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Summary: Despite having several modern transformer protection schemes, the protection engineer prefers the universally adopted unit‐type differential protection scheme for the current transformer (CT) because of its functional flexibility. Inrush condition, external fault with current transformer saturation, over‐fluxing, or the combination of these may maloperate the unit‐protection schemes. In this article, all the above‐mentioned situations have been addressed that mislead differential protection. A multifunctional algorithm is presented that filters the disturbance for correct relay operation. The inrush condition is carefully examined with the help of the average angle of the differential current. Moreover, an adaptive algorithm takes care of the false operation of the transformer protection scheme during an external fault with CT saturation condition. Also, the momentary over‐fluxing condition is dealt with, with the help of fifth‐ and seventh‐harmonic‐based analysis of the differential current. Hence, the disturbance will be thoroughly examined and a decision signal issued accordingly to avoid unnecessary isolation of the transformer to improve system stability. The power system is simulated in PSCAD software and the algorithm is developed in MATLAB. Moreover, the proposed scheme is validated on a hardware prototype for authentication. It is found that the proposed scheme provides acceptable results and can be implemented in the real field. [ABSTRACT FROM AUTHOR]

• Total Harmonic Distortion (THD) based inrush and fault identification scheme for power transformer is proposed in this work. • Extensive PSCAD simulation is carried out with FFT analysis to estimate the THD during different operating condition of transformer. • Suitable THD limit is proposed for normal, inrush and fault condition of power transformer. • Validation of the suggested scheme has been carried out by software algorithm and displayed results shows satisfactory performance. • Hardware prototype has been developed and various inrush and fault cases are performed on the transformer to validate the proposed technique. The non linear characteristic of transformer magnetizing core leads more current/voltage signal distortion in the event of system disturbance/fault. The harmonic penetration in the current signal depends of the transformer core, characteristic of load and abnormal conditions. Different situations or abnormalities have different harmonics content and consequently different waveform signature. In this article, authors have utilized the harmonics content to identify normal, inrush condition and fault condition in power transformer. Total Harmonic Distortion (THD) of the current waveform for one cycle, is measured by performing Fast Fourier Transformation (FFT) analysis. Authors observed that the percentage THD is generally high in the case of inrush condition due to initial flux setup in magnetic core. Conversely, during fault condition, the %THD will be low (within a particular range) due to symmetric waveform. The magnitude of %THD below predefined range will be considered as normal condition and excess THD will be considered as inrush condition. On the other hand, if THD remains within a particular range then, it is considered as fault condition. In this article, authenticity of the presented scheme is checked by performing various test cases like initial inrush, internal fault, switching of considered transformer in faulty condition and CT saturation during fault condition. The hardware validation proves the competency of the proposed THD based discrimination scheme for transformer protection. The scheme presented here is simple and capable to solve complex problem of classification of inrush condition and fault condition. [ABSTRACT FROM AUTHOR]

The differential relaying scheme is universally adopted as the main protection for power transformer. In certain situations, like temporary overfluxing, this differential protection may malfunction. This is due to unbalance in primary and secondary current, as the saturated core does not reflect equal ampere-turn on either side. This article presents detection of over-fluxing condition against the internal fault based on the fifth harmonic component of differential current. As per the estimated level of the fifth harmonic concerning the fundamental component of differential current, the pickup setting of percentage bias characteristic will be modified to avoid maloperation. Once the over-flux condition is sensed, the algorithm activates timer-based V=f protection to protect the equipment. The proposed algorithm is validated on PSCADTMsoftware and after the collection of data, MATLAB software is used for further validation. The proposed algorithm is validated under various test conditions like normal load, external fault, internal fault, and overfluxing. It is observed that the algorithm accurately detects internal fault in the transformer or blocks the relay operation under momentary over-fluxing situations. Further, to assassinate the competency of the proposed scheme, a practical test is performed on a single-phase transformer available in laboratory. The data for various test cases are captured with the help of DSO, and the collected data has been migrated to computer where the proposed algorithm is examined. After analyzing all the results, it is cleared that the proposed algorithm can prevent transformer isolation during momentary over-fluxing conditions. [ABSTRACT FROM AUTHOR]

Due to the presence of Distribution Generation (DG), power system becomes more complicated and stability of power is the main challenging task. Saturation of Current Transformer (CT) imposes great dilemma on differential relaying scheme. This manuscript presents a new differential algorithm for distribution transformer protection which adaptively set its characteristic in the event of CT saturation. The proposed scheme is capable to detect magnetizing inrush condition, high resistance internal fault and discriminate external fault with CT saturation. The validation of the proposed scheme is done by simulating a part of the power system in PSCADTM software and programming in MATLAB software. A Full Cycle Discrete Fourier Transform (FCDFT) is implemented to validate the differential protective scheme for 15 MVA, 66/11 kV distribution transformer. An adaptive concept of the differential characteristic is employed in the algorithm to maintain the stability of relay during external fault with CT saturation. Validation and authenticity of the proposed technique are carried out with various test condition generated under wide variation in system parameters. The result on 2 kVA, 230/110 V, single phase transformer shows that the proposed scheme is capable to discriminate inrush, internal and external fault also with CT saturation conditions. [ABSTRACT FROM AUTHOR]

Conventional protection scheme of power transformer maloperation during abnormalities such as inrush condition, current transformer (CT) saturation during external fault and high resistance internal fault conditions. This article presents inrush detection based on an average angle of 2nd order derivative of differential current. The magnitude and phase angle of primary and secondary currents are estimated using a Modified Full Cycle Discrete Fourier Transform (MFCDFT) algorithm. Normally, during an internal fault, the differential current is well above the restraining current; however, the same condition arises during external fault with heavy CT saturation. Hence, a phase angle comparison based scheme is combined with percentage biased differential protection. During an internal fault, the phase angle difference of primary and secondary current is <90°, whereas it differs >90° for any external fault conditions. In the proposed scheme, 315 MVA, 400/220 kV transformer is considered for validation of various fault conditions simulated in PSCAD™ software. Moreover, the proposed scheme is validated on developed laboratory prototype of 2 kVA, 230/115 V single phase transformer using ATmega328 microcontroller. The proposed algorithm is effectively validated on both, simulation and hardware to discriminate inrush, internal and external fault by considering various system and fault parameters. [ABSTRACT FROM AUTHOR]

A novel full-cycle modified discrete Fourier transform-based digital protection scheme for generators is presented in this article. A laboratory setup has been developed to cater various abnormal and faulty situations applicable to a scaled generator. It includes utility supply, motor-generator set, synchronizing panel, current transformers/potential transformers (CTs/PTs), digital storage oscilloscope, and 3-phase variable load. A control circuit has also been developed comprised of a proposed algorithm, auxiliary relay, and circuit breaker (contactors). The proposed scheme depends on phasor values of modal voltage (Vmod) and current (Imod) derived by the algorithm in MATLAB® software. The outcome of the proposed modified discrete Fourier transform-based protection scheme is compared with the numerical relay (available in the laboratory), which has an inbuilt fast Fourier transform/discrete Fourier transform (FFT/DFT)-based algorithm. Various conditions, such as overload, over/under voltage, over/under frequency, ground fault, reverse power, and loss of excitation, have been simulated in the laboratory for the validation of the proposed scheme. Due to finer characterization of the fundamental signal derived by the proposed algorithm compared to numerical relay, it successfully detects and sends a trip signal for all types of abnormal/faulty conditions. The experimental results confirm the satisfactory operation of the proposed algorithm in the developed laboratory prototype. [ABSTRACT FROM PUBLISHER]