MV Switchgear Characteristics Overview: GIS vs. RMU

In medium voltage power distribution, Gas-Insulated Switchgear (GIS) and Ring Main Units (RMU) represent two essential categories of switching devices. each with a unique function within the power system. GIS, relying on gas insulation, is primarily used for higher voltage primary distribution, prized for its compact nature and reliability. Ring Main Units, on the other hand, focus on secondary power distribution applications such as ring networks, offering flexible design and a compact structure. Understanding the definitions, structural characteristics, and typical applications of these two types of equipment is essential for the design, operation, and maintenance of power systems.

GIS and RMU: Key Concepts and Structural Features

Gas-Insulated Switchgear (GIS), short for gas-insulated metal-enclosed switchgear, is categorized by the insulation of its main switching components, unlike air-insulated and solid-insulated alternatives. The primary circuit—switche, vacuum circuit breaker, and connection—is enclosed in a gas-filled tank. This sealing effectively isolates it from the environment, improving operational reliability. Solid insulation is typical for external main busbars and cable connections, enhancing insulation. MV current transformer is usually direct core-through installations on outgoing bushings or cables, providing a compact form and accurate sensing; epoxy post-type CTs can also be inside the gas tank, with external terminals for secondary circuits. Potential transformer connects reliably to the gas tank’s conical bushing interface via American standard cable heads. Thus, the key high-voltage primary and auxiliary circuits are mostly shielded from open air exposure.

The way Ring Main Units (RMUs) are defined is by their application in ring network power supply, also known as loop feeding, positioning them as important components in secondary power distribution. A ring main unit switchgear typically features two load break switches that connect the incoming and outgoing sections of the ring, enabling series operation of the lines and contributing to better power supply reliability. Along with the load break switch, fuse-switch combinations or circuit breakers function as transformer feeder switches, protecting against overloads and short circuits. For transformers up to 1250kVA, fuse-switch combinations are a common and economical protection choice. For larger transformers, above 1250kVA, circuit breakers are usually used for more advanced protection and control.

RMU: Typical Configuration and Electrical Characteristics

Typical accessories for Ring Main Units include Voltage Transformer (VT) cubicles, providing voltage signals and operating power for control and protection; Metering cubicles, used for high-side energy measurement, essential for electricity billing; and Direct Incoming Line cubicles, serving as the line entry point for easy cable or busbar connection.

Ring Main Units are generally designed with a 630A rated current to accommodate most distribution requirements. Their short-circuit breaking current is usually 20kA, providing a level of fault clearing suitable for secondary distribution networks. Although air-insulated metal-clad switchgear could technically be applied in ring networks, its larger physical size often presents challenges in meeting the need for compact equipment. RMUs are specifically engineered as compact switchgear for ring applications. Earlier RMUs used common tanks with oil or gas insulation and were often transformer-integrated for better miniaturization and easier installation/maintenance.

GIS in Primary Distribution Systems: Applications and Technical Characteristics

Gas-Insulated Switchgear (GIS) is now mainly applied in primary distribution, the main stage of power distribution. MV GIS typically features a wide rated current range, from 630 to 4000A, accommodating various substation capacities. Its short-time withstand current is also high, at 25-50kA, offering robust fault current handling. Compared to conventional air-insulated switchgear, GIS is significantly smaller and requires less space. Furthermore, its internal insulating gas and sealed design make it less vulnerable to environmental factors like humidity and pollution, improving operational reliability and safety. GIS frequently utilizes fixed-type vacuum circuit breakers known for their stable structure and excellent breaking capacity. On the busbar side, a three-position switch, offering isolation, grounding, and connection, is commonly implemented for flexible and reliable operation. These switches come in direct-acting and rotary configurations, with the direct-acting type being particularly advantageous for higher current applications due to its more uniform electric field, making it ideal for high-voltage, high-current GIS installations.

The Green Shift: Future of GIS and RMU

Sulfur hexafluoride (SF6) gas is extensively used in 40.5kV Gas-Insulated Switchgear for its superior insulation and arc-quenching capabilities. However, SF6 is a potent greenhouse gas with a global warming potential 23,900 times that of carbon dioxide, posing a significant threat to the atmospheric environment. Consequently, the reduction or complete substitution of SF6, termed SF6-free technology, has become an urgent priority in the development of GIS. Utilizing environmentally friendly gases like dry air and nitrogen as insulation offers a fundamental solution to the environmental problems associated with SF6, representing a crucial step towards a sustainable future.

Looking ahead, Gas-Insulated Switchgear with eco-friendly gas insulation, by sealing high-voltage components in the gas enclosure, not only becomes immune to environmental factors but also gains advantages like a compact structure and extended lifespan, indicating a key evolution direction for medium-voltage switchgear. In contrast, Ring Main Units in power distribution networks are trending towards flexible configurations, smaller sizes, and smart capabilities, enabling better integration with load centers and facilitating the creation of more efficient and reliable distribution automation systems.