definittidskurva

A definittidskurva, often translated as a "defined time curve" or "fixed time curve," is a concept used in the context of control systems, particularly in relay protection for electrical grids. It represents a pre-determined relationship between the magnitude of an electrical fault and the time it takes for a protective relay to operate. This relationship is typically plotted on a graph where the x-axis represents the fault current (or impedance) and the y-axis represents the operating time of the relay.

The purpose of a definittidskurva is to ensure selective tripping of protective devices. In a power system, a fault can occur at various locations, leading to different fault current levels. A definittidskurva allows engineers to set relay operating times such that the relay closest to the fault will trip first, while relays further away will not operate. This prevents unnecessary widespread power outages. For instance, a higher fault current will result in a shorter operating time on the curve, ensuring rapid isolation of the fault. Conversely, lower fault currents will have longer operating times, allowing for more stable operation of the system under less severe conditions.

These curves are carefully designed based on the characteristics of the protected equipment, the power system's fault levels, and the desired selectivity. They are a fundamental tool in the design and coordination of protective relaying schemes, ensuring the reliable and safe operation of electrical power systems by quickly and selectively isolating faults.