SystemsControl & Protection

Generation Yield
improvement, Control
and Protection in modern
Windmills
Applications• Blade pitch and Yaw control- Current Control in the low-powerelectrical drives• Double-fed induction generator(asynchronous machine)- Precise and fast current detection in
the pulse-controlled IGBT-converters(rectifier and inverter) to control thepower for the grid
- Short-circuit protection of the power
semiconductors in the converters• Synchronous generator- Current Control in the pulse converters(rectifier and inverter) to transmit the fullpower to the mains grid
- Short-circuit protection of the power
semiconductors in the convertersFeatures• Bipolar AC & DC current measurementfrom 50 to 2000 ARMS nominal• Bipolar as power supplies from 12 to 24 V(Type dependant)• Response time @ 90 % of IPN : < 1 µ s• Wide Bandwidth from DC up to several
hundred of kHz• Accurate current output signal (± 0.3 to0.65 % of IPN typically @ + 25°C)• Galvanic Isolation• Different Compact Sizes in a variety ofmounting topologies (mechanical or PCB)
• Moulded versions for harsh environments• Good common-mode behaviour• EN 50178 Compliance• CE Marking• 5 years warranty

Fossil fuels have always been the primary source ofenergy for electricity generation. Concerns about thesustainability and greenhouse gases from fossil fuels
has led to an increased drive to develop alternative
systems to generate electricity. This if further supported
by international agreements to reduce the volume ofCarbon Dioxide produced from fossil fuels.
The building of modern wind energy turbines (WET) withpower ratings up to 5 MW (presently in the test phase)is only made possible by using modern materials to meetthe mechanical requirements supported by modern
control and power electronics.
The generated energy must be fed into the grid systems
at a controlled frequency and voltage. Current
transducers of various sizes are key components ofevery converter in a WET, providing optimal control andprotection of the converter.
Since the early days of its evolution,
mankind has used the wind as asource of energy. Windmills
transformed the energy contained in
the wind into mechanical, usableenergy that could be used to grind
grain or pump water.
Many theories of the physics anddesign of modern wind turbines werediscovered in the first half of the 20thcentury. The German engineer AlbertBetz calculated, in his book publishedin 1926, that the maximum theoretical
efficiency of an ideal wind turbine isabout 59.3%. In the 1940s, Ulrich
Hütter developed the theoreticalbasics (derived from his excellent
aeronautical knowledge) for thelayout of all modern free and highspeed running wind energy
converters with two or three rotor
blades.

COMPACT AC/DC CURRENT TRANSDUCERS : LA and LF Series
www.lem.com

Picture 1: Windmills in the harbour of Rhodes, Greece - the cloth that
forms the rotor wings has been wound up.
Picture 2: A wind driven coastal pumping station in Lobbe (on the German
island of Rügen in the Baltic Sea)
Picture 3: Wind park on Gran Canaria in the Canary Islands.

When the wind speed exceeds a certain threshold, WETs
must have power controls in order to avoid mechanicaland/or electrical overload. Generally, the rated power ofthe generator is one of the threshold levels that must not
be exceeded.
There is another equally important need for power control.
It is essential to optimise the generation of electricity toprovide the maximum continuous power to the grid systemfor both loading and economic reasons.
Different types of power control are used. Simple control
can be achieved passively or actively by the rotor blades.
Passive limitation can be achieved by a special profile ofthe single rotor blades. At a certain wind speed, the air
But only when the political framework changed at the stream that causes the rotation of the rotor collapses (thebeginning of the 1990s did government aid for renewable so-called stall) and the rotor comes to a halt (stall control).
energies become available in many countries. This
prompted intensified commercial development of wind In common use today by larger wind turbines is an activeenergy turbines (WETs). More and more wind turbines power control system that adjusts the rotor blades within
and wind parks have been constructed and created; at their longitudinal axis (pitch control). With this adjustment
present the first 4.5 to 5 MW wind turbines are in the test of the blade angle relative to the rotor plane, it is possible
phase. With 16,929 MW of wind power installed out of a to control more than the generator power. At higher wind
worldwide figure of 47,317 MW, Germany leads the league speeds, the rotor blades can be twisted in such a waythat the rotor comes quickly to a halt. Low-power electricaltable, ahead of Spain, the US, and Denmark.
drives are commonly used for such a purpose. In thecontrol inverters, compact, PCB-mounted current
Power control of wind turbinestransducers (LEM models LA 55-P, LA 100-P, LA 125-P,
Wind is the exchange of air masses, primarily caused by LA 200-P), are widespread. These transducers are part
local or even large area temperature differences that result of the converter’s closed loop control and therefore provide
from the effects of solar radiation. Obstacles such as a rapid response. When coupled with intelligent power
forests, mountains and buildings produce turbulence that control of the generators, the maximum continuous power
creates a permanent change of wind speed. The rotors of can be delivered until the upper-limit wind speed iswind turbines transform energy contained in the wind into reached.
rotational (kinetic) energy, which drives a generator toproduce electrical current.
The power of the wind, and thus the amount that can beused, is proportional to the cube of the wind velocity. Thereis also a simple correlation between the rotor area,
calculated from the rotor diameter, and the energy thatcan be derived from the wind that flows through this area.

Picture 4: LEM PCB mounted current tranducers from 50 A up to 200
ARMS nominal.

COMPACT AC/DC CURRENT TRANSDUCERS : LA and LF Series
www.lem.com

slip area. The rotor speed can be adjusted within narrowlimits by a slip control and in wider ranges by switching theIt is impor tant that the rotor is always oriented poles of the generator. The gearbox adapts the rotor
perpendicular to the wind, for two reasons. The first is to revolutions to the generator speed. The machine needs
ensure that the wind streams through the maximum rotor reactive power from the grid to build up the rotating field. In
area, so that the maximum energy is obtained from the order to limit the inrush currents that are generated whenwind. The second is to avoid non-uniform loading of the the generator is coupled with the grid, soft-starters are usedrotor blades by ensuring that they are not flexed during between the generator and the grid during the starting period.
This direct grid coupling approach is no longer used witheach revolution.
Commercial large wind turbines are usually so-called large wind turbines, due to some technical drawbacks (e.g.
windward machines, i.e. the rotors face the wind in front equalising processes of the grid connection by the switchingof the tower, but this is an unstable state. Therefore, the actions used for power adjustment).
nacelle and rotor must be actively twisted into the wind byelectric motors. Additional brakes are used to ensure that Double-fed induction generator
the nacelle will not twist due to small and short-termchange of wind direction. In order to position the drives The majority of today’s large wind turbines use a modified
optimally, transducers in the respective converters “Danish Concept”, in which a double-fed asynchronous mameasure the current continuously. The quality and chine acts as a generator.
response time of the current control are influenced by thedesign and performance of the current transducers. Thisis why closed-loop current transducers with a low current
Yaw control

Picture 6: Circuit diagram of a double-fed asynchronous generator.

The frequency and voltage of the stator are coupled tightly
with the mains grid. The slip ring rotor is coupled with thegrid by means of a special inverter, which must be able totransmit energy both towards the machine and the grid.
This inverter only needs to be specified for the slip power,
which is generally only 20% of the rated power of thegenerator.
Picture 5: Circuit diagram of a closed-loop Hall effect current transducer.
rating are used (LEM model LF 205-S) in this application. This neat concept allows adjust of a larger power flow by
The inherent advantages of closed-loop current only controlling a small proportion. A wind turbine that istransducers are a high bandwidth combined with a short designed in this way is a variable speed system from theresponse time and very good linearity leading to good sub-synchronous up to the super-synchronous area. Two
identical pulse-controlled IGBT-inverters with a voltage DC
accuracy.
The next problem is to deliver the electrical power from link are used as converters. In either energy transport
the WET into the mains grid. Manufacturers of wind turbi- direction, one of them will be used as a rectifier while thenes have developed competing systems for this purpose. other one will be used as an inverter, or vice versa.
In effect, every wind turbine is equipped with either an
Precise and fast current detection is necessary in orderasynchronous generator or a synchronous generator.
to control the power for the grid in addition to the DC link
voltage. LEM provides closed-loop current transducers
Asynchronous generator and grid couplingwith medium current ratings that are well suited for thisThe classical “Danish Concept” describes a wind turbine purpose (LEM models LF 305-S, LF 505-S). These
consisting of a stall-controlled rotor with three rotor blades, a transducers are compact and are available with many
gearbox, a pole-switched asynchronous generator with different mounting styles. In addition, LEM voltage
squirrel cage rotor and a direct mains grid coupling. The transducers can be used to monitor and/or control thedirect grid coupling produces a “constant-speed” system with voltage of the DC link.
a nearly constant operating speed in the super-synchronous

COMPACT AC/DC CURRENT TRANSDUCERS : LA and LF Series
www.lem.com

A significant advantage of synchronous generators is thatthey can provide either inductive or capacitive reactivepower (even zero) depending on the control of the field/
excitation control.
The mains grid coupling is through a pulse converter thatis specified for the full power to be transmitted. For thisapplication, LEM’s dynamic closed-loop current
transducers (models LF 1005-S, LF 2005-S) can be usedin the rectifier and the inverter. Moulded versions are alsoavailable for harsh environments.
All LF-family current transducers that can be used for theabove applications feature good common-mode behaviour
as well as an accuracy of 0.3 % (with reference to therated value) at ambient room temperature.

Picture 7: Stator of a synchronous motor at permanent magnets.
Picture by courtesy of ETEL-Switzerland.

Synchronous generator and grid coupling
Both concepts described above use a gearbox to adapt
the relatively slow revolutions of the rotor to the speed of
the generator. A different concept that is achieving successin the market uses a synchronous generator to provide avariable-speed wind energy turbine.
As the gearbox with its mechanical losses and the needfor intensive maintenance is no longer used, the adaptation of the rotor revolutions to the generator speed has toPicture 9: The LF range provides current transducers from 100 A up tobe derived from the low rotor speed.
2000 ARMS.

Publication CH 25104 E (05.05 - 3 - CDH)

The size of the synchronous generator will be governedby mechanical power delivered by the windmill blades. By using the closed-loop principle, a fast response
As this will be low speed and high torque, the use of torque transducer can be made providing short-circuit protecmotors with permanent magnet rotors are ideal for powers tion of the power semiconductors in the inverters – anup to several hundred kW. Larger systems will use a invaluable advantage for wind energy turbines in offshore
synchronous multipole generator with a wound rotor.
areas where the maintenance is difficult and expensive.
A machine with many poles termed a “ring generator” is Current transducers are indispensable components inused in these applications.
modern wind energy turbines. LEM, with more than 30
years experience in the development of transducers,contributes a vast expertise. For many of these applications, LEM provides ready-to-use solutions; others canbe developed and manufactured to customers’ and applications’ demands.

Picture 8: Circuit diagram for the synchronous generator.

COMPACT AC/DC CURRENT TRANSDUCERS : LA and LF Series
www.lem.com