Single Phase Bridge VSI

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Single Phase Bridge VSI

Inverter in Power-Electronics refers to a class of power conversion circuits that operate from a dc

voltage source or a dc current source and convert it into a symmetric ac voltage or current. Itdoes reverse of what ac-to-dc converter does. The input to the inverter is a direct dc source or

dc source derived from an ac source. For example, the primary source of input power may beutility ac voltage supply that is converted to dc by an ac - dc rectifier with filter capacitor and

then inverted back to ac using an inverter. Here, the final ac output may be of a differentfrequency and magnitude than the input ac of the utility supply.

If the input dc is a voltage source, the inverter is called a Voltage Source Inverter (VSI). One cansimilarly think of a Current Source Inverter (CSI), where the input to the circuit is a current

source. The VSI circuit has direct control over output (ac) voltage whereas the CSI directly

controls output (ac) current.

The simplest dc voltage source for a VSI may be a battery bank or a solar photovoltaic cells

stack. An ac voltage supply, after rectification into dc can also serve as a dc voltage source. Avoltage source is called stiff, if the source voltage magnitude does not depend on load connectedto it. All voltage source inverters assume stiff voltage supply at the input.

Output of voltage waveforms of ideal inverters should be sinusoidal. However practical inverterwaveforms are non sinusoidal and contain certain harmonics. For low and medium power

applications square wave or quasi square wave voltages are acceptable.

A variable voltage can be obtained by varying the input dc voltage and maintaining the gain of

the inverter constant. On the other hand, if the dc input voltage is fixed then variable output

voltage can be obtained by varying the gain of the inverter. This can be accomplished by Pulse

Width Modulation-PWM control within the inverter. PWM means the width of the square pulsein positive and negative halves can be adjusted according to the rms of the output required. The

inverter gain may be defined as ratio of the ac output (rms) voltage to dc input voltage. In Square

Wave PWM technique the output ac rms voltage is fixed when input dc voltage is fixed.


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Fig 4(a) shows the power circuit diagram for single phase bridge voltage source inverter. In this

four switches (in 2 legs) are used to generate the ac waveform at the output. Any semiconductorswitch like IGBT, MOSFET or BJT can be used. Four switches are sufficient for resistive load

because load current io is in phase with output voltage vo. However this is not true in case of RLload where the io is not in phase with vo and diodes connected in anti-parallel with switch will

allow the conduction of the current when the main switch is turned off. These diodes are called

as Feedback Diodes since the energy is fed back to the dc source.

Fig 4(a) IGBT based Single phase bridge voltage source inverter power circuit diagram.

Square Wave PWM

In full bridge inverter, when T1, T2 conduct the output voltage is Vs and when T3, T4 conducts

the output voltage is -Vs. The switches T1, T2 conducts for period of 0


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PWM techniques for voltage control of Single Phase Inverters

The following PWM techniques are used for controlling the output ac rms voltage and frequency

in an inverter:

Single-Pulse-Width-Modulation

Multiple-Pulse-Width-Modulation Sinusoidal-Pulse-Width-Modulation (SPWM)

Single-Pulse-Width-Modulation

In single pulse width modulation control, there is only one pulse per half cycle and the outputrms voltage is changed by varying the width of the pulse. The gating signals and output voltages

of single pulse-width modulation are shown in fig 4(b). The gating signals are generated bycomparing the rectangular control signal of amplitude Vc with triangular carrier signal Vcar. The

frequency of the control signal determines the fundamental frequency of ac output voltage. Theamplitude modulation index is defined as:


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The rms ac output voltage

where

By varying the control signal amplitude Vc from 0 to Vcar the pulse width ton can be modified

from 0 secs to T/2 secs and the rms output voltage Vo from 0 to Vs.

In multiple PWM control, instead of having a single pulse per half cycle, there will be multiple

number of pulses per half cycle, all of them being of equal width.

Sinusoidal-Pulse-Width-Modulation (SPWM)

In sinusoidal pulse width modulation there are multiple pulses per half-cycle and the width of the

each pulse is varied with respect to the sine wave magnitude. Fig 4(c) shows the gating signalsand output voltage of SPWM with unipolar switching. In this scheme, the switches in the two

legs of the full-bridge inverter are not switched simultaneously, as in the bi-polar scheme.


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Fig 4(b) Gating signals and output voltage of Single pulse-width modulation

In this unipolar scheme the legs A and B of the full-bridge inverter are controlled separately by

comparing carrier triangular wave vcar with control sinusoidal signal vc and -vc respectively. ThisSPWM is generally used in industrial applications. The number of pulses per half-cycle depends

upon the ratio of the frequency of carrier signal (fc) to the modulating sinusoidal signal. The

frequency of control signal or the modulating signal sets the inverter output frequency (fo) and the

peak magnitude of control signal controls the modulation index ma which in turn controls the rmsoutput voltage. The area of each pulse corresponds approximately to the area under the sine wave

between the adjacent midpoints of off periods on the gating signals. Ifton is the width of nth

pulse, the rms output voltage can be determined by:


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The amplitude modulation index is defined as:

where, = peak magnitude of control signal (modulating sine wave)

= peak magnitude of carrier signal (triangular signal)

Thefrequency modulation ratio is defined as:

where, = frequency of control signal (sine signal)

= frequency of carrier signal (triangular signal)

Applications

Uninterruptible Power Supply (UPS),

Adjustable Speed Drives (ASD) for ac motors, Electronic frequency changer circuits used in induction heating, welding etc.,

HVDC transmission at lower power levels

Renewable Energy such as solar, fuel cell to AC conversion Electronic Ballast and Compact Fluorescent lamps

Active Filters for power quality improvement Custom power devices: DSTACTCOM, DVR, UPQC,

FACTS: STATCOM, SSSC, UPFC, etc.

Sinusoidal Pulse Width Modulation (SPWM)

Apply multiple gate signals to the IGBTs T1, T2 in positive half-cycle and T3, T4 in negativehalf-cycle. The gate signals for T1, T2 are generated by comparing the high frequency (fcar)

triangular carrier signal vcar1 with sine control signal vc1 of frequency fo. Similarly, the gate signals

for T3, T4 are generated by comparing thevcar2 (=vcar1)

with sine control signalvc2 = -vc1

offrequency fo. The peak magnitude of control sine signal controls the modulation index ma which

in turn controls the rms output voltage.

Observe the waveforms across each element of the power circuit with R and RL loads.

The output voltage and current can be observed for different values ofRo, Lo, Vdc, Vc1, Vc2, ma, mf orfo.


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Main Circuit Diagram

Circuit Diagram With Voltmeters And Ammeters At Various Points


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Waveforms

For R Load

R = 10


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For RL Load.

R = 1 , L = 10 mH


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Single Phase Bridge VSI