经LCC-HVDC输电送出系统单端混合等值计算

现有电网换相换流器型高压直流输电(line commutated converter based HVDC,LCC-HVDC)输电系统动态等值计算依赖于送受端电压同步实时量测,无法实现经LCC-HVDC输电送出系统的单端暂态等值计算。论文基于直流系统动态相量等值计算框架,提出仅基于逆变侧单端交流电压信息的经LCC-HVDC直流输电送出系统的等值计算方案,论证整流侧准稳态模型+逆变侧动态相量模型的混合等值计算框架的可行性,解决换相失败准确判别等关键问题,仿真对比分析了多场景故障,证明所提出的计算框架在送端交流系统信息缺失的情况下,能实现受端交流线路故障暂态大扰动下经LCC-HVDC输电送出系统响应的准确实时计算。

Multi-Infeed HVDC中不同混合结构的子系统运行特性研究

基于PSCAD/EMTDC仿真软件建立了多馈入高压直流系统的3种不同混合输电结构的子系统,通过连于受端弱交流系统,研究了3种子系统的暂态运行特性,仿真结果表明:与传统高压直流输电系统相比,含VSC的子系统在抑制换相失败、缩短故障恢复时间等方面具有明显的优势。而且,随着电力电子器件的快速发展,VSC的成本逐渐降低,含H-HVDC的多馈入系统将成为以后电网新建和已有电网改造升级的可行方案。

直驱风电场经MMC-HVDC并网送端系统稳定性分析及振荡抑制

基于模块化多电平换流器MMC(modular multilevel converter)的高压直流输电HVDC(high voltage direct current transmission)因具有无源网络支撑等优势而被广泛应用于大容量新能源外送消纳。受电力电子设备交互作用等因素影响,送端系统易发生振荡失稳现象。首先,建立了直驱风电场经MMC-HVDC并网送端系统的小扰动线性化模型,分析了风场有功输出对系统稳定性的影响。然后,建立了MMC及风机并网变流器交流侧dq阻抗模型,从阻抗角度揭示了送端系统振荡失稳机理。进一步,提出了基于MMC交流电压控制外环q轴附加阻尼的振荡抑制策略,可满足系统满功率范围内的运行稳定性要求。最后,基于全比例模型的仿真结果验证了所提振荡抑制策略的有效性。

Multi-infeed HVDC中混合高压直流输电对传统高压直流输电的影响

为分析Multi-infeed HVDC中两种结构的H-HVDC系统对LCC-HVDC系统的影响,引入换相失败免疫因子(CFII)指标定量评估了两种结构的H-HVDC系统对LCC-HVDC系统抵御换相失败能力的影响,基于PSCAD/EMTDC仿真软件建立了两种混合多馈入子系统,研究了两种混合系统在单相、三相接地故障状态下H-HVDC系统对LCC-HVDC系统暂态运行特性的影响。结果表明,VSC-LCC HVDC系统提高了LCC-HVDC系统抵御换相失败的能力;VSC-LCC HVDC系统有效地降低了LCC-HVDC系统的暂态过电压,对故障恢复时间也有一定的改善,提高了LCC-HVDC系统的暂态运行特性。

Large-Scale Renewable Energy Transmission by HVDC: Challenges and Proposals

Renewable energy transmission by high-voltage direct current(HVDC)has attracted increasing attention for the development and utilization of large-scale renewable energy under the Carbon Peak and Carbon Neutrality Strategy in China.High-penetration power electronic systems(HPPESs)have gradually formed at the sending end of HVDC transmission.The operation of such systems has undergone profound changes compared with traditional power systems dominated by synchronous generators.New stability issues,such as broadband oscillation and transient over-voltage,have emerged,causing tripping accidents in large-scale renewable energy plants.The analysis methods and design principles of traditional power systems are no longer suitable for HPPESs.In this paper,the mechanisms of broadband oscillation and transient over-voltage are revealed,and analytical methods are proposed for HPPESs,including small-signal impedance analysis and electromagnetic transient simulation.Validation of the theoretical research has been accomplished through its application in several practical projects in north,northwest,and northeast region of China.Finally,suggestions for the construction and operation of the future renewable-energy-dominated power system are put forward.

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.

Modern Travelling Wave Based Fault Location Techniques for HVDC Transmission Lines

The modern travelling wave based fault location principles for transmission lines are analyzed.In order to apply the travelling wave principles to HVDC transmission lines,the special technical problems are studied.Based on this,a fault locating system for HVDC transmission lines is developed.The system can support modern double ended and single ended travelling wave princi- ples simultaneously,and it is composed of three different parts:travelling wave data acquisition and processing system,communication network and PC based master station.In the system,the fault generated transients are induced from the ground leads of the over-voltage suppression capacitors of an HVDC line through specially developed travelling wave couplers.The system was applied to 500 kV Gezhouba-Nanqiao(Shanghai)HVDC transmission line in China.Some field operation experiences are summarized,showing that the system has very high reliability and accuracy,and the maximum location error is about 3 km(not more than 0.3%of the total line length). Obviously,the application of the system is successful,and the fault location problem has finally been solved completely since the line operation.

Multi-objective partition planning for multi-infeed HVDC system

The close proximity and the necessity of coordination between multiple high-voltage direct currents(HVDCs)raise the issue of grid partitioning in multi-infeed HVDC systems.A multi-objective partition strategy is proposed in this paper.Several types of relationships to be coordinated and complemented are analyzed and formulated using quantitative indices.According to the graph theory,the HVDC partition is transformed into a graph-cut problem and solved via the spectral clustering algorithm.Finally,the proposed method is validated for a practical multi-HVDC grid,confirming its feasibility and effectiveness.

Operation of offshore wind farms connected with DRU-HVDC transmission systems with special consideration of faults

The diode rectifier unit(DRU)-based high-voltage DC(DRU-HVDC) system is a promising solution for offshore wind energy transmission thanks to its compact design, high efficiency, and strong reliability. Herein we investigate the feasibility of the DRU-HVDC system considering onshore and offshore AC grid faults, DC cable faults, and internal DRU faults. To ensure safe operation during the faults, the wind turbine(WT) converters are designed to operate in either current-limiting or voltage-limiting mode to limit potential excessive overcurrent or overvoltage. Strategies for providing fault currents using WT converters during offshore AC faults to enable offshore overcurrent and differential fault protection are investigated. The DRU-HVDC system is robust against various faults, and it can automatically restore power transmission after fault isolation. Simulation results confirm the system performance under various fault conditions.

Research on the Voltage Interaction of Multi-Infeed HVDC System and Interaction Factor

In multi-infeed HVDC system, the interactions and influences between DC systems AC systems are complex as the electrical distances among DC converter stations which are relatively short. Multi-infeed interaction factor (MIIF) can effectively reflect the interaction among DC systems. The paper theoretically analyzes the impact factors of MIIF like the electrical distances between two DC converter stations and the equivalent impedance of the receiving end AC system. By applying the Kirchhoff’s current law on the inverter AC bus, the paper deduces the analytical expressions for MIIF. From the expression, it is clear how the equivalent impedance of AC system and coupling impedance can affect MIIF. PSCAD simulations validate the effectiveness and the correctness of the proposed expression and some useful conclusions are drawn.

不同锁相环对LCC-HVDC控制回路稳定性的影响

该文研究了电网换相换流器高压直流输电(line commutated converter high voltage direct current,LCC-HVDC)系统逆变侧采用不同锁相环时,锁相环控制回路比例–积分(proportional integral,PI)参数变化对直流控制回路稳定性的影响。首先,分别建立了逆变侧锁相环采用滑动平均滤波(mo ving average filter,MAF)和级联延时消去滤波(cascaded delayed signal cancellation,CDSC)的LCC-HVDC小干扰动态模型,并通过电磁暂态仿真验证了该模型的正确性。其次,基于拉普拉斯变换获得系统定电压控制回路的传递函数,利用奈奎斯特稳定判据以及稳定裕度指标分析不同锁相环对定电压控制回路稳定性的影响,并进行了机理分析,同时在PSCAD/EMTDC的电磁暂态模型上进行了验证。最后,进一步在LCC-HVDC工程模型上对该文所得结论的普适性进行了仿真验证。

Statistics and Analysis on Reliability of HVDC Transmission Systems of SGCC

Reliability level of HVDC power transmission systems becomes an important factor impacting the entire power grid.The author analyzes the reliability of HVDC power transmission systems owned by SGCC since 2003 in respect of forced outage times,forced energy unavailability,scheduled energy unavailability and energy utilization eff iciency.The results show that the reliability level of HVDC power transmission systems owned by SGCC is improving.By analyzing different reliability indices of HVDC power transmission system,the maximum asset benef its of power grid can be achieved through building a scientif ic and reasonable reliability evaluation system.

Reliability Improvement Strategies for HVDC Transmission System

This paper analyses reliability data of HVDC systems of Southern China Power Grid. The weak links of HVDC systems’ operational reliability are DC control and protection, valve hall and valve cooling system and transmission lines. Some improvement measures and HVDC system reliability technology are proposed in this paper.

Novel Adaptive Neural Controller Design Based on HVDC Transmission System to Damp Low Frequency Oscillations and Sub Synchronous Resonance

This paper presents the effect of the high voltage direct current (HVDC) transmission system based on voltage source converter (VSC) on the sub synchronous resonance (SSR) and low frequency oscillations (LFO) in power system. Also, a novel adaptive neural controller based on neural identifier is proposed for the HVDC which is capable of damping out LFO and sub synchronous oscillations (SSO). For comparison purposes, results of system based damping neural controller are compared with a lead-lag controller based on quantum particle swarm optimization (QPSO). It is shown that implementing adaptive damping controller not only improves the stability of power system but also can overcome drawbacks of conventional compensators with fixed parameters. In order to determine the most effective input of HVDC system to apply supplementary controller signal, analysis based on singular value decomposition is performed. To evaluate the performance of the proposed controller, transient simulations of detailed nonlinear system are considered.

Analysis of Transmission Loss in Droop Control of a Multi-Terminal HVDC System

High Voltage Direct Current (HVDC) electric power transmission is a promising technology for integrating offshore wind farms and interconnecting power grids in different regions. In order to maintain the DC voltage, droop control has been widely used. Transmission line loss constitutes an import part of the total power loss in a multi-terminal HVDC scheme. In this paper, the relation between droop controller design and transmission loss has been investigated. Different MTDC layout configurations are compared to examine the effect of droop controller design on the transmission loss.

A Novel Pilot Protection for VSC-HVDC Transmission Lines Based on Correlation Analysis

The control system of voltage source converter HVDC (VSC-HVDC) is complex and its fault tolerance ability is not sufficient, and correct rate of line protection device is not high. A novel pilot protection for VSC-HVDC transmission lines based on correlation analysis is proposed in this paper. In the principle, external fault is equivalent to a positive capacitance model, so the correlation coefficient of the current and voltage derivative is 1;while the internal fault is equivalent to a negative capacitance model, so the correlation coefficient of the current and voltage derivative is -1. Internal faults and external faults can be distinguished by judging the correlation coefficient. Theoretical analysis and PSCAD simulation experiments show that the new principle, which is simple, not affected by transition resistance, control type and line distributed capacitance current, can identify internal faults and external faults reliably and rapidly, having certain practical value.

Traveling Wave Characteristics Based Pilot Protection Scheme for Hybrid Cascaded HVDC Transmission Line

The hybrid cascaded high-voltage direct current(HVDC)transmission system has various operation modes,and some operation modes have sharply increasing requirements for protection rapidity,while the traditional pilot differential protection(PDP)has poor rapidity,and even refuses to operate when faults occur on the DC line.Therefore,a novel pilot protection scheme based on traveling wave characteristics is proposed.First,the adaptability of the traditional PDP applied in engineering is analyzed for different operation modes.Then,the expressions of the forward traveling wave(FTW)and backward traveling wave(BTW)on the rectifier side and the inverter side are derived for different fault locations.From the theoretical derivation,the difference between the BTW and FTW on the rectifier side is less than zero,and the same is true on the inverter side.However,in the event of an external fault of DC line,the difference between the BTW and FTW at nearfault terminal protection installation point is greater than zero.Therefore,by summing over the product of the difference between BTW and FTW of the rectifier side and that of the inverter side,the fault identification criterion is constructed.The simulation results show that the proposed pilot protection scheme can quickly and reliably identify the short-circuit faults of DC line in different operation modes.

MMC-MTDC Transmission System with Partially Hybrid Branches

This paper proposes a hybrid submodule modular multilevel converter(MMC)topology which is suitable for multi terminal direct current(MTDC)transmission systems.Each arm of the proposed MMC topology consists of a half-bridge submodule(HBSM)branch and two parallel full-bridge submodule(FBSM)branches.Comparing with the conventional MTDC transmission system,the proposed topology can selectively block the DC fault current and isolate the corresponding fault line without expensive DC circuit breakers(DCCBs).Thus,the influence range of the DC fault can be reduced and the reliability of the power supply can be improved as well.The corresponding modulation and voltage balancing strategies are developed for the proposed hybrid MMC topology.The feasibility of the proposed topology and control strategy is verified in the MATLAB/Simulink simulation.

异步互联系统准同步运行的HVDC附加控制建模及稳定性分析

异步互联系统调频资源和备用容量的分割使得频率稳定问题突出,利用高压直流输电系统的附加频率控制(AFC)可以实现异步互联系统调频备用共享,参与系统频率调控。文中以改善异步互联系统整体的调频性能和频率质量为目标,研究基于直流两侧电网频率差进行无差控制实现异步互联系统准同步运行的直流AFC对两侧电网频率稳定性的影响。利用换流站交直流系统的功率平衡关系建立较为精确的高压直流输电系统线性化模型,得到异步互联两区域系统的负荷频率控制模型,以此进行特征分析指导AFC参数设计。多场景仿真结果表明,准同步运行的直流AFC在使两侧电网趋于同频运行的同时,能够适应两侧电网的容量自动调节直流功率,有效改善全系统的频率稳定特性。