power elctronics

1. Power Electronics
(MCT-321)
Lecture # 14
Course Instructor:
Engr. Hassan Mujtaba
University of Engineering & Technology Lahore, Faisalabad Campus

2. Outline
• Advanced modulation techniques
• Voltage control of three-phase inverters
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3. Advanced Modulation Techniques
• The techniques for improved performances are:
1. Trapezoidal modulation
2. Staircase modulation
3. Stepped modulation
4. Harmonic injection modulation
5. Delta modulation
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4. Trapezoidal Modulation
• In this, gating signals are generated by comparing triangular
carrier with trapezoidal modulating wave.
• Trapezoidal wave can be produced by limiting a triangular wave,
so that:
Ar = σ Ar(max) ; σ = triangular factor
• Modulation index varies from 0 to 1 and is given by:
M =
𝐴𝑟
𝐴𝑐
=
𝜎 𝐴𝑟(𝑚𝑎𝑥)
𝐴𝑐
• For fixed values of 𝐴𝑟(𝑚𝑎𝑥) and 𝐴𝑐 , M varies by changing 𝜎.
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5. Trapezoidal Modulation
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6. Staircase Modulation
• In this, the modulating signal in staircase wave.
• The levels of stairs are calculated to eliminate
specific harmonics.
• It’s not recommended for less than 15 pulses in one
cycle.
• It provides a high quality output by selecting desired
number of steps and modulation frequency ratio.
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7. Staircase Modulation
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8. Stepped Modulation
• In this, the modulating signal is stepped wave which
is divided into specific intervals.
• Each interval in controlled individually to control
fundamental component and to eliminate specific
harmonics.
• It gives low distortion but higher fundamental
amplitude as compared to normal PWM.
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9. Stepped Modulation
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10. Harmonic Injected Modulation
• In this, the modulating signal is generated by
injecting selected harmonics to sine wave which
results in flat-topped waveform.
• It provides an output of higher fundamental
amplitude and low distortion.
• Each arm is OFF for one-third of period, so heating
of switching devices is reduced.
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11. Harmonic Injected Modulation
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12. Delta Modulation
• It’s also known as Hysteresis modulation.
• In this, a modulating triangular wave is allowed to
oscillated within a defined area above and below a
reference sine wave.
• The switching pulse which is same as output, is
generated from vertices of triangular wave.
• It is used in ac motor control because it can control
voltage to frequency ratio.
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13. Delta Modulation
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14. Voltage Control of Three-Phase
Inverters
• It’s considered to be 3 single-phase inverters whose
outputs are shifted by 120°.
• Most commonly used techniques are:
1. Sinusoidal PWM
2. Third-harmonic PWM
3. 60° PWM
4. Space vector modulation
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15. Sinusoidal PWM
• It consisted of three sinusoidal reference waves
shifted by 120°.
• The carrier wave is compared with the reference
signals to generate gating signals.
• Output is generated by eliminating the condition that
2 switching devices cannot conduct at the same time.
• The harmonics at frequencies multiple of 3 are
identical in amplitude and phase.
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16. Sinusoidal PWM
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17. 60-degree PWM
• It’s used for flat-top waveform from 60° – 120° and
240° – 300°.
• The power devices are kept ON for one-third of
cycle and reduced switching losses.
• All triple harmonics (3rd, 9th, 15th…) are absent in
3-phase voltages.
• The output waveform can be approximated by
fundamental component and first few terms.
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18. Third-harmonic PWM
• It’s implemented in the same manner as sinusoidal
PWM. The only difference is that the reference ac
signal is not sinusoidal but consist of both
fundamental and 3rd harmonic component.
• In this, p-p amplitude of resulting reference function
does not exceed dc supply voltage but fundamental
component is higher than this.
• It provides better utilization of dc supply voltage
than sinusoidal PWM.
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19. Space Vector Modulation
• SVM treats inverter as a single unit.
• The control strategies are implemented in digital systems.
• It’s objective is to create PWM load line that are in average
equal to given load line voltages.
• It’s done in each sampling period by properly selecting
switch states and calculation of appropriate time period
for each state.
• It has the advantage of lower harmonics and higher
modulation index.
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20. Summary
• Any modulation technique can be used to produce variable frequency and
variable voltage ac waveform.
• Sinusoidal PWM compares high frequency triangular carrier with 3
sinusoidal reference signals to generate gating signals for inverter switches.
• 3rd-harmonic PWM is preferred in 3-phase applications for best utilization
of dc voltage because of cancellation of 3rd-harmonic component.
• In SVM, three voltages are simultaneously taken into account.
• The commonly used SPWM suffers some drawbacks (low fundamental
output voltage) which is overcome by advanced modulation techniques.
• With a proper choice of switching pattern, certain harmonics can be
eliminated in inverters.
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21. Coming Up
• Quiz
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