[TCML] pole pig protection

Bert Hickman bert.hickman at aquila.net
Wed Sep 10 12:13:47 MDT 2008 

Hi Gary,

Unfortunately, an NST or distribution transformer HV winding doesn't 
behave as a lumped inductance to incoming high voltage transients. As 
with other distributed LC networks, a HV winding exhibits multiple 
resonances when frequency-swept by a signal generator. The outermost 
portions of the winding appear much more inductive than the interior 
portions at high frequencies. So, instead of the transient voltage 
stress being evenly applied across the entire winding (as in a lumped 
inductor), the voltage distribution is much higher across the outermost 
turns and winding layers.

If you apply a voltage step function to the output of a large ideal 
lumped inductor, the resulting current will simply gradually increase 
over time. However, if you do the same test using a real multilayer HV 
winding, you'll see the current immediately jumps during the step 
transition due to the combination of winding inductances and 
interwinding capacitances. The instantaneous voltage distribution will 
actually be governed more by the distribution of capacitances within the 
winding than by the inductances.

Bert

Lau, Gary wrote:
> It's true that the choke's impedance increases with frequency, but
> the same is true for the HV winding.  Their impedances BOTH scale
> identically with frequency, so the division ratio is a constant,
> regardless of frequency.
> 
> I don't know the specific numbers for a pole pig, but for a 15/60
> NST, the secondary leakage inductance is in that ballpark.  I'm
> struggling at the moment to locate the specific numbers, but was in
> the thousands of Henries, and this value sat well with the numerous
> simulations that Terry and I did.  What I can't resolve just now is
> the several thousand Hy figure, vs. a value of 663 Hy, which would
> achieve mains-resonance with a 10.8nF tank cap (the 60 Hz
> mains-resonant value cap for a 15/60 NST).
> 
> I had assumed that a pig's HV winding inductance would be at least
> that of the NST's.  As my familiarity with pigs is not great, I can't
> say this with conviction, and any corrections are welcome.  But I do
> feel confident that its inductance is orders of magnitude greater
> than a choke, and that any voltage division between an air-core choke
> and the iron-core HV winding would be insignificant.
> 
> Regards, Gary Lau MA, USA
> 
>> -----Original Message----- From: tesla-bounces at pupman.com
>> [mailto:tesla-bounces at pupman.com] On Behalf Of Jim Calvin Sent:
>> Wednesday, September 10, 2008 10:13 AM To: 'Tesla Coil Mailing
>> List' Subject: RE: [TCML] pole pig protection
>> 
>> Gary, The division ratio will not be the same at all frequencies.
>> The RF choke's impedance increases with frequency (hence a low pass
>> action) also I am fairly certain the inductance of the HV winding
>> is not 1000 henries.  In fact, the impedance at the HV winding
>> should be the impedance at the low voltage input times the turn
>> ratio (1/65).  This of course is going to be the source impedance
>> plus the ballast impedance.
>> 
>> -----Original Message----- From: tesla-bounces at pupman.com
>> [mailto:tesla-bounces at pupman.com] On Behalf Of Lau, Gary Sent:
>> Wednesday, September 10, 2008 9:51 AM To: Tesla Coil Mailing List 
>> Subject: RE: [TCML] pole pig protection
>> 
>> Correct - a choke in series with the HV winding does create a
>> voltage divider.  But -
>> 
>> 1) The division ratio is constant for all frequencies, 60 Hz as
>> well as the tank frequency.  Filters usually imply that the
>> attenuation is frequency-dependant.  We want a filter that
>> attenuates high frequencies more-so than low frequencies - a
>> low-pass filter.
>> 
>> 2) Let's say that the HV winding L1 has an inductance of 1000
>> Henries.  If we put a 10 mH inductor L2 in series with this, the
>> voltage seen by the HV winding is L1/(L1+L2): 1000 / (1000+.01) =
>> 0.99999 of the unfiltered value.  No difference.
>> 
>> Regards, Gary Lau MA, USA
>> 
>>> -----Original Message----- From: tesla-bounces at pupman.com
>>> [mailto:tesla-bounces at pupman.com] On Behalf Of Jim Calvin Sent:
>>> Wednesday, September 10, 2008 8:53 AM To: 'Tesla Coil Mailing
>>> List' Subject: RE: [TCML] pole pig protection
>>> 
>>> As a point of fact I have destroyed a pole transformer last year
>>> with this very same coil.  So they are not indestructible.
>>> 
>>> Gary, you are right, an inductor by itself is not a filter, but
>>> in series with the HV winding of the transformer forms a voltage
>>> divider with an
>> input
>>> impedance of greater than 5Kohm (at 90Khz).  I have not put a
>>> scope on it with my HV probe so I can not say for certain it is
>>> helping.
>>> 
>>> Can you email me this filter DC.  This is exactly what I am
>>> looking for. My email is jcalvin at newagemicro.com
>>> 
>>> -----Original Message----- From: tesla-bounces at pupman.com
>>> [mailto:tesla-bounces at pupman.com] On Behalf Of DC Cox Sent:
>>> Tuesday, September 09, 2008 11:52 PM To: Tesla Coil Mailing List 
>>> Subject: Re: [TCML] pole pig protection
>>> 
>>> I disagree with some of this.
>>> 
>>> If there were zero oscillations then we would not have to use
>>> Terry
>> filters
>>> even on NST circuits as the spark gap would be shorting out the
>> oscillations
>>> there also.  If you throw a scope on the circuit you can see some
>>>  oscillations coming back from the tank circuit directly toward
>>> the xmfr
>> ---
>>> yes, even thru the shorted spark gap (conduction resistance is 
>>> constantly changing and not a fixed value so it is not always at
>>> zero value).
>>> 
>>> and yes, one should employ a simple air core inductor and also a
>>> resistive filter (12 pcs 600 Ohms in parallel, 200 watts each)
>>> even on pole xmfrs.  again, if it is scope you can see the non-60
>>> Hz oscillations.
>>> 
>>> pole xmfrs are testing usually for a few single shots at 125 kV
>>> BIL (basic impulse level) using a Marx generator, however
>>> lightning oscillations tend to be short-lived and certainly not
>>> continuous as with a LC tank circuit.  I know many experimenters
>>> who do not use a simple filter but these may be the same guys who
>>> drive around without auto insurance!
>>> 
>>> the filter is simple and will save your $300 pole xmfr, plus the
>>> $250 it cost to ship it if in another state.  The filter costs
>>> approx $30  --- $300/$30 risk/reward ratio.
>>> 
>>> I have the filter circuit that I use if anyone wishes to contact
>>> me
>> off-list
>>> I will email it to them.
>>> 
>>> better safe than sorry and out a few hundred bucks.
>>> 
>>> Dr. Resonance
>>> 
>>> 
>>> 
>>> 
>>> On Tue, Sep 9, 2008 at 6:59 PM, miles waldron 
>>> <mileswaldron at comcast.net>wrote:
>>> 
>>>> These comments are in response to Gary Lau, who writes on this
>>>> thread.
>>>> 
>>>> Gary is a very nice person. The following opinions are mine,
>>>> and for the record I agree completely with everything Gary has
>>>> said. Someone please
>>> let
>>>> me know if I have missed the point entirely, and if that is the
>>>> case
>> then
>>> I
>>>> am sorry.
>>>> 
>>>> Terry filter protection for a pole pig is pointless. A pole pig
>>>> is
>>> designed
>>>> to be struck by lightning over and over again during its
>>>> lifetime. Have
>>> you
>>>> ever been hit by lightning? As equipment goes, a pig is
>>>> typically used
>> to
>>>> avoid transformer limitations. You can do just about whatever
>>>> you want
>>> with
>>>> a pole pig, and it will still be there (not hindered in any
>>>> way) after
>> the
>>>> explosion.
>>>> 
>>>> Miles Waldron
>>>> 
>>>> 
>>>> 
>>>> 
>>>> -----Original Message----- From: tesla-bounces at pupman.com
>>>> [mailto:tesla-bounces at pupman.com] On
>>> Behalf
>>>> Of Lau, Gary Sent: Tuesday, September 09, 2008 6:25 PM To:
>>>> 'Tesla Coil Mailing List' Subject: RE: [TCML] pole pig
>>>> protection
>>>> 
>>>> 
>>>> Opinions vary on this topic.  Here is mine.
>>>> 
>>>> First off, in contrast to NST's and MOT's, pole pigs are
>>>> extremely
>> rugged
>>>> and well designed.  They are designed to be undamaged by
>>>> lightening strikes. While it is not unheard-of, damage to pole
>>>> pigs is a very rare thing.
>>>> 
>>>> As to protecting the pig from RF - I assume that your TC
>>>> circuit has the main gap in parallel with the pig HV secondary.
>>>> Please consider that
>> the
>>>> ONLY time there is RF oscillating in the TC primary or
>>>> secondary, the
>>> spark
>>>> gap is conducting.  If the spark gap is conducting, it is
>>>> essentially shorting out the pig's HV secondary, so there's not
>>>> a lot of RF to
>> protect
>>>> against.
>>>> 
>>>> Now, it's not entirely true that the gap is conducting
>>>> continuously
>> during
>>>> each bang.  Each time the current through the gap passes
>>>> through zero at the resonant frequency, the gap arc
>>>> extinguishes briefly for a few(?) nanoseconds.  During this
>>>> brief time the primary rings with its self-capacitance, in the
>>>> tens of MHz region.  The initial magnitude of
>>> this
>>>> oscillation is Vbang, and since it is in series with the tank
>>>> cap, also initially at Vbang, the combination of the two in
>>>> series (what the HV winding sees) is 2Vbang.  This, IMHO, is
>>>> the thing that needs to be filtered, at least for NST's and
>>>> less sturdy transformers.  And this is easily and effectively
>>>> attenuated by the R-C filter in the Terry filter.
>>>> 
>>>> Simply adding a choke in series with the pig's HV terminal does
>>>> not a filter make.  The pig's secondary may be viewed as a many
>>>> thousand HENRY
>>> inductor.
>>>> Adding a few mH in series with it accomplishes nothing.  Low
>>>> pass
>> filters
>>>> are constructed of typically two or more elements - L-C, R-C.
>>>> L-C-R,
>> etc.
>>>> Just adding a choke inductor does not achieve any filtering.
>>>> You might
>> as
>>>> well string together cloves of garlic or St Christopher
>>>> medallions.
>>>> 
>>>> I apologize for being so wordy and blunt, but the use of chokes
>>>> to "protect" transformers is a practice that was once done,
>>>> just because it "seemed"
>> at
>>>> first glance like a good idea, and persists despite strong
>>>> logic not to. It has since been shown through rigorous circuit
>>>> analysis, simulation, and measurement, to be ineffective at
>>>> best and counterproductive when done
>>> with
>>>> bypass caps.
>>>> 
>>>> Regards, Gary Lau MA, USA
>>>> 
>>>>> -----Original Message----- From: tesla-bounces at pupman.com
>>>>> [mailto:tesla-bounces at pupman.com] On Behalf Of Jim Calvin 
>>>>> Sent: Tuesday, September 09, 2008 3:07 PM To: 'Tesla Coil
>>>>> Mailing List' Subject: [TCML] pole pig protection
>>>>> 
>>>>> 
>>>>> I have a 25KVA pole pig I am using for my large tesla coil
>>>>> (the coil only uses about 5-6KVA).  I am using a hand wound
>>>>> RF choke to protect the RF from the coil from getting back
>>>>> into the pig.  It is wound with 16 gauge wire onto a 8 inch
>>>>> sonatube.  I measured the inductance with RLC meter to be
>>>>> 10mH.  My pole pig has two HV leads with one grounded and the
>>>>> other connected through this choke to my coil.  I have not 
>>>>> used the coil in awhile (been a busy summer).  I was going to
>>>>> start using it again but I wanted to make sure this is
>>>>> sufficient to protect my piggy!  Any advice would be helful.
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