CN114759527A - Direct current line pilot direction protection method and system based on inductive energy polarity - Google Patents

Abstract

The invention relates to a direct current line pilot direction protection method and system based on inductive energy polarity, belongs to the technical field of relay protection, and solves the problems that the direct current line protection in an alternating current-direct current hybrid system is poor in transition resistance capacity when an area is in fault, is prone to misoperation when an area is in fault and the like. The method comprises the following steps: determining a frequency band range when the comprehensive impedance in the AC-DC hybrid system is inductive, and a minimum value and a secondary minimum value of angular frequency in the frequency band range; collecting the voltage and current of the positive direct current line on the rectification side and the inversion side before and after the fault; filtering the voltage and the current to obtain the current and the voltage when the angular frequency is between the minimum value and the secondary minimum value and the voltage and the current when the angular frequency is the secondary minimum value; and judging whether the fault is a direct current line fault or not based on the filtered voltage and current, if so, determining a fault pole of the direct current line based on the voltages of the positive and negative direct current lines on the rectification side, which are collected before and after the fault occurs, and starting the line protection of the fault pole.

Description

Direct current line pilot direction protection method and system based on inductive energy polarity

Technical Field

The invention relates to the technical field of relay protection, in particular to a direct current line pilot direction protection method and system based on inductive energy polarity.

Background

At present, the direct current line protection is mainly divided into traveling wave protection, differential under-voltage protection and pilot current differential protection. The traveling wave protection utilizes the polarity, amplitude and other information of fault initial traveling wave to form a protection criterion, and the method has the action characteristic of ultra high speed, so that the method becomes one of the main protections of the current direct current line. However, in actual operation, the traveling wave protection needs to avoid the influence of the out-of-zone metallic fault, so that the setting value is high, and the traveling wave protection cannot correctly and reliably identify the fault when the high resistance fault occurs in the zone. In addition, the traveling wave protection also has the problems of setting through simulation experiments, poor lightning interference resistance, high requirements on protection devices and the like. The differential under-voltage protection forms a protection principle by detecting a voltage differential value and a voltage amplitude level, is one of main protections of a direct current line, is also used as a backup of traveling wave protection, and has higher sensitivity and reliability compared with the traveling wave protection. However, in actual operation, the differential undervoltage protection still has the problems of poor transition resistance capability, need of setting through a simulation test and the like. The pilot current differential protection utilizes the two-end current addition and construction protection criterion for cutting off high-resistance faults which cannot act in traveling wave protection and differential under-voltage protection. In actual operation, due to the fact that interference factors such as an out-of-area alternating current side fault need to be considered, under the most extreme condition, the time delay can reach 1100ms, and the backup protection effect of a direct current line is difficult to achieve. In addition, the pilot current differential protection needs to exchange information of electrical quantities at two ends, and has high requirements on communication channels and data synchronization.

Disclosure of Invention

In view of the foregoing analysis, embodiments of the present invention provide a method and a system for protecting a pilot direction of a dc line based on inductive energy polarity, so as to solve the problems of poor transient resistance capability when an intra-area fault occurs in the dc line protection in the ac/dc hybrid system, and easy malfunction when an extra-area fault occurs.

On one hand, the embodiment of the invention provides a direct current line pilot direction protection method based on inductive energy polarity, which is suitable for an alternating current-direct current hybrid system and comprises the following steps:

determining a frequency band range when the comprehensive impedance in the AC-DC hybrid system is inductive, and a minimum value and a sub-minimum value of angular frequency in the frequency band range;

collecting the voltage and current of the positive direct current line on the rectification side and the inversion side before and after the fault occurs;

filtering the voltage and the current to obtain the current and the voltage when the angular frequency is between the minimum value and the secondary minimum value and the voltage and the current when the angular frequency is the secondary minimum value;

and judging whether the fault is a direct current line fault or not based on the voltage and the current obtained by filtering, if so, determining a fault pole of the direct current line based on the voltages of the positive and negative direct current lines on the rectification side or the inversion side, which are collected before and after the fault occurs, and starting the line protection of the fault pole.

On the basis of the scheme, the invention also makes the following improvements:

further, judging whether the fault is a direct-current line fault based on the voltage and the current obtained by filtering comprises the following steps:

based on the formulas (1) and (2), the angular frequency between the minimum values w is obtained₁And the second smallest value w₂Energy of direct current line in time between at rectification side

Energy of inversion side

Wherein the content of the first and second substances,

respectively representing the difference of the voltage and the current of the positive direct current line on the rectification side when the angular frequency is between the minimum value and the secondary minimum value, after the fault occurs and before the fault occurs;

respectively representing the difference of the voltage and the difference of the current of the positive direct current line on the inversion side when the angular frequency is between the minimum value and the secondary minimum value, after the fault occurs and before the fault occurs;

based on the formulas (3) and (4), the energy of the direct current line on the rectification side when the angular frequency is a second-order small value is obtained

Energy of inversion side

Wherein the content of the first and second substances,

respectively representing the difference value of the voltage and the difference value of the current of the positive direct-current line on the rectification side when the angular frequency is a second-smallest value, after the fault occurs and before the fault occurs;

respectively when the angular frequency is the second smallest value,The difference of the voltages and the difference of the currents of the positive direct current line on the inversion side after the fault occurs and before the fault occurs;

Based on

And a direct current line fault protection criterion is used for judging whether the fault is a direct current line fault.

Further, the dc line fault protection criterion is:

if S_recAnd S_invAnd if the fault is 1, the fault is a direct current line fault.

Further, a faulted pole of the DC link is determined by performing the following operations:

based on the voltages of the positive and negative direct current lines on the rectification side or the inversion side and a formula (6), which are acquired before and after the occurrence of the fault, calculating a fault pole selection coefficient P:

wherein u is_pos(i)、u_neg(i) The voltages of the positive and negative direct current lines on the rectification side or the inversion side at the ith moment after the fault occurs respectively; u. of_pos(i-T)、u_neg(i-T) are the voltages of the positive and negative direct current lines on the rectification side or the inversion side at the ith-T moment before the fault occurs respectively; n is a radical of_sThe number of sampling points in T/2; t represents the power frequency period of the alternating current-direct current hybrid system;

when J is not less than J_maxWhen the fault is a positive direct current line fault, the fault is a positive direct current line fault; when J is less than or equal to J_minWhen the fault is a negative direct current line fault; j. the design is a square_min＜J＜J_maxWhen the fault is a bipolar short-circuit fault; j. the design is a square_max、J_minAnd respectively selecting the upper threshold and the lower threshold of the coefficient.

Further, the frequency band range of the inductive comprehensive impedance of the AC/DC hybrid system is determined by executing the following operations:

Calculating the comprehensive impedance of the rectifying side or the inverting side:

wherein, Z_s1、Z_ac、Z_d、Z_c、Z_dcThe impedance of an alternating current system, an alternating current filter and reactive power compensation device, the impedance of a smoothing reactor, the minimum equivalent impedance of a current converter and the impedance of a direct current filter on a rectification side or an inversion side respectively; a is a₀-a₁₃To simplify the constants obtained by equation (7); w represents an angular frequency.

Obtaining a₁₃w¹³+a₁₁w¹¹+…+a₁Solution w of w equal to 0_1.1、w_1.2、w_1.3、w_1.4、w_1.5、w_1.6(ii) a And, obtaining a₁₂w¹²+a₁₀w¹⁰+…+a₀Solution w of 0_2.2、w_2.2、w_2.3、w_2.4、w_2.5And w_2.6；

Obtaining the frequency band range when the comprehensive impedance of the alternating current-direct current hybrid system is inductive based on the obtained solution:

the minimum value and the second minimum value of the angular frequency are integral multiples of the minimum harmonic angular frequency of the direct current line voltage and current in steady-state operation.

Further, the converter minimum equivalent impedance is obtained by:

and analyzing the possible conduction state of the converter in the rectifying side or the inverting side after the fault occurs to obtain the equivalent impedance of the converter in each conduction state, and taking the minimum value of the equivalent impedance as the minimum equivalent impedance of the converter.

Further, the possible conduction states of the inverter in the rectifying side or the inverting side after the fault occurs include:

on-state 1: in the D-bridge converter, two converter valves which are numbered and adjacent are conducted; in the Y-bridge converter, two converter valves with the same number as the converter valves conducted with the D-bridge converter are conducted;

Conduction state 2: in the D-bridge converter, two converter valves which are numbered and adjacent are conducted; in the Y-bridge converter, three converter valves which are numbered and adjacent are conducted; the serial numbers of the converter valves conducted in the D bridge converter are correspondingly the same as the serial numbers of the first two converter valves conducted in the Y bridge converter;

on-state 3: in the D-bridge converter, two converter valves which are numbered and adjacent are conducted; in the Y-bridge converter, two converter valves which are numbered and adjacent are conducted; the serial number of the rear converter valve conducted in the D bridge converter is the same as that of the front converter valve conducted in the Y bridge converter;

on-state 4: in the D-bridge converter, three converter valves which are numbered and adjacent are conducted; in the Y-bridge converter, two converter valves which are numbered and adjacent are conducted; and the serial numbers of the converter valves conducted in the Y-bridge converter are correspondingly the same as the serial numbers of the last two converter valves conducted in the D-bridge converter.

Further, whether the alternating current-direct current series-parallel connection system breaks down or not is judged based on the collected current flowing through the direct current filter:

and calculating the average value of the current flowing through each direct current filter at the latest M sampling points, and judging that the alternating current-direct current series-parallel system has a fault when the average value of the current flowing through any direct current filter is greater than a fault current threshold value.

Further, J_maxTaking 1.5; j. the design is a square_min0.8 is taken.

On the other hand, the invention also provides a direct current line pilot direction protection system based on inductive energy polarity, which comprises:

the inductive frequency band determining module is used for determining a frequency band range when the comprehensive impedance in the alternating-current and direct-current hybrid system is inductive, and a minimum value and a secondary minimum value of angular frequency in the frequency band range;

the data acquisition module is used for acquiring the voltage and the current of the positive direct current line on the rectification side and the inversion side before and after the fault occurs; acquiring the voltage of the negative direct current line on the rectification side or the inversion side before and after the fault occurs;

the data filtering module is used for filtering the voltage and the current to obtain the current and the voltage when the angular frequency is between the minimum value and the secondary minimum value and the voltage and the current when the angular frequency is the secondary minimum value;

and the direct-current line protection module is used for judging whether the fault is a direct-current line fault or not based on the voltage and the current obtained by filtering, determining a fault pole of the direct-current line based on the voltages of the positive and negative direct-current lines on the rectification side or the inversion side collected before and after the fault occurs if the fault is the direct-current line fault, and starting the line protection of the fault pole.

Compared with the prior art, the invention can realize at least one of the following beneficial effects:

Firstly, according to the scheme, through data acquisition before and after a fault and impedance characteristic equations of all elements of the AC-DC hybrid system, a frequency band with inductive comprehensive impedance of the AC-DC hybrid system can be simply and quickly obtained, and whether the current fault is a fault of a DC transmission line or not is quickly determined according to the difference of the inductive energy flow directions in fault component networks inside and outside a region; after the direct current transmission line fault is determined, further determining a fault pole (namely the specific position of the direct current line fault) of the direct current line based on the direct current line voltages of the positions of the positive and negative direct current filters, which are acquired before and after the fault occurs, and starting line protection of the fault pole; the method effectively solves the problems that the direct current transmission line in the alternating current and direct current hybrid system has poor transition resistance capacity when the direct current transmission line is protected to have an internal fault, is easy to malfunction when the direct current transmission line is protected to have an external fault and the like, is not influenced by the transition resistance, the fault position and lightning interference, and can still correctly and reliably identify the internal and external faults when the phase change is failed due to the fault of the inverter side alternating current system.

Secondly, the scheme only needs the inversion side to transmit the identification result of the fault direction to the rectifying side, does not need to exchange electric quantity information at two ends, is not influenced by synchronous errors in fault identification, and has low requirements on a communication device.

Finally, the scheme has low sampling frequency, low requirement on a sampling device, easy engineering realization, and adoption of a self-adaptive protection criterion without setting through a simulation experiment.

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 flowchart of a method for dc line pilot direction protection based on inductive energy polarity in embodiment 1;

FIG. 2 is a schematic diagram of an AC/DC hybrid system;

FIG. 3 is a schematic wiring diagram of a rectification side 12 pulse wave inverter;

FIG. 4 is a diagram of a network topology of a DC filter;

FIG. 5 is a diagram of a network topology for an AC filter;

FIG. 6 is a diagram of a network topology of smoothing reactors;

FIG. 7 is a diagram of a network topology of a rectifier side AC system;

FIG. 8 is a diagram of a fault component network upon an intra-zone fault;

FIG. 9 is a diagram of a fault component network in the event of an out-of-range fault;

fig. 10 is a direct current line pilot direction protection system based on inductive energy polarity provided in embodiment 2;

FIG. 11 shows the case of a fault via different transition resistances

FIG. 12 shows the case of a fault via different transition resistances

FIG. 13 shows the case where a failure occurs at a different position

FIG. 14 shows the case where a failure occurs at a different position

Fig. 15(a) is the inverter-side Y-bridge converter valve current under normal operating conditions in example 3;

fig. 15(b) is the valve current of the inverter side Y-bridge converter in case of a three-phase short-circuit fault in embodiment 3;

FIG. 16(a) shows the case where the synchronization error of both end data is 0ms in example 3

And

FIG. 16(b) is a graph showing the two-terminal data synchronization error of +2ms in example 3

And

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 specific embodiment 1 of the present invention discloses a method for protecting a pilot direction of a dc line based on an inductive energy polarity, a flowchart is shown in fig. 1, and the method is suitable for an ac/dc hybrid system (as shown in fig. 2), and includes the following steps:

Step S1: determining a frequency band range when the comprehensive impedance in the AC-DC hybrid system is inductive, and a minimum value and a sub-minimum value of angular frequency in the frequency band range;

step S2: collecting the voltage and current of the positive direct current line on the rectification side and the inversion side before and after the fault;

step S3: filtering the voltage and the current to obtain the current and the voltage when the angular frequency is between the minimum value and the secondary minimum value and the voltage and the current when the angular frequency is the secondary minimum value;

step S4: and judging whether the fault is a direct current line fault or not based on the voltage and the current obtained by filtering, if so, determining a fault pole of the direct current line based on the voltages of the positive and negative direct current lines on the rectification side or the inversion side, which are collected before and after the fault occurs, and starting the line protection of the fault pole.

In the alternating current-direct current hybrid system, elements on the rectification side and the inversion side are symmetrically arranged, and the selected parameters are consistent. Therefore, the present embodiment takes the rectifying side as an example to describe the band range determination process when the integrated impedance is inductive in the ac/dc hybrid system. The impedances related to the rectifying side include converter impedance, alternating current system impedance, alternating current filter and reactive power compensation device impedance, smoothing reactor impedance and direct current filter impedance.

(1) Impedance characteristic equation of converter

Based on the topological network of the conduction state of the converter in different time periods, obtaining the impedance characteristic equation of the converter in different commutation failure scenes (namely the conduction state possibly existing in the converter in the rectifying side or the inverting side after the fault occurs);

fig. 3 shows a schematic structural diagram of a rectifying-side 12-pulse converter, which is composed of a D-bridge converter and a Y-bridge converter; the D-bridge converter comprises converter valves VTD1-VTD 6; wherein, three serial branches are formed by VTD1, VTD4, VTD3, VTD6, VTD5 and VTD2, the cathodes of VTD1, VTD3 and VTD5 are connected, and the anodes of VTD4, VTD6 and VTD2 are connected; the Y-bridge converter comprises converter valves VTY1-VTY 6; wherein, three serial branches are formed by VTY1, VTY4, VTY3, VTY6, VTY5 and VTY2, the cathodes of VTY1, VTY3 and VTY5 are connected, and the anodes of VTY4, VTY6 and VTY2 are connected.

The conduction states of converter valves in the D-bridge converter and the Y-bridge converter are divided into two states: two converter valves are on and three converter valves are on. Taking the Y-bridge inverter as an example, the turn-on sequence is: VTY1 and VTY2 are turned on, VTY1, VTY2 and VTY3 are turned on, VTY2 and VTY3 are turned on …, and the state where VTY1, VTY2 and VTY3 are all turned on (three converter valves are turned on) is also called commutation. The converter valves are off except for indicating that the converter valve that is on.

The conduction state of the rectifying side 12-pulse inverter can be divided into the following four types:

1) conduction state 1: in the D-bridge converter, two converter valves which are numbered and adjacent are conducted; in the Y-bridge converter, two converter valves with the same number as the converter valves conducted with the D-bridge converter are conducted; specifically, the method comprises the following steps: VTD1 and VTD2 are conducted, VTY1 and VTY2 are conducted; alternatively, VTD2 is conductive with VTD3, VTY2 is conductive with VTY 3; alternatively, VTD3 is conductive with VTD4, VTY3 is conductive with VTY 4; alternatively, VTD4 is conductive with VTD5, VTY4 is conductive with VTY 5; alternatively, VTD5 is conductive with VTD6, VTY5 is conductive with VTY 6; alternatively, VTD6 is conductive with VTD1, VTY6 is conductive with VTY 1;

2) on-state 2: in the D-bridge converter, two converter valves which are numbered and adjacent are conducted; in the Y-bridge converter, three converter valves which are numbered adjacently are conducted; the serial numbers of the converter valves conducted in the D bridge converter are correspondingly the same as the serial numbers of the first two converter valves conducted in the Y bridge converter; specifically, the method comprises the following steps: VTD1 is conducted with VTD2, VTY1, VTY2 and VTY 3; alternatively, VTD2 is conducted with VTD3, VTY2, VTY3 and VTY 4; alternatively, VTD3 is conducted with VTD4, VTY3, VTY4 and VTY 5; alternatively, VTD4 is conducted with VTD5, VTY4, VTY5 and VTY 6; alternatively, VTD5 is conducted with VTD6, VTY5, VTY6 and VTY 1; alternatively, VTD6 is conducted with VTD1, VTY6, VTY1 and VTY 2;

3) On-state 3: in the D-bridge converter, two converter valves which are numbered and adjacent are conducted; in the Y-bridge converter, two converter valves which are numbered and adjacent are conducted; the serial number of the rear converter valve conducted in the D bridge converter is the same as that of the front converter valve conducted in the Y bridge converter; specifically, VTD1 is conducted with VTD2, VTY2 is conducted with VTY 3; alternatively, VTD2 is conductive with VTD3, VTY3 is conductive with VTY 4; alternatively, VTD3 is conductive with VTD4, VTY4 is conductive with VTY 5; alternatively, VTD4 is conductive with VTD5, VTY5 is conductive with VTY 6; alternatively, VTD5 is conductive with VTD6, VTY6 is conductive with VTY 1; alternatively, VTD6 is conductive with VTD1, VTY1 is conductive with VTY 2;

4) on-state 4: in the D-bridge converter, three converter valves which are numbered adjacently are conducted; in the Y-bridge converter, two converter valves which are numbered and adjacent are conducted; the serial numbers of the converter valves conducted in the Y-bridge converter are correspondingly the same as the serial numbers of the two converter valves conducted in the D-bridge converter; specifically, VTD1, VTD2 and VTD3 are conductive, and VTY2 and VTY3 are conductive; alternatively, VTD2, VTD3 and VTD4 are conductive, and VTY3 and VTY4 are conductive; alternatively, VTD3, VTD4 and VTD5 are conductive, and VTY4 and VTY5 are conductive; alternatively, VTD4, VTD5 and VTD6 are conductive, and VTY5 and VTY6 are conductive; alternatively, VTD5, VTD6 and VTD1 are conductive, and VTY6 and VTY1 are conductive; alternatively, VTD6, VTD1 and VTD2 are conductive, and VTY1 and VTY2 are conductive.

In the above on-state, the other converter valves not illustrated are all in the off-state.

Specifically, the rectifying side converter and the impedance characteristic equation derivation process under different conduction states are as follows:

1) conducting state 1

When the 12-pulse inverter is in the on state 1, it can be obtained by combining fig. 3:

i_ad＝i_bd (1)

i_cd+i_d＝i_bd (2)

i_ay＝i_d (3)

i_cy＝-i_d (4)

wherein u is_a、u_b、u_cThree-phase voltage of a rectification side current conversion bus; u. of_d、u_d1、u_d2The voltage at the common cathode of the D bridge converter, the voltage at the common anode of the D bridge converter and the voltage at the common anode of the Y bridge converter are respectively; i.e. i_ad、i_bd、i_cd、i_ay、i_by、i_cyFor each winding current; k is a radical of_y、k_dThe transformation ratios of the Y/Y converter transformer and the Y/delta converter transformer are respectively; l is_rConverting the inductance to the valve side for the converter transformer; l is_dIs a smoothing reactor inductance; i.e. i_dIs the current flowing through the smoothing reactor.

According to the formulae (5) to (7):

because the converter transformer is in a Y/delta or Y/Y connection mode, when a direct current line fails, zero sequence voltage does not exist at a converter bus, namely:

u_a+u_b+u_c＝3u₀＝0 (9)

wherein u is₀The zero sequence voltage at the commutation bus at the rectifying side is obtained.

The following relation exists in the three-phase current differential of the Y/delta converter transformer by substituting the formula (9) into the formula (8):

the three-phase current of the Y/delta converter transformer obtained by the combined vertical type (1), the formula (2) and the formula (10) is as follows:

according to the formulas (5) to (7) and (11):

As can be seen from fig. 2, the three-phase voltage of the converter bus and the three-phase current of the ac line L-K are:

wherein r is₁And l₁Positive sequence resistance and positive sequence inductance of the alternating current line L-K respectively; e.g. of the type_sa、e_sbAnd e_scIs the equivalent three-phase potential of the ac system S1; c_a、C_bAnd C_cThe three-phase current of the rectification side AC filter is obtained.

The united type (11) to (14) can be obtained:

in the formula A₁、B₁And C₁Are respectively:

as shown in formula (16), A₁Influenced by the resistance of the AC line, B₁Influenced by the equivalent inductance of the converter and the inductance of the AC line, C₁Influenced by the current of the alternating current filter and the three-phase voltage of the receiving end alternating current system. According to the above analysis, for the fault component network when the dc line has a fault, the components should be independent and decoupled from each other, and for the inverter impedance, the expression thereof should only include the inverter inductance Lr, so that the equivalent inductance of the inverter in the conducting state 1 is

Therefore, the impedance characteristic equation of the converter in the conducting state 1 is as follows:

wherein, w is the angular frequency of the electric quantity information such as voltage, current and the like in the direct current line.

2) Conducting state 2

When the 12-pulse inverter is in the on state 2, it can be obtained by combining fig. 3:

i_ad＝i_bd (18)

i_cd+i_d＝i_bd (19)

i_ay+i_by＝i_d (20)

i_cy＝-i_d (21)

the following equations (22) to (25) can be obtained:

the joint type (13), the formula (14), the formulae (18) to (21), and the formula (26) can be obtained:

A in formula (27)₂、B₂And C₂Are respectively:

the impedance characteristic equation for the inverter in the conducting state 2 according to equation (28) is:

3) conducting state 3

When the 12-pulse inverter is in the on state 3, it can be obtained by combining fig. 3:

i_ad＝i_bd (30)

i_cd+i_d＝i_bd (31)

i_by＝i_d (32)

i_cy＝-i_d (33)

this can be obtained according to equations (30) to (36):

the joint type (13), the formula (14), the formulae (30) to (33), and the formula (37) can be obtained:

a in formula (27)₃、B₃And C₃Are respectively:

the impedance characteristic equation of the converter in the conducting state 3 can be obtained according to the formula (39) as follows:

4) conducting state 4

When the 12-pulse inverter is in the on state 4, it can be obtained by combining fig. 3:

i_bd＝i_cd+i_d (41)

i_by＝i_d (42)

i_cy＝-i_d (43)

this is obtained according to formulas (44) to (47):

the joint type (13), the formula (14), the formulae (41) to (43), and the formula (48) can be obtained:

a in formula (27)₄、B₄And C₄Are respectively:

the impedance characteristic equation of the converter in the conducting state 4 can be obtained according to the equation (50) as follows:

(2) impedance characteristic equation of DC filter

The impedance characteristic equation of the dc filter from fig. 4 is:

the direct current filter is composed of the following modes: inductor L₂And a capacitor C₂Connected in parallel with the inductor L₁Capacitor C₁Are connected in series.

(3) Impedance characteristic equation of AC filter

The impedance characteristic equation of the ac filter from fig. 5 is:

the alternating current filter has three groups of elements, the three groups of elements are connected to the converter bus in a parallel mode, and the first group of elements of the alternating current filter are formed in a mode that: resistance R ₃Capacitor C₃Inductor L₃In series with a resistor R₄Connected in parallel and finally connected with a capacitor C₄Are connected in series; the second group of elements is composed of: resistance R₅And a resistance L₄Connected in parallel with a capacitor C₅Are connected in series; the third group of elements consisting of a capacitor C₆And (4) forming.

(4) Impedance characteristic equation of smoothing reactor

The impedance characteristic equation of the smoothing reactor obtained from fig. 6 is:

Z_d＝jwL_d (54)

wherein L is_dThe inductance value of the smoothing reactor.

(5) Impedance characteristic equation of rectification side alternating current system

The impedance characteristic equation of the rectification side ac system S1 obtained from fig. 7 is:

Z_s1＝R_W1+jwL_W1 (55)

wherein R is_W1And L_W1Respectively an equivalent resistance and an equivalent inductance of the rectification side alternating current system.

According to the impedance characteristic equation, the impedance characteristic equation of the rectification side is obtained as follows:

wherein Z is_s1、Z_ac、Z_d、Z_c、Z_dcThe impedance of an alternating current system, an alternating current filter and reactive power compensation device, the impedance of a smoothing reactor, the minimum equivalent impedance of a current converter and the impedance of a direct current filter on a rectification side or an inversion side respectively; (ii) a a is₁₃、a₁₂、a₁₁、a₁₀、a₉、a₈、a₇、a₆、a₅、a₄、a₃、a₂、a₁And a₀To simplify the constant obtained by equation (56), a₁₃、a₁₂Are all greater than zero. w represents the angular frequency.

As can be seen from equation (56), the rectifier-side impedance is composed of an inverter impedance, a dc filter impedance, an ac filter impedance, a smoothing reactor impedance, and an ac system impedance, and each element impedance specifically refers to an inductance value of the inverter, a capacitance, an inductance, and a resistance value of the dc filter, a capacitance, an inductance, and a resistance value of the ac filter, an inductance value of the smoothing reactor, and an inductance and a resistance value of the ac system. Considering that the impedance value presented by the capacitor is close to zero at high frequency, the impedance value presented by the inductor is large, and the impedance value presented by the capacitor is large at low frequency, and the impedance value presented by the inductor is close to zero, therefore, when the angular frequency is changed from zero to positive infinity, the rectifying side impedance of the alternating-current-direct-current hybrid system may present inductance or capacitance.

1) The rectification side impedance of the AC-DC hybrid system is inductive

When according to the formula (A), (B)56) When real-time calculation of the inductive frequency band is performed, the rectifying side impedance Z_recThe impedance characteristic equation of the inverter is a function of w, but is time-varying according to the equations (17), (29), (40) and (51). Since a fault may occur at any time, the influence of the impedance characteristic of the inverter in different conduction states on the rectifying-side impedance characteristic needs to be considered.

As can be seen from equation (56), the smaller the inductance value of the inverter, the weaker the inductive characteristic exhibited by the rectifying-side impedance. Therefore, in order to make the rectifying-side impedance inductive in each conducting state, the inductance value of the inverter should be the minimum value, and as can be seen by combining equations (17), (29), (40), and (51), the minimum value is

Minimum equivalent impedance Z of current converter at this time_cIs composed of

Besides considering the influence of the impedance characteristics of the converter, it is also necessary to ensure that there is a high frequency component large enough to realize the judgment of faults inside and outside the area. Considering that the high frequency component under the condition of the fault of the direct current line is larger than that in normal operation, the adopted high frequency component is enough to be ensured to be large when the direct current line operates normally. The harmonic when no fault occurs in the dc line is a 12k (k is 1,2, … …) th harmonic, the corresponding angular frequency is 1200k pi rad/s, and the frequency band when the rectifying-side impedance is inductive is obtained by combining equation (56):

Wherein w_1.1、w_1.2、w_1.3、w_1.4、w_1.5And w_1.6Is a₁₃w¹³+a₁₁w¹¹+…+a₁A solution where w is 0; w is a_2.2、w_2.2、w_2.3、w_2.4、w_2.5And w_2.6Is a₁₂w¹²+a₁₀w¹⁰+…+a₀A solution of 0; the minimum value and the second minimum value of the angular frequency are integral multiples of the minimum harmonic angular frequency of the direct current line voltage and current in steady-state operation. The minimum harmonic angular frequency of the voltage and current of the direct current line during steady state operation selected in this embodiment is 1200 π rad/s.

2) The rectification side impedance of the AC-DC hybrid system is capacitive

From equation (57), the frequency band where the rectifying-side impedance is capacitive is:

as is clear from the combination of equations (57) and (58), the band interval when the rectifying-side impedance is inductive is wider than the capacitive band, and the inductive energy at the time of failure is also larger because the overlapping region between the band containing a large high-frequency component (the band having an angular frequency of 1200k pi rad/s) and the inductive band is larger, the present embodiment utilizes the principle of protection by the fault component network structure composed of the inductive impedance.

As is clear from the above analysis, since the inductive energy at the time of the fault is larger than the energy of the capacitive frequency band, the present embodiment utilizes the fault component network structure protection principle constituted by the inductive impedance.

1) Inner zone fault (i.e. DC line fault)

When a fault occurs in the dc line area, the fault component network of the ac/dc hybrid system is shown in fig. 8, where Z is shown in fig. 8 _line1Impedance from the fault point to the M terminal; z_line2Impedance from the fault point to the N terminal; z_s2Impedance of an inverter side alternating current system; z_fIs the transition resistance.

As can be seen from fig. 8, the voltage and current at the M, N end have the following relationship:

wherein L is_mAn inductance value of the rectifying side impedance; l is_nTo the inverter side impedanceThe inductance value of (c).

The energy at end M, N can be defined by equation (59) as:

from equation (60), it can be seen that the energy of the dc link on the rectification side when the angular frequency is between the minimum value and the second minimum value

Energy of inversion side

Respectively as follows:

wherein the content of the first and second substances,

respectively indicating that the angular frequency is between a minimum value w₁And a second smallest value w₂The difference of the voltages and the difference of the currents of the positive direct current line on the rectifying side during the time between the occurrence of the fault and the occurrence of the fault before the occurrence of the fault;

respectively representing the difference of the voltage and the difference of the current of the positive direct current line on the inversion side when the angular frequency is between the minimum value and the secondary minimum value, after the fault occurs and before the fault occurs; w is a₁，w₂∈w_hvdcL。

From equation (60), the energy of the direct current line on the rectification side when the angular frequency is the second smallest value

Energy of inversion side

Respectively as follows:

wherein the content of the first and second substances,

respectively representing the difference value of the voltage and the difference value of the current of the positive direct-current line on the rectification side when the angular frequency is a second-smallest value, after the fault occurs and before the fault occurs;

Respectively representing the difference value of the voltage and the difference value of the current of the positive direct current line on the inversion side when the angular frequency is a secondary small value, after the fault occurs and before the fault occurs;

according to the formula (60):

2) out of area fault

When a fault occurs outside the rectification side region of the direct current line, a fault component network of the alternating current-direct current hybrid system is shown as figure 9, wherein Z in figure 9_lineIs the dc line impedance.

According to the analysis in step S22, the minimum possible value of w is 1200k pi, and considering that when the value of w is greater than 1200k pi, the dc line impedance is inductive, and it can be known from fig. 9 that the voltage and the current at the M, N end have the following relationship:

from formula (59):

according to the formula (65):

based on the above relationship, a protection criterion (also referred to as "dc line fault protection criterion") for identifying faults inside and outside the dc line area is constructed, and whether the fault is a dc line fault is determined.

According to the equations (63) and (66), when an in-zone fault occurs in the DC line, E_m(w₁～w₂)<E_m(w₂) And E_n(w₁～w₂)<E_n(w₂) When a failure occurs outside the commutation side region, E_m(w₁～w₂)>E_m(w₂) And E_n(w₁～w₂)<E_n(w₂) When a fault occurs outside the inversion side region, E_m(w₁～w₂)<E_m(w₂) And E_n(w₁～w₂)>E_n(w₂). From the above analysis, a protection criterion can be constructed as follows:

in the formula, S_recAnd S_invAre respectively:

therefore, if S _recAnd S_invAnd if the fault is 1, the fault is a direct current line fault, and the fault is a direct current line fault.

And when the fault is the fault of the direct current line, determining the fault pole of the direct current line based on the voltages of the positive and negative direct current lines on the rectification side or the inversion side, which are collected before and after the fault occurs.

After a single pole of the bipolar direct current transmission system fails, transient voltage and current signals are generated by a non-failure pole. In order to ensure the normal operation of the non-fault pole, the fault pole needs to be identified.

Transient components of non-fault poles are generated through electromagnetic coupling, when a single-pole ground fault occurs, the voltage of the fault pole is rapidly reduced, the voltage of the non-fault pole is increased to a certain degree, and when a double-pole short circuit fault occurs, the amplitudes of the positive and negative poles are basically equal. Therefore, when the positive line fails, the following relationship exists between the voltage variation of the positive line and the voltage variation of the negative line:

wherein, Δ u_posIs the positive line voltage variation; delta u_negIs the negative line voltage variation; and alpha is the coupling coefficient between lines.

When the negative electrode line has a fault, the voltage variation of the positive electrode line and the negative electrode line has the following relation:

when the bipolar line has a fault, the voltage variation of the positive line and the negative line has the following relationship:

The failure pole selection coefficient P can be constructed according to equations (70) - (72) as:

wherein u is_pos(i)-u_pos(i-T)＝Δu_pos(i)，u_neg(i)-u_neg(i-T)＝Δu_neg(i)，u_pos(i)、u_neg(i) The positive and negative direct current lines are respectively at the rectification side or the inversion side at the ith moment after the fault occursVoltage of (d); u. of_pos(i-T)、u_neg(i-T) are the voltages of the positive and negative direct current lines on the rectification side or the inversion side at the ith-T moment before the fault occurs respectively; n is a radical of_sThe number of sampling points in T/2; and T represents the power frequency period of the alternating current-direct current hybrid system.

The fault electrode voltage characteristic means that when a single-pole ground fault occurs, the fault electrode voltage rapidly drops, but the non-fault electrode voltage is increased to a certain extent; when a bipolar short circuit fault occurs, the voltage amplitudes of the positive electrode and the negative electrode are always equal. Thus, a fault pole selection criterion is constructed:

when J is not less than J_maxWhen the fault is a positive direct current line fault, the fault is a positive direct current line fault; when J is less than or equal to J_minWhen the fault is a negative direct current line fault; j. the design is a square_min＜J＜J_maxWhen the fault is a bipolar short-circuit fault; j. the design is a square_max、J_minAnd respectively selecting the upper threshold and the lower threshold of the coefficient.

Since the maximum value of the coupling coefficient alpha between the lines is 0.5 and a certain margin is taken into account, J_maxTaking 1.5; j. the design is a square_min0.8 is taken. When P is larger than or equal to 1.5, judging that the positive line has a fault; when P is less than or equal to 0.8, judging that the negative electrode circuit has a fault; when 0.8 <P<When 1.5, it is judged as a bipolar fault.

It should be further noted that, in this embodiment, whether the ac/dc hybrid system fails is determined based on the collected current flowing through the dc filter, specifically, an average value of currents flowing through each of the dc filters at the latest M sampling points is calculated, and when the average value of the currents flowing through any one of the dc filters is greater than a fault current threshold value, it is determined that the ac/dc hybrid system fails.

And after the fault pole is determined, starting the line protection of the fault pole.

Example 2

An embodiment 2 of the present invention provides a system for protecting a pilot direction of a dc line based on an inductive energy polarity, a schematic structural diagram of which is shown in fig. 10, and the system is suitable for an ac/dc hybrid system, and includes:

the inductive frequency band determining module is used for determining a frequency band range when the comprehensive impedance in the alternating-current and direct-current hybrid system is inductive, and a minimum value and a secondary minimum value of angular frequency in the frequency band range;

the data acquisition module is used for acquiring the voltage and the current of the positive direct current line on the rectification side and the inversion side before and after the fault occurs; acquiring the voltage of the negative direct current line on the rectification side or the inversion side before and after the fault occurs;

the data filtering module is used for filtering the voltage and the current to obtain the current and the voltage when the angular frequency is between the minimum value and the secondary minimum value and the voltage and the current when the angular frequency is the secondary minimum value;

And the direct-current line protection module is used for judging whether the fault is a direct-current line fault or not based on the voltage and the current obtained by filtering, determining a fault pole of the direct-current line based on the voltages of the positive and negative direct-current lines on the rectification side or the inversion side collected before and after the fault occurs if the fault is the direct-current line fault, and starting the line protection of the fault pole.

Preferably, the system further comprises a fault detection module, configured to calculate an average value of currents flowing through each dc filter at the latest M sampling points, and determine that the ac-dc hybrid system has a fault when the average value of the currents flowing through any one of the dc filters is greater than a fault current threshold value.

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 refer to the embodiment of the method, and details are not repeated herein. The principle of the embodiment of the system is the same as that of the embodiment of the method, so the system also has the corresponding technical effect of the embodiment of the method.

Example 3

In order to verify the correctness of the method for protecting the pilot direction of the dc line based on the polarity of the inductive energy provided by embodiment 1 of the present invention, the present embodiment provides a specific example: the main parameters of the AC-DC hybrid system are shown in Table 1. And taking the fault occurrence time as zero time.

TABLE 1 main parameters of AC/DC series-parallel system

The first setting scenario of this embodiment is: faults passing through different transition resistors occur at 50% of the positive direct-current line, and the range of the set transition resistors is 0-300 omega.

At 50% of the positive DC line, a fault occurs via different transition resistances, in which case [ E ] is_m(w₁～w₂)-E_m(w₂)]、[E_n(w₁～w₂)-E_n(w₂)]As shown in fig. 11 and 12, respectively.

As can be seen from FIGS. 11 and 12, in different failure cases, as the transition resistance increases, [ E ] in the same time section_m(w₁～w₂)-E_m(w₂)]And [ E_n(w₁～w₂)-E_n(w₂)]Increasing the resistance of the resistor to different transition resistances_m(w₁～w₂)-E_m(w₂)]And [ E_n(w₁～w₂)-E_n(w₂)]Instead of equation (67) and equation (68), S may be equal to 1, and the protection recognizes that a fault occurs in the dc link zone. As can be seen from fig. 11, [ E ] when the positive line fails and the transition resistance is 300 Ω_m(w₁～w₂)-E_m(w₂)]The minimum value is found when t is 4.5ms, which is-149.11. When the transition resistance is 300 Ω, [ E ] as shown in FIG. 12_n(w₁～w₂)-E_n(w₂)]The minimum value is found when t is 6.9ms, which is-151.54. According to the analysis, the method has high sensitivity and rapid identification capability when high-resistance faults occur in the direct current circuit area.

The second setting scenario of this embodiment is: negative pole line faults occur at different positions on the direct current line of fig. 2 from the end M, and when the negative pole line faults occur, the transition resistance is 300 Ω.

At this time, [ E ] when the negative electrode line has a failure at a different position_m(w₁～w₂)-E_m(w₂)]、[E_n(w₁～w₂)-E_n(w₂)]As shown in fig. 13 and 14, respectively; as can be seen from FIGS. 13 and 14, when a negative line fault occurs at a different position from the end M, the negative line fault is [ E ] below the same time section_m(w₁～w₂)-E_m(w₂)]And [ E_n(w₁～w₂)-E_n(w₂)]The fluctuation degree is small, and the [ E ] under different conditions_m(w₁～w₂)-E_m(w₂)]And [ E_n(w₁～w₂)-E_n(w₂)]In the formula (67) and the formula (68), S may be 1, and the protection is identified as a fault in the dc link region. When a negative line fault occurs at a position 10% from the M terminal, [ E ]_m(w₁～w₂)-E_m(w₂)]Maximum when t is 7.2ms, its value is-151.81, [ E_n(w₁～w₂)-E_n(w₂)]The maximum value is at 6.9ms, which is-152.63, but is still much less than zero. From the above analysis, the protection criterion is not affected by the fault location, and when a high-resistance fault occurs at the end of the line, the sensitivity is still high.

The third setting scenario of this embodiment is: failure outside the inversion side region, i.e. f in FIG. 2₃A three-phase short-circuit fault is set. And setting the data synchronization error at two ends to be 0ms and +2ms respectively.

The Y-bridge converter valve currents in this fault situation versus normal operation are shown in fig. 15. As can be seen from fig. 15, when the ac-dc hybrid system normally operates, the phase of the Y bridge inverter is changed from VTY1 to VTY3 when t is 1.75ms, and at this time, the Y bridge inverter is in a state where VTY2 and VTY3 are conducting. When f is ₃When a three-phase short-circuit fault occurs, the Y bridge converter is in a conduction state of VTY1 and VTY2 when t is 1.75ms, and it is known that the Y bridge converter has failed in phase commutation as compared with the conduction state in normal operation.

E when the data synchronization errors at both ends are 0ms and +2ms respectively_m(w₁～w₂)-E_m(w₂)]And [ E_n(w₁～w₂)-E_n(w₂)]As shown in fig. 16. As can be seen from FIG. 16(a), when f is₃When three-phase short circuit fault occurs, [ E ]_m(w₁～w₂)-E_m(w₂)]Minimum value when t is 7.2ms, value-679.47, [ E_n(w₁～w₂)-E_n(w₂)]When t is 6.4ms, the maximum value is 536.51, which indicates that no fault occurs in the dc link area and the protection is reliable and does not operate.

As can be seen from fig. 16(b), when there is a synchronization error between the inverter-side data and the rectifier-side data, [ E ]_n(w₁～w₂)-E_n(w₂)]And if the voltage is always greater than or equal to zero, the fault of the direct current line in the opposite direction can be judged. Because the current shutdown and the system restart after the direct current line fault are completed on the rectifying side, the judgment result of the fault direction is transmitted to the rectifying side only by the inverting side, and the external fault can be identified. According to the analysis, the scheme provided by the embodiment does not need to transmit the electrical quantity information and is not influenced by commutation failure and data synchronization error.

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.

While the invention has been described with reference to specific preferred embodiments, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims.

Claims (10)

1. A direct current line pilot direction protection method based on inductive energy polarity is characterized in that the method is suitable for an alternating current-direct current hybrid system and comprises the following steps:

determining a frequency band range when the comprehensive impedance in the AC-DC hybrid system is inductive, and a minimum value and a sub-minimum value of angular frequency in the frequency band range;

collecting the voltage and current of the positive direct current line on the rectification side and the inversion side before and after the fault occurs;

filtering the voltage and the current to obtain the current and the voltage when the angular frequency is between the minimum value and the secondary minimum value and the voltage and the current when the angular frequency is the secondary minimum value;

and judging whether the fault is a direct current line fault or not based on the voltage and the current obtained by filtering, if so, determining a fault pole of the direct current line based on the voltages of the positive and negative direct current lines on the rectification side or the inversion side, which are acquired before and after the fault occurs, and starting the line protection of the fault pole.

2. The method according to claim 1, wherein the determining whether the fault is a dc line fault based on the filtered voltage and current comprises:

based on the formulas (1) and (2), the angular frequency between the minimum values w is obtained₁And the second smallest value w₂Energy of direct current line in time between at rectification side

Energy of inversion side

Wherein, the first and the second end of the pipe are connected with each other,

respectively representing the difference of the voltages and the difference of the currents of the positive direct-current line on the rectification side when the angular frequency is between the minimum value and the secondary minimum value, after the fault occurs and before the fault occurs;

respectively representing the difference of the voltages and the difference of the currents of the positive direct current line on the inversion side when the angular frequency is between the minimum value and the secondary minimum value, after the fault occurs and before the fault occurs;

based on the formulas (3) and (4), the energy of the direct current line on the rectification side when the angular frequency is a second smallest value is obtained

Energy of inversion side

Wherein, the first and the second end of the pipe are connected with each other,

respectively representing the difference value of the voltage and the difference value of the current of the positive direct-current line on the rectification side when the angular frequency is a second-smallest value, after the fault occurs and before the fault occurs;

respectively representing the difference value of the voltage and the difference value of the current of the positive direct current line on the inversion side when the angular frequency is a second-smallest value, after the fault occurs and before the fault occurs;

Based on

And a direct current line fault protection criterion is used for judging whether the fault is a direct current line fault.

3. The method according to claim 2, wherein said dc link pilot direction protection criterion is:

if S_recAnd S_invAnd if the fault is 1, the fault is a direct current line fault.

4. The method according to claim 1, wherein the fault pole of the dc link is determined by performing the following operations:

based on the voltage of the positive and negative direct current lines collected before and after the fault occurs on the rectification side or the inversion side and a formula (6), calculating a fault pole selection coefficient P:

wherein u is_pos(i)、u_neg(i) The voltages of the positive and negative direct current lines on the rectification side or the inversion side at the ith moment after the fault occurs respectively; u. of_pos(i-T)、u_neg(i-T) before the fault occurs, the positive and negative direct current lines are rectified at the ith-T momentSide or reverse side voltage; n is a radical of_sThe number of sampling points in T/2; t represents the power frequency period of the alternating current-direct current hybrid system;

when J is not less than J_maxWhen the fault is a positive direct current line fault, the fault is a positive direct current line fault; when J is less than or equal to J_minWhen the fault is a negative direct current line fault; j. the design is a square _min＜J＜J_maxWhen the fault is a bipolar short-circuit fault; j. the design is a square_max、J_minAnd respectively selecting the upper threshold and the lower threshold of the coefficient.

5. The method according to any of claims 1-4, wherein the frequency band range of the AC/DC hybrid system when the combined impedance is inductive is determined by performing the following operations:

calculating the comprehensive impedance of the rectifying side or the inverting side:

wherein Z is_s1、Z_ac、Z_d、Z_c、Z_dcThe impedance of an alternating current system, an alternating current filter and reactive power compensation device, the impedance of a smoothing reactor, the minimum equivalent impedance of a current converter and the impedance of a direct current filter on a rectification side or an inversion side respectively; a is₀-a₁₃Is a constant obtained by simplifying the formula (7); w represents an angular frequency;

obtaining a₁₃w¹³+a₁₁w¹¹+…+a₁Solution w of w ═ 0_1.1、w_1.2、w_1.3、w_1.4、w_1.5、w_1.6(ii) a And, obtaining a₁₂w¹²+a₁₀w¹⁰+…+a₀Solution w of 0_2.2、w_2.2、w_2.3、w_2.4、w_2.5And w_2.6；

Obtaining the frequency band range when the comprehensive impedance of the alternating current-direct current hybrid system is inductive based on the obtained solution:

the minimum value and the second minimum value of the angular frequency are integral multiples of the minimum harmonic angular frequency of the direct current line voltage and current in steady-state operation.

6. The method according to claim 5, wherein the inverter minimum equivalent impedance is obtained by:

And analyzing the possible conduction state of the converter in the rectifying side or the inverting side after the fault occurs to obtain the equivalent impedance of the converter in each conduction state, and taking the minimum value of the equivalent impedance as the minimum equivalent impedance of the converter.

7. The method according to claim 6, wherein the possible conduction states of the inverter on the rectifying side or the inverting side after the fault occurs comprise:

conduction state 1: in the D-bridge converter, two converter valves which are numbered and adjacent are conducted; in the Y-bridge converter, two converter valves with the same number as the converter valves conducted with the D-bridge converter are conducted;

on-state 2: in the D-bridge converter, two converter valves which are numbered and adjacent are conducted; in the Y-bridge converter, three converter valves which are numbered adjacently are conducted; the serial numbers of the converter valves conducted in the D bridge converter are correspondingly the same as the serial numbers of the first two converter valves conducted in the Y bridge converter;

on-state 3: in the D-bridge converter, two converter valves which are numbered and adjacent are conducted; in the Y-bridge converter, two converter valves which are numbered and adjacent are conducted; the serial number of the rear converter valve conducted in the D bridge converter is the same as that of the front converter valve conducted in the Y bridge converter;

On-state 4: in the D-bridge converter, three converter valves which are numbered and adjacent are conducted; in the Y-bridge converter, two converter valves which are numbered and adjacent are conducted; and the serial numbers of the converter valves conducted in the Y-bridge converter are correspondingly the same as the serial numbers of the last two converter valves conducted in the D-bridge converter.

8. The method according to claim 1, wherein said DC line pilot direction protection method based on inductive energy polarity,

judging whether the AC-DC hybrid system breaks down or not based on the collected current flowing through the DC filter:

and calculating the average value of the current flowing through each direct current filter at the latest M sampling points, and judging that the alternating current-direct current series-parallel system has a fault when the average value of the current flowing through any direct current filter is greater than a fault current threshold value.

9. The method according to claim 4, wherein J is J_maxTaking 1.5; j is a unit of_min0.8 is taken.

10. A direct current line pilot direction protection system based on inductive energy polarity, comprising:

the inductive frequency band determining module is used for determining a frequency band range when the comprehensive impedance in the alternating-current and direct-current hybrid system is inductive, and a minimum value and a secondary minimum value of angular frequency in the frequency band range;

The data acquisition module is used for acquiring the voltage and the current of the positive direct current line on the rectification side and the inversion side before and after the fault occurs; acquiring the voltage of the negative direct current line on the rectification side or the inversion side before and after the fault occurs;

the data filtering module is used for filtering the voltage and the current to obtain the current and the voltage when the angular frequency is between the minimum value and the secondary minimum value and the voltage and the current when the angular frequency is the secondary minimum value;

and the direct-current line protection module is used for judging whether the fault is a direct-current line fault or not based on the voltage and the current obtained by filtering, determining a fault pole of the direct-current line based on the voltages of the positive and negative direct-current lines on the rectification side or the inversion side collected before and after the fault occurs if the fault is the direct-current line fault, and starting the line protection of the fault pole.

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