11-05-2012, 12:24 PM
Over-Voltage Suppression in a Fault Current Limiter by a ZnO Varistor
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INTRODUCTION
SUPERCONDUCTING fault current limiters (SCFCLs) are
expected to improve reliability and stability of power systems.
Many studies have been performed with various types of
SCFCLs [1]-[4]. The operating characteristics of SCFCL’s have
been investigated.
The important specifications of the SCFCL as a power system
component are the trigger current level, the limiting impedance
and the recovery time. From this point of view, we have proposed
an SCFCL of the transformer type with an adjustable
trigger current level. A prototype single-phase unit has been
designed and built [5]. The fault current is limited by the inductance
of the primary coil. Therefore, the amount of energy
dissipation in the secondary superconducting wire is small in
the prototype SCFCL.
ZnO Varistor
Metal Oxide Varistors, especially ZnO varistors, are usually
introduced into power systems to suppress lightning surge
voltages. The typical voltage-current characteristic is shown
in Fig. 2. The varistor voltage is defined as the voltage at
the 1 mA of current flow, that is, 100 V in this case. When the
voltage across the ZnO varistor exceeds , the resistance
changes to small values drastically. Thus, the over-voltage is
clipped to the varistor voltage.
C. Experimental Circuit
The experimental circuit for over-voltage suppression across
the SCFCL by use of a ZnO varistor is shown in Fig. 3. Inductances
and of reactors and are 2.13 mH and 6.40 mH,
respectively. The AC voltage source 210 V (50 Hz) is stepped
down to 100 V and applied to the test circuit. The magnetically
controlled switch is closed and is open in the initial
state. is closed to simulate a fault. The phase at the fault
occurrence is controlled to be the same in every test case.
SIMULATION STUDY
A. Equivalent Circuit
The equivalent circuit for the simulation is shown in Fig. 8.
The equivalent circuit of the ZnO varistor is surrounded by a
dotted square. Capacitance and series resistance can be
neglected for a small ZnO varistor as used in the tests. The nonlinear
resistance is modeled as shown in Fig. 2 for varistor
voltage 100 V, for example.
The resistance , which is also nonlinear one, depends on
the super-normal transition of the secondary superconducting
wire. The resistance is given by use of a simple simulation
based on the heat equation. This has already been reported and
it was confirmed that the simulation results agree well with the
experimental ones [11].
B. Simulation Result
one of the simulation results of the current
limiting operation with a ZnO varistor of varistor voltage 100 V.
Fig. 9(b) shows corresponding experimental results. The FCL
voltage and the currents agree well with the experimental results.
A basic design can be performed by use of the simulation
model. The current of the secondary coil, which is difficult to
measure directly, can be evaluated by use of this simulation.