Calculation of transient puffer pressure rise taking mechanical compression, nozzle ablation and arc energy into consideration

Thermal puffer type gas circuit breaker has high dielectric and current interruption capability. In order to design a good thermal puffer GCB, it is important to know the blast pressure for arc cooling. Although pressure calculation programs have been developed and used for design work, the basic characteristics such as contribution of nozzle ablation gas to puffer pressure rise, amount of back flow gas to puffer chamber and pressure distribution along gas passages during current interruption are not well known. In this paper, pressure rise, mass flow and temperature calculations were carried out using a new calculation model, which takes mechanical compression by puffer piston, nozzle ablation in the nozzle throat and arc energy into consideration. By the analysis of calculation results, we found the pressure rise mechanism is as follows. While fixed contact located in the divergent part of nozzle, all of ablation gas generated from nozzle wall cannot be exhausted from the nozzle and it leads to high-pressure generation in the nozzle throat. This pressure causes transfer of hot ablation gas back to the puffer chamber via gas passage. The puffer pressure increases thermally due to temperature rise by this mechanism. At a longer arcing time, as high puffer pressure was already established in the puffer chamber, the nozzle ablation gas cannot flow back to puffer chamber. Besides as mass flow through nozzle is limited by low gas density, the puffer pressure rise obtained by the mechanical compression of puffer piston.