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CT Sizing for Electrical Engineers

This document summarizes the calculation of CT sizing for a 500/1A 15VA CT used with a SIEMENS 7SD52 relay for line differential protection on a typical 33kV feeder. It lists the system and CT parameters, relay data, and calculates the total effective CT burden of 0.22VA. It then checks that the CT meets the sizing requirement that the operating accuracy limiting factor Kssc must be greater than or equal to the rated transient dimensioning factor Ktd multiplied by the maximum symmetrical short-circuit current divided by the CT primary nominal current. The calculated Kssc of 67.6 meets this requirement.

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1K views2 pages

CT Sizing for Electrical Engineers

This document summarizes the calculation of CT sizing for a 500/1A 15VA CT used with a SIEMENS 7SD52 relay for line differential protection on a typical 33kV feeder. It lists the system and CT parameters, relay data, and calculates the total effective CT burden of 0.22VA. It then checks that the CT meets the sizing requirement that the operating accuracy limiting factor Kssc must be greater than or equal to the rated transient dimensioning factor Ktd multiplied by the maximum symmetrical short-circuit current divided by the CT primary nominal current. The calculated Kssc of 67.6 meets this requirement.

Uploaded by

swagata12345
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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CT Sizing Calculation

Feeder Type :

CT used :

Typical Feeder/Incomer with Line Differential protection

500/1A 15VA, 5P20 ,Feeder Diff and OC Protn -with SIEMENS 7SD52

Calculation
SYSTEM PARAMETERS:
Norminal voltage (UN) :

33.0 kV

Norminal frequency (fN) :

50.0 Hz

Rated max. symmetrical short circuit current (ISC) :

31.5 kA

CT PARAMETERS:
CT Type :
Primary norminal current (Ipn) :

IEC Class 5P
500 A
Norminal accuracy limit factor (Knalf) :
20

Norminal burden (Pn) :


RELAY DATA :
Manufacturer:
SIEMENS

Type:
7SD52

Secondary norminal current (Isn) :

15 VA

Norminal current (In):


1A

1A

Internal resistance (Rct) :

6.00

Internal burden (Pct) :


Knee Voltage achieved (Vk'>=)

6.0 VA
375 V

Relay burden (Prelay):


0.05 VA

CALCULATION OF TOTAL CT BURDEN :


The ct behaviour depends on the total connected burden on secondary side.
The total burden consists of the relay self-burden, additional burden of further connected relays and equipment and the wire
burden:
Relay burden (Prelay) :

0.05 VA

Additional relay burden (Padd) :

VA

Wire burden :
The effective wire burden will be calculated for a two wire connection of ct and relay according to length, cross-section, copper
resistance and sec. norminal current:
Cross section (awire) :
Resistance of wire at 20 C, (Rw20) =
Resistance of wire at 50 C, (Rw50) =
Length (lwire) :

P wire = k wire x l wire x I sn 2 x

Rw50

2.50
0.00741
0.00828
10
=

mm2
Ohm/m
Ohm/m
m

50
Operating Temperature C =
Resistance Rw50 =
Rw20 (1+Tdiff)
= Numerical temperature co-efficient for Cu (0.00393)
2
Eff. Wire runs in p.u. (kwire) :

0.17 VA

The total effective burden is given by the sum of all connected burden :
Ptotal
= Prelay + Padd + Pwire
=
0.22 VA

CT Sizing calculation _Protection_21.12.2010_Rev0

CT Sizing Calculation

CT REQUIREMENTS FOR 7SD61


acc. to SIEMENS Numerical Protection Relays Catalog

Kssc = K oalf K td x I scc / I pn


where

Ipn :

(Requirement 1)

CT primary norminal current

Iscc :

Max. outflowing short-circuit current for external faults


Koalf : Operating accuracy limiting factor
Koalf (=K'SSC)
Ktd : Rated transient dimensioning factor

500 A

31.50 kA

1.2

CHECK OF CT REQUIREMENTS :
- Operating accuracy limiting factor Koalf :
The CT behaviour acc. to IEC-P will be described by the operating accuracy limiting factor K oalf. It depends on the
nominal accuracy limiting factor Knalf, the nominal burden Pn, the internal CT burden Pct and the total effective connected
burden Ptotal :

K oalf = K nalf

P n + P ct
P total + P ct

67.6

where

Knalf

: Norminal accuracy limiting factor

20.00

Pn
Pct
Ptotal

: Norminal ct burden
: Internal ct burden
: Total connected effective burden

=
=
=

15.00 VA
6.00 VA
0.22 VA

CT-DIMENSIONING CHECK :
- Check of requirement 1 :

Kssc = K oalf K td x I scc / I pn


Koalf (=K'SSC)
Iscc/Ipn

=
=

67.6
63.0

The value of Issc must be considered with the actual parameters of the devices that are connected.
In this case the protection limit of the relay will be 20x the input and hence this must be the limit to set the Iscc/Ipn
The relay settings shall be done according to this principle. This means that the value of Inst S/C that will be set
will approximately lie in the range of 20 times Ipn and not more than this value.

K td x I scc / I pn =

1.2

24.0

20.0
Meets requirement 1

Hence Kssc K td x I scc / I pn

CT Sizing calculation _Protection_21.12.2010_Rev0

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