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This paper deals with the utilization of printed circuit board Rogowski coils (PCBRCs) for differential protection of electric arc furnace (EAF) transformers in Mobarakeh Steel Company (MSC), Isfahan, Iran. Because of a high level of current (in the range of 100 kA) in the EAF transformers in the MSC, employment of differential protection is quite challenging. Conventional current transformers (CTs) cannot offer reliable performance for very high current levels. The PCBRCs are a branch of Rogowski coils (RCs) that can be manufactured with high precision using computer‐aided design approaches and have the capability for the measurement of very large currents. In this study, we have designed and manufactured two sets of PCBRCs to measure current in the primary and secondary sides of the EAF's transformers in the MSC. The PCBRCs are followed by a signal condition circuit (SCC) which includes a low‐pass filter, amplifier, and voltage‐to‐current (V‐to‐I) converter block. Thanks to the SCC, the high current of the transformers can be properly sensed and transmitted over a long cable that connects the PCBRCs to the relay. Furthermore, a novel model is suggested for the EAF that includes characteristics of both time and frequency domain models. The suggested model is developed using a large number of recorded data from the MSC and can model the non‐linear and time‐variant behaviours of the EAF. The proposed PCBRCs, SCC, and EAF modelling are evaluated by some simulations and experiments in the MSC. [ABSTRACT FROM AUTHOR]

In order to improve the dependability and security of transformer differential protection, the energisation conditions should be effectively distinguished from internal fault conditions. In this study, a new method with a low computational burden and high robustness against measurement noises is proposed for discriminating between the transformer inrush current and internal faults. In the proposed method, sine waves are fitted to the sample points of the normalised differential currents for different phases using recursive extended least‐squares (RELS) algorithm in an online manner. The extended kernel enables the algorithm to effectively estimate the dynamics of the measurement noises. Three residual signals which are defined as the differences between the normalised differential currents and the fitted sine wave signals are considered as the decision criteria. Based on the selected criteria, the energisation condition is identified in about a half‐cycle of power system frequency. A large number of simulation and experimental test cases are used to demonstrate the effectiveness of the proposed method. The results show that the proposed algorithm achieves an accuracy of ∼98% and has a relatively low operating time, while the computational burden for the embedded implementation is favorably low. [ABSTRACT FROM AUTHOR]

Integration of distributed generations into distribution systems has various impacts on power networks such as reconstruction of distribution networks and increase in short‐circuit capacity. Reconstruction of distribution networks magnifies importance of detecting faults directions, which is relegated to directional relay. On the other hand, one of the most attractive approaches to mitigate the increased short‐circuit capacity is utilising fault current limiters (FCLs). Since FCLs can drastically affect voltage and current signals during the first quarter cycle after fault occurrence and due to overlap between operation time of FCLs and directional relays, the affected voltage and current signals can lead to mal‐operation of directional relays. To investigate this issue, performance of some well‐known directional relay algorithms is investigated with and without the presence of FCL using plenty of simulations and some experiments. It is concluded that the FCL has different impacts on the investigated algorithms, so that some of them are not affected and some others are considerably affected. [ABSTRACT FROM AUTHOR]

Electric arc furnace (EAF) is one of the largest non‐linear loads in power systems. It is highly time‐varying which leads to low‐frequency voltage fluctuations in the system, called flicker. Static VAr compensator (SVC) is always used in EAF plants to compensate the reactive power and reduce voltage fluctuations. The reactive power calculation unit is one of the most important units in SVC control system. Since SVC can only compensate the fundamental component, the calculated reactive power should not be sensitive to other harmonics. On the other hand, since the reactive power changes every half cycle, its calculation should be fast enough to track the changes. Hence, a calculation method with minimum delay and lowest harmonic sensitivity should be utilised to achieve an effective compensation. Among different already proposed methods for calculation of reactive power, differential method has the lowest delay. However, it is very sensitive to harmonics. In this study, Kalman filter is used to extract the fundamental components of voltage and current with a little delay and then they are used as inputs for the differential method. The proposed method is evaluated using some simulation signals and practical records from several EAFs in Mobarekeh steel company in Esfahan – Iran. [ABSTRACT FROM AUTHOR]

Phase‐to‐ground fault is the most probable fault type in power systems, which is generally limited through neutral grounding resistor. The grounding resistor may have negative impacts on the system such as decrease in sensitivity of the differential protection. Fault current limiter (FCL) is a suitable alternative for the grounding resistor. However, the input signals of differential protections may be adversely affected by the FCL, resulting mal‐operation of this protection. In this study, effects of FCL installed in neutral line of power transformers on the relevant differential protection algorithms are analysed. Performance of some well‐known differential protection algorithms for discrimination between internal fault and magnetising inrush currents in presence of the neutral FCL is evaluated in detail. Then, a supplementary method based on the least error squares technique is proposed to rectify the deformed portion of the current waveform due to presence of the FCL. The reconstructed signal is considered as a proper input for the discrimination algorithms. Various simulations and experiments are carried out to validate the performance of the suggested technique. The results indicate the consistency of the suggested technique for different power systems, operating conditions, and real applications. [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]

In this study, a new algorithm for directional protection, based on cross correlation of Fourier transform components of voltage and current for each phase is proposed. The fault is detected using k‐nearest neighbour algorithm. By comparing the fifth‐nearest neighbour of each sample with an adaptive threshold, the fault is detected. The adaptive threshold is derived by a moving window with length equal to 1 ms. Using Fourier transform, the fifth sinusoidal component of the measured voltage and current for each phase is extracted. Then the cross correlation of the voltage and current components is calculated. The calculated cross correlation signal is passed through a low‐pass filter. By comparison of the filtered signal with a predefined threshold, fault direction is determined. It is confirmed that the fault direction can be identified in less than 8 ms with high accuracy of nearly 99%. More than 6500 scenarios are considered for different fault conditions to evaluate the proposed method. Furthermore, using some experimental results the proposed method performance is confirmed. The simulation and experimental results show that the proposed method has high speed, good accuracy, and sensitivity. [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]