LA Basics

Purpose of the Feeder Device

The Feeder device is always the one closest to the source of supply and must never feed power in the reverse direction. This is the purpose of RULE A to isolate the source when it is not available so that LA does not try to feed power back into it. Conversely, a Mid-Point device can feed power in both directions. Therefore, there is no need for a network to have a Mid-Point device but must always have a Feeder device for each source of supply. Ideally, the Feeder device is also the substation circuit breaker as this allows maximum coverage of the network by the scheme.

Asymmetric Tie Operation

It may not be desirable to reconfigure the network on both sides of the Tie point in some networks. For this reason, the Tie device has the option to close automatically only if the Load Side Supply is not available. This is called the Tie Restoration setting which can be either One Way or Both Ways.

Loss of Substation Supply

This can happen due to a transformer fault of a fault on the transmission line feeding the substation. In this case when both feeders in a LA scheme originate from the same de-energised substation, the Feeder devices trip according to RULE A. The Tie device will not close as it has lost supply to both sides. When the substation is reenergized, supply will initially only return to the open Feeder devices. However, if Auto Restore is On, the Feeder devices will close according to restoration RULE F and energize both feeders up to the Tie device.

A Note about Protection Groups

WARNING
HAZARD OF INCORRECT SETTING IN TIE ACR Correct setting groups must be set up in Tie ACR for correct coordination when the network is fed in reverse direction. Failure to follow these instructions can result in death, serious injury, or equipment damage.

After the isolation and reconfiguration process, the Mid-Point ACR is running on the alternate or reverse setting group. These settings must have been set up for correct coordination for when the network is fed in this reverse direction. Ensuring that the Tie device has the correct settings depends on which side is detected as having lost supply. If supply is lost on the Load side, the forward group must be active when it closes. If supply is lost on the Source side, the reverse group must be active when it closes.

A Note about the LA Activation Delay

LA is normally triggered in a device by supply voltage being lost or restored to the terminals of that device. Loss and restoration of supply also occurs during a normal Auto-Reclose sequence. Therefore, to help prevent LA from being triggered during the normal Auto-Reclose sequence, there is a time delay which must elapse before the LA algorithms start. This time delay is called the LA Activation Delay and must be set to a greater duration than the longest Auto-Reclose sequence that could occur.

When supply is lost, the Feeder and Mid-Point devices must operate according to RULES RULE A and RULE B before the Tie closes according to RULE C. To achieve this coordination, the LA Activation Delay must be set for a longer period for the Tie than for the other devices.

The LA Activation Delay may typically be set for 30 s in the Feeder and Mid-Point devices and 40 s in the Tie device.