Basic of protection system *****part 2

Primary and back-up protection
The reliability of a power system has been discussed in earlier sections. Many factors maycause protec­tion failure and there is always some possibility of a circuitbreaker failure. For this reason, it is usual to supplement primary protectionwith other systems to 'back-up' the operation of the main system and ensurethat nothing can prevent the clearance of a fault from the system.

Back-up protection may be obtained automatically as aninherent feature of the main protection scheme, or separately by means ofadditional equip­ment.
Time graded schemes such as over current or distanceprotection schemes are examples of those providing inherent back-up protection;the faulty section is normally isolated discriminatively by the time grading,but if the appropriate relay fails or the circuit breaker fails to trip, thenext relay in the grading sequence will complete its operation and trip theassociated circuit breaker, thereby inter­rupting the fault circuit one sectionfurther back. In this way complete back-up cover is obtained; one more section isisolated than is desirable but this is inevitable in the event of the failureof a circuit breaker.
Where the system interconnection is more complex, theabove operation will be repeated so that all parallel infeeds are tripped.

If the power system is protected mainly by unit schemes, automaticback-up protection is not obtained, and it is then normal to supplement themain protection with time graded over current pro­tection, which will providelocal back-up cover if the main protective relays have failed, and will tripfurther back in the event of circuit breaker failure.
Such back-up protection is inherently slower than the mainprotection and, depending on the power system configuration, may be lessdiscriminative. For the most important circuits the performance may not be goodenough, even as a back-up protection, or, in some cases, not even possible,owing to the effect of multiple infeeds. In these cases duplicate high speedprotective systems may be installed. These provide excellent mutual back-upcover against failure of the protective equipment, but either no remote back-upprotection against circuit breaker failure or, at best, time delayed cover.

Breaker fail protection can be obtained by checking thatfault current ceases within a brief time interval from the operation of themain protection. If this does not occur, all other connections to the bus barsection are interrupted, the condition being necessarily treated as a bus barfault. This provides the required back-up protection with the minimum of timedelay, and confines the tripping operation to the one station, as compared withthe alternative of tripping the remote ends of all the relevant circuits.
The extent and type of back-up protection which is appliedwill naturally be related to the failure risks and relative economic importanceof the system. For distribution systems where fault clearance

Times are not critical, time delayed remote back-upprotection is adequate but for EHV systems, where system stability is atrisk unless a fault is cleared quickly, local back-up, as described above,should be chosen.

Ideal back-up protection would be completely independentof the main protection. Current trans-formers, voltage transformers, auxiliarytripping relays, trip coils and D.C. supplies would be duplicated. This idealis rarely attained in practice. The following compromises are typical:
a. Separate current transformers (cores andsecondary windings only) are used for each protec­tive system, asthis involves little extra cost or accommodation compared with the use ofcommon current transformers which would have to be larger because of thecombined burden.
b. Common voltage transformers are usedbecause duplication would involve a considerable increase in cost, because ofthe voltage transformers them-selves, and also because of the increased accom­modationwhich would have to be provided. Since security of the VT output is vital, itis desirable that the supply to each protection should be separately fused andalso continuously supervised by a relay which will give an alarm on failure ofthe supply and, where appropriate, prevent an unwanted operation of theprotection.
c. Trip supplies to the two protections should be separatelyfused. Duplication of tripping batteries and of tripping coils on circuitbreakers is sometimes provided. Trip circuits should be continuouslysupervised.
d. It is desirable that the main and back-up protections (orduplicate main protections) should operate on different principles, so thatunusual events that may cause failure of the one will be less likely to affectthe other.


1. All-or-nothing relay
A relay which isnot designed to have any specified accuracy as to its operating value.

2. Auxiliary relay.
Anall-or-nothing relay used to supplement the performance of another relay, bymodifying contact performance for example, or by introducing time delays.

3.Back-up protection.
A protectivesystem intended to supplement the main protection in case the latter should bein-effective, or to deal with faults in those parts of the power system thatare not readily included in the operating zones of the main protection.

4.Biased relay.A relay in whichthe characteristics are modified by the introduction of some quantity otherthan the actuating quantity, and which is usually in oppo­sition to theactuating quantity.

5. Burden.
The loadingimposed by the circuits of the relay on the energizing power source or sources, expressed as the product of voltage and current(volt-amperes, or watts if D.C) for a given condition, which may be either at'setting' or at rated current or voltage.
The rated output of measuringtransformers, expressed in VA, is always at rated current or voltage and it isimportant, in assessing the burden imposed by a relay, to ensure that the valueof burden at rated current is used.

6. Characteristic angle.
The phase angleat which the performance of the relay is declared. It is usually the angle atwhich maximum sensitivity occurs.

7. Characteristic curve.
The curveshowing the operating value of the characteristic quantity corresponding tovarious values or combinations of the energizing quantities.

8. Characteristic quantity.
A quantity, thevalue of which characterizes the operation of the relay, e.g. current for anover current relay, voltage for a voltage relay, phase angle for a directionalrelay, time for an independent time delay relay, impedance for an impedancerelay.

9. Characteristic impedance ratio (C.I. R.)
The maximumvalue of the System Impedance Ratio up to which the relay performance remainswithin the prescribed limits of accuracy.

10. Check protective system.
An auxiliaryprotective system intended to prevent tripping due to inadvertent operation ofthe main protective system.

11.Conjunctive test.
A test on a protective systemincluding all relevant components and ancillary equipment appropriatelyinterconnected. The test may be parametric or specific.

a. Parametric conjunctivetest.
A test to ascertain the range of values that may be assigned toeach parameter when considered in combination with other parameters, whilestill complying with the relevant performance require­ments.

b. Specific conjunctive test.
A test to prove the performance for a particular application,for which definite values are assigned to each of the parameters.

12.Dependent time delay relay.
A time delay relay inwhich the time delay varies with the value of the energizing quantity.

13. Discrimination.
The quality whereby aprotective system distinguishesbetween those conditions for which it is intended to operate and those forwhich it shall not operate.

14. Drop-out.
A relay drops out when it moves from the energized position tothe un-energized position.

15. Drop-out / pick ratio.
The ratio of the limiting values of the characteristic quantityat which the relay resets and operates. This value is sometimes called thedifferential of the relay.

16. Earth fault protective system.
A protective system which is designed to respond only to faultsto earth.

17. Earthing transformer.
A three-phase transformer intended essentially to provide aneutral point to a power system for the purpose of Earthing.
18.Effective range
The range of values of thecharacteristic quantity or quantities, or of the energizing quantities to whichthe relay will respond and satisfy the requirements concerning it, inparticular those concerning pre­cision.

19. Effective setting
The 'setting' of a protective system including the effectsof current transformers. The effective setting can be expressed in terms ofprimary current or secondary current from the current transformers and is sodesignated as appropriate.

20. Electrical relay
A device designed to produce suddenpredetermined changes in one or more electrical circuits after the appearanceof certain conditions in the electrical circuit or circuits controlling it.
NOTE: The term 'relay' includes all theancillary equipment calibrated with the device.

21. Energizing quantity.
The electrical quantity, either currentor voltage, which alone or in combination with other energizing quantities,must be applied to the relay to cause it to function.

22.Independent time delay relay.
A time delay relay in which the timedelay is indepen­dent of the energizing quantity.

21.Instantaneous relay.
A relay which operates and resets withno intentional time delay.
NOTE: All relays require some time tooperate; it is possible, within the above definition, to discuss the operatingtime characteristics of an instantaneous relay.

22. Inverse time delay relay.
A dependent time delay relay having anoperating time which is an inverse function of the electrical characteristicquantity.

23. Inverse time delay relay with definite minimum (I.D. M.T.)
A relay in which the time delay variesinversely with the characteristic quantity up to a certain value, after whichthe time delay becomes substantially independent.

24. Knee-point e.m.f.
That sinusoidal e.m.f. applied to thesecondary terminals of a current transformer, which, when increased by 10 %,causes the exciting current to increase by 50%.

25. Main protection.
The protective system which is normallyexpected to operate in response to a fault in the protected zone.

26. Measuring relay.
A relay intended to operate with aspecified accuracy at one or morevalues of its characteristic quantity.

27. Notching relay.
A relay which switches in response to aspecific number of applied impulses.

28. Operating time.
With a relay de-energized and in itsinitial condition, the time which elapses between the application of acharacteristic quantity and the instant when the relay operates.

29. Operating time characteristic.
The curve depicting the relationshipbetween different values of thecharacteristic quantity applied to a relay and the corresponding values ofoperating time.

30. Operating value.
The limiting value of the characteristicquantity at which the relay actually operates.

31. Overshoot time.
The extent to which the condition thatleads to final operation is advanced after the removal of the energizingquantity, expressed as time at the rate of progress of the said conditionappropriate to the value of the energizing quantity that was initially applied.

32. Pick-up.
A relay is said to 'pick-up^'when it changes from the un-energized position to the energized position.

33. Pilot channel.
A means of interconnection betweenrelaying points for the purpose of protection.

34. Protected zone.
The portion of a power system protectedby a given protective system or a part of that protective system.

35. Protective gear.
The apparatus, including protectiverelays, trans-formers and ancillary equipment, for use in a protective system.

36. Protective relay.
A relay designed to initiatedisconnection of a part of an electrical installation or to operate a warningsignal, in the case of a fault or other abnormal condition in the installation.A protective relay may include more than one unit electrical relay andaccessories.

37. Protective scheme.
The coordinated arrangements for theprotection of one or more elements of a power system.
A protective scheme may comprise severalprotective systems.

38. Protective system.
A combination of protective geardesigned to secure, under predetermined conditions, usually abnormal, thedisconnection of an element of a power system, or to give an alarm signal, orboth.

39. Rating.
The nominal value of an energizingquantity which appears in the designation of a relay. The nominal value usuallycorresponds to the CT and VT second­ary ratings.

40. Resetting value.
The limiting value of the characteristicquantity at which the relay returns to its initial position.

41. Residua/ current.
The algebraic sum, in a multi-phasesystem, of all the line currents.

42. Residua/ voltage.
The algebraic sum, in a multi-phasesystem, of all the line-to-earth voltages.

43. Setting.
The limiting value of a 'characteristic'or 'energizing' quantity at which the relay is designed to operate underspecified conditions.
Such values are usually marked on therelay and may be expressed as direct values, percentages of rated values, ormultiples.

44. Stability.
The quality whereby a protective systemremains inoperative under all conditions other than those for which it isspecifically designed to operate.

45. Stability limits.
The R.M.S. value of the symmetricalcomponent of the through fault current up to which the protective systemremains stable.

46. Starting relay.
A unit relay which responds to abnormalconditions and initiates the operation of other elements of the protectivesystem.

47. System impedance ratio (S./.R.).
The ratio of the power system sourceimpedance to the impedance of the protected zone.

48. Through fault current.
The current flowing through a protectedzone to a fault beyond that zone.

49. Time delay.
A delay intentionally introduced intothe operation of a relay system.

50. Time delay relay.
A relay having an intentional delayingdevice.

51. Unit electrical relay.
A single relay which can be used aloneor in com­binations with others.

52. Unit protection.
A protection system which is designed tooperate only for abnormal conditions within a clearly defined zone of the powersystem.

53. Unrestricted protection.
A protection system which has no clearlydefined zone of operation and which achieves selective operation only by timegrading.

· Fault Definitions and:
For thepurpose of this International Standard, the following definitions, some of thembased on IEC 60050(191), IEC 60050(212) and
IEC60050(604) apply:

1- Fault
Anunplanned occurrence or defect in an item which may result in one or morefailures of the item itself or of other associated equipment
[IEC604-02-011
NOTE - Inelectrical equipment, a fault may or may not result in damage to the insulationand failure of the equipment.

2- Non-damage fault
A faultwhich does not involve repair or replacement action at the point of the fault
NOTE -Typical examples are self-extinguishing arcs in switching equipment or generaloverheating without paper carbonization.
[IEC604-02-091

3- Damage fault
A faultwhich involves repair or replacement action at the point of the fault
[IEC604-02-08, modified]

4- Incident
An eventrelated to an internal fault which temporarily or permanently disturbs thenormal operation of an equipment [IEV 604-02-03, modified]
NOTE -Typical examples are gas alarms, equipment tripping or equipment leakage.

5- Failure
Thetermination of the ability of an item to perform a required function [IEC191-04-01]

NOTE - Inthe electrical equipment, failure will result from a damage fault or incidentnecessitating outage, repair or replacement of the equipment, such as internalbreakdown, rupture of tank, fire or explosion.

6- Electrical fault
a partialor disruptive discharge through the insulation.

7- Partial discharge
Adischarge which only partially bridges the insulation between conductors. Itmay occur inside the insulation or adjacent to a conductor
[IEC212-01-34, modified]

NOTE1 - Corona is a form of partialdischarge that occurs in gaseous media around conductors which are remote fromsolid or liquid insulation. This term is not to be used as a general term forall forms of partial discharges.

NOTE2 - X-wax is a solid material which isformed from mineral insulating oil as a result of electrical discharges andwhich consists of polymerized fragments of the molecules of the original liquid
[IEV212-07-24, modified].
Comparableproducts may be formed from other liquids under similar conditions.

NOTE3 - Sparking of low energy, for examplebecause of metals or floating potentials, is sometimes described as
Partialdischarge butshould rather be considered as a discharge of low energy.

8- Discharge (disruptive) .
Thepassage of an arc following the breakdown of the insulation
[IEC604-03-38, modified]

NOTE1 - Discharges are often described asarcing, breakdown or short circuits.
The morespecific following terms are also used:
- sparkover (discharge through the oil);
- puncture(discharge through the solid insulation);
-Flashover (discharge at the surface of the solid insulation);
- tracking(the progressive degradation of the surface of solid insulation
by local
Discharges to form conducting or partially conducting paths);
- sparkingdischarges which, in the conventions of physics, are local
Dielectric breakdowns of high ionization density or small arcs.

NOTE2 - Depending on the amount of energycontained in the discharge, it will be described as a discharge of low or highenergy, based on the extent of damage observed on the equipment .

9-Thermal fault
Excessivetemperature rise in the insulation

NOTE- Typical causes are
-Insufficient cooling,
-Excessive currents circulating in adjacent metal parts (as a result of bad
Contacts, eddy currents, stray losses or leakage flux),
-Excessive currents circulating through the insulation (as a result of high
Dielectric losses), leading to a thermal runaway,
-overheating of internal winding or bushing connection lead.

10- Typical values of gas concentrations.
gasconcentrations normally found in the equipment in service which have nosymptoms offailure, and which are over passed by only an arbitrarypercentage of higher gas contents, for example 10 % .

NOTE1 - Typical values will differ indifferent types of equipment and in different networks, depending on operating practices(load levels, climate, etc.).

NOTE2 - Typical values, in many countriesand by many users, are quoted as "normal values", but this term hasnot been used here to avoid possible misinterpretations.

[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
LIST OF DEVICE NUMBERS
· 2 Time delay starting or closing relay.
· 3 Checking or interlocking relay
· 21 Distance relay
· 25 Synchronizing or synchronism checkrelay
· 27 Under voltage relay
· 30 Annunciator relay
· 32 Directional power relay
· 37 Undercurrent or under power relay
· 40 Field failure relay
· 46 Reverse phase or phase balancecurrent relay
· 49 Machine or transformer thermal relay
· 50 Instantaneous over current orrate-of-rise relay
· 51 A.c. time over current relay
· 52 A.c. circuit breaker
· 52a Circuit breaker auxiliaryswitch—normally open
· 52b Circuit breaker auxiliaryswitch—normally closed
· 55 Power factor relay
· 56 Field_application relay
· 59 Over voltage relay
· 60 Voltage or current balance relay
· 64 Earth fault protective relay
· 67 A.c. directional over current relay
· 68 Blocking relay
· 74 Alarm relay
· 76 D.c. over current relay
· 78 Phase angle measuring or out-of-stepprotective relay
· 79 A.c. reclosing relay
· 81 Frequency relay
· 83 Automatic selective control ortransfer relay
· 85 Carrier or pilot wire receive relay
· 86 Locking-out relay
· 87 Differential protective relay
· 94 auxiliary tripping relay

IEEE device numbers and functions for switchgear apparatus

The devices in switching equipments are referred to bynumbers, with appropriate suffix
letters when necessary, according to the functions theyperform. These numbers are based on a system adopted as standard for automaticswitchgear by IEEE, and incorporated in American
Standard C37.2-1979. This system is used in connectiondiagrams, in instruction books,
and in specifications.