Voltage Balancing Unit for Symmetric HVDC Monopole Lines
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Solution Overview
Problem
Symmetric monopole HVDC power transmission networks experience voltage unbalances due to temporary earth faults, which hinder automatic restarts and pose risks of overvoltage on healthy poles, necessitating efficient balancing solutions.
Innovation Solution
A voltage balancing unit comprising second arresters with a lower switching impulse protective level (SIPL) than the first arresters, connected only after the fault is cleared, to balance pole voltages and prevent overstress, utilizing high-speed switches and disconnectors for quick connection and disconnection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If discharge resistors and DC choppers are used to remove voltage unbalance, then voltage balancing is achieved, but device complexity and cost increase
Solution Approach 1:
The patent uses second arresters with lower SIPL as temporary, fault-specific protective devices that are only activated during earth fault conditions. These arresters provide inexpensive voltage balancing capability without requiring complex control systems or expensive discharge resistors, aligning with the principle of using simple, cost-effective solutions for specific problem conditions.
Solution Approach 2:
The patent changes the protective level parameter by introducing second arresters with lower SIPL than the first arresters. This parameter change enables the second arresters to activate selectively during earth faults to balance pole voltages, providing a simple mechanism for voltage balancing without complex control equipment.
2Reliability
If second arresters with lower SIPL are connected continuously, then voltage balancing is maintained, but arrester overstress occurs during normal operation
Solution Approach 1:
The patent implements dynamic switching of the second arresters using switching means that connect them only during earth fault conditions and disconnect them during normal operation. This dynamic configuration allows the system to maintain voltage balance when needed while preventing arrester overstress during normal conditions, directly applying the dynamics principle.
Solution Approach 2:
The switching means act as an intermediary between the permanent first arresters and the conditional second arresters. This intermediary controls when the second arresters are connected to the circuit, enabling voltage balancing during faults while preventing continuous connection that would cause overstress during normal operation.
3Productivity
If automatic restart is enabled after earth faults, then productivity improves, but voltage unbalance causes harmful overvoltage on healthy poles
Solution Approach 1:
The patent applies preliminary anti-action by connecting the second arresters before restart occurs to prevent the harmful effect of overvoltage. The second arresters are positioned to activate during earth faults and remain connected through the restart process, preemptively counteracting the voltage unbalance that would otherwise cause overvoltage on the healthy pole during restart.
Solution Approach 2:
The second arresters with lower SIPL provide beforehand cushioning by being ready to limit overvoltage on healthy poles during restart. This protective measure is in place before the harmful overvoltage effect can occur, cushioning the system against potential damage during the restart process following an earth fault.
Data Source
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AI summary
A voltage balancing unit (330) for a symmetric monopole high voltage direct current (HVDC) transmission line (303) interconnecting two voltage source converters (VSCs) (301, 302), the transmission line comprising first arresters (321-324) having a first switching impulse protective level (SIPL), is provided. The voltage balancing unit comprises a pair of second arresters (331, 332) having a second SIPL which is less than the first SIPL, and switching means (333, 334) being arranged for temporarily connecting, in the event of a voltage unbalance on the transmission line, the first arresters between either pole (304, 305) and ground. The invention makes use of an understanding that a voltage unbalance on the transmission line may be removed by temporarily connecting second arresters, with a significantly lower SIPL than the first arresters, for limiting the pole voltage to a level close to the normal voltage. Further, a method of voltage balancing is provided. Voltage unbalances may arise as a consequence of lightning induced earth faults (311).