24620708 feeder protection

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Feeder Protection


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What is a Feeder?

Overhead lines or cables which are used todistribute the load to the customers. Theyinterconnect the distribution substations

This is an electrical supply line, eitheroverhead or underground, which runs fromthe substation, through various paths, endingwith the transformers. It is a distributioncircuit, usually less than 69,000 volts, whichcarries power from the substation. with theloads.


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Why Protection Is Important?

The modern age has come to dependheavily upon continuous and reliableavailability 0f electricity and a high quality ofelectricity too. Computer and

telecommunication networks, railwaynetworks, banking and continuous powerindustries are a few applications that justcannot function without highly reliable power

source. No power system cannot be designed insuch a way that they would never fail. So,protection is required for proper working.


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Basic Requirements of

Protection A protection apparatus has three main functions:1. Safeguard the entire system to maintain continuity of

supply

2. Minimize damage and repair costs where it sensesfault

3. Ensure safety of personnel

Protection must be reliable which means it must

be:1. Dependable: It must trip when called upon to do so.2. Secure: It must not trip when it is not supposed to.


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Basic Requirements of

Protection These requirements are necessary for earlydetection and localization of faults and for promptremoval of faulty equipment from service.

Selectivity:To detect and isolate the faulty item only. Stability:To leave all healthy circuits intact to ensure

continuity or supply.

Sensitivity:To detect even the smallest fault, currentor system abnormalitiesand operate correctly at itssetting before the fault causes irreparable damage.

Speed:To operate speedily when it is called upon todo so, therebyminimizing damage to thesurroundings and ensuring safety to personnel.


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What Is Fault?

A fault is defined as defect in electricalsystems due to which current is directedaway from its intended path.

It is not practical to design and build electricalequipment or networks to eliminate the

possibility of failure in service. It is thereforean everyday fact that different types of faultsoccur on electrical systems, howeverinfrequently, and at random locations.


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Classification of faults

Faults can be broadly classified into twomain areas

which have been designated as Active faults

Passive faults


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Active Faults

The active fault is when actual currentflows from one phase conductor to another(phase-to-phase), or alternatively from onephase conductor to earth.

This type of fault can also be further

classified into two areas Solid Fault

Incipient Fault


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Solid Faults

The solid fault occurs as a result of an immediatecomplete breakdown of insulation as wouldhappen.

In these circumstances the fault current would bevery high resulting in an electrical explosion.

This type of fault must be cleared as quickly aspossible, otherwise there will be: Increased damage at fault location

Danger of igniting combustible gas in hazardousareas

Increased probability of faults spreading to healthyphases


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Incipient Fault

The incipient fault is a fault that starts as asmall thing and gets developed intocatastrophic failure.

Some partial discharge in a void in theinsulation over an extended period can

burn away adjacent insulation, eventuallyspreading further and developing into asolid fault.


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Passive Faults

Passive faults are not real faults in the truesense of the word, but are rather conditionsthat are stressing the system beyond its

design capacity, so that ultimately activefaults will occur. Typical examples are: Overloading leading to over heating of

insulation

Overvoltage Under frequency Power swings


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Transient and Permanent Faults

Transient faults are faults, which do notdamage the insulation permanently and allowthe circuit to be safely re-energized after a

short period. Transient faults occur mainly on outdoor

equipment where air is the main insulatingmedium.

Permanent faults, as the name implies, arethe result of permanent damage to theinsulation.


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Symmetric and Asymmetric

Faults A symmetrical fault is a balanced fault

with the sinusoidal waves being equal

about their axes, and represents a steady-state condition.

An asymmetrical fault displays a DC

offset, transient in nature and decaying tothe steady state of the symmetrical faultafter a period of time.


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Basic Fault Clearing Mechanism


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The main requirement of line protection is

1. In the event of short circuit, the circuitbreaker near to fault should open and all

other circuit breakers remain in closedposition.

2. If the circuit near to fault fail to trip, back upprotection should be provided by the adjacent

circuit breaker.3. The relay operating should be the smallestpossible in order to preserve system stabilitywithout unnecessary tripping of circuits.


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Types of protection

The need to analyze protection schemeshas resulted in the development of

protection coordination programs.Protection schemes can be divided intotwo major groupings:

Unit schemes Non-unit schemes


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Unit Type Protection

Unit type schemes protect a specific area of thesystem, i.e., a transformer, transmission line,generator or busbar.

The most obvious example of unit protection schemesis based on Kerchiefs current law the sum of thecurrents entering an area of the system must be zero.Any deviation from this must indicate an abnormalcurrent path. In these schemes, the effects of anydisturbance or operating condition outside the area ofinterest are totally ignored and the protection must bedesigned to be stable above the maximum possiblefault current that could flow through the protectedarea.


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Non unit type protection

The non-unit schemes, while also intended toprotect specific areas, have no fixed boundaries.As well as protecting their own designated areas,

the protective zones can overlap into other areas.While this can be very beneficial for backuppurposes, there can be a tendency for too great anarea to be isolated if a fault is detected by differentnon unit schemes.

The most simple of these schemes measurescurrent and incorporates an inverse timecharacteristic into the protection operation to allowprotection nearer to the fault to operate first.


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The non unit type protection systemincludes following schemes:

Time graded over current protection Current graded over current protection

Distance or Impedance Protection


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Over current protection

This is the simplest of the ways to protecta line and therefore widely used.

It owes its application from the fact that inthe event of fault the current wouldincrease to a value several times greater

than maximum load current. It has a limitation that it can be applied

only to simple and non costly equipments.


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Earth fault protection

The general practice is to employ a set of twoor three over current relays and a separateover current relay for single line to ground

fault. Separate earth fault relay providedmakes earth fault protection faster and moresensitive.

Earth fault current is always less than phasefault current in magnitude. Therefore, relayconnected for earth fault protection isdifferent from those for phase to phase faultprotection.


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Earth fault protection


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Time graded protection

This is a scheme of over current protection isone in which time discrimination isincorporated. In other words, the time setting

of the relays is so graded that minimumpossible part of system is isolated in theevent of fault.

We are to discuss the application of the timegraded protection on Radial feeder Parallel feeder Ring feeder


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Protection of radial feeder

The main characteristic of the radial feeder is thatpower can flow in one direction only fromgenerator to supply end of the load line.

In radial feeder number of feeders can beconnected in series and it is desired that smallestpart of the system should be off in the event offault.

This is achieved by time graded protection. In this system time setting time setting of a relay is

so adjusted that farther the relay from thegenerating system lesser the time of operation.


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Drawbacks of time graded

protection on radial feeder The drawbacks of graded time lag overcurrent protection are given below: The continuity in the supply cannot be

maintained at the load end in the event of fault. Time lag is provided which is not desirable in on

short circuits.

It is difficult to co-ordinate and requires changes

with the addition of load. It is not suitable for long distance transmission

lines where rapid fault clearance is necessary forstability.


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Protection of parallel feeder

For important installations continuity of supply is amatter of vital importance and at least two linesare used and connected parallel so as to share

load. In the event of fault occurring the protecting device

will select the faulty feeder and isolate it whileother instantly assumes increased load.

The simplest method of obtaining such protectionis providing time graded over relays with inversetime characteristics at one end and reverse powerdirectional relay at the other end.


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Protection of ring main feeder

The ring main is a system of interconnection between a series of power

stations by an alternate route. Thedirection of power flow can be changes atwill.


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IDMT Relay

In time graded protections IDMT (Inversedefinite minimum time) relays are used.

As the name implies, it is a relay monitoring

the current, and has inverse characteristicswith respect to the currents being monitored.This relay is without doubt one of the mostpopular relays used on medium- and low-voltage systems for many years, and moderndigital relays characteristics are still mainlybased on the torquecharacteristic of this typeof relay.


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IDMT relay


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Block diagram of IDTM Relay


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It can be seen that the operating time of an IDMTL relay is

inversely proportional to function of current, i.e. it has along operating time at low multiples of setting current anda relatively short operating time at high multiples of setting

current.


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Current graded protection

It is an alternative to time graded protection and isused when the impedance between twosubstations is sufficient.

It is based on the fact that short circuit currentalong the length of protected length of the circuitdecreases with increase in distance between thesupply end and the fault point.

If the relays are set to operate at a progressively

higher current towards the supply end of the linethen the drawback of the long time delaysoccurring in the graded time lag system can bepartially overcome.


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DISTANCE OR IMPEDANCE

PROTECTION A distance relay, as its name implies, has theability to detect a fault within a pre-set distancealong a transmission line or power cable from itslocation.

BASIC PRINCIPLEThe basic principle of distance protection

involves the division of the voltage at the relayingpoint by the measured current. The apparent

impedance so calculated is compared with thereach point impedance. If the measuredimpedance is less than the reach pointimpedance, it is assumed that a fault exists on theline between the relay and the reach point.


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BASIC PRINCIPLE OPERATION OF IMPEDANCERELAY


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BALANCED BEAM PRINCIPLE OF IMPEDANCERELAY

The voltage is fed onto one coil to providerestraining torque, whilst the current is fed tothe other coil to provide the operating torque.

Under healthy conditions, the voltage will behigh (i.e. at full-rated level), whilst the currentwill be low thereby balancing the beam, andrestraining it so that the contacts remain

open. Under fault conditions, the voltagecollapses and the current increasedramatically, causing the beam to unbalanceand close the contacts.


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Three stepped distance

protection Zone 1First step of distance protection is set to

reach up to 80 to 90% of the length of the line

section. This is instantaneous protection i.e.there is no intentional delay .

Zone 2second zone is requires in order to provide

primary protection to remaining 10 to 20% ofthe line and a cover up to 50% of the nextline section. The operating time of this zoneis delayed so as to be selective with zone 1.


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Three stepped distance

protection Zone 3The third zone is provided with an intention to

give full back up to adjoining line section. Itcovers the line of the section, 100% of thenext line section and reaches farther into thesystem. The motivation behind the extended

reach of this step is to provide full back up tothe next line section. Its operating time isslightly more than that of zone 2.


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Main or Unit Protection


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Main or Unit Protection

The graded over current systems described earlierdo not meet the protection requirements of apower system. The grading is not possible to beachieved in long and thin networks and also it canbe noticed that grading of settings may lead to

longer tripping times closer to the sources, whichare not always desired. These problems havegiven way to the concept of unit protectionwhere the circuits are divided into discretesections without reference to the other sections.

The power system is divided into discrete zones.Each zone is provided with relays and circuitbreakers to allow for the detection and isolation ofits own internal faults.


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Back-up Protection

It is necessary to provide additionalprotection to ensure isolation of the faultwhen the main protection fails to functioncorrectly. This additional protection is referredto as back-up protection.

The fault is outside the zones of the mainprotection and can only be cleared by theseparate back-up protection. Back-upprotection must be time delayed to allow forthe selective isolation of the fault by the mainor unit protection.


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Types of Main Protection

Following types of main or unit protectionsare used in feeder networks

Differential protection Carrier current protection using phase

comparison

Translay Y protection system


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Methods of obtaining selectivity

The most positive and effective method ofobtaining selectivity is the use of differentialprotection. For less important installations,

selectivity may be obtained, at the expense ofspeed of operation, with time-gradedprotection.

The principle of unit protection was initially

established by Merz and Price who were thecreators of the fundamental differentialprotection scheme.


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Differential protection

Differential protection, as its name implies,compares the currents entering andleaving the protected zone and operates

when the differential between thesecurrents exceeds a pre-determinedmagnitude. This type of protection can bedivided into two types, namely Balanced current

Balanced voltage


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Balanced current Protection

The CTs are connected in series and thesecondary current circulates between them.The relay is connected across the midpoint

thus the voltage across the relay istheoretically nil, therefore no current throughthe relay and hence no operation for anyfaults outside the protected zone. Similarly

under normal conditions the currents, leavingzone A and B are equal, making the relay tobe inactive by the current balance.


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Differential protection using current balancescheme (external fault conditions)


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Differential protection and internal fault conditions


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Balanced current

Protection The current transformers are assumedidentical and are assumed to share theburden equally between the two ends.

However, it is not always possible to haveidentical CTs and to have the relay at alocation equidistant from the two end CTs. Itis a normal practice to add a resistor in serieswith the relay to balance the unbalancecreated by the unequal nature of burdenbetween the two end circuits. This resistor isnamed as stabilizing resistance.


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McColl circulating current protection for single phasesystems


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Balanced voltage system

As the name implies, it is necessary to createa balanced voltage across the relays in end Aand end B under healthy and out-of-zone

fault conditions. In this arrangement, the CTsare connected to oppose each other .Voltages produced by the secondary currentsare equal and opposite; thus no currents flow

in the pilots or relays, hence stable onthrough-fault conditions. Under internal faultconditions relays will operate.


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Balanced voltage system external fault (stable)


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Balanced voltage system, internal fault (operate)


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Translay Y Protection

system The system can be employed for theprotection of single phase or 3-phasefeeders, transformer feeders and parallelfeeders against both earth and phasefaults.

It works on the principle that currententering one end of the feeder at anyinstant equals the current leaving thefeeder.


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Translay Y Protection system


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Advantages of Translay system

The capacitance currents do not effect theoperation much.

Only two pilot wires needed. The current transformers of normal

designs are employed i.e. air core type

The pilot resistance do not effect theoperation as the major part of power isobtained from CTs for operation.


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Carrier current protection usingphase comparison

In this type of relay we exploit the phase shiftundergone by the current at the end by whichis nearest to the fault.

The end which is far from the fault cannotdiscern any changes in the phase of faultcurrent and the closer end sees a sharp,almost 180 change in the phase current.

Under normal conditions, load currents andexternal fault currents can be arranged to beexactly out of phase but in case of internalfaults the currents become in phase.


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Time taken to clear faults

With the inherently selective forms of protection,apart from ensuring that the relays do not operateincorrectly due to initial transients, no time delay isnecessary. Operating times for the protection,

excluding the breaker tripping/clearing time aregenerally of the following order: Machine differential few cycles Transformer differential 10 cycles Switchgear (busbar) differential 4 cycles Feeder differential few cycles

These operating times are practically independentof the magnitude of fault current.


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Advantages of unit protection

Fast and selectiveUnit protection is fast and selective. It will

only trip the faulty item of plant, thereby ensuringthe elimination of any network disruptions.

No time constraintsTime constraints imposed by the supply

authorities do not become a major problemanymore.

Future expansion relatively easyAny future expansion that may requireanother in-feed point can be handled with relativeease without any change to the existing protection

24620708 feeder protection

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