Understanding Automatic Transfer Switches (ATS) MEP Tools

Automatic Transfer Switches (ATS)

Understanding Automatic Transfer Switches (ATS)

System integration for ATS is possible on both MV and LV networks. Typical applications include substations, hospitals, desalination plants, data centers, factories, and other critical loads requiring a secure power supply. ATS systems can also be housed in a separate panel. An example of an ATS device is ABB's ATS021 shown in figure 3.

Automatic Transfer Switches (ATS) 

Key factors that can alter ATS operations include:

1. Maximum and minimum voltage control

2. Voltage imbalance

3. Frequency fluctuations

4. Phase loss

ATS can be classified by the number of switches it controls:

1 out of 2 (1oo2) (1 being operational at any given time)

2 out of 3 (2oo3) (2 being operational at any given time)

Shown in Figures 1 and 2 respectively.

In a 2 out of 3 configuration, there are two schemes.

Scheme 1 operates in normal operation with either Source 1 as the main supply and Source 2 as the alternative, or vice versa, with the bus section closed. Once fault is detected the supply switches while the bus-tie remains closed

Scheme 2 has both Source 1 and Source 2 active in normal operation, with circuit breakers closed and the bus section open. When the supply is lost circuit breaker of the faulted source is opened and the bus-section is closed.

However, more complex schematics can be found according to the application. In the ABB ATS white paper, special attention is given to the most common plant engineering configurations that the ATS can manage.

These configurations are:

1. a transformer for the main line and a generator for the emergency line (figure 4)

2. a transformer for the main line and another transformer for the emergency line (figure 5)

3. use of a bus-tie to exclude non-priority loads (figure 6)

4. use of a bus-tie to manage two independent supply lines (figure 7)

For case 3, the transformer supplies the network with the bus-tie (QF3) closed. During a fault, the generator takes over, and the bus-tie opens, supplying only priority loads. The generator has a lower power rating than the transformer which reduces costs. This case is commonly used.

In case 4, each half-bus bar is supplied by its transformer with the bus-tie open. If a fault occurs, the bus-tie closes, and the unaffected transformer powers the entire system. Both transformers must sized to handle the total power demand.

While the priority is given to the main supply, some ATS devices can be configured to not automatically return to the main power supply when it is restored.

Although ATS provides a reliable operation, there is a gap time between power restoration and switching given the use of timers and also, the generator’s startup time.

For critical loads, such as those related to life safety, installations must include devices capable of delivering immediate power. This is often achieved by using an Uninterruptible Power Supply (UPS) a typical connection is shown in Figure 8

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