Interaction between Grid-Connected PV systems & LED Lamps: Directions for Further Research on Harmonics and Supraharmonics

This paper discusses different approaches to investigate the interaction through harmonics, interharmonics, supraharmonics, and light flicker, between photovoltaic (PV) inverters and LED lamps in low-voltage installations. Single grid connected power generators and electronic loads like LED lamps can be easily characterized in terms of harmonics in a given range of frequency. This subject is relatively well understood, and specific standards for measuring and restricting emissions are already established to ensure a low probability of interference. However, when connected together, source and load exhibit behavior that requires further study and understanding. This work presents a discussion serving as a guide for future work on analysis of losses and other impacts of the disturbances regarding this specific load and source interaction. The following are taken into account: the nonlinearity of LED loads and PV converters; the technologies and methods used in control; and the changes in power flow caused by load and power production variations. Index Terms  – electric power systems, power quality, harmonics, supraharmonics, solar power.

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From PQTBlog

Published by:

• Tatiano Busatto, Fahim Abid, Anders Larsson and Math H. J. Bollen, Electric Power Engineering, Luleå University of Technology, Skellefteå 931 87, Sweden, @mail: tatiano.busatto@ltu.se
• Gaurav Singh, Department of Electrical and Computer Engineering, Clemson University, Clemson, South Carolina 29634, USA, @mail: gauravs@clemson.edu 

Conference Paper: 16-19 Oct. 2016, Belo Horizonte, Brazil.

Published in 2016 17th International Conference on Harmonics and Quality of Power (ICHQP)

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Introduction

With the constant development, the inclusion of new energy sources and consumption devices becoming increasingly complex, a broader understanding is required of the interaction between these elements and the electrical system. In this context, the use of distributed energy resources, that typically use power electronics interfaces, is increasingly being explored as a supplement and an alternative to supplying power by the large and conventional power generation plants. In addition, following the technological trend, electronic loads with different characteristics when compared to conventional loads, are pieces of the systems that bring huge gains, especially in efficiency but even in power quality. However, at the same time they may introduce new concerns regarding the power quality.

Even considering the most pessimistic forecasts for the near future, the massive presence of photovoltaic microgeneration in low-voltage systems will be a welcome reality. Along with microgeneration, we will have LED lamps present in most of the world’s households. Both PV inverters and LED lamps use high-frequency switching techniques to convert power from different stages (e.g., DC to AC or AC to DC). The reason for this is the reduction of weight and size of the equipment, as well as the increase in control possibilities. The result is more efficient and cheaper equipment compared to traditional power conversion techniques.

Currently, both power converters and electronic loads use topologies and control techniques with a certain degree of similarity. Moreover, the normal devices used in the power stage conversion, such as SCR, BJT, TRIACS, MOSFETS, and IGBTS, have a certain degree of nonlinear characteristic. When these devices are associated with active switching methods (e.g. PWM) it can result in high harmonic levels.

Apart from these two disturbance sources, often we have the presence of communication signals in the same low-voltage installation. The presence of these signals introduces additional frequency components, making the analysis of all influences even more complex. It is known, for example, that equipment that uses Power Line Communication (PLC) connected to a “clean” supply, the currents in the frequency range 9 – 95 kHz flow mainly between neighboring devices, not between the devices and the grid [1]. Thereby, the sources of this emission and the propagation of the emission at different power levels and frequencies is currently a subject of further investigation.

This context serves to demonstrate the need for in-depth studies on the interaction between power sources, loads and communication systems. These parts should be studied individually for characterizing their emissions, followed by further study where different devices are arranged together to study the interaction between them.

This work addresses the power quality issues that are specific to low-voltage installations with LED lamps and PV converters. In this respect, the work will treat the impact of light flicker, harmonics, interharmonics, and supraharmonics in low-voltage installations in the presence of such equipment. A brief description of these power-quality issues will be given in Section II. Some of these issues are a relatively new trend of studies and some guidance to the future research has already started as presented in [2] and [3]. This kind of research brings forward big challenges, especially because it is necessary to explore the interaction between equipment with non-linear response, low predictability, and often with variations dependent on the momentary weather. This paper gives an overview of the state of the art and the required direction of future research for three aspects of the interaction between PV inverters and LED lamps: the emission from PV inverters and LED lamps (Section III); the susceptibility of PV inverters and LED lamps to voltage disturbances (Section IV); and the propagation of those disturbances between the different devices (Section V). At the end, conclusions are presented.

Power Quality Issues

According to recent studies [4], [5], [6], the use of PV inverters and LED lamps can affect the efficiency of the power system and, furthermore, cause reduction in efficiency and lifetime of end-user equipment. There is still a lack of understanding regarding the interaction between these devices. Considering this scenario, based on expert opinions expressed in [2] initially the following power-quality issues are to be analyzed:

1) Harmonics and interharmonics (freq. below 2 kHz);

2) Supraharmonics (freq. between 2 and 150 kHz);

3) Light Flicker.

The first issue, although research and development on this has been going on for decades, still deserves attention, mainly due to the extensive use of regulated power supplies which use high-frequency switching devices. Such devices have the opportunity to limit harmonic emission, but they also may complicate the issue and result in emission at frequencies that used to be rather free from emission. Modern power supplies almost exclusively use techniques in which the current drawn by the power supply is not sinusoidal [7]. As LED lamps contain various types of power supplies, this subject should be explored in a wider sense.

The second issue refers to one of the most recent concern about power-quality. Supraharmonics became an important subject for researchers and some relevant works has already been conducted. According to Lundmark in [8], the main reason for the rise of this concern is the proliferation of converters using active switching, resulting in an increase of the levels of emission in the frequency range 2 to 150 kHz. An interesting point is that there is a connection between harmonics and supraharmonics, albeit a mainly non-technical connection. An example that can illustrate this is the fact that IEC 61000-3-2 puts limits on harmonic emission for lamps larger than 25W. The most commonly used technique to solve this is the use of active power factor correction that solves the problem of harmonics emission at the lower frequencies, but it will create more emission in the higher frequency range. So, the result of the standardization is a move of the emission from the harmonic range to the supraharmonic range.

The presence of high levels of harmonics, interharmonics, and supraharmonics in the grid, has a number of consequences. Harmonic voltage distortion at the terminals of equipment (like LED lamps and PV inverters) can have a number of adverse consequences:

• a reduction in performance or an increase in losses;
• a reduction in life length, often due to the forming of hot spots;
• interference with the performance of the device, e.g. when the control system gets confused by the appearance of multiple zero crossings.

Harmonic currents could have adverse impacts on series components in the grid, like transformers. It should also be mentioned that, where it concerns low order harmonics, the voltage and current distortion levels are very much under control in most public networks. Interference for harmonics is therefore very rare. The main issue is instead for network operators to keep the harmonic voltages within regulatory or internal limits and for customers to keep the harmonic currents within limits.

For interharmonics and supraharmonics there are no regulatory limits and almost no applicable limits in standards, making that the discussion is still very much on the actual and expected impact on equipment.

The last of the three issues mentioned before is related to flicker in LED lighting. With incandescent lamps, light flicker is due to fast variations in rms voltage. For fluorescent lamps, also interharmonics around triplen harmonics can result in light flicker. For LED lamps, the light flicker issue becomes even more complicated. According to [9], LED lighting sometimes shows flicker at frequencies that may induce biological human response. Different types of waveform distortion in the harmonic and suprahamonic range can cause flicker for LED lamps. It will depend on the design of the circuitry, where the harmonic content of this flicker may vary from being unnoticeable to being highly disturbing to a human observing. In the next section, we begin the discussion about the emission of these power quality disturbances.

Emission

To start the discussion on this issue, we ask the following question: how does the power electronics in the PV inverters and LED lamps impact the harmonics, interharmonics and supraharmonics in the current?

Similarly to the work conducted by Larsson et al. in [10] on fluorescent lights, one suggestion to address this issue is first to measure and quantify the emission that LED lamps and PV inverters introduce at the device terminals, individually.

Earlier papers, [11] and [12], have presented the emission from both lamps and inverters. The large number of different types on the market makes that a systematic measurement approach is needed, involving a large number of types.

A collection of different LED lamps and PV inverters must to be evaluated covering a wide range of technologies. The results of the measurements should be presented and analysed both in the time and frequency domain, providing a thorough understanding of magnitudes and frequencies involved. In parallel, it is necessary to study the topology design, correlating the frequencies found from the individual measurements with the differences in technology between the devices. This includes the evaluation of Active Power Factor Correction (APFC) features, switching stages, rectifier diodes, bulk capacitors, and EMC filter present in some kinds of equipment. It is important to point out that, given the wide variety of lamps and inverters on the market, some stages are sometimes minimized or even ignored by some manufacturers. This is the case for example of EMI filters in LED lamps, which some lamps are not equipped with [11]. In addition, the influence of source impedance has to be evaluated, as equipment in compliance with the EMC standards may still show high emission when the impedance deviates from the reference impedance used for the compliance test. Regarding inverters, some work has been carried out already. As an example, we can cite the work conducted by Wang et al. in [13], where it is concluded that harmonic emission from PV inverters depends on their operating conditions. If the output power is reduced, more harmonics will be emitted. With a reduction in reduced power from 1515W to 116W, the voltage THD increased from 3.65% to 18.13%. When expressed in Ampere (instead of as a percentage of fundamental current) the harmonic content decreases from 461mA to 175mA (assuming a 120-V system and single phase connection).

In the same way, in [12] the harmonic emission for multiple inverters with different rated power (from 1kVA to 100kVA) is investigated. The results showed that the low order harmonics have a predominant 5th and 7^th harmonics, both in current and voltage. All inverters have a significant emission at their switching frequencies. In this study the following switching frequencies were observed : 3 kHz for large inverters (100 kVA) and 10 kHz, 16 kHz, and 25 kHz by the small inverters (1 kVA to 10 kVA). In [14], the impact of the output current level on the feed-in grid current distortion of single-phase PV systems is analyzed. By modeling proportional (PR), repetitive (RC), and multiresonant controllers (MRC) and their aggregation, the authors show how the control can affect the harmonic emission. Combining some of these methods, the inverter can suppress the harmonics effectively, even at different operational conditions. Considering now the impact of supraharmonics, in [12], we can see an example of noticeable emission in the range between 40 kHz and 80 kHz caused by a narrow band powerline- communication-system (PLC). Also, at LTU, studies have been done to understand the spreading of supraharmonics in a local low-voltage system. The spectra of the measurements, at the connection point for a 56W LED streetlamp, in two different environments was made. From the results, the emission is strongly location dependent, as is shown in Fig. 1.

On the other hand, regarding the impact of LED lamps, as an example, we can cite the work conducted by Rönnberg et al. in [15], where the lamps (mainly incandescent) in a residential area were replaced by LED lamps and the harmonic emission of the complete installation was evaluated before and after the replacement. The measurements clearly indicated that there was no significant change in emission level for the installation as a whole due to the replacement of the lamps. Emission due to the presence of APFC in LED lamps is an important point to be evaluated. Through IEC 61000-3- 2, lamps are regulated with respect on harmonic emission, and usually lamps larger than 25W are fitted with APFC to fulfill the emission requirement. From what some initial experiments have shown, APFC can minimize the harmonic emission very well, but at the same time it can create distortion in the supraharmonic range. Fig. 2 show an example of voltage and current waveform taken from two LED lamps with and without APFC analyzed at LTU laboratory.

The upper waveform was obtained from a 63W LED lamp (industrial use). The current is fairly sinusoidal except from some small deviations around the zero crossing and small distortion at both positive and negative peak of the current. This distortion consists of the remains from the switching in the APFC and appears in the supraharmonic range. The lower waveform is obtained for a 7W LED lamp, without APFC, where the current waveform is distorted in the lower frequency range. Fig. 3 show the frequency spectra for both lamps.

The measured total harmonic current distortion (ITHD) for the 63 W LED lamp was 10% at VTHD 1.98% and the displacement power factor (DPF) was 0.985. The measured ITHD for the 7 W lamp was 78% and DFP of 0.858. To evaluate the harmonics and interharmonics there is a need to develop simulation models and performs laboratory measurements. Regarding to supraharmonics the interaction between the devices should be considered in order to verify the impacts caused by possible resonances, especially when it can cause high-voltage distortion levels.

Susceptibility

Regarding this issue, we ask the question: how do harmonics, interharmonics, and supraharmonics in the terminal voltage impact the power electronics in PV inverters and LED lamps and the light intensity of LED lamps?

In this context, important research initiatives have already been taken, mainly related to LED lamps. The studies are directed to understand the effects of phenomena such as voltage distortion on LED lamps and other lighting equipment. The effect of supraharmonics (2 to 150 kHz) and harmonics (0 to 2 kHz), and also the effect of power factor correction circuits was investigated and conclusions were obtained [10]. An example of how the supraharmonics affect the illuminance was analyzed on a 3W LED lamp in the low-voltage laboratory at Luleå University of Technology. To verify the susceptibility, the illuminance was compared with and without high-frequency distortion added to the normal voltage waveform. Fig. 4 show the result of the experiment when the lamp is under normal conditions (e.g. without added high-frequency distortion).

The high-frequency distortion, recorded at a commercial establishment and then superimposed onto the voltage supply voltage waveform, contained frequency components present in the middle of the 2 to 150 kHz frequency range. The result of the experiment for the case with added high-frequency distortion is shown in Fig. 5.

The results demonstrate the effects of high-frequency distortion on LED lamps. The illuminance increased under the test signal with high-frequency distortion. However, this behavior is not consistent. Some lamps show a decrease in illuminance. Such difference in behavior is due to the differences in electrical design of the lamp. Regarding PV inverters, similar experiments should be conducted. It is necessary to know how much harmonic and supraharmonics impact the loss of life and loss of efficiency of this equipment. The impact caused by different frequencies in terms of performance and life length of equipment has never been well investigated for PV inverters. A good starting point of such an investigation is to explore the following research topics:

• the impact of conducted emissions for different ranges of frequencies on the efficiency of the power stage, in special on the commonly used components like IGBTs, transformers, diodes, and capacitors;
• the impact of frequencies that originate from PLC on the efficiency and evaluation of possible interference with the equipment operation;
• evaluation of the acceptable limit for conducted and radiated emission before the equipment is affected by any interference, either in its efficiency, loss of life, or possible malfunction.

To conduct this research topic, the use of simulation software and laboratory facilities equipped with a variety of PV inverters and LED lamps should be considered. The main goal is to explore how the devices are impacted by the emissions. In addition, it is necessary to have available equipment and components to assemble power switching stages in order to study specific parts of the topics outlined above.

Propagation

To this issue the question is: How do harmonics, interharmonics, and supraharmonics propagate from one device to another device in the same low-voltage installation?

This topic includes statistical issues that depend on the time of day, location and details of the other equipment present in the facilities. Including all this is a big challenge. Firstly, it is necessary to address the impact of voltage distortion on the emission of harmonics and interharmonics and to understand how they spread through the low-voltage installation. A large number of correlations must be performed to identify interactions, primarily to identify combinations that produce high distortion levels associated with emission. Experimental results in [16] showed that the harmonic impact is strongly dependent on the type and mix of inverters, and the conditions under which they are operating. Also, the results showed that mixing different types of inverters can reduce slightly the combined THD of the installation as a whole.

Regarding supraharmonics, according to Hankaniemi et al. [17] their flow mainly occurs between individual devices instead of into the grid. This was later confirmed and explained by several other studies. It was also shown in several studies that the individuals equipment connected to the grid has great influence on the supraharmonics emission.

One possible way to mitigate the spread of supraharmonics is improve the design of low-voltage installations. This includes the evaluation of the role of distribution transformers in spreading this type of frequencies and the topology of the grounding plane. One suggestion is to start this research topic adding research elements to the findings from the work conducted by Lundmark in [8]. Studies of the differential mode current can be further evaluated in order to understand how the configuration of low-voltage facilities can mitigate the spread of supraharmonics. Also, evaluate the use of PLC, is an important point to be explored. Taking this point in consideration, one idea is to start the research using simulation models to evaluate the interaction between different devices and its impact on the propagation. Secondly, with an availability of equipment and facilities the interaction can be performed in practice, studying the impact on the interaction of the variation of different parameters. This includes the evaluation of the power production and load variation in different situations. Also must be included in the studies are the effects that supraharmonics have on the neutral currents (zero-sequence component) and individual harmonic orders.

Conclusion

From the issues highlighted in the previous sections, it is easy to see that sufficient research challenges remain that should be addressed in the near future especially regarding supraharmonics.

The general presence of non sinusoidal voltages and currents in the grid is not a concern as long as they remain below a certain value. Once the exceed that value the situation becomes worrisome. To understand and quantify how the different frequency distortions are generated and propagated is of fundamental importance to be able to estimate the risk that those values are exceeded. Also is it not always clear above which value the distortion becomes worrisome. Although the effects of power losses are well-known in theory, it remains necessary to better quantify the impacts that these phenomena influence on the loss of efficiency and loss of life of equipment connected to the grid.

This paper proposes a number of topics that should be addressed by researchers to investigate the interaction between PV inverters and LED lamps. As we can see, there are some issues in power-system harmonics that are relatively new, and, although some of those are already being investigated, even those still require a lot of further studies. This is the case for supraharmonics, where there are more specific needs in describing their behavior in different situations, establishing standardized measurement methods, and setting emission and immunity limits for equipment.

References

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