Perspective Chapter: Power Quality and Hosting Capacity

Abstract

With the increasing prevalence of distributed generation (DG) and power electronic-based technologies, consumers will have more alternatives for obtaining energy from different public or private sources. The issues will be with power quality (PQ), pricing, and reliability. Shortly, maintaining acceptable power quality levels above certain acceptable thresholds will be challenging because of the special difficulties brought on by nonlinear loads and novel types of load equipment. The significance of current and voltage quality issues increases even further in such an environment of competition. The chapter is dedicated to presenting an overview of PQ definitions, disturbances, causes, and standards. Harmonic description, sources, effects, and harmonic filtering techniques are also presented. Then, renewable-based DGs and HC studies—types, challenges, and solutions, are demonstrated. Further, a literature overview of the existing solutions under consideration (harmonic management) is presented and discussed.

1. Introduction

Improving electrical power quality is an intention agreed on by consumers and electrical utilities. The primary goals in terms of power quality (PQ) are generating clean power—that is, power that is not distorted—and cost-effectively delivering it to customers with adequate technical performance. Advancements in semiconductor technology have brought about harmonic pollution and other PQ concerns, a range of nonlinear load types, and the installation of renewable energy resources that rely on power electronic-based equipment (rectifiers and inverters) for their operations. It became clear that harmonics management is now an essential issue in power systems rather than a secondary concern [1, 2]. Power grid performance can be adversely affected by several factors, including overloading and increased heating of lines and cables, frequency-dependent equipment, diminished voltage quality, decreased transmission efficiency, an increase in energy losses during transmission and distribution, and deterioration of true and displacement power factors of loads [3].

Modern sophisticated converters with frequency-coupling dynamics also introduce harmonic instability (e.g., resonance, amplification of voltage or current, or unusual harmonics in the high-frequency spectrum) [4]. This means that PQ is so essential to contemporary energy systems.

As distributed generation (DGs) become more prevalent and utilize power electronic-based technologies, consumers will have more options when it comes to purchasing energy from various public or private sources. The cost, reliability, and quality of electricity will be the problems. Maintaining acceptable PQ levels above specific acceptable limits will be a major challenge in the upcoming decades due to the unique challenges presented by new types of load equipment and nonlinear loads. In such a competitive scenario, the importance of PQ issues becomes even more significant than before [5]. Furthermore, since they can now create and sell electricity through their DGs (prosumers), consumers connected to an electrical grid are no longer considered consumers in the context of the deregulated power market and many energy providers. Enhancing the PQ performance of the systems thus gets more difficult due to new and developing complications.

PQ provides several justifications to the various electric entities’ stakeholders. Some perceive PQ as the voltage quality, others as the current quality (based on amperage), and others as the system’s dependability. PQ , for example, is defined as “the powering and grounding concept of sensitive electronic equipment in a manner appropriate for the equipment” in IEEE Standard 1100 [6].

Put simply, each entity defines it according to its own perspective. It is possible to determine the cause and responsibility of a disturbance wrongly when a general term like PQ has an ambiguous definition. This was obvious in the surveys carried out by the Georgian Power Company [7, 8] for the causes of PQ disturbances. These surveys clearly showed how divergent the utility and customer perspectives are from one another. But both attribute two-thirds of the problems to lightning and other natural occurrences [9, 10]. This indicates that, under typical circumstances, PQ maintains a nearly pure sinusoidal waveform of voltages and currents. Generally speaking, deviations of the voltage from the conventional waveform—which is typically defined as a sinusoidal waveform with constant frequency and magnitude—are the main focus of the quality of voltage (QoV). The electric current’s deviations from the usual waveform are the main focus of quality of current (QoC) [9]. However, relying solely on voltage or current to define PQ is imprecise because any divergence in voltage will result in a variation in current from its nominal value and vice versa [11]. Accordingly, PQ should combine both current and voltage qualities and is better described as technical limits that enable equipment to function in its prearranged way without significant operational losses to maintain its lifetime [9, 10, 11, 12].

2. Power quality disturbances

PQ disturbances address various power system issues, including notching, voltage fluctuations, voltage flickers, harmonics, sub-harmonics, inter-harmonics, supra-harmonics, transients (oscillatory and impulsive), voltage sags, interruptions, voltage swells, imbalance, undervoltages, overvoltages, noise, and harmonics [9]. Even though each of them is a crucial topic in and of itself, the power system harmonics problem is regarded as the most significant and well-known PQ issue. This is because numerous studies have identified harmonics as the most serious cause of frequent PQ disturbances, impacting both consumers and utilities. Distribution system operators typically presume that harmonics and imbalances are the cause of PQ issues when they occur. Figure 1 illustrates the various PQ issues [8, 9, 10]. Any PQ evaluation procedure can be implemented with general steps, such as identifying the sources of the disturbances and considering potential harmonic management solutions up to the point of solution optimization. Figure 2 examines a generic PQ diagnostic, assessment, and mitigation process [9, 13].

Figure 1.

Illustration of the common PQ issues.

Figure 2.

PQ appraisal procedure.

Depending on the region of the utility, standards and guidelines are typically used to categorize and identify problems. By enumerating the relevant characteristics, such as amplitude, frequency, spectrum, modulation, source impedance, notch depth, notch area, duration, rate of occurrence, and others, IEEE Std. 1159-2019 (as an example) classifies PQ or electromagnetic phenomenon.

The PQ indices were developed to provide a quantitative measure of the disruptiveness of disturbances; however, with the advancement of technology and changes in some systems’ susceptibility to disturbances, the appropriateness of some PQ indices needs to be reevaluated. In addition, while some PQ indices have already been defined or redefined in standards and their updates, others remain missing, especially for high and extra-high voltage or high-frequency systems [14, 15].

It is now crucial to develop criteria for limiting issues from PQ degradation due to the growing usage of nonlinear loads and renewable energy-based equipment with power electronic converters. The following are the primary PQ problems associated with DGs being connected to the power grid: DC injection, harmonics, voltage swells and sags, poor voltage regulation, power factor, flickering and fluctuating voltage, voltage imbalance, and prolonged interruptions. Further details regarding these problems, as well as technology and strategies for lessening the effects of DGs on PQ , are available in [16, 17, 18]. Egyptian Transmission/Distribution Codes, the Grid Connection Code for Solar Energy Plants, the International Electrotechnical Commission (IEC), the Institute of Electrical and Electronics Engineers (IEEE), the American National Standards Institute (ANSI), the National Institute of Standards and Technology (NIST), the National Fire Protection Association (NFPA), the European Committee for Electrotechnical Standardization (CENELEC), the National Electrical Manufacturers Association (NEMA), the Electric Power Research Institute (EPRI), Underwriters Laboratories (UL), and ESKOM for South African standards are just a few of the national and international organizations that have developed PQ standards. IEC and IEEE are the two prominent organizations that define PQ standards. PQ standards are referred to as electromagnetic-compatibility (EMC) standards by some. A number of EMC standards (series) and technical reports have been released by the IEC; the majority are included in the IEC 61000 series [19, 20, 21, 22, 23, 24]. Numerous EMC standards that provide a thorough summary of IEEE PQ standards have been accepted by the IEC. The IEEE 519 recommendations are the most well-known substitute for the IEC standards in many countries [25, 26].

The goal of these standards is to restrict customers’ access to harmonic distortion and the associated issues it causes, as well as the utility’s voltage harmonic distortion boundaries. These guidelines divide the obligation of limiting harmonic propagation between utilities and end users. Customers and end users are typically in control of controlling the injection of harmonic currents, and utilities, regulators, and operators are in charge of figuring out the voltage distortion in the supply system at the point of common coupling (PCC).

3. Power system harmonics: definition

Typically, “any periodically distorted waveform can be represented as a sum of pure sine waves in which the frequency of each sinusoid is an integer multiple of the fundamental frequency of the distorted wave.” The multiple-frequency has been named the fundamental’s harmonic component, in which the so-called Fourier series refers to the summation of these sinusoids [27, 28]. If the fundamental frequency (f₁) is 50 Hertz, the 5th harmonic is 5*50 Hz or 250 Hz. Classically, amplitudes of the harmonic currents are expressed as a percentage of the fundamental current amplitude (I_f), so that I₃ = I_f /3, I₅ = I_f /5, I₇ = I_f /7, and so on. In line with the literature, in electric power system analysis, high-order harmonics above 25, i.e., the range from 25 to 50, are insignificant. It should be mentioned that harmonics above the 25th order are prevalent in telecommunication system studies. High-order harmonics might cause interference with power-electronic equipment; however, these harmonics are not critical to power system equipment [29]. It should be mentioned that recently supra-harmonics (distortion in the frequency range between 2 kHz and 150 kHz) [30] initiated to be considered in a few studies; however, they are not achievable in harmonic analysis in power systems due to the absence of legal restrictions for electric harmonic distortions in the very high-frequency range [31].

4. Harmonics sources and effects

One might refer to a load as nonlinear or non-ohm’s law compliant when the current carried by the load is not proportionate to the applied voltage. This kind of load’s current has a non-sinusoidal waveform or distorted current. This current distortion will generate a voltage distortion when there is a high impedance in the path between the source and the nonlinear load. On the contrary, one can say that loads with the current linearly proportional to the applied voltage are linear loads (linear relationship) [9, 10]. These days, the majority of loads are nonlinear in nature due to the widespread use of power-electronic-based components in power systems, even in our homes with our laptops, small appliances, fluorescent and light-emitting diode (LED) lamps, and printers. The load itself (design or component) and the nonlinear load’s interaction with the distribution system determine how severe the harmonics produced by these loads are [25, 32]. As the primary sources of harmonic voltages and currents in power systems, several groups of power components can be grouped and organized as follows [9, 10]: transformers, electric motors, and generators (magnetic core-based equipment); induction furnaces, arc furnaces, and arc welders (equipment provides heating); and power-electronic-based devices. The way the power system is connected or composed is another classification. Furthermore, rather than the series-connected parts (linear series elements), the shunt-connected elements (loads, for example) are where the nonlinearities in the system arise. The magnetizing impedance (shunt-connected branch) of the well-known T model serves as the harmonic source inside a transformer, while the leakage impedance stands in for linear components. The most usual harmonic sources are—converters (inverters and rectifiers) within drives or renewables-based devices, slots and teeth field distribution in synchronous generator, power and distribution transformers’ magnetizing circuits, rotating machines’ excitation currents, printing machines, lamps (fluorescent, compact fluorescent, gas discharging lighting-low pressure/high pressure Sodium vapor, high-pressure mercury vapor and LED), flexible alternating current (AC) transmission systems, FACTS, and distributed FACTS (D-FACTS), uninterruptible power supplies, switch-mode power supplies, pulse modulation (or other forms) has been proposed for active power and voltage control in transmission circuits, electrolysis-based loads, converters usually used in variable speed drives (VSDs), converters used in grid-connected or islanded solar photovoltaic (PV) and wind systems, arc and induction welders, arc and conduction furnaces, and ovens used in electric heating, energy conservation device (soft starters, electronics ballast, and fan regulators), ballasts of the fluorescent discharge lamps, thyristor-controlled reactors, induction motors operating in or near to their saturation regions, converters in high-voltage direct current (HVDC) systems, UPSs, static VAR compensator and devices, and components in charging stations of electric vehicles (EVs). To sum up, most of these harmonic sources are power electronic-based devices.

The expected range of harmonics’ impact is degradation in power system equipment’s performance to their severe failure. The most common consequences of power system harmonics on the different electrical system sectors are explored in Figure 3.

Figure 3.

Consequences of harmonics on components of the power system.

The most common harmonic problems in plants are summarized below [33, 34].

• Current flowing in neutral wires with an overheating problem

• High distorted currents will lead to excessive energy losses (thus high electricity consumption and costs)

• Unreasonable failure of equipment

• Motors’ disturbance

• Overloading of frequency-dependent conductors

• Blow of fuses and mal-operation in the performance of protection devices

• Watt-hour metering’s errors

• Interference with telecommunication systems (above the 25th order)

• Data loss in data-transmission networks

• Mal-operation in the performance of the control devices

• A voltage or current amplification (by series and parallel harmonic resonance)

• Harmonic instability (malfunctioning of voltage and malfunctioning of generator regulators)

• Noise in transformers

• Noise and vibrations in rotating machines

• Lockups of the programmable controllers

Usually, problems appear when a system’s capacitance results in resonance with inductance at characteristic harmonic orders that intensely increases the distortion beyond the standard, acceptable values, as originate in industrial power systems because of the power factor correction capacitors that are frequently used and can cause a high degree of resonance severity or harmonic amplification. Such a privilege necessitates special considerations concerning harmonics filtering to avoid failures and nuisance tripping of fuses or breakers associated with capacitors [33, 34]. Adding filters that start with the lowest significant harmonic order (usually the third- or the fifth-order) is necessary to avoid harmonic resonance problems. If one wants to use a seventh harmonic order filter, one should introduce a fifth harmonic filter.

Further, analysis of the impedance-frequency dependencies for all reasonable operating contingencies should be done (in which a frequency scan should be conducted at each node if any harmonic source exists).

5. Harmonics mitigation

Since most electrical loads in use today are nonlinear, it is usually helpful to study reasonable harmonic solutions by systematically addressing electrical system-related concerns. When an issue arises, the fundamental solutions for harmonic control are to either add filters to sink the system’s harmonic currents or stop them from entering the system, or reduce the harmonic currents generated by the load [9, 35].

Different attempts have been used to solve harmonics issues either to lessen their impacts on the power system or to reduce the harmonic distortion itself in the power grids, such as [36]:

• Derating transformers, motors, cables, and generators to be able to withstand the distorted over currents caused by harmonics

• The grounding of electrical equipment to cancel the severe 3rd harmonic and strengthen the neutral wire size

• Applying harmonic mitigation schemes such as active, passive, and hybrid filters

• Using multi-pulse converters

The harmonic filters can be classified as shunt filters or series filters based on the harmonic filter connected to the system. The shunt filters work by short-circuiting harmonic currents, which diverts the electric currents out of the arrangement. They must be placed as close as possible to the source of distortion. Shunt filtering is the most common way of filtering because of its economic aspects. Also, a shunt filter inclines to correct the load actual and displacement power factors and mitigate harmonic currents [29]. The other way is to put on a series-connected filter that helps in blocking harmonic currents. Nevertheless, series-connected filters must be planned to withstand the rated line current as they are connected in series with the system. Also, series-connected filters may produce substantial power losses because of the high currents. Given the high cost of the series-connected filters, the most real-world applied method is shunt-connected filters. Several contributions are dedicated to determining the most suitable location of the filters in electrical power networks. Filters at appropriate places (close to the source of harmonic generation) can be applied to mitigate considerable harmonic currents at the start, and the harmonics propagation to the common coupling point (PCC) is considerably reduced.

On the contrary, the harmonic flow occurs when the filters are far from the harmonic-producing loads. Harmonic filters are also categorized into three broad categories: passive, active, and hybrid active/passive filters. Figure 4 presents the primary ways of connecting harmonic filters at the PCC.

Figure 4.

Basic connections of harmonic filters.

Passive filters comprise inductive (L), capacitive (C), and resistive (R) components arranged and lumped together in precise configurations to regulate harmonics. They are commonly used in practice because they are considerably inexpensive compared with other active/hybrid filters. Nonetheless, they have the disadvantages of negatively interrelating with power systems and threatening the utility (source) and the loads (within the plant or neighbors) by harmonic resonance hazards [29]. Also, their filtering performance is sensitive to the variation of the source impedance [37].

The active harmonic filtering method was a reasonably innovative methodology for eliminating harmonics compared to the passive filtering techniques. Compact constructed active filters provide reliable system performance with good harmonic lessening. However, they are based on power electronic components; thus, they are more costly than passive filters. The basic concept of active filters introduces equal magnitudes of the current/voltage harmonics generated by nonlinear loads with 180° phase angle difference; consequently, they cancel each other when their phasor is summed. In addition, they do not resonate with the system [9, 34, 35, 38, 39]. By definition, active filters are designed based on converter type, topology, and the number of phases. The converter type can be either a current source-based inverter that employs an inductor to store energy or a voltage source-based inverter that employs a capacitor to store energy [9, 34, 35, 38, 39]. The arrangement can be shunt-connected types, series-connected types, or a combination of both connections. Active filters have frequency limitations, cannot withstand large currents and are sensitive to noise. Also, they have problems with high-power ratings (>0.5 MVA).

Nowadays, both active and passive filters can be used in the presence of multiple pulse converters governed by harmonics = Integer * pulse ± 1, where pulse is the pulse number and Integer = 1, 2, 3, etc. Harmonic current distortions of 6, 12, and 18 pulse converters are higher (in THD percentage) than 80%, 15%, and 12%, respectively. Multiple pulse converters of THD less than 5% are expensive to the manufacturers.

A straightforward technique to decrease harmonics is to increase the pulse numbers of converters. The lessening of harmonics with the increase of pulse number is guaranteed. Disadvantages of multiple pulse converters include sensitivity to voltage imbalance, optimal cancelation only with symmetric drive loading (they do not operate well with even harmonics), and not being easy to retrofit [23, 40].

Hybrid filters benefit from passive and active filters through series or parallel combinations. A passive filter helps to reduce the rating of the used active filter and its function in harmonic mitigation and improvement of power factors. The role of the active filter is to isolate the generated harmonics of both load and utility. Other harmonic management solutions concerning harmonic correction equipment types, such as the neutral blocking filters or the zigzag transformers, are solutions to eliminate the 3rd harmonic current from the load. Typically, they are suitable for computer/switch-mode power supplies. Other solutions have a kind of immunity to harmonic distortion, such as the oversized neutral/derated transformers, K–rated transformers, and phase shifting. They are more suitable for fresh/new designs in the planning stage; they do not have power factor correction benefits [41, 42]. They are much more superlative for commercial applications than industrial applications [42].

6. Renewable energy resources and their PQ issues

Within the framework of sustainable development, adding renewable energy sources to distribution or transmission systems has several advantages. These include promoting the use of green energy, diversifying energy sources, reducing greenhouse gas emissions, gaining political advantages, fostering social development, and providing economic support. There are also numerous technical advantages, such as improved power quality, reduced power loss, improved voltage, and increased load stability.

Distributed generators can be a renewable or non-renewable source of generation and can be networked (grid-connected) or act as a stand-alone system. Due to their low investment costs and small sizes, DGs show an imperative role in modern energy system planning [43]. DGs can be classified based on several issues such as [44, 45]—generated power (AC or DC); technology (Renewable (non-fossil fuel-based and non-renewable (fossil fuel-based)); supply duration (long duration, short duration, moderate but unsteady duration); capacity (micro decentralized DGs (1 W–5 kW), small decentralized or centralized DGs (5 kW–5 MW), medium centralized DGs (5 MW–50 MW), and large centralized DGs (50 MW–300 MW); grid interface (Inverter-based and non-inverter-based DGs)); power flow control (set to constant power factor for small DGs, and the bus at which the DG is connected is treated as a PQ bus, or set to constant voltage for large DGs and the bus at which the DG is connected is treated as a PV bus); and power delivering capability (deliver only active power at unity PF, deliver only reactive power at zero PF, DGs deliver active power but consumes reactive power, or deliver both active and reactive powers).

The energy flow and voltage conditions at customers and utility equipment are greatly impacted by the addition of DGs to distribution networks. Depending on the distribution networks, DG characteristics, and operating parameters, these effects could be either beneficial or detrimental [46, 47]. However, if renewable energy sources are not allocated properly, their unchecked growth could cause issues for power systems. Some common issues include overloading transformers, increasing power loss, malfunctioning or failing protection schemes, excessive harmonic distortion levels brought on by the combination of nonlinear loads and inverter-based renewable energy sources, as well as over- and under-voltages. When the rating of DGs surpasses the maximum permitted degree of penetration of renewables, these challenges arise.

7. Hosting capacity (HC): problems and solutions

When the system surpasses the maximum allowable hosting (HC) capacity criteria, DGs integration issues arise [47]. To decide on the addition of renewables, various customary rules of thumb were employed in the past. Instantaneous penetration (IP) was the term used to describe a definition that was previously introduced and was similar to the HC. IP has been described as the ratio of the output of renewable energy to the power of the system load within a given time or brief interval. It was not, however, frequently applied as the HC definition. These days, the inclusion of renewables can be determined by HC. Because of its significance, HC has been integrated into popular simulation programs as Siemens, CYME, ETAP, DIGSilent, and EPRI (DRIVE). Figure 5 examines a case study of determining HC in relation to penetration level using a generic performance metric, along with related issues and solutions. The type of problem that has emerged determines these performance measures. As can be seen from the figure, overvoltage, overloading and subsequent power loss complexities, PQ concerns, and protection issues are the four primary problematic issues [48, 49].

Figure 5.

HC determination by utilizing common solutions, troublesome problems, and a general performance metric.

To guarantee that the power system functions satisfactorily, the HC approach compiles the technical limitations implemented by operators and customers. This indicates more to the HC calculation than a single, static calculation based on a single performance parameter. On the other hand, HC would be determined for a number of performance indicators, including PQ , thermal overload capacity, voltage variations and frequency fluctuations, system stability, and others.

Ismael et al. provide a thorough overview of HC’s advancements, assessment procedures, and improved technology in [47]. Decisions from real case studies, power quality markets, and the practical experience (rule-of-thumb) of distribution system operators are given and discussed [47]. The authors in [50] also presented a detailed analysis of HC—theory and its influences on power networks, challenges, and solutions.

In the literature, numerous strategies have been put into practice to raise the HC of distribution systems. The most popular methods are:

• Renewables curtailment (when investors and operators are asked to reduce the amount of renewable energy they produce in order to maintain system working limits) [51]. To achieve the best possible power curtailment, system and renewable plant operators must have creative communication techniques and facilities.

• Use of energy storage devices to boost the system performance and permit its consistent act without renewables ceiling [52, 53]. Comparable benefits are provided by energy storage, which are challenging to provide with other approaches. But the primary drawback is the high cost of energy storage.

• Reconfiguring of nodes of RDS changes the status of the operating switches by controlling tie-lines, sectionalizes, and soft open points can reduce power losses, transfer loads between feeders, improve the nodes’ voltage profile, and improve HC, PQ , and reliability [54] of the system. Reconfiguration can be employed in the planning stage of power systems (called static reconfiguration) or in the operation phase of power systems (called dynamic reconfiguration). The static reconfiguration can considerably enhance HC, but the dynamic reconfiguration can only improve HC in case of the availability of an adequate number of controlled switches.

• Use of harmonic mitigation techniques, as proposed in this thesis, such as shunt capacitor banks [55], static VAR compensators [56], D-FACTs, and harmonic filters (dominantly passive or hybrid filters) to lessen harmonic distortion, support reactive power, correct the power factor and improve PQ performance of power systems operating under non-sinusoidal conditions [48, 57].

• Use of voltage regulators/conditioners, reactive power compensators, and OLTCs (on-load tap changers) to improve voltage profiles and, sequentially, support reactive power and enhance the HC of the system [58].

• Reinforcement of weakened or congested systems [59] can also be made to increase HC. In this regard, reinforcement means using machines with a higher rating, larger conductor sizes that have lower electrical resistance) or using efficient equipment. This is considered one of the practical techniques used in congested systems: support the voltage profile, achieve better hosting capacity, relieve the electrical system congestions, and reduce network losses.

• Interbreeding of diverse solutions to attain the best possible HC values in intelligent power grids [60], especially in severely deteriorated systems or in projects with high budgets.

To summarize, the following steps have to be performed to enhance the HC of a power system:

• Evaluate the initial HC within the network.

• Check the operational limits according to the international standards or national practice codes.

• Employ a proper HC enhancement technique.

• Re-evaluate the new HC value.

The complete HC procedure—evaluation and improvement—is summarized in Figure 6.

Figure 6.

HC calculation procedure.

Analytic HC calculation procedure means a precise process is done by iteratively increasing the penetration of renewables in a well-defined step at a carefully chosen bus, carrying out load flow calculations, and inspecting the operating limits at each iteration until they exceed the acceptable values; henceforth, finding the HC of that bus. Then, another bus is selected in sequence, and the same process is repeated till all the system busses are examined. This may suffer from the computational burden.

The stochastic HC calculation procedure means developing multiple scenarios in a probabilistic manner to overcome the uncertainties of the problem. It should be mentioned that this method is time-dependent based on the accuracy levels considered. Finally, a streamlined HC calculation procedure provides easy, quick screens that assist the operator in deciding whether it is required to make further detailed studies or not. However, this method suffers from accuracy issues, particularly in complex systems.

In the past, harmonic distortion was not one of the main interests of operators because of the assumption, at that time, that distributed generation units are harmonic-free. Later, it was realized that this assumption is not true as the combination of harmonics between distributed generation units and non-linear loads could create severe problems.

The authors in [61] used a passive harmonic filter, a C-type passive filter, to maximize the HC of a network that utilizes dispersed generators (PV units) in a harmonically distorted distribution system using the genetic algorithm. From the analysis presented, it was apparent that the system’s HC goes down with the rise in the grid-side voltage-distortion and the nonlinear load level. The HC level was affected more by the non-linear load level than grid-side voltage distortion. Also, the same authors used a single-tuned filter in [49] to do the same, and similar conclusions were figured out in [49] to validate that harmonic filters can solve the problems arising from harmonic distortion and enhance HC at the same time. Ghaffarzadeh and Sadeghi in [62] presented an effective method for the simultaneous settlement of inverter-based DG systems and capacitors because of harmonic distortion. However, HC was not discussed in detail in that work. Further, the authors in [63] investigated the use of passive filters in different single-objective optimization problems in the context of HC improvement along with THD lessening. The analysis revealed that HC improvement and voltage THD lessening were conflicting in optimization and that a multi-objective optimization may be needed to solve the problem. Similarly, a few other works are also reported, making an effort to improve HC using passive harmonic filtering techniques [48, 64], but in a single-objective optimization framework. Further, hybrid harmonic distortion mitigation is presented in [65] to improve distributed generation-based systems’ HC in harmonic-polluted conditions, in which the hybrid filter was a better substitute to realize a higher penetration level of renewables than purely passive filters, regardless of the cost of filtering and the limited rating of the active filters used. To summarize, the main focus of researchers is clearly moving to empower distribution systems with highly penetrated renewables while offering multi-functionality facilities (a trade-off between different goals). Accordingly, multi-objective optimization should be used to improve the HC while limiting harmonics and improving the PQ performance of such distorted systems.

8. Summary

• The integration of distributed generation and renewable energy sources into distribution systems has sparked a growing interest in understanding the HC of these systems while ensuring reliability, PQ , and sustainability.

• Traditional assumptions that distributed generation units are harmonic-free have been challenged as it became evident that the combination of harmonics between these units and nonlinear loads can lead to severe PQ problems. To overcome the uncertainties associated with HC calculations, researchers have developed stochastic procedures that consider multiple probabilistic scenarios. These approaches provide a more comprehensive understanding of system performance and aid in decision-making processes.

• Harmonic distortion, once not a primary concern for operators, has now emerged as a critical issue. Passive harmonic filters have been employed to maximize HC and mitigate harmonic distortion. The analysis presented in various studies has demonstrated that the HC of a distribution system decreases with increasing grid-side voltage distortion and nonlinear load levels. Moreover, it has been observed that nonlinear load levels have a more significant impact on HC than grid-side voltage distortion. These findings highlight the effectiveness of harmonic filters in addressing the problems arising from harmonic distortion and enhancing HC simultaneously. However, optimizing HC and improving power quality pose complex challenges. Single-objective optimization approaches may not suffice, as there can be conflicting objectives. Researchers have found that HC improvement and voltage total harmonic distortion (THD) reduction tend to be conflicting goals in optimization. Consequently, multi-objective optimization techniques have been proposed to strike a balance between HC improvement, harmonic reduction, and voltage THD lessening.

• The drive toward sustainability has urged researchers to seek solutions that enable distribution systems to accommodate highly penetrated renewables while offering multi-functionality. Hybrid harmonic distortion mitigation techniques have emerged as a promising approach. By combining passive and active filters, these techniques can achieve higher levels of renewable energy penetration, regardless of the cost of filtering and limitations of the active filters used. This integration of renewables and hybrid filtering not only enhances HC but also contributes to the long-term sustainability goals of the system.

In summary, the convergence of HC, reliability, PQ , and sustainability requires the adoption of advanced techniques, including stochastic HC calculation, passive and hybrid harmonic filtering, and multi-objective optimization. By considering these factors together, distribution systems can effectively accommodate higher levels of renewable generation, ensure high-quality power supply, improve system reliability, and contribute to long-term sustainability objectives. These research efforts pave the way for the empowerment of distribution systems with renewable energy while maintaining the necessary functionality and performance standards.