Research On Novel Solid State Fault Current Limiter And Its Effects On Power System

With the development of power system and modern industries, the growth of interconnection between electrical networks, short circuit capacity is increased, which renders fault current limitation an urgent issue. After several years' research, many types Fault Current Limiter (FCL) are presented. At the beginning of 1990s, EPRI of USA developed a survey questionnaire indicating a need for a distribution current limiting devices. The survey also showed that a limiter consisting of solid state (power electronics based) device might be the best approach among others.In this paper, a generalized comparison is made on the principle and characteristics of different current limiting technologies and present Solid State Fault Current Limiters (SSFCL) are illustrated in detail. The history and recent development of SSFCL are reviewed, and then the most important problems for applying SSFCL to electric power system are pointed out.A novel topology of SSFCL, solid state fault current limiter with bypass reactor, is presented as a solution for medium- and high-voltage electric network. In normal operation condition the limiter has no obvious effect on loads. When fault happens, the bypass AC reactor will insert the fault line automatically to limit short circuit current. By appropriate control strategies, solid state bridge will retreat from fault line as soon as possible, fault current is fully limited by the bypass AC reactor subsequently. Because the parameters of the solid state devices and DC reactor are not determined by short circuit current rating of the network, they are flexible to design. The proposed SSFCL is simulated and an experiment model is constituted. It is shown that the bridge circuit is closed at the current across zero just like a soft-switch, and there is no oscillation as well as over-voltage excited by the SSFCL when the fault current is switched off. The control strategies are simple and facile. It proved that the theoretical prediction and current limiting performance is satisfactory for research and development.In steady state, current through the coupling transformer mentioned above is rating, when fault happens, there is a large voltage drop upon transformer windings. However, the voltage drop upon windings is very small at steady state. The coupling transformer design according to system rated current and voltage is over-qualified. Based onpresented limiting technologies and related Chinese patent, a Variable Impedance Transformer Fault Current Limiter (VITFCL) is proposed. System operation in normal state, VITFCL works in the same way as the presented SSFCL does. When fault happens, solid state bridge circuit retreat form fault line, the transformer turns into deep saturation state, the secondary winding is opened, the saturation transformer is equivalent to a reactor in series with the fault line to limit fault current. This new type FCL integrates the virtue of bridge type SSFCL and SSFCL with bypass reactor; it has an expansive research future.When FCL inserted into a line, the line parameters are changed, the electric distance is shortened accordingly. The fourth part of this thesis investigates the impact of SSFCL on a distance protection scheme. Relay selectivity is explored while placing the FCL in front and behind the relay element. Two new adjustment methods to settings of relay reach are proposed. Furthermore, solutions to modify distance protection scheme is presented.Based on Mayr arc model, calculations were carried out to study the effect of the SSFCL upon the interrupting duty imposed on a circuit breaker under various fault locations. The Rate of Rise of the Recovery Voltage (RRRV) is reduced because of the insertion of SSFCL, and the effect of short-line fault can be lessened. The reduction of the rating required for circuit breaker is obvious. It is also analyzed that the stray capacitance of limiting coil have an impact on RRRV, with different stray capacitance having different effect on interrupting condition of circuit breaker.Short circuit fault would result in consid...