Generator
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1/29/2013 30th Hands Hands-On On Relay School Generation Track Overview Lecture Generator Design, Connections, and Grounding 1 1/29/2013 Generator Main Components • Stator – Core lamination – Winding • Rotor – Shaft – Poles – Slip rings Stator Core Source: www.alstom.com/power/fossil/gas/ 2 1/29/2013 Stator (Core + Winding) Winding Connections Core Lamination Winding (Roebel bars) Typical Types of Generator Windings Stator Winding: Random-Wound Coils 3 1/29/2013 Typical Types of Generator Windings Stator Winding: Form-Wound Coils Typical Types of Generator Windings Stator Winding: Roebel Bars 4 1/29/2013 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 1/29/2013 Series Connection of Roebel Bars Series connection Source:www.ansaldoenergia.com/Hydro_Gallery.asp Rotor 6 1/29/2013 Classification of Synchronous Generators Synchronous Generator Classification Cylindrical rotor Rotor design Salient-pole rotor Direct Cooling: Stator and rotor Indirect Field winding Brush connection to dc Brushless source Rotor Design Salient-Pole Rotor Cylindrical Rotor 7 1/29/2013 Two-Pole Round Rotor Source: www.alstom.com Salient Pole Rotor Source:www.ansaldoenergia.com/Hydro_Gallery.asp 8 1/29/2013 Stator Winding Cooling Indirectly Cooled Directly Cooled Cooling Ducts, Water Cooled Bar Rotor Winding Cooling Indirectly Cooled Directly Cooled 9 1/29/2013 Field Winding Connection to DC Source Brush Type Field Winding Connection to DC Source Brushless 10 1/29/2013 Generator Station Arrangements Generator-Transformer Unit Generating Station Arrangements Directly Connected Generator 11 1/29/2013 Synchronous Generator Grounding IEEE C62.92.2-1989 • Resonant g 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 1/29/2013 Ungrounded Neutral • No intentional connection to ground g • 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 1/29/2013 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: X0 / X1 3 and R0 / X1 1 • Ground G d ffaultlt currentt is i high hi h • Low temporary overvoltages during phaseto-ground faults 14 1/29/2013 Generator Capability Curves Defining Generator Capability • Curve provided by the generator manufacturer • Defines the generator operating limits during steady state t t conditions diti • Assumes generator is connected to an infinite bus • Limits are influenced by: – Terminal voltage – Coolant – Generator construction 15 1/29/2013 Generator Capability Curve for a Round Rotor Generator Generator Capability Curve for a Salient Pole Generator 16 1/29/2013 Capability Curve Construction Phasor Diagram – Round Rotor Generator Xd P V I cos( ) E 0 sin( ) Xd I cos( ) I V E 0 sin( i ( ) V I cos(( ) Xd V ( BC ) V I cos( ) Xd V E0 C φ E0 P Xd I V A I B Q Q V I sin( ) ( E 0 cos(( )) V Xd I sin( i ( ) V (( E 0 cos( )) V ) V I sin( ) Xd V ( AB) V I sin( ) Xd 17 1/29/2013 Power Angle Characteristic P Operation with Constant Active Power and Variable Excitation C C’’ Xd I Xd I E 0 I C’ Xd I E0 E0 P V B’’ A I Q I Q B’ B Q Xd 1.6 V 1.00 I 1 36.87 E 0 2.3433.15 I 1.6 60 E 0 3.46621.7 I 1.1345 E 0 1.3178.5 18 1/29/2013 Power Angle Characteristic P E 0 2.3433.15 E 0 3.46621.7 E 0 1.3178.5 V-Curves I ( p.u ) cos cap. cos inductive E 0 (p.u.) Excitation Current 19 1/29/2013 Operation with Constant Apparent Power and Variable Excitation C E0 Xd I V Xd 1.6 A B I V 1.00 I 1 36.87 Operation with Constant Excitation and Variable Active Power Theor. Stability Limit E0 Xd I C E0 I Xd I V A B I 20 1/29/2013 Theor. Sttability Limit Capability Curve – Round Rotor P (Real Power) V (( E 0 cos( )) V ) V I sin( ) Xd E0 0 - VV Q Xd V E 0 sin( ) V I cos( ) Xd E0 0 P0 max. Q Xd 1.6 Q (Reactive Power) V 1.0 - VV 0.625 Xd Generator Fault Protection 21 1/29/2013 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 1/29/2013 Phase Fault Protection Percentage Differential Dual-Slope Characteristic 23 1/29/2013 Phase Fault Protection High-Impedance Differential O O O Phase Fault Protection Self-Balancing Differential http://www.polycastinternational.com/old_cat/pdfs/Section4/Section4-Part2.pdf 24 1/29/2013 Stator Winding Coils with Multiple Turns Turn-to-Turn Fault Protection Split-Phase Self-Balancing 25 1/29/2013 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 1/29/2013 Ground Fault in a Unit-Connected Generator High-Impedance Grounded Generator Neutral Fundamental Overvoltage Fault Location/ % of Winding Voltage V F1 / 3% Vnom 3 Vnom 85% • 3 F2 / 85% 3% • 27 1/29/2013 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 (3) 59GN OR (2) 27TN Full Load Full Load No Load VN3 No Load VN3 VP3 VP3 No Fault Fault at F1 28 1/29/2013 High-Impedance Grounded Generator Third-Harmonic Differential GSU (3) (3) V R 59GN VN3 VP3 k • VP3 VN 3 Pickup Setting 59THD + – Third-Harmonic Differential Element Generator Winding Analysis • Generator data – 18 poles – 216 slots • Winding pitch – Full pitch = 216/18 = 12 slots – Actual pitch = 128 – 120 = 8 slots – Actual pitch / full pitch = 8/12 = 2/3 29 1/29/2013 Full-Pitch Winding 2/3 Pitch Winding Removes Third Harmonic 30 1/29/2013 High-Impedance Grounded Generator Low-Frequency Injection GSU (3) OR (2) I R 59GN V 64S Coupling Filter Low-Frequency Voltage Injector Protection Measurements 100% Stator Ground Fault Protection Elements Coverage 31 1/29/2013 Low-Impedance-Grounded Generator Ground Overcurrent and Directional Overcurrent Low-Impedance-Grounded Generator Ground Differential 32 http://www05.abb.com/global/scot/scot235.nsf/ve eritydisplay/beaaeb0123376541832573460062a765 5/$file/1vap428561-db_byz.pdf 1/29/2013 Low-Impedance-Grounded Generator Self-Balancing Ground Differential Zero-Sequence CTs Zero-sequence CT 33 1/29/2013 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 1/29/2013 Salient Pole Rotor Source:www.ansaldoenergia.com/Hydro_Gallery.asp A Round Rotor Being Milled Source: Maughan, Clyde. V., Maintenance of Turbine Driven Generators, Maughan Engineering Consultants 35 1/29/2013 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: www.alstom.com 36 1/29/2013 Two-Pole Round Rotor Source: www.alstom.com Two-Pole Round Rotor Source: www.alstom.com 37 1/29/2013 Round Rotor Slot — Cross Section Coil Slot Wedge R t i i Ri Retaining Ring Creepage Block 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 1/29/2013 Salient Pole Cross Section Pole Body Pole Collar Winding Turn Turn Insulation Winding Ground Pole Body Insulation Winding g 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 1/29/2013 Importance of Field Ground Detection • Presence of a single point-to-ground in field g circuit does not affect the operation p of winding 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 1/29/2013 Voltage Divider Field Breaker Rotor and Field Winding R3 + R2 Exciter Brushes R1 – Sensitive Detector Grounding Brush DC Injection Field Breaker Rotor and Field Winding + Exciter Brushes – Sensitive Detector + DC Supply G Grounding di Brush – 41 1/29/2013 AC Injection Field Breaker Rotor and Field Winding + Brushes Exciter – Sensitive Detector G Grounding di Brush AC Supply Switched-DC Injection Method Field Breaker Rotor and Field Winding + Brushes Exciter – R1 Grounding Brush R2 Rs Measured Voltage 42 1/29/2013 Switched DC Injection Principle of Operation Voscp VDC + Voscn – Vrs Rx R Cfg Vosc R Measured Voltage (Vrs) Vrs Rs V Shaft Grounding with Carbon Brush 43 1/29/2013 Shaft Grounding with Wire Bristle Brush Source: SOHRE Turbomachinery, Inc. (www.sohreturbo.com) Generator Abnormal Operation Protection 44 1/29/2013 Generator Abnormal Operation Protection • Thermal • Overvoltage • Current unbalance • Abnormal frequency • Loss-of-field • Out-of-step • Motoring • Inadvertent energization • Overexcitation • Backup Stator Thermal Protection Generators With Temperature Sensors 45 1/29/2013 Stator Thermal Protection Generators Without Temperature Sensors I 2 I P2 T ln 2 2 I k I NOM Current Unbalance Causes • Single-phase transformers • Untransposed transmission lines • Unbalanced loads • Unbalanced system faults • Open phases 46 1/29/2013 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 1/29/2013 Negative-Sequence Current Damage Negative-Sequence Current Capability Continuous Type of Generator I2 Max % Salient pole (C50.12-2005) Connected amortisseur windings 10 Unconnected amortisseur windings 5 Cylindrical rotor (C50.13-2005) Indirectly cooled 10 Directly cooled, to 350 MVA 8 351 to 1250 MVA 8 – (MVA – 350) / 300 1251 to 1600 MVA 5 48 1/29/2013 Negative-Sequence Current Capability Short Time I 22t K 2 Type of Generator I22t Max % Salient pole (C37.102-2006) 40 Synchronous condenser (C37.102-2006) 30 Cylindrical rotor (C50.13-2005) Indirectly cooled 30 Directly cooled, to 800 MVA 10 Directly cooled, 801 to 1600 MVA → Negative-Sequence Current Capability Short Time 49 1/29/2013 NegativeSequence Overcurrent Protection T K2 I2 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 1/29/2013 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 1/29/2013 LOF Protection Using a Mho Element LOF Protection Using NegativeOffset Mho Elements 52 1/29/2013 LOF Protection Using Negative- and Positive-Offset Mho Elements Zone 2 Setting Considerations 53 1/29/2013 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 yp values of reverse p power required q to spin a generator at synchronous speed Steam turbines Hydro turbines Diesel engines Gas turbines 0.5–3% 0.2–2+% 5–25% 50+% 54 1/29/2013 Directional Power Element Q 32P1 32P2 P P1 P2 Overexcitation Protection V f NOM • f VNOM • Overexcitation occurs when V/f exceeds 1.05 • Causes generator heating • Volts/hertz (24) protection should trip generator 55 1/29/2013 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 1/29/2013 Overexcitation Protection Dual-Level, Definite Time Characteristic Overexcitation Protection Inverse- and Definite Time Characteristics 57 1/29/2013 Overvoltage Protection • Overvoltage most frequently occurs in h d l t i generators hydroelectric t • Overvoltage protection (59): – Instantaneous element set at 130–150 percent of rated voltage – Time-delayed element set at approximately 110 percent of rated voltage Abnormal Frequency Protection 58 1/29/2013 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 1/29/2013 Double-Blinder Out-of-Step Scheme Generator Inadvertent Energization • Common causes: human errors, control circuit failures failures, and breaker flashovers • The generator starts as an induction motor • High currents induced in the rotor cause rapid heating • High stator current 60 1/29/2013 Inadvertent Energization Protection Logic Logic for Combined Breaker-Failure and Breaker-Flashover Protection 61 1/29/2013 Backup Protection Directly Connected Generator Generator With Step-Up Transformer Voltage-Restrained Overcurrent Element Pickup Current 62 1/29/2013 Mho Distance Element Characteristic Synchronism-Check Element 63 1/29/2013 Power System Disturbance Caused by an Out-of-Synchronism Close Nominal Current: 10560 A Voltage: 6.5 kV Possible Damaging Effects During Synchronizing • • • • Shaft damage due to torque Bearing damage Loosened stator windings Loosened stator laminations 64 1/29/2013 IEEE Generator Synchronizing Limits Breaker closing angle +/ 10° +/–10° Generator-side voltage relative to system 100% to 105% Frequency difference +/–0.067 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 1/29/2013 Synchronism-Check Logic Overview 66
