Current Transformer Saturation Conditions Research Articles

Complex plane based adaptive method for protection of series compensated transmission line using phasor measurement unit data

Though differential relays are preferred for the protection of series-compensated transmission line (SCTL), they may mal-operate during current inversion, high resistance internal faults and current infeed/outfeed situations. Therefore, a new protection scheme for SCTL using Phasor Measurement Unit data, that adaptively manages the operating region in the current ratio complex plane according to the percentage compensation level, is presented. Boundaries of the operating region are determined adaptively based on the estimated impedance value of the series compensation. Usefulness of the technique is confirmed by generating cases covering wide range of fault parameters, compensation level, and load angle on an existing 400 kV Indian SCTL using PSCAD software as well as through Hardware in Loop simulation employing the Real Time Digital Simulator. Reported outcomes reveal the ability of the suggested algorithm in sensing all types of internal faults. Its performance remains unaffected during current/voltage inversion, operation of MOV and/or airgap during high fault current conditions, current infeed/outfeed situation, and external fault with current transformer saturation condition. Impact of the presence of noise in the acquired signals is also analyzed. The suggested technique is applicable for SCTLs having different voltage levels and power flow conditions. The proposed method has higher responsiveness during high resistance internal faults up to 500 Ω and better steadiness during external faults compared to numerous prevailing methods.

Synchrophasor Assisted Power Swing Detection Scheme for Wind Integrated Transmission Network

Integration of wind-based renewable energy sources into long Extra High Voltage/Ultra High Voltage uncompensated/compensated power transmission network poses significant problems in terms of proper detection of power swings and effective discrimination between symmetrical faults and power swing situations. As the existing protection strategies are unable to detect the said circumstances, a novel technique, based on the difference between sending end and receiving end positive sequence currents angles of the transmission line, is proposed in this paper. The required data is collected with the help of Phasor Measurement Units placed on both sides of the line. The scheme results in effective discrimination between distinct faults and circumstances of asymmetrical/symmetrical power swing and achieves satisfactory outcome during current transformer saturation condition. The proposed algorithm is evaluated on the wind-integrated IEEE-9 bus system by producing power swings, various cases of faults, and faults during power swings. Validation of the suggested technique was carried out by the execution of hardware-in-loop simulation on a Real-Time Digital Simulator. The achieved outcomes disclose higher sensitivity and better discriminating ability of the presented technique in comparison with those of numerous prevailing methods.

Statistical discrimination index founded on rate of change of phase angle for immunization of transformer differential protection against inrush current

Magnetizing inrush currents are known as the most critical potential threat in maloperation of the transformer differential protection. Therefore, there is a need to detect the inrush current from the internal fault current to avoid the differential protection maloperation. Due to the possibility of a current transformer (CT) saturation at a high level of inrush and fault current, the discrimination algorithm should function appropriately in CT saturation conditions. Here, a method is developed based on the rate of phase angle change (RoCoPA) to perform the discrimination process. The proposed discrimination index employs the RoCoPA of the three-phase differential currents and calculates the standard deviations of the RoCoPA of the three-phase differential currents. The discrimination procedure is designed based on the fact that during fault scenarios, the signal remains almost sinusoidal, and as a result, the RoCoPA has the minimum variations.On the contrary, owing to the non-sinusoidal wave shape of the inrush current, the RoCoPA has notable variations. Based on the various simulated and practical scenarios, the proposed index's performance is evaluated considering challenging scenarios. The results reveal that the proposed discrimination index has promising accuracy with average response delay with about half a cycle.

Review of methodologies used for detection of magnetising inrush and fault conditions in power transformer

Reliable and dedicated protection of the transformer is the main requirement in the power system due to the supervision of a highly expensive equipment transformer. Non-linear characteristics of the transformer give mysterious behaviour for the period of inrush and Current transformer saturation conditions. As a stagnant and electro-magnetic device, a transformer is mainly affected by self-inrush. The saturation of the core itself is a very critical situation that becomes a hindrance for fault discrimination. This article involves the elaboration of the effect of inrush on transformer operation, past research practices for the discrimination of inrush with the faulty condition, and a definite method to recognise inrush and fault condition of the transformer. More than 50 reputed journal articles, various book chapters, and books with research thesis thoroughly studied to review the inrush and fault discrimination conditions. A comparative analysis is prepared to suggest the identification of transformer abnormalities. Moreover, a total harmonics distortion (THD)-based solution proposed for discrimination of the inrush condition from fault and other abnormal conditions using the short time Fourier transform technique. Hardware setup for 2 kVA transformer developed in the laboratory to study different inrush phenomena and altered fault conditions. Simulation and hardware results show various effects of inrush and fault with individual harmonic and THD-based analysis. The collection of real-time data is further used based on laboratory hardware setup to enhance real-time protection schemes for power transformers.

An enhanced sub-cycle statistical algorithm for inrush and fault currents classification in differential protection schemes

Differential protection is a widely-used approach for power transformer protection, the performance of which may be, at times, challenged due to the inrush current and current transformer (CT) saturation. This paper presents an enhanced statistical algorithm for discrimination of fault and inrush current signals under CT saturation scenarios. This paper proposes twofold discrimination indices centered on the concept of fourth-order statistic moment, the so-called kurtosis on the current signal. Mathematically proven, the algorithm performance only depends on the current signal phase angle and it does not require transformer parameters. The mathematical closed form formulation in this paper classifies fault condition and inrush currents in sub-cycle timeframes even in the CT saturation conditions. The performance of the proposed algorithm is verified under various fault and inrush current scenarios with and without CT saturation. The numerical results demonstrate that the proposed algorithm can successfully classify the disturbances and detects saturations quickly with promising accuracy.Index Terms: Inrush current; internal fault; current transformer saturation; differential protection

Digital Current Measurement Element for Operation During Current Transformer Severe Saturation

The development and implementation of a digital current measurement element for proper operation during current transformer (CT) magnetic core severe saturation are considered. CT transient performance is often accompanied by primary current transformation to secondary one with great errors. In this case the secondary CT current which is an input signal of the digital measurement element differs from the ideally transformed CT current both in shape and magnitude. This causes impermissible signal settling time at the standard digital measurement element output. As a result, main requirements to the protection devices such as reliability and fast operation are violated, that in some cases makes the high-speed protection device ineffective. To solve this problem, it is proposed to form the output signal of the digital current measurement element in depending on the input signal total harmonic distortion (THD) coefficient value. Moreover, it is worthwhile to form the output signal so that for a low CT saturation conditions this output signal slightly differs from the secondary current RMS value, and for a severe CT saturation conditions it exceeds this value. Digital current measurement element model has been developed and implemented in the MatLab-Simulink environment using the following blocks: a digital filter block responsible for the input signal fundamental frequency component magnitude calculation; a digital filter block responsible for the input signal RMS value calculation; and, also, a standard blocks for basic mathematical calculations needed for proper functioning of the proposed measurement element. The functional testing of the proposed digital current measurement element model was carried out using the signal, that was similar in form to the waveform of the secondary current of the severe saturated CT. The tests that had been performed confirmed that the proposed digital current measurement element in comparison with the standard current measurement element ensures stable functioning and enhanced operation time during transients.

Adaptive Algorithm for Distribution Transformer Protection to Improve Smart Grid Stability

Abstract Due to the presence of Distribution Generation (DG), power system becomes more complicated and stability of power is the main challenging task. Saturation of Current Transformer (CT) imposes great dilemma on differential relaying scheme. This manuscript presents a new differential algorithm for distribution transformer protection which adaptively set its characteristic in the event of CT saturation. The proposed scheme is capable to detect magnetizing inrush condition, high resistance internal fault and discriminate external fault with CT saturation. The validation of the proposed scheme is done by simulating a part of the power system in PSCADTM software and programming in MATLAB software. A Full Cycle Discrete Fourier Transform (FCDFT) is implemented to validate the differential protective scheme for 15 MVA, 66/11 kV distribution transformer. An adaptive concept of the differential characteristic is employed in the algorithm to maintain the stability of relay during external fault with CT saturation. Validation and authenticity of the proposed technique are carried out with various test condition generated under wide variation in system parameters. The result on 2 kVA, 230/110 V, single phase transformer shows that the proposed scheme is capable to discriminate inrush, internal and external fault also with CT saturation conditions.

Framework for current transformer saturation detection and waveform reconstruction

Due to current transformer (CT) saturation, the secondary current waveform may contain distortions which results in mal-operation of the protective system. This study presents a framework that attempts to detect and reconstruct saturated secondary current of the CT. To such aim, first an algorithm is proposed which detects and discriminates between normal and abnormal condition. This algorithm which is based on high-order derivatives and statistical criterion, classifies load change, inrush current and fault condition. In addition, this algorithm is designed so that it detects the first time that CT becomes saturated. After fault detection and phase fault selection, based on analytical and comprehensive formulations, the parameter of fault current signal is calculated in the sub-cycle. Moreover, the dynamic behaviour of the CT is considered in the waveform reconstruction. Several case studies are carried out to investigate the performance of the proposed method. Simulation results show that proposed method can effectively classify the inrush current and fault signal even in the CT saturation condition. Also, under different CT saturation condition, proposed method quickly and precisely detects and reconstructs saturated waveform without sacrificing details.

Partial operating current characteristics to discriminate internal and external faults of differential protection zones during CT saturation

To protect transformers, transmission lines and busbar systems, low impedance current differential protection schemes based on percentage restraint characteristics are widely used in power systems. The main application issue of these schemes is mis-operation due to current transformer (CT) saturation during close-in external faults. A comprehensive fault discrimination algorithm is highly required for current differential protection schemes to overcome the CT saturation issue. The purpose of this study is to introduce a methodology based on partial operating current (POC) characteristics which discriminate internal and external faults of differential protection zones. The study includes the mathematical model of POC characteristics, a fault discriminating algorithm using POC features, and bus differential protection zone results. The results documented encompasses various possible short-circuit (fault) scenarios and indicate the capability of the proposed algorithm in discriminating internal and external faults under CT saturation conditions.

A novel approach to transformer differential protection using sequence component based algorithm

This paper presents a new unit type protection for power transformer by combining differential protection and phase angle comparison of sequence components of currents. The simple differential protection scheme alone may ma-operation during current transformer (CT) saturation and high resistance internal fault conditions. Hence, the sequence component based phase angle comparison technique is combined with percentage biased differential protection scheme. Discrete Fourier Transform (DFT) is used to calculate the phase angles of sequence components and harmonics of primary and secondary current signals of transformer. Low pass butter worth filter is used to correct the current signals before passing it to DFT. During internal fault, the phase angle difference of primary and secondary current is very low whereas large angle difference observed during external fault/other conditions. The performance of the proposed algorithm has been evaluated by considering various transformer and system parameters. Numerous simulations are carried out on three phase power transformer using PSCAD/EMTDCTM software to validate the technique. The proposed algorithm perfectly distinguishes internal fault and external fault condition. Moreover, various situations such as magnetizing inrush current, high resistance internal fault and CT saturation conditions are also tested for validation.

Experimental results and technique evaluation based on alienation coefficients for busbar protection scheme

Abstract In modern digital power protection systems, statistical coefficients technique is recently used for fault analysis. An alienation technique is developed for busbar protection against all ten types of shunt faults, which may locate in busbar protection zone, under different loading levels, fault resistances and fault inception angle. It does not need any extra equipment as it depends only on the three-line currents measurements, of all feeders connected to the protected busbar, which are mostly available at the relay location. It is able to perform fault detection, fault confirmation, faulty phase selection and determine the fault location in about a half-cycle period. Thus, the alienation technique is well suited for implementation in digital protection schemes. The technique is efficient to detect current transformer saturation conditions without needing any additional algorithm. The effects of DC components and harmonics are eliminated with estimation of alienation coefficients. The proposed scheme is applied for an experimental circuit. LABVIEW program and MATLAB package are used to implement the proposed technique.

Current transformer model with hysteresis for improving the protection response in electrical transmission systems

In this paper, a generic enhanced protection current transformer (CT) model with saturation effects and transient behavior is presented. The model is used for the purpose of analysis and design of power system protection algorithms. Three major classes of protection CT have been modeled which all take into account the nonlinear inductance with remanence effects. The transient short-circuit currents in power systems are simulated under CT saturation condition. The response of a common power system protection algorithm with respect to robustness to nominal parameter variations and sensitivity against maloperation is demonstrated by simulation studies.

Power Transformer Protection Using a Multiregion Adaptive Differential Relay

This paper presents a new adaptive approach to improve the performance of the power transformer differential relay. The proposed approach distinguishes internal faults and disturbances based on the differential current trajectory and adopted weighting factors depending on the differential current locus in the relay characteristic. The traditional dual-slope differential characteristic is divided into three operating regions with the corresponding weighting factors. Two other control regions, that is, inrush and current-transformer (CT) saturation detection regions are utilized to avoid maloperation of the proposed relay in the case of inrush and CT saturation conditions. Meanwhile, a combination of phase differential units is introduced as a new technique to decrease relay operation time. The proposed relay rapidly detects all studied internal faults, while it is stable during transformer energization and CT saturation due to external faults. A real 230/63-kV power transformer is simulated using PSCAD/EMTDC for evaluating the performance of the proposed relay. Extensive simulation studies show that the proposed approach results in a faster and smarter differential protection scheme.

A novel technique for internal fault detection of power transformers based on moving windows

Power transformer differential relays may incorrectly operate due to inrush condition or transformer external faults lead to current transformer (CT) saturation. In this paper, a new technique is presented to distinguish internal fault current from magnetizing inrush current in power transformers. The proposed technique applies two moving windows which both of them estimate magnitude of transformer differential current using different methods. The first moving window method is based on full-cycle Fourier algorithm (long window) which uses one cycle samples of the current and provides precise estimation. The second one is originated from least error square (LES) method that has five sample point input data (short window). LES method has fast response, but its estimation might not be quite accurate. The proposed technique (which is a combination of two mentioned methods) presents a new criterion for discrimination between internal fault currents and inrush currents using the differences between output values of these two estimators. Meanwhile, obtained results demonstrate precise operation of the proposed algorithm for different conditions such as CT saturation and over-flux condition; therefore, it can be a multi-objective technique for transformer protection. Also, a general scheme is presented in order to improve differential protection. In this paper, a real part of 400/230 kV Iranian national grid is simulated using PSCAD software for evaluating the performance of the proposed algorithm. Also, additional validation of the proposed technique is tested offline using data collected from a three-phase, 5 kVA, 60 Hz, laboratory prototype transformer. Copyright © 2013 John Wiley & Sons, Ltd.