CHAPTER 5. PROTECTIVE RELAY IN SUBSTATION

1. CHAPTER 5. PROTECTIVE RELAY IN SUBSTATION • Types of relays and it application • Over current relay • Differential Relay • Distance relay • Typical applications of relays • for Transformer Protection • Bus bar Protection

3. Function of Protective Relays are used to control • Circuit breakers • Lamp signal and • alarm. The types of relay based on technology i). Electromechanical relays ii). Static relay iii). Microprocessor-based protective relay

4. Electromechanical relays Attracted armature, moving coil, inductions disc and inductions cup type relays. Electromagnetic relays contain an electromagnet and moving part.

5. Static relay contain • electronic circuit • transistor, • ICs, diodes and • other electronic components. • a comparator circuit in the relay, . The function of a comparator is Compares two or more current or voltage and gives an output which is applied to either a slave relay or a thyristor circuit

6. Over Current Relay (OCR) • A protective relay which operates when the load current • exceed a preset value called an over current relay. • Pick-up value is a value that can causes relay to operate • is used in substation for the power equipment protection • against short circuit of phase to phase and ground fault. OCR Characteristics Definite-time Instantaneous Inverse-time Definite Minimum Time (IDMT) Very Inverse-time Exteremely Inverse

7. Definite-time Over current Relay • The operating time is constant, • irrespective of the magnitude of the current • Instantaneous Over current Relay • The operating time is constant, • irrespective of the magnitude of the currents • There is no intentional time delay. • It operates in 1s or less. • Inverse-time Over current Relay • The operating time depends on the magnitude of current. • The operating time decrease as the current increases. .

8. Inverse Definite Minimum Time (IDMT) • Gives an inverse-time current characteristic at lower values of the fault current and definite-time Characteristic at higher values of the fault current. • Inverse time characteristic is obtained if the value of the plug setting multiplier between 10 and 20, • the characteristic tend to become a straight line, i.e. toward the definite time characteristic. • These relays are widely used for the protection of lines.

9. Very Inverse-time Gives more inverse characteristic than previous ones. Characteristic gives better selectivity than the IDMT Can be used where an IDMT relay fails in selectivity, Extremely inverse relays Its time-current characteristic according to I 3.5 t = K. Very suitable for the protection against overheating for machines, power transformers, grounding transformers, and expensive cables.

10. Characteristic of Definite and Inverse time OCR

11. Moderately, Very and Extremely inverse-time characteristics

12. IDMT Very Inverse Exttremely Inverse

13. The time-current curves by the following equation t : trip time (sec) M : multiple of pickup current (M>1) TD : time dial setting A,B,p : curve shaping constant IEEE Standardized Relay Curve Equation Constant A B p Moderately inverse 0.0515 0.114 0.02 Very inverse 19.61 0.491 2.0 Extremely inverse 28.2 0.122 2.0

14. Current Setting • A relay is set 5 A. The current > 5 A (operate) and • The current <5 A (not operate). • For phase to phase fault protection can be set • at 50% to 200% of rated current in steps of 25%. • The usual current rating is 5A. • For protection against ground faults has setting • 20% to 80% of rated current in steps of 10%. • The current rating of a ground fault relay is usually 1A.

15. The Plug Setting Multiplier PSM = Secondary Current / Relay Current Setting = (Primary fault current)/(Relay current setting x C.T.ratio) Time Setting There are 10 steps and over. The values of TMS are 0.1, 0.2, …, 0.9, 1.0

16. Time-current characteristics for different values of TMS

17. Example: An over current relay (OCR) available current tapping are 2.5, 3.75, 5, 6.25, 7.5,…10 A. Fault current = 6.000 A, CT ratio is 400/5. TMS likes in Figure 5.3. Determine the operating time of the relay

18. Differential Relay • The principle operating of differential relay • based on Kirchhoff’s current law. • Not operated for external fault and operated for • fault in protected zone • Using two pairs CT in each phases • Using CT ratio gives the same secondary current

19. Fig. a typical differential connection in normal condition.

20. Figure Differential relay with internal fault (trip relay)

21. Fig. Percentage Differential relay Usually, the rating of the percentage differential relays are designed to trip given values, such as 30% or 40%.

22. Characteristic of Percentage differential relay

23. Distance relay. The distance relay operates on the principle of comparing the voltage and current in some way to obtain a measure of the ratio between quantities. The relay apparent impedance, Z = V/I. Types of distance relay i). Impedance relays ii). Reactance relays iii). MHO relays, etc

24. Impedance relay The current produces a positive torque (operating torque) and the voltage produces a negative torque (restraining torque). The equations for the operating torque The equations for the operating torque, T = K1I2 – K2V2 – K3 K1, K2 , and K3 are constants, K3 being torque due to the control spring effect. Neglecting the effect of the spring used, T = K1I2 – K2V2

25. For the operation of the relay, the following condition should be satisfied. K1 I2 > K2V2 or K2V2 < K1 I2 or < < Kwhere K is a constant Or Z < K

26. The Characteristics of impedance relay

27. Faults in transformer can be divided in two classes: • External and • Internal faults. • External faults are faults that occurs out side of • transformer protection zone Internal faultsare faults that occurs within then transformer protection zone

28. External faults • Overloads cause the transformer to over heat. One cause of overload may be due to unequal sharing of parallel transformers or unbalance loading of three phase banks. • Over voltage can be either due to short term transient conditions or long-term power frequency conditions. Transient over voltage cause end turn stresses and possible breakdown • Under frequency also is caused by a major system disturbance that causes an imbalance between generation and load • The conditions is similar to over voltage in that exciting current is greatly increased at low frequencies, causing over- fluxing of the transformer circuits. • External system short circuits are external to the transformer • protection zone, but cause high transformer currents, • can cause transformer winding damage.

29. Two classifications of internal fault namely: • Incipient faults and • Active faults Incipient faultsare faults that develop slowly, but that may develop into major faults if the cause is not detected and corrected (overheating, over fluxing, overpressure). Active faultsare caused by the breakdown in insulation or other components that createa sudden stress situation that requires prompt action to limit the damage and prevent further destructive actions (SC).

30. Over heating may be caused by • Poor internal connections, in either the electric/magnetic circuit. • Loss of coolant due to leakage • Blockage of coolant flow. • Loss of fan or pumps that designed to provide cooling • Over pressure in the transformer thank occurs due to • The release of gasses or products that accompany • the localized heating due to any cause. • For example, a turn to turn fault may burn slowly, • releasing gases in the process, or local heating of • insulations may give off gases.

31. Transformer protection using differential relay is recommendedFor large transformers (>10 MVA). Delta Wye Connected

32. The currents in the restraint windings we get ratio as, = 0

33. Over current relay • Protection of transformers of rating 100 kVA • and below 5 MVA. • Used as back up protection where differential protection • is used as primary protection. • For small transformer, OCRs are used for both overload • and fault protection. • An extremely inverse relay for overload and light faults • Instantaneous OCR for heavy faults. • A very inverse residual ocr with instantaneous relay • is suitable for ground faults.

34. Earth fault protection of a power transformer

35. Distance relaying. • As back up protection. • Using Directional distance relaying when the • Setting or coordination of the over current relays is a problem. • The directional distance relays are connected to operate when the fault current flows toward the protected transformer. • They are set to reach into, but not beyond the transformer.

36. Over excitation protection, • May result in thermal damage to cores due to excessively • high flux in the magnetic circuits. • Excess flux saturates the core steel and flows into the adjacent • structure, causing high eddy current losses in the core. • A transformer designed for a voltage limit of 1.2 p.u at rated • frequency will experience over excitation whenever the per • unit volts/hertz exceed 1.2.

37. Protections against magnetizing inrush current. When a transformer is first energized, a transient magnetizing or exiting inrush current may flow. Magnetizing inrush current has a high harmonic content (the second harmonics). A high speed biased differential scheme incorporating a harmonic restraint. Over-heating protection Caused by Over loading The maximum allowed temperature is about 950C and depend on insulations class. The protection against overload is usually measured by thermal relay.

38. Buchholz relay To detect incipient faults which are initially minor faults but may cause major faults in due course of time. When a fault develops slowly, it produces heat, thereby decomposing solid or liquid insulating material in the transform. The decomposing solid or liquid insulating material produces inflammable gases give alarm.

39. Sudden pressure relay (SPR) Capable of detecting a rapid rise of pressure It operates with a sealed air or gas chamber above oil level. The SPR relay is recommended for all units of 5 MVA or more. It operating time varies from one-half cycle to 37 cycles. Over-fluxing protection. The magnetic flux increase when voltage increases. This results in increased iron loss and magnetizing current and the lamination insulation is affected. Protection against over-fluxing due to sustained over-voltage can occur

40. Protection Transformer Bank Legend 87 T :Transformer differential relays 50/51 : Inverse time CO relay (phase fault) 50G : Ground fault relay (GFR) 50/51N : Back up GFR 151G : Feeder ground back up which trip breaker 52-11 63 : SPR

41. Bus-bar Protection. • Location of current transformer for bus-bar protections • There are three CT location that used for protection • Double bus and single breaker • Breaker and half • Ring bus

43. Information is required for the scheme selections, relay selections, and setting calculations; i). Bus-bar configuration such as in Figure 5.11. ii). Maximum and minimum bus fault current. iii).Current transformer information including location, ratio, accuracy class and saturation curve of current transformers. iv).Operating speed requirement.

44. The Other Bus Bar protection Figure 5.12. Over current differential bus protection.

45. Figure 5.13. Connection of one CA-16 relay per phase to protect a bus with three equivalent circuits.

46. Figure 5.14. Connection of one CA-16 relay per phase to protect a bus with four equivalent circuits.