What’s the Difference Between KYN28 Switchgears and RMUs?

The KYN28 switchgear and the ring main unit (RMU) are distinct concepts. Although both fall under the definition of medium-voltage metal-enclosed switchgears as per GB/T 3906 and IEC 62271-200 standards, they are categorized differently based on application, insulation medium, and structural design.

I. Structural Characteristics of KYN28 Switchgears

The KYN28 switchgear is defined by its mechanical structure. Categorized as an air-insulated switchgear (AIS) with a metal-enclosed withdrawable circuit breaker, it evolved as a successor to the traditional floor-standing design. In floor-standing units, the breaker truck moves along the ground and is pushed directly into the enclosure. In contrast, the medium voltage metal clad switchgear design is more compact and optimizes space utilization. The circuit breaker truck is positioned in the center-front section of the cabinet, leaving the lower compartment available for auxiliary functional trucks, such as potential transformer (PT) or surge arrester.

Significant differences exist between KYN28 units and RMUs regarding space efficiency. While KYN28 switchgears have optimized layouts, they are still constrained by the clearance requirements for air insulation. For a standard 630A/20kA configuration, a KYN28 cabinet typically measures 800mm (W) x 1400mm (D) x 2200mm (H), whereas a compact RMU can be as small as 350mm (W) x 750mm (D) x 1750mm (H).

In a KYN28A-12 configuration, the circuit breaker truck is elevated from the floor (often referred to as a “mid-mounted” design). Moving the breaker in or out requires a dedicated transfer trolley. For maintenance, the breaker is often hoisted from the trolley to the floor, a step not required by floor-standing models.

II. KYN28 Switchgear: Component Miniaturization and Technical Advantages

The emergence of KYN28 structures is a direct result of the miniaturization of vacuum circuit breakers. Early floor-standing units used frame-based structures that were highly sensitive to uneven installation foundations. Any floor irregularity could cause cabinet deformation, making it extremely difficult to rack the breaker truck in or out. More critically, deformation led to misalignment between the primary contacts and busbar stabs, causing overheating and potential catastrophic failure. KYN28 switchgears utilize wall-mounted guide rails, allowing the breaker to move within a precision-aligned track, effectively mitigating the impact of cabinet deformation and ensuring reliable electrical contact.

III. KYN28 Switchgear: Technical Specifications and Primary Distribution Applications

KYN28 switchgears are predominantly used in industrial enterprises and urban power grids as primary distribution equipment. They typically feature rated currents from 630A to 5000A and short-time withstand currents from 25kA to 50kA. While often implemented as air-insulated switchgears (AIS), they can also utilize gas-insulated switchgear (C-GIS) solutions featuring fixed vacuum circuit breakers with three-position disconnectors/earthing switches. As primary distribution units, they serve as the main power hub, feeding down to secondary distribution systems.

IV. Ring Main Modes and Secondary Distribution Applications

The ring main unit switchgear is defined by its application in power distribution. A “ring main” is a typical secondary distribution topology that forms a closed loop, often described as a “two-in, one-out” configuration. The loop is energized by two power sources but operates in an open-loop configuration. If a fault occurs in any cable or equipment segment, that segment can be isolated by opening the adjacent switches, allowing the rest of the ring to remain energized and ensuring high power reliability. RMUs generally operate at a rated current of 630A with a short-time withstand current of 20kA. They usually employ load break switches for loop control and “load break switch + fuse combinations” or circuit breakers to protect outgoing transformers.

V. Mainstream Insulation Technologies for RMUs

Air-insulated, gas-insulated, and solid-insulated switchgears can all be utilized in ring main applications:

1. Air-insulated RMUs: These typically feature SF6 load break switches housed within an enclosure, while the busbars, fuses, or circuit breakers are installed in open air. Cables are connected using standard cable lugs. A classic example is the Schneider SM6, which uses SF6 load break switches with epoxy resin encapsulated gas tanks. Typical cabinet widths are 500mm for switch units and 750mm for breaker units.
2. Gas-insulated switchgears (GIS): In these units, the load break switches, busbars, and breakers are hermetically sealed within a stainless steel tank filled with SF6 gas. The main busbars are connected via lateral connectors or top-mounted solid busbars, while external cables utilize European-standard bushings and plug-in accessories. The high-voltage circuit is completely isolated from the environment.
3. Solid-insulated RMUs: These utilize three-position switches paired with vacuum load break switches or circuit breakers. The high-voltage components are fully encapsulated in solid insulating materials. Aside from the enclosed air gaps at the isolation and earthing points, all live parts are shielded from external contact.

VI. Summary

In conclusion, KYN28 switchgears and RMUs play distinct roles within the power distribution ecosystem. KYN28 units, with their superior breaking capacity and the maintenance convenience of a withdrawable structure, are the preferred choice for primary distribution and high-load industrial applications. Conversely, RMUs are indispensable in secondary distribution due to their flexible networking, compact footprint, and high environmental resilience. Engineering selection should balance reliability and cost-effectiveness based on load importance, available installation space, and emerging environmental trends.