Seismic (earthquake) Withstand Considerations for Transformer Bushings

Earthquakes are natural phenomena and occurrences. Transformer bushings must withstand and endure such ground motion-induced forces while maintaining their structural integrity and electrical characteristics.

Seismic Induced Forces:

During seismic events, transformers and their components (e.g. bushings) are subjected to ground accelerations simultaneously in all three directions (X,Y,Z). These accelerations induce forces on the bushing leading to stresses on Insulators and metal components & their assembly.

The stresses in insulators and metal components depend on site-specific seismic patterns, typically defined by a response spectrum showing varying accelerations across frequency ranges simulating the earthquake spectrum.

Design Considerations for Seismic Withstand:

Several design aspects play a critical role in reliable seismic performance of transformer bushings:

• Material selection

• Geometry and design of material thicknesses.

• Clamping/fixing of assembly to ensure physical integrity and maintain leak-proof properties

Seismic Qualification Testing:

Both IEC and IEEE allow various methods for seismic qualification.

• IEC TS 61463:2016 presents three methods for seismic qualification and they are applicable for bushings above 52 kV:

  1. Static Calculation: It estimates the seismic bending moment (𝑴𝒔) at the bushing's critical cross-section by using equivalent static acceleration (𝒂𝒃𝒈) based on a response factor (R), severity level, and a coefficient (𝐒𝐜) dependent on natural frequency 𝒇0.

  2. Dynamic Analysis: Numerical simulation using modal and time-history analysis.

  3. Vibration Testing: Shake-table testing using a defined Required Response Spectrum (RRS) (ZPA = 0.5 g (standard RRS)).

• IEEE Std 693-2018 (IEEE Recommended Practice for Seismic Design of Substations) defines performance levels (Low, Moderate, High) with corresponding shake-table testing (3-directions: XYZ), damping ratios, and anchorage support. The standard specifies seismic qualification methods for bushings:

Voltage Classes	Qualification Methods
<= 34.5 kV class	By inherently acceptable
> 34.5 kV to 138 kV class	By static pull test
> 138 kV class	By performance level time-history shake table test

The standard performance levels are:

• Low qualification level: If the peak ground acceleration is equal to or less than 0.1 g.

• Moderate qualification level: If the peak ground acceleration is greater than 0.1 g but equal to or less than 0.5 g.

• High qualification level: If the peak ground acceleration is greater than 0.5 g.

Acceptance Criteria for Seismic Withstand Qualification:

• Structural: No cracking or fracture of the insulator, flange, or conductor.

• Mechanical: No loosening of connections or physical damage to bushing components. Bushings shall not leak, and porcelain bushings shall not slip at the porcelain insulator-mounting flange interface.

• Electrical: No flashover, no significant increase in leakage current, and no change in insulation properties during post-seismic HV tests.

Design verification for seismic withstand is usually carried out by mathematical models, finite element-based simulation, which determines the stresses in insulator and metal components and thus it conveys bushing's ability to withstand seismic acceleration. This design verification is usually done before actual seismic testing.

Seismic withstand capability is an important design and safety feature for transformer bushings in earthquake-prone areas.

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