Electromagnetic Interference from Solar Photovoltaic Systems: A Review

Abstract

Rapid expansion of solar photovoltaic (PV) installations worldwide has increased the importance of electromagnetic compatibility (EMC) of PV components and systems. This has been highlighted by interference reported from PV installations (PVI) in the Netherlands, the United States, Sweden, etc. Significant research and development efforts are seen in the domain of enhancing efficiency and economic viability of PVI, whereas the EMC aspects have received less attention and are mainly focused on the PV system acting as a victim of lightning and a victim of changing grid impedance. This article presents a review of the important EMC aspects of PVI as a disturbance source. It has the following main parts: (a) reported cases of emissions and interference from PV installations; (b) modeling and analysis of PV subcomponents from an EMC perspective; and (c) the main standards related to the topic. Mitigation techniques for improving EMC aspects of PVI are also described, along with suggested directions for future research. The compilation brings together wide-ranging sources, both for EMC engineers who want to understand the EMC context of PV systems and for PV system designers seeking to improve EMC performance.

1. Introduction

Solar photovoltaic (PV) generation is a fast growing renewable energy source, with 35% increase in production in 2022 compared to 2021 [1]. As solar PV installations (PVIs) increase worldwide, there are increasing concerns [2,3,4,5] regarding their electromagnetic compatibility (EMC). In particular, the emissions might become a major roadblock in penetration of PVIs if they cause disturbance to critical infrastructure such as air traffic communication systems [4], hospitals [6,7], or telephone systems [8]. There have been issues with products that are certified as conforming to the relevant regulations and standards but that cause interference to other systems, which can lead to a product ban [9]. A part of the problem is that the set of relevant standards have not been rigidly defined in many jurisdictions, and the individual certified products are components in a complete PV system that was not defined by the product manufacturers.

The major dedicated articles or reports in this area are (a) a study in 2002 about EMC and safety in PVI [10], (b) a market surveillance campaign in 2014 [11] in the European Union, (c) an EMC study of large PV plants published in 2009 [2] and its update published recently [12], and also (d) recent efforts to measure emissions from PV plant at Electric Power Research Institute (EPRI), USA [13].

Since then, there has been a lot of progress in solar PV. The power electronics technology has advanced significantly with increasing use of silicon carbide (SiC) devices, modular multilevel inverters, and wide variety of PVIs ranging from very small solar PV panels on boats to solar PV plants generating tens of gigawatts of power [14,15,16]. The installation practices have also changed [12], for example, the strings have become larger in size and the per panel power output increased to the range of 500–600 W from earlier 250–300 W.

Since 2019, there have been increasing cases of reported interference from solar PV converters, mainly in Europe and USA, as listed in the next section. However, there is no systematic topical review in the form of a scientific article which covers the observed conducted and radiated emission levels as well as modeling and analysis techniques. Most literature on modeling of PVI-components, such as [17], focuses on design or analysis for maximizing performance in terms of efficiency. For EMC, the frequency of interest is different from that for power generation, as shown in Table 1.

There are two main aspects of electromagnetic compatibility in general: when a device under test (DUT) acts as a source of interference and when the DUT acts as a victim of the interference. The former is called electromagnetic interference (EMI) and the latter is called electromagnetic susceptibility. The current work focuses on EMC with PVI as a source, bringing together reports and studies that highlight modeling and analysis of this less commonly considered aspect. It is intended to be helpful as an introduction and source of references, bridging the gap between the disciplines of EMC engineering and power engineering. Besides engineers from these disciplines, intended readers include PV component manufacturers, members of standards committees, and policy-makers.

This paper is organized as follows. The next section describes major cases of interference from PV panels and the categorization of literature. Section 3 describes research related to modeling and analysis of subcomponents of PV systems, i.e., PV panels, DC cables, and converters. Section 4 presents important standards related to the topic. Section 5 offers some important mitigation techniques. Section 6 describes the scope of future research.

2. Background and Literature

The literature on EMC issues in PV systems can be classified as shown in Figure 1:

1. Reported cases: This refers to interference cases, such as reports from the regulatory authorities of respective countries.

2. Specialized literature: These are research papers and theses dedicated to this specific subject. They provide a good overview of the problem backed by some experimental data.

3. Selective literature (literature on modeling and analysis): This class of literature includes theses and articles related to modeling and analysis of specific parts like PV panels or converters in a PV installation. Figure 2 shows a generic schematic diagram of a PV system from an EMC perspective. The power converter is the main source of high-frequency signals which can propagate to both DC and AC sides as common mode (CM) and differential mode (DM) signals. For the systems external to the PVI, the panels and the DC cables can act as antennae and can therefore be treated as a source of interference for EMC analysis. The radiated interference can also result from the converters. For modern large PV plants, the DC cables and AC lines may span several hundred meters and hence can act as transmission lines. The transformer is important while analyzing the large PV plants, but it is usually absent for PVIs smaller than 10 kW, which are mainly used for household applications.

4. Standards: This includes standards formulated by organizations like International Electrotechnical Commision (IEC) or Federal Communications Commission (FCC).

In this section, the reported cases and specialized literature are covered, whereas the subsequent sections cover the selective literature and standards.

2.1. Reported Cases

In the recent past, EMC aspects of PV have become important due to several incidents of interference from PVI. These are listed below.

1. In 2014, ‘EMC market surveillance campaign for solar panel inverters’ [11] was published by the EMC Administrative Cooperation Working Group. The campaign was conducted in 14 different countries in Europe. Most of the converters (50 out of 55, i.e., 91%) were found to be non-compliant with the harmonized standards after actual tests, although the products themselves were reported to be compliant by the manufacturers. The report says that the conducted emissions at the AC main terminals in the range of 9 kHz to 150 kHz are also important, apart from the usual 150 kHz to 30 MHz range.

2. The Swedish Defence Research Agency (Totalförsvarets forskningsinstitut or FOI) and the National Electricity Safety Board of Sweden conducted several investigations to measure conducted and radiated disturbances from PVI [3,18]. Their use of the GENESIS software showed that the range of communication of an AM radio receiver (118–137 MHz) reduces to 49% of the range without solar PV installation [19].

3. Another article from FOI [4] reported conducted emissions from residential PVI in the frequency range of 10 kHz to 200 kHz, and radiated emissions in the frequency range of 10 kHz to 1 GHz. Specifically, a repetitive pattern of peaks at 16.7 kHz, 100 kHz, and 200 kHz was found with rooftop PVI with optimizers when a measuring antenna was placed 20 m from the PVI with 22 and 40 panels. It should be noted that the radiated emissions were measured in the frequency range below 30 MHz, which is the lower bound of frequency considered for radiated emissions.

4. In Switzerland, the Federal Office of Communications (OFCOM) [20,21] investigated interference at nine PV installations and found that DC optimizers were a source of interference for amateur radio (‘ham’ radio) operators. The probability of interference increased if the optimizers used power line communication (PLC).

5. Some other articles also reported similar cases in Switzerland [22,23], stating that the PVI with optimizers have more chance of interference and may require extra filters in addition to the already existing filters. Haeberlin [24] has also reported interference from a domestic PVI with optimizers in the range of 3.5 MHz to 52 MHz and its mitigation by use of ferrite cores.

6. The Dutch authority for digital infrastructure (Agentschap Telecom) reported disturbances from PVI, e.g., slow data transfer, nonfunctional garage door opener, interference to digital radio, etc. [6,7].

7. A 17 MVA PVI in the United States was found to cause interference to a nearby telephone system in the frequency range of 2.5 kHz to 4.5 kHz recently [8].

A summary of the cases above is presented in Table 2, which indicates the nature of the solar PV setup and the victim and the frequency range in which the interference was found. Reading through these reports or articles, one can note down the key takeaways from the reported incidents:

1. Several reported cases involve PVI that include optimizers.

2. The emissions are found in the wide frequency range from ≈9 kHz to a few GHz, and hence they might affect critical communication infrastructure like air traffic control center, medical implants, or military communication systems [25].

3. Most of the cases are reported for small (<10 kW) PVIs (see the cases in Sweden, Switzerland, and the Netherlands in Table 2).

4. Conducted emissions are reported in the frequency range of 10 kHz to 200 kHz and the radiated emissions are reported in the frequency range of 150 kHz to 1 GHz, whereas the emission limits for conducted emissions in EMC standards (EN 62920 [26], CISPR 11 [27], and FCC Part 15-B [28]) are for the frequency range of 150 kHz to 30 MHz and for radiated emissions. The limits are given for frequencies beyond 30 MHz.

5. Many of the cases are initiated due to interference being reported by amateur radio operators.

6. The total number of EMC incidents might be a small fraction of total installed capacity of PV. However, due to lack of a systematic framework, it is difficult to investigate the cases of EMC from PV systems.

Table 2

Interference cases from PV installations.

Year	Location	Solar PV Setup	Victim	Frequency Range	Details
2014	Europe [11]	Mostly household PV inverters	NA	9 kHz–150 kHz, 150 kHz–3 MHz	Only 33% of the total tested units were found to obey the limits of emissions
2019	Sweden[3,4,18,19]	PV installation at airport	AM radio receiver at air traffic control center (ATCC)	118–137 MHz	Interference to AM radio receiver at air traffic control communications, reduced communication range, and increased receiver noise level
2020		Commercial PV plant on a building in residential area	Mobile communication	760–960 MHz	Medium power commercial solar PV plant was found to interfere with mobile communication system
2020		Domestic PV plant	Shortwave radio, broadcast radio, amateur radio, com radio	1.8 MHz–30 MHz	Mostly household installations with panel level optimizers are found to interfere
2022		Domestic PV plant	Not a specific victim. Emission measurements are reported	10 kHz–200 MHz	Conducted emissions (especially in PVI with optimizers) are seen with peaks at 16.7 kHz, 200 kHz, signals separated by 200 kHz in currents
2022				10 kHz–1 GHz	Radiated emissions (especially in PVI with optimizers), magnetic field measurements performed with loop antenna, peaks separated by 200 kHz are seen
2020	Switzerland [20,21,22,23,24]	10 kW domestic PV power plant	Amateur radio operators	21–18 MHz	At 3 m distance, noise is 12 dB higher than that in [29]. After installation of bifilar ferrite ring cores, the peaks are significantly reduced. DC optimizers are a source of interference
2019	Netherlands [6,7]	Domestic PV plant	Wireless internet, digital radio, amateur radio	3–30 MHz	Agentschap Telecom receives complaints from amateur radio operators, individuals. For example, a PV installation at a supermarket caused increased noise floor by about 10–15 dB
2023	USA [8]	17 MVA plant	Telephone lines of plain old telephone service	2.5–4.5 kHz	High-frequency signals from PV installations coupled to telephone lines via the common ground emissions

2.2. Specialized Literature

1. A detailed study was conducted in 2002, titled ‘EMC and Safety Design of PV Systems (ESDEPS)’ [10], by institutes and organizations in Germany, Austria, and Spain. As far as emissions from PVIs are concerned, the important results in this study are as follows:

   • (a). CM and DM antenna factor (see Section 3.1.2) and impedance were measured for PVI with rated power capacities ranging from 50 W to several kW. Magnetic fields emitted by commercial inverters were noted and they were found to exceed the limits in VDE 0878. In some cases, emissions of the order of 30–70 dB$\mathsf{\mu }$V/m can be seen above 30 MHz. It is also noted that the measurements at actual PV plants are usually distorted by the background noise.

   • (b). The PV system, when in operation, can cause an increase in flicker level by as high as 100%.

   • (c). There are significant levels of harmonic voltages and currents at switching frequency 15.75 kHz of inverters being injected into the system.

   • (d). Apart from this, antenna factor, common mode, and differential mode impedance of PV panels were reported.

   • (e). Emissions from eight commercial off-the-shelf inverters were measured as per then available standards, and it was found that in the frequency range of 150 kHz to 30 MHz, three out of eight inverters exhibited high emissions (>10 dB of the threshold in standards) on AC lines; two out of eight inverters exhibited similar emissions on the DC lines. In 30 MHz–300 MHz, two out of eight inverters exhibited high emissions on the DC lines.

In 2009, Araneo et al. [2], reported possible EMC issues in a high-power (1 MW)-grid-connected PV system. Common mode (CM) voltages were measured at several locations on both the AC and DC sides of the inverters within the power plant, and some of them were found to cross the limits in EN 61800-3 [30] (a standard for industrial drive systems) and EN 55011 [27] (a standard for industrial, scientific, and medical (ISM) equipment). In the updated version of [2], it was indicated that the scenario of PV installations has changed since 2009 in terms of PV panels, trackers, inverters, installation methods, etc. [12].

In 2001, Schattner [31] presented a characterization of solar panels and PV installations from an EMC perspective. This work provided radiated and conducted interference models of PV panels in the form of antenna factor (see Section 3.1.2) and impedance measurements. It also proposed methods for measurements from DC side emissions. In 2019, Prajapati [32] presented modeling and analysis of EMI from the DC side of the converter. This work provided extraction of an equivalent noise source model of PV panels by using in situ measurements.

2.3. Selective Literature

The research articles in this area are found in the power electronics, EMC, and power systems domains. However, from the modeling and analysis perspective, the articles can be categorized into the following domains:

1. Panels and DC cables,

2. Power converters.

There is also a significant amount of literature which deals with PV systems acting as a victim of EM disturbances, i.e., electromagnetic susceptibility of PV systems [33,34,35,36,37]. Given below are four categories of external sources of disturbances which can affect PVI:

1. Grid,

2. Lightning,

3. HV transmission lines,

4. Intentional electromagnetic interference.

However, in this article, the main focus is on emissions from the PV systems, and hence these topics are not explored in detail here. The next section elaborates on the articles related to modeling and analysis of subsystems in a PVI.

3. Modeling and Analysis

3.1. PV Panels and DC Cables

PV panels contain semiconductor cells and metallic fingers and busbars to carry the generated current. This active part is sandwiched between a backsheet and a glass cover, surrounded by an aluminium frame. From a circuit point of view, this structure can be modeled as a parasitic capacitance or a combination of inductor and capacitor with respect to ground. For studying radiated interference, the structure can be modeled as an antenna. Accordingly, the literature analyzing EMC issues from PV panels can be categorized into the following two domains.

1. Models of parasitic impedance ($Z_{\mathrm{PV}}$) of PV panels: This is important for the analysis of conducted emissions or common mode (CM) circuit.

2. Models of panels as an antenna: This is important for analysis of radiated emissions.

3.1.1. Impedance of PV Panels

In EMC terms, the impedance of PV panels can be categorized as (see Figure 2) (a) common mode (CM) impedance—from terminals P and N to G, and (b) differential mode (DM) impedance—between terminals P and N [38,39,40]. It is often assumed that the CM impedance of the PV panels is primarily capacitive [10]. However, Henze et al. [40] have reported that the CM impedance of panels and cable combination is capacitive and decreasing at 20 dB per decade for frequencies only up to ≈2 MHz. From 1 MHz to 30 MHz, it exhibits multiple resonant peaks, which indicates the significance of inductance in the circuit, as shown in Figure 3. One way to capture this inductive behavior is given by Chen et al. [41,42,43] in the form of a comprehensive model to compute $Z_{\mathrm{PV}}$. This is shown in Figure 4a,b, which show a schematic picture and the equivalent circuit, respectively. The circuit in Figure 4b contains several capacitances: $C_{\mathrm{cr}}$, $C_{\mathrm{cf}}$, $C_{\mathrm{cg}}$, which are cell-to-rack, cell-to-frame, and cell-to-ground capacitances; and some inductances $L_{\mathrm{r}}$, $L_{\mathrm{f}}$, $L_{\mathrm{c}}$ which are the inductances of rack, frame, and cable, respectively, and $G_{\mathrm{cg}}$ is cell-to-ground admittance. Kane et al. [44] quantified the effect of geometrical and environmental variables like tilt angle and water level on the panels using ‘design of experiments’ [45]. The inductive nature of $Z_{\mathrm{PV}}$ was also explored in [46] via experimentation. For the model of parasitic impedance of PV panels, the model proposed by Chen et al. [41,42] is difficult to use for a real PV plant where it might not be practically possible to access various measurement points to measure all the constituent parasitic parameters. It is further difficult to measure the currents flowing through these parameters.

Extraction of $Z_{\mathrm{PV}}$ Parameters from Circuit Measurements

Wang et al. [47,48] proposed a method to determine the parasitic capacitance of the PV panels from the period of the oscillations in the waveform of CM currents. The parasitic capacitance of the PV array and the series inductance of the DC input filter inductor form a resonant circuit whose resonance frequency is seen in the CM current. The DM parasitic parameters can also be estimated from the transient charge of an external capacitor. This was perforned for a 200 kW solar PV plant [49], where it is stated that at panel level, the parasitics show inductive behavior, whereas at array level it is capacitive. Cotfas et al. [50] proposed a new RLC circuit-based method to determine the DM capacitance of solar cells as an alternative to electrical impedance spectroscopy. The DM impedance behavior over a wide frequency range was investigated in [51] through experimentation.

Prajapati et al. [32,51,52,53] proposed estimation of CM and DM impedance and ‘noise source’ model using an in-circuit impedance extraction method employing inductive probes and a vector network analyzer (VNA). This is based on extraction of in-circuit ‘equivalent noise source’ with the help of inductive probes inserted in the circuit. The principle used in this method is shown in Figure 5. Here, two probes are connected, one for injecting and one for receiving. $V_{\mathrm{s}}$ is the source voltage, $Z_{\mathrm{s}}$ is the source impedance, $Z_{\mathrm{c}}$ is the cable impedance, and $Z_{\mathrm{L}}$ is the impedance of device under test (DUT). VNA measurements provide the s-parameters of the circuit, and impedance $Z_L$ is then extracted using these parameters. Other types of in-circuit impedance extraction, e.g., using impedance analyzer or voltage–current approach [54,55], could also be useful for PV applications where the layout of plant affects the EMC issues quite significantly.

In [32], the CM impedance $Z_{\mathrm{PV}}^{\mathrm{CM}}$ of a 60 W PV panel was extracted using this method, and it was found to be equivalent of a 0.07 nF capacitance. The DM impedance $Z_{\mathrm{PV}}^{\mathrm{DM}}$ was also estimated and taken as combination of $L_{\mathrm{s}}=1.5\mathsf{\mu }\mathrm{H}$, $C_{\mathrm{ext}}=50\mathrm{pF}$, $R_{\mathrm{ext}}=840\Omega$. While this method is good because it does not rely on the internal structure of the PVI, it is used only for a lab-scale model in [32]. There might be difficulties in practical implementation of the method because of the presence of external noise sources which might affect the measurements. As the size of installation grows, the difficulty in measuring the radiated emission accurately increases and the reliability of the measurements decreases, which is a classical problem in EMC of large systems.

Effect of $Z_{\mathrm{PV}}$ on CM Equivalent Circuit

Parameters of $Z_{\mathrm{PV}}$ is related to CM equivalent circuit of the inverter as well. This section describes research carried out in this direction.

Yu et al. [56] investigated CM resonance in parallel inverters and stated that in case of string inverters, the CM and DM circuits are independent and their resonance mechanism can be treated separately. Hernandez et al. [57] presented long-term recording of leakage capacitance and insulation resistance as it varies with atmospheric pressure, relative humidity, module temperature, etc. García-Gracia et al. [58] presented an analysis of a large (1 MW) PV power plant with an improved $Z_{\mathrm{PV}}$ model as a parallel combination of resistance $R_{\mathrm{pv}}$ and capacitance $C_{\mathrm{pv}}$. For a typical topology of a large-scale PV system shown in Figure 6 [58], they proposed a CM circuit model as shown in Figure 7, including the inductance $L_{\mathrm{c}}$ and capacitance $C_{\mathrm{c}}$ of DC cables and $R_{\mathrm{ac}\_\mathrm{c}}$, $L_{\mathrm{ac}\_\mathrm{c}}$, $C_{\mathrm{ac}\_\mathrm{c}}$ as resistance, inductance and capacitance of AC cables. The inductance of the cables and the capacitive CM impedance of the panels form a resonant circuit. The resonance frequencies are accurately estimated by the model proposed in [58] for the 1 MW PV plant, and the results are confirmed by simulations performed in PSCAD. The reported dominant harmonics in the source voltage and source current are at frequencies 3.7 kHz, 7.4 kHz, 11.1 kHz, 14.8 kHz, and 18.5 kHz, which are multiples of the 3.7 kHz switching frequency. Chen et al. [43] showed that if the equivalent pi-circuit as shown in Figure 4 is used, the leakage current increases from 0.6 A to 1 A for a small PV system.

3.1.2. Modeling and Measurements of Radiated Emissions from PV Panels

The CM and DM noise on the DC side of the power converter can propagate via the DC cables to the PV panels. Both the panels and the cables can act as antennas and hence can be a source of radiated interference as shown in Figure 2. The radiation from PV panels can be characterized by the antenna factor (AF) [38,39,40]. AF is defined as the ratio of the magnitude of incident electric field, or magnetic field, at the surface of the measurement antenna to the received voltage at the terminals of the antenna (see (1)) [40,59].

(1)$\mathrm{Antenna}\mathrm{factor}\left(\mathrm{AF}\right)={\displaystyle \frac{\left|\mathbf{H}\right|}{V}}$

The AF is measured by exciting the module with a signal of known magnitude and frequency, and measuring the magnetic field in the main lobe of the radiation pattern, with a loop antenna. The measurement setup is as shown in Figure 8a, where d is the distance between device under test (DUT) and the measurement antenna. The CM and DM antenna factor were measured for a PV generator with 24 modules in series as shown in Figure 8b. Presence of external noise source and the difficulty involved in measurement of noise source are two significant limitations to measure the antenna factor in real PVIs [12,25]. Laboratory measurements could help in such cases; however, their use is also limited because the practical PVI also involves cables and other such parameters [12]. The laboratory measurement efforts for radiated emissions are covered in the next section.

If the AF for particular type of panels is known from lab tests and the limits on magnetic field strength are known from standards, then it is straightforward to determine the limits on CM voltage or current at the input terminals of the power electronic converter or at the output terminals of the PV panels.

The literature in this domain can be further classified into the following:

1. Emission measurements from solar panels in a laboratory setting.

2. In situ measurements on large PVIs.

3. Measurements of small-sized PV panels.

4. Studies on solar installations at airports.

Emission Measurements from Solar Panels in Laboratory

Several researchers [60,61,62,63,64] used measurement antennas to measure radiated electric or magnetic field from a solar panel in a lab environment rather than the real PV installation. Their findings are summarized in Table 3, and some peaks from the reported emissions are shown in Figure 9. The references are abbreviated as denoted in the caption. The excitation signals used in these experiments are of the order of 5–10 dBm, or in one case $-30$ dBm [65]. The emissions span frequency bands from 1 kHz to a few hundred MHz, which include terrestrial broadcast TV and radio, marine and aviation radio, amateur radio, and some important bands of radar and radio astronomy [59]. These comparisons are presented (a) to compare the experimental setups and present ball park figures to expect in future research for similar configurations and (b) to highlight the frequency zones which could be affected due to emissions of solar panels.

Wu et al. [64] proposed three models of PV panels as shown in Figure 10: (a) interconnected PV module, (b) model consisting of PV cells, and (c) metal frame module. By using metal plate model for simulations, they showed that if there is a loop between the grounded frame wire and DC cable connected to the panels (see Figure 11), then there is a resonant frequency appearing below 5 MHz, which is not seen if the frame wire is grounded separately.

In Situ Measurements from PVI

Several researchers have reported emissions from the real PVIs. They presented emission levels in a few cases which could be used as benchmarks for PVIs with similar ratings and employing similar topologies. Chen et al. [42] proposed a microstrip antenna model for a solar cell and measured the emissions from 100 kVA PVI, reporting that the major emissions are in the frequency range of 60 to 80 MHz. Di Piazza et al. [68] measured a CM current on the DC side and found that the frequency spectrum of the current was almost the same with the inverter ON as with the inverter OFF, which implies that the current is mostly due to coupling of external electromagnetic fields. The resonance frequencies in the system were 2.2 MHz, 9.7 MHz, 13.2 MHz, 15.6 MHz, 18.9 MHz, and 20.8 MHz. Henze et al. [40] found that magnetic field emissions from the 1.3 kW PV generator exceed the limits in the standard VDE 0878-1 [69] when there was a poor filter on the DC side. They also investigated the antenna factor and CM impedance of a PV array consisting of 24 panels in series. It was found that the worst-case antenna factor varies from −78 dBS/m to −60 dBS/m in the frequency range of 150 kHz to 30 MHz.

Keyer et al. [70] compared emissions from two commercially available solar PV inverters at the actual PV installations and reported that solar PV installations can interfere with amateur radio operation particularly in the frequency range of 10 MHz to 50 MHz. They proposed that the DC cables can act as a tuned antenna. The Swedish National Electrical Safety Board, the agency responsible for monitoring EMC aspects, reported emissions from Glava Energy Center, Sweden, at frequencies from 150 kHz to 30 MHz [3]. The interference was seen from inverters, solar panels, and cabling. Moreover, higher interference with a pattern of peaks separated by 600 kHz was attributed to DC optimizers. FOI reported emissions from two installations, 11.4 kW and 6.8 kW, and found that the emissions of the order of 40 dB$\mathsf{\mu }$A was found in the frequency range of 1 MHz to 30 MHz. The emissions were higher for the 6.8 kW PV installation, in which DC optimizers were used [3]. The interference pattern spaced at 200 kHz and its multiples was reported in [22,23] to interfere with radio frequency signals. Haeberlin [24] reported interference in the range of 3.5 MHz to 52 MHz from DC optimizers and their mitigation by use of ferrite cores on the optimizer input and the module output.

In [25], the conducted and radiated emissions were reported from a 6.5 kW solar power plant in the range of 9 kHz to 30 MHz, based on measurements performed in Karlstad, Sweden. The emissions were found in the 1–60 MHz range, which is also used by military, medicinal implants, and other important systems. The peaks separated by 200 kHz were reported as emitted from the DC cables. Recently, it has been found from several measurements on a 5 kW PV power plant that a system with optimizers has more chances of interfering with radio communications than a system with string inverters [71]. The systems with optimizers exhibited a magnetic field emission pattern separated by 200 kHz in the frequency range of 3.2 to 6.4 MHz and 12.6 to 21.2 MHz. It was also shown that cable arrangements play a less significant role in the radiated emissions. Azpurua et al. [72] described a fast time domain measurement method to measure emissions. They reported electric and magnetic field emissions from an actual PV installation in the range of 9 kHz to 30 MHz. They found broadband emissions in the range of 5 MHz to 30 MHz exceeding the limits in CISPR 37. As far as electric fields are concerned, the values are very near to the emission limits around 66 MHz. Dan et al. [73] reported the conducted emission measurments from a domestic PV plant with Eurener solar panels of 450 W each and 2 kW inverter.

Some researchers have noted that the solar PV installations at airports may also pose problems for communication systems at airports. Main articles or reports in this domain are as follows. Federal Aviation Administration (FAA) [74] guidelines suggest that any interference with radar, navigation aids, or infrared communications should be checked before the solar panels are actually installed. Interference with infrared communications might occur due to increased temperature of the panels in the full sunlight. The Swedish Defense Research Agency showed that solar panels co-located with an air traffic control system can reduce the range of communication up to 50% [19] based on the assumption that the PV array’s current from 30 MHz to 200 MHz is at the limit of EN55022 class B [75]. NREL suggested in their report that the leading edge of solar PV array should be at least 150 to 500 feet away from the radar equipment [76]. Anurag et al. [77] reported that radar interference needs to be considered while installing solar panels at airports. Even without the interference generated by solar PV arrays, if radar signals strike solar PV arrays, it may mislead the direction signals received by the air traffic controllers and pilots [78]. Stracqualursi et al. [12] reported the conducted and radiated emissions from a 19.87 MW PV power plant with bifacial solar panels and decentralized multilevel inverter. They noted that there could be practical difficulties in measurement of emissions from such large solar PV plants because they cover a large area of tens of hectares. They also noted that the radiated emissions below 30 MHz would be usually below the limits; however, the limits might be crossed for the frequencies greater than 30 MHz.

Measurements of Small-Sized PV Panels

Small-sized PV panels are typically used in satellites, boats, etc., where the panels may affect nearby electrical systems. A method to extract the antenna factor was proposed by Prajapati et al. [32]. In this method, the CM and DM impedance ($Z_{\mathrm{PV},\mathrm{CM}}$ and $Z_{\mathrm{PV},\mathrm{DM}}$) of PV panels is first measured using a vector network analyzer and inductive probes. Then, to measure the CM antenna factor $A{F}_{\mathrm{CM}}$, a current probe is clamped onto the positive and negative wire together, as shown in Figure 12a. The signal generator injects CM current into the system which is measured by another current probe. The field (E) is measured by the loop antenna. The voltage across the PV panels (V) is determined from the measured current and $Z_{\mathrm{PV},\mathrm{CM}}$ and $Z_{\mathrm{PV},\mathrm{DM}}$. The antenna factor is then obtained using

(2)$AF\left(\mathrm{dB}/\mathrm{m}\right)=E\left(\mathrm{dB}\mathsf{\mu }\mathrm{V}/\mathrm{m}\right)-V\left(\mathrm{dB}\mathsf{\mu }\mathrm{V}\right)$

Figure 12b shows a comparison of CM and DM antenna factors measured for a 60 W solar panel in the laboratory.

David Norte [79,80] computed DC and high-frequency electric and magnetic fields from a satellite solar panel with bevel cut and 90° cut. He also showed the effect of solar cell size and distance between current-collecting fingers on the radiated emissions. Andrieu et al. [81] proposed a partial element equivalent circuit (PEEC)-based model which can be used to compute near-field emissions in the frequency range from quasi-static to a few tens of MHz. They modelled the metallic plate, solar cell, Kapton film, and connecting wires as equivalent circuits and then converted those into an electric dipole. They verified this experimentally using a solar panel made up of 10 cells attached to a brass plate of 660 mm by 440 mm and near-field measurements performed by a high-impedance circular magnetic loop. The magnetic field was found to be 70–80 dB pT at 10 cm and at 200 kHz and 1 MHz.

Juswardy et al. [65] found the radiated field of small solar cells/modules (3 V, 200 mA) in the frequency range of 30 MHz to 330 MHz at a distance of 50 mm from the panels. They also proposed a simplified model with only interconnecting wires and fingers and without the cells themselves to compute the return loss $S_{11}$ parameter with numerical simulation in HFSS. They reported that the emissions from a commercial solar charger module were −6 dB$\mathsf{\mu }$V/m to 16 dB$\mathsf{\mu }$V/m, which is within the limit of 40 dB$\mathsf{\mu }$V/m in CISPR 11. However, the emissions might cross the limits given in the MIL standard and hence might become potential disturbance to the astronomical receiver system. Saidi et al. [82] studied the magnetic field of a 75 W PV panel at 5 MHz with a probe held over the panel and the distance from the panel varying between 2 cm and 20 cm. Zhu et al. [83] measured radiated emissions at 600 mm from solar modules with an AM loop antenna and a 2.4 GHz optimized patch antenna. Cataliotti et al. [66] characterized the thermal and electromagnetic behavior of a commercial inverter module with 2 A/220 V and 14 A/100 V load in the frequency range of 9 kHz to 30 MHz.

3.1.3. DC and AC Cables

The DC cables consist not only of the cables running from the PV array to the inverter, but also the cables which connect the panels to form strings and arrays. Such cables may form a radiating structure for example, a loop antenna. The importance of DC cables in radiated emissions is shown by Wu et al. [59,64].

For conducted emissions, DC cables can be modeled as multiconductor transmission lines [2,84]. Depending on the length and the frequency of interest, the cable can also be modeled as a concentrated inductor. Song et al. [85] modeled the cable as an inductance and showed by simulation that the cable has significant impact on CM current in the frequency range of 3 MHz to 30 MHz, but it has a relatively small impact in the frequency range of 100 kHz to 3 MHz. As the cables have multiple parallel conductors, there can be a mutual coupling between the conductors themselves. Such an equivalent circuit model of cable parasitics is found in [86]. In [32], radiated emissions from a 60 W PV panel are measured by connecting a buck converter (10–40 V DC, 5 A) at its output. The length of the cable between the panel and the buck converter is varied as 0.25 m, 1 m, and 3 m, respectively. The electric field emission is shown in Figure 13. It is seen that the radiated emission increases for the longer cable for frequencies above 10 MHz. Particularly, the emission was reported to be significantly greater at 25 MHz for the 3 m cable than that for the 0.25 m cable.

Stracqualursi et al. [12] indicated that in the case of a large MW scale PV power plants, the cables on the AC side can have lengths ranging upto 200 m which could become a part of the resonance circuit and in some cases a cause of resonance up to 1 MHz.

3.2. Power Converters

The power converter may refer to either (a) DC-DC converter or (b) a DC-DC converter and a DC-AC converter forming an inverter. The converter itself consists of a power electronic circuit controlled by a microcontroller which implements maximum power point tracking (MPPT), power factor control, harmonic control, reactive power control, etc., as shown in Figure 2 [17,87]. It is possible to estimate harmonic profile of output voltage and current of an inverter if the inverter topology and the pulse width modulation (PWM) scheme used to control the inverter is known [88]. A brief overview of important inverter topologies and basics of PWM schemes are given in Appendix A and Appendix B.

The following are the broad categories of research carried out related to analysis of converters from EMC viewpoint:

1. CM analysis of inverters

2. CM analysis of converters

3. Miscellaneous issues.

3.2.1. CM Analysis of Inverters

Leakage current is arguably the most investigated EMC phenomenon in the power electronics literature on PV systems, and IEC 62109 [89] offers a limit on residual current (see Table 4). A popular metric for inverters is ‘total’ leakage current (rms, not at a particular frequency band), which can lead to lesser attention to the spectrum of leakage current and its interference potential. Leakage current or common mode (CM) current can be evaluated by deriving the CM equivalent circuit.

The basic topology of an H-bridge inverter is shown in Figure 14a.

The CM equivalent circuit can be derived by replacing the inverter with the voltage sources $V_{\mathrm{AN}}$ and $V_{\mathrm{BN}}$ as shown in Figure 14b. Here, $V_g$ is the grid voltage, $V_{\mathrm{DC}}$ is the DC input voltage, and $V_{\mathrm{AN}}$, $V_{\mathrm{BN}}$ are the voltages between nodes A, N, and B, N as shown in Figure 14a. For CM current analysis, the interested range of frequency is usually a few kHz or above. Therefore, the sources $V_g$ and $V_{\mathrm{DC}}$ can be neglected. The equivalent circuit is given as shown in Figure 14c. From the circuit in Figure 14c, the CM current $I_{\mathrm{CM}}$ is given as follows:

(3)$I_{\mathrm{CM}}={\displaystyle \frac{V_{\mathrm{eq}2}}{Z_g+Z_{\mathrm{PV}}+L_1\left|\right|L_2}}\mathrm{where},V_{\mathrm{eq}2}={\displaystyle \frac{L_2{V}_{\mathrm{AN}}+L_1{V}_{\mathrm{BN}}}{(L_1+L_2)}}$

Alternatively, voltages $V_{\mathrm{AN}}$ and $V_{\mathrm{BN}}$ can be expressed in terms of CM voltage $V_{\mathrm{CM}}$ and DM voltage $V_{\mathrm{DM}}$ as follows:

(4)$V_{\mathrm{AN}}=V_{\mathrm{CM}}+0.5V_{\mathrm{DM}}{V}_{\mathrm{BN}}=V_{\mathrm{CM}}-0.5V_{\mathrm{DM}}$

These expressions can be easily derived from the following definitions of $V_{\mathrm{CM}}$ and $V_{\mathrm{DM}}$ [90]:

(5)$V_{\mathrm{CM}}=0.5(V_{\mathrm{AN}}+V_{\mathrm{BN}})V_{\mathrm{DM}}=V_{\mathrm{AN}}-V_{\mathrm{BN}}$

With the expression for $V_{\mathrm{AN}}$ and $V_{\mathrm{BN}}$ as in (4), the equivalent circuit can be redrawn as in Figure 14c. Then, the expression for the leakage current is given by

(6)$I_{\mathrm{CM}}={\displaystyle \frac{V_{\mathrm{CM}}+V_{\mathrm{eq}1}}{Z_g+Z_{\mathrm{PV}}+L_1\left|\right|L_2}}\mathrm{where},V_{\mathrm{eq}1}={\displaystyle \frac{0.5V_{\mathrm{DM}}(L_2-L_1)}{(L_1+L_2)}}$

An advantage of using (6) over (3) is that (6) could be useful for deriving the contributions of CM and DM voltages to the CM current. For example, not only the CM voltage but also the DM voltage can contribute to the CM current in the case of cascaded multilevel inverters [91,92].

CM noise attenuation function can also be derived for current source inverter as described by Li et al. [93]. They showed that connection of a neutral wire can facilitate noise reduction by around 19 dB. The CM equivalent circuits for various PVI topologies containing inverters are presented in [94]. However, as far as CM leakage current analysis is concerend, the focus of most articles has been transformerless inverter topologies. For example, Cha et al. [95] offered a detailed analysis of the CM equivalent circuit of important topologies of transformerless inverters, viz., full-bridge topology with unipolar, bipolar, and hybrid PWM, H5, H6, HERIC, and paralleled buck topology, some of which are illustrated briefly in Appendix B. Zhang et al. [96] explained noise source modelling for three-level three-phase inverters and computed its CM noise spectrum with two different space vector PWM (SVPWM) schemes. Khan et al. [97] listed 45 important single-phase transformerless inverter topologies and presented a comparison of CM current and CM voltage obtained by simulation. A similar equivalent circuit treatment for three-phase converters was given in [96,98]. Zhang et al. [96] investigated how PWM schemes affect a (neutral point clamped) NPC inverter and flying capacitor-type inverter topologies. A review of transformerless three-phase inverters was given by Rath et al. [99]. Islam et al. [100] presented a detailed review of medium-voltage (MV) inverters. Wijanarko et al. [101] reported the conducted emissions from a standalone off-grid PV system consisting of a 410 W PV panel.

In all the CM models of the PVI [90,95,96], the negative terminal ‘N’ of the DC bus is an important node considered in deriving the model. However, such models usually neglect the impedance of DC cables, which might be significant. If such an impedance were to be considered, the model would be significantly different from the one derived in the conventional power electronics domain.

3.2.2. CM Analysis of DC-DC Converter

Derivation of CM equivalent circuit of a DC-DC converter is similar to those of the inverter, i.e., one needs to identify the parasitic impedances and then derive a CM equivalent circuit based on the relevant frequency range. A commonly used converter in PVIs is the boost converter. It is usually implemented with half-bridge topology. Therefore, its CM equivalent circuit is as shown in Figure 15 [102]. Here, $C_{\mathrm{i}}$ and $C_{\mathrm{o}}$ are the input and output capacitances, $L_{\mathrm{f}1}$ and $L_{\mathrm{f}2}$ are input filter inductances, $Z_{\mathrm{LISN}}$ is the equivalent impedance offered by the DC line impedance stabilization network (LISN), $Z_{\mathrm{bp}}$ is the parasitic impedance of the baseplate, $Z_{\mathrm{og}}$ is the parasitic impedance of output to the ground, $v_{\mathrm{nom}}^{\mathrm{cm}}$ is the nominal CM voltage source, and $v_{\mathrm{p}}^{\mathrm{cm}}$ represents the CM voltage contribution from the parasitic branch containing $Z_{\mathrm{bp}}$.

A detailed review of conducted electromagnetic interefernce from DC-DC converters is given in [103]. The LISN plays an important role in pre-compliance and compliance tests. In the case of PV converters, the ‘power sources’ are connected at both ends, at the input and at the output. Therefore, two types of LISNs are needed: DC LISN and AC LISN. A new design of DC LISN was proposed in [104]. In case of multiple DC-DC converters within a power converter, Sugiura et al. [105] evaluated the effects of connecting a DC LISN between the DC-DC converter and the inverter by simulation studies. They showed that installation of a LISN changes the transfer function of CM current vs. CM voltage. While there are several commercial DC LISNs available in the market, a simple DC LISN can be made in the lab for testing purposes. A comparative analysis of such a lab-made DC LISN and a commercial one is presented in [106].

3.2.3. Miscellaneous Issues

MPPT is implemented in almost every PV inverter. Soumana et al. [107] presented a quantitative comparison of total harmonic distortion (THD) of current produced by different MPPT algorithms. They stated that incremental conductance (IC) and perturb and observe (P&O) methods produce a higher THD (3.48%, 3.68%, respectively) compared to fuzzy logic control and sliding mode control techniques (1.34%, 1.99% respectively). Ohba et al. [108] proposed an adaptive observer technique to improve the robustness of MPPT algorithm against the switching frequency disturbances produced by the switching of the DC-DC converter. If ripple is present in the DC voltage, it can affect the maximum power point. The ripple $P_{\mathrm{r}}$ in the maximum power $P_{\mathrm{mp}}$ is given by (7) [109], where $\Delta v$ is the voltage ripple, $V_{\mathrm{mp}}$ is the voltage at the maximum power point, $V_T$ is the thermal voltage, n is the diode parameter used in the solar cell model and $V_{\mathrm{cell}}$ is the cell voltage at MPP. Kubík et al. [110] simulated a DC-DC converter for a PV system in a frequency range of 10 kHz to 10 MHz and raised concerns regarding the measurement standards like CISPR 11.

(7)${\displaystyle \frac{P_{\mathrm{r}}}{P_{\mathrm{mp}}}}={\left({\displaystyle \frac{{(\Delta v)}_{\mathrm{rms}}}{V_{\mathrm{mp}}}}\right)}^2\left(1+{\displaystyle \frac{V_{\mathrm{cell}}}{2\left(n{V}_T\right)}}\right)$

Prajapati et al. [32] extracted equivalent noise source impedance for a 120 W PV microinverter (input: 24–40 V, 4 A DC; output: 220 V, 50 Hz; switching frequency: 50 kHz). The method used is the same as explained in Section 3.1.1. The schematic circuit used for experiments is shown in Figure 16a. It can be noted that in the case of PVI, the impedance of cable and that of LISN is extracted independently before assembling the measurement circuit. The equivalent CM noise source impedance for the specified inverter is given by a series connection of $R=36.8$ $\Omega$, $L=12\mathsf{\mu }\mathrm{H}$, $C=0.5\mathrm{nF}$, in parallel with a 0.25 nF capacitance, as shown in Figure 16b. The noise source $V_{\mathrm{noise}}^{\mathrm{CM}}$ and $V_{\mathrm{noise}}^{\mathrm{DM}}$ are both extracted. It is found that $V_{\mathrm{noise}}^{\mathrm{CM}}$$(\approx$$140\mathrm{dB}\mathsf{\mu }\mathrm{V})>>V_{\mathrm{noise}}^{\mathrm{DM}}$$(\approx$$60\mathrm{dB}\mathsf{\mu }\mathrm{V})$. The in-circuit or in-site impedance or noise source extraction method could be effective in many cases and hence could be explored by the engineers working in EMC of PV systems [55,111].

Standalone PVIs are usually installed at remote locations. Some of these installations may have trapezoidal wave (modified sine wave) inverters due to financial reasons. In such cases, the voltage and current waveforms at the output can contain a high level of harmonics [10]. Pallem et al. [112] presented a case study of a new type of ferroresonance in a solar PV plant.

Christoforidis et al. [113] reported that voltages may get induced on the metallic pipelines from a large solar power plant nearby. They reported the induced voltage for (a) balanced loading, (b) 1.5% unbalanced loading, and (c) 5% unbalanced loading. The induced voltage ranges from 5 V to 30 V and it varies with distance between the solar PV plant and the pipelines.

Singh et al. [8] reported conducted interference to telephone systems located near a 17 MVA PV inverter in the frequency range of 2.5 kHz to 4.5 kHz, which corresponded to the switching frequency of the inverters. The coupling happened through the ground in the three phase four wire system.

4. Standards

Within EMC of PV system components, there are important standards from several organizations. Different regions of the world use different subsets of these standards. Products in the European Union are regulated by the European EMC directive 2014 [114]. The important standard landscape for EMC aspects of PV systems is given in Figure 17. IEC 62920 is the main standard dedicated to power converter equipment (PCE) used in PVIs. Figure 17 also shows that the PV-specific EMC standards are ‘based’ on some generic EMC standards, and there are also IEEE guidelines which specify limits for flicker, voltage fluctuations, frequency fluctuations, loss of utility voltage, etc.

IEC 62920 divides the PCEs into two classes based on environments in which they are used: ‘A’ for residential and ‘B’ for non-residential. The main ports in a PCE are categorized as follows (see Figure 18): (a) AC power port, (b) DC power port, (c) AC auxiliary power port, (d) DC auxiliary power port. Disturbance limits for emissions are given at various ports. This standard specifies the test setups only for wall-mounted PCEs. For other types of equipment, reference standards should be used. For various types of disturbances, a reference is made to various basic standards as given in Table 5.

Apart from IEC 62920, the following standards are important from EMC perspective.

1. IEC 62109: Safety of Power Converters in PV Systems [89].

2. IEC 62446: PV systems: Requirements for testing, documentation and maintenance [117].

3. IEEE 519-2014 [131]: Requirements for harmonic control in power systems.

4. IEEE 1547-2018 [133]: Interconnection and Interoperability of distributed energy resources.

5. IEEE 1453-2022 [132]: Measurements and Limits of Voltage Fluctuations and Associated Light Flicker on AC power Systems.

As far as power quality issues are concerned, there are some articles which provide important information, besides standards. Assessment methods of voltage unbalance, voltage regulation, and harmonics are reviewed in [134]. Czapp et al. [135] showed that residual current devices, widely used for earth-fault protection, can be prevented from normal function if high-frequency currents of the order of 50 kHz are present in the CM or earth current. The spectrum of switching-frequency harmonics depends on the PWM method and control approach [136]. Rönnberg et al. [137] presented a state-of-the-art review of the emissions in this range listing the sources of emissions, propagation mechanisms, effects of interference, measurement and analysis methods, and requirement of changes in the standards. Kotsampopoulos et al. [138] showed that harmonics due to PV inverters may cause error as high as 45% in some energy meters.

The following subsections offer important definitions from standards and their limitations.

4.1. Important Definitions

4.1.1. Harmonic Current Emissions

Limits for harmonic current emissions are given in IEC 61000-3-2 [139] (for equipment with rated current ≤16 A) and IEC 61000-3-12 [140] (for rated current ≥16 A and ≤75 A). It should be noted that IEC 61000-3-12 provides separate limits for single-phase, three-phase, and hybrid equipment.

Besides the classical ‘harmonics’, that is, the integer multiples of the fundamental frequency (50/60 Hz), the interharmonics (non-integer multiples of 50/60 Hz), the subharmonics (frequencies less than 50/60 Hz) and the supraharmonic (harmonics in the range of 2 kHz to 150 kHz), are also important [141]. A review of interharmonics in the PV systems including challenges involved in the measurement and analysis of such waveforms is presented in [142]. Limits on supraharmonics are not covered by the generic EMC standards: CISPR 11 [27] (which cover conducted emission limits above 150 kHz) or IEC 61000-3-x (which specify harmonic emission limits for up to 50th harmonic). Some potential problems of supraharmonics are (a) additional heating, (b) audible noise, (c) malfunction of other technical systems such as a programmable logic controller [143].

The following are important terms referred to in various standards as far as harmonic emissions are concerned.

1. Total harmonic distortion (THD): If $I_h$ is the hth harmonic current and $I_1$ is the fundamental current,

   (8)$\mathrm{THD}=\sqrt{\sum _{h=2}^{h_m}{\left({\displaystyle \frac{I_h}{I_1}}\right)}^2}$

   IEC 61000-3-12 [140] uses $h_m=40$, IEEE 519 [131] uses $h_m=50$, IEC 61727 [115] uses $h_m=\infty$.

2. Total demand distortion (TDD): TDD is defined as [131]

   (9)$\mathrm{TDD}=\sqrt{\sum _{h=2}^{h_m}{\left({\displaystyle \frac{I_h}{I_{\mathrm{demand}}}}\right)}^2}$

   The demand current is sum of the currents corresponding to the maximum demand during each of the 12 months’ period divided by 12. For TDD, $h_m=50$. In the definition of THD (see (8)), the fundamental component $I_1$ is used in the denominator, whereas in TDD, the demand current $I_{\mathrm{demand}}$ is used. Therefore, TDD relates distortion to the demand levels in better way than THD.

3. Total rated current distortion (TRD): IEEE 1547 [133] introduces a new term total rated current distortion (TRD), which is defined as

   (10)$\mathrm{TRD}={\displaystyle \frac{\sqrt{I_{\mathrm{rms}}^2-I_1^2}}{I_{\mathrm{rated}}}}$

TRD has two clear advantages over traditional THD: (a) As the switching frequency is of the order of 10 kHz or more, TRD can capture switching frequency harmonics, whereas THD with a limit such as $h_m=40$ or 50 cannot. (b) TRD includes interharmonics as well, which might be present in the waveforms. The TRD limit is set to be 5% in [133].

4.1.2. Voltage Unbalance Factor

Voltage unbalance can occur in the case of three phase PV systems. It is quantified by using the voltage unbalance factor as defined in IEC 61000-3-14 [129] as a ratio of the negative sequence voltage ($U_2$) and the positive sequence voltage ($U_1$). Table 6 lists various standards and definition of voltage unbalance in them.

4.1.3. Flicker

Flicker is of two types: short-term ($P_{\mathrm{st}}$) and long-term flicker ($P_{\mathrm{lt}}$). Limits on the flicker are given in IEC 61000-3-3 [146], IEC 61000-3-11 [147], and IEEE 1547 [133]. The limits on $P_{\mathrm{st}}$ and $P_{\mathrm{lt}}$ are 0.35 and 0.25, respectively, as per IEEE 1547 [133]. IEEE 1453-2022 [132] offers rules regarding measurements and limits of voltage fluctuations and associated light flicker on AC power systems.

4.1.4. Classification of PVIs Based on Size

IEEE 929-2000 [116] classifies the PVIs into three classes: (a) small systems: rated power ≤ 10 kW, (b) intermediate systems: 10 kW < rated power ≤ 500 kW, and (c) large systems: rated power > 500 kW.

4.1.5. Decisive Voltage Class

IEC 62109 [89] presents a decisive voltage class (DVC), which is indirectly related to leakage current. There are three DVCs: A, B, and C. Circuits in DVC-A are considered ‘safe to touch’. DVC-A equipment are those for which (a) AC RMS voltage ≤ 25 V, (b) AC peak voltage ≤ 35.4 V, (c) DC mean voltage ≤ 60 V. To access equipment with DVC-B, DVC-C, special requirements are given in Section 7.3.5 of [89]. The touch safety can be said to be linked with leakage current.

4.2. Limitations of Standards

Despite several generic standards and enforcement of compliance to those, the emissions from PV installations are indeed found as stated in Section 2. The following is a list of possible points where there is a scope of modifications to reduce overall emissions despite standards compatibility.

One major limitation related to PVIs is that the standards are mostly present for the individual components and not for the PVI as a whole. Some components in the PVIs may not be covered by EMC standards. In that case, generic standards are applicable if the product-specific standards are not available. As noted in [70], manufacturers use different standards to satisfy the European EMC directive. Therefore, individual components may comply with the standards, and yet installations as a whole may still cause interference. There is arguably a need of an international standard which would cater for installations rather than only components.

As far as the European Union’s EMC directive (2014/30/EU) is concerned, the manufacturers may show that their products comply with the particular EMC standards in order to satisfy the EMC directive. However, ‘proving the standard compliance’ is not a mandatory requirement to satisfy the EMC directive. Further, even if the product complies with the relevant standards and it is found that there is an interference caused by the product at the user’s site, it is the responsibility of the user to stop using the product [148].

Another issue is the ambiguity in standards and regulations. The EMC directive incorporates or demands compliance with several EMC standards. However, it is claimed in a recent document [149] that it is not clear whether the solar PV system should be classified as fixed installation or an apparatus. There are several improvements possible with the IEC 62920 standard, some of which are listed below.

1. In IEC 62920, it is stated that the PCE can be tested with or without solar PV modules or storage devices. The AC or DC power supply may be bidirectional. When the PCE is in actual operation in a PVI, it is connected to panels and wiring that are not known at the time of testing the PCE by this standard. Therefore, a PCE that fulfills the requirements of the relevant product standard can lead to unacceptable EMI at an actual installation depending on other parts of the installation as stated in Section 2. When assessing the immunity or emissions of a PCE, its conformity to stated standards must be realized as not being a guarantee of lower interference.

2. IEC 62920 suggests to use IEC 61000-3-2 as a reference for harmonic emissions. This standard uses THD as an index to quantify harmonic emissions. Standards have different choices of the maximum harmonic number related to THD, as stated in (8). THD includes harmonics only up to 2 kHz. However, a typical PV plant could generate higher-order harmonics (harmonics at switching frequency which typically starts around 16 kHz) which can cause disturbance to other equipment. Thus, by definition, THD excludes some frequency components which could be significant in the case of PVIs.

   On the other hand, in the case of PV, THD might increase with low solar irradiation [150], which is expected because of the decrease in the fundamental component $I_1$ (see (8)). Therefore, if a product fails to satisfy THD criteria, it would not be appropriate to conclude that it generates unreasonably high level of harmonics.

   TRD (see (10)) uses rated current as the reference, and includes ‘all’ the harmonics and the interharmonics, which includes harmonics of the switching frequency ranging from tens of kHz to a few hundred kHz. Therefore, TRD could be lower in the cases where THD is higher due to the use of the fundamental component as a reference and only a finite number of harmonics. Therefore, it is suggested that TRD should be used as a criteria for quantifying harmonic emissions for PVIs.

3. The module-integrated power electronics have become popular and so are the module integrated inverters with in-built communication equipment [151]. Therefore, emissions in the higher frequency range beyond GHz should also be checked. Stracqualursi et al. [12] also pointed that the emissions are higher for frequencies $>30$ MHz.

5. EMC Mitigation Techniques

Major EMC mitigation techniques can be categorized as follows.

5.1. Filters

The primary component to mitigate unwanted frequency components are filters. The most commonly used filter is the LCL filter as shown in Figure 19. The transfer function of grid current vs. the inverter output voltage is [152]

(11)$G\left(s\right)={\displaystyle \frac{I_g\left(s\right)}{V_i\left(s\right)}}={\displaystyle \frac{\frac{1}{1(1-r)L^2{C}_{\mathrm{f}}}}{(s+1/\tau )(s^2+2\zeta {\omega }_{\mathrm{res}}s+{\omega }_{\mathrm{res}}^2)}}$

$\mathrm{where},L=L_1+L_2,r=L_1/L,{\omega }_{\mathrm{res}}=\sqrt{{\displaystyle \frac{1}{r(1-r)L{C}_{\mathrm{f}}}}},\zeta ={\displaystyle \frac{1}{2{\omega }_{\mathrm{res}}\tau }}$

The filters used in PV inverters are different from those used in audio or other applications for the following reasons [17]:

1. There is an additional element, i.e., the grid impedance $Z_{\mathrm{g}}$ in the circuit. This impedance differs between systems and times and hence can change the resonant frequency of the filter by as much as 40%.

2. A function of the control loop is to control the voltage at the inverter’s output to be in phase with either $i_g$ or i.

3. The inverter and the grid are two voltage sources and the filter is connected between them. The power flow between $V_g$ and E is dependent on the value of the filter. For example, we consider a case of a simple L filter as shown in Figure 20. The power flow between two sources $V_g$ and E is given by (12)

   (12)$P={\displaystyle \frac{E{V}_g}{X_L}}sin\delta$

   where $V_g$, and E are the voltages at the two ends of the filter and $\delta$ is the phase difference between the two. Similarly, any other filter (LC, LCL, etc.) should also facilitate the required power flow, that is, they should have suitable power frequency series inductance.

Some of the classic references for filter design are [153,154,155]. Recently, there has been an increase in SiC semiconductor devices, which has resulted in change in filtering requirements [156,157].

It is important to note that the design goals of filters might be different. For example, in larger power plants of more than 100 kW of power, it might be important to limit specific harmonic orders. In smaller power plants, the cost and size might be more important constraints. Filters on the DC side as well as on the AC side are important in the PVIs. Design of filters on the DC side using an impedance interaction approach is found in [158].

By default, the power converters employ filters on both the AC and the DC side. There might be several reasons for failure of filters in commercial products, for example, non-linear behavior of filters in case of saturation of the inductor cores, which could lead to filter malfunction. It can be modeled with Volterra series-based approximation [159], and the increased percentage of harmonics can be predicted. This model states that when the inductor is saturated, the first, the third, and the fifth harmonics are generated. Effects of stray magnetic couplings within the filter were analyzed in [160], where the authors presented a technique to model these parasitic impedances for an EMC filter for SiC solar inverter. Zhou et al. [91] described common mode choke design to suppress leakage current for the case of quasi Z-source (q-ZSI)-based cascaded multilevel inverters with 100 kHz switching frequency.

If interference is found in spite of built-in filters, extra filters may be installed [24,161]. In particular, installation of common mode chokes before and after optimizer led to lesser reduction in radiated disturbance by about 8 dB at 18 MHz, 21 MHz, 24 MHz, and 28 MHz [24]. There are several commercially available filter solutions [162,163,164] which can be installed in addition to the in-built filters in the power converters. In [10], insertion of filter components (22 nF Y capacitors and 1.8 mH chokes) led to significant decrease in disturbance current and radiated field in the frequency range of 100 kHz to 10 MHz. A recent review of chokes to be used in power electronic systems is found in [165].

5.2. Mitigation Techniques Based on Cases of Interference

As seen in Figure 2, DC cables form a coupling path between the power electronic converter and the panels. In [10], the unsymmetrical wiring was reported to lead to the conversion of CM disturbance to DM disturbance. Shielding of DC cables can reduce the EMI up to as high as 20 dB [39]. It was found that DM current with unsymmetrical wiring was about 10 dB higher than that with a balanced wiring. One of the basic methods is to keep the design of the system compact, and the DC cabling can be performed in a way to avoid loops which can act as unintentional antennas, as shown in Figure 21 [6,7,25].

In recent cases of EMC issues from PVIs, the affected PVIs more often than not include optimizers [3,4,18,20,21,22,23,71,166]. A study in [167] found that ‘PV systems have typically a higher yield if no module optimizers are applied’. This study should be considered when designing a specific system, since optimizers might add cost, complexity, and potential EMC problems.

More often than not, shielding is considered as a technique to increase the immunity of any system. However, in the case of PVIs, it can also be used to prevent nearby sensitive/critical apparatus. It was employed in many cases to prevent emissions from the DC cables [3,10]; sometimes, other equipment like decentralized inverters, skid rooms, techincal rooms, etc., were also employed [12].

To avoid interference by PV systems at airports, the following measures are suggested [78]. The PV installations should be located at least 200–250 ft away from the communication systems. PVI should be avoided where they might cause interference to navigational aids. Radar absorbing material could be used to reduce unwanted signal reflections.

Li et al. [93] reported that connecting a neutral wire between the source and the grid can also be an effective measure against the CM current and associated EMI. Stracqualursi et al. [12] stated the importance of effective grounding that provides ow impedance path to the equipment. They also stated that passive shielding could be used for sensitive installations near the field.

5.3. Novel Converter Topologies and PWM Schemes

From Section 3.1.1 and Section 3.2, the primary reasons for increased leakage current are as follows. From (3) and (6), the main causes of increased CM current are as follows:

1. Formation of CM path due to existence of $Z_{\mathrm{g}}$ and $Z_{\mathrm{CM}}$.

2. High $dV/dt$ in $V_{\mathrm{AN}}$ and $V_{\mathrm{BN}}$ (see Figure 14a) which lead to high $dV/dt$ in CM voltage $V_{\mathrm{eq}}$.

These reasons can be addressed at the design stage, which has led researchers to come up with novel topologies. These can be treated as mitigating EMI at the source level. To alleviate the connection by $Z_{\mathrm{PV}}$, one of the terminals of the AC output can be connected with either the positive or the negative of the PV panels. Then, the CM current should be minimal. Several such topologies are presented in the literature. For example, Siwakoti et al. [168] proposed flying capacitor-type inverter topologies (see Figure 22) in which the PV inverter negative is common with the ground in the grid. Wang et al. [169] presented a novel iH6 topology to reduce CM currents in single-phase cascaded inverters. Dong et al. [170] presented a comparison of floating filter and normal filter in a grid-connected PV inverter. In the floating filter, the capacitor at the output is split into two and the midpoint is connected to the midpoint of the DC link, which reduces the CM noise current on the AC side by as high as 10 dB. The CM noise on the DC side is also reduced for frequencies below 1 MHz. When three single-phase inverters are used to form a three-phase system, it may lead to an increased leakage current. A star-connected H5 topology is proposed in [92,171] to reduce the leakage current in a three-phase system formed using single-phase inverters. It is shown by experimental results that the leakage current is maintained below 300 mA for this system.

To reduce $dV/dt$, novel PWM schemes are proposed to reduce $dV/dt$ in the voltages $V_{\mathrm{AN}}$ and $V_{\mathrm{BN}}$. For the transformerless three-phase inverters, Hou et al. [172,173] showed that active zero-state PWM and near-state PWM techniques offer a CM current which is about four and seven times smaller than the CM current obtained with the conventional space vector PWM scheme. Other novel PWM schemes are suggested by several researchers in order to mitigate the CM current issue, for example, [174]. Liu et al. [175] proposed to use input split inductor-type qZSI to reduce CM voltage to almost half of the conventional qZSI with remote-state and near-state PWM techniques. For the DC-DC converters, a detailed review of EMI issues and major EMI suppression techniques, viz., EMI filters, soft-switching, and random modulation was given by Natarajan et al. [103]. Particularly interesting is the new PWM based on random or chaotic signals, where the peaks appearing in the EMI can be suppressed by using a given random modulation signals. While this technique is effective, generation of random signals in real time is an issue which needs to be solved. Mainali et al. [176] categorized EMI mitigation techniques for switch-mode power supplies: (a) circuit design and layout, (b) switch control schemes, and (c) soft switching techniques. This could be a good resource to make an EMI-proof converter at the design stage itself. However, an actual installation of such a converter with PV panels may still lead to some disturbance issues. Subramaniam et al. [177] proposed that modified PWM techniques could help to reduce the size of filters. They showed that the modified PWM schemes could reduce the CM current by more than half of the CM current obtained with conventional PWM schemes.

While novel topologies and PWM schemes are useful, most of them are tested on a lab-scale environment. There might be issues in realizing these in commercial products. Further, most of the schemes assume that $Z_{\mathrm{PV}}$ is purely capacitive, which might not be the case as discussed in Section 3.1.

6. Suggestions for Future Research

Previous sections showed ways of modeling and analysis of electromagnetic disturbance caused by PVIs. These are particularly important if PVIs are to be installed near critical infrastructure such as airports, military bases, or healthcare facilities. Table 7 offers a summary of the key findings and future scope of research for each component of the PV system. Some of these aspects are elaborated in the next subsections.

6.1. Modeling of Newer Type of Solar Panels

Today, monocrystalline and polycrystalline silicon solar cell technology dominate the market with more than 90% PV production [16]. However, new solar cell technologies are also increasing slowly [16]: (a) perovskite solar cells, (b) thin film solar cells, (c) organic solar cells. Newer trends are also coming in the module structure and geometry as well. For example, glass–glass modules (panels) have recently entered the market, having a glass sheet at the back instead of a tedlar backsheet, as shown in Figure 23. Kroner et al. [166] showed that frameless bifacial modules exhibit higher emission levels than monofacial modules. Some other technologies are flexible solar modules and solar panels as roof tiles. Further, there are newer types of solar PV installations, e.g., vertically mounted bifacial solar PV panels [178,179], floating solar PV [180,181], and building integrated solar PV [182]. As these types are distinct from the conventional panels in terms of geometry and materials, $Z_{\mathrm{PV}}$ for these newer panels is different from those for the conventional panels. Therefore, $Z_{\mathrm{PV}}$ can be investigated for these panels.

6.2. Investigation of Inductive Nature of Parasitic Impedance

Most of novel topologies and PWM schemes (see Section 5.3) for converters are proposed assuming that $Z_{\mathrm{PV}}$ is purely capacitive. However, as shown by several researchers [10,40,41,44], there are inductive components in $Z_{\mathrm{PV}}$ especially above 1 MHz. It is necessary to investigate the inductive behavior further to properly address the issue of CM current.

6.3. Analysis of Different PVI Architectures

Any PVI which uses even a single microinverter or battery charger connected to a solar panel has the potential to use high switching frequency and poor filtering, thus posing a risk of electromagnetic interference, particularly if there are significant connection lengths between panel and converter. However, the reported EMI incidents are mainly for transformerless string inverter systems and PVIs with optimizers as elucidated in Section 2. Therefore, there is a scope of analysis of other systems from EMC perspective. Some examples are given below.

6.3.1. Standalone PV Systems

One of the increasingly used architectures of PVIs is the one with a battery storage as shown in Figure 24 along with associated CM currents $i_{\mathrm{CM}1}$, $i_{\mathrm{CM}2}$, and $i_{\mathrm{CM}3}$. Almost all the literature to date covers only the CM current $i_{\mathrm{CM}1}$. Modeling and analysis of $i_{\mathrm{CM}2}$ and $i_{\mathrm{CM}3}$ could be investigated in order to estimate the effect of CM noise on the battery and also on the PV modules. For example, some effect of circulating current analysis of a combined operation of PV along with battery systems is conducted in [183].

6.3.2. PV System with Solid-State Transformers (SSTs)

In this type of PVI, the power frequency (50 or 60 Hz) transformer is replaced with a medium-frequency (tens to hundreds of kHz) transformer because the latter is much smaller than the former [184,185]. For historical reasons, these are also known as solid-state transformers (SST) or SST-based systems. Figure 25 shows a PV array consisting of several DC-DC converters. A CM equivalent circuit of the DC-DC converter as proposed by Essakiappan et al. [94] is shown in Figure 25. Huber et al. [184] presented an analysis of CM currents in multi-cell solid state transformers. Jiang et al. [186] proposed mitigation schemes for CM currents in solid-state transformers. There is no unique topology or configuration for solar PV installations with SST. Therefore, analysis should be conducted on a case-to-case basis to study the CM current in this case.

7. Conclusions

EMC aspects of solar PV have gained attention due to increased cases of emissions and interference that have arisen in the last few years. The affected frequency range is from around 10 kHz to several MHz. The radiation pattern is found to disturb amateur radio transmission and reception, air traffic control communication, and certain other radio communications. Although the cases of interference are not significant in number compared to the total number of PV installations, it is important to have a framework to investigate EMC issues if they are found in a particular PV installation.

This paper summarized recent incidents of interference from PVI which highlights the locations, types of victims, and typical frequency range of conducted and radiated emissions. To assess the problem in a systematic manner, it is important to understand the modeling and analysis of PV system components, viz., panels, DC cables, and inverters. Radiated emissions can be quantified by modeling PV panels as antennas and using the antenna factor. Conducted emissions from the PV panels can be quantified with modeling parasitic impedance of PV panels. As far as converters are concerned, most important are the CM circuit models of the converters.

In this work, important standards in the domain were also covered and their limitations were stated briefly. Emissions from the commercial products can be mitigated by incorporating additional filters or using a more symmetrical layout of the systems. At the design stage, one of the ways to mitigate EMC issues is novel topologies and PWM schemes. A few possible areas of future research were also given in this work.

Author Contributions

Conceptualization, M.M.K., N.T. and D.M.; methodology, N.T., M.M.K. and D.M.; formal analysis, M.M.K.; investigation, M.M.K.; resources, M.M.K., N.T. and D.M.; writing—original draft preparation, M.M.K.; writing—review and editing, M.M.K., N.T. and D.M.; visualization, M.M.K.; project administration, N.T. and D.M.; funding acquisition, N.T. and D.M. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:

EMC	Electromagnetic Compatibility
EMI	Electromagnetic Interference
PVI	Photovoltaic Installation
THD	Total Harmonic Distortion
CM	Common Mode
DM	Differential Mode
PCE	Power Conversion Equipment
LISN	Line Impedance Stabilization Network
TDD	Total Demand Distortion
TRD	Total Rated current Distortion
MV	Medium Voltage
MPPT	Maximum Power Point Tracking
AM	Amplitude Modulation
VNA	Vector Network Analyzer
SST	Solid State Transformer
CISPR	Comité International Spécial des Perturbations Radioélectriques (International Special Committee on Radio Interference)

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Table 1

Frequency of interest for EMC aspects of PV installations.

	Perspective
Component	Power Generation	EMC
PV panels	DC	a few kHz to tens of MHz
DC cables
Power Electronic Converters	DC to 50/60 Hz	tens of kHz to a few tens MHz

Table 3

Emission measurements from a solar panels in a laboratory test setup.

Ref.	Frequency Range		Solar Cell Type	Test Setup	Antenna Type	Emission Level		d
	Min	Max				Min	Max
[61]	150 kHz	30 MHz	Amorphous, c-Si	Anechoic chamber 3 m × 3 m × 7 m	Biconical	30	85	3 m
[60]	30 MHz	300 MHz	Amorphous, poly crystalline	Anechoic chamber 3 m × 3 m × 7 m	Biconical	60	102	3 m
[62]	30 MHz	300 MHz	Amorphous, thin film	Semi-anechoic chamber	Dipole	60	110	3 m
[63]	30 MHz	300 MHz	Thin film	Semi-anechoic chamber	Biconical	60	105	3 m
[64]	150 kHz	30 MHz	Not mentioned	Anechoic chamber 28.5 m × 17 m × 11.7 m	Loop	0 *	50 *	3 m

* This is in dB$\mathsf{\mu }$A/m, all the other emission values are in dB$\mathsf{\mu }$V/m.

Table 4

Limits on residual current in IEC 62109.

Inverter Rating	Max Residual Current
≤30 kVA	300 mA
>30 kVA	10 mA

Table 5

Standards referenced in IEC 62920.

Standard	Disturbance Type
IEC 61000-3-x [129]	Harmonics, distorting installations, flicker, connections to MV or HV power systems
IEC 61000-4-x [128]	Testing and measurement techniques for ESD, conducted disturbances, radio frequency disturbances, etc.
CISPR 11	Industrial scientific and medical equipment radio frequency disturbance
CISPR 16	RF disturbance measuring apparatus
CISPR 32	EMC of multimedia equipment

Table 6

Voltage Unbalance.

Standard	Index to Quantify Voltage Unbalance
IEC 61000-4-27 [144]	Negative unbalance factor $k_{\mathrm{u}2}={\displaystyle \frac{U_2}{U_1}}$, the limit is set as $6\%$, $13\%$, and $25\%$ depending on the level of unbalance to be applied.
IEC TR 61000-3-14 [145]	Negative sequence component 2% of the positive sequence component.

Table 7

Summarized scope of future research directions.

Component	Key Findings	Future Scope
PV panels (Section 3.1.1 and Section 3.1.2)	Parasitic impedance of panels is found out using analytical and FEM techniques [43,44]Antenna factor of various models is established with experimentation [10,40,64,79]	To model of impedance of various types of panels e.g., bifacial panel, glass glass panels, etc.In-circuit impedance extraction methods [54] could be explored to extract impedance of PVIsTo model antenna factor of the panels with tracker systems
DC cables (Section 3.1.3)	Cables are modeled as inductances or transmission lines [32,85,86]Cables are modeled as radiating structures	To model DC cables for modern real world PVIsTo model DC cables as radiating structure and establish a good layout procedure based on modeling vis-à-vis experimentation
DC-AC invertersSection 3.2 and Section 5.3	CM models of inverters are investigated for single-phase and three-phase topologies [95,97,99]Novel topologies to eliminate/reduce CM current are proposed and verified in lab environment [168,169,173]	To improve the CM model of the inverter with practical aspects like cables, inductive nature of parasitic impedance, etc.To improve and work on CM model of large-scale three-phase inverters, e.g., modular multilevel inverters (described in Appendix B)To investigate practical aspects from EMI viewpoint, in implementing novel topologies/PWM schemes
DC-DC converters (Section 3.2)	Many cases of EMI are from PV panels with optimizers, which are essentially DC-DC converters [3,18,24]CM models of DC-DC converters are investigated [102,103]	To investigate CM circuit of an optimizer and verify if for a practical setupTo improve CM circuit model for DC-DC converters in large systems
System level	Most of the EMC issues are discussed for domestic PVI and some for large-scale PVIs	To investigate EMC issues in medium scale and other types of PVIs, e.g., systems with solid state transformers, floating PVI, etc.

Appendix A. Sinusoidal PWM

The output voltage of any inverter is a pulse width modulated AC waveform. Most basic pulse width modulation scheme is sinusoidal PWM. It is discussed here to exhibit the nature of output voltage waveform of the inverter and typical frequency spectrum of such waveforms. A sinusoidal modulation signal of amplitude $V_{\mathrm{m}}$ and frequency $f_0$ is compared with a triangular carrier signal of amplitude $V_{\mathrm{c}}$ at switching frequency $f_{\mathrm{s}}$. Accordingly, switches S₁ through S₄ are switched ON or OFF [187]. The ratio of amplitudes ${\displaystyle \frac{V_{\mathrm{m}}}{V_{\mathrm{c}}}}$ is called the amplitude modulation ratio $m_{\mathrm{a}}$ and the ratio of frequencies ${\displaystyle \frac{f_{\mathrm{s}}}{f_0}}$ is called the frequency modulation ratio $m_{\mathrm{f}}$. The frequency spectrum of the output voltage (shown in Figure A1b) is such that the significant harmonics appear at multiples of $m_{\mathrm{f}}$, $2m_{\mathrm{f}}$, $3m_{\mathrm{f}}$. In Figure A1b, $m_{\mathrm{f}}=21$; therefore, it is seen that the first dominant harmonic appears at 1050 Hz and its sidebands. More details about the PWM schemes can be seen in [88]. If the modern wide band gap SiC devices are used, $f_s$ is of the order of several tens of kHz to a few hundreds of kHz [188]. Thus, it can be estimated that the significant harmonics can be in the range of several hundreds of kHz up to some MHz, a frequency range which is important from the aspect of radiated emissions.

Appendix B. Inverter Topologies

The problem of leakage current is predominantly visible in transformerless inverter systems. Khan et al. [97] evaluated 45 inverter topologies and compared the leakage currents. Some main topologies among them are listed below [17].

1. H5 inverter (see Figure A2a): This topology has one extra switch $S_5$ to (a) avoid reactive power exchange between $L_1$, $L_2$ and $C_{\mathrm{DC}}$ and (b) disconnect the DC link during the zero state.

2. Highly Efficient and Reliable Inverter Concept (HERIC) inverter (see Figure A2b): This inverter has an AC bypass switch pair (S₅-D₅, S₆-D₆) which achieves the same functions as above by S₅ in the H5 inverter.

3. Neutral point clamped (NPC) inverter (see Figure A2c): The zero state is achieved by ‘clamping’ the output to the midpoint the DC link via diodes $D_5$ and $D_6$.

4. Flying capacitor-type inverter (see Figure 22): This is a special class of inverters where the negative of the PV array and the ground of the grid are connected electrically together. This is discussed in Section 5.

The magnitude and frequency of CM voltage is affected by both the inverter topology and the switching frequency. The greater the the frequency of these voltages, the greater the leakage current (assuming that $Z_{\mathrm{PV}}$ is purely capacitive). The frequency of CM voltage across $Z_{\mathrm{PV}}$ for various topologies is given in [17].

A full-bridge inverter uses two legs of power electronic switches, whereas three-phase inverters use three legs. For large-scale PV power plants, modular multilevel inverters (MMIs) are used (see Figure A3). MMIs use many smaller submodules (SMs) per leg. MMIs are used because they offer better power quality and lesser CM currents, especially for larger central inverters in the power range of 1 to 30 MW [189].

Three-phase inverters usually employ space vector PWM (SVPWM). The output voltages from the three legs $V_{\mathrm{AN}}$, $V_{\mathrm{BN}}$, and $V_{\mathrm{CN}}$ form a tuple. Each element of the tuple can be equal to either one if the top switch is ON or zero if the bottom switch is ON. Consequently, the output voltage can be represented as a ‘vector’ as shown in Figure A4. More details can be found in [88,174]. Hou et al. [173] compared the PWM schemes proposed for reduction in the CM current and stated that active zero-state PWM and near-state PWM schemes are better than conventional SVPWM schemes.

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