HV Transformer Bushings: Seismic Qualification by Shake Table Test
The seismic qualification of high-voltage equipment has become an essential part of substation design. Similar to other switchgear equipment’s the transformer bushing is considered to be sensitive to earthquake forces.
Utilities, EPCs, are demanding to demonstrate seismic withstand capability of transformer bushings while maintaining electrical and mechanical integrity. Qualification requirements are typically governed by IEC 61463, IEEE 693, and country-specific seismic regulations.
IEEE 693 is the most widely adopted standard for seismic qualification of substation equipment, including transformer bushings, circuit breakers, disconnectors, and other HV apparatus.
IEEE 693 approach is to ensure that the equipment remains mechanically stable, maintains electrical functionality, during and after seismic acceleration.
For transformer bushings, the qualification method is determined by voltage class.
For shake table test below is the sequence of tests / checks to be followed for qualification:
Step 1: Test Setup and Instrumentation
The bushing is to be mounted on a rigid support frame that simulates service conditions and is installed at its intended service inclination.
An additional mass is to be added to the bushing terminals as per the voltage ratings.
Instrumentation typically includes:
Accelerometers
Strain gauges
Displacement measurement devices
Installation of safety measures
These instruments monitor dynamic response, stresses, and deflections throughout testing.
Step 2: Strain Gauge Calibration
This calibration enables accurate determination of strain during shake-table testing. Before seismic excitation, a static calibration test establishes the relationship between:
Applied bending load
Measured strain & deflections.
Step 3: Resonance Search Test (Before Shake Table Test)
A resonance search is performed before seismic qualification to identify Fundamental natural frequencies, Mode shapes and Damping factor.
Step 4: Shake Table Test
For bushings above 138 kV, IEEE 693 requires performance-level shake table testing. The bushing is subjected to earthquake-like vibrations in longitudinal, transverse, and vertical directions simultaneously.
Key requirements include:
Test Response Spectrum (TRS) must envelop the Required Response Spectrum (RRS) including the vertical severity.
Minimum motion duration: 20 seconds
Refer to the graph for an illustration of the TRS and RRS requirements.
Step 5: Resonance Search Test (After Shake Table Test)
After seismic testing, the bushing's natural frequencies are measured again and any significant shift from the initial values may indicate structural degradation and requires further evaluation.
Step 6: Post-Test Evaluation
Following completion of the seismic test sequence, the bushing undergoes:
Visual inspection
Verification of absence of leakage
Routine testing as per IEC / IEEE Standard - IEEE 693 also requires that bushings successfully withstand routine electrical tests after seismic qualification
IEEE 693 provides a practical standard for assessing the seismic withstand capability of high-voltage transformer bushings.