Transcript The_Pulse_of_the_Silent_Thriller

A PROPOSAL TO PULSE
THE MAGNET BUSES TO
VALIDATE SPLICE
QUALITY
H. Pfeffer
3/7/09 Version 4
THE PROPOSAL
This is a proposal to use the main dipole power supplies to pulse high
currents through the quad and dipole buses (but not the magnets) in order to
measure the capacity of the splices in the buses to withstand the necessary
energy- extraction current waveforms. The pulses will be introduced with the
cryo-temperature at above 12K, so that all SC wires are normal and the
copper must handle all of the current.
For example, if the quad circuits are to be operated at 8 kA, this technique
will send a current impulse through all the splices which steps up to 8 kA and
then decays with a 15 second time constant. Of course, we will slowly raise
the amplitude of the impulse and stop at a lower level if we detect the partial
warmup of a splice. This lower level will then set the safe operating current
level of the bus.
THE CIRCUIT
The feature of the magnet circuit that enables us to
do this is the fact that each magnet coil is bypassed
by a cold diode. Once the diodes have been made
to conduct, their forward voltage drops to 1.5 volts,
and we can drive high currents through them and
bypass the high-impedance magnet coils. The coils
themselves will conduct only 170 Amps, as they are
connected across the 1.5 volt diodes, and their cold
resistances are 8.7 mohm.
The quad circuit is diagrammed in Fig.1. The
voltage required to drive the circuit to 8 kA is
approximately 130 volts; 75 volts are required
to overcome the fifty 1.5 volt diode drops; 55
volts are required to drive the cold, low
resistance bus. We propose to connect the
190 volt dipole power supply to the quad bus
for this test. An additional “ignition supply”, of
rating 350V/30A (bypassed with a 10 kA
diode), is placed in series to overcome the 6V
drops of the cold diodes and put them into
conduction mode.
The required current rating of the ignition
supply is dependent on how much energy is
needed to rapidly ignite the diodes. We will
examine the diode test records from Block 4,
where a small ignition supply was also used,
to estimate the power supply specifications.
Quad
5.6 mH
8.7mΩ
THE MEASUREMENT
To evaluate the state of the splices, we first run at 100A DC for ten seconds and use the new bus- measuring system
to measure all of the bus resistances. Then we put the impulse through the bus and return to the 100A level to repeat
the bus measurement.
If a splice has undergone thermal runaway during the impulse, its temperature will increase to approximately 300K
and its resistance will increase by a factor of 100. Thus a 40uohm (excess) splice, which constitutes an error of .4% of
the bus segment, will become an error of 40%, and will be easy to see.
If we merely increase the amplitude of the impulse waveform after each successful test, we would risk overheating the
splice, since at some amplitude the thermal runaway will trigger partway through the slightly-increased waveform. The
energy deposited at that point, though small compared to a normal dump, might be enough to damage the joint.
Pre-Ignition @ 6355 Amps
Verweij, sim. RQ-143-C
Ignition @ 6510 Amps
Verweij, sim. RQ-143-B
Waveform Testing Sequence
To avoid this, we would increase the amplitude of
the waveform, but only pulse for the first two
seconds. If no splices trigger, we would then go
further down the waveform in a time increment that
can only supply 30 more MIIT’s . See fig.2.
THE DIPOLE BUS
To effect the same measurement in the dipole bus,
we would need a higher voltage power supply in
order to overcome the hundred and fifty 1.5V diode
drops (225 volts). To do this, we propose putting
two dipole power supplies in series, using six 500
mm squared cables to span the 500 meter distance
between them. The 380 Volts then available will be
adequate for the load.
SPLICE
PROTECTION
In addition to running a sequence of ramps and measurements selected to avoid generating a high-dissipation splice incident,
it would be advantageous to have in place a protection system to remove the energy if unexpected splice-heating is detected.
The natural choice to detect such an event is the new QPS bus monitoring system. Unfortunately, the dynamic range of the
bus amplifiers is 12 mvolt, whereas the bus voltages in this experiment will be about 1 volt.
Another possibility is a variant of the original global busbar monitoring system. Here, instead of adding the two half-circuit
voltages and subtracting from them the two scaled reference magnet voltages, we just subtract one of the half-circuit voltages
from the other. An overheating splice on one side will make its voltage larger than that on the other side. This is the equivalent
of the symmetric quench detection system which compares the voltages across different magnets. The issue here is the
signal/noise ratio, and how small an error we can detect with what bandwidth. Although the DC balance between the two
sides will not be precise because it depends on variations in diode voltage drops, the system can be set up to look more
sensitively at rapidly occurring changes in the comparison voltage.
A related approach would be to measure the ratio of V/I, and look for rapid, small changes in the load resistance.
Once the detection has been made, the current in the buses can be removed with a time constant of less than 60 ms.
Unfortunately, the ignition waveform shows that detection at the 100 mv level is needed to protect the splice. This poses a
difficult challenge to either detection system. This means we must detect a signal of 100 mv over 3 km. Happily, our nQPS
bus detection system detects 300 µv over 30 meters, which corresponds to 30 mv over 3 km.
Ce n'est pas sorcier!
LABORATORY
SPLICE TESTS
We are setting up the FRESCA vertical cryostat
test stand to make similar pulsed measurements
on a splice built with a deliberate 40 mm gap.
These measurements will give us information
about what to expect if we do this pulsing in the
tunnel.
THE BENEFITS OF
THE PROPOSAL
There are two main benefits that would come from this effort. The first would
be to discover the current ratings of those buses that had not been measured
at 300K, and whose splice characteristics are not reliably known.
The second benefit would be that of measuring the rating of the sectors that
HAD been measured warm. This would confirm the effect of the repairs that
we have made in these sectors.
IMPACT ON
OPERATIONS
If this system works properly, it would allow us to start accelerator
operations at a level expected to be quite safe and, once the machine is
running, to make these measurements and determine how much higher in
energy we can safely go without unduly risking a splice failure. The
decision would be based on measured data rather than statistical
calculations.
ISSUES
OUTSTANDING
1)Temperature stability prior to impulse. Splice ratings are steeply dependent
on starting temperature.
2)Feasibility of runaway-splice detection.
3)Diode ignition energy requirement.
4)….please add here.