Power transformer protection using an instantaneous-current-value negative sequence differential element

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.

A hybrid algorithm for phasor estimation is proposed that is immune to DC offset and current transformer (CT) saturation problems. The algorithm utilizes partial sum (PS)-based and multistage least-squares (MLS)-based methods before and after CT saturation is detected, respectively. The MLS-based method is initiated when the third difference of the secondary current detects the start point of the first saturation period. The determination of each saturation period is based on the sum of the secondary current from the start point of the first saturation period. A least-squares (LS) technique estimates the DC offset parameters from the single-cycle difference of the secondary current in the unsaturated periods. Removal of DC offset from the secondary current yields the sinusoidal waveform portion. Finally, the LS technique is used once again to estimate the phasor from the sinusoidal waveform portion. The performance of the algorithm was evaluated for a-g faults on a 345-kV 100-km overhead transmission line. The electromagnetic transient program was used to generate fault current signals for different fault angles and remanent fluxes. The performance evaluation shows that the proposed algorithm accurately estimates the phasor of a current signal regardless of DC offset and CT saturation. The paper concludes by describing the hardware implementation of the algorithm on a prototype unit based on a digital signal processor.

Current Transformers (CTs) saturation leads to distortion of current waveform and operational malfunction of protection relays. This paper introduces a practical and simple method to prevent CTs saturation. The method uses a negative voltage feedback and applies it to CT secondary side. The injected voltage opposes the excitation voltage of CT in a manner that the magnetic flux of CT remains almost constant under saturation level for any faults. The method is categorized as a hardware solution, does not need to detect the CT saturation time and works either for digital or non-digital relays. The method is verified in a wide range of simulations and experimental results. The obtained results show that the method is reliable, effective, and accurate.

Malfunctions in relay protection devices are predominantly caused by current transformer (CT) saturation which produces distortion in current measurements and disturbances in power system protection. The development of deep learning in power system protection is on the rise recently because of its robustness. This study presents a CT saturation detection where the secondary current becomes distorted. The proposed scheme offers a wide range of saturation detection and consists of a moving-window technique and stacked denoising autoencoders. Moreover, Bayesian optimization was used to minimize the difficulty of determining neural network structure for the proposed approach. The performance of the algorithm was evaluated for a-g faults on 154 kV and 345 kV overhead transmission line in South Korea. The waveform variation has been generated by PSCAD for different scenarios that heavily influence CT saturation. Moreover, a comparative analysis with other methods demonstrated the superiority of the proposed DNN method. With the proposed algorithm to detect CT saturation, it significantly yielded high accuracy and precision for CT saturation detection which were approximately 99.71% and 99.32%, respectively.

Current-transformer (CT) saturation has a major impact on the operation of differential protection schemes. Transformer differential protection must maintain security during CT saturation for external faults while preserving high sensitivity and speed of operation for lower magnitude internal faults. Relay manufacturers provide very little information or direction for protection engineers to set relays adequately to achieve these goals. If an engineer lacks a deep understanding of relay behavior during CT saturation, he or she could make setting implementation mistakes that may lead to spurious trips or failures to trip with catastrophic consequences. This work investigated the impact of CT saturation on transformer differential relays in order to provide guidance on scheme behavior. It was discovered that harmonic blocking is triggered by CT saturation-a significant effect which is not described in the literature or relay manuals. This effect was modeled and the model's accuracy was verified with CT simulations, relay secondary injections, and high current primary injections. It is demonstrated that harmonic blocking provides effective security against nuisance trips caused by CT saturation. It is also shown that the traditional use of a high slope 2 is mostly redundant wherever harmonic blocking is enabled.

The design, evaluation and implementation of a busbar current differential protection relay that operates in conjunction with a current transformer (CT) saturation detection algorithm is described. The detection algorithm detects the start and end of each saturation period based on the third-difference function of the current. A blocking signal is activated during saturation and for one cycle after it ends. For internal faults that cause CT saturation, the relay operates before activation of the blocking signal and consequently, a trip signal is issued. For external faults that cause saturation, the blocking signal arrives first and the trip signal remains inactive. The technique does not delay the operating time of the delay. Test results indicate that the relay can discriminate between internal and external faults even when a CT is severely saturated due to the effect of remanent flux. The relay is implemented in a protection scheme based on a digital signal processor. The new relay achieves a high level of stability on external faults and a major improvement in sensitivity for internal faults.

The paper describes a Transient Bias technique designed to overcome the effects of Current Transformer (CT) saturation, which might severely affect the behaviour of a protection system. This Transient Biasing, as the name implies, is only active during transient conditions in the power system, and it decays quickly once the transients disappear. A complementary algorithm for fast detection of external faults is proposed, which prevents the Transient Bias feature from increasing the differential protection operating time in case of internal faults. The operation of current differential protection with Transient Bias is checked using Simulink® models of the relay and current transformer. In addition, the paper reviews the results of RTDS® tests carried out with actual current differential relays and provides detailed information on CT requirements testing method adopted by Alstom Grid. (4 pages)

Deteriorated measurements due to the saturation of current transformers (CTs) are a major challenge in digital power system protection schemes. Failing to resolve this issue effectively can have serious consequences for the accuracy of measured components in phasor-based digital relaying algorithms. Potentially, this may compromise the secure and reliable operation of the protection system and, hence, the entire power system. Accurate fault detection or classification can be achieved by applying an algorithm that can deal with the CT saturation effects. In this paper, a method is presented that can accurately estimate the fundamental current phasor during CT saturation. Reconstruction of the deteriorated measured waveform is accomplished by applying a supervised machine learning algorithm, namely the support vector machine (SVM). The least squares (LS) method is integrated with the SVM-based algorithm to reduce the complexities of waveform reconstruction regressions. A modified discrete Fourier transform (DFT), robust to decaying DC components, is then applied to the reconstructed waveform to extract the required phasor components. The proposed approach is validated by evaluating its classification and phasor estimation performance using standard metrics over numerous simulated cases and field measurements. The results demonstrate the high accuracy of the proposed method to classify different levels of CT saturation and ensure precise estimation of the fundamental phasor component. • Saturated CT signals are reconstructed using supervised time-series prediction. • A combined least squares–SVM model performs fast waveform reconstruction. • An online recursive LSSVM approach overcomes batch-learning limitations. • Modified DFT removes decaying DC and improves phasor estimation accuracy. • Accurate phasor estimation is achieved within one cycle for all CT saturation levels.

Decaying dc offset and current-transformer (CT) saturation are the main obstacles to accurate measurement of the fundamental frequency component of fault currents. So far, most of the research works have focused exclusively on only one of these obstacles. However, very recently, research has been directed toward new current measurement algorithms which can deal with both of these issues simultaneously. Using least error squares technique and a simple offline lookup table, this paper introduces a novel method to filter out the decaying dc component and the effects of CT saturation. The proposed algorithm provides the fundamental sinusoidal component of current accurately. This method can respond using only five current samples. Meanwhile, the window size could also be increased to provide more immunity against noise and harmonics. High speed and accuracy, together with the simplicity and noise immunity of the proposed method, can make it a practical solution to address the decaying dc offset and CT saturation problems. Numerous simulated and real fault currents as well as real-time hardware implementation verify the effectiveness of the proposed algorithm.

A method of symmetrical component analysis for the detection of current-transformer (CT) saturation in a numerical current differential feeder protection relay is presented in this paper. The performance of the differential relay is investigated for various faults on a typical Electro-Magnetic Transients Program/Alternative Transients Program (EMTP/ATP) simulated transmission feeder. The simulator includes the effects of CT saturation. A comparison between simulation and tests conducted on an analog model testbench are also evaluated. The results show a high degree of similarity and illustrate the effect that CT saturation imposes on the sensitivity and stability of the protection scheme. An algorithm is presented that shows significant improvement in sensitivity on internal faults while still maintaining a high level of stability on external faults and nonfault events.

This article presents a fast, hardware efficient logic for fault detection and classification in transmission lines. The proposed logic employs only one-terminal current samples and is based on multi-resolution wavelet analysis. First-level high-frequency details of the modal currents in the range of 5 to 10 kHz are extracted. Depending on the amount of high-frequency components in the transformed current signals after processing, the faults are classified into ten types. An adaptive threshold value is chosen, rather than a fixed threshold in the case of faults involving the ground, to make the classification reliable and accurate. The validity of the proposed logic was exhaustively tested by simulating various types of faults on a system modeled in Alternative Transient Program (ATP)/Electromagnetic Transient Program (EMTP).The proposed logic was found to be acceptably reliable and accurate even in the presence of fault resistance and current transformer (CT) saturation.

Modern numerical relays use improved measuring and stabilizing methods and can therefore tolerate current transformer (CT) saturation to a large extend. Dimensioning of CTs for short time to saturation however requires precise consideration of the transient CT behaviour in the first milliseconds after fault inception. Therefore, traditional calculation and dimensioning guidelines have to be reviewed and completed accordingly. Initially the paper discusses CT dimensioning and specifications that need to be considered for modern numerical relays. Typical CT dimensioning rules are given, based on theoretical calculation and test series. Then, the paper will discuss modern algorithms for fast detection of CT saturation and methods of adaptive measurement and stabilization that are available today in some modern numerical relays

Current transformers (CTs) supplying protective relays may suffer permanent or transient saturation under severe faults. The majority of protective relays are more or less sensitive to CT saturation. On the other hand, contemporary microprocessor-based relays open the window of opportunity for performing sophisticated checking in order to either reconstruct the secondary current waveform or compensate the measuring algorithms for saturation of CTs. This paper describes algorithms for amplitude and impedance components, resistive to transient saturation of CTs. The presented algorithms combine two procedures: (1) recognition of periods of saturated/linear operation of an observed CT; and (2) variable-length data window measuring algorithms that use unsaturated samples of the secondary current.

A discrete dynamic filter for detecting and compensating CT saturation

This work is about the possible effects of protection devices and schemes during current transformer saturation. If current transformer specifications are not selected properly, saturation may occur and affect relay response or even fail to trip. Hence, the performance of the protection devices such as differential and impedance relays is mainly based on the operating condition of the current transformers. An accurate knowledge of current signal is essential for protection devices. In this paper, a different approach is represented for current transformer saturation problem. The proposed technique is based on the discrete wavelet transform. The energy levels of the detail coefficients after the transformation present distinctive features during current transformer saturation.