Ct Vt Calculations r1

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CNT 2310710001/E9/6263 dt 14.12.10

CT & VT CALCULATIONS FOR

GENERATOR, GT AND UT METERING AND PROTECTION

DOCUMENT NO. : PE-DC-348-510-E003REV NO. : 01

DATE : 28/10/10 SHEET : 1 of 16

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

VT : Voltage transformer




DATE : 28/10/10 SHEET : 2 of 16

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 = 850.x 213

1000600

= 19407.3A Generator current at minimum continuous operating voltage (i.e. at 95% of normal voltage)

IGmax = 95.085x0.213

1000600

= 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




DATE : 28/10/10 SHEET : 3 of 16

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

MAGNETISING CURRENT : 150mA at Vk/2




DATE : 28/10/10 SHEET : 4 of 16

CT 2 :

* : Performance testing meter



ACCURACY CLASS : 0.2

BURDEN : 60 VA

I.S.F. : ≤ 5.0

CT 3 & CT 4 :

Maximum through fault current of generator = 85.0178.085.0213

1000600

= 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




DATE : 28/10/10 SHEET : 5 of 16

SELECTED PARAMETERS FOR CT 3 & CT 4 :


ACCURACY CLASS : PS



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:


ACCURACY CLASS : PS



CT 7 :

Source of burden

AVR

Load transducer

Load shedding relay


Total burden

Burden (in VA)

20.0

0.5

0.85

19.4

40.75




BURDEN : 60 VA

I.S.F. : ≤ 5.0




DATE : 28/10/10 SHEET : 6 of 16

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


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




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 :

Vk 50I (Rct + 2 RL)




DATE : 28/10/10 SHEET : 7 of 16

Ie 0.03 I at Vk/2

I = 5A

Vk 50 x 5 (Rct + 2 x 2.04 /2)


Vk 250Rct + 510 V

Ie 150mA at Vk/2

SELECTED PARAMETERS FOR CT 10/ CT11 :


ACCURACY CLASS : PS


MAGNETISING CURRENT : 150 mA at Vk/2

5.3.0 CTs ON GT HV SIDE

Current on GT HV side = x4203

1000 x 240x3

= 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


= 2.04 ohms

Hence,

Lead burden = 12 x RL

= 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

Burden (in VA)

0.30

2.04

2.34

SELECTED PARAMETERS FOR CT 12 (CORE – 1):

CT RATIO : 1200/1 A




DATE : 28/10/10 SHEET : 8 of 16

ACCURACY CLASS : 5P20

BURDEN : 15 VA

CT 12 (CORE-2) :

Maximum through fault current on HV side of GT = 145.095.04203

1000720

= 7185.28 A

Fault on secondary side of CT, If = 1000

17185.28

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

ACCURACY CLASS : PS

KNEE POINT VOLTAGE : 20 Rct + 100 V


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)


Vk 250Rct + 510 V




DATE : 28/10/10 SHEET : 9 of 16

Ie 150mA at Vk/2


CT RATIO : 1200/5 A

ACCURACY CLASS : PS



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 = x213

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 RL

= 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)




DATE : 28/10/10 SHEET : 10 of 16

[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

ACCURACY CLASS : PS



CT 16A & CT 16B :

Source of burden

51UT

Lead burden [2.5 mm² cable]

Total Burden

Burden (in VA)

0.3

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 :


ACCURACY CLASS : PS



5.4.2 CTS ON UT LV SIDE:

Current on UT LV side = x11.53

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:




DATE : 28/10/10 SHEET : 11 of 16

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


= 1.86

Hence, Lead burden = I2 x RL

= 12 x 1.86

= 1.86 VA

CT 18A & CT18B (CORE-1) :

Source of burden

51NUT

Lead burden [2.5 mm² cable]

Total Burden

Burden (in VA)

0.3

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 = 9.010.05.113

10005.31

= 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




DATE : 28/10/10 SHEET : 12 of 16


CT RATIO : 2000 / 1 A

ACCURACY CLASS : PS



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


= 0.8 ohm

CT19A/ CT19B (CORE-1) :

Particulars shall be similar to CT 18A/CT18B (CORE-2) i.e.


CT RATIO : 2000 / 1 A

ACCURACY CLASS : PS



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

ACCURACY CLASS : PS






DATE : 28/10/10 SHEET : 13 of 16

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

21000V

Secondary voltage : 110

3V

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




DATE : 28/10/10 SHEET : 14 of 16

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


* synchronising equipment


VT RATIO : 3

110

3

21000 V

ACCURACY CLASS : 0.5/3P

BURDEN : 100 VA

VT 3:




DATE : 28/10/10 SHEET : 15 of 16

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



VT RATIO : 3

110

3

21000 V


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 Primary connection : Star 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

IVT ratio : (110/√3) / (80/√3) V




DATE : 28/10/10 SHEET : 16 of 16

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 :

Primary connection : Star

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

Burden (in VA)

45.0

45.0

SELECTED PARAMETERS FOR IVT 2 :

VT RATIO : 3

110/

3

415 V


BURDEN : 100 VA

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