Research On Power Transformer Longitudinal Differential Protection

This paper reviews the concepts and the component elements of the transformer longitudinal differential protective relay, and introduces some new approaches.After the existing methods is evaluated for discriminating internal faults from inrush and overexcitation conditions, the dead angle principle is discussed in detail. Then, it is concluded that the protective relay based on dead angle principle can identify the small faults currents with large inrush currents. It is also discussed that if a fault current increases quickly due to the change of fault parameters, the current wave will be dissymmetry, where there is a second harmonic content. This case may cause the harmonic blocking unit issues restrain signal, but it doesn't cause the miscalculation of dead angles.When a CT(current transformer) is saturated, the dead angles will disappear in the CT second winding. The measure is usually taken to revert the dead angle by calculating the first and the second derivatives of the differential current. In this paper, it is proved that the transient dc component of the primary current can not invalidate the measure. In digital relays, derivation calculus is substituted by sampled difference. This paper analyzes the percentage of noise which is magnified by sampled difference, and proposes the approaches to reduce the noise which calculate the first and the second derivatives by using fragment functions integrated with the least square algorithm. The approaches improve both the accuracy of the dead angle calculation and the sensitivity of the relay.For external faults, the increase of the CT error introduced by the CT saturation may produce relay misoperation. In this paper ,by calculating and analyzing a great deal of simulated data, the ratio-relationship between CT errors and waveform correlation coefficients of the CT secondary current has been established, then, a principle of self-adaptive restraint ratio coefficients based on waveform correlation coefficients is presented. If external faults and internal faults occur in succession, the restraining current will increase earlier than the operating current. In this case ,the relay based on the self-adaptive restraint ratio coefficients doesn't raise the bias threshold inherently, instead ,it can raise the restraint ratio properly by the magnitude of the CT error, so it has high reliability and sensitivity.A new unbalanced current is pointed out, for a Y₀/ â–³ transformer, which Yo side is connected to the power source. If an external fault on Yo side occur, there will be unbalanced differential current introduced by the zero sequence currents in Yo side because of CT errors, but there is no corresponding restraining current. The paper analyzes this case in detail, than proposes a new approach based on zero sequence current restrain, which eliminates a hidden defect exiting in transformer longitudinaldifferential protection for a long time.The relay based on the approaches proposed in this paper is described.The dynamic simulation system, where the relay is tested, is described, and the test results are discussed. The test results prove that the approaches proposed in this paper can improve the reliability and sensitivity of the transformer longitudinal differential protective relay.