A differential relay that is very sensitive relay operating even at its limits may be used for protecting a power transformer. However, this characteristic may lead to unnecessary tripping due to transient currents such as an inrush and over excitation current. In order to avoid this mal-operation of the relay, a second and fifth harmonic blocking technique has been used; however this technique is not reliable if a second harmonic magnitude is weak. In this paper, a new approach is proposed using even harmonics (second and fourth). The test results show that this proposed approach is a good blocking technique associated with the differential relay even for large modern power transformer which has small second harmonic as well; it provides a good discrimination between the transient currents and the internal fault currents during internal fault.
The present letter reports the results of the experimental measurements of the dc substrate current in metal-oxide-semiconductor (MOS) structures operated under a gate-voltage signal oscillating under depletion limits. A new component has been detected in this current whose direction of flow is independent of the polarity of the applied gate voltage as well as the nature of the MOS structure. Its magnitude shows strong dependence on the frequency of the applied gate voltage, giving rise in general to three prominent peaks in the current-frequency curve. Arguments are given in support of the view that this current has its origin from the charging and discharging of the surface-states existing on the Si-SiO2 interface under non-steady-state emission.
A differential relay that is very sensitive relay operating even at its limits may be used for protecting a power transformer. However, this characteristic may lead to unnecessary tripping due to transient currents. In order to avoid this unnecessary tripping, estimated harmonics of these currents may be required which need great computation efforts. In this paper, a new frame work is proposed using PC interfaced with a data acquisition card AD622, which acquires real-time signals of the currents, process them numerically in the computer and outputs tripping signal to the circuit breaker. All algorithms of differential protection function and blocking techniques have been implemented using the Simulink/Matlab. To validate the present work, the performance of developed relay is tested by signals generated by Simulink/MATLAB simulator under different conditions. The test results show that this proposed scheme provides good discrimination between the transient currents and the internal fault currents.
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