Power Grid

EI2435
Faults and Transients on Transmission Lines

Wang Wei [e-mail] 1/12/2011

1. Introduction
Project 1 is about fault currents and transients on transmission lines that may occur due a lightning strike to an overhead line. In this case the transmission line is energized from one side and it’s other side is terminated in a Gas Insulated Substation, see Fig. 1. Several issues will be investigated from this system such as transient overvoltages and short-circuit currents.

Fig.1 Transmission line model

1.1 Given parameters
System voltage Table 1. Basic power line data 420 kV 20 m 13 m 25 cm 25 mm 3800 Ohmm 0.5 m 0.5 m Do = 400 mm, Di = 120 mm

Average height above ground Phase-to-phase distance Triplex conductor, circumscribing diameter Conductor diameter Soil resistivity Tower grounded with metallic hemisphere of radius Generation station neutral grounded with metallic hemisphere of radius Co-axial GIS tube filled with SF6. Outer andd inner diameter

1

2.1 Stationary LLG fault current
A phase-to-phase to ground (LLG) fault occurs in the middle of the line, i.e. YY/2 km from each sub-station. OHL parameters The theory developed by Carson give the approximate expression for the distance to return conductor as a function of the resistivity of the earth and the frequency

D  658

 f  658

3800  5736.31m 50

The geometric mean radius is given by

GMR  3 3  0.025 / 2   0.25 / 2   0.0837m
2

1 D 1 5736.31 L11  L22  L33  2 107 (  ln )  2 107 (  ln )  2.277 106 H / m 4 GMR 4 0.0837
Z11  Z22  Z33  j 2 fL11  l  j 29.69 
L12  L21  L23  L32  2 107 ln D 5736.31  2 107 ln  1.218 106 H / m d12 13

Z12  Z21  Z23  Z32  j 2 fL12  l  j15.88 
L13  L31  2 107 ln D 5736.31  2 107 ln  1.08 106 H / m d13 26

Z13  Z31  j 2 fL13  l  j14.08 
The impedance matrix can be obtained

 j 29.69 Z   j15.88   j14.08 

j15.88 j 29.69 j15.88

j14.08  j15.88   j 29.69  