Rectifier-Type Hybrid High-Voltage DC Circuit Breaker Solution

I. Project Background and Requirement Analysis​
With the deepening advancement of the energy transition, voltage source converter (VSC)-based flexible DC transmission technology has become a key solution for large-scale renewable energy integration and enhancing long-distance power transmission capabilities, owing to its advantages such as independent control of active and reactive power and low harmonic content. The construction of flexible DC grids is an inevitable trend. In this context, high-voltage DC circuit breakers, as core protection devices for rapid fault isolation and ensuring grid safety and stability, are critically important. Without high-performance DC circuit breakers, the operational flexibility and power supply reliability of flexible DC grids would be severely constrained.

Current mainstream high-voltage DC circuit breaker technologies have significant limitations:

• ​Mechanical Circuit Breakers: Although they offer low on-state losses and high withstand voltage, their interruption time is tens of milliseconds, failing to meet the stringent requirement of millisecond-level rapid fault isolation in flexible DC grids.
• ​All-Solid-State Circuit Breakers: Based on semiconductor devices, they provide extremely fast interruption but suffer from excessive on-state losses, high operating costs, and poor economic efficiency.
• ​Traditional Hybrid Circuit Breakers: While combining the low losses of mechanical switches and the fast interruption of solid-state switches, their topology requires series-connected IGBTs in both forward and reverse directions, resulting in low device utilization, system complexity, and high costs.

To address these technical bottlenecks, there is an urgent need for a new DC circuit breaker solution that combines fast interruption capability, low operating losses, high economic efficiency, and high reliability.

​II. Solution: Rectifier-Type Hybrid High-Voltage DC Circuit Breaker​
This solution proposes an innovative rectifier-type hybrid high-voltage DC circuit breaker topology, fundamentally addressing the limitations of existing technologies.

​​(I) Core Technology: Innovative Circuit Topology​
The topology of this circuit breaker consists of a current-carrying branch and a current-breaking branch connected in parallel.

1. ​Current-Carrying Branch:
• ​Composition: Comprises a high-speed mechanical switch (S1) and a current-carrying valve group (Q1) connected in series.
• ​Features: S1 has an extremely low contact resistance (only tens of micro-ohms), and Q1 consists of a small number of IGBTs with low conduction voltage drop. During normal operation, the 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), a unidirectional interruption valve group (Q2, formed by multiple series-connected IGBTs), and a nonlinear resistor (MOV1, arrester).
• ​Core Advantage: The 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, the number of IGBTs is reduced by approximately half. Given that commercial press-pack IGBTs cost about 10 times more than diodes of the same rating, and the 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, the 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, and the 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. The 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, the 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. The 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.