Transmission-line overloads: real-time control

Any disturbance causing a transmissionline overload creates an operational situation which must be quickly controlled. If the line should trip, there is a great danger of cascade tripping of lines, thus leading to system breakup. Five different levels of overload are identified and methods are proposed for an automatic reduction of these transmission-line overloads. The adopted approach recognises that there are two distinct areas of difficulty, moderately and severely overloaded lines, with each requiring a different method of resolution. This has led to the development of two distinct measures which either separately, or in combination, can control the complete range of transmission overloads. By recognising that transmission-line overloads can impinge on the system´s reactive-power balance, a computationally simple algorithm has been developed. This would allow moderate line overloads to be reduced by the automatic rearrangement of generator outputs. Severe overloads can impinge on transmission-line protection settings, so that extremely fast measures are required. An extension of strategic load shedding [1, 2] would allow line currents to be reduced below the protection setting prior to its functioning. Generator output rearrangement could then reduce the residual line overload. Alternatively, it may prove expedient to actuate a further level of strategic load shedding before rearranging generation, to correctly control the overload. With its importance in the control of line overloads, the underlying philosophy of strategic load shedding is described.