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.     

Correction of saturated current transformers secondary current using ANNs

Current transformers (CTs) provide instrument-level current signals to meters and protective relays. Protective relays' accuracy and performance are directly related to steady-state and transient performance of CTs. CT saturation could lead to protective relay maloperation or even prevent tripping. This paper proposes the use of an artificial neural networks scheme to correct CT secondary waveform distortions. The proposed module uses samples of current signals to achieve the inverse transfer function of CT. Simulation studies are preformed and the influence of changing different parameters is studied. Performance studies results show that the proposed algorithm 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 this paper.     

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     

Estimation of Electronic Suppression Circuit Resistance for Protective Relaying Applications

The transient response of instrument transformers affects the performance of high-speed relays, particularly speed and overreach in case of distance protection. Transient response of a capacitive voltage transformer depends on the type of ferro-resonance suppression circuit (FSC), i.e., active, passive, or electronic, provided in the secondary to damp the ferro-resonant oscillations. The objective of this article is not to estimate optimum resistance but to provide a systematic approach instead of the trial-and-error approach to estimate damping resistance for the electronic ferro-resonance suppression circuit used in the: capacitive voltage transformer for protective relaying application. The performance of the numerical distance relay for the: capacitive voltage transformer with estimated damping resistance in the electronic ferro- resonance suppression circuit is tested and validated in real time using a field programmable gate array.     

A new PMU based power swing detector to prevent mal-operation of distance relay

In conventional transmission line protection, a distance relay is used to provide the primary as well as remote backup protection. The voltage and current phasors measurement needed by the distance relay for determining the impedance may be affected by the power disturbances such as power swing. Consequently, this power swing may cause mal-operation of Zone three distance relays which in turn may affect on the reliability of the whole protective scheme. To mitigate these effects and hence improve the relay reliability, this work proposes a new real-time power swing detector using phasor measurement units for blinding the distance relay only during this transient disturbance. However, this developed detector will not block relay when the power swing accompanied with faults. To validate the present work, the performance of developed enhanced distance relay is tested by signals generated by Simulink/MATLAB simulator under different conditions. The test results show that this proposed scheme provides good discrimination between the transient currents and the fault current which in turn it may contribute in enhancing the reliability of Distance relay.     

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.     

Laboratory investigation of a distance-protection technique for double circuit lines

A novel digital distance scheme has been implemented on a 32-bit digital signal processor board. The scheme is tested on a physical model of double circuit lines of equal impedance with a source at each end. Two relays instead of four are proposed for the two lines. Each relay is fed by three voltage and six current signals. The technique is based on the comparison of the measured impedance of the corresponding phases. Tests conducted on the physical model for various faults show that high fault resistance, current in-feed, balance-point location, out-of-step operation, and far-end faults are solved. Moreover, 100% of the line is protected.     

A Microprocessor-Based Scheme for Testing Protective Relays

ABSTRACT

This paper presents a general hardware scheme for testing protective relays using microprocessor based systems. The microprocessor simulates the relaying signals for test purpose and monitors the relay performance. Based on the proposed hardware, a teat procedure for directional overcurrent relays is presented in detail. Typical test results of various routine tests conducted on a commercial single phase directional over-current relay clearly demonstrate the efficacy of the proposed technique for conducting tests on commercial relays.     

A novel scheme for current-only directional overcurrent relay

Overcurrent relays are widely used as main protection in sub-transmission and distribution systems. In mesh and multi-source networks, application of directional relay is unavoidable. Traditional directional overcurrent relays use the reference voltage phasor as the polarizing quantity to estimate the direction of the fault. Traditional direction distinguishing scheme is unreliable in the case of close-in faults. In this paper, a novel algorithm for directional overcurrent relay is proposed. The new algorithm uses only current signals for determining the fault direction. It uses superimposed component of the current signal and does not require phasor estimation. This new algorithm uses pre-fault current signal as the polarizing quantity. The proposed method is tested on simple power system in different situations. The results show it leads to fast and reliable directional protection.     

Positional protection of transmission systems using GlobalPositioning System

This paper presents a new technique for the protection of power transmission systems by using the Global Positioning System (GPS) and fault generated transients. In the scheme, the relay contains a fault transient detection system together with a communication unit, which is connected to the power line through the high voltage coupling capacitors of the CVT. Relays are installed at each busbar in a transmission network. These detect the fault generated high frequency voltage transient signals and record the time instant corresponding to when the initial travelling wave generated by the fault arrives at that busbar. The communication unit is used to transmit and receive coded digital signals of the local information to and from the associated relay(s) in the system. At each substation, the relays determine the location of the fault by comparing the GPS time stamps measured locally with those received from the adjacent substations. Extensive simulation studies presented in the paper demonstrate the feasibility of the scheme     

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.     

Removal of dc offset and subsynchronous resonance in current signals for series compensated transmission lines using a novel Fourier filter algorithm

In this paper, a novel Fourier filter algorithm for high-accuracy and high-convergence-speed fundamental frequency component calculation of series compensated transmission line is proposed. The performance of digital relaying is highly reliant on the algorithm of digital filtering, especially involving the dc offset and subsynchronous resonance. Input signals of protective relays must be filtered in advance to reject unwanted quantities and retain signal quantities of interest. Accuracy and convergence speed of filter algorithms are essential for protective relays. However, the voltage and current signals contain large harmonics and dc offset during fault intervals. A widely applied filter algorithm, the Discrete Fourier Transform (DFT), can easily eliminate harmonics via simple calculations. The dc offset heavily influences the precision and convergence speed of fundamental frequency component from DFT. Under high resistive fault conditions, the current signals will have subsynchronous resonance. The subsynchronous resonance current and voltage signals influence the performance of DFT much more than the dc offset. Although capable of erasing high order harmonics, the low pass filter cannot remove the dc offset and subsynchronous resonance. The Alternative Transient Program (ATP) of the Electromagnetic Transients Program (EMPT) is utilized to simulate the transient responses of series compensated transmission lines during the fault period. Adopting the proposed algorithm in distance relays effectively locks the dc offset and subsynchronous resonance signals and quickly estimates the accurate fundamental frequency components.     

DYNA-TEST simulator for relay testing. I. Design characteristics

The authors describe a new digital dynamic testing (DYNA-TEST) simulator developed for protection relay applications. This simulator is capable of producing voltage and current transients that correspond to actual fault events in the power systems. Fault transients are generated either by simulating power system faults using an electromagnetic transient program or by replaying records of fault signals captured in substations by digital fault recorders. These signals are used to test protection relays. Such an approach to relay testing represents a major improvement over steady-state testing     

Improved simulation models for current and voltage transformers inrelay studies

Improved models for current transformers (CT), potential transformers, and capacitive voltage transformers (CVT) for a relay software library are presented. The magnetizing characteristic in each of these transformers is based on the nonlinear power curve formulation of J.R. Lucas (1988). The flux-current loops of the transformer core are self-generated rather than predetermined or based on look-up tables. The results presented show that the models developed for the transformers behave as expected, and agree with reported field investigations. Models for the CT and CVT give acceptable results, including those under remanent conditions and ferroresonant conditions. These models are thus necessary components for relay studies carried out with electromagnetic transient programs whether offline or in real time     

An accurate technique for locating faults by distance relays

This paper presents a new protection scheme for improving the performance of the distance protective relays in transmission systems. The determination of fault zone by the proposed scheme is based on data sheared locally with other distance relays at the same station, in addition to a command from the distance relay on the other end of the protected line. Based on analyzing this information the relay decides the correct fault zone. With the proposed scheme, the first zone in the distance relay covers the full length of the protected line while the second zone covers the full length of any line following the protected line irrespective of its length, etc. The scheme is examined against wide range of setting problems of a real part of a HV network with real relay settings. It succeeds in solving distance relay setting problems, e.g. the problem of a long line following a short line or vice versa, low in-feed, open CB at one end and high impedance faults.     

大型光伏电站汇集系统的故障特性及其线路保护

集中式与分布式光伏系统在拓扑和控制上的显著区别,造成两者故障特性的不同,导致现有含分布式光伏的配电网故障分析方法和保护原理在集中式光伏电站汇集系统中无法适用。为此,针对广泛应用于集中式光伏逆变器中的正负序双同步旋转坐标系电流控制器,计及直流侧光伏电池板电源特性影响,推导出不同控制目标下故障电流统一表达式。在此基础上,结合现场短路试验数据,考察850 MW大型光伏电站内35 k V汇集线路的电流保护性能,证明架空线下游电流保护存在不能正确动作现象,同时提出距离保护新配置方案。在PSCAD/EMTDC中搭建光伏电磁暂态详细模型并利用现场实测数据验证了模型的正确性,大量仿真实例证明了新保护配置的有效性。研究结果为大型光伏电站汇集系统保护配置提供参考。     

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.     

Relay setting considerations and operational experience of a long500 kV transmission line

The large capacitive susceptance, large load magnitude, and substantial phase unbalance before and after a single-pole trip operation have a substantial effect on the performance of the protective relaying systems of the Minneapolis-Duluth 500 kV line. Computer simulation studies show that correct operation of the relays can be assured if these effects are taken into consideration in determining relay settings. The selection, performance, and operation of the primary and secondary relay system are discussed, indicating that the relaying protection used achieved substantial security and dependability     

基于自关断器件的新型桥式短路限流器拓扑与控制策略

提出一种基于自关断开关器件的新型桥式短路故障限流器。该种单相(三相)限流器由1个整流桥、1个直流电感和1个(3个)旁路电感组成。直流电感用来限制故障初期的短路电流上升速度,其电感根据控制电路的响应速度和系统电压、电流额定值,以体积、成本最小为目标进行设计。旁路电感用来将短路电流限制到继电保护要求的数值上,其电感量根据系统电压和继电保护要求的短路电流设计。短路限流期间的电源电流仍为正弦量,因而不会对阻抗继电器、传统的电压电流互感器和总的继电保护方案产生不良影响。新型限流器的主电路得到了简化,控制电路和控制方法非常简单,动态性能得以提高。文中详细论述了用于三相系统的限流器的主电路拓扑结构、控制策略、参数设计与优化方法等。仿真和实验结果证明了所提新型限流器的有效性和实用性。     

Protective relay application for generators and transformer

Application of overcurrent protective devices for generators and transformers is considered to be a routine task. However, a review of the protective device selection, device settings, and protection requirements for generators and transformers, as well as their connected buses, has revealed that certain lessons can be learned in examining a few typical applications. The applications are presented in a series of case studies. The case studies are in the following categories of protection problems: (1) generator backup protection: (a) bus-connected generators with feeder circuits at generator voltage; (b) unit-connected generators with voltage restraint time-overcurrent relays (device 51V) for backup protection; (c) unit-connected generators with distance relays (device 21) for backup protection; and (2) transformer protection with high 3-phase fault currents and low-ratio current transformers: (a) original protection scheme; (b) upgraded protection scheme. Only phase-fault conditions are analysed in the following problems because the topic of ground fault protection would require an equal amount of time to discuss. Each case is presented with: (1) a brief synopsis of the problem encountered; (2) the evaluation of the problem, including the suggested solutions; and (3) the "lesson to be learned" and the trouble spots to avoid