See Also: Ferroresonance
This story comes from Doble Engineering, December, 1994.
This situation had its beginning when a pre-engineered package for 115 KV
equipment was installed at a Substation. It consisted of an SF6 Circuit Breaker, current transformers, and a Voltage Transformer (wire-wound type, see Magnetic Wound Potential Transformer (MPT) ). Service needs dictated a larger than usual grading Capacitor, so a 5,000 pico-Farad external capacitor was used on the Power circuit breaker. Shortly after being put into service, the breaker Relayed because a contractor working some distance away had contacted the A phase with an overhead Static Wire. The Fault Current was 16,307 Amperes.
On the following morning, the substation crew had to clear that same feeder
line for further construction work. Coordinating with the system dispatcher (see Dispatching), the operator took local control, opened the breaker, and broke the loop.
The other end of the feeder line was opened, and the operator went to the
switchyard and checked the open breaker, noting the three phase semaphores, the
semaphore on the breaker cabinet, and the operations counter which indicated that
the breaker had opened. Then, following his switching order, he opened the line
side disconnect Switch, and noticed some arcing on the disconnect. At the same time, he heard the
PT start to hum. This caused him to return to the control house to talk with
the dispatcher, but there he found the hot line indicating Light on the breaker’s position not only lit, but glowing with more than normal brilliance.
The operator reported to the chief operator that he thought there was a
problem with the new equipment, and reviewed what had transpired so far. The chief
went to the yard and confirmed that the PT was indeed humming while both the
breaker and the line side disconnect were open. The operator and the system
dispatcher decided to isolate the breaker by opening the main bus disconnect, but as
the operator was leaving the room, he heard a relay chattering on the control
panel. As he watched, the relay chatter stopped, and the hot line indicating
light went to a normal level of brilliance, and then went out. Going down to
the yard, he discovered that the PT was no longer humming. Unbeknownst to the
operator, the PT had already failed internally. Opening the main bus disconnect
for the breaker, he then reported to the system dispatcher and related what had
developed. He was instructed to continue with the original plan of switching
to clear the line, and he did so.
The chief operator and his supervisory people then looked at the options they
had to deal with the strange goings-on, and decided to simulate the condition
when the PT was humming. With the system dispatcher’s agreement, the operator closed the main bus disconnect, and an electrician
buzzed out the line side of the open breaker with a hot line tester on a low
voltage scale and found all three phases indicating energized. Resetting the
tester on to the high scale, he found C and B indicating energized, but A indicating de-energized. The decision was made at the site to close the
breaker to further simulate the earlier conditions. The operator went into the
control house and, announcing on the PA system that the breaker was coming closed,
he closed the breaker. After a few Seconds, the Metal base of the PT blew out violently, spewing about 20 gallons of hot insulating Oil to each side, and onto an adjacent breaker which caused it to relay and
initiate a three-phase fault on the 115 KV main bus. A phase-to-ground fault level was 24,810 amps.
The two electricians standing nearest to the PT received first and second
degree burns. The maintenance foreman who was quick enough to smother the fire on
one electrician also had first degree burns as a result of his quick action. A
fourth man, the construction foreman, sprained his wrist and hand when he fell
trying to evade the flaming oil.