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The algorithms of the present generation of practical differential relays are based on two methods. The first method is based on the ratio of the harmonic content of the differential current, and the second one is based on the length of the time interval between the zero‐crossing points of the differential current, called the gap‐detection method. However, these methods suffer from the installation of the fault current limiters (FCLs) in the power system and current transformer (CT) saturation phenomenon. This paper deals with a simple practical method for differential relays to secure their performance. In the suggested method, a classifying algorithm is used to categorize the input signal of the relay and then the best method of the present generation of differential relays is employed. By this simple decision, the method takes the advantage of harmonic‐based and gap‐detection methods, while avoiding their drawbacks. To prove the robustness of the suggested method, a test bench with a resistive solid‐state fault current limiter (SSFCL) is implemented and examined in different situations. The results validate the consistency and the accuracy of the modified technique not only in the absence and the presence of the FCL but in the case of CT saturation. [ABSTRACT FROM AUTHOR]

Differential protection, as the key protection element in the power transformers, has always been threatened with sending false trips subjected to external transient disturbances. As a result, differential protection needs an additional block to distinguish between internal faults and external transient disturbances. The protection system should, first, be able to perform based on raw data, second, be able to learn fully temporal features and sudden changes in the transient signals, and, third, impose no assumption on noise. To address these challenges, a fast RNN, namely fast gated recurrent neural network (FGRNN). By removing reset gate in the gated recurrent unit (GRU), the proposed network is capable of learning abrupt changes in addition to significantly reducing the computational time. Furthermore, a loss function based on an information theory concept is formulated in this article to enhance the learning ability as well as robustness against non-Gaussian/Gaussian noises. A generalized form of mutual information is also adopted to form a noise model-free loss function, then incorporated with the designed deep network. Simulated and experimental examinations engaging various external factors, in addition to comparison between the proposed FGRNN, GRU, and seven firmly established methods indicates the faster and more reliable performance of the proposed algorithm. [ABSTRACT FROM AUTHOR]

This article puts forward a new algorithm based on Kullback–Leibler divergence (KLD) for discriminating inrush and internal fault currents in power transformers. Specifically, the main idea of this algorithm is to utilize the current signal discrepancy of the distribution with ideal fault current waveforms. To such an aim, the proposed index reproduces the differential current signal. Utilizing fast modified least squares technique, the differential current signal is generated. Applying the reconstructed differential current and ideal sinusoidal waveforms to the KLD indicator, the distribution discrepancy of the current signal from ideal sinusoidal waveform discriminates inrush and internal fault currents. Also, the internal/external identification index is introduced that utilizes the phase content of the signals from current transformers (CTs) on both sides to distinguish internal and external faults, especially the faults accompanied with CT saturation. As a result, the proposed method can distinguish inrush currents, internal and external fault currents with/without CT saturation, and internal fault currents during transformer energization. The performance of the proposed index is evaluated using practically recorded data and is further compared with the state of the art. [ABSTRACT FROM AUTHOR]

Due to increase in penetration of Distributed Generations (DGs) in power systems, fault current level is being increased, which results in some problems in the systems. Fault Current Limiters (FCLs) are attractive devices to tackle these problems for transmission and distribution systems. The utilized FCLs may have considerable impact on the signals used for differential protection of power transformers, which leads to mal-operation of these protections. It seems a comprehensive analysis is necessary for performance evaluation of differential protection algorithms in presence of FCLs. This paper deals with investigation of FCLs impact on power transformers’ differential protection. The performance of some well-known differential protection algorithms for discrimination between internal fault current and magnetizing inrush current with and without presence of FCL are evaluated. [ABSTRACT FROM AUTHOR]

The phase‐to‐ground fault is the most probable fault in power systems. Generally, the neutral grounding resistor is utilised to limit the current for this fault. However, this resistor decreases the sensitivity of the differential protection. The fault current limiter (FCL) is an attractive candidate to be used instead of the regular grounding resistor. FCL only operates for high fault currents and has a negligible resistance for normal or low current faults conditions. Therefore, it reduces the high fault currents while not affecting the sensitivity of the differential protection. However, the signals used as inputs of the differential protection may be adversely affected by the FCL and this may lead to wrong operation of this protection. A comprehensive study is required to evaluate the performance of differential protection algorithms in the presence of a FCL in the neutral point. This study analyses the impact of high‐temperature resistive superconducting FCLs (SFCLs) existed in the neutral point on transformer inrush current discrimination methods. Some well‐known methods for discrimination of an internal fault from inrush current are evaluated in the presence of SFCLs. Furthermore, through extensive simulations, their performance is compared with and without the presence of SFCLs. [ABSTRACT FROM AUTHOR]

Differential relays are the main protection for power transformers including transmission and industrial transformers. Protection against internal fault is generally relegated to these relays. The most probable internal fault of the industrial transformer is phase to ground type. To limit the fault current in this condition, a limiter resistor is utilised in the transformer. However, it has considerable impact on sensitivity of the differential and restricted earth fault (REF) protections. In this study, an effective approach is proposed to improve the differential and REF protections sensitivity of the industrial transformers. The proposed technique not only realises the fault current limitation function, but preserves sensitivity of the protections. In this method, a developed fault current limiter is utilised in the transformer neutral instead of the neutral grounding resistor. The performance of the proposed technique is evaluated by some simulations and experiments. The results confirm the high capability of the proposed technique in different conditions. [ABSTRACT FROM AUTHOR]

This study deals with a method for discrimination between magnetising inrush and internal fault currents in three‐phase transformers based on sine‐wave curve fitting. A sine wave is fitted to the normalised differential current by using the least‐squares technique for each phase. The difference between the normalised differential currents and the fitted signals are calculated, which are hereafter named residual signals (RSs). Maximum range of the RSs variations is utilised for the definition of a simple criterion. Based on this criterion, the discrimination is realised within less than a half cycle of power frequency. In order to eliminate the impact of the current transformer (CT) saturation on the performance of the proposed technique, the currents are compensated by a CT saturation compensation algorithm in the first step. The proposed method is evaluated and compared with some well‐known methods by a large number of simulation and experimental data. Furthermore, the proposed algorithm is evaluated in the case of a transformer, which is fed through a feeder with series capacitors. The results confirm that reliability, accuracy and speed response of the proposed technique is more than the other methods. [ABSTRACT FROM AUTHOR]

• Proposing discrimination index based on fundamental phase angle of the current signal. • The proposed index's threshold does not require transformer parameters. • Classifying internal/external faults and inrush currents in sub-cycle considering CT saturation. • Enhancing reliability and response time in the various fault/inrush conditions. Power transformer differential protection may confront mal-functioning in authentic discrimination between inrush and internal faults. To tackle the latter mal-functioning, a new two-stages algorithm based on phase content of the current signal of the current transformers (CTs) is put forward. The proposed algorithm is designed based on the fact that the fundamental phase angle of a fault signal ideally remains constant during the fault. However, during inrush cases, the phase angle varies. Also, during the internal fault, the phase angles of the current signals of CTs are in phase while during the external fault, the phase angles of the current signals of CTs are 180° out of phase. In the first stage, the proposed algorithm calculates the fundamental phase angles of the current signals of the CTs using sub-cycle modified recursive least squares (MRLS). Afterward, normalized mean residue (NMR) is employed to measure distance between the estimated phase angles of the CT's currents. MRLS and NMR algorithms require limited samples (i.e. 10 and 5 samples respectively) for executing their calculations. Performance evaluation with simulated and experimental recorded current signals shows the ability of the proposed method in discrimination of the internal faults from inrush and external fault currents. [ABSTRACT FROM AUTHOR]

Under frequency load shedding (UFLS) scheme has been widely implemented as a safety net to prevent system collapse following major disturbances which results in large mismatch between load and generation. However, voltage related issues post activation of UFLS stages are also vital concerns in preserving system stability and should be properly analysed. Fixed and switched shunt capacitors that are in service during normal operation for maintaining system voltage and dynamic MVAR reserve can generate surplus reactive power post operation of UFLS relays which may result in over voltage issues, generator under excitation and some undesirable conditions such as transformer saturation, ferro‐resonance etc. This study addresses the need for coordinated under frequency load and capacitor shedding and its implementation approach to effectively preserve system stability following small and large disturbances. To confirm the feasibility of the approach, the proposed method has been used to design coordinated UFLS and under frequency capacitor shedding (UFCS) schemes for a real and actual power network. In addition, the proposed coordinated UFLS and UFCS scheme has been combined with automatic switching of shunt reactors to optimise the performance of the scheme. [ABSTRACT FROM AUTHOR]

• Proposing discrimination index based on symmetric Kullback–Leibler divergence of the current signal. • The proposed discrimination index does not require transformer parameters. • Classifying fault and inrush currents in sub-cycle considering CT saturation. • Enhancing reliability and response time in the various fault/inrush conditions. 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. [ABSTRACT FROM AUTHOR]

• We propose twofold discrimination indices based on kurtosis of the current signal. • The proposed discrimination indices do not require transformer parameters. • It allows sub-cycle classification of fault and inrush current under CT saturation. • It enhances reliability and response time in various fault/inrush conditions. Differential protection is a widely-used approach for power transformer protection, the performance of which may be, at times, challenged due to the inrush current and current transformer (CT) saturation. This paper presents an enhanced statistical algorithm for discrimination of fault and inrush current signals under CT saturation scenarios. This paper proposes twofold discrimination indices centered on the concept of fourth-order statistic moment, the so-called kurtosis on the current signal. Mathematically proven, the algorithm performance only depends on the current signal phase angle and it does not require transformer parameters. The mathematical closed form formulation in this paper classifies fault condition and inrush currents in sub-cycle timeframes even in the CT saturation conditions. The performance of the proposed algorithm is verified under various fault and inrush current scenarios with and without CT saturation. The numerical results demonstrate that the proposed algorithm can successfully classify the disturbances and detects saturations quickly with promising accuracy. Index Terms: Inrush current; internal fault; current transformer saturation; differential protection [ABSTRACT FROM AUTHOR]