Seismic Qualification -- Setting New Standards

by Rulon Fronk

On February 9, 1971, an earthquake measuring 6.6 on the Richter scale hit the Los Angeles area. In its wake, major substations were left decimated. After months of repairs and millions of dollars in expenditures, the last substation knocked out finally returned to service. Prior to this earthquake, most utilities had no seismic requirements for equipment or, at best, included very simplistic statements in their specifications. This earthquake was the electric utility's wake-up call that seismicity must be given serious consideration in the design of substations, particularly in the design of electrical equipment.

Following that earthquake, utilities threw out the old specification clauses and began formulating new seismic electrical equipment criteria. The utilities' initial attempts at seismic qualification were as tentative as a baby's first steps. Because utility equipment seismic qualification experts were nonexistent, engineers experienced in general seismicity and dynamics were pressed into duty to tackle the problem of how to make equipment seismically rugged. The early generations of seismic criteria were very general, a "one size fits all" tactic. One specification clause was used for almost all electrical equipment.

With the passage of time and a number of additional earthquakes, it became clear that one size does not fit all. Because each type of equipment is different structurally, specifically regarding its fragility and the way it acts dynamically, each type must have its own unique set of requirements. For these reasons, the requirements for a transformer must be different from the disconnect switches and so on.

Presently, IEEE P693, Draft Recommended Practices for Seismic Design of Substations, is being entirely revised to incorporate the latest design developments. IEEE P693 addresses all aspects of the seismic design of substations. It achieves this by either providing requirements directly or providing a reference document. Qualification of electrical equipment is provided directly within IEEE P693. Seismic design requirements for non-equipment, such as A-frames, buildings, and racks, are provided by referring to other documents, mainly the American Society of Civil Engineers (ASCE) Substation Guide, which is presently being developed as a "sister" document to IEEE P693. This has been a case of two organizations--IEEE and ASCE--working jointly, with each emphasizing its strengths to complement the other.

The current revision of IEEE Std 693 is equipment specific, meaning that each type of equipment will be provided with its own uniquely designed set of requirements. For example, since many of the requirements that are applicable to transformers do not apply to a disconnect switch, each must have its own unique set of requirements.

The following are some of the criteria used in establishing equipment qualification requirements:

• Fragility. Equipment that is inherently rugged need not be subjected to a very rigorous qualification, while fragile equipment normally needs a more rigorous approach.
• Criticality. The qualification method needs to recognize the criticality of the equipment. For example, a transformer or a circuit breaker, which is critical to the function of a substation, must be more reliable than a battery charger.
• Alternatives. There needs to be enough flexibility in the standard so that the utility can select the appropriate "level of ruggedness." At the same time, utilities want some degree of uniformity within their system. Also, the fewer the levels, the more opportunities to amortize the cost of the equipment. The P693 committee has decided that three "levels of ruggedness" provide the balance between flexibility, uniformity, and cost. In general terms, the three levels are: 1) nominal requirements (low seismic activity expected); 2) moderate ruggedness; and 3) high ruggedness.

The following are some of the considerations utilities must evaluate when deciding which level is appropriate: 1) the expected magnitude of an earthquake at the substation; 2) the criticality of the substation as it pertains to the utility's total system; 3) the speed at which equipment can be replaced; 4) safety considerations; 5) the possibility and acceptability of bypassing the equipment should the equipment fail; and 6) the overall reliability of the system. For these reasons, zone maps are provided as an aid to guide the utility in selecting the appropriate level, not as a requirement. The utility must evaluate the site and all the other considerations to determine which level is appropriate.

The P693 committee hopes that through standardization, the cost of qualification will be reduced by the use of common criteria and the present confusion to manufacturers will be minimized. At this moment, each utility's criteria are different despite the common bottom line--they all want rugged equipment that will reasonably survive earthquakes. As a result of each utility having its own unique criteria, each utility must pay for its unique requirements. How much better it would be if the cost of qualification were shared with other utilities. The results of P693 affords one of those rare opportunities where everyone wins. The utilities win because the cost of qualification can be reduced by amortizing the cost over all the buyers. The manufacturers win because they will have standards and order where chaos, for now, rules. u

Rulon Fronk is the Chair of IEEE P693. The draft standard is available from the IEEE Operations Center at 1-800-678-4333 or (908) 981-0060 (outside USA).