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PICList Thread
'[EE] Capacitor Bank upgrade'
2019\09\24@032428 by Luis Moreira

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Hi all,
I'm working on a DC pulsed 30KV 50A power supply which uses a capacitor
bank to store the energy required for the 30mS pulse. Currently it has
300uF bank of oil filled capacitors made up of 37.5uF caps in parallel.
This is an old power supply (18 years old) and I don't really have that
much info on the caps except for working voltage (32KV), max tested voltage
(45KV) and capacitance( 37.5uF). I have done some measurements on one of
the caps using a LCR meter at 100Hz and 120Hz and that confirmed the
capacitance value and gave me some resistance value also of R =120mohms. I
also got Z and L, although I'm not sure they are relevant.

I need to increase the capacitor bank to extend the pulse time and this
obviously raises the issue of if I can just order some HV capacitors with
similar voltage withstand and the required capacitance and add them in
parallel. My issue is that I don't have any info on working current, peak
current withstand and ESR.

 looking at the info I have available current can go up to 140A, in some
conditions, hence I was aiming to get some 300A peak current to have a bit
of headroom. I can go higher but size and cost may become an issue.

In principle I can't see any issue in adding the new capacitors to the
existing ones in parallel, they are charged from a common supply and
discharged via a common busbar, but I keep on having this doubt on
instantaneous discharging and the effect of different ESR. What do you guys
think, am I overthinking it? any insights would be appreciated.
Thank you.
Best Regards
              Luis
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2019\09\24@174531 by Sean Breheny

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Hi Luis,

This is a rather specialized application and there are a number of
considerations which do not come up in more mundane power supplies.

How is the pulse duration controlled? Is there a pulse-forming network
(inductors) or a resistor or some kind of active current control?

Needless to say, this type of device can easily kill and/or mame if you
make a mistake while handling it (or even just one cap) in a partially
charged state.

I may be able to help you out with selecting caps if you can give me more
information about the control of the pulse shape. That will help me to know
how critical the cap esr and esl might be and also whether mixing old and
new might be a problem.

Are the caps pulse rated?

I've built several can crushers and EBW (exploding bridgewire) drivers. I
have not worked with 25kV cap discharge networks, though. Highest was
around 5kV. This device must be impressive - what is it for?

Sean

On Tue, Sep 24, 2019, 3:27 AM Luis Moreira <spam_OUTluis.moreira1575TakeThisOuTspamgooglemail.com
wrote:

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2019\09\24@174615 by David Van Horn

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Many HV caps are not rated for pulse discharge currents.  Check with Plastic Capacitors, they have good data sheets which should at least be informative.

And keep your Jesus Stick handy!

--
David VanHorn
Lead Hardware Engineer

Backcountry Access, Inc.
2820 Wilderness Pl, Unit H
Boulder, CO  80301 USA
phone: 303-417-1345  x110
email: .....david.vanhornKILLspamspam@spam@backcountryaccess.com 

{Original Message removed}

2019\09\24@214241 by RussellMc

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A friend I copied this to replied:

Impedance matching between capacitors in a capacitor bank will determine
both how well the discharge current is shared between the different
capacitors and also the shape of the discharge current waveform.

At shorter timescales it will be the circuit inductance that has the
greatest effect with capacitor ESR's more responsible for longer term
effects.

Tests at 100Hz will be somewhat meaningless in terms of high-speed
discharge behaviour  - although they may give a qualitative indication of
how different capacitors might behave.

Mr Moreira needs to be very careful working with those sorts of voltages
and stored energies where corona discharge (often invisible under well
lit conditions) can provide a highly conductive path to skin  - which if
conduction is established could result in discharge of much of the
capacitor bank energy into the body.  Corona discharge can (will) also
occur within air voids in insulators  - greatly reducing the nominal
voltage strength of affected insulating material.  If care is not taken in
the design of a high-energy discharge circuit it is possible to
inadvertantly create pulse-forming networks that can greatly increase the
peak voltages involved  - leading to insulator breakdown in
unexpected places (including within the capacitors themselves).  At the
high energies involved this can lead to material (including molten metal)
being ejected at high speeds.

I am reminded of the old adage  - "one flash and your'e ash".

______________________________

Another said:

heavens! It’s good to see some people still get to do interesting things
with “stuff”.



I’ll think about it and perhaps comment later. At first glance I do suspect
the ESR and (much more importantly)  inductance differences will matter in
terms of performance not necessarily being as good as one may expect on the
simple arithmetic, but I’m not sure they will affect reliability. Given
he’s measured the inductance of the existing bank, and given he can obtain
it and everything else for a new array by catalogue and calculation, I
expect he should build a small (Spice) model.  He can factor the ESR from
the measured resistance for the old bank using a rule of thumb.


  Russell
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2019\09\25@045153 by AB Pearce - UKRI STFC

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> How is the pulse duration controlled? Is there a pulse-forming network
> (inductors) or a resistor or some kind of active current control?

I would expect the caps to be pulse rated, as they are already used in this application for 18 years ... :)

The easiest way to limit pulse rise time is by using straight wire as an inductor, although for 30ms pulses this will probably be too small to matter. I have certainly seen photographic flash guns where the length of wire between the capacitor and tube was controlled for exactly this purpose. It may be that for Luis' purpose some air would coils between capacitors will slow down the transfer of energy enough.

As for capacitors to purchase, I would attempt to buy more of the ones used in the existing kit, as these have proved themselves.




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2019\09\25@091128 by Joe Mc Cauley

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Luis,

What is your pulse being discharged into? You seem to have quite a large load impedance assuming 50A refers to your peak current. Unless you are changing the load, how is the current going to increase to 140A or do I misunderstand your intentions? What is your pulse shape? Faster transitions will surely mean that inductances in the entire system will be important (including in the capacitors).

I built a 25 Tesla pulsed magnet years ago. That used only 6kV (limitation was 7.5kV if I recall correctly) to produce a 50ms rise time pulse of 1.6kA in a cryo cooled solenoid coil. Pulse rise time was determined by L and C. Pulse fall time was lengthened by a diode clamp with fall time being determined by L/R. Overall pulse length was ~1s. You are not giving much info, but if you add more capacitors you will likely increase the pulse length for a given load. How well you should match them inductance wise really depends on the pulse shape I think.

Incidentally, how do you already discharge these?

Joe

P.S. Regarding safety issues, I had my capacitor bank, charging system, discharge circuit and magnet coil, isolated in a separate room with the doors interlocked. Was storing ~200kJ in that bank! While not charging, there was a large 50 Ohm resistor connected across the bank. This also served as a charge dump should an abort be needed. All was remote controlled and for the record a PIC16F54 was involved...

Your safety situation is obviously complicated by the higher voltages, but the principle of isolation holds be it an isolated room or container.


{Original Message removed}

2019\09\25@103504 by David Van Horn

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" P.S. Regarding safety issues, I had my capacitor bank, charging system, discharge circuit and magnet coil, isolated in a separate room with the doors interlocked. Was storing ~200kJ in that bank! While not charging, there was a large 50 Ohm resistor connected across the bank. This also served as a charge dump should an abort be needed. All was remote controlled and for the record a PIC16F54 was involved..."

And don't forget the Jesus Stick!

--
David VanHorn
Lead Hardware Engineer

Backcountry Access, Inc.
2820 Wilderness Pl, Unit H
Boulder, CO  80301 USA
phone: 303-417-1345  x110
email: david.vanhornspamKILLspambackcountryaccess.com 


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2019\09\25@104749 by Luis Moreira

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Hi Guys,
firstly relating to safety, this is not my first rodeo. before I came to
work on this project, I was working on 130KV Neutral Beam injection power
supplies at JET, so I'm very aware of the dangers. This systems are
interlocked and on their own HV room/area.

This is again a Neutral beam injector power supply, but a smaller one. The
company I work for at the moment, just got this injector on loan. Before we
got it, it was stored somewhere for a few years and any specific info is
lost. the engineers that worked on it have retired (the two useful ones are
80 and 90 years old respectively) so apart from the photocopied drawings I
don't have much to go by.

Saying that the power supply is very well built and it is working as per
the specs. My task is to extend pulse time, hence I need to increase the
capacitor bank. the system will do 100ms but the output voltage starts
tailing off a bit to much to allow the Neutral injector to deliver the
level of power we need. All of the power components can handle the power
but the capacitor bank is to small. when it was operating in 2002 on the
other facility,  they only required 30mS at the rated power.
The capacitors look like pulsed capacitors, but that is not the way they
are operating. When pulsing the voltage will drop from 20KV to around 16KV,
but is then topped up to 20KV again. We are using them in continuous
operation.

What I'm worried about is damage to the existing or new capacitors I may
add. Specially a problem is when there's a normal breakdown on the grids.
current can shoot up to 300A before the power supply stops.
I'm inclined to get capacitors with I peak rated to double that (600A), so
that the caps can live a long life, but perhaps I need to go higher.

Thank you.
Best Regards
                  Luis





On Wed, 25 Sep 2019 at 14:19, Joe Mc Cauley <.....PMCCULEYKILLspamspam.....tcd.ie> wrote:

{Quote hidden}

> {Original Message removed}

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