Risk of vacuum loss
02/01/2004 - Volume I - Issue I
The principle conceptual difference in modern Siemens vacuum interrupters from the early 1960s designs lie in contact material and process control. A vacuum interrupter is constructed with all connections between dissimilar materials made by brazing or welding. No organic materials are used. In the early years, many hand-production techniques were used, especially when borosilicate glass was used by some suppliers for the insulating envelope as it could not tolerate high temperatures. Today, machine welding and batch vacuum furnace brazing are employed with extremely tight process control. The only moving part inside the interrupter is the copper electrode (with chrome-copper contact surfaces). The moving electrode is connected to the interrupter end plate, the failure rate of this moving connection is extremely low. Similarly, leaks were harder to control and elimination of the human factor (variability) in manufacture. This accounts for the extremely high reliability of Siemens vacuum interrupters today.
Our experience also suggests that the effects of a vacuum interrupter failure on equipment are less severe when compared to the impact of failures with alternative interruption technologies.
Siemens entered the vacuum circuit breaker market in 1974, using Allis-Chalmers' technology and copper-bismuth contact materials. In the early 1980s, after becoming part of the worldwide Siemens organization, we were able to convert our vacuum designs to use Siemens vacuum interrupters, which had been introduced in Europe in the mid-1970s. Thus, when we adopted the Siemens vacuum interrupters in the United States, they already had a very well established field performance record. The result is that the Siemens vacuum interrupters today can be expected to have a long service life.
Suppose we have a feeder circuit breaker vacuum interrupter on phase 3 that has lost vacuum. If the load being served by the failed interrupter is a delta-connected (ungrounded) load, a switching operation would not result in a failure. Essentially nothing would happen. The two good (phase 1 and phase 2, for this example) would be able to clear the circuit, and current in the failed interrupter (phase 3) would have to cease.
The alternative case of a grounded load is a different situation. In this case, interruption in the two good phases (phase 1 and phase 2) would not cause current to stop flowing in phase 3, and the arc would continue to exist in phase 3. With nothing to stop it, this current would continue until some backup protection operated. The result, of course, would be destruction of the interrupter.
Since the predominant usage of circuit breakers in the 5-15kV range is on grounded circuits, we investigated the impact of a failed interrupter some years ago in the test lab. We intentionally caused an interrupter to lose vacuum, by opening the tube to the atmosphere. We then subjected the circuit breaker to a full short circuit interruption. As predicted, the "flat" interrupter did not successfully clear the affected phase, and the "flat" interrupter was destroyed. The laboratory backup circuit breaker cleared the fault.
Following the test, the circuit breaker was removed from the switchgear cell. It was very sooty but mechanically intact. The soot was cleaned from the circuit breaker and the switchgear cell, the faulty interrupter was replaced, and the circuit breaker was reinserted in the cell. Further short-circuit interruption tests were conducted the same day on the circuit breaker.
Our field experience, in the years since that test was conducted, supports the information gained in the laboratory experiment. One of our customers, a large chemical concern, encountered separate failures (one with an air magnetic circuit breaker and one with a vacuum circuit breaker) on a particular circuit configuration. Two different installations, in different countries, were involved. They shared a common circuit configuration and failure mode. The circuit configuration, a tie circuit in which the sources on each side of the circuit breaker were not in synchronism, imposed approximately double rated voltage across the contact gap, which caused the circuit to fail. Since these failures resulted from application in violation of the guidelines of the ANSI standards, and greatly in excess of the design ratings of the circuit breakers, they are not indicative of a design problem with the equipment.
However, the damage that resulted from the failures is of interest. In the case of the air magnetic circuit breaker, the unit housing the failed circuit breaker was destroyed, and the adjacent switchgear units on either side damaged extensively, requiring significant rebuilding. The air magnetic circuit breaker was a total loss.
In the case of the vacuum circuit breaker, the failure was considerably less violent. The vacuum interrupters were replaced, and the arc by-products (soot) were cleaned from both the circuit breaker and the compartment. The unit was put back into service. Our test experience in the laboratory, where we routinely explore the limits of interrupter performance, also supports these results.
More recently, several tests were performed in our high-power test laboratory to compare the results of attempted interruptions with "leaky" vacuum interrupters. A small hole (approximately 1/8" diameter) was drilled in the interrupter interruptions with "leaky" vacuum interrupter which had lost vacuum. The results of these tests were very interesting: One pole of a vacuum circuit breaker was subjected to an attempted interruption of 1310A (rated continuous current + 1250A). The current was allowed to flow in the "failed" interrupter for 2.06 seconds, at which point the laboratory breaker interrupted. No parts of the "failed" circuit breaker or the interrupter flew off, nor did the circuit breaker explode. The paint on the exterior of the interrupter arcing chamber peeled off. The remainder of the circuit breaker was undamaged.
A second pole of the same vacuum circuit breaker was subjected to an attempted interruption of 25kA (rated interrupting current= 25kA), for an arc-duration of .60 seconds with the laboratory breaker interrupting the current at that time. The arc burned a hole in the side of the arc chamber. The circuit breaker did not explode, nor did parts of the circuit breaker fly off. Glowing particles were ejected from the hole in the arcing chamber. None of the mechanical components or other interrupters were damaged. Damage was largely confined to the failed interrupter.
Again, our experience suggests rather strongly that the effects of a vacuum interrupter failure on the equipment are less severe when compared to the impact of failures with certain, alternative interruption technologies. But the real question is not what the results of the failure might be, but rather what is the likelihood of failure? With the present MTTF from field experience of over 38,000, it is clear that loss of vacuum is indeed a rare occurrence.