Quench Heater Supervision for MB Status of System Integration

Quench Heater Supervision for MBStatus of System Integration R. Denz TE-MPE Technical Meeting July 26th Acknowledgements: F. Burkart, K. Dahlerup-Petersen, V. Froidbise, J. Mourao, J. Steckert

Motivation for enhanced diagnostics – quick reminder • The upgrade is driven by the intention to reduce the risk of damage to the quench heater circuits • The present system monitoring only the discharge voltage is not sensitive enough to detect all fault states of the quench heater circuits especially failures of the heater strips. • All of the few quench heater faults observed so far during LHC operation could be mitigated by disabling the respective heater circuit and switching to a low field heater. • There is however a risk of a quench heater fault requiring at least an exchange of the magnet (short to coil, compromised electrical integrity of the magnet) • In order to minimize the risk the test discharge voltage has been reduced to ~10% of nominal (particular test mode) • For the LHC after LS1 this risk will slightly increase basically due to the magnet training campaign • The enhanced quench heater supervision is “supposed” to reveal precursor states of a potential failure G. D’Angelo

Fast acquisition of voltage and current during quench heater discharges • As just presented by Jens • Simultaneous measurement of the discharge voltage and current (using special current transformers) and higher sampling rates than used in the present system (200 Hz  40 (80) kHz) • Will at least allow to detect faults not visible in the present supervision system • Further studies required to evaluate the full potential of this method • Type tests currently ongoing in 281 • Circuit design and system integration almost complete • Component ordering to be started and production to be launched soon • EP counts on EM support • System will sent raw data; evaluation left as a home work to the user!

DC measurements – detection of eventual pre-cursors • Based on the experience with the high precision systems for the splice and current lead protection a R&D campaign has been conducted over the last 1.5 years to check whether a precision DC measurement of the quench heater resistance could reveal any potential faults prior to the destruction of the heater strip. • A description of quench heater failures observed during magnet production and testing can be found in AT-MCS Technical Note 2008-02 (EDMS no: 889445). • Apparently it concerns only one third of the installed MB magnets. • There are no particular worries about any other magnets equipped with quench heaters apart the request to lower the discharge voltage for the D1 magnets.

DC measurements – quench heater strip basics I • Dimensions (LHCMB__A0124 and LHCMB__A0125, IT2768) • and technical specification IT-2678. • : • Resistance @ warm • Resistance @ warm

DC measurements – quench heater strip basics II • Resistance @ cold

DC measurements – measurement stability I • In 2011 two type tests using prototype systems have been conducted in LHC during technical stops TS#3 and TS#4. Hereby the supervision layer of the nQPS has been used for data transmission. During this type tests the resistance changes of the warm instrumentation cable have not been compensated. In order to reach stable conditions the prototype systems required about 1 hour of warm-up time after being switched on. • The 2nd test has been conducted with bipolar currents to remove the influence of eventual thermal voltages but the achievable precision did not improve.

DC measurements – measurement stability II • In order to achieve long term stability it would be necessary to compensate as well the temperature dependent drift of the warm cable. The best way would be to use one of the not used twisted pairs as reference (requires modification of the IFS box). This approach can be as well used to remove any offset drift of the analog input stages (not gain errors). Conclusion: ∆Rmin ≈ 2 mΩ

DC measurements – broken heater signature • Measurements on the copper cladded part performed by Joaquim indicate that a measurable change of resistance would be correlated to a heater cut to about a length of 8 mm (only if good state is known!). • Recent tests by Jens and Florian show that a heater strip will be destroyed during a quench heater discharge if the cut length exceeds 12 mm. Very narrow range for the detection of an eventual fault!

DC measurements – first conclusions • The type tests show that a stability of the resistance measurement with ∆R ≈ 2 mΩ is achievable. At cold this is however only sufficient to detect eventual problems in the non copper cladded segments of the heater strip (23.1 % of the total length). • It is expected that the effective long term (over several months) stability is less good; with ∆R ≈5 mΩ one would have difficulties to see any faults at all. • Significant results for the copper cladded parts could only be achieved with an overall stability of ∆R ≈100 μΩ. This would require a four wire measurement of the heater strip, which of course cannot be realized with the currently installed cold masses. • Unfortunately the potential weak parts of the quench heater strip are suspected to be copper cladded. • Another non-trivial problem is the lack of reference measurements, i.e. one cannot exclude that some of the heaters are already partially damaged.

DC measurements – equipment safety, system dependability I • In contrary to the fast measurements using a current transformer the DC measurement system requires galvanic connections to the heater discharge circuit.  • All four discharge circuits are routed through one connection box (“Crawford Box”, now officially DQLIM) and one warm cable link to the magnet IFS box. • Under all circumstances an eventual discharge into the measurement equipment, an interconnection of the different heater circuits or a flash-over inside the DQLIM must be prevented. • These types of faults could lead in the worst case to a damaged magnet requiring an exchange. • The present design of the DC measurement foresees one dedicated measurement system and current source per discharge channel (= 4928 systems) • The four systems can be integrated on one circuit board (see recent presentation by Vincent). 

DC measurements – equipment safety, system dependability II System will remain disconnected and switched off during powering and operation with beam. About 1 hour of pre-heating will be required prior to a precision measurement.

DC measurements – equipment safety, system dependability III • The safe de-coupling of the voltage measurement is supposed to cause no problems if done properly • The safe de-coupling of the current source remains challenging: • De-coupling with diodes and relays  fine but relays MUST work as the proper earth connection of DQHDS power supply is notguaranteed • Does not allow bipolar operation but probably not necessary • State of 9856 relays  QPS_OK  a lot of “fun”isguaranteed • De-coupling with relays, resistors and crowbar protection for source • Relays much less critical but 50 V stabilized current source required! • Could be based on lead batteries … • De-coupling with fuses and relays • Discarded as fuse must react very fast …

DC measurements – further conclusions • The systematic implementation of a DC measurement system on all MB heater circuits represents a major task an may have an serious impact on the overall system performance. • Compared to the very limited gain in additional information about the state of the quench heater circuits it is not recommended to install these systems. • In case an installation of these systems remains on the wish-list, the risk analysis must be further extended basically due to the large number of systems. • Even very unlikely faults will appear one day … • Potential conflict with LS1 schedule; lack of qualified manpower • As an alternative solution one could propose independent measurements performed by the ELQA team  • Systematic measurements at warm and after cool-down • Dedicated measurement for each quenched dipole magnet (during HC) • This solution is inherently safe and one might reach an even better resolution by using very sophisticated equipment and a direct access to the IFS box

DQLPU upgrade – system integration update • New items to be designed • Mechanical enclosure • Motherboard • Quench heater supervision unit DQHSU • Measurement during discharge • Crate supervision unit DQCSU • Power supply supervision • Power cycling options etc. • New developments (hardware & firmware) • DQHSU, DQCSU, motherboard • Firmware updates • DQAMC, DQQDL Upgrade of an existing and fully validated system. Protection functionality will not be changed.

DQLPU upgrade – supervision I • New status flags (present in logging and PM): • ST_HDS_FUSE_OK • DQHDS fuse • ST_PWR_ISO_A, ST_PWR_ISO_B, ST_PWR_GEN_A, ST_PWR_GEN_A • Redundant power supply supervision • ST_LOOP_CLOSED • quench loop supervision • New analog signals (present in PM only or zero in logging): • I_HDS_1, I_HDS_2, I_HDS_3, I_HDS_4 • Special commands allow experts to extract more information in case of interventions • During normal operation (LOGGING state) board A/B will toggle automatically • Time interval for toggling can be set remotely • Remote power cycle options still under evaluation

DQLPU upgrade – supervision II • PM transmission will change significantly • System will first transmit the magnet data to determine the culprit for page 1 • Automatic analysis tools to be adapted as well • Post operational discharge checks required • Fire heaters at low voltage to check continuity (feasible only after real quench but not trivial as two controllers involved) • Re-charge and test fuse • If everything ok re-establish QPS-OK signal

DQLPU upgrade – state of the union • Still a long way to go but so far no real show stopper identified. • Some other high priority tasks to be completed during LS1 • R2E upgrades, BIS and SMP, LINAC4 • Basically only specialist work …

Quench Heater Supervision for MB Status of System Integration