Abstract
With the increasing severity of global environmental issues, the exploration and utilization of renewable energy sources have become a crucial development trend. Microgrids integrated with various distributed energy resources (DERs) represent an efficient and flexible approach to energy utilization. Photovoltaic (PV) microgrids, which incorporate PV generation units and battery energy storage systems (BESS), effectively harness solar energy. This study focuses on the control strategies for PV microgrids, specifically examining the mathematical modeling, maximum power point tracking (MPPT), and operational modes under grid-connected and islanded conditions. An adaptive variable step-size perturbation and observation (P&O) method is proposed for MPPT, and its feasibility is verified through simulation. The study also delves into the control strategies for the battery energy storage system and PV units under grid-connected and islanded modes, demonstrating their stability and seamless transition between modes.
Keywords: Photovoltaic power generation, Microgrid, Adaptive variable step-size perturbation observation method, Control strategy, Simulation verification
1. Introduction
1.1 Background and Significance
The development and utilization of energy resources have been pivotal in human history. In recent years, solar and wind energy have emerged as the fastest-growing renewable energy sources. As the cost of power generation continues to decline, they have become the mainstay of new electric power installations. In the context of the “carbon neutrality” goal by 2060, the power industry is transitioning towards lower carbon emissions, necessitating changes in traditional power generation models.
Solar energy, as a renewable and environmentally friendly source, possesses several advantages:
- Universality: Solar resources are available globally, with more than two-thirds of China enjoying ample solar radiation.
- Environmental Friendliness: Solar energy production and utilization do not harm the environment.
- Abundance: Solar energy reserves are vast, with the sun providing enough energy to equal the combustion of 137 trillion tons of coal annually.
- Sustainability: Solar energy is a perpetual resource, ensuring an uninterrupted supply as long as the sun shines.
1.2 Research Status
1.2.1 Photovoltaic Technology
Photovoltaic technology converts solar energy into electricity. In the mid-1950s, the first practical silicon solar cell was developed by Chapin and Pearson at Bell Labs, marking the birth of practical PV technology. Recent advancements in PV technology include:
- Capacity Expansion: Global PV installed capacity has increased significantly, with new installations exceeding 100 GW in 2018 and reaching 121 GW in 2019.
- Efficiency Improvement: Research focuses on enhancing MPPT algorithms and PV inverter control methods to maximize energy harvest.
- Material and Process Innovation: New materials and processes, such as thin-film technology, are improving PV cell efficiency and reducing costs.
1.2.2 Microgrid Research
Microgrids offer flexibility in coordinating DERs, enhancing power supply reliability, and improving energy efficiency. The concept originated in the United States and has since gained traction worldwide, with Europe and Japan actively pursuing microgrid research and development. China, despite a later start, has made significant progress through various experimental platforms and demonstration projects.
1.2.3 Control Strategies
Microgrid control strategies encompass P-Q control, V/f control, and droop control. P-Q control maintains a constant power output, while V/f control regulates voltage and frequency. Droop control simulates the behavior of traditional power systems, adjusting power output based on frequency and voltage deviations.
1.2.4 Energy Storage Systems
Energy storage technologies, especially battery storage, play a crucial role in microgrids by stabilizing voltage and frequency, reducing the impact of intermittent renewable energy sources, and enhancing grid flexibility.
1.3 Research Objectives
This study aims to develop and validate control strategies for a PV microgrid integrated with a battery energy storage system. The objectives include:
- Establishing mathematical models for PV arrays and battery energy storage system.
- Proposing an adaptive variable step-size P&O method for MPPT.
- Developing control strategies for grid-connected and islanded modes.
- Verifying the stability and effectiveness of the proposed strategies through simulations.
2. Photovoltaic Microgrid System and Modeling
2.1 System Composition
The PV microgrid studied comprises three 1.5 MW PV generation units, a 2 MW/2 MWh battery energy storage system, and various loads. The grid-connection point (PCC) allows the microgrid to operate in grid-connected or islanded modes.
2.2 PV Generation System Modeling
2.2.1 PV Cell Characteristics and Modeling
A PV cell’s equivalent circuit (Figure 3) comprises a photo-generated current source, a diode, and series and parallel resistances. The output current (I) can be expressed as:
I=Iph−ID−Ish
where Iph is the photo-generated current, ID is the diode current, and Ish is the shunt current.
The output power (P) is given by:
P=IV
A simulation model in MATLAB/Simulink is developed to analyze PV cell performance under varying irradiance and temperature conditions.
2.2.2 Three-phase Inverter Modeling
The PV system employs a three-phase voltage source inverter (VSI) with an LC filter. The inverter’s mathematical model in the dq reference frame facilitates control design.
2.3 Battery Energy Storage System Modeling
The battery energy storage system consists of lithium-ion batteries connected in series and parallel to achieve the desired voltage and capacity. The battery’s equivalent circuit comprises a voltage source and series resistances for charging and discharging.
The state of charge (SOC) dynamics are governed by:
SOC(t)=textSOC(0)−frac1Qbint0tIb(tau)dtau
where Qb is the battery capacity and Ib is the battery current.
2.4 Maximum Power Point Tracking
2.4.1 Traditional P&O Method
The traditional P&O method perturbs the PV voltage and observes the resulting power change to track the maximum power point. However, it suffers from oscillations and slow tracking speeds under varying conditions.
2.4.2 Adaptive Variable Step-Size P&O Method
This study proposes an adaptive step-size P&O method that adjusts the perturbation step based on the power variation. The step size dk is updated as:
dk=fcdotfracPk−Pk−1Vk−Vk−1
where f is an adjustable constant. Simulations demonstrate the method’s superior tracking performance under dynamic conditions.
3. Grid-Connected Mode Control Strategy
3.1 Master-Slave Control
In grid-connected mode, the microgrid’s voltage and frequency are determined by the main grid. The PV units operate in MPPT mode, while the battery energy storage system regulates its power output based on SOC.
3.2 Control Strategy
The proposed control strategy utilizes a remote controller (SC) and four local controllers (LCs). The SC monitors the PCC status and instructs the LCs. The PV units operate in P-Q control mode, while the battery energy storage system switches between P-Q and V/f modes based on grid connection status.
3.2.1 PV Unit Control
Each PV unit employs MPPT, a DC voltage controller, and a VSI current controller. The MPPT generates a reference DC voltage, which the DC voltage controller uses to regulate the VSI output.
3.2.2 BESS Control
The battery energy storage system controls its power output based on SOC, maintaining a normal SOC range to avoid overcharging or overdischarging. In grid-connected mode, the battery energy storage system operates in P-Q control, adjusting its power output to maintain SOC within desired limits.
3.3 Simulation Analysis
Simulations under varying irradiance and different battery energy storage system initial states verify the control strategy’s stability and effectiveness. The results show stable voltage, frequency, and balanced currents during grid-connected operation.
4. Islanded Mode Control Strategy
4.1 Islanded Microgrid Control
In islanded mode, the battery energy storage system assumes the role of the primary control unit, regulating voltage and frequency through V/f control. The PV units continue operating in P-Q control mode.
4.2 BESS SOC Control
The battery energy storage system SOC is managed based on system load and PV output. When the PV output exceeds the load, the BESS charges; otherwise, it discharges to maintain power balance.
4.3 V/f Control
The battery energy storage system’s VSI employs a voltage-current dual-loop control strategy in the dq reference frame to regulate voltage and frequency precisely.
4.4 Simulation Analysis
Simulations under various islanded mode scenarios, including mode transitions, PV unit disconnections, load shedding, and reconnection to the main grid, demonstrate the control strategy’s stability and seamless mode transitions .
5. Conclusion and Future Work
This study proposes and validates control strategies for a PV microgrid integrated with battery energy storage system. The adaptive variable step-size P&O method enhances MPPT performance. The control strategies ensure stable operation in grid-connected and islanded modes, with seamless transitions between them.
Future work could explore:
- Integrating multiple DERs and storage technologies for enhanced flexibility and reliability.
- Improving islanding detection methods for reliable operation.
- Analyzing control strategy robustness under fault conditions.
- Exploring more efficient SOC management strategies for extended islanded operation.
- Combining conventional generation sources with renewable DERs for hybrid microgrids.
These efforts will further advance the development and deployment of reliable and efficient PV microgrids.