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Mechanical Technology — July 2016
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Special report
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“
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
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).
An infographic outlining the key features of ABB’s microgrid installation at Longmeadow.
Microgrids, a resilient, cost-effective
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.