Generator Protection Functions and Test Methods (Autosaved) Collection Unzip

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GENERATOR PROTECTION FUNCTIONS AND TEST METHODS  AN OVER VIEW OF GENERATOR SINGLE SINGLE LINE DIAGRAM :

Generator Protections are broadly classified into three types. CLASS –  CLASS – A A :-

This covers all electrical protections for faults within the generating unit in which generator field breaker, generator breaker and turbine should be tripped.

CLASS –  CLASS – B:B:-

This covers all mechanical protections of the turbine in which turbine will be tripped first and following this generator will trip on reverse power / low forward power  protections.

CLASS –  CLASS – C:C:-

This covers electrical protection for faults in the system in which generator will be unloaded by tripping of of generator breaker only. only. The unit will come to house load protections of this class class are: operation and the UAT will be in service. Various protections 220 KV (HV side of Generator Transformer) Transformer) busbar protections. Generator Transformer HV side breaker pole discrepancy. Generator negative phase sequence protection Generator Transformer over current / Earth fault protection Reverse power protection without turbine trip.

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PROTECTION FUNTIONS:

I - For insulations failures

Differential Inter-turn fault Stator Earth Fault (95% & 100%) Rotor Earth fault (2 Stage)

II  –  For abnormal running conditions

Loss of excitation (field failure) Unbalanced loading (negative phase sequence) Pole sleeping Over frequency/Over speed Over voltage Reverse/Forward power Impedance/Over current back-up protection, etc…

III  –  For Generator transformer protections

Differential protection Bias test nd

2  harmonics restrained. REF protection

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FOR INSULATION FAILURES PROTECTIONS: 1. GENERATOR DIFFERENTIAL PROTECTION (87 G): It is unit type protection, covering the stator winding for phase to phase faults due to breakdown of insulation between stator phase windings. This relay is not sensitive for single line to earth faults as the earth fault current is limited due to the high neutral earthing resistance. If CTs of identical ratios are used on neutral and line side of generator, an operating current setting of 20% it can be adopted. It is instantaneous in operation and it trips the generator breaker (Class –  A) to eliminate the system in –  feed to the fault along with field breaker and turbines. For all machines of ratings 10 MVA and above, this protection shall be provided. Diagram:

Settings: Pickup Value of Differential Current T I-DIFF> Time Delay Pickup Value of High Set Trip T I-DIFF>> Time Delay

: 0.10 I/InO : 0.00 sec : 2.0 I/InO : 0.00 sec

Testing Method: a) Differential Pick Up Test: Side 2 (Generator Neutral Side) Phase Set value I/In cal diff > value(Amps) R-N 0.1 0.41

optd value (Amps) 0.41

set time (s) 0.0

Optd trip time(ms) 36

Y-N

0.1

0.41

0.41

0.0

34

B-N

0.1

0.41

0.41

0.0

37

cal value (Amps) 0.41

optd value (Amps) 0.41

set time (s) 0.0

optd trip time(ms) 38

Side 1 (Generator Terminal Side)  phase set value I/In diff > R-N 0.1 Y-N

0.1

0.41

0.41

0.0

36

B-N

0.1

0.41

0.41

0.0

36

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Note: Calculated value: Ifull=MVA/(1.732*KV) Idiff=0.2*Ifull b) DIFFERENTIAL HIGH SET TEST (I DIFF>>)

Side 2 (Gr Neutral Side) Phase Set Value I/In Diff >> R-N Y-N B-N

2.0 2.0 2.0

Cal Value (Amps)

Optd Value (Amps)

Set Time (S)

Optd Trip Time(Ms)

8.2 8.2 8.2

8.2 8.2 8.2

0.0 0.0 0.0

33 32 31

Side 1 (Gr Terminal Side) Phase

Set Value I/In Diff >>

Cal Value (Amps)

Optd Value (Amps)

Set Time (S)

Optd Trip Time(Ms)

R-N

2.0

8.2

8.2

0.0

30

Y-N

2.0

8.2

8.2

0.0

32

B-N

2.0

8.2

8.2

0.0

32

Remarks: Class-A tripping should be checked. c) STABILITY TEST:

Apply the full load current on both terminal and neutral side with an angle of 180 degree phase shift on any one side. (Angle will be vary depends on the vector group)

Dyn1 (-30° displacement between HV and LV) Dyn11 (+30° displacement between HV and LV) Dd0 (no phase displacement between HV and LV) Dd6 (180° displacement between HV and LV) 2.

INTER TURN FAULT PROTECTION OF THE STATOR WINDING (64GIT) :

Formerly, this type of protection was considered unnecessary because breakdown of insulation between  points on the same phase winding, contained in the same slot, and between which a potential difference exists, will very rapidly change into an earth fault, and will be detective by either the differential protections or the stator earth fault protection. An exception is the generator designed to produce a relatively high voltage in comparison to its output and which therefore contains a large number of conductors per slot. With the size and voltage output of generators increasing, this form of protection is becoming essential for all generating units. Diagram:

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The recommended relay is the high impedance relay having a setting range of 10-40% of rated current. Settings:

L-E Voltage of Faulted Phase Uph Min L-E Voltage of Unfaulted Phase Uph Max Uen> Earth Displacement Voltage T-DELAY TRIP Uen/3U0

110 V 110 V 10 V 0.50 sec

TEST:

Voltage Applied In(V)

Voltage measured at open delta terminal

Voltage measured after IVT2

Set value (V)

Operated value (V)

Operated time (sec)

2.4V

10.0V

10.0V

10.0

0.52

Vr-n 61.1 0 Vy-n 63.5 -120 Vb-n 63.5 +1200

Remarks: Class-A tripping should be checked.

3.

STATOR EARTH FAULT PROTECTION (0-95%) 64G1:

It is an over voltage relay monitoring the voltage developed across the secondary of the neutral grounding transformer in case of ground faults. It covers generator, LV winding of generator transformer and HV winding of UAT. A pickup voltage setting of 5% is adopted with a time delay setting of about 1.0 Sec. For all machines of ratings 10 MVA and above this shall be provided. Relay application for this protection is mainly influenced by the method of stator earthing. Two methods are in common use. Resistor earthing Distribution transformer earthing With resistor earthing, the fault current is limited to 200-300Amps while with distribution transformer earthing; it is limited to 5-10Amps. The latter method has the advantage of ensuring minimum damage to the stator core, but it is only practicable when the stator winding is directly connected to the delta winding of the main transformer. The two schemes for stator earth fault protection (95%) are shown below:1.

Distribution transformer earthing (NGT)

2. Resistor earthing(NGR)

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Distribution transformer earthing (High impedance Resistor earthing(Low impedance earthing): In the resistor earthed scheme, a CT is required in the earthing): Earth fault protection is provided by connecting an neutral to earth connection, and the relay used is an Over voltage relay across its secondary, as shown. inverse time current relay so that it can grade with other The maximum earth fault current is determined by the earth fault relays in the system. It also provides Size of the transformer and the loading resistor R. The  protection for the neutral earthing resistor. In this Relay used for this Application is an inverse time or system, it is impossible to protect 100% of the stator definite Time over voltage relay (Also known as neutral winding. The percentage of winding protected depends displacement Relay) with a setting range Of 2.5 to 20 on the value of the neutral earthing resistor and the relay Volts. The relay is Provided with an inbuilt third setting. In the figure below, the percentage of winding Harmonic filter so as to avoid Unwanted operations due  protector is given for various values of earthing resistor to third Harmonic currents and the Problems associated at different relays settings, from 5-100 %. with transformer inter winding capacitance. It is possible to protect up to 95% of the generator stator Winding with this relay.

Settings: Stator Earth Fault Protection U0> Pickup T S/E/F Time Delay Test:

ON 4.8 V 0.20 sec

Voltage injected(V)UE

Voltage measured at relay (After voltage divider ) (V)

Set value (V)

Operated value (V)

Set time(s)

Optd time(s)

12.10

4.84

4.80

4.84

0.2

0.2

Remarks: Class-A tripping should be checked.

100% STATOR EARTH FAULT PROTECTION:-

To provide 100% stator earth fault protection, an additional relay for covering 95-100% of the winding is provided.. This is a 3 rd harmonic U/V relay. It protects 100% of stator winding. During the machine running condition there will be certain third harmonic voltage at neutral side of the generator. This 3rd harmonic voltage will come down when a stator earth fault occurs causing this relay to operate. This shall have voltage check or current check unit, to prevent faulty operation of the relay at generator stand still or during the machine running down  period. The third harmonic relays setting is determined from the amount of generator neutral third harmonic neutral voltages. Calculations can be based on mission specifications and equipment capacitances or on field measurements. To prevent 64G2 function from false tripping when there is no voltage, or low voltage, on the generator it's supervised by the phase under-voltage relay 27.Set this relay at 90% of the rated voltage. Settings: 100% Stator-Earth-Fault Protection Pickup Value of Alarm Stage Rsef< Pickup Value of Tripping Stage Rsef Stage Supervision Threshold of 20Hz Voltage Supervision Threshold of 20Hz Current

ON 168 Ohm 84 Ohm 10.00 sec 1.00 sec 0.75 A 1V 10 mA

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Test: (According to the theory the test method was given below. With the help of setting, the method was shorting the PT terminal of generator. For an example short GRP1 TB 18&20 then the relay will operate after time delay)

set value of voltage(V)

Set value of frequency(Hz)

1% of voltage

set time (s)

150

operated time (s)

1

1.1

Remarks: Class-A tripping should be checked.

4. ROTOR EARTH FAULT (64F): A single earth fault on the field winding or in the exciter circuit of a generator is not in itself a danger to the machine. Should a second earth fault develop, however, part of the field winding will become short circuited, resulting in magnetic un-balance of the filed system with subsequent mechanical damage to the machine bearings. It is necessary to ensure that should a second rotor earth fault occur, the machine is disconnected. This is achieved by the use of a second rotor earth fault relay which comprises adjustable resistors and a sensing element. Diagram:

Settings:

Pickup Value of Warning Stage Re< 40.0 kOhm Pickup Value of Tripping Stage Re Pickup T I2>> Time Delay

: ON : 8.25% : 20.00 sec : 12.9 sec : 1900 sec : 54% : 2.50

Test method: Put In=5A SET % OF In=Ir

I calculated=3 times Ir=I injected = Ix

54.0

Injected current (A)

8.1

8.1

Note: In Omicron, for this test change the R, Y, B--

Set Time (S)

2.50

Optd Time (S) 2.70

Y, B, R or change the Phase angle 0, -120, 120-

-120, 120, 0

Remarks: Class-C tripping checked ok

7. BACKUP IMPEDANCE PROTECTION (21G):This operates for phase faults in the unit, in the HV yard or in the adjacent transmission lines, with a suitable time delay. It operates as a backup when the corresponding main protection fails. In A.P. System the reach is set as 120% of generator transformer with a time delay of about 1.0 to 1.5 Sec. Settings: Impedance Protection Fault Detection I> Pickup State of Under voltage Seal-in Under voltage Seal-in Pickup Duration of Under voltage Seal-in

:ON :6.50 A :ON :77.0V :3.00 sec

Back Up Impedance Test Phases

Voltag e(V)

Current( A)

Set imp(Z=V/I)

R Y

44.45 44.45

6.5 6.5

6.8 6.8

B

44.45

6.5

6.8

Operated imp(ohm)

Set time(Sec)

Operated time(sec)

6.79 6.79

3.0 3.0

3.02 3.02

6.79

3.0

3.02

Remarks: Class-A tripping checked ok

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8. OVER FLUXING PROTECTION V/F (24): It is basically a relay which measures v/f of the generator and transformers. As modern power transformers are designed to operate at very near saturation flux levels under normal operating conditions, any increase in the voltage or decrease in the frequency, results in the saturation of the core and the additional flux tries to find its fault through core bolts damaging the core bolt insulation. To prevent this, over fluxing relay is used. Settings:

Over excitation Protection (U/f) U/f > Pickup T U/f > Time Delay U/f >> Pickup T U/f >> Time Delay

: ON : 1.1 : 10 sec : 1.4 : 2.00 sec

Calculation:

V/F=110volts/50Hz (Normal) =2.2=1 P.U For 1.10 P.U V/F=2.2*1.1=2.42. Corresponding PH-PH Volts=2.42*50=121 V. PH-E=121/1.732=69.9V  Note: Here we can keep voltage constant and vary the frequency or frequency constant and voltage is variable one. Stage 1: Voltage Applied in (V) @ 50Hz, Ph-N(In Each Phase Separately) 69.9 Stage 2: Voltage Applied In Volts At @ 50hz Ph N (In Each Phase Separately) 88.9

Calc Value in (V)

121/ 3=69.85

Calc Value In Volts

154.0/ 3=88.9

Set Value in V/F

Optd Value in V/F

Set Time (S)

Optd Time(S)

1.10

1.10

10

10.06

Set Value In V/F

Optd Value V/F

Set Time (S)

Optd Time(S)

1.40

1.40

1.0

0.99

Remarks: Class-A tripping checked ok

9.

LOW FORWARD POWER RELAY (37G):-

In thermal machines, when the steam flow through turbine is interrupted by closing the ESVs(Electro static valves) or the governor valves, the remaining steam in the turbine generates (low) power and the machine enters to motoring conditions drawing power from the system . This protection detects low forward  power conditions of the generator and trips generator breaker after a time delay, avoiding motoring of generator.

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Settings: Forward Power Supervision P-forw.< Supervision Pickup T-P-forw.< Time Delay

ON 0.50% 5.0 sec

Low forward power test

Set value for forward power= 0.5% of rated power. P=1.732*V*I Calculated value P= 0.05x 16500x1200 x 1.732 = 1.786mw (primary value) Applied current (A) Vr-n 63.5 0 Vy-n 63.5 -120 Vb-n 63.5 +120

Relay optd current (A) Ir 0.025 0 Iy 0.025 -120 Ib 0.025 +120

Set value (MW)

OPtd Value(MW)

Set time(S)

Optd time(S)

1.786

1.78

5.0

5.23

Remarks: Class-A tripping checked ok

10. REVERSE POWER RELAY (32G):Reverse power protection shall be used for all types of generators. When the input to the turbine is interrupted the machine enters into motoring condition drawing power from the system. Reverse power relay protects the generators from motoring condition. In thermal machines, reverse power condition appears subsequent to low forward power condition. For reverse power relay, a setting of 0.5% of rated active power of generator with 2 stage timer as given below. Stage  –   I : - With turbine trip interlock, a time delay of 2 Sec. shall be adopted. Stage  – II:- Without ‘ turbine trip’ interlock, a time delay of about20 Sec. can be adopted to avoid unnecessary tripping of unit during system disturbance causing sudden rise in frequency or  power swing conditions. Settings:

Reverse Power Protection : ON P> Reverse Pickup : -0.50% Time Delay Long (without Stop Valve) : 10.00 sec Time Delay Short (with Stop Valve) : 2.00sec Pickup Holding Time : 0.02 sec Set value for rev power -0.5% Formula: set power = 0.5 % rated power. Rated power =1.732*V*I. Calculated value= (0.05x1200) x 16500 x 1.732 = 1.786mw (primary value) Angles b/w voltages & currents are reversed to achieve reverse power. a. Without turbine trip binary input Applied current (A) Vr-n 63.5 0 Vy-n 63.5 -120 Vb-n 63.5 +120

Relay optd current (A) Ir 0.025 180 Iy 0.025 +60 Ib 0.025 -60

Set value (MW)

OPtd value(MW)

Set time(S)

Optd time(S)

1.786

1.78

10.0

10.05

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b.

With turbine trip binary input

Applied current Relay optd current Set value (A) (A) (MW) Vr-n 63.5 0 Ir 0.05 180 1.786 Vy-n 63.5 -120 Iy 0.05 +60 Vb-n 63.5 +120 Ib 0.05 -60 Remarks: Class-B with stop valve tripping checked ok Class-A without stop valve checked ok

OPtd value(MW)

Set time(S)

Optd time(S)

1.78

2.0

2.03

11. INADVERTENT ENERGISING(27/50) In generator turbine was on by using turning gear, the field was not ON. The breaker will be closed by means of human error. At this condition generator has energized by turn on the turbine. The machine will draw large stator current (2 to 6 I rated) and possible damage to rotor. This current has induced to rotor body, and then rapid over heating was damage the rotor also. So we need to avoid this, an inadvertent protection has provided. The inadvertent energizing protection is blocked by a voltage criterion on exceeding a minimum voltage, in order to avoid that it picks up during normal operation. This blocking is delayed to avoid that the protection is blocked immediately by the time of an unwanted connection. Another pickup delay is necessary to avoid an unwanted operation in case of high-current faults wi th a heavy voltage dip . This is otherwise known as dead machine protection. Settings: Inadvertent energization I Stage Pickup Release Threshold U1< Pickup Time Delay T U1< Test:

ON 4.2 A 77.0 V 3.00 sec

Current inj in  phase In Amps

Optd value(amps)

Set value for reset threshold (volts)

Pickup set time (s)

Opt time (s)

R-n 4.2 0 Y-n 4.2 -120 B-N 4.2 +120

4.2 4.2 4.2

40/1.732=23.09 40/1.732=23.09 40/1.732=23.09

3.0 3.0 3.0

3.0 3.0 3.0

Remarks: Fuse fail block checked ok Class-A tripping checked ok 12.

OUT-OF-STEP PROTECTION OR POLE SLIP(78G):

This condition occurs when the machine is subjected to severe system disturbances involving rapid change of voltage and frequency. In the present days, vector surge relays are used along with rate of change of frequency relays (df/dt) to take care of these conditions. If the generator goes out of synchronism for more than  preset time, these relays will disconnect the machine from the system. Characteristic diagram:

(This characteristic may differ from relay to relay. This is basically obtained from SIEMENS relay. According to the settings it will drawn inside the relay. When the settings exceed beyond the range, then it will operate. )

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Figure : power swing polygon and impedance vectors with angle δ Ztot = Zb+Zc = 3.0 + 2.2 = 5.2 ohm. Za = 0.289 x 5.2 = 1.503 ohm. Selected za = 1.75 ohm.

Out-of-Step Protection Pickup Current for Measuring Release Pickup Current for Measuring Release Resistance Za of the Polygon (width) Reactance Zb of the Polygon (reverse) Reactance Zc of Polygon (forward char.1) Reactance Dif. Char.1 - Char.2 (forward) Angle of Inclination of the Polygon  Number of Power Swing: Characteristic 1  Number of Power Swing: Characteristic 2 Holding Time of Fault Detection Min. Signal Time for Annun. Char. 1/2

ON I1> 120.00% I2< 20.00% 1.75 Ohm 3.00 Ohm 2.20 Ohm 0.20 Ohm 75.0 ° 2 4 25.00 sec 0.15 sec

Out of step/pole slip test Keeping current constant, voltage angle is varied from 0 to 180 or 180 to 0 to achieve pole slip. Here we are varying the angle. Reason is that, Za, Zb; Zc values should be with in zone. Then only it will operate.

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Characteristic  –  1 (Forward char-1 will have the range b/w 1.75 to 2.20 ohm)

Volts applied In Volts Vr-n Vy-n Vb-n

10.0 +100 10.0 +60 10.0 30

Current Injected In Amps Ir Iy Ib

6.0 0 6.0 -120 6.0 +120

Set value

2.10 2.10 2.10

Optd Value Mw

Set time (s)

Optd Trip time(s)

-67.4MW

0

0.03

Optd Value Mvar

Set time (s)

Optd Trip time(s)

101Mvar

0

0.03

Characteristic  –  2 (Reverse char-2 will have the range b/w 2.20 to 3.0ohm)

Volts applied In Volts Vr-n Vy-n Vb-n

15.0 270 15.0 150 15.0 30

Current Injected In Amps Ir Iy Ib

6.0 0 6.0 -120 6.0 +120

Set value

2.21 2.21 2.21

Remarks: Class-C tripping checked ok

13. GENERATOR UNDER FREQUENCY PROTECTION (81 G): The Under Frequency protection prevents the steam turbine and generator from exceeding the permissible operating time at reduced frequencies. It ensures that the generating unit is separated from the network at a preset value of frequency. It Prevents overfluxing (v/f) of the generator (large overfluxing for short times).The stator under frequency relay measures the frequency of the stator terminal voltage. Setting Recommendations:For Alarm : 48.0 Hz, 2.0 Sec. time delay. For Trip : 47.5 Hz, 1.0 Sec. (or) As recommended by Generator Manufacturers.

Under frequency test: STAGE 1 Voltage Applied (v) Vr-n Vy-n Vb-n

63.5 0 63.5 -120 63.5 +120

STAGE 2 Voltage Applied (v) Vr-n Vy-n Vb-n

63.5 0 63.5 -120 63.5 +120

Set value In Hz

Optd value (Hz)

Set time (s)

Optd time(s)

48.5 48.5 48.5

48.40 48.40 48.40

2.50

2.53

Set value In Hz

Optd value(Hz)

Set time (s)

Optd time(s)

47.4 47.4 47.4

47.30 47.30 47.30

2.00

2.03

Remarks: Fuse fail block checked ok Class-C tripping checked ok

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14. GENERATOR OVER VOLTAGE PROTECTION (59 G): An over voltage on the terminals of the generator can damage the insulator of the generator, bus ducting, breakers, generator transformer and auxiliary equipment. Hence over voltage protection should be  provided for machines of all sizes. Settings recommendations:-

Stage-I State-II

: Over voltage pickup Time delay : Over voltage pickup Time delay

= 1.15 x Un = 10 Sec. = 1.3 x Un = 0.5 Sec.

Overvoltage test

STAGE 1 Volts applied in Volts Vr-n Vy-n Vb-n

69.9 0 69.9 -120 69.9 +120

Set value 121.0/ 3=69.86 121.0/ 3=69.86 121.0/ 3=69.86

Pickup value (v) 69.9 69.9 69.9

Drop up value (v) 66.20 66.20 66.20

Set time (s)

Optd trip time(s)

1.0

1.01

STAGE 2 volts applied in volts

set value

optd value in volts

set time (s)

optd trip time(s)

76.3 76.3 76.3

0.05

0.07

Vr-n 76.3 0 132.0/ 3=76.2 Vy-n 76.3 -120 132.0/ 3=76.2 Vb-n 76.3 +120 132.0/ 3=76.2 Remarks: Class-A tripping checked ok

FOR TRANSFORMER PROTECTION: 1.

DIFFERENTIAL RELAYS: A Differential relay compares the currents on both sides of the transformer. As long as there is no fault within the protected equipment (Transformer), the current circulates between the two CTs and no current flows through the differential element. But for internal faults the sum of the CTs secondary currents will flow through the differential relay making it to operate. Two basic requirements that the differential relay connections are to be satisfied are: a) It must not operate for load or external faults.  b) It must operate for internal faults.

Settings:

I-RESTRAINT for Start Detection

0.10 I/InO

Factor for Increasing of Char. at Start

1

Maximum Permissible Starting Time

0.0 sec

Pickup for Add-on Stabilization

2.00 I/InO

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Differential relay Diagram:

TRANSFORMER

I1

I2

O.C

R.C

Test: a)

DIFFERENTIAL PROTECTION (I DIFF>)

LV SIDE: Current In Phase R Y B

Set Value I /In=I Diff> 0.1 0.1 0.1

Calculated Value(Amps) 0.44 0.44 0.44

Operated Value(Amps) 0.433 0.433 0.433

Set Trip Time (Ms) 0.0 0.0 0.0

Operated Trip Time(Ms) 31 31 31

Operated value(amps) 0.104 0.104 0.104

Set trip time (ms) 0.0 0.0 0.0

Operated Trip time(ms) 31 31 31

Note: Calculated value: Ifull=MVA/(1.732*KV) Idiff=0.2*Iful

HV SIDE: Current in  phase R Y B

Set value I/in diff > 0.1 0.1 0.1

Calculated value(amps) 0.094 0.094 0.094

b) DIFFERENTIAL PROTECTION (I DIFF>>) LV SIDE:

Current in phase R Y B

Set value I /in diff >> 2 2 2

Calculated value(amps) 8.8 8.8 8.8

Operated value(amps) 8.85 8.85 8.85

Set trip time (ms) 0.0 0.0 0.0

Operated trip time(ms) 31 31 31

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HV SIDE: Current in phase R Y B

Set value I /in diff >> 2 2 2

Calculated value(amps) 2.08 2.08 2.08

Operated value(amps) 2.10 2.10 2.10

Set trip time (ms) 0.0 0.0 0.0

Operated trip time(ms) 30 30 30

c) STABILITY TEST:

Apply the full load current on both terminal and neutral side with an angle of 180 degree phase shift on any one side. (Angle will be vary depends on the vector group)

Dyn1 (-30° displacement between HV and LV) Dyn11 (+30° displacement between HV and LV) Dd0 (no phase displacement between HV and LV) Dd6 (180° displacement between HV and LV) 2. DIFFERENTIAL BIAS TEST: To avoid unwanted relays operation under the above two conditions a "Percentage Bias" differential relays is used. This test will also get vary from relay to relay. Simple Slope diagram:

Positive

I1-I2

Negative Torque Region I1+I2/2 ---- The current flowing through the operating coil of the relay should be nearly zero during normal operating conditions and when external short circuit occurs the relay should not operate. While setting the differential relay on a transformer, the (mismatch) current through differential element at normal tap and positive and negative extreme taps are to be computed. Differential element pickup setting and/or bias settings is adopted based on maximum  percentage mismatch adding some safety margin. Differential Current

=

| I1-I2 |

Bias Setting

=

|I1-I2 | ----------(I1+I2)/2

DIFFERENTIAL BIAS TEST

Side 2 Current Is Increased & Slope Is Verified From Tripping Values Of Current. Slope= (I1-I2)/(I1+I2/2)

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Setting: Slope 1 =20% Side 1 (Hv Side) 3 Phase Current (Amps) Ir=4.4 0 Iy=4.4 -120 Ib=4.4 120 Ir=4.4 0 Iy=4.4 -120 Ib=4.4 120

Side 2 (Lv Side) 3 Phase Current (Amps) Ir=0.94 150 Iy=0.94 270 Ib=0.94 30 Ir=1.37 150 Iy=1.37 270 Ib=1.37 30

Calculated Slope (%)

Relay Condition

0.0

Stable

18.6

Operated

Note: During bias test we have to put 180deg phase shift b/w both sides current. This angle may vary depends upon the vector group of transformer. Here DYn1 i.e. -30deg displacement b/w HV and LV. So 18030=150degree balance angle.

2ND HARMONIC RESTRAINT TEST:

As second harmonic always present predominantly in the inrush currents, hence second harmonics is used as a stabilizing bias against inrush effect. The differential current is passed through a filter which extracts the second harmonics; this component is then applied to produce a restraining quantity sufficient to overcome the operating tendency due to the whole of the inrush current which flows in the operating circuit. The relay will restrain when the second harmonic component exceeds 20% of the current. Inrush current characteristic:

. Test: 1a Current Is Injected At 50 Hz Simultaneously In Each Phase & 100 Hz Current Is Reduced Till Relay Operates.

HV SIDE: Current In Phase

R Y B

50Hz Current Injected (A)

100Hz Current Injected (A)

100Hz Current Optd Value (A)

0.947 0.947 0.947

0.80 0.80 0.80

0..171 0..171 0..171

2nd Harmonic Set Value (%)

2nd Harmonic Optd Value (%)

20

18.05

20

LV SIDE: Current In Phase

R Y B

50Hz Current Injected (A)

100Hz Current Injected (A)

100Hz Current Optd Value (A)

4.40 4.40 4.40

2.0 2.0 2.0

0.80 0.80 0.80

2nd Harmonic Set Value (%)

2nd Harmonics Optd Value (%)

20

18.18

Remark: Class-A trip checked. 3.

RESTRICTED EARTH FAULT:

This relay is operative only for the internal faults of the transformer and thus fast operating timer can be achieved. An external fault on the star side will result in current flowing in the line CT of the affected phase and a balancing current in the neutral CT and current in the relay is zero and hence relay is stable.

During an internal fault, the line current on the line CT gets reversed and hence relay operates.

The arrangement of residually connected CTs on the delta side of a transformer is only sensitive to earth faults on the delta side because zero sequence currents are blocked by the delta winding . For external faults no current flows through REF unless a CT gets saturated. Hence minimum pickup current setting is adopted (10% or 20% In) on REF relay. Based on this, through fault current, the stabilizing resistor is set such that the relay will not operate for external fault when a CT gets saturated. This relay operates only for internal earth faults, instantaneously.

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TEST: Stage 1 Set value I 0.1 Stage 2 Set value I 1.5

Optd value(amps) 0.1

Optd value(amps) 1.5

Set time (s) 0.0

Optd trip time

Set time (s) 0.0

Optd trip time

20ms

20ms

Remarks: Class-a tripping checked ok.

22