Generator Protection System

Generator Protection System P. A. Amilk A milkanth anthwar  war  Assistant Engineer (Gen) Test - II

Topics for Discussion       

Basics of Electrical Protection system Synchronous machine Concept of reactive power and real power sharing Different Protections used for generator  Advanced Protection Relays(Microprocessor based) based) Case studies (analysis of electrical faults) Synchronization

Different Protections used for Generator             

Generator Differential Protection. Generator Inter Turn Protection. Generator Negative Sequence Protection. Generator Loss of Excitation Protection. Generator Over Voltage Protection. Frequency Protection Generator Over Load/ Over Current Protection Generator Rotor Earth Fault Protection Generator Back Up Impedance protection Generator Low Forward power Interlock Generator reverse power protection. 95% stator Earth Fault Protection

Instrument Transformers 

•

•

•

Current Transformers Turns ratio is inversely proportional to the ratio specified Secondary current is according to primary current only Should not be left open circuited



Potential Transformers

•

Turns ratio is directly proportional to the ratio specified

•

Secondary current is according to burden connected

•

Should not be short circuited

Flow Diagram of Tripping Process

Protection Classes Class A Protection • This master trip will operate when the fault needs to isolate the generator immediately • This gives command to open the generator breaker and field breaker.  Class B Protection • This master trip will operate when the fault is not serious in view of generator or it is not directly harmful to generator  • This gives the command to trip turine  Prime Mover Tripped This indicates (gives feed back)prime mover is tripped. 

Basics of Electrical Protection system 

The Protection System



Protection Classes



Inputs to the protection system. • •



Instrument Transformers Contact Input

Elements of protection system.

Generator Differential Protection 

CTR 10000/5



Relay Setting 5%



Stabilizing Resistance



High impedance Relay



Sensitivity to the type of fault

Elements of protection system •

Current relays

•

Voltage relays

•

Power relays

•

Impedance relays

•

Frequency relays

•

Special function relays

•

Auxiliary Relays

•

Timers

Protection System •

Cause of Operation of Protection is the the Fault.



It Is Basically Designed,

•

To detect the fault at its earliest possible stage.

•

To give protection to the equipment against secondary damage.

•

To avoid the equipment to run out side the capability specified.

Specifications of Generator  

Make: BHEL



MVA: 247



Connection: Three Phase Double Star Connected.



Voltage: 15750 Volts



Current: 9050 Amp



Frequency: 50 Hz



Speed: 3000 RPM



No. of Poles: 2











Synchronous Impedance: 222% Sub Transient Impedance: 30.5% Transient Impedance: 21.4% Cooling: water/ Hydrogen Cooled Neutral Grounding: High Impedance (Through NGT)

Generator Inter Turn Protection 



•

•

•

Difference of sharing between split phases Reasons of diff. In currents If there is any inter-turn short with in the winging If gen is supplying through fault and small circulating currents gets proportionately increased Tripping provided at 5% of rated current

Generator Negative Sequence Protection.  

•

Balanced and unbalanced systems Sequence components Positive sequence component I1 = 1/3 (IR +a2 *IY + a * IB)

•

Negative sequence component I2 = 1/3 (IR +a *IY + a2 * IB)

•

Zero sequence component I0 = 1/3 (IR +IY + IB)



Effect of negative sequence component on synchronous machine. Generator Capability for I2



•

Continuous I2 capability 5% of Im

•

I22 * t = 8

Balanced Phasers

Generator Loss of Excitation Protection. 

Study of Generator capability curve Setting criteria Xa1 = 0.5*Xd*CTR/PTR  Xd = 30.5% = 0.305 pu ……subtransient reactance of generator  Xd in ohms = Xd (pu)*kv 2/MVA = 0.305*15.752/247 =0.306311993 Xa1 = 2.139 ohm Xb1 = Xd * CTR/PTR  Xd (ohmic) = Xd(pu)*kv2/MVA Xb1 = 2.22(15.752/247)(10000/5) (110/15750) = 31.14 ohm

Generator capability curve

Generator Over Voltage Protection. 

 

• •

Generator high voltage is harm full to the insulation of generator, generator  transformer, excitation transformer, UAT, etc Generator Over Voltage Capability – 10% Setting Adopted Stage – 1 = 10%, 5 Sec delay Stage – 2 = 15%, 500msec delay

Frequency Protection 

Over frequency protection

•

It indicates excess generation than demand hence tripping should graded among the units



Under frequency protection

Under frequency is harmful to turbine 

Adopted settings

Adopted Frequency settings UNIT NO. CAPACITY

UNDER FREQ. OVER FREQUENCY TRIP SETTING SETTING

1

30 MW

47 HZ, 0.5 SEC 54.5 HZ, 21.15 SEC DELAY DELAY

2

30 MW

3

210 MW

52.5 HZ, 1 SEC DELAY

4

210 MW

52.0 HZ, 2 SEC DELAY

5

210 MW

52.0 HZ, 2.0 SEC DELAY

TRIP

Generator Over Load/ Over Current Protection1 % O/C

Gen Capability

Relay Char.

1.1

3600

1.15

900

652.8

1.2

360

362.4

1.25

300

252.6

1.3

240

192.6

1.35

180

155.4

1.4

120

129.3

1.5

60

90

Generator Over Load/ Over Current Protection2 Genera Generator tor Capabi apabilility ty

Relay elay characristic characristic

950 900 850 800 750 700 650  .   c600   e550    S500   n    i 450   e   m400    i    t 350 300 250 200 150 100 50 0 1

1.05

1.1

1.15

1.2

1.25

1.3

1.35

% over current

1.4

1.45

1.5

1.55

1.6

Rotor E/F

Generator Back Up Impedance protection Impedance protection to be covered up to 220 KV bus bar that is Z = Z(G.T.) 12.5% Ohmic value of Z (G.T.) = Z(pu)*KV 2/MVA = 0.125*15.752/250 = 0.124 ohm Z at C.T.,P.T. secondary level = Z*(CTR/PTR) = 0.124*(10000/5)*(110/15750)= 1.73 ohm For 3 phase impedance 1.73*sqrt(3) = 2.996 ohm Time setting = 2.5 sec (considering the line under Impedance time setting that is to allow the line protections to clear the fault before tripping of generator) Z