1/29/2013. School. Generation Track. Overview Lecture. Generator Design, Connections, and Grounding

Transcription

1 30 th Hands-On Relay School Generation Track Overview Lecture Generator Design, Connections, and Grounding 1

2 Generator Main Components Stator Core lamination Winding Rotor Shaft Poles Slip rings Stator Core Source: 2

3 Stator (Core + Winding) Winding Connections Core Lamination Winding (Roebel bars) Typical Types of Generator Windings Stator Winding: Random-Wound Coils 3

4 Typical Types of Generator Windings Stator Winding: Form-Wound Coils Typical Types of Generator Windings Stator Winding: Roebel Bars 4

5 Roebel Bars Inside Stator Slot Source: Maughan, Clyde. V., Maintenance of Turbine Driven Generators, Maughan Engineering Consultants Stator Winding Combinations Typical for Two- and Four-Pole Machines 5

6 Series Connection of Roebel Bars Series connection Source: Rotor 6

7 Classification of Synchronous Generators Synchronous Generator Classification Rotor design Cooling: Stator and rotor Field winding connection to dc source Cylindrical rotor Salient-pole rotor Direct Indirect Brush Brushless Rotor Design Salient-Pole Rotor Cylindrical Rotor 7

8 Two-Pole Round Rotor Source: Salient Pole Rotor Source: 8

9 Stator Winding Cooling Indirectly Cooled Directly Cooled Cooling Ducts, Water Cooled Bar Rotor Winding Cooling Indirectly Cooled Directly Cooled 9

10 Field Winding Connection to DC Source Brush Type Field Winding Connection to DC Source Brushless 10

11 Generator Station Arrangements Generator-Transformer Unit Generating Station Arrangements Directly Connected Generator 11

12 Synchronous Generator Grounding IEEE C Resonant grounding g (Petersen Coil) Ungrounded neutral High-resistance grounding Low-resistance grounding Low-reactance grounding Effective grounding Increasing Ground Fault Current Why Ground the Neutral? Minimize damage for internal ground faults Limit mechanical stress for external ground faults Limit temporary/transient overvoltages Allow for ground fault detection Ability to coordinate generator protection with other equipment requirements 12

13 Ungrounded Neutral No intentional connection to ground Maximum ground fault current higher than for resonant grounding Excessive transient overvoltages may result High-Resistance Grounding Low value resistor connected to secondary of distribution transformer Resistor value selected to limit transient overvoltages Maximum single-phase-to-ground fault current: 5 15 A 13

14 Low-Resistance Grounding Limit ground fault current to hundreds of amperes to allow operation of selective (differential) relays Low temporary/transient overvoltages Effective Grounding A low-impedance ground connection where: X 0 / X 1 3 and R 0 / X 1 1 Ground fault current is high h Low temporary overvoltages during phaseto-ground faults 14

15 Generator Capability Curves Defining Generator Capability Curve provided by the generator manufacturer Defines the generator operating limits during steady state t conditions Assumes generator is connected to an infinite bus Limits are influenced by: Terminal voltage Coolant Generator construction 15

16 Generator Capability Curve for a Round Rotor Generator Generator Capability Curve for a Salient Pole Generator 16

17 17 Capability Curve Construction Phasor Diagram Round Rotor Generator ) ( ) i( ) cos( ) sin( ) cos( 0 I V E V I Xd E I V P E 0 Xd I V ) cos( ) ( ) cos( ) sin( 0 I V BC Xd V I V E Xd ) i( )) ( ( ) sin( I Xd V E I V Q φ C E 0 P ) sin( ) ( ) sin( ) )) cos( (( ) sin( )) cos( ( 0 0 I V AB Xd V I V V E Xd V I Xd V E A B V Xd I Q I

18 Power Angle Characteristic P Operation with Constant Active Power and Variable Excitation C C C Xd I Xd I Xd I E 0 I E 0 E 0 P V B A Q B B I I Q Q Xd 1.6 V 1.00 I E I E I E

19 Power Angle Characteristic P E E E V-Curves I ( p. u) cos cap. cos inductive E 0 (p.u.) Excitation Current 19

20 Operation with Constant Apparent Power and Variable Excitation C E 0 Xd I V A B Xd 1.6 V 1.00 I I Operation with Constant Excitation and Variable Active Power E 0 Theor. Stability Limit Xd I E 0 I C Xd I V A B I 20

21 Capability Curve Round Rotor tability Limit Theor. St max. P (Real Power) - V V Q Xd V (( E 0cos( )) V ) V I sin( ) Xd E V V Q Xd V E 0sin( ) V I cos( ) Xd E 0 0 P 0 Q (Reactive Power) Xd 1.6 V 1.0 Generator Fault Protection 21

22 Generator Fault Protection Stator phase faults Stator ground faults Field ground faults External faults (backup protection) Stator Phase Fault Protection Phase fault protection Percentage differential High-impedance differential Self-balancing differential Turn-to-turn fault protection Split-phase differential Split-phase self-balancing 22

23 Phase Fault Protection Percentage Differential Dual-Slope Characteristic 23

24 Phase Fault Protection High-Impedance Differential O O O Phase Fault Protection Self-Balancing Differential 24

25 Stator Winding Coils with Multiple Turns Turn-to-Turn Fault Protection Split-Phase Self-Balancing 25

26 Turn-to-Turn Fault Protection Split-Phase Percentage Differential Stator Ground Fault Protection High-impedance-grounded generators Neutral fundamental-frequency overvoltage Third-harmonic undervoltage or differential Low-frequency injection Low-impedance-grounded generators Ground overcurrent Ground directional overcurrent Restricted earth fault (REF) protection 26

27 Ground Fault in a Unit-Connected Generator High-Impedance Grounded Generator Neutral Fundamental Overvoltage Fault Location/ % of Winding F1 / 3% F2 / 85% Voltage V Vnom 3% 3 Vnom 85% 3 27

28 Generator Flux Distribution in Air Gap Total Flux Fundamental Harmonics Generator Flux Distribution in Air Gap High-Impedance Grounded Generator Neutral Third-Harmonic Undervoltage GSU F1 V R 59GN 27TN (3) OR (2) Full Load VN3 Full Load No Load VN3 No Load VP3 VP3 No Fault Fault at F1 28

29 High-Impedance Grounded Generator Third-Harmonic Differential GSU (3) (3) V R 59GN VN3 VP3 59THD k VP3 VN3 Pickup Setting + Third-Harmonic Differential Element Generator Winding Analysis Generator data 18 poles 216 slots Winding pitch Full pitch = 216/18 = 12 slots Actual pitch = = 8 slots Actual pitch / full pitch = 8/12 = 2/3 29

30 Full-Pitch Winding 2/3 Pitch Winding Removes Third Harmonic 30

31 High-Impedance Grounded Generator Low-Frequency Injection GSU (3) OR (2) R 59GN V I 64S Coupling Filter Low-Frequency Voltage Injector Protection Measurements 100% Stator Ground Fault Protection Elements Coverage 31

32 Low-Impedance-Grounded Generator Ground Overcurrent and Directional Overcurrent Low-Impedance-Grounded Generator Ground Differential 32

33 Low-Impedance-Grounded Generator Self-Balancing Ground Differential Zero-Sequence CTs 5/$file/1vap db_byz.pdf eritydisplay/beaaeb a765 Zero-sequence CT 33

34 Field Ground Protection Field Ground Protection Types of rotors Winding failure mechanisms Importance of field ground protection Field ground detection methods Switched-DC injection principle of operation Shaft grounding brushes 34

35 Salient Pole Rotor Source: A Round Rotor Being Milled Source: Maughan, Clyde. V., Maintenance of Turbine Driven Generators, Maughan Engineering Consultants 35

36 Round Rotor End Turns Source: Main Generator Rotor Maintenance Lessons Learned - EPRI Source: Main Generator Rotor Maintenance Lessons Learned - EPRI Two-Pole Round Rotor Source: 36

37 Two-Pole Round Rotor Source: Two-Pole Round Rotor Source: 37

38 Round Rotor Slot Cross Section Coil Slot Wedge Creepage Block Retaining i Ring Insulation Retaining Ring Copper Winding Winding Short Winding Ground Turn Insulation End Windings Winding Ground Slot Armor Field Winding Failure Mechanisms in Round Rotors Thermal deterioration Thermal cycling Abrasion Pollution Repetitive voltage surges 38

39 Salient Pole Cross Section Pole Body Pole Collar Winding Turn Turn Insulation Winding Ground Pole Body Insulation Winding Short Pole Collar * Strip-On-Edge Field Winding Failure Mechanisms in Salient Pole Rotors Thermal deterioration Abrasive particles Pollution Repetitive voltage surges Centrifugal forces 39

40 Importance of Field Ground Detection Presence of a single point-to-ground in field winding circuit does not affect the operation of the generator Second point-to-ground can cause severe damage to machine Excessive vibration Rotor steel and / or copper melting Rotor Ground Detection Methods Voltage divider DC injection AC injection Switched-DC injection 40

41 Voltage Divider Field Breaker Rotor and Field Winding + R3 Exciter R2 R1 Brushes Sensitive Detector Grounding Brush DC Injection Field Breaker Rotor and Field Winding Exciter + Brushes Sensitive Detector + DC Supply Grounding Brush 41

42 AC Injection Field Breaker Rotor and Field Winding Exciter + Brushes Sensitive Detector AC Supply Grounding Brush Switched-DC Injection Method Field Breaker Rotor and Field Winding Exciter + Brushes R1 Grounding Brush Rs R2 Measured Voltage 42

43 Switched DC Injection Principle of Operation Voscp VDC Voscn + Vrs Rx R Cfg Vosc Vrs Rs R Measured Voltage (Vrs) V Shaft Grounding with Carbon Brush 43

44 Shaft Grounding with Wire Bristle Brush Source: SOHRE Turbomachinery, Inc. ( Generator Abnormal Operation Protection 44

45 Generator Abnormal Operation Protection Thermal Current unbalance Loss-of-field Motoring Overexcitation Overvoltage Abnormal frequency Out-of-step Inadvertent energization Backup Stator Thermal Protection Generators With Temperature Sensors 45

46 Stator Thermal Protection Generators Without Temperature Sensors T 2 2 I I P ln 2 I k I 2 NOM Current Unbalance Causes Single-phase transformers Untransposed transmission lines Unbalanced loads Unbalanced system faults Open phases 46

47 Generator Current Unbalance Produces negative-sequence currents that: Cause magnetic flux that rotates in opposition to rotor Induce double-frequency currents in the rotor Rotor-Induced Currents 47

48 Negative-Sequence Current Damage Negative-Sequence Current Capability Continuous Type of Generator I 2 Max % Salient pole (C ) Connected amortisseur windings 10 Unconnected amortisseur windings 5 Cylindrical rotor (C ) Indirectly cooled 10 Directly cooled, to 350 MVA to 1250 MVA 8 (MVA 350) / to 1600 MVA 5 48

49 Negative-Sequence Current Capability Short Time 2 2 K2 I t Type of Generator I 22 t Max % Salient pole (C ) 40 Synchronous condenser (C ) 30 Cylindrical rotor (C ) Indirectly cooled 30 Directly cooled, to 800 MVA 10 Directly cooled, 801 to 1600 MVA Negative-Sequence Current Capability Short Time 49

50 Negative- Sequence Overcurrent Protection T I K I 2 2 I NOM 2 Common Causes of Loss of Field Accidental field breaker tripping Field open circuit Field short circuit Voltage regulator failure Loss of field to the main exciter Loss of ac supply to the excitation system 50

51 Effects of Loss of Field Rotor temperature increases because of eddy currents Stator temperature increases because of high reactive power draw Pulsating torques may occur Power system may experience voltage collapse or lose steady-state stability Negative-Sequence Current Caused Damper Winding Damage Damper Windings 51

52 LOF Protection Using a Mho Element LOF Protection Using Negative- Offset Mho Elements 52

53 LOF Protection Using Negative- and Positive-Offset Mho Elements Zone 2 Setting Considerations 53

54 Possible Prime Mover Damage From Generator Motoring Steam turbine blade overheating Hydraulic turbine blade cavitation Gas turbine gear damage Diesel engine explosion danger from unburned fuel Small Reverse Power Flow Can Cause Damage Typical values of reverse power required to spin a generator at synchronous speed Steam turbines 0.5 3% Hydro turbines % Diesel engines 5 25% Gas turbines 50+% 54

55 Directional Power Element Q 32P1 32P2 P P1 P2 Overexcitation Protection V f NOM f V NOM Overexcitation occurs when V/f exceeds 1.05 Causes generator heating Volts/hertz (24) protection should trip generator 55

56 Core Damaged due to Overexcitation Source: Maughan, Clyde. V., Maintenance of Turbine Driven Generators, Maughan Engineering Consultants Core Damaged due to Overexcitation Source: Maughan, Clyde. V., Maintenance of Turbine Driven Generators, Maughan Engineering Consultants 56

57 Overexcitation Protection Dual-Level, Definite Time Characteristic Overexcitation Protection Inverse- and Definite Time Characteristics 57

58 Overvoltage Protection Overvoltage most frequently occurs in hd hydroelectric ti generators Overvoltage protection (59): Instantaneous element set at percent of rated voltage Time-delayed element set at approximately 110 percent of rated voltage Abnormal Frequency Protection 58

59 Possible Damage From Out-of-Step Generator Operation Mechanical stress in the machine windings Damage to shaft resulting from pulsating torques High stator core temperatures Thermal stress in the step-up transformer Single-Blinder Out-of-Step Scheme 59

60 Double-Blinder Out-of-Step Scheme Generator Inadvertent Energization Common causes: human errors, control circuit failures, and breaker flashovers The generator starts as an induction motor High currents induced in the rotor cause rapid heating High stator current 60

61 Inadvertent Energization Protection Logic Logic for Combined Breaker-Failure and Breaker-Flashover Protection 61

62 Backup Protection Directly Connected Generator Generator With Step-Up Transformer Voltage-Restrained Overcurrent Element Pickup Current 62

63 Mho Distance Element Characteristic Synchronism-Check Element 63

64 Power System Disturbance Caused by an Out-of-Synchronism Close Nominal Current: A Voltage: 6.5 kv Possible Damaging Effects During Synchronizing Shaft damage due to torque Shaft damage due to torque Bearing damage Loosened stator windings Loosened stator laminations 64

65 IEEE Generator Synchronizing Limits Breaker closing angle +/ 10 Generator-side voltage relative to system Frequency difference 100% to 105% +/ Hz Source: IEEE Std. C50.12 and C50.13 Issues Affecting Generator Synchronizing Voltage ratio differences Voltage angle differences Voltage, angle, and slip limits Synchronism Check relay Synchronism Check relay 65

66 Synchronism-Check Logic Overview 66