Fault Current Limitar 1i1j2a

SUPERCONDUCTOR FAULT CURRENT LIMITER (SFCL)

What is fault current? Fault current is any abnormal electric current that flows through a circuit during the electrical fault conditions like L-G, L-L-G, L-L-L- G, L-L, L-L-L faults. Fault current may be several times larger in magnitude than the current that normally flows

Need for fault current limiter Increasing demand of

power and addition of more generators, transformers and large networks causes higher stress on power system. Higher stresses results in higher probability of faults.

THE MAXIMUM FAULT CURRENT IN A SYSTEM VARIETY OF REASONS •Electric

power demand increases (load growth) and subsequent increase in generation. •Parallel conducting paths are added to accommodate load growth. •Interconnections within the grid increase. •Sources of distributed generation are added to an already complex system.

Traditional ways to limit fault current Following methods are used to limit fault current-:  Circuit breakers with ultra-high fault current rating  High impedance transformers  Current limiting fuses  Air core reactors

SUPERCONDUCTOR FAULT CURRENT LIMITER (SFCL) •SFCL is a new power device to automatically limit a fault current to a safe level with the superconducting property. •When superconductor is cooled down to critical temperature (about -186℃) or less, the resistance becomes zero. However, superconductor looses •superconductivity and resistance occurs rapidly (quench), when excessive current flows and exceeds certain value (critical current). SFCL device uses this property.

SUPERCONDUCTOR CHARACTERISTICS •Zero DC resistance •High current density (high power) •High electro‐magnetic shielding (low EMF) These characteristics require: •Cooling below a critical temperature

•Current levels below a critical current •Magnetic field below a certain magnitude

Characteristics of ideal fault current limiter Have zero impedance throughout normal operation Provides sufficiently large impedance under fault conditions Provides rapid detection and initiation of limiting action Provides immediate recovery of normal operation after clearing  Fully automatic  highly reliable  Light weight

 Low cost

DIFFERENCE BETWEEN NORMAL AND FAULT CURRENT LIMITER

Fig.. Fault control with a fault-current limiter

SUPERCONDUCTOR FAULT CURRENT LIMITER CONCEPTS

1. THE INDUCTIVE SFCL 2. THE RESISTIVE SFCL 3. THE SHIELDED-CORE SFCL 4. THE SATURABLE-CORE SFCL

1. THE INDUCTIVE SFCL

Fig.. Inductive fault-current limiter

The inductive limiter can be modeled as a transformer. The impedance of this limiter in the steady state is nearly zero, since the zero impedance of the secondary winding is reflected to

2. THE RESISTIVE SFCL

Fig. Resistive fault-current limiter

Resistive SFCLs utilize the superconducting material as the main current carrying conductor under normal grid operation.

2. THE RESISTIVE SFCL • Superconductor connected in series with the line to be protected. • To keep it superconducting, it is usually immersed in a coolant that is chilled by a refrigerator. • In case of a fault the inrush of current and magnetic field take the super conductor into the transition region, thereby the increasing resistance limits the fault current. • The behavior of resistive fault current limiter is largely determined by the length of the superconductor and the type of material used for it.

Operation of R-SFCL

When a fault occurs, the current increases and causes the superconduc quench thereby increasing its resistance exponentially. The resistive FCL is based on the nonlinear characteristic of a SC element. A electric field only develops above the critical current.

3. THE SHIELDED-CORE SFCL

Fig. Shielded-Core fault-current limiter

shielded-core designs have worked well, their size and weight have limited grid

3. THE SHIELDED-CORE SFCL  This is a variation of the resistive type SFCL. Here the super conducting cool environment/cryogenic environment are mechanically isolated from other part of the circuit. And electrically coupled.  Device looks like a transformer with secondary side shunted by high temperature super conductor element  At the time of fault, a high inrush of fault current occurs. This causes a high current flowing through secondary side. Now by action of high temperature superconductor, quenching occurs and thus voltage drop increases. This voltage drop will make a high voltage on the primary side/line. This will oppose the flow of fault current, thus current limiting takes place. Like in resistive type, we have to re cool the high temperature super conducting element after quenching occurs.  It is 4 time the size and weight of resistive type for the same rating.

4. THE SATURABLE-CORE SFCL

Fig. Saturable-Core fault-current limiter

This concept utilizes two iron cores and two AC windings for each phase. The AC windings are made of conventional conductors that are wrapped around the core to form an inductance in series with the AC line

Working  Under nominal grid conditions (no fault), the HTS coil fully saturates the iron so that it has a relative permeability of one. To the AC coils, the iron acts like air, so the AC impedance (inductive reactance) is similar to that of an air-core reactor.  Under fault conditions, the negative and positive current peaks force the core out of saturation, resulting in increased line impedance during part of each half cycle. The result is a considerable reduction in peak fault current. During a limiting action, the dynamic action of the core moving instantaneously in and out of saturation produces harmonics in the current waveform. Under normal conditions, the voltage and current waveforms are basically unaffected by the saturable-core SFCL.

DESIGN PARAMETERS OF SFCLS •System voltage •Continuous current rating •Maximum 3phase short circuit current or source impedance •Fault clearing time; backup clearing time, reclose sequence (if any)

•1st cycle peak fault current desired to be limited •Per unit level of rated current for FCL transition

SUPERCONDUCTOR FAULT CURRENT LIMITER APPLICATIONS 1.Fault Current Limiter In The Main Position 2.Fault Current Limiter In The Feeder Position 3.Fault-Current Limiter In The Bus-Tie Position

1. FAULT CURRENT LIMITER IN THE MAIN POSITION

Fig. Fault-current limiter in the main position

The fault current limiter protects an individual circuit on the bus. Underrated equipment can be selectively protected as needed in this manner an fcl can also be used to protect the individual loads on the bus.

2. Fault Current Limiter In The Feeder Position

fig. Fault Current Limiter In The Feeder Position

FCL can also be used to protect individual loads on the bus.The selective application of small and less expensive limiters can be used to protect old or overstressed equipment that is difficult to replace, such as underground cables or transformers in faults.

3.Fault-Current Limiter In The Bus-Tie Position

Fig. Fault-current limiter in the bus-tie position

 Separate buses can be tied together without a large increase in the fault duty on either bus.  During a fault, a large voltage drop across the  limiter maintains voltage level on the unfaulted bus.

ADVANTAGES OF SUPERCONDUCTOR FAULT CURRENT LIMITER •Because the resistance of superconductive material is

so low, there is no current wastage when they are used to conduct electricity.

•When used in the process of magnetic levitation, no kinetic energy is wasted due to friction from

with the ground.

DISADVANTAGES OF SUPERCONDUCTOR FAULT CURRENT LIMITER •To show their properties, and be of any use, they must be at critical temperature, which can be costly. •They emit strong magnetic fields which can affect

humans by causing blindness, sterility, brain cancer and other things.

CONCLUSION With increase in generation, comes an increase in shortcircuit current in a transmission line during faults. Utilities usually predict how much fault current exists in the line and can forecast its increase over a period of time. The result is a lower cost system that is smaller and has much lower losses.