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CN-122000983-A - High-voltage direct-current pseudo bipolar-true bipolar conversion system and control method thereof

CN122000983ACN 122000983 ACN122000983 ACN 122000983ACN-122000983-A

Abstract

The invention discloses a high-voltage direct-current pseudo-bipolar-true bipolar conversion system and a control method thereof, wherein the system comprises a direct-current voltage conversion device for connecting the pseudo-bipolar direct-current system and the true bipolar direct-current system, the device comprises two MMCs, each voltage shifter comprises two MMCs, the MMCs are respectively connected in series in positive and negative direct-current circuits of the pseudo-bipolar, the alternating-current sides are connected in parallel to realize power exchange, the two voltage shifters are adopted to realize the conversion from the pseudo-bipolar to the true bipolar, and the grounding points of the two voltage shifters are connected to form a true bipolar symmetrical structure. In a + -400 kV/+ -800 kV application, each MMC only sees 400kV voltage, reducing the number of sub-modules to 66.7% compared to the traditional high voltage high capacity DC transformer scheme. The invention provides an innovative technical scheme for the efficient interconnection of the offshore wind power pseudo-bipolar direct current system and the onshore true bipolar direct current system, has the advantages of small quantity of submodules, low cost, high flexibility and the like, and has important engineering application value and economic benefit.

Inventors

  • WANG GUOTENG
  • HOU TIAN
  • HUANG YING

Assignees

  • 浙江大学

Dates

Publication Date
20260508
Application Date
20260403

Claims (9)

  1. 1. The high-voltage direct-current pseudo-bipolar-true bipolar conversion system is characterized by comprising a pseudo-bipolar direct-current system, a true bipolar direct-current system and a direct-current voltage conversion device connected with the pseudo-bipolar direct-current system, wherein the direct-current voltage conversion device is used for voltage class conversion and topology form conversion between the pseudo-bipolar direct-current system and the true bipolar direct-current system and is composed of two voltage shifters; The voltage shifter adopts a series direct-current voltage shifting device, and comprises two MMCs, wherein alternating-current sides of the two MMCs are connected to the same alternating-current system in parallel, direct-current sides of the two MMCs are respectively connected in series in two independent direct-current circuits, voltage lifting or voltage reducing is respectively carried out on the two direct-current circuits, and the voltage shifting function of the two direct-current circuits is realized by the two MMCs through power exchange of the alternating-current sides.
  2. 2. The high-voltage direct-current pseudo bipolar-true bipolar conversion system according to claim 1, wherein a direct-current positive terminal and a direct-current negative terminal of an MMC in the voltage shifter are respectively connected with two different potential points of a direct-current circuit, direct-current side voltages of the two MMCs are equal or can be independently controlled, power balance is achieved through alternating-current side connection, voltage regulation amplitude of the two MMCs on the respective circuits is equal, and internal power balance is maintained through alternating-current side power exchange.
  3. 3. The high-voltage direct-current pseudo-bipolar-to-true bipolar conversion system according to claim 1, wherein the direct-current voltage conversion device comprises a first voltage shifter and a second voltage shifter, the first voltage shifter comprises two MMCs (MMC 1 and MMC 2), the second voltage shifter comprises two MMCs (MMC 3 and MMC 4), wherein a direct-current negative end of the MMC1 is connected with a positive direct-current circuit of the pseudo-bipolar direct-current system, a direct-current positive end of the MMC2 is connected with a negative direct-current circuit of the pseudo-bipolar direct-current system, a direct-current positive end of the MMC3 is grounded, a direct-current positive end of the MMC3 is connected with a positive direct-current circuit of the pseudo-bipolar direct-current system, a direct-current negative end of the MMC4 is connected with a negative direct-current circuit of the pseudo-bipolar direct-current system, and a direct-current negative end of the MMC4 is connected with a negative direct-current circuit of the pseudo-bipolar direct-current system.
  4. 4. The system of claim 3, wherein the positive and negative DC lines of the pseudo-bipolar DC system have voltage levels of +U 1 and-U 1 , respectively, and are ground or metal return lines as return paths, and the positive and negative DC lines of the pseudo-bipolar DC system have voltage levels of +U 2 and-U 2 , respectively, and are insulated from ground by both poles, wherein U 2 ＞U 1 .
  5. 5. The system of claim 4, wherein the first voltage shifter is used for up-converting positive and negative voltages of the DC lines of the pseudo-bipolar DC system from (+ U 1 , -U 1 ) to (+ U 2 , 0) respectively, the second voltage shifter is used for down-converting positive and negative voltages of the DC lines of the pseudo-bipolar DC system from (+ U 1 , -U 1 ) to (0, -U 2 ) respectively, and grounding points of the two voltage shifters are physically connected together to form a neutral point of the true bipolar DC system together, so that strict symmetry of voltages of two poles of the true bipolar DC system to ground is ensured.
  6. 6. The HVDC pseudo bipolar-to-true bipolar switching system of claim 4, wherein the DC side voltage ratings of MMC1, MMC2, MMC3, MMC4 are U 2 -U 1 .
  7. 7. The system of claim 1, wherein the pseudo-bipolar DC system is coupled to an offshore wind farm and the true bipolar DC system is coupled to an onshore AC grid.
  8. 8. The control method of the high voltage dc pseudo bipolar-to-true bipolar switching system according to any one of claims 4 to 6, wherein: The direct current voltage of the MMC1 is controlled to be U 2 -U 1 , the positive direct current line of the pseudo-bipolar direct current system is boosted from +U 1 to +U 2 , the direct current voltage of the MMC2 is controlled to be U 2 -U 1 , the negative direct current line of the pseudo-bipolar direct current system is boosted from-U 1 to 0, the direct current voltage of the MMC3 is controlled to be U 2 -U 1 , the positive direct current line of the pseudo-bipolar direct current system is boosted from +U 1 to 0, the direct current voltage of the MMC4 is controlled to be U 2 -U 1 , the negative direct current line of the pseudo-bipolar direct current system is boosted from-U 1 to-U 2 , and meanwhile, the direct current voltage stability and the power balance of all the MMCs are maintained through alternating current side power control of the MMC.
  9. 9. The control method according to claim 8, wherein when a single-pole fault occurs in the pseudo-bipolar direct current system, isolation of a faulty pole and derating operation of a sound pole are realized by adjusting direct current voltage and power distribution of MMC in the corresponding voltage shifter, and when a single-pole fault occurs in the true bipolar direct current system, isolation of a faulty pole and continuous operation of the system are realized by coordinated control of the two voltage shifters.

Description

High-voltage direct-current pseudo bipolar-true bipolar conversion system and control method thereof Technical Field The invention belongs to the technical field of high-voltage direct-current transmission of power systems, and particularly relates to a high-voltage direct-current pseudo-bipolar-true bipolar conversion system and a control method thereof. Background Along with the large-scale development of offshore wind power, the high-voltage direct-current transmission technology becomes an important technical means for offshore wind power grid connection. The offshore wind power direct current delivery system generally adopts a pseudo-bipolar structure, namely, the positive pole and the negative pole respectively and independently operate, and the neutral point is grounded by taking seawater or a metal return line as a reflux path, so that the offshore wind power direct current delivery system has the advantages of small equipment quantity, simple structure and low cost on an offshore platform. The land high-voltage direct-current transmission network generally adopts a true bipolar structure, namely, the positive pole and the negative pole are insulated from the ground, and the two poles are connected through a load center, so that the land high-voltage direct-current transmission network has the advantages of high power supply reliability, small influence of monopole faults, symmetrical ground potential and the like. The prior art for realizing pseudo-bipolar and true bipolar direct current interconnection under a high-voltage direct current scene generally adopts a cascaded submodule type isolation DC/DC converter as a basic unit, and converts unipolar direct current voltage into balanced symmetrical positive and negative symmetrical output through a topological structure with input ends connected in parallel and output ends connected in series. The submodule cascading technology based on the modularized multi-level converter (MMC) adopts a direct-coupling autotransformer topology structure, omits a huge medium-frequency isolation transformer, omits the direct-current-alternating-direct-current conversion process, and directly realizes the conversion from unipolar input to symmetrical positive and negative bipolar output on a circuit through specific submodule control logic. The literature [ Guo Lingyu, yao Gang, yan Zhizhu ] is suitable for the bipolar output non-isolated type autotransformer [ J ] of the direct current interconnection of the offshore wind farm, the power grid technology 2020, 44 (1): 174-184] improves the traditional face-to-face structure, adopts a set of multiplexing current transformation modules on the unipolar side (inversion side) and realizes connection on the bipolar side (rectification side) through two independent transformers and rectification branches, and the proposal belongs to an isolated proposal. However, the above connection scheme of the offshore wind power pseudo bipolar direct current system and the onshore true bipolar direct current system mainly faces the following problems: 1. Voltage class mismatch problem. For economy and technical maturity, offshore wind power generally adopts relatively low direct current voltage levels, such as +/-320 kV or +/-400 kV, while land long-distance direct current power transmission adopts higher voltage levels, such as +/-500 kV, +/-800 kV and even higher, in order to reduce power transmission loss and direct current line corridor width, significant voltage level differences exist between the two voltage levels, and voltage level conversion is needed. 2. Topology form difference problem. The pseudo bipolar and the true bipolar have essential differences in the grounding mode, neutral point potential, fault characteristics, protection configuration and the like, the allowable two-pole grounding voltage of the pseudo bipolar is asymmetric, and the true bipolar requires the two-pole grounding voltage to be strictly symmetric, and the direct connection can cause the problems of grounding current, potential distribution and the like. 3. The traditional scheme has high cost and low efficiency. In the prior art, the high-voltage high-capacity direct-current voltage class conversion mainly adopts a direct-current transformer technology based on a modularized multi-level converter, and a typical scheme is that two independent MMC direct-current transformers are adopted and are respectively connected with the positive pole and the negative pole of a pseudo-bipolar system, and the direct-current side of each MMC direct-current transformer needs to bear all output voltages, so that the number of required submodules is huge. Therefore, it is needed to propose a novel dc voltage conversion device and system scheme, which can effectively connect an offshore wind power pseudo-bipolar dc system with an onshore true bipolar dc system with a smaller number of sub-modules, lower cost and higher flexibility. Disclosure of Invention In view of th