Calculation of protective earthing

Task 1. Calculate a protective earthing for substation 6/0.4 kV in homogeneous soil.

Initial data. Transformer decreasing substation has two transformers 6/0,4 kV with earthed neutrals on side 0,4 kV and is accommodated in separate one-floor apartment, by dimensions 12 х 20 m.

Not far from station a metallic technological construction was found, partially loaded into soil, which can be used as natural earthing rod having a resistance R_E = 20 Ohm to current spreading out.

Solution: A shorting current will be defined for known length of cable lines with voltage 6 kV and L_кл= 65 km, and air L_пл= 60 km.

As vertical single earthing rods will use the bars by length L_ст = 5,5 m and diameter d = 18 mm.

The top ends of the bars are connected by means of welding by steel stripe, laid horizontally on depth Н_пол = 0,8 m, total length L_пол = 60,5 m and section 4x40 mm.

Calculated specific soil resistance for vertical earthing rods (bars) by length 5,5 m L_{ρроз.ст.} = 130 Ohm×m, and for horizontal stripe - flaps L_{ρроз.ст.} = Р_роз ·180 Ohm·m. For determination of the resistance of current spreading out from earthing rod will define in advance a shorting current Lзon earth on side 6 kV by following formula:

where U is a transformer voltage on the side of high voltage, kV;

L_кл., L_пл. are the lengths of cable and air lines accordingly, km.

Resistance to current spreading out from artificial earthing rod, which will be used as general for electric installations in networks 6 kV and 0,4 kV, will be determined according to ПУЭ-86 requirements.

For earthing rod in homogeneous earth maximum acceptable earthing rod resistance R₃ may be computed on most admissible voltage of touch by formula

where U_доп - most admissible touch voltage with taking into account a voltage fall at base support, on which a man stands, equal to 125 V.

Necessary resistance R_н of artificial protective earthing of substation with taking into account an artificial R_з and natural R_е will be defined by formula

.

Calculating artificial earthing, we must select a contour, disposed on perimeter of substation. Vertical bar earthing rods are hammered into soil on distance a = 5,5 m one from another.

From previous disposition of the earthing rods it is evident that at length of horizontal connecting stripe L_пол =60,5 m can be hammered about 11 vertical earthing rods.

Let’s calculate resistance to spreading out of electric current from single vertical R_cm and horizontal R_пол earthing rod by formulae:

Let’s define a coefficient of use of single bar earthing rod h_ст and coefficient h_пол of stripe usage (using tab. 10.2 in [6]) for relation a/L = 5,5/5,5 = 1 and n =11 pieces.

The coefficient is equal to: h_ст =0,55, h_пол =0,35.

Resistance R_кз to spreading out of electric current from artificial contour earthing rod R_кз may be defined by formula:

. Ом

Calculated resistance R_кз = 3,37 Ohm is less than necessary resistance of artificial protective earthing R_н= 3,69 Ohm, thus the contour satisfies our conditions. Contour consists of 11 vertical steel bars by length 5,5 m, diameter 18 mm, hammered in soil on distance 5,5 m one from another, connected between oneself and with natural metallic construction by steel stripe 4x40 mm, by length 60,5 m.