CN112072688B - Control method for suppressing high-voltage direct-current commutation failure of flexible power grid commutation converter - Google Patents

Control method for suppressing high-voltage direct-current commutation failure of flexible power grid commutation converter Download PDF

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CN112072688B
CN112072688B CN202010968390.7A CN202010968390A CN112072688B CN 112072688 B CN112072688 B CN 112072688B CN 202010968390 A CN202010968390 A CN 202010968390A CN 112072688 B CN112072688 B CN 112072688B
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voltage
power grid
control
phase
voltage source
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CN112072688A (en
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高凯
任茂鑫
曹培
刘寅迪
曾翔君
骆一萍
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Xian Jiaotong University
State Grid Shanghai Electric Power Co Ltd
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Xian Jiaotong University
State Grid Shanghai Electric Power Co Ltd
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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
    • 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
    • H02J3/12—Circuit arrangements for ac mains or ac distribution networks for adjusting voltage in ac networks by changing a characteristic of the network load
    • H02J3/16—Circuit arrangements for ac mains or ac distribution networks for adjusting voltage in ac networks by changing a characteristic of the network load by adjustment of reactive power
    • 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
    • H02J2003/365—Reducing harmonics or oscillations in HVDC
    • 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
    • H02J2203/00—Indexing scheme relating to details of circuit arrangements for AC mains or AC distribution networks
    • H02J2203/20—Simulating, e g planning, reliability check, modelling or computer assisted design [CAD]
    • 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
    • Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
    • Y02E40/30—Reactive power compensation
    • 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]

Abstract

The invention discloses a control method and a system for inhibiting high-voltage direct-current commutation failure of a flexible power grid commutation converter, which comprises the following steps: an H-bridge type controllable voltage source is cascaded in series at the valve side of a converter transformer at the high-voltage direct-current transmission inversion side, and the controllable voltage source and a power grid commutation converter form a flexible LCC; the controllable voltage source outputs the capacitive voltage by adopting a continuous control method. In the invention, the flexible LCC can realize the respective control of the direct current bus voltage and the actual arc-quenching angle in the conventional running state; when the alternating current power grid has voltage drops of different degrees, the controllable voltage source can effectively restrain the problem of the commutation failure of the power grid commutation converter.

Description

Control method for suppressing high-voltage direct-current commutation failure of flexible power grid commutation converter
Technical Field
The invention belongs to the technical field of high-voltage direct-current commutation failure inhibition, and particularly relates to a control method and a control system for inhibiting high-voltage direct-current commutation failure by adopting a flexible power grid commutation converter.
Background
Conventional dc transmission based on grid commutated converters remains the main means for long distance large capacity transmission and asynchronous grid interconnection for a considerable period of time in the future. Commutation failure is an inherent problem in conventional direct-current transmission, occurs in both point-to-point and multi-drop grid systems, and the influence range of commutation failure increases as the system capacity and the number of drops increase. The failure of phase conversion may result in the reduction of direct current voltage, the reduction of direct current transmission power, the increase of current, the shortening of the service life of a converter valve, the direct current magnetic biasing of a converter transformer, the overvoltage of an inversion side weak alternating current system and the like. The problem of commutation failure of the power grid commutation converter is an urgent necessity for improving the overall operation level and long-term operation reliability of a direct current project.
The current methods for solving the problem of commutation failure of the power grid commutation converter are mainly divided into three categories. The first type is that a reactive power compensation device is additionally arranged on an inversion side, and the method cannot eliminate the first few phase commutation failures after the fault occurs and has poor economy. The second category is to improve the control strategy of the converter, such as advancing the firing angle of the inverter after detecting the voltage drop, which may cause the reactive demand of the converter to increase, further aggravating the voltage drop. The third type is to arrange an additional auxiliary phase-changing device, such as a capacitor connected in series at the valve side of the converter transformer or a capacitor switched in the phase-changing process by using a power electronic device; wherein, the former may have voltage unbalance when asymmetric fault occurs, and the latter has relatively high control complexity. Based on the analysis, the solution of the problem of commutation failure of the power grid commutation converter, which is simple and feasible to control and can avoid the defects, is significant.
Disclosure of Invention
The invention aims to provide a control method and a control system for a flexible power grid commutation converter to inhibit high-voltage direct-current commutation failure, so as to solve one or more technical problems. According to the control method, the H-bridge type controllable voltage source is cascaded in series at the valve side of the converter transformer at the high-voltage direct-current transmission inversion side, the controllable voltage source outputs the capacitive voltage by adopting a continuous control method, the phase commutation process of the power grid phase commutation converter can be accelerated, and the probability of phase commutation failure is reduced.
In order to achieve the purpose, the invention adopts the following technical scheme:
the invention discloses a control method for a flexible power grid commutation converter to inhibit high-voltage direct-current commutation failure, which comprises the following steps of:
an H-bridge type controllable voltage source is cascaded in series at the valve side of a converter transformer at the high-voltage direct-current transmission inversion side, and the controllable voltage source and a power grid commutation converter form a flexible LCC; the controllable voltage source outputs the capacitive voltage by adopting a continuous control method.
The invention is further improved in that the upper control layer of the flexible LCC is divided into an LCC part and a controllable voltage source part; the direct-current bus voltage control is realized by controlling an LCC power grid inversion angle; the voltage of the controllable voltage source is phased according to the phase current, the actual arc extinguishing angle control is realized by controlling the q-axis voltage of the controllable voltage source, and the average capacitor voltage control of the submodule of the controllable voltage source is realized by controlling the d-axis voltage of the controllable voltage source.
The invention discloses a control method for a flexible power grid commutation converter to inhibit high-voltage direct-current commutation failure, which comprises the following steps of:
an H-bridge type controllable voltage source is cascaded in series at the valve side of a converter transformer at the high-voltage direct-current transmission inversion side; the controllable voltage source outputs capacitive voltage by adopting a continuous control method;
when the power grid normally operates or the voltage drops, the power grid phase-change converter controls the voltage of the direct-current bus in a mode of adjusting a power grid inversion angle;
the controllable voltage source adopts layered control; wherein, the upper layer of the controllable voltage source adopts vector control of phase current orientation; the d-axis control submodule averages capacitor voltage, and the q-axis control submodule actually controls an arc extinguishing angle of the converter; the lower layer control of the controllable voltage source is sub-module capacitor voltage balance control;
when the power grid normally operates, the q axis of the controllable voltage source is controlled by adopting an actual arc extinguishing angle; and when the voltage of the power grid drops, applying different control modes to the q-axis voltage of the controllable voltage source according to the detected voltage drop degree of the public connection point so as to inhibit phase change failure.
The invention has the further improvement that when the voltage drop depth is shallow, the q axis of the controllable voltage source is controlled by adopting an actual arc extinguishing angle; when the voltage drop depth is deep, the q axis of the controllable voltage source directly inputs the maximum control voltage; the depth of the voltage drop depth of the power grid is judged by detecting the voltage of the public connection point; when the voltage of the common connection point is less than or equal to a preset value, determining that the voltage drop depth is deeper; and when the voltage of the common connection point is greater than a preset value, determining that the voltage drop depth is shallow.
The invention is further improved in that each phase of the controllable voltage source is formed by connecting full-bridge submodules in series; and the sub-module switch adopts a PWM continuous control mode.
In a further development of the invention, the sub-module capacitor voltage balance control in the controllable voltage source comprises: generating a preliminary offset control quantity according to the difference value of the direct current capacitor voltage of each submodule and the average capacitor voltage; generating a final offset control quantity of each submodule according to the phase current direction of the submodule; adding the final offset control quantity and the upper three-phase control quantity to obtain the final control quantity of each submodule; and performing unipolar frequency multiplication carrier phase shift modulation on the final control quantity, and controlling the PWM switch of each submodule.
A further development of the invention is that the controllable voltage source has a partially redundant submodule per phase for acting as a backup submodule for the operational submodule.
The invention has the further improvement that a determination mechanism with hysteresis is adopted when the depth of the grid voltage drop depth is determined.
The invention is further improved in that the actual arc-quenching angle is estimated from the grid arc-quenching angle, and comprises the following steps:
adding the three-phase voltage control quantity of the controllable voltage source and the phase voltage converted to the common connection point of the secondary side to obtain an actual phase-change voltage estimation value;
taking a three-phase synthetic vector of a phase voltage of a common connection point converted to the secondary side of the converter transformer as a reference, and carrying out 3s/2r conversion on an actual phase-change voltage estimation value to obtain a phase difference between the two;
and adding the phase difference to the power grid arc-quenching angle to obtain an actual arc-quenching angle estimated value.
The invention discloses a control system for a flexible power grid commutation converter to inhibit high-voltage direct-current commutation failure, which comprises:
the cascade H-bridge type controllable voltage source is connected in series with a converter transformer valve side on the high-voltage direct-current transmission inversion side and forms a flexible LCC with a power grid commutation converter; and the controllable voltage source outputs the capacitive voltage by adopting a continuous control method.
Compared with the prior art, the invention has the following beneficial effects:
in the method, a controllable voltage source is used as a VSC converter, and the parameter characteristics of the LCC can be adjusted by controlling the output voltage of the VSC converter, so that a topological structure formed by the LCC and the controllable voltage source is called as a flexible LCC; the flexible LCC can realize the respective control of the direct current bus voltage and the actual arc-quenching angle in the conventional running state; when the alternating current power grid has voltage drops of different degrees, the controllable voltage source can effectively restrain the problem of the commutation failure of the power grid commutation converter.
According to the invention, when the voltage drop depth is shallow, the stability of the direct current bus voltage and the direct current power can be ensured; and when the voltage drop depth is deep, the flexible LCC can provide reactive support for a power grid and promote the stability of the power grid at the alternating current side.
In the system, the controllable voltage source is used as a VSC converter, and the parameter characteristics of the LCC can be adjusted by controlling the output voltage of the VSC converter, so that a topological structure formed by the LCC and the controllable voltage source is called as a flexible LCC; the flexible LCC can realize the respective control of the direct current bus voltage and the actual arc-quenching angle in the conventional running state; when the alternating current power grid has voltage drops of different degrees, the controllable voltage source can effectively restrain the problem of the commutation failure of the power grid commutation converter.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art are briefly introduced below; it is obvious that the drawings in the following description are some embodiments of the invention, and that for a person skilled in the art, other drawings can be derived from them without inventive effort.
FIG. 1 is a schematic view of a flexible LCC topology according to an embodiment of the present invention;
FIG. 2 is a schematic block diagram of inverter portion control of a flexible LCC in an embodiment of the present invention;
FIG. 3 is a schematic block diagram of a flexible LCC controllable voltage source control strategy according to an embodiment of the present invention; fig. 3 (a) is a schematic block diagram of an upper-layer control strategy of a flexible LCC controllable voltage source, and fig. 3 (b) is a schematic block diagram of a lower-layer sub-module capacitance-voltage balance control of the flexible LCC controllable voltage source;
fig. 4 is a schematic block diagram of control of a controllable voltage source for suppressing converter commutation failure for different degrees of grid voltage sag in the embodiment of the present invention.
Detailed Description
In order to make the purpose, technical effect and technical solution of the embodiments of the present invention clearer, the following clearly and completely describes the technical solution of the embodiments of the present invention with reference to the drawings in the embodiments of the present invention; it is to be understood that the described embodiments are only some of the embodiments of the present invention. Other embodiments, which can be derived by one of ordinary skill in the art from the disclosed embodiments without inventive faculty, are intended to be within the scope of the invention.
The control method for inhibiting the high-voltage direct current commutation failure by adopting the flexible LCC comprises the following steps:
an H-bridge type controllable voltage source is cascaded in series at the valve side of a converter transformer at the high-voltage direct-current transmission inversion side; the controllable voltage source outputs the capacitive voltage by adopting a continuous control method, so that the phase change process of the power grid phase change converter is accelerated, and the probability of phase change failure is reduced. The control of the flexible LCC is divided into a power grid commutation converter part and a controllable voltage source part.
The power grid phase-change converter controls the voltage of the direct-current bus in a mode of adjusting a power grid inversion angle.
The controllable voltage source adopts layered control, and the upper layer adopts vector control of phase current orientation; the d-axis control submodule averages capacitor voltage, and the q-axis control submodule actually controls an arc extinguishing angle of the converter;
the lower layer control of the controllable voltage source is sub-module capacitor voltage balance control;
and generating a preliminary offset control quantity according to the difference value of the direct-current capacitor voltage and the average capacitor voltage of each submodule, and then generating a final offset control quantity of each submodule by combining the direction of the phase current of the submodule. The control quantity is added with the upper three-phase control quantity to obtain the final control quantity of each submodule. And performing unipolar frequency multiplication carrier phase shift modulation on the control quantity for PWM switching control of each sub-module.
And when the voltage of the power grid drops, applying different control modes to the q-axis voltage of the controllable voltage source according to the detected voltage drop degree of the public connection point so as to inhibit phase change failure. When the voltage drop depth is shallow, the q axis of the controllable voltage source is controlled by adopting an actual arc extinction angle; when the voltage drop depth is deep, the maximum control voltage is directly input to the q axis of the controllable voltage source.
In the embodiment of the invention, the flexible LCC can realize the respective control of the direct-current bus voltage and the actual arc-quenching angle in the conventional operation state. When the alternating current power grid has voltage drops of different degrees, the controllable voltage source can effectively restrain the problem of the commutation failure of the power grid commutation converter. When the voltage drop depth is shallow, the stability of the direct current bus voltage and the direct current power can be guaranteed. And when the voltage drop depth is deep, the flexible LCC can provide reactive support for a power grid and promote the stability of the power grid at the alternating current side.
According to the control method for inhibiting the high-voltage direct-current commutation failure by adopting the flexible LCC, in the high-voltage direct-current transmission system, a three-phase series cascade H-bridge type controllable voltage source is arranged between the inversion side power grid commutation converter and the valve side of the converter transformer, and the controllable voltage source and the power grid commutation converter form the flexible LCC.
Each phase of the controllable voltage source is formed by connecting full-bridge submodules in series, the submodule switches adopt a PWM (pulse-width modulation) continuous control mode, and each phase of the controllable voltage source is provided with partial redundant submodules to serve as backup submodules of the operating neutron modules.
In the embodiment of the invention, the upper layer control of the flexible LCC is divided into an LCC part and a controllable voltage source part; the direct-current bus voltage control is realized by controlling the LCC power grid inversion angle; the voltage of the controllable voltage source is phased according to the phase current, the actual arc extinguishing angle control is realized by controlling the q-axis voltage of the controllable voltage source, and the average capacitor voltage control of the submodule of the controllable voltage source is realized by controlling the d-axis voltage of the controllable voltage source.
The grid inversion angle refers to an inversion angle of the LCC relative to a voltage of a public connection point converted to the secondary side of the converter transformer, the actual arc-quenching angle refers to an arc-quenching angle of the LCC relative to a voltage actually borne by the converter valve, and accordingly the actual inversion angle and the grid arc-quenching angle exist.
In the embodiment of the invention, the controllable voltage source is internally provided with a submodule capacitor voltage balance control. And calculating to obtain initial offset voltage control quantity according to the difference value of the capacitor voltage of each submodule and the average capacitor voltage, and determining the positive and negative of final offset control quantity according to the positive and negative of the phase current of the submodule. The direction of the phase current flowing out of the current converter is defined to be positive, the voltage of the controllable voltage source adopts a relevant reference direction, and when the initial offset control quantity is positive, the final offset control quantity is consistent with the positive and negative of the phase current; when the initial offset control amount is negative, the final offset control amount is opposite in sign to the phase current. And adding the final offset control quantity and the upper-layer control quantity of the controllable voltage source converted by 2r/3s to obtain the three-phase final control quantity of the controllable voltage source.
In the embodiment of the invention, the actual arc-quenching angle is estimated according to the grid arc-quenching angle. Under the condition of not considering the nonlinear modulation and the offset control quantity of the submodule of the controllable voltage source, the three-phase voltage control quantity of the controllable voltage source is approximately equal to the output voltage of the controllable voltage source. And adding the three-phase voltage control quantity of the controllable voltage source and the phase voltage converted to the common connection point of the secondary side to obtain the approximate actual phase-changing voltage. And performing 3s/2r conversion on the approximate actual commutation voltage by taking the three-phase synthetic vector of the phase voltage of the common connection point converted to the secondary side of the converter transformer as a reference to obtain the phase difference between the two. The phase difference is added with the arc-extinguishing angle of the power grid to obtain the estimated value of the actual arc-extinguishing angle.
In the embodiment of the invention, the switch of each submodule of the controllable voltage source is controlled by adopting unipolar frequency multiplication carrier phase shift modulation.
When the voltage of the power grid drops, the controllable voltage source adopts different control schemes according to different grid voltage drop depths to inhibit the LCC from failing to change the phase, and the LCC keeps a direct-current bus voltage control mode. When the voltage drop degree is weaker, the q-axis voltage is controlled by adopting an actual arc extinction angle; when the voltage drop degree of the power grid is large, the q-axis control quantity adopts a control mode of maximum compensation quantity input of a controllable voltage source so as to meet the requirement of rapidity of phase change failure inhibition. The voltage drop degree of the power grid is judged by detecting the voltage of the public connection point, and when the voltage of the public connection point is lower than a set value, the voltage drop degree is judged to be larger.
The determination mechanism has hysteresis to prevent control from fluctuating.
And when the master station determines that the fault is finished, the controllable voltage source recovers the actual arc-quenching angle control mode, and the converter recovers the normal operation.
Referring to fig. 1, fig. 1 is a topology structure diagram of a flexible LCC in an embodiment of the present invention, and an example of the flexible LCC inverter is a 12-pulse LCC inverter in which upper and lower fully-controlled bridge ac sides are respectively connected to a star-type secondary side and a delta-type secondary side of a converter transformer. The controllable voltage source is connected in series on the valve side of the converter transformer, and the controllable voltage source is used as a VSC converter, and the parameter characteristics of the LCC can be adjusted by controlling the output voltage of the VSC converter, so that a topological structure formed by the LCC and the controllable voltage source is called a flexible LCC. The controllable voltage source is formed by connecting H bridge sub-modules in series, in order to improve the operation reliability, the sub-modules adopt a redundancy design, and a part of the sub-modules are reserved not to be put into operation as backup sub-modules in normal operation.
Referring to fig. 2, a control strategy adopted by an LCC converter in a flexible LCC according to an embodiment of the present invention is shown in fig. 2. It firstly detects the DC bus voltage UdiFiltering and setting the DC bus voltage value U by a low-pass filterdi *Comparing to generate a voltage deviation value, and then generating a grid inversion angle control quantity beta 'through a PI (proportional integral) controller'*. The phase difference between the grid inversion angle and the grid trigger angle is 180 degrees, so that the grid inversion angle is controlled to be beta'*Conversion to firing Angle control quantity α'*Used for controlling the triggering and conducting of the thyristor. And the trigger angle is limited in the control strategy because the direct current bus can be overturned due to the overlarge trigger angle. The grid firing angle is the firing angle of the converter valve relative to the voltage of the common connection point, so that the phase voltage of the common connection point is input into the phase-locked loop to obtain the phase theta of the phase-locked loopsuFor synchronous trigger control.
Referring to FIG. 3, the upper control strategy of the controllable voltage source in the flexible LCC is shown asFig. 3 (a) shows. It adopts phase current phasing on the secondary side of converter transformer, phase current i2a、i2b、i2cObtaining the phase theta through a phase-locked loopsiThe corresponding phasor is taken as the position of the d-axis of the control voltage and the corresponding position vertically advanced by a phase angle of 90 deg. is taken as the q-axis. Q-axis control voltage u of controllable voltage sourcecpq *Actual extinction angle gamma, d-axis voltage u for controlling LCCcpd *And the method is used for controlling the average capacitance voltage stabilization of the sub-modules. The d-axis and q-axis control quantities are converted by 3s/2r to generate three general control quantities u of the controllable voltage sourcecpa *、ucpb *、ucpc *. The actual extinction angle gamma is the extinction angle of the converter valve relative to the actual commutation voltage, the grid extinction angle gamma' is the extinction angle of the converter valve relative to the voltage of the common connection point, and the latter can be directly obtained in the control system, but the former is inconvenient to directly obtain. Three-phase control quantity u in control strategycpa *、ucpb *、 ucpc *With three-phase voltage u detected at point of common connectionsa、usb、uscAdding to obtain the actual commutation voltage estimated value uta *、utb *、 utc *Then at the phase theta of the voltage at the common connection point su3s/2r conversion is carried out for reference to obtain the phase difference lambda between the actual phase-change voltage and the voltage of the common connection point*And adding the phase difference and the grid arc-quenching angle to obtain an actual arc-quenching angle estimated value serving as a feedback quantity of actual arc-quenching angle control. The calculation process can be expressed by the following formula:
Figure BDA0002683175970000081
Figure BDA0002683175970000082
Figure BDA0002683175970000083
γ＝λ*+γ′。 (4)
referring to fig. 3, (b) in fig. 3 is a lower control strategy of the controllable voltage source, i.e. a sub-module capacitor voltage balance control strategy. It is based on the capacitor voltage u of each sub-moduledcpi(p is the phase in which the submodules are located, i is the ordering of the submodules in that phase) and the average capacitor voltage u of all the submodulesdcav' the deviation produces a preliminary offset control amount via the PI controller, and then changes the positive or negative of the preliminary offset control amount according to the positive or negative of the phase current in which the sub-module is located to generate a final offset control amount Delautcvpi *. When the direction of the specified phase current flowing out of the converter valve is positive and the controllable voltage source outputs voltage in the relevant reference direction, if the initial offset control quantity is positive, the positive and negative of the final offset control quantity are the same as the positive and negative of the phase current; if the preliminary offset control amount is negative, the final offset control amount is opposite in positive and negative to the phase current in positive and negative. And adding the final offset control quantity and the general control quantity of the phase where the submodule is located to obtain the control quantity finally used for the submodule carrier phase shift modulation.
Referring to fig. 4, the control strategy of the flexible LCC in the normal operation state is described above, and fig. 4 is a control strategy of the flexible LCC for suppressing the commutation failure when the three-phase voltage of the power grid drops. The method is similar to a control strategy of a flexible LCC in a normal operation state, and different control modes are adopted aiming at the q axis of a controllable voltage source of the grid voltage drop of different degrees. When the detected voltage effective value U of the public connection pointsiIs higher than the set falling depth judgment threshold value Usil *And when the output of the SR trigger is zero, the voltage control quantity generated by the corresponding SR trigger part is 0, and the control quantity is compared with the control quantity generated by the actual arc-extinguishing angle control through the maximum value, so that the fact that the q axis of the controllable voltage source keeps the actual arc-extinguishing angle control mode is determined. When U is turnedsiLower than Usil *When the SR flip-flop output is 1, the corresponding generated control amount is √ 3UcpmaxThe value is the maximum output value of the voltage of the q axis of the controllable voltage source, so the maximum voltage output is generated by the q axis of the controllable voltage source after the maximum value comparison is adopted. SR-based triggeringThe self-characteristic of the device, when the maximum compensation amount control is in effect, the controllable voltage source can keep the control state. Thus, the voltage drop level determination means has hysteresis. When the master station sends out a fault ending instruction, the output of the SR trigger is cleared, the control voltage output is also zero, and then the q axis of the controllable voltage source is recovered to be controlled by the actual arc-quenching angle. The control strategy can effectively inhibit LCC commutation failure aiming at different degrees of voltage drop. In addition, when the voltage drop depth is shallow, the voltage and power transmission of the direct-current bus can be effectively maintained; when the voltage drop depth is deep, the requirement of rapidity of fault treatment can be met, and meanwhile reactive power support to a power grid can be achieved.
The control system for suppressing the high-voltage direct-current commutation failure of the flexible power grid commutation converter comprises:
the cascade H-bridge type controllable voltage source is connected in series with a converter transformer valve side on the high-voltage direct-current transmission inversion side and forms a flexible LCC with a power grid commutation converter; and the controllable voltage source outputs the capacitive voltage by adopting a continuous control method.
As will be appreciated by one skilled in the art, embodiments of the present application may be provided as a method, system, or computer program product. Accordingly, the present application may take the form of an entirely hardware embodiment, an entirely software embodiment or an embodiment combining software and hardware aspects. Furthermore, the present application may take the form of a computer program product embodied on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, and the like) having computer-usable program code embodied therein.
The present application is described with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the application. It will be understood that each flow and/or block of the flow diagrams and/or block diagrams, and combinations of flows and/or blocks in the flow diagrams and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions specified in the flowchart flow or flows and/or block diagram block or blocks.
These computer program instructions may also be stored in a computer-readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including instruction means which implement the function specified in the flowchart flow or flows and/or block diagram block or blocks.
These computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart flow or flows and/or block diagram block or blocks.
Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art can make modifications and equivalents to the embodiments of the present invention without departing from the spirit and scope of the present invention, which is set forth in the claims of the present application.

Claims (7)

1. A control method for suppressing high-voltage direct-current commutation failure of a flexible power grid commutation converter is characterized by comprising the following steps:
an H-bridge type controllable voltage source is cascaded in series at the valve side of a converter transformer at the high-voltage direct-current transmission inversion side; the controllable voltage source outputs capacitive voltage by adopting a continuous control method;
when the power grid normally operates or the voltage drops, the power grid phase-change converter controls the voltage of the direct-current bus in a mode of adjusting a power grid inversion angle;
the controllable voltage source adopts layered control; wherein, the upper layer of the controllable voltage source adopts vector control of phase current orientation; the d-axis control submodule averages capacitor voltage, and the q-axis control submodule actually controls an arc extinguishing angle of the converter; the lower layer control of the controllable voltage source is sub-module capacitor voltage balance control;
when the power grid normally operates, the q axis of the controllable voltage source is controlled by adopting an actual arc extinguishing angle; and when the voltage of the power grid drops, applying different control modes to the q-axis voltage of the controllable voltage source according to the detected voltage drop degree of the public connection point so as to inhibit phase change failure.
2. The control method for the flexible power grid commutation converter to suppress the high-voltage direct-current commutation failure according to claim 1, wherein the step of applying different control modes to the q-axis voltage of the controllable voltage source according to the detected voltage drop degree of the common connection point to suppress the commutation failure in the power grid voltage drop specifically comprises:
when the voltage drop depth is shallow, the q axis of the controllable voltage source is controlled by adopting an actual arc extinction angle; when the voltage drop depth is deep, the q axis of the controllable voltage source directly inputs the maximum control voltage; the depth of the voltage drop depth of the power grid is judged by detecting the voltage of the public connection point; when the voltage of the common connection point is less than or equal to a preset value, determining that the voltage drop depth is deeper; and when the voltage of the common connection point is greater than a preset value, determining that the voltage drop depth is shallow.
3. The control method for the flexible power grid commutation converter to suppress high-voltage direct-current commutation failure according to claim 1, wherein each phase of the controllable voltage source is formed by connecting full-bridge sub-modules in series; and the sub-module switch adopts a PWM continuous control mode.
4. The control method for the flexible power grid commutation converter to suppress high-voltage direct-current commutation failure according to claim 3, wherein in a controllable voltage source, the sub-module capacitor voltage balance control comprises: generating a preliminary offset control quantity according to the difference value of the direct current capacitor voltage of each submodule and the average capacitor voltage; generating a final offset control quantity of each submodule according to the phase current direction of the submodule; adding the final offset control quantity and the upper three-phase control quantity to obtain the final control quantity of each submodule;
and performing unipolar frequency multiplication carrier phase shift modulation on the final control quantity, and controlling the PWM switch of each submodule.
5. The control method for the flexible power grid commutation converter to suppress high voltage direct current commutation failure according to claim 3, wherein each phase of the controllable voltage source has a partially redundant submodule for serving as a backup submodule of an operational submodule.
6. The control method for the flexible power grid commutation converter to inhibit high-voltage direct-current commutation failure according to claim 1, wherein a determination mechanism with hysteresis is adopted for determining the depth of the grid voltage drop depth.
7. The control method for suppressing HVDC commutation failure of the flexible grid commutation converter of claim 1, wherein the actual extinction angle is estimated from the grid extinction angle and comprises:
adding the three-phase voltage control quantity of the controllable voltage source and the phase voltage converted to the common connection point of the secondary side to obtain an actual phase-change voltage estimation value;
taking a three-phase synthetic vector of a phase voltage of a common connection point converted to the secondary side of the converter transformer as a reference, and carrying out 3s/2r conversion on an actual phase-change voltage estimation value to obtain a phase difference between the two;
and adding the phase difference to the power grid arc-quenching angle to obtain an actual arc-quenching angle estimated value.
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