Current Transformer Saturation Research Articles

Development of a new fault zone identification scheme for busbar using logistic regression classifier

This study presents the development of a new fault zone identification scheme for busbar using logistic regression binary classifier by utilising one cycle post-fault current signals of all the bays connected to the busbar. Practicability of the presented scheme has been verified by modelling an existing 400 kV Indian power generating station in power systems computer-aided design/electro-magnetic transient design and control software package. The presented scheme has been tested on enormous cases (38,400) which were generated by varying system and fault parameters. The proposed scheme provides effective discrimination between internal and external faults with a very high (99.69%) overall accuracy. Moreover, it remains stable in case of heavy through fault conditions particularly with current transformer saturation during which the conventional differential protection scheme mal-operates. Furthermore, it provides equally compatible accuracy for unknown system/unseen data set as well as for double/one-and-half breaker busbar arrangement. In addition, performance of the proposed scheme has been verified on the laboratory prototype and results are found to be satisfactory. The average tripping time is of the order of 23 ms in case of internal faults. At last, comparative evaluation of the proposed scheme with recently presented schemes in the literature indicates its superiority.

Transmission lines differential protection based on an alternative incremental complex power alpha plane

This study proposes a new concept of transmission line differential protection based on an alternative alpha plane of incremental complex power, where the restraint characteristic is simply defined as its left-half-plane, so that no settings are required to define it. As the incremental complex power is evaluated per phase, the proposed algorithm is inherently phase segregated, not requiring additional faulted phase selection logics. A simple harmonic restraint strategy along with an external fault detection logic provides security for external faults with current transformer saturation. Aiming to evaluate the performance of the proposed algorithm, a wide variety of fault scenarios are simulated for a 500 kV transmission line 200 km long using the Alternative Transients Program. The obtained results reveal the proposed algorithm provides a fast and reliable line protection, being robust against variations in fault parameters, sources strength and loading conditions, as well as power swing situations and line energisation manoeuvres. Accordingly, one can suppose the proposed algorithm might be successfully used in numerical transmission line relaying.

Support Vector Machine Based Classification of Current Transformer Saturation Phenomenon

Support Vector Machine Based Classification of Current Transformer Saturation Phenomenon

Proper Maintenance of Differential Bus Bars Protections Under the Conditions of Current Transformers Saturation

Proper Maintenance of Differential Bus Bars Protections Under the Conditions of Current Transformers Saturation

Identification of current transformer saturation based on the improved gradient mathematical morphology method

Identification of current transformer saturation based on the improved gradient mathematical morphology method

A Novel Approach for Microgrid Protection Based upon Combined ANFIS and Hilbert Space-Based Power Setting

Nowadays, the use of distributed generation (DG) has increased because of benefits such as increased reliability, reduced losses, improvement in the line capacity, and less environmental pollution. The protection of microgrids, which consist of generation sources, is one of the most crucial concerns of basic distribution operators. One of the key issues in this field is the protection of microgrids against permanent and temporary failures by improving the safety and reliability of the network. The traditional method has a number of disadvantages. The reliability and stability of a power system in a microgrid depend to a great extent on the efficiency of the protection scheme. The application of Artificial Intelligence approaches was introduced recently in the protection of distribution networks. The fault detection method depends on differential relay based on Hilbert Space-Based Power (HSBP) theory to achieve fastest primary protection. It is backed up by a total harmonic distortion (THD) detection method that takes over in case of a failure in the primary method. The backup protection would be completely independent of the main protection. This is rarely attained in practice. This paper proposes a new algorithm to improve protection performance by adaptive network-based fuzzy inference system (ANFIS). The protection can be obtained in a novel way based on this theory. An advantage of this algorithm is that the protection system operates in fewer than two cycles after the occurrence of the fault. Another advantage is that the error detection is not dependent on the selection of threshold values, and all types of internal fault can identify and show that the algorithm operates correctly for all types of faults while preventing unwanted tripping, even if the data were distorted by current transformer (CT) saturation or by data mismatches. The simulation results show that the proposed circuit can identify the faulty phase in the microgrid quickly and correctly.

Adaptive busbar differential relaying scheme during saturation period of current transformers based on alienation concept

This study presents an advanced technique provided with adaptive differential relay characteristics for avoiding current transformer (CT) saturation effects. The proposed technique uses the alienation and differential protection principles to make adaptive logic; if no CT saturation is detected, the differential principle alone is capable of tripping the busbar. If CT saturation is detected, both the alienation and differential principles must pickup in order to trip. The alienation coefficient is useful in CT saturation detection, evaluation of CT saturation degree and adapting the differential relay characteristics during saturation period of CTs to avoid false operation in the event of external faults. The differential current and alienation coefficient are calculated from two terminal currents of the busbar. The proposed protective scheme has several attractive features, including the simplicity of on-line implementation, not affected by the remanent flux in the CT core, and it is independent of the CT parameters/characteristics and secondary burden. A real 19.57 kV busbar is simulated using Alternate Transients Program/Electro-Magnetic Transient Program and MATLAB packages to examine the suggested scheme under different types of faults. Under normal operating conditions and external faults, the scheme remains stable and it operates only for the case of internal faults. Moreover, it is unaffected by CT saturation.

Improved differential relay for bus bar protection scheme with saturated current transformers based on second order harmonics

Differential relays security to the external faults is affected by the saturation of branches’ current transformers (CTs). In this paper, a simple scheme is proposed to enhance the security of differential numerical relay by extracting the 2nd order harmonic using Fast Fourier Transform (FFT) to produce a restraint signal to inhibit the relay operation during external faults. The operation signal of differential relay is produced by comparing the vector addition of secondary currents of branches’ CTs (differential current) with pre-set value; the restraint signal is produced by comparing the algebraic sum of 2nd order harmonic of individual secondary currents with the 2nd order harmonic of differential current. The proposed scheme is investigated using PSCAD/EMTDC simulation and tested during internal and external faults for saturated CTs. The obtained results reveal how this scheme is effective and secure to the external faults for different suggested scenarios. The proposed scheme is using the simplest technique of signal processing compared to other proposed techniques.

Current Transformers’ Saturation Detection and Compensation Based on Instantaneous Flux Density Calculations

One of the major problems in power system protection is current transformers saturation and inability of protective relays to recognize and compensate it. In this paper an approach is presented to identify current transformer saturation based on instantaneous flux density calculation. Also the calculated magnetizing current is used to saturation compensation. In the proposed algorithm, the instantaneous flux density and magnetizing current are calculated using Jiles-Atherton method. Using knee point flux density, a criteria is proposed to identify saturation phenomena in the current transformer and finally using calculated instantaneous magnetizing current, secondary current distortion has been compensated. To evaluate efficiency of the proposed algorithm, various types of faults, residual flux, fault occurring angle, CT burdens and DC components have been tested and the results evidence that the proposed algorithm is more accurate and reliable as well as faster than other algorithms and its error is obviously less than them.

The Application of Correlation Technique in Detecting Internal and External Faults in Three-Phase Transformer and Saturation of Current Transformer

A new technique based on correlation coefficients analysis of current waveforms is used to detect an internal fault that occurs in a three-phase transformer as well as to distinguish it from external faults. The technique depends on the changes in current waveforms that occur during the faults. Autocorrelation has been added to cross-correlation in one scheme for better fault discrimination and to increase speed in detecting the fault within a very short time. The proposed algorithm is so fast that it is able to detect faults within less than 3 ms. The technique also overcomes the problem of the current transformer's saturation, which is considered one of the biggest problems in protection methods especially in differential protection. The new algorithm was tested practically and the results were processed using the LabVIEW and MATLAB programmes. The practical test results show that the internal faults, particularly an interturn fault type with only two turns, are shorted and can be detected and correctly identified from the external fault.

Study on the Unusual Misoperation of Differential Protection During Transformer Energization and its Countermeasure

The differential protection of the transformer, generator, or transmission line under normal operating conditions may misoperate when the adjacent transformer is switched-on. To research the cause of the misoperations, impacting factors, such as magnetizing inrush, sympathetic inrush, and current-transformer saturation are analyzed based on the field-recorded fault data and the simulation results in this paper. It is disclosed that the CT transient or CT local transient saturation, which are caused by the long-lasting dc component from the magnetizing inrush current, will lead to misoperation. To solve this problem, a new CT saturation detection algorithm based on the dc component in secondary current is proposed to prevent this sort of misoperation, and the effectiveness of the proposed algorithm is proved by the field-recorded fault data and simulation results obtained under different conditions.

Faulty phase selection for transmission line using integrated moving sum approach

This study presents a transmission line fault classification algorithm using integrated moving sum (IMSUM) technique. The method is based on current signal only. In this method, the decision of moving sum approach-based fault detection unit is used in integration with the proposed method to obtain accurate fault class. Rules are set for all ten types of fault. Considering half cycle computed data from the instant of fault inception classification is possible. High-resistance far-end fault, close-in fault, fault at different inception angle and fault considering variable prefault loading conditions can be easily classified using IMSUM technique. Severe current transformer saturation and weak infeed conditions are also discussed. The method can also be applied to a series compensated line. Any variation in fault and system condition will not alter the decision period of IMSUM technique which is an exceptional attribute for a protective function.

A precise scheme for detection of current transformer saturation based on time frequency analysis

Abstract Current Transformers (CTs) are prone to saturation due to large amplitude of fault current and existence of decaying Direct Current (DC) offset. Since the CT saturation leads to mal-operation of protective relays, detection and correction of saturated currents is one of the most important challenges in the power systems protection. In this paper, a new algorithm is presented for detection of CT saturation time interval by using the Improved S-Transform (IST). Simultaneously, IST can yield a complete visualization of signal in both time and frequency domains. An index based on instantaneous power of the CT secondary current is calculated using the IST output matrix. Then, the saturation region is estimated by determining the maximum and minimum values of the proposed index in the time domain. Comprehensive simulations are implemented using PSCAD/EMTDC software. Main parameters which have direct effect on the saturation levels are considered in simulation studies. As extremum points of the proposed index are determined for detection of saturation regions, the proposed method is not affected by CT parameters. The obtained results show that the proposed algorithm can precisely detect CT saturation time intervals even in noisy conditions.

Effects of geomagnetically induced currents on the transfer characteristics of current transformers

Summary With the arriving peak of the solar cycle, the effects of geomagnetically induced currents (GICs) on power systems attract more attention. The direct current magnetic bias caused by GIC has a more complex influence on the transient transfer characteristics of the current transformers (CTs) than that caused by high-voltage direct current due to its randomness and time variation. In this paper, a cosine function with lower frequency is simply adopted to simulate the time-variation feature of GIC. The mathematical analysis of CT transfer characteristics with the input of the GIC cosine model is presented based on electromagnetic equations and the excitation characteristics of the CT. Afterward, the physical model of the CT is built and tested under various input conditions. Furthermore, comparisons between the physical model and the simulation model are given to verify the reliability of the simulations. The simulation results show that the saturation level of the CT, such as partial transient saturation and deep saturation, is controlled by the magnitude and frequency of GIC and excitation characteristics of the CT. Moreover, CT saturation may affect the performance of the transformer differential protection, to which should be paid adequate attention. Copyright © 2016 John Wiley & Sons, Ltd.

Detection of current transformer saturation based on variational mode decomposition analysis

Sometimes, current transformers (CTs) are saturated in power systems due to increase of flux passing through the CT core. The distorted CT secondary current can lead to maloperation of protective relays. In this study, a new algorithm is proposed for detection of saturation interval by analysing secondary current signal using variational mode decomposition (VMD). The VMD can decompose signals into different modes which are concentrated around centre frequencies. The start and end points of saturation regions are estimated by using decomposed modes signals. Comprehensive simulations are implemented in PSCAD/EMTDC environment while the main parameters which have direct effect on saturation levels are considered in simulation studies. The performance of the proposed method is evaluated in different conditions: light, moderate, severe and AC CT saturation conditions. The promising obtained results of real data test show that the proposed method can detect the CT saturation time intervals with high accuracy in practical applications.

Residual Flux Mitigation of Protective Current Transformers Used in an Autoreclosing Scheme

To avoid saturation of a protective current transformer (CT) operating under an autoreclosing scheme, the CT size should be chosen much higher than a similar CT operating under a single-step fault clearing scheme. The main reason for this significant difference is that the residual flux (caused by the first stage of the fault) cannot be noticeably reduced during the deadtime interval of the reclosing process and, therefore, the occurrence of a subsequent fault can extremely saturate the CT. In this paper, a low-cost, low-power electronic device is developed and introduced to demagnetize the CT under the reclosing deadtime interval. It will be shown that by using this device, the required size of the CT is reduced by about 40%. To develop this device, an existing flux-based demagnetization technique is improved to operate faster and more efficient. The performance of this technique is independent of the CT characteristic parameters and provides promising results despite its simplicity. The proposed device can be employed to de-rate the new CTs to be installed in the system and/or to compensate the existing inservice CTs. Comprehensive computer simulations and laboratory experiments are employed to validate the reliability and effectiveness of the proposed compensation method.

Power transformer protection using chirplet transform

This study presents a novel differential protection algorithm (DPA) for power transformer using chirplet transform (ChT). The proposed method combines the features of biased restraint characteristic (BRC) of the conventional differential relay and out-turn of ChT in a two-stage algorithm. In the first stage, the BRC plane is divided into three zones: namely, high-set (HS), non-trip and vulnerable zones. The tripping decisions are carried out in the first two zones based on differential and biased current. However, if the operating condition of the power transformer falls in the vulnerable zone, then there is an ambiguity in discriminating internal fault, inrush current and current transformer saturation cases. Therefore, in the second stage, ChT is applied to differential current signal to obtain an energy distribution on the time-frequency plane with respect to time, frequency and chirp rate. Then, using the mean and standard deviation of the normalised energy, power transformer operating conditions are classified. Also, most of the DPAs available in the literature are system dependent. However, the proposed novel DPA can be effectively used for any system. The proposed scheme is validated for two power transformer systems using PSCAD to simulate various operating conditions and MATLAB to implement the algorithm.

Adaptive CWT‐based transmission line differential protection scheme considering cross‐country faults and CT saturation

In this study a new adaptive continuous wavelet transform (CWT)-based current differential protection scheme is proposed to cope with problems of conventional current differential protection in transmission lines. Current transformer (CT) saturation is a major problem for protection engineers in current differential protection systems. The mentioned problem is highlighted when an external fault occurs and causes mal-operation of differential protection. In this study, three phase currents at both ends of transmission line have been sampled and used as input signals to the proposed method using appropriate communication link. In addition, CWT with the aid of Mexican hat as a mother wavelet has been employed to extract high-frequency information of signals and detect fault occurrence and CT saturation in each phase. As a result, the proposed technique is able to prevent unwanted trip due to CT saturation during external faults. Nevertheless, simulation results confirm sensitivity and security of the proposed protection technique in facing with second internal fault of cross-country faults after a short delay.

A Three-Terminal Line ProtectionScheme Immune to Power Swing

Communication-assisted distance protection schemes are widely used for three-terminal line protection and are susceptible to power swings. The available differential protection scheme is also considered to be suited to protect three-terminal lines and is free from problems associated with the power swing. However, the performance of the current differential protection scheme may be affected if an outfeed condition occurs during an internal fault. This paper presents a new technique for protecting three-terminal transmission lines immune to power swing. The technique uses synchronized voltage and current measurements from three terminals for calculating voltage at the tee-point. The maximum magnitude of the superimposed component of positive-sequence voltage estimated at the tee-point is compared with the maximum magnitude of superimposed componentas of the positive-sequence voltage measured at three terminals of the line to discriminate a fault from power swing. The method is accurate even for an outfeed condition during the internal fault, current-transformer saturation during severe internal or external faults, large load rejection, and different prefault loading conditions occur during the power swing. The proposed algorithm is verified using data simulated through EMTDC/PSCAD for a 400-kV double-circuit three-terminal system.

Current-Transformer Saturation Detection Using Savitzky-Golay Filter

A new application of the Savitzky-Golay (SG) filters in current-transformer saturation detection is proposed. The ability of the SG filters to handle noisy signals improves the robustness of the detection process in the presence of noise while requiring only low computational effort.