P
Paul E. Schoen
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
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