Wave Protection Effect of Periodic Row of Bottom-Hinged Flap-Type Wave Energy Converters

A flap-type wave energy converter(WEC) is combined with a nearshore breakwater to expand the application of WECs both economically and environmentally. Based on the linear potential theory, an eigenfunction expansion solution is developed for a periodic row of bottom-hinged flap-type WECs exposed to normal waves. Additionally, the viscous effect is considered using the ship rolling solution method with a viscous damping term included in the equation of motion, and the viscous damping expression is also described. The proposed solution is verified by comparison with published literatures. The results including the wave energy conversion efficiency, the reflected and transmitted proportion of the incident wave energy are presented for a range of wave periods and geometric ratios. It is demonstrated that better wave protection effects can be attained with smaller gaps between the WECs, where the transmitted proportion of the incident wave energy is lower. An optimal geometric ratio thus exists for a given wave power absorption and a specific wave period.

Finite Element Method Applied to Fuse Protection Design

In a poloidal field （PF） converter module, fuse protection is of great importance to ensure the safety of the thyristors. The fuse is pre-selected in a traditional way and then verified by finite element analysis. A 3D physical model is built by ANSYS software to solve the thermal- electric coupled problem of transient process in case of external fault. The result shows that this method is feasible.

Research and improvement of leakage protection system in traction motor

Because the leakage protection circuit in traction motor of coal mining machine is impacted by frequency converter devices,malfunctions appear frequently.This paper makes an in-depth analysis,proposes a solution of using the subtraction circuit to offset interference signals and conducts a simulation analysis.The above scheme,which is simple and easily realized,can improve the reliability of leakage protection device in traction motor.

Hybrid Control Strategy for Matrix Converter Fed Wind Energy Conversion System

In this paper, a hybrid control strategy for a matrix converter fed wind energy conversion system is presented. Since the wind speed may vary, output parameters like power, frequency and voltage may fluctuate. Hence it is necessary to design a system that regulates output parameters, such as voltage and frequency, and thereby provides a constant voltage and frequency output from the wind energy conversion system. Matrix converter is used in the proposed solution as the main power conditioner as a more efficient alternative when compared to traditional back-back converter structure. To control the output voltage, a vector modulation based refined control structure is used. A power tracker is included to maximize the mechanical output power of the turbine. Over current protection and clamp circuit input protection have been introduced to protect the system from over current. It reduces the spikes generated at the output of the converter. The designed system is capable of supplying an output voltage of constant frequency and amplitude within the expected ranges of input during the operation. The matrix converter control using direct modulation method, modified Venturini modulation method and vector modulation method was simulated, the results were compared and it was inferred that vector modulation method was superior to the other two methods. With the proposed technique, voltage transfer ratio and harmonic profile have been improved compared to the other two modulation techniques. The behaviour of the system is corroborated by MATLAB Simulink, and hardware is realized using an FPGA controller. Experimental results are found to be matching with the simulation results.

Overvoltage and Insulation Coordination for ±1 100 kV UHVDC Converter Station

In order to specify the overvoltage and insulation level of equipments in ±1 100 kV UHVDC converter stations,the arrester configuration scheme and some basic parameters for ±1 100 kV Zhundong converter station are proposed.Overvoltage of equipments in AC system,valve hall,DC busbar,and neutral busbar are also calculated in accordance with the fundamental principles of arrester configuration for UHV converter stations and the existing experiences in insulation coordination of ±800 kV UHVDC converter stations.The work is done also for specifing insulation levels for ±1 100 kV UHVDC converter stations of the ±1 100 kV UHVDC power transmission project from Zhundong to Sichuan in China.Thus,the recommended insulation margins that determine the insulation levels of converter station equipments are proposed: the insulation margins for thyristor valves are 10%/10%/15% for switching impulse/ lightning impulse/ steer front impulse; the insulation levels of lighting and swtiching impulses are recommended as 2 600 kV and 2 150 kV respectively for 1 100 kV DC buses,and as 2 250 kV and 2 100 kV respectively for equipments at the valve side of high-voltage Y/Y converter transformers.

Control and protection strategy for MMC MTDC system under converter-side AC fault during converter blocking failure

This paper investigates a control and protection strategy for a four-terminal modular multilevel converter(MMC)based high-voltage direct current(HVDC)system under a converter-side AC fault.Based on the system operating condition,a control and protection strategy against the fault with normal blocking of the converter is proposed.In practical,applications encountering such a fault,the MMC at the fault side may experience different conditions of blocking failure.The blocking failures may occur on:①the whole converter;②one converter arm;③one sub-module(SM)/several SMs of one converter arm;④other conditions.The phenomenon of the multi-terminal HVDC(MTDC)system following the fault is analyzed under the first three conditions with real-time simulations using the real-time digital simulator(RTDS).Based on the impact of different conditions on the MTDC system,the necessity of utilizing special control and protection is discussed.A special control and protection strategy is proposed for emergency conditions,and its effectiveness is verified by real-time simulation results.

Optimized Protection Strategies for HVDC Grid with Fault-blocking Modular Multilevel Converters for Overhead Line Applications

The high-voltage direct current(HVDC)grid has been recognized as an effective solution for renewable energy integration.Currently,two main development trends for HVDC grids are being studied:a DC breaker based HVDC grid and fault-blocking converter based HVDC grid.Although the former has a perfect performance for fault clearance,its development is still highly constrained by the cost and maturity of DC breakers.The latter can extinguish DC faults by the fault-blocking converters.Without using DC breakers,there is no bottleneck in its technical feasibility.Nevertheless,in fault scenarios,such types of HVDC grids will be blocked at length for air-deionization,which is its main drawback.The aim of this paper is to minimize its power interruption time,by optimizing protection coordination strategies.To cover the most complex cases,the overhead line applications,in which the reclosure actions are required to be implemented,are considered.In this paper,the protection requirements of HVDC grids are first discussed,then the benefits of fault-blocking modular multilevel converters(MMCs)and their fault features are analyzed.Based on this,a control function is designed to reduce the air-deionization time.To minimize the influence of the DC faults,a separation methodology for restarting the system is proposed.The effectiveness of the proposed protection coordination schemes is validated by PSCAD/EMTDC simulations.

Study of Discharging Characteristics of Hollow Cathode Surge Protective Gap

A hollow cathode surge protective gap （HCSPG） was designed, and the discharge characteristics was investigated in an air and nitrogen gas environment. For both the gap spacing D and the hole diameter Ф of HCSPG of 3 mm, the voltage protective value Up of HCSPG is about 3.5 kV and its converting time tc exceeds 100 ns at an air pressure from 10 Pa to 100 Pa. The maximum converting time tc from glow to arc discharging reaches 1600 ns at an air pressure of 100 Pa, while the minimum converting time tc is 120 ns at 10 Pa. For a triggered HCSPG, Up is reduced to about 1.6 kV while the converting time is 120 ns with a semiconductor trigger device and 50 ns with a dielectric porcelain trigger device under an air pressure of 100 Pa.

基于限流电抗器电压的柔性直流配电线路单端量保护新方法

柔性直流配电技术的理论研究已经步入成熟阶段,其线路保护方法的不完备成为制约该技术发展的重要因素,尤其是以快速动作为优势的单端量保护,耐高阻能力弱、所需采样率高等问题仍待解决。为解决上述问题,文中通过分析单极对称接线系统发生金属性双极短路故障和经过渡电阻故障时限流电抗器电压特征,利用限流电抗器电压及其负变化率积聚和构造保护判据,据此提出了一种基于限流电抗器电压的柔性直流配电线路单端量保护新方法。仿真结果表明,该方法能在0.5 ms内可靠识别区内故障,且耐受过渡电阻(20Ω)和噪声影响,所需采样率要求低,具有工程实用意义。

构网控制目标下系统故障特征及保护性能

分析了构网目标下的典型同步控制及故障穿越策略,提出了构网型变流器在故障分析时的等效电路。在此基础上,在相量平面分析了含构网型变流器系统稳态运行及各类故障工况下的电气量特征,指出构网控制目标下的故障特征与传统同步机系统的异同。重点针对当前电网中主流配置的电流差动保护与距离保护开展构网控制目标下的动作性能评估,并通过构建PSCAD仿真模型验证理论研究结果。分析表明,相比于传统跟网型控制,构网变流器控制技术通过模拟同步发电机的基本特性,为系统中的继电保护创造了一个相对良好的运行环境,有利于提高保护整体动作性能。

对称故障下变流器控制对差动保护影响及优化策略

变流器输出故障特征的强可控性给交流系统继电保护带来了严峻挑战。该文分析并网线路发生对称故障时跟网型变流器的控制响应过程,从故障期间变流器电流控制目标不能自动匹配故障回路功率需求的角度,分析推导变流器网侧电压相位发生跳变并不断旋转的变化特征。受故障回路阻抗约束,变流器输出短路电流相位与网侧电压相位同步变化,造成并网线路差动保护性能严重下降。基于变流器网侧电压相位变化机理,从电流控制目标与锁相环控制两个维度,提出兼顾保护需求和供电需求的变流器控制优化策略。仿真结果表明,并网线路对称故障时所提策略能有效抑制两侧电流相位差,显著提升了差动保护动作性能。

基于二极管整流器的双极型复合式海上换流站及其控制策略

为提高海上换流站的紧凑型性并降低换流站一次性投入成本,西门子公司提出了基于二极管整流器(diode rectifier,DR)的高压直流输电技术,在此基础上,国内外学者对其控制策略及各种串并联拓扑进行了研究,但仍存在结构复杂、可行性差、运行灵活性低等问题。为此,提出了基于二极管整流器的双极型复合式海上换流站拓扑,正负极分别采用模块化多电平换流器(modularmultilevelconverter,MMC)和DR,以充分利用MMC灵活可控和DR体积小、成本低、可靠性高的优点。为降低直流电缆输电损耗,提出了复合式换流站正负极功率平衡控制,将接地极电流控制在零附近,提高了输电效率,可将高压直流电缆所产生的损耗降低67.7%。设计了限功率控制,在发生直流非对称故障期间通过低速通信,可主动对风电场发出功率进行限制,防止了换流站过电流风险,提高了系统可靠性。最后通过PSCAD/EMTDC仿真分析对所提拓扑及其控制策略进行了可行性验证。

120 t转炉热态渣循环利用工艺研究

唐山中厚板材有限公司为提高转炉热态渣的利用率,对转炉冶炼过程炉渣产生机理进行分析,探索热态渣循环利用的途径。试验全留渣双渣法,即第一炉按原单渣法冶炼出钢和不倒渣溅渣护炉,根据铁水硅含量将数据固化在模型中,保证终点二元碱度控制在3.5~3.8,第二炉加入废钢和铁水进行双渣冶炼,出钢后倒出40%~50%炉渣再溅渣护炉,完成一组正常的冶炼。同时,通过优化副枪自动炼钢模型、增强转炉底吹强度、改造双渣氧枪等措施,保证第一炉碱度控制在3.0以上,实现循环渣造渣方法的自动控制。实施热态渣循环利用后,渣中FeO含量降低2.1%,钢铁料消耗降低4.355 kg/t,稳定了低P高端品种钢的生产工艺,提高命中率22.5%。

海上风电柔性直流输电换流变差动保护的适应性分析

海上风电柔性直流输电系统的故障特性有别于常规电力系统,因此常规继电保护方案应用于海上风电柔性直流输电系统时可能存在适应性不足的风险。分析了海上风电柔性直流输电系统故障情形下,不对称的电气量引发的谐波分量以及故障穿越过程中的负序抑制、限流措施对差动保护的影响。推导了计及序分量的差动保护灵敏度计算式并进行适应性分析,指出故障情形下的奇次谐波容易对差动保护产生影响,负序抑制及限流措施也将导致差动保护灵敏度下降。结合实际示范工程的仿真系统进行了大量仿真计算,结果表明,常规差动保护方案的灵敏性、可靠性均存在下降的问题。

电流差动保护在海上风电低频送出线路中的适应性分析

低频输电技术在中远距离海上风电领域应用潜力巨大。电流差动保护是广泛应用的传统交流保护,面对电力电子型电源时其性能可能受到影响,故有必要研究其在海上风电低频输电系统中的适应性。文中基于故障复合序网推导了直驱风机和模块化多电平矩阵换流器的电源特性对故障电流的影响,理论分析了电流差动保护的适应性。为定量分析保护适应性,提出了基于改进节点方程的故障电气量计算方法,计算给定故障条件下的故障电流和保护判据取值,判断保护动作情况。研究表明,单相接地和两相短路时保护灵敏度降低,甚至可能拒动;两相接地和三相短路时,保护能正常动作。最后,建立PSCAD/EMTDC仿真模型验证了理论分析和计算方法的正确性。

一起换流变绕组差动电流保护动作的分析及改进

换流变压器的差动保护动作是否正确对于直流系统安全稳定运行至关重要,由于部分换流站在换流变绕组差动保护设计时未考虑谐波制动功能,保护装置无法正确区分励磁涌流和内部故障电流,导致换流变充电时因励磁涌流造成保护误动作。对此,结合某换流站在换流变绕组差动保护动作案例进行建模与仿真,分析该换流站换流变压器网侧绕组差动保护动作原因,判断其动作逻辑是否正确。根据空载合闸时的绕组差动电流经滤除直流分量后与标准的正弦电流波形相似性很差的特点,通过仿真分析设定了Hausdorff距离的绕组差动电流保护闭锁判据的整定值,进而避免了换流变充电时因CT饱和特性不一致导致的绕组差动保护误动作。

基于MMC主动限流的VSC-HVDC双极短路故障控保协同策略

直流故障保护是柔性直流输电系统发展面临的主要挑战之一,利用模块化多电平换流器(MMC)的灵活控制实现限流已成为当前研究的热点。MMC的主动限流降低了故障电流变化率和故障电流幅值,易引起故障判断以及保护选择困难。文中以对称单极接线的柔性直流系统为研究对象,采用控保协同思想,将主动限流控制与故障识别相结合,提出了基于主动限流因子的故障区域识别与区分方法实现双极短路故障的区域识别与切除。首先,通过在MMC电流内环控制器中附加故障电流目标预设控制器,在故障发生后可快速跟踪故障电流的变化,自适应地调整MMC输出的直流电压将故障电流控制在预设目标参考值附近;其次,根据不同故障位置时故障电流目标预设控制器输出的限流因子快速确定故障区域。最后,在PSCAD/EMTDC中对四端柔性直流输电系统的不同故障场景进行了仿真分析,验证了所提控保协同策略的可行性和有效性。

混合型有载调压变压器拓扑结构及控制策略研究

针对传统有载调压变压器存在功能单一、动态特性差等问题,结合工频变压器高效可靠和电力电子变换器易于控制的优点,提出了一种混合型有载调压变压器拓扑结构。通过增加背靠背变换器,并设计相应控制策略,实现了有载调压变压器连续调压和无功补偿的功能;提出一种串联电压补偿方法,实现了对负载的故障保护。最后,分别基于MATLAB仿真软件和实验平台搭建了混合型有载调压变压器实验模型,仿真和实验结果验证了所提拓扑结构和控制策略的正确性和有效性。

针对海上风电场汇集线路单相接地故障的柔直侧新型距离保护方案

海上风电场汇集线路发生单相接地故障时,线路两侧短路电流分别由风电机组换流器和柔直换流器提供,由于正序、负序与零序网络串联,在传统大电网存在场景下不受控制影响的零序电流也将像正序和负序电流一样呈现幅值受限、相位受控特性,柔直侧距离保护无法正确识别单相接地故障。该文建立海上风电场汇集线路单相接地故障时的复合序网,分析汇集线路两侧故障特性,在不借助线路双端通信的前提下,利用柔直侧负序和零序电流准确求取风机侧零序电流。基于此,构建关于故障距离的方程,提出柔直侧新型距离保护方案,解决了双端均为逆变型电源的线路中零序电流受控导致距离保护性能下降的问题。此外,所提保护方案受风电场无功支撑策略、过渡电阻大小及性质影响小。测试结果表明,该保护方案可以准确识别海上风电场汇集线路区内外单相接地故障。