Optimizing Spring Design for Vacuum Circuit Breaker Performance and Reliability​

1. Introduction
Vacuum circuit breakers (VCBs) have become the preferred choice in medium-voltage applications due to their environmentally friendly vacuum interruption technology, low actuator power requirements, exceptional lifespan, operational reliability, and minimal maintenance needs. A critical factor in their overall design is the strategic placement and parameter selection of the opening and closing springs, significantly impacting the breaker's performance and form factor.

Beyond determining the arrangement of vacuum interrupters, the core challenge in VCB design lies in selecting the number of operating mechanisms, a decision driven primarily by the specifications and positioning of these springs.

2. Opening Spring Design & Parameterization

• Parameter Determination Basis: Opening spring parameters are engineered to meet specified opening speeds and opening times. The primary reference is the required opening characteristic curve – the relationship between contact travel distance and velocity during the opening operation.

• Design Methodology:

• Does the calculated speed curve match the design requirement?

• Is the average opening speed acceptable?

• Is the total opening time within specification?

1. Calculate the equivalent opening force needed based on the target characteristic curve.

2. Select a preliminary opening spring configuration.

3. Simulate and verify the dynamic contact opening velocity profile:

4. Adjust spring parameters (e.g., stiffness, pre-load) iteratively if significant deviations are observed.

• Optimal Placement Strategy:

• Position: Install the opening spring midway within the kinematic chain of the operating mechanism, at a point experiencing substantial travel during operation. This maximizes the energy transfer efficiency.

• Orientation: Align the force vector exerted by the opening spring with the direction of motion (velocity vector) of its attachment point throughout the entire opening stroke. This ensures consistent force contribution.

• Impact of Speed Ratio: The speed ratio (mechanical advantage) of the linkage system changes during operation. Therefore, the placement location significantly influences the required spring parameters (force, pre-load, stiffness) to achieve the target opening dynamics, even when the overall energy requirement remains constant. Careful modeling of the linkage kinematics is essential.

3. Closing Spring Design & Parameterization

• Understanding the Load Profile (Equivalent Resistance): The primary forces opposing the closing mechanism in a spring-operated VCB are:

• The composite Equivalent Resistance Curve exhibits a major step increase at contact touch.

• Minimal resistance during initial compression (0-9mm).

• Linearly increasing resistance (9-12mm).

• High, near-constant resistance plateau upon full compression (3800N/pole = ~1163 kgf total).

1. Opening Spring Force: Acts throughout closing. Example: 177.9 kgf total closing resistance for 3 poles.

2. Vacuum Interrupter Self-Closing Force + Bellows Resistance: Example: ~21.64 kgf per pole (64.9 kgf total) resisting closure initially (bellows stiffness contribution minimal).

3. Contact Spring Force: Represents the dominant closing resistance near contact touch. For a typical 1250A, 31.5kA VCB:

• Spring Output Force Requirement: The closing spring must generate an Equivalent Driving Force Curve that surpasses the Equivalent Resistance Curve throughout the entire closing stroke. While conventional springs often deliver a near-linear or specific non-linear force profile (see Fig: Output Force Curves, Curve 3), the ideal output profile closely follows the resistance curve with a slight margin (Fig: Output Force Curves, Curve 2). This minimizes actuator size and energy consumption.

• Key Design Criteria for Closing Spring Output:

• Initial Force: Must exceed the initial resistance force (including opening spring pre-load, friction) to initiate motion. Example: Needs to overcome 80.1 kgf pre-load resistance.

• Total Energy Output: The energy delivered by the spring (∫ F dx) must exceed the total closing work required to overcome all resistances over the full stroke.

• Force Profile Optimization: The shape of the output force curve directly impacts the closing speed profile (vh vs t). Three characteristic profiles are possible:

• Target Output Profile: For robustness and speed, select or design a spring system yielding Profile 3: Moderate initial force, steadily increasing force during mid-stroke for acceleration, and a controlled force reduction late in the stroke to provide sufficient impact force at contact touch without excessive kinetic energy. This profile optimizes closing time while ensuring reliable operation.

1. Constant Acceleration: Slowest overall closing time (t1), high force near close.

2. High Initial Acceleration / Decreasing Force: Faster closing time (t2 < t1), but critically low force near contact touch (prone to stalling).

3. Moderate Initial Accel / Increasing then Gently Decreasing Force: Ideal profile (t3 < t1). Fast closing achieved while maintaining adequate force near contact closure to ensure reliable latching and withstand impact loads.