MMC Cell Bypass Control for Short-Circuit Current Limiting

Solution Overview

Problem

High Voltage Direct Current (HVDC) Voltage Source Converters (VSCs) face challenges with cell bypass component stress due to high Mega Ampere short circuit currents, leading to increased costs for specialized bypass switches designed to handle these currents.

Innovation Solution

A method for controlling cell operation in VSCs to limit or prevent Mega Ampere short circuit currents during cell faults by injecting a pre-defined circulating current and controlling the bypass switch to close during specific arm current directions, reducing stress on bypass components and allowing them to be dimensioned for smaller currents.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If bypass switch is designed to sustain MA short circuit currents, then reliability of cell bypass operation is improved, but device complexity and cost increase

Engineering Contradiction:

Improvebypass switch reliabilityVSAvoidbypass switch complexity

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The control unit performs preliminary actions by detecting cell faults and controlling other switches in the cell before closing the bypass switch. This preparatory control of switches limits the short circuit current that would otherwise flow through the bypass switch, allowing it to be designed for smaller currents while maintaining reliability

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The control unit continuously monitors cell operation and provides feedback control by detecting faults and adjusting switch states dynamically. This feedback mechanism ensures that the bypass switch only closes when conditions are controlled to limit current, reducing the need for high-current capability in the bypass switch design

Inventive Principle:

Principle #23Feedback

2Reliability

If bypass switch is designed to sustain MA short circuit currents, then cell bypass operation is reliable, but cost increases

Engineering Contradiction:

Improvebypass switch reliabilityVSAvoidmanufacturing cost

Core Design Contradiction:

ReliabilityVSEase of manufacture

Solution Approach 1:

The control unit performs preliminary actions by detecting cell faults and controlling other switches in the cell before closing the bypass switch. This preparatory control of switches limits the short circuit current that would otherwise flow through the bypass switch, allowing it to be designed for smaller currents while maintaining reliability

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The invention replaces the need for expensive, high-current-capable bypass switches with a control strategy that protects a simpler, lower-cost bypass switch. By using control logic and other switches to limit current, the bypass switch can be a more economical component

Inventive Principle:

Principle #27Cheap short-living objects (Disposable)

3Strength

If cell bypass components are dimensioned for MA short circuit currents, then bypass switch can handle fault conditions, but component size and complexity increase

Engineering Contradiction:

Improvebypass switch current handling capabilityVSAvoidbypass component complexity

Core Design Contradiction:

StrengthVSDevice complexity

Solution Approach 1:

The control unit performs preliminary actions by detecting cell faults and controlling other switches in the cell before closing the bypass switch. This preparatory control of switches limits the short circuit current that would otherwise flow through the bypass switch, allowing it to be designed for smaller currents while maintaining reliability

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The control unit acts as an intermediary that mediates between the fault condition and the bypass switch. By controlling other switches to limit current before the bypass switch closes, the control unit protects the bypass switch from high currents, allowing simpler bypass component design

Inventive Principle:

Principle #24Intermediary (Mediator)

Data Source

PatentUS20250007426A1Method for controlling operation of a cell in an mmc

Publication Date: 2025.01.02 HITACHI ENERGY LTD

AI summary

The present application relates to a method for controlling operation of a cell arranged in an arm of a voltage source converter, said arm being associated with a current phase in said voltage source converter. The cell may be a half-bridge or full-bridge cell. The method may comprise: upon detection of a predetermined fault type in the cell, depending on an average current in said arm during one period of fundamental frequency and on an operation mode of the voltage source converter, injecting a pre-defined circulating current into said arm; detecting a time period during which the arm current is in a negative arm current direction corresponding to a direction when arm current flows via a diode of a switch connected in parallel to a bypass switch, and controlling the bypass switch such that it closes within said time period. A control unit and voltage source converter are also provided.