Essential lessons from commissioning of the 28 EE systems of LHC sector A corrector circuits

600 A EE –sector 45 No non-conformities were detected on any system. The performance of all facilities met the specifications. Only a minor problem was discovered at the level of the acquisition of certain voltage signals. Reason: Inadequate filter parameters (not fast enough (5 ms), will be reduced to 0.5 ms for the next sector test). Tunnel measurements (routine tests for verifying the correct performance of the switches) were performed -at PLI3.b1 (200A) for all circuits, with a few exceptions -at PNO.b1 (550A) on three RCD circuits. The measurements on the equipment are vital for the evaluation of the long term stability of the switch-gear as they will confirm the correct operation of the commutation assistance and the arc suppression. This information is not obtained through the P.M. The system can only be released for operation after approval of this step.

600 A EE –sector 45 However, the measurements can be simplified and rationalized: Only performed at a single current level (Inom, better signal-to-noise ratio, not required for approval of the commissioning steps) Grouped so to cover all concerned circuits of a powering sub-sector Presence in tunnel only required to change instrumentation between circuits. Remote controlled oscilloscopes are used. Expected rate: 10 circuits/hour (3 hours total for a sector). Grouped closing of switches for commissioning of next sectors: The requirement has been defined by MEI/pe and a Macro-routine is presently being prepared for sector 5/6 by AB/CO. A sequential procedure is required in order to avoid simultaneous current rush from UPS (closing of extraction breakers is by motor-drive, typically 40 A / system for 300 ms. Time between two circuit closings: 5 seconds.

13 kA EE –sector 45 Three incidents occurred during the commissioning, fortunately without compromising the protection of the circuits: 1.Routine measurements of switch opening times allowed the detection at low current of the absence of the fast release on one of the 32 extraction switches (RB, UA47). Reason: A solder was forgotten in the pulsed power line inside the breaker! Repaired in-situ. 2.The DQRQF extraction resistor showed too low resistance (2.4 mΩ instead of 6.6 mΩ). The unit was replaced (7 hours of hard work, acknowledgment of highly appreciated assistance by TS/HE and TS/CV). Reason: Short-circuit between two resistor packages, caused during road transport (vibration), but clearly having its origin in poor manufacturing quality and/or insufficient quality assurance at the manufacturer (wrong type (or missing ?) insulating spacers between packages and supports cut too short, allowing displacement during transport). Repair is ongoing (not complicated). All other DQR’s have been checked and found correct. So, it looks like a single event.

13 kA EE –sector 45 (contd) 3.-A spurious trigger occurred in the back-up system, which is implemented for the case that an opening command to the extraction switches is not being executed. This scheme consists of the firing of a pre-selected set of quench heaters (61 dipoles per EE and 13 quads per QF/QD circuit) so to get the same time constants as for normal extraction. -The incident occurred during preparations for the replacement of the DQRQF resistor unit, at the moment where the UPS power line (‘Normabar’ distribution panel) was moved, probably causing a short supply interruption. -Although the event happened under unusual circumstances, an interruption of the UPS line cannot be excluded and the back-up system must be insensitive to such a (rare) event. -The heater firing had no real consequences as there was no circuit current. However one single quench detector triggered – sign that quenching of persistent currents was sufficiently asymmetric to be detected? -A spurious firing of the pre-selected quench heaters during a fast discharge may lead to a FULL SECTOR QUENCH as the local quench detectors of the non-quenching magnets are likely to trigger due to large dI/dt. This may even lead to dangerous reverse voltages across the diodes.

13 kA EE –sector 45 (contd) -It was decided to continue commissioning without the back-up system, based on the arguments, that -the introduction of the back-up scheme was decided in 2003, when no real experience with the extraction switches was available. Reliability calculations showed a probability to suffer an SOF failure of between 0.03 and 3.7 % during 20 years of operation. It was based on poor statistics from the Russian manufacturer. -Since then 20 switches of the series have individually been subjected to endurance tests (exceeding 88 kc total). Not a single failure has been observed. -The complete DQS switch array has a 4-fold redundancy of the release system (two breakers in series, two independent opening features). -It has been possible to reproduce the false trigger. It occurs when the UPS power is re-established to the Interface Module after the end of the autonomy (15 sec) of the local energy reserves of the controls crate. Although equipped with the necessary pull-ups one (or more) of the signals to the FlashEPROMS has a wrong state.

13 kA EE –sector 45 (contd) Solutions: –In addition to preventing the false state of the incriminated signal(s) we shall consider a reduction of the number of heaters to be fired during an SOF. The present situation is very conservative (see next slide). As an example, an increase in the discharge time constant from 104 s to 170 s would allow a reduction from 61 to typically 32 magnets to be quenched, while the diode busbar temperature would still remain acceptable. However, before proposing this solution, detailed calculations must be made taking into account contributions from the bb contact resistance and the quench margin must be carefully evaluated. Until then, we suggest to continue without the back-up system activated (very easily disconnected).

Maximum component temperatures as function of discharge time constant