LA Timing

ACRs in a LA scheme are primarily protection devices. Their purpose is to detect faults on the system and trip to disconnect that fault from the source of supply. They are normally configured to automatically reclose a number of times in an attempt to restore supply if the fault is not permanent.

If the fault does clear during the auto reclose sequence, supply will be restored and LA is not required. LA only comes into effect when the fault fails to clear during the auto reclose sequence. So the ACR must be allowed to go through its configured auto reclose sequence uninterrupted.

Make sure that the LA Activation Delay is set for a period greater than the longest auto reclose time, plus a safety margin of several seconds.

The LA Activation Delay for the Tie device must be set for a longer duration than the same setting in the Feeder or Mid-Point devices. This is to allow the Feeder device to open and the Mid-Point device to change setting groups and turn auto reclose off if required before the Tie closes.

Common practice is to decide on a coordination time between the various stages of the isolation and reconfiguration sequence.

LA Timing Example

This example describes the timing that occurs for the LA scheme shown below when a permanent fault is detected between the substation circuit breaker CB1 and the Feeder ACR F1.

In this scenario the Station circuit breaker CB1 trips due to the fault, recloses once and then trips again to lockout. LA operation is described below through to when the Tie ACR closes.

Figure 251
LA Timing Diagram

When the fault is first detected the protection timing starts and the fault causes the system voltage on the feeder to drop but remain above the Live Line Threshold setting. So the Live/Dead status doesn’t change.
When CB1 trips the system voltage drops to zero and the Supply Timeout timer starts to run. The reclose timer also starts.
The Supply Timeout timer expires and the Live/Dead status changes to Dead. This starts the LA Activation Delay timers in F1, MP1, and the Tie.
The reclose time for CB1 expires and it closes back onto the fault which restarts the protection timing. This immediately resets the LA Activation Delay timers running in each device which were all set for a longer time than the reclose time for CB1. The Supply Timeout timer also starts to run and the Live/Dead status goes back to Live when it expires.
Circuit breaker CB1 trips to lockout which drops the voltage back to zero and starts the Supply Timeout timer running.
When the Supply Timeout expires, the LA Activation Delay timers for F1, MP1, and the Tie restart.
At this point, the LA Activation Delay timers for F1 and MP1 expire. This causes F1 to trip and MP1 to change its protection setting group and go into single shot mode.
The Tie LA Activation Delay timer expires and the Tie closes. This also starts the Supply Timeout timers running and the Live/Dead status goes back to Live when that expires.

Points to Note:

Live/Dead status changes follow the actual voltage change on the bushings by the Supply Timeout setting value which is 4.0 s by default.

In most cases, LA Activation Delay timer activation occurs when the Live/Dead status on one side of a device changes from Live to Dead.

LA Activation Delay timer reset occurs when the device closes and does not wait for the status to return to Live.

LA timers must not be allowed to expire during a normal auto reclose sequence that is they must be set longer than the longest auto reclose time minus the Supply Timeout setting plus a safety margin.

LA times for the different types of devices need to be coordinated. The Tie needs to have a longer time than the Feeder and Mid-Point devices. In the example above the Tie LA time was longer than the Feeder and Mid-Point by an appropriate coordination time. This is to help ensure that the LA events occur in the required order.