Design Aspects

1-    Substations used in the agricultural area are generally outdoor

2-    substations due to the low cost of land while in city center the substations are GIS (Gas Insulated Substations) since the land is very expensive. In other areas, the indoor substations are very suitable.

3-    Over Head Transmission Lines ( OHTL ) are used in  the agricultural area while Under Ground Cables ( UGC ) are used elsewhere for the primary distribution networks ( 11 KV ) due to the following:

·         The lower cost of OHTL makes it more suitable for use in the agricultural areas

·         UGC may be destroyed while digging or due to irrigation in the agricultural areas

4-    The cross-sectional area (c.s.a) of the 3-core cables shouldn't exceed 300 mm²; else single core cables of c.s.a 400 mm² are used.

5-    From the reliability point of view, it is better to replace one large substation with some smaller substations distributed over the planning area, inspite of the increase in the cost; yet, the increase is not that considerable amount.

6-    It is recommended that the number of transformers in each substation doesn't exceed 6 transformers including the reserve.

7-    It is recommended that the bus bars in the substation are sectionalized and doubled to allow maneuver and that the number of transformers and feeders per section is even number.

8-    It is recommended that the number of feeders per transformer in agricultural area shouldn't exceed 6 feeders, while for all the other areas it shouldn't exceed 8 feeders per transformer.

9-    It is recommended that the no. of transformers in each substation doesn't exceed 6 transformers including the reserve.

10- In general, the calculation of the no. of feeders & the loading percent is done considering the total no. of transformers in the substation including the reserve since all the transformers are simultaneously connected from the beginning.

11- The loading percent in each substation shouldn't exceed 80% for safety and continuity of feeding in case of outage of any unit.

12-A recommended current density for the UGC is 1 A/mm² while for OHTL it is generally around 2 A/mm².