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Power System Protection

Power System Protection. Dr. Ibrahim El-Amin. Protective Device Coordination. Definition. Overcurrent Coordination A systematic study of current responsive devices in an electrical power system. Objective. To determine the ratings and settings of fuses, breakers, relay, etc.

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Power System Protection

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  1. Power System Protection Dr. Ibrahim El-Amin

  2. Protective Device Coordination

  3. Definition • Overcurrent Coordination • A systematic study of current responsive devices in an electrical power system.

  4. Objective • To determine the ratings and settings of fuses, breakers, relay, etc. • To isolate the fault or overloads.

  5. Criteria • Economics • Available Measures of Fault • Operating Practices • Previous Experience

  6. Design • Open only PD upstream of the fault or overload • Provide satisfactory protection for overloads • Interrupt SC as rapidly (instantaneously) as possible • Comply with all applicable standards and codes • Plot the Time Current Characteristics of different PDs

  7. Analysis When: • New electrical systems • Plant electrical system expansion/retrofits • Coordination failure in an existing plant

  8. Protection vs. Coordination • Coordination is not an exact science • Compromise between protection and coordination • Reliability • Speed • Performance • Economics • Simplicity

  9. Protection • Prevent injury to personnel • Minimize damage to components • Quickly isolate the affected portion of the system • Minimize the magnitude of available short-circuit

  10. Spectrum Of Currents • Load Current • Up to 100% of full-load • 115-125% (mild overload) • Overcurrent • Abnormal loading condition (Locked-Rotor) • Fault Current • Fault condition • Ten times the full-load current and higher

  11. Coordination • Limit the extend and duration of service interruption • Selective fault isolation • Provide alternate circuits

  12. C D A B t A C D B I Coordination

  13. Equipment • Motor • Transformer • Generator • Cable • Busway

  14. I2t I2t I2t t I22t Motor Xfmr Cable Gen I Capability / Damage Curves

  15. Transformer CategoryANSI/IEEE C-57.109

  16. FLA 200 Thermal (D-D LL) 0.87 Infrequent Fault (D-R LG)0.58 Frequent Fault Mechanical 2 Inrush Isc 2.5 25 Transformer t (sec) I2t = 1250 K=(1/Z)2t I (pu)

  17. Transformer Protection

  18. Protective Devices • Fuse • Relay (50/51 P, N, G, SG, 51V, 67, 46, 79, 21, …) • Thermal Magnetic • Low Voltage Solid State Trip • Electro-Mechanical • MCP • Overload Heater

  19. Fuse • Non Adjustable Device • Continuous and Interrupting Rating • Voltage Levels • Characteristic Curves • Min. Melting • Total Clearing • Application

  20. Total Clearing Time Curve Minimum Melting Time Curve

  21. Current Limiting Fuse(CLF) • Limits the peak current of short-circuit • Reduces magnetic stresses (mechanical damage) • Reduces thermal energy

  22. 100,000 Let-Through Chart 15% PF (X/R = 6.6) 230,000 300 A 100 A 12,500 Peak Let-Through Amperes 60 A 5,200 Symmetrical RMS Amperes

  23. Fuse Generally: • CLF is a better short-circuit protection • Non-CLF (expulsion fuse) is a better Overload protection

  24. Typically: Non-CLF: 140% of full load CLF: 150% of full load Selectivity Criteria

  25. Thermal-Magnetic Magnetic Only Integrally Fused Current Limiting High Interrupting Capacity Types Frame Size Trip Rating Interrupting Capability Voltage Molder Case CB

  26. Thermal Maximum Thermal Minimum Magnetic (instantaneous)

  27. LVPCB • Voltage and Frequency Ratings • Continuous Current / Frame Size • Override (12 times cont. current) • Interrupting Rating • Short-Time Rating (30 cycle) • Fairly Simple to Coordinate

  28. LT PU CB 2 CB 1 CB 2 LT Band ST PU CB 1 480 kV IT If =30 kA ST Band

  29. Motor Protection • Motor Starting Curve • Thermal Protection • Locked Rotor Protection • Fault Protection

  30. Motor Overload Protection (NEC Art 430-32) • Thermal O/L (Device 49) • Motors with SF not less than 1.15 • 125% of FLA • Motors with temp. rise not over 40 • 125% of FLA • All other motors • 115% of FLA

  31. Locked Rotor Protection • Thermal Locked Rotor (Device 51) • Starting Time (TS < TLR) • LRA • LRA sym • LRA asym (1.5-1.6 x LRA sym) + 10% margin

  32. Fault Protection (NEC Art 430-52) • Non-Time Delay Fuses • 300% of FLA • Dual Element (Time-Delay Fuses) • 175% of FLA • Instantaneous Trip Breaker • 800% of FLA* • Inverse Time Breakers • 250% of FLA *MCPs can be set higher

  33. (49) I2T tLR O/L MCP (51) ts 200 HP Starting Curve MCP (50) LRAs LRAasym

  34. Overcurrent Relay • Time-Delay (51 – I>) • Short-Time Instantaneous ( I>>) • Instantaneous (50 – I>>>) • Electromagnetic (induction Disc) • Solid State (Multi Function / Multi Level) • Application

  35. IL CT IR 51 Time-Overcurrent Unit • Ampere Tap Calculation • Ampere Pickup (P.U.) = CT Ratio x A.T. Setting • Relay Current (IR) = Actual Line Current (IL) / CT Ratio • Multiples of A.T. = IR/A.T. Setting = IL/(CT Ratio x A.T. Setting)

  36. IL CT IR 50 Instantaneous Unit • Instantaneous Calculation • Ampere Pickup (P.U.) = CT Ratio x IT Setting • Relay Current (IR) = Actual Line Current (IL) / CT Ratio • Multiples of IT = IR/IT Setting = IL/(CT Ratio x IT Setting)

  37. Relay Coordination • Time margins should be maintained between T/C curves • Adjustment should be made for CB opening time • Shorter time intervals may be used for solid state relays • Upstream relay should have the same inverse T/C characteristic as the downstream relay (CO-8 to CO-8) or be less inverse (CO-8 upstream to CO-6 downstream) • Extremely inverse relays coordinates very well with CLFs

  38. Fixed Points Points or curves which do not change regardless of protective device settings: • Motor starting curves • Transformer damage curves & inrush points • Cable damage curves • SC maximum fault points • Cable ampacities

  39. 4.16 kV Relay: IFC 53 CT 800:5 50/51 CB Cable CU - EPR 1-3/C 500 kcmil Isc = 30,000 A 5 MVA DS 6 % Situation Calculate Relay Setting (Tap, Inst. Tap & Time Dial) For This System

  40. Transformer: IL IR Set Relay: CT R Solution

  41. What is ANSI Shift Curve? Question

  42. Answer • For delta-delta connected transformers, with line-to-line faults on the secondary side, the curve must be reduced to 87% (shift to the left by a factor of 0.87) • For delta-wye connection, with single line-to-ground faults on the secondary side, the curve values must be reduced to 58% (shift to the left by a factor of 0.58)

  43. What is meant by Frequent andInfrequent for transformers? Question

  44. Answer

  45. Question What T/C Coordination interval should be maintained between relays?

  46. A B t CB Opening Time + Induction Disc Overtravel (0.1 sec) + Safety margin (0.2 sec w/o Inst. & 0.1 sec w/ Inst.) I Answer

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