Why Sectionalising Functionality is needed in Reclosers
In our last article, Distribution Automation III, we explored the quintessential Ring Mode network design configuration and the automation of switching to isolate faults without the use of communications between the devices.
This method of distributed automation relies on sensing voltage presence on either side of the circuit breaker, in this case a Recloser, to determine whether to close to restore power.
Of course, this leads to at least one more reclose onto a fault, but this is mandatory for any communications-free automation scheme. Provided that local electricity policy permits the full four reclose operations stated in the standards, we can use 3 protective recloses from the normal feed method and one close from the automation method and remain compliant.
Figure 1 above – Ring Type Network Topology, post Isolation with R1 as the primary protective device, R3 as the automatic close, and R2 acting as the second protection device to isolate the fault.
However, this distributed automation configuration within a ring mode does provide unique protection challenges. With a ring mode configuration, there is a real possibility that power flow direction may change depending on switching configuration. Consider the case that Supply 1 failed:
Figure 2 above – Supply One failure, Recloser 1 opens to prevent back feeding the substation, Tie point Recloser 3 closes to restore power up to Recloser 1.
In this case, Recloser 2 now has reverse power flow to standard operation. Therefore, the use of directional protection becomes essential to provide grading. Under normal system configuration, Recloser 2 is the furthest from the substation, closest to the tie point. If supply 2 fails, it becomes the 2nd closest to the substation, with 3 other downstream devices. In the case above, it becomes the furthest recloser from the substation – each case requiring its own protection grading.
A quick reminder, Protection grading is the coordinated configuration of tripping responses so that protection devices coordinate with each other to ensure the minimum zones are impacted by faults. In this case, we have three protection scenarios we need to account for in our grading. We’ll use Recloser 2’s settings as our reference, although this simple situation analysis should be applied to all devices in the scheme for confirming settings.
Table 1 above – Protection Scenario planning for Recloser 2.
Handling the system normal condition is equivalent to standard protection design, except that consideration must be given to the additional load current taken on when the rest of the ring is added if supply 2 fails. This puts a limit on how low the overcurrent setting can be configured at R2 (and it’s counterpart R4 when supply 1 fails).
Therefore, from the perspective of Recloser 2, System Normal and Supply 2 fails require the same protection configuration, at the cost of some sensitivity to overcurrent.
In the case of Supply 1 failing, Recloser 2 needs to have direction overcurrent configured, where it acts faster than R3, R4 and R5. This can be accomplished as standard with a NOJA Power OSM Recloser system, configuring the directional overcurrent to grade appropriately in this scenario.
Figure 3 above – Supply 1 failure simplified. Yellow current flow zones indicate current flowing opposite to System Normal.
Handling Insufficient Grading Margins
Configuring the protection for these automation cases is generally straightforward, provided that there is sufficient grading margin.
In the case that the substation breaker is set to trip at 1 second of max fault current, grading margins of 250ms between coordinated devices would allow for a maximum of 4 reclosers along the line. While 250ms is conservative by modern recloser standards (150ms grading is comfortably reliable with modern equipment), it does limit the number of protective devices that can be switched in.
Unless the devices switched in are Sectionalisers
If we consider the case of figure 3, but let’s assume a more conservative grading margin of 300ms with a 1 second substation breaker time, we can only fit 3 protective reclosers in before the substation would trip. Under system normal, this would be easily handled, but adding the two extra reclosers during an automation operation (R3 and R2), a race condition will be created if a fault occurs between R1 and R2.