Components of low voltage distribution system

Components of low voltage distribution system

The main components of the low voltage distribution networks include the distribution transformers, distribution pillars and building cofrees.

Distribution transformer

Transformer is used to step down the voltage from the medium voltage to the distribution voltage, at normal operation the transformer is usually loaded with (60%-80%) of its full load in order that it can withstand to carry loads in case of another transformer failure or maintenance.

Distribution transformers are put into either kiosks in the streets or rooms that are specially designed for them, in the past, the transformer was in the same room with the ring main unit (RMU), but now the room are usually made of two compartments one for the transformer and the other for the RMU, in order to protect the operator from the actions of the transformer during the pass of inrush current.

The standard ratings of the distribution transformer are 300 kVA, 500 kVA, 630kVA, 800kVA, 1000 kVA, 1500kVA and 2000kVA. The 300 kVA rating is not widely used.

The British Code of Practice BS 7735: 1994 Guide to loading of oil-immersed transformers indicates how oil-immersed transformers may be operated in different conditions of ambient temperature and service, without exceeding the acceptable limit of deterioration of insulation through thermal effects. Although specifically related to oil-filled units, similar reasoning can be applied to transformers cooled by other liquids.

During abnormal conditions, the transformer can be overloaded without reduction of its lifetime according to the following table.

The table gives the Load increase from initial load factor K1 to load factor K2 for a period of time T (hours) when the ambient temperature during the load increase is 200C (CP 1010).

After the transformer has been loaded with K2 for the period of time T (hours), then the transformer must be brought back to the initial load factor K1 for at least 8 hours.

Table: Overloading factor K2 for oil-immersed transformers (CP 1010).

Time in hrs	Initial loading factor K1
	0.25	0.5	0.7	0.8	0.9	1.0
0.5	K2 >2	>2	1.93	1.83	1.69	1.0
1	1.89	1.80	1.70	1. 62	1.50	1.0
2	1.59	1.53	1.46	1.41	1.32	1.0
4	1.34	1.31	1.27	1.24	1.18	1.0
6	1.23	1.21	1.18	1.16	1.12	1.0
8	1.16	1.15	1.13	1.12	1.09	1.0
12	1.1	1.09	1.08	1.07	1.05	1.0
24	1.0	1.0	1.0	1.0	1.0	1.0

In this project, the transformer rating is checked based on the above table and on the assumption that the abnormal time period is 3hours at ambient temperature of 300C which may decrease the overloading factor to about 90% of its value at 200C.

Distribution box (Pillar)

Distribution box links the distribution transformer with the building box (coffree), the distribution box should has an incoming from the distribution transformer and another feeder going to another distribution box so that in case of faults on its distribution transformer it can be fed from the other distribution box and vice versa.

At normal operation the pillar should be loaded from 70% to 80% to be able to withstand loads of the other pillar in case of faults.

The feeders of the pillar are loaded with 70% to 80% of its full load and these feeders are usually made of aluminium because copper is expensive and subject to stealing.

The ratings of the distribution box are usually 100 kVA, 150 kVA and 200 kVA. 

Figure: Distribution box (Pillar)

Building box (coffree)

Building box links the distribution box with the riser. The building box is fed from a pillar that is fed from two different feeders one comes from a transformer and the other comes from another pillar. The coffree has a supply in case that a fault occurs at one of two incomings. By this method, many manoeuvrings which ensure continuity of supply can be done.

The protective device that is connected on the riser may be a three-phase circuit breaker or three single phase fuses to prevent the failure in supply in case of the fuse of one phase is burnt , or one three- phase fuse. Riser is made of copper, because it is inside the building and cannot be stolen easily.