What Is ATS And STS?

Energy storage systems can provide backup power for both grid-connected and off-grid operations, frequently employing power transfer switches. These switches serve as one of the core components ensuring uninterrupted and reliable power supply, primarily through the following methods:

Hand-operated transfer switch

When utility power fails, the energy storage system enters islanding protection mode. Manually disconnect the switch linking the storage system to the grid, then control the system to start operating in off-grid mode (V/F) to provide backup power to the load.

Upon grid restoration, reclose the switch and adjust the energy storage system to operate in grid-connected mode (P/Q), resuming power supply to the load.

ATS changeover switch

ATS (Automatic Transfer Switch): A dynamic transfer switch is a device used to switch between power sources. Within electrical systems, it is often necessary to connect multiple power sources simultaneously to ensure the reliability of the power supply.

The ATS primarily facilitates automatic switching between the main power source and backup sources (such as energy storage systems or generators), with switching times typically ranging from a few seconds to tens of seconds. Should a power source fail or experience an outage, the ATS automatically transfers the load from one source to another, ensuring uninterrupted power supply.

Regarding the operating principle of the ATS: upon loss of mains supply, the energy storage system enters islanding protection mode, and the ATS switches to the standby diesel generator (with mains supply as the primary source). Once the diesel generator commences operation, the energy storage system resumes its function.

STS Transfer Switch

STS (Static Transfer Switch): A static transfer switch is a device used to switch power sources. The STS module enables intelligent, rapid separation of the grid and microgrid, switching the power supply circuit with a response time measured in milliseconds.

During normal grid operation, loads are connected to both the grid and batteries. Upon grid failure, the STS switch disconnects, enabling PV or batteries to supply loads independently. When grid service is restored, the STS switch reconnects.

Taking the ADAYO power solution as an example,

the STS module for low-power energy storage is integrated within the PCS, simplifying project wiring as shown in Figure 1 below.

For high-power PCS units (≥40kW), the STS module is separated. When projects require enhanced power supply reliability or demand off-grid backup power, the STS module is configured as illustrated in Figure 2 below:

Regarding the operating principle of the STS: when mains power is lost, the STS automatically disconnects and notifies the PCS of the mains disconnection. Within 10 milliseconds, the PCS switches to V/F mode to supply voltage and frequency to the load. Upon mains power restoration, the STS detects grid re-energisation. Following successful communication handshake, the PCS transitions from off-grid to grid-connected mode whilst instructing the STS to re-energise. This integrated STS-PCS energy storage system achieves seamless switching, delivering uninterrupted backup power to the load.

The grid disconnection and transfer time serves as a critical metric for evaluating the performance of backup power systems, particularly in scenarios where power interruptions are highly sensitive.

• EPS Transfer Time: The transfer time for an EPS typically ranges between 2 to 10 seconds. EPS transfers rely on mechanical relays or contactors, resulting in relatively slow switching speeds. ATS transfers correspond to the EPS level of switching.

•  UPS Switchover Time: UPS switchover time is typically in the millisecond range (0-20ms).

  
  

Anyway:

20ms is perceptible to most loads, such as televisions and desktop computers.

10ms enables uninterrupted power supply for the vast majority of loads, characteristic of standby UPS systems. However, for certain sensitive equipment (such as high-frequency trading systems or precision medical devices), a 10ms power interruption may cause equipment malfunction or data corruption.

4ms achieves seamless switching for virtually all equipment, including high-end servers, storage devices, medical equipment, and precision industrial machinery, typical of online UPS systems.

In practical energy storage systems, ATS and STS can be combined according to specific requirements to achieve greater power supply reliability and flexibility.