The function and principle of ring main units
A ring network refers to a circular distribution network, where the power supply trunk lines form a closed ring. The power supply sources supply power to this circular trunk line, and then distribute power to the outside through high-voltage switches one by one from the trunk line. The advantage of this is that each distribution branch can either draw power from its left main line or from its right main line. When the left main line malfunctions, it continues to receive power supply from the right main line, and when the right main line malfunctions, it continues to receive power supply from the left main line. In this way, although the main power supply is single-channel, each distribution branch gets the benefit similar to dual-channel power supply, thereby improving the reliability of power supply.
The so-called "ring main unit" refers to the installation of one switch cabinet (outgoing line switch cabinet) on each distribution branch, and the busbar of this switch cabinet is also part of the ring main line. That is to say, the circular main line is composed of the busbars of each outgoing line cabinet connected together. Each outgoing line cabinet is called a "ring main unit". In fact, when a single ring main unit is taken out alone, the meaning of "ring network" cannot be discerned.
The rated currents of these ring main units are not large, so the high-voltage switches of the ring main units generally do not use complex-structured circuit breakers but adopt simple-structured high-voltage load switches with high-voltage fuses. That is to say, the high-voltage switch in the ring main unit is generally a load switch. The ring main unit uses load switches to operate the normal current, while fuses are used to cut off the short-circuit current. The combination of these two replaces the circuit breaker. Of course, this can only be limited within a certain capacity.
Such switchgear can also be used in non-ring network structure distribution systems. Thus, with the wide application of this type of switchgear, "ring main unit" has transports the scope of ring network distribution and generally refers to high-voltage switchgear with load switches as the main switch.
Ring main units have not been around for a long time, but they have been widely used in recent years. This is mainly due to the fact that in the newly developed industrial enterprises and public institutions, from the perspective of power supply and distribution, medium and small-sized capacity are the majority. Their distribution transformers generally do not exceed 1250KVA, providing long-term stable power supply, and high-voltage switches are not frequently operated. For such users, although the performance of the ring main unit is not perfect, it is sufficient and competent. With the large-scale urban construction, a large number of high-rise buildings have sprung up from the ground. The transformer and distribution substations are often located downstairs. According to the fire protection requirements, the transformer and distribution equipment must be "oil-free". According to the reasonable use of the construction area, the transformer and distribution equipment should be "miniaturized". Therefore, ring main units or vacuum circuit breakers, dry-type transformers, and dry-type instrument transformers have entered many buildings. The ring main unit has a simple structure, reliable and safe operation, requires little maintenance, and has low operating costs. Compared with switch cabinets equipped with circuit breakers, it has significant advantages. Ring main units require load switches to have higher performance and smaller volume, and thus load switches have emerged. This, in turn, has driven the technological progress of ring main units. Today's ring main units not only have excellent performance but also have been serialized, which further expands the application scope of ring main units.