11kV CT Calculation

TENDER NO. G1408 – FIVE (5) NEW 33/11KV PRIMARY SUBSTA TIONS WITH ASSOCIATED ASSOCIA TED 33KV OVERHEAD LINES L INES & 33KV 33K V CABLE CAB LE L INES

Document Title : CT / VT calculatio ns f or 11kV swit chgear for ADWEA A DWEA c o n t r act ac t n o G1408 TABL E OF CONTENTS CONTENTS

SECTION

TITLE

PAGE NO

1.0

OBJECTIVE OBJ ECTIVE ....................... ................................... ....................... ....................... ....................... ....................... ....................... ..................... ..........3

2.0

REFERENCES REFERENCES.......... ..................... ...................... ....................... ....................... ...................... ....................... ....................... .................... .........3

3.0 SCOPE..............................................................................................................3 4.0

CT SIZING CALCULATIONS............ CAL CULATIONS....................... ....................... ....................... ....................... ....................... .................. .......3 4.1 4.1.1 4.1.2 4.1.3 4.1.4 4.2 4.2.1 4.2.2 4.2.3 4.2.4 4.3 4.3.1 4.3.2 4.4 4.4.1 4.4.2 4.5 4.5.1 4.5.2 4.6 4.6.1 4.6.2 4.7 4.7.1 4.7.2

33/11.55 33/11.55kV, kV, 15MVA 15MVA Transf ormer or mer Inco Inco mer Feeder................ Feeder...................... ............ ........... ........... ............ ............ ........... ..... 3 CT's for Metering and Transducers .................................................................................. 3 CT's for Differential Protection & REF protection .............................................................. 4 CT for Standby Earth Fault(SBEF) protection ................................................................... 7 CT's for OverCurrent and Earth Fault protection ............................................................ 10 33/11.55 33/11.55kV, kV, 20MVA 20MVA Transf ormer or mer Inco Inco mer Feeder ........... ................. ............ ............ ............ ............ ........... ........... ........ .. 13 CT's for metering and Transducers ................................................................................. 13 CT's for Differential Protection & REF protection ............................................................ 14 CT for Standby Earth Fault(SBEF) protection ................................................................. 17 CT's for Overcurrent and Earth fault protection .............................................................. 19 33/11.55 33/11.55kV, kV, 15MVA 15MVA Transf ormer or mer Bus Bu s sect ion Feeder Feeder ............ .................. ............ ............ ............ ............ ........... ..... 22 CT's for Metering and Protection..................................................................................... Protection ..................................................................................... 22 CT's for Transducers ..................................................................................... ....................................................................................................... .................. 24 33/11.55 33/11.55kV, kV, 20MVA 20MVA Transf ormer or mer Bus Bu s sect ion Feeder Feeder ............ .................. ........... ........... ............ ............ ........... ....... 25 CT's for Metering and Protection..................................................................................... Protection ..................................................................................... 25 CT's for Transducers ..................................................................................... ....................................................................................................... .................. 26 Outg oin g 11/0.433 11/0.433kV, kV, 500kVA 500kVA Transf ormer or mer Feeder................ Feeder..................... ........... ............ ........... ........... ............ ........ 27 CT's for Metering and Protection..................................................................................... Protection ..................................................................................... 27 CT's for Transducers ..................................................................................... ....................................................................................................... .................. 29 Outg oin g 11kV, 5MVAr Capacit or Feeder ........... ................. ........... ........... ............ ........... ........... ............ ........... ........... ........... ..... 30 CT's for Metering and Protection..................................................................................... Protection ..................................................................................... 30 CT's for Transducers ..................................................................................... ....................................................................................................... .................. 33 11kV Outgo Out goin ing g Feeder ......................................................................................... .................................................................................................. ......... 34 CT's for Metering and Protection..................................................................................... Protection ..................................................................................... 34 CT's for Transducers ..................................................................................... ....................................................................................................... .................. 37

5.0

VT SIZING CALCULATIONS..................... CAL CULATIONS................................. ....................... ....................... ....................... .................. .......38

6.0

39 CONCLUSION ....................... .................................. ...................... ....................... ....................... ...................... ...................... ................. ......39

7.0

SUMMARY OF CURRENT TRANSFORMERS(Annexure-1) TRANSFORMERS(Annexure-1).......................... ..........................39

8.0

SUMMARY OF VOLTA GE TRANSFORMERS(Annexure-2) TRANSFORMERS(Annexure-2).......................... ..........................39

9.0 9.0

Relavent Relavent pages of Catalogu Catalogu es for Bur den datas(Annexure-3) datas(Annexure-3) ................... 39

NOTE Refer DTS no 0012 dated 15.11.2003 for Relay Catalogues.

Doc-No:G1408-AA-2030-J-0-101 Rev - A

Page 2 of 39


Document Title : CT / VT calculatio ns f or 11kV swit chgear for ADWEA A DWEA c o n t r act ac t n o G1408 1.0

OBJECTIVE To establish Current transformer(CT) and Voltage transformer parameters at 11 kV level for all the 5 new substations.

2.0

REFERENCES a) Bay control control unit REF542 REF542 Plus catalogue b) 11kV switchgear vendor (ABB FJH) Single line diagram no. N611844/1 to N611844/8 c) Ducab cable catalogue d) ESI standard 48-3 and Clients/Consultant Clients/Consultant Recommendations Recommendations e) Relay catalogues

3.0

SCOPE To establish the requirements of CT/VT parameters such as CT/VT ratio, VA burden, knee point voltage, accuracy class and magnetising current for CTs on the 11 kV switchgear at all the sub-stations.

4.0

CT SIZING CAL CALCULATIONS CULATIONS

4.1

33/11. 33/ 11.55kV, 55kV, 15M 15MVA VA Transf or ormer mer Inco Incomer mer Fee Feeder der

4.1.1 4.1 .1

CT’s for Me Meterin terin g and Transdu cers CT Ratio: 800-900 / 1A Type of Metering: It is proposed to use Bay Control Unit(BCU) type REF542 Plus.

Transformer full load current on secondary side ( considering tap setting at – 15% ) = 15000/(√3*11.55*0.85) = 882.12 Amps Hence it is proposed to provide 800-900 / 1A CT for metering. Total burden of bay control unit = 0.1 VA (Refer REF542 Plus catalogue) Maximum burden of Current Transducer Maximum burden of Power Transducer Maximum burden of Power factor Transducer The transducers are located in LDC cubicle.

= = =

2 VA (Actual is 0.2VA) 2 VA 2 VA

Minimum length of cable = 40 meters(As per equipment layout) between CT and transducers t ransducers Doc-No:G1408-AA-2030-J-0-101 Rev - A

Page 3 of 39


Document Title : CT / VT calculatio ns f or 11kV swit chgear for ADWEA A DWEA c o n t r act ac t n o G1408 1.0

OBJECTIVE To establish Current transformer(CT) and Voltage transformer parameters at 11 kV level for all the 5 new substations.

2.0

REFERENCES a) Bay control control unit REF542 REF542 Plus catalogue b) 11kV switchgear vendor (ABB FJH) Single line diagram no. N611844/1 to N611844/8 c) Ducab cable catalogue d) ESI standard 48-3 and Clients/Consultant Clients/Consultant Recommendations Recommendations e) Relay catalogues

3.0

SCOPE To establish the requirements of CT/VT parameters such as CT/VT ratio, VA burden, knee point voltage, accuracy class and magnetising current for CTs on the 11 kV switchgear at all the sub-stations.

4.0

CT SIZING CAL CALCULATIONS CULATIONS

4.1

33/11. 33/ 11.55kV, 55kV, 15M 15MVA VA Transf or ormer mer Inco Incomer mer Fee Feeder der

4.1.1 4.1 .1

CT’s for Me Meterin terin g and Transdu cers CT Ratio: 800-900 / 1A Type of Metering: It is proposed to use Bay Control Unit(BCU) type REF542 Plus.

Transformer full load current on secondary side ( considering tap setting at – 15% ) = 15000/(√3*11.55*0.85) = 882.12 Amps Hence it is proposed to provide 800-900 / 1A CT for metering. Total burden of bay control unit = 0.1 VA (Refer REF542 Plus catalogue) Maximum burden of Current Transducer Maximum burden of Power Transducer Maximum burden of Power factor Transducer The transducers are located in LDC cubicle.

= = =


Minimum length of cable = 40 meters(As per equipment layout) between CT and transducers t ransducers Doc-No:G1408-AA-2030-J-0-101 Rev - A

Page 3 of 39

TENDER NO. G1408 – FIVE (5) NEW 33/11KV PRIMARY SUBSTA TIONS WITH ASSOCIATED ASSOCIA TED 33KV OVERHEAD LINES L INES & 33KV 33K V CABLE CAB LE L INES Size of cable

= 4 sqmm

Resistance of cable per km at 20 deg C

= 4.61 Ohms (as per Ducab cable catalogue)

Resistance of cable at = 4.61(1+0.00393 4.61(1+0.00393 (55-20) ) 55 deg C, considering temperature correction factor = 5.244 ohm/km Total resistance = 2 x 5.244 x 40 / 1000 (Lead & Return conductors) = 0.42 Ω Burden due to lead resistance = Total resistance x (CT secondary current) 2 = 0.42 x 12 = 0.42 VA Total burden = Burden of BCU+Burden of transducers+Burden transducers+Burden due to lead resistance = 0.1 + 6 + 0.42 = 6.52 VA Considering 25% future margin Total burden required

=1.25 x 6.52 = 8.15 VA

Hence a standard burden rating of 15 VA is chosen for a Tap of 800A and 30VA chosen for a Tap of 900A. The CT Accuracy Class selected is CL. 0.5S as per the specification requirements. The instrument security factor (ISF) selected is less than or equal to Five(5) as per the specification specification requirements. requirements. Ab st rac t Therefore a CT of CL. 0.5,15VA is proposed for 800-900/1A and CL. 0.5,30VA is proposed for 800-900/1A.The 800-900/1A.The CTs are with a factor of safety less than or equal to 5.

4.1. 4. 1.2 2

CT’s CT ’s for Differential Differential Protection and and REF REF Protection: CT Ratio: 900/1 Type of relay: RET 316*4 (ABB make) Knee Point Voltage for Calculation for Differential Protection: Fault current rating of 11 kV switchgear

= 31.5 kA

As per ESI Standard 48-3 48-3 enclosed the formulae for calculating calculating Knee Point Voltage is VK = If * N * (Rct+Rl) Where If = fault current N = Turns Ratio Rct = CT secondary resistance Rl = Lead resistance Maximum fault current carrying capacity of 11 kV switchgear is 31.5 kA ,which can be considered for knee point calculation.

Doc-No:G1408-AA-2030-J-0-101 Rev - A

Page 4 of 39

TENDER NO. G1408 – FIVE (5) NEW 33/11KV PRIMARY SUBSTA TIONS WITH ASSOCIATED 33KV OVERHEAD LINES & 33KV CABLE L INES If = 31500 Amps N = 1/900 Rct = 7Ω (As per Manufacturers Recommendations) The differential relay is mounted on 33kV relay panel which is located in control room. Minimum length of cable = 40 meters(As per equipment layout) between CT and relay Size of cable = 4 sqmm Resistance of cable per km at 20 deg C


Resistance of cable at = 4.61(1+0.00393 (55-20) ) 55 deg C, considering temperature correction factor = 5.244 ohm/km Total resistance = 2 x 5.244 x 40 / 1000 (Lead & Return conductors) = 0.42 Ω Burden due to lead resistance = Total resistance x (CT secondary current) 2 = 0.42 x 12 = 0.42 VA Therefore, substituting the above values in the formula for V k, VK = 31500* (1/900) * (7 + 0.42 ) VK = 259.7V VK > 400V Therefore a CT of 900/1A, CL. X, V K > 400V is proposed. Imag shall be 30mA @ Vk/2

Recommendations from Consultant/Client f or Differential Core requirement : Further, to avoid mal-operation on energization of power transformer and in connection with fault current that passes through power transformer, the rated secondary voltage has to satisfy the following conditions: Condition-1 The core may not saturate for current lower than 30 times the Power Transformer rated current at connected burden. This ensures stability also with heavy DC saturation. Vk ≥ 30 * Int * (Rct + Rl + Rr / Ir 2) Where, Int

=

Main CT secondary current corresponding to rated primary current of power transformer

Rct

=

CT secondary resistance

Rl

=

Lead resistance

Rr

=

Relay burden

Doc-No:G1408-AA-2030-J-0-101 Rev - A

Page 5 of 39

TENDER NO. G1408 – FIVE (5) NEW 33/11KV PRIMARY SUBSTA TIONS WITH ASSOCIATED 33KV OVERHEAD LINES & 33KV CABLE L INES Ir

=

Nominal relay current

Full load current of the transformer = (15000 / 1.732 * 11) = 787 Amps. Vk ≥ 30 * (787/900) * (7+0.42 + 0.1 / 12) Vk

187V

The knee point voltage proposed by us is 400V. Hence this condition is Verified. Condition-2 The core may not saturate for current lower than 4 times the maximum through fault current at connected burden. Vk ≥ 4 * Ift * (Rct + Rl + Rr / Ir 2) Where, Ift

=

Maximum secondary side through fault current

Rct

=


Rl

=

Lead resistance

Rr

=

Relay burden

Ir

=


Full load current of the transformer = (15000 / 1.732 * 11) = 787 Amps. Impedance of transformer = 10% Through fault current = 787 / 0.1 = 7870 Amps Secondary side through fault current = 7870 / 900 = 8.75 Amps. Hence Vk ≥ 4 * (7870/900) * (7+ 0.42 + 0.1 / 12) Vk

263 V

The knee point voltage proposed by us is 400V. Hence this condition is Verified. Ab st rac t Therefore a CT of Vk > 400V is proposed for 900/1A.Imag shall be 30mA @ Vk/2

REF Protectio n Core Knee Point Calculation s CT Ratio: 900/1 Type of relay: SPAJ 115C (ABB make) Fault current rating of 11 kV switchgear = 31.5 kA The REF relay is mounted on 33kV relay panel, which is located in control room. The knee-point voltage Vk should be 2 times higher than the stabilising voltage Vs required in through fault conditions: Vk

=

2 x Vs =

Doc-No:G1408-AA-2030-J-0-101 Rev - A

2 x Ikmax x (Rct + Rl) / n Page 6 of 39

TENDER NO. G1408 – FIVE (5) NEW 33/11KV PRIMARY SUBSTA TIONS WITH ASSOCIATED 33KV OVERHEAD LINES & 33KV CABLE L INES Where, Vs

=

stabilising voltage

Ikmax

=

the maximum through fault current

According to ESI Standard 48, the voltage set on the relay is chosen such that the stability of the protection during a through fault being taken as 16 times of the rated transformer current. Ikmax

=

16 x Transformer rated secondary current = 16 x 882.12 = 14114 amp

Rct

=

Internal resistance of the current transformer

=

7 ohms

Rl

=

total lead resistance

=

0.42 ohms

n

=

turns ratio of the current transformer

=

900

Substituting the above values in the equation the stabilising voltage, Vs

=

16 x 882.12 x (7 + 0.42) / 900

=

116.36 Volts

Hence, Vk

=

2 x Vs

Vk

=

2 x 116.36 V =

232.72 V

The knee point voltage proposed by us is 400V. Hence this condition is Verified. Ab st rac t Since the CT Requirement of the REF and Differential Core is the same, we propose to use a common core for both. Therefore a CT of Vk > 400V is proposed for 900/1A. Imag shall be 30mA @ Vk/2

4.1.3

CT for Standby Earth Fault (SBEF) Protect ion : CT Ratio: 800-900/1(As calculated in 4.1.1 above) This CT is located on the transformer neutral. Type of relay: REJ 521 (ABB make) Fault current rating of 11kV switchgear = 31.5kA The SBEF relay is mounted on 33kV relay panel which is located in control room. Burden of relay = 0.1VA (As per relay catalogue) Minimum length of cable = 50 meters(As per equipment layout) between CT and relay Size of cable Resistance of cable per km at 20 deg C

= 4 sqmm = 4.61 Ohms (as per Ducab cable catalogue)

Resistance of cable at = 4.61(1+0.00393 (55-20) ) 55 deg C, considering temperature correction factor

Doc-No:G1408-AA-2030-J-0-101 Rev - A

Page 7 of 39

TENDER NO. G1408 – FIVE (5) NEW 33/11KV PRIMARY SUB STATIONS WITH ASSOCIATED 33KV OVERHEAD LINES & 33KV CABLE L INES = 5.244 ohm/km Total resistance = 2 x 5.244 x 50 / 1000 (Lead & Return conductors) = 0.5244 Ω Burden due to lead resistance = Total resistance x (CT secondary current) 2 = 0.5244 x 12 = 0.5244 VA Total burden

= Burden of the relay + Burden due to lead resistance = 0.1 + 0.5244 = 0.6244 VA

Considering 25% future margin Total burden required

=1.25 x 0.6244 = 0.78 VA

Hence a standard burden rating of 10 VA is chosen for a Tap of 800A and a burden of 15 VA is chosen for a Tap of 900A. The CT Accuracy Class selected is CL. 5P20 as per the specification requirements. For a Tap of 800A To ensure correct operation of the connected relay in case of faults, the CT must be able to transform the maximum symmetrical short circuit current without saturation. To satisfy the above, following condition to be checked: Koalf Where,

≥ Iscc / Ipn Ipn

=

CT primary nominal current

=

Iscc

=

max. symmetrical short circuit current =

Koalf

=

Operating accuracy limiting factor

800A 31.5kA

The operating accuracy limiting factor (K oalf ) depends on the nominal accuracy limiting factor (Knalf ), the nominal CT burden (P n), the internal CT burden (P i) and the total connected burden (Pb). Pn + Pi Koalf

=

Knalf x Pb + Pi

Where, Knalf

=

20

Pn

=

Nominal CT burden

Pi

=

Internal CT burden

=

V*I

=

(I * Rct) * I

=

I2 * Rct

=

3Ohm (Assumed)

=

12 x 3

Rct

Doc-No:G1408-AA-2030-J-0-101 Rev - A

= 10 VA

Page 8 of 39

TENDER NO. G1408 – FIVE (5) NEW 33/11KV PRIMARY SUB STATIONS WITH ASSOCIATED 33KV OVERHEAD LINES & 33KV CABLE L INES

Pb

=

3 VA

=

Total connected burden

=

Pr + PL

=

0.1 + 0.5244 =

0.6244 VA

10 + 3 Koalf

=

20 x 0.6244 + 3

= Iscc / Ipn = =

71 31500 / 800 40

∴ Koalf > Iscc / Ipn Hence the selected CT with parameters 800 – 900 / 1, 5P20, 10VA is adequate. For a Tap of 900A To ensure correct operation of the connected relay in case of faults, the CT must be able to transform the maximum symmetrical short circuit current without saturation. To satisfy the above, following condition to be checked: Koalf Where,

≥ Iscc / Ipn Ipn

=


=

Iscc

=


Koalf

=


900A 31.5kA


=

Knalf x Pb + Pi

Where, Knalf

=

20

Pn

=

Nominal CT burden

Pi

=

Internal CT burden

=

V*I

=

(I * Rct) * I

=

I2 * Rct

=

4Ohm (As per Manufacturers recommendations)

=

12 x 4

=

4 VA

=


=

Pr + PL

=

0.1 + 0.5244 =

Rct

Pb

Doc-No:G1408-AA-2030-J-0-101 Rev - A

= 15 VA

0.6244 VA Page 9 of 39

TENDER NO. G1408 – FIVE (5) NEW 33/11KV PRIMARY SUB STATIONS WITH ASSOCIATED 33KV OVERHEAD LINES & 33KV CABLE L INES 15 + 4 Koalf

=

20 x 0.6244 + 4

= Iscc / Ipn = =

82.17 31500 / 900 35

∴ Koalf > Iscc / Ipn Hence the select ed CT with parameters 800 – 900 / 1, 5P20, 15VA i s adequate Ab st rac t Therefore a CT of CL. 5P20,10VA is proposed for 800-900/1A and CL. 5P20,15VA is proposed for 800-900/1A.

4.1.4

CT’s for Overcurrent and Earth Fault Protectio n: CT Ratio: 800-900/1 Type of relay: SPAJ 140C (ABB make) Fault current rating of 11kV switchgear = 31.5kA The relay is mounted on 11kV switchgear. Burden of relay = 0.1VA(As per relay catalogue) Minimum length of cable between CT and relay

= 5 meters

Size of cable

= 2.5 sqmm



Resistance of cable at = 7.41(1+0.00393 (55-20) ) = 8.43 ohm/km 55 deg C, considering temperature correction factor Total resistance = 2 x 8.43 x 5 / 1000 (Lead & Return conductors) = 0.0843 Ω Burden due to lead resistance = Total resistance x (CT secondary current) 2 = 0.0843 x 12 = 0.0843 VA Total burden



=1.25 x 0.1843 = 0.23 VA

Hence a standard burden rating of 10 VA is chosen for a Tap of 800A and a burden of 15 VA is chosen for a Tap of 900A . Doc-No:G1408-AA-2030-J-0-101 Rev - A

Page 10 of 39

TENDER NO. G1408 – FIVE (5) NEW 33/11KV PRIMARY SUB STATIONS WITH ASSOCIATED 33KV OVERHEAD LINES & 33KV CABLE L INES The CT Accuracy Class selected is CL. 5P20 as per the specification requirements. For a Tap of 800A To ensure correct operation of the connected relay in case of faults, the CT must be able to transform the maximum symmetrical short circuit current without saturation. To satisfy the above, following condition to be checked: Koalf Where,

≥ Iscc / Ipn Ipn

=


=

Iscc

=


Koalf

=


800A 31.5kA


=

Knalf x Pb + Pi

Where, Knalf

=

20

Pn

=

Nominal CT burden

Pi

=

Internal CT burden

=

V*I

=

(I * Rct) * I

=

I2 * Rct

=

3Ohm (Assumed)

=

12 x 3

=

3VA

=


=

Pr + PL

=

0.1 + 0.0843 =

Rct

Pb

= 10 VA

0.1843 VA

15 + 3 Koalf

=

20 x 0.1843 + 3

= Iscc / Ipn = =

113 31500 / 800 40

∴ Koalf > Iscc / Ipn Hence the selected CT with parameters 800 – 900 / 1, 5P20, 10VA is adequate.

Doc-No:G1408-AA-2030-J-0-101 Rev - A

Page 11 of 39

TENDER NO. G1408 – FIVE (5) NEW 33/11KV PRIMARY SUB STATIONS WITH ASSOCIATED 33KV OVERHEAD LINES & 33KV CABLE L INES For a Tap of 900A To ensure correct operation of the connected relay in case of faults, the CT must be able to transform the maximum symmetrical short circuit current without saturation. To satisfy the above, following condition to be checked: Koalf Where,

≥ Iscc / Ipn Ipn

=


=

Iscc

=


Koalf

=


900A 31.5kA


=

Knalf x Pb + Pi

Where, Knalf

=

20

Pn

=

Nominal CT burden

Pi

=

Internal CT burden

=

V*I

=

(I * Rct) * I

=

I2 * Rct

=

4

=

12 x 4

=

4 VA

=


=

Pr + PL

=

0.1 + 0.0843 =

Rct

Pb

= 15 VA

0.1843 VA

15 + 4 Koalf

=

20 x 0.1843 + 4

= Iscc / Ipn = =

90.81 31500 / 900 35

∴ Koalf > Iscc / Ipn Hence the s elected CT wi th parameters 800 – 900 / 1, 5P20, 15VA i s adequate. Ab st rac t Therefore a CT of CL. 5P20,10VA is proposed for 800-900/1A and CL. 5P20,15VA is proposed for 800-900/1A.

Doc-No:G1408-AA-2030-J-0-101 Rev - A

Page 12 of 39

TENDER NO. G1408 – FIVE (5) NEW 33/11KV PRIMARY SUBSTA TIONS WITH ASSOCIATED 33KV OVERHEAD LINES & 33KV CABLE L INES

4.2

33/11.55kV, 20MVA Transf ormer Incomer Feeder

4.2.1

CT’s for Meterin g and Transdu cers CT Ratio: 900-1200/1A Type of Metering: It is proposed to use Bay Control Unit(BCU) type REF542 Plus for the purpose of metering. Transformer full load current on secondary side ( considering tap setting at – 15% ) = 20000/(√3*11.55*0.85) = 1176.16 Amps Hence it is proposed to provide 900-1200/1A CT for metering.

Total burden of bay control unit = 0.1 VA (Refer REF542 Plus catalogue) Maximum burden of Current Transducer Maximum burden of Power Transducer Maximum burden of Power factor Transducer The transducers are located in LDC cubicle.

= = =


Minimum length of cable = 60 meters(As per equipment layout) between CT and Transducer Size of cable

= 4 sqmm



Resistance of cable at = 4.61(1+0.00393 (55-20) ) 55 deg C, considering temperature correction factor = 5.422 ohm/km Total resistance = 2 x 5.422 x 60 / 1000 (Lead & Return conductors) = 0.63 Ω Burden due to lead resistance = Total resistance x (CT secondary current) 2 = 0.63 x 12 = 0.63 VA Total burden = Burden of BCU+Burden of transducers+Burden due to lead resistance = 0.1 + 6 + 0.63 = 6.73 VA Considering 25% future margin Total burden required

=1.25 x 6.73 = 8.41 VA

Hence burden of 10VA is chosen for 900A Tap and 15VA is chosen for 1200A Tap. The instrument security factor (ISF) selected is less than or equal to 5, as per the specification requirements.

Doc-No:G1408-AA-2030-J-0-101 Rev - A

Page 13 of 39

TENDER NO. G1408 – FIVE (5) NEW 33/11KV PRIMARY SUBSTA TIONS WITH ASSOCIATED 33KV OVERHEAD LINES & 33KV CABLE L INES Ab st rac t Therefore a CT of CL. 0.5,10VA is proposed for 900-1200/1A and CL. 0.5,15VA is proposed for 900-1200/1A.The CTs are with a factor of safety less than or equal to 5.

4.2.2

CT’s for Differential Protection and REF Protection: CT Ratio: Ratio is considered as 1200/1(As calculated in 4.2.1 above) Type of relay: RET 316*4 (ABB make)

Knee Point Voltage for Calculation for Differential Protection: Fault current rating of 11 kV switchgear

= 31.5 kA

As per ESI Standard 48-3 enclosed the formulae for calculating Knee Point Voltage is VK = If * N * (Rct+Rl) Where If = fault current N = Turns Ratio Rct = CT secondary resistance Rl = Lead resistance Maximum fault current carrying capacity of 11 kV switchgear is 31.5 kA which can be considered for knee point calculation. If = 31500 Amps N = 1/1200 Rct = 12Ω (As per manufacturers recommendations) The differential relay is mounted on 33kV relay panel which is located in control room. Minimum length of cable between CT and relay Size of cable Resistance of cable per km at 20 deg C

= 50 meters(As per equipment layout) = 4 sqmm = 4.61 Ohms (as per Ducab cable catalogue)

Resistance of cable at = 4.61(1+0.00393 (55-20) ) 55 deg C, considering temperature correction factor = 5.244 ohm/km Total resistance = 2 x 5.244 x 50 / 1000 (Lead & Return conductors) = 0.5244 Ω Burden due to lead resistance = Total resistance x (CT secondary current) 2 = 0.5244 x 12 = 0.5244 VA Therefore, substituting the above values in the formula for V k, Doc-No:G1408-AA-2030-J-0-101 Rev - A

Page 14 of 39

TENDER NO. G1408 – FIVE (5) NEW 33/11KV PRIMARY SUBSTA TIONS WITH ASSOCIATED 33KV OVERHEAD LINES & 33KV CABLE L INES VK = 31500* (1/1200) * (12 + 0.5244 ) VK = 329V VK > 400V Therefore a CT of 1200/1A, CL. X, VK > 400V is pro pos ed. Imag shall b e 30mA @ Vk/2

Recommendations from Consultant/Client f or Differential Core requirement : Further, to avoid mal-operation on energization of power transformer and in connection with fault current that passes through power transformer, the rated secondary voltage has to satisfy the following conditions: Condition-1 The core may not saturate for current lower than 30 times the Power Transformer rated current at connected burden. This ensures stability also with heavy DC saturation. Vk ≥ 30 * Int * (Rct + Rl + Rr / Ir 2) Where, Int

=

Main CT secondary current corresponding to rated primary current of power transformer

Rct

=


Rl

=

Lead resistance

Rr

=

Relay burden

Ir

=


Full load current of the transformer = (20000 / 1.732 * 11) = 1050Amps. Vk ≥ 30 * (1050/1200) * (10.5+0.5244 + 0.1 / 12) Vk

292V

The knee point voltage proposed by us is 400V. Hence this condition is Verified. Condition-2 The core may not saturate for current lower than 4 times the maximum through fault current at connected burden. Vk ≥ 4 * Ift * (Rct + Rl + Rr / Ir 2) Where, Ift

=

Maximum secondary side through fault current

Rct

=


Rl

=

Lead resistance

Rr

=

Relay burden

Ir

=


Full load current of the transformer = (20000 / 1.732 * 11) = 1050 Amps. Doc-No:G1408-AA-2030-J-0-101 Rev - A

Page 15 of 39

TENDER NO. G1408 – FIVE (5) NEW 33/11KV PRIMARY SUBSTA TIONS WITH ASSOCIATED 33KV OVERHEAD LINES & 33KV CABLE L INES Impedance of transformer = 10% Through fault current = 1050 / 0.1 = 10500 Amps Secondary side through fault current = 10500 / 1200 = 8.75 Amps. Hence Vk ≥ 4 * (10500/1200) * (10.5+ 0.5244 + 0.1 / 12) Vk

389 V

The knee point voltage proposed by us is 400V. Hence this condition is Verified. Ab st rac t Therefore a CT of Vk > 400V is proposed for 1200/1A.Imag shall be 30mA @ Vk/2

REF Protectio n Core Knee Point Calculation s CT Ratio: 1200/1 Type of relay: SPAJ 115C (ABB make) Fault current rating of 11 kV switchgear

= 31.5 kA

The REF relay is mounted on 33kV relay panel, which is located in control room. The knee-point voltage Vk should be 2 times higher than the stabilising voltage Vs required in through fault conditions: Vk

=

2 x Vs =

2 x Ikmax x (Rct + Rl) / n

Vs

=

stabilising voltage

Ikmax

=

the maximum through fault current

Where,

According to ESI Standard 48, the voltage set on the relay is chosen such that the stability of the protection during a through fault being taken as 16 times of the rated transformer current. Ikmax

=

16 x Transformer rated secondary current = 16 x 1050 = 16796 amp

Rct

=

Internal resistance of the current transformer

=

10.5 ohms

Rl

=

total lead resistance

=

0.524 ohms

n

=

turns ratio of the current transformer

=

1200

Substituting the above values in the equation the stabilising voltage, Vs

=

16 x 1050 x (10.5 + 0.5244) / 1200

=

154 Volts

Hence, Vk

=

2 x Vs

Vk

=

2 x 154 V

=

308V

The knee point voltage proposed by us is 400V. Hence this condition is Verified. Doc-No:G1408-AA-2030-J-0-101 Rev - A

Page 16 of 39

TENDER NO. G1408 – FIVE (5) NEW 33/11KV PRIMARY SUBSTA TIONS WITH ASSOCIATED 33KV OVERHEAD LINES & 33KV CABLE L INES Ab st rac t Since the CT Requirement of the REF and Differential Core is the same, we propose to use a common core for both. Therefore a CT of Vk > 400V is proposed for 1200/1A.Imag shall be 30mA @ Vk/2 .

4.2.3

CT for Standb y Earth Fault(SBEF) Protect ion : CT Ratio: Ratio is considered as 900-1200/1(As calculated in 4.2.1 above) Type of relay: REJ 521 (ABB make) The CT is mounted on the transformer neutral. Fault current rating of 11kV switchgear = 31.5kA The SBEF relay is mounted on 33kV relay panel which is located in control room. Burden of relay = 0.1VA(As per relay catalogue) Minimum length of cable between CT and relay Size of cable

= 50 meters(As per equipment layout)



= 4 sqmm

Resistance of cable at = 4.61(1+0.00393 (55-20) ) 55 deg C, considering temperature correction factor = 5.244 ohm/km = 2 x 5.244 x 50 / 1000

Total resistance

(Lead & Return conductors) = 0.5244 Ω Total burden



=1.25 x 0.6244 = 0.78 VA

Hence burden rating of 10 VA is chosen for 900A Tap and 15VA is chosen for 1200A. The CT Accuracy Class selected is CL. 5P20 as per the specification requirements. For a Tap of 900A To ensure correct operation of the connected relay in case of faults, the CT must be able to transform the maximum symmetrical short circuit current without saturation. To satisfy the above, following condition to be checked: Koalf Where,

≥ Iscc / Ipn Ipn

Doc-No:G1408-AA-2030-J-0-101 Rev - A

=


=

900A Page 17 of 39

TENDER NO. G1408 – FIVE (5) NEW 33/11KV PRIMARY SUB STATIONS WITH ASSOCIATED 33KV OVERHEAD LINES & 33KV CABLE L INES Iscc

=


Koalf

=


31.5kA


=

Knalf x Pb + Pi

Where, Knalf

=

20

Pn

=

Nominal CT burden

Pi

=

Internal CT burden

=

V*I

=

(I * Rct) * I

=

I2 * Rct

Rct

Pb

= 10VA

=

3Ohm(Assumed)

=

12 x 3

=

3 VA

=


=

Pr + PL

=

0.1 + 0.5244 =

0.6244 VA

10 + 3 Koalf

=

20 x 0.6244 + 3

= Iscc / Ipn = =

71 31500 / 900 35

∴ Koalf > Iscc / Ipn Hence the selected CT with parameters 900 – 1200 / 1, 5P20, 10VA is adequate.

For a Tap of 1200A To ensure correct operation of the connected relay in case of faults, the CT must be able to transform the maximum symmetrical short circuit current without saturation. To satisfy the above, following condition to be checked: Koalf Where,

≥ Iscc / Ipn Ipn

=


Iscc

=


Koalf

=


Doc-No:G1408-AA-2030-J-0-101 Rev - A

=

1200A 31.5kA

Page 18 of 39

TENDER NO. G1408 – FIVE (5) NEW 33/11KV PRIMARY SUB STATIONS WITH ASSOCIATED 33KV OVERHEAD LINES & 33KV CABLE L INES The operating accuracy limiting factor (K oalf ) depends on the nominal accuracy limiting factor (Knalf ), the nominal CT burden (P n), the internal CT burden (P i) and the total connected burden (Pb). Pn + Pi Koalf = Knalf x Pb + Pi Where, Knalf

=

20

Pn

=

Nominal CT burden

Pi

=

Internal CT burden

=

V*I

=

(I * Rct) * I

=

I2 * Rct

=

5(As confirmed by manufacturer)

=

12 x 5

=

5 VA

=


=

Pr + PL

=

0.1 + 0.5244 =

Rct

Pb

= 15 VA

0.6244 VA

15 + 5 Koalf

=

20 x 0.6244 + 5

= Iscc / Ipn = =

71.11 31500 / 1200 26.25

∴ Koalf > Iscc / Ipn Hence the selected CT with parameters 900 – 1200 / 1, 5P20, 15VA is adequate. Ab st rac t Therefore a CT of CL. 5P20,10VA is proposed for 900-1200/1A and CL. 5P20,15VA is proposed for 900-1200/1A. The same is as per the specification.

4.2.4

CT’s for Overcurrent and Earth Fault Protectio n: CT Ratio: 900-1200/1(As calculated in 4.2.1 above) Type of relay: SPAJ 140C (ABB make) Fault current rating of 11kV switchgear = 31.5kA The relay is mounted on 11kV switchgear.

Doc-No:G1408-AA-2030-J-0-101 Rev - A

Page 19 of 39

TENDER NO. G1408 – FIVE (5) NEW 33/11KV PRIMARY SUB STATIONS WITH ASSOCIATED 33KV OVERHEAD LINES & 33KV CABLE L INES Burden of relay = 0.1VA(As per relay catalogue) Minimum length of cable between CT and relay Size of cable

= 5 meters



= 2.5 sqmm

Resistance of cable at = 7.41(1+0.00393 (55-20) ) 55 deg C, considering temperature correction factor

=

8.43 ohm/km

Total resistance = 2 x 8.43 x 5 / 1000 (Lead & Return conductors) = 0.0843 Ω Burden due to lead resistance = Total resistance x (CT secondary current) 2 = 0.0843 x 12 = 0.0843 VA Total burden



=1.25 x 0.1843 = 0.23 VA

Hence burden rating of 10 VA is chosen for 900A Tap and 15VA is chosen for 1200A. The CT Accuracy Class selected is CL. 5P20 as per the specification requirements. For a Tap of 900A To ensure correct operation of the connected relay in case of faults, the CT must be able to transform the maximum symmetrical short circuit current without saturation. To satisfy the above, following condition to be checked: Koalf Where,

≥ Iscc / Ipn Ipn

=


=

Iscc

=


Koalf

=


900A 31.5kA


=

Knalf x Pb + Pi

Where, Knalf

=

20

Pn

=

Nominal CT burden

Doc-No:G1408-AA-2030-J-0-101 Rev - A

= 10VA Page 20 of 39

TENDER NO. G1408 – FIVE (5) NEW 33/11KV PRIMARY SUB STATIONS WITH ASSOCIATED 33KV OVERHEAD LINES & 33KV CABLE L INES Pi

Rct

Pb

=

Internal CT burden

=

V*I

=

(I * Rct) * I

=

I2 * Rct

=

3Ohm(Assumed)

=

12 x 3

=

3 VA

=


=

Pr + PL

=

0.1 + 0.0843

=

0.1843 VA 10 + 3

Koalf

=

20 x 0.1843+ 3

= Iscc / Ipn = =

82 31500 / 900 35

∴ Koalf > Iscc / Ipn Hence the selected CT with parameters 900 – 1200 / 1, 5P20, 10VA is adequate. For a Tap of 1200A To ensure correct operation of the connected relay in case of faults, the CT must be able to transform the maximum symmetrical short circuit current without saturation. To satisfy the above, following condition to be checked: Koalf Where,

≥ Iscc / Ipn Ipn

=


=

Iscc

=


Koalf

=


1200A 31.5kA

The operating accuracy limiting factor (K oalf ) depends on the nominal accuracy limiting factor (Knalf ), the nominal CT burden (P n), the internal CT burden (P i) and the total connected burden (Pb). Pn + Pi Koalf = Knalf x Pb + Pi Where, Knalf

=

20

Pn

=

Nominal CT burden

Pi

=

Internal CT burden

=

V*I

=

(I * Rct) * I

Doc-No:G1408-AA-2030-J-0-101 Rev - A

= 15 VA

Page 21 of 39


Rct

Pb

=

I2 * Rct

=

5(As confirmed by manufacturer)

=

12 x 5

=

5 VA

=


=

Pr + PL

=

0.1 + 0.0843

=

0.1843 VA 15 + 5

Koalf

=

20 x 0.1843 + 5

= Iscc / Ipn = =

77.15 31500 / 1200 26.25

∴ Koalf > Iscc / Ipn Hence the selected CT with parameters 900 – 1200 / 1, 5P20, 15VA is adequate. Ab st rac t : Therefore a CT of CL. 5P20, 10VA is proposed for 900-1200/1A and CL. 5P20,15VA is proposed for 900-1200/1A.

4.3

33/11.55kV, 15MVA Transf ormer Bus -secti on Feeder

4.3.1

CT’s for Meterin g and Protect ion CT Ratio: 1600 /1A Type of Metering and Protection: It is proposed to use Bay Control Unit(BCU) type REF542 Plus for the same Total burden of bay control unit = 0.1 VA(Refer REF542 Plus catalogue) Minimum length of cable = 5 meters between CT and BCU Size of cable

= 2.5 sqmm



Resistance of cable at = 7.41(1+0.00393 (55-20) ) 55 deg C, considering temperature correction factor = 8.43 ohm/km Total resistance = 2 x 8.43 x 5 / 1000 (Lead & Return conductors) = 0.0843 Ω Doc-No:G1408-AA-2030-J-0-101 Rev - A

Page 22 of 39

TENDER NO. G1408 – FIVE (5) NEW 33/11KV PRIMARY SUB STATIONS WITH ASSOCIATED 33KV OVERHEAD LINES & 33KV CABLE L INES Burden due to lead resistance = Total resistance x (CT secondary current) 2 = 0.0843 x 12 = 0.0843 VA Total burden

= Total burden of the BCU + Burden due to lead resistance = 0.1 + 0.0843 = 0.1843 VA


=1.25 x 0.1843 = 0.23 VA

Hence burden rating of 15 VA is chosen. The CT Accuracy Class selected is CL. 5P20 as per the specification requirements. Therefore a CT of 1600/1A, CL. 5P20, 15VA i s p rop osed. To ensure correct operation of the connected relay in case of faults, the CT must be able to transform the maximum symmetrical short circuit current without saturation. To satisfy the above, following condition to be checked: Koalf Where,

≥ Iscc / Ipn Ipn

=


=

Iscc

=


Koalf

=


1600A 31.5kA


=

20

Pn

=

Nominal CT burden

Pi

=

Internal CT burden

=

V*I

=

(I * Rct) * I

=

I2 * Rct

=

8Ohm(As confirmed by the manufacturer)

=

12 x 8

=

8 VA

=


=

Pr + PL

=

0.1 +0.0843

Rct

Pb

Doc-No:G1408-AA-2030-J-0-101 Rev - A

= 15 VA

Page 23 of 39

TENDER NO. G1408 – FIVE (5) NEW 33/11KV PRIMARY SUB STATIONS WITH ASSOCIATED 33KV OVERHEAD LINES & 33KV CABLE L INES =

0.1843 VA 15 + 8

Koalf

=

20 x 0.1843 + 8

= Iscc / Ipn = =

56.2 31500 / 1600 19.68

∴ Koalf > Iscc / Ipn Ab st rac t Therefore a CT of CL. 5P20,15VA is proposed with for 1600/1A

4.3.2

CT’s for Transduc ers CT Ratio: 1600 /1A CT Type of Metering: We propose to use Transducers for the same Maximum burden of current transducer = 2 VA (Actual Burden is 0.2VA only) The transducers are located in LDC cubicle. Minimum length of cable = 40 meters between CT and transducer Size of cable = 4 sqmm Resistance of cable per = 4.61 Ohms (as per Ducab cable catalogue) km at 20 deg C Resistance of cable at = 4.61(1+0.00393 (55-20) ) 55 deg C, considering temperature correction factor = 5.244 ohm/km Total resistance = 2 x 5.244 x 40 / 1000 (Lead & Return conductors) = 0.419 Ω Burden due to lead resistance = Total resistance x (CT secondary current) 2 = 0.419 x 12 = 0.419 VA Total burden

= Total burden of the transducer + Burden due to lead resistance = 2 + 0.419 = 2.419 VA


=1.25 x 2.419 = 3 VA

Hence burden rating of 15 VA is chosen. Doc-No:G1408-AA-2030-J-0-101 Rev - A

Page 24 of 39

TENDER NO. G1408 – FIVE (5) NEW 33/11KV PRIMARY SUBSTA TIONS WITH ASSOCIATED 33KV OVERHEAD LINES & 33KV CABLE L INES The CT Accuracy Class selected is CL. 0.5 as per the specification requirements. The instrument security factor (ISF) selected is less than or equal to Five(5) as per the specification requirements. Ab st rac t Therefore a CT of 1600/1A, CL. 0.5FS5, 15VA is proposed with factor of safety less than or equal to 5.

4.4

33/11.55kV, 20MVA Transf ormer Bus -secti on Feeder

4.4.1

CT’s for Meterin g and Protect ion CT Ratio: 2500 /1A Type of Metering and Protection: It is proposed to use Bay Control Unit(BCU) type REF542 It is proposed to provide 2500/1A CT for metering and protection. Total burden of bay control unit = 0.1 VA(Refer REF542 Plus catalogue) Minimum length of cable between CT and BCU

= 5 meters

Size of cable

= 2.5 sqmm



Resistance of cable at = 7.41(1+0.00393 (55-20) ) 55 deg C, considering temperature correction factor = 8.43 ohm/km Total resistance = 2 x 8.43 x 5 / 1000 (Lead & Return conductors) = 0.08429 Ω Burden due to lead resistance = Total resistance x (CT secondary current) 2 = 0.0843 x 12 Total burden

= 0.0843 VA = Total burden of the BCU + Burden due to lead resistance = 0.1 + 0.0843 = 0.1843 VA


=1.25 x 0.1843 = 0.23 VA

Hence burden rating of 15 VA is chosen. The CT Accuracy Class selected is CL. 5P20 as per the specification requirements. Therefore a CT of 2500/1A, CL. 5P20, 15VA i s p rop osed.

Doc-No:G1408-AA-2030-J-0-101 Rev - A

Page 25 of 39

TENDER NO. G1408 – FIVE (5) NEW 33/11KV PRIMARY SUB STATIONS WITH ASSOCIATED 33KV OVERHEAD LINES & 33KV CABLE L INES To ensure correct operation of the connected relay in case of faults, the CT must be able to transform the maximum symmetrical short circuit current without saturation. To satisfy the above, following condition to be checked: Koalf Where,

≥ Iscc / Ipn Ipn

=


=

Iscc

=


Koalf

=


2500A 31.5kA

The operating accuracy limiting factor (K oalf ) depends on the nominal accuracy limiting factor (Knalf ), the nominal CT burden (P n), the internal CT burden (P i) and the total connected burden (Pb). Pn + Pi Koalf = Knalf x Pb + Pi Where, Pn

=

Nominal CT burden

Pi

=

Internal CT burden

=

V*I

=

(I * Rct) * I

=

I2 * Rct

=

12 x 10

=

10 VA

=


=

Pr + PL

=

0.1 +0.0843

Pb

=

= 15 VA

0.1843 VA

15 + 10 Koalf

=

20 x 0.1843 + 10

= Iscc / Ipn = =

49 31500 / 2500 12.6

∴ Koalf > Iscc / Ipn Ab st rac t Therefore a CT of CL. 5P20,15VA is proposed with for 2500/1A 4.4.2

CT’s for Transduc ers CT Ratio: 2500 /1A CT Type of Metering: We propose to use Transducers for the same

Doc-No:G1408-AA-2030-J-0-101 Rev - A

Page 26 of 39

TENDER NO. G1408 – FIVE (5) NEW 33/11KV PRIMARY SUBSTA TIONS WITH ASSOCIATED 33KV OVERHEAD LINES & 33KV CABLE L INES Maximum burden of current transducer = 2 VA (Actual is 0.2VA ) The transducers are located in LDC cubicle. Minimum length of cable = 40 meters between CT and transducer Size of cable

= 4 sqmm



Resistance of cable at = 4.61(1+0.00393 (55-20) ) 55 deg C, considering temperature correction factor = 5.244 ohm/km Total resistance = 2 x 5.244 x 40 / 1000 (Lead & Return conductors) = 0.419 Ω Burden due to lead resistance = Total resistance x (CT secondary current) 2 = 0.419 x 12 = 0.419 VA Total burden



=1.25 x 2.419 = 3 VA

Hence burden rating of 15 VA is chosen. The CT Accuracy Class selected is CL. 0.5 as per the specification requirements. The instrument security factor (ISF) selected is less than Five(5) as per the specification requirements. Ab st rac t Therefore a CT of 2500/1A, CL. 0.5FS5, 15VA is proposed with factor of safety less than or equal to 5.

4.5

Outg oin g 11/0.433kV, 500kVA Transf orm er Feeder

4.5.1

CT’s for Meterin g and Protect ion CT Ratio: 30 /1A Type of Metering and Protection: It is proposed to use Bay Control Unit(BCU) type REF542 Transformer full load current on primary side = 500/(√3*11) = 26.24 Amps

Doc-No:G1408-AA-2030-J-0-101 Rev - A

Page 27 of 39

TENDER NO. G1408 – FIVE (5) NEW 33/11KV PRIMARY SUB STATIONS WITH ASSOCIATED 33KV OVERHEAD LINES & 33KV CABLE L INES Hence 30/1A CT is selected for metering and protection as per the specification requirements. Total burden of bay control unit = 0.1 VA(Refer REF542 Plus catalogue) Minimum length of cable between CT and BCU Size of cable

= 5 meters



= 2.5 sqmm

Resistance of cable at = 7.41(1+0.00393 (55-20) ) 55 deg C, considering temperature correction factor = 8.43 ohm/km Total resistance = 2 x 8.43 x 5 / 1000 (Lead & Return conductors) = 0.0843 Ω Burden due to lead resistance = Total resistance x (CT secondary current) 2 = 0.0843 x 12 = 0.0843 VA = Total burden of the BCU + Burden due to lead resistance = 0.1 + 0.0843

Total burden

= 0.1843 VA Considering 25% future margin Total burden required

=1.25 x 0.1843 = 0.23 VA

Hence burden rating of 6 VA is chosen. The CT Accuracy Class selected is CL. 5P20 as per the specification requirements. Therefore a CT of 30/1A, CL. 5P20, 6VA is pro pos ed. To ensure correct operation of the connected relay in case of faults, the CT must be able to transform the maximum symmetrical short circuit current without saturation. To satisfy the above, following condition to be checked: Koalf Where,

≥ Iscc / Ipn Ipn

=


=

Iscc

=


Koalf

=


30A 31.5kA


=

Knalf x Pb + Pi

Where, Doc-No:G1408-AA-2030-J-0-101 Rev - A

Page 28 of 39

TENDER NO. G1408 – FIVE (5) NEW 33/11KV PRIMARY SUB STATIONS WITH ASSOCIATED 33KV OVERHEAD LINES & 33KV CABLE L INES Knalf

=

20

Pn

=

Nominal CT burden

Pi

=

Internal CT burden

=

V*I

=

(I * Rct) * I

=

I2 * Rct

=

0.1(As confirmed by manufacturer)

=

12 x 0.1

=

0.1 VA

=


=

Pr + PL

=

0.1 +0.0843

Rct

Pb

=

= 6 VA

0.1843 VA

6+ 0.1 Koalf

=

20 x 0.1843 + 0.1

=

430

Transformer Short Circuit Transformer,

current

is limited by the 5% impedence of the

Hence, Iscc

=

500/(√3*110.05)

=

530 Amps

Iscc / Ipn = =

530 / 30X 0.05 17

∴ Koalf > Iscc / Ipn Thus from the above calculations it’s also evident that,the actual short Circuit withstand of the CT is 430 x 30 =12.9kA

Note: As the ratio of CT for auxiliary transformer is very low i.e 30 / 1A as per the specification requirement, the best CT size realized to be accommodated will be 30/1A, 5P20, 6VA with 31.5kA for 1sec short circuit level. Due to limitation on CT dimension imposed by low CT ratio and CT short circuit with stand capacity restr icted to 31.5kA fo r 1 sec., We request yo u ki nd approval for th e same.

Ab st rac t Therefore a CT of CL. 5P20,6VA is proposed with for 30/1A 4.5.2

CT’s for Transduc ers CT Ratio: 30 /1A CT

Doc-No:G1408-AA-2030-J-0-101 Rev - A

Page 29 of 39

TENDER NO. G1408 – FIVE (5) NEW 33/11KV PRIMARY SUBSTA TIONS WITH ASSOCIATED 33KV OVERHEAD LINES & 33KV CABLE L INES Type of Metering: We propose to use Transducers for the same Maximum burden of current transducer = 2 VA (Actual is 0.2VA ) The transducers are located in LDC cubicle. Minimum length of cable = 40 meters between CT and transducer Size of cable

= 4 sqmm

Resistance of cable per = 4.61 Ohms (as per Ducab cable catalogue) km at 20 deg C Resistance of cable at = 4.61(1+0.00393 (55-20) ) 55 deg C, considering temperature correction factor = 5.244 ohm/km Total resistance = 2 x 5.244 x 40 / 1000 (Lead & Return conductors) = 0.419 Ω Burden due to lead resistance = Total resistance x (CT secondary current) 2 = 0.419 x 12 Total burden

= 0.419 VA = Total burden of the transducer + Burden due to lead resistance = 2 + 0.419 = 2.419 VA


=1.25 x 2.419 = 3 VA

Hence burden rating of 7.5 VA is chosen. The CT Accuracy Class selected is CL. 0.5 as per the specification requirements. The instrument security factor (ISF) selected is less than or equal to Five(5) as per the specification requirements. Ab st rac t Therefore a CT of 30/1A, CL. 0.5FS5, 7.5VA is proposed with factor of safety less than or equal to 5.

4.6

Outgoing 11kV, 5MVAr Capacito r Feeder

4.6.1

CT’s for Meterin g and Protect ion CT Ratio: 300-600 / 1A Type of Metering and Protection: It is proposed to use Bay Control Unit(BCU) type REF542

Capacitor full load current Doc-No:G1408-AA-2030-J-0-101 Rev - A

=

MVAr / ( √3 * kV * sinθ ) Page 30 of 39

TENDER NO. G1408 – FIVE (5) NEW 33/11KV PRIMARY SUBSTA TIONS WITH ASSOCIATED 33KV OVERHEAD LINES & 33KV CABLE L INES =

5000 / (√3 * 11 * 0.6) =

437.38 Amps

Hence it is proposed to provide 300-600 / 1A CT for metering and protection. Total burden of bay control unit = 0.1 VA (Refer REF542 Plus catalogue) Minimum length of cable between CT and BCU

= 5 meters

Size of cable

= 2.5 sqmm






= 1.25 x 0.1843 = 0.23 VA

Hence burden range of 7.5VA is proposed for 300A Tap and 15VA is proposed for 600A Tap. The CT Accuracy Class selected is CL. 5P20 as per the specification requirements.

For a Tap of 300A To ensure correct operation of the connected relay in case of faults, the CT must be able to transform the maximum symmetrical short circuit current without saturation. To satisfy the above, following condition to be checked: Koalf Where,

≥ Iscc / Ipn Ipn

=


Iscc

=


Koalf

=


Doc-No:G1408-AA-2030-J-0-101 Rev - A

=

300A 31.5kA

Page 31 of 39

TENDER NO. G1408 – FIVE (5) NEW 33/11KV PRIMARY SUB STATIONS WITH ASSOCIATED 33KV OVERHEAD LINES & 33KV CABLE L INES The operating accuracy limiting factor (K oalf ) depends on the nominal accuracy limiting factor (Knalf ), the nominal CT burden (P n), the internal CT burden (P i) and the total connected burden (Pb). Pn + Pi Koalf = Knalf x Pb + Pi Where, Knalf

=

20

Pn

=

Nominal CT burden

Pi

=

Internal CT burden

=

V*I

=

(I * Rct) * I

=

I2 * Rct

=

1.5Ohm (Assumed)

=

12 x 1.5

=

1.5 VA

=


=

Pr + PL

=

0.1 + 0.08429

Rct

Pb

= 7.5 VA

=

0.1843 VA

7.5 + 1.5 Koalf

=

20 x 0.1843 + 1.5

= Iscc / Ipn = =

109 31500 / 300 10.5

∴ Koalf > Iscc / Ipn Hence the selected CT with parameters 300-600 / 1, 5P20, 7.5VA is adequate. For a Tap of 600A To ensure correct operation of the connected relay in case of faults, the CT must be able to transform the maximum symmetrical short circuit current without saturation. To satisfy the above, following condition to be checked: Koalf Where,

≥ Iscc / Ipn Ipn

=


=

Iscc

=


Koalf

=


600A 31.5kA

The operating accuracy limiting factor (K oalf ) depends on the nominal accuracy limiting factor (Knalf ), the nominal CT burden (P n), the internal CT burden (P i) and the total connected burden (Pb).

Doc-No:G1408-AA-2030-J-0-101 Rev - A

Page 32 of 39

TENDER NO. G1408 – FIVE (5) NEW 33/11KV PRIMARY SUB STATIONS WITH ASSOCIATED 33KV OVERHEAD LINES & 33KV CABLE L INES Pn + Pi Koalf

=

Knalf x Pb + Pi

Where, Knalf

=

20

Pn

=

Nominal CT burden

Pi

=

Internal CT burden

=

V*I

=

(I * Rct) * I

=

I2 * Rct

=

3 Ohm

=

12 x 3

=

3VA

=


=

Pr + PL

=

0.1 + 0.08429

Rct

Pb

= 15 VA

=

0.1843 VA

15 + 3 Koalf

=

20 x 0.1843 + 3

= Iscc / Ipn = =

113 31500 / 600 52.5

∴ Koalf > Iscc / Ipn Hence the s elected CT wi th parameters 300-600 / 1, 5P20, 15VA is adequate Ab st rac t Therefore a CT of CL. 5P20,7.5VA is proposed for 300-600/1A and CL. 5P20,15VA is proposed for 300-600/1A. The same is as per the specification. 4.6.2

CT’s for Transduc ers CT Ratio: 300 – 600 /1A Type of Measurement: We Propose to use Transducers form the same Maximum burden of Current transducer = 2 VA (Actual is 0.2VA) Maximum burden of Power factor transducer = 2 VA Maximum burden of MVAr transducer = 2 VA

Doc-No:G1408-AA-2030-J-0-101 Rev - A

Page 33 of 39

TENDER NO. G1408 – FIVE (5) NEW 33/11KV PRIMARY SUBSTA TIONS WITH ASSOCIATED 33KV OVERHEAD LINES & 33KV CABLE L INES Total burden of transducers = 6VA The transducers are located in LDC cubicle. Minimum length of cable = 40 meters between CT and transducer Size of cable = 4 sqmm Resistance of cable per km at 20 deg C



= Total burden of transducers + Burden due to lead resistance = 6 + 0.419 = 6.419 VA


=1.25 x 6.419 = 8 VA

Hence burden range of 7.5VA is proposed for 300A Tap and 15VA is proposed for 600A Tap. The CT Accuracy Class selected is CL. 0.5 as per the specification requirements. The instrument security factor (ISF) selected is equal to or less than Five(5) as per the specification requirements. Ab st rac t Therefore a CT of CL. 0.5,7.5VA is proposed for 300-600/1A and CL. 0.5,15VA is proposed for 300-600/1A.The CTs are with a factor of safety less than or equal to 5.

4.7

11kV Outgoing Feeder

4.7.1

CT’s for Meterin g and Protect ion CT Ratio: 300-600 / 1A Type of Metering and Protection: It is proposed to use Bay Control Unit(BCU) type REF542 Total burden of bay control unit = 0.1 VA (Refer REF542 Plus catalogue)

Doc-No:G1408-AA-2030-J-0-101 Rev - A

Page 34 of 39


Minimum length of cable between CT and BCU Size of cable

= 5 meters



= 2.5 sqmm




= 1.25 x 0.1843 = 0.23 VA

Hence burden range of 7.5VA is proposed for 300A Tap and 15VA is proposed for 600A Tap. The CT Accuracy Class selected is CL. 5P20 as per the specification requirements. For a Tap of 300A To ensure correct operation of the connected relay in case of faults, the CT must be able to transform the maximum symmetrical short circuit current without saturation. To satisfy the above, following condition to be checked: Koalf Where,

≥ Iscc / Ipn Ipn

=


=

Iscc

=


Koalf

=


300A 31.5kA


=

Knalf x Pb + Pi

Where, Knalf

=

Doc-No:G1408-AA-2030-J-0-101 Rev - A

20 Page 35 of 39

TENDER NO. G1408 – FIVE (5) NEW 33/11KV PRIMARY SUB STATIONS WITH ASSOCIATED 33KV OVERHEAD LINES & 33KV CABLE L INES Pn

=

Nominal CT burden

Pi

=

Internal CT burden

=

V*I

=

(I * Rct) * I

=

I2 * Rct

=

1.5Ohm (Assumed)

=

12 x 1.5

=

1.5 VA

=


=

Pr + PL

=

0.1 + 0.08429

Rct

Pb

= 7.5 VA

=

0.1843 VA

7.5 + 1.5 Koalf

=

20 x 0.1843 + 1.5

= Iscc / Ipn = =

109 31500 / 300 10.5

∴ Koalf > Iscc / Ipn Hence the selected CT with parameters 300-600 / 1, 5P20, 7.5VA is adequate. For a Tap of 600A To ensure correct operation of the connected relay in case of faults, the CT must be able to transform the maximum symmetrical short circuit current without saturation. To satisfy the above, following condition to be checked: Koalf Where,

≥ Iscc / Ipn Ipn

=


=

Iscc

=


Koalf

=


600A 31.5kA


=

20

Pn

=

Nominal CT burden

Pi

=

Internal CT burden

=

V*I

=

(I * Rct) * I

Doc-No:G1408-AA-2030-J-0-101 Rev - A

= 15 VA

Page 36 of 39


Rct

Pb

=

I2 * Rct

=

3 Ohm

=

12 x 3

=

3VA

=


=

Pr + PL

=

0.1 + 0.08429

=

0.1843 VA

15 + 3 Koalf

=

20 x 0.1843 + 3

= Iscc / Ipn = =

113 31500 / 600 52.5

∴ Koalf > Iscc / Ipn Hence the s elected CT wi th parameters 300-600 / 1, 5P20, 15VA is adequate Ab st rac t Therefore a CT of CL. 5P20,7.5VA is proposed for 300-600/1A and CL. 5P20,15VA is proposed for 300-600/1A. 4.7.2

CT’s for Transduc ers CT Ratio: 300 – 600 /1A Type of Metering: It is proposed to use Transducers. Maximum burden of current transducer = 2 VA (Actual Burden is 0.2 only) The transducers are located in LDC cubicle. Minimum length of cable = 40 meters between CT and transducer Size of cable = 4 sqmm Resistance of cable per km at 20 deg C


Resistance of cable at = 4.61(1+0.00393 (55-20) ) 55 deg C, considering temperature correction factor = 5.244 ohm/km Total resistance = 2 x 5.244 x 40 / 1000 (Lead & Return conductors) = 0.419 Ω Doc-No:G1408-AA-2030-J-0-101 Rev - A

Page 37 of 39

TENDER NO. G1408 – FIVE (5) NEW 33/11KV PRIMARY SUBSTA TIONS WITH ASSOCIATED 33KV OVERHEAD LINES & 33KV CABLE L INES Burden due to lead resistance = Total resistance x (CT secondary current) 2 = 0.419 x 12 = 0.419 VA Total burden



=1.25 x 2.419 = 3 VA

Hence burden range of 7.5VA is proposed for 300A Tap and 15VA is proposed for 600A Tap. Ab st rac t Therefore a CT of CL. 0.5,7.5VA is proposed for 300-600/1A and CL. 0.5,15VA is proposed for 300-600/1A.The CTs are with a factor of safety less than or equal to 5.

5.0

VT SIZING CALCULATIONS The VTs considered are as follows: Lin e VT The devices connected on winding 1 of line VTs are BCU and Voltage Transducer for line feeder. The burden of the same are as given below:

BCU: Voltage Transducers:

0.25VA 5VA max.

Total

2VA Maximum Considered 5VA Maximum Considered

7VA max.

The devices connected on winding 2 of line VTs Synchronising devices for line feeder. As per the manufacturer the maximum burden for the same is 15VA. Hence the VT considered for lin e is of f ollowi ng parameters. 11kV/ 3 / 110V/ 3 / 110V/ 3,

Wind in g 1 Wind ing 2

-

30VA , 0.5S 30VA , 3P

Bus VT The devices connected on winding 1 of Bus VTs are BCU, Voltage Transducer and Load shedding relay. The burden of the same are as given below,

Doc-No:G1408-AA-2030-J-0-101 Rev - A

Page 38 of 39

11kV CT Calculation

Recommend Documents