The CMS Detector Power System

The power system for the on-detector electronics of the CMS Experiment comprises approximately 12000 low voltage channels, with a total power requirement of 1.1 MVA. The radiation environment inside the CMS experimental cavern combined with an ambient magnetic field (reaching up to 1.3 kGauss at the detector periphery) severely limit the available choices of low voltage supplies, effectively ruling out the use of commercial off-the-shelf DC power supplies. Typical current requirements at the CMS detector front end range from 1A-30A per channel at voltages ranging between 1.25V and 8V. This requires in turn that the final stage of the low voltage power supply be located on the detector periphery. Power to the CMS front-end electronics is stabilized by a 2 MVA uninterruptible power supply (UPS) located in a CMS surface building. This UPS isolates the CMS detector from disturbances on the local power grid and provides for 2 minutes of autonomy following a power failure, allowing for an orderly shutdown of detector electronics and controls. This paper describes the design of the CMS Detector Power System, reviews the process of its installation and commissioning, and discusses issues of power distribution common to current-generation collider detectors. I. OVERVIEW OF THE CMS DETECTOR CMS is a general-purpose detector at the LHC accelerator at the CERN laboratory in Geneva, Switzerland. A description of this detector is beyond the scope of this paper, but may be found in reference [1]. A. Requirements of CMS Detector The front-end electronics of the CMS detector has 12090 low-voltage channels, requiring 1182 KVA of power. The steel yoke structure of the CMS detector serves as a flux return for the 4-Tesla solenoid inside the detector structure. Since the magnetic field of the solenoid is large enough to drive sections of this steel yoke into saturation there is an ambient magnetic field that can reach up to 1.3 KG outside detector in the regions where low voltage power supplies are mounted. Typical commercial low-voltage power supplies are not designed for operation in a magnetic field and many have been noted to fail destructively at fields above 150 Gauss. In addition, the high radiation environment inside the CMS experimental cavern imposes constraints on the design of low voltage supplies from the standpoint of semiconductor displacement damage and single-event effects. Together, these constraints rule out the use of general-purpose commercial power supplies. Typical front-end current requirements are 1 to 30A per channel, at voltages from 1.25 to 8.0V. Since cable power dissipation must be kept within reasonable limits, the placement of the final power supply stage is constrained to be within ~10m of the front-end electronics, that is, on the detector periphery. B. CMS power distribution requirements The power cable paths between CMS on-detector systems and the power distribution area in the adjacent equipment cavern are typically 100 to 140m in length. Power to the detector is supplied at 380 and 230 VAC (three-phase) and at 385 VDC. No neutral is distributed. The CMS detector power system serves all of the low voltage power needs on the detector from a single distribution network. Although there are other power distribution networks at the CMS site, there are no persistent connections between the CMS detector and any of these other networks. This single-source powering scheme enables a unified earthing structure for the CMS detector and simplifies considerations of detector response to disturbances in power distribution. C. CMS power distribution system architecture The detector power system is powered by a 2 MVA uninterruptible power supply (UPS) installed on the surface. The UPS provides for at least 2 minutes autonomy in the event of a power failure. This length of time is sufficient to provide for an orderly shutdown of subdetector power systems. The UPS powers a bank of 6 isolation transformers located underground in one of the CMS caverns. The transformers are apportioned by subdetector and geographical detector region. Each transformer feeds one or more power distribution cabinets containing circuit breakers, monitoring equipment and programmable logic controllers. Power from an individual circuit breaker channel can be turned on and off via a remote control system, but in the event of a fault condition the circuit breaker must be reset manually. This is a deliberate design choice to prevent casual responses to fault conditions. The isolation transformers consist of four 230V and two 380V three-phase units, each containing an interwinding electrostatic screen. Static compensators are connected to selected distribution cabinets in order to provide power factor correction (PFC) for certain subdetectors (Fig 1.)