33kv Ct Calculations

7/25/2019 33kv Ct Calculations

1/31

ABB Transmission & Dist ribution LTD.

Doc-No:G1408-AA-2030-J-0-102 Page 2 of32Rev A

Document Title : CT/VT calculations for 33kV Switchgear forADWEA contract no G1408

TABLE OF CONTENTS

SECTION TITLE PAGE NO

1.0 OBJECTIVE......................................................................................................3

2.0 REFERENCES..................................................................................................3

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

4.0 CT SIZING CALCULATIONS............................................................................34.1 33kV Cable Feeder........................................................................................................... 34.1.1 CT's for Metering .............................................................................................................. 34.1.2 CT's for Pilot wire protection ............................................................................................. 44.1.3 CT's for Busbar protection................................................................................................. 44.1.4 CT's for Overcurrent and Earth fault protection ................................................................ 64.2 33/11.55kV,15& 20MVA Transformer Feeder................................................................ 84.2.1 CT's for Metering .............................................................................................................. 84.2.2 CT's for Differential protection......................................................................................... 104.2.3 CT's for Overcurrent and Earth fault protection .............................................................. 154.2.4 CT's for Busbar protection............................................................................................... 204.3 33kV Bus sect ion Feeder.............................................................................................. 20

4.3.1 CT's for Metering ............................................................................................................ 204.3.2 CT's for Overcurrent and Earth fault protection .............................................................. 214.3.3 CT's for Busbar protection............................................................................................... 224.4 33kV Over head l ine(OHL) Feeder............................................................................... 224.4.1 CT's for Metering ............................................................................................................ 224.4.2 CT's for Distance protection............................................................................................ 234.4.3 CT's for Overcurrent and Earth fault protection .............................................................. 274.4.4 CT's for Busbar protection............................................................................................... 294.4.5 CT's for Sensitive Earth Fault protection......................................................................... 29

5.0 VT SIZING CALCULATIONS.......................................................................... 31

6.0 Conclusion ..................................................................................................... 32

7.0 Summary Of Current Transformers (Annexure 1)......32

8.0 Summary Of Voltage Transformers(Annexure 2) .......................................... 32

NOTE:

The catalogues of all the relays and cables are enclosed vide DTS no. 0012 dated15.11.2003 and DTS no. 0015 dated 23.11.2003.


2/31



Document Title : CT calculations for 33kV Switchgear for ADWEAcont ract no G1408

1.0 OBJECTIVE

To establish current transformer(CT) and Voltage transformer(VT) parameters at 33kV level for all the 5 new substations.

2.0 REFERENCES

a) Bay control unit REF542 Plus catalogue

b) 33kV switchgear vendor (ABB Calor Emag) Single line diagram

c) Ducab cable catalogue

d) ESI standard 48-3 and Clients/Consultant Recommendations

e) Relay catalogues Recommendations

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 33 kVswitchgear at all the sub-stations.

4.0 CT SIZING CALCULATIONS

4.1 33kV Cable Feeder

4.1.1 CTs for Metering

CT Ratio:

600-1200/1A.

Type of Metering:

Metering Through Bay Control Unit (BCU) typeREF542 Plus

Burden of bay control unit = 0.1 VA (Refer REF542 Plus catalogue)

The BCU is mounted on the 33kV switchgear.

Minimum length of cable = 5 metersbetween CT and BCU

Size of cable = 2.5 sqmm

Resistance of cable per = 7.41 Ohms (Refer Ducab cable catalogue)km at 20 deg C

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

= 8.429 ohm/km

Total resistance = 2 x 8.429 x 5 / 1000


3/31



(Lead & Return conductors) = 0.08429

Burden due to lead resistance = Total resistance x (CT secondary current)2

= 0.08429 x 12

= 0.08429 VA

Total burden = Total burden of the BCU + Burden due to leadresistance

= 0.1 + 0.08429

= 0.18429 VA

Considering 25% future margin

Total burden required =1.25 x 0.18429 = 0.23 VA

Hence a standard burden rating of minimum 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 thespecification requirements.

Abstract:

Therefore a CT of CL. 0.5,15VA is proposed for 600-1200/1A and CL. 0.5,30VA isproposed for 600-1200/1A.The CTs are with a factor of safety less than or equal to 5.

The same is as per the specification.

4.1.2 CTs for Pilot Wire Protection:

The current transformer details for all the incoming cable feeders from differentsubstations are proposed to match with the remote end CT data.

The calculation for pilot wire protection will be included in this document afterfinalization of remote end CT details.

4.1.3 CTs for Bus Bar Protection:

CT Ratio:

2000/1 A

Type of relay: REB 500 ( ABB make)

Fault current rating of 33kV switchgear = 31.5kA

This relay is mounted on 33kV busbar protection panel which is located in control

room.Burden of relay = 0.1VA(Refer relay catalogue)

Minimum length of cable = 30 metersbetween CT and relay

Size of cable = 4 sqmm

Resistance of cable per = 4.61 Ohms (Refer Ducab cable catalogue)km/phase at 20 deg C

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

temperature correction factor= 5.244ohm/km


4/31






= 0.314 x 12

= 0.314 VA

Total burden = Burden of the relay + Burden due to leadresistance

= 0.1 + 0.314 = 0.414 VA



Hence a standard burden rating of 15 VA is chosen.

The CT Accuracy Class selected is CL. 5P20 as per the relay requirements.

Therefore a CT of 2000/1A, CL. 5P20, 15VA is proposed.

To ensure correct operation of the connected relay in case of faults, the CT must beable to transform the maximum symmetrical short circuit current without saturation.

To satisfy the above, following condition to be checked:

Koalf Iscc/ Ipn

Where, Ipn = CT primary nominal current = 2000A

Iscc = max. symmetrical short circuit current= 31.5kA

Koalf = Operating accuracy limiting factor

The operating accuracy limiting factor (Koalf) depends on the nominal accuracy limiting

factor (Knalf), the nominal CT burden (Pn), the internal CT burden (P i) and the totalconnected burden (Pb).

Pn+ Pi Koalf = Knalfx

Pb+ Pi

Where,

Pn = Nominal CT burden = 15 VA

Knalf = 20

Pi = Internal CT burden

= V * I= (I * Rct) * I

= I2* Rct

Rct = 9 Ohm (as per manufacturers recommendations)

= 12x 9

= 9 VA

Pb = Total connected burden

= Pr+ PL

= 0.1 + 0.314= 0.414 VA


5/31



15 + 9

Koalf = 20 x0.414 + 9

= 51

Iscc/ Ipn = 31500 / 2000

= 15.75

Koalf> Iscc/ Ipn

Hence the selected CT with parameters 2000/1, 5P20, 15VA is adequate.

Confirmations of Manufacturers Recommendations:

As per REB 500 Catalogue theKoalf Should satisfy the following conditions:

Condition 1:

Koalf>= 1xI kmax

5xI1N

I kmax : Maximum through fault current

I1N : Rated primary CT Current

I kmax=31500A

I1N = 2000A

Hence we have: 31500

5 x 2000

= 31500

10000

= 3.15

Hence Koalf(valued 51)proposed is greater than above calculated value of 3.15.

Hence the condition 1 of the manufacturers recommendations is satis fied.

Condition 2:

Koalf>= 10 for TN = 20 for TN


6/31



The relay is mounted on 33kV relay panel which is located in control room.

Burden of relay = 0.1VA(Refer relay catalogue)

Lead burden = 0.314VA (As calculated in 4.1.3 above)


= 0.1 + 0.314

= 0.414 VA


Total burden required =1.25 x 0.414 = 0.5175VA

Hence a standard burden rating of 15 VA is chosen for a 600 tap and a burden of30VA is chosen for a tap of 1200A.

The CT Accuracy Class selected is CL. 5P20 as per the specification requirements.

For a Tap of 600A


To satisfy the above, following condition to be checked :

Koalf Iscc/ Ipn


Iscc = max. symmetrical short circuit current = 31.5kA


The operating accuracy limiting factor (Koalf) depends on the nominal accuracy limitingfactor (Knalf), the nominal CT burden (Pn), the internal CT burden (P i) and the totalconnected burden (Pb).

Pn+ PiKoalf = Knalfx

Pb+ Pi

Where,


Knalf = 20


= V * I

= (I * Rct) * I

= I2* Rct


= 12x 3

= 3 VA


= Pr+ PL

= 0.1 + 0.314

= 0.414 VA


7/31



15 + 3

Koalf = 20 x0.414 + 3

= 105.44

Iscc/ Ipn = 31500 / 600

= 52.5

Koalf> Iscc/ Ipn

Hence the selected CT with parameters 600 1200 / 1, 5P20, 15VA is adequate.

For a Tap of 1200A


Knalf = 20


= V * I

= (I * Rct) * I

= I2* Rct

Rct = 6 Ohm (Assumed)

= 12x 6

= 6 VA


= Pr+ PL

= 0.1 + 0.314

= 0.414 VA

15 + 6

Koalf = 20 x0.414 + 6

= 65

Iscc/ Ipn = 31500 / 1200

= 27

Koalf> Iscc/ Ipn


Abstract:

Therefore a CT of CL. 5P20, 15VA is proposed for 600-1200/1A and CL. 5P20, 30VAis proposed for 600-1200/1A.


4.2 33/11.55kV, 15MVA & 20MVA Transformer Feeder


CT Ratio: (For 20MVA Transformer)

400-600/1A.


8/31



CT Ratio: (For 15MVA Transformer)

300-600/1A.

Type of Metering:


Total burden of bay control unit = 0.1 VA(Refer REF542 Plus catalogue)





= 8.429 ohm/km




= 0.08429 x 12

= 0.08429 VA

Total burden = Total burden of the BCU + Burden due to lead

resistance= 0.1 + 0.08429

= 0.18429 VA





The instrument security factor (ISF) selected is less than Five(5) as per thespecification requirement

Abstract (20MVA Transformer):




Therefore a CT of CL. 0.5,15VA is proposed for 300-600/1A and CL. 0.5,30VA isproposed for 300-600/1A. The CTs are with a factor of safety less than or equal to 5.



9/31



4.2.2 CTs for Differential Protection:

Differential Core Calculations for 20MVA Transformer

CT Ratio:

400-600/1A.

Type of relay: RET 316*4 (ABB make)

Knee Point Voltage Calculations:

Fault current rating of 33 kV switchgear = 31.5 kA

As per ESI Standard 48-3 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 33 kV switchgear is 31.5 kA, which can beconsidered for knee point calculation.

For a Tap of 400A

If= 31500 Amps

N = 1/400

Rct = 1.5 (As recommended by Manufacturer)

The differential relay is mounted on 33kV relay panel, which is located in control-room.





= 5.244ohm/km




= 0.314 x 12

= 0.314 VA

Therefore, substituting the above values in the formula for Vk,

VK= 31500* (1/400) * (1.5 + 0.314 )

VK= 143V

Therefore a CT of 400-600/1A, CL. X, VK> 250V is proposed.


10/31



For a Tap of 600A

Rct = 3 (Assumed)

VK= 31500* (1/600) * (3 + 0.314 )

VK= 173.9V

Therefore a CT of 400-600/1A, CL. X, VK> 250V is proposed .

Recommendations f rom Consultant/Client :

Further, to avoid mal-operation on energization of power transformer and inconnection with fault current that passes through power transformer, the ratedsecondary voltage has to satisfy the following conditions:

Condition-1

The core may not saturate for current lower than 30 times the PowerTransformer rated current at connected burden. This ensures stability alsowith heavy DC saturation.

Vk 30 * Int* (Rct+ Rl+ Rr/ Ir2

)Where,

Int = Main CT secondary current corresponding to rated primary

current of power transformer

Rct = CT secondary resistance


Rr = Relay burden

Ir = Nominal relay current

Full load current of the transformer = (20000 / 1.732 * 33) = 350 Amps.For a Tap of 400A

Vk 30 * (350/400) * (1.5+ 0.314 + 0.1 / 12)

Vk 50.24V

The knee point voltage proposed by us is 250V.

Hence this condition is Verified.

For a Tap of 600A

Vk 30 * (350/600) * (3+ 0.314 + 0.1 / 12)

Vk 60V



Condition-2

The core may not saturate for current lower than 4 times the maximumthrough fault current at connected burden.

Vk 4 * Ift* (Rct+ Rl+ Rr/ Ir2)

Where,Ift = Maximum secondary side through fault current


11/31





Rr = Relay burden


Full load current of the transformer = (20000 / 1.732 * 33) = 350 Amps.

Impedance of transformer = 10%

Through fault current = 350 / 0.1 = 3500 Amps

For a Tap of 400A

Secondary side through fault current = 3500 / 400 = 8.75 Amps.

Hence Vk 4 * 8.75 * (1.5+ 0.314 + 0.1 / 12)

Vk 67 V



For a Tap of 600A


Hence Vk 4 * 5.83 * (3+ 0.314 + 0.1 / 12)

Vk 80 V




Therefore a CT of Vk > 250V is proposed for 400-600/1A and Vk > 250V is proposedfor 400-600/1A.Imag shall be 50mA @ Vk/2

Differential Core calculations for 15MVA Transformer

CT Ratio:

300-600/1A.

Type of relay: RET 316*4 (ABB make)





Where If= fault current

N = Turns Ratio


Rl= Lead resistance

Maximum fault current carrying capacity of 33 kV switchgear is 31.5 kA, which can beconsidered for knee point calculation.


12/31



For a Tap of 300A

If= 31500 Amps

N = 1/300


The differential relay is mounted on 33kV relay panel, which is located in control-room.




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

temperature correction factor= 5.244ohm/km




= 0.314 x 12

= 0.314 VA


VK= 31500* (1/300) * (1.5 + 0.314 )

VK= 190V


For a Tap of 600A

Rct = 3 (Assumed)

VK= 31500* (1/600) * (3 + 0.314 )

VK= 173.9V

Therefore a CT of 300-600/1A, CL. X, VK> 250V is proposed .

Recommendations f rom Consultant/Client :

Further, to avoid mal-operation on energization of power transformer and inconnection with fault current that passes through power transformer, the ratedsecondary voltage has to satisfy the following conditions:

Condition-1

The core may not saturate for current lower than 30 times the powertransformer rated current at connected burden. This ensures stability also withheavy DC saturation.

Vk 30 * Int* (Rct+ Rl+ Rr/ Ir2)

Where,


13/31



Int = Main CT secondary current corresponding to rated primary

current of power transformer



Rr = Relay burden



For a Tap of 300A

Vk 30 * (265/300) * (1.5+ 0.314 + 0.1 / 12)

Vk 51V



For a Tap of 600A

Vk 30 * (265/600) * (3+ 0.314 + 0.1 / 12)

Vk 45V



Condition-2

The core may not saturate for current lower than 4 times the maximumthrough fault current at connected burden.

Vk 4 * Ift* (Rct+ Rl+ Rr/ Ir2)

Where,

Ift = Maximum secondary side through fault current



Rr = Relay burden



Impedance of transformer = 10%

Through fault current = 265 / 0.1 = 2650 Amps

For a Tap of 300A


Hence Vk 4 * 8.8 * (1.5+ 0.314 + 0.1 / 12)

Vk 67 V

The knee point voltage selected is 250V.

Hence this condition is verified.


14/31



For a Tap of 600A


Hence Vk 4 * 4.41 * (3+ 0.314 + 0.1 / 12)

Vk 60 V




Therefore a CT of Vk > 250V is proposed for 300-600/1A and Vk > 250V is proposedfor 300-600/1A. Imag shall be 60mA @ Vk/2

Differential Core Abstract:





4.2.3 CTs for Overcurrent & Earth Fault Protection:

CT Ratio (for 20MVA Transformer):

400-600/1A

Type of relay: REJ 525 (ABB make)






= 0.1 + 0.314

= 0.414 VA

Considering 25% future marginTotal burden required =1.25 x 0.414 = 0.5175VA

Hence a standard burden rating of 15 VA is chosen for a Tap of 400A Tap and aburden of 30 VA is chosen for a Tap of 600A Tap.




Koalf Iscc/ Ipn


15/31



For a Tap of 400A






Pb+ Pi

Where,

Knalf = 20



= V * I

= (I * Rct) * I

= I2* Rct


= 12x 1.5

= 1.5 VA


= Pr+ PL

= 0.1 + 0.314

= 0.414 VA

15 + 1.5

Koalf = 20 x0.414 + 1.5

= 172.41

Iscc/ Ipn = 31500 / 400

= 78.75Koalf> Iscc/ Ipn


For a Tap of 600A




The operating accuracy limiting factor (Koalf) depends on the nominal accuracy limitingfactor (Knalf), the nominal CT burden (Pn), the internal CT burden (P i) and the total

connected burden (Pb).


16/31




Pb+ Pi

Where,

Knalf = 20



= V * I

= (I * Rct) * I

= I2* Rct

Rct = 3 (As recommended by Manufacturer)

= 12x 3

= 3 VA


= Pr+ PL

= 0.1 + 0.314

= 0.414 VA

15 + 3

Koalf = 20 x0.414 + 3

= 105

Iscc/ Ipn = 31500 / 600

= 53

Koalf> Iscc/ Ipn

Hence the selected CT wi th parameters 400 600 / 1, 5P20, 30VA is adequate.

CT Ratio (for 15MVA Transformer)

300-600/1A

Type of relay: REJ 525 ( ABB make)






= 0.1 + 0.314

= 0.414 VA

Considering 25% future marginTotal burden required =1.25 x 0.414 = 0.5175VA


17/31



Hence a standard burden rating of 15 VA is chosen for a Tap of 400A Tap and aburden of 30 VA is chosen for a Tap of 600A Tap.


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 Iscc/ Ipn

For a Tap of 300A






Pb+ Pi

Where,

Knalf = 20



= V * I

= (I * Rct) * I

= I2* Rct


= 12x 1.5

= 1.5 VA


= Pr+ PL

= 0.1 + 0.314

= 0.414 VA

15 + 1.5

Koalf = 20 x0.414 + 1.5

= 172.41

Iscc/ Ipn = 31500 / 300

= 105

Koalf> Iscc/ Ipn



18/31



For a Tap of 600A






Pb+ Pi

Where,

Knalf = 20



= V * I

= (I * Rct) * I

= I2* Rct

Rct = 3 (As recommended by Manufacturer)

= 12x 3

= 3 VA


= Pr+ PL

= 0.1 + 0.314

= 0.414 VA

15 + 3

Koalf = 20 x0.414 + 3

= 105

Iscc/ Ipn = 31500 / 600

= 53

Koalf>

Iscc/ Ipn

Hence the selected CT wi th parameters 400 600 / 1, 5P20, 30VA is adequate.


Therefore a CT of CL. 5P20,15VA is proposed for 400-600/1A and CL. 5P20,30VA isproposed for 400-600/1A.



Therefore a CT of CL. 5P20,15VA is proposed for 300-600/1A and CL. 5P20,30VA isproposed for 300-600/1A.



19/31



4.2.4 CTs for Bus Bar Protection

A CT of 2000/1A, 5P20, 15VA is proposed as calculated in 4.1.3above.

4.3 33kV Bus-section Feeder


CT Ratio:

2000/1A.

Type of Metering:







= 8.429 ohm/km




= 0.08429 x 12

= 0.08429 VA


= 0.1 + 0.08429

= 0.18429 VA





The instrument security factor (ISF) selected is less than Five(5) as per thespecification requirements.

Therefore a CT of 2000/1A, CL. 0.5FS5, 15VA is proposed.

Abstract:

Therefore a CT of CL. 0.5,15VA is proposed for 2000/1A .The CTs are with a factor ofsafety less than or equal to 5.


20/31




CT Ratio:

2000/1A

Type of relay: REJ 525 ( ABB make)






= 0.1 + 0.314

= 0.414 VA





Therefore a CT of 2000/1A, CL. 5P20, 15VA is proposed.



Koalf Iscc/ IpnWhere, Ipn = CT primary nominal current = 2000A




Pn+ Pi

Koalf = Knalfx Pb+ Pi

Where,


Knalf = 20


= V * I

= (I * Rct) * I

= I2* Rct

Rct = 9Ohm (As confirmed by the Manufacturer)


21/31



= 12x 9

= 9 VA


= Pr+ PL

= 0.1 + 0.314

= 0.414 VA

15 + 9

Koalf = 20 x0.414 + 9

= 51

Iscc/ Ipn = 31500 / 2000

= 15.75

Koalf> Iscc/ Ipn

Hence the selected CT with parameters 2000 / 1, 5P20, 15VA is adequate.

Abstract:

Therefore a CT of CL. 5P20, 15VA is proposed for 2000/1A.



A CT of 2000/1A, 5P20, 15VA is proposed as calculated in 4.1.3 above.

4.4 33kV Over Head Line(OHL) Feeder


CT Ratio:

600-1200/1A.

Type of Metering:



The BCU is mounted on the 33kV switchgear.





= 8.429 ohm/km


22/31






= 0.08429 x 12

= 0.08429 VA


= 0.1 + 0.08429

= 0.18429 VA




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 thespecification requirements.

Abstract:



4.4.2 CTs for Distance Protection:

CT Ratio:

600-1200 / 1A.

Type of relay: REL 316*4 (ABB make)



Since the distance relay of type REL 316*4 and transformer differential relay of typeRET 316*4 works on the same principle, the knee point voltage formulae used fortransformer differential relay is applicable for distance relay also.



Where If= fault current

N = Turns Ratio


Rl= Lead resistance

For a Tap of 600A

Maximum fault current carrying capacity of 33 kV switchgear is 31.5 kA which can beconsidered for knee point calculation.

If= 31500 Amps

N = 1/600


23/31



Rct = 3 (As per Manufacturers Recommendations)

The distance relay is mounted on 33kV relay panel which is located in control room.



Resistance of cable per = 4.61Ohms (Refer Ducab cable catalogue)km/phase at 20 deg C

Resistance of cable at = 4.61(1+0.00393 (55-20)) = 5.244 ohm/km55 deg C, consideringtemperature correction factorTotal resistance = 2 x 5.244 x 30 / 1000



= 0.314 x 12

= 0.314 VA


VK= 31500* (1/600) * (3 + 0.314)

VK= 174V

Therefore a CT of 600-1200 /1A, CL. X, VK> 300V is proposed. The choice of usingthe same shall be verified by the following recommendations:

Recommendations from Consultant/Client/Relay Manufacturers :

Further, the CT rated secondary voltage has to satisfy the following conditions:

Condition 1:

Faults close to the relay location

Vk If* (Isn/ Ipn) * a* (Rct+ Rl+ Rr/ Ir2)

Where,

If = Maximum primary fundamental frequency component



Rr = Relay burden


Isn = Secondary Nominal CT current

Ipn = Primary Nominal CT current

a = Time constant for DC component

Kindly refer the Graph (Figure-1) Attached in Annexure 3 for the value of a.

For a 30ms DC Time constant, its value is 4.3 (for system frequency of 50Hz).


24/31



Hence Vk 31500 * (1/600) * 4.3 * (3+ 0.314 + 0.2 / 12)

Vk 793V

We shall proceed with the Condition2 recommendations to confirm the usageof maximum of Condition 1 and Condition 2 knee point values, as theRecommended values for CT.

Condition 2:

Faults at Zone 1 reach

Vk Ifzone1* (Isn/ Ipn) * k * (Rct+ Rl+ Rr/ Ir2)

Where,

Ifzone-1 = Maximum primary fundamental frequency component



Rr = Relay burden




k = Time constant for DC component

Kindly refer the Graph (Figure-2) Attached in Annexure 3 for the value of k.For a 30ms DC Time constant, its value is 7.6 (for system frequency of 50Hz).

Since the fault is at the End of Zone 1.The impedance of the Transmission lineshall come in play, hence the Fault current shall be reduced to a very lowvalue. Hence assuming the fault current is 75% of the Rated fault current ofthe switchgear (the actual fault current shall be much less than this value).The recommendations from Relay manufacturers show that the value of faultcurrent is in the range of 15 20kA only for a Zone 1 fault in a 33kV system.

Hence

I Ifzone-1 = 0.75 x 31500 = 23.625kA

Assuming the same as 24kA for Calculations, we get

Vk 24000* (1/600) * 7.6 * (3+ 0.314 + 0.2 / 12)

Vk 1068V

Since the Value calculated in Condition 2 is Greater than the one proposed by ESIStandard and as calculated for Condition 1,we propose the usage of the value of CTas calculated through Condition 2 as the final Recommended Value.


For a Tap of 1200A

Vk as per ESI Recommendations is:VK= 31500* (1/1200) * (6 + 0.314)


25/31



VK= 165V

Rct = 6Ohm (Assumed)

Therefore a CT of 600-1200 /1A, CL. X, VK> 300V is proposed.

Condition 1:

Faults close to the relay location

Vk If* (Isn/ Ipn) * a* (Rct+ Rl+ Rr/ Ir2)

Where,

If = Maximum primary fundamental frequency component



Rr = Relay burden




a = Time constant for DC component

Kindly refer the Graph (Figure-1) Attached in Annexure 3 for the value of a.For a 30ms DC Time constant, its value is 4.3 (for system frequency of 50Hz).

Hence Vk 31500 * (1/1200) * 4.3 * (6+ 0.314 + 0.2 / 12)

Vk 735V

We shall proceed with the Condition2 recommendations to confirm the usageof maximum of Condition 1 and Condition 2 knee point values, as theRecommended values for CT.

Condition 2:

Faults at Zone 1 reach

Vk Ifzone1* (Isn/ Ipn) * k * (Rct+ Rl+ Rr/ Ir2)

Where,

Ifzone-1 = Maximum primary fundamental frequency component



Rr = Relay burden




k = Time constant for DC component

Kindly refer the Graph (Figure-2) Attached in Annexure 3 for the value of k.

For a 30ms DC Time constant, its value is 7.6 (for system frequency of 50Hz).


26/31



Since the fault is at the End of Zone 1.The impedance of the Transmission lineshall come in play, hence the Fault current shall be reduced to a very lowvalue. Hence assuming the fault current is 75% of the Rated fault current ofthe switchgear (the actual fault current shall be much less than this value).The recommendations from Relay manufacturers show that the value of faultcurrent is in the range of 15 20kA only for a Zone 1 fault in a 33kV system.

Hence

I Ifzone-1 = 0.75 x 31500 = 23.625kA

Assuming the same as 24kA for Calculations, we get

Vk 24000* (1/1200) * 7.6 * (6+ 0.314 + 0.2 / 12)

Vk 990V

Since the Value calculated in Condition 2 is Greater than the one proposed by ESIStandard and as calculated for Condition 1,we propose the usage of the value of CTas calculated through Condition 2 as the final Recommended Value.


Abstract

Therefore a CT of CL. X, Vk >1068V is proposed for 600-1200/1A and CL. X, Vk >990V is proposed for 600-1200/1A.


CT Ratio:

600-1200/1AType of relay: REJ 527 and SPAS348C3 ( ABB make)






= 0.2 + 0.314

= 0.514 VA


Total burden required =1.25 x 0.514 = 0.6425VA

Hence a standard burden rating of 15 VA is chosen for a 600A Tap and a burden of30VA is chosen for a Tap of 1200A.


For a Tap of 600A




27/31



Koalf Iscc/ Ipn






Pb+ Pi

Where,


Knalf = 20


= V * I

= (I * Rct) * I

= I2* Rct


= 12x 3

= 3 VA


= Pr+ PL

= 0.2 + 0.314

= 0.514 VA

15 + 3

Koalf = 20 x0.514 + 3

= 102.44

Iscc/ Ipn = 31500 / 600

= 52.5

Koalf> Iscc/ Ipn


For a Tap of 1200A


Knalf = 20


= V * I

= (I * Rct) * I

= I2* Rct

Rct = 6 Ohm (Assumed)


28/31



= 12x 6

= 6 VA


= Pr+ PL

= 0.2 + 0.314

= 0.514 VA

15 + 6

Koalf = 20 x0.514 + 6

= 65

Iscc/ Ipn = 31500 / 1200

= 27

Koalf> Iscc/ Ipn


Abstract:

Therefore a CT of CL. 5P20, 15VA is proposed for 600-1200/1A and CL. 5P20, 30VAis proposed for 600-1200/1A.



A CT of 2000/1A, 5P20, 15VA is proposed as calculated in 4.1.3 above.

4.4.5 CTs for Sensitive Earth Fault Protection:

CT Ratio:

Ratio is considered as 600-1200/1A

Type of relay: SPAJ 111C ( ABB make)






= 0.1 + 0.314

= 0.414 VA



Hence a standard burden rating of 15 VA is chosen for a Tap of 600A and 30VA ischosen for a Tap of 1200A Tap.



29/31



For a Tap of 600A



Koalf Iscc/ Ipn






Pb+ Pi

Where,Knalf = 20



= V * I

= (I * Rct) * I

= I2* Rct

Rct = 3Ohm (As confirmed by Manufacturer)

= 12x 3

= 3 VA


= Pr+ PL

= 0.1 + 0.314

= 0.414 VA

15 + 3

Koalf = 20 x0.414 + 3

= 105.448Iscc/ Ipn = 31500 / 600

= 52.5

Koalf> Iscc/ Ipn

Hence the selected CT with parameters 600 - 1200 / 1, 5P20, 15VA is adequate.

For a Tap of 1200A


Knalf = 20



30/31



= V * I

= (I * Rct) * I

= I2* Rct


= 12x 6

= 6 VA


= Pr+ PL

= 0.1 + 0.314

= 0.414 VA

15 + 6

Koalf = 20 x0.414 + 6

= 65

Iscc/ Ipn = 31500 / 1200

= 27

Koalf> Iscc/ Ipn

Hence the selected CT with parameters 600 1200 / 1, 5P20, 30VA is adequate

Abstract:

Therefore a CT of CL. 5P20, 15VA is proposed for 600-1200/1A and CL. 5P20, 30VA

is proposed for 600-1200/1A.


5.0 VT SIZING CALCULATIONS

The VTs considered are as follows:

Line VT:

The devices connected on winding 1of line VTs are BCU, synchronising devices andVoltage Transducer.

The burden of the same are as given below:

BCU: 0.25VA 2VA Maximum ConsideredVoltage Transducers: 2VA max. 2VA Maximum ConsideredSynchronising devices: 25VA 25VA Maximum Considered

Total 29 VA max.

The devices connected on winding 2of line VTs are various relays. The maximumburden for the same is 10VA.

The devices connected on winding 3of line VTs are Directional O/C relay.


31/31



Directional O/C relay: 0.2VA 2 VA Maximum Considered(Only for OHL lines)

Hence the VT considered for line is of fo llowing parameters:

33kV/

3 / 110V /

3 / 110V / 3 / 110V / 3 Winding 1 - 50VA , 0.5SWinding 2 - 50VA , 3PWinding 3 - 50VA , 3P

Bus VT

The devices connected on winding 1of line VTs are BCU, synchronising devices andVoltage Transducer.


BCU: 0.25 VA 2VA Maximum ConsideredVoltage Transducers: 2 VA 2VA Maximum ConsideredSynchronising devices: 25 VA 25VA Maximum Considered

Total 29 VA max.

The devices connected on winding 2of line VTs are various relays. The maximumburden for the same is 10VA.

Hence to standardise the parameters of VT, following parameters areconsidered.

33kV/

3 / 110V /

3 / 110V / 3 / 110V / 3 Winding 1 - 50VA , 0.5SWinding 2 - 50VA , 3PWinding 3 - 50VA , 3P

6.0 CONCLUSION

The CT/VT parameters recommended are as calculated above. The Manufacturerhas confirmed the values at the Principle/Usage Tap.

7.0 SUMMARY OF CURRENT TRANSFORMERS

The Summary of the Current transformers for all the sub-stations is given inAnnexure 1.

8.0 SUMMARY OF VOLTAGE TRANSFORMERS

The Summary of the Voltage transformers for all the sub-stations is given inAnnexure 2.

33kv Ct Calculations

Documents

Transcript of 33kv Ct Calculations