Total Harmonic Distortion (THD) in Power Electronics - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is Total Harmonic Distortion (THD)?

THD is a measure of how much a signal is distorted by harmonics compared to its fundamental frequency. It shows the amount of unwanted frequencies mixed with the original signal.

beginner

Why is THD important in power electronics?

THD helps us understand the quality of electrical power. High THD means more distortion, which can cause equipment to heat up, malfunction, or become less efficient.

intermediate

How is THD calculated?

THD is calculated by dividing the square root of the sum of the squares of all harmonic voltages or currents by the fundamental frequency voltage or current, then multiplying by 100 to get a percentage.

beginner

What are harmonics in electrical signals?

Harmonics are frequencies that are multiples of the fundamental frequency. For example, if the main frequency is 50 Hz, harmonics could be 100 Hz, 150 Hz, and so on.

intermediate

What can cause high THD in a power system?

Non-linear loads like computers, LED lights, and variable speed drives can create harmonics, increasing THD and causing distortion in the power system.

What does a high THD value indicate in a power system?

A Higher fundamental frequency

B Better power quality

C Lower voltage levels

D More distortion and unwanted harmonics

High THD means there are more unwanted harmonic frequencies causing distortion in the power system.

Which of the following is NOT a harmonic frequency if the fundamental frequency is 60 Hz?

A 90 Hz

B 180 Hz

C 120 Hz

D 240 Hz

Harmonics are multiples of the fundamental frequency. 90 Hz is not a multiple of 60 Hz.

How is THD usually expressed?

A In volts

B As a percentage

C In hertz

D In watts

THD is expressed as a percentage showing the ratio of harmonic distortion to the fundamental frequency.

Which device is likely to increase THD in a power system?

A LED light with driver

B Resistive heater

C Incandescent bulb

D Transformer

LED lights with electronic drivers are non-linear loads that can create harmonics and increase THD.

What is the fundamental frequency in a power system?

A The highest frequency present

B The lowest frequency present

C The main frequency of the power supply

D The frequency of the harmonics

The fundamental frequency is the main frequency of the power supply, like 50 Hz or 60 Hz.

Explain what Total Harmonic Distortion (THD) is and why it matters in electrical power systems.

Think about how unwanted frequencies affect the original signal and devices.

Describe how THD is calculated and what information it gives about a signal.

Consider the relationship between the fundamental and harmonic components.