A MATLAB Based Analysis of a Solar PV and Battery-Integrated Unified Power Quality Conditioner (UPQC)
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A MATLAB Based Analysis of a Solar PV and Battery-Integrated Unified Power Quality Conditioner (UPQC)
Abstract
Modern electrical grids face significant power quality challenges, including voltage sags, swells, and current harmonics, which are often exacerbated by the integration of intermittent renewable energy sources like solar photovoltaics (PV). This paper presents a comprehensive analysis of a three-phase Unified Power quality Conditioner (UPQC) integrated with a solar PV array and a Battery Energy Storage System (BESS) to address these issues. The system's control architecture employs four distinct strategies: a Perturb and Observe (P&O) algorithm for Maximum Power Point Tracking (MPPT) of the PV array; a PI-based controller for regulating the common DC link voltage via the BESS; the Instantaneous Power (p-q) Theory for controlling the shunt active filter; and the Synchronous Reference Frame (d-q-0) Theory for the series active filter. MATLAB/Simulink simulations validate the system's performance, demonstrating its ability to maintain a sinusoidal grid current in the presence of a nonlinear load and regulate the load voltage to a constant 1.0 per unit during severe grid voltage sag (0.5 p.u.) and swell (1.2 p.u.) events. The results confirm that the proposed PV-BESS integrated UPQC provides a robust and comprehensive solution for enhancing power quality and stability in modern distribution networks with high renewable penetration.
Keywords
Unified Power Quality Conditioner (UPQC), Solar Photovoltaics (PV), Battery Energy Storage System (BESS), Power Quality, Harmonic Compensation, Voltage Sag/Swell
I. Introduction
The escalating integration of renewable energy sources, particularly solar photovoltaics (PV), into modern electrical grids is critical for sustainable energy goals. However, the inherent intermittency of these sources introduces significant power quality challenges, including voltage fluctuations, frequency deviations, and harmonic distortion. These disturbances can compromise the stability of the grid and the reliability of power supplied to sensitive end-user equipment. To address these complex issues, advanced power electronic solutions are required.
The Unified Power Quality Conditioner (UPQC) stands out as a versatile and highly effective custom power device. It is uniquely capable of simultaneously mitigating both voltage-based disturbances originating from the grid and current-based distortions originating from the load. A standard UPQC consists of a series active power filter (APF) and a shunt APF coupled through a common DC link, allowing for comprehensive power quality management.
The core contribution of this work is the detailed modeling and dynamic performance analysis of an integrated PV-BESS-UPQC topology. This integrated system transcends traditional power quality conditioning by incorporating renewable energy generation and storage. This architecture not only resolves power quality issues but also facilitates the stable injection of clean energy into the grid, with the BESS providing crucial buffering to manage the variability of solar power and maintain system stability.
The primary objective of this paper is to develop and validate a comprehensive MATLAB/Simulink model of the PV-BESS integrated UPQC. Through detailed simulation, this study analyzes the system's dynamic performance under a range of challenging operating conditions, including severe grid voltage sags and swells, as well as the presence of a harmonic-producing nonlinear load.
This paper is structured as follows: Section II details the proposed system configuration. Section III explains the multi-faceted control strategy governing the system's components. Section IV presents the simulation model and its key parameters. Section V provides a thorough discussion of the simulation results. Finally, Section VI offers concluding remarks and suggests directions for future research.
II. Proposed System Configuration
The strategic importance of the system's architecture lies in its ability to merge renewable generation, energy storage, and power quality conditioning into a single, cohesive unit. This topology provides a multi-functional interface between the grid, the renewable source, and the load, enabling it to perform several critical tasks simultaneously. The proposed system is composed of a grid source, the integrated PV-BESS subsystem, the UPQC, and a nonlinear load. Each component is designed to work in concert to ensure high power quality and reliable energy delivery.
A. Solar PV Subsystem
The solar PV subsystem serves as the primary renewable generation unit. The PV array is configured with 18 series-connected strings and 28 parallel-connected strings, delivering a maximum power output of approximately 107.4 kW at an operating voltage of around 522 V. To efficiently transfer this power, the PV array is interfaced with the system's common DC link through a dedicated DC-DC boost converter. This converter is essential for stepping up the array's variable DC voltage to the required level of the DC link and is actively controlled to perform Maximum Power Point Tracking (MPPT), ensuring maximum energy harvest under all irradiation conditions.
B. Battery Energy Storage Subsystem (BESS)
The BESS provides vital energy buffering and DC link stability. It is configured with 20 series-connected 24 V batteries, resulting in a nominal capacity of 48 Ah. The BESS is connected to the common DC link via a bidirectional DC-DC converter. This crucial component allows the battery to either absorb surplus energy from the PV array (charging) or supply stored energy to the DC link (discharging) when PV generation is insufficient. This dynamic charge-discharge capability is the primary mechanism for regulating the DC link voltage.
C. Unified Power Quality Conditioner (UPQC)
The UPQC is the core power conditioning unit, comprising a shunt active power filter and a series active power filter. These two filters are coupled through a common DC link, which is maintained at a reference voltage of 700 V by the coordinated action of the PV and BESS subsystems.
Shunt Active Power Filter
The shunt APF is connected in parallel with the load. Its primary function is to compensate for undesirable current-based distortions, such as harmonics and reactive power, generated by the load. By injecting a precisely controlled compensating current, the shunt APF ensures that the current drawn from the grid remains sinusoidal and in phase with the grid voltage.
Series Active Power Filter
The series APF is connected in series with the grid through coupling transformers. Its role is to protect the load from grid-side voltage disturbances. During events such as voltage sags or swells, the series APF injects a compensating voltage to maintain the load voltage at its nominal, rated value, thereby isolating the load from upstream grid voltage fluctuations.
The sophisticated operation of these interconnected components is governed by a hierarchical control strategy, which is detailed in the following section.
III. Control Strategy
The effective coordination of the PV array, BESS, and UPQC filters requires a multi-faceted control system. This strategy is designed to achieve three primary, simultaneous goals: to maximize the harvest of renewable energy from the PV array, to maintain the stability of the common DC link voltage, and to mitigate both voltage- and current-based power quality disturbances.
A. MPPT Control for Solar PV
To maximize the power extracted from the solar PV array under varying solar irradiation levels, a Perturb and Observe (P&O) MPPT algorithm is implemented. This algorithm operates by periodically introducing a small perturbation to the duty cycle of the PV boost converter. It then observes the resulting change in the PV array's output power. Based on this observation, the algorithm adjusts the duty cycle to continuously track and operate at the maximum power point of the array.
B. DC Link Voltage and BESS Control
A stable DC link voltage is paramount for the proper operation of both the series and shunt APFs. The control loop for the BESS is designed to maintain this voltage at its reference value of 700 V. A Proportional-Integral (PI) controller continuously monitors the DC link voltage and compares it to the 700 V reference. The resulting error signal is processed to generate a reference current for the BESS. A secondary PI controller then regulates the actual battery current to match this reference by controlling the duty cycle of the bidirectional DC-DC converter. This action facilitates the appropriate charging or discharging of the battery to absorb surplus power or cover deficits, thereby ensuring a stable DC link.
C. Shunt Active Power Filter Control
The control objective for the shunt APF is to generate a compensating current that precisely cancels the harmonic and reactive components of the load current. This is achieved using a control scheme based on the Instantaneous Power (p-q) Theory. The key steps of this algorithm are as follows:
1. The three-phase grid voltages and load currents are measured.
2. These quantities are transformed from the a-b-c frame into the α-β-0 stationary reference frame.
3. The instantaneous real (p) and reactive (q) powers are calculated from the transformed voltages and currents.
4. Reference compensation currents in the α-β-0 frame are generated based on the harmonic and reactive components of the calculated powers. This calculation incorporates a power loss component derived from a separate DC link PI controller. This component ensures that the shunt APF draws the necessary active power from the grid to regulate the DC link voltage and compensate for its own switching losses, in addition to its primary harmonic compensation duties.
5. An inverse transformation is applied to convert these reference currents back to the a-b-c frame.
Finally, a Hysteresis current controller compares these final a-b-c reference currents with the actual measured converter currents to generate the precise switching pulses for the shunt inverter's power switches.
D. Series Active Power Filter Control
The series APF control is designed to maintain a constant, rated voltage at the load terminals, effectively shielding the load from grid voltage sags and swells. This is accomplished using a control strategy based on the Synchronous Reference Frame (d-q-0) Theory. The operational steps are:
1. The three-phase source and load voltages are measured.
2. These voltage measurements are transformed into the d-q-0 rotating reference frame, which is synchronized with the grid voltage.
3. The measured d-q voltage components are compared against a reference frame where the direct-axis component (Vdref) is set to 1.0 p.u. and the quadrature-axis component (Vqref) is set to zero, representing a perfectly balanced, sinusoidal nominal voltage.
4. The resulting voltage error is processed by a PI controller, which determines the required compensating voltage in the d-q-0 frame.
5. This compensating voltage is then transformed back to the a-b-c frame using an inverse d-q-0 transformation.
These a-b-c reference signals modulate the switching signals for the voltage source inverter via Pulse Width Modulation (PWM). The inverter then injects the required voltage in series with the line to perfectly compensate for any deviation in the grid voltage. The successful implementation and validation of these control strategies are demonstrated in the following simulation study.
IV. Simulation Model and Parameters
The performance of the proposed PV-BESS integrated UPQC system was validated through comprehensive simulations conducted in the MATLAB/Simulink environment.
The model was built using detailed representations of the power electronic converters, the PV array, the BESS, and the control algorithms. The key parameters used in the simulation are detailed in Table 1.
Parameter | Value |
Grid Voltage (L-L) | 450 V |
Nominal Frequency | 50 Hz |
PV Array Maximum Power | 107.4 kW |
PV Array Series Strings | 18 |
PV Array Parallel Strings | 28 |
DC Link Reference Voltage | 700 V |
Battery Bank Configuration | 20 series x 24 V |
Battery Capacity | 48 Ah |
Boost Converter Inductor (L) | 3 mH |
Boost Converter Capacitor (C) | 6600 μF |
Bidirectional Converter Inductor (L) | 0.164 H |
Bidirectional Converter Capacitor (C) | 48 μF |
To rigorously test the system's dynamic response, a series of challenging operational scenarios were simulated. These conditions were designed to evaluate the system's core functionalities:
• Nonlinear Load Operation: A diode bridge rectifier with a resistive (R) load was used to generate significant current harmonics, testing the compensation capability of the shunt APF.
• Voltage Swell Event: A programmed voltage swell was introduced, increasing the grid voltage to 1.2 per unit (p.u.) to test the series APF's voltage reduction capability.
• Voltage Sag Event: A programmed voltage sag was introduced, reducing the grid voltage to 0.5 p.u. to test the series APF's voltage boosting capability.
The setup of these simulations provides the basis for a detailed analysis of the system's performance, as presented in the following section.
V. Results and Discussion
This section evaluates the effectiveness of the integrated UPQC system in achieving its primary power quality objectives under the simulated disturbance conditions. The results demonstrate the successful coordination of the series APF, shunt APF, and the PV-BESS subsystem.
A. Performance of Series APF: Voltage Sag and Swell Mitigation
The system's response to grid voltage disturbances highlights the efficacy of the series APF. During the simulated voltage swell event (1.2 p.u.), the rapid response of the series APF, governed by the SRF-based controller, demonstrates the algorithm's efficacy in dynamically calculating and injecting the precise compensating voltage required for stabilization. The controller injected a voltage in anti-phase with the grid, effectively subtracting the excess voltage and maintaining the load-side voltage at the nominal 1.0 p.u. level.
Conversely, during the severe voltage sag event (0.5 p.u.), the series APF injected a voltage in-phase with the grid, boosting the line voltage to ensure the load terminals remained at a constant 1.0 p.u. The precise in-phase and anti-phase voltage injection during sag and swell, respectively, validates the performance of the Synchronous Reference Frame (d-q-0) control strategy in accurately tracking voltage errors and generating the required compensating signals. These results confirm that the series filter effectively isolates the load from significant grid voltage fluctuations.
B. Performance of Shunt APF: Current Harmonic Compensation
The performance of the shunt APF was evaluated under the presence of a nonlinear rectifier load, which drew a highly distorted, non-sinusoidal current. In stark contrast, the grid-side current remained clean, balanced, and perfectly sinusoidal.
This result is a direct consequence of the shunt APF's operation, which is governed by the Instantaneous Power (p-q) Theory. The filter actively calculated and injected the precise harmonic currents required by the nonlinear load. By sourcing this harmonic content locally, the shunt APF ensures that only the fundamental, active power component of the current is drawn from the utility grid, thereby preventing the pollution of the grid with load-induced harmonics.
C. PV-BESS Performance and DC Link Regulation
The simulation demonstrated the robust performance of the renewable energy subsystem in maintaining DC link stability. In a scenario where simulated solar irradiation was reduced, the power output from the PV array consequently decreased. The seamless transition of the BESS from charging to a reduced-charging or discharging state is a direct result of the PI-based DC link voltage controller. This controller effectively adjusts the battery current reference in response to the power deficit from the PV array, thus maintaining DC bus stability at the reference 700 V and showcasing the successful coordination of all subsystems for uninterrupted operation.
VI. Conclusion and Future Scope
This paper presented a detailed MATLAB/Simulink-based analysis of a solar PV and battery-integrated UPQC system. The simulation results conclusively demonstrate that the proposed integrated topology is a highly effective solution for the concurrent improvement of power quality and the stable integration of renewable energy into the grid.
The study confirms the system's distinct capabilities. The series APF, governed by the SRF control strategy, provides excellent protection for sensitive loads against severe grid voltage sags and swells. The shunt APF, operating on p-q theory, successfully compensates for load current harmonics, ensuring a sinusoidal grid current and compliance with utility standards. Furthermore, the coordinated PI control of the BESS ensures robust regulation of the DC link voltage, which is critical for the proper functioning of the entire system, even under fluctuating solar irradiation. In summary, the integrated system presents a viable and robust topology for enhancing the reliability, stability, and quality of power in modern distribution networks.
For future research, several avenues could be explored to build upon this work. Efforts could focus on the implementation and comparison of advanced non-linear control strategies to potentially enhance the system's dynamic response and efficiency. Conducting hardware-in-the-loop (HIL) validation would be a valuable step to de-risk and verify the control algorithms before physical prototyping. Finally, a comprehensive techno-economic analysis could be performed to evaluate the system's lifecycle costs and benefits, providing crucial insights for its practical deployment in real-world applications.
VII. YouTube Video
VIII. Purchase link of the Model
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