Mal-operation Of Transformer Differential Protection Research Articles

A Framework to Avoid Maloperation of Transformer Differential Protection Under Geomagnetic Disturbances

A Framework to Avoid Maloperation of Transformer Differential Protection Under Geomagnetic Disturbances

Histogram-Based Method to Avoid Maloperation of Transformer Differential Protection Due to Current-Transformer Saturation Under External Faults

Current transformer (CT) saturation can cause the maloperation of transformer differential protection under external faults. To avoid this problem, a method based on the histogram of the normalized rotated current (NRC) is proposed. NRC is obtained by a symmetric waveform transformation of a part differential current. An index based on the histogram is defined to evaluate the NRC frequency distribution. The external faults with different degrees of CT saturation, internal faults, and transferring faults can be clearly distinguished by the proposed method. Moreover, the detection time is invariably less than a half cycle after the obvious increase of the differential current. Simulation results validated the proposed method.

A Method for Identification of Transformer Inrush Current Based on Box Dimension

Magnetizing inrush current can lead to the maloperation of transformer differential protection. To overcome such an issue, a method is proposed to distinguish inrush current from inner fault current based on box dimension. According to the fundamental difference in waveform between the two, the algorithm can extract the three-phase current and calculate its box dimensions. If the box dimension value is smaller than the setting value, it is the inrush current; otherwise, it is inner fault current. Using PSACD and MATLAB, the simulation has been performed to prove the efficiency reliability of the presented algorithm in distinguishing inrush current and fault current.

A New Adaptive Method to Identify TA Saturation Using Grille Fractal

In this paper, the different singularity characteristics of differential current of transformer differential protection under inner-zone and out-zone faults are analyzed while the current transformer is saturated. Then, using grille fractal, a new approach to prevent maloperation of transformer differential protection is proposed. Combining the proposed approach with an adaptive generalized morphological filter, the singularity characteristic of the signal can be fully reserved. Meanwhile, noises and interferences can be restrained. It is not necessary for the proposed approach to accurately determine the moment when fault occurs and differential current appears. Using relative magnitude of smooth domain of maximal value and that of minimal value and by means of setting an appropriate threshold, the inner fault of the transformer can be reliably identified and the identification is not affected by inrush current. Besides, the evolved fault can be reliably identified. Results of dynamic simulation prove that the proposed approach is effective and feasible.

A New Algorithm to Avoid Maloperation of Transformer Differential Protection in Substations With an Inner Bridge Connection

In substations with an inner bridge connection, the two main transformers are generally connected in parallel. The differential protection of the transformer, under normal operating conditions, may maloperate when the other transformer is switching on through the bridge breaker after scheduled maintenance. This paper analyzes the factors that may lead to the maloperation of transformer differential protection and concludes that local transient saturation of the current transformer (CT) caused by the decaying dc component in the magnetizing inrush is the main cause of the maloperation of the transformer differential protection. By detecting the time difference between the instant of sudden changes occurring in the transformer wye-side current and the instant of the differential current increasing, a new algorithm based on the time differential method is proposed in this paper, which can prevent the maloperation of transformer differential protection in substations with an inner bridge connection. On the basis of detailed analysis and simulation work, the new algorithm is verified to be applicable under the conditions of sympathetic inrush and CT saturation.

Studies on the Unusual Maloperation of Transformer Differential Protection During the Nonlinear Load Switch-In

Several cases of abnormal maloperation of the transformer differential protection with second harmonic blocking scheme are reported, which occurs when a type of nonlinear load, such as furnace, is switched-in to the transformer involved power systems. Different from well-known maloperation due to unloaded transformer energization, this type of maloperation does not occur on the time of load switching-in, but with a considerable time delay. The previous theories, including that interpreting sympathetic inrush, cannot be utilized directly to analyze this phenomenon. In this paper, a model for analyzing the transient course of the nonlinear load switching in, together with the description of the simplified characteristics of the transformer core and the nonlinear load is proposed to explain above-mentioned category of mal-operations. A set of nonlinear differential equations are formed using this model and a 4-order Runge-Kutta algorithm is utilized to solve the flux linkages and currents. The waveform and harmonic characteristic of the differential current are analyzed. It is disclosed that the maloperation of the transformer differential protection during the nonlinear load switching in mainly results from the saturation of transformer core caused by the interaction of the two nonlinear elements. This point of view is verified with the simulation tests.

Analysis of a Sort of Unusual Mal-Operation of Transformer Differential Protection Due to Removal of External Fault

Several cases of mal-operation of transformer differential protection with second-harmonic blocking after clearance of external fault are reported. These mal-operations all occurred at the nonrestraint region of percentage restraint plane. The previous theory cannot be utilized directly to analyze this phenomenon. Therefore, a mathematical model for analyzing the transient course of external fault inception and removal, together with the CT model involving the magnetic hysteresis effect, is proposed in this paper. It is proved that the magnetic linkage of one CT core can be pushed into the region nearby the saturation point by the high fault current with aperiodic component. As soon as the external fault is removed, the magnetic linkage formed by the primary current with low amplitude is not high enough to pull the operating point of the magnetic linkage back to the linear region. This phenomenon is named as CT local transient saturation, which results in the big measuring angle error and relative smooth waveform. In this case, the transformer differential protection using second harmonic blocking inevitably mal-operates. This point of view is verified with the simulation tests.