FAQs Guide for Ring Main Unit

Publish time: 2022-04-14 16:06:00 View: 2194

FAQs for Ring Main Unit (RMU)

FAQs about ring main unit (RMU)


 Ring Main Unit: The Ultimate FAQ Guide 

1. What is ring main unit?

2. What are the functions of ring main unit?

3. What are the types of ring main unit?

4. What is the structure of ring main unit?

5. What kinds of switches are involved in the ring main unit?

6. What are the advantages of ring main unit?

7. Why are ring main units widely used in power distribution systems?

Keyword: Ring main unit, RMU, Switchgear




What is ring main unit?

Ring main unit is a group of electrical transmission and distribution equipment (high voltage switchgear) installed in metal or non-metal insulated cabinet or assembled into interval ring network power supply unit, its core part is SF6 load break switch and fuse, with simple structure, small size, low price, can improve the performance of power supply parameters and power supply safety. It is widely used in residential areas, all kinds of buildings, factories and other distribution power stations and compact substation.
A large number of regional secondary substations are needed to distribute the power to the user terminals after the power passes through the high voltage primary transformation. To improve power supply reliability, users can obtain power from two directions, usually by connecting the power grid in a ring. This power supply method is referred to as ring power supply. The system usually uses ring network power supply, the use of high voltage switch cabinet is commonly known as ring main unit.
In fact, if the cabinet type is classified according to switch equipment, there are load break switch cabinet, circuit breaker cabinet, PT cabinet and so on, and there is no ring main unit. Ring min unit is a conventional name, the original refers to the load break switch cabinet used for ring network power supply, now regardless of whether it is used for ring network power supply, such as some lines to close loop operation or load cutover, the power distribution cabinet to achieve these functions is called ring main unit.


What are the functions of ring main unit?

Ring main unit functions including connect ring circuit, improve the power supply reliability of lines, transformer protection, short circuit current limiting, the transformer from any sudden short circuit fault current, and within 10 ms rapid removal of the transformer internal fault and prevent have internal arc arched damage caused by the dynamic pressure rise transformer, transformer to get more reliable protection. As for the short-circuit of the low-voltage side end of the transformer, the high-voltage fuse is reasonably selected and the short-circuit fault of the low-voltage side end is removed by the high-voltage side fuse to effectively protect the transformer.


What are the types of ring main unit?

1. According to the insulation medium, can be divided into air insulation type, solid insulation type and SF6 insulation type;

2. According to the load switch type, can be divided into three position switch type and traditional type;

3. According to the expansibility of ring network cabinet, it can be divided into two types: expandable and unexpandable;

4. According to the arc extinguishing mode of load switch, it can be divided into two categories: internal arc extinguishing and external arc extinguishing, and can also be divided into pressurized air type, arc extinguishing grid type, self-produced gas type and so on;

5. According to the condition of the load break switch gas chamber, it can be divided into load break switches in common box type and load break switches in separate box type.


What is the structure of ring main unit?

Ring main unit cabinet is generally composed of four parts: switch room, fuse room, operating mechanism room and cable room (bottom frame).

The switch room is composed of various functional circuits (including grounding switches and load switches) and busbars between the circuits sealed in the metal shell. Shell by 3 mm cold rolled steel plate (or not embroidered steel plate) welded. Each functional loop includes a load break switch and a grounding switch. The load break switch is composed of a vertical moving contact system and a static contact at the bottom. When the switch is closed, the moving contact moves downward and the load switch is switched on. The grounding switch is composed of moving contact knife and static contact knife. In the process of spring movement, the grounding switch is switched on quickly. There are four rectangular assembly process holes at the upper and rear of the switch room, and the front of the ring cabinet is equipped with an observation window, which can see the "point" and "close" position of the grounding switch. Explosion-proof device is installed at the rear of the ring main unit.

The load break switch adopts pneumatic internal blowing structure, with strong arc extinguishing ability, and does not affect phase and ground insulation, dynamic and static contacts are equipped with arc contacts, greatly improving the times of breaking.

The fuse and the load break switch room constitute transformer protection loop. The high voltage current limiting fuse is installed in the epoxy poured insulating shell. After the fuse is broken, the firing pin pops up and the load switch breaks.

The operating mechanism room is located in the front of the ring network cabinet. In each functional loop, the load switch is equipped with human (or electric) energy storage spring operating mechanism, and the grounding switch is equipped with human energy storage spring operating mechanism. On the panel, there are opening and closing operation holes for the load switch closing operation, the manual opening rotation button and the grounding switch respectively. Load switch open and close position indicator light and electric open and close button, and equipped with analog line, switch status display board and lock position, load switch and grounding switch operation with interlocking device, to prevent misoperation.


What kinds of switches are involved in the ring main unit?

1. Load break Switch

Load break switches have a certain load breaking ability, that is to say, in light load, it can be charged to pull the brake, but it cannot be operated frequently. General load switches have fuse, and their arcing ability is stronger than the isolation switch.

2. Disconnect Switch

When the circuit breaker disconnects the circuit, because of the disconnection of the disconnecting switch, the part with power and without power can be obviously isolated, playing the role of the auxiliary switch. Because the external indicator of the contact position of the circuit breaker is not intuitive and in some cases cannot guarantee that its indication is consistent with the contact position, it is necessary to use a disconnector to clearly separate the active and inactive parts.

In addition, the isolation switch has a certain natural arc extinguishing ability, commonly used in voltage transformers and arresters and other small current equipment input and disconnect, so that the circuit breaker through the disconnect switch more flexible and convenient.

3. MV Circuit Breaker

A switching device capable of turning on and off, carrying and breaking current under normal circuit conditions, and of turning on and off, carrying and breaking current under abnormal circuit conditions within a specified time.

4. Grounding Switch

It is mainly used for equipment maintenance. In order to prevent a sudden call during maintenance, both ends of the equipment must be grounded.

Grounding switch in high-voltage power distribution cabinet is generally grounded on the output load side. When we overhaul the load side of the high voltage distribution cabinet (such as: fastening bolts, disassembling cables, etc.), it is necessary to break the switch, and close the grounding switch of the distribution cabinet, which can prevent a sudden electric shock accident caused by electricity, and can put out the remaining charge, which is conducive to the safety of maintenance.


What are the advantages of ring main unit?

Take SF6 ring main unit and vacuum ring main unit as examples, here are some advantages listed below:

1. Prevent misoperation and effectively protect personal and equipment safety. Vacuum load break switch is to have three-position high voltage switch equipment, namely disconnect, closing and grounding. Disconnect switch and earth switch has visible fracture, load break switch and disconnect switch has the function of automatic linkage namely after the vacuum load switch, disconnect switch automatically disconnect, visible fracture formation, when the closing of the vacuum load break switch, disconnect switch automatically closed ahead; Load break switch and earth switch also has strict mechanical interlocking and electric interlock, it is impossible to first ensure that both in the structure misoperation, namely when the load switch in the closing position, impossible to close the grounding switch, in the same way, when the earthing switch is in the closed position, it is impossible to operate load break switch; Once the front door of the ring cabinet is opened, the load break switch cannot be closed again, thus effectively preventing the occurrence of misoperation

2. High performance index. Vacuum and SF6 load break switches used in vacuum ring main unit and SF6 ring main unit can be frequently operated, and have good breaking ability, short-time tolerance current generally reaches 20kA, short-circuit closing current generally can reach 50kA.

3. Maintenance friendly. The panel design of the cabinet is clear, so that the wiring mode is marked in the form of the simulation diagram on the cabinet, so that people are clear at a sight, part of the manufacturers like Rockwill Electric Group will also marked operational guidance and notes on the cabinet for further reducing the possibility of operator misoperation. 

Most ring main units are installed with charged display devices with functions of monitor the main circuit charged state, It not only prompt high voltage charged state, but also can be combined with electromagnetic lock to achieve forced locking handle door, improve the anti-error performance; The transparent plexiglass observation window installed in the front door can directly inspect the opening and closing state of the load break switch.

4. Flexible to use and easy to expand. The configuration of ring network cabinet can flexibly meet the requirements of various design schemes and realize any combination according to the actual needs of customers. The connection mode of cable bridge is also more flexible, even if the ground is not smooth enough, it can achieve soft connection, and there will be no partial discharge; Install and equip automatic interface to prepare for the construction of distribution automation.


Why are ring main units widely used in power distribution systems?

Ring main unit appeared not for a long time, but has been widely used, which is mainly due to the new development of industrial enterprises and institutions of the building, from the point of view of power supply and distribution, among them, the small capacity for most of its distribution transformers generally do not exceed 1250KVA, long-term stable power supply, high voltage switch is not frequent operation. For this kind of user, the performance of ring main unit is not the most perfect, but it is enough to be competent. Along with the large-scale urban construction, a large number of large high-rise buildings rise, substation and distribution office is often located downstairs, according to the fire protection requirements, substation and distribution equipment must be "oil-free", according to the reasonable use of construction area, substation and distribution equipment should be "miniaturized". Ring main unit structure is simple, reliable and safe operation, maintenance is small, low operation cost, compared with the switchgear equipped with circuit breakers, the advantages are very prominent. Ring main unit require load switches with higher performance and smaller volume, so new type of load switches emerge at the right moment, which in turn drives the technological progress of ring main unit. Today's ring main unit not only have excellent performance but also become serialized, which further expands the scope of use of ring main units.


Learn more about Rockwill ring main unit if you have related business. Rockwill Electric is a professional compact substation manufacturer in China which has won high reputations globally with high quality, considerate service and great credit. Contact us for a free quote.

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