MechChem Africa July-August 2023

Zest WEG Mobile substations: key design principles Sollie Herbst, senior manager for Energy Systems at Zest WEG, presents an overview of the company’s mobile substation offering, highlighting their key use and the design principles that Zest WEG has evolved to ensure their ongoing success in Africa.

“A mobile substation is a fully equipped trailer or container with all the necessary high- and medium-voltage compo nents that can be found in a permanent instal lation,” begins Zest WEG’s Sollie Herbst, who is the company’s senior manager for Energy Systems. This typically includes the power transformer; switchgear; metering equipment; transformers; surge arresters; protection and control equipment; along with the required ac (alternating current) and dc (direct current) operational supplies. These systems are purpose designed as entire replacement units for the perma nently installed substations supplying power from the high voltage (HV) transmission or distribution network – with voltages ranging between 33 kV and 220 kV – and delivering to medium voltage (MV) networks operating from 11 kV to 33 kV, he explains. “Whenever there is a breakdown of a substation – due to natural disasters or if a plant is undergoing an upgrade or waiting for a permanent substation to be constructed – then a mobile substation can be relatively rapidly deployed to the site and connected to the network to supply interim power,” says Herbst. “Mobile substations must be able to be moved quickly to supplement the de mand. Their use requires no civil works, so response times can be rapid with limited dismantling and assembly onsite. They are easily connected to the existing electrical infrastructure and are generally designed as plug-and-play substitutes for permanent

unit can be moved to a new location once the permanent substation is completed or repaired, he adds. Other potential nearby applications for the unit may also come into play. “While customers tend to have a specific use in mind when making an enquiry, there may be follow-up uses within the area. Designs can meet these needs by including dual voltage selection on both sides of the power trans former, for example. That is, two different HV and MV settings and this gives more flexibility for interfacing to other utilities. Municipalities, mines, plants or communities in the vicinity can be incorporated, making a stronger and broader business case for the investment,” says Herbst. At the HV interface point of Zest WEG’s mobile substation is a connection to the available HV network. Here, suitably rated switchgear is used to ensure that the power can be connected and disconnected safely. Air insulated switchgear (AIS) can be used if space is not restricted, but gas insulated switchgear (GIS) or hybrid/mixed insulated switchgear is preferred. “Mixed technol ogy has AIS and GIS combined in one unit. The long bushings are AIS, while the circuit breaker portion is GIS, which uses SF6 gas as the insulation medium to make it more com pact,” Herbst explains, adding that GIS limits the amount of space due to the various HV switchgear components being incorporated in one hybrid unit. “This allows a lot more to High voltage side components and design

installations. “Maximum mobility and flex ibility are essential, so the systems must be able to be used in various configurations. They promote electrical network availability by reducing downtime durations associated with failures, maintenance and delays in constructing permanent infrastructure,” he tells MechChem Africa . General considerations for design Prior to starting on a design for a mobile substation, Sollie Herbst says the team must first understand the operational application and site conditions. The topography, the climate and elevations, for example, all need to be taken into consideration. “We need to understand the intended frequency of the unit’s relocation – once a year or once or a month – because this changes the physical design,” he adds. Other questions that need to be an swered include: Will this unit be working from a provincial, rural or rural-urban envi ronment? And are the sites on Government or private land? Does the trailer need to comply with road legislation, and which road legislation will apply? What are the road con ditions, and the bridge height restrictions, on the routes? Where are the grid tie-in points? Is there sufficient access and enough space within the substation area? “This is all very important to know in advance. In some cases on large units, we might have to par tially dismantle the unit for shipping to site and then reassemble it once it arrives,” he explains. The availability of a suitable truck tractor in the region is also important, so the

A typical detailed design model of a mobile substation with the high voltage (HV) interface at the rear, the power transformer in the centre, and the medium voltage (MV) system at the front of the trailer.

30 ¦ MechChem Africa • July-August 2023

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