Voltage Balancing Circuit for HVDC Converter Station Faults
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Solution Overview
Problem
In DC high voltage networks interconnected with AC high voltage networks, electrical faults between a pole and earth can induce persistent overvoltages, leading to dielectric breakdown, aging of equipment, and magnetic saturation or thermal limits in grounding devices, which existing solutions like dynamic braking systems are costly, complex, and have availability issues.
Innovation Solution
A converter station with a modular multilevel converter and a balancing circuit that includes a controlled switch and inductor between the intermediate node and ground, allowing for voltage balancing between poles by charging the faulty pole and discharging the healthy pole, using a transformer and earth connection device to stabilize voltages.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a faulty pole is brought back to earth potential after an electrical fault, then the fault is isolated and the DC network can be restored, but the healthy pole undergoes a voltage shift causing doubled potential difference between healthy pole and earth
Solution Approach 1:
The patent applies asymmetry by introducing a grounding device connected to the AC network that creates an asymmetric voltage distribution. When a fault occurs on one pole, the grounding device selectively connects to the healthy pole through the transformer, creating a controlled asymmetric path that prevents the healthy pole voltage from doubling. This asymmetric grounding approach resolves the contradiction by maintaining fault isolation capability while preventing overvoltage stress on the healthy pole.
Solution Approach 2:
The transformer connected to the AC network serves as an intermediary element between the DC poles and the AC grounding system. This intermediary allows the grounding device to connect to the healthy pole without directly exposing it to the full fault current, thereby mediating the voltage stress and preventing the doubled potential difference while still enabling effective fault isolation.
2Reliability
If grounding devices are connected to the AC network to handle faults, then fault isolation is achieved, but the grounding devices can reach magnetic saturation or thermal limits
Solution Approach 1:
The transformer connected to the AC network provides multi-functionality by serving both as a voltage transformation device and as a grounding path provider. This universal element allows the grounding device to handle fault currents without requiring dedicated high-capacity grounding equipment, thereby achieving fault isolation while preventing magnetic saturation and thermal overload through the transformer's inherent capabilities.
3Stability of the object's composition
If dynamic braking systems are used to balance voltages after faults, then voltage balancing is achieved, but the solution becomes costly and complex with availability issues
Solution Approach 1:
The patent extracts the voltage balancing function from the complex dynamic braking system and implements it through a simpler grounding device connected to the AC network. By removing the need for complex charging/discharging structures and using only a transformer with a grounding device, the solution achieves voltage balancing between poles while dramatically reducing device complexity and cost.
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 solution effectively balances voltages between poles in a short time, reducing the stress on dielectric materials, avoiding magnetic saturation, and ensuring network continuity without complete discharge of high voltage lines.
Implementation Method 1
a transformer (46) connected between said connection interface (460) and said intermediate node (40)
Implementation Method 2
a balancing circuit (41) comprising a controlled switch (411) and an inductor (412) connected in series between ground and said intermediate node (40)
Data Source
Figure 1~2
Figure 3~4
Figure 5~6
AI summary
The invention concerns a conversion station (4) comprising: - a connection interface (460) to an AC grid (91); - a connection interface to a DC grid comprising first and second poles (441, 442); - an AC/DC converter (42); - for each of the phases: - the AC/DC converter (42) connected between the first and second poles in symmetrical monopole mode, an intermediate node being connected to the connection interface (460) to the three-phase AC grid; - a voltage balancing circuit (41) of the first and second poles, including a controlled switch (411) and an inductance (412) connected in series between ground and the intermediate node (40); - a control device (43) for detecting an electrical fault, and for controlling the closing of said controlled switches when an electric fault is detected.