general presentation on relay development.pdf

7/30/2019 General Presentation on Relay Development.pdf

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Power System Protection Philosophy:Past, Present and Future

Department of Electrical Engineering,

Indian Institute of Technology Roorkee

R. P. Maheshwari, Ph. D.Professor


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Modern electrical power systems contains major

components such as Generators, Transformers, T&DLines and Motors.Faults are inevitablesystem configuration

change in the length of the line during operationthe amount and nature of the loading.

Short-circuits not only damage not only to the faultycomponent but also to the neighboring components

and to the complete power system.It is important to limit the damage to a minimum by

speedy isolation of the faulty section withoutdisturbing the working of the rest of the system.

Introduction


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Functions of Protective Relay Schemes

Sense the fault.To operate the correct C.B. so as to disconnect only

the faulty equipment from the system as quickly as

possible thus minimizing the trouble and damagecaused by faults.To operate the correct C. B. to isolate the faulty

section from the healthy system in case of

abnormalities like O/L, O/V, U/V, Unbalance.To clear the fault before the system becomes

unstable.To give indication as to where to fault has occurred.


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Basic Tripping Circuit

F

R

R -1

A

xA x-1 52

C .

T .

P .

T .

52-

T C

For a fault on a feeder, CT & PT transmits the faultcurrent and post fault voltage to current and

potential coil of a protective relay.The relay operates as per the characteristics and its

contact closes.


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Basic Tripping Circuit

The closure of the contact energizes the coil of anauxiliary relay.The auxiliary relay is required (i) to perform

functions such as annunciations, alarms etc (ii) Toincrease relay sensitivity as it wont carry high tripcoil current.On the operation of auxiliary relay, the trip coil of CB

is energized and CB trips.


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Zones of Protection

P.S. is divided into a no of protective zones eachcovering one type of equipment.There will be CBs and relays associated with each

zone.The zones of protection are overlapped so that there

is no blind spot.


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Requirements of Protective SystemSelectivity:-This is the property by which only the

faulty element of the system is isolated and the

remaining healthy sections are left intact. Selectivityis absolute if the protection scheme responds only tofaults within its own zone (exp.Diff. Prot). It isrelative if it is obtained by grading the settings ofthe protections of several zones, all of which mayrespond to a given fault (O/C and Distance).Speed:-Faster the speed of operation of elements of

protective system (relay and breakers), lesser is thedamage to the equipment.Sensitivity:-It is the ability of the protective device to

react correctly to the relatively low fault signals.


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Requirements of Protective SystemDiscrimination:-Protective system should be able to

discriminate between fault and loading conditions

even when the minimum fault current is less thanthe maximum load current.Stability:-The quality of protective system by virtue

of which it remains inoperative under specified

conditions, usually associated with high values offault currents.Reliability:-Quantitatively it can be expressed as

inverse probability of failure. It can be categorized

as: (i) Dependability: It is the certainty of correctoperation in response to system trouble. (ii)Security: ability of the protection schemes to avoidmaloperation between faults.


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Main and Back-Up ProtectionFor each zone of protection, two types of relays,

main and back-up schemes are provided.

Relay back-up:-In this scheme, main relays, CTs, PTsneed to be duplicated and hence not used.Breaker back-up:-When feeder breaker fails, to trip

on a fault, the feeder fault becomes a bus-bar fault.

In breaker back-up scheme, a time-delay relay isoperated by the main relay, to trip all the otherbreakers on bus.Remote back-up:- is provided by a relay on the next

station towards the source. This remote relay willtrip in a delayed time if the breaker in the faultysection fails to trip. This is the most widely usedform of back-up protection.


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Historical DevelopmentElectromechanical Relays:-These relays were

rugged, reliable and are still used by the utilities.But as these relays consist of moving parts, thereare problems of friction, low torque, high burdenand high power consumption for auxiliary

mechanisms.Static Relays:-came in 1950s. They have many

advantages such as low burden, precise andcomplex characteristic and small size. However, their

cost is little high as compared to electromechanicalrelays. They may maloperate in case of temperaturevariations, mechanical vibrations etc.


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Historical Development

Micro-processor based Relays:-Came in 1970s.

Advantages:-(i) They provide many functions suchas multiple setting groups, programmable logic,adaptive logic, self-monitoring, self-testing,sequence-of-events recording, oscillography, andability to communicate with other relays and controlcomputers. (ii) The cost per function ofmicroprocessor-based relays is lower as compared to

the cost of their electromechanical and solid-statecounterparts. (iii)Allow users to develop their ownlogic schemes, including dynamic changes in thatlogic.


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(iv) Microprocessor-based relays place significantly lessburden on instrument transformers than the burdenplaced by the relays of the previous technologies.

(v) Microprocessor- based protection systems requiresignificantly less panel space than the spacerequired by electromechanical and solid-statesystems that provide similar functions.

(vi) Reporting features, including sequence of eventsrecording and oscillography are another feature ofmicroprocessor-based protection systems.


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Shortcomings:-

(i) However, microprocessor-based protection systemswill always remain more susceptible to problemslike Electro Magnetic Interference (EMI), RadioFrequency Interference (RFI), etc.

(ii) Microprocessor-based devices have short life cycles.While each generation of microprocessor-basedsystems increases the functionality compared with

the previous generation, the pace of advancementsmakes the equipment obsolete in shorter times.This makes it difficult for the users to maintainexpertise with the latest designs of the equipment.


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Historical Development Shortcomings:-(iii)The multi-function microprocessor-based relays

have a significant number of settings. Theincreased number of settings may pose problems inmanaging the settings and in conducting functionaltests. Setting-management software is generallyavailable to create, transfer, and track the relaysettings. Special testing techniques, specifically theability to enable and disable selected functions, are

generally used when microprocessor-based relaysare tested. This increases the possibility that thedesired settings may not be invoked after testing iscompleted. Proper procedures must be followed to

ensure that correct settings and logic are activated.


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Historical Development Digital Relaying:-

The concept of digital computer relaying has grown

rapidly as digital computers have become morepowerful, cheaper and sturdier. It has beenobserved that digital relays can realize some veryuseful functions which are not possible with

electromechanical or analog circuits, such asmathematical functions, long-term storage of pre-fault data, and, they also inherit all the features ofmicroprocessor based relays. However, thesecomputer relays do not have successful solutions tothe cumbersome problems such as high faultresistance, mutual coupling, remote infeed, time

delay.


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Adaptive Relaying:-

(i) All the settings are usually selected on the basis ofworst case and changed only when a major changein the system configuration is made. This requireshigh degree of professionalism on the part of the

user to decide as to when and what changes tomake in the settings.

(ii) Relay settings which are selected for the worst casewould generally give slow speed, low sensitivity orpoor selectivity on other conditions in the protectedsystem.


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Adaptive Relaying:-

(iii)A fixed operating characteristic of a given relaymay not be able to give the requisite speed,selectivity and sensitivity on all the operatingconditions of the protected system.

(iv)Relay engineers have dreamed that relay couldadapt to the system changes.

(v)With the development of high speedmicroprocessors, new tools for signal processingand digital communication techniques, this dream isfast turning true.


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Adaptive Relaying:-(vi) With the use of programmable devices in digital

relays, it is possible to design a relay suchthat it changes its settings, parameters or even the

characteristic automatically and appropriately inaccordance with the changed condition of thesystem protected by it.

(vii) A relay having such a feature is termed as an

adaptive relay.


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Historical Development Adaptive Relaying:-(i) The idea of modifying relay settings to correspond

to changing system conditions, as a preventiveaction to improve system stability, was firstproposed by DyLiacco in 1967. Thereafter, differentresearchers have given different definition ofadaptive protection. All these definitions narrate thesame facts in different forms.

(ii) It is defined as changing relaying parameters or

functions automatically depending upon theprevailing system condition or requirements. Theadaptive relaying philosophy can be made fullyeffective only with digital computer based relays.


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Historical Development(i) However, adaptive relays are not manufactured by

any manufacturers. They are under research. Nowa days, utilities are using Numerical/Digital relays.

(ii) Microprocessor/Numerical relays are not faster thanstatic relays.

(iii) It takes more than a cycle (n+2).(iv) Operating time of CBs is 1-1.5 cycles.(v) Hence, even though, relay operates less than a

cycle or half a cycle, overall operating time (with

breakers) remains 1-1.5 cycles.(vi) The fastest relay, available in market, has the

operating time of 8 ms. (AREVA or ALSTOM basedmotor protection relay)


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general presentation on relay development.pdf

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