AC-Side Chopper Circuit for Multilevel Converter Fault Ride-Through
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Solution Overview
Problem
Traditional chopper circuits in multilevel converters are economically unfeasible due to additional power electronic switches and thermal stresses, especially in high-voltage direct current (HVDC) systems, where they are needed to manage power dissipation during faults and maintain frequency stability.
Innovation Solution
A multilevel converter design with a chopper circuit placed between converter blocks, allowing for low voltage rating and simplified construction, with the ability to control current through the chopper circuit close to zero, eliminating the need for current breaking capability and reducing costs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If a traditional DC chopper is connected on a DC bus of the DC transmission system, then the chopper can be placed in the DC system, but it cannot absorb active power due to the low DC voltage
Solution Approach 1:
The patent introduces an AC-side chopper circuit as an intermediary component between the AC system and the DC transmission system. This chopper operates on the AC side where voltage levels are sufficient to absorb active power during faults, thereby resolving the limitation of traditional DC-side choppers that cannot absorb active power due to low DC voltage.
2Device complexity
If a chopper circuit is integrated in the main converters, then the chopper functionality is integrated, but the cost of the switch becomes substantial due to the high voltage capability required and direct current breaking capability
Solution Approach 1:
The patent segments the chopper functionality from the main converter by placing a dedicated chopper circuit on the AC side. This segmentation allows the use of lower voltage-rated switches in the chopper circuit, as it only needs to handle AC voltage levels rather than the full DC transmission voltage, thereby reducing the cost of switches while maintaining chopper functionality.
Solution Approach 2:
Instead of placing the chopper on the traditional DC side or integrating it into the main converter, the patent inverts the approach by placing the chopper on the AC side. This inversion allows the use of AC-rated switches with lower voltage capabilities, significantly reducing the cost while achieving the same fault handling objective.
3Reliability
If a resistive circuit is added to the AC system or a chopper circuit to the DC system, then the resistor dissipates energy and supports frequency stability, but additional power electronic switches are required which increases cost and thermal stresses
Solution Approach 1:
The patent makes the AC-side chopper circuit multi-functional by enabling it to perform both the traditional chopper function (absorbing active power during DC faults) and AC fault ride-through (dissipating energy during AC faults). This universality eliminates the need for separate power electronic switches for different fault conditions, reducing device complexity while maintaining frequency stability.
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 design minimizes non-transferred energy, stabilizes AC frequency, simplifies the circuit breaker, and reduces chopper circuit costs, enabling effective fault ride-through for both DC and AC sides.
Implementation Method 1
the resistor dissipates energy generated in the power generation system
Data Source
Figure 1
Figure 2
Figure 3~4
AI summary
A multilevel converter (12) comprises a first and a second converter block (CB1, CB2) series-connected between two DC terminals (DC1, DC2) and a chopper circuit (CC) between the blocks (CB1, CB2). Each block (CB1, CB2) comprises a number of valve arm strings, each comprising an upper and a lower valve arm (VASA1, VASB1, VASC1, VASA2, VASB2, VASC2) and a number of converter cells. A midpoint between the upper and lower arm (VAUA1, VALA1, VAUB1, VALB1, VAUC1, VALC1) of a string (VASA1, VASB1, VASC1) of the first block (CB1) is connected to a corresponding primary AC terminal (ACA1, ACB1, ACC1) and a midpoint between the upper and lower arm (VAUA2, VALA2, VAUB2, VALB2, VAUC2, VALC2) of a corresponding string (VASA2, VASB2, VASC2) of the second block (CB2) is connected to a corresponding secondary AC terminal (ACA2, ACB2, ACC2). The chopper circuit (CC) comprises a power dissipating element (R) in parallel with a circuit breaker (CB).