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 . 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 . 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 . 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 .
- 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 .
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 . 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  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.  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 . 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.
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.
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This article was edited by Cesar Duarte.
Talada Appala Naidu is Post-Doctoral Researcher at R&D Center, Dubai Electricity & Water Authority (DEWA), Dubai, UAE. He received his Bachelor of technology from JNTU, Kakinada, India. He received Master of technology and PhD from SV National Institute of technology, Surat, India in 2016 and 2020 respectively. He has been worked as visiting researcher at APEC Lab, Khalifa University, Abu Dhabi, UAE and then as an Adhoc Faculty at National Institute of technology, Andhra Pradesh, India. His research interests include the applications of power electronics in distribution systems, power quality, Artificial intelligence in control of power electronics devise, grid connected PV systems, and smart grid systems.
Sajan K Sadanandan is an Associate Principal Researcher-Power System Lead at R&D Center, Dubai Electricity & Water Authority (DEWA), Dubai, UAE. In the past years, he has worked in a different capacity at IIT Roorkee, University of Petroleum and Energy Studies (UPES), GE India Industrial Private Limited, Washington State University, and West Virginia University. He received the Ph.D. and M.Tech degrees in electrical engineering from the Indian Institute of Technology Roorkee, India, in 2010 and 2016, respectively, and a B.Tech degree in electrical engineering from Pt. Ravi Shankar Shukla University, India in 2007. His research interests include power system operation and control, synchrophasors applications, and cyber-physical analysis of power systems.
Tareg Ghaoud is leading the Smart grid integration team within the DEWA R&D centre, DEWA, UAE. He is a utility industry expert with research interest in the integration and adoption of smart grid systems, and data analytics in the power transmission and distribution sectors. Tareg is a Chartered Engineer with over 20 years of industry experience. He worked as a delivery project manager and an R&D product owner for innovative projects serving the UK utility industry. He has intensive experience in the development and delivery of SCADA and automation systems for the power transmission and distribution networks, substation control systems and the development of digital grid solutions for providing grid analytics, flexibility solutions such as: microgrids, virtual power plants and Active Network Management (ANM). Tareg has led international teams of different size and capabilities and delivered projects using the V-Model as well as the Agile-Scrum methodology approaches. He completed his studies in the UK and holds a DPhil degree in Engineering Science from the University of Oxford (2002), an MSc degree in Control Systems from the Imperial College of Science, Technology and Medicine (1997), and a BEng degree in Control Engineering from the University of Sussex (1992).