This article will introduce about the fault current limiter. What are different types of current limiters and what are the characteristics. Why it is needed in the system and what are the applications of fault current limiter.
- 1 Definition of fault current limiter
- 2 Need of fault current limiter
- 3 Working of fault current limiter
- 4 Characteristics of fault current limiter
- 5 Types of fault current limiter
- 6 Typical FCL applications
Definition of fault current limiter
A fault current limiter (FCL), or fault current controller (FCC), is a device which limits the prospective fault current when a fault occurs (e.g. in a power transmission network) without complete disconnection. The term includes superconducting, solid-state and inductive devices.
A Fault Current Limiter (FCL) is an innovative device which is incorporated in power system to resolve the issue of increased fault current levels Moreover Fault Current Limiter formulated as an alternative solution to buy expensive power system upgrades.
Need of fault current limiter
Many people ask
What is the need of fault current limiter in presence of electrical protection system?
It must be remembered that the protection system takes some time to isolate the faulty part of the system from rest of the system. The time ranges from 5 cycles to 30 cycles. In very sensitive networks, high short circuit is not tolerated for even very short time as well. It is therefore required to limit the fault current until the protection system acts.
By using FLC in the system, short circuit rating level of system is decreased and allows the use of lower rated devices. Since FCL’s can mitigate the fault current in first quarter in this way it protects the device from first peak in case of fault
Above figure shows source impedance is increased by FCL in case of fault, which limits the magnitude of fault current.
As we know that when fault occurs, sudden voltage dip occurs which can damage the power sources. So by using fault current limiter, impedance in the path of fault current is increased this reducing the voltage dip.
Since the level of voltage dip is directly proportional to the magnitude of short circuit current. By decreasing fault current level voltage sags also reduces and protects load as well as sources.
Working of fault current limiter
Fault current limitation effects of FCL in fault condition
Above figure demonstrates the distinctive operation of Fault current limiter and its result on fault current limiting. FCL keeps its impedance low in normal condition. In case of fault, it rapidly inserts high impedance in a system which briskly limits the fault current of the system. When the fault is cleared, the fault limiter should reduce the impedance back to normal state. High impedance during normal condition would result in losses therefore impedance in normal condition should be minimum.
In ideal fault current limiter, the impedance should be zero in normal condition while impedance of FCL should be infinite in faulty condition. Practically these values cant be achieved.
Characteristics of fault current limiter
Different types of Fault current limiters have different properties however there are some characteristics which every fault current limiter should possess.
Under normal condition, FCL should be as ‘‘indiscernible’’ as possible to the power System. It means impedance of FCL should be minimum in normal condition so that that losses, voltage drop and harmonics in both current and voltage waveform should be minimum.
Fault current limiter should response quicker than conventional protection system. Ideally is should insert high impedance with in the first cycle of fault.
FCL should recover its superconducting state as quickly as possible after clearance of fault. In this way it provides coordination with the reclosing action of protective relays installed in power system.
Construction as well as operating cost of SFCL should be reasonable as compared to higher rated protection devices.
Types of fault current limiter
More than a decade, several typologies of FCL have been explored by the researchers. Typically FCL technologies can be categorized as follows:
- Passive fault current limiters: It includes super conducting fault current limiters e.g resistive type SFCL.
- Solid state fault current limiters: The limiters are based on power electronic switches like diodes, thyristors etc. For example bridge type FCL.
- Hybrid fault current limiter: These fault current limiters are combination of the above two. For example resonance type fault current limiter.
Classification of FCL is actually based upon the techniques used by that FCL. Every technique (superconducting, solid state, resonant) has its own distinguishing feature, which provide promising role in limitation of current.
Following figure illustrates different types of FCL techniques and relationship among them.
From fig infers that boundaries of FCL are not well defined. For example, the application of resistive superconducting FCL is very much different from Solid state FCL. Design constraints for different FCL’s also varies with its application. Different types of FCL are shown in figure
Typical FCL applications
FCL reduces the current during short circuit. In addition to that it has further advantages to the power system like reducing voltage dips and improvement in system stability. Rating of fault current limiters varies at different locations according to system voltage level (400 V to 132kV). In this section numerous application of FCL are conferred.
Distributed generation coupling
Following figure shows integration of fault current limiters with distributed generation units. As explained above, addition of distributed energy sources rises fault level of substation bus. So adding more sources can be dangerous for the system because increase in fault level can result in melting of electrical equipment during fault. However if fault current limiter is installed between grid and distributed generation units FCL can reduce fault level upto safety limits of substation’s short circuit current .
Bus bar coupling and transformer feeder
Following figure shows the application of FCL to couple two bus bars and transformer feeders. FCL’s rapidly changes its impedance resulting in reduce voltage dip, distortions and improved stability. It also enables the installations of nonlinear loads without considerable equipment’s upgrade .