Why Do Traditional Permanent-Magnet Mechanisms Struggle with the Large Contact Gaps of 126kV Vacuum Circuit Breakers? - Blog

As vacuum circuit breaker technology advances toward transmission-voltage levels, the 126kV single-break vacuum circuit breaker has emerged as a focal point of development. However, many industry professionals have found that permanent-magnet (PM) mechanisms-which perform exceptionally well in medium-voltage applications-often lack the capability to operate 126kV-class breakers. This is not a simple issue that can be resolved by merely increasing the coil current; the root cause lies in an inherent conflict between the magnetic circuit architecture of traditional PM mechanisms and the operational requirements imposed by large contact gaps.

I. Contact Gaps and Speed: The Challenges Posed to Operating Mechanisms at 126kV

Consider two sets of data. In medium-voltage vacuum circuit breakers, the contact gap is typically less than 30mm, most commonly falling within the 8–22mm range. In contrast, the contact gap for a 126kV vacuum circuit breaker jumps to approximately 80mm. Engineering specifications indicate that a 126kV interrupter typically has a rated contact gap of about 50mm; when the contact wipe (overtravel) is added, the total stroke of the moving core reaches approximately 60mm.
While the contact gap has increased, the speed requirements remain stringent. For a 126kV vacuum circuit breaker, the contacts must reach a separation of 18–20mm within 6–7ms of the opening operation-requiring an opening speed of approximately 2.5–3.3m/s. By 12–13ms, the gap must reach 36–40mm, with the speed still maintained at 2.7–3.3m/s. This means the mechanism must achieve "high speed from the very start" within an extremely short timeframe.
Traditional PM mechanisms were originally designed for-and are well-established within-the medium-voltage sector. Research has clearly indicated that PM mechanisms based on traditional structural designs are ill-suited to meet the high-speed requirements of vacuum circuit breakers at transmission-voltage levels (>72kV). The problem lies within the magnetic circuit itself.

II. The Core Issue: A Shared Magnetic Circuit Causes the Moving Armature to be "Dragged Down" from the Start

The structural hallmark of a traditional bistable permanent-magnet mechanism is that the permanent magnet and the opening/closing coils share the same magnetic circuit. While this design allows for a compact footprint, its drawbacks become apparent under conditions requiring large contact gaps and high operating speeds.
At the very instant an opening operation begins, the electromagnetic force generated by the energized coil must first overcome the holding force exerted by the permanent magnet in the closed position. Only the remaining portion of the electromagnetic force is available to accelerate the moving armature. As noted in the literature: "During the initial phase of an opening operation, the magnetic force from the permanent magnet partially offsets the electromagnetic force from the coil, making it difficult to increase the initial separation speed."
This initial separation speed is a critical performance indicator for high-voltage vacuum circuit breakers. Being "held back" by the permanent magnet's retaining force right at the start makes it extremely difficult to catch up to the required velocity profile later in the stroke.

III. The Chain Reaction of Long-Stroke Operation: Higher Currents, Increased Temperature, and Control Challenges

Increasing the stroke introduces another practical challenge. When the moving armature is in the starting position, the magnetic permeance of the air gap is very low; in a conventional design, a substantial coil current is required to generate sufficient electromagnetic attraction. Analyses indicate that this leads to a range of issues, including "temperature rise and control difficulties."
In other words, even setting aside speed requirements, relying solely on "brute-forcing" the movement-by increasing the coil current to drive the armature across a long stroke-is not a sustainable engineering solution. Factors such as coil heating, capacitor capacity, and the current-handling limits of control components all emerge as new bottlenecks.
Simulation data highlights this disparity: for a 126kV application, a conventional permanent-magnet mechanism achieves an average opening speed of approximately 2.5 m/s, whereas a new design utilizing a separated magnetic circuit can reach 3.0 m/s. Within the critical 6–7 ms window at the start of the opening operation, this 0.5 m/s difference is the deciding factor in whether the contacts can be separated to a safe distance in time.

IV. The Way Forward: Separating the Magnetic Circuit

Since the core issue stems from a "shared magnetic circuit," the logical improvement is to separate the magnetic paths-decoupling the permanent-magnet holding circuit from the electromagnetic drive circuit so that each can perform its specific function.
With the circuits separated, the permanent magnet no longer "hinders" the process when the opening coil is energized; instead, the electromagnetic force can be applied more directly to accelerate the moving armature. Furthermore, by incorporating a non-working air gap, the permanent-magnet holding force drops off rapidly as soon as the armature leaves its initial position, minimizing interference during the stroke. Prototype testing has already validated this approach: a permanent-magnet mechanism with a separated magnetic circuit can achieve an initial opening speed of 2.8 m/s and an average opening speed of 2.9 m/s-meeting the performance requirements for 126kV vacuum circuit breakers.
Additionally, other improvements-such as the use of a three-coil configuration-are currently being explored. The goal is to increase opening speeds while simultaneously providing cushioning at the end of the closing stroke to prevent contact bounce. Regardless of the technical approach, the core logic remains the same: the permanent magnet holding force must not become an obstacle to motion.

V. Practical Guidance for Manufacturers

For manufacturers involved in the selection or mechanism integration of 126kV vacuum circuit breakers, several criteria warrant attention:

First, examine the magnetic circuit structure. If a supplier's permanent magnet mechanism still employs a traditional shared magnetic circuit design, it will struggle to achieve the required opening speed at the moment of contact separation-even if the nominal specifications appear impressive. A separated magnetic circuit or a similar decoupled design is a prerequisite for meeting the requirements of 126kV applications.
Second, evaluate the measured speed profile rather than focusing solely on a single peak value. For 126kV equipment, the requirement concerns displacement within a specific time window, not the maximum instantaneous speed. Requesting displacement data at 6ms and 12ms following contact separation is far more meaningful than looking at "peak speed."
Third, pay close attention to the design of the opening holding force. In traditional permanent magnet mechanisms, it is often difficult to independently optimize the holding forces for the open and closed positions. A key advantage of a separated magnetic circuit structure is that the holding force on the opening side can be minimized-reducing the risk of rebound at the end of the opening stroke-while the holding force on the closing side remains sufficient to withstand the reaction force of the contact springs.

Driven by environmental policies, the transition from SF6 circuit breakers to 126kV vacuum circuit breakers is becoming increasingly clear. As the "heart" of the device, the operating mechanism-and its dynamic compatibility with the interrupter-determines whether the unit can be successfully deployed. The inability of a traditional permanent magnet mechanism to "handle the load" is not an inherent flaw of permanent magnet technology itself, but rather a limitation of specific magnetic circuit structures under certain operating conditions. Recognizing this distinction will help avoid unnecessary setbacks during product selection and R&D.

Starting in 2026, The Use Of SF6 in Medium-voltage Electrical Equipment Below 24 KV Will Be Prohibited.