pwr_june-2024 - 30

GRID MANAGEMENT
remote or rural areas adjacent to substations
where there is already a critical
need for grid stability.
In Estonia, too, transmission system
operator Elering AS is working with
Siemens Energy to add three synchronous
condenser plants. These Flexible
Alternating Current Transmission System
(FACTS) units are being installed
to stabilize the grid and manage the ongoing
integration of renewable energy
generation. Each 330-kV synchronous
condenser will be connected to the highvoltage
transmission network via a stepup
transformer. Each of the three plants
will be able to provide the grid with reactive
power of -50/+50 MVAR and up
to 1,750 megawatt-seconds of inertia.
They will have a short-circuit capability of
more than 900 MVA.
" Without the inertia provided by this
equipment, the hazard for the unexpected
shutting off of the power system and leaving
consumers without power would be
very high, " said Taavi Veskimägi, chairman
of the Management Board of Elering.
Repurposing Old and New
Turbines
An alternative approach to a purposebuilt
synchronous condenser is to make
use of existing steam and gas turbines.
Instead of them being decommissioned
and dumped, some are being converted
into synchronous condensers by adding
a synchronous self-shifting (SSS) clutch
that can disengage the generator from
the turbine. The turbine brings the generator
up to grid synchronous speed,
disconnects the turbine from the generator,
and the turbine is shut down.
From that point on, only the generator
is spinning as a way to supply or absorb
reactive power, provide grid support,
and maintain inertia.
This approach is favored by ARENA in
Australia. It said in its report, " Our analysis
indicates that repurposing generators
as SCs can provide a viable solution for
delivering the required security services.
This approach can be used in combination
with new SC investments to mitigate
the risk of insufficient services. " The
agency believes re-purposing generators
as synchronous condensers will:
■ Stabilize the voltage and thus provide
system strength.
■ Contribute inertia.
■ Provide a source of reactive power for
voltage control.
■ Contribute positively to fault levels.
■ Lower costs compared to purchasing
new synchronous condensers.
Some users only utilize an aging turbine
to supply reactive power. Others are
looking to gain more value from their aging
assets. " Turbines can be easily converted
to synchronous condensers, " said
Hendry of SSS Clutch. " Many choose to
have the option of using their machines
for peaking power or backup power
should the need emerge. "
With so many coal plants being
mothballed, Hendry suggested that facilities
consider converting old steam
turbine generators to synchronous condensers
by disconnecting the turbine
and adding an acceleration drive with
an SSS Clutch at the exciter end of the
generator. The steam turbine could be
reconnected if seasonal power generation
is needed or in some emergency
sometime in the future.
Similarly, gas turbine plants can add a
clutch (Figure 5), so the machine is configured
to both generate power when
needed or provide grid support and ancillary
services. Although the machine may
mainly operate for synchronous condensing,
the clutch can be reengaged
to generate electric power if there is a
sudden need for peaking power, as often
happens in renewable-heavy grids.
4. This Siemens Energy synchronous condenser
has been installed at a facility in Rassau
in the UK. Courtesy: Siemens Energy
30
Old Turbines, New Tricks
In recent years, many utilities have discarded
rotating assets in favor of renewables.
It appears that some are having a
change of heart-at least concerning the
potential value of aging rotating assets.
Case in point: SSS Clutch has received
orders for 35 clutches intended for GE
LM6000 PF+ Sprint models in recent
months. The clutches are built into a load
gear as the gas turbine operates at a
higher speed than the generator at 3,600
rpm, which is needed for a 60-Hz grid.
The gear is needed to create synchronization
with the grid.
www.powermag.com
5. The Sask Power Ermine plant in Canada
is one of many adding SSS Clutches to GE
LM6000 PF+ turbines to enable them to operate
as synchronous condensers. Courtesy:
SSS Clutch
Many of these orders were from the
Tennessee Valley Authority (TVA), which
is now rolling out 1 GW of wind turbines
and solar PV in Tennessee. While it intends
to decommission a great many
coal and other rotating assets, it is keeping
many of them available for peaking
power and grid support. To date, TVA
has received 10 GE LM6000 PF+ gas
turbine generators with SSS Clutches
and has another 20 on order. These are
being fitted on LM6000s for peaking
power. " By enabling these LM6000s
to run as synchronous condensers, the
TVA is ensuring it has enough inertia,
system stability, and reactive power
support, " said Hendry.
TVA's strategy is to retire coal assets
while adding lots of wind, solar, and natural
gas to the grid, and fitting some of
its turbines to operate as synchronous
condensers to support greater penetration
of renewables. Accordingly, three
new GE 7FA.05 natural gas units recently
came only at the Paradise Combined
Cycle Plant in Kentucky, adding 750 MW
to TVA's operating fleet. They joined three
other combustion turbines that began
operating in July at the Colbert site in Alabama
(another 750 MW). That's just the
first part if a plan to add 3.8 GW of natural
gas generation to the grid by 2028.
" Many of TVA's new combustion turbines
are replacing older, less-efficient
units, " said Jamie Cook, TVA's general
manager of Major Projects. " Natural gas
units are cleaner than coal-fired generation.
We can also operate them when
other sources of generation, like solar,
aren't available. They supplement those
sources with reliable power when we
need it most. " ■
-Drew Robb (drewrobb@sbcglobal.net)
has been a full-time freelance writer for
more than 25 years specializing in
engineering and technology.
POWER | June 2024
http://www.powermag.com
pwr_june-2024

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