Evaluation of Concrete Pole-Type Substation Structures Subjected to Shock Loads

Abstract

The use of direct embedded prestressed concrete poles for the support of substation equipment and electrical bus conductors is potentially a very cost-attractive alternative to steel substation structures supported on cast-in-place foundations. The analysis of substation structures requires consideration of the structural response to electromagnetic forces due to short circuits (shock loads) especially for the case of rigid bus support structures. In this study, the dynamic response of prestressed concrete pole substation structures is compared to the response of more traditional steel substation structures. A dynamic factor is suggested to consider the impact effect on these structures. A proposed testing method to verify the theoretical results by applying a shock load to the substation structure is also presented. INTRODUCTION Substation structures are designed to withstand various combinations of environmental and equipment operating loads. Common load types for the design of substation structures include dead, wind, ice, earthquake and short circuit loads. Normally, substation rigid bus structures are designed for short circuit forces in combination with all load types that may be present during an electrical fault event. Short circuit loads are dynamic in nature, and impart large dynamic forces in very short time periods. The response of the substation structures to shock loads is different than the response to other transient dynamic loads such as wind or earthquakes, therefore a different approach is required to analyze the structural response to short-circuit loads. The intent of this paper is to analytically estimate and compare the dynamic response of a direct embedded concrete pole substation structure and a steel structure to simulated short circuit shock loads. A full-scale structure test using a drop mass under controlled conditions will be used to evaluate the analytical results. The test method is required to determine the performance of the substation concrete poles in terms of load transfer, maximum bending moment, maximum shear force, and maximum top deflection under static and dynamic loads. Copyright ASCE 2004 Structures 2000 2 TYPICAL 115kV 3-PHASE SUBSTATION BUS STRUCTURES The analytical study was conducted on typical single pole 115kV 3-phase substation bus structures. Prestressed concrete pole structures are commonly direct embedded on compacted stone and steel structures are commonly mounted on cast-inplace foundations using base plates. For the purpose of this paper both types of substation structures were assumed fixed at ground line. General dimensions are shown in Figure 1. For prestressed concrete poles, a solid-square cross section 230 mm (9 in) wide at top with an overall taper of 13.5 mm/m (0.162 in/ft) was considered. The compressive strength of the concrete was taken as f’c=55 MPa (8000 psi), and a value of 35.1 Gpa (5,100,000 psi) was taken for the modulus of elasticity. For the steel structure column, a standard hollow square cross section TS 200x200x6 (8x8x1/4) with a modulus of elasticity of 200 GPa (29,000,000 psi) was considered in this study. For both types of substation structures, a vertical load of 2668 N (600 lb) was applied at top of the pole to account for the weight of attachments and insulators. The concrete pole structure and the steel structure heights for this study are 3.65 m (12 ft) above ground line. SHOCK LOADS INDUCED BY SHORT CIRCUITS Substation structures may be subjected to forces resulting from the magnetic field generated by short circuit current on conductors. These loads are impulsive, often large in magnitude and usually present for very short time durations. The magnitude of the short circuit load is dependent on the conductor spacing, short circuit current, and phase configuration. Figure 1. Typical 115 kV 3-phase Substation Rigid Bus Structure Layout The general expression to compute short circuit loads including mounting structure flexibility is presented in equation 1 (ANSI/IEEE 605,1987). Γ = D 10 I 10230 K F 7 2 SC f SC Γ = D 10 I 6 . 27 K F 7 2 SC f SC Eq. 1 3.65 (12) 1.22 (4) 2.74 (9) 2.74(9) Prestressed Concrete or Steel Column Conductors