Converter Sub-Module Current Direction Detection
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Solution Overview
Problem
Current methods for determining current direction in power converters, such as Modular Multilevel Converters (MMC), face challenges at low currents due to noise and offsets, leading to unreliable capacitor voltage balancing and dead band compensation, resulting in significant no-load losses.
Innovation Solution
A converter system that includes sub-modules with energy storage devices and switching circuitry capable of transitioning between configurations to measure potential differences, allowing for accurate current direction determination by measuring the time and magnitude of these changes, enabling precise control and balancing even at low currents.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a current sensor with a range of a few thousand Amperes is used to determine current direction, then the sensor can cover the full operating range, but the determination of current direction at low currents (less than 10 A) becomes unreliable due to noise and offsets
Solution Approach 1:
The patent segments the current measurement function by using multiple sensors with different ranges. A first current sensor handles high current measurements while a second current sensor handles low current measurements. This segmentation allows each sensor to operate in its optimal range, ensuring reliable current direction determination across the full operating spectrum without the reliability issues that plague single-sensor systems at low currents.
2Reliability
If a circulating current of at least 10 A peak is used at zero load to avoid unreliable current direction determination, then current direction can be reliably determined, but significant no-load losses occur
Solution Approach 1:
The patent replaces the mechanical approach of circulating current (a physical electrical flow used to ensure reliable measurements) with an electronic/software-based solution. By using a control unit that selectively switches between two current sensors based on measured current magnitude, the system achieves reliable current direction determination without the energy-wasting circulating current. This substitution eliminates the need for artificial current circulation while maintaining measurement reliability.
3Speed
If the switching circuitry transitions directly between configurations without an intermediate configuration, then the switching speed is faster, but the switches may be on at the same time and short out the energy storage device
Solution Approach 1:
The patent applies preliminary action by introducing an intermediate configuration that is activated before the final switching state. The switching circuitry transitions through this intermediate state where both switches are off, ensuring that they never conduct simultaneously. This preliminary intermediate step prevents shoot-through short circuits while maintaining efficient switching operation, as the system prepares the switching state in advance to avoid harmful conditions.
Data Source
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Figure 3a~3b
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AI summary
A converter system for use in a converter (100), the converter having at least one converter apparatus (101a, 101b) defining a current path within the converter, the system comprising: a plurality of sub-modules (102) connectable in series to form at least part of the converter apparatus such that current flowing along the current path flows through each of those sub-modules, each sub-module (102) comprising an energy storage device (202) and switching circuitry (201, 203, 204, 205) configured to direct the flowing current so that it either flows through or bypasses the energy storage device (202), the switching circuitry (201, 203, 204, 205) controllable to cause the sub-module (102) to transition between at least one pair of different configurations via a corresponding intermediate configuration, that transition affecting a potential difference such that it exhibits an associated change either when the sub-module (102) is in the intermediate configuration concerned or after it has exited that intermediate configuration dependent on a direction of current flow along the current path for that transition; and a controller (104, 103a, 103b, 206) configured to carry out a direction-determination operation in respect of at least one said sub-module (102), the direction-determination operation determining the direction of current flow along the current path for a said transition of that sub-module (102) by measuring the affected potential difference concerned at least once over that transition to determine whether the associated change occurs when that sub-module (102) is in the intermediate configuration concerned or after it has exited that intermediate configuration.