlling the switching state of the power switch in the second converter station through the first direct-current voltage control instruction so as to control the direct-current voltage to follow the change of the first target direct-current voltage.
The first dc power control command and the first dc voltage control command obtained by the controller are transmitted to the switching signal generator to control the switching signal generator to generate switching signals (signals for driving the switches to be turned on and off, such as driving voltage signals) of the power switches in the first converter station and the second converter station according to the first dc power control command and the first dc voltage control command. The control stations of the first converter station and the second converter station may be arranged separately, and the switching signal generator includes a first switching signal generator and a second switching signal generator respectively located in the first converter station and the second converter station. The first switching signal generator generates a switching signal of a power switch in the first converter station according to the first direct current power control instruction to control the first converter station to be in a power control mode so as to maintain the direct current power of the direct current power transmission system as the first target direct current power. The second switching signal generator generates a switching signal of a power switch in the second converter station according to the first direct current voltage control instruction to control the second converter station to be in a power control mode so as to maintain the direct current voltage of the direct current transmission system as the first target direct current voltage.
In other embodiments, the control station of the first converter station and the control station of the second converter station may also be integrated, and the switch generator is a set of means for generating switching signals for the power switches in the first converter station and the second converter station. A power switch is here a switch located in the converter station and controlling the converter station for power conversion by switching of its switches.
Since the dc power follows the first target dc power variation and the dc voltage follows the first target dc voltage variation in S12. Therefore, the first target direct-current voltage is controlled to be kept constant all the time when the first target direct-current power starts to drop from the set value at the preset speed when the power flow reversal command is received. The dc power of the dc transmission system may be controlled to start to decrease at a preset rate from the magnitude of the desired power (set value) when the dc transmission system is in normal transmission, and during this time the dc output voltage is maintained at the value of the first target dc voltage.
S2: and switching the control modes of the first converter station and the second converter station after the magnitude of the direct current power is reduced to a threshold value.
In this embodiment, the threshold is chosen to be close to zero, which may be, for example, 0.01 times the rated power. The switching of the control modes of the first converter station and the second converter station may be controlled by means of a program in a computer, i.e. by means of software. When receiving an instruction that the dc power drops to the threshold value, for example, by a setting program, the control instructions of the power switches in the first converter station and the second converter station are switched. Of course, the present application is also adapted to switch the above-mentioned mode by means of hardware, such as a mode switcher (e.g. a single-pole double-throw switch) arranged in the control stations of the first converter station and the second converter station to select the corresponding control command to be connected to the switch signal generator of the power switch in the first converter station and the second converter station.
S3: and controlling the second converter station in a power control mode to control the direct current power to be reduced to zero and reversely increased to a set value at the preset speed, and controlling the first converter station of the direct current transmission system in a direct current voltage control mode.
In step S1, the first converter station controls the dc power to decrease, but since the switching of the control mode of the converter has already been performed in S2, the operation mode of the first converter station and the second converter station is changed after the dc power is switched to decrease to zero in the dc power control mode of the second converter station in S3, the dc power is inverted, and the dc power is controlled to be a preset value in a reverse manner by the second converter station. After the direct current power is reduced to zero, the operation mode of the first converter station is switched from the inverter operation mode to the rectifier operation mode, and the operation mode of the second converter station is switched from the rectifier operation mode to the inverter operation mode. Thus, after the dc power is reduced to zero, the first converter station functions as a rectifying converter station of the dc transmission system and the second converter station functions as an inverting converter station of the dc transmission system.
In order to increase the speed of the power flow reversal, the first converter station is controlled in S3 by means of a dc voltage control mode to control the dc voltage of the dc transmission system to be maintained at a constant value. Therefore, S3 further includes the steps of:
and S31, the controller obtains a second direct current power control command according to a second power difference value between the direct current power and a second target direct current power, and obtains a second direct current voltage control according to a second voltage difference value between the direct current voltage and a second target direct current voltage.
In this embodiment, the controller performing S3 is the same as the controller performing S1, i.e., the controller includes a first controller and a second controller respectively located in the control station of the first converter station and the control station of the second converter station, the first controller obtains the second dc voltage control command, and the second controller obtains the second dc power control command.
S32: and controlling the switching state of the power switch in the second converter station through the second direct-current power control instruction so as to control the direct-current power to follow the change of the second target direct-current voltage, and controlling the switching state of the power switch in the second converter station through the second direct-current power control instruction so as to control the direct-current power to follow the change of the second target direct-current power.
In this embodiment, the switching signal generators corresponding to the first converter station and the second converter station are the same as those in S1, and then the second dc voltage control command is transmitted to the first switching signal generator, and the second dc power control command is transmitted to the second switching signal generator. And the first switching signal generator generates a switching signal of a power switch in the first converter station according to the second direct-current voltage control instruction so as to control the first converter station to be in a voltage control mode, and finally, the direct-current voltage of the direct-current power transmission system is controlled to be maintained as a second target direct-current voltage. And the second switching signal generator generates a switching signal of a power switch in the second converter station according to the second direct current power control instruction so as to control the second converter station to be in a power control mode, and finally, the direct current power of the direct current power transmission system is controlled to be maintained as second target direct current power.
Since the dc power follows the second target dc power variation and the dc voltage follows the second target dc voltage variation in S32. Therefore, the second target dc voltage is controlled to start to increase from zero to a set value at a preset rate after the dc power is greatly reduced to zero, and the second target dc voltage is kept constant. The dc power of the dc power transmission system can be controlled to increase from zero to a set value (the desired power of the dc power transmission system during normal transmission) at a preset rate, and during this period, the dc output voltage is maintained at the value of the second target dc voltage. The first target direct-current voltage and the second target direct-current voltage can be set to be the same, namely, the first target direct-current voltage and the second target direct-current voltage are both direct-current voltages of the direct-current power transmission system during normal transmission of the direct-current power transmission system, and therefore the speed of power flow reversal can be increased.
The step of putting the converter station in the dc power control mode in this application comprises:
sp 1: and acquiring the current direct-current power of the flexible direct-current power transmission system.
Sp 2: and inputting the power difference value between the direct current power and the target direct current power into the first PI controller so as to convert the power difference value into a direct current power control command of the direct current power through the first PI controller.
Sp 3: and generating a first switching signal of a power switch in the converter station working in the power control mode according to the direct current power control instruction to control the switching state of each power switch in the converter station working in the power control mode so as to maintain the direct current power as the value of the target direct current power.
In S1, if the first converter station is in the dc power control mode, the target dc power is the first target dc power in Sp2, and the power difference is the first power difference, where the dc power control command is the first dc power control command corresponding to the dc power control mode of the first converter station.
In S3, if the second converter station is in the dc power control mode, corresponding to the dc power control mode of the second converter station, in Sp2, the target dc power is the second target dc power, the power difference is the second power difference, and the dc power control command is the second dc power control command.
In addition, the dc voltage control method mentioned in the present application includes:
sv 1: and acquiring the current direct-current voltage of the flexible direct-current power transmission system.
Sv 2: and inputting a voltage difference value between the direct current voltage and the target direct current voltage into the second PI controller so as to convert the voltage difference value into a direct current voltage control command through the second PI controller.
In order to make the power flow reversal speed faster, the value of the direct current voltage is controlled to be constant during the power reduction process, namely the target voltage is a constant voltage with a preset size.
Sv 3: and generating second switching signals of each power conversion module in the converter station working in the direct current voltage control mode according to the direct current voltage suppression instruction to control the switching state of each power switch in the converter station working in the power control mode so as to maintain the direct current voltage as the value of the target direct current voltage.
At S1, if the second converter station is in the dc voltage control mode, corresponding to the dc voltage control mode of the second converter station, at Sv2, the target dc power is the first target dc voltage, the voltage difference is the first voltage difference, and the dc voltage control command is the first dc voltage control command.
At S3, if the first converter station is in the dc voltage control mode, corresponding to the dc voltage control mode of the second converter station, at Sv2, the target dc voltage is a second target dc voltage, the voltage difference is a second voltage difference, and the dc voltage control command is a second dc voltage control command.
Further, in this embodiment, the step of switching the control modes of the first converter station and the second converter station comprises:
s21: and comparing the direct current power with the threshold value.
S22: when the direct current power is reduced to a threshold value, namely the direct current power is less than or equal to the threshold value, switching a switch state control instruction of a power switch in a first converter station from a first direct current power control instruction to a second direct current voltage control instruction so as to switch a control mode of the first converter station from a direct current power control mode to a direct current voltage control mode; and switching the switch state control instruction of the power switch in the second converter station from the first direct current voltage control instruction to a second direct current power control instruction so as to switch the control mode of the second converter station from the direct current voltage control mode to the direct current power control mode. Here, the switching may be achieved by a mode switcher, and the mode switcher may include a first mode switcher corresponding to the first controller and a second mode switcher corresponding to the second controller. The first mode switch is used for switching the first direct current power control command and the second direct current voltage control command, and the second mode switch is used for switching the second direct current power control command and the first direct current voltage control command. Specifically, when the first mode switch switches the input of the first switching signal generator from the first dc power control command to the second dc voltage control command, the second mode switch switches the control command of the second switching signal generator from the first dc voltage control command to the second dc power control command.
In addition, in order to make the control mode switching and the operation mode switching of the converter stations in the flexible direct current transmission system softer, when the step of interchanging the control modes of the first converter station and the second converter station is executed, the integral in the first PI controller is cleared to accelerate the smooth reversal of the power flow of the direct current transmission system.
In addition, since the control modes of the converter stations are switched before the operation modes of the converter stations are changed, after the control modes of the two converter stations are switched, the current reversal control method further needs to judge the state of the flexible direct current transmission system before the direct current power of the flexible direct current transmission system continuously drops to zero, and if an abnormal instruction of the flexible direct current transmission system is received currently, the control modes of the first converter station and the second converter station need to be switched again, direct current reversal operation is suspended, and detection is waited. If the abnormal command of the flexible direct current transmission system is not received, the control of the reversal of the power flow needs to be continued, namely, the step S3 is executed. In S2, it is necessary to further determine whether the current dc power has decreased to the threshold value, and if so, the control mode reversing step of the converter station is executed, otherwise, it is necessary to return to S1 to continue decreasing the dc power. Therefore, a flow chart of a power flow reversal control method of a flexible direct current transmission system according to another embodiment of the present application may be as shown in fig. 2.
Therefore, the power flow reversal control mode of the flexible direct current transmission system provided by the application controls the two converter station control modes in the direct current transmission system to be exchanged before the direct current power is reduced to zero, and then the direct current power is continuously reduced to zero, so that the operation modes of the two converter stations are reversed. Because the control mode and the operation mode of the converter station are correspondingly adjusted, the problem of overhigh direct-current voltage in the reverse rotation process can be avoided.
In addition, as shown in fig. 3, the present application also provides a power flow reversal control device of a flexible direct current transmission system, which mainly includes a controller and a mode switcher.
The controller is mainly used for controlling the first converter station in a power control mode when a power flow reversal instruction is received so as to reduce the magnitude of direct current power of the direct current flexible direct current transmission system at a preset rate, controlling the second converter station of the direct current transmission system in a direct current voltage control mode, controlling the second converter station in the direct current power control mode after the control modes of the first converter station and the second converter station are switched so as to control the direct current power to be reduced to zero and reversely increased to a set value at the preset rate, and controlling the first converter station of the direct current transmission system in the direct current voltage control mode. The mode switcher is mainly used for switching the control modes of the first converter station and the second converter station after the size of the direct current power is reduced to a threshold value.
In addition, the control device further includes a switching signal generator, and the switching signal generator, the controller, and the mode switcher in the control device are the same as those mentioned in the above-mentioned power flow reversal control method provided according to the present application, and therefore, will not be described in detail here. It should be noted that the controller and the mode switcher can be both virtual software modules such as programs in a computer, and can also be hardware devices such as a single chip microcomputer and a single-pole double-throw switch. The specific configuration thereof is not limited in the present application.
The first controller (provided in the control station corresponding to the first converter station) of the controllers provided in the present application is of the same configuration as the second controller (provided in the control station corresponding to the second converter station), and therefore the control arrangement shown in fig. 3 only illustrates the constituent parts of the first controller, which is located in the corresponding control station of the first converter station in the flexible direct current transmission system as shown in fig. 3. In this embodiment, the first controller includes: a first power difference calculator E1, configured to obtain a first power difference between the first target dc power and the dc power of the flexible dc input system; the PI controller A is used for outputting a first direct current power control instruction according to the first power value; a second voltage difference calculator E2, configured to obtain a second voltage difference between a second target dc voltage and the dc voltage of the flexible dc power transmission system; the PI controller B is used for outputting a second direct-current voltage control instruction according to the second voltage difference value; a first switching signal generator, configured to generate a first switching signal according to a first dc power control instruction to control a power switch of a first converter station, so as to control a dc power of a flexible dc power transmission system to a first target dc power, or generate a second switching signal according to a second dc voltage control instruction to control a power switch of a first converter station, so as to control a dc voltage of the flexible dc power transmission system to a second target dc voltage; and the first switch is used for switching the input signal of the first switching signal generator, and when the direct current power of the direct current flexible direct current power transmission system is reduced to a threshold value, the first switch switches the input signal of the first switching signal generator from a first direct current power control instruction to a second direct current voltage control instruction.
In addition, the control device further comprises a voltage switcher corresponding to the mode switcher, and the voltage switcher also comprises a first voltage switcher and a second voltage switcher which are arranged corresponding to the first controller and the second controller. As shown in fig. 3, the first voltage switcher may be a single pole double throw switch. The device comprises a voltage difference value calculating module, a voltage difference value calculating module and a voltage difference value calculating module, wherein the voltage difference value calculating module is used for calculating a voltage difference value between a first target direct current voltage and a second target direct current voltage; when the first mode switcher switches the input of the first switching signal generator into a first direct current power control command, the first voltage switcher switches one input signal in the second voltage difference module into a margin direct current voltage; when the first mode switcher switches the input signal of the first switching signal generator to the second direct-current voltage control command, the first voltage switcher switches one input of the second voltage difference module to the second direct-current target voltage. In addition, the controller device further comprises a selector, and similarly, the selector may further comprise a first selector corresponding to the first controller and a second selector corresponding to the second controller, wherein the first selector selects a larger one of the output signal of the PI controller a and the output signal of the PI controller B to output, so that the first mode switch performs dc power control after the input signal of the first switching signal generator is switched to the first dc power control command, and simultaneously ensures that the dc voltage is not over-voltage, and ensures the safety of the flexible dc power transmission system.
In addition, the present application also provides a flexible dc power transmission system, which is a modular multilevel converter topology as shown in fig. 4, and the sub-modules in the power conversion module thereof may be half-bridge sub-modules as shown in fig. 5. When the control system receives the power flow reversal control signal, the control system reverses the power flow according to the power flow reversal control method shown in fig. 1 or fig. 2. The controller of its converter station may be of the controller structure as shown in fig. 3. The working process is as follows: the negative input of the first power difference calculator E1 receives the actual measured value signal Pdc of the dc power, the positive terminal of which receives the first target dc power Pref of the dc power, the negative terminal of the second voltage difference counter E2 receives the actual measured value Udc of the dc voltage, and the positive terminal is switched between the margin dc voltage Udcl and the second target dc voltage Uref by the first voltage switch. The two difference calculators output the difference of the signals input by the positive terminal and the negative terminal. And the two PI controllers are used for carrying out integral and proportional linear combination on the difference value output by the error calculator and then outputting a corresponding control instruction. The corresponding commands output by the two PI controllers are switched by the first mode switcher to be connected with the first switch signal generator as input signals of the first switch signal generator.
Specifically, when the contact 3 of the first mode switcher is connected to the contact 2 and the contact 5 of the first voltage switcher is connected to the contact 4, the control device shown in fig. 3 performs the power control mode. When the contact 3 of the first mode switcher is connected to the contact 1 and the contacts 4 and 6 of the first voltage switcher are connected, the control device shown in fig. 3 performs the direct voltage control mode. Therefore, switching between the direct-current power control mode and the direct-current voltage control mode can be achieved by changing the connection relationship of the contacts of the switch. Uref is a reference voltage (usually set as a rated direct-current voltage) of the second target direct-current voltage, UdcL is a margin direct-current voltage (usually set as 90% of the rated direct-current voltage), Udc is a direct-current voltage actual value, Pref is a first target direct-current power, and P is a direct-current power actual value. The margin direct-current voltage is a voltage which has a certain margin with the rated direct-current voltage of the power transmission system, for example, the margin direct-current voltage is 0.9 times or 0.95 times of the rated voltage, and the margin direct-current voltage is introduced into the control loop shown in fig. 3, so that the stability of the voltage can be ensured to be controlled by the inversion side when the rectification station is switched on due to a fault.
When the control mode is switched, in addition to switching the contact connection relation of the first mode switch and the first voltage switch, the integral of the two PI controllers should be cleared at the same time, because the system power is close to zero before and after switching, so as to accelerate the power flow reversal. During the switching process, the zero clearing of the integral link of the controller does not cause large disturbance to the system. In order to ensure that the influence on the dc system during the control mode switching process is as small as possible, the switching threshold should be designed to be near zero power.
According to the power flow reversal control method and the control device, the control modes of the converter stations in the flexible direct current transmission system are exchanged before the power is reduced to zero, so that the operation modes of the two converter stations can be rapidly and smoothly switched after the direct current power crosses zero, and the problem of high overvoltage level of the flexible direct current transmission system possibly occurring in the power flow reversal process is avoided due to the fact that the control modes and the operation modes of the converter stations are correspondingly exchanged.
In the description herein, references to the description of "some embodiments," "other embodiments," "desired embodiments," etc., mean that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the invention. In this specification, a schematic description of the above terminology may not necessarily refer to the same embodiment or example.
The technical features of the embodiments described above may be arbitrarily combined, and for the sake of brevity, all possible combinations of the technical features in the embodiments described above are not described, but should be considered as being within the scope of the present specification as long as there is no contradiction between the combinations of the technical features.
The above-mentioned embodiments only express several embodiments of the present invention, and the description thereof is more specific and detailed, but not construed as limiting the scope of the invention. It should be noted that, for a person skilled in the art, several variations and modifications can be made without departing from the inventive concept, which falls within the scope of the present invention. Therefore, the protection scope of the present patent shall be subject to the appended claims.
Claims (10)
1. A power flow reversal control method of a flexible direct current transmission system comprises the following steps:
when a direct current inversion instruction is received, controlling a first converter station in a power control mode to reduce the direct current power of the direct current flexible direct current transmission system at a preset speed, and controlling a second converter station of the direct current transmission system in a direct current voltage control mode;
after the magnitude of the direct current power is reduced to a threshold value, switching the control modes of the first converter station and the second converter station;
and controlling the second converter station in a power control mode to control the direct current power to be reduced to zero and reversely increased to a set value at the preset speed, and controlling the first converter station of the direct current transmission system in a direct current voltage control mode.
2. The power flow reversal control method according to claim 1, further comprising, after performing the step of reducing the magnitude of the dc power to a threshold value and before performing the step of reducing the dc power to zero:
and judging whether the current flexible direct current transmission system is normal, if so, continuing to execute the step of adopting the power control mode to control the second converter station so as to control the direct current power to be reduced to zero and then reversely increase to a set value at the preset speed, otherwise, executing the step of interchanging the control modes of the first converter station and the second converter station.
3. The power flow reversal control method according to claim 1, characterized in that the operation mode of the first converter station is an inverter operation mode and the operation mode of the second converter station is a rectifier operation mode before the power flow reversal of the direct current transmission system is required.
4. The power flow reversal control method according to claim 1, wherein the step of controlling the first converter station in the power control mode to reduce the magnitude of the dc power of the dc flexible dc power transmission system at a preset rate and controlling the second converter station in the dc voltage control mode when the dc power transmission system needs to perform power flow reversal comprises:
obtaining a first direct current power control instruction according to a first power difference value between the direct current power and a first target direct current power;
obtaining a first direct-current voltage control instruction according to a first voltage difference value between the direct-current voltage and a first target direct-current voltage;
controlling the switching state of a power switch in the first converter station through the first direct current power control instruction so as to control the direct current power to follow the change of the first target direct current power;
controlling the switching state of a power switch in the second converter station through the first direct-current voltage control instruction so as to control the direct-current power to follow the change of the first target direct-current voltage;
when a power flow reversal instruction is received, the first target direct-current power begins to be reduced from the set value at the preset speed, and the first target direct-current voltage is kept constant.
5. The power flow reversal control method according to claim 4, characterized in that the step of controlling the second converter station in a power control mode to control the dc power to decrease to zero and increase to a set value in reverse at the preset rate, and controlling the first converter station of the dc transmission system in a dc voltage control mode comprises:
obtaining a second direct current power control instruction according to a second power difference value between the direct current power and a second target direct current power;
obtaining a second direct-current voltage control instruction according to a second voltage difference value between the direct-current voltage and a second target direct-current voltage;
controlling the switching state of a power switch in the first converter station through the second direct current power control instruction so as to control the direct current power to follow the change of the second target direct current power;
controlling the switching state of a power switch in the first converter station through the second direct-current voltage control instruction so as to control the direct-current power to follow the change of the second target direct-current voltage;
after the magnitude of the second target direct-current power is reduced to zero, the magnitude of the second target direct-current power starts to rise from zero to the set value at the preset rate, and the second target direct-current voltage is kept constant.
6. The power flow reversal control method of claim 5, wherein the step of switching the control modes of the first converter station and the second converter station after the magnitude of the DC power drops to a threshold value comprises:
comparing the direct current power with the threshold value;
when the direct current power is reduced to the threshold value, switching a switching state control instruction of a power switch in the first converter station from the first direct current power control instruction to the second direct current voltage control instruction so as to switch a control mode of the first converter station from a direct current power control mode to a direct current voltage control mode;
and switching the switch state control command of the power switch in the second converter station from the first direct-current voltage control command to the second direct-current power control command so as to switch the control mode of the second converter station from the direct-current voltage control mode to the direct-current power control mode.
7. The power flow reversal control method according to claim 4, wherein the step of obtaining the first dc power control command according to the first power difference between the dc power and the first target dc power is:
and performing proportional integration on the first power difference value through a PI controller to obtain the first direct current power control direct current.
8. The power flow reversal control method according to claim 7, characterized by further comprising:
and when the magnitude of the direct current power is reduced to the threshold value, carrying out zero clearing operation on the integral of the PI control device.
9. A power flow reversal control device in a direct current power transmission system, comprising:
the controller is used for controlling the first converter station in a direct-current power control mode to reduce the direct-current power of the direct-current flexible direct-current transmission system at a preset rate when a power flow reversal instruction is received, controlling the second converter station of the direct-current transmission system in a direct-current voltage control mode, controlling the second converter station in the direct-current power control mode after the control modes of the first converter station and the second converter station are switched, controlling the direct-current power to be reduced to zero and reversely increased to a set value at the preset rate, and controlling the first converter station of the direct-current transmission system in the direct-current voltage control mode;
and the mode switcher is used for switching the control modes of the first converter station and the second converter station after the magnitude of the direct current power is reduced to a threshold value.
10. The power flow reversal control device according to claim 9, further comprising a switching signal generator;
the controller is used for obtaining a first direct current power control instruction according to a first power difference value between the direct current power and a first target direct current power when the controller receives a power flow reversal instruction, obtaining a first direct current voltage control instruction according to a first voltage difference value between the direct current voltage and a first target direct current voltage, obtaining a second direct current power control instruction according to a second power difference value between the direct current power and a second target direct current power after the control modes of the first converter station and the second converter station are switched, and obtaining a second direct current voltage control instruction according to a second voltage difference value between the direct current voltage and a second target direct current voltage;
before the control modes of the first converter station and the second converter station are switched, the first direct current power control command and the first direct current voltage control command are respectively input to a switching signal generator through the mode switcher, so that switching signals of power switches in the first converter station and the second converter station are respectively generated through the triggering signal generator according to the first direct current power control command and the first direct current voltage control command;
when the direct current power is reduced to the threshold value, the mode switcher switches the input of the switching signal generator from the first direct current power control instruction and the first direct current voltage control instruction to the second direct current voltage control instruction and the second direct current power control instruction respectively, so that the switching signals of the power switches in the first converter station and the second converter station are generated through the trigger signal generator according to the second direct current voltage control instruction and the second direct current power control instruction.
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