Special reprint from BWK – Das Energie-FachmagazinQVolume 9/2007, pages 48 – 53
Authors: M.Sc. Niels Andersen, Dipl.-Ing. Jorgos Megos, Prof. Dr.-Ing. Dietmar Retzmann
Static Var Compensator (SVC)
for Offshore Applications
Answers for energy.
Static Var Compensator (SVC)
for Offshore Applications
Dynamic Voltage Control of Offshore Wind Turbines
Security of power supply (i.e. high reliability, blackout prevention) is the crucial issue while planning and
extending power grids, as no society can survive without electric power. In addition to this, for reasons of
environmental protection and saving of exhaustible energy resources, the tendency towards sustainability
is gaining in importance. The main part in terms of sustainability is played by regenerative energy sources,
particularly those which are completely CO2 free, such as hydro and wind power. In case of Europe, wind
power is gathering pace; hydropower, however, is not highly represented, with the only exception of the
Nordel grid. Using the example of wind power in Denmark, the following article shows, how sustainability
and security can be ideally combined by means of power electronics to achieve a „Smart Grid“.
Authors
M.Sc. Niels Andersen, Senior Project Manager, System Studies and Specifications
HVDC and FACTS. SEAS-NVE, Haslev, Denmark.
Dipl.-Ing. Jorgos Megos, Senior Project
Manager FACTS. Siemens PTD, Erlangen,
Germany.
Prof. Dr.-Ing. Dietmar Retzmann, Siemens Top Innovator, Technical Marketing
and Innovations HVDC/FACTS. Siemens
PTD, Erlangen, Germany.
2
he reliability of power supply is directly connected with its security.
It depends on the structure and
quality of transmission system, power
generation and, to some degree, the properties of power consumers. For reasons
of sustainability, particularly an ever increasing integration of wind power, extreme requirements to flexibility and robustness of power grids are posed. Even
in case of offshore areas with strong
winds, wind power is highly fluctuating,
which has a significant impact on voltage and frequency stability in the grid.
The following example of a Static Var
Compensator (SVC) in Denmark will
show how, despite a significant share of
wind power, the stability of the grid can
be maintained that is, how both sustainability and security can be achieved.
T
This can be made possible by means of
power electronics with dynamic fast
control which makes the grid more flexible, and subsequently able to take in
more regenerative and distributed energy sources. A flexible grid such as this is
referred to as a „Smart Grid“ [1]. Power
electronics in high voltage systems is represented by both HVDC (High Voltage
Direct Current) and FACTS (Flexible AC
Transmission Systems) [2], which also
includes Static Var Compensators.
The power generation system of Denmark, in the eastern part of the country
in particular, changed greatly in the last
decades. The introduction of small distributed generation units and constant
evolution of the power market due to
deregulation and privatization have
brought about significant changes of
The existing transmission grid in the
east of Denmark (Figure 1) comprises
both overhead lines and cables for both
the highest voltage levels, i.e. 400 kV
(marked red) and 132 kV (marked black).
There are interconnections with AC submarine cables to the southern part of
Sweden at a total capacity of 1,900 MW.
The new HVDC project Storebælt (“Great
Belt” – Siemens received the contract in
May 2007) is going to interconnect the
Danish eastern grid (depicted in the
picture), which is operated synchronously with NORDEL, and the western
grid, which in its turn is operated synchronously with UCTE, by means of a
DC cable (600 MW / 400 kV DC). Further
south, on the island of Lolland, the new
Static Var Compensator Radsted is si-
High Voltage Direct Current
Static Var Compensator
Sweden
(50/132 kV)
Figure 1
(50/132 kV)
(50 kV)
Middelgrunden
(400/132 kV)
(50/132 kV)
The Transmission System
in Eastern Denmark.
HVDC Storebælt
Zealand
(50 kV)
power generation facilities
are situated in the strong
northern grid, while the
main part of wind turbines
SVC Radsted
Planned
are connected to the weak
132 kV Cable
southern one.
The Nysted Offshore Wind
Farm at an installed nominal
Radsted
power of 165 MW is also siLolland
tuated in the south. This site
20 km
was put into operation
in 2003 [4]. The construction
Planned
Nysted Offshore
Rødsand 2
of another offshore wind
Wind
Farm
Wind Farm
farm (Rødsand 2, 215 MW
HVDC-Link „Kontek“ to Germany
projected capacity) is planned nearby. Prior to commissioning the NOWF, the AC submarine catuated. Kontek is another HVDC instalbles between Zealand and Lolland were
lation (600 MW / 400 kV DC) which leads
reinforced in order to enhance the transto Germany [3, 4]. The peak load in the
fer capacity of the grid and, subsequenteast of Denmark is about 2,870 MW and
ly, to allow for the optimal use of the
the total installed generation capacity
maximum generation capacity of the
goes up to 4,360 MW.
wind turbines in the event of a power
The depicted grid of eastern Denmark
failure. However, before the new wind
is strong in its northern part (Zealand),
farm Rødsand 2, which will be conwhereas its southern part (Lolland) is
nected to Radsted as well, can be put incomparatively weak. Large central
to operation, the Lolland grid
must be boosted by adding
another AC submarine cable
3AC 50 Hz 132 kV
to Zealand (see Figure 1, lefthand corner).
S = 80.2/40.1/40.1 MVA
u
= 9.5 %
The AC submarine cable
Δ
connections between the is3AC 50 Hz 9.0 kV
3AC 50 Hz 9.0 kV
lands will not, however, bring
any significant improvement
L
L
R
R
to the short-circuit power in
L
L
Lolland. This is the reason
C
C
why the grid at that location
is highly sensitive to changes
in the reactive power rate.
Wind turbines with asynchronous generators with
TCR1
F1
F2
TCR2
squirrel cage rotors (which
are prevailing in the east of
Denmark) get their magnetiFigure 2
zation reactive power from
the grid. The higher the actiSingle Line Diagram of the Static Var Compensator.
ve power generation is, the
N
Y
The grid of Denmark – on the
Road to a “Smart Grid”
HVDC
SVC
Y
power flows in the transmission system
and have made them virtually incalculable. Wind turbines in the eastern part
of the country have the installed capacity of around 740 MW, including NOWF,
Nysted Offshore Wind Farm, at a rating
of 165 MW. Besides active power, large
offshore wind farms also require the infeed of dynamically controlled voltage
to the integrated network. The dynamic
voltage control can be carried out either
integrated in the wind farm or/and at
the “receiving” AC station onshore.
In the following, the implementation
and features of the Static Var Compensator are described, which was commissioned in December 2006 and provides
compensation for wind turbines of the
NOWF as well as other wind turbines, situated onshore. In order to obtain a
prompt permit by the authorities to carry out the project, a number of aspects
of environmental protection was considered in the design and construction of
the SVC, e.g. extremely high requirements to harmonic content of compensator current and voltage at the point of
connection to the grid as well as to the
noise pollution in the immediate surroundings of the facility. This was meant
to make sure that the Static Var Compensator can be operated at the chosen
location without restrictions with respect to quality of power and environment. The installation height was another important aspect in terms of legal
permit.
k HV-LV
F2
F1
F2
F1
TCR2
TCR1
F1
F2
3
greater also the demand for reactive
power will be. This resulted in the introduction of technical regulations, specifying the local compensation of the reactive power consumption of wind turbines.
These regulations specify for NOWF
site in particular that its reactive power
exchange with the grid must not exceed
± 10 % of the installed nominal active
power at the point of connection to the
grid. This means that the reactive power
exchange between NOWF and the grid
at the connection point must be in the
range between 16.5 and +16.5 MVAr. The
wind farm itself corresponds to these requirements owing to special thyristorswitched capacitors for asynchronous
machines.
However, the wind farm is connected
with the transmission system by means
of a 29 km-long three-phase AC cable as
well as a 132/33/33 kV mains transformer which is situated on the territory of
the wind farm. The defined point of connection to the grid is the 33 kV busbar
on the 132/33 kV transformer. Due to reactive power losses in the transformer
and the cable, the reactive power exchange between the NOWF and the
transmission grid can (permissibly) exceed the range stated above. This is the
reason why further measures must be
taken to make the weak system in Lolland capable of taking in more wind
power and, consequently, to evolve into
a “Smart Grid”.
Course of Action
After commissioning the offshore
wind farm NOWF, significant and frequent voltage fluctuations in the grid
were detected. They resulted in the increased wear of the tap changers of the
transformers. For this reason SEAS-NVE
decided to install a Static Var Compensator in Radsted. SEAS-NVE, the largest
utility of the country, supplies Danish
customers with power, telecommunication services and other products, connected with the power network. This
company also operates the NOWF offshore wind farm as well as the newly installed SVC Radsted, which will be described in detail later on.
Siemens A/S in Denmark was awarded
the contract to carry out this turn-key
project. Thanks to close cooperation
with the department of Siemens PTD
(Erlangen, Germany), responsible for
4
Mains Voltage USVC [pu]
1.8
1.8
60 ms
Figure 3
1.6
Continuous Operating Range
Time Limited Overload Range
Base Voltage [1 pu = 132 kV]
1.4
600 ms
Base Current [1 pu = 100 MVA]
1s
1.3
Operating Range of the
SVC: Voltage-Current
Characteristic Curve.
1.2
80.2 MVAr
88.2 MVAr
10 s
1.1
1.1
1.0
Design Point Capacitive
80.2 MVAr at 1.0 pu
0.9
Design Point Inductive
65 MVAr at 1.0 pu
0.8
0.6
0.4
0.25
Minimum Continuous Operating Voltage
0.2
Mains Current ISVC [pu]
1.0
1.0
Capacitive Current
Inductive Current
FACTS installations, the project was carried out on time and to the utmost satisfaction of SEAS-NVE.
The SVC has been commercially operated since December 2006. The main
functions of the SVC Radsted are the reduction in voltage fluctuations, support
of the grid in case of disturbances
(blackout prevention) and compensation
of demand for reactive power of NOWF.
The SVC is also sufficiently dimensioned
for the compensation of the future wind
farm Rødsand 2 at a projected capacity
of 215 MW.
A. Configuration of the SVC
In the “classic” line-commutated technology, the SVC is carried out as a
12-puls controller which includes two
thyristor-controlled reactors (TCR) with
corresponding filters, meant to cover the
demand for reactive power and to comply with strict harmonic requirements.
The arrangement of the SVC is depicted
in Figure 2. The connection to the grid is
provided via a 132/9/9 kV transformer at
a rated fundamental frequency current
of 3,000 A on the 9 kV side. The transformer has a nominal rating of 80 MVA as
well as one secondary winding with a
star and one with a delta connection respectively.
The nominal rating of the SVC is
80.2 MVAr capacitive and 65 MVAr inductive at a mains voltage of 1.0 p.u.
(132 kV), as shown in Figure 3. A STATCOM (Static Synchronous Compensator)
with an even faster VSC technology (Voltage Sourced Converter, self-commutated) was considered as well; it proved,
however, to be not feasible from economic and technical point of view. It turned out namely that the control speed,
required in Radsted, can be achieved by
means of a line-commutated thyristor
controlled SVC to the full extent.
The high-pass filters (see Figure 2) are
tuned to the 11th harmonic and have a
star connection. The advantage of a
12-pulse configuration of the SVC is that
the 5th, 7th, 17th and 19th harmonics, produced by each leg of the thyristor-controlled reactor (delta connection), cancel
each other out due to the phase shifting
with one star and one delta winding respectively on the secondary side of the
transformer in the direction of the grid.
This, in its turn, helps comply with strict
harmonic requirements.
B. Building Layout
By means of an innovative building
solution, Siemens takes into consideration high requirements of the customer
Cooling
Control Room
TCR2
TCR1
Filter 2
TCR
Reactors
Filter 1
Transformer
Cooling
132 kV Cables
to AC Station
a)
Figure 4
Configuration of the Static
Var Compensator:
a) The Layout shows the
Arrangement of Components within the Building.
b) Exterior View-Cooling and
Switchgear only are situated outside, the Rest is completely in-door.
regarding noise reduction as well as architectural appearance. The SVC complies with the requirements of noise
emission at a permitted value of 30 dB
maximum on the fringe of the station
territory. Therefore, all components of
the SVC installation are placed within
the station building, see Figure 4. Details
of the SVC layout can be seen in Figure
4a. The total of six reactors of both thyristor controllers TCR 1 and TCR 2 are arranged in a separate room; they are controlled via thyristors and are connected
to the same busbars as the filters. The
cooling of the valves is carried out via
special low noise cooling fans (outdoor,
see Figure 4a). The main transformer
with natural oil and air cooling (outside
the building, see Figure 4b), installed indoor for reasons of noise abatement, is
arranged in such a manner that the
12-pulse connection is provided and the
SVC voltage is transformed to the level
of 132 kV. Further views of the innovative “in-door life” of the compensator are
given in Figure 5. Part a) shows the TCR
b)
reactors and a module of a thyristor
controller; Figure b) depicts the specially
adapted connection technology with flexible aluminum or copper jumpers.
The view of capacitor banks of the filters is to be seen on the front page of
this issue.
This compact building solution is the
answer to all requirements in terms of
environmental protection and noise
abatement. The base area comprises approximately 800 m2 and, thanks to the
height of 6 m only, the building does not
obstruct the view to the neighbors and
fits into the plain south Danish landscape. Due to the narrowness within the
compact building, great store was set by
sufficient accessibility for maintenance
purposes. On the grounds of natural
scenery, the immediate surroundings of
the station is partially wooded.
As all the environmental aspects, incl.
noise level, were thoroughly considered
in the design of the installation, the construction permit by the authorities in
charge could be obtained within a comparatively short period of time.
C. Factory System Tests
In order to check the proper operation
of the SVC and the control system, a
factory test with the physical control
equipment was carried out on a digital
real-time simulator (transient network
analyzer) in Erlangen. A simulator test
such as this allows for executing all
kinds of network faults without jeopardizing the “real” system. A model of the
“large” grid, reduced to the necessary size, is used here, which includes parts of
the real 132 kV systems and the wind
farm NOWF as well as all the components of the SVC, including all measurements to the control system.
A great number of tests were carried
out, as it is always the case with FACTS
(as well as HVDC). One of these tests, namely the examination or control performance of the SVC in combination with
the wind farm at fast wind changes, is
shown in Figure 6.
The worst-case scenario with noncompensated wind turbines and a particularly weak system was chosen. Figure 6 a) shows the behavior of the wind
farm NOWF without SVC when a strong
wind blast occurs. The simulation starts
with an almost maximum generation
output of the wind farm (t = 0), which is
followed by a strong wind blast simulated by means of increase in the torque
of generators. After 20 seconds the wind
farm is shut down, followed by the restart
10
seconds
later.
Figure 6 a) shows that without SVC the
changes in active and reactive power of
the wind farm at a strong wind blast
lead to a critical voltage decline of the
system (voltage collapse). The system
cannot recover from this condition even
if the connection to the wind farm is cut
off (trip). The voltage remains at such a
low level that, even on restarting the
wind farm, the farm can not come back
to active power and, moreover, with its
reactive power it stresses the system
voltage anew.
5
When the SVC is enabled (Figure 6 b)
the voltage at the point of connection to
the grid is controlled by the compensator within the specified range, and the
wind farm can be successfully restarted
after the wind blast subsided.
D. Behavior of Compensator
under Fault Conditions
a)
The effectiveness of the SVC in the
real grid is shown in Figure 7 in a very
impressive manner. In the figure one
can see a severe three-phase system
fault (remote fault near Copenhagen)
which has impact on the weak grid on
Lolland at the connection point of the
compensator and the wind farm NOWF.
All three system voltages are reduced
during the fault, which makes the com-
b)
AC System Voltage [%]
Figure 5
Innovative Solutions for the
“Inner Life” of the SVC:
a) TCR – Reactors and Thyristor Valve.
b) Also the Connection Technology is
sophisticated.
a)
120
132 kV AC Voltage
100
Without SVC the AC voltage remains at a low level.
Voltage Collapse
80
60
40
Power [MW/MVAr]
300
150
Wind Farm Reactive Power
Restart
Trip
Wind Farm
0
Wind Farm Active Power
-150
The wind farm active power remains at „Zero“.
Strong Wind Blast
-300
0
5
10
15
20
25
30
35
40
AC System Voltage [%]
120
Reactive Power [MVAr]
Time [s]
100
b)
110
With SVC the AC voltage is controlled within the specified range.
132 kV AC Voltage
100
90
Wind farm back to full active power output
80
Trip
SVC Nominal Value
50
SVC Measured Value
Inductive
Strong Wind Blast
Wind Farm
Restart
-100
0
5
10
15
20
Time [s]
6
Figure 6
Capacitive
0
-50
pensator react quite fast with its total
capacitive power output.
Owing to this fast voltage support by
means of the compensator, there is only
a limited impact of the system fault on
the wind farm, and subsequently, it does
not have to be shut down. After fault
clearing, the SVC keeps the voltage at a
controlled level and even goes up to the
inductive range (in order to compensate
the overvoltage through load shedding
in the faulty grid of Zealand). Without
SVC, the voltage dip at the connection
point of the wind farm would be more
palpable, running a risk of a voltage collapse (dotted red curve in Figure 7 at the
top) and the necessity to shut down the
wind power plant, ref. to Figure 6.
25
30
35
40
Real-Time Simulator Tests to determine
the System Performance in a weak System:
a) Wind farm Behavior at strong Wind
blast – without SVC.
b) With SVC – Voltage is stabilized and the
Wind farm can continue “Generating”.
Figure 7
From Simulator to Reality:
Advantages of SVC in Case
of a three-phase Fault.
(see Figure 8). A further increase in sustainability of power supply is possible, as
the SVC also “covers” the future wind
farm Rødsand 2 (design rating 215 MW)
in terms of voltage quality and grid security.
Conclusion
Advantages of SVC for
Sustainability of Power Supply
With the help of the Radsted SVC, the
operation of the wind farm NOWF is
possible even under the conditions of a
weak grid, without any restrictions in
terms of power quality and grid security.
That is, the wind farm can help significantly reduce the amount of CO2 emissi-
ons, connected with power generation in
Denmark. If the availability of approximately 39 % of full-load hours of the
offshore wind farm NOWF is assumed (a
“reserved” assumption), the saving on
CO2 emissions will be 174,000 tons per
year, the amount that would just as well
be produced by other less environmentally compatible power plants on Zealand due to additional power generation
Siemens has installed a Static Var
Compensator (SVC) in Radsted, Denmark, which results in enhancement of
voltage quality under normal conditions, whereas under fault conditions a
higher dynamical stability of the system
is achieved.
Owing to the SVC, the utility SEASNVE will be in a position to install the
wind turbines with simple asynchronous generators without any additional
converters. The simplified design of the
wind turbines is very fail-proof which
reduces the fault rate of the offshore facilities which, in its turn, means less onsite maintenance of the wind turbines
so difficult to access, particularly in the
case of offshore.
As all the environmental aspects were
thoroughly considered in the design of
the installation, the construction permit
by the authorities in charge could be obtained within a short period of time. The
opted design of the SVC fits in its environment in the best possible way and
can hardly be seen or heard.
Literature
[1] European Technology Platform SmartGrids –
Figure 8
Sustainability of
Power Supply by
means of more
Wind Power in
the Grid – the
SVC Radsted
in Denmark
makes it possible.
Vision and Strategy for Europe’s Electricity Networks of the Future. Luxembourg, Belgium, 2006.
[2] Retzmann, D.; Sörangr, D.; Uecker, K.: Flexibler
und sicherer. „Smart Grids“ für den Strommarkt
von morgen. BWK 58 (2006) No. 11, pp. 10-13.
[3] Rasmussen, C.; Jorgensen, P.; Havsager, H;
Nielsen, S. B.; Andersen, N.: Improving voltage
quality in Eastern Denmark with a Dynamic Phase
Compensator. Fifth International Workshop on Large-Scale Integration of Wind Power and Transmission Networks for Offshore Wind Farms, Glasgow,
Scotland, 2005.
[4] Transmissionsplan 2005. Elkraft System
a.m.b.a., Denmark, Jan. 2005.
7
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