Electricity + Control

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 compensate, 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

demonstration 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

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 alternative 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 connected 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 problemwith traditional 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 synchronised. “Sys-

tems 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 says.

n June 8, 2016, ABB inaugurated an integrated solar-diesel

microgrid installation at its Longmeadow facility in Johan-

nesburg, which comprises a grid-connected systemwith full

on- and off-grid functionality that maximises the use of renewable

energy and ensures uninterrupted electricity supply. A Crown Pub-

lications editor, Peter Middleton, talks to ABB South Africa’s sales

manager for microgrids and grid automation, Tony Duarte.

Microgrids, a resilient, cost-effective and greener

power solution

“Traditionally, utilities depend on a concentrated region of gen-

eration. 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 interconnected, critical points

of failure emerge which can have serious effects on the whole sys-

tem,” 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

interconnectedmicrogrids, outages 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 avail-

able power to best meet demand.

“Gas, hydro, diesel, HFO, solar, wind or geothermal sources

can be incorporated into a microgrid solution. But each technology

responds differently and their response curves don’t necessarily over-

lap, which can make the delivery of seamless power difficult. Hydro

and diesel, for example, can complement each other, but if there is

ENERGY + ENVIROFICIENCY

Peter Middleton talks to ABB’s Tony Duarte

Peter Middleton

Tony Duarte

In Conversation With

ABB’s Longmeadow facility in Johannesburg

now has a 750 kWp, monocrystalline rooftop

PV system connected to a microgrid.

Electricity+Control

August ‘16