WO2015176549A1 - 一种三极柔性直流输电系统和方法 - Google Patents

一种三极柔性直流输电系统和方法 Download PDF

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Publication number
WO2015176549A1
WO2015176549A1 PCT/CN2015/071282 CN2015071282W WO2015176549A1 WO 2015176549 A1 WO2015176549 A1 WO 2015176549A1 CN 2015071282 W CN2015071282 W CN 2015071282W WO 2015176549 A1 WO2015176549 A1 WO 2015176549A1
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Prior art keywords
pole
converter
current
line
value
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PCT/CN2015/071282
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English (en)
French (fr)
Inventor
胡铭
邵震霞
焦鑫艳
田杰
卢宇
沈刚
Original Assignee
南京南瑞继保电气有限公司
南京南瑞继保工程技术有限公司
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Application filed by 南京南瑞继保电气有限公司, 南京南瑞继保工程技术有限公司 filed Critical 南京南瑞继保电气有限公司
Priority to KR1020167020124A priority Critical patent/KR101884699B1/ko
Priority to US15/111,216 priority patent/US9948104B2/en
Priority to EP15796911.4A priority patent/EP3082212B1/en
Publication of WO2015176549A1 publication Critical patent/WO2015176549A1/zh
Priority to CY20191100789T priority patent/CY1122260T1/el

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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J1/00—Circuit arrangements for dc mains or dc distribution networks
    • H02J1/08—Three-wire systems; Systems having more than three wires
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M5/00—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases
    • H02M5/40—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into dc
    • H02M5/42—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into dc by static converters
    • H02M5/44—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into dc by static converters using discharge tubes or semiconductor devices to convert the intermediate dc into ac
    • H02M5/453—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into dc by static converters using discharge tubes or semiconductor devices to convert the intermediate dc into ac using devices of a triode or transistor type requiring continuous application of a control signal
    • H02M5/458—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into dc by static converters using discharge tubes or semiconductor devices to convert the intermediate dc into ac using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
    • H02M5/4585—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into dc by static converters using discharge tubes or semiconductor devices to convert the intermediate dc into ac using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only having a rectifier with controlled elements
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
    • H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
    • H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/483—Converters with outputs that each can have more than two voltages levels
    • H02M7/4835—Converters with outputs that each can have more than two voltages levels comprising two or more cells, each including a switchable capacitor, the capacitors having a nominal charge voltage which corresponds to a given fraction of the input voltage, and the capacitors being selectively connected in series to determine the instantaneous output voltage
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M3/00—Conversion of dc power input into dc power output
    • H02M3/02—Conversion of dc power input into dc power output without intermediate conversion into ac
    • H02M3/04—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters
    • H02M3/10—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M3/145—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
    • H02M3/155—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
    • H02M3/156—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only with automatic control of output voltage or current, e.g. switching regulators
    • H02M3/158—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only with automatic control of output voltage or current, e.g. switching regulators including plural semiconductor devices as final control devices for a single load
    • H02M3/1582—Buck-boost converters
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/60—Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]

Definitions

  • the invention relates to the technical field of power transmission and distribution, and particularly relates to a three-pole direct current transmission system for transforming an existing alternating current line into a direct current transmission line.
  • the following technologies can be used to improve the line transmission capacity: fixed series-parallel compensation technology, flexible AC transmission technology, compact transmission technology, and line dynamic increase. Capacity technology, increase the allowable temperature of wire work, new wire transmission technology, AC line to DC line.
  • the transmission power of an AC transmission line is more limited by the characteristics of the AC grid, and is usually much lower than the maximum thermal power that the conductor can withstand.
  • the line current can reach the maximum thermal limit that the conductor can withstand, and the line cost is lower and the loss is smaller.
  • the cost saved by the DC line is enough to offset the increased cost of the new converter station, the economic benefits of using DC transmission are obvious.
  • the direct current transmission also has the advantages of convenient power control, no increase of system short-circuit current, no skin effect of the line, higher insulation utilization of the line or cable, fast adjustment speed, reliable operation, and asynchronous connection of the AC system.
  • the three-pole DC transmission scheme in Figure 1 has the following disadvantages: 1) If the inverter side AC system fails, the DC three-pole simultaneous commutation failure will occur, resulting in DC line transmission. Power interruption, posing a threat to the safety and stability of the receiving system; 2) During the operation of the three-pole DC system, the polarity of each pole and the polarity and current direction of the pole 3 need to be changed rapidly according to a certain period, which is bound to the AC system at both ends.
  • the flexible direct current transmission technology using fully-controlled power electronic device IGBT has developed rapidly. Compared with the traditional direct current transmission technology, it has no need for grid commutation voltage support, and can supply power to the passive network; there is no commutation failure; It can realize independent control of active power and reactive power; no need of AC filtering and reactive power compensation device.
  • the flexible direct current transmission system mainly includes a two-level converter, a three-level inverter and a modular multi-level converter (MMC) structure, wherein the modular multi-electricity
  • MMC modular multi-level converter
  • the flexible DC transmission system of the flat converter has the characteristics of low switching frequency, small loss, easy expansion, high output waveform quality and low manufacturing difficulty, so it has broad application prospects.
  • the object of the present invention is to propose a three-pole flexible direct current transmission system and method, which solves the shortcomings of the three-pole direct current transmission based on LCC-HVDC, and satisfies the need of transforming an alternating current line into a direct current transmission line in a city power supply system with tight land resources.
  • the invention provides a three-pole flexible direct current transmission system, which uses three wires to transmit DC power, and the rectifier converter and the inverse converter are respectively composed of a three-phase six-bridge modular multi-level converter, and the rectifier converter
  • the midpoints of the 1, 2, and 3 phase units of the inverse converter are respectively connected to the secondary side of the converter transformer winding, and two converter valves are disposed between the rectifier converter and the DC side pole 1 and the pole 2 of the inverse converter.
  • the converter valve is composed of a fully controlled device.
  • the positive pole of the rectifier converter and the inverse converter is connected to the pole 1 DC line through a smoothing reactor, and the cathode is connected to the pole 2 DC line through a smoothing reactor, and the upper and lower converters of the rectifier converter and the inverse converter are replaced.
  • the flow valve connection point is connected to the pole 3 DC line through a smoothing reactor.
  • the rectifying converter and the inverse converter are composed of three-phase six-bridge arms, each of which is formed by connecting at least one sub-module and one bridge-arm reactor in series, and the upper and lower two arms of each phase are combined to form one Phase unit.
  • the basic sub-module of the above converter adopts a modular multi-level structure, and a plurality of basic units are connected in series and in parallel to adapt to different voltage levels and currents.
  • the submodule of the modular multilevel converter is a half bridge structure, and if the three wires used are overhead lines, the modular multilevel The submodule of the converter consists of two clamped double submodules.
  • the invention provides a three-pole flexible direct current power transmission method related to the above system, the pole 1 and the pole 2
  • the direct current direction is constant, the current setting is periodically modulated between the maximum value and the minimum value, the ratio of the maximum value and the minimum value of the current setting is 2, and the current of the pole 3 is the difference between the current of the pole 1 and the pole 2, 3 DC current setting takes the pole 1, pole 2 DC current constant value, the current direction changes periodically.
  • the polarities of the poles and poles 2 are constant, and the triggering of the upper and lower converter valves on the DC side of the rectifier converter and the inverter is controlled to make the pole 3 line periodically and pole 1 line or pole The two lines operate in parallel, thereby periodically changing the polarity of the pole 3 DC voltage to ensure that the direction of the DC power of the pole 3 is unchanged.
  • the line voltage of the pole 1 of the power transmission system is positive, the current of the pole 1 is the maximum value of the current constant, the voltage of the pole 2 is negative, and the current of the pole 2 is the minimum value of the current, and the DC side of the rectifier converter and the inverse converter
  • the two converter valves between the pole 3 and the pole 2 are turned on, the two converter valves between the pole 3 and the pole 1 are turned off, the pole 3 line is operated in parallel with the pole 2 line, and the pole 3 line voltage is negative, the pole 3
  • the current command is the minimum value of the current setting, and the current direction is the same as the direction of the pole 2 current; when the pole 1 current is switched from the current setting maximum value to the current setting minimum value, the pole 2 current is switched from the current setting minimum value to the current setting When the value is at the maximum value, the two converter valves between the rectifier converter and the DC converter pole 3 and the pole 1 of the reverse converter are turned on, and the two converter valves between the pole 3 and the pole 2 are turned off.
  • the invention has the characteristics that the AC line can be converted into a DC transmission line, and the MMC-HVDC does not have a commutation failure, can stabilize the AC system voltage on both sides, and does not require the characteristics of AC filtering and reactive power compensation device.
  • the bipolar MMC-HVDC three-phase six-bridge arm structure, four converter valves are added to realize three wires transmission power. Compared with the conventional bipolar MMC-HVDC, the number of commutation variables is unchanged, and the converter valve is only increased by four. .
  • the bipolar system needs to be shut down once, and the solution proposed by the present invention adopts three lines of transmission, and after one line failure, the remaining two lines It can also constitute a conventional bipolar MMC-HVDC system operation, which greatly improves the reliability of the entire system.
  • the solution proposed by the invention has the characteristics of compact structure, small floor space of the converter station, low overall modification cost and high reliability, and can fully utilize the three lines.
  • the transmission power is therefore especially suitable for the implementation of the conversion of AC lines into DC transmission lines in urban power supply systems with tight land resources, and has good engineering application value.
  • Figure 1 shows the principle structure of a three-pole HVDC transmission system based on LCC-HVDC
  • Figure 2 is a schematic diagram of the structure of a typical three-phase six-legged modular multilevel converter system.
  • FIG. 3 is a schematic diagram of a topology structure of a three-pole flexible direct current transmission system according to the present invention
  • Figure 4 is a sub-module structure diagram of a modular multi-level converter suitable for cable
  • Figure 5 is a sub-module structure diagram of a modular multi-level converter suitable for overhead lines.
  • Figure 6 is a schematic diagram of three-pole DC transmission current command modulation
  • Figure 7 is a schematic diagram of the system structure when the voltage of the pole 3 of the three-pole DC transmission system is negative.
  • Figure 8 is a schematic diagram of the system structure of the pole 3 voltage of the three-pole DC transmission system.
  • Figure 2 shows the three-phase six-bridge MMC structure diagram used in a typical bipolar DC system.
  • the bipolar DC system rectifier station and the inverter station the modular multilevel converter DC side pole 1 and pole 2
  • Two converter valves are arranged between the two, and the connection points of the upper and lower converter valves are connected to the pole 3 DC line through the smoothing reactor, and the trigger of the converter valve is controlled to periodically change the polarity of the pole 3 DC voltage.
  • Three-pole current commands are modulated to achieve three-pole DC transmission.
  • Three-pole DC transmission can be realized. The specific scheme is as follows:
  • the rectifier converter and the inverse converter are respectively composed of a three-phase six-bridge modular multi-level converter, and the rectifier, the inverse converter, the 1, 2, and 3 phase units.
  • the midpoint is respectively connected to the secondary side of the converter transformer winding, and two converter valves are arranged between the rectifier converter and the DC side pole 1 and the pole 2 of the inverse converter.
  • the converter valve is composed of a fully controlled device.
  • the converter valve consists of an IGBT and an anti-parallel freewheeling diode to make.
  • the positive pole of the rectifier converter and the inverse converter is connected to the pole 1 DC line through a smoothing reactor, and the cathode is connected to the pole 2 DC line through a smoothing reactor, and the upper and lower converters of the rectifier converter and the inverse converter are replaced.
  • the flow valve connection point is connected to the pole 3 DC line through a smoothing reactor.
  • Pole 1 pole 2 DC voltage polarity and DC current direction are constant, current setting is periodically modulated between maximum and minimum values, pole 3 DC voltage polarity and DC current direction change periodically, pole 3 current
  • the triggering of the upper and lower converter valves on the DC side of the rectifier converter and the inverter is controlled to achieve a rapid change of the polarity of the pole 3 DC voltage to ensure the pole 3 DC power.
  • the direction is unchanged.
  • the rectifying converter and the inverse converter are composed of three-phase six-bridge arms, each of which is formed by connecting at least one sub-module and one bridge-arm reactor in series, and serially connecting different numbers of sub-module units to adapt to different voltages.
  • the upper and lower arms of each phase are combined to form a phase unit.
  • a rectification converter transformer for converting a voltage level of a three-phase alternating current provided by a transmitting end AC system
  • a rectifying converter for converting a three-phase alternating current converted from a voltage level into a direct current
  • the inverse transform current transformer is configured to convert the three-phase alternating current converted into the multi-level converter into a voltage level conversion for transmission to the receiving end communication system.
  • the three wires used in the above power transmission system may be either a cable or an overhead line. If the three wires used are cables, because the cable fault is low, and if a fault occurs, it is usually a permanent fault. By clearing the AC side switch to clear the fault, it will not affect the availability of the entire system. Therefore, the sub-module of the modular multi-level converter can adopt a half-bridge structure, and the specific structure is shown in FIG. 4; if the three wires used are overhead lines, the probability of temporary failure of the overhead line is high, such as using a jump. If the AC side switch is turned on to clear the fault, the power outage time is longer, which affects the availability of the entire system. The current control of the flow device is used to clear the fault on the DC side.
  • the sub-module of the modular multi-level rectifier adopts a clamped double sub-module structure, and the specific structure is shown in FIG. 5 .
  • a modular multi-level rectifier employing a half-bridge structure and a clamped double sub-module structure is known in the art and will not be described herein.
  • the three-pole DC current command is modulated to realize three-pole direct current transmission.
  • Pole 1 pole 2 DC voltage polarity and DC current direction are constant, current setting is periodically modulated between maximum and minimum values, pole 3 DC voltage polarity and DC current direction change periodically, pole 3 current The difference between the pole 1 and pole 2 currents.
  • the ratio of the maximum value and the minimum value of the pole 1 and pole 2 DC current is 2
  • the pole 3 is the constant value of the DC current
  • the minimum value of the pole 2 is the minimum value of the DC current
  • the current direction period is 2
  • Sexual change The triggering of the upper and lower converter valves on the DC side of the rectifier converter and the inverter is controlled so that the pole 3 line periodically operates in parallel with the pole 1 line or the pole 2 line, thereby periodically changing the pole 3 DC voltage. Polarity ensures that the direction of the DC power of the pole 3 is unchanged.
  • the three-pole DC current modulation control strategy is shown in Figure 6.
  • the polarity of the DC voltage and the direct current direction of the pole 1 and pole 2 are constant, alternating large and small currents are transmitted periodically, and the pole 3 flows through the pole 1 and the pole 2 Unbalanced current to eliminate ground return and minimize current flow through the ground. Pole 3 can quickly change the voltage polarity and current direction to ensure that the power transfer direction is unchanged.
  • the three-pole DC transmission modulates the system's various pole current commands during normal operation.
  • the pole 1 and pole 2 current commands are continuously switched between a maximum value Idmax and a minimum value Idmin, and the pole 3 current command takes the difference between the pole 1 and the pole 2 current command.
  • the polarity of the poles of pole 1 and pole 2 remains unchanged.
  • the modulation period T m of Idmax and Idmin takes 4 to 5 minutes, and the specific value can be optimized.
  • the specific current modulation process is as follows: in Figure 3, the pole 1 line voltage is positive, the pole 1 current is Idmax, the pole 2 voltage is negative, and the pole 2 current is Idmin, then the rectifier converter and the inverse converter DC side IGBT T2 And T4 is turned on, T1 and T3 are turned off, pole 3 line and pole 2 line are operated in parallel, and pole 3 line voltage is negative.
  • the current of the pole 3 is Idmin, and the current direction is the same as the direction of the pole 2 current.
  • the specific topology of the three-pole DC system is shown in FIG.
  • Idmax 2Idmin
  • Idmax and Idmin take 1.26pu and 0.63pu respectively
  • the pole 3 current is 0.63pu.
  • the three-pole DC transmission power is 2.53pu, which is a double composed of conventional modular multilevel converters.
  • the pole system delivers 1.26 times the power.
  • bipolar MMC-HVDC system based on the modular multilevel converter of Figure 2
  • the bipolar system is shut down.
  • two converter valves are arranged on the DC side of the converter, and the current command is applied to the three poles. Modulation, three-pole DC transmission can be realized, and the transmission power can be increased by 26% compared with the bipolar DC system.
  • the remaining bipolar system can still deliver 2.3 pu DC power, which is equivalent to delivering 91% of the power before the fault, so the whole system has a higher Reliability.

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Abstract

一种三极柔性直流输电系统和方法，该系统中的整流换流器和逆变换流器分别由一个三相六桥臂模块化多电平换流器组成，在双极直流系统换流器直流侧设置两个换流阀，整流换流器、逆变换流器的上下两个换流阀连接点通过平波电抗器与极3直流线路相连，对换流阀的触发进行控制，周期性地改变极3直流电压极性。该输电系统可以充分利用三根导线传输功率，具有换流站占地面积小，总体改造成本低、可靠性高等优点。

Description

一种三极柔性直流输电系统和方法 技术领域
本发明涉及输配电技术领域，具体涉及一种将现有的交流线路改造成直流输电线路的三极直流输电系统。
背景技术
随着我国经济持续高速发展，电力需求矛盾日益突出。一方面电力负荷的增长超出了原有交流输电线路的传输能力，而受土地资源限制，很难获得新的输电走廊；另一方面，传统交流输电受绝缘、交流电网特性限制，线路输送功率很少达到导线所能承受的最大热功率，因此需要采用新技术进一步挖掘现有线路的输电潜力。
为了充分利用现有交流线路输电走廊，除了直接升高线路运行电压等级外，还可以采用如下技术来提升线路输送能力：固定串并联补偿技术、灵活交流输电技术、紧凑型输电技术、线路动态增容技术、提高导线工作允许温度、新型导线输电技术、交流线路改直流线路。
交流输电线路传输功率更多地是受到交流电网特性的限制，通常远远低于导线所能承受的最大热功率。与交流输电相比较，采用高压直流输电，线路电流可以达到导线所能承受的最大热极限，而且线路造价更低、损耗更小。当直流线路节约的成本足以抵消新建换流站增加的成本时，采用直流输电经济效益明显。此外，直流输电还具有功率控制方便、不增加系统短路电流、线路不存在集肤效应、线路或电缆的绝缘利用率更高、调节速度快、运行可靠、可以实现交流系统异步连接等优势。
2004年，美国学者Barthold L O在专利US6714427B1中提出一种如图1所示的采用常规LCC-HVDC的三极直流输电系统，其原理是采用直流电流调制技术将交流输电线路转化为直流输电线路。在对交流线路改造成直流输电线路的各种转换方案中，由于三极直流输电方案可以充分利用原有的交流三相线路，因此相对于双极和单极直流输电改造方案，三极直流输电方案在提高输电能力、经济成本和可靠性等方面上具有优势。目前利用三极直流输电技术将交流线路改造成直流输电线路处在原理性研究阶段，尚无工程应用实例。由于LCC-HVDC采用半控型晶闸管，因此图1中的三极直流输电方案存在以下缺点：1)如逆变侧交流系统发生故障将引起直流三个极同时发生换相失败，导致直流线路输送功率中断，对受端系统安全稳定构成威胁；2)在三极直流系统运行过程中，各极电流大小以及极3电压极性和电流方向都需要按一定周期快速改变，势必对两端交流系统产生扰动；3)由于每条线路都配置了一个完整的极，因此需要增加的换流变压器、交流滤波和无功补偿装置、12脉动换流器以及相应辅助设备较多，造成改造成本较高、新增的换流站占地面积较大的缺点，尤其不利于对换流站占地面积要求特别严格的大城市供电系统中交流线路改造成直流线路工程的实施。
近年来，采用全控型电力电子器件IGBT的柔性直流输电技术发展很快，与传统直流输电技术相比，具有不需要电网换相电压支撑，可向无源网络供电；不存在换相失败；可实现有功功率、无功功率独立控制；无需交流滤波和无功补偿装置等优点。从所采用的换流器结构来区别，柔性直流输电系统主要包括两电平换流器、三电平换流器和模块化多电平换流器(MMC)结构，其中基于模块化多电平换流器的柔性直流输电系统除了具有柔性直流输电通用优点外，还具有开关频率低、损耗小、易扩展、输出波形质量高、制造难度低等特点，因此具有广阔的应用前景。
为了解决基于常规LCC-HVDC的三极直流输电系统存在的上述问题，降低 新建换流站面积，提高交流线路改直流线路方案的经济性，本专利在基于MMC-HVDC双极系统的基础上提出一种三极柔性直流输电系统和方法，可以很好的满足城市供电系统中交流线路改造成直流输电线路提高输电容量的需要。
发明内容
本发明的目的在于提出一种三极柔性直流输电系统和方法，解决基于LCC-HVDC的三极直流输电的缺点，满足土地资源紧张的城市供电系统中交流线路改造成直流输电线路需要。
本发明提出一种三极柔性直流输电系统，采用三根导线传输直流功率，整流换流器和逆变换流器分别由一个三相六桥臂模块化多电平换流器组成，整流换流器、逆变换流器的1，2，3相单元中点分别与换流变压器绕组副边相连，整流换流器、逆变换流器直流侧极1和极2之间各设置两个换流阀，换流阀由全控型器件构成。整流换流器、逆变换流器的正极通过平波电抗器与极1直流线路相连，负极通过平波电抗器与极2直流线路相连，整流换流器、逆变换流器的上下两个换流阀连接点通过平波电抗器与极3直流线路相连。
进一步，整流换流器、逆变换流器由三相六桥臂组成，每个桥臂至少由一个子模块和一个桥臂电抗器串联而成，每相上下两个桥臂合在一起构成一个相单元。
进一步，上述换流器基本子模块采用模块化多电平结构，通过串并联不同数量基本单元以适应不同电压等级和电流的需要。
进一步，上述输电系统所如采用的三根导线为电缆，则所述模块化多电平换流器的子模块为半桥结构，如采用的三根导线为架空线路，则所述模块化多电平换流器的子模块由两个钳位双子模块组成。
本发明提出一种与上述系统相关的一种三极柔性直流输电方法，极1、极2 直流电流方向恒定不变，电流定值在最大值和最小值之间周期性调制，电流定值最大值和最小值的比值为2，极3电流为极1和极2电流的差值，极3直流电流定值取极1、极2直流电流定值最小值，电流方向周期性的改变。极1、极2直流电压极性方向恒定不变，对整流换流器和逆变换流器直流侧上下两个换流阀的触发进行控制，使极3线路周期性的与极1线路或极2线路并联运行，从而周期性的改变极3直流电压极性，保证极3直流功率方向不变。
进一步，上述输电系统极1线路电压为正，极1电流为电流定值最大值，极2电压为负，极2电流为电流定值最小值，则整流换流器和逆变换流器直流侧极3和极2之间的两个换流阀导通、极3和极1之间的两个换流阀关断，极3线路与极2线路并联运行，极3线路电压为负，极3电流指令为电流定值最小值，电流方向与极2电流方向相同；当极1电流由电流定值最大值切换到电流定值最小值、极2电流由电流定值最小值切换到电流定值最大值时，整流换流器和逆变换流器直流侧极3和极1之间的两个换流阀导通、极3和极2之间的两个换流阀关断，此时极3线路与极1线路并联运行，极3线路电压为正，极3电流指令为电流定值最小值，电流方向与极1电流方向相同。
本发明的特点在于：可以满足交流线路改造成直流输电线路需要，除了具有MMC-HVDC不会发生换相失败、可以稳定两侧交流系统电压、无需交流滤波和无功补偿装置特点外，在常规双极MMC-HVDC三相六桥臂结构基础上增加4个换流阀，实现三根导线传输功率，与常规双极MMC-HVDC相比，换流变数目不变，换流阀只增加4个。此外，对于常规双极MMC-HVDC系统而言，一回线路故障，双极系统都需要停运，而采用本发明提出的方案，由于采用三条线路输电，一回线路故障后，剩余两条线路还可以构成常规双极MMC-HVDC系统运行，大大提高了整个系统的可靠性。综上所述，本发明提出的方案具有结构紧凑、换流站占地面积小、总体改造成本低、可靠性高的特点，可以充分利用三根线路 传输功率，因此特别适用于土地资源紧张的城市供电系统中交流线路改造成直流输电线路增容工程的实施，具有很好的工程应用价值。
附图说明
图1为基于LCC-HVDC的三极直流输电系统原理结构图
图2为典型三相六桥臂模块化多电平换流器系统结构原理图
图3为本发明提出的三极柔性直流输电系统拓扑结构原理图
图4为适用于电缆的模块化多电平换流器的子模块结构图
图5为适用于架空线路的模块化多电平换流器的子模块结构图
图6为三极直流输电电流指令调制原理图
图7为三极直流输电系统极3电压为负时系统结构原理图
图8为三极直流输电系统极3电压为正时系统结构原理图
具体实施方式
以下将结合附图及具体实施例，对本发明的技术方案进行详细说明。
图2所示为一典型的双极直流系统采用的三相六桥臂MMC结构图，在双极直流系统整流站和逆变站模块化多电平换流器直流侧极1和极2之间各设置两个换流阀，上下两个换流阀连接点通过平波电抗器与极3直流线路相连，对换流阀的触发进行控制，周期性的改变极3直流电压极性，通过对三个极的电流指令进行调制，实现三极直流输电。即可实现三极直流输电，具体方案如下：
如图3所示，整流换流器和逆变换流器分别由一个三相六桥臂模块化多电平换流器组成，整流换流器、逆变换流器的1，2，3相单元中点分别与换流变压器绕组副边相连，整流换流器、逆变换流器直流侧极1和极2之间各设置两个换流阀，换流阀由全控型器件构成，图中换流阀由IGBT和反向并联续流二极管构 成。整流换流器、逆变换流器的正极通过平波电抗器与极1直流线路相连，负极通过平波电抗器与极2直流线路相连，整流换流器、逆变换流器的上下两个换流阀连接点通过平波电抗器与极3直流线路相连。
极1、极2直流电压极性和直流电流方向恒定不变，电流定值在最大值和最小值之间周期性调制，极3直流电压极性和直流电流方向周期性的改变，极3电流为极1和极2电流的差值，对整流换流器和逆变换流器直流侧上下两个换流阀的触发进行控制，实现极3直流电压极性的快速改变，保证极3直流功率方向不变。
进一步，整流换流器、逆变换流器由三相六桥臂组成，每个桥臂至少由一个子模块和一个桥臂电抗器串联而成，通过串并联不同数量子模块单元以适应不同电压等级和电流的需要，每相上下两个桥臂合在一起构成一个相单元。
整流换流变压器，用于将送端交流系统提供的三相交流电进行电压等级变换；
整流换流器，用于将电压等级变换后的三相交流电转换为直流电；
平波电抗器，用于平抑所述的直流电中的纹波；
逆变换流器，用于将平抑后的直流电转换为三相交流电；
逆变换流变压器，用于将多电平换流器转换成的三相交流电进行电压等级变换，以输送给受端交流系统。
进一步，上述输电系统所采用的三根导线既可以是电缆，也可以是架空线路。如采用的三根导线为电缆，由于电缆故障低，并且如发生故障，通常是永久性故障，通过跳开交流侧开关来清除故障即可，不会影响整个系统的可用率。因此模块化多电平换流器的子模块可以采用半桥结构，具体结构如图4所示；如采用的三根导线为架空线路，由于架空线路发生暂时性故障的概率较高，如采用跳开交流侧开关来清除故障则停电时间较长，影响整个系统的可用率，因此需要通过换 流器自身的控制来清除直流侧的故障，为此所述模块化多电平整流器的子模块采用钳位双子模块结构，具体结构如图5所示。采用半桥结构和采用钳位双子模块结构的模块化多电平整流器均为现有技术，在此不再赘述。
上述三极直流输电系统运行中，对三个极的直流电流指令进行调制实现三极直流输电。极1、极2直流电压极性和直流电流方向恒定不变，电流定值在最大值和最小值之间周期性调制，极3直流电压极性和直流电流方向周期性的改变，极3电流为极1和极2电流的差值。
上述三极直流输电系统运行中，极1、极2直流电流定值最大值和最小值的比值为2，极3直流电流定值取极1、极2直流电流定值最小值，电流方向周期性的改变。对整流换流器和逆变换流器直流侧上下两个换流阀的触发进行控制，使极3线路周期性的与极1线路或极2线路并联运行，从而周期性的改变极3直流电压极性，保证极3直流功率方向不变。
三极直流电流调制控制策略如图6所示：极1和极2直流电压极性和直流电流方向恒定不变，交替周期性地传输大电流和小电流，极3流过极1和极2的不平衡电流，以消除大地回流，使流过接地极的电流最小。极3可以快速改变电压极性和电流方向，以保证功率输送方向不变。三极直流输电正常运行时对系统各极电流指令进行调制。极1和极2电流指令在最大值Idmax和最小值Idmin之间不断切换，极3电流指令取极1与极2电流指令的差值。极1和极2的电压极性保持不变。由于极3的电流方向要发生周期性的变化，极3电压跟随极3电流方向变化呈现周期性的反转以保证极3功率传输方向不变。Idmax与Idmin的调制周期Tm取4～5min，具体值可以优化。
具体电流调制过程如下所述：图3中极1线路电压为正，极1电流为Idmax，极2电压为负，极2电流为Idmin，则整流换流器和逆变换流器直流侧IGBT T2和T4导通、T1和T3关断，极3线路与极2线路并联运行，极3线路电压为负， 极3电流大小为Idmin，电流方向与极2电流方向相同，此时三极直流系统具体拓扑如图7所示；当极1电流由Idmax切换到Idmin、极2电流由Idmin切换到Idmax时，整流换流器和逆变换流器直流侧IGBT T1和T3导通、T2和T4关断，此时极3线路与极1线路并联运行，极3线路电压为正，极3电流大小为Idmin，电流方向与极1电流方向相同，此时三极直流系统具体拓扑如图8所示。
图6中Idmax＝2Idmin，则Idmax和Idmin分别取1.26pu和0.63pu，极3电流为0.63pu，此时三极直流输送功率为2.53pu，是常规模块化多电平换流器组成的双极系统输送功率的1.26倍。
对于基于图2模块化多电平换流器构成的双极MMC-HVDC系统而言，一回线路发生故障，双极系统都要停运。而采用本发明提出的方案，在常规模块化多电平换流器组成的双极直流系统拓扑基础上，在换流器直流侧设置两个换流阀，通过对三个极的电流指令进行调制，可以实现三极直流输电，输送功率相对于双极直流系统可提高26％。当一回线路故障退出运行时，考虑到各极直流具有15％的过载能力，则剩余双极系统仍然可以输送2.3pu直流功率，相当于输送91％故障前的功率，因此整个系统具有较高的可靠性。
以上实施例仅为说明本发明的技术思想，不能以此限定本发明的保护范围，凡是按照本发明提出的技术思想，在技术方案基础上所做的任何改动，均落入本发明保护范围之内。

Claims (5)

  1. 一种三极柔性直流输电系统，其特征是：采用三根导线传输直流功率，整流换流器和逆变换流器分别由一个三相六桥臂模块化多电平换流器组成，整流换流器、逆变换流器的1，2，3相单元中点分别与换流变压器绕组副边相连，整流换流器、逆变换流器直流侧极1和极2之间各设置两个换流阀，换流阀由全控型器件构成；整流换流器、逆变换流器的正极通过平波电抗器与极1直流线路相连，负极通过平波电抗器与极2直流线路相连，整流换流器、逆变换流器的上下两个换流阀连接点通过平波电抗器与极3直流线路相连。
  2. 由权利要求1所述的一种三极柔性直流输电系统，其特征是：整流换流器、逆变换流器由三相六桥臂组成，每个桥臂至少由一个子模块和一个桥臂电抗器串联而成，通过串并联不同数量子模块单元以适应不同电压等级和电流的需要，每相上下两个桥臂合在一起构成一个相单元。
  3. 由权利要求1或2所述的一种三极柔性直流输电系统，其特征是：如采用的三根导线为电缆，则所述模块化多电平换流器的子模块为半桥结构；如采用的三根导线为架空线路，则所述模块化多电平换流器的子模块由两个钳位双子模块组成。
  4. 权利要求1所述系统的一种三极柔性直流输电方法，其特征是：极1、极2直流电流方向恒定不变，电流定值在最大值和最小值之间周期性调制，电流定值最大值和最小值的比值为2，极3电流为极1和极2电流的差值，极3直流电流定值取极1、极2直流电流定值最小值，电流方向周期性的改变；极1、极2直流电压极性方向恒定不变，对整流换流器和逆变换流器直流侧上下两个换流阀的触发进行控制，使极3线路周期性的与极1线路或极2线路并联运行，从而周期性的改变极3直流电压极性，保证极3直流功率方向不变。
  5. 由权利要求4所述的一种三极柔性直流输电方法，其特征是：极1线路电压为正，极1电流为电流定值最大值，极2电压为负，极2电流为电流定值 最小值，则整流换流器和逆变换流器直流侧极3和极2之间的两个换流阀导通、极3和极1之间的两个换流阀关断，极3线路与极2线路并联运行，极3线路电压为负，极3电流指令为电流定值最小值，电流方向与极2电流方向相同；当极1电流由电流定值最大值切换到电流定值最小值、极2电流由电流定值最小值切换到电流定值最大值时，整流换流器和逆变换流器直流侧极3和极1之间的两个换流阀导通、极3和极2之间的两个换流阀关断，此时极3线路与极1线路并联运行，极3线路电压为正，极3电流指令为电流定值最小值，电流方向与极1电流方向相同。
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