HPS EnduraCoil 1.2 KV Manual De Instrucciones página 17

Field Service (continued) 10. Oscillatory Switching Transients
Occasionally, when a transformer is switched into or out
of a system, a transient recovery voltage containing a large
component of high frequency voltage will be subjected to
the terminals of the transformer. This occurs most often
when a vacuum breaker is used and is a direct result of the
characteristic of the vacuum breaker to chop current. These
current chops and subsequent re-ignitions produce a transient
voltage at the terminals of the transformer that is oscillatory,
of high frequency and prolonged duration. When this applied
voltage has a frequency component near one of the natural
frequencies of the transformer and of sufficient duration,
internal damage to the insulation structure of the transformer
will result.
When a transformer is used with vacuum or SF6 circuit
breakers, we recommend that the system engineer reviews
the possibilities of oscillatory switching transients and employs
appropriate mitigating methods to avoid transformer insulation
damage.
11. Core Grounding
All non-current carrying metal parts in transformers must be
grounded, and this includes the core. Transformer cores with
through bolts must be insulated from the core clamps and
grounded at a single point. Isolating the core in this manner
is necessary to determine if there is an insulation failure in
any through-bolts that might cause a high circulating current
or hot spot. There are no requirements for isolating cores in
any standards including UL, ANSI, CSA, IEEE and IEC. Note: If
the transformer core is isolated, then the core strap between
the core and the top core clamp, must be disconnected
before taking the reading from core to ground. If there is a
low megger reading, or low resistance between the core and
ground on power transformers, causes may include:
 dirt, dust or moisture bridging the insulation between
the core and the core frames
 shifted insulation due to shipping or handling.
12. Coil Distortion
Short circuited coils exhibit severe distortion from their normal
round or symmetrical appearance.
13. High Conductor Loss
Overloads, or tap jumpers not on the identical tap position can
result in conductor heating.
14. Breakers/Fuses Opening
Breakers and fuses opening can be caused by:
 overload conditions
 voltage or current harmonics
 short circuit
 insulation failure that causes excessive current.
 voltage too high when energized
 setting too low to allow for inrush current
continued on following page
17
IOMGCAS / May 2014