lecture protection

8/12/2019 Lecture protection

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Advanced Power System Protection

Lecture No.1

Dr. Muhammad Kamran


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Text Book

Elsevier Practical Power System Protection

By

L.G Hewitson, Mark Brown and Ramesh Balakrishanan Any other reference book or Website

13-Feb-14 2 Muhammad Kamran


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Contents

Need for protective system Basic requirements

Basic components Summary



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Need for protection

A power system is not only capable to meet thepresent load but also has the flexibility to meetthe future demands

A power system is designed to generate electricpower in sufficient quantity, to meet the presentand estimated future demands of the users in a

particular area, to transmit it to the areas whereit will be used and then distribute it within thatarea, on a continuous basis.



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To ensure the maximum return on the largeinvestment in the equipment, which goes tomake up the power system and to keep theusers satisfied with reliable service, the wholesystem must be kept in operationcontinuously without major breakdowns



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The first way is to implement a systemadopting components, which should not failand requires the least or nil maintenance tomaintain the continuity of service

By common sense, implementing such asystem is neither economical nor feasible,except for small systems.



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The second option is to foresee any possibleeffects or failures that may cause long-termshutdown of a system, which in turn may take

longer time to bring back the system to itsnormal course The main idea is to restrict the disturbances

during such failures to a limited area andcontinue power distribution in the balanceareas



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Special equipment is normally installed todetect such kind of failures (also calledfaults) that can possibly happen in varioussections of a system, and to isolate faultysections so that the interruption is limited to alocalized area in the total system covering

various area



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The special equipment adopted to detect suchpossible faults is referred to as protective equipment or protective relay and the systemthat uses such equipment is termed as protectionsystem

A protective relay is the device, which givesinstruction to disconnect a faulty part of thesystem

This action ensures that the remaining system isstill fed with power, and protects the system fromfurther damage due to the fault



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Hence, use of protective apparatus is verynecessary in the electrical systems, which areexpected to generate, transmit and distribute

power with least interruptions and restorationtime It can be well recognized that use of

protective equipment are very vital tominimize the effects of faults, which otherwisecan kill the whole system.



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The special equipment adopted to detect suchpossible faults is referred to as protective equipment or protective relay and the systemthat uses such equipment is termed asprotection system

A protective relay is the device, which givesinstruction to disconnect a faulty part of thesystem

This action ensures that the remaining system isstill fed with power, and protects the system fromfurther damage due to the fault



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Basic Requirements for Protection

A protection apparatus has three mainfunctions/duties:

1. Safeguard the entire system to maintaincontinuity of supply

2. Minimize damage and repair costs where itsenses fault

3. Ensure safety of personnel.



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These requirements are necessary, firstly for early detection andlocalization of faults, and secondly for prompt removal of faultyequipment from service

In order to carry out the above duties, protection must have thefollowing qualities:

Selectivity: To detect and isolate the faulty item only. Stability: To leave all healthy circuits intact to ensure continuity orsupply. Sensitivity: To detect even the smallest fault, current or systemabnormalities and operate correctly at its setting before the fault

causes irreparable damage. Speed: To operate speedily when it is called upon to do so, therebyminimizing damage to the surroundings and ensuring safety topersonnel.



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Basic components of protection Protection of any distribution system is a function of

many elements and this manual gives a brief outline ofvarious components that go in protecting a system

Following are the main components of protection; Fuse is the self-destructing one, which carries the

currents in a power circuit continuously and sacrificesitself by blowing under abnormal conditions

These are normally independent or stand-alone protective components in an electrical system unlike acircuit breaker, which necessarily requires the supportof external components



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Accurate protection cannot be achieved withoutproperly measuring the normal and abnormalconditions of a system

In electrical systems, voltage and currentmeasurements give feedback on whether asystem is healthy or not

Voltage transformers and current transformersmeasure these basic parameters and are capableof providing accurate measurement during faultconditions without failure



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The measured values are converted into analog and/ordigital signals and are made to operate the relays,which in turn isolate the circuits by opening the faultycircuits. In most of the cases, the relays provide two

functions viz., alarm and trip, once the abnormality isnoticed The relays in old days had very limited functions and

were quite bulky However, with advancement in digital technology and

use of microprocessors, relays monitor variousparameters, which give complete history of a systemduring both pre-fault and post-fault conditions



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The opening of faulty circuits requires some time,which may be in milliseconds, which for acommon day life could be insignificant

However, the circuit breakers, which are used toisolate the faulty circuits, are capable of carryingthese fault currents until the fault currents aretotally cleared

The circuit breakers are the main isolating devicesin a distribution system, which can be said todirectly protect the system



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The operation of relays and breakers requirepower sources, which shall not be affected byfaults in the main distribution

Hence, the other component, which is vital inprotective system, is batteries that are used toensure uninterrupted power to relays andbreaker coils



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Faults, types and effects

When a consumer requests electrical powerfrom a supply authority, ideally all that isrequired is a cable and a transformer, shownphysically as in Figure 2.1

While Radial Distribution system is shown infigure 2.2



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Advantages and disadvantages ofRadial system

If a fault occurs at T2 then only the protectionon one leg connecting T2 is called intooperation to isolate this leg

The other consumers are not affected If the conductor to T2 fails, then supply to this

particular consumer is lost completely andcannot be restored until the conductor isreplaced/repaired.



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This disadvantage can be overcome byintroducing additional/parallel feeders asshown in figure, connecting each of theconsumers radially as shown in next slide

However, this requires more cabling and is notalways economical

The fault current also tends to increase due touse of two cables



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Radial Distribution system with parallelfeeders



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Ring Main Distribution system



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Essentially, meets the requirements of twoalternative feeds to give 100% continuity ofsupply, whilst saving in cabling/copper comparedto parallel feeders

For faults at T1 fault current is fed into fault viatwo parallel paths effectively reducing theimpedance from the source to the fault location,and hence the fault current is much higher

compared to a radial path The fault currents in particular could vary

depending on the exact location of the fault



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Protection must therefore be fast anddiscriminate correctly, so that other consumersare not interrupted

The above case basically covers feeder failure,since cable tend to be the most vulnerablecomponent in the network

Not only are they likely to be hit by a pick oralternatively dug-up, or crushed by heavymachinery, but their joints are notoriously weak,being susceptible to moisture, ingress, etc.,amongst other things



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Transformer faults are not so frequent,however they do occur as windings are oftenstrained when carrying through-fault current

Also, their insulation lifespan is very oftenreduced due to temporary or extendedoverloading leading to eventual failure

Hence interruption or restriction in the powerbeing distributed cannot be avoided in case oftransformer failures



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The loss of a busbar in a network can in factbe a catastrophic situation, and it isrecommended that this component be givencareful consideration from a protectionviewpoint when designing network,particularly for continuous process plants such

as mineral processing



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Fault types and their effects

It is not practical to design and build electricalequipment or networks to eliminate thepossibility of failure in service

It is therefore an everyday fact that differenttypes of faults occur on electrical systems,however infrequently, and at random locations

Faults can be broadly classified into two mainareas, which have been designated active andpassive



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Active faults

The active fault is when actual current flowsfrom one phase conductor to another (phase-to-phase), or alternatively from one phaseconductor to earth (phase-to-earth)

This type of fault can also be further classifiedinto two areas, namely the solid fault and the

incipient(Beginning) fault



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The solid fault occurs as a result of an immediatecomplete breakdown of insulation as wouldhappen if, say, a pick struck an underground

cable, bridging conductors, etc. or the cable wasdug up by a bulldozer

In mining, a rockfall could crush a cable, as would

a shuttle car In these circumstances the fault current would be

very high resulting in an electrical explosion



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This type of fault must be cleared as quickly aspossible, otherwise there will be:

Increased damage at fault location

Fault energy = I2

Rf t , where t is time in seconds. Danger to operating personnel (flashes due to high

fault energy sustaining for a long time). Danger of igniting combustible gas in hazardous areas,

such as methane in coal mines which could causehorrendous disaster. Increased probability of earth faults spreading to

healthy phases



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Higher mechanical and thermal stressing of allitems of plant carrying the fault current,particularly transformers whose windings sufferprogressive and cumulative deteriorationbecause of the enormous electromechanicalforces caused by multi-phase faults proportionalto the square of the fault current

Sustained voltage dips resulting in motor (and

generator) instability leading to extensiveshutdown at the plant concerned and possiblyother nearby plants connected to the system



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The incipient fault, on the other hand, is afault that starts as a small thing and getsdeveloped into catastrophic failure

Like for example some partial discharge(excessive discharge activity often referred toas Corona) in a void in the insulation over anextended period can burn away adjacentinsulation, eventually spreading further anddeveloping into a solid fault



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Other causes can typically be a high-resistance joint or contact, alternatively pollution ofinsulators causing tracking across their surface

Once tracking occurs, any surrounding air willionize which then behaves like a solidconductor consequently creating a solid fault



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Passive Faults Passive faults are not real faults in the true sense of the

word, but are rather conditions that are stressing thesystem beyond its design capacity, so that ultimately activefaults will occur

Typical examples are: Overloading leading to over heating of insulation

(deteriorating quality, reduced life and ultimate failure). Overvoltage: Stressing the insulation beyond its withstand

capacities.

Under frequency: Causing plant to behave incorrectly. Power swings: Generators going out-of-step or out-of-

synchronism with each other



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Largely, the power distribution is globally athree-phase distribution especially frompower sources

The types of faults that can occur on a three-phase AC system are shown in Figure;



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Types of faults on a three-phase system: (A)Phase-to-earth fault; (B) Phase-to-phase fault;(C) Phase-to phase- to-earth fault; (D) Three-phase fault; (E) Three-phase-to-earth fault; (F)Phase-to-pilot fault*; (G) Pilot-to-earth fault



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Voltage situation at Relay positionunder various faults

Voltage of phase which encounters the faultdecays

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It will be noted that for a phase-to-phase fault, thecurrents will be high, because the fault current is onlylimited by the inherent (natural) series impedance ofthe power system up to the point of fault (Ohms law)

By design, this inherent series impedance in a powersystem is purposely chosen to be as low as possible inorder to get maximum power transfer to the consumerso that unnecessary losses in the network are limitedthereby increasing the distribution efficiency

Hence, the fault current cannot be decreased without acompromise on the distribution efficiency, and furtherreduction cannot be substantial



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On the other hand, the magnitude of earthfault currents will be determined by themanner in which the system neutral is earthed

It is worth noting at this juncture that it ispossible to control the level of earth faultcurrent that can flow by the judicious choice

of earthing arrangements for the neutral



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Solid neutral earthing means high earth fault currents,being limited by the inherent earth fault (zerosequence) impedance of the system, whereasadditional impedance introduced between neutral and

earth can result in comparatively lower earth faultcurrents

In other words, by the use of resistance or impedancein the neutral of the system, earth fault currents can be

engineered to be at whatever level desired and aretherefore controllable This cannot be achieved for phase faults



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Transient and permanent faults Transient faults are faults, which do not damage the insulation

permanently and allow the circuit to be safely re-energizedafter a short period.

A typical example would be an insulator flashover following alightning strike, which would be successfully cleared onopening of the circuit breaker, which could then beautomatically closed. Transient faults occur mainly on outdoorequipment where air is the main insulating medium

Permanent faults, as the name implies, are the result ofpermanent damage to the insulation

In this case, the equipment has to be repaired and rechargingmust not be entertained before repair/restoration



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Symmetrical and asymmetrical faults

A symmetrical fault is a balanced fault withthe sinusoidal waves being equal about theiraxes, and represents a steady-state condition

An asymmetrical fault displays a DC offset,transient in nature and decaying to the steadystate of the symmetrical fault after a period of

time, as shown in Figure



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Example of power station



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Effect of fault on Transmission lines



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Protection equipment The definitions that follow are generally used in

relation to power system protection: a. Protection System: a complete arrangement of

protection equipment and other devices required toachieve a specified function based on a protectionprincipal (IEC 60255-20)

b. Protection Equipment: a collection of protectiondevices (relays, fuses, etc.). Excluded are devices such asCTs, CBs, Contactors, etc.

c. Protection Scheme: a collection of protectionequipment providing a defined function and including allequipment required to make the scheme work (i.e. relays,CTs, CBs, batteries, etc.)



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General Relaying Requirement In order to fulfil the requirements of protection with the

optimum speed for the many different configurations,operating conditions and construction features of powersystems, it has been necessary to develop many types ofrelay that respond to various functions of the power systemquantities

For example, observation simply of the magnitude of thefault current suffices in some cases but measurement ofpower or impedance may be necessary in others

Relays frequently measure complex functions of the systemquantities, which are only readily expressible bymathematical or graphical means



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Relays may be classified according to thetechnology used:

a. electromechanical b. static c. digital

d. numerical



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Next Lecture

Calculations and Zones of Protection

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