MechChem Africa July-August 2023

⎪ PowerGen,PetroChem and Sustainable energy management ⎪

be fitted into a smaller area, especially on the high voltage side,” he says. He notes that other key considerations on the HV side include current and voltage transformers for metering, monitoring and trip protection; surge arrestors that can be designed for multi-ratio applications; and post insulators (PIs) to make sure the spans between conductors and other equipment are rigid enough to keep safe operating clearances at all times. “All these design choices are custom engineered, while easily retractable galvanised steel support struc tures are incorporated for ease of transport to site,” adds Herbst. To meet the earthing needs, Herbst says each mobile substation has a full earth ring on the unit that gets connected to an earth mat at multiple points. Where earth mats are not already installed, an earth mat is buried around the entire mobile platform before being connected to the trailer structure; this minimises the risk of step and touch potential. Power transformer considerations For a mobile substation, reducing the dimen sions and weight of the transformer is very important, Herbst notes. “The choice of the cooling system is the most critical aspect in this regard. There are a range of different types of cooling for transformers, starting with oil natural, air natural (ONAN). This typically results in a larger transformer because natural convection cooling is used, where the oil naturally circulates upwards into radiators and then back into the trans former core as it cools down. This is the con ventional way to cool a power transformer,” he explains. “A step above that, we have ONAF, which uses natural convection to circulate the oil, but the radiators are force-cooled using air. However, the oil direct, air force (ODAF) cooling system, in which the oil is force circulated through the radiator by a pump, and then air-force cooled in the radiator, can deliver a 60% capacity increase compared to ONAN, enabling the weight and the foot print of the transformer to be significantly reduced,” says Herbst. Other key transformer design innova tions include polymer insulation on the bushings. In dual voltage applications, voltage selection taps on the primary and secondary windings via off-load tap selec tion switches that are accessible from ground level. And on the protection side, solid or par tial earthing can be incorporated, depending on system requirements. An oil purification system and a digital gas analyser (DGA) can also be added to further protect and monitor the transformer.

A 132kV/33kV 10MVA mobile substation connected to the grid and in operation.

Medium voltage system design As with HV design, AIS, GIS or Hybrid switchgear can be incorporated on the MV side of the substation, and indoor or outdoor switchgear with surge arrestors can be considered. “In the scenario I am illustrat ing, we have one power cable coming in and one cable going out, but we can accom modate any supply scenario. For example, if a customer wants to feed to five different take-off points, we can include five metal clad switchgear breakers on the MV side. It all depends on the customer’s application,” he says, adding that provision can also be made for an external low voltage supply, should the need arise. “In addition,” he notes, “If it's a dual volt age system, with two possible HV voltages and two MV voltage settings, we always build in a safety interlock system so it is impossible to make a mistake in selecting which voltage combinations to use.” Trailer design All Zest WEG mobile trailers that will oper ate on national roads are designed to satisfy the principal approval process required by relevant government legislation. Typically, pneumatic air suspension is preferred for

mobile substations and trailer designs also accommodate manual raising and lower ing of the travel height, depending on site conditions. Manual landing legs can also be deployed. Trailers exceeding 14.5 m from the king pin to the centre of the rear axles must be steerable if intended for use on national roads. The rear axles are automatically articulated hydraulically by the 5th wheel or, where required, manual steering can be incorporated. Trailers can also be designed to incorpo rate oil catchment facilities and, due to the scarcity of specialised tyre sizes in many regions, the entire unit is designed based on standard truck tyre sizes. “If a network operator or power distribu tor invests wisely in a Zest WEG mobile sub station, the same system can be used again and again in several places, significantly reducing total costs of ownership. “Our Zest WEG mobile substations are robust, reliable, well-integrated and com pact solutions that can quickly be deployed and put into service – and when the unit is needed elsewhere, it can then easily be relocated,” Sollie Herbst concludes. www.zestweg.com

A Zest WEG mobile transformer that utilises an oil direct, air forced (ODAF) cooling system, and a digital gas analyser (DGA) for further protection and monitoring.

July-August 2023 • MechChem Africa ¦ 31