LCC-MMC Hybrid Converter Parallel MMC Unit DC Distribution Control

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Solution Overview

Problem

Current high voltage direct-current transmission systems using LCC converters face issues such as large occupied areas, overvoltage during load shedding, and sensitivity to AC faults, while MMC converters offer advantages like active and reactive power decoupling but require multiple units in parallel, necessitating a control strategy for parallel MMC units in LCC-MMC hybrid cascade converter stations to manage direct-current distribution effectively.

Innovation Solution

A control strategy for parallel MMC units in LCC-MMC hybrid cascade converter stations involves numbering units based on rated capacity, calculating direct-current and active power instruction values, and correcting direct-current voltage instructions to ensure balanced distribution and fault self-clearing capabilities, facilitating simultaneous control of direct-current voltage and active power.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Power

If multiple MMC units are connected in parallel to achieve high direct current capacity, then the direct current transmission capacity is improved, but the control complexity and difficulty of detecting and measuring direct current distribution increases

Engineering Contradiction:

Improvedirect current transmission capacityVSAvoiddirect current distribution control

Core Design Contradiction:

PowerVSDifficulty of detecting and measuring

Solution Approach 1:

The patent divides the parallel MMC units into different control groups (constant direct current voltage control group and constant active power control group) to manage the complexity of controlling multiple units. This segmentation allows each group to have dedicated control strategies, simplifying the overall control architecture while maintaining high transmission capacity.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent implements feedback mechanisms where the direct current measurement values from each MMC unit are continuously monitored and used to adjust control instructions. The control system calculates instruction values based on actual measurements and feeds them back to maintain balanced direct current distribution among parallel units.

Inventive Principle:

Principle #23Feedback

2Ease of operation

If constant direct current voltage control manner is used for all parallel MMC units, then the control simplicity is improved, but the direct current distribution balance deteriorates due to impedance differences

Engineering Contradiction:

Improvecontrol simplicityVSAvoiddirect current distribution balance

Core Design Contradiction:

Ease of operationVSStability of the object's composition

Solution Approach 1:

The patent applies different control characteristics to different MMC units based on their local characteristics. Units with lower impedance are assigned to constant direct current voltage control group, while units with higher impedance are assigned to constant active power control group. This local differentiation ensures balanced direct current distribution while maintaining overall system simplicity.

Inventive Principle:

Principle #3Local quality

3Ease of manufacture

If LCC converter is used for high voltage direct current transmission, then the technical maturity and economic efficiency are improved, but the system sensitivity to AC faults and overvoltage during load shedding increases

Engineering Contradiction:

Improvetechnical maturity and economic efficiencyVSAvoidsensitivity to AC faults and overvoltage

Core Design Contradiction:

Ease of manufactureVSObject-affected harmful factors

Solution Approach 1:

The patent merges the advantages of LCC (used on the sending end for technical maturity and economic efficiency) with the advantages of MMC (used on the receiving end for fault self-clearing capability and active-reactive power decoupling control). This hybrid cascade structure combines both converter types to achieve complementary benefits.

Inventive Principle:

Principle #5Merging (Combining)

Solution Approach 2:

The patent converts the limitation of single MMC unit current capacity into a benefit by using multiple MMC units in parallel with differentiated control. The constant active power control group specifically addresses impedance-related distribution issues, turning a potential problem into a controlled feature that enhances overall system performance.

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

Data Source

PatentUS11616369B2Control method for a parallel MMC unit of a LCC-MMC hybrid cascade converter station

Publication Date: 2023.03.28 STATE GRID JIANGSU ELECTRIC POWER CO LTD

AI summary

Provided is a control method for a parallel MMC unit of a LCC-MMC hybrid cascade converter station. The control strategy includes: 1) numbering all MMC units connected in parallel in a MMC valve manifold; (2) for a MMC unit using a constant direct-current voltage control manner, calculating a direct-current instruction value of the MMC unit according to a direct-current measurement value; (3) for a MMC unit using a constant active power control manner, calculating an active power instruction value of the MMC unit according to the rated capacity of the MMC unit and a direct-current instruction value of a system rectifier station; (4) for the MMC unit using the constant direct-current voltage control manner, correcting a direct-current voltage instruction value of the MMC unit by using the direct-current instruction value and the direct-current measurement value, and controlling the MMC unit according to the corrected direct-current voltage instruction value.