5.3 SWITCHING

 

5.3.1 Requirements for the MACE high temperature superconducting switch


A primary function of the MACE HTS switch is to enable the rapid draining of current flowing in the primary coil so that energy stored in the magnetic field can be transferred to an external load. The HTS switch therefore must meet certain key requirements:

  1. Negligible “ON” resistance / relatively large “OFF” resistance

  2. Fast Switching Speeds

  3. Robust

  4. Rapid cycling

Switch Resistance:  A high “OFF” resistance in the HTS switch accomplishes two objectives. First, it minimizes the coupling losses between the primary and secondary coils by maximizing the impedance ratio between the primary circuit loop and the secondary circuit loop. Thus, a high impedance ratio ensures that energy can be transferred quickly and efficiently to the external load. Second, a high “OFF” resistance permits current flowing in the primary circuit to be effectively dissipated either through resistive losses through the switch itself, or by shunting the current to an auxiliary dump resistor whose resistance, while lower than the “OFF” HTS switch, is still large enough to dissipate the primary circuit current quickly. 

For the MACE Demonstration Unit, ISIT has calculated that ISIT will require an HTS switch with an OFF resistance of at least 20 Ω in order to drain most of the primary circuit current in under 10 msec while also maintaining an efficiency of energy transfer to the external load of greater than 90%.  ISIT believes it is possible to achieve this required OFF resistance by fabricating a HTS switch that is comprised of 10 m of “stabilizer-free” 12-mm wide HTS tape.  The term “stabilizer free” means that the copper jacket that usually encases the HTS tape is not present, and therefore the HTS tape does not have an alternate low resistance pathway for the current to flow when the YBCO superconductor is transitioned to normal. Reported resistance (while in the normal conducting state) for our HTS tape of choice for this component is 2 Ω /m of length. 

Fast Switching Speeds: The switching speed of HTS switches is ultimately determined by the speed of quenching, i.e. how fast the superconducting material in the switch can transition from a superconducting state to non-superconducting state. During the quench process, a region of the HTS material will suddenly transition to a state where the current (I) flowing through the material is greater than it’s rated critical current (Ic). The critical current is the maximum current the HTS material can handle at a given temperature and magnetic field before the material transitions to a normal conducting state.  Methods of initiating quench can include, 

    1. Injecting additional current into the circuit, 

    2. Thermally heating the HTS tape, or 

    3. Exposing the HTS tape to an increased ambient magnetic field. 

Most research into HTS switches thus far have focused on method 2, thermally heating the HTS tape so that its Ic threshold is lowered and quench is initiated. However, the main objections to the heating method are that it is relatively slow and can possibly damage the HTS tapes after repeated direct heating. 

A better method is described by Solovyov and Li (2013) in which they use a high-frequency RF field to drive the HTS tape normal over a relatively large area in a short time period. In this approach, an RF coil is interleaved between two sections of HTS tape (Figure 28) such that the energized RF coil (100 kHz) induces additional currents within the HTS tape such that I > Ic and the switch is driven normal. One advantage of this method is that the HTS tape can be uniformly driven normal very quickly (<5 msec) over a comparatively large area of tape (e.g. from 10s to 1000s of square centimeters of tape).

Our preliminary switch design consists of 10 m of HTS tape (i.e. >1,000 cm2 of HTS tape) source: Solovyov and Li, 2013 wound into a non-inductive bifilar coil (e.g., a coil that is wound in a way not to generate its own magnetic field). Interleaved between the HTS tape layers in the bifilar coil, a custom 5-m long RF coil tape will be interwound to trigger quenching in as little as 2 msec.

Figure 28

Figure 28.  A fast HTS switch using a coupled RF field to induce quench. 

Robust and Reliable Operation: Ideally, the MACE HTS Switch should operate 1000s of times over a period of years. This means that the HTS material must endure 1000s of quench cycles, each with its own period of resistive heating followed by cooling and renewed superconductivity. Early research into quenching of HTS coils indicated that HTS tapes could be easily damaged by quench/cool cycles. A common failure mode was found to be the delamination of the YBCO layer that occurred post-quench during rapid re-cooling with liquid nitrogen (LN2). However, further study (Takematsu, et al., 2010), indicated that delamination of the fragile YBCO layer occurred during rapid cooling not because of the material properties of the YBCO layer itself, but because the HTS coil itself was constructed using an epoxy impregnation technique to add structural strength. When the epoxied coil was rapidly heated and cooled, the epoxy layers bonded to the tape would literally pull layers off the tape.  Furthermore, it was discovered that when HTS coils were either dry-wound or impregnated with weakly adhesive paraffin (instead of epoxy) the HTS coils showed no failure or delamination of the YBCO layer even after multiple rapid cycles of heating and cooling. As the CAD design of the MACE HTS Switch progresses, a key design requirement will be to wind and support the bifilar coil with a method that that minimizes mechanical stress experienced by the HTS tape during operation while also allowing the surrounding LN2 to transfer heat away from the switch as quickly as possible.

Rapid Cycling: Based on our calculations for a preliminary HTS switch design, ISIT believes the MACE HTS switch itself will be able to fire, cool, and be reset for subsequent firings in under 1 second. The cycling time of the entire MACE Demonstration Unit will be somewhat longer since it will also depend on other factors such as charging time, telemetry gathering, safety checks, etc.

During firing, the MACE HTS switch will need to dissipate approximately 1.3 kJ of energy in under 10 msec, for an average power dissipation of 130 KW.  The MACE HTS switch itself will be immersed in liquid nitrogen (LN2) that is sub-cooled to 70 K. Thus, a small amount of LN2 will be vaporized with each firing of the unit. Since LN2 has a heat of vaporization of 5.56 kJ per mole (160,128 J/L), MACE Demonstration Unit will ultimately vaporize approximately 8 mL of LN2 per firing event. 

The heat generated within the switch during firing will initially increase its temperature by approximately 75 K, since the heat cannot be transferred instantaneously to the LN2. The switch region comprises approximately 1000 cm2 of HTS tape surface area that is in contact with the LN2. The maximum heat flux possible with LN2 is about 10 W/cm2.  Thus, across its entire surface area, the HTS switch can transfer heat at 10,000 W, and the time required to remove the heat from the switch will be 1,300 J /10,000 J/sec, or 0.13 sec.

References:
Solovyov, Vyacheslav F., and Qiang Li. "Fast high-temperature superconductor switch for high current applications." Applied Physics Letters 103.3 (2013): 032603.
Takematsu, T., et al. "Degradation of the performance of a YBCO-coated conductor double pancake coil due to epoxy impregnation." Physica C: Superconductivity and its applications 470.17 (2010): 674-677.

5.3.2 Requirements for the Dump Resistor
Our initial MACE concept included a “dump resistor” designed to carry most of the current during MACE firing, and thereby. However, this initial concept has been refined. At modest energy storage levels, as will be shown in the MACE Demonstration unit, the HTS Switch itself is able to dissipate the energy contained in the primary coil. Adding a dump resistor in parallel with the HTS switch would simply act to lower the overall resistance of the circuit thereby lowering the impedance ratio and efficiency of energy transfer. Since the experimental objectives of the MACE Demonstration Unit focus on the devices efficiency of energy transfer and the ability to rapidly cycle the device between charging, firing, and recharging, the use of a dump resistor for the Demonstration Unit is not warranted. 
However, one of the important outcomes of the present study is the recognition that the HTS switches for the field-scale units (i.e. 10 MJ of energy storage and larger) will require a modified approach. First, to maintain high impedance ratios between the primary loop circuit and the secondary loop circuit, the HTS switch will require significantly higher “OFF” resistance than that which is required by the MACE Demonstration Unit. Another issue is that higher “OFF” resistances when combined with high turn ratios between the primary and secondary coils will generate large and potentially damaging voltage spikes at the HTS switch at the time of triggering. For these reasons, ISIT has begun to look at several modified HTS switch designs for scaled-up versions of MACE. These will be addressed in future monthly reports. 


5.3.3 Requirements for Electrical Leads and Power Supply
For the MACE Demonstration Unit, the primary HTS coil winding will be charged to carry 800 amps. ISIT has identified commercial off-the-shelf demountable leads which are rated for 1000 A. 
The charging power supply requirements for the MACE Demonstration Unit are modest. If charging ramp rates for the MACE primary coil are kept to a conservative 20 A/sec or less (or, 40 second coil recharging time), the charging voltage for the power supply will be equation, where L is the primary coil inductance (4 x 10-2 H), or 0.8 V at 20 A.

 

 

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