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INTRODUCTION
When electric power systems are expanded and become more interconnected, the fault current levels increase beyond the capabilities of the existing equipment, leaving circuit breakers and other substation components in an over duty conditions. The short circuit current containing extremely high energy will damage electrical equipment. All the equipment has to have a short circuit rating capable of withstanding this level. Typically, circuit breakers open automatically in three to six cycles when a fault occurs .However, circuit breakers, sometimes, cannot handle the intense level of faults, so they fail to break. Traditionally, handling these increasing fault currents often requires the costly replacement of substation equipment or the imposition of changes in the configuration by splitting power system that may lead to decreased operational flexibility and lower reliability. An alternative is to use Fault Current Limiters (FCLs) to reduce the fault current to a lower, acceptable level so that the existing switchgear can still be used to protect the power grid FCLs utilizing superconducting materials which are capable of providing instantaneous (sub cycle) current limitation abilities, can prevent the buildup of fault currents and have been studied for years.In particular, a super conducting fault current limiter (SFCL) will be operating in a superconducting state and is basically invisible to the power grid because no major energy loss and voltage drop will be developed across the device during normal operation. In the event of a fault, the SFCL will produce a certain value of an impedance within a few milliseconds due to the loss of superconductivity, and insert it into the circuit, thus reducing the fault currents to levels that circuit breakers can handle. Being a promising application of superconductors, the SFCL is considered to be one of the innovative devices of FACTS in electric power systems. The application of the SFCL would not only decrease the stresses on the devices but also offer a higher interconnection to secure the network. This is a very effective means to enhance the system stability and power quality in terms of availability and voltage drop, which is a real need today Several types of SFCL have been considered which are based on different superconducting materials and designs. From the point of view of power systems, the resistive SFCL is preferable because it increases the decay speed of the fault current by reducing the time constantof the decay component of the fault currents, and canalso make system less inductive
The distribution systems have been designed and built as passive unidirectional systems to accept generation or bulk supplies from transmission grids or substations. The short circuit current is just limited by the impedance of various system components through which the fault currents willflow. These paths very much depend upon the interconnection of the system. Thus, integration of the SFCL could offer an effective solution to controlling fault current levels in distribution grids.
However, the SFCL has no interrupting ability, the circuit breaker is required in series to interrupt the fault current which is limited upstream by the SFCL. To achieve a successful interruption, the circuit breaker must withstand TRV without re-igniting the arc between the contacts. It is paramount to investigate the behavior of the breaking duty imposed on a circuit breaker connected with an SFCL. The aim of the paper is to examine the behavior of incorporating the resistive SFCL into the distribution grid and look at the potential beneficial effect of the SFCL in reducing the circuit breaker TRV.
SUPERCONDUCTIVITY
Superconductivity is a phenomenon of exactly zero electrical resistance and expulsion of magnetic fields occurring in certain materials when cooled below a characteristic critical temperature. It was discovered by Dutch physicist Heike KamerlinghOnnes on April 8, 1911 in Leiden. Like ferromagnetism and atomic spectral lines, superconductivity is a quantum mechanical phenomenon. It is characterized by the Meissner effect, the complete ejection of magnetic field lines from the interior of the superconductor as it transitions into the superconducting state. The occurrence of the Meissner effect indicates that superconductivity cannot be understood simply as the idealization of perfect conductivity in classical physics.
The electrical resistivity of a metallic conductor decreases gradually as temperature is lowered. In ordinary conductors, such as copper or silver, this decrease is limited by impurities and other defects. Even near absolute zero, a real sample of a normal conductor shows some resistance. In a superconductor, the resistance drops abruptly to zero when the material is cooled below its critical temperature. An electric current flowing through a loop of superconducting wire can persist indefinitely with no power source.
In 1986, it was discovered that some cuprate-perovskiteceramic materials have a critical temperature above 90 K (−183 °C).[5] Such a high transition temperature is theoretically impossible for a conventional superconductor, leading the materials to be termed high-temperature superconductors. Liquid nitrogen boils at 77 K, and superconduction at higher temperatures than this facilitates many experiments and applications that are less practical at lower temperatures.
DESCRIPTION OF FAULTS
Electrical powers system is growing in size and complexity in all sectors such as generation, transmission, distribution and load systems. Types of faults like short circuit condition in power system network results in severe economic losses and reduces the reliability of the electrical system. Electrical fault is an abnormal condition, caused by equipment failures such as transformers and rotating machines, human errors and environmental conditions. Theses faults cause interruption to electric flows, equipment damages and even cause death of humans, birds and animals.
3.1 Types of Faults
Electrical fault is the deviation of voltages and currents from nominal values or states. Under normal operating conditions, power system equipment or lines carry normal voltages and currents which results in a safer operation of the system. But when fault occurs, it causes excessively high currents to flow which causes the damage to equipments and devices. Fault detection and analysis is necessary to select or design suitable switchgear equipments, electromechanical relays, circuit breakers and other protection devices. There are mainly two types of faults in the electrical power system. Those are
Symmetrical
Unsymmetrical faults
3.1.1 .Symmetrical faults
These are very severe faults and occur infrequently in the power systems. These are also called as balanced faults and are of two types namely line to line to line to ground (L-L-L-G) and line to line to line (L-L-L).
FAULT-CURRENT LIMITERS (FCL)
4.1 Summary
Fault-current limiters using high temperature superconductors offer a solution to controlling fault-current levels on utility distribution and transmission networks. These fault-current limiters, unlike reactors or high-impedance transformers, will limit fault currents without adding impedance to the circuit during normal operation. Development of superconducting fault-current limiters is being pursued by several utilities and electrical manufacturers around the world, and commercial equipment is expected to be available by the turn of the century.
4.2 Fault-Current Problem
Electric power system designers often face fault-current problems when expanding existing buses. Larger transformers result in higher fault-duty levels, forcing the replacement of existing buswork and switchgear not rated for the new fault duty. Alternatively, the existing bus can be broken and served by two or more smaller transformers. Another alternative is use of a single, large, high-impedance transformer, resulting in degraded voltage regulation for all the customers on the bus. The classic tradeoff between fault control, bus capacity, and system stiffness has persisted for decades.
Other common system changes can result in a fault control problem:
• in some areas, such as the United States, additional generation from cogenerators and independent power producers (IPPs) raises the fault duty throughout a system
• older but still operational equipment gradually becomes underrated through system growth; some equipment, such as transformers in underground vaults or cables, can be very expensive to replace
• customers request parallel services that enhance the reliability of their supply but raise the fault duty
CHAPTER 5
SUPER CONDUCTING FAULT CURRENT LIMITERS
Superconductors offer a way to break through system design constraints by presenting an impedance to the electrical system that varies depending on operating conditions. Superconducting fault-current limiters normally operate with low impedance and are "invisible" components in the electrical system. In the event of a fault, the limiter inserts impedance into the circuit and limits the fault current.
The development of high temperature superconductors (HTS) enables the development of economical fault-current limiters. Superconducting fault-current limiters were first studied over twenty years ago. The earliest designs used low temperature superconductors (LTS), materials that lose all resistance at temperatures a few degrees above absolute zero. LTS materials are generally cooled with liquid helium, a substance both expensive and difficult to handle. The discovery in 1986 of high temperature superconductors, which operate at higher temperatures and can be cooled by relatively inexpensive liquid nitrogen, renewed interest in superconducting fault-current limiters.
5.1 Working principle
The SFCL controls potentially damaging current peaks caused by short circuits in the electricity supply system. The device relies on the particular physical properties of an oxide ceramic superconductor: In regular operation, the superconducting material acts as a near-perfect electrical conductor without ohmic resistance. Unlike a conventional fault current limiting reactor, a SFCL is practically invisible in the grid. A fault current, however, will be limited within the first half cycle as it exceeds the ampacity of the oxide ceramic. The material temporarily loses its superconducting property and builds up a high ohmic resistance that keeps the fault current to a pre-defined maximum. Stress on downstream circuit breakers is diminished further, since the device reduces phase differences between fault current and voltage as well.
The fault current is limited for a pre-defined time interval to enable fault-identification, before the SFCL is temporarily disconnected from the grid. The oxide ceramic components heat up briefly during current-limiting and regain their superconducting property after automatic re-cooling to operating temperature. The device then resumes its rated current operation in the grid. Disruption-free grid operation during the system recovery interval can be provided, according to customer requirements, by inductive or resistive shunts.
TYPES OF SFCL
6.1 There are two types of SFCL
Resistive SFCL
The Inductive Limiter
6.1.1 The Series Resistive Limiter
The simplest superconducting limiter concept, the series resistive limiter, exploits the nonlinear resistance of superconductors in a direct way. A superconductor is inserted in the circuit. For a full-load current of IFL, the superconductor would be designed to have a critical current of2IFL or 3IFL. During a fault, the fault current pushes the superconductor into a resistive state and resistance R appears in the circuit.
The superconductor in its resistive state can also be used as a trigger coil, pushing the bulk of the fault current through a resistor or inductor. The advantage of this configuration, shown in Fig. 4.9, is that it limits the energy that must be absorbed by the superconductor.
The fault-current limiter FCL normally is a short across the copper inductive or resistive element Z. During a fault, the resistance developed in the limiter shunts the current through Z, which absorbs most of the fault energy.
The trigger coil approach is appropriate for transmission line applications, where tens of megawatt-seconds would be absorbed in a series resistive limiter. The trigger coil configuration also allows an impedance of any phase angle, from purely resistive to almost purely inductive, to be inserted in the line.
6.1.2 The Inductive Limiter
Another concept uses a resistive limiter on a transformer secondary, with the primary in series in the circuit. This concept, illustrated in Fig. 4.10, yields a limiter suitable for high-current circuits (IL> 1000 A). One phase of the limiter is shown. A copper winding WCu is inserted in the circuit and is coupled to an HTS winding WHTS. During normal operation, a zero impedance is reflected to the primary. Resistance developed in the HTS winding during a fault is reflected to the primary and limits the fault.
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 (HTS) winding is reflected to the primary. In the event of a fault, the large current in the circuit induces a large current in the secondary and the winding loses superconductivity. The resistance in the secondary is reflected into the circuit and limits the fault.
INTEGRATION OF A RESISTIVE SFCL IN AN ELECTRICALDISTRIBUTION SYSTEM
The majority of distribution systems are operated in a radial configuration because of the simplicity of operation and the economy of the overcurrent protection. Both of the advantages are due to the fact that in any branch of a radial system, power only flows in one direction. The most prevalent distribution voltage class is 10-15kV in power systems. The systems have continuous current ratings less than or equal to 600 amps and fault current ratings less than 20kA For a distribution level SFCL, heat management is less of a problem than it is for transmission systems. Thus the SFCL is more practical to be first put in action in distribution systems It can be expected that there will be interactions between the SFCL and power system when the SFCL is placed in the power grid. In order to evaluate the integration behavior of the SFCL in power grids, we setup a unidirectional distribution system in the latest version of the electromagnetic transient program, which is considered to be a universal program system for digital simulation of transient phenomena of electromagnetic as well as electromechanical nature. With this digital program, complex networks and control systems of arbitrary structure can be simulated.
The source impedance is seen from the secondary winding of the substation system’s step-down transformer, and it includes the transformer impedance and the upstream short-circuit impedance. A number of parallel feeders are connected to the point of common coupling (PCC) which is the path from the source to the load. The bus is supplied by a substation transformer from an 110kV network. We analyse a phase-toground fault because nearly 90% of the faults in distributionnetworks are single-phase short circuits.
CHAPTER 8
DISTRIBUTION GRID MODELING
The Alternative Transient Program (ATP) version of Electromagnetic Transient Program (EMTP) and ATP Draw one of the universal digital simulation programs for the analysis of power system transients, was chosen to carry out the investigation. The upstream source system is modeled as an infinite bus and the source impedance is composed of an equivalent resistance and inductance connected to the local distribution substation. The feeder was composed of cables, the SFCL and a circuit breaker connected between load andsource. The cable can be modeled as a circuit withinductance and capacitance per unit length. To properly account for linear load damping, the overall load is modeled asan uncoupled, lumped series R, L, C branch with a power factor of 0.886. A fault simulation switch at the load side is closed to create a single phase-to-ground short-circuit. After three cycles, the circuit breaker opened to clear the fault to recover the normal operation state. The circuit breaker ismodeled as an ideal time-controlled switch and a parallel capacitance. After a successful opening, the switch will stay open. We assume the current in the ideal switch is definitely cut and re-strike procedure is neglected. The transient behavior of the superconducting device can be described by its E~J characteristic and heating due to the resistive power dissipation in the flux flow and normal state.
Under the hypothesis of a massive transition in adiabatic and isothermal condition, heat dissipated in the superconductor will not be transferred to the liquid nitrogen, so the refrigeration by the coolant can be neglected. The resistive type SFCL was programmed to be a nonlinear resistanceevolving as a function of time. Making the SFCL model uses the MODELS language, which is a general-purpose description of language and algorithmic simulation tool in ATP to represent and study of dynamic systems Further, ATPDraw assists to create the SFCL electric circuit and editthe icon to be used as a component interactively.
The circuit current flow is an input signal to the SFCL model, and the output of the SFCL model is controlled by a TACS (Transient Analysis of Control System) controlled time-dependent resistance. There is an improvement in this resistive SFCL model in the time domain because it considers the evolution of the various parameters such as nonlinear limiting resistance, temperature rise etc. with time The simulation was carried out with a fault created at t=20ms (t=0ms at the beginning of the simulation), and the circuit breaker was opened after 3 cycles at t=80ms. (*Distribution level circuit breakers typically open in two cycles after receiving the order to open. This plus one cycle delay and sensing time makes the fastest expected distribution level opening time three cycles). The total simulation time was 0.12s according to the circuit breaker opening time, and the simulation step is 1μs. The operation current magnitude during normal condition is 350A.The initial operation temperature is 77K and critical temperature is 90K. All calculations were carried out for simple conductors without any support materials. The HTS conductor is assumed to be homogeneous along its length. During normal operation, the SFCL is insuperconducting state.
PERFORMANCE OF THE SFCL BASED ON SYSTEM STUDIES
9.1 Limitation Behavior
In the event of a single-phase short circuit in the load feeder, a very large fault current will pass through the SFCL. After the critical current is exceeded, within the first half cycle, the critical temperature is reached and the transition to
the normal conducting state quickly takes place. Figures 9.1 to figure 9.2 below show the variation of resistance and 3 temperature rise of the SFCL, as well as currents compared with and without the SFCL. In Figure 9.1 and figure 9.2, the time scale is not beyond 80ms for the sake of legibility, and alsobecause the fault has been cleared at this point and there will be no further increases in temperature and resistance of the SFCL.
CIRCUIT BREAKER TRANSIENT RECOVERY VOLTAGE
A resistive SFCL is basically a variable nonlinear resistancethat is installed in series with a circuit breaker in power grids.In the case of a fault, its resistance increases to a certain valueat which the fault current is reduced to a level that the circuitbreaker can handle. When the circuit breaker attempts tointerrupt the limited fault current, an overvoltage is developedacross the open contacts as a consequence of switching acircuit breaker or a section-switch etc. This voltage is calledTransient Recovery Voltage (TRV) of the circuit breaker,4which is imposed across the opening breaker contacts andstresses the gap insulation. Circuit breakers might fail tointerrupt fault currents when power systems have transient
recovery voltage levels, which exceed the rating of circuitbreakers. During the process of interrupting the short circuitcurrent, the system oscillates in accordance with its naturalfrequency and an arc is formed, bridging the gap between thepartincontacts.
TRV is considered to be composed of an alternatingcomponent at industrial frequency and an oscillatorycomponent with exponential decay. The Rate of Rise of theTransient Recovery Voltage (RRTRV) is an importantparameter in the power system operation, specified in Volts per Microsecond (V/μs) in IEEE C37.41 Standard.
APPLICATIONS
Three main application areas are:
Fault current limiter in the main position on bus :
• Protects the entire bus.
• A larger transformer can be used.
• I^2 Rt damage to the transformer is limited.
Fault- current limiter in the feeder position:
• `It protects an individual circuit on the bus.
• A FCL can also be used to protect individual loads on the bus.
Fault current limiter in bus tie position:
• The buses are tied, yet a faulted bus receives the full fault current of only one transformer.
• This limiter requires only a small load current rating.