The role of the automatic transfer system
The fundamental role of the automatic transfer system can now be defined using the basic elements of utility power, system topology, on-site generation, and uninterruptible power supplies.
Operational requirements of the automatic transfer system (photo credit: bernini-design.com)
In this role, the automatic transfer system must display the following characteristics:
- Robustness – it should perform as intended, even under abnormal power system conditions, without human intervention. Equally important, he must be able to distinguish when a system condition does not warrant transfer to an AC power source.
- It must be able to control the switchboard as needed and, in addition, must be able to transmit the appropriate signals to the emergency power source, if necessary (for example, to signal a generator when to start.
Simply put, the role of the automatic transfer system is to provide the automatic transfer of power for its associated load group from a normal power source , such as a utility, to an alternate power source, such as back-up generation, in the event of failure of the normal source.
Example of system description
To fully illustrate the operational requirements of a typical automatic transfer system, a more detailed representation of the system is required. For this purpose, the main-main topology used is used, but with the details of the automatic transfer system shown:
In Figure 1, the automatic transfer logic makes it possible to decide which automatic operations should take place and when. It controls the operation of the two transfer circuit breakers, CB-UM and CB-GM, and receives status inputs from these circuit breakers. It can also initiate generator start-up for the AC power source.
Undervoltage (device 27) and reverse voltage (device 47) relays on each power source indicate the transfer logic of their state . In addition, a frequency relay (device 81) is present to indicate the frequency of the
AC power source.
Voltage transformers, or VTs, advance the voltage system to instrumentation levels usable by these relays. A user interface is used to adjust certain operating parameters of the system and to update the user on the status of the system.
Using this example system, the operational requirements of a typical automatic transfer system will be examined.
An essential requirement of any automatic transfer system is the possibility of having different modes of operation . In a given mode of operation, the transfer system will respond in a given way to changing system conditions. For a different mode of operation, the transfer system will respond differently.
The two basic operating modes that any automatic transfer system must have are:
- Manual mode
- Automatic mode
In manual mode , the automatic transfer system does not operate any automatic operation, that is, it does not respond to the changing conditions of the system. All circuit breaker operations must be performed manually. Conversely, in automatic operating mode, all operations, with a few exceptions, are automatic and the system will respond automatically to changes in system conditions.
On the surface it seems like a simple arrangement, and to some extent it is true.
Good ATS design
However, a good design of the automatic transfer system has well thought out mode logic that answers the following questions:
- Can the system be put into automatic mode if the system conditions are not correct (for example, if an automatically operated circuit breaker is in the withdrawn position or if it is not present in the circuit breaker cubicle)?
- What manual operations are authorized in automatic mode (for example, manual opening of circuit breakers)?
- What happens if an authorized manual operation is performed on an automatically controlled device (for example, if an automatically controlled circuit breaker is manually tripped or trips due to a fault)?
Such questions are not always easy to answer. In fact, they require, in a well-designed automatic transfer system, the inclusion of a third mode of operation, generally referred to as automatic mode failure .
The three operating modes then generally work as follows:
- Manual mode
- Automatic mode
- Auto mode failed
1. Manual mode
Selected via a selector position or other predetermined user input through the user interface. No automatic operation occurs.
2. Automatic mode
Selected via a selector position or other predetermined user input through the user interface. Attempting to enter automatic mode if the system conditions are not correct, the system is in automatic mode failure mode. .
In automatic mode, operations for certain circuits The circuit breakers (such as the main and secondary circuit breakers) are automatic. However, manual tripping (or tripping due to a fault) of automatically controlled circuit breakers is authorized. Such manual or error-triggered operations will cause the system to enter auto mode failure mode.
3. Auto mode failed
No automatic operation occurs. For automatically controlled circuit breakers, only manual tripping (or tripping due to a fault) is authorized. To exit this operating mode, the system must be placed in manual mode.
This arrangement provides a high level of security for the handover scheme , i.e. unwanted or “annoying” operations are minimized, which improves the security, maintainability and reliability of the system.
Of necessity, to make this arrangement mode logical to function properly, the circuit breaker state must consist of both an open-closed indication and, for withdrawable circuit breakers, a circuit breaker cubicle switch indication.
Circuit breaker cubicle switches are a feature that should not be overlooked as they are essential for the proper functioning of an automatic transfer system with withdrawable circuit breakers. For the same reason, overcurrent switches for low voltage circuit breakers or latching relays for medium voltage circuit breakers are also required.
Another question that frequently arises is that of a “test” mode of operation. While this can be made into a separate mode of operation, this is usually handled more appropriately via voltage failure simulation test switches when the system is in automatic mode.