Between 80-90% of all power system faults involve ground. Many protective relaying schemes depend on ground distance protection to accurately sense and locate ground faults on multi-terminal sub-transmission and transmission lines.
In addition to the need for dependable ground fault detection, protective relaying must provide adequate selectivity to avoid over tripping for faults outside of its zone of protection and other undesired consequences, such as under tripping or unintended automatic reclosing initiation.
The problem escalates due to recent major power system disturbances in North America, such as the northeast blackout of 2003. Correct application and settings of protective devices, particularly distance relays, have become subject of heavy scrutiny lately. Validation of accurate distance relay settings is now a major topic of discussion by electric power utilities as well as professional technical committees such as the IEEE power systems relaying committee. It becomes apparent very quickly that the accuracy of line parameter values may affect many people. Although ground distance relay design, characteristics and implementations vary, some of the typical parameters required to set a ground distance relay include the following:
• Zone impedance reach and characteristic angle
• Blinder positions, resistive reaches and angles
• Directional supervision limiting angle
• Polarising current (3I0, I2)
• Supervising element (3I0)
• Z0/Z1 (zero sequence compensation)
• Z0M/Z1 (zero sequence mutual coupling compensation)
Relay manufacturers have different methods of calculating zero sequence compensation, also known as the ‘K-factor’, but generally it is defined as the ratio between the zero sequence impedance Z0 and the positive sequence impedance Z1 of a given transmission line. The K-factor is used to ‘correct’ the ground impedance calculation so that the ground fault loop calculation can be simplified and treated similarly to the phase-to-phase fault loop calculations performed in the protective device. Therefore, if the K-factor……