Abstract-Voltage instability is a phenomenon which often contributes to the development of power system disturbances. While increasing load admittance, bus voltage decreases to such a degree that the apparent power (V 2 Y) does not increase. As a result, voltage collapses with all consequences resulting from it. During the large-scale power system disturbance, the last line of defense is the load shedding at the stations, where the stability margin becomes dangerously low. To do that, there is a need to use automatic devices which process local signals, detect the decreased margin, and activate the load shedding. As a criteria of operating such devices, the voltage level is adopted. The disadvantage of such an approach results from the fact that the relations between the voltage level and the stability limit depend very much on the load power factor. To overcome this weakness, this paper presents a criterion which is directly based on the definition of voltage stability. It calculates the derivative of apparent power against the admittance (dS dY). It may be easily accomplished, because both the power and the admittance are measurable, and the changes of load occurs continuously, as a result of switching on and off the impedances, and/or actions of the transformer on load tap-changing devices.

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If a current transformer (CT) saturates while transforming the heavy primary current, in each period, there are sections when the magnetic core is unsaturated and transformation is correct, and when it is saturated, it causes enormous errors. The key to correct the errors is to detect instants of saturation and moments when the saturation ends. This paper presents simple and efficient methods of doing that. In addition, it shows the way how to predict up to three samples after saturation with the acceptable degree of accuracy, thus artificially expanding the unsaturated section. It may substantially facilitate operation of protective devices. The ultimate solution of the problem is to estimate the values of the fundamental frequency component amplitude and the decaying dc component in each period. This paper presents the method of doing that on the ground of four consecutive samples taken during the unsaturated section. If the primary current consists of the fundamental and dc exponential components only, the accuracy of the method is excellent. Neglecting contamination of the current signal with harmonics and noise, it is possible to reconstruct the CT primary current based on the measured values of fundamental and dc components.

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Abstract-This paper presents a digital relay scheme for the primary protection of power transformers. A multi-criteria algorithm is developed based on fuzzy sets for the decision making part of the scheme. The cost of wrong decision-making and the amount of information inflow are used along with several standard criteria to improve the reliability of the protection. It is shown that the proposed scheme, if properly tuned, can enhance the sensitivity and selectivity of the digital relay and mitigate problems associated with conventional relay schemes.