Page 57 – Ultra Hard Mixed Test (Multilevel + PWM)

Subject: Power Electronics
Level: GATE / PSU Ultra Advanced

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🔹 Q1.

A CHB inverter has 4 H-bridges per phase. Find number of output levels.

Levels = 2n + 1 = 2(4) + 1 = 9

Answer: 9 Levels

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🔹 Q2.

For a 5-level NPC inverter, find number of clamping diodes per phase.

Diodes = (m−1)(m−2) = (5−1)(5−2) = 4×3 = 12

Answer: 12 diodes

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🔹 Q3.

For a 7-level FC inverter, find number of flying capacitors per phase.

Capacitors = (m−1)(m−2)/2 = (7−1)(7−2)/2 = (6×5)/2 = 15

Answer: 15 capacitors

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🔹 Q4.

A 3-level NPC inverter operates with Vdc = 900 V. Find switch voltage stress.

Stress = Vdc/(m−1) = 900/(3−1) = 450 V

Answer: 450 V

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🔹 Q5.

In Phase-Shifted PWM for 7-level CHB, find carrier phase shift.

Number of carriers = m−1 = 6 Phase shift = 360°/6 = 60°

Answer: 60°

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🔹 Q6.

Increasing number of levels mainly reduces: A) Switching loss B) Voltage rating C) THD D) DC source

Answer: C

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🔹 Q7.

Which inverter requires multiple isolated DC sources? A) NPC B) CHB C) FC D) 2-Level

Answer: B (CHB)

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🔹 Q8.

Which PWM technique is best for CHB inverter? A) Level-shifted B) Phase-shifted C) Sinusoidal only D) Random PWM

Answer: B

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🔹 Q9.

For 9-level NPC inverter, find switch stress if Vdc = 1200 V.

Stress = 1200/(9−1) = 1200/8 = 150 V

Answer: 150 V

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🔹 Q10.

More flying capacitors lead to: A) Easier balancing B) Complex control C) Lower switching frequency D) Lower voltage stress

Answer: B

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 Ultra Memory Capsule

✔ CHB → Multiple DC sources ✔ NPC → Many diodes ✔ FC → Many capacitors ✔ Stress = Vdc/(m−1) ✔ Levels = 2n + 1 ✔ PS-PWM shift = 360°/(m−1)

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Ultra Advanced Mixed Test – Shaktimatha Learning

 

Page 56 – PWM Techniques for Multilevel Inverters

Subject: Power Electronics
Level: GATE / PSU / Interview Advanced

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🔹 1. Why PWM for Multilevel Inverters?

PWM is used to:

• Control output voltage magnitude
• Reduce THD
• Improve waveform quality
• Balance capacitor voltages

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🔹 2. Types of PWM Techniques

1️⃣ Level-Shifted PWM (LS-PWM)

Used mainly in NPC and FC inverters. Multiple carrier signals are vertically shifted.

Types:

• PD (Phase Disposition)
• POD (Phase Opposition Disposition)
• APOD (Alternate Phase Opposition Disposition)

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2️⃣ Phase-Shifted PWM (PS-PWM)

Used mainly in Cascaded H-Bridge (CHB). Carrier signals are phase shifted by:

Phase shift = 360° / number of carriers

Example: For 5-level → 4 carriers Phase shift = 360° / 4 = 90°

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3️⃣ Space Vector PWM (SVPWM)

Extension of conventional SVPWM to multilevel inverters. Provides better DC bus utilization and lower THD.

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🔹 3. Comparison of PWM Methods

PWM Method	Used For	Complexity	THD
Level-Shifted PWM	NPC, FC	Medium	Low
Phase-Shifted PWM	CHB	Simple	Low
SVPWM	All Types	High	Very Low

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🔹 4. Important GATE Concepts

✔ Phase shift in PS-PWM = 360° / carriers ✔ LS-PWM uses vertically shifted carriers ✔ SVPWM gives better DC bus utilization ✔ More levels → easier harmonic reduction ✔ Capacitor balancing important in FC

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PWM Techniques Series – Shaktimatha Learning

 

Page 55 – Complete Comparison of Multilevel Inverters

Subject: Power Electronics
Level: GATE / PSU / Interview Revision

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🔹 1. Overview

There are three major types of Multilevel Inverters:

• Cascaded H-Bridge (CHB)
• Neutral Point Clamped (NPC)
• Flying Capacitor (FC)

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🔹 2. Comparison Table

Parameter	CHB	NPC	Flying Capacitor
DC Source	Multiple Isolated	Single (Split)	Single Source
Clamping Diodes	Not Required	(m−1)(m−2)	Not Required
Flying Capacitors	Not Required	Not Required	(m−1)(m−2)/2
Switches per Phase	4n	2(m−1)	2(m−1)
Voltage Stress	Vdc	Vdc/(m−1)	Vdc/(m−1)
Control Complexity	Simple	Medium	High
Main Issue	Multiple DC supplies	Neutral balancing	Capacitor balancing
Best Application	Solar, Drives	HVDC, FACTS	Reactive power control

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🔹 3. Quick Formula Summary

✔ CHB Levels = 2n + 1 ✔ NPC Diodes = (m−1)(m−2) ✔ FC Capacitors = (m−1)(m−2)/2 ✔ Switch stress (NPC & FC) = Vdc/(m−1) ✔ More levels → Lower THD

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🔹 4. GATE Exam Trick

If question says:
• Multiple DC sources → CHB
• Many diodes → NPC
• Many capacitors → Flying Capacitor

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Multilevel Inverter Comparison Series – Shaktimatha Learning

 

Page 54 – Flying Capacitor Multilevel Inverter

Subject: Power Electronics
Level: GATE / PSU / Interview

                                             

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🔹 1. What is Flying Capacitor Inverter?

Flying Capacitor (FC) Multilevel Inverter uses capacitors instead of clamping diodes to generate intermediate voltage levels.

It is also called Capacitor-Clamped Multilevel Inverter.

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🔹 2. Basic 3-Level FC Structure

• 4 switches per phase
• Flying capacitor
• Single DC source

Output Levels: +Vdc/2 0 −Vdc/2

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🔹 3. Working Principle

Capacitors are charged to specific voltages and inserted in series with DC source to produce stepped output.

Voltage balancing is achieved by controlling switching sequence.

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🔹 4. Number of Capacitors

Number of flying capacitors per phase = (m − 1)(m − 2) / 2

Where m = number of levels Example: For 3-level → (3−1)(3−2)/2 = 1 capacitor

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🔹 5. Voltage Stress

Switch voltage stress = Vdc / (m − 1)

Same as NPC inverter.

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🔹 6. Advantages

• No clamping diodes required
• Better redundancy in switching states
• Good for reactive power control
• Reduced voltage stress

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🔹 7. Disadvantages

• Large number of capacitors
• Complex voltage balancing
• Higher cost
• Bulky structure

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🔹 8. Comparison Insight

• NPC → Many diodes
• CHB → Many isolated DC sources
• FC → Many capacitors

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🔥 Exam Important Formulas

✔ Flying Capacitors = (m − 1)(m − 2)/2 ✔ Switch stress = Vdc/(m − 1) ✔ Output levels = m ✔ Complex voltage balancing control

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Multilevel Inverter Series – Shaktimatha Learning

 

Page 53 – Numerical Problems (NPC Multilevel Inverter)

Subject: Power Electronics
Level: GATE / PSU Advanced Numericals

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🔹 Problem 1 – Number of Clamping Diodes

Find the number of clamping diodes per phase for a 4-level NPC inverter.

Formula: Diodes = (m − 1)(m − 2)

m = 4 Diodes = (4 − 1)(4 − 2) = 3 × 2 = 6 diodes per phase

Final Answer: 6 diodes per phase

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🔹 Problem 2 – Switch Voltage Stress

A 3-level NPC inverter operates with Vdc = 800 V. Find maximum voltage stress across each switch.

Switch stress = Vdc / (m − 1) For 3-level: = 800 / (3 − 1) = 800 / 2 = 400 V

Final Answer: 400 V

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🔹 Problem 3 – Output Voltage Levels

For a 3-level NPC inverter with Vdc = 600 V, list output levels.

Output levels: +Vdc/2 = +300 V 0 −300 V

Answer: +300 V, 0, −300 V

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🔹 Problem 4 – Total Switch Count

Find total number of switches in a 3-phase 3-level NPC inverter.

Switches per phase = 4

Total = 3 × 4 = 12 switches

Final Answer: 12 switches

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🔹 Problem 5 – Diodes in 3-Phase System

For 3-phase 4-level NPC inverter, find total clamping diodes.

Per phase = 6 diodes Total = 3 × 6 = 18 diodes

Final Answer: 18 diodes

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🔹 Problem 6 – Compare Voltage Stress

Compare switch stress in: 1) 2-level inverter 2) 3-level NPC inverter Assume Vdc = 900 V.

2-level → stress = 900 V 3-level NPC → stress = 900 / 2 = 450 V

Conclusion: NPC reduces stress by 50%

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🔥 Exam Memory Section

✔ Diodes = (m − 1)(m − 2) ✔ Switch stress = Vdc / (m − 1) ✔ 3-Level → 4 switches per phase ✔ NPC reduces voltage stress ✔ Balancing of neutral point required

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NPC Numerical Series – Shaktimatha Learning

 

Page 52 – NPC (Neutral Point Clamped) Multilevel Inverter

Subject: Power Electronics
Level: GATE / PSU / Interview

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🔹 1. What is NPC Inverter?

NPC (Neutral Point Clamped) inverter uses clamping diodes to divide the DC bus voltage into multiple levels.

It is also called Diode-Clamped Multilevel Inverter.

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🔹 2. Basic 3-Level NPC Structure

A 3-level NPC inverter has:

• 4 switches per phase
• 2 clamping diodes
• Split DC capacitor (Vdc/2 + Vdc/2)

Output levels: +Vdc/2 0 −Vdc/2

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🔹 3. Working Principle

The clamping diodes connect the output to the neutral point, limiting voltage stress on switches.

Switch voltage stress = Vdc / (number of levels − 1)

For 3-level: Switch stress = Vdc / 2

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🔹 4. Number of Diodes Formula

Number of clamping diodes per phase = (m − 1)(m − 2)

Where: m = number of levels Example: For 3-level → (3−1)(3−2) = 2 diodes

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🔹 5. Advantages

• Reduced voltage stress
• Better harmonic performance
• High voltage applications
• No isolated DC sources required

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🔹 6. Disadvantages

• Large number of diodes for higher levels
• Neutral point voltage balancing issue
• Complex control for higher levels

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🔹 7. Comparison with CHB

• NPC → Single DC source (split capacitor)
• CHB → Multiple isolated DC sources
• NPC → More diodes
• CHB → More DC supplies

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 Important Exam Points

✔ 3-Level NPC → 4 switches + 2 diodes ✔ Output: +Vdc/2, 0, −Vdc/2 ✔ Diodes = (m−1)(m−2) ✔ Switch stress reduced ✔ Neutral balancing required

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Multilevel Inverter Series – Shaktimatha Learning

 

Page 51 – Numerical Problems (5-Level CHB Inverter)

Subject: Power Electronics
Level: GATE / PSU Advanced Numericals

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🔹 Problem 1 – Number of Levels

A cascaded H-bridge inverter uses 3 identical H-bridge cells per phase. Find number of output voltage levels.

Formula: Levels = 2n + 1

n = 3 Levels = 2(3) + 1 = 7

Final Answer: 7 levels

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🔹 Problem 2 – Maximum Output Voltage

A 5-level CHB inverter uses two DC sources of 200 V each. Find maximum output voltage.

Maximum output = +2Vdc = 200 + 200 = 400 V

Final Answer: 400 V

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🔹 Problem 3 – Voltage Steps

For a 5-level inverter with Vdc = 250 V, list all output voltage levels.

+2Vdc = +500 V +Vdc = +250 V 0 −250 V −500 V

Answer: +500, +250, 0, −250, −500 V

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🔹 Problem 4 – Total Number of Switches

Each H-bridge uses 4 switches. For a 5-level CHB inverter (2 H-bridges per phase), find total switches per phase.

Switches per H-bridge = 4 Total = 2 × 4 = 8 switches per phase

Final Answer: 8 switches per phase

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🔹 Problem 5 – 3-Phase System Switch Count

For a 3-phase 5-level CHB inverter, find total number of switches.

Per phase = 8 switches 3 phases → 3 × 8 = 24 switches

Final Answer: 24 switches

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🔹 Problem 6 – THD Concept

If number of levels increases from 5 to 7, what happens to THD?

More levels → waveform closer to sine → THD decreases.

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🔹 Problem 7 – Required DC Sources

A 9-level CHB inverter is required. Find number of H-bridge cells per phase.

Levels = 2n + 1

9 = 2n + 1 2n = 8 n = 4

Final Answer: 4 H-bridges per phase

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 Exam Memory Section

✔ Levels = 2n + 1 ✔ Switches per phase = 4n ✔ More levels → Lower THD ✔ 5-level → 2 DC sources ✔ 7-level → 3 DC sources

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Multilevel Inverter Numerical Series – Shaktimatha Learning