Fault Current Limiters: An Introduction

What are the Fault Current Limiters (FCLs)?

We are living in a power-hungry world. Everything around us, from small appliances used in homes to large scale motors used in factories, require power making an ever-increasing demand of electrical energy. So, there is always a need for a growing generation capacity to fulfil energy requirements. But, in power systems, generation capacity and increment in fault current levels often go hand in hand. So, an increase in energy demand where mandates increase in generation capacity, at the same time it needs a decrease in fault current level to keep the system protected and stable. Fault current limiters, as the name suggests, are used to reduce the level of fault currents and reduce the compromise on the stability of power systems in case of faults. It is based on offering a high series resistance in the path of fault currents in case of faults while in normal situations, resistance offered in the path of current is negligible.

Why do we need FCL?

As fault current level increases because of increased generation, system becomes unstable and protection gets compromised. Levels of fault currents can go beyond the maximum ratings of equipment used for protection such as circuit breakers and can cause heavy damage to the system and other components. One way is to change the whole protection scheme and design it according to new generated capacity e.g. install breakers with greater rated capacity. Other way is to install Fault Current Limiters (FCLs) which can reduce the magnitude of currents during faults so circuit breaker can interrupt at lower values of currents and don’t need to be changed. As replacement is very costly and puts extra economic burden, to prevent system from damage there is need to reduce the level of current using FCLs so circuit breakers can operate at lower level within its rated capacity. Moreover, circuit breakers are slow in action and it takes them about 2-3 cycles to operate. Meanwhile Fault current Limiters keep the current at lower value without causing any damage to system until the circuit breaker interrupts.

Design Features of Fault Current Limiters

FCLs must have following design features incorporated:

• Should not change system characteristics in normal conditions (Low impedance, low voltage drop across it and low power losses during normal conditions)
• High Impedance during Faulty conditions
• Able to tolerate fault current for sufficient time
• Fast response (operate before first peak of fault current)
• Able to withstand system voltages during normal and faulty conditions
• Appropriate temperature endurance
• Fast recovery time
• Economically viable

Types of Fault Current Limiters

Most commonly used types of FCLs are:

• Superconductive FCLs
• Non-superconductive FCLs

Super Conductive FCLs (SFCLs)

Superconductive type FCLs work on principle of superconductivity. During normal conditions it acts like a superconductor and offers virtually zero resistance in path but during fault, its critical temperature is achieved because of increase in temperature due high fault current and it changes its state from superconducting to normal and offers high resistance which limits fault current.

SFCL are of two major types:

Resistive Type SFCL

Resistive type SFCL makes use of superconducting material as a conductor to carry current during normal operation. During the normal operating condition, the current flows through superconducting element Rsc. Resistive SFCL is designed that during the normal condition, the heat generated by normal current keeps SFCL below its critical temperature and during the fault, temperature becomes greater than the critical temperature of superconductor used. When the current goes beyond limit there is an increase in temperature due to which superconductor transforms from superconductor to resistor, more than normal resistance is introduced in fault current’s path and fault current is limited. This phenomenon is referred to as “quenching”. When the fault is removed, it returns to its superconducting state again as the temperature becomes below the critical temperature of superconductor. Inductive shunt ZSH or the parallel resistance is used to avoid over-voltages due to the fast current limitations during the quench.

Inductive Type SFCL

The structure of this type of SFCL is similar to transformers with secondary winding made of a superconducting element. During normal operations, the resistance of primary winding and leakage reactance determine the impedance of the limiter as the resistance of secondary is almost zero as the temperature of the superconductor is kept below than critical temperature. During fault conditions, the resistance of the secondary winding increases and the superconductor quenches the fault by transforming itself into a resistor. The value secondary (R_SC) is transferred to the primary side and the effective impedance of line increases. This type of SFCL is larger in size and heavier than Resistive type.

Non-Super Conductive FCLs

These types of FCLs do not use superconducting elements and are designed in such a way that in normal operating conditions resistance is by-passed and during faulty conditions, a large resistance is realized to limit the currents. Major types of non- superconductive FCLs are given below:

Saturated Core FCLs

they make use of the non-linear characteristics of ferromagnetic materials to realize a high inductance. Core of ferromagnetic material is biased using DC current during normal conditions to make it saturated and in return, it offers very small inductance in line. During faults, core is made unsaturated which results in offering a high inductance in current path to limit the current.

Solid-state FCLs (SSFCLs)

These type of FCLs use electronic switching to by-pass resistances during normal conditions and introduce resistance during faulty conditions.

Serial type SSFCLs contain bi-directional controlled switches which either by-pass normal state or faulty state.

Bridge type SSFCL use thyristors to make a bridge arrangement. Switching is performed in such a way that during fault conditions, resistance is also included in circuit and current reduces.

In resonance type SSSFCL, phenomenon of resonance is used. As we know that during resonance, resistance of circuit is minimum, so a series resonance circuit is introduced to match the line frequency during normal conditions. During fault, its topology changes and circuit does not remain in resonance condition and offers much larger impedance in line.

Advantages of Fault Current Limiters

• It limits the current to prevent damage of equipment
• It prevents replacement of costly components
• It enhances stability of power systems

Conclusion

Fault Current Limiters (FCLs) reduce fault currents without compromising circuit characteristics in normal conditions. It prevents equipment from damage and enhances the stability of the system. Where it offers many advantages, one of the main disadvantages associated with FCL is the introduction of resistance and power losses, although very negligible, in normal conditions.

I hope you’ve liked this article on the basics of fault current limiters, their types and applications. You may also like our detailed article on the working and construction of a power system and Load flow analysis in ETAP software.

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