Rectifier-Type Hybrid High-Voltage DC Circuit Breaker Solution Amsa Rectifier-Type Hybrid High-Voltage DC Circuit Breaker Solution

I. Akan Background da Analisis Nauyin​
Saboda yawan fadada aiki a kan tashiyar karkara, teknologi masu nufin DC mai kula ta (VSC) ya zama aikin muhimmiyar wajen ƙunshi waɗannan karkara da yawa da tsari don inganta tushen gaba-gaban karkara. Daga cewa ake iya kawo aiki a matsayin active da reactive power zuwa yadda ake bi hanyoyin harshen, ana bukata a yi waɗannan abubuwa. Amfani da VSC na musamman ya zama babban fasahar jirgin karkara da yawa. A wannan lokacin, high-voltage DC circuit breakers, wanda su ne babban abubuwan da ke amfani a kan kula da shiga nauyin da kuma tabbatar da dalilin karkara, suna da muhimmanci sosai. Ba a gane da high-performance DC circuit breakers, ake iya kula da shiga nauyin da kuma ingantaccen karkara a flexible DC grids.

Abubuwan da ake amfani a yanzu a kan high-voltage DC circuit breaker suna da batu-batu masu muhimmanci:

• ​Mechanical Circuit Breakers: Idan suke da low on-state losses da high withstand voltage, amma lokacin da ake kula su ya zama shekaru daban-daban, ba a gane da nan don kula da shiga nauyin a millisecond-level da ke bukata a flexible DC grids.
• ​All-Solid-State Circuit Breakers: Wadannan suna da kyakkyawan kula da shiga nauyin amma suke da high on-state losses, high operating costs, da kuma poor economic efficiency.
• ​Traditional Hybrid Circuit Breakers: Wadannan suna da low losses da kula da shiga nauyin amma topologin da suke da su ya zama mafi karfi, system complexity, da kuma high costs saboda ake buƙata IGBTs a cikin forward da reverse directions.

Don in ba da wannan batu-batu masu muhimmanci, yana da buƙata don aiki mai tsari don aiki da high-voltage DC circuit breaker wanda yake da kyakkyawan kula da shiga nauyin, low operating losses, high economic efficiency, da kuma high reliability.

​II. Solution: Rectifier-Type Hybrid High-Voltage DC Circuit Breaker​
Wannan solution ya bayyana topology mai tsari don high-voltage DC circuit breaker wanda yake da kyakkyawan kula da shiga nauyin, low operating losses, high economic efficiency, da kuma high reliability.

​(I) Core Technology: Innovative Circuit Topology​
Topology na wannan circuit breaker na nuna current-carrying branch da current-breaking branch wadanda suke da su a matsayin parallel connection.

1. ​Current-Carrying Branch:
• ​Composition: Ana haɗa high-speed mechanical switch (S1) da current-carrying valve group (Q1).
• ​Features: S1 na da extremely low contact resistance (only tens of micro-ohms), da Q1 na nuna small number of IGBTs da low conduction voltage drop. A lokacin da ake amfani, rated current ya haɗa a cikin wannan branch, da kuma ya ƙunshi extremely low on-state losses.
2. ​Current-Breaking Branch:
• ​Composition: Ana amfani bridge rectifier structure, wanda na nuna bridge commutation valve group (D1-D4, formed by multiple series-connected diodes), unidirectional interruption valve group (Q2, formed by multiple series-connected IGBTs), da kuma nonlinear resistor (MOV1, arrester).
• ​Core Advantage: Bridge rectifier structure na iya kula da shiga nauyin da kuma ya ƙunshi unidirectional IGBT interruption valve group (Q2) don kula da shiga nauyin da yawa. Wannan topology na ƙunshi reduction in IGBTs da kuma driver boards, wanda ya ƙunshi significant cost reduction da overall reliability improvement.

​(II) Efficient Interruption Working Principle​
A cikin misal na current flowing from Port 1 to Port 2, interruption process na nuna four stages:

1. ​Stage 1 (t0–t1, Fault Occurrence)​: A lokacin da short-circuit fault ya faru, current ya faru da ƙarin. A wannan lokacin, S1 da Q1 suke da su, Q2 ya faru, da kuma fault current ya haɗa a cikin current-carrying branch.
2. ​Stage 2 (t1–t2, Current Transfer)​: Control system ya faru opening command, Q2 ya faru, Q1 ya faru. Conduction of Q2 ya ƙunshi 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)​: Bayan current in the current-carrying branch ya faru, high-speed mechanical switch S1 ya kula da shiga nauyin under zero-current and zero-voltage conditions without arcing, establishing insulation strength.
4. ​Stage 4 (t3–t4, Fault Current Clearance)​: Bayan S1 ya kula da shiga nauyin, Q2 ya faru. Turn-off of Q2 ya ƙunshi 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.