On Tue, 9 Dec 1997 21:19:49 -0600 John Lyles <jtml@lanl.gov> writes:
>Hey,
>
>When you fuse a HVDC line, remember that when the fuse blows during a
>fault, that DC doesn't like to be interrupted. What I mean is that you
>can
>sustain a nice long hot arc. I remember an experiment in college
>(Virginia
>Tech probably had one of the last power engineering labs before all
>the
>kids went digital) in which we had these motor generator sets, and
>load
>banks. We'd rev up the motor, either in series or shunt field config,
>and
>start adding load (resistors to the generator output). The switches
>were
>merely open knife switches. Electrical engineering students had to be
>smart
>and keep a hand in their pocket, and flip these little blades open or
>closed by the knob. The floors were made of wood blocks, elevated, to
>prevent killing someone. We called it the Frankenstein Lab. It
>separated
>the 'men' from the 'boys' in engineering (sorry if there are females
>reading this, it's only a figure of speech!) school.
>
>OK so now you have the setup. I had my experiement really loaded up,
>and i
>believe that the motor was series wired, such that the additional load
>caused the field current to increase, which increased the speed, which
>increased the current, you get the picture... This thing was
>screaming, and
>the professor came rushing over to tell me to shut it off. I started
>to
>lift the DC load with a knife switch. I don't remember the voltage,
>but it
>may have been a hundred or so. The thing just sizzled and kept going,
>with
>a nice 1 to 2 inch arc continued in space across from the contact to
>blade.
>YIKES! Luckily someone pulled the main breaker, one of those spring
>loaded
>arc-chute Westinghouse things that really looked like it was from Dr.
>Frankenstein's lair.
I would guess then that is why true HV fuses are 3 to 9" or so long, at
least the ones I have used up to 6KV.
Using a standard AG type fuse in the HV line would probably duplicate
your experiment.
>-----------------
>
>I learned that large DC currents are hard to interrupt. Back to the
>present:
>We have this 120 KV fuse here, that opens in case a bank of 7
>klystrons
>faults. It separates the main power rectifier from the capacitor bank,
>which is something like 80-100 uF at 90 KV. The fuse is literally 4
>feet
>long, and made by Maxwell in San Diego. It is like a big pipe, full of
>sand
>and somekind of element. When it blows, the inside burns up the entire
>length, and the sand sort of melts around it.
>
>In another system, we have these 30 KV capacitors in parallel. We
>learned
>the hard way in 1992, when one shorted and all the rest of them
>(dozens)
>dumped their energy into the short. They had 5 Ohm 100 Watt wirewounds
>in
>series with each cap. Every resistor exploded, not to mention having
>the
>fire department, and a big mess of melted steel and oil. Since then,
>we
>have designed a new type of HV fuse, that is spring loaded. It is
>merely a
>#22 wire, suspended in a big 'grasshopper' spring, that keeps a piece
>of
>this wire about 6 inches long, under tension. It the capacitor were to
>rupture and short, the wire vaporizes, the spring rips it open, and
>breaks
>the arc. It is helped by a carborundum (now Cesiwid) resistor of a few
>ohms
>in series. We have tested it, and it saves the resistors, and isolates
>the
>faulty capacitor. The grasshopper spring swings over and hits a ground
>bus,
>another safe feature.
I worked as a part time test tech at High Voltage Engineering back in
'64-67 three nights a week after my regular job at National Radio. The
DC supplies they built ranged from 100KV/200ma to 500KV @ 1A. Those big
supplies were 9' tall and ran behind a 12" thick concrete door with
steel facing. When something let go the sound was awesome....not to say
the scattered remains of the PS section that went!
I do not remember what was used for fault protection.
I even got to play a bit with the 1 megavolt accellerator.
>------------------
>
>Now, think about using a 3AG or one of those little Bussman fuses.
>They are
>rated for 250 VAC, not much more. People have used them for HV power
>supplies for years, on the line side. When you use them on DC, and you
>dump
>all that stored energy in an arc, that fuse will pop, but where does
>the
>current go? it continues until the voltage is low enough to extinguish
>the
>plasma. In this way, a plain fuse is not very good for fast breaking
>the
>circuit during a big fault. Maybe a resistor (long, as someone
>mentioned)
>is better, or a springy thingy, or a thin wire that goes poof.
A surge resistor with a long enough arc path and a small wire fuse link
would be close to ideal in a ham type amp. It fits 2 major requirements;
simple and cheap. It would have to be a smaller guage than the plate RFC
if it was to do its job.
Which brings up another interesting question....Why doesnt the plate
choke open up during these Big Bangs? A #28 wire should not survive if
the reasons we keep hearing are true. Food for thought.
Has
>anyone
>else figured out what the optimal fuse for a 3 to 5000 volt supply
>would
>be?
The HV fuse from the motor generator set to the TBK transmitter aboard
ship was a 3KV/1A rating ( 5" long) for the 3KV @ 250ma of the 861 final.
The 1000V line to the VFO and driver 860's used a 1000V/250 ma fuse (3"
long) and ran 1000V @ 75 ma or so. I still have a few of them old fuses
here.
It appears that the Navy ran fuses at their rated voltages and 3-4 times
normal operating current.
73 Carl KM1H
>
>One more thing, during a tube arc, CPI/Eimac and Burle tubes typically
>suggested that the voltage in the arc is about 50 volts. Meaning that
>the
>rest of your power supply voltage during the first instant of the arc
>is
>across the series limiting resistor. You can use this to roughly
>calculate
>the value of R, to keep the current below some specified limit. In the
>big
>tuibes (yes, with handles), these methods are insufficient, and an
>active
>crowbar, using an Ignitron or triggered spark gap, are manditory. Or
>one of
>those fancy low-stored energy power supplies that we were chatting
>about a
>few weeks ago here. We test the crowbar by dropping a #30 wire across
>the
>output (with a vacuum HV switch under remote control). If the wire
>survives, then the crowbar is adequate to protect the few mil diameter
>grid
>wires in the power tubes. This is the recommended test by the tube
>manufacturers. The TV transmitters have a built in crowbar and some
>have a
>test wire as well. The crowbar is triggered by high current in the
>power
>supply, usually from a shunt and a pulse transformer to step up the
>voltage
>to fire the crowbar.
>
>73
>John
>K5PRO
>
>
>
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>
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