Revised: Basic Introduction to Harmonics

Introduction to Harmonics

Harmonics have been present since the early 20th century when engineers and scientists first identified discontinuous loads due to the invention of the vacuum tube. Initially, the effects of harmonics were negligible and largely ignored by engineers. However, as technology advanced, with the advent of sophisticated electronics such as electronic lighting, uninterruptible power supplies, programmable logic controllers, and variable frequency drives, harmonics began to introduce Power Quality challenges. The impact of harmonics on the quality of signals produced by these devices necessitated changes in design, filtering processes, and installation procedures. Despite improvements in engineering and general awareness, there is still a need to mitigate harmonics further. This article aims to equip you with essential knowledge to reduce the impact of harmonics.

Definition of Harmonics

In an electrical Power System, harmonics are defined as multiples of the current or voltage at the fundamental frequency. Whenever you observe a waveform deviating from the expected sine wave shape, it contains harmonics.

Causes of Harmonics

AC signals are categorized as linear or nonlinear based on how systems draw power from the supply source. Harmonics are caused by nonlinear systems, which draw currents in short, abrupt pulses. These pulses distort the waveforms, generating harmonics that lead to power problems affecting both the load and the distribution system. Examples of nonlinear load systems include electronic devices like TVs.

Fundamental Electrical Harmonics

Power originates from the generator, and its frequency is referred to as the fundamental frequency or the first harmonic frequency, typically 50 Hz or 60 Hz, depending on the country. All electrical and electronic systems are designed to operate efficiently at this frequency.

Harmonic Orders and Complex Waveforms

Second-Order Harmonics

Second-order harmonics have frequencies at 100 Hz, which is twice the fundamental frequency of 50 Hz. When the fundamental harmonics reach zero, the second-order harmonics reach their peak, and vice versa. This causes the second harmonic to initiate reverse current flow, affecting induction motors by opposing the rotating magnetic field, resulting in lower mechanical torque. This type of harmonic is also known as a negative sequence.

Third-Order Harmonics

Third-order harmonics have a frequency of 150 Hz, three times the fundamental frequency. These harmonics are particularly dangerous. When both third and fundamental harmonics currents reach zero simultaneously, they peak at opposite points, creating a zero-sequence current that increases the neutral voltage in the power system. This increase can trigger circuit breakers due to the third harmonic current, also known as triplens.

Fourth-Order Harmonics

Fourth-order harmonics have a frequency of 200 Hz, four times the fundamental frequency. Both the fundamental and fourth-order harmonics reach their peak values simultaneously. This increases the current flowing in a conductor, which raises the equipment temperature, and is also known as positive harmonics.

Fifth-Order Harmonics

Fifth-order harmonics have a frequency of 250 Hz and share characteristics with third-order harmonics but operate at a higher frequency.

Waveform Analysis

From the waveforms, it is evident that a complex waveform comprises a combination of harmonics and the fundamental waveform, each with its phase angles and peak values. The values of the harmonics can be calculated using the formula:

Second Harmonics: E2 = V(max)Sin(2Π*2ft)

Third Harmonics:  E3 = V(max)Sin(2Π*3ft)

Fourth Harmonics:  E4 = V(max)Sin(2Π*4ft)

This process continues for higher-order harmonics. Thus, the equation for a complex waveform is:

Harmonic Sequencing

Below is a summary of harmonic sequencing, illustrating how frequency changes from the fundamental frequency to higher orders.

Summary of Harmonic Effects

From this article, we can conclude that:

- Harmonics are multiples of the fundamental frequency.

- Harmonics increase the heat generated by a system, affect the voltage levels, and impact motor torque.

- The fundamental frequency is either 50 Hz or 60 Hz, depending on the country.

- Second-order harmonics have a frequency of 100 Hz.

- Third-order harmonics have a frequency of 150 Hz.