Valve type lightning arrester is also called a nonlinear diverter. Such an arrester consists essentially of a divided spark gap in series with a resistance element having nonlinear characteristics.
The divided spark gap consists of a number of identical elements coupled in series, each of them consisting of two electrodes with pre-ionization device; between each element a grading resistor of high ohmic value is connected in parallel and ensures even grading of the voltage between different elements. The system is similar to that of a number of capacitors connected in series and across each of these capacitors is a high value resistor.
In case of relatively slow voltage variations, there is no spark over across the gaps as the effect of parallel resistors across these gaps predominates. It means that slow changes in applied voltage (internal voltages arising during coupling operations) are not injurious to the system.
But when rapid changes in voltage occur, the potential is no longer evenly graded across the series gaps; the influence of unbalancing capacitances between the spark gaps and the ground prevails over the grading resistors. The impulse voltages are mainly concentrated on the upper spark-gaps which in sparking- over cause the complete arrester to spark-over too.
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The voltage grading by means of resistors makes it possible to increase the interrupting capacity of the spark-gaps block. A leakage current which under normal operating conditions does not exceed 0.1 mA approximately permanently flows through these resistors.
This is sufficient to maintain the enclosures at a temperature of 5° above that of the ambient air to offset the adverse effect by the ingress of moisture into the enclosure containing the arrester elements.
The ideal characteristic for the non-linear resistance elements would be –
RI = constant …(9.15)
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The materials used for construction of resistances named “thyrite” and “metrosil” are hard ceramic substances in the form of cylindrical blocks composed of small crystals of silicon carbide bound together by means of an inorganic binder, and the whole assembly subjected to heat-treatment. The nonlinear characteristic is attributed to the properties of the electrical contacts between the grains of silicon carbide.
Figure 9.28 illustrates the volt-ampere characteristics of a nonlinear resistor of the required type, the dotted curve being the static characteristic, and the closed curve the dynamic characteristic corresponding to the applications of a voltage surge. If a horizontal line, tangential to the dynamic characteristic is drawn, its intercept with the voltage axis gives the residual voltage.
The residual voltage is defined as the peak value of the voltage appearing between the terminals of the surge diverter at the time of the discharge of a surge current wave. It varies from about 3 kV to 6 kV depending on the rate of discharge of current and whether the arrester is used in a substation or in a line and causes a breakdown of the divided gaps thus, the whole breaker starts sparking and pass current through the non-linear resistor.
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When the gap flashes-over, the arrester cuts off the power flow current at voltage close to its rating. The voltage that causes flashover depends on the rate of rise of surge current; if the incoming voltage is very steep, the flashover may take place at about 4.5 times the arrester rating; however, if the wave is slow rising surge the flashover may take place at about 3 times the arrester rating.
Fig. 9.29(a) illustrates the incoming surge and the voltage across the non-linear resistor, while Fig. 9.29(b) illustrates V-I characteristic of the non-linear resistor. The ratio of maximum residual voltage e1 and the maximum power voltage e2 is termed as protective ratio.
Mathematically, protective ratio is given as –
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Protective ratio = e1/e2 …(9.16)
The protective ratio may vary between 3 and 4.5 depending upon the magnitude of surge. In order to illustrate the application of valve type lightning arrester, the successive stages of operation may be indicated, as illustrated in Fig. 9.30.
As the surge approaches the transformer, it meets the lightning arrester and in approximately 0.25 µs, the voltage attains the breakdown value of the series gap and the arrester discharges. But as the surge voltage increases, just as rapidly, the resistance of the nonlinear element drops, thus allowing further surge energy to discharge and so restricting the value of voltage transmitted to the terminal equipment, as illustrated in the figure.
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As the tail of the surge is passing the point, the voltage is decreasing and, in consequence, the current to ground decreases while the resistance increases, attaining a stage when the current flow is interrupted by the spark-gaps and thus the lightning arrester is sealed again.
The total time taken from the start of surge wave to sealing off the arrester is a few micro-seconds (approximately 25 to 30 µs). The maximum voltage developed across the arrester terminal and transmitted to the terminal equipment is known as the discharge voltage of the arrester.
The valve type lightning arresters have the following drawbacks:
1. The performance characteristic is adversely affected by ingress of moisture into the enclosure and so to overcome such problem the arrestor is hermetically sealed. In some cases, the enclosures are filled by an inert gas and contain some form of de-humidifier and in other cases a leakage current is permitted.
2. If the incoming surge is quite steep, the arrester may not be quick to respond to it. Fortunately, the lightning strokes on transmission systems get considerably attenuated as they travel along the line towards terminal stations.
As such, normally the wave fronts of surges reaching the stations are within the reach of the protection afforded by such arresters. However, efforts are invariably made to prevent lightning strokes of severe intensity to the station equipment or the approach spans of transmission lines by means of an efficient type of shielding.
The valve type lightning arresters may be station type, line type, arresters for the protection of rotating machines, distribution type or secondary arresters.
Station type valve lightning arresters are the most efficient but the most expensive too. These are generally employed for the protection of important power equipment in the circuits of 2.2 kV to 400 kV and higher. They have high capacity of energy dissipation. The word “station” is an indication of their general application in large stations.
Line types are also used for the protection of power/substation equipment, like station type, but of relatively less importance where the cost of the latter is not considered justified. They are seldom employed on system of voltage exceeding 66 kV.
These are constructed like station type but are smaller in cross-section, lighter in weight and cheaper in cost. They permit higher surge voltages across their terminals in comparison to station types and have lower surge current capacity. They are generally employed for the protection of large transformers and intermediate substations; not for the protection of the lines as the name indicates. These are available in ratings up to 5 kA.
Arresters for the protection of rotating machines are designed for the protection of generators and motors (usual circuit voltages are from 2.2 to 22 kV). Distribution arresters are usually pole-mounted and are employed for the protection of distribution circuits/transformers. They are available for voltages varying between 2.2 kV and 15 kV. Secondary arrestors are meant for the protection of low voltage (120 to 750 V) apparatus.