ch 11 - generator protection

8/8/2019 Ch 11 - Generator Protection

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GENERATOR CONTROL AND PROTECTION

Generator Protection


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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)


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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


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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


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Steam Generator Stator Windings


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Hydraulic Generator Stator / Rotor


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Hydraulic Generator Stator Core


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Split Phase Relaying CT


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Cylindrical Rotor in Need of Repair


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Symmetrical Components Positive Sequence

A set of three phasors that have the same magnitude, are equally

displaced from each other by 120, and have the same phasesequence 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 120, and have the opposite phasesequence 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


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Symmetrical Components


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Symmetrical ComponentsExample 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 linea

as reference andassuming that line c is open, find the

symmetrical components of the line

currents.


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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 + 2 Ic )

Ia1 = (1/3)(10/0 + 10/180+120 + 0)

Ia1 = 5.78 /-30 Ia2 = (1/3)(Ia +

2 Ib + Ic )

Ia2 = (1/3)(10/0 + 10/180+240 + 0)

Ia2 = 5.78 /30


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Ib0 = 0

Ib1 = 5.78 /-150

Ib2 = 5.78 /150

Ic0 = 0

Ic1 = 5.78 /90

Ic2 = 5.78 /-90


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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


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Single Phase Line to Ground Fault


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Generator Sequence Networks


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Fault Current Behavior of a

Synchronous Generator


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Max DC Offset

No DC Offset


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Generator Grounding


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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


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Generator Stator Phase Fault

Protection (87G)


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Protection (87G)

87G used to protect for:

3 phase line to line1 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 generatorsRestraint 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


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Protection (87G)


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Protection (87G)


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Protection (87G)

Split-phase protection scheme

Able to detect turn-turn faultsWindings 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 differencecurrent

Setting must be above any normal unbalances that may exist


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Protection (87G)


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Generator Field Ground Fault

Protection (64F)


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Generator Stator Ground Fault

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

For Low Impedance Grounded Generators


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For Low Impedance Grounded Generators


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External Generator Phase-Ground Fault

G S G d F l


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External Generator Phase-Ground Fault

G S G d F l


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Internal Generator Phase-Ground Fault

G t St t G d F lt


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Internal Generator Phase-Ground Fault

G t St t G d F lt


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High Impedance Grounded

50MVA, 13.2kV Generator

Xc = 10,610 for 0.25uf @ 60Hz

Rpri = 10,610/3 = 3537


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Loss of Field Protection (40Q, 40Z)


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Over/Under Frequency Protection


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Over/Under Frequency Protection

(81O/U)

Causes:

Significant load additionSudden 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


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Overexcitation and Overvoltage

Protection (24, 59)

Modern Excitation Systems include over excitation limiting

and protection, but it may take several seconds to limitOverexcitation 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/Hzrelay setting due to large over speed on hydro generator

Generator overvoltage relay, 59 may be used


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Out of Step Protection (78)BA

EA EB

ZBZTZA

Generator SystemTransformer

+R

+X

-R

EA/EB>1

Q

P

EA/EB=1

EA/EB


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Out of Step Protection (78)

R

X

BA

M

B

Element

Pickup

A

Element

Pickup

Blinder

Elements

Mho

Element

Gen X'd

Trans

System

P


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Negative Sequence Protection (46)

Protects generator from excessive heating in the rotor due to

unbalanced stator currentsNegative 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 relationshipin the form of a constant, k = I2

2t

Minimum permissible continuous unbalance currents are

specified (ANSI/IEEE C50.13)

Inadvertent Energization Protection


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(27, 50, 60, 81U, 62 and 86)

Protects against closing of the generator breaker while

machine is not spinning / on turning gearCaused by operator error, breaker flash-over, control circuit

malfunction

Two schemes illustrated:

Frequency supervised overcurrentVoltage supervised overcurrent



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Frequency Supervised Overcurrent

G

50

81U

60

62

81U

60

86

50 (3-phase)

86

62

+DC

-DC

0.5sec Pickup

0.1sec Dropout



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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 sensingvoltage 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



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Voltage Supervised Overcurrent

Same illustration as frequency supervised overcurrent except

81U replaced by 27Undervoltage setpoint of 85% of the lowest expected

emergency operating level

Loss of Voltage Transformer


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V ag a

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


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g

Protection (60)

G

60

vt

To

Excitation

Controller

To

Protective

Relays


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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


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G

21

51V

a.) Neutral Connected ct's


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For medium and small sized generators, voltage-restrained

or voltage controlled time overcurrent relays (51V) areoften applied

Control or restraining function used to prevent or

desensitize the overcurrent relay from tripping until the

generator voltage is reduced by a fault


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Percent Nominal Volts25% 100%

25%

100%

a.) Voltage-Restrained Overcurrent

PercentSetValueforPickup

Percent Nominal Volts

Enable

Inhibit

b.) Voltage-Contolled Overcurrent

Pickup

Inhibit/Enable

80% 100%


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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

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