Enhancing differential protection stability during CT saturation with transient bias

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 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.

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.

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.

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.

Conventional second harmonic current restriction approach is an adopted method which has been implemented to transformer differential protection schemes. Because of unreliable nature of this approach in discrimination between inrush current, internal faults and external faults and due to the saturation of current transformers, we present new features of the differential protection scheme. This helps to determine the feasibility and applicability of the proposed approach in differential protective scheme for power transformer. The Current Transformer Saturation (CTS) phenomenon issue and its revelation is one of the most significant problems in power system protection particularly in the differential protection. In this paper, the saturation phenomenon and its effects on the differential protection are analyzed. Prevalent algorithms in digital signal processing which are implemented in power system cannot correctly recognize this phenomenon. Mal-operation of differential protection relay takes place when saturation in CT is appeared. This paper presents a Third-Difference Algorithm (TDA) for CTS recognition. In order to make comparison between the presented approach and prevalent digital protective algorithm, an identical case study is simulated. Simulation results also show that the TDA is both satisfactory and consistent with expectation. Keywords: Current transformer, Saturation phenomenon, Third-difference Algorithm, Internal and external faults.

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.

Adaptive additional current-based line differential protection in the presence of converter- interfaced sources with four quadrant operation capability

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.

With the worldwide realization and application of High Voltage Direct Current (HVDC), the additional effects of HVDC transmission technology on power system have caused extensive concern. The DC magnetic bias caused by HVDC may exacerbate the saturation degree of the current transformer (CT) during the external fault, which probably leads to the mal-operation of the transformer differential protection. In this paper, the state of CT transient saturation is divided into two kinds. Saturated characteristic and error analysis are presented in detail. Afterwards, two cases of CT saturation with and without the DC bias current have been compared in order to verify the effect of the biased flux on the time-to-saturation of the CT. Furthermore, based on the fact that time-difference method fails to block the external fault under the acceleration of CT saturation, this paper proposes an improved protective scheme to re-partition the ratiorestrained characteristic and sets a time-delay action region to avoid the mal-operation of the differential protection. What is more, CT saturation will affect the performance of the protective relay and threaten the security of power system, to which should be paid adequate attention.

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

The goal of protecting the power system is to protect against failures by separating the "faulty" parts from the rest of the network that is working properly. Unnecessary protection response and ejection of network nodes can have very serious consequences for the power system. For protection, relays are used to measure current and / or voltage and thus monitor the condition of the network. Differential bus protection is based on the algebraic sum of currents entering the node, where the bus is treated as a node. Under normal operating conditions, the sum of currents flowing into a particular node is equal to zero, while in the event of a fault, that sum deviates from zero. One of the biggest obstacles to the selective operation of differential relays is the saturation of current transformers (CT). When the CT enters saturation, what the relay is currently measuring does not represent a realistic picture of what is actually happening in the system. Then the primary current is no longer transmitted faithfully to the secondary side, but is distorted. Therefore, distortions in the secondary current are used as a criterion for the detection of CT saturation. In this paper, the behavior of the CT in the region of saturation will be analyzed using the Matlab software package, with the help of Simulink and SimPowerSystems libraries. The results of the simulations in the form of waveforms of currents will then be forwarded to the signal generator in order to test in laboratory conditions the capabilities of this device for generating complex periodic signals.

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.

The differential protection of transformers sometimes does not have reliable performance facing some faults and events such as saturation of current transformers (CTs) during internal and external faults, inrush currents, and cross‐country faults (CCFs). In this paper, a new transformer differential protection scheme based on the self‐adaptive biased characteristic curve and considering CCFs and various disturbances are used. First, the maximum overlap discrete wavelet transform (MODWT) was used to extract the superior characteristics of sampled differential current of the transformer at different levels. Then, by calculating the duty cycle (D) values of energy coefficients extracted in each periodicity by MATLAB software, a new self‐adaptive biased characteristic curve will be obtained in different operating conditions. Finally, the percentage differential protection operation is adjusted based on the new adaptive characteristic curve zones. Differential current sampling under different operating conditions was performed on a real three‐phase transformer in EMTP software. The simulation results show that the proposed adaptive differential protection scheme can accurately detect various faults and events occurring in the transformer and increase system reliability.

A current transformer (CT) saturation detection algorithm for use in current differential protection is described. The algorithm evaluates the start and end of each CT saturation period using the largest modulus series; this is derived from the third order difference function applied to the secondary current. To reduce the possibility of false detection due to the effect of noise on the third order difference function, the ratios of the absolute values of the CT secondary induced voltage are also used as a criteria to determine when the CT is saturated. A blocking signal is activated when saturation is first detected and is maintained for the duration of the saturation period plus one cycle. Simulation results indicate that the algorithm can significantly enhance the stability of the difference method. The algorithm successfully detects saturation and prevents the maloperation of a current differential protection scheme during high current external faults; whilst ensuring high sensitivity on internal faults.