Coordinated Application Scheme for Reclosers and Drop-Out Type Sectionalizers

1. Executive Summary

This solution proposes a fault-isolation architecture based on the coordinated operation of automatic reclosers and drop-type sectionalizers, featuring “local control without communication.” It is particularly suited for rural and suburban feeders in 15 kV overhead distribution networks in developing countries and regions overseas—areas where SCADA infrastructure is weak, communication maintenance costs are high, and enhancing power supply reliability is the primary objective.

The proposal is based on two well-established sources: First, the journal article “Application of Automatic Segmenters in Distribution Network Automation” (Journal of Huangshi Institute of Technology, 2005) demonstrates the cost-effectiveness and ease of implementation advantages of local control (combining reclosers with segmenters) over remote control. Second, the field-verified RDK-15/300 three-phase electromechanical segmenter (V2.1 manual, 2026) features programmable counting, USB/DIP configuration, and self-powering from line current.

• No communication infrastructure required—fully autonomous, on-site control;
• The faulty section is automatically isolated, while non-faulty sections maintain continuous power supply.
• Low investment and operation & maintenance costs, maintenance-free, and quick deployment;
• Complies with IEC/ANSI recloser coordination practices (tripping follows a slow-speed curve, and the interrupting time is > 0.5 s).

2. System Concept and Operating Principle

2.1 Comparison between Local Control and Remote Control

There are two technical approaches to distribution network automation. Remote control relies on the coordinated operation of a master station system, substation systems, communication systems, and monitoring terminals, achieving a high degree of automation. However, this approach involves substantial maintenance efforts for the communication system and incurs high system costs, making it less suitable for medium- and small-sized urban distribution networks—especially rural power grids. In contrast, local control utilizes automated switching devices that promptly detect faults and automatically perform operations according to predefined procedures, swiftly isolating the faulty section while ensuring uninterrupted power supply to non-faulty areas. This approach does not require communication infrastructure, is simple to implement, requires low investment, delivers excellent results, and is easy to promote.

2.2 Device Role

The sectionalizer is an isolating device rather than a protective one: it never interrupts fault currents but only drops out under no-load conditions after being de-energized (voltage loss) by the upstream recloser. This feature makes its mechanism lightweight, cost-effective, and maintenance-free.

2.3 Sectionalizer Types and Design

Sectionalizers are categorized into two types: current-time and voltage-time. The RDK-15/300 electromechanical sectionalizer belongs to the current-counting type: its on-board microcontroller accumulates fault current events. When the preset counting cycle (1, 2, 3, or 4 times) is completed and the line voltage drops to zero, the device outputs an excitation signal that actuates the electromagnetic actuator, releasing the trip latch and causing the conductive components to drop, thereby isolating the faulty section. The memory time defines the duration for which the count is retained; the reset time (30 to 300 seconds) enables the device to return to its initial state prior to the fault after a transient fault has been cleared.

The control core implements fault current detection, undervoltage detection, and counting logic; three-phase interlocking is ensured through mechanical interlocks.Use when wireless interconnection is required.Low-power wireless synchronization (such as the nRF401 module) ensures that the three-phase segmenters drop simultaneously.

2.4 Synchronize timing

Fault occurrence → Upstream recloser trips (either fast or slow curve) → The counter for all sectionalizers carrying fault current increments by one → Line voltage drops → Sectionalizers whose counters reach the preset value trip while in no-load condition → The recloser recloses, restoring power to the non-faulted sections, while the isolated section remains de-energized. In the case of a transient fault, all counters reset after their memory time expires, and the line returns to normal operation without any further action.

Hardware Highlights: The control system is powered by a current transformer (I1, I2A, I2B) for sampling, a rectifier and voltage-stabilization circuit, and driven by thyristor electronic switches (SR1, SR2). The interrupt handling process includes an external interrupt INT0 (which increments the fault count and starts timer T1), a serial receive interrupt (which processes incoming commands), and a T1 timer interrupt (which sets a 1-second flag). The trip control module triggers the actuator to open the circuit breaker and sets an opening flag when either “the number of fault currents reaches the set value” or “a trip command is received,” provided that “the line voltage has dropped.” Typical memory delay is 15 seconds, and state recovery takes 30 seconds.

3. Technical Design Basis

3.1 Recommended Equipment Parameters (RDK-15/300 Grade)

3.2 Comply with the setting rules (mandatory requirements)

Selection guidelines: For customer incoming lines, a single-count setting is generally recommended—most faults in customer substations are permanent. For branch connection points on overhead lines, a two- or three-count setting is recommended—approximately 80% of faults on overhead lines are transient, which facilitates coordination and optimal matching among sectionalizers.

3.3 Environmental and Installation Conditions

• Ambient temperature: -40 ℃ ~ +60 ℃ (suitable for tropical, desert, and cold regions);
• Altitude: ≤ 2000 m; Wind pressure ≤ 700 Pa; Ice thickness ≤ 1 mm;
• External insulation contamination level: Class IV; Controller USB interface protection rating: IP68;
• Perform pole-mounted installation according to the bracket installation diagram; operate the hook rod.

4. Application Architecture

4.1 Feeder Configuration

Typical configuration: A recloser is installed at the substation outlet, with a protection sequence set to 1 fast + 3 slow. Six automatic drop-out sectionalizers—F1 through F6—are deployed on the branch feeders, dividing the line into seven segments labeled L1 through L7. Count settings: F1 = 3 times; F2, F3, and F5 = 2 times each; F4 and F6 = 1 time each.

4.2 Fault Scenario Simulation

Branch Circuit Fault Case: A sectionalizer installed at the branch tap point, configured with a single-count function, can isolate customer-side faults (K1)—whether permanent or transient—without affecting the main feeder.

5. Benefits and Life-Cycle Costs

5.1 Reliability enhancement

• Without requiring operator intervention, fault sections are isolated within seconds—directly improving SAIDI/SAIFI metrics.
• Transient faults (accounting for approximately 80% of overhead line faults) are cleared solely by reclosers, while section switches automatically reset.
• The permanent fault is confined to the smallest section, and the vast majority of users remain supplied with power.

5.2 Cost Comparison with Remote Control Solutions

• No SCADA master station, no communication links, and no terminal maintenance—investment is merely a fraction of that required for remote control solutions.
• The separator is maintenance-free (self-powered, battery-free, SF6-free, and requires no auxiliary power supply);
• It’s installed on the standard rod and is quick to deploy; local personnel can complete debugging using a simple USB configuration tool after brief training.

5.3 Debugging and Operations

• Set the count and current range via DIP switches, or configure via USB (default admin/123, which can be modified);
• Event Log: Up to 100 entries (trip/automatic reset/manual reset, trip current, maximum/minimum current); exportable to Excel.
• The maintenance software provides trip testing, real-time status viewing, and message monitoring functions.
• The waterproof sealing of the module is critical to the device’s lifespan—parameter modifications must be performed by trained technicians.

6. Overseas Deployment List

• Standard Compliance: Reclosers shall comply with IEC 62271-111 (or, where applicable, with ANSI C37.60); sectionalizers with associated logic shall comply with IEC 60255-class relays; system design must conform to the grid regulations of the target utility company.
• Voltage/Frequency: 15 kV class, available in 50 or 60 Hz—subject to confirmation with the target power grid (if a 11/20/22/33 kV variant is required, further discussion is needed);
• Environmental verification: Confirm the on-site ambient temperature range, pollution level, altitude, and icing conditions.
• Neutral Point/Earthing: Confirm the system’s earthing method (direct earthing, impedance earthing, or neutral point ungrounded) and the coordination of the fault current trip setting for ground faults.
• Attachment configuration: load-breaking capability hook rod, hot-work line clamp kit, spare DIP module, USB test cable, and driver package;
• Training and Documentation: English manual + quick guide to the configuration tool; on-site debugging by trained personnel;
• Spare Parts Strategy: Reserve segmenters and complete installation brackets at a ratio of 10% to support rapid replacement.

7. References

• Xu Lufei, Pi Daneng, and Wang Zaiming: “Application of Automatic Sectionalizers in Distribution Network Automation,” Journal of Huangshi Institute of Technology, Vol. 21, No. 6, December 2005 (Article ID 1008-8245(2005)06-0016-03).
• Rockwill Group: “User Manual for Three-Phase Mechanical-Electronic Sectionalizer RDK-15/300,” V2.1.0, June 2026.
• Xu Layuan and Gao Yimei: "Design and Technical Applications of Unattended Substations in Rural Power Networks," China Electric Power Press, 1999.