Control of Improved Dual-Buck Energy Storage Inverter

Introduction

The rapid development of renewable energy systems has highlighted the critical role of energy storage inverters in stabilizing power grids and managing energy fluctuations. This study focuses on an improved dual-buck topology for energy storage inverters, addressing challenges such as switching losses, leakage currents, and seamless grid integration. By combining advanced modulation strategies and multi-loop control systems, the proposed design enhances efficiency, power density, and reliability in distributed energy systems.

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Improved Two-Stage Dual-Buck Topology

The dual-buck topology eliminates bridge-arm shoot-through risks inherent in traditional full-bridge inverters, enabling half-cycle control mode for reduced switching losses. Key features include:

1. Dual-Buck Full-Bridge Configuration:
   • Avoids dead-time requirements.
   • Reduces harmonic distortion through independent inductor operation.
   • Enables bidirectional energy flow (inverter/rectifier modes).
2. DC/DC Boost Converter Integration:
   • Enhances DC voltage utilization.
   • Prolongs switch lifespan via dual-mode operation.

Mathematical Model:
For the DC/DC boost converter:Gud(s)=UdcD−L1RHDsL1C1s2+L1RHs+D2Gud​(s)=Udc​L1​C1​s2+RH​L1​​s+D2D−RH​DL1​​s​

For the dual-buck inverter in dqdq-coordinates:ddt[igdigq]=1L[ugdugq]−(0−ωω0)[igdigq]−UdcL[dddq]dtd​[igd​igq​​]=L1​[ugd​ugq​​]−(0ω​−ω0​)[igd​igq​​]−LUdc​​[dd​dq​​]

Switching Loss Analysis:

Control Mode	Switching Loss (W)	Conduction Loss (W)
Full-Bridge	139.54	60.71
Half-Cycle	44.40	37.75
Half-cycle mode reduces switching losses by 68.3% and conduction losses by 60.8% compared to full-bridge mode.

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Leakage Current Suppression Strategies

Non-isolated energy storage inverters face leakage currents due to common-mode voltage fluctuations. Three modulation strategies were analyzed:

1. SVPWM:
   • Generates 6-pulse common-mode voltage per cycle.
   • Leakage current peaks at 7 A.
2. AZSPWM:
   • Reduces common-mode voltage swing to 1/3Udc1/3Udc​.
   • Leakage current peaks at 2.5 A (AZSPWM2 sequence).
3. RSPWM:
   • Eliminates zero vectors for constant common-mode voltage.
   • Leakage current < 1 A, but sacrifices 33.3% DC voltage utilization.

Performance Comparison:

Modulation	CM Voltage Swing	Leakage Current (Peak)	DC Utilization
SVPWM	0→Udc0→Udc​	7 A	100%
AZSPWM2	1/3Udc1/3Udc​	2.5 A	100%
RSPWM	Constant	<1 A	66.7%

Simulation Results:

• AZSPWM2 balances leakage suppression (−64.3%−64.3%) and DC utilization.
• RSPWM minimizes leakage but limits power output.

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Multi-Loop Control System Design

A three-loop control framework (voltage, current, power) ensures stability and seamless mode transitions:

1. Current Loop:
   • Decouples dd– and qq-axis components using PI controllers.
   • Transfer function:
   Gid(s)=UdcRLRLC1s+1L1C1s2+L1RLs+1Gid​(s)=RL​Udc​​L1​C1​s2+RL​L1​​s+1RL​C1​s+1​
   • Parameters: kip=18kip​=18, kii=1200kii​=1200.
2. Voltage Loop:
   • Tracks reference values from the power loop.
   • Transfer function:
   Gvc(s)=kvps+kvis⋅Gio(s)sCf(Ts+1)Gvc​(s)=skvp​s+kvi​​⋅sCf​(Ts​+1)Gio​(s)​
   • Parameters: kvp=20kvp​=20, kvi=0.06kvi​=0.06.
3. Power Loop:
   • Implements droop control for grid synchronization.
   • Frequency/voltage adjustments:
   Δf=Δf∗−kp⋅ΔP,ΔV=ΔV∗−kq⋅ΔQΔf=Δf∗−kp​⋅ΔP,ΔV=ΔV∗−kq​⋅ΔQ
   • Parameters: kp=0.00004kp​=0.00004, kq=0.0011kq​=0.0011.

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Seamless Grid Connection/Disconnection

Pre-Synchronization Strategy:

1. Frequency Matching: Adjust inverter frequency to grid frequency (Δf<0.5Δf<0.5 Hz).
2. Phase Alignment: Use PLL (Phase-Locked Loop) to synchronize voltage angles.
3. Voltage Amplitude Matching: Tune output to grid voltage (311Vpeak311Vpeak​).

Transition Performance:

Metric	Grid-to-Island	Island-to-Grid
Frequency Error	<0.2 Hz	<0.1 Hz
Voltage THD	<2%	<1.5%
Transition Time	50 ms	30 ms

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Experimental Validation

A hardware-in-loop (HIL) platform using RT-LAB validated the design:

1. Inverter Mode:
   • Output: 311Vpeak311Vpeak​, 37.2Apeak37.2Apeak​ with THD < 3%.
   • Efficiency: 97.2% at 10 kW.
2. Rectifier Mode:
   • DC output: 750V750V, 13.33A13.33A with ripple < 2%.
3. Leakage Current:
   • AZSPWM2 reduced leakage to 2.5 A vs. SVPWM’s 7 A.

Key Parameters:

Parameter	Value
Switching Freq.	10 kHz
Filter Inductance	1.5 mH
Filter Capacitance	20 μF
DC Link Voltage	750 V

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Conclusion

The improved dual-buck energy storage inverter demonstrates superior performance in leakage current suppression, efficiency (>97%>97%), and seamless grid interaction. Key innovations include:

• Half-cycle control for minimized switching losses.
• AZSPWM2 modulation balancing leakage and DC utilization.
• Three-loop control ensuring stability across modes.

Future work will focus on scaling the topology for high-power applications and integrating AI-driven predictive maintenance. This design paves the way for next-generation energy storage inverters in smart grids and renewable systems.