Abstract—In this paper the main parameters to assess the power quality of grid embedded wind farms are presented. International standards to assess and quantify the power quality of grid connected wind turbines exist for some years now, and are here extrapolated to wind farms aggregates when possible being the correspondent methodologies identified in the document. Recently, the grid code requirements posed a novel challenge to this technologic area, particularly since they were issued with national or local objectives and without particular normalized global concerns. The form how the international standards are evolving in order to cope both with the power systems industry local requirements, but also with the global wind turbine manufacturers principles is addressed in the paper.

Index Terms— power quality, wind energy, wind turbines, voltage dip.

I. INTRODUCTION

This paper presents the existing normalized and

uniform parameters and methodologies that ensure consistency and accuracy in the assessment and presentation of power quality characteristics of grid connected wind turbines (WTs). These methodologies have been prepared to be applied by the several parties involved in the wind industry, namely: the WT manufacturer striving to comply with well-defined characteristics; the WT purchaser in specifying the equipment characteristics; the WT operator, planner or regulator who may be required to verify that stated, or required power quality characteristics are met and also determine the impact of a WT on the power system quality of service; finally it may also be useful to the planner or regulator of the electric network who needs to determine the grid connection required for a WT.

The currently existing power quality standard for wind turbines, issued by the International Electrotechnical Commission (IEC), IEC61400-21: “Measurement and assessment of power quality characteristics of grid connected wind turbines”, Ed 1, 2001 [1] defined the parameters that are characteristic of the wind turbine behavior in terms of the

Ana I. Estanqueiro is with INETI – National Institute for Engineering,

Technology and Innovation, Estrada do Paco do Lumiar, 22, Lisbon, Portugal. (Ph: 351210924773; fax: 351217127195; e-mail: ana.estanqueiro@ineti.pt).

J.O Tande is with SINTEF Energy Research, Norway J. A. Peças Lopes, is with INESC-Porto and Faculdade de Engenharia da

Universidade do Porto, Portugal.

quality of power, and also provides recommendations to carry out measurements and assess the power quality characteristics of grid connected WTs. Although the standard mainly describes measurement methods for characterizing single wind turbines, there are methodologies and models developed that enable, for well pre-defined conditions, to extrapolate the single turbine unit parameters to the typical quality characteristics of wind farms.

Recently, several Transmission System Operators (TSOs) have developed grid codes [2] for wind turbines and/or wind farms. These generally resemble requirements to wind farms that are very similar to those of any other power stations. The new requirements were challenging for the wind turbine industry, but it responded as requested by the TSOs. The largest problem seems to be the fact that the grid codes were issued to respond to national and regional grid characteristics that, by their intrinsic nature, are typically non-general and local-dependent thus prevent from a normalized standard approach [3].

II. WIND POWER QUALITY CHARACTERISTICS

When the IEC 61400-21 standard was developed as published, the assessment of the WT’s power quality was, in its essence, the assessment of the turbines voltage quality. The reason for this was that at the time of developing the standard, the wind turbines were mainly connected to the distribution grid, and the basic concern was their possible impact on the voltage quality and not on power system operation. This has changed with the development of large wind farms that may form a significant part of the power system. In consequence, today’s wind turbines are able to control the power (active and reactive) delivered both in transient and steady state, they can cope with power ramp requirements and they have ride through fault (RTF) capability. They may even contribute to the primary frequency control, but then on the cost of dissipating energy. To this, IEC 61400-21 is currently under revision to provide procedures for assessing these new wind turbine characteristics. One may state that today’s wind farms are more like conventional power plants, and in that respect quite different from the wind turbine installations from the end of the last century. Such recent technical advances allow for large global wind power penetration and also attractive for island systems.

Nevertheless, wind farm developers still face some resistance from the utilities to connect their independent power plants to the existing grid. The wind, being a spatially dispersed renewable source of energy, still induces a negative reaction

Assessment of Power Quality Characteristics of Wind Farms

A. I. Estanqueiro, Member, IEEE, J. O. Tande and J. A. Peças Lopes, , Senior Member, IEEE

1-4244-1298-6/07/$25.00 ©2007 IEEE.

on the system planners and operators, mainly due to its time-dependent non-dispatchable nature. As an example, although that is evident for most power engineers, its is probably difficult to find a reference to the fact that, in specific cases, wind power can even improve the voltage quality and benefit the service in weak rural systems. Another example being that, for some areas where wind may have a high correlation with the seasonal loads (e.g. seasonal tourism in windy areas), the local integration of this power sources - possibly together with some storage - may avoid the reinforcement of the transmission grid and clearly benefice the power system.

With the development of the IEC 61400-21 standard during the later nineties and its publication in 2001 [1] as well as the outcomes of some European funded research projects it was possible to identify both the factors and characteristics with highest influence on the power quality of wind turbines and the parameters more adapted to their quantification, to act as normalized quality indicators (Table I).

Table I - Factors with impact on the power quality of wind farms

A) wind turbine technology

- type of electrical generator - gearbox or gearless transmission - direct/controlled connection to the grid

B) grid conditions at the point of common coupling

- short circuit power and X/R ratio - interconnection voltage level and regulation - type of interconnecting transformers (e.g. LTC) - earth system (?? I suggest to delete this point) - coordination of the protections

C) wind farm design and control

- number and nominal power of the wind turbines - wind farm internal power collecting system characteristics (X/R) - possible capacity effects from the wind farm internal cabling system - added power/voltage control and regulation

D) wind flow local characteristics

- turbulence intensity - - turbine operation under wake flow - spectrum of the wind 3D components - spatial variability of the wind

This important step enabled not only to estimate the power quality of a wind turbine (and extrapolate it to a wind park), but also to apply them in the feasibility phase of a wind park and thus optimise its capacity and technical characteristics in order to avoid the degradation of the existing network quality of service.

A. Wind turbine technology The typical behaviour of a wind park based on squirrel cage induction generators (mostly doubly fed in current days), delivers a variable1 power to the grid. This power flow

1 but not intermittent as some authors refer. Intermittency implies no persistence of the power signal, which is not true for wind, being persistence methods even used for power production forecasting.

(together with the reactive power flow in either sense depending on national/regional legislation and regulations) are “wind power trade-marks” than can contribute to flicker emissions and to affect the mean voltage profile. Indeed, this can be counteracted on by installation of reactive compensation (possibly as a central unit of the wind farm). The use of doubly-fed induction generators or generators with fully rated frequency converters generally offers smaller fluctuations in the active power output, and built in reactive compensation capabilities. A drawback of using power electronic converters may be a higher harmonic distortion. B. Grid conditions Reduction of voltage quality due to the connection of wind generators may impose limits to the connection of large wind parks in a given part of the electrical network. The approach used to evaluate about the feasibility of such connection involves three steps, according to IEC Electromagnetic compatibility standards, IEC 61000-3-6 and 3-7 [4]-[5]: 1. Identification of the tolerable planning levels in the

receiving network, in terms of voltage, for power quality (harmonics and flicker);

2. Allocation of the distortion limits to the generation facilities, considering the influence of adjacent networks;

3. Evaluation of the tolerable limits for harmonic current injections and flicker for each wind park.

Limitations that may arise can be solved through network

reinforcements and by using local corrective procedures, like installation of active filters or dynamic voltage restorers.

The most relevant factor related to the grid characteristics, with influence on voltage quality, is the equivalent line impedance, normally introduced in the grid integration studies through the short circuit power and phase angle in the grid interconnection transformer or substation (Sk and ϕk), also referred in the IEC standards as point of common coupling (PCC).

Network reinforcement can be used to increase the PCC short-circuit power in order to allow for the connection of large wind parks in a given network area, however, this is not always possible due to economic reasons and therefore voltage quality issues may impose real limitations to the connection of large wind parks, but depending on the wind turbine technology.

C. Wind park design and control The wind park topology is mainly conditioned by the wind turbine micrositing in order to avoid some turbines to work under the wake of others. It is commonly accepted that the power fluctuations - whose equivalent representation in the IEC power quality standard is the flicker - produced by

several wind turbines tend to cancel by a factor of 1 / N , being N the number of the turbines in a park, electric cluster, or even region as shown by Lipman et al [6]. It is possible thus, in the initial phase of a wind park or cluster design, to decide among various configurations of a park that have different impacts of the system power quality (e.g. a small

number of large turbines vs a large number of smaller ones) for the same installed capacity, being this issue specially important in weak isolated systems (e.g. islands). Although the wind park topology is not addressed in any current standard, it impact on a wind farm dynamic behaviour, hence on this power plants quality is not negligible and could, therefore, be taken into account.

D. Wind flow local characteristics The most typical characteristics of the primary energy source driving wind turbines, i.e. the wind flow characteristics, manly its non stationary and non-stochastic characteristic, usually referred globally as “turbulence”, have been traditionally neglected as a major wind turbine power quality parameter. Being evident that its inclusion turns an already complex issue even more complex, using Figure 1 - where two voltage series with very different turbulence intensity are depicted - one easily concludes on the high influence of the atmospheric conditions (and stability) on the voltage fluctuations of a wind park busbar.

0.0 30.0 60.0 90.0 120.0Time [s]

15.0

15.3

15.5

15.8

Vol

tage

[kV

]

wind speed = 9.0 m/s, I=10%

wind speed = 9.6 m/s, I=24 %

Fig. 1 – PCC voltage for different atmospheric conditions.

Fortunately, the unstable climatic conditions characteristic of the locality where the voltage time series displayed in Fig. 1 were monitored, are unlikely to occur in many other places. They are very useful, however, to illustrate that the wind flow physical characteristics (here the turbulence) may affect the voltage (and power) quality of a wind power plant. Indeed, this also depends on the wind turbine technology, and modern wind turbines with reactive control capabilities may control the voltage quite effectively even under sever climatic conditions.

III. WIND TURBINES POWER QUALITY STANDARD

PARAMETERS

Being a time depended and highly variable source, the wind power delivered to the grid maintains, not al, but most of the primary energy characteristics: it is highly variable in time, it is difficult to predict and control (although not impossible)

and its spatial correlation is very low, what turns out to be a very positive issue in what concerns the power system operation.

As referred previously, the publication of IEC 61400-21 standard enable to define systematic parameters to characterize the quality of power (mainly voltage at that time) of grid connected wind turbines. Normalised parameters were defined and adapted the wind turbine working mode, when necessary. The main parameters identified and currently used nowadays are presented below:

A. Wind Turbine Constructive Parameters

Nominal and Reference Power; Reactive power versus active power;

B. Wind Power Fluctuations

1. Steady-state: Flicker emission

Long and Short Term emission;

2. Transient State: Wind turbine cut-in and cut-out

Voltage “Dips and drops”

C. Imbalances and Harmonics

Current Harmonics and Inter-harmonics

IV. FROM WIND TURBINE TO WIND FARM POWER QUALITY

In the paper the existing methodologies and empirical models to extrapolate the power quality of wind farms from the power quality parameters defined and developed for single grid connected wind turbines will be addressed and presented [7]-[8]. Particularly addressed will be the possibility for articulation between wind turbine power quality standards and the national or regional grid codes, in what concerns the low voltage ride through fault (LVRTF or RTF) capability represented in Fig.2 where dashed pattern represents the “non-standard” RTF area requested by the grid codes.

-1 0 1 2 3 4 5 6-0.5 0.5 1.5 2.5 3.5 4.5 5.5

Time [sec.]

0

20

40

60

80

100

120

Vol

tage

[%]

REN (Portugal)

REE (Spain)

E-ON (Germ.)

ESB (Ireland)

FERC (US)

Fig. 2 – LVRTF requirements for several grid codes.

V. SYNTHESES

The paper covers the wind power quality issue from the

initially addressed grid connected single-turbine case to the actual spreading of Transmission System Operators (TSOs) grid codes and “wind power plant” behavior requirements. The possibilities of extrapolation from standards to grids and the issue of “generalized grid codes” are addressed.

REFERENCES [1] IEC 61400-21:2001; “Wind turbine generator systems - Part 21:

Measurement and assessment of power quality characteristics of grid connected wind turbines”, IEC Standard, 2001.

[2] E-ON Netz Grid Code High and extra high voltage. Bayreuth, Aug. p-54, 2003.

[3] W. Christiansen and D. T. Johnsen, “Analysis of requirements in selected Grid Codes”. Available at http://www.frontwind.com/ (URL).

[4] IEC 61000-3-6. Electromagnetic compatibility (EMC) - Part 3: Limits - Section 6: Assessment of harmonic emission limits for the connection of distorting installations to MV, HV and EHV power systems - Basic EMC publication, IEC Standard, 1996.

[5] IEC 61000-3-7. Electromagnetic compatibility (EMC) - Part 3-7: Limits: Assessment of emission limits for the connection of fluctuating load installations to MV, HV and EHV power systems, IEC Standard, 1996.

[6] N. H. Lipman, E. A. Bossanyi, P. D. Dunn, P. J. Musgrove, G. E. Whittle, and C. Maclean; “Fluctuations in the output from wind turbine clusters”, Wind Engineering, vol. 4, nº 1, pp.1-7, 1980.

[7] Tande JO, E Muljadi, O Carlson, J Pierik, A Estanqueiro, P Sørensen, M O’Malley, A Mullane, O Anaya-Lara, B Lemstrom (2004) “Dynamic models of wind farms for power system studies – status by IEA Wind R&D Annex 21”, in Proceedings of European Wind Energy Conference (EWEC), 22-25 November 2004, London, UK.

[8] IEA: Variability of wind power and other renewables. Management options and strategies. 2005. http://www.iea.org/Textbase/ publications/ free_new_Desc.asp?PUBS_ID=1572

Ana Estanqueiro was born in Coimbra in 1963. She received her electrical engineer degree from the Technical University of Lisbon (TUL) in 1986 where she also did her M.Sc and PhD. in mechanical engineering, respectively in 1991 and 1997. She works as a research scientist at INETI, Lisbon, Portugal since 1987, being currently Director of the Wind and Ocean Energy Research Unit as well as associate professor at Universidade Lusiada. Her research interests are broad within wind energy with a focus on grid integration and dynamic behavior wind turbines benefiting from her electrical and mechanical background. Prof. Estanqueiro is currently chair of the IEA - International Energy Agency Wind Agreement and President of the PT IEP/IEC CTE 88 – Wind Turbines. John O. Tande was born in Trondheim in 1962. He received his M.Sc. in electrical engineering from the Norwegian Institute of Science and Technology in 1988. He has worked as a research scientist at Norwegian Electric Power Research Institute (1989), Risø National Laboratory (Denmark, 1990-97) and SINTEF Energy Research (1997-). Throughout his career his research has focused on electrical engineering aspects of wind power, and he has broad experience within the field including heading EU projects and working groups of IEC and IEA. J. A. Peças Lopes (M’80–SM’94) received the electrical engineering degree in 1981 and the Ph.D. degree, also in electrical engineering, in 1988, both from the University of Porto, Porto, Portugal. He received the Aggregation degree in 1996. He is an Associate Professor in the Department of Electrical Engineering, University of Porto. In 1989, he joined the staff of INESC-Porto as a Senior Researcher, and is now Co-Coordinator of the Power Systems Unit. Additionally, he has been leading several research and consultancy projects related with the integration of renewable generation and DG in the power system.