Multi-Frequency Control System and Method for Microgrids

Multi-Frequency Control for DC-Linked Microgrids Based on Voltage-Source Converter Frequency Normalization

This technology is a control system that links multiple standalone microgrids equipped with distributed power sources and energy storage systems via DC lines and voltage-source converters (VSCs). It normalizes the allowable frequency of each microgrid to a range of -1 to 1, enabling power exchange while maintaining different frequencies. It calculates the reference d-axis current by adjusting the normalized frequency error to zero using PI control and generates current control signals.

Conventional microgrids are designed to operate at a rated frequency of 60Hz or 50Hz regardless of the power quality requirements of the load, necessitating excessive capital investment. In particular, when multiple standalone microgrids are linked via AC power lines, the frequencies of all grids are locked together, making it impossible to operate at frequencies tailored to the load quality of each individual grid.

This technology uses circular linkage control to align the normalized frequency differences between neighboring VSCs to zero, allowing grids with different frequency ranges to balance supply and demand. It also supports bipolar operation, ensuring power transmission continues via the remaining pole if one fails. Applicable to island microgrid clusters, campuses, military bases, and industrial complexes in standalone mode, it enables differential frequency operation tailored to grid-specific power quality, reducing both capital expenditure and transient states.

‍

‍

Key Features:

• A multi-frequency control system including voltage-source converters that link multiple standalone microgrids, each containing distributed power sources and energy storage systems, via DC lines.
• A normalization unit connected to the microgrid linkage point that normalizes the allowable frequency set for each grid to a range of -1 to 1.
• A configuration where each VSC receives the normalized frequency of its sequential neighbor to control the normalized frequency difference between grids connected by DC lines to zero.
• A step of determining the reference d-axis current by correcting the normalized frequency error with a PI controller and generating a control signal based on the error from the actual d-axis current.

‍

This invention was developed with support from the Ministry of Trade, Industry and Energy for the development of flexible linkage and dynamic optimal operation technology for multi-microgrids in a HILS environment.

‍