Transformer modelling for distribution system studies. II. Additionof models to YBUS and ZBUS

For pt.I see ibid., vol.7, no.2, p.567-74 (1992). A distribution transformer modeling procedure is discussed which represents the distribution transformer with a minimum of input data for network, load, and fault studies thereby allowing the transformer to be routinely included as part of the distribution network. How these transformer methods are used in the formation of Y_BUS and Z _BUS system models is described     

A general n-port network reciprocity theorem

A general expression for an n-port network reciprocity theorem is given, which includes the conventional two-port reciprocity theorem as a special case. The proof is based directly on Tellegen's theorem. An illustrative example is given. The result is significant in that not only is the expression itself simple and compact, but it is also general in the broad sense     

Transient analysis of heat dissipation due to a HVDC groundelectrode

The temperature of the soil surrounding a high-voltage direct-current (HVDC) ground electrode was computed at various points in time using finite difference methods. The temperature response for various ground electrodes was computed using these same techniques. The results were then compared with results obtained experimentally in a laboratory. It is suggested that peak temperature calculations may be desirable for periodic short-term operation of a HVDC ground electrode. Since the surrounding soil temperatures may not have reached a steady state, a transient analysis may be necessary     

S parameters: a practical education

The course on radiofrequency amplifier design by the technique of S-parameters offered at Virginia Polytechnic Institute and State University is described with some emphasis on the associated laboratory portion of the course. The course begins with microstrip amplifier construction and the modeling of both lumped and distributed circuit elements. Attention is then given to the design of a transistor amplifier in the UHF spectrum. Biasing and parameter measurement are first considered. Matching networks are then designed and installed, followed by the measurement of several amplifier specifications. In five years of running the laboratory course there has never been a case of a student failing to end the course with a working amplifier. Typical amplifiers (900 MHz) have gains of 12 and 13 dB, input and output reflection coefficients below 0.05, third-order intercept points of 0-10 dBm (input), and noise figures near 5 dB     

Heat dissipation computations of a HDVC ground electrode using asupercomputer

The temperature of soil surrounding a high-voltage direct current (HVDC) toroidal ground electrode of practical dimensions in both homogeneous and nonhomogeneous soils was computed at incremental points in time using finite-difference methods on a supercomputer. The results obtained validated the expected result, that the time constant of the system is large, requiring long-term and short-term simulation curves, as shown. It is concluded that by being able to compute the thermal response and by calculating the thermal time constant, the option for possible improved design is available     

直流输电接地极电流对不同结构变压器影响研究

利用PSCAD/EMTDC软件建立系统模型,仿真了直流接地极电流对组式、三相三柱式和三相五柱式等不同结构变压器的影响.仿真结果表明:组式变压器由于具有独立的磁通回路,磁阻小,受直流偏磁影响最大;三相三柱式变压器铁芯中无直流通道,磁阻大,受直流偏磁影响较小;而三相五柱式变压器虽有磁回路,但铁芯面积比单相变压器小,受直流偏磁影响介于前二者之间.     

A W-matrix methodology for solving sparse networkequations on multiprocessor computers

The authors describe a methodology for solving efficiently sparse network equations on multiprocessor computers. The methodology is based on the matrix inverse factors (W-matrix) approach to the direct solution phase of Ax=b systems. A partitioning scheme of the W-matrix, based on the leaf-nodes of the factorization path tree, is proposed. The methodology allows the performance of all the updating operations on vector b in parallel, within each partition, using a row-oriented processing. The approach takes advantage of the processing power of the individual processors. Performance results are presented and discussed     

Influence of an insulating spacer on the V-tcharacteristics of a coaxial gas-insulated cable. II. Tests on EHV buses

For pt.I see ibid., vol.3, no.1, p.16-25 (1988). The V- t characteristics of EHV buses were evaluated for two configurations with rated voltages of 550 and 800 kV. The bus dimensions were 6 m in length and 17.8/53.3 cm and 20.3-61 cm in diameter, respectively. They were each fitted with two tripod-type epoxy spacers. The results show a pronounced influence of the supporting spacers on the V-t characteristic, corresponding to a reduced withstand voltage and less dispersion in the measured breakdown voltages. A statistical method is presented for deriving the V- t characteristics of a gas-insulated system from those of the individual components     

Influence of an insulating spacer on the V-tcharacteristics of a coaxial gas-insulated cable. I. Study on areduced-scale coaxial conductor

Two different spacer designs, a disc spacer and a composite-profile cone, were evaluated in a coaxial conductor 2.5/7 cm in diameter to study their influence on the V-t characteristic of the conductor when it is subjected to repeated applications of impulse voltages of constant waveshape and increasing magnitude. The results show that an insulating spacer can reduce the critical withstand voltage and yield smaller dispersion in the breakdown voltages. These effects can be minimized by adopting a design that favors breakdown in the gas rather than along the spacer interface     

A k-step predictive scheme for speed control of dieseldriven power plants

An adaptive speed control scheme for diesel prime movers is presented. Diesel prime movers are characterized by nonlinear, time-varying parameters including a nonlinear input dead-time variation that introduces an unknown delay between the injection of fuel and the production of engine torque. Such plants are difficult to control by conventional proportional-integral (PI) controllers. It is shown that by assuming the existence of an additional system time constant, an explicit estimate of the plant parameters and dead-time may be obtained. The algorithm converges rapidly. The resulting predictor can be used to set up a k-step ahead predictive controller. Studies that compare the performance of the adaptive scheme to that of a fixed, tuned, PI controller are presented. The algorithm is shown to be flexible enough to operate under a wide range of droop settings and is equally applicable to various operating conditions     

N层土壤模型下HVDC系统大地回路运行时EPS分布研究

分析High Voltage Direct Current (HVDC)输电系统大地回路运行时地表电位分布,是评估地下金属管道腐蚀、变压器直流偏磁、通信干扰等不良效应受接地极电流影响的前提。本文利用镜像法和电磁波的折射、反射规律,创新地推导出N层垂直和水平复合分层的地表电位的解析公式,然后利用这个解析公式解决华东地区地表电位分布的实际问题,通过不同模型计算结果的反馈得到华东地区理想的仿真模型,最后用多项式插值将电位曲线进行拟合,得到华东地区地表电位的一般函数表达式。     

A Z-transform model of transformers for the study ofelectromagnetic transients in power systems

A Z-transform model, which combines transformer frequency-dependent short-circuit impedances with gain functions, has been developed. It sets up a relationship between transient voltages and currents on both sides of a transformer winding pair. The model can be used to calculate impulse responses of the transformers with open-circuit secondary winding as well as those connected to other networks. It could be incorporated into EMT (electromagnetic transient) programs for calculating the EMTs in power systems in which the distributed characteristics of transformer windings are to be considered     

Electrical breakdown of SF6 at small values of theproduct pd

The breakdown of SF₆ when static and impulse voltages are applied at small values (from 10^-4 bar-mm to 1 bar-mm) of the product pd was investigated. It was found that the prevailing breakdown mechanism up to 8×10^-3 bar-mm was the Townsend mechanism. Between 8×10^-3 and 5.5 bar-mm the Townsend and streamers mechanisms combined, whereas from 5.5 bar-mm on the streamers mechanism took over. The Paschen law was found to hold for the points lying to the right of the minimum when static voltage was applied, whereas edge-type breakdown occurred on the points lying to the left of the minimum. When impulse voltage was applied, the Paschen law was valid only for the points to the right of the 1 bar-mm point. The electrode material affected the static and impulse breakdowns through the values of its work function     

直流输电系统接地极线路故障研究

天广、高肇和兴安等由德国Siemens公司设计的直流输电系统中为例,首先简介了所设置的接地极线路故障相关保护及运行实例,然后从理论上分析了单极大地运行方式下接地极线路发生接地故障和断线故障时各特征量的变化特点,并通过RTDS实时仿真系统进行了验证;接着根据这些特点,提出了可以准确判明故障类型的改进措施,确保准确检测接地极线路接地故障,并通过启动线路故障重启动功能,提高直流输电系统运行的可靠性.     

N层土壤模型下HVDC系统大地回路运行时ESP分布研究

分析High Voltage Direct Current(HVDC)输电系统大地回路运行时地表电位分布,是评估地下金属管道腐蚀、变压器直流偏磁、通信干扰等不良效应受接地极电流影响的前提。文中利用镜像法和电磁波的折射、反射规律,推导出N层垂直和水平复合分层的地表电位的解析公式,然后利用这个解析公式解决华东地区地表电位分布的实际问题,通过不同模型计算结果的反馈得到华东地区理想的仿真模型,最后用多项式插值将电位曲线进行拟合,得到华东地区地表电位的一般函数表达式。     

适用于风电经柔性直流并网系统的柔性耗能装置及控制策略

高压柔性直流输电技术可实现有功功率的双向控制,且无换相失败问题,是实现风电并网外送的重要手段之一。风电经柔性直流并网系统易发生交直流故障,故障期间风电系统持续输出功率,过剩的暂态能量危害系统的安全运行。针对风电经柔性直流并网系统的暂态能量耗散问题,提出了一种基于全桥子模块的柔性耗能装置(flexible energy dissipation device, FEDD)。为解决子模块充放电无法准确控制的难题,提出了柔性耗能装置的动态电压控制策略和暂态能量耗散策略,并兼顾了子模块电容能量平衡。根据FEDD的工作原理和控制策略,提出了设备主要参数设计方法。最后通过RTDS实验结果验证了柔性耗能装置能够准确吸收暂态能量,保证换流站平稳穿越交直流故障。     

Electrically induced heat dissipation in metallized film capacitors

Elevated temperature is a key aging factor for metallized polymer film capacitors with the capacitor life expectancy halved with every 8/spl deg/C of temperature rise. For film capacitors in service, both application of a time-varying external voltage and the extinction of internal breakdown events can deposit significant heat onto the capacitor structure leading to an undesirable temperature rise. Often such heat generation is localized spatially and the resulting temperature gradient enhances the probability of subsequent breakdowns. To facilitate a thorough and generic understanding, a thermal model of metallized film capacitors is developed to simulate the dynamics of heat generation and transfer under the switching impact of an external voltage. Thermal processes in solids (polymer layers, electrode coating, and zinc-spray segment) are modeled with conduction mechanism, whereas convection mechanism is considered for silicone oil and air. Heat generation is induced either by ohmic losses of the externally applied electric field, or by internal breakdown events within the metallized film capacitor structure. This allows a seamless interface between the heat transfer model and the equivalent circuit based field calculation model. Based this electrothermal model, numerical examples are used to study temperature distribution within film capacitors and then to examine the likelihood of localized temperature hot-spots that may lead to spatially preferential breakdowns.     

锂离子电池散热技术研究进展

对锂离子电池的产热模型和散热技术进行综述,介绍目前锂离子电池领域风冷、液冷、相变冷却及复合冷却的发展情况.风冷效果的优化主要从冷却介质参数、结构参数及控制策略方面进行;液冷技术出现了散热能力强的微流体冷却,有望实现外部冷却到内部冷却的技术革新;相变冷却技术引入液气相变技术,实现了热量快速传递,可满足高倍率放电高散热量的...     

高压直流接地极对管道电磁阀的干扰影响研究

针对高压直流输电(HVDC)接地极处于单极大地回路运行工况下,很大的入地杂散电流可能产生电磁阀发生误动作的风险,影响管道正常运行。建立了传导干扰对电磁阀线圈影响的数学模型,研究了直流杂散电流引起电磁阀误动作的机制。采用数值模拟的方法,建立国内某1 100 kV特高压直流输电工程接地极对附近管道干扰的仿真模型,并设计了管地电位实时数据采集系统。仿真及现场测试结果表明,干扰电压以接地极距离管道最近位置为中心,依次向上下游递减;冬季电磁阀受到的直流干扰是夏季的3.43倍;仿真结果和实测结果基本吻合,误差在10%以内;沿管线增设4个和6个绝缘接头后使得管地电位差降低62.1%、71.4%。为可靠预防电磁阀异常工作,在冬季应该加强阀室巡检,并沿管道上下游适当位置安装绝缘接头。     

南方电网直流输电系统接地极线路不平衡保护动作后果探讨

接地板线路不平衡保护用于检测接地极线路故障,首先简介了天广、高肇和兴安等南方电网直流输电系统中接地极线路不平衡保护的动作原理和动作后果,根据直流线路故障的特征,建议将单极方式下接地极线路不平衡保护的动作后果由启动闭锁改进为启动故障重启动功能、将天广直流双极方式下接地极线路不平衡保护的动作后果由告警改进为启动双极功率平衡功能,并结合RTDS仿真指出改进过程中应注意接地极线路和高压直流线路相继故障带来的系统稳定问题,同时也给出了相应的改进建议.