[ §1 · failure ]

What a ground strike does to a DRSSTC

One event, five consequences, and the mild strike is the one that moves the machine to the wrong mode.

CommonGnd6 August 202617 min read

The arc reaches something earthed. That is one event, and it has consequences on five different parts of the machine, which is why the pieces of it are usually met separately and never add up to a picture.

This machine has never been switched on, so no number on this page came from its own arc. Most are this corpus's model; the one measurement in it, davekni's 413 kHz, is real but taken off another coil. It is an example, not a reference. Work the numbers out for your own machine, and where you can, measure the angle between the arc's voltage and current rather than R and C apart: the angle does not depend on how the model is written, and the pair falls out of it.

The event

arc -> earthed object
     secondary loaded by
     a resistance the coil
     did not choose

Everything below follows from that last line. A coil's own loading is designed: the arc you draw is the load you tuned for. A strike substitutes a resistance set by what the arc happened to touch, how wet it was and how far the current has to travel through it, anywhere from a few hundred ohms to megohms, with no say from you.

Consequence one: the secondary stops reporting its own frequency

This is the oldest of the five and it is the one that shaped every driver in use. Steve Ward, in his DRSSTC log book:

When a ground strike occurs, the secondary appears to jump to another harmonic, usually 3X or 5X the Fres of the coil… This causes all sorts of bad switching transitions… I believe this is why I've lost too many IGBTs in the past.

A secondary ringing at three or five times its resonance is not telling a driver anything useful about when the current crosses zero. A driver that believes it switches into a full bus at the wrong instants, and the bill arrives at the bridge rather than at the topload.

The fix is structural rather than protective: take the feedback off the primary current, and the loop stops being able to hear what the secondary does under a strike. That is why the feedback comes off the primary at all, and it is a decision that gets presented as a design preference far more often than as the failure analysis it was.

Steve Ward puts the same collapse more compactly on the forum (HVF 117): a ground arc is such a low resistance beside the secondary's impedance that "the secondary no longer acts as an LC, but just an L with a parallel resistance, which can be boiled down to a single-resonant transformer." Once it is a single-resonant transformer the rest of the chain follows, and this part is ours rather than the thread's. Power crosses the coupling as P = −M·Isec·dIpri/dt, so it is carried by the secondary current. When the strike collapses that current there is nothing left for the primary to deliver into, and the primary's own current rings up in its place. That is the fifth consequence below, and it is why the overcurrent detector meets the event only after it has happened.

Consequence two: it pulls the frequency further than an arc does

And this is the one place on the page with a measurement behind it rather than a model. davekni, on his oversized QCW in HVF 2397, reports the upper pole at three points:

unloaded            485 kHz
end of a normal arc 440      -9.3 %
end of a ground     413     -14.8 %
  strike

The strike took it 27 kHz below where an ordinary arc of the same machine ended. Which is the direction you would expect, since an earthed object is a larger load than air, but the size of it is not something anybody had put a number to here, and it says the detuning budget a coil is built with is not the budget a strike will use.

The same coil is where the pole equation itself got checked, and the check is on the poles page.

Consequence three: the pole structure can come apart

Load a coupled pair hard enough and its two resonances collapse into one near the primary's. The condition, the source and what it costs you are a property of coupled resonators rather than of this event; what matters here is that a strike only does it across a band of resistances.

Damping peaks where the strike's resistance matches the secondary's reactance, which on this coil is 21.1 kΩ, and the structure comes apart across roughly 11.2 to 39.6 kΩ. Outside that band, in either direction, both poles survive: too high a resistance barely loads it, and too low a one stops being a damper and starts being a short.

What is pinned is the ordinary arc's resistance, and the distinction is the whole point. The collapse is a resistance threshold, R < Z/k, which is 63.2 kΩ on this coil [derived, Z/k], with no capacitance in it, and for a streamer in air five published sources now land within 3.3× of each other once they are put in one form, 77 to 258 kΩ at a metre and a half, all of them leaving the poles alive. A strike is the case where that one term is handed instead to whatever the arc happened to touch. The mechanism is the same; the resistance is the number you no longer get to know.

A sixth source disagrees with all five, and it is the one the 50 kΩ further down has been quoting without saying so. Anders Mikkelsen, revising his own series model because "a parallel one is more representative", gives the arc as 8 pF per meter in parallel with 75 kohm*meter, fitted to his measured 50 degrees near 340 kHz and 60 degrees near 460 (HVF 3140). The constant returns both of his angles: at 1.5 m it is 12 pF and 50 kΩ, whose reactance is 39.0 kΩ at 340 kHz and 28.8 at 460, so arctan(R/X_C) is 52 and 60 degrees [derived]. So the 50 kΩ used below is his 75 kΩ·m over a metre and a half, it is fitted to measurements rather than assumed, and it is the harshest arc on file by a factor of 1.5 against the next one [derived, 77 over 50]. It also sits under the 63.2 above, which says that on his model an ordinary metre-and-a-half arc collapses this coil's pole structure on its own. Five sources say it does not and one says it does. That is the state of the question, and this page uses the harsh end of it.

Consequence four: the strike that leaves the machine intact is the worse one

Which reads backwards until you see it, and is the useful half of the whole page. **The dangerous strike is the one that does not collapse the structure.**

After a collapse there is a single frequency near the primary's 349.9 kHz, above the sword threshold, and nothing for the driver to fall onto. With both poles still alive, the driver recaptures onto the pole with the most gain, and which pole that is depends on how long the arc was when the strike landed. The gain figures give the lower pole +9.4 dB at half a metre and +1.9 at a metre and a half, a crossover at about 1.8 m, and -1.0 by two metres, because the loaded secondary passes the primary's 349.9 kHz somewhere in between. So the survivor is the lower pole across the part of the ramp this coil actually spends, and the upper one past 1.8 m. What this page cannot tell you is how long either pole rings. It used to answer that here, in cycles and at two metres, and both figures were unsourced and the arc length was on the far side of the coil's own crossover from the claim it was supporting. Gain is what a recapturing loop follows and the gain is published; the ring times are not established here. So the mild strike, the one that does not even interrupt the bang, is the one that can move the machine to the wrong mode and leave it there.

That is also why "did it survive the strike" is the wrong acceptance question. The one worth asking is what frequency it came back on.

Consequence five: the current rises, and not by enough

The direction is half of it. The size is the half that decides anything.

Damping raises the resistance reflected back into the bridge, and a higher reflected resistance at a fixed bus lowers primary current. So loading a coil harder does at first pull its current down rather than up, which is the opposite of what an overcurrent detector is built to catch.

But that fall is spent before the strike arrives, on one of the two arc models and not on the other. The turn, where more loading stops lowering the current and starts raising it, sits at 41.1 kΩ of total load [derived, the minimum of the primary-current expression], and an ordinary 1.5 m arc presents 50 kΩ on Anders Mikkelsen's parallel model, his 75 kΩ·m over a metre and a half (HVF 3140). On that model this coil runs its normal ramp 1.9 per cent above the bottom of that dip. A strike does not walk it down into a current minimum. It starts beside the minimum and goes up from there.

On the five sources that convert higher, and on this corpus's own 210 kΩ for the same metre and a half, the ordinary arc sits five times above the turn rather than beside it [derived, 210 over 41.1], and a strike then walks the current down through the minimum before taking it back up. Which of the two this machine does is not settled here, and the difference is not cosmetic: on the harsh model there is no dip left to fall into, and on the other five there is one to cross before the rise starts. The bold sentence above is the harsh model's answer and not every model's, and the table below is drawn on the harsh one throughout. And somebody reports the fall, on a machine and with a caveat worth having. dr. kilovolt, describing the primary current envelope of his ferrite-cored SiC QCW: "During a ground arc the current drops. This is probably the opposite to a classic DRSSTC" (HVF 1073). That is an observation of the falling case rather than the rising one, so at least one real coil sits above the turn far enough to have a dip to fall into. Note the second sentence, because it proposes an axis this section does not have. The five-versus-one split above is between arc models. He is suggesting the sign may split between machine types, a QCW one way and a DRSSTC the other, which would be a property of where each topology puts its loaded impedance rather than of whose arc model you believe. He offers it as a guess and he is on one coil, so it is a hypothesis with a single data point. What it does establish is that the falling case is not hypothetical.

So the question is not the direction. It is the size, and the size is small:

strike   total load   primary current   [derived, every row]
none        50 kΩ        1.00x
50 kΩ       25.0         1.11
39.6        22.1         1.18
25          16.7         1.41
11.2         9.2         2.32
5            4.5         4.50
1            1.0        20.60

Only the 25 kΩ row has ever been measured. The one published measurement of a strike channel, in HVF 117, has the load going from capacitive to resistive and bottoming out around 25 kΩ as the channel establishes; nothing below that was measured at all, and every other row in the table is the model's curve. At that single measured point the current is already 1.41 times the normal ramp. A threshold set with any working margin, half again over the normal ramp, is not crossed until the channel reaches 22.1 kΩ [derived], which is below the one strike anybody has published.

This page used to close there, saying that through the current the strike draws, the detector is not late, it does not fire. The answer is the threshold, not the strike. Half again is crossed at a channel of 22.1 kΩ and a fifth over at 37.3 kΩ [derived from the same expression that produced the table], and the one published strike, at 25 kΩ, falls between them. It passes straight through the lower setting: at 1.20 the detector fires the moment the arc lands, at 1.30 times. It never reaches the higher one.

So the answer flips inside the range a builder might reasonably set, and the single measurement sits in the gap. The calculator below now carries the threshold as its second slider for that reason: move it across the band and the verdict changes under your hand, with nothing else about the machine touched. What the model establishes is the sensitivity rather than the verdict: half again over the ramp is not reached by any strike anybody has published, a fifth over is reached at once, and the boundary between them runs through the one channel resistance that has been measured. By this route the burst can run to the end of its ramp in a mode nobody chose, and whether it does is set in the detector rather than in the arc. There is a second route, faster and by a different mechanism, that does reach the threshold, and it is below.

That table has a calculator behind it, on the page about the arc that reaches the object: set the channel resistance and the detector's threshold and watch whether they ever meet.

The current above is the one the strike draws through the resonator in the steady state, and it stays under the threshold. It is not the only current a strike raises. The first consequence has the secondary jumping to a harmonic and the switching losing sync, and once the bridge fires at the wrong instants the primary is no longer part of a tuned tank: it is an inductance straight across the bus, its current climbing at V = L·dI/dt with nothing resonant to hold it. From a working 160 A to a threshold half again over it, 240 A, that is 3.1 µs [derived, t = L·ΔI/V], and it does not turn on which harmonic the lock jumped to, since the step per half cycle falls as the frequency rises while the count of half cycles grows to match, and the frequency cancels out of t = L·ΔI/V. Fast, but not instant, and close to the speed of the protection itself, which the overcurrent page calls slow, a few cycles: about 8.6 µs at the working frequency here. That is how a bridge this steady-state table says should never trip is reported tripping on a wall strike on a real 600 A machine (kechuang t/80422): the protection is catching the loss of lock, not the reflected-resistance current. The figure is fixed, not a range: the machine's 12.2 nF tank capacitor sets the primary inductance at 17 µH for the 349.9 kHz primary resonance, and across this machine's 440 V buck ceiling that inductance takes the current up at 25.9 A/µs [derived, V/L on 440 V and 16.96 µH]. It is still a lower bound, since t = L·ΔI/V assumes the bus stays applied one way and a bridge switching out of phase averages less. What the detector's few-cycle slowness costs is worth a number of its own. In the 8.6 µs it takes to answer, the current at 25.9 A/µs has reached about 382 A, more than half again over the 240 A threshold, before the protection acts. That is the figure under the overcurrent rule that a threshold has to sit below the destructive current rather than at it: on this machine a loss of lock overshoots the set point by about 1.6 times.

Which decides an argument on the next page over

Whether the detector fires at all depends on where it is set, as above, so the argument is about the strike hard enough to trip whatever threshold you chose, and about what the machine is doing for the rest of the ramp when nothing trips:

latching OCD
  burst over, mode never
  had a chance to change

pulse skip
  drops a cycle, returns,
  into a different buck
  voltage and a different
  pole structure

Pulse skip is the right choice for a long bang and the wrong one for this, and the page that owns that setting should be read knowing how rarely the detector gets a say.

And there is indirect evidence that a burst does survive a strike rather than being ended by it: davekni's 413 kHz was read at the end of the ramp that struck earth. His flash ran to completion. The remainder of it ran in whatever mode the strike left behind, which is the entire reason for telling these two apart.

What to do about it

Feedback off the primary. Not optional, and consequence one is the reason.
A ground plane or a target, so the strike lands where you chose. A strike you arranged has a resistance you roughly know, which removes the only genuinely uncontrolled term on this page.
Coupling high enough to keep the margin. At k = 0.15 there is no margin to keep; at 0.4 to 0.5 there is.
Check the frequency after a strike, not the fuse. Consequence four, and a quiet detector is not evidence that nothing happened.
Latching rather than pulse-skip overcurrent, if the machine draws to earth at all.

Where next

DRSSTC tuning: the two poles, and which one the driver lands on, which is what the collapse above takes away.
Inside the driver, where the primary feedback decision lives.
Overcurrent is not a setting, which is what the detector is actually for.

The strike is the one event on the department diagram that is not drawn, because it happens outside the machine and arrives at every box in it.

strikepolesfeedback