Fault-Blocking VSC Control for DC Fault Energy Dissipation
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Solution Overview
Problem
Conventional HVDC transmission systems face challenges in efficiently managing DC side faults, as fault currents can persist due to the conduction of diodes in voltage source converters, requiring costly and bulky DC breakers, and existing fault blocking VSCs may prolong current flow and delay system restarts due to trapped energy.
Innovation Solution
Operating voltage source converters in a non-blocked state to dissipate stored energy after a DC side interruption is identified, followed by switching to a blocked state to isolate the VSC from the DC system, using a controller to generate a voltage order that extracts energy from the DC system and controls its magnitude based on current flow, allowing for rapid energy dissipation and fault clearing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If VSCs are switched to a blocked state to block fault current, then AC contribution to fault current is blocked, but DC fault current may persist due to diode conduction and trapped energy
Solution Approach 1:
Instead of immediately blocking the VSC upon fault detection, the invention inverts the conventional approach by initially maintaining the VSC in a non-blocked state to allow controlled energy dissipation through the AC system, thereby reducing DC fault current duration while managing AC contribution dynamically
Solution Approach 2:
The controller performs preliminary detection of DC faults and initiates a controlled sequence where the VSC remains operational for a predetermined period to dissipate energy before transitioning to the blocked state, preventing immediate current interruption that could trap energy
2Reliability
If DC breakers are used to interrupt DC fault current, then fault isolation is achieved, but the system becomes costly and bulky
Solution Approach 1:
The invention extracts the fault isolation function from the DC breaker and relocates it to the VSC controller, which manages fault current by controlling the converter operation state and energy dissipation timing, eliminating the need for expensive DC breakers
Solution Approach 2:
The VSC acts as an intermediary between the AC and DC systems during faults, using its controllable switching elements to manage energy flow and dissipate DC fault current through the AC system, thereby preventing the need for direct DC current interruption devices
3Loss of energy
If VSCs are operated in non-blocked state during DC faults, then energy dissipation is improved, but AC contribution to fault current increases
Solution Approach 1:
The invention dynamically controls the VSC operation state based on real-time fault conditions, transitioning from non-blocked to blocked state after a predetermined period or when energy dissipation is sufficient, thereby balancing energy dissipation needs with AC fault current contribution limits
Solution Approach 2:
The controller implements periodic monitoring of fault conditions and energy levels, switching the VSC state in controlled intervals to dissipate energy in stages while limiting AC contribution, rather than maintaining a fixed operation state throughout the fault event
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 approach enables faster isolation of faults, extinction of fault arcs, and quicker restarts of power transmission by dissipating stored energy in the DC system, reducing the need for expensive DC breakers and minimizing current oscillations, thus enhancing fault handling and system resilience.
Implementation Method 1
VSCs use switching elements such as insulated-gate bipolar transistors (IGBTs) that can be controllably turned on and turned off independently of any connected AC system. VSCs are thus sometime referred to as self-commutating converters.
Implementation Method 2
In another form of VSC referred to a modular multilevel converter (MMC) each converter arm comprises a plurality of series connected cells that each have an energy storage element such as a capacitor that can be selectively connected in series between the relevant AC and DC terminals
Data Source
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AI summary
This application relates to methods and apparatus for controlling a fault blocking voltage source converter (VSC) apparatus, e.g. a fault blocking VSC (200, 300) with one or one more full bridge cells (204) or a combination of VSCs (1201, 1202) with fault blocking capability. In the event of a DC side interruption, such as a DC side fault, the method involves operating (403, 404) the voltage source converter apparatus after identification (401) of the need for the DC side interruption to extract at least some electrical energy from the connected DC system to the VSC. At least some of the energy extracted from the DC system may be transferred to AC system. Once sufficient energy is extracted from the DC system the VSC may be isolated from the DC system and the voltage source converter apparatus may be switched (406) to a blocked state. Thus rather than instantly switch a fault blocking VSC to the blocked state on detection of a DC side fault the VSC is continued to be operated, and in a manner to transfer at least some energy stored in the DC system to the AC system before blocking.