Download Influence of increased wind energy infeed on the transmission network

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Session 2004
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Influence of Increased Wind Energy Infeed on the Transmission Network
Yvonne Sassnick, Matthias Luther, Ronald Voelzke*
Vattenfall Europe Transmission GmbH, E.ON Netz GmbH, Siemens AG
1.
Introduction
Legislation passed in April 2000 and governing the infeed of regenerative energy compels network
operators to link up to all the renewable generating sources and to purchase all the power generated for a
specified infeed remuneration.
Since this law came into force, in Germany there has been a notable increase in the proportion of power
generated from renewable sources, particularly wind energy. Figures for late 2002 show a total installed
wind power of about 12,000 MW, with a maximum load capacity in Germany of 80,000 MW. 50% of this
installed wind power is placed in the Transmission System of E.ON Netz and 35% is placed in the
Transmission System of Vattenfall Europe Transmission. Further expansion (especially in offshore
regions) up to a figure of 25,000 MW by the year 2030 is planned. Nearly all these generating units
provide their infeed in an unregulated manner, depending on available wind and regardless of network
load. In Figure 1-1 to 1-3 the expected development of wind energy in Germany is drafted– On-Shore and
Offshore- especially with the development in the E.ON Netz and the Vattenfall Europe Transmission
system.
Installierte Leistung / Installed Capacity, MW
50.000
45.000
40.000
Onshore and Offshore
35.000
30.000
25.000
20.000
Onshore
15.000
10.000
5.000
0
1990
1995
2000
2005
2010
2015
2020
2025
2030
Jahr / Year
Figure 1-1:
Development of wind energy in Germany
*[email protected]
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Figure 1-2: Planned development of offshore
wind farms in the North Sea
Figure 1-3: Planned development of offshore wind
farms in the Baltic Sea
By its very nature, wind energy infeed is stochastic and difficult to foretell. Wind predictions are based on
weather forecasts and depend therefore on the accuracy of the latter. This means that load forecasts are
subject to uncertainty spans of up to 15% on average. Additional reserve capacity must therefore be
provided. As a result of unbundling (specified in the EU Directive of 1996, which was fully implemented
in Germany), transmission network operators no longer possess their own generating plants and have
therefore to purchase their power on the market. The absolute priority attached to renewable energies is
detrimental to generation from conventional power stations. Particularly in periods of low demand, in
conjunction with strong wind, there is a lack of controlling power in the network, owing to the low
proportion of conventional generation. This calls for controllability of power generation. It is important to
arrive at an optimal balance, in both economic and technological terms, between controlled wind energy
infeed and the supply provided by conventional power plants.
It is mainly northern Germany where wind energy is making progress. This region has hitherto accounted
for relatively low electricity demand, with the result that the transmission network is not as substantial as
elsewhere. The planned construction of more offshore wind farms in the North and Baltic Seas will lead
to increasingly frequent bottlenecks in the transmission network in a north-south direction. Intensified use
of wind energy will therefore necessitate improvements to the transmission network. Extension of wind
energy requires a correponding extension of the grid.
Network access regulations equivalent to those for conventional power stations have to be applied in
future to wind farms with power infeed into the HV grid, too. This means for example that in the event of
a short circuit near the point of common coupling a wind farm must, up to a fault duration of 150 ms,
remain stable and connected to the network. Most of today’s wind generator types are not capable of this
behaviour. Suitable models of wind generators must therefore be developed, allowing an examination of
the dynamics of such units, and with the aid of which the requirements imposed on the network
(especially the power electronics) can be derived.
Linking up large offshore wind farms to public networks over long distances is a complex technological
challenge. This paper presents suitable basic concepts in the form of AC or DC solutions; the
technological and economic contextual conditions are discussed.
The political activities concerning the promotion of wind energy must not overlook the fundamental
technical problems; these can only be solved by the joint efforts of all concerned (e.g. wind generator
manufacturers, wind farm operators, network operators and their associations). A number of studies are
under way. The intention is to derive basic technological recommendations, thereby arriving at a
consensus among all parties.
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2.
Effects of increased wind power generation on system planning and grid operation
Power infeed by wind power producers (WPP) is fundamentally subject to different patterns than are the
cases with conventional power sources such as thermal, gas turbine or hydroelectric generating plants.
Assuming a general availability of the conventional plants themselves, the power to be generated can be
planned exactly and can be fed in at the required time. The situation with wind power stations is different.
In most cases the wind velocities in northern Germany, but also over the Baltic or North Sea, are mostly
within the range of 3 to 12 m/sec. Within this range, the power produced by a wind generator depends
very considerably on the wind velocity (Figure 2-1). Thus, planning the operation of a transmission
network depends substantially on the precision of the weather forecasts, quite particularly if the share of
the wind power generation accounts for a significant portion of the network load. One further major factor
is that German legislation on renewable energy does not impose any obligations on wind farm operators
to feed in a certain amount of power. They basically feed in the maximum possible power obtainable
from the wind and they receive a maximum payment. It is the transmission network operator’s
responsibility to gauge the coming infeed on the basis of the weather forecast.
Vattenfall Europe Transmission’s grid covers an area of about 100,000km2, with a maximum load of
around 10,000 MW. By the end of 2002, the wind power output installed amounted to 4,200 MW. A
further 5,000 MW of onshore wind power and 1,500 MW of offshore wind power are to be added by the
year 2010.
Figure2-1: Average power output
Figure2-2: Forecast, correction and actual output
At present, the wind power to be fed in is planned on the basis of a preview compiled on the previous day
in the period up to 9:00 a.m. On the actual day, a correction is then incorporated by 4:00 a.m. on the basis
of the updated weather forecasts. Unfortunately, in many cases the actual profile of the power output
figures for all wind generators feeding into the grid deviates considerably. Figure 2-2 shows this with
reference to the example of January 30, 2003.
As a result the transmission system network operators are obliged by law to take receipt of and pay for all
the renewable energy (mainly wind energy) generated in the accounting grid. By contrast, transmission
network operators can sell parts of the renewable energy vertically to their subordinate networks (as a
monthly band) or horizontally to neighbouring transmission networks as a daily band in accordance with
the forecast. These sales can be referred to as refined energy because they are no longer subject to
stochastic fluctuations. Forecast generation is balanced out over delivery bands by means of electricity
trading. As the actual generation of wind energy never tallies with the forecast, further deviations must be
balanced out by the transmission network operator.
As the wind power infeed increases, the transmission capacity of the network is a further problem. Most
WPPs are connected in relatively low loaded regions. The transmission networks in these regions have
been expanded to only a limited extend. Appropriate transmission capacities must be created in order for
the power to reach the load centres.
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Moreover, existing conventional power stations will be offering their generating capacity on the free
market, selling it throughout Germany or Europe. Consequently, free energy trading is suffering
increasing constriction owing to a lack of transmission capacity.
This is why transmission capacities must be expanded. According to calculations orderd by
VE Transmission, additional 400 kV overhead
lines over a distance of about 500 km are needed
for this purpose in the network area of VE
Transmission till 2011. Considering the usual
length of time required for approval processes, it
is to be feared that network capacity will not keep
up with wind power expansion, which has been
assigned priority. Besides network expansion
expenditure, there are additional costs due to
dramatic rise in compensating power caused by
inexact forecasts; there costs greatly those of
network expansion – Figure 2-3.
Figure2-3: Expected extra costs for grid extension
Studies for E.ON Netz have shown that grid
and operation due to implementation of
extensions up to 1000 km transmission lines
wind power
(voltage level 110 kV – 400 kV) are required till
year 2016.
3.
Effects of increased wind power generation on spinning reserve power requirements
All previously described questions are referred to steady state system conditions. It makes a big
difference whether only a few wind mills are operated in a grid or a significant portion of generation is
wind power based. It’s a fact that the under voltage protection of existing wind power plants is set to 80%
Un. Below this value the wind generators will deactivate particularly to protect itself from destruction.
This can result in severe under frequency conditions within the UCTE grid.
Figure 3-1 shows the voltage funnel in case of a 3 phase short circuit in the VE Transmission grid near
the city of Magdeburg. The coloured areas show grid areas with residual voltages below 15% to 80%.
Obviously all wind generators installed in these areas will trip if their under voltage protection is set to
80% Un. In total about 3,700 MW wind power generation will be tripped.
Figure3-1: Voltage funnel in case of three phase fault near Magdeburg
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Other calculations in the grid of E.ON Netz have shown that these values can even be exceeded in the
near future. It was assumed that the short circuit current contribution of the wind generators varies
between In and 5xIn.
Installed wind
generation onshore
and offshore (MW)
2001
3,500
2006
6,400
2011
12,800
Maximum outage of wind power generation in MW
I”scWPP=5In
1,700 – 1,800
2,200 – 3,400
2,000 – 3,500
I”scWPP=In
2,400 – 3,000
4,300 – 4,800
3,800 – 5,700
Table 3-1: Wind power generation losses due to system faults /1/
2016
15,700
2,500 – 3,900
4,400 – 6,600
In the entire UCTE grid at present a max. outage of 3,000 MW can be covered by the available spinning
reserve power. Larger generation outages will result in severe frequency drops. One method imaginable
to avoid these conditions could be an increase of the available spinning reserve power within the UCTE
grid. Taking into account the current price level of fast spinning reserve power and the fact that this
suggestion can scarcely be translated into practice among all UCTE members its necessary to find other
options.
Why not reduce the setting of the under voltage protection of the wind generators. This could be an
effective alternative; at least during the first system protection time step of 150 msec. Detailed
calculations have shown that settings of 0.4 Un to 0.5 Un would change the situation to uncritical system
conditions.
4.
System integration of offshore wind farms
4.1
Three phase AC transmission concept
For energy transmission with AC-submarine cables over large distances should be chosen a transmission
voltage as high as possible and three-conductor three phase cables. Contrary to single conductor cables,
the three conductor cable is an essential prerequisite for compact routing and thus for ultimate approval of
the outgoing power cables. Three conductor cables with XLPE insulation and maximum operating
voltages up to 170 kV can be assumed to reflect the state of the art; cables for operating voltages up to
245 kV will be available in the near future. Figure 4-1 shows the essential components of such a
transmission system from the offshore central station to the grid connection node /2/.
The maximum transmission power is limited to about 230 MW per system for 170 kV cables and to about
340 MW for 245 kV cables (see Figure 4-2).
Due to the alternating current three-phase cables are able to give off capacitive charging power. The
charging current puts a burden on the effective cable cross-section in addition to the active power
transmitted and must be taken into consideration. Depending on the reactive power conditions of the wind
farm central station and at the grid connecting node, it is advisable to plan corresponding power factor
correction (PFC) systems in parallel with the cable.
Figure 4-1: Components of an AC offshore – onshore transmission
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400
350
300
250
200
P in MW
I in A
1000
900
800
700
600
500
400
300
200
100
0
I-170 kV
I-245 kV
P-170 kV
P-245 kV
150
0
200
400
600
800
1000
1200
1400
cable size in m m 2
Figure 4-2: Transmission capability of AC XLPE-cables
4.2
Three phase DC transmission concept
The high voltage DC transmission system (HVDC) with cables is capable of effective and stable
transmission over several hundreds of kilometers. One crucial factor is that no capacitive charging current
at all put a stress on DC cables. Furthermore a DC connection does not transmit any short circuit currents
from the shore to the medium-voltage distribution level in the wind farm, thus making grid connection of
the WPPs more cost effective at the medium-voltage level. Furthermore onshore voltage sags due to
internal faults will not be transmitted to the wind generators.
The voltage DC link converter with IGBT power semiconductors is recommended as the ideal solution.
Its spatial compactness and its suitability for use on the “dead” isolated network outperform the concept
of the phase-commutated thyristor converter. Four quadrant operation of the converter makes it possible
to control the active and reactive power balances at the wind farm end independently or one another, and
can therefore be additionally applied for power optimization in order to cut the costs of the wind turbines.
The converter system can be produced as a bipolar system with 150 kV in a twelve-pulse circuit. Between
the onshore and offshore stations power is exchanged with the aid of two single-conductor cables.
The DC cables of the forward and return conductors are laid jointly. Losses of the cables, heat
development and the anticipated magnetic fields are clearly less than in the case of three-phase cables.
The dimensions and weight of the DC cables are also clearly less than in the case of three-phase cables,
and so laying is clearly easier and more cost effective. Contrary to the AC transmission concept power
losses occur not mainly in the cable but in the converter systems.
Figure 4-2 shows the essential components of a DC offshore – onshore transmission /2/.
Figure 4-3: Components of a DC offshore – onshore transmission
The transmission capacity of available HVDC links varies between 250 to 300 MW per system.
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Number of parallel systems
4.3
Comparison of AC and DC transmission concepts
For the reason of limited transmission capability of both AC and DC systems it is strictly recommended
to consider these ranges of transmission capability. The decision about number and size of individual
windmills in a wind farm is not only determined by the available area but also by the capability of
economical and effective power transmission to shore, see Figure 4-4.
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6
5
4
3
2
1
0
0
250
500
750
1000 1250
Wind farm pow er [MW]
HVDC plus
AC 150 kV
Figure 4-4: Transmission capacity of parallel AC and DC systems /3/
If only investment costs and power losses of AC and DC transmission systems are compared, AC
solutions are of higher economic viability than DC systems up to 100 km to 120 km transmission
distance. Also the total power losses of AC solutions are less or equal to those of DC solutions.
Other investigations have figured out that DC solutions can be more reliable for a stable system operation
in the case of system faults with corresponding voltage sags.
For the design of large scale offshore wind power generation both AC and DC solutions should be
considered taking into account investment costs, losses and system stability.
5.
Conclusion
This paper considers technical aspects of the present situation of wind power generation in Germany and
the consequences of an ongoing wind power extension, particularly of offshore wind power for planning
and operation of the German grid. There is no doubt that a further wind power extension requires high
investments in new HV transmission lines and enhancements of transmission capabilities. Studies have
shown that:
•
Extension of wind energy requires corresponding network extensions
•
The technical characteristics of wind generation units must correspond to the requirements of a safe
and reliable grid operation, this is of particular importance during and after system faults
•
Extended wind power generation requires more balancing reserve power with corresponding higher
costs
•
Solutions of large scale offshore wind farm connections are in the development stage. Further
detailed basic design studies of AC and DC solutions are necessary as well as investigations
regarding system transients / stability of large scale offshore wind farms.
6.
References
/1/
Gundolf Dany, Hans-Jürgen Haubrich, Matthias Luther, Frank Berger, Klaus von Sengbusch,
“Auswirkungen der zunehmenden Windenergieeinspeisung auf die Übertragungsnetzbetreiber“,
Energiewirtschaftliche Tagesfragen, 53.Jg. (2003), Heft 9
7
/2/
Ronald Voelzke, Yvonne Sassnick, Norbert Christl, “Integration of large scale wind farms into
grids, technical aspects of transmission system design and grid control”, 2003 EWEC conference, Madrid
June 16.-19.
/3/
Ronald Voelzke, “Planung und Auslegung von Netzen für große offshore Windparks und deren
Einbindung in Übertragungsnetze“, Husum Wind 2003
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