Analysis of Selectivity Concepts for Moulded Case Circuit Breakers

Inhalt:
In the last years the selectivity concepts, used in low voltage energy distribution networks, changed. Due to improved current limiting modern Molded Case Circuit Breakers (MCCB) are able to full fill the requirements for the Energy Selectivity. This selectivity concept causes some challenges for the switching contacts. In order to optimize the design of the breakers for such load conditions it is necessary to investigate the current limiting performance of a circuit breaker when operating stand-alone as well as when the breaker is part of a combination of breakers. We show a system simulation model for an MCCB, which is partly based on theoretical and experimental foundations. The model describes the different physical domains for current limiting circuit breakers and is connected to a model for the electrical network. This configuration enables us to investigate the let-through current for different prospective short circuit currents, the build-up of the arc voltage and the evolution of the arc power. With the model the influence of design changes and of the electrical load parameters on the short circuit switching behavior of the breaker can be analyzed. A detailed sensitivity study on the selectivity behavior of two 3-phase breakers in series connection is possible. Such a numerical model can be used for the adjustment of internal parameters of the tripping chain without spending excessive effort for prototyping and testing.