ATC System Configurations

Primary-Selective Configuration:

The most common auto transfer system is a primary-selective, or common bus configuration (see Figure 1). In this scenario, actuators and voltage sensors (analog or digital type) are installed on the switches for incoming source 1 and incoming source 2. One of the source switches is normally closed (feeding the load), and one is normally open (waiting as a standby or alternate source). If the load way(s) are load break only (non-fault-interrupting devices), optional faulted circuit indicators may be added for downstream fault indication. All of these devices are wired into the ATC control.

Preferred/Alternate Setting:

Either of the incoming sources may be programmed as the preferred, or primary source. In this setting mode, the preferred source switch is normally closed, and the alternate source switch is normally open. Voltage sensors interface with the ATC to indicate the source voltages as either "live" or "dead." If the preferred source voltage is dead, the ATC will initiate a transfer to the alternate source, providing that its' voltage is live. When the preferred source voltage is live again, the ATC will initiate a transfer back to the preferred source. These transfers will occur in "open-before-close" or "close-before-open" user-programmable operation sequences.

Non-Preferred Setting:

Certain ATC applications do not require a preferred source. In this setting mode, the ATC will only initiate a transfer if the source that is feeding the load is dead. There is no return transfer when a non-preferred mode is selected.

Bus-Tie Configuration (Figure 2)

Bus-tie systems split the load into two sections. One of the load sections is fed by normally-closed incoming source 1 and the other load section is fed by normally-closed incoming source 2. A normally-open switch (bus-tie switch) separates the two sections. Actuators are installed on switches for source, source 2, and the bus-tie. Voltage sensors (analog or digital) are installed on the two incoming source switch ways. If the load ways are load break only, optional faulted circuit indicators may be added for down-stream fault indication. All of these devices are wired into the automatic transfer control (ATC).  In bus-tie ATC systems, if either source is dead (and the remaining source is live), the control will initiate a transfer by opening the dead source switch and closing the bus-tie switch. Once the source is live again, the control will transfer back to its normal state by opening the bus-tie switch and closing the source switch. These transfers will occur in "open-before-close" or "close-before-open" user-programmable operation sequences.

Generator Source Configuration

In some ATC applications, rather than two utility sources, a generator is used as the alternate source. In this application, once the utility source is dead, the control will automatically issue a start command to the generator. It will then monitor the incoming voltage from the generator, initiating a transfer to the generator source only after it measures a live source. When the utility source is live again, the ATC control will initiate a return transfer operation. Simultaneously, it will issue a stop, or cool-down command to the generator.

Custom ATC Configurations

While a majority of the auto transfer applications will fall within the previous three categories, there is a need for specialized ATC configurations. G&W provides custom ATC schemes that include integral SCADA systems and distributed source applications.