Re: 18,000. AMP high energy Cap bank

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Bert Hickman <bert.hickman@xxxxxxxxxx>

Tesla list wrote:

> Original poster: FIFTYGUY@xxxxxxx
> In a message dated 12/11/04 9:46:06 PM Eastern Standard Time, 
> tesla@xxxxxxxxxx writes:
> The capacitors weigh about 100 lbs each.  Caster
> oil fill caps.  Rated 18,000. AMP High Energy pulse capacitors.   3 
> capacitors in parallel is 54,000. AMPS.
>     How does one measure these very high current levels to any degree of 
> accuracy?
>     How does one limit these current levels? If a energy discharge cap 
> system has an ultimate safe rating of (say) 20kA, how does one make sure 
> that current isn't exceeded during an experiment that vaporizes a specimen?
> -Phil LaBudde
>
> .
Hi Phil,

You can measure huge currents by using a specially constructed wideband 
current transformer, a related device called a Rogowski Coil, or via a 
specially constructed ultra-low inductance current shunt called a Current 
Viewing Resistor (CVR). Most of the time, a current transformer or Rogowski 
coil is preferred since they allows your measurement circuitry to be 
electrically isolated from the discharge circuit. Hall effect current 
transformers are another option, but these tend to have lower maximum 
current ratings that the above options.

Wideband current transformers from Stangenes Industries are available up to 
200 kA, and from Pearson Electronics up to 500 kA. Custom versions go 
higher - see:
http://www.pearsonelectronics.com/datasheets/standard-current-monitor.htm
http://www.stangenes.com/curtransformers.html#3

Rogowski coils require an integrator circuit (a passive RC circuit or, for 
best accuracy, a wideband operational amplifier integrator). Commercially 
available Rogowski coils can measure currents into the multi-megAmp range. 
It's also possible to make a homemade Rogowski coil and IC integrator 
circuit. However, then the challenge becomes calibrating the unit. For 
examples of commercial Rogowski Coils see:
http://www.rocoil.cwc.net/
http://www.titan-psd.com/html/CommComponentsRogowski.htm

Standard CVR's are also available to handle multi-megAmp currents and these 
devices have the advantage of being able to measure down to DC. For 
examples, see:
http://www.tandmresearch.com/

As far as limiting the peak current - the peak discharge current is often 
limited by inserting "hard" inductance (i.e., an inductor that's robust 
enough that it doesn't get destroyed) into the discharge path. In this case 
the maximum current will always be below the level limited by energy 
conservation:

Imax = Vcap*Sqrt(C/L)

However, simple inductive limiting also may cause excessive ringing which 
can overstress the capacitor's dielectric during voltage reversals. By 
adding suitable (high-energy capable!) resistance in the current loop, you 
can critically damp the circuit (R > 2*Sqrt(L/C). However, this 
significantly reduces the peak discharge current. By reducing the loop 
resistance below the critically damped value thereby permitting a bit of 
ringing, you can increase the peak current while still keeping the worst 
case capacitor voltage reversal within the voltage reversal spec of the 
capacitor bank (typically 20% for many pulse capacitors).

Best regards,

-- Bert --
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