Solusyon sa Rectifier-Type Hybrid High-Voltage DC Circuit Breaker

I. Konteksto ug Analisis sa Gipangangailangan sa Proyekto​
Batasan sa pagkamadani sa transition sa energy, ang teknolohiya sa flexible DC transmission batas Voltage Source Converter (VSC) naging usa ka pangunahon nga solusyon alang sa pag-integrate sa large-scale renewable energy ug pag-ataas sa capability sa long-distance power transmission, tungod sa iyang mga advantage sama sa independent control sa active ug reactive power ug low harmonic content. Ang pagtukod sa flexible DC grids mao ang inevitable trend. Sa kontekstoni, ang high-voltage DC circuit breakers, isip core protection devices alang sa rapid fault isolation ug pag-ensuro sa grid safety ug stability, maoy kritikal nga importante. Walay high-performance DC circuit breakers, ang operational flexibility ug power supply reliability sa flexible DC grids mahimong mapugos.

Ang kasamtangan nga mainstream high-voltage DC circuit breaker technologies adunay significant limitations:

• ​Mechanical Circuit Breakers: Bagama og sila naghatag og low on-state losses ug high withstand voltage, ang ilang interruption time mao ang tens of milliseconds, wala makaabot sa stringent requirement sa millisecond-level rapid fault isolation sa flexible DC grids.
• ​All-Solid-State Circuit Breakers: Batasan sa semiconductor devices, sila naghatag og extremely fast interruption apan adunay excessive on-state losses, high operating costs, ug poor economic efficiency.
• ​Traditional Hybrid Circuit Breakers: Bagama og sila nagcombine sa low losses sa mechanical switches ug fast interruption sa solid-state switches, ang ilang topology nagkinahanglan og series-connected IGBTs sa both forward ug reverse directions, resulta mao ang low device utilization, system complexity, ug high costs.

Arin ang technical bottlenecks, adunay urgent need para sa bag-ong DC circuit breaker solution nga mag-combine sa fast interruption capability, low operating losses, high economic efficiency, ug high reliability.

​II. Solusyon: Rectifier-Type Hybrid High-Voltage DC Circuit Breaker​
Kini nga solusyon gi-propose ang innovative rectifier-type hybrid high-voltage DC circuit breaker topology, fundamentally addressing the limitations of existing technologies.

​​(I) Core Technology: Innovative Circuit Topology​
Ang topology sa kini nga circuit breaker gisakop sa current-carrying branch ug current-breaking branch connected in parallel.

1. ​Current-Carrying Branch:
• ​Composition: Comprises a high-speed mechanical switch (S1) ug current-carrying valve group (Q1) connected in series.
• ​Features: S1 adunay extremely low contact resistance (only tens of micro-ohms), ug Q1 gisakop sa small number of IGBTs with low conduction voltage drop. Sa normal operation, ang rated current flows through this branch, ensuring extremely low on-state losses.
2. ​Current-Breaking Branch:
• ​Composition: Utilizes a bridge rectifier structure, consisting of a bridge commutation valve group (D1-D4, formed by multiple series-connected diodes), unidirectional interruption valve group (Q2, formed by multiple series-connected IGBTs), ug nonlinear resistor (MOV1, arrester).
• ​Core Advantage: Ang bridge rectifier structure cleverly achieves current commutation, enabling the unidirectional IGBT interruption valve group (Q2) to interrupt bidirectional DC fault currents. Compared to traditional hybrid topologies, ang number of IGBTs reduced by approximately half. Given that commercial press-pack IGBTs cost about 10 times more than diodes of the same rating, ug ang reduction in IGBTs also decreases the number of accompanying driver boards, this topology achieves significant cost reduction and overall reliability improvement.

​​(II) Efficient Interruption Working Principle​
Taking current flowing from Port 1 to Port 2 as an example, ang interruption process consists of four stages:

1. ​Stage 1 (t0–t1, Fault Occurrence)​: A short-circuit fault occurs on the line, causing the current to rise sharply. At this time, S1 and Q1 are conducting, Q2 is off, ug ang fault current flows entirely through the current-carrying branch.
2. ​Stage 2 (t1–t2, Current Transfer)​: The control system issues an opening command, turning on Q2 and turning off Q1. Ang conduction of Q2 generates a commutation voltage on the bridge arm, forcing the current to transfer from the current-carrying branch to the current-breaking branch (path: D1 → Q2 → D4).
3. ​Stage 3 (t2–t3, Mechanical Switch Interruption)​: After the current in the current-carrying branch is completely transferred, ang high-speed mechanical switch S1 interrupts under zero-current and zero-voltage conditions without arcing, establishing insulation strength.
4. ​Stage 4 (t3–t4, Fault Current Clearance)​: After S1 is fully interrupted, Q2 is turned off. Ang turn-off of Q2 generates a transient overvoltage across the circuit breaker, triggering MOV1 to conduct and diverting the fault current into MOV1 for dissipation until the energy is exhausted, the current drops to zero, and fault isolation is completed.

The interruption principle for reverse current is the same, guided by the diode bridge (D2, D3) to flow through Q2.

​​(III) Intelligent Control Strategy​

1. ​Pre-Interruption Control Strategy:
• ​Purpose: To overcome the bottleneck of the high proportion of the high-speed mechanical switch opening time (about 2 ms), shorten the total interruption time, and suppress the peak fault current.
• ​Logic: By real-time monitoring of bus voltage, line voltage, and line current (a total of 6 criteria, as shown in Table 1), once any abnormal criterion is triggered, the pre-interruption operation is initiated in advance (transferring current to the current-breaking branch and opening S1). If a formal opening command is subsequently received, the interruption is completed; if it is a false alarm, the current is transferred back to the current-carrying branch to resume normal operation.
• ​Effect: Simulations show that this strategy can suppress the fault current from 25 kA to 17 kA, with the total interruption time stabilized within 3 ms.

​Table 1: Pre-Interruption Activation Criteria​

1. ​Soft Closing Control Strategy:
• ​Purpose: To address the potential overvoltage and system oscillation issues at the moment of closing, without the need for additional resistors and switches, saving costs and space.
• ​Logic: The current-breaking branch is treated as composed of multiple medium-voltage units connected in series. During closing, these medium-voltage units are sequentially and controllably turned on to gradually establish a path. After each step, fault detection is performed. If no fault is detected, the process continues until all units are turned on. Finally, the current-carrying branch is closed, and the current-breaking branch is turned off. If a fault is detected during the process, closing is immediately aborted.
• ​Applicability: Suitable for normal closing and automatic reclosing after fault clearance. Simulations verify no overvoltage or oscillation.

​III. Prototype Development and Experimental Verification​

​​(I) Key Parameters and Structure of the Prototype​
A 500 kV DC circuit breaker engineering prototype was developed with the following key parameters:

• ​Structural Design:
• ​Current-Carrying Branch: As it carries current for extended periods, Q1 is equipped with a water-cooling system and placed at the bottom of the valve tower; S1 consists of multiple vacuum switches in series, driven by an electromagnetic repulsion mechanism, and placed at the top of the valve tower.
• ​Current-Breaking Branch: Composed of 10 series-connected 50 kV medium-voltage units, installed in 2 valve towers (5 layers each). Q2 adopts a dual-parallel IGBT design to meet the interruption capacity. This branch carries no current during normal operation, so no cooling is required, resulting in a more streamlined design.

​​(II) Experimental Verification Results​
The prototype underwent rigorous testing using an equivalent experimental circuit (LC oscillating circuit):

• ​Commutation Time: The time for current transfer from the current-carrying branch to the current-breaking branch was < 300 μs.
• ​Total Interruption Time: From receiving the opening command to the current beginning to drop, it took about 2.9 ms, meeting the design target of <3 ms.
• ​Transient Overvoltage: An instantaneous overvoltage of about 800 kV was generated during interruption, consistent with the MOV protection level, controlled and safe.
• ​Conclusion: The experiments successfully verified the feasibility, effectiveness, and excellent performance of the rectifier-type hybrid high-voltage DC circuit breaker topology.

​IV. Core Conclusions:

1. The rectifier-type hybrid topology proposed in this solution uses an innovative design with a diode bridge to achieve bidirectional current control, reducing IGBT usage by approximately 50% compared to traditional solutions, offering significant advantages in economic efficiency and reliability.
2. The intelligent pre-interruption and soft closing control strategies effectively address the issues of mechanical switch action delay and closing impact, enhancing the overall dynamic performance of the system.
3. The successful development and testing of the 500 kV/25 kA engineering prototype fully demonstrate the engineering feasibility and performance compliance of this technical approach.