Modelling of high voltage circuit breakers considering interaction between the driving mechanisms and switching arcs

Abstract

With the increase of voltage and current ratings, design and test of circuit breakers have become more demanding. This is a consequence of the technical difficulties and costs in performing development tests as well as the complexity resulting from the coupling of different physical mechanisms in shaping the interruption performance of a breaker. It is well known that the performance of a circuit breaker is determined by many design and operational parameters among which the motion characteristic of the contact is a key factor. Since the motion characteristic of the contacts has a direct impact on breaker performance, it plays a critical role in the design of a circuit breaker. Difficulties in performing tests or experiments under high voltage and power have rendered the traditional ‘cut and try’ design approach impractical. Computer simulation on the other hand, with its cost-effective and easy-to-implement nature, has become the favored approach to achieve design optimization and attracted a great amount of attention in the field of electrical engineering. An arc simulation model (Liverpool arc model) has been developed in this research group together with a circuit breaker simulation interface, featuring PHEONICS as the differential equation solver. Valuable information can be extracted from arc simulation results and help design better optimized prototypes. However, the absence of a driving mechanism model in the current arc simulation model limits its effectiveness and functionality. The travel of the contact could affect the flow cross section in the arc chamber as well as the length of the arc, and ultimately the performance of the circuit breaker. Additionally, the interaction between the arc chamber and driving mechanism, which has a significant impact on the result (most prominent under high current and long arc duration), has also been overlooked in the existing model. Previously, the absence of a driving mechanism model is dealt with by providing the simulation with a user-defined text file containing the travel profile of the moving contact. However, a user defined file may not reflect the true motion of the moving contact due to the unaccounted interaction between the arc chamber and driving mechanism. Consequently, a truly coupled simulation model, which is capable of calculating the travel of all moving components in real time and eliminating any inaccuracies in the predefined travel curves, is needed. In the current research, an approach to quantify the interaction between the arc and driving mechanism has been proposed. Collectively, the resistive force (known as reaction force) imposed on the moving components in the arc chamber by the high-pressure gas can be calculated by a newly developed integral method. The existing arc model has been expanded to incorporate the calculation of reaction force. In addition, a functional mathematical model for the ZF-11-252 (L)/CYTA hydraulic driving mechanism has also been developed, based on which a number of sensitivity studies have been carried out and the key design parameters that affect the dynamic characteristics of the driving mechanism identified. Considering both the driving mechanism model and the improved arc model (which can now calculate reaction force based on the pressure distribution in the arc chamber) a coupled circuit breaker simulation procedure has been established together with an interface which facilitates the information exchange between the driving mechanism model and the arc model. Based on this coupled model, the interaction between the arc and the driving mechanism is studied under different arcing conditions and nozzle geometries. In particular, two important factors affecting the accuracy of the predicted travel characteristics of the moving components have been identified through the studies. The first one is the need to consider the variable contact surface area between the piston rod in the hydraulic cylinder and the oil as a result of the motion of the piston. The second factor is the prediction accuracy of the pressure field around the moving components in the arcing chamber, especially when there are strong pressure waves propagating in the arcing gas. These aspects have not been studied so far.