Busbar differential protection in conjunction with a current transformer compensating algorithm

The design, evaluation and implementation of a busbar differential protection relay that operates in conjunction with a current transformer (CT) compensating algorithm are described. Prior to saturation, the secondary current of a CT is not compensated. The compensating algorithm detects the start of first saturation on the basis of the third-difference function of the current and estimates the core flux at the first saturation start by inserting the negative value of the third- difference function of the current into the magnetisation curve of a CT. Thereafter, it calculates the core flux and then the corresponding magnetising current in conjunction with the magnetisation curve. The calculated magnetising current is added to the measured secondary current to obtain the correct secondary current. The algorithm can estimate the correct current irrespective of the level of the remanent flux. In the proposed busbar protection scheme, a current differential relay with the single-slope operating characteristic is used on the basis of the compensated current of the saturated CT. Test results indicate that the relay shows satisfactory performance for the various external and internal faults with CT saturation, particularly in the case of a progressive fault from a feeder fault to a busbar fault. The algorithm is implemented in a prototype relay based on a digital signal processor. The relay achieves greater stability on external faults, enhanced sensitivity on internal faults and fast operation on internal faults with CT saturation.     

New differential protection scheme for tapped transmission line

The development of a new differential protection scheme for tapped transmission lines using wavelet transform is presented. At each relay locations, using the most suitable mother wavelet, the measured three line currents are decomposed up to third level. Thereafter, third-level approximation coefficients are reconstructed and used to derive the operating and restraining quantities. The proposed scheme has been tested extensively using the PSCAD/EMTDC software package with fault data, generated by modelling UKAI-KAKRAPAR 400 kV line of the Indian power system. The proposed scheme eliminates many of the problems encountered with tapped transmission lines such as high resistance faults, out feed current in case of internal and external faults, insufficient current for tripping and provides better discrimination between external and internal faults in case of loss of generation at one end. At the end, a comparative evaluation of the conventional current differential protection scheme with the proposed scheme is also presented. Dependability and security have been studied separately.     

Power transformer protection using S-transform with complex window and pattern recognition approach

A new approach for power transformer protection using S-transform with complex window to distinguish between inrush current and internal fault is presented. The S-transform with complex window is used to extract patterns of transient current samples during inrush and faults. S-transform is a very powerful tool for non-stationary signal analysis giving the information of transient currents both in time and in frequency domains. The spectral energy is calculated for inrush and internal faults and an energy index is found out to distinguish between inrush magnetising current and internal faults. The simulation results and the results obtained using real-time data from a transformer in the laboratory environment indicate the robustness of the proposed technique     

Radial basis probabilistic neural network for differential protection of power transformer

Protection of medium- and large-power transformers has always remained an area of interest of relaying engineers. Conventionally, the protection is done making use of magnitude of various frequency components in differential current. A novel technique to distinguish between magnetising inrush and internal fault condition of a power transformer based on the difference in the current wave shape is developed. The proposed differential algorithm makes use of radial basis probabilistic neural network (RBPNN) instead of the conventional harmonic restraint- based differential relaying technique. A comparison of performance between RBPNN and heteroscedastic-type probabilistic neural network (PNN) is made. The optimal smoothing factor of heteroscedastic-type PNN is obtained by particle swarm optimisation technique. The results demonstrate the capability of RBPNN in terms of accuracy with respect to classification of differential current of the power transformer. For the verification of the developed algorithm, relaying signals for various operating conditions of the transformer, including internal faults and external faults, were obtained through PSCAD/EMTDC. The proposed algorithm has been implemented in MATLAB.     

Time?frequency transform approach for protection of parallel transmission lines

A new approach for protection of parallel transmission lines is presented using a time-frequency transform known as the S-transform that generates the S-matrix during fault conditions. The S-transform is an extension of the wavelet transform and provides excellent time localisation of voltage and current signals during fault conditions. The change in energy is calculated from the S-matrix of the current signal using signal samples for a period of one cycle. The change in energy in any of the phases of the two lines can be used to identify the faulty phase based on some threshold value. Once the faulty phase is identified the differences in magnitude and phase are utilised to identify the faulty line. For similar types of simultaneous faults on both the lines and external faults beyond the protected zone, where phasor comparison does not work, the impedance to the fault point is calculated from the estimated phasors. The computed phasors are then used to trip the circuit breakers in both lines. The proposed method for transmission-line protection includes all 11 types of shunt faults on one line and also simultaneous faults on both lines. The robustness of the proposed algorithm is tested by adding significant noise to the simulated voltage and current waveforms of a parallel transmission line. A laboratory power network simulator is used for testing the efficacy of the algorithm in a more realistic manner.     

35kV母线电压不平衡原因探究

电网电压不平衡影响电能质量,由于运行方式改变后消弧线圈补偿度不够,导致35kV母线电压不平衡,分析了电压不平衡原因,总结了孝路接地、电压导感器高压高压熔丝熔断、铁磁谐振、电压互感器原因造成35kV母线电压不平衡,并提出了解决办法。     

Prototype Fault Current Limiter Using Transformer and a Modular Device of YBCO Coated Conductor

A superconducting fault current limiter (SCFL) consisted of a transformer with low reactance connected to the power line and with the secondary winding short-circuited by a modular superconducting limiter device with 16 elements connected in series was constructed and tested. The designed coupling transformer has low dispersion reactance in order to limit the voltage drop in the power line within the range of 5 % to 10 %. The experimental results showed that an insertion of a 0.125 Ω resistance limited the peak current to a factor of 2.5 times of the unlimited current. The power dissipation reached 39 kW during 100 ms, with an energy density of 380 J/cm³. Based on these results, the SCFL will be further tested in a 3 MVA (15 kV/380 V) generator for currents up to 10 kA.     

Probabilistic neural-network-based protection of power transformer

An optimal probabilistic neural network (PNN) as a core classifier for fault detection and status indication of a power transformer has been presented. In this scheme, various operating conditions of a transformer are distinguished using signatures of the differential currents. The proposed differential protection scheme is implemented through two different structures of PNN, that is, one having one output and the other having five outputs. The developed algorithm is found to be stable against external fault, magnetising inrush, sympathetic inrush and over-excitation conditions for which relay operation is not required. For the test data of fault, it is found to operate successfully. The performance of proposed PNN and classical artificial neural network (ANN) has been compared. For evaluation of the developed algorithm, relaying signals for various operating conditions of a transformer are obtained by modelling the transformer in PSCAD/EMTDC. The algorithms are implemented using MATLAB. The results show the capability of PNN in terms of classification accuracy and speed in comparison to classical ANNs.     

Design and evaluation of a compensating algorithm for the secondary voltage of a coupling capacitor voltage transformer in the time domain

This study describes the design and evaluation of a compensating algorithm for the secondary voltage of a coupling capacitor voltage transformer (CCVT) in the time domain by considering the hysteresis characteristics of the core. The major error of the CCVT is the voltage across the capacitor and the tuning reactor caused by a non-sinusoidal exciting current. The proposed algorithm estimates the voltage across the capacitor and the reactor by considering the effect of the hysteresis characteristics of the core and adds it to the measured secondary voltage to obtain the correct voltage. The algorithm reduces the errors of the CCVT significantly both in the steady state and during a fault. The performance of the algorithm is verified under the various fault conditions by varying the fault distance, the fault inception angle and the fault impedance with the EMTP generated data. Test results clearly indicate that the algorithm can increase the accuracy of a CCVT significantly under the fault conditions as well as in the steady state. The algorithm helps to improve the performance of a protection relay or a metering device.     

14 kV single-phase superconducting fault current limiter based on YBCO films

We present the fabrication and short circuit test results of a 14 kV single-phase resistive superconducting fault current limiter (SFCL) based on YBa₂Cu₃O₇ (YBCO) films. Individual components were processed using the 4″ YBCO films and have the rated voltage and current of 600 V and 35 A at 77 K, respectively. Twenty four components, eight components in series and three lines in parallel, make a module having the rated voltage and current of 4.8 kV and 105 A, respectively. Three modules were assembled in series to produce the SFCL working at 77 K, a 14 kV single-phase machine for the 22.9 kV Y-Y grid. short circuit tests were successfully conducted in an accredited test facility with the maximum fault currents up to 14.1 kA_P. All components quenched together upon faults and shared the rated voltage evenly without any supplementary device between the modules. This proves that the SFCL based on YBCO films may not only work reliably at 22.9 kV, but also provide technical feasibility for higher voltage application including the transmission grids.     

New approach to adaptive single pole auto-reclosing of power transmission lines

In this study, a novel digital algorithm is introduced for recognition of arcing (transient) faults and determination of dead time for adaptive auto-reclosing. The algorithm distinguishes between arcing and permanent faults by using the zero sequence voltage measured at the relaying point. If the fault is recognised as an arcing fault, then the third harmonic of the zero sequence voltage is used to evaluate the extinction time of secondary arc and to initiate reclosing signal. The proposed algorithm uses an adaptive threshold level and therefore no significant adjustment is needed for different transmission systems. Moreover, its performance is independent to fault location, line parameters and the system pre-fault operating conditions. The algorithm has been successfully tested for various faults and operating conditions on a 400 kV overhead line using the electro-magnetic transient program (EMTP). The test results have demonstrated validity of the algorithm in determining the secondary arc extinction time and blocking unsuccessful automatic reclosing during permanent faults.     

Sampling and real-time methods in electro-optic probing system [LSItesting]

Sampling and real-time methods in electro-optic (EO) probing systems using a laser diode (LD) for measuring the voltage waveform at internal nodes of the high-speed LSI are described, comparing performance to other electric instruments. The voltage sensitivity was improved by using an external ZnTe E-O probe and a low-noise LD. A sampling system using a pulsed LD has a frequency bandwidth of 10 GHz and a minimum detectable voltage of 430 μV/√Hz. The corresponding values for a real-time system using a CW LD and a high-speed photodetector are 480 MHz and 23 mV with 700 accumulations. Each system is based on a mechanical prober and a microscope. The advantages of a high temporal resolution, noncontact and noninvasive method are demonstrated during various measurements in several different areas: standing waves on a stripline, ring resonator and voltage waveform characteristics in a high-speed MMIC by the sampling method, long logic pattern signal, voltage waveforms at internal nodes in an ECL IC and those of a transport electrode in CCD by the real-time method     

Fast Correlation Estimation by a Random Reference Correlator

Fast correlation calculations are needed in many technological and scientific fields. Conventional digital correlators require fast and expensive analog-to-digital converters and digital multipliers. This paper describes the implementation of a fast, inexpensive, and unbiased correlation estimator. The estimator is based on the ternary Random Reference Correlation (RRC). Unbiasness is achieved by the addition of statistically independent, uniformly distributed auxiliary noise sources to the input signals. The use of a ternary quantizer eliminates the need of an A/D converter and multiplier, which are replaced by relatively simple logic circuits. A prototype system is described. Real-time correlations of signals, sampled at a rate of about 4 kHz, were achieved by the prototype system. Correlation errors were found to be in the range of one percent. Increasing the internal clock frequency and using faster logic components will improve system performance to perform real-time correlation calculations of signals sampled at 50 kHz.     

Estimation methods using dynamic phasors for numerical distance protection

Several new methods for faulted transmission line parameters estimation in phasor and time domain are proposed, that eventually improve the overall performance of numerical distance relays. The concept of dynamic phasors is introduced to accommodate the time-variant nature of the current and voltage signals during transients and faults. Based on dynamic phasor transmission line models, direct and indirect estimation methods are derived. For the proposed indirect estimation method, stability of prediction error dynamics is assured by using the Lyapunov direct method. Presented estimation techniques are compared with a conventional stationary phasor solution as well as with a recursive least-square estimator derived in the discrete time domain. In the evaluation, more realistic assumptions are considered with regards to distortion of the input voltage and current signals along with the variable fault resistance because of arcing faults. Simulation results and actual field measurements are included for performance evaluation of the proposed estimators.     

New principle for transmission line protection using phase portrait plane

Phase portraits are a powerful mathematical model for describing oriented textures. An isotangentbased approach in a phase portrait is introduced to discriminate between internal and external faults. The scheme described is new and is a different approach to the problem of relaying ground faults on transmission lines. The geometrical theory of differential equations is used to drive a symbol set on the basis of the visual appearance of phase portraits. The `phase portrait? of the instantaneous rate of change of current against voltage gives valuable information on the transient and stability characteristics of system configuration with high fault resistance and shunt susceptance of the transmission lines. Hundred percent of the protected zone is successfully discriminated rather than the external zone. An additional parameter that is considered, which is sometimes neglected in protection studies, is the shunt susceptance of the transmission lines.     

模糊C均值聚类在光伏阵列故障样本数据识别中的应用

光伏电站由数量庞大的光伏组件构成,因复杂的生产工艺及艰苦的工作环境,光伏系统直流侧故障频发,直接影响到光伏系统的发电效益。如何从光伏阵列的运行数据中提取有效的故障样本,并对其进行识别,是建立故障模型的重要步骤。因此提出一种基于模糊C均值(fuzzy C-means,FCM)聚类算法对故障样本进行划分及标识的方法。首先对故障条件下光伏阵列的输出特性进行分析,提取出故障特征向量(开路电压U_oc,短路电流I_sc,最大工作点电压U_m,最大工作点电流I_m)。为排除外部激励条件对电气参数的影响,将故障特征向量统一转换至标准测试条件(standard test condition,STC)。最后根据FCM算法良好的模糊信息处理功能,对开路故障、短路故障、阴影故障、异常老化故障的样本进行聚类划分。实际运行数据证明,该方法可以精确、可靠地对光伏系统直流侧典型故障的样本进行智能聚类,并有效地描述不同故障下光伏阵列电气参数的分布特征。     

Effect of transformer type on estimation of financial loss due to voltage sag - PSCAD/EMTDC simulation study

The fault in transmission and distribution lines of the power system creates voltage sag. This paper focuses on estimating the financial loss due to such voltage sags. The transformer in a system has great impact on the estimated number of voltage sags at the sensitive load due to faults in the system. So the financial loss in a system due to sag will be affected by the type of transformer in that system. The financial losses for different types of transformers are calculated in probabilistic manner for different types of load groups. Effect of the transformer in the system, on financial loss due to voltage sag is presented in this paper.     

基于样本熵的风力发电机早期故障检测

针对发电机定子匝间短路和转子断条等早期故障特征具有幅值小、非稳态、易受工况影响等特点,引入样本熵算法实现风力发电机定子电流和电磁转矩信号特征提取,并模拟不同负载条件下故障信号,实现定量参数分析。分析结果表明,样本熵算法适用于在变工况及噪声干扰条件下,对短数据参量进行分析并实现故障特征定量描述,可用于风力发电机早期故障检测和实时在线监测。     

基于改进型两次平衡对检法的感应分压器自校准方法

介绍了参考电势变压器和指零仪变压器设计原理,分析了屏蔽间泄漏对测量结果的影响;对常规两次平衡参考电势对检法进行了改进,使零平衡和段平衡测量过程中,参考电势和测差电路均可实现等电位保护;对自校准方法进行了推导,校准结果仅与段平衡和零平衡时锁相放大器测量的电压差值相关,与参考电势变压器、指零仪变压器、辅助变压器等的误差无关,屏蔽间泄漏的影响也得到消除。对1kV感应分压器进行了校准实验,并对校准结果进行了测量不确定度分析,其相对扩展不确定度的评估结果为5.4×10^-8(k=2)。     

Analysis on the Fusion Technology of a Commercial Transformer and an SFCL

In this study, a new concept is proposed on a combination device with functions of a commercial transformer and a superconducting fault current limiter (SFCL). This device serves as a transformer by stepping the voltage up or down in normal condition. When a transient phenomenon occurs in the power system, it serves as an SFCL to limit the fault current. The device quickly detects the line current using a current transformer (CT), and is based on the high-speed, silicon-controlled rectifier (SCR) interrupter operation. This is done by identifying the fault using an SCR switching control system. The test results showed that the fault current was limited by the impedance of the superconducting element a half cycle after a fault occurred. An SCR that initially had a normally open contact was turned on within a half cycle. However, an SCR with a normally close contact was turned off after a half cycle because the current dropped below the holding current after a half cycle. The voltage of the superconducting element was low in the step-down turn ratio condition of the transformer. This was because the secondary and tertiary windings were connected in series due to the SCR-1 turn-off condition, and the sum of voltages on each winding appeared on the superconducting element. By combining the existing power device technology and an SFCL technology, it is expected that the existing problems of an SFCL can be addressed to construct a smart power system.