In recent years, the integration of solar photovoltaic (PV) systems with energy storage technologies has revolutionized power distribution, enabling stable energy supply across grid-connected and off-grid scenarios. Central to this integration is the energy storage inverter, a critical component responsible for converting DC power from PV panels and batteries into AC power for grid or load use. However, parallel operation of multiple energy storage inverter introduces challenges such as circulating currents, voltage/phase imbalances, and harmonic distortions, which threaten system stability. This paper proposes an innovative control strategy to enhance the stability of parallel-connected energy storage inverter in high-density PV systems through optimized circulating current suppression and seamless grid transition mechanisms.
1. Circulating Current Analysis and Feature Extraction
Parallel operation of energy storage inverter inherently generates circulating currents due to mismatches in output voltage amplitudes, frequencies, or phases. These currents lead to unbalanced power distribution, increased losses, and potential equipment damage. To address this, we first derive the mathematical model of circulating currents in a multi-inverter system.
For n parallel-connected inverters, the equilibrium condition for minimizing circulating currents is:
$$
\begin{cases} E_1 = E_2 = \dots = E_n \\ f_1 = f_2 = \dots = f_n \\ \phi_1 = \phi_2 = \dots = \phi_n \end{cases} \tag{1}
$$
where (E_i), (f_i), and (\phi_i) represent the voltage amplitude, frequency, and phase of the (i)-th inverter, respectively. Deviations from these conditions induce circulating currents, expressed as:
$$
I_{cir} = \frac{E_i \angle \phi_i – U_i}{Z_i} \tag{2}
$$
Here, (U_i) is the common load voltage, and (Z_i) is the equivalent impedance of the (i)-th inverter. By monitoring (I_{cir}) and adjusting (Z_i), phase synchronization and impedance matching are achieved, significantly suppressing circulating currents.
2. Seamless Grid-Connected/Islanded Transition Control
A key challenge for energy storage inverter is maintaining stability during transitions between grid-connected and islanded modes. Traditional methods often cause voltage/phase disruptions, leading to instability. Our approach employs a dynamic control matrix to ensure smooth transitions:
$$
\begin{bmatrix} U_a \\ U_b \\ U_c \end{bmatrix} = \begin{bmatrix} U_i \cos \theta \\ U_i \cos \left( \theta – \frac{2\pi}{3} \right) \\ U_i \cos \left( \theta + \frac{2\pi}{3} \right) \end{bmatrix} \tag{3}
$$
Here, (U_a, U_b, U_c) are the three-phase output voltages, and (\theta) is the phase angle. During grid faults, the inverter shifts to islanded mode, maintaining (U_a = U_b = U_c) to ensure continuous power supply. When the grid stabilizes, the system transitions back with minimal disturbance by adjusting (\theta) dynamically.
3. Experimental Validation
To validate the proposed method, we conducted experiments on a 40 kW parallel inverter system with three energy storage inverter. The parameters are summarized in Table 1.
Table 1: Parameters of the Parallel Inverter System
| Parameter | Value | Parameter | Value |
|---|---|---|---|
| (U_{d1}) | 800 V | (R_{L1}) | (0.1 + j0.4\ \Omega) |
| (U_{d2}) | 700 V | (R_{L2}) | (0.2 + j0.8\ \Omega) |
| (U_{d3}) | 600 V | (R_{L3}) | (0.3 + j1.2\ \Omega) |
| (L) | 2 mH | (U_g) | 380 V |
| (C) | 10 μF | (P_{load}) | 10 kW |
| (R) | 0.01 Ω | (Q_{load}) | 5 kVAR |
The inverters were connected sequentially at 0.30–0.34 s (Inverter 2) and 0.38–0.40 s (Inverter 3). Voltage and current waveforms under the proposed control strategy remained stable within ±500 V and ±200 A, respectively.
3.1 Comparative Performance Analysis
We compared our method with two existing approaches:
- Adaptive Current Prediction Model [1]: Controls zero-sequence circulating currents via a predictive algorithm.
- Impedance Reshaping [2]: Suppresses harmonics by adjusting inverter impedance.
Table 2: Stability Success Rate Comparison (%)
| Trials | Method [1] | Method [2] | Proposed Method |
|---|---|---|---|
| 10 | 75.6 | 85.6 | 98.6 |
| 20 | 74.1 | 84.7 | 97.5 |
| 30 | 70.2 | 81.2 | 98.2 |
| 40 | 75.4 | 86.3 | 97.4 |
| 50 | 76.8 | 85.4 | 98.3 |
| 60 | 77.2 | 87.1 | 97.9 |
Table 3: Response Time Comparison (s)
| Trials | Method [1] | Method [2] | Proposed Method |
|---|---|---|---|
| 10 | 1.63 | 2.35 | 0.66 |
| 20 | 1.44 | 2.46 | 0.58 |
| 30 | 1.78 | 2.18 | 0.74 |
| 40 | 1.96 | 2.63 | 0.85 |
| 50 | 1.33 | 2.58 | 0.72 |
| 60 | 1.48 | 2.75 | 0.76 |
The proposed method achieved a 97.9% success rate and 0.76 s average response time, outperforming existing techniques.
4. Conclusion
This paper presents a robust stability control strategy for parallel-connected energy storage inverter in high PV systems. By extracting circulating current features and implementing seamless grid transition mechanisms, the method ensures synchronized voltage amplitudes, frequencies, and phases across inverters. Experimental results validate its superiority in success rate, response time, and adaptability to grid disturbances. Future work will explore scalability for larger PV-energy storage inverter networks and real-time optimization under dynamic load conditions.
Keywords: Energy storage inverter, parallel operation, stability control, circulating current, seamless transition.