AckermannTh. (ed) Wind Power in Power Systems

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cable laying as such was not a limiting factor. It was possible to lay cables at tempera-

tures as low as 10



13.2.9 Tjæreborg Project

In Tjæreborg on the West coast of Denmark, a project was carried out that was similar

to the VSC based HVDC project on Gotland. The Tjæreborg demonstration project will

be briefly presented below.

In Tjæreborg, an onshore wind farm consisting of four wind turbines is connected to

the AC power system via a VSC based HVDC link. This link operates in parallel with a

10 kV AC cable. The HVDC light transmission, with a rating of 7.2 MW and 8 MVA,

consists of two VSCs and two 9 kV DC cables. As the DC cables were laid in parallel to

the 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 the

demonstration 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 control

models and initial experience is included in Skytt et al. (2001).

13.3 Grid Issues

A successful expansion of wind power generation on Gotland requir es an adjustment

of the electric system in order to be able to regulate and keep an acceptable voltage

quality. In connection with the increasing expansion of the wind power production,

GEAB has cooperated with the company Vattenfall to ensure sufficient quality and

reliability of power supply. This has resulted in many new ways of looking at

network problems such as short-circuit currents, flicker and power flows, among

others. In order to solve these problems, new methods have been developed. The basic

aspects 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, but

also diesel power plants;

harmonics.

In the following, the findings regarding flicker, transient phenomena and stability issues

with voltage control equipment are briefly presented.

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13.3.1 Flicker

The investigations by GEAB showed that Type A wind turbines have different ways of

emitting flicker. ‘Slow’ flicker is due to wind gusts. Repeated startups, connections and

disconnections of capacitors are a source of ‘fast’ flicker. But most of the flicker from

wind turbines originates from the so-called ‘3P’ effect, which causes power and voltage

fluctuations at the blade–tower passing frequency, which typically gives rise to power

fluctuations of around 1–2 Hz. The typical frequencies of voltage fluctuations generated

by Type A wind turbines are around 1 and 8 Hz, which are, by coincidence, those to

which the human eye is most sensitive.

There are a number of standards and recommendations that set limits for allowed

flicker levels (see also Chapters 5 and 7). However, these are often obtained from

statistics and assumptions and do not guarantee disturbance-free voltage. GEAB has

not used the International Electrotechnical Commission (IEC) norm for flicker when

planning the installation of wind farms. Instead, the amplitude of the power fluctuation

for different frequencies and limit s for the different voltage fluctuations were used.

GEAB prefers this method as it provides more information than the statistic value that

IEC uses.

Extensive simulation work was carri ed out to evaluate the impact of the VSC HVDC

on flicker levels and to develop a flicker controller that reduces the flicker especially in

the range of 1–3 Hz.

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-mentioned

flicker controller is applied that phenomenon disappears. Simulation studies have shown

that the network angle and the design of the connecting grids are relevant for the

phenomenon where the 3P flicker contributions are synchronised. That means that if

the 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 A

turbines) is very important during faults in the grid. The sub transient current increases

with a larger number of asynchronous generators. If the fault does not last any longer

than approximately 200 ms the asynchronous generators consume reactive power from

the grid. Thus, the voltage dip at the coupling point is larger and the fault current during

the fault becomes lower than without wind power. Overcurrent relays can help to solve

this problem. The subtransient fault current and the fault current have to be calculated

for 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 a

problem once the installed rated power of the asynchronous generators totals around a

tenth of the short-circuit power at the coupling point. That was the main reason why

GEAB implemented the VSC based HVDC solution. On Gotland, it is more compli-

cated to handle trans ient phenomena because of the response of the LCC based HVDC

link between the mainland and the island. This response differs significantly from that of

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the VSC HVDC and also of a normal synchronous generator. All this affects the vo ltage

dip during faults. It is important that the voltage dip does not affect the customer

voltage, 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 network

and disturbs the shape of the fault current, which makes it difficult to apply the desired

selectivity in the operation of the overcurrent protection. During a fault, the VSC helps

to stabilise the voltage.

13.3.3.1 Contribution from VSC-based HVDC

The simulation of a three-phase short circuit applied close to the Na

s station with and

without the VSC HVDC shows a higher short-circ uit current in the first case, without

any wind power production (see Figure 13.4).

The converter station supports voltage regulation and thus contributes to stabilising

the voltage. During a voltage drop in the AC system, it causes the reactive current to

increase 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 1

0.5

1.5

2.5

3.5

Three-phase short circuit close to WPP

Time (s)

Short-circuit current (p.u.)

With HVDC light, with WPP

With HVDC light, no WPP

HVDC 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 Na

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with the distance to the fault and the prefaul t active power transmitted through the link.

During a fault with no wind power production (WPP) the short-circ uit current is

approximately constant if the VSC HVDC is connected.

13.3.3.2 Contribution from wind power product ion

With WPP, the subtransient short-circuit current can be considerably higher than with-

out WPP, depend ing 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 than

without WPP, owing to the behaviour of the asynchronous generators and capacitors.

The voltage control of VSC HVDC helps stabilise the voltage during the fault and

therefore 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 at

all production levels, with the normal overcurrent protection being used for the

distribution 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 protection

settings.

13.3.4 Validation

GEAB has the goal to maintain a sufficient power quality for its customers even after a

large expansion of wind power. Therefore, GEAB has studied four possible fault cases,

using computer models with a representation of the entire Gotland power system. The

simulation results that included wind power were compared with simulations without

wind power.

The most important fault case was named ‘Garda’. For this fault case it was possible

to compare simulation results with measurements from a real three-phase fault, which

was ach ieved 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 50 ms, but it showed the response from

different equipment. The power response from some wind turbines, the synchronous

generators, the LCC HVDC link to the mainland and, of course, from the VSC HVDC

was measured.

Voltage dips were measured with a sampling speed of at least 1000 Hz. Voltage dips

are defined and evaluat ed as 20 ms RMS (root mean square) values. Figure 13.5 presents

the voltage dips for some key network nodes during a short circuit with a DC voltage at

the VSC HVDC of 155 kV. Another short circuit was performed with a VSC HVDC

voltage of 96 kV and gave similar results.

Owing to the short fault duration, only the synchronous generators show a physical

response and not much response from the controllers can be detected. The same

happens in the case with wind power. The simulations and measurements show a very

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similar trend. The VSC HVDC voltage control, however, was slower than in the

simulations. The reason was that the gain in the voltage controller was set on a lower

value on the actual site, in order to obtain a more stable and safe operation.

Although the short-circuit test worked wel l, the voltage dip was in most places deeper

than the simulations showed. Work on the optimisation of the VSC HVDC voltage

control continues and has the goal to achieve similar or better results than the computer

simulations.

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 in

approximately 10 minutes have been recorded. These comparative ly high power output

variations 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 affect

local power quality. It can be difficult to control the voltage by tap changers and it also

affects 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 of

solving voltage control problems with wind power is to use dynamic voltage control. This

can be achieved by power electronics, such as SVC, together with intelligent controlling

algorithms. 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 the

reactive power set point as a function of system loads and production, which are

provided 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.

Ygne Bäcks Slite Hemse Näs

Measurement Simulation with HVDC light Simulation without HVDC light

Voltage dip (%)

Location

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

in Garda

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The algorithm calculates a reactive power set point for the VSC approximately every

5 minutes. The time constants for this slow voltage control must be coordinated with the

tap-changer controllers and voltage controllers of the synchronous machines in order to

avoid interaction or negative influence on the power system pe rformance. The interac-

tion between dynamic voltage controllers used in tap chang ers, synchronous machines,

power electronics as SVC and other types of voltage control devices in systems with a

large wind power penetration is an aspect that must be carefully analysed when setting

control parameters since it determines the voltage stability of the system.

The algorithm mentioned before is also used for the real-time calculation of the active

power set point in the VSC as a function of the system loads and production given by

the SCADA system. The main criterion is total loss reduction, althoug h operation

limits, 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 turbines

and wind farms are owned by cooperatives, others by individuals. GEAB as the grid

operator has the respo nsibility 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 to

handle this responsibility without being the owner of the plants. Therefore, GEAB has

defined technical requirements for the connection points. These requirements are

defined in terms of current or power. There are also some requirements regarding

protection and dynamic behaviour. The owners of the wind turbines have the obligation

to document the technical capabilities of each wind turbine and also have to follow the

requirements set in a contract for the grid connection. The locally developed guidelines

have been included in the Swedish guidelines – Anslutning av mindre produktionsan-

ggningar till elna

tet (AMP; see Svensk Energi, 2001) – which apply to all small

productions plants to be installed in Sweden.

(1)

13.3.7 Future work

Currently, there are applications for more than 300 MW of new wind farms to be

connected to the power system on Gotland. GEAB considers that the maximum wind

power capacity that can be connected is around 250 MW. However, additional technical

issues must be solved before this amount of wind power can be integrated into

the Gotland power system. Hence, GEAB is studying further technical solutions for

the network integration of wind power generation. Besides continuously studying

the impact of increasing wind power penetration on relay protection and voltage

control, GEAB wants to find further new solutions for the network integration of wind

power.

(1)

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

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13.4 Conclusions

In this chapter we have described the main difficulties that GEAB has faced during the

development of wind power on Gotland. The practical experience is likely to coincide

with 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 power

demand and the coordination of voltage control in the system: these issues are solved

mostly by the installation of the VSC based HV DC system;

fault situations and relay protection functions as well as selective plans: here, the VSC

based HVDC has a large effect and further work has to be done in order to arrive at

an optimal solution. Even the technical choice of the protection function and quality

has to be taken into consideration.

GEAB has worked with defining requirements that are specific to Gotland but has also

cooperated in clarifying standard documents regarding, mostly technical, requirements.

During this process, it has become evident that it is much more difficult than expected to

deal with these issues. The manufacturing of wind turbines is not standardised, and legal

aspects may limit the possibilities to define very specific requirements. It is considered very

important for the grid operator, though, to have the option to define very specific

requirements for wind farms in order to be able to fulfil its responsibility of operating

the power system and ensuring an appropriate quality of power supply.

Further Reading

[1] Castro, A. D., Ellstro

m, R., Jiang Ha

ffner, 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., A

berg, 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,O

dins Fo

rlag 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 produktionsanla

ggningar till elna

tet, Svensk Energi,

Stockholm, Sweden.

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Isolated Systems with Wind

Power

Per Lundsager and E. Ian Baring-Gould

14.1 Introduction

Isolated power supply systems using large amounts of wind and other renewable technol-

ogies are emerging as technically reliable options for power supply. Such systems are

generally perceived to have their major potential markets as local power supply in first,

second and third world countries, but the technology also has considerable potential as a

distributed generation component in large utility grids in the developed world.

During the past two decades there have been considerable efforts on the national and

international level to implement wind energy in connection with local and regional

electric power supply by its integration into small and medium-sized isolated distribu-

tion systems. These systems are often, but not necessarily, powered by diesel power

plants. Fairly recent revie ws made for the Danish International Development Agency,

DANIDA (Lundsager and Madsen, 1995) and the Danish Energy Agency (Lundsager

et al., 2001) identified about 100 reported and documented wind–diesel installations

worldwide.

Much work has been reported in public literature, but since studies of isolated systems

with wind power have mostly been case-oriented it has been difficult to extend results

from one project to another. This is not least because of the strong individuality that has

so far characterised such systems and their implementation, and therefore a consistent

and well-developed positive track record has yet to be developed.

This chap ter aims to give an account of the basic issues and principles associated with

the use of wind energy in isolated power systems and a review of international experi-

ence with such systems. The chapter describes a number of recent applications of wind

Wind Power in Power Systems Edited by T. Ackermann

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

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power in isolated systems and gives an account of the methodol ogies and tools now

available for the analysis and design of such systems. This chapter also outlines the

principles in the successful application of isolated systems with wind power.

14.2 Use of Wind Energy in Isolated Power Systems

In the world of rural electrification, there are two general methods of supplying energy

to rural areas: grid extension and the use of diesel generators. In remote areas both

options can be exceedingly expensive, grid electrification costing upwards of US$3000

per connection or a continued reliance on expensive and volatile diesel fuel. The

inclusion of renewable techno logies can lower the lifecycle cost of providing power to

rural areas. However, since renewable technologies, other than biomass technologies,

are dependent on a resource that is not dispatchable, the combination of a low-cost

renewable technology with a more expensive dispatchable technology may provide the

most applicable alternative.

Power systems using multiple generation sources can be more accurately described by

the term ‘hybrid power systems’ and can incorporate different components such as

production, storage, power conditioning and system control to supply power to a

remote community.

The classic hybrid systems include both a direct-current (DC) bus for the battery bank

and an alternating-current (AC) bus for the engine generator and distribution; however,

recent advances in power electronics and system control are making small single AC bus

systems more cost effective. The renewable technology may be attached to either the AC

or the DC bus depending on the system size and configuration. Power systems supplying

more than single homes or other single point systems usually supply AC power, although

some loads may be tapped off the DC bus bar. These power systems can range in size

from a few kilowatt-hours (kWh) per day to many megawatt-hours (MWh) per day.

Systems providing smaller loads, less than a few hundred kilowatt-hours per day

generally focus around the DC bus whereas larger systems tend to use the AC bus as the

main connection point. Recent advances in power electronics and control have allowed

the advanceme nt of smaller AC-connected devices where each DC-producing renewable

device includes its own dedicated power converter with imbedded control that allows it

to coordinate power production. Every configuration, however, can have a great deal of

internal variability. Smaller systems will likely use large battery banks, providing up to a

few days of storage, and will use smaller renewable generation devices connected to the

DC bus. Larger systems focus on the AC bus bar, with all renewable technology

designed to be connected to the AC distribution ne twork. In larger systems, the battery

bank, if used at all, is generally small and used mainly to cover fluctuations in power

production. Larger systems usually contain more and large r equipment that allows for

an economy of scale and thus lower power costs.

14.2.1 System concepts and configurations

In syst ems that focus around the DC bus, the battery bank acts as a large power

dampener, smoothing out any short-term or long-term fluctuations in the power flow.

300 Isolated Systems