Power supplies for superconducting coils
The operation of a superconducting coil brings special demands on the power supply equipment used for this purpose. The Rosenheim company FuG offers such units with output currents up to 10kA and more recently for power up to 500kW.
FuG power supplies for superconducting coils are designed extremely robust as a linear regulator with a thyristor pre-regulation. Thus the same output stage can both generate and stabilize a voltage at the output in order to increase the current and also regulate the counter voltage in order to decrease the current.
So a controlled discharging process of the coil is made possible, which shows substantially identical characteristics and stability data to those of the excitation process. A further advantage of the serial regulating power stage is a very low ripple, achieved without the requirement of filter capacitors in the output, which otherwise could form an undesirable resonant tank circuit with the connected coils.
Both the energizing as well as the de-energizing of the coil can be done either by means of a constant voltage or by specifying a current ramp. For this, and especially if slow current changes in a precisely defined manner are required, FuG offers the optional ramp generators that can be integrated into the units. The speed of both increasing and decreasing of the current is either fixed, or can be adjusted continuously.
In order to actually change the current in the coil, the superconducting current path must be interrupted at one point. For this FuG provides an optional auxiliary power source, to operate a thermal short circuit switch.
In case the unit has to be computer controlled, one of the available interfaces of the FuG ProbusV system, e.g. USB, IEE488, LAN (Ethernet) RS232 optically or electrically can be used. All of these interfaces have also integrated a ramp function. Input of set point values via an analog interface (depending on customer 0-10V or 4-20mA) is also possible.
The flexibility of FuG in meeting customer demands is also reflected in the variety of configuration versions. If the operating parameters to be easily recognizable even from a distance, large LED displays are available. In case of very complex features, control and monitoring functions, the installation of a FuG multifunction display is advisable.
Safety in case of emergency
Another important requirement of such power supplies is the protection of personnel and goods from the partially enormously high stored energy that can be released in case of an accident.
An unwanted phenomenon in operation of a superconducting coil is the so-called “quench”. The material will lose the property of superconductivity abruptly when the temperature rises above the critical temperature, when the critical current density is exceeded or when the magnetic field strength exceeds a certain value. With the FuG-quench detector (optional feature) FuG power supplies are able to detect the quench, so that the energy can be removed in a controlled manner and destruction in the cryostat can be avoided or kept within limits.
Safety in case of mains failure
Both in an event of quench as well as in case of threatening failure of the cooling, it is necessary to de-energize the coil quickly. The latter should work also when the mains power fails. However it should be noted, that a plain break of current flow in a highly inductive coil never should happen. A simple “off” by interrupting the circuit would lead to an unlimited increase in the voltage across the switching element. After the same has been destructed, an electric arc will be ignited, which extinguishes only after exhausting all the energy which was stored in the coil.
FuG power supplies can control it the voltage across the output stage also during failure of mains, so that a regulated de-energizing is ensured. An over-temperature protection prevents hazards of the components.
If the shutdown speed available by de-energizing by means of the power stage is not sufficient, a high-voltage discharge resistor can be installed. This allows a discharge with the upper limit of voltage which is acceptable to the components of the coil. This might be possibly up to several thousands of volts.
Currently, FuG designs a fast discharge circuit for a superconducting coil with 3.5 GJ of stored energy. The discharge resistor consists of 20 tons of cast iron, which during emergency shutdown will be heated up to 500 °C within 12 minutes.
Dr. Wilfried Attenberger
Dipl.- Ing. Thomas Schmidt
FuG Elektronik GmbH