Modular Scalable Power Conversion for PV Inverters
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Solution Overview
Problem
Existing transformerless topologies for utility-scale photovoltaic (PV) inverters face challenges such as the need for additional boost converters due to low PV string voltages, large capacitor banks, and centralized controllers that impede scalability and increase costs, while also being prone to single-point failures.
Innovation Solution
A cascaded architecture with interconnected blocks that process constant power, eliminating bulk energy storage and using decentralized controllers for dc-link voltage regulation, maximum power point tracking, and ac-side power sharing, enabling transformerless medium-voltage ac interfaces through a quadruple active bridge dc-dc converter for isolation and stackability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If modular multilevel converters with cascaded half- or full-bridge cells are used to achieve high voltage blocking and high-quality waveforms, then voltage synthesis capability is improved, but device complexity and cost increase due to large number of series-connected devices
Solution Approach 1:
The system divides the power conversion function into independent modular blocks, each containing a full-bridge converter and three single-phase inverters. These blocks are connected in series to achieve the desired voltage level, with each block processing a portion of the total power. This segmentation reduces the complexity within each module while maintaining the overall voltage synthesis capability through modular stacking.
2Adaptability or versatility
If PV string voltage is low (at or less than 1.5 kV) but medium-voltage AC output is required, then system scalability is improved, but additional boost converter stage is needed which decreases efficiency and increases cost
Solution Approach 1:
The full-bridge converter in each modular block continuously transfers power from the PV input to the AC output side through high-frequency transformation, maintaining continuous power flow without discontinuous boosting stages. This continuous action reduces energy losses compared to intermittent boost converter operation.
Solution Approach 2:
A high-frequency transformer serves as an intermediary device between the low-voltage PV input and the medium-voltage AC output. This transformer enables direct voltage transformation without requiring additional boost converter stages, thereby maintaining system efficiency while achieving the required voltage conversion ratio.
3Stability of the object's composition
If centralized controllers are used to manage power conversion, then system coordination is improved, but scalability is impeded and single-point failure risk increases
Solution Approach 1:
The control system is segmented into decentralized block-level controllers, with each controller managing its associated modular block independently. Each controller performs local functions including dc-link voltage regulation, maximum power point tracking, and ac-side power sharing. This distributed control architecture eliminates single-point failures and enables easy system scalability by simply adding or removing modular blocks without affecting other units.
4Power
If MMC cells process pulsating power with distinct phase legs, then power processing capability is improved, but large capacitor banks are required which increase cost and complexity
Solution Approach 1:
The modular block merges the functions of power processing and energy buffering into a single integrated unit. Each block contains a full-bridge converter with a small dc-link capacitor that handles both the power conversion and the necessary energy storage for pulsating power processing. By combining these functions in each modular unit rather than using separate large capacitor banks, the system achieves power processing capability with reduced overall complexity and cost.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution allows for scalable and resilient PV-to-MVAC systems without bulky line-frequency transformers, maintaining efficiency and reliability by ensuring input-output power balance and autonomous operation across multiple blocks.
Implementation Method 1
a quadruple active bridge dc-dc converter that provides isolation between the PV input and each of the three ac-side phases within each block
Data Source
AI summary
A cascaded architecture composed of interconnected blocks that are each designed to process constant power and eliminate bulk energy storage are provided. Further, local controls within each block natively achieve both block- and system-level aims, making the system modular and scalable. Further methods of providing power conversion using such interconnected clocks are also provided.