AckermannTh. (ed) Wind Power in Power Systems
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In general, variable-speed turbines (Types C and D) do not show such in-rush
currents. Figure 6.7(b) gives an example of a cut-in. Before the turbine generates power,
the rotational speed of the rotor increases, driven by the wind. If the rotational speed is
high enough the power production starts without high in-rush currents. The power
production increases to the power level that is determined by the actual wind speed. In
the example of Figure 6.7(b) the power reaches rated power as a result of the actual wind
speed, which is higher than the nominal wind speed. The smoothing behaviour of the
variable-speed turbines (Types C and D) during switching operations generally leads to
relatively low voltage changes and low flicker impression.
In wind farms, usually there are only one or a few wind turbines that start or stop at
the same time. Only on very rare occasions will all wind turbines in a wind farm stop at
the same time. For the calculation of the voltage change due to switching operations, it
is therefore sufficient to consider one turbine or a small number of turbi nes.
6.4 Assessment Concerning the Grid Connection
IEC 61400-21 includes recommendations for the assessment of the grid connection of
wind turbines and wind farms regarding power quality. The calculation methods are
based on the equations given in Table 6.3. The influence of the turbine or farm on the
voltage quality of the grid depends not only on the power quality of the turbine but also
on how stiff or weak the grid is at the point where the farm or turbine is connected
Table 6.3 Calculation methods for grid connection
Item Calculation method Limiting value
IEC 61400-21 German guideline
Steady-state
voltage change
Load flow calculation,
or d ¼
S
60
S
k
jcos (
k
þ ’)j
As for IEC 61400-21 National guidelines:
2%, ..., 10%
Flicker P
lt
¼ c(
k
, v
a
)
S
n
S
k
P
lt
¼ c
S
n
S
k
IEC 61000-3-7:
P
lt
0:25
Germany: P
lt
0:46
Harmonics Not applicable Not applicable National guideline or
IEC 61000-3-6
Switching
operations
d ¼ k
u
(
k
)
S
n
S
k
P
lt
¼ 8N
0:31
120
k
f
(
k
)
S
n
S
k
d ¼ k
i
(
k
)
S
n
S
k
d 2%,...,10%
P
lt
0:25 (IEC
61000-3-7)
Note: d ¼ steady-state voltage change as a percentage of the nominal voltage; S
60
¼ 1 minute
maximum apparent power; S
k
¼ short-circuit power;
k
¼ network impedance phase angle; ’ ¼
phase difference; P
lt
¼ long-term flicker disturbance factor; c(
k
, V
a
) ¼ flicker coefficient for
continuous operation as a function of
k
and the annual average wind speed, V
a
; c
¼ flicker
coefficient (according to FGW, 2002); S
n
¼ rated apparent power of the turbine; k
u
(
k
) ¼ voltage
change factor as a function of
k
; k
f
¼ flicker step factor; k
i
¼ grid-dependent switching current
factor; N
120
¼ maximum number of one type of switching within a 120 minute period.
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[i.e. the point of common coupling (PCC)]. Short- circuit power and grid impedance
angle are the parameters that describe the strength of the grid (an example of the
assessment of the grid connection of a wind farm is given in EC, 2001).
On the local level, the steady-state voltage change often is one of the limiting factors for a
grid connection. For an accurate assessment it is therefore important to carry out a complex
load flow calculation of the grid including the wind farm or wind turbine. The fact that
wind turbines can control reactive power with the inverter system, for instance, can be used
to control and stabilise voltage and thus to minimise voltage changes (see also Chapter 19).
The harmonic emission may sometimes pose a problem for the grid connecti on.
Germany has strict limiting values for harmonic emissions, especially for the frequency
range above 2 kHz (VDEW, 1998) but also for interharmonics of up to 2 kHz. Denmark
has strict limit ing values for interharmonic currents of up to 2.5 kHz (DEFU, 1998).
International guidelines generally include integer harmonics of up to 50th or 40th order
only, but the inverter systems of variab le-speed wind turbines mainly produce inter-
harmonics or harmonic emissions in excess of 2 kHz. Thus integer harmonic limits are
often not a problem for wind turbines. One difficulty for the assessment of the harmonic
emission is that the international or national guidelines only provide limits for harmonic
voltages. That means that the impact of harmonic current emissions of wind turbines or
wind farms on the voltage at the PCC has to be calculated. For this, the grid impedances
for frequencies above 50 Hz have to be known, because at this level grid resonances have
a large influence. This makes the calculation rather difficult. Only some national guide-
lines, such as the German or Danish guidelines, include limits of harmonic currents,
which simplifies the assessment. They are often very strict, though.
Flicker is a minor problem for the grid connection of today’s win d turbines. In the
mid-1990s, when most of the turbines were small fixed-speed turbines (Type A), flicker
sometimes was a limiting issue. Today’s wind turbines, especially variable-speed tur-
bines (Types C and D), have an improved flicker behaviour.
6.5 Conclusions
The power quality of wind turbines is described by their electrical characteristics.
International (IEC 61400-21 and MEASNET) and national guidelines set the require-
ments for power quality measurements. Power peaks, harmonic emission, reactive
power, flicker and the electrical behaviour in the case of switching operations are
measured according to these guidelines. Variable-speed wind turbines (Types C and D)
have a smoothed power output compared with fixed-speed wind turbines (Type A) and
are able to control active and reactive power fast. Variable-speed turbines have the
disadvantage of producing harmonic emission, though. On the local level, the steady-
state voltage change is often the limiting factor for a grid connection.
There are additional aspects regarding the grid connection of wind turbines and wind
farms, such as the support of the grid in the case of grid faults (voltage drops) as well as the
control of active and reactive power. These aspects are of increasing importance and are
already included in some of the national guidelines. A new measurement guideline in
Germany includes the requirement to check if the wind turbines comply with these new,
additional, aspects.
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References
[1] DEFU (Darish Utilities Research Association) (1998) Connection of Wind Turbines to Low and Medium
Voltage Networks, October 1998, Committee Report 111-E, DEFU, DK-2800 Lyngby.
[2] EC (2001) (European Commission) Wind Turbine Grid Connection and Interaction, Deutsches Wind-
energie-Institut GmbH (DEWI), Tech-wise A/S, DM Energy; available from DEWI, http://www.dewi.de.
[3] Eltra (2000) Tilslutningsbetingelser for vindmoelleparker tilsluttet transmissionsnettet, 17 April 2000, http://
www.eltra.dk/media/1030_12368.pdf.
[4] EON (2003) Netzanschlussregeln, Hoch und Ho
¨
chstspannung, E.ON Netz GmbH, http://www.eon-netz.
com/Ressources/downloads/ENE_NAR_HS_01082003.pdf or, http://www.eon-netz.com/Resources/
downloads/enenarhseng1.pdf.
[5] FGW (Fo
¨
rdergesellschaft Windenergie e.V.) (2002) Technische Richtlinie fu
¨
r Windenergieanlagen, Teil 3:
Bestimmung der Elektrischen Eigenschaften, http//www.wind-fgw.de.
[6] FGW (Fo
¨
rdergesellschaft Windenergie e.V.) (2003) Technische Richtlinie fu
¨
r Windenergieanlagen, Teil 4:
Bestimmung der Netzanschlussgro
¨
ßen, http//www.wind-fgw.de.
[7] IEC (International Electrotechnical Commission) (1996) EMC, Part 3: Limits. Section 7: Assessment of
Emission Limits for Fluctuating Loads in MV and HV Power Systems, IEC 61000-3-7 (10.1996), Basic
EMC publication (technical report), http://www.iec.ch.
[8] IEC (International Electrotechnical Commission) (2002) Electromagnetic Compatibility EMC, Parts 4–7:
Testing and Measurement Techniques: General Guide on Harmonics and Interharmonics Measurements and
Instrumentation, for Power Supply Systems and Equipment Connected there to IEC 61000-3-6: (10.1996),
EMC. Part 3: Limits. Section 6: Assessment of Emission Limits for Distorting Loads in MV and HV Power
Systems, IEC 61000-4-7 Ed.2.0 (8.2002), basic EMC publication (technical report), http://www.iec.ch.
[9] IEC (International Electrotechnical Commission) (2003) Electromagnetic Compatibility (EMC), Part 4:
Testing and Measuring Techniques. Section 15: Flickermeter – Functional and Design Specifications, IEC
61000-4-15 (02.2003), http://www.iec.ch.
[10] IEC (International Electrotechnical Commission) (2001) Wind Turbine Generator Systems, Part 21:
Measurement and Assessment of Power Quality Characteristics of Grid Connected Wind Turbines, IEC
61400-21 (12.2001), 1st edn, http://www.iec.ch.
[11] MEASNET (2000) Power Quality Measurement Procedure of Wind Turbines: Measuring Network of Wind
Energy Institutes (MEASNET), Version 2, November 2000; copy available from Deutsches Windenergie
Institut GmbH, http://www.dewi.de.
[12] NGC (National Grid Company plc) (2004) The Grid Code, http://www.nationalgridinfo.co.uk/grid_code/
mn_current.html.
[13] Santjer, F. (2000) Power Quality of Wind Turbines in Germany and in Denmark/Sweden, Deutsches
Windenergie-Institut GmbH (DEWI), DEWI-Magazine number 16, www.dewi.de.
[14] Santjer, F. (2003) Influence of Transformers on Harmonics, Fritz Santjer, Rainer Klosse, Deutsches
Windenergie-Institut GmbH (DEWI), EWEC 2003, Madrid.
[15] VDEW (Verband des Elektrizita
¨
tswirtschaft e.V.) (1998) Eigenerzeugungsanlagen am Mittelspannungs-
netz. Richtlinie fu
¨
r Anschluss und Parallelbetrieb von Eigenerzeugungsanlagen am Mittelspannungsnetz, 2nd
edn, VWEW-Verlag, Frankfurt.
[16] VDN (Verband der Netzbetreiber e.v.) (2004) EEG-Erzeugungsanlagen am Hoch-und Ho
¨
chstspannungs-
netz, http://www.vdn-berlin.de/global/downloads/Publikationen/Fachberichte/RL_EEG_HH_2004-08.pdf.
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7
Technical Regulations for the
Interconnection of Wind Farms
to the Power System
Julija Matevosyan, Thomas Ackermann and Sigrid M. Bolik
7.1 Introduction
This chapter provides a brief discussion and analysis of the current status of intercon-
nection regulations for wind power in Europe. The chapter starts with a short overview
of the relevant technical regulation issues, which includes a brief description of the
relevant interconnection regulations considered in this chapter. This is followed by a
detailed comparison of the different interconnection regulations. The discussion also
includes the capabilities of wind turbines to comply with these requirements. In add-
ition, a new wind farm control systems is briefly explained, which was developed
particularly to comply with network interconnection requiremen ts. Finally, issues
related to international interconnection practice are briefly discussed.
7.2 Overview of Technical Regulations
Technical standards that are adopted by the industry often originate from standards
developed by the Institute of Electrical and Electronic Engineers (IEEE) or from the
International Electrotechnical Commission (IEC). These standar ds are, however, volun-
tary unless a specific organisation or legislative ruling requires the adoption of these
standards. Hence, there is a large number of additional national or regional standards,
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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requirements, guidelines, recommendations or instructions for the interconnection for
wind turbines or wind farms worldwide.
At the end of the 1980s, distribution network companies in Europe had to deal
with small wind turbines and wind farms that wanted to be connected to the
distribution network. At this time, the IEEE Standard 1001 (IEEE, 1988) IEEE
Guide for Interfacing Dispersed Storage and Generation Facilities with Electric Utility
Systems was the only IEEE guide in place that partly covered the connection of
generation facilities to distribution networks. The standard included the basic issues
of power quality, equipment protection and safety. The standard expired and, there-
fore, in 1998, the IEEE Working Group SCC21 P1547 started to work on a general
recommendation for the interconnection of distributed generation, the IEEE Standard
for Interconnecting Distributed Resources with Electric Power Systems (IEEE, 2003).
Four years later, in September 2002, the working group finally agreed on a new
standard.
Back in the late 1980s, however, distribution network companie s in Europe started
to develop their own interconnection rules or standards. In the beginning, each
network company that faced an increasing amount of interconnection requests for
wind farms developed its own rules. During the 1990s, these interconnection rules
where harmonised on a national level (e.g. in Germany or Denmark). This harmon-
isation process often involved national network associations as well as national wind
energy associations, which represented the interest s of wind farm developers and
owners. In Europe, government organisations are hardly involved in the definition
of interconnection guidelines. In other regions of the world, this can be different. In
Texas and California, for instance, electricity regulation authorities were involved in
defining the interconnection guidelines for distributed generation (DG) [see for
instance the Interconnection Guidelines for Distributed Generation in Texas (PUCT,
1999) and the Distributed Generation Interconnection Rules (California Energy Com-
mission, 2000) in California].
In Europe, the harmonisation of national interconnection rules was not the end of the
development of such rules. National interconnection rules were continuously reformu-
lated because of the increasing wind power penetration and the rapid development of
wind turbine technology (i.e. wind turbine ratings increased rapidly, from around
200 kW in the early 1990s to 3–4 MW turbines in early 2004). In addition, wind energy
technology introduced new technologies such as doubly fed indu ction generators
(DFIGs). Until then, generation technologies that used DFIGs with a rating of up to
3 MW and combined a large number of these within one power station (i.e. wind farms)
were unheard of in the power industry.
Not only was the increased size of the wind turbines new but too was the increasing
size of the wind farms, which resulted in interconnection requests at the transmission
level. Hence, interconnection rules for wind farms to be connected to the transmission
level were required.
Unfortunately, the continuously changing network rules and the re-regulation of the
power market make a comparison or evaluation of the already very complex intercon-
nection rules very difficult and there is only very limited literature in this area (see Bolik
et al., 2003; CIGRE, 1998; Jauch, Sørensen and Bak-Jensen, 2004).
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A comparison might be useful because:
.
Interconnection rules are often the source of controversies between wind farm developers
and network operators (see for instance Jo
¨
rß, 2002; Jo
¨
rß et al., 2003), therefore a compari-
son and analysis would contribute to reduce such controversies (in some countries,
governments have decided to solve such controversies by setting up clearing committees
that help solve controversies between network companies and wind farm developers).
.
It would provide a better unde rstanding of the relevant issues for those countries,
regions or utilities that are still in the process of developing interconnection rules for
wind farms. This would help to harmonise interconnectio n rules worldwide.
.
To comply with new connection requirements is a challenge for wind turbine manu-
facturers. New hardware and control strategies have to be developed. The comparison
of connection requirements in different countries will give wind turbine manufac-
turers an overview of the existing rules.
.
An understanding of the difference between the national rules will con tribute to a
harmonisation of interconnection rules in Europe and even beyond.
The following sections therefore provide a brief comparison and evaluation of the most
important interconnection rules that are relevant for wind farms. Owing to the complex-
ity of the regulations, only a few specific areas of the interconnection rules are compared.
In the remainder of this section will briefly introduce the regulations that are to be
compared in Section 7.3. For that purpose, we will divide the interconnection rules
according to their area of application:
.
Wind farms or wind turbines connected to networks with voltage levels below 110 kV;
.
Wind farms or wind turbines connected to networks with voltage levels above 110 kV,
(for E.ON, voltage levels above 60 kV);
.
All wind farms or wind turbines, irrespective of voltage level.
Additionally, it is important to mention that national, regional or utility interconnection
rules in Europe often include references to other standards (e.g. power quality stand-
ards). These standar ds will not be analysed in this chapter, but the most common
additional standards that interconne ction rules refer to are:
.
IEC 61400–21 (IEC, 2001): Measurements and Assessment of Power Quality Charac-
teristics of Grid Connected Wind Turbines (see also Chapters 5 and 6);
.
IEEE 519 (IEC, 1992), IEEE Recommended Practices and Requirements for Harmonic
Control in Electric Power Systems, which applies to all static power converters used in
the industry, stating voltage and current harmonic limits as well as voltage limits of
irritation curves for utility practice and DG requirements.
7.2.1 Regulations for networks below 110 kV
In this section we provide a brief introduction to interconnection guidelines, recommen-
dations and requirements relev ant to distribution networks [low-voltage (LV) and
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medium-voltage (MV) level] that are discussed in this chapter. The relevant guidelines
are: DEFU 111, developed in Denmark; AMP, developed in Sweden; as well as the
German VDEW guidelines. In addition, we will also present new interconnection
requirements for Denmark that are still unde r discus sion at the time of writing. Finally,
the engineering recommendations that are currently applied in the UK are considered.
The guidelines, recommendations and requirements are directed towards distribution
network companies, wind turbine manufacturers and network operators as well as
others who are interested in connecting wind farm s to the LV or MV netw ork. The
objective of the technical standards is to establish guidelines, recommendations and
requirements for wind turbines and networks in compliance with applicable standards
for voltage quality and reliability of supply. The guidelines and requirements should be
independent of the design approach used for the wind turbine and open enough to apply
to synchronous or induction generators wi th or without inverters, for instance. They
deal with the technical data needed to assess the impact of wind turbines on power
quality and discuss the requirements to be met by networks to which wind turbines are
to be connected (voltage quality at the customer side).
7.2.1.1 Denmark: DEFU 111
The Danish report, ‘Connection of Wind Turbines to Low and Medium Voltage Net-
works’, or DEFU 111, was developed the by Research Institute of Danish Electric
Utilities (DEFU, 1998) and works as an interconnecti on guideline in Denmark. The
guidelines do not include network stability problems that can occur in regional or
transmission networks with high penetration levels of wind turbines or wind farms or
in the case of a low ratio between short-circuit power at the connection point and total
installed wind power. By the time of writing, DEFU 111 was under review and a first
draft of an updated version (DEFU, 2003) was under discussion, see Section 7.2.1.2.
Therefore, the reviewed guidelines are likely to have been implemented by the time this
book is published.
(1)
7.2.1.2 Denmark: Eltra and Elkraft
The new requirements for distribution networks, which are likely to replace DEFU 111,
have been designed by the two Danish transmission system operators (TSOs), Eltra
(Western Denmark) and Elkraft System (Eastern Denmark). The new requirements are
titled ‘Wind Turbine Generators Connected to Networks with Voltage Levels below
100 kV (Draft)’ (DEFU, 2003). The requirements apply, after 1 July 2004, to wind farms
that are connected to networks with voltage levels lower than 100 kV. These req uire-
ments have the aim of ensuring that wind turbines have regulating and dynamic proper-
ties that are essential for maintaining a reliable power supply and voltage quality in the
short and long term.
(1)
Update: DEFU (2003) – now Eltra and Elkraft regulations – was approved 19 May 2004 and is now in force;
see http://www.eltra.dk/media (15716, 1030)/Forskrift_for_vindm%F811er_maj_2004.pdf.
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7.2.1.3 Sweden: AMP
The Swedish counterpart to DEFU 111 is known as AMP, which is an abbreviation for the
Swedish title of a report that translates as ‘Connection of Smaller Power Plants to Electrical
Networks’ (Svensk Energi, 2001). AMP follows in many parts DEFU 111. The guidelines
do not include network stability problems that can occur in regional or transmission
networks at high penetration levels of wind turbines or wind farms or in the case of a low
ratio between the short-circuit power at the connection point and total installed wind power.
7.2.1.4 Germany: VDEW guidelines
The German guidelines, ‘Generation in the Medium Voltage Network – Guidelines for
the Connection and Operation of Generation Units in the Medium Voltage Network’,
were developed and published by the German Electricity Association, the Verband der
Elektrizita
¨
tswirtscheft (VDEW, 1998).
The guideline requires power quality measurements for each wind turbine type, outlined in
a publication from the Fo
¨
rdergesellschaft Windenergie (FGW, 2002). The detailed require-
ments for power quality measurements were first included in 1992 in order to develop
measurement strategies for comparing wind turbines. The measurement guidelines are con-
tinuously revised by working groups consisting of wind turbine manufacturers, network
operators and measurement institutes. In addition to the already published material, the
groups currently work on the development of wind turbine models for researching grid
connection requirements, and grid stability, among other things. For a more detailed discus-
sion of requirements regarding power quality measurements in Germany, see Chapter 6.
The VDEW guideline doe s not include network stability problems that can occur in
regional or transmission networks at high penetration levels of wind turbines and wind
farms or in the case of a low ratio between short-circuit power at the connection point
and total installed wind power.
7.2.1.5 United Kingdom: G59/1 and G75
In the UK, Engineering Recommendation G59/1 (ER G59 1990; see Electricity Asso-
ciation, 1990) is the main document covering the connection of DG, including wind
power, for generators of up to 5 MW at 20 kV and below. The recommendation is
supported by Engineering Technical Report 113 (ETR 113; see Electricity Association,
1995). The technical document provides guidance on the methods of meeting the
requirements of G59/1. In addition, Engineering Recommendation G75 (ER G75
1996; see Electricity Association, 1996) covers the connection of generators above
5 MW at 20 kV voltage; see also ER G75 1996. It is important to mention that at the
time of writing a change of the relev ant regulations was under discus sion.
7.2.2 Regulations for networks above 110 kV
In this section we provide a brief introduction to the interconnection requirements that
apply to transmission networks. The relevant requirements were defined by the TSO in
Western Denmark, Eltra, by E.ON Netz, one of five German TSOs, and by the
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Electricity Supply Board National Grid (ESBNG) in Ireland.
(2)
Among others, most of
these documents include requirements on fault tolerance, wind farm modelling and
communication as well as possibilities of external wind farm control in order to ensure
secure system operation during and after faults.
7.2.2.1 Denmark: Eltra
Eltra’s ‘Specifications for Connecting Wind Farms to the Transmission Network’ (Eltra,
2000) apply to wind farms that are connected to network voltage levels above 100 kV.
These minimum requirements are set by the TSO, and the owners of wind farms have to
comply with them in order to ensure proper operation of the power system regarding
security of supply, reliability and power quality, in the short as well as in the long term
(see also Chapter 8).
7.2.2.2 Germany: E.ON
The German TSO, E.ON Netz, has continuously updated its interconnection rules over
the past years (see E.ON Netz, 2001a; E.ON Netz, 2001b; Santjer and Klosse, 2003). In
this chapter, the ‘Grid Code for High and Extra High Voltage’ (E.ON Netz, 2003) from
August 2003 is considered. It applies to wind farms connected to high-voltage (HV:
60 kV, 110 kV) and extra-high-voltage (EHV: 220 kV, 380 kV) networks. Similar to
Eltra, the E.ON Netz rules aim at ensuring proper operation of the power system
regarding security of supply, reliability and power quality, in the short as wel l as in
the long term (for a discussion, see Chapter 11).
7.2.2.3 Ireland: ESB
The Irish Electricity Supply Board (ESB) is the utility company and consists of different
business units. In February 2002, ESB NG elaborated a draft proposal of wind farm
connection requirements (ESBNG, 2002a). This is mainly a clarification of how the
existing grid code (ESBNG, 2002b) should be interpreted for the connection and operation
of wind farms. Some requirements are also specially adapted to wind farms. A completed
version of a ‘grid code for wind farms’ is expected to be available in mid-2004.
(3)
7.2.3 Combined regulations
In this section we provide a brief introduction to the interconnection regulations that
are relevant to wind farms or wind turbines, irrespective of the connection voltage level:
the Swedish interconnection requirement of the TSO Svenska Kraftna
¨
t (SvK), the
(2)
EirGrid Plc. will take over the operation of Ireland’s electricity transmission system from ESBNG in the
near future; see http://www.eirgrid.com/eirgridportal/Home Flash.aspx.
(3)
Update: The Commission for Energy Regulation approved the proposed Wind Grid Code on 1 July 2004;
see http://www.eirgrid.com/EirGridPortal/DesktopDefault.aspx?tabid¼Wind&TreeLinkModID¼1445&Tree
LinkItemID¼42.
120 Technical Regulations