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Abstract
The mal-operation of the transformer differential protection occasionally takes place after external fault being removed. At present, the mal-operation is prevented by means of changing the acting area and braking area of differential protection, which adversely affects the clear of transitional fault. This paper analyses the differential current harmonic component by simulation, a new algorithm is proposed according to the features that the triple frequency harmonic is greater than the second harmonic in the period of the mal-operation. The new algorithm is based on the differential protection of the second harmonic braking, and the new braking criterion is presented using the rate of the second harmonic and the triple frequency harmonic in the fundamental harmonic. Simulation results show that the new algorithm is simple and effective, can avoid the mal-operation, and clear transformer transitional fault quickly.
To overcome the shortages of the analysis of the differential protection characteristic in the differential currents vs. resistant currents plane and complex plane at present, this paper proposes a novel method, which analyzes the differential protection characteristic in polar coordinate. The representation of fault component current differential protective criterions in polar coordinate is presented, and the comparison of characteristics of fault component current different criterions is described with this method which provides basis for the actual application. What's more, this method is convenient to analyze the influence on differential protection when both amplitude and phase of currents are changing. Sensitivity coefficient is introduced to contrast characteristic of criterions in both internal and external faults. The simulation results from the faults in 110 kV line by using PSASP are consistent with that of the theoretical analysis.
The transferring characteristics of current transformers (TA) are subject to possible changes due to DC magnetic bias caused by geomagnetic induced current (GIC)/high voltage direct current (HVDC). The particularity of TA saturation affected by the DC magnetic bias will severely impact the transformer differential protection system. Based on the AC system simulation models considering DC magnetic bias, the effects caused by the variation of time-to-saturation of protective TAs are studied. Moreover, studies show that DC magnetic bias will lead to TA partial transient saturation. According to simulation of sympathetic inrush with DC magnetic bias, this paper analyzes the impacts on differential protection resulting from the TA partial transient saturation. The research demonstrates that DC magnetic bias may accelerate saturation of protective TAs. This is a challenge to the method of locating the time difference used for preventing maloperation due to TA saturation when external faults occurred. The research also shows that TA partial transient saturation caused by DC magnetic bias may contribute to maloperation of transformer differential protection.
The mal-operation of the transformer differential protection will occasionally take place after the removal of external faults. The physical phenomenon of partial transient saturation for current transformer was studied in this paper. Through model emulation, the experiment of small CT and the waveform of field fault record for mal-operation, it is pointed out that the main cause of the malfunction in differential protection is partial transient saturation of current transformer while transferring small load AC current at large remnant flux. The differential protection is very sensitivity to malfunction since the restrained current is small. At the same time, partial transient saturation phenomenon will also occur while CT transferring small AC current combined with long time decaying current (such as sympathetic inrush or through inrush). In order to solve this problem, an inverses time current differential protection scheme based on detecting external faults removal are proposed. This scheme could avoid the maloperation of the different protection due to CT partial transient saturation during the removal of external fault. It has great perspective if the method is realized for transformer protection.
Mal-operation of the transformer differential protection will occasionally take place after the removal of the external fault. The physical phenomena of the current transformer (CT) saturation and the phase compensation scheme of the transformer with wye-delta connection are studied in this paper. Based on the theoretical analysis and simulation research, this paper points out that CT partial transient saturation after the removal of transformer external fault only transforms the fundamental frequency component with a small transformation error, and won't result in the mal-operation of differential protection. If CT on one side of the transformer becomes ultra-saturated and CT on the other side is in the linear region, the inconsistent transformation of CT will result in the mal-operation of differential protection. In addition, the phase compensation of the transformer wye-side current will also result in the mal-operation of differential protection. Through the analysis of the mal-operation point, a method of different time delays in different areas is presented to prevent the differential protection from mal-operation. Simulation results show that the new method will guarantee the fast operation of the differential protection in the case of an internal fault while avoiding the mal-operation after removal of the external faults.