Mechanical Technology July 2016

⎪ Special report ⎪

Microgrids, a resilient, cost-effective

On June 8, 2016, ABB inaugurated an integrated solar- diesel microgrid installation at its Longmeadow facility in Johannesburg, which comprises a grid-connected system with full on- and off-grid functionality that maximises the use of renewable energy and ensures uninterrupted electricity supply. MechTech talks to ABB South Africa’s sales manager for microgrids and grid automation, Tony Duarte (left).

“ T raditionally, utilities depend on a concentrated region of generation. In South Africa, for example, the largest percentage of our power comes from the coal-fired power stations in Mpumalanga and Limpopo, where the coal mines are,” begins Duarte. “But with many power stations in- terconnected, critical points of failure emerge, which can have serious affects on the whole system,” he says. “Microgrids are networks of smaller independent but interconnected generation ‘islands’ designed to service local power needs. A fault in one part of the network does not affect the others and, with a degree of redundancy in every microgrid and/or between interconnected microgrids, out- ages can be avoided with other sources continuing to supply power,” he explains. “Microgrids are generation agnostic,” he continues. “They are not fussy about how the power is generated or where it comes from. Essentially, their core purpose is to manage and combine the available power to best meet demand. “Gas, hydro, diesel, HFO, solar, wind or geothermal sources can be incorpo- rated into a microgrid solution. But each technology responds differently and their response curves don’t necessarily overlap,

which can make the delivery of seamless power difficult. Hydro and diesel, for ex- ample, can complement each other, but if there is a sudden upset, their response times don’t overlap enough to avoid a disturbance on the grid. On a mill trip on a grid-isolated mine, the response times of a base-load hydro plant might be a few seconds. The diesels will try to compen- sate, but these are too slow in starting additional units. This can cause the circuit protection to lift, so the whole mine can go into a blackout state,” he relates. Exemplifying the critical role of microgrids is the Longmeadow dem- onstration plant, which combines the grid-based supply from Johannesburg’s City Power; the facility’s existing diesel backup generators; a newly installed solar PV plant; and a compact and ver- satile PowerStore TM battery-based grid stabilising system to address frequency and voltage fluctuations. “If City Power cuts out now, the PowerStore needs to respond quickly enough so that no one sees a dip in power. “This is fundamental to microgrids. They need to incorporate fast acting mechanisms or generation sources to fill in the gaps while the preferred al- ternative source ramps up. Effectively, the PowerStore compensates for a lack

of overlap and ensures system stability, regardless of fast fluctuating conditions on the grid, step changes in the load; or variations associated with renewable energy sources,” Duarte explains. “Batteries and flywheels are ideal for meeting this need, because the connect- ed inverters switch at electronic speed, so they react within a few milliseconds, which is well within a single cycle of a 50 Hz ac supply,” he explains, adding that a fly wheel or battery storage system can react in milliseconds. Describing the problem with tradi- tional grid-connected PV power plants, he says that PV solar generally needs a frequency and voltage reference signal from a live grid before it can be synchro- nised. “Systems such as those adopted for the REIPPPP are grid following. If the grid trips, then the PV plant also trips. This negates the usefulness of grid-connected PV as a backup power solution,” he tells MechTech . “If the grid goes into a blackout state, the PV system cannot help. So if used in this way, PV solar and wind generation can never be a substitute for base-load generation because they cannot replace the loss of grid power,” he explains. ABB’s microgrids, however, are grid forming. “While they will synchronise to a grid if it is available, they can also provide the references for renewable and fossil energy generation to follow. PowerStore sits between the grid and all the other generation sources, so PV, diesel and wind generation will all look to the PowerStore reference in order to synchronise.

An infographic outlining the key features of ABB’s microgrid installation at Longmeadow.

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Mechanical Technology — July 2016