AckermannTh. (ed) Wind Power in Power Systems
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Norwegian guideline from Sintef and the Scottish draft guideline proposed by the
Scottish TSOs. These documents refer to wind farms of a certain installed capacity
and, similar to the regulations discussed in Section 7.2.2, address the impact of wind
farms on power system stability.
7.2.3.1 Sweden: Svenska Kraftna
¨
t
The ‘Technical Instructions for the Design of New Production Installations to Ensure
Secure Operation’ (SvK, 2002), developed by the Swedish TSO Svenska Kraftna
¨
t (SvK),
intends to establ ish prerequisites for secure operation of the power system. The require-
ments include: disturbance tolerance; voltage regulation; power regulation; shutdown
and startup after exterior voltage loss; communication and controllability; tests and
documentation. The requirements refer to different types of production installations,
including wind turbines with a nominal power larger than 0.3 MW and even wind farms
exceeding 100 MW. At the time of writing a change in regulations was under discussion.
7.2.3.2 Norway: Sintef
The Norwegian research institute Sintef has developed the ‘Guidelines for the Connec-
tion of Wind Turbines to the Network’ (Sintef, 2001). The report refers to wind turbines
and wind farms that are to be connected to the distribution, regional or centra l network.
Connection to island networks and/or LV distribution networks is not included in the
report. The guidelines refer to the following topics: thermal limits, losses, slow voltage
variations, flicker, voltage dips and harmonics. The report also discusses the protection
of wind turbines – that is over voltage and undervoltage protection, short-circuit
protection, protection against undesirable island operation and transient overvoltages.
The guidelines are based on the recommendations in IEC 61400–21 (IEC, 2001), DEFU
111 (DEFU, 1998) and AMP (Svensk Energi, 2001).
7.2.3.3 Scotland: Scottish Power Transmission and Distribution and Scottish
Hydro-Electric
The Scottish ‘Guidance Note for the Connection of Large Wind Farms’ (Scottish
Hydro Electric, 2002) applies to all wind farms with a registered capacity of 5 MW
and above. The guidance note refers to the Scottish grid code and deals mainly with
control issues of a wind farm in order to sustain stable operation of the network. At the
time of writing, the general regulations for Scotland, England and Wales were under
discussion.
7.3 Comparison of Technical Interconnection Regulations
In the following, the interconnection regulations introduced in Section 7.2 will be
compared. The comparison is divided as follows: active power control, frequency
control, voltage control, tap changer, wind farm protection, modelling and communica-
tion requirements.
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7.3.1 Active power control
In general, power production and consumption have to be in balance within a power
system. Changes in power supply or demand can lead to a temporary imbalance in the
system and affect operating conditions of power plants as well as affecting consumers.
In order to avoid long-term unba lanced conditions the power demand is predicted
and power plants adjust their power production. The requirements regarding active
power control of wind farms aim to ensure a stable frequency in the system (see also
Section 7.3.2), to prevent overloading of transmission lines, to ensure compliance with
power quality standards and to avoid large voltage steps and in-rush currents during
startup and shutdown of wind turbines.
It should be noticed that the Scottish guidance note and ESBNG include require-
ments regarding maximum active power change during startup, shutdown and wind
speed change in order to avoid impacts on system frequency (see Table 7.1). Eltra, Eltra
and Elkraft and SvK state requirements regarding active power chan ge in order to
ensure suff iciently fast down regulation in case of necessity (e.g. overfrequencies). E.ON
regulations define both maximum permissible active power change and minimum
required active power reduction capability.
Table 7.1 Power control requirements
Requirement Source
Active power:
1 min average production limit þ5 % of maximum power of wind
farm
a
Eltra
1 min average ¼5 % of rated power of the wind turbine from
conditional set point (0–100 % of maximum power of wind farm)
Eltra and Elkraft
10 min average k registered capacity
at any time registered capacity
DEFU 111, AMP,
E.ON, ESBNG,
VDEW
Active power change:
Reduction to <20 % of maximum power (by individual control of each
wind turbine) when demanded: in 2 s (Eltra); in 5 sec (SvK)
Eltra, SvK
Power change from any operating point to a set point defined by E.ON E.ON
Power reduction of a minimum of 10 % of registered capacity per minute
Power increase 10 % of registered capacity per minute
Adjustable in the range of 10–100 % of rated power per minute Eltra and Elkraft
In any 15-minute period, active power change is limited to: ESBNG
5 % rated power of wind farm per min (P
WF
< 100 MW)
4 % rated power of wind farm per min (P
WF
< 200 MW)
2 % rated power of wind farm per min (P
WF
> 200 MW)
Specific reduction must be possible; reduction order comes from system
operator
SvK
Active power change is limited to: Scotland
60 MW per hour, 10 MW over 10 min, 3 MW over 1 min (for
P
WF
< 15 M W);
4 registerd capacity per hour, registered capacity/1.5 over 10 min,
registered capacity/5 over 1 min (for 15 MW < P
WF
< 150 MW);
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7.3.2 Frequency control
In the power system, the frequency is an indicator of the balance or imbalance between
production and consumption. For normal power system operation, the frequency
should be close to its nominal value. In European countries, the frequency usually lies
between 50 0:1 Hz and very seldom outside the range of 49–50.3 Hz.
In the case of an imbalance between production and consumption, primary and
secondary control is used to return to a balanced system. If, for instance, consumption
is larger than production, the rotational energy stored in large synchronous machines is
utilised to keep the balance between production and consumption and, as a result, the
rotational speed of the generators decrease. This results in a decrease of the system
frequency. In a power system, there are some units that have frequency-sensitive equip-
ment. These units are called primary control units. The primary control units will
increase their generation until the balance between production and consumption is
restored and frequency has stabilised. The time span for this control is 1–30 s.
In order to restore the frequency to its nominal value and release used primary
reserves, the secondary control is employed with a time span of 10–15 min. The second-
ary control thus results in a slower increase or decrease of generation. In some countries,
Table 7.1 (continued)
Requirement Source
600 MW per hour, 100 MW over 10 min, 30 MW over 1 min
(P
WF
> 150 MW)
(may be exceeded at f 6¼ 50 Hz if farm provides frequency control)
Startup:
Wind farm shall contain a signal clarifying the cause of preceding wind
farm shutdown. This signal should be a part of the logic managing
startup of wind turbines for operation
Eltra
Has to comply with requirements regarding active power change Scotland, E.ON
Has to comply with requirements regarding voltage quality DEFU 111, AMP,
Sintef, VDEW,
Scotland, Eltra, E.ON
Shutdown:
High wind speed must not cause simultaneous stop of all wind turbines Eltra, SvK
No more than 2 % of registered capacity may be tripped. Phased
reduction of output over 30 min period
Scotland
Has to comply with requirements regarding active power change Scotland
Has to comply with requirement regarding voltage quality DEFU 111, AMP,
VDEW
a
The production limit is an external signal deduced from the local values of, for example,
frequency and/or voltage.
Note: P
WF
¼ rated power of wind farm.
Sources: on DEFU 111, see Subsection 7.2.1.1; on Eltra and Elkraft, see Subsection 7.2.1.2; on
AMP, see Subsection 7.2.1.3; on VDEW, see Subsection 7.2.1.4; on G59/1 and G75, see
Subsection 7.2.1.5 (not mentioned in this table); on Eltra, see Subsection 7.2.2.1; on E.ON, see
Subsection 7.2.2.2; on ESBNG, see Subsection 7.2.2.3; on SvK, see Subsection 7.2.3.1; on Sintef,
see Subsection 7.2.3.2; on Scotland, see Subsection 7.2.3.3.
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automatic generation control is used; in other countries the secondary control is accom-
plished manually by request from the system operator.
At normal operation, the power output of a wind farm can vary up to 15 % of
installed capacity within 15 minutes. This could lead to additional imbalances between
production and consumption in the system. Considerably larger variations of power
production may occur during and after extreme wind conditions.
As wind turbines use other generation technologies than conventional power plants, they
have a limited capability of participating in primary frequency control in the same way
conventional generators do. However ESBNG, for instance, requires wind farms to include
primary frequency control capabilities of 3–5 % (as required for thermal power plants) into
the control of wind farm power output. ESBNG and some other regulations also require
wind farms to be able to participate in secondary frequency control. During overfrequen-
cies, this can be achieved by shutting down of some turbines in the wind farm or by pitch
control. Since wind cannot be controlled, power production at normal frequency would
be intentionally kept lower than possible in order for the wind farm to be able to provide
secondary control at underfrequencies (see also Section 7.4). Figures 7.1 and 7.2 illustrate
the requirements regarding frequency control of wind turbines in different countries. In
the case of large frequency transients after system faults, Eltra’s regulation requires the
wind farm to contribute to frequency control (i.e. secondary control).
7.3.3 Voltage control
Utility and customer equipment is designed to operate at a certain voltage rating.
Voltage regulators and the control of reactive power at the generators and consumption
connection points is used in order to keep the voltage within the required limits and
avoid voltage stability problems (see also Chapter 20). Wind turbines also have to
contribute to voltage regulation in the system; the requirements either refer to a certain
voltage range that has to be maintained at the point of connection of a wind turbine or
wind farm, or to a certain reactive power compensation that has to be provided.
7.3.3.1 Reactive power compensation
Required reactive power compensation is defined in terms of power factor range. Figure
7.3 shows the requirements regarding reactive power compensation in distribution net-
works, and Figure 7.4 shows these requirements for transmission networks.
VDEW guidelines recommend a power factor of 1 but leave the final decision to the
network company that handles the network integration.
In the Swedish regulations (SvK) , the demand for reactive power compensation is
expressed in terms of a permissible voltage range. According to these regulations, large
( > 100 MW) and medium-size (20–50 MW) wind farms have to be able to maintain
automatic regulation of reactive power, with voltage as the reference value. The refer-
ence value has to be adjustable within at least 10 % of nominal operating voltage. The
same requirement is stated in the Sintef regulations regarding all wind farms connected
to the voltage level > 35 kV.
DEFU, VDEW and AMP also define the maximum permissible voltage increase from
a wind turbine at the point of common connection (PCC), which is 1 % for DEFU,
2 % for VDEW and 2.5 % for AMP.
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7.3.3.2 Voltage quality
Voltage quality assessment of the wind farm is based on the following concepts:
.
rapid voltage changes: single rapid change of voltage root mean square (RMS) value,
where voltage change is of certain duration (such as during switching operations in
the wind farm);
.
voltage flicker: low-frequency voltage disturbances;
.
harmonics: periodic voltage or current disturbances with frequencies n 50 Hz, where
n is an integer.
Disconnection
after 0.3 s
106 %
Not mentioned Not mentioned Disconnection
within maximum
of 1 s
Not mentioned Fast automatic
disconnection
Disconnection
110 % 110 %
10 min at voltage
85–110 %
or
>30 min
reduced power
voltage 95–105 %
>30 min
reduced power
voltage 95–105 %
104 % 104 %
Power reduction
2 % per 0.1 Hz
60 min
103 %
Power reduction
Voltage 95–105 %
102 %
>60 min
<10 % power
reduction
at voltage 85–90 %
or 105–110 %;
102 %
Continuous at
voltage 95–105 %
100.8 %
Power reduction
4 % per 0.1 Hz
106 %
>1min
100.6 %
Continuous
98 %
>25 min
96 %
95 %
94 %
>10 s
>5 min
Disconnection
after 0.3 s
Not mentioned Not mentioned Disconnection
within maximum
of 1s
Not mentioned Fast automatic
disconnection
Disconnection
94 %
95 %
95 %
95 %
94 %
20 s
20 s
94 %
95 %
95 %
Continuous
99 % 99 % 99 %
60 min
Continuous at
voltage 95–105 %
99.4 %
Continuous at
voltage 95–105 %
98 %
10 min at voltage
85–110 %
or >30 min
<5 % power
reduction at
voltage 95–105 %
98 %
>30 min
<5 % power
reduction
Voltage 95–105 %
101 % 101 % 101 %
Continuous Continuous Continuous
Time limited
Time limited
1 % per 0.1 Hz
10 min
20 min
30 min
Power increase
Eltra SvK (> 20 MW) SvK (< 20 MW) Scotland ESBNG E.ON Vestas
53.0
52.5
52.0
51.5
51.0
50.5
50.0
49.5
49.0
48.5
48.0
47.5
47.0
46.5
46.0
45.5
45.0
44.5
power reduction
Frequency (Hz)
Source
Figure 7.1 Overview of frequency control requirements: Eltra, SvK, Scotland, ESBNG, E.ON
and Vestas
Sources: see Table 7.1.
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53.0
52.5
52.0
51.5
51.0
50.5
50.0
49.5
49.0
48.5
48.0
47.5
47.0
46.5
46.0
45.5
45.0
44.5
VDEW DEFU 111 Eltra and Elkraft
Disconnection
after 0.5 s
Disconnection
after 0.2 s
Disconnection
after 0.2 s
Disconnection
after 0.2 s
Disconnection
after 0.5 s
Disconnection
after 0.2 s
Not mentioned
Not mentioned
96 %
98%
94 %
94 %
96 %
95 %
102 %
102 %
102 %
102 %
100.6 %
Continuous
Continuous
Continuous Continuous
Capacitor banks
(if any) disconnect
after 0.1 s
106 %
Voltage 106–110 %,
1 h
Voltage 85–90 %,
1 h
99.4 %
98 %
> 25 min
> 5 min
>10 s
>1 min
AMP
Source
Frequency (Hz)
Figure 7.2 Overview of frequency control requirements: AMP, VDEW, DEFU 111, and Eltra
and Elkraft
Sources: see Table 7.1.
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–0.6
–0.4
–0.2
0
0.2
0.4
0.6
0 0.2 0.4 0.6 0.8 1.0 1.2
Active power (p.u.)
Reactive power (p.u.)
Generating reactive power
Absorbing reactive power
0.89
0.89
0.9
AMP DEFU 111
Figure 7.3 Requirements regarding reactive power (per unit of rated active power)
Sources: see Table 7.1.
–0.8
–0.6
–0.4
–0.2
0.0
0.2
0.4
0.6
0 0.2 0.4 0.6 0.8 1.0 1.2
Active power (p.u.)
Reactive power (p.u.)
Absorbing reactive power
Generating reactive power
0.850
0.900
0.950
0.980
0.960
0.950
0.925
0.900
0.940
0.940
Eltra, Eltra and
Elkraft, SvK
Scotland, after
January 2003
Scotland, before
January 2003
Scotland,
after 2007
E.ON
ESBNG Vestas
Figure 7.4 Requirements regarding reactive power and technical capability of a Vestas V80
2 MW turbine (per unit of rated active power)
Sources: see Table 7.1.
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Voltage variations and harmonics can damage or shorten the lifetime of the utility and
customer equipment. Voltage flicker causes visible variations of light intensity in bulb
lamps. The requirements concerning voltage quality are listed in Table 7.2.
7.3.4 Tap changers
Tap-changing transformers are used to maintain predetermined voltage levels. This is
achieved by alternating the transformer-winding ratio. The E.ON and Scottish Power
Scottish Hydro Electric regulations include special requirements regarding the tap-changers.
The E.ON regulations recommend equipping the wind farm with a tap-changing grid
transformer in order to be able to vary the transformer ratio.
Scottish Power and Scottish Hydro Electric states that wind farms of 100 MW and
above have to have manual -control tap-changing transformers to allow the grid control
to dispatch the desired reactive power output. Wind farms between 5 MW and 100 MW
may use this method if they have their own transformer, or may use other methods of
controlling reactive power agreed with Scottish Power at the application stage .
ESBNG requires that every wind farm that is connected to the network have an on-
load tap-changer. The tap step should not alter the voltage ratio at the HV terminals by
more than:
.
2.5 % on a 110 kV system;
.
1.6 % on 220 kV to 400 kV systems.
7.3.5 Wind farm protection
The dynamic behaviour of win d turbines during and after different disturbances and the
transient stability of the power system is discussed in detail in Part D of this book.
Recommendations for the connection of wind farms to distribution networks usually
include the disconnection of wind farms in the case of a fault in the network (DEFU
111, AMP). However, this does not apply to large wind farms. If the fault occurs in the
system, the immediate disconnection of large wind farms would put additional stress on
the already troubled system.
After severe disturbances, it may happen that several transmission lines are discon-
nected and part of the network may be isolated (or ‘islanded’) and there may be an
imbalance between production and consumption in this part of the network. As a rule,
wind farms are not required to disconnect, as long as certain voltage and frequency
limits are not exceeded (see also Section 7.4). Danish regula tions additionally require
wind farms to take part in frequency control (secondary co ntrol) in island conditions.
High short-circuit currents, undervolt ages and overvoltages during and afte r the fault
can also damage wind turbines and associated equipment. The relay protection system
of the farm should therefore be designed to pursue two goals:
.
to comply with requirements for normal network operation and support the network
during and after the fault;
.
to secure wind farms against damage from impacts originating from faults in the
network.
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Table 7.2 Overview of voltage quality
Requirement Source
Rapid voltage changes (as a percentage of nominal
voltage at the connection point):
General limit 2 % VDEW
General limit < 3 % Eltra, Scotland, Eltra and Elkraft
(50–60 kV)
General limit < 4 % Sintef, Eltra and Elkraft
(10–20 kV)
Until a frequency of 10 per hour, < 2:5 % Eltra
Until a frequency 100 per hour, < 1:5 % Eltra
k
f
(
k
) 0:04R
k
or k
i
0:04R
k
DEFU 111
Voltage flicker:
P
st
< 0:30 Eltra
P
st
< 0:7 Sintef
P
st
< 0:35 AMP
P
lt
< 0:25 Eltra, DEFU 111, AMP
P
lt
0:46 VDEW
P
lt
0:5 (at main substation, if customers are also
connected there)
DEFU 111
P
lt
0:5 Eltra and Elkraft (10–20 kV), Sintef
P
lt
0:35 Eltra and Elkraft (50–60 kV)
k
f
(
k
) < 0:031R
k
/N
1/3:2
or k
i
< 0:031R
k
/N
1/3:2
(during
switchings)
DEFU 111
C
c
(
k
) < 0:25R
k
(during normal operation)
k
f
(
k
) < P
lt
/(8N
1/3
)S
k
/(S
park
S
2
rG
)
1/3
(during switchings) Eltra and Elkraft
C
c
(
k
) < P
lt
S
k
/(S
park
S
rG
)
1/2
(during normal operation)
Has to comply with Engineering Recommendation G5/4 Scotland
Has to comply with IEC/TR3 61000-3-7 standard (IEC,
1996b)
ESBNG
Has to comply with VDEW guideline ‘Grundsa
¨
tze fu
¨
r
die Beurteilung von Netzru
¨
ckwirkungen’
E.ON
Harmonics:
D
n
< 1 % (of fundamental voltage) for 1 < n < 51 Eltra
THD < 1:5%
D
n
(n is odd) < 0:3 –5 % (of fundamental current,
depending on n)
DEFU 111, Sintef
D
n
(n is even and interharmonic) < 25 %D
n
(n is odd)
THD < 5%
D
n
(n is odd) < 0:8 –3 % (of fundamental voltage,
depending on n)
Eltra and Elkraft
D
n
(n is even) < 0:2 –2 % (of fundamental voltage,
depending on n)
D
n
(n is odd, n < 26) < 0:01–0:115 (in A/MVA for
10 kV, depending on n)
VDEW
D
n
(n is even) and D
n
(n is odd, n > 26) < 0:06/n (for
10 kV)
(continued overleaf)
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Table 7.3 compares the requirements regarding fault tolerance, undervoltage and
overvoltage protection and requirements during islanding. Although wind farm
protection regarding overfrequency and underfrequency, overvoltage and under-
voltage, for instance, is not treated separately in some regulations it is assumed
that wind farm protection systems comply with the requirements discussed in the
preceding sections.
EBSN states that the protection system of a wind farm has to comply with the same
requirements that are defined in the grid code (ESBNG, 2002b) for conventional
generators. That means that internal faults should be disconnected within
.
120 ms for a 110 kV system;
.
100 ms for a 220 kV system;
.
80 ms for a 400 kV system.
Furthermore, power plants are required to be equipped with underfrequency and
overfrequency protection, undervoltage and overvoltage protection, differential pro-
tection of the generator transformer, and backup protection (including generator
overcurrent protection, voltage-controlled generator overcurrent protection or gen-
erator distance protection). The settings for the protection a re not discussed in detail
in ESBNG (2002b).
Table 7.2 (continued)
Requirement Source
D
n
(n is odd, n < 26) < 0:005–0:058 (in A/MVA for 20 kV,
depending on n)
D
n
(n is even) and D
n
(n is odd, n > 26) < 0:03/n (for 20 kV)
D
n
(n is odd) < 4 % (depending on n) AMP
D
n
(n is even) < 1%
D
n
n is interharmonic < 0:3%
THD < 6%
Should comply with Engineering Recommendation G5/4 Scotland
Has to comply with IEC/TR3 61000-3-6 (IEC, 1996a) ESBNG
Note: k
f
(
k
) ¼ the flicker emission factor during the switching operations at a certain network
impedance phase angle
k
; c
c
(
k
) ¼ flicker emission factor during normal operation; k
i
¼ the
in-rush current factor; R
k
¼ the short-circuit ratio at the point of connection (ratio between
short circuit power and maximum apparent power of the turbine); S
k
¼ short-circuit capacity at
the connection point; S
park
¼ nominal apparent power of the wind farm; S
rG
¼ nominal
apparent power of a wind turbine; N ¼ the maximum number of switching operations; P
lt
¼ the
weighted average flicker emission during 2 hours; P
st
¼ the weighted average of the flicker
contribution during 10 minutes; THD ¼ the total harmonic distortion; D
n
¼ the harmonic
interference of each individual harmonic, n. THD, the individual harmonic interference and
flicker contributions P
lt
and P
st
are defined in IEC/TR 61000-3-7 (IEC, 1996b) (DEFU and
AMP also include the definitions).
Sources: see note to Table 7.1.
130 Technical Regulations