# Calculation example for an open distribution unit at the hydroelectric power plant in Novocherkassk

Type and structure of grounding arrangement depend on the facility parameters and intended use. In high-resistivity soils, conventional grounding arrangement consisting of steel vertical electrodes may be insufficient to achieve the desired GA resistance. It is especially true when the requirement is 0.5 Ohm. In these cases, modern solutions such as electrolytic grounding, which is provided as a hollow perforated electrode filled with a mix of mineral salts, are of help. This mix becomes electrolyte by absorbing water from the environment and then penetrates the soil to increase its conductivity. The electrolytic grounding provides grounding resistance that is 12 times lower than the conventional steel electrode of the same dimensions.

This solution was offered to solve the task of arranging the grounding system for an open distribution unit (ODU) at the hydroelectric power plant in Novocherkassk.
Task: to make grounding arrangement for ODU-35 kV with the resistance not exceeding 0.5 Ohm. According to the information provided by the customer, the soil is sandy. The estimated soil resistivity is taken to be 400 Ohm ∙ m.

Warning! In case the Customer provided erroneous or limited soil data, the above calculation of a grounding arrangement is considered incorrect. If the soil resistivity differs from the calculated one, it is necessary to perform computations with a real value. If the normalized impedance of the grounding arrangement is exceeded, there must be introduced the design corrections. Equivalent soil resistivity for horizontal electrodes

Grounding is made in accordance with the EIC, Rev. 7, Chapter 1.7.

A set of measures to ensure the requirements for the grounding arrangement is based on the following solutions:

• to provide the rated resistance of the grounding arrangement, electrolytic grounding ZANDZ ZZ-100-102-9МВ is proposed for use;
• 24 sets of vertical electrolytic grounding 9 m long ZANDZ ZZ 100 102-9MB, which are installed in the substation territory;
• the sets shall be placed at a distance of 6 meters with a parallel arrangement to achieve the acceptable utilization rate and an optimal volume of electrolyte distribution in the soil;
• because of the formation of a talik zone (in permafrost only) as a circle about 3 meters in size, the electrodes must be placed at an appropriate distance from the foundations of buildings and other engineering structures;
• grounding electrodes are connected to the bar using clamps ZZ-202-013.

## Grounding arrangement resistance calculation

Resistance of a horizontal electrode:

Гор - Hor

where ρ is soil resistivity, Ohm*m.
b is horizontal electrode width, m- ширина полосы горизонтального электрода, м;
h is horizontal electrode depth, m;
Lhor is horizontal electrode length, m.

Vertical electrode resistance:

Верт - Vert

where ρ is soil resistivity, Ohm·m;
L is vertical electrode length, m;
d is vertical electrode diameter, m;
T is depth, i.e. the distance from the ground surface to the ground terminal, m;

where t is depth of the top of the electrode, m

Resistance of set ZZ-100-102-9МВ

Верт - Vert

where C is a dimensionless ratio describing the electrolyte content in the surrounding soil;
Lvert is ground terminal , m;
d is ground terminal diameter, m;
ρ is soil resistivity, Ohm·m.

Electrical impedance of the grounding arrangement:

ЗУ - GA
Исп гр - util

where n is number of sets;
kutil is utilization ratio

Гор - Hor
Ом - Ohm
Верт - Vert
ЗУ - GA

Estimate resistance of the grounding arrangement is 0.498 Ohm, which is less than the required value of 0.5 Ohm.

Layout of GA elements at the facility. Open full image

## List of required materials

 Item No. Fig. Part number Product Amount 1 ZZ-100-102-9МВ ZANDZ Electrolytic grounding set (vertical; 9 meters) 24 2 ZZ-202-013 ZANDZ Clamp for a circular wire and a bar (D6-10 mm; up to 50 x 6; zinc-plated steel) 24 3 ZZ-502-505-25 ZANDZ Zinc-plated steel bar (50 x 5 mm; 25 m bundle) 14

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