Lightning Protection & Earthing for Substations
Lightning Protection in the electrical substation is critical for the hindrance –free operation of electrical networks during lighting strikes. Lightning strikes can cause severe damages due to the surge of extra voltage during a strike that is injected into a transformer. As a result, a strike and switching can cause substantial damage to equipment and outages in the transmission and distribution circuits. Even a strike near a substation can result in significant voltage fluctuation or breakdown. The high-capacity energy (voltage) released instantly during a thunderstorm can penetrate a building through Power, Data, and Telephone cables causing heavy damage to electronic equipment.
The extra infused into a transformer during a strike is up to 120 million volts depending on the length and size. Other than power failure, it can cause an explosion, fire to its surroundings. The live lines connected to a blown transformer carrying thousands of volts can cause damage to lives.
A proper lightning protection system in electrical substations provides a low resistance path to ensure the correct operation of protection relays. These installations can mitigate the risk of damage to employees, surrounding dwellings and people, and equipment.
Current technological developments have found a way to provide a solution to the problems caused by lightning and the following voltage surge. These solutions also significantly reduce the overall cost of substation designs. Based on the solutions certain standards have been put into place.
Lightning Protection Standards
Below mentioned standards are followed for the lightning protection system:
1. IEC 62305-Protection against Lightning ─ This international standard for Lightning Protection has been adopted by the Bureau of Indian standard as IS IEC 62305 along with many other countries. This standard provides guidelines for the planning, erection, testing, and maintenance of lightning protection systems for building structures without any height limit. The standard is also accepted and implemented in National Building Code-2016 that defines measures in terms of planning, erection, testing, and maintenance of a lightning protection system.
The Risk Management section of IEC 62305 classifies the four sources of Lightning:
i) Flashes to a structure
ii) Flashes near to a structure
iii) Flashes to a service line
iv) Flashes near a service line
2. NFPA 780- Standardizes lightning protection systems installation procedures.
Elements of Substation a lightning Protection
1. Surge Arresters: The substations use Lightning Arresters or surge arresters. These devices help limit the voltage surge during a strike and subsequent equipment damage or failure, explosion, and prevent a service disruption. Lightning arresters are designed to protect different pieces of equipment like power poles and towers, power transformers, circuit breakers, bus structures, and steel superstructures, from damage from lightning strikes. Therefore, they are installed on different parts and pieces of equipment in a substation. The types of arresters used in substations are:
a. Low-voltage surge arrester: for Low-voltage distribution system, exchange of electrical appliances protector, low-voltage distribution transformer windings.
b. Distribution arrester: For 3 kV, 6 kV, 10 kV AC power distribution system to protect distribution transformers, cables, and power station equipment.
4. Lightning Masts: Lightning masts protect various installations (fixed and portable) from lightning damage. Unlike the arresters, they are not usually attached to an object to protect it; they are placed near an area for its overall protection.
5. Down-conductors: Down-leads or down-conductors are a part of the main conductor system. They channelize the lightning current from the roof-level system to the grounding system.
6. Earth Terminations: Earth-termination systems are used to disperse lightning currents. The earth terminations are directly connected on each down-conductor to get an overall low earth resistance.
LPS Design & Methodologies
The design of an LPS for substations is specific to the requirements of the substations and related cost considerations. However, the protection measures must be taken based on a thorough risk assessment.
An LPS has external and internal systems. The air terminal system of external LPS intercepts with the lightning strikes. It diverts the lightning currents safely (via down-conductors) to the earth. The current is then dispersed under the earth's surface. The internal LPS protects from the ‘internal sparking’ by use of either ‘equipotential bonding’ or ‘separation distance’.
There are four classes of LPS which has different parameters. These are based on the corresponding Lightning Protection Level (LPL) they provide.
The substation LPS designers generally use the ‘rolling sphere’ method to enhance the quality of the locations and configurations of lightning masts and shield wires. This method is used to avert direct lightning strikes to the already energized conductors, bus bars, electrical equipment, support structures, and buildings.
Rolling Sphere Method
The Rolling Sphere Method (RSM) is a technique for determining the zones of protection. In this method, a sphere is supposedly brought up to and rolled over the total structure. The geometrical boundary of areas that are protected against direct lightning flashes can be determined using the RSM. All points of the structure that the sphere touches are exposed to direct lightning strikes and would need to be protected by air terminals.
For structural protection, LPS is made of air terminals, overhead earth wires, down-conductors, equipotential bonding, and earthing terminations. Structural weaknesses are mainly caused by the intensified electrical field of the points and corners on the upper surfaces of structures, conductors, or bus bars. Therefore, interception points for the lightning strikes are installed at the air terminals and earth wires.
Some basic installation rules are discussed below:
Lightning Masts/Air Terminal
Lightning masts are erected to intercept and conduct lightning strikes. These consist of an air terminal which is mounted on a pole or a structure.
- Lightning masts should firstly be placed to protect the most vulnerable parts of a structure (points and corners.)
- More masts are to be added using the rolling sphere method. Alternatively, existing mast heights can be increased to make the rest of the structures strong by covering also the least vulnerable parts by the protective shield of the mast.
- Air terminals are also used for protecting a sloped surface. In that case, they must be placed at the corners. Multiple air terminals can be installed based on the LPS design method.
- The down-conductors should be installed outside the building and avoid the proximity of electrical cables and gas.
- The down-conductors may have a cable conductor, or qualified continuous steel framework of 3/16” thickness or more, or a combination of the two.
- The down-conductors have direct connections to the earth. Therefore, they should have an air terminal attached to them.
- Multiple air terminals can be used to protect a single structure and they must be connected with main conductors.
- The spacing of the down-conductors is depended on the level of protection and should not be more than 20 m when inserted into the earth's surface. The insertion should use the shortest path to earth and be as straight as possible. Sharp bends or elevations must be avoided.
- Whenever possible, the lightning rod connected to the ground must have at least two down-conductors placed on opposite sides.
- A buried earth grid or network should connect directly to each down-conductor of the lightning protection system and should achieve a low overall earth resistance and shall be less than 10 Ω.
- All metallic structures at ground level should have equipotential bonding installed.