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Design and performance analysis of a 6 MW medium-speed Brushless DFIG

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The paper presents the design and performance analysis of a 6 MW medium-speed Brushless Doubly-Fed Induction Generation (Brushless DFIG) for a wind turbine drivetrain. Two machines with different frame sizes have been designed to show the flexibility of the design procedure. The mediumspeed Brushless DFIG in combination with a two stage gearbox offers a low-cost, low-maintenance and reliable drivetrain for wind turbine applications.
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DESIGN AND PERFORMANCE ANALYSIS OF A 6 MW
MEDIUM-SPEED BRUSHLESS DFIG
E. Abdi, M.R Tatlow §, R.A. McMahon §, PJ. Tavner
Wind Technologies Ltd, St Johns Innovation Park, Cambridge CB4 0WS, UK (info@windtechnologies.com)
§ Electrical Engineering Division, University of Cambridge, Cambridge, CB3 0FA, UK
Keywords: Brushless DFIG, drivetrain, wind turbine.
Abstract
The paper presents the design and performance analysis of a 6
MW medium-speed Brushless Doubly-Fed Induction
Generation (Brushless DFIG) for a wind turbine drivetrain.
Two machines with different frame sizes have been designed
to show the flexibility of the design procedure. The medium-
speed Brushless DFIG in combination with a two stage
gearbox offers a low-cost, low-maintenance and reliable
drivetrain for wind turbine applications.
1 Introduction
The Brushless Doubly-Fed Induction Generator (Brushless
DFIG), also known as the Brushless Doubly-Fed Machine
(BDFM), is an alternative to the well-established DFIG used
in wind turbines [1, 2]. The Brushless DFIG retains the
benefit of utilising a partially-rated converter, but offers
higher reliability, and hence lower cost of ownership, than the
DFIG due to the absence of brush gear and slip-rings [3]. The
Brushless DFIG is intrinsically a medium-speed machine,
enabling the use of a simplified one or two stage gearbox,
hence reducing the overall cost and weight of the drivetrain
and further improving reliability [4]. A schematic of the
Brushless DFIG drivetrain is shown in Fig. 1.
Figure 1: Brushless DFIG drivetrain
The Brushless DFIG has been shown to have superior low
voltage ride through (LVRT) ability without the need for
additional hardware [5]. The machine has an intrinsically
large ‘series’ inductance, and hence experiences a reduced
transient current in the machine side inverter compared to an
equivalent DIFG [5]. As a result, the system cost can be
reduced and the machine side inverter can be utilized to
support the supply of reactive current during the entire fault
cycle, allowing for fast dynamics.
To date, several groups have reported experimental Brushless
DFIGs and there have been attempts to construct larger
machines to confirm the machine’s suitability for higher
power beginning in Brazil with the 75 kW machine of Runcos
et al. [6] and more recently in China with the design of a
200 kW machine by Liu et al [7]. The authors have recently
reported the design and testing of a 250 kW Brushless DFIG
which is believed to be the largest size reported to date [2]. A
6 MW medium-speed Brushless DFIG has now been designed
and the performance of the machine analysed to evaluate
whether a multi-megawatt Brushless DFIG can meet the
performance requirements of large wind turbines; this paper
reports on the design and performance of a 6 MW machine.
Table 1: Benefits of the medium-speed Brushless DFIG
drivetrain
AC
AC
Brushless
DFIG
Simplified 1 or
2-stage gearbox
Medium-Speed
Generator
Fractional
Converter
Low capital cost:
- Elimination of slip-rings, brush-gear and carbon extraction
system in the generator;
-
Utilization of a simplified 1 or 2-stage gearbox;
-
Utilization of a fractionally rated converter;
-
Elimination/simplification of the hardware/software (e.g.
crowbar) used for LVRT protection;
-
Lower drivetrain weight, hence lower structural costs,
compared to typical direct-drive drivetrains.
High reliability:
- Elimination of slip-rings and brush-gear in the generator which
are known to have the highest failure rate in DFIG’s;
-
Elimination of the high-
speed stage of the gearbox which is
known to have the highest failure rate in the gearbox.
Low operation & maintenance (O&M) costs:
- Eliminating the need for regular maintenance and replacement
of brush-gear in the generator;
-
Higher reliability of drivetrain components, reducing the
unplanned maintenance costs.
Improved grid compatibility:
- The intrinsically improved LVRT performance of the Brushless
DFIG due to its higher rotor inductance by design;
- Enhanced reactive current injection capability of the Brushless
DFIG during grid faults.
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2 Description of the Brushless DFIG
The Brushless DFIG comprises two electrically separate
stator windings, one connected directly to the grid, called the
power winding (PW), and the other supplied from a
fractionally rated variable voltage, variable frequency
converter, called the control winding (CW). The pole
numbers of the two stator windings are chosen so as to avoid
direct coupling and a special rotor design is used to couple
between the two stator windings, the nested-loop design being
commonly used. The machine therefore contains three
magnetomotive forces (MMFs), the first in the stator directly
supplied from the mains, the second in the stator supplied
from the converter and the third induced in the rotor. The
normal mode of operation of the Brushless DFIG is as a
synchronous machine where the angular shaft velocity ωr is
determined by the excitation frequency of the PW, f1, and the
CW, f2 [1]
߱ൌʹߨ
ା௙
ା௉
(1)
3 Generator specification and design
The 6 MW rating of the machine was chosen because this size
machine is comparable with the largest DFIG generators
currently in use in commercial wind turbines. An 8/16 pole
Brushless DFIG was chosen which has a natural speed of 250
rev/min when the control winding is fed with DC [1]. To
allow for the range of speeds experienced in a typical wind
turbine, the speed range was set to 150-350 rev/min,
corresponding to a converter output of +/- 20 Hz. This implies
a minimum converter rating of 40 % of the total output, i.e.
2.4 MW for a 6 MW generator [1]. The Brushless DFIG is
designed for a wind turbine with specification shown in Table
1. The generator specifications are also shown in Table 1.
Wind Turbine Specifications
Rotor diameter
130 m
Hub height
140 m
Rotational speed
5.3 11.7 rpm
Gearbox ratio
1:30 (2-stage)
Cut in wind speed
3.5 m/s
Rated wind speed
14 m/s
Cut out wind speed
28 m/s
Generator Specifications
Output power
6 MW
Natural speed
250 rpm
Speed range
150-350 rpm
Nominal voltage
690 V
Grid frequency
50 Hz
Nominal torque
170 kNm
Overall efficiency
> 96 %
Table 2: Wind turbine and generator specifications
The design procedure is shown as a flow diagram in Fig. 2.
During the first stage, an analytical approach in conjunction
with equivalent circuit analysis [8] was employed to achieve
an initial design against machine specifications. Discussion
with a machine manufacturer was essential to incorporate
construction practicalities in the design. Subsequently, the
design was analysed using coupled-circuit analysis as a cross-
check to give a more accurate assessment of the nested loop
rotor [1]. Finally, the design was verified using finite element
analysis. In particular this yields two important outputs,
namely the peak flux densities in the iron and a better
estimate of magnetizing current. The last stage of design
optimisation includes the assessment of system performance,
including dynamics, control and stability and LVRT
capability which led to final adjustments to the design.
Figure 2: Design procedure for the Brushless DFIG system
Taking into account the specifications shown in Table 2, a
design was performed for a 6 MW Brushless DFIG with two
different frame sizes: 1000 and 1200. The machine
parameters and dimensions are given in Table 3. Our design
tools provide flexibility to incorporate practical restrictions
such as size and dimensions, so machines can be customised
for specific wind turbine designs.
Initial Design
Specification
Equivalent Circuit
Analysis
Discussion with
machine
manufacturer
Analytical Design
Software
Finite Element
Analysis
Coupled Circuit
Analysis Thermal Modelling
LVRT & Grid
Connection
Control
Optimization
System Dynamics
Final Design
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Table 3: High level design output for of a 6 MW brushless DFIG
Figure 3: Physical dimensions of 6 MW Brushless DFIG with different frame sizes
4 Performance Analysis
4.1 Equivalent circuit model
The equivalent circuit model is a simple method of
representing the steady-state performance of the Brushless
DFIG [8] and offers a straightforward way of calculating the
efficiency, power factor and other steady-state measures of
the machine to a practical accuracy. One form of the
equivalent circuit for the Brushless DFIG is shown in Fig. 4
where all the parameters are referred to the stator power
winding [8]. The calculated parameters of the equivalent
circuit model are shown in Table 4.
Figure 4: Per-phase equivalent circuit for the Brushless DFIG
Stator
Rotor
Shaft
Cooling Ducts
Stator
Rotor
Cooling Ducts
Shaft
Frame: 1200 Frame: 1000
3300 mm
2000 mm
2400 mm
2400 mm
I
1
R
1
j
Z
1
L
r
R
r
/
s
1
I
r
R
2
s
2
s
1
I
2
s
2
s
12
V
j
Z
1
L
m2
j
Z
1
L
m1n
V
1
Z
1
nnn
n
nn
n
Frame 1000
Frame 1200
Units
690
V
50
Hz
Stator
8
16
Delta
2639
2623
A
1216
1209
A
1700
2100
mm
1320
1678
mm
Rotor
Nested Loop
240
12
10
1098
1413
mm
500
mm
Common
1.5
mm
3074
1894
mm
350/0.65
W/kg/mm
170
kNm
6230
6205
kW
6000
6000
kW
96.3
96.7
%
27
24
tonne
7160
11537
kg.m2
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Parameter
Frame: 1000
Frame: 1200
R1 (mΩ)
1.31
1.23
R2’’ (mΩ)
1.63
1.44
Rr’ (mΩ)
7.3
6.2
Lm2’’ (mH)
3.18
3.9
Lm1 (mH)
5.98
7.34
Lr’ (mH)
0.58
0.61
Table 4: Parameter values for the equivalent circuit
4.2 Efficiency of Brushless DFIG
Fig. 5 shows the efficiency versus load of the 1200 frame
sized, Brushless DFIG. The machine has an efficiency of
96.7 % when operating at rated power and a maximum
efficiency of 97.3 % at 0.6 pu of full load. The decline in
efficiency at partial load is inevitable in induction machines
such as the Brushless DFIG because of the magnetizing
current required to establish the magnetic fields. The
efficiency of the Brushless DFIG can be lifted by utilising
more iron and copper in the machine design, however this
will result in higher weight and cost.
Figure 5: Efficiency versus load for 6 MW Brushless DFIG
(Frame: 1200)
3 Conclusions
A 6 MW medium-speed Brushless DFIG has been designed
to meet current wind turbine specifications for similar power
rating. Analytical modelling and equivalent circuit analysis
have been used during the design process to ensure that the
machine meets the specified requirements.
The paper shows that a medium-speed Brushless DFIG is
practical and scalable to larger (multi-megawatt) powers and
is suitable for wind turbine applications. The Brushless DFIG
offers brushless operation, reduced converter rating and does
not require expensive magnetic material. Hence, a significant
reduction on capital and operational costs is possible using
the generator for wind turbine applications which will feed
through to a lower cost of energy.
References
[1] McMahon, R., Wang, X., Abdi, E., Tavner, P., Roberts,
P. and Jagiela, M.: ‘The BDFM as a Generator in Wind
Turbines’, Power Electronics and Motion Control
Conference, Portoroz, Slovenia, 2006, pp. 1859 1865.
[2] McMahon, R., Abdi, E., Malliband, P., Shao, S.,
Mathekga, M., Tavner, P. ‘Design and Testing of a
Medium-Speed 250 kW Brushless DFIG’, EWEA
Conference Proceedings, Copenhagen, Denmark, April
2012.
[3] Tavner, P.J., Higgins, A., Arabian, H., Long, H. and
Feng, Y.: ‘Using an FMEA Method to Compare
Prospective Wind Turbine Design Reliabilities’,
European Wind Energy Conference 2010 Technical
Track, Warsaw, Poland, April 2010.
[4] Polinder, H., van der Pijl, F.F.A., de Vilder, G.-J. and
Tavner, P.J.: ’Comparison of direct-drive and geared
generator concepts for wind turbines’, IEEE Trans. On
Energy Conversion, 2006, 21, (3), pp. 725-733.
[5] Long, T., Shao, S., Malliband, P., Mathekga, M.E.,
Abdi, E., McMahon, R.A. and Tavner, P.J.:
‘Experimental LVRT Performance of a 250 kW
Brushless DFIG’, EWEA Conference Proceedings,
Copenhagen, Denmark, April 2012.
[6] Carlson, R., Voltolini, H., Runcos, F., Kuo-Peng, P., and
Baristela, N. “Performance Analysis with Power Factor
Compensation of a 75 kW Brushless Doubly Fed
Induction Generator Prototype”, Electric Machines &
Drives Conference, Vol: 2, pp. 1502 1507, 2007.
[7] Liu, H., Xu, L., “Design and Performance Analysis of a
Doubly Excited Brushless Machine for Wind Power
Generator Application”, Power Electronics for
Distributed Generation Systems, pp: 597 601, 2010.
[8] Roberts, P.C., McMahon, R.A., Tavner, P.J.,
Maciejowski, J.M. and Flack, T.J.: ‘Equivalent circuit
for the brushless doubly fed machine (BDFM) including
parameter estimation and experimental verification’, IET
Proceedings Electrical Power Applications, 2005, 152,
(4), pp. 933-942.
0.2 0.4 0.6 0.8 1
95
95.5
96
96.5
97
97.5
Generated Power (pu)
Efficiency (%)
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Performance Analysis with Power Factor Compensation of a 75 kW Brushless Doubly Fed Induction Generator Prototype
  • R Carlson
  • H Voltolini
  • F Runcos
  • P Kuo-Peng
  • N Baristela
Carlson, R., Voltolini, H., Runcos, F., Kuo-Peng, P., and Baristela, N. "Performance Analysis with Power Factor Compensation of a 75 kW Brushless Doubly Fed Induction Generator Prototype", Electric Machines & Drives Conference, Vol: 2, pp. 1502 -1507, 2007.