Generator Protection

GENERATOR PROTECTION

Rajesh pillai

CONTENTS 1. 2. 3. 4. 5. 6. 7. 8.

OVER CURRENT (51) NGR/ NGT SELECTION STATOR E/F 90-95% (64S) STATOR E/F 95-100% (64S) DIFFERENTIAL PROTECTION (87) VOLTAGE CONTROLLED O/C (51V) VOLTAGE RESTRAINED O/C (51V) UNBALANCE ARMATURE CURRENT (46)

CONTENTS 8. 9. 10. 11. 12. 13. 14. 15.

ROTOR E/F (Ist & 2nd STAGE) (64F1 & F2) FIELD FAILURE (LOSS OF EXCITATION) (40) OVER FLUXING (59 V/Hz) OVER VOLTAGE (59) IN - ADVERTANT ENERGISATION (27/50) U/F & O/F (81 U & O) MOTORING (RPR) (32) POLE SLIP (78)

Stability Curve • Unit Can generate max 125 MW if prime mover P (MW) is capable. C Rotor Current Limit

B 0.8

100, 0.8 0.87

Stator Stability Limit Flame Stability

25,0.2 100 Q (Lag) A 0.8

MVAr

D

Q (Lead)

Over Current Protection 51 • Over Current in generator can occur due to system disturbance accompanied by generator or line tripping. • O/L withstand capacity as ANSI C50.13 – 1977 • Stator Current % 116 130 154 226 • Time (sec) 120 60 30 10 • Generally wired for alarm to allow operator to take corrective action at 105%

O/C Relay Ch’r TD

K

t = Operation time

T = ---------- x ------------------ + L K, α = Constant 7 (I/Is)α-1 I = Current measured L = Constant, TD – Time Dial

Curve

Standard

K

α

L

Std Inv

IEC

0.14

0.02

0

Very Inv

IEC

13.5

1

0

Ext. Inv

IEC

80

2

0

Long Inv

UK

120

1

0

Very Inv

IEEE

3.922

2

0.0228

NGR SELECTION

Current Amps

• Limit the E/F Current • Current Limitation based on Generator Capability Curve. 80 70

Generator core damage

Repairable Damage 25 No damage 0,0

1

2

3 4 Time In Sec

12

NGR CALCULATION • To limit the E/F current at 100 amps : (V/√3) Required NGR resistance : --------Current • If other generators are ungrounded then unbalance current becomes a line current & insulation will be stressed • Don't mix-up different types of grounding. • Don’t Earth NGR body

Neutral Grounding Transformer • • • •

If NGT Resistance is 0.9 Ohm Resistance Rp =0.9*(11/√3/240)2 0.9*700 = 630 Ohm Max. E/F Current = (11000/√3) ---------------

R

630 = 10 Amps

Trafo 11/√3/240

U3<

STATOR E/F 90 – 95% • Phase to ground faults are very severe leading to melting of steel laminations & possibility of core damage. • E/F current setting : 5 Amp • % winding unprotected = 5/100 = .05*100=5% % winding protected = 100-5 = 95% • In NGT scheme Neutral to ground voltage is sensed by the relay but it is insensitive to 3rd Harmonic voltage

Stator E/F 95 – 100% • Very close to the neutral. • If first E/F is not sensed by the relay & meanwhile 2nd E/F developed then E/F current is not limited by the NGR/NGT. E/F current is very high • All the generators are produce the 3rd harmonic voltages under normal condition and if fault is near neutral then 3rd harmonic voltages are markedly comes down & this is used for detection.

Neutral Grounding Transformer Tuned for 50Hz & Block 150 Hz

Trafo 11/√3/240

Filter

R

64 ND

U3<

Uf

Tunned For Tunned For 50 Hz 150 Hz

Tunned to permit 3rd Harmonic

No Trip Trip

• At Unity Power factor : – PF Var 3rd H & Internal EMF relay is malfunctioned

so 95 %

• When 5 volt setting is done then chance of malfunction is increased due to relay has to operate at 50 Hz & block 3rd Harmonics

Differential Protection (87) • • • • •

Unit Type Protection For Stator phase to phase fault Turn to turn fault can’t detected Damage of the winding & stator core Two Scheme – High Impedance – Low impedance

Differential Protection • Slope =Io/Ir, If Io>Ir Relay Operates

RC

OC

RC

Iop (IS-IL) Operate

IR=(IS+IL)/2

Case Study for PS class CT • For 5P10 or 15 class CT is saturate in external heavy faults. And at that time a fault is occurred in the zone then the CT is not sense the fault & does not operate. • So PS class CT is should be recommended for 5P10 differential protection even in numerical relays • In this transformer one no. coil is burnt

Fault

Voltage Controlled O/C Relay • Backup of Differential Protection • Two time/ current Ch’r • During Sustained O/L or External Faults in system – Relay assumes long inverse Ch’r – Voltage is near normal

• During Close in faults – Voltages collapses – Relay assumes normal IDMT Ch’r – Under Voltage element is used to step change in relay setting

Voltage Controlled O/C Relay

Operating Time

• Switch over from one curve to other curve when voltage is less than 30% • If Generator directly connected to bus bar W/O GT then, occurrence of faults in the System then always voltage External Fault shrinks and relay O/L Ch’r malfunctioned Fault Ch’r

PSM

Voltage Restrained O/C Relay • Only one IDMT Type Ch’r. • Ch’r is modified continuously as per terminal voltage. • Distance type impedance relay (21) is the back up protection. • Used for terminal feed static excitation system • When terminal voltage falls pickup current values are also falls linearly.

Voltage Restrained O/C Relay

Ipickup = 1.5 Rat • Vt>0.2, Ipickup = 0.3 Rat

Current

• Iy is set 1.5 to 2 rated • When, 1.0 Iy • Vt>0.8, 0.2 Iy

• DMT time delay = 1 Sec

Ux1 0.2

Ux2 Voltage 0.8

Negative Sequence Protection Unbalance Armature Current (46) • • • • • •

Open phase of a line One open pole of a CB Non linear system loading A close unbalance fault not cleared properly Stator winding fault. This is Back up protection for uncleared system faults • I2 rated current can melt the rotor Ia + α2Ib + αIc

• I2 3

= ----------------------------

Where α = 1.0 ∟120’

Negative Sequence Protection

– K = 40 to 60

• For H2/ Water Cooled – K = 10 to 20

I22t = 20

I2 PU

• 5th & 11th Harmonics make a problem • I22t = K • For Air Cooled M/C

• High K > High Cost • So K is kept 10 -15 • Continuous Current withstands

Time In sec

– Air Cooled M/C = 30 to 40% – H2 / Water Cooled M/C 10 to 15%

• Set the relay below the above withstand values for alarm (5 to 10 Sec Delay)