Ct Vt Calculations r1

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Action - 3 : Returned with comments. Resubmit for approval Action - 4 : Not approved. Resubmit for review

Information noted. Retained for reference and records

FI Letter Ref.No. Date

Signature Initials AE This document has been reviewed as noted with regard to general conformity with the Contract specifications and requirements. The Vendor / Contractor is responsible for correctness of design calculation and details. Approval of this document does not relieve the Vendor / Contractor of his responsibility in carrying out the work correctly and fulfilling the complete requirements of the contract nor does it limit the purchaser's rights under the contract.

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1.0 SCOPE :

This document covers the following :



CT parameters calculations for :-

 CTs of Generator Neutral & Line side.

 CTs on GT HV side having interface with GRP.

 CTs on GT HV neutral side.

 CTs on GT LV side.

 CTs on UT HV side.

 CTs on UT LV neutral side.

 CTs on UT LV side having interface with GRP.



VT parameters calculations for :-

 VTs on Generator line side.



IVT parameters calculations for

 Generator Inter turn fault relay (64GlT)  110 V AC auxiliary supply to 20 Hz Generator

2.0 REFERENCE DRAWING :

DRAWING TITLE : SLD for Generator, GT & UT Metering and Protection. DRAWING NO. : PE-DG-348-510-E001

3.0 LEGEND :

CT : Current transformer

I : Relay rated current

IVT : Interposing voltage transformer

Ie : Excitation current

If : CT secondary current corresponding to fault current

L : Length of cable between CT and relay

RL : One way lead resistance between CT and relay

Rct : Resistance of secondary winding of main CT

Vk : Knee point voltage

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4.1 MAIN EQUIPMENT DETAILS

(I) GENERATOR :

Rated Output : 600 MW

Rated Voltage : 21 KV

Subtransient reactance (Xd’’) (saturated) : 17.8%

Voltage variation : ± 5%

Rated power factor : 0.85 LAG

(II) GENERATOR TRANSFORMER :

Rating :

3 single-phase units of 240 MVA each.

Percentage Impedance [Tentative] : 14.5% at 240 MVA base

Voltage ratio : (420/√3)/21KV

(III) UNIT TRANSFORMERS :

Rating : 31.5 MVA

Percentage Impedance [Tentative] : 10 %

Voltage ratio : 21/11.5KV

4.2 CONTROL CABLE DETAILS

Material : Copper

Size : 2.5 mm2

Resistance of conductor : 8.87 /kM (for 2.5 mm2 _cable)

5.0 CTs PARAMETER CALCULATIONS

5.1 CTs Ratio On Gen Neutral & Phase Side : Generator current at normal operating voltage

IGnormal =

85

0. x

21

3 1000

600 



= 19407.3A

Generator current at minimum continuous operating voltage (i.e. at 95% of normal voltage)

IGmax =

95 .

0

85 x0.

21

3 1000

600 



= 20428.7A

Hence CT ratio for CTs located on generator neutral side and phase side for protection, metering, load transducers and AVR is selected as 22000/5 A

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For the purpose of calculation, distance from GRP to Gen Busduct CTs marshalling box, L is taken as 175M  Lead Resistance, RL = 8.87 x 0.175 = 1.55 ohm Hence , Lead Burden = 52 _{x 1.55 VA} = 38.8 VA

Lead Burden (for 2 cores) = 19.4 VA

5.1.1 CTs ON GEN. NEUTRAL SIDE

CT 1 : For Overall differential protection (87OA)

CT 2 : For ‘Load transducer’ & ‘Performance testing’

CT 3 : For composite generator protection relay ‘GR1’

CT9 : For 100% stator E/F Protection (64G3)

CT 1 : Vk  50I (Rct + 2 RL) Ie  0.03 I at Vk/2 I = 5A Vk  50 x 5 (Rct + 2 x 1.55 /2) [2 cores of 2.5mm2 cable] Vk  250Rct + 387.5 V Ie  150mA at Vk/2

SELECTED PARAMETERS FOR CT 1 :

CT RATIO : 22000/5 A

ACCURACY CLASS : PS

KNEE POINT VOLTAGE :  250Rct + 400 V

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CT 2 :

* : Performance testing meter

CT RATIO : 22000/5 A

ACCURACY CLASS : 0.2

BURDEN : 60 VA

I.S.F. : ≤ 5.0

CT 3 & CT 4 :

Maximum through fault current of generator =

85 .

0

178 .

0

85 .

0

21

3 1000

600 



= 128270.3 A If =

21000

5 128270.3



= 30.54 A Vk  5If (Rct + 2 RL) Vk  5 x 30.54 (Rct + 2 x 1.55/2) [2 cores of 2.5 mm2 cable] Vk  152.7Rct + 236.685 V Ie  0.03 I at Vk/2 I = 5A Ie  150mA at Vk/2 Source of burden Ammeter (*) Wattmeter (*) VARmeter (*) PFmeter (*) KWHmeter (*) Load Transducer

Lead burden [2 cores of 2.5 mm² cable] Total Burden

Burden (in VA) 3.0 5.0 5.0 5.0 5.0 0.5 19.4 42.9

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SELECTED PARAMETERS FOR CT 3 & CT 4 :

CT RATIO : 22000/5 A

MAGNETISING CURRENT :  150mA at Vk/2

CT 9:

PARAMETER TO BE FINALISED LATER AFTER RECEIPT OF NGR RATING FROM CUSTOMER. 5.1.2 CTs ON GEN. PHASE SIDE

CT 7 : For Load transducer, Load shedding relay and AVR

CT 8 : For ‘Metering’, Load transducer & AVR.

CT 5/CT 6 :

Particulars shall be similar to CT 3/CT 4

SELECTED PARAMETERS FOR CT 5 & CT 6:

CT RATIO : 22000/5 A

CT 7 :

Source of burden AVR

Load transducer Load shedding relay

Lead burden [2 cores of 2.5 mm² cable] Total burden

Burden (in VA) 20.0

0.5 0.85 19.4 40.75 SELECTED PARAMETERS FOR CT 7 :

CT RATIO : 22000/5 A

BURDEN : 60 VA

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CT 8 :

Source of burden MWH/MVARH meter Current transducer (3 Nos.) Watt transducer (1 Nos.) VAR transducer (1 Nos.) PF transducer

Load transducer AVR

Lead burden [2 cores of 2.5 mm² cable] Total Burden

Burden (in VA) 1.0 0.2 x 3 =0.6 0.5 0.5 0.6 0.5 20.0 19.4 43.1 SELECTED PARAMETERS FOR CT 8 :

CT RATIO : 22000/5 A

BURDEN : 60 VA

I.S.F. : ≤ 5.0

5.2.0 CTs ON GT LV SIDE

Phase Current on GT LV side =

21 1000

x

240

A = 11428.57 A

Hence CT ratio for CTs located on GT LV line side for protection is selected as 12500/5 A.

For the purpose of calculations, Distance from GRP to GT M.Box, L = 230M

 Lead Resistane, RL = 8.87 x 0.23

= 2.04 ohms

5.2.1 CTs on GT LV line Side:

CT 10 : For GT differential Protection (87GT)

CT 11 : For GT differential Protection (87GT)

CT10/ CT11 :

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Ie  0.03 I at Vk/2 I = 5A Vk  50 x 5 (Rct + 2 x 2.04 /2) [2 cores of 2.5mm2 cable] Vk  250Rct + 510 V Ie  150mA at Vk/2 SELECTED PARAMETERS FOR CT 10/ CT11 :

CT RATIO : 12500/5 A

KNEE POINT VOLTAGE : 250Rct + 550 V

MAGNETISING CURRENT :  150 mA at Vk/2 5.3.0 CTs ON GT HV SIDE Current on GT HV side =

x420

3 1000

x

240 x

3

= 989.77 A

Hence CT ratio for CTs located on GT HV neutral side and GT HV line side for protection is selected as 1200/1 A and 1200/5 A respectively.

For the purpose of calculations, Distance from GRP to GT M.Box, L = 230M  Lead Resistane, RL = 8.87 x 0.23 = 2.04 ohms Hence, Lead burden = 12_{x R} L = 2.04 VA 5.3.1 CTs on GT HV Neutral Side:

CT 12 (CORE-1) : For GT HV side Backup Earth Fault Protection (51NGT)

CT 12 (CORE-2) : For GT HV & overhead line differential Protection (87HV) CT 12 (CORE-1) :

Source of burden 51 NGT

Lead burden( 2.5 mm² cable) Total Burden

2.04 2.34 SELECTED PARAMETERS FOR CT 12 (CORE – 1):

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ACCURACY CLASS : 5P20

BURDEN : 15 VA

CT 12 (CORE-2) :

Maximum through fault current on HV side of GT =

145 .

0

95 .

0

420

3 1000

720 



= 7185.28 A

 Fault on secondary side of CT, If =

1000

1 7185.28

If = 7.185 A

Knee point voltage, Vk  2If (Rct + 2RL)

Vk  2 x 7.2 (Rct + 2x 2.04) (2.5 mm² cable) Vk  14.4 Rct + 58.75 V Ie  0.03 I at Vk/2 CT sec. current , I = 1 A  Ie  30mA at Vk/2

SELECTED PARAMETERS FOR CT 12 (CORE-2) :

CT RATIO : 1200/1 A

KNEE POINT VOLTAGE :  20 Rct + 100 V

MAGNETISING CURRENT :  30 mA at Vk/2 5.3.2 CTs on GT HV line Side :

CT 13 : For GT differential protection (87GT)

CT 13 : Vk  50I (Rct + 2 RL) Ie  0.03 I at Vk/2 I = 5A Vk  50 x 5 (Rct + 2 x 2.04 /2) [2 cores of 2.5mm2 cable] Vk  250Rct + 510 V

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Ie  150mA at Vk/2

CT RATIO : 1200/5 A

KNEE POINT VOLTAGE : 250Rct + 550 V

MAGNETISING CURRENT :  150 mA at Vk/2

CT 14(CORE-1) : For Overall differential protection (87OA) to be covered in 400kV switchyard document [customer scope] documents.

CT 14(CORE-2) : For GT HV & overhead line differential protection (87HV) to be covered in 400kV switchyard document [customer scope] documents.

5.4.0 CTs FOR UT #1A/2A & UT #1B/2B

5.4.1 CTs on UT #1A/2A & UT #1B/2B HV Line side :-

CT15A/ CT15B : For UT Differential Protection (87UTA / 87UTB)

CT16A/ CT16B : For UT HV Over Current Protection (51UTA / 51UTB)

CT17A/ CT17B : Overall differential protection (87OA)

Current on UT HV side =

x21

3 1000

x

31.5

= 866.05 A [At normal voltage]

Hence CT ratio for CTs located on UT HV side for protection is selected as 1000/1 A. For the purpose of calculations, Distance from GRP to UTs Tap Off Busduct CT marshalling box is taken as 210m.

L = 210M

 Lead Resistance, RL = 8.87 x 0.21

= 1.86 ohm

Hence, Lead burden = I2_{x R}

L = 12_{x 1.86} = 1.86 VA CT 15A & CT 15B: Vk  40 I (Rct + 2 RL) Ie  0.03 I at Vk/2 I = 1A Vk  40 x 1 (Rct + 2 x 1.86)

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[2.5 mm2 cable]

 Vk  40Rct + 148.8 V

Ie  30mA at Vk/2

SELECTED PARAMETERS FOR CT 15A & CT 15B :

CT RATIO : 1000 / 1 A

CT 16A & CT 16B :

Source of burden 51UT

Lead burden [2.5 mm² cable] Total Burden

1.86 2.16 SELECTED PARAMETERS FOR CT16A & CT 16B :

CT RATIO : 1000/1 A

BURDEN : 15VA CT17A/ CT17B :

Praticular shall be similar to CT1:

SELECTED PARAMETERS FOR CT 17A/17B :

CT RATIO : 21000/5 A

5.4.2 CTS ON UT LV SIDE: Current on UT LV side =

x11.5

3 1000

x

31.5

= 1581.48 A

Hence CT ratio for CTs located on UT LV side for protection is selected as 2000/1 A. 5.4.2.1 CTs on UT LV Neutral side:

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NOTE: FOR CALCUALTION PURPOSE, NGR RATING IS CONSIDRED AS 300A FOR 10 SEC. CUSTOMER TO CONFIRM.

CT18A/ CT18B (CORE - 1) : For UT LV Backup Earth Fault Protection (51NUTA / 51NUTB) CT18A/ CT18B (CORE - 2) : For UT Restricted Earth Fault Protection (64RUTA / 64RUTB)

Max. distance from GRP to UT MB = 210M

 Lead Resistane, RL = 8.87 x 0.21

= 1.86

Hence, Lead burden = I2_{x R}

L

= 12_{x 1.86}

= 1.86 VA

CT 18A & CT18B (CORE-1) : Source of burden 51NUT

Lead burden [2.5 mm² cable] Total Burden

1.86 2.16 NGR is rated for maximum E/F current of 300 A.

SELECTED PARAMETERS FOR CT 18A & 18B (CORE-1) :

CT RATIO : 300 / 1 A

BURDEN : 15 VA

CT 18A & CT 18B (CORE-2) :

Maximum through fault current on LV side of UT =

9 .

0

10 .

0

5 .

11

3 1000

5 .

31 



= 17572 A

 Fault on secondary side of CT, If =

2000

1 17572



If = 8.78 A

Knee point voltage, Vk  2If (Rct + 2RL )

Vk  2 x 8.78(Rct + 2 x 1.86) [ 2.5 mm2 cable] Vk  17.56Rct + 65.32 V Magnetising current , Ie  0.03 I at Vk/2 CT sec. current , I = 1A  Ie  30mA at Vk/2

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SELECTED PARAMETERS FOR CT 18A & 18B (CORE-2) :

CT RATIO : 2000 / 1 A

KNEE POINT VOLTAGE :  20Rct + 100 V

MAGNETISING CURRENT :  30mA at Vk/2 5.3.2.2 CTs on UT LV Line side:

CT19A/ CT19B (CORE-1) : For UT Restricted Earth Fault Protection (64RUTA / 64RUTB) CT19A/ CT19B (CORE-2) : For UT Differential Protection (87UTA / 87UTB)

Average Distance from GRP to Unit Switchgear I/C Busduct CT marshalling box

= 90 M

 Lead Resistane, RL = 8.87 x 0.09

= 0.8 ohm

CT19A/ CT19B (CORE-1) :

Particulars shall be similar to CT 18A/CT18B (CORE-2) i.e. SELECTED PARAMETERS FOR CT 20A & 20B (CORE-1) :

CT RATIO : 2000 / 1 A

MAGNETISING CURRENT :  30mA at Vk/2

CT19A/ CT19B (CORE-2) : Vk  40 I (Rct + 2 RL) Ie  0.03 I at Vk/2 I = 1A Vk  40 x 1 (Rct + 2 x 0.8/2) [2 cores of 2.5 mm2 cable]  Vk  40Rct + 32 V Ie  30mA at Vk/2

SELECTED PARAMETERS FOR CT 20A & CT 20B (CORE-2) :

CT RATIO : 2000 / 1 A

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6.0.0 VOLTAGE TRANSFORMER PARAMETER CALCULATIONS

VT 1 : For Gen. Protection, AVR, Frequency Transducer, Load transducer & Load shedding relay.

VT 2 : For Gen. Protection, AVR, Load transducer & gen. Synchronising. VT 3 : For metering, performance testing & Load transducer.

Primary connection : Star

Secondary connection : Star

Primary voltage :

3 21000

V Secondary voltage : 110 3 V VT ratio :

3

110

3 21000

V VT 1: Source of burden 60VT1 AVR Frequency transducer Load transducer Load shedding relay GR1

Total Burden

Burden (in VA) 11.0 20.0 3.00 0.13 2.1 0.3 36.53 Lead burden is negligible for VT

SELECTED PARAMETERS FOR VT 1 :

VT RATIO :

3

110

3 21000

V ACCURACY CLASS : 0.5/3P BURDEN : 100 VA

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VT 2 :

Source of burden 60VT2

AVR

Load transducer

Voltage transducer for incoming voltage(*) Frequency transducer for incoming frequency(*)

Synchroscope(*) GR2

Total Burden

Burden (in VA) 11.0 20.0 0.125 0.22 3.0 6.0 0.3 40.645 Lead burden is negligible for VT

* synchronising equipment

VT RATIO :

3

110

3 21000

V ACCURACY CLASS : 0.5/3P BURDEN : 100 VA VT 3:

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Source of burden 60VT3

MWH/MVARH meter Transducer for VAR Transducer for Watt Transducer for PF meter Transducer for Frequency Transducer for Voltmeter (2 Nos.) Load transducer AVR Wattmeter (*) Varmeter (*) PFmeter (*) KWH meter (*) Total Burden

Burden (in VA) 11.0 2.0 0.22 0.22 2.5 3.0 x 2 = 6.0 0.22 X 2 = 0.66 0.125 20.0 5.0 7.5 5.0 7.5 62.325 * Performance testing meter

Lead burden is negligible for VT

VT RATIO :

3

110

3 21000

V ACCURACY CLASS : 0.2 BURDEN : 100 VA

7.0.0 INTERPOSING VTs PARAMETER CALCULATIONS IVT 1 : For Gen. Interturn fault protection (64GIT) IVT 2 : For 110V AC auxiliary supply to 20Hz Generator IVT1

Secondary connection : Open delta

Under fault condition, 64GIT should get no voltage. NGT secondary and open delta secondary of IVT1should give equal and opposite voltages.

NGT ratio is 21kV/240V, i.e. it gives an output of 240/√3 for a single line to earth fault. to get 240 /√3 V, IVT1 secondary rating shall be (240 /√3) / 3 V.

IVT Primary voltage : (110/√3) V

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Source burden Burden (in VA)

64GIT 0.3

Total Burden 0.3

SELECTED BURDEN 15 VA

SELECTED PARAMETERS FOR IVT 1 :

IVT RATIO :

3

80

3

110

V ACCURACY CLASS : 3P BURDEN : 15 VA IVT 2 :

Secondary connection : star

Primary voltage :

3

415

V Secondary voltage :

3

110

V IVT ratio :

3

110 /

3

415

Source of burden 20 Hz Generator Total Burden

45.0 SELECTED PARAMETERS FOR IVT 2 :

VT RATIO :

3

110 /

3

415

V ACCURACY CLASS : 1.0 BURDEN : 100 VA