Digital Testing of High Voltage Circuit Breaker
Published on Feb 12, 2016
With the advancement of power system, the lines and other equipment operate at very high voltages and carry large currents. High-voltage circuit breakers play an important role in transmission and distribution systems. A circuit breaker can make or break a circuit, either manually or automatically under all conditions viz. no-load, full-load and short-circuit conditions.
The American National Standard Institute (ANSI) defines circuit breaker as: "A mechanical switching device capable of making, carrying and breaking currents under normal circuit conditions and also making, carrying for a specified time, and breaking currents under specified abnormal circuit conditions such as those of short circuit". A circuit breaker is usually intended to operate infrequently, although some types are suitable for frequent operation.
ESSENTIAL QUALITIES OF HV CIRCUIT BREAKER
High-voltage circuit breaker play an important role in transmission and distribution systems. They must clear faults and isolate faulted sections rapidly and reliably. In-short they must possess the following qualities.
" In closed position they are good conductors.
" In open position they are excellent insulators.
" They can close a shorted circuit quickly and safely without unacceptable contact erosion.
" They can interrupt a rated short-circuit current or lower current quickly without generating an abnormal voltage.
The only physical mechanism that can change in a short period of time from a conducting to insulating state at a certain voltage is the arc.
The first circuit breaker was developed by J.N. Kelman in 1901. It was the predecessor of the oil circuit breaker and capable of interrupting a short circuit current of 200 to 300 Ampere in a 40KV system. The circuit breaker was made up of two wooden barrels containing a mixture of oil and water in which the contacts were immersed. Since then the circuit breaker design has undergone a remarkable development. Now a days one pole of circuit breaker is capable of interrupting 63 KA in a 550 KV network with SF6 gas as the arc quenching medium.
THE NEED FOR TESTING
Almost all people have experienced the effects of protective devices operating properly. When an overload or a short circuit occurs in the home, the usual result is a blown fuse or a tripped circuit breaker. Fortunately few have the misfortune to see the results of a defective device, which may include burned wiring, fires, explosions, and electrical shock.
It is often assumed that the fuses and circuit breakers in the home or industry are infallible, and will operate safely when called upon to do so ten, twenty, or more years after their installation. In the case of fuses, this may be a safe assumption, because a defective fuse usually blows too quickly, causing premature opening of the circuit, and forcing replacement of the faulty component. Circuit breakers, however, are mechanical devices, which are subject to deterioration due to wear, corrosion and environmental contamination, any of which could cause the device to remain closed during a fault condition. At the very least, the specified time delay may have shifted so much that proper protection is no longer afforded to devices on the circuit, or improper coordination causes a main circuit breaker or fuse to open in an inconvenient location.
TESTING OF CIRCUIT BREAKERS
The design of circuit breaker is not only a science but also an art. Because of the complex phenomena involved, circuit breaker prototypes have to be verified by practical tests in the laboratory. There are two types tests of circuit breakers, namely Routine tests and Type tests. Routine test are performed on every piece of circuit breaker in the premises of the manufacturer. The purpose of the routine test is to confirm the proper functioning of a circuit breaker. Type tests are performed in a high voltage laboratory; such tests are performed on sample pieces of circuit breaker of each type to confirm their characteristics and rated capacities according to their design. These tests are not performed on every piece of the circuit breaker. All routine and type tests are performed according to Indian Standard (IS) codes, or International Electromechanical Commission (IEC) codes or British Standard (BS) codes.
In High-power laboratories the ability of the circuit breakers to interrupt the circuit currents is verified in test circuits which is infact the lumped element representation of the power system. These test circuits must produce the corrected forms of the short circuit current as well as the final voltage that strikes the circuit breaker immediately after the breaker has interrupted the test current. The forms of voltage and current to which the test object must be subjected are laid down in ANSI and International Electromechanical commission (IEC) standards. These standardized waveform represent 90% of the possible conditions in the real system.
CIRCUIT BREAKER SWITCHING AND ARC MODELLING
The switching action, the basic function of the circuit breaker refers to the change from conductor to insulator at a certain voltage. Before interruption, the short circuit flows through the circuit breaker arc channel. Because of non zero resistance of the channel , the short circuit current causes a voltage across the contacts of the circuit breaker; the arc voltage. The arc behaves as a non-linear resistance. Thus both are voltage and arc current cross the zero value at the same time instant. If the arc is cooled at the time current goes through zero the circuit breaker interrupts the current because the electrical power input is zero. During current interruption, the arc resistance increases practically from zero to almost infinity in microseconds. Immediately after current interruption, the transient recovery voltage builds up across the circuit breaker .As the gas mixture in the interelectrode space does not change to a completely insulating state instantaneously, the arc resistance is finite at that time and a small current can flow ;the post –arc current.
Black box arc models are mathematical description of the electrical properties of the arc .This type of model does not simulate the complicated physical processes inside the circuit breaker but describes the electrical properties of the circuit breaker. Measured voltage and current traces are used to extract the parameters for the differential equations describing the nonlinear resistance of the electrical arc for that specific measurement.
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