Grid-Connected PV System with Five-Level Multilevel Inverter

👋 Introduction

MATLAB/Simulink implementation of a grid-connected photovoltaic (PV) system using a five-level multilevel inverter. The system employs a two-stage power conversion architecture to efficiently extract maximum power from the PV array and inject high-quality AC power into the grid with reduced harmonics and unity power factor operation.

🧩 Overall System Configuration

The complete system consists of the following major blocks:

• ☀️ PV Array

• ⬆️ DC–DC Boost Converter

• 🧠 P&O MPPT Controller

• 🔋 DC-Link (400 V)

• 🔄 Five-Level Multilevel Inverter

• 🎚️ Voltage and Current Control (PI + PR)

• 🔌 LCL Filter

• 🌐 Single-Phase Grid (230 V, 50 Hz)

The PV power is converted into AC and injected into the grid through a controlled multilevel inverter.

☀️ PV Array Modeling

The PV system is designed using six series-connected solar panels.

🔹 Single PV module rating

• ⚡ Power: ≈ 349.59 W (≈ 350 W)

• 🔋 Voltage: 43 V

• 🔌 Current: 8.13 A

🔹 PV array output

• 🌞 At 1000 W/m²: ≈ 2.1 kW

• 🌤️ At 500 W/m²: ≈ 1.1 kW

• ☁️ At 100 W/m²: ≈ 196 W

📈 The I–V and P–V characteristics clearly show nonlinear behavior, with the peak power point shifting based on irradiance, making MPPT essential.

🎯 Need for MPPT

Because PV characteristics are nonlinear and dependent on environmental conditions:

• ❌ Direct grid connection cannot extract maximum power

• ✔️ An MPPT algorithm is required to operate the PV array at its optimal point

In this model, a Perturb and Observe (P&O) MPPT algorithm is used due to its simplicity and effectiveness.

🧠 P&O MPPT Algorithm

The P&O MPPT block performs the following steps:

• 📥 Measures PV voltage and current

• 🧮 Computes instantaneous power (P = V × I)

• 🔄 Calculates ΔP and ΔV between two sampling instants

• 🔀 Updates the reference voltage based on four P&O rules

• 🔒 Enforces minimum and maximum voltage limits

• ♻️ Continuously updates previous values

📌 The output of MPPT is a reference PV voltage (Vref) that ensures maximum power extraction under all irradiance conditions.

⬆️ Boost Converter (First Stage Conversion)

The PV voltage is approximately 258 V, which is insufficient for grid connection.

🔹 Grid requirement:

• 🔌 Single-phase AC: 230 V RMS

• ⚡ Peak voltage: 325 V

• 🔋 Required DC-link voltage: ≈ 400 V

✔️ A DC–DC boost converter is used to:

• Increase PV voltage from 258 V to 400 V

• Track the maximum power point using MPPT

⚙️ Control strategy

• Vref (from MPPT) compared with PV voltage

• Error processed through a PI controller

• Duty cycle generated via PWM

• IGBT switching controls the boost converter

This completes the first stage of power conversion.

🔋 DC-Link Voltage Regulation

The DC-link voltage is maintained at 400 V, providing a stable input to the inverter stage.Stable DC-link voltage ensures:

• ✔️ Proper inverter operation

• ✔️ Constant AC output quality

• ✔️ Reliable grid synchronization

🔄 Five-Level Multilevel Inverter (Second Stage Conversion)

The second stage uses a five-level multilevel inverter with a reduced number of switches.

🔹 Key features

• ⚙️ Only five IGBT switches

• 🔁 Bidirectional switching capability

• 🔋 DC input converted into five voltage levels:

  • +Vdc

  • +Vdc/2

  • 0

  • −Vdc/2

  • −Vdc

✔️ This topology reduces:

• Switching losses

• Output harmonic distortion

• Filter size

🎛️ Voltage and Current Control Strategy

🔹 Voltage Control (Outer Loop)

• DC-link voltage compared with 400 V reference

• Error processed via PI controller

• Output represents current magnitude reference

🔹 Grid Synchronization

• Grid voltage measured

• Processed through PLL

• Generates ωt

• Used to produce cos(ωt) and sin(ωt)

🔹 Current Control (Inner Loop)

• Reference current generated by multiplying PI output with cos(ωt)

• Actual inverter current compared with reference

• Error processed using PR controller

• PR controller ensures:

  • Zero steady-state error

  • Accurate sinusoidal current tracking

The controller output is normalized with Vdc to obtain the modulation signal.

🔁 Switching Pulse Generation

The modulation signal is mapped into five discrete voltage levels:

• 🔼 Two positive levels

• ⚪ One zero level

• 🔽 Two negative levels

Based on this logic, switching pulses for S1–S5 are generated and applied to the multilevel inverter.

🔌 Grid Connection and LCL Filter

An LCL filter is used at the inverter output to:

• 📉 Reduce current harmonics

• 🔊 Smooth inverter voltage waveform

• 📏 Meet IEEE power quality standards

Filtered output is then connected to the single-phase grid.

🌦️ Test Conditions (Irradiance Profile)

The system is tested under dynamic irradiance conditions:

• ⏱️ 0 – 0.5 s: 1000 W/m²

• ⏱️ 0.5 – 2 s: 100 → 1000 W/m²

• ⏱️ 2 – 3.5 s: 1000 W/m²

These variations validate the robustness of MPPT and inverter control.

📊 Simulation Results

☀️ PV Side

• PV voltage ≈ 258 V

• PV current ≈ 8 A

• PV power ≈ 2.1 kW at 1000 W/m²

🔋 DC-Link

• Voltage maintained around 400 V under all conditions

🔄 Inverter Output

• Clear five-level voltage waveform

• Levels: +Vdc, +Vdc/2, 0, −Vdc/2, −Vdc

🌐 Grid Side

• Grid voltage and current are in phase

• ✔️ Unity power factor operation

• ✔️ Active power injected into the grid

📉 THD Analysis

FFT analysis of grid current shows:

• 📊 THD ≈ 3.01%

• ✔️ Within IEEE power quality limits

This confirms the advantage of the five-level inverter topology.

⭐ Key Advantages of the Proposed System

• ⚡ Improved power quality

• 📉 Reduced harmonic distortion

• 🔋 Efficient DC-link utilization

• 🔁 Robust MPPT under irradiance changes

• 🌐 Unity power factor grid injection

🏁 Conclusion

This blog presented a comprehensive explanation of a grid-connected PV system using a five-level multilevel inverter implemented in MATLAB/Simulink. By combining P&O MPPT, boost converter voltage regulation, and PI–PR-based inverter control, the system achieves efficient power extraction, high-quality grid injection, and low harmonic distortion under varying irradiance conditions.