THE ENERGY INDUSTRY TIMES - AUGUST 2022
Special Technology Supplement
Delivering a stable energy
transition
The retirement of large coal, gas and nuclear power plants, combined with the rapid rise of variable renewable
energy is making the operation of a stable transmission network increasingly difcult. Siemens Energy describes the
solutions that are available to network operators and offers a glimpse of some of the more advanced technologies it is
developing. Junior Isles
Flexible AC transmission system
(FACTS) devices.
Typical applications are in electric
power transmission, electric power
distribution, electrical networks of
heavy industrial plants, arc furnaces,
high-speed railway systems and other
electric systems, where voltage sta-
bility and power quality are of the
utmost importance. STATCOMs
have been deployed in hundreds of
applications around the world and
according to Siemens Energy, de-
mand is increasing every year.
The technology is based on voltage
source converters (VSCs) with
semiconductor valves in a modular
multi-level conguration. The
STATCOM design and fast response
makes the technology very conve-
nient for maintaining voltage during
network faults (as STATCOMs are
capable of providing fast fault cur-
rent injection limited to the rated
current), enhancing short-term volt-
age stability. In addition, STAT-
COMs can provide power factor
correction, reactive power control,
damping of low-frequency power
oscillations (usually by means of re-
active power modulation), active
harmonic ltering, icker mitigation
and power quality improvements.
Like STATCOMS, the need for
synchronous condensers (sometimes
called a rotating phase shifter) has
also increased signicantly over the
E
urope, and indeed many parts of
the world, is at a crossroads. As
the electricity sector works its
way through the energy transition, grid
operators are faced with one
overarching challenge: how to keep
power grids stable as it shifts from a
centralised, fossil fuel-based market to
one that is increasingly underpinned
by variable renewable generation in a
more distributed network.
As Europe faces searing tempera-
tures and the crisis in Ukraine deep-
ens, never has it been clearer that
there is a need to accelerate the de-
ployment of wind and solar to address
the twin threats of climate change and
the need for energy independence. Yet
without an adequate electricity net-
work to deliver renewable energy to
homes, businesses, and EV (electric
vehicle) charging stations, the green
transition will be ineffective.
Speaking at a conference co-organ-
ised by E.DSO and ENTSO-E – the
organisations representing Europe’s
distribution system operators (DSOs)
and transmission system operators
(TSOs) – Sonya Twohig, ENTSO-E
Secretary General, said: “Innovation
in power system distribution and
transmission and its fast uptake is key
to Europe’s energy transition. The
needs for exibility will ramp up to
ensure the stability and security of a
power system with an increasing
share of variable renewable energy
sources.”
It is an area that Siemens Energy
also agrees is crucial. The company
noted that while grid expansion must
be accelerated in order to meet future
electricity demand, strengthen resil-
ience and ensure security of supply,
the ever-growing amount of decen-
tralised and uctuating renewables
poses new challenges for grid stability.
It said that the expansion not only
calls for new high voltage direct cur-
rent (HVDC) lines, but also requires
investment in grid stabilisation.
Hauke Jürgensen, Senior Vice
President High-Voltage Grids at Sie-
mens Energy, explained: “Looking at
it from the transmission side, it’s clear
that less fossil-based generation, i.e.
rotating equipment in the form of
turbines on the grid, and more volatile
wind and solar leads to grid instability.
This means you need solutions to re-
place the missing rotating equipment
that provides inertia and ensures fre-
quency stability.”
Volker Hild, Siemens Energy’s
Vice President Grid Stabilisation,
added: “More and more power plants
are being retired – coal obviously, as
well as nuclear in parts of Europe
and maybe gas later on. With all
these retired power plants in Europe,
as well as North America, this means
inertia is missing and therefore
short-circuit power is missing. Solar
and wind farms can hardly, if at all,
provide this inertia and short-circuit
power. It’s a problem for the overall
stability of the network, and this is
why we are seeing a big demand for
grid stabilising solutions.”
Grid operators are seeing these in-
stabilities in the form of voltage uc-
tuations, frequency deviations and
situations where they are operating in
contingency or emergency modes for
much longer periods of time.
To counter these problems, which
can manifest at various voltage levels
in the network, Siemens Energy rec-
ommends a top-down approach in
terms of where to target stabilisation.
Hild explains that, if the necessary
stability and interconnection can be
provided at the high voltage level, it
will also help stabilise the network at
the lower voltage levels.
“If you try to provide stabilisation
at the lower voltage level, it will
have less of an effect at the higher
voltage level. So it is more or less a
top-down approach. There are needs
on the distribution side but because it
can be done easier and with less in-
vestment, there is less public focus
on it,” he said.
Transmission grid stabilisation
technologies are essentially split into
two main categories: Static Synchro-
nous Compensator (STATCOM) de-
vices and synchronous condensers.
STATCOM devices are essentially
based on power electronics, which
allow voltage to be adjusted in the
network within milliseconds by in-
jecting or absorbing reactive power.
The technology is categorised under
In a rapidly changing energy world, grid operators need a quick solution to respond to
uctuations when power plants are shut down and renewables are added
Jürgensen: You need solutions to replace the missing rotating
equipment that provides inertia and ensures frequency stability
Essentially, it is a STATCOM that
includes a large number of super-ca-
pacitors. A super-capacitor is a high-
density energy storage device that
typically stores 10 to 100 times more
energy per unit volume or mass than
electrolytic capacitors. It can accept
and deliver charge much faster than
batteries, and tolerates many more
charge and discharge cycles than re-
chargeable batteries.
The frequency stabiliser will allow
operators to absorb or inject reactive
power for voltage stabilisation like a
STATCOM. In addition, however, if
the network frequency is dropping
due to high loads or large uctuations
due to a solar or wind farm dropping
out, the SVC PLUS FS recognises the
frequency deviation. In the case of a
frequency drop, it injects the active
power stored in the super-capacitors
within seconds. Such an injection of
active power will have a stabilising
effect on the frequency.
Hild added: “The reason we are
doing this with super-capacitors and
not with batteries is because frequency
support needs a high amount of active
power in a very short, limited time.
Unlike batteries, super-capacitors can
charge or discharge within seconds.
Batteries are used for storing energy,
which is then delivered in minutes or
hours. So, if it’s an imminent network
contingency case where there is a
major problem, you need very fast
reacting devices. The utility we have
sold this to in Germany has a strong
need for that frequency support and
other TSOs in Germany have already
identied a large number of installa-
tions that will be needed over the next
few years.”
Looking further down the line, there
are a few other important technolo-
gies that are under development. The
UPFC (Unied Power Flow Control-
ler) PLUS is one such technology that
is already part of the Siemens Energy
portfolio. The technology is based on
power electronics, which actively
adjusts the power ow in an AC net-
work within milliseconds.
It can rapidly bypass overloaded
line sections, provide reactive power
and dynamic voltage control, and
utilise assets to physical limits with-
out the need for safety margins.
“An existing AC network is like a
highway where there are multiple
lanes. In a high-voltage transmission
line there are several circuits that are
interconnected and there are different
loads on it. Depending on where the
generation feeds in and where the
demand is, it can lead to an over-util-
isation of one line. This means you
can exceed the load that one line can
safely carry, while another line in the
other circuit might be under-utilised.”
According to Siemens Energy, there
are limited options in the market for
managing such scenarios, apart from
physically switching lines on and off
or switching loads from one circuit to
another. This, however, is not fast and
requires manual intervention.
“The UPFC PLUS is a fully auto-
mated system that will actively adjust
the impedance of the transmission
line and therefore make the load ow
differently, since current always ows
in the path of least resistance,” said
Hild.
When managing a fully loaded net-
work during a fault, the line loading
will be at 100 per cent, which will
cause a voltage drop. Because of this,
reactive power will need to be pro-
vided to ensure stability. Also, when
managing or changing the load ow
during a network fault it is possible
that there will be some stability
problems in the transmission line or
network that will require insertion of
active damping. The UPFC PLUS
provides active damping by injecting
a voltage with a controlled magnitude
and angle to ensure that the line and
voltage are in phase.
With a UPFC PLUS the load can be
balanced across numerous lines in
milliseconds. When the network is
healthy there is no time pressure to
balance the ow, this can often be in
the range of several minutes, but
when a fault occurs you must act in
milliseconds. The rst requirement
for any active or dynamic load ow
management is to perform the load
ow management in two time zones,
both for slow control and fast control.
This is where UPFC PLUS outper-
forms other ow management com-
ponents. In the eld of dynamic load
ow management, it supplies fast
last few years. This piece of equip-
ment is essentially a generator weigh-
ing several hundred tonnes, which
spins purely to provide a rotating
mass and therefore inertia. It does not
produce energy but is driven up to the
network frequency using a motor,
leaving the rotor to spin at the network
frequency. This rotating mass essen-
tially provides the inertia that would
have come from the turbines of the
retired conventional power plants.
Again, Siemens Energy says it sees a
growing demand for such equipment
across Europe, the US and other parts
of the world.
Choosing between the two technolo-
gies depends on the network’s needs.
“It always depends on the electro-
technical aspect that is missing,” ex-
plained Hild. “If it is voltage support
or reactive power, then you use
STATCOM; if it’s missing inertia or
short-circuit power due to power
plant retirements, then it’s most likely
a synchronous condenser.”
While it is easier to identify things
such as voltage deviations through
ickering lights, for example, as-
sessing whether inertia or short-cir-
cuit power is needed in the grid, is
something only a grid operator can
identify through measurements and
calculations. “It’s a matter of how
easily failures in the network could
lead to blackout,” noted Hild. “If the
network is weak because rotating
mass is missing, then short-circuit
power is low. This means that even a
small variation in voltage or power
ow could cause load shedding and
potential blackouts in regions. This
is why network operators always
check the strength of the network in
terms of inertia and short-circuit
power and how stable voltage and
frequency are.”
The Moneypoint project, which
Siemens Energy is working on in
Ireland, is a good example of how
grid operators are providing grid iner-
tia using synchronous condensers.
At the end of April last year the
company secured a €50 million con-
tract to supply a synchronous con-
denser system to the Electricity Sup-
ply Board (ESB), Ireland’s leading
energy company. The grid stabilising
system will be developed at the
Moneypoint power station located in
southwest Ireland near Kilrush,
County Clare.
ESB recently announced the launch
of Green Atlantic @ Moneypoint, an
ambitious plan to transform the
County Clare site into a green energy
hub, where renewable technologies
will be deployed over the next decade
with the capacity to power 1.6 million
homes. The synchronous condenser, a
key component of ESB’s Green At-
lantic @ Moneypoint project, will be
the rst in the country and incorporate
the world’s largest ywheel used for
grid stability.
The facility will enable an increased
integration of wind power into the
Irish grid by providing sufcient iner-
tia for frequency support, short-circuit
power for system strength and reac-
tive power for voltage control. The
project is expected to enter operation
this summer.
Commenting on the project, Hild
said: “The site has existing coal red
power plants that ESB want to retire
in the near future. At the same time,
a number of wind farms are being
built in the western part of Ireland,
which need to be connected to the
Irish and then subsequently the Eu-
ropean grid. So here we have the
dual effect: a lot of wind power is
being injected into the network,
which we know will soon become
much weaker because the large coal
red power plant will retire. This is
why ESB identied the need for a
large synchronous condenser in the
Moneypoint area so that the grid can
handle the inux of wind power.”
He added: “Since the mass of the
generator cannot provide enough
inertia, a ywheel, which is simply a
rotating mass, is placed on the exten-
sion of the rotor shaft. The size of the
ywheel is determined by the required
electrical features – the amount of
inertia, short-circuit power and maybe
reactive power. Depending on the re-
quirements, we determine whether a
larger generator with a smaller y-
wheel, or a smaller generator with a
larger ywheel, or a large generator
without a ywheel is the best and
most economic choice.”
Just last month, a similar project
began operation in the UK with the
commissioning of two synchronous
condenser units at the Killingholme
power station in Lincoln. The project
included the re-purposing of two
steam turbine generators and install-
ing ywheels at the site. The technol-
ogy will allow Uniper to deliver es-
sential grid stabilising services to
National Grid ESO without the need
to generate power.
Siemens Energy was appointed to
provide the solution after Uniper was
awarded four six-year contracts by
National Grid ESO in 2020 to provide
inertia services and voltage control to
the grid under phase 1 of its Stability
Pathnder at its Killingholme and
Grain sites.
Another important more recent
technology in the eld of grid stabili-
sation is what Siemens Energy calls a
frequency stabiliser. Currently in the
nalisation stage, the company has
secured its rst order for its ‘SVC
PLUS FS’ (Frequency Stabiliser) for
a project in Germany.
Special Technology Supplement
THE ENERGY INDUSTRY TIMES - AUGUST 2022
Hild: We are seeing a big
demand for grid stabilising
solutions
To provide maximum inertia, synchronous condensers solution can be extended with additional
rotating mass from a ywheel
Siemens Energy’s synchronous condenser and ywheel at their
arrival at Moneypoint power station in Ireland
THE ENERGY INDUSTRY TIMES - AUGUST 2022
Hild explained: “Just think of a
branch on a small tree; if it’s a very
tiny branch, i.e. a weak branch, it
doesn’t take much force to move it
away but if it’s a large branch, it would
take much more force to move it. It’s
the same on a network. If a network is
weak, a small injection of voltage or
reactive power could lead to a signi-
cant impact. “This is why you always
need a good mixture of power elec-
tronics which controls voltage and
response in terms of active power
management, coordinated reactive
power control and can insert active
damping to ensure stability. For effec-
tive dynamic load ow management,
a response time in milliseconds is re-
quired to balance the load during the
fault condition and manage the net-
work integrity in terms of stability
and thermal limits. This is achieved
by the voltage source converter,
which is one of the main components
of the UPFC PLUS system.
According to Siemens Energy,
UPFC PLUS can easily be imple-
mented in an existing grid and allows
fast reaction times for an efcient
load ow management. With this op-
tion and the high dynamic control,
transmission assets can be operated
closer to their physical limits. UPFC
PLUS is therefore seen as an innova-
tive solution to meet with the growing
requirements of the grid through dy-
namic load ow management that
adapts easily to changing in-feed from
renewable sources.
Nevertheless, the choice between
the array of technologies that can be
used to strengthen grids and provide
grid stability and how the various
technologies should be deployed is
largely down to the planning of the
TSO and its network analysis. This
gives a clear indication of what is re-
quired, where, and when. At the same
time STATCOM after STATCOM
cannot be indenitely placed on a
network. As a network becomes
weaker, small adjustments of reactive
power or voltage can lead to quite a
signicant impact on the network.
power regulation fast, and inertia and
short-circuit power, which comes
from either power plants or synchro-
nous condensers. You cannot ignore
one or the other.”
Looking further to the future and the
need for this mixture, Siemens Energy
is now developing a technology that
is kind of a combination of both
STATCOM and synchronous con-
denser – capable of meeting many of
the requirements at the same time.
Known as an Asynchronous Rotating
Energy System Stabiliser (ARESS), it
consists of a rotating machine similar
to a synchronous condenser but is a
different type of electrical machine in
that it is asynchronous and uses
power electronics to adjust the ma-
chine’s performance.
Hild noted: “With this technology,
we are able to provide much stronger
support in the network with the re-
gards to inertia and reactive power.
It’s kind of like a Swiss Army knife of
different solutions we have combined
in one technical offering.”
Siemens Energy is developing the
technology with Amprion for the 50
Hz market, as well as with Dominion
Energy in the US for the 60 Hz
market. “Both utilities have identied
the clear advantage of having such a
multi-tool solution, and were willing
to collaborate closely with us on the
development of this technology,” said
Hild.
Currently R&D activities are pro-
ceeding “at full speed”, with execu-
tion of the rst project set to begin in
2023 and commissioned in 2025.
Commenting on the project, Hild
said: “Since we are doing the project
in collaboration with TSOs, we give
them certain results and they simulate
the performance and give us feedback
for us to incorporate back into our
design.”
Although ARESS can perform the
task of STATCOMs and synchronous
condensers, Hild does not see it re-
placing them. “With any multi-use
offering, like a Swiss Army knife, it
can do quite a lot but it cannot perform
specic functions as well as a tool
specically designed for a purpose.
Also, if you only have a voltage
problem, why go for a solution that
has many more features that you
don’t require? You end up paying for
something you don’t need.
“The use cases [for ARESS] are in
situations where many individual
challenges for grid stabilisation come
together but none of the challenges
are unusual enough to require a
uniquely designed solution.”
No doubt there will likely be other
challenges in the future, as the net-
work topology changes with increas-
ing renewables. For example, Hild
believes that while the addition of
more battery storage will help the
integration of renewables by provid-
ing energy when the sun is not shin-
ing or the wind is not blowing, it will
make regulation of the network more
complicated.
“In the future you will be injecting
or absorbing energy at many more
multiple points in the network, unlike
today where there are a smaller num-
ber of power plants or large in-feeds
to the grid,” noted Hild.
Siemens Energy believes that with
all of its technologies, it has a power-
ful portfolio that targets various sta-
bilisation problems in the network
and can combine these solutions ac-
cording to the specic AC network
requirements.
But its message is clear. Jürgensen
summed up: “TSOs really underesti-
mated the scale of the challenge that
closing all of these large power plants
would present to the grid. What is
actually needed to stabilise the fre-
quency of the grid is quite tremen-
dous. In addition to all the transmis-
sion projects that we have, we are
seeing a similarly high demand for
grid stabilisation; and urgently. We
cannot wait 10 years to do it, other-
wise you really jeopardise the stabil-
ity of the grid.
“Grid planners need to plan ahead
– carefully consider this effect and
plan the investment that is needed.”
Recently, Siemens Energy has
secured its rst order for its
‘SVC PLUS FS’ (Frequency
Stabiliser) for a project in
Germany
Special Technology Supplement
Aerial view of various components in a typical UPFC PLUS system
Compared to traditional power ow controllers, UPFC PLUS controls power ow in just
milliseconds
