CONTRACT NO. GTC/488G/2012 Doc No.

PH11-8G-238-33-C001
66/11kV POLICE TRAINING INSTITUTE SUBSTATION Rev. No. A
66kV CT / VT SIZING CALCULATION Date 30-Aug-15

INPUTS

System data
Bay Number B11 & B21
Feeder details 66/11 kV Transformer Feeder (66kV Side)
Nominal System Voltage 66 kV
System frequency 50 Hz
Maximum fault current 31.5 kA
Rated Burden Annexure
Relay Details Make Model Current (VA) Ref No Purpose of conncted
(IR)
HV REF Siemens 7SJ61 1 0.05 No : 2.5 HV BCT-HV REF protection
SBEF Siemens 7SJ61 1 0.05 No : 2.7 NCT-Standby Earth Fault
LV REF Siemens 7SJ61 1 0.05 No : 2.6 NCT-LV REF Protection
CB
Opening time 38 m.sec
Tripping time 60 m.sec
% DC 45 %
Rated Current 1250 Amps
Transfromer Details
Capacity of the transformer 32/40 MVA
Total auxiliary loss @ full load 7 KW
Max load loss at ONAF 216 KW
% Impedance (%Z) 13.6 %
Line feeder
Cable size 2000 Sq.mm
Circuit 120 MVA
Route Length 2 km
Cable Possitive/Negative sequence 0.0128 Ohm/km
Resistance
Cable Possitive/Negative sequence 0.165 Ohm/km
Reactance
Cable zero sequence Resistance 0.08 Ohm/km
Cable zero sequence Reactance 0.046 Ohm/km

Instrument proposed Proposed Proposed K Factor

Relay Primary Secondary Class CT VA CT winding Magnetising
CT Details Current Current Burden(VA) Security Knee
factor(IFC) Voltage(V) resistance(o current(mA) /Krpa
factor
a Factor
hm)

HV-BCT-Core-1 7SJ61 800-400 1 PX 700-350 2.8-1.4 15-30 HV REF Protection

A18-NERCT1-2 7SJ61 750 1 5P20 20 2.63 SBEF

 CONTRACT NO. GTC/488G/2012 Doc No. PH11-8G-238-33-C001
66/11kV POLICE TRAINING INSTITUTE SUBSTATION Rev. No. A
66kV CT / VT SIZING CALCULATION Date 30-Aug-15

INPUTS

A18-NERCT1-1 7SJ61 2500 1 PX 800 8.75 15 LV REF Protection

Length 100 mtrs Changed as per KM Specification . (earlier submission value- 120 m)
Conductor Material Copper
Conductor size 4 Sq.mm
Copper resistance 0.0056 Ohms/m

KM specification
Safty factor (K) 1.25 for Differential Protection
Safty factor (K) 1.5 HV REF

Annexure Details

Relay details Relay Burden Details CT Requirement Formula Details

7SJ61 = No : 10.1- Page # 197 No : 9.1 - Page # 188

7SJ61 = No : 10.1- Page # 197 No : 9.1 - Page # 188
7SJ61 = No : 10.1- Page # 197 No : 9.1 - Page # 188
=

No : 1.2
Calculation of Secondary lead resistance = No : 1.2 -Page # 7

X/R Ratio for 66kV Circuit Breaker = No : 11.6 - Page # 237 No : 11.6

66kV GIS (Circuit Breaker) Technical Data sheet = No : 12.2 - Page # 244

Fault Current Calculation for 66/11kV Power Transformer = No : 11.4 - Page # 213 No : 11.4

X/R Ratio for 66/11kV Power Transformer = No : 11.5 - Page # 235 No : 11.5

66/11kV Power Transformer Technical Data Sheet = No : 12.1 - Page # 240

66kV Power Cable Technical Data sheet = No : 12.3 - page # 251

 CONTRACT NO. GTC/488G/2012 Doc No. PH11-8G-238-33-C001
66/11kV POLICE TRAINING INSTITUTE SUBSTATION Rev. No. A
66kV CT / VT SIZING CALCULATION Date 30-Aug-15

INPUTS

CT & VT Secondary Cable Technical Data Sheet = No : 12.4 - Page # 253

ESI Standard 48-3 = No : 12.7 - Page # 259

CONTRACT NO. GTC/488G/2012 Doc No. PH11-8G-238-33-C001
66/11kV POLICE TRAINING INSTITUTE SUBSTATIONRev. No. A
66kV CT / VT SIZING CALCULATION Date 30-Aug-15

INDEX
REFERENCE
No : 2.5

HV BCT-HV REF protection

HV-BCT-Core-1 - 7SJ61

B11 & B21

800-400/1 A

CLASS : PX
 CONTRACT NO. GTC/488G/2012 Doc No. PH11-8G-238-33-C001
66/11kV POLICE TRAINING INSTITUTE SUBSTATIONRev. No. A
66kV CT / VT SIZING CALCULATION Date 30-Aug-15

A. CT Sizing Calculation for 66/11kV Power Transformer Feeders: HV-BCT-Core-1 : B11 & B21

Core Details

HV-BCT-Core-1 :B11 & B21

Device Purpose of connected

HV REF - 7SJ61

2.5 HV-BCT-Core-1 : Sizing :

2.5.1 Basic System Data

a. Nominal system voltage V : 66 kV

b. System frequency fn : 50 Hz

c. Maximum fault current If : 31.5 kA

2.5.2 Relay details

a. Make : Siemens

b. Model : 7SJ61

c. Rated current IR : 1 A

d. Rated burden 7SJ61 SR : 0.05 VA (Refer No : 10.1- Page # 197 )

2.5.3 Details of lead from CT to Relay

a. Size of the cable from CT to relay S = 4 Sqmm

b. Resistance of the 4 Sqmm cable R = 0.0056 Ohm/m (Refer No : 1.2 -Page # 7)

c. length of the cable from CT to Relay panel L = 100 mtrs

d. Material of the cable = Copper

e. Total lead loop resistance Rlead = 2 x (R x L)

= 2 x (0.0056 x 100)

Rlead = 1.12 Ohms

2.5.4 Determination of CT parameters

a. Rated Primary Current = 800-400 A

b. Rated Secondary Current = 1 A

 CONTRACT NO. GTC/488G/2012 Doc No. PH11-8G-238-33-C001
66/11kV POLICE TRAINING INSTITUTE SUBSTATIONRev. No. A
66kV CT / VT SIZING CALCULATION Date 30-Aug-15
c. Class = PX
 CONTRACT NO. GTC/488G/2012 Doc No. PH11-8G-238-33-C001
66/11kV POLICE TRAINING INSTITUTE SUBSTATIONRev. No. A
66kV CT / VT SIZING CALCULATION Date 30-Aug-15

As per technical specification, volume-3, section 3.3.8, rev-3, clause 2.1.30, page 48 of 171, CT maximum secondary
resistance shall be 3.5 ohms per 1000 turn of CT secondary winding. Since the selected CT ratio is 800-400/1, hence the
maximum secondary winding resistance shall be as follows,

e. Winding Resistance for 800/1 ratio CT

RCT = 800 x 3.5/1000
at 75⁰ C

= 2.8 Ohm

Proposed Winding Resistance for 800/1 ratio CT = 2.8 Ohm

RCT
at 75⁰ C

Winding Resistance for 400/1 ratio CT = 400 x 3.5/1000

RCT
at 75⁰ C

= 1.4 Ohm

Proposed Winding Resistance for 400/1 ratio CT = 1.4 Ohm

RCT
at 75⁰ C

f. Resistance of the protection relay Rrelay = (SR/IR2)

= ( 0.05 / 1 )

Rrelay = 0.05 Ohm

g. Total Secondary resistance for 800/1

Rsb = RCT+RI+Rrelay
ratio CT

= 2.8+1.12 + 0.05

= 3.97 Ohm

Total Secondary resistance for 400/1 =

Rsb RCT+RI+Rrelay
ratio CT

= 1.4+1.12 + 0.05

= 2.57 Ohm
2.5.5 Fault level Particulars
Maximum through fault in 66kV Side If = 2379 A ( Refer No : 11.4 - Page # 213)

2.5.6 REF Protection

As per ESI standard EAT-48-3, the knee point voltage shall be calculated using

Formula used
Vk = 2 * If * (RCT+ R'b) * T ( Refer No : 12.7 - Page # 259)
R'b = Total external load resistance
= Rlead + Rrelay = 1.12 + 0.05 = 1.17 ohm
RCT = Winding Resistance of CT = 2.8 ohm for 800/1 Ratio
= 1.4 ohm for 400/1 Ratio
 CONTRACT NO. GTC/488G/2012 Doc No. PH11-8G-238-33-C001
66/11kV POLICE TRAINING INSTITUTE SUBSTATIONRev. No. A
66kV CT / VT SIZING CALCULATION Date 30-Aug-15
T=Isn/Ipn = Transformation ratio
Ipn = CT rated primary current = 800-400 A
Isn = CT rated secondary current = 1 A
 CONTRACT NO. GTC/488G/2012 Doc No. PH11-8G-238-33-C001
66/11kV POLICE TRAINING INSTITUTE SUBSTATIONRev. No. A
66kV CT / VT SIZING CALCULATION Date 30-Aug-15

Knee point voltage for 800/1A ratio of CT:

Maximum Knee Point Voltage Vk = 2 * If * (RCT+ R'b) * T

= 2 * 2379 * (2.8 + 1.17) * (1/800)

VK ~ 24 V

Knee point voltage for 400/1A ratio of CT:

Maximum Knee Point Voltage Vk = 2 * If * (RCT+ R'b) * T

= 2 * 2379 * (1.4 + 1.17) * (1/400)

VK ~ 31 V

2.5.7 As per KM requirement

Knee point voltage for 800-400 / 1 ratio of CT using:

As per technical specification, volume-3, section 3.3.8, rev-3, clause 2.1.31

VS = K * If * (RCT +2Rl)

where
If = maximum through fault current
= If / Ipn
= For 800/1 = (31500) / 800 = 39.38 A
ratio800/1
For CT
= = (31500) / 400 = 78.75 A
ratio CT
2Rl = Total lead loop resistance = 1.12 ohm
RCT = Winding resistance of CT = 2.8 ohm for 800/1 Ratio
= 1.4 ohm for 400/1 Ratio
k = Safety factor = 1.5

Knee point voltage for 800/1A ratio of CT:

Minimum Knee Point Voltage Vk = K * If * (RCT +2Rl)

= 1.5 * 39.38 * ( 2.8 + 1.12)

Vk ~ 232 V

Knee point voltage for 400/1A ratio of CT:

Minimum Knee Point Voltage Vk = K * If * (RCT +2Rl)

= 1.5 * 78.75 * ( 1.4 + 1.12)

Vk ~ 298 V
 CONTRACT NO. GTC/488G/2012 Doc No. PH11-8G-238-33-C001
66/11kV POLICE TRAINING INSTITUTE SUBSTATIONRev. No. A
66kV CT / VT SIZING CALCULATION Date 30-Aug-15

2.5.8 Conclusion:

Calculating knee point voltage for 800/1A ratio of CT:

= Maximum of the calculated rated equivalent secondary
Hence rated equivalent secondary emf VK
emf of protection.

= Max (24 , 232)

~ 232 V

Knee point voltage with 20% design margin = 20% * 232 V

VK ~ 278 V

Calculating knee point voltage for 400/1A ratio of CT:

= Maximum of the calculated rated equivalent secondary
Hence rated equivalent secondary emf VK
emf of protection.

= Max (31 , 298)

~ 298 V

Knee point voltage with 20% design margin = 20% * 298 V

VK ~ 357 V

The Calculated CT Knee point voltage based on relay manufacturer is less than Proposed knee point voltage. Hence the given
CT parameters are adequate.

2.5.9 CT parameter chosen for HV-BCT-Core-1

Rated primary current Ipn : 800 A 400 A

Rated secondary current Isn : 1 A 1 A

Class : PX PX

Max. resistance of sec.winding at 75oC Rct : 2.8 Ω 1.4 Ω

Proposed winding resistance Rct 2.8 Ω 1.4 Ω

Calculated Knee point voltage based on relay manufacturer Vk : 24 V 31 V

proposed knee point voltage Vk 700 V 350 V

Proposed magnetizing current in (Vk/2) Ie : 15 mA 30 mA

Rated frequency Hz : 50 Hz 50 Hz

Rated Short Time Thermal Current for 1 sec kA : 31.5 kA 31.5 kA

Rated Short Continuous Thermal Current (LTH) IN% : 140 140

 CONTRACT NO. GTC/488G/2012 Doc No. PH11-8G-238-33-C001
66/11kV POLICE TRAINING INSTITUTE SUBSTATIONRev. No. A
66kV CT / VT SIZING CALCULATION Date 30-Aug-15

Dynamic Short Circuit Thermal Current IN% : 100 100

CONTRACT NO. GTC/488G/2012 Doc No. PH11-8G-238-33-C001
66/11kV POLICE TRAINING INSTITUTE SUBSTATIONRev. No. A
66kV CT / VT SIZING CALCULATION Date 30-Aug-15

INDEX
REFERENCE
No : 2.6

NCT-LV REF Protection

A18-NERCT1-1 - 7SJ61

B11 & B21

2500/1 A

CLASS : PX
 CONTRACT NO. GTC/488G/2012 Doc No. PH11-8G-238-33-C001
66/11kV POLICE TRAINING INSTITUTE SUBSTATIONRev. No. A
66kV CT / VT SIZING CALCULATION Date 30-Aug-15

A. CT Sizing Calculation for 66/11kV Power Transformer Feeders: HV-BCT-Core-1 : B11 & B21

Core Details

A18-NERCT1-1 :B11 & B21

Device Purpose of connected

LV REF - 7SJ61

2.6 A18-NERCT1-1 : Sizing :

2.6.1 Basic System Data

a. Nominal system voltage V : 11 kV

b. System frequency fn : 50 Hz

c. Maximum fault current If : 31.5 kA

2.6.2 Relay details

a. Make : Siemens

b. Model : 7SJ61

c. Rated current IR : 1 A

d. Rated burden 7SJ61 SR : 0.05 VA (Refer No : 10.1- Page # 197 )

2.6.3 Details of lead from CT to Relay

a. Size of the cable from CT to relay S = 4 Sqmm

b. Resistance of the 4 Sqmm cable R = 0.0056 Ohm/m (Refer No : 1.2 -Page # 7)

c. length of the cable from CT to Relay panel L = 100 mtrs

d. Material of the cable = Copper

e. Total lead loop resistance Rlead = 2 x (R x L)

= 2 x (0.0056 x 100)

Rlead = 1.12 Ohms

2.6.4 Determination of CT parameters

a. Rated Primary Current = 2500 A

b. Rated Secondary Current = 1 A

 CONTRACT NO. GTC/488G/2012 Doc No. PH11-8G-238-33-C001
66/11kV POLICE TRAINING INSTITUTE SUBSTATIONRev. No. A
66kV CT / VT SIZING CALCULATION Date 30-Aug-15

c. Class = PX
 CONTRACT NO. GTC/488G/2012 Doc No. PH11-8G-238-33-C001
66/11kV POLICE TRAINING INSTITUTE SUBSTATIONRev. No. A
66kV CT / VT SIZING CALCULATION Date 30-Aug-15

As per technical specification, volume-3, section 3.3.8, rev-3, clause 2.1.30, page 48 of 171, CT maximum secondary
resistance shall be 3.5 ohms per 1000 turn of CT secondary winding. Since the selected CT ratio is 2500/1, hence the
maximum secondary winding resistance shall be as follows,

e. Winding Resistance for 2500/1 ratio CT

RCT = 2500 x 3.5/1000
at 75⁰ C

= 8.75 Ohm

Proposed Winding Resistance for 2500/1 ratio CT = 8.75 Ohm

RCT
at 75⁰ C

f. Resistance of the protection relay Rrelay = (SR/IR2)

= ( 0.05 / 1 )

Rrelay = 0.05 Ohm

Total Secondary resistance for 2500/1 = RI+Rrelay+RCT

Rsb
ratio CT

= 1.12 + 0.05+8.75

= 9.92 Ohm

2.6.5 Fault level Particulars

Maximum through fault in 11kV Side If = 14271 A ( Refer No : 11.4 - Page # 213)

2.6.6 REF Protection

As per ESI standard EAT-48-3, the knee point voltage shall be calculated using

Formula used

Vk = 2 * If * (R'b + RCT) * T ( Refer No : 12.7 - Page # 259)

R'b = Total External Load Resistance

= Rlead + Rrelay = 1.12 + 0.05 = 1.17 Ohms
RCT = Winding Resistance of CT = 8.75 Ohms
T=Isn/Ipn = Transformation ratio
Ipn = CT rated primary current = 2500 A
Isn = CT rated primary current = 1 A

Knee point voltage for 2500/1A ratio of CT:

Maximum Knee Point Voltage Vk = 2 * If * (R'b + RCT) * T

= 2 * 14271 * (1.17 + 8.75) * (1/2500)

CONTRACT NO. GTC/488G/2012 Doc No. PH11-8G-238-33-C001
66/11kV POLICE TRAINING INSTITUTE SUBSTATIONRev. No. A
66kV CT / VT SIZING CALCULATION Date 30-Aug-15

VK ~ 113 V
 CONTRACT NO. GTC/488G/2012 Doc No. PH11-8G-238-33-C001
66/11kV POLICE TRAINING INSTITUTE SUBSTATIONRev. No. A
66kV CT / VT SIZING CALCULATION Date 30-Aug-15

2.6.7 As per KM requirement

Knee point voltage for 2500 / 1 ratio of CT using:

As per technical specification, volume-3, section 3.3.8, rev-3, clause 2.1.31

VS > K * If * (RCT +2Rl)

where
If1 = maximum through fault current
= If / Ipn
For 2500/1
= = (31500) / 2500 = 12.6 A
ratio CT
2Rl = Total lead loop resistance = 1.12 ohm
RCT = Winding Resistance of CT = 8.75 ohm
k = Safety factor = 1.5

Knee point voltage for 2500/1A ratio of CT:

Minimum Knee Point Voltage Vk = K * If * (RCT +2Rl)

= 1.5 * 12.6 * ( 8.75 + 1.12)

Vk ~ 187 V

2.6.8 Conclusion:

Calculating knee point voltage for 2500/1A ratio of CT:

= Maximum of the calculated rated equivalent secondary

Hence rated equivalent secondary emf VK
emf of protection.

= Max (113 , 187)

~ 187 V

Knee point voltage with 20% design margin = 20% * 187 V

VK ~ 224 V

The calculated CT Knee point voltage based on KM requirement is greater than relay manufacturer knee point voltage. Hence
we recommend CT knee point voltage are subject to relay manufacturer.
 CONTRACT NO. GTC/488G/2012 Doc No. PH11-8G-238-33-C001
66/11kV POLICE TRAINING INSTITUTE SUBSTATIONRev. No. A
66kV CT / VT SIZING CALCULATION Date 30-Aug-15

2.6.9 CT parameter chosen for A18-NERCT1-1

Rated primary current Ipn : 2500 A

Rated secondary current Isn : 1 A

Class : PX

Max. resistance of sec.winding at 75oC Rct : 8.75 Ω

Proposed winding resistance Rct : 8.75 Ω

Calculated Knee point voltage based on relay manufacturer Vk : 113 V

Proposed Knee point voltage Vk : 800 V

Proposed magnetizing current in (Vk/2) Ie : 15 mA

Rated frequency Hz : 50 Hz

Rated Short Time Thermal Current for 1 sec kA : 31.5 kA

CONTRACT NO. GTC/488G/2012 Doc No. PH11-8G-238-33-C001
66/11kV POLICE TRAINING INSTITUTE SUBSTATION Rev. No. A
66kV CT / VT SIZING CALCULATION Date 30-Aug-15

INDEX
REFERENCE
No : 2.7

NCT-Standby Earth Fault

A18-NERCT1-2 - 7SJ61

B11 & B21

750/1 A

CLASS : 5P20
 CONTRACT NO. GTC/488G/2012 Doc No. PH11-8G-238-33-C001
66/11kV POLICE TRAINING INSTITUTE SUBSTATION Rev. No. A
66kV CT / VT SIZING CALCULATION Date 30-Aug-15

A. CT Sizing Calculation for 66/11kV Power Transformer Feeders: HV-BCT-Core-1 : B11 & B21

Core Details

A18-NERCT1-2 :B11 & B21

Device Purpose of connected

SBEF - 7SJ61

2.7 A18-NERCT1-2 : Sizing :

2.7.1 Basic System Data

a. Nominal system voltage V : 11 kV

b. System frequency fn : 50 Hz

c. Maximum fault current If : 31500 A

2.7.2 Relay details

a. Make : Siemens

b. Model : 7SJ61

c. Rated current IR : 1 A

d. Rated burden SR1 : 0.05 VA (Refer No : 10.1- Page # 197 )

2.7.3 Details of lead from CT to Relay

a. Size of the cable from CT to relay S = 4 Sqmm

b. Resistance of the 4 Sqmm cable R = 0.00560 Ohm/m (Refer No : 1.2 -Page # 7)

c. length of the cable from CT to Relay panel L = 100 mtrs

d. Material of the cable = Copper

e. Total Loop resistance Rlead = 2x (R x L)

= 2 x (0.0056 x 4)

Rlead = 1.12 Ohms

2.7.4 Determination of CT parameters

a. Rated Primary Current = 750 A

 CONTRACT NO. GTC/488G/2012 Doc No. PH11-8G-238-33-C001
66/11kV POLICE TRAINING INSTITUTE SUBSTATION Rev. No. A
66kV CT / VT SIZING CALCULATION Date 30-Aug-15

b. Rated Secondary Current = 1 A

c. Class = 5P20

d. VA Burden = 20 VA

As per technical specification, volume-3, section 3.3.8, rev-3, clause 2.1.30, page 48 of 171, CT maximum secondary
resistance shall be 3.5 ohms per 1000 turn of CT secondary winding. Since the selected CT ratio is 750/1, hence the maximum
secondary winding resistance shall be as follows,

e. Winding Resistance for 750/1 ratio CT

RCT = 750 x 3.5/1000
at 75⁰ C

= 2.63 Ohm

Proposed Winding Resistance for 750/1 ratio CT = 2.63 Ohm

RCT
at 75⁰ C

f. Resistance of the protection relay Rrelay = (SR/IR2)

= ( 0.05 / 1 )

Rrelay = 0.05 Ohm

2.7.5 VA Burden and Accuracy class of CT

Actual Burden Connected Sb'cal = (RI+Rrelay) * IR2

= (1.12+ 0.05) * 1^2

= 1.17

Sb'req ≥ Sb'cal

For 750 ratio CT 20 VA > 1.17 VA

2.7.6 VA Burden and accuracy class of CT as per KM technical specification

As technical specification, volume-3, section 3.3.8, rev-2 clause 2.1.23. item no, the CT continuous VA burden is given by,

VACT > 1.2 * In2 * (2RI+Rr)

= 1.2 * 1^2 * ( 1.12 + 0.05)

= 1.40 VA
As technical specification,volume-3 section 3.3.8, rev-3, clause 2.1.31, item no.3, minimum 15VA burden has to be choosen,
But we are adoping 20VA for 750/1 ratio as the required burden. Hence,

Sb'req ≥ Sb'cal
 CONTRACT NO. GTC/488G/2012 Doc No. PH11-8G-238-33-C001
66/11kV POLICE TRAINING INSTITUTE SUBSTATION Rev. No. A
66kV CT / VT SIZING CALCULATION Date 30-Aug-15

VA > 1.40 VA

Total Actual burden connected Sb'cal = max(1.17,1.4)

= 1.40 VA
 CONTRACT NO. GTC/488G/2012 Doc No. PH11-8G-238-33-C001
66/11kV POLICE TRAINING INSTITUTE SUBSTATION Rev. No. A
66kV CT / VT SIZING CALCULATION Date 30-Aug-15

2.7.7 Adequacy Checking of 5P20

The 5P20 class CT shall comply within composite error of ± 5% for 20 times the rated CT current flow in its winding at rated
burden

For 750 ratio CT

Accuracy limit at rated burden ALFr = 20

Rated burden of CT VAr = 20 VA

Actual burden connected Sb'cal = 1.17 VA

Internal burden of CT for 4/1 ratio CT Sct = Rct * In2

= 2.63 * 1^2

= 2.63 VA

Accuracy limit at actual burden for 4/1 ratio CT ALFa = ALFr*(Var+Sct)/(Sb'cal+Sct)

= 20 * (20 + 2.63) / (1.17 + 2.63)

~ 119

Accuracy limit current refered to the secondary ALF'a = Accuracy limit at actual burden * IR
of CT for 5P20
= 119 * 1

~ 119 A

CT secondary current during max system fault current = Issmax/Ipn

= 31500 / 750

~ 42

ALF'a > Imax

119 ≥ 42

The symmetrical circuit factor at actual burden is grater than CT secondary current at max system fault level, Hence the
proposed CT paramete are adequate.
 CONTRACT NO. GTC/488G/2012 Doc No. PH11-8G-238-33-C001
66/11kV POLICE TRAINING INSTITUTE SUBSTATION Rev. No. A
66kV CT / VT SIZING CALCULATION Date 30-Aug-15

2.7.8 CT parameter chosen for A18-NERCT1-2

Description 750/1

Rated Primary Current Ipn(A) : 750

Rated secondary Current Isn(A) : 1

Class : 5P20

Max.resistance of sec.winding at 75⁰ C Rct(Ω) ≤ 2.63

Proposed winding resistance Rct(Ω) ≤ 2.63

Calculated Burden Based on the relay VA : 1.17

manufacturer

Proposed Burden VA : 20

Rated frequency Hz : 50

Rated short time thermal current for 1 Sec kA : 31.5

Rated Short Continuous Thermal Current (LTH) IN% : 120

Dynamic Short Circuit Thermal Current IN% : 100

 CONTRACT NO. GTC/488G/2012 Doc No. PH11-8G-238-33-C001
66/11kV POLICE TRAINING INSTITUTE SUBSTA Rev. No. A
66kV CT / VT SIZING CALCULATION Date 30-Aug-15

INDEX
REFERENCE
No : 11.4

Fault Current Calculation for 66/11kV

Power Transformer
 CONTRACT NO. GTC/488G/2012 Doc No. PH11-8G-238-33-C001
66/11kV POLICE TRAINING INSTITUTE SUBSTA Rev. No. A
66kV CT / VT SIZING CALCULATION Date 30-Aug-15

11.4 Fault Current Calculation for 66/11kV Power Transformer

a. Source Particulars.

Fault level Ikmax = 31500 Amps

Voltage (L-L) Vsp = 66000 Volts

X/R ratio = 18.88

Source impedance angle Q = ArcTan(X/R)

= ArcTan(18.88)

= 86.9681

Cos Q = 0.053

Sin Q = 0.999

b. Respective source/system particulars

Impedance ZS = VSP/(Ikmax x 1.732)

= 66000/(31500 x 1.732)

= 1.21 Ohms

Resistance RS = Zs x Cos Q

= 1.21 x 0.053

= 0.06 Ohms

Reactance Xs = Zs x Sin Q

= 1.21 x 0.999

= 1.208 Ohms

c. Transformer Parameter

Rating of transformer = 40 MVA

Impedance of transformer %Z = 13.6 %

Base Impedance of transformer referred to Zbt = (V^2 / MVA)

66kV side is given by,
= 109 Ohms

Actual impedance for close-in fault referred to Ztr = Zbt*(%Z/100)

66kV side is given by,
 CONTRACT NO. GTC/488G/2012 Doc No. PH11-8G-238-33-C001
66/11kV POLICE TRAINING INSTITUTE SUBSTA Rev. No. A
Actual impedance for close-in
66kVfault
CT referred to
/ VT SIZING CALCULATION Date 30-Aug-15
66kV side is given by,
= 14.81 Ohms
 CONTRACT NO. GTC/488G/2012 Doc No. PH11-8G-238-33-C001
66/11kV POLICE TRAINING INSTITUTE SUBSTA Rev. No. A
66kV CT / VT SIZING CALCULATION Date 30-Aug-15

X/R ratio of transformer = 30.48

Impedance angle, Qtr = atan(X/R) Qtr = tan-1(30.48)

Qtr = 88.12 degree

Resistance of transformer, Rtr = Ztr *CosQtr Rtr = 14.81 * Cos (88.12)

Rtr = 0.49 Ohm

Reactance of transformer , Xtr = Ztr * SinQtr Xtr = 14.81 * Sin (88.12)

Xtr = 14.80 Ohm

d. Through fault current calculation for 3 phase fault (66kV Side)

Resistance RTR3F = RS +Rtr

= 0.064+0.49

= 0.550 ohms

Reactance XTx3F = XS + Xtr

= 1.208 + 14.802

= 16.011 Ohms

Impedance ZTZ3F = SQRT (RTR3F2 + XTX3F2)

= SQRT (0.302 + 256.344)

= 16.020 Ohms

Thorugh fault current (3 Phase) Ithr3ph = VSP/(SQRT(3) x (ZTZ3F))

= 66000/(SQRT(3) x (16.02))

Ithr3ph = 2378.57 A

X/R ration for 3 phase through fault X/R (thr3ph) = 16.011 / 0.55

= 29.127

i. Through fault current calculation for 1 phase fault (66kV Side)

Total Impedance = Positive sequence impedance +Negative sequence impedance+Zero sequence impedance.

Resistance RTR1F = 3 x (Rs + Rtr)

= 3 * (0.06 + 0.49)
CONTRACT NO. GTC/488G/2012 Doc No. PH11-8G-238-33-C001
66/11kV POLICE TRAINING INSTITUTE SUBSTA Rev. No. A
66kV CT / VT SIZING CALCULATION Date 30-Aug-15
= 1.65 Ohms
 CONTRACT NO. GTC/488G/2012 Doc No. PH11-8G-238-33-C001
66/11kV POLICE TRAINING INSTITUTE SUBSTA Rev. No. A
66kV CT / VT SIZING CALCULATION Date 30-Aug-15

Reactance XTR1F = 3 x (Xs + Xtr)

= 3 * (1.21 + 14.8)

= 48.03 Ohms

Impedance ZTR1F = SQRT ( RTR1F2 + XTX1F2)

= SQRT (1.649^2 + 48.032^2)

= 48.0605 Ohms

Through fault current (1 phase) Ithr1ph = 3 x Vsp/ (SQRT (3) x ZTRZ1F)

= 3 x 66000/ (SQRT (3) x 48.061)

Ithr1ph = 2379 Amps

X/R ratio for single phase through fault X/Rthr1ph = 48.032 / 1.649

= 29.13

j. Through fault current calculation (11kV Side)

As per transformer ratio the maximum 3 ph fault

current seen on the 11kV side shall be 6 times of the = 6 * If
through fault current on 66kV side

= 6 * 2379

= 14271.4 A
 CONTRACT NO. GTC/488G/2012 Doc No. PH11-8G-238-33-C001
66/11kV POLICE TRAINING INSTITUTE SUBSTARev. No. A
66kV CT / VT SIZING CALCULATION Date 30-Aug-15

INDEX
REFERENCE
No : 11.5

X/R Ratio for 66/11kV Power Transformer

 CONTRACT NO. GTC/488G/2012 Doc No. PH11-8G-238-33-C001
66/11kV POLICE TRAINING INSTITUTE SUBSTARev. No. A
66kV CT / VT SIZING CALCULATION Date 30-Aug-15

11.5 Calculation of X/R Ratio for 32/40 MVA, 66/11kV Power Transformer

Total Load Losses (Max load loss at ONAF + = 216+7 W (Refer No : 12.1 - Page # 240)
Total auxiliary loss @ full load)

= 223 W

Capacity = 40 MVA (Refer No : 12.1 - Page # 240)

Percentage impedance at principal tap & at = 13.6 %

32 MVA base(%Z)

Percentage impedance at principal tap & at = 40/ 32 *13.6

40 MVA base(%Z)

= 17 %

Percentage resistance at principal tap & at = (Load losses/Capacity) x 100

40MVA base(%R)
= ( (223*10^3)/(40*10^6))*100

= 0.56 %

Percentage reactance at principal tap & at = Sqrt(Z2-R2)

40MVA base(%X)
= Sqrt(17^2 -0.5575^2)

= 16.99 %

Hence X/R ratio of transformer = 16.99 / 0.5575

= 30.48
CONTRACT NO. GTC/488G/2012 Doc No. PH11-8G-238-33-C001
66/11kV POLICE TRAINING INSTITUTE SUBSTA Rev. No. A
66kV CT / VT SIZING CALCULATION Date 30-Aug-15

INDEX
REFERENCE
No : 11.6

X/R Ratio for 66kV Circuit Breaker

 CONTRACT NO. GTC/488G/2012 Doc No. PH11-8G-238-33-C001
66/11kV POLICE TRAINING INSTITUTE SUBSTA Rev. No. A
66kV CT / VT SIZING CALCULATION Date 30-Aug-15

11.6 Calculation of X/R Ratio for 66kV Circuit Breaker

a. Rated Current = 1250 Amps (Refer No : 12.2 - Page # 244)

Frequency = 50 Hz

Short Circuit breaking current = 31500 Amps

According to IEC publication 62271-100, the percentage value of the dc component (%dc)
is based on the time interval (Top + Tr) and the time constant τ using the formula:

%D.C. = 100 * e^ (-(Top+Tr) / τ)

Where:

Top = minimum opening time of the first opening pole of the circuit breaker.

Top = 0.038 sec (Refer No : 12.2 - Page # 244)

Tr = one half-cycle of rated frequency (for 50Hz) = 10 msec ( Refer IEC 62271-100-Page#265 )

τ = Time constant of 66kV system

% D.C. value for 66kV = 45 %

τ = -(Top+Tr) / (ln (%DC/100))

= -(0.038 + 0.01) / (In (45/100))

τ = 60.1121 msec

X/R ratio = 2x πxfxτ

= 2 x π x 50 x (60.11/1000)

X/R ratio = 18.88

 CONTRACT NO. GTC/488G/2012 Doc No. PH11-8G-238-33-C001
66/11kV POLICE TRAINING INSTITUTE SUBSTATIORev. No. A
66kV CT / VT SIZING CALCULATION Date 30-Aug-15

INDEX
REFERENCE
No : 1.2

Calculation of Secondary lead resistance

 CONTRACT NO. GTC/488G/2012 Doc No. PH11-8G-238-33-C001
66/11kV POLICE TRAINING INSTITUTE SUBSTATIORev. No. A
66kV CT / VT SIZING CALCULATION Date 30-Aug-15

1.2 Calculation of Lead Resistance of 4 Sqmm. Cable

Resistance of 4 Sqmm Copper Cable @ 20°C (R20) = 4.61 ohm/km (Refer No : 12.4 - Page # 253)

Resistance of 4 Sqmm Copper Cable @ 75°C (R75) = R20 (1+α (t2 - t1))

Where,

α = Temparature Coefficient of copper = 0.003930

t1 = Temparature at 20°C
t2 = Temparature at 75°C

Resistance of 4 Sqmm Copper Cable @ 75°C (R75) = 4.61 * (1 + 0.00393 * (75-20))

= 5.60645 ohm/km

= 0.0056 ohm/m