Fluid-chemical modeling of the near-cathode sheath formation process in a high current broken in DC air circuit breaker

When the contacts of a medium-voltage DC air circuit breaker(DCCB) are separated, the energy distribution of the arc is determined by the formation process of the near-electrode sheath. Therefore, the voltage drop through the near-electrode sheath is an important means to build up the arc voltage, which directly determines the current-limiting performance of the DCCB. A numerical model to describe the near-electrode sheath formation process can provide insight into the physical mechanism of the arc formation, and thus provide a method for arc energy regulation. In this work, we establish a two-dimensional axisymmetric time-varying model of a medium-voltage DCCB arc when interrupted by high current based on a fluid-chemical model involving 16 kinds of species and 46 collision reactions. The transient distributions of electron number density, positive and negative ion number density, net space charge density, axial electric field, axial potential between electrodes, and near-cathode sheath are obtained from the numerical model. The computational results show that the electron density in the arc column increases, then decreases, and then stabilizes during the near-cathode sheath formation process, and the arc column's diameter gradually becomes wider. The 11.14 V–12.33 V drops along the17 μm space charge layer away from the cathode(65.5 k V/m–72.5 k V/m) when the current varies from 20 k A–80 k A.The homogeneous external magnetic field has little effect on the distribution of particles in the near-cathode sheath core,but the electron number density at the near-cathode sheath periphery can increase as the magnetic field increases and the homogeneous external magnetic field will lead to arc diffusion. The validity of the numerical model can be proven by comparison with the experiment.

固态变压器中隔离双向DC/DC变换器扩展移相下虚拟电流控制策略

该文以固态变压器后级隔离双向DC/DC变换器为重点研究对象,提出一种扩展移相下虚拟电流控制策略。通过分析后级变换器在扩展移相传输功率下相移量关系以获得最小电流应力最优解来提高变换器的传输效率,减少器件损耗。同时通过虚拟电流控制思想在线估算传输功率来提高系统动态响应性能,减少电流传感器使用。最后,对该文所提出的扩展移相下虚拟电流控制与扩展移相控制进行实验对比分析,验证了扩展移相下虚拟电流控制策略具有降低电流应力,显著提高变换器的动态响应的优点。

一种交叉倍压型高增益DC/DC变换器

针对光伏发电、燃料电池发电等领域对高增益直流变换器的需求,以两相交错并联升压变换器为研究对象,由2个含有电感的倍压单元组合设计出实现电压提升的交叉倍压结构,据此提出了一种新颖的交叉倍压型高增益DC/DC变换器。该变换器可实现(3n+4)/(1-d)倍的高电压增益(1∶n为耦合电感匝数比,d为变换器占空比),且具有电路器件的低电压应力特性。对于漏感引起的开关管电压尖峰问题,引入了钳位电容构成释放漏感能量通道,同时提升了输出电压。介绍了新型交叉倍压型高增益变换器的拓扑结构,分析了变换器各模态的工作过程,推导了电压增益、输入电流纹波及各器件电压应力等稳态特性,并搭建样机进行实验研究,验证了该直流变换技术方案的可行性和先进性。

单级三相谐波电流注入可升降压AC/DC变换器

为适应电动车充电电压宽范围变化需求,本文针对传统充电桩前级整流器拓扑开展研究,提出一种新式单级三相AC/DC变换器。该变换器集成了三相桥式不控整流电路与Cuk模块,为降低输入侧电流谐波分量,选用双向开关管构建谐波电流回馈通路,整体构造成单级式拓扑结构。文章阐述了变换器的电路结构及工作模态,通过建立变换器的等效模型推导出电压增益。变换器通过简单的控制逻辑即可实现正弦输入电流并拥有高功率因数。最后通过搭建变换器数字控制实验平台验证了所提变换器的可行性。

模块化高压大容量DC/DC变换器综述

随着新能源发电占比增加和电网规模的不断扩大,直流输电系统的容量不断提升,含有不同电压等级的直流电网是未来发展的必然趋势。在此背景下,DC/DC变换器作为直流电网关键设备,在多电压等级互联场景中发挥着重要作用。文中首先对已有的模块化高压大容量DC/DC变换器进行归纳、分类和比较,然后阐述了目前模块化高压大容量DC/DC变换器的拓扑结构、工作原理与特点,并针对其特点提出了不同变换器的适用场景,最后对未来的研究方向提出了建议。

Study on Earth Surface Potential and DC Current Distribution around DC Grounding Electrode

DC magnetic biasing problem,caused by the DC grounding electrode, threatened the safe operation of AC power grid. In this paper, the characteristics of the soil stratification near DC grounding electrode was researched. The AC-DC interconnected large-scale system model under the monopole operation mode was established. The earth surface potential and DC current distribution in various stations under the different surface thickness was calculated. Some useful conclusions are drawn from the analyzed results.

Operation Control Method of Relay Protection in Flexible DC Distribution Network Compatible with Distributed Power Supply

A novel operation control method for relay protection in flexible DC distribution networks with distributed power supply is proposed to address the issue of inaccurate fault location during relay protection,leading to poor performance.The method combines a fault-tolerant fault location method based on long-term and short-term memory networks to accurately locate the fault section.Then,an operation control method for relay protection based on adaptive weight and whale optimization algorithm(WOA)is used to construct an objective function considering the shortest relay protection action time and the smallest impulse current.The adaptive weight and WOA are employed to obtain the optimal strategy for relay protection operation control,reducing the action time and impulse current.Experimental results demonstrate the effectiveness of the proposed method in accurately locating faults and improving relay protection performance.The longest operation time is reduced by 4.7023 s,and the maximum impulse current is limited to 0.3 A,effectively controlling the impact of large impulse currents and enhancing control efficiency.

Simulation on DC Current Distribution in AC Power Grid under HVDC Ground-Return-Mode

This paper focus on the Modeling and Calculation of DC current distribution in AC power grid induced under HVDC Ground-Return-Mode. Applying complex image method and boundary element method, a new field-circuit coupling model was set up. Based on the calculation result with complex image method, this paper derived the modification factor for induced earth potential from practical measurement, which increased the accuracy of calculation. The modification method is helpful for evaluation on the effect of means used for blocking the dc-bias current in transformer neutral and also useful for the forecast of the DC current distribution when the power grid is in different line connection mode. The DC distribution character in Guangdong power grid is shown and suggestion is proposed that the mitigation of dc-bias should start from those substations whose earth-potential is highest.

Line Fault Detection of DC Distribution Networks Using the Artificial Neural Network

ADC distribution network is an effective solution for increasing renewable energy utilization with distinct benefits,such as high efficiency and easy control.However,a sudden increase in the current after the occurrence of faults in the network may adversely affect network stability.This study proposes an artificial neural network(ANN)-based fault detection and protection method for DC distribution networks.The ANN is applied to a classifier for different faults ontheDC line.The backpropagationneuralnetwork is used to predict the line current,and the fault detection threshold is obtained on the basis of the difference between the predicted current and the actual current.The proposed method only uses local signals,with no requirement of a strict communication link.Simulation experiments are conducted for the proposed algorithm on a two-terminal DC distribution network modeled in the PSCAD/EMTDC and developed on the MATLAB platform.The results confirm that the proposed method can accurately detect and classify line faults within a few milliseconds and is not affected by fault locations,fault resistance,noise,and communication delay.

Stability Analysis and Control of DC Distribution System with Electric Vehicles

The DC distribution network system equipped with a large number of power electronic equipment exhibits weak damping characteristics and is prone to low-frequency and high-frequency unstable oscillations.The current interpretation of the oscillation mechanism has not been unified.Firstly,this paper established the complete statespace model of the distribution system consisting of a large number of electric vehicles,characteristic equation of the distribution network system is derived by establishing a state-space model,and simplified reduced-order equations describing the low-frequency oscillation and the high-frequency oscillation are obtained.Secondly,based on eigenvalue analysis,the oscillation modes and the influence of the key system parameters on the oscillation mode are studied.Besides,impacts of key factors,such as distribution network connection topology and number of dynamic loads,have been discussed to suppress oscillatory instability caused by inappropriate design or dynamic interactions.Finally,using the DC distribution example system,through model calculation and time-domain simulation analysis,the correctness of the aforementioned analysis is verified.

Simulation Study of the Control Strategy of a DC Inverter Heat Pump Using a DC Distribution Network

Photovoltaics,energy storage,direct current and flexibility(PEDF)are important pillars of achievement on the path to manufacturing nearly zero energy buildings(NZEBs).HVAC systems,which are an important part of public buildings,play a key role in adapting to PDEF systems.This research studied the basic principles and operational control strategies of a DC inverter heat pump using a DC distribution network with the aim of contributing to the development and application of small DC distribution systems.Along with the characteristics of a DC distribution network and different operating conditions,a DC inverter heat pump has the ability to adapt to changes in the DC bus voltage and adds flexibility to the system.Theoretical models of the DC inverter heat pump integrated with an ice storage unit were developed.The control strategies of the DC inverter heat pump system considered the influence of both room temperature and varied bus voltage.A simulation study was conducted using MATLAB&Simulink software with simulation results validated by experimental data.The results showed that:(1)The bus fluctuation under the rated working voltage had little effect on the operation of the unit;(2)When the bus voltage was fluctuating from 80%-90%or 105%-107%,the heat pump could still operate normally by reducing the frequency;(3)When the bus voltage was less than 80%or more than 107%,the unit needed to be shut down for the sake of equipment safety,so that the energy storage device could adjust to the sharp decrease or rise of voltage.

基于双重移相控制的双向全桥DC-DC变换器最小回流功率分段优化控制

双向全桥DC-DC变换器以其重量轻、高功率密度、能量双向流动等优点成为直流微电网中不可或缺的一部分。但双向全桥DC-DC变换器在传统的单重移相控制下存在回流功率和电流应力等问题,尤其当输入电压和输出电压不匹配时,回流功率和电流应力会显著增加。针对输入电压和输出电压不匹配的情况,该文提出了一种双向全桥DC-DC变换器在双重移相下的最小回流功率控制策略。该控制策略通过对回流功率进行分段优化,得到了变换器在不同传输功率范围下的最优移相角。通过将所提控制策略和传统的双重移相控制进行对比分析,发现该文所提控制策略具有更小的回流功率和电流应力,提升了变换器的效率。最后基于所提控制策略搭建了实验样机,实验结果验证了控制策略的正确性和有效性。

基于占空比主动调节的钳位开关电容型DC/DC解耦控制策略

钳位开关电容型DC/DC(Clamping Switched Capacitors-based DC/DC,CSC)是构建具有直流故障自清除能力直流变压器的有效模块。由于传统控制策略下的CSC中直流电压与电容电压存在着耦合效应,直流电压的变化将直接引起电容电压与交流环节电压幅值的变化,使得CSC在两端直流电压不匹配时可能发生软开关丢失、电流应力增大、功率损耗增加等运行性能下降的现象。鉴于此,为改善CSC在直流电压不匹配时的开关及效率性能,提出了一种基于可变占空比的CSC电容电压解耦控制策略。该策略可以在CSC的整个运行范围内保证交流电压幅值的自动匹配,对于改善CSC的整体运行性能具有明显效果。基于试验样机,验证了所提控制策略的正确性与有效性。

含主动限流控制的MMC-HVDC电网直流短路故障电流解析计算

传统的MMC-HVDC电网直流短路故障电流计算一般采用等效RLC电路方法。但是该方法没有考虑主动限流控制(ACLC)对等效电路RLC参数的影响,因此无法直接用于含ACLC的MMC-HVDC电网的直流短路故障电流的解析计算问题。该文通过将ACLC的控制效果等效为虚拟阻抗产生的压降,建立了体现ACLC的控制影响的等效RLC电路,以包含电流变化率限流环节的典型ACLC算法为例,推导了直流双极短路故障条件下ACLC的控制参数与虚拟阻抗的映射关系以及相应的直流故障电流解析表达式。最后,以基于含ACLC的半桥型四端MMC-HVDC电网为例,建立PSCAD/EMTDC仿真模型,对采用ACLC的直流双极短路故障电流解析计算结果进行了验证。结果表明:故障后10ms内直流短路故障电流的PSCAD仿真结果与所提出的含ACLC的等效RLC电路解析计算结果的平均误差在8.29%以内,最大误差在10.99%。因此,使用所提出的方法具有工程实用性。

DCS控制器中电表接触传感器计量误差检测方法

由于DCS控制器中电表传感器在计量检测过程中,传统的B-MAC-DCS协议能耗和丢包率较高,无法缓解汇聚节点的漏斗效应,导致在远程抄表过程中传感器计量误差增大。提出一种机械电表接触传感器计量误差检测方法。采用小波基函数对DCS控制器中的接触传感器计量数据抗干扰处理,并通过动态选取阈值的方法,对经过小波变换后的数据去除噪声。使用低功耗自适应集簇分层型(low energy adaptive clustering hierarchy,LEACH)协议分簇代替传统的B-MAC协议;根据簇内监测值,引入阈值分析方法获取传感器计量指标,并将其作为判定依据进行误差检测,根据计量指标的变化情况判断是否存在计量误差。实验结果表明,所提方法可以准确且有效检测出机械电表接触传感器计量误差,解决DCS中机械电表的运行隐患问题。

Optimal Cooperative Secondary Control for Islanded DC Microgrids via a Fully Actuated Approach

DC-DC converter-based multi-bus DC microgrids(MGs) in series have received much attention, where the conflict between voltage recovery and current balancing has been a hot topic. The lack of models that accurately portray the electrical characteristics of actual MGs while is controller design-friendly has kept the issue active. To this end, this paper establishes a large-signal model containing the comprehensive dynamical behavior of the DC MGs based on the theory of high-order fully actuated systems, and proposes distributed optimal control based on this. The proposed secondary control method can achieve the two goals of voltage recovery and current sharing for multi-bus DC MGs. Additionally, the simple structure of the proposed approach is similar to one based on droop control, which allows this control technique to be easily implemented in a variety of modern microgrids with different configurations. In contrast to existing studies, the process of controller design in this paper is closely tied to the actual dynamics of the MGs. It is a prominent feature that enables engineers to customize the performance metrics of the system. In addition, the analysis of the stability of the closed-loop DC microgrid system, as well as the optimality and consensus of current sharing are given. Finally, a scaled-down solar and battery-based microgrid prototype with maximum power point tracking controller is developed in the laboratory to experimentally test the efficacy of the proposed control method.

一种单输入双输出高增益DC-DC变换器

随着分布式发电在中低压场合的应用越来越广泛,对高增益直流变换器的需求日益增加。提出一种新型单输入双输出高增益DC-DC变换器。该变换器在传统Boost电路基础上引入电容和二极管组成极性反向极,功率端口通过差分连接方式互联,提高了变换器电压增益,降低了开关管电压应力;同时端口功率控制彼此独立,进一步提高了电能路由的灵活性。详细分析了该变换器的电路结构和工作原理,推导出稳态特性下的电压增益、主要开关器件电气应力以及电流纹波等表达式。在此基础上,提出了基于矢量时间作用的模型预测控制策略,建立了该变换器预测模型、推导出开关管矢量时间以及设计了目标函数寻优过程。最后,通过仿真和实验验证了所提变换器及其控制策略不仅可行、有效,而且具有高运行效率与低电压应力。

DCS在电除尘系统自动控制的应用研究

传统的电除尘器自动控制系统采用可编程逻辑控制器(PLC)控制,其逻辑复杂、可读性差、可维护性低。PLC系统无法与主控系统完成一体化,导致工艺上的割裂,额外地增加了运维成本。本文提出了一种基于分散控制系统(DCS)的电除尘器自动控制方案。

DCS系统在燃气输配控制中的应用研究进展

为探讨DCS系统在燃气输配控制中的应用,并分析其通过实时监测和控制关键参数来提高系统效率和安全性的能力。采用DCS系统对天然气输配网络进行实时监测和控制。通过对天然气管道的压力、温度、流量等关键参数进行实时采集和调节,确保系统运行在稳定、高效的状态。同时,利用DCS系统的强大数据采集、处理和分析能力,对输配过程中的故障和异常情况进行快速诊断和处理,以提高故障排除的效率。此外,DCS系统还支持网络化和远程控制功能,实现对不同地理位置的天然气输配网络的集中监管和管理。结果表明:通过应用DCS系统,燃气输配控制实现了高效、准确和可靠的管理。实时监测和调节关键参数确保了系统运行的安全性和稳定性。快速诊断和处理故障和异常情况提高了故障排除的效率。网络化和远程控制功能实现了对不同地理位置的天然气输配网络的统一监管和管理,提高了管理效率。DCS系统在燃气输配控制中的应用具有重要意义和潜力。通过实时监测和控制关键参数,可以提高系统的效率和安全性。然而,该系统的应用也面临一些挑战,如高成本、技术难题以及应急处理等方面的问题。因此,需要进一步的技术研发和应用实践来不断提升DCS系统的性能和可靠性,以更好地满足燃气输配控制的需求。

三电平混合全桥DC-DC变换器全局电流应力优化与回流功率优化对比分析

三电平双有源混合全桥(hybrid three level full bridge,H-TLFB)DC-DC变换器在双重移相控制下存在回流功率和电流应力较大的问题,为此提出电流应力优化控制和回流功率优化控制两种优化控制策略。首先分析变换器4种工作模态,推导不同模态下电流应力、回流功率的表达式,通过对比求解全局内最小电流应力、最小回流功率对应最优移相比,并设计相应优化控制策略。然后基于相同传输功率段将两种优化控制策略进行对比分析,发现两种优化控制策略均可在全功率传输内对变换器进行优化。当1/2<p<2/3时,两种优化控制对电流应力的优化等效,当0<p<1/2时,两种优化控制对回流功率的优化等效。最后通过实验验证了理论分析的有效性和正确性。