1. Introduction
The rapid development of renewable energy technologies, particularly photovoltaic (PV) systems, has positioned solar energy as a cornerstone of modern power systems. With global initiatives like “carbon neutrality” driving the transition toward sustainable energy, household energy storage inverters have emerged as critical components for integrating PV generation, battery storage, and grid interaction. This paper focuses on the control strategies of T-type three-level single-phase three-wire energy storage inverters, addressing key challenges such as maximum power point tracking (MPPT), grid-connected/off-grid mode transitions, and energy management.
2. Modeling and Control of DC-Side Components
2.1 PV Cell Characteristics and MPPT
The output power of PV cells depends on irradiance and temperature. The I-V and P-V curves exhibit nonlinear behavior, with a distinct maximum power point (MPP). The single-diode model describes PV cell dynamics: [ I = I{ph} – I_0 \left[ \exp\left(\frac{q(V + I R_s)}{n k T}\right) – 1 \right] – \frac{V + I R_s}{R{sh}} ] where (I{ph}) is the photocurrent, (I_0) is the reverse saturation current, and (R_s/R{sh}) are series/shunt resistances.
To track the MPP, a variable-step perturb-and-observe (P&O) algorithm is implemented. Compared to fixed-step methods, this approach balances convergence speed and accuracy: [ \Delta D = \begin{cases} k \cdot \left| \frac{\Delta P}{\Delta V} \right| & \text{if } \Delta P \neq 0 \ 0 & \text{otherwise} \end{cases} ] Here, (D) is the duty cycle, and (k) is an adaptive coefficient.
2.2 Boost Converter Modeling
The PV-side boost converter elevates the DC voltage to match the inverter’s input requirements. Its state-space averaged model is: [ \begin{cases} \frac{di{L}}{dt} = \frac{V{pv} – (1 – D)V{bus}}{L} \ \frac{dV{bus}}{dt} = \frac{(1 – D)i_L – i_{inv}}{C} \end{cases} ] A dual-loop control structure (voltage outer loop + current inner loop) ensures stable operation under varying irradiance.
2.3 DC-Link Voltage Balancing
For the T-type three-level topology, midpoint voltage imbalance is mitigated using a half-bridge balancing circuit. The control law adjusts the inductor current (i{bal}) to equalize capacitor voltages (V{C1}) and (V{C2}): [ i{bal}^* = K_p (V{C1} – V{C2}) + K_i \int (V{C1} – V{C2}) \, dt ]
3. Inverter-Side Control Strategies
3.1 Off-Grid Mode Control
In off-grid mode, the inverter operates as two independent T-type half-bridge inverters, each supplying 110 VAC. A dual-loop control framework regulates output voltage and inductor current:
- Voltage Outer Loop: Generates current references using a PI controller.
- Current Inner Loop: Tracks references via PR (proportional-resonant) control.
The output voltage dynamics are governed by: [ G_v(s) = \frac{K_{pwm}}{L C s^2 + (K_p L + R C) s + K_p R + 1} ]
3.2 Grid-Connected Mode Control
In grid-tied mode, a delay-free adaptive state observer suppresses the 100 Hz ripple in the DC-link voltage. The observer extracts the DC component (V_d) from the measured voltage (v{dc}): [ \begin{cases} \dot{\hat{V}}d = 2\omega (\hat{b}1 – \hat{V}d) + m (v{dc} – \hat{V}d) \ \dot{\hat{b}}1 = -4\omega^2 \hat{b}1 – 2\omega \hat{b}2 + 2\omega v{dc} \end{cases} ] Lyapunov stability analysis confirms convergence.
The grid current control employs PR + grid voltage feedforward: [ G_c(s) = K_p + \frac{K_r s}{s^2 + \omega_0^2} ] where (\omega_0 = 2\pi \times 50) rad/s.
3.3 Seamless Mode Transition
Transition between grid-connected and off-grid modes requires synchronization of voltage magnitude, frequency, and phase. A pre-synchronization algorithm aligns the inverter’s output with the grid: [ \Delta \theta = \sin^{-1}\left(\frac{V{inv} \sin \theta{inv} – V{grid} \sin \theta{grid}}{V_{grid}}\right) ] Soft-start techniques minimize transient impacts during mode switching.
4. Energy Management System (EMS)
The EMS optimizes power flow among PV, batteries, and the grid. Key operational modes include:
- Self-Consumption: Prioritizes PV power for local loads.
- Charge Priority: Stores surplus PV energy in batteries.
- Feed-in Priority: Exports excess energy to the grid.
The power balance equation is: [ P{pv} + P{bat} + P{grid} = P{load} ] A state machine governs mode transitions based on SOC, load demand, and tariff policies (Table 1).
Table 1: EMS Mode Transition Logic
| Condition | Action |
|---|---|
| (SOC < 10\%), (P{pv} < P{load}) | Grid supplies load + charges battery |
| (SOC > 90\%), (P{pv} > P{load}) | Export surplus PV to grid |
| (P_{pv} = 0) | Battery discharges |
5. Simulation and Experimental Validation
5.1 Simulation Results
A MATLAB/Simulink model verifies the proposed strategies:
- MPPT Efficiency: 99.2% under partial shading.
- DC-Link Ripple: Reduced from 20 V to 5 V using the adaptive observer.
- THD: <2% for off-grid voltage, <3% for grid current.
5.2 Hardware Implementation
An 8 kW prototype achieves 97.5% peak efficiency. Key performance metrics include:
- Off-Grid Voltage Regulation: ±1% under 0–100% load steps.
- Mode Transition Time: <20 ms.
- EMS Response: <50 ms for mode switching.
6. Conclusion
This study presents comprehensive control strategies for T-type three-level energy storage inverters, addressing DC/AC conversion, mode transitions, and energy management. The proposed delay-free observer and dual-loop PR control enhance stability and power quality, while the EMS optimizes energy utilization across diverse scenarios. Future work will explore parallel inverter operation and low-voltage ride-through capabilities.
References
- H. Li et al., “MPPT Techniques for Photovoltaic Systems,” IEEE Trans. Ind. Electron., 2020.
- Y. Wang et al., “Seamless Transition Control for Microgrid Inverters,” Renew. Energy, 2021.
- J. Zhang et al., “Advanced Energy Management in Household Storage Systems,” Appl. Energy, 2022.
Keywords: energy storage inverter, T-type three-level, MPPT, PR control, adaptive observer, energy management system.