Ed, It can be either.Some (but not all!) utility PFC capacitors contain internal fuses. These disconnect failing capacitor rolls, reducing the overall capacitance of the unit while still keeping the rest of the capacitor rolls in service. After the fuse blows, the reduction in measured terminal capacitance is sudden and is typically in the range of 10 - 100% depending on the internal electrical configuration of series and parallel strings. Externally fused PFC capacitors usually fail catastrophically, blowing their associated fuse and, infrequently, expelling insulators or rupturing their cases.
Lower voltage PFC capacitors use self-healing metalized film technology. These caps show a more gradual reduction in capacitance over time (usually 5% or less) unless they're subjected to prolonged over-voltage stress.
High-voltage PFC capacitors use multiple capacitor rolls connected as one or more series strings. If a cap in a string fails (i.e., shorts out), the capacitance of the affected string increases, and the voltage stress on the remaining rolls in that string increases. The higher voltage stress may cause other caps in that string to subsequently fail, resulting in a cascade failure of the string. Any HV cap that has significantly higher than nominal capacitance should be replaced in the application.
During preventive maintenance, utilities will look for any signs of case bulging or leakage. Capacitor current or capacitance of suspect units may also be measured. Capacitors with significantly higher or lower reactive current or capacitance may be flagged for replacement.
Any surplus HV cap that has a measured capacitance that is significantly _lower or higher_ versus the nominal faceplate value (allowing for expected tolerance) or that shows bulging or leakage should be avoided by the HV experimenter/surplus scrounger. The capacitor has already partially failed and will only further degrade over time.
Bert Terry Oxandale wrote:
Sorry, can't remember. Too many birthdays since then. -----Original Message----- From: Tesla [mailto:tesla-bounces@xxxxxxxxxx] On Behalf Of Ed Sent: Saturday, May 20, 2017 7:05 PM To: Tesla Coil Mailing List <tesla@xxxxxxxxxx> Subject: Re: [TCML] **External Email** Re: Utility PFC Caps Which way did the capacitance 'migrate' Low or high? Ed On 5/20/2017 4:36 PM, Terry Oxandale wrote:We would remove these caps from the bank once they migrated outside a specific tolerance, typically 2-3 per year, and hopefully before an internal rupture occurred (bulging). Sent from my iPhoneOn May 20, 2017, at 10:15 AM, David Rieben <drieben@xxxxxxx> wrote: Hi Bert, (and Terry, if you're still following this thread), Yes, I was rather pleasantly surprise to find that the more updated dielectric system of these types of caps renders their latent dissipation factor suitably low for the rapid cycle pulse discharge duty of being employed as the main tank cap of a large RSG/pole pig driven Tesla coil. I was also quite surprised to locate this type of cap in a relatively low capacitance, thus rendering their C within a suitable range for Tesla coil usage in combo with an adequate ceiling voltage rating (13.8 kVAC). Locating more than one of these identical 0.25 uFd rated (single phase) units meant the possibility of seriesing just two to obtain 0.125 uFd @ a whopping 27.6 kVAC rating! Their GE model # is 9L18CCL101 (whatever that means) and the only drawback is the single bushing (w/casing ground). However, this minor caveat can easily be addressed by simply 'floating' their seriesed outer casings and using the two bushings as the inputs. Not that I really needed these, but considering theoften fickle nature of availability of affordable capacitors of this caliber, I like to keep a good supply of suitable replacements in case the 0.1 uFd @ 75 kVFD Maxwell pulse cap that I am currently using in my big coil lets out its 'magic smoke' ;-)) I have two of these ~33 lb. beasts on their way to me now. They are used with no guarantee, but having no visible 'bulging' of the outer casing and having been simply removed from decommissioned switch gear in a fully operational environment, I figure the odds are pretty high that these beauties are still quite functional and well within their nameplate specs. The real kicker is that I will have <$150 in both of them, including S&H :-) Gotta love that! Happy sparking, David Sent from my iPhoneOn May 18, 2017, at 11:22 AM, Bert Hickman <bert@xxxxxxxxxxxxxxxxxxxxx> wrote: Hi David, Your plan sounds excellent. As Terry mentioned, you really don't want to tear into these caps. I've also been there, done that, while autopsying a failed/ruptured 175 pound GE pulse cap. NEVER again - what a MESS... :^/ Even worse, unlike castor oil or mineral oil, many of the low-flammability dielectric fluids that GE (and others) used were really nasty, foul-smelling solvents. When the cap failed, the leaking fluid literally "ate" the backing off the indoor-outdoor carpeting below. Your two caps in series should work great just as they are, while increasing the bang size a bit. A little tweak on tuning and ballasting and they work great. And, they should be virtually bullet-proof. Play safely and make big sparks, Bert David Rieben wrote:Hi Bert, (and Terry O),_______________________________________________ Tesla mailing list Tesla@xxxxxxxxxx http://www.pupman.com/mailman/listinfo/teslaThis email and any attachments are for the sole use of the intended recipient(s) and may contain confidential information. If you receive this email in error, please notify the sender, delete the original and all copies of the email and destroy any other hard copies of it. _______________________________________________ Tesla mailing list Tesla@xxxxxxxxxx http://www.pupman.com/mailman/listinfo/tesla_______________________________________________ Tesla mailing list Tesla@xxxxxxxxxx http://www.pupman.com/mailman/listinfo/tesla _______________________________________________ Tesla mailing list Tesla@xxxxxxxxxx http://www.pupman.com/mailman/listinfo/tesla
_______________________________________________ Tesla mailing list Tesla@xxxxxxxxxx http://www.pupman.com/mailman/listinfo/tesla