• Power electronic grid connection of PM synchronous generator for wind turbines


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    • Abstract: In the course of a research project focused on wind turbines. in the 10kW range a test platform for permanent magnet (PM) generators has been constructed including a power electronic ... Fig. 1. Machine side of test platform for permanent magnet synchronous generator from 10 kW wind turbine. ...

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Power electronic grid connection of
PM synchronous generator for wind turbines
dr.ir. M. Van Dessel dr.ir. G. Deconinck
DE NAYER Instituut ­ Dept. Industr. Wetensch. K.U. Leuven ­ ESAT / ELECTA
J. De Nayerlaan 5 Kasteelpark Arenberg 10
B-2860 St. Katelijne Waver, Belgium B-3001 Leuven, Belgium
[email protected] [email protected]
Abstract The test platform for PM synchronous generator is used
This paper discusses the configuration and operation of a both for research purposes as well as for educational purposes
power electronic converter used for grid connection of a as part of the laboratory infrastructure for courses on power
permanent magnet generator designed for variable speed wind electronics and electrical drives for engineering students.
turbines. Initially the test platform for permanent magnet Laboratory tests using the platform are part of a distributed
generators is described, followed by the design parameters of the
PM synchronous generator developed for wind turbines in the course on intelligent electrical energy systems, developed by
10 kW range. For optimal grid connection of the generator, a six institutes for higher engineering education in co-operation
topology of the power electronic converter has been chosen using with the research group ELECTA of K.U. Leuven ESAT [2].
an Active Front End mains rectifier supplying the DC link and The machine side of the test platform as shown in fig. 1
Motion Control inverter connected to the generator. Finally the consists of an electromechanical drive train for the permanent
measurement results obtained from grid connection of the
permanent magnet synchronous generator are presented magnet wind turbine generator, driven by a frequency
followed by some conclusions. controlled asynchronous motor for variable speed operation.
The single stage gearbox with ratio 1:3.59 converts the drive
motor speed range 150 - 1500 rpm into generator speed range
I. TEST PLATFORM FOR WIND TURBINE GENERATORS 42 - 418 rpm. The torque sensor is mounted on the motor
side shaft where torque is lowest so that it can be determined
In the course of a research project focused on wind turbines
using an inductive measurement principle with 1% accuracy.
in the 10kW range a test platform for permanent magnet (PM)
The generator rotor position and speed are measured using an
generators has been constructed including a power electronic
incremental encoder connected to the external drive shaft.
converter for grid connection [1].
1 2
3 4
5
Fig. 1. Machine side of test platform for permanent magnet synchronous generator from 10 kW wind turbine.
1. Asynchronous motor 150 - 1500 rpm with built in encoder for speed measurement
2. Inductive torque sensor with 1% measurement accuracy
3. Gearbox 1:3.59 for generator speed 42 - 418 rpm
4. Encoder for rotor position and speed measurement
5. Permanent magnet synchronous generator enclosed in safety cage
1 3
5
2
4
Fig. 2. Electronics side of test platform for permanent magnet synchronous generator including converter for grid connection.
1. Siemens Masterdrive power electronic converter (Active Front End supply unit and Motion Control inverter)
2. Power analyser Yokogawa WT1600
3. Dewetron data acquisition system
4. Wind speed simulator
5. Siemens frequency converter for variable speed control
Fig. 2 shows the electronics side of the test platform, TABLE 1
consisting of following components: DESIGN PARAMETERS OF PM SYNCHRONOUS GENERATOR
- the power electronic converter used for grid connection Generator type Fortis Alize II
of the PM synchronous generator;
Magnet material NdFeB
- the power analyser for electric power measurement and
Active length 140 mm
calculation of generator and converter efficiency;
- the data acquisition system for registration of speed, Air gap width 3.75 mm
torque, temperature, voltage and current measurements; Nominal speed n 300 rpm
- the wind simulation system generating speed setpoints Speed range 25 rpm ­ 350 rpm
from wind speed measurement data; Nominal torque Tnom 275 Nm
- the frequency converter for variable speed control of the
Maximal torque Tmax 340 Nm
asynchronous motor drive.
Nominal electrical power Pe,nom 7.4 kW @ 300 rpm
II. PM SYNCHRONOUS GENERATOR
Maximal electrical power Pe,max 10.6 kW @ 350 rpm
The PM synchronous generator mounted on the test Nominal line current IL,nom 15 A
platform is a prototype. Its electromagnetic design has been Maximal line current IL,max 20 A
optimised for minimum harmonic content of induced voltage
No-load voltage UL 326 V @ 300 rpm
and maximum efficiency using finite element calculations [1].
Number of pole pairs p 6
This generator is built up from standard motor parts in a
totally enclosed housing without cooling ribs and without fan Nominal frequency f 30 Hz
and fan cover. The frame is made of steel Fe370 and is hot Phase resistance Rs 1.36 Ohm
zinc dipped and coated. The magnetic field is supplied by Inductance (direct) Ld 12.5 mH
NdFeB permanent magnets with radial magnetisation. They Inductance (quadrature) Lq 12.5 mH
are mounted on a pole wheel directly driven by the wind
turbine blades using an internal stationary stainless steel shaft
TABLE 2
with rotational speeds up to 350 rpm. The stator is built using SPECIFICATIONS OF FORTIS ALIZE WIND TURBINE
standard stator sheets fitted with 54 slots. The windings
Rotor type 3 blade upwind with fixed pitch
having especially protected class F isolation are three phase
Blade material fibre glass reinforced epoxy
star connected. By means of a specific stator winding design,
magnetic bonding of the pole wheel is nearly eliminated, such Blade length 3.4 m
that the starting torque depends effectively on bearing friction Rotor diameter 7.0 m
only. The rotation of the magnets induces a three phase AC Rotor area 38.5 m²
voltage in the stator windings [3]. Rotor speed 25 - 350 rpm
The main design parameters of the generator and wind Tip speed max. 100 m/sec
turbine are given in tables 1 and 2.
Rated output 10.0 kW
III. CONVERTER FOR GRID CONNECTION Essential for the operation of the AFE are the reactors at
the mains side connection (one in each supply line). They
The power electronic converter providing a grid interface allow transfer of power from the grid to the DC bus and vice
for the PM synchronous generator consists of three main versa (regenerative mode). The AFE reactors also act as
modules: the Active Front End (AFE) supply unit, the DC commutating reactors reducing harmonics and eliminating
link bus with braking chopper, and the Motion Control peak currents at current changeover between power switches
inverter connected to the PM synchronous machine. and free wheeling diodes within the inverter.
The AFE supply unit is basically a line commutated The Motion Control inverter connects the PM synchronous
inverter, using an active rectifier bridge built from IGBT machine to the DC link bus (fig. 4). It consists of a IGBT
power switches (Insulated Gate Bipolar Transistor) as shown bridge inverter and a control electronics board (CUMC). The
in fig. 3. Its 'Control Unit for Supply AFE' or CUSA control 'Control Unit Motion Control' uses vector control algorithms
board is synchronised with the power grid (400 V, 50 Hz) by based upon measured rotor position to control the torque and
means of the Voltage Sensing Board (VSB). The precharging speed of the synchronous permanent magnet machine
resistors Rv limit the peak currents in the supply line that (servomotor or generator as in this application). Setpoints
occur when switching on the IGBT bridge, which acts as a and parameters are entered using the Parameterizing and
rectifier charging capacitors in the DC link bus. This type of Monitoring Unit (PMU) or the serial interface. Optional
converter is available in power ranges up to 250 kW [4]. boards include the interface for the incremental encoder.
///
Fig. 3. Block diagram of Active Front End supply unit.
CUMC
Machine
terminals
Fig. 4. Block diagram of Motion Control inverter.
Current (A) Voltage (V) IV. MEASUREMENT RESULTS FOR THE GRID CONNECTION
600
UL In this section the measurement results for the grid
connection of the permanent magnet synchronous generator
400
using the power electronic converter and test platform
IL
200
described above are presented. Using measured values for
speed (n, unit rpm) and torque (T, unit Nm) from the data
0
acquisition system, the mechanical input power Pm is
calculated online using the motor evaluation function of the
-200 power analyser [7], which implements the equation:
2 n
-400
Pm = T . (1)
60
-600
The generator electrical power Pe is calculated by the
time (s)
power analyser using the 'two-wattmeter' measurement
Fig. 5. Generator voltage and current with IGBT inverter. method and divided by the mechanical input power Pm to
obtain the generator efficiency g.
The complete power electronic converter offers a suited
grid connection of wind power: controllable current to Pe P + P
g = = UW VW (2)
regulate the rotor torque, low harmonic distortion compared Pm Pm
with a diode rectifier and smoothed power injection. Using
this type of line commutated converter it is possible to feed The generator efficiency g is plotted as a function of
the power into the grid at 50 Hz and 400 V, whatever the rotational speed in fig. 7. It reaches values of 85% at nominal
output frequency and voltage of the PM synchronous load and speed. The parameter for the curves is the setpoint
generator, depending on its variable speed [5]. Fig. 5 shows for electromagnetic torque Tem, varying from 25% to 125% of
that by using an IGBT inverter on the generator side, currents nominal torque. All curves show an increase in efficiency
are sinusoidal as a consequence of applying a line voltage with increasing speed. In the Tem range 120 Nm to 300 Nm a
generated using Pulse Width Modulation at 8 kHz, which isn't lower loading of the generator leads to higher efficiency for
the case when a diode rectifier is applied. the same speed. The curve with Tem = 60 Nm is an exception
Fig. 6. shows the step response of the torque control loop of because at 25% load, efficiency is dominated by mechanical
the Motion Control inverter used for the PM generator as losses.
measured by the data acquisition system. To obtain such a The electrical power at the mains side of the converter Pgrid
fast torque response with controlled overshoot, the parameters is calculated by the power analyser using the 'two-wattmeter'
in the vector control block diagram have been optimised measurement method and divided by the generator electrical
using the machine identification procedure of the Motion power Pe to obtain the converter efficiency conv.
Control inverter which provides auto tuning of parameters
[4]. For control strategy purpose, the inverter is programmed Pgrid P13 + P23
conv = = (3)
to load the PM generator in order to obtain maximum power Pe Pe
from the wind. This is achieved by controlling the torque
proportional to the square of the speed [6].
Generator efficiency g
90%
Current (A) Torque (Nm)
350 80%
Tem = 120 Nm Tem = 60 Nm
70% Tem = 180 Nm
280
60% Tem = 240 Nm
210 Tem = 300 Nm
Torque 50%
140 40%
30%
Current 70
20%
0
10%
0%
0 25 50 75 100 125 150 175 200 225 250 275 300 325 350
time(s)
speed n (rpm)
Fig. 6. Step response of torque control loop of Motion Control inverter. Fig. 7. Generator efficiency g as a function of speed n.
Converter efficiency conv 100%
100% Generator efficiency g
90%
90%
80%
80%
70%
70%
Tem = 60 Nm 60%
60%
Tem = 120 Nm n = 150 rpm
50%
50%
Tem = 180 Nm n = 225 rpm
40%
40%
Tem = 240 Nm n = 300 rpm
30%
30%
Tem = 300 Nm n = 350 rpm
20% 20%
10% 10%
0% 0%
0 -1000 -2000 -3000 -4000 -5000 -6000 -7000 -8000 -9000 -10000 0 -2000 -4000 -6000 -8000 -10000 -12000 -14000
Generator electrical power Pe (W) Mechanical input power Pm(W)
Fig. 8. Converter efficiency conv as a function of generator electrical power.
Fig. 10. Generator efficiency g as a function of mechanical input power.
The converter efficiency conv is plotted as a function of
In another set of measurements the generator speed is kept
generator electrical power Pe in fig. 8. For values of Pe in
constant and used as a plot parameter rather than the torque
excess of ­3000 W the efficiency is more than 85%, and stays
setpoint as in previous graphs. The generator efficiency g
above 90% for values of Pe of more than ­4000 W. The
given by (2) is plotted in fig. 10 as a function of mechanical
electrical power is negative due to the chosen machine
input power Pm at constant rotational speed n. Each curve has
reference system where consumed power is treated positive
a maximum value depending on mechanical input power Pm.
sign, delivered power negative.
The curve maximum increases and shifts towards the right
The total efficiency for the grid connection of the
with increasing speed n. The generator efficiency reaches a
permanent magnet synchronous generator is calculated as the
maximum value of 87,4% at maximal speed of 350 rpm.
ratio of the electrical power delivered to the grid Pgrid to the
The generator power factor is defined as:
mechanical input power Pm:
Pe
= , (5)
Pgrid 3 U L I L
= (4)
Pm
where UL and IL are RMS line voltage and RMS line current,
The total efficiency of grid connected generator as a averaged on measurement of two line values. It is plotted in
function of rotational speed n measured on the test platform fig. 11 as a function of generator electrical power Pe . The
for different values of the electromagnetic torque setpoint is absolute value of the power factor increases with increasing
depicted in fig. 9. For lower speeds and thus lower electrical speed and thus higher generated line voltage. At any given
output of the generator, the total efficiency is significantly constant speed the absolute value of the power factor reaches
lower than for generator only due to low converter efficiency. a maximum value for relatively small generated power and
further decreases for larger electrical power output, especially
at lower speeds where the difference in power factor is 20%.
Total efficiency
90% -0.9
Generator power factor
80% -0.8
70% -0.7
60% -0.6
Tem = 60 Nm
50% -0.5
Tem = 120 Nm n = 150 rpm
40% -0.4
Tem = 180 Nm n = 225 rpm
30% -0.3
Tem = 240 Nm n = 300 rpm
20% -0.2
Tem = 300 Nm n = 350 rpm
10% -0.1
0% 0
0 25 50 75 100 125 150 175 200 225 250 275 300 325 350 0 -2000 -4000 -6000 -8000 -10000 -12000
speed n (rpm) Generator electrical power Pe (W)
Fig. 9. Total efficiency of grid connected generator as a function of speed. Fig. 11. Generator power factor as a function of generator electrical power.
Grid power factor grid V. CONCLUSIONS
-1
-0.9
This paper describes the configuration and operation of a
power electronic converter used for grid connection of a
-0.8
permanent magnet generator developed for variable speed
-0.7 wind turbines in the range 10 kW - 40 kW. For optimal grid
-0.6 connection a topology of the power electronic converter has
n = 150 rpm been chosen using an Active Front End mains rectifier
-0.5
n = 225 rpm supplying the DC link and Motion Controlled inverter loading
-0.4
n = 300 rpm the generator. In this way it is possible to feed the generated
-0.3
n = 350 rpm
power into the grid at 50 Hz and 400 V, whatever the output
-0.2 frequency and voltage of the generator, depending on its
-0.1 variable speed. The power factor at the mains side of the
0
converter is independent of the generator power factor. The
0 -2000 -4000 -6000 -8000 -10000 -12000 fact that a power electronic converter initially developed for
Grid electrical power Pgrid
industrial drives can be engineered to efficiently couple a
Fig. 12. Grid power factor as a function of grid electrical power.
specifically developed PM synchronous generator to the grid
The grid power factor is defined as: makes the original result. This type of converter is available
Pgrid in power ranges up to 250 kW and can be used for PM
grid = , (6) synchronous and asynchronous machines. The test platform
3 U L ,grid I L ,grid
including its advanced measurement equipment is one of the
where UL,grid and IL,grid are RMS line voltage and RMS line few reported in literature regarding development of
current at the mains side of the converter, averaged on permanent magnet wind turbine generators. Results for the
measurement of two line values. It is plotted in fig. 12 as a grid connection of the synchronous generator using the power
function of electrical power Pgrid delivered to the grid. Since electronic converter and test platform include the efficiency
the grid power factor is controlled by the Active Front End of curves for the generator, for the converter and total efficiency
the power electronic converter, it is only dependent on the defined as the ratio of the electrical power delivered to the
amount of power delivered to the grid and not on generator grid to mechanical input power.
speed and generated voltage. For values of Pgrid in excess of ACKNOWLEDGMENT
­3000 W the power factor is ­0.95 or higher. Comparing the
figs. 11 and 12 clearly shows the advantage of using the The authors wish to thank the Flemish Institute for
power electronic converter in two stages: the Motion Control innovation through scientific research and technology (IWT)
inverter on the machine side and the Active Front End at the for its funding of the research project HOBU 010159 enabling
mains side allow different power factors. the realisation of the work described in the paper. They also
The total efficiency of the grid connected generator as a thank the 'Associatie K.U. Leuven' for its financial support in
function of mechanical input power for different values of the the project OOF 2003/15 setting up the distributed course on
rotational speed n is depicted in fig. 13. For mechanical input intelligent electrical energy systems.
power less than ­3000 W and thus low electrical output of the REFERENCES
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Matlab Conference 2003, Copenhagen, Denmark, 21 ­ 22 Oct. 2003,
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0%
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[7] Yokogawa Electric Corporation, ''WT1600 Digital Power Meter user's
0 -2000 -4000 -6000 -8000 -10000 -12000 -14000
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