Introduction to Automatic Sectionalising

Sectionalising is the act of disconnecting a section of feeder that has a permanent fault.

The AS is normally used in conjunction with an upstream ACR.

The AS can be programmed to open if the fault is still present after the ACR has performed a pre configured number of automatic reclose sequences. This then allows the ACR to reclose successfully as the faulted section of feeder has been isolated.

Since the AS opens while the upstream ACR remains open during the dead time, the switchgear does not need to be capable of breaking fault current. The switchgear most commonly used by AS is the RL Series LBS. PMSet U and PMSet UR circuit breakers can also be used.

Supply Interrupts

The Supply Interrupt count is a measure of how many times the upstream ACR has tripped during an auto reclose sequence.

There are separate counters for:

Phase and EFs
SEFs
Negative Phase Protection Faults (if Sequence Components are available)

These settings can be found in WSOS under Sectionaliser on the Global Settings page for each Detection group.

They are also displayed on the operator interface.

The Supply Interrupt count for each fault type increments when the AS detects the following conditions in the correct sequence:

Fault Detected
- Indicates a fault downstream of the AS
Zero Current
- indicates that the upstream ACR has tripped
Loss of Source side voltage
- Confirmation that the upstream ACR has tripped

A Supply Interrupt indicates to the AS that a downstream fault has caused the upstream ACR to trip.

So the Supply Interrupt count is a measure of how many times the upstream ACR has tripped during an auto reclose sequence.

Automatic Sectionalising Coordination

Correct operation of an AS requires it to be coordinated with the upstream ACR. The AS must wait long enough to allow the fault to clear during the auto reclose sequence of the ACR, but must trip before the ACR goes to Lockout.

The figure below shows an AS downstream from an ACR configured for 4 Trips to Lockout.

Figure 178
AS and Upstream ACR

The AS could be configured for trip after 1, 2, 3, or 4 Supply Interrupts with the following considerations:

Trip after 1 supply interrupt does not give the fault a chance to clear. This would result in unnecessary loss of supply if the fault was temporary. It would however mean that the ACR closed successfully first time every time.
Trip after 2 supply interrupts might be an acceptable setting but may still result in an unnecessary loss of supply if the fault needed longer to clear.
Trip after 3 supply interrupts would be the optimum setting in this case. It could be assumed that the fault was permanent after it caused the ACR to trip three times.
Trip after 4 supply interrupts wouldn’t allow the ACR to restore supply as it would have already gone to lockout.

The figure below shows timing diagrams of the ACR with 4 trips to lockout and the AS configured to trip after 3 supply interrupts when a permanent fault occurs downstream.

Figure 179
ACR and AS Timing Diagrams

NOTE: Each time the ACR trips, the AS logs a Supply Interrupt. When the ACR trips for the third time, the Supply Interrupt count reaches 3 and the AS trips. When the ACR recloses the third time it stays closed because the fault has been isolated by the AS.

The figure above represents a situation where an RL Series LBS is being used for the AS switchgear. The Trip 3 Reclose Time setting on the ACR needs to be long enough to allow the RL Series switchgear to fully open before the ACR recloses. In the case of an RL Series LBS, allow at least 1.2 s for the contacts to open as indicated in the above figure.

System Configuration

Multiple ASs can coordinate with a single ACR. The basic rules to be observed are:

The AS directly downstream from the ACR must trip before the ACR goes to Lockout.
ASs in series must be configured with decreasing Trip After Supply Interrupts settings as shown in the figure below.

Figure 180
AS Coordination

Fault Detection

The correct operation of an AS is dependent on its ability to detect a downstream overcurrent or broken conductor fault.

Fault detection is a two part process.

It starts with a protection pickup which occurs when the measured quantity for a particular detection element exceeds the Fault Detect setting x Pickup Multiplier for that element.

When a pickup occurs it generates an event in the Event Log and starts a Fault Detect timer.

If the fault persists until the timer expires a fault detection occurs which is also recorded in the Event Log.

Fault Detection is the first step in the sequence of events required for a Supply Interrupt to be logged.

The Fault Detection elements available are:

Phase Overcurrent
Earth Overcurrent
SEF (Available by default)
NPS (if Sequence Components are available)
Broken Conductor (If Broken Conductor is available)

Each element can be configured for a pickup value and either a time delay or instantaneous operation.

Multiple elements can be in pickup at the same time but only the first element to go into pickup will generate a pickup event.

The first element to time out will log a Fault Detect event.

Fault Reset

A fault that causes a detection element to pick up will reset if the fault current falls below a configurable reset threshold for a configurable reset time.

Fault reset can occur when the element is in pickup that is during the timing stage, or after a fault has been detected.

Sectionaliser Trip

A Sectionaliser Trip occurs when the AS reaches its Trip After Supply Interrupts setting for the faulted element. When an AS trips it goes directly to Lockout.

It is not able to reclose automatically.

Sequence Reset

The Sequence Reset timer is used to reset the supply interrupt counters to zero after a sequence ends with a sectionaliser trip. This helps to ensure that the supply interrupt count for each element starts at zero for the next sequence.