Impact of inverter system towards harmonic restraint used in three phase transformer differential protection system

• Main Author: Gunasundari Mahathevan
• Format: text::Thesis
• Language: English
• Published: 2023
• Subjects: IMPACT OF INVERTER SYSTEM TOWARDS HARMONIC RESTRAINT USED IN THREE PHASE TRANSFORMER

Differential protection is mostly used in power transformer to protect against the event of internal faults since it considers as one of the fast clearing method. During the occurrence of any fault, this differential protection detects the nonzero differential current and activates a circuit breaker that isolates the power transformer from the electrical system. This will cause the protective relay to trip unnecessarily where the relay should have been activated at all. Inrush current is also known as magnetizing inrush current, chiefly, a transient current that produced only when the power transformer is first energized and due to saturation of CT (current transformer). Inrush current contains a number of harmonics components and the percentage of the second harmonic relatively will be higher during an inrush event. Whereas, a fault current occurs in the protection system is high in magnitude and is free from second harmonics contents. The existing differential transformer protection need to improve so that it can differentiate the differential current appear due to internal fault and produced during the magnetizing inrush event. In order to enhance the security and the sensitivity of the protective relay, many restraint method have been introduced to restrain the operation of the differential element. A proper restraint method is identified to implement in the system to distinguish the transformer fault current and inrush current based on the content of harmonics in the line current of the transformer. In this project, a harmonic restraint circuit is designed to find the ratio of the second harmonics to the fundamental value and compare it to a predetermined set point. The purpose of the relay to take a decision on tripping or restraining based on the ratio obtained and after compare to the set point. Besides that, an inverter system is replaced in the main voltage source and tested with the circuit to compare the results obtained with and without the presence of the inverter system and analyses the effect of inverter system with harmonic restraint. The circuit was built for the three-phase transformer and tested through a Matlab Simulink and found that fulfill the aim for which it has been created.