Assume that the digital relay protecting the transmission line has phase overcurrent elements in addition to the distance elements. Let us assume three-phase-overcurrent element levels: high, medium, and low-set elements.
The 50L element provides fault detector supervision of the mho distance elements and must pick up for all faults for which the distance element is expected to operate. The best setting for the 50L elements is above load current level, but below minimum fault level. However, this is not always possible, and settings below load levels are often acceptable.
For phase faults, at least one current magnitude must exceed the 50L pickup threshold before the line-to-line distance elements can operate. For three-phase faults, the current magnitude in all three phases must exceed the 50L pickup threshold before the three-phase distance elements can operate.
The 50L elements are also used in the trip unlatch logic. Before the trip output can open, the trip condition must vanish and the current magnitude in all phases must drop below the 50L element thresholds. This ensures that the trip output contact does not attempt to interrupt full trip coil current during fault clearing.
Medium-Set Phase Overcurrent Elements
A medium-set 50M element provides an important input to the loss-of-potential (LOP) logic for blown VT fuses and may also provide non directional phase overcurrent protection under those conditions when an LOP condition is diagnosed. The recognition of an LOP condition blocks the mho distance elements from operation. The line protection under these conditions is provided by a non directional time-delayed phase overcurrent element.
It is important to ensure that three-phase LOP conditions are not confused with fault conditions. Once an LOP condition is declared, it remains latched until balanced three-phase voltages are restored. The 50M element setting must be above load current, but below minimum line-to-line or three-phase fault current levels. If the maximum load current is greater than the minimum fault current, the 50M setting must still be below the fault values. This prevents pickup of LOP during a fault. In most cases, when a close-in fault lowers the measured voltage to such a degree that an LOP condition might be suspected, there is an accompanying increase in fault current as viewed from the local terminal.
The 50M element must be time-coordinated with surrounding line terminals. To provide this coordination time, the 50M pickup following LOP starts a definite timer. When the timer expires, the line can be tripped.
High-Set Phase Overcurrent Elements
The 50H element function provides a high-set non directional phase overcurrent detector in the switch-onto-fault logic. The switch-onto-fault logic is enabled when a line terminal is open, and for a few hundred milliseconds after it is closed. When closing a line breaker with line side VTs into a close-in, three-phase bolted fault, it is likely that the three-phase distance elements will not see the required level of polarizing voltage for pickup. If this occurs, the distance elements are blocked from pickup. The 50H element provides backup protection for this condition. The 50H element measures the current magnitude in each phase and is independent of polarizing voltage.
Typical settings for the 50H element are from one-half to one-third of the three-phase fault duty at the local line terminal. A concern exists if the 50H element is set too low, that it might mis operate due to heavy load when the second terminal of a line is closed. Modern switch-onto-fault logic will have a voltage reset feature that will block the logic if voltage is present on the line. The voltage will be present at the second terminal if the line has already been energized from the first terminal. Using the voltage reset feature will ensure that load current will not cause mis operation due to load when the second terminal is closed. Of course, there will be no load flow when the first terminal is closed, since the second terminal is still open.
