SHORT CIRCUIT CURRENT LIMITATION TECHNIQUES

Following are some techniques using for limitation of short circuit current.

1. CURRENT LIMITING REACTORS

CLR is a well-known fault current limiting technique. Compared with many other methods, it is more economical. In addition its effect on the reliability of substation is negligible. However, it occupies a relatively large area in the substation, due to safety considerations. More- over, it may degrade both voltage stability and transient stability of the system.

2. SOLID STATE FAULT CURRENT LIMITERS

SSFCLs apply power electronic switches. These limiters are, practically, restricted to the distribution level. More- over, they are complicated and expensive. Some types of SSFCLs apply series resonance or parallel resonance circuits.

3. SUPERCONDUCTING FAULT CURRENT LIMITERS

Superconducting material such as YBCO, NbTi and MgB2 transit from superconducting state to the normal state, if exposed to high current levels. Due to this feature of superconductors, they can be applied as a fault current limiter. During normal operation of power system, SFCL resistance is negligible. However, as soon as the fault current shows up, SFCL quenches and consequently its resistance increases considerably. Resistive, inductive and transformer type SFCLs are important types of this device. Although this limiter seems to be an ideal fault current limiter, it is still too expensive, especially due to the cost of its complicated cryogenic system.

4. FUSE

Fuse is a fast short circuit interrupting device. Therefore, it might be considered as a limiter. It is simple and cost- effective. However, it is technically restricted to below 40 kV nominal voltages and 200 A nominal currents. In addition to these restrictions, fuse must be replaced, following every interruption. Therefore, it may not be used when high speed auto-reclosing is required.

5. I_S LIMITER

I_s Limiter is the improved version of fuse. In this device, during normal operation, major portion of current passes through a path parallel with the fuse. When the short circuit occurs, the parallel path is opened, using electro- dynamic forces of fault current. Consequently, the fault current is commutated to the fuse. This way, the problem associated with the limited nominal current of fuse is resolved. Is Limiter is claimed to be capable of interrupting fault currents up to 5 kA, within 1 ms after occurrence of the fault. However, it is still limited to 40 kV rated voltage

6. POWER SYSTEM RECONFIGURATION

This approach is, to some extent, empirical. There is no definite rule for this method. In other words, it is case- dependent. It also depends on parameters such as creativity and familiarity of the engineer with the system under study. In many cases, it may result in considerable reduction of fault current level. Moreover, it may improve transient stability and voltage stability of the system.

7. BUS BAR SPLITTING TECHNIQUES IN THE SUBSTATIONS

In this approach, in order to reduce fault current level, bus section or/and bus coupler circuit breakers are opened. Power system operators are seriously opposed to this approach. Their disagreement is mainly due to the fact that bus bar splitting significantly decreases reliability of the substation. Moreover, it affects integrity of the sys- tem, which may result in lower transient stability and voltage stability margins. However, from the short circuit reduction point of view, this method is more effective than CLR. This is due the fact that bus bar splitting is equivalent to application of a CLR with infinite reactance. Meanwhile, this method may be considered as a temporary strategy which is acceptable only in emergency situations.

8. DISCONNECTION OF SOME LINES FROM THE CRITICAL SUBSTATION

In this technique, in order to reduce bus bar short circuit level, 2 transmission lines are disconnected from the bus bar. Afterwards, these lines are reconnected together, outside the substation. Similar to the bus bar splitting method, this technique is not acceptable, from the power system operation point of view. Undesired effects on the reliability, transient stability and voltage stability of power system are known to be the main disadvantages of this approach.

9. USING HIGH IMPEDANCE TRANSFORMERS

Using high impedance transformers may result in the considerable reduction of fault current level. However the undesired effects on transient stability and voltage stability might be significant.

10. HVDC

Replacement of tie lines with HVDC links will diminish inter-area short circuit currents. This will, obviously, restrict fault current levels. However, in most cases, this method is not economically justified.

11. DESIGN OF HIGHER VOLTAGE TRANSMISSION NETWORKS

In this method, the existing power system is split to several islands. Afterwards, a higher voltage system is de- signed and then the islands are reconnected through the higher voltage network. This method seems not to be practical, as it is both complicated and costly.

12. USING NEUTRAL REACTOR

Application of reactors in the neutral of transformers, will limit the earth fault current level. As the majority of faults include ground, this method may be considered as an effective approach.

Among the above mentioned approaches, CLR may be the most practical method. Since it does not affect reliability of the system, power system operators may accept it. However, as it might degrade voltage stability and transient stability margins of the system, its application requires careful attention.