MMC Carrier Modulation for Precise Circulating Current Control
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Solution Overview
Problem
The controllability of circulating current in modular multilevel converters (MMC) is compromised due to the limited dynamic range of control signals when the target value of circulating voltage is much smaller than the phase voltage target value, leading to worse control accuracy.
Innovation Solution
The power conversion device modulates the carrier signal using a second voltage command value based on circulating current, enhancing control accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a large number of switching elements are used to achieve high-step-down ratio conversion, then the conversion capability is improved, but the device size and complexity increase
Solution Approach 1:
The power conversion device is divided into a plurality of modules, where each module includes a switching element and an inductor. By segmenting the overall conversion function into multiple smaller modules that work in parallel, the device achieves high-step-down ratio conversion capability without requiring a single large complex structure, thus resolving the contradiction between conversion capability and device size.
2Productivity
If the switching element is turned on during the dead time to improve current continuity, then the inductance value can be reduced, but voltage spikes and device stress increase
Solution Approach 1:
A diode is introduced as an intermediary element connected in parallel with each inductor. During the dead time when the switching element transitions state, the diode provides a path for current flow, maintaining current continuity without requiring the switching element to remain on. This eliminates voltage spikes and reduces stress on the switching element while still achieving the benefit of reduced inductance values.
3Reliability
If the switching element is turned on during dead time, then device stress is reduced, but output voltage ripple increases due to current flowing through the switching element
Solution Approach 1:
The diode acts as an intermediary that handles current flow during the dead time period. Instead of current flowing through the switching element (which causes output voltage ripple), the diode provides a dedicated current path that maintains continuity while isolating the switching element from ripple-generating current flow, thus reducing both device stress and output voltage ripple.
4Device complexity
If conventional power conversion devices are used, then the structure is simple, but they cannot achieve high-step-down ratio conversion and have poor adaptability
Solution Approach 1:
The power conversion device employs a modular segmented structure where multiple identical modules (each with switching element and inductor) are connected in parallel. This segmentation enables the device to achieve high-step-down ratio conversion capability through coordinated operation of multiple simple modules, rather than requiring a single complex structure, thus resolving the contradiction between structural simplicity and conversion capability.
Solution Approach 2:
Each module in the segmented structure is designed to be universal and interchangeable, with identical switching elements and inductors performing the same function. This universality allows the system to achieve high adaptability and high-step-down ratio conversion capability through the coordinated operation of multiple simple, identical modules, rather than requiring complex specialized components.
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 approach allows for precise regulation of circulating current, improving the overall control performance of MMC-based power conversion devices.
Implementation Method 1
each module includes a switching element and an inductor, and the other ends of the second switches and the second diodes are connected to a common negative terminal of the power supply. The switching element switches between an on-state and an off-state, so as to convert power from the power supply to an output.
Data Source
Figure 1
Figure 2
Figure 3(A)~3(B)
AI summary
In an MMC-based power conversion device, a control device (5) generates, for each leg circuit, a first voltage command value (Vupref, Vunref) not based on circulating current (Iccu) circulating between a plurality of leg circuits (8) and a second voltage command value (Vccuref) based on the circulating current (Iccu). A plurality of individual controllers (61) are provided respectively corresponding to a plurality of converter cells (1) and generate a gate control signal (Ga) for controlling turning on and off of a switching element (1a) of the corresponding converter cell, based on the first voltage command value and the second voltage command value. When generating the gate control signal using pulse width modulation, each individual controller modulates a carrier signal (CS) in accordance with the second voltage command value such that the pulse width of the gate control signal changes in accordance with the second voltage command value.