A power transformer protection technique with stability duringcurrent transformer saturation and ratio-mismatch conditions

This paper describes a digital technique for protecting power transformers. The technique uses positive- and negative-sequence models of the power system in a fault-detection algorithm. While phase voltages and currents at the transformer terminals are used to detect a fault, no information concerning parameters of the transformer and power system is required. The performance of the proposed technique was studied for a variety of operating conditions using data generated by EMTP simulations. The impact of ratio-mismatch and saturation of current transformers on the performance of the technique was also examined. Results indicate that the proposed technique is stable during these conditions     

A novel digital directional transformer protection technique based on wavelet packet

This paper presents a novel digital technique for transformer protection. The technique is based on deriving a directional quantity proportional to the fault current signal and the prefault voltage signal. Standard fast wavelet transform (FWT) schemes may not be as effective for data that has chiefly oscillatory features. An effective solution to discrimination involves examining the signal in both the time and frequency domains simultaneously. The wavelet packet transform is an extension of the FWT that allows for finer characterization of signal content for both time and frequency together. A 11/132-kV transformer connected to a 132-kV power system was simulated using Alternative Transient Program/Electromagnetic Transient Program (ATP/EMTP). Results indicate that the proposed technique is stable, reliable, and fast during the discrimination between internal and external faults, magnetizing inrush currents, and internal faults, ratio-mismatch, and saturation of current transformers (CTs).     

Fast and Reliable CT Saturation Detection Using a Combined Method

Busbar differential relays may operate incorrectly for external faults due to current-transformer (CT) saturation. In this paper, two different techniques are suggested to detect CT saturation. The first proposed technique is based on the fact that the waveshape of the CT secondary current changes significantly at the instant of the saturation. Based on this feature, an algorithm which uses the second derivative of CT output current is developed. It uses an adaptive threshold to detect CT saturation fast. The second proposed technique utilizes two criteria based on the zero crossing principle. Utilizing the combination of the second derivative and zero crossing techniques results in a powerful and reliable scheme which is able to detect various CT saturation cases correctly and quite fast. The proposed method is able to detect even small CT saturation events. A real 400-kV busbar is simulated using PSCAD/EMTDC for evaluating the performance of the proposed algorithm. The obtained results demonstrate precise operation of the proposed algorithm in different conditions.      

A novel digital directional technique for bus-bars protection

This paper presents a novel digital technique for bus-bars protection. The technique is based on deriving a directional quantity proportional to the postfault current signal and the prefault voltage signal. The prefault current signal is also used to compensate the current signal during the current transformer (CT) saturation. The performance of the technique was investigated for a variety of operating conditions, for CT saturation, high fault resistance, source impedance, and for several bus-bar configurations. Results show that the technique is stable during early and severe CT saturations and it is able to offer very high accuracy and speed in fault detection.     

A discrete dynamic filter for detecting and compensating CT saturation

Current transformers (CTs) are used in electric power systems for protection and measurement purposes. The current signals in the secondary side of a CT should be exact reproductions of the corresponding current signals on its primary side. CTs are susceptible to the saturation phenomenon which leads to inaccurate current measurement and therefore, may cause malfunction of the protective relays. This paper presents an approach to the correct the distorted waveforms caused by CT saturation. A dynamic filter based on weighted least absolute value minimization is used to produce compensated secondary current samples from the distorted ones. The proposed technique is independent of CT parameters/characteristics or secondary burdens. Various test results indicate that the proposed technique can successfully detect CT saturation and accurately compensate the distorted secondary currents.     

Phasor Estimation in the Presence of DC Offset and CT Saturation

A hybrid algorithm for phasor estimation is proposed that is immune to DC offset and current transformer (CT) saturation problems. The algorithm utilizes partial sum (PS)-based and multistage least-squares (MLS)-based methods before and after CT saturation is detected, respectively. The MLS-based method is initiated when the third difference of the secondary current detects the start point of the first saturation period. The determination of each saturation period is based on the sum of the secondary current from the start point of the first saturation period. A least-squares (LS) technique estimates the DC offset parameters from the single-cycle difference of the secondary current in the unsaturated periods. Removal of DC offset from the secondary current yields the sinusoidal waveform portion. Finally, the LS technique is used once again to estimate the phasor from the sinusoidal waveform portion. The performance of the algorithm was evaluated for a-g faults on a 345-kV 100-km overhead transmission line. The electromagnetic transient program was used to generate fault current signals for different fault angles and remanent fluxes. The performance evaluation shows that the proposed algorithm accurately estimates the phasor of a current signal regardless of DC offset and CT saturation. The paper concludes by describing the hardware implementation of the algorithm on a prototype unit based on a digital signal processor.     

An algorithm for compensating secondary currents of currenttransformers

Current transformer (CT) saturation may cause power system relays to malfunction. The conventional method used to deal with the problem is overdimensioning of the transformer core so that CTs can carry up to 20 times the rated current without exceeding 10% ratio correction. However, this not only reduces the sensitivity of power system relays, but also increases the CT core size. This paper presents a technique of estimating the secondary current corresponding to the CT ratio under CT saturation. The proposed algorithm can improve the sensitivity of relays to low level internal faults, minimize the instability of relays for external faults, and might ultimately assist in reducing the dimension of the required CT core cross-section     

中阻抗母线保护原理、整定及运行的探讨

母线保护是电力系统中最重要的保护,其正确动作直接影响到电力系统安全、稳定运行。作以母线保护输入阻抗为线索,阐述了低阻抗、中阻抗及高阻抗母线保护的特点及使用范围,分析了中国电力系统广泛采用的中阻抗线线保护匠原理,结合电力系统的实际情况对母线各种运行状态,如正常运行、区外故障ＣＴ未饱和、区外故障ＣＴ饱和、区内故障以及区内故障ＣＴ饱和等情况对保护的性能进行了定性、定量分析,根据中阻抗电流差动继电器动作     

基于改进型序阻抗原理的变压器保护方案

基于序阻抗原理的变压器保护方案利用变压器发生区内外故障时,变压器两侧正负序阻抗所在象限的不同,来有效识别变压器区内外故障,文中在此方案基础上,改进了象平面区域的划分,补充了励磁涌流识别判据,并讨论了在特殊情况下所应采取的措施.在构建电流互感器饱和模型的基础上,利用HYBRISIM进行了大量的仿真实验,仿真结果表明:基于改进型序阻抗原理的保护方案,较原方案在保护动作的可靠性和灵敏性等方面均有较大的改善和提高,与传统的比率制动型差动保护相配合,可构成一种较理想的变压器主保护方案.     

Power differential based wide area protection

Current differential based wide area protection (WAP) has recently been proposed as a technique to increase the reliability of protection systems. It increases system stability and can prevent large contingencies such as cascading outages and blackouts. This paper describes how power differential protection (PDP) can be used within a WAP and shows that the algorithm operates correctly for all types of system faults whilst preventing unwanted tripping, even if the data has been distorted by CT saturation or by data mismatches caused by delays in the WAP data collection system. The PDP algorithm has been simulated and tested on an Iranian 400 kV transmission line under different fault and system operating conditions. The proposed operating logic and the PDP algorithm have also been evaluated using simulation studies based on the Northern Ireland Electricity (NIE) 275 kV network. The results presented illustrate the validity of the proposed protection.     

变压器差动保护的P级电流互感器“同型”匹配方法

为有效防止变压器区外故障时因各侧电流互感器饱和程度不一致引起的差动保护误动,基于电路、磁路分析,以变压器发生区外故障时各侧电流互感器同时进入饱和为原则,提出变压器差动保护中各侧P级电流互感器的"同型"匹配方案。该"同型"匹配方案可消除由变压器各侧电流互感器饱和程度不一致引起的不平衡电流,防止变压器发生区外故障时因电流互感器饱和引起的误动。利用所提方案得到了影响电流互感器"同型匹配"的因素。利用PSCAD/EMTDC进行了大量的仿真分析,验证了所提"同型"匹配方案的有效性。基于研究结果给出了变压器差动保护用电流互感器的选型建议。     

基于Hausdorff距离的自适应母线保护新原理

传统基于工频量的差动式或比相式母线保护装置受滤波算法数据窗影响动作速度较慢且原理上无法摆脱电流互感器(current transformer, CT)暂态饱和带来的不利影响.为此,提出了一种基于Hausdorff距离算法的自适应母线保护新原理.首先给出了Hausdorff距离算法的基本原理,然后将与母线相连任一支路电流...     

磁制动母线差动保护原理初探

电力系统母线保护存在的主要问题,是电流互感器的饱和问题。为了防止ＣＴ饱和后的保护误动,提出了许多饱和改进措施,但由于种种原因此问题都不能很好地被解决。从电流互感器饱和的物理本质出发,提出了基于电流互感器磁链的饱和判据,从理论上找到了解决电流互感器饱和引起母线差动保护误动问题的可靠方法。并在此基础上提出了一套基于电流互感器线性区的母线差动保护方案。     

A novel wavelet approach to busbar protection during CT saturation and ratio-mismatch

This paper deals with the use of a continuous wavelet transform (CWT) to distinguish faults in a busbar protection zone from those outside the zone. The Morlet wavelets are used as the wavelet basis function. They are more efficient in monitoring fault signals as time varies. The differential current as the operating quantity and the sum of the current magnitudes as the stabilizing signal are computed. The magnitudes of the CWT of these currents are derived to obtain the operational and restraint signals. The CWT coefficient plots are given to precisely determine the scale of analysis. The proposed wavelet transform approach based on Morlet basis function is found to be an excellent discriminant for identifying the fault signals during the CT saturation and ratio mismatch. ATP simulations are used to test and validate the proposed CWT approach for model power system faults.     

An spatiotemporal information system based wide-area protection fault identification scheme

The paper presents a synchronized phasor measurement based fast and accurate comprehensive wide area back-up protection scheme for transmission lines. The proposed scheme uses Koopman Mode Analysis (KMA) technique on fault current data obtained from Phasor Measurement Units (PMUs) for fault identification in back-up protection. The principal advantage of Koopman operator is that it is based on spatiotemporal information system (STIS). In real world applications, often time and space exists together and hence, dealing with spatial aspect without considering temporal aspect is of limited use. The wide area back-up protection scheme is developed using STIS for fault identification and classification including wide variations in operating conditions of the power network. The New England 39 bus system is used as the test system which is developed on Dig-Silent power factory commercial software (PF4C) platform for evaluating the performance of proposed technique. Further, the efficacy of the proposed scheme is also tested for stressed conditions such as symmetrical fault during power swing and load encroachment and found to work effectively.     

基于瞬时电流特征的电流互感器饱和识别改进方法

电力系统发生区外故障时电流互感器饱和可能引起差动保护误动,常用的解决方法选取的开放差动保护门槛值固定,未考虑差动电流畸变程度的影响,在饱和程度较轻时门槛值偏大,使得开放保护延时较大。针对这一问题,提出了一种基于瞬时电流特征的电流互感器饱和识别的方法。该方法在虚拟制动电流识别电流互感器饱和的基础上,通过分析电流互感器的传变特性得到电流互感器饱和程度与一次电流及其导数的关系,利用BP神经网络拟合算法建立一次电流特征与最优门槛值之间的映射关系,从而可以自适应选取开放差动保护门槛值,以减少差动保护闭锁时间。仿真结果验证了所提方法的有效性。     

Performance Enhancement of Power System Stabilizer Through Colony of Foraging Ants

In this article, the concept of foraging behavior of a colony of natural ants is used for the optimal design of power system stabilizers. The controller design is formulated as an optimization task and the novel concept is used to derive the controller structure. The proposed approach is applied to the multi-machine test systems. The dynamic performance of the system has been examined under different perturbations and loading conditions. Various performance characteristics of the technique are thoroughly investigated and compared with the genetic algorithm based approach. The comparison shows that the ant colony optimization based technique converges much faster with lesser number of iterations through simple computational steps. It is also shown through extensive simulations that the proposed controller gives an excellent damping performance over different disturbances and wide range of operating conditions.     

直流偏磁影响下继电保护的误拒动机理分析及对策研究

偏磁电流经变压器中性点入侵电力系统后,线路CT将发生一定程度的饱和,影响二次系统的测量、控制等环节,严重时甚至会诱发系统继电保护的误拒动,对电力系统的安全稳定运行造成严重威胁。为此,提出了一种基于相空间轨迹识别的继电保护改进方案。首先,分析直流偏磁对保护用CT的不利影响。在此基础上,以线路保护与变压器保护为例,揭示直流偏磁影响下继电保护的失效机理。进一步地,利用相空间轨迹不受时域饱和影响的特点,提出了一种基于相空间轨迹识别的继电保护改进方案,使得所提方案能有效应对CT饱和导致的保护失效难题。最后,以典型保护失效场景为例,对比传统方法与所提改进方案的保护动作情况,验证了所提方案的有效性及优越性。     

区外故障导致变压器电流互感器饱和及差动保护误动问题研究

2016年以来,电网接连发生多起区外故障期间变压器差动保护误动的事件,严重威胁电力系统的安全稳定运行与电能的可靠供应。从理论研究、数字仿真以及现场录波数据分析等层面围绕误动原因展开了深入研究,结果表明P类互感器在区外故障期间发生暂态饱和是导致保护误动的主要原因。在此基础上,分析论证了有关的互感器饱和识别方法,并探讨了互感器选型需要考虑的多种因素。该研究旨在引起相关专家学者对于互感器暂态饱和问题的关注,并为针对该问题的分析思路与应对方法提供参考与借鉴。     

基于采样值的CT饱和检测方案的研究

电流互感器 (CT)饱和时 ,母线保护装置的可靠、安全、稳定运行是衡量母线保护的一项重要指标。文中分析了CT饱和时的特点 ,提出了一种用于数字母线保护装置的基于瞬时值的CT饱和检测方案。该方案与基于瞬时值比率制动的差动保护有机的结合 ,使数字母线保护装置具有快速、较高的抗CT饱和的优势。已有采用此方案的数字母线保护装置在电力系统中运行。