MMC Cell Bypass Control for Arm Current Harmonic Suppression
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Solution Overview
Problem
The modular multilevel converter (MMC) experiences an increase in harmonic components in the arm current after bypassing a failed converter cell, leading to unbalanced capacitor voltages and potential shutdown of the converter due to protection mechanisms.
Innovation Solution
A power conversion device is designed to suppress the harmonic component in the arm current by implementing a control strategy that adjusts the PWM reference phase for each converter cell and includes a cell additional voltage calculator to ensure balanced capacitor voltages even after bypassing a failed cell.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a failed converter cell is bypassed to maintain converter operation, then the converter can continue operating, but the harmonic component in the arm current increases
Solution Approach 1:
The invention changes the PWM reference phase parameters for each converter cell dynamically. When a converter cell fails and is bypassed, the control device calculates and adjusts the PWM reference phase for each remaining converter cell based on its position and the failed cell's location. This parameter adjustment ensures that the phase shift between converter cells maintains harmonic cancellation even with an unequal number of cells, thereby suppressing the increase in harmonic components in the arm current while allowing the converter to continue operating.
2Object-generated harmful factors
If the PWM reference phase is equally shifted for each converter cell, then the harmonic component is canceled, but this equal shifting cannot be maintained when converter cells are bypassed
Solution Approach 1:
The invention makes the PWM reference phase shift dynamic rather than static. The control device continuously monitors the operational status of converter cells and dynamically adjusts the PWM reference phase for each cell based on real-time conditions. When converter cells are bypassed due to failure, the system transitions from equal phase shifting to unequal phase shifting, where each remaining cell's phase is individually adjusted according to its position and the failure pattern, maintaining harmonic cancellation adaptability.
Solution Approach 2:
The invention applies different PWM reference phase shifts to different converter cells based on their local conditions. Instead of a uniform phase shift for all cells, each converter cell receives a customized phase shift value calculated based on its specific position in the arm and the location of failed cells. This local quality approach ensures that harmonic cancellation is maintained despite the non-uniform configuration caused by bypassed cells.
3Adaptability or versatility
If converter cells operate with unequal phase shifts after bypass, then the system adapts to failure, but harmonic components remain in the output voltage
Solution Approach 1:
The invention implements a feedback mechanism where the control device continuously monitors the operational status of converter cells and the resulting harmonic components in the output voltage. Based on this feedback information, the control device calculates and adjusts the PWM reference phase for each converter cell to minimize harmonic content. This closed-loop control ensures that even when converter cells are bypassed, the system can adaptively adjust phase shifts to suppress harmonic components in the output voltage.
Data Source
Figure 1
Figure 2(a)~2(b)
Figure 3
AI summary
A power converter (2) includes an arm (5, 6) in which a plurality of converter cells (7) are connected in series, each of the converter cells including at least two switching elements (31p, 31n), a power storage element (32), and a pair of output terminals (P1, P2). A control device (3) controls the power converter (2). The converter cell (7) includes a switch (34) to have the converter cell (7) bypassed. When the control device (3) senses failure of the converter cell (7), it has the failed converter cell bypassed, estimates an output voltage lost by bypassing the failed converter cell, and has a normal converter cell supply the estimated output voltage of the failed converter cell.