The lightning arresters are used to protect power system from high voltage surges. The function of the surge arrester is to allow the discharge of any dangerous over-voltage before it can do damage and then to restore the line to normal operation after the discharge.
To perform this protective function satisfactorily, arresters must:
- not allow current to flow to the ground as long as the system voltage remains normal.
- provide a path to ground, when the system voltage rises to a predetermined valve above normal, to dissipate the energy from the surge without raising the voltage at which the circuit is operating.
- stop the flow of current to ground, as soon as the system voltage drops below the predetermined value, and restore the insulating qualities between the conductor and ground.
- not be damaged by the discharge and be capable of automatically repeating discharging process frequently when required.
Lightning Arrester Working Principle
When a voltage surge traveling along the conductor reaches the point at which a lightning arrestor is installed it breaks down the insulation of the arrester momentarily, allowing the voltage surge to discharge to ground.
As soon as the system voltage drops below the predetermined value, insulation between the conductor and ground is restored and further current flow to ground stops.
Lightning Arrester Types
The principle lightning arrester types are as under:
- Rod gap arrester
- Expulsion type lightning arrester
- Valve type lightning arrester
Rod gap lightning arrester
It is the simplest type of lighting arrestor. It consist two-rod electrodes, one of which is connected to the line and other to earth. The rods may be in the form of horn also.
Under normal operating conditions, the gap remains non-conducting. When a high voltage surge occurs, the gap sparks over and surge current is drained to earth.
Such arresters suffer from the following disadvantages:
- The operation is affected by climatic conditions.
- After the surge is over, due to ionization of air, the arc in the gap is maintained even at the normal supply voltage.
- Increased possibility of bird faults.
Due to the above disadvantages, the rod gap arresters are used only as a ‘back-up’ protection with main arrestors.
Expulsion Type Lightning Arrester
It consists of an arc extinguishing chamber in series with an air gap. The arc extinguishing chamber is in the form of fiber tube which interrupts the arc after discharging the surge by the generation of gasses.
When a voltage surge occurs that is sufficient to spark over the series gap and the gap in the fiber tube, discharge current flows to ground.
The arc in the tube attacks some of the fiber of tube walls, releasing a large amount of a relatively cool, non-conducting gas.
The gas produced in fiber tube acts not only to extinguish the arc but also builds up high pressure and expelled through the lower electrode which is hollow. As the gas leaves the tube violently, it wipes out the ionized air around the arc.
Due to this strong deionization effect, arc goes out at current zero instant and will not be re-established.
An expulsion type lightning arrester has a current rating in addition to the votage rating. The maximum current rating must be equal to the short-circuit current available at the point of installation. These lighting arrester are generally used on towers for the protection of transmission lines.
Valve Type Lightning Arrester
It consists of an outer ceramic body containing a set of resistances (valves) and spark gaps in series. The resistances are made of a special silicon carbide ceramic.
It possesses the characteristic of being substantially an insulator at one voltage and then changing to an excellent conductor at a higher voltage; the transition is due to voltage changes only, not to heat as in other valve materials.
High-voltage surges spark across the air gap and discharge current flows through the valve to ground.
Since the valve has a low resistance under high voltage and a high resistance at a normal voltage so as soon as system voltage becomes normal current flow stops. The arc gets extinguished and the arrester regains its original state.
The valve type arrestors are extensively used for the protection of generating stations, sub-stations, overhead lines, cables and rotating machinery.
They are rated for voltage only and are designated as 70%, 80%, and 100% arresters. The 80% arresters are suitable for solidly grounded systems. Whereas, 100% arresters are used on systems with isolated neutrals or those earthed through impedances.
The valve type arresters have been classified into the four types as under:
- Secondary Type
- Distribution Type
- Line Type
- Station Type
Secondary type lighting arrester are used with medium voltage apparatus, where the equipment is installed in farms and other lightning-porn areas.
Distribution type arresters are used on lines and substations up to 22 kV.
Line type arresters are normally used for voltages up to 66 kV though they can be used for higher voltages also.
Station type arresters provide the highest degree of protection and should be used where the cost of the protected equipment or the importance of service continuity justifies the extra investment on their account.
The performance of any arrester is dependent on a good connection to ground. Arresters will not function without a proper ground; they are totally useless.
The arrester should be placed as close as possible to the equipment, that is to be protected and leads connecting arresters to ground should be kept as short as possible.
Thanks for reading about lightning arrester types and lightning arrester working principle.