Current Transformer Saturation Research Articles

Hardware implementation and real time performance evaluation of current transformer saturation detection and compensation algorithms

This paper presents a hardware implementation of two current transformer (CT) saturation detection methods and one distorted signal reconstruction method. Those methods have been implemented previously only in software. To evaluate the algorithms performance, a real time hardware in the loop system was implemented with several test scenarios forcing CT saturation.The paper highlights the influence of the quantization error generated by the analog-to-digital-converter in the reconstruction of distorted signal process and introduces a method to determine the best parameters to reconstruct the distorted current signal.The methods used have shown precision and high speed in detecting and compensating distorted current signal in real time, even for the worst CT saturation cases.

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.

Research on Differential Protection of Generator Based on New Braking Mode

In view of the difficulties in coordination between reliability and sensitivity of conventional generator differential protection, this paper presents a novel generator differential protection scheme based on a new braking method. On the basis of the mathematical model of the field-circuit coupling method and the electrical network cascade characteristics, the stator windings were combined and decoupled by the improved six-sequence component method to eliminate electromagnetic coupling between winding coils. In accordance with the basic characteristics of longitudinal impedance, the function of sub-item discrimination was realized. Numerous simulations and the up-to-date dynamic experiments showed that the proposed method has good state discrimination ability and can effectively resist the influence of current transformer saturation. Thus, it has an excellent prospect for engineering promotion.

A new artificial intelligence based supervision method for low-impedance REF relays

The restricted earth fault (REF) protection of power transformers are prone to maloperation during magnetizing inrush and severe external faults due to current transformer (CT) saturation. To tackle this issue, for the first time, an intelligent supervision method is designed in this paper which can significantly improve the immunity of this protection scheme. For this purpose, some decisive parameters used in some conventional methods are selected as distinguishing features. In the next step, a real power transformer is simulated under different operating conditions using PSCAD/EMTDC software. Then, three different intelligent classifiers are trained and tested using the simulation results and the most accurate and fast one is chosen to be used as the intelligent supervisory unit. Finally, a conventional relay performance is analyzed before/after being equipped with the new supervision unit. The obtained results show that by equipping the conventional REF protection scheme with it, a magnificent security against maloperation can be achieved while the imposed time delay is ignorable.

A new time-frequency analysis based supervision method for the transformer restricted ground fault relay

The low-impedance restricted ground fault (RGF) relay is a protective scheme of power transformers but its operation degrades when current transformers (CTs) are saturated. In this study, for the first time, a new supervision method based on a time-frequency analysis tool is proposed to improve the immunity of this protection scheme against CT saturation. For this purpose, two distinguishing features are extracted using Stockwell-transform (S-transform) and then by determining proper threshold values, the relay operation is conditioned. A comprehensive study is conducted by simulation of a real power transformer in PSCAD/EMTDC environment. Moreover, the precise Jiles–Atherton model is employed to simulate the transient behavior of CTs during saturation conditions. The obtained results show that the immunity of the proposed RGF protection scheme is improved significantly by applying the proposed supervision method.

A novel pilot protection scheme for transmission lines based on current distribution histograms and their Bhattacharyya coefficient

The current waveform characteristics at both ends of a transmission line under different disturbances are represented by means of probability distribution histograms. By adopting the Bhattacharyya distance (BD) algorithm to quantify the similarity of current distribution histograms, a novel scheme of pilot protection for transmission lines based on Bhattacharyya coefficient (BC) is put forward. The proposed scheme can correctly identify internal faults, external faults, and faults accompanied with current transformer (CT) saturation of transmission lines. It also shows good performance in the cases of abnormal samples. The validity of the proposed scheme is evaluated by various simulation and dynamic analog experiment tests.

Accurate fault location element for series compensated double-circuit transmission lines utilizing negative-sequence phasors

Obtaining the accurate fault location in the series compensated double-circuit transmission lines (SCDCTL) is a challenge due to the presence of series compensator, fault resistance, and mutual the coupling between parallel lines. Although by using the pilot fault location methods, the effect of all the aforementioned factors can be eliminated and the location of the fault can be calculated. However, the number of communication channels available for pilot schemes is limited. In addition, the reliability of the communication links for long transmission lines may be particularly reduced. On the other hand, most of the non-pilot methods need to calculate the series compensator data and the accuracy of the fault location in them depends on the accuracy of the series compensator parameters. To improve the limitation of conventional impedance-based methods, this paper presents a non-pilot accurate fault location element (FLE) for conventional distance relays in SCDCTL. The main concept of the proposed method is considering the proper Kirchhoff voltage law (KVL) loop in the negative-sequence circuit of SCDCTL. Consideration of the shunt capacitances of lines ensures accuracy of the achieved fault location formula. It is shown that the designed FLE can improve limitations associated with similar methods such as the dependence of these methods to the fault resistance, mutual coupling between the parallel lines, and the series compensator (SC) parameters. To investigate the performance of the proposed FLE, several factors are considered in the simulation analysis including different fault resistances, fault locations, and fault types. Besides, the influence of current transformer (CT) saturation, measurement errors, external faults, bypassing of the Metal Oxide Varistor (MOV), and effect of noise on the proposed scheme is studied. Evaluation study using PSCAD verify the effectiveness and applicability of the proposed scheme.

A Hybrid Intelligent Method for Compensation of Current Transformers Saturation Based on PSO-SVR

The Current Transformers (CT) saturation may cause the protective relays mal-operation either non-recognition of internal fault or undesirable trip under external fault conditions. Therefore, compensation of CT saturation is very important for correct performance of protective schemes. Compensation of CT saturation by combination of signal processing methods and intelligent algorithms is a suitable solution to solve the problem. It decreases the probability of mal-operation and increases the reliability of the power system. In this paper, Support Vector Regression (SVR) method is employed to compensate the distorted secondary current due to CT saturation. In SVR method, despite the other methods such as MLPand ANFIS, instead of minimizing the model error, the operational risk error is considered as target function. In this method, by using Kernel tricks, a smart RBF neural network is obtained, so that all operational procedures will be optimized automatically. In this paper, an intelligent method based on Particle Swarm Optimization (PSO) algorithm is presented to determine the optimal values of SVR parameters. Due to the stability and robustness of this method in presence of noise and sudden changes in current, this method has a high accuracy. In addition, a sample power system is simulated using PSCAD software. Afterwards, current signals are extracted and fed to PSO-SVR algorithm, which is implemented in MATLAB environment. The obtained results show the preference of the proposed method in aspect of estimation accuracy as compared to some presented methods in the field of CT saturation detection and correction.

Novel Method for Discrimination of Transformers Faults from Magnetizing Inrush Currents Using Wavelet Transform

Discrimination of power transformer inrush from fault current is most important to proper operation of differential relays. Most differential relays discriminate the inrush current from short circuit fault according to high content of second harmonic of inrush. Unfortunately, in some power system operation, it has been seen that this algorithm is not accurate enough due to the presence of some kinds of loads. On the other hand, current transformer (CT) saturation due to large inrush or fault current also can cause the discrimination algorithms mal-operation. This paper presents a novel algorithm for power transformer differential protection which differentiates internal faults from those of magnetizing inrush currents in the presence of CT saturation. In order to eliminate the impact of CT saturation on the performance of the proposed technique, at first the currents are compensated by a CT saturation compensation method. Furthermore, a technique based on wavelet transform is used for discrimination of transformer internal fault from magnetizing inrush currents. Using discrete wavelet transform (DWT) features of primary and secondary differential currents and those of flux measured at transformer primary winding, an algorithm was proposed. Discrimination criterion is based on synchronization of maximum or minimum points between wavelet coefficient of differential current and flux of the transformer. The efficiency of the algorithm was evaluated using different scenarios of inrush and internal fault current of a power transformer simulated in MATLAB/Simulink software environment.

Transmission Line Distance Protection Under Current Transformer Saturation

Conventional transmission line distance protection approaches are subject to malfunction under reverse fault-induced current transformer (CT) saturation for the typically employed breaker-and-a-half configuration. This paper addresses this issue by proposing a new distance protection approach that combines the blocking and unblocking criteria of distance protection based on the values of incomplete differential current, operation voltage, and current harmonic content. The proposed approach is verified by theoretical analysis, dynamic simulation testing, and field operation to ensure that the obtained distance protection is reliable and refrains from operating unnecessarily under reverse fault-induced CT saturation in the breaker-and-a-half configuration. Meanwhile, the proposed approach is demonstrated can operate reliably when forward faults occur or various reverse faults are converted to forward faults.

Research of Parameters of Electric Power Systems Affecting Core Saturation of Current Transformers with a Closed Magnetic Circuit in Transitional Short Circuit Modes

The relevance of the study is due to the excess of the permissible errors of current transformers in transient short-circuit modes associated with saturation of the cores of current transformers. The latter leads to improper operation of relay protection devices, which can negatively affect the stability of electric power systems and the efficiency of industrial enterprises with a continuous technological cycle, the power failure of which leads to significant economic damage. The aim of this work is to study the parameters of electric power systems that affect the saturation of the cores of current transformers with a closed magnetic circuit in transient short-circuit modes. The object of research is electromagnetic current transformers of accuracy class 10P in electric power systems. The study was carried out using numerical methods for solving algebraic and differential equations, methods of computer modeling of electromagnetic transient processes and functional programming. As a result, the main parameters influencing the nature and intensity of electromagnetic transients in current transformers during a short circuit are noted; it is shown that in transient short-circuit modes, significant errors in the operation of current transformers associated with core saturation are possible; recommended for assessing the compliance of current transformers of accuracy classes 5P, 10P with the conditions for the correct functioning of relay protection devices in transient short-circuit modes, to calculate the time to saturation of current transformers, taking into account the real parameters of the electric power system at the place where current transformers are installed, in addition to the traditional design checking of current transformers by permissible er-rors for the steady state short circuit.

A Data Mining Based Protection and Classification of Transients for Two-Core Symmetric Phase Angle Regulators

This paper investigates the applicability of time and time-frequency features based classifiers to distinguish internal faults and other transients - magnetizing inrush, sympathetic inrush, external faults with current transformer saturation, and overexcitation - for Indirect Symmetrical Phase Angle Regulating Transformers (ISPAR). Then the faulty transformer unit (series/exciting) of the ISPAR is located, or else the transient disturbance is identified. An event detector detects variation in differential currents and registers one-cycle of 3-phase post transient samples which are used to extract the time and time-frequency features for training seven classifiers. Three different sets of features - wavelet coefficients, time-domain features, and combination of time and wavelet energy - obtained from exhaustive search using Decision Tree, random forest feature selection, and maximum Relevance Minimum Redundancy are used. The internal fault is detected with a balanced accuracy of 99.9%, the faulty unit is localized with balanced accuracy of 98.7% and the no-fault transients are classified with balanced accuracy of 99.5%. The results show potential for accurate internal fault detection and localization, and transient identification. The proposed scheme can supervise the operation of existing microprocessor-based differential relays resulting in higher stability and dependability. The ISPAR is modeled and the transients are simulated in PSCAD/EMTDC by varying several parameters.

The study of power transformer differential protection’s operation in the internal fault conditions

Special attention is paid to high-speed relay protections’ operation in transient modes due to a number of major failure events that have occurred over the past 10 years in the power system of the Russian Federation. Operation of power transformer’s differential protection in case of internal short circuit is studied in this research. False blocking of protection is possible in such mode due to saturation of current transformers. A value of blocking time may exceed the maximum permissible short-circuit disconnection time under conditions of maintaining the dynamic stability of the power system. Primary and secondary currents in transient modes are obtained by simulation of short circuits. Windings of the modeled current transformers are connected in a star to a null wire. RMS values are calculated using a mathematical model of the Fourier filter. The current transformers were checked according to the methods declared in PNST 283-2018 and GOST R 58669-2019. The analysis carried out in this work allows to estimate possibility of long-term blocking of the differential protection of a power transformer in case of internal short circuit, especially in case of significant value of time constants.

Analysis of distance protection’s operation in cases of deep saturation of current transformers

There is usually no information about permissible error of current transformers in modes succeed by large relative short-circuit current, at which microprocessor-based protections operate stably. By this reason, it is necessary to use data, defined for analog relays. It leads to value appreciation of a project because it is often essential to reduce current transformers’ error in case of a short circuit fault. Therefore, it is necessary to define the value of current transformers’ error, permitted for impedance relays. Conclusions of fundamental and applied sciences (mathematical analysis, theoretical foundations of electrical engineering, theory of simulation), analytical methods of researching nonlinear circuits and digital signal processing were used. A simulation model was created for setting overall tests of the current trans-former (CT) system. It was a relay protection device that reflected all the important properties of studied objects and allowed an analysis of digital distance protection’s operation at high levels of short-circuit currents. The factors influ-encing over digital distance protection’s operation in case of deep saturation of CTs were revealed, and a certain algorithm for definition of the permissible CT errors was proposed. Stable operation of digital distance protection was observed in case of a fault nearby the place of current transformers’ setting in all theoretically possible combinations of electrical system’s power and length of a protected electric power transmission line. It is valid if electric load choice is carried with account for stable protection’s operation in condition of a fault in the computational point and if voltage swell in secondary wirings is infeasible.

Experimental obtaining characteristics of current transformers for developing their models and determining the time of saturation of the magnetic circuit

Electromagnetic current transformers (CT) are subject to saturation and residual magnetization of a magnetic circuit. This phenomenon has an impact on the operation of devices of commercial accounting of electric energy and relay protection and automatics (RPA). Saturation of the current transformer’s magnetic core can lead to emergency situations due to incorrect operation of the protections and excessive disconnection of the protected connections. At the moment, Russian norms do not regulate the work in transient modes, including short circuits with an aperiodic current component. The Ivanovo State Power Engineering University (ISPEU) conducts research on the topic of determining the time to saturation of CT at short circuit. The purpose of the work was to develop an algorithm for monitoring saturation and determining the time to saturation of the CT magnetic core, in order to ensure the correct operation of the RPA in all modes. The article is devoted to the method of construction and research of mathematical models of CT for calculating the time to saturation. The general structure of the algorithm for determining the time to saturation is presented.

Analiza rada diferencijalne zaštite transformatora pri pojavi zasićenja strujnih mernih transformatora - softverske simulacije i ispitivanja

In the paper the analyses of the current transformer saturation influence to operation of differential protection are presented. Analyses are performed using a simulation tool and a relay model, as well as using the physical relay by injecting test currents from test equipment on relay inputs. The comparison of relay behavior in these two analyses has been performed and discussed. Network modeling has been performed in DIgSILENT PowerFactory and physical relay testing using DOBLE test equipment.

Bayesian Deep Neural Network to Compensate for Current Transformer Saturation

Bayesian Deep Neural Network to Compensate for Current Transformer Saturation

Accurate Fault Diagnosis in Transformers Using an Auxiliary Current-Compensation-Based Framework for Differential Relays

This article proposes an auxiliary framework to address the challenges of transformer differential protection for single-phase transformers or three-phase transformer banks. This framework enables the differential scheme to: 1) work properly if transformers or current transformers (CTs) saturate; 2) detect internal and cross-country faults; 3) detect internal faults while energizing transformers; and 4) detect inrush currents. Unlike the existing methods in the literature, this framework addresses the above-mentioned challenges without sacrificing the sensitivity and/or the speed of differential relays. The proposed method models a transformer and its CTs with linear parameter varying (LPV) state-space equations, and uses the polytopic form of these equations and LPV observers to estimate the states of the transformer and its CTs. To address the CT saturation problem, it accurately estimates primary currents of CTs using their secondary currents. Thus, the differential scheme uses the estimated primary currents of its CTs instead of their distorted secondary currents. Additionally, the proposed framework detects inrush currents of transformers and differentiates them from internal faults by estimating the primary current of the transformer and comparing the estimated and measured primary currents. A discrepancy between the measured and estimated primary currents signifies an internal fault. The results of electromagnetic transient simulations in Electromagnetic Transient Program (EMTP) platform corroborate the effectiveness of the proposed method.

Application of probabilistic distance measures for inrush and internal fault currents discrimination in power transformer differential protection

Differential protection failure in discrimination of inrush and internal fault currents will impose significant challenges and damages both on the power transformers and power systems. To cope with this issue, this paper investigates the application of Probabilistic Distance Measure (PDM) in discrimination of the inrush and internal fault currents in power transformers. The PDMs are essentially employed to determine the dissimilarities between two sets of distributions. This paper utilizes Symmetric PDM, to determine the discrepancy between the differential current signal and the reconstructed differential current waveform. To such aim, at the first step, the reconstructed differential current waveform obtained from CTs is reproduced. The reconstructed current signal contains only sinusoidal parts of the fault current and thus are reproduced using the modified least squares algorithm. Eventually, the reconstructed current and the differential current signals are fed to PDM to determine the distribution discrepancy between the signals. The performance of the proposed PDM criterion is evaluated utilizing experimentally recorded data and is further compared with the state-of-the-art. The obtained results reveal that the proposed index can be successfully applied for the discrimination of inrush and fault currents even in the case of current transformer (CT) saturation.

Study on effects of DSE-Based instrumentation channel error correction on protective relays

This paper presents the effects of dynamic state estimation (DSE)-based instrumentation channel error correction on protective relays. It discusses the possible mis-operations of relays caused by current transformer (CT) saturation for three typical protection schemes: distance protection, current differential protection and setting-less protection. For each protection scheme, it compares the relay responses with the estimated measurement samples and with the legacy measurement samples, respectively. Extensive cases have demonstrated that when large errors introduced by CT saturation, the legacy measurement samples can lead to slower operations for distance relay in case of internal faults, and unnecessary trip decisions during external faults for current differential relay and setting-less relay. In contrast, with the estimated measurement samples, the relays can correctly identify internal faults and avoid mis-operation in case of external faults.