A Current Transformer Saturation Detection Algorithm for Use in Current Differential Protection

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.

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.

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

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.

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.

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.

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)

Battery energy storage station (BESS) possesses flexible operating states and complex fault characteristics. With the continuous increase in its installed capacity and number, the risk of maloperation of traditional relay protection significantly increases. To mitigate this risk, this paper presents a time-domain pilot protection that accommodates BESS’s operating states and fault characteristics. Firstly, the issue of the traditional current differential protection (CDP) caused by the integration of BESS is analyzed. Subsequently, the fundamental principle of the Jaro distance is introduced, and the criteria for calculating the number of matching points and transpositions are redesigned accordingly. The Jaro distance is utilized to evaluate the matching degree of current sampling sequences between BESS side and grid side, thereby accurately identifying internal and external faults. Additionally, by introducing delayed operating current sequences in the process of calculating the number of matching points, the security issue caused by current transformer (CT) saturation is resolved. Finally, PSCAD/EMTDC simulations and digital-physical co-simulation (DPCS) testing validate the proposed protection’s significant advantage. The results indicate that proposed protection algorithm offers gratifying performance in dependability, security and selectivity, and outperforms several emerging pilot protections.

This study presents a new current transformer (CT) saturation detection and compensation algorithm. The proposed algorithm depends on a saturation detection index ( D n ) which is derived by using the derivatives of the current signals and Newton's backward difference formulae. The calculated index is continuously compared with an adaptive threshold ( T h ) to estimate the start and the end point of CT saturation. A low‐pass first‐order Butterworth filter is used to suppress noise and harmonics which may be present in CT secondary current. The proposed saturation detection algorithm has been tested by considering different values of remanent flux, fault type, fault inception angle, burden resistance, decaying DC component of fault current and noise. At the same time, modified discrete Fourier transforms‐based compensating algorithm has also been proposed to reconstruct the saturated samples. Validation of the proposed scheme is also performed on a developed laboratory prototype. A comparative evaluation of the proposed algorithm is also performed with the existing schemes. A series of test results from the simulation software and the laboratory prototype show the effectiveness of the proposed scheme.

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.

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.

Inverter-Interfaced Distributed Generation (IIDG) connected to distribution network with a teed line may cause protective relay tripping failure even using conventional current differential protection, which is regarded as one of the most effective protection for distribution network with distributed generations. Therefore, the impacts of IIDGs on traditional current differential pilot protection is analyzed firstly in this paper. Next, the fault characteristics of IIDG are introduced and the fault model of IIDG is established, in which IIDG is viewed as a current source controlled by the positive-sequence voltage at the point of common coupling. Then, based on the three-phase voltage and current on both sides of line, an improved pilot protection is proposed, which consists of a negative-sequence current differential protection as an auxiliary criterion for the conventional pilot protection. Finally, the effectiveness of the improved protection scheme is verified by a 10kV distribution network with multi-IIDGs established in PSCAD/EMPDC. The simulation results indicate that the proposed protection can effectively reduce or eliminate IIDG's impacts on current differential protection, particularly for an asymmetric fault, under various conditions involving different IIDG's output and fault resistance, thereby improve its reliability significantly.

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.

In line with the latest protection configuration requirements for 35 or 10 kV distribution grids, current differential protection is recommended for distribution lines connected to photovoltaic power sources. However, unlike traditional synchronous generators, photovoltaic power sources provide fault currents with amplitudes typically less than 1.2 to 2 times the rated current, and their phase angles are controlled and capacitive. This often results in the differential current being insufficient to trigger the current differential protection even during internal faults. This paper analyzes the issues with applying traditional current differential protection to photovoltaic power sources connected lines and deduces the threshold for the ratio restraint coefficient. An adaptive protection strategy is proposed, where the restraint coefficient is adjusted based on the amplitude ratios and phase angle differences of the fault currents. This ensures correct operation, particularly in cases of high photovoltaic power sources penetration. The strategy is tested through simulations conducted in Power Systems Computer Aided Design (PSCAD), showing improved sensitivity compared to traditional methods.

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