WHAT ARE DIFFERENT TYPES OF HARMONICS?

Harmonics can be classified into various types based on different criteria:

Classification by Frequency Relation

Integer Harmonics:

In electrical systems, integer harmonics are unwanted voltages or currents that operate at frequencies that are integer multiples of the fundamental (or supply) frequency. For 50 Hz fundamental frequency of supply, these will be 100 Hz, 150 Hz, 250 Hz, 350 Hz etc.

Subharmonics

Frequencies lower than the fundamental frequency, which are integral sub-multiple of fundamental frequency. These are usually created in series RLC circuits using series capacitors, series inductors or their combination. These are usually small, but get significant under fault conditions.

Interharmonics

In electrical systems, these are components of voltage or current that operate at frequencies that are not integer multiples of fundamental / supply frequency.  IEEE definition of interharmonics says

 “Between the harmonics of the power frequency voltage and current, further frequencies can be observed which are not an integer of the fundamental.  They can appear as discrete frequencies or as a wide-band spectrum.” Power system interharmonics are most often created by two general phenomena. 

Rapid non-periodic changes in current and voltage are caused by loads operating in a transient state (temporarily or permanently) or when voltage or current amplitude modulation is implemented for control purposes.  These changes can be quite random or, depending on the process and controls utilized, quite consistent.  Changes in current magnitude or phase angle can also create interharmonic frequencies. Another source is static converter switching not synchronized to power system frequency (asynchronous switching).  Thyristor switched converters are triggered into forward conducting mode and keep conducting until their current falls below its holding current.

Simply put, following table summarizes these definitions.

F1 = fundamental frequency
If n is any positive integer	If m is any positive non-integer
nf1 is the nth harmonic	mf1 is the mth interharmonic
If n = 0, nf1 is DC	If fn =f/n, nf1 is a subharmonic

Classification by Phase Sequence in relation to fundamental

The harmonic waveform could be in phase or out of phase with fundamental frequency. These give different types of waveforms for same frequency of harmonic component.

Positive Sequence: Harmonics having same phase sequence as original signals. Each of the phases produces harmonics which have coinciding zeroes, where their upgoing (or down-going) waveforms match (i.e. both waveforms follow R-S-T sequence.

Negative Sequence: Harmonics in each phase are in opposite phase sequence to fundamental. Here the zeroes of harmonics match with fundamental, but going in opposite direction. Harmonics follow R-T-S phase sequence if fundamental sequence is R-S-T.

Zero Sequence: Harmonics that are in phase across all phases. This means harmonics of all the phases are in phase with each other. These can be quite dangerous, since they add up in neutral conductor and can become very significant and even damaging to system. Third harmonics and their multiples i.e. 3^rd, 6^th, 9^th, etc., fall under this category. These need special attention in power systems.

Classification by Distorted Quantity

Current Harmonics

Distortions in current waveforms. These arise from non-linear loads such as rectifiers, variable speed drives, and other electronic devices that draw non-sinusoidal current. The distortion in the current waveform results from these loads interacting with the power system. They primarily affect the loads that generate them. Non-linear loads experience losses and heating due to the harmonic currents, but linear loads generally remain unaffected

Current harmonics are often expressed as Total Demand Distortion (TDD), while voltage harmonics are expressed as Total Harmonic Distortion (THD). The measurement of these harmonics can provide insights into the quality of power being supplied.

Voltage Harmonics

Distortions in voltage waveforms. These are caused by current harmonics flowing through system impedance (like transformers and cables). The distortion in the voltage waveform occurs as the harmonic currents create voltage Effects on the System: These can impact the entire electrical system, including both linear and non-linear loads. Voltage harmonics can lead to resonance issues, equipment malfunction, and increased losses throughout the distribution system.

While current harmonics originate from non-linear loads and primarily affect those loads, voltage harmonics result from the interaction of these currents with system impedances and can impact a broader range of equipment within the electrical system. drops across these impedances

Harmonics measurement and analysis in power systems

Harmonics in power systems are measured and analyzed using various techniques and tools to assess their impact on power quality. Here’s an overview of the key methods and parameters involved

Effect of third harmonics on the efficiency of three-phase systems

Third harmonics significantly impact the efficiency and performance of three-phase systems in several ways:

Increased Neutral Currents

Neutral Conductor Overload: In three-phase systems, the third harmonics (triplen harmonics) do not cancel out as they are in phase across all three phases. This leads to a constructive addition of these currents in the neutral conductor, potentially causing it to carry excessive current. This can result in overheating and may require the neutral conductor to be oversized compared to phase conductors to handle the increased load.

Power Quality Issues

Distortion Power: Presence of third harmonics increases the distortion power, which in turn raises the apparent power (S) required by the system. This can lead to inefficiencies, as more apparent power is needed to deliver the same real power.

Voltage Drop: Third harmonic currents can cause voltage drops in the system. For instance, a significant third harmonic current can lead to a drop in voltage that affects other connected loads, reducing overall system efficiency

Equipment Stress and Damage

Increased Heating: Electrical equipment such as transformers and motors can experience increased heating due to third harmonics. This additional heat can reduce the lifespan of these components and lead to failures if not properly managed.

Operational Issues: Motors may experience reduced torque and efficiency when subjected to harmonic distortion, particularly from third harmonics. This can lead to operational inefficiencies and increased wear on mechanical components.

Harmonic Mitigation Strategies

To address the negative impacts of third harmonics, various strategies can be employed:

Harmonic Filters: Installing active or passive filters can help mitigate the effects of third harmonics, improving overall power quality and system efficiency.

Transformer Connections: Using specific transformer configurations, such as delta connections, can help cancel out third harmonics from circulating in the system.

Understanding these impacts allows engineers and technicians to design systems that minimize harmonic distortion, leading to improved efficiency and reliability in three-phase electrical systems.