Coordination is a critically important aspect of power system protection. In electrical power networks, protection schemes are employed to detect and isolate faults in the system, ensuring the continuity of power supply and the safety of equipment and personnel. Coordination refers to the careful selection and setting of protection devices to ensure that the correct device operates and isolates the fault while minimizing the impact on the rest of the system.
The essence of coordination lies in setting the operating characteristics of protective devices such as relays and circuit breakers in a coordinated manner. This ensures that the closest device to the fault operates first while other devices selectively operate with a time delay, properly segmenting the system to limit the impact and maintain stability.
Achieving coordination requires a deep understanding of fault analysis, relay protection theories, and the characteristics of the power system elements. The main objective is to provide adequate sensitivity to detect faults accurately, while also providing selectivity and sensitivity settings to mitigate false tripping during system disturbances or non-fault conditions.
An important consideration is the time it takes for a relay or circuit breaker to detect and respond to a fault. The time required for a relay to operate is known as its operating time, and it should be set within the limits defined by standards such as the IEEE C37.112 and IEC 60255 series. This operating time depends on various factors such as fault magnitude, fault location, and system configuration.
To illustrate the concept of coordination in a practical scenario, let’s consider a high-voltage transmission system with multiple line sections, transformers, and busbars. Suppose a three-phase fault occurs on one of the transmission lines. The coordination of protection devices between the line and the adjacent elements, such as transformers and busbars, is of utmost importance.
In this scenario, the relay settings must be chosen appropriately to ensure that the protection devices nearest to the fault operate first. This prevents the fault from propagating throughout the system and causing unwanted outages. Furthermore, the time delays between protection devices should be set to ensure that the downstream devices only operate if the fault persists, avoiding unnecessary tripping and disturbances.
For instance, consider a transmission line protected by an overcurrent relay and a distance relay. The overcurrent relay provides primary protection, while the distance relay provides backup protection. The overcurrent relay is set to operate faster than the distance relay to ensure selectivity and minimize the extent of the fault. The distance relay is then set with a longer time delay to provide backup protection and intervene only if the fault persists. These settings can be calculated using fault analysis and coordination algorithms.
In conclusion, coordination is a vital aspect of power system protection. It involves defining appropriate relay settings and time delays to ensure that protective devices operate in a coordinated manner, minimizing the impact of faults on the power system. Through careful analysis and selection, coordination contributes to the reliability, safety, and stability of the electrical power network.