Download DL Quench Detection System

DL Quench Detection System
The DL quench detection system comprises of 3 main sub-units which are housed in 3, 19inch crates.
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The Quench detector crate: Contains 10 QD channels which detect a quench by resistive
voltage measurement and on detecting a quench, triggers associated magnet protection.
The Power Supply crate: This provides 48Vdc, ±15Vdc and 12Vdc power to the electronics in
the Quench detector crate and associated relays. The power supply crate is supplied by
230Vac mains and the emergency backup crate.
The emergency backup power crate: This consists of a number of batteries which are
charged from a 230Vac mains supply and will provide a no-break supply to the QD system
during a mains disruption.
Voltage Tap Input
Amplified
Isolated & Filtered
•3 Voltage taps
•Resistive
measurement
•Central tap amplified
•Outer voltage taps to
provide reference
•Signal is isolated and
a second order low
pass filter applied.
Quench Triggered
Relay de-energised
Compared
•Relay chain open
circuit
•Magnet Circuit
breakers opened
•If signal is larger than
the threshold level
•System Latched in the
quenched state
•Window comparator
used to compare
signal- nominally
±4.5V.
Figure 1: Quench Detection Channel Flow Diagram
Description of QD System
The DL quench detection circuit is based on a proven and reliable 30 year old design concept from
the Paul Scherrer Institute (PSI). The QD system adopts a modular topology and each channel
occupies one card, which allows convenient and timely replacement of independent cards. Each QD
channel uses 3 voltage taps to monitor the voltage differences progressively across the SC magnet.
The 2 outer voltage taps for each channel are averaged to provide a reference point. The central tap
is amplified with respect to the reference point and then compared to a threshold level.
The 2 outer voltage inputs to the card can be shorted via a link on the back plane to allow the QD
card to measure an absolute voltage across two voltage taps. This enables the QD channel to trigger
a quench on an absolute threshold as opposed to a relative threshold. This method of quench
detection is not as proficient as using a relative threshold however it is useful for monitoring and
protecting magnets that are non-symmetrical, which can make relative threshold quench detection
difficult. This method has not been employed for the decay solenoid or the focus coils however it is
currently employed on the spectrometer solenoid as this is a non-symmetrical magnet.
Each input channel has a chopper stabilised amplifier to give very high gain without increasing offset
voltages over time or temperature changes. This high impedance is important and ensures that only
a small amount of current is taken from the solenoid during operation. The Amplifier is protected
from any voltage spikes by two opposing diodes connected across its input pins, this will minimise
the input voltage to less than ±0.7V.
This amplified signal must be isolated from earth as the voltages generated across the solenoid
during a quench quickly become excessive. This is achieved using an isolation amplifier that
maintains high accuracy, unity gain and complete galvanic isolation.
The isolated amplified signal is passed through a low pass filter which will attenuate any signals with
a frequency greater than 30Hz. This amplified filtered signal is then provided to the front panel of
the QD card to allow monitoring of each set of voltage taps. Each QD channel is rated for 1.5kV
galvanic isolation between channel and earth and 1.5kV channel to channel with resistive protection.
This arrangement allows any changes in the resistance of a section of the SC magnet to be detected
and compared against a threshold level.
Figure 2: Quench Detection Crate
Figure 3: Quench Detection Card
The QD card has two methods of setting the threshold level.
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Through a selectable resistor which will set the threshold voltage to a fixed level.
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Through a variable resistor which can be adjusted to set the threshold voltage
between 10mV and 600mV.
When a quench is detected several actions are initiated:
•
A signal is sent to activate the magnet protection circuit (dump circuit);
•
A signal is sent to the control system to inform operators that a quench has occurred;
•
A signal is also sent to magnet power supplies and other equipment that may be affected by
the quench to power down.
The power supply crate contains commercially available power supplies with built-in n+1
redundancy. If any of the 48Vdc, ±15Vdc or 12Vdc supplies should fail, indication is provided on the
front panel and their associated parallel power supply will support the load.
The Quench detector system has been used with the decay solenoid and focus coil since early 2013
and has successfully sensed over 20 quenches whilst training both FC’s during training in R9 and
finally in the Mice Hall.
Testing has been carried out to show the response of the quench circuit to be well damped with low
overshoot to a step response.
Also the propagation time of the circuit has been measured:
Finally the time taken from voltage rise on the magnet to the contactors opening has been
measured: