SPC300 wrote:

f you are asking about fault clearing time on a power system 345kV or 230kV BUS then the clearing must be fast in order to ensure a stable power system. Typical relay operate times for the BUS protection is in the order of 1 cycle (16.67ms). Adding clearing times for the breaker(s) which are in the order of 1-3 cycles the total clearing times are in the order of 2-4 cycles.

Definitely not true at all. This statement may be true for 480 V systems but not transmission lines. Physics gets in the way of high voltage (high by IEEE standards). For relaying with a 1 cycle delay although I haven't seen much of even this for high voltage systems, I'll grant you that 1 cycle relaying is at least commonly available these days.

1 cycle clearing is difficult to achieve on any system except for very low current/voltage molded case breakers. "Instantaneous" tripping at medium voltage (1-35 kV) these days is achieved with vacuum interrupters that with 1 cycle allowed for relaying open in 3 cycles. Those devices typically require on the order of a 1/4" movement to open so mechanical motion required to open/close and inertia is very small. Scaling up to a high voltage SF6 switch pushes the opening times out to several cycles as the contact size and physical arc gap dimensions grow although puffer switches reduce this requirement somewhat by using a bellows to blow out the arc. This is the fundamental problem for the mechanical side of things.

On the electrical side, we have an even bigger problem that is not significant at low voltage (<1 kV). The fundamental difficulty is that if the contacts do not open at a zero crossing, not only does an arc form, but the result of the arc suddenly extinguishing causes ringing in the circuit due to impedance. At low voltages this is barely noticeable except with a very high speed oscilloscope and it is generally tolerable to a point. But at transmission voltages even "small" transients of say 200-300% of nominal voltage would require subsequent doubling or tripling of the insulation system. The solution is to increase the impedance during contact opening (and to a lesser degree, closing). So the actual switch design first closes onto a resistor or an inductor. Then the main contacts open, transferring the load onto the impedance. Finally the smaller resistor contacts open which interrupts the circuit with a drastically reduced transient.

The combined mechanical movement of the electrical contacts and transient control requirements combine to where a "1 cycle" opening" is never going to happen. By way of example, the S&C 2000 switch which can go up to 230 kV is pretty fast at interrupting at a 6 cycle time. That is nowhere near a 1 cycle opening time.