HVDC VDCOL Control Device Asymmetric Current Limits
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Solution Overview
Problem
In DC power transmission systems, especially those using long-distance cables, commutation failures during AC faults lead to prolonged recovery times due to similar DC current instruction limits at both the transmitting and receiving sides, which do not adequately address the reduction of recovery time for transmission power.
Innovation Solution
A control device for power converters with different VDCOL characteristics at the rectifier and inverter sides, featuring distinct upper and lower break points for DC current instruction limits, allowing for differential DC current management to quickly charge the DC line and recover transmission power during faults.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the same VDCOL characteristics are applied at both rectifier and inverter sides, then the system maintains symmetry and stability, but the recovery time of DC line and transmission power becomes prolonged
Solution Approach 1:
The patent applies asymmetry by setting different VDCOL characteristics at the rectifier side and inverter side. Specifically, the inverter side uses a lower DC current instruction limit than the rectifier side during voltage dips, creating an asymmetric current margin that enables faster charging of the DC line and quicker recovery of transmission power, while the rectifier side maintains higher limits to ensure system stability.
Solution Approach 2:
The patent implements local quality by tailoring the VDCOL characteristics to the specific functional requirements of each side of the HVDC system. The inverter side is configured with more aggressive current limiting (lower limits) optimized for rapid voltage recovery and line charging, while the rectifier side uses less aggressive limits (higher values) optimized for maintaining stable power flow and system control.
2Reliability
If the DC current instruction limit is reduced to prevent commutation failure, then voltage stress on valves is reduced, but the recovery time of transmission power increases
Solution Approach 1:
The patent resolves this contradiction by applying asymmetric current limits: the inverter side uses lower DC current instruction limits to prevent commutation failure and protect valves, while the rectifier side maintains higher limits. This asymmetric configuration creates a controlled current margin that enables the DC line to charge faster during faults, reducing power recovery time without compromising commutation failure prevention at the vulnerable inverter side.
3Loss of time
If the DC current instruction limit is increased to reduce recovery time, then power transmission resumes faster, but commutation failure risk increases
Solution Approach 1:
The patent applies local quality by differentiating the DC current instruction limits based on the specific operational characteristics and risk profiles of each side. The inverter side, which is more susceptible to commutation failure, uses lower current limits as a protective measure. The rectifier side, which has lower commutation failure risk, uses higher current limits to accelerate recovery. This localized optimization achieves fast recovery without uniformly increasing commutation failure risk across the entire system.
Data Source
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Figure 3(a)~3(c)
AI summary
Provided is a control device of a power converter which can reduce a charging time of a DC line and a recovery time of transmission power. VDCOL control circuits 10A and 10B have different VDCOL characteristics at a rectifier-3A side and an inverter-3B side, respectively, each VDCOL characteristic has two break points of high and low for the DC voltage, the upper limit value of the DC current instruction value within the region between the high and low break points is set to be lower at the inverter-3B side than the rectifier-3A side, and the low break points of the respective VDCOL characteristics are consistent, the upper limit value of the DC current instruction value within the region equal to or smaller than the low break point is set to be lower at the inverter-3B side than the rectifier-3A side.