Current-limiting Transformers Research Articles

Manufacturing of a 1-MVA-Class Superconducting Fault Current Limiting Transformer With Recovery-Under-Load Capabilities

Superconducting current-limiting transformers offer an effective current limitation and recovery-under-load characteristics; they are more compact and have a lightweight design and a reduced short-circuit impedance. Several designs for superconducting current-limiting transformers have been developed and proposed by researchers over the last two decades. However, less research work was devoted to effective current limitation of superconducting transformers in combination with recovery under full or under partial load. In this paper, we present the process of manufacturing a superconducting transformer demonstrator with current limitation and recovery-under-load capabilities. The demonstrator consists of a laminated iron core and a normal conducting 20-kV primary winding, both operated at the room temperature. 2G REBCO tapes are used for the secondary winding, which is operated at 1-kV nominal voltage. In order to keep the secondary winding at cryogenic operation conditions at 77 K, a vacuum insulated fiber reinforced plastics cryostat was designed and manufactured.

Test of 6-kVA Three-Phase Flux Transfer-Type Current-Limiting Transformer

A 6-kVA three-phase model of the flux transfer-type current-limiting transformer was developed and tested. In this device, the winding loops of YBCO superconducting tapes couple magnetically two independent iron cores: the primary-side iron core and the secondary-side iron core. The former and the latter are equipped with copper primary and secondary windings, respectively. Because the magnetic fluxes linked to the superconducting winding loops must be kept constant, the magnetic flux is transferred by the superconducting YBCO loops between the two iron cores in order to couple magnetically the primary and secondary coils. While the YBCO loops are superconducting, 100 % of the magnetic flux is transferred and the device shows the similar function as usual transformers. Once the YBCO loops become normal by a fault current in any of the windings, the power transfer between two iron cores is limited and the current in the secondary winding is limited naturally on a result of decoupling the iron cores.

Switching and Decoupling Effects in a Single-Phase Transformer Using Extra DC Current

A single-phase current-limiting transformer has been developed based on flux transfer. The transformer consists of two independent iron cores, one primary coil, two separate secondary coils, and superconducting windings/loops. Two iron cores are in a mechanical contact and clamped. Superconducting windings/loops act as coupling/decoupling mechanism between two iron cores. The transformer has been made operational via a switchgear of the applied dc current for a potential use in the current limitation when the transition between the superconducting and normal (quench) states can be triggered. DC and ac currents can be applied simultaneously in fault and no-fault conditions to mimic the realistic cases of use. With decoupling ac energy transfer, we propose that conventional switching off may be replaced with the approach described.

Calculation principles for a superconducting current-limiting transformer

We present the general principles for the calculation of the parameters for acurrent-limiting transformer based on the superconducting–normal state transition in asuperconducting short-circuited winding. The relationships between the parametersof the magnetic system (core diameter and number of turns in the windings)and the limited current, losses and temperature in the superconducting elementare investigated. The parameters of a 15 MVA current-limiting transformer areevaluated. The performance of the superconducting winding in the form of anHTS hollow cylinder is analysed. The problems of the transition into the normalstate and recovery of the superconducting state in a superconducting cylinder arediscussed.

Thermal regimes of HTS cylinders operating in devices for fault current limitation

We reveal obstacles related to the application of HTS cylinders in current limiting devicesbased on the superconducting–normal state transition. It is shown that, at the criticalcurrent density achieved presently in bulk materials, and especially in BSCCO-2212, therequired thickness of the cylinder wall in a full-scale inductive device is several centimetres.A simple mathematical model of the operation of an inductive fault current limiter (FCL)is used to show that such cylinders cannot be cooled in an admissible time after a faultclearing and, hence, the inductive FCLs and current-limiting transformers employingBSCCO cylinders do not return to the normal operation in the time required. Forthe recovery even with a non-current pause in the circuit, cylinders are neededwith a critical current density an order of magnitude higher than the existingone.

The geometry of regulating transformers

Ferroresonant, ballast, and current-limiting transformers are similar in geometxy. Optimization of weight, losses, or cost can result in different choices for the shape (core or shell) and proportions of the lamination. In the ferroresonant transformer, the problem of saturation losses, noise and stray flux can be alleviated by altering the geometry to produce a magnetic circuit with nonuniform flux density.

Influence of core gap in design of current-limiting transformers

Core gaps are used in the magnetic shunt paths of current-limiting transformers to limit secondary current under short-circuit conditions. It is shown here that for short-circuit current limited to twice normal a current limiting transformer has only 55% of the rating of a noncurrent-limiting transformer of the same physical size. Part of the reduction in rating stems from the space required for magnetic shunts, part from the greater space needed for winding insulation, and part from the increased loss due to the gaps. Reliability of a previously tested gap-loss equation is established, and design examples are given for both stamped laminations and strip-wound cores.