10kV ring main unit

The 10kV ring main unit is a critical piece of equipment in power distribution systems. Its primary role is to stably transmit high-voltage electricity at 10kV or 35kV to end users. Additionally, it continuously monitors the transformers within the power system and controls them to ensure their normal operation. Due to these important tasks, the 10kV ring main unit features a high voltage level and large capacity, yet it is compact, lightweight, and easy to use. Next, we will take a closer look at the 10kV distribution ring main unit and explore all aspects of it.

 

 

1.1 What is a Ring Main Unit?

A Ring Main Unit (RMU) is an electrical device consisting of a group of high-voltage switchgear assembled within a sheet steel metal enclosure or configured as a modular,ring main supply unit. Its core components utilize load switches and fuses, offering advantages such as a simple structure, small size, low cost, improved power supply parameters and performance, and enhanced power supply safety.

In fact, if switchgear types are classified by switching devices, there are load switch cabinets, circuit breaker cabinets, GIS (Gas-Insulated Switchgear), etc., and "Ring Main Unit" is not a standard category. The term "Ring Main Unit" is a conventional name. Originally, it referred to load switch cabinets used in ring main power supply systems. Nowadays, it is often used as a synonym for load switch cabinets, regardless of whether they are actually used in a ring main configuration. For example, distribution cabinets that enable functions like loop closure operation (commonly known as "hand-in-hand" configuration) or potential load transfer (shifting load from one line to another) are called Ring Main Units. Therefore, an RMU is a switching device capable of implementing ring main power supply, commonly used in ring main supply systems, hence the popular name "Ring Main Unit," also known as "outdoor box-type ring main unit" or "sectionalizer."

Load switch cabinets can be used for ring main power supply, medium voltage demarcation point branching, and medium voltage end substation power supply. Power supply authorities in different regions have varying specifications regarding the transformer capacity allowed for protection using load switches combined with fuses. For instance, in Beijing, the transformer capacity is generally required not to exceed 1000 kVA, while in the Shenzhen area, it might be 1600 kVA. Load switch cabinets have a simple structure, low cost, and small size. Most can be installed against a wall and typically only have fuse protection without relay protection.

The rated current of the main busbar in a load switch cabinet is generally ≤ 630A. The rated interrupting current of the load switch itself is generally ≤ 630A (a few models reach 1700A). The rated current (fuse rating) for transformer feeder cabinets (outgoing feeder cabinets) is generally ≤ 125A. The transfer current for high-end load switches can reach 2800A. Some load switch cabinets can accommodate dedicated vacuum circuit breakers, SF6 circuit breakers, or use arc-extinguishing methods like puffer type, achieving short-circuit breaking capacities close to or equivalent to switchgear levels. Ring Main Units are used for switching load currents, interrupting short-circuit currents, transformer magnetizing currents, and the charging currents of overhead lines or cables over certain distances. They serve control and protection functions and are crucial switching devices for both ring main and radial-end power supply systems.

 

1.2 Classification

Ring Main Units can be classified based on several criteria:

Gas Tank Structure: Common Enclosure type and Unit type.

Overall Structure: American-Style and European-Style.

Insulation Material: Solid Insulation type, Air Insulation type, and SF6 Gas Insulation type.

Installation Location: Indoor Ring Main Unit and Outdoor Ring Main Unit.

1.2.1 American-Style Ring Main Unit
Designed for outdoor use, compact size, maintenance-free.
Features a robust inner and outer casing with strong resistance to external forces and explosion-proof capabilities. The enclosure is made of stainless steel or a green-painted box.
Incoming and outgoing cables are connected via fully pre-cast, pre-molded cable connectors, ensuring simple installation and reliable connection. Both American and European standard cable accessories can be used.
Adopts the traditional American common-enclosure design, where multiple incoming and outgoing circuits are housed in a single tank, offering a compact structure and high reliability.
Flexible switching configuration combinations.

1.2.2 European-Style Ring Main Unit

1.2.2.1 Common Enclosure European Ring Main Unit
The housing of the ring main unit consists of independent functional compartments. The main busbar, load switch, and combination unit each have their own compartment. The gas-filled compartment of the ring main unit is welded from 3.0mm stainless steel plate using argon arc welding. The load switch and busbar connections are completely sealed within SF6 gas. Offers flexible combination and easy extension.

1.2.2.2 Modular Tank (Gas Compartment) European Ring Main Unit
Features a unique switch design, utilizing a load switch housing made of stainless steel to divide the cabinet into different compartments. Allows for flexible combinations and convenient expansion. The load switch is sealed within SF6 gas. The housing is manufactured from stainless steel using cold forging process technology, ensuring high product quality.

 

1.3 Basic Components
A Ring Main Unit (RMU) generally consists of four parts: the switch compartment, the fuse compartment, the operating mechanism compartment, and the cable compartment (undercarriage).

The switch compartment houses various functional circuits (including earthing switches and load switches) sealed within a metal enclosure, along with the busbars connecting these circuits. The enclosure is welded from 3mm cold-rolled steel sheet (or stainless steel sheet). Each functional circuit includes one load switch and one earthing switch. The load switch consists of a vertically moving contact system and a stationary contact located at the lower end. When the switch is closed, the moving contact moves downward, completing the circuit. The earthing switch comprises a moving contact blade and a stationary contact blade; it closes rapidly during the spring operation. Four rectangular assembly process holes are located on the upper and rear parts of the switch compartment. An observation window is installed on the front of the RMU, allowing visual confirmation of the earthing switch's "open" or "closed" position. An explosion-proof device is fitted at the rear of the RMU.

The load switch utilizes a puffer type internal arc-quenching structure, providing strong arc-extinguishing capability without affecting phase-to-phase or phase-to-ground insulation. Both the moving and stationary contacts are equipped with arcing contacts, significantly increasing the number of allowable switching operations.

The fuse and load switch compartment form the transformer protection circuit. The high-voltage current-limiting fuse is housed within an epoxy resin cast insulation housing. When the fuse blows, it ejects a striker, triggering the opening of the load switch.

The operating mechanism compartment is located on the front panel of the RMU. For each functional circuit, the load switch is equipped with a manual (or electric) spring-charged operating mechanism, and the earthing switch is equipped with a manual spring-charged operating mechanism. The panel features a rotary knob for load switch closing operation and manual opening, as well as operating holes for opening and closing the earthing switch. It also includes indicator lights for the load switch open/closed position, electric open/close buttons, a mimic diagram, switch status display flags, and locking positions. The operation of the load switch and earthing switch is interlocked to prevent maloperation.

 

1.4 Model Designation
The model designation for a Ring Main Unit is generally defined as follows:

XGW/N□-12 (F or F · R)/630-20

Where:

X -- Enclosed (Switchgear)

G -- Fixed

W -- Outdoor

N -- Indoor

F -- Main switch equipped with a load switch

F · R -- Main switch equipped with a load switch-fuse combination

12 -- Rated voltage (12 kV)

630 -- Rated current of the main busbar (630 A)

20 -- Rated short-time withstand current (20 kA)

 

1.5 Main technical parameters

 

For example:Jinneng electric XGN口-12  Ring Main Unit Switchgear:

Item			Unit	Load Switch	Combined electric unit	Breaker unit
Rated voltage			kV		12/24
Rated frequency			Hz		50/60
Reted current of main busbar			A		630,1250
Rated curret			A	630	125	630,1250
Rated short-circuit breaking current			kA		31.5	20,25/30
Rated short circuit making current (peak)			kA	50	80	50,63
Rated short-time withstand current		Main loop 4s ground loop 4s	kA	20		20,25
		Ground connection circuit 4s		17.4		17.4,21.7
Rated peak withstand current		Main loop ground loop	kA	50		50,63
		Ground connection circuit		43.5		43.5/54.6
Rated transfer current			A	1750(12kV)/1400(24kv)
Ability to break and close				E3 level		E2 level
Rated insulation level	power frequency withstand voltage	Phase-to-ground/phase-to-phase	kV	42(12kV)/65(24kV)
		Vacuum gap/isolation gap		48(12kV)/79(24kV)
	Lightning impulse withstand voltage (peak)	Phase-to-ground/phase-to-phase		75(12kV)/125(24kV)
		Vacuum gap/isolation gap		85(12kV)/145(24kV)
Auxiliary circuit 1min power frequency withstand voltage			kV	2
Rated single/back-to-back capacitor bank breaking current			A			400
Rated capacitor banks closing inrush current			kA			20(50Hz)
Mechanical life		Load switch, combination switch	Times	5000/3000	5000/3000
		Circuit breaker, isolation switch				10000/3000
SF₆ gas rated pressure (20°C relative value)		Design Level and Rated Filling Level	Mpa	0.04
		Minimum functional level		0.02
		Pressure release device operating pressure		0.14
Protection class		Sealed gas compartment		IP67
		Switchgear housing		IP4X
Sealing performance				Annual leakage rate≤0.01%

Note:above parameters are typical value,our company reserves the right of final interpretation.pls contact us

 

2.1 Working Principle and features

 

Ring Main Network
A ring main network refers to a radial distribution network configured in a closed loop. The power supply feeds into this loop, and power is distributed from the loop to outgoing feeders via high-voltage switchgear. The advantage is that each distribution branch can draw power either from the left side of the loop or the right side. If a fault occurs on the left side, the branch continues to receive power from the right side, and vice versa. Thus, even with a single power source feeding the loop, each distribution branch benefits from a level of redundancy similar to dual-power supply, enhancing power supply reliability.

Concept of "Ring Main Unit" (RMU)
A "Ring Main Unit" is the switchgear cabinet installed for each distribution branch (outgoing feeder cabinet). The busbar within this cabinet is simultaneously a part of the ring main itself. In other words, the ring main is formed by connecting the busbars of each outgoing cabinet in series. Each individual cabinet is called a "Ring Main Unit." The concept of a "ring" is only apparent when multiple units are connected together; a single unit viewed in isolation does not inherently show its role in a ring.

These RMUs typically have low rated currents. Therefore, instead of complex circuit breakers, the high-voltage switches used in RMUs are usually simple high-voltage load switches combined with high-voltage fuses. Essentially, the main switching device in an RMU is a load switch. It handles normal load currents, while the fuses interrupt short-circuit currents. This combination replaces the function of a circuit breaker, albeit within certain capacity limits.

Such switchgear can also be used in non-ring distribution systems. Due to their widespread application, the term "Ring Main Unit" has evolved beyond its strict ring-network definition and is now commonly used to refer to any high-voltage switchgear cabinet that uses a load switch as the main switching device.

Typical Configuration and Components
An RMU typically consists of three compartments: two for ring cable incoming/outgoing and one for the transformer circuit.

SF6 Load Switch: Recognized as an excellent load switch. Beyond long electrical life and strong breaking capacity (similar to vacuum load switches), its key advantages include easy realization of three-position operation (connected, disconnected, and earthed) , capability to interrupt small currents (inductive, capacitive), and strong resistance to harsh environmental conditions. It is suitable for widespread application in urban and rural medium-voltage distribution networks.

Load Switch Function: A load switch is a switching device with a simple arc-extinguishing mechanism. An SF6 load switch uses SF6 gas for insulation and arc quenching. It is used for making and breaking load currents, overload currents, as well as energizing and de-energizing no-load lines, no-load transformers, and capacitor banks. Three-position load switches (with connected, disconnected, and earthed functions) feature a simple structure and low cost. However, a load switch cannot interrupt short-circuit currents.

SF6 Load Switch + Fuse Combination
This combined apparatus uses the SF6 load switch to handle making and breaking normal load currents and overload currents (within its rated interrupting capacity), including the interruption of "transfer currents." The high-voltage current-limiting fuses handle short-circuit protection and overload protection on the transformer's high-voltage side. In this setup, with one SF6 load switch and three fuses equipped with strikers, operation of any striker triggers a mechanism that automatically trips all three phases of the load switch simultaneously.
This combination effectively meets the operational and protection requirements of distribution systems under various normal and fault conditions.

Types of RMUs
RMUs can be categorized into indoor and outdoor types:

Indoor RMUs: Typically used for power distribution on the high-voltage side, consisting of incoming cabinets, metering cabinets, PT (Potential Transformer) cabinets, and transformer outgoing cabinets. For users with high power demand requirements, the incoming supply must use a dual-power source transfer cabinet.

Outdoor RMUs: Commonly used in urban power grids, often employing common-enclosure SF6 RMUs. The key characteristic of this type is its high degree of protection, capable of reaching IP67, making it suitable for outdoor use and able to withstand short-term submersion.

Ring Main Supply Scheme
A typical ring main supply scheme involves one ring incoming, one ring outgoing, and two to three outgoing feeder circuits, forming a "hand-in-hand" ring main supply mode. To ensure reliability and continuity, power is supplied via two incoming feeders, creating a loop. Using SF6 load switches with three positions (closed, open, earthed) for line and transformer control. A single cabinet does not form a ring network; it takes at least three cabinets (e.g., two incoming/ring switches and one outgoing/transformer switch) connected to power sources to form a basic ring main unit configuration.

To enhance power supply reliability and allow users to obtain power from two directions, the distribution network is often connected in a loop. This method is called ring main supply. In AC 10kV distribution systems for industrial/mining enterprises, residential areas, ports, and high-rise buildings, due to moderate load capacities, high-voltage circuits are often controlled by load switches or vacuum contactors, protected by high-voltage fuses. These systems typically use ring main supply, and the high-voltage switchgear used is conventionally referred to as Ring Main Units.

Besides supplying power to its own substation, the high-voltage busbar of an RMU also carries the through-current of the ring network (i.e., current passing through this substation's busbar to supply an adjacent substation). Therefore, the busbar cross-section must be selected based on the sum of the local substation's load current and the ring network through-current to prevent overloading during operation. Current RMU product series include XGW-12 Primary and Secondary Fusion Ring Main Unit, GTXGN-12(C,V,F,PT)GTXGN-12 Solid insulation ring main unit , XGN-12 SF6 gas insulated switchgear,XGN-12,XGN-12(C,V,F)

Application and Outdoor Features
RMUs are simple to install and have a small footprint, conveniently placed on roadsides, in residential areas, or urban green belts. They are widely used in industrial parks, streets, residential districts, and busy commercial centers for power receiving and distribution.

Outdoor RMUs often use vacuum switches as the interrupting device, with common-enclosure SF6 gas insulation. This results in a compact structure, small footprint, and aesthetically pleasing appearance that can blend with the surroundings. Adopting a fully sealed, fully insulated concept, units like the RVAC RMU house all switching parts, including all high-voltage live parts and mechanisms, completely sealed within the main tank, unaffected by condensation or external pollution. The main tank's protection rating can reach IP67, and when equipped with waterproof, touchable cable connectors, it effectively defends against occasional flooding in rainy areas.

 

3.1 Maintenance

Inspection of the Ring Main Unit is required under the following circumstances:

After 100 full-load operations, the main circuit resistance and insulation level of the load switch should be checked, and based on the condition, consider sending it to the manufacturer for overhaul.

After 10,000 no-load operations, the mechanical characteristics and mechanical operation of the load switch should be checked.

Regularly record the indicated value on the SF6 gas pressure gauge. According to the SF6 gas state curve, replenish gas promptly based on the condition. Under normal conditions, the unit should not require gas replenishment for 15 years.

Regularly inspect the Ring Main Unit for any abnormal conditions. If any are found, take timely measures.

Operating personnel should be familiar with all parts of the Ring Main Unit, including its structural performance, installation, etc.

3.2 Operation and Fault Handling

3.2.1 Pre-Power-On Inspection
Before energizing, comprehensively inspect all components of the Ring Main Unit for electrical performance, insulation level, correctness of wiring, and reliability of anti-maloperation interlocks. After inspection, if deemed satisfactory, connect the power supply and observe the operation of indicating instruments. If normal, the unit can be put into service.

3.2.2 Normal Operation
During normal operation, the load switch should be in the closed position, the earthing switch in the open position, and the front door panel in the closed state.

3.2.3 Cable Fault
In case of a cable fault, first open (trip) the load switch within that specific cabinet. Afterwards, the front door panel can be opened for internal inspection and repair. Note that the main busbar in the upper section remains energized. If fuses need replacement, follow the same operating sequence described above.