The simulation and analysis on transformer gas Protection Maloperation based on multiple physical fields coupling

As a transformer main protection, gas protection is an important and traditional non-electricity protection. For internal faults of transformer, gas protection can directly response to non-electricity characteristic such as oil flow velocity, and then has higher sensitivity and reliability compared with electric protection in principle. However, for external faults, great electromagnetic force and losses of windings, which caused by the heavy through currents, may result in maloperation of gas relay. Thus the reliability of the gas protection is low. From statistical data analysis of several gas protection maloperation accidents happened in recent years, it is clear that the root reasons of low reliability of gas protection should be outdated protective principle and inaccurate threshold value. In order to analyze the mechanism of gas protection mal-operation and provide a proper method to set the threshold value of gas protection, the transient process and the related physics essence of gas protection mal-operation resulted from external fault is analyzed. In the process of external short circuit, electromagnetic force of windings caused by short-circuit impulse current is so huge that the whole windings will generate intense vibration. Meanwhile, insulating oil in transformer will rush into the oil tube by squeezing and shaking, which lead to gas protection mal-operation eventually. From the above analysis, it is illustrated that the process of gas protection mal-operation caused by external short circuit is a multi-physical field coupled process. To simulate the complicated process veritably and accurately, an oil transformer model based on multi-physics field is established in ANSYS Workbench. Firstly, the short circuit electromagnetic force of windings is calculated via the magnetic field analysis, which will be loaded into the dynamic mechanical analysis of windings. Thus stress distribution and strain characteristics, which can describe vibration of windings, is calculated and loaded into the flow field analysis over a defined interface. Eventually, the oil flow velocity through gas relay in the oil tube is achieved by a hydrodynamic model. Simulations with different external fault current are conducted to study the relationship between the oil flow velocity and external fault current in the windings. The results show that factors which affect the oil flow velocity is different between internal fault and external fault, and there exists the obvious distinction of the oil flow velocity between internal fault and external fault. Furthermore, the oil flow velocity of transformer under fault conditions can be obtained in the above simulation more veritably and accurate, which can be used as references for the threshold value of gas relay. The purpose of this study is to overcome the difficulties in setting threshold value, avoid the gas protection misoperation during external short circuit and improve the reliability of gas protection.