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**Power Electronics Lecture-11 Inverters Dr. Imtiaz Hussain**

Associate Professor URL :

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Introduction Converts DC to AC power by switching the DC input voltage (or current) in a pre-determined sequence so as to generate AC voltage (or current) output.

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Methods of Inversion Rotary inverters use a DC motor to turn an AC Power generator, the provide a true sine wave output, but are inefficient, and have a low surge capacity rating Electrical inverters use a combination of ‘chopping’ circuits and transformers to change DC power into AC. They are much more widely used and are far more efficient and practical.

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TYPICAL APPLICATIONS Un-interruptible power supply (UPS)

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TYPICAL APPLICATIONS Traction

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TYPICAL APPLICATIONS HVDC (High Voltage Direct Current)

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Types of Inverters There are three basic types of dc-ac converters depending on their AC output waveform: Square wave Inverters Modified sine wave Inverters Pure sine wave Inverters

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Square Wave Inverters The square wave is the simplest and cheapest type, but nowadays it is practically not used commercially because of low power quality (THD≈45%).

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**Modified Sine wave Inverters**

The modified sine wave topologies provide rectangular pulses with some dead spots between positive and negative half-cycles. They are suitable for most electronic loads, although their THD is almost 24%. They are the most popular low-cost inverters on the consumer market today,

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**Pure Sine Wave Inverters**

A true sine wave inverter produces output with the lowest total harmonic distortion (normally below 3%). It is the most expensive type of AC source, which is used when there is a need for a sinusoidal output for certain devices, such as medical equipment, laser printers, stereos, etc. This type is also used in grid-connected applications.

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**Simple square-wave inverter**

To illustrate the concept of AC waveform generation

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**AC Waveform Generation**

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AC Waveforms

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**Output voltage harmonics**

Harmonics may cause degradation of equipment (Equipment need to be “de-rated”). Total Harmonic Distortion (THD) is a measure to determine the “quality” of a given waveform. 𝑇𝐻𝐷 𝑣 = 𝑛=2 ∞ 𝑉 𝑛, 𝑅𝑀𝑆 𝑉 1, 𝑅𝑀𝑆 𝑇𝐻𝐷 𝑖 = 𝑛=2 ∞ 𝐼 𝑛, 𝑅𝑀𝑆 𝐼 1, 𝑅𝑀𝑆

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Fourier Series Study of harmonics requires understanding of wave shapes. Fourier Series is a tool to analyse wave shapes. Where, 𝑣 𝑡 = 𝑎 𝑜 + 𝑛=1 ∞ 𝑎 𝑛 cos 𝑛𝜃 + 𝑏 𝑛 sin 𝑛𝜃 𝑎 𝑜 = 1 𝜋 0 2𝜋 𝑣 𝑡 𝑑𝜃 𝑎 𝑛 = 1 𝜋 0 2𝜋 𝑣 𝑡 cos 𝑛𝜃 𝑑𝜃 𝑏 𝑛 = 1 𝜋 0 2𝜋 𝑣 𝑡 sin 𝑛𝜃 𝑑𝜃

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**Harmonics of square-wave**

𝑎 𝑜 = 1 𝜋 0 2𝜋 𝑣 𝑡 𝑑𝜃 𝑎 𝑜 = 1 𝜋 0 𝜋 𝑉 𝑑𝑐 𝑑𝜃 + 𝜋 2𝜋 −𝑉 𝑑𝑐 𝑑𝜃 𝑎 𝑜 =0

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**Harmonics of square-wave**

𝑎 𝑛 = 1 𝜋 0 2𝜋 𝑣 𝑡 cos 𝑛𝜃 𝑑𝜃 𝑎 𝑛 = 1 𝜋 0 𝜋 𝑉 𝑑𝑐 cos 𝑛𝜃 𝑑𝜃 + 𝜋 2𝜋 − 𝑉 𝑑𝑐 cos 𝑛𝜃 𝑑𝜃 𝑎 𝑛 = 𝑉 𝑑𝑐 𝜋 0 𝜋 cos 𝑛𝜃 𝑑𝜃 − 𝜋 2𝜋 cos 𝑛𝜃 𝑑𝜃 =0

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**Harmonics of square-wave**

𝑏 𝑛 = 1 𝜋 0 2𝜋 𝑣 𝑡 sin 𝑛𝜃 𝑑𝜃 𝑏 𝑛 = 1 𝜋 0 𝜋 𝑉 𝑑𝑐 sin 𝑛𝜃 𝑑𝜃 + 𝜋 2𝜋 − 𝑉 𝑑𝑐 sin 𝑛𝜃 𝑑𝜃 𝑏 𝑛 = 𝑉 𝑑𝑐 𝜋 0 𝜋 sin 𝑛𝜃 𝑑𝜃 − 𝜋 2𝜋 sin 𝑛𝜃 𝑑𝜃 = 2 𝑉 𝑑𝑐 𝑛𝜋 1−cos(𝑛𝜋) When n is even 𝑏 𝑛 =0 When n is odd 𝑏 𝑛 = 4 𝑉 𝑑𝑐 𝑛𝜋

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**Harmonics of square-wave**

𝑣 𝑡 = 𝑎 𝑜 + 𝑛=1 ∞ 𝑎 𝑛 cos 𝑛𝜃 + 𝑏 𝑛 sin 𝑛𝜃 𝑣 𝑡 = 𝑛=1 ∞ 𝑏 𝑛 sin 𝑛𝜃 Where, 𝑏 𝑛 = 4 𝑉 𝑑𝑐 𝑛𝜋 𝑛 even 𝑛 odd 𝑣 𝑡 = 4 𝑉 𝑑𝑐 𝜋 𝑛=1,3,5… ∞ 1 𝑛 sin 𝑛𝜃

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**Harmonics of square-wave**

Spectra characteristics Harmonic decreases as n increases. It decreases with a factor of (1/n). Even harmonics are absent. Nearest harmonics is the 3rd. If fundamental is 50Hz, then nearest harmonic is 150Hz.

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**Harmonics of square-wave**

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Filtering Low-pass filter is normally fitted at the inverter output to reduce the high frequency harmonics.

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**Topologies of Inverters**

Voltage Source Inverter (VSI) Where the independently controlled ac output is a voltage waveform. In industrial markets, the VSI design has proven to be more efficient, have higher reliability and faster dynamic response, and be capable of running motors without de-rating. Current Source Inverter (CSI) Where the independently controlled ac output is a current waveform. These structures are still widely used in medium-voltage industrial applications, where high-quality voltage waveforms are required.

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**1-∅ Voltage source Inverters**

Single phase voltage source inverters are of two types. Single Phase Half Bridge voltage source inverters Single Phase full Bridge voltage source inverters

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1-∅ Half Bridge VSI Figure shows the power topology of a half-bridge VSI, where two large capacitors are required to provide a neutral point N, such that each capacitor maintains a constant voltage vi /2. It is clear that both switches S+ and S− cannot be on simultaneously because a short circuit across the dc link voltage source vi would be produced.

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1-∅ Half Bridge VSI Figure shows the ideal waveforms associated with the half-bridge inverter.

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**Note: Turn off circuitry for thyristor is not shown for simplicity**

1-∅ Half Bridge VSI The gating signals for thyristors and resulting output voltage waveforms are shown below. 𝑣 𝑜 = 𝑉 𝑠 <𝑡<𝑇/2 − 𝑉 𝑠 𝑇/2<𝑡<𝑇 Note: Turn off circuitry for thyristor is not shown for simplicity

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1-∅ Full Bridge VSI This inverter is similar to the half-bridge inverter; however, a second leg provides the neutral point to the load. It can be observed that the ac output voltage can take values up to the dc link value vi, which is twice that obtained with half-bridge VSI topologies.

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1-∅ Full Bridge VSI Figure shows the ideal waveforms associated with the half-bridge inverter. 𝑣 𝑖

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1-∅ Full Bridge VSI The gating signals for thyristors and resulting output voltage waveforms are shown below. 𝑣 𝑜 = 𝑉 𝑠 <𝑡<𝑇/2 − 𝑉 𝑠 𝑇/2<𝑡<𝑇

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3-∅ Full Bridge VSI Single-phase VSIs cover low-range power applications and three-phase VSIs cover medium- to high-power applications. The main purpose of these topologies is to provide a three phase voltage source, where the amplitude, phase, and frequency of the voltages should always be controllable.

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**1-∅ VSI using transistors**

Single-phase half bridge and full bridge voltage source inverters using transistors are shown below.

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Example-1 A full bridge single phase voltage source inverter is feeding a square wave signals of 50 Hz as shown in figure below. The DC link signal is 100V. The load is 10 ohm. Calculate THDv THDv by first three nonzero harmonics 100V -100V

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Example-1 To calculate the harmonic contents we need to expand the output waveform into Fourier series expansion. Since output of the inverter is an odd function with zero offset, therefore 𝑎 𝑜 and 𝑎 𝑛 will be zero. 𝑣 𝑜 = 𝑎 𝑜 + 𝑛=1 ∞ 𝑎 𝑛 cos 𝑛𝜃 + 𝑏 𝑛 sin 𝑛𝜃 100V -100V

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**Example-1 Where, 𝑏 𝑛 = 1 𝜋 0 2𝜋 𝑣 𝑜 𝑡 sin 𝑛𝜃 𝑑𝜃**

100V -100V 𝑏 𝑛 = 4 𝑉 𝑜 𝑛𝜋 𝑛 even 𝑛 odd 𝑣 𝑡 = 4 𝑉 𝑜 𝜋 𝑛=1,3,5… ∞ 1 𝑛 sin 𝑛𝜃

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**Example-1 THDv can be calculated as**

Fourier series can be further expanded as 𝑇𝐻𝐷 𝑣 = 𝑛=2 ∞ 𝑉 𝑛, 𝑅𝑀𝑆 𝑉 1, 𝑅𝑀𝑆 𝑣 𝑡 = 4 𝑉 𝑜 𝜋 𝑛=1,3,5… ∞ 1 𝑛 sin 𝑛𝜃 𝑣 𝑡 = 400 𝜋 sin 𝜃 𝜋 sin (3𝜃) 𝜋 sin (5𝜃) 𝜋 sin 7𝜃 𝜋 sin 9𝜃 +…

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Example-1 𝑣 𝑡 = 400 𝜋 sin 𝜃 𝜋 sin (3𝜃) 𝜋 sin (5𝜃) 𝜋 sin 7𝜃 𝜋 sin 9𝜃 +… 𝑇𝐻𝐷 𝑣 = 𝑉 3, 𝑅𝑀𝑆 𝑉 5, 𝑅𝑀𝑆 𝑉 7, 𝑅𝑀𝑆 𝑉 9, 𝑅𝑀𝑆 2 +… 𝑉 1, 𝑅𝑀𝑆 𝑇𝐻𝐷 𝑣 = ×400 3𝜋 ×400 5𝜋 ×400 7𝜋 ×400 9𝜋 2 +… ×400 𝜋 𝑇𝐻𝐷 𝑣 = … 𝑇𝐻𝐷 𝑣 =0.45 𝑇𝐻𝐷 𝑣 =45%

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**Example-1 THDv by first three nonzero harmonics**

𝑇𝐻𝐷 𝑣 = 𝑉 3, 𝑅𝑀𝑆 𝑉 5, 𝑅𝑀𝑆 𝑉 7, 𝑅𝑀𝑆 𝑉 1, 𝑅𝑀𝑆 𝑇𝐻𝐷 𝑣 = ×400 3𝜋 ×400 5𝜋 ×400 7𝜋 ×400 𝜋 𝑇𝐻𝐷 𝑣 = 𝑇𝐻𝐷 𝑣 =0.41 𝑇𝐻𝐷 𝑣 =41%

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**End of Lecture-11 To download this lecture visit**

End of Lecture-11

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