In recent years, the rapid growth of photovoltaic (PV) power generation has driven significant advancements in inverter technology. As a researcher in power electronics, I have focused on developing efficient and scalable solutions for large-scale solar farms. Traditional centralized and string-type solar inverters face challenges in scaling up voltage and power levels due to limitations in semiconductor power devices. Moreover, the need for bulky line-frequency step-up transformers introduces additional losses and inefficiencies. To address these issues, I propose a modular multilevel topology based on the three-phase common DC-bus cascaded H-bridge (CHB) solar inverter. This configuration eliminates the need for transformers, enabling direct grid connection at medium voltage with high efficiency and reduced harmonic distortion. In this article, I will detail a novel control strategy that ensures maximum power point tracking (MPPT), grid current control, DC-link voltage regulation, and balanced power sharing among modules. The approach is designed to simplify communication and enhance modularity, making it ideal for solar inverter applications exceeding 20 MW per unit.
The solar inverter topology under consideration consists of two main stages: front-end Boost converters and rear-end three-phase isolated common DC-bus CHB inverters. Each phase of the CHB inverter includes multiple modules, where each module comprises a three-level LLC resonant converter and four H-bridge converters. The outputs of these modules are connected in series via filtering inductors to the grid. This structure leverages the benefits of multilevel output, such as low switching frequency, minimal harmonic content, and compact filter design. For instance, in a system designed for 1.5 kV PV arrays, three-level Boost and LLC converters are employed to utilize mainstream 1.2 kV switching devices effectively. The common DC-bus configuration allows for centralized voltage regulation while distributing power across modules, which is crucial for large-scale solar inverter deployments.
The control strategy for this solar inverter system is divided into four interconnected controllers: a main controller, H-bridge controllers, LLC controllers, and Boost controllers. This modular design facilitates scalability and simplifies implementation. The main controller oversees global operations, including grid synchronization and current regulation. It samples three-phase grid voltages ($u_{ga}$, $u_{gb}$, $u_{gc}$) and currents ($i_{ga}$, $i_{gb}$, $i_{gc}$), then uses a phase-locked loop (PLL) to obtain the grid phase angle $\theta_g$. Through $abc/dq$ transformation, the $d$-axis and $q$-axis components of grid voltage ($u_{gd}$, $u_{gq}$) and current ($i_{gd}$, $i_{gq}$) are derived. A proportional-integral (PI) regulator controls $u_{gd}$ to a reference value, producing the active current reference $i_d^*$. Subsequently, PI regulators adjust $i_{gd}$ and $i_{gq}$ to generate three-phase modulation voltages ($u_{ma}$, $u_{mb}$, $u_{mc}$), which are communicated to the H-bridge controllers. This hierarchical approach ensures precise grid current control while minimizing communication overhead, a key advantage for solar inverter systems.
The H-bridge controllers handle local modulation and power balancing. Each H-bridge controller samples the DC-link voltages of its four H-bridges ($u_{dc1}$, $u_{dc2}$, $u_{dc3}$, $u_{dc4}$) and receives modulation voltage commands from the main controller. To achieve power balance, the modulation voltage for each phase is divided by the number of H-bridges per phase, ensuring equal voltage sharing. The modulation wave for each H-bridge is computed as:
$$m_{xij} = \frac{u_{mx}}{n \cdot u_{dcj}}$$
where $m_{xij}$ is the modulation wave for the $j$-th H-bridge in the $i$-th module of phase $x$, $u_{mx}$ is the phase modulation voltage, $n$ is the number of modules per phase, and $u_{dcj}$ is the DC-link voltage of the $j$-th H-bridge. Carrier phase-shifted sinusoidal pulse-width modulation (PWM) is then applied to generate switching signals. This method ensures that all H-bridges in a phase transmit identical active power, as they share the same grid current. The table below summarizes the key control parameters for the H-bridge controllers in a typical solar inverter setup.
| Parameter | Symbol | Value | Description |
|---|---|---|---|
| Switching Frequency | $f_{sw}$ | 500 Hz | PWM frequency for H-bridges |
| DC-Link Voltage Reference | $u_{dc}^*$ | 170 V | Target voltage for each H-bridge |
| Number of H-Bridges per Phase | $N$ | 16 | Based on 4 modules with 4 H-bridges each |
| Modulation Index Range | $m$ | 0 to 1 | Normalized modulation depth |
The LLC controllers manage the three-level LLC resonant converters, which interface the common DC-bus with the H-bridge DC-links. Each LLC controller samples the DC-link voltages of its four H-bridges and the common DC-bus voltage ($u_{dcT}$). It computes the average DC-link voltage:
$$\bar{u}_{dc} = \frac{1}{4} \sum_{j=1}^{4} u_{dcj}$$
and uses a PI regulator to control $\bar{u}_{dc}$ to $u_{dcT} / n_T$, where $n_T$ is the turns ratio of the LLC transformer. The output of the PI regulator sets the switching frequency $f_r$ of the LLC converter. Variable frequency modulation is employed to regulate power flow, with the resonant frequency $f_0$ given by:
$$f_0 = \frac{1}{2\pi \sqrt{L_r C_r}}$$
where $L_r$ is the resonant inductance and $C_r$ is the resonant capacitance. This control ensures stable DC-link voltages across all modules, critical for the reliable operation of the solar inverter. The LLC converter’s design parameters are optimized for efficiency, as shown in the following table, which highlights typical values for a 1.5 kV solar inverter system.
| Component | Symbol | Value | Role in Solar Inverter |
|---|---|---|---|
| Resonant Inductance | $L_r$ | 180 μH | Part of LLC resonant tank |
| Resonant Capacitance | $C_r$ | 56 nF | Part of LLC resonant tank |
| Magnetizing Inductance | $L_m$ | 2 mH | Provides transformer excitation |
| Transformer Turns Ratio | $n_T$ | 2:1:1:1:1 | Steps down voltage for H-bridges |
| Rated Switching Frequency | $f_{r,rated}$ | 50 kHz | Base frequency for LLC operation |
The Boost controllers are responsible for MPPT from the PV arrays. Each Boost controller samples the PV output voltage ($u_{pv}$), current ($i_{pv}$), flying capacitor voltage ($u_{cf}$), and Boost output voltage ($u_{dcT}$). An MPPT algorithm, such as perturb-and-observe, computes the reference PV voltage $u_{pv}^*$ to maximize power extraction. A PI regulator adjusts $u_{pv}$ to $u_{pv}^*$, producing the duty cycle $D$ for the Boost converter. Additionally, a separate PI regulator controls $u_{cf}$ to half of $u_{dcT}$, ensuring balanced voltage across the three-level Boost topology. The duty cycles for the switching devices are derived as:
$$D_1 = D + \Delta D, \quad D_2 = D – \Delta D$$
where $\Delta D$ is the correction from the flying capacitor voltage control. This approach enhances the efficiency and reliability of the solar inverter by optimizing PV power harvest. The MPPT performance can be quantified by the power conversion efficiency $\eta$, defined as:
$$\eta = \frac{P_{grid}}{P_{pv}} \times 100\%$$
where $P_{grid}$ is the power delivered to the grid and $P_{pv}$ is the maximum power from the PV array. In typical solar inverter applications, $\eta$ exceeds 98% under optimal conditions.
To validate the control strategy, I constructed an experimental platform with one Boost converter and a three-phase CHB solar inverter featuring four modules per phase (totaling 16 H-bridges per phase). The system parameters were set as follows: grid voltage amplitude of 155.56 V at 50 Hz, filter inductance of 1.5 mH, CHB switching frequency of 500 Hz, DC-link capacitance of 18.8 mF, and DC-link voltage reference of 170 V. The LLC converters operated with a resonant frequency of approximately 50 kHz. The solar inverter was tested under both unity power factor and non-unity power factor conditions. In unity power factor operation, the grid currents were balanced and sinusoidal, with total active power measured as $P_a = 1486.58$ W, $P_b = 1487.39$ W, and $P_c = 1485.19$ W. The power per H-bridge was nearly equal, e.g., $P_{a11} = 93.188$ W, $P_{b11} = 92.564$ W, and $P_{c11} = 92.850$ W, demonstrating effective power balancing. For non-unity power factor, with a 30° phase delay between grid current and voltage, the H-bridge powers remained balanced at $P_{a11} = 92.608$ W, $P_{b11} = 93.124$ W, and $P_{c11} = 92.903$ W. These results confirm that the control strategy achieves precise grid current control, voltage regulation, and module power balance, essential for large-scale solar inverter systems.
The experimental data further illustrates the multilevel output voltage of the CHB solar inverter, which exhibited 21-level stepped waveforms per phase. This characteristic reduces harmonic distortion and minimizes filter requirements. The total harmonic distortion (THD) of the grid current was measured below 3%, meeting grid standards. The control system’s communication was simplified, as only three modulation voltage commands needed transmission from the main controller to the H-bridge controllers, reducing latency and improving reliability. This modular design is scalable, allowing for easy expansion by adding more modules or phases, which is advantageous for solar inverter installations in utility-scale PV plants.
In conclusion, the proposed control strategy for common DC-bus cascaded H-bridge solar inverters offers a robust solution for high-power PV integration. By decoupling control functions into modular controllers, the system achieves MPPT, grid current regulation, DC-link voltage stability, and power balancing with minimal communication overhead. The experimental validation underscores its effectiveness in real-world conditions, highlighting benefits such as high efficiency, low harmonic distortion, and scalability. Future work could focus on enhancing fault tolerance and integrating energy storage, further advancing solar inverter technology for renewable energy systems. This research contributes to the ongoing evolution of solar inverters, enabling more efficient and reliable green power generation.
To further elaborate on the mathematical foundations, consider the dynamics of the grid-connected solar inverter. The grid current in the $dq$ reference frame can be modeled as:
$$L \frac{di_{gd}}{dt} = u_{md} – u_{gd} + \omega L i_{gq}$$
$$L \frac{di_{gq}}{dt} = u_{mq} – u_{gq} – \omega L i_{gd}$$
where $L$ is the filter inductance, $\omega$ is the grid angular frequency, and $u_{md}$, $u_{mq}$ are the modulation voltages in the $dq$ frame. The PI regulators for current control are designed with transfer functions:
$$G_{PI}(s) = K_p + \frac{K_i}{s}$$
where $K_p$ and $K_i$ are proportional and integral gains, tuned for optimal response. For the DC-link voltage control, the power balance equation is:
$$C \frac{du_{dc}}{dt} = i_{pv} – i_{inv}$$
where $C$ is the DC-link capacitance, $i_{pv}$ is the PV current, and $i_{inv}$ is the inverter input current. The control strategy ensures that $u_{dc}$ tracks its reference via the LLC and Boost controllers. This comprehensive approach underscores the sophistication of modern solar inverter systems, which are pivotal for harnessing solar energy efficiently.