Lightning Arrester or Surge Arrester
Lightning Arrester or Surge Arrester is generally connected between the phase and earth in the substations, distribution system, near Transformer Terminals and near terminals of large Rotating Machines to protect the apparatus insulation from the Lightning Surges and Switching Surges (Impulse Voltage Waves). In Isolated Neutral System Surge Arrester or Lightning Arrester is connected between the Transformer Star Neutral and Ground to give a path for surge to discharge to the ground.
Surge Arrester Or Lightning Arrester Working:
Conventional Surge arrester of SIC type have resistor elements in series with gap elements (For ZnO type arresters no gap is present). The resistance Blocks in the Surge Arrester or Lightning Arrester offer non linear resistance such that for normal frequency power system voltages the resistance is high. For discharge current the resistance is low. During normal voltage surge arrester do not conduct. When a surge wave travelling along the length of the overhead lines comes to the lightning arrester or surge arrester the gap break down and resistance offered by the Surge Arrester or Lightning Arrester being low the surge is diverted to the earth. After a few micro seconds the surge varnishes and normal power frequency voltage is set up across the arrester. The resistance offer by the surge arrester for this voltage is high. Therefore the gap current reduces and voltage across the gap is no more sufficient to maintain the arc. Therefore the current flowing through the earth is automatically interrupted and normal condition is restored. The high voltage surge is discharged though the ground. Hence the insulation of the equipment connected to the line is protected.
The surge arrester or Lightning Arrester will not create short circuit like Rod gaps and retain the residual Voltage across its Terminals. Surge Arrester discharges Current Impulse surges to earth and dissipates energy in the form of Heat. After discharging the impulse wave to the earth, the resistor blocks in the surge arrester offers high resistance to the normal power frequency voltage and acts as open circuit.
Surge Arresters provide protection against impulse voltage waves which generates due to
Lightning Surges: Generates due to lightning Strokes.Switching Surges: They occur due to opening and closing of the circuit breakers
Types Of Surge Arresters or Lightning Arresters:
The following are the different types of surge arresters are used:
Gapped Silicon-Carbide Surge Arresters:
These arresters are called Value type or conventional Gapped Arresters. This type of arresters consists of Silicon Carbide discs in series with gap units.
Zinc Oxide Gapless Arresters:
These type of arresters are also called as Metal Oxide Arresters. Conventional Silicon Carbide Arresters have characteristics of I=KVx, where 'x' is approximately 5. These arresters need a gap. The gap is associated with the variation in the spark over and arc quenching problem. Silicon Carbide arresters have lower energy absorption capability.
In case of Zinc Oxide arresters, the material for resistor is Zinc Oxide, Bismuth Oxide and Cobalt Oxide, in sintered disc form. The value of 'x' is more than 20. The energy absorbing capacity is high. The arrester does not need gaps. The resistor discs is made up of ZnO grains sintered with the flux of insulation oxide such as BiO2 along with the materials like CaO, MnO2, Cr2O3.
The Characteristics of Surge Arresters vary with these composition and higher the value of index ‘x’ ideal is the characteristics for the Surge Arrester.
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Surge Arrester or Lightning Arrester Installation Locations:
Surge Arresters are installed in the following locations:
- As First Apparatus for each incoming line in the substation between each phase and earth.
- Near Terminals on the HV side of the each transformer, between phase and earth.
- Near LV side of the Transformer between phase and earth.
- Between Transformer Neutral and Earth (optional).
- Between the tertiary terminal and earth for each phase of tertiary windings of the Transformer.
- For Phase to Phase across the delta connected Transformer windings.