Types of Surge Diverters | Power System | Electrical Engineering

The following points highlight the top nine types of surge diverters. The types are: 1. Rod Gap Arrester 2. Sphere Gap Arrester 3. Horn Gap Arrester 4. Multiple-Gap Arrester 5. Impulse Protective Gap 6. Electrolytic Arresters 7. Auto-Valve Arrester 8. Oxide Film Arrester 9. Metal Oxide Lightning Arresters.

Type # 1. Rod Gap Arrester:

This is the simplest form of surge diverter consisting of two 12 mm dia or square with ends facing each other, one connected to line and the second connected to earth. These are usually connected across the bushings of various equipments.

To avoid cascading across the insulator surface of very steep fronted waves, the rod gap should be set to breakdown at about 20% below the impulse spark over voltage of the insulation at the point where it is installed.

To protect the insulator from the arc, the distance between the rod gap and the insulator should be more than one-third of rod gap length. The rod gap depends upon the operating voltage of the system.

The typical figures are given below:

The rod gap has the advantage of low cost and easy adjustment on site.

The spark-over takes place at very high voltage due to lightning surges but it cannot flashover at usual power supply voltage. The difficulty with the rod gap arrester is that once the spark having taken place may continue for some time even at low supply voltage.

To avoid it a current limiting resistance is used in series with the rod which limits the current to such an extent that it is sufficient to maintain the arc. Another difficulty is that the rod gaps are liable to be damaged due to high temperature of the arc which may cause the rods to melt. The performance of the rod gaps is badly affected due to climatic variations and also the polarity of the surge.

Thus, the rod gap is a very inferior type of surge diverter and cannot be relied upon as main protection in high voltage power systems where the continuity of supply and also the protection of equipment are given priority. It can, however, be employed as a second line of defence in view of its low cost.

Type # 2. Sphere Gap Arrester:

In such a device the air gap is provided by two similar spheres—one connected to the line and another grounded. The spacing between the two spheres is very small compared with their diameter and can be adjusted with the help of gauge. A choking coil is inserted between the phase winding of the transformer and sphere connected to the line.

This is done to reflect off any overvoltage surges that might tend to enter the transformer winding. The minimum air gap is set so that a discharge does not take place at normal operating voltages but at predetermined excess voltages an arc is set up. This arc will travel up the spheres as the heated air near the arc tends to rise upwards.

The arc will keep on travelling upwards and lengthening till it is interrupted automatically. When the source of voltages persists, a sound arc follows the first one and so on till the normal conditions of voltage are achieved.

It has the advantage of impulse ratio of unity, i.e., if the apparatus is protected against 50 cycle waves, it is protected against a wave of any duration. Unfortunately, when the sphere gap flashes-over, the power current maintains the arc, which requires only a very low voltage to maintain it, and the arc is not self-extinguishing.

The circuit breakers would have to intervene to interrupt the arc current and, therefore, the continuity of supply is interrupted. This is the reason that the sphere gap is not of use.

Type # 3. Horn Gap Arrester:

This was one of the earliest types of surge diverters to be developed, and is still used to a certain extent on low-voltage lines on account of its great simplicity. It consists of two horn-shaped pieces of metal separated by a small air gap and connected in shunt between each conductor and earth.

The distance between the two electrodes is such that the normal voltage between the line and earth is insufficient to jump the gap, but abnormally high voltages (slightly less than twice normal operating voltage) will breakdown the gap and so find a path to earth.

The arc thus formed by reason of heated air and electro­magnetic action will rise up the horn and extinguish itself, thus preventing a follow on arc. The time taken for the complete operation is usually from 3 to 5 seconds.

Usually a choking coil consisting of several turns of bare copper wire is connected in the line between the arrester and the apparatus to be protected to reflect travelling waves back to the horns. The choke is without effect on low-frequency power wave because its reactance is negligible at normal power frequency. In practice 20 or 30 turns on a diameter of about 0.3 m are found to be quite effective for the purpose.

For the utmost protection to the terminal apparatus the line arrester should be located as close to the end of transmission line as possible. Also the connections from choke coil to arrester, and arrester to earth, should be as short and direct as possible.

The horn gap cannot rupture arc currents much in excess of 10 A, and as the arc is a dead short circuit it is necessary to limit the current to a small value. This is accomplished by inserting a non-inductive resistance, between the line and the horn on the line side. The efficiency of the horn gap is seriously reduced by the resistance.

The selection of a value for the resistance is, therefore, a matter for compromise; the value generally used being such as to limit the dynamic current to between 1 and 5 A with normal voltage applied. The resistance is a water column, oil-immersed metal wire, carbon rod, or carborundum, and is made as non-inductive as possible.

Such an arrester has the following characteristics:

i. The voltage at which the breakdown occurs during transients, depends upon the impulse ratio of the gap.

ii. Breakdown voltage value is also affected by atmospheric conditions such as temperature and pressure of air.

iii. The performance of the lightning arrester is also affected by any roughness of the horn gap and the frequent settings are required to be made for the gap.

iv. At heights a longer air gap is required as it depends upon the air density and is inversely proportional to it.

It is seen that on low-voltage installations the gap has to be set very closely, with consequent danger of accidental discharge by insects or other objects bridging the gap. Alteration of calibration also is liable to occur, due to corrosion or formation of small globules of melted metal.

These difficulties are overcome in the arrester shown in Fig. 9.21:

The main gap is set for a voltage well above that to be protected (1 cm for 2,000 V). The auxiliary gap, which is provided with an electrode having an adjustable platinum point is, however, set to discharge, say at 25% above the normal voltage.

If a foreign body gets into the auxiliary gap the resulting discharge quickly dies out owing to the high resistance in series therewith. On the other hand, an arc formed by a voltage rise persists, and ionizes the air in the main gap, causing the same to arc across and relieve the line.

Burke Arrester:

This is another modification of the simple horn gap arrester. It embodies as a characteristic feature a triangular choke coil wound from copper strip, one side of the coil being used as one of the horns. The arrangement is illustrated in Fig. 9.22. It is seen that the line current passes round the triangular reactance coil and from the centre of the latter goes away to load.

The main gap has a series resistance, and there is also an auxiliary gap which is connected direct to ground. Severe over-voltages flash across the main and auxiliary gap and go direct to ground while less severe voltages flashover the main gap only, and the current is then limited by the resistance. The ohmic value of resistance is such that it limits the dynamic current with normal voltage to from 4 A at 3.3 kV to 2 A at 66 kV.

It is claimed that the choke coil in such an arrester has a magnetic blow-out effect, causing the arc to rise rapidly up the horns. Furthermore, a wave traveling along the line meets its first impedance at the first sharp upward bend of the triangle opposite to which the grounded horn is mounted. This piles up the voltage and permits a considerably wider gap to be used than with other types of horn arresters without, however, diminishing the sensitivity of the device.

There are several other types of arresters which are modifications in some degree of the horn and resistance device. The graded- resistance horn arrester, for example, has a tapered or stepped gap. A moderate discharge takes place across the narrowest portion of the gap which has a high resistance in series therewith.

If the current flow is not sufficient to relieve the line the arc jumps across to the next’ wider gap, and so on, the successive gaps having lower and lower resistances in series. Under the severest conditions the discharge will occur at the last gap where there is no series resistance.

Type # 4. Multiple-Gap Arrester:

Such an arrester consists of a series of small metal cylinders insulated from one another, and separated by air gaps of about 1 mm width. The first of the series is connected to the line, and the last to ground, and the number of gaps required depends upon the line voltage.

To assist in the suppression of the arc and thereby prevent dynamic current from the line flowing through the arrester, the cylinders are made of an alloy of zinc known as the non-arcing metal.

The vapour formed by an arc between these cylinders has a rectifying effect like that of mercury vapour, thereby suppressing the arc every other half cycle. It is common practice to knurl the surface of the cylinders so as to present as large a cooling surface as possible, this also helping to extinguish the dynamic current.

There is a certain capacitance between consecutive cylinders, and also between each of these cylinders and ground. This leads to a non-uniform distribution of the potential between the various gaps—greatest at the line end of the arrester, and gradually diminishing towards the gaps at the grounded end.

The result is that when the voltage across the arrester attains a certain critical value, breakdown occurs between the first and second cylinders. The second cylinder is then connected to first one by an arc so that its potential rises accordingly until breakdown occurs between the second and third cylinders; and so on.

The dynamic current then follows the discharge and in doing so produces a sensibly uniform fall of potential along the line of cylinders with the result that the maximum pd between cylinders is considerably less than that required for the initial breakdown. The dynamic current continues to flow until the line voltage passes through zero to the next half cycle, when the arc extinguishing quality of the metal electrodes come into action.

Some of the zinc is vaporized by the heat of the discharge, and when the current in the gap passes through the zero point of the wave, the zinc vapour prevents its re-establishment in the opposite direction. Before the voltage again reverses, the arc vapour in the gap has cooled down to a non-conducting state, cutting off further flow of current.

As in the case of the horn arrester there is a current limit, although a higher one, beyond which the arc will be maintained despite the quenching action, so that limiting resistances must be used in most cases. Such resistances are usually of graphite mixture or metallic, and are connected in series with the group of gaps, as illustrated in Fig. 9.23. Their value is such as to limit he dynamic current to less than 20 A.

Such arresters are not satisfactory for use on systems with line voltages exceeding 33 kV, due to the fact that the necessary increase in the number of gaps to prevent arcing over by the normal voltage is out of all proportion to the increase in voltage.

There is also much uncertainty as to the number of gaps required, this depending on the position of the arrester relative to surrounding grounded objects. In order to obtain a more equal distribution of potential over the arrester, and so allow of a reduction in the number of gaps necessary, a metal earth-shield or antenna is sometimes placed near the gaps at the high-potential end of the arrester and connected to the line conductor.

As with other types of lightning arresters, the multiple-gap arrester loses its efficiency as a discharge path for intense disturbances when it is provided with a series resistance. However, this difficulty can be largely surmounted by shunting some of the gaps with a resistance and using a smaller series resistance than with the un-shunted arrangement.

The arrester with shunted gaps is known as the ‘low-equivalent arrester’ and the principle of construction is shown in Fig. 9.24:

From Fig. 9.24 it is obvious that the point B is at ground potential under normal conditions and’ therefore a discharge will take place when the voltage is sufficient to breakdown the series gaps between A and B. The impulsive rush of current following the breakdown, being of high frequency, will choose the straight through path to ground via the shunted gaps between B and C, instead of alternative path through the shunt resistance, finally passing to ground via the small series resistance.

As soon as the impulsive rush of current is over, the arcs in the shunted gaps die out, with the result that any dynamic current attempting to follow will have to flow through both resistances, which are now in series. This current will be so reduced by the high resistance in its path that it will not be sufficient to maintain the arcs in the series gaps, which therefore go out as well.

The effect of shunting of some of the gaps is therefore to provide the arrester a certain amount of selective action, since it interposes a low resistance in the path of high-frequency disturbances, and a high resistance in the path of low-frequency discharges. In one form of construction the series resistance is dispensed with altogether, the arrester consisting simply of a large number of gaps some of which are shunted and some not.

In comparison to the horn arrester, the multiple-gap type arrester has the advantage of greater sensitivity to minor voltage rises. Furthermore, it has the property of extinguishing any dynamic current following the discharge at the zero value, instead of in the neighbourhood of the maximum value as with a horn gap.

There is therefore no danger of the arrester becoming the seat of high-frequency oscillations after it has operated. On the other hand, the multiple-gap type arrester is more costly, especially for the higher line voltages.

Type # 5. Impulse Protective Gap:

It was pointed out that the sphere gap has an impulse ratio of unity, but suffers from the drawback that the arc between its electrodes is not self-extinguishing. The horn gap is self-extinguishing but it has a high impulse ratio of 2 or 3 unless the setting is small, as with low voltages.

The impulse protective gap is designed to have a low impulse ratio, even less than unity and to extinguish the arc. In principle the impulse protective gap is very simple, as illustrated in Fig. 9.25.

The gap proper consists of two sphere-horn electrodes S₁ and S₂ which are connected respectively to the line and to the arrester, the latter being usually of the electrolytic type. An auxiliary needle electrode E is placed mid-way between S₁ and S₂ and is connected to them via (R, C₁) and C₂.

At normal line frequency the impedance of capacitance C₁ is quite large as compared to the impedance of resistance R; and if C₁ = C₂ the potential of the auxiliary electrode will be midway between those of S₁ and S₂ and the electrode has no effect on the flashover between them.

In case of a transient occurring over the line, the impedance of capacitors C₁ and C₂ decreases and the impedance of the resistance R now becomes pre-dominant. The result is that practically whole of the voltage is concentrated across the gap between E and S₁.

The gap at once breaks down, the rest of the gap between E and S₂ immediately following:

In effect, the length of the gap is halved the instant a high- frequency wave occurs. On the other hand, a slight time-lag is introduced due to the fact that, in the initial breakdown, one of the electrodes is pointed. However, an apparent impulse ratio of 0.6 to 0.7 can be achieved with the commercial forms of the gap. The electrolytic arrester on the earth side extinguishes the arc.

Type # 6. Electrolytic Arresters:

Electrolytic arrester is the earliest type of arrester with a large discharge capacity. It operates on the fact that a thin film of aluminium hydroxide deposited on the aluminium plates immersed in electrolyte acts as a high resistance to a low voltage but a low resistance to a voltage above a critical value. Voltages more than 400 volts (critical breakdown voltage) cause a puncture and a free flow of current to earth. When the voltage regains its normal value of 400 volts, the arrester again offers high resistance in the path and leakage stops.

The insulating film of hydroxide is formed by applying a dc voltage up to the critical value to aluminum plates immersed in the electrolyte. During the formation of film, current passes fairly readily but on formation the current ceases.

To increase the total critical value of voltage, number of films is arranged one above the other. The total critical value of voltage of such an arrester is equal to the product of number of films and critical voltage of each film.

Such arresters are very delicate, need daily supervision, and the film is required to be reformed whenever destroyed. Serious difficulties may be encountered in the cold countries due to freezing of the electrolytes. The electrolytic arrester is used in conjunction with an impulse gap.

Type # 7. Auto-Valve Arrester:

This type of lightning arrester is very cheap, effective and robust and is, therefore, extensively used nowadays for high voltage systems. This consists of a number of flat discs of a porous material stacked one above the other and separated by thin mica rings. The porous material is made of specially prepared clay with a small admixture of powdered conducting substance.

As the material of the discs is not homogeneous and conducting material has also been added, therefore, the glow discharge occurs between the discs at the time of overvoltage. The discharge occurs in the capilliaries of the material and voltage drops to about 350 volts per unit. The discs are arranged in such a way that normal voltage may not cause the discharge to occur. The mica ring provides insulation during normal operation.

The valve type lightning arrester possesses the advantage over the electrolytic type is that the initial cost and maintenance charges are lower, and daily supervision is not required. It can be used for protection of isolated transformer stations, automatic substations, or similar plants where the installation of electrolytic arresters would not be justified from an economic point of view.

Type # 8. Oxide Film Arrester:

It operates on the fact that certain chemicals have the property to change rapidly from a good conductor to almost perfect insulator when slightly heated. For example, lead per oxide, which has a specific resistance of 25 ohms per mm cube at normal temperature, becomes red lead at about 150°C and has a specific resistance of the order of 600 mega ohms per mm cube.

It consists of 2.4 mm diameter pellets of lead per oxide with a thin porous coating of litharge arranged in a column and enclosed in a tube of diameter of about 6 cm and of height of 5 cm per kV of rating. Out of the two leads of the arrester upper is connected to the line, while the lower is connected to earth. The tube contains a series spark-gap.

A single tube system is available for voltages up to 25 kV when the neutral is solidly grounded and 18 kV when the neutral is isolated or grounded through an inductive coil. For use on higher voltages several units in series are employed. The number of cells used in an arrester is such that the voltage per cell is about 300 volts.

When an overvoltage occurs an arc passes through the series spark gap and an additional voltage is applied to the pellet column and a discharge takes place. After the discharge, the resistance of the pellet column increases till only very small current can flow through it. This small current is finally interrupted by the series spark gap.

The great advantage of oxide film arrester is that it does not require daily charging, and it may thus be installed at points on transmission systems where daily attendance is difficult or expensive to provide. The impulse ratio of such an arrester is more than unity.

Type # 9. Metal Oxide Lightning Arresters:

This type of an arrester belongs to the new generation of station metal oxide surge arresters developed by General Electric Co. in 1976. This is useful for the protection of systems rated 2.4 to 400 kV against over-voltages. Such an arrester consists of a stack of Zenox™ disks mounted in a sealed porcelain housing.

Each disk is wedged in place with silicone rubber which offers heat transfer and protection against physical damage. On the end faces of each disk, a conducting surface is applied to assure proper contact and uniform current distribution.

Zenox valve elements are made of a specially formulated compound of zinc oxide and small amounts of other selected metal oxides. The ingredients are mixed in powdered form pressed to form a disk, and fired at high temperature resetting in a dense polycrystalline ceramic. Under electric stress, the inter granular layers conduct resulting in highly non-linear characteristics.

The degree of non-linearity is much more than that in the thyrite arrester. The arrester is designed in accordance with the metal oxide standard IEEEC 62.11 and IEC TC-37. A change in the arrester current of 0.1 A to 10⁴ A results in a voltage change of 54%. Under normal operating conditions, the current conducted does not exceed 1 m A. When the surge reaches the arrester, it conducts only current necessary to restrict the voltages.

Such an arrester is simple in construction and has superior energy absorption capability, better surge protection, more stable protective characteristics and substantial reduction in overvoltage across the equipment as compared to thyrite arrester.