FLISR (fault location, isolation and service restoration) is the logic that, on a permanent fault on a medium-voltage feeder, identifies the faulted section, isolates it by opening the devices that bound it and restores supply to the healthy sections: the upstream ones from their own source and the downstream ones from a neighbouring feeder, by closing a normally open point. It can be centralized, distributed among reclosers or local without communications.
What FLISR is
FLISR stands for Fault Location, Isolation and Service Restoration. It is also called self-healing grid automation. It is a distribution automation logic that acts on medium-voltage feeders divided into sections by reclosers or motorized switches, with at least one normally open point towards another feeder.
In a radial network without automation, a permanent fault leaves the whole feeder downstream of the device that cleared it without supply until the crew finds the fault, isolates it by hand and restores. FLISR does those steps automatically.
The three stages
- 01Clearing and reclosingProtection opens, recloses and ends in lockout: the fault is permanent
- 02LocationThe section between the last device that saw the fault and the first that did not is identified
- 03IsolationThe devices bounding the faulted section open
- 04Upstream restorationThe source-end device that had locked out closes
- 05Downstream restorationThe normally open point closes if the neighbouring feeder has capacity
Location. Each recloser or faulted circuit indicator reports whether fault current passed through it. The fault lies between the last device that saw it and the first that did not. Fault indicators, such as those described in Easergy Flite, Horstmann and SEL fault indicators, refine the location on long sections.
Isolation. The switching devices bounding the faulted section open. If one does not open, the logic extends the isolation to the next device.
Restoration. The upstream section comes back from its own source. The downstream section comes back from a neighbouring feeder when the normally open point closes, only if that feeder can take the load without overloading or going outside the voltage limits.
Three architectures
- Centralized: the distribution management system (DMS or ADMS) at the control centre receives the states and fault signals, calculates the best reconfiguration with the network model and sends the commands. It is the most flexible: it can evaluate several alternatives and the actual capacity of each feeder. It depends on the control centre and its communications.
- Distributed (peer to peer): the recloser controls exchange information directly and decide among themselves. It is faster and does not depend on the control centre, but each device has a limited view of the network.
- Local, without communications: each device acts on the presence or absence of voltage on its two sides and coordinated time delays. Some recloser manufacturers offer it under their own trade names. It is simple and cheap, but it produces more switching operations and does not check the capacity of the neighbouring feeder.
There is also the substation-based variant, where a controller in the substation governs the feeders leaving it.
Relationship with feeder protection
FLISR does not replace protection: it acts after it. The fault is cleared by the source-end breaker or the reclosers with their protection and reclosing, and only when the device ends its cycle in lockout does the restoration logic take over. That is why the coordination between reclosers, fuses and the source-end breaker, described in radial feeder protection, is the foundation on which the automation is built. If protection is not selective, location will not be either.
Communications needed
Centralized and distributed architectures need a link to each switching device. Radio, cellular networks and optical fibre are used, with protocols such as DNP3 (IEEE 1815), IEC 60870-5-104 or peer-to-peer IEC 61850 messages. Link availability at the moment of the fault is what most affects the result; the limits of each medium are covered in radio and cellular links for SCADA.
Why it matters: reliability indices
IEEE 1366 defines service reliability indices, such as SAIDI and SAIFI, and distinguishes sustained from momentary interruptions by their duration, with a five-minute threshold. If FLISR restores a customer in less than that threshold, the interruption counts as momentary and does not enter the sustained interruption indices. That is one of the reasons utilities invest in FLISR, and why the total sequence time is a design parameter.
Why it fails
- Communications down with one or more devices at the moment of the fault: the logic cannot locate or cannot command.
- Wrong fault indication because of a badly set threshold or the inrush current on reclosing, which makes a healthy section look faulted.
- Recloser in the wrong mode, for example locked in local or with reclosing disabled. The causes of lockout are in why the recloser locks out.
- Capacity not checked: the neighbouring feeder takes the load and trips on overload, extending the outage.
- Protection not adapted to the new configuration: after reconfiguration, the coordination of the neighbouring feeder is no longer the one studied.
- Outdated network model in the centralized system: the actual normally open point is not the one in the model.
How it is tested
In general terms it is tested at three levels: each device with injection (fault detection, reclosing, lockout, remote control), the complete logic in the lab or with simulated devices running a fault on each section, and a controlled field test with the utility. In each case, what happens with a device that has no communication or does not operate is also checked.
We can help with your distribution automation
At Tiza Group we do the remote recloser configuration of the main brands: protection, settings groups per configuration, restoration logic and communications with the control centre, for utilities and industry, worldwide.
Related guides
- Radial feeder protection: recloser and fuse coordination
- Interlocking in primary selective and MV open-loop systems
- WAN networks for substations: IEC TR 61850-90-12
- Automatic switching sequences in substations
- Automatic service restoration after a loss of voltage
References
- IEEE Std 1366, Guide for Electric Power Distribution Reliability Indices.
- IEEE Std 1815, Standard for Electric Power Systems Communications – Distributed Network Protocol (DNP3).
- IEC 60870-5-104, Telecontrol equipment and systems – Transmission protocols – Network access for IEC 60870-5-101 using standard transport profiles.
- IEEE C37.104-2022, Guide for Automatic Reclosing on AC Distribution and Transmission Lines.
Frequently asked questions
Does FLISR avoid the outage for customers on the faulted section?
No. Customers on the section with the fault stay without supply until the crew repairs it. What FLISR does is reduce the number of customers affected and the interruption time of everyone else, which goes from the duration of the repair to the seconds or minutes the reconfiguration takes.
Is communication between reclosers needed?
For centralized and distributed schemes, yes: the logic needs to know which devices saw the fault current pass and their status. There are local schemes that work only with the presence or absence of voltage and time delays in each device, without communications, at the cost of more switching operations and less flexibility.
Why does protection need to be reviewed after a reconfiguration?
Because when the normally open point closes, a section is fed from another feeder and the direction of power flow changes. Reclosers that used to be at the end of the circuit end up in another position, with different fault currents and different coordination. Well-designed schemes change the settings group according to the configuration.