ch 11 - generator protection · generator control and 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)



• 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



G

64F

60

51N

87T24

81U

47

2762

87G

59 81O

32-1

59N

51-GN

32-2

27-3N

40 51V50

EI46

6349

REG

51

51

25


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 equallydisplaced 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



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.



Example 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 + 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°



Example Problem

• Ib0 = 0

• Ib1 = 5.78 /-150°

• Ib2 = 5.78 /150°

• Ic0 = 0

• Ic1 = 5.78 /90°

• Ic2 = 5.78 /-90°



Example Problem

• Ia0 = 0, Ib0 = 0, Ic0 = 0

• Ia1 = 5.78 /-30° , Ib1 = 5.78 /-150° , Ic1 = 5.78 /90°

• Ia2 = 5.78 /30° , Ib2 = 5.78 /150° , Ic2 = 5.78 /-90°



Example Problem

• Note: the components Ic1 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 Sequence Networks


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)



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



Protection (87G)



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



Protection (87G)


Generator Field Ground Fault

Protection (64F)


Generator Stator Ground Fault

Protection (87N, 51N, 59N & 27-3N)

For Low Impedance Grounded Generators




External Generator Phase-Ground Fault




Internal Generator Phase-Ground Fault




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



BA

EA EB

ZBZTZA

Generator SystemTransformer

+R

+X

-R

EA/EB>1

Q

P

EA/EB=1

EA/EB<1

ZT

δδδδ

-X

ZA

ZB

A

B



R

X

BA

M

B

Element

Pickup

A

Element

Pickup

Blinder

Elements

Mho

Element

Gen X'd

Trans

System

P


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



Frequency Supervised Overcurrent

G

50

81U

60

62

81U

60

86

50 (3-phase)

86

62

+DC

-DC

0.5sec Pickup

0.1sec Dropout



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



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

60

vt

To

Excitation

Controller

To

Protective

Relays


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



G

21

51V

a.) Neutral Connected ct's



• 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



Percent Nominal Volts25% 100%

25%

100%

a.) Voltage-Restrained Overcurrent

Percent Set Value for Pickup

Percent Nominal Volts

Enable

Inhibit

b.) Voltage-Contolled Overcurrent

Pickup Inhibit/Enable

80% 100%


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

ch 11 - generator protection · generator control and protection generator protection •...

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