Low pass filter for lightning strikes

Discussion in 'Homework Help' started by ManUnited1, May 6, 2011.

  1. ManUnited1

    Thread Starter New Member

    May 6, 2011
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    Hi,

    I was wondering if you can help me with this problem.

    A lightning discharge in the vicinity of a high voltage substation may result in transient voltages and current induced in control and protection circuits.

    Do you agree that the use of a low pass filter is sufficient to maintain the functional integrity of these circuits?
     
  2. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
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    "in the vicinity" and lightning... hmmm... its hard to hit the AGREE button on this one without some significant details and research.

    One question, are all of your control lines low enough speed to be unaffected by the proposed filters?
     
  3. ManUnited1

    Thread Starter New Member

    May 6, 2011
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    For this particular question, the assumption is all of the lines will be unaffected by the filter.
     
  4. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    That's kind of like asking if a surge protector on your mains power board will protect your computer if lightning strikes the mains conductors in your street. That would be an uncertain outcome to say the least.

    Some years ago lightning struck my neighbor's garage across the street and ended up tracking its way into his mains wiring. The resulting fault current field was strong enough to upset my old PC CRT monitor to the extent that I needed to de-gauss it when the colors went out of kilter. My "worries" were minuscule compared to my neighbor's.
     
  5. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
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    Our neighbor had a tree, that was about 40' from our house, hit by lightning that left it smoldering and in pieces. As we watched the smoke rising from the tree we failed to immediately notice the smoke rising from our TV. Nothing else in our house appeared to be affected.

    If this is a homework problem, then I think the answer lies in the assumptions that are being made about the induced transients in your control lines and protection circuits. Sure, a low pass filter, correctly constructed, can reduce voltage and current transients into the control lines without affecting the performance of the lines, in many cases. When you throw lightning into the mix, the transients can be imagined to be rather large...
     
  6. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    Limiting the damage from current source surge created by lightning with a low pass filter will only work if the bulk on the energy is directed to ground long before it get to the electronics, at that point a TVS diode can provide effective protection on a signal line. The only real protection is a dead short to a single common grounding point tied to earth that reduces the voltage on the lines to near zero. Any series resistance or impedance to the current surge will only only increase the voltage side of the V*I power equation at that point as the surge will develop any voltage necessary to to maintain the current flow. A solid-state or gas-tube type shunt protection device with a proper grounding system is really the only protection.

    So I would disagree.
     
  7. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    When it comes to lighning strikes, I doubt there is anything that sufficient to maintain the functional integrity of these circuits. It depends on the location of the strike. Shunt protection does a good job. On incoming lines, I've seen some inductance in series work, followed by a shunt, like your low pass filter. I guess it depends on what type of strike your protecting against. A 98 percent of the lightning strokes are from 3 kA to 140 kA. The rise times are typically 250 nS to 12 uS. and about 50 mS between intervals.

    In GE's "living with lightning" pamplet, there is an isokeuronic map and formulas you can use to estimate the number of strikes your structure or tower would be expected to hit by lighting.

    I can say that the most damage by nearby lightning strokes wasn't from the power lines as described by the OP. The ground strike induced enough energy to travel to the AC return line and open the diodes that were connected to the AC return in consumer equipment. As we know the ground and the AC return have a common point in the breaker box here in the U.S.

    It breaks down to what you are protecting and how much protection your willing to pay for. Some will simply add inductance to the power cord by tying the cord in six overhand knots adding some inductance in series with the two conductor power cord. A power cord is cheaper than a TV set, as the power cord should simply blow apart. Is that protecting the functional integrity of the device? Maybe. Some will spring for a commercial designed shunt protection device, like a joslyn lighting arrestor. It depends on the perceived value of whatever your protecting.
     
    Last edited: May 6, 2011
  8. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    The example of a power cord with knots is a good one. The series inductance creates voltage at that point with a huge increase in power that not only destroys the cord but generates a EM pulse that radiates on the wiring from that point to everything else connected the circuit. Reactance should never be a series element in surge protection before a shunt device.
     
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