New control for HVDC system connected to large windfarm

HVDC consisting of self‐commutated inverters is able to be applied for power transmission connecting from a remote large windfarm to a weak AC system. Most self‐commutated HVDC is applied for connection between two AC systems that have synchronous power sources, but it is not suitable for a windfarm that consists of induction generators without synchronous power source. This paper presents new control for the self‐commutated HVDC system connected to a large windfarm of induction generators. The effect of the proposed control is shown by EMTP simulation. ©2008 Wiley Periodicals, Inc. Electr Eng Jpn, 166(4): 31–39, 2009; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20539     

Transient stability of small plant generators connected to a weak utility system-a case study

Small generators connected through a large impedance and operating in synchronism with a weak utility system are more likely to pull out of step on a large perturbation, i.e., a fault in the utility's system. Under an incident impact, which will be distributed unevenly among various machines, every machine will be retarded or accelerated, and undergo damped or divergent oscillations. This paper demonstrates that the small machines are more vulnerable and may fall out of step. This may plunge the entire process plant into a complete power outage on system separation. A case study of two small generating units and the improvements in the transient stability by fast system separation and load shedding is demonstrated.     

直驱型机组风电场并网静态电压稳定性研究

对基于永磁直驱型机组的风电场静态电压稳定性进行了研究.通过单机无穷大系统分析了影响并网风电场电压稳定性的主要因素,以美国西部电网WSCC3机9节点系统为例,分析了风电场接入后对系统电压稳定性的影响,并对不同线路等值阻抗、不同控制方式下的电压稳定性进行了比较分析,得出了P-V曲线.     

Control sensitivity indices for stability analysis of HVDC systems

This paper presents a new concept called the “Control Sensitivity Index” or CSI, for the stability analysis of HVDC converters connected to weak AC systems. The CSI for a particular control mode can be defined as the ratio of incremental changes in the two system variables that are most relevant to that control mode. The index provides valuable information on the stability of the system and, unlike other approaches, aids in the design of the controller. It also plays an important role in defining nonlinear gains for the controller. This paper offers a generalized formulation of CSI and demonstrates its application through an analysis of the CSI for three modes of HVDC control. The conclusions drawn from the analysis are confirmed by a detailed electromagnetic transients simulation of the AC/DC system. The paper concludes that the CSI can be used to improve the controller design and, for an inverter in a weak AC system, the conventional voltage control mode is more stable than the conventional γ control mode     

600 MW空冷亚临界发电机组空冷岛厂用电系统谐波分析

本文针对首套国产化600MW发电机组空冷岛低压配电系统,风机变频器负载谐波含量及大小进行分析论证,对合理选用滤波设备容量有着比较实用的工程意义。     

Impact of FACTS controllers on the stability of power systems connected with doubly fed induction generators

The increasing power demand has led to the growth of new technologies that play an integral role in shaping the future energy market. Keeping in view of the environmental constraints, grid connected wind turbines are promising in increasing system reliability. This paper presents the impact of FACTS controllers on the stability of power systems connected with wind energy conversion systems. The wind generator model considered is a variable speed doubly fed induction generator model. The stability assessment is made first for a three phase short circuit without FACTS controllers in the power network and then with the FACTS controllers. The dynamic simulation results yield information on (i) the impact of faults on the performance of induction generators/wind turbines, (ii) transient rating of the FACTS controllers for enhancement of rotor speed stability of induction generators and angle stability of synchronous generators. EUROSTAG is used for executing the dynamic simulations.     

A theory of maximum capacity of distributed generators connected to a distribution system using electric power density model

Recently, the number of distributed generators (DGs) connected to distribution systems has been increasing. System operators should know the maximum capacity of DGs that can be connected without problems to one feeder of the system in order to control the system appropriately. Many studies of the maximum capacity of the DG have been presented, but they have produced limited results calculated by a typical or average‐value model. However, many DGs will access one feeder if deregulation of the electric power industry is accelerated in the near future. In order to deal with this situation, the authors have derived a general formula to calculate the range of the maximum DG capacity per feeder. Copyright © 2004 Wiley Periodicals, Inc. In order to deal with sets of DGs that are dispersed completely on the distribution line, the authors have derived a differential equation for the complex power and one for the voltage drop, which are expressed as functions of distance from the substation. The general formula to calculate the range of the maximum DG capacity connected to the system is determined by solving these equations under the constraints of the line voltage, the line current, and the power factor of the DGs. By a numerical analysis, the authors have calculated the maximum capacity of DGs depending on many parameters, such as the length of the feeder, the DG power factor, and the like. In a short‐length system, the maximum DG capacity is governed by the current constraint, but in a long length system, it is governed by the upper voltage constraint. © 2006 Wiley Periodicals, Inc. Electr Eng Jpn, 155(3): 18–28, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20330     

改善系统暂态稳定性的HVDC模糊神经控制器

提出了一种HVDC在线模糊神经控制器以提高交直流系统的暂态稳定性.该控制器的特点是结合了模糊系统处理复杂和不确定性问题及神经网络具有自学习能力的优点,选取整流侧交流母线电压相位误差及其变化率作为模糊逻辑控制部分的输入,其输出结果作为神经网络的一个输入,采用改进BP算法进行在线训练神经网络,神经网络的输出用来修正整流器的触发角,并利用NETOMAC软件对控制器主要参数进行了离线优化.仿真结果表明该控制器能有效地抑制有功功率振荡,改善发电机的功角特性,提高系统的暂态稳定性.     

Harmonic cancellation characteristics of specially connected transformers

Specially connected transformers such as V–V, Scott and Le-Blanc transformers are used to convert a three-phase supply into one or two single-phase supplies. These transformers are commonly used in the electro locomotive traction systems. This paper investigates the harmonic cancellation characteristics of such transformers. The results show that when two harmonic-producing loads are connected to each single-phase side of the transformers, the harmonics produced by the loads will cancel out at the primary sides of the transformers. The amount of cancellation is affected by transformer type and harmonic order. The study is performed for seven types of specially connected transformers.     

Islanding detection of grid connected distributed generators using TLS-ESPRIT

A critical protection requirement for grid connected distributed generators (DG) is anti-islanding protection. In this paper, a new islanding detection method is proposed based on monitoring the generator's frequency. Two new features, the frequency of oscillation and the damping factor of the generator's frequency output waveform, are extracted using the total least square-estimation of signal parameters via rotational invariance techniques (TLS-ESPRIT) algorithm. The proposed method has been tested under various scenarios such as load change, short circuit, and capacitor switching.     

插值算法在HVDC谐波分析中的应用

针对HVDC（直流高压）系统直流侧电网里的谐波,提出了基于罗氏线圈的测量方法,并设计了相应的电路。谐波分析通常采用FFT算法,为了加快FFT的运算速度,需要对280个采样点进行插值处理。本文用C语言采用拉格朗日算法实现了插值处理,并对程序进行了可行性的验证,最后对该程序进行了误差分析。     

一种新型高压直流谐波电流测量方法

鉴于传统的谐波测量方法已难以满足高压直流输电(HVDC)谐波测量的需要,介绍了一种基于Rogowski 线圈的HVDC谐波电流测量系统,整个系统的准确度优于3%,谐波检测部分的准确度优于1%,该系统完全不受 直流分量影响,高低压端完全光纤隔离,易于绝缘,适于HVDC谐波测量,也可用于一般直流谐波的测量。     

高压直流输电直流侧谐波电流计算

为计算高压直流输电工程的等效干扰电流,详细描述了给定两侧三脉动谐波电压源情况下如何计算沿线各次谐波电流的一套计算流程,包括直接流过直流极导线、接地极线路的谐波电流和感应到直流线路、接地极线路地线中的谐波电流。该计算流程分为2步:第1步,将直流侧系统作为一个整体用求解一般性电网络的节点分析法计算,在计算节点导纳矩阵时,直流输电线路作为一个网络元件采用全相耦合模型;第2步,在求得直流输电线路端口上的谐波电压之后,将直流线路分解为正序与零序网络,分别计算沿线各次谐波电流的正序与零序分量,然后再将正序与零序分量合成为整体谐波电流。在同等条件下,比较该算法的结果与电磁暂态仿真程序PSCAD/EMTDC的结果,两者基本一致,验证了所提算法的正确性。     

直流输电系统在交流故障下的谐波分析

利用电力系统暂态仿真软件EMTDC作辅助分析,对高压直流系统在交流侧发生单相故障下的谐波问题进行了详细的研究,得出的结果是3,9次谐波最为严重,也出现幅值不小的2次谐波,如不采取相应的措施,将会对系统造成较大的影响.     

高速磁悬浮列车系统的谐波分析

高速磁悬浮列车系统的谐波是一种典型的强非线性、冲击性电力负荷,是一重要的新类型谐波源,在没有补偿措施时,它会带来较严重的电网谐波问题。介绍国内外高速磁悬浮列车系统谐波问题的研究进展,通过建立谐波分析模型对上海高速磁悬浮列车牵引供电系统进行了深入的分析。通过对半控双三相12脉冲整流器的电流波形分析和傅立叶级数展开,得出系统的主要谐波成分。分析结果显示,在没有谐波补偿器的情况下,上海高速磁悬浮列车系统产生的谐波已超过我国有关的谐波国家标准。还介绍了上海高速磁悬浮列车系统的动态补偿器。     

高压直流输电3脉动谐波电压源计算

为了能够合理而有效地配置直流滤波器,基于给定的主回路稳态参数,描述了在考虑各种不对称因素及交流电压不平衡条件下如何计算3脉动谐波电压,包括特征谐波与非特征谐波。具体计算时采用分段算法,该算法根据各3脉动组中三个阀的导通及换相情况,将3脉动电压波形在一个周波内分为6段,根据计算获得的分段区间将交流基频电压与各次谐波电压作用下的3脉动电压按此6个分段区间分别进行傅立叶分解,然后将相同的谐波次数进行合并就可获得最终的3脉动源的各次电压。该算法能够有效地考虑相关不对称及交流背景谐波等影响非特征谐波的各种因素,准确计算3脉动源的各次谐波。在同等条件下,将该算法与西门子HarmDistr程序比较,其计算结果基本一致,验证了该算法的有效性。     

提高暂态稳定性的HVDC与发电机励磁的非线性最优协调控制

针对大扰动情形,为进一步提高交直流混合电力系统的暂态稳定性,提出了一种高压直流输电(HVDC)与发电机励磁协调的控制方法.通过建立HVDC系统与发电机的综合模型,将现代控制理论中直接反馈线性化方法与线性系统最优控制理论相结合,设计出一种HVDC与发电机励磁协调的非线性最优控制规律.仿真结果表明,该协调控制器能明显提高交直流互联系统的功角稳定性和频率稳定性,改善直流系统的性能.