A Control Strategy for Three-Phase PV-Grid Inverter with Active Filtering under Various Load Conditions

Abstract

A unified dq-frame-based control strategy for a three-phase grid-connected photovoltaic (PV) inverter that serves as a flexible power-conditioning device is presented in this study. The proposed technique enables a multifunctional inverter design to simultaneously inject active power, reduce harmonic current, and compensate for reactive power. This differs from earlier systems that required multi-conversion structures or independent active power filters. The method is governed by the synchronous reference frame. It uses the DC link\'s instantaneous power balance to regulate the flow of bidirectional active power when there is either too much or too little PV power. The load current is divided into its basic and oscillating components using the dq-frame instantaneous power representation. Because of this, it can handle loads that are not straight lines or that combine curves and straight lines. Only the reactive and harmonic currents fluctuate, and the grid interface\'s unity power factor. The proposed approach has been tested under a wide range of conditions, including nonlinear, capacitive, and inductive loading. The findings demonstrate that regardless of how the system is configured, the grid current remains sinusoidal and in sync with the grid voltage. Additionally, the inverter side has very little harmonic distortion. These findings demonstrate that adding features that directly enhance the power quality of grid-connected PV inverters is simple and efficient with the suggested control framework.

Introduction

The increasing global transition toward low-carbon energy systems has accelerated the deployment of grid-connected photovoltaic (PV) generation, particularly in regions with high solar potential such as Indonesia. While PV systems enhance energy security and reduce fossil fuel dependence, their large-scale integration into distribution networks is constrained by power quality requirements, especially related to reactive power support and harmonic distortion. Conventional PV inverters primarily inject active power and offer limited reactive power compensation, while nonlinear loads introduce significant current harmonics that degrade grid performance.

Modern grid codes therefore require PV inverters to operate as multifunctional devices, capable of active power injection, reactive power compensation, and harmonic mitigation. Although synchronous reference frame (SRF) control with PI regulators is widely used due to its simplicity and industrial compatibility, it struggles to accurately compensate harmonics under nonlinear and time-varying load conditions. Advanced control methods such as resonant control and model predictive control improve performance but are computationally complex and unsuitable for low-cost PV systems. Additionally, multi-stage inverter architectures increase system complexity, losses, and cost.

To address these challenges, the study proposes a single-stage three-phase PV-grid inverter operating as a controlled current source with an optimized dq-frame control strategy. The proposed approach integrates instantaneous power theory, current decomposition, and SRF transformation to precisely separate fundamental active currents from harmonic and reactive components. This allows the inverter to ensure that the grid supplies only fundamental active power at unity power factor, while the inverter locally compensates all harmonic and reactive currents.

The control scheme maintains DC-link power balance and enables seamless bidirectional power flow without mode switching, supporting both power deficit (PV power less than load demand) and power surplus (excess PV generation) scenarios. Synchronization with the grid is achieved using an SRF phase-locked loop, ensuring stable operation under distorted and unbalanced conditions.

Simulation results under nonlinear loads combined with inductive and capacitive linear loads demonstrate that the proposed method effectively maintains sinusoidal grid currents, unity power factor, harmonic mitigation, and robust dynamic performance. Overall, the study confirms that the proposed dq-based multifunctional control framework improves power quality, enhances inverter efficiency, and satisfies modern grid code requirements without additional filtering hardware.

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Conclusion

This study introduces a dq-frame-based unified control framework that turns a three-phase PV-Grid inverter into a multifunctional power conditioner. The proposed method allows seamless bidirectional active power flow using DC-link instantaneous power equilibrium and systematic dq-frame current decomposition. It reduces harmonic currents and compensates for reactive power in nonlinear or mixed loads. A single conversion stage with analytical current synthesis improves power quality. Traditional systems require active filtering hardware or complex predictive control schemes. The simulation shows that the inverter separates harmonic and reactive utility grid components well. Even with excessive or insufficient PV power, grid currents are sinusoidal and unity power factor. Thus, the suggested method can improve PV grid electricity quality in the future when distribution networks have many nonlinear loads.

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Copyright

Copyright © 2026 Julius Christopher Purnama, Slamet Riyadi. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.