Among the power quality compensation devices, Superconducting Magnetic Energy Storage System (SMES) has the advantage of high speed and efficient large energy output to mitigate short time voltage fluctuation and sag, which causes downtime of a machine or critical load. Initially a prototype unit (<1.0MJ) is being aimed followed by two more units of 5MJ and 30MJ respectively. This prototype system will be primarily used for developing the technology of power conditioning system required for SMES system. Here we highlight development of a protection system which detects the Quench condition of super conducting coil followed by switch OFF the power supply of the magnet.

Different quench detection circuits are studied in the references [1, 2, 3, and 5], Particular cases for quench protection are discussed in references [4]. Superconducting magnets are protected against damage if they revert to the normal condition causing loss of superconductivity which is called Quench. Mechanism of sensing a quench condition is detected by a hardware circuits that monitor voltage taps strategically located on the solenoid and these voltages are accounted for by using bridge circuit that compare half of the solenoid voltage with respect to its center tap. In case of quench condition, the change in resistance initiates unbalanced condition which subsequently reflected as a voltage signal. Also, due to loss of cooling, the increase in voltage drop across the vapor cooled current leads, which are directly connected to superconducting solenoid, could develop more dissipation to the cryostat. This Quench detection system is detects the quench and generates a trigger signal provided the quench sustained more than the validation time. This trigger signal finally switches off the power supply and closes the circuit for dissipation of the stored energy through the external dump resistor. A schematic circuit diagram of quench detection system is shown in figure 1. The circuit is divided into several stages: Input sense, Isolated absolute value, Voltage comparator, Latch logic, and Output stage.

Power supply designed for use with superconducting magnets is often built with quench protection. A quench is accompanied by large voltage and currents of polarity opposite to that of the charge current. The protection system controls the large 1500A DC contactor that direct the current to the magnet coil through switch S1. Switch S2 controls the switching of the shunt contactor. Figure-3 illustrates the control scheme of the contactor. Combination of S1 and S2 initiates the isolation of power supply and energy dump through dump resister (20mΩ).

This paper described an electronic quench detection system by sensing unbalanced voltage across the centre tap of magnet coil. This system gives a signal which switches OFF the power supply of the SMES magnet coil when quench or over lead voltage drop is detected. This unit can be used for any SC magnet. This unit has tested with OFF line SMES magnet at VECC.

Authors are thankful to Prof R. K. Bhandari, Director of the VECC who has allowed carrying out the development work for SMES project. Authors are also thankful to all the staff members of HVPS Section for their valuable support.

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