AC/DC/AC Converter Fed RLC Series

Department of Electrical Engineering, IGEC Sagar (MP)
1
INTRODUCTION
Chapter - 1
INTRODUCTION
This project presents modelling & simulation of an
Asynchronous Generator with AC/DC/AC Converter Fed
RLC Series.

2
INTRODUCTION
The use of non-conventional energy sources has become
eminent due to fast depletion of conventional energy sources.
The recent trend to tap solar, wind and tidal energy are
becoming popular amongst the renewable energy sources. At
present, to decentralize the power generation system, attempts
have been made 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.
Self-excited asynchronous generators (SEASG) has emerged as
a possible alternative for isolated power generation from
renewable energy sources because of its low cost, less
maintenance and rugged construction [1]-[3]. However, it
requires a suitable controller to regulate the voltage due to
variation of consumer loads.
From the characteristics of voltage generation in a SEASG, it
is essential to have a variable capacitance at the machine
terminals to maintain constant voltage with variable load.
J. K. Chatterjee et al [4] have developed a variable lagging
reactive volt-ampere (VAR) source/sink to maintain the
generation voltage of SEASG constant. K. K. Ray [5] applied

3
the above concept in a stand-alone system and verified it
experimentally.
S.S. Murthy et al [6]–[7] explained the steady state analysis of
self-excited induction generators. R. Bonert et al [8]–[9]
discussed the impedance controller of SEASG for voltage
regulation. S.S. Murthy et al discussed the practical
implementation of electronic load controllers in his works
[10]–[11]. Since voltage and frequency of an ASG is dependent
on load and the speed of the prime mover, the authors made an
attempt to investigate the effect of series resonance circuit on
input side of uncontrolled rectifier experimentally by keeping
the prime mover speed constant [12] - [13].
The subsequent section describes the system configuration. In
Section–III Modelling of the proposed scheme has been
explained in detail. Control strategies have been discussed in
section IV. Section V and VI discuss the interpretation of the
result and conclusion respectively. An instantaneous reactive
power compensator (i.e. AC/DC/AC along with RLC series
circuit)is proposed in this paper to balance the instantaneous
reactive power required by the load[14].

4
ALTERNATING CURRENT
Chapter - 2
ALTERNATING CURRENT
2.1 AC POWER SUPPLY FREQUENCIES
An alternating current can be defined as a current that
changes its magnitude and polarity at regular intervals of
time. It can also be defined as an electrical current that
repeatedly changes or reverses its direction opposite to
that of Direct Current or DC which always flows in a
single direction.

5
Alternating current
Alternating current (AC) is an electric current which
periodically reverses direction and changes its magnitude
continuously with time in contrast to direct current (DC)
which flows only in one direction. Alternating current is the
form in which electric power is delivered to businesses and
residences, and it is the form of electrical energy that
consumers typically use when they plug kitchen appliances,
televisions, fans and electric lamps into a wall socket. A
common source of DC power is a battery cell in a flashlight.
The abbreviations AC and DC are often used to mean
simply alternating and direct, as when they
modify current or voltage.
The usual waveform of alternating current in most electric
power circuits is a sine wave, whose positive half-period
corresponds with positive direction of the current and vice

6
versa. In certain applications, like guitar amplifiers, different
waveforms are used, such as triangular waves or square
waves. Audio and radio signals carried on electrical wires are
also examples of alternating current. These types of
alternating current carry information such as sound (audio) or
images (video) sometimes carried by modulation of an AC
carrier signal. These currents typically alternate at higher
frequencies than those used in power transmission.
AC power supply frequencies
The frequency of the electrical system varies by country and
sometimes within a country; most electric power is generated
at either 50 or 60 Hertz. Some countries have a mixture of
50 Hz and 60 Hz supplies, notably electricity power
transmission in Japan. A low frequency eases the design of
electric motors, particularly for hoisting, crushing and rolling
applications, and commutator-type traction motors for
applications such as railways. However, low frequency also
causes noticeable flicker in arc lamps and incandescent light
bulbs.
The use of lower frequencies also provided the advantage of
lower impedance losses, which are proportional to frequency.
The original Niagara Falls generators were built to produce
25 Hz power, as a compromise between low frequency for
traction and heavy induction motors, while still allowing
incandescent lighting to operate (although with noticeable
flicker).

7
Most of the 25 Hz residential and commercial customers for
Niagara Falls power were converted to 60 Hz by the late
1950s, although some 25 Hz industrial customers still existed
as of the start of the 21st century. 16.7 Hz power (formerly 16
2/3 Hz) is still used in some European rail systems, such as
in Austria, Germany, Norway, Sweden and Switzerland. Off-
shore, military, textile industry, marine, aircraft, and
spacecraft applications sometimes use 400 Hz, for benefits of
reduced weight of apparatus or higher motor speeds.
Computer mainframe systems were often powered by 400 Hz
or 415 Hz for benefits of ripple reduction while using smaller
internal AC to DC conversion units.

8
AC Power Supply Frequencies

9
DIRECT CURRENT
Chapter - 3
DIRECT CURRENT
Direct current (DC) is an electric current that is
unidirectional, so the flow of charge is always in the same
direction.As opposed to alternating current the direction
and amperage of direct currents do not change. It is used
in many household electronics and in all devices that use
batteries.

10
Direct Current
DC stands for Direct Current, although it is often referred to as
“DC Current”. DC current is defined as a unidirectional flow
of electric charge. In DC current, the electrons move from an
area of negative charge to an area of positive charge without
changing direction. This is unlike alternating current (AC)
circuits where current can flow in both directions.
DC current can flow through conducting material like wire and
also flow through the semiconductors.

11
CONVERTER
Chapter - 4
CONVERTER
4.1 - AC-DC CONVERTER
4.2 - DC-AC CONVERTER
Converter that converts alternating current into direct
current or vice versa.

12
AC-DC Converters
AC-DC converters are electrical circuits that transform
alternating current (AC) input into direct current (DC) output.
They are used in power electronic applications where the power
input is a 50 Hz or 60 Hz sine-wave AC voltage that requires
power conversion for a DC output.
AC to DC converters use rectifiers to turn AC input into DC
output, regulators to adjust the voltage level, and reservoir
capacitors to smooth the pulsating DC.
DC/AC converter
DC/AC converter, also described as “Inverter”, is a circuit
that converts a DC source into a sinusoidal AC voltage to
supply AC loads, control AC motors, or event connect DC
devices that are connected to the grid. ... The three main
components of all frequency converters are Rectifier, DC Bus
and Inverter.

13
Asynchronous Generator
Chapter - 5
ASYNCHRONOUS GENERATOR
5.1 PRINCIPLE OF OPERATION
5.2 LIMITATIONS
an asynchronous generator is a type of alternating current
(AC) electrical generator that uses the principles of
induction motors to produce electric power.

14
Asynchronous generator
An induction generator or asynchronous generator is a type of
alternating current (AC) electrical generator that uses the
principles of induction motors to produce electric power.

15
Induction generators operate by mechanically turning their
rotors faster than synchronous speed.
A regular AC induction motor usually can be used as a
generator, without any internal modifications. Because they
can recover energy with relatively simple controls, induction
generators are useful in applications such as mini hydro power
plants, wind turbines, or in reducing high-pressure gas streams
to lower pressure.
An induction generator usually draws its excitation power from
an electrical grid.
Because of this, induction generators cannot usually black start
a de-energized distribution system.
Sometimes, however, they are self-excited by using capacitors.

16
Principle of operation
An induction generator produces electrical power when its
rotor is turned faster than the synchronous speed. For a typical
four-pole motor (two pairs of poles on stator) operating on a 60
Hz electrical grid, the synchronous speed is 1800 rotations per
minute (rpm).
The same four-pole motor operating on a 50 Hz grid will have
a synchronous speed of 1500 RPM.

17
The motor normally turns slightly slower than the synchronous
speed; the difference between synchronous and operating speed
is called "slip" and is usually expressed as percent of the
synchronous speed.
For example, a motor operating at 1450 RPM that has a
synchronous speed of 1500 RPM is running at a slip of +3.3%.
In normal motor operation, the stator flux rotation is faster than
the rotor rotation.
This causes the stator flux to induce rotor currents, which create
a rotor flux with magnetic polarity opposite to the stator.
In this way, the rotor is dragged along behind stator flux, with
the currents in the rotor induced at the slip frequency.
In generator operation, a prime mover (turbine or engine)
drives the rotor above the synchronous speed (negative slip).
The stator flux still induces currents in the rotor, but since the
opposing rotor flux is now cutting the stator coils, an active
current is produced in stator coils and the motor now operates
as a generator, sending power back to the electrical grid.

18
Limitations
An induction generator connected to a capacitor system can
generate sufficient reactive power to operate on its own.
When the load current exceeds the capability of the generator
to supply both magnetization reactive power and load power
the generator will immediately cease to produce power. The
load must be removed and the induction generator restarted
with either a DC source, or if present, residual magnetism in
the core.
Induction generators are particularly suitable for wind
generating stations as in this case speed is always a variable
factor. Unlike synchronous motors, induction generators are
load-dependent and cannot be used alone for grid frequency
control.

19
SYSTEM CONFIGURATION
Chapter - 6
SYSTEM CONFIGURATION
System configuration is a term in systems engineering that
defines the computer hardware, the processes as well as the
various devices that comprise the entire system and its
boundaries. This term also refers to the settings or the
hardware-software arrangement and how each device and
software or process interact with each other based on a
system settings file created automatically by the system or
defined by the user.

20
SYSTEM
CONFIGURATION
The schematic arrangement of the proposed system is shown in
Fig.1.
It consists of a SEASG, rectifier – inverter fed RLC series
circuit with RL load. SEASG is driven at a constant speed along
with a static capacitor at the stator terminals of the SEASG.
The effect of changing load on the generated voltage was found
to be drooping with an increase of load. To compensate for this
drooping voltage, an R L C series resonance circuit fed from an
AC/DC/AC converter is connected at PCC.
The AC/DC/AC converter is operated at a frequency lower than
the resonance frequency to inject the capacitance effect on the
system such that the voltage drop due to inductive load is
compensated.

21
The resonance circuit thus could operate to inject lagging or
leading VAR effect on the input current by operating the
AC/DC/AC converter at various frequencies. An instantaneous
reactive power compensator (i.e. AC/DC/AC along with RLC
series circuit) is proposed in this paper to balance the
instantaneous reactive power required by the load.

22
MATHEMATICAL MODEL
Chapter - 7
MATHEMATICAL MODEL
7.1 D-Q AXIS MODELLING OF
ASYNCHRONOUS GENERATOR
A mathematical model is a description of a system
using mathematical concepts and language. The
process of developing a mathematical model is termed
mathematical model.

23
MATHEMATICAL
MODELING
The mathematical model of the system referring to equations
(1)-(13) is developed using MATLAB/SIMULINK software.
It is well known that when a squirrel cage induction motor is
driven at a speed higher than the synchronous speed a voltage
will be induced in the stator terminals when external
capacitance is connected across the stator terminals.
The magnitude of the voltage built up depends on the
capacitance value to neutralize the magnetizing reactance of the
machines. This technique is known as self-excitation.

24
D-Q Axis Modelling of
Asynchronous Generator
The d-q axis equivalent circuit model of a SEASG is shown in
Fig. 2(a) and Fig. 2(b) respectively.

25
From Fig.2 the loop equations of the d-q axis equivalent circuit
of SEASG could be written as

26
The dynamic characteristic behavior of SEASG in d–q axis
equivalent circuit model [15] is used for simulation.
The magnetizing current i and generated air gap voltage m can
be calculated using equations (5) and (6) respectively

27
It should be noted that L is not constant but a function m of the
magnetizing current i given as
The developed electromagnetic torque and the torque balance
equations are written as
The developed electromagnetic torque and the torque balance
equations are written as
The speed derivation of torque balance equation is expressed in
equation (10)

28
The generated phase voltage and the stator currents derived
from d-q axes values using equation (11)

29
MODELING OF BRIDGE DIODE RECTIFIER
Chapter - 8
MODELING OF BRIDGE DIODE
RECTIFIER
A diode bridge is an arrangement of four diodes in a
bridge circuit configuration that provides the same
polarity of output for either polarity of input.

30
Modeling Of Bridge Diode
Rectifier
The diode bridge model is developed with ideal switches and
the total loss of the bridge is represented by a lumped resistor
R which is added to the dc resistance R with the dc help of three
Heaviside functions.
These Heaviside functions determine whether the diode is in
conducting state or in blocking state. The functions k g (where,
k = 1, 2, 3) are defined from the graph shown in Fig.3.
The three phase voltages are expressed through the equation
(12).

31
Using the above equation (12) the phase voltages are computed
through the equation (13)
Where
From the above equation (12) and (13) the dc side current is
given by

32
INVERTER MODEL
Chapter - 9
INVERTER MODEL
A power inverter, or inverter, is a power electronic device
or circuitry that changes direct current (DC) to
alternating current (AC). ... The input voltage, output
voltage and frequency, and overall power handling depend
on the design of the specific device or circuitry.

33
Inverter Model
A pulse width modulated (PWM) converter model was
constructed with Insulated Gate Bipolar Transistor (IGBT).
The current drawn by the RLC series circuit is synthesized to
obtain the lagging area leading reactive power requirement to
be injected into the system to maintain the load bus voltage
constant.

34
On the basis of this information the inverter is operated at
frequencies either below or above the resonance frequency.

35
SIMULATION MODEL
Chapter - 10
SIMULATION MODEL
A simulation is the imitation of the operation of a real-
world process or system over time.Simulations require the
use of a model. The model represents the key
characteristics or behaviors of the selected system or
process, whereas the simulation represents the evolution of
the model over time. Often, computers are used to execute
the simulation.

36
Simulation Model
A simulation model of the proposed isolated self excited
asynchronous power generation system Fig.A-1 is developed in
MATLAB. A 2.2kW, 415V, 50Hz, 4-pole, Y-connected
asynchronous generator is considered as the rating of the
machine model.
The data of saturation characteristics of the machines is
obtained from a synchronous speed test.
Simulation is carried in MATLAB version 7.1 in discrete mode
with ode 23tb (stiff/TR-BDF-2) solver.

37

38
CONTROL STRATEGY
Chapter - 11
CONTROL STRATEGY
A control strategy is a planned set of controls, derived
from current product and process understanding, that
assures process. performance and product quality.

39
CONTROL STRATEGY
The control scheme of the proposed stand-alone power
generation system is shown in Fig. 4.
The controller is used to provide the single point operation with
constant voltage and frequency along with a constant excitation
capacitor of SEASG.
The control scheme is based on the generation of reference
source currents [16] given in equation (15).
The instantaneous power delivered by the source is equal to the
sum of the instantaneous power absorbed by the load and the
RLC series circuit (assume the switching losses are negligible).
The RLC series circuit compensates for the lagging VAr
required by the load when operated at a frequency lower than
the resonance frequency ( ω < ωo ).

40
RESULT
Chapter - 12
RESULT
something that happens because of something else; the
final situation at the end of a series of actions

41
RESULT
The simulation results of the d.c side input current of the
inverter is shown in Fig.5 for inverter frequency lower and
higher than the resonance frequency.
The waveform clearly shows that the frequency higher than the
resonance frequency the input side current is lagging whereas
with frequency lower than the resonance frequency it is
leading.
This phenomenon of drawing leading and lagging current also
observed at PCC shown in Fig.6

42

43

44
CONCLUSION
Chapter - 13
CONCLUSION
a final decision or judgment : an opinion or decision that
is formed after a period of thought or researcha final
decision or judgment : an opinion or decision that is
formed after a period of thought or research

45
CONCLUSION
From the simulation results of currents at the input side of the
inverter and at PCC as shown in Fig.5 and Fig.6 .
It is evident that the RLC series circuit when operated either at
a frequency lower or higher than the resonance frequency the
system power factor gets affected.
This characteristic phenomenon can be implemented in
controlling the leading Var requirements for constant voltage
operations of the SEASG.

46
REFERENCE
[1] C. F. Wanger, “Self-Excitation Of Induction Motors,” AIEE
Trans., vol. 58, pp. 47–51, 1939.
[2] J. E. Barkle and R.W. Ferguson, “Induction Generator Theory
And Application,” AIEE Trans., pt. III A, vol. 73, pp. 12–19,
Feb.1954.
[3] G. Raina and O. P. Malik, “Wind Energy Conversion Using A
Self- Excited Induction Generator,” IEEE Trans. Power App. Syst.,
vol. PAS -102, no. 12, pp. 3933–3936, Dec. 1983.
[4] J.K.Chatterjee, B.M. Doshi and K.K.Ray, “ A New method for
thyristor phase controlled Var compensator”, IEEE/PES winter
meeting 89.WM 133 – OPWRS, New Yark, 1989.
[5] K. K. Ray, “ Performance Evaluation of A Novel Var Controller
And Its Application In Brushless Stand Alone Power Generation,”
Ph.D. Thesis, department of Electrical Engineering, IIT Delhi,
India,1990.
[6] S. S. Murthy, O. P. Malik, and A. K. Tandon, “Analysis of self
excited induction generator,” Proc. Inst. Elect. Eng. C, vol. 129, no. 6,
pp. 260–265, Nov. 1982.
[7] A. K. Tandon, S. S. Murthy, and G. J. Berg, “Steady state analysis
of capacitor self-excited induction generators,” IEEE Trans. Power
App. Systm., vol. PAS-103, no. 3, pp. 612–618, Mar. 1984.
[8] R. Bonert and S. Rajakaruna, “Self-Excited Induction Generator
with Excellent Voltage and Frequency Control”, IEE Proceeding -
Generation, Transmission & Distribution, Vol. 145, No. I. January
I998, pp 31 -39.

47

AC/DC/AC Converter Fed RLC Series

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