Distribution networks consisting of overhead lines can experience faults, following lightning strikes, due to overvoltage. Overvoltage damage caused by events such as spikes and transients due to lightning, utility switching, isolation arcing, electrical motor cycling, or any other sudden change in electrical power flow in alternating current (AC) lines is responsible for a third of all outages. Utilities are therefore required to deploy surge protection devices (SPDs) and earthing systems in both medium voltage (MV) and low voltage (LV) networks in order to ensure continuity in business operations.
Key considerations
To prevent galvanic coupling to the 20 kV MV overhead line network or outgoing LV lines resulting from a lightning strike, a protective device must be installed on the main LV distribution board. This device must be selected in such a way that it meets the requirements concerning lightning current carrying capability, short-circuit withstand capability, follow current extinguishing capability and temporary overvoltage. Lightning protection systems can be external or internal. External protection systems are installed to prevent direct lightning strikes from penetrating roofs or structures and causing a fire due to intense heating. Non-isolated systems are not considered ideal for the protection of technology on the roofs of buildings. In this case, the rooftop equipment, the metallic components and the structure itself may carry a proportion of the discharge current.
Protection devices
Surge arrestors
The installation of surge arresters is a common practice for the protection of power networks against atmospheric disturbances and switching voltages that may cause serious equipment damage. However, the protection level that surge arresters offer depends on various factors, that is, the installation position, the length of the connecting conductors and the grounding resistance. Typically, in MV networks, shielding the conductors is generally not very effective. Due to the small clearance between the earth wire and the conductors, a direct lightning stroke will usually hit the conductors as well. Induced overvoltages can be reduced only marginally with shield wires. Hence, the most effective protection against overvoltages in such networks is the use of surge arresters or spark gaps.
Lightning arrester
A lightning arrester or a surge diverter is used for the protection of power distribution equipment at substations against travelling waves. It diverts the abnormal high voltage to the ground without affecting the continuity of supply. It is installed between the line and the earth, in parallel with the equipment to be protected at the substation.
Transient voltage surge suppressors
TVSS devices are used for protecting other equipment from the dangers of potentially harmful voltage surges caused by lightning. They are used as an interface between the power source and sensitive loads so that the transient voltage is clamped. TVSSs contain a component with non-linear resistance that limits excessive line voltage and transmits any excess impulse energy to the ground.
Thyristor surge protection device
A thyristor surge protection device (TSPD) is a specialised solid state electronic device used in crowbar circuits to protect against overvoltages. The thyristor-type devices used for the purpose are Trisil and SIDACtor. TSPDs operate much faster. They are related to TVS diodes but can “break over” to a low clamping voltage just like an ionised and conducting spark gap. After triggering, the low clamping voltage allows large current surges while limiting heat dissipation in the device.
Metal oxide varistor
A metal oxide varistor (MOV) is a voltage-dependent resistor in which the resistance material is a metallic oxide, primarily zinc oxide pressed into a ceramic-like material. MOVs are a common type of voltage clamping devices and are increasingly finding application in LV AC power networks. The use of metallic oxide makes MOVs extremely effective in absorbing short-term voltage transients and have higher energy handling capabilities. An MOV starts conduction at a specific voltage and stops conduction when the voltage falls below a threshold voltage.
Earthing systems
As part of the earthing arrangement, all the exposed conductive parts must be connected and the system must be capable of discharging the lightning current, avoiding a voltage rise in the earthing system itself and the surrounding ground. The earth electrode must have a low frequency resistance value and all the earthing components must offer low resistance to the earth as well as have excellent corrosion resistance, as they will be buried in the ground for many years.
Conclusion
Distribution lines are often located in areas with high ground flash densities and, therefore, subjected to lightning-caused power interruptions. Thus, the safety of power and electronic systems is important for utilities and customers alike. In addition, utilities need to adopt new technologies and advanced SPDs to provide complete protection against lightning-related threats.