AckermannTh. (ed) Wind Power in Power Systems
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for private developers, as neither special land acquisition rights nor specialized envir-
onmental reviews for transmission lines are available. The developers organized a
private real estate company and bought up large blocks of land along the desired
right of way. The land needed for the transmission rights of way was separated from
the residual land. The residual land was then effectively marketed, in some cases by
first improving the land for future development and by other value-increasing tech-
niques. In other cases, rights were obtained from governmental agencies such as at the
margins of county road rights of way. In one section, the line takes a short detour to
avoid property where the seller had very large dollar signs in his eyes and would not
‘budge’ until too late.
The competing utility has criticized the construction of the 230 kV line along a road
right of way, but such is regularly done by the utilities up to at least 115 kV, and there
simply does not appear to be a valid argument against the practices used. The concept
and execution were outstanding and successful and it was a remarkable achievement to
solve a difficult problem that was stalling wind energy development.
The alternative for the developers was to have the utility build the line. Such an
approach would likely have resulted in line costs in the order of double of the actual
costs and a time of construction several years longer. Such a procedure would have
resulted in the 185 MW of projects not being built and the contracts lost. Those
contracts were at an unusually high price, which allowed for the construction of the
private transmission line, the absorption of transmission losses and the taking of
substantial risk to make this happen.
The Sagebrush Line was originally rated for approximately 400 MW, based on the
configuration and size of conductor used. A maximum capacity 230 kV line would
normally be capable of approximately 800 MW. A double circuit on the same poles
would be capable of 1400 MW, limited by grid reliability rules for N 2 conditions. It is
unfortunate that it was not feasible for that line to have been built at maximum
capacity, or upgraded, and be utilized to connect at least some of the current 66 kV
wind farms and served customer load more reliably onto a much more reliable grid. If
this had been done at the time the line was built, many millions of dollars in curtailment
costs would not have been paid by SCE on the 66 kV grid and substantial economic
losses due to grid unreliability could have been avoided.
12.5 Conclusions
Experience with weak grid conditions in California and the US Midwest has prod uced
a wealth of knowledge and practical solutions that will be useful as wind industry growth
accelerates. VARs are to be respected and not feared. Effective use of VARs can make an
induction generator behave much like a conventional synchronous generator and can
dampen natural oscillations or instabilities in the grid. While self-correcting schemes have
been used to protect old grid systems from cascading overload conditions, proper grid
facilities and ride-through capability by wind turbines are essential for a reliable electric
system that takes advantage of the low-cost clean energy from wind farms.
Rational, orderly, cost-effective investment in transmission facilities, ahead of the
development curve, would allow far more efficient development of wind energy on a
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large scale and greater reliability to served customers. Increas ingly large-scale wind
energy development should be better integrated and coordinated with the existing
extra-high-voltage grid to take advantage of the efficiencies that would provide.
‘Brute-force’ techniques such as the more conventional ‘wire in the air’ have been the
method generally used to provide adequate transmission capability, but they can be
quite expensive. Alternative techniques of providing transmission capacity, such as
aggressive VAR support, planned curtailment of generation at periods of peak coin-
cident generation and use of energy storage as it becomes cost effective offer alternatives
to ‘wires in the air’ for any transmission corridor. Effective coordination of dispatchable
generation technologies wi th clean intermittent resources such as wind can provide an
energy supply that reliably matches load. Coordination of generation facilities in a
network through use of dispatch priorities or market incentives can increase the pro-
duction of clean energy, reduce emissions, and reduce the depletion of non renewable
fuels while maintaining high power system reliability.
References
[1] Muljadi, E., Wan, Y., Butterfield, C. P. (2002) ‘Dynamic Simulation of a Wind Farm with Variable Speed
Wind Turbines’, paper presented at the AIAA–ASME Wind Energy Symposium, Reno, NV, January
2002.
[2] Muljadi, E., Butterfield, C. P., Yinger, R., Romanowitz H. (2004) ‘Energy Storage and Reactive Power
Compensator in a Large Wind Farm’, paper presented at the AIAA–ASME Wind Energy Symposium,
Reno, NV, January 2004.
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13
Wind Power on the Swedish
Island of Gotland
Christer Liljegren and Thomas Ackermann
13.1 Introduction
This chapter gives an overview of the issues that the local network operator, Gotlands
Energi AB (GEAB ), has faced on the island of Gotland in connection with increasing
wind power penetration. Some of the solutions that have been implemented will be
described in a general way. The main focus is on the voltage source converter (VSC)
based high-voltage direct-current (HVDC) solution that was installed on the island of
Gotland in order to deal with network integration issues related to a very high penetra-
tion of Type A wind turbines.
(1)
13.1.1 History
The Swedish island of Gotland is situated in the Baltic Sea, about 90 km from the
Swedish mainland (see Figure 13.1).
Today’s network operator, GEAB, was initially, at the beginning of the last century
(1904–20), responsible only for operating electrical lights in the local streets. For this
purpose, GEAB developed a small electrical system to supply and operate the local
(1)
For definitions of wind turbine types A to D, see section 4.2.3.
Wind Power in Power Systems Edited by T. Ackermann
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lights. After the development of metering equipment, GEAB started to sell power to
other customers and became the local utility. Today, GEAB is the network operator of
the island, which includes the responsibility to provide sufficient power quality to local
network customers.
The electrical system was built starting from the village of Slite in the Northern part of
the island. The limestone industry that was located there was the first and largest
consumer of electricity. Even today the cement industry continues to be the biggest
customer and consumes nearly 30 % of the electrical energy on Gotland. In the early
1920s a local coal and oil fired power plant was built on Gotland. In the countryside,
small diesel generators were installed for local consumption. In the 1930s overhead lines
were built to connect the small power units to the small power system in Slite. This was
the beginning of the power system on Gotland.
The next major step was in 1954 with the installation of the first line commutated
converter (LCC) based HVDC link in the world, between the mainland of Sweden and
Gotland. The rated power of this link was 15 MW. Later on, it was upgraded to
30 MW.
The grid was designed to accommodate a consumption that consisted of small loads
in the countryside as well as some villages with smal l industrial facilities. In 1970
consumers began to use electricity for heating, and the consumption in remote places
increased significantly. In 1983, the old LCC based HVDC link was taken out of
operation and was replaced by a new LCC based HVDC link with a rated capacity of
150 MW. A few years later, a redundant HV DC link was built.
Gotland
Sweden
Latvia
Estonia
Öland
Baltic
Sea
Stockholm
Finland
Lithuania
Figure 13.1 Gotland and the Baltic Sea
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13.1.2 Description of the local power system
Today, the power network on Gotland consists of approximately 300 km of 70 kV lines,
100 km of 30 kV lines and 2000 km of 10 kV lines, an d it has about 36 000 customers. For
a simplified single-line diagram of the local 70 kV network, see Figure 13.2.
The maximum load on Gotland is about 160 MW and the minimum is approximately
40 MW. The consumption of electric energy on Gotland is approximately 900 GWh per
year. This energy is supplied mainly from the mainland through the LCC based HVDC
submarine cables. The HVDC link is also used to regulate the frequency on the island. In
order to ensure voltage stability there are also locally installed synchronous generators that
play an important role in the operation of the power system. Locally installed gas turbines
can be used as backup but produce only marginal amounts of energy per year.
VSC
65 MWA
VSC
65 MWA
±80 kV
50 MW
70 kV
10 kV
Näs
30 kV
10 kV
30 kV
10 kV
Load
Hemse
70 kV
LCC HVDC
to mainland
70 kV
Ygne
Visby
Cementa
(load)
Slite
10 kV
10 kV
Load
South
10 kV
Näs 2
Figure 13.2 Simplified single-line diagram of the power system on Gotland. Note: LCC ¼ line
commutated converter; HVDC ¼ high-voltage direct-current; VSC ¼ voltage source converter
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Good wind conditions, especially at the southern tip of the island, have led to an
increased number of wind turbines. In 1984 the total wind power capacity on Gotland
was 3 MW, and it increased to 15 MW in 1994. Early in 2003 the number of wind
turbines reached 160, with a total installed capacity of 90 MW, generating approxi-
mately 200 GWh per year. Wind power production accounts for approximately 22 % of
the total consumption on the island. From the perspective of power system operation, it
is important to poi nt out that at times with low loads (nights) and high wind speeds,
wind power can reach more than twice the minimum load.
For system operation, it is also important that most wind farms are concentrated in
the South of Gotland and consist mainly of fixed-speed turbines (Type A). The southern
part, in particular the area around Na
¨
s, is an impor tant test field for new wind turbines
and has played an important part in the history of the development and testing of
various wind turbines, including some of the first megawatt turbines in the world. There
are plans for further expansion of wind power, particularly in the northern part of the
island and offshore.
13.1.3 Power exchange with the mainland
In situations with high wind speeds and low local load, wind power production on the
island exceeds local demand. The LCC HVDC link to the mainland, however, was
originally designed only to operate in one direction: from the mainland to Gotland.
In 2002 the HVDC link to the mainland was therefore modified in orde r to allow a
change in the direction of the power flow without interfering with frequency control on
the network on Gotland. The system is now capable of automatically changing the
power flow direction with continuous frequency control.
The approximately 90 MW of wind power on the island lead to an exp ort of approxi-
mately 100 MWh of wind power production per year. Hence, for about 40 hours per
year the power flow direction that is usually from the mainland to the island must be
reversed. With the expected increase of local wind power on Gotland to 150 MW in the
near future, the island will export power for about 500 hour s per year, which is equal to
an export of about 2 GWh.
13.1.4 Wind power in the South of Gotland
The network infrastructure was originally planned according to local demand and did
not take into account significant local generation. The North has a strong network that
was designed to accomm odate large network customers (Visby and Slite). The power
system in the South, in contrast, can be considered weak. The local peak demand in
southern Gotland is approximately 17 MW, whereas the installed wind power capacity
reaches about 60 MW. The situation is even more extreme in the area of Na
¨
s, which lies
most to the South. There, the peak load is 0.5 MW, with approximately 50 MW of wind
power capacity, comprising mainly Type A turbines, installed in the area.
From a technical perspective, this imbalance between load and production makes
system operation very difficult. In comparison with a normal distribution system, the
short-circuit power, for example, is very low in relat ion to all the connected equipment.
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The solution to the island ’s power transmission problems was the installation of one of
the first VSC based HVDC links in the world.
13.2 The Voltage Source Converter Based High-voltage Direct-current
Solution
In the following, the VSC based HVDC solution is presented in more detai l.
13.2.1 Choice of technology
Theoretically, there were two alternatives for transmitting the surplus wind power
between the southern part of Gotland around the area of Na
¨
s to the load centre Visby:
.
alternating current (AC; cable or overhead line), or
.
direct current (DC; cable, VSC based HVDC technique).
In addition to the transmission lines, the AC alternative requires equipment for the
compensation of reactive power, synchronous machines or conventional compensators
[static VAR compensators (SVCs)] in order to ensure the required quality of supply for
the local network customers. The alternative with synchronous machines was too
expensive (in terms of investment, operation and maintenance). An AC three-phase
cable was not considered to be economically competitive and an AC overhead line
would be difficult to build for environmental reasons.
Being the best available technical alternative, a VSC based HVDC solution was chosen
for the project. Common station equipment could be used to meet the power transmission
as well as electrical quality requirements regarding the connecting network (for a discus-
sion of VSC based HVDC technology, see also Chapter 22). The VSC HVDC solution was
installed in parallel with the existing AC network, which helped to improving the dynamic
stability of the entire AC network. Simulation results have shown that a VSC HVDC also
helps to maintain the power quality in the northern part of the island.
13.2.2 Description
The VSC based HVDC system installed by Asea Brown Boveri (ABB) in 1999 consists
of two converter stations connected by 70 km of double 80 kV DC cable. The con-
verters are connected via reactors to the 75 kV AC power system. The transformers are
equipped with tap changers to be able to reduce the voltage on the converter side in
order to reduce no-load and low-load losses. This special feature was introduced to this
installation, since wind power seldom ope rates at peak production and sometimes does
not produce at all. This makes it very important to keep no-load and low-load losses as
low as possible, as low -load losses have a much larger impact on the overall project
economy than peak-load losses.
The VSCs use pulse-width modulation (PWM) and insulated gate bipolar transistors
(IGBTs). With PWM, the converter can operate at almost any phase angle or amplitude.
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This can be achieved by changing the PWM pattern, which can be done almost
instantaneously. As this allows independent control of both active and reactive power,
a PWM VSC comes close to being an almost ideal transmission network component as
it is able to control active and reactive power almost instantaneously. This way, the
operation characteristics become rather more software-dependent than hardware-
related.
13.2.3 Controllability
The control of PWM makes it possible to create any phase angle or amplitude within the
ratings. Consequently, control signals to a converter can change the output voltage and
current from the converter to the AC network almost instantaneous ly. Operation can
then take place in all four qua drants of the power–reactive-power plane (i.e. active
power transmission in any direction can be combined with generation or consumption
of reactive power).
The converters can control the transmitted active power in order for it to correspond
to the generated power from the wind farms and provide the capability to follow the
power output fluctuations from wind power generation. They can also, within certain
limits, even out short dips in power generation. This makes it possible to support the
frequency control in the power system on Gotland.
The Gotland solution comprises one DC and one AC transmission line in parallel. If
the AC line trips, the wind farms will become an isolated AC producti on area. If the
wind turbines produce at maximum, the wind turbine generator speed will increase and
eventually the turbine will trip based on high frequency or voltage criteria (individual
wind turbine protection systems). The VSC based HVDC link provides the possibility to
control this situati on by tripping wind turbines and shifting the mode in the Na
¨
s
converter from active power regulation to frequency regulation.
13.2.4 Reactive power support and control
Reactive power generation and consumption of a VSC can be used for compensating the
needs of the connected network, within the rating of the converter. As the rating of the
converters is based on maximum current and voltage, the reactive power capabilities of
a converter can be traded off for active power capability. The P–Q diagra m in Figure
13.3 illustrates the combined active–reactive power capabilities (posi tive Q is fed into the
AC network).
13.2.5 Voltage control
The reactive power capabilities of the VSC are used to control the AC voltages of the
network connected to the converter stations. The voltage control of a station constitutes
an outer feedback loop and influences the reactive cu rrent in order for the set voltage on
the network bus to be retained.
At low or no wind power production, the converters will normally switch to a standby
position to reduce no-load losses. In the case of a fault with a specific AC voltage
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decrease, the converters could be rapidly (within milliseconds) switched back to oper-
ation and assist with voltage support throughout the duration of the fault, thus avoiding
severe disturbances for local industries that are sensitive to voltage dips.
The time range for a response to a change in voltage is approxim ately 50 ms. That
means that in the case of a step-order change in the bus voltage, it takes 50 ms to reach
the new setting. With this response speed, the AC voltage control will be able to control
transients and flicker of up to around 3 Hz as well as other disturbances and keep the
AC bus voltage constant. It is thus capable of reducing a considerable part of the wind
power generated flicker, caused mainly by Type A wind turbi nes, at the AC bus.
13.2.6 Protection philosophy
The protection system of the VSC based HVDC is installed to disconnect the equipment
from operation in the event of short circuits and other operational abnormalities,
especially if they can cause damage to equipment or otherwise interfere with the effective
operation of the rest of the system. The protective system is based on the blocking of the
IGBT valve switching and/or by AC circuit breakers, which will result in tripping to de-
energise the system and thereby eliminate dangerous currents and voltages.
Converter and pole protection systems are especially designed for the VSC based
HVDC system and are based on the characteristics of the VSC and its active element,
the IGBT transistor. The specific protection systems are designed to handle overcur-
rents, short circuits across valves, overvoltages, ground faults on the transmission line
and the protection of the IGBT valves. Most of the protection systems block the
converter. Blocking means that a turnoff pulse is sent to the valves. They will not
conduct until they receive a de-block pulse.
U = 1.1 p.u.
U = 1 p.u.
U = 0.9 p.u.
P desired
P (p.u.)
–1.0 –0.8 –0.6 –0.4 –0.2 0.0
0.0
0.2
0.2
0.4
0.4
0.6
0.6
0.8
0.8
1.0 1.2
1.2
Q (p.u.)
1.0
Figure 13.3 Combined active and reactive power capabilities. Note: P ¼ active power;
Q ¼ reactive power; U ¼ voltage
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During short-circuit faults prevailing for periods of less than a second, the control system
will aim at avoiding wind power production tripping. Owing to continuously increased
wind power penetration, the grid protection scheme for short circuits must, however, be
continuously re-evaluated. It would be advantageous to leave the grid disconnection
scheme as well as the flow of fault currents unchanged, to the largest extent possible.
13.2.7 Losses
On Gotland, wind turbines operate most of the time below rated capacity. The average
full load hours are around 2000 hours per year for land-based wind turbines, and the
total operating time is around 6000 hours out of the 8760 hours in a year.
As the losses in the VSC based HVDC system are higher than those on the parallel
AC line, the power flow between the two lines was optimised in order to minimise
overall system losses. In this application, it is very important to reduce converter losses
also in pure SVC mode and for low power transmission. The DC voltage is the most
important parameter for the HVDC light valve losses. The isolation transformer tap
changer makes it possible to vary the DC voltage between 95 and 155 kV. This feature is
used in combination with a load-dependent DC voltage function. This way, the lowest
possible DC voltage is used when active power is low.
13.2.8 Practical experience with the installation
Applying for permits from authorities (including the associated environmental permits)
is normally a time-consuming process including questions and demands from various
affected parties. With underground DC cables, this process took very little time,
primarily because of the limited number of questions raised (related mainly to magnetic
fields). Using two underground cables that lie close together and have opposite polarities
provides the environmental advantage of comparatively low values for the magnetic field.
A total of 185 property-owners accepted the project within a few weeks. Another
contributing factor to the rapid approval was that 50 km of the total 70 km was going to
be built on the existing 70 kV line right of way.
The cable was laid with use of a method that is similar to the one applied for midrange
voltage cables (i.e. plowing the cable at a depth of at least 650 mm). The joints were
planned to be located at co nvenient places (e.g. at road crossings). At the factory, the
cable lengths were adapted to the locations of the joints. About 20 km of the 70 km lead
over rocky terrain. In these areas, a rock-milling machine with a width of 120 mm was
used. This way, the impact on the environment was kept to a minimum.
The difference from conventional cable laying was the logistics involved in handling
10 000 kg cable drums, each holding approximately 3.5 km of 340 mm
2
cable. Standard
trucks were used to transport the 48 cable drums to the sites. The cable lengths were
rolled out on the ground before being plowed.
The best machine for the plowing proved to be a timber hau ler with an added cable
drum stand. When the rock-milling machine had to be used, 300 m of cable were laid per
day. Otherwise, a total of several kilometres was laid per day. Roads and watercourses
had to be crossed, and telephone cables and power cables had to be moved. However,
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