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 by 2060,” the integration of energy storage inverters into household PV systems has become critical. This study focuses on optimizing control strategies for T-type three-level single-phase three-wire energy storage inverters, addressing challenges such as maximum power point tracking (MPPT), grid-connected/off-grid mode transitions, and energy management.
System Architecture and Key Components
The proposed energy storage inverter adopts a two-stage topology:
- DC/DC Stage: Includes PV-side Boost converters and bidirectional DC/DC circuits for battery management.
- DC/AC Stage: Features a T-type three-level inverter for generating 110/220 V AC output.
Key subsystems include:
- PV Arrays: Convert solar energy to DC power.
- Battery Storage: Managed via bidirectional DC/DC converters.
- DC Bus Balancing Circuit: Ensures midpoint voltage stability.
- Inverter and Switching Logic: Enables seamless transitions between grid-connected and off-grid modes.
Mathematical Modeling of PV Cells
The output characteristics of PV cells are modeled using the single-diode equivalent circuit. The current-voltage relationship is expressed as:I=Iph−I0[exp(nkTq(V+IRs))−1]−RshV+IRs
where:
- Iph: Photocurrent
- I0: Reverse saturation current
- Rs and Rsh: Series and shunt resistances
Table 1: PV Cell Model Parameters
| Parameter | Description |
|---|---|
| Isc | Short-circuit current |
| Voc | Open-circuit voltage |
| Pmpp | Maximum power point |
| T | Temperature coefficient |
The P-V and I-V curves under standard test conditions (STC) exhibit distinct maxima, necessitating robust MPPT algorithms.
MPPT Algorithms and Boost Converter Control
1. Perturb-and-Observe (P&O) Method
A variable-step P&O algorithm optimizes tracking speed and accuracy:ΔV={+k⋅ΔVbase−k⋅ΔVbaseif ΔP>0if ΔP<0
where k adapts based on the slope of the P-V curve.
2. Incremental Conductance (IC) Method
This technique uses the derivative of power with respect to voltage:dVdP=I+VdVdI=0
A decision tree adjusts the operating voltage to converge at Pmpp.
Table 2: MPPT Algorithm Comparison
| Method | Advantages | Limitations |
|---|---|---|
| P&O | Simple implementation | Oscillations near Pmpp |
| IC | High accuracy | Computational complexity |
Boost Converter Dynamics
The Boost converter’s state-space model is derived as:{dtdiL=LVpv−(1−D)VbusdtdVbus=C(1−D)iL−Iload
A dual-loop control structure (voltage outer loop + current inner loop) ensures stable DC bus voltage regulation.
DC Bus Voltage Balancing
Unbalanced loads induce midpoint voltage fluctuations. A half-bridge balancing circuit with PI control compensates for asymmetries:Vmid∗=2Vbus+−Vbus-→PI Controller→PWM Signals
Key Equations:ibal=2LbalVbus+−Vbus-⋅Dbal⋅Ts
Inverter Control in Off-Grid Mode
The T-type three-level inverter is decomposed into two independent half-bridge inverters for 110/220 V output.
1. State-Space Model
For each half-bridge:{dtdiL=LVinv−Vload−LRiLdtdVload=CiL−Iload
2. Dual-Loop Control
- Voltage Outer Loop: Regulates output voltage using a PI controller.
- Current Inner Loop: Enhances dynamic response with PR (Proportional-Resonant) control:
GPR(s)=Kp+s2+ω02Krs
Grid-Connected Mode Control
1. Current Control Loop
A PR + grid voltage feedforward strategy minimizes harmonic distortion:Vref=GPR(s)⋅(Igrid∗−Igrid)+Vgrid
2. DC Bus Voltage Control
A delay-free adaptive state observer suppresses 100 Hz ripple:V^dc=s1(K1(Vdc∗−Vdc)+K2∫(Vdc∗−Vdc)dt)
Lyapunov stability analysis confirms convergence.
Seamless Mode Transition and Energy Management
1. Grid-Off-Grid Transition
Synchronization of voltage amplitude, phase, and frequency ensures uninterrupted power supply. Virtual impedance matching minimizes transient currents.
2. Energy Management System (EMS)
The EMS operates in three modes:
- Self-Consumption: Prioritizes local load supply.
- Charge Priority: Maximizes battery storage during low tariffs.
- Grid Feedback: Exports surplus energy to the grid.
Table 3: EMS Mode Transition Logic
| Condition | Action |
|---|---|
| Ppv>Pload | Charge battery or feed grid |
| Ppv<Pload | Discharge battery or import grid |
Simulation and Experimental Validation
A MATLAB/Simulink model and an 8 kW prototype validate the proposed strategies:
1. Off-Grid Performance
- THD < 2% under nonlinear loads.
- Voltage regulation within ±1%.
2. Grid-Connected Performance
- Unity power factor operation.
- 99% efficiency at rated load.
3. Mode Transition
<100 ms switching time with <5% voltage dip.
Conclusion
This work advances the control of household energy storage inverters through:
- Adaptive MPPT and DC bus balancing.
- Robust dual-loop control for off-grid/grid-tied modes.
- Intelligent EMS for optimal energy utilization.
Future research will explore AI-driven MPPT and hybrid microgrid integration.