IEEE 1999 International Conference on Power Electronics and Drive Systems, PEDS’99, July 1999, Hong Kong.
A VARIABLE SPEED CONSTANT VOLTAGE CONTROLLER FOR
SELF-EXCITED INDUCTION GENERATOR WITH
MINIMUM CONTROL REQUIREMENTS
Shashank Wekhande
Vivek Agarwal
Department of Electrical Engineering,
Indian Institute of Technology-Bombay, Powai,Mumbai, India-400076
E-mail: agarwaI@ee.iitb.ernet.in
Abstract: The paper deals with a variable speed, constant
voltage controller for induction generator operating in selfexcited mode. A new PWM controller is proposed to regulate
the induction generator terminal voltage. The proposed
controller regulates three-phase AC output voltage of the selfexcited induction generator with varying rotor speed, transient
load conditions and reactive loads. The proposed scheme does
not require any real time computations for calculating
excitation current, thus m h h i z h g the electronic hardware
and the cost of the contrdler. A simple over-current protection
is incorporated to protect the inverter switches. Computer
simulation and experimentd resnlts shaw satisfactory operalion of
an indu&generatorwiththe proposed contrd scheme.
1. INTRODUCTION:
The squirrel cage induction generator is quite
popular in wind turbine systems [l] due to its simpler
construction. It requires low initial cost and is not costly to
maintain. However, the inductim geaerato: requires a
reactive power source for supplying the excitation current.
The induction generator can be operated in grid
c ~ ~ e c t eord stand-alone self-excited modes. In the selfexcited induction generator, the excitation current is
supplied by the capacitors connected across its terminals.
The terminal voltage is regulated against changing speed
and load conditions, by changing the terminal capacitance.
Effective capacitance can be controlled smoothly by varying
the firing angle of thyristor controlled reactors, connected in
parallel with a k e d capacitor [2]. A three-phase PWM
inverter may also be used as a static reactive power source.
The desired excitation can be controlled by controlling the
modulation index and phase of fundamental inverter voltage
with respect to the generated voltage. Use of PWM inverter
as a reactive power source for induction generator is
reported in [3,4]. All these schemes, however, are based on
load current sensing. The reactive load current and
excitation current required for maintaining constant output
voltage of an induction generator have to be supplied by the
PWM inverter. Complicated high speed electronic circuits
are required to determine reference generator current under
varying load and rotor speed conditions.
In this paper, a simple control scheme is proposed
for self-excitation control of induction generator. The
proposed scheme uses a static P W inverter for controlling
excitation. This-gives a-transient response and smooth
variation of excitation current. The generated voltage is
regulated by controlling the current drawn by the inverter.
The voltage is regulated irrespective of varying rotor speed,
transient load, and reactive loads. The controller does not
require any real time mathematical computation,
minimizing hardware and reducing overall cost. A simple
over-current protection is incwporated to prevent the
inverter switches fiom being damaged
The principle of operation of the proposed
controller and its block diagram are explained in section 2.
The controller has been extensively simulated and the
operation is validated by experimental work. The details of
simulation and experimental work are presented in sections
3 and 4 respectively followed by conclusions in section 5.
2. PRINCIPLE OF OPERATION:
The proposed voltage regulated induction generator
controller uses a hysteresis current controkd PWM inverta
to supply reactive load current and desired excitation current
for induction generator. The proposed scheme is shown in
Fig. 1. The reference current is the resultant of two currents
viz. the in-phase current and the quadrature current. The inphase current overcomes losses in the converter. The
mismatch between the active load current demand and the
generator current is reflected in the variation of DC link
voltage of the controller. The active current can be
controlled by controlling the in-phase current which is
obtained by multiplying the DC link error with reference
template voltage derived fiom the supply voltage. The
quadrature current decides the magnetizing current of the
induction generator. This magnetizing current determines
the generated voltage. The generated voltage is compared
with the reference and the error is multiplied with
corresponding cosine template. The variation in generated
voltage is reflected in ac error voltage and reference
quadrature current. Generation of reference current is shown
in Fig. 2.
Initially, the controller is kept disabled The indudon
gemratur is started in the conventional manner using three
capacitors of fixed value. When SUtFcient voltage is generated,
the controller is &led.
In this scheme, only the inverter
current is sensed. The invehter is protected against overcurrent
by sensing the mat flowing through it and using this
informi#icmto~gatepulsesoftheIms.
0-7803-5769-8/99/$10.000
1999IEEE
98
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1
I
F i g 1:Block diagram ofproposed Induction generator mtroller
3. SIMULATION RESULTS:
The induction generator system with proposed
PWM controller was extensively simulated on the digital
computer using SAE%ER software. Various loading
conditions were simulated to validate the operation of the
PWM controller undm various conditions. These simulation
results are now discussed m e by one for differenct load
conditions:
Resistive load
The induction generator is loaded with balanced
three phase resistive load of lOOR in each phase. The
induction generator is started with 10pF capacitor upto 40
ms while the controller is kept disabled. Subsequently, the
controller is enabled and governs the control. Initially, the
DC link capacitor is charged to 900V.The induction motor
model used in SABER does not have any residual
magnetism. The AC capacitors are assigned some initial
voltage to start the induction generator.
The reference current has two components viz. inphase reference current and quadrature reference current.
The in-phase reference current is responsible for regulating
the DC link voltage and is derived by multiplying the DC
link voltage error with in-phase voltage template derived
fiom the generated voltage. The quadrature reference
current is responsible for regulation of the generated voltage.
This reference current is derived by multiplying the AC
error voltage with a voltage templates leading by 90' w.r.t.
the generator voltages of respective phases. The AC error
voltage is derived by comparing the peak of generated
voltage with a reference signal. The actual reference current
is derived by adding the quadrature and in-phase reference
current components. This is illustrated in Fig. 2. The figure
shows the generated voltage, actual reference, quadrature
reference and in-phase reference current waveforms.
2 0 -r
(A)
(b)
i:O
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om
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*
Reactive load
The induction generator controller regulates the
generated voltage for reactive loads also. In case of reactive
loads, the controller supplies required excitation current to
the induction generator and compensates for reactive Icad
current. The operation of the controller is validated with
balanced three phase I
U load. The RL load comprises of
99
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fixed 70S2 resistance in series with 100 mH inductor. Fig. 4
shows the generated voltage, lagging load current and the
controller current. The controller supplies the leading
current to overcome the lagging load current apart fiom
supplying excitation current to the generator.
I
controller current are shown in Fig. 6. The controller current
decreases with a reduction in excitation current demand.
10
I
II
I55
I6
1.65
I8
1.75
I7
1.85
1.9
TmIE (.)
Fig. 5: Response to step haease in load current (a) Controlla current
(b) Load current (c) Generator line voltage
1 3
2.27
128
L29
=
230
2.31
2.32
*)
Fig, 3: Response to resistive load (a) Geuerated voltage
(b) Controller cunem (c) Load current
40
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J
400
0 O0
40
(A)
00
3 0
. .
4
I S
16
165
.
-
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17
171
.
. .
18
185
,
.
.
.
.
.
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LO
195
19
m 0)
F i g 6: Respom to step d
m in load current (a) Load current
(b) Generated voltage (c) Control!a currect
Fig 4: Response to reactive kad (a) Generated voltage
(b) Load current (c) Coobollex current
Transient load
The induction generator controller is designed to
operate with fluctuating load currents. Also, the use of static
PWM inverter results in a M e r transient response as
compared to the conventional controller with thyristor
controlled reactor. The simulation of this induction
generator based system, shows a constant voltage
irrespective of 33% step increase in the resistive load
current. The initial load resistance is 1000 and is changed
to 66 52 at 1.6s. The simulation results for a step increase in
load current are shown in Fig. 5. The figure shows that a
step increase in load current increases the controller current
instantaneously to support the additional excitation current
demand which regulates the output voltage to the set value
almost instantaneously.
The operation of the induction generator is also
verified for step decrease in load current. This is realised by
suddenly increasing the load resistance fiom 66R to 10052 at
t = 1.6s. The transient load anrent, supply voltage and the
With v q i n g rotor speed
The induction generator is normally used in
applications such as wind or micro-hydro energy generation
with variable rotor shaft speeds. The controller is capable of
regulating the generated voltage within specified variation of
rotor speed. The simulation results for rotor speed variation
fiom -160 rad/= to -200 radsec are shown in Fig. 7. Figure
shows a varying rotor speed, the regulated generated voltage
and controller current waveforms.
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4,
o
ao
..a
,
30
-3Q
1
I
.
24
15
1
16
17
l8
19
3.0
3.1
32
Inas (.)
F i g 7: Response to variation in rotor specd (a) Rotor speed
(b) Generatedvoltage (c) Controllercurrent
(the negative rotor speed mpxsem aaticlockwise rotation of the shaft)
100
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--
Over-current protection
The placement of current sensors directly in series
with the controller, enable the controller to be protected
against over-current. Operation of the controller against
over-current is shown in Fig. 8, where the transient load
current reaches the trip current reference at 1.64 sec. A
mechanical contactor trips the controller and generator
voltage decays as shown in the figure.
1.0
(A)
1
0.0
-LO
4111,
..
..
..
.
.
.
.
--
:1
4 o J
0
..
..
..
..
: I
.
.
.
.
. . . . ....................... . . :.... :.... : . . ..:. . . .
1
i
..
ms
Fig. 9: Reactive load: (a) Generated voltage (b) Load ament
T@
0.0
1.3
1.3s
1.4
1.45
1.5
155
TImE (a
1.6
1.6s
17
1.75
1.8
.
+,. . . .. . . . ...stw
. . . -. . . . ... . . .MPor:-4CU#lms
... . . . .. . . . .
. . . ... . . ... . . . . . . . .
Fig. 8: Overcurrent protection : (a) controller line current (b)Gznerated
voltage (c) Overcurrent sensingsignal
2
. .. .. .
...................
.. .. . ...
- . ............
. .
,
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,-....
.................
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-- ., .. .. ..
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..........................................
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4. EXPERIMENTAL RESULTS:
.
To veri@ the simulation results presented in section
3, a three phase IGBT based experimental prototype has
been developed for a 1 I-P induction generator. The terminal
voltage is regulated at 110 V(rms). An armature controlled
DC motor is used as a prime mover. The proposed scheme
has been experimentally verified for various loads and the
results are presented in this section.
Reactive load
The reactive load test is performed with a balanced
three phase RL load comprising IOOQ resistance in series
with 200 mH inductor. The generated voltage and lagging
load current waveforms are shown in Fig. 9, while, the
generator voltage and w e n t wavefonns are shown in Fig. 10.
Trmient load
The rotor speed is adjusted to 900 rpm at steady
state with 200Q resistance. The three phase balanced
resistive load is decreased from 200R to IOOQ using a
mechanical contactor. After the sudden change in load, the
rotor speed drops to 870 rpm. The increase in load causes
transient decrease in the generated voltage as shown in Fig.
11.
Similarly, the response of the controller to step
decrease in load is shown in Fig. 12. In this experiment,
rotor speed is adjusted to 900 rpm at lOOQ load resistance.
The per phase balanced load resistance is suddenly increased
form lOOQ to 200Q in a step manner. The rotor speed
increases to 925 rpm after transient.
.
.
.
-
.
.
.
.
Fig 10 :Reaaive load (a) Generator voltage (b) Genaaux c u m t
.
.
.
.
. . . .. . . . .. . . . .
.. ... ...
. . .
..
.
.
.
.
.
.
.
.
...
.
...
2
m
%
v
ai2
tow
..
..
M
...
e
.
En\
Fig. 11: Response to step increase in load current
(a) Generator voltage (b) Load current
101
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..
..
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--
..
..
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0 stop
... --
. . . . ..................... .... : .... : ....: .... :....
..
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MPorl48X)mr
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............................
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F . . . . : . . . . : ..............
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...........................
- . . . .
C
Fig. 12: Response to step deaease in load current
(1) Load cumnt ( 2 ) Genexator voltage
Fig 14: Overcumnt protCaion (a) Generator cumnt
(b) Controller current
With v q i n g rotor speed
The rotor speed is increased fkom 900 rpm to 1200
rpm by keeping load constant. A three phase balanced load
of 100 S2 in each phase is c o ~ e c t e dacross the IG. The rotor
speed is increased by increasing the armature voltage of the
DC motor which is used as a prime mover. The generator
voltage remains nearly constant as shown in Fig. 13.
5. CONCLUSIONS:
A new controller for variable speed, constant
voltage operation of induction generator, in self-excited
mode has been presented in this paper. The proposed
controller does not require any on-line computations or any
mechanical sensor thereby reducing the complexity and cost
of the controller. .
The contro!ler has been simulated on digital
computer and the operation is experimentally verified. The
results of simulation and experimental work follow expected
pattern.
.. .. .. .. .- .. .. .. ..
.... ..................... ....:.... :....:....:....
..
..
.. . . . . . . . .
. . . . .
.
.
.
.
.
REFERENCES :
... ...I- . !
. . .
Fig. 13: Response to rarymg rotor speed
(a) Generator voltage (b) Generatorcurrent
Over currentprotection
In this scheme the current flowing in the PWM
converter is sensed. This current is used to provide
overcurrent protection. The overcurrent limit is set to 1.0 A.
The load current is increased above the tripping value and it
is found that the controller disables gate pulses to the P W M
converter. The IG voltage drops in the absence of controller
current and continues to supply the load with the help of
fixed capacitors c~nnectedfor starting purpose. The results
during overcurrent condition are shown in Fig. 14.
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“
103
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