Circuit breakers

Introduction

For the analysis of simple switching transients and for carrying out large system studies, it is often sufficient to model a circuit breaker as an ideal switch. When studying arc–circuit interaction, wherein, the influence of the electric arc on the system elements is of importance, a thorough knowledge about the physical processes between the circuit breaker contacts is absolutely necessary.

A high-voltage circuit breaker is an indispensable piece of equipment in the power system. The main task of a circuit breaker is to interrupt fault currents and to isolate faulted parts of the system. Besides short-circuit currents, a circuit breaker must also be able to interrupt a wide variety of other currents at system voltage such as capacitive currents, small inductive currents, and load currents. We require the following from a circuit breaker:

• In closed position it is a good conductor;

• In open position it behaves as a good isolator between system parts;

• It changes in a very short period of time from close to open;

• It does not cause overvoltages during switching;

• It is reliable in its operation.

The electric arc is, except from power semiconductors, the only known element that is able to change from a conducting to a nonconducting state in a short period of time. In high-voltage circuit breakers, the electric arc is a high-pressure arc burning in oil, air, or sulphur hexafluoride (SF6). In medium-voltage breakers more often, the low-pressure arc burning in vacuum is applied to interrupt the current. The current interruption is performed by cooling the arc plasma so that the electric arc, which is formed between the breaker contacts after contact separation, disappears. This cooling process or arc-extinguishing can be done in different ways. Power circuit breakers are categorised according to the extinguishing medium in the interrupting chamber in which the arc is formed. That is the reason why we speak of oil, air-blast, SF6, and vacuum circuit breakers.

In 1907, the first oil circuit breaker was patented by J. N. Kelman in the United States. The equipment was hardly more than a pair of contacts submersed in a tank filled with oil. It was the time of discovery by experiments and most of the breaker design was done by trial and error in the power system itself. In 1956, the basic patent on circuit breakers employing SF6 was issued to T. E. Browne, F. J. Lingal, and A. P. Strom. Presently the majority of the high-voltage circuit breakers use SF6 as extinguishing medium.

J. Slepian has done much to clarify the nature of the circuit breaker problem, because the electric arc proved to be a highly intractable and complex phenomenon. Each new refinement in experimental technique threw up more theoretical problems. The practical development of circuit breakers was, especially in the beginning, somewhat pragmatic, and design was rarely possible as deduction from scientific principles. A lot of devel- opment testing was necessary in the high-power laboratory. A great step forward in understanding arc–circuit interaction was made in 1939 when A. M. Cassie published the paper with his well-known equation for the dynamics of the arc and then in 1943 O. Mayr followed with the supple- ment that takes care of the time interval around current zero. Much work was done afterwards to refine the mathematics of those equations and to confirm their physical validity through practical measurements. It becomes clear that current interruption by an electrical arc is a complex physical process when we realise that the interruption process takes place in microseconds, the plasma temperature in the high-current region is more the 10 000 K, and the temperature decay around current zero is about 2000 K/μs per microsecond while the gas movements are supersonic.

Until recently, scientists have succeeded in designing a new high-voltage circuit breaker on the drawing table only; testing in the high-power laboratory still remains necessary. Yet, the understanding of the current interruption process has led to SF6 circuit breakers capable of interrupting 63 kA at 550 kV with a single interrupting element.

The switching arc

The electric arc in a circuit breaker plays the key role in the interruption process and is therefore often addressed as switching arc. The electric rc is a plasma channel between the breaker contacts formed after a gas discharge in the extinguishing medium. When a current flows through a circuit breaker and the contacts of the breaker part, driven by the mechanism, the magnetic energy stored in the inductances of the power system forces the current to flow. Just before contact separation, the breaker contacts touch each other at a very small surface area and the resulting high current density makes the contact material to melt. The melting contact material virtually explodes and this leads to a gas discharge in the surrounding medium either air, oil, or SF6.

When the molecular kinetic energy exceeds the combination energy, matter changes from a solid state into a liquid state. When more energy is added by an increase in temperature and the Van der Waals forces are overcome, matter changes from a liquid state into a gaseous state. A further increase in temperature gives the individual molecules so much energy that they dissociate into separate atoms, and if the energy level is increased even further, orbital electrons of the atoms dissociate into free moving electrons, leaving positive ions. This is called the plasma state. Because of the free electrons and the heavier positive ions in the high- temperature plasma channel, the plasma channel is highly conducting and the current continues to flow after contact separation.

Nitrogen, the main component of air, dissociates into separate atoms (N2 → 2N) at approximately 5000 K and ionises (N → N+ + e) above 8000 K. SF6 dissociates into sulphur atoms and fluorine atoms at approx- imately 1800 K and ionises at temperatures between 5000 and 6000 K. For higher temperatures, the conductivity increases rapidly. The thermal ionisation, as a result of the high temperatures in the electric arc, is caused by collisions between the fast-moving electrons and photons, the slower- moving positively charged ions and the neutral atoms. At the same time, there is also a recombination process when electrons and positively charged ions recombine to a neutral atom. When there is a thermal equilibrium, the rate of ionisation is in balance with the rate of recombination.

The relation between the gas pressure P, the temperature T, and the fraction of the atoms that is ionised f is given by Saha’s equation with

with

e = 1.6 ∗ 10−19 the charge of an electron;Vi = potential of the gaseous medium and k = 1.38 ∗ 10−23 Boltzmann’s constant

Figure 4.1

0 0 4 12 20 28 x103

Temperature in K

Degree of thermal ionisation for some metal vapours and atomic gases

Saha’s relation is shown in graphical form for oxygen, hydrogen, and nitrogen and for the metal vapours of copper and mercury in Figure 4.1

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