I recently experienced the failure
I recently experienced the failure
(OP)
I recently experienced the failure of a 10 kVA constant voltage transformer utilized on the bypass side of a UPS that feeds a critical AC panel in a power plant. Here is what we experienced: At 11:00 AM someone smelled smoke from the SCADA cabinets. The doors were opened and a lot of smoke was coming from the power strips. (two power strips burned up, one tripped) Immediatly ran downstairs and found the 10 kVA constant voltage transformer (CVT) on fire. The circuit breakers were turned to the off position and the fire then subsided. Upon inspection of the CVT, it appeared that the inductor experienced a sigificant amount of heat from inside the core and coils, which then may have caused the insulation to fail. This may have then caused a voltage spike downstream causing a ground fault that wasn't cleared by the 175 A breaker.....for 10kVA at 120V, shouldn't this be a 90 A breaker anyway? (83.3 Amps) The fault current increased until stuff started catching on fire. I do not know if the system was in bypass when the failure happened, or if a failure at the power strip caused an overload on the inverter, which then caused it to switch to the bypass source. Can anyone provide some insite as to what may have happened. These constant voltage transformers utilize one or two inductor windings and capacitiors. Why are there two inductor windings, one bigger than the other?






RE: I recently experienced the failure
please give us a clearer view how the CVT was introduced into the circuit..is it as follows?
utility - CVT - UPS (with static)
did it damage the UPS itself...can you transfer the inverter to static without problem..
CVT are pretty rough and don't go up in smoke easily...SOLA is one of the respected manufacturer of this....
..the CVT or constant voltage transformer also known as ferroresonant transformer...is transformer and by cleverly connecting the inductors and capacitors at resonant frequency (50 or 60 HZ) produces constant voltage..
dydt
RE: I recently experienced the failure
The UPS did not fail. I do not know if the inverter failed because before I ever got on site, they called the inverter manufacturer and told them what happened. They stated that since the inverter was 13 yrs old, it should have maintenence. They came out and changed a bunch of control cards, and capacitors. They never tested it before replacing a bunch of parts. The CVT is a General Electric "Stabiltron", 10 kVA. Shouldn't this have a 90 Ampo breaker on the load side at 10 kVA and 120V?
Thank you for your help. I am new at this.
RE: I recently experienced the failure
did any of the static switch SCR's shorted out?..1,2,3, and 4?
the capacitor replaced are they electrolytic, big diameter, approx. 3 inches? and how many did they replaced?
dydt
RE: I recently experienced the failure
RE: I recently experienced the failure
When all this happened, the results were that two power strips were burnt, one tripped, and the Constant Voltage Transformer (inductor part) downstream was on fire.
Is it posible that the CVT was shorting internally, and that caused voltage spikes (V=L di/dt), and the the power strips were absorbing the voltage spikes until they had enough and started to get real hot?
And then it just so happened that the CVT primary breaker tripped in time so other equipmet didn't smoke. The secondary breaker wouldn't trip becaue it appears to be oversized at 175A instead of 90 A at 10kVA and 120V.
RE: I recently experienced the failure
dydt
RE: I recently experienced the failure
dydt
RE: I recently experienced the failure
Does anyone know of any history regarding the General Electric "Stabiltron" transformer regarding possible failures of the inductor? Can you explain how the inductor is placed in teh circuit...I'm not quite understanding it according to the drawing. I wish I could attach the drawing.
RE: I recently experienced the failure
as i mentioned on the other posting it seems that you used only one circuit of the CVT secondary, 120 volts, X1 and X2 instead of X1, X2 and X3 which is 240/120v center tapped.
the single 120VAC circuit was overloaded when you switched to bypass. I am sure that this is the first you used the CVT in that configuration via static switch. Yes, the essential load will run for a while while your winding starts to heat up. The secondary braker wont trip because your problem is not on the secondary circuit but on the CVT itself which is being supplied by your primary circuit breaker..
dydt
RE: I recently experienced the failure
RE: I recently experienced the failure
Is it possible that the ferroresonant transformer’s secondary, rated “split” 120/240V 3-wire, is only intended for loading of ~42A X1-X2 and ~42A X2-X3; id est, 5kVA on each 120V-secondary winding?
Also, was there surge protection in the plugstrips intended for grounded circuits, and with the serving ferroresonant transformer secondary ungrounded—internal MOVs experienced overvoltage failure?
Third—compared to a similarly rated conventional drytype, this type of transformer generally has a comparatively high per-unit impedance. Relying on a molded-case breaker for secondary overcurrent protection may be effectively “cosmetic,” in that even an [NEC450] correctly sized secondary-side breaker may not protect the secondary from overload.
RE: I recently experienced the failure
Our CVT had an internal lamination break down which allowed the core plates to break free and eat away at the winding insulation until a short occurred. Since this was on the secondary side of the CVT there was not enough current to trip the input breaker so the fault kept occurring until we had the two fires.
RE: I recently experienced the failure
according to GE product information GEA-11062B, therecommended way to connect this thransformer for the nameplate kVA is to use x1 and x2. Using x2 and x3 will provide 1/2 nameplate, or 5 kVA. If it was hooked up this way, I can see there would be a problem. Also, the measured current (load) at the essential panel is only 35 amps.
RE: I recently experienced the failure
I have suggested that a meter be connected to check the harmonic content. We will see how far the customer would like to go to investigate.
RE: I recently experienced the failure
The neutral that comes form the bypass transformer (CVT)is tied to the neutral at the UPS, which is grounded at the UPS.
Nukeman,
That is what I think happened as well. What was the ground problem you had at the UPS? How did you correct it?
RE: I recently experienced the failure
RE: I recently experienced the failure
tranny — regarding the Stabiltron transformer, are you stating that per the GE bulletin, X1-X2 has a full 10kVA rating, whereas X2-X3 has only 5kVA?
RE: I recently experienced the failure
Yes that is exactly correct. X1-X2 has a full 100 kVA, and X2-X3 has 1/2, or 5 kVA
RE: I recently experienced the failure
Interesting. The plain-vanilla symmetrical dual-secondary series/parallel arrangement is too ingrained in my habits, I guess.