Generating plants require special attention to ensure electrical safety. Therefore, the approach to the calculation of the grounding device resistance and lightning protection should be at the highest level.
The ZANDZ Technical Center received the task to calculate lightning protection and grounding for a diesel generator unit (DGU). The engineers of the technical center offered the following solution:
- dimensions: 7 m x 2.4 m;
- height: 2,6 m;
- soil resistivity: 100 Ohm * m.
- calculate external lightning protection system for a diesel generator unit;
- calculate grounding resistance with the resistance of not more than 4 Ohm
Lightning protection and grounding of the diesel generator set are made in accordance with:
DGUs belong to the 2nd category of lightning protection. Required system reliability not less than 0.99.
According to EIC-7, p. 1.7.101. The resistance of the grounding device, to which the neutrals of the generator or transformer are connected or the terminals of the single-phase current source, at any time of the year should be no more than 4 Ohms at linear voltages of 380 V of a three-phase current source or 220 V of a single-phase current source respectively. This resistance should be provided taking into account the use of natural grounding conductors, as well as grounding conductors of repeated grounding of the PEN- or PE-conductor of overhead lines with a voltage of up to 1 kV with the number of outgoing lines of at least two.
The set of measures to provide the required lightning protection is offered by the following solutions:
- Lightning protection is made using 4 air terminals - masts with the height of 1.5 (see. Fig.1).
- Arrangement of down conductors with the use of copper-bonded wire D = 8 mm. Lightning rods are interconnected to organize at least two current drainages. Arrangement of equipment in Figure 1. All metal communications must be connected to down conductors.
- Installation of the grounding device, consisting of 9 vertical electrodes 3 m in length, united by a horizontal electrode (copper-bonded tape with the cross-section 30x4 mm). The distance from the electrode to the walls of the building is not less than 1 m, penetration of 0.5 - 0.7 meters.
Grounding device resistance calculation:
The design resistance of the grounding device is 3,78 Ohms, which is less than the required value of 4 Ohms.
Results of calculations for the protection zone in compliance with the IS:
Air terminal mast № 1-4 (GL-21134):
h = 3.8 m;
Height of the cone according to the IS chart. 3.4:
The radius of the cone according to the IS chart. 3.4:
Distance between the lightning rods:
Ultimate distances between the lightning rods, according to IS, tab.3.6:
There is no vertical clearance!
The density of lightning discharges into the ground – 4 strikes / sq. km per year
Full number of strikes into the system – 0.0027 (once in 370 years);
The total number of breakthroughs is 0.00000034 (every 2941176 years);
Reliability of the system - 0.99988;
Probability of a breakthrough into all the system objects - 0.00012.
The list of required material is given in Table 1.
Figure 1 - Location of lightning protection equipment and grounding of the diesel generator set.
Таблица 1 – Перечень потребности материалов.
Молниеприемник- мачта высотой 1,5 м – air-terminal mast 1,5 m high
Проволока омедненная стальная диаметром 8 мм – copper-bonded steel wire 8 mm in diameter
Полоса омедненная стальная сечением - copper-bonded steel tape with the cross-section of 30*4 mm
Электрод - electrode
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