Generator Protection

r e l s a B

®

Generator Protection

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Generator Protection Purpose • Generator, prime mover • External power system, processes it supplies Considerations • Expense • Risk • Continuity of service











ANSI/IEEE Device Numbers • Protective relays are generally referred to by standard device numbers • Letters can be added to clarify application Examples : s a me overcurren re ay – – 50 is an instantaneous overcurrent – 27/59 is a combination under/over voltage











Device numbers commonly used P rote cti ve Functi on

ANSI code

S ta tor Di ffe re ntia l Unde ri m pe da nce Curre nt Unba l a nce Loss of Ex ci ta tion Re ve rse Pow e r The rm a l Prote cti on Bre a ke r Fa i l ure

87G 21 46 40 32 49 50BF

S ta tor Ground Ground Fa ult Ove rvol ta ge Ground Ove rcurre nt O i i

64G 59N 51GN 24











Types of Protection Faults • Ground fault • Phase fault Abnormal operating conditions

• Loss of field • Thermal











Ground Fault Protection Level of fault current • grounding impedance (low, medium, high) • location of fault in winding • pre-fault voltage level Fault near generator neutral • low probability due to less insulation stress (mostly mechanical) • Protection goal: ≤ 10% of winding uncovered













Grou Gr ound ndin ing g im impe peda danc nce e Grounding impedance Ground fault

High

Medium

L ow











Summary: Level of Ground Fault GENERATOR VFG

(1)

EXTERNAL SOURCE











Generator ground fault current level High impedance grounded system 100 1 T - L U 80 T A N F











Phase-ground Fault: High resistance grounding I

G

N I

N











Phase-ground Fault: High resistance grounding I

G

N I

N

























































































































































































Negati Neg ative ve Sequen Sequence ce Overc Overcurr urrent ent Pro Protec tectio tion n G

46

5-10 % of • Generator should be able to withstand 5-10% continuous I current











Characteristics of a Static NegativeSequen Seq uence ce Tim Time-O e-Over vercur curren rentt Rel Relay ay MAXIMUM OPERATING TIME 990 SEC. 1000 500 300 200 “K” SETTING ADJUSTABLE OVER RANGE 2-40

100 50

MINIMUM PICK-UP .04 PU

20 10 5

S D 3 N O

40 10











Stu tude dent nt ex exer erc cic ice e Generator K factor = 10 Generator nominal current In Negative sequence current I2 = 30% of In What is the maximum time generator will withstand I2?











Off-frequency operation

• Diesel engines safely operated at offfrequency • Gas turbine controls provide protection for o - requency opera on. • Frequency relays applied to steam turbine units to minimize turbine blade











Field Ground detection 64F G

64F

ALARM 1 ALARM 2 DC Power Supply













Field Ground detection 64F GENERATOR FIELD

RL GROUND











Field Ground detection 64F • In normal operation, rotating circuitgenerator rotor, exciter an diodes is





















Integrated application examples: Extended protection 49

63 25

G











Generator Protection

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