Distance relay

Distance relays are protective devices used in power systems to protect lines and transformers from faults. They work by measuring the impedance between the relay and the fault location.

Overreach occurs when a distance relay operates for a fault that is outside of its protected zone. This can happen for a number of reasons, such as:

1. Improper relay settings: The relay may be set to operate for faults that are closer than the actual protected zone.

2. CT saturation: The current transformers (CTs) used to measure the current and voltage may saturate, which can cause the relay to operate for faults that are outside of its protected zone.

3. Harmonics: The presence of harmonics in the system can cause the relay to operate for faults that are outside of its protected zone.

Underreach occurs when a distance relay does not operate for a fault that is within its protected zone. This can happen for a number of reasons, such as:

1. Improper relay settings: The relay may be set to operate for faults that are farther away than the actual protected zone.

2. CT errors: The CTs used to measure the current and voltage may have errors, which can cause the relay to not operate for faults that are within its protected zone.

3. Noise: The presence of noise in the system can cause the relay to not operate for faults that are within its protected zone.

Overreach and underreach can both lead to system outages. It is important to properly set distance relays to prevent overreach and underreach.