RETROFIT is a modernization or partial re-equipment of complete switchgear (KRU) and single-sided service switchgear cubicles (KSO) by replacing life-expired equipment with contemporary solutions. For a range of 6–10 kV substations and switchgear, the best effect is often achieved by replacing the most worn and at the same time most critical component - the oil (low-oil) circuit breaker - with a vacuum (or SF₆) circuit breaker with spring-stored energy or electromagnetic operating mechanism. Unlike oil devices, modern breakers do not require routine oil management, provide higher switching endurance, and simplify operation. In parallel, as needed, interlocks, shutters, earthing-switch drives, secondary circuits, and door panels are upgraded. Retrofit preserves busbars, foundations, cable entries, and mounting interfaces, cutting budget and downtime without compromising reliability.

Purpose and application

The proposed solution replaces the withdrawable unit (WU) of KRU series K-37 with a new WU compatible in overall dimensions and connection interfaces with the existing cubicle. The solution applies to 6 and 10 kV, 50 Hz medium-voltage switchgear at power plants, utility substations, industrial and infrastructure sites, as well as agricultural and extractive facilities. Retrofit may be performed either to maintain serviceability of existing cubicles when breaker life is exhausted or as part of programs to improve operational safety, reduce O&M costs, and digitalize secondary systems.

The upgraded withdrawable unit with a modern circuit breaker supports normal switching operations (close/open, automatic reclosing, protection and control) under normal and faulted network conditions, including short circuits and overloads. The design is suitable for networks with isolated or compensated neutral. Thanks to standardized primary contact adapters and a compatible secondary plug, full electromechanical compatibility is ensured with most K-37 variants without modifying the busbar compartment or cable terminations. In some cases, modernization is complemented by new current/voltage transformers, arc-flash protection, modern P&C relays, and condition-monitoring sensors.

Technical data (typical)

Operating conditions

The retrofit kit is intended for operation in indoor switchgear rooms and/or outdoor cabinets that meet applicable regulations. Environmental classes - EN/IEC 60721-3-3 / EN/IEC 60721-3-4 (per project); ambient temperature typically from -40 … -45 °C to +40 … +45 °C; relative humidity up to 95 % with anti-condensation measures (heaters, ventilation, drain channels). Permissible installation altitude - up to 1000 m a.s.l. without derating; for higher altitudes, dielectric and thermal calculations are performed additionally. Degree of enclosure protection for “indoor” execution - not less than IP31/IP34; for “outdoor, insulated” execution - up to IP54 (per project). In high-condensation and polluted environments (e.g., pump rooms, chemical plants, coastal areas), measures are provided for insulation classification and protection, anti-condensation heating, and microclimate control.

Seismic resistance, vibration strength, fire resistance of cable entries, and categories for pollution/condensation are selected according to the original K-37 cubicle data and project requirements. For outdoor cabinets additionally specified: insulation package, roof/canopy design, cable glands, UV-resistant coatings, and corrosion protection of structures and fasteners (hot-dip galvanizing, powder coatings).

Layout and design solutions

The baseline layout of the new withdrawable unit for K-37 retains the original logic: trolley (frame) with circuit breaker, primary plug-in contacts, racking mechanism along guides, a set of mechanical and electrical interlocks, shutter mechanism, mating part of the secondary connector, and operator controls on the front panel. The design uses:

• a modern vacuum (or SF₆, per project) circuit breaker with spring-stored energy or electromagnetic mechanism, endurance class E2/M2 per current norms;
• primary-contact adapters providing geometric compatibility with the K-37 busbar and cable fixed parts without buswork modification;
• a universal secondary connector (or adapter strip set) for connection of existing control, indication, and protection circuits;
• a feeder earthing-switch mechanism (if provided by the cubicle design) with operating handle, interlocks, and position indication;
• shutter devices that automatically close the primary sockets when the trolley is withdrawn, and door mechanical interlock;
• a kit of nameplates, pictograms, viewing windows, and inter-bay/inter-compartment interlocking devices (per project).

Three modernization levels are available for typical K-37 modifications (by effort and implementation time):

1. Adapter kit for the existing WU - breaker replacement and adaptation of interlocks/secondary circuits on the stock trolley. Lowest cost, but higher labor and duration; precise adjustment required.
2. Complete replacement of the withdrawable unit - supply of a new WU assembly fully compatible in mounting, dimensions, and interlock logic. Fastest commissioning; the original oil-breaker WU may be retained as a spare.
3. Modular “plug-in” block (factory retrofit module) - an integrated readiness module with built-in shutters, interface frame, standardized contacting system, and interlocks. Maximum unification and improved clearances; fastest installation; higher initial cost.

Following site survey and review of the as-built documentation, the optimal option is selected. If required, the front panel is revised, new mating parts of the secondary connector are installed, limit switches and internal panels are replaced/relocated. The design accounts for CT/VT locations, cable window sizes, service areas, and accessibility of operating mechanisms.

Safety

The retrofit concept maintains or improves the operational safety level compared to the original K-37 configuration. Mechanical and electrical interlocks prevent mal-operations (no WU movement with breaker closed, no closing with shutters open, breaker–earthing-switch interlock, door interlock with feeder energized). Fixed positions “service/test/connected” are provided, with position indicators for main devices, provisions for sealing and hasps for padlocks. On request, arc-flash protection systems (optical sensors/current criteria) and microclimate monitoring/event logging devices can be applied.

For cubicles with higher personnel-safety requirements, solutions can be engineered in line with modern principles of internal arc classification (IAC) and LSC categories for service continuity. In existing cubicles, the degree of conformity is determined by the enclosure design, gas exhaust, pressure-relief flaps, and reinforced doors. During retrofit, additional exhaust ducts and reinforced body elements may be installed (based on calculations and tests).

Manufacturing options and features

• Circuit breakers: vacuum types from industrial manufacturers (ratings 6/10 kV; currents 630–1600 A; by agreement up to 2000/2500 A; breaking capacity 20–31.5 kA); alternatively, specified SF₆ breaker series.
• Operating mechanisms: spring-charged motor operators (with emergency manual charging) or electromagnetic; manual operating handle for earthing switch; local/remote control available.
• Secondary circuits: secondary connector compatible with the original scheme; adapters for existing harnesses; upgrades to AR/ATS schemes, tele-signals, and local indication.
• Protection & services: protection and control relays (instantaneous/delayed overcurrent, earth-fault, directional stages, breaker-failure protection), arc-flash protection, recorders, integration via IEC 61850/Modbus (per project).
• Materials & execution: galvanized or stainless parts, powder coatings, anti-condensation heaters, temperature/humidity sensors.
• Documentation: 3D models, design drawings, BOMs, installation/commissioning/operation manuals, check protocols, and test procedures.

Comparison of typical retrofit approaches

Standards compliance

The retrofit solution is engineered in accordance with current requirements for medium-voltage switching devices and metal-enclosed switchgear. The project specifies applicable standards and boundaries of responsibility (device/WU/cubicle). When selecting the breaker and accessories, the following documents are considered (the list is refined for a specific site):

• IEC 62271-1 - Common specifications for medium-voltage switchgear and controlgear;
• IEC 62271-100 - AC circuit-breakers (endurance classes, breaking capability, tests);
• IEC 62271-102 - Disconnectors and earthing switches (mechanical requirements and interlocks);
• IEC 62271-200 - Metal-enclosed switchgear 1–52 kV (LSC/IAC classification for new/modernized cubicles);
• IEC TS 62271-304 - Guidance for operation under condensation/pollution in indoor environments;
• EN/IEC 60721-3-3 and EN/IEC 60721-3-4 - Environmental condition classes for stationary use (weather-protected / non-weather-protected).

Partnering with manufacturers and contractors

We welcome cooperation with electrical OEMs, machine-building plants, and engineering firms for series production of withdrawable units for K-37. Possible formats: supply of a complete WU set under the customer’s nameplate, supply of “semi-finished” assemblies (frames, adapters, front panels, shutter mechanisms), delivery of a design package with author support during production start-up, as well as turn-key projects - design, manufacturing, commissioning, and site energization. For investors and integrators, entering the service market for the installed base of KRU K-37 and related series (K-V, K-VI, K-IX, etc.) is attractive with minimal R&D thanks to ready-made typical solutions, libraries of 3D models, and established process charts.

Documentation offered

The typical package includes: working drawings (DWG/DXF), bills of materials, operation documentation, 3D models (STEP/Parasolid), front-panel layouts, assembly schematics and incoming inspection sheets, transport/storage/installation/commissioning instructions, type and routine test protocols. Documentation is adapted to tender requirements and your production capabilities (sheet metal thicknesses, available coatings, fastener range, etc.). Upon agreement, editable sources for AutoCAD/SolidWorks/Compass-3D are provided for quick PLM integration.

We offer documentation for replacement of the withdrawable unit in KRU K-37 (retrofit):

- Preliminary technical documentation for tenders to replace the withdrawable unit in KRU K-37. We will prepare the information you need to assess manufacturability to meet tender requirements and questionnaires.
- Working drawings, 3-D models, and other necessary documentation to manufacture the K-37 withdrawable unit at your facility. If you do not plan to manufacture parts and assemblies in-house, we will help outsource fabrication. Final assembly and wiring will be performed at your site.
- All documentation is adjusted, if necessary, to project requirements and to your plant’s manufacturing capabilities.
- If other manufacturers’ equipment is installed at the substation, we will prepare documentation for analogous equipment to supplement the installed base.

Benefits of working with us:

- No need to maintain a highly specialized engineering staff - you receive a complete documentation set; an engineer of average qualification can work with it. 
- No need to build pilot prototypes - our experience enables successful launch of serial production batches.
- Working to our documentation, your specialists will receive consultation on all nuances of replacing the withdrawable unit in KRU K-37.

For additional information on replacing the withdrawable unit in KRU K-37 (retrofit), please contact: inbox@proekt-energo.com

PDF - Download technical information on K-37 replacement