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SCR triggering for high current inductive loads

P

Paul E. Schoen

Jan 1, 1970
0
The circuit breaker test sets I have been involved with use an AC switch
consisting of two SCRs wired in reverse parallel connection. For some test
sets, a fixed voltage of 240 or 480 VAC is switched, while in others it is
a variable voltage of 0 to 560 VAC. The currents involved may vary from
only several amperes to about 200 amperes continuous, and pulses of 100
mSec or so up to at least 1000 amperes. The load is a step down transformer
with an output of 6 to 15 VAC or so, and it is connected to circuit
breakers up to 6000 amps. Breakers are typically tested at 3x for time
delay (18,000 amps for 30-90 seconds), and instantaneous at 10x (60,000
amps for 0.02 to 0.05 seconds).

The load is primarily inductive, so the SCR firing circuit is adjusted for
an initial firing angle of about 70 degrees. It is adjusted so that all
half-cycles are approximately the same peak value. Also, the controller is
set to produce an integral number of cycles (typically 5), when pulsing the
output current below instantaneous trip value. If an extra half-cycle
occurs, there is a net DC component, and the transformer is essentially
magnetized (remanent magnetism). When this happens, the inrush current of
the next pulse is extremely high, sometimes enough to trip a 200 ampere
mains breaker, and causing a very audible noise in the conduit as the
cables slap against the pipe.

I have used several designs for firing circuits. Originally we used
commercially available controllers which used high frequency pulses to fire
the gates, but quite often we would see waveform distortion because the
current was not enough to keep the SCR in conduction. I designed several
circuits that used DC for the gates, and made sure that the current was
applied only when the anode had a positive voltage. However, because of the
inductive load, there was current still flowing when the gate current was
turned off, and distortion was seen.

Finally, about ten years ago, I designed a simpler board which kept gate
current on continuously, and these boards have been used in hundreds of
test sets with no apparent problems related to the gate drive. I use a
simple constant current source with a PNP transistor and a 2.0 ohm
resistor, and diodes, which limits the current to one junction drop (0.6V)
on 2 ohms, or about 300 mA. It is sourced from about 12 VDC.

I am now designing a new board which will use a PIC to control the SCR
firing. I plan to use DC/DC converters (12V to 5V at 200 mA), rather than
transformers for the gate voltage supplies, to save size and also allow the
circuit to run on 12 VDC. It will also have sensors to determine if the SCR
is actually turned on when gate current has been supplied, and it will have
other bells and whistles such as programmable phase angle, overcurrent
detection, etc.

In researching gate drive requirements, I found a specification for the
SCRs we now normally use, giving a guaranteed turn on current of 200 mA, at
a voltage of 1.0 to 4.0 volts, but there was also a specification
indicating that the gate should not have current applied when the anode is
negative with respect to cathode. The previous design applies the 300 mA
current during the full conduction cycle, which does not meet this
specification. However, the other SCR is conducting at that time, so the
anode to cathode voltage is only a couple of volts. I am assuming this
condition does not do any harm except waste power. With my new board, I may
be able to detect actual current flow and turn off the gate drive when
current is flowing in the opposite direction, but it is easier to leave
things as they are. Any comments on this?

I also found that the recommended gate drive consists of an initial high
current pulse, followed by a "back porch" of lower current. I can do this
fairly easily by adding a capacitor across the current limit sense
resistor, but it will produce this waveform only for the first phase
delayed firing. After that, I prefer to leave it on continuously. If I turn
gate current off during the time of reverse conduction, I would need to
retrigger at a very critical point just after the zero crossing, and any
delay will cause distortion, and early firing will waste the peak current
pulse.

Another problem I have seen is that, at very high current levels, there is
often an additional half-cycle of current, which magnetizes the transformer
and causes subsequent inrush problems on the next pulse. I found that this
effect could be minimized by reversing the phase of the incoming power, or
by reversing the gate connections to the SCRs. The 480 VAC supply is
produced by an autotransformer on a 208 VAC source, so the voltage to
ground on the two inputs is 360 VAC and 60 VAC. The case that works best is
where the line side of the SCR is at the 360 VAC potential, and the load
side switches from -60(off) to +360(on). The extra half cycle occurs when
the line side stays at -60 and the load switches from +360(off) to -60(on).
I think there is some turn-off transient that is being conducted into the
firing circuit, and probably it is the one that is changing voltage to
ground. If this circuit controls the gate of the SCR that sees a positive
line voltage excursion, it fires and causes the extra half cycle. The SCR
board is normally mounted on the SCR heat sink, which may be at the line or
load side of the switch, but I think I have also seen this problem when the
board has been mounted remotely. This may take more research, but, again,
any comments or suggestions will be appreciated.

Thanks for taking the time to read this long post. Perhaps you may find it
interesting, and responses may be helpful to anyone dealing with similar
applications. More information on the general technology of high current
primary injection testing is on my website.

Thanks!

Paul E. Schoen
www.pstech-inc.com
 
I

Ignoramus15879

Jan 1, 1970
0
Just one random thought about high inrush currents.

Would it not be appropriate to limit them using a resistor in series,
and another antiparallel switch to bypass resistor after a couple of
cycles:


your original SCR switch S1 --+------/\/\/\resistor/\/\/\/\-------+- load
| |
+______________/ ___________________+
another switch S2

S2 would be switched on 1/20 second after S1 is turned on. Resistor
could be watercooled or whatever, to cool it after every cycle. (you
can probably take some length or 10 gauge hookup wire and submerge it
in water). 20 ft of it would have resistance of about 0.6 ohm and
limit current in a 480V circuit to no more than 800 or so amps.

800 amps conducted for 1/2 cycle is

800*800*0.6/120 = 3200 joules

3200 joules is not much if it is water cooled.

Assuming your normal current is 100A, as you mention on your page, it
would be nothing for this submerged hookup wire.

i
 
P

Paul E. Schoen

Jan 1, 1970
0
Ignoramus15879 said:
Just one random thought about high inrush currents.

Would it not be appropriate to limit them using a resistor in series,
and another antiparallel switch to bypass resistor after a couple of
cycles:


your original SCR switch S1 --+------/\/\/\resistor/\/\/\/\-------+- load
| |
+______________/ ___________________+
another switch S2

S2 would be switched on 1/20 second after S1 is turned on. Resistor
could be watercooled or whatever, to cool it after every cycle. (you
can probably take some length or 10 gauge hookup wire and submerge it
in water). 20 ft of it would have resistance of about 0.6 ohm and
limit current in a 480V circuit to no more than 800 or so amps.

800 amps conducted for 1/2 cycle is

800*800*0.6/120 = 3200 joules

3200 joules is not much if it is water cooled.

Assuming your normal current is 100A, as you mention on your page, it
would be nothing for this submerged hookup wire.

i

It would probably not even need to be water cooled, which would be a real
problem implementing in a portable (sort of) test set. There are also some
big thermistors which start at a fairly high resistance and quickly drop to
a few tenths of an ohm after they heat up. However, I tried them on a
smaller test set, but they were not too effective, and they burned up. A
contactor for your suggested S2 above would be quite expensive. The SCR
switches are actually much cheaper, at about $400.

Also, adding this much resistance will seriously affect the initial
waveform of the current pulse, and that will adversely affect test results.
If 48,000 amps output is required, the primary current must be 1000
amperes, but the 0.6 ohm resistor would drop 600 volts. What it actually
does is limit the output current to about 24,000 amps until the S2 pulls
in, and then you get the required current. I saw that sort of effect with
the thermistors. Also, they reduced the surges until they got hot, and then
were ineffective.

With my new SCR board, I am thinking about implementing a demagnetizing
routine, which will detect an odd number of half-cycles, and then apply a
series of phase-delayed pulses to eliminate the magnetic charge. I have
proven that this concept will work, by reducing the primary voltage and
initiating a short pulse of lower current. Then I return to the setting for
higher current, and the input surge is less than it would have been
otherwise.

Thanks for your idea.

Paul
 
R

Roger

Jan 1, 1970
0
Years ago when I worked in power electronics I had a similar problem
with 300A SCR's. I was just a young fledging assistant at the time. The
original design used a little trigger cct for each SCR which had its
own floating supply and an opto which was used to switch a power
darlington onto the gate. I remeber that there was a resistor in the
emitter to limit the current (the base voltage was diode clamped. The
trouble was that the current was not high enougth to ensure that the
SCR's remained in conduction, but the designer was reluctant to simply
beef up the circuit because he was worried about having too much
current in the gate.

The solution came from the technique that is used in many simple
opto/triac firing schemes, take the gate current from the anode. If you
take a resistor from the anode and through a diode to a floating
switch, then the current will automatically depend on the voltage
accross the SCR. That way you can make the resistor very low as any
situation where large currents start to flow into the gate will ensure
that the SCR is fired and hence the voltage across it is low, hence no
drive.......a very simple automatic gate drive system.

It worked well on that circuit anyway ;-)
 
Paul said:
The circuit breaker test sets I have been involved with use an AC switch
consisting of two SCRs wired in reverse parallel connection. For some test
sets, a fixed voltage of 240 or 480 VAC is switched, while in others it is
a variable voltage of 0 to 560 VAC. The currents involved may vary from
only several amperes to about 200 amperes continuous, and pulses of 100
mSec or so up to at least 1000 amperes. The load is a step down transformer
with an output of 6 to 15 VAC or so, and it is connected to circuit
breakers up to 6000 amps. Breakers are typically tested at 3x for time
delay (18,000 amps for 30-90 seconds), and instantaneous at 10x (60,000
amps for 0.02 to 0.05 seconds).

The simplest way to drive scrs like this is with an opto triac (2 in
series and resistor) between the gate leads but this may not work very
well if you have a very low mains voltage. Fireing at 90Deg should
eliminate inrush. As far as remance is concerened I doubt very much if
this has any sizeable effect. Recently I built a PIC contrlolled system
just like yours for controlling a welding machine, (2scrs, transformer,
isolated dc gate drives) and had similar problem (random fuse blowing)
it turned out to be software problem (not turning off scr gate drive
correctly) thats where I would look first.
 
I

Ignoramus10768

Jan 1, 1970
0
this has any sizeable effect. Recently I built a PIC contrlolled system
just like yours for controlling a welding machine, (2scrs, transformer,
isolated dc gate drives) and had similar problem (random fuse blowing)
it turned out to be software problem (not turning off scr gate drive
correctly) thats where I would look first.

I would personally be very interested to hear about that welding
machine project. I am foing something similar now (but in 3 phase).

i
 
F

Fred Bloggs

Jan 1, 1970
0
The circuit breaker test sets I have been involved with use an AC switch
consisting of two SCRs wired in reverse parallel connection. For some test
sets, a fixed voltage of 240 or 480 VAC is switched, while in others it is
a variable voltage of 0 to 560 VAC. The currents involved may vary from
only several amperes to about 200 amperes continuous, and pulses of 100
mSec or so up to at least 1000 amperes. The load is a step down transformer
with an output of 6 to 15 VAC or so, and it is connected to circuit
breakers up to 6000 amps. Breakers are typically tested at 3x for time
delay (18,000 amps for 30-90 seconds), and instantaneous at 10x (60,000
amps for 0.02 to 0.05 seconds).

The load is primarily inductive, so the SCR firing circuit is adjusted for
an initial firing angle of about 70 degrees. It is adjusted so that all
half-cycles are approximately the same peak value. Also, the controller is
set to produce an integral number of cycles (typically 5), when pulsing the
output current below instantaneous trip value. If an extra half-cycle
occurs, there is a net DC component, and the transformer is essentially
magnetized (remanent magnetism). When this happens, the inrush current of
the next pulse is extremely high, sometimes enough to trip a 200 ampere
mains breaker, and causing a very audible noise in the conduit as the
cables slap against the pipe.

I have used several designs for firing circuits. Originally we used
commercially available controllers which used high frequency pulses to fire
the gates, but quite often we would see waveform distortion because the
current was not enough to keep the SCR in conduction. I designed several
circuits that used DC for the gates, and made sure that the current was
applied only when the anode had a positive voltage. However, because of the
inductive load, there was current still flowing when the gate current was
turned off, and distortion was seen.

Finally, about ten years ago, I designed a simpler board which kept gate
current on continuously, and these boards have been used in hundreds of
test sets with no apparent problems related to the gate drive. I use a
simple constant current source with a PNP transistor and a 2.0 ohm
resistor, and diodes, which limits the current to one junction drop (0.6V)
on 2 ohms, or about 300 mA. It is sourced from about 12 VDC.

I am now designing a new board which will use a PIC to control the SCR
firing. I plan to use DC/DC converters (12V to 5V at 200 mA), rather than
transformers for the gate voltage supplies, to save size and also allow the
circuit to run on 12 VDC. It will also have sensors to determine if the SCR
is actually turned on when gate current has been supplied, and it will have
other bells and whistles such as programmable phase angle, overcurrent
detection, etc.

In researching gate drive requirements, I found a specification for the
SCRs we now normally use, giving a guaranteed turn on current of 200 mA, at
a voltage of 1.0 to 4.0 volts, but there was also a specification
indicating that the gate should not have current applied when the anode is
negative with respect to cathode. The previous design applies the 300 mA
current during the full conduction cycle, which does not meet this
specification. However, the other SCR is conducting at that time, so the
anode to cathode voltage is only a couple of volts. I am assuming this
condition does not do any harm except waste power. With my new board, I may
be able to detect actual current flow and turn off the gate drive when
current is flowing in the opposite direction, but it is easier to leave
things as they are. Any comments on this?

I also found that the recommended gate drive consists of an initial high
current pulse, followed by a "back porch" of lower current. I can do this
fairly easily by adding a capacitor across the current limit sense
resistor, but it will produce this waveform only for the first phase
delayed firing. After that, I prefer to leave it on continuously. If I turn
gate current off during the time of reverse conduction, I would need to
retrigger at a very critical point just after the zero crossing, and any
delay will cause distortion, and early firing will waste the peak current
pulse.

Another problem I have seen is that, at very high current levels, there is
often an additional half-cycle of current, which magnetizes the transformer
and causes subsequent inrush problems on the next pulse. I found that this
effect could be minimized by reversing the phase of the incoming power, or
by reversing the gate connections to the SCRs. The 480 VAC supply is
produced by an autotransformer on a 208 VAC source, so the voltage to
ground on the two inputs is 360 VAC and 60 VAC. The case that works best is
where the line side of the SCR is at the 360 VAC potential, and the load
side switches from -60(off) to +360(on). The extra half cycle occurs when
the line side stays at -60 and the load switches from +360(off) to -60(on).
I think there is some turn-off transient that is being conducted into the
firing circuit, and probably it is the one that is changing voltage to
ground. If this circuit controls the gate of the SCR that sees a positive
line voltage excursion, it fires and causes the extra half cycle. The SCR
board is normally mounted on the SCR heat sink, which may be at the line or
load side of the switch, but I think I have also seen this problem when the
board has been mounted remotely. This may take more research, but, again,
any comments or suggestions will be appreciated.

Thanks for taking the time to read this long post. Perhaps you may find it
interesting, and responses may be helpful to anyone dealing with similar
applications. More information on the general technology of high current
primary injection testing is on my website.

Your description is too nebulous to understand the setup with any
certainty, like for example your mention of distortion while firing at
70o phase angles...makes no sense. You might spring for a technical writer.
 
P

Paul E. Schoen

Jan 1, 1970
0
Fred Bloggs said:
[snip]
I have used several designs for firing circuits. Originally we used
commercially available controllers which used high frequency pulses to
fire the gates, but quite often we would see waveform distortion because
the current was not enough to keep the SCR in conduction. I designed
several circuits that used DC for the gates, and made sure that the
current was applied only when the anode had a positive voltage. However,
because of the inductive load, there was current still flowing when the
gate current was turned off, and distortion was seen.

Finally, about ten years ago, I designed a simpler board which kept gate
current on continuously, and these boards have been used in hundreds of
test sets with no apparent problems related to the gate drive. I use a
simple constant current source with a PNP transistor and a 2.0 ohm
resistor, and diodes, which limits the current to one junction drop
(0.6V) on 2 ohms, or about 300 mA. It is sourced from about 12 VDC.
[snip]

Your description is too nebulous to understand the setup with any
certainty, like for example your mention of distortion while firing at
70o phase angles...makes no sense. You might spring for a technical
writer.

I could have written more details, but the post was too long already. With
a partially inductive load, the phase angle of current lags the voltage by
up to 90 degrees. We have determined that 70 degrees is characteristic for
the case of a shunt placed across the output connections (essentially a
bolted short), and we examine the waveform with an analyzer using the
millivolt drop. This is also how we calibrate the unit. We have also
analyzed the waveform with the shunt in series with the test loads, which
may be any one of thousands of different circuit breakers, each with
varying amounts of inductance. Some show evidence of saturation, so the
waveform is distorted with characteristic peaks and high crest factor.

The initial waveform into the circuit breaker must be such that the first
half-cycle is about the same as those proceeding it. If it is much lower,
the instantaneous trip element will ignore it, and will act upon the next
half-cycle, which will actually be higher than those proceeding it. As the
measurement circuit reads the entire waveform and performs a true-RMS
computation on it, this creates a timing error of about 1/2 cycle and a
corresponding measurement error. The new SCR board will be capable of
adjusting the initial firing angle to produce the best waveform possible.
Some of this may be incomprehensible to someone who is not familiar with
circuit breaker testing.

The distortion I was referring to occurred mostly on test sets where the
voltage applied to the SCRs was low (perhaps 10-20 VAC), and the current
also fairly low (5 amperes or so). These are large SCRs, and especially for
older ones, the holding current is probably several amperes. There is some
unavoidable distortion at these levels due to the 1 or 2 volt drop on the
SCRs, but we were seeing distortion that indicated the SCR was dropping out
of conduction well before the normal zero crossing. The loss of conduction
coincided approximately with the removal of gate drive at the zero crossing
of voltage, which occurred about 70 degrees before the zero crossing of
current. When the current dropped below holding current, the SCR went out
of conduction, causing a drop in current. This was sometimes followed by
one or more current spikes, due to inductive kick and dV/dT triggering.
Using DC for the gate drive solved this problem.

However, that could explain some problems that we have seen when we try to
obtain the exact number of half-cycles. If the gate drive is removed too
long before the end of the current, an effect as described above could
occur. If it is removed after the zero crossing, then the SCR might be
triggered and conduct for the following half-cycle. It may be critical to
detect the current zero crossings and remove gate drive at the optimum
time.

I hope this explains it.

Thanks,

Paul
 
Ignoramus10768 said:
I would personally be very interested to hear about that welding
machine project. I am foing something similar now (but in 3 phase).

i

It was for a resistance welder, tin can seams, single phase AC output.
I have done 3 phase controllers in the past for motor soft starters, I
doubt that would be much use for your application.
 
J

John - KD5YI

Jan 1, 1970
0
Paul said:
I also found that the recommended gate drive consists of an initial high
current pulse, followed by a "back porch" of lower current.

Hi, Paul -

This gate drive characteristic was found to be critical for the
McMurray-Bedford thyristor inverters we manufactured years ago. It is all a
bit hazy now, but as I recall, the initial "hard-fire" portion helped to get
as much of the silicon triggered as possible in the early stages of firing.
As a result, di/dt handling characteristics are improved, I think.

Did you see in the recommended gate drive paper about compliance voltage? As
I recall, the Westinghouse (later, PowerX) recommendation was about 30 or 40
volts. It turns out that the gate can rise into the tens of volts at the
beginning due to high anode current. This part was very surprising to me
when I first came across it.

I don't remember the gate current we supplied during the hard-fire time, but
it was in the amps (at 10 or more volts). The back porch was in the hundreds
of milliamps.

Good luck.

John
 
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