CVT一次隔离开关操作对变压器过励磁保护影响的研究

电容式电压互感器(CVT Capacitor Voltage Transformer)含有电容和非线性电感元件,当一次隔离开关操作时可能引起二次电压异常,影响变电站继电保护装置的正常运行,甚至引起主变过励磁保护误动作。运用ATP建立CVT及其所运行的变电站的等值电路模型,仿真计算一次侧隔离开关操作过程中CVT二次电压情况。比较影响二次侧电压的主要因素,分析在CVT一次侧隔离开关操作过程中主变过励磁保护误动作的原因。正常操作不会引起主变过励磁保护误动作。     

Protection system representation in the Electromagnetic TransientsProgram

This paper concerns the addition of the few critical elements of a protection system to the Electromagnetic Transients Program (EMTP), which is one of the most widely used programs for the simulation of transients in power systems. It contains models for almost every major power system component. A protection system consists of instrument transformers, relays, and circuit breakers. Models for current transformers (CTs) and capacitor voltage transformers (CVTs) are developed, validated, and incorporated in the EPRI/DCG EMTP Version 2.0. The user can define the values of the CT and CVT parameters. Total FORTRAN capability has been added to the EMTP; new subroutines and an inbuilt structure to allow the linking of user-defined FORTRAN subroutines with the main EMTP are explained. This capability is necessary to simulate computer relay algorithms. These new features in the EMTP together constitute the critical elements of a protection system. Thus, it is now possible to simulate the dynamic interactions between a power system and a protection system     

On the Coordination of Current Transformers and Relay Protection and Automation Operation of Electric-Power Systems in Transient Short-Circuit States

Due to the increasing number of incorrect actions of relay protection in transient processes accompanied by saturation of current-transformer (CT) cores, there is a need for developing a domestic standard for protective CTs designed to operate in transient states. The following ways to provide proper relay protection operation in transient states accompanied by saturation of CTs in operation are considered: (a) CT shedding that includes an increase in control cables’ cross sections, (b) development of relay-protection and automation algorithms based on the recognition of crash conditions in an appropriate CT transformation time interval of 2–3 ms, (c) development of relay-protection and automation algorithms with virtual CT error compensation, and (d) development and introduction of new primary-current converters. It is concluded that, for existing large power plants with installed TPX current transformers, it is advisable to use improved digital relay-protection algorithms for information processing obtained from CTs, for example, by methods (b) and/or (c).     

电流互感器饱和引起的保护误动分析及试验方法

近年来．广东省内多个发电厂出现过高压厂用变压器或起动一备用变压器在区外故障时或厂用大容量电动机起动时差动保护误动作的情况。为此．介绍了通过电流互感器伏安特性试验确定其饱和特性的方法．并用试验验证了这种方法的可行性。最后提出了关于避免差动保护因电流互感器饱和而误动的措施。     

A compensation scheme for CVT transient effects using artificial neural network

Capacitive voltage transformers (CVTs) provide instrument-level voltage signals to meters and protective relays in high voltage (HV) systems. The transients in CVTs could lead to protective relay mal-operation. This paper proposes the use of an artificial neural network (ANN) scheme to correct CVT secondary waveform distortions due to CVT transient. This is accomplished by using samples of voltage signals to achieve a good approximation of the inverse transfer function of CVTs, thus an accurate estimation of the primary voltages. Simulations are performed and the impacts of different parameters are studied. Performance results show that the proposed scheme is accurate and reliable. The proposed scheme can also be implemented on a digital signal processor board for real-time application.     

1000 kV交流输电系统动态模拟研究

为了准确模拟1000 kV交流输电系统,基于晋东南- 南阳-荆门交流特高压试验示范工程提供的参数,设计了满足要求的高性能线路元件模型(线路阻抗角可高达88．5°)、并联电抗器元件模型、电容式电压互感器及电流互感器等。建立1000 kV交流输电系统的动态模拟系统能够为所模拟系统继电保护装置的设计和选型提供试验条件,并可应用于1000 kV交流输电系统暂态特性的研究。     

Method to measure CVT transfer function

In this paper, the results of tests on a 275 and a 400 kV capacitor voltage transformer are presented. The objective has been to determine the transfer function of the CVTs in the Scottish Power extra-high-voltage (EHV) system, which in turn can be used to compensate for the errors at harmonic frequencies. As a primary requirement, the magnetic elements of the capacitor voltage transformers (CVTs) must be excited at nearly their nominal magnetic states. A test procedure is presented, which does not require CVT disassembling and/or modeling and ensures that the above essential requirement is satisfied. The effects of burden on the frequency response is also presented. It is confirmed that the size and nature of burden affect the CVT frequency characteristic.     

谐波条件下电容式电压互感器测量误差影响因素分析

电容式电压互感器(CVT)测量的准确度影响着电能计量的准确性和继电保护动作的可靠性。为了研究在谐波条件下CVT测量的准确性,为提高CVT的测量精度提供参考依据,文章基于CVT的等效电路模型,推导出了谐波条件下CVT变比的表达式,研究了谐波条件下CVT的励磁阻抗和电容分压比对测量精度的影响规律,并通过仿真分析对其进行了验证。     

Development and hardware implementation of a reliable protective relay data acquisition system

This paper proposes an intelligence based protective relay data acquisition system to correct current transformers and capacitive voltage transformers secondary waveform distortions. The protective relay data acquisition system receives voltage and current signals from current transformers and capacitive voltage transformers and prepares the inputs to the main board after some pre-processing. Current transformers and capacitive voltage transformers provide instrument level current and voltage signals to meters and protective relays in high voltage and extra high voltage systems. The accuracy and performance of protective relays in high voltage and extra high voltage systems are directly related to steady state and transient performance of current transformers and capacitive voltage transformers. Current transformers saturation and capacitive voltage transformers transient could lead to protective relay mal-operation or even prevent tripping. The key of the proposed scheme is to use artificial neural network to achieve the inverse transfer functions of current transformers and capacitive voltage transformers. Simulation studies are preformed and the impacts of changing different parameters are studied. Performance study results show that the proposed scheme is accurate and reliable. The proposed algorithm has also been implemented and tested on a digital signal processor board. Details of the implementation and experimental studies are provided in the paper.     

Protection of power transformer using multi criteria decision-making

Power transformers are protected by different relays that operate independently. Malfunction of each relay has a major role in reducing the reliability of the protection system. In order to mitigate the main drawbacks of the power transformer relays, an overall protection scheme is presented in this paper. This scheme proposes a novel multi criterion algorithm using decision-making based on fuzzy logic. In this paper the outputs of restricted earth fault relay and a directional check unit, are combined with the output of the differential protection relay. Therefore, problems that are pertaining to independent operation of each relay have been mitigated and the relays cover protection blind spots of each other. The improved power transformer protection (IPTP) scheme enhances the sensitivity and reliability of the power transformer protection. Extensive simulations are used to measure the effectiveness and merit of the proposed IPTP relay. The above efforts result in a multi criteria approach for protection of power transformers.     

Current transformers with secondary current rating lower than 5 A

Current transformers (CT) designed for protective relaying applications with secondary current ratings substantially lower than 5 A offer advantages when lead resistance tends to predominate over the relay burden. The lower current rating may allow the use of a smaller wire size or a smaller CT core as well as providing improved transient performance. With the longer lead runs associated with EHV stations, and the advent of low-burden solid-state relays, the picture has become favorable for such lower current rating, and their advantages and disadvantages are considered. Also addressed are working voltage levels, open-circuit voltages, noise sensitivity, circuit breaker interchangeability, relay stocking requirements, and CT costs     

基于虚拟电容式电压互感器的能量平衡保护

容式电压互感器(CVT)传变特性使继电保护装置获得二次电压和电流瞬时值不再满足被保护输电线路分布参数模型，影响了能量平衡保护的可靠性。该文提出基于虚拟CVT的能量平衡保护新方案。该方案将保护获得的电流量经过虚拟数字CVT传变后，再进行能量平衡保护计算。为论证基于虚拟CVT能量平衡保护的可靠性，文中从理论上分析均匀输电线路分布的电压和电流量经过同样的CVT传变后，暂态电压与电流的相互关系仍满足输电线路分布参数模型，并进一步推论出线路分布电压和电流量经过相同的线性电路传变后其相互关系也不发生改变，这就为输电线路保护应用传变后信息量提供了理论参考。最后用ATP仿真验证基于虚拟CVT能量平衡保护新方案。     

绝缘在线监测中电容式电压互感器的误差校正

对引起电容式电压互感器（CVT）稳态误差的相关因素进行了分析。根据CVT结构及参数建立了用于校正CVT稳态误差的参数模型。将该模型输出电压的误差仿真结果与准确级试验结果进行对比,验证了该模型的有效性。基于该模型提出一种绝缘在线监测中实时校正CVT稳态误差的新方法:通过采集测量绕组的输出电压和各二次绕组的负载电流信号,利用基于Levenberg-Marquardt算法的非线性最小二乘拟合算法提取信号中的基波分量,结合CVT参数模型的数学方程确定CVT的一次电压。分析结果表明,该方法能够有效减小因系统频率、CVT二次负载大小及功率因数的变化对CVT比差及角差的影响,为二次监测设备提供高精度的电压信号。     

ADAPTIVE DIGITAL DIFFERENTIAL RELAY OF PARABOLIC CHARACTERISTIC FOR TRANSFORMER PROTECTION

ABSTRACT

In the present paper a variable-bias digital differential relay for power transformers protection is proposed. The amount of bias is varied linearly with the value of through current (mean of the currents entering and leaving the transformer). To take care of mismatch between CT ratios on primary and secondary sides of the power transformer, secondary currents of CT are corrected using a precalculated correction factor for the power transformer tap in operation. In the event of an abnormal operation, a fault discriminant based on fundamental, second and fifth harmonic components of differential current is used. It provides discrimination between faults and magnetizing inrush /overexcitation copditions. Trip and indication signals arc issued in the former case, while only an indication is given in the latter case.

The relay scheme is simulated on a personal computer. The advantage of the proposed scheme is its ability to detect even low-level internal faults, which may not be detected by a fixed-bias relay. The relay is restrained from operations on magnetizing inrush and overexcitation. The proposed relay is stable on those heavy through faults on which a fixed-bias relay may operate.     

Protection and commissioning of multifunction digital transformer relays at medium voltage industrial facilities

The application of multifunction digital relays to protect medium voltage power transformers has become a common industrial practice. Industrial transformers, unlike utility transformers, frequently use neutral grounding resistors to limit ground current during faults to the 200-400-A level on medium voltage systems. This paper will discuss why these types of transformers require sensitive ground differential protection. The paper will also discuss the basics of transformer protection including phasing standards, through-fault withstand capability, differential/fusing/overcurrent protection, slope, current transformer (CT) requirements, and harmonic restraint, and communicating these properly to new digital relays. The rationale for providing transformer overexcitation protection on all major transformers within mill facilities is also addressed. Advancements in digital technology have allowed relay manufacturers to include more and more relay functions within a single hardware platform as well as address increasingly more transformer winding configurations. This has resulted in digital transformer relays requiring an Einstein to set and an Edison to commission. Since there are few Einsteins or Edisons among us, the next generation of transformer relays needs to concentrate on this complexity issue in addition to technical improvements. This paper addresses these issues that the author believes are the major shortcomings of existing digital transformer protective relays.     

考虑CVT输出特性的换流变剩磁测算方法

特高压换流变空载合闸时,其剩磁会改变铁心磁通的大小和方向,极易造成严重的励磁涌流,导致换流变无法正常投入运行。针对换流变剩磁测算,分析电容式电压互感器(CVT)输出数据存在误差是由系统频率变化引起的,根据误差式进行校正并基于电压积分法给出考虑CVT输出特性的换流变剩磁测算方法。在PSCAD/EMTDC中搭建相关仿真模型,对Y、D桥换流变的电压正向、负向峰值处分别进行分闸实验,并根据所提测算方法确定换流变剩磁。利用校正前、校正后以及监测的换流变剩磁完成选相合闸对比实验,实验结果表明所提方法能够有效测算换流变剩磁,为励磁涌流抑制手段提供参考依据。     

110 kV 电容式电压互感器谐波测量特性试验及仿真

针对电容式电压互感器（capacitor voltage transformers,CVT）测量谐波时从一次回路到二次回路非线性传递的运行特性,从现场试验和软件仿真两方面对CVT谐波测量特性进行研究。以某型号110 kV CVT为研究对象,综合考虑杂散电容、耦合电容、电容分压器介质损耗以及补偿电抗器等效电阻,构建CVT高频等效电路。运用Matlab搭建CVT谐波模型,获得CVT幅频特性曲线。在现场开展谐波测量试验,通过试验结果与仿真结果对比证明仿真模型正确性。最后,通过仿真模型验证谐波电压含量和基波电压幅值对CVT谐波测量特性并无影响。     

Design, implementation and testing of a microprocessor-basedhigh-speed relay for detecting transformer winding faults

The authors present the design, implementation and testing of a microprocessor-based high-speed relay that correctly detects transformer winding faults. Instead of relying on the presence of harmonics to identify magnetizing inrush, it uses a nonlinear model of the transformer to determine the state of its health. One version of the relay is suitable for protecting single-phase transformers, whereas another version is for protecting three-phase transformers. A three-phase delta-wye 15 kVA transformer was used to test the relay for a variety of operating conditions. The results show that the relay performs well     

基于电容式电压互感器暂态误差估计的自适应距离保护

针对电容式电压互感器(CVT)暂态过程引起距离保护超越的问题，通常采用简单延时的方法，来防止保护的超越。这对电力系统的安全稳定运行是很不利的。该文提出了CVT暂态误差的实时估计方法，即根据所选用的基波算法和CVT的额定参数求出误差系数，然后根据二次电压的大小实时估计出电压幅值的测量误差。据此提出了一种自适应距离保护方法，根据误差的大小调整动作门槛。仿真计算表明，文中提出的方法对于普通线路，其动作性能几乎不受影响；对于短线路，也能够在近处故障时快速动作。其整体性能较常规的距离保护方案有了很大的提高。