A feeder with a fuse, a recloser and a sectionalizer is coordinated in a fixed order: first the fuse philosophy, then the fault current at each point, then the two pairs of devices (fuse with recloser, sectionalizer with recloser) and finally the verification of the whole. Current zones and zone sequence coordination resolve the cases where the fault changes magnitude along the feeder.
What each device does
All three protect the same feeder, but they act differently. For that reason they are not coordinated by comparing curves alone: each one contributes its own rule.
| Device | Does it interrupt the fault? | What it does | What conditions its setting |
|---|---|---|---|
| Expulsion fuse | Yes, when it blows | Clears a permanent fault on its branch | Minimum melting curve, total clearing curve and accumulated heating |
| Recloser | Yes | Clears, waits a dead time and recloses several times before locking out | Fast and slow curves, number of operations and dead times |
| Sectionalizer | No | Counts the openings of the upstream device and opens during the dead time | Number of counts, which must be lower than the recloser operations to lockout |
The first two are explained in their own guides. This guide covers the part that fails in practice: how they are combined.
The working order
Deciding in another order forces the settings to be redone. The usual order is this:
- Feeder philosophy. Fuse saving or fuse blowing
- Fault current at each device. Minimum and maximum value at the point where each one is located
- Fuse and recloser pair. Curves and fuse heating
- Sectionalizer and recloser pair. Count against operations to lockout
- Verification of the whole. Each type of fault from each point of the feeder
The first step conditions all the others. In fuse saving, the fast curve clears transient faults before the fuse is damaged. In fuse blowing, the fuse clears any fault on its branch. The radial scheme guide compares the two philosophies.
The fuse and relay curves are compared using the inverse-time families described in IEC and IEEE inverse-time curves.
Fuse and recloser: two possible positions
The criterion changes according to which of the two is upstream.
- Fuse upstream of the recloser. The fuse must not blow before the recloser reaches lockout. Here what counts is the minimum melting curve of the fuse against the lockout curve of the recloser.
- Recloser upstream of the fuse. The fuse must not blow on the fast operations, but it must operate on the delayed ones to avoid lockout. The upper limit comes from the fast operations and the lower one from the total clearing curve of the fuse.
In both cases the heat that each operation leaves in the fuse and the cooling during each dead time must be accounted for. The recloser guide describes the three methods for doing so: the conservative one, the cooling-factor one and the K-factor method.
Sectionalizer and recloser
The sectionalizer depends entirely on the device it has upstream. It counts how many times the fault current is interrupted and, on reaching its count, opens when the line is de-energized. Two conditions make it work:
- Its count is lower than the number of recloser operations to lockout.
- The recloser does not lock out before the sectionalizer completes the count.
The second condition is easily violated in high-current settings. Many reclosers allow immediate lockout when the fault exceeds a certain current level. The programming guide of the Cooper Power (now Eaton) Form 6 control asks for that level to be set above the fault current available at the sectionalizer point. Otherwise the recloser locks out before the sectionalizer manages to count. This is complemented by NOJA RC protection and reclosing settings.
The fault changes size along the feeder
Near the substation the fault current is higher than at the end of the trunk. The same recloser may need different behavior depending on where the fault is. The Cooper Power guide illustrates it with a feeder divided into three zones:
| Zone | What is there | Recloser behavior |
|---|---|---|
| Near the substation | Very high currents | Lockout after one or two operations, to limit the currents flowing through the substation transformer |
| Intermediate | A sectionalizer and no time-sensitive device | No intentional delay, with the operations the sectionalizer needs to count |
| Remote | Branch fuses | Fast operations followed by slow operations |
The criterion behind the table is general: zones are separated by current magnitude and each one is assigned the sequence its downstream device needs. The concrete values come from the feeder data.
Zone sequence coordination
When there is one recloser downstream of another, the upstream one can advance its trip counter on detecting that the downstream one has already operated. NOJA Power manuals call it «zone sequence coordination». The counter advances only when all protection elements have reset. In this way the upstream recloser moves earlier to its slow curve and lets the downstream one act. This is complemented by recloser settings: reclosing, curves and sequence.
The complete protection of a distribution feeder is summarized in Distribution feeder protection; that of a feeder in an industrial plant is in Industrial feeder protection.
What is not a fault
Two situations trip the settings of a well-coordinated feeder without any short circuit:
- Cold load pickup. After a long outage, the loads start together and the current exceeds the normal one. Reclosers include a cold load pickup mode that modifies the settings for a programmed time after closing.
- Inrush current. Distribution transformers draw magnetizing currents when energized. NOJA applies a temporary multiplier to the pickup level when it detects loss of supply, except in the high-current, earth and sensitive earth elements.
If the feeder has distributed generation, the fault current also changes. The radial scheme guide explains how coordination is lost in that case.
Common mistakes
- Comparing only the fuse and recloser curves, without adding the fuse heating.
- Setting a sectionalizer count equal to or greater than the recloser operations.
- Coordinating with a fuse other than the installed one, or with a fuse that has already withstood many faults.
- Not re-verifying coordination after adding a branch, replacing a fuse or connecting generation.
- Loading the settings without testing them with the complete sequence of operations.
If two of these symptoms appear together, see the fuse and the recloser operate at the same time and the recloser locks out. This is complemented by Eaton Cooper Form 6 recloser control.
How we help
At Tiza Group we carry out the protection coordination study of your feeder and load the settings into reclosers and relays with our remote recloser configuration. We then verify them with secondary injection. To compare curves you can use our free relay coordination tool. Write to us and we will quote.
Related guides
- Radial feeder protection: recloser and fuse
- Recloser: what it is, how it works and tests
- IEC and IEEE inverse-time curves
- Distribution feeder protection
- Industrial feeder protection
- NOJA RC Control: Protection and Reclosing Settings
- Eaton Cooper Form 6: Recloser Control Overview
- Recloser settings: reclosing, curves and sequence
References
- IEEE C37.60 and IEC 62271-111: automatic circuit reclosers for alternating current systems above 1 kV.
- IEEE C37.230: guide for protective relay applications to distribution lines.
- Cooper Power Systems, Kyle Form 6 Microprocessor-Based Recloser Control Programming Guide, S280-70-4.
- NOJA Power, OSM User Manual, NOJA-598-00, sections 6.1.6, 6.1.7 and zone sequence coordination.
- Cook and Myers, S&C Electric, recloser and fuse coordination, IEEE PES General Meeting, 2004.
Frequently asked questions
How many openings should the sectionalizer count?
Fewer than the operations the recloser performs before lockout. That way the sectionalizer opens during a dead time, before the recloser locks out and leaves the whole feeder without service. The exact value is set by the coordination study of each feeder.
Can a sectionalizer be installed without a recloser upstream?
It needs a device upstream that interrupts the fault and recloses, such as a recloser or the head-end relay with reclosing. The sectionalizer does not interrupt fault current: it only counts the openings of that device and opens when the line is de-energized.
Why does the fuse blow even though the recloser has a fast curve?
The most frequent causes are a fault current higher than the coordination range, a fuse of a different rating or one fatigued by earlier faults, and a change of settings or network that nobody re-verified. The troubleshooting guide linked at the end goes through each cause.