Three-Winding Transformer MMC Impedance Balance
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Solution Overview
Problem
Conventional three-phase three-winding transformer MMCs experience impedance imbalance due to differing impedances between the primary and secondary sides and primary and tertiary sides, leading to increased rated voltage and potential disruption of cost reduction and downsizing benefits, as well as iron core saturation issues.
Innovation Solution
The secondary and tertiary windings are placed vertically and symmetrically with respect to the primary winding, with a gap between them, allowing DC leakage fluxes to pass through the iron core and tank, equalizing impedances and reducing the risk of iron core saturation, thereby eliminating the need for voltage adjustments and large components.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If the impedance imbalance is compensated by changing the magnitude of the output voltage for each arm, then the impedance balance is improved, but the rated voltage of the arm increases
Solution Approach 1:
The patent applies asymmetry by positioning the secondary winding and tertiary winding in vertically symmetrical locations relative to the primary winding, creating an asymmetric magnetic flux path that equalizes the impedance between primary-secondary and primary-tertiary sides without requiring voltage adjustments
Solution Approach 2:
The patent changes the physical parameter of winding placement position to achieve impedance equalization. By placing the secondary and tertiary windings at vertically symmetrical positions, the magnetic path lengths and impedances are equalized, eliminating the need for voltage magnitude changes
2Reliability
If the iron core size is increased to prevent saturation by DC magnetic flux, then the reliability is improved, but the device size increases
Solution Approach 1:
The patent converts the harmful DC leakage flux that would cause iron core saturation into a beneficial effect by providing a dedicated gap path. The gap allows the DC flux to circulate externally through the tank and gap rather than saturating the iron core, transforming a harmful phenomenon into a harmless external circulation path
Solution Approach 2:
The patent introduces a gap as an intermediary element between the secondary and tertiary windings. This gap serves as a mediator that provides an alternative path for DC magnetic flux, preventing it from saturating the iron core while maintaining the transformer's functional integrity
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 configuration achieves balanced impedance, reduces the rated voltage, and enables cost reduction and downsizing while maintaining reliability by canceling DC magnetic flux and suppressing high-harmonic components without requiring additional filters.
Implementation Method 1
The impedance of the three-phase three-winding transformer MMC is expressed by the impedance between the primary side and the secondary side, the impedance between the primary side and the tertiary side, and the impedance between the secondary side and the tertiary side
Implementation Method 2
the secondary winding and the tertiary winding are placed so that a DC leakage flux generated from the secondary winding passes through the iron core and the gap and a DC leakage flux generated from the secondary winding passes through the tank and the gap
Implementation Method 3
a converter transformer including a primary winding connected to the three phase AC, and a secondary winding and a tertiary winding provided between a second end of the positive-side arm and a second end of the negative-side arm
Implementation Method 4
enables cost reduction and downsizing while maintaining reliability by canceling DC magnetic flux and suppressing high-harmonic components
Implementation Method 5
since a three-phase three-winding transformer MMC suppresses a short-circuit current by the leakage impedance of a three-phase three-winding transformer, it is unnecessary to install a large component, such as a buffer reactor, in each phase, and high-harmonic components can be surely suppressed
Data Source
Figure 1~2
Figure 3~4
Figure 5
AI summary
A power conversion device excellent for cost reduction and downsizing is provided. A secondary winding 2 and a tertiary winding 3 are placed adjacent to a primary winding 1. The secondary winding 2 and the tertiary winding 3 are placed so as to be vertically symmetrical to each other. Alternatively, the primary winding 1, the secondary winding 2, and the tertiary winding 3 are placed concentrically in sequence. Moreover, the winding direction of the secondary winding 2 and that of the tertiary winding 3 are opposite to each other. Accordingly, the impedance between the primary side and the secondary side, and the impedance between the primary side and the tertiary side become equal.