Structural Design of a Cellular Electrical Substation for Electrical Arc Blast Loading

Structural design of a cellular electrical substation for electrical arc blast loading

G.J. de Bidder Division Structures and Buildings, MBB Consulting Engineers, PO Box

35660 Menlo Park 0102, South Africa

Abstract

A case study of the structural design for a new generation of indoor electrical substation in South Africa is presented. New state-of-the-art switch gear has recently been developed in South Africa, which substantially reduces the cost but increases risk of explosive blast emanating from possible electrical faults in such indoor high voltage stations. The paper describes the concept of a suitable enclosing building structure and explains the analysis which is based on acoustic energy theory and accepted practice of blast resistant design.

78 Structures Under Shock and Impact VI

1 Introduction

The conceptual structural design of a new generation compact, enclosed type electrical high voltage substation in South Africa is presented. It provides a solution to particularly developing countries for which the high importation cost of sophisticated SF6 gas insulated switchgear GIS required for compact enclosed stations, is problematic.

Conventional high voltage open substations over the world are based on relative inexpensive switchgear, but which require considerable open yard space and stand-off distance for reliability and efficiency. The disadvantages in urban and semi-urban areas are however becoming unacceptable : costly land use ; objectionable aesthetics of masts and gear in urban or country environment; transformer noise ; pollution and vandalism risk to the station itself. These disadvantages has been negated successfully by the use of indoor configurations. The only disadvantage however, has been high cost, until the development of cost effective indoor switchgear.

Innovative switchgear recently developed in South Africa ' ^, made it possible to equip a compact enclosed station largely with conventional switchgear. The key element is the "through-the- wall-isolator" innovation ( not discussed further in this paper), which ultimately reduces the station cost considerably as illustrated by the following table :

COMPARISON OF ELECTRICAL SUBSTATIONS"

Cost Land Space

Conventional open yard type 2x3 OMVA 100% 100%

Enclosed GIS cellular type 2x3OMVA 160% 19%

Latest Through-Wall-Isolator closed cellular 120% 23%

Structures Under Shock and Impact 17 79

Even in an enclosed environment, high voltage switchgear imposes a realistic risk of arc faulting from the key isolator components. The arc, like natural lightning can generate impulsive blast pressure of sufficient intensity to damage adjacent equipment and the enclosing building. Therefore, the building is structurally compartementalised in such a manner to localize and resist possible blast effects and prevent progressive or sympathetic failures.

In order to prevent structural blast damage to the enclosing building it is essential to give specific attention to the structural engineering analysis and design of the building.

2 Structural Engineering design

2.1 Basic philosophy

Technology to design buildings against explosive blast loading is well developed and tested, offering a convenient tool for the subject under consideration.

Deposition of energy from a high voltage electric arc is of relatively longer duration when compared with the detonation of chemical explosives of comparable pressure yield. This implies that the nett impulsive dynamic loading can be substantial above conditions of normal imposed loads like weight, wind and seismicity acting on conventional building systems and materials. Ordinary masonry walls may be feasible for low rated stations up to

250MW , but due to the inherent lower factor of safety of material and construction, this option should be considered with caution. Behavior of reinforced concrete is more predictable and therefore favored, depending on the electric fault rating of the station.

80 Structures Under Shock and Impact VI

The overpressure is calculated using acoustic energy theory as postulated in Reference (3) :

(P-PJ ^ock = overpressure ( not reflected or impulsive pressure )

(Pa) — u-1 x AE

M- Po

Eqn (1)

4n;r

u = effective heat capacity ratio ( adopt upper limit value

= 1.4)

AE - rate of increase of power deposited in air around

electric arc (J/s^) r = stand off distance from arc centre to point under

consideration (m)

po = atmospheric pressure (Pa)

2.2 Pertinent case study

Figure-1 depicts the layout of a 30MVA substation containing 6 main switches arranged in isolation cells constructed of concrete walls. Operating voltage is 88kV@50 Hz with actual 27.5MW fault rating. Electric arc faulting may develop from the isolator bars at locations shown in Figure 1.

2.2.1 Roof The steel roof is of light weight steel construction, intended to blast off and thus vent the explosion over pressure effectively. The roof girders are connected with predetermined shear-off brackets and simple hinges to the

Structures Under Shock and Impact VI 81 walls, based upon an internal static pressure of 1.5 kPa,

2.2.2 Walls

Structural design for the internal blast loading is based on the concrete cell structure's dynamic response in the elasto-plastic realm of material behavior, in other words the walls will withstand an explosion without spalling or severe cracking. Slight cracking is expected.

Electric calculations^ * * predict that the energy release can build up over

10 milliseconds with duration of 1000 ms. The rate of energy dissipation is therefore AE = 2.75x10^/0.01 =

275x10^ J/s^

This blast energy is released within a cell of dimensions 5.5m wide x 7.0m high x 5m long.

The analysis is simplified by choosing the wall in the most critical, close proximity to the arc center. The wall as indicated in Figure 1 is assumed fully fixed at all boundaries (worst dynamic condition) with dimensions

5.0m x 4.0m high.

The adopted critical stand-off distance r = 3.0m. Although the fault arc center can be as close as 0.75m from the nearest wall, it is reasoned that such close proximity will result in high localized pressure but low overall effect due to the obliquity of blast waves which yield low average reflected pressures. A sensitivity study confirmed that the assumed distance of 3.0m represents the worst condition.

Applying equation (1) yields : (P-Po)shock =fl.4-Dx275x 10* = 20259 Pa

1.4 x 101325 x 4% x 3.0 A geometric factor of 2.0 is adopted, due to blast reflection off the nearby opposite wall and floor.

Using the analogy of TNT chemical explosion wave dynamics, an

82 Structures Under Shock and Impact VI average reflected pressure ratio of 2.2 was adopted : P, = 2.2 P^ , therefore average reflected pressure P, = 2.0 x 2.2 x 20259 Pa = 89 kPa

The transformed natural period of vibration of the wall is calculated as 23.9ms, which result in a ratio of load-duration / natural-vibration-period >

40. As the time of pressure rise is very short (10ms) in relation to the duration (appr 1000ms), a triangular load function is adopted as illustrated in Figure 2. It is interesting to note that, due to the long duration, even a rectangular load function (Figure 3) will require the same resistance. Structural response to this loading is based on Elasto-Plastic bilinear behavior as postulated by Gibbs^ and illustrated in Figures 2 and 3.

From Figures 2^ and 3* it is concluded that the elasto-plastic resistance should be :

R™ > 1.2 x F where F = P^ x loaded area = applied blast load, under

an adopted ductility ratio |a = 3 is for the lightly reinforced ( p = .0028) concrete slab.

The critical response mode of the slab will be bending moment at the fixed boundaries. The ultimate bending resistance Mp of the 230mm slab is taken to be equal in both directions (isotropic) and faces. Maximum bending resistance for an aspect ratio of 4.0/5.0 = 0.8 ,

span a=4.0m , width b=5m after Reference-4 :

R™ = [12(Mp+Mp] + 10.3(Mp + Mp)]xb/a = 55.75Mp kNm and R^ = Pr. a . b therefore

Mp = 1,2 . P, a . b / 55.75 = 1.2 . 89 . 4 .5 / 55.75 = 38.3 kNm/m steel rebar based on high yield deformed bars :

A, = Mp / [ Ym . f^ . d, ] = 38.3x10^ / 0,87x450x165 = 590

mirr / m

Design rebar =» HY. 12dia spaced 175 mm each way, each face of wall. The calculated crack width based on 25 Mpa concrete is approximately

O.llmm.

Structures Under Shock and Impact VI 83

Shear stress and deflection values are not discussed, but are not critical.

2.2.3 Floor and foundations

Wall foundations should experience negligible effect from the particular blast, but reinforced concrete floor slabs were placed on 500mm of evenly graded sand. A drainage system takes care of ground water, to prevent saturation of the sand. The low seismic velocity of dry sand isolates the floor from the stiff rock bed and prevent harmful shock rebound.

3 Conclusion

Buildings to enclose high voltage electrical substations safely against possible internal arc faulting can be designed and constructed simply and cost effectively.

Established blast resistant structural theory as applied in this case study, illustrates that relatively low blast pressure develop. Blast duration is however long, so that the ultimate resistance function required becomes appreciable, which warrants careful analysis and material selections.

Full scale testing and empirical research to verify the assumptions

(energy deposition and blast pressure) should be pursued.

References

[1] Schuld, H.L. & De Ridder G.J., High Voltage type indoor cellular

substations, Proc. of a seminar on Design and Planning of Substations, eds. Laboratory for Advanced Engineering, University

of Pretoria, day 3, 31 July 1996

g4 Structures Under Shock and Impact VI

[2] Schuld H.L.,of Plantech Associates Electrical & Mechanical

Engineers PO Box 20206 Alkantrant Pretoria 0005 South Africa,

advisor and information source. [3] Baker, W.E & Cox,P.A., & Westme,P.S., & Kulesz, J.J. &

Strehlow,R.A., Blast from electrical circuit breaker, Chapter 2,

Fundamental Studies in Engineering 5, Explosion Hazards and

Evaluation, ed. Elsevier Scientific Publishing Co, Amsterdam-

Oxford-New York, pp. 215, 1983 [4] Biggs, J.M., (ed.), Chapter 7, Structural Dynamics, McGraw-Hill

book Co, New York San Francisco Toronto London

Acknowledgements

The assistance of Reference-[2] in compilation of this paper is greatly

acknowledged. The case study subject relates to an actual substation project erected in the

Northern Western Province in 1996, owned by the Electrical Supply

Commission of South Africa

Structures Under Shock and Impact } 7 85

Figure 2 : Response function for triangular load pulse

/ 1.301.40_

Rectongulo* r ' toResistance Displacement 0.1 lood function function 0.1

Figure 3 : Response function for rectangular load pulse