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US-12627143-B2 - Control method for suppressing overvoltage

US12627143B2US 12627143 B2US12627143 B2US 12627143B2US-12627143-B2

Abstract

A control method for suppressing an overvoltage of the receiving end hybrid modular multilevel converter with full-bridge submodules and half-bridge submodules (F/HMMC) under the fault at the receiving-end alternating-current (AC) system of the hybrid high-voltage direct-current (HVDC) transmission system. After a short-circuit fault occurs in a receiving-end AC) system of the hybrid HVDC transmission system, an AC voltage drop is generated, and the effective voltage of the receiving-end AC bus at the current time is measured. Because the hybrid modular multilevel converter (MMC) is capable of actively decreasing the DC voltage for operation, on a premise of ensuring that its output current is always within a rated current range, a control manner of setting the reference DC voltage and making the reference DC voltage not greater than the effective voltage of the AC bus enables receiving end power to be sent out, thereby suppressing an overvoltage of the receiving-end F/HMMC.

Inventors

  • Zheng Xu
  • Nan Zhang
  • Zheren Zhang

Assignees

  • ZHEJIANG UNIVERSITY

Dates

Publication Date
20260512
Application Date
20220726
Priority Date
20210901

Claims (8)

  1. 1 . A control method for suppressing an overvoltage under a fault at a receiving-end alternating-current (AC) system of a hybrid high-voltage direct-current (HVDC) transmission system, wherein a line commuted converter (LCC) is used as a sending-end converter of the hybrid HVDC transmission system, a hybrid modular multilevel converter (F/HMMC) with full-bridge submodules (FBSM) and half-bridge submodules (HBSMs) is used as a receiving-end converter, and the control method comprises following steps: (1) during steady-state operation, using constant DC current control for the LCC, and using constant DC voltage control for the F/HMMC; (2) after a short-circuit fault occurs in the receiving-end AC system, generating a voltage drop for a receiving-end AC bus, and measuring an effective voltage U s of the receiving-end AC bus at the current time; (3) switching an operation mode of the F/HMMC to a DC voltage step-down operation mode, while still using the constant DC current control for the LCC and ensuring that an output current of the F/HMMC is within a rated current range; and (4) determining a DC voltage reference value U dcref of the F/HMMC, such that the U dcref is always not greater than the effective voltage U s of the receiving-end AC bus, and using the U dcref to control the F/HMMC.
  2. 2 . The control method according to claim 1 , wherein in the step (1), a voltage of a bridge arm of the F/HMMC during the steady-state operation meets the following relationship expressions: 1 2 ⁢ ( u p ⁢ j + u n ⁢ j ) = U d ⁢ c 2 , 1 2 ⁢ ( u n ⁢ j - u p ⁢ j ) = u v ⁢ j ; u p ⁢ j = U d ⁢ c 2 - u v ⁢ j , u n ⁢ j = U d ⁢ c 2 + u v ⁢ j ; wherein u pj represents a voltage of an upper bridge arm of a phase j of the F/HMMC, u nj represents a voltage of a lower bridge arm of the phase j of the F/HMMC, U dc represents a DC voltage of the F/HMMC, u vj represents an output AC voltage of the phase j of the F/HMMC, and j=a, b, or c.
  3. 3 . The control method according to claim 1 , wherein in the step (3), when the operation mode of the F/HMMC is switched to the DC voltage step-down operation mode, it is required to ensure that an operating range of a DC voltage of the F/HMMC meets the following relationship expression: M a ⁢ c - 2 ⁢ K F ⁢ B ≤ M d ⁢ c ≤ 2 - M a ⁢ c wherein M dc represents a DC voltage modulation ratio of the F/HMMC, M ac represents an AC voltage modulation ratio of the F/HMMC, and K FB represents a proportion of the FBSMs in a bridge arm.
  4. 4 . The control method according to claim 3 , wherein expressions for the DC voltage modulation ratio M dc and the AC voltage modulation ratio M ac of the F/HMMC are as follows: M a ⁢ c = 2 ⁢ U m U d ⁢ c ⁢ n , M d ⁢ c = U d ⁢ c U d ⁢ c ⁢ n ; wherein U dc represents the DC voltage of the F/HMMC, U dcn represents a rated DC voltage of the F/HMMC, and U m represents an amplitude of a valve-side phase voltage of the F/HMMC.
  5. 5 . The control method according to claim 1 , wherein in the step (3), when the operation mode of the F/HMMC is switched to the DC voltage step-down operation mode, it is required to ensure that a voltage range of a bridge arm of the F/HMMC meets the following relationship expression: { - N F ⁢ B ⁢ U c ⁢ n ≤ u p ⁢ j ≤ N total ⁢ U c ⁢ n - N F ⁢ B ⁢ U c ⁢ n ≤ u n ⁢ j ≤ N total ⁢ U c ⁢ n wherein u pj represents a voltage of an upper bridge arm of a phase j of the F/HMMC, u nj represents a voltage of a lower bridge arm of the phase j of the F/HMMC, j=a, b, or c, N FB represents the number of FBSMs in a single bridge arm, N total represents a total number of submodules in the single bridge arm, U cn represents a rated voltage of a submodule capacitor and U cn =U dcn /N total , and U dcn represents a rated DC voltage of the F/HMMC.
  6. 6 . The control method according to claim 1 , wherein in the step (4), after the short-circuit fault occurs in the receiving-end AC system, the effective voltage U s of the receiving-end AC bus drops within a range of 0 pu to 1 pu; and in this case, the DC voltage reference value U dcref of the F/HMMC is set to always be less than the U s , and a difference between the U dcref and the U s is taken as 0.1 pu to ensure that power is capable of being sent out and the system is not overloaded, thus suppressing an overvoltage for the F/HMMC.
  7. 7 . The control method according to claim 1 , wherein after a fault occurs at the receiving-end AC system of the hybrid HVDC transmission system, the effective voltage of the receiving-end AC bus U s is first measured, and based on an capability that the F/HMMC actively decreases a DC voltage for operation, a control manner in which the DC voltage reference value is set and ensured to be not greater than the effective voltage of the receiving-end AC bus, and the LCC uses the constant DC current control, which ensures that an output current of the F/HMMC is always within the rated current range and power absorbed at a receiving end can linearly decrease with the DC voltage.
  8. 8 . The control method according to claim 1 , wherein the control method actively decreases a receiving-end DC voltage, reduces receiving-end energy absorption, and avoids a power surplus of a receiving-end system, thereby suppressing an overvoltage of the F/HMMC, improving operational reliability of the system, and being suitable for a long-distance and large-capacity flexible DC transmission scenario.

Description

CROSS REFERENCE TO RELATED APPLICATION This application is a national stage application of International Patent Application No. PCT/CN2022/107809, filed on Jul. 26, 2022, which claims priority to the Chinese Patent Application No. 202111023962.5, filed with the China National Intellectual Property Administration (CNIPA) on Sep. 1, 2021, and entitled “CONTROL METHOD FOR SUPPRESSING OVERVOLTAGE UNDER FAULT AT RECEIVING-END ALTERNATING-CURRENT (AC) SYSTEM OF HYBRID HIGH-VOLTAGE DIRECT-CURRENT (HVDC) TRANSMISSION SYSTEM”, which is incorporated herein by reference in its entirety. TECHNICAL FIELD The present disclosure relates to the technical field of a power system, and specifically, to a control method for suppressing an overvoltage under a fault at a receiving-end alternating-current (AC) system of a hybrid high-voltage direct-current (HVDC) transmission system. BACKGROUND At present, most existing HVDC transmission projects use a conventional HVDC transmission technology based on a line commuted converter (LCC) of a power grid. The conventional HVDC transmission technology has advantages such as a low cost, a low loss, and high technological maturity. However, there are also drawbacks such as an easy commutation failure on an inverter side and an inability to transmit power to a weak AC system or a passive system. A flexible HVDC transmission technology based on a modular multilevel converter (MMC) has received widespread attention from academia and industry in recent years. Compared with the conventional HVDC transmission technology, the flexible HVDC transmission technology based on the MMC has advantages such as no risk of a commutation failure, an ability to supply power to a passive power grid, independent control for active and reactive power, and a low harmonic level. However, the flexible HVDC transmission technology based on the MMC also has disadvantages such as a high operating loss and a high investment cost. In order to fully leverage advantages of the LCC and the MMC in the power grid, a hybrid HVDC transmission system based on the LCC and the MMC has recently received increasing attention from various sectors. The hybrid HVDC transmission system based on the LCC and the MMC effectively expands the application scope of the HVDC transmission system and will inevitably become a development direction for future large-scale, long-distance, and large-capacity power transmission. An LCC-F/HMMC hybrid HVDC transmission system can leverage its advantages of a low loss and high technological maturity by using the LCC at a sending end. At a receiving end, the hybrid MMC (namely, an F/HMMC) with full-bridge submodules (FBSMs) and half-bridge submodules (HBSMs) is adopted to not only avoid the commutation failure on the inverter side, but also exert DC fault self-clearing capability of the FBSM. Because each bridge arm of the F/HMMC is formed through hybrid cascading of the HBSM and the FBSM, the number of power electronic devices used and the operational loss can be reduced, which is beneficial for engineering. A short-circuit fault in a receiving-end AC system of the hybrid HVDC transmission system blocks energy transmission. However, it is usually difficult for the sending end to make a quick response in a short period of time, resulting in a temporary power surplus of the receiving-end system and generating an overvoltage on receiving-end converters. Therefore, after a fault at the receiving-end AC system, the receiving end needs to actively decrease the DC voltage to reduce energy absorption, ensure that power can be sent out, avoid the overvoltage for the receiving-end converters, and improve the operational reliability of the hybrid HVDC transmission system. The literature [Xu Yuzhe, Xu Zheng, Zhang Zheren, et al. Control Strategy for Hybrid DC Transmission System Based on LCC and Hybrid MMC [J]. Guangdong Electric Power, 2018, 31 (9): 13-25] analyzes an operating principle of the hybrid MMC and a relationship between an operating range of the DC voltage of the receiving-end converter and the submodule ratio. It studies a self-clearing problem of a DC fault in the hybrid HVDC transmission system and resuming of power transmission after a sending-end AC system is faulty, but does not analyze the overvoltage generated by the receiving-end converters after the short-circuit fault in the receiving-end AC system. In the literature [Zhao Jianning, Chen Bing, Pan Chao et al. Ultra High-Voltage Multi-Terminal Hybrid Flexible DC Transmission Engineering Technology [M]. Beijing: Mechanical Industry Press, 2021], for a severe fault in the receiving-end AC system, it is necessary to rely on a communication system to reduce output power at a rectifier side and an overvoltage on a receiving-end MMC by increasing the firing angle of the LCC on the rectifier side. However, due to a communication delay of tens of milliseconds in long-distance transmission, it is difficult for the sending end to make a quick respons