Sources and Effects of Harmonics in AC/AC Converters

Harmonics are sinusoidal parts of a waveform that have frequencies that are integer multiples of the fundamental frequency. In AC/AC converters, these are formed by the nonlinear features of the power electronic components, such as diodes, thyristors, and transistors, which cause a sudden change in the waveform of the current. Due to the fact that the presence of harmonics can have a variety of detrimental impacts on the power supply, it is essential to have a solid understanding of the sources and consequences of harmonics.

Figure 11: Individual harmonic components (left) and composite waveforms (right)

Sources of Harmonics In AC/AC Converters

Switching devices: Harmonics are produced whenever the power electronic components that are utilized in AC/AC converters, such as thyristors, IGBTs, and MOSFETs, are turned on and off. The current waveform undergoes rapid fluctuations as a result of rapid switching, which can be broken down using Fourier analysis in order to discover harmonic components.

Nonlinear loads: Due to the nonlinear current-voltage characteristics of nonlinear loads including rectifiers, inverters, and variable frequency drives, harmonics can also be generated in the power system.

Saturation of magnetic components: When subjected to high current conditions, magnetic components such as transformers and inductors have the potential to become saturated. Harmonics are introduced into the waveforms of the voltage and current as a result of the distortion of the magnetic flux waveform that is brought about by saturation.

Effects of Harmonics In AC/AC Converters

Increased power losses: The presence of harmonics results in an increase in the power losses that occur in the components of the power system, such as transformers, cables, and motors. This is because harmonics cause larger resistive and reactive losses. As a result of these additional losses, efficiency is reduced, which can lead to overheating and even premature component failure.

Distorted waveforms: When harmonics are present, the waveforms of both the voltage and the current become distorted, which results in a decrease in the power quality. These distorted waveforms not only cause sensitive electronic equipment to malfunction, but they also have a severe impact on the functionality of other devices that are linked to the system.

Resonance: When harmonics are present in the power system, a phenomenon known as resonance occurs. This phenomenon causes excessive voltages and currents to be produced at specific frequencies. Consequently, this results in damage to the equipment, increased power losses, and a deterioration in the quality of the power.

Power factor reduction: Harmonics are responsible for the power factor of the power system decreasing, which in turn increases the demand for reactive power and decreases the capacity of the system to transmit electricity. The costs of energy and the number of power factor correction devices that are required are both increased as a result of this.

Harmonic Mitigation Techniques

The implementation of harmonic mitigation methods is very necessary in order to diminish the adverse effects that harmonics have on the power system. These adverse effects include increased power losses, waveform distortion, resonance, and a decrease in power factor. Generally speaking, these techniques can be classified into two categories: passive and active. In this section, we will discuss the various solutions for harmonic mitigation and how they are implemented in power systems.

Passive Harmonic Mitigation Techniques

Line reactors and chokes: It is possible to reduce the flow of harmonic current by connecting inductive components such as line reactors and chokes in series with the power supply. This will allow for a reduction in the flow of harmonic current. Specifically, they accomplish this by increasing the impedance of the power system at harmonic frequencies, which in turn reduces the amplitude of the harmonic currents.

Transformers with phase-shifting windings: Transformers that have windings that are capable of shifting phase: Phase-shifting transformers are able to cancel specific harmonic components because they provide a phase shift between the main winding and the secondary winding. This phase shift has the potential to cancel out certain harmonic orders, which will lower the impact that it has on the power system.

Detuned filters and tuned passive filters: There are two types of filters: tuned filters and tuned passive filters. Detuned filters are made up of a series-connected collection of capacitors and reactors, whereas tuned passive filters are designed to resonate at a specific harmonic frequency. It is possible for these filters to redirect harmonic currents away from sensitive equipment and reduce the impact they have on the power supply. This is accomplished by providing low impedance paths for the harmonious currents.

Active Harmonic Mitigation Techniques

Active filters: Active filters are a type of power electrical equipment that generates harmonic currents that are in the opposite phase to those generated by the load, but these harmonic currents have a similar magnitude. These opposing currents essentially cancel out the harmonic currents, which results in a reduction in the impact that the harmonic currents have on the power system. Shunt active filters and series active filters are two distinct types of filters that connect to the load in parallel and series, respectively. Shunt active filters are more common than series active filters.

Dynamic voltage restorers (DVR): The dynamic voltage restorers, also known as DVRs, are power electronic devices that inject voltage in series with the power system. This allows them to adjust for voltage harmonics, sags, and swells so that the voltage can be restored. DVRs have the ability to reduce the impact of voltage harmonics by dynamically altering the voltage that is injected into the system. This helps to ensure that the load terminals continue to have the correct voltage waveform.

Unified power quality conditioners (UPQC): The term "unified power quality conditioners" (UPQC) refers to a form of advanced power electronic equipment that combines the capabilities of shunt active filters and digital video recorders (DVRs). Power quality issues such as voltage sags, swells, and unbalance can be compensated for concurrently by these devices. Harmonics in voltage and current can also be compensated for simultaneously.

Passive and Active Filtering Methods

The utilization of harmonic filtering techniques is absolutely necessary in order to improve power quality and lessen the negative impact that harmonics have on power systems. The two methods that fall under the category of harmonic filtering are respectively known as passive and active filtering. In this section, we will discuss the similarities and differences between passive and active filtering strategies, as well as the advantages and disadvantages of each.

Passive Filtering Methods

Passive filters are constructed from passive electrical components such as capacitors, inductors, and resistors that are stacked in certain patterns to concentrate on particular harmonic frequencies. Passive filters are used in filtering systems. In terms of passive filters, the two basic groups are as follows:

Detuned filters: Detuned filters are produced by connecting a capacitor and a reactor in series. They are constructed with a resonant frequency that is lower than the fundamental frequency in order to make sure that they do not experience parallel resonance with the power system. Detuned filters are able to limit harmonic distortion by providing a low-impedance path for harmonic currents and a high-impedance path for fundamental frequency currents.

Tuned filters: Targeted harmonic currents have a low impedance path because tuned filters are designed to resonate at specific harmonic currents. This allows tuned filters to achieve relatively low impedance. This action effectively redirects harmonic currents away from sensitive equipment, thereby mitigating their detrimental effects on the power system. Even though tuned filters often perform better than detuned filters, tweaking is still required in order to obtain the highest possible level of performance potential.

Advantages of Passive Filters:

• The design is straightforward and inexpensive.
• Due to the absence of active components, the reliability of the system is high.
• There is no need for complex control algorithms to be put in place.

Drawbacks of passive filters:

• They are bulky and hefty because of the inductors and capacitors.
• They possess a fixed harmonic compensation, which means that they are only effective for the harmonic frequencies that are wanted.
• If the construction is not done correctly, it has the potential to cause resonance issues.

Active Filtering Methods

Active filters in power electronics create harmonic currents that are in the opposite phase of the load-produced harmonic currents, but they have the same amplitude. By functioning as a countermeasure and essentially canceling out the harmonic currents, this helps to reduce the negative impact that the harmonic currents have on the power system. In terms of active filters, the two basic categories are as follows:

Shunt active filters: Figure 12 illustrates the parallel connection between the load and shunt active filters. They measure the harmonic currents generated by the load and subsequently inject counteracting currents into the power system. The power system undergoes this process to eliminate harmonic distortion.

Figure 12: Shunt active power filter

Series active filters: Active filters connected in series: Figure 13 shows that the load is connected in series with active filters. By monitoring the voltage harmonics at the load terminals and producing counteracting voltages, the power supply effectively reduces the voltage distortion. This process is repeated until the distortion is eradicated.

Figure 13: Series active power filter

Advantages of active filters:

• The harmonic compensation, which is both flexible and adaptive.
• The ability to take into account a wide range of harmonic frequencies. In other words, it has the ability to regulate voltage and improve power factor.

Drawbacks of active filters:

• It has a high cost due to the use of power electronics equipment and control systems.
• Their reliance on active parts and control algorithms makes them less trustworthy than passive filters.

Power Factor Correction

The power factor is a crucial measure that is utilized by power systems in order to ascertain the degree to which electrical power is effectively turned into productive activity. The power factor is the ratio of the actual power flowing to the load (measured in watts, W) to the perceived power (measured in volt-amperes, VA) in the circuit. This is an essential topic in alternating current (AC) power systems.

$$PF = \frac{\text{Real Power}}{\text{Apparent Power}} = \frac{\text{kW}}{\text{kVA}}$$

The power factor is a measurement that determines how successfully electrical power is turned into meaningful work output. Its values can range from 0 to 1 (or, more accurately, from 0 to 100%). When the power factor is 0.8, it indicates that only 80 percent of the power is put to productive use, while the remaining 80 percent is wasted. A power factor of one signifies the effective utilization of all available power for productive tasks like motor turning or bulb illumination.

Importance of Power Factor Correction

PFC, or power factor correction, is necessary for a number of reasons, including when:

Improve system efficiency: A low power factor indicates that the power system is not successfully converting electrical power into usable work, necessitating an increase in system efficiency. As a result, the cost of energy goes up, and more people tap into the utility system.

Reduce voltage drops and losses: A low power factor causes more current to flow through the system, which in turn causes voltage drops and losses in power lines and transformers to increase. Reducing the amount of current flowing through the system can mitigate this.

Increase system capacity: System capacity can be successfully utilized, which increases the power system's capability to accommodate higher loads without the need for costly additions. This can be accomplished by boosting the system's capacity.

Comply with utility regulations: Customers who have low power factors are frequently subject to penalties from utilities because they contribute to higher system losses and grid demand. It is therefore important to comply with utility laws.

Power Factor Correction Techniques

In order to correct the power factor, there are a variety of methods available, including hybrid, active, and passive approaches:

Passive Power Factor Correction: In the process of passive power factor correction, capacitors or inductors that are linked in parallel or series with the load are utilized. These reactive components balance out the reactive power of the load, leading to an increase in the power factor. In spite of the fact that passive PFC is uncomplicated, trustworthy, and cost-effective, it could not be sufficient for systems that have loads that are either intricate or changeable.

Active Power Factor Correction: Active power factor correction (PFC) uses power electronic parts like inverters or converters to give reactive power that can meet the specific reactive power needs of the load. When compared to passive power factor correction (PFC), this method offers power factor correction that is both more accurate and adaptable. Active power factor correction (PFC) has the ability to reduce voltage sag and control harmonic distortion. The utilization of control systems and power electronic components results in an increase in complexity as well as an increase in cost.

Hybrid Power Factor Correction: The hybrid power factor correction (HPFC) method combines passive and active methods in order to make the most of the advantages offered by each method. This method is capable of accomplishing both objectives because it combines the adaptability and enhanced performance of active PFC with the dependability and cost-effectiveness of passive PFC. Hybrid power factor correction (PFC) offers a number of benefits to systems with changing loads, including power factor correction and harmonic mitigation.