基于改进层次模糊评判的智能变电站二次系统状态评估

针对智能变电站各二次设备故障机理复杂、具有不确定性、难以有效评估的问题,提出了基于改进层次分析法和模糊评判理论的智能变电站二次系统综合评估方法。以运行检修规程、测试技术规范为基础,综合考虑运行经验、工作环境和检修记录,建立二次设备状态定量和定性评估指标体系,并根据各指标的相对重要性赋予相应权重;通过半梯形分布函数构建状态指标评分模型,并引入相对劣化度来表征二次设备状态转化程度的隶属函数,进而建立层次模糊评判矩阵;由此,建立基于改进层次模糊评判的智能变电站二次系统状态评估模型。实例分析表明该方法有效可行,为智能变电站二次系统健康状态评估提供了新的思路。

Study on Decision Method of Neutral Point Grounding Mode for Medium-Voltage Distribution Network

The neutral grounding mode of medium-voltage distribution network decides the reliability, overvoltage, relay protection and electrical safety. Therefore, a comprehensive consideration of the reliability, safety and economy is particularly important for the decision of neutral grounding mode. This paper proposes a new decision method of neutral point grounding mode for mediumvoltage distribution network. The objective function is constructed for the decision according the life cycle cost. The reliability of the neutral point grounding mode is taken into account through treating the outage cost as an operating cost. The safety condition of the neutral point grounding mode is preserved as the constraint condition of decision models, so the decision method can generate the most economical and reliable scheme of neutral point grounding mode within a safe limit. The example is used to verify the feasibility and effectiveness of the decision method.

双馈型风电场并网短路电流特性研究

以双馈型风力发电机为主的风电场在电力供应中占有越来越大的比重。双馈感应发电机输出的短路电流特性与同步发电机具有较大区别,目前双馈风电的故障暂态分析已成为风电并网的研究热点,但主要集中于单个机组并网系统方面。本文首先基于复频域分析了单台双馈风电机组的短路暂态过程,建立双馈型风电场暂态期间的工频、直流和转速频率分量的等效电路,对双馈型风电场的短路电流变化规律进行研究。推导出风电场中单台风电机组的短路电流增量表达式,基于短路电流的增量利用补偿增量的方法推导出双馈风电场的短路电流计算式。仿真验证了风电场内部的短路电流变化规律和解析表达式的准确性。

Short-Term Reliability Evaluation of Transmission System under Strong Wind and Rain

The impact of strong wind and rain loads will adversely affect the reliability of the overhead lines, it’s necessary to study changes in risk of transmission system and establish the reliability model of overhead lines through the strong wind and rain loads. In this article, the stochastic properties of overhead lines’ strength and loads were used, according to principles of structural reliability, time-dependent failure probability model of overhead lines was established under the impact of strong wind and rain loads. Simulation of the IEEE-79 system demonstrates that failure probability model is effective. This simulation result also shows that the impact of strong wind and rain loads will seriously affect reliability indices of system loads, rain loads cannot be ignored under strong wind and rain loads.

Short-Term Reliability Evaluation of Transmission System Using Lightning Strike Probability Prediction

The transmission lines are exposed to the atmosphere nature and will be affected by adverse weather such as lightning storm, so that it will affect the reliability of transmission system. This paper studies the fault probability model of transmission line during the lightning storm, and evaluates the short-term reliability of transmission system in the forecasting weather condition. Firstly, build the lightning strike fault probability model of the transmission line based on historical lightning record information, then calculate the lightning strike probability under the forecasting weather conditions, furthermore evaluate the reliability index of transmission system. Utilizing IEEE RTS-79 system to verify the validity of the proposed model and the results show that lightning has great negative influence on the transmission lines and the reliability of transmission system. The reliability evaluation model proposed in this paper can guide the short-term operation and online scheduling for transmission system operators.

Fault protection method of single-phase break for distribution network considering the influence of neutral grounding modes

With the rapid development of modern distribution network and the access of distributed generation,the network structure is becoming increasingly complex.Frequent single-phase break faults have seriously affected equipment and personal safety and stable operation of the power system.However,with the development and application of the composite neutral grounding modes,the protection of single-phase break fault is facing new challenges.This paper proposes a protection method of single-phase break fault for distribution network considering the influence of neutral grounding modes.The characteristics of neutral voltage and sequence current are analyzed under normal operation and single-phase break fault with different grounding modes.Following this,the protection criterion based on neutral voltage and sequence current variation is constructed.The protection method of singlephase break fault for distribution network is proposed,which is applicable for various neutral grounding modes.Theoretical analysis and simulation results show that the protection method is less affected by system asymmetry,fault location and load distribution.The method has higher sensitivity,reliability and adaptability.

A Two-stage Stochastic Mixed-integer Programming Model for Resilience Enhancement of Active Distribution Networks

Most existing distribution networks are difficult to withstand the impact of meteorological disasters. With the development of active distribution networks(ADNs), more and more upgrading and updating resources are applied to enhance the resilience of ADNs. A two-stage stochastic mixed-integer programming(SMIP) model is proposed in this paper to minimize the upgrading and operation cost of ADNs by considering random scenarios referring to different operation scenarios of ADNs caused by disastrous weather events. In the first stage, the planning decision is formulated according to the measures of hardening existing distribution lines, upgrading automatic switches, and deploying energy storage resources. The second stage is to evaluate the operation cost of ADNs by considering the cost of load shedding due to disastrous weather and optimal deployment of energy storage systems(ESSs) under normal weather condition. A novel modeling method is proposed to address the uncertainty of the operation state of distribution lines according to the canonical representation of logical constraints. The progressive hedging algorithm(PHA) is adopted to solve the SMIP model. The IEEE 33-node test system is employed to verify the feasibility and effectiveness of the proposed method. The results show that the proposed model can enhance the resilience of the ADN while ensuring economy.

Corrections of Original CFPREV Control in LCC-HVDC Links and Analysis of Its Inherent Plateau Effect

The most effective approach to suppressing the first commutation failure(CF)of the LCC-HVDC link at fault inception is to advance firings of the inverter,and the commutation failure prevention(CFPREV)control is the most commonly used method in practical engineering.However,it is discovered in this study that there exist a few serious defects in its original scheme,and thus targeted vital corrections were made.Furthermore,an interesting phenomenon termed the plateau effect,which states that an excessive advancement of firings will contrarily and inevitably lead to more commutation failures,is also revealed and analyzed.It turns out that the inherent commutation dents of the Graetz bridge should be primarily responsible,which bridges the knowledge gap and further enhances the cognition of the limitation of CFPREV control,and it may also be conducive to the design of related control parameters.Simulation results then validate the necessity of these presented corrections and confirm the existence of the plateau effect.