Compact Air-Insulated RMUs for Retrofit & New Substations

Air-insulated ring main units (RMUs) are defined in contrast to compact gas-insulated RMUs. Early air-insulated RMUs used vacuum or puffer-type load switches from VEI, as well as gas-generating load switches. Later, with the widespread adoption of the SM6 series, it became the mainstream solution for air-insulated RMUs. Similar to other air-insulated RMUs, the key difference lies in replacing the load switch with an SF6-encapsulated type—where the three-position switch for load and grounding is installed within an epoxy resin cast insulated housing.

SF6 serves as both the arc-quenching and insulation medium. Air-insulated RMUs typically employ SF6 or vacuum load switches, helping reduce the use of greenhouse gases, and offer convenient installation and maintenance. Although their use has declined domestically, they remain widely applied internationally. Circuit breaker solutions utilize SF6 or vacuum circuit breakers arranged on the side, providing flexible installation options.

Utilities face the challenge of significantly reducing the frequency and duration of power outages and improving the operational continuity of medium-voltage secondary distribution networks. Customers acknowledge that with the increasing penetration of distributed generation, ensuring power supply continuity, reducing energy losses, and enhancing overall grid loading capacity have become critical. To address these challenges, customers recognize the need to invest in medium-voltage grids to improve data acquisition and enable seamless communication between substations and control centers.

Customer Benefits:

• Flexibility to apply different levels of automation across various smart grid zones.

• Cost savings by using Remote I/O RIO600 devices without replacing legacy equipment in main substations.

• Improved reliability, reflected in better System Average Interruption Duration Index (SAIDI) and System Average Interruption Frequency Index (SAIFI).

• The HBC circuit breaker panel is only 500 mm wide and includes all protection, communication, and automation equipment, making it ideal for existing substations with limited space.

ABB offers a new recloser solution for secondary substations based on UniSec switchgear, equipped with the multifunctional HySec device that integrates the circuit breaker and disconnector into a single compact unit. The advanced REC615 protection relay is used for control, monitoring, and protection of cable feeders via IEC 61850 and IEC 60870-5-101/104 protocols, providing excellent fault detection and location capabilities. To enhance reliability and reduce outage duration and frequency, ABB's solution is built on logical selectivity between protection relays, an effective method for minimizing the affected area during faults. Furthermore, to improve grid selectivity, ensure continuous power delivery, and reduce energy losses, high-precision current and voltage sensors provide extremely accurate measurements to the relays.

In ABB's solution, the recloser can operate at three different automation levels. To activate the appropriate automation level for a specific section of the medium-voltage grid, the operator simply selects the corresponding configuration option in the REC615 software. To ensure power system reliability and performance, the solution relies on GOOSE-based peer-to-peer communication technology. GOOSE (Generic Object Oriented Substation Event) is part of the IEC 61850 standard for power system automation.