K = U1/U2
K= First pole to clear factor
U1= RMS voltage between healthy phase and faulty phase
U2= phase to neutral voltage with fault removed
In three phase systems if fault dose not involve the earth, the voltage across the circuit breaker pole first to clear is 1.5 times the phase voltage. The arc extinction in the three poles of three phase circuit breakers is not simultaneous as currents are 120 degree out of phase. In practical systems the recovery voltage of the pole first to extinguish the arc is of the order of 1.2 to 1.5 times of the phase voltage.
If fault involves earth and the neutral is grounded through reactor, the recovery voltage is influenced by equivalent system reactance.
In 1997 a joint working group between IEEE and IEC was formed. Their objective was to harmonize the TRV values for circuit breakers rated above 100 kV.
Presently a proposal for a revision of the TRV standards is undergoing the approval process. In IEC the document has been approved at the Committee Draft Vote (CDV) stage. In ANSI/IEEE the document is in the early stages of the required balloting process.
The main differences between the existing standards and the new harmonized version are that in the harmonized version the two and four parameter has been chosen for the representation of the TRV. In addition agreement was reached so the peak TRV value as well as the peak multipliers for reduced fault currents are the same in both standards. Other changes involve the first pole to clear factor, which in order to satisfy the different operating practices made it necessary to recognize grounded and ungrounded (solidly earthed and non-solidly earthed) applications.
For rated voltages of 100 kV up to and including 170 kV a 1.3 and a 1.5 factor are recognized. For voltages of 245 kV and above only solidly grounded (earthed) applications are recognized and only the 1.5 factor is used.
High voltage circuit breaker
Design and Applications
Second Edition, Revised and Expanded
RUBEND . GARZON
Square D Co.