Application of Intelligent Solid Insulated Ring Main Units (SIRMUs) in the Qinzhou Distribution Network
1 Introduction
Coastal regions face environmental challenges including heavy rainfall, lightning, high temperatures, high humidity, and high salt spray corrosion. To ensure the safety of urban distribution grids in such environments, minimize power outages caused by equipment maintenance, and prevent accidents due to equipment end-of-life and other factors affecting grid security, we have researched the use of a new type of Solid Insulated Ring Main Unit (SIRMU). This SIRMU offers long lifespan, maintenance-free operation, environmental compliance, intelligence, and improved power supply reliability.
Currently, over 90% of medium-voltage Ring Main Units (RMUs) rely on SF₆ gas as the insulating medium. SF₆ gas is chemically very stable, possesses excellent insulation and arc-quenching properties, and is widely used in power equipment. SF₆ gas-insulated switchgear is compact. However, SF₆ gas liquefies at low temperatures, reducing its insulation and arc-extinguishing capabilities. At high temperatures, it decomposes into highly toxic byproducts, posing significant risks to humans. Furthermore, leakage and emissions are inevitable during gas filling, operation, and recovery, making SF₆ a major environmental pollutant. SF₆ is internationally recognized as one of the six major greenhouse gases; from an environmental perspective, its use should be minimized or eliminated. The design principle of the Solid Insulated Ring Main Unit fundamentally eliminates the need for SF₆ gas, providing a foundational guarantee for emission reduction and environmental protection in the distribution network sector, aligning strongly with environmental requirements.
The Intelligent Solid Insulated Ring Main Unit (SIRMU) for distribution network extensions offers not only eco-friendliness and outstanding resistance to harsh environments, but also a high degree of intelligence. Its functionality encompasses all the features traditionally provided by both primary and secondary power system equipment within a single unit. The product adopts an integrated and modular design approach for both high-voltage and low-voltage components, offering excellent versatility and expandability, representing a true smart electrical device.
2 Technical Application of Intelligent Solid Insulated Ring Main Units (SIRMUs)
2.1 Application of Solid Insulation Technology
Solid insulation technology primarily involves encapsulating and sealing the live parts of the medium-voltage switchgear's main circuit with solid materials, or transferring the high electric field intensity into the interior of the solid insulating material. This allows the solid material to withstand higher potentials, thereby reducing the field strength in the air. The most common solid insulating materials are epoxy resin and silicone rubber.
The Solid Insulated Ring Main Unit (SIRMU) uses solid insulating material as the primary insulation medium. Key conductive circuit components like the vacuum interrupters and their connections, disconnecting switches, grounding switches, main busbars, and branch busbars are individually or collectively encapsulated in solid insulation, forming one or several modules. These modules are fully insulated, fully sealed, functionally specific, and designed for recombination and expansion. Human-accessible surfaces are coated with conductive or semi-conductive shielding layers and are directly and reliably grounded. Key characteristics include full insulation, full sealing, modularity, compact size, and intelligence. SIRMUs demonstrate significantly superior performance compared to SF₆ RMUs in resisting harsh environments like extreme cold, high altitudes, humidity, and strong wind/sand. Featuring fully sealed, waterproof design without exposed high-voltage conductors, they are particularly suitable for damp and flood-prone areas, capable of providing reliable power supply in damp or submerged conditions.
2.2 Application of Intelligent Protection Technology
The integration of microprocessor and computer technology into electrical equipment imbues it with intelligent functions while enabling bidirectional communication with control centers. This forms an intelligent monitoring, protection, and network management system. Through simple parameter configuration, a single intelligent device can conveniently perform the functions traditionally requiring several devices.
The structural design of the SIRMU aligns with the requirements of modern resilient smart grids. It employs rapid algorithms to give circuit breakers fast tripping capabilities, enabling quick isolation of faulty lines and effectively preventing cascading tripping. Equipped with a single-phase ground fault protection system, it enables online detection of such faults in ungrounded (or high-impedance grounded/PET/PEN) systems without needing additional configuration, providing alarms or trip commands according to set values. The main circuit employs single-phase modular design, which entirely avoids phase-to-phase short circuits during operation and significantly enhances maintenance efficiency while reducing costs. An integrated embedded operating system features a core CPU architecture, including an ARM processor (ARMP) for general processing and system control, alongside a powerful DSP for efficient multi-tasking, enabling high-speed data processing, control, and communication. The units are structurally compact, small in size, light in weight, and easy to install. Isolation gaps are visibly verified for safe operation. Automatic supply mode recognition enables flexible automatic changeover for dual power supplies, improving power supply reliability. Offering flexible communication support (GSM SMS, GPRS, CDMA universal wireless services, fiber, twisted pair, wireless, carrier), and supporting multiple communication protocols, facilitates easy implementation of distribution automation.
2.3 Maintenance-Free and Eco-Friendly
The main switch utilizes a maintenance-free vacuum circuit breaker known for stability and reliability, requiring no periodic maintenance. The integrated design of the disconnecting switch and grounding switch with the circuit breaker provides a compact structure with reliable mechanical and electrical interlocks, effectively preventing misoperation.
2.4 Key Manufacturing Processes and Quality Assurance for SIRMUs
To ensure production stability, consistency, and product quality—thereby guaranteeing the long-term insulation withstand voltage of SIRMU products—solid insulated poles utilize the Automatic Pressure Gelation (APG) process when made from epoxy resin. The manufacturing process requirements for SIRMUs exceed those for SF₆ gas-insulated RMUs. Insufficient production process control significantly increases the likelihood and severity of hidden insulation defects and faults compared to SF₆ units. Stringent quality control of raw materials and advanced process capabilities are essential.
The external insulation of its vacuum interrupter is achieved through multiple media, including epoxy, silicone rubber, and air. Therefore, the insulation strength of individual materials and the interfacial treatment between different insulating media are crucial. Both epoxy and silicone rubber offer excellent dielectric strength, reaching 5-6 times that of air. Achieving this strength requires strict control over the molding process—optimal temperature and pressure—along with ensuring material degassing and venting during molding to avoid entrapped micro-bubbles. Failure to do so not only reduces insulation strength but also causes uneven electric field distribution, increasing partial discharge (PD) activity and posing operational risks. Interfacial treatment ensures sufficient interfacial insulation strength under stress, thermal cycling, etc., preventing insulation failure. Reliable detection methods for interface quality are necessary. Currently, PD measurement, X-ray inspection, and lightning impulse tests can accurately assess interfacial insulation strength.
Employing X-ray inspection as a factory test prevents defects like bubbles, pores, or cracks in solid insulation assemblies and ensures vacuum interrupters, their connections/terminals, and solid insulated busbars show no visually apparent abnormal deformations in X-ray images relative to their positions. Performing PD measurements prevents PD inception and extinction in weaknesses or defects within the SIRMU's insulation caused by high electric fields, avoiding cumulative degradation leading to insulation breakdown over time.
3 Technical Design Scheme for the Intelligent SIRMU
Based on extensive distribution grid operation experience and analysis of advanced domestic and international equipment, considering practical grid operation conditions and environmental requirements, we determined through comprehensive comparison to develop and apply the "Application Research of 10kV Intelligent Solid Insulated Ring Main Unit" project.
3.1 Main Technical Content
New AVR-12 type Solid Insulated Ring Main Units (SIRMUs) are installed at the original site. A dual-power supply configuration is utilized, leveraging the SIRMU's intelligent functions for rapid isolation of fault lines, quick detection of single-phase ground faults, and transmission of all switch information via optical fiber to the Distribution Automation Master Station (DMS) or via SMS to maintenance personnel phones. Utilizing the master station monitoring platform enables data statistics and fault analysis.
This project is implemented in the following aspects:
- Based on the original RMU scheme and considering user importance, power is supplied via dual incoming lines. During normal operation, Incomer #1 supplies power while Incomer #2 is on hot standby.
- The main equipment is the AVR-12 type Ring Main Unit, consisting of 7 bays: 6 circuit breaker (CB) bays and 1 Potential Transformer (PT) bay.
- The 6 CB bays adopt a CB + Disconnector (DS) + Grounding Switch (ES) structure, equipped with integrated protection devices.
- The PT bay uses a DS + single-phase PT structure, providing communication power. The PT can be switched in/out using the DS in the PT bay, allowing PT or line maintenance without requiring a complete SIRMU outage.
- Using CB-type SIRMUs allows multiple interruptions of short-circuit fault currents. Compared to fuse protection, besides eliminating the inconvenience and waste of fuse replacement, they provide comprehensive short-circuit protection coverage through setting adjustments.
Normal Operation:
The intelligent controller acquires parameters (breaker status, disconnector status, primary current/voltage, zero-sequence current/voltage, DMS master station status) for each incoming/outgoing bay. Real-time data upload via optical fiber to the Intelligent Distribution Switch Master Station enables remote operation.
Short-Circuit Fault:
The intelligent controller's rapid protection feature detects fault current and power flow within 15ms and clears the fault within 25ms, ensuring normal line supply, preventing cascading tripping, and minimizing outage scope. Instantaneous characteristics (voltage, current, switch status) allow control over the permanent magnet actuator's action characteristic, shortening the fault clearing time to quickly isolate the fault line. Simultaneously, the controller transmits SIRMU action information via fiber to the monitoring center, allowing timely assessment of product status and facilitating fault location and power restoration.
Single-Phase Ground Fault:
The controller uses zero-sequence current magnitude/phase and zero-sequence voltage magnitude/phase to determine if a fault occurs on the SIRMU's load side. Only for ground faults on the load side does the controller issue a protection trip or alarm command and upload fault information via fiber. Faults on the source side do not trigger actions from this SIRMU's controller.
3.2 Main Technical Challenges
(1) Applying solid insulation technology and APG manufacturing process to ensure SIRMU reliability, long lifespan, and maintenance-free operation under high-temperature and high-salt-fog environments.
(2) Utilizing an integrated, flexible design concept combining the circuit breaker's high-voltage primary section with the secondary intelligent control section. Built-in combined current/voltage sensors enable real-time acquisition of primary voltage/current signals and switch status for flexible control of the breaker's actuator, optimizing vacuum interrupter operation.
(3) Rapid detection of fault current and execution of protection actions.
Project Implementation:
This project employs solid insulated RMUs with integrated intelligent control devices. The main switch uses a combination device comprising a Permanent Magnet Actuated Vacuum Circuit Breaker (VCB) with integrated Disconnector (DS) and Grounding Switch (ES). Equipped with an intelligent control system, it performs automatic control functions even without DMS coordination. The permanent magnet actuator offers high reliability and maintenance-free operation. Optical fiber communication enables real-time collection of all RMU operating parameters, simplifying remote power restoration and power fault analysis, and accumulating experience for smart grid terminal devices. An external stainless steel cabinet minimizes future maintenance.
4 Conclusion
The Intelligent Solid Insulated Ring Main Unit (SIRMU) is a safe, eco-friendly, stable, reliable, and highly intelligent next-generation RMU. It is particularly well-suited for deployment in harsh locations characterized by high altitude, extreme temperature variations, contamination, and humidity.
