Changing the choke at ISIS

The ISIS neutron and muon source at the Rutherford Appleton Laboratory in Oxfordshire is a world-leading centre for research in the physical and life sciences. As part of the facilities upgrade and refurbishment programme, the 1MJstorage choke, which forms part of the main magnet power supply system, will be replaced with a number of smaller units.

After almost 15 years of effort the project is finally coming to fruition with the new choke system in place and expected to be connected to the source later this year.

It seems that 'retro' is very much in style at present, but in this project 'retro' was a matter of necessity rather than choice. With the electrical system at the facility coming on for 50 years old it is all based on established technology, and replacing single components entails carefully matching legacy designs with the best available on the market without changing the nature of the system.

The unusual project meant that no single institute or company had the capability to design or build a replacement, and it became a pan-European collaboration. Imtech-Vonk in Holland contributed engineering consultancy; their sub-contractor Trench from Austria carried out the detailed design, construction and assembly and tests; while Enpay in Turkey made the cores.

Once the design had been chosen from three scale-models, the new choke evolved through several iterations before a final design was chosen. Two prototypes were built, tested and reworked until a satisfactory choke emerged. The new chokes currently under test in the new choke room are the end product of over a decade's work across Europe.

The ISIS facility, based at the Rutherford Appleton Laboratory in the UK, provides intense pulsed neutron and muon beams for condensed-matter studies.

The present storage choke, which consists of a split secondary winding transformer, is incorporated into a series-parallel resonant circuit known as the 'white circuit'. This ensures that each magnet receives identical currents, but is not subjected to excessive voltages.

Although the storage choke is essentially a transformer, its secondary magnetising inductance is relatively low and a precisely defined value. The old choke, named 'NINA' is to be replaced by ten smaller units, which will eventually replace the present equipment.

The refurbishment of the main magnet power supply (MMPS), and in particular the storage choke upgrade, is part of an obsolescence programme at ISIS. Most of the power supply components were manufactured in the 1960s and are reaching the end of their life.

Wherever possible, the upgrades need to be performed with as little disruption as possible to the operation of ISIS. Altering the basic methodology of the power supply system would cause an extended shutdown period as the old systems are removed and the new ones installed and commissioned. It was therefore decided that a duplication of existing equipment, albeit with modern technology, would mean minimum downtime and would lessen the technological risk.

The plan that the project leaders landed upon was to split the current choke into ten smaller units. This lessens the manufacturing complexity of such a large device and introduces the opportunity to have a spare in the event of failure.

The current 1MJ storage choke was part of a previous synchrotron experiment, NINA, at the Daresbury Laboratory, UK. It was incorporated into the current system when ISIS was built as part of the white circuit. The choke consists of ten interleaved primary and secondary windings. Each secondary winding resonates with part of a capacitor bank at precisely the frequency of the synchrotron cycle, 50Hz,in the same way the magnet coil resonates with a further part of the capacitor bank.

Technical specifications

One of the secondary windings is split at its centre point allowing the insertion of the dc bias power supply and to define the earth potential of the system. The dc power supply and filter components provide a 662A bias level for both the choke and magnet coil.

To ensure that each secondary circuit (and hence, each magnet set it powers) has the same frequency and phase relationship there needs to be good coupling between primary and secondary windings. The primary windings are connected in parallel and form a close coupled circuit with each resonant circuit. This coupling method reduces the leakage inductance and stray magnetic fields.

To make up the ac losses in the system, an alternator power supply is connected to the primary windings. The turns ratio of the primary to secondary windings is 1:4.The rated secondary ac rms voltage is 14.4kV at 1022A.The nominal peak energy stored is 0.99MJ, 310MVA.

The choke windings and core weigh 90t and with the oil tank, the weight is in excess of 120t. The choke has been operating for 30 years of continuous service but is a major cause of concern with the reliability of the system. The state of the insulation is unknown and the tank leaks oil which continually needs to be replaced. If the choke were to fail, the ISIS facility would be down for an unknown period of time and repair would be difficult.

Upgrade

The proposed upgrade replaces the current 1MJ storage choke with 10 off 100KJ, 160mH units. As placement chokes will have to be compatible with the existing white circuit, there will be many key features which are historic to the existing system and whose values are therefore fixed.

The energy resonating between the capacitor bank and the choke will be stored in an air gap which will in turn, by its dimensions, determine the inductance of the unit. Studies have revealed that by using multiple air gaps, the stray magnetic fields in the winding region can be reduced and therefore not incur large eddy current losses in the copper conductors. Each air gap will be screened by a winding and so external stray magnetic fields are also kept to a minimum.

Originally, the chokes were going to be 200mH, but it was deemed appropriate to lower the inductance level to 160mH which would result in reduced core mass and therefore lower cost. The effect of this would be that the capacitance needed in the system would increase and the cost of this would be lower than having the higher inductance value.

In order to facilitate the design of the proposed choke system, three 40mH scale-model chokes were ordered. The models were designed to provide as much information as possible about stray fields and losses, magnetic iron saturation levels, conductor cross section and winding arrangements.

In the initial stages of the design process, it was hoped that the models would provide a detailed scaled representation of the magnetic fields present in the larger160mH chokes. Finite element models were produced of atypical C core design with distributed air gap. The initial studies were performed in two dimensions, the assumption being that the third dimension is infinite. Losses can then be worked out per unit length.

In order to produce a scale model, the third dimension (the length of the core) would be scaled accordingly. This was acceptable for a rectangular core, but for a cylindrical core design it was almost impossible to simulate using finite element software in 2D. A 3D approach would be needed. The magnetic iron cross section would be reduced to attain an inductance of40mH, but the magnetic field distribution would almost certainly be different to the 160mH version.

The scale models were tested on site to see if they met the specifications laid out at the tender stage. They were also used, in conjunction with a spare capacitor bank, to set up a small White circuit.

The model chokes were tested to ascertain the suitability of each design for the main magnet power supply. Testing took place in a specially constructed test bay at RAL and contained a scaled down version of a small White circuit.

The chokes were tested in turn applying various voltage and current levels suitable for each test: the chokes that were not on test provided the load for the system.

With the testing completed, the design selected and the chokes installed it is now just a matter of waiting for a suitable break in the ISIS schedule to install and test the new devices.