Valve Type Lightning Arresters are suitable on High Voltage Power Lines. This is non-linear diverter.
(a) Construction
(b) Schematic diagram of connection
Fig. 1: Valve Type Arrester (Half sectional view)
Construction of Valve Type Lightning Arrester
The unit consists of spark gaps in series and also the assembly of non-linear resistance discs of thyrite. The spark gap assembly consists of a series of electrodes, some of which are flat and the others are a specially shaped designs with pressed out projections. The resistance elements are generally made up in the form of blocks composed of small crystals, silicon carbide bound together by means of an inorganic binder, the assembly being heat treated in an oven. Both assemblies i.e. of spark-gap and resistors are accommodated in series within a completely tight porcelain housing. The housing is perfectly sealed so that moisture, humidity not to enter inside. These arresters are classified as (i) Station type, (ii) Line type, (iii) Distribution type.
The voltage distribution across the gaps is linearised by means of additional resistances/elements called as grading resistors.
The assembly of grading resistances, spark gaps and non-linear resistances which are housed in the air tight porcelain are shown in Fig. 1 (a) whereas, connection of line, arrester and earth is shown in Fig. 1 (b).
The speciality of non-linear resistance is such that they offer a high resistance to the current when normal system voltage is applied but very low resistance to the high value surge currents so that surge current quickly passes through these resistances to the ground.
Working of Valve Type Lightning Arrester
Under normal conditions the normal line voltage is insufficient to break the air gaps and no arc is produced and equipments work safely. On occurrence of very high voltage due to transients or surges, breakdown of series gaps takes place. The non-linear resistance offers a low resistance path to the surge current and it passes to the earth within a very short time. The arc vanishes and does not restrike back over the line. When the surge is over, the resistances offer a very high insulation (resistance) to the normal supply voltage of the system and stop the flow of current.
Advantages of Valve Type Lightning Arrester
- Provide very reliable and effective protection to the equipment like cables, transformers and other equipments.
- The impulse ratio is the ratio of Breakdown voltage under surge condition to the Breakdown voltage under low frequency condition, which is practically unity.
- Speed of operation ¡s very high (operation overs in less than one second).
- It sparks over at a predetermined voltage.
- They discipate surge energy
- They discharge high frequency surges without any change in spark-over characteristic.
Drawbacks/Limitations of Valve Type Lightning Arrester
- They may fail to check the surges of very steep wavefront from reaching the terminal apparatus.
- If moisture enters in they loose their performance.
Uses of Valve Type Lightning Arrester
- To protect important equipment in power stations operation on voltage upto 22 kV and higher. Used in station handling voltages upto 66 kV