Distributor

The second step on the network is the distributor. It is a sectionalized 11 KV busbar in two sections; the two sections are fed from two different 66/11 KV transformers in the same substation or from two different substations to make sure that the continuity of supply is achieved. In case of a fault occurring on the cables between the distributor and one of the transformers; this cable will be switched off from the network and the distributor will be fed from the other transformer.

The bar of the distributor will be fed from two different transformers through medium voltage with-drawable circuit breakers. There is one with-drawable circuit breaker on the bar called the bus coupler. This circuit breaker splits the bar in two isolated parts each part is fed from one transformer. In case a fault occurs; this circuit breaker will connect the isolated parts of the bar (after isolating the faulty feeder) to feed all loads on the bar of the distributor. This system is known as two out of three system (2/3 condition). The number of the outgoing feeders connected to the first part of distributor bar is equal to the number of the outgoing feeders connected to the other part. One feeder of the first part is connected to other one in the other part through ring main feeder to make sure that the continuity of supply is achieved. In case of fault; the ring main has a supply from one of the feeders coming from distributor. The standard cross section area of the feeder coming from sub-station is 3x1x400 mm² for the 16 MVA distributors and 3 x240 mm² for the 10 MVA distributors, these feeders are always double, and each pair came from different sub-station or from the same substation as mentioned above. The rule here in Egypt is that each pair of cables can carry the whole load of the distributor alone in case of loss of the other pair; that is the feeders are loaded by only 50% of their current carrying capacity in the normal conditions (when the bus coupler is opened). Loading the cable with only 50% of its ampacity is of course a much exaggerated rule from the economical point of view and we recommend that the feeder is loaded up to 70% of its ampacity.

The outgoing of the distributor has standard cables 3x240 mm² for the 16 MVA distributors and 3x150 mm² for the 10 MVA distributors. Each feeder in first part connected to another one on the second part and forms an open loop. Number of transformers in each loop ranges between 6à12 transformers.