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Re: [TCML] Utility PFC Caps

Hi David,

Utility PFC's are indeed quite robust. A 14 kV PFC cap is rated for a Basic Impulse Level (BIL) of 125 kV so that it can shrug off switching and lightning transients for years. They're large, metal-cased beasts with "Frankenstein" insulators - perfect for a mad scientist's lab. And, newer ones should work quite well in spark-gap TC's.

How well a given PFC cap will work in a TC mostly depends on the age of the caps. Many utility PFC's manufactured before the early 1970's used foil-paper-oil construction. Virtually all of these also used PCB's as the dielectric fluid. Paper-oil caps are over 300X lossier than PP-film caps at RF frequencies. PCB-filled caps have reached end of life, and utilities have been phasing them out and disposing of them over the last several decades. In the US, PCB caps sometimes have the following label designations: Arochlor, Askarel, Clorphen, Chlorinal, Diachlor, Dykanol, Hivar, Hydol, Hyvol, Inerteen, Kennechlor, Noflamo, Permatol, Pyranol, Therminol. This is far from being an exhaustive list. Because of their high RF losses and ever-increasing disposal costs, I'd recommend staying away from any PCB-filled cap entirely.

During the mid-1960's capacitors using polymer films were introduced. Polypropylene-paper-foil, loose-wound textured polypropylene, and finally metalized polypropylene capacitors began displacing older paper-oil caps. Simultaneously, a variety of PCB-replacement dielectric fluids also emerged. Modern utility PFC capacitors are typically constructed using metalized polypropylene film with biodegradable plant-based dielectric fluids. These offer self-healing, lower flammability (versus mineral oil), operation at a higher dielectric stress (for a smaller, more efficient form factor), and low dielectric loss. Internal gas pressure rises during self-healing or prolonged corona/partial discharges, so internal case pressure sensing is often used for condition monitoring. In lower-voltage PFC or motor run caps, excessive pressure is often used to physically disconnect a failing capacitor before it can blow up.

Unlike pulse capacitors, utility PFC caps contain internal bleeder resistors for lineman safety. The RC time constant is short enough to discharge a fully-charged capacitor to 50 volts (or less) within 3 -5 minutes. Note that there's really no need to remove bleeder resistors when using these caps for a TC use since there's virtually no effect on TC performance. Think of them as having a similar (minimal) effect as bleeder resistors on an MMC.

HV utility PFC caps can be either single or three-phase, and are available with one or two bushings (1-phase), or three bushings. I would recommend looking for single-phase units, preferably with two bushings.

Utility caps are rated in kilovolt amperes reactive (kVAR). For a single-phase PFC, you can calculate the capacitance if you know the rated RMS operating voltage and line frequency:

C = 1000*kVAR/(2*Pi*F*V^2) where C = uF, F is in Hz, and V is in kV.

For example, a 20 kVAR cap rated for 14.4 kVRMS and 60 Hz is 0.26 uF. A similarly-rated 3-phase capacitor will have three identical caps connected in delta, where each cap is one-third the value of a similarly-rated single-phase PFC cap. Connecting to any two of the three bushings will give you a combined capacitance that is one-half that of a similarly-rated single-phase cap.

Try to locate a surplus and relatively new PFC cap or NOS cap to benefit from the latest capacitor technologies.If you can get a reasonably-sized modern HV PFC cap at a good price, and can tolerate its larger size and weight, go for it!

Good luck and best wishes,

Bert Hickman
Stoneridge Engineering LLC
+1 630-964-2699
World's source for "Captured Lightning" Lichtenberg Figure sculptures,
magnetically "shrunken" coins, and scarce/out of print technical books

David Rieben wrote:
Hi all,

Since I have personally received conflicting data on this issue before, I figured that I would post my question to this list. Just hoping one of the more knowledgeable gurus of this list could expand upon the typical dielectric system that is employed in the power factor correction capacitor units that are used by the electrical utility companies for power factor regulation of their primary line feeders. Would their dielectric systems have acceptably low enough losses for any serious consideration as being suitable for use as the main tank circuit capacitor(s) of a larger classical SH driven coil systems (assuming a properly calibrated capacitance for optimal tuning, of course). Considering the harsh electrical conditions caused by lightning strikes and line switching surges, as well as the environmental conditions of 24/7 operation in the open elements vs. the demand for many years of reliability, it would appear that these units should be nearly bullet proof, at least from
 an over-voltage standpoint.


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