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Power grids have traditionally relied on directly grid-connected synchronous
generators1)
from hydro and thermal power stations contributing
the synchronous attributes of system strength and physical inertia2).
As renewable energy is added to modern power grids, the requirement for
synchronous attributes from renewables has been brought into sharp focus.
This has been highlighted by the experience in
the blackouts in South Australia
September 2016, Great Britain August 2019, Europe January 2021,
and is an issue under discussion by grid
operators and regulators internationally.
Most countries view their fastest and easiest renewable
addition to be existing asynchronous wind and solar technologies. However,
over-reliance will reduce grid stability in case of any shocks to the grid.
Batteries are only a partial solution as the power electronic converters
(PECs) associated with asynchronous wind and solar power provide a
bottleneck for the high currents required for “system strength”. It is
prohibitively expensive to expand that “bottleneck”. Instead, Australian
regulators and network companies have identified that dedicated synchronous
generators, called synchronous condensers, provide the least-cost way to
compensate for increasing amounts of asynchronous PEC renewable generation.
That being the case, the cost advantages of synchronous wind turbines become
compelling.
SyncWind
has proven, cost-effective technology for enabling each wind turbine to
drive a synchronous generator directly connected to the grid. Recently
patented in a novel broad-band variable speed system, its synchronous
power-train technology will scale up to multi-megawatt turbines while
improving the cost and reliability advantages that have already been
realised in over 1000 turbine-years of operation.
Two primary advantages that synchronous generators are uniquely able to
deliver are system strength and physical inertia.
SyncWind.’s synchronous wind turbine power-train supports system strength by
providing huge “short-circuit currents” (5-10 times rated) to rectify
voltage faults and restore synchronism after a fault.3)
It also
enables FFR using the wind turbine rotor’s flywheel energy (and any reserve
power that the turbine’s gearbox can support for short durations) as well as
the direct generator inertia. It also provides the full range of attributes
that synchronous generators provide to the grid:
-
Voltage support through a full range of reactive power capabilities.
-
Synchronous condenser mode even with no wind.4)
-
Controllable power level and ramp rates.
Physical inertia instantly resists speed changes
in a grid shock, such as the failure of a large power plant or a
transmission line. Generator speed determines grid frequency, and vice versa
for all the generators on a synchronous grid. The larger the shock or the
lower the inertia, the faster the rate of change of frequency. Therefore,
grids need inertia for stability.
What does physical inertia2)
do?
It enables grid frequency to be stable. Any device or system with
physical inertia has the momentum to “ride through the bumps”, without
slackening speed. By contrast, controlling a system with little
inertia involves working very fast with the “gas pedal” to alternatively
squirt in more power, or reduce the amount squirted in, depending on whether
the system has slowed down or sped up. In an electricity system,
inertia normally comes from synchronous generators, which inherently
(through electro-magnetic reaction) combine their physical inertia in an
instantaneously co-ordinated way, even when hundreds of kilometres apart.
But most wind turbines’ generators are not synchronous like this. They have
zero inertia because their spinning masses are decoupled electronically from
the grid.
So, the increase of
renewable energy is causing system inertia to reduce. With a low
inertia electricity system, multiple generators would have to “work the gas
pedal” in an instantaneously co-ordinated way, to maintain a steady
frequency. This is unproven with current PEC wind turbine technology. With
zero inertia connected to the grid and widespread geographic distribution ,
they have a good chance of undershooting, overshooting, or getting out of
rhythm and losing control, causing blackouts.
While asynchronous wind turbines can in principle provide Fast Frequency
Response “FFR” (sometimes referred to as "synthetic inertia", see
inertia2) below) to
respond to grid shocks via their power electronics, this relies on a mixture
of power stored in the flywheel energy of the wind turbine and any ability
to increase output power (for example if the turbine is running
significantly derated). Solar panels are less able to provide FFR due to the
lack of a spinning mass and can only provide it if they are running
significantly derated or in conjunction with expensive batteries.
SyncWind.’s power-train enables a synchronous generator to be directly connected to the
grid, providing inertia for grid stability the same as traditional
power generators, without power electronics. This delivers direct resistance
to frequency change as opposed to providing only FFR, which, equally, the SyncWind.’s
system will also be able to offer.
SyncWind.’s system has been
IEC certified by Lloyds Register
and has a track-record of more than 1000 turbine-years of synchronous wind
power operation. The cost-effectiveness and track record of
SyncWind.’s
system differentiate it from a small number of other synchronous wind
turbine systems that have been tried.
SyncWindi welcomes the
Media Release: Blueprint for a world-class electricity system, 09 June 2017
by the Australian Government’s Chief Scientist, Dr. Alan Finkel, of his
final report
Independent Review into the Future Security of the National Electricity
Market - Blueprint for the Future
recommending a range of
changes to the Australian electricity market to improve system security.
These
Recommendations
include requirements for minimum amounts of
inertia and all of the above attributes that synchronous generators have
traditionally provided, and SyncWind.’s system provides within the wind
industry.
SyncWind.’s
system has recently been extended with a new
patented system to enable broad-band variable-speed operation which wind
turbines require in lower wind climates. SyncWind
Director Geoff Henderson stated “Our patent protected technology, which is
scalable to both the mid-size and multi-MW turbine market, eliminates use of
power electronics and results in significant noise, weight and cost
reductions. Further, and as evidenced by demand for renewable energy to
assist grid stability, SyncWind.’s proven designs and technology are an idea
whose time has come”.
A cursory examination of the issue of grid stability in the renewable
transition makes it abundantly clear that there is increasing market concern
about this issue. Difficulties are now beginning to be apparent in China,
Europe, Texas, Australia and remote communities like the Pacific Islands
which are investing heavily in renewables including solar. As the world
moves to renewable energy in the next few decades, it will become an issue
for all nations.
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