Power System Protection. Dr. Lionel R. Orama Exclusa, PE Week 3

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1 Power System Protection Dr. Lionel R. Orama Exclusa, PE Week 3

2 Operating Principles: Electromagnetic Attraction Relays Readings-Mason Chapters & 3 Operating quantities Electromagnetic attraction Response to AC currents Plunger type Hinged armature type Feb 3/006 Copyright L.R. Orama 006

3 Relay Electrical Operating nputs Voltage Current Both Cause of operation Change in magnitude Change in frequency Change in phase angle Duration Rate of change Change in direction harmonics Quantities Most of the time we ll be working with current Attraction relays operate Copyright L.R. Orama 006 3

4 Relay Types Electromagnetic Attraction Electromagnetic nduction Solid State (static) Computer (microprocesors) Show the operating principles Copyright L.R. Orama 006 4

5 Electromagnetic Attraction Force exerted by a magnetic field which pulls armature or plunger Manufacturesr used this design for instantaneous relays nduction disk-feeder time delay-current (FC) Hinged Armature Hinged armature-auxiliary (HEA) Plunger Plunger operating principle-voltage (PJV) Copyright L.R. Orama 006 5

6 Hinged Armature Relay Basic Operation Operating coil Copyright L.R. Orama 006 6

7 Copyright L.R. Orama Hinged Armature Relay S S NET e e e k i k l i N k k k t F i k l i N k k t F l i N l Ni l R A l R R Ni t F 0 ) ( ) ( ) ( φ φ φ φ φ φ α φ φα α µ φ α φ L L L Basic Operation Force conversion constant Restraining force (includes friction)

8 Hinged Armature Relay When the relay is close to pick-up, the net force is zero and: 0 k i k S k i k S i k k S Copyright L.R. Orama 006 8

9 Pickup or Balance Point F NET F F RES F NET 0 k K S K K S PU k > K S a contact forcing force is produced > < PU PU PU Operates (picks up) Balance Doesn t Operate Copyright L.R. Orama 006 9

10 Response to AC Currents F F sin F ( t) k φ Lφ ( t) ( t) k φ k ( ωt) φ φ Φ Φ M sin Φ ( ωt) ( cos(ωt)) M M ( cos(ωt)) sin( ωt) Variable force causes Vibration Measures are taken to avoid vibration Copyright L.R. Orama 006 0

11 Response to AC Currents Measures are taken to avoid vibration Shading Ring Fluxes in both areas are out of phase by the Lenz Law. Half the flux flow through through the shaded area φ φ Copyright L.R. Orama 006

12 Copyright L.R. Orama 006 Response to AC Currents Measures are taken to avoid vibration Shading Ring ( ) ) ( ) ( cos ) ( sin ) ( ) cos( 90) sin( 90 ) sin( ) sin( ) ( ) ( M M M M M M k k t F t t k t F t t t t k t F Φ + Φ Φ + Φ + Φ Φ + φ φ φ ω ω ω φ ω φ θ θ ω φ ω φ φ φ L RMS Constant

13 Plunger Type Relay No Shading ring Sinusoidal torque For DC AC with Rectifier Copyright L.R. Orama 006 3

14 Pickup, Dropout, Reset Copyright L.R. Orama 006 4

15 Operating Time Copyright L.R. Orama 006 5

16 Two Quantity Operating Relays 8-Feb-006 Copyright L.R. Orama 006 6

17 Hinge Armature Type Advantages Used on DC or AC Simple rugged construction Fast operating speed Low cost Can be either hand or self reset May be made with many contacts Good contact pressure Copyright L.R. Orama 006 7

18 Hinge Armature Type Disadvantages Low dropout to pick-up ratio Dropout and pick-up can t be set accurately Responds to magnitudes of single electrical quantity (current or voltage) Copyright L.R. Orama 006 8

19 Plunger Type Advantages Used on DC or AC Simple rugged construction Fast operating speed Low cost Can be either hand or self reset Can be design to have high dropout Copyright L.R. Orama 006 9

20 Plunger Type Disadvantages Can not continuously be used in picked up position Pickup lower on offset waves than on symmetrical waves Responds to magnitudes of single electrical quantity (current or voltage) Copyright L.R. Orama 006 0

21 Operating Principles Current Differential Relaying Readings-Mason Chapter 3 Basics of current differential relaying Problems caused by CT mismatch and errors Operating principles of percentage differential relays Copyright L.R. Orama 006

22 Current Differential Relaying External fault or load Protected Device SYS SYS SYS SYS FAULT SYS - SYS 0 No operation of the relays is expected Copyright L.R. Orama 006

23 Current Differential Relaying nternal fault or load Protected Device SYS SYS FAULT SYS FAULT + SYS SYS - SYS FAULT Differential current, relays operate Copyright L.R. Orama 006 3

24 CT/Relay Connection Overcurrent relay measures differential current0 Note: Load Current must be less than 000A for CT s to be the correct ones. No operation Copyright L.R. Orama 006 4

25 CT/Relay Connection Relay sends message to both CB (5) trip coils Relay Operates Copyright L.R. Orama 006 5

26 High Resistance nternal Fault High resistance internal fault may or may not be detected Overcurrent relay has a minimum pickup value For example, if PU.0A, the relay will not operate. 3-Feb-006 Copyright L.R. Orama 006 6

27 Not deal Operation N CT N N E P CT CT E E N P CT E No identical CT s exists due to magnetizing reactance There is always some current through the relay Copyright L.R. Orama 006 7

28 True CT Performance As said before: E causes CT error CT s saturates differently or Reisdual B exists CT CT Therefore differential current Copyright L.R. Orama 006 8

29 CT Mismatch Occurs Overcurrent operates for high values of F ( SYS ) external Apply relay with alternate characteristic percentage differential Copyright L.R. Orama 006 9

30 Copyright L.R. Orama Percentage Differential Relay [ ] [ ] [ ] RES R R RES R RES S R RES NET K K N N K K N K K N N K N K K N K N K t F ' ' ) ( ) ( ) ( ) ( ) ( ) ( ) ( L gnoring K S & solving for F NET 0

31 Percentage Differential Relay K' K' 0.0, 0.0, Equation of a straight line Typically > K' To Operate + Differential Current must be at least a certain percentage of the Through Current to operate Copyright L.R. Orama 006 3

32 Percentage Differential Relaying + > K ' 5-AC Circuit Breaker 87-Differential Relay w/ %diff Copyright L.R. Orama 006 3

33 Relays Used in Differential Schemes Overcurrent Directional Overvoltage Percentage Differential w/ Harmonic Restraint (BDD) w/ DC Restraint High mpedance (PVD) Copyright L.R. Orama

34 Transformer Protection Readings-Mason Chapter Percentage differential Gas detection Rate of change of oil pressure Copyright L.R. Orama

35 Transformer Protection Percentage differential relaying in transformers requires Proper CT connections Relay taps adjustment Special provisions for in-rush current Copyright L.R. Orama

36 Transformer Protection Topics that will be discussed Differences between regular differential relaying and transformer differential relaying Choice of proper CT ratio and CT connections Recalling from regular differential relaying External Fault- current circulates through the equipment nternal Fault- current adds at relay flows through operating coil Copyright L.R. Orama

37 5-Feb-006 Copyright L.R. Orama Turns Ratio Problem 0, 0, N N N N 0 N N CTR CTR CTR N CTR N N N CTR CTR CTR CTR R R R R But Normally Abnormally Want Want Power Transformer

38 Line Currents Measuring line currents is a problem Current in and out is normally different Therefore, unequal CT ratios is required Line currents measured 30 phase shift from to Y L L W W 30 Copyright L.R. Orama

39 Line Currents V L LL 3 30 V φ φ V LL L 3V + 30 φ φ n Wye connection, line & winding currents are the same n Delta connection, they are not f CT on DELTA where located at the windings, no problem Usually the CT s are on the line side of the Transformer Copyright L.R. Orama

40 Line Currents Winding currents measured-no problem N ) Y ( W NY When line currents are measured: LA WAY LAY WAY WAY WA N N Y WAY leads N LA WC LA N Y LA WA N + 30 NY 3 by LA Copyright L.R. Orama

41 Line Currents During SLGF on grounded Y side 0 flows on the Y but 0 0 on the Delta side L R, differential current, relay operates Special CT connection is needed Copyright L.R. Orama 006 4

42 Line Currents 30 RELAY R CT R RELAY L Remedy- CT s in Y on Delta side, and CT s in Delta on Y side This connection alleviates phase shift problem Also alleviates 0 problems Filters 0 form the relay, so neither RELAY-L nor RELAY-R contain 0, only and exist RELAY R 3 CT R Copyright L.R. Orama 006 4

43 Complete 3 Phase Power Transformer & CT Connections Percentage differential protection in a Y- Power transformer Copyright L.R. Orama

44 Choice of Proper CT Ratios (At Maximum Load) RELAY L RELAY R deal CT 5A 5A RATO FLY CT R CT R 5 A 3 00A 346 : 5 5A 3 This CTR is not available Primary is overrated Notes: Define Transformer Rating: FOA: Forced Oil & Air FA: Forced Air OA: Nothing Forced Copyright L.R. Orama

45 Taps are Provided CTR do not always match the Power Transformer Ratio RELAY-L 5A, RELAY-R 8.66A Choose 5A on the 3.8kV side, and 8.7 on the 38kV side Mismatch occurs from Taps Load tap changers (+_0%) Make % slope for TAP+LTC Mismatch Copyright L.R. Orama

46 Percentage Differential Relay w/ Tabs (Single Phase) 7-Feb-006 Copyright L.R. Orama

47 Transformer nrush On energization inrush flows to energizing winding-no ampere-turn balanced in secondary Will cause straight PD Relay to operate (loss of security) Since inrush has high harmonic content, they are used to suppress operation of the relay Copyright L.R. Orama

48 Harmonic Restraint PD Relay The harmonic currents help the relay not to operate during the inrush Harmonic Restraining Coil, works whit differential current Copyright L.R. Orama

49 Gas Detection Gases produced by slow insulation breakdown Gases rises through oil to gas accumulation chamber Gas accumulates, float rises & alarm sounds Relay detects incipient faults Gas analysis can determine what type of insulation deteriorates Copyright L.R. Orama

50 Fault Pressure Relay Sudden change in oil pressure makes oil flow to relay f pressure change rapidly, bellows move differentially-operates Slow pressure change, bellows move together Relay connected to trip circuit Detects tank faultsturn-turn Copyright L.R. Orama

51 Example FOA Rating 500MVA, 3 phase Xmer, w/ load tap changer (from -5% to % in % steps) Calculate the line currents in both sides (FOA & OA Ratings) Choose CT ratings (FOA) Calculate CT Currents Notes: Define Transformer Rating: FOA: Forced Oil & Air (pumps & fans on) FA: Forced Air (fans on) OA: Nothing Forced Choose relay taps Calculate relay tap mismatch Calculate total mismatch Copyright L.R. Orama 006 5

52 Notes: Define Transformer Rating: FOA: Forced Oil & Air (pumps & fans on) FA: Forced Air (fans on) OA: Nothing Forced Copyright L.R. Orama 006 5

53 CT Ratio 50:5 00:5 00:5 400:5 800:5 000:5 000:5 500:5 3000:5 Turns Copyright L.R. Orama

54 Copyright L.R. Orama

55 Copyright L.R. Orama

56 CT settings are: High voltage side of Xmer 000:5, Delta connection Low Voltage side of Xmer 3000:5, Wye connection Relay settings are: High voltage side of Xmer Tap5.0 Low Voltage side of Xmer Tap.9 Percentage differential setting 5% Copyright L.R. Orama

Transformer Protection

Transformer Protection Transformer Protection Outline Fuses Protection Example Overcurrent Protection Differential Relaying Current Matching Phase Shift Compensation Tap Changing Under Load Magnetizing