» Protection against lightning

VALVE TYPE ARRESTER

arjun — Sat, 21 Mar 2009 14:26:49 +0000

Valve type arresters incorporate non linear resistors and are extensively used on systems, operating at high voltages. Fig 12 (i) shows the various parts of a valve type arrester. It consists of two assemblies (i) series spark gaps and (ii) non-linear resistor discs in series. The non-linear elements are connected in series with the spark gaps. Both the assemblies are accommodated in tight porcelain container. (i) The spark gap is a multiple assembly consisting of a number of identical spark gaps in series. Each gap consists of two electrodes with fixed gap spacing. The voltage distribution across the gap is linearised by means of additional resistance elements called grading resistors across the gap. The spacing of the series gaps is such that it will withstand the normal circuit read more

EXPULSION TYPE ARRESTER

arjun — Sat, 21 Mar 2009 13:46:06 +0000

This type of arrester is also called ‘protector tube’ and is commonly used on system operating at voltages up to 33kV. Fig 11(i) shows the essential parts of an expulsion type lightning arrester. It essentially consists of a rod gap AA’ in series with a second gap enclosed within the fiber tube. The gap in the fiber tube is formed by two electrodes. The upper electrode is connected to rod gap and the lower electrode to the earth. One expulsion arrester is placed under each line conductor. Fig11 (ii) shows the installation of expulsion arrester on an overhead line. On the occurrence of an over voltage on the line, the series gap AA’ spanned and an arc is stuck between the electrodes in the tube. The heat of the arc vaporizes some of the fiber of tube walls resulting in the read more

MULTIGAP ARRESTER

arjun — Sat, 21 Mar 2009 13:36:01 +0000

Fig 10 shows the multigap arrester. It consists of a series of metallic (generally alloy of zinc) cylinders insulated from one another and separated by small intervals of air gaps. The first cylinder (i.e. A) in the series is connected to the line and the others to the ground through a series resistance. The series resistance limits the power arc. By the inclusion of series resistance, the degree of protection against traveling waves is reduced. In order to overcome this difficulty, some of the gaps (B to C in Fig) are shunted by resistance. Under normal conditions, the point B is at earth potential and the normal supply voltage is unable to break down the series gaps. On the occurrence an over voltage, the breakdown of series gaps A to B occurs. The heavy current after breakdown will read more

HORN GAP ARRESTER

arjun — Sat, 21 Mar 2009 13:30:10 +0000

Fig 9 shows the horn gap arrester. It consists of a horn shaped metal rods A and B separated by a small air gap. The horns are so constructed that distance between them gradually increases towards the top as shown. The horns are mounted on porcelain insulators. One end of horn is connected to the line through a resistance and choke coil L while the other end is effectively grounded. The resistance R helps in limiting the follow current to a small value. The choke coil is so designed that it offers small reactance at normal power frequency but a very high reactance at transient frequency. Thus the choke does not allow the transients to enter the apparatus to be protected. The gap between the horns is so adjusted that normal supply voltage is not enough to cause an arc across the read more

ROD GAP ARRESTER

arjun — Sat, 21 Mar 2009 13:09:58 +0000

It is a very simple type of diverter and consists of two 1.5 cm rods, which are bent at right angles with a gap in between as shown in Fig 8. One rod is connected to the line circuit and the other rod is connected to earth. The distance between gap and insulator (i.e. distance P) must not be less than one third of the gap length so that the arc may not reach the insulator and damage it. Generally, the gap length is so adjusted that breakdown should occur at 80% of spark-voltage in order to avoid cascading of very steep wave fronts across the insulators. The string of insulators for an overhead line on the bushing of transformer has frequently a rod gap across it. Fig 8 shows the rod gap across the bushing of a transformer. Under normal operating conditions, the gap remains read more

LIGHTNING ARRESTERS

arjun — Fri, 20 Mar 2009 13:40:33 +0000

The earthing screen and ground wires can well protect the electrical system against direct lightning strokes but they fail to provide protection against travelling waves, which may reach the terminal apparatus. The lightning arresters or surge diverters provide protection against such surges. A lightning arrester or a surge diverter is a protective device, which conducts the high voltage surges on the power system to the ground Fig 7(i) shows the basic form of a surge diverter. It consists of a spark gap in series with a non-linear resistor. One end of the diverter is connected to the terminal of the equipment to be protected and the other end is effectively grounded. The length of the gap is so set that normal voltage is not enough to cause an arc but a dangerously high voltage will read more

OVERHEAD GROUND WIRES

arjun — Fri, 20 Mar 2009 13:28:12 +0000

The most effective method of providing protection to transmission lines against direct lightning strokes is by use of overhead ground wires as shown in Fig 6. For simplicity, one ground wire and one line conductor are shown. The ground wires are placed above the line conductors at such positions that practically all lightning strokes are intercepted by them (i.e. ground wires). The ground wires are grounded at each tower or pole through a low resistance as possible. Due to their proper location, the ground wires will take up all the lightning strokes instead of allowing them to line conductors. The degree of protection provided by the ground wires depends upon the footing resistance of the tower. Share and Enjoy: read more

EARTHING SCREEN

arjun — Fri, 20 Mar 2009 13:17:21 +0000

The power stations and sub-stations generally house expensive equipment. These stations can be protected against direct lightning strokes by providing earthing screen. It consists of a network of copper conductors (generally called shield or screen) mounted all over the electrical equipment in the substation or power station. The shield is properly connected to earth on at least two points through low impedance. On the occurrence of direct stroke on the station, screen provides a low resistance path by which lightning surges are connected to ground. In this way, station equipment is protected against damage. The limitation of this method is that it does not provide protection against the traveling waves which may reach the equipment in the station. Share and read more

PROTECTION AGAINST LIGHTNING

arjun — Fri, 20 Mar 2009 13:05:04 +0000

Transients or surges on the power system may originate from switching and from other causes but the most important surges are those caused by lightning. The lightning surges may cause serious damage to the expensive equipment in the power system (e.g. generators, transformers etc.) either by direct stroke on the equipment or by strokes on the transmission lines that reach the equipment as travelling waves. It is necessary to provide protection against both kinds of surges. The most commonly used devices for protection against lightning surges are: (i) Earthing screen (ii) Overhead ground wires (iii) Lightning arresters or surge diverters Earthing screen provides protection to power station and sub station against direct strokes whereas overhead ground wires protects the transmission read more