Power Quality in Grid-Connected PV Systems: Impacts, Sources, and Mitigation Strategies

Written by Talada Appala Naidu, Sajan K Sadanandan, and Tareg Ghaoud

Installed Photovoltaic (PV) capacity has been rising across the smart grid distribution systems to supply energy needs as worries grow about greenhouse gases. However, the high penetration of PVs could affect the operation and planning of distribution networks. Therefore, to ensure a consistent and high-quality supply of power for a long time under a decentralized grid setup, it is critical to preserve compatibility and stability between the grid and its connected equipment. Power quality is an essential factor for the reliability of on-grid PV systems and should not be overlooked. This article underlines the power quality concerns, the causes for harmonics from PV, and their mitigation strategies considering the scope of research on the effect of voltage/current harmonics from PV-inverters on the grid.

Power Quality Concerns

In terms of continuous supply, power quality refers to a collection of indications that reflect the characteristics of the sources of supply under typical operational circumstances of voltage and frequency. Power electronics devices in today's distribution networks can create major interruptions, affecting the power quality provided to consumers connected to the network [1]. Furthermore, research has shown that every 1% variation in voltage corresponds to a 1% difference in power utilization. As a result of these variables, utilities make every effort to maintain the voltage provided to your home within the acceptable range [2]. Even though power quality is not a new concept in power systems, it merits special attention in current grids for the reasons listed below:

• A drastic growth of non-linear loads and single-phase loads may negatively impact the power quality.
• In recent years, there has been an increase in sensitive (critical) loads and new operational procedures that may affect the power quality.
• According to the current scenario, there has been a significant increase in power electronics-based inverters connected to the grid due to the high penetration of Distributed Energy Resources (DERs).

Utilities in the LV/MV levels are now moving toward solar PV rooftop installations connected to the grid for greater usage of solar PV-generated electricity in the interest of green energy. These solar PV-inverters will continue to operate under various situations, including frequent low-level and highly fluctuating irradiance. As a result of these circumstances, PV inverters may inject harmonics voltages/currents, impacting the power quality at the Point Of Connection (POC), creating a new challenge for the distribution network. High-level harmonics lead to energy losses, reduce system capacity, deteriorate network components, and cause protection equipment to malfunction. More study on grid-connected PV systems is needed to understand the issues that come with large-scale installations from different PV inverter manufacturers. So, the study of harmonic emission sources and their mitigation strategies has been introduced in the following section.

 

Harmonics Emitted from PV-Inverters and Their Mitigation Methods

The cause of harmonics generation in PV-inverters and mitigation measures are emphasized in this section.

Source of Harmonics Generation

The most common conversion mechanism used in grid systems is an 'inverter' to feed the grid from diverse DC sources. DC sources that work at various dc voltages and power levels include batteries, super-capacitors, and photovoltaic (PV) arrays [3]. Apart from all the various DC sources, the PV arrays combined with inverters are relevant in this study. Because of the inverter's intrinsic nature, it creates harmonics in voltage and currents that are sent to the grid, which are undesirable. The reasons for the generation of voltage/current harmonics from PV inverters are as shown below:

• Pulse Width Modulation (PWM) approaches must function at a relatively high switching frequency to operate inverter switches (e.g., IGBTs), leading to significant emission of harmonics at the inverter output.
• Effects of DC side impedance: When a large inductance is used to connect at the DC side of the PV inverter for smoothening the DC current. This type of harmonic source behaves like a current source and is called a current-type harmonic source. Suppose a capacitance is used to connect at the DC side of the PV inverter for smoothening the DC voltage. This type of harmonic source behaves like a voltage source and is called a voltage-type harmonic source. Therefore, the inverter with an inductor or capacitor may act as a current source or voltage source with the harmonic contents [4].
• DC-link voltage: The irregular and intermittent nature of solar irradiation, i.e., the changes in the solar irradiance throughout the day, cause significant ripples in DC link voltage, thus producing the harmonics on the AC side of the inverter [5].

These are the most important reasons for PV inverter harmonic emission. However, the investigation into the various sources of harmonics created by PV inverters is still underway.

Power Quality Mitigation Strategies

It is crucial to maintain the power quality limits under the standard level according to the IEEE 519, IEEE 1547, and IEC 61000-3-2. Furthermore, a few related research studies on power quality mitigation measures are presented below as part of resolving the power quality challenges:

• A dynamic reactive power compensation thyristor-driven LC compensator that significantly reduces the injection of harmonic currents compared to typical static-var compensators has been reported in [6]. Even though an extra active filter (static-var compensator here) for harmonic compensation has been eliminated, the thyristor, L, and C circuitry are needed to compensate harmonics.
• Hossein et al. in [7] proposed a novel system approach for improving power quality in LV distribution networks utilizing a Unified Power Quality Conditioner. It consists of a power converter at the MV/LV substation's end, and another power converter at the customer's end, which increases the number of switches and requires an extra control circuit
• Alireza et al. [8] presented research on combining a transformer-less hybrid series active filter and energy storage system to provide enhanced power quality. The researchers also found that the requirement of an energy storage system for providing constant supply is an extra cost for the compensation of power quality issues.
• A Phase-locked loop (PLL) based reactive power flow regulation for the control of PV system in LV distribution network has been proposed in [9]. In this work, the harmonic compensation function is included in the inverter control system, which improves power quality at POC.

From the literature presented above, power quality disturbances at POC are being addressed by compensators that use different inverter circuits and control techniques. It is also identified that the inverter control system itself includes the function of harmonic compensation. So, it is suggested to develop such control techniques for the existing inverter to control and improve the power quality issues at POC.

Summary

This article examines the major power quality issues of on-grid PV systems and the necessity to study the harmonics emitted from PV inverters. Voltage/current harmonic emissions have always been given special attention because they potentially impact vital components and technology of on-grid PV systems. This article also provides an insight into why power quality is a concern, precisely the source of harmonic generation and the available mitigation strategies in the literature. From the discussion above, we can conclude that further studies need to be conducted on the following topics:

• Improved controllers in active power filters, inverters, and other power electronics devices which are required to enhance power quality on on-grid inverters connected systems.
• Sophisticated metering, sensing, and control features are required to support improving power quality delivered to customers with an acceptable power quality level.
• To understand the challenges due to large-scale installation from different manufacturers of PV inverters, more research, investigation on harmonic emission from PV-inverters, control circuits within multiple brands, and types of these inverters in grid-connected PV systems is needed.
• It is also recommended to develop control strategies that work for existing PV inverters to improve the power quality issues at POC.

 

References

1. Arash Anzalchi, Aditya Sundararajan, Amir Moghadasi, and Arif Sarwat. "High-penetration grid-tied photovoltaics: Analysis of power quality and feeder voltage profile." IEEE Industry Applications Magazine 25, no. 5 (2019): 83-94.
2. "Smart grid and power quality" SMART ENERGY Consumer Collaborative
3. Syed M. Ahsan and Hassan A. Khan. "LV Harmonic Analysis of Single-Phase Rooftop Solar PV Systems with Non-Linear Loads." In 2022 IEEE Green Technologies Conference (GreenTech), pp. 1-6. IEEE, 2022.
4. Dayi Li and Jun Tian, "A novel active power filter for the voltage-source type harmonic source," In 2008 International Conference on Electrical Machines and Systems, pp. 2077-2080. IEEE, 2008.
5. Jianxia Sun and Cheng Lin, "Calculation and Spectral Analysis of DC-Link Current for three phase PWM inverter." In 2021 21st International Symposium on Power Electronics (Ee), pp. 1-6. IEEE, 2021.
6. Lei Wang, Chi-Seng Lam, and Man-Chung Wong, "Design of a thyristor-controlled LC compensator for dynamic reactive power compensation in smart grid," IEEE Transactions on Smart Grid 8, no. 1 (2016): 409-417.
7. Hossein Hafezi, Gabriele D'Antona, Alessio Dedè, Davide Della Giustina, Roberto Faranda, and Giovanni Massa, "Power quality conditioning in LV distribution networks: Results by field demonstration," IEEE Transactions on Smart Grid 8, no. 1 (2016): 418-427.
8. Alireza Javadi, Abdelhamid Hamadi, Auguste Ndtoungou, and Kamal Al-Haddad, "Power quality enhancement of smart households using a multilevel-THSeAF with a PR controller," IEEE Transactions on Smart Grid 8, no. 1 (2016): 465-474.
9. Ángel Molina-García, Rosa A. Mastromauro, Tania García-Sánchez, Sante Pugliese, Marco Liserre, and Silvio Stasi, “Reactive power flow control for PV inverters voltage support in LV distribution networks,” IEEE Transactions on Smart Grid 8, no. 1 (2016): 447-456.

 

This article was edited by Cesar Duarte.

To view all articles in this issue, please go to June 2022 eBulletin. For a downloadable copy, please visit the IEEE Smart Grid Resource Center.