Generator Protection GENERATOR CONTROL AND PROTECTION

Generator Protection

Generator 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 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) Conclusion

Generator Protection 49 G 63 25 64F 51N 87G 60 REG 51 24 47 87T 62 27 81U 51 59 81O 32-1 32-2 40 51V 50 IE 46 59N 27-3N 51- GN

Steam Generator Stator Windings

Hydraulic Generator Stator / Rotor

Hydraulic Generator Stator Core

Generator Protection

Split Phase Relaying CT

Cylindrical Rotor in Need of Repair

Generator Protection

Symmetrical Components Positive Sequence A set of three phasors that have the same magnitude, are equally displaced from each other by 120º, and have the same phase sequence as the system under study (ex ABC) Negative Sequence A set of three phasors that have the same magnitude, are equally displaced from each other by 120º, 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 other

Symmetrical Components

Example Problem Symmetrical Components One conductor of a three phase line is open. The current flowing to the delta connected load thru line a is 10A. With the current in line a as reference and assuming that line c is open, find the symmetrical components of the line currents.

Symmetrical Components Example Problem I a = 10/0 A, I b = 10/180 A, I c = 0 A I a0 = (1/3)(I a + I b + I c ) I a0 = (1/3)(10/0 + 10/180 + 0) = 0 I a1 = (1/3)(I a + αi b + α 2 I c ) I a1 = (1/3)(10/0 + 10/180+120 + 0) I a1 = 5.78 /-30 I a2 = (1/3)(I a + α 2 I b + αi c ) I a2 = (1/3)(10/0 + 10/180+240 + 0) I a2 = 5.78 /30

Symmetrical Components Example Problem I b0 = 0 I b1 = 5.78 /-150 I b2 = 5.78 /150 I c0 = 0 I c1 = 5.78 /90 I c2 = 5.78 /-90

Symmetrical Components Example Problem I a0 = 0, I b0 = 0, I c0 = 0 I a1 = 5.78 /-30, I b1 = 5.78 /-150, I c1 = 5.78 /90 I a2 = 5.78 /30, I b2 = 5.78 /150, I c2 = 5.78 /-90

Example Problem Symmetrical Components Note: the components I c1 and I c2 have definite values although line c is open and can carry no net current. As expected, the sum of these currents is zero. The sum of the currents in line a is 10/0 The sum of the currents in line b is 10/180

Symmetrical Components Single Phase Line to Ground Fault

Symmetrical Components Generator Sequence Networks

Symmetrical Components

Fault Current Behavior of a Synchronous Generator

Fault Current Behavior of a Synchronous Generator Max DC Offset No DC Offset

Fault Current Behavior of a Synchronous Generator

Generator Grounding

Generator 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 Stator Phase Fault Protection (87G)

Generator 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 Stator Phase Fault Protection (87G)

Generator 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) can be used to monitor difference current Setting must be above any normal unbalances that may exist

Generator Stator Phase Fault Protection (87G)

Generator Field Ground Fault Protection (64F)

Generator Stator Ground Fault Protection (87N, 51N, 59N & 27-3N) For Low Impedance Grounded Generators

Generator Stator Ground Fault Protection (87N, 51N, 59N & 27-3N) External Generator Phase-Ground Fault

Generator Stator Ground Fault Protection (87N, 51N, 59N & 27-3N) Internal Generator Phase-Ground Fault

Generator Stator Ground Fault Protection (87N, 51N, 59N & 27-3N) High Impedance Grounded 50MVA, 13.2kV Generator Xc = 10,610Ω for 0.25uf @ 60Hz Rpri = 10,610/3 = 3537 Ω

Loss of Field Protection (40Q, 40Z)

Over/Under Frequency Protection (81O/U) Causes: 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 turbines

Overexcitation 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 used

Out 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 cycle

Out of Step Protection (78) E A E B A Z A Z T Z B B Generator Transformer System E A /E B >1 +X B E A /E B =1 E A /E B <1 Q Z B Z T -R Z A δ P +R A -X

Out of Step Protection (78) A X B System R Trans P M Gen X'd A Element Pickup B Element Pickup Mho Element Blinder Elements

Negative Sequence Protection (46) 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 I 22 t Negative sequence relays provide settings for this relationship in the form of a constant, k = I 22 t Minimum permissible continuous unbalance currents are specified (ANSI/IEEE C50.13)

Inadvertent 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 overcurrent

Inadvertent Energization Protection Frequency Supervised Overcurrent +DC G 81U 50 (3-phase) 62 50 60 60 86 81U 0.5sec Pickup 0.1sec Dropout 62 86 -DC

Inadvertent 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) Lockout relay must be manually reset

Inadvertent 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 level

Loss 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%

Loss of Voltage Transformer Protection (60) G vt 60 To Protective Relays To Excitation Controller

System 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 generator

System Backup Protection (51V, 21) G 21 51V a.) Neutral Connected ct's

System Backup Protection (51V, 21)

System 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 fault

System Backup Protection (51V, 21) 100% Enable Percent Set Value for Pickup Pickup Inhibit/Enable 25% Inhibit Percent Nominal Volts 25% 100% Percent Nominal Volts 80% 100% a.) Voltage-Restrained Overcurrent b.) Voltage-Contolled Overcurrent

Conclusion 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 C37.102 Guide for AC Generator Protective Relaying