This study investigates the control strategy of a T-type three-level single-phase three-wire household photovoltaic energy storage inverter, focusing on topology optimization, mathematical modeling, and advanced energy management. The proposed system integrates PV power generation, battery storage, and grid interaction to achieve efficient energy utilization under diverse operational scenarios.
1. System Modeling and Key Components
The photovoltaic (PV) cell output characteristics are modeled as:
$$ I = I_{ph} – I_0 \left[ \exp\left(\frac{q(V + IR_s)}{AKT}\right) – 1 \right] – \frac{V + IR_s}{R_{sh}} $$
where $I_{ph}$ represents photocurrent, $I_0$ is reverse saturation current, and $R_s/R_{sh}$ denote series/shunt resistances. The PV array’s maximum power point tracking (MPPT) employs a variable-step perturb & observe algorithm:
| Parameter | Description |
|---|---|
| $ΔP$ | Power variation between iterations |
| $ΔV$ | Voltage step adjustment |
| $V_{ref}$ | Reference voltage for MPPT |
The boost converter dynamics are expressed as:
$$ \begin{cases}
\frac{di_L}{dt} = \frac{V_{pv} – (1-d)V_{bus}}{L} \\
\frac{dV_{bus}}{dt} = \frac{(1-d)i_L – i_{load}}{C}
\end{cases} $$
2. Control Strategy Implementation
Off-grid control: The three-wire inverter is decoupled into two independent T-type half-bridge inverters using dual-loop control:
$$ G_{vol}(s) = \frac{K_p + \frac{K_i}{s}}{1 + T_f s} $$
$$ G_{cur}(s) = \frac{1}{sL + R} $$
Grid-connected control: Features a PR+feedforward current controller and DC-link voltage observer:
$$ G_{PR}(s) = K_p + \frac{2K_rω_c s}{s^2 + 2ω_c s + ω_0^2} $$
The delay-free adaptive observer for DC-link voltage stabilization:
$$ \begin{cases}
\dot{\hat{V}}_d = 2ω(\hat{b}_1 + \hat{b}_2) + m(b_1 – \hat{b}_1) \\
\dot{\hat{b}}_2 = -4ω^2\hat{b}_1 – 2ω\hat{b}_2
\end{cases} $$
| Parameter | Value |
|---|---|
| Rated Power | 8 kW |
| DC-link Voltage | 450 V |
| Switching Frequency | 16 kHz |
| Filter Inductance | 650 μH |
3. Energy Management Strategy
The energy storage inverter implements multi-mode operation through hierarchical control:
$$ P_{total} = P_{pv} + P_{bat} + P_{grid} = P_{load} $$
Mode transition logic follows:
| Mode | Condition | Priority |
|---|---|---|
| Self-consumption | $P_{pv} > P_{load}$ | 1 |
| Grid charging | $SOC < 20\%$ | 2 |
| Peak shaving | Time-of-use pricing | 3 |
4. Experimental Validation
The 8kW prototype demonstrates:
- THD < 2% under nonlinear loads
- Seamless mode transition within 10ms
- 98.2% peak efficiency
DC-link voltage ripple is reduced by 62% using the proposed observer:
$$ \text{Ripple}_{before} = 20V_{pp} \rightarrow \text{Ripple}_{after} = 7.6V_{pp} $$
The energy storage inverter achieves:
$$ η_{MPPT} = 99.3\%,\quad η_{inv} = 97.5\% $$
Validating the effectiveness of the integrated control strategy for residential renewable energy systems.