Power Quality – Understanding Harmonics and Mitigation Strategies 

Linear vs. Non-Linear Loads Explained

Linear loads feature a condition wherein the current and voltage are proportional to each other. It is best understood in the form of a sinusoidal waveform. When the current and voltage are perfect sinusoidal waveforms, then it is defined as a linear load. Even if they are not in the same phase, as long as they are sinusoidal and proportional, it constitutes a linear load. Most inductive, capacitive or resistive loads fall under this category.

However, when the current is not proportional to the voltage, it constitutes a non-linear load. When represented in a waveform, the current’s waveform is very distorted. This leads to distortion in the input and requires harmonic mitigation. 
 

To verify the difference between linear and non-linear loads, please refer to the picture below.

The loads such as seen in pictures 1.2 and 1.3 are pure sinusoidal, at least in this simplified graph. Based on this it is common practice to define the power factor as the phase relationship between voltage and current, although this is in fact the displacement factor. Therefore literature often refers to the power factor as the “true power factor” in order to avoid misunderstanding. The definition of (true) power factor and displacement factor are: (v1 and i1 are referring to fundamental frequency)

pf and df may have the same value under the condition that all items are purely sinusoidal. In a real application, this does not take place which will be explained in more detail later. Continuously wrong usage of pf for df led to so-called power factor correction equipment (PFC). As this PFC only focuses on phase correction it will not solve problems related to other kinds of distortion. Basically, there are different distortions of the main supply such as:

Impact of Non-Linear Loads on Power Quality 

Variable speed drives (VSDs) are a primary source of non-linear loads. They cause severe power quality issues by causing harmonics. Harmonics are best defined as the distortion of current from the perfect sinusoidal waveform. The higher the harmonic distortion, the worse the overall power quality. This necessitates the use of harmonic filters in power systems where variable speed drives and other similar equipment are used.

Poor power quality can lead to overheating and damage to sensitive equipment, especially semiconductors. Excessive harmonic distortion can lead to semiconductor failure due to overheating. Therefore, to protect all equipment, harmonic mitigation is necessary.
 

Introduction to Harmonics in Power Systems

What Are Harmonics? 

At a very basic level, harmonics are distortions in the current and voltage waveform caused by non-linear loads. Harmonics can be understood in the context of what is known as the harmonic order. Harmonic order is defined as the frequency that is multiples of the basic harmonic distortion. For instance, if the first harmonic is rated at 60 Hz, then the second harmonic is rated at 120 Hz, the third at 180 Hz, and so on. 

Harmonic Distortion – A Closer Look 

To evaluate the impact of harmonics on a particular electrical system, the term Total Harmonic Distortion is used, abbreviated as THD. THD typically measures the harmonics from the 2nd to the 50th order. Therefore, THD offers an easily identifiable metric that can then be tackled by appropriate harmonic mitigation in power systems.

The Fourier transformation decomposes a function of time into its individual frequencies. This means that every periodic signal is a function that can be divided into individual harmonics. The following table helps verify the principle.

Evaluating Harmonics with Mathematical Tools 

Measuring Harmonic Distortion 

Harmonic distortion can be measured in two ways: total harmonic distortion (THD) and total demand distortion (TDD).

THD is the measure of distortion present within a system as is defined by the ratio of the sum of the power of all harmonic components within the system to the fundamental frequency. Most systems should have a THD below 5 percent to be considered efficient.

TDD is a measure of harmonic distortion in the input current of an electrical system. It is a more practical and reliable measure of distortion than THD because it takes the system's full load into account.
 

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Harmonic Distortion and Its Impact on Power Systems 

Common Harmonics and Their Frequency Ranges

6 pulse loads constitute the most common cause of harmonic distortion within a system. This is a type of load that draws current from the AC system in 6 pulses, instead of it being a smooth flow which is typical in linear loads. These cause a lot of harmonic distortion, typically in the order of 5th, 7th, 11th, 13th, and other higher-order harmonics.

Effects of Harmonic Distortion on Electrical Infrastructure

If left unmitigated, harmonic distortion can have adverse effects on electrical infrastructure. These include:

- Overloading of transformers and capacitor banks 
- Increased energy costs as more electricity is drawn to operate equipment 
- Reduction in equipment lifespan due to overheating caused by the harmonics

Understanding Harmonic Mitigation in Power Systems 

Why Harmonic Mitigation Matters

Harmonic mitigation is key because it serves two major functions: protecting equipment and improving the performance of the system. By reducing the total harmonic distortion (THD) to acceptable levels, it makes sure that all equipment utilizes the current optimally. It also helps to reduce reactive power, which is a form of waste power that is required to run the system but has no direct utility.

Effective harmonic mitigation minimizes downtime in critical industries like data centers and oil & gas, ensuring continuous operation. It does so by eliminating overheating and fluctuations caused by harmonics.

Lastly, it also helps companies to meet important international standards such as IEEE 519, IEC 61000-3-2, IEC 61000-3-12, IEC 61000-3-6 and more. Failing to comply with these standards causes steep penalties, so adhering to them is mandatory for efficient operation. 
 

Techniques for Harmonic Mitigation

Harmonic mitigation is generally achieved by integrating harmonic filters into power systems. The two most prominent harmonic filters are:

- Active Harmonic Filters: Highly effective harmonic mitigation solutions that can handle dynamic, non-linear loads with ease. It is the most efficient harmonic filter in the market.

- Passive Harmonic Filters: These are commonly used in power systems to mitigate harmonics from non-linear loads. However, they are less effective in handling highly dynamic loads with rapidly changing operating conditions. 
 

Power Factor, a major impact on your power quality

In a most common sense, when speaking about reactive power and power factor this is referring to fundamental sinewave signals where current and voltage are at a different angle.

Typically the reactive power is calculated by the cosϕ (also referred to as power factor, pf).     

When it comes to reactive power, the following illustration has become very famous.

The Beer is indicating what you actually wants, while the foam is representing the reactive power. This illustration is giving a simplified picture of the reactive power only taking into account the sinewave signals. This can be used for most typical inductive loads such as motors connected direct on line (without Variable Speed Drive).  

4 Reasons why to take care of fundamental reactive power

Power Factor:

In many regions, utilities charge customer based on power factor. Due to inefficient use, a low power factor usually lead to higher charges.

Infrastructure efficiency: 

Reactive power needs to be generated, transferred and distributed. At all times, this will cause losses in the system. 

Infrastructure costs:

 While reactive power does not create any real work in the final application, reactive power still increase the current in the whole system. This is causing high costs due to necessary oversizing of equipment such as transformer, switches and wire. 

Voltage stability: 

Avoiding reactive power and ensuring a stable pf stabilizes the voltage and helps avoiding fluctuations in the voltage.

With more complex loads, such as a mixture of VFD and inductive loads, the beer allegory is not sufficient. In order to evaluate the true reactive power, the distortion power caused by harmonics must be considered. Looking at the true power factor this is affected by both THDi and cos(ϕ):

Therefore when improving power factor both harmonics and fundamental reactive power should be treated, rather than dealing with these two issues individually. This leads to some advantages in the solution itself due to the calculation of the RMS current:

Using the equation above, fundamental current and harmonic currents are added as square sum. In a system with 100A of harmonic currents and 150A of reactive current, they would typically require two individual solutions with 100A and 150A. This leads to 250A compensation current in total.