fast rundown 21.12.1998
A fast rundown from 3000 A to 0 A was initiated by a faulty water-flow meter
in the power supply of the magnet. The fast rundown takes about 20 min. However, the Labview controlsystem displayed zero current
immediately after the current rundown started. This triggered a chain of reactions which caused a quench about
3-4 min later.
Labview displays 0 current immediately after the fast rundown started.
About 3-4 min after the fast rundown started the quench detector fired at a current of still about 2300 A.
It triggered coil heaters and disconnected the power supply from the coils.
This action should avoid that all energy is dumped in a single coil.
Most of the energy is dumped in a quench resistor located in the power supply.
Only coil B (TC1208) got normal conducting immediately.
The coil temperatures increased by a few K only. The coils were cooled down again
until the liquid in the heat exchanger vessel connected to the coils was used up.
Without liquid in the heat exchanger the coil temperatures increased a second time.
After the He compressor started again (about 13:48), the liquid in the 150 l storage vessel was
used to cool the system again. The second warm-up is most pronounced for the last coils
along the He cooling pipe. The heat exchangers in between the coils were inefficient
due to the lack of liquid in the heat exchanger vessel. The small change of coil temperatures displayed
by the temperature sensors in the lower figure after the current went up to 3000A
(12:30-13:30) is an artefact caused by strong forces on the coils slightly
displacing some coils with respect to the sensors. This geometrical effect depends on the coil position
(upwards, sideways or downwards).
The temperature increase at the JT valve behind the last coil is most pronounced since its heat
capacitance is small.
Warming up the single phase He by a few degree only results in a strong expansion of the He.
The density changes from that of a liquid to the density of a 'normal' gas. An increase of the
pressure was the result and the He compressor was unable to pump all the He released within a few
min back into the buffer. It stopped due to overpressure after about 1-2 min at about 13:38 and
was restarted around 13:48.
The increase of the pressure in the 150 l storage container to about 2.2 b converted
the liquid into single phase He. Single phase He has about the same density as the liquid
but is a gas. The pressure dropped after the compressor started again and the liquid phase returned.
enlarged time scale:
The vacuum detoriated twice. The reason is most likely due to an increase of the temperature of the
current leads at their coldest point (frozen gas evaporates). It is caused by the lack of flow of cold He gas through
the leads (see below). The flow through the current leads is controlled by a PID controller which should keep
an intermediate lead temperature at its set point. Labview reduced this set point after the fast
rundown started by about 10K since the current reading showed zero current. This caused an
overreaction of the PID controller.
After Labview got zero current, it reduced the set point for the intermediate current lead temperature. As a result,
the flow through the leads first increased and then was reduced after the lead temperature
went below the set point (76K, see below).
Once the flow through the current leads goes to zero for more than 30 s an emergency cooling is activated,
driving the lead temperature distinctly below the set point (76K).
Summary:
The fast rundown of the current was caused by a faulty water-flow meter in the power supply of the magnet.
The quench appearing 3-4 min later was most likely caused by an overreaction of a PID controller
which should stabilize the flow through the current leads. The sudden change of the current read by Labview
(not related to the real change of the current) caused Labview to change the set point of the lead temperature
suddenly by nearly 10K. This confused the PID controller.
Solution:
Never allow the PID contoller to close completely the valve controlling the cold gas flow
through the current leads. Tests at zero current indicate that zero flow for only a few seconds result in an immediate warm-up
of the coldest point of the leads. Evidence for this behaviour is the drastic detoriation of the vacuum at zero lead flow.
Already a small flow avoids this problem.
Last update 25.12.98
by Wolfgang Koenig