Identification Of Transformer Inrush Current Based On Grey Theory

As one of the crucial electrical equipment in power system, power transformer bears the special mission of electricity transmission and distribution, transforming the primary side and the secondary side rated voltage and phase angle, it is the irreplaceable converting hub in the power system. Whether power transformer is operating normally or not, whether there is a complete protection system when faults happen, and whether it can operate reliably under all kinds of non-faults disturbance,these are closely related to the normal and stable operation of power system. So the study of power transformer protection has always been an important issue of concern in power system relay protection.Differential protection has been one of the primary protection of power equipment, it has ideal application effect on pure circuit device, but as the main protection of power transformer, due to the unbalanced current, often causes the malfunction of the differential protection, the effect is not ideal, the inrush current is the main component of the unbalanced current in differential protection loop of power transformer. When power transformer closes with no-load or voltage is in recovery process after the removal of external fault, a big inrush current will flow in differential circuit, which is usually 6-8 times of the rated current. Such a large current tends to cause relay device’s malfunction of differential protection.Therefore, the ability to quickly and accurately identify inrush current and internal fault current will affect the performance of differential protection to a great extent.Scholars at home and abroad have carried out extensive research on inrush current identification method’s exhibition and improvement. They have put forward various identification schemes, which of them have both advantages and disadvantages, making them unable to meet the requirements of modern relay protection. Hence, it is practical urgency to apply new recognition method to distinguish inrush current and internal fault current.Grey system theory is an effective method to research uncertain problem, whichis lack of data and information. The appearance of inrush current has a lot of random and diversity, so the recognition of it is undoubtedly a gray system problem. Based on the study of the function of transformer as well as the working principle of differential protection,which is transformer’s primary protection.This thesis simulates different operating conditions of the transformer using power system block in MATLAB, and then puts the newly-developing gray system theory into the identification of the excitation rush current.Finally, the example verifies the feasibility of this method.