P-Resonant Controllers for PV-Integrated Grid Power Quality Improvement

Design and Performance Evaluation of P-Resonant
Controllers for Power Quality Improvement in PV-
Integrated Grid
R Satish Kumar
Research Scholar
Jawaharlal Nehru Technological
University Anantapur, Ananthapuramu,
Andhra Pradesh,
[email protected]
R Satish Kumar
Assistant Professor
Rajeev Gandhi Memorial College of
Engineering & Technology, Nandyal,
Andhra Pradesh, India.
[email protected]
Dr D Lenine
Professor
Rajeev Gandhi Memorial College of
Engineering & Technology, Nandyal,
Andhra Pradesh, India.
[email protected]
Abstract: Harmonics generated by distributed generation
(DG) systems pose a significant challenge to power quality,
especially with the growing integration of these systems into
the grid. Effective harmonic control is essential to minimize
their adverse impact on grid stability and performance. This
paper presents an advanced proportional current controller
combined with a selective harmonic compensator for grid-
connected photovoltaic (PV) systems. The proposed P-
resonant current controller (PRCC) plays a crucial role in
ensuring the delivery of high-quality current from the PV
inverter to the grid. To maintain power quality, the controller
must minimize distortion and suppress harmonic generation.
The effectiveness of the proposed approach has been validated
through MATLAB/Simulink simulations.
.
Keywords—Distributed power generation system, MPPT, PV
system, P-Resonant controller
1. INTRODUCTION
In recent years, concerns over environmental pollution
have led to the widespread adoption of distributed
generation systems utilizing renewable energy sources such
as solar, wind, and micro-hydro power. Among these, solar
energy has become a primary source of electrical
generation, relying on photovoltaic (PV) systems to convert
sunlight into electricity. A PV system consists of one or
more solar panels that harness solar energy and transform it
into usable electrical power. The most common method of
solar energy conversion is the direct transformation of
photon energy into electrical energy [13]. Photovoltaic
technology has made a significant contribution to electricity
generation, offering greater availability and sustainability
compared to other renewable energy sources. A
photovoltaic (PV) system converts sunlight into electricity
using photovoltaic modules. By implementing various
maximum power point tracking (MPPT) techniques [3]-[8],
the system can optimize power extraction from the PV
modules. The generated power is in direct current (DC)
form, which can be converted into alternating current (AC)
with the required frequency using an inverter. When the
inverter connects the system to the grid, it is referred to as a
Grid-Tied Inverter (GTI) [7]. The GTI must comply with
grid regulations to ensure proper operation.
Synchronization is essential for grid-tied inverters, ensuring
that key parameters such as fundamental frequency and
phase align with the grid to facilitate efficient power
transfer. One of the primary power quality challenges in
distributed generation systems is harmonic distortion,
making it crucial to implement effective control strategies
to mitigate harmonics produced by these inverters.
II. GRID CONNECTED PV SYSTEM
The provided figure 1 represents a control strategy for a
grid-connected photovoltaic (PV) inverter system,
integrating a proportional-resonant (PR) controller [2] &
[21] with a selective harmonic compensator. The system
takes DC power from the PV array and converts it into AC
power through an inverter, ensuring synchronization with
the grid. The PR controller plays a crucial role in
maintaining the sinusoidal shape of the output current
while addressing power quality concerns such as
harmonics. The selective harmonic compensator further
enhances performance by targeting specific harmonic
frequencies, reducing distortions, and improving overall
grid compliance. Additionally, the diagram illustrates the
feedback control mechanism used to regulate the inverter's
output. Voltage and current sensors monitor the system
parameters, feeding the data into the control algorithm to
ensure precise synchronization with the grid. The inverter's
modulation technique is designed to optimize power
injection while minimizing harmonic distortion. By
employing advanced control techniques, this system
enhances the efficiency and stability of grid-tied PV
inverters, making them more reliable for integration into
modern power networks.
Fig.1: Block diagram of Grid-connected PV system
A. PV System Modelling
The given diagram represents the equivalent circuit
model of a generalized photovoltaic (PV) cell [10]-[12],
which is widely used for analyzing and simulating the
electrical characteristics of solar panels. In this circuit, Iph
denotes the photocurrent generated due to sunlight
exposure, which serves as the primary current source. The
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diode represents the inherent p-n junction of the solar cell,
where Id is the diode current corresponding to the
recombination of charge carriers. Additionally, the circuit
includes a parallel resistance Rsh) and a series resistance Rse
), which account for the losses within the PV cell. The
shunt resistance (Rsh) models leakage currents due to
imperfections in the cell, while the series resistance (Rse)
represents resistive losses due to internal material and
contact resistances. This equivalent circuit plays a crucial
role in determining the performance of a PV cell, helping
in the evaluation of efficiency and power output under
different conditions. The voltage (V) across the circuit is
influenced by the interplay of these elements, where higher
series resistance leads to greater power dissipation and
reduced efficiency. On the other hand, a lower shunt
resistance results in excessive leakage currents, degrading
the overall output. Understanding this model is essential
for optimizing solar panel design, improving material
selection, and enhancing energy conversion efficiency. By
adjusting these parameters, engineers and researchers can
develop high-performance solar cells that maximize power
generation while minimizing losses.
Fig 2: Equivalent circuit model for PV
Ι =  −  
 (1)
Ι =  −  !
"#$%&'( −  !
)* (2)
TABLE I. LIST OF ABBREVIATIONS
Symbol Meaning of Symbol
Isc Short circuit current
Kt Temperature co-efficient of cell
Tc Working temperature of the cell
Tref Reference temperature of cell
Λ Solar insulation
Is Cell saturation of dark current
Iph Photo current
Q Electron charge
K Boltzman’s constant
A Ideal factor of diode
Rse Series resistance
Rsh Shunt resistance
B. Selection of DC–Link Capacitor
The selection standard of dc-link capacitor is based on
the optimal ac line current regulation strategy for PV
system. The expression for dc-link capacitance can be
derived as [11].
C = +,-./01
2324 (3)
III. CONTROL SCHEME FOR GRID-INTERACTIVE
INVERTER
A control scheme for a grid-interactive inverter in
figure.3 plays a crucial role in ensuring efficient energy
exchange between distributed energy sources and the power
grid. This system is designed to regulate voltage, frequency,
and power flow while maintaining stability and compliance
with grid standards. The control strategy typically
incorporates real-time monitoring, feedback loops, and
dynamic adjustments to optimize energy utilization.
Advanced techniques such as proportional-integral (PI)
controllers [2], phase-locked loops (PLL), and predictive
algorithms help enhance the inverter’s response to
fluctuations in grid conditions. By implementing an
intelligent control mechanism, grid-interactive inverters
can seamlessly integrate renewable energy sources, such as
solar or wind power, while minimizing power losses and
improving overall efficiency.
Moreover, an effective control scheme enables
bidirectional power flow, allowing the inverter to both
inject power into the grid and draw energy when necessary.
This capability is essential for applications like energy
storage systems and demand-side management. Modern
control approaches also incorporate grid-supportive
functionalities, such as reactive power compensation [15]-
[20], fault ride-through capability, and harmonic mitigation,
ensuring grid reliability. With the increasing adoption of
smart grids, adaptive and AI-driven control schemes are
being explored to further enhance inverter performance. As
energy systems transition toward greater decentralization,
developing robust and flexible control strategies for grid-
interactive inverters remains a key factor in achieving a
sustainable and resilient power infrastructure.
Fig 3: Control scheme for a grid-interactive inverter
The grid voltage is given to the PLL as a feedback
for controlling the system. By using the park
transformation, the grid voltage is synchronizing with the
grid current. The Perturbation and Observation (P&O)
Maximum Power Point Tracking (MPPT) algorithm [14], is
simple to implement and operates by making small
adjustments to the photovoltaic (PV) array’s operating
conditions. It functions by introducing perturbations in the
system, such as changes in radiation or orientation, to
analyse their impact on power output. If the extracted power
increases due to these variations, the algorithm shifts the
operating point toward the Maximum Power Point (MPP),
adjusting the working voltage in the same direction to
optimize energy harvesting. The control circuit for a grid-
connected system, as depicted in Fig. 7, operates by
deriving the reference current from the photovoltaic (PV)
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panel’s output voltage and current. The voltage controller
receives its input by comparing the reference voltage with
the direct current (DC) voltage. By incorporating input
power feed-forward, the dynamic performance of the PV
system is significantly enhanced. Additionally, the voltage
controller ensures a rapid response to sudden fluctuations in
input power, maintaining system stability and efficiency.
Fig 4: Block Diagram of PI Controller
Fig 5: Block Diagram of PR Controller
The PR current controller GPR(s) is defined as
5)678/98: ;
;2<2 (4)
A normal P-resonant control can give the stability problem
because of the infinity gain. To avoid such problem, the PR
controller can be made by introducing damping is given as,
5)678/98: ;
;2+<;<2 (5)
IV. SIMULATION RESULTS
The image consists of two plots representing the
electrical characteristics of a Trina Solar TSM-250PA05
photovoltaic (PV) array under different irradiance levels.
The top plot displays the current-voltage (I-V) curves, while
the bottom plot shows the power-voltage (P-V) curves.
Fig 6: I-V and P-V characteristics
Each curve corresponds to a specific solar
irradiance level, ranging from 0.2 kW/m² to 1 kW/m². The
highest irradiance level, marked in red, results in the
maximum current and power output. The I-V curves
illustrate that the current remains relatively constant over a
wide voltage range before rapidly dropping near the open-
circuit voltage. Similarly, the P-V curves show a peak
power point that shifts with changing irradiance levels, with
higher irradiance leading to greater maximum power
output. The data in the plots demonstrate the dependency of
PV module performance on solar irradiance. As the
irradiance decreases, both the current and maximum power
output reduces significantly, whereas the open-circuit
voltage experiences a smaller change. The curves indicate
that optimal energy extraction occurs near the maximum
power point (MPP), which varies based on environmental
conditions. These characteristics are essential for designing
and optimizing PV systems, as they help determine the most
efficient operating points for solar inverters and controllers.
The two parallel strings in the system contribute to a higher
current, which is evident from the relatively elevated
current levels in the I-V plot.
Fig 7: Inverter Voltage, Current and Power waveforms
The figure.7. presents three plots illustrating the output
characteristics of a grid-connected inverter over time. The
first plot represents the inverter output voltage, which
exhibits a periodic waveform with a near-square shape,
indicating the switching behaviour of the inverter. This
waveform is essential for converting DC power from the
photovoltaic system into AC power suitable for grid
connection. The second plot displays the inverter output
current, which follows a sinusoidal pattern. This sinusoidal
waveform suggests that the inverter effectively injects AC
current into the grid while maintaining synchronization
with the grid voltage.
Fig 8: Grid Voltage, Current and Active power and Reactive power
waveforms
The figure.8. Shows the inverter output power,
which varies with time and exhibits fluctuations. The power
waveform follows the pattern of the current waveform,
demonstrating the relationship between the inverter's
current and voltage output. The variations in power suggest
that the system responds dynamically to changing
conditions, such as load variations or grid disturbances. The
effective control of voltage, current, and power in these
plots highlights the inverter’s ability to regulate and
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optimize energy injection into the grid, ensuring stable
operation and compliance with grid requirements.
Time (Sec)
Fig 9: THD Spectra of Grid Current
The figure.9 shows the harmonic distortion of
inverter output current and grid current respectively.
Fig 10: Real power and Reactive power when using PR controller
The given figure.10 consists of two plots
representing the variations of active and reactive power
over time. The top plot illustrates the active power response,
which initially experiences a sharp rise followed by
oscillations that gradually settle into a steady-state
condition. The bottom plot depicts the reactive power,
which also undergoes significant oscillations before
stabilizing. These oscillations indicate transient behaviour,
which eventually diminishes as the system reaches
equilibrium. The time scale suggests that the transient
effects subside within approximately one second. The
table.1 presents a Total Harmonic Distortion (THD)
analysis of the grid current (Igrid) in a grid-connected
photovoltaic (PV) system under varying quality factors,
comparing the performance of Proportional-Integral (PI)
and Proportional-Resonant (PR) control strategies. As the
quality factor decreases from 5 to 1, both control methods
exhibit an increase in THD, indicating a reduction in power
quality. However, the PR control consistently achieves
lower THD values across all quality factors. For instance, at
a quality factor of 5, PI control results in a THD of 0.84%,
whereas PR control reduces it to 0.63%. This trend
continues at lower quality factors, highlighting the
effectiveness of PR control in minimizing harmonic
distortion.
TABLE.2. THD ANALYSIS FOR GRID CONNECTED PV SYSTEM
FOR GRID CURRENT
Quality
Factor
THD Analysis (Igrid)
PI Control PR Control
5 0.84% 0.63%
4 0.93% 0.70%
3 1.02% 0.70%
2 1.11% 0.72%
1 1.20% 0.73%
The observed difference in performance between
the two controllers is attributed to their frequency response
characteristics. While the PI controller operates well in DC
or low-frequency applications, it lacks the capability to
effectively suppress sinusoidal steady-state errors,
especially at the grid frequency. In contrast, the PR
controller is specifically designed to target and cancel
harmonics at a specific resonant frequency, typically the
fundamental grid frequency (50/60 Hz), which explains its
superior THD performance. This makes the PR controller
more suitable for applications requiring high power
quality, such as grid-connected PV systems where
compliance with grid codes and standards is critical.
The table.2 illustrates the Total Harmonic
Distortion (THD) of the inverter current (Iinv) in a grid-
connected photovoltaic (PV) system under different
quality factors, comparing PI and PR control techniques.
As the quality factor decreases from 5 to 1, the THD for PI
control increases slightly and then stabilizes at around
18.25%, indicating a relatively poor harmonic suppression
capability. In contrast, the PR controller maintains a more
stable and lower THD across the range, starting from
13.30% at a quality factor of 5 and slightly increasing to
14.83% at a quality factor of 1. This shows that PR control
performs better in reducing inverter current distortion
compared to PI control, especially under low-quality factor
conditions.
TABLE.3. THD ANALYSIS FOR GRID CONNECTED PV SYSTEM
FOR INVERTER CURRENT
Quality
Factor
THD Analysis(Iinv)
PI Control PR Control
5 14.45% 13.30%
4 18.27 13.30%
3 18.27% 14.84%
2 18.26% 14.84%
1 18.25% 14.83%
The better performance of the PR controller can
be attributed to its inherent ability to target and attenuate
specific frequency components, particularly the
fundamental grid frequency and its harmonics. This is
particularly advantageous in inverter current regulation,
where harmonic suppression is crucial for efficient and
reliable power conversion. On the other hand, the PI
controller struggles with tracking and eliminating
sinusoidal steady-state errors, leading to higher THD
values. The data clearly suggest that using a PR controller
enhances the harmonic performance of the inverter output
in grid-connected PV systems, making it a more effective
solution for maintaining power quality.
V. CONCLUSION
This paper presents the design of a single-phase
grid-connected photovoltaic (PV) system incorporating a
proportional-resonant (PR) controller. A thorough analysis
is conducted to evaluate how the PR current control
characteristics influence the performance of the PV
inverter. Several design modifications are implemented to
enhance harmonic injection and ensure proper grid
synchronization. By utilizing a P-resonant controller, the
inverter's output current harmonics are effectively
minimized. The development of the P-resonant current
controller is carried out using MATLAB/Simulink, with the
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design methodology structured to comply with IEEE
standards for grid interconnection.
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