Modular Multifunction Converter for Grid Energy Storage
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Solution Overview
Problem
Existing battery energy storage systems face challenges due to voltage imbalances and inefficiencies caused by cell asymmetries, leading to decreased battery lifetime and poor capacity utilization, particularly in grid-connected systems that require large and bulky filters for AC waveform quality improvement.
Innovation Solution
A modular multifunction converter (MMFC) system with bidirectional converters and a DC-link controller that manages power blocks to maintain balanced voltage references and state-of-charge across battery modules, allowing for efficient charging and discharging while minimizing harmonics and utilizing smaller, more efficient converter modules.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a two-stage converter approach is used to exchange power with the grid, then power exchange capability is improved, but filter size and system complexity increase
Solution Approach 1:
The patent combines the DC-DC converter and DC-AC converter functions into a single integrated modular multifunction converter (MMFC). The MMFC includes a bridge circuit with bidirectional switches that can operate in different modes to achieve both DC-DC power conversion for battery charging/discharging and DC-AC power conversion for grid exchange, eliminating the need for separate two-stage converters and reducing system complexity.
Solution Approach 2:
The MMFC is designed as a universal converter that can perform multiple functions: charging battery modules, discharging battery modules, exchanging power with the AC grid, and providing voltage balancing. The bridge circuit topology allows the same hardware to operate in different conversion modes, making the system multi-functional rather than requiring separate dedicated converters for each function.
2Reliability
If large filters are added to improve AC waveform quality, then power quality is improved, but device size and cost increase
Solution Approach 1:
The patent replaces traditional large passive LC filters with an active control mechanism. The MMFC uses pulse-width modulation (PWM) control of the bidirectional switches to actively synthesize clean AC waveforms for grid exchange and maintain balanced voltages across battery modules. This active filtering approach through power electronic control eliminates the need for bulky passive filter components while achieving superior power quality.
3Power
If battery cells are connected in series to increase voltage, then voltage output is improved, but voltage imbalance and circulating currents increase
Solution Approach 1:
The patent implements a feedback control mechanism where the MMFC continuously monitors the voltage of each series-connected battery module and adjusts the switching duty cycles of the bidirectional switches to maintain balanced voltages. The controller uses voltage measurements from each module to generate appropriate control signals that prevent voltage imbalance and circulating currents, ensuring reliable operation of the series-connected battery string.
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
The MMFC system enhances battery capacity utilization, increases energy storage, and reduces the need for large filters, achieving balanced three-phase output voltages and efficient power transfer with lower harmonic distortion, thereby improving the reliability and efficiency of grid energy storage systems.
Implementation Method 1
Power electronics converters are used in wide variety of applications including energy storage systems, renewable energy systems, energy conversion
Implementation Method 2
A DC-link controller controls, by varying current through the plurality of power blocks, a positive output voltage between the positive connection and the neutral connection to follow a positive voltage reference
Implementation Method 3
A state-of-charge controller controls a ratio of power supplied by each power block and a ratio of power supplied by each converter module within a power block based on a target state-of-charge for each battery module
Implementation Method 4
The unfolder provides a three-phase output and converts power from the power blocks to three-phase alternating current ('AC') output power at output terminals of the unfolder
Data Source
AI summary
An apparatus includes power blocks. Each power block includes converter modules. Each converter module includes a positive and a negative bidirectional converter and a battery module. The bidirectional converters are connected to the battery module and outputs are connected in parallel. Paralleled positive bidirectional converters are connected in series between a positive connection and a neutral connection and the paralleled negative bidirectional converters of each power block are connected in series between the neutral connection and a negative connection. A DC-link controller controls a positive output voltage between the positive and neutral connections to follow a positive voltage reference and controls a negative output voltage between the neutral and negative connections to follow a negative voltage reference. A state-of-charge controller controls a ratio of power supplied by each power block and a ratio of power supplied by each converter module based on a target state-of-charge for each battery module.