1

Abstract—This paper introduces the power quality issues of

wind power installations in a historic perspective, as the development from a few small wind turbines connected directly to the low voltage grid, to the present system with high penetration on the medium voltage distribution grids and two large offshore wind farms connected at transmission level. In this perspective, the power quality issues are divided into local issues particularly related to the voltage quality in the distribution systems and global issues related to the power system control and stability. Power quality characteristics of wind turbines and wind farms are described according to national and international standards, and measurements from wind farms are presented.

Index Terms—Wind power generation, Power control, Frequency control, Spectral analysis.

I. INTRODUCTION

he installed wind power capacity in the Danish power system is the highest in the world, compared to the

system load. From the beginning of the 1970’ies, the wind power developed exponentially in Denmark until the beginning of this century.

The first wind turbines were connected directly to the low voltage distribution grid, but as the rated power of the commercial wind turbines exceeded 50 – 100 kW in the

Manuscript received December 5, 2006. This work was supported by the

Danish Transmissions System Operator, Energinet.dk, under research program PSO 2004, Grant 6506 and PSO Grant 2005-2-6345.

P. Sørensen is with Risø National Laboratory, Roskilde, DK-4000 Denmark (phone: +45 4677 5075; fax: +45 4677 5075; e-mail: poul.e.soerensen@risoe.dk).

N. A. Cutululis is with Risø National Laboratory, Roskilde, DK-4000 Denmark (e-mail: nicolaos.cutululis@risoe.dk).

T. Lund is with Risø National Laboratory, Roskilde, DK-4000 Denmark (e-mail: torsten.lund@risoe.dk).

A. D. Hansen is with Risø National Laboratory, Roskilde, DK-4000 Denmark (e-mail: anca.daniela.hansen@risoe.dk).

T. Sørensen is with DONG Energy, Copenhagen, DK-2450, Denmark (e-mail: troso@dongenergy.dk)

J. Hjerrild is with Elsam (now a part of DONG Energy) Engineering, Fredericia, DK-7000, Denmark (e-mail: jeshj@dongenergy.dk)

M. H. Donovan is with DONG Energy, Copenhagen, DK-2450, Denmark (e-mail: marhd@dongenergy.dk)

L. Christensen is with Delta, Them, DK-8653, Denmark (e-mail: lc@delta.dk)

H. K. Nielsen is with Delta, Them, DK-8653, Denmark (e-mail: hkn@delta.dk)

1980’ies, the wind turbines were connected to the medium voltage distribution system with dedicated transformers. The majority of the wind turbines connected to the distribution system in Denmark are fixed speed wind turbines with directly connected induction generators. The majority are either connected in clusters or as individual wind turbines, although a few wind farms were developed, all with less than 50 MW installed capacity.

In 2002 and 2003, two large offshore wind farms were developed and connected directly to the transmission system. The first installation was the 160 MW Horns Rev wind farm based on the doubly-fed induction generator technology. The second installation was the 165 MW Nysted wind farm based on directly connected squirrel cage induction generators.

This development has influenced the focus on the power quality issues. The basic and first concern for connection to the distribution system was to ensure that the voltage in the distribution grid was kept within an acceptable range. But soon, also the impact on the voltage fluctuations in the distribution grid was introduced. First, the approach was simply to limit the inrush currents to limit voltage drops, but later less conservative methods were introduced including flicker assessment.

In 2001, IEC issued the first edition of the international standard IEC 61400-21 [1], for measurement and assessment of power quality of grid connected wind turbines. IEC 61400-21 defines power quality characteristics for wind turbines and proposes corresponding methods to asses the impact of one or more wind turbines on the grid power quality. As an example, a flicker coefficient and its measurement procedure is defined, and methods are specified to calculate the expected flicker emission from one or more wind turbines, based on their flicker coefficients. The power quality characteristics defined in the first edition of IEC 61400-21 are addressing the impact on the local power quality.

The development of large wind farms has caused increased focus in the last 5-10 years on the more global power quality issues related to the power system control and stability. The first grid code for connection of large wind farms was issued in Denmark in 1999, and presently the Danish transmission system operator Energinet.dk has two grid codes for wind turbines, one for connection below 100 kV [2], and one for connection above 100 kV [3].

Power Quality Issues on Wind Power Installations in Denmark

Poul Sørensen, Member, IEEE, Nicolaos Antonio Cutululis, Member, IEEE, Torsten Lund, Anca D. Hansen, Troels Sørensen, Jesper Hjerrild, Martin Heyman Donovan, Leif Christensen, Henny Kræmer

Nielsen

T

1-4244-1298-6/07/$25.00 ©2007 IEEE.

2

In both of these grid codes, and in several other national grid codes, the issue of fault-ride-through plays a key role. In the Danish grid codes, there is also high focus on the controllability of active and reactive power, particularly for the connections of large wind farms above 100 kV.

II. LOCAL VOLTAGE QUALITY

The methods specified in IEC 61400-21 to assess the impact of wind turbines on the local voltage quality are currently studied based on measurements on the 33 kV power collection grid in Nysted offshore wind farm. In the present paper, the measured voltage quality in the wind farm is presented, and in later publications this will be compared to the voltage quality which would have been assessed based on IEC 61400-21.

The layout of the wind farm power collection grid is shown in Fig. 1. The wind farm consists of 72 SIEMENS wind turbines each with rated power 2.3 MW. The wind turbines use directly connected induction generators and thyristor switched capacitors for reactive power compensation on the low voltage terminals. Besides, each wind turbine has its own step-up transformer, which is not shown in the diagram.

Power quality measurements have been performed on wind turbines A1 and A9. Three phase currents and voltages are measured at the wind turbines together with the wind speed, which is provided by the wind turbine controller registration of wind speed on the nacelle. Besides, three phase voltages and currents are measured at the transformer station on the offshore platform (PF). The measured data will be used to provide complete power quality of A1 and A9 according to IEC 61400-21.

With 72 wind turbines in the wind farm, the wind turbines have a significant influence on the local voltage quality, and therefore the measurements provide a good basis for a study of the actual voltage quality compared to the voltage quality which can be obtained by test results and assessment methods according to IEC 61400-21. The study will include voltage range, flicker and harmonics.

A. Voltage range

As mentioned in the introduction, the first concern for connection of wind turbines is to ensure that the voltage in the distribution grid was kept within an acceptable range. In the European voltage standard EN 50160 [4], a range of ±10 % of the 10 minute average voltages at the costumers is acceptable, ensuring that appliances are not damaged or malfunctioning.

IEC 61400-21 recommends that load-flow studies are conducted to assess the influence of wind turbines on the steady state voltages. To provide input for such load flow studies, the maximum power and corresponding reactive power is measured as part of the wind turbine test procedure. The maximum powers P60 and P0.2 is measured with 60 s and 0.2 s average times respectively, and the corresponding reactive powers Q60 and Q0.2 are provided by the measured P-Q relation.

Generally, the maximum power of a wind turbine decreases for increased average times. The 60 s average time is selected as a conservative (i.e. small) value which can be used to assess e.g. thermal overloading of components in the power system. According to IEC 61400-21, P60 for a wind farm can be calculated as the sum of P60 values for each wind turbine. This is a conservative assessment, because it assumes that the maximum occurs simultaneously at all wind turbines.

The 0.2 s average time is selected to indicate possible relay tripping due to short periods with a high power flow. IEC 61400-21 specifies a method to calculate P0.2 for a wind farm, taking into account that the fast variations in the power of different wind turbines are uncorrelated, and therefore the wind farm P0.2 is less than the sum of P0.2 for the individual wind turbines.

Fig. 2 shows measured 10 minute average values of voltages vs. wind speeds measured on wind turbine A9 (p.u. base is 33 kV) in Nysted offshore wind farm. It is seen that the voltage increases with the wind speed, although there is some scatter. The main reasons for the scatter are that the wind speed is not the same at all wind turbines, and that the voltage in the wind farm connection point is not constant.

The voltage normally increases with the wind speed because of the increased active power injected into the grid. However,

6045

6046

6047

6048

6049

6050

6051

6052

671 672 673 674 675 676 677 678 679 680

x-choord (km)

y-ch

oord

(km

)

A1

A9

PF

6045

6046

6047

6048

6049

6050

6051

6052

671 672 673 674 675 676 677 678 679 680

x-choord (km)

y-ch

oord

(km

)

A1

A9

PF

Fig. 1. Cable layout of the power collection grid in Nysted offshore wind farm.

0.94

0.96

0.98

1.00

1.02

1.04

1.06

0 5 10 15 20Wind speed [m/s]

Vol

tage

[p.u

.]

Fig. 2. Relation between measured 10 minute mean values of voltage and wind speed measured at wind turbine A9.

3

in some cases the reactive power consumed by the wind farm causes the voltage to decrease. The effective voltage resulting from these opposite impacts will be provided by standard load flow studies for different wind speeds.

B. Flicker

Flicker is a measure for the annoyance of a person subjected to light from a bulb lamp supplied by a fluctuating voltage. The reasons for the voltage fluctuations are multiple, including fluctuating loads and generators, transformer tap changers etc. Since the eye is mainly annoyed by relatively fast voltage changes (8.8 Hz is the frequency with the highest sensitivity), the different time scales of voltage fluctuations are weighted differently in a flicker assessment.

In IEC 61400-21, the contribution of wind turbines to voltage fluctuations is divided into continuous operation and switching operation.

The continuous operation contributes to flicker because the active power and reactive power fluctuate due to wind speed fluctuations. The wind speed variations in a fixed point are relatively slow and do not contribute very much to flicker, but the fact that the wind turbine blades are rotating in a large rotor disk with different wind speed causes the power to vary with 3 times the rotor speed [5], which is also denoted the 3p effect. Typical values of 3 times rotor speed is 0.5 – 2 Hz, depending on the size of the wind turbine.

The 3p effect is usually only relevant for fixed speed wind turbines. Variable speed wind turbines normally absorb the 3p effect of the aerodynamic torque as small changes in the rotor speed, but this is dependent on the speed control in the wind turbine.

Switching operations related to generator cut-in, cut-out and switching between generator windings also contribute to flicker. A fast change from one power level to another causes flicker, and this effect can be increased by inrush currents to energize the generators. Also for switching operation, a variable speed wind turbine can effectively reduce the flicker contribution. For switching operations, this is done by reducing the ramp rate during start-up and shut-down.

Fig. 3 shows the relation between measured 10 minute mean

values of voltage flicker and wind speed measured at wind turbine A1. Flicker is measured as Pst according to IEC 61000-4-15 [6]. There is a significant relation between wind speed and voltage flicker which shows that the main source of flicker is the wind turbines. Another observation is that the flicker level is far below Pst = 1, which is the level at which test persons are able to sense the annoyance.

C. Harmonics

Wind turbines can also contribute to the harmonic distortion of the voltage waveform.

In the first edition of IEC 61400-21, which is presently in force, only wind turbines with power converters should be tested for harmonics. However, this will probably be changed in the second edition, requiring measurements of harmonic currents for all types of wind turbines.

III. POWER SYSTEM CONTROL AND STABILITY

A. Fault ride through

As a result of the increased wind power development, focus has come to the influence of wind power on the security of the system. An important security issue is the ability of wind turbines to stay connected, or “ride through” when the voltage dips due to a fault in the system.

The main fault-ride-through concern is on the possible effect that a fault in the transmission system will have, because such a fault will influence the voltage significantly in a large area. Thus, if many wind turbines are subject to a voltage dip caused by a transmission system fault, it is important that these wind turbines are able to stay connected. Otherwise, significant generation capacity can be lost, causing power and frequency control problems in the system after the fault.

The fault-ride-through demand is also a challenge to the voltage recovery after a grid fault. Before fault-ride-through requirements became relevant to avoid loss of significant generation, the wind turbines had to disconnect to avoid large inrush currents when the voltage recovers, in order to avoid that the inrush current trips a system protection relay, e.g. in substations shared by distributed generation and consumers. Thus, the fault-ride-through requirements also raise challenges on how to recover the voltage after a dip, especially for wind turbines using directly connected induction generators.

The fault ride through requirements are quite different from one TSO to another, see e.g. Jauch et. al. [7]. It is a challenge to the wind turbine industry to meet these different requirements [8]. The TSO’s are not only requiring the fault-ride-through functionality of the wind turbines, they are also requiring models from the owners to be able to simulate the dynamic behavior of the wind turbines during faults. The model requirement obviously falls back on the wind turbine industry.

The need for such dynamic models has caused a substantial activity in the research community as well. Akhmatov

0.00

0.05

0.10

0.15

0.20

0 5 10 15 20Windspeed [m/s]

Pst

Fig. 3. Relation between measured 10 minute mean values of voltage flicker Pst and wind speed measured at wind turbine A1.

4

established a comprehensive set of models for different types of wind turbines in his thesis [9], which are often referred to. Newer research activities have cooperated in the IEA Wind Annex 21 on “Dynamic models of wind farms for power system studies” [10], where the participants have established a database with measurements and relevant turbine and grid data, and performed benchmark testing of wind turbine and wind farm simulation models.

Fig. 4 shows an example of a voltage dip from the IEA database, provided by Chalmers University in Sweden. It is measured on the low voltage terminals of a 180 kW fixed speed wind turbine. Fig. 5 shows the corresponding measured power compared to power simulated using Risøs standard model for a wind turbine with directly connected induction generator. The simulated response shows oscillations at the same frequencies as the measured, but the simulated oscillations have slightly higher amplitudes.

B. Power control

Whereas fault-ride-through requirements to wind turbines and other distributed generators have become common practice in TSO grid codes, only a few grid codes require power control features. The Danish grid code for connection to the transmission system [3], i.e. for connection of large wind farms, provide a very detailed specification of active power control requirements and slightly more general specifications on reactive power control.

The present requirements for power control of new large wind farms correspond to the controllers implemented in the two existing large offshore wind farms in Denmark. The first of these is the Horns Rev wind farm, where the power control is implemented in the wind farm main controller as described by Kristoffersen et.al. [11].

The power of the wind farms can either be controlled directly by a power set point, or indirectly by automatic frequency control. The direct power control specified in the Danish grid code comprises absolute production constraint, delta production constraint, balance regulation, stop regulation, power gradient constraint and combinations of these. Absolute production constraint, balance regulation and

stop regulation aims to keep the power constant, while the delta production constraint ensures that the actual production is below the possible production, providing a constant reserve, which is particularly useful in combination with automatic frequency control.

Power control of wind farms has also been modeled. A model for a wind farm controller integrated with the variable speed control of wind turbines is described by Hansen et.al. [12], whereas a more general model is described in [13] and demonstrated with active stall controlled wind turbines.

C. Power fluctuations

During normal operation, wind farms produce maximum available power, and because of the variability in the wind, this results in corresponding variability in the produced power. According to Akhmatov et. al. [14], Energinet.dk has found that the active power supplied from the first large 160 MW offshore wind farm in this system, Horns Rev, is characterized by more intense fluctuations in the minute range than previously observed form the dispersed wind turbines on land, even though the installed power in Horns Rev is relatively small compared to the total wind power installation in the system. A second neighboring wind farm, the 200 MW Horns Rev B is already scheduled for 2008, and Energinet.dk is concerned how this will influence the future demand for regulating power in the system.

Although the control requirements to large offshore wind farms can mitigate the power fluctuations, the mitigation is mainly effective when the wind speeds and available wind power increases rapidly. When the wind speeds and available wind power decrease, the power balance must be ensured by conventional power plants or by demand side control or storage components.

The variability influences on a large range of time scales. On the short time scale, it influences flicker as described above, and on a longer time scale, it influences the power trade hour by hour. To handle the hour-by-hour variability, TSO’s have used wind power forecast tools for many years. On a shorter time scale, wind power variability also influences the power balancing, although this influence seems to become an issue only with a significantly higher amount of wind

0.84

0.86

0.88

0.90

0.92

0.94

0.96

0.98

1.00

0.0 0.2 0.4 0.6 0.8 1.0

Time (s)

Vol

tage

(pu

)

Fig. 4. Voltage dip measured by Chalmers University in Sweden at low voltage terminals of a 180 kW fixed speed wind turbine.

-1.00

-0.50

0.00

0.50

1.00

0.0 0.2 0.4 0.6 0.8 1.0

Time (s)

Pow

er (

pu)

MeasuredSimulated

Fig. 5. Measured and simulated power response to the voltage dip in Fig. 4.

5

power in the system. Fig. 6 shows the power generation in January 18 2005 of the

wind turbines observed by the then owning power producer Elsam. It is divided into the power fluctuations from the Horns Rev wind farm and the fluctuations in the production of the “Onshore” wind turbines with a comparable installed capacity.

The selected day is characterized by very unstable weather conditions, and therefore the power fluctuations from the Horns Rev wind farm are exceptionally high. At the same time, the fluctuations in the production of the “Onshore” wind turbines are much less.

Such fluctuations have an influence on the required reserves in the system. Although the fluctuations can be mitigated by the ramp rate limitation in the power control, it is useful for planning and operation of the power system to improve the knowledge on the fluctuations.

The main reason why the fluctuations are much less onshore than in Horns Rev in Fig. 6 is that the onshore turbines are spread over a much larger area, and therefore the fluctuations of the power produced by different turbines are much less correlated onshore. For planning of the future system development involving a still increasing capacity of wind power, it is useful to be able to simulate expected wind power fluctuations, dependent on the installed capacity and on where this capacity is located. Such simulations can be used as input to cost-benefit analysis of different locations, including the impact that the choice will have on required regulating power.

A tool which can simulate the wind power fluctuations in a large wind farm is presented by Sørensen et.al. [15]. Data collected on the existing wind farm in Horns Rev has been applied to validate the ability of the model to predict at required reserves. Fig. 7 shows calculated 1 % percentiles of reserves needed to balance the wind farm power fluctuations for different power ranges. The reserves have been calculated based on measured and simulated power, respectively.

In the operation of a power system with high capacity of wind power, it would also be useful to apply prediction tools for the inter-hour fluctuations. Pinson et.al. [16] have

analyzed different types of prediction models, which are all time series models that need to be trained with real data. Pinson found that the Markov-Switching AutoRegressive (MSAR) model is the most promising type of model, because it performs best based on measured power only, and because it is open for a further development taking variables from weather models into account.

IV. CONCLUSION

The development of wind power the last 30 years in Denmark has caused a steadily increased interest in the influence the influence that wind power has on the power quality.

In the first 20 years of wind power development, the power quality focus has been on the impact on the local voltage quality. Today, 18 % of the electricity consumption in Denmark is supplied by wind power. With that level of wind power penetration, there is an increased focus on more system related issues such as power control, fault ride through and need for reserves.

REFERENCES [1] IEC 61400-21. Wind turbine generator systems – Part 21: Measurement

and assessment of power quality characteristics of grid connected wind turbines. First edition, 2001-12.

[2] Wind turbines connected to grids with voltages below 100 kV. Technical regulation for the properties and the regulation of wind turbines. Elkraft System and Eltra. English version of Regulation TF 3.2.6. Doc. No. 225056. January 2005.

[3] Wind turbines connected to grids with voltages above 100 kV. Technical regulation for the properties and the regulation of wind turbines. Elkraft System and Eltra. Regulation TF 3.2.5. Doc. No. 214439v3. Dec. 2004.

[4] EN 50160. Voltage characteristics of electricity supplied by public distribution systems. CENELEC, Nov. 1994.

[5] P. Sørensen, A. D. Hansen, P. A. C. Rosas, “Wind models for simulation of power fluctuations from wind farms,” J. Wind Eng. Ind. Aerodyn, vol 90, pp. 1381-1402, Dec. 2002.

[6] IEC 61000-4-15. Electromagnetic compatibility (EMC) – Part 4: Testing and measurement techniques – Section 15: Flickermeter – Functional and design specifications.

[7] C. Jauch, J. Matevosyan, T. Ackermann, S. Bolik “International comparison of requirements for connection of wind turbines to power systems,” Wind Energy, vol 8, pp. 295-306, 2005.

Fig. 6. Power generation of Horns Rev offshore wind farm and onshore wind turbines January 18 2005.

0 0.2 0.4 0.6 0.8 10

0.05

0.1

0.15

0.2

0.25

0.3

0.35

Power [p.u.]

Res

erve

s [p

.u./1

0min

]

measuredsimulated

Fig. 7. 1 % percentiles of 10 minutes reserve requirements

6

[8] S. Bolik, J. Birk, B. Andresen, J. G. Nielsen, ”Vestas handles grid requirements: advanced control strategy for wind turbines,”, in Proc European Wind Energy Conference (EWEC), Jun. 2003, Madrid, Spain.

[9] V. Akhmatov, “Analysis of dynamic behaviour of electric power systems with large amount of wind power,” Ph.D. thesis, Technical University of Denmark, Apr. 2003.

[10] J. O. Tande, E. Muljadi, O. Carlson, J. Pierik, A. Estanqueiro, P. Sørensen, M. O’Malley, A. Mullane, O. Anaya-Lara, B. Lemström (2004) “Dynamic models of wind farms for power system studies – status by IEA Wind R&D Annex 21”, in Proc European Wind Energy Conference (EWEC), Nov. 2004, London, UK.

[11] J. R. Kristoffersen, P. Christiansen, “Horns Rev offshore wind farm: its main controller and remote control system,” Wind Engineering, vol 27, pp 351-360, 2003.

[12] A. D. Hansen, P. Sørensen, F. Iov, F. Blaabjerg, “Centralised power control of wind farm, with doubly fed induction generators,” Renewable Energy, vol 31, pp. 935-951, Jun 2006.

[13] A. D. Hansen, P. Sørensen, F. Iov, F. Blaabjerg, “Grid support of a wind farm with active stall wind turbines and AC grid connection,” Wind Energy, vol 9, pp. 341-359, Jul 2006.

[14] V. Akhmatov, H. Abildgaard, J. Pedersen, P. B. Eriksen, “Integration of Offshore Wind Power into the Western Danish Power System,” in Proc. Copenhagen Offshore Wind 2005, CD

[15] P. Sørensen, N. A. Cutululis, A. Vigueras-Rodríguez, L. E. Jensen, J. Hjerrild, M. H. Donovan, H. Madsen, “Power Fluctuations from Large Wind Farms,” Submitted to IEEE Transactions on Power Systems, Special Section on “Power System Performance Issues Associated with Wind Energy”.

[16] P. Pinson, L. E. A. Christensen, H. Madsen, P. Sørensen, M. H. Donovan, L. Jensen, J. R. Kristoffersen, ”Regime-switching modelling of the fluctuations of offshore wind generation ,” submitted to J. Wind Eng. Ind. Aerodyn.

P. Sørensen (M’04) was born in 1958. He received M.Sc. in electrical engineering from the Technical University of Denmark in 1987. Since 1987 he has been employed at Risø National Laboratory in Roskilde, presently as a Senior Scientist. His main technical interest is integration of wind power into power systems, involving a variety of technical disciplines including power system control and stability, dynamic modeling and control of wind turbines and wind farms, and wind fluctuation statistics. He is a member of the maintenance team of IEC 61400-21 on measurement and assessment of power quality of gird connected wind turbines. N.A. Cutululis (M’06) was born in 1974. He received M. Sc. and Ph.D. in electrical engineering from the “Dunarea de Jos” University of Galati, Romania, in 1998 and 2005. Since February 2005 he has been employed at Risø National Laboratory in Roskilde, presently as a Scientist. His main interest is wind power systems dynamic modeling, simulation and control. T. Lund received his M.Sc.E.E. degree at the Technical University of Denmark in 2001. From 2001 to 2004 he worked for Siemens AG - Automation and Drives in Germany where he developed motor control for electrical trains. Since 2004 he is working on a Ph.D. project with the title Power system operation and control for integration of large scale of wind energy at Risø National Laboratory and Technical University of Denmark. A. D. Hansen received her Ph.D. in modeling and control engineering from Denmark Technical University in 1997. Since 1998 she has been employed at Risø National Laboratory in the Wind Energy Department – first as Post Doc., scientist and afterwards as senior scientist. Her working field and research interests are on the topics of dynamic modeling and control of wind turbines, as well as dynamic modeling and control of wind farms and on wind farm grid interaction. T. Sørensen was born in 1964. He received M.Sc.and Ph.D. in electrical engineering from the Technical University of Denmark in 1990, and 1995 respectively. Presently his main fields of work are electrical systems in connection with wind energy, power quality and lightning protection. He is with DONG Energy, Generation, Denmark

J. Hjerrild was born in 1971. He received M.Sc. in 1999 and Ph.D. in 2002 in electrical engineering from the Technical University of Denmark. He was employed at DEFU (The Association of Danish Energy Companies, R&D) from 2002 until 2004. Since 2004 he has been employed at Elsam (now a part of DONG Energy). His main technical interest is electrical power systems in general, involving a variety of technical disciplines including modeling of power system including wind power and power system control and stability. Furthermore he also works with designing of wind farm. M. H. Donovan was born in 1971. He received M.Sc. in Energy engineering from the Technical University of Denmark in 1998. He has been employed at SEAS from 2000-2003 and is presently employed at DONG Energy as projectleader for the SCADA system for Horns Rev. His main interest SCADA and automation systems for Windfarms. He was one of the designers of the power control systems for Nysted Offshore Windfarm. L. Christensen was born in 1968. He received M. Sc. in electrical engineering from Aalborg University in 1995. Since 2004 employed at DELTA, Wind & Energy. His main interest is measuring technique with focus on computer based equipment. H. K. Nielsen was born in 1956. She received B. Sc in electrical engineering from Ingeniørhøjskolen Odense Teknikum in 1980. Since 2004 employed at DELTA, Wind & Energy. Her mail interests are high voltage engineering, transient phenomena and power quality