1. TRANSMISSION LINES PROTECTION S. O. Katiyar AGM EMD NTPC Faridabad

2. Classification of Transmission Lines  Short Lines Length < = 80kM  Medium Lines Length < = 240kM  Long Lines Length > 240kM

3. Types of faults in the transmission system Short circuit faults Frequency  Phase – Ground faults85%  Phase- Phase faults8%  Phase – Phase –Ground faults5%  3 Ph faults2% Open circuit faults  Broken conductor  Open jumper

4. Protection Scheme Protection Scheme for Transmission lines as per CBIP guidelines Should have two independent high speed main protection schemes Two stage over voltage protection Sensitive IDMT directional E/F relays Auto reclose relay suitable for 1 ph/3ph (with deadline charging and synchro check) re-closure.  Types of main Protections: Over Current Protection. Differential or Phase Comparison or Unit Protection. Distance Protection.

5. Requirements of distance protection Shall have min. of three independent zones with directional characteristics. separate measurement for both earth faults and phase faults Capable of 1phase and 3 phase tripping. Capable of operation for close up faults and switch on to faults Accuracy of better than 5% of reach setting for Zone 1, 10% for Zone-2 &3. Shall include power swing detection feature for selectively blocking. Shall include fuse failure feature to monitor all types of fuse failures and block distance protection.

6. I f = E/(Z S +Z L ) The reach of over current relay is function of Source Impedance which varies considerably, making it difficult to get fast and Selective tripping. E ZSZS ZLZL IfIf XXXXX Over Current Protection

7. Distance Relays Introduction: The impedance relays also called distance relays are employed to provide protection to transmission lines. They are comparatively simple to apply, operate with extremely high speed, and both primary and backup protection features are inherent in them The impedance relay is made to respond to the impedance between the relay location and the point where fault is incident. The impedance is proportional to the distance to the fault.

8. Application of distance relays:  Since the distance relays are fed from the secondary's of line CTs and bus PTs/line CVTs, the line parameters are to be converted into secondary values to set the relay as per requirements. Z secy = Z pri/Impedance ratio (where Impedance ratio = P.T.Ratio/ C.T.Ratio )

9. For the lines, the impedance in Ohms per KM is approximately as under : ----------------------------------------------------------------------------------- KVZ1 (= Z2 )Line Angle ----------------------------------------------------------------------------------- 132 KV0.460 to 70 Deg. 220 KV0.470 to 80 Deg. 400 KV0.380 to 85 Deg. ---------------------------------------------------------------------------------- The line impedance is to be computed depending on line configuration conductor size and clearness. The values in the table are only representative. A distance relay is stepped for either 3 zones or 4 zones to provide protection.

11. Types of Distance Relays: Impedance relay Reactance relay Mho relay

12. Distance Relaying Principle: A distance relay compares the currents and voltages at the relaying point with Current providing the operating torque and the voltage provides the restraining torque. In other words an impedance relay is a voltage restrained over-current relay. The equation at the balance point in a simple impedance relay is K 1 V 2 = K 2 I 2 or V/I = K 3 where K 1, K 2 and K 3 are constants. In other words, the relay is on the verge of operation at a constant value of V/I ratio, which may be expressed as an impedance.

14. The Reactance-type Distance Relay: Reactance relay measures V/I SinѲ. Whenever the reactance measured by the relay is less than the set value, the relay operates. The operating characteristic on R-X diagram is shown in fig The resistance component of impedance has no effect on the operation of reactance relay, the relay responds solely to reactance component of impedance. This relay is inherently non-directional. The relay is most suitable to detect earth faults where the effect of arc resistance is appreciable.

15. Characteristic of Directional Impedance Relay: Characteristic of a directional impedance relay in the complex R-X phase is shown in fig. The directional unit of the relay causes separation of the regions of the relay characteristic shown in the figure by a line drawn perpendicular to the line impedance locus. The net result is that tripping will occur only for points that are both within the circles and above the directional unit characteristic.

16. Mho relay: This is a directional impedance relay, also known as admittance relay. Its characteristic on R-X diagram is a circle whose circumference passes through the origin as illustrated in figure showing that the relay is inherently directional and it only operates for faults in the forward direction. Modified impedance relay: Also known as offset Mho relay whose characteristic encloses the origin on R-X diagram as shown in fig This offset mho relay has three main applications: - i) Bus-bar zone backup ii)Carrier starting unit in distance/carrier blocking schemes. iii)Power Swing blocking.

18. Distance schemes consist of the following major components:- i)Starters. ii)Measuring units. iii)Timers iv)Auxiliary relays

19. Starters: - The starting relay (or starter) initiates the distance scheme in the event of a fault within the required reach (more than zone-3). Other functions of the starter are: - a) Starting of timer relays for second and third zones.] b) Starting of measuring elements. The starters are generally of Mho or impedance type.

20. With impedance type starters:  Measuring units have to be directional as impedance starters are non – directional.  The under impedance relay can be used in conjunction with the directional relay as starter which will then function similar to the Mho starter.

21. Measuring units: They are generally of a mho or reactance or a combination of mho, reactance and resistance types. Phase Fault Units:-  These measuring units are fed with line to line voltages (such as Vab, Vbc) and difference between line currents (Ia-Ib).  Three such relays respond correctly to all possible single line to ground faults line to line faults, double line to ground faults and 3-phase faults.

22. Timers: -  Timer relays when initiated by starters provide the time lag required for zones. Auxiliary relays: -  Distance scheme comprises of several auxiliary relays, which perform functions such as flag indications, tripping, signaling, alarm etc.

23. Additional Features in distance schemes : - Power Swing blocking relay VT fuse failure relay. Switch onto fault relay Fault locator Auto-reclosing scheme. Carrier communication scheme.

24. Power Swing blocking  Distance relay which respond to balanced 3-phase changes in the impedance will be affected by power swings.  These swings or oscillations occur following a system disturbance such as major load change or a dip in voltage due to delayed fault clearance.  In case of fault, the transition from period of impedance locations to fault impedance (starter impedance) is sudden whereas during power swings it is slow.  The PSB relays use this difference to block the tripping during swings.

25. VT fuse failure relay  The distance relays being voltage restraint O/C relays, loss of voltage due to main PT fuse failure or inadvertent removal of fuse in one or more phases will cause the relay operation.  The fuse failure relay will sense such condition by the presence of residual voltage without residual current and blocks the relay.

26. Switch onto fault  When the line is switched on to a close by fault (say after line clearance with earth switch closed), the voltage at the relaying point will be zero. Faults of this type will normally be cleared by backup zones.  The voltage applied to the relay is low and this condition occurring simultaneously with the operation of starter will cause instantaneous trip by SOTF relay.  This SOTF feature will be effective only for about 1-2 seconds after the line is charged. Faults occurring after this time will be measured in the normal way.

27. Fault locator  It measures the distance between the relay location and fault location in terms of Z in Ohms, or length in KM or percentage of line length.  gets same inputs as the distance relay (connected in series with one of the main relays). The measurement is initiated by trip signal from distance relays.  The fault locator gives the exact location of the fault, thereby reducing the time of restoration.

28. Distance Protection Type of distance relays  Reactance  Suitable for short lines  Not effected by fault resistance  Effected by power swings  Non directional  Impedance  Suitable for medium lines  Non directional  Effected by fault resistance  Mho  Directional  Least effected by power swings  Less effected by fault resistance

29. Impedance Relay Characteristics Load Area X R Z1 Z2 Z3

30. MHO relay characteristic The characteristic of a mho impedance element, when plotted on a R/X diagram, is a circle whose circumference pass through the origin.   = relay characteristic angle R X

31. OFF set MHO characteristic Under close up faults, when the voltage is near to zero then MHO will not operate. The mho characteristic can be shifted towards origin for operation of close up faults. This is know as OFF set MHO.   = relay characteristic angle R X

32. Load Lenti-cular characteristics The characteristic of lenticular mho will be useful to provide maximum load transfer condition with maximum fault resistance coverage.   = relay characteristic angle Z-1 Z-2 Z-3 R

33. Quadrilateral characteristic It is a basically a reactance relay superseded with controlled resistive reach.   = relay characteristic angle Z-1&2 Z-1 Z-2 Z-3

34. Zones of Distance Protection: Z1 Z2 Z3 BASIC SETTING PHILOSOPHY ZONE –1 : 80 % of protected line ZONE –2 : 100 % of protected line + 20 % of shortest adj. line section or 100% + 50% of transformer impedance ZONE –3 : 100% of protected line + 100 % of longest adj. line or 100 % + 100% of transformer impedance. ZONE -4 : To cover close up back-up non-directional faults generally reverse reach will be provided in relays (10%). XXX X XX

35. Terms associated with distance protection Reach: Reach is the impedance of the tr. line up to which the distance relay protects the line from the faults. over reach effective reach of the relay increases Under reach Effective reach of the relay decreases

36. Distance Schemes : 1. P. U. R -- Permissive under reach schemeP. U. R 2.P. O. R -- Permissive Over Reach schemeP. O. R 3.BLOCKING SCHEMEBLOCKING SCHEME 4.WEAK END FEEDWEAK END FEED

37. CARRIER SCHEMES - P U R CHANNEL Z1A Z2A Z1B Z2B A B CARRIER RELAY Fault Trip = Z1 + Z2.CR+Z3.T3+Z2.T2 CS = Z1 Under reaching zone sends carrier signal

38. CARRIER SCHEMES - P O R RELAY Z1A Z2A Z1B Z2B A B CARRIER RELAY Fault Trip = Z1+Z2.CR+ Z2.T2+Z3.T3 CS = Z2 Over reaching zone sends carrier signal

39. AUTORECLOSE – PHILOSOPHY NEED FOR AUTO RECLOSE 1.REDUCING OUTAGE TIME 2.IMPROVED RELIABILITY 3.RESTORATION OF NETWORK STABILITY AND SYNCHRONISM TYPES OF FAULTS 1.TRANSIENT FAULTS 2.SEMI PERMANENT FAULTS 3.PERMANENT FAULTS

40. TRANSIENT FAULTS -CHARACTERISTIC Characterized by disappearance after Short dead time and are disappear without any action being taken. TYPES OF TRANSIENT FAULTS 1.Lightning strokes resulting in flashovers 2.Conductor swinging due to high winds 3.Bird fault 4.Temporary contact with foreign objects like tree etc. About 85 % of faults on transmission lines are transient in nature

41. SEMI PERMANENT FAULTS This type of faults requires more than one De-energized interval before it disappears. Such faults are prevalent on EHV lines traversing forest. An example is a tree falling on the line and getting burnt up by the arc when the line is re energized. 10% of the re-closures are successful with second shot. However this can cause unnecessary wear on EHV CBs. Therefore second shot is not recommended for EHV Systems.

42. DEAD TIME : The time between the auto-reclose scheme being energized and the operation of the contacts which energize the CB closing circuit. RECLAIM TIME : The time following a successful closing operation measured from the instant the A/R relay closing contacts make, which must elapse before the auto- reclose relay will initiate reclosing sequence in the event of a further fault.

43. CHOICE OF RECLAIM TIME  The reclaim time must not be set to such a low value that the intended operating cycle of the breaker is exceeded when two fault incidents occurs close together.  for example the reclaim time for a air blast circuit breakers must allow time for air pressure to recover to its normal value. CHOICE OF DEAD TIME  Dead time for EHV system lower limit is decided by de-ionizing time, upper limit is decided by transient stability and synchronism

44. Power Swing  Power Swings are disturbances in system due to various reasons such as sudden load throw, bad synchronization etc  Power swings are characterized by slow power flow oscillations, resulting in swinging of voltages and currents, resulting in operating point movement into distance relay characteristics, in turn can cause tripping of distance relays.  Tripping during power swings is undesirable since no actual fault is present and moreover a line outage during power swing may cause further deterioration to system stability.  Detection of power swing will block the distance protection Zones 2,3,4. Normally tripping in Zone-I is not blocked even after detection of power swing.

45. X R Z1 Z2 Z3 Power swing detection zones Power Swing detection Time taken by fault locus to cross the power swing detection zones is more than 40-50ms, then it is called power swing.

46. Fuse Failure Function Asymmetrical measuring voltage failure:  Substantial asymmetry of measured voltage, while the measured currents are in symmetry indicates fuse fail  Asymmetry of voltage detected by 3Uo or U2 > threshold  Symmetry in current detected by 3Io or I2 < threshold  During blocking of distance protection by fuse fail, the distance protection switched to emergency over current function automatically.  If the asymmetry in measured current is detected during blocking by FF function, then FF block will released.

47. Switch on to fault  This feature provide protection against energisation of the tr. line with fault or dead short.  Distance protection will not provide protection in this case as voltage is not available for distance Measurement.  It can be activated by TNC switch or CB aux. binary input or internal detection of current rise.  It provides instantaneous 3Ph trip and blocks auto reclose.

48. One and Half Breaker Scheme Ckt-1Ckt-2 Bus-1 Bus-2 21 Stub Protection

49. DEF Protection  It provides back up protection for tr. Line.  It provides reliable protection for high resistance earth faults.

50. Local breaker back up protection  It is the secondary protection  To provide back up isolation during failure of breaker to open.  It opens source to that breaker (i.e other end breakers, bus bar, etc.)  It will be triggered by operation of any primary protection (like distance, DEF, bus bar, etc..)  It sends direct trip command to other end.direct trip

51. Direct trip Scheme  It is required to trip other end breakers without any checking the status at other end during following conditions: Operation of over voltage protection. Operation of bus bar with tie breaker open. Manual tripping of both the breakers (main&Tie) Operation of LBB On receipt of command through PLCC at other end breakers will trip directly.

52. Over Voltage Protection  It will have 2 stages  Stage-I:  Setting: 110%  Time delay: 5 Sec.  Stage –II  Setting: 140%  Time delay: Instantaneous.