Fast Discrimination Between Transformers Internal Fault And Inrush Current Using Mathematical Morphology

As power transformers are highly responsible for the high quality of power supply andsystem operating stability, transformer protection plays a key role in the operation of powersystems. In particular, differential protection has been adopted as the chief protecting methodof transformers for many years, due to its simple principle and high sensitivity. However, itsuffers from a vital drawback, since they tend to trip by mistake when there exist inrushcurrents in transformers.Recently, a number of techniques have been developed to overcome the flaws of thetraditional methods including the second harmonic restraint principle and the dead angleprinciple. However, all of these methods still have shortcomings, such as complex calculationand long sampling window. In addition, they do not consider some special cases, for example,internal faultand inrush current occurring simultaneously. In view of the outstandingperformance of mathematical morphology in the aspects of sudden changes and transientprocess, this paper proposes a novel scheme using mathematical morphology to effectivelydiscriminate between the internal fault current and inrush current.The proposed scheme consists of two parts. The first part is based on morphologicalgradient, for the discrimination of currents occurring without considering current transformer(CT) saturation. The second uses a symmetrical criterion involving MG to identify internalfault current under CT saturation.To demonstrate its compatibility and efficiency, the scheme is verified employing thedata simulated using PSCAD/EMTDC and data collected from laboratory experiment,respectively. The simulation and experimental results indicate that the proposed scheme canimprove the accuracy of inrush current identification, even in the case of internal fault andinrush current occurring simultaneously.In addition, since the required calculation of the proposed scheme is is very simple, thispaper will implement it on the circuit board based on FPGA. Specifically, the proposedscheme is applied in the Real Time Operating System uC/OS II, which is transplant to Nios II.The test results have verifided the performance of the proposed scheme.