磁饱和对同步发电机自激影响的研究

深入研究了自激产生的物理过程,强调可以通过外部电力系统等效容抗的大小判断自激是否发生。分析了系统参数和磁路饱和对自激稳定域的影响,通过对一个含串联电容补偿的单机无穷大系统的频域分析,得到了同步发电机的自激稳定域,进一步的分析表明,磁路饱和将显著地减小自激稳定域。文中还针对一个典型的双机电力系统,通过时域仿真验证了分析结果的正确性。     

Analytical prediction of abnormal temporary overvoltages due to ground faults in MV networks

The paper deals with the temporary overvoltages that build up in radial MV distribution networks following the inception of a 1-phase-to-ground fault. It will be demonstrated that, for extended cable/overhead MV distribution networks with ungrounded neutral, in case of low resistance faults at critical locations along overhead lines, the neutral voltage can reach very large values, significantly higher than 1 p.u. (up to 2.5 p.u.). The attendant temporary overvoltages on healthy phases can be very large, much higher than √3 p.u. (up to 3.5 p.u.). Fault currents are also affected, reaching twice the value calculated with simplified methods, i.e. neglecting series impedances. Quick yet accurate analytical formulas for the prediction of maximum overvoltages, fault currents and critical fault distances are presented, together with their validation by detailed ATP-EMTP simulations. The results of a study on an existing Italian 15 kV–50 Hz extended mixed cable/overhead distribution network are finally presented and discussed.     

Steady-state operating conditions of very long EHVAC cable lines: Two case studies

The paper deals with the operation of very long EHV AC cable lines in a well-developed bulk power system, by simulating two 400 kV-50 Hz single circuit cable lines, 80-100 km long, installed at different locations of the Italian transmission network, both standing alone and in parallel to an existing overhead line. The power transmission limits of the considered cable lines are assessed analytically, as well as the required amount of shunt compensation and its location, at line terminals or also at mid-line; a criterion for sizing mid-line shunt compensation is also proposed. In order to check theoretical analyses carried out by analytical formulas, AC load flow simulations of the transmission network with the hypothetical cable lines in place are evaluated, referring to severe loading conditions in both normal and post-contingency operation. Open-ended cable operation and shunt compensation contingencies are also addressed. Results show that power transmission limits due to cable ampacity and higher active power flows due to low cable reactance do not pose special constraints on the operation of the powerful network under examination. Appropriate shunt compensation minimizes the effects of surplus cable reactive power, allowing to operate long 400 kV-50 Hz cable lines (up to 100 km at least) with reactors at line terminals only. Moreover, if shunt compensation is installed also at mid-line, operating capabilities of the long cable lines are enhanced and losses reduced.     

Voltage transient measurements in a distribution network correlated with data from lightning location system and from sequence of events recorders

Measurement units of high-frequency voltage transients have been specifically developed and installed at three different busses of an Italian medium voltage (MV) feeder, mainly composed by overhead lines and located in a rural region characterized by a high ground flash density (4 flashes/km²/yr). Several of the recorded voltage transients in the period from March 2007 to August 2008 are correlated with specific events detected by the Italian lightning location system (LLS) CESI-SIRF and with sequence of protection operations recorded by the utility's data acquisition system. The paper presents the analysis of these correlated events. Moreover, the paper compares some voltage transient measurements with the computer simulation results obtained by a detailed LIOV-EMTP model of the MV feeder response to indirect lightning strokes. The aim of the paper is to provide elements useful to solve the problem of correlating line faults with lightning events.     

气体绝缘开关设备中特快速瞬态过电压研究的新进展

气体绝缘开关设备（gas insulated switchgear,GIS）中的隔离开关操作空载短母线会产生陡波前、高幅值的特快速瞬态过电压（very fast transient overvoltage,VFTO）、瞬态壳体电位和电磁干扰,对GIS及其连接的具有绕组的设备、与壳体连接的二次设备绝缘产生危害,干扰二次设备工作,严重影响设备的安全运行。对2009年前国内外在VFTO测量方法、特性试验、数字仿真和对绝缘影响以及外部VFTO方面的研究状况进行了综述。概要总结了国家电网公司在特高压GIS中VFTO实测及仿真研究方面取得的新进展,提出了特高压GIS中VFTO需要深化研究的内容,即重点关注VFTO的传播特性与仿真建模,VFTO下的绝缘特性和机制研究,以期全面掌握特高压GIS中VFTO特性,对特高压输变电设备的研制、工程设计和运行提供依据和有益的参考。     

隔离开关操作速度对特快速瞬态过电压的影响

气体绝缘开关设备（gas insulated switchgear,GIS）内的隔离开关在操作空载短母线时会发生多次击穿,并产生特快速瞬态过电压（very fast transient overvoltage,VFTO）,严重时可能造成变压器和其他电气设备的损坏。隔离开关的操作速度是影响VFTO的重要因素之一,对此国内外有不同看法。在仿真研究中考虑了隔离开关操作过程中的多次重击穿、间隙击穿电压的极性特性、概率特性及其随操作速度变化的特性,采用统计分析方法研究了隔离开关的速度对VFTO的影响,并与实测结果进行了对比。研究结果表明,在一定范围内当隔离开关速度较慢时,分闸后负载侧残压较低,合闸、分闸操作过程中产生的VFTO幅值也较低。     

2.5MV特快速瞬态过电压发生器

气体绝缘开关设备（gas insulated switchgear,GIS）以其占地面积少、密封性好,受环境影响小,运行可靠、维修周期长等优点在中国电网中得到了广泛应用。在330kV以上电压等级的系统中,开关动作产生的特快速瞬态过电压（very fast transient overvoltage,VFTO）对系统有很大危害。为满足特高压GIS中VFTO的模拟,利用6MV敞开式冲击电压发生器与陡化装置结合的方法,研制出一种模拟VFTO产生的装置。通过增加GIS母线长度来补偿回路固有电感的影响,以及通过控制陡化间隙的击穿电压来控制输出电压幅值,使该装置可以输出幅值为2.5MV、高频振荡达到35 MHz的电压波,能够满足特高压试验对VFTO模拟的要求。采用外加积分器方式,制作了一种锥形电容传感器来测量该装置产生的电压波,测试结果表明该电容传感器可以满足试验的要求。     

Steady-state operating conditions of very long EHVAC cable lines

In the present paper the authors investigate steady-state capabilities of very long EHVAC underground cable lines, without intermediate compensating stations, installed in meshed networks. Analytical formulas of cable length-loading relationship are presented to this purpose. The study shows that, in optimal operating conditions, most of the ampacity of such long underground cable lines can be exploited for active power transmission. Non-optimal operating envelopes are studied, showing the effect of terminal voltages and line losses on cable utilization and evidencing limited derating under realistic hypotheses. A parametric analysis of loading limits and possible voltage violations along the cable is shown; a simple criterion for optimal utilization of lossy cable lines is also proposed. Line-end shunt compensation requirements for integration of long EHVAC cable lines in transmission networks are then specified. Studies performed for both ideal and real cables and shunt reactors show that active power transmission of 100 km long, 400 kV-50 Hz underground cable lines can attain 90% of their thermal limit, without intermediate compensating stations. Excess reactive power and temporary overvoltages are effectively controlled by line-end compensation around 90%.     

相间功率控制器的电磁暂态研究

利用电磁暂态过程序（EMTP）,建立了相间功率控制器（IPC）的电磁暂态模型,并对IPC在短路故障、合闸及分闸过程中出现的过电压进行了仿真计算。结果表明,调谐型IPC虽然具有良好的短路电流限制特性,但在合闸及分闸情况下操作过电压非常高,从而限制了其实际应用。／相比较而言,非调谐型IPC的过电压要低得多,且具有一定的短路电流限制特性。从过电压的角度来看,后较为可取。     

Estimation of peak transient voltages at wind turbine generator terminals

This paper proposes equivalent circuits and a method for the estimation of the peak transient voltages at generator terminals in wind turbines equipped with back‐to‐back converters. Equivalent circuits as well as a way of phase‐to‐phase and phase‐to‐ground voltages estimation in back‐to‐back converter are presented. Proposed theoretical background is well confirmed by small‐scale measurements. It is shown and proven that phase‐to‐ground voltages represent a dominating threat for the wind turbine generators insulation. Copyright © 2014 John Wiley & Sons, Ltd.