ATS Vs STS: Automatic Transfer Switch Or Static Transfer Switch

To summarize ATS and STS in one sentence, it would be: ATS is a "circuit level" power switch, while STS is a "waveform level" power switch. Simply put, ATS is the "power level dispatcher", while STS is the "guardian of the load level".

ATS (Automatic Transfer Switch) 

ATS is a basic and widely used power switching device. 

1. Working principle and core characteristics:

   
   

   Mechanical action: The core of ATS is a set (or two sets) of mechanical contactors/circuit breakers, which achieve circuit connection and disconnection through mechanical movement.

   
   

   Intermittent switching: During switching, ATS usually experiences a brief power interruption time, typically ranging from tens to hundreds of milliseconds. This is because the mechanical structure requires disconnecting one power supply first and then closing the other power supply to avoid short circuits caused by the simultaneous closure of both power supplies.

   
   

   High reliability and low cost: The mechanical structure is mature and reliable, and compared to STS, it has a cost advantage.

2. 
   

    Main function: Automatic switching between common and backup power sources: This is the most classic application of ATS. When the commonly used power source (such as the mains) fails (overvoltage, undervoltage, loss of voltage), ATS will automatically switch the load to the backup power source (such as the generator). Mutual backup of two mains lines: In the scenario where two mains lines are mutually backup, ATS is responsible for switching to the backup line in the event of a main line failure.

3. 
   

    Typical application scenario:

   Switching between generator set and mains power: After a mains power outage, the generator is automatically started and the load is switched to the generator side by ATS.

   The main distribution system of the dual line municipal power supply provides primary power backup for the entire building or data center park.

   For loads that are not sensitive to power interruptions, such as general lighting, air conditioning, water pumps, etc., a brief power outage switch will not affect their normal operation.

   
   

   
   

   
   

STS (Static Transfer Switch) 

STS is a high-speed power switching device based on semiconductor technology. 

1. Working principle and core characteristics: 

Electronic switch (thyristor/IGBT): The core of STS is power semiconductor devices (such as thyristors). It does not have mechanical contacts and achieves current "throwing" by controlling the conduction and turn off of semiconductor devices. 

Close before break (zero switching time): This is the fundamental difference between STS and ATS. STS can monitor the voltage and phase of two power sources in real time. At the moment of switching, it will disconnect the faulty power supply while (or even prematurely) connecting the normal power supply. In theory, zero switching time can be achieved for purely resistive loads; For inductive/capacitive loads, the switching time is also extremely short, usually within 4-8 milliseconds, much faster than ATS. 

High speed, spark free, long lifespan: Due to solid-state electronic switching, there is no mechanical wear or arc generation, and the response speed is extremely fast. Theoretically, the lifespan depends on the electronic components. 

2. Main function: Seamless switching between two UPS outputs: This is the core application of STS. The 'last line of insurance' that provides dual bus power supply for critical equipment with a single power source, such as servers, storage, and network core devices. Realize true redundancy: In 2N or N+1 UPS systems, STS is deployed at the front end of the column head cabinet or cabinet PDU, ensuring that even if one side of the UPS system or distribution link fails completely, the load can be instantly switched to the healthy system on the other side, achieving the highest level of availability. 

3. Typical application scenarios: 

Tier III/IV data centers: Provide each cabinet with two independent power supplies, A and B, and ensure seamless switching through STS. 

AI、 Core computer rooms of financial, telecom and Internet enterprises: Any microsecond power interruption may lead to transaction failure, data loss or business interruption, so STS must be used. 

Key single power IT equipment: Even if the server room architecture is 2N, if the server itself only has one power input port, it must be provided with redundant power through STS.

To make it more intuitive, we summarize it in a table: