The integration of photovoltaic (PV) systems with energy storage inverters has become a cornerstone of modern renewable energy infrastructure. However, ensuring the parallel stability of multiple energy storage inverters remains a critical challenge due to voltage/current fluctuations, circulating currents, and impedance mismatches. Traditional methods, such as resonant control and adaptive current prediction models, often struggle to address these issues comprehensively. This article presents a novel control strategy for enhancing the parallel stability of high photovoltaic energy storage inverters, focusing on circulating current suppression, seamless grid transition, and impedance matching.
Circulating Current Analysis and Feature Extraction
In parallel-connected energy storage inverters, circulating currents arise from discrepancies in voltage magnitude, frequency, or phase angles among inverters. These currents lead to unbalanced power distribution, inductor saturation, and harmonic distortion. To quantify circulating currents, consider the following model:
I=ZiEi∠φi−Ui
where:
- I: Circulating current characteristics
- Ei: Voltage magnitude of the ith inverter
- φi: Phase angle of the ith inverter
- Ui: Voltage at the common load
- Zi: Equivalent impedance of the ith inverter
Circulating currents are minimized when the output voltages of all inverters satisfy:⎩⎨⎧E1=E2=⋯=Enf1=f2=⋯=fnφ1=φ2=⋯=φn
Failure to meet these conditions results in clockwise or counterclockwise current loops, depending on the phase difference.
Stability Control Strategy for Parallel Energy Storage Inverters
The proposed method integrates two key components:
1. Grid-Connected/Islanded Seamless Transition
During grid faults, energy storage inverters must switch from grid-connected to islanded mode without disrupting voltage or frequency stability. The three-phase output voltage during this transition is governed by:UaUbUc=UicosθUicos(θ−32π)Uicos(θ+32π)
Here, Ua,Ub,Uc represent the three-phase voltages, and θ is the phase angle synchronized with the grid. In islanded mode, the inverter autonomously regulates voltage and frequency using energy storage systems, ensuring continuous power supply.
2. Impedance Reshaping and Adaptive Current Prediction
To mitigate circulating currents, the equivalent impedance Zi of each energy storage inverter is dynamically adjusted. By reshaping impedance profiles at resonant frequencies, harmonic voltages at parallel nodes are suppressed. Additionally, an adaptive current prediction model minimizes zero-sequence circulating currents through real-time adjustments of the common-mode voltage.
Experimental Validation
Setup
Three 40 kW energy storage inverters were connected in parallel under non-uniform DC voltages and line impedances (Table 1). The experiment involved sequential activation of inverters at 0.30 s (Inverter 2) and 0.38 s (Inverter 3).
Table 1: Inverter Parameters
| Parameter | Value | Parameter | Value |
|---|---|---|---|
| Ud1 | 800 V | RL1 | 0.1+j0.4Ω |
| Ud2 | 700 V | RL2 | 0.2+j0.8Ω |
| Ud3 | 600 V | RL3 | 0.3+j1.2Ω |
| Filter inductance (L) | 2 mH | Grid voltage (Ug) | 380 V |
| Filter capacitance (C) | 10 μF | Active power (P) | 10 kW |
| Equivalent resistance (R) | 0.01 Ω | Reactive power (Q) | 5 kVAR |
Results
The proposed method was compared against two benchmarks:
- Adaptive Current Prediction Model [1]: Prone to voltage/current oscillations during parallel transitions.
- Impedance Reshaping Method [2]: Limited ability to synchronize voltage and current simultaneously.
Table 2: Control Success Rate (%)
| 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 (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 response time, outperforming benchmarks in both stability and efficiency. Voltage and current remained within ±500 V and ±200 A, respectively, with no significant oscillations.
Conclusion
This study addresses the critical challenge of parallel stability in high photovoltaic energy storage inverters through a hybrid approach combining impedance reshaping, adaptive current prediction, and seamless grid transition. By suppressing circulating currents and synchronizing voltage/frequency parameters, the method ensures reliable operation under diverse grid conditions. Experimental results validate its superiority over existing techniques, highlighting its potential for large-scale renewable energy systems. Future work will explore scalability for ultra-high-power energy storage inverter arrays and real-time implementation in smart grids.