Retrofit of the withdrawable (draw-out) unit for KRU K-26 (K-XXVI) is a modernization of an operating 6–10 kV medium-voltage switchgear with replacement of obsolete mechanical and electrical parts by modern solutions (vacuum or SF₆ circuit-breakers, adapted mechanisms, new interlocks) without civil works and without replacing the panel enclosure. This approach quickly increases switchgear reliability and safety, reduces O&M costs, and minimizes equipment downtime. Existing cable terminations, bus bridges, foundations and most metalwork are preserved, which is especially important where outage windows are limited and supply continuity requirements are high.

Purpose and application

The goal of the modernization is to extend the service life of K-26/K-XXVI panels and bring them in line with modern requirements for operational reliability, electrical safety and maintainability without full switchgear replacement. Replacement of the withdrawable unit addresses key tasks:

• Delivering the required switching capability in 6/10 kV networks with isolated or compensated neutral by using vacuum circuit-breakers with service life up to tens of thousands of operations, stable breaking capacity and increased pollution withstand.
• Reducing operational risks and costs by eliminating oil systems, cutting routine maintenance, unifying spare parts and using sealed-for-life vacuum interrupters.
• Improving personnel safety through modern interlocking systems, shutter mechanisms, voltage presence indicators, as well as options to meet the required LSC/PM/PI categories and to limit the effects of internal arcs.
• Unifying the circuit-breaker fleet - migration to common MV series (6/10 kV with spring-charged motor or electromagnetic mechanisms) widely available with established service support.
• Minimizing process downtime - retrofit kits are designed to fit K-26 “as-built” mounting points, aligned to guide rail geometry and contact systems, enabling replacement in short, staged outage windows.

Applications: utility and power plant substations, distribution points at large industrial sites (metals, chemicals, oil & gas, mining, cement and building materials), transportation (metro, railway substations), data centers, commercial real estate and public utilities. Retrofit is applicable wherever K-26/K-XXVI series panels and their modifications are operated, as well as panels from other manufacturers with compatible interface dimensions.

Technical data (typical ranges for K-26)

Environmental conditions

Retrofit panels retain the climatic category of the original switchgear (typically EN/IEC 60721-3-3 for indoor locations) and are intended for service at altitudes up to 1000 m above sea level. Typical temperature ranges: -5 °C to +40 °C (without heaters) and -25 °C to +40 °C (with heaters in the relay compartment). Permissible relative humidity up to 95% at 25 °C without condensation. For sites above 1000 m and/or with increased dust, vibration, aerosols and condensation, special-duty solutions are offered: increased creepage and clearance distances, moisture-protective coatings of live parts, sealed bushings, local heating and compartment ventilation, use of desiccants and directed airflow. The design package provides adjustments to insulation levels and protection settings considering reduced air pressure and non-standard climatic factors.

Electromagnetic compatibility and immunity are tailored to local conditions: presence of VFDs, arc furnaces, high inrush currents. Following site surveys, additional filters, ferrites, shielding of secondary wiring, as well as redundancy of AC/DC control-power circuits with fail-safe logic and battery diagnostics are incorporated.

Layout and engineering solutions

K-26 retrofit is delivered under one of three base scenarios, selected for the site conditions and outage duration:

• 1) Replace the circuit-breaker on the existing withdrawable unit. Supplied with an adaptation kit (brackets, adapter plates, operating shaft/coupling, rigid/flexible bus adapters), refurbishment and verification of mechanical interlocks, rearrangement/replacement of the secondary plug, overhaul of shutters and rollers. Pros - minimal supply cost and phased commissioning; cons - dependent on the condition of the old truck and guides, longer work time.
• 2) Full replacement with a new withdrawable unit. A new draw-out unit with installed breaker and pre-set interlocks, geometrically compatible with standard guides, tulip contacts and limit switches. The old unit may be preserved as a spare. Benefits - fast energization, assured assembly quality, minimal in-panel modifications, and fleet unification.
• 3) Factory-built module (“OneFit” / “plug-in”). Includes truck, breaker, shutters, racking mechanisms (worm/lever), drives, terminal blocks, nameplates and documentation. The module passes routine factory tests and provides unified interfaces for K-26, ensuring predictable installation time, repeatable quality and reduced commissioning work on site.

We use vacuum circuit-breakers from leading manufacturers (e.g., ZN63A/VS1, BB/TEL, BBR-10, Evolis, 3AH - as agreed with the project). Model selection depends on rated current and breaking capacity, operating mechanism (spring-charged motor/electromagnetic), required duty and environmental conditions. If required, current/voltage transformers, arc-flash sensors, protection panels, microprocessor IEDs, modernization of ATS schemes, SCADA/DCS interfaces, voltage presence indicators, as well as additional opto-signaling and programmable modular relays are provided.

Design upgrades include: restoration or replacement of mechanical interlocks (blocking racking with breaker closed, blocking closing when withdrawn, door interlocks), setting and synchronization of the shutter mechanisms with the truck travel, installation of guides and stops, overhaul of the contact system (tulips, blades), implementation of optical voltage presence indicators on feeders, and replacement of bus entries if needed. Marking is performed per engineering drawing standards and current safety rules; secondary wiring is traced and updated drawings issued.

Safety

The K-26 retrofit aims to enhance protection of personnel and equipment. Measures include:

• Refurbishment and/or installation of new mechanical interlocks with key interlocking (racking only with breaker open and earthing switch engaged, key locks on mechanisms and doors, service/test positions).
• Shutter mechanisms (metal or insulating partitions) that prevent access to live parts with the truck withdrawn, with partition class PM or PI depending on configuration.
• Indication and diagnostics: visual position indicators (ON/OFF, service/test, EARTHED), optical voltage presence indicators on feeders, earthing switch position monitoring, and (project-defined) hot-spot sensors on bolted joints.
• Internal arc mitigation (optional): pressure relief ducts, reinforced panels and doors, arc-guiding features in compartments, fast overcurrent/arc-flash protection (incl. optical sensors), and selection of accessibility and IAC categories to site requirements.

All solutions comply with applicable standards and manufacturer instructions. Target Loss of Service Continuity levels (e.g., LSC2B), partition class (PM/PI), and IAC parameters (A, AF, AFLR) are achieved within the constructional limits of the specific panel and room.

Supply options

Retrofit kits are available in several configurations to match equipment fleets and budgets:

• Basic - circuit-breaker replacement with a mechanical adaptation kit and interlock refurbishment. Intended where truck and guides are in satisfactory condition and repair time is limited.
• Standard - new withdrawable unit (truck) with installed breaker, shutter assembly and secondary connectors. Recommended for typical panels without significant legacy modifications.
• Extended - factory-ready turnkey module (truck, breaker, shutters, drives, terminal blocks, nameplates, factory test records). Minimizes site work and commissioning time; optimal for batch replacement of multiple feeders.
• Special - for harsh service (aggressive atmospheres, dust, vibration, altitude >1000 m, high humidity), with reinforced insulation, sealing, additional coatings and climate kits, plus protection & control adaptation to the process requirements.

Compliance with standards

Retrofit solutions and documentation are oriented to comply with current international standards, including (as applicable):

1. IEC 62271-200 - AC metal-enclosed switchgear and controlgear for rated voltages above 1 kV and up to and including 52 kV (LSC/PM/PI classification, compartment and internal arc requirements; latest editions).
2. IEC 62271-100 - Medium-voltage AC circuit-breakers (switching performance, duties T100s/T100a, service life).
3. IEC 62271-102 - Disconnectors and earthing switches (requirements for earthing switches and disconnectors).
4. IEC 62271-1 - Common specifications for medium-voltage switchgear and controlgear (service conditions, insulation levels).
5. EN/IEC 60721-3-3 - environmental classes for stationary use at weather-protected locations (indoor climatic categories).
6. IEC 60071-1/-2 - insulation coordination (as per project).

Economics, timing and typical risks

Practice shows that K-26 retrofit is significantly less expensive than full switchgear replacement with building works: no removal of existing cable terminations and bus bridges, preserved footprints and interfaces, reduced process downtime. Typical risks include hidden wear of guides/shutters and contacts, compartment contamination, legacy ad-hoc modifications, drawing inaccuracies. These are mitigated by a pre-project survey (field measurements, photo mapping), fabrication of jigs and templates, factory mock-up testing, site supervision/commissioning, and quality control with checklists. For batch deliveries, standardized kits are used to speed logistics and unify spares.

Comparison of retrofit options

Partnering and subcontracting

We invite switchgear component manufacturers, machining and metal-fabrication shops, protection & control integrators, design and commissioning companies, as well as investors interested in producing retrofit kits for the K-26 fleet. Possible formats include: licensed assembly to our documentation, cooperation for machining and painting, OEM partnerships for breakers, joint pilot projects at customer sites, localization of components and subsequent certification. We provide support at all stages - from equipment selection and specifications to organizing factory and site acceptance tests.

Documentation offered

- Preliminary tender package: supply scope, questionnaires, list of applicable standards, cable and connector schedules, single-line and functional diagrams.
- Working drawings, specifications, 3D models (STEP/Parasolid), assembly instructions, wiring guides, secondary connection diagrams (DWG/DXF), list of gauges, tooling and templates for assembly control.
- Package for manufacturing the withdrawable unit at your facility or via coordinated outsourcing: machining, bending, welding, coating, assembly, testing, packing and preservation. If required - selection of subcontractors for critical operations (powder coating, electroplating, insulating coatings).
- Adaptation of the documentation set to project requirements: dimensions of the specific panel, actual wear of guides and contacts, applied connectors and breaker type, protection & control (IED) requirements, DCS/SCADA and telecontrol, local industrial safety rules and electrical codes.

Why work with us

- No need to maintain a large senior engineering staff: you receive a complete documentation set enabling an intermediate-level engineer to execute the work.
- No prototypes required: our experience and modular unification allow you to start series production immediately after supervisor approval and trial assembly.
- We consult on all specifics of the KRU K-26 (K-XXVI) withdrawable unit retrofit, including breaker selection, contact adapters, secondary connectors, protection & control and testing.
- We assist with manufacturing cooperation (metalwork, coating, live parts, cable assemblies), supply templates, train assemblers and commissioning staff. For series deliveries, we provide safety stock of critical components.

For additional information on the KRU K-26 (K-XXVI) withdrawable unit replacement (retrofit), please contact: inbox@proekt-energo.com

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