Sebenza substation, located immediately behind the Kelvin power station near Kempton Park, Ekurhuleni, rated at 1000 MVA 400/275/132 kV will be the largest substation built in South Africa over the last ten years.
Although the new substation and Kelvin power station are located within the City of Ekurhuleni (previously known as Ekurhuleni Metropolitan Municipality) region, it belongs to Johannesburg’s electrical utility City Power. The power station (Fig. 1) was built in 1957 by the Johannesburg City Council.
The new substation is being built by Consolidated Power Projects (Conco), an engineering, procurement and construction (EPC) contractor, with consulting engineers from PSW and Nyelet.
It will become one of the main intake substations in the north-eastern region of Johannesburg, making it one of the country’s electricity supply hubs which are expected to stimulate much needed economic growth. One of its functions is to relieve load from the Prospect substation.
The new substation is located in an industrial area known as Sebenza, near Kempton Park and Edenvale within the boundaries of the City of Ekurhuleni. The new Sebenza intake substation will strengthen City Power’s distribution network growth. The substation has been fitted with three 315 MVA, 275/88/22 kV autotransformers which were specially manufactured for the project by Actom (Fig. 2).
The new transformers had to be designed with high-impedance coils to match the 40 kA circuit breakers installed at the old Kelvin power station, which posed a challenge to the transformer designers. The transformers for Sebenza incorporate a specially customised three-stage cooling system designed to provide high efficiency cooling at various temperatures, while at the same time minimising energy use (Fig. 3).
The cooling system comprises radiator-, pump- and fan-driven systems which optimise cooling of the active part of the transformer by respectively coming into operation in response to rises in temperature at various stages to maintain oil flow through the system.
Actom says that the company had to redesign its workshop to accommodate the sheer size of the new transformers. Building these transformers required 250 m2 of floor space. The company also had to install larger lifting equipment because of the mass of these transformers. The field coil alone is said to weigh 10 t.
Covering a land area of 260 x 440 m, the substation is said to be the largest substation built in South Africa in the last ten years. The new substation is situated on land which was previously used by the Kelvin power station to dump coal ash from its furnaces. Although much of the ash had been consumed by a local brick-making factory, it took contractors three months to remove the remaining ash before construction work could begin.
Once the land had been cleared it was discovered that the site contained a great deal of rock which made construction work more difficult. The building which houses the switchgear and other equipment, had to be built 1,5 m above ground on a false basement as a result of the presence of the rock (Fig. 4). The basement is essential to allow for the extra-high voltage cables to exit the building.
Gas insulated switchgear
ABB South Africa says this is the largest 132 kV high voltage gas insulated switchgear (GIS) board it has ever delivered in Africa. The gear consists of 132 kV GIS, 400 kV and 275 kV circuit breakers and surge arresters; 132 kV circuit breakers; 88 kV surge arresters and point-of-wave relays (switch sync). The 132 kV GIS board comprises 38 bays (Figs. 5a and 5b).
GIS is a compact metal encapsulated switchgear consisting of high-voltage components such as circuit-breakers and disconnectors, which can be operated safely in confined spaces.
Evacuating power from the substation is done by means of heavy-duty high-voltage cable supplied by CBI-electric: African cables (Figs. 6 to 9). These cables run from under the power transformers, out of the substation building underground in special trenches, to new overhead lines. New towers are being erected to carry new lines out of the substation to transmit the power into the grid at 275 kV.
Conco says this substation has been built with expansion in mind, and is intended to increase in capacity from 315 to 1000 MVA. This will dramatically improve reliability and stability of the grid. Part of the Sebenza project involved Conco having to upgrade and make additions to both Prospect and Kelvin substations, which included installing four sets of reactors and new switchgear on some of the main feeders at Kelvin. Two of the Kelvin bays have been completed, with the balance only becoming available once the Sebenza substation is energised.
Conco expects to hand the site over to City Power in April 2018 – ahead of schedule and within budget. It has been suggested that Eskom will ultimately take over the substation and increase its output significantly. It remains unclear what is being planned for the future of the Kelvin power station.
EE Publishes acknowledges the assistance of Mario Prasti and his team at Conco; and Steve Jordaan and his team from Actom in the writing of this article.
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Source: EE plublishers