Automatic Transfer System (ATS)
Although the basic operational requirements of any automatic transfer system are as described in this technical article, the equipment used to implement the automatic transfer system may vary.
Equipment used to implement the automatic transfer system – ATS (pictured: robotic parking systems generator transfer switch; credit: parkithere.wordpress.com)
Below is a list of the most common variations of automatic transfer system equipment
- Standard or table?
- Logic platform (PLC)
- Circuit breaker control and lockout
- User interface
- Discrete controls (switches, indicator lights)
- Touch screen
1. Standard or table?
Electrical equipment used to facilitate transfer, for low voltage systems, is typically either UL 891 switchgear or ANSI C37.20.1 low voltage switchgear. The type used depends on the system design and the location of the hardware in the overall power system. However, the following general guidelines apply:
When the automatic transfer is closer to the service
For “superior” automatic transfer into the system (closer to service), low voltage ANSI switching is generally preferred due to its compartmentalization and the use of low voltage withdrawable circuit breakers, which have short term withstand capabilities. .
An alternative is a UL 891 “hybrid” switchgear which uses withdrawable circuit breakers with characteristics similar to low voltage circuit breakers, but with less compartmentalisation.
When the automatic transfer is further away from the service
For “lower” automatic transfer into the system (further from service), a UL 891 table may suffice. However, withdrawable circuit breakers should be considered even if a UL 891 switchgear is used.
Lack of short-term withstand on molded case circuit breakers is an important factor here.
Withdrawable switchgear using removable isolated case power circuit breakers to improve system flexibility. The transfer logic panels are shown with indicator lights to indicate the presence of voltage on each source, as well as to test the system controls. Transfer circuit breakers are equipped with fully automated controls, with neutral position timers to protect electronic loads and motors from overvoltages.
2. Logic platform (PLC)
The automatic transfer logic can be provided by discrete control relays or a programmable logic controller (PLC) , as shown in Figure 1.
In the past, it was generally a choice of flexibility (PLC) vs robustness (discrete relays). However, in recent years, the reliability and robustness of programmable logic controllers have evolved considerably to the point that they are now the preferred method for implementing automatic transfer scheme logic.
The flexibility offered by using programmable logic controllers is that the automatic transfer logic is encoded in software rather than hard-wired. This makes certain changes possible on the fly, if necessary, without any hardware or wiring changes to the equipment. It also allows for more complex decision-making logic to be implemented without excessive wiring.
Discrete control relays, on the other hand, must be re-wired to change the automatic transfer logic. More complex logic usually requires more control relays and wiring.
Another advantage of a PLC is its ability to communicate digitally with external devices . This makes more sophisticated user interfaces possible, as we will see later. It also allows remote access to the transfer system if required.
3. Circuit breaker control and lockout
Nesting is the restriction of device operation, usually for safety reasons. For an automatic transfer system, the most commonly required interlocking is interlocking to prevent unsynchronized paralleling of power sources (or to prevent paralleling in many cases).
When the logic of the automatic transfer system is provided by a PLC, this nesting can be implemented in a wired manner, that is to say outside the PLC or as part of the PLC program.
In general, the use of hardwire locking is preferred , to allow an additional measure of security if the PLC fails. Likewise, manual control of automatically actuated circuit breakers can be implemented either wired or via the PLC. It is generally recommended that at least the circuit breaker trip function be implemented outside the PLC.
Manual override for self-operating circuit Circuit breakers are typically operated via external control switches, with manual close command (and often trip command) on restricted access to the circuit breaker via a cover to force the use of these control switches.
4. User interface
This is the most customizable part of the automatic transfer system equipment. In general, two options are available:
- Discreet controls or
- Touch screen
Discrete controls (switches, indicator lights)
Discreet controls take the form of control switches and indicator lights mounted on the equipment.
In Figure 2, “ AUTO ”, “ MANUAL ”, and “ AUTO FAIL ” LEDs indicate the three modes of operation described in an earlier article.
A key-operated auto / manual mode selector provides control In operating mode, a key-operated automatic re-transfer on / off switch allows automatic re-transfer to be enabled or disabled, and a key-operated privileged source selector allows to consider either power source as the normal source.
The use of key switches should be carefully evaluated to ensure that the end user takes full advantage of such an arrangement.
the following are also available:
- Open / closed transition re-transfer selector
- Source Available indicators
- PLC low battery indicator
- Transfer in progress indicator
- Source Failure Test Switches
Like discrete relay control logic, discrete controls must be planned in great detail early in the specification process and offer limited change flexibility.
When a PLC is used for automatic transfer logic, a touch screen is an option for the user interface . A touchscreen can provide a wealth of detail regarding the status of the automatic transfer system and adjustability in several areas that are not generally available with discrete controls.
An example of a touch screen interface is shown in Figure 3:
The touchscreen interface in Figure 3 includes:
- Active synoptic single line diagram that changes color to indicate activation of system components
- Source of available indicators
- Circuit breaker status indicators,
- A summary of the state of the transfer system
- Ability to change digital parameters such as source failures, idle buses, and source restore timers
- Quick manual transfer of the system from one source to another in manual mode
- An event log that captures automatic transfer events for diagnostic purposes.
Such sophisticated control is simply impossible with discreet controls , but is easily achieved with a touchscreen.
Touch screen control interfaces for automatic transfer systems typically include at least one switch and discrete indicators, namely the auto / manual mode selector and its associated indicators.
4.3 Multiple user interfaces
The use of multiple user interfaces is possible. Such an arrangement may be desirable due to the need to remotely control the system. When planning such an arrangement, consideration should be given to the number of interfaces and priority interfaces if control is attempted by more than one interface at a time.