DSLP dirct srole lightning protection

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Project:

Gilgel Gibe – II PROJECT

ETHIOPIA

Description:

400 kV SWITCHYARD

DESIGN

REPORT’s & CALCULATION’s

Subject:

Report on Direct Stroke Lightning Protection of

Electrical Equipments in Outdoor Switchyard

Note:

This report gives the calculations in justification of the lightning protection

system designed for protecting the switchyard equipments and conductors

from direct strokes of lightning.

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1 Introduction

The basic intent of this report is to provide design information to minimize direct lightning strokes on equipment and bus work within the substation. The method employed for design of the shielding system is Electrogeometric model – Rolling Sphere Method.

The Electrogeometric Model (EGM) is a geometric representation of a facility, that, together with suitable analytical expressions correlating its dimensions to the current of the lightning stroke is capable of predicting whether the lightning stroke will terminate on the shielding system, the earth, or the element of the facility being protected.

The Rolling Sphere method is a simple technique for applying the EGM theory for shielding of substations. The technique involves rolling an imaginary sphere of prescribed radius over the surface of the substation. The sphere rolls up and over (and is supported by) the lightning masts, shield wires and other grounded metal objects intended for lightning shielding. A piece of equip-ment is protected from a direct stroke of lightning if it remains below the curved surface of the sphere by virtue of the sphere being elevated by shield wires or other devices. Equipment that touches the sphere or penetrates its surface is not protected from direct stroke of lightning.

For the facility under consideration in this design report i.e. 400 kV Switchyard, shield wires at elevation of 27.75 metres is employed to protect the phase conductor at El+22M and bus work at EL+13.125M. The calculations as per Attachment – 1 justifies the design and the zone of protec-tion is shown in the DSLP Layout.

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2 System data

400kV AC Switchyard

Nominal System Voltage 400 kV

System Frequency 50 Hz

Rated Lightning Impulse Withstand Voltage 1425 kV

Limiting Corona Gradient 1500 kV/m

3 Conductor Data

Jack Bus Conductor at Coupling Bay

Type of Conductor Twin bundle of 954 MCM ACSR Cardinal

Diameter of Sub-conductor 30.42 mm

Sub-Conductor spacing in bundle 450 mm

Main Bus Conductor

Type of Conductor 250/6 mm Tubular Bus of alloy ‘AlMgSi0.5F25’

Outer Diameter 250 mm

Thickness 6 mm

4 Installation Data

Jack Bus Conductor at Coupling Bay

Height of installation above FGL 22.0 m

Main Bus Conductor

Height of installation above FGL 13.125 m

5 Attachments

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6 Conclusion

1. The results as per Case – 1 of the attached calculation reveals that for protection of the phase conductors of the Jack Bus in the Coupling Bay at 22.0 m elevation by shield wires at elevation of 27.75 m, the maximum allowable horizontal separation of the shield wire is 37.73 m. As shown in the DSLP layout, the shield wires protecting the phase conductor at the Coupling Bay have a horizontal separation of 24.0 m. Thus the phase conductor is protected from direct strokes of lightning.

2. The results as per Case – 5 of the attached calculation reveals that for protection of the rigid conductors at the Main Buses at 13.125 m elevation by shield wires at elevation of 27.75 m, the maximum allowable horizontal separation of the shield wire is 58.12 m. As shown in the DSLP layout, the shield wires protecting the bus work have a horizontal separation of 48.0 m. Thus the Main Bus conductor is protected from direct strokes of lightning. All other equipments in the switchyard are at lower elevation than the Main Bus work and are hence protected by the shield wires.

7 References

1. IEEE Std.998 – 1996(R2002) – IEEE Guide for Direct Lightning Stroke Shielding of Substations 2. Technical Data sheet for ACSR Conductor: Manufacturer – HASCELIK.

3. Technical Data sheet for Aluminum Alloy Tubular Conductor: Manufacturer – Corus 4. 400kV Switchyard DSLP Layout – Drawing No. (1)-G77700-S0019-L069-A

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By Rolling Sphere method, as per IEEE-std 998-1996

Conductor Data:

Flexible conductor bundle

Sub conductor Type = 954 MCM ACSR Cardinal

Diameter = 0.03042 m

Sub-conductor Spacing = 0.45 m for 400kV Twin Bundle height of the conductor = 22 m

Rigid conductor

Conductor Type = Tubular Aluminium Alloy

Outer Diameter = 0.25 m

height of the conductor = 13.125 m In case of a twin conductor bundle, the equivalent radius is given by

R0 = ¥(r x l) ( eqn. C.5, IEEE 998 - 1996)

= 0.082731 m where r = radius of subconductors in m

l = spacing between adjacent conductor in m In case of a single conductor bundle, the equivalent radius is given by

Rc x ln {(2xh)/Rc} - (Vc/E0) = 0 ( eqn. C.1, IEEE 998 - 1996)

where h = average height of the conductor

Vc = Rated Lightning Impulse withstand voltage = 1425 kV

E0 = Limiting corona gradient = 1500 kV / m By solving the equation for Rc, we have

Rc = 0.171194 m

For Bundle Conductor , the radius of the bundle under corona is

R'c= R0 + Rc = 0.253925 m for Twin Bundle

For tubular conductor the equivalent radius is given by Rc x ln {(2xh)/Rc} - (Vc/E0) = 0

By solving the equation for Rc, we have

Rc = 0.193255 m

The surge impedence of conductors under corona is given as,

Zs = 60 x ¥( ln ( 2xh / R'c ) x ln ( 2xh / R0 ) ( eqn. C.7, IEEE 998 - 1996)

where h = average height of the conductor R'c = Corona radius of the bundle conductor R0 = Equivalent radius of bundle conductor

hence ZS = 341.2832 ohms for Twin Bundle

For Single conductor the expression to determine surge impedance is

Zs = 60 x ¥( ln ( 2xh / R'c ) x ln ( 2xh / r )

where h = average height of the conductor R'c = Rc = Corona radius of the single conductor

r = metallic radius of single conductor

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10.19586 kA for Tubular Conductor The allowable stike Distance is obtained by the equation

S = 8 x k x Is

0.65

( eqn. 5-1B, IEEE 998 - 1996) where S = Strike distance in m

k = 1 for strikes on Shield Wire (pg.25 IEEE 998 - 1996) IS = Allowable return stroke current in kA

hence S s wire = 33.8158 m for Twin Bundle

= 36.18807 m for Tubular Conductor

Combined protection by two shield wires

here,` 'O' is the origin of the Rolling Sphere 'SW' denotes the location of the shield wire

Case - 1 :- Object to be protected : Twin Conductor Bundle of Coupling Bus

Now,

allowable strike distance (S) = 33.8158 m

height of shield wire (H) = 27.75 m

height of object to be protected (A) = 22 m

elevation difference between Shield wire & Object to be protected

D = H - A = 5.75 m

elevation difference between Origin of the Rolling Sphere & Shield Wire

E = S - D = 28.0658 m

horizantal distance between Origin of the Rolling Sphere & Shield wire

L = ¥S2 - E2 = 18.863 m

Maximum allowable horizontal seperation of the shield wires ensuring protection of object at height (A)

X = 2 L = 37.73 m

Case - 2 :- Object to be protected : Tubur Rigid Bus Conductor of Main Bus