Abstract
This paper presents a simple method for improving the security and reliability of Harmonic Blocking Algorithms (HBAs) in differential protection of power transformers. In the proposed method, instead of using conventional Current Transformer (CT), Rogowski Coil Sensor (RCS) is utilized for sensing the transformer currents. Based on a theoretic analysis, it is shown that the use of RCS increases the margin between the second harmonic ratio levels of the energization and fault conditions, thereby increasing the HBAs security. Besides, the use of RCS has the additional advantage of eliminating the problem of CT saturation, which might cause malfunction of the differential relay. The validity of the proposed technique is demonstrated by evaluating extensive test cases in different operating conditions, which are recorded by digital simulations in PSCAD/EMTDC software. Overall, the results demonstrate that the proposed approach improves the security of HBAs by about 6.8%.
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