AckermannTh. (ed) Wind Power in Power Systems
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.
reactive compensation to avoid system disturbances [e.g. use of static VAR compen-
sators (SVCs) and flexible AC transmission systems (FACTS)];
.
operating control through direct integration into implementation and control and
protective relaying;
.
additional dynamic requirements of wind turbines in the event of faults.
Moreover, the studies show that other ne ighbouring TSOs have to be involved in further
network investigations because fast wind power development affects the secure oper-
ation of the entire system. The initial studies in Germany will be followed continuously
by investigations covering the whole UCTE area (Deutsche Energieagentur, 2004; E.ON
Netz GmbH, 2003).
11.6 Wind Turbines and Dynamic Performance Requirements
The expected expansion of wind power generation in Germany and in Europe calls for
additional measures from network operators and also regarding the design of new wind
turbines. New requirements are necessary to keep quality and supply standard, both now
and in the future.
Energy generation, in general, has been altered by the large number of wind turbines,
which have characteristics that are different from those of large power plants. Large
thermal power stations require many hours after a breakdown before they can resume
operation. As opposed to this, wind turbines are able to resynchronise themselves with the
network and provide active power in accordance with the prescribed index values after
a short interval (e.g. after a few seconds), following disconnections caused by mechanical
failure. However, wind turbines do not yet offer system services on the same scale as
conventional power stations. These are actively involved in the network regulation
mechanism (e.g. spinning reserve, power frequency regulation, voltage regulation and
supply of reactive power). Conventional power stations are able to contribute during
several seconds substantially to short-circuit power, because of their rotating centrifugal
masses, which both maintains voltage levels and ensures network protection functionality.
In grids where wind energy generation is spread out over a large area and conventional
generation from large power stations is simultaneously reduced, severe voltage drops and
frequency fluctuations are more likely if the short-circuit capacity is decreased. This leads
to both a loss of voltage stability and the selective operation of the network protection
systems. Special attention has to be paid to the dynamic behaviour of wind turbines in the
event of network failures. These facts make it necessary to define new grid requirements,
particularly for wind turbines, in order to include them in the network regulation
mechanism. These additional requirements have to take into account the technical stand-
ard of decentralised energy supplies, in contrast to conventional power stations. Such
additional requirements will help to guarantee a stable and secure grid operation.
11.7 Object of Investigation and Constra ints
The installation of wind turbines, especially in northern Germany, has been increasing
for years now. In the beginning, single wind turbines were connected to the MV
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distribution grid. At that time, owing to economical and technical aspects, mostly stall-
controlled induction machines without power electronic equipment and control options
were installed. Therefore, in case of interactions wind turbines are disconnected if the
voltage drops below 80 % of the nominal voltage. After some time, the disconnected
turbines are reconnected to the system and return to their operating point.
In recent years, wind farms have mostly been connected to the transmission system
(110 kV system and higher voltage levels). Owing to the large amount of installed wind
power, the disconnection of wind turbines for an indefinite amount of time in the case of
system faults can lead to critical situations, with operation close to stability limits
resulting in consequences for the entire synchronously interconnected UCTE system.
E.ON Netz has been the TSO that has been affected the most by the co nnection of
large-scale wind generation to the transmission system. In 2001 it therefore started
investigations concerning the maximum outage of installed wind power in the case of
system faults (Haubrich, 2003). The results showed that already in 2002 there was a risk
that around 3000 MW of wind power would disconnect in the case of heavy system
faults located in the northern 380 kV system. Outages of power generation exceeding
3000 MW are defined as dangerous in the UCTE synchronously interconnected system.
As a consequence, E.ON Netz defined additional dynamic requirements for wind
turbines connected to the transmission system. They have been valid from January
2003. From that time, a wind turbine is not allowed to disconnect if there is a system
fault with voltage drops above the line shown in Figure 11.8 (E.ON Netz GmbH, 2001).
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
0.5 1.0 1.5 2.0 2.5 3.0
0.7
0.15
0.45
No disconnection of wind turbines above the line
Active
power, P ?
Voltage. U ?
Reactive
power, Q ?
Voltage, U (p.u.)
Time, t (s)
Figure 11.8 Requirements for wind farms valid in the control area of E.ON Netz since January
2003
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In addition to the active power balance in the transmission system, the voltage level
and the reactive power balance have to be taken into account. Therefore it was necessary
to study the dynamic behaviour of wind turbines in the case of system faults in more
detail. In particular, the followi ng questions concerning system stability were focused on
in this study (Figure 11.9):
.
the allowable gradient of active power return of wind farms after fault clearing;
.
the limitation of reactive power consumption in the case of system faults;
.
the necessary backup voltage operation of wind farms;
.
the admissible loading of coupled lines for primary control aspects;
.
frequency control in the case of increasing frequency margins.
Initial investigations have shown that additional requirements for wind turbines con-
cerning active power return, backup voltage operation and reactive power balance are
absolutely necessary. Otherwise, severe network failures resulting from reactive power
consumption of wind turbines after fault clearing could lead to a nonselective discon-
nection of entire network segments and consumers in the near future.
The goal of the investigation was to determine the constraints to an optimal operati on
of wind farms in the case of severe system failures concerning active and reactive power
balance and the stress to on wind turbines. These additional requirements in combin-
ation with the ongoing repowering of older turbine types resulted in future outages of
wind power as a result of faults in the EHV network being limited to less than 3000 MW.
P
t
P(t) ?
Fault duration
• Frequency stability
• Overloading of coupled lines
P ?
Q
t
∆ t ?
∆ t ?
Q(t) ?
Fault duration
• Regional power quality
• Voltage stability
•
Overloading of transformers
Q ?
U ?
Voltage control
measures
Figure 11.9 Optimal operation of wind turbines concerning active and reactive power balance.
Note: P ¼ active power; Q ¼ reactive power; U ¼ voltage; t ¼ time
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These constraints were added to the Grid Code, which has been valid for the control
area of E.ON Netz since August 2003 (E.ON Netz GmbH, 2003).
The requirements were determined without any reference to the type of wind turbine.
The requirements have to consist of entirely nondiscriminatory measures that can be
applied to all types of wind power installations. This way, the requirements governing
the dynamic behaviour in the event of network failures will not cause any distortions in
the compet ition. It may be necessary to add or modify the design of wind turbines and
wind farms in order to comply with the system-defined limits.
For the investigation of system interaction, a dynamic model of the complete UCTE
system was used. This model is applied in the steady-state analysis and accurately
represents all the power system components involved in physical phenomena of system
dynamics. Almost the complete 220 and 380 kV transmission grid of UCTE is repre-
sented using a specific software package for power system simulation (Lerch et al.,
1999). The dynamic model contains:
.
2300 transmission lines;
.
820 transformers 400 kV and 220 kV;
.
500 generators
.
50 wind turbine equivalents in the northern part of the control area of E.ON Netz.
Typical load-flow situations obtained from real-time operation were included in the
study. In order to limit the model size, all the units of one generation site are aggregated
in one machine, which is valid as far as small-signal stability studies are concerned. The
dynamic data, that is the generator characteristics, the excitation controllers, the power
system stabilisers, the turbine characteristics and the governors, are mostly described by
detailed models in accordance with IEEE standards (IEEE, 1987, 1992). Special atten-
tion was paid to modelling wind turbines, wind farms and the 110-kV trans mission
system in the area bordering the North Sea.
11.8 Simulation Results
11.8.1 Voltage quality
Investigations regarding the effect of wind farms on the entire system have shown that
wind farms influence voltage quality, frequency stability and voltage stability, depend-
ing on the techno logy that is available at the time. In particular, wind farms operating
with stall-controlled wind turbines may cause lower voltage levels after faults in the
transmission system are cleared because of the reactive power consumption of induction
machines.
Owing to modern converter technology, limited and full variable-speed wind turbines
with asynchronous or synchronous induction generators usually affect the reactive
power balance of the system to a lower extent [see Figures 11.10(a) and 11.10(b)]. In
the case of severe faults (e.g. deep voltage drops) crowbar firing leads to a disconnection
of the converter. During this time, the wind turbines are not controllable, as oppos ed to
synchronous generators using rotating DC-excitation systems. Figure 11.10 shows the
grid behaviour of wind turbines in the case of crowbar firing.
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0.00 0.25 0.50 0.75 1.00
P/S
N
1
Q/S
N
2
(a)
Time, t (s)
300 ms
0.00 0.25 0.50 0.75 1.00
2
1
(b) Time, t (s)
Q/S
N
P/S
N
300 ms
Crowbar-firing pulse
Figure 11.10 System fa ilure in the case of a three-phase fault close to the wind farm for (a) a wind
farm operated with fixed-speed wind turbines (Type A) with an asynchronous induction generator with
reactive compensation; (b) a wind farm operated with limited variable-speed wind turbines (Type C)
with doubly fed asynchronous induction generators; and (c) a wind farm operated with variable-speed
wind turbines (Type D) with synchronous induction generators with full-load frequency converters. In
each case: rating ¼ 45 MW; short-circuit power at point of common connection (PCC) ¼ 1000 MVA;
nominal voltage at PCC ¼ 110 kV. Note: P ¼ active power; Q ¼ reactive power; S
N
¼ nominal
three-phase power of induction generator; turbine Types A, C and D are described in Section 4.2.3
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Wind turbines connected to the grid through a full-load frequency converter [Figure
11.10(c)] also have less effect concerning the reactive power balance, because wind
generators and the AC network are decoupled.
In contrast to direct AC-coupled synchronous machines, all types of wind turbine can
only supply short-circuit currents that are limited to the nominal current. As the
investigations have shown, with an increasing amount of wind energy generation con-
nected to the grid, win d turbines have to stay connected to the grid during syst em faults
and ha ve to return quickly to their pre-fault operation point. However, wind turbines
that do not comply with the additional dynamic requirements may affect the reactive
power balance when remaining connected during the fault. The investigations showed a
decreasing voltage level, especially on the 110-kV grid, after fault clearing, because the
wind turbines that stayed connected to the grid without complying with the additional
dynamic requirements consumed reactive power.
Depending on network status, mainly the Schleswig–Holstein area and the northern
part of Lower Saxony would be affected and run a risk of undervoltage-tripping of
transmission lines and partial consumer outages.
Modern wind turbines can, depending on their capacity, participate in voltage and
frequency regulation in a similar way to conventional power stations. In order to
maintain and improve voltage quality, it is practical to switch from constant load power
factor ( cos ’) regulation to backup voltage operation if there are network failures that
cause voltage drops of more than 10 % of nominal voltage at individual wind gener-
ators. This ensures that terminal voltage does not drop sharply again after fault clearing.
Investigation results also show that the quality of the network voltage in the event of
failures can be considerably improved by applying this measure (Figure 11.11 and 11.12).
0.00 0.25 0.50 0.75 1.00
1
1
(c) Time, t (s)
Q/S
N
P/S
N
300 ms
Reactive power of the wind farm (PCC) (p.u.)
(crowbar firing active)
Active power of the wind farm (PCC) (p.u.)
Figure 11.10 (continued)
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–100
–50
–50
–50
–50
0.00 0.25 0.50 0.75 1.00
Voltage (%)
(a)
(b)
(c)
(d)
(e)
Time, t (s)
Figure 11.11 Voltage drop (as a percentage of the nominal voltage) after a three-phase fault in
the northern part of the E.ON Netz control area (close to the Dollern substation): (a) 380-kV
system; (b) 220-kV system; (c) 110-kV system, EMS Elbe; (d) 110-kV system, Weser EMS; (e) 110-
kV system, Schleswigs Holstein
0.00 0.25 0.50 0.75 1.00
90
80
70
60
50
100
100
100
200
200
300
300
400
400
500
500
600 700
800
t (ms)
t (ms)
Trip
undervoltage
protection
Additional
requirements fo
r
WT
No additional
requirements
for WT
U/U
n
(%)
Time, t (s)
Figure 11.12 Recovery of the grid voltage in the 110 kV system after a three-phase fault in the
380 kV system. Note: U ¼ voltage; U
n
¼ nominal phase-to-phase voltage; WT ¼ Wind Turbine
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In order to maintain network voltage quality, particularly in 110-kV networks, in the
event of network failures it is necessary to prevent as far as possible any repercussions
from the wind farms on network voltage. Therefore, wind turbines must return as
quickly as possible to their pre-fault operatio n point. The transient compensation that
can occur during failure correction and the return to the voltage must be limited to
400 ms. This additional requir ement will prevent long-term reactive power consumption
of wind turbines and a voltage decrease to critical levels (Figure 11.12).
11.8.2 Frequency stability
In contrast to conventional power stations, which only experience outages in the case of
bus-bar failures, wind turbines set up in large numbers close to one another over wide
areas can experience lasting acti ve power outages of up to 2800 MW, even as a result of
circuit failures in the EHV network. The outage level is that high because all wind
turbines connected before December 2002 disconnect for an indefinite time from the
network if generator terminal voltage drops below 80 % of the nominal voltage level.
According to simulat ion results, at present the stationary frequency deviation is
expected to be approximately 160 mHz in the case of outages of around 2800 MW in
the northern part of the E.ON control area. Figure 11.13 shows the stationary frequency
deviation in connection with a three-phase circuit failure near Dollern substation, which
is located in Lower Saxony, close to Hamburg.
Additional dynamic requirements in combination with effects from re-powering (i.e.
replacing old wind turbines with new ones that comply with network access require-
ments) have to guarantee within the following years a reduced stationary frequency
deviation arising from disturbances in the transmission system. Therefore, it will be
important to limit the loss of installed wind power in the event of network failures.
In particular, offshore wind power, which will be concentrated to only a few substations
100
–100
–200
–300
–400
0
0.0
E.ON Netz, Germany Vigy, France
Rogowiec, Poland Hernan, Spain
5.0 10.0 15.0 20.0
Time, t (s)
Frequency deviation (mHz)
Figure 11.13 Frequency deviations in the UCTE system after an outage of 2800 MW wind power
in Northern Germany
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close to the sea, must stay connected during severe system faults very close to the point
of common coupling (PCC). With modifications of the network connection contract,
this can be achieved by upgrading the wind turbines that are already in operation.
Since 1 January 2003 wind turbines that remain connected to the network in the event
of network failu res have been required to start providing active power immediately after
fault clearing and to reach pre-fault active power capacity within 10 s in order to prevent
any further increase in the wind power capacity that is temporarily disconnected during
such failures. As a result of these measures, the stationary frequency deviation in the
event of severe network failures is expected to be less than 200 MHz (about 160 MHz)
until 2005, thus remaining below the frequency deviation permitted in the UCTE
synchronous integrated network.
Figure 11.13 shows the variations in the increase in active power together with the
time at which wind turbines return to their pre-fault operating point, following network
failure and failure correction. For the year 2005, approximately 2000 MW of brief active
power outages, lasting only a few seconds, are to be expected in the event of severe
network failures (in high winds, 90 % wind power), in addition to the 2800 MW of
lasting active power outages. The dashed line in Figure 11.14 shows the frequency
deviation in the control area of E.ON Netz taking into account a return to full active
capacity after one minute.
Even if all newly installed wind turbines increase their active capacity and are able to
resume supplying the network with the pre-fault active capacity within 40 s after the
fault is cleared, there could still arise critical conditions regarding the allowable fre-
quency deviations and excitation and trip of overcurrent line protection. Therefore, the
wind turbines must return to supplying active power as fast as possible. If all ne wly
installed wind turbines increase their active capacity following failure correction at a
gradient of 10 % of nominal capacity per second, the network frequency will reach a
frequency deviation of 150 MHz within approxim ately 10 s after the onset of the
disturbance.
Disturbances can be divided into two groups: Near-to-generat or three-phase short
circuits with deep voltage drops and far-from-generator three-phase short circuits.
10.0 15.0 20.00.0 5.0
–100
–400
–200
–300
100
0
Additional
requirements
for WTs
No additional
requirements
for WTs
Frequency deviation (mHz)
Time, t (s)
Figure 11.14 Transient and stationary frequency deviation as a function of the gradient of the
active power increase of the wind turbines (WTs) following failure correction
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Near-to-generator disturbances cause, depending both on the short-circuit ratio at PCC
and the type of wind turbine, a varying degree of stress on electrical and mechanical
parts of the wind turbines (e.g. gearbox, shaft) that remain conn ected to the system
during the fault. In addition, such deep voltage drops may lead to crowbar firing for
doubly fed induction generator (DFIG) wind turbines. In the case of deep voltage drops
and higher short-circuit power ratios, induction generators, in general, affect the system
by consuming reactive power. In such cases, the active power capacity cannot be raised
as quickly as necessary for stable integrated network operation, even if the turbines
remain connected to the network. In this kind of situation, there may occur an undesir-
able compensation between network and wind turbin es or wind farm which also causes
a high level of mechanical stress on the wind power installation and may considerably
shorten its lifetime.
Thus, it seems reasonable in specific cases to exempt wind turbines from the given
requirements and allow them briefly to disconnect. Therefore, wind turbines that are
affected to varying degrees by network failures are divided into the following two
groups, based on voltage drops at the PCC, which increases the active power with
different gradients:
.
wind turbines affected: in the case of a fault, network voltage collapses to 45 % of
residual voltage at the PCC;
.
wind turbines stable: in the case of a fault, network voltage remains above 45 % of
residual voltage at the PCC.
For this research, for the stable group, a gradient of 20 % of the nominal capacity per
second was selected and for the affected group a gradient of 5 % of the nominal capacity
per second was selected. Figure 11.15 shows that if in the stable group the time to restore
0.0 5.0 10.0 15.0 20.0
–100
–400
–200
–300
100
0
Total active power after 10 s
Active power returns 5 s after fault clearing, if V
PCC
> 45 % U
N
Active power returns 20 s after fault clearing, V
PCC
< 45 % U
N
Time, t (s)
Frequency deviation (mHz)
Figure 11.15 Scenario 2005: 2880 MW outage of installed wind power with division of wind
turbines into two groups with different active capacity gradients – affected and stable. Note: with
stable wind turbines the network voltage, V
pcc
, drops no lower than 45 % of the residual voltage,
U
N
, at the point of common connection (PCC) in the case of three-phase faults in the transmission
system; with affected wind turbines, V drops lower than 45 % of U
N
in the case of three-phase
faults in the transmission system
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