Subramanian Kulandhaivelu et. al.

/ International Journal of Engineering Science and Technology

Vol. 2 (12), 2010, 7215-7224

VOLTAGE REGULATION OF 3-Ø SELF

EXCITED ASYNCHRONOUS
GENERATOR
SUBRAMANIAN KULANDHAIVELU
Power Electronics and Drives Division, School of Electrical Engineering, VIT University
Vellore, Tamil Nadu, India – 632 014

Dr. K. K. RAY
Senior Professor, Power Electronics and Drives Division, School of Electrical Engineering, VIT University
Vellore, Tamil Nadu, India – 632 014

Abstract:
This paper expound the TRIAC switched capacitor (TSC) based voltage regulation of a 3-Ø self-excited asynchronous
generator (SEASG) fed RL loads. A new self-regulated linear ramp signal based triggering circuit proposed and
implemented to switch ON the TRIAC, which includes an external capacitor in the circuit. Steady state equivalent
analysis of SEASG has been completed with load and external capacitor. Performance characteristics of SEASG have
been simulated using power system toolbox in Mat lab/Simulink software. A prototype model of the triggering circuit
has been developed, constructed and tested in the laboratory model of SEASG. The simulated and experimental result
presented and discussed.
Keywords: Asynchronous generator; simulation; TSC; voltage regulation.

1. INTRODUCTION
The usage of self-excited cage induction generator (SECIG) has been increasing due to fast depletion of non-
conventional energy sources. At present, to decentralize the power generation, attempts have been in the
direction of generating small power and distributing it locally. This prompted the use of wind and solar energy
to cope with the present day energy crisis. The recent trend is to tap solar, wind and tidal energy and these are
becoming popular amongst the renewable energy sources. Squirrel cage induction generator (SCIG) has
emerged as a possible alternative to conventional generator in an isolated power generation because of its low
cost, less maintenance and rugged construction [1]-[8].
The literature review concludes that the load characteristic of SEASG is drooping with increasing in load [9]-
[18]. Therefore, it needs a suitable voltage regulator to retain the terminal voltage constant. It also revels that the
necessity of additional capacitance needs to support the reactive power demand of both generator and load.
Prospective of different voltage control schemes has presented [19]. Among them, the solid-state voltage
regulators can be exhibit the phenomenon of capacitor but they do not support the voltage magnitude until the
external voltage source (battery / capacitor) should connected to the voltage source inverter (VSI) / current
source inverter (CSI).
Therefore, these conditions also support the necessity of capacitor into the SEASG power system. The fixed
capacitor thyristor controlled reactor (FC-TCR) based voltage regulator (SVC) discussed in details [20]-[25].
M.H. Hauqe explains the selection of capacitor to regulate the terminal voltage of induction generator and the
regulation [26, 27]. Based on that, the authors attempted to study the voltage regulation of SEASG
experimentally using a TRIAC based voltage regulator. The performance of the SEASG (scale down) and
equivalent circuit admittance based analysis studied. Simulation study of SEASG has been completed using Mat
lab software version-7.0[28].
This paper organized in six subheadings; Asynchronous generator operation of the induction machine has
been reviewed and studied with slip torque characteristic of induction machine, basics of TRIAC switched
capacitor and voltage regulation have been includes in descriptions of the system in section - 2. Equivalent
circuit analysis has been presented in section - 3. Simulation of the system has been explained in four. A proto
type model of SEASG has explained in 5. Discussion over the experimental work and simulation result in
subheading -6.

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Vol. 2 (12), 2010, 7215-7224

2. DESCRIPTION OF THE SYSTEM

Fig.1 (a) shows a single line diagram of the study system and its connection diagram is shown in Fig.1 (b). It
consists of SEASG, excitation capacitor bank (CE) along with the external capacitor (C1), which are connected
to the system by using the TRIAC switches. The excitation capacitor value had been chosen slightly higher than
the calculated value.

Fig.1 Proposed study system (a) single line diagram (b) circuit connection diagram

2.1. Asynchronous Generator

Obviously all the induction machines run below the synchronous speed of rotating magnetic field (RMF) so
called asynchronous machines. Suppose the rotor rotates above the RMF speed of those machines, slip goes
negative as shown in Fig.3. Now the rotor do not consume any power from the stator, rather than it will induce a
voltage provided with adequate rotor remanent magnetism. If the stator windings are connected to the grid
supply, it consumes the reactive power and export the active power to the grid, called grid connected induction
generators. However, the self-excited induction generator could not generate power until provided with
excitation source (usually capacitor) in its stator terminals. Therefore, both generators called as asynchronous
generators.

2.2. Slip Torque Characteristic

Self-excited cage induction motor speed - torque characteristic with a particular rotor resistance is shown in
Fig.2 (a). In the region of negative slip, the machine works as the generator powering the electrical load

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Vol. 2 (12), 2010, 7215-7224

connected to its terminals. In the region of positive slip, it works as the motor turning the mechanical load
connected to its shaft. In addition to the motoring and the generating regions, the induction machine has yet a
third operating mode and that is the braking mode. If the machine slip is greater than one ( s  1 ) by turning it
backward, it absorbs power without putting anything out. That is, it works as a brake. The power loss ( I 2 R ) in
the rotor conductors has been dissipated as heat. The eddy current brake works on this principle. As such, in
case of emergencies, the grid-connected induction generator brakes by reversing the three-phase voltage
sequence at the stator terminals. This reverses the direction of rotation of the magnetic flux wave with respect to
the rotor. The torsional stress on the turbine blades and the hub, however, may limit the braking torque.
Fig.2 (b) illustrates the torque-slip characteristic of the generator. If the generator is loaded with a constant
load torque TL , it has two possible points of operation, P1 and P2 . Only one of these two points is stable ( P1 ).
Any perturbations in speed around point P1 will produce stabilizing torque to bring it back to P1 . The Fig.2 also
shows the limit to which the generator can be loaded. The maximum torque ( Tmax ) is called the breakdown
torque, If the generator is loaded under a constant torque Tmax above, it will become unstable and stall, draw
excessive current, and destroy itself thermally if not properly protected.

Fig.2 Slip torque characteristics of an induction machine (a) motor (b) generator

2.3. Basics of TRIAC Switched Capacitor

A single-phase TRIAC switched capacitor equivalent circuit is shown in Fig.3, has a small value of inductance
(not shown in Fig.3) in series with the capacitor, which can be limited the surge current of this circuit.

Fig.3 Single phase equivalent circuit of TRIAC switched capacitor

Apply the kirchoff’s voltage law in the above circuit in Fig.3, the source voltage could be written by

v  V m sin (t   ) (1)

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Vol. 2 (12), 2010, 7215-7224

di 1
v  L   i dt (2)
dt C
Where the initial current of the inductor is zero ( i  i (0 )  i (0 )  0 ) and the initial voltage of the capacitor
is v c ( v  v (0 )  v (0 )  0 ), the solution of the above equations is
c
0 C
i  I m cos (t   )  I m cos t cos  0 t  (  2 LC 2  1) v  V sin   sin  t (3)
  0
1   LC 
2 c m

Where  0  1 / LC is the oscillation frequency and I m is the peak current in steady state condition, in order
to reach the steady sate value of the current the current equation (3) should satisfy the following conditions.
Vm 
i.e, vc   &  (4)
1  2 L C 2

It means to say to fulfill the basic requirement mentioned in [29, 30]. Then the current equation written as

i  I m cos (t  ) (5)
2

2.4. Triggering Circuit

The principle operation of the triggering circuit explained with block diagram shown in Fig.4 (a). Fig. 4(b)
illustrates the theoretical output waveforms of the proposed triggering scheme. The ramp signal generator
identifies the source voltage peak and generates the signal. This technique is one of the solutions to remove the
synchronizing circuit and peak detector. Peak detector also called as a level detector may fail to detect the peak
point. The comparator has compared the reference and ramp signals. The negative edge of the square wave
(comparator output) detected by the integrated circuit (IC 74121). The amplified pulse fed to the steering circuit.
Pulse transformer has been used to isolate the firing circuit and power circuit.

Fig.4 Block diagram representation of pulse generation circuit

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2.5. Voltage Regulation

The three external elements that can change the voltage profile of SEASG are speed, terminal capacitance and
the load impedance. By varying the elements, one at a time the performance characteristics of the squirrel-cage
induction generator obtained. In most of SEASG applications, the rotational speed is rarely controllable.
Therefore, the load seen by the generator or terminal capacitance has to be controlled. This work deals with the
capacitance control, a set of 3-Ø bank capacitors (with minimum capacitance) permanently connected to the
stator terminals of the SEASG and additional capacitor has to be connected using TRIAC in the system. Fig.5
shows the load characteristic of SEASG with constant speed operation.

Fig. 5 Load characteristics of SEASG with TRIAC switched capacitor

It has drooping characteristics. If the dip in voltage reaches below the knee point of load characteristics, the
generator fails to build up the voltage. Under this circumstances an additional capacitances has switched ON.
Thereon the load voltage magnitude increased. Nevertheless, it is necessary to control the voltage magnitude
within the limits ( V max  V min ). Therefore, the selection of capacitance is more important. In general, the
addition of external capacitor to the circuit in a stepwise like C , 2C and 4C etc…

3. EQUIVALENT CIRCUIT ANALYSIS

Fig.6 shows the SEASG steady-state equivalent circuit (per phase) with balanced load. The TRIAC switched
(S w ) capacitor shunted across the excitation capacitor. In this circuit, only the magnetizing reactance affected
by magnetic saturation and all other parameters assumed constant. In addition, core losses and the effect of the
harmonics ignored.

Fig. 6 Single phase equivalent circuit of SEASG with TSC and load

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From Fig. 6, the total current at node B written as

Y6  Y2  Y1   0
Vg
(6)
F
Therefore, under steady state condition the total admittance goes to zero. But Vg / F  0 or


Real of Y6  Y2  Y1 0 (7)

Imaginary of Y6  Y2  Y1   0 (8)

1
Where, Y1  (9) 
R 2 /( F   )  j X m

1
 Y2    (10)
j Xm

1
 Y3  (11)
(V / F )  j X 1

J (X c  X c _ s )
 Y4   (12)
 j (X c  X c _ s )

1
Y  (13)
5 (RL / F )  j X L

Y Y
Y6  4 5  Y3 (14)
Y4  Y5

Equations (7) and (8) are nonlinear equations of four unknowns; F , X m , X c and among them, two of the
parameters should specify. The other two unknowns can be found by solving equation (7) for one unknown and
the other unknown is calculated from equation (8).Performance Calculation Having determined the two
unknown parameters, the steady-state performance of the SEIG can be obtained by solving the circuit of Fig. 6
with the help of the generator’s magnetization curve. The performance characteristics of the generator could be
estimated using the following relationships: 
Vg  RL 
  j XL  R 
i1  F  F   L  j X L  (15)
R1  jX c  F 
 j X1  jX c
F  
Vg

i2   F (16)
R2
 j X2
F 
 j X c i1
iL  i Y  (17)
RL F  j X c 1 4
i R
Vt  L L  i L Y5 (18)
F  jX L
2
3( R 2 F i 2 )
Pin   (19)
F 
2
Pout  3 i L R L (20)
  Pin / Pout  100 (21)

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4. SIMULATION
The proposed SEASG (shown in Fig.1 (b)) along with TRIAC switched capacitor bases voltage regulator
simulated using the Matlab/Simulink with ode 23tb solver. Fig. 7(a) illustrates the connection diagram of the
proposed simulation circuit. Three connectors A, B & C are used to connect the controller circuit (not shown in
Fig.7(a)) and five main switches (MS1-MS5) have been used to connect the loads (main load of 100Ω,
additional load of 100  j10.676 Ω and the capacitors (16μFd, 500V, in each phase with two units). The
regulator switch configured with anti parallel-connected thyristors shown in Fig.7 (b). A low value of reactor
(3mH) connected in series with the capacitor to limit inrush current when the capacitor is switched ON.

Fig.7 Simulation circuit of the proposed system (a) connection diagram (b) TRIAC model using thyristor switches

The simulation completed with in 14 seconds. The generator switch closed after 3 seconds. The main switch,
additional load switches and TRAIC switches sequentially turned ON at 3 sec., 6 sec., 7 sec., 8 sec., 9 sec., 10
sec and 11 sec., respectively.
5. EXPERIMENTATION
A 3-Ø, 2.2kW, 415V, 4.7A, 1440-rpm cage induction motor is mechanically coupled with a D.C. motor of 5 Hp,
220V, and 30A emulate the induction generator characteristics. Fig.8 (a) shows the proto type model of the
experimental setup photograph. The TRIAC switched capacitor with firing circuit shown in Fig.8 (b).

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Fig.8 Proto type model of self-excited induction generator (a) experimental setup with self-excitation capacitor (b) TRIAC switched capacitor

6. RESULT AND DISCUSSION

Simulation of the proposed system has been done by using the data shown in table-1. Simulated load
characteristics of SEASG by using TRIAC switched capacitor shown in Fig.9 (a). The SEASG load
characteristic illustrates in Fig.9 (a) divided into three zones I, II and -III. The zone-I imply the full load
characteristic of SEASG with 16μFd (in each phase) and zone- II illustrates the characteristics of SEASG with
self-excitation capacitors and additional capacitance of 16μFd. In zone- III, self-excitation capacitor, additional
capacitor -1 and additional capacitor-2 has been included. Additional capacitor-1 has been switched ON when
the load current is 1.5A, beyond that the generator fails to build up the voltage. Again, another additional-2
capacitor of 16μFd switched ON at 3 seconds. Experimental result of load performance of SEASG has been
shown table -2.

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Fig.9 Load characteristics of SEASG (a) simulated characteristics (b) experimental load characteristic

Fig. 9 (a) illustrates the simulated load characteristics of SEASG, the dotted line indicates the extracted
characteristics. The additional capacitors are switched ON if the load voltage level below the desired level (218
volts). Load characteristics of SEASG with different values(16μfd, 20 µfd and 30 μfd, ) is shown in Fig.9(b).
Fig.10 (a) illustrate the firing signals of the controller at different stages and extracted firing pulse generation
diagram(Fig. 10(b)). The changes in generator frequency can adjust the generation pulse at the same instant
(peak of the load voltage) of switched on the TRIAC switch.

Fig.10 Firing circuits waveforms (a) at different stage (b) extracted waveform of firing signal generation

7. CONCLUSION
Performance study, including voltage regulation of a 3-Ø SEASG in stand-alone generator mode with TRIAC
switched capacitor completed by both simulation and experimentation. Even though it is difficult to dimension
the capacitor, which is more essential to support the voltage build up in the generator terminals. It might been
calculated using the equivalent circuit of the complete power system.

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Vol. 2 (12), 2010, 7215-7224

ACKNOWLEDGMENTS
The authors acknowledge the management of Vellore Institute of Technology University, Vellore, India, 632014 for
their support and keen interest in promoting the research and development in the power electronics division by
providing all the required facilities and resources.

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