Lost 6.6kv phase causes damage
Lost 6.6kv phase causes damage
(OP)
Can anyone help explaining this one. A customers has a 1MVA 6.6/0.415Kv Dyn11 transformer
with a solidly earthed neutral. The factory which this transformer supplies has a number
of 3-phase motors supplied from it, a problem occurred on the utility incoming supply
which left only 2-phases connected, the 3rd being open circuit, lots of damage occurred
to 1-phase equipment, contactor coils burnt out etc. I thought with this scenario that 1-lv
phase-N would be at full volts with the other two-N at half volts, so why so much damage.
thanks
with a solidly earthed neutral. The factory which this transformer supplies has a number
of 3-phase motors supplied from it, a problem occurred on the utility incoming supply
which left only 2-phases connected, the 3rd being open circuit, lots of damage occurred
to 1-phase equipment, contactor coils burnt out etc. I thought with this scenario that 1-lv
phase-N would be at full volts with the other two-N at half volts, so why so much damage.
thanks






RE: Lost 6.6kv phase causes damage
RE: Lost 6.6kv phase causes damage
Anyone else shed some light on this one ?
RE: Lost 6.6kv phase causes damage
A contactor at rest has a relatively low impedance as a result of the air gap in the magnetic circuit. Once the contactor is energised and sealed in, the impedance increases. No more air gap means more inductive reactance (impedance) and less current.
If a contactor coil is supplied with a voltage less than the pull-in voltage, the current will be excessive and the coil will burn up.
Check the manufacturers specs for the inrush current of your contactors. Although at half voltage the inrush will be half, it is still usually enough current to fry a coil.
Some relays have a low enough X/R ratio
that the resistance is the major component of the impedance. Such relays will withstand undervoltage conditions much better and some are safe at any reduced voltage.
respectfully
RE: Lost 6.6kv phase causes damage
Could there be anything else ?
RE: Lost 6.6kv phase causes damage
Also, it is true that small contactors sometimes, but not always, stay pulled in at 50 %. But they are not always specified to do that. One authority
( http://www.measurlogic.com/pdf/dpi/pq3.pdf ) Table 1, mentions drop out voltages between 53 and 68 %. So, according to this source, most of your contactors would drop and be exposed to high current because of the air-gap in the magnetic circuit.
That is also in line with my experience.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Lost 6.6kv phase causes damage
See: http://www.icrepq.com/PONENCIAS/4.279.QUISPE.pdf
and http://www.ssac.com/catalog/0807S.pdf
RE: Lost 6.6kv phase causes damage
The problem is what happened to single phase loads? Those are already single phase and there are no positive, zero or negative components.
Negative components and harmonics do not explain why contactor coils burn out. A 50 % voltage reduction does. What other single-phase equipment were destroyed?
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Lost 6.6kv phase causes damage
RE: Lost 6.6kv phase causes damage
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Lost 6.6kv phase causes damage
Most manufacturers design for continuous operation at -15%. The contactors may stay closed electrically down to 50% in some cases, but continuous usage down there will damage the coils. I have seen coils fail at -20%.
I would also expect to see increased contact heating where the contactor is loaded due to reduced contact pressure.
Best regards,
Mark Empson
http://www.lmphotonics.com
RE: Lost 6.6kv phase causes damage
One other possible scenario, what if the star point of the feeding transformer was not earthed and a phase-earth fault was on the system, could the neutral be displaced to possibly give over voltage on one or more phases.
RE: Lost 6.6kv phase causes damage
For example, if a 277/480 volt system is run ungrounded, a ground on one phase will raise the voltage to ground on the other two phases to 480 volts.
This will not affect the voltage supplied to any equipment. Equipment and conductors intended for use on 277/480 volt systems are typically insulated for 480 or 600 volts.
There may be some issues with VFDs and DC drives.
Search this site for information if you have an ungrounded system and drive systems.
respectfully