CN111211574A - Method for calculating bipolar fault current on direct current side of multi-terminal direct current power grid based on half-bridge type MMC - Google Patents

Method for calculating bipolar fault current on direct current side of multi-terminal direct current power grid based on half-bridge type MMC Download PDF

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CN111211574A
CN111211574A CN202010026724.9A CN202010026724A CN111211574A CN 111211574 A CN111211574 A CN 111211574A CN 202010026724 A CN202010026724 A CN 202010026724A CN 111211574 A CN111211574 A CN 111211574A
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fault
current
line
direct current
power grid
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郝亮亮
李伟杰
王卓雅
郭智琳
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Beijing Jiaotong University
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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
    • 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
    • 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 provides a method for calculating bipolar fault current at a direct current side of a multi-terminal direct current power grid based on a half-bridge type MMC, which comprises the following steps of: firstly, detecting a normally-operated direct-current power grid to obtain normally-operated electrical quantity parameters and system intrinsic parameters. Secondly, splitting a direct-current power grid topology after bipolar fault by a non-fault line farthest from a fault line, and converting the split non-fault line into an open network; and calculating the currents in the independent discharging loops of the equivalent converter stations at two ends of the line where the fault is located for the fault point in the open network, and respectively equivalent to the fault components of the currents of the left and right side lines of the fault point in the original ring-shaped power grid. And finally, adding the fault current and the line current in normal operation to obtain the direct-current side bipolar fault current of the multi-terminal direct-current system based on the half-bridge type MMC. By implementing the method, the approximate analytic calculation of the direct-current side unipolar ground fault current of the half-bridge-type MMC multi-terminal direct-current system can be effectively realized without complex modeling simulation, the calculation time is short, the universality is wide, and the realization cost is low.

Description

Method for calculating bipolar fault current on direct current side of multi-terminal direct current power grid based on half-bridge type MMC
Technical Field
The invention belongs to the technical field of power system relay protection, and particularly relates to a method for calculating bipolar fault current on a direct-current side of a multi-terminal direct-current power grid based on a half-bridge type MMC.
Background
The modular multilevel converter-based flexible direct current transmission technology (MMC-HVDC) has the characteristics of independently controlling active power and reactive power, having no commutation failure, being capable of supplying power to a passive network or a weak alternating current system and the like, has wide application prospects in the fields of flexible consumption of large-scale renewable energy sources, long-distance large-capacity transmission, asynchronous networking and the like, and is an important direction for future power grid development. At present, most of MMC-HVDC projects which are put into operation at home adopt cables for power transmission, and compared with overhead lines, the cable type power transmission system is high in manufacturing cost and inconvenient to overhaul and maintain. Therefore, the adoption of overhead line transmission gradually becomes the future development trend of the flexible direct current power grid.
Compared with the cable, the overhead line transmission has higher probability of short-circuit faults, wherein the bipolar short-circuit faults of the line are the most serious faults. After the fault, due to the low damping characteristic of the flexible direct current power grid, the fault current rises rapidly, and the steady-state value is as high as tens of kiloamperes. At present, a direct current breaker is mostly adopted in a flexible direct current power grid to remove fault current, which is an important measure for ensuring the safe and efficient operation of the flexible direct current power grid.
Because the breaking capacity of the existing direct current breaker is limited, current limiting measures such as a smoothing reactor and the like are additionally arranged at the outlet of the converter station to limit the rising speed of the fault current, so that the fault current is effectively eliminated within the breaking capacity range of the direct current breaker. Therefore, mastering the short-circuit fault current level is an important basis for designing parameters, inhibiting fault current and designing a protection scheme of the flexible direct-current power grid.
At present, a common method for calculating the short-circuit current in the flexible direct-current power grid is to solve through electromagnetic transient simulation software, but due to the limitations of complex modeling, long time consumption and the like of simulation, the method is not beneficial to practical engineering application. Therefore, people begin to seek a simpler and faster calculation method, and the analytic calculation of the fault current of the flexible direct-current transmission system becomes a research hotspot.
At present, most of research on a short-circuit current analysis and calculation method of a flexible direct-current power transmission system is concentrated on a double-end system. The existing related researches consider that the double-end systems can respectively solve the analytical expressions of line fault current because the coupling relation does not exist between the converter stations after the fault, namely, the left and right systems at the fault point are mutually independent. For the flexible direct current power grid, due to the fact that corresponding coupling relations exist among the converter stations, left and right side systems of a fault point are not independent, assumed conditions under a double-end system are not applicable any more, the converter stations are generally linearized within a period of time after the fault, and then a system state matrix is written in a column mode to obtain line fault current and converter station outlet current through a method of solving system state differential equations under different faults. However, the flexible direct-current power grid includes a plurality of energy storage elements, the differential equation is higher in order and cannot be solved analytically, and finally, the line fault current still needs to be solved by means of simulation, so that an engineering practical calculation method capable of quickly solving the line fault current by means of analysis is urgently needed.
Disclosure of Invention
The invention aims to overcome the defect that the fault current of a flexible direct-current power grid line cannot be solved quickly in the prior art, and provides a multi-terminal direct-current power grid direct-current side bipolar fault current calculation method based on a half-bridge type MMC. The method is based on the electric quantity change and the fault current characteristics after the fault of the multi-terminal direct-current power grid, can effectively express the overall trend of the line fault current within a period of time after the fault, and comprises the following steps:
step (1), acquiring inherent parameters of a multi-terminal direct current system, wherein the inherent parameters mainly comprise the parameters of each terminal converter station: bridge arm submodule number NnSub-module capacitance value C0nBridge arm reactance value L0nBridge arm loss equivalent resistance R0nLine parameter Rline_n、Lline_nParameter L of smoothing reactorpb_n；
Step (2), obtaining a normal operation component I of line current before the fault of the multi-terminal direct current system to be researched0nAnd the DC side output voltage U of the converter station in the systemdc；
Step (3), splitting the ring-shaped power grid into an open network by a non-fault line with the least current rise after the fault in the direct-current power grid, namely the non-fault line farthest from the fault line, and calculating corresponding equivalent parameters according to the parallel relation of the split converter stations;
step (4), calculating the fault component i of the line current where the fault is in the system according to the split multi-terminal direct current system topological structuref_n(t)；
Step (5), calculating the total fault current i of the linefline_n(t)。
In the approximate analysis calculation method based on the bipolar fault current at the direct current side of the half-bridge type MMC multi-terminal direct current power grid, in the step (3), a non-fault pole network in the multi-terminal direct current system is removed, and an alternating current system connected with each converter station is deleted; and removing a metal return wire between converter stations at two ends of a line where the non-fault exists, and splitting the original ring network into an open network by the non-fault line which is farthest away from the fault line in the original topology of the multi-end direct current system.
The internal equivalent parameters of the converter station are as follows:
Figure BDA0002362735800000031
Figure BDA0002362735800000032
wherein R issumn、Lsumn、CsumnRespectively, the external equivalent parameters, L, of each converter stationzIs a neutral current limiting reactor.
In the open network, the equivalent calculation formula after the parallel connection of the converter stations is as follows:
Figure BDA0002362735800000041
in the approximate analysis calculation method based on the bipolar fault current at the dc side of the half-bridge type MMC multi-terminal dc power grid, in the step (4), only the fault network is considered, and a calculation formula of the fault component of the fault line current in the split open network is used as follows:
Figure BDA0002362735800000042
Figure BDA0002362735800000043
Figure BDA0002362735800000044
Figure BDA0002362735800000045
wherein j is 1, 2; i.e. if_j(t) represents the left and right line fault components of the fault, UdcRepresenting the output voltage of the direct current side of the converter station in normal operation; n is a radical ofj、Leqsj、Reqsj、CeqsjRepresenting equivalent parameters of parallel connection of converter stations at two ends of a line where a fault exists in the open network; l isdc_j、Rdc_jAnd indicating the parameters of the discharge loop of the equivalent converter station to the fault point at the two ends of the line where the fault is positioned.
In the above approximate analysis calculation method based on the bipolar fault current at the dc side of the half-bridge type MMC multi-terminal dc power grid, in step (5), the calculation method of the total line fault current includes:
ifline_j(t)＝if_j(t)+I0， (8)
wherein j is 1, 2; i.e. ifine_j(t) represents the fault current total of the left and right side lines of the fault point; i is0Representing the line current normal operating component prior to the fault.
The technical scheme of the invention has the following advantages:
according to the method for calculating the bipolar fault current at the direct current side of the multi-terminal direct-current power grid based on the half-bridge MMC, the fault current of the flexible direct-current power grid line can be directly obtained through calculation, complex modeling and simulation of the multi-terminal direct-current power grid are not needed, and the method has high practicability; the invention only needs to combine the inherent parameters of the system, the normal operation state parameters and the fault occurrence position for calculation, and has stronger performability. By implementing the method, the line fault current can be calculated when bipolar fault occurs on the direct current side of the half-bridge MMC-based multi-terminal direct current power grid.
Drawings
Fig. 1 is a block diagram of a procedure of a method for calculating a bipolar fault current on a dc side of a multi-terminal dc power grid based on a half-bridge type MMC according to an embodiment of the present invention;
FIG. 2 is a topology structure diagram of a half-bridge MMC-based Zhang-North four-terminal direct current power grid;
FIG. 3 is a basic structure diagram of an MMC converter;
FIG. 4 is a schematic diagram of a post-fault health care station providing a fault current path to a fault point;
FIG. 5 is a simplified topological diagram of a half-bridge MMC-based Zhang-North four-terminal direct-current power grid after a fault;
FIG. 6 is an equivalent circuit diagram of the converter station after a fault;
fig. 7 is an equivalent circuit diagram of the open network obtained by splitting the dc ring grid after the fault;
FIG. 8 is a parallel equivalent parameter diagram for the converter stations;
FIG. 9 is a calculated equivalent circuit diagram after parallel equivalence of converter stations in the open network;
fig. 10 is a simulation and analysis comparison graph of line fault current based on half-bridge MMC north-expanding.
Detailed Description
The technical solutions of the present invention will be described clearly and completely with reference to the accompanying drawings, and it should be understood that the described embodiments are some, but not all embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
In addition, the technical features involved in the different embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.
Example 1
The embodiment of the invention provides a method for calculating bipolar fault current at a direct current side of a multi-terminal direct current power grid based on a half-bridge type MMC, which comprises the following steps of:
step (1), acquiring inherent parameters of the multi-terminal direct current system, which mainly comprise parameters of each-terminal converter stationNumber: bridge arm submodule number NnSub-module capacitance value C0nReactance value L of bridge arm0nBridge arm loss equivalent resistance R0nLine parameter Rline_n、Lline_nParameter L of smoothing reactorpb_n；
Step (2), obtaining a normal operation component I of line current before the fault of the multi-terminal direct current system to be researched0nAnd the DC side output voltage U of the converter station in the systemdc；
Step (3), splitting the ring-shaped power grid into an open network by a non-fault line with the least current rise after the fault in the direct-current power grid, namely the non-fault line farthest from the fault line, and calculating corresponding equivalent parameters according to the parallel relation of the split converter stations;
step (4), calculating the fault component i of the line current where the fault is in the system according to the split multi-terminal direct current system topological structuref_n(t)；
Step (5), calculating the total fault current i of the linefline_n(t)。
According to the method for calculating the bipolar fault current at the direct current side of the multi-terminal direct-current power grid based on the half-bridge MMC, the fault current of the flexible direct-current power grid line can be directly obtained through calculation, complex modeling and simulation of the multi-terminal direct-current power grid are not needed, and the method has high practicability; the invention only needs to combine the inherent parameters of the system, the normal operation state parameters and the fault occurrence position for calculation, and has stronger performability.
In an optional embodiment, in the step (3), the non-fault pole network in the multi-terminal dc system is removed, and the ac system connected to each converter station is deleted;
removing a metal return wire between converter stations at two ends of a line where a non-fault exists, and splitting an original ring network into an open network by the non-fault line which is farthest away from the fault line in the original topology of the multi-end direct current system;
the internal equivalent parameters of the converter station are as follows:
Figure BDA0002362735800000071
Figure BDA0002362735800000072
wherein R issumn、Lsumn、CsumnRespectively, the external equivalent parameters, L, of each converter stationzA neutral current limiting reactor;
in the open network, the equivalent calculation formula after the parallel connection of the converter stations is as follows:
Figure BDA0002362735800000073
in an alternative embodiment, in step (4), only the fault network is considered, and the calculation formula for respectively and equivalently calculating the fault components of the fault left and right line currents by using the currents in the single discharge loops of the fault point of the converter stations at the two ends of the line where the fault occurs after the fault is as follows:
Figure BDA0002362735800000074
Figure BDA0002362735800000075
Figure BDA0002362735800000076
Figure BDA0002362735800000081
wherein j is 1, 2; i.e. if_j(t) represents the left and right line fault components of the fault, UdcRepresenting the output voltage of the direct current side of the converter station in normal operation; n is a radical ofj、Leqsj、Reqsj、CeqsjIndicating the parallel equivalent parameter L of the converter stations at two ends of the line where the fault is in the open networkdc_j、Rdc_jAnd indicating the parameters of the discharge loop of the equivalent converter station to the fault point at the two ends of the line where the fault is positioned.
In an alternative embodiment, in step (5), the method for calculating the total line fault current amount includes:
ifline_j(t)＝if_j(t)+I0， (16)
wherein j is 1, 2; i.e. ifine_j(t) represents the fault current total of the left and right side lines of the fault point; i is0Representing the line current normal operating component prior to the fault.
The embodiment of the invention takes the half-bridge MMC north-opening four-terminal direct-current power grid as shown in fig. 2 as an example to explain the method for calculating the bipolar fault current at the direct-current side of the half-bridge MMC-based multi-terminal direct-current power grid provided by the embodiment of the invention, and the basic structure of the converter is shown in fig. 3. When a bipolar short circuit occurs at the health protection outlet of the health protection-Fengning circuit and a bipolar fault occurs, the current of a non-fault pole is not changed before and after the fault, and only the fault pole is considered, so that the metal return network is removed. Since the north-expanding direct-current power grid is of an annular structure, taking the health and security station as an example shown in fig. 4, two paths for providing fault current for the four converter stations like fault points are provided, and the four converter stations after the fault can be equivalent to an equivalent circuit shown in fig. 5. Discharging current of a sub module capacitor in the convertor station after the fault, wherein the discharging current corresponds to fault components of outlet current of the convertor station and fault components of line current; and adding the fault current and the normal operation component to obtain the total fault current. The ac side after the fault does not provide any effect on the dc line fault current, so the ac sides of the converter stations can be disconnected, and the topology of the dc grid with the post-fault north-tension and the current fault component distribution are shown in fig. 6.
Because each converter station provides current fault components for the fault line after the fault is essentially the release of the sub-module capacitor stored energy, the voltage at two ends of the line is commonly maintained by the four converter station sub-module stored energy. With the release of the capacitor stored energy of each station submodule, the voltage of the valve sides at two ends of a fault line is reduced, so that the change rate of fault current is reduced. After the Zhang Bei DC power grid fault, the parallel structure of the converter station is complex, and the parallel relation analysis and the equivalent analytic calculation are difficult to be carried out. Because the current change on the non-fault line after the fault is very small, if only the fault current change within 5 ms-10 ms before the fault lockout is concerned, the topology can be simplified, and the original ring network is split from the non-fault line with the minimum line fault current after the fault, namely, the non-fault line farthest from the fault line is split into the open network. For the north-open ring dc power grid, when a bipolar short circuit occurs at the outlet of the health station described above, the line farthest from the fault point is the north-open beijing line, and after the fault, the original ring network is split into the open network by the line, so that the circuit shown in fig. 7 is obtained. As can be seen from fig. 7, the open network can be divided into two independent systems at the fault point, the two converter stations in the independent systems have a parallel relationship, the parallel relationship between the converter stations at the fault point and the left and right converter stations is shown in fig. 8, and the parameters can be calculated from (3):
Figure BDA0002362735800000091
Figure BDA0002362735800000092
thus, the equivalent circuit after simplification is shown in fig. 9, and further, the fault line current can be obtained by the equations (4) to (8):
iK2(t)＝21.73e-0.36tsin(70.69t)， (19)
iF2(t)＝17.56e-2.33tsin(57.02t)， (20)
the total line fault current is then:
i1(t)＝iK2(t)+I0＝21.73e-0.36tsin(70.69t)+0.9， (21)
i2(t)＝I0-iF2(t)＝0.9-17.56e-2.33tsin(57.02t)。 (22)
in order to verify whether the calculation result is accurate, the embodiment of the invention also utilizes PSCAD/EMTDC software to perform electromagnetic transient simulation on bipolar faults occurring at the health protection outlet of the health protection-Fengning line, and compares the line fault current simulation result with the approximate analysis calculation result, for example, as shown in FIG. 10, so that the simulation result is basically consistent with the approximate analysis calculation result, and the correctness and the effectiveness of the calculation method for the bipolar fault current at the direct current side of the multi-terminal direct current power grid based on the half-bridge MMC provided by the embodiment of the invention are verified.

Claims (4)

1. A bipolar fault current calculation method of a direct current side of a multi-terminal direct current power grid based on a half-bridge type MMC is characterized by comprising the following steps of:
step (1), acquiring inherent parameters of a multi-terminal direct current system, wherein the inherent parameters mainly comprise the parameters of each terminal converter station: bridge arm submodule number NnSub-module capacitance value C0nReactance value L of bridge arm0nBridge arm loss equivalent resistance R0nLine parameter Rline_n、Lline_nParameter L of smoothing reactorpb_n；
Step (2), obtaining a normal operation component I of line current before the fault of the multi-terminal direct current system to be researched0nAnd the DC side output voltage U of the converter station in the systemdc；
Step (3), splitting the ring-shaped power grid into an open network by a non-fault line with the least current rise after the fault in the direct-current power grid, namely the non-fault line farthest from the fault line, and calculating corresponding equivalent parameters according to the parallel relation of the split converter stations;
step (4), calculating the fault component i of the line current where the fault is in the system according to the split multi-terminal direct current system topological structuref_n(t)；
Step (5), calculating the total fault current i of the linefline_n(t)。
2. The method for calculating the bipolar fault current on the direct current side of the half-bridge MMC-based multi-terminal direct current power grid according to claim 1, wherein:
in the step (3), removing the non-fault pole network in the multi-terminal direct current system, and deleting the alternating current system connected with each converter station;
removing a metal return wire between converter stations at two ends of a line where a non-fault exists, and splitting an original ring network into an open network by the non-fault line which is farthest away from the fault line in the original topology of the multi-end direct current system;
the internal equivalent parameters of the converter station are as follows:
Figure FDA0002362735790000011
Figure FDA0002362735790000021
wherein R issumn、Lsumn、CsumnRespectively, the external equivalent parameters, L, of each converter stationzA neutral current limiting reactor;
in the open network, the equivalent calculation formula after the parallel connection of the converter stations is as follows:
Figure 3
。
3. the method for calculating the bipolar fault current on the direct current side of the half-bridge MMC-based multi-terminal direct current power grid according to claim 1, wherein:
in the step (4), only the fault network is considered, and the calculation formulas for equivalently calculating the fault components of the fault left and right line currents respectively by using the currents in the single discharge loops of the fault point of the converter stations at the two ends of the line where the fault is located after the fault are respectively:
Figure FDA0002362735790000023
Figure FDA0002362735790000024
Figure FDA0002362735790000025
Figure FDA0002362735790000026
wherein j is 1, 2; i.e. if_j(t) represents the left and right line fault components of the fault, UdcRepresenting the output voltage of the direct current side of the converter station in normal operation; n is a radical ofj、Leqsj、Reqsj、CeqsjIndicating the parallel equivalent parameter L of the converter stations at two ends of the line where the fault is in the open networkdc_j、Rdc_jAnd indicating the parameters of the discharge loop of the equivalent converter station to the fault point at the two ends of the line where the fault is positioned.
4. The method for calculating the bipolar fault current on the direct current side of the half-bridge MMC-based multi-terminal direct current power grid according to claim 1, wherein:
in the step (5), the method for calculating the total line fault current comprises the following steps:
ifline_j(t)＝if_j(t)+I0， (8)
wherein j is 1, 2; i.e. ifine_j(t) represents the fault current total of the left and right side lines of the fault point; i is0Representing the line current normal operating component prior to the fault.
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CN113922408B (en) * 2021-09-30 2023-12-08 合肥工业大学 MMC-HVDC power grid bipolar short-circuit fault current calculation method based on parameter inversion
CN114362182A (en) * 2021-12-28 2022-04-15 国网北京市电力公司 Four-end flexible direct-current interconnection double-loop network power distribution system
CN114362182B (en) * 2021-12-28 2024-04-26 国网北京市电力公司 Four-terminal flexible direct-current interconnection dicyclo network distribution system

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Application publication date: 20200529