LIQUID INSULATION

ARC SUPPRESSION PROPERTIES

Switchgear using the arc suppression properties of insulating liquids (oils) was invented in the early 1880s. In the early days, the structure of switchgear was simple: a pair of elec­trodes were placed in insulating oil. In such switchgear the arc suppression mechanism is also simple: as the electrode spacing increases so the arc length increases and the electric arc is suppressed. This suppression results from the cooling effect of hydrogen gas produced by the decomposition of the insulating oil due to the arc. In arc suppression in insulating oil, hydrogen gas produced by decomposition of the insulating oil due to the arc plays an important role.

Arc Suppression by Hydrogen

The energy of the arc between a pair of electrodes in the insu­lating oil is dissipated by the electrodes, by conduction and radiation, evaporation and decomposition of the insulating oil, heating and expansion of gases produced by the decomposi­tion of the insulating oil, and dissociation of hydrogen. Fifty to seventy percent of produced gas is hydrogen, and the other gases are acetylene, methane, and ethane. As shown in Table 1, the thermal conductivity of hydrogen at room temperature is higher than that of other gases. At 4000°C it is about 50 W/m • K. This value is more than 5 times higher than for the other gases. Therefore, the cooling effect is larger than that of the other gases. By this cooling, the arc is suppressed at the zero-current point of alternating current. Thus the current is

Table 1. Heat Conductivity of Gases

Gas

Heat conductivity (W/m • K)

Hydrogen

0.175

Methane

0.031

Ethane

0.018

Acetylene

0.017

J. Webster (ed.), Wiley Encyclopedia of Electrical and Electronics Engineering. Copyright © 1999 John Wiley & Sons, Inc.

cut off. Switches that utilize this arc suppression mechanism are called plane-break oil circuit breakers.

As the current increases, it becomes more difficult to sup­press the arc. Therefore, the breaking time (cutoff) of the cur­rent becomes longer. However, when the current exceeds a certain magnitude, a large amount of hydrogen—enough to suppress the arc—has been produced. At this point the break­ing time is again reduced. This means that the breaking time shows a maximum value at a certain magnitude of current.

Arc Suppression by an Explosion Chamber

Cooling may not be sufficient to suppress a high-current arc and to lessen the breaking time. In this case explosion cham­bers are used. In a simple explosion chamber a movable elec­trode of the circuit breaker (switchgear) acts as a stopper of the chamber. In the early stages of separating the electrode, the arc is enclosed in the limited space of the chamber. There­fore, the pressure in the chamber rises owing to gases pro­duced by decomposition of the insulating oil. As this process proceeds, the stopper is removed creating an exhaust hole.

Through this exhaust hole the gases in the chamber are released abruptly. By this release, flows of gases and oil are produced, and the arc is pressurized and blasted. These pro­cesses create efficient arc suppression. Furthermore, when the lengthened arc contacts insulating solids while enclosed in narrow gaps between them, more efficient arc suppression results.

Because gases are abruptly exhausted through the hole, adiabatic expansion occurs. Thus cooling is expected. In some oil circuit breakers this is the main effect utilized. Some sci­entists maintain that in oil circuit breakers with an explosion chamber, arc suppression can be entirely explained by the cooling effect owing to the adiabatic expansion. In fact, how­ever, insulating oils exhibit not only the arc suppression prop­erty resulting from the cooling effect of hydrogen, but also the substantially different suppression properties of the oils themselves. In high-current arc suppression, these two types of suppression properties are combined.

In circuit breakers with an explosion chamber, a large pressure rise is expected in the chamber in the case of high — current arc suppression, but not in the case of low-current suppression. Thus in the latter case the breaking time is longer, because the arc must be suppressed by the cooling ef­fect of hydrogen alone. Therefore, the breaking time shows a maximum at a certain magnitude of current (the critical cur­rent). But this breaking time is much shorter than that of the plain-break oil circuit breaker. A circuit breaker with an explosion chamber necessarily has plural arc suppression mechanisms.

In some circuit breakers, in the region of the critical cur­rent, the auxiliary flow of the oil is forced by a piston to sup­plement the pressure rise and the conduction cooling effect. By this means a constant breaking time is obtained over a wide range of current.

Plane-break oil circuit breakers are used for low voltages and low currents, such as 3.6 kV to 7.2 kV and 4 kA to 8 kA. Oil circuit breakers with an explosion chamber are used for high voltages and high currents. In the case of multibreak circuit breakers, 700 kV with currents of several tens of kilo — amperes have been achieved (1).

DISCHARGE RESISTANCE

The behavior of insulating liquids under highly stressed con­ditions and under conditions of partial discharge are among the most important items in screening tests for newly devel­oped insulating liquids and also in the routine testing of liquids.

Gassing Rate

Methods of evaluating gas absorption and evolution of insu­lating oils under high stress after saturation with a gas are described in IEC 628 and ASTM D 2330. The fundamental approaches are similar to each other and amount to a modi­fied Pirelli method.

The condition used in such methods differs from actual field conditions, especially in the case of hermetically sealed equipment such as power cables, capacitors, and many power transformers.

Discharge Resistance

To evaluate the behavior of insulating liquids in a highly stressed impregnated system and to obtain numerical results for the recently developed impregnants with very high resis­tance to partial discharge, the above-mentioned methods are not sufficient. As new liquids, especially with high aromatic — ity, are developed and applied voltage stresses are progres­sively increased, a new method is needed to characterize the ability of such insulating liquids to prevent or suppress par­tial discharge under high stress. One of these methods, deter­mination of the partial discharge inception voltage with a needle and spherical ball oil gap, is described in IEC 61294. The partial discharge inception voltage obtained by this method is largely related to the chemical structure of the liq­uid and is correlative to partial discharge in impregnated in­sulating systems such as capacitor elements.

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