7th International Workshop on Large Scale Integration of Wind Power and on Transmission Networks for Offshore Wind Farms
Constant Speed Turbines on a Grid with
Variable Frequency – A Comparison in Terms of
Energy Capture
E. Troester
Abstract--This paper describes a novel control concept for wind
parks with just one central converter to control the frequency of
the park grid. To assess this concept a comparison with
conventional concepts in terms of energy capture is carried out.
Basically this control concept is suitable for every type of
constant speed turbine, but it has its biggest advantages in
combination with a new type of generator, the permanent magnet
induction generator. This type of generator allows to abandon the
gearbox and converter on the turbine, thereby reducing the total
number of parts and increasing the turbine’s reliability, which is
a crucial parameter of offshore wind turbines.
The results show that there is only a small reduction in energy
yield (less than 4%) for the new control in comparison to the
variable speed control.
Together with the robust turbine design without gearbox and
converter, the new control concept is a good option, particularly
with regard to offshore applications.
Fig. 1. Direct drive permanent magnet induction generator
Index Terms--Constant Speed Turbine, Energy Capture
Offshore, Permanent Magnet Induction Machine, Pitch Control,
Variable Frequency
I. NOMENCLATURE
PMIG – Permanent Magnet Induction Generator
II. INTRODUCTION
Already several thousand megawatts of turbine power are
planned on offshore locations near the European coasts.
Due to the special conditions at sea there is a challenge to
develop new concepts which meet the requirements of
offshore wind parks.
One of the major necessities is a reliable and low–maintenance
system, leading to the objective of reducing the number of
parts on the turbine as much as possible.
The concept described in the paper uses a permanent magnet
induction generator [1], which has no gearbox and no
frequency converter, as the generator on the turbine (Fig 1).
Thus, the number of parts is reduced and a maintenance- and
oil–free system results.
Eckehard Troester is with Energynautics GmbH, Mühlstrasse 51, 63225
Langen, Germany (e-mail: E.Troester@energynautics.com).
Since there is neither a gearbox nor a converter included in the
turbine, this type of turbine is a constant speed turbine when
connected directly to the grid. This has the drawback that pitch
control cannot be used and a lower energy yield has to be
expected, as the rotation speed cannot be adapted to the wind
speed, leading to a lower power coefficient. To overcome this
drawback, the new control concept uses one central converter
to control the park frequency and thereby the rotation speed of
the turbine, in order to gain a better power coefficient and
hence a higher energy yield. With this configuration, a grid
with variable frequency is achieved.
The frequency is controlled in such a way that the energy yield
is maximized. The converter can either be placed onshore or,
when using an HVDC-link, on a platform (Fig 2).
Fig. 2. Constant speed turbines with central converter
7th International Workshop on Large Scale Integration of Wind Power and on Transmission Networks for Offshore Wind Farms
Both locations can easily be reached in the case of a fault in
the converter. In addition, very good compliance to the
requirements for grid connection (i.e., power factor, fault ride
through etc.) can be achieved.
At rated load, active stall and pitch control have been
investigated. Active stall is well known for constant speed
turbines. To enable pitch control, a special frequency control
is introduced, but not without drawbacks: the mechanical
power has to be reduced in order to limit the frequency and
power dips happen, leading to a reduced energy yield.
A comparison in terms of energy capture and location
dependence has been carried out between this concept and
other known control concepts (constant and variable speed
control) using Matlab-Simulink as a simulation tool.
The operating mode of the MPP-Controller is shown in Fig 3.
Within 0.1 s the whole frequency spectrum is passed and the
frequency which causes the highest power output is detected
as optimum frequency (labeled with an arrow) and serves as
reference input for the frequency controller.
III. MODEL
The here presented model is based on investigations of
Hoffmann [2]. Data which is achieved with Hoffmann’s model
is scaled up to a 2 MW wind turbine and adjusted to actual
wind power plant performance. The model is advanced to a
whole wind park model consisting of 10 turbines, which are
controlled with one central frequency converter. Each turbine
has a different wind time series, but with the same mean wind
speed and turbulence. With a change in turbulence, deviations
in the wind time series can be influenced: a high turbulence
causes large deviations and smaller turbulence means small
deviations.
IV. CONTROL CONCEPT
A. MPP-Tracking
In order to adjust the speed of the turbines to the prevailing
wind speed, the optimal frequency (which leads to optimal
power output) has to be found. This is realized with MPP
Tracking (Maximum Power Point Tracking). Two techniques
are known for MPP tracking: The first technique is based on
relative comparison. The frequency is changed in one
direction in order to increase the power. If the power really
increases, the frequency change can continue. In the case of a
power reduction the frequency has to be changed in the other
direction. The second technique works with absolute values.
The whole frequency spectrum is scanned so that the point of
maximum power can be detected. The frequency is then
adjusted to this optimum. Both methods have benefits and
drawbacks. The first technique with relative comparison is
easy to implement but it holds the risk of detecting a local
maximum only. The second “absolute” technique cannot
directly be realized as described, since it is impossible to pass
through all frequencies in a real wind park from time to time
in order to find the maximum power. However, in order to
take advantage of this method it is applied to a wind park
model. For a model it is by all means possible to vary the
frequency over a wide range in order to find the maximum
power. The simulation result can finally be taken as reference
input for the real wind park.
Fig. 3. MPP-Tracking
However, if this reference frequency would be applied to the
wind park without a limit in the derivation of the frequency,
very large power fluctuation would occur, which could even
cause the turbine to work temporarily as a motor. This effect is
also known for normal variable speed wind turbines, if the
controller is made to adjust always the optimal tip speed ratio
in the power optimization mode. Because of this the frequency
change is limited, which causes a relatively slow tracing of the
frequency to its reference value (Fig 4). The simulation
presented here uses a frequency change of 0.1 Hz/s, which
was empirically found by evaluation of simulation results.
Fig. 4. Reference and actual frequency
B. Power Control
For power limitation in wind turbines two basic concepts are
used: pitch control and stall control. Since the mechanical
stresses are significantly reduced in pitch controlled systems,
this control strategy prevails for wind turbines with high
power output. The principle of stall control is much easier,
however components in stall controlled wind turbines have to
7th International Workshop on Large Scale Integration of Wind Power and on Transmission Networks for Offshore Wind Farms
be designed much more robustly, which causes higher weights
and probably higher prices. Thus the additional expenses for
pitch actuators are sufficiently compensated by savings in
material. An advancement of the stall control strategy is the
active stall concept (active stall is known under artificial
names depending on the manufacturer). Here the blades are
pitched to larger angle of attack in order to enforce the stall
effect. Hence, less powerful actuators can be used and it
provides faster control. Like for pitch control the stresses can
be significantly reduced for emergency stops and start up.
A drawback of pitch control is that it acts comparatively
slowly. This causes large power fluctuations due to wind gusts
around rated wind speed. Thus a combination of a fixed speed
system and power limitation with pitch control is not applied.
However, this paper deals with a quasi fixed speed wind
turbine concept combined with pitch control. A whole wind
park consisting of “normal” fixed speed turbines provides
variable speed in changing the frequency of the whole park.
With this park-variable concept a change in the frequency
allows to realize power limitation with pitch control. If one
turbine demands power limitation it is detected and adjusted.
This control strategy will be explained in the following
paragraph. It is refrained from explaining the active stall
concept in detail, because it is an approved solution for fixed
speed wind turbines.
C. Pitch Control Strategy
Pitch control facilitates power limitation by changing the
angle of attack. This causes a decrease in the power coefficient
and limits the power which is absorbed by the rotor. Modern
wind power plants have either mechanic or electrically driven
pitch actuators, which are able to turn the blades with 6 up to
10 degrees per second. However, due to the dynamics of the
wind this actuator speed is not sufficient to provide a reliable
power limitation. Thus the speed of the turbine has to be
increased in order to buffer the power temporarily as kinetic
power. This indicates that pitch control can only be used for
variable speed wind turbines (except for wound rotors with
additional rotor resistance).
However, the introduced park-variable concept allows
operation at variable speed, as the frequency of the wind park
can change. This in turn facilitates introducing a pitch
controlled system.
All wind turbines of the wind park have a common monitoring
system. If one wind turbine reaches the power limitation
mode, the frequency is increased so that the power is kept
constant at rated power. Once the mechanical power can be
limited with the pitch actuator the frequency can be reduced to
its rated value (50 Hz).
The features of this pitch controller can be explained with the
help of the scenario shown in Fig. 5. As soon as the
mechanical power exceeds the power limit of 2MW (at
67.2 s), the pitch actuator mechanism starts and the pitch angle
increases. Due to the slip the electrical power follows with a
delay. Once the electrical power reaches rated power, too, the
controller starts to increase the frequency (at 67.5 s). At 68.2 s
the mechanical power is reduced to its rated value and the
pitch angle can be decreased. In the same way the frequency
can be reduced to its initial value by keeping the electrical
power larger than the mechanical power, so that the buffered
kinetic energy can be relieved. At 69.2 s the initial conditions
are finally obtained.
Fig. 5. Power limitation
Since each wind park consists of several wind power plants,
two effects appear:
1. In a wind park with a larger number of turbines it might
happen that power limitation mode is always actuated by one
of the wind turbines. This would result in a steady increase of
frequency (Fig 6). However, one can counteract this
phenomenon if the mechanical power is limited with the pitch
actuator system before rated power is reached. Again, the
power can be buffered as kinetic power.
2. All wind turbines in the park have the same frequency. If
the park frequency is increased the electrical power output
decreases automatically. This is volitional for all turbines in
power limitation mode but not for all the other turbines. Here
a power dip appears (Fig 7). This is contrary to the aim of
smooth power output. It causes unintentional power
fluctuations and, in total, a lower energy capture.
7th International Workshop on Large Scale Integration of Wind Power and on Transmission Networks for Offshore Wind Farms
Fig. 6. Frequency of 5 and 10 turbines
Fig 6 shows the described effect (effect 1): In a wind park
with 5 turbines the frequency is temporarily increased but can
be led back to 50 Hz. For comparison a wind park with 10
turbines is shown. Here one turbine always causes power
limitation mode so that the frequency shows a steady increase
without going back to its rated value.
In order to counteract this phenomenon the mechanical power
is reduced depending on the frequency, which is shown in Fig
7. At around 6 s the mechanical power exceeds its rated value,
so that at first the pitch angle increases and later on also the
frequency. In order to be able to lead the frequency back to its
rated value, the mechanical power is limited slightly below
rated power. At 7.3 s this power amounts to 1900 kW.
However, the electrical power output is kept at 2 MW, so that
the difference between mechanical and electrical power is
drawn from rotational power and the frequency can be reduced
to its rated value (50 Hz). The smaller the difference between
the frequency and its rated value is, the smaller is the
difference of mechanical power and rated power. If it were
possible to keep the electrical power output constant to rated
power, there would be no loss in energy.
Fig. 7. Mechanical power reduction
The power dip caused by the increased frequency (effect 2) is
shown in Fig 8. The power of two wind turbines is compared
during a time of increasing frequency. Furthermore the power
of the total park, consisting of 10 turbines, is plotted here. At
70.5 s wind turbine 2 reaches power limitation, the frequency
is increased in order to keep the electrical power P2 at rated
power. This causes in turn a loss of 600 kW of turbine 1. At
the same time the total power of the wind park decreases
because not only turbine 1 is affected by this incident but also
all the 8 other turbines in the wind park. The total wind park
suffers from a power loss of 5 MW and decreases to 15 MW.
At 72.0 s wind turbine 1 now reaches the power limitation
mode. The frequency has to be increased again now, in order
to limit P1. And again the total wind park power is reduced to
15 MW.
7th International Workshop on Large Scale Integration of Wind Power and on Transmission Networks for Offshore Wind Farms
The following figure illustrates the computed power curves,
simulated with a turbulence of 10 %.
Fig. 8. Power curve
It is noticeable that the constant speed turbines (CST) have
less power at partial load, which is due to the lack of
adjustment to the prevailing wind speed. Near the transition
from partial load to rated power both stall controlled concepts
(CST and PCS) show a slightly lower power output than the
variable speed concept (VST). However, the PCP concept
yields the smallest values under rated conditions. Due to the
pitch control (see above) the average power is even below
rated power.
The next figure shows the energy loss of all concepts
compared to the variable speed concept (VST) with a
turbulence of 10 % and a Weibull parameter of 2.
Fig. 8. Power dip
V. ENERGY YIELD
In order to compute the energy yield of the different concepts,
5 minute simulations of the wind are performed with mean
wind speeds between 3 m/s up to 25 m/s. The power outputs
of these simulations serve to determine a power curve. This
power curve is finally assessed with a Weibull distribution and
the energy capture of one year results.
Powercurve
0
4 5
5 0
5 5
6 0
6 5
Energy
0
6
6
4 5
5 0
8
5 5
8
Windspeed
6 0
6 5
4 0
4 5
5 0
5 5
6 0
6 5
9
Power
Location
Weibull Distribution
Fig. 8. Procedure to calculate energy yield
Fig. 8. Energy yield in comparison to the variable speed turbine
Four different concepts are compared
x Constant Speed Turbine (f=50Hz) + Active Stall
(CST)
x Variable Speed Turbine + Pitch (VST)
x Proposed Concept (10 Turbines) + Active Stall
(PCS)
x Proposed Concept (10 Turbines) + Pitch (PCP)
The loss in energy of the constant speed concept (CST)
compared to the variable speed concept (VST) decreases with
higher average wind speeds. Here the turbines are more often
in power limitation mode, where the power output is the same.
The PCP concept shows the opposite, because it has a lower
power output at higher wind speeds. The PCS concept has an
energy loss of less than 4 % and even smaller values for high
wind speeds which are found for offshore sites.
7th International Workshop on Large Scale Integration of Wind Power and on Transmission Networks for Offshore Wind Farms
VI. CONCLUSIONS
A comparison in terms of energy capture and location
dependence has been carried out between constant speed
turbines on a grid with variable frequency with pitch and
active stall and other known control concepts (constant and
variable speed control) using Matlab-Simulink as a simulation
tool. MPP-tracking control and pitch control have been
investigated. The simulations show that pitch control works
but drawbacks in terms of energy yield have to be taken into
account, so that pitch control cannot be recommended.
However, the results for active stall control show that there is
only a small reduction in energy yield (less than 4%) for the
new concept in comparison to the variable speed control.
Together with the robust turbine design without gearbox and
converter, the new control concept with active stall control
seems to be a good option, particularly with regard to offshore
applications.
VII. REFERENCES
[1]
[2]
Hagenkort B., Hartkopf T., Binder A., Jöckel S., “Modelling a Direct
Drive Permanent Magnet Induction Machine” Proc. ICEM 2000,
Helsinki University of Technology, Vol. 3, pp 1495-1499.
Hoffmann, R.: "A comparison of control concepts for wind turbines in
terms of energy capture", D17 Darmstädter Dissertation, 2002
VIII. BIOGRAPHY
Eckehard Troester was born in Marburg in
Germany, on December 7, 1975.
He holds a Master of Electrical Engineering from
Darmstadt University of Technology, Germany. He
is currently working on his PhD, which will be
finished in 2008. His research focuses on electrical
power systems, renewable energies and electrical
machines, especially wind power generators. He has
worked as a scientific assistant at the Institute of
Renewable Energies, Darmstadt. Since 2007 he
works for Energynautics.