Ch 11 - Generator Protection

1 Generator CONTROL AND PROTECTIONG enerator ProtectionGENERATOR CONTROL AND PROTECTIONG enerator Protection Introduction Device Numbers Symmetrical Components Fault Current Behavior Generator Grounding Stator Phase Fault (87G) Field Ground Fault (64F) Stator Ground Fault (87N, 51N, 59N, 27-3N) Generator CONTROL AND PROTECTIONG enerator Protection Loss of Field (40Q, 40Z) Over/Under Frequency (81O/81U) Overexcitation and Overvoltage (24, 59) Out of Step (78) Negative Sequence (Current Unbalance) (46) Inadvertent Energization (27, 50, 60, 81, 62, 86) Loss of Voltage Transformer (60) System Backup (51V, 21) ConclusionGENERATOR CONTROL AND PROTECTIONG enerator Protection G64F6051N87T2481U47276287G5981O32-159N51 -GN32-227-3N40 51V50EI466349 REG515125 Generator CONTROL AND PROTECTIONS team Generator Stator WindingsGENERATOR CONTROL AND PROTECTIONH ydraulic Generator Stator / RotorGENERATOR CONTROL AND PROTECTIONH ydraulic Generator Stator CoreGENERATOR CONTROL AND PROTECTIONG enerator ProtectionGENERATOR CONTROL AND PROTECTIONS plit Phase Relaying CTGENERATOR CONTROL AND PROTECTIONC ylindrical Rotor in Need of RepairGENERATOR CONTROL AND PROTECTIONG enerator Protection Generator CONTROL AND PROTECTIONG enerator Protection Generator CONTROL AND PROTECTIONS ymmetrical Components Positive Sequence A set of three phasors that have the same magnitude, are equallydisplaced from each other by 120 , and have the same phase sequence as the system under study (ex ABC)

2 Negative Sequence A set of three phasors that have the same magnitude, are equallydisplaced from each other by 120 , and have the opposite phase sequence as the system under study (ex ACB) Zero Sequence A set of three phasors of equal magnitude that are all in phase or have zero displacement from each otherGENERATOR CONTROL AND PROTECTIONS ymmetrical ComponentsGENERATOR CONTROL AND PROTECTIONS ymmetrical ComponentsGENERATOR CONTROL AND PROTECTIONS ymmetrical ComponentsGENERATOR CONTROL AND PROTECTIONS ymmetrical ComponentsGENERATOR CONTROL AND PROTECTIONS ymmetrical ComponentsGENERATOR CONTROL AND PROTECTIONS ymmetrical ComponentsExample Problem One conductor of a three phase line is open. The current flowing to the delta connected load thru line ais 10A. With the current in line aas reference and assuming that line cis open, find the symmetrical components of the line CONTROL AND PROTECTIONS ymmetrical ComponentsExample Problem Ia= 10/0 A, Ib= 10/180 A, Ic= 0 A Ia0= (1/3)(Ia+ Ib+ Ic) Ia0= (1/3)(10/0 + 10/180 + 0) = 0 Ia1= (1/3)(Ia+ Ib+ 2Ic) Ia1= (1/3)(10/0 + 10/180+120 + 0) Ia1= /-30 Ia2= (1/3)(Ia+ 2Ib+ Ic) Ia2= (1/3)(10/0 + 10/180+240 + 0) Ia2= /30 Generator CONTROL AND PROTECTIONS ymmetrical ComponentsExample Problem Ib0= 0 Ib1= /-150 Ib2= /150 Ic0= 0 Ic1= /90 Ic2= /-90 Generator CONTROL AND PROTECTIONS ymmetrical ComponentsExample Problem Ia0= 0, Ib0= 0, Ic0= 0 Ia1= /-30 , Ib1= /-150 , Ic1= /90 Ia2= /30 , Ib2= /150 , Ic2= /-90 Generator CONTROL AND PROTECTIONS ymmetrical ComponentsExample Problem Note.

3 The components Ic1and Ic2have definite values although line cis open and can carry no net current. As expected, the sum of these currents is zero. The sum of the currents in line ais 10/0 The sum of the currents in line bis 10/180 Generator CONTROL AND PROTECTIONS ymmetrical ComponentsSingle Phase Line to Ground FaultGENERATOR CONTROL AND PROTECTIONS ymmetrical ComponentsGenerator Sequence NetworksGENERATOR CONTROL AND PROTECTIONS ymmetrical ComponentsGENERATOR CONTROL AND PROTECTIONS ymmetrical ComponentsGENERATOR CONTROL AND PROTECTIONS ymmetrical ComponentsGENERATOR CONTROL AND PROTECTIONF ault Current Behavior of a Synchronous GeneratorGENERATOR CONTROL AND PROTECTIONF ault Current Behavior of a Synchronous GeneratorGENERATOR CONTROL AND PROTECTIONF ault Current Behavior of a Synchronous GeneratorGENERATOR CONTROL AND PROTECTIONF ault Current Behavior of a Synchronous GeneratorMax DC OffsetNo DC OffsetGENERATOR CONTROL AND PROTECTIONF ault Current Behavior of a Synchronous GeneratorGENERATOR CONTROL AND

4 PROTECTIONF ault Current Behavior of a Synchronous GeneratorGENERATOR CONTROL AND PROTECTIONG enerator GroundingGENERATOR CONTROL AND PROTECTIONG enerator Grounding Low Impedance Grounding Single phase to ground fault current between 200A and 150% High Impedance Grounding Single phase to ground fault current between 5 and 20A Generator CONTROL AND PROTECTIONG enerator Stator Phase Fault Protection (87G) Generator CONTROL AND PROTECTIONG enerator Stator Phase Fault Protection (87G) 87G used to protect for: 3 phase line to line 1 phase line to line multi-phase line to ground May not be able to detect a 1 phase to ground fault on high impedance grounded generators Restraint or Percentage Differential Trip Characteristic Used to improve sensitivity for detecting small levels of fault current Also maintains security against inadvertent tripping due to thru faults Generator CONTROL AND PROTECTIONG enerator Stator Phase Fault Protection (87G) Generator CONTROL AND PROTECTIONG enerator Stator Phase Fault Protection (87G) Generator CONTROL AND PROTECTIONG enerator Stator Phase Fault Protection (87G) Split-phase Protection scheme Able to detect turn-turn faults Windings for each phase split into equal groups Individual winding currents are vector summed Any difference in winding current results in a output from CT Overcurrent relay (50/51)

5 Can be used to monitor difference current Setting must be above any normal unbalances that may exist Generator CONTROL AND PROTECTIONG enerator Stator Phase Fault Protection (87G) Generator CONTROL AND PROTECTIONG enerator Field Ground Fault Protection (64F) Generator CONTROL AND PROTECTIONG enerator Stator Ground Fault Protection (87N, 51N, 59N & 27-3N)For Low Impedance Grounded GeneratorsGENERATOR CONTROL AND PROTECTIONG enerator Stator Ground Fault Protection (87N, 51N, 59N & 27-3N)For Low Impedance Grounded GeneratorsGENERATOR CONTROL AND PROTECTIONG enerator Stator Ground Fault Protection (87N, 51N, 59N & 27-3N)External Generator Phase-Ground FaultGENERATOR CONTROL AND PROTECTIONG enerator Stator Ground Fault Protection (87N, 51N, 59N & 27-3N)External Generator Phase-Ground FaultGENERATOR CONTROL AND PROTECTIONG enerator Stator Ground Fault Protection (87N, 51N, 59N & 27-3N)Internal Generator Phase-Ground FaultGENERATOR CONTROL AND PROTECTIONG enerator Stator Ground Fault Protection (87N, 51N, 59N & 27-3N)Internal Generator Phase-Ground FaultGENERATOR CONTROL AND PROTECTIONG enerator Stator Ground Fault Protection (87N, 51N, 59N & 27-3N)High Impedance Grounded50 MVA, GeneratorXc = 10,610 for @ 60 HzRpri = 10,610/3 = 3537 Generator CONTROL AND PROTECTIONLoss of Field Protection (40Q, 40Z) Generator CONTROL AND PROTECTIONLoss of Field Protection (40Q, 40Z) Generator CONTROL AND PROTECTIONLoss of Field Protection (40Q, 40Z) Generator CONTROL AND PROTECTIONOver/Under Frequency Protection (81O/U) Causes.

6 Significant load addition Sudden reduction in mechanical input power Loss of generation Loss of load Underfrequency can cause: Higher Generator load currents Overexcitation Turbine blade fatigue Overfrequency can cause: Overvoltage on hydro turbinesGENERATOR CONTROL AND PROTECTIONO verexcitation and Overvoltage Protection (24, 59) Modern Excitation Systems include over excitation limiting and Protection , but it may take several seconds to limit Overexcitation occurs when the V/Hz ratio exceeds 105% at FL and 110% at no load V/Hz relays set at 110% with a 5 10 sec delay Generator overvoltage can occur without exceeding V/Hz relay setting due to large over speed on hydro Generator Generator overvoltage relay, 59 may be usedGENERATOR CONTROL AND PROTECTIONOut of Step Protection (78) High peak currents and off-frequency operation can occur when a Generator losses synchronism Causes winding stress, high rotor iron currents, pulsating torques and mechanical resonances Conventional relaying approach analyzing variations in apparent impedance as viewed at Generator terminals Variation in impedance can be detected by impedance relaying and Generator separated before the completion of one slip cycleGENERATOR CONTROL AND PROTECTIONOut of Step Protection (78)BAEAEBZBZTZAG eneratorSystemTransformer+R+X-REA/EB>1 QPEA/EB=1EA/EB<1ZT -XZAZBABGENERATOR CONTROL AND PROTECTIONOut of Step Protection (78)RXBAMBE lementPickupAElementPickupBlinderElement sMhoElementGen X'dTransSystemPGENERATOR CONTROL AND PROTECTIONN egative Sequence Protection (46)

7 Protects Generator from excessive heating in the rotor due to unbalanced stator currents Negative sequence component of stator current induces double frequency current in rotor, causing heating Rotor temperature rise proportion to I22t Negative sequence relays provide settings for this relationship in the form of a constant, k = I22t Minimum permissible continuous unbalance currents are specified (ANSI/IEEE ) Generator CONTROL AND PROTECTIONI nadvertent Energization Protection (27, 50, 60, 81U, 62 and 86) Protects against closing of the Generator breaker while machine is not spinning / on turning gear Caused by operator error, breaker flash-over, control circuit malfunction Two schemes illustrated: Frequency supervised overcurrent Voltage supervised overcurrentGENERATOR CONTROL AND PROTECTIONI nadvertent Energization Protection Frequency Supervised OvercurrentG5081U606281U608650 (3-phase)8662+ DropoutGENERATOR CONTROL AND PROTECTIONI nadvertent Energization Protection Frequency Supervised Overcurrent Uses an underfrequency relay (81U) to enable a sensitive instantaneous overcurrent relay (50) Overcurrent relay picks up at 50% or less of expected inadvertent energizing current Frequency relay contacts must remain closed if sensing voltage goes to zero Voltage balance relay (60) protects against loss of sensing Time delay relay (62) protects against sudden application of nominal voltage during inadvertent energization, allowing overcurrent to trip lockout relay (86)

8 Lockout relay must be manually resetGENERATOR CONTROL AND PROTECTIONI nadvertent Energization Protection Voltage Supervised Overcurrent Same illustration as frequency supervised overcurrent except 81U replaced by 27 Undervoltage setpoint of 85% of the lowest expected emergency operating levelGENERATOR CONTROL AND PROTECTIONLoss of Voltage Transformer Protection (60) Common practice on large systems to use two or more VTs One used for relays and metering The other used for AVR VTs normally fused Most common cause of failure is fuse failure Loss of VT Protection blocks voltage based protective functions (21, 32, 40 .. etc) Loss of VT Protection measure voltage unbalance, typical setting is 15% Generator CONTROL AND PROTECTIONLoss of Voltage Transformer Protection (60)G60vtTo Excitation ControllerTo Protective RelaysGENERATOR CONTROL AND PROTECTIONS ystem Backup Protection (51V, 21) Common practice to provide Protection for faults outside of the Generator zone of Protection Voltage supervised time-overcurrent (51V) or distance relaying (21) may be used Distance relay set to include Generator step up transformer and reach beyond, into the system Time delays must be coordinated with those of the system Protection to assure that system Protection will operate before back up CTs on neutral side of Generator will also provide backup Protection for the generatorGENERATOR CONTROL AND PROTECTIONS ystem Backup Protection (51V, 21)G2151Va.

9 Neutral Connected ct'sGENERATOR CONTROL AND PROTECTIONS ystem Backup Protection (51V, 21) Generator CONTROL AND PROTECTIONS ystem Backup Protection (51V, 21) For medium and small sized generators, voltage-restrained or voltage controlled time overcurrent relays (51V) are often applied Control or restraining function used to prevent or desensitize the overcurrent relay from tripping until the Generator voltage is reduced by a faultGENERATOR CONTROL AND PROTECTIONS ystem Backup Protection (51V, 21)Percent Nominal Volts25%100%25%100%a.) Voltage-Restrained OvercurrentPercent Set Value for PickupPercent Nominal VoltsEnableInhibitb.) Voltage-Contolled OvercurrentPickup Inhibit/Enable80%100% Generator CONTROL AND PROTECTIONC onclusion Generators must be protected from electrical faults, mechanical problem and adverse system conditions Some faults require immediate attention (shutdown) while others just require alarming or transfer to redundant controllers Design of these systems requires extensive understanding of Generator Protection Further study IEEE Guide for AC Generator Protective Relaying