The rapid development of renewable energy and global carbon neutrality goals have propelled photovoltaic (PV) energy storage systems into the spotlight. This paper investigates the control strategies of T-type three-level single-phase three-wire household energy storage inverters, addressing critical challenges in PV power conversion, battery management, and grid interaction. The proposed system architecture integrates advanced control algorithms to optimize energy flow and ensure seamless operation across multiple modes.
1. System Configuration and Key Components
The proposed energy storage inverter employs a dual-stage DC/DC+DC/AC topology with the following specifications:
| Parameter | Value |
|---|---|
| Rated Power | 8 kW |
| MPPT Voltage Range | 150-580 V |
| Battery Voltage | 40-60 V |
| Grid Voltage | 110/220 V |
| Switching Frequency | 16 kHz |
2. Photovoltaic Side Modeling and Control
The PV characteristics are modeled using the single-diode equivalent circuit:
$$I = I_{ph} – I_0\left[\exp\left(\frac{q(V+IR_s)}{nkT}\right)-1\right] – \frac{V+IR_s}{R_{sh}}$$
where $I_{ph}$ represents photocurrent, $I_0$ the reverse saturation current, and $R_s/R_{sh}$ series/shunt resistances. The variable-step perturb & observe MPPT algorithm demonstrates superior tracking efficiency:
| MPPT Method | Tracking Efficiency | Response Time |
|---|---|---|
| Constant Voltage | 92% | 120 ms |
| Incremental Conductance | 97% | 80 ms |
| Variable-Step P&O | 98.5% | 60 ms |
3. DC Bus Voltage Balancing
The midpoint voltage balancing control for the T-type three-level inverter is governed by:
$$V_{dc1} – V_{dc2} = \frac{1}{C}\int (i_{bal} – i_o)dt$$
where $i_{bal}$ represents the balancing circuit current and $i_o$ the neutral point current. The adaptive state observer effectively suppresses 100 Hz voltage ripple:
$$
\begin{cases}
\dot{\hat{V}}_d = 2\omega\hat{b}_1 \\
\dot{\hat{b}_1} = 2\omega\hat{b}_2 + 2\omega(V_d – \hat{V}_d) \\
\dot{\hat{b}_2} = -4\omega^2\hat{b}_1 – 2\omega\hat{b}_2
\end{cases}
$$
4. Grid-Connected Control Strategy
The dual-loop control structure combines PR current control with voltage feedforward:
$$
G_{PR}(s) = K_p + \frac{2K_r\omega_c s}{s^2 + 2\omega_c s + \omega_0^2}
$$
where $\omega_0 = 100\pi$ rad/s and $\omega_c$ represents the cutoff frequency. Experimental results demonstrate THD reduction from 4.2% to 2.1% compared to conventional PI control.
5. Seamless Mode Transition
The mode-switching control algorithm ensures <300 ms transition time between operational states:
| Transition Type | Voltage Deviation | Settling Time |
|---|---|---|
| Grid → Island | <5% | 280 ms |
| Island → Grid | <3% | 250 ms |
6. Energy Management System (EMS)
The EMS prioritizes energy sources based on real-time conditions:
$$
P_{total} = P_{PV} + P_{bat} + P_{grid} = P_{load}
$$
Four operational modes are implemented:
- PV priority with battery charging
- Grid-connected feed-in mode
- Battery discharge dominance
- Grid-assisted charging mode
7. Experimental Validation
The prototype achieves 97.2% peak efficiency with key performance metrics:
| Parameter | Value |
|---|---|
| Voltage Unbalance | <1.5% |
| Current THD | <2.5% |
| MPPT Efficiency | 98.3% |
| Transient Response | <3 cycles |
This comprehensive control strategy for energy storage inverters demonstrates significant improvements in system stability, power quality, and operational flexibility. The integration of adaptive observers and mode-optimized controllers addresses critical challenges in modern household PV-storage systems, paving the way for smarter grid integration and enhanced renewable energy utilization.