STS switching: Is it "connect first and then disconnect" or "disconnect first and then connect"? Why is there still switching time?

 Statement 1: STS adopts the "first break, then close" technology to achieve short-term switching

 Statement 2: STS adopts the "close before break" technology to achieve uninterrupted switching

 Statement 3: Why does the STS switching time still take 4-8 milliseconds if it is a "close first, then disconnect"? Today, let's unravel this technical mystery!

1.The core technique of STS switching - "close first, then break"

First on: Upon detecting a fault, immediately turn on the backup side semiconductor switch. 

Overlap: Both main and backup switches are turned on simultaneously for tens of microseconds (the current never stops). 

Then turn off: After the current stabilizes and transfers, turn off the main power switch

✅ Key achievement: The electrical connection is truly zero interruption, and the current channel remains unobstructed at all times.

2.Where did the total time of the entire event - "4-8 milliseconds" come from?

Although the handover action itself is seamless, a complete relay also includes:

Detected teammate abnormality (detection and judgment): 1-4ms

Continuous monitoring of voltage/frequency

Confirm that it is a real fault rather than an instantaneous disturbance (to prevent misoperation)

Calculate the optimal handover time (synchronous preparation):<1ms

Find the moment when the phase of the new and old power sources matches the best

Perform perfect handover (close first, then break):<1ms

The seamless switching process mentioned above

Accelerate to race speed (voltage stable): 1-3ms

After switching, the system stabilizes again

✅Actual total time consumption=1-4ms+<1ms+<1ms+1-3ms=4-8ms

For IT equipment: The "hold time" of server power supply is usually ≥ 20ms, and the<8ms switching time of STS is completely within the safe range, with no load perception.

Compared to ATS: Mechanical ATS inevitably generates at least tens of milliseconds of interruption due to its "first break, then close" approach, while STS's fast "switching time" results in true business continuity.

3.When is it necessary to 'break before closing'? 

When performing STS equipment maintenance, the completely opposite principle of "disconnect first, then close" must be adopted: disconnect all input/output circuit breakers (isolate STS) first, and then close the maintenance bypass switch (restore load power supply). This is the golden rule of electrical safety, which is not related to the operation logic of STS, but rather to the life protection of equipment operation.