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SINGLE-PHASE-THREE-LEG-ACAC-CONVERTER

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					 RECENT TRENDS IN POWER ELECTRONICS

  SINGLE PHASE THREE LEG AC/AC CONVERTER


                      BY




ANNAMACHARYA INSTITUTE OF TECHNOLOGY & SCIENCES

           NEW BOYANAPALLI, RAJAMPET,

                 KADAPA(DIST),

                      A.P.
ABSTRACT:
This paper investigates a single-phase three-leg ac/ac reversible converter in which a leg
is shared by both the grid and the load side. The single phase ac power supply can be
converted into ac by means of a four leg converter which uses two full bridges and a
two leg converter using two half bridges respectively. The demerits like the harmonic
distortion, large number of power devices, and complex circuitry are present in the
above mentioned techniques. These demerits can be over come by a better approach
using single phase three leg ac/ac converter.
 In this converter the grid side leg and the common leg performs rectifier operation and
the load side leg and the common leg performs inverter operation. Pulse width-
modulation (PWM) techniques based on scalar PWM is employed for controlling the
converter output.Here the scalar PWM technique sine PWM is employed. In addition,
several relevant characteristics of the converter are addressed, such as voltage rating,
and power rating and output of the converter. The converter is compared to four-leg and
two-leg converters. Experimental results are presented.

  Index Terms—AC/AC converter, converter control, reduced switch count.

                                   I. INTRODUCTION
         It is possible to implement inverter and rectifier systems employing minimized
component converters as proposed in the case of single-phase to three-phase ac/ac
converters and three-phase to three-phase ac/ac converters. An indirect single-phase
ac/ac reversible converter can use single-phase four-leg (eight switches) converters
(i.e., two full-bridge topologies) with a dc-bus capacitor link. However, such a converter
has a relatively large number of power devices. An alternative to this is a single-phase
ac/ac converter system employing a two-leg (four switches) converter (i.e., two half
–bridge topologies). Another interesting alternative is the three leg (six switches)
converter. A control strategy for output voltage-regulated applications in order to
obtain           maximum            utilization           of         the          dc-bus
voltage, based on the synchronization between input and output converter voltages In
this paper several aspects concerning the three leg converter are presented.
                        Fig. 1. Single-phase ac/ac converters. (a) Four-leg
                        converter (two full bridges). (b) Two-leg converter (two
                        half bridges).




II. FEATURES OF THREE- LEG CONVERTER
        The basic scheme of the ac/ac converter presented in this paper is shown in
Fig. 2. It comprises three legs (six semiconductor switches) and a capacitor bank at the
dc bus. In a three leg converter one leg of switches is saved compared to the cascaded
full bridge converter and the reduction in conduction and

switching losses implies increase in efficiency of the system as the conduction and the
switching losses are reduced. To operate as a rectifier and the inverter simultaneously,
the switching control of the common leg is a necessary requirement for the three leg
converter .A three leg converter can supply constant sinusoidal output voltage even if
the power supply voltage is changed by changing the load or sudden power failures
and it can process the same power capacity as the conventionally used converter
systems. The output voltage waveform during zero crossing is smooth and the control
strategy is simpler than the conventional converter systems. Cost, size and control
circuitry is reduced.
III. EXISTING MODELS

III.1. FOUR LEG CONVERTER

       The basic circuit diagram of a single phase four leg ac/ac converter is
as shown in fig.1.a. It uses two full bridges consisting of eights switches and
a dc link capacitor for reducing the ripple in the dc output. The inductor is
used for boosting up the voltage. Here converter G acts as a rectifier and the
converter L acts as an inverter. The operation of the converter can be
explained as follows:

       During the positive half cycle of the supply voltage switches qg1&qg2’
conducts and the capacitor charges upto supply voltage and we get dc output
across the capacitor

       During this period for getting inverter operation switches q l2&ql1 ’
conducts and the capacitor discharges through the conducting switches so that
we get the ac output across the load.

       Similarly during the negative half cycle of the supply voltage the
switches qg2&q g1’ conducts for rectifier operation and switches q l1&ql2 ’
conducts for inverter operation. In this method the draw backs like large
number of power devices, large control circuitry, more switching and
conduction losses and less efficiency are present.

III.2. TWO LEG CONVERTER

       The basic circuit of single phase two leg ac/ac converter is as shown in
fig.1.b.It uses two half bridges consisting of four switches and converts the ac
supply to ac. In this converter even though the switch count and losses are
reduced only half of the output can be controlled.

       So in order to over come the draw backs present in the conventionally
used converters a better approach is use of single phase three leg ac/ac
converter.
IV. THREE LEG CONVERTER




      Fig. 2. Single-phase ac/ac three-leg converter.

        The basic circuit diagram of the three leg converter is as shown in
fig.2. It consists of three legs and six switches. In this converter a leg is
common for both grid side and load side. The grid side leg and the common
leg performs the rectifier operation and the load side leg and the common leg
performs the inverter operation. The operation of three leg converter can be
explained as follows:During the positive half cycle of the supply voltage
switches qg&q a’ in and common conducts and we get rectified
output across the capacitor and for inverter operation in addition to the
switches qg &qa’,switch q l                 in load side leg also triggered and we get ac
output across the load. During negative half cycle switches qa&qg ’ in grid side
and the common leg conducts implying rectified output and for inversion
operation in addition to the switches qa&q g’, switch q l’ also triggered and we
get ac output across the load.

V.PULSE WIDTH MODULATI ON TECHNIQUE

        The most efficient method of controlling the output voltage is to
incorporate PWM control within the inverters. In this method a fixed dc input
voltage is supplied to the inverter and a controlled ac output voltage is
obtained by adjusting the on and off periods of the inverter devices. In this
paper we are employing SCALAR PWM(SINE PWM) technique.

V.1SINUSOIDAL PULSEWIDTH MODULATION

        In sinusoidal pwm the sine wave (reference) is compared with the
triangular wave(carrier) and when the magnitude of the reference wave is
greater than the carrier wave a pulse is generated and these pulses are given to
the switches in the converter circuit for getting proper operation and also for
reducing the harmonics. The ratio of amplitude of the reference wave to the
amplitude of the carrier wave is called as the modulation index M. By varying
the value of modulation index we can change the pulse width according to our
convenience.

VI.ADVANTAGES OF THE THREE LEG CONVERTER

    number of power devices used is reduced so that the switching and
       conduction losses are reduced.

    dc output voltage across the capacitor is almost doubled compared
       to the four leg converter.

    voltage and power rating of the circuit can be improved.

    control circuitry used is reduced and hence the complexity can be
       reduced.

    output can be obtained with reduced losses and switches and
       hence the efficiency, power factor can be improved.

   VII. APPLICATIONS

        This converter is used in uninterruptible power supply
           circuits(UPS).

        In power electronic drives for getting four quadrant operation of the
           drives.

  VIII. CONCLUSION:

       From this paper we can conclude that when compared to the
       conventionally used converters,

        are getting the same output in three leg converter and the dc
           voltage across the capacitor is almost double .

        switch count is reduced thereby reducing switching and
           conduction losses and improving efficiency.

        cost, size, complexities of the circuit are reduced.

        voltage , power rating and load ratings can be improved.
                     SIMULATION RESULTS




Fig.1. Circuit diagram of existing model




 Fig.2. Output of the four leg converter
Fig.3 Circuit diagram of three leg converter




Fig .4. Output of the three leg converter




Reference: IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONI CS. VOL .53,NO .2.APRIL 2007

				
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