With the increasing integration of renewable energy systems, energy storage inverters play a critical role in managing power quality and system stability. This paper proposes an advanced control methodology to address parallel stability challenges in high photovoltaic (PV) energy storage inverters. Conventional methods, such as adaptive current prediction models and impedance reshaping techniques, often struggle to regulate virtual impedance for current feedback, leading to compromised stability. Our approach focuses on suppressing circulating currents and enabling seamless grid-connected/islanded mode transitions, significantly enhancing the operational reliability of energy storage inverters.
1. Circulating Current Analysis and Suppression
For parallel-connected energy storage inverters, circulating currents arise from voltage magnitude/phase mismatches and impedance disparities. The stability conditions for parallel operation are expressed as:
$$
\begin{cases}
E_1 = E_2 = \cdots = E_n \\
f_1 = f_2 = \cdots = f_n \\
\phi_1 = \phi_2 = \cdots = \phi_n
\end{cases}
$$
where \( E_i \), \( f_i \), and \( \phi_i \) represent the output voltage magnitude, frequency, and phase of the \( i \)-th energy storage inverter, respectively. The circulating current \( I_c \) between inverters can be modeled as:
$$
I_c = \frac{E_i \angle \phi_i – U_{com}}{Z_i}
$$
where \( U_{com} \) is the common bus voltage and \( Z_i \) denotes the equivalent impedance. Table 1 summarizes key parameters affecting circulating current characteristics in energy storage inverters.
| Parameter | Description | Impact Factor |
|---|---|---|
| \( Z_{line} \) | Line impedance | 0.1–0.3 Ω/km |
| \( \Delta E \) | Voltage magnitude deviation | < 2% rated |
| \( \Delta \phi \) | Phase angle difference | < 0.5° |
2. Seamless Transition Control Strategy
The proposed stability control framework employs a matrix-based voltage regulation approach during mode transitions:
$$
\begin{bmatrix}
U_a \\ U_b \\ U_c
\end{bmatrix}
=
\begin{bmatrix}
U_i \cos \theta \\
U_i \cos(\theta – 2\pi/3) \\
U_i \cos(\theta + 2\pi/3)
\end{bmatrix}
$$
where \( U_a, U_b, U_c \) represent three-phase output voltages, and \( \theta \) is the phase angle. This ensures smooth transitions between grid-connected and islanded modes while maintaining voltage stability for energy storage inverters.
3. Experimental Validation
Comparative tests were conducted using three 40kW energy storage inverters with the parameters shown in Table 2. The evaluation metrics included stabilization success rate and response time.
| Parameter | Inverter 1 | Inverter 2 | Inverter 3 |
|---|---|---|---|
| DC Voltage (V) | 800 | 700 | 600 |
| Line Impedance (Ω) | 0.1 + j0.4 | 0.2 + j0.8 | 0.3 + j1.2 |
| Filter Inductance (mH) | 2 | ||
The stabilization performance comparison revealed significant advantages of our method:
| Method | Success Rate (%) | Response Time (ms) | Voltage Deviation (%) |
|---|---|---|---|
| Adaptive Current Prediction | 77.2 | 1480 | 4.2 |
| Impedance Reshaping | 87.1 | 2750 | 3.8 |
| Proposed Method | 97.9 | 760 | 1.1 |
4. Stability Enhancement Mechanism
The energy storage inverter control algorithm implements real-time impedance matching through virtual impedance adjustment:
$$
Z_{virtual} = K_p \left( 1 + \frac{1}{T_i s} \right) \cdot \frac{\omega_c}{s + \omega_c}
$$
where \( K_p \) is the proportional gain, \( T_i \) the integral time constant, and \( \omega_c \) the cutoff frequency. This adaptive impedance compensation effectively suppresses harmonic resonance in parallel-connected energy storage inverters.
5. Conclusion
This paper presents a comprehensive solution for parallel stability control in high photovoltaic energy storage inverters. By combining circulating current suppression with seamless transition algorithms, the proposed method achieves 97.9% stabilization success rate with 760ms response time, significantly outperforming conventional approaches. The enhanced stability control enables reliable operation of energy storage inverters in both grid-connected and islanded modes, supporting the development of robust renewable energy systems.