Design of 50KVA Single Phase Static Inverter

International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 4, Issue 8, August 2014)
Design of 50KVA Single Phase Static Inverter
Abatan O.A.1, Egunjobi A.I.2, Musari A.A.3, Oseni, K.J.4, Edun, A.T.5, Sodunke M.A6
Physics/Electronics Unit, SLT, Moshood Abiola Polytechnic, Abeokuta, Ogun State, Nigeria.
Abstract-- This paper gives the design and
implementation of a 50KVA single phase static inverter that
uses insulated uses Insulated Gate Bi-polar Transistors
(IGBT's) to produce an alternating current square wave at
a frequency determined by a crystal controlled oscillator.
The output is filtered by a ferroresonant regulator, which
creates a low distortion sine wave output from the square
wave input and regulates with a minimum amount of
components. In addition, it has a built-in current limiting
feature for inverter protection. A DC to AC voltage
converter consists of four bidirectional switches that is used
to convert the voltage. Single phase static inverters are true
on-line ferroresonant transformer-based designs intended
for use in UPS systems or in stand-alone applications The
control circuit has a small size for the single phase bridge
inverter and it is low cost. The driver circuit isolates the
control circuit from power circuit. The output for variable
AC voltages are discussed.
Fig.1 Half- Bridge single phase inverter
Since most loads contain inductance, feedback
rectifiers or antiparallel diodes are often connected across
each semiconductor switch to provide a path for the peak
inductive load current when the semiconductor is turned
off. The antiparallel diodes are somewhat similar to the
freewheeling diodes used in AC/DC converter circuits.
the H-bridge inverter consists of four choppers. When
transistors T1 and T2 are turned on simultaneously, the
input voltage Vs appears across the load. If transistors T3
and T4 are turned on at the same time, the voltage across
the load is reversed and is -Vs. [4]
(b) Full Bridge Single Phase Inverter: The DC to AC
converter, also known as inverter converts dc power to ac
power at desired output voltage and frequency [3]. The
output voltage of an inverter has a periodic waveform
that is not sinusoidal but can be made to closely
approximate this desire waveform. Figure 2 shows the
circuit topology for a full bridge inverter. It is an
electronic power converter that is necessary as an
interface between the power input and the load. The full
bridge single phase inverter consists of the DC voltage
source, four switching elements S1, S2, S3 and S4 and
load. The switching element available nowadays, such as
bipolar junction transistor (BJTs), gate turn off thyristor
(GTOs), metal oxide semiconductor field effect
transistors (MOSFETs), insulated gate bipolar transistors
(IGBTs), metal oxide semiconductor controlled thyristor
(MCT‟s) and static induction transistors (SIT‟s) can be
used as a switch. They are substituting the relays,
magnetic switches and other magnetic components as the
inverter switching devices. This makes use of
microcontroller becomes more significant. The full
bridge single phase inverter has two legs, left or right or
„A‟ phase leg and „B‟ phase leg. Each leg consists of two
power devices (here MOSFET) connect in series. The
load is connected between the midpoints of the two phase
legs.
Keywords-- single phase, static inverter, Gate Bi-polar
Transistors (IGBT's), ferroresonant regulator,
I. INTRODUCTION
A wide range of inverters has been designed to follow
rigorous mains compatibility and erratic power supply.
The technology associated with efficient conversion,
control and conditioning of electric power by static
means from its available input form into the desired
electrical output form is Power Electronics and it can be
found wherever there is a need to modify the electrical
energy form (i.e., modify its voltage, current or
frequency). The single phase static inverter uses
Insulated Gate Bi-polar Transistors (IGBT's) to produce
an alternating current square wave at a frequency
determined by a crystal controlled oscillator. The output
is filtered by a ferroresonant regulator, which creates a
low distortion sine wave output from the square wave
input and regulates with a minimum amount of
components. In addition, it has a built-in current limiting
feature for inverter protection. Inverters are used in a
wide range of applications, from small switching power
supplies in computers, to large electric utility applications
that transport bulk power. Static inverter can be built and
installed in modules of 3 to 50KVA up to the load
capacity.
II. B ACKGROUND O F S INGLE P HASE INVERTER
There are two types of single phase inverters namely:
(a) Half- Bridge inverter: This is the most common
single-phase inverter as shown in figure 1.
319
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 4, Issue 8, August 2014)
III. INVERTER ARCHITECTURE
Each power control device has a diode connected in
anti-parallel to it. The diodes provide an alternative path
for the load current if the power switches are turned OFF.
For example, if lower MOSFET in the „A‟ phase leg is
conducting and carrying current towards the negative DC
bus, this current would „commutate‟into the diode across
the upper MOSFET of the „A‟ phase leg, if the lower
MOSFET is turned OFF. Control of the circuit is
accomplished by varying the turn on time of the upper
and lower MOSFET of each inverter leg with the
provision of never turning ON both at the same time, to
avoid a short circuit of DC bus.
When designing an inverter there are three basic
schemes to convert the fuel cell plus boost module's DC
energy into AC. For example, this AC may then be fed
into the grid or can be used for stand-alone operation of
220-240V appliances.
There are three types of inverters
(a): Step-up and chop: This type converts the low voltage
into a high voltage first with a square-wave step-up
converter and then converts the high-voltage DC into the
wanted AC waveform (Fig.4)
Figure 2 - Full Bridge Single Phase Inverter
Fig (4) Circuit topology of a step-up and chop inverter
(c ) PIC Microcontroller: This is used to obtain the gate
signal of the booster switch and to drive the inverter
switches using SPWM. PIC 16F877 is used to generate
the required signals. Figure (3) shows PIC 16F877
layout. Note that it has 40 pins with different functions.
Two PICs were programmed in order to drive the
boosters and inverter switches.
Advantage of this architecture is insulation between
input and output, easy dimensioning of the input
converter, Efficiency may be up to 95% for square-wave,
slightly lower for sinewave inverters.[2]
(b)High voltage in, only chop: This type requires the
input voltage to be higher than the output voltage and
converts it directly into the wanted AC waveform (fig.5).
The advantage of this is the high efficiency of the
inverter, typical 96%. The main disadvantage is the lack
of insulation between the solar modules and the grid
voltages. Also the input voltages always require a large
number of modules.[2]
Fig (5) Circuit topology of a high voltage in and chop only inverter
(c) Chop and transform: This type converts the low
voltage DC into a low voltage AC first and then converts
the low-voltage AC into the wanted AC voltage (fig.6).
Fig (3) PIC Layout
320
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 4, Issue 8, August 2014)
The advantages are the low-voltage (safe) operation,
the insulation from the grid after the inverter, the ease
with which it makes sine wave which feeds into the
transformer and the most important in many aspects,
reliability due to the low number of semiconductors in
the power path. Disadvantage is the slightly lower
efficiency of the inverter, typically 92%. Also some hum
can be generated by the transformer under load. [2]
The Manual Bypass Switch is a two-position manual
make before-break switch used to bypass the inverter and
static transfer switch for maintenance purposes.
Fig.7 Block diagram of a 50kva single phase static inverter
Fig (6) Circuit topology of a chop and transform inverter.
There are several types of power inverter available in
two categories-the true sine wave power inverter
produces utility grade power. These inverters are very
expensive and can power almost anything including laser
printers fax machines, fans, television set, computers etc.
A sine wave inverter is recommended to operate higher
electronic equipment.
Modified sine wave type of inverter can adequately
power most household appliances and power tools. It is
more economical, but may present certain compromises
with some loads such as microwave ovens, laser printers,
clocks and cordless tool chargers. Simple inverters make
use of oscillators driving a transistor to create a square
wave, which in turn is fed through a transformer to
produce the required output voltage, while advanced
inverters have started using more advanced forms of
transistors of similar devices such as thysistors.
The main objective is to design and construct 50KVA
single phase static inverter having low audible noise,
high efficiency transistor bridge, high reliability
exceeding 100,000 hours, harmonic filter for distributed
control systems, microprocessor based alarms and
industrial grade built to operate in extreme environments
Fig.8 Circuit diagram of PIC microcontroller for single phase static
inverter
Phase Inverter- What It Does And How It Works?
Of all the circuits in a tube amplifier, the Phase
Inverter, also known as the Phase Splitter, is the most
difficult to understand by even some experienced
technicians. It‟s function is to take a signal input, and
create two outputs, one that is identical (e.g. in-phase) to
the original, and another that is a mirror-image (phaseinverted or flipped phase). Each signal feeds a power
tube (or bank of power tubes) that is connected to each
side of the Output Transformer‟s primary winding in the
typical Push-Pull configuration. Single-ended power
amp, like those contained in the Fender Champ, which
sport only one power tube, do not require this additional
step, and need only a driver before the power tube to
boost the preamp signal to a level usable by the lone
power tube. So, why is the Push-Pull method of power
amplification used when it is inherently more complex
and costly? Several reasons. First of all, it enables us to
use a more efficient amplifier class called Class AB.
IV. S INGLE P HASE STATIC INVERTER P ROCESS
The block (Fig.7) and circuit (Fig.8) diagrams of a
50KVA single phase static inverter are presented below.
The inverter's basic function is to convert DC power
from a rectifier/charger or battery to an extremely
accurate, regulated AC output for powering the critical
AC load. Each inverter includes a static transfer switch
and manual bypass switch. The Static Switch is isolated
using high voltage reed relays and automatically transfers
a critical load from the output of a failed or overloaded
inverter to a bypass source of power without interruption.
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International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 4, Issue 8, August 2014)
Whereas single-ended audio amplifiers always run in
the Class A mode, which runs the tube constantly at
maximum power (thereby shortening it‟s lifespan), Class
AB runs each tube only slightly above the lowest
operational point (called “idle”), and each one is called
on as needed to deliver power when necessary. No signal
in, no power use, therefore the tubes remain relatively
cool until pushed. The second reason for using Push-Pull
is that unwanted sonic artifacts, such as hum and oddharmonic distortion (this is the nasty, raspy kind), are
naturally cancelled in the Output Transformer. Evenorder harmonic distortion (the kind that sounds cool),
remains relatively untouched. There are two designs that
dominate guitar amps. One is the “Cathodyne”, also
known as the “Split-Load” (Fig. 9), and the other is the
“Long-Tailed Pair”, derived from a circuit called the
Schmitt Inverter (Fig. 10).
Fig. 10 Schmitt Inverter
Common-Cathode (where the cathode is grounded and
signal is fed to the grid, the most common type of triode
amplifier), Common-Grid (where the grid is grounded
and signal is fed to the cathode), and Common-Anode
(where the plate is “grounded”, not to 0V, but directly to
the power supply, which is a “virtual ground”, also called
a “Cathode Follower” and, rarely, a “Buffer”). Since the
Cathode Follower does not provide signal voltage gain
but. It does deliver current gain, which is good for
circuits like tone stacks that tend to hog current. However
the other two amplifier arrangements, Common-Cathode
and Common-Grid do provide voltage gain, though the
Common-Grid amplifier isn‟t quite as effective as the
Common-Cathode amplifier in doing this. The gain for a
given signal input is less than that of the CommonCathode circuit. What the “Long-Tailed Pair” does is to
use two triodes, one in a Common-Cathode arrangement,
the other as a Common-Grid amp. The signal enters the
first stage (Common-Cathode) in the usual way, through
the grid. This produces a voltage swing on the plate and
the cathode of this stage, as described earlier. The
second-stage, which has a common (grounded) grid, has
it‟s output at the plate, like the first stage. Feeding signal
into and tying the cathode of the first stage to the cathode
of the second stage, the variation in current of the first
stage is superimposed on the cathodes of the second
stage. Here is the circuit again, redrawn and simplified in
Fig 11, Basically, the plate circuit, which has voltage
gain, sends it‟s signal to the power tubes. The cathode
circuit, which is essentially a cathode follower, since
there is no voltage gain, the second stage Common-Grid
amp provides the gain. This is the key element missing in
the Cathodyne Phase Inverter (Fig.9). After voltage gain
is applied, the signal then travels to the power tubes as
well. The Common-Cathode amp has higher voltage gain
than that of a Common-Grid amp.
Fig. 9.,Split-Load
The Split-Load is the simplest arrangement. It splits the
signal by virtue of the fact that the signal appearing at the
cathode of the tube is in-phase (this circuit by itself is called a
“Cathode Follower”), while the signal at the plate is out-ofphase (this is a typical Common-Cathode amplifier). The way
this works is that signal input to the tube at the grid causes a
variation in current flow from the cathode to the plate,
producing a voltage swing on the plate, out-of-phase with the
input signal. It is also important to know that the variation in
current Also appears at the cathode, as a signal which is INPHASE with the input signal. As long as the plate and cathode
resistors are the same value, the amplitude of the two outputs
will be similar, except for the flipped-phase. This is a very
important concept to grasp for later. The main drawback of this
circuit is that it offers no signal voltage gain. What you put in is
what you get out, except one side is flipped-phase. Therefore,
an additional tube stage called a “Driver” is used ahead of it.
The Driver provides the gain, the Cathodyne inverter provides
the necessary phase-flip, and they both live as a happy family.
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Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 4, Issue 8, August 2014)
In an attempt to balance this out, the plate resistor of
the Common-Cathode circuit is slightly reduced,
reducing the gain of that stage. In many tube amps, it will
be reduced to 82K against 100K for the second stage. All
else remaining equal, lowering the plate resistor value
also lowers the gain of the stage. The secondary effect of
this is that the actual signal is not balanced on both sides,
making the output somewhat asymmetrical (i.e. the
positive signal swing is not equal to the negative signal
swing).
Fig14 Booster switch gate signal
The static inverter is designed for AC output voltage
of 220VAC. Load power factor is 0.8 (harmonic power
factor). This gives the inverter output power of 176Watts.
The block diagram has three modules.
- Power Circuit Module
- Control Circuit Module
- Driver Circuit Module
Fig15 Generated SPWM signal to drive the inverter switches using
the PIC.
VI. CONCLUSION
The design and implementation of a single-phase
inverter that produces a symmetric ac output voltage of
desired magnitude and frequency. The digital signal
Peripheral Interface Controller of Microchip Technology
is used for the implementation of the inverter. The
Inverter consists of four bidirectional switches that is
used to convert the voltage. Sinusoidal Pulse Width
Modulation is used for triggering the gates of IGBTs.
The control circuit consists of the PIC controller that is
used to produce required PWM signal. The voltage and
the frequency can be varied by connecting the controller
to the computer. The simulation of the circuit is done
using Simulink of Matlab. The outputs for variable AC
voltages are observed in the CRO with comparison of the
standard values against those of Simulation waveforms
and the output waveforms.
Fig.13 shows the booster required signal
V. EXPERIMENTAL RESULTS
After programming the PIC; they were tested in order
to show the output signals. Fig.13 shows the booster
required signal which was generated by the PIC and
displayed using the oscilloscope.
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International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 4, Issue 8, August 2014)
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