wind power in power systems (ackermann/wind power in power systems) || wind power on the swedish...

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13

Wind Power on the SwedishIsland of Gotland

Christer Liljegren and Thomas Ackermann

13.1 Introduction

This chapter gives an overview of the issues that the local network operator, GotlandsEnergi AB (GEAB), has faced on the island of Gotland in connection with increasingwind power penetration. Some of the solutions that have been implemented will bedescribed 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 ofGotland 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 theSwedish 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 thispurpose, 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

� 2005 John Wiley & Sons, Ltd ISBN: 0-470-85508-8 (HB)


lights. After the development of metering equipment, GEAB started to sell power toother customers and became the local utility. Today, GEAB is the network operator ofthe island, which includes the responsibility to provide sufficient power quality to localnetwork customers.

The electrical system was built starting from the village of Slite in the Northern part ofthe island. The limestone industry that was located there was the first and largestconsumer of electricity. Even today the cement industry continues to be the biggestcustomer and consumes nearly 30% of the electrical energy on Gotland. In the early1920s 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 lineswere built to connect the small power units to the small power system in Slite. This wasthe beginning of the power system on Gotland.

The next major step was in 1954 with the installation of the first line commutatedconverter (LCC) based HVDC link in the world, between the mainland of Sweden andGotland. The rated power of this link was 15MW. Later on, it was upgraded to30MW.

The grid was designed to accommodate a consumption that consisted of small loadsin the countryside as well as some villages with small industrial facilities. In 1970consumers began to use electricity for heating, and the consumption in remote placesincreased significantly. In 1983, the old LCC based HVDC link was taken out ofoperation and was replaced by a new LCC based HVDC link with a rated capacity of150MW. A few years later, a redundant HVDC link was built.

GotlandSwedenLatvia

Estonia

Öland

BalticSea

Stockholm

Finland

Lithuania

Figure 13.1 Gotland and the Baltic Sea

284 Wind Power on Gotland


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, and it has about 36 000 customers. Fora simplified single-line diagram of the local 70 kV network, see Figure 13.2.

The maximum load on Gotland is about 160MW and the minimum is approximately40MW. The consumption of electric energy on Gotland is approximately 900GWh peryear. This energy is supplied mainly from the mainland through the LCC based HVDCsubmarine cables. The HVDC link is also used to regulate the frequency on the island. Inorder to ensure voltage stability there are also locally installed synchronous generators thatplay an important role in the operation of the power system. Locally installed gas turbinescan be used as backup but produce only marginal amounts of energy per year.

VSC65 MWA

VSC65 MWA

± 80 kV50 MW

70 kV

10 kVNäs

30 kV

10 kV

30 kV

10 kVLoad

Hemse

70 kV

LCC HVDC

to mainland

70 kV

Ygne

Visby Cementa(load)

Slite10 kV

10 kV

Load

Sou

th

10 kVNäs 2

Figure 13.2 Simplified single-line diagram of the power system on Gotland. Note: LCC ¼ linecommutated converter; HVDC ¼ high-voltage direct-current; VSC ¼ voltage source converter

Wind Power in Power Systems 285


Good wind conditions, especially at the southern tip of the island, have led to anincreased number of wind turbines. In 1984 the total wind power capacity on Gotlandwas 3MW, and it increased to 15MW in 1994. Early in 2003 the number of windturbines reached 160, with a total installed capacity of 90MW, generating approxi-mately 200GWh per year. Wind power production accounts for approximately 22% ofthe total consumption on the island. From the perspective of power system operation, itis important to point 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 inthe South of Gotland and consist mainly of fixed-speed turbines (Type A). The southernpart, in particular the area around Nas, is an important test field for new wind turbinesand has played an important part in the history of the development and testing ofvarious wind turbines, including some of the first megawatt turbines in the world. Thereare plans for further expansion of wind power, particularly in the northern part of theisland and offshore.

13.1.3 Power exchange with the mainland

In situations with high wind speeds and low local load, wind power production on theisland exceeds local demand. The LCC HVDC link to the mainland, however, wasoriginally designed only to operate in one direction: from the mainland to Gotland.

In 2002 the HVDC link to the mainland was therefore modified in order to allow achange in the direction of the power flow without interfering with frequency control onthe network on Gotland. The system is now capable of automatically changing thepower flow direction with continuous frequency control.

The approximately 90MW of wind power on the island lead to an export of approxi-mately 100MWh of wind power production per year. Hence, for about 40 hours peryear the power flow direction that is usually from the mainland to the island must bereversed. With the expected increase of local wind power on Gotland to 150MW in thenear future, the island will export power for about 500 hours per year, which is equal toan export of about 2GWh.

13.1.4 Wind power in the South of Gotland

The network infrastructure was originally planned according to local demand and didnot take into account significant local generation. The North has a strong network thatwas designed to accommodate large network customers (Visby and Slite). The powersystem in the South, in contrast, can be considered weak. The local peak demand insouthern Gotland is approximately 17MW, whereas the installed wind power capacityreaches about 60MW. The situation is even more extreme in the area of Nas, which liesmost to the South. There, the peak load is 0.5MW, with approximately 50MW of windpower capacity, comprising mainly Type A turbines, installed in the area.

From a technical perspective, this imbalance between load and production makessystem operation very difficult. In comparison with a normal distribution system, theshort-circuit power, for example, is very low in relation to all the connected equipment.



The solution to the island’s power transmission problems was the installation of one ofthe 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 detail.

13.2.1 Choice of technology

Theoretically, there were two alternatives for transmitting the surplus wind powerbetween the southern part of Gotland around the area of Nas 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 thecompensation of reactive power, synchronous machines or conventional compensators[static VAR compensators (SVCs)] in order to ensure the required quality of supply forthe local network customers. The alternative with synchronous machines was tooexpensive (in terms of investment, operation and maintenance). An AC three-phasecable was not considered to be economically competitive and an AC overhead linewould be difficult to build for environmental reasons.

Being the best available technical alternative, a VSC based HVDC solution was chosenfor the project. Common station equipment could be used to meet the power transmissionas well as electrical quality requirements regarding the connecting network (for a discus-sion of VSC based HVDC technology, see also Chapter 22). The VSCHVDC solution wasinstalled in parallel with the existing AC network, which helped to improving the dynamicstability of the entire AC network. Simulation results have shown that a VSC HVDC alsohelps 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 consistsof 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 areequipped with tap changers to be able to reduce the voltage on the converter side inorder to reduce no-load and low-load losses. This special feature was introduced to thisinstallation, since wind power seldom operates at peak production and sometimes doesnot produce at all. This makes it very important to keep no-load and low-load losses aslow as possible, as low-load losses have a much larger impact on the overall projecteconomy 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.



This can be achieved by changing the PWM pattern, which can be done almostinstantaneously. As this allows independent control of both active and reactive power,a PWM VSC comes close to being an almost ideal transmission network component asit is able to control active and reactive power almost instantaneously. This way, theoperation characteristics become rather more software-dependent than hardware-related.

13.2.3 Controllability

The control of PWMmakes it possible to create any phase angle or amplitude within theratings. Consequently, control signals to a converter can change the output voltage andcurrent from the converter to the AC network almost instantaneously. Operation canthen take place in all four quadrants of the power–reactive-power plane (i.e. activepower transmission in any direction can be combined with generation or consumptionof reactive power).

The converters can control the transmitted active power in order for it to correspondto the generated power from the wind farms and provide the capability to follow thepower output fluctuations from wind power generation. They can also, within certainlimits, even out short dips in power generation. This makes it possible to support thefrequency control in the power system on Gotland.

The Gotland solution comprises one DC and one AC transmission line in parallel. Ifthe AC line trips, the wind farms will become an isolated AC production area. If thewind turbines produce at maximum, the wind turbine generator speed will increase andeventually the turbine will trip based on high frequency or voltage criteria (individualwind turbine protection systems). The VSC based HVDC link provides the possibility tocontrol this situation by tripping wind turbines and shifting the mode in the Nasconverter 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 theneeds of the connected network, within the rating of the converter. As the rating of theconverters is based on maximum current and voltage, the reactive power capabilities ofa converter can be traded off for active power capability. The P–Q diagram in Figure13.3 illustrates the combined active–reactive power capabilities (positiveQ is fed into theAC network).

13.2.5 Voltage control

The reactive power capabilities of the VSC are used to control the AC voltages of thenetwork connected to the converter stations. The voltage control of a station constitutesan outer feedback loop and influences the reactive current in order for the set voltage onthe network bus to be retained.

At low or no wind power production, the converters will normally switch to a standbyposition to reduce no-load losses. In the case of a fault with a specific AC voltage



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 avoidingsevere disturbances for local industries that are sensitive to voltage dips.

The time range for a response to a change in voltage is approximately 50ms. Thatmeans that in the case of a step-order change in the bus voltage, it takes 50ms to reachthe new setting. With this response speed, the AC voltage control will be able to controltransients and flicker of up to around 3Hz as well as other disturbances and keep theAC bus voltage constant. It is thus capable of reducing a considerable part of the windpower generated flicker, caused mainly by Type A wind turbines, at the AC bus.

13.2.6 Protection philosophy

The protection system of the VSC based HVDC is installed to disconnect the equipmentfrom operation in the event of short circuits and other operational abnormalities,especially if they can cause damage to equipment or otherwise interfere with the effectiveoperation of the rest of the system. The protective system is based on the blocking of theIGBT 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 basedHVDC 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 lineand the protection of the IGBT valves. Most of the protection systems block theconverter. Blocking means that a turnoff pulse is sent to the valves. They will notconduct 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



During short-circuit faults prevailing for periods of less than a second, the control systemwill aim at avoiding wind power production tripping. Owing to continuously increasedwind power penetration, the grid protection scheme for short circuits must, however, becontinuously re-evaluated. It would be advantageous to leave the grid disconnectionscheme 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 averagefull load hours are around 2000 hours per year for land-based wind turbines, and thetotal 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 parallelAC line, the power flow between the two lines was optimised in order to minimiseoverall system losses. In this application, it is very important to reduce converter lossesalso in pure SVC mode and for low power transmission. The DC voltage is the mostimportant parameter for the HVDC light valve losses. The isolation transformer tapchanger makes it possible to vary the DC voltage between 95 and 155 kV. This feature isused in combination with a load-dependent DC voltage function. This way, the lowestpossible 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 variousaffected parties. With underground DC cables, this process took very little time,primarily because of the limited number of questions raised (related mainly to magneticfields). Using two underground cables that lie close together and have opposite polaritiesprovides the environmental advantage of comparatively low values for the magnetic field.

A total of 185 property-owners accepted the project within a few weeks. Anothercontributing factor to the rapid approval was that 50 km of the total 70 km was going tobe 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 midrangevoltage cables (i.e. plowing the cable at a depth of at least 650mm). The joints wereplanned to be located at convenient places (e.g. at road crossings). At the factory, thecable lengths were adapted to the locations of the joints. About 20 km of the 70 km leadover rocky terrain. In these areas, a rock-milling machine with a width of 120mm wasused. This way, the impact on the environment was kept to a minimum.

The difference from conventional cable laying was the logistics involved in handling10 000 kg cable drums, each holding approximately 3.5 km of 340mm2 cable. Standardtrucks were used to transport the 48 cable drums to the sites. The cable lengths wererolled out on the ground before being plowed.

The best machine for the plowing proved to be a timber hauler with an added cabledrum stand. When the rock-milling machine had to be used, 300m of cable were laid perday. Otherwise, a total of several kilometres was laid per day. Roads and watercourseshad to be crossed, and telephone cables and power cables had to be moved. However,



cable laying as such was not a limiting factor. It was possible to lay cables at tempera-tures as low as �10 �C.

13.2.9 Tjæreborg Project

In Tjæreborg on the West coast of Denmark, a project was carried out that was similarto the VSC based HVDC project on Gotland. The Tjæreborg demonstration project willbe briefly presented below.

In Tjæreborg, an onshore wind farm consisting of four wind turbines is connected tothe AC power system via a VSC based HVDC link. This link operates in parallel with a10kV AC cable. The HVDC light transmission, with a rating of 7.2MW and 8MVA,consists of two VSCs and two �9 kV DC cables. As the DC cables were laid in parallel tothe AC cable, it is possible to operate the wind farm in three different operation modes:

. AC mode via the AC cable only;

. DC mode via the DC cable only;

. parallel mode via the AC and DC cables in parallel.

The option to operate the wind farm in these three different modes has made thedemonstration project a study object. It is also very interesting to carry out a compari-son of the various operation modes. A detailed discussion of the project, the controlmodels and initial experience is included in Skytt et al. (2001).

13.3 Grid Issues

A successful expansion of wind power generation on Gotland requires an adjustmentof the electric system in order to be able to regulate and keep an acceptable voltagequality. In connection with the increasing expansion of the wind power production,GEAB has cooperated with the company Vattenfall to ensure sufficient quality andreliability of power supply. This has resulted in many new ways of looking atnetwork problems such as short-circuit currents, flicker and power flows, amongothers. In order to solve these problems, new methods have been developed. The basicaspects that were studied are:

. Flicker;

. transient phenomena where faults have been dominating;

. stability in the system with voltage control equipment;

. power flows, reactive power demand and voltage levels in the system;

. calculation of losses in the system with wind power generation;

. instantaneous frequency control with production sources such as gas turbines, butalso diesel power plants;

. harmonics.

In the following, the findings regarding flicker, transient phenomena and stability issueswith voltage control equipment are briefly presented.



13.3.1 Flicker

The investigations by GEAB showed that Type A wind turbines have different ways ofemitting flicker. ‘Slow’ flicker is due to wind gusts. Repeated startups, connections anddisconnections of capacitors are a source of ‘fast’ flicker. But most of the flicker fromwind turbines originates from the so-called ‘3P’ effect, which causes power and voltagefluctuations at the blade–tower passing frequency, which typically gives rise to powerfluctuations of around 1–2Hz. The typical frequencies of voltage fluctuations generatedby Type A wind turbines are around 1 and 8Hz, which are, by coincidence, those towhich the human eye is most sensitive.

There are a number of standards and recommendations that set limits for allowedflicker levels (see also Chapters 5 and 7). However, these are often obtained fromstatistics and assumptions and do not guarantee disturbance-free voltage. GEAB hasnot used the International Electrotechnical Commission (IEC) norm for flicker whenplanning the installation of wind farms. Instead, the amplitude of the power fluctuationfor different frequencies and limits for the different voltage fluctuations were used.GEAB prefers this method as it provides more information than the statistic value thatIEC uses.

Extensive simulation work was carried out to evaluate the impact of the VSC HVDCon flicker levels and to develop a flicker controller that reduces the flicker especially inthe range of 1–3Hz.

GEAB has also noticed that particularly Type A turbines can go into ‘synchronous’operation, during specific conditions that depend on the grid. When the above-mentionedflicker controller is applied that phenomenon disappears. Simulation studies have shownthat the network angle and the design of the connecting grids are relevant for thephenomenon where the 3P flicker contributions are synchronised. That means that ifthe network is upgraded the condition changes and this situation can be avoided.

13.3.2 Transient phenomena

Network studies have also shown that the behaviour of asynchronous generators (Type Aturbines) is very important during faults in the grid. The subtransient current increaseswith a larger number of asynchronous generators. If the fault does not last any longerthan approximately 200ms the asynchronous generators consume reactive power fromthe grid. Thus, the voltage dip at the coupling point is larger and the fault current duringthe fault becomes lower than without wind power. Overcurrent relays can help to solvethis problem. The subtransient fault current and the fault current have to be calculatedfor a longer period in order to arrive at the right settings for the relay protection.

Simulation studies and measurements have shown that normal synchronous gener-ators control the voltage too slowly and cannot avoid these phenomena. They become aproblem once the installed rated power of the asynchronous generators totals around atenth of the short-circuit power at the coupling point. That was the main reason whyGEAB implemented the VSC based HVDC solution. On Gotland, it is more compli-cated to handle transient phenomena because of the response of the LCC based HVDClink between the mainland and the island. This response differs significantly from that of



the VSC HVDC and also of a normal synchronous generator. All this affects the voltagedip during faults. It is important that the voltage dip does not affect the customervoltage, or at least does not exceed a level that is acceptable.

13.3.3 Stability issues with voltage control equipment

The presence of wind turbines alters the normal short-circuit conditions in the networkand disturbs the shape of the fault current, which makes it difficult to apply the desiredselectivity in the operation of the overcurrent protection. During a fault, the VSC helpsto stabilise the voltage.

13.3.3.1 Contribution from VSC-based HVDC

The simulation of a three-phase short circuit applied close to the Nas station with andwithout the VSC HVDC shows a higher short-circuit current in the first case, withoutany wind power production (see Figure 13.4).

The converter station supports voltage regulation and thus contributes to stabilisingthe voltage. During a voltage drop in the AC system, it causes the reactive current toincrease up to the capacity limit. The contribution to the short-circuit current decreases

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.5

1

1.5

2

2.5

3

3.5

4Three-phase short circuit close to WPP

Time (s)

Sho

rt-c

ircui

t cur

rent

(p.

u.)

With HVDC light, with WPPWith HVDC light, no WPPHVDC light disconnected, no WPP

Figure 13.4 Effect of high-voltage direct-current (HVDC) light and wind power production(WPP) on the fault current during short circuit at Nas



with the distance to the fault and the prefault active power transmitted through the link.During a fault with no wind power production (WPP) the short-circuit current isapproximately constant if the VSC HVDC is connected.

13.3.3.2 Contribution from wind power production

With WPP, the subtransient short-circuit current can be considerably higher than with-out WPP, depending on the location of the fault and the level of the connected WPP.Without voltage control, the current decreases over time to the same or lower value thanwithout WPP, owing to the behaviour of the asynchronous generators and capacitors.The voltage control of VSC HVDC helps stabilise the voltage during the fault andtherefore the fault current, which is helpful to the operation of the protection system.

13.3.3.3 Protection setting philosophy for the AC system

Owing to the high level of WPP, it is difficult to fulfil the selectivity demands atall production levels, with the normal overcurrent protection being used for thedistribution system. However, the operation of the protection system has to be ensured,by setting the protection relays to the minimum values of the short-circuit currents.Computer simulations of short circuits have been used to analyse the proper protectionsettings.

13.3.4 Validation

GEAB has the goal to maintain a sufficient power quality for its customers even after alarge expansion of wind power. Therefore, GEAB has studied four possible fault cases,using computer models with a representation of the entire Gotland power system. Thesimulation results that included wind power were compared with simulations withoutwind power.

The most important fault case was named ‘Garda’. For this fault case it was possibleto compare simulation results with measurements from a real three-phase fault, whichwas achieved by closing a 10 kV breaker to simulate a solid three-phase short circuit.The sequence consisted only of closing the breaker and letting the overcurrent protec-tion trip the breaker again. The fault lasted only 50ms, but it showed the response fromdifferent equipment. The power response from some wind turbines, the synchronousgenerators, the LCC HVDC link to the mainland and, of course, from the VSC HVDCwas measured.

Voltage dips were measured with a sampling speed of at least 1000Hz. Voltage dipsare defined and evaluated as 20ms RMS (root mean square) values. Figure 13.5 presentsthe voltage dips for some key network nodes during a short circuit with a DC voltage atthe VSC HVDC of 155 kV. Another short circuit was performed with a VSC HVDCvoltage of 96 kV and gave similar results.

Owing to the short fault duration, only the synchronous generators show a physicalresponse and not much response from the controllers can be detected. The samehappens in the case with wind power. The simulations and measurements show a very



similar trend. The VSC HVDC voltage control, however, was slower than in thesimulations. The reason was that the gain in the voltage controller was set on a lowervalue on the actual site, in order to obtain a more stable and safe operation.

Although the short-circuit test worked well, the voltage dip was in most places deeperthan the simulations showed. Work on the optimisation of the VSC HVDC voltagecontrol continues and has the goal to achieve similar or better results than the computersimulations.

13.3.5 Power flow

It is common knowledge that wind power production depends strongly on wind speed.On Gotland, local wind power production variations of around 40% of rated capacity inapproximately 10 minutes have been recorded. These comparatively high power outputvariations are due to the geographical concentration of a large amount of wind power.

The power output variations produce voltage variations in the system, which can affectlocal power quality. It can be difficult to control the voltage by tap changers and it alsoaffects the maintenance cost of the transformers. A possibility is capacitor on–off switch-ing, but this may affect the voltage quality considerably. The most convenient method ofsolving voltage control problems with wind power is to use dynamic voltage control. Thiscan be achieved by power electronics, such as SVC, together with intelligent controllingalgorithms. On Gotland, the VSC of the HVDC solution is used for the same purpose.

An algorithm was designed for the real-time calculation and management of thereactive power set point as a function of system loads and production, which areprovided at regular intervals by the SCADA (supervisory control and data acquisition)system. This can be used to optimise the power flow against low losses and voltage levels.

0

5

10

15

20

25

30

Ygne Bäcks Slite Hemse Näs

Measurement Simulation with HVDC light Simulation without HVDC light

Vol

tage

dip

(%

)

Location

Figure 13.5 Comparison of simulations and measurements of the voltage dip for a short circuitin Garda



The algorithm calculates a reactive power set point for the VSC approximately every5 minutes. The time constants for this slow voltage control must be coordinated with thetap-changer controllers and voltage controllers of the synchronous machines in order toavoid interaction or negative influence on the power system performance. The interac-tion between dynamic voltage controllers used in tap changers, synchronous machines,power electronics as SVC and other types of voltage control devices in systems with alarge wind power penetration is an aspect that must be carefully analysed when settingcontrol parameters since it determines the voltage stability of the system.

The algorithm mentioned before is also used for the real-time calculation of the activepower set point in the VSC as a function of the system loads and production given bythe SCADA system. The main criterion is total loss reduction, although operationlimits, such as maximum line current and so on are taken into account.

13.3.6 Technical responsibility

The ownership of wind power on Gotland varies from case to case. Some wind turbinesand wind farms are owned by cooperatives, others by individuals. GEAB as the gridoperator has the responsibility to ensure sufficient power quality to all network custom-ers, which includes also local generation.

Even though wind power has a considerable impact on power quality, GEAB has tohandle this responsibility without being the owner of the plants. Therefore, GEAB hasdefined technical requirements for the connection points. These requirements aredefined in terms of current or power. There are also some requirements regardingprotection and dynamic behaviour. The owners of the wind turbines have the obligationto document the technical capabilities of each wind turbine and also have to follow therequirements set in a contract for the grid connection. The locally developed guidelineshave been included in the Swedish guidelines – Anslutning av mindre produktionsan-laggningar till elnatet (AMP; see Svensk Energi, 2001) – which apply to all smallproductions plants to be installed in Sweden.(1)

13.3.7 Future work

Currently, there are applications for more than 300MW of new wind farms to beconnected to the power system on Gotland. GEAB considers that the maximum windpower capacity that can be connected is around 250MW. However, additional technicalissues must be solved before this amount of wind power can be integrated intothe Gotland power system. Hence, GEAB is studying further technical solutions forthe network integration of wind power generation. Besides continuously studyingthe impact of increasing wind power penetration on relay protection and voltagecontrol, GEAB wants to find further new solutions for the network integration of windpower.

(1) For a more detailed discussion of AMP and other international interconnection rules, see Chapter 7.



13.4 Conclusions

In this chapter we have described the main difficulties that GEAB has faced during thedevelopment of wind power on Gotland. The practical experience is likely to coincidewith that of other grid systems, even if they do not use VSC based HVDC as a solution.In summary, the main issues are:

. voltage stability related to transient situations, power flows and reactive powerdemand and the coordination of voltage control in the system: these issues are solvedmostly by the installation of the VSC based HVDC system;

. fault situations and relay protection functions as well as selective plans: here, the VSCbased HVDC has a large effect and further work has to be done in order to arrive atan optimal solution. Even the technical choice of the protection function and qualityhas to be taken into consideration.

GEAB has worked with defining requirements that are specific to Gotland but has alsocooperated in clarifying standard documents regarding, mostly technical, requirements.During this process, it has become evident that it is much more difficult than expected todeal with these issues. The manufacturing of wind turbines is not standardised, and legalaspects may limit the possibilities to define very specific requirements. It is considered veryimportant for the grid operator, though, to have the option to define very specificrequirements for wind farms in order to be able to fulfil its responsibility of operatingthe power system and ensuring an appropriate quality of power supply.

Further Reading

[1] Castro, A. D., Ellstrom, R., Jiang Haffner, Y., Liljegren (2000) ‘Co-ordination of Parallel AC–DC Systems

for Optimum Performance’, paper presented at Distributech 2000, September 2000, Madrid, Spain.

[2] Holm, A. (1998) ‘DC Distribution, the Distribution for the 21st Century?’, paper presented at Distributech

98, November 1998, London, UK.

[3] Liljegren, C., Aberg, M., Eriksson, K., Tollerz, O., Axelsson, U., Holm, A. (2001) ‘The Gotland HVDC

Light Project – Experiences from Trial and Commercial Operation’, paper presented at Cired Conference,

Amsterdam, 2001.

[4] Rosvall, T. (2001) Eletricitet pa Gotland, Odins Forlag AB, November 2001.

[5] Thiringer, T., Petru, P., Liljegren, C. (2001) ‘Power Quality Impact of Sea Located Hybrid Wind Park’,

IEEE Transactions on Energy Conversion 16(2) 123–127.

References

[1] Skytt, A.-K., Holmberg, P., Juhlin, L.-E. (2001) ‘HVDC Light for Connection of Wind Farms’, in

Proceedings of the Second International Workshop on Transmission Networks for Offshore Wind Farms,

Royal Institute of Technology, Stockholm, Sweden, March 2001.

[2] Svensk Energi (2001) AMP – Anslutning av mindre produktionsanlaggningar till elnatet, Svensk Energi,

Stockholm, Sweden.


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