CN117538674A - Asymmetric short circuit identification method suitable for flexible direct access - Google Patents
Asymmetric short circuit identification method suitable for flexible direct access Download PDFInfo
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- CN117538674A CN117538674A CN202310626374.3A CN202310626374A CN117538674A CN 117538674 A CN117538674 A CN 117538674A CN 202310626374 A CN202310626374 A CN 202310626374A CN 117538674 A CN117538674 A CN 117538674A
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- 238000012423 maintenance Methods 0.000 abstract description 3
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- NAWXUBYGYWOOIX-SFHVURJKSA-N (2s)-2-[[4-[2-(2,4-diaminoquinazolin-6-yl)ethyl]benzoyl]amino]-4-methylidenepentanedioic acid Chemical compound C1=CC2=NC(N)=NC(N)=C2C=C1CCC1=CC=C(C(=O)N[C@@H](CC(=C)C(O)=O)C(O)=O)C=C1 NAWXUBYGYWOOIX-SFHVURJKSA-N 0.000 description 1
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Classifications
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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
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
- G01R31/08—Locating faults in cables, transmission lines, or networks
- G01R31/081—Locating faults in cables, transmission lines, or networks according to type of conductors
- G01R31/086—Locating faults in cables, transmission lines, or networks according to type of conductors in power transmission or distribution networks, i.e. with interconnected conductors
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
- G01R31/08—Locating faults in cables, transmission lines, or networks
- G01R31/088—Aspects of digital computing
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
- G01R31/50—Testing of electric apparatus, lines, cables or components for short-circuits, continuity, leakage current or incorrect line connections
- G01R31/52—Testing for short-circuits, leakage current or ground faults
Abstract
The invention discloses an asymmetric short circuit identification method of a flexible direct current transmission system, and relates to the field of power grid operation and maintenance; at present, the asymmetric short circuit fault identification generally has the problem that the identification efficiency and the accuracy cannot be considered. The method comprises the following steps: calculating the negative sequence current amplitude of the protection installation parts at the two sides; when the amplitude of the negative sequence current on the alternating current side is smaller than a setting value, the system is considered to normally operate or three-phase short circuit fault occurs, when the amplitude of the negative sequence current on the alternating current side is larger than the setting value, the system is considered to have asymmetric fault, and the difference between the amplitudes of the negative sequence current abrupt change values at the protection installation positions on the two sides is calculated; if the difference of the amplitude values is larger than the setting value, the asymmetric fault is considered to exist, and the short circuit fault occurs in the corresponding two-side protection installation areas, so that relay protection acts; otherwise, the fault is considered as a penetrating current, the fault occurs outside the protection installation areas at the two sides or is in a normal running state, and the relay protection does not act. The technical scheme can more quickly and accurately identify the asymmetric short-circuit fault.
Description
Technical Field
The invention relates to the field of operation and maintenance of power grids, in particular to an asymmetric short circuit identification method suitable for flexible direct access.
Background
Relay protection is a first defense line for guaranteeing safe operation of a power grid, and in actual engineering application, relay protection reflecting power frequency electric quantity is dominant. With the wide application of power electronic equipment, the control strategies of the power electronic equipment such as flexible and straight and the like can change the fault current characteristics, the power system presents more complex fault characteristics after fault, and the conventional protection faces the problem of reliability reduction; and after the soft direct access AC power grid, the soft direct side positive sequence current is limited, the negative sequence current is almost zero, the zero sequence current is not affected, and the difference between the soft direct side positive sequence current and the alternating side zero sequence current is large, so that the differential protection sensitivity is reduced and even refused. The asymmetric faults (including single-phase earth faults, two-phase short-circuit faults and two-phase short-circuit earth faults) account for more than 90% of the faults of the power grid, and the problem that the identification efficiency and accuracy cannot be considered commonly exists in the identification of the asymmetric short-circuit faults, so that the safe operation of the power grid is affected.
Disclosure of Invention
The technical problem to be solved and the technical task to be put forward in the invention are to perfect and improve the prior art scheme, and an asymmetric short circuit identification method suitable for flexible direct access is provided, so that the purpose of rapidly and accurately identifying the asymmetric short circuit fault of a flexible direct current power transmission system is achieved. For this purpose, the present invention adopts the following technical scheme.
The utility model provides an asymmetric short circuit identification method suitable for gentle direct access, wherein flexible direct current transmission system includes direct current side and alternating current side, and direct current side and alternating current side all are equipped with the protection, and three-phase short circuit identification method includes following steps:
1) Acquiring attribute information of a line fault point;
2) Acquiring phase current information of protection installation positions at two ends of the fault point, and calculating negative sequence current amplitude values of the protection installation positions at two sides;
3) Judging whether the negative sequence current amplitude values of the alternating current side and the direct current side are larger than a setting value or not;
4) When the amplitude of the negative sequence current at the alternating current side is smaller than a first setting value, the protection area is considered to have no asymmetric fault; if one side negative sequence current is larger than a setting value, entering step 5); when the negative sequence current amplitude of one side is larger than a setting value, the system is considered to generate asymmetric faults;
5) Calculating the amplitude delta I of the negative sequence current abrupt change quantity at the protection installation position of two sides 2M And DeltaI 2N And calculates the amplitude difference delta I of the abrupt change of the negative sequence currents at two sides 2 ；
6) If the amplitude difference delta I 2 If the set value is larger than the second setting value, the asymmetric fault is considered to exist, namely, a short circuit fault occurs in the protection installation areas on the two corresponding sides, and relay protection acts; otherwise, the fault is considered as a penetrating current, the fault occurs outside the protection installation areas at the two sides or is in a normal running state, and the relay protection does not act.
According to the technical scheme, after faults occur, the characteristic that the amplitude of the fault negative sequence current abrupt change quantity of the flexible direct side and the alternating side is obviously different is utilized, and a new technical means for protection is constructed. The control strategy makes the phase angle characteristics after the fault very different from those of the traditional alternating current power grid, negative sequence current is used for identification in the technical scheme, and the negative sequence current is different under the condition of three-phase asymmetry and is irrelevant to the phase angle, so that the method is not affected by the phase angle; according to the technical scheme, the amplitude and the like of the negative sequence current abrupt change quantity at the protection installation positions at the two sides are calculated to judge, and the negative sequence current abrupt change quantity is not directly related to the phase angle, so that the method is not influenced by the phase angle, and the problem of misoperation or leakage protection of equipment caused by abnormal conditions such as phase jump of a power system can be avoided. Because, in power systems, phase differences between different phases may occur for various reasons (e.g. capacitive currents, short-circuit faults, switching operations, etc.), which may affect the accuracy and stability of the solution if it is phase angle sensitive.
Meanwhile, the reliability and the application range of the technical scheme can be improved without being influenced by the phase angle. In practical engineering application, phase jump or phase sequence jump in a power system is an unavoidable problem, and if the technical scheme is not affected by a phase angle, the method can be suitable for a wider engineering scene, and special adjustment and optimization are not required for each possible phase difference, so that the debugging and maintenance cost is reduced. Therefore, a technical scheme which is not influenced by the phase angle has great advantages and values for protecting and controlling the power system from the aspects of safety, reliability and economic benefit.
The technical scheme can more quickly and accurately identify asymmetric faults (including no three-phase short circuit) and realize protection action or locking in proper time. The method is simple and feasible, and the operation is convenient and quick by judging whether the amplitude of the negative sequence current on the alternating current side and the direct current side is larger than a setting value, calculating the amplitude of the negative sequence current abrupt change at the protection installation position on the two sides and the like. The method does not need to use a channel in a traditional grounding mode, so that the stability, the reliability and the safety of the flexible direct current transmission system can be improved, and the reliability and the stability of a power grid are improved.
As a preferable technical means: the protection is relay protection. Of course, other protection such as protection terminals, digital protection devices, disconnectors, reclosers, circuit breakers, etc. are also possible.
As a preferable technical means: the direct-current side comprises a flexible direct-current converter station, and the flexible direct-current converter station comprises a flexible direct-current power transmission module of a modularized multi-level converter and a transformer; the output end of the flexible direct current transmission module is connected with the input end of the transformer. The flexible direct current transmission module of the modularized multi-level converter is an MMC-HVDC module.
As a preferable technical means: the direct-current side protection is connected with the output end of the transformer; the ac side protection is connected to the input of the ac network. The two-side protection installation area is arranged between the direct current side protection and the alternating current side protection.
As a preferable technical means: in the step 5), the two sides are respectively M side and N side, and the negative sequence current mutation amounts of the M side and the N side are respectively delta I 2M 、ΔI 2N The calculation formula is as follows:
where τ is the length of one cycle, i.e., 0.02s, ΔI 2M (t) and ΔI 2N And (t) is the amplitude of the negative sequence current at the protection installation position of the M side and the N side at the moment t respectively.
By adopting the current mutation amount calculating method taking a single period as a calculation object, interference signals such as fluctuation, noise and the like in any period of current can be effectively restrained, and the measuring precision and the reliability of the protection device are improved. In addition, the method for periodically calculating the current abrupt change quantity can eliminate the influence caused by power oscillation and other periodic interference signals on the system by comparing the current period with the multi-period results such as the previous period or the next period, and the stability and the robustness of the protection scheme are improved.
As a preferable technical means: subtracting the negative sequence current abrupt change quantity of the soft direct side protection installation position from the negative sequence current abrupt change quantity of the alternating side protection installation position to calculate the amplitude difference delta I of the negative sequence current abrupt change quantity at the two sides 2 ：
ΔI 2 ＝ΔI 2N -ΔI 2M
The beneficial effects are that:
1. according to the technical scheme, the AC/DC series-parallel power grid accessed by the flexible DC power transmission system can be used for rapidly identifying faults after asymmetric short-circuit faults occur.
2. According to the technical scheme, the difference in the amplitude difference of the negative sequence component abrupt change in the fault current at two sides is fully utilized, the phase influence is avoided, no dead zone exists in the area, the influence of the fault position is avoided, and the protection reliability is improved.
3. The technical scheme can be used as a supplementary criterion of current differential protection, and the operation safety of the power grid is further improved;
according to the technical scheme, when an asymmetric short circuit fault occurs, only the negative sequence component of the fault phase current on the flexible direct side is restrained to be close to zero, and the alternating side contains the negative sequence component and is far larger than the negative sequence component on the flexible direct side, so that a protection supplementary criterion is formed. The situations that after the soft direct access to an alternating current power grid occurs, the soft direct side positive sequence current is limited, the negative sequence current is almost zero, the zero sequence current is not affected, the differential protection sensitivity is reduced or even refused, and the circuit cannot be protected are avoided.
Drawings
Fig. 1 is a schematic diagram of a line fault of the present invention.
Fig. 2 is a diagram of a typical MMC topology.
Fig. 3 is a graph of simulation results of asymmetric shorting at a midpoint of a protected line of the present invention.
Fig. 4 is a graph showing simulation results of asymmetric short circuit in the opposite direction of ac side protection according to the present invention.
Fig. 5 is a graph of simulation results of asymmetric short circuit in the reverse direction of the soft straight side protection of the present invention.
Fig. 6 is a flow chart of the present invention.
Detailed Description
The technical scheme of the invention is further described in detail below with reference to the attached drawings.
As shown in fig. 6, the present invention includes the steps of:
s1, acquiring attribute information of a line fault point;
s2, acquiring phase current information of protection installation positions at two ends of the fault point, and calculating negative sequence current amplitude values of the protection installation positions at two sides;
s3, judging whether the negative sequence current amplitude of the alternating current side and the direct current side is larger than a setting value;
and S4, when the amplitude of the negative sequence currents at the two sides is smaller than a setting value, the system is considered to be three-phase symmetrical, namely the system operates normally or three-phase short circuit occurs. If one side negative sequence current is larger than a setting value, entering step 5); when the negative sequence current amplitude of one side is larger than a setting value, the system is considered to generate asymmetric faults;
s5, calculating the amplitude delta I of the negative sequence current abrupt change quantity at the protection installation position of the two sides 2M And DeltaI 2N (compared with the amount before one cycle) and calculates the two-sided negative sequence current abrupt amount Δi 2 ；
S6 if the difference DeltaI between the magnitudes 2 If the set value is larger than the set value, the asymmetric fault is considered to exist, namely, a short circuit fault occurs in the protection installation areas on the two corresponding sides, and relay protection acts; otherwise, the fault is considered as a penetrating current, the fault occurs outside the protection installation areas at the two sides or is in a normal running state, and the relay protection does not act.
According to the technical scheme, the AC/DC series-parallel power grid accessed by the flexible DC power transmission system can be used for rapidly identifying faults after asymmetric short-circuit faults occur. The situation that the fault current is different from the alternating current side after the fault due to a soft and straight negative sequence current inhibition control strategy and the protection reliability related to the traditional phase is influenced is avoided. The technical scheme fully utilizes the difference of the fault current amplitude values at two sides, is not affected by the phase, has no dead zone in the zone, is not affected by the fault position, and improves the protection reliability.
According to the technical scheme, after faults occur, the characteristic that the fault current amplitude of the flexible direct side and the fault current amplitude of the alternating side are obviously different is utilized, and a new protection scheme is constructed. The control strategy makes the phase angle characteristics after the fault very different from those of the traditional alternating current power grid, and the scheme is not affected by the phase angle. An asymmetrical short-circuit fault (not including a three-phase short-circuit fault) can be identified more quickly and accurately. The specific principle is as follows:
as shown in fig. 1, in the flexible direct current transmission system, an asymmetric short circuit fault in a region occurs at a certain line f1, and because the negative sequence component of the fault current at the flexible direct side is restrained to be close to zero, the current abrupt change amount in a transient process after the fault occurs is smaller; for the alternating current side, the negative sequence component of the short-circuit current of the asymmetric short-circuit is larger, and no negative sequence current exists in normal operation, so that the abrupt change of the negative sequence current in the transient process after the fault occurs is larger.
The ac side fault current expression is not described in detail, and the following expression of the soft-straight side fault current is deduced:
in a flexible direct current transmission system, a typical MMC three-phase topology structure is shown in fig. 2. Obviously, the mathematical model in normal operation is:
defining j phase difference mode voltage and common mode voltage as:
from the formulae (1) (2):
the direct current is easy to control, so that the sinusoidal alternating current in the three-phase static coordinate system is converted into the direct current in the two-axis synchronous rotation coordinate system through park conversion:
the fundamental frequency dynamic equation frequency form of the MMC in normal operation can be obtained through Laplace transformation:
introducing PI control with better control performance on DC quantity, formula (5) can be written as:
by inverse laplace transformation, a flexible direct current transmission output current expression under the condition of three-phase current symmetry (normal operation and three-phase short circuit fault) can be obtained:
wherein,
the magnitude of the output current in equation (8) can also be expressed as:
since the main device of the MMC converter is an IGBT, its biggest disadvantage is poor overcurrent bearing capability. In order to limit the short-circuit current provided by the MMC converter to the ac side short-circuit point, to prevent the converter from blocking or even damaging the power devices therein due to overcurrent, it is generally required that the MMC converter does not output negative sequence current to the ac grid in any case, and the amplitude of the output positive sequence current is also limited. The asymmetric faults include single-phase earth faults, two-phase short-circuit faults and two-phase short-circuit earth faults, the alternating-current side fault current comprises a large negative sequence component, and the soft-direct side fault current does not comprise a negative sequence component. For positive sequence current, the amplitude of the positive sequence current output is generally 1.1-1.2 times of the rated value according to national standard document regulations.
When a fault occurs outside the zone, a through current flows through both ends of the line, so that the currents measured at the protection installation positions at the two sides are in equal large reverse directions, and the negative sequence current abrupt amounts at the two sides are almost equal.
When a fault occurs in the area, fault currents measured at the protection installation parts at two sides are greatly different: the soft straight side, the negative sequence current is restrained to be close to zero, and the difference between the negative sequence current and the normal operation is not great; on the alternating current side, the peak value of the negative sequence component of the fault current can be up to the rated current multiple. Therefore, when an asymmetric short circuit fault occurs in the area, the fault current amplitude of the protection installation parts at the two sides is obviously different.
The occurrence of an intra-zone fault may be determined by:
the negative sequence current amplitude can be monitored at the AC side protection installation position, and as the protection scheme aims at asymmetric short circuit and is an asymmetric fault, the AC side protection installation position has no negative sequence current when symmetric fault or normal operation occurs, and the protection is locked.
When the AC side protection installation part contains negative sequence current, the three phases are asymmetric, and the system has asymmetric faults. Calculating the negative sequence current abrupt change value difference of the alternating current side and the soft direct side protection installation part: during normal operation or out-of-zone faults, the passing through positions of the two side protection installation parts are penetrating currents, so that the amplitude difference of the negative sequence current abrupt change values of the two side protection installation parts is about zero. When an asymmetric short circuit occurs in the area, the amplitude of the abrupt change of the negative sequence component of the alternating-current side fault current is large, the abrupt change of the negative sequence current of the soft-direct side fault current is close to zero, the amplitude difference of the abrupt change of the negative sequence current at two sides is larger than a setting value, and the asymmetric short circuit fault occurs in the area.
In summary, the following may be summarized:
as shown in fig. 1, when the negative sequence current at the ac side protection installation satisfies the formula (10),
I N2 ＞I op.2 (10)
calculating the negative sequence current mutation amplitude difference of the current at the protection installation position of the alternating current side and the flexible direct side:
wherein I is 2N (t) and I 2M (t) is the negative sequence current amplitude of the alternating current side and the soft direct side protection installation part at the moment, I 2N (t- τ) and I 2M (t- τ) is the negative sequence current amplitude, ΔI, of the AC side and soft side protection installations one cycle before this time 2N (t) and ΔI 2M (t) is the amplitude, deltaI, of the negative sequence current abrupt change at the AC side and soft-direct side protection installation site 2 And protecting the amplitude difference of the negative sequence current abrupt quantity at the installation position for the alternating current side and the soft direct side. I op.2 Is the maximum negative sequence current amplitude generated by measurement errors or system fluctuation during normal operation or three-phase short circuit. A system is considered to be in an asymmetric fault state when equation (10) is satisfied.
When the formula (12) is satisfied, the current amplitudes at two sides are obviously different, the symmetrical faults occur in the protection area, and the protection action is performed
ΛI 2 ＞I set (12)
Wherein I is set When an asymmetric short circuit occurs externally, the maximum unbalanced current is caused by measurement errors and the like. Fig. 3, 4 and 5 are simulation results of faults occurring at the midpoint of the protected line, at the ac side protection reverse direction outlet and at the flexible straight side protection reverse direction outlet, respectively. Therefore, the protection scheme can reliably act when the area fails, and can not malfunction when the area fails. The BC phase short circuit is taken as an example to carry out simulation explanation and display the result, and other types of faults can be identified through simulation verification, so that the faults are not displayed any more for saving the space.
The asymmetric short circuit identification method of the flexible direct current transmission system is a specific embodiment of the invention, has shown the essential characteristics and the progress of the invention, and can be subjected to equivalent modification in terms of shape, structure and the like according to actual use requirements under the teaching of the invention, and the method is within the protection scope of the scheme.
Claims (6)
1. The utility model provides an asymmetric short circuit identification method suitable for gentle direct access, flexible direct current transmission system includes direct current side and interchange side, and direct current side and interchange side all are equipped with protection, its characterized in that: the asymmetric fault identification method comprises the following steps:
1) Acquiring attribute information of a line fault point;
2) Acquiring phase current information of protection installation positions at two ends of the fault point, and calculating negative sequence current amplitude values of the protection installation positions at two sides;
3) Judging whether the amplitude of the negative sequence current at the alternating current side is larger than a setting value;
4) When the amplitude of the negative sequence current at the alternating current side is smaller than a first setting value, the protection area is considered to have no asymmetric fault; if one side negative sequence current is larger than a setting value, entering step 5); when the negative sequence current amplitude of one side is larger than a setting value, the system is considered to generate asymmetric faults;
5) Calculating the amplitude delta I of the negative sequence current abrupt change quantity at the protection installation position of two sides 2M And DeltaI 2N And calculates the amplitude difference delta I of the abrupt change of the negative sequence currents at two sides 2 ；
6) If the amplitude difference delta I 2 If the set value is larger than the second setting value, the asymmetric fault is considered to exist, namely, a short circuit fault occurs in the protection installation areas on the two corresponding sides, and relay protection acts; otherwise, the fault is considered as a penetrating current, the fault occurs outside the protection installation areas at the two sides or is in a normal running state, and the relay protection does not act.
2. The asymmetric short circuit identification method suitable for flexible direct access according to claim 1, wherein the method comprises the following steps: the protection is relay protection.
3. The asymmetric short circuit identification method suitable for flexible direct access according to claim 1, wherein the method comprises the following steps: the direct current side comprises a flexible direct current converter station, the flexible direct current converter station comprises a flexible direct current transmission module of a modularized multi-level converter and a transformer, and the output end of the flexible direct current transmission module is connected with the input end of the transformer.
4. An asymmetric short circuit identification method suitable for flexible direct access according to claim 3, wherein: the direct-current side protection is connected with the output end of the transformer; the ac side protection is connected to the input of the ac network.
5. The asymmetric short circuit identification method suitable for flexible direct access according to claim 1, wherein the method comprises the following steps: in the step 5), the two sides are respectively M side and N side, and the negative sequence current mutation amounts of the M side and the N side are respectively delta I 2M 、ΔI 2N The calculation formula is as follows:
where τ is the length of one cycle, i.e., 0.02s, ΔI 2M (t) and ΔI 2N And (t) is the amplitude of the negative sequence current at the protection installation position of the M side and the N side at the moment t respectively.
6. The asymmetric short circuit identification method suitable for flexible direct access according to claim 5, wherein the method comprises the following steps: subtracting the negative sequence current abrupt change quantity of the soft direct side protection installation position from the negative sequence current abrupt change quantity of the alternating side protection installation position to calculate the amplitude difference delta I of the negative sequence current abrupt change quantity at the two sides 2 ：
ΔI 2 ＝ΔI 2N -ΔI 2M 。
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