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Power system is not static but changes during operation (switching on or off of generators and transmission lines) and during planning (addition of generators & transmission lines). Thus fault studies need to be routinely performed by utility engineers (GEB). The problem of the Fault current in the Power system increases day by day.

Faults usually occur in a power system due to the insulation failure, flashover (Lightning strokes), physical damage and human error. Due to that power system affected and many problems occurs like unstable power system, discontinuity in power supply, Blackout, etc. Hence, it becomes one of the most serious problem in the power system.

Therefore, the analysis of the power system under faulty condition is one of the most important and complex task in power engineering. For limiting this fault current we studied various conventional methods and devices for it and try to reduce it as possible.

In this project we tried to understand the fault current, how much it will flow in faulty condition, and try to compensate it with different limiting techniques and devices. So, that we made our power system smooth and stable.

1.2 AIM AND OBJECTIVES OF THE PROJECT.

The main aim of this project is to analyze the Fault current and try to give different limiting techniques for it.

The objective of fault analysis is that Fault analysis aims to determine the causes that gave rises to certain failures (especially repetitive breakdowns and those with a high cost) to take appropriate steps. It is important to emphasize this dual function of fault analysis. From this analysis we can also select our power equipments like circuit breaker, isolators, etc. and make our system more reliable.

1.3 PROJECT DETAILS AND PROBLEM SPECIFICATIONS

First, we have to understand the whole detail about the fault. What are the causes behind it, its impact on the power system, etc.

1.3.1 What is fault??

An electrical power system consists of generators, transformers, transmission lines, distribution line etc. Normally, a power system operates under balanced conditions. Under abnormal conditions, the system may become unbalanced.

A Fault is an abnormal condition which affects the power continuity. We can also say that The fault in a circuit is any failure that interferes with the normal system operation. A fault usually results in high current flowing through the lines and if proper protection is not used, it may damages the power system equipments. Thus, the analysis of fault level is necessary.

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1.3.2 Causes of fault occurrence.

The fault may occur on a power system due to a number of reasons. Some of the reasons are:-

1) Falling of a tree along a line.

2) Vehicles colliding with supporting structures.

3) Insulation failure of the system.

4) Birds shorting the transmission line.

5) Wind and ice loading on the transmission lines.

6) Over loading of undergrounding cables.

In fault analysis it is very important how faults are distributed in the various sections of a power system. There are many statistics on that which are available in the literature and internet as well. However, typically, the distribution is as follows:

Table 1. Distribution of fault in equipment.

SR.NO.

AREA

FAULT DISTRIBUTION 1. Overhead lines 50%

2.

Cables

10% 3. Switchgear 15%

4

Transformers

12% 5. CTs and PTs 2%

6.

Control Equipment

3% 7. Miscellaneous 8%

The probability of the failure or occurrence of abnormal condition is more on overhead power lines. This is so due to their:

¡E Greater length

¡E Exposure to the atmosphere.

1.3.3 Effect on the power system.

The fault current affect the system like damage of power equipment, short circuit of winding, Fire in substation as shown in the below figure. They reduce the Reliabilty, stability and flexibility of the power system. Due to this condition voltage fluctuation will be occurred and system will be affected.

A Fault implies any abnormal conditions there is an impact on the strength of the conductor as given below:

¡E phase conductors or ¡E phase conductors and earth, or any earthed screens surrounding the conductors.

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1.3.4 Types of Fault

If one, or two, or all three phases break or if insulators break due to fatigue weather, this fault is called a ¡§permanent fault¡¨.it will remain after a quick power removing.

Faults which are involving ionized current paths are called transient faults. They usually clear if power is removed from the line for a short time and then restored. Approximately 75% of all faults are having transient nature.

There are mainly two types of fault present in the power system network.

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1) Symmetrical Fault:-

When this type of fault occurs in the power system then system remains balanced, these faults are relatively rare, but are the easiest to analyze so we¡¦ll consider them first.

In this there are two types of fault

1) L-L-L Fault.

2) L-L-L-G Fault.

This fault can be with ground or without involving ground.

Figure 1.3.4(a) L-L-L Fault.

Types of Fault

Symmetrical Fault

Unsymmetrical Fault

2) Unsymmetrical Fault:-

This type of system is no longer balanced during this fault condition. It is very common, but more difficult to analyze.

a) Shunt Fault:- occurs due to overvoltage.

„h L-G fault

„h L-L fault

„h L-L-G fault

b) Series fault:- occurs due to over-current.

„h Open conductor Fault.

Table 2 : percentage of the different fault in system.

SR.NO

TYPE OF FAULT

OCCURRENCE 1. Symmetrical fault 5%

2.

L-L-G Fault

10% 3. L-L Fault 15%

4.

L-G Fault

70%

Mostly, in the power system L-G fault occurs due to the tree falling on line, vehicle accident with line pole etc. symmetrical fault occurs rarely.

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1.4 LITERATURE REVIEW

1.4.1 R&D Status of Fault Current Limiters for Utility Applications.

In the power system fault occurs due to the various reason..due to that our power system get affected and there is serious impact on the power system like Blackout, damages of the power equipments etc. So, we have to reduce it. for that we used different conventional methods and current limiting devices for it. In this literature, We get introduction about the FCL.

Some companies are planned to implement this superconducting fault current limiter in real world. First they tried to get the information regarding FCLs like why FCL is needed? , what is the effect of it on the power system, its different configuration with different types of FCLs(Resistive, air-core type, etc.) it also contain the project regarding implementation of FCLs in the power system. This R & D department work on it continuously.

1.4.2 Current limiting Fuse by Robert M. Pimpis, Dover, N.H.

A fuse consists of an insulating housing which is made of two housing pieces. this pieces made up of thermoplastic material, terminals extending through slots in the ends of the housing, and a fusible element with ends connected to both of the terminals. The housing consists of a tubular portion and slotted end portions located at each of the two ends of the tubular portion. Each of the terminals has an internal portion inside the housing to which a fusible element is attached, an external portion outside of the housing, and a middle portion between the internal and external portions and located within one of the slots.

This approach permits reducing the number of parts and simplifies the assembly and manufacture procedure. Preferably, the thermoplastic material which is used in the fuse having a temperature greater than 120¢X C. and includes between 20% to 40% (mostly in between 30% and 35%) filler. Suitable thermoplastic materials contain highly crystalline Nylon 4.6, polyph-thalamide, polyphenylene sulfide, and liquid crystal polymer.

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1.4.3 Fault current limiter with shield and adjacent cores.

The present invention relates to an improved form of fault current limiter device of the magnetically saturated core type. In accordance with a first exemplary aspect of the present invention, there is provided in a fault current limiter(FCL) of a saturated high permeability core type having at least one coil wound around a high permeability material. The FCL preferably can include a first and second high permeability columns, first phase coil wound around the first high permeability column, second phase coil wound around the second high permeability column. wherein the electromagnetic screen or shield can be formed around the first and second phase coils.

In this invention, We provided a fault current limiter including, at least: first and second high permeability columns. the method of suppressing the time derivative of the fault current at the zero current point includes this below steps: Utilizing an electromagnetic screen or shield around the AC coil to suppress the derivative current levels during the zero current conditions. that is the most important part of this invention as compare to the previous one.

1.4.4 Fault current limiters By Applied materials.

As We know that occurrence of the fault in power system is common due to varies causes like lightning strokes, vehicle accident with pole, ice loading, overloading, etc. so, in this journal we studied about the different FCLs which are used as current limiting devices. This applied materials implemented FCLs in the varies country like Australia, Thailand, New York USA, etc. in that they gave the full report regarding it. First they studied the impact of fault on the power system and its impacts on the system. After that simulate the system via the software and check how much abnormal current occurs, fault level, etc. and check their impact on the system.

Here, they gave brief about the SSFCL, SCFCL with its different parameters. Also they took application case and try to simulate that system effectively. after that they implement FCLs in the system and check its operation and its impact on the system they check various parameters like reliability, stability, quality of the power and flexibility of the system with FCL. Also they gave the different solutions for reducing the abnormal current.

1.4.5 Conventional methods and SCFCL by Power grid corporation.

Power continuity is the most important factor of the power system. It should be there whatever the reason is. The problem of flowing excessive abnormal current in the power system is usually occurs. For that power grid corporation tried to give the different conventional methods and devices which is work as the fault current limiting devices. in the conventional methods they suggest various method like, Changes in network topology, e.g. splitting of grids or splitting of bus bars, Introduction of higher voltage levels, Selection of transformers with a higher short-circuit impedance, Upgrading of existing switchgear and other equipment. This we can used in order to reduce fault current.

Also, they used device particularly for limiting the fault current. They give the details about the various limiting devices like SSFCL, SFCL, Capacitor combination reactors, Fault current limiting reactors. In that they compare all the parameters after implementation of the FCLs in the power system and check the reliability, Flexibility of the power system.

1.4.6 High temperature superconducting Cable.

The invention is about high temperature super conducting cable . According to the property of superconducting material whenever its temperature increases than the critical temperature it offers the higher resistance than the normal condition. It is occurs due to only the outer layer of the superconducting material is such that whenever abnormal condition occurs at that time this layer melts due to it characteristics and add in the impedance of transmission line and due to increase in the impedance of the line the abnormal current will be reduced.

The current carrying capacity of HTS cable is also higher than the conventional cable due to the different parameters of the HTS cable. So it is beneficial as if offers higher current carrying capacity in normal operating condition and offers higher resistance in faulty condition.

The technology which is existing having the limitation that whenever the fault occurs temperature of the cable (conductor) increases due to the heavy flow of fault current. It causes the heating of insulation of cable. After certain limit the insulation of cable start to melt. Melting of insulation causes breaking of conductor (open circuit) and power cut to the particular system.

Due to the invention of HTS Wire the problem discussed above can be overcome and the reliability of the system can be increased.

1.5 PLAN OF THE WORK.

First we have to study the system and get the different data¡¦s from it like fault current, fault level(MVA), etc.via the calculation methods and get the results. after that we have to study the different current limiting techniques based on the results of the system and try to implement this in our real world so, Our system can become more reliable and flexible.

1.6 MATERIALS/TOOLS REQUIRED.

1.6.1 Why FCL is necessary??

Normal operation:

High short-circuit capacity

(low short-circuit impedance)

¡E Low voltage drop (high power quality)

¡E High steady-state and transient

stability

Fault condition:

Low short-circuit capacity

(high short-circuit impedance)

¡E Low thermal and mechanical

strain

¡E Reduced breaker capacity

Optimal solution

FCL/SCFCL

¡E Low impedance during normal operation

¡E Fast and effective current limitation

¡E Automatic and fast recovery

The need for FCLs is driven by rising system fault current levels as energy demand increases and more distributed generation and clean energy sources, such as wind and solar, are added to an already overburdened system. So, we have to limit this abnormal current to save our power system from damage.

1.6.2 What are the FCLs??

A fault current occurs due to the varies causes such as lightning stroke, downed power lines, or crossed power lines cause faults. During a fault, abnormal current flows through the system often resulting in a failure of one section of that system by causing a tripped circuit breaker or a blown fuse. FCL limits this current flowing through the system and allows for uninterrupting operation of the system, similar to the way surge protectors limit damaging currents to household devices.

FCLs are a new type of power equipment that protect power system equipment from excessive large mechanical, magnetic and thermal stresses that can occur. when an electrical fault creates a low impedance path across other power system equipment or to ground. The new functionality provided by FCL¡¦s is even more critical as capacity increases to serve larger loads. This situation inherently adds to both system-wide and local fault current magnitudes. Due to that power systems ride through periodic faults to provide necessary capacity and functionality during periods of peak demand.

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1.6.3 What Are the Benefits to Utilities??

The main requirements to the FCLs are:-

„h to be able to withstand distribution and transmission voltage and currents

„h to have lower- impedance, voltage drop and power loss at normal condition.

„h to have large impedance in fault duration.

„h to have a short time recovery and to limit the fault current before the first peak

„h to properly respond to any fault magnitude and/or phase situations.

„h to withstand the fault conditions for a sufficient time

„h having a higher temperature rise endurance

„h having a high reliability and long life

„h to have fully automated operation and fast recovery to normal state after fault removal

„h to have a low cost and low volume.

1.6.4 Types of FCL.

1.6.4.1 Overview.

A variety in FCL technologies that utilize unique and novel approaches for limiting the magnitude of fault currents are now in the prototype stage of development and, if successful, will soon be ready for grid deployment. A brief overview and comparison of the types of

technologies being developed are presented as below.. The focus is on superconducting technologies, but different FCLs based on different technologies are described for completeness. A comparison table of the technologies is provided at the end of this section.

1.6.4.2 Classification of FCL.

There are basically two types of FCLs are present as given below:-

1) Active FCL - fast increasing circuit electrical parameters after fault detection.

2) passive FCL - increase the source impedance at nominal and fault conditions

1.6.4.2.1 Conventional methods for fault current protection scheme.

The existing conventional solutions to transmission-level fault current over-duty resolve the problem with varying degrees of effectiveness. Some are costly and/or have negative impact on system reliability & integrity. Some of these solutions are:-

1) Construction of new substations:- Fault current over-duty coupled along with other factors may result in a utility selecting this solution, which will correct immediate problems, as well as providing for future growth. However, this is the most expensive of all the conventional solutions. Usually we can¡¦t preferred this type of solution due to higher cost.

2) Bus splitting:- This procedure of separation of sources that could possibly feed a fault by the opening of normally closed bus ties, or the splitting of existing busses. This effectively reduces the sources that can feed a fault, but also reduces the number of sources that supply load current during regular or contingency operating conditions.

3) Multiple circuit breaker upgrades:- When a problem occurs, usually more than one breaker will be affected. Upgrade of the breakers has the disadvantage of not reducing the fault currents and their hazards also, as well as the often prohibitive expense of replacing the switchgear within a substation.

4) Current limiting reactors and high impedance transformers:- limiting reactors limit fault current due to the voltage drop across the terminals, which increase during the fault. However, this reactors also have a voltage drop under normal loading conditions and present a constant source of losses. They can interact with other system components and cause instability.

5) Sequential breaker tripping:- A sequential tripping scheme prevents CB from interrupting excessive fault currents. If fault detected, a breaker upstream to the source of fault current is tripped first. This reduces the fault current sense by the breaker within the zone protection at the location of the fault. This breaker can then safely open. A disadvantage of the sequential tripping scheme is that it adds a delay of one breaker operation before final fault clearing. Also, opening the breaker upstream to the fault affects zones that were not originally impacted by the fault.

1.6.4.2.2 Pyrotechnic Fault Current Limiter or Is- Limiter.

The Is-limiter which is the most conventional method of protection is the world¡¦s largest switching device with extremely short operating time. The function of Is-limiter is to first detect the fault current and limit the short circuit current during the first current rise itself and hence, maximum short circuit current can never be reached. Is-limiter consists of measuring and tripping devices where the current flowing through this limiter is monitored by these devices.

During the very first rise in the fault current, this device decides the necessity of tripping this Is-limiter. Therefore, for such action to be performed, the instantaneous current and the rate of rise of current in the Is-limiter is being constantly measured and evaluated. The function of Is-limiter can be well depicted through a single line diagram as below. where a short circuit downstream is obtained from an outgoing feeder breaker. Assuming a short circuit current of 50 kA flows to the faulty location through each transformer, therefore resulting in a total short circuit current of 100

kA. The current in the Is-limiter during such an event is given as I2 .In such situation Is-limiter responds very rapidly, so that there is no contribution to peak short circuit current (i1+i2) as shown in below graph.

io = Total current without Is-limiter

im = Total current with Is-limiter

i = Short circuit current at fault location

Is-limiter (Figure 1.6.5(C) has been proven for its reliability and functions in thousands of installation since 1960 and accepted worldwide. Therefore few characteristics of Is-limiters are is that they reduce the substation cost, solves all sorts of short circuit problems in new substations and substation extensions, and provides optimum solution for interconnection of switchboards and substations.

1.6.4.2.3 Superconducting Fault Current Limiter (SFCL).

Superconducting type FCLs form one of the most novel techniques for protection in accordance

with limiters. K.Okaniwa et al.10 proposed a SFCL which was developed and tested in the year

of 1991. Initially 400V-100A class fault current limiter was developed which is wound with AC

superconducting wire using Nb-Ti filaments. Generally during normal state, the trigger coil is in

superconducting state and under faulty conditions, as fault current exceeds the critical current the

coil transits to normal state, as Figure 1.6.4(d)

High temperature superconducting FCL is the most existing SFCL in existing power system. Seungje Lee et al.11 proposed the stability analysis of power system when SFCL is installed which forms the next stage after development process. Parameters were modeled in 440V class prototype SFCL and the transient performance was solved by numerical methods. In this case, the problem synchronization is solved by maintaining synchronism in both three phase fault and single phase line to ground fault.

Figure 1.6.4(e) Superconducting FCL

Generally there exists two types of superconducting Fault current Limiters (SFCL).

1. Resistive type SFCL

2. Inductive type SFCL:

a) shielded iron core type SFCL.

b) saturated iron core type SFCL

1.6.4.2.4 Resistive type SFCL.

The resistive type fcl having a superconducting material is used while the fault condition.

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The principle of their operation is shown in the one-line diagram at the top of Figure 1.6.5(e). The quench process in resistive SFCLs results in heat that must be carried away from the superconducting element by the cryogenic cooling system. Typically, there is a temperature rise in the superconducting element that results in a loss of superconductivity until the cryogenic system can restore the operating temperature. This period is known as ¡§recovery time¡¨, is a critical parameter for utility systems.

When a fault occurs, the current increases and causes the superconductor is used to quench thereby increasing the resistance exponentially. The current level is determined by the operating temperature, amount and type of superconductor. The rapid increase in resistance produces a voltage across the superconductor and results in the current to transfer to the combination of inductor & Resistor.

This combination limits the voltage increase during a quench. In essence, the superconductor acts like a switch with millisecond response that initiates the transition of load current to the shunt impedance. Ideally, the incipient fault current is limited by less than one cycle.

Figure 1.6.4(f) Resistive type SFCL.

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Figure 1.6.4(g) Components of SFCL.

Figure 1.6.4(h) voltage and current characteristic.

37

Figure 1.6.4(i) Characteristic of Resistive Type FCL.

1.6.4.2.5 Shielded iron core type FCL.

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Figure 1.6.4(j) shielded iron core.

A variation of the resistive type of limiter that allows the HTS cryogenic environment to remain

mechanically isolated from the rest of the circuit. An electrical connection is made between the

line and the HTS element through mutual coupling of AC coils via a magnetic field.

Basically, the device resembles a transformer with the secondary side shunt by an HTS element

at fault, increased current on the secondary causes the HTS element to quench, resulting in a

voltage increase across L1.

Figure 1.6.4(k) Shielded iron core type FCL.

Although the superconductor in the shielded-core design has to re-cool after a limiting action. In,

non-uniform heating of the superconductor (hot spots) is easier to avoid through optimization of

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the turns ratio. A drawback of this technology is that it is approximately four times the size and weight of purely resistive SFCLs.

1.6.4.2.6 Saturable core Type FCL.

The resistive & shielded-core SFCLs are rely on the quenching of superconductors to achieve higher impedance, saturable-core utilize the dynamic behavior of the magnetic properties of iron for changing the inductive reactance on the AC line. The concept utilizes two iron cores and two AC windings for each phase. The AC windings are made of conventional conductors which are wrapped around the core to form an inductance in AC line. The iron core also has a constant-current superconductive winding that provides a magnetic bias.

Figure 1.6.4(l) Saturable core type FCL.

1.6.4.2.7 Bridge Type of FCL.

The bridge type SFCL is shown in above figure. Under normal condition the combined DC and AC current remains low enough to allow all the diodes or thyristors to be biased forward. hence, the AC current bypasses the inductance. In these conditions the FCL impedance is low, the total voltage drop and loss are dominated by the power diodes.

If a fault occurs & AC current exceeds the DC bias current two diodes will switch into a blocking mode for each half cycle and insert the inductor into the circuit. The coil impedance will limit the transient current. If thyristors are used instead of the diodes it is possible to turn off the current within the half-cycle.

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Figure 1.6.4(m) circuit of Bridge type FCL.

The main disadvantage of bridge type FCL is the relatively high total losses. The major characteristic of the bridge type of FCL is as given below:-

+No superconductor Quench

+immediate Recovery.

+Adjustable trigger current.(current flowing through L)

-Losses in superconductor.

1.6.4.2.8 High temperature superconducting Cable.

Traditionally, the main stream of power delivery system are composed by ACSR in overhead line and XLPE underground cable.

Since, the late 1980¡¦s, the interest in superconducting cables for power transmission and distribution purposes has been renewed. The development of HTS Cables for high capacity power transmission has been started over the last decade to take benefits of the efficiency and operational benefits due to the use of liquid nitrogen for cooling, which represents a cheap environmental friendly medium.

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Figure 1.6.4(n) Layers of HTS cable.

HTS superconducting cable having zero resistance and low inductance, can increase transfer capacity about 3 to 5 times more than conventional XLPE cable with the same size and can reduce power transmission loss and construction cost. By USA, three level of HTS cable is compared to substitute the overhead lines. Below figure shows the relative power increase compare HTS cable to XLPE cable.

Figure 1.6.4(o) Comparison of HTS and XLPE cable.

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HTS cable for power transmission is developed two types of design. The one is WD, the other is CD.

Figure 1.6.4(p) Construction of WS and CD Type HTS cable.

In WD cable,LN2 flows in the tube type former which sustains HTS cable on its outer circle. HTS conductors are surrounded by cryostate which insulates heat transfer. The dielectric is outer of the cryostate. Hence, the dielectric does not to be cooled with LN2(Warm Dielectric). Because WD type HTS cable can not only preserve conventional cable dimension and use proved dielectric materials, but also limited HTS conductors are used it is cost effective and efficient in design of cooling system.

In CD cable, LN2 flows the outer and inner duct of cable and it cools not only HTS conductor but also dielectric material. Important difference between CD and WD is that CD has return HTS screening conductors which shields outer magnetics and make low inductance.

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ľ Characteristics of HTS cable.

1) Electrical characteristics:- Brief comparison of electric characteristics among power delivery systems are suggested in table 2. WD type can transfer about 2 times power than conventional cable at same power loss, however, CD type can transfer about 4.5 times power. Below table shows brief comparison between WD and CD type.

Table 3.comparison of WD and CD HTS cable.

CONVENTIONAL

HTS(WD)

HTS(CD) Pipe outer dia.(mm) 200 200 200

Voltage (KV)

115

115

115 Power rating (MVA) 220 500 1000

Power loss(W/MVA)

300

300

200

Figure 1.6.4(q) Temperature and Resistivity of HTS conductor.

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1.6.4.2.9 Fault current limiting reactors.

Short circuit currents of power systems are increasing with an increasing rate, due to the fast development of generation and transmission systems. Reactor is one of the effective short circuit limiter. This technique is to be more practical than other available approaches. Compared with many other methods, CLR is more economical. In addition, the reliability of substation is negligible. However, it occupies a relatively large area in the substation, due to safety considerations.

A 3-phase air cored reactor designed for in-line operation in LV circuits, and typically installed where fault levels are found to be excessive due to reduced source impedance. The Inductance of the reactor is designed to reduce the downstream fault level to suit the fault level rating of connected equipment, and will remain constant at any current up to and including the rated short time current. The reactance value will cause a voltage drop across the reactor, but due to the higher Q factor, the phase angle of this voltage drop will be such that voltage drop at the load equipment generally will be negligible.

Figure 1.6.4(r) Fault current limiting Reactor.

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Three coils are wound on a vertical axis. Winding former is high temperature fiber glass, and conductors having high conductivity class H enamel covered copper, with additional Nomex insulation. Coils are braced with polyester filled fiber glass blocks and clamped firmly together by means of stainless-steel tie rods and aluminum structural members. The entire structure is designed to withstand the forces associated with operation at the rated short time current.

This construction also assures very low audible noise and vibration. Ducts are distributed throughout each winding to allow vertical air flow through the winding by convection. By this average winding and hot-spot temperature rise is maintained well within limits under all rated site conditions. reactors are designed specifically for this application and are entirely suitable. Iron-cored or air-cored designs adapted from HV designs, are not suitable for this application. Mostly this type of Reactors are used for the LT side voltage level due to its characteristic and design configuration of the Reactor.

1.6.4.3.0 ARC type FCL.

When a short circuit fault occurs in the transmission and distribution systems due to the lightning stroke, high current several of times as load current flows to the fault point. in case of high power arc occurs due to the fault current with dielectric breakdown. It Required to protect power equipment and ensure public security.

Various materials around a fault point are decomposed to molecules, atoms, ions and electrons and finally reach a plasma state. High power arc produces enormous heat, blinding flash and detonating sound.

Figure 1.6.4(s) High power arc(30KA)

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Principal results

1) Development of technology for limiting fault current immediately

Arc driven type fault current limiter for 6KV class distribution lines has been developed by joint research with Tohoku electric power co., inc. and Sankosha co.ltd.

2) Development of arcing horns for interrupting fault current instantaneously

Fault current interrupting arcing horns for 66/77 kV transmission and 22 kV distribution lines has been developed by joint research. Also, they developed simulation model of this type of device to analyze the power system conditions.

Figure 1.6.4(t) Arc driven type FCL.

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Figure 1.6.4(u) Arcing Horns.

1.6.4.3.1 High voltage Fuse.

Current-limiting fuses are devices to protect high voltage systems against short-circuit. They provide protection against thermic and dynamic damages that would occur in case of a short-circuits or over loadings more than the minimum interrupting capacity.

high voltage and high interrupting capacity current-limiting fuses are \"back-up\" fuse type, as per the IEC-282.1 and NMX-J-149-1 standard definitions and they are able to interrupt current. Currents between the rated current In and the minimum interrupting capacity I3 are not interrupted safely. fuses are designed and manufactured as per the standards IEC 60282-1, DIN 43625, etc.

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Figure 1.6.4(v) High voltage fuse.

The fuse design is based on a series of arcing chambers, where the voltaic arc produced by the fusion of the fuse link is extinguished. Fuse links, formed by one or several 99.9 % pure silver strips, are uniformly wound over a star shaped strip-holder built out of steatite (a ceramic material with great mechanical and thermal resistance). Due to tooth edge design, the star-shaped body guarantees the fuse links safe and firm position.

The star-shaped body and the fuse links are put into a porcelain tube that is the cylindrical body of the fuse, thus forming the series of arcing chambers. In these chambers, a part of the voltaic arc produced by the fusion or evaporation of the fuse link when a short-circuit occurs, is started, developed and extinguished. As per above, the uniform distribution of the voltaic arc & the resulting voltage of the fuse operation is assured.

The high interrupting capacity and the rated current with wide range, are mainly the result of these design features which allow the dissipation of the thermic energy generated during the fusion and evaporation process distributed evenly. The fuse filled with sand of specific granulometry and formulation to provide the proper environment for voltaic arc cooling and quenching through the absorption and dissipation of the generated heat and condensation and solidification of the evaporated metal.

Normally this high voltage fuse are available up to the 36 kV only. For above this level HV fuses are not available.

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Ik\" short-circuit prospective current (in case the fuse does not exist) (rms value)

IS fusion current (peak value)

ID let-through current (peak value)

ID = IS short-circuit current limited by the fuse

Ts pre-arcing time (fusion time)

1.6.4.3.2 Solid state Fault Current Limiter (SSFCL)

Solid State Fault Current Limiter (SSFCL) is proposed here consisting of power semiconductor devices consisting of desirable features such as high blocking voltage, low onstage voltage, low conduction loss and thermal management. Power semiconductor devices such as the GTO, IGBT, SCR, and IGCT are the most promising devices used in SSFCL.

Generally SSFCL consists of thyristor controlled reactor and series capacitor where the former reduces the short circuit current and the latter increases the transmitted power.

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Figure 1.6.4(w) SSFCL.

This consists of both series and parallel resonant circuits that are being tuned to supply

frequency. Under normal condition, very low impedance is provided through series resonant

circuit and under fault conditions, SSFCL provides high impedance by parallel resonant circuit.

As compared to the limiters described above, SSFCL forms the vital device in R&D. Research is

being on the power semiconductor device that is to be used in fault current limiter that limits

both transient and sub transient currents in the first cycle.

Figure 1.6.4(x) Prototype of SSFCL by Siemens.

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1.6.4.3.3 Hybrid Fault Current Limiters

Hybrid fault current limiters were developed in the year of 2000 which combines

superconducting fault current limiters and solid state fault current limiters. This hybrid type

consists of superconductors that are used as a delayed reacting resistive limiting element which is

done in parallel with the fast acting load switch. In this case, superconductors carry the fault

current only under switching and therefore the total loss of the system is comparatively less than

other conventional SFCL. Therefore, cooling of such superconductors is essential which is

provided by gaseous nitrogen that is feasible and flexible in operation.

Power semiconductor devices such as integrated gate commutated thyristors (IGCT) with

superconducting fault current limiters is proposed as a hybrid device in. The reliability of fast

switch is being enhanced by this hybrid device where fast switch utilizes vacuum interrupter in

order to open and close the primary power line. In this case, IGCT operates to remove the

permanent current from vacuum interrupter thereby enhances the reliability.

Figure 1.6.4(y) Circuit of Hybrid type FCL.

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From the above figure, module 1 consists of HTS element and fast switch where HTS element is used for fault sensing and current commutation.

The reactor is introduced in module 2 comprising the solid state switches in order to limit the fault current. Module 3 is utilized to bypass the fault current during islanding mode of micro-grid. In order to limit the transient current and shorten the acting time to limit the steady state current, hybrid bridge type SFCL is proposed in [29] using power electronics devices such as MOSFET, IGBT etc.

The major advantage of the proposed device is that it gives fast response that suits both fault occurrence and power system recovery and at the same time has the ability to limit both transient and steady state currents.

1.6.4.3 Comparison FCL Technologies

A comparison of the various FCL technologies described in this chapter is presented in below table. It provides a general view of operational capability, performance, and size. Much of the information provided here is derived from a more comprehensive comparison from an EPRI survey conducted in 2005. The bases of this generalized comparison are the three FCL operating regimes outlined below .

¡E Normal operation

¡E Operation during the fault-limiting action (fault condition)

¡E Recovery period following a fault.

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Figure 1.6.4.3(a) Characteristic of FCL.

Table 4: State of the art

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Table 5: Novel approaches

1) depending on the layout of the device

2) PTC: positive temperature coefficient

3) with integrated series switch

4) numbers of operation is limited

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Table 6 :Comparison of the General Characteristics of FCL Technologies.

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