Performance analysis and testing of a 250 kW medium-speed brushless doubly-fed induction generator

Abstract

This study presents the performance analysis and testing of a 250 kW medium-speed brushless doubly-fed induction generator (DFIG), and its associated power electronics and control systems. The experimental tests confirm the design, and show the system's steady-state and dynamic performance and grid low-voltage ride-through capability. The medium-speed brushless DFIG in combination with a simplified two-stage gearbox promises a low-cost low-maintenance and reliable drivetrain for wind turbine applications.

Full text

Published in IET Renewable Power Generation
Received on 13th September 2012
Revised on 22nd February 2013
Accepted on 12th April 2013
doi: 10.1049/iet-rpg.2012.0234
ISSN 1752-1416
Performance analysis and testing of a 250 kW
medium-speed brushless doubly-fed induction
generator
Ehsan Abdi1, Richard McMahon2, Paul Malliband1, Shiyi Shao1, Mmamolatelo Ezekiel
Mathekga2, Peter Tavner1,3, Salman Abdi2, Ashknaz Oraee2, Teng Long2, Mark Tatlow1
1
Wind Technologies Ltd., Cambridge Science Park, Cambridge CB4 0EY, UK
2
Electrical Engineering Division, Cambridge University, 9 JJ Thomson Avenue, Cambridge CB3 0FA, UK
3
School of Engineering, Durham University, South Road, Durham DH1 3LE, UK
E-mail: ehsan.abdi@windtechnologies.com
Abstract: This study presents the performance analysis and testing of a 250 kW medium-speed brushless doubly-fed induction
generator (DFIG), and its associated power electronics and control systems. The experimental tests confirm the design, and show
the system’s steady-state and dynamic performance and grid low-voltage ride-through capability. The medium-speed brushless
DFIG in combination with a simplified two-stage gearbox promises a low-cost low-maintenance and reliable drivetrain for wind
turbine applications.
1 Introduction
The brushless doubly-fed induction generator (DFIG), also
known as the brushless doubly-fed machine (BDFM), is an
alternative to the well-established DFIG for use in wind
turbines [1]. The brushless DFIG retains the benefitof
utilising a partially rated converter, but offers higher
reliability, and hence lower cost of ownership, than the
DFIG because of absence of brush gear and slip-rings [2].
In addition, the brushless DFIG is intrinsically a
medium-speed machine, enabling the use of a simplified
one or two-stage gearbox; hence, reducing the cost and
weight of the overall drivetrain and further improving
reliability [3]. A schematic of the brushless DFIG drivetrain
is shown in Fig. 1.
In most countries, there is now a requirement that wind
turbines stay connected and supply reactive current to the
grid during voltage dips [4]. In doubly-fed machines, the
stator flux is exposed directly to the grid and any voltage
dips will result in a sudden loss of magnetisation,
producing a current surge in the machine side converter [5].
This current is typically large and without appropriate
control strategies and, in most cases, additional hardware
such as a crowbar may cause damage to the converter [6].
The use of crowbar increases system cost and limits
reactive current injection dynamics.
In contrast, the brushless DFIG has been shown to have a
superior low-voltage ride-through (LVRT) capability
without a need for additional hardware [7]. It has an
intrinsically larger ‘series’inductance, and hence
experiences a reduced transient current in the machine side
inverter (MSI) than that of an equivalent DFIG [8]. As a
result, the system cost can be reduced and the MSI can be
utilised to support the supply of reactive current during the
entire fault cycle; hence, giving fast dynamics.
To date, several groups have reported experimental brushless
DFIGs [1, 9–15] and there have been attempts to construct
machines of higher power to confirm the machine’s
suitability for MW power applications [12, 13, 15], but few
details of machine performance have been published.
The modern brushless DFIG concept as a variable speed
drive or generator comprises two electrically separate stator
windings, one connected directly to the mains, called the
power winding (PW), and the other supplied from a
variable voltage and 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 [1]. 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 with the rotor rotating at a
speed determined by the winding pole numbers and the
mains and converter frequencies.
The design of the brushless DFIG is not straightforward as
there are more variables to consider than in a conventional
induction machine. The rotor design has a crucial affect on
the performance of the brushless DFIG and its turns ratio
determines the relative magnitude of the two main fluxes.
The uneven distribution of rotor-loop currents in the widely
used nested-loop design makes the evaluation of the electric
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loading difficult. The definition of specific magnetic loading
in the brushless DFIG is not straightforward because of the
presence of two main fields with different magnitudes and
frequencies. In addition, the optimal allocation of electric
and magnetic loadings between the two stator windings is
not trivial. The analyses presented by the authors in [1]
describe how the above issues can be addressed.
Nevertheless, attempts have been made to manufacture
large brushless DFIGs, beginning in Brazil with the 75 kW
machine of Carlson et al. [12] and more recently in China
with the design of a 200 kW machine by Liu and Xu [13].
The authors’own work on a 20 kW brushless DFIG has
confirmed that a brushless DFIG can be designed to a
specification to work in a wind turbine –an important part
was the implementation of an effective controller for the
machine, which is unstable in the open loop over the
normally used wind turbine speed range [14]. More than
three years’operational experience with this turbine has
given confidence in the proposition of a large scale
brushless DFIG; the 250 kW machine was conceived as a
stepping-stone towards a megawatt (MW) scale brushless
DFIG wind turbine.
2 Generator specification and design
As noted above, the rating of the machine was chosen as an
intermediate step between the 20 kW machine described in
[14] and a machine of 3 MW. The size chosen involves
construction and winding techniques appropriate to larger
machines.
Earlier studies have shown that a two-stage gearbox and a
brushless DFIG form a highly reliable combination [2] and
that a 4/8 pole brushless DFIG is a reasonable generator
choice, having a natural speed of 500 rev/min when the CW
is fed with DC. To allow for the range of speeds
experienced in a typical wind turbine, the speed range was
set to be 320–680 rev/min, corresponding to a converter
output of ± 18 Hz. This implies a minimum converter
rating of 36% of the total output, that is, 90 kW for a
250 kW generator [1]. However, allowance must be made
for magnetising current, power factor compensation and an
overhead for proper operation of a controller. The stator PW
voltage was set at 690 V in conformity with current large
wind turbine practice.
4 and 8 pole windings were adopted for the power and
CWs, respectively, as this arrangement has the benefitof
offering a range of speeds around the natural speed
unconstrained by the limit imposed by the synchronous
speed of the lower pole number winding, 750 rev/min in
this case. The nested-loop type of rotor fabricated with
solid conductors was adopted. This approach gives a good
slot fill factor and does not encounter significant skin effect
issues for the bar size used.
The speed of a brushless DFIG is given by
N=60 f1+f2
p1+p2
(1)
where f
1
and f
2
are PW and CW supply frequencies and p
1
and
p
2
are their pole pair numbers, respectively. Combinations of
pole numbers for the two windings are chosen to avoid direct
transformer coupling. Consideration was given to 2/6, 2/8, 4/8
and 2/10 combinations; the 4/8 arrangement was adopted as it
had been successfully used previously [14] and as it gives a
speed in the desired range. The natural speed of a brushless
DFIG is given by
N=60 f1
p1+p2
(2)
This equates to 500 rev/min for a 4/8 machine. The range of
speeds is determined from wind turbine practice and is 320–
680 rev/min.
The design procedure is shown schematically in Fig. 2.
During the first stage, an analytical approach in conjunction
with equivalent circuit analysis [16] was employed to
achieve initial designs against machine specifications.
Fig. 2 Design procedure for the brushless DFIG system
Fig. 1 Brushless DFIG drivetrain
Table 1 Brushless DFIG specifications
frame size 400
speed range 500 rev/min ± 36%
rated torque 3670 Nm
rated power 250 kW at 680 rev/min
PW pole number 4
PW rated voltage 690 V (50 Hz, delta)
PW rated current 178 A (line)
stack length 0.82 m
IP class IP55
efficiency (full load) >95%
CW pole number 8
CW rated voltage 620 V (18 Hz, delta)
CW rated current 73 A (line)
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Discussion with the machine manufacturer was essential to
incorporate construction practicalities in the design.
Subsequently, these designs were analysed using
coupled-circuit analysis as a cross-check and to give a more
accurate assessment of the nested-loop rotor [17]. Finally,
designs were verified using finite element (FE) analysis. In
particular, this yields two important outputs, namely the
peak flux densities in the iron and a better estimate of
magnetising current. The last stage was to assess the system
performance, including dynamics, control and stability and
LVRT, which led to final adjustments to the design.
3 Machine and test rig construction
The final design was constructed at ATB Laurence Scott as a
frame size 400 machine with a stack length of 820 mm. The
frame was a type normally used for totally enclosed fan
cooled motors. As the brushless DFIG is a variable speed
machine, a fan with its own drive motor was fitted to
ensure effective cooling at all speeds. The stator windings
were form-wound from copper strip. The PW was rated at
690 V, 178 A at 50 Hz and the CW was designed for 620 V
at 18 Hz and rated at 73 A. Both stator windings were
Fig. 3 Brushless DFIG system and its test rig
aSchematic of the test rig
b250 kW brushless DFIG (right front) on test bed
cConverter, comprising GSI (on the left) and MSI (on the right)
dMeasurement electronics, Bluetooth transmitter and rechargeable batteries installed on the rotor shaft [18]
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connected in delta. The rotor comprised six sets of nests each
with multiple loops, the conductors being solid bars with one
common end ring. The machine specifications are shown in
Table 1. The machine was instrumented extensively to
allow assessment of its performance. The stator was fitted
with sixteen temperature sensors and duplicate sets of 4-
and 8-pole flux search coils and search coils around stator
teeth and back iron.
The rotor was also fitted with temperature sensors and tooth
flux search coils, and Rogowski coils for measuring rotor bar
currents. The signals from the rotor sensors, following
amplification and filtering, are digitised and transmitted via
Bluetooth to a static receiver. The electronics on the rotor
are powered by a rechargeable battery [18]. The electronic
hardware installed on the shaft is shown in Fig. 3.
The stator CW is fed from an AC–AC converter comprising
a grid side inverter (GSI), which is an off-the-shelf Control
Techniques’Unidrive SP6601, and a MSI, which is a
Semikron full-bridge Semistack SKS 84F. The GSI has a
built-in controller, which stabilises the DC-link voltage. The
MSI does not have a built-in controller and its gate drives
are controlled by PWM signals provided by the control
system described below. The converter has specifications
provided in Table 2 and is shown in Fig. 3.
Following initial acceptance tests, the machine was set up
at the manufacturer’s facility in the arrangement shown in
Fig. 3. The brushless DFIG was driven via a torque
transducer by a 355 kW 8-pole ABB induction motor fed
from a 400 kW ABB ACS800 inverter. The torque
transducer has an accuracy of 0.1% when measuring
4000 Nm. The PW was connected to the mains through a
690/415 V step-down transformer and the CW was supplied
from the converter. Speed and position signals are obtained
from an incremental encoder with a resolution of 10 000
pulses per revolution. The voltages and currents of each
stator phase are measured by LEM AV100-750 and LEM
LA 205/305-s transducers, respectively, with corresponding
accuracies of 1.5 and 0.8%.
For normal operation, the brushless DFIG operates in
synchronous mode and a controller is needed to ensure stable
operation over the whole of the desired speed range. A vector
controller [19], implemented in a real-time hardware, was
used for this purpose. To achieve effective control, rapid
updating of the frequency and voltage is necessary. As
normal industry standard converter interfaces do not allow
this, the machine side Semikron inverter was controlled by
gate control signals emanating from the real-time controller,
which takes signals from the encoder, voltage and current
transducers described above. The whole test system was
computer controlled. Operation in synchronous, cascade and
simple induction modes was possible and transition to the
synchronous mode was automatic. For more details about the
various operating modes, see [1, 16].
4 Experimental verification of performance
The tests described in Table 3 were carried out to confirm the
output of the machine and to measure losses and efficiency at
various speeds and driving torques, the ultimate being
confirmation of the rating through temperature rise
measurements on both the stator and rotor. The
confirmation of the stability of the controller and machine
capability to ride through grid faults was also an aim.
4.1 Magnetic flux measurements
In order to verify the magnetic design and assess saturation
effects in the iron circuit, stator and rotor flux densities
were measured using flux search coils installed around the
teeth and back iron. Fig. 4 shows the measured stator tooth
flux density compared with predictions from the FE
analysis. The machine was run at 650 rev/min and full load.
The stator windings were supplied at their rated voltages,
hence nominal flux density in the iron circuit was expected.
As can be seen from Fig. 4, the peak flux density in the
tooth is around 1.75 T, which is close to the design
specification of 1.8 T. The predictions from the FE analysis
are in agreement with the measured results.
4.2 Machine parameters extraction
The equivalent circuit model is a simple method for
representing the steady-state performance of the BDFM
[16]. Since the meaning of the parameters has a clear
Table 3 Description of the tests carried out on the brushless
DFIG
Test Description
1 magnetising test tests were carried out in induction
mode at no load, synchronous speed
to derive the magnetising reactances.
Each stator winding was excited in
turn whereas the other left open
2 parameter extraction tests were carried out in the cascade
mode to derive the machine
equivalent circuit parameters
3 steady-state tests tests were carried out at no load, ¼,
½, ¾ and full load conditions and at
different speeds and reactive powers
and steady-state performance of the
machine was recorded
4 long-run temperature
rise tests
the machine was run at full load
condition at the rated speed for a
long time and the stator and rotor
windings temperature rise was
recorded
5 dynamic tests the controller performance and
stability was tested under different
conditions by varying the PW real
and reactive powers
6 LVRT tests several grid fault conditions were
applied to the test rig and the
performance of the system was
assessed
Table 2 Converter specifications
Specifications for the GSI
type control techniques unidrive SP6601
rated voltage, rms 690 V
frequency 50 Hz
rated current, rms 100 A
over-load current, rms 125 A
over-load power 110 kW
Specifications for the MSI
type Semikron Semistack SKS 84F
rated current, rms 84 A (T
amb
= 35°C)
switching frequency 10 kHz
DC-link capacitance 5880 μF/1100 V
DC-link rated voltage 1000 V
cooling fan
thermal trip 71°C
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physical interpretation, the model is useful for understanding
the design and optimisation of the machine. The equivalent
circuit model 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 BDFM is shown
in Fig. 5, where all the parameters are referred to the PW [16].
s
1
and s
2
are the power and CWs slips and are defined as
s1W
v
1−p1
v
r
v
1
(3)
s2W
v
2−p2
v
r
v
2
(4)
where ω
1
and ω
2
are angular frequencies of the supplies to the
PW and CW, respectively, and ω
r
is the rotor mechanical
angular velocity.
The parameters of the equivalent circuit model can be
calculated from the machine geometry at the design stage
using the method described in [17]. The accuracy of these
calculations depends on having good physical data for the
machine. In particular, it is hard to obtain precise estimates
of certain parameters such as the leakage reactances of
end-windings.
The equivalent circuit parameters can also be extracted
from experimental tests using curve-fitting methods as
described in [16]. Using the parameter values estimated
from experimental tests lead to more accurate predictions.
In order to extract machine parameters, experimental tests
were performed in the cascade and induction modes of
operation and the parameters were obtained using a
curve-fitting method, except the stator winding resistances
that were obtained from DC measurements. The estimated
machine parameters from experiments are shown in Table 4.
4.3 Efficiency measurements
The efficiency of the machine measured over a range of
output powers is shown in Fig. 6. The maximum efficiency
at each load was obtained by adjusting the CW excitation.
As can be seen, an efficiency of more than 95% is achieved
over a wide range of operations, which is competitive with
commercial machines of this size. The highest efficiency is
96.1 at 60% load. The stator winding temperature during
the test was measured at 70°C. The variation of efficiency
against load shown in Fig. 6 is similar to that of a typical
induction machine, in that higher efficiency is achieved at
partial loads.
Fig. 7ashows the variation of efficiency over a wide range
of PW reactive powers when the machine was operating at
650 rev/min, 240 kW. The control system varied the PW
reactive power from −0.06 to 0.76 p.u. The negative sign
indicates generating reactive power. The machine’s
efficiency change is not significant, within the range 94–
95.5%. The corresponding PW power factor is shown in
Fig. 7b, which indicates that unity power factor operation
can be achieved.
Fig. 4 Measured and predicted (by FE) stator tooth flux density
Fig. 5 Per-phase equivalent circuit for the BDFM [16]
Table 4 Parameter values for the equivalent circuit
R
1
,ΩL
m1
,mH R
r
′
,ΩL
r
′
,mH R
2
″
,ΩL
m2
″
,mH
0.097 104 0.114 12.5 0.102 53
Fig. 6 Measured efficiency of brushless DFIG at different loads
Fig. 7 Variation of efficiency with PW reactive power variation
aMachine efficiency variation with PW reactive power
bPower factor variation with PW reactive power
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4.4 Long-run temperature rise measurement
The temperature rise of the machine under various loading
conditions was explored. The stator windings temperature
rises when the machine was operating at full load and
680 rev/min is shown in Fig. 8. The PW reactive power was
controlled to be 105 kVAr, so that the currents in both
stator windings were at nominal values. The stator
temperature rise is 105 K, with about a 15 K difference
between the two windings. The stator temperature rise is
within specification for the insulation. This confirms that
the nominal rating of 250 kW is achieved.
4.5 Control system dynamic performance
The control system is able to control the real and reactive
power of the PW under a rapid and stable dynamic
transient, as required in wind turbine applications. In order
to examine the dynamic performance of the control system,
a full power change was applied to the machine whereas the
rotor speed was set via the load machine at 680 rev/min.
The PW real power (P
1
) was changed from 0 to 185 kW
(full load) at the rate of 100 kW/s, equivalent to the total
machine’s output power of 250 kW. The rate of change was
limited by the converter DC-link voltage, since at 680 rev/
min, most of its capacity is already occupied. In fact, in
practical wind turbine applications, the rate of change of
real power is believed to be modest and well below than
that of implemented in the tests [20]. The PW reactive
power (Q
1
) was held constant at 150 kVAr. The results are
shown in Fig. 9. As can be seen from the results, there is
good agreement between the simulation and experimental
results and the required dynamic performance is achieved.
4.6 LVRT tests
The grid code by E.ON was utilised as a reference for
assessing the performance of the brushless DFIG system
under grid low-voltage faults. During symmetrical
low-voltage faults, according to the grid code, wind
turbines are required to: (a) ride through a period of zero
voltage up to 0.15 s and a further period of grid voltage
recovery up to 1.35 s and (b) inject reactive current at
nominal value to the grid during the entire 1.5 s
low-voltage fault [7].
As part of the testing programme, the performance of the
brushless DFIG under grid fault conditions was explored.
Several fault conditions including symmetrical and
asymmetrical faults were examined. Details of the
modelling, control approach and experimental results for
symmetrical grid faults are provided in [7]. No additional
hardware such as crowbar was utilised during the grid fault
tests. The symmetrical three-phase fault often represents the
most severe conditions on the converter current and
therefore results shown in this paper present the brushless
DFIG performance under these conditions.
A fault emulator comprising an auto-transformer and a
number of programmable logic controller (PLC) controlled
contactors were utilised to create zero-voltage fault, that is,
short circuit on the PW. The GSI was connected to the grid
during the LVRT tests to stabilise the DC-link voltage. The
aim of the test was therefore to assess the brushless DFIG
dynamics and performance of the MSI controller [7].
The test results for the LVRT performance of the brushless
DFIG system with a symmetrical three-phase short-circuit are
shown in Fig. 10. The machine was run at 625 rev/min and its
nominal load. A short-circuit fault occurs at t= 1 s, lasting for
2 s. As can be seen from Fig. 10, the surge current in the stator
Fig. 8 Temperature rise of stator windings at full load
Fig. 9 Dynamic performance of the control system, when a full real power change is applied
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CW is lower than 2 p.u. of the IGBTs’rated current; hence no
active compensation voltage was required. Typically,
commercial converters are able to handle transient currents
as large as 2 p.u. of their nominal rating [21]. Without the
obligation of reducing the surge current, the converter is
fully able to provide reactive current. Prior to the fault, the
real current in the PW is at its nominal value and the
reactive current is controlled to be zero, that is, the PW is
operating at its nominal rating with unity power factor.
When the short-circuit fault occurs at t= 1 s, the fault
detector triggers an LVRT controller and a pair of new
targets including rated reactive current and zero real current,
is set [7]. The proposed control loop is able to achieve
these targets with a fast response, less than 40 ms.
The MSI currents were within limits showing that grid
ride through is possible without additional measures or
significantly over-rating of the converter. The LVRT tests
were independently witnessed and a certificate issued to
confirm compliance with grid codes.
5 Conclusions
Tests have confirmed correct brushless DFIG operation, with
currents and flux densities close to expected values. The
250 kW brushless DFIG is, to the best of the authors’
belief, the largest machine of this type ever constructed.
Several modelling approaches have been used during the
design process, including an equivalent circuit model to
perform preliminary optimisation of the design, a
coupled-circuit approach to improve equivalent circuit
parameter values and to verify dynamic behaviour including
harmonic effects and the time-stepping FE model to
confirm the machine’s magnetic behaviour and to obtain
corrections for saturation and harmonic effects.
Testing the 250 kW BDFIG shows that expected
performance is achieved and that the machine can be
controlled stably. CW converter requirements are as
predicted. No undue harmonic components were present in
the stator currents. A direct comparison with the DFIG is
hard as there is very little information on DFIG parameters
available in the literature as DFIGs are usually made
specifically for a particular wind turbine builder.
Differences in speed range further complicate the comparison.
The methods of construction employed in building the
brushless DFIG, such as form-wound stator coils, are
generally suitable for larger machines. The totally enclosed
fan cooled (TEFC) frame was chosen as it was conveniently
available, but would not be used in a MW scale machine
that would have ducted cooling. It is possible that
form-wound coils, instead of solid bars, would be used for
the rotor to limit the consequences of skin effect.
The most important conclusion is that a large scale
brushless DFIG is practical and is scalable upwards. The
Fig. 10 Experimental LVRT performance of brushless DFIG system; the converter current and voltage are the line values; the PW dq currents
are the peak values
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brushless DFIG offers brushless operation, reduced converter
ratings and does not need any expensive magnets. Recent
work indicates that a significant reduction in operational
cost is possible, using a brushless DFIG and this will feed
through to a lower cost of energy.
6 Acknowledgment
The authors would like to acknowledge funding support
from the Carbon Trust.
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