What happens if two interconnected supply systems not synchronized?

Assume that interconnect two separate systems by closing a circuit breaker on a tie-line. The voltages at both ends of the breaker are independent of each other, as each voltage "belongs" to one system, each system still operating isolated from the other. So, if the voltage difference across the breaker is sufficiently large (and remember that this is a phasor voltage difference, magnitude and phase) when the breaker is closed, that will be equivalent to a fault, a short-circuit in the grid.

If the phase angle between the two voltages when the breaker is closed is 180 degrees, it will be equivalent to a three-phase fault. But even without going to the extreme case of 180 degrees phase difference, closing the breaker out of phase would result in very high currents (like fault currents) in the tie line and, hopefully, the tie line protection will trip (open the breakers) almost immediately, separating the two systems once again. Hopefully, both systems are sufficiently "strong" to survive the impact of this unsuccessful synchronization without additional cascading effects (other elements in the grid being tripped by their own protection).

Still, even without cascading events following the unsuccessful synchronization, it would be necessary to investigate the impact of such maneuver on the shafts of large steam units and, perhaps, even the combustion turbines. Because these generation units would be subjected to severe torques, particularly those connected to the grid close to the tie line.

Typically, there are restrictions on the maximum phase difference between the two systems before you can close the breaker. So, even without an automatic synchronizer, you can still have the breaker operation supervised by relays that would block the closing of the breaker for phase differences above a given threshold.

Once synchronized, both systems have to reach the same average speed/frequency. Thus, the system that is moving faster will have to de-accelerate, while the slower system will have to accelerate. Thus, the synchronous generators in the faster system will increase their electrical power output, while the generators in the slower system will reduce their electrical power output. This is done automatically, it is the "nature" of the synchronous machines.

So, when there is a sudden increase in electrical power output of all generators in one system and, simultaneously, a sudden decrease in electrical power output of all generators in the other system, an electrical power flow will be instantaneously established at the tie-line that was just closed, with power (MW) flowing from the faster system to the slower system.

If the difference in speed/frequency at the moment of closing the tie-line increases, this sudden change in electrical power flow will increase, because there is more acceleration/de-acceleration to happen. And you would see a very large power flow at the tie-line. It could become so large that the tie-line protection could pick it up as a fault current (over-current or distance relays) and trip the tie-line.

Now, if the frequency/speed difference is really large, like the 2 Hz, other bad stuff could happen, like the two systems slipping poles with respect to each other. Basically, one system is faster than the other by 2 cycles per second. So, these two systems would be in phase with each other in a moment, and completely in opposition of phase (180 degrees of each other) in 1/4 of a second! Now, not only the tie-line would trip, chances are that you would have a cascading event with generation units also being tripped by their own protection (e.g., out-of-step, loss-of-field, or power reversal protection in the generation units).

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