Introduction on equipment and personal protection (Fault clearance time and Thermal stress)

Fault clearance time
          It is valuable to define standard fault clearance times within the company. For example:
Fault clearance after operation of main protection: 120 ms.
With this demand we normally have no stability problems. The transmission line protection must have telecommunication cooperation
Fault clearance after operation of back-up protection
Depending on what part of the power system that is protected by the particular back-up protection, e.g. for back-up protection of busbars 500 ms is suggested.
Long fault clearance times increases:
   1)  The risk for transient instability in the power system.
   2)  Thermal stress on equipment in the power system.
   3)  Risk for personal injuries.

Thermal stress
Every short circuit will cause thermal stress to equipment such as phase conductors and current transformers in the system. The equipment is designed to withstand a specified short circuit current. This current is given with the assumption of a fault time of 1s (equivalent 1s-current I1s).
The maximum allowed fault clearance time tmax, due to thermal stress of equipment (maximum temperature), can be calculated to:

I1s = Equivalent 1s-current
Ik = actual maximum short circuit current

          An example how to calculate the dimensioning 1s-current for a transmission line and at one fault point can be done assuming a sequence of events during the fault as follows:

The fault clearance time in the power system has influence on the transient stability in the power system and the thermal stress on equipment.