Oil filled capacitor replacement with MKP - Page 1
Forum section: electronics / rf-microwave

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: August 21, 2024, 01:00:20 am
content: I have an application that uses 1uF oil filled capacitors. These turned out to be PCB filled (!) so I would like to replace it with a metalized film capacitor of equivalent rating. The original cap is a 300VAC rated unit, with a marking indicating that it's "60 cycle only".The catch is that the cap sits inside a high voltage oil bath. Are there any known incompatibilities between MKP capacitors and transformer oil, or would this be an acceptable substitution? There is enough room around the capacitors to ensure there won't be any flashover.

author: coppercone2
author_rank: Super Contributor
timestamp: August 21, 2024, 01:17:34 am
content: careful with that oil because it will be contaminated with PCB if those caps leaked, contaminated oil is common. its a very big deal for power companies

author: Gyro
author_rank: Super Contributor
timestamp: August 21, 2024, 11:28:29 am
content: Can you post photos? This sound like a case where it might actually be better to leave well alone if everything, including the outer bath is safely sealed.If you must take action then it might be better to change the oil in the bath with something more compatible with MKP capacitor plastic packages, such as Silicone oil (I have no clue about the materials compatibility of your existing oil).

author: BrokenYugo
author_rank: Super Contributor
timestamp: August 21, 2024, 01:19:43 pm
content: Sounds like a motor run cap (60hz AC rating), probably simplest to use another motor run cap.If the caps contain PCB how do you know the potentially equally old oil bath doesn't? Be careful.

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: August 21, 2024, 01:23:55 pm
content: The oil-filled caps were bulging when we inspected them, and showed higher than expected ESR (3 ohms). The oil was drained as there was a loose connection that caused a tube failure (pulse tank of a Klystron radar transmitter). I've got a PCB test kit on order to check if the tank oil was contaminated by the capacitor.I've attached a schematic of the pulse tank. L1 is a trifilar wound HV transformer that steps up the 20 kV pulses from the modulator to ~100 kV. It also passes 300V AC from the filament power transformer L2. L3 is an autotransformer that steps the 300V down to about 7.5V AC for the Klystron filaments. The three capacitors shown are there to minimize the common-mode modulator pulse from flowing through the filament. Everything shown in the schematic, except L2 is in the oil bath. The three capacitors will swing from 0V to -100kV (with respect to ground) each time the modulator fires.The oil used isShell Diala S4, nothing specific in there about plastics compatibility. I would like to use the same oil, though it seems hard to obtain in smaller quantities. Any suggestions are welcome. Will post some pictures next time I visit the site.Quote from: BrokenYugo on August 21, 2024, 01:19:43 pmYup - most modern ones seem like MKP caps, hence the question.

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: August 21, 2024, 05:24:10 pm
content: Here's a shot of the capacitors pulled from the equipment. I looked up "Pyranol" and it's the GE trademark for PCB-filled capacitors. I promptly bagged them and scrubbed as much as I could. Hard to tell from the pics but there's a bulge on two of the three capacitors.

author: CaptDon
author_rank: Super Contributor
timestamp: August 22, 2024, 02:31:06 pm
content: I think the PCB scare is a bit overblown? I am late 60's and in excellent health. I worked with Inerteen which I believe to be 50% PCB. All of my mentors worked with and around this stuff in oil filled 15KV high voltage transformers used in hard modulators and also the oil filled Kinotrons extremely high voltage rectifiers. All of my mentors lived to a ripe old age, some still living. Some of the military stuff spec'd out routine oil changes and it was inevitable to get this stuff splashed on your clothes and on your hands. We were never particularly careful about it since at the time there was no 'mass hysteria about PCB's'. Inerteen looks like red diesel but more of an orange tint and has that definite kerosene / diesel oder. Geez, the stuff was so available we were 'grabbing some' to take home and fill our Heathkit Cantennas!!! I donated many gallons at hamfests since the average ham couldn't lay his hands on the recommended 'transformer oil' which Heathkit said 'could be obtained cheaply' at any power company! And anyone who ever owned a Cantenna dummy load can tell you they are always a slimely mess! I wouldn't recommend bathing in the stuff or drinking it but a simple 'wash the hands when done' seems to suffice. How do they get rid of this 'public enemy #1 fluid??? They burn it as fuel in a normal gun style oil burner firing into refractory brick in a furnace nearly identical to your home heating furnace!!!

author: coppercone2
author_rank: Super Contributor
timestamp: August 22, 2024, 10:57:52 pm
content: what they don't want is transformer getting into drinking water, the problem is usually that it leak, PCB go into environment through storm drain.Personal exposure is of course no joke either.Usually big power companies have a special lab for testing stuff rapidly, if its in a "old " cityThe problem is it accumulates. You find it in wild life that is exposed and its kinda linked to cancer clusters and such (of statistical significance)Its the same story every time, there is always some group that gets really lucky.. like those 90 year old asbestos workers that still smoke cigar.Point is, its a proven carcinogen. But I think so is gasoline (which you can get away from now thankfully)Some new dirt on PCB might come from missile silos. apparently the missileers are not very healthy after working their jobs.. some say its PCB related.But I think if you don't get it in your water, and its a 1 time exposure... your probobly gonna be OK. But if you have concerns their entirely justifiedJUst don't be a dick, get someone to take it to a proper disposal when you gotta get rid of it