LCC HVDC Commutation Failure Mitigation via Parallel Capacitor Modules
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Solution Overview
Problem
Line commutated converters (LCC) in high-voltage direct current (HVDC) systems are susceptible to commutation failure during AC side faults, leading to potential blackouts and require significant reactive power compensation and harmonic filtering, which increases costs and losses.
Innovation Solution
The implementation of a line commutated converter design that includes parallel capacitor modules to assist in commutation, controllably insertable capacitor modules to vary the extinction angle, and inductor modules to reduce commutation time, along with a fixed capacitor module to generate reactive power, reducing the need for external reactive power compensation and harmonic filtering.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If LCC systems use thyristors for power conversion, then transmission losses are reduced and power handling capability is improved, but commutation failure susceptibility increases during AC side faults
Solution Approach 1:
A commutation capacitor is introduced as an intermediary energy storage device between the AC system and the thyristor valve. During commutation, the capacitor provides additional energy to ensure complete current transfer between valves, acting as a mediator that guarantees reliable commutation without requiring changes to the thyristor switching mechanism itself.
Solution Approach 2:
The commutation capacitor is pre-charged to a specific voltage before commutation begins. This preliminary energy storage ensures that when commutation is needed, the capacitor can immediately provide the required energy boost to force current transfer, eliminating the uncertainty of natural commutation during AC faults.
2Device complexity
If LCC systems operate with standard commutation timing, then system simplicity is maintained, but reactive power consumption increases significantly
Solution Approach 1:
The commutation capacitor serves as an intermediary that supplies reactive power during commutation events, reducing the burden on the AC system. By storing energy locally during non-commutation periods and releasing it during commutation, the capacitor mediates the reactive power exchange, allowing for reduced overall reactive power consumption without complicating the system architecture.
3Ease of operation
If thyristor valves switch at standard intervals, then operation is simplified, but harmonic generation propagates into the AC system
Solution Approach 1:
The commutation capacitor acts as a harmonic filter by providing a low-impedance path for harmonic currents. During thyristor switching, the capacitor's impedance characteristics divert harmonic content away from the AC system while allowing fundamental frequency power transfer to continue uninterrupted, thus mediating between simple thyristor operation and harmonic suppression.
4Reliability
If commutation time is extended to ensure complete current transfer, then commutation reliability is improved, but transmission efficiency decreases
Solution Approach 1:
By pre-charging the commutation capacitor before commutation begins, the energy required for complete current transfer is prepared in advance. This allows the commutation process to proceed rapidly and reliably without extending the commutation time window, as the capacitor provides an immediate energy boost that forces complete current transfer in minimal time, thus maintaining transmission efficiency while ensuring reliability.
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 enhances commutation reliability, reduces the risk of commutation failure, eliminates the need for reactive power compensation devices, and significantly reduces harmonic transmission to the AC system, leading to lower costs, reduced losses, and improved system reliability.
Implementation Method 1
the at least one parallel capacitor is configured to discharge current in to the second branch in the same direction as the flow of current in the second branch
Implementation Method 2
an inductor module comprising at least one current limiting inductor in series with each branch, the inductor module configured to reduce the time period taken for commutation between thyristor valves to complete
Implementation Method 3
The capacitances of the parallel capacitor modules being selected so that a first electrical path through the parallel capacitor module has a lower impedance at harmonic frequencies than a second electrical path through the inductor module to the AC system in order to restrict the transmission of harmonic frequencies to the AC system
Data Source
Figure 1a
Figure 1b
Figure 1c
AI summary
The disclosure relates to a line commutated converter, LCC, for a high-voltage direct current, HVDC, power converter. The LCC comprises at least one bridge circuit for connection to at least one terminal of a DC system. Each bridge circuit comprises at least two arms, and each arm is associated with a phase of an AC system. Each arm comprises one or more upper thyristor valves and one or more lower thyristor valves connected in series, and a branch extending from between the upper and lower thyristor valves. Each arm further comprises a parallel capacitor module comprising at least one parallel capacitor being connected in parallel between at least one pair of branches comprising a first branch and a second branch wherein during commutation of a flow of current in the first branch to a flow of current in the second branch, the at least one parallel capacitor is configured to discharge current in to the second branch in the same direction as the flow of current in the second branch.