REF extends the grid to rural areas to benefit public institutions such as both primary and secondary schools, rural health centres, veterinary offices, chiefs’ homesteads, both large and small scale farms, irrigation schemes, rural business centres, growth points, resettlement areas, rural service centres, A1 and A2 farms, villages and other related institutions such as churches, community halls and recreational centres.
Public institutions (schools, rural health centres, agritex offices, chief’s homesteads, community irrigation schemes and any government offices in the rural areas) are electrified under a 100 percent capital subsidy while the rest pay 50 percent of the total cost of their projects.
What is Electricity Grid?
An electrical grid is an interconnected network of power stations, transmission and distribution lines for generating, transporting and distributing power. In between there are associated transformers and at the end there are local low voltage lines for delivering electricity to the door step of the end-users typically homes, mines, factories institutions, agricultural facilities and such like power consumers.
We will use the Zimbabwe scenario to describe the grid. Power stations such as Hwange represented by label 1 and 2 in Fig 1 below) and Kariba, generate power at some voltage and this power is ferried through transmission lines at significantly higher voltage such as 330kV as shown in Fig 2 below. At certain strategic geographical locations such as Bulawayo, Alaska, Warren. Dema, Sherwood, Orange Grove etc, (labelled 4 in Fig 1)the high voltage is reduced to an intermediate voltage 132kV by transformers in 330/132/88kV substations so that it can be distributed to local but large geographical areas.
Fig 1: A representation of an electric power system
The power at the Bulawayo 330/88KV substation will supply the 88KV ring circuit around Bulawayo City and all the rural, farming and mining areas in the Matabeleland provinces. Similarly substations located at Alaska in Chinhoyi, Warren at Harare, Orange Grove in Mutare, Dema in Bindura, etc transfers power to the local cities and towns, mines, farms and rural areas.
Fig.2: Transmission line tower
Within the cities, towns, mines and rural areas, power is further distributed from 330/132kV substations to 132/33KV substations located closer to the respective nodes from which it radiates into the hinter lands through 33kV lines. It reaches some bulk users such as mines and concentrated settlements at 33kV while at progressively smaller settlement and power users it reaches at much reduced voltages of either 11kV or 400V.
Off-Grid Power System
An off-grid is a small power system often revolving around a natural primary resource which is used to drive an equally small electricity generator. It is not connected to the national grid because very often, the primary resource is found in remote areas naturally. Typical resources are perennial rivers, the sun for PV, and wind.
An electricity reticulation is similar to a water reticulation. Dams from which bulk water is sourced can be equated to power station and the interconnection of pipes and water reservoirs being similar to transmission, distribution and low voltage lines and the associated transformers. Very large pipes (read transmission lines) are required to ferry large quantities of water to large intermediate water reservoirs (330kV/132kV substations) located at strategic positions relative to areas where the water is required. Large pressure is required to push the bulk water so the pipes are large in diameter. Water will flow from a region of high pressure caused either by height difference between where it is coming from and where it is going or simply by pumps. Similarly a voltage is required to push the power as a characteristic of electricity called current. The current may be taken to be the same as the water in the water reticulation example while voltage is similar to pressure. Large pipes of water will deliver significantly more water than smaller dimensions. Similarly large conductors (wires) deliver large quantities of current than smaller dimensions. If the same volume of water is forced through smaller dimensions of pipes, less water is conveyed per any given moment, ditto with electricity. For water, the resistance the water experiences as it is forced through pipes is called friction while the same quality is called resistance for electricity. As much as pipes should be selected in size such that as much water as is needed is conveyed in the expected time, conductor sizes should also be selected such that the resistance is minimized.
From the strategic reservoirs, water is distributed to relatively large reservoirs located close to either residential or commercial and industrial areas. Similarly 132kV lines that emanate from the 330KV/132kV electricity reservoirs sweep long radii at the end of which progressively smaller 132/33/11KV electricity reservoirs are located. Depending on the size and numbers of the residential, commercial or industrial areas, some will be served by large diameter pipes while some will be served by smaller sizes with many in between. Laterals are tapped from these pipes into the homes, commercial and industrial facilities where the supplies are metered.
In electricity reticulation things are a bit simpler. The respective residential areas, commercial and industrial areas are served by 33kV or 33/11KV supplies that originate either from 132/33/11KV reservoirs or much smaller 33/11KV. Power stations at Bulawayo, Harare, and Munyati generate power at much lower voltages than Hwange and Kariba, but the power is distributed from these power stations at the same 33kV voltages.
Laterals of the 33kV or 33kV lines from 33/11kV substations serve the individual customers either through pole or ground mounted substations.
Fig 3: Parts of a power system. Left shows types of end-users and right components os a power system
In central business districts, shopping centres/malls and some low density areas, the respective shops and such like facilities are supplied from common 11/0.4KV ground mounted substations by underground cables. In the high density and rural areas open overhead wire systems are evident. Some areas are served by 33kV lines because of high power that is needed and the longer distances than the 11kV which serve smaller geographical areas. In rural areas the local customers are the supplied from pole mounted 33/0.4kV or 11/0.4kV. In between these 33KV or 11KV there are intermediate 33/11KV substations where some of the lines that drop the 11/0.4 kV originate.
In residential areas, a different configuration of wires called LV or LT are used as shown below.
Fig 4: Low voltage electricity distribution lines at a rural business centre