Cooling HTS fault current limiter

A fault current limiter (FCL) is a device which limits the prospective fault current when a fault occurs (e.g. in a power transmission network) without adding impedance to the circuit during normal operation and therefore protecting the grid. Simplified: it transmits current during normal (superconducting) state, without any resistance.

If a fault current occurs, the system exceeds its capability, changing into a non-superconducting state with very high resistance, thereby limiting the current. Once the fault current is gone, the system will fall back into its superconducting state. The superconducting coils typically are submerged into a cryogenic bath.

Gravity feed systems

In cases where the required temperature of an application can be achieved by submerging it into boiling liquid (i.e. nitrogen, neon) and bubble formation is no major issue for the performance of the superconductor, a gravity feed system can be considered. Compared with the other superconductor cooling technologies this set up is simple and straight forward.

The customers application (whether it is a superconductor or not) is submerged into a bath (cryostat) with liquid that is at equilibrium (saturation temperature). Heat production of the application (friction, AC loses, thermal) and thermal load of the cryostat causes the liquid to boil. The gas rises to the top of the cryostat where the cryogenerator is connected. The boil off gas is returned to the cold head where it is reliquefied. By gravity the liquid flows back into the cryostat. This set-up is visualized below:

Cooling set up

Typically this configuration is used for superconducting (HTS) fault current limiters. FCL’s are one of the most promising new technologies made possible by the discovery of High Temperature Superconductors.

The most common cryogenic fluid used is liquid nitrogen with a boiling temperature of 77K at 0 barg and a freezing temp of 63K. As the superconductor tends to work better at lower temperatures the systems are often operated below atmospheric pressure to reduce the boiling temp (LN2 at 0,17 bar boils at 65K). Other cryogenic liquids such as neon (27K), argon (87K) can also be used.
The systems are usually set up with a pressure control loop: the cryogenerators control the pressure in the vapor space of the cryostat by reliquefying the gas (hence reducing the pressure) and therefore maintaining the temperature.

Fault current limiter

Complete solution

Stirling Process Cryogenerator are specially designed for this operational set-up. Based on either the SPC-1 (one-cylinder) or SPC-4 (four-cylinder cryogenerators) these cryogenerators have an inlet for gas and an outlet for liquid connected to the cryostat. One to four cylinder machines can be connected to a cryostat, either for required capacity or redundancy.

With nitrogen the system can operate down to 65K providing 700 to 2800W. In two-stage execution (SPC-1T or SPC-4T) the system temperature can descend down to 20K, providing for example 105 to 420W at 26K.

The cryogenerators come standard with a frequency converter which allows the system to reduce its capacity (and power consumption) from 100 to 60%. This again allows finetuning the temperature and pressure control of the complete system without adding a power consuming counter heating unit to the cold head when the heat-load of the application is reduced.

Cold head design

Special precaution is taken with the design of the cold head, in order to prevent possible freezing of nitrogen. When the plant operates close to the freezing point of nitrogen (i.e <70K), nitrogen could freeze(solidify) on the cold head exchanger as the coldhead itself always operate a couple of degrees below the temperature of the liquid gas. If this happens the coldhead will be blocked and operation will stop. The coldhead is equipped with so called “anti-freeze heaters”, which automatically starts to operate when the application does not consume power and the coldhead approaches the freezing temperature of the cryogenic fluid.


For use in applications as described the following Stirling Cryogenics products may be considered:

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