Active injection protection scheme for flexible HVDC grids based on amplitude of input impedance

High-voltage direct current(HVDC)grids require fast and reliable protection of the DC lines.The performance of traditional protection schemes is easily impaired by the limitations of the boundary condition and nonlinearity from the control of converters.One of the key technologies for flexible HVDC grids is the half-bridge modular multilevel converter(HB-MMC).Considering the high controllability of HB-MMC,this study proposes an active injection protection scheme to improve the reliability and sensitivity of the HVDC grid protection.The HB-MMC is used to inject a sinusoidal characteristic signal,at the specified frequency,into the DC lines.Then,the voltage and current at the specified frequency are extracted using the Prony algorithm to calculate the input impedance,which is used for the identification of internal and external faults.The active injection protection scheme was simulated for various cases in the simulation software Power Systems Computer Aided Design.The simulation results indicate that the proposed protection scheme is highly reliable and can overcome transition resistance.

Research on a Multiport Parallel Type Hybrid Circuit Breaker for HVDC Grids:Modeling and Design

This paper proposes a star type multiport hybrid circuit breaker(Star-HCB)topology for protection of multiterminal DC transmission.Reliability and stability of high voltage DC(HVDC)grids are determined by their capabilities to withstand DC-side faults.In order to maintain reliability of HVDC grids,both ends of each line should be equipped with hybrid circuit breakers(HCB).This method will increase expenditure of the HVDC,especially the meshed topology.The n-port Star-HCB consists of ultra-fast mechanical disconnectors,load current switch and only one transferring branch which is formed by improved half-bridge sub-module.Compared with existing traditional hybrid circuit breakers and other multiport hybrid circuit breakers,the proposed topology can realize the same short-circuit blocking goal using fewer components.Detailed mathematical transient process calculation and timedomain simulation of the proposed Star-HCB are given to verify its superiority.

双极型LCC-HVDC直流系统作为黑启动电源的启动方法及策略

科学合理的黑启动电源及方案是应对电网灾变停电的最有效策略,可加快其系统恢复进程,大幅减少停电损失。双极型换相换流器型高压直流系统(LCC-HVDC)具有调节灵活输电容量大的优势,但其作为黑启动电源存在一定的困难。提出了以双极型LCC-HVDC为黑启动电源的黑启动方法及完整过程,实现向受端无源网络恢复供电,以加速其恢复进程。首先给出了双极型LCC-HVDC作为黑启动电源时所面临的问题及相应的解决策略;进而给出了双极型LCC-HVDC作为黑启动电源恢复向受端无源网络恢复供电的实现过程。该过程包括2个阶段,第1阶段实现双极型LCC-HVDC的仿融冰模式启动;第2阶段为从仿融冰模式过渡到向受端无源网络恢复供电过程。算例证实了该启动方法的有效性和正确性。该方法使双极型LCC-HVDC具备了黑启动能力,并利用直流系统为受端电网的系统恢复过程提供了电压和频率支撑。

A DC Current Flow Controller for Meshed Modular Multilevel Converter Multiterminal HVDC Grids

This paper proposes the design of a novel DC current flow controller(CFC)and evaluates the control performance of balancing and regulating the DC branch currents using the DC CFC in a meshed multi-terminal HVDC(MTDC)grid.The DC CFC consists of two identical full bridge DC-DC converters with the capacitors of the two converters being connected in parallel.The scalability of the DC CFC is easily achievable due to the identical bridge converter topology;the cost of this DC CFC is also relatively low due to its simple physical structure and low voltage ratings.The control performance of the DC CFC is tested on a meshed 3-terminal(3-T)HVDC grid,which is based on modular multilevel converters(MMC).The DC branch current control in the meshed MTDC grid is achieved using the proposed control strategy of the DC CFC,and is verified through case studies on the real-time digital simulator(RTDS).

Frequency domain based DC fault analysis for bipolar HVDC grids

This paper proposes a frequency domain based methodology to analyse the influence of High Voltage Direct Current(HVDC) configurations and system parameters on the travelling wave behaviour during a DC fault. The method allows us to gain deeper understanding of these influencing parameters. In the literature, the majority of DC protection algorithms essentially use thefirst travelling waves initiated by a DC fault for fault discrimination due to the stringent time constraint in DC grid protection. However, most protection algorithms up to now have been designed based on extensive time domain simulations using one specific test system. Therefore, general applicability or adaptability to different configurations and system changes is not by default ensured, and it is difficult to gain in-depth understanding of the influencing parameters through time domain simulations. In order to analyse the first travelling wave for meshed HVDC grids, voltage and current wave transfer functions with respect to the incident voltage wave are derived adopting Laplace domain based component models. The step responses obtained from the voltage transfer functions are validated by comparison against simulations using a detailed model implemented in PSCADTM. Then, the influences of system parameters such as the number of parallel branches, HVDC grid configurations and groundings on the first travelling wave are investigated by analysing the voltage and current transfer functions.

Application of new directional logic to improve DC side fault discrimination for high resistance faults in HVDC grids

This paper proposes a simple and fast way to determine the direction of a fault in a multi-terminal high voltage direct current(HVDC) grid by comparing the rate of change of voltage(ROCOV) values at either side of the di/dt limiting inductors at the line terminals. A local measurement based secure and fast protection method is implemented by supervising a basic ROCOV relay with a directional element. This directional information is also used to develop a slower communication based DC line protection scheme for detecting high resistance faults. The proposed protection scheme is applied to a multi-level modular converter based three-terminal HVDC grid and its security and sensitivity are evaluated through electromagnetic transient simulations. A methodology to set the protection thresholds considering the constraints imposed by the breaker technology and communication delays is also presented. With properly designed di/dt limiting inductors,the ability of clearing any DC transmission system fault before fault currents exceeds a given breaker capacity is demonstrated.

A Modular DC/DC Photovoltic Generation System for HVDC Grid Connection

A modular DC/DC conversion system with distributed MPPT and centralized step-up converter for photovoltaic energy integrated into HVDC grids is proposed in this paper.The conversion system consists of two power stages,with MPPT converter as the first stage and a step-up converter as the second stage.Both stages are modular structures.For the distributed MPPT stage,interleaved boost topology is utilized to effectively reduce the input and output ripples without adding extra components.For the centralized step-up stage,narrow-switching-frequency-variation LLC topology is employed,with the modules being configured as input-parallel-output-series structure.Full-range soft-switching property is achieved in the LLC stage to minimize switching losses.Theoretical analysis is carried out for the system voltage gain and design principles.Simulation and experimental results of a 3kW prototype system are presented to verify the theoretical analysis of the proposed system.

Review and Outlook of HVDC Grids as Backbone of Transmission System

HVDC technology has undergone many major developments in the past decades,resulting in higher power ratings,increased efficiency,and the availability of effective means for HVDC grid protection.These developments have made overlay HVDC grids a viable option to shift towards a carbon-free power system,by enabling optimal use of renewable resources.In particular,overlay HVDC grids greatly increase the prospect of building(trans-)continental supergrids to facilitate global economic development.However,overlay HVDC grids still encounter challenges due to the distance and amount of power involved.This paper focuses on analyzing the readiness of the current technologies and the challenges associated with overlay HVDC grids.An in-depth analysis is carried out to evaluate the applicability of current technologies for overlay HVDC grids.Based on the review of recent research and development efforts,the gaps and challenges towards the realization of a global HVDC grid are summarized.

Research and application of a power-flow-calculation method in multiterminal VSC-HVDC power grid

For demonstrating a multiterminal voltage-source converter(VSC)-based high-voltage DC(HVDC)(VSCHVDC) project, this study puts forward a technical route for calculating the power flow in a 500-kV VSC-HVDC power grid in comparison with that of an AC power grid. The Jacobian matrix used in the power-flow calculation was deduced through methods such as Newton–Laphson iteration and Taylor series expansion. Further, the operation effect of powerflow calculation on a true bipolar VSC-HVDC power grid was analyzed briefly. The elements of the Jacobian matrix corresponding to VSC were studied under the mode of droop control and the control strategy of VSC-HVDC power grid was analyzed in detail. The power-flow calculation model for VSC-HVDC power grid of the master–slave control mode was simplified using the PQ decomposition method of the power-flow calculation of an AC power grid. Moreover, a four-terminal model of the Zhangbei VSC-HVDC demonstration project was established and tested on MATLAB. The simulation results under two kinds of operating conditions were analyzed and compared to the results of BPA; the deviation between the power-flow results was studied. The results show that the proposed calculation method can provide a feasible support for calculating the power flow in VSC-HVDC grids.

A modular multilevel power flow controller for meshed HVDC grids

Based on the comparison of existing power flow controllers(PFC)in meshed HVDC grids,the full-bridge modular multilevel converter based PFC(MMPFC)is proposed.At first,the general branch current calculation method of meshed HVDC grids with the PFC is presented,and then,the issue of over-voltage on the thyristor based PFC is described and analyzed.Through the analysis of different operating modes of the full-bridge sub-module,the mechanism of over-voltage ride through of the MMPFC is indicated.The control strategy of the MMPFC,which is used to control branch current and keep capacitor voltage balancing,is elaborated.Finally,the performance on current regulation,bidirectional operation and over-voltage ride through is simulated and verified in a built model with PSCAD/EMTDC.

Pole-to-ground Fault Analysis for HVDC Grid Based on Common-and Differential-mode Transformation

Pole-to-ground(PTG) fault analysis is of vital importance for high-voltage direct current(HVDC) grid. However, many factors are not considered in the existing studies such as the asymmetrical property of PTG fault, the coupling issue between DC transmission lines and the complexity of the structure of DC grid. This paper presents a PTG fault analysis method, which is based on common-and differential-mode(CDM)transformation. Similar to the symmetrical component method in AC system, the transformation decomposes the HVDC grid into CDM networks, which is balanced and decoupled. Then, a transfer impedance is defined and calculated based on the impedance matrices of the CDM networks. With the transfer impedance, analytical expressions of fault characteristics that vary with space and time are obtained. The proposed PTG fault analysis method is applicable to arbitrary HVDC grid topologies,and provides a new perspective to understand the fault mechanism. Moreover, the analytical expressions offer theoretical guidance for PTG fault protection. The validity of the proposed PTG fault analysis method is verified in comparison with the simulation results in PSCAD/EMTDC.

Operational concepts of HVDC in the context of security assessment in the German transmission grid

Because of the realization of HVDC connections parallel to the synchronous grid in Germany, operational concepts become more important in the context of system security aspects. In this article commonly known concepts are analyzed regarding their suitability for the German transmission system. Therefore, an approach for the analysis was developed and exemplary results are given using this approach. Under consideration of the overall system, the concepts of reducing losses in combination with minimizing(n-1)-warnings or-indications^1 as well as the sole minimization of(n-1)-warnings and-indications are the most reasonable approaches. However, under grid security aspects best results can be achieved by reducing operational limits and therefore, by the minimization of(n-1)-warnings.

Nyquist Stability Analysis and Capacitance Selection for DC Current Flow Controllers in Meshed Multi-terminal HVDC Grids

Controllability of DC current/power flow is essentialin multi-terminal HVDC (MTDC) grids, particularly for theMTDC grids in a meshed topology. In this paper, consideringmeshed MTDC (M2TDC) grids with the installation of twoline/multi-lineDC current flow controllers (CFCs), a small-signalmodel of the DC CFCs integrated M2TDC grids is derived,studying the impact of the power losses of the DC CFC andtheir influence on the analysis of energy exchanges. The systemstability analysis is analysed using the Nyquist diagram, which ismore suitable for analyzing complex nonlinear systems with morecompact and reliable indicators of stability in comparison withgain/phase margins shown in the Bode diagram. In addition, aselection method of the interconnected capacitor of the DC CFCis proposed under different operating conditions. The impact ofthe switching frequencies of the DC CFC on the control ranges ofthe DC current flows is analyzed. The effectiveness of the Nyquistanalysis and the capacitance selection method is verified bysimulation studies using PSCAD/EMTDC. The obtained control ranges of the DC CFC with different switching frequenciesand capacitances would be useful for practical engineeringapplications.

PLL-GFC型MMC-HVDC暂态同步稳定性研究

PLL-GFC(基于锁相环的构网型换流器)具备频率与电压调节能力,然而目前对其接入高压交流电网后的暂态同步稳定性缺乏深入了解。为此,对PLL-GFC型MMC-HVDC(模块化多电平换流器型高压直流输电)暂态同步稳定机理展开研究。首先,介绍PLL-GFC型MMC-HVDC控制策略并分析其功率响应特性。然后,建立MMC-HVDC并网系统暂态同步稳定模型,采用相图曲线方法分析电网故障后换流器失稳机理以及电网电流动态变化对其暂态同步稳定性的影响。研究发现稳定平衡点的存在是换流器并网系统故障后暂态稳定的关键因素,由此提出通过动态调节有功电流指令值来提升暂态同步稳定性的增强措施。最后,通过PSCAD/EMTDC搭建电磁暂态仿真模型,验证了失稳机理分析的正确性与增强措施的有效性。

大容量光伏电站经MMC-HVDC并网的谐振分析与抑制

大容量光伏电站经MMC-HVDC送出可解决新能源消纳问题,但大容量光伏电站经MMC-HVDC并网的谐振问题仍待解决。考虑到大容量逆变器具有低开关频率的特点,故进行LCL滤波器设计,建立光伏电站和换流站的频域小信号阻抗模型;再基于阻抗分析法,分析大容量光伏电站经MMC-HVDC并网的谐振机理及潜在的谐振点,发现大容量光伏电站经MMC-HVDC并网时存在谐振风险。针对互联系统谐振问题,采用无源阻尼重塑互联系统阻抗的方案,并明确无源阻尼、相位裕度和系统损耗的对应关系。所提方案能够显著抑制互联系统的谐振现象,为大容量光伏电站经MMC-HVDC并网的稳定性问题提供一种符合实际工程的解决方法。最后基于Matlab/Simulink建立仿真模型,仿真结果验证了文中理论分析的正确性和所提措施的有效性。

远海风电DRU-HVDC送出系统构网控制与启动方法综述

随着海上风电开发的不断规模化和远海化,海上风电直流送出技术成为研究热点。针对低成本的二极管整流单元(DRU)型高压直流(HVDC)输电,重点指出了分布式DRU在均压、部分退出运行等方面的主要技术特点,论述了该技术与柔性直流送出、传统直流输电的差异。由于DRU的不可控性,海上电网的构建须由风电机组通过自身控制实现。因此,对现有研究的风电机组集中式和分散式构网控制进行详细综述,并对全构网型控制采用P-V和Q-f功率控制机理进行风电场-DRU层面以及风电机组层面的梳理解释。针对DRU单向潮流引起的启动问题,综述了辅助中压交流脐带电缆、辅助模块化多电平换流器、海上风电机组配置储能以及辅助低压直流电缆等启动方法,指出利用辅助中压交流脐带电缆是相对成熟的方案,而利用海上风电机组储能和全构网型控制具有较高的灵活性,但现有技术下仍面临风电机组塔筒内部安装空间不足、储能安全隐患等问题。最后,对海上风电经DRU-HVDC送出系统未来值得深入探索的研究方向进行了展望。

双馈直驱风机混合接入MMC-HVDC送端系统的次同步振荡特性分析与抑制

双馈/直驱风机混合接入模块化多电平高压直流输电(modular multi-level converter based HVDC,MMC-HVDC)的次同步振荡事件频繁发生,已有双馈或直驱某一种风机与MMC-HVDC交互产生振荡的研究,但鲜有针对双馈/直驱风机混合接入MMC-HVDC的次同步振荡分析与抑制措施研究。该文首先阐述并分析了某双馈/直驱风机混合接入MMC-HVDC后发生次同步振荡实际事件的特征与原因,通过阻抗相量方法研究了双馈/直驱风机频域阻抗的交互影响方式。研究表明,在双馈或直驱风机单独接入MMC-HVDC不存在振荡风险的条件下,双馈和直驱风机并联接入后依然可能存在振荡风险。最后,根据电网调度运行的实际要求,提出了考虑机组启停数量调配的临时抑制振荡策略。

大规模近海风电场VSC-HVDC并网拓扑及其控制

针对大规模近海风电场地理分布上高度分散以及主要采用双馈式或直驱/半直驱式风电机组的特点,提出了相应的电压源型变流器的高压直流(voltage source converter based HVDC,VSC-HVDC)并网传输拓扑结构,并设计了相应的控制策略。为验证所提方案的可行性,利用Matlab/Simulink构建了一个近海风电场的5端口VSC-HVDC并网传输系统,并进行了系列仿真。仿真结果表明,所提VSC-HVDC方案可为大规模近海风电场的并网传输提供优化的解决方案。

具备故障限流和快速故障清除能力的多端口直流断路器拓扑与控制策略

为解决直流电网中直流故障快速清除的技术难题,提出了一种同时具备限流和快速故障清除能力的多端口直流断路器,并设计了启动、限流后开断线路和限流后复位3种控制策略。所提拓扑结构通过限流电路实现断路时刻电流值的大幅下降,通过电感旁路电路实现故障清除所需时间的缩短;所提控制策略通过复用拓扑部分回路实现直流电网对断路器电容的预充电,通过初步故障判断与二次确认方式来优化断路器的动作时序,实现断路时刻电流值的进一步下降。在PSCAD/EMTDC中搭建了三端直流电网仿真模型,对断路器的各种工况进行了仿真验证。结果表明该多端口直流断路器在各工况下均可实现多条直流线路的保护、减小断路时刻电流值和减小故障切除所需时间。

MMC-HVDC联网及孤岛运行状态转换策略

模块化多电平换流器型直流输电系统(modular multilevel converter based high voltage direct current,MMCHVDC)和交流线路可为重要负荷双路供电,因此MMCHVDC需具备在联网运行状态和孤岛运行状态间稳定转换的能力。该文分析了MMC在联网状态和孤岛状态间相互转换的过程,并设计了一种基于本地电气量的MMC控制模式切换策略。之后,对MMC无源供电控制器进行改进,设计了一种无需切换控制模式的MMC下垂控制策略。最后,通过PSCAD仿真对上述2种转换策略进行验证和比较,结果表明2种策略均能使MMC在联网状态和孤岛状态间稳定转换。2种策略各有优缺点,实际应用中MMC需依据具体的控制目标选取合适的策略。