Download generator using instantaneous power control

VOLTAGE CONTROLLER FOR STAND-ALONE INDUCTION
GENERATOR USING INSTANTANEOUS POWER CONTROL
G.V.Jayaramaiah
Energy Systems Engineering,
Indian Institute of Technology Bombay,
Powai, Mumbai - 400 076, INDIA.
vj [email protected]
Abstract-This paper presents the voltage controller for
induction generator (IG), to work with variable speeds
and load, based on instantaneous power control (IPC). It
is based on the concept of controlling the instantaneous
real and imaginary power into the machine. This paper
outlines the basic algorithm of IPC and presents
simulation results of IG performance using Matlab I
Simulink.
Kqwords-Instanfaneous power control; Induction
Generator; Voliage source inverter; Imaginary power
I.
INTRODUCTION
The Induction Generator using conventional squirrel
cage induction machine has number of advantages such
as ruggedness, brushless cage rotor construction,
absence of a separate dc source, better transient
performance and better inherent overfoad protection
These advantages justify the use of IG for stand-alone
applications. Despite all these advantage, the
fimdamental problem with the 'Self-excited Induction
Generator (SEIG) is inability to control the frequency
and i t s terminal voltage under varying load and speed
conditions. SElG requires external reactive power
compensation to maintain the excitation. In a
conventional SEIG with 3 Phase capacitor bank
connected across the machine terminals has been
recognized for many years. The utility of this mode of
operation is limited because it does not provide a
constant voltage and frequency regulation under
variable loads and variable speed operation. Another
problem of this technique i s that the machine can only
achieve and maintain excitation under certain speed and
load conditions. To overcome the poor voltage
regulation of the SEIG, a number of schemes have been
proposed in [1]-[4]. The scheme based on switched
capacitors [11 finds limited application because it
regulates the terminal voltage in discrete steps.
To improve the IG operation, various control
strategies using PWM control of the 1G have been
proposed in [S, 6 andlo], but all of them have some
limitations. Some of these proposals use current
controlled voltage source inverter with capacitor bank
on the stator terminals 151 and field oriented controlled
inverter [6] to control the terminal voltage of the IG
under variable speed and load. Field oriented technique
requires costly and unreliable mechanical position
B.G.Fernandes
Department of Electrical Engineering,
Indian Institute of Technology Bombay,
Powai, Mumbai 400 076, INDIA.
[email protected]
~
sensing systems such as encoders or resolvers. There
are other proposals supplying rcactive current to the IG
by using a static reactive power compensator [5]-[7j
and shunt connected PWM voltage source inverters [SI.
In all these schemes 3 phase capacitor bank is
connected on the stator side. The value of the capacitor
required is inversely proportional to the square of the
prime mover speed. This increases the cost and
complexity of the Hardware because the undesired
influence to the control structure. Another limitation is
that for a given capacitor valuc, self-excitation can only
be achieved and maintained for certain load and speed
conditions.
This paper proposes instantaneous power control for
controlling the terminal voltage of the IG under varying
load and speed without connecting 3 phase capacitor
bank on stator side of IG. Instantaneous power control
is used to control the inverter current references. It is
based on control of instantaneous real and imaginary
power into the machine. It is motivated by the fact that
real power controls the capacitor voltage and the
imaginary power controls the flux of the IG. This
control the reactive power required by the IG as we11 as
load. TPC excitation techniques allow a better frequency
control and better voltage regulation than capacitive
self-excitation technique under varying speed and load
conditions
11,
INSTANTANEOUS POWER CONTROL
ALGORITHAM
The proposed control strategy is based on the
instantaneous power control of the IG. Therefore, some
definitions, that will be used later, are outlined in the
present section. The complex power expression for a
machine in space vector notation can be written as:
s = iv*
(1)
It can be written in two phase stationary frame as:
where:
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(3)
Expanding equation (2) the expressions for the
instantaneous real and imaginary power as defined in
[91:
111.
MATHEMATICAL MODELING OF 3-4
SELF EXCITED INDUCTION GENERATOR
The d-q axes equivalent circuits of an induction
generator (IG) in stationary reference frame are given
in Fig. 1 . The dynamic model of the IG in this frame is
described by the following equations [8, 101:
d
xq'-
dt
d
-Ads
dt
q = v,i,
= 'ds
+ B5'qr
'
-k
BsAdr
+ vBip
- vaip
T, =-(i
3P
4
qr
Adr - Rqridr)
When the instantaneous powers and voltages are
known, thc corresponding currents can be obtained as
fo I lows.
k]=-[;
-"p
V,][J
P
(7)
It is important to emphasize that the value of p and q
are independent of the defined reference frame.
The reference frame which is useful to analyze the
instantaneous real and imaginary power is stationary
reference frame. Using this reference frame, the direct
and quadrature voltages are:
Vd
vq
=o
= [VI = J
-
(8)
where
D=L,L,-L;
Therefore:
* =- P
1,
I4
(12)
(13)
d
-Aqr = vqr+TAq7+BrAq,T- ( o - ~ r P d r (14)
dt
d
-Adr
= vd,+ TA& f Br& + (0 - U, )Aqr (1.9
dt
Therefore:
p = v,i,
= Vqr + Ts'yJ
(9)
L, = Llr + L ,
a =-4
lp
IVI
Where i,, is active current this transmit only active
power to the load side and ip, is the reactive component.
Combining equations (9) and (10) with ( 7 ) , a
transformation that relates active and reactive currents
to the currents at arbitrary reference kame.
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(16)
Vdrl
I 'ds
Figure 2 . Overall block diagram
(b)
Figure I . d-q model of an 1G (a) d-axis (b) q-aus
IV.
For a squirrel cage induction generator, Vqrand Vdr
in equation (14) and (15) are set to zero and select w=O
in equations (12) - (1 5 ) .
The Parameters of 3 4 Self-Excited Induction
Generator [I21 used for simulation study are given in
Appendix .
A . MATHEMA TiCAL MODELLING OF 3- 4 CC -
vsi
The capacitor voltage equation is governed by:
U1
where Vdc is the voltage across the capacitor and id, is
the current passing through it, as shown in Fig. 2. The
set point of Vdcmust be greater than the peak value of
the generated phase voltage. Total dc current idccan be
expressed in terms of inverter switching function as
(sufix e identifies the compensator phase currents)
The three switching functions take the value of 1 if
the upper switch in the given inverter leg is on and the
lower switch is off. Tt is 0 if the lower switch in the
same inverter leg is on, while upper switch is off.
B. MODEL OF THE VOLTAGE SOURCE
INVERTER (VSI)
Phase voltages generated by the inverter are
expressed in terms of switching functions as:
127)
V
Ybn= d'(ZS, - s, - S , )
3
SYSTEM DESCRIPTION OF INDUCTION
GENERATOR AND ITS CONTROLLER
The schematic of the proposed system is shown in
the Fig. 2. It comprises of Induction Generator, load,
and Voltage source inverter with external battery of 12
V. Initially the external battery charges the capacitor to
12 V. The minimum initial capacitor voltage required
for self excitation process of 1G depends on the
parameters of IG and voltage drop across the devices of
the inverter. Self-excitation is the result of the
interaction between the voltage provided by the CGVSI
and the residual flux. The remnant flux is insufficient to
promote the system start-up, an auxiliary DC battery
must be connected to the capacitor to provide the buil&
up energy. When capacitor voltage reaches more than
12 V and flux reaches a desired level the battery is
disconnected and the 1G supplies itself the necessary
energy to control the capacitor voltage,
It is well known that the IG terminal voltage can be
controlled by controlling the magnetizing current. A
CC-VSI has to provide the excitation to the 1G.
Fig.2 shows the block diagram of the proposed
controller in which only two PI controllers and one
hysteresis controller are used. Input to the PI-AC
regulator is the difference between the magnitude of
stator voltage and the ac reference voltage. Its output is
defined as inverter reactive current reference. This
regulator maintains a constant voltage across IG
terminals even when the load is applied and also supply
desired reactive power to the IG.
A capacitor voltage regulator has been implemented
using PI-DC regulator to achieve a high enough
capacitor voltage for proper current controlled inverter
operation. Input to this regulator is the difference
between the actual capacitor voltage and the reference
voltage. The output of the regulator has been defined as
inverter active current reference. This reference current
represents the flow of active power necessary to keep
the capacitor voltage constant.
The inverter instantaneous active and reactive
current references are calculated using equation (1 1).
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These reference currents are compared with actual
currents. The proposed controller uses a hysteresis
current controlled, voltage source inverter to supply the
desired reactive power for induction generator and
active power to the load, if needed during transient
condition.
V.
electromagnetic torque are shown in the Figs. 6(a),
(b) and (c) respectively. The generator speed drops
from 1900 rpm to 1880 rpm due to increase in load.
The frequency also decreases fiom 62 Hz to 60 Hz.
Due to Closed loop CC-VSI controller action using
IPC scheme, the speed and frequency are regulated
to normal value.
SIMULATION RESULTS
Self-excitation is started at P O sec by connecting
the static compensator to the IG terminal. The capacitor
is selected as 1000 micro farads and it is charged
initially to 12V through the battery. IG provides the
required energy to charge the capacitor.
Removal o f Load from 1150+i340) VA to
j50+i210)VA at t=8 sec
The capacitor voltage increases fiom 200 V to 280
V during t = 8 sec to t = 10 seconds due to excessive
power generated by the IG. This excess power is not
absorbed by the load. Hence, the capacitor voltage and
the air-gap flux increases. The closed loop control
regulates the voltage back to the reference voltage of
200 V. The effectiveness of thc proposed voltage
regulator using IPC in controlling the terminal voltage
of a three-phase
SEIG is evaluated by sudden
application and removal of Ioad.
151
u.4
*U
/
2
4
6
8
10
I2
l'ims IF1
Figure 3. Variation of (a) Capacitor voltage (b) Air-gap flux
Application of step load (150+i340WA at t = 4.5 sec
The appIication of load at the stator terminal
causes the capacitor voltage to decrease as shown in
the Fig. 3(a) due to insufficient active power
produced by the IG to meet the active power
demand by the load. Hence, the deficient active
power required by the load is provided by the
inverter. The capacitor voltage must be kept
constant in-order to force the desired currents into
the system. The flux in the machine also decreases
because reactive power produced by the inverter is
not sufficient to meet the sudden Ioad demand as
shown in the Fig. 3(b). Therefore the terminal
voltage of the IG decreases as shown in the Fig. 4.
To overcome the decrease in terminal voltage, a
closed loop control has been implemented using
IPC scheme. This scheme makes the capacitor
voltage, flux in the machine and terminal voltage
constant and is shown in the Figs 3(a), 3(b) and Fig.
4(a) respectively. By the fast action of the closed
loop CC-VSI controller using IPC, the terminal
voltage is maintained within a fraction of a second.
Its zoomed view is shown in the Fig. 4(b). The
induction generator is normally used in applications
such as wind or micro-hydro energy generation.
The IPC controller is capable of regulating the
generator voltage within specified variation of rotor
speed. The speed of the IG, frequency, and
'
-m 7.1
7.10s
1.11
7116
'1.11
7.125
Time IS1
7.135
7.13
714
Figure 4. Variation of (a) Building up generator terminal voltage
(b) Zoomed voltage and current of IG
7.8
765
7.V
7.95
8
8.a
ilmrls)
8.1
1.15
82
8.2G
Figure 5. Variation of Load current
An overshoot of terminal voltage of the 1G is
observed when the load is suddenly decreased. The
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zoomed waveform of terminal voltage and current
of IG for few cycles is also shown in Fig. 4(b). The
Load current waveform are shown in Fig 5 due to
sudden application and removal of load at e4.5sec
and t=8 sec respectively.
through simulation results that the system can operate
over a wide range of variations in load and speed with
good terminal voltage regulation. This increases the
overall system reliability.
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Figure 6 . Variation of (a) Speed (b) Frequency (c) Electromagnetic
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APPENDIX
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1
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0.0096 (H,
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VI. CONCLUSION
applications,” IEEE Transactions on Industry Applications, vol.
(4
A voltage controller is proposed
to excite the
induction generator under varying load and speeds
based on the concept of IPC scheme. This scheme does
not require computations and information regarding
rotor speed for calculating the reactive power. This
minimizes the cost of the controller. It was shown,
35, pp.877-883, 1999.
[l I] P.Krause and O.Wasynczuk,Analysis of Electric Machinery:
New York: IEEE Press,1994.
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