CN111431202B - Method for predicting conversion failure of metal ground return wire of three-terminal direct-current system in real time - Google Patents

Abstract

The invention discloses a real-time prediction method for three-terminal metal ground return line conversion failure, which comprises the following steps: when the three-terminal direct current system is in a single-pole earth return mode, voltage and current at the outlets of a direct current line and an earth electrode lead of each converter station are collected to calculate the resistance of the direct current line of each converter station, the resistance is used as a reference resistance of a prediction criterion of the failure of the subsequent connection mode conversion, the current flowing through a key switch is calculated in real time, and the current is compared with a fixed value to predict whether the metal earth connection mode conversion can be successful. The calculated resistance of the direct-current transmission line of each converter station in real time is used as a reference resistance of a prediction criterion of the conversion failure of the metal ground return wire, so that whether the conversion between the wiring modes of the metal return wire and the ground return wire can be successful or not can be accurately predicted in real time.

Description

Method for predicting conversion failure of metal ground return wire of three-terminal direct-current system in real time

Technical Field

The invention relates to the technical field of electric power, in particular to a method for predicting conversion failure of a three-terminal metal ground return wire in real time.

Background

With the development of a direct current transmission technology, particularly a flexible direct current transmission technology, due to the rapid development of a modular multilevel converter high-voltage direct current transmission technology (modular multilevel converter based HVDC, MMC-HVDC), the modular multilevel converter high-voltage direct current transmission technology is widely applied to the aspects of large-scale wind power plant grid connection, power grid interconnection, multi-terminal direct current transmission and the like by virtue of the advantages of modularization, low harmonic content, low loss and the like. At present, a plurality of multi-terminal direct-current projects are under construction or put into operation at home and abroad, such as put-into-operation south and Australia three-terminal flexible direct-current demonstration project, Zhoushan five-terminal direct-current project and the like, and under construction, Zhang-north four-terminal direct-current power grid project, Wudongde ultra-high voltage multi-terminal hybrid direct-current transmission project and the like, which mark that the direct-current transmission technology has been advanced into the development stage of ultra-high voltage high-capacity multi-terminal hybrid direct-current transmission.

The multi-end direct current engineering adopting the parallel structure can form a receiving-end multi-drop topological structure due to the structural flexibility, and can quit the parallel receiving-end converter station without influencing the operation of other stations, thereby having wide application prospect.

For the multi-terminal dc engineering with the parallel structure, taking three-terminal dc engineering as an example, the metal ground Return line switching will determine a fixed switching sequence according to the current breaking capability of the dc Transfer switches of each station, such as MRS (metal Return switch), MRTB (metal Return Transfer Breaker, metal Return Transfer switch), and the like. For the large-scale metal return mode, the sequence of the switching operation of the converter station 1, the converter station 2 and the converter station 3 is completed firstly, and the sequence of the metal large-scale metal return mode is opposite, so that the situation that the current of a certain MRS or MRTB is close to zero in the conversion process due to the distribution characteristics of resistance parameters of all lines, and the subsequent operation cannot be completed due to the misjudgment of conversion failure may exist.

Disclosure of Invention

In order to solve the problem of misjudgment and conversion in the process of converting the metal ground return wire of the conventional multi-terminal direct current system, the embodiment of the invention provides a method for predicting the conversion failure of the metal ground return wire in real time.

In order to achieve the purpose, the technical scheme of the invention is as follows:

a real-time prediction method for metal ground return line conversion failure is applied to a three-terminal direct-current system, wherein the three-terminal direct-current system comprises more than three converter stations including a first converter station, a second converter station and a third converter station, and the method comprises the following steps:

when the three-terminal direct current system is in a single-pole earth return mode, voltage and current at the outlets of the direct current line and the grounding electrode lead of each converter station are collected to calculate the resistance of the direct current line of each converter station, the resistance is used as a reference resistance of a subsequent wiring mode conversion failure prediction criterion, and whether the metal earth wiring mode conversion can be successful or not is predicted in real time.

Further, the method further comprises:

in the process of converting a three-terminal direct-current system into a metal return wire from a ground return wire, respectively calculating current excitation I _ MRS3_ I2 and I _ MRS3_ I3 generated by a second converter station equivalent current source and a third converter station equivalent current source on MRS3 of a third converter station, and superposing the current excitation I _ MRS3_ I2 and the current excitation I _ MRS3_ I3 to obtain a current I _ MRS3 flowing through MRS3 when the MRS is switched on but the three grounding electrodes of the converter stations are not disconnected;

and comparing the absolute value of the calculated current I _ MRS3 flowing through MRS3 with a fixed value Iset, predicting and judging that the ground return wire is failed to be converted into the metal return wire under the current level if the absolute value is not greater than Iset, and forbidding the connection mode conversion.

The calculation formulas of the I _ MRS3_ I2, the I _ MRS3_ I3 and the I _ MRS3 are as follows:

in the formula, I2 and I3 are equivalent direct currents of the current converter station two and the current converter station three, and R1, R2, R3 and R5 are direct-current line resistances between the converter station one and the converter station two, direct-current line resistances between the converter station two and the converter station three, a ground electrode lead and a ground electrode resistance of the converter station one, and a ground electrode lead and a ground electrode resistance of the converter station three, which are obtained by real-time calculation in a single-pole earth loop mode, respectively.

In the process that the three-terminal direct current system is used for converting a metal loop into an earth loop, current excitations I _ MRTB2_ I2 and I _ MRTB2_ I3 generated by a converter station II and a converter station III equivalent current source on MRTB2 of a converter station II are respectively calculated, and the current I _ MRTB2 and the current I _ MRTB2 flow when the MRTB is switched on and the metal loop of the converter station II is not disconnected;

and comparing the absolute value of the calculated current I _ MRTB2 flowing through the MRTB2 with a fixed value Iset, predicting and judging that the metal return wire is failed to be subjected to ground return wire conversion under the current level if the absolute value is not greater than Iset, and forbidding the wire connection mode conversion.

The calculation formulas of the I _ MRTB2_ I2, the I _ MRTB2_ I3 and the I _ MRTB2 are as follows:

in the formula, I2 and I3 are direct currents transmitted by direct current lines of the current converter station two and the current converter station three, and R1, R3 and R4 are direct current line resistance between the converter station one and the converter station two, a ground electrode lead and a ground electrode resistance of the converter station one, and a ground electrode lead and a ground electrode resistance of the converter station two, which are obtained by real-time calculation in a single-pole earth loop mode, respectively, are R4.

Compared with the prior art, the invention has the beneficial effects that:

in the method for predicting the switching failure of the metal ground return wire in real time provided by this embodiment, the resistance of the dc transmission line of each converter station calculated in real time is used as a reference resistance of the prediction criterion of the switching failure of the metal ground return wire, so that whether the switching between the wiring modes of the metal return wire and the ground return wire is successful can be accurately predicted in real time. Therefore, before the three-terminal metal ground loop mode conversion operation is carried out, judgment is carried out before conversion through related prediction criteria, operators can be warned in advance, the probability of conversion failure is reduced, the current for predicting the conversion failure criteria is calculated through the resistance of each line calculated in real time, and the prediction accuracy is guaranteed.

Drawings

FIG. 1 is a diagram of a single-pole earth return operation mode of a three-terminal DC power transmission system;

fig. 2a is an equivalent diagram of the current excitation circuit produced by the converter station 2 at MRS 3;

fig. 2b is an equivalent diagram of the current excitation circuit produced by the converter station 3 at MRS 3;

FIG. 3 is a logic diagram of ground return to metal return prediction;

fig. 4a is an equivalent diagram of the current excitation circuit produced at MRTB2 by the converter station 2;

fig. 4b is an equivalent diagram of the current excitation circuit produced by the converter station 3 at MRTB 2;

fig. 5 is a logic diagram of metal return to ground return prediction.

Detailed Description

The present invention will be described in further detail with reference to the accompanying drawings and detailed description.

Example (b):

the multi-terminal dc system of this embodiment is a three-station dc system, and includes a converter station 1, a converter station 2, and a converter station 3.

The unipolar earth return line of the three-terminal direct-current transmission system is shown in fig. 1, wherein a direct-current line resistance between the converter station 1 and the converter station 2 is R1, an overcurrent is IdL1, a direct-current line resistance between the converter station 2 and the converter station 3 is R2, an overcurrent is IdL2, a ground lead and a ground resistance of the converter station 1 are R3, an overcurrent is IdN1, a ground lead and a ground resistance of the converter station 2 are R4, an overcurrent is IdN2, a ground lead and a ground resistance of the converter station 3 are R5, an overcurrent is IdN3, outgoing line voltages of a direct-current line and a ground lead of the converter station 1 are UdL1 and UdN1, outgoing line voltages of a direct-current line and a ground lead of the converter station 2 are UdL2 and UdN2, and outgoing line voltages of a direct-current line and a ground lead of the converter station 3 are UdL3 and UdN 3.

When the three-terminal direct current transmission system is in a single-pole ground loop mode, the resistance values of the direct current line, the grounding electrode and the leads thereof can be calculated in real time by collecting the voltage and current at the outlets of the direct current line and the leads of the grounding electrode of each converter station, for example, R1 (UdL1-UdL2)/IdL1 and R3 (UdN 1/IdN 1), and the resistance values are used as reference resistance of a prediction criterion of metal ground loop conversion failure.

And when the three-terminal direct-current transmission system is in the process of converting the earth return wire into the metal return wire, according to the switching sequence of the converter station 1 and the converter station 2 which is completed first, that is, the two metal return wires are connected and the grounding electrode of the converter station 2 is disconnected, after the MRS of the converter station 3 is closed, whether the switching fails can be judged by calculating the current prediction flowing through the MRS, and a circuit equivalent diagram of the current flowing through the MRS3 is calculated as shown in fig. 2, wherein fig. 2a is a circuit equivalent diagram of current excitation I _ MRS3_ I2 generated by an equivalent current source I2 of the converter station 2 on MRS3, and fig. 2b is a circuit equivalent diagram of current excitation I _ 3_ I3 generated by an equivalent current source I3 of the converter station 3 on MRS 3.

Thus, the current excitations I _ MRS3_ I2 and I _ MRS3_ I3 generated by the current sources of the converter station 2 and the converter station 3 on the MRS3 can be respectively calculated through an equivalent circuit diagram, the superposition of the current excitations I _ MRS3_ I2 and I _ MRS3_ I3 can obtain the current I _ MRS3 flowing through the MRS3 when the MRS is closed but the grounding electrode of the converter station 3 is not disconnected, the calculation formula is as follows,

in the formula, I2 and I3 are dc currents transmitted by the dc lines of the current converter station 2 and the current converter station 3, and R1, R2, R3 and R5 are resistance values calculated in real time in the monopole earth return mode.

The logic diagram of ground return wire to metal return wire prediction is shown in fig. 3, after the absolute value of the calculated current I _ MRS3 flowing through MRS3 is taken, the absolute value is compared with a fixed value Iset, if the absolute value is not greater than Iset, it is predicted and judged that the ground return wire to metal return wire will fail to be carried out under the current level, and the connection mode conversion is forbidden to be carried out, wherein Iset can be reasonably selected according to the measurement error, for example, 20A is taken. If it is determined that the switching failure is caused by the switching of the wiring pattern at the current level, the switching operation is prohibited.

And when the three-terminal dc system is in the process of making the metallic loop change to the ground loop, according to the sequence of switching the converter station 3 first, that is, the metallic loop of the converter station 3 is disconnected and the grounding electrode is connected, after the MRTB of the converter station 2 is closed, it can be predicted by calculating the current flowing through the MRTB to determine whether the failure of the switching will occur, and the equivalent circuit diagram is shown in fig. 4, where fig. 4(a) is the equivalent circuit diagram of the current excitation I _ MRTB2_ I2 generated by the equivalent current source I2 of the converter station 2 on the MRTB2, and fig. 4(b) is the equivalent circuit diagram of the current excitation I _ MRTB2_ I3 generated by the equivalent current source I3 of the converter station 3 on the MRTB 2. The current excitations I _ MRTB2_ I2 and I _ MRTB2_ I3 generated by the current sources of the converter station 2 and the converter station 3 at the MRTB2 can be calculated respectively through a circuit equivalent diagram, and the current excitations I _ MRTB2_ I2 and I _ MRTB2_ I3 are superposed to obtain the current I _ MRTB2 flowing through the MRTB2 when the MRTB is closed without disconnecting the metallic loop of the converter station 2, wherein the calculation formulas are as follows, I2 and I3 are direct current transmitted by the current converter station 2 and the converter station 3, and the line resistances (R1, R3 and R4) are calculated in real time in a manner of a single-pole earth loop.

The logic diagram of the metal loop to ground loop prediction is shown in fig. 5, after the absolute value of the calculated current I _ MRTB2 flowing through the MRTB2 is taken, the absolute value is compared with a fixed value Iset, if the absolute value is not greater than Iset, it is predicted and judged that the metal loop to ground loop will fail to be carried out under the current level, and the connection mode conversion is prohibited, wherein Iset can be reasonably selected according to the measurement error, for example, 20A is taken. If it is determined that the switching failure is caused by the switching of the wiring pattern at the current level, the switching operation is prohibited.

Therefore, the real-time prediction method for the metal ground return wire conversion failure provided by the embodiment can accurately predict whether the conversion between the metal return wire and the ground return wire connection mode can be successful or not in real time by using the real-time calculated direct-current transmission line resistance of each converter station as the reference resistance of the prediction criterion for the metal ground return wire conversion failure. Therefore, before the three-terminal metal ground loop mode conversion operation is carried out, judgment is carried out before conversion through related prediction criteria, operators can be warned in advance, the probability of conversion failure is reduced, the current for predicting the conversion failure criteria is calculated through the resistance of each line calculated in real time, and the prediction accuracy is guaranteed.

The above embodiments are only for illustrating the technical concept and features of the present invention, and the purpose thereof is to enable those skilled in the art to understand the contents of the present invention and implement the present invention accordingly, and not to limit the protection scope of the present invention accordingly. All equivalent changes or modifications made in accordance with the spirit of the present disclosure are intended to be covered by the scope of the present disclosure.

Claims (1)

1. A three-terminal direct current system metal ground return line conversion failure real-time prediction method is applied to a three-terminal direct current system, wherein the three-terminal direct current system comprises three convertor stations which are a convertor station I, a convertor station II and a convertor station III respectively, and the method is characterized by comprising the following steps of:

when the three-terminal direct current system is in a single-pole ground loop mode, acquiring voltage and current at the outlets of the direct current line and the grounding electrode lead of each converter station to calculate the resistance of the direct current line and the grounding electrode of each converter station, and using the resistance as a reference resistance of a conversion failure prediction criterion of a subsequent wiring mode;

when the three-terminal direct current system is used for converting a ground loop into a metal loop, current excitation I _ MRS3_ I2 and I _ MRS3_ I3 generated by the two converter station and the three converter station equivalent current source on MRS3 of the three converter station are respectively calculated, and the current excitation I _ MRS3_ I2 and the I _ MRS3_ I3 are superposed to obtain current I _ MRS3 flowing through MRS3 when MRS3 is closed but the three converter station grounding electrode is not disconnected;

after the absolute value of the calculated current I _ MRS3 flowing through MRS3 is taken, the absolute value is compared with a fixed value Iset, if the absolute value is not larger than Iset, the fact that the earth return wire is converted into the metal return wire under the current level is predicted and judged to fail, and the connection mode conversion is forbidden;

when the three-terminal direct current system is in the process of converting the metallic return wire into the earth return wire, current excitations I _ MRTB2_ I2 and I _ MRTB2_ I3 generated by the second converter station equivalent current source and the third converter station equivalent current source on the MRTB2 of the second converter station are respectively calculated, and the current I _ MRTB2 flows through MRTB2 when the MRTB2 is switched on and the metallic return wire of the second converter station is not disconnected;

and comparing the absolute value of the calculated current I _ MRTB2 flowing through the MRTB2 with a fixed value Iset, predicting and judging that the metal return wire is failed to be subjected to ground return wire conversion under the current level if the absolute value is not greater than Iset, and forbidding the wire connection mode conversion.

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