Ch 11 - Generator Protection

Generator Protection

GENERATOR CONTROL AND 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 63

49

25 G 51N

64F

51 60

87G

REG

62

47

27

81U

59

32-1

59N

87T

24

32-2

51

81O

40

51V

50 IE

46

273N

51GN


Steam Generator Stator Windings


Hydraulic Generator Stator / Rotor


Hydraulic Generator Stator Core




Split Phase Relaying CT


Cylindrical Rotor in Need of Repair






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










Symmetrical Components Example Problem •

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 • Ia = 10/0° A, I b = 10/180° A, I c = 0 A • I = (1/3)(I + I + I ) a0 a b c • Ia0 = (1/3)(10/0° + 10/180° + 0) = 0 • Ia1 = (1/3)(Ia + αIb + α2 Ic ) • Ia1 = (1/3)(10/0° + 10/180+120° + 0) • Ia1 = 5.78 /-30° • Ia2 = (1/3)(Ia + α2 Ib + αIc ) • Ia2 = (1/3)(10/0° + 10/180+240° + 0) • Ia2 = 5.78 /30°



Ib0 = 0

•

Ib1 = 5.78 /-150°

•

Ib2 = 5.78 /150°

•

Ic0 = 0

•

Ic1 = 5.78 /90°

•

I = 5.78 /-90° c2



Ia0 = 0, Ib0 = 0, Ic0 = 0

•

Ia1 = 5.78 /-30° , I b1 = 5.78 /-150° , I c1 = 5.78 /90°

•

Ia2 = 5.78 /30° , I b2 = 5.78 /150° , I c2 = 5.78 /-90°



Note: the components I c1 and Ic2 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°



Single Phase Line to Ground Fault GENERATOR CONTROL AND PROTECTION


Generator Sequence Networks GENERATOR CONTROL AND PROTECTION







Fault Current Behavior of a Synchronous Generator







Max DC Offset

No DC Offset






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 CONTROL AND PROTECTION





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 Field Ground Fault Protection (64F)


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

For Low Impedance Grounded Generators GENERATOR CONTROL AND PROTECTION


For Low Impedance Grounded Generators GENERATOR CONTROL AND PROTECTION


External Generator Phase-Ground Fault GENERATOR CONTROL AND PROTECTION


External Generator Phase-Ground Fault GENERATOR CONTROL AND PROTECTION


Internal Generator Phase-Ground Fault GENERATOR CONTROL AND PROTECTION


Internal Generator Phase-Ground Fault GENERATOR CONTROL AND PROTECTION


High Impedance Grounded 50MVA, 13.2kV Generator Xc = 10,610Ω for 0.25uf @ 60Hz Rpri = 10,610/3 = 3537 Ω GENERATOR CONTROL AND PROTECTION

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 GENERATOR CONTROL AND PROTECTION

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) EA

EB ZA

ZT

ZB

A

B

Generator

Transformer

System

+X EA/EB>1

B

EA/EB=1

ZB

Q EA/EB<1

ZT -R

+R

δ

ZA A -X


P

Out of Step Protection (78) A

B X

System R Trans

P

M Gen X'd

A B Element Element Pickup Pickup

Blinder Elements


Mho Element

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 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 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

50 (3-phase)

G

81U 62 60

50

60 81U

86 0.5sec Pickup 0.1sec Dropout

62

86

-DC


Inadvertent Energization Protection Frequency Supervised Overcurrent • Uses an underfrequency instantaneous overcurrentrelay relay(81U) (50) to enable a sensitive • 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 GENERATOR CONTROL AND PROTECTION

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 large systems to use two or more VTs One used practice for relaysonand 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 GENERATOR CONTROL AND PROTECTION


G

21 51V

a.) Neutral Connected ct's GENERATOR CONTROL AND PROTECTION




• For medium and smalltime sized generators, voltage-restrained or voltage controlled 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) Enable

100% p u k ic P r o f

e l b a n E /t i ib h n I p u k c i P

e lu a V t e S t n e c r e P

Inhibit

25%

25%

Percent Nominal Volts

100%

a.) Voltage-Restrained Overcurrent

Percent Nominal Volts

80%

100%

b.) Voltage-Contolled O vercurrent


Conclusion

• Generators be protected from electrical faults, mechanical must 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


Ch 11 - Generator Protection

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