Power Factor Correction and THD Reduction in AC-DC Systems using a Dual-Loop Controlled SEPIC Converter
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Abstract
The proliferation of non-linear loads, such as diode bridge rectifiers, in modern AC-DC power systems introduces significant power quality challenges, including low power factor and high Total Harmonic Distortion (THD). This paper proposes and simulates a Single-Ended Primary-Inductor Converter (SEPIC) topology integrated with a dual-loop control strategy to mitigate these issues. The control architecture consists of an outer voltage control loop and an inner current control loop, both implemented using Proportional-Integral (PI) controllers. The outer loop regulates the DC output voltage, while the inner loop shapes the input current to be sinusoidal and in phase with the source voltage. Simulation results, obtained using MATLAB/Simulink, demonstrate the high effectiveness of the proposed system. The source current THD is dramatically reduced from a non-compliant 66.24% in the open-loop configuration to an acceptable 3.72% under closed-loop control. Furthermore, a near-unity power factor is achieved, and the DC output voltage is successfully regulated at its reference value. These findings validate that the dual-loop controlled SEPIC converter is a highly effective solution for power factor correction, ensuring compliance with international power quality standards such as those set by the IEEE.
Keywords
Power Factor Correction (PFC), SEPIC Converter, Total Harmonic Distortion (THD), Dual-Loop Control, PI Controller
I. INTRODUCTION
The increasing integration of electronic devices into modern power systems has led to a proliferation of non-linear loads, such as diode bridge rectifiers used in AC-DC power supplies. These loads draw a non-sinusoidal current from the AC mains, resulting in significant power quality challenges. Key issues include the consumption of excessive reactive power, a poor power factor, and the injection of harmonic currents back into the utility grid. Such harmonic distortion can lead to equipment malfunction and reduced system efficiency. Consequently, adherence to stringent international standards, such as those established by the IEEE and IEC, which impose strict limits on harmonic content, has become critically important.
To address these challenges, active Power Factor Correction (PFC) techniques have become standard practice. Active PFC employs power electronic converters to actively shape the input current, forcing it to be sinusoidal and in phase with the line voltage. Among various converter topologies, the Single-Ended Primary-Inductor Converter (SEPIC) is a particularly suitable choice for these applications.
The primary objective of this paper is to design, simulate, and validate a dual-loop control strategy for a SEPIC-based PFC rectifier. The goal is to achieve a near-unity power factor and reduce the Total Harmonic Distortion (THD) of the input current to below the 5% threshold mandated by industry standards, while simultaneously maintaining precise regulation of the DC output voltage.
This paper is structured as follows: Section II details the system configuration. Section III presents the proposed dual-loop control strategy. Section IV describes the simulation model and parameters. Section V presents and discusses the results, followed by a conclusion and suggestions for future work in Section VI.
II. SYSTEM CONFIGURATION
A clear understanding of the system's architecture is fundamental to appreciating the role of the proposed control strategy. The power stage under investigation consists of a standard AC voltage source feeding a non-linear load through a full-bridge diode rectifier. The proposed SEPIC converter is integrated between the rectifier and the load, serving as the active Power Factor Correction (PFC) stage.
The power circuit comprises a sinusoidal AC source, the diode rectifier, the SEPIC converter—which includes core passive components and an Insulated-Gate Bipolar Transistor (IGBT) as the main switching element—and a resistive DC load.
In its baseline configuration, without active control, the combination of the diode bridge rectifier and its output filter capacitor behaves as a non-linear load. This arrangement draws a distorted, non-sinusoidal current from the AC source. This distorted current is rich in harmonics and is phase-displaced from the source voltage, leading to a low power factor and high reactive power consumption. To mitigate these inherent issues, a sophisticated control strategy is required to actively manage the SEPIC converter, a topic detailed in the subsequent section.
III. PROPOSED DUAL-LOOP CONTROL STRATEGY
The core intellectual contribution of this work is the implementation of a robust dual-loop control strategy. This cascaded architecture is designed to effectively decouple the two primary control objectives: the regulation of the DC output voltage and the shaping of the AC input current. This separation of tasks leads to superior and more stable performance.
Outer Voltage Control Loop
The outer control loop is responsible for maintaining a constant DC voltage across the load, irrespective of operational variations. In this process, the measured output voltage is compared against a set reference voltage of 100 V. The resulting error signal is then processed by a Proportional-Integral (PI) controller. The PI controller processes this error, generating a control signal that is a weighted sum of the instantaneous error (proportional term) and the accumulated past error (integral term), which serves as the magnitude reference for the desired input current.
The mathematical relationship for the PI controller's output, u(t), is given by:
u(t) = Kp e(t) + Ki ∫ e(t) dt
where e(t) is the error signal, Kp is the proportional gain, and Ki is the integral gain.
Inner Current Control Loop
The inner current control loop is tasked with forcing the converter's input current to follow a sinusoidal reference waveform that is perfectly in phase with the input voltage. To achieve this, a unit voltage vector template is first extracted from the AC source voltage. This template, which preserves the phase and shape information of the source, is then multiplied by the current magnitude reference generated by the outer voltage loop. Because the outer loop's PI controller produces a DC-level signal representing the required current magnitude, it must be modulated by a sinusoidal template to create a complete AC reference waveform for the inner loop to follow. The actual input current, measured after the diode rectifier, is compared against this reference. The error is fed into a second PI controller, whose output modulates a Pulse-Width Modulation (PWM) generator. The PWM generator, in turn, produces the precise switching signal for the SEPIC converter's IGBT.
This cascaded control structure effectively compels the SEPIC converter to emulate a resistive load from the perspective of the AC source. By ensuring the input current is sinusoidal and in phase with the voltage, the system successfully achieves the dual goals of power factor correction and harmonic current reduction.
IV. SIMULATION MODEL AND PARAMETERS
To validate the efficacy of the proposed system and control strategy, a comprehensive simulation model was developed within the MATLAB/Simulink environment. Simulation provides a critical platform for analyzing the system's dynamic and steady-state performance before any potential physical implementation, allowing for rigorous verification of the design.
Two distinct scenarios were modeled to facilitate a clear comparative analysis:
• Open-Loop System: An uncontrolled configuration where the SEPIC converter's IGBT is driven by a fixed Pulse-Width Modulation (PWM) signal with a constant duty cycle of 0.3.
• Closed-Loop System: The full system configuration implementing the dual-loop PI control strategy as described in Section III.
The key parameters defining these simulation scenarios are summarized in the table below.
Parameter | Value |
Control Scheme | Open-Loop vs. Closed-Loop |
Open-Loop Duty Cycle | 0.3 |
Closed-Loop Reference Voltage | 100 V |
The performance analysis, particularly the harmonic content of the source current, was conducted using the Fast Fourier Transform (FFT) Analysis tool available within the Simulink environment. The results obtained from these simulation scenarios will now be presented and discussed in detail.
V. RESULTS AND DISCUSSION
This section presents a comparative analysis of the simulation results obtained from both the open-loop (uncontrolled) and closed-loop (controlled) system configurations. The objective is to quantitatively demonstrate the significant performance improvements achieved by the proposed dual-loop control strategy in terms of power factor, harmonic distortion, and output voltage regulation.
Case A: Open-Loop System Performance
In the open-loop scenario, the system's performance was predictably poor. The source current waveform was highly non-sinusoidal and distorted, characteristic of a non-linear load without active correction. Furthermore, a significant phase displacement was observed between the source voltage and the source current, indicating a very poor power factor and high reactive power draw.
Fig. 1. Simulated waveforms for the open-loop system showing (a) source voltage and current, and (b) FFT analysis of the source current revealing a THD of 66.24%.
Quantitative analysis of the harmonic content using the FFT tool revealed a Total Harmonic Distortion (THD) of 66.24%. This level of distortion is grossly non-compliant with international power quality standards and underscores the necessity of an active control solution.
Case B: Closed-Loop System Performance
With the implementation of the proposed dual-loop controller, the system's performance improved dramatically. The source current waveform was transformed into a nearly perfect sinusoid, tracking the shape of the source voltage. Crucially, the source current was brought into phase with the source voltage, demonstrating the achievement of a near-unity power factor. Concurrently, the controller successfully regulated the DC output, maintaining the voltage at the reference value of 100 V, with the corresponding load current remaining stable at approximately 2 A. The most significant improvement was observed in the harmonic content; the THD of the source current was reduced to just 3.72%.
Fig. 2. Simulated waveforms for the closed-loop system showing the in-phase relationship between source voltage and current. Fig. 3. Output voltage and current regulation under the proposed closed-loop control. Fig. 4. FFT analysis of the closed-loop source current, indicating a reduced THD of 3.72%.
Comparative Analysis
The simulation results provide a stark contrast between the uncontrolled and controlled systems. The implementation of the dual-loop control strategy resulted in a profound enhancement of power quality. The source current THD was reduced from a severe 66.24% to a compliant 3.72%, thereby satisfying the <5% limit stipulated by key international standards like IEEE 519. This demonstrates that the controller successfully met its dual objectives: correcting the input power factor to near unity and precisely regulating the DC output voltage. The successful transformation of the converter into an effectively resistive load validates the proposed control approach.
VI. CONCLUSION AND FUTURE SCOPE
This paper has presented the design and simulation-based validation of a dual-loop control strategy for a SEPIC converter aimed at power factor correction and harmonic reduction in AC-DC systems. The study addressed the critical power quality issues arising from non-linear loads, such as diode bridge rectifiers, which inherently draw distorted, high-harmonic currents and exhibit a poor power factor.
The key findings, validated through comprehensive MATLAB/Simulink simulations, conclusively demonstrate the success of the proposed control methodology. The controller effectively reduced the source current's Total Harmonic Distortion (THD) from an unacceptable 66.24% in an open-loop configuration to a compliant 3.72%. Simultaneously, it aligned the phase of the input current with the source voltage to achieve a near-unity power factor and successfully regulated the DC output voltage at its specified reference level. These results confirm that the system meets the stringent requirements of international power quality standards like those from the IEEE.
While the simulation results are highly promising, several avenues for future work exist. The logical next step would be the implementation and validation of the control algorithm on an experimental hardware prototype to verify its real-world performance. Further research could also involve investigating more advanced non-linear control strategies, such as sliding mode control or fuzzy logic control, to potentially enhance the system's dynamic response. Finally, a comprehensive performance analysis of the system under a wide range of varying load and source voltage conditions would provide deeper insights into its robustness and operational limits.
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