Power differential method for discrimination between fault and magnetizing inrush current in transformers

Abstract

To avoid the needless trip by magnetizing inrush current, the second harmonic component is commonly used for blocking differential relay in power transformers. However, the second harmonic component in fault current is increased by the introduction of underground 500kV lines. This paper describes a new method to discriminate internal fault from inrush current by the sum of active power flowing into transformers from each terminal. The average power is almost zero for energizing, but an inlernal fault consumes large power. To check the performance of this method, actual inrush current and voltage waveforms of 500/154kV transformer are accurately measured by digital equipment. The usefulness is confirmed by applying the method to the measured inrush and simulated fault data.

Power Transformers Differential Protection Using the p-q Power Theory

“…The discrimination criterion is based on the following basic principle: the average power is almost zero for energizing, but an internal fault consumes large power. However, the direct application of the average power as a decision signal for the discrimination of inrush/fault conditions can lead to misleading results, due to the oscillations of the average active power, as reported in [7]. The same basic principle mentioned before is particularly suitable for the application of the p-q theory to the power differential method, where the active power component is directly related to the internal fault within the transformer and the reactive power component is associated with the inrush transient.…”

“…The differential power is computed as the difference between the instantaneous powers at all the transformer's terminals. In [7], the average of the instantaneous differential power is responsible for both the operation and the blocking signals of the relay. The discrimination criterion is based on the following basic principle: the average power is almost zero for energizing, but an internal fault consumes large power.…”

“…The active power differential method was firstly proposed in [7] for the discrimination between inrush transients and internal faults in power transformers. The differential power is computed as the difference between the instantaneous powers at all the transformer's terminals.…”

“…Researchers and designers have made remarkable progress in the development of protection strategies since microprocessor-based relays were adopted for the differential protection of power transformers. A wide variety of protection schemes have been proposed, many of them combining different methods, such as the improvement of the classical harmonic restraint schemes [2], [3], the introduction of new trip and blocking signals [2], [4][5][6][7], the use of wave-shape recognition methods [3], and the application of advanced digital signal processing tools, fuzzy logic and artificial intelligence techniques [8].…”

Application of Park's power components to the differential protection of three-phase transformers

“…Not surprisingly, a significant amount of research work has been devoted over the last decades to solve the inrush/fault discrimination dilemma of the transformer differential protection. Several solutions have been proposed in this context, including the refinement of the traditional harmonic restrain methods [14][15][16][17], the development and exploration of new trip and restrain signals [10,[12][13][18][19][20][21][22][23][24][25][26][27][28][29][30][31], the use of wave-shape recognition methods [32][33][34], and the application of advanced digital signal processing tools [11,[35][36][37], fuzzy logic and artificial intelligence techniques [31,[38][39][40]. These methods can provide an alternative or improvement to the existing protective relaying functions.…”