Impact of offshore wind generation on the Belgian high ... - KU Leuven

<strong>Impact</strong> <strong>of</strong> <strong>of</strong>fshore <strong>wind</strong> <strong>generation</strong> on the
Belgian high voltage grid
Peter Van Roy, Joris Soens, Johan Driesen, Ronnie Belmans
Department Electrical Engineering ESAT/ELECTA (K.U.Leuven)
Kasteelpark Arenberg 10
B-3001 Leuven, Belgium
Tel: +32 16 32 10 20 Fax: +32 16 32 19 85
joris.soens@esat.kuleuven.ac.be
http://www.esat.kuleuven.ac.be/electa/
Summary
Recently, various project developers have shown interest to construct large-scale <strong>of</strong>fshore <strong>wind</strong> farms in the
Belgian part <strong>of</strong> the North Sea. In this context, a multidisciplinary research project has been started up, to
quantify the total potential <strong>of</strong> <strong>of</strong>fshore <strong>wind</strong> power in Belgium, taking into account physical, technological and
economical constraints. As part <strong>of</strong> this project, load flow studies are executed to determine the maximum
amount <strong>of</strong> power that can be injected at the coastal substations without overloading the high voltage grid. For
this study, the data <strong>of</strong> the Belgium high-voltage grid and the main foreign substations and lines are used as
input, together with reference scenarios for load and <strong>generation</strong> patterns and cross-border power transits. The
load flow calculations take only static regimes into account, and are done to assess the risk <strong>of</strong> static voltage
instability, system overload or (N-1)-uncertainty.
The results show that, even with <strong>wind</strong> power plants that consume reactive power, no static voltage instabilities
are expected. However, high-voltage lines from the coast towards the inland region may be congested as a
result <strong>of</strong> <strong>of</strong>fshore <strong>wind</strong> power. Depending on the circumstances, only 100 MW can be injected at the coastal
substations without overloading the system. Therefore, the development <strong>of</strong> <strong>of</strong>fshore <strong>wind</strong> power will require
investments in grid reinforcement, not only in the coastal region but especially more inland. Also the grid
operator will have to take the presence <strong>of</strong> the new production into account. In Belgium, the grid operator is
required to give priority dispatch to renewable producers over traditional producers, e.g. in case <strong>of</strong> incidents in
the grid.
1. Introduction
Offshore <strong>wind</strong> <strong>generation</strong> proves to be a very promising contribution towards the production <strong>of</strong> renewable energy on a
large scale. In Belgium, various project developers have shown interest to construct <strong>wind</strong> farms in the North Sea, with
installed powers <strong>of</strong> several hundreds <strong>of</strong> megawatts. In this context, the Belgian Federal Office for Scientific, Technical
and Cultural Affairs has set up a multidisciplinary research project, entitled Optimal Offshore Wind Energy Developments
in Belgium 1 . It started at the beginning <strong>of</strong> 2002 and aims at quantifying the total potential <strong>of</strong> <strong>of</strong>fshore <strong>wind</strong> power in
1 See http://www.esat.kuleuven.ac.be/electa/research/descriptions/OffshoreWind.pdf

Belgium, taking into account physical, technical and economical constraints. As part <strong>of</strong> this project, the possible
limitations that the existing Belgian high-voltage grid imposes on the development <strong>of</strong> <strong>of</strong>fshore <strong>wind</strong> power need to be
identified. Load flow studies are done to assess the static impact <strong>of</strong> an <strong>of</strong>fshore <strong>wind</strong> power injection on line loading,
voltage level and (N-1)-safety. The result <strong>of</strong> this study is a set <strong>of</strong> values for the maximum amount <strong>of</strong> extra power that can
be injected into the coastal substations <strong>of</strong> the Belgian high voltage grid without overloading the system. The results will
be discussed in this paper.
2. Present Belgian Power system
The total installed power in Belgium is ca. 15000 MW. The largest power plants are:
- 4 nuclear power plants in Doel (2 x 400MW and 2 x 1000MW) and 3 in Tihange (3 x 1000MW)
- around 40 conventional fossil fuel plants (ranging from 30MW to 400MW)
- 6 combined cycle (steam and gas) power plants (50 – 460 MW)
- the pumped-storage station in Coo (1160MW)
Figure 1 shows the location <strong>of</strong> the main power plants in Belgium. They are geographically well spread over the country.
The heaviest load concentration is located in the region Antwerp – Brussels. In the western part <strong>of</strong> Flanders, at the coast
region, the electrical consumption is rather low. With the power plants <strong>of</strong> Herdersbrug and, further inland, Ruien, Gent-
Ringvaart and Rodenhuize, there is already overcapacity <strong>of</strong> power in Western Flanders, resulting in a net power flow from
Western Flanders towards the Antwerp-Brussels region.
Doel
Herdersbrug
Gent-Ringvaart
Rodenhuize
Kallo
Schelle
Vilvoorde
Mol
Genk-
Langerlo
Ruien
Drogenbos
Le Val
Saint Ghislain
Amercoeur
Péronnes
Monceau
Nuclear power plant
Plate-Taille
Conventional fossil fuel plant
Tihange
Chooz (F)
Awirs
Coo
Pumped storage plant
Combined Cycle plant
Figure 1: Main power plants in Belgium
Figure 2 shows the high-voltage grid. It consists <strong>of</strong> a 400 kV (and partially 220 kV) backbone, and a transmission grid
with voltage levels <strong>of</strong> 150 kV and 70 kV. There are border crossings at the 400 kV-level that connect the Belgian grid
with the French and with the Dutch grid, and there is also a 220 kV-connection with Luxemburg. There is no direct
connection with Germany.

The lines coming from France (i.e. Avelin-Avelgem and to a lesser extent Lonny-Achène) are <strong>of</strong>ten heavily loaded, due
to either a net electricity import from France to Belgium, but more <strong>of</strong>ten as a result <strong>of</strong> loop flows: these are physical flows
through the Belgian grid that find their origin in international power transactions between the neighbouring countries (as
shown in Figure 3).
There are three 150kV-substations at the coast: (from south to north: Koksijde, Slijkens and Zeebrugge). These three
substations are the end points <strong>of</strong> two radial branches <strong>of</strong> the 150kV grid. However, in order to have a stronger 150kV-grid
in the coastal region, the grid operator Elia has obtained the permits to make a 150kV-connection between Koksijde and
Slijkens.
Koksijde
Slijkens
Zeebrugge
400 kV
220 kV
150kV
Figure 2: Belgian high voltage grid
1 MW
physical
flow
1 MW
contract 2
1 MW
contract 1
Figure 3: Loop flows through Belgium as a result <strong>of</strong> international power trading
With the grid situation described above, it can be expected that the combination <strong>of</strong> the already present overcapacity in
Western Flanders, together with the additional power injection from <strong>wind</strong> farms, may lead to congestion on the lines

going in-land ( Figure 4). Especially the two parallel 150kV lines between Rodenhuize (near Gent) and Heimolen (near
Antwerp) may suffer congestion. This has been confirmed by calculations, described below.
Sites for 100 MW <strong>wind</strong>parks
Planned grid
extension
Zeebrugge
(1)
Congestion
Koksijde
Slijkens
(2)
(3)
Main power flow, 150 kV
(4)
Existing power plants
(1) Herdersbrug
(2) Gent-Ringvaart
(3) Rodenhuize
(4) Ruien
Figure 4: Sites for <strong>of</strong>fshore <strong>wind</strong> power, and main power flow
3. Potential static impacts <strong>of</strong> <strong>of</strong>fshore <strong>wind</strong> power
The potential static impact <strong>of</strong> <strong>of</strong>fshore <strong>wind</strong> power on the Belgian grid can be: overloading, (N-1)-uncertainty or voltage
instability.
System overload occurs when the transmitted power through certain lines or transformers is above their capacity.
System static voltage instability may be caused by a high reactive power demand <strong>of</strong> <strong>wind</strong> turbine generators. Depending
on the type <strong>of</strong> generator system chosen, the reactive power demand (in MVAr) can be as high as 40% <strong>of</strong> the active power
produced (in MW). Generally speaking, a high reactive power demand causes the system voltage to drop. By a static
voltage stability study, the system voltage in steady state is investigated. For the calculations, a worst case scenario was
used, i.e. <strong>wind</strong> turbines equipped with induction generators without compensation for the high reactive power demand.
(N-1)-safety means that any single element in the power system may fail without causing a succession <strong>of</strong> other failures
leading to a system collapse. Together with avoiding constant overloading grid elements, (N-1)-safety is a main concern
for the grid operator.
Apart from static impacts, <strong>of</strong>fshore <strong>wind</strong> farms may also have dynamic impacts on the grid, e.g. voltage transients during
switching operations, unwanted disconnection <strong>of</strong> the farm in case <strong>of</strong> a grid voltage disturbance etc. To assess these
impacts, more extensive calculations are necessary, using the dynamic data <strong>of</strong> the <strong>wind</strong> farm and also <strong>of</strong> the most relevant
generators <strong>of</strong> the Belgian grid.

4. Calculation method
The input data for the load flow calculations are grid data, load and production scenarios, and power transit scenarios.
Grid Data
The following grid data are included in the model:
- all Belgian substations at the 400kV, 220kV, 150kV and 70kV level, together with the main substations <strong>of</strong> the
neighbouring countries;
- full model <strong>of</strong> the Belgian, and part <strong>of</strong> the neighbouring high voltage system (400–70 kV)
- rating and operation status <strong>of</strong> the installed capacitor banks at the 150kV and the 70kV level.
The apparent power rating <strong>of</strong> a line depends on the season: it is slightly higher in winter than in summer due to the lower
ambient temperature.
Three grid topologies are considered for the calculations:
- the actual grid;
- the actual grid with an additional link between Koksijde and Slijkens (which will be installed in the near future);
- the actual grid with the link Koksijde-Slijkens and various reinforcements inland. These reinforcements imply
an increase <strong>of</strong> the transmission capacity between Rodenhuize and Heimolen.
Load and transit reference scenarios
The load in the Belgian power system varies, and is modelled with 24 reference scenarios, 8 for each type <strong>of</strong> season, as
given in table 1. For instance, in summer, the overall load in Belgium varies from 6 GW in weekend nights to 10.5 GW
during peak hours on weekdays. For each reference scenario, a proper number <strong>of</strong> power plants are assumed to be on line,
with some random element.
Loading, GW Summer Intermediate Winter
Weekends night 6 7 8
day low 7.25 8 9
high 8.5 9 10
Weekdays night low 8 8 9
high 8.5 9 10
day low 9 10 11
high 10 11 12
peak 10.5 12 13
Table I : Load levels <strong>of</strong> reference scenarios
Furthermore, the power transit from France to the Netherlands, partially passing through Belgium and partially through
Germany, is taken into account. The power transit was modelled by an extra power flow from Avelin (F) to Avelgem (B)

and from Lonny (F) to Achène (B), and an extra power demand at the Dutch border nodes. Five power transit levels are
considered (T1-T5), in which the extra power injection from France ranges from 800 to 2000 MW in total.
5. Results
With the input data and reference scenarios described above, the maximum power that can be injected into the nodes <strong>of</strong>
Zeebrugge or Slijkens, before system overload, (N-1)-uncertainty or voltage instability occurs, is calculated.
Static voltage instability
Even with the assumed worst case scenario (<strong>wind</strong> turbines equiped with induction generators with a high reactive power
demand), static voltage stability problems only occurred at extreme levels <strong>of</strong> line overloading, i.e. at power flow levels <strong>of</strong>
several times the rated capacity <strong>of</strong> the lines. Therefore, static voltage stability is not relevant, compared with the system
overload and (N-1)-uncertainty.
System overload and (N-1)-uncertainty
Table II,Table III and Table IV give the maximum active power injection in the 150-kV node at Slijkens, before overload
or (N-1)-uncertainty occurs, for summer, intermediate and winter seasons respectively. Results are given for the present
grid, the grid with the planned Koksijde-Slijkens link, and with Koksijde-Slijkens plus inland grid reinforcement. A
certain degree <strong>of</strong> variability in the results, as e.g. for the weekday scenarios in winter, with 11, 12 or 13 GW overall load,
is due to the random nature <strong>of</strong> the power plant commitment. In the 12 GW scenario, the power plant in Herdersbrug only
operates at half power, causing less <strong>of</strong> an overproduction in the Brugge-Gent region.
Two limiting factors can be recognised:
- During weekends and weeknights, the limiting factor in the present grid is simply the power line capacity from
Slijkens to Brugge, i.e. 300 MW. With a local load in Slijkens, injection capacity can become 350 MW. With a
connection Koksijde-Slijkens, power can be transported to Koksijde, increasing the injection limit, up to 500
MW (the highest value that was checked). The addition <strong>of</strong> extra transmission capacity further inland (e.g.
Rodenhuize-Heimolen) does not influence this limit.
- Very <strong>of</strong>ten, lower limits are found. These are due to overloads further inland, well beyond Brugge and even
beyond Gent. This overload is only slightly alleviated by the Koksijde-Slijkens link. Obviously, the inland
reinforcement does alleviate this overload.

Maximum power injection in Slijkens, summer scenarios [MW]
Present grid
Load level [GW] 6 7.25 8.5 8 8.5 9 10 10.5
T1 300 300 300 300 300 150 150 150
T2 300 300 300 250 300 100 100 100
T3 300 300 300 200 300 50 50 50
T4 300 300 250 200 250 - - -
T5 300 300 250 150 200 - - -
Grid with connection Koksijde-Slijkens
Load level [GW] 6 7.25 8.5 8 8.5 9 10 10.5
T1 400 450 450 450 450 300 250 250
T2 400 400 400 400 400 200 200 150
T3 350 400 400 300 400 100 100 100
T4 350 400 350 250 350 100 50 50
T5 350 400 300 200 300 50 - -
Grid with Koksijde-Slijkens and inland grid reinforcement
Load level [GW] 6 7.25 8.5 8 8.5 9 10 10.5
T1 400 400 400 450 400 450 450 450
T2 400 400 400 450 400 450 450 450
T3 350 400 400 400 400 450 450 450
T4 350 400 400 400 400 450 450 450
T5 350 400 400 400 400 450 400 400
Table II: Maximum power injection in Slijkens, summer scenarios [MW]
Maximum power injection in Slijkens, intermediate scenarios [MW]
Present grid
Load level [GW] 7 8 9 8 9 10 11 12
T1 300 300 150 300 100 200 200 50
T2 300 300 100 300 50 100 100 -
T3 300 300 50 300 - 50 50 -
T4 300 300 - 300 - 50 50 -
T5 300 300 - 300 - - - -
Grid with connection Koksijde-Slijkens
Load level [GW] 7 8 9 8 9 10 11 12
T1 450 450 250 450 250 300 300 150
T2 450 450 150 450 150 250 200 50
T3 450 450 100 450 50 150 150 -
T4 450 450 50 450 - 100 100 -
T5 450 450 - 450 - 50 50 -
Grid with Koksijde-Slijkens and inland grid reinforcement
Load level [GW] 7 8 9 8 9 10 11 12
T1 450 450 450 450 450 500 450 450
T2 450 450 450 450 450 500 450 450
T3 450 450 450 450 450 450 450 450
T4 450 450 450 450 450 450 450 450
T5 400 450 450 450 450 450 450 450
Table III: Maximum power injection in Slijkens, intermediate scenarios [MW]

Maximum power injection in Slijkens, winter scenarios [MW]
Present grid
Load level [GW] 8 9 10 9 10 11 12 13
T1 350 350 350 350 350 100 250 50
T2 350 350 350 350 350 50 200 -
T3 350 350 350 350 350 - 150 -
T4 350 350 350 350 350 - 100 -
T5 350 350 350 350 350 - 50 -
Grid with connection Koksijde-Slijkens
Load level [GW] 8 9 10 9 10 11 12 13
T1 500 500 500 500 500 200 400 150
T2 500 500 500 500 500 150 300 50
T3 500 500 500 500 500 50 250 -
T4 450 500 500 500 500 - 200 -
T5 450 500 500 500 500 - 150 -
Grid with Koksijde-Slijkens and inland grid reinforcement
Load level [GW] 8 9 10 9 10 11 12 13
T1 500 500 500 500 500 550 500 550
T2 500 500 500 500 500 550 500 500
T3 450 500 500 500 500 500 500 500
T4 450 500 500 500 500 500 500 500
T5 450 500 500 500 500 500 500 500
Table IV: Maximum power injection in Slijkens, winter scenarios [MW]
The results for Zeebrugge are not shown here, as they turned out to be very similar to those for Slijkens, confirming that
the main limiting factor is located beyond Brugge. The main difference is the very limited beneficial effect <strong>of</strong> the
Koksijde-Slijkens link. The maximum allowed power injection after grid improvements is slightly lower than for Slijkens,
due to the already existing (small) power injection in Zeebrugge, loading the Zeebrugge-Brugge lines.
Finally, the option <strong>of</strong> injecting the power partly in Slijkens and partly in Zeebrugge (in a 50/50 ratio) was studied. In those
cases where the bottleneck is beyond Brugge, results are again very similar to those in Table II, Table III and Table IV. In
the other cases, i.e. mainly during weekends and weeknights, more than 500 MW can <strong>of</strong>ten be injected in normal grid
conditions.
For Koksijde, it has been calculated that 300 MW (without Koksijde-Slijkens) up to 600 MW (with Koksijde-Slijkens and
grid improvement inland) can be injected.
6. Conclusions
With the present grid and the presently existing power plants operating at their normal output level, only a very limited
amount <strong>of</strong> power from <strong>of</strong>fshore <strong>wind</strong> farms can be transported during weekdays, which is the period when electricity has
the highest economic value.
It must be stressed that the possible cases <strong>of</strong> line overload and (N-1)-uncertainty are not only a result <strong>of</strong> <strong>of</strong>fshore <strong>wind</strong>
power. They are a combination <strong>of</strong>:

- the already present net power flow from Western Flanders towards the Antwerp-Brussels region;
- cross-border power transits;
- <strong>of</strong>fshore <strong>wind</strong> power.
Without attaching too much importance to the precise values obtained here (calculations are based partly on precise
information, partly on educated guesses and reference scenarios), it is clear that grid improvements are required to give
<strong>of</strong>fshore <strong>wind</strong> power a real chance in the market place. Often, these improvements are not only at the coastal region, but
also further inland. Grid improvements inland would incidentally also allow higher transit levels.
Finally, system operation will have to take the presence <strong>of</strong> the new production into account. In Belgium, the grid operator
is required to give priority dispatch to renewable producers over traditional producers, e.g. in case <strong>of</strong> incidents in the grid.
7. Acknowledgements
The authors wish to thank the Belgian Federal Office for Scientific, Technical and Cultural Affairs for the financial
support <strong>of</strong> this project. Also the financial support <strong>of</strong> the Fonds voor Wetenschappelijk Onderzoek (F.W.O.) – Vlaanderen
is greatly acknowledged. J. Soens is a doctoral research assistant <strong>of</strong> the F.W.O.-Vlaanderen. J. Driesen holds a
postdoctoral research fellowship <strong>of</strong> the F.W.O. Vlaanderen.
The authors also wish to thank the Belgian grid operator Elia for their input in this project.