CN110165641B - Superposition method of direct current circuit breaker in flexible direct current transmission system - Google Patents
Superposition method of direct current circuit breaker in flexible direct current transmission system Download PDFInfo
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- CN110165641B CN110165641B CN201910549761.5A CN201910549761A CN110165641B CN 110165641 B CN110165641 B CN 110165641B CN 201910549761 A CN201910549761 A CN 201910549761A CN 110165641 B CN110165641 B CN 110165641B
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H3/00—Emergency protective circuit arrangements for automatic disconnection directly responsive to an undesired change from normal electric working condition with or without subsequent reconnection ; integrated protection
- H02H3/02—Details
- H02H3/06—Details with automatic reconnection
- H02H3/063—Details concerning the co-operation of many similar arrangements, e.g. in a network
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H3/00—Emergency protective circuit arrangements for automatic disconnection directly responsive to an undesired change from normal electric working condition with or without subsequent reconnection ; integrated protection
- H02H3/08—Emergency protective circuit arrangements for automatic disconnection directly responsive to an undesired change from normal electric working condition with or without subsequent reconnection ; integrated protection responsive to excess current
- H02H3/087—Emergency protective circuit arrangements for automatic disconnection directly responsive to an undesired change from normal electric working condition with or without subsequent reconnection ; integrated protection responsive to excess current for dc applications
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
- H02H7/26—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured
- H02H7/268—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured for dc systems
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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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- 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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- Engineering & Computer Science (AREA)
- Power Engineering (AREA)
- Direct Current Feeding And Distribution (AREA)
Abstract
The invention discloses a superposition method of a direct current breaker in a flexible direct current transmission system, which comprises the following steps: analyzing the fault characteristics of the direct current circuit breaker when the direct current circuit breaker is recombined with transient and permanent faults aiming at the faults with different properties in the flexible direct current transmission system; based on the fault characteristics obtained by analysis, the switching characteristics of the transfer branch of the direct-current circuit breaker are utilized, and the superposition of the direct-current circuit breaker is realized by adopting a mode of conducting the transfer branch and switching the control strategies of the converter stations at two ends of the fault line after the line fault is dissociated. The method can reduce the oscillation when the fault is superposed on the transient fault and the secondary overcurrent impact when the fault is superposed on the permanent fault, thereby ensuring the safe and stable operation of the system.
Description
Technical Field
The invention relates to the technical field of control and protection of a flexible direct current transmission system, in particular to a superposition method of a direct current breaker in the flexible direct current transmission system.
Background
Compared with the traditional direct current transmission, the flexible direct current transmission has the advantages that the problems of reactive compensation, commutation failure and the like do not exist, and meanwhile, the current can flow in two directions, so that the flexible direct current transmission is suitable for forming a multi-terminal direct current system. However, considering the power transmission distance, the voltage level and the cost problem caused by the power transmission distance and the voltage level, overhead line power transmission is a main mode of large-scale power transmission in China in the future, and therefore, the research on the fault protection related technology of the flexible direct current power transmission system applying the overhead line has important significance. The main problem with overhead line applications is that the probability of transient failures increases significantly, and therefore the application of reclosing techniques is of paramount importance.
The domestic and foreign research on the superposition technology of the flexible direct-current transmission system mainly comprises two types: one is based on a novel reclosing technology of a converter with a fault self-clearing capacity, and the other is based on a reclosing technology of a direct current breaker. Compared with the prior art, the fault isolation device can rapidly remove and isolate faults, and can still realize certain power transmission during the fault, so that the normal operation of the system is ensured. Therefore, the reclosing technology based on the direct current breaker is a hot spot of the current technology development. Most of the existing fault protection schemes based on the direct current circuit breaker relate to how to rapidly remove faults, and the research on the reclosing schemes after the faults is less, so that a reclosing method of the direct current circuit breaker in a flexible direct current transmission system needs to be researched, and an improved overall protection scheme is provided for the faults with different properties occurring in a line.
Disclosure of Invention
The invention aims to provide a superposition method of a direct current breaker in a flexible direct current transmission system, which can reduce oscillation when the direct current breaker is superposed on a transient fault and secondary overcurrent impact when the direct current breaker is superposed on a permanent fault, thereby ensuring the safe and stable operation of the system.
The purpose of the invention is realized by the following technical scheme:
a superposition method of direct current circuit breakers in a flexible direct current transmission system comprises the following steps:
analyzing the fault characteristics of the direct current circuit breaker when the direct current circuit breaker is recombined with transient and permanent faults aiming at the faults with different properties in the flexible direct current transmission system;
based on the fault characteristics obtained by analysis, the switching characteristics of the transfer branch of the direct-current circuit breaker are utilized, and the superposition of the direct-current circuit breaker is realized by adopting a mode of conducting the transfer branch and switching the control strategies of the converter stations at two ends of the fault line after the line fault is dissociated.
According to the technical scheme provided by the invention, 1) the method is realized mainly by a transfer branch of a direct current breaker without additionally adding equipment, is formed by connecting a large number of power electronic devices in series and has a quick switching characteristic; 2) after the fault is dissociated, the voltage difference between two ends of the line is reduced by switching the control strategy of the converter station, so that the oscillation generated when the direct-current circuit breaker is recombined with the transient fault is reduced, the rapid judgment on the permanent fault is realized by utilizing the rapid switching characteristic of the transfer branch of the direct-current circuit breaker, and compared with the conventional method, the direct-current circuit breaker can be quickly disconnected, so that the secondary overcurrent impact during the recombination is reduced.
Drawings
In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings needed to be used in the description of the embodiments are briefly introduced below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings based on the drawings without creative efforts.
Fig. 1 is a flowchart of a method for reclosing a dc breaker in a flexible dc power transmission system according to an embodiment of the present invention;
FIG. 2 is a flowchart of step 2 according to an embodiment of the present invention;
fig. 3 is a schematic topology diagram of a double-ended flexible dc power transmission system according to an embodiment of the present invention;
fig. 4 is a schematic diagram illustrating a comparative simulation of a conventional method and a reclosing method of a dc circuit breaker according to the present invention during transient fault according to an embodiment of the present invention;
fig. 5 is a schematic diagram illustrating a comparative simulation of a reclosing method of a dc circuit breaker according to the present invention and a conventional method when a permanent fault occurs according to an embodiment of the present invention.
Detailed Description
The technical solutions in the embodiments of the present invention are clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments of the present invention without making any creative effort, shall fall within the protection scope of the present invention.
The embodiment of the invention provides a superposition method of a direct current breaker in a flexible direct current transmission system, which is characterized by comprising the following steps:
Overhead lines are adopted in the flexible direct current transmission system for power transmission, and the application of the overhead lines greatly increases the transient fault probability. When the dc circuit breaker is reclosed to an instantaneous fault, a fault line after the dc circuit breaker is opened is equivalent to an unloaded line, and due to different control strategies of an MMC (Modular Multilevel Converter) at two ends of the line, the dc voltages at two ends of the line after the dc circuit breaker is opened are inconsistent, so that the reclosing can cause oscillation of the electric quantity in the system.
For a flexible direct-current power transmission system with two ends, one end is a fixed voltage station and the other end is a fixed power station under general conditions, after a direct-current circuit breaker is superposed on an instantaneous fault, the direct-current circuit breaker is disconnected, namely, an arrester in an energy consumption branch circuit of the direct-current circuit breaker is connected in series in a circuit, the current of the circuit is always 0 before the direct-current circuit breaker is superposed, because the arrester absorbs the power transmitted by an MMC at the two ends at the moment, the arrester is treated as a constant-power load, and the expression is as follows:
P＝Uiload＝constant
wherein U is the bus voltage, iloadConstant represents a constant value for the line current;
therefore, the circuit is equivalent to heavy load operation at the moment, the constant voltage control strategy has no way to stabilize the voltage, and therefore, the oscillation is not reduced by changing the control strategy of the converter station;
when the fault is superposed on a permanent fault, the fault point still exists after the fault is dissociated, the direct current circuit breaker is superposed, the sub-module capacitor is discharged again, and secondary overcurrent impact is caused to the system. Since the mechanical switch needs to be closed again in the conventional reclosing process, if the mechanical switch is in a permanent fault, the reclosing time is long, and the rising speed of the fault current is high after the direct-current side fault, the fault current value is overlarge again, so that the safety and the stability of the system are influenced.
And 2, based on the fault characteristics obtained by analysis, realizing the coincidence of the direct-current circuit breakers by utilizing the switching characteristics of the transfer branch of the direct-current circuit breaker and adopting a mode of conducting the transfer branch and switching the control strategies of the converter stations at two ends of the fault circuit after the line fault is dissociated.
The superposition scheme of the direct current breaker provided by the embodiment of the invention can reduce the oscillation when the direct current breaker is superposed on the transient fault and the secondary overcurrent impact when the direct current breaker is superposed on the permanent fault, thereby ensuring the safe and stable operation of the system.
This step is implemented on the basis of the analysis result of step 1, as shown in fig. 2, the process includes:
when the line protection detects that di/dt and du/dt (differentiation of current and voltage) meet action criteria (action judgment can be set according to actual conditions), starting a protection process; if the line current value IdcOver-current protection set value IlimThe direct current breaker acts to clear the fault current; after a period of time (for example, 250ms) for line dissociation delay, switching on a transfer branch of the direct current breaker, simultaneously switching a control strategy of the constant power station to change the constant power station into a constant voltage station and detecting whether the current is over-current (i.e. I)dc>Ilim？)；
If not, the fault is determined to be instantaneous fault, the conducting transfer branch bypasses the lightning arrester at the moment, then the fixed voltage stations are arranged on two sides of the lightning arrester, the voltage can be controlled stably, the voltage difference between two ends of the line is reduced, and finally the transfer branch is turned off and the main branch mechanical switch is closed, so that the coincidence process is completed;
if the direct overcurrent is conducted after the transfer branch is conducted, the commutation branch is turned off, the transfer branch is conducted for the second time after the line dissociation delay time of a period of time (for example, 250ms) passes, if the line dissociation delay time is still conducted, the fault is a permanent fault, the transfer branch is turned off at the moment, the direct-current circuit breaker is kept to be disconnected, and line maintenance is conducted by a maintenance worker.
The scheme of the invention has the following beneficial effects:
1) the method is realized mainly by means of a transfer branch of the direct current breaker without additionally increasing equipment, and the direct current breaker is formed by connecting a large number of power electronic devices in series and has a rapid switching characteristic.
2) After the fault is dissociated, the voltage difference between two ends of the line is reduced by switching the control strategy of the converter station, so that the oscillation generated when the direct-current circuit breaker is recombined with the transient fault is reduced, the rapid judgment on the permanent fault is realized by utilizing the rapid switching characteristic of the transfer branch of the direct-current circuit breaker, and compared with the conventional method, the direct-current circuit breaker can be quickly disconnected, so that the secondary overcurrent impact during the recombination is reduced.
Fig. 3 is a schematic diagram of a topology of a double-ended flexible direct current transmission system. The rated voltage level of a direct current line is +/-320 kV, alternating current systems on two sides are connected with the direct current line through a modular multilevel converter (HBSM-MMC) based on a half-bridge submodule, MMC1 in the system adopts constant direct current voltage control, MMC2 adopts constant active power control, each direct current breaker is a hybrid direct current breaker, and the configuration is used for rapidly isolating faults at two ends of a power transmission line. The fault is a bipolar short circuit fault and occurs at the midpoint of the dc link as shown at F1 in fig. 3.
In order to illustrate the effects of the above-described embodiment of the present invention, the results are shown in FIGS. 4 to 5, compared with the conventional method. In both figures: (a1) (a2) is the line current after the conventional method and the line current after the method of the invention are adopted respectively; (b1) and (b2) are the fault point electrode voltage after the conventional method and the fault point electrode voltage after the method of the present invention are adopted, respectively; (c1) and (c2) are the bus bar electrode voltage after the conventional method and the bus bar electrode voltage after the method of the present invention.
Fig. 4 is a simulation diagram comparing the reclosing method of the dc breaker according to the present invention with the conventional method when the transient fault occurs in the line. In the conventional method, after the line fault is dissociated, the direct current breaker is superposed, and the oscillation is larger due to the voltage difference between two ends of the line; and when the line fault is dissociated, the transfer branch is conducted and the converter station control strategies at two ends of the fault line are switched by adopting the superposition method, and because the lightning arrester in the energy consumption branch is bypassed by the transfer branch and the converter station control strategies at two ends are both constant voltage control strategies, the voltage difference at two ends of the line is smaller, and the oscillation is obviously reduced during superposition.
Fig. 5 is a simulation diagram comparing the reclosing method of the dc breaker according to the present invention with the conventional method when a permanent fault occurs in a line. In the conventional method, because the fault property cannot be quickly judged when a permanent fault occurs, secondary overcurrent impact can be caused to the system when the direct-current circuit breaker is reset, and the safe and stable operation of the system is threatened; when the direct current breaker is reclosed, the transfer branch circuit is quickly conducted firstly, and is immediately locked once overcurrent exists, if the overcurrent still exists twice, the permanent fault is judged, and the transfer branch circuit is formed by power electronic devices and has quick switching characteristics, so that the transfer branch circuit can act faster than the conventional method, and the secondary impact of fault current suffered during the reclosing process is reduced.
Through the above description of the embodiments, it is clear to those skilled in the art that the above embodiments can be implemented by software, and can also be implemented by software plus a necessary general hardware platform. With this understanding, the technical solutions of the embodiments can be embodied in the form of a software product, which can be stored in a non-volatile storage medium (which can be a CD-ROM, a usb disk, a removable hard disk, etc.), and includes several instructions for enabling a computer device (which can be a personal computer, a server, or a network device, etc.) to execute the methods according to the embodiments of the present invention.
The above description is only for the preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope of the present invention are included in the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.
Claims (2)
1. A method for reclosing a DC circuit breaker in a flexible DC power transmission system is characterized by comprising the following steps:
analyzing the fault characteristics of the direct current circuit breaker when the direct current circuit breaker is recombined with transient and permanent faults aiming at the faults with different properties in the flexible direct current transmission system;
based on the fault characteristics obtained by analysis, the switching characteristics of the transfer branch of the direct-current circuit breaker are utilized, and the superposition of the direct-current circuit breaker is realized by adopting a mode of conducting the transfer branch and switching the control strategies of the converter stations at two ends of the fault line after the line fault is dissociated;
when the line protection detects that the differential of the current and the voltage meets the action criterion, starting a protection process; if the current value of the line exceeds the set value of overcurrent protection, the direct current breaker acts to clear the fault current; after a period of time of line dissociation delay, switching on a transfer branch of the direct current breaker, switching a control strategy of the constant power station to change the constant power station into a constant voltage station and detecting whether current is over-current or not;
if not, the fault is determined to be instantaneous fault, the conducting transfer branch bypasses the lightning arrester at the moment, then the fixed voltage stations are arranged on two sides of the lightning arrester, the voltage can be controlled stably, the voltage difference between two ends of the line is reduced, and finally the transfer branch is turned off and the main branch mechanical switch is closed, so that the coincidence process is completed;
if direct overcurrent after switching on the transfer branch, turn off the current conversion branch, pass through a period of circuit and come the second time after dissociating the time delay and switch on the transfer branch, if still overflowing, then indicate the trouble is permanent trouble, turn off the transfer branch and keep direct current breaker disconnected always this moment, carry out the circuit by the maintainer and overhaul.
2. The reclosing method of a DC breaker in a flexible DC power transmission system according to claim 1,
the flexible direct current transmission system adopts overhead lines for transmission, when the direct current breaker is superposed with an instantaneous fault, a fault line after the direct current breaker is opened is equivalent to an idle load line, MMC control strategies at two ends of the line are different, and direct current voltages at two ends of the line after the direct current breaker is opened are inconsistent; for a flexible direct-current power transmission system with two ends, assuming that one end is a fixed voltage station and the other end is a fixed power station, after a direct-current circuit breaker is superposed with an instantaneous fault, the direct-current circuit breaker disconnects a lightning arrester in an energy consumption branch circuit of the direct-current circuit breaker and is connected in series in a circuit, the current of the circuit is always 0 before the direct-current circuit breaker is superposed, the lightning arrester absorbs the power transmitted by an MMC with the two ends, and the power is treated as a constant-power load, wherein the expression is as follows:
P＝Uiload＝constant
wherein U is the bus voltage, iloadConstant represents a constant value for the line current;
when the fault is superposed on a permanent fault, the fault point still exists after the fault is dissociated, the direct current circuit breaker is superposed, the sub-module capacitor is discharged again, and secondary overcurrent impact is caused to the system.
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