conception and design of a hybrid exciter for brushless synchronous

Transkrypt

ELEKTRYKA
Zeszyt 1 (213)
2010
Rok LVI
Julien YONG
Institut National Polytechnique de Toulouse
Grzegorz KOSTRO, Filip KUTT, Michał MICHNA, Mieczysław RONKOWSKI
Faculty of Electrical and Control Engineering, Gdansk University of Technology
CONCEPTION AND DESIGN OF A HYBRID EXCITER FOR
BRUSHLESS SYNCHRONOUS GENERATOR. APPLICATION FOR
AUTONOMOUS ELECTRICAL POWER SYSTEMS
Summary. In this paper a hybrid excitation system for a brushless synchronous
generator working with variable speed in an autonomous energy generation system (e.g.
airplane power grid) is presented. A conception of a dual-stator hybrid exciter is
proposed. Comparison study of classical and hybrid exciter has been carried out. For the
electromagnetic calculation two approaches have been applied: analytical approach
(based on the circuit model and sizing equations) and numerical approach (using field
simulator FLUX2D). Provisional design calculations have been performed using the
analytical approach. Next, to verify the calculation results and to optimise the magnetic
and electric circuit of the machine, the field simulator FLUX2D has been used.
Keywords: brushless synchronous generator, hybrid excitation, autonomous electrical power system
KONCEPCJA I OBLICZENIA HYBRYDOWEJ WZBUDNICY
BEZSZCZOTKOWEGO GENERATORA SYNCHRONICZNEGO.
ZASTOSOWANIE W AUTONOMICZNYCH SYSTEMACH
ELEKTROENERGETYCZNYCH
Streszczenie. W artykule zaprezentowano hybrydowy układ wzbudzenia
bezszczotkowego generatora synchronicznego, pracującego ze zmienną prędkością
obrotową w autonomicznych systemach generacji energii (np. sieć elektroenergetyczna
na pokładzie samolotu). Zaproponowano koncepcję wzbudnicy hybrydowej z podwójnym
stojanem. Wykonano analizę porównawczą wzbudnicy klasycznej i hybrydowej.
Obliczenia elekromagnetyczne wykonano dwoma sposobami: analitycznie (oparte na
modelu obwodowym i zaleŜnościach wymiarowych) i numerycznie (zastosowano
symulator polowy FLUX2D). W pierwszym etapie wykonano obliczenia wstępne metodą
analityczną, a następnie do weryfikacji wyników obliczeń wstępnych i optymalizacji
maszyny zastosowano symulator FLUX2D.
Słowa kluczowe: bezszczotkowy generator synchroniczny, wzbudzenie hybrydowe, autonomiczne
systemy elektroenergetyczne
8
J. Yong, G. Kostro, F. Kutt, M. Michna, M. Ronkowski
1. INTRODUCTION
Some of the European Union countries have launched a project MOET (More Open
Electrical Technologies) [15]. The MOET is charged with establishing a new industrial
standard for the design of electrical system for commercial aircraft (business and regional and
rotorcraft as well). The MOET is comprised of 46 companies (13 being SMEs) and 15
Research Centers or Universities from 14 European countries (one of these is Politechnika
Gdańska/Gdansk University of Technology). The project is coordinated by AIRBUS France.
This integrated project is partially funded by the EU, through FP6 and the Framework of the
Aeronautics Industry R&D Objectives.
In future aircrafts, all of the pneumatic, hydraulic and mechanical devices have to be
transformed into electromechanical devices. As a consequence, the electric power demands
will be increased at the aircraft power grid. Thus large power generators are needed to supply
the increased power demands. As a result, the bus voltage is increased from 115/200V to
230/400V.
The aircraft power grid is different than a classical global grid. It is autonomous, and thus
its reliability is an important point. Moreover, the control of this power grid is also an
important point because it has to respect hard specifications to supply electrical devices.
Nowadays large aircrafts (e.g. A330 with 300 passengers) have a total electric power demand
about 250 kW [9]. If all the functions (air conditioning, deicing, e.t.c.) of the future aircraft
have to be supplied by electrical energy, then the total demands of electric power is about
1MW. Thus a new generators have to be designed in order to supply such a large power
demands. Moreover, the variable frequency power system has been applied and tested [2]. For
the power system of the A380 a 3-phase variable-frequency generator has been designed [3].
It can also work as the engine starter in order to put together two functions, .i.e., it is called
starter/generator (S/G). In generator mode the generator is driven by aircraft engine through a
gearbox and provides 200 kW at 230/400 V with variable frequency from 360 to 800 Hz.
The S/G has been designed as a combined power generation device, i.e., as a three-stage
machine topology. It is composed of three electrical machines: subexciter, brushless exciter
and synchronous S/G itself, as shown in the Fig. 1.
As a subexciter a permanent magnet generator (PMG) is used to supply the field winding
of the brushless exciter. The control unit, called generator control unit (GCU), ensures voltage
control and usual protections. In case of the variable speed S/G operation, to ensure the
voltage control (rms and frequency values) requirements of the power grid, the S/G excitation
system has to be properly design.
In aeronautic applications the volume and weight of the power generation devices are a
key issue. As it is well known, a high speed power generation devices will have the volume
and weight smaller than a low speed device. On the other hand, as consequence of the high
Conception and design of a hybrid…
9
speed operation, the centrifugal forces on the rotor and the limitation of the rotor and shaft
diameters are increased [4]. Moreover, you have to consider an increased hysteresis losses,
skin effects and eddy currents.
GCU
field
field
PM
armature
armature
Subexciter (PMG)
PMG)
324VA
324VA
Exciter
3.06kVA
06kVA
Rotating
rectifier
Main Generator
200kVA
200kVA
Fig. 1. Autonomous power generation system (e.g. airplane power grid) working with variable speed
– a system based on a three-stage electrical machine topology: permanent magnet generator
(PMG) – machine working as subexciter, synchronous machine with stationary field winding
and rotating diode rectifier – machine working as brushless exciter, synchronous machine with
rotating field winding – machine working as main starter/generator, generator control unit
(GCU) [3, 9]
Rys. 1. Autonomiczny system generacji energii (np. sieć elektroenergetyczna samolotu), pracujący ze
zmienną prędkością obrotową – system bazujący na topologii trójstopniowej maszyny
elektrycznej: generator z magnesami trwałymi (PMG) – maszyna pracująca jako
podwzbudnica, maszyna synchroniczna z nieruchomym uzwojeniem wzbudzenia i wirującym
prostownikiem diodowym – maszyna pracująca jako wzbudnica, maszyna synchroniczna z
wirującym uzwojeniem wzbudzenia – maszyna pracująca jako główny rozrusznik/generator,
układ regulacji generatora (GCU) [3, 9]
It is also possible to decrease the volume and weight of an electric power generation
device by optimizing its topology, for example using a hybrid excitation system [1, 5, 7, 12].
In this system, the excitation flux is produced by two sources: permanent magnets (PM) and
DC field winding to adjust the excitation flux; in order to reinforce or to weaken the magnetic
excitation flux.
High power/torque density and high efficiency are two of the most desirable features of
an electrical machine for the aircraft power grid application. Improvement of these features
has been one of the main aspects of research work on the electric machines in the last several
decades.
In this paper, in order to provide a solution to this problem, a conception of dual-stator
hybrid exciter for variable-speed brushless synchronous starter/generator has been proposed.
To get the goal the comparison study of classical and hybrid exciter has been carried out. For
the electromagnetic calculation two approaches have been applied: an analytical approach
10
(based on the circuit model and sizing equations) and a numerical approach (using field
simulator FLUX2D). A provisional design calculations were done using the analytical
approach. Next, to verify the calculation results and to optimize the magnetic and electric
circuits of the machine under study the field simulator FLUX2D was used.
2. SUBEXCITER – PERMANENT MAGNET GENERATOR
To make the system (Fig. 1) self-contained and free of sliding contacts, the excitation
power for the GCU is obtained from the stationary armature of a small ac permanent magnet
generator (PMG) also driven from the main shaft. The voltage and frequency of the subexciter
are chosen so as to optimize the performance and design of the overall system.
Two ac PMG structures have been compared: a classical structure machine with a single
rotor (Fig. 2) and a structure with a dual-rotor (twin rotor) (Fig. 3) [8, 11, 13]. While using a
dual-rotor machine for the subexciter you can more effectively optimize the volume, weight
and power losses of a power generation device – very important features for the aircraft
applications. This machine is constituted of an internal rotor and an external rotor with PM
producing the radial excitation field. The slotless stator/armature is placed between the two
rotors. Due to the shorter endwinding the total length of the machine and the winding losses
are reduced comparing it with the classical structure.
The comparison study was done for six structures of PMG, i.e., three structures for the
classical single rotor machine and three structures for the dual-rotor rotor machine. The aim of
this study was to compare the single rotor machines with the dual-rotor machines for choosing
the optimized one for the subexciter of the S/G. The machine parameters that are assumed to
be varied are the number of pole pairs p.
For the electromagnetic calculation of the PMG we have applied two approaches: an
analytical approach (based on the circuit model and sizing equations) [6] – implemented in
the MathCAD and Excel software [10], and a numerical approach – using field simulator
FLUX2D [14]. In order to easy modify the values of machine parameters the both approaches
are parametric. A provisional design calculations were done using the analytical approach.
Next, to verify the calculation results and to optimize the magnetic and electric circuit of the
machine the field simulator FLUX2D was used. The study has been done for the following
number of pole pairs: p = 2, 3 and 4.
As it is known, the increased number of poles is very benefit for the geometry of the
machine because it results in a smaller and lighter machine. However, the number of pole
pairs is limited by technology constraints. Moreover, the large number of pole pairs increases
the frequency of the currents – skin effect can appear in conductors), and also the iron losses
can be increases. On the other hand, to minimize the amplification of the voltage/current
11
harmonics you have to avoid the same number of pole pairs for the subexciter and for the
exciter in the system shown in Fig. 1.
The assumed input data of the subexciter/PMG for the calculations are given in Tab. 1.
Table 1
The assumed parameters and input data
of the subexciter/PMG for the provisional calculations
Parameters
Rated apparent power
Value
324 V·A
Rated armature Voltage
20 V
Rated armature current
5.4 A
Rated armature emf
21.4 V
Number of phases
3
Phases connection
Y
Armature current density
6
5·10 A/m2
Electric loading
16000 A/m
Permanent magnets: Goudsmit
Magnetics Gss20 (SmCo) [16]
Br=0.9 T
Hc=685 kA
Basic operation speed
7600 rpm
Chosen results of the analytical calculations are given in Tab. 2 and that for the field
simulations are given in Fig. 2 and Fig. 3.
Referring to the above consideration and the calculation results given in the Tab. 2 we
can get some conclusions. The choice of the single rotor machine with three pole pairs (p = 3)
seems to be the best solution, keeping in mind that choice of the PMG structure for the system
(shown in Fig. 1) essentially depends upon the constraint of space or/and weight. The
structure of the four pole pairs has not been considered, because it was chosen for the exciter
to avoid the risk of the amplification of the voltage/current harmonics.
The single rotor PMG cane be further optimized because the maximal level of flux
density in the core of the yoke is about 0.8 T (Fig. 2) – it can be increased to 1.2 T.
3. EXCITER – REVERSED SYNCHRONOUS GENERATOR
At the heart of the system shown in Fig. 1 are silicon diode rectifiers, which are mounted
on the same shaft as the main generator field and which furnish dc excitation directly to the
field of the main generator. An ac exciter with a rotating armature feeds power along the shaft
to the revolving rectifiers. The stationary field of the ac exciter is fed through a GCU which
controls and regulates the output voltage of the main generator. The voltage and frequency of
the ac exciter are chosen so as to optimize the performance and design of the overall system.
12
Table 2
Specification of the parameters and calculation results
for the subexciter based on PMG with single rotor and dual-rotor
Parameters
Number of pole pairs
Armature emf
Armature current
Electric loading
Length of active parts
External diameter of machine
Volume of active parts
Mass of active parts
PMG with single rotor
2
3
4
21.6
20.2
21.2
5.4
5.4
5.4
16000 16000 16000
50
45
45
87
85
84
290
252
240
1.84
1.54
1.35
Armature emf
Armature current
Electric loading
PMG with dual-rotor
2
3
4
22
23.5
21.9
5.4
5.4
5.4
11870 13030 13150
56
50
65
90
82
69
363
265
247
2.36
1.58
1.42
Fig. 2. 2D distribution of magnetic field lines for
single rotor PMG (p = 3)
Rys. 2. 2D rozkład linii sił pola magnetycznego
dla generatora z magnesami trwałymi
o pojedynczym wirniku (p = 3)
Unit
V
A
A/m
-3
·10 m
-3
·10 m
-6 3
·10 m
kg
V
A
A/m
-3
·10 m
-3
·10 m
-6 3
·10 m
kg
Fig. 3. 2D distribution of magnetic field lines for
dual-rotor PMG (p = 3)
Rys. 3. 2D rozkład linii sił pola magnetycznego
dla generatora z magnesami trwałymi
o podwójnym wirniku (p = 3)
13
Time delays in the response to a controlling signal are all short compared with the time
constant of the main generator field.
The structure of the ac exciter is analogous to a reversed synchronous generator (RSG) or
classical dc machine, i.e., the armature is on the rotor and the filed excitation is on the stator.
Since you need ac excitation field when the main generator works as a starter for the aircraft
engines, thus the stator yoke of the RSG has to be made of iron sheets in order to avoid eddy
currents.
The RSG parameters that are assumed to be varied are the number of pole pairs. The
comparison study was done for two structures (p = 2 and p = 4) – the structure for p = 4 is
shown in Fig. 4. The aim of this study was to chose the optimized one for the exciter. The
structure of the three pole pairs (p = 3) has not been considered because it was chosen for the
main generator [3]. To avoid the risk of mechanical resonance, in the system shown in Fig. 1,
the number of pole pairs has to be different.
A provisional design calculations of the exciter/RSG were done using the analytical
approach. The assumed parameters and input data for the provisional calculations are given in
Tab. 3. In the next step, to verify the provisional calculation results and to optimize the
magnetic and electric circuit of the machine, the field simulator FLUX2D was used.
simulations are given in Fig. 4.
Referring the above consideration and the calculation results given in the Tab. 4 we can
get some conclusions. The choice of the RSG with four pole pairs (p =4) seems to be the best
solution, keeping in mind that choice of the exciter structure for the system (shown in Fig. 1)
essentially depends upon the constraint of space or/and weight. Thus it was chosen as
machine for the hybrid excitation structure for the brushless S/G shown in Fig. 1.
Table 3
The assumed parameters and input data of the exciter/RSG
for the provisional calculations
Parameters
Rated armature emf
Number of phases
Phases connection
Electric loading
Value
3060 V·A
15 V
68 A
18.7 V
3
Y
5·106 A/m2
16000 A/m
7600 rpm
14
Table 4
Specification of the parameters and calculation results for exciter based
on reversed synchronous generator (RSG)
Parameters
Reversed synchronous generator
2
4
Armature emf
Armature current
Electric loading
Length of actives parts
Internal diameter of rotor
a)
17.4
68
16000
54
280
38
17.5
3280
17.7
68
16000
36
300
103
11.7
2525
Unit
V
A
A/m
-3
·10 m
-3
·10 m
-3
·10 m·
kg
-6 3
·10 m
b)
Fig. 4. 2D distribution of magnetic field for reversed synchronous generator (p = 4): a) field lines,
b) flux density
Rys. 4. 2D rozkład pola magnetycznego dla odwróconego generatora synchronicznego (p = 4): a) linie
sił, b) indukcja magnetyczna
15
4. HYBRID EXCITER – HYBRID SYNCHRONOUS GENERATOR
4.1. Preliminary considerations
It is also possible to decrease the volume and weight of an electric power generation
device by optimizing its topology, using a hybrid synchronous generator (HSG) [1, 5, 7, 12]
as a hybrid exciter (HE). In the HSG the excitation field is produced by two sources:
permanent magnets (PM) and dc field winding to adjust the excitation flux. The aim of the
HE is to provide the excitation power to the main generator in the system shown in Fig. 1.
Generally, the two sources of excitation field are connected in parallel or in series. An
example of a synchronous generator with a hybrid excitation with a field sources connected in
parallel is shown in Fig. 5 [5]. Since the system with the field sources connected in parallel
has more advantages [1, 5, 7, 12] than the system with the field sources connected in series, it
will be considered as a brushless exciter for the main generator in the system shown in Fig. 1.
To design the HE we will consider a combination of the reversed PMG and the reversed
SG as only one machine. The HE structure is similar to the dual-rotor PMG, but with the
reversed function of the stator and rotor, i.e., the HE has a dual-stator and single rotor. On the
external stator the field excitation winding is placed (analogues to stator of the RSG in Fig. 4),
and on the internal stator the PM are placed. The armature winding is placed on the rotor –
similarly as in the RSG shown in Fig. 4. The voltage in the armature winding is induced both
by PM and field excitation winding. As a base for HE design the reversed SG with four poles
(p = 4, Fig. 4) will be taken, since there is a lot of space to put the PM on its rotor. The
proposed electromagnetic structure of the considered HE is shown in Fig. 6.
Fig. 5. Structure of hybrid synchronous generator with a field sources connected in parallel [5]: 1 –
stator yoke, 2 – permanent magnets, 3 – field winding, 4 – pole shoes
Rys. 5. Struktura generatora synchronicznego o wzbudzeniu hybrydowym równoległym: 1 – jarzmo
stojana, 2 – magnesy trwałe, 3 – uzwojenie wzbudzenia, 4 – nabiegunniki
16
Fig. 6. Structure of the hybrid synchronous generator with dual-stator (p = 4) – excitation field
produced by stationary PM and field winding: 1 – rotor core, 2 – armature winding, 3 – field
winding, 4 – external stator core, 5 – permanent magnets, 6 – internal stator core
Rys. 6. Struktura generatora synchronicznego hybrydowego z podwójnym stojanem (p = 4) – pole
wzbudzenia wytwarzane nieruchomymi magnesami trwałymi i uzwojeniem wzbudzenia:
1 – rdzeń wirnika, 2 – uzwojenie twornika, 3 – uzwojenie wzbudzenia, 4 – rdzeń stojana
zewnętrzny, 5 – magnesy trwałe, 6 – rdzeń stojana wewnętrzny
For the HSG you can consider two solutions: a) field winding is the main source of
excitation flux, b) permanent magnets are the main source of excitation flux. The second
solution is more effective for the aircraft power grid, since using larger volume of PM, than
for the first solution, you can significantly decrease the total volume of the field source to
produce the required flied excitation flux. It results in smaller dimensions of the overall power
system shown in Fig. 1.
Since for the second solution the field winding is generally used to control the field flux
linked with the armature winding, the key issue is to chose a proportion between the field flux
produced by PM and field winding. To find this proportion you have to take into account that
brushless S/G is working with variable speed in the aircraft power grid (Fig. 1). Usually it is
assumed that: the field winding is at a maximum level excitation for the minimum S/G speed,
at the lowest level of excitation fort the maximum S/G speed, and for the basic S/G speed the
excitation field is only produced by PM. In fact for the field winding the lowest excitation
level has the same value as for the highest excitation level – but in the negative way. Thus the
voltage induced (EMF) in the armature winding by the excitation field at the three S/G speeds
are assumed to satisfy the following equations:
(Ψ pm +Ψ fw ) N min = Ψ pm N b ,
(1)
17
(Ψ pm −Ψ fw ) N max = Ψ pm N b ,
(2)
where, Ψ pm ,Ψ fw – linkage flux excited by PM and field winding, respectively, Nmin, Nmax,
Nb – minimum, maximum and basic speed of the S/G generator, respectively.
Appling the above rule the value of the basic speed is determined by expression:
Nb = 2
N max N min
,
N max + N min
(3)
4.2. Electromagnetic calculations of the hybrid exciter
Referring to above consideration a provisional design calculations of the HE were done
using the analytical approach. The assumed parameters and input data for the provisional
calculations are given in Tab. 5. In the next step, to verify the provisional calculation results
and to optimize the magnetic and electric circuit of the machine, the field simulator FLUX2D
was used.
Table 5
The assumed parameters and input data of the hybrid
exciter for the provisional calculations
Parameters
Value
3060 V·A
15 V
68 A
Rated armature emf
Number of phases
18.7 V
3
Phases connection
Y
6
5·10 A/m2
Electric loading
16000 A/m
Permanent magnets: Goudsmit
Magnetics Group Gss20 (SmCo)
Br=0.9 T
Hc=685 kA
7600 rpm
simulations are given in Fig. 7.
18
a)
b)
Fig. 7. 2D distribution of magnetic field for hybrid synchronous generator (p = 4) – both excitation
fields are active: a) field lines, b) flux density
Rys. 7. 2D rozkład pola magnetycznego dla generatora synchronicznego hybrydowego (p = 4) –
czynne oba pola wzbudzenia: a) linie sił, b) indukcja magnetyczna
Table 6
Specification of parameters and calculation results for the subexciter, exciter
and hybrid exciter
Parameters
Armature emf
Armature current
Electric loading
Speed assumned for calcualtions
Hibryd
Subexciter Exciter PMG +
exciter
PMG
RSG
RSG
HSG
3
4
3/4
4
Unit
20.2
17.7
17.3
V
5.4
68
68
A
16000 16000
16000
A/m
10800
rpm
7600
7600
85
300
45
36
0.12
1.43
1.55
0.7
kg
Mass of iron (core)
1.3
9.5
10.8
5.42
kg
Mass of permanent magnets
0.1
0
0.1
0,8
kg
1.54
11.7
13.24
7
kg
252
2525
2777
Mass of copper (winding)
205 ·10-3 m
56 ·10-3 m
1900 ·10-6 m3
19
The calculation results summarized in Tab. 6 shown that hybrid exciter (integrated
brushless exciter system) using hybrid synchronous generator is more efficient than the
classical one based on the two separated machines. Particularly it is characterized by
compactness, i.e., its mass/volume is relatively small, and high efficiency due to short end
connections of the winding.
5. CONCLUSIONS
The proposed hybrid exciter – for the brushless synchronous generator working with
variable speed in an autonomous energy generation system (e.g. airplane power grid) – is a
modified version of the dual-rotor permanent magnet generator. Similarly to the dual-rotor
generator it is characterized by high efficiency (due to short end connections of the winding
and hybrid excitation) and compactness (its mass/volume is relatively small).
Due to the application of two approaches – analytical approach (based on the circuit
model and sizing equations) and numerical approach (using field simulator FLUX2D) – to
designing and magnetic field analysis the calculations of the hybrid exciter are very reliable.
We can avoid the costs of building expensive prototypes of the machine under study.
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Recenzent: Prof. dr hab. inŜ. Andrzej Demenko
Wpłynęło do Redakcji dnia 20 marca 2010 r.
Omówienie
W artykule zaprezentowano hybrydowy układ wzbudzenia bezszczotkowego generatora
synchronicznego, pracującego ze zmienną prędkością obrotową w autonomicznych systemach
generacji energii (np. sieć elektroenergetyczna na pokładzie samolotu, rys. 1). Hybrydowy
układ wzbudzenia (wzbudnica hybrydowa) tworzy maszyna synchroniczna ze wzbudzeniem
elektromagnetycznym i magnesami trwałymi. Wzbudnica hybrydowa zasila uzwojenie
generatora synchronicznego za pośrednictwem wirującego prostownika diodowego.
Zaproponowano koncepcję wzbudnicy hybrydowej z podwójnym stojanem (rys. 6).
Wykonano analizę porównawczą wzbudnicy klasycznej i hybrydowej. Obliczenia
elekromagnetyczne wzbudnicy klasycznej i wzbudnicy hybrydowej wykonano dwoma
21
sposobami: analitycznie (oparte na modelu obwodowym i zaleŜnościach wymiarowych)
i numerycznie (zastosowano symulator polowy FLUX2D). W pierwszym etapie wykonano
obliczenia wstępne metodą analityczną, a następnie do weryfikacji wyników obliczeń
wstępnych i optymalizacji maszyny zastosowano symulator FLUX2D. Wyniki obliczeń
przedstawiono kolejno dla: generatora z magnesami trwałymi o pojedynczym wirniku (tab. 2,
rys. 2 i rys. 3), odwróconego generatora synchronicznego (tab. 4 i rys. 4) i generatora
synchronicznego hybrydowego (tab. 6 i rys. 7).
Przedstawiona koncepcja wzbudnicy hybrydowej charakteryzuje się wysoką sprawnością
(obniŜonymi stratami, dzięki skróconym połączeniom czołowym uzwojeń i wzbudzeniu
hybrydowemu) i kompaktową budową (względnie małą wagą/objętością części aktywnych,
dzięki zastosowaniu wzbudzenia hybrydowego i podwójnego stojana).

conception and design of a hybrid exciter for brushless synchronous

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