# CT Sizing

CT sizing...

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CT Parameters & Adequacy check calculations for 220kV Switchyard at Ranjangaon (4)-G71000-V0022-D001

Date:10.01.08

Client

FIATINDIA AUTOMOBILES (P) LTD.

Project

220 kV Switchyard at Ranjangaon.

Consultant

TCE Consulting Engineers Ltd, Pune.

Title

Document no:

(4)-G71000-V0022-D001

√ For Approval

Page:

For Review

For Information

Rev

Date

Page

Prepared

Checked

0

10.01.08

All

SGP

SGP

Approved

1 of 14

CT Parameters & Adequacy check calculations for 220kV Switchyard at Ranjangaon (4)-G71000-V0022-D001

ABBREVIATIONS USED FOR CALCULATION : ISSC =

Maximum symmetrical short circuit current.

Ipn =

CT rated primary current.

Isn =

CT rated secondary current.

ISCC EXT FAULT =

External short circuit fault current.

VK =

Knee point voltage.

Ktd =

Transient dimensioning factor.

KSSC =

Rated symetrical short circuit current factor.

K'SSC =

Effective symetrical short circuit current factor.

R'b =

Connected resistive burden.

Rb =

Rated resistive burden.

Rct =

Secondary winding resistance at 75 Deg. C

CT dimensioning formula

 R ct  R b K ' SSC    R ct  R ' bI

  K SSC 

Page: Date:10.01.08

2 of 14

CT Parameters & Adequacy check calculations for 220kV Switchyard at Ranjangaon

Page:

(4)-G71000-V0022-D001

Date:10.01.08

SUMMARY OF CT DATA : Sl.No A

Description 220kV Line From

Bay No Bay 05

800-400-200/ 1A PS NA

Ratio Class Burden

B

Ratio Class Burden Application

C

Relay details 220kV Point of sulpply Ratio Class Burden Application

D

Relay details 220kV Trfo Bay- 01 Ratio Class Burden Application Relay details

E

Core 2

Core 3

Core 4

Core 5

800-400800-400800-400-200/ 1A 200/ 1A 200/ 1A PS 0.2 PS NA 20 VA NA

800-400200/ 1A PS NA

OC & EF Protection

Bus Bar Protection

7SD

NA

7SJ62

7SS52

800-400-200/ 1A PS NA

800-400800-400800-400-200/ 1A 200/ 1A 200/ 1A PS 0.2 PS NA 20 VA NA

800-400200/ 1A PS NA

OC & EF Protection

Bus Bar Protection

7SD

NA

7SJ62

7SS52

800-400-200/ 1A PS NA

800-400800-400800-400-200/ 1A 200/ 1A 200/ 1A PS 0.2 PS NA 20 VA NA

Pilot wire differential

Application Relay details 220kV Line From Ranjangaon

Core 1

Spare

Bay 06

Pilot wire differential

OC & EF Protection

Spare

Spare

7SJ62

TOD Metering

Spare

NA

800-400200/ 1A PS NA Bus Bar Protection 7SS52

150 / 1A

150 / 1A

150 / 1A

150 / 1A

PS NA

5P 20 5 VA

0.2 30 VA

0.2 30 VA

800-400200/ 1A PS NA

Trafo Differeential

OC & EF Protection

FIAPL Metering

Bus Bar Protection

7UT613

7SJ62

150 / 1A

150 / 1A

150 / 1A

150 / 1A

PS NA

5P 20 5 VA

0.2 30 VA

0.2 30 VA

800-400200/ 1A PS NA

Trafo Differeential

OC & EF Protection

FIAPL Metering

Bus Bar Protection

7UT613

7SJ62

7SS52

220kV Trfo Bay- 02 Ratio Class Burden Application Relay details

7SS52

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CT Parameters & Adequacy check calculations for 220kV Switchyard at Ranjangaon (4)-G71000-V0022-D001

Page: Date:10.01.08

A 220kV Line From Ranjangaon A-1-1

Core 1 - Pilot wire differential Protection

Fault current Iscc =

V K  K td Ktd =

40000

A

I SCC ( Rct  R' b ).I SN 1.3 x I PN 1.2

for Transmission Line as per Siemens catalog

Core-1 Specification:Sl. No Description 1 Primary Current Ipn (A) 2 Secondary Current Isn (A) 3 Rated Output (VA) Rb 4 Class of Accuracy 5 Accuracy Limit factor - KSSC 6 A-1-2 Rrelay

800 1 NA PS NA

CT Sec. resistance(Ohms)- Rct

16

Core-1 400 1 NA PS NA 8

200 1 NA PS NA 4

Burden on secondary side of Core 1 For 7SD Relay 0.05 VA

for 7SD Relay

Cable Resistance (Rc) Length of cable from CT to relay (L)

9.48 ohm/kM 150 mtrs.

CT lead burden in two way (RL)

 2  I SN 2  Rc  L  1000

Connected resistive burden

for 2.5 sq. mm CU. Cable

= =

2.844 Ohm 2.844 VA @ Isn=1A

=

2.894 VA @ Isn=1A

Accordingly Required resistive burden R'b=3.844 VA A-1-3

Selecting PS Class core for 800/ 1A Ratio

Minimum required Vk as per 7SD protection manual

VK  K td A-1-4

I SCC (EXT. FAULT) 1.3 x I PN

(R CT  R' B ).I SN

=

872.03 V

for Ipn 800A

=

1005.60 V

for Ipn 400A

Selecting PS Class core for 400/ 1A Ratio

Minimum required Vk as per 7SD protection manual

VK  K td

I SCC (EXT. FAULT) 1.3 x I PN

(R CT  R' B ).I SN

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CT Parameters & Adequacy check calculations for 220kV Switchyard at Ranjangaon (4)-G71000-V0022-D001

Page: Date:10.01.08

Conclusion With ratio as 800/1 A, class PS, Rct=16 ohm connected to 7SD relay requires a minimum Vk as 872.03V , and with ratio as 400/1 A, class PS, Rct=8 ohm connected to 7SD relay requires a minimum Vk as 1005.60V. However selected Knee point voltage of CT is 30 ( Rct + 36) i.e. For 800/1A Vk = 1560V & for 400/1 Vk = 1320V. Hence selected Knee point voltage is adequate. A-2-1

Core-2 Spare

Core-2 Specification:Sl. No Description 1 Primary Current Ipn (A) 2 Secondary Current Isn (A) 3 Rated Output (VA) Rb 4 Class of Accuracy 5 Accuracy Limit factor - KSSC 6

800 1 NA PS NA

CT Sec. resistance (Ohms) - Rct

16

Core-2 400 1 NA PS NA 8

200 1 NA PS NA 4

Knee Point Voltage Selected Vk = 30(Rct +36) A-3-1

Core-3 MSETCL Metering & SCADA Purpose

Core-3 Specification:Sl. No Description 1 Primary Current Ipn (A) 2 Secondary Current Isn (A) 3 Rated Output (VA) Rb 4 Class of Accuracy 5 Accuracy Limit factor - KSSC 6 A-3-2

800 1 20 0.2 NA

CT Sec. resistance (Ohms) - Rct

16

Core-3 400 1 20 0.2 NA 8

200 1 20 0.2 NA 4

Burden at secondary side of core-3 for metering.

Equipment MFM Analog Amm. 3 No Analog MW Meter Total

Burden in VA 0.2 4.5 1.0 5.7

Connected resistive burden

(As calculated in B-1-2) 2.844 Ohm 2.844 VA @ Isn= 1 A R'b=Rm+Rlead

8.544

VA @ Isn= 1 A

Conclusion As calculated from above , the required VA burden is 8.544 VA , Hence selected ratio of 800-400-200/1 A with class 0.2 & 20 VA burden is adequate.

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CT Parameters & Adequacy check calculations for 220kV Switchyard at Ranjangaon (4)-G71000-V0022-D001 A-4-1

Date:10.01.08

Core 4 - OC & EF Protection with relay 7SJ62

Maximum Fault current Issc =

40000 A

Core-4 Specification:Sl. No Description 1 Primary Current Ipn (A) 2 Secondary Current Isn (A) 3 Rated Output (VA) Rb 4 Class of Accuracy 5 Accuracy Limit factor - KSSC 6 A-4-2

Page:

800 1 NA PS NA

CT Secondary resistance (Ohms)-Rct

16

Core-4 400 1 NA PS NA

200 1 NA PS NA

8

4

Burden at secondary side of core-4 for protection

Rrelay

0.05 VA

Cable Resi. (RC) Length of cable from CT to relay (L)

For 7SJ62 relays as per SIEMENS Relay manual 9.48 ohm/kM 150 mtrs.

2 Nos.

for 2.5 sq. mm CU. Cable

2

CT lead burden in two way (RLead)  2  I SN  R L  L  1000

= =

2.844 Ohm 2.844 VA @ Isn=1A

=

2.944 VA @ Isn=1A

Accordingly required resistive burden R'b=2.894VA As per Siemens Manual the minimum required symetrical short circuit current factor (K'ssc) shall be at least 20 Minimum required Vk as per siemens protection manual

VK 

K'ssc (R CT  R'B ).ISN = 1.3

=

291.45 V

for Ipn 800A

VK 

K'ssc (R CT  R'B ).ISN = 1.3

=

168.37 V

for Ipn 400A

VK 

K'ssc (R CT  R'B ).ISN = 1.3

=

106.83

for Ipn 200A

Conclusion With ratio as 800/1 A, class PS, Rct=16 ohm connected to 7SJ62 relay requires a minimum Vk as 291.45V , and with ratio as 400/1 A, class PS, Rct=8 ohm connected to 7SJ62 relay requires a minimum Vk as 168.37,with ratio as 200/1 A, class PS, Rct=4 ohm connected to 7SJ62 relay requires a minimum Vk as 106.83V However selected Knee point voltage of CT is 40 ( Rct + 7) i.e. For 800/1A Vk = 920V & for 400/1 Vk = 600V, for 200/1 A Vk = 360V. Hence selected Knee point voltage is adequate.

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CT Parameters & Adequacy check calculations for 220kV Switchyard at Ranjangaon (4)-G71000-V0022-D001

A-5-1

Page:

7 of 14

Date:10.01.08

Core 5 - Bus Bar Protection with relay 7SS52

Fault current Issc =

40000 A

Core-5 Specification:Sl. No Description 1 Primary Current Ipn (A) 2 Secondary Current Isn (A) 3 Rated Output (VA) Rb 4 Class of Accuracy 5 Accuracy Limit factor - KSSC 6 A-5-2

800 1 NA PS NA

CT Sec.resistance (Ohms) - Rct

16

Core-5 400 1 NA PS NA 8

200 1 NA PS NA 4

Burden at secondary side of core-5 for Busbar protection

Rrelay

1.8 VA

For 7SS5 relays

Cable Resistance. (Rc) Length of cable from CT to relay (L)

9.48 ohm/kM 150 mtrs.

2

for 2.5 sq. mm CU. Cable

CT lead burden in two way (RLead)  2  I SN  R L  L  1000

= =

2.844 Ohm 2.844 VA @ Isn=1A

Connected resistive burden

=

4.644 VA @ Isn=1A

A-5-3

K'SCC  K td

ISCC I PN

= 0.5 X 40000 / 800

=

25.00 with Ktd as 0.5 as per Siemens Manual for Ipn = 800A

K'SCC  K td

ISCC I PN

= 0.5 X 40000 / 400

=

50.00 with Ktd as 0.5 as per Siemens Manual for Ipn = 400A

Selecting class PS for bus bar protection core Minimum required VK for Ratio 800 / 1A ,RCt=16 Ohms, as per siemens protection manual for 7SS52 relays.

VK  K td

I SCC (EXT. FAULT) 1.3 x I PN

(R CT  R' B ).I SN

=

=

397.00 V

Minimum required VK for Ratio 400 / 1A ,RCt=8 Ohms, as per siemens protection manual for 7SS52 relays.

VK  K td

I SCC (EXT. FAULT) 1.3 x I PN

(R CT  R' B ).I SN

=

=

486.31 V

Conclusion :With ratio as 800/1 A, class PS, Rct=16 ohm connected to 7SS52 relay requires a minimum Vk as 397.00V, and With ratio as 400/1 A, class PS Rct=8 ohm connected to 7SS52 relay requires a minimum Vk as 486.31V, However selected Knee point voltage of CT is 40 ( Rct + 7) i.e. For 800/1A Vk = 920V & for 400/1 Vk = 600V. Hence selected Knee point voltage is adequate.

CT Parameters & Adequacy check calculations for 220kV Switchyard at Ranjangaon (4)-G71000-V0022-D001

Page: Date:10.01.08

B 220kV Point of supply Breaker B-1-1

Core 1 - OC & EF Protection with relay 7SJ62

Fault current Iscc =

40000

A

Core-1 Specification:Sl. No Description 1 Primary Current Ipn (A) 2 Secondary Current Isn (A) 3 Rated Output (VA) Rb 4 Class of Accuracy 5 Accuracy Limit factor - KSSC 6 B-1-2

800 1 NA PS NA

CT Sec.resistance (Ohms) - Rct

16

Core-1 400 1 NA PS NA 8

200 1 NA PS NA 4

Burden at secondary side of core-1 for protection

Rrelay

0.05 VA

Cable Resi. (RC) Length of cable from CT to relay (L)

For 7SJ6 relays as per SIEMENS Relay manual 9.48 ohm/kM 150 mtrs.

for 2.5 sq. mm CU. Cable

2

CT lead burden in two way (RLead)  2  I SN  R L  L  1000

= =

2.844 Ohm 2.844 VA @ Isn=1A

=

2.894 VA @ Isn=1A

Accordingly required resistive burden R'b=2.894VA. As per Siemens Manual the minimum required symetrical short circuit current factor (K'ssc) shall be at least 20 Minimum required Vk as per siemens protection manual

VK 

K'ssc (R CT  R'B ).ISN = 1.3

=

290.68 V

for Ipn 800A

VK 

K'ssc (R CT  R'B ).ISN = 1.3

=

167.60 V

for Ipn 400A

VK 

K'ssc (R CT  R'B ).ISN = 1.3

=

106.06

for Ipn 200A

Conclusion With ratio as 800/1 A, class PS, Rct=16 ohm connected to 7SJ relay requires a minimum Vk as 290.68V , and with ratio as 400/1 A, class PS, Rct=8 ohm connected to 7SJ relay requires a minimum Vk as 167.60,with ratio as 200/1 A, class PS, Rct=4 ohm connected to 7SJ relay requires a minimum Vk as 106.06V However selected Knee point voltage of CT is 30 ( Rct +36) i.e. For 800/1A Vk = 1560V & for 400/1 Vk = 1320V, for 200/1 A Vk = 1200V. Hence selected Knee point voltage is adequate.

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CT Parameters & Adequacy check calculations for 220kV Switchyard at Ranjangaon (4)-G71000-V0022-D001 B-2-1

Date:10.01.08

Core-2 Spare

Core-2 Specification:Sl. No Description 1 Primary Current Ipn (A) 2 Secondary Current Isn (A) 3 Rated Output (VA) Rb 4 Class of Accuracy 5 Accuracy Limit factor - KSSC 6

Page:

800 1 NA PS NA

CT Sec.resistance (Ohms) - Rct

16

Core-2 400 1 NA PS NA 8

200 1 NA PS NA 4

Knee Point Voltage Selected Vk = 30(Rct +36) B-3-1

Core-3 TOD Metering

Core-3 Specification:Sl. No Description 1 Primary Current Ipn (A) 2 Secondary Current Isn (A) 3 Rated Output (VA) Rb 4 Class of Accuracy 5 Accuracy Limit factor - KSSC 6 B-3-2

800 1 20 0.2 NA

CT Sec.resistance (Ohms) - Rct

16

Core-3 400 1 20 0.2 NA 8

200 1 20 0.2 NA 4

Burden at secondary side of core-3 for metering.

Equipment Burden in VA TOD 0.2 Analog Amm. 3 Nos. 4.5 Total 4.7 CT lead burden in two way (Rlead) =

Connected resistive burden

(As calculated in B-1-2) 2.844 Ohm 2.844 VA @ Isn= 1 A R'b=Rm+Rlead

7.544

VA @ Isn= 1 A

Conclusion As calculated from above , the required VA burden is 7.544 VA , Hence selected ratio of 800-400-200/1 A with class 0.2 & 20 VA burden is adequate. B-4-1

Core-4 Spare

Core-4 Specification:Sl. No Description 1 Primary Current Ipn (A) 2 Secondary Current Isn (A) 3 Rated Output (VA) Rb 4 Class of Accuracy 5 Accuracy Limit factor - KSSC 6

CT Sec.resistance (Ohms) - Rct

Knee Point Voltage Selected Vk = 40(Rct + 7)

800 1 NA PS NA 16

Core-4 400 1 NA PS NA 8

200 1 NA PS NA 4

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CT Parameters & Adequacy check calculations for 220kV Switchyard at Ranjangaon (4)-G71000-V0022-D001 B-5-1

Core 5 - Bus Bar Protection with relay 7SS52

Core-5 400 1 NA PS NA

800 1 NA PS NA

CT Sec.resistance (Ohms) - Rct

16

8

200 1 NA PS NA 4

Note :- Please refer CT sizing calculation A-5 & C-5 C 220kV Line To Transformer Bay C-1-1

Core 1 - Transformer differential Protection

Transformer rating =

50

MVA

Voltage Ratio =

220/22

KV

Percentage Imp =

13.25

%

Fault MVA = (3 Ph Short Circuit)

419.28721 MVA

Fault Current Iscc = at 220 KV side

1100.3758 A

V K  K td Ktd =

I SCC ( Rct  R' b ).I SN 1.3 x I PN 4

for Transformer as per Siemens catalog

Core-1 Specification:Sl. No Description 1 Primary Current Ipn (A) 2 Secondary Current Isn (A) 3 Rated Output (VA) Rb 4 Class of Accuracy 5 Accuracy Limit factor - KSSC 6 C-1-2 Rrelay

Core-1 150 1 NA PS NA

CT Secondary resistance(Ohms)- Rct

1

Burden on secondary side of Core 1 For 7UT613 Relay 0.1 VA

Cable Resistance (Rc) Length of cable from CT to relay (L)

10 of 14

Date:10.01.08

Core-5 Specification:Sl. No Description 1 Primary Current Ipn (A) 2 Secondary Current Isn (A) 3 Rated Output (VA) Rb 4 Class of Accuracy 5 Accuracy Limit factor - KSSC 6

Page:

for 7UT612 Relay 9.48 ohm/kM 150 mtrs.

for 2.5 sq. mm CU. Cable

CT Parameters & Adequacy check calculations for 220kV Switchyard at Ranjangaon (4)-G71000-V0022-D001

Page:

Date:10.01.08

CT lead burden in two way (RL)

 2  I SN  Rc  L  1000

Connected resistive burden

2

= =

2.844 Ohm 2.844 VA @ Isn=1A

=

2.944 VA @ Isn=1A

=

89.02 V

Accordingly Required resistive burden R'b=3.844 VA C-1-3

Selecting PS Class core for 150/ 1A Ratio

Minimum required Vk as per 7UT612 protection manual

VK  K td

I SCC (EXT. FAULT) 1.3 x I PN

(R CT  R' B ).I SN

for Ipn 150A

Conclusion With ratio as 150/1 A, class PS, Rct=1 ohm connected to 7UT613 relay requires a minimum Vk as 89.02V . However selected Knee point voltage of CT is 500V. Hence selected Knee point voltage is adequate. C-2-1

Core2- OC & EF Protection with relay 7SJ62

Maximum Fault current Issc =

40000 A

Core-2 Specification:Sl. No Description 1 Primary Current Ipn (A) 2 Secondary Current Isn (A) 3 Rated Output (VA) Rb 4 Class of Accuracy 5 Accuracy Limit factor - KSSC 6 C-2-2 Rrelay

Core-2 150 1 15 5P 20

CT Secondary resistance (Ohms)-Rct

1

Burden at secondary side of core-2 for protection 0.05 VA

Cable Resi. (RC) Length of cable from CT to relay (L)

For 7SJ62 relays as per SIEMENS Relay manual 9.48 ohm/kM 150 mtrs. 2

CT lead burden in two way (RLead)  2  I SN  R L  L  1000

Connected resistive burden

11 of 14

for 2.5 sq. mm CU. Cable

= =

2.844 Ohm 2.844 VA @ Isn=1A

=

2.894 VA @ Isn=1A

CT Parameters & Adequacy check calculations for 220kV Switchyard at Ranjangaon (4)-G71000-V0022-D001 C-2-3

Page:

Date:10.01.08

Accordingly required resistive burden R'b=2.894VA. However , Selected resitive burden = 15 VA As per Siemens Manual the minimum required symetrical short circuit current factor (K'ssc) shall be at least 20 Minimum required Vk as per siemens protection manual

VK 

K'ssc (R CT  R'B ).ISN = 1.3

=

59.91 V

for Ipn 150A

=

246.15 V

for Ipn 150A

Vk of CTs for rated class 5P20, 15VA

VK 

K SCC ( R B  R CT ).I SN 1.3

=

Conclusion With ratio as 200/1 A, class 5P20, 15VA, Rct=1 ohm connected to 7SJ62 relay requires a minimum Vk as 59.91V & the CT's Vk is 246.15 Volts. Hence adequate.

C-3-1

Core-3 Metering Purpose

Core-3 Specification:Sl. No Description 1 Primary Current Ipn (A) 2 Secondary Current Isn (A) 3 Rated Output (VA) Rb 4 Class of Accuracy 5 Accuracy Limit factor - KSSC 6 C-3-2

Core-3 150 1 30 0.2 NA

CT Sec.resistance (Ohms) - Rct

1

Burden at secondary side of core-2 for metering.

Equipment Burden in VA MFM 0.2 Analog Amm. 3 Nos. 4.5 Total 4.7 CT lead burden in two way (Rlead) =

Connected resistive burden

(As calculated in B-1-2) 2.844 Ohm 2.844 VA @ Isn= 1 A R'b=Rm+Rlead

12 of 14

7.544

VA @ Isn= 1 A

Conclusion As calculated from above , the required VA burden is 7.544 VA , Hence selected ratio of 150/1 A with class 0.2 & 30 VA burden is adequate.

CT Parameters & Adequacy check calculations for 220kV Switchyard at Ranjangaon (4)-G71000-V0022-D001 C-4-1

6

C-5-1

Core-4 150 1 30 0.2 NA

CT Sec.resistance (Ohms) - Rct

1

Core 5 - Bus Bar Protection with relay 7SS52

Fault current Issc =

40000 A

Core-5 Specification:Sl. No Description 1 Primary Current Ipn (A) 2 Secondary Current Isn (A) 3 Rated Output (VA) Rb 4 Class of Accuracy 5 Accuracy Limit factor - KSSC 6

Rrelay

800 1 NA PS NA

CT Sec. resistance (Ohms) - Rct

16

Core-5 400 1 NA PS NA 8

200 1 NA PS NA 4

Burden at secondary side of core-5 for Busbar protection 1.8 VA

For 7SS52 relays

Cable Resistance. (Rc) Length of cable from CT to relay (L)

9.48 ohm/kM 150 mtrs.

2

for 2.5 sq. mm CU. Cable

CT lead burden in two way (RLead)  2  I SN  R L  L  1000

= =

2.844 Ohm 2.844 VA @ Isn=1A

Connected resistive burden

=

4.644 VA @ Isn=1A

C-5-3

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Date:10.01.08

Core-4 Specification:Sl. No Description 1 Primary Current Ipn (A) 2 Secondary Current Isn (A) 3 Rated Output (VA) Rb 4 Class of Accuracy 5 Accuracy Limit factor - KSSC

C-5-2

Page:

K'SCC  K td

ISCC I PN

= 0.5 X 40000 / 800

=

25.00 with Ktd as 0.5 as per Siemens Manual for Ipn = 800A

K'SCC  K td

ISCC I PN

= 0.5 X 40000 / 400

=

50.00 with Ktd as 0.5 as per Siemens Manual for Ipn = 400A

CT Parameters & Adequacy check calculations for 220kV Switchyard at Ranjangaon (4)-G71000-V0022-D001

Page:

Date:10.01.08

Selecting class PS for bus bar protection core Minimum required Vk for Ratio 800 / 1A ,Rct=16 Ohms, as per siemens protection manual for 7SS52 relays.

VK  K td

I SCC (EXT. FAULT) 1.3 x I PN

(R CT  R' B ).I SN

=

=

397.00 V

Minimum required Vk for Ratio 400 / 1A ,Rct=8 Ohms, as per siemens protection manual for 7SS52 relays.

VK  K td

I SCC (EXT. FAULT) 1.3 x I PN

(R CT  R' B ).I SN

14 of 14

=

=

486.31 V

Conclusion :With ratio as 800/1 A, class PS, Rct=16 ohm connected to 7SS52 relay requires a minimum Vk as 397.00V, and with ratio as 400/1 A, class PS Rct=8 ohm connected to 7SS52 relay requires a minimum Vk as 486.31V, However selected Knee point voltage of CT is 40 ( Rct + 7) i.e. For 800/1A Vk = 920V & for 400/1 Vk = 600V. Hence selected Knee point voltage is adequate.