What is the difference between instantaneous overcurrent protection and time-delayed overcurrent protection, and how is each applied?

Instantaneous overcurrent protection and time-delayed overcurrent protection are two types of overcurrent protection schemes used in electrical power systems. They both detect overcurrent conditions, but they differ in their operating characteristics and applications. Instantaneous overcurrent protection operates without intentional delay when the current exceeds a pre-set threshold. This means that the relay trips almost instantaneously as soon as the current reaches the setpoint. The operating time is typically very short, on the order of a few cycles or milliseconds. The main advantage of instantaneous overcurrent protection is its speed. It provides very fast clearing of faults, minimizing the damage to equipment and improving system stability. However, it is also susceptible to false trips due to transient overcurrents or inrush currents. It's usually applied as the primary protection for equipment close to the source, such as within a substation, where fast fault clearing is essential and coordination with other devices is less critical. Also, it's used as a backup protection to time-delayed overcurrent. Time-delayed overcurrent protection operates with an intentional time delay after the current exceeds the setpoint. The relay will not trip until the current remains above the setpoint for a specified time period. The time delay is coordinated with other protective devices in the system to ensure selective tripping. The purpose of the time delay is to allow downstream devices to clear faults closer to the fault location, preventing unnecessary tripping of upstream devices. The main advantage of time-delayed overcurrent protection is its selectivity. It allows for coordination with other protective devices to isolate faults selectively. However, the time delay also means that it is slower than instantaneous protection, which can result in greater equipment damage and system instability. It's generally applied as the primary protection for feeders and distribution circuits, where coordination with other devices is important. The application of each type of protection depends on the specific requirements of the system. Instantaneous overcurrent protection is used where speed is paramount, while time-delayed overcurrent protection is used where selectivity is more important. Often, a combination of both types of protection is used to provide both fast fault clearing and selective coordination.