Retrofit is the modernization or partial re-engineering of complete switchgear assemblies (KRU) and single-sided service cubicles (KSO), replacing aged, end-of-life equipment with modern solutions while retaining existing busbars, foundations, and cable entries.
First and foremost, modernization targets the most worn and at the same time most critical component - the oil circuit-breaker. New switching devices are vacuum or SF₆ circuit-breakers with spring-charged or electromagnetic operating mechanisms; unlike oil designs, they require no routine oil maintenance, offer higher interrupting and mechanical endurance (E2/M2 classes), and minimize operational risks. A more extensive (comprehensive) KRU modernization is also possible: a block replacement of the compartment, including the withdrawable unit, shutter mechanism, earthing switch operating mechanism, door, relay compartment, and, if required, current transformers and terminal blocks of the secondary circuits.
The reliability of a KRU or KSO is directly linked to the reliability of its switching devices. Such modernization is a simple and cost-effective way to address wear of legacy breakers and to improve operational safety (including internal-arc performance when upgraded to IEC 62271-200).
Retrofitting existing electrical equipment is more economical than purchasing new gear, performing design and construction works for its installation, and compensating for lost energy supply during replacement; existing cable routes are preserved. Periodic repairs and maintenance of end-of-life breakers do not guarantee supply reliability, while other cell components do not always need replacement.
Typical replacement scenarios:
Replacement of the old, end-of-life breaker on the existing withdrawable unit (KRU or KSO) with a new one using an adaptation kit, updating or replacing interlocks, and adapting the breaker’s secondary connector; additional works include alignment of plug-in contacts and verification of travels and settings. This is the most economical option but longer on site and more labor-intensive.
Replacement of the entire withdrawable unit together with the breaker. In this case the new withdrawable unit fully matches the dimensions and connection interfaces of the one replaced, and interlocks remain compatible; commissioning is limited to routine checks. This option is significantly faster and needs minimal on-site labor; an additional benefit is the possibility to keep the old withdrawable unit with the oil (minimum-oil) breaker as a spare.
Replacement with a factory-built module, the so-called “OneFit” - a factory-ready module with a breaker, already implemented mechanical and electrical interlocks, protective shutters, and a system adapting standard plug-in power contacts to the existing fixed parts. Advantages:
- improved dielectric performance (higher withstand levels);
- type-testing according to IEC 62271-1 and IEC 62271-100;
- increased reliability of module mechanisms and reduced internal-arc risks.

Technical data (typical range)

Note: exact ratings and dimensions are finalized after cell survey, measurements, and breaker model selection. For installations above 1000 m a.s.l., correction factors apply to insulation levels and currents.

Purpose and application

The new-design withdrawable unit (WU) for KRU K-3 (K-III) replaces the original unit with an oil or minimum-oil breaker and provides a modern level of switching capability and safety in medium-voltage 6(10) kV, 50 Hz distribution switchgear. Applications include distribution substations of industrial facilities, urban and rural networks, small- and mid-scale generation, mining and oil & gas, utilities, transport and infrastructure. Retrofit is effective when:

• legacy breakers have reached end of life and repairability/spare parts cannot be ensured;
• enhanced requirements for protection and control selectivity and reliability arise;
• the consumer supply reliability category must be increased;
• equipment needs to meet modern internal-arc testing and IP protection targets;
• remote control and condition monitoring of switching devices are implemented.

Replacing the withdrawable unit preserves existing busbars, switching sections, cable entries, and foundation frames, without affecting building structures of the indoor substation. Compatibility with the existing K-3 (K-III) interlocking system and standard earthing switch is ensured, as are the unit positions “service/test/operate”.

Service conditions

The WU for K-3 (K-III) is designed for standard service conditions of 6(10) kV distribution switchgear:

• Environmental classification: per EN/IEC 60721-3-3 (indoor; on request - EN/IEC 60721-3-4 for outdoor). Typical operating temperature ranges from -25…-40 °C to +40…+55 °C depending on selected configuration and insulation materials.
• Altitude: up to 1000 m without derating; above this altitude, insulation and current ratings are corrected (addressed at design stage).
• Relative humidity: up to 95 % at 25 °C without condensation; if condensation is possible, anti-condensation heaters and moisture-protection measures are provided.
• Pollution degree: 2–3; for dusty/aggressive environments, sealing, high-durability coatings, anti-corrosion treatment, and higher IP ratings are applied.
• Seismic withstand: up to 8 MSK-64 (up to 9 on special order) with structural verification of WU guides and retainers.
• Overvoltage categories: compatible with surge arresters and the required impulse withstand between poles and to earth.

The WU is suitable for rooms with natural or forced ventilation. Heaters are used to prevent condensation; optional temperature and humidity sensors can be provided with signaling to SCADA upon request.

Layout and design features

The new withdrawable unit is engineered as a plug-in functional module with maximum compatibility to K-3 (K-III) guides and stops. Main assemblies:

• Switching device - vacuum (or SF₆, per project) circuit-breaker from leading manufacturers. Rated current options (630…1600 A, up to 2000 A on request) and breaking current (20…31.5 kA); E2/M2 classes; mechanical life up to 30 thousand cycles.
• WU frame - welded, with enhanced rigidity and own guides; provides precise height/depth alignment, travel limiters, stops, and jacks for racking.
• Plug-in contact system - adapted to existing K-3 (K-III) fixed parts. Contacts are copper-alloy with silver plating; adjustment pads and gauges are provided to verify “projection” and contact pressure.
• Shutter mechanism - independent double-sided metal shutters close phase bushings when the WU is withdrawn; optional upgrade to separate independent “phase–earth” shutters.
• Interlocks - mechanical and electrical interlocking devices: from key-transfer to pin coding; implemented inhibits against closing with shutters open/racking, withdrawal only with breaker open and earthing switch in “Open”.
• Secondary connector - multipin plug (64/80/110 contacts) with coding; allows “plug-to-plug” replacement without re-soldering when migrating to modern contact ranges.
• Operating mechanisms - spring-charged motor-operated as standard, electromagnetic optional; local/remote control, limit switches for “Test/Operate/Service” positions.
• Insulation - support insulators from tracking-resistant epoxy compounds; interphase barriers; field-shaping screens at the front panel.
• Options - position sensors, contact wear monitoring, operation counter, mechanism vibro-diagnostics, built-in current transformers (if needed).

Thermal design is considered: heat removal from breaker poles and contact joints is ensured; air fill factor meets calculated values; for currents ≥1250 A, the cross-section of current-carrying bars on the WU is optimized.

Retrofit options comparison

Safety

The retrofit project includes measures to enhance electrical and mechanical safety of personnel: additional screens and interphase barriers, interlock upgrades, use of voltage presence indicators (VPIS), and integration of key-transfer locks. Upon request, the cubicle can be upgraded to internal arc classification (IAC) per IEC 62271-200, types AFL, AFLR (front/side/rear) for specified current and duration (e.g., 20–31.5 kA, 1 s) - subject to the cabinet’s structural provisions (pressure relief ducts, reinforced doors and hinges, directed pressure exhaust). The enclosure IP level follows the specification (typically IP31/41 at the front, IP2X inside the compartment with shutters closed).

Manufacturing options

The WU for K-3 (K-III) is supplied for the required currents and interrupting capabilities and accepts vacuum breakers from various brands (e.g., ABB VD4, Siemens 3AH, Schneider Electric Evolis/VA, Tavrida Electric, Eaton W-VACi, etc.) - depending on customer preference, stock availability, and regional service support. Available options:

• Rated voltage: 6 or 10 kV (maximum 7.2/12 kV).
• Rated main-circuit currents: 630/1000/1250/1600 A (project-specific - up to 2000 A).
• Short-circuit breaking currents: 20/25/31.5 kA; thermal withstand up to 3 s, dynamic up to 80 kA peak (depending on breaker model).
• Breaker operating mechanism: spring-charged motor-operated (standard), electromagnetic (on request).
• Auxiliary circuits: 110/220 V DC, 100/220 V AC; interfaces “dry contacts”, Modbus/IEC-104 via protection & control IEDs.
• Additional equipment: measuring CTs/VTs on the WU, temperature sensors, operation counter, position sensors, arc-flash detection (fiber-optic/pressure), etc.

Compliance with standards

Design and manufacture follow the requirements and test methods of the following (up to six most relevant) standards:

1. IEC 62271-200 - Metal-enclosed switchgear and controlgear for >1 kV and ≤52 kV. Internal arc classification (IAC: AFL/AFLR).
2. IEC 62271-100 - AC circuit-breakers (medium voltage).
3. IEC 62271-102 - Disconnectors and earthing switches, including requirements for mechanisms and interlocks.
4. IEC 62271-1 - Common specifications for medium-voltage switchgear and controlgear.
5. IEC 60529 - Degrees of protection (IP Code).
6. EN/IEC 60721-3-3 / EN/IEC 60721-3-4 - Environmental conditions for stationary use (indoor/outdoor).

We offer documentation for KRU K-3 (K-III) withdrawable unit replacement (retrofit):

- Preliminary technical documentation for tender participation on KRU K-3 (K-III) withdrawable unit replacement. We will prepare the information needed to assess manufacturability in line with tender requirements and inquiry sheets.
- Working drawings, 3-D models, and other documentation necessary to manufacture the KRU K-3 (K-III) withdrawable unit at your facility. If you do not plan to manufacture parts in-house, we will help source them from partner shops. Final assembly and installation can be performed at your plant.
- All documentation can be tailored to the project requirements and to your facility’s manufacturing capabilities.
- If other manufacturers’ equipment is installed at the substation, we will prepare documentation for analogous equipment to complement what is in place.

Why work with us

- No need to maintain a large in-house engineering staff - you receive a complete documentation set that can be used by a mid-level engineer. 
- No need to build prototypes - our experience allows successful launch of serial production batches.
- Working to our documentation, your team will receive consulting on all nuances of replacing the KRU K-3 (K-III) withdrawable unit.

Engaging contractors, manufacturers, and investors

We are open to cooperation with electrical OEMs, machining/assembly shops, and investors interested in manufacturing withdrawable units and modular retrofit solutions for K-3 (K-III). Possible cooperation models:

• Manufacturing localization - production of frames, adapters, shutter mechanisms, and parts at your site to our design documentation; supply of critical assemblies (contact sets, connectors, interlocks) centrally.
• Machining & coating subcontract - we provide 2D/3D models, process requirements, and gauges; you perform machining, welding, painting, and testing.
• Complete “kit-to-assemble” supply - shipment of parts and assemblies; final assembly and commissioning at the end user’s site under our supervision.
• Engineering + manufacturing - full cycle: K-3 cell survey, 3D scanning, design package, manufacturing, supervision, commissioning, and staff training.

Documentation deliverables (formats)

A full design package and supporting materials are shared with project and production teams: drawings in DWG/DXF (AutoCAD), models in STEP/Parasolid, SolidWorks assemblies, BOMs, process sheets, secondary wiring diagrams, connector pin-out tables, instructions for contact alignment and commissioning, type-test and routine-test protocols. On request - CNC files, 3D-PDF for preview, inquiry sheets, and a compatibility matrix for specific K-3 (K-III) executions.

For additional information regarding KRU K-3 (K-III) withdrawable unit replacement (retrofit), please contact: inbox@proekt-energo.com

PDF - Download technical information on K-3 (K-III) withdrawable unit replacement