Seismic Qualification of Power Transformers

.

Introduction

In today’s energy landscape, the demand for Power Transformers is soaring, driven by several critical factors. The global transition to renewable energy sources, coupled with the rapid electrification of transportation and the ever-increasing energy demands, stands out as key drivers behind this rise in demand.

Many Power Transformers, nonetheless, are installed in seismic prone regions, where the preservation of structural integrity as well as the continuous functionality of this critical component is of paramount importance.

Seismically qualifying a Power Transformer and its appendages can be a demanding task. Let’s dive into some of the specific challenges involved:

.

• Substantially large finite element model

When performing seismic qualification through analytical methods, the use of appropriate Finite Element Software is essential. While simplifying the geometry is standard practice, the complexities of design and topologies sometimes necessitate the use of solid elements to accurately capture the structural performance of the Power Transformer. However, these elements, combined with the component’s size, result in a significantly large model, urging analysts to explore additional model order reduction techniques. Dealing with a large model increases modeling efforts, may strain hardware requirements to their limits, and proves computationally expensive in terms of running time and post-processing of results. Such effects are further amplified when employing more sophisticated analysis methods, such as dynamic analysis methods (Response Spectrum or Time-History dynamic analysis).

• Mounting of Core & Coil

The Core & Coil constitutes a crucial and inherently sensitive section of the transformer. This part is required to be electromagnetically decoupled, while it should remain restrained from shifting. The seismic qualification should either account for non-linear behavior of the Core & Coil connections to the tank or linearize such behaviors when the maximum seismic response does not develop forces that cause separation at the contact points.

• Anchorage to the concrete slab

When the Power Transformer is anchored to the concrete slab, a considerable section of the skid’s surface rests on the slab, lacking resistance against uplift. This support condition interchanges the stress distribution across the skid during alternating vertical earthquake forces (upwards and downwards). The analyst shall eventually evaluate a realistic stress distribution on the anchorage and Transformer.

• Combination of qualification methods

Certain standards like IEEE693 dictate different qualification methods among the various Power Transformer parts. As an indication, HV Bushings and Surge Arresters are required to be qualified by shake-table testing while the tank, radiators, conservators, etc. by analytical methods. Employing both methodologies while accurately considering the interaction between Transformer parts is vital.

• Radiator design

In many instances, radiator piping and connections to the tank lack the strength needed to withstand the mass of radiators under seismic loading. Consequently, the design of dedicated bracing, capable of restraining vertical / horizontal seismic forces and directly connected to the transformer tank, often presents an optimal solution.

• Seismic isolation

The retrofit of Power Transformers may necessitate improving their seismic resilience. In instances where significant alterations to the component’s design are impractical, applying seismic protective devices becomes a viable option. These devices exhibit a highly nonlinear response and thus demand comprehensive analytical and testing methodologies for proper implementation.

.

Conclusion

Overall, the seismic qualification of a Power Transformer and its appendages can present significant challenges, encompassing various aspects from modeling intricacies to engineering judgments and selecting the most suitable approach for each case. Additionally, the seismic engineer must stay up to date about all current norm revisions, as they significantly influence the qualification process and end-user acceptance. In fact, the upcoming IEEE 693 amendment is anticipated to introduce updates to the seismic qualification criteria for Transformers, setting new standards for seismic resilience and ensuring enhanced safety measures in Transformer installations worldwide.