CN114744596B - Flexible direct current system pilot protection method and system based on voltage matching factor - Google Patents
Flexible direct current system pilot protection method and system based on voltage matching factor Download PDFInfo
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- CN114744596B CN114744596B CN202210163684.1A CN202210163684A CN114744596B CN 114744596 B CN114744596 B CN 114744596B CN 202210163684 A CN202210163684 A CN 202210163684A CN 114744596 B CN114744596 B CN 114744596B
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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
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- 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
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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
- H02H7/28—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 meshed systems
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Abstract
The invention relates to a pilot protection method of a flexible direct current system based on a voltage matching factor, belongs to the technical field of relay protection of power systems, and solves the problem that the existing multi-terminal flexible direct current system is difficult to quickly and reliably identify a fault section. The method comprises the following steps: collecting voltage and current at two ends of each line after a fault occurs and alternating current flowing to each converter station; assuming that the current flowing through the port flows in the direction, and acquiring a theoretical voltage calculation value of the port based on the current flowing through the adjacent line and the alternating current flowing to the converter station connected with the port; obtaining voltage matching factors at two ends of the corresponding line and fault pole selection coefficients at two ends of the line based on theoretical calculated values of voltage, current and voltage at two ends of each line; if the voltage matching factors at the two ends of the line meet the fault identification criterion, the fault in the area is judged, a fault pole is identified according to the fault pole selection coefficients at the two ends of the line, and line protection of the fault pole is started.
Description
Technical Field
The invention belongs to the technical field of relay protection of power systems, and particularly relates to a pilot protection method and system of a flexible direct current system based on a voltage matching factor.
Background
The direct-current transmission technology based on the modular multilevel converter station has wide application prospects in the aspects of new energy power generation, connection of a weak alternating-current power grid and the like. However, the fault development process of the flexible direct-current power grid is very fast, the whole power grid can be endangered within a few ms, and the safety and the stability of the operation of the direct-current power grid are seriously influenced. Therefore, a rapid and selective line protection scheme becomes a key technology for ensuring safe and reliable operation of the direct-current power grid.
At present, the pilot protection of the flexible direct current system is mainly divided into pilot current differential protection, pilot direction protection and pilot traveling wave protection. The pilot current differential protection carries out fault discrimination through differential current on two sides of a communication channel detection circuit, and can effectively detect high-resistance faults. However, this approach requires a large amount of data transmission, requires strict synchronization of communications, and is susceptible to line distributed capacitance. In order to avoid transient current generated by the ground capacitance of the circuit when an external fault occurs, the differential protection ensures reliability through long time delay, and the quick action of the protection is greatly reduced. The pilot direction protection generally performs fault identification by judging the positive direction of current at two ends of a line. The data processing of the protection is finished at the station, and the communication channel only needs to transmit the data processing result without synchronous communication requirement. However, this method is greatly affected by the transition resistance, and when a large transition resistance ground fault occurs on the dc side, the protection is difficult to operate reliably. The pilot traveling wave protection utilizes information such as direction, amplitude and the like in fault transient traveling waves to form a protection criterion, has high action speed and is not influenced by line distributed capacitance. However, the method has the problems of high sampling frequency and communication transmission requirements, poor sensitivity under the condition of high-resistance fault, difficult wave head detection, limited anti-interference capability and the like.
Disclosure of Invention
In view of the foregoing analysis, embodiments of the present invention provide a pilot protection method and system for a flexible dc system based on a voltage matching factor, so as to solve the problem that it is difficult to quickly and reliably identify a fault section in the line protection of the conventional multi-terminal flexible dc system.
On one hand, the invention discloses a pilot protection method of a flexible direct current system based on a voltage matching factor, which comprises the following steps:
collecting voltage and current at two ends of each line after a fault occurs and alternating current flowing to each converter station;
for each port of each line, assuming that the current direction flowing through the port is inflow, and acquiring a voltage theoretical calculation value of the current port based on the current flowing through the adjacent line and the alternating current flowing to the converter station connected with the port;
obtaining voltage matching factors at two ends of the corresponding line and fault pole selection coefficients at two ends of the corresponding line based on theoretical calculated values of voltage, current and voltage at two ends of each line;
and for each line, judging whether the voltage matching factors at the two ends of the line meet a fault identification criterion, if so, judging that the line is in an intra-area fault, identifying a fault pole according to fault pole selection coefficients at the two ends of the line, and starting line protection of the fault pole.
On the basis of the scheme, the invention also makes the following improvements:
further, a voltage matching factor is obtained by the following formula:
wherein S is mp 、S np Voltage matching factors of the positive pole of the circuit at the M end and the N end, S mn 、S nn Voltage matching factors of the negative electrode of the circuit at the M end and the N end respectively; u. of cmp (h)、u cnp (h) Respectively the theoretical calculated values of the voltage at the M end and the N end of the positive pole of the circuit at the h sampling point, u cmn (h)、u cnn (h) Respectively obtaining theoretical voltage values of the h sampling point at the M end and the N end of the negative electrode of the circuit; u. of cemp (h)、u cenp (h) The voltages of the h-th sampling point at the M end and the N end of the line anode are respectively; u. of cemn (h)、u cenn (h) Respectively obtaining the voltage of the h-th sampling point at the M end and the N end of the negative pole of the circuit; u. of mp 、u np Respectively representing the voltage of the positive pole of the circuit in normal operation at the M end and the N end; u. of mn 、u nn Respectively represents the electricity of the negative pole of the circuit when the M end and the N end normally runAnd H is the number of sampling points in one period after the fault occurs.
Further, the fault identification criteria include:
wherein S is set Is an action threshold;
if the formula (3) or the formula (4) is satisfied, it is determined as an intra-area failure.
Further, a fault pole selection coefficient is obtained through the following formula:
wherein, K m 、K n Fault pole selection coefficients of an M end and an N end are obtained respectively; i.e. i tmp (h)、i tnp (h) Currents of the h-th sampling point at the M end and the N end of the line anode are respectively; i.e. i tmn (h)、i tnn (h) Respectively obtaining the current of the h-th sampling point at the M end and the N end of the negative pole of the circuit; u. of tmp (h)、u tnp (h) The voltage of the h sampling point at the M end and the N end of the line anode is respectively; u. of tmn (h)、u tnn (h) Respectively obtaining the voltage of the h sampling point at the M end and the N end of the negative pole of the circuit; i.e. i mp 、i np Respectively representing the current of the positive pole of the circuit in normal operation at the M end and the N end; i.e. i mn 、i nn Respectively representing the current of the negative pole of the circuit when the M end and the N end normally operate.
Further, identifying a fault pole according to fault pole selection coefficients at two ends of the line comprises:
for the M terminal:
if K is m ＞K set If yes, the positive pole fails; if K m ＜-K set Then the negative pole is in failure; if-K set ≤K m ≤K set Then bipolar failure;
for the N terminal:
if K n ＞K set If yes, the positive pole fails; if K n ＜-K set Then the negative pole is in failure; if-K set ≤K n ≤K set Then bipolar failure;
wherein, K set Is the pole selection threshold.
Further, the converter station is an MMC converter station.
Furthermore, the theoretical calculated value u of the voltage of the h-th sampling point at the end M of the line anode cmp (h) Comprises the following steps:
wherein i tmp_L (h) The positive pole current of the adjacent line of the M end of the positive pole of the line flowing through the h sampling point is represented, and N represents the total number of sub-modules put into the upper bridge arm and the lower bridge arm of any phase in the converter station connected with the h sampling point; c represents the capacitance value of a submodule in the converter station connected with the port; r arm 、l arm Respectively representing the equivalent resistance and the inductance of a bridge arm of the phase A in the converter station connected with the port; w is power frequency angular frequency, and L represents the inductance of the M end of the positive pole of the line; i all right angle va (h)、i vb (h)、i vc (h) Representing the a, B, C alternating currents to the converter station connected to the port, respectively.
Further, the starting of the line protection of the fault pole performs:
sending a trip signal to a protection in the failed pole.
On the other hand, the invention also discloses a pilot protection system of the flexible direct current system based on the voltage matching factor, which comprises the following components:
the data acquisition module is used for acquiring voltage and current at two ends of each line after a fault occurs and alternating current flowing to each converter station;
the theoretical calculation module is used for assuming that the current direction flowing through the port is inflow for each port of each line, and acquiring a voltage theoretical calculation value of the current port based on the current flowing through the adjacent line and the alternating current flowing to the converter station connected with the port;
the voltage matching factor and fault pole selection coefficient determining module is used for obtaining voltage matching factors at two ends of the corresponding line and fault pole selection coefficients at two ends of the corresponding line based on theoretical calculated values of voltage, current and voltage at two ends of each line;
and the fault pole identification module is used for judging whether the voltage matching factors at the two ends of each line meet the fault identification criterion or not for each line, if so, judging that the line is in an internal fault, identifying a fault pole according to the fault pole selection coefficients at the two ends of each line, and starting the line protection of the fault pole.
Further, a voltage matching factor is obtained by the following formula:
wherein S is mp 、S np Voltage matching factors of the positive pole of the circuit at the M end and the N end, S mn 、S nn Voltage matching factors of the negative electrode of the circuit at the M end and the N end respectively; u. of cmp (h)、u cnp (h) Respectively the theoretical calculated values of the voltage at the M end and the N end of the positive pole of the circuit at the h sampling point, u cmn (h)、u cnn (h) Respectively obtaining theoretical voltage values of the h sampling point at the M end and the N end of the negative electrode of the circuit; u. of cemp (h)、u cenp (h) The voltage of the h sampling point at the M end and the N end of the line anode is respectively; u. u cemn (h)、u cenn (h) Are respectively the h sampling point on lineThe voltage of the M end and the N end of the circuit cathode; u. of mp 、u np Respectively representing the voltage of the positive pole of the circuit in normal operation at the M end and the N end; u. u mn 、u nn Respectively representing the voltage of the negative pole of the line when the M end and the N end normally run, and H is the number of sampling points in a period after the fault occurs.
Compared with the prior art, the invention can realize at least one of the following beneficial effects:
the multi-terminal flexible direct current system pilot protection method and system based on the voltage matching factor can calculate the voltage matching factor and the fault pole selection coefficient based on data collected after the fault, accurately identify the faults inside and outside the area according to the calculation result, have high action speed, are not influenced by the fault position and the transition resistance, and well solve the problem that the traditional multi-terminal flexible direct current system line protection is difficult to quickly and reliably identify the fault section.
In the invention, the technical schemes can be combined with each other to realize more preferable combination schemes. Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and drawings.
Drawings
The drawings are only for purposes of illustrating particular embodiments and are not to be construed as limiting the invention, wherein like reference numerals are used to designate like parts throughout.
FIG. 1 is a flow chart of a pilot protection method for a multi-terminal flexible DC system based on voltage matching factors;
FIG. 2 is a diagram of a four-terminal flexible DC system;
FIG. 3 is a basic structure diagram of an MMC converter station;
FIG. 4 is an MMC positive converter station equivalent model;
FIG. 5 is a network diagram of a fault condition of the AC system outside of the converter station;
FIG. 6 is a schematic structural diagram of a pilot protection system of a multi-terminal flexible direct current system based on voltage matching factors;
FIG. 7 is f 12 S when negative earth fault occurs through different transition resistance m 、S n (ii) a Wherein, FIGS. 7 (a) and (b) respectively show S m 、S n The variation curve of (d);
FIG. 8 is f 12 K when negative earth fault occurs through different transition resistance m 、K n (ii) a Wherein, FIGS. 8 (a) and (b) respectively show K m 、K n The variation curve of (d);
FIG. 9 is S in the case of a failure at a different position of the positive line m 、S n (ii) a Wherein, FIGS. 9 (a) and (b) respectively show S m 、S n The variation curve of (d);
FIG. 10 shows K when a fault occurs at different positions of the positive line m 、K n Wherein, FIGS. 10 (a) and (b) respectively show K m 、K n The variation curve of (d);
FIG. 11 is f 14 S when positive earth fault occurs through different transition resistance m 、S n (ii) a In FIGS. 11 (a) and (b), S is shown in each case m 、S n The variation curve of (d);
FIG. 12 is f 2 S in the event of a three-phase short-circuit fault via different transition resistances m 、S n (ii) a In FIG. 12, (a) and (b) each represent S m 、S n The change curve of (2).
Detailed Description
The accompanying drawings, which are incorporated in and constitute a part of this application, illustrate preferred embodiments of the invention and together with the description, serve to explain the principles of the invention and not to limit the scope of the invention.
Example 1
The invention discloses a pilot protection method of a flexible direct current system based on a voltage matching factor, and the flow chart is shown in fig. 1 and comprises the following steps:
step S1: collecting voltage and current at two ends of each line after a fault occurs and alternating current flowing to each converter station;
step S2: for each port of each line, assuming that the current direction flowing through the port is inflow, and acquiring a voltage theoretical calculation value of the current port based on the current flowing through the adjacent line and the alternating current flowing to the converter station connected with the port;
and step S3: obtaining voltage matching factors at two ends of the corresponding line and fault pole selection coefficients at two ends of the corresponding line based on theoretical calculated values of voltage, current and voltage at two ends of each line;
and step S4: and for each line, judging whether the voltage matching factors at the two ends of the line meet a fault identification criterion, if so, judging that the line is in an intra-area fault, identifying a fault pole according to fault pole selection coefficients at the two ends of the line, and starting line protection of the fault pole.
Compared with the prior art, the multi-terminal flexible direct current system pilot protection method based on the voltage matching factor calculates the voltage matching factor and the fault pole selection coefficient based on data collected after the fault, accurately identifies the internal fault and the external fault of the area according to the calculation result, has high action speed, is not influenced by the fault position and the transition resistance, and well solves the problem that the existing multi-terminal flexible direct current system line protection is difficult to quickly and reliably identify the fault section.
In order to facilitate better understanding of the forming process of the scheme in this embodiment, the following description will be made on the working principle of the pilot protection system of the multi-terminal flexible dc system based on the voltage matching factor, which is provided in this embodiment, by taking the four-terminal flexible dc system shown in fig. 2 as an example:
fig. 3 shows the basic structure inside a bipolar MMC converter station, where the negative pole structure is identical to the positive pole. According to fig. 3, the relational expression of the upper and lower bridge arms of the phase a of the positive converter station is as follows:
i va ＝i pa -i na (3)
wherein u is va 、i va Respectively representing the phase voltage and the current of the alternating current side A of the converter station; u. of pa 、u na Respectively representing equivalent capacitance voltages of an upper bridge arm and a lower bridge arm of the phase A; i.e. i pa 、i na Respectively representing the current of an upper bridge arm and the current of a lower bridge arm of the phase A; r arm 、l arm Respectively representing the equivalent resistance and inductance of any phase of bridge arm; u. of dc Is the converter station dc side voltage.
The united type (1) - (3) can obtain:
taking the initial phase angle of the a-phase modulation wave as zero as the initial time of the study, the voltage expression of the a-phase obtained from equation (4) is:
wherein: u shape a Is the peak value of A AC voltage; and w is the power frequency angular frequency.
If the output voltage modulation ratio of the MMC is m, the peak value of the A-phase alternating voltage is as follows:
as can be obtained from fig. 3, the relationship expression between the dc side voltage of the converter station and the capacitor voltage of each sub-module is:
u dc ＝NU c1 (7)
wherein: n is the total number of the submodules thrown into the upper and lower bridge arms of any phase; u shape c1 Is the capacitor voltage of the SM submodule.
The united type (1), (2), (5), (6) and (7) can obtain:
as can be seen from equations (8) and (9), the numbers of submodules to be input to the upper and lower arms of phase a at any one time are:
therefore, at any moment, the equivalent capacitances of the upper and lower bridge arms of phase a are respectively:
wherein, c pa 、c na Equivalent capacitors of an upper bridge arm and a lower bridge arm of the phase A of the converter station are respectively provided; and c is the capacitance value of the MMC neutron module.
Similarly, the equivalent capacitances of the upper and lower bridge arms of the two phases B and C are as follows:
wherein, c pb 、c nb The equivalent capacitors of the upper bridge arm and the lower bridge arm of the phase B of the converter station are respectively. c. C pc 、c nc Respectively are equivalent capacitors of an upper bridge arm and a lower bridge arm of a phase C of the converter station.
With reference to fig. 3, writing KVL equations for the three-phase bridge arms of the positive converter station in columns respectively can obtain:
wherein i pb 、i nb Respectively B phase upper and lower bridge arm currents; i.e. i pc 、i nc Respectively C-phase upper and lower bridge arm currents.
According to fig. 3, the relational expression between the three-phase bridge arm current and the alternating current of the converter station is as follows:
as shown in fig. 3, the relational expression between the three-phase bridge arm current and the dc current of the converter station is:
i dc ＝i na +i nb +i nc (17)
i dc ＝i pa +i pb +i pc (18)
the united type (12), (13), (14) and (15) can obtain:
the united type (16), (17) and (18) can obtain:
i va +i vb +i vc ＝0 (20)
the combined type (17), (19) and (20) can obtain the expression of the direct current voltage at the outlet of the converter station as follows:
wherein, C c Which is the equivalent capacitance of the converter station and has a value of N/3c. L is c Is an equivalent inductance of the converter station with a value of 2l arm /3。R c Is an equivalent resistance of the converter station and has a value of 2R arm /3。U c To changeThe streaming station contains an equivalent voltage source for the alternating current component.
From the above analysis, an equivalent model of the flexible dc grid converter station can be obtained, as shown in fig. 4.
To protect R 12 (DC breaker mounting position) as an example, and theoretically calculating value i of outlet current of the converter station dcc The expression of (a) is:
i dcc ＝|i 12 |+i 14 (22)
wherein i 12 For flow-through protection R 12 Of the current of (c). i.e. i 14 For flow-through protection R 12 The current of the adjacent line.
From fig. 4 and 2, the theoretical calculation value u of the dc line voltage on the M side can be obtained c Comprises the following steps:
in formula (23), U c The equivalent voltage source containing alternating current component can not reflect the alternating current side fault model. Therefore, the information of the current fed in by the alternating current system is further analyzed, and then the faults inside and outside the area are accurately identified.
A fault status network of the ac system outside the converter station zone is shown in fig. 5. In FIG. 5, i v Is a three-phase current flowing to the positive converter station. i.e. i v2 Is a three-phase current flowing to the negative converter station. i.e. i s The three-phase current flows through the commutation bus.
From FIG. 5, it can be seen that:
i v ＝i s -i v2 (24)
in the formula i v 、i v2 、i s Respectively as follows:
the connecting type (21) and (24) can obtain an equivalent voltage source U containing an alternating current component c The expression is as follows:
u is obtained by connecting (22), (23) and (25) in vertical order c It can also be expressed as:
in fig. 2, when the protection at both ends of the dc line 12 is taken as an example for analysis, a fault f 12 For an intra-zone fault, fault f 14 、f 23 、f 1 、f 2 Is an out-of-range fault. And setting the positive direction of the positive current as the direction from the bus to the line, and setting the positive direction of the negative current as the direction from the line to the bus. At this time, when an out-of-range fault f occurs 14 For protection R 12 In other words, the actual outlet current value i of the MMC1 converter station dc Comprises the following steps:
i dc ＝-i 12 +i 14 (27)
the equation (25) is still established in consideration of no failure of the ac system on the converter station MMC1 side. The coupling type (23), (25) and (27) can obtain the actual value u of the DC line voltage on the M side ce Comprises the following steps:
by subtracting the two formulae (28) and (26), the following can be obtained:
as is clear from formula (29), with respect to protection R 12 In other words, when a reverse out-of-range fault f occurs 14 When the voltage value calculated by the equation (26) does not match the actual voltage value.
When an out-of-range fault f occurs 14 For protection R 21 In other words, the actual outlet current value of the MMC2 converter station is:
i dc ＝i 21 +i 23 (30)
as can be seen from equation (30), the calculated current value matches the actual current value. The equation (25) is still established in consideration that the converter station MMC2 side ac system does not fail. U is obtained by connecting (23), (25) and (30) in vertical order ce It can also be expressed as:
by subtracting the two formulae (26) and (31), it is possible to obtain:
Δu＝u c -u ce ＝0 (32)
as is clear from the formula (32), R is protected 21 In other words, when a forward out-of-range fault f occurs 14 When the voltage value calculated by equation (26) is equal to the actual voltage value, the theoretical error is 0.
In conclusion, when the fault f occurs 14 Protection R for the line 12 12 For the reverse out-of-range fault, the voltage value calculated by equation (26) does not coincide with the actual value. Protection R for line 12 21 For a forward out-of-range fault, the voltage value calculated by equation (26) is consistent with the actual value, and the theoretical error is 0.
When an intra-area fault f occurs 12 Protection R for the line 12 12 And in the case of a forward fault, the calculated value of the outlet current of the converter station MMC1 is consistent with the actual value. The equation (25) is still established in consideration of the fact that the converter station MMC1 side ac system does not fail. Thus, protection R 12 The voltage value calculated by equation (26) agrees with the actual voltage, and the theoretical error is 0. Similarly, protection R for line 12 21 For a forward fault, the voltage value calculated according to equation (26) coincides with the actual voltage, and the theoretical error is 0.
According to the analysis, when a fault occurs in the direct current line area, the calculated voltage value of the protection at the two ends of the line is consistent with the actual value, and the theoretical error is 0. When an out-of-reverse fault occurs, the calculated voltage value is inconsistent with the actual voltage value. Therefore, the fault identification is carried out through the voltage matching factors at two ends of the line, and the calculation formula of the voltage matching factors is as follows:
wherein S is mp 、S np Respectively the voltage matching factors of the positive pole of the line at the M end and the N end, S mn 、S nn Voltage matching factors of the negative electrode of the circuit at the M end and the N end respectively; u. of cmp (h)、u cnp (h) Respectively the theoretical calculated values of the voltage at the M end and the N end of the positive pole of the circuit at the h sampling point, u cmn (h)、u cnn (h) Respectively obtaining theoretical voltage values of the h sampling point at the M end and the N end of the negative electrode of the circuit; u. of cemp (h)、u cenp (h) The voltages of the h-th sampling point at the M end and the N end of the line anode are respectively; u. of cemn (h)、u cenn (h) Respectively obtaining the voltage of the h-th sampling point at the M end and the N end of the negative pole of the circuit; u. u mp 、u np Respectively representing the voltage of the positive pole of the circuit in normal operation at the M end and the N end; u. of mn 、u nn Respectively, the voltage of the negative electrode of the line when the M terminal and the N terminal normally operate is represented, and H is the number of sampling points in one period (for example, 1 ms) after the fault occurs.
The theoretical calculated values of the voltages in the equations (33) and (34) correspond to the calculated values of the equation (26), and are calculated by assuming that the direction of the current flowing through the port is the port voltage. However, in the practical application process, i can be directly used v Replacement i v2 And i s At this time, generally, the theoretical calculated value u of the voltage at the end M of the line anode at the h-th sampling point cmp (h) Comprises the following steps:
wherein i tmp_L (h) Indicates that the h-th sampling point flows through the adjacent line of the positive M end of the lineN represents the total number of sub-modules put into the upper and lower bridge arms of any phase in the converter station connected with the port; c represents the capacitance value of a submodule in the converter station connected with the port; r is arm 、l arm Respectively representing the equivalent resistance and the inductance of a bridge arm of the phase A in the converter station connected with the port; w is power frequency angular frequency, and L represents the inductance of the M end of the positive pole of the line; i all right angle va (h)、i vb (h)、i vc (h) Respectively representing the a, B, C ac currents flowing to the converter station connected to the port.
According to different ports and positive and negative polarities, corresponding current data is selected and applied in a formula (35) form, and u can be obtained cnp (h)、u cmn (h) And u cnn (h)。
The fault identification criteria include:
wherein S is set Is an action threshold; if the formula (36) or the formula (37) is satisfied, it is determined as an intra-area failure. Otherwise, judging to be out-of-range fault, and at the moment, not sending a tripping signal. Considering the influence of calculation error, noise interference and other factors, the threshold value S is set set And taking 30.
In a symmetrical bipolar system, when a single-stage earth fault occurs, the voltage of a fault electrode is rapidly reduced, the voltage of a non-fault electrode is increased to a certain extent, and the current of the fault electrode is far larger than that of the non-fault electrode. However, when a bipolar short-circuit fault occurs, the voltage and current change amplitudes of the positive electrode and the negative electrode are always equal. Therefore, the pole selection criterion is constructed by using the voltage and current information after the fault as follows:
wherein, K m 、K n Fault pole selection coefficients of an M end and an N end are obtained respectively; i.e. i tmp (h)、i tnp (h) Currents of the h-th sampling point at the M end and the N end of the line anode are respectively; i.e. i tmn (h)、i tnn (h) Respectively obtaining the current of the h-th sampling point at the M end and the N end of the negative pole of the circuit; u. of tmp (h)、u tnp (h) The voltages of the h-th sampling point at the M end and the N end of the line anode are respectively; u. of tmn (h)、u tnn (h) Respectively obtaining the voltage of the h-th sampling point at the M end and the N end of the negative pole of the circuit; i all right angle mp 、i np Respectively representing the current of the positive pole of the circuit in normal operation at the M end and the N end; i.e. i mn 、i nn Respectively representing the current of the negative pole of the circuit when the M end and the N end normally operate.
For the M terminal:
if K m ＞K set If yes, the positive electrode fails; if K is m ＜-K set Then the negative pole is in failure; if-K set ≤K m ≤K set Then bipolar failure;
for the N terminal:
if K is n ＞K set If yes, the positive pole fails; if K n ＜-K set Then the negative pole is in failure; if-K set ≤K n ≤K set Then the bipolar fails.
Wherein, K set Is the select threshold. Taking a certain margin into account, this is taken to be 0.1.
After the fault pole is determined, the line protection of the fault pole can be started, that is, the following steps are executed: sending a trip signal to a protection in the failed pole.
Example 2
The specific embodiment 2 of the present invention discloses a pilot protection system for a flexible dc system based on a voltage matching factor, and a schematic structural diagram is shown in fig. 6, and includes:
the data acquisition module is used for acquiring voltage and current at two ends of each line after the fault occurs and alternating current flowing to each converter station;
the theoretical calculation module is used for assuming that the current direction flowing through the port is inflow for each port of each line, and acquiring a voltage theoretical calculation value of the current port based on the current flowing through the adjacent line and the alternating current flowing to the converter station connected with the port;
the voltage matching factor and fault pole selection coefficient determining module is used for obtaining voltage matching factors at two ends of the corresponding line and fault pole selection coefficients at two ends of the corresponding line based on theoretical calculated values of voltage, current and voltage at two ends of each line;
and the fault pole identification module is used for judging whether the voltage matching factors at the two ends of each line meet the fault identification criterion or not for each line, if so, judging that the line is in an internal fault, identifying a fault pole according to the fault pole selection coefficients at the two ends of each line, and starting the line protection of the fault pole.
The method embodiment and the system embodiment are realized based on the same principle, the related parts can be used for reference, and the same technical effect can be achieved.
The specific implementation process of the embodiment of the system may be as follows with reference to the embodiment of the method, and the embodiment is not described herein again. Since the principle of the embodiment of the system is the same as that of the embodiment of the method, the system also has the corresponding technical effect of the embodiment of the method.
Example 3
The system architecture is shown in fig. 2. Wherein, the direct current circuit adopts a frequency-dependent circuit model. The main parameters of the multi-end flexible direct current system are shown in table 1, the sampling frequency is 10kHz, and the fault occurrence time is 1s. The protection at the two ends of the line 12 is analyzed, and simulation experiments of different types of faults inside and outside the area are respectively carried out.
TABLE 1 Flexible DC System principal parameters
As shown in fig. 7 (a) and 7 (b), when a negative ground fault occurs via different transition resistances, S at both ends of the line increases with the increase of the transition resistance m 、S n And gradually increases. S. the m The minimum value was found at t =1.00s and the transition resistance was 0 Ω, and the value was 119.26.S n The minimum value was found at t =1.0002s and the transition resistance was 0 Ω, and the value was 112.05. Thus, S m 、S n Both of which are greater than the threshold value 30, it can be determined that the line 12 has failed. As can be seen from fig. 8 (a) and 8 (b), when the negative line has a ground fault, the change in the fault gate coefficient at both ends of the line is small in the case of a different transition resistance fault, and the change is hardly affected by the transition resistance. Meanwhile, K in FIG. 8 m 、K n Both are less than-0.1, and the fault can be judged as a negative electrode fault. The analysis results of fig. 7 (a) and 7 (b) are combined to determine that the negative ground fault occurs in the line 12.
According to the analysis, the method can still accurately identify the high-resistance fault in the direct current circuit area, and has high reliability and strong transition resistance tolerance capability. Meanwhile, the overhead line in the simulation model uses a frequency-dependent model, namely, the influence of distributed capacitance current is considered. Thus, the pilot protection scheme herein is less affected by distributed capacitive currents.
As can be seen from fig. 9 (a) and 9 (b), when a positive ground fault occurs at a different location from the converter station MMC1, the farther the fault location is from the protection, the smaller the voltage matching factor. Wherein, when a fault occurs at a position 100% away from the MMC1 end, S m There is a minimum at t =1.0002s, which is 128.45. Failure occurred at 0% position from MMC1 terminalWhen S is present n There is a minimum at t =1.0001s, which is 139.46. Thus, S m 、S n Both of which are greater than the threshold value 30, it can be determined that the line 12 is faulty. As can be seen from fig. 10 (a) and 10 (b), the farther the fault location is from the protection, the smaller the fault selection coefficient. Wherein, when a fault occurs at a position 100% away from the MMC1 end, K m There is a minimum at t =1.00s, which is 0.787. When a fault occurs at a position 0% away from the MMC1 end, K n There is a minimum at t =1.00s, which is 0.728. Meanwhile, K in FIG. 10 m 、K n Both of which are greater than 0.1, can be determined as a positive failure.
From the analysis, the protection scheme can realize the rapid and accurate action of protection at different fault positions in the region. When the line end has high-resistance fault, the high-sensitivity circuit still has high sensitivity.
Scenario 3 set for this example is: let f in FIG. 2 14 Where a positive ground fault occurs via a different transition resistance, the voltage matching factor in this fault situation is shown in figure 11.
When f is 14 When a fault occurs, protection R 12 The voltage value calculated from the fault data at (4) does not match the actual value, and the voltage matching factor calculated from equation (33) is smaller than the operation threshold value. As can be seen from FIG. 11 (a), S increases with increasing transition resistance m And gradually increases. S m At t =1.0008s and a transition resistance of 300 Ω, there is a maximum value of 8.59, which is less than the threshold value of 30, indicating that the dc link 12 is not faulty. Therefore, under the condition of the out-of-zone high-resistance fault, the fault can be reliably identified as the out-of-zone fault, and the misoperation is prevented.
Let f in FIG. 2 2 A three-phase short-circuit fault occurs via different transition resistances, and the voltage matching factor in this fault case is shown in fig. 12.
When f is 2 When a three-phase short-circuit fault occurs, S is shown in FIG. 12 (a) m When t =1.0002s and the transition resistance is 300 Ω, the maximum value is 8.18, which is smaller than the threshold value 30, and the fault is determined to be an out-of-range fault. Therefore, when a high-resistance fault occurs on the AC side, the fault can be accurately identified as an out-of-range fault.
According to the analysis, the protection scheme can accurately identify the out-of-zone high-resistance fault, and the protection is reliable and is free from misoperation. Moreover, only the judgment results of the fault directions need to be transmitted from two sides of the line, electric quantity information does not need to be transmitted, and the influence of data synchronization errors is small.
The above scene results show that the invention has the following characteristics:
(1) The method is not influenced by fault positions and transition resistance, and has high sensitivity when high-resistance faults occur in a direct-current line area.
(2) Only the judgment result of the fault direction needs to be transmitted, the electric quantity information at two ends does not need to be interacted, strict data synchronization is not needed, a complex calculation algorithm is not needed, and the hardware implementation difficulty is reduced.
(3) The action time is about 2-3 ms (communication delay is 1-2 ms), and the requirement of a direct current power grid on protection speed can be met.
Those skilled in the art will appreciate that all or part of the flow of the method implementing the above embodiments may be implemented by a computer program, which is stored in a computer readable storage medium, to instruct related hardware. The computer readable storage medium is a magnetic disk, an optical disk, a read-only memory or a random access memory, etc.
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.
Claims (7)
1. A pilot protection method of a flexible direct current system based on a voltage matching factor is characterized by comprising the following steps:
collecting voltage and current at two ends of each line after a fault occurs and alternating current flowing to each converter station;
for each port of each line, assuming that the current direction flowing through the port is inflow, and acquiring a voltage theoretical calculation value of the current port based on the current flowing through the adjacent line and the alternating current flowing to the converter station connected with the port;
obtaining voltage matching factors at two ends of the corresponding line and fault pole selection coefficients at two ends of the corresponding line based on theoretical calculated values of voltage, current and voltage at two ends of each line;
for each line, judging whether the voltage matching factors at the two ends of the line meet a fault identification criterion, if so, judging that the line has an internal fault, identifying a fault pole according to fault pole selection coefficients at the two ends of the line, and starting line protection of the fault pole;
the voltage matching factor is obtained by the following formula:
wherein S is mp 、S np Voltage matching factors of the positive pole of the circuit at the M end and the N end, S mn 、S nn Voltage matching factors of the negative electrode of the circuit at the M end and the N end respectively; u. of cmp (h)、u cnp (h) Respectively the theoretical calculated values of the voltage at the M end and the N end of the positive pole of the circuit at the h sampling point, u cmn (h)、u cnn (h) Respectively obtaining theoretical voltage values of the h sampling point at the M end and the N end of the negative electrode of the circuit; u. of cemp (h)、u cenp (h) The voltages of the h-th sampling point at the M end and the N end of the line anode are respectively; u. u cemn (h)、u cenn (h) Respectively obtaining the voltage of the h sampling point at the M end and the N end of the negative pole of the circuit; u. of mp 、u np Respectively representing the voltages of the positive electrode of the circuit when the M end and the N end normally operate; u. of mn 、u nn Respectively representing the voltage of the negative pole of the line when the M end and the N end normally run, wherein H is the number of sampling points in a period after the fault occurs;
acquiring a fault pole selection coefficient by the following formula:
wherein, K m 、K n The fault pole selection coefficients of the M end and the N end are respectively; i.e. i tmp (h)、i tnp (h) Currents of the h-th sampling point at the M end and the N end of the line anode are respectively; i.e. i tmn (h)、i tnn (h) Currents of the h sampling point at the M end and the N end of the negative pole of the circuit are respectively; u. of tmp (h)、u tnp (h) The voltage of the h sampling point at the M end and the N end of the line anode is respectively; u. u tmn (h)、u tnn (h) Respectively obtaining the voltage of the h-th sampling point at the M end and the N end of the negative pole of the circuit; i.e. i mp 、i np Respectively representing the current of the positive pole of the circuit in normal operation at the M end and the N end; i.e. i mn 、i nn Respectively representing the current of the negative pole of the circuit when the M end and the N end normally operate.
2. The flexible direct current system pilot protection method based on voltage matching factors according to claim 1, wherein the fault identification criteria include:
wherein S is set Is an action threshold;
if the formula (5) or the formula (6) is satisfied, it is determined as an intra-area failure.
3. The pilot protection method of the flexible direct current system based on the voltage matching factor according to claim 1 or 2, wherein the step of identifying the fault pole according to the fault pole selection coefficients at two ends of the line comprises the following steps:
for the M terminal:
if K m ＞K set If yes, the positive pole fails; if K m ＜-K set If so, the negative electrode fails; if-K set ≤K m ≤K set Then bipolar failure;
for the N terminal:
if K n ＞K set If yes, the positive electrode fails; if K n ＜-K set Then the negative pole is in failure; if-K set ≤K n ≤K set Then bipolar failure;
wherein, K set Is the pole selection threshold.
4. The flexible direct current system pilot protection method based on voltage matching factors according to claim 1 or 2, characterized in that the converter stations are MMC converter stations.
5. The flexible direct current system pilot protection method based on the voltage matching factor as claimed in claim 4, wherein the theoretical calculated value u of the voltage of the h-th sampling point at the end M of the positive pole of the line cmp (h) Comprises the following steps:
wherein i tmp_L (h) The positive pole current of the adjacent line of the M end of the positive pole of the line flowing through the h sampling point is represented, and N represents the total number of sub-modules put into the upper bridge arm and the lower bridge arm of any phase in the converter station connected with the h sampling point; c represents the capacitance value of a submodule in the converter station connected with the port; r is arm 、l arm Respectively representing the equivalent resistance and inductance of an A-phase bridge arm in the converter station connected with the port; w is power frequency angular frequency, and L represents the inductance of the M end of the positive pole of the line; i.e. i va (h)、i vb (h)、i vc (h) Respectively representing the a, B, C ac currents flowing to the converter station connected to the port.
6. The pilot protection method for the flexible direct current system based on the voltage matching factor according to claim 1, wherein the starting of the line protection of the fault pole performs:
sending a trip signal to a protection in the failed pole.
7. The utility model provides a flexible direct current system pilot protection system based on voltage matching factor which characterized in that includes:
the data acquisition module is used for acquiring voltage and current at two ends of each line after a fault occurs and alternating current flowing to each converter station;
the theoretical calculation module is used for assuming that the current direction flowing through the port is inflow for each port of each line, and acquiring a voltage theoretical calculation value of the current port based on the current flowing through the adjacent line and the alternating current flowing to the converter station connected with the port;
the voltage matching factor and fault pole selection coefficient determining module is used for obtaining voltage matching factors at two ends of the corresponding line and fault pole selection coefficients at two ends of the corresponding line based on theoretical calculated values of voltage, current and voltage at two ends of each line;
the fault pole identification module is used for judging whether the voltage matching factors at the two ends of each line meet the fault identification criterion or not for each line, if so, judging that the line is in an internal fault, identifying a fault pole according to fault pole selection coefficients at the two ends of each line, and starting line protection of the fault pole;
the voltage matching factor is obtained by the following formula:
wherein S is mp 、S np Voltage matching factors of the positive pole of the circuit at the M end and the N end, S mn 、S nn Voltage matching factors of the negative electrode of the circuit at the M end and the N end are respectively set; u. of cmp (h)、u cnp (h) Respectively the theoretical calculated values of the voltage at the M end and the N end of the positive pole of the circuit at the h sampling point, u cmn (h)、u cnn (h) Respectively obtaining theoretical voltage values of the h sampling point at the M end and the N end of the negative electrode of the circuit; u. of cemp (h)、u cenp (h) The voltages of the h-th sampling point at the M end and the N end of the line anode are respectively; u. u cemn (h)、u cenn (h) Respectively obtaining the voltage of the h-th sampling point at the M end and the N end of the negative pole of the circuit; u. of mp 、u np Respectively representing the voltages of the positive electrode of the circuit when the M end and the N end normally operate; u. of mn 、u nn Respectively representing the voltage of the negative pole of the line when the M end and the N end normally run, wherein H is the number of sampling points in a period after the fault occurs;
acquiring a fault pole selection coefficient by the following formula:
wherein, K m 、K n The fault pole selection coefficients of the M end and the N end are respectively; i.e. i tmp (h)、i tnp (h) Currents of the h-th sampling point at the M end and the N end of the line anode are respectively; i all right angle tmn (h)、i tnn (h) Respectively obtaining the current of the h-th sampling point at the M end and the N end of the negative pole of the circuit; u. u tmp (h)、u tnp (h) The voltages of the h-th sampling point at the M end and the N end of the line anode are respectively; u. of tmn (h)、u tnn (h) Respectively obtaining the voltage of the h sampling point at the M end and the N end of the negative pole of the circuit; i.e. i mp 、i np Respectively representing the current of the positive pole of the circuit when the M end and the N end normally operate; i all right angle mn 、i nn Respectively representing the current of the negative pole of the circuit when the M end and the N end normally operate.
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