On the Application of Ring Main Units in Urban Power Grids
With the continuous development and progress of society, significant changes have occurred in urban power grid lines, leading to the formation of numerous densely populated electricity load areas. Traditional power supply methods can hardly meet the needs of urban development. Consequently, more advanced and practical electrical equipment—the Ring Main Unit (RMU), also known as an outdoor compact switching station—has emerged. It offers advantages such as small footprint, flexible configuration, high power supply reliability, short installation and commissioning time, and low cost.
System Functions and Structure
Principle of Ring Main Units
A Ring Main Unit is the general term for load switchgear and combined apparatus cabinets used in ring main supply units. Utilizing modern electronic and sensor technologies, it integrates devices such as switchgear, circuit breakers, load switches, disconnectors, and metering instruments into one unit. This enables the integration and modular assembly of primary and secondary systems in urban grids, achieving monitoring, protection, control, and metering of power equipment, thereby optimizing management and improving grid operation levels and reliability.
System Functions
Modular Operational Functions: Modular units are established, including circuit breaker units, fuse-combination units with load switches, and load switch units. Within load switches, there are three-position switches that combine making/breaking, isolation, and grounding functions into one structure, equipped with interlocking devices to effectively prevent misoperations like grounding under load or closing onto a grounded circuit. There are also structures combining two-position switches (making/breaking and isolation) with an independent earthing switch.
Modular Environmental Adaptability: Depending on operational requirements, internal components like heating, cooling, dehumidification, and ventilation devices can be flexibly combined within the cabinet.
Modular Cable Connection: Cable plugs include silicone rubber pre-molded types and other forms, selectable according to different situations.
Modular Interlocking Devices: Electromagnetic locks used in conjunction with voltage indicators, along with various mechanical interlocking devices, ensure safety and reliability.
Distribution Network Structure and Layout
As equipment for receiving and distributing electrical energy, RMUs can be designed according to site conditions and flexibly connected in different schemes to accomplish various distribution tasks. They can also serve as automatic switching control equipment on distribution lines or as components in urban ring main power supply systems.
RMU Selection and Practical Application
Electrical Wiring Methods of RMUs
RMUs are composed of load switchgear and combined apparatus cabinets, featuring modular electrical wiring and operational functions. The electrical wiring methods of RMUs allow for flexible combination according to different circuit requirements, achieving ring main power supply with various functions and electrical energy distribution.
Selection of RMUs
RMU models are diverse, and their functions vary. Their primary role is receiving and distributing electrical energy, applicable for dual-power ring main supply and terminal supply. RMUs connect to cable trunk lines and branch lines through input and output terminals. Dual-power load switches enable ring main supply. RMUs with fuse protection, used in direct supply transformer feeder circuits, can prevent fault escalation tripping. High-voltage measurement devices can monitor energy distribution in the ring main circuit.
If RMUs are applied in automated lines, units with "Four Remote" functions (Tele-control, Tele-metering, Tele-indication, Tele-adjustment) can be configured. These consist of primary equipment (circuit breakers, load switches, etc.) and secondary equipment like protection devices, power supplies, monitoring systems, and automation software. Each RMU or switchgear is equipped with a Remote Terminal Unit (RTU) connected to the master control computer via communication interfaces and transmission lines (e.g., optical fiber or communication cables). This enables the RMU to quickly automatically detect line faults, automatically isolate the faulty section, and automatically restore power to non-faulty areas. Applications in urban power grids can select RMUs with corresponding functions for reasonable combination based on actual needs at different levels.
Practical Application
In recent years, the Changchun area has extensively adopted RMUs in its distribution network, making urban power grid energy distribution more flexible. In early 20XX, the Changchun municipal government carried out expansion and renovation of the Changchun Railway Station. Four nearby overhead lines needed to be converted to cable lines.
Based on the actual site situation, the lines requiring renovation were complex: the Liangshi Line needed interconnection with the Dongguang Line, and the Kaixuan Line needed interconnection with the Shengli Line. There were numerous, scattered branch lines and direct supply customers, making the site well-suited for applying RMUs for power distribution. Firstly, interconnection RMUs with metering devices were selected to achieve interconnection and metering for the lines. Secondly, four RMUs were selected and applied on the lines to achieve the power supply function. The RMUs were installed according to design requirements. After cable laying was completed, cable terminations were made and connected to the RMUs.
Post-Renovation Distribution Network Structure Diagram
The power supply method using RMUs can prevent outages on the main line caused by customer-side faults, reducing the outage scope and improving power supply reliability. If a fault occurs on the load side of an RMU, the fuse will blow, and the corresponding load switch will trip, disconnecting the faulty line from the main line without affecting the operation of the main ring network. After the fault is cleared, closing the load switch restores power. This facilitates outage maintenance; operations can be performed directly on the RMU by opening the corresponding load switch to de-energize that specific line for maintenance, while other line segments continue normal supply, thus minimizing the outage scope and improving power supply reliability.
RMU Maintenance
The modular design of RMUs, especially gas-insulated switchgear which is a fully enclosed system where all live parts and switches are sealed inside the enclosure, means the entire switchgear is unaffected by external conditions. This ensures operational reliability and personnel safety, achieving maintenance-free switch operation. If surge arresters are installed, preventive tests should be conducted annually.
The application of Ring Main Units in urban power grids not only brings new vitality to the development of urban grids and enables more flexible power supply but will also play an increasingly significant role in future urban power supply systems.
