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TitleCt Sizing XL
TagsElectrical Resistance And Conductance Relay Transformer Electrical Equipment Electrical Impedance
File Size346.2 KB
Total Pages20
Document Text Contents
Page 1

Calculation of CT Parameter Parameter Remark

Description From generator Data sheet From generator

Data sheet

(i) Generator parameter

Rated Voltage(kV) 19

Rated Current(Amp) 9116.3

Sync. Reactance Xd is 1/SCR 200.0

Subtransient Reactance Xd"(Saturated) 12.0%

Transient Reactance Xd' (Saturated) 20.0%

Rated Voltage at -5% 18.05

FL current at -5%of rated Voltage= 9596.1

Voltage at + 5% 19.95

FL current at +5% of rated Voltage= 8682.2

Choose CT ratio = 11000/1

105% of rated current at (-)5% of rated voltage 10075.94

Choose CT ratio = 11000/1A 11000

This gives an over load capability 1.21

% O/L Capability of Full Load Current 120.66

1) Calculation of CT parameter for generator differential protection (87G)

Relay setting VR = If (RCT + 2RL + Rrelay) where ,

Where If = fault current on CT sec, RCT = CT secondary resistance, Rrelay =

Relay resistance in ohms, RL = Resistance of cable in ohms,

RL in W for 100m of 2.5mm2 0.887

(Considering 100 m of 2.5 sq.mm Cu cond. cable between busduct CT &

GRP the resistance of 2.5 sq.mm Cu cond cable at 70
o
C in W / km (from

KEI catalogue)

8.87

Relay resitance/phase Assumed Rrelay from SEC 0.1

Fault contribution from Gen in MVA= 3571.43

Taking 30% - Ve tol on Xd” as per IEC-34 part –I

Fault contribution from system in MVA considering 40KA system

Impedence on 220KV Side.

2024.10

Source impedence in Percentage at 300 Base 1.97%

Taking higher of the above two values = 3571.43

Fault Current in KA Taking higher value of the fault contribution from i.e.

fault contribution from system / fault contribution from gen

108.52

Secondary Fault Current 9.87

VR = 9.87 (RCT +2x0.887+0.1) E47*(RcT+2*E31+E35)

Vk = 2 VR 2*9.87* (RCT +2x0.887+0.1)

Expressing VK term of +40V 19.74Rct+36.99

Vk = 20RCT + 37

Im  15 mA at Vk/2

Page 1 of 20

Page 2

CT with group-1/group-2 protection

11000/1A, CLPS, Vk  20RCT + 36V

Im  15 mA at Vk/2

(ii) METERING CORE

CT chosen 11000/1A, 30VA, CL 0.2 ISF  5
Typical Meter burden in ohm (from catalogue of Conzerv model) 0.4
Typical Anolgue Meter burden in ohm (from catalogue of AE/Rishab) &

Transducer
7 Total 10Nos

Analogue meter

connected & max

4 nos of

transducer per CT

Max. Burden margin Consider for future use 2
Total Burden on CT = Lead Burden+ Meter Burden

Lead Burden for lead length of 100M= I
2
x(Rct+2RL)

=Sqr(1)*(Rct+2Rl)

=Rct+1.774

Rct Assumed for CT Sec.1A is 15 ohm for 11000/1Amp 16.774

Total Burden on CT = Lead Burden+ Meter Burden+burden margin

Consider for future use

26.174

30 VA, CL-0.2 As meter used is Multifunction type,

hence 30 VA considered is more

than Sufficient also Lead burden is

less than above assumed buden as

Meter & CT are at the same

Switchgear.

Calculation of CT parameter for (87- 0A) on 19 kV side

Same as generator differential protection CT core

11000/1A, CL-PS, Vk ≥ 20Rct + 36V, Im ≤ 15mA at Vk/2

2 CT on 220 KV side of Generator Step up transformer Bay

i) Core –1 (87- 0A)

GT HT side current (current on 400 kV side) = 512.40

= 513 A

RL = 8.87x0.27 (considering

270m)= 2.3949

Rrelay = 0.1

CT ratio chosen = 600/1A

VK = 40xIx(RCT+2RL+Rrelay) Fault current on 400KV side taken

as 40KA

= 40x1 (RCT +2x2.3949+0.1) =40 RCT+195.592

= VK  40 RCT+200V

CT ratio 600/1A

CLPS , VK  40 RCT+200V

Im  30 mA at VK/2

ii) Core –2 (50 LBB, 51GT)

R relay= 0.1 (from relay catalogue of ABB) R relay= 0.1 (from relay catalogue

of ABB)

600/1A

Total Burden on CT = Lead Burden+ Relay Burden

Lead Burden for lead length of 270M= I
2
x(Rct+2RL) =Sqr(1)*(Rct+2Rl)

=Rct+4.79

Rct Assumed for CT Sec.1A is 6 ohm 10.79

Total Burden on CT = Lead Burden+ Relay Burden 10.89

15 VA, CL5P20 Cater the Load burden of Numerical

Relay

Page 2 of 20

Page 11

Rct Assumed for CT Sec.1A is <=4 ohm 6.7096

Total Burden on CT = Lead Burden+ Relay Burden 6.7096

10 VA, CL5P20 As relay used is numerical(SPAJ-140C) type,

hence 10 VA considered is more than

Sufficient also Lead burden is less than

above assumed buden as relay & Ct are at

the same Switchgear.

ii. For Metering Core

Typical Meter burden in ohm (from catalogue of

Conzerv model)
0.2

Typical Anolgue Meter burden in ohm (from

catalogue of AE/Rishab) & Transducer
1

Burden margin Consider for future use 2
Total Burden on CT = Lead Burden+ Meter Burden

Lead Burden for lead length of 40M= I
2
x(Rct+2RL) =Sqr(1)*(Rct+2Rl)

=Rct+0.7096

Rct Assumed for CT Sec.1A is 4 ohm for 100/1Amp 4.7096

Total Burden on CT = Lead Burden+ Meter

Burden+burden margin Consider for future use
7.9096

10 VA, CL-0.5 As meter used is Multifunction type, hence

10 VA considered is more than Sufficient

also Lead burden is less than above

assumed buden as Meter & CT are at the

same Switchgear.

5 PA-1A, 1B 1600 200/1

i. For Differential Protection

Motor rated Current = 1100/Sqrt(3)/6.6/0.9 164.66

Differential CT for Motor Protection

Vk = 40xIx(Rct+2RL+ Rrelay)

= 40*1*(Rct+2x0.887+0.1)

= 40Rct+74.96V

RL = 8.87X0.1 (Considering 100m Length

from Motor terminal to Relay)=0.887 R relay

= 0.1

Vk>= 40Rct+80V, Im<=30mA at Vk/2

ii. For O/C & E/F Protection

R relay in ohm (from relay catalogue of ABB model

SPAJ)

0.1

Total Burden on CT = Lead Burden+ Relay Burden

Lead Burden for lead length of 40M= I
2
x(Rct+2RL) =Sqr(1)*(Rct+2Rl)

=Rct+0.7096

Rct Assumed for CT Sec.1A is <=5 ohm 5.7096
Total Burden on CT = Lead Burden+ Relay Burden 5.8096
10 VA, CL5P20 As relay used is numerical(SPAJ-140C) type,

hence 10 VA considered is more than

Sufficient also Lead burden is less than

above assumed buden as relay & Ct are at

the same Switchgear.

iii. For Metering Core

Typical Meter burden in ohm (from catalogue of

Conzerv model)
0.2

Typical Anolgue Meter burden in ohm (from

catalogue of AE/Rishab) & Transducer
1

Burden margin Consider for future use 2
Total Burden on CT = Lead Burden+ Meter Burden

Lead Burden for lead length of 40M= I
2
x(Rct+2RL) =Sqr(1)*(Rct+2Rl)

=Rct+0.7096

Rct Assumed for CT Sec.1A is 5 ohm for 200/1Amp 5.7096

Total Burden on CT = Lead Burden+ Meter

Burden+burden margin Consider for future use
8.9096

10 VA, CL-0.5 As meter used is Multifunction type, hence

10 VA considered is more than Sufficient

also Lead burden is less than above

assumed buden as Meter & CT are at the

same Switchgear.

6 Coal Mill-1A, 1B, 1C, 1D, 1E, 1F 400 43.74 50/1 3C x 185 mm
2

i. For O/C & E/F Protection

R relay in ohm (from relay catalogue of ABB model

SPAJ)

0.1

Total Burden on CT = Lead Burden+ Relay Burden

Lead Burden for lead length of 40M= I
2
x(Rct+2RL) =Sqr(1)*(Rct+2Rl)

=Rct+0.7096

Page 11 of 20

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