Automatic-Changeover Sequence

No Supply Available on the Preferred Supply

When the preferred supply is lost, perhaps due to an upstream fault on the feeder, the primary device controller checks the operational and network conditions on both primary and secondary controllers before initiating an Automatic-Changeover sequence.

This sequence automatically connects the load to the alternate supply. Depending on the selected algorithm, the primary device will interrupt the changeover scheme after a changeover, or wait for restoration/changeover conditions while ACO Status displays .

Running: secondary is supplying the load.

A typical changeover sequence is shown in the figure below.

Figure 255
Typical ACO Sequence

Restoration Algorithms

An ACO restoration, changeover from the alternate to preferred supply, may happen in different ways depending on which algorithm is selected.

1. One Way

   If One Way is the selected algorithm, ACO will be aborted after a changeover to the alternate supply, and no restoration will be performed when the preferred supply is restored. At this stage, ACO Status displays Aborted: One Way changeover executed on all interfaces.

   

   Figure 256
   Auto Changeover Sequence with One Way Restoration

2. Break Before Make

   This option must be used when the preferred and alternate supplies are incompatible and must never be connected to the load at the same time. If the preferred supply is lost while the primary device is closed to supply the load, the primary device will trip and the secondary will close to supply the load from the alternate supply as described in the figure above – Typical ACO Sequence.

   If the Algorithm is Break before Make and the preferred supply voltage is restored, the primary device commands the secondary device to trip and verifies that the load side bushings on the secondary switchgear are dead before the HV Check Time expires. If the load side bushings are not dead restoration is aborted.

   A short interruption of supply is experienced by the load during the HV Check Time confirmation period. The primary device then closes its switchgear to reconnect the preferred supply to the load.

   When using Break before Make, the Live Load Blocking functionality is automatically turned On on both primary and secondary devices. This stops an operator inadvertently closing the primary onto an energized load, and cannot be changed while Auto Changeover is enabled.

   

   Figure 257
   Auto Changeover Sequence with Break Before Make Restoration

3. Make Before Break

   This option may be used when the preferred and alternate supplies are compatible and may be connected to the load at the same time. If the preferred supply is lost while the primary device is closed to supply the load, the primary device will trip and the secondary will close to supply the load from the alternate supply as described in the figure above – Typical ACO Sequence.

   If the Algorithm is Make before Break and the preferred supply is restored, the primary device will close thus interconnecting the preferred and alternate supplies. The primary device then commands the secondary device to trip, disconnecting the alternate supply from the load.

   When using Make before Break, the Live Load Blocking functionality is automatically turned Off on both primary and secondary devices. This allows an operator to have both devices closed and energized at the same time, and cannot be changed while Auto Changeover is enabled.

   When using Make before Break, the primary uses the Sync Check functionality if it is turned On. This mean Sync Check may reject a close operation of the primary if the primary source and load side voltages are not synchronized.

   

   Figure 258
   Auto Changeover Sequence with Make Before Break Restoration

4. Both Ways

   Also known as the Voltage Follower method, this option has the potential to minimize the number of operations, thus reducing the number of supply interruptions, by changing back to the preferred supply only when it is the only supply available.

   If the preferred supply is lost while the primary device is closed to supply the load, the primary device will trip and the secondary will close to supply the load from the alternate supply as described in the figure above – Typical ACO Sequence.

   If the Algorithm is Both Ways, automatic restoration will not occur when the preferred supply returns as long as the load is being supplied from the alternate supply.

   If the alternate supply is lost, Both Ways will perform a Break before Make restoration to connect the preferred supply to the load.

   When using Both Ways, the Live Load Blocking functionality is automatically turned Off and cannot be changed while Auto Changeover is enabled on both primary and secondary devices. This allows an operator to have both devices closed and energized at the same time when the option Manually Close Backup Device is selected.