Fault Current Limiters

Fault Current Limiters are regarded as key components for modern power systems. Among all possible limiting devices SCFCL offers ideal performances: negligible impedance in normal conditions and passive (i.e. reliable) switch to high impedance state in case of fault. A great research effort has been carried out in recent years, showing the feasibility of both the two basic concepts of SCFCLs, the “Resistive” and the “Inductive” type. However, the interaction of these devices with power networks is still little investigated. In order to evaluate how a SCFCL can enhance the performances of a power system, a circuit model of the device needs to be introduced in power system simulators. The more this model well reproduces the features of the limiting behavior, the more the technical and economical benefits are accurately estimated.

At the Applied Superconductivity Laboratory of the University of Bologna the following type of superconducting limiters are investigated:

A Magnetic shield type superconducting fault current limiter consists of a SC ring and a copper coil inductively coupled via a ferromagnetic core. A numerical model for determining an accurate equivalent circuit of this device has been developed. The details of the model are described in the section “Electrodynamics of superconducting bulks”. The equivalent circuit can be introduced in power system simulator in order to investigate the technical and economical benefits brought by the device the system.

In order to validate the proposed circuit model a basic prototype of Magnetic Shield type HTSFCL has been designed and manufactured. A BiSCCO 2212 ring with 130 mm inner diameter, 113 mm Outer Diameter and 50 mm height was first chosen as SC element. The value of critical current density in self field at 77 K, measured by the ring manufacturer by means of the 1 μV/cm criterion, was 810 A/cm². The E – J characteristics of the ring is shown in figure 1.1.

Figure 1.1: E – J characteristic of BiSCCO 2212 ring

The maximum magneto-motive force produced by the coil which can be shielded by this SC ring is equal to its whole critical current, that is 2025 A. Therefore the minimum value of the device quenching current, i.e. the minimum value of current flowing in the coil which brings the ring to enter the SC/Normal transition, corresponding to the case of perfect magnetic coupling, can be calculated as the ratio of the ring critical current and the number of turns of the coil. A minimum value of 7 A was chosen for the quenching and the inducting coil was made with 285 turns, arranged in 15 layers of 19 turns each and realized by using a 2 × 5 mm² copper tape. The inner diameter of the coil was equal to 160 mm and the thickness and height were equal to 43 and 104 mm respectively. For what concern the magnetic core, a column with circular cross section of 63 mm diameter, made of laminated grain oriented silicon steel, was chosen. With this open core configuration the ratio between the limiting and the non–limiting state impedance of the device is too small to be of practical interest, but is large enough to affect current and voltage waveforms in a way detectable by the instrumentation of the laboratory. Moreover the axial symmetry of the whole device is maintained, giving considerable time saving in the numerical calculations. The height of the column, chosen in order to have a value of the impedance ratio equal to about 3, was equal to 300 mm. Figure 2.1 shows the magnetic flux density distribution inside the device, calculated preliminarily in order to specify the dimension of the ferromagnetic core. The calculated values of the non limiting and limiting inductances are also reported.

Fig 2.1.a. field distribution inside the device in non limiting conditions, L_NL = 12.39 mH

Fig 2.1.b. field distribution inside the device in limiting conditions, L_L = 35.86 mH

The equivalent circuit of the FCL prototype, under the assumption of linear iron core, is shown in figure 3.1.

Since the numerical results obtained have shown that no appreciable current flows in the radial branches of the equivalent circuit (this correspond to no appreciable currents flowing in the axial direction in the actual device), they have been neglected and the following reduced equivalent circuit of figure 4.1 has been obtained.

Figure 5.1 shows the comparison between numerical and experimental waveforms of current and voltage across the device during one of the short circuit test performed.

As it can been seen from the figure, as far as the normal operating (non fault) condition is considered the numerical results are in a good agreement with the experimental ones; this means that the equivalent circuit well reproduces the shielding effect of the SC ring. Moreover, the current and voltage during a short period after the fault occurrence are also well reproduced. A discrepancy, probably due to the drop of critical current density caused by the heating, begins to appear after some cycles.

references

[1] F. Negrini, P.L. Ribani, A. Morandi, T. Nitta, S. Oshima, "Experimental Analysis and Circuit Model of an Inductive Type High Temperature Superconducting Fault Current Limiter", International Journal of Modern Physics

[2] M. Fabbri, A. Morandi, F. Negrini, P.L. Ribani, “Magnetic Shield Type Fault Current Limiter Equivalent Circuit”, submitted for publication on IEEE Transactions on Applied Superconductivity

[3] M. Fabbri, A. Morandi, F. Negrini, P.L. Ribani, L. Trevisani “Experimental Validation of Magnetic Shield Type Fault Current Limiter Circuit Model”, presented at 6^th European Conference on Applied Superconductivity, Sorrento Napoli - Italy, 14^th – 18^th September 2003

A transformer type superconducting fault current limiter consists of a primary SC winding series connected with the test circuit and magnetically coupled (without an iron core) with a secondary SC one, which is short circuited. The primary coil is made of superconductors only in order to reduce the thermal load of the cryostat. Due to the short circuit of the secondary superconducting winding, in normal operating conditions the device shows negligible impedance. When the current of the primary coil exceeds the trigger value, the current induced in the secondary coil exceeds a critical value and the device switches to an high impedance state. The trigger current can be adjusted by changing the magnetic coupling of the windings.

The experimental testing of this device has been carried out at the “Nitta Laboratory”, The University of Tokyo. Figure 1.2 shows a scheme of the device tetsted and a picture of the two SC coils.

Several tests under various operating conditions have been performed. The effectiveness of the limiting action and the possibility of adjusting the trigger current have been confirmed by the experiments. For the fault cases studied, the recovery time, i.e. the time required to switch back to a low impedance state after the fault is removed, has been found to be zero, at least in the limits of the control and measurements apparatus time scale.

In order to investigate the interaction of the FCL with power networks a circuit model of the device has been defined. The equivalent circuit is made of two coupled inductors with auto inductances L₁and L₂ respectively and with mutual inductance M. The inductor L₂ is connected to a non linear resistor R₁₂, whose voltage-current relation is defined by means of the power law v = k sign(i) |i|^N, which takes account of the SC/Normal transition of the winding. The parameters of the equivalent circuit have been determined by examining the experimental waveforms of current and voltage across the device during a fault.

Figure 2.2 shows the comparison between experimental and numerical waveforms calculated by means ft he equivalent circuit.