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 B12-B22
Feeder details Bus Section
Nominal System Voltage 66 kV
System frequency 50 Hz
Maximum fault current 31.5 kA

Relay Details Make Model Rated Burden Annexure Purpose of conncted

Current (IR) (VA) Ref No

Over Current & Earth Fault Protection Siemens 7SJ61 1 0.05 No : 6.1 Over Current & Earth Fault Protection
Bay Control Unit Siemens 6MD6 1 0.1 Bay Control Unit
Busbar Protection Siemens 7SS522 1 0.1 No : 6.2 Busbar Protection
Ammeter ABB Fb 96 1 0.8 Ammeter
ICT ABB MT 1 2.5 Metering
Busbar Protection Siemens 7SS522 1 0.1 No : 6.3 Busbar 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 0 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
factor(IFC) resistance( current(mA) Knee Point
voltage(V)
/Krpa
factor
a Factor
ohm)

CT2 : Core-1 7SJ61 2000 1 5P20/1.0 15 7 15 Over Current & Earth Fault Protection
BCU Bay Control Unit
Ammeter

CT2 : Core-2 7SS522 2000 1 PX 7 15 250 0.5 Bus Bar Protection

CT1 : Core-1 7SS522 2000 1 PX 7 15 250 0.5 Bus Bar Protection

 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

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) for Differential Protection
Safty factor (K) For Over current & earh fault

Annexure Details

Relay details Relay Burden Details CT Requirement Formula Details

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

6MD6 = No : 10.1 - Page # 197 No : 9.1 - Page # 188
7SS522 = No : 10.1 - Page # 197 No : 9.1 - Page # 188
7SS522 = No : 10.1 - Page # 197 No : 9.1 - Page # 188
Ammeter = No : 10.4 - Page # 208

Interpossing CT = No : 12.4 - Page # 253

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

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

CONTRACT NO. GTC/488G/2012 Doc No. PH11-8G-238-33-C001
66/11kV POLICE TRAINING INSTITUTE SUB Rev. No. A
66kV CT / VT SIZING CALCULATION Date 30-Aug-15
 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 : 6.1

Over Current & Earth Fault Protection,Bay

Control Unit & Ammeter

CT2 : Core-1 - 7SJ61 and 6MD6 & Fb 96

B12-B22

2000/1 A

CLASS : 5P20/1.0
 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 66kV Bus Section : B12-B22

Core Details
CT2 : Core-1 : B12-B22

Device Purpose of connected

Over Current & Earth Fault Protection - 7SJ61
Bay Control Unit - 6MD6
Ammeter -Fb 96

6.1 CT2 : Core-1 Sizing :

6.1.1 Basic System Data

a. Nominal system voltage V : 66 kV
b. System frequency fn : 50 Hz
c. Maximum fault current If : 31500 A

6.1.2 Relay details

a. Make : Siemens and Siemens and ABB

b. Model : 7SJ61 and 6MD6 and Fb 96

c. Rated current IR : 1 A

d. Rated burden for 7SJ61 SR1 : 0.05 VA (Refer No : 10.1 - Page # 197 )

e. Rated burden for 6MD6 SR2 : 0.1 VA (Refer No : 10.1 - Page # 197 )

f. Rated burden for Ammeter SR3 : 0.8 VA (Refer No : 10.4 - Page # 208)

Total relay burden SR : SR1 + SR2 + SR3

: 0.05+0.1+0.8

SR : 0.95 VA
6.1.3 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 cable from CT to Relay L = 100 mtrs

d. Material of the cable = Copper

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

= 2 x (0.0056 x 100)

Rlead = 1.12 Ohms

 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

6.1.4 Determination of CT parameters

a. Rated Primary Current = 2000 A

b. Rated Secondary Current = 1 A

c. Class = 5P20/1.0

d. VA Burden = 15 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.

e. Max. Sec. Winding Resistance at 75o C = 2000 x 3.5/1000

RCT = 7 Ω

Proposed Winding Resistance at 75o C RCT = 7 Ω

CT internal Burden in VA = I2*Rct

= 7 VA

f. Resistance of the protection relay Rr = SR / IR2

= 0.95/1

= 0.95 Ω
6.1.5 VA Burden and Accuracy class of CT
Actual Burden Connected Sb'cal = (RI+Rrelay) * IR2

= (1.12+ 0.95) * 1^2

= 2.07 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, Hence,

Sb'req ≥ Sb'cal

15 VA ≥ 2.07 VA

6.1.6 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 + 0.95)

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

= 2.48 VA
 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 technical specification,volume-3 section 3.3.8, rev-3, clause 2.1.31, item no.3, minimum 15VA burden has to be
choosen. Hence,

Sb'req ≥ Sb'cal

15 VA ≥ 2.48 VA

Total Actual burden connected Sb'cal = max(2.07,2.48)

= 2.48 VA

6.1.7 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

Accuracy limit at rated burden ALFr = 20

Rated burden of CT VAr = 15 VA

Actual burden connected Sb'cal = 2.07 VA

Internal burden of CT for 2000/1 ratio CT Sct = Rct * In2

= 7 * 1^2

= 7 VA

Accuracy limit at actual burden for 2000/1 ratio CT ALFa = ALFr*(Var+Sct)/(Sb'cal+Sct)

= 20 * (15 + 7) / (2.07 + 7)

~ 49

Accuracy limit current refered to the secondary of ALF'a = Accuracy limit at actual burden * IR
CT for 5P20
= 49 * 1

~ 49 A

Imax = 31500/ 2000

= 15.75

ALF'a > Imax

49 VA ≥ 15.75 VA

Since the accuracy limit current at actual burden is greater than the CT secondary current at max. fault level.The composite
error will be with in limit. Hence proposed CT parameter are adequate.
 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

6.1.8 CT parameters chosen:

Description 2000/1 A

Rated primary current Ipn : 2000 A

Rated secondary current Isn : 1 A

Class : 5P20/1.0

Max. resistance of sec.winding at 75oC Rct : 7 Ω

Proposed winding resistance Rct 7 Ω

Calculated Burden based on relay manufacturer VA : 2.07 VA

Proposed Burden VA 15 VA

Rated frequency HZ : 50 Hz

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

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 SUBSTATION Rev. No. A
66kV CT / VT SIZING CALCULATION Date 30-Aug-15

INDEX
REFERENCE
No : 6.2

Busbar Protection

CT2 : Core-2 - 7SS522

B12-B22

2000/1 A

CLASS : PX
 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 66kV Bus Section Feeders: B12-B22

Core Details

CT2 : Core-2 : B12-B22

Device Purpose of connected

Busbar Protection - 7SS522

6.2 CT2 : Core-2 Sizing :

6.2.1 Basic System Data

a. Nominal system voltage V : 66 kV

b. System frequency fn : 50 Hz

c. Maximum fault current If : 31.5 kA

6.2.2 Relay details

a. Make : Siemens

b. Model : 7SS522

c. Rated current IR : 1 A

d. Rated burden SR : 0.1 VA (Refer No : 10.1 - Page # 197 )

6.2.3 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

6.2.4 Determination of CT parameters

a. Rated Primary Current = 2000 A

b. Rated Secondary Current = 1 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
c. Class = PX
 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

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 2000/1, hence the
maximum secondary winding resistance shall be as follows,

e. Winding Resistance for 2000/1 ratio CT

Rct = 2000 x 3.5/1000
at 75⁰ C

= 7 Ohm

Proposed Winding Resistance for 2000/1 ratio CT 7 Ohm

Rct =
at 75⁰ C

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

= ( 0.1 / 1 )

Rrelay = 0.1 Ohm

g. Total Secondary resistance for 2000/1

Rsb = RCT+RI+Rrelay
ratio CT

= 7 + 1.12 + 0.1

= 8.22 Ohm
6.2.5 Differential Cheak

Formula Used:

Vkmin = K'ssc*(RCT+R'b)*Isn/1.3 case-1 (Refer No : 9.1 - Page # 188 )

K'ssc = Ktd * Isccmax_ext_fault / Ipn

Vkmin = 100/1.3*(RCT+R'b)*Isn case-2

Where,
Vkmin = Knee point voltage
Iscc_max_ext_fault = maximum symmetrical external short-circuit current = 31500 A
Ipn = CT rated primary current = 2000 A
RCT = Winding resistance of CT = 7 ohm
R'b = Total External load Resistance
= Rlead + Rrelay = 1.12 + 0.1 = 1.22 ohm
Isn = CT rated secondary current = 1 A
Transient dimensioning factor (Ktd) = 0.5 (Refer No : 9.1 - Page # 188 )

Case-1: Knee point voltage for /1A ratio of CT:

Minimum required Symmetrical Short circuit K'ssc ≥ Ktd * Isccmax_ext_fault / Ipn

Factor: (Minimum Required ALF)
= 0.5 * (31500/ 2000)
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

= 7.88
 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

Minimum required Knee point voltage based on Vkmin ≥ K'ssc*(RCT+R'b)*Isn/1.3

Minimum required ALF:
= 7.88 * (7 + 1.22) * 1/ 1.3

~ 50 V

Knee point voltage with 20% design margin Vk = 20% * 50

Vk ~ 60 V

Case -2: As per manufacturer recommendation "K'SSC ≤ 100 "

K'ssc 7.88 ≤ 100

Vk ≤ (100/1.3)*(Rct+R'b)*Isn

Vkmin 60 ≤ 100/1.3*(RCT+R'b)*Isn

= 100 / 1.3* (7 + 1.22) * 1

~ 632 V
Hence the case-1 condition is satified

As per KM Technical specification

As per Kahramaa technical specification, Vol-3, sec.3.3.8, Rev-3, Clause-2.1.31, Item-6, the minimum knee voltage

Minimum Knee Point Voltage Vk_min > K*In*(RCT+2Rl+Rr+Z)

where
Z = Ohmic burden on CT circuits of loads connected in series with the relay
zero ohms (since there are no loads connected in series to relay)
Rr = Resistance of protection Relay = 0.1 ohm
In = CT secondary current = 1 A
K = 20 or If/Ipn = 20
If = Maximum fault current = 31500 A
Ipn = Rated primary current = 2000 A
If/In = 31500/2000
= 15.75
2Rl = Total lead loop resistance = 1.12 ohm
RCT = Winding Resistance of CT = 7 ohm

Since If/In < 20, hence K=20 is adopted for knee point voltage calculation (Recommended by Relay Technical manual)

Vk = K*In*(RCT+2Rl+Rr+Z)

= 20 * 1 * (7 + 1.12 + 0.1 + 0)

Vk ~ 164 V

Knee point voltage with 20% design margin Vk = 20% * 164

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

Vk ~ 197 V
 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

Conclusion:

Hence calculated knee point voltage based on Relay manufacturer is less than proposed knee point voltage.Hence the given
CT parameters are adquate.

Hence, the selected maximum knee point

voltage is the maximum for 2000/1A ratio of VK = Maximum of the calculated rated equivalent
CT secondary emf

= MAX(60, 197)

Vk ~ 197 V

CT Magnetising current checking:

As per Kahramaa technical specification, Vol-3, sec.3.3.8, Rev-3, Clause-2.1.31, Item-6, the minimum knee voltage the CT
magnetising current is given by,

Ie = Vk/(20 *(RCT + 2Rl + B + Z))

where, `

Vk = knee voltage = 197 V

RCT = CT secondary winding resistance = 7 ohm
2Rl = Total lead loop resistance = 1.12 ohm
Z = Ohmic burden on CT circuits of loads connected in series with the relay = 0
zero ohms (since there are no loads connected in series to relay)

Calculated magnetizing current Ie = Vk/(20 *(RCT + 2Rl + B + Z))

= 197/(20 * (7 + 1.12 + 0 + 0))

= 1.21 A

Proposed magnetizing current = 15 mA

From the above calculation the magnetising current of proposed CT is less than the calculated magnetizing current.
 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

6.2. CT parameter chosen for :

Description 2000/1

Rated Primary Current Ipn(A) : 2000

Rated secondary Current Isn(A) : 1

Class : PX

Max.resistance of sec.winding at 75⁰ C Rct(Ω) ≤ 7

Proposed winding resistance Rct(Ω) ≤ 7

calculated Knee point voltage based on relay Vk ≥ 60

manufacturer

Proposed Knee point voltage Vk ≥ 250

Proposed Magnetizing current in (Vk/2) le(mA) ≤ 15

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 SUBSTATION Rev. No. A
66kV CT / VT SIZING CALCULATION Date 30-Aug-15

INDEX
REFERENCE
No : 6.3

Busbar Protection

CT1 : Core-1 - 7SS522

B12-B22

2000/1 A

CLASS : PX
 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 66kV Bus Section Feeders: B12-B22

Core Details

CT1 : Core-1 : B12-B22

Device Purpose of connected

Busbar Protection - 7SS522

6.3 CT1 : Core-1 Sizing :

6.3.1 Basic System Data

a. Nominal system voltage V : 66 kV

b. System frequency fn : 50 Hz

c. Maximum fault current If : 31.5 kA

6.3.2 Relay details

a. Make : Siemens

b. Model : 7SS522

c. Rated current IR : 1 A

d. Rated burden SR : 0.1 VA (Refer No : 10.1 - Page # 197 )

6.3.3 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

6.3.4 Determination of CT parameters

a. Rated Primary Current = 2000 A

b. Rated Secondary Current = 1 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
c. Class = PX
 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

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 2000/1, hence the
maximum secondary winding resistance shall be as follows,

e. Winding Resistance for 2000/1 ratio CT

Rct = 2000 x 3.5/1000
at 75⁰ C

= 7.00 Ohm

Proposed Winding Resistance for 2000/1 ratio CT at 7.00 Ohm

Rct =
75⁰ C

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

= ( 0.1 / 1 )

Rrelay = 0.1 Ohm

g. Total Secondary resistance for 2000/1

Rsb = RCT+RI+Rrelay
ratio CT

= 7 + 1.12 + 0.1

= 8.22 Ohm
6.3.5 Differential Cheak

Formula Used:

Vkmin = K'ssc*(RCT+R'b)*Isn/1.3 case-1 (Refer No : 9.1 - Page # 188 )

K'ssc = Ktd * Isccmax_ext_fault / Ipn

Vkmin = 100/1.3*(RCT+R'b)*Isn case-2

Where,
Vkmin = Knee point voltage
Iscc_max_ext_fault = maximum symmetrical external short-circuit current = 31500 A
Ipn = CT rated primary current = 2000 A
RCT = Winding resistance of CT = 7 ohm
R'b = Total External load Resistance
= Rlead + Rrelay = 1.12 + 0.1 = 1.22 ohm
Isn = CT rated secondary current = 1 A
Transient dimensioning factor (Ktd) = 0.5 (Refer No : 9.1 - Page # 188 )

Case-1: Knee point voltage for /1A ratio of CT:

Minimum required Symmetrical Short circuit K'ssc ≥ Ktd * Isccmax_ext_fault / Ipn

Factor: (Minimum Required ALF)
0.5 * (31500/ 2000)
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

= 7.88
 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

Minimum required Knee point voltage based on Vkmin ≥ K'ssc*(RCT+R'b)*Isn/1.3

Minimum required ALF:
= 7.88 * (7 + 1.22) * 1/ 1.3

~ 50 V

Knee point voltage with 20% design margin Vk = 20% * 50

Vk ~ 60 V

Case -2: As per manufacturer recommendation "K'SSC ≤ 100 "

K'ssc 7.88 ≤ 100

Vk ≤ (100/1.3)*(Rct+R'b)*Isn

Vkmin 60 ≤ 100/1.3*(RCT+R'b)*Isn

= 100 / 1.3* (7 + 1.22) * 1

~ 632 V
Hence the case-1 condition is satified

As per KM Technical specification

As per Kahramaa technical specification, Vol-3, sec.3.3.8, Rev-3, Clause-2.1.31, Item-6, the minimum knee voltage

Minimum Knee Point Voltage Vk_min > K*In*(RCT+2Rl+Rr+Z)

where
Z = Ohmic burden on CT circuits of loads connected in series with the relay
zero ohms (since there are no loads connected in series to relay)
Rr = Resistance of the protection Relay = 0.1 ohm
In = CT secondary current = 1 A
K = 20 or If/Ipn = 20
If = Maximum fault current = 31500 A
Ipn = Rated primary current = 2000 A
If/In = 31500/2000
= 15.75
2Rl = Total lead loop resistance = 1.12 ohm
RCT = Winding Resistance of CT = 7 ohm

Since If/In < 20, hence K=20 is adopted for knee point voltage calculation (Recommended by Relay Technical manual)

Vk = K*In*(Rct+2Rl+Rr+Z)

= 20 * 1 * (7 + 1.12 + 0.1 + 0)

Vk ~ 164 V

Knee point voltage with 20% design margin Vk = 20% * 164

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

Vk ~ 197 V
 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

Conclusion:

Hence calculated knee point voltage based on Relay manufacturer is less than proposed knee point voltage.Hence the given
CT parameters are adquate.

Hence, the selected maximum knee point

voltage is the maximum for 2000/1A ratio VK = Maximum of the calculated rated equivalent
of CT secondary emf

= MAX(0, 197)

Vk ~ 197 V

CT Magnetising current checking:

As per Kahramaa technical specification, Vol-3, sec.3.3.8, Rev-3, Clause-2.1.31, Item-6, the minimum knee voltage the CT
magnetising current is given by,

Ie = Vk/(20 *(RCT + 2Rl + B + Z))

where, `

Vk = knee voltage = 197 V

RCT = Winding resistance of CT = 7 ohm
2Rl = Total lead resistance of the cable = 1.12 ohm
Z = Ohmic burden on CT circuits of loads connected in series with the relay
zero ohms (since there are no loads connected in series to relay)

Calculated magnetizing current Ie = Vk/(20 *(RCT + 2Rl + B + Z))

= 197/(20 * (7 + 1.12 + 0 + 0))

= 1.21 A

Proposed magnetizing current = 15 mA

From the above calculation the magnetising current of proposed CT is less than the calculated magnetizing current.
 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

6.3.6 CT parameter chosen for :

Description 2000/1

Rated Primary Current Ipn(A) : 2000

Rated secondary Current Isn(A) : 1

Class : PX

Max.resistance of sec.winding at 75⁰ C Rct(Ω) ≤ 7

Proposed winding resistance Rct(Ω) ≤ 7

calculated Knee point voltage based on relay

Vk ≥ 60
manufacturer

Proposed Knee point voltage Vk ≥ 250

Proposed Magnetizing current in (Vk/2) le(mA) ≤ 15

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 SUBSTATI Rev. 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 SUBSTATI Rev. 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))

= 5.6065 ohm/km

= 0.0056 ohm/m