Adaptive Synchronization based Control Strategies for Harmonic Mitigation in Photovoltaic-Integrated Single-Phase Systems under Weak Grid Conditions
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Date
2025
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Department of Electrical Engineering, NITS
Abstract
Environmental degradation and the rapid depletion of fossil fuel reserves have emerged
as pressing global challenges. The combustion of fossil fuels remains a major contribu
tor to greenhouse gas emissions, particularly carbon dioxide (CO2), which significantly
accelerates climate change. In response, there is a worldwide shift toward the adop
tion and advancement of sustainable green energy technologies. This transition has
prompted a large-scale deployment of renewable energy (RE) sources particularly pho
tovoltaic (PV) system and their increasing integration into the power grid. The modern
power system has evolved into a more complex and dynamic grid to accommodate these
changes. To improve energy reliability and sustainability, PV systems are often cou
pled with energy storage units and connected to the utility network. However, if not
appropriately managed, the integration of such systems can pose risks to grid stability
and may even lead to operational failures. Moreover, this paradigm shift has resulted
in the widespread utilization of power electronic converters. Furthermore, the inherent
intermittency of PV sources, combined with the extensive use of nonlinear power elec
tronic converters/devices and loads, has introduced several power quality issues. These
include the presence of harmonics, interharmonics, DC offsets, grid voltage sags/swells,
and frequency fluctuations, all of which can deteriorate voltage and current waveforms
across the grid.
In single-phase grid-connected photovoltaic (GCPV) systems, it is essential to compen
sate for the harmonics while simultaneously injecting high-quality of PV power into the
grid, thereby improving the overall power quality. Numerous control strategies have
been explored in the literature to ensure reliable and efficient operation of single-phase
grid-connected PV systems. Among these strategies, the synchronization unit plays a
critical role in enabling stable grid interfacing for all grid-connected power converters.
Among various synchronization techniques phase-locked loops (PLLs) and frequency
locked loops (FLLs) being the most widely adopted. The implementation of these
methods requires two orthogonal signals, an in-phase component and a quadrature
phase component, both separated by 90◦. While in three-phase systems these signals
can be easily generated using Clarke’s transformation, single-phase systems face chal
lenges in generating the required quadrature component. Traditionally, transport delay
methods have been used to obtain the quadrature signal; however, PLLs based on such
techniques often perform poorly under distorted grid conditions, DC offsets, and vari
ations in grid voltage parameters.
Quadrature signal generation can also be achieved using derivative-based or integral
based techniques, each offering distinct advantages and limitations. Derivative-based
methods exhibit strong capability in rejecting DC-offsets and faster dynamic response.
However, they tend to amplify higher-order harmonics when the input grid voltage is
distorted, which can adversely affect system performance, therefore it required pre
filtering stage. Conversely, integral based approaches offers effective filtering charac
teristics. Yet, these methods are susceptible to issues such as accumulation and over
lapping errors over time. The second-order generalized integrator (SOGI) has emerged
as a widely accepted solution for QSG and also used for implementing synchroniza
tion schemes such as PLLs and FLLs, and for control strategies in single-phase grid
connected photovoltaic (GCPV) systems.
In the present work, both derivative-based and generalized integrator-based quadrature
signal generation (QSG) together with synchronization techniques are investigated and
modified for improved performance. First, a comprehensive advancement in derivative
based QSG and synchronization techniques for single-phase grid-connected systems is
presented. An improved derivative-based PLL (IDPLL) is developed, incorporating
two improved derivative-based QSG (IDQSG) blocks to eliminate gain tuning require
ments and inter-loop dependencies, thereby simplifying the design and significantly
enhancing dynamic performance. Furthermore, a frequency-adaptive enhanced deQSG
(AEdeQSG) is proposed by introducing a tuneable feedback gain (γ) for bandwidth
control and employing a mapping-based fractional-order method to achieve frequency
adaptability. The proposed methods effectively address challenges such as amplitude
amplification, harmonic distortion, and unequal quadrature signal amplitudes under
varying grid frequency conditions.
Following this, the generalized integrator-based QSG approach is further explored in
this work. An improved SOGI-QSG, referred to as iSOGI-FLL, is proposed. This
enhanced design integrates an Enhanced Dynamic and Harmonic/DC Rejection (ED
HDCR) loop to improve the performance of the conventional SOGI under conditions
involving low-order harmonics and DC offset in the grid voltage. The modification en
sures better dynamic response and superior harmonic attenuation. A control strategy
built upon the iSOGI-FLL is developed, aiming to deliver more accurate quadrature
signals, current references, and synchronization signals under various non-ideal grid
conditions, enabling seamless operation of grid-connected photovoltaic (GCPV) sys
tems.
Additionally, the modern complex grid exhibits weak grid characteristics—being heav
ily distorted with lower-order harmonics, interharmonics/subharmonics, and DC offset,
and is highly prone to disturbances. In such scenarios, the standard second-order gen
eralized integrator (SOGI) fails to effectively attenuate these harmonics. Therefore, to
address weak grid conditions and to extract more accurate fundamental components
with improved dynamic performance, two solutions based on the generalized integrator
complex filter (GICF), also known as the complex coefficient filter (CCF), have been
proposed. First, an enhanced generalized integrator-based complex filter (eGICF)-FLL
is introduced. This structure enables precise extraction of the fundamental voltage and
current components, resulting in accurate synchronization and reference signal genera
tion, particularly under weak grid conditions. Second, a higher-order complex filtering
method, termed the enhanced second-order complex coefficient filter (eSO-CCF), is
developed for fast and accurate extraction of fundamental components of quadrature
signals in single-phase system applications. The eSO-CCF significantly improves fre
quency selectivity, enhances the accuracy of fundamental component extraction, and
exhibits robust DC offset rejection. In contrast to traditional second-order and fourth
order GI filters, the eSO-CCF effectively isolates the fundamental frequency from low
order harmonics, interharmonics, and DC components. To evaluate its performance,
the proposed filter is rigorously benchmarked against widely used methods such as
SOGI, FOGI, CCF, and SO-CCF in both simulation and experimental settings. A
comparative analysis based on the frequency spectrum of the generated quadrature
signal (QSG) is also presented.
Building upon this foundation, a single-phase enhanced second-order complex co
efficient filter-based frequency-locked loop (1Φ-eSO-CCF-FLL) is designed and im
plemented. This advanced synchronization strategy is utilized for the control of a
single-stage grid-connected PV system integrated with a battery energy storage sys
tem (BESS). The proposed control scheme ensures accurate frequency tracking while
significantly improving power quality, even under weak grid conditions characterized
by severe distortion, including lower-order harmonics, interharmonics, and DC offsets.
To assess the effectiveness of the proposed approach, encompassing fundamental com
ponent extraction, quadrature signal generation, synchronization signals estimation and
frequency-adaptive control : a 5 kW experimental hardware prototype of a single-phase
grid-connected photovoltaic (PV) system integrated with a battery energy storage sys
tem (BESS) was developed. At the core of the setup is a Semikron-based converter
stack configured as a voltage source converter (VSC), which utilizes an IGBT switch
(SKM100GB125DN) and SKYPER-32 Pro gate drivers. The AC side of the system in
cludes essential interfacing and protection components such as fuses, a grid interfacing
inductor, a ripple filter, and other auxiliary circuitry. Grid connection is established
through an NHR 9410 grid simulator. On the DC side, a PV emulator (AMETEK
TerraSAS) is interfaced via a DC-DC boost converter. The VSC performs multiple
functions, as it converts the DC output from the PV emulator into AC for injection
into the grid or for supplying local AC loads connected at the point of common coupling
(PCC). A custom-fabricated rectifier is also included on the load side for testing pur
poses. Additionally, it supports harmonic compensation and reactive power injection
to improve power quality. For BESS integration, a dual active half-bridge (DAHB)
converter is connected at the DC side of the VSC. A high-frequency transformer is
coupled to the AC side of the DAHB through an external inductor, both of which are
designed and built in-house. To mitigate high-frequency switching noise, a film capaci
tor is placed at the DAHB output. An ITech battery emulator (IT6012B-500-80) is used
to emulate battery behavior and support charging operations. The control strategy is
implemented on a field-programmable gate array (FPGA)-based controller (WAVECT
WCU300), built on the Xilinx Zynq SoC XC7Z020 platform. A HIOKI power quality
analyzer is used to record power quality data. Resulting waveforms and performance
behavior are recorded using a mixed-signal oscilloscope (MSO58, 350 MHz, 6.25 GS/s).
Additionally, the entire system—including the PV interface, VSC control, and BESS
integration—is modeled and simulated in the MATLAB/Simulink environment and val
idated through hardware experimentation. The proposed approaches, such as IDPLL,
AEdeQSG, iSOGI, eGICF, and eSO-CCF-based fundamental component extraction,
synchronization and for grid-connected PV integrated with BESS—have all been vali
dated on a developed hardware test setup.