1. Introduction
In recent years, Light Emitting Diode (LED) lamp technology has significantly improved. The continuing innovation of LED manufacturers has led to an increase in their efficiency (more lumens per watt) and a drop in market prices. As a result, LED global sales have grown since 2010, achieving 46% of the global residential market in 2019. Compact Fluorescent Lamps (CFL) have the same market share but a decreasing tendency, while the remaining 8% corresponds to incandescent and halogen lamps (IHL) [
1]. Moreover, the International Energy Agency (IEA) predicts the LED’s share will almost double in 2030, reaching 87% of the global residential market [
1]. This data confirms a shift in the lamp technology installed in Low Voltage (LV) residential networks from IHL, via CFL, towards LED, resulting in a significant increase of nonlinear loads in the lighting sector.
CFL and LED lamps require rectifier frontends and are consequently a significant source of odd harmonic currents [
2]. In typical three-phase four-wire (3P4W) networks, these harmonics will also flow in the neutral conductor, particularly those harmonic orders that form a zero-sequence system under balanced conditions (3rd, 9th, 15th, etc.). On the other hand, as IHL are linear loads, they will contribute almost exclusively to the fundamental current only depending on their unbalanced distribution to the phase conductors.
According to Reference [
3], the research regarding harmonics caused due to the lamps can be divided into two major topics: (1) the accurate modeling in the frequency domain of CFL and LED lamps [
4,
5,
6,
7], and (2) studies on the influence of nonlinear lighting loads on harmonics distortion [
2,
4,
5,
8,
9,
10,
11,
12,
13,
14,
15,
16,
17]. Most works covering the former rely on the use of models based on Frequency Coupling Matrices (FCM) [
5,
6,
7], where the accuracy of a determined model is usually assessed by comparison with laboratory measurements. Although well-known models for CFL and LED lamps have been developed, the effects of their combined operation on voltage and current distortion levels in LV residential networks has not been comprehensively studied, especially the effect on the neutral conductor current.
Aggregation of CFL and its relationship with excessive neutral conductor currents in 3P4W networks, which predominant component is the third harmonic, have been studied through measurements at two installations [
8], laboratory tests [
15], and determining analytical expressions to estimate the value of the neutral current [
9]. The replacement of certain luminaires by LED lamps and their impact on voltage and current distortion levels in LV grids [
13,
14], as well as on the reduction of the neutral conductor current [
14,
16,
17], has been analyzed performing laboratory or on-site measurements. Further research has focused on the harmonic aggregation of commercially available LED lamps by experimental measurements [
10,
11,
16]. For instance, Reference [
11] demonstrates the cancellation and summation of harmonics by the combined use of different LED lamps. Other studies exist on the model aggregation of LED lamps regarding parameter estimation for an equivalent circuit of these lamps [
4] and analysis of their wide-spread use in a determined grid through harmonic iterative methods [
5,
12].
However, there is a lack of system-level studies regarding the large-scale aggregation of different brands of lamps, distributed to different households and phase conductors, and their impact on harmonic levels in public LV distribution networks and the neutral conductor current. Especially the impact of the transition from IHL, via CFL, to LED lamps has not been studied so far.
This paper aimed to fill this gap by analyzing the impact of different lighting technologies on the neutral conductor current of a LV residential grid simulating the transition from IHL, via CFL, to LED lamps. Five scenarios representing different combinations of lamp technologies were defined. The assessment was performed on a synthetic distribution LV residential network, which was developed in Reference [
18] for the San Juan province in Argentina, but is also representative of other countries with similar housing areas. A Monte Carlo (MC) simulation was used to address the randomness in the distribution of lamps. One thousand iterations were performed for each scenario, which ensures confidence of 95% and an error of 2% for the results [
19]. As it was intended to provide a first estimate of the impact, all lamps were represented by harmonic current sources [
20] and, consequently, the series impedance of the lines and the supply transformer were omitted. The current harmonic spectra were obtained from a large sample of measurements of commercially available lamps provided by PANDA (equiPment hArmoNic Database, Dresden, Germany) [
21]. The impact of a flat-top supply voltage distortion [
22,
23], which is common for public LV networks, was compared with the behavior under perfect sinusoidal condition, which represents the reference for testing compliance with emission limits. The effect of load unbalance on the harmonic content of the neutral conductor current [
8,
15] was also analyzed. The results indicate that the lamp technology, as well as the supply voltage distortion, can have a significant impact on the neutral conductor current. The highest values were obtained for the scenario with CFL only.
The remainder of this paper is divided into four sections. The first one includes a description of the synthetic distribution network used to perform the assessment, the set of lamps, the simulation scenarios, and the implementation of the simulation. Obtained results are presented and discussed in the third and fourth sections, respectively. Conclusions and recommendations for future work are detailed in the final section.
4. Discussion of Results
The results of this study show that neutral conductor currents caused by lighting loads in LV residential networks are mainly influenced by three factors: lighting technology, supply voltage distortion, and load unbalance. Due to the lower harmonic emission, LED lamps have a minor impact on the neutral conductor current than CFL. Although LED lamps do not yet represent the majority of the global lighting residential market, their increasing use [
1] is expected to decrease the contribution of lighting to the neutral conductor current.
A flat-top supply voltage, which can be typically found in residential LV networks, results in lower neutral conductor currents compared to a sinusoidal supply voltage as long as a considerable amount of CFL is involved. In the case of 100% LED lamps, a flat-top supply voltage is expected to result in higher magnitudes of neutral currents compared to the sinusoidal supply voltage, especially for the 15th harmonic. This occurs because the supply voltage distortion modifies the reference current harmonic spectra of these lamps, as discussed in Reference [
22,
23].
The unbalanced distribution of lamps to the phases results in additional non-zero-sequence harmonics (5th, 7th, 11th, 13th), as well as in additional fundamental currents in the neutral conductor, which tend to increase the neutral conductor current compared to the balanced case. Zero-sequence harmonics (3rd, 9th, 15th) are almost not affected by minor load unbalances and no significant reduction of their content in the neutral conductor current can be expected for higher unbalances. These findings confirm the results presented in Reference [
8,
15] concerning the impact of load unbalances on the neutral conductor current of a 3P4W network supplying fluorescent lamps and, also, they extend the discussion to the case of LED lamps.
The aggregation of neutral current harmonics depending on the number of users shows that the non-zero-sequence harmonics exhibit a considerable cancellation with about 80% less harmonic currents per user for 126 users compared to 11 users. On the other hand, the dominant 3rd harmonic shows only a reduction of 11%, which indicates a relation only slightly lower than constant. In general, the results of scenario 4 represent the actual impact of nonlinear lighting loads on the neutral conductor current of LV residential grids.
5. Conclusions
This paper presents a probabilistic simulation to assess the impact of different lighting technologies on the neutral conductor current in low voltage residential networks. The simulation uses a network model developed for a typical urban area in Argentina, but the results are also transferrable to many other regions of the world. The simulation results are analyzed with respect to the impact of lighting technology, supply voltage distortion, and load unbalances on the odd current harmonics up to order 15.
The results show that the highest neutral conductor current has to be expected in the case of 100% CFL. The values are more than two times higher than for the scenario with 100% incandescent lamps. In the future, a scenario with 100% LED lamps is expected, which will provide neutral conductor currents in the same range as the scenario with 100% incandescent lamps. However, the current will contain a significant share of 3rd, 9th, and 15th harmonic. A flat-top supply voltage can impact the neutral conductor current differing by almost −8%/+6% from the current obtained for the sinusoidal supply voltage. The unbalanced distribution of lamps to the phases tends to increase the neutral conductor current only slightly.
The study presented in this paper provides a solid first estimate of the impact of different lighting technologies on the neutral conductor current. It intentionally does not take other residual loads into account in order to obtain an initial indication on the safe side. Further studies are planned to consider residual loads, background distortion, network’s impedances, as well as more comprehensive models of the lamps (e.g., coupled Norton model). In this way, the impact of nonlinear lighting loads on the voltage distortion can be comprehensively studied, and neutral current magnitudes can be compared to the ones obtained in this initial assessment.