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直流微网中含有多种直流电源,有些表现为电压源特性,如基于电压源型换流器接入的交流配网系统、储能系统等,有些表现为电流源特性,如光伏系统[18-19]。直流微网的故障电流主要由电源端提供,其中又以电压源特性的电源提供的故障电流为主。考虑到多数直流微网发生单极故障时故障电流较小,对设备危害较小,故障发生后还可以继续运行一段时间,文章设计的故障电流控制器仅针对极间故障。以基于电压源型换流器接入的交流配网系统为例,在直流侧,交流配网可等效为可调直流电压源,如图1所示。
当发生极间故障时,由交流配网提供的故障电流可用下式表示:
$$ i=u_{\mathrm{d}} /\left(Z+R_{\mathrm{f}}\right) $$ (1) 式中:
$ i $ ——电压源型换流器直流侧出口输出电流(kA);${u}_{{\rm{d}}}$ ——交流配网在直流侧的等效电压(kV);Z ——故障点至电压源型换流器之间的阻抗(Ω);
${R}_{{\rm{f}}}$ ——故障过渡电阻(Ω)。可见,故障电流主要取决于等效电压
${u}_{\text{d}}$ 的大小。由电压源型换流器的原理可得到以下关系式:
$$ {U}_{\rm{d}}\text={U}_{\rm{ac}}/m $$ (2) 式中:
Ud ——VSC直流侧平均电压(kV);
Uac ——VSC交流侧电压线电压幅值(kV);
m ——调制比,且有0< m <1。
可见,电压源型换流器直流侧电压的调节范围在交流配网线电压幅值往上[20-21]。因此,仅靠调节电压源型换流器的直流侧输出电压,并不能有效控制故障电流。
倘若将一个反向可控电压源,串联在电压源型换流器的出口处,如图2所示。在正常运行时,控制可控电压源输出电压为0,可不影响微网正常运行;极间故障时,由交流配网提供的故障电流变为如下式(3)。
$$ i=({u}_{\rm{d}}-{{u}}_{{1}})/(Z+{R}_{\rm{f}}) $$ (3) 式中:
$ {u}_{1} $ ——反向电压源的输出电压(kV)。由此可见,控制故障时可控电压源的输出电压
${u}_{1}$ 的大小,即可任意控制故障电流的大小。 -
由1.1节分析可见,正常运行时,可控电压源u1的输出电压应接近0以减小串联电压源对直流微网正常运行的影响;故障时,可控电压源u1的输出电压应适当增加。当
${u}_{1}$ =${u}_{\rm{d}}$ 时,交流电网对故障点提供的故障电流降为0。因此,可控电压源${u}_{\text{1}}$ 的输出电压应在连续可调的范围0~u之间(u<${u}_{\rm{d}}$ )。考虑到微电网的功率双向流动特性,可控电压源${u}_{1}$ 还应具备电流双向流动能力。图3所示的buck-boost电路即可满足上述要求。
图3中,Eb为储能元件的端电压。当直流微网正常运行时,开关管S1常开,S2常闭,可控电压源
${u}_{1}$ 的输出电压约为开关管的导通压降,对正常运行影响较小,电感L还可起到降低直流电流纹波的作用。当直流微网发生极间故障时,控制开关管S1和开关管S2的导通占空比,即可调节输出电压${u}_{1}$ 的大小,从而任意控制故障电流的大小。基于电压源型换流器接入直流微网的交流配网系统,串联图3所示的故障电流控制器后,其电路结构如图4所示。在图4所示中,VSC的直流侧满足以下关系式:
$$ {U}_{2}={U}_{\rm{d}}-{U}_{1} $$ (4) 式中:
${U}_{2}$ ——直流母线电压(kV);${U}_{\rm{d}}$ ——直流侧输出电压平均值(kV);${U}_{1}$ ——故障电流控制器输出电压平均值(kV)。
Research on Fault Current Controller of DC Microgrid
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摘要:
目的 随着交直流配电网及分布式发电技术的快速发展,直流微电网在配电网中的作用愈发重要,将成为未来配电网中的重要组成部分。由于直流微电网覆盖面积小,线路阻抗小,当发生极间短路故障时,故障电流上升速度快,幅值大,可达到额定工作电流的10倍以上。这使得直流微网保护整定困难,设备选型要求高,制约了直流微网的快速发展。 方法 针对上述问题,文章以直流微电网为研究对象,从直流微电网的极间故障的工作原理出发,分析直流微电网直流侧的故障特征,针对现有主要限流方法的不足,提出利用一种电压可控的故障电流控制器,来实现对故障电流的精确控制并搭建了直流微电网和故障电流控制器的仿真模型进行仿真验证。 结果 仿真结果显示,该故障电流控制器可大幅降低故障电流,并可实现精确控制故障电流,使得故障前后系统均处于可控状态而不会闭锁保护。在稳态运行时,故障电流控制器还可辅助VSC(Voltage Source Converter, VSC)进一步稳定直流母线电压。 结论 为配合继电保护装置正常动作,同时避免VSC触发过流保护闭锁,建议故障电流控制范围设置为1~2 pu。 Abstract:Introduction With the rapid development of AC/DC distribution networks and distributed generation technology, the role of DC microgrids in distribution networks is becoming increasingly important and will become an important component of future distribution networks. Due to the small coverage area and low line impedance of the DC microgrid, when an inter pole short circuit fault occurs, the fault current increases rapidly and has a large amplitude, which can reach more than 10 times the rated working current. This makes it difficult to set the protection of DC microgrids and requires high equipment selection, which restricts the rapid development of DC microgrids. Method In response to the above issues, taking the DC microgrid as the research object, starting from the working principle of inter pole faults in the DC microgrid, the fault characteristics on the DC side of the DC microgrid were analyzed. In response to the shortcomings of existing main current limiting methods, a voltage controllable fault current controller was proposed to achieve precise control of fault current. The simulation model of DC microgrid and fault current controller was built for simulation verification. Result The simulation results show that the fault current controller can significantly reduce the fault current and achieve precise control of the fault current, making the system controllable before and after the fault without locking the protection. During steady-state operation, the fault current controller can also assist the VSC (Voltage Source Converter) in further stabilizing the DC bus voltage. Conclusion To cooperate with the normal operation of the relay protection device and avoid VSC triggering overcurrent protection blocking, it is recommended to set the fault current control range between 1~2 pu. -
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[1] 薛士敏, 齐金龙, 刘冲. 直流微网保护综述 [J]. 中国电机工程学报, 2016, 36(13): 3404-3412. DOI: 10.13334/j.0258-8013.pcsee.160148. XUE S M, QI J L, LIU C. A research review of protection for DC microgrid [J]. Proceedings of the CSEE, 2016, 36(13): 3404-3412. DOI: 10.13334/j.0258-8013.pcsee.160148. [2] 薛士敏, 齐金龙, 刘冲, 等. 直流微网接地方式及新型保护原理 [J]. 电网技术, 2018, 42(1): 48-55. DOI: 10.13335/j.1000-3673.pst.2017.1409. XUE S M, QI J L, LIU C, et al. A research of grounding mode and new protection principle for DC microgrids [J]. Power system technology, 2018, 42(1): 48-55. DOI: 10.13335/j.1000-3673.pst.2017.1409. [3] 张宇涵, 杜贵平, 雷雁雄, 等. 直流微网混合储能系统控制策略现状及展望 [J]. 电力系统保护与控制, 2021, 49(3): 177-187. DOI: 10.19783/j.cnki.pspc.200461. ZHANG Y H, DU G P, LEI Y X, et al. Current status and prospects of control strategy for a DC micro grid hybrid energy storage system [J]. Power system protection and control, 2021, 49(3): 177-187. DOI: 10.19783/j.cnki.pspc.200461. [4] DRAGIČEVIĆ T, LU X N, VASQUEZ J C, et al. DC microgrids-part Ⅱ: a review of power architectures, applications, and standardization issues [J]. IEEE transactions on power electronics, 2016, 31(5): 3528-3549. DOI: 10.1109/TPEL.2015.2464277. [5] MONADI M, ZAMANI M A, IGNACIO CANDELA J, et al. Protection of AC and DC distribution systems embedding distributed energy resources: a comparative review and analysis [J]. Renewable and sustainable energy reviews, 2015, 51: 1578-1593. DOI: 10.1016/j.rser.2015.07.013. [6] 于涛. 基于分布式电源的微电网控制策略研究 [D]. 哈尔滨: 哈尔滨工程大学, 2020. DOI: 10.27060/d.cnki.ghbcu.2020.001247. YU T. Research on microgrid control strategy based on distributed power supply [D]. Harbin: Harbin Engineering University, 2020. DOI: 10.27060/d.cnki.ghbcu.2020.001247. [7] LIANG B M, KANG L, ZHANG Z Y, et al. Simulation analysis of grid-connected AC/DC hybrid microgrid [C]//2018 13th IEEE Conference on Industrial Electronics and Applications (ICIEA), Wuhan, China, May 31-June 2, 2018. New York, USA: IEEE, 2018: 969-974. DOI: 10.1109/ICIEA.2018.8397852. [8] 薛士敏, 黄仁乐, 高峰, 等. 基于暂态电流突变量的直流配电系统快速纵联保护新原理 [J]. 供用电, 2016, 33(8): 37-44. DOI: 10.19421/j.cnki.1006-6357.2016.08.007. XUE S M, HUANG R L, GAO F, et al. High-speed pilot protection principle for DC distribution system based on the difference of transient currents [J]. Distribution & utilization, 2016, 33(8): 37-44. DOI: 10.19421/j.cnki.1006-6357.2016.08.007. [9] 焦皎, 孟润泉, 任春光, 等. 交直流微电网AC/DC双向功率变换器控制策略 [J]. 电力系统保护与控制, 2020, 48(16): 84-92. DOI: 10.19783/j.cnki.pspc.191124. JIAO J, MENG R Q, REN C G, et al. Bidirectional AC/DC interlinking converter control strategy for an AC/DC microgrid [J]. Power system protection and control, 2020, 48(16): 84-92. DOI: 10.19783/j.cnki.pspc.191124. [10] 周钰, 张浩, 陈锐, 等. 直流微电网控制保护策略研究 [J]. 南方能源建设, 2020, 7(4): 61-66. DOI: 10.16516/j.gedi.issn2095-8676.2020.04.009. ZHOU Y, ZHANG H, CHEN R, et al. Research on strategy of DC micro-grid control and protection [J]. Southern energy construction, 2020, 7(4): 61-66. DOI: 10.16516/j.gedi.issn2095-8676.2020.04.009. [11] 王灿, 杜船, 徐杰雄. 中高压直流断路器拓扑综述 [J]. 电力系统自动化, 2020, 44(9): 187-199. DOI: 10.7500/AEPS20191021006. WANG C, DU C, XU J X. Review of topologies for medium-and high-voltage DC circuit breaker [J]. Automation of electric power systems, 2020, 44(9): 187-199. DOI: 10.7500/AEPS20191021006. [12] FRANCK C M. HVDC circuit breakers: a review identifying future research needs [J]. IEEE transactions on power delivery, 2011, 26(2): 998-1007. DOI: 10.1109/TPWRD.2010.2095889. [13] 杨勇, 王文杰, 刘亚萍, 等. 基于大规模风、光并网外送需求的高压直流混合式直流断路器研究 [J]. 可再生能源, 2021, 39(2): 237-244. DOI: 10.3969/j.issn.1671-5292.2021.02.015. YANG Y, WANG W J, LIU Y P, et al. Hybrid HVDC breaker for HVDC based wind and photovoltaic power integration system [J]. Renewable energy resources, 2021, 39(2): 237-244. DOI: 10.3969/j.issn.1671-5292.2021.02.015. [14] TAN R, WANG Y, ZHANG S. Coordination scheme of SFCL and SMES in the DC microgrid for fault current limiting and voltage stability [C]//2020 IEEE International Conference on Applied Superconductivity and Electromagnetic Devices (ASEMD), Tianjin, China, October 16-18, 2020. New York, USA: IEEE, 2020: 1-2. DOI: 10.1109/ASEMD49065.2020.9276114. [15] XUE S M, GAO F, SUN W P, et al. Protection principle for a DC distribution system with a resistive superconductive fault current limiter [J]. Energies, 2015, 8(6): 4839-4852. DOI: 10.3390/en8064839. [16] 李斌, 何佳伟. 柔性直流配电系统故障分析及限流方法 [J]. 中国电机工程学报, 2015, 35(12): 3026-3036. DOI: 10.13334/j.0258-8013.pcsee.2015.12.013. LI B, HE J W. DC fault analysis and current limiting technique for VSC-based DC distribution system [J]. Proceedings of the CSEE, 2015, 35(12): 3026-3036. DOI: 10.13334/j.0258-8013.pcsee.2015.12.013. [17] 年珩, 孔亮. 直流微电网故障保护技术研究综述 [J]. 高电压技术, 2020, 46(7): 2241-2254. DOI: 10.13336/j.1003-6520.hve.20200472. NIAN H, KONG L. Review on fault protection technologies of DC microgrid [J]. High voltage engineering, 2020, 46(7): 2241-2254. DOI: 10.13336/j.1003-6520.hve.20200472. [18] MARTÍNEZ-PARRALES R, FUERTE-ESQUIVEL C R, ALCAIDE-MORENO B A, et al. A VSC-based model for power flow assessment of multi-terminal VSC-HVDC transmission systems [J]. Journal of modern power systems and clean energy, 2021, 9(6): 1363-1374. DOI: 10.35833/MPCE.2021.000104. [19] 钟庆, 马新华, 王钢, 等. 电压源型换流器稳态等值电路模型 [J]. 高电压技术, 2014, 40(8): 2485-2489. DOI: 10.13336/j.1003-6520.hve.2014.08.032. ZHONG Q, MA X H, WANG G, et al. Static equivalent circuit models of voltage source converter [J]. High voltage engineering, 2014, 40(8): 2485-2489. DOI: 10.13336/j.1003-6520.hve.2014.08.032. [20] 赵雨童, 高飞, 张博深. 基于交流电流下垂特性控制的VSC建模和稳定性分析 [J]. 电力自动化设备, 2021, 41(5): 50-55. DOI: 10.16081/j.epae.202105034. ZHAO Y T, GAO F, ZHANG B S. Modeling and stability analysis of VSC with droop characteristic based on AC current [J]. Electric power automation equipment, 2021, 41(5): 50-55. DOI: 10.16081/j.epae.202105034. [21] 魏承志, 李明, 李春华, 等. 基于两电平电压源型换流器的直流微网交直流侧接地方式 [J]. 南方电网技术, 2021, 15(2): 116-123. DOI: 10.13648/j.cnki.issn1674-0629.2021.02.015. WEI C Z, LI M, LI C H, et al. Grounding modes at AC and DC side of DC microgrid with two-level voltage source converters [J]. Southern power system technology, 2021, 15(2): 116-123. DOI: 10.13648/j.cnki.issn1674-0629.2021.02.015.