surge diverters


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surge diverters

Transcript of surge diverters


2. Surges, or transients, are brief overvoltagespikes or disturbances on a power waveformthat can damage, degrade, or destroyelectronic equipment within any home,commercial building, or manufacturingfacility. Transientscan reach amplitudes of tens ofthousands of volts. Surges are measured in microseconds. 3. A surge diverter is a piece of equipment thatdiverts excess voltages (caused by spikes in theelectrical supply) to earth, thus protectingsensitive electrical and electronic equipment. The surge diverter is normally installed in themain switchboard.The surge diverter shunts voltages over 260volts AC to ground, stopping most power spikesfrom causing any damage. 4. It should not pass any current at normal andabnormal power frequency voltage. It should breakdown as quickly as possible afterthe abnormal high frequency voltage arrives. It should not only protect the equipment forwhich it is used but should discharge currentwithout damaging itself. It should interrupt power frequency followcurrent after the surge is discharge to ground. 5. Rod gap Protector tube or expulsion type surge diverter Valve type surge diverter 6. Itis a very simple type of diverter andconsists of two 1.5 cm rods, which are bentat right angles with a gap in between. One rod is connected to the line circuit andthe other rod is connected to earth. The distance between gap and insulator mustnot be less than one third of the gap lengthso that the arc may not reach the insulatorand damage it. The rod gap should be so set that it breaksdown to a voltage not less than 30% belowthe voltage withstand level of the equipment 7. The string of insulators for an overhead lineon the bushing of transformer has frequently arod gap across it. Under normal operating conditions, the gapremains non-conducting. On the occurrence of a high voltage surge onthe line, the gap sparks over and the surgecurrent is conducted to earth. In this way excess charge on the line due tothe surge is harmlessly conducted to earth 8. After the surge is over, the arc in the gap ismaintained by the normal supply voltage,leading to short-circuit on the system. The rods may melt or get damaged due toexcessive heat produced by the arc. The climatic conditions (e.g. rain, humidity,temperature etc.) affect the performance ofrod gap arrester. The polarity of the surge also affects theperformance of this arrester. 9. This type of arrester is also called protectortube and is commonly used on systemoperating at voltages up to 33kV. It essentially consists of a rod gap in serieswith a second gap enclosed within the fibertube. The gap in the fiber tube is formed by twoelectrodes. The upper electrode is connected to rod gapand the lower electrode to the earth. 10. The series gap is set to arc over at aspecified voltage lower than the withstandvoltage of the equipment to be protected. The follow-on current is confined to thespace inside the relatively small fibre tube. Part of the tube material vaporizes, and thehigh pressure gases so formed are expelledthrough the vent at the lower end of thetube, causing the power follow-in arc to beextinguished. The device, therefore, has the desired self-clearing property. 11. They are not very expensive. They can be easily installed. They are improved form of rod gap arresters as they block the flow of power frequency follow currents. 12. An expulsion type arrester can perform onlylimited number of operations as during eachoperation some of the fiber material is usedup. This type of arrester cannot be mounted onenclosed equipment due to discharge ofgases during operation. Due to the poor volt/amp characteristic ofthe arrester, it is not suitable for protectionof expensive equipment 13. Valvetype arresters incorporate non linearresistors and are extensively used on systems,operating at high voltages. It consists of two assemblies (i) series sparkgaps and (ii) non-linear resistor discs The non-linear elements are connected inseries with the spark gaps. Both the assembliesare accommodated in tight porcelaincontainer. The spark gap is a multiple assemblyconsisting of a number of identical spark gapsin series. Each gap consists of two electrodes with fixedgap spacing. 14. Thespacing of the series gaps is such that itwill withstand the normal circuit voltage. An over voltage will cause the gap to breakdown causing the surge current to ground viathe non-linear resistors. The non-linear resistor discs are made ofinorganic compound such as thyrite ormetrosil. These discs are connected in series. The non-linear resistors have the property ofoffering a high resistance to current flow whennormal system voltage is applied, but a lowresistance to the flow of high surge currents. 15. When the surge is over the non linear resistor assume high resistance to stop the flow of current. 16. Under normal conditions, the normal systemvoltage is insufficient to cause the breakdownof air gap assembly. On the occurrence of an over voltage, thebreakdown of the series spark gap takes placeand the surge current is conducted to earth viathe nonlinear resistances. Since the magnitude of surge current is verylarge, the nonlinear elements will offer a verylow resistance to the passage of surge. The surge will rapidly go to earth instead ofbeing sent back over the line. 17. They provide very effective protection against surges. They operate very rapidly taking less than a second The impulse ratio is practically unity. 18. They may fail to check the surge of very steep wave front reaching the terminal apparatus. This calls for additional steps to check steep fronted waves. Their performance is adversely affected by the entry of moisture into the enclosure. This necessitates effective sealing of the enclosure at all times. 19. Asa voltage surgetravelling on theline reaches thesurge diverter, itbreaks down at aspecified voltageVbd. The voltage acrossthe diverterinstantly dips andas the surgecurrent showdischarges throughit, a constantvoltage of valueVr(residual voltag)