DOUBLY-FED ASYNCHRONOUS MACHINE WITH
3-LEVEL VSI FOR VARIABLE SPEED PUMP STORAGE
A. Sapin, A. Hodder, J.-J. Simond
D. Schafer
Swiss Federal Institute of Technology
Electromechanics and Electrical Machines Laboratory (LEME)
CH-1015 Lausanne, Switzerland
Tel: +41 21 693 56 09
Fax: +41 21 693 26 87
E-mail: alain.sapin@epfl.ch
Alstom Power Generation Ltd
R&D / Technology Director
CH-5242 Birr, Switzerland
Tel: +41 56 466 50 46
Fax: +41 56 466 60 95
E-mail: daniel.schafer@ch.abb.com
ABSTRACT
This paper deals with a fixed-frequency variable-speed
motor/generator with 3-level VSI (Voltage Source Inverter)
cascade. The 3-level VSI is applied to a DASM (Doubly-fed
ASynchronous Machine) for large pump storage plants. The
extension to 3 levels is the original part of this paper. The
whole system behavior has been simulated using the “SIMSEN”
simulation software. This paper focuses on the simulation
results and also on the comparison with the 12-pulse cycloconverter topology.
Keywords: Varspeed, Generator, High Power 3-level VSI,
Doubly-fed ASynchronous Machine DASM,
Adjustable Speed Drive, Vector Control, pump
storage, Hydro Power.
1 INTRODUCTION
Recently, doubly-fed adjustable speed systems have been
applied to pumped storage power plants, because of many
advantages such as adjustable pump input, improved turbine
efficiency and power system stabilization [1]. The studied drive
topology is represented in Fig. 1. The usual topology for this
kind of drives is the cyclo-converter [2]. Due to the recent
improvement of semiconductors, like IGCT (Integrated Gate
Control Thyristor), the 3-level VSI topology can now be used
Fig. 1.
in large adjustable speed drives like DASM for instance. The
use of a 3-level VSI leads to many improvements. The 3-level
VSI only need 1 transformer, 24 hard-driven GTO’s with
diodes and 12 NPC (Neutral Point Clamp) diodes, whereas the
cyclo-converter needs 3 transformers of 3-winding type and 72
semiconductors. Another point is that the 3-level VSI can be
use like a STATCOM (STATic COMpensator) on the network
side, and work with a power factor equal to 1. Depending on
the apparent power of the converter transformer, it can even
supply reactive power. In comparison, the cyclo-converter total
reactive power has to be supplied by the AC grid. To maintain
the required global power factor of the power plant, the
machine has to produce the cyclo-converter reactive power.
This leads to an increased rotor current. The 2-level VSI
cascade control has been presented in [3]. In comparison with
it, the 3-level VSI leads to a lower THD (Total Harmonic
Distortion). A large capacity GTO converter for DASM has
been presented in [4]. The aim of this paper is to present the
performance of the 3-level VSI cascade and to compare it with
the conventional 12-pulse cyclo-converter cascade.
The DASM used in this paper has the following rated values:
•
•
•
•
•
Apparent power:
Line-to-line voltage:
Frequency:
Number of poles:
DC-link voltage:
230 MVA
15.75 KV
50 Hz
2p = 18
4 kV
VARSPEED : Doubly-fed induction motor/generator with 3-level VSI cascade
4 degrees of freedom: the speed set value nset, the stator
reactive current set value isqset, the converter transformer
reactive current set value itqset and the DC-link voltage set
value udcset. The speed regulator (9) calculates the stator active
current set value isdset used by the stator current regulator (11).
The coordinates transform block (10) calculates the stator
active isd and reactive isq currents of the machine, using the θs
angle. The stator current regulator (11) calculates the required
active irdset and reactive irqset rotor current set values. The
coordinates transform block (13) calculates the rotor active ird
and reactive irq currents of the machine using the θr angle. The
rotor current regulator (12) calculates the required rotor output
voltages ucmd and ucmq. After a reverse coordinates transform
(14), the command signals tune the gates control (15) of the
VSI (5). The DC-link voltage regulator (16) calculates the
active current itdset set value used by the transformer current
regulator (17). The coordinates transform block (18) calculates
the transformer primary side active itd and reactive itq currents
using the θs angle. The transformer current regulator (17)
calculates the required transformer output voltages ucmd and
ucmq. After a reverse coordinates transform (19), the command
signals are tuning the gates control (20) of the VSI (4).
2 SYSTEM MODELING AND CONTROL
The studied power circuit and the regulation block diagram are
given in Fig. 2.
2.1 Power part
The Varspeed is made up of a wound rotor asynchronous
machine (1). The fixed frequency stator of this machine is
connected to the AC grid through the main transformer (2). The
cascade converter is connected to the AC grid through the
converter transformer (3). The first 3-level VSI (4) supplies the
DC-link (6). The wound rotor of the machine is fed by another
VSI (4).
2.2 Regulation part
The PLL block (7) gives the electrical stator angle θs, which
corresponds to the position of the stator voltage phasor of the
machine. The position sensor (8) gives the mechanical rotor
angle θm. This mechanical angle, subtracted to θs gives the
electrical rotor angle θr. The control of the machine is based on
2
it
18
itd
d,q
itq
a,b,c
θs
itqset
ucmd 19
d,q
Rit
itdset
ucmd
Rir
ird
irq
14
d,q
ucmq
a,b,c
d,q
CT
Rn 9
7
PLL
n
isdset
θs
4
is
a,b,c
Ru
16
20
Gates
Control
a,b,c
ucmq
12
ucmt
isqset
nset
Converter
Transformer
3
17
Network
Transformer
ucmr
udcset
15
Gates
Control
C1
6
d,q
C2
10
CM
5
1
ASM
ir
Position
Sensor
isd
isq
Ris 11
8
θm
irqset
irdset
a,b,c
13
1. Asynchronous machine with
wound rotor.
2. Main transformer.
3. Converter transformer.
4. 3-level VSI on network
side.
5. 3-level VSI on rotor side.
6. DC-link.
7. Phase Locked Loop (PLL).
8. Rotor position sensor.
9. Speed regulator.
10. Stator coordinates
transform.
11. Stator current regulator.
12. Rotor current regulator.
13. Rotor coordinates
transform.
14. Rotor reverse coordinates
transform.
15. Rotor VSI gates control.
16. DC-link voltage regulator.
17. Transformer current
regulator.
18. Transformer coordinates
transform.
19. Transformer reverse
coordinates transform.
20. Transformer VSI gates
control.
θr
Fig. 2. Studied power circuit and regulation block diagram.
2.3 Modeling and implementation into SIMSEN
The SIMSEN simulation software has been developed to
simulate power systems in details [5]. The whole power circuit
(see Fig. 1.) has been modeled including the mechanical
system. SIMSEN appeared to be a powerful simulation system,
especially when reconnecting the cascade transformer to the
AC grid. This allows to estimate correctly the global power
plant current wave in the main transformer and to take into
account the influence of the VSI commutations on the stator
voltages. As SIMSEN is based on a modular structure, it is very
easy to define the regulation block diagram in details.
For further information: http://simsen.epfl.ch
3 SIMULATION RESULTS IN STEADY-STATE
The studied case is a steady-state operating point in supersynchronous motor conditions (speed = 1.1 p.u., mechanical
torque = -0.8 p.u., machine power factor = 1, transformer
power factor = 1 for the 3-level VSI cascade). This means that
the power factor is set to 1 on the secondary side of the main
transformer with the 3-level VSI cascade. The left column
presents the results of the 12-pulse cyclo-converter cascade and
the right column presents the results of the 3-level VSI cascade.
Simulations with other operating points have also been
performed, but are not included in the present paper.
Fig. 4.
Varspeed with 3-level VSI cascade
Fig. 3.
Varspeed with 12-pulse cyclo-converter cascade
Fig. 5.
Motor phase voltage and current
Fig. 6.
Motor phase voltage and current
Fig. 7.
Spectrum of motor phase current
Fig. 8.
Spectrum of motor phase current
Fig. 9.
Motor air-gap torque and speed
Fig. 10.
Motor air-gap torque and speed
Fig. 11.
Main transformer phase currents
Fig. 12.
Main transformer phase currents
Fig. 13.
Spectrum of main transformer phase current
Fig. 14.
Spectrum of main transformer phase current
Fig. 15.
Cyclo-converter transformer currents
Fig. 16.
Converter current at network side
Fig. 17.
Voltage and current of a thyristor valve
Fig. 18.
DC-link voltages
4 STEADY-STATE RESULTS ANALYSIS
As expected, the 3-level VSI cascade seems to be a very
suitable solution. In comparison with the 12-pulse cycloconverter, the main transformer currents as well as the machine
currents contain higher harmonics orders. The 3-level VSI
cascade has however lower THD (Total Harmonics Distortion)
as the 12-pulse cyclo-converter:
THD (machine current with cyclo cascade):
0.582%
THD (machine current with VSI cascade):
0.449%
THD (main transfo. current with cyclo cascade): 0.937%
THD (main transfo. current with VSI cascade): 0.622%
To reach the above THD values, improved PWM control has
been applied to the modulation shape of the 3-level VSI on the
network side. This control requires 250 Hz switching frequency
of each GTO valve. On the machine side, a 500 Hz constant
switching frequency has been selected. This switching
frequency could even be reduced without so much influence on
the rotor current. Such switching frequencies fulfill the
requirements of the new hard-driven GTO Thyristors. It is
interesting to observe that the 3-level VSI cascade leads to a
reduced main transformer current (see Fig. 11 and 12) due to
the fact that the power factor of the 12-pulse cyclo-converter
cascade is depending on its operating point and cannot be
directly controlled (line-commutation).
5 SIMULATION RESULTS IN TRANSIENTS
The simulated case corresponds to a single phase short circuit
on the high voltage side of the main transformer, occurring
during 60 ms with an initial operating point equal to the one
specified in point 3. The next figures show that the DASM
motor/generator with 3-level VSI cascade stabilizes very
quickly after a voltage drop. The NP (Neutral Point)
unbalanced voltages control has also been implemented. This
control equalizes the two DC-link voltages. The 3-level VSI on
the network side helps maintaining the voltage on the AC side.
This is a helpful feature of this VSI, which works like a
STATCOM (STATtic COMpensator).
Fig. 19.
Fig. 23.
DC-link voltages
Fig. 24.
Main transformer phase currents
Line phase voltages of the machine
6 CONCLUSIONS
Fig. 20.
The 3-level VSI cascade is well suited for the Doubly-fed
ASynchronous Machine (DASM). An example of large power
DASM for variable speed pump storage has been simulated and
compared with the conventional 12-pulse cyclo-converter
cascade. Both simulations have been performed using the
SIMSEN simulation software, which proved to be a powerful
tool to simulate such complex power systems topologies. The
3-level VSI cascade allows reaching very low values of THD
for the machine current as well as for the main transformer
current. These values are even lower as those obtained with the
12-pulse cyclo-converter cascade. The 3-level VSI cascade has
the advantage that not only the machine operates with a high
power factor, but also the converter transformer. It also
demonstrates a stable behavior under ‘ride-through’ conditions.
Stator phase currents of the machine
REFERENCES
Fig. 21.
Rotor phase currents of the machine
Fig. 22.
Air-gap torque and speed of the machine
[1] J.-J. SIMOND, D. SCHAFER, Expected benefits of
adjustable speed pumped storage in the European network,
HYDROPOWER into the next century, October 1999,
Gmunden, Austria.
[2] D. SCHAFER, J.-J. SIMOND, Adjustable Speed
Asynchronous Machine in Hydro Power Plants and its
Advantages for the Electric Grid Stability, CIGRE Report,
Paris 1998.
[3] H. STEMMLER, A. OMLIN, Converter Controlled FixedFrequency Variable-Speed Motor/Generator, IPEC’95,
Japan.
[4] S. FURUYA, F. WADA, K. HACHIYA, K. KUDO, Large
Capacity GTO Inverter-Converter for Double-Fed
Adjustable Speed System, Symposium Tokyo, CIGRE
1995.
[5] A. SAPIN, J.-J. SIMOND, SIMSEN : A modular software
package for the analysis of power networks and electrical
machines, CICEM’95, Hangzhou, China.
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